Spectroelectrochemical (IR, UV/Vis) Determination of the Reduction Pathways for a Series of [Re(CO)3(.alpha.-diimine)L']0/+ (L' = Halide, OTf-, THF, MeCN, n-PrCN, PPh3, P(OMe)3) Complexes

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Abstract

The reduction pathways were investigated for a series of [Re(C0)3(a-diimine)L'lo'+ (L' = C1-, Br-, I-, Otf-, THF, MeCN, n-PrCN, PPh3, P(OMe)3; a-diimine = bpy, i-Pr-PyCa, dapa, dpp, abpy) complexes.The individual one-electron reduction steps were studied mainly in THF, by a combination of cyclic voltammetry and in situ FTIR and W / v i s spectroscopy.The stability of the reduced [Re(CO)3(a-diimine)L'Y-'' complexes depends on the ability of the a-diimine radical ligand to accommodate the unpaired electron in its lowest empty n* orbital, which increases in the order bpy < i-Pr-PyCa < dapa < dpp < abpy.The higher the energy of this n* orbital, the more pronounced is the donor character of the a-diimine radical.Concomitant polarization of the radical complex and increased electron density within the Re-(a-diimine) chelate ring labilize the Re-L' bond and may cause dissociation of L' and dimerization of the Re radicals, or substitution of L' by a better ~t acceptor.The kinetic stability of the Re-L' bond increases in the order halide << THF < PPh3, n-PrCN < P(OMe)3.The labilizing effect of the strong donating power of the reduced a-diimine ligand on the Re-L' bond becomes much more apparent upon the second one-electron reduction step.For a-diimine = bpy, i-Pr-PyCa, dpp, the products of the first one-electron reduction were converted in this step into the five-coordinated anionic [Re(CO)s(a-diimine)l-complexes, in which the n-electron density is largely delocalized over the Re-(a-diimine) chelate bond, as indicated by their low v(C0) frequencies and average C=O force constants.For a-diimine = dapa, the anionic [Re(C0)3(~,N,(N)-dapa)]species was produced, in which the third imine nitrogen atom interacts only weakly with the Re center.Importantly, for L' = P(OMe)3, the strongest n acceptor employed, the stable six-coordinated [Re(C0)3(N,N-dapa)(P(OMe)3)3anion was formed.The only stable six-coordinated [Re(C0)3(a-diimine)L'l-anions with L' = THF or PPh3 in the axial position were obtained for a-diimine = abpy.The abpy ligand is a strong n acceptor due to its very low-lying n* LUMO of the azo bond character.In this case, even a third one-electron reduction step was observed, which is probably more localized on the aromatic rings of abpy and converted the abpy ligand into a strong donor.As a result, the interaction of the ligand L' = THF, PPh3 with the Re center in the dianionic product { [Re(CO)3(abpy)12-* -L'} is very weak.

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