Copper(I) Complexes of Tripodal Tris(imidazolyl) Ligands:  Potential Mimics of the Cu(A) Site of Hydroxylase Enzymes

Copper(I) complexes of tripodal tris(N-methyl-4,5-diphenyl-imidazolyl)methane ligands, N₃CR (1a−c, R = OH, OMe, H), have been prepared as models for the Cu(A) site of copper hydroxylase enzymes. In the absence of additional donors, the ligands 1 react with [Cu(CH₃CN)₄]PF₆ (2) to produce dinuclear complexes [(N₃CR)₂Cu₂](PF₆)₂ (3) in which the tripodal ligands bridge two trigonal Cu centers; the structures of 3b and 3c are established by X-ray diffraction. Mononuclear adducts [(N₃CR)CuL]Z are produced with L = acetonitrile (4), carbon monoxide (5), and t-BuNC (6, 7). The carbonyl complexes 5 are in dynamic equilibrium with the dimeric complexes 3, but 5c (R = H) can be isolated. The structures of the isocyanide derivatives depend critically on the tripod methane substituent, R. Thus, the X-ray structures of 6 (R = OMe) and 7 (R = H) show trigonal and tetrahedral geometries, respectively, with bi- or tridentate coordination of the tripod. A trinuclear complex [Cu₃(N₃COH)₂(t-BuNC)₂](PF₆)₃ (8) is formed from N₃COH (1a) which features both three-coordinate and two-coordinate Cu atoms and bidentate tripod coordination. Reactions of dioxygen with dinuclear 3c or mononuclear [(N₃CR)CuL]Z are sluggish, producing from the latter in acetone [(N₃CH)Cu^II(L)(L‘)](PF₆)₂ (9, L = acetone, L‘ = H₂O).