3[(dx2-y2, dxy)(pz)] excited states of binuclear copper(I) phosphine complexes: Effect of copper - ligand and copper - copper interactions on excited state properties and photocatalytic reductions of the 4,4′-dimethyl-2,2′-bipyridinium ion in alcohols

Abstract

A comprehensive study of the structural and spectroscopic properties of two-, three-, and four-coordinate copper(I) complexes with aliphatic phosphine ligands is presented. All complexes described in this work are characterized by X-ray crystallography. The intramolecular Cu⋯Cu separations in [Cu2(dcpm)2]X2, [Cu2(dcpm)2-(CH3CN)2] X2, and [Cu2(dmpm)3]-(ClO4)2 (dcpm=bis(dicyclohexylphosphino)methane; dmpm=bis(dimethylphosphino)methane; X=ClO4 - and PF6 -) are in the range 2.639(2)-3.021(2) Å. The anion⋯CuI interaction is weak, as evidenced by the nearest O⋯Cu separation of 2.558(6) Å in [Cu2(dcpm)2](ClO4)2 and the closest Cu⋯F separation of 2.79(1) Å in [Cu2(dcpm)2](PF6)2. The absorption bands of [Cu2(dcpm)2]X2 and [Cu2(dcpm)2(CH3CN)2]X2 (X=ClO4 - and PF6 -) at λmax 307-311 nm in CH2Cl2 are assigned as 1[3dσ* → 4pσ] transitions; this has been confirmed by resonance Raman spectroscopy. The triplet emissions in the visible region from these complexes exhibit long lifetimes and are sensitive to the environment. The lowest emissive excited state is tentatively ascribed as 3[(dx2-y2, dxy)(pz)] in nature. For [Cu2(dcpm)2]2+ salts in CH3CN, the emissive species is postulated to be [Cu2(dcpm)2(CH3CN)n]2+ (n≥3). Efficient photocatalytic reduction of MV2+ (4,4′-dimethyl-2,2′-bipyridinium) to MV+ in alcoholic solutions by using [Cu2(dcpm)2](PF6)2 or [Cu2(dppm)2(CH3CN)4] (ClO4)2 (dppm= bis(diphenylphosphino)methane) as a catalyst has been observed. The addition of CH3CN or use of [Cu2(dmpm)3]-(ClO4)2 as a catalyst did not allow photocatalytic reduction processes to occur.