Clusters with molybdenum iron(3) sulfur(4) subunits and molybdenum-coordinated polycarboxylate ligands: Structural and reactivity models for the iron-molybdenum cofactor of nitrogenase.

The use of $\rm(Et\sb4N)\sb2\lbrack MoFe\sb3S\sb4Cl\sb3(Cl\sb4$-cat)$\rm(CH\sb3CN)\rbrack$ () as starting material in the synthesis of $\lbrack\rm MoFe\sb2S\sb4\rbrack\sp{3+}$ clusters with Mo-bound polycarboxylate ligands is exploited. Reaction of 1 with oxalic acid affords $\rm(Et\sb4N)\sb2\lbrack(MoFe\sb3S\sb4Cl\sb4)(\mu$-$\rm C\sb2O\sb4)\rbrack$ double cubane (). Reaction of with $\rm(Et\sb4N)\sb2(C\sb2O\sb4)$ gives $\rm(Et\sb4N)\sb3\lbrack(MoFe\sb3S\sb4Cl\sb4)(C\sb2O\sb4)\rbrack$ (), which undergoes substitution reaction with CN$\sp-$ at the Mo to afford $\rm (Et\sb4N)\sb3\lbrack(MoFe\sb3S\sb4Cl\sb3)$-$\rm (C\sb2O\sb4)(CN)\rbrack$ () and further coupling with S$\sp{2-}$ to yield $\rm(Et\sb4N)\sb5\{\lbrack(MoFe\sb3S\sb4Cl\sb2)$-$\rm(C\sb2O\sb4)\rbrack \sb2(\mu$-$\rm CN)(\mu$-$\rm S)\rbrack$ (). also reacts with other acids to give $\rm \lbrack(L)MoFe\sb3S\sb4Cl\sb3\rbrack\sp{n-}(L=citrate\sp{3-}, n=3,$; L = citrate$\sp{2-},$ n = 2,; L = citramalate, n = 2,; L = methyliminodiacetate, n = 2, 1) products. Compounds - and 1 have been structurally characterized. All clusters were studied by infrared, electronic and EPR spectroscopy and cyclic voltammetry. reacts with mercapto carboxylic acids to give the $\rm(Et\sb4N)\sb4\lbrack(MoFe\sb3S\sb4Cl\sb3)$-(SCH(R)$\rm CH\sb2COO)\rbrack\sb2$ (R = H, 2; R = CH$\sb3,$ 3; R = CH$\sb2$COOH, 4) end products in excellent yields. also reacts with ligands such as thio-diglycolic acid, 3,4-dimercapto toluene and thiosalicylic acid to give products 5, 6 and 7, respectively, that have the ligand anions coordinated to the Mo atom. All clusters have been studied by infrared and EPR spectroscopy and cyclic voltammetry. Clusters 2, 3 and 5 have been structurally characterized. Most of the above clusters catalytically reduce $\rm N\sb2H\sb4$ to NH$\sb3$ in CH$\sb3$CN with Co(Cp)$\sb2$ as reductant and $\rm Lut\cdot HCl$ as proton source. Alteration of the Mo-bound polycarboxylate influences the catalytic efficiency, which is proportional to the number of -COOH and -OH moieties available. Kinetic treatment for 3 gives values of K$\sb{m}=7.7$ mM and $\rm V\sb\max=2.20$ mM/min. $\rm PEt\sb3$ and CO inhibit $\rm NH\sb3$ production. Implications regarding the involvement of the Mo heterometal and the polycarboxylate ligand in the catalytic process are discussed and the possible relevance to the Mo-bound homocitrate of FeMo-co is presented. Cluster reacts with NaBPh$\sb4$ in the presence of excess PEt$\sb3$ to give $\rm Mo\sb2Fe\sb6(\mu\sb3$-$\rm S)\sb6(\mu\sb4$-$\rm S)\sb2(Cl\sb4$-cat)(PEt$\sb3$) (2). Structural determination shows 2 as a double cubane with the Mo atoms at the periphery of the aggregate. Cluster 2 is composed of two $\rm\lbrack MoFe\sb3S\sb4\rbrack\sp{2+}$ edge-linked reduced subunits through $\mu\sb4$-S moieties. Structural details, spectroscopic studies and reactivity of 2 towards small molecules (CO, Bu$\sp{\rm t}$NC, NO$\sp+$ and PEt$\sb3)$ are presented. The new $\rm MoFe\sb3S\sb4$-polycarboxylate clusters are the first examples of synthetic models that have Mo environment very similar to the one of the FeMo-co and also show catalytic reactivity towards the reduction of $\rm N\sb2H\sb4$ to $\rm NH\sb3.$.PhDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/104764/1/9610109.pdfDescription of 9610109.pdf : Restricted to UM users only