METAL METAL MULTIPLE BONDS IN ORDERED ASSEMBLIES .1. TETRANUCLEAR MOLYBDENUM AND TUNGSTEN CARBOXYLATES INVOLVING COVALENTLY LINKED M-M QUADRUPLE BONDS - MOLECULAR-MODELS FOR SUBUNITS OF ONE-DIMENSIONAL STIFF-CHAIN POLYMERS

Tetranuclear compounds of formula [M2(O2C-t-Bu)3]2(mu-L), where M = Mo and W, have been prepared by either the reaction between M2(O2C-t-Bu)4 and a dicarboxylic acid or from the reaction between M2(O2C-t-Bu)3(CH3CN)x+ and an anionic bridging ligand, L(2-). Specific complexes studied are for L = oxalate, perfluoroterephthalate, 1,8-anthracenedicarboxylate, 9,1 0-dihydro- 1,8-anthracenedicarboxylate, 1, l'-ferrocenedicarboxylate, 2,3-dioxyquinoxaline, and 2,7-dioxynaphthyridine. Within this class the M4-M bonds are covalently linked by the rigid bridges L in either a perpendicular or parallel mode as represented by oxalate and 1,8-anthracenedicarboxylate, respectively. These complexes have been studied by NMR spectroscopy, UV-visible spectroscopy, cyclic voltammetry, X-ray diffraction (L = 2,7-dioxynaphthyridine), and approximate MO calculations employing the Fenske-Hall method. The oxidized radical cations have been prepared for L = oxalate and perfluoroterephthalate, and these were studied by ESR and near-IR spectroscopy. Collectively these studies show that the electronic coupling between the two quadruply bonded M2 centers can be mediated by the ligand bridge to yield class I, II, or III behavior in both the perpendicular and parallel modes. In the absence of a suitable ligand bridge, the alignment of the M2 units in the parallel mode does not favor electronic coupling, and MO calculations reveal little splitting of the delta-orbitals These results are compared with studies of ligand bridged dinuclear systems such as Ru(II/III)-pyrazine-Ru(II/III) and are discussed in terms of perpendicular and parallel stiff-chain polymers incorporating M4-M bonds.