Tripodal Bis(imidazole) Thioether Copper(I) Complexes: Mimics of the Cu(M) Site of Copper Hydroxylase Enzymes

Tripodal bis(imidazole) thioether ligands, (N-methyl-4,5-diphenyl-2-imidazolyl)2C(OR)C(CH3)2SR' (BIT(OR,SR'); R = H, CH3; R' = CH3, C(CH3)3, C(C6H5)3), have been prepared, offering the same N2S donor atom set as the CuM binding site of the hydroxylase enzymes, dopamine beta hydroxylase and peptidylglycine hydroxylating monooxygenase. Isolable copper(I) complexes of the type [(BIT(OR,SMe))Cu(CO)]PF6 (3a and 3b) are produced in reactions of the respective tripodal ligands 1a (R = H) and 1b (R = Me) with [Cu(CH3CN)4]PF6 in CH2Cl2 under CO (1 atm); the pyramidal structure of 3a has been determined crystallographically. The infrared (IR) nu(CO)'s of 3a and 3b (L = CO) are comparable to those of the Cu(M)-carbonylated enzymes, indicating similar electronic character at the copper centers. The reaction of [(BIT(OH,SMe))Cu(CH3CN)]PF6 (2a) with dioxygen produces [(BIT(O,SOMe))2Cu2(DMF)2](PF6)2 (4), whose X-ray structure revealed the presence of bridging BIT-alkoxo ligands and terminal -SOMe groups. In contrast, oxygenation of 2b (R = Me) affords crystallographically defined [(BIT(OMe,SMe))2Cu2(mu-OH)2](OTf)2 (5), in which the copper centers are oxygenated without accompanying sulfur oxidation. Complex 5 in DMF is transformed into five-coordinate, mononuclear [CuII(BIT(OMe,SMe))(DMF)2](PF6)2 (6). The sterically hindered BIT(OR,SR') ligands 9 and 10 (R' = t-Bu; R = H, Me) and 11 and 12 (R' = CPh3; R = H, Me) were also prepared and examined for copper coordination/oxygenation. Oxygenation of copper(I) complex 13b derived from the BIT(OMe,SBu-t) ligand is slow, relative to 2b, producing a mixture of (BIT(OMe,SBu-t))2Cu2(mu-OH)2-type complexes 14b and 15b in which the -SBu-t group is uncoordinated; one of these complexes (15b) has been ortho-oxygenated on a neighboring aryl group according to the X-ray analysis and characterization of the free ligand. Oxygenation of the copper(I) complex derived from BIT(OMe,SCPh3) ligand 12 produces a novel dinuclear disulfide complex, [(BIT(OMe,S)2Cu2(mu-OH)2](PF6)2 (17), which is structurally characterized. Reactivity studies under anaerobic conditions in the presence of t-BuNC indicate that 17 is the result of copper(I)-induced detritylation followed by oxygenation of a highly reactive copper(I)-thiolate complex.