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) Å) 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.