Doubly Bridged Double Cubanes Containing MFe3S4 Clusters (M = Mo, W). Synthesis, Structure, and Conversion to Spin-Quartet Single Clusters in Solution

Article abstract of DOI:10.1021/ja00380a011

Reaction of the Fe(III)-bridged double-cubane clusters ^3- with the 3,6-disubstituted catechols R'2catH2 in acetonitrile solution at ambient temperature results in bridge cleavage and formation of the new set of clusters ^4- (M = Mo, W; R' = n-Pr, CH2CH=CH2), which are isolated in good yields as Et4N⁺ salts.The structure of the monoclinic form of (Et4N)4 is reported.It crystallizes in space group C2/c with a = 31.474 (7) Angstroem, b = 19.544 (5) Angstroem, c = 25.182 (6) Angstroem, β = 141.77 (1) grad, and Z = 4; the structure was refined to R = 4.8percent by using 4227 unique data (F_o² > 3?(F_o²)).The structure of the centrosymmetric anion is that of a doubly bridged double-cubane cluster.Two MoFe3S4(SEt)2(R'2cat) subclusters, of unexpectional dimensions and containing a Mo-R'₂cat chelate ring, are joined by two Mo(μ-SEt)Fe bridges.Bridge bond distances are 0.04 - 0.09 Angstroem longer than comparable terminal distances.In solution the clusters undergo complete thiolate ligand substitution with arylthiols RSH affording ^4- (R = Ph, p-tolyl, p-C6H4Cl), several of which have been isolated as Et4N⁺ salts.In coordinating solvents such as Me2SO, acetonitrile, and DMF at ambient temperature the double cubanes are cleaved to the single cluster solvates ^2-.On the basis of ¹H NMR evidence, all thiolate ligands are present as terminal Fe-SR groups; M atom coordination is completed by a solvent molecule, which is in rapid exchange with bulk solvent.Magnetic susceptibility and EPR spectral properties confirm a S = 3/2 ground state.EPR parameters resemble those of the S = 3/2 spin system of FeMo proteins and the FeMo cofactor of nitrogenase, suggesting an electronic similarity.Solvate clusters undergo a single one-electron reduction at ca. -1.1 to -1.5 V (vs.SCE).This property together with the lability of coordinated solvent molecules leads to the possibility that (reducible) nitrogenase substrates may be bound and activated at the M atom site.