Cyanide Addition to Diiron and Diruthenium Bis-Cyclopentadienyl Complexes with Bridging Hydrocarbyl Ligands

: We conducted a joint synthetic, spectroscopic and computational study to explore the reactivity towards cyanide (from Bu 4 NCN) of a series of dinuclear complexes based on the M 2 Cp 2 (CO) 3 scaffold (M = Fe, Ru; Cp = η 5 -C 5 H 5 ), namely [M 2 Cp 2 (CO) 2 ( µ -CO){ µ , η 1 : η 2 -CH=C=CMe 2 }]BF 4 ( 1Fe-Ru ), [Ru 2 Cp 2 (CO) 2 ( µ -CO){ µ , η 1 : η 2 -C(Ph)=CHPh}]BF 4 ( 2Ru ) and [M 2 Cp 2 (CO) 2 ( µ -CO){ µ -CN(Me)(R)}] CF 3 SO 3 ( 3Fe-Ru ). While the reaction of 1Fe with Bu 4 NCN resulted in prevalent allenyl deprotonation, preliminary CO-NCMe substitution in 1Ru enabled cyanide addition to both the allenyl ligand (resulting in the formation of a h 1 :h 2 -allene derivative, 5A ) and the two metal centers (affording 5B1 and 5B2 ). The mixture of 5B1-2 was rapidly converted into 5A in heptane solution at 100 ◦ C, with 5A being isolated with a total yield of 60%. Following carbonyl-chloride substitution in 2Ru , CN − was incorporated as a terminal ligand upon Cl − displacement, to give the alkenyl complex 6 (84%). The reactivity of 3Fe and 3Ru is strongly influenced by both the metal element, M, and the aminocarbyne substituent, R. Thus, 7aRu was obtained with a 74% yield from cyanide attack on the carbyne in 3aRu (R = Cy, cyclohexyl), whereas the reaction involving the diiron counterpart 3aFe yielded an unclean mixture of the metastable 7aFe and the CO/CN − substitution product 8aFe . The cyano-alkylidene complexes 7aRu (R = Cy) and 7bFe (R = Me) underwent CO loss and carbene to carbyne conversion in isopropanol at 60–80 ◦ C, giving 8aRu (48%) and 8bFe (71%), respectively. The novel compounds 5A , 5B1-2 , 6 and 7aRu were characterized by IR and NMR spectroscopy, with the structure of 7aRu further elucidated by single crystal X-ray diffraction analysis. Additionally, the DFT-optimized structures of potential isomers of 5A , 5B1-2 and 6 were calculated.


Introduction
Dinuclear metal complexes enable unique reactivity patterns on bridging ligands, arising from the cooperativity of the two closely situated metal centers, which are generally not attainable in related mononuclear species [1][2][3][4][5].The readily available and cost-effective diiron compound [Fe 2 Cp 2 (CO) 4 ] (Cp = η 5 -C 5 H 5 ) serves as an ideal platform to explore this chemistry, and it has indeed been utilized as a starting material to build a diverse array of bridging organometallic architectures [6][7][8][9][10].Typically, these reaction pathways initiate with the substitution of one carbonyl ligand, induced by either thermal or photolytic treatment.The parallel reactivity of the diruthenium homolog [Ru 2 Cp 2 (CO) 4 ], though relatively less explored, exhibits substantial similarities compared to its diiron counterpart.However, the stronger Ru-Ru bond, compared to Fe-Fe, permits, in specific cases, the modification of bridging hydrocarbyl fragments, avoiding detrimental fragmentation pathways that may be favored with the diiron complexes (vide infra) [11].Notably, the benchmark organometallic species [M 2 Cp 2 (CO) 4 ] (M = Fe, Ru) has been extensively studied in the past decades to explore new routes for carbon-carbon bond formation, aiming to model the heterogeneously catalyzed Fischer-Tropsch process [6,[12][13][14].Essentially, these studies relied on the principle that a dimetallic framework may represent the simplest system suitable for modeling a metal surface [15].
Inorganics 2024, 12, x FOR PEER REVIEW 2 of 22 in specific cases, the modification of bridging hydrocarbyl fragments, avoiding detrimental fragmentation pathways that may be favored with the diiron complexes (vide infra) [11].Notably, the benchmark organometallic species [M2Cp2(CO)4] (M = Fe, Ru) has been extensively studied in the past decades to explore new routes for carbon-carbon bond formation, aiming to model the heterogeneously catalyzed Fischer-Tropsch process [6,[12][13][14].Essentially, these studies relied on the principle that a dimetallic framework may represent the simplest system suitable for modeling a metal surface [15].The present work focuses on the reactivity of selected derivatives of [M2Cp2(CO)4] (M = Fe, Ru) featuring distinct hydrocarbyl ligands occupying a bridging coordination site, namely allenyl (1), alkenyl (2) and aminocarbyne (3), see Scheme 1.These air-stable compounds can be prepared using synthetic methodologies involving the initial displacement of one CO ligand with, respectively, 2-methyl-3-butyn-2-ol (leading to 1Fe-Ru [16,17]), diphenylacetylene (2Ru [18,19]) and isocyanides CNR (3Fe-Ru [20][21][22]).The diruthenium allenyl complex 1Ru exhibits a rich and versatile reactivity once a coordination site becomes available.This can be achieved using trimethylamine N-oxide (Me3NO = TMNO) in acetonitrile as a typical solvent, resulting in the selective substitution of one CO (eliminated as CO2) with a labile NCMe ligand [23,24].This lability is equivalent to a coordination vacancy, facilitating the entry of unsaturated substrates (e.g., alkynes, alkenes), which then couple with the bridging hydrocarbyl ligand to generate diverse organometallic motifs [25,26].However, the parallel chemistry of 1Fe is not accessible due to prevalent Fe-Fe bond cleavage induced by CO removal (see above) [16].Similar considerations apply to related alkenyl complexes, with the diruthenium 2Ru (and similar compounds) providing access to diverse structures upon reaction with small unsaturated organic units [27,28].Concerning the aminocarbyne complexes, 3Fe can be easily obtained, even in multigram scales, exploiting a straightforward and quite general synthetic route [20,21].The successive CO-NCMe substitution takes place with preservation of the dinuclear structure, allowing for several derivatization reactions that have been documented in the literature [7,8].Conversely, the synthesis of diruthenium aminocarbyne complexes (3Ru) is more challenging, and the N-cyclohexyl derivative (R = Cy in Figure 1) is the only one producible in a scale practically suitable for exploratory chemistry [22].The diruthenium allenyl complex 1Ru exhibits a rich and versatile reactivity once a coordination site becomes available.This can be achieved using trimethylamine N-oxide (Me 3 NO = TMNO) in acetonitrile as a typical solvent, resulting in the selective substitution of one CO (eliminated as CO 2 ) with a labile NCMe ligand [23,24].This lability is equivalent to a coordination vacancy, facilitating the entry of unsaturated substrates (e.g., alkynes, alkenes), which then couple with the bridging hydrocarbyl ligand to generate diverse organometallic motifs [25,26].However, the parallel chemistry of 1Fe is not accessible due to prevalent Fe-Fe bond cleavage induced by CO removal (see above) [16].Similar considerations apply to related alkenyl complexes, with the diruthenium 2Ru (and similar compounds) providing access to diverse structures upon reaction with small unsaturated organic units [27,28].Concerning the aminocarbyne complexes, 3Fe can be easily obtained, even in multigram scales, exploiting a straightforward and quite general synthetic route [20,21].The successive CO-NCMe substitution takes place with preservation of the dinuclear structure, allowing for several derivatization reactions that have been documented in the literature [7,8].Conversely, the synthesis of diruthenium aminocarbyne complexes (3Ru) is more challenging, and the N-cyclohexyl derivative (R = Cy in Figure 1) is the only one producible in a scale practically suitable for exploratory chemistry [22].
Cyanide addition serves as a valuable strategy for generating C-C bonds in organometallic chemistry [29,30], with tetrabutylammonium cyanide (Bu 4 NCN) being a convenient reagent due to its good solubility in common organic solvents [31,32].To date, the reactivity of diiron and diruthenium µ-allenyl and µ-alkenyl complexes (1-2, and their acetonitrile derivatives) with Bu 4 NCN remains unexplored.Conversely, this chemistry has been investigated for a few compounds of type 3, showing a significant influence of the specific R substituent [20,33,34].Moreover, diiron derivatives with a terminal cyanide ligand can be prepared from 3Fe in two steps, via the intermediate formation of labile acetonitrile adducts (Scheme 1) [35,36].Cyanide addition serves as a valuable strategy for generating C-C bonds in organometallic chemistry [29,30], with tetrabutylammonium cyanide (Bu4NCN) being a convenient reagent due to its good solubility in common organic solvents [31,32].To date, the reactivity of diiron and diruthenium µ-allenyl and µ-alkenyl complexes (1-2, and their acetonitrile derivatives) with Bu4NCN remains unexplored.Conversely, this chemistry has been investigated for a few compounds of type 3, showing a significant influence of the specific R substituent [20,33,34].Moreover, diiron derivatives with a terminal cyanide ligand can be prepared from 3Fe in two steps, via the intermediate formation of labile acetonitrile adducts (Scheme 1) [35,36].
Expanding our understanding of the reactivity of 1-3 with cyanide is motivated by two primary reasons.First, the unsaturation within the bridging hydrocarbyl ligand offers opportunities to increase the complexity of the organic moiety [37-39], with the cyano group potentially acting as a nitrogen donor towards one of the two iron centers [40][41][42].Second, the possibility of placing cyanide to occupy an iron coordination site deserves consideration for potential implications in catalysis [43].In particular, previous studies have shown that diiron bis-cyclopentadienyl complexes with terminal cyanide and bridging carbyne ligands behave as models of [FeFe] hydrogenase [44,45], promoting the electrocatalytic production of dihydrogen from acetic acid [46].It is hypothesized that the unsaturated carbon (carbyne) and nitrogen (CN) carbons bind the hydrogen atoms prior to H-H bond formation [47,48].Herein, we present a synthetic, spectroscopic and computational study providing new insights into the chemistry of 1-3, and some of their related acetonitrile derivatives, with Bu4NCN.

Results and Discussion
We started investigating the reactivity of the diiron allenyl complex 1Fe with Bu4NCN.This reaction predominantly yielded the diferracyclopentenone complex 4, resulting from the deprotonation of one methyl group [16] (Scheme 2).Given the ability of the cyanide ion to behave as a Brønsted base towards the allenyl ligand, we turned our attention to the acetonitrile derivative 1Ru-NCMe, which was prepared from 1Ru using the literature procedure [25].The subsequent reaction with Bu4NCN produced a mixture of products (Scheme 2), which could be partially separated via careful column chromatography on alumina.Expanding our understanding of the reactivity of 1-3 with cyanide is motivated by two primary reasons.First, the unsaturation within the bridging hydrocarbyl ligand offers opportunities to increase the complexity of the organic moiety [37-39], with the cyano group potentially acting as a nitrogen donor towards one of the two iron centers [40][41][42].Second, the possibility of placing cyanide to occupy an iron coordination site deserves consideration for potential implications in catalysis [43].In particular, previous studies have shown that diiron bis-cyclopentadienyl complexes with terminal cyanide and bridging carbyne ligands behave as models of [FeFe] hydrogenase [44,45], promoting the electrocatalytic production of dihydrogen from acetic acid [46].It is hypothesized that the unsaturated carbon (carbyne) and nitrogen (CN) carbons bind the hydrogen atoms prior to H-H bond formation [47,48].Herein, we present a synthetic, spectroscopic and computational study providing new insights into the chemistry of 1-3, and some of their related acetonitrile derivatives, with Bu 4 NCN.

Results and Discussion
We started investigating the reactivity of the diiron allenyl complex 1Fe with Bu 4 NCN.This reaction predominantly yielded the diferracyclopentenone complex 4, resulting from the deprotonation of one methyl group [16] (Scheme 2).Given the ability of the cyanide ion to behave as a Brønsted base towards the allenyl ligand, we turned our attention to the acetonitrile derivative 1Ru-NCMe, which was prepared from 1Ru using the literature procedure [25].The subsequent reaction with Bu 4 NCN produced a mixture of products (Scheme 2), which could be partially separated via careful column chromatography on alumina.
The first fraction eluted was complex 5A, comprising a µ-η 2 :η 2 coordinated allene ligand resulting from CN − addition to the α carbon of the allenyl moiety.This product was isolated with a yield of approximately 30% and identified by IR and NMR spectroscopy.The infrared spectrum of 5A (in CH 2 Cl 2 solution) exhibits the characteristic pattern of analogous diruthenium and diiron compounds [49,50], with two bands ascribable to the terminal carbonyls falling at 1948 and 1927 cm −1 , the latter being more intense than the former.Additionally, the weak absorption at 2200 cm −1 accounts for the cyanide incorporated within the allene moiety.The NMR spectra of 5A (in CDCl 3 ) show two sets of resonances in a molar ratio of 1.7, attributed to the two stereoisomers (5A-is1 and 5A-is2) differing in the spatial orientation of the C α substituents (i.e., CN and H), with the Cp ligands adopting a mutual trans geometry (with respect to the Ru-Ru axis).Remarkably, the related complex [Ru 2 Cp 2 (CO) 2 {µ,η 2 :η 2 -CH 2 =C=CMe(Ph)}] was reported to exist in CD 2 Cl 2 solution as two stereoisomers related to the spatial arrangement of Me and Ph [49].We note that the trans configuration for the Cp rings has been recognized in the solid-state structures of all crystallographically characterized complexes based on the M 2 Cp 2 (CO) 2 core (Cp = C 5 H 5 or substituted cyclopentadienyl) and containing a bridging (substituted) µallene ligand [50][51][52].The first fraction eluted was complex 5A, comprising a µ-η 2 :η 2 coordinated allene ligand resulting from CN − addition to the α carbon of the allenyl moiety.This product was isolated with a yield of approximately 30% and identified by IR and NMR spectroscopy.The infrared spectrum of 5A (in CH2Cl2 solution) exhibits the characteristic pattern of analogous diruthenium and diiron compounds [49,50], with two bands ascribable to the terminal carbonyls falling at 1948 and 1927 cm −1 , the latter being more intense than the former.Additionally, the weak absorption at 2200 cm −1 accounts for the cyanide incorporated within the allene moiety.The NMR spectra of 5A (in CDCl3) show two sets of resonances in a molar ratio of 1.7, attributed to the two stereoisomers (5A-is1 and 5A-is2) differing in the spatial orientation of the Cα substituents (i.e., CN and H), with the Cp ligands adopting a mutual trans geometry (with respect to the Ru-Ru axis).Remarkably, the related complex [Ru2Cp2(CO)2{µ,η 2 :η 2 -CH2=C=CMe(Ph)}] was reported to exist in CD2Cl2 solution as two stereoisomers related to the spatial arrangement of Me and Ph [49].We note that the trans configuration for the Cp rings has been recognized in the solid-state structures of all crystallographically characterized complexes based on the M2Cp2(CO)2 core (Cp = C5H5 or substituted cyclopentadienyl) and containing a bridging (substituted) µallene ligand [50][51][52].
The optimized geometries of 5A-is1 and 5A-is2 were DFT calculated and are shown in Figure S1.The structure with the cyano group pointing far away from the Ru2Cp2 scaffold (5A-is1) was found to be more stable than the other one by approximately 1.4 kcal/mol; a view of this structure is depicted in Figure 1 along with the main calculated bonding parameters.The lower stability of the cis configuration for the Cps was confirmed on theoretical grounds (Figure S1).
The optimized geometries of 5A-is1 and 5A-is2 were DFT calculated and are shown in Figure S1.The structure with the cyano group pointing far away from the Ru 2 Cp 2 scaffold (5A-is1) was found to be more stable than the other one by approximately 1.4 kcal/mol; a view of this structure is depicted in Figure 1 along with the main calculated bonding parameters.The lower stability of the cis configuration for the Cps was confirmed on theoretical grounds (Figure S1).
The most abundant fraction collected from the column chromatography of the reaction mixture from 1Ru-NCMe and Bu 4 NCN consisted of a mixture of the diruthenium complexes 5B1 and 5B2 (in a ratio of ≈3.3 according to 1 H NMR) that could not be separated from the ammonium salt by-product (Scheme 2).The identity of the isomers 5B1 and 5B2 was determined based on IR and 1 H NMR spectra, with the assistance of DFT calculations and literature data.In both isomers, the infrared wavenumber for the cyanide group (2104 cm −1 ) is lowered by ca. 100 cm −1 compared to 5A, indicative of coordination to a low valent ruthenium center [35,[53][54][55].The predominant isomer (5B1, see the DFT-optimized structure in Figure 2) displays one terminal CO (1983 cm −1 ) and one semibridging CO ligand (1894 cm −1 ), with the cyano group bound to Ru1 and the Cp ligands adopting a trans configuration.The same geometry was previously recognized for the closely related chloride complex [Ru 2 Cp 2 (Cl)(CO)(µ-CO){µ,η 1 :η 2 -CH=C=CMe 2 }], 1Ru-Cl (Figure 2), crystallographically characterized [49].In the IR spectrum of 1Ru-Cl (in CH 2 Cl 2 ), the carbonyl absorptions fall at 1982 and 1882 cm −1 .The semibridging coordination of one CO ligand is evident from the computational data obtained for 5B1, with the Ru1-µCO distance (1.904 Å) considerably shorter than Ru2-µCO (2.376 Å), analogous to what was reported for 1Ru-Cl [experimental distances of 1.886(3) and 2.403(2) Å, respectively].Complex 5B2 features the cyanide ligand bound to Ru2, and terminal and classical bridging carbonyl ligands, with corresponding IR stretching vibrations occurring at 1983 and 1827 cm −1 .The calculated Ru-µCO bond lengths are 2.022 and 2.083 Å (Figure 2).
complexes 5B1 and 5B2 (in a ratio of ≈3.3 according to 1 H NMR) that could not be separated from the ammonium salt by-product (Scheme 2).The identity of the isomers 5B1 and 5B2 was determined based on IR and 1 H NMR spectra, with the assistance of DFT calculations and literature data.In both isomers, the infrared wavenumber for the cyanide group (2104 cm −1 ) is lowered by ca. 100 cm −1 compared to 5A, indicative of coordination to a low valent ruthenium center [35,[53][54][55].The predominant isomer (5B1, see the DFToptimized structure in Figure 2) displays one terminal CO (1983 cm −1 ) and one semibridging CO ligand (1894 cm −1 ), with the cyano group bound to Ru1 and the Cp ligands adopting a trans configuration.The same geometry was previously recognized for the closely related chloride complex [Ru2Cp2(Cl)(CO)(µ-CO){µ,η 1 :η 2 -CH=C=CMe2}], 1Ru-Cl (Figure 2), crystallographically characterized [49].In the IR spectrum of 1Ru-Cl (in CH2Cl2), the carbonyl absorptions fall at 1982 and 1882 cm −1 .The semibridging coordination of one CO ligand is evident from the computational data obtained for 5B1, with the Ru1-µCO distance (1.904 Å) considerably shorter than Ru2-µCO (2.The 1 H NMR spectrum of the mixture 5B1/5B2 exhibits the typical low-field resonance for the C α -H (9.06 ppm for 5B1, 9.44 ppm for 5B2) [49,56,57], confirming that the structure and coordination of the bridging allenyl ligand are not affected by the incorporation of the cyano group in the complex.The 1 H NMR signals for the Cp rings of 5B1-2 fall in the range 5.08 to 5.30 ppm, indicating the same Cp arrangement in these two complexes.Since these chemical shift values are quite close to those reported for 1Ru-Cl (4.91, 5.26 ppm) and other trans-Ru 2 Cp 2 (CO) 2 structures with bridging hydrocarbyl ligands [58], it is plausible that the trans geometry occurs in 5B1-2.
The computed Gibbs free energy difference between trans-5B1 and trans-5B2 is small (<1 kcal/mol), justifying the occurrence of both these geometric isomers.However, computer outcomes do not rule out the potential existence of cis structures (see Figure S2).
It appears that the formation of 5A and 5B1-2 takes place with a rearrangement of the Ru 2 Cp 2 core, transitioning from the cis geometry observed in 1Ru [25] to the trans one.This phenomenon, well described for various CO-substituted bimetallic complexes, is believed to proceed according to the Adams-Cotton mechanism [8,[59][60][61].In this mechanism, cis and trans isomers may interconvert in solution via a bridge-opened structure, where bridging ligands move to terminal positions, followed by rotation around the metal-metal bond.The coordination switch from terminal to bridging sites and vice versa is likely responsible for the cyanide ligand in 5B1-2 binding to different ruthenium atoms.Notably, fast mobility of cyanide, moving from one metal center to another, was previously observed in [Fe 2 Cp 2 (CN)(CO) 3 ] − [62] and trinuclear platinum clusters [63].
To assess the thermodynamic stability of 5A and 5B1-2, we subjected these compounds to heating in heptane solution at ca. 100 • C. While 5A, and its stereoisomeric ratio, remained unchanged after 2 h, selective conversion of 5B1-2 to 5A was complete in approximately 10 min.Complex 5A was subsequently purified by alumina chromatography, providing a total yield of this compound from 1Ru of 60%.The cyanide migration reaction converting 5B1-2 to 5A mirrors the migration of hydride from bridging metal coordination to the C α allenyl carbon in a closely related diruthenium system [49].
The diphenyl-alkenyl diruthenium complex 2Ru-NCMe was prepared using the TMNO strategy [18], then 2Ru-NCMe was allowed to react with Bu 4 NCN in dichloromethane solution.This reaction cleanly resulted in acetonitrile-cyanide substitution, giving rise to 6 (Scheme 3).However, all our attempts to isolate 6 from tetrabutylammonium tetrafluoroborate, formed as the by-product of the substitution reaction, were unsuccessful.Consequently, an alternative route was devised to obtain pure 6.Initially, 2Ru-NCMe was converted to the chloride derivative 2Ru-Cl following a literature procedure.Subsequently, the reaction of 2Ru-Cl with Bu 4 NCN proceeded smoothly at room temperature affording 6 which could be effectively separated from Bu 4 NCl by alumina chromatography.The novel complex 6 was finally isolated with an 84% yield.
According to DFT calculations, the most stable structure of 6 features the cyano group bound to Ru2, with the Cp rings in the trans configuration (Figure 3).The bridging CO ligand is almost equidistant between the two ruthenium centers, the calculated Ru-µCO distances being 2.016 and 2.079 Å.Similarly, the alkenyl C α carbon is nearly equidistant from the two ruthenium centers, with Ru-C α distances of 2.097 and 2.169 Å.
Alternative isomers, respectively bearing Cp ligands in cis or the cyano bound to the other ruthenium (Ru1), appear significantly less probable on theoretical grounds (Figure S3).In particular, similar to the µ-allenyl complexes 5B1-2, the binding of cyanide to Ru1 would force one CO ligand to adopt a semibridging coordination fashion.Computed Ru-µCO distances in trans-6-1 are 1.902 and 2.347 Å, see Figure S3.
The spectroscopic data collected for 6 are in full agreement with the DFT outcomes.The IR spectrum, recorded in dichloromethane solution, exhibits three main absorptions accounting for a ruthenium-bound cyanide (2104 cm −1 ), a terminal carbonyl (1978 cm −1 ) and a bridging carbonyl ligand (1826 cm −1 ).The NMR spectra reveal a single species in CDCl 3 solution, with the Cp rings resonating at 5.38 and 4.97 ppm ( 1 H) and 93.3 and 93.0 ppm ( 13 C).In the 1 H spectrum, the alkenic CH appears at 4.92 ppm, while, in the 13 C NMR spectrum, the alkenic carbons, C a and C b , have been detected, respectively, at 154.4 and 72.4 ppm, and the cyanide at 132.6 ppm.The values for C a and C b are quite close to those reported for the crystallographically characterized trans-2Ru-Cl [27], and in particular, the low-field chemical shift of C a aligns with its bridging alkylidene character [7,8,27,64].According to DFT calculations, the most stable structure of 6 features the cyano group bound to Ru2, with the Cp rings in the trans configuration (Figure 3).The bridging CO ligand is almost equidistant between the two ruthenium centers, the calculated Ru-µCO distances being 2.016 and 2.079 Å.Similarly, the alkenyl Cα carbon is nearly equidistant from the two ruthenium centers, with Ru-Cα distances of 2.097 and 2.169 Å.According to DFT calculations, the most stable structure of 6 features the cyano group bound to Ru2, with the Cp rings in the trans configuration (Figure 3).The bridging CO ligand is almost equidistant between the two ruthenium centers, the calculated Ru-µCO distances being 2.016 and 2.079 Å.Similarly, the alkenyl Cα carbon is nearly equidistant from the two ruthenium centers, with Ru-Cα distances of 2.097 and 2.169 Å.We extended our study to the reactivity of the diiron and diruthenium aminocarbyne complexes 3 with Bu 4 NCN (Scheme 4).The reaction involving the diruthenium complex 3aRu yielded the bridging µalkylidene derivative 7aRu, resulting from the selective cyanide addition to the carbyne center.This outcome aligns with the previous reaction involving the N-benzyl substituted homologue of 3aRu, 72.4 ppm, and the cyanide at 132.6 ppm.The values for Ca and Cb are quite close to those reported for the crystallographically characterized trans-2Ru-Cl [27], and in particular, the low-field chemical shift of Ca aligns with its bridging alkylidene character [7,8,27,64].
We extended our study to the reactivity of the diiron and diruthenium aminocarbyne complexes 3 with Bu4NCN (Scheme 4).The reaction involving the diruthenium complex 3aRu yielded the bridging µalkylidene derivative 7aRu, resulting from the selective cyanide addition to the carbyne center.This outcome aligns with the previous reaction involving the N-benzyl substituted homologue of 3aRu, [Ru2Cp2(CO)2(µ-CO){µ-CN(Me)(CH2Ph)}]CF3SO3, affording [Ru2Cp2(CO)2(µ-CO){µ-C(CN)NMe(CH2Ph)}] [34].Complex 7aRu was isolated after alumina chromatography and isolated with a 74% yield.X-ray quality crystals of this compound were obtained from a dichloromethane/hexane mixture settled aside at −30 • C, and the molecular structure was subsequently determined through X-ray diffraction analysis (Figure 4).A few dinuclear complexes containing a bridging cyano-aminoalkylidene ligand have been crystallographically characterized, all based on the M 2 Cp 2 (CO) 2 core (M = Fe, Ru) [34,38,[65][66][67][68]. Bonding parameters of 7aRu resemble those previously reported for the closely associated compound [Ru 2 Cp 2 (CO) 2 (µ-CO){µ-C(CN)NMe(CH 2 Ph)}], featuring a benzyl group in place of cyclohexyl [34].In both structures, the cyano group points towards the side with the Cp rings, which are in relative cis orientation.In 7aRu, the C( hexyl [34].In both structures, the cyano group points towards the side with the Cp rings, which are in relative cis orientation.In 7aRu, the C(4)-N( 1  The IR spectrum of 7aRu (in CH2Cl2) exhibits the pattern typical of a Ru2Cp2(CO)2(µ-CO) core, consisting of three carbonyl bands (2004,1968 and 1800 cm −1 , respectively).Additionally, the absorption at 2146 cm −1 accounts for the carbene-bound cyano moiety.The NMR spectra (CDCl3) reveals the presence of a single species in solution, likely corresponding to the same geometry observed in the solid-state structure, thereby indicating the absence of stereoisomerism arising from the orientation of the Cp ligands or the alkylidene substituent.The 13 C NMR signal for the alkylidene carbon falls at 139.8 ppm, consistent with data on related diruthenium complexes [34,68].
Surprisingly, the reaction of the diiron counterpart of 3aRu, namely 3aFe, with cyanide revealed significant differences between these two homologous compounds (Scheme 4).A mixture of products was obtained from 3aFe, as indicated by the IR spectrum of the The IR spectrum of 7aRu (in CH 2 Cl 2 ) exhibits the pattern typical of a Ru 2 Cp 2 (CO) 2 (µ-CO) core, consisting of three carbonyl bands (2004,1968 and 1800 cm −1 , respectively).Additionally, the absorption at 2146 cm −1 accounts for the carbene-bound cyano moiety.The NMR spectra (CDCl 3 ) reveals the presence of a single species in solution, likely corresponding to the same geometry observed in the solid-state structure, thereby indicating the absence of stereoisomerism arising from the orientation of the Cp ligands or the alkylidene substituent.The 13 C NMR signal for the alkylidene carbon falls at 139.8 ppm, consistent with data on related diruthenium complexes [34,68].
Surprisingly, the reaction of the diiron counterpart of 3aRu, namely 3aFe, with cyanide revealed significant differences between these two homologous compounds (Scheme 4).A mixture of products was obtained from 3aFe, as indicated by the IR spectrum of the reaction mixture.Through careful chromatography under a strictly inert atmosphere, the alkylidene complex 7aFe and the aminocarbyne derivative 8aFe were separated.The unprecedented 7aFe proved to be strongly air-sensitive, converting upon air contact into 3aFe (detected by IR and 1 H analyses) and a paramagnetic mixture of unidentified carbonyl species.Moreover, 7aFe is unstable in CH 2 Cl 2 or CDCl 3 , where it slowly underwent cyanide loss to recover 3aFe, presumably via cyanide-chloride exchange with the solvent [69].Thus, the identification of 7aFe relied on the solution IR spectrum and key NMR data, but a full spectroscopic characterization was not possible.
Complex 8aFe was previously synthesized from 3aFe using the acetonitrile substitution route (see Scheme 1) [36,70].Diiron and diruthenium complexes with the general formula [M 2 Cp 2 (L)(CO)(µ-CO){µ-CN(Me)(R)}] 0/+ (R ̸ = Me, L = anionic or neutral ligand) can exhibit both cis/trans isomers, with reference to the geometry of the Cps, and α/β isomers, differing in the relative orientation of R and L [7,8,70], as shown in Scheme 5.The α/β isomerism arises from the inhibited rotation around the µ-(C-N) bond, which possesses a significant iminium character [8].When L is a halide or pseudohalide ligand, the IR spectrum serves as a strongly diagnostic tool for detecting cis and trans forms.Typically, the bridging CO stretching wavenumber (around 1800 cm −1 ) is almost coincident in such two isomers, while the terminal CO stretching significantly differs, occurring at ca. 1980 and 1960 cm −1 in the cis and trans isomers, respectively [7,8,70].
Complex 8aFe was previously synthesized from 3aFe using the acetonitrile substitution route (see Scheme 1) [36,70].Diiron and diruthenium complexes with the general formula [M2Cp2(L)(CO)(µ-CO){µ-CN(Me)(R)}] 0/+ (R ≠ Me, L = anionic or neutral ligand) can exhibit both cis/trans isomers, with reference to the geometry of the Cps, and α/β isomers, differing in the relative orientation of R and L [7,8,70], as shown in Scheme 5.The α/β isomerism arises from the inhibited rotation around the µ-(C-N) bond, which possesses a significant iminium character [8].When L is a halide or pseudohalide ligand, the IR spectrum serves as a strongly diagnostic tool for detecting cis and trans forms.Typically, the bridging CO stretching wavenumber (around 1800 cm −1 ) is almost coincident in such two isomers, while the terminal CO stretching significantly differs, occurring at ca. 1980 and 1960 cm −1 in the cis and trans isomers, respectively [7,8,70].
The IR and 1 H NMR spectra of 8aFe indicate the presence in solution of α-trans and β-trans isomers, as previously reported [70].
The outcome of the reaction involving 3aFe contrasts with previous findings where 3bFe reacted with tetrabutylammonium cyanide yielding the cyano-alkylidene derivative 7bFe as the sole, stable product [33].The steric hindrance introduced by the cyclohexyl group in 3aFe presumably plays a crucial role in disfavoring cyanide binding to the carbyne [20], although electronic factors should also be invoked, given the discrepancy observed between the reactivities of 3aFe and 3aRu.
To test the thermodynamic stability of the cyano-alkylidene complexes 7bFe and 7aRu, these were subjected to thermal treatment in various solvents.Interestingly, 7bFe Scheme 5. α/β and cis/trans isomers observed in asymmetric diiron and diruthenium aminocarbyne complexes.R = aryl or alkyl ̸ = Me.L = anionic or neutral ligand, corresponding to neutral and cationic complexes, respectively.
The IR and 1 H NMR spectra of 8aFe indicate the presence in solution of α-trans and β-trans isomers, as previously reported [70].
The outcome of the reaction involving 3aFe contrasts with previous findings where 3bFe reacted with tetrabutylammonium cyanide yielding the cyano-alkylidene derivative 7bFe as the sole, stable product [33].The steric hindrance introduced by the cyclohexyl group in 3aFe presumably plays a crucial role in disfavoring cyanide binding to the carbyne [20], although electronic factors should also be invoked, given the discrepancy observed between the reactivities of 3aFe and 3aRu.
To test the thermodynamic stability of the cyano-alkylidene complexes 7bFe and 7aRu, these were subjected to thermal treatment in various solvents.Interestingly, 7bFe underwent quantitative conversion to 8bFe when heated at around 60 • C in isopropanol, acetonitrile or tetrahydrofuran solutions.This conversion involved CO elimination and intramolecular cyanide migration.Isopropanol proved to be the most effective solvent for this transformation, yielding 8bFe in a 71% yield.
The IR spectrum of the reaction mixture revealed that the cyanide migration was nonselective when conducted in THF solution, resulting in the production of 8bFe in combination with minor, unidentified species.Note that the CO and Cp rings are potential sites of the addition of carbon nucleophiles to dinuclear complexes of type 3 [71,72].
Complex 8bFe, previously obtained via either CO-NCMe substitution (Scheme 1) [35] or selenocyanate decomposition [70], was identified by comparing spectroscopic data with the literature.The structure of 8bFe (as the bis-aqua species trans-8bFe•2H 2 O) was confirmed by X-ray diffraction (Figure S4).The structure of trans-8bFe was previously reported as a solvent-free crystal [70].Bonding parameters and stereochemistry are almost identical and will not be commented on any further.Hydrogen bonds are present involving the cyanide ligand and the water molecules.The only other example of crystallographically characterized dimetallic bis-cyclopentadienyl carbonyl complex featuring a terminal CN ligand is the homolog of 8bFe exhibiting two cis-oriented methylcyclopentadienyl ligands (Cp ′ ) [73].
The IR spectrum of 8bFe is diagnostic for a mixture of cis and trans isomers, with a prevalence of the former (CO bands at 1980, 1958 and 1803 cm −1 , vide infra) [8,70].Consistently, the 1 H NMR spectrum revealed two sets of signals in an approximate 4.5 ratio.
The diruthenium complex 7aRu was sluggish to the CN − migration-CO removal process described for 7bFe.As a matter of fact, this reaction reached approximately 50% conversion after 24 h in isopropanol at reflux.Adding TMNO to the reaction mixture significantly accelerated the process, reaching completion after 30 min in isopropanol at 60 • C.However, the favorable action of TMNO was at the expense of selectivity, resulting in significant amounts of a secondary product.Following alumina chromatography, 8aRu was finally isolated in a moderate yield.We explored the alternative possibility of obtaining 8aRu via chloride-cyanide replacement, similar to the process described for the synthesis of 6 from 2Ru-Cl (see Scheme 3).However, this route proved impracticable, as 3aRu-Cl showed inertness towards Bu 4 NCN (Scheme 4).
The carbonyl pattern in the IR spectrum of 8aRu suggests the presence of trans and cis isomers, with a large prevalence of the former, revealing a prevalent cis to trans rearrangement of the Ru 2 Cp 2 scaffold ongoing from 7aRu to 8aRu.The infrared absorption for the cyano group falls at 2098 cm −1 , suggestive of a Ru-CN linkage.Moreover, the aminocarbyne µ-C-N bond manifests itself with a medium intensity band at 1540 cm −1 , consistent with its partial double bond character [8,21].
The 1 H NMR spectrum of 8aRu exhibits two sets of signals for each cis/trans species, attributable to the α and β forms (Scheme 5).The overall trans to cis ratio is approximately 6.In the 13 C NMR spectrum, the carbyne center resonates in the typical low-field region characteristic of dinuclear µ-aminocarbyne complexes [8,[74][75][76].A comparative view of spectroscopic features of 8aRu and 8aFe (Table 1) highlights important electronic effects provided by the distinct metal centers.Regarding the IR signals, they are slightly shifted to lower wavenumbers in 8aFe compared to 8aRu.This observation is coherent with the generally higher degree of π-backdonation occurring to π-acceptor ligands from 3d soft metal centers compared to their 4d congeners, correlated with the lower electronegativity of the 3d elements, resulting in stronger 3d metal-ligand bonds [77][78][79][80].

General Details
Complexes [M 2 Cp 2 (CO) 4 ] (M = Fe, Ru) were purchased from Merck, while organic reagents were purchased from Merck or TCI Europe, and were of the utmost available purity.Complexes 1Ru-NCMe [25], 1Fe [16], 2-NCMe [27], 2Ru-Cl [27], 3aRu [22], 3aFe [21], 3bFe [20], 7bFe [33] were prepared according to the literature.Solvents were obtained from Merck (petroleum ether with a boiling point range of 40-60 • C).Dichloromethane, acetonitrile, tetrahydrofuran and hexane underwent drying using the solvent purification system mBraun MB SPS5.Reactions were carried out under N 2 atmosphere using standard Schlenk techniques and anhydrous solvents, and were monitored through liquid infrared spectroscopy.Chromatographic separations were conducted on columns of deactivated alumina (Merck, 4% w/w water) under N 2 atmosphere, using solvents from the bottle.Infrared spectra of solutions were recorded on a Perkin Elmer Spectrum 100 FT-IR spectrometer with a CaF 2 liquid transmission cell (2300-1500 cm −1 range).IR spectra were processed with Spectragryph software [81].NMR spectra were recorded at 298 K on a Jeol JNM-ECZ 400 MHz or a Jeol JNM-ECZ500R instrument, both equipped with Royal HFX Broadband probe.Chemical shifts (expressed in parts per million) are referenced to the residual solvent peaks ( 1 H, 13 C) [82].NMR spectra were assigned with the assistance of 1 H-13 C (gs-HSQC and gs-HMBC) correlation experiments [83].NMR signals due to secondary isomeric forms (where detectable) are italicized.Elemental analyses were performed on solid samples washed with pentane and prolongedly dried under vacuum, using a Vario MICRO cube instrument (Elementar).The isolated products were conserved under N 2 atmosphere.A solution of 1Fe (107 mg, 0.223 mmol) in CH2Cl2 (10 mL) was treated with Bu4NCN (66 mg, 0.246 mmol) and then left to stir for 3 h.The final solution was loaded on top of an alumina column, and the brown fraction corresponding to 4 was collected using a mixture of CH2Cl2 and THF (1:1 v/v) as the eluent.This product was obtained as a brown solid upon solvent evaporation under vacuum.Yield 44 mg, 50%.IR (CH2Cl2): ῦ/cm −1 = 1975vs (CO), 1796s (µ-CO), 1748m (C=O), 1611m (C=C).

Reaction of Diruthenium
A dark-orange solution of 2Ru-Cl (25 mg, 0.041 mmol) in CH 2 Cl 2 (10 mL) was treated with Bu 4 NCN (14 mg, 0.052 mmol).The resulting mixture was left to stir for 1.5 h, and the final light-orange solution was loaded on top of an alumina column.The fraction corresponding to 6 was collected using a CH 2 Cl 2 /THF mixture (3/1 v/v).The title compound was obtained as a yellow solid upon solvent evaporation under vacuum.Yield         Bu4NCN (58 mg, 0.22 mmol) and stirred for 1 h.The resulting solution was analyzed by IR spectroscopy [ῦ/cm −1 = 2142w, 2090w, 2068m, 2002vs, 1945s, 1796m, 1739s] and then loaded on top of an alumina column.Elution with neat dichloromethane afforded a purple fraction corresponding to 7aFe.Subsequently, a dark-green fraction corresponding to 8aFe was collected using acetonitrile as the eluent.The solvent was evaporated from each solution under vacuum.A mixture of 7aRu (55 mg, 0.097 mmol) and Me3NO•2H2O (TMNO•2H2O; 12 mg, 0.11 mmol) in deaerated i PrOH (10 mL) was heated at 80 °C for 2 h.Afterwards, the volatiles were removed under vacuum, and the resulting residue was dissolved in the minimum volume of dichloromethane.This solution was loaded on top of an alumina column.Impurities were eluted using dichloromethane.Subsequently, the yellow fraction corresponding to the title product was collected using a CH2Cl2/THF mixture (1:1 v/v).The solvent was then evaporated under vacuum, affording a yellow solid.Yield 25 mg (48%).volume of dichloromethane.This solution was loaded on top of an alumina column.Impurities were eluted using dichloromethane.Subsequently, the yellow fraction corresponding to the title product was collected using a CH2Cl2/THF mixture (1:1 v/v).The solvent was then evaporated under vacuum, affording a yellow solid.Yield 25 mg (48%).

Attempt to Prepare 8aRu via Chloride-Cyanide Substitution
The chloride complex 3aRu-Cl was prepared using a procedure analogous to that reported for the synthesis of the homologous diiron compound [21].A mixture of 3aRu (70 mg, 0.10 mmol), Me3NO•2H2O (TMNO•2H2O; 23 mg, 0.20 mmol) and LiCl (13 mg, 0.31 mmol) was refluxed in i PrOH (5 mL) for 2 h.The resulting red solution was allowed to cool to room temperature and taken to dryness under vacuum.Subsequently, 3aRu-Cl was recovered from alumina column chromatography using THF as the eluent.The eluate was taken to dryness under vacuum and the resulting orange solid was washed with  The chloride complex 3aRu-Cl was prepared using a procedure analogous to that reported for the synthesis of the homologous diiron compound [21].A mixture of 3aRu (70 mg, 0.

X-ray Crystallography
Crystal data and collection details for 7aRu•CH 2 Cl 2 and trans-8bFe•2H 2 O are reported in Table 2. Data were recorded on a Bruker APEX II diffractometer equipped with a PHOTON2 detector using Mo-Kα radiation.Data were corrected for Lorentz polarization and absorption effects (empirical absorption correction SADABS) [84].The structures were solved by direct methods and refined by full-matrix least-squares based on all data using F 2 [85].Hydrogen atoms were fixed at calculated positions and refined by a riding model, excepts those of the water molecules of trans-8bFe•2H 2 O which were located in the Fourier difference map and refined isotropically with restraints on the O-H and H•••H distances.All non-hydrogen atoms were refined with anisotropic displacement parameters.

Details of DFT Calculations
All geometries were optimized with ORCA 5.0.3 [86] using the BP86 functional with the zero-order regular approximation (ZORA) to take relativistic effects into account and in conjunction with a triple-ζ quality basis set (ZORA-def2-TZVP) and the auxiliary basis set SARC/J.For ruthenium, the basis set "SARC-ZORA-TZVP" [87] was used.The dispersion corrections were introduced using the Grimme D3-parametrized correction and the Becke-Johnson damping to the DFT energy [88].All the structures were confirmed to be local energy minima (no imaginary frequencies).The solvent was considered through the continuum-like polarizable continuum model (C-PCM, dichloromethane).

Conclusions
Dimetallic compounds offer uncommon reactivity enabled by cooperative effects provided by the interconnected metal centers, and diiron and diruthenium complexes based on the M 2 Cp 2 (CO) 3 scaffold serve as versatile substrates to explore reaction patterns and build new organometallic ligands.In this work, we explore the reactivity of a series of these types of complexes, featuring different hydrocarbyl ligands (C x H y ) on one bridging site, towards the cyanide ion.We demonstrate that cyanide addition may be favored by the prior extrusion of one CO ligand, and can be directed to the metal centers or the C x H y fragment, depending on the cases.However, intramolecular cyanide migration, from one site to another, can be promoted thermally, and is facilitated by the flexibility of the M 2 Cp 2 (CO) n framework, where the Cp and CO ligands easily exchange their positions and spatial arrangements adapting to structural changes on the hydrocarbyl moiety.Interestingly, the reactivity of aminocarbyne complexes highlights a significant influence of the metal type, with the aminocarbyne moiety manifesting enhanced stability in diiron complexes compared to diruthenium homologues.Overall, our findings expand the knowledge on the reactivity of easily accessible organometallic platforms and may provide useful insights for future synthetic design and catalytic studies.

Scheme 3 .
Scheme 3. Synthetic routes leading to a diruthenium µ-alkenyl complex with a cyanide ligand.Novel products/pathways are denoted in blue, while wavy bonds indicate cis-trans isomers.

Scheme 3 .Scheme 3 .
Scheme 3. Synthetic routes leading to a diruthenium µ-alkenyl complex with a cyanide ligand.Novel products/pathways are denoted in blue, while wavy bonds indicate cis-trans isomers.