Crystal structures and vibrational and solid-state (CPMAS) NMR spectroscopic studies in the tris(triphenylphosphine)-copper(I) and -silver(I) formate systems

Abstract

The complexes [Cu(PPh3)3O2CH]·HCO2H (1), [Cu(PPh3)3O2CH]·0.5EtOH (2), [Ag(PPh3)3O2CH]·HCO2H (3) and [Ag(PPh3)3O2CH] (4) have been prepared and studied by infrared and solid-state 13C and 31P cross-polarization magic-angle-spinning (CPMAS) NMR spectroscopy. Crystal structures of 1, 2 and 3 show increasing degrees of disorder, but all are shown to contain [M(PPh3)3O2CH] molecular units containing a four-coordinate metal centre with the three crystallographically independent PPh3 ligands occupying three of the coordination sites and the formate anion coordinated through one oxygen atom to the fourth site. The formic acid molecule in [Cu(PPh3)3O2CH]·HCO2H is hydrogen-bonded to the formate ion (O⋯O = 2.529(5) A) to form an assembly that is best described as a biformate anion [H(HCO2)2]−. Unlike the corresponding silver(I) complex, [Cu(PPh3)3O2CH] is not stable in its unsolvated form; the loss of the solvent of crystallization results in conversion of the complex to [Cu(PPh3)2O2CH] and PPh3. Bands in the IR spectra due to the formate and formic acid groups are assigned, and these are consistent with the silver complexes having structures analogous to those of the corresponding copper complexes. The 13C CPMAS NMR spectra clearly resolve resonances from the formate and formic acid carbon atoms, while the 31P CPMAS NMR spectra exhibit complex, partially resolved multiplets arising from inequivalence of the phosphine ligands and the presence of 1J(M,31P) and 2J(31P,31P) scalar (or spin–spin) coupling. These spectra were interpreted with the aid of the 2D 31P CPCOSY technique.