Synthesis, X-ray Structure, and Catalytic Properties of the Unprecedented Tetranuclear Copper(I) Species [Cu(dppz)]4 (Hdppz = 3,5-Diphenylpyrazole)

The reaction of [Cu(CH3CN)4](BF4) with 3,5-dipbenylpyrazole (Hdppz) in the presence of triethylamine yields quantitatively the new cooper(I) complex [Cu(dppz)]4(1). Reaction of 1 with stoichiometric amounts of cyclohexyl isocyanide (RNC) in diethyl ether affords the dimeric complex [Cu(dppz)(RNC)]2 (2). For both complexes 1 and 2 an X-ray crystal structure analysis has been performed. Crystals of 1 are triclinic, space group P1BAR (No. 2), with a = 19.423(9) angstrom, b = 11.627(1) angstrom, c = 11.905(2) angstrom, alpha = 85.75(1)degrees, gamma = 72.92(2)degrees, gamma = 88.92(2)degrees, and Z = 2. The final R = 0.047 for 3839 reflections having I > 3sigma(I). Compound 1 consists of tetrameric [Cu(dppz)]4 molecules with Cu...Cu interactions (average 3.12 angstrom) supported by dppz ligands in the exo-bidentate mode. Crystal of 2 are triclinic, space group P1BAR (No. 2), with a = 9.246(2) angstrom, b = 9.859(4) angstrom, c = 12.543(2) angstrom, alpha = 103.82(2)degrees, beta = 102.25(2)degrees, gamma = 110.75(2)degrees, and Z = 1. The final R = 0.049 for 1545 reflections having I > 3sigma(I). Complex 2 exhibits a Cu-[N-N]2-Cu ring in a rather unusual chairlike conformation. Complex 1 is catalytically active in the oxidative coupling of primary aromatic amines leading to the corresponding azobenzenes in the presence of molecular oxygen at atmospheric pressure. The reaction proceeds with a selectivity of 100%, no other oxidation products being formed. For this reaction, spectroscopic and chemical evidence seems to indicate the intermediacy of a copper(II) imido species.