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 Cu_M 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) ν(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(μ-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 [Cu^II(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(μ-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(μ-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.