Dinuclear oxomolybdenum(V) complexes which show strong electrochemical interactions across bis-phenolate bridging ligands: a combined spectroelectrochemical and computational study

A UV/VIS/NIR spectroelectrochemical study has been carried out on a series of dinuclear complexes of the type [{Mo(Tp(Me,Me))(O)Cl}(2){mu-OO}], where 'OO' denotes a bis-phenolate bridging ligand and Tp(Me,Me) is tris(3,5-dimethylpyrazolyl)hydroborate. The bridging ligands are 1,4-[O(C6H4)(n)O](2-)(n=1 1, 2 2, 3 3 or 4 4), 1,4-[O(C(6)H(3)Me2)(2)O](2-) 5, 1,3-[O(C6H4)O](2-) 6 and 1,4-[OC6H4XC6H4O](2-) (X=CH2 7, S 8 or SO2 9). Thus 1-4 have oligophenylene spacers; in 5 the biphenyl bridge is twisted by the presence of the Me substituents, in contrast to 2 which has a normal biphenyl spacer; 6 has a meta-substituted phenylene bridge in contrast to the para-substituted analogue 1; and 7-9 have single-atom spacers between the two phenyl rings. All complexes undergo two one-electron oxidations and two one-electron reductions, apart from 6 whose oxidation is irreversible. The effects of the different spacer groups on the electrochemical interactions in the complexes were examined by voltammetric determination of the redox splittings, the thioether spacer of 8 proving particularly effective at transmitting electronic interactions compared to the SO2 bridge of 9. UV/VIS/NIR Spectroelectrochemical studies on the mono- and di-oxidised complexes showed the presence of intense, low-energy phenolate --> Mo-VI charge-transfer bands; for example for [4](2+), lambda(max) = 1033 nm (epsilon = 50 000 dm(3) mol(-1) cm(-1)). The assignments of these as LMCT transitions were confirmed by spectroelectrochemical studies on mononuclear model complexes [Mo(Tp(Me,Me))(O)Cl(OC6H4R)] (R=H 10 or OMe 11) and by molecular orbital (ZINDO) calculations. Experimental and computational evidence indicate that the large separation between the two oxidations of 1-4 is ascribable in part to a near-planar bridging ligand conformation. The reduced forms of 1 and 6 were also examined by spectroelectrochemistry; whereas [1](-) [(MoMoV)-Mo-IV state] shows low-energy intervalence charge-transfer transitions across the para-substituted bridge, no such transitions are detectable across the meta-substituted bridge of [6](-).