Chiral Tripodal Ligand Containing Three N-Heterocyclic Donor Functions and Its Copper Complexes: Crystallization of [LCuI]22+/[L2CuII]2+ Stereoisomers and Tyrosinase Activity

A novel chiral ligand system L containing one pyridyl and two imidazolyl donor functions has been synthesized and investigated with respect to its Cu-I and Cu-II coordination chemistry. Reaction with [Cu(MeCN)(4)]PF6 and [Cu(MeCN)(4)]OTf led to the dimeric complexes [LCu](2)X-2 (1, X = PF6; 2, X = OTf) with the ligands L in different configurations (R,S). The ligand matrix formed in these complexes can also host a Cu-II ion instead of two Cu-I ions so that mixed crystals of [L2Cu]X-2 and [LCu](2)X-2 can be produced. The pure compounds [L2Cu]X-2 (3, X = PF6; 4, X = OTf) can be obtained by treatment of 1 and 2 with O-2 in acetonitrile, respectively. From the corresponding solution 3 crystallizes with the two L molecules in different configurations, while 4 crystallizes with the ligands in (S,S) or (R,R) configurations, respectively. Crystals containing the analogous diastereomers of 3 were obtained, besides those isolated previously, when this compound was synthesized by reaction of 1 with AgPF6. On treating 2 with O-2 as the oxidant in acetonitrile, besides formation of 4, additional evidence for oxygenation of L to L-ox, where one of the original phenyl units corresponds to a phenolate function, was found: The dinuclear complex [(LCu)-Cu-ox(OH)(OTf)CuL](OTf) (5) was isolated as the final product of O-2 activation and conversion, which resembles the one of tyrosinase. In acetonitrile 5 reacts further to give 4 and [(L2Cu2)-Cu-ox](OTf2 (6), and hence, product mixtures are obtained. In CH2Cl2 decomposition can be avoided, and hence, changing the solvent from acetonitrile to CH2Cl2 leads to selective formation of 5.