Chiral tripodal ligand containing three N-heterocyclic donor functions and its copper complexes: crystallization of [LCu(I)]2(2+)/[L2Cu(II)](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]2X2 (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]X2 and [LCu]2X2 can be produced. The pure compounds [L2Cu]X2 (3, X = PF6; 4, X = OTf) can be obtained by treatment of 1 and 2 with O2 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 O2 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 L(ox)Cu(OH)(OTf)CuL (5) was isolated as the final product of O2 activation and conversion, which resembles the one of tyrosinase. In acetonitrile 5 reacts further to give 4 and L(ox)2Cu22 (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.