Molybdenum Atom Ligand Substitution Reactions of MoFe3S4 Cubane-Type Clusters: Synthesis and Structures of Clusters Containing Mo-Bound Pseudosubstrates of Nitrogenase

Article abstract of DOI:10.1021/ja00321a019

The ligand substitution chemistry of the title clusters, containing a Mo atom in a coordination environment similar (by EXAFS criteria) to that in nitrogenase, has been further developed.Reaction of ^3- with tetrachlorocatechol (Cl4catH4) yields the doubly bridged double cubane ^4- (1).This and analogous clusters form the solvated, spin-quartet, single cubanes *)(MeCN)>^2- (2) in acetonitrile solution.These species undergo a number of Fe and Mo site-specific ligand substitution reactions, which are summarized and include catecholate exchange at the Mo site.Reaction of 2 (R=p-C6H4Cl; cat^*=3,6-diallylcatecholate) with excess Cl4catH2 and catechol (catH2) yields *)2>^4- (cat^*=Cl4cat, cat (3)). (Et4N)4 (3) crystallizes in monoclinic space group P2₁/n with a=18.25(2) Angstroem, b=22.08(3) Angstroem, c=26.21(3) Angstroem, β=99.43(10) deg, and Z=4.The anion consists of two MoFe3S4(S-p-C6H4Cl)2(cat) subclusters joined by two Mo-S(R)-Fe bridges whose elongated bonds contribute to the ready cleavage to 2 in acetonitrile.The ligand binding reactions of 2 and its W analogue were investigated by 1H NMR spectroscopy.In ca.10 mM solutions reactions of 1 equiv of the nitrogenase-related ligands L=CN^-, N3^-, N2H4, NH3, PhNHNH2, and piperidine were found to be nearly quantitative at 298 K.Spectra at 240 K unambiguously demonstrated binding from the appearance of inequivalent R-substituent resonances in a 2:1 intensity ratio, consistent with *)L>^2-,3- clusters (4, M=Mo, W) of C_s symmetry.By variation of R and cat^* groups using substitution reactions, diffraction quality crystals of 4 with L=CN^- and N3^- were obtained. (Et4N)3 crystallizes in orthorhombic space group Pnma with a=22.793(5) Angstroem, b=19.274(3) Angstroem, c=12.456(2) Angstroem, and Z=4. (Et4N)3 crystallizes in space group P2₁/n with a=19.158(9) Angstroem, b=18.56(1) Angstroem, c=20.15(1) Angstroem, β=105.96(5) deg, and Z=4.The Mo-L portions of these single cubanes are linear (L=CN^-) or bent (L=N3^-, Mo-N-N=125(1) deg), and Mo-C (2.19(3) Angstroem) and Mo-N (2.17(2) Angstroem) bond distances are consisitent with an intermediate oxidation state (III, IV) of the Mo atom.The results of this and prior investigations have defined means for systematic and specific manipulation of all noncore ligands in ³⁺ single cubanes, results that should be pertinent to the development of substrate-reducing systems based on reduced (²⁺) single-cubane clusters.