CO2 Derivatives of Molecular Tin Compounds. Part 2: Carbamato, Formato, Phosphinoformato and Metallocarboxylato Complexes

Single-crystal X-ray diffraction structures of organotin compounds bearing hemicarbonate and carbonate ligands were recently reviewed by us—“CO2 Derivatives of Molecular Tin Compounds. Part 1: Hemicarbonato and Carbonato Complexes”, Inorganics 2020, 8, 31—based on crystallographic data available from the Cambridge Structural Database. Interestingly, this first collection revealed that most of the compounds listed were isolated in the context of studies devoted to the reactivity of tin precursors towards carbon dioxide, at atmospheric pressure or under pressure, thus highlighting the suitable disposition of Sn to fix CO2. In the frame of a second part, the present review carries on to explore CO2 derivatives of molecular tin compounds by describing successively the complexes with carbamato, formato, and phosphinoformato ligands, and obtained from insertion reactions of carbon dioxide into Sn–X bonds (X = N, H, P, respectively). The last chapter is devoted to X-ray structures of transition metal/tin CO2 complexes exhibiting metallocarboxylato ligands. As in Part 1, for each tin compound reported and when described in the original study, the structural descriptions are supplemented by synthetic conditions and spectroscopic data.


Introduction
More than ever and everywhere in the world, the recovery and utilization of carbon dioxide have become a real priority, mobilizing many actors, from academic research to industry, involved in various domains and specialties. Long considered simply as abundant and chemically inert waste, but with devastating effects, carbon dioxide is now differently viewed by chemists, considered and accepted as a promising C1 building block, useful for a wide range of reactions and leading to a wide diversity of organic compounds. In this quest to transform CO 2 into chemicals with higher added value, coordination and organometallic chemistry played-and continue to play-an important and determining role in facilitating its activation [1]. Since the first resolution of an X-ray crystal structure highlighted the metal coordination of a CO 2 molecule-it was in 1975 that M. Aresta's group published the characterization of a (carbon dioxide)bis(tricyclohexylphosphine) nickel complex [2]-advances and knowledges relative to carbon dioxide binding modes have continued to progress, and have proved particularly beneficial in catalysis and organic synthesis. Much later in 2019, for instance, Galindo et al. reported the X-ray structures of cis-M(C 2 H 4 ) 2 (CO 2 )(PNP) compounds (M = Mo and W; PNP = 2,6-bis(diphenylphosphinomethyl)pyridine), which are the first examples of stable metal complexes with coordinated ligands of ethylene and carbon dioxide [3]. In conclusion of this recent work, the authors aim, as perspectives, for the conversion of C 2 H 4 and CO 2 into acrylate derivatives.
Since the 1990s, the chemical transformation of CO 2 and its coordination on metal centers have been regularly reviewed and updated [4][5][6][7][8][9][10][11][12][13][14]. During the last decade, the reactivity of main group elements, and in particular metals and metalloids from group 14 (Si, Since the 1990s, the chemical transformation of CO2 and its coordination on metal centers have been regularly reviewed and updated [4][5][6][7][8][9][10][11][12][13][14]. During the last decade, the reactivity of main group elements, and in particular metals and metalloids from group 14 (Si, Ge, Sn, Pb), toward small molecules also aroused a growing interest [15,16]. In this context and based on our previous work in this field, we have recently reviewed molecular organotin compounds bearing hemi-carbonate and carbonate ligands and which result from the reactivity of tin precursors towards carbon dioxide [1]. In this second part, we continue to explore from a structural point of view the molecular derivatives of tin which result from the insertion of carbon dioxide into the Sn-X bonds (X = N, H, P). Thus, the formation and structures of carbamato, formato and phosphinoformato tin derivatives are successively described and discussed. The last chapter is devoted to metallocarboxylato tin complexes exhibiting a bridging CO2 ligand. Although these latter compounds are not derived from the direct reactivity of tin precursors with carbon dioxide, we found appropriate to include them in this review. Regarding the general method used to carry out this inventory, the compounds described below were identified from the data available on the online portal of the Cambridge Structural Database (CSD) web interface (2017) [17]. In addition to the structural description, comparison tables including selections of structural and spectroscopic data are also displayed at the end of each section (Tables 1-8).
Among the useful chemicals with higher added-value which can be produced by the transformation of CO2, the derivatives involving C-N bonds occupy an important place, historically and economically. Thus, from 1922, the German Bosch-Meiser process allowed the industrial use of carbon dioxide (together with ammonia) as raw material for the synthesis of urea [(NH2)2CO]. It is still today the most important industrial chemical process in terms of CO2 amounts converted (115 Mt) [18]. Since then, the use of carbon dioxide as C1 synthon has been extended to the synthesis of other types of nitrogen compounds such as oxazolidinones, quinazolinediones, ureas, carbamates, isocyanates and polyurethanes (Scheme 1). These molecules constitute intermediates and derivatives of great importance for the chemical, agrochemical and pharmaceutical industry. The domain was firstly reviewed by He et al. in 2012 [19], and recently updated by Song et al. [20] and then by Dalpazzo et al. [21], both in 2019. In most cases, the intervention of a catalytic precursor-organic or organometallic, is required to achieve the targeted product. Scheme 1. Molecular representations of nitrogen-containing molecules accessible from CO2 used as a C1 building block.

Tin-Promoted Insertion Reaction of CO2 into C-N Bonds
Regarding more specifically the use of main group metals as catalytic species for the insertion of CO2 into the C-N bond, the beneficial role of organotin compounds was regularly emphasized in the past, being the subject of several studies. In 2002, Tominaga and Sasaki found that di-n-butyltin oxide, n-Bu2SnO, was an effective catalyst for the dehydrating condensation of 1,2-aminoethanols with carbon dioxide to lead to the formation of 2-oxazolidinones [22] (Scheme 2). Reactions were carried out under 5 MPa of CO2, at 180 °C, for 16 h, in N-methyl-2-pyrrolidone (NMP) as solvent and gave satisfactory yields in the range of 53 to 94%. The use of NMP has been found to be more effective than the use of protic and non-polar solvents. From a mechanistic point of view, the authors proposed the formation of a seven-membered cyclic tin carbamate complex (Scheme 3) by the Scheme 1. Molecular representations of nitrogen-containing molecules accessible from CO 2 used as a C1 building block.

Tin-Promoted Insertion Reaction of CO 2 into C-N Bonds
Regarding more specifically the use of main group metals as catalytic species for the insertion of CO 2 into the C-N bond, the beneficial role of organotin compounds was regularly emphasized in the past, being the subject of several studies. In 2002, Tominaga and Sasaki found that di-n-butyltin oxide, n-Bu 2 SnO, was an effective catalyst for the dehydrating condensation of 1,2-aminoethanols with carbon dioxide to lead to the formation of 2-oxazolidinones [22] (Scheme 2). Reactions were carried out under 5 MPa of CO 2 , at 180 • C, for 16 h, in N-methyl-2-pyrrolidone (NMP) as solvent and gave satisfactory yields in the range of 53 to 94%. The use of NMP has been found to be more effective than the use of protic and non-polar solvents. From a mechanistic point of view, the authors proposed the formation of a seven-membered cyclic tin carbamate complex (Scheme 3) by the insertion of CO 2 into the Sn-N bond of a stannaoxazolidine species and whose in-situ formation was supported by electrospray mass spectrometry. However, to our knowledge, such a species has not been yet structurally isolated. From this intermediate, In 2013, Ghosh et al. revisited the previous reaction using chlorostannoxane derivatives, 1,3-dichloro-1,1,3,3-tetraalkyldistannoxanes (Scheme 4), as catalytic precursor, in methanol under 1.72 MPa of CO 2 pressure and at 150 • C [23]. Chlorostannoxanes were selected because of (i) the presence of multi-active catalytic centers, and (ii) the ability to control Lewis acidity by changing the substituents on the metal centers. In comparison to the previous Tominaga's study using n-Bu 2 SnO [22], an improvement of turnover numbers was recorded using chlorostannoxanes, from 9 to 138. Furthermore, it has been demonstrated that the activity of the catalytic precursors depends on the nature of the R and R substituents, the n-butyl derivative (R and R' = n-Bu) being the most efficient.
such a species has not been yet structurally isolated. From this intermediate, the i lecular elimination of one molecule of 2-oxazolidinone was suggested to explain generation of the starting complex, n-Bu2SnO. Scheme 2. Synthesis of 2-oxazolidinones from CO2 and 1,2-aminoalcohols catalyzed by ndata from [22]. Scheme 3. Key step requiring the insertion of CO2 into the Sn-N bond of the stannaoxazo species formed in situ, data from [22].
In 2013, Ghosh et al. revisited the previous reaction using chlorostannoxane tives, 1,3-dichloro-1,1,3,3-tetraalkyldistannoxanes (Scheme 4), as catalytic precu methanol under 1.72 MPa of CO2 pressure and at 150 °C [23]. Chlorostannoxan selected because of (i) the presence of multi-active catalytic centers, and (ii) the a control Lewis acidity by changing the substituents on the metal centers. In compa the previous Tominaga's study using n-Bu2SnO [22], an improvement of turnov bers was recorded using chlorostannoxanes, from 9 to 138. Furthermore, it h demonstrated that the activity of the catalytic precursors depends on the nature and R′ substituents, the n-butyl derivative (R and R' = n-Bu) being the most effici Scheme 4. Molecular representation of chlorostannoxane compounds used as tin precurso the synthesis of 2-oxazolidinones by direct condensation of 2-aminoalcohols with carbon d (R = n-Bu, Ph; R′ = n-Bu, Ph), data from [23].
The direct synthesis of carbamates from carbon dioxide has also aroused r interest in the past two decades [24]. The state of the art was well established in Calderazzo et al. [25] and recently updated by Marchetti et al. [26]. Indeed, car can be viewed as a potential alternative to access to isocyanates, which are his employed for the production of polyurethanes (PU). Industrially, isocyanates duced by reacting primary amines with phosgene, but handling of this reactan Scheme 4. Molecular representation of chlorostannoxane compounds used as tin precursors for the synthesis of 2-oxazolidinones by direct condensation of 2-aminoalcohols with carbon dioxide (R = n-Bu, Ph; R = n-Bu, Ph), data from [23].
The direct synthesis of carbamates from carbon dioxide has also aroused renewed interest in the past two decades [24]. The state of the art was well established in 2003 by Calderazzo et al. [25] and recently updated by Marchetti et al. [26]. Indeed, carbamates can be viewed as a potential alternative to access to isocyanates, which are historically employed for the production of polyurethanes (PU). Industrially, isocyanates are produced by reacting primary amines with phosgene, but handling of this reactant is not without risks and requires drastic precautions. Therefore, the recent route based on the thermal decomposition of carbamates into isocyanates can be considered as a safer and greener synthetic pathway [27,28].
In the past, several groups investigated the action of organotins as catalyst precursors for the direct conversion of CO 2 into carbamates. In 2001, Sakakura et al. first published the synthesis of carbamates by reaction of amines and alcohols catalyzed by tin complexes (n-Bu 2 SnO and Me 2 SnCl 2 ) [29] (Scheme 5). Optimal reaction conditions require a pressure of 30 MPa (300 bar) at 200 • C for 24 h. Moreover, the addition of acetals (2,2-diethoxy-or 2,2dimethoxypropane) acting as a dehydrating agent significantly improves the conversion, and the increase in pressure promotes the urethane selectivity.
Inorganics 2021, 9, x FOR PEER REVIEW 4 of 33 without risks and requires drastic precautions. Therefore, the recent route based on the thermal decomposition of carbamates into isocyanates can be considered as a safer and greener synthetic pathway [27,28].
In the past, several groups investigated the action of organotins as catalyst precursors for the direct conversion of CO2 into carbamates. In 2001, Sakakura et al. first published the synthesis of carbamates by reaction of amines and alcohols catalyzed by tin complexes (n-Bu2SnO and Me2SnCl2) [29] (Scheme 5). Optimal reaction conditions require a pressure of 30 MPa (300 bar) at 200 °C for 24 h. Moreover, the addition of acetals (2,2-diethoxy-or 2,2-dimethoxypropane) acting as a dehydrating agent significantly improves the conversion, and the increase in pressure promotes the urethane selectivity.

Scheme 5.
Halogen-free process for the conversion of carbon dioxide to carbamates, data from [29].
Concomitantly and supported by density functional theory (DFT) calculations, Schaub et al. confirmed that dialkyltin(IV) dialkoxides can be used as reagents for the synthesis of aromatic carbamates from aromatic amines and carbon dioxide [32]. The process takes place between 60 and 70 bar of pressure, at 135 °C and in pentane as solvent. A NMR yield of 92% was recorded for the synthesis of methyl phenylcarbamate from aniline using three molar equivalent of n-Bu2Sn(OMe)2. Neither methylaniline nor diphenylurea was chromatographically detected. Extension of the reaction conditions to the synthesis of diurethanes from 4,4′-methylenedianiline and 2,4-diaminotoluene was successful with yields of 60 and 77%, respectively. In the perspective of an industrial development, the authors have demonstrated the possibility of regenerating and re-using the dialkyltin(IV) dialkoxides, by using an excess of dimethylcarbonate. Shortly after, in a new study, the same group improved the reaction yield (up to 97%) by combining with dialkyltin oxide, Scheme 5. Halogen-free process for the conversion of carbon dioxide to carbamates, data from [29].
In 2016, Choi et al. reported the synthesis of N-phenylcarbamates from CO 2 and aniline promoted by several di-n-butyltin dialkoxides: n-Bu 2 Sn(OMe) 2 , n-Bu 2 Sn(OEt) 2 , n-Bu 2 Sn(OPr-n) 2 , and n-Bu 2 Sn(OBu-n) 2 (Scheme 6) [30]. The best yield of N-phenylcarbamate (41%) was obtained under 5 MPa (5 bar) of CO 2 for 20 min at 150 • C using n-Bu 2 Sn(OMe) 2 as tin precursor. The catalytic activity changes depending on the nature of the alkoxide substituents linked to the tin atom [n-Bu 2 Sn(OMe) 2 > n-Bu 2 Sn(OPr-n) 2 > n-Bu 2 Sn(OEt) 2 > n-Bu 2 Sn(OBu-n) 2 ]. N,N -diphenylurea is formed as byproduct (4%). From a mechanistic point of view, the hemicarbonato tin complex, n-Bu 2 Sn(OMe)(OCO 2 Me), resulting from the reactivity of CO 2 with n-Bu 2 Sn(OMe) 2 [1,31], is claimed as the key intermediate of the reaction. In the case of reactions involving n-Bu 2 Sn(OBu-n) 2 , the authors also showed that the starting tin complex could be regenerated after the first catalytic run (by reacting with n-BuOH) and could thus be recycled without loss of activity.
Inorganics 2021, 9, x FOR PEER REVIEW 4 of 33 without risks and requires drastic precautions. Therefore, the recent route based on the thermal decomposition of carbamates into isocyanates can be considered as a safer and greener synthetic pathway [27,28]. In the past, several groups investigated the action of organotins as catalyst precursors for the direct conversion of CO2 into carbamates. In 2001, Sakakura et al. first published the synthesis of carbamates by reaction of amines and alcohols catalyzed by tin complexes (n-Bu2SnO and Me2SnCl2) [29] (Scheme 5). Optimal reaction conditions require a pressure of 30 MPa (300 bar) at 200 °C for 24 h. Moreover, the addition of acetals (2,2-diethoxy-or 2,2-dimethoxypropane) acting as a dehydrating agent significantly improves the conversion, and the increase in pressure promotes the urethane selectivity.

Scheme 5.
Halogen-free process for the conversion of carbon dioxide to carbamates, data from [29].
Concomitantly and supported by density functional theory (DFT) calculations, Schaub et al. confirmed that dialkyltin(IV) dialkoxides can be used as reagents for the synthesis of aromatic carbamates from aromatic amines and carbon dioxide [32]. The process takes place between 60 and 70 bar of pressure, at 135 °C and in pentane as solvent. A NMR yield of 92% was recorded for the synthesis of methyl phenylcarbamate from aniline using three molar equivalent of n-Bu2Sn(OMe)2. Neither methylaniline nor diphenylurea was chromatographically detected. Extension of the reaction conditions to the synthesis of diurethanes from 4,4′-methylenedianiline and 2,4-diaminotoluene was successful with yields of 60 and 77%, respectively. In the perspective of an industrial development, the authors have demonstrated the possibility of regenerating and re-using the dialkyltin(IV) dialkoxides, by using an excess of dimethylcarbonate. Shortly after, in a new study, the same group improved the reaction yield (up to 97%) by combining with dialkyltin oxide, Scheme 6. Tin-promoted synthesis of N-phenyl carbamate from aniline and carbon dioxide (R = Me, Et, n-Pr, and n-Bu), data from [30].
Concomitantly and supported by density functional theory (DFT) calculations, Schaub et al. confirmed that dialkyltin(IV) dialkoxides can be used as reagents for the synthesis of aromatic carbamates from aromatic amines and carbon dioxide [32]. The process takes place between 60 and 70 bar of pressure, at 135 • C and in pentane as solvent. A NMR yield of 92% was recorded for the synthesis of methyl phenylcarbamate from aniline using three molar equivalent of n-Bu 2 Sn(OMe) 2 . Neither methylaniline nor diphenylurea was chromatographically detected. Extension of the reaction conditions to the synthesis of diurethanes from 4,4 -methylenedianiline and 2,4-diaminotoluene was successful with yields of 60 and 77%, respectively. In the perspective of an industrial development, the authors have demonstrated the possibility of regenerating and re-using the dialkyltin(IV) dialkoxides, by using an excess of dimethylcarbonate. Shortly after, in a new study, the same group improved the reaction yield (up to 97%) by combining with dialkyltin oxide, used as a tin precursor, tetraalkyl orthosilicates, which promote the generation of dialkyltin(IV) dialkox-ides [33]. This positive effect had been initially shown for the tin-promoted synthesis of organic carbonates from carbon dioxide and alcohols [34].

Early Works
Historically, the pioneering works on carbamates of tin derivatives dates back to the 1960s and are to be credited to A. J. Bloodworth and A. G. Davies who conducted extensive and fundamental investigations applying a systematic approach. From 1965, they described the synthesis of N-organo-N-trialkylstannylcarbamates resulting from the addition of trialkyltin alkoxides to isocyanates [35]. The reactions were reported as rapid and exothermic and taken place at room temperature (Scheme 7). Alkyl and aryl isocyanates exhibit a comparable reactivity. used as a tin precursor, tetraalkyl orthosilicates, which promote the generation of dialkyltin(IV) dialkoxides [33]. This positive effect had been initially shown for the tin-promoted synthesis of organic carbonates from carbon dioxide and alcohols [34].

Early Works
Historically, the pioneering works on carbamates of tin derivatives dates back to the 1960s and are to be credited to A. J. Bloodworth and A. G. Davies who conducted extensive and fundamental investigations applying a systematic approach. From 1965, they described the synthesis of N-organo-N-trialkylstannylcarbamates resulting from the addition of trialkyltin alkoxides to isocyanates [35]. The reactions were reported as rapid and exothermic and taken place at room temperature (Scheme 7). Alkyl and aryl isocyanates exhibit a comparable reactivity. Scheme 7. Formation of organo ester of N-organo-N-trialkylstannylcarbamic acid from trialkyltin alkoxides and isocyanates, data from [35].
From a coordination point of view, the authors preferentially recommended the representation of the carbamato ligand as N-coordinated to tin, in amido form, but did not exclude an O-coordination, in imido form, via an interconversion process. Subsequently, this structural consideration was the subject of several subsequent works, involving other research groups, and based on studies carried out by 119 Sn Mössbauer spectroscopy [37], as well as more recently by computational calculations [38].

Reactivity
In terms of reactivity, N-organo-N-trialkylstannylcarbamates have been also involved in various organic reactions. In their initial article [35], Bloodworth and Davies showed in particular their reactivity towards acetic anhydride, ethylamine and ethanol, which lead, according to a metathetical process, to the corresponding urethanes. In 1968, the same authors reported the decarboxylation of trialkyltin esters of N-organo-N-trialkylstannylcarbamic acids by isocyanates and isothiocyanates providing an alternative route to the synthesis of carbodi-imides [39]. In 1992 and then in 1993, Shibata et al. un-Scheme 7. Formation of organo ester of N-organo-N-trialkylstannylcarbamic acid from trialkyltin alkoxides and isocyanates, data from [35].
Inorganics 2021, 9, x FOR PEER REVIEW 5 of 33 used as a tin precursor, tetraalkyl orthosilicates, which promote the generation of dialkyltin(IV) dialkoxides [33]. This positive effect had been initially shown for the tin-promoted synthesis of organic carbonates from carbon dioxide and alcohols [34].

Early Works
Historically, the pioneering works on carbamates of tin derivatives dates back to the 1960s and are to be credited to A. J. Bloodworth and A. G. Davies who conducted extensive and fundamental investigations applying a systematic approach. From 1965, they described the synthesis of N-organo-N-trialkylstannylcarbamates resulting from the addition of trialkyltin alkoxides to isocyanates [35]. The reactions were reported as rapid and exothermic and taken place at room temperature (Scheme 7). Alkyl and aryl isocyanates exhibit a comparable reactivity. Scheme 7. Formation of organo ester of N-organo-N-trialkylstannylcarbamic acid from trialkyltin alkoxides and isocyanates, data from [35].
From a coordination point of view, the authors preferentially recommended the representation of the carbamato ligand as N-coordinated to tin, in amido form, but did not exclude an O-coordination, in imido form, via an interconversion process. Subsequently, this structural consideration was the subject of several subsequent works, involving other research groups, and based on studies carried out by 119 Sn Mössbauer spectroscopy [37], as well as more recently by computational calculations [38].

Reactivity
In terms of reactivity, N-organo-N-trialkylstannylcarbamates have been also involved in various organic reactions. In their initial article [35], Bloodworth and Davies showed in particular their reactivity towards acetic anhydride, ethylamine and ethanol, which lead, according to a metathetical process, to the corresponding urethanes. In 1968, the same authors reported the decarboxylation of trialkyltin esters of N-organo-N-trialkylstannylcarbamic acids by isocyanates and isothiocyanates providing an alternative route to the synthesis of carbodi-imides [39]. In 1992 and then in 1993, Shibata et al. un-Scheme 8. Formation of trialkyltin ester of N-organo-N-trialkylstannylcarbamic acid from bis(trialkyltin) oxides and isocyanates, data from [36].
From a coordination point of view, the authors preferentially recommended the representation of the carbamato ligand as N-coordinated to tin, in amido form, but did not exclude an O-coordination, in imido form, via an interconversion process. Subsequently, this structural consideration was the subject of several subsequent works, involving other research groups, and based on studies carried out by 119 Sn Mössbauer spectroscopy [37], as well as more recently by computational calculations [38].

Reactivity
In terms of reactivity, N-organo-N-trialkylstannylcarbamates have been also involved in various organic reactions. In their initial article [35], Bloodworth and Davies showed in particular their reactivity towards acetic anhydride, ethylamine and ethanol, which lead, according to a metathetical process, to the corresponding urethanes. In 1968, the same authors reported the decarboxylation of trialkyltin esters of N-organo-N-trialkylstannylcarbamic acids by isocyanates and isothiocyanates providing an alternative route to the synthesis of carbodi-imides [39]. In 1992 and then in 1993, Shibata et al. underlined the positive and efficient role of methyl ester of N-ethyl-N-tributylstannylcarbamic acid as promoter for the Darzens reaction [40] and the addition of Michael [41] (Scheme 9). Inorganics 2021, 9, x FOR PEER REVIEW 6 of 33 derlined the positive and efficient role of methyl ester of N-ethyl-N-tributylstannylcarbamic acid as promoter for the Darzens reaction [40] and the addition of Michael [41] (Scheme 9).

X-ray Crystal Structures
Regarding the solid-state characterization of metal carbamates by single-crystal Xray diffraction, the area was first reviewed by Calderazzo et al. [25] and then recently updated by Marchetti et al. [26]. To date, numerous examples of metal carbamates have thus been isolated and characterized, demonstrating a rich coordination chemistry and highlighting a great diversity of structures. Herein, we will focus exclusively on X-ray structures involving tin moieties. To the best of our knowledge, six CSD depositions of carbamato tin complexes have been identified up to now (CSD entries and deposition numbers are reported in Table 1).
The first X-ray crystallographic investigation on a tin complex bearing a carbamato ligand was reported by Zakharov et al. in 1980, as part of studies on organotin isocyanate derivatives [42]. Thus, the Russian group characterized at the solid-state the structure of the trimethyltin(IV) ester of N-trimethylstannylcarbamic acids, Me3SnNHC(=O)-OSnMe3 (1), which describes a polymeric arrangement displaying a zigzag one-dimensional infinite chain along the c-axis ( Figure 1). The organization of 1 consists of SnMe3 moieties bridged by carbamate dianions. Two distinct sites of tin atoms are distinguished: (i) those located in the main chain exhibiting a trigonal bipyramidal geometry (TBP) whose equatorial plane is occupied by three methyl substituents and the apical positions by two oxygen atoms; (ii) SnMe3 pendant groups, connected to the main chain by nitrogen atoms (Sn-N = 2.04(2) Å) and positioned in a syndiotactic arrangement, and whose tin atoms exhibit a tetrahedral geometry. This type of architecture is comparable to the chain-like structures reported for the organotin(IV) selenite complexes, (Me3Sn)2SeO3·H2O and (Ph3Sn)2SeO3 [43], as well as for the organotin(IV) carbonato complexes, (Me3Sn)2CO3 and (i-Bu3Sn)2CO3 [44]. However, the authors pointed out that, despite the presence of a hydrogen atom on the nitrogen of carbamato groups, no hydrogen bonds are observed between the chains, which is explained by the proximity of bulky Me3Sn groups. Moreover, we consider that compound 1 is the only example of an N-coordinated tin carbamate, structurally characterized to date.

X-Ray Crystal Structures
Regarding the solid-state characterization of metal carbamates by single-crystal X-ray diffraction, the area was first reviewed by Calderazzo et al. [25] and then recently updated by Marchetti et al. [26]. To date, numerous examples of metal carbamates have thus been isolated and characterized, demonstrating a rich coordination chemistry and highlighting a great diversity of structures. Herein, we will focus exclusively on X-ray structures involving tin moieties. To the best of our knowledge, six CSD depositions of carbamato tin complexes have been identified up to now (CSD entries and deposition numbers are reported in Table 1).
The first X-ray crystallographic investigation on a tin complex bearing a carbamato ligand was reported by Zakharov et al. in 1980, as part of studies on organotin isocyanate derivatives [42]. Thus, the Russian group characterized at the solid-state the structure of the trimethyltin(IV) ester of N-trimethylstannylcarbamic acids, Me 3 SnNHC(=O)-OSnMe 3 (1), which describes a polymeric arrangement displaying a zigzag one-dimensional infinite chain along the c-axis (Figure 1). The organization of 1 consists of SnMe 3 moieties bridged by carbamate dianions. Two distinct sites of tin atoms are distinguished: (i) those located in the main chain exhibiting a trigonal bipyramidal geometry (TBP) whose equatorial plane is occupied by three methyl substituents and the apical positions by two oxygen atoms; (ii) SnMe 3 pendant groups, connected to the main chain by nitrogen atoms (Sn-N = 2.04(2) Å) and positioned in a syndiotactic arrangement, and whose tin atoms exhibit a tetrahedral geometry. This type of architecture is comparable to the chain-like structures reported for the organotin(IV) selenite complexes, (Me 3 Sn) 2 SeO 3 ·H 2 O and (Ph 3 Sn) 2 SeO 3 [43], as well as for the organotin(IV) carbonato complexes, (Me 3 Sn) 2 CO 3 and (i-Bu 3 Sn) 2 CO 3 [44]. However, the authors pointed out that, despite the presence of a hydrogen atom on the nitrogen of carbamato groups, no hydrogen bonds are observed between the chains, which is explained by the proximity of bulky Me 3 Sn groups. Moreover, we consider that compound 1 is the only example of an N-coordinated tin carbamate, structurally characterized to date.
Thereafter, with the aim of applications in heterogeneous catalysis, Calderazzo et al. reported the preparation of two N,N-dialkylcarbamato tin(IV) complexes, Sn(O 2 CNi-Pr 2 ) 4 (2) and Sn(O 2 CNEt 2 ) 4 (3), which were used as precursors for the chemical implantation of metal cations on a silica support [45]. The grafting method consisted of promoting reactivity of N,N-dialkylcarbamates with silanol groups of silica. Compounds 2 and 3 were synthesized by treating anhydrous Sn(IV) chloride with diethylamine and di-isopropylamine, respectively, in toluene at room temperature, under atmospheric pressure of carbon dioxide (Scheme 10). Yields higher than 90% are reported. Inorganics 2021, 9, x FOR PEER REVIEW 7 of 33 (2) and Sn(O2CNEt2)4 (3), which were used as precursors for the chemical implantation of metal cations on a silica support [45]. The grafting method consisted of promoting reactivity of N,N-dialkylcarbamates with silanol groups of silica. Compounds 2 and 3 were synthesized by treating anhydrous Sn(IV) chloride with diethylamine and di-iso-propylamine, respectively, in toluene at room temperature, under atmospheric pressure of carbon dioxide (Scheme 10). Yields higher than 90% are reported. Scheme 10. Reaction scheme leading to Compounds 2 and 3, data from [45].
Recrystallized from heptane, suitable single crystals of 2 were analyzed by X-ray diffraction leading to the resolution of the crystallographic structure. In 2002, in the frame of metal-organic chemical vapor deposition (MOCVD) investigations, Molloy et al. described a new synthetic route to 3, starting from a solution of tetrakis(N,N-diethylamino)tin(IV) complex in hexane that was bubbled with carbon dioxide (Scheme 11) [46]. Scheme 11. Alternative route leading to compound 3, data from [46]. X-ray structures of 2 and 3 revealed comparable mononuclear molecules. In both compounds, the tin atom is eight-coordinated by four chelating N,N-dialkylcarbamato ligands, which are exclusively O-donor ( Figure 2). For 3, the authors suggest that the geometry can be viewed as a distorted square antiprism. From a spectroscopic point of view, similar 119 Sn NMR chemical shifts were also recorded for 2 and 3, at −920.8 and −930.0 ppm, respectively ( Table 2). In 13 C NMR, the carbamato carbons show one resonance at 164.9 ppm for 2, and 166.2 ppm for 3. In the case of 2, CP/MAS 13 C NMR measurements confirm the chemical shift recorded in solution. A thermogravimetric analysis was also  (2) and Sn(O2CNEt2)4 (3), which were used as precursors for the chemical implantation of metal cations on a silica support [45]. The grafting method consisted of promoting reactivity of N,N-dialkylcarbamates with silanol groups of silica. Compounds 2 and 3 were synthesized by treating anhydrous Sn(IV) chloride with diethylamine and di-iso-propylamine, respectively, in toluene at room temperature, under atmospheric pressure of carbon dioxide (Scheme 10). Yields higher than 90% are reported. Scheme 10. Reaction scheme leading to Compounds 2 and 3, data from [45].
Recrystallized from heptane, suitable single crystals of 2 were analyzed by X-ray diffraction leading to the resolution of the crystallographic structure. In 2002, in the frame of metal-organic chemical vapor deposition (MOCVD) investigations, Molloy et al. described a new synthetic route to 3, starting from a solution of tetrakis(N,N-diethylamino)tin(IV) complex in hexane that was bubbled with carbon dioxide (Scheme 11) [46]. Scheme 11. Alternative route leading to compound 3, data from [46]. X-ray structures of 2 and 3 revealed comparable mononuclear molecules. In both compounds, the tin atom is eight-coordinated by four chelating N,N-dialkylcarbamato ligands, which are exclusively O-donor ( Figure 2). For 3, the authors suggest that the geometry can be viewed as a distorted square antiprism. From a spectroscopic point of view, similar 119 Sn NMR chemical shifts were also recorded for 2 and 3, at −920.8 and −930.0 ppm, respectively ( Table 2). In 13 C NMR, the carbamato carbons show one resonance at 164.9 ppm for 2, and 166.2 ppm for 3. In the case of 2, CP/MAS 13 C NMR measurements confirm the chemical shift recorded in solution. A thermogravimetric analysis was also  (2) and Sn(O2CNEt2)4 (3), which were used as precursors for the chemical implantation of metal cations on a silica support [45]. The grafting method consisted of promoting reactivity of N,N-dialkylcarbamates with silanol groups of silica. Compounds 2 and 3 were synthesized by treating anhydrous Sn(IV) chloride with diethylamine and di-iso-propylamine, respectively, in toluene at room temperature, under atmospheric pressure of carbon dioxide (Scheme 10). Yields higher than 90% are reported. Scheme 10. Reaction scheme leading to Compounds 2 and 3, data from [45].
Recrystallized from heptane, suitable single crystals of 2 were analyzed by X-ray diffraction leading to the resolution of the crystallographic structure. In 2002, in the frame of metal-organic chemical vapor deposition (MOCVD) investigations, Molloy et al. described a new synthetic route to 3, starting from a solution of tetrakis(N,N-diethylamino)tin(IV) complex in hexane that was bubbled with carbon dioxide (Scheme 11) [46]. Scheme 11. Alternative route leading to compound 3, data from [46]. X-ray structures of 2 and 3 revealed comparable mononuclear molecules. In both compounds, the tin atom is eight-coordinated by four chelating N,N-dialkylcarbamato ligands, which are exclusively O-donor ( Figure 2). For 3, the authors suggest that the geometry can be viewed as a distorted square antiprism. From a spectroscopic point of view, similar 119 Sn NMR chemical shifts were also recorded for 2 and 3, at −920.8 and −930.0 ppm, respectively ( Table 2). In 13 C NMR, the carbamato carbons show one resonance at 164.9 ppm for 2, and 166.2 ppm for 3. In the case of 2, CP/MAS 13 C NMR measurements confirm the chemical shift recorded in solution. A thermogravimetric analysis was also Scheme 11. Alternative route leading to compound 3, data from [46]. X-ray structures of 2 and 3 revealed comparable mononuclear molecules. In both compounds, the tin atom is eight-coordinated by four chelating N,N-dialkylcarbamato ligands, which are exclusively O-donor ( Figure 2). For 3, the authors suggest that the geometry can be viewed as a distorted square antiprism. From a spectroscopic point of view, similar 119 Sn NMR chemical shifts were also recorded for 2 and 3, at −920.8 and −930.0 ppm, respectively ( Table 2). In 13 C NMR, the carbamato carbons show one resonance at 164.9 ppm for 2, and 166.2 ppm for 3. In the case of 2, CP/MAS 13 C NMR measurements confirm the chemical shift recorded in solution. A thermogravimetric analysis was also carried out on 3 showing a continuous weight loss between room temperature and 300 • C (and resulting in the recovery of SnO 2 as final residue). Beyond their interest as precursors for the deposition of tin oxide films [4,5], tin tetracarbamates were also used as versatile reactants for various organometallic and organic syntheses leading to alkoxystannanes [47], tetraalkynylstannanes [48] (Scheme 12), and O-silylurethanes [49]. carried out on 3 showing a continuous weight loss between room temperature and 300 °C (and resulting in the recovery of SnO2 as final residue). Beyond their interest as precursors for the deposition of tin oxide films [4,5], tin tetracarbamates were also used as versatile reactants for various organometallic and organic syntheses leading to alkoxystannanes [47], tetraalkynylstannanes [48] (Scheme 12), and O-silylurethanes [49].  [51]. In particular, they focused on the reaction involving [(Me3Si)2N]2Sn with CO2, which led to the isolation of the dimeric bisalkoxide complex, [Sn(OSiMe3)2]2. However, in this case and compared to the Kemp's report, the tin carbamate is not stable but leads to the formation of [Sn(OSiMe3)2]2, trimethylsilyl isocyanate, and 1,3-bis(trimethylsilyl)carbodiimide, respectively, according to a ligand metathesis process. carried out on 3 showing a continuous weight loss between room temperature and 300 °C (and resulting in the recovery of SnO2 as final residue). Beyond their interest as precursors for the deposition of tin oxide films [4,5], tin tetracarbamates were also used as versatile reactants for various organometallic and organic syntheses leading to alkoxystannanes [47], tetraalkynylstannanes [48] (Scheme 12), and O-silylurethanes [49].  [51]. In particular, they focused on the reaction involving [(Me3Si)2N]2Sn with CO2, which led to the isolation of the dimeric bisalkoxide complex, [Sn(OSiMe3)2]2. However, in this case and compared to the Kemp's report, the tin carbamate is not stable but leads to the formation of [Sn(OSiMe3)2]2, trimethylsilyl isocyanate, and 1,3-bis(trimethylsilyl)carbodiimide, respectively, according to a ligand metathesis process.  [51]. In particular, they focused on the reaction involving [(Me 3 Si) 2   In 2011, continuing their study on the reactivity of silyl-substituted tin(II) amides towards heteroallenes, Kemp et al. reported the interaction of CO2, OCS and CS2 with (Me2N)2Sn leading to the formation of insertion products characterized as bis-(N,N-dimethylcarbamato)tin(II), bis(N,N-dimethylthiocarbamato)tin(II), and bis(N,N-dimethyldithiocarbamato)tin(II), respectively [52]. Thus, (Me2N)2Sn in hexane solution reacts quickly with carbon dioxide, at room temperature and under atmospheric pressure, to give with a yield almost quantitative, the new complex [(Me2NCO2)2Sn]2 (5) (Scheme 14). The authors describe the reaction as exothermic. In the solid-state, compound 5 is organized as a dimer involving two types of carbamato ligands: bridging and terminally (η 2 -chelating) coordinated to Sn. From a supramolecular point of view, the existence of intermolecular interactions Sn···O [2.8499(18) Å] between neighboring dimers and involving one oxygen atom of terminal carbamato ligands, leads to the propagation of a polymer chain ( Figure  4). The coordination geometry of tin atoms can be viewed as a highly distorted trigonal bipyramid. In solution, the 119 Sn NMR spectrum exhibits only one resonance at δ -613 ppm. In the 13 C { 1 H} NMR spectrum, the carbamato carbon also displays only one signal, not making it possible to differentiate the bridging and terminal modes highlighted in crystals of 5. Scheme 14. Reaction scheme leading to Compound 5, data from [52]. In 2011, continuing their study on the reactivity of silyl-substituted tin(II) amides towards heteroallenes, Kemp et al. reported the interaction of CO 2 , OCS and CS 2 with (Me 2 N) 2 Sn leading to the formation of insertion products characterized as bis-(N,Ndimethylcarbamato)tin(II), bis(N,N-dimethylthiocarbamato)tin(II), and bis(N,Ndimethyldithiocarbamato)tin(II), respectively [52]. Thus, (Me 2 N) 2 Sn in hexane solution reacts quickly with carbon dioxide, at room temperature and under atmospheric pressure, to give with a yield almost quantitative, the new complex [(Me 2 NCO 2 ) 2 Sn] 2 (5) (Scheme 14). The authors describe the reaction as exothermic. In the solid-state, compound 5 is organized as a dimer involving two types of carbamato ligands: bridging and terminally (η 2 -chelating) coordinated to Sn. From a supramolecular point of view, the existence of intermolecular interactions Sn···O [2.8499(18) Å] between neighboring dimers and involving one oxygen atom of terminal carbamato ligands, leads to the propagation of a polymer chain ( Figure 4). The coordination geometry of tin atoms can be viewed as a highly distorted trigonal bipyramid. In solution, the 119 Sn NMR spectrum exhibits only one resonance at δ -613 ppm. In the 13 C { 1 H} NMR spectrum, the carbamato carbon also displays only one signal, not making it possible to differentiate the bridging and terminal modes highlighted in crystals of 5.

Scheme 13.
Reaction scheme leading to Compound 4, data from [50]. In 2011, continuing their study on the reactivity of silyl-substituted tin(II) amides towards heteroallenes, Kemp et al. reported the interaction of CO2, OCS and CS2 with (Me2N)2Sn leading to the formation of insertion products characterized as bis-(N,N-dimethylcarbamato)tin(II), bis(N,N-dimethylthiocarbamato)tin(II), and bis(N,N-dimethyldithiocarbamato)tin(II), respectively [52]. Thus, (Me2N)2Sn in hexane solution reacts quickly with carbon dioxide, at room temperature and under atmospheric pressure, to give with a yield almost quantitative, the new complex [(Me2NCO2)2Sn]2 (5) (Scheme 14). The authors describe the reaction as exothermic. In the solid-state, compound 5 is organized as a dimer involving two types of carbamato ligands: bridging and terminally (η 2 -chelating) coordinated to Sn. From a supramolecular point of view, the existence of intermolecular interactions Sn···O [2.8499(18) Å] between neighboring dimers and involving one oxygen atom of terminal carbamato ligands, leads to the propagation of a polymer chain ( Figure  4). The coordination geometry of tin atoms can be viewed as a highly distorted trigonal bipyramid. In solution, the 119 Sn NMR spectrum exhibits only one resonance at δ -613 ppm. In the 13 C { 1 H} NMR spectrum, the carbamato carbon also displays only one signal, not making it possible to differentiate the bridging and terminal modes highlighted in crystals of 5. Scheme 14. Reaction scheme leading to Compound 5, data from [52]. Scheme 14. Reaction scheme leading to Compound 5, data from [52].
To our knowledge, the latest tin carbamate structure resolved to date by X-ray diffraction is to be credited to Fulton et al., who published in 2011, the reactivity of β-diketiminate tin derivatives with carbon dioxide [53].  Figure 5). This mode of coordination is unusual for carbamates and is similar to that observed in the case of hemicarbonato derivatives [1]. The X-ray structure of complex 7 has not been reported but the spectroscopic data are very similar to 6 ( Table 2) and the authors mention that for both compounds, the insertion reaction is irreversible, even under reduced pressure. To our knowledge, the latest tin carbamate structure resolved to date by X-ray diffraction is to be credited to Fulton et al., who published in 2011, the reactivity of β-diketiminate tin derivatives with carbon dioxide [53]. (7), respectively (Scheme 15). In the solid-state, 6 consists of a mononuclear complex describing a trigonal pyramidal arrangement of the ligands around the tin atom and in an endo configuration. The carbamate moiety is terminally O-bonded to Sn [Sn−O = 2.1346(16) Å] and its orientation can be viewed almost perpendicular to the NCCCN plane of the β-diketiminate ligand ( Figure 5). This mode of coordination is unusual for carbamates and is similar to that observed in the case of hemicarbonato derivatives [1]. The X-ray structure of complex 7 has not been reported but the spectroscopic data are very similar to 6 ( Table 2) and the authors mention that for both compounds, the insertion reaction is irreversible, even under reduced pressure. Scheme 15. Reaction scheme leading to Compounds 6 and 7, data from [53].  To our knowledge, the latest tin carbamate structure resolved to date by X-ray diffraction is to be credited to Fulton et al., who published in 2011, the reactivity of β-diketiminate tin derivatives with carbon dioxide [53].  Figure 5). This mode of coordination is unusual for carbamates and is similar to that observed in the case of hemicarbonato derivatives [1]. The X-ray structure of complex 7 has not been reported but the spectroscopic data are very similar to 6 ( Table 2) and the authors mention that for both compounds, the insertion reaction is irreversible, even under reduced pressure. Scheme 15. Reaction scheme leading to Compounds 6 and 7, data from [53]. Scheme 15. Reaction scheme leading to Compounds 6 and 7, data from [53].   (7) 119.9(6)   a Measured in chloroform-d; b measured in benzene-d 6 .

Early Works
To our knowledge, pioneering work on formato tin complexes dates back to 1927.

X-ray Crystal Structures of Formato Tin Complexes Resulting from Reactivity with CO2
While the formation of the above compounds required the use of formic acid in their preparation, there are also a few examples of formato tin complexes directly isolated under carbon dioxide atmosphere, resulting from the insertion of CO2 into the Sn-H bond. In our opinion, six complexes, published mainly during the last decade and characterized by X-ray diffraction, correspond to this category of compounds (Table 3). In 2009, Roesky et al. reported the structural and spectroscopic characterization of several complexes resulting from hydrostannylation reactions of carbon dioxide, ketones, aldehydes, alkynes and carbodiimides with the tin(II) hydride precursor, [HC{(Me)CN(2,6-i-Pr2C6H3)}2]SnH [62]. In the case of the reaction involving CO2, bubbling at room temperature very quickly causes a change in color (from yellow to colorless) of the reaction mixture (toluene solu-

X-ray Crystal Structures of Formato Tin Complexes Resulting from Reactivity with CO2
While the formation of the above compounds required the use of formic acid in their preparation, there are also a few examples of formato tin complexes directly isolated un-

X-Ray Crystal Structures of Formato Tin Complexes Resulting from Reactivity with CO 2
While the formation of the above compounds required the use of formic acid in their preparation, there are also a few examples of formato tin complexes directly isolated under carbon dioxide atmosphere, resulting from the insertion of CO 2 into the Sn-H bond. In our opinion, six complexes, published mainly during the last decade and characterized by X-ray diffraction, correspond to this category of compounds (Table 3 infrared was confirmed by single-crystal X-ray diffraction. The molecular structure shows moreover a distorted pseudo-tetrahedral environment for the tin atom, completed by one electron lone pair ( Figure 11).

Scheme 16.
Reaction scheme leading to Compound 8, data from [62]. infrared was confirmed by single-crystal X-ray diffraction. The molecular structure shows moreover a distorted pseudo-tetrahedral environment for the tin atom, completed by one electron lone pair ( Figure 11).

CSD Entry Deposition Number
Ref. [

Foreword
This type of compound, rarely described so far, results from the insertion of CO 2 into the Sn-P bond then leads to the formation of the Sn-OC(O)P moiety which can be qualified as a phosphinoformato derivative, by analogy with phosphino formic acid (H 2 PCOOH) according to the recent work published by Kaiser et al. [68]. To the best of our knowledge, only two examples of this type of complex have been described so far (Table 5), and their characterization is quite recent (last decade).

CSD Entry Deposition Number
Ref.

X-Ray Crystal Structures
The first example of the phosphinoformato tin complex, characterized by X-ray diffraction as [(i-Pr 2 P) 2 N·CO 2 ] 2 Sn (14), was resolved in 2011 by Kemp et al. [69]. Complex 14 was isolated as a white precipitate by bubbling CO 2 at room temperature (for 10 min) through a pentane solution of [(i-Pr 2 P) 2 N] 2 Sn, which caused a color change of the solution from orange to yellow (Scheme 21). Structural analysis of single-crystals confirmed the capture of two molecules of CO 2 by [(i-Pr 2 P) 2 N] 2 Sn forming a six-membered ring complex in which CO 2 is inserted into one Sn-P bond of each ligand ( Figure 17). Moreover, the authors studied the stability of 14 in particular by thermogravimetric analysis and showed the easy release of CO 2 from 90 • C. They showed also the possibility of recovering the starting complex by simply heating a solid sample of 14 under argon. Compound 14 is stable for several months stored under CO 2 at room temperature or under argon at −25 • C. On the basis of these observations, the incorporation of CO 2 as an adduct is preferentially considered rather than as a real insertion. Inorganics 2021, 9, x FOR PEER REVIEW 21 of 33 Scheme 21. Reaction scheme leading to Compound 14, data from [69]. More recently, in 2019, Mitzel et al. reported the reactivity of the geminal frustrated Lewis pair (F5C2)3SnCH2P(t-Bu)2 with a variety of small molecules and, in particular, with CO2 [70]. The formation of the CO2 adduct of (F5C2)3SnCH2P(t-Bu)2, characterized as (F5C2)3SnCH2P(t-Bu)2·CO2 (15), was first demonstrated by NMR at −70 °C in THF-d8 ( Table  6). The authors mention the instability of the complex at room temperature causing the release of CO2 (Scheme 22). The CO2 molecule is bonded to tin and phosphorus atoms, thus leading to the formation of a five-membered ring with an exocyclic C-O bond. The PCO2 unit is planar ( Figure 18). Other adducts resulting from the reactivity of (F5C2)3SnCH2P(t-Bu)2, in particular with SO2 and CS2, were also isolated in the solid state and describe comparable solid-state structures.  More recently, in 2019, Mitzel et al. reported the reactivity of the geminal frustrated Lewis pair (F5C2)3SnCH2P(t-Bu)2 with a variety of small molecules and, in particular, with CO2 [70]. The formation of the CO2 adduct of (F5C2)3SnCH2P(t-Bu)2, characterized as (F5C2)3SnCH2P(t-Bu)2·CO2 (15), was first demonstrated by NMR at −70 °C in THF-d8 ( Table  6). The authors mention the instability of the complex at room temperature causing the release of CO2 (Scheme 22). The CO2 molecule is bonded to tin and phosphorus atoms, thus leading to the formation of a five-membered ring with an exocyclic C-O bond. The PCO2 unit is planar ( Figure 18). Other adducts resulting from the reactivity of (F5C2)3SnCH2P(t-Bu)2, in particular with SO2 and CS2, were also isolated in the solid state and describe comparable solid-state structures.

Scheme 22.
Reaction scheme leading to Compound 15, data from [70]. More recently, in 2019, Mitzel et al. reported the reactivity of the geminal frustrated Lewis pair (F 5 C 2 ) 3 SnCH 2 P(t-Bu) 2 with a variety of small molecules and, in particular, with CO 2 [70]. The formation of the CO 2 adduct of (F 5 C 2 ) 3 SnCH 2 P(t-Bu) 2 , characterized as (F 5 C 2 ) 3 SnCH 2 P(t-Bu) 2 ·CO 2 (15), was first demonstrated by NMR at −70 • C in THF-d 8 ( Table 6). The authors mention the instability of the complex at room temperature causing the release of CO 2 (Scheme 22). The CO 2 molecule is bonded to tin and phosphorus atoms, thus leading to the formation of a five-membered ring with an exocyclic C-O bond. The PCO 2 unit is planar ( Figure 18). Other adducts resulting from the reactivity of (F 5 C 2 ) 3 SnCH 2 P(t-Bu) 2 , in particular with SO 2 and CS 2 , were also isolated in the solid state and describe comparable solid-state structures.  More recently, in 2019, Mitzel et al. reported the reactivity of the geminal frustrated Lewis pair (F5C2)3SnCH2P(t-Bu)2 with a variety of small molecules and, in particular, with CO2 [70]. The formation of the CO2 adduct of (F5C2)3SnCH2P(t-Bu)2, characterized as (F5C2)3SnCH2P(t-Bu)2·CO2 (15), was first demonstrated by NMR at −70 °C in THF-d8 ( Table  6). The authors mention the instability of the complex at room temperature causing the release of CO2 (Scheme 22). The CO2 molecule is bonded to tin and phosphorus atoms, thus leading to the formation of a five-membered ring with an exocyclic C-O bond. The PCO2 unit is planar ( Figure 18). Other adducts resulting from the reactivity of (F5C2)3SnCH2P(t-Bu)2, in particular with SO2 and CS2, were also isolated in the solid state and describe comparable solid-state structures. Inorganics 2021, 9, x FOR PEER REVIEW 22 of 33  na [70] a Measured in benzene-d6; b measured in tetrahydrofuran-d8; c determined at 208 K; d determined at 203 K.

Foreword
The last part of this collection is devoted to bimetallic transition-metal/tin complexes with bridging carbon dioxide, of general formula M(CO2)Sn, also named metallocarboxylato tin complexes. Compared to the categories previously described, their synthesis does not involve the direct use of CO2 but is based on the reactivity between metallocarboxylic acids and halogenated precursors of organotins. However, we have found it interesting and complementary to also include this class of compounds in this review. Metallocarboxylato complexes and in particular tin derivatives began to be studied in the mid-1980s, often considered as suitable models of intermediates for the catalytic conversion of carbon dioxide [4]. Most of the structures shown below were resolved in the 1990s. The D. Gibson's group in Louisville (USA) was one of the most active in this field characterizing several of these compounds. Metallocarboxylato-tin complexes have also been the subject of specific infrared considerations [71]. Indeed, νOCO adsorption bands constitute well-suited probes for orienting and determining the coordination modes of the CO2 ligand.

Foreword
The last part of this collection is devoted to bimetallic transition-metal/tin complexes with bridging carbon dioxide, of general formula M(CO 2 )Sn, also named metallocarboxylato tin complexes. Compared to the categories previously described, their synthesis does not involve the direct use of CO 2 but is based on the reactivity between metallocarboxylic acids and halogenated precursors of organotins. However, we have found it interesting and complementary to also include this class of compounds in this review. Metallocarboxylato complexes and in particular tin derivatives began to be studied in the mid-1980s, often considered as suitable models of intermediates for the catalytic conversion of carbon dioxide [4]. Most of the structures shown below were resolved in the 1990s. The D. Gibson's group in Louisville (USA) was one of the most active in this field characterizing several of these compounds. Metallocarboxylato-tin complexes have also been the subject of specific infrared considerations [71]. Indeed, ν OCO adsorption bands constitute well-suited probes for orienting and determining the coordination modes of the CO 2 ligand.

X-Ray Crystal Structures
To the best of our knowledge, in 1987, Gladysz et al. reported the first X-ray characterization of a transition metal/tin bridging CO 2 complex by isolating (η-C 5 H 5 )Re(NO)(PPh 3 ) (CO 2 SnPh 3 ) (16) [72]. Compound 16 was obtained in high yield (86%) by reacting (η-C 5 H 5 )Re(NO)(PPh 3 )(CO 2 K) with 1.1 equivalent of Ph 3 SnCl at −78 • C in THF (Scheme 23). Yellow crystal prisms of 16 grew from a mixture of CH 2 Cl 2 /hexane. Compound 16 is air-and water-stable, and could also be alternatively prepared from (η-C 5 H 5 )Re(NO)(PPh 3 )(CO 2 H) and (Ph 3 Sn) 2 O at 25 • C in THF. Moreover, the decarboxylation of 16 occurs by heating at 180 • C for 10 min (from solid) or at 140 • C, for 20 h in solution in xylene. Using Me 3 SnCl, the authors claim the formation of the analogue (η-C 5 H 5 )Re(NO)(PPh 3 )(CO 2 SnMe 3 ) with a yield of 95%. The study was also extended to the synthesis of rhenium/germanium and rhenium/lead bridging CO 2 complexes using Ph 3 GeBr and Ph 3 PbCl, as precursors, respectively. The crystallographic structure of 16 confirms the presence of two moieties based on rhenium and tin, respectively, linked by a CO 2 bridging ligand. The tin atom of 16 adopts a distorted trigonal bipyramid geometry in which the equatorial positions are occupied by the two oxygen atoms of the CO 2 ligand and supplemented by a phenyl group (Figure 19). The CO 2 ligand can be regarded as a carboxylato fragment, which is corroborated by infrared analysis, bidentally bound to the tin atom and defined as µ(n 1 -C: Inorganics 2021, 9, x FOR PEER REVIEW 23 of 33 in xylene. Using Me3SnCl, the authors claim the formation of the analogue (η-C5H5)Re(NO)(PPh3)(CO2SnMe3) with a yield of 95%. The study was also extended to the synthesis of rhenium/germanium and rhenium/lead bridging CO2 complexes using Ph3GeBr and Ph3PbCl, as precursors, respectively. The crystallographic structure of 16 confirms the presence of two moieties based on rhenium and tin, respectively, linked by a CO2 bridging ligand. The tin atom of 16 adopts a distorted trigonal bipyramid geometry in which the equatorial positions are occupied by the two oxygen atoms of the CO2 ligand and supplemented by a phenyl group (Figure 19). The CO2 ligand can be regarded as a carboxylato fragment, which is corroborated by infrared analysis, bidentally bound to the tin atom and defined as μ(n 1 -C: n 2 -O,O′) ligand.

Scheme 23.
Reaction scheme leading to Compound 16, data from [72]. In 1991, Gibson et al. published the solid state structure of (η-C5H5)Fe(CO)(PPh3)(CO2SnPh3) (17) prepared by mixing, under nitrogen and at low temperature, Ph3SnCl to a THF solution of CpFe(CO)(PPh3)COOK·3H2O [73]. The structure was found to be isomorphous to 16, with the tin atom occupying a trigonal bipyramid arrangement ( Figure 20). Compared to the Gladysz complex, the authors underline for 17 a greater difference between the lengths of the Sn-O bonds, corresponding to a value of 0.219 Å (Table 7). However, in the literature devoted to triaryltin organocarboxylate monomers this difference is generally much more marked in the range of 0.48 to 0.81 Å [74]. Scheme 23. Reaction scheme leading to Compound 16, data from [72].
Inorganics 2021, 9, x FOR PEER REVIEW 23 of 33 in xylene. Using Me3SnCl, the authors claim the formation of the analogue (η-C5H5)Re(NO)(PPh3)(CO2SnMe3) with a yield of 95%. The study was also extended to the synthesis of rhenium/germanium and rhenium/lead bridging CO2 complexes using Ph3GeBr and Ph3PbCl, as precursors, respectively. The crystallographic structure of 16 confirms the presence of two moieties based on rhenium and tin, respectively, linked by a CO2 bridging ligand. The tin atom of 16 adopts a distorted trigonal bipyramid geometry in which the equatorial positions are occupied by the two oxygen atoms of the CO2 ligand and supplemented by a phenyl group (Figure 19). The CO2 ligand can be regarded as a carboxylato fragment, which is corroborated by infrared analysis, bidentally bound to the tin atom and defined as μ(n 1 -C: n 2 -O,O′) ligand.

Scheme 23.
Reaction scheme leading to Compound 16, data from [72]. In 1991, Gibson et al. published the solid state structure of (η-C5H5)Fe(CO)(PPh3)(CO2SnPh3) (17) prepared by mixing, under nitrogen and at low temperature, Ph3SnCl to a THF solution of CpFe(CO)(PPh3)COOK·3H2O [73]. The structure was found to be isomorphous to 16, with the tin atom occupying a trigonal bipyramid arrangement ( Figure 20). Compared to the Gladysz complex, the authors underline for 17 a greater difference between the lengths of the Sn-O bonds, corresponding to a value of 0.219 Å (Table 7). However, in the literature devoted to triaryltin organocarboxylate monomers this difference is generally much more marked in the range of 0.48 to 0.81 Å [74].  (Table 7). However, in the literature devoted to triaryltin organocarboxylate monomers this difference is generally much more marked in the range of 0.48 to 0.81 Å [74]. Two years later in 1993, the same group reported the solid state isolation of a new Fe/Sn CO2-bridged complex, which was characterized by single crystal X-ray diffraction analysis as the indenyl derivative (η 5 -C9H7)Fe(CO)(PPh3)(CO2SnPh3) (18). Compound 18 was prepared under nitrogen, in THF solution and at 273 K, from (η 5 -C9H7)Fe(CO)2(PPh3) + ·I − and Ph3SnCl [75]. As shown by the structure displayed in Figure  21, the Sn atom geometry consists of a distorted trigonal bipyramid. However, the steric hindrance of the indenyl ligand and the resulting interactions with the phosphine ligand are suspected to cause notable structural differences compared to complexes 16 and 17:  The Gibson's group continued to be very prolific in this field until the end of the 1990s, structurally characterizing several new iron and rhenium/tin CO2 bridged complexes. In 1994, the reaction of Cp*Re(CO)(NO)COOH with Ph3SnCl in the presence of Na2CO3 afforded the formation of Cp*Re(CO)(NO)(CO2SnPh3) (19) showing a μ2-η 3 coordination mode for the CO2 ligand ( Figure 22) [76]. As reflected by the lengths of the Sn-O Two years later in 1993, the same group reported the solid state isolation of a new Fe/Sn CO 2 -bridged complex, which was characterized by single crystal X-ray diffraction analysis as the indenyl derivative (η 5 -C 9 H 7 )Fe(CO)(PPh 3 )(CO 2 SnPh 3 ) (18). Compound 18 was prepared under nitrogen, in THF solution and at 273 K, from (η 5 -C 9 H 7 )Fe(CO) 2 (PPh 3 ) + ·I − and Ph 3 SnCl [75]. As shown by the structure displayed in Figure 21  The Gibson's group continued to be very prolific in this field until the end of the 1990s, structurally characterizing several new iron and rhenium/tin CO 2 bridged complexes. In 1994, the reaction of Cp*Re(CO)(NO)COOH with Ph 3 SnCl in the presence of Na 2 CO 3 afforded the formation of Cp*Re(CO)(NO)(CO 2 SnPh 3 ) (19) showing a µ 2 -η 3 coordination mode for the CO 2 ligand (Figure 22) [76]. As reflected by the lengths of the Sn-O and C-O bonds (Table 7), an asymmetric bonding mode of carboxylic oxygens to carbon and tin atoms was observed.  (Table 7), an asymmetric bonding mode of carboxylic oxygens to carbon and tin atoms was observed. In 1995, Gibson et al. resolved by X-ray diffraction, the structure of four new CO2bridged transition metals/tin complexes [77]. CpFe(CO)(PPh3)(CO2SnMe3) (20) and CpFe(CO)(PPh3)(CO2Sn(n-Bu)3) (21) were synthesized by reaction in THF between CpFe(CO)(PPh3)CO2 − K + and ClSnMe3 and ClSnPh3, respectively. In both cases, the bridging carboxyl ligand adopts a μ2-η 3 coordination mode in which the carboxyl atom is bound to the iron atom and both oxygens are bound to the tin atom. Cp*Fe(CO)2(CO2SnPh3) (22) was prepared from Cp*Fe(CO)3 + BF4 − and Ph3SnCl in CH2Cl2 at 0 °C in the presence of KOH. Compound 22 displays a similar structure to that of complexes 20 and 21 in which tin atoms are five-coordinated and adopt a distorted trigonal-bipyramidal geometry. The CO2-bridged rhenium-tin complex Cp*Re(CO)(NO)(CO2SnMe3) (23) was isolated as crystalline solid from Cp*Re(CO)(NO)COOH and ClSnMe3 in the presence of KOH. The X-ray structure of 23 highlighted a μ2-η 2 coordination mode of the bridging CO2 ligand in which the carbonyl carbon and one oxygen atom are bonded to the rhenium atom and the tin atom, respectively. As a result, the tin atom adopts a distorted tetrahedral geometry (Figure 23).   (Table 7), an asymmetric bonding mode of carboxylic oxygens to carbon and tin atoms was observed. In 1995, Gibson et al. resolved by X-ray diffraction, the structure of four new CO2bridged transition metals/tin complexes [77]. CpFe(CO)(PPh3)(CO2SnMe3) (20) and CpFe(CO)(PPh3)(CO2Sn(n-Bu)3) (21) were synthesized by reaction in THF between CpFe(CO)(PPh3)CO2 − K + and ClSnMe3 and ClSnPh3, respectively. In both cases, the bridging carboxyl ligand adopts a μ2-η 3 coordination mode in which the carboxyl atom is bound to the iron atom and both oxygens are bound to the tin atom. Cp*Fe(CO)2(CO2SnPh3) (22) was prepared from Cp*Fe(CO)3 + BF4 − and Ph3SnCl in CH2Cl2 at 0 °C in the presence of KOH. Compound 22 displays a similar structure to that of complexes 20 and 21 in which tin atoms are five-coordinated and adopt a distorted trigonal-bipyramidal geometry. The CO2-bridged rhenium-tin complex Cp*Re(CO)(NO)(CO2SnMe3) (23) was isolated as crystalline solid from Cp*Re(CO)(NO)COOH and ClSnMe3 in the presence of KOH. The X-ray structure of 23 highlighted a μ2-η 2 coordination mode of the bridging CO2 ligand in which the carbonyl carbon and one oxygen atom are bonded to the rhenium atom and the tin atom, respectively. As a result, the tin atom adopts a distorted tetrahedral geometry (Figure 23).  In 1997, two new specimen of CO 2 -bridged rhenium/tin complexes were characterized at the solid-state by the Gibson's group [78]. The transmetalation reaction involving Cp*Re(CO)(NO)(CO 2 SnMe 3 ) (23) and Me 2 SnCl 2 led to the formation in good yield of Cp*Re(CO)(NO)(CO 2 Sn(Cl)Me 2 ) (24) jointly with Me 3 SnCl (Scheme 24). The X-ray structure of complex 24 shows the presence of a µ 2 -η 3 CO 2 ligand bridging the two moieties of rhenium and tin. The tin atom exhibits a five-coordinate geometry adopting an edgecapped tetrahedral arrangement rather than a distorted trigonal bipyramid. In 1997, two new specimen of CO2-bridged rhenium/tin complexes were characterized at the solid-state by the Gibson's group [78]. The transmetalation reaction involving Cp*Re(CO)(NO)(CO2SnMe3) (23) and Me2SnCl2 led to the formation in good yield of Cp*Re(CO)(NO)(CO2Sn(Cl)Me2) (24) jointly with Me3SnCl (Scheme 24). The X-ray structure of complex 24 shows the presence of a μ2-η 3 CO2 ligand bridging the two moieties of rhenium and tin. The tin atom exhibits a five-coordinate geometry adopting an edgecapped tetrahedral arrangement rather than a distorted trigonal bipyramid.  In 1997, Komiya et al. reported the synthesis of the CO2 complex of iron(0), Fe(CO2)(depe)2 [depe = 1,2-bis(diethylphosphino)ethane)], resulting from the replacement of N2 in Fe(N2)(depe)2 with CO2. Fe(CO2)(depe)2 reacts then with Ph3SnCl in Et2O at −78°C to lead to the formation of FeCl(CO2SnPh3)(depe)2 (26) (Scheme 26) [79]. The same reaction, using Me3SnCl as tin precursor, gives rise to the analogous complex FeCl(CO2SnMe3)(depe)2. The X-ray structure of 26 shows that iron and tin atoms are linked by a CO2 molecule, acting as μ(n 1 -C: n 2 -O,O′) ligand ( Figure 25).

Scheme 26.
Reaction scheme leading to Compound 26, data from [79].  (27) and Os3(CO)10(μ-OMe)(μ-OH) (with yields of 18% and 7%, respectively) [80]. The tiphenylgermyl homologue of 27, Os3(CO)10(μ-η 2 -O=COGePh3)(μ-OMe), was also obtained from the reaction of Ph3GeOH with Os3(CO)12 and using a comparable synthetic protocol (in the presence of [Bu4N][OH] and in methanol). The authors described the structure of 27 as being a trinuclear cluster of three osmium atoms bearing one bridging methoxy ligand and one bridging triphenlystannylcarboxylate ligand. Remarkably, the Sn−O distances show a large difference in length (2.075 (5) Å and 2.858 (5) Å, respectively), which results from an unusual mode of coordination showing the organotin fragment in a pendant arm arrangement. The carbon atom and one oxygen atom of O=COSnPh3 are bonded to two distinct osmium atoms, while the tin atom is connected via the second oxygen atom of the ligand (Figure 26).  (27) and Os 3 (CO) 10 (µ-OMe)(µ-OH) (with yields of 18% and 7%, respectively) [80]. The tiphenylgermyl homologue of 27, Os 3 (CO) 10 (µ-η 2 -O=COGePh 3 )(µ-OMe), was also obtained from the reaction of Ph 3 GeOH with Os 3 (CO) 12 and using a comparable synthetic protocol (in the presence of [Bu 4 N][OH] and in methanol). The authors described the structure of 27 as being a trinuclear cluster of three osmium atoms bearing one bridging methoxy ligand and one bridging triphenlystannylcarboxylate ligand. Remarkably, the Sn−O distances show a large difference in length (2.075 (5) Å and 2.858 (5) Å, respectively), which results from an unusual mode of coordination showing the organotin fragment in a pendant arm arrangement. The carbon atom and one oxygen atom of O=COSnPh 3 are bonded to two distinct osmium atoms, while the tin atom is connected via the second oxygen atom of the ligand (Figure 26). Inorganics 2021, 9, x FOR PEER REVIEW 28 of 33  Figure 27). The possible formation of the metallocarboxylate group is explained by the nucleophilic addition of H2O to a terminal CO ligand, followed by deprotonation. Alternatively, the authors also suggest that [(Cl2SnOCOSnCl2)] 2-could be considered as a carbonite ion [CO2] 2- [82], stabilized by coordination at one Pt atom and two Sn atoms. A selection of spectroscopic data (NMR and IR) relating to metallocarboxylatotin complexes is reported in Table 8.   4 ]. The authors described the unprecedented structure of 28 as consisting of a formally neutral Pt 9 core bonded to eight carbonyls, three µ 4 -SnCl 2 , two µ 3 -[SnCl 3 ]and the six electron donor bis-stannyl-carboxylate [(Cl 2 SnOCOSnCl 2 )] 2− ligand ( Figure 27). The possible formation of the metallocarboxylate group is explained by the nucleophilic addition of H 2 O to a terminal CO ligand, followed by deprotonation. Alternatively, the authors also suggest that [(Cl 2 SnOCOSnCl 2 )] 2− could be considered as a carbonite ion [CO 2 ] 2− [82], stabilized by coordination at one Pt atom and two Sn atoms. A selection of spectroscopic data (NMR and IR) relating to metallocarboxylato-tin complexes is reported in Table 8.  Figure 27). The possible formation of the metallocarboxylate group is explained by the nucleophilic addition of H2O to a terminal CO ligand, followed by deprotonation. Alternatively, the authors also suggest that [(Cl2SnOCOSnCl2)] 2-could be considered as a carbonite ion [CO2] 2- [82], stabilized by coordination at one Pt atom and two Sn atoms. A selection of spectroscopic data (NMR and IR) relating to metallocarboxylatotin complexes is reported in Table 8.    a Measured in chloroform-d; b methylenchloride-d 2 ; c measured in tetrahydrofuran-d 8 ; d measured in benzene-d 6.

Conclusions
In the conclusion of the first part of this inventory, "CO 2 Derivatives of Molecular Tin Compounds. Part 1: Hemicarbonato and Carbonato Complexes", we previously claimed that the CO 2 derivatives of molecular tin compounds could truly be considered as a class of compounds in their own right. The additional solid-state structures described in this second part and focusing more specifically on carbamato, formato, phosphinoformato and metallocarboxylato complexes, contribute assuredly to reinforce this point of view. Once again, a rich diversity of architectures, including discrete, dimeric, polynuclear and polymeric structures, and exhibiting various modes of coordination, sometimes totally unusual, were highlighted. Thus, this collection of compounds supports again the great reactivity of molecular tin complexes toward carbon dioxide and underlines the facile insertion of CO 2 into Sn-X bonds (X = OR, N, H, P). This reactivity, which has been known for a long time, has already been efficiently used for catalytic applications involving CO 2 and organotins, in particular as has been shown, with the aim of accessing carbamate compounds and their derivatives. However, the latest advances recently published are very interesting and promising, suggesting new perspectives in terms of reactivity of organotins with respect to small molecules and in particular for the activation of carbon dioxide. This is especially the case of studies relating to the reductive hydroboration of CO 2 or those involving the formation of CO 2 -Sn/phosphorus frustrated Lewis pairs (FLP) adducts, which start new insights. In the field of inorganic chemistry, the isolation of a new cluster incorporating a bis-stannyl-carboxylate group, also viewed as the coordination of a carbonite fragment, constitutes an innovative result which can be considered as unprecedented. Thus, the second half of the last century was incontestably a period of strong developments and fundamental advances in organotin chemistry, but the coming decades promise to be just as fruitful and exciting in terms of progress and inventiveness, and in particular, in the quest to activate carbon dioxide.