Spectroscopic and electrochemical properties of heteroleptic cationic copper complexes bis-(diphenylphosphino)alkane-(2,2′-biquinoline)copper(I). Crystal structure of bis(diphenylphosphino)ethane-(2,2′-biquinoline)copper(I) perchlorate

A series of Cu^(I)(2,2′-biquinoline)(L)](ClO₄) complexes (L = bis(diphenylphosphino)methane (bppm), 1,2-bis(diphenylphosphino)ethane (bppe), 1,4-bis(diphenylphosphino)butane (bppb)) have been synthesized and characterized by elemental analysis, conductivity, ESI-mass, NMR and UV-Vis spectroscopies, cyclic voltammetry, X-ray diffraction (Cu^(I)(2,2′-biquinoline)(bppe)](ClO₄)) and DFT calculations. These compounds are monometallic species in a distorted tetrahedral arrangement, in contrast with related compounds found as dinuclear according to diffraction studies. The spectroscopic properties are not directly correlated with the length of alkyl chain bridge between the bis-diphenylphosphine groups. In this way, the chemical shift of some 2,2′-biquinoline protons and the metal to ligand charge transfer (Cu to 2,2′-biquinoline) follows the order Cu(2,2′-biquinoline)(bppm)](ClO₄), Cu(2,2′-biquinoline)(bppb)](ClO₄), Cu(2,2′-biquinoline)(bppe)](ClO₄). The same dependence is followed by the potentials to Cu(II)/Cu(I) couple. These results are discussed in terms of inter-phosphorus alkane chain length and tetrahedral distortions on copper.