DINUCLEAR BIS(PHOSPHANE)COPPER(I) COMPLEX ES WITH 2,2'-BIPYRIMIDINE AS BRIDGING LIGAND - STRUCTURAL ANALOGY TO BLUE COPPER CENTERS AND REDUCTION TO PARAMAGNETIC SPECIES

Dinuclear complexes [(PR3)2Cu(mu:eta4-bpym)Cu(PR3)2](BF4)2 with the bi s-chelating bridging ligand 2,2'-bipyrimidine (bpym) and various monod entate and bidentate phosphanes were synthesized via a four component ''self assembly'' reaction. The reversible electrochemical reduction t o monocationic radical complexes was studied by cyclic voltammetry and electron spin resonance (ESR). Highly resolved ESR spectra revealed t he presence of the bpym anion radical ligand via resolved hyperfine co upling with H-1, N-14, P-31, Cu-63 and Cu-65 nuclei. The variation of phosphane ligands was accompanied by significant changes of electroche mical potentials which were rationalized considering electronic and st ructural effects. The configurational flexibility of Cu(I) in this env ironment is evident from the X-ray crystal structure analysis of [(PPh 3)2Cu(mu:eta4-bpym)Cu(PPh3)2](BF4)2 . H2O . CH3OH for which the coordi nation at the metal can be described as 3 + 1 (NN'P + P'), qualitative ly similar to the (NN'S + S') arrangement around the type 1 Cu centers of ''blue'' copper proteins. Intramolecular cooperative pi donor/pi a cceptor/pi donor ''sandwich'' interactions provide parts of the drivin g force for this highly distorted coordination geometry which illustra tes the proclivity of Cu(I) towards such an ''entatic state'' arrangem ent.