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-dime thylpyrazolyl)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 oligophe nylene 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 an alogue 1; and 7-9 have single-atom spacers between the two phenyl rings. Al l complexes undergo two one-electron oxidations and two one-electron reduct ions, apart from 6 whose oxidation is irreversible. The effects of the diff erent spacer groups on the electrochemical interactions in the complexes we re examined by voltammetric determination of the redox splittings, the thio ether spacer of 8 proving particularly effective at transmitting electronic interactions compared to the SO2 bridge of 9. UV/VIS/NIR Spectroelectroche mical 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 spectroele ctrochemical studies on mononuclear model complexes [Mo(Tp(Me,Me))(O)Cl(OC6 H4R)] (R=H 10 or OMe 11) and by molecular orbital (ZINDO) calculations. Exp erimental and computational evidence indicate that the large separation bet ween the two oxidations of 1-4 is ascribable in part to a near-planar bridg ing ligand conformation. The reduced forms of 1 and 6 were also examined by spectroelectrochemistry; whereas [1](-) [(MoMoV)-Mo-IV state] shows low-en ergy intervalence charge-transfer transitions across the para-substituted b ridge, no such transitions are detectable across the meta-substituted bridg e of [6](-).