Extended Hydrogen-Bonded Molybdenum Arrays Derived from Carboxylic Acids and Dianilines: ROP Capability of the Complexes and Parent Acids and Dianilines

: From reactions involving sodium molybdate and dianilines [2,2 ′ -(NH 2 )C 6 H 4 ] 2 (CH 2 ) n (n = 0, 1, 2) and amino-functionalized carboxylic acids 1,2-(NH 2 )(CO 2 H)C 6 H 4 or 2-H 2 NC 6 H 3 -1,4-(CO 2 H) 2 , in the presence of Et 3 N and Me 3 SiCl, products adopting H-bonded networks have been characterized. In particular, the reaction of 2,2


Introduction
Despite the intensive research on imido compounds over the last few decades, driven partly by their presence in metathesis catalysts [1], examples of complexes bearing highly functionalized imido ligation remain somewhat limited [2].With this in mind, we have been interested in exploiting amino-functionalized carboxylic acids, and our early work focused on the use of anthranilic acid [3][4][5], and in separate studies we have reported the use of potentially chelating dianilines, to access new structural motifs [6,7].Our entry into such chemistry is via sodium molybdate, which is combined with anilines in the presence of triethylamine and trimethylsilyl chloride, using refluxing dimethoxyethane as the solvent [8][9][10][11].The use of dianilines or amino-functionalized carboxylic acids in combination with molybdate under such conditions has the potential to afford products with extensive intra-and/or inter-molecular hydrogen bonding.For example, use of [(2-NH 2 C 6 H 4 ) 2 O] led to the salt [Et 3 NH][MoCl 4 [(2-NH 2 C 6 H 4 )(2-NC 6 H 4 )O] comprising discrete units of two anions and two cations linked by a series of H-bonds [11].We also note that chiral diimido complexes (of Mo and W) have been reported by Sundermeyer et al. [12].Herein, we combine the two sets of 'ligands' in one pot, i.e., an amino-functionalized carboxylic acid and a chelating dianiline, with the aim of further increasing the degree of hydrogen bonding present.In particular, use of the dianilines [2,2 ′ -(NH 2 )C 6 H 4 ] 2 (CH 2 ) n (n = 0, 1, 2), in combination with either anthranilic acid or 2-aminoterephthalic acid, in the 'molybdate' preparation led to products (Scheme 1) with extensive intra-and intermolecular interactions resulting in the formation of H-bonded pairs, chains, or a 3D network.
Given the interest in alternatives to petroleum-derived plastics [13], t much research devoted to seeking catalysts that are capable of producin mers via ring opening polymerization (ROP) of cyclic esters.Both tran metal-free ROP systems are attracting interest [ [14][15][16][17][18][19][20][21][22][23][24].Ideally, such cat highly active, inexpensive, and non-toxic.In the case of metals, the use of metals is now attracting attention [25].In terms of molybdenum-based s ited number that have thus far been reported tend to exhibit activity only peratures, with early examples based on ammonium decamolybdate (me or bis-(salicylaldhydato)dioxomolybdenum (110 °C in mesitylene) [27].amples include oxo and imido complexes derived from chelating pheno oxydianiline [(2-NH2C6H4)2O] [11], or a family of iodoanilines [30].For m lybdenum-based systems, trans-esterification is evident.Given the some senal of molybdenum-based ROP catalysts, we have investigated the herein (shown in Scheme 1) for their potential in the ROP of ε-capr valerolactone.Moreover, given the aforementioned interest in metal-f ROP, we have also examined the use of the precursor anilines and acids ployed in the synthesis of 1-3 as ROP catalysts.We also note that there is in the ability of hydrogen bonding to promote ROP, particularly in carbo systems [31] and amine/amide-containing systems [32][33][34][35][36][37][38][39].Given the interest in alternatives to petroleum-derived plastics [13], there is currently much research devoted to seeking catalysts that are capable of producing greener polymers via ring opening polymerization (ROP) of cyclic esters.Both transition metal and metal-free ROP systems are attracting interest [ [14][15][16][17][18][19][20][21][22][23][24].Ideally, such catalysts should be highly active, inexpensive, and non-toxic.In the case of metals, the use of earth-abundant metals is now attracting attention [25].In terms of molybdenum-based systems, the limited number that have thus far been reported tend to exhibit activity only at elevated temperatures, with early examples based on ammonium decamolybdate (melt at 150 • C) [26] or bis-(salicylaldhydato)dioxomolybdenum (110 • C in mesitylene) [27].More recent examples include oxo and imido complexes derived from chelating phenols [28,29], or the oxydianiline [(2-NH 2 C 6 H 4 ) 2 O] [11], or a family of iodoanilines [30].For many of these molybdenum-based systems, trans-esterification is evident.Given the somewhat limited arsenal of molybdenum-based ROP catalysts, we have investigated the three complexes herein (shown in Scheme 1) for their potential in the ROP of ε-caprolactone and δ-valerolactone.Moreover, given the aforementioned interest in metal-free catalysts for ROP, we have also examined the use of the precursor anilines and acids (Scheme 2) employed in the synthesis of 1-3 as ROP catalysts.We also note that there is growing interest in the ability of hydrogen bonding to promote ROP, particularly in carboxylic acid-based systems [31] and amine/amide-containing systems [32][33][34][35][36][37][38][39].Scheme 2. Dianilines and acids employed herein.

Synthesis and Characterization of Hydrogen-Bonded 2,2′-Diaminobiphenyl-Derived Molybdenum Complex
Use of the established sodium molybdate/aniline/Et3N/Me3SiCl preparation [8][9][10][11] but employing a combination of 2,2′-diaminobiphenyl [2,2′-NH2(C6H4)]2 and 2-aminoterephthalic acid H2NC6H3-1,4-(CO2H)2 as the amine sources, following work-up (MeCN), led to the isolation of small orange/red prisms in moderate yield.In the IR spectrum, a broad peak at 3444 cm −1 is assigned to vNH (Figure S1, SI).Single crystals suitable for Xray diffraction were obtained from a saturated acetonitrile solution at 0 °C.The molecular structure is shown in Figure 1, with selected bond lengths and angles given in the caption; alternative views are given in Figure S2   , led to the isolation of small orange/red prisms in moderate yield.In the IR spectrum, a broad peak at 3444 cm −1 is assigned to vNH (Figure S1, SI).Single crystals suitable for X-ray diffraction were obtained from a saturated acetonitrile solution at 0 • C. The molecular structure is shown in Figure 1, with selected bond lengths and angles given in the caption; alternative views are given in Figure S2, SI.The asymmetric unit comprises one molybdenumcontaining complex, two 2,2 ′ -diaminobiphenyl molecules, and three and a half acetonitrile solvent molecules of crystallization, i.e., [  Catalysts 2024, 14, x FOR PEER REVIEW Scheme 2. Dianilines and acids employed herein.

Synthesis and Characterization of Hydrogen-Bonded 2,2′-Diaminobiphenyl-Derived Molybdenum Complex
Use of the established sodium molybdate/aniline/Et3N/Me3SiCl preparation but employing a combination of 2,2′-diaminobiphenyl [2,2′-NH2(C6H4)]2 and 2-ami ephthalic acid H2NC6H3-1,4-(CO2H)2 as the amine sources, following work-up (M led to the isolation of small orange/red prisms in moderate yield.In the IR spectr broad peak at 3444 cm −1 is assigned to vNH (Figure S1, SI).Single crystals suitable ray diffraction were obtained from a saturated acetonitrile solution at 0 °C.The mol structure is shown in Figure 1, with selected bond lengths and angles given in the ca alternative views are given in Figure S2, SI.The asymmetric unit comprises one m denum-containing complex, two 2,2′-diaminobiphenyl molecules, and three and a h etonitrile solvent molecules of crystallization, i.The only NH hydrogen atom that does not act as an H-bond donor is H(8B), presumably due to the lack of a suitably positioned acceptor.The structure adopts a 3D H-bonded network overall (Figure 3).

D-H•••
The MALDI-ToF spectrum of 1•2[2,2′-NH2(C6H4)]2•3.5MeCN is given in Figure S3, SI; for assignments see experimental Section 4.1.Complex 1 (and 2 and 3 below) despite being recrystallized from MeCN proved to be insoluble in common organic solvents, which not The only NH hydrogen atom that does not act as an H-bond donor is H(8B), presumably due to the lack of a suitably positioned acceptor.The structure adopts a 3D H-bonded network overall (Figure 3).only hampered characterization in solution (e.g., only a weak 1 H NMR spectrum of 1 was obtained, Figure S4, SI for the aromatic region), but also restricted our ROP studies to the melt phase, vide infra.

Synthesis and Characterization of the Methylene-Bridged Dianiline-Derived Molybdenum Complex
We then investigated the use of the methylene-bridged dianiline   S3, SI; for assignments see experimental Section 4.1.Complex 1 (and 2 and 3 below) despite being recrystallized from MeCN proved to be insoluble in common organic solvents, which not only hampered characterization in solution (e.g., only a weak 1 H NMR spectrum of 1 was obtained, Figure S4, SI for the aromatic region), but also restricted our ROP studies to the melt phase, vide infra.

Synthesis and Characterization of the Methylene-Bridged Dianiline-Derived Molybdenum Complex
We then investigated the use of the methylene Following work-up (extraction into MeCN), orange crystals were isolated, which were subjected to a singlecrystal X-ray determination.The molecular structure is shown in Figure 4, with selected bond lengths and angles given in the caption.The asymmetric unit comprises one molybdenum complex and three molecules of non-coordinated acetonitrile.The dianiline-derived ligand binds in imido/amine fashion about the octahedral metal center, whilst the 2aminoterephthalic acid binds in carboxylate/amide fashion.In the IR spectrum (Figure S5, SI), there are peaks at 3501, 3446, and 3385 cm −1 , which are assigned to vNH.Interestingly, a trimethylsilylation has occurred at the 2-aminoterephthalic acid, as has been observed previously in the system [Et

Synthesis and Characterization of the Ethylene-Bridged Dianiline-Derived Molybdenum Complex
The length of the bridge in the dianiline was then extended to an ethylene linkage, namely [

Synthesis and Characterization of the Ethylene-Bridged Dianiline-Derived Molybdenum Complex
The length of the bridge in the dianiline was then extended to an ethylene linkage, namely [2,2′-(NH2)C6H4]2CH2CH2, and, to avoid the previous silylation issue, anthranilic acid 1,2-(NH2)(CO2H)C6H4 was employed rather than 2-aminoterephthalic acid.Work-up as before led to the isolation of purple crystals of X-ray diffraction quality.In the IR spectrum (Figure S8, SI), there are peaks at 3452/3348 cm −1 , which are assigned to vNH.The molecular structure is shown in Figure 6, with selected bond lengths and angles given in the caption; alternative views are given in Figure S9

D-H•••
In the packing of 3•MeCN, the two unique Mo complexes H-bond in a head-to-tail fashion via the NH 3 groups to a coordinated carboxylate oxygen atom in the case of H(3C) to O(3), or via a bifurcated pair of interactions between H(6B) and O(1) and Cl(1); this is not over a crystallographic inversion center.These pairs of molecules then H-bond further to other molecules via the NH 3 groups to the non-coordinated carboxylate, resulting in H-bonded chains running parallel to c (Figure 7).These diamond-shaped interactions again occur in head-to-tail pairs not dictated by crystallographic inversion symmetry, involving O(2) and O(4).

Ring Opening Polymerization of ε-Caprolactone (ε-CL)
Complexes 1-3 have been screened for their ability to act as catalysts in the presence of benzyl alcohol (BnOH) for the ROP of ε-caprolactone, and the results are presented in Table 4. Results for 1-3 are compared with the related molybdenum-containing complexes I and II [11] (see Figure 8).Based on previous molybdenum ROP studies in our group [11,[28][29][30], we selected the conditions of 130 • C with a ratio of ε-CL to complex of 500:1 in the presence of one (for 2 and 3) or two (for 1) equivalents of benzyl alcohol per metal over 24 h, i.e., a [CL]:[catalyst]:[BnOH] ratio of 500:1:1 or 500:1:2.However, given the aforementioned problematic issues with solubility, the complexes herein were only screened as melts under either N 2 or air (see Table 4).All complexes were found to be active under these polymerization conditions with similar monomer conversions (>95%, e.g., Figures S11-S14, SI), affording relatively high molecular weight polymers, with 3 under N 2 (Entry 5, Table 4) affording the highest at ca. 31,650 Da, albeit with poor control (Ð = 3.66); selected gpc traces are given in Figures S15-S21.End group analysis by 1 H NMR spectroscopy reveals signals at 3.63, 5.09, and 7.34 ppm consistent with the presence of a BnO end group, which indicates that the polymerization proceeds through a coordination-insertion mechanism, whereby the monomer coordinates to the metal followed by the acyl oxygen bond cleavage of the monomer and chain propagation.Interestingly, consistent with the wide Ð values, the MALDI-TOF spectra revealed several families of products including OH terminated polymers, OBn terminated polymers, and cyclic polymers (e.g., Figures 9 and 10; expansions are given in the SI, Figures S22-S25).There was evidence of trans-esterification, and all observed M n values were significantly lower than the calculated values.4); right (Entry 2, Table 4).For Entry 1, Table 4 4); right using 3 (Entry 5, Table 4).For Entry 3,  S39 and S40).

Ring Opening Polymerization of δ-Valerolactone (δ-VL)
As in the case of ε-CL, the complexes herein were only screened for the ROP of δ-VL as melts under either N2 or air (see Table 5) using a ratio of [VL]:[catalyst]:[BnOH] of 500:1:2 (for 1) or 500:1:1 (for 2, 3, and I, II).All complexes were found to be active under these polymerization conditions with monomer conversions (>75%, Figures S28-S35, SI), affording relatively high molecular weight polymers; selected gpc traces are given in Figures S36-S42, SI.As for CL, the conversions observed for 3 were somewhat lower than for the other systems.The highest molecular weights were afforded by 2 under air (Entry 4, Table 5) and II under N2 (Entry 9, Table 5), although in each case there was evidence of bimodal behavior. 1H NMR spectra of the PVL again indicated that the products were catenulate.The MALDI-TOF spectra revealed several families of products including OHterminated polymers and cyclic polymers (e.g., Figures 11 and 12; expansions are given in Figures S43-S46, SI).As for PCL, there was evidence of trans-esterification, and all observed PVL Mn values were significantly lower than the calculated values.

Ring Opening Polymerization of δ-Valerolactone (δ-VL)
As in the case of ε-CL, the complexes herein were only screened for the ROP of δ-VL as melts under either N 2 or air (see Table 5) using a ratio of [VL]:[catalyst]:[BnOH] of 500:1:2 (for 1) or 500:1:1 (for 2, 3, and I, II).All complexes were found to be active under these polymerization conditions with monomer conversions (>75%, Figures S28-S35, SI), affording relatively high molecular weight polymers; selected gpc traces are given in Figures S36-S42, SI.As for CL, the conversions observed for 3 were somewhat lower than for the other systems.The highest molecular weights were afforded by 2 under air (Entry 4, Table 5) and II under N 2 (Entry 9, Table 5), although in each case there was evidence of bimodal behavior. 1H NMR spectra of the PVL again indicated that the products were catenulate.The MALDI-TOF spectra revealed several families of products including OHterminated polymers and cyclic polymers (e.g., Figures 11 and 12; expansions are given in Figures S43-S46, SI).As for PCL, there was evidence of trans-esterification, and all observed PVL M n values were significantly lower than the calculated values.5); right using 2 (Entry 4, Table 5).
TGA and DSC results (Figures S52-S55, SI) for the molybdenum catalysts employed herein indicate that at the temperature utilized for the ROP procedure, there is no degradation to other by-products.

Materials and Methods
All manipulations were carried out under an atmosphere of nitrogen using standard Schlenk line and cannula techniques or a conventional N 2 -filled glovebox.Solvents were refluxed over the appropriate drying agents and distilled and degassed prior to use, i.e., dimethoxyethane was refluxed over Na-benzophenone/ketyl, and acetonitrile and triethylamine were refluxed over calcium hydride.Trimethylsilylchloride (TCI, Oxford, UK), 2-aminoterephthalic   The pre-catalyst (0.010 mmol) was added to a Schlenk tube in the glovebox at room temperature.The appropriate equivalent of BnOH (from a pre-prepared stock solution of 1 mmol BnOH in 100 mL toluene) was added, and the system was stirred for 5 min and then the solvent was removed in vacuo.The appropriate amount of ε-CL was added, and the reaction mixture was then placed into a sand bath pre-heated at 130 • C and heated for the prescribed time (24 h) under either N 2 or air.The polymerization mixture was quenched on addition of an excess of glacial acetic acid (0.2 mL) in methanol (50 mL).The resultant polymer was then collected on filter paper and was dried in vacuo.GPC (in THF) was used to determine molecular weights (M n and PDI) of the polymer products.

X-ray Crystallography
In all cases, crystals suitable for an X-ray diffraction study were grown from a saturated MeCN solution at 0 • C. Diffraction data for 1•2[2,2 ′ -NH 2 (C 6 H 4 )] 2 •3.5MeCN and 2•3MeCN were collected on pixel array detector-equipped Rigaku diffractometers using a rotating anode X-ray source, while that for 3•MeCN was collected on a Bruker SMART 1K CCD (Bruker AXS, Madison, WI, USA) diffractometer equipped with a sealed-tube X-ray source [52].Data were corrected for absorption, polarization, and Lp effects.All of the structures were solved and refined routinely [53][54][55][56].H atoms were included in a riding model except the NH hydrogens in 2•3MeCN and 3•MeCN, where the coordinates were refined with mild distance restraints.U iso (H) was set to 120% of that of the carrier atoms except for OH, NH 3 , and CH 3 (150%).Further details are presented in Table 6.CCDC 2,286,024-2,286,026 contain the supplementary crystallographic data for this paper.These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures (accessed 20 March 2024).

Figure 2 .
Figure 2. View of H-bonded chains running parallel to c.

Figure 2 .
Figure 2. View of H-bonded chains running parallel to c.
[2,2′-(NH2)C6H4]2CH2 in combination with 2-aminoterephthalic acid H2NC6H3-1,4-(CO2H)2.Following work-up (extraction into MeCN), orange crystals were isolated, which were subjected to a single-crystal X-ray determination.The molecular structure is shown in Figure 4, with selected bond lengths and angles given in the caption.The asymmetric unit comprises one molybdenum complex and three molecules of non-coordinated acetonitrile.The dianiline-derived ligand binds in imido/amine fashion about the octahedral metal center, whilst the 2-aminoterephthalic acid binds in carboxylate/amide fashion.In the IR spectrum (Figure S5, SI), there are peaks at 3501, 3446, and 3385 cm −1 , which are assigned to vNH.Interestingly, a trimethylsilylation has occurred at the 2-aminoterephthalic acid, as has been observed previously in the system [Et3NH][MoCl3{2-(HN)C6H4CO2}{2-Me3SiO2CC6H4N}] [2].Moreover, a search of the Cambridge Structural Database (CSD) reveals four other occurrences of this type of PhCO2SiMe3 motif [42-45].The coordination around molybdenum is completed by two chloride ligands, which are trans to the amine and amide functions.The formula can thus be written as [MoCl2(O2CC6H3NHCO2SiMe3)(NC6H4CH2C6H4NH2)]•3MeCN (2•3MeCN).
2,2 ′ -(NH 2 )C 6 H 4 ] 2 CH 2 CH 2 , and, to avoid the previous silylation issue, anthranilic acid 1,2-(NH 2 )(CO 2 H)C 6 H 4 was employed rather than 2-aminoterephthalic acid.Workup as before led to the isolation of purple crystals of X-ray diffraction quality.In the IR spectrum (Figure S8, SI), there are peaks at 3452/3348 cm −1 , which are assigned to vNH.The molecular structure is shown in Figure 6, with selected bond lengths and angles given in the caption; alternative views are given in Figure S9, SI.This purple complex was identified as [MoCl 3 {1,2-(NH)(CO 2 )C 6 H 4 }{NC 6 H 4 CH 2 CH 2 C 6 H 4 NH 3 }]•MeCN (3•MeCN) and formed in good yield (ca. 65%).Single crystals suitable for X-ray diffraction were obtained from a saturated acetonitrile solution at 0 • C. In the asymmetric unit, there are two independent Mo complexes and two MeCN molecules of crystallization.The two molybdenum-containing zwitterionic molecules differ slightly in their conformations in the solid state.For example, the two rings in the N-N groups are twisted relative to each other by different amounts: C(8) > C(13) vs. C(16) > C(21) = 49.81(13) • and C(29) > C(34) vs. C(37) > C(42) = 30.87(13) • .In both unique complexes, there are intra-molecular bifurcated H-bonds between one of the NH 3 H atoms and the coordinated chlorides Cl(1) and Cl(2) or Cl(5) and Cl(6).The two MeCN solvent molecules of crystallization both act as H-bond acceptors from the N-H groups at N(1) and N(4) ( , SI.This purple complex was identified as [MoCl3{1,2-(NH)(CO2)C6H4}{NC6H4CH2CH2C6H4NH3}]•MeCN (3•MeCN) and formed in good yield (ca. 65%).Single crystals suitable for X-ray diffraction were obtained from a saturated acetonitrile solution at 0 °C.In the asymmetric unit, there are two independent Mo complexes and two MeCN molecules of crystallization.The two molybdenum-containing zwitterionic molecules differ slightly in their conformations in the solid state.For example, the two rings in the N-N groups are twisted relative to each other by different amounts: C(8) > C(13) vs. C(16) > C(21) = 49.81(13)° and C(29) > C(34) vs. C(37) > C(42) = 30.87(13)°.In both unique complexes, there are intra-molecular bifurcated H-bonds between one of the NH3 H atoms and the coordinated chlorides Cl(1) and Cl(2) or Cl(5) and Cl(6).The two MeCN solvent molecules of crystallization both act as Hbond acceptors from the N-H groups at N(1) and N(4) (Table3).

Figure 8 .
Figure 8. Known complexes I and II.

Figure 8 .
Figure 8. Known complexes I and II.

Figure 8 .
Figure 8. Known complexes I and II.