Coordination chemistry of mixed pyridine-phenol ligands; Synthesis and crystal structure of [Cu2L2{mu-PhB(OH)O}(2)] [HL=6-(2-hydroxyphenyl)-2,2'-bipyridine], a dinuclear copper(II) complex with phenylboronate bridges

Reaction of Cu(BF4)(2) with 6-(2-hydroxyphenyl)-2,2'-bipyridine (HL) followed by precipitation with aqueous NaBPh(4) affords the unexpected product [Cu(2)L(2){mu-PhB(OH)O}(2)] in which the bridging groups-which are the anions of phenylboric acid, PhB(OH)(2)-are derived from decomposition of the [BPh(4)](-) counter-ion. The complex contains two five-coordinate Cu-II centres in an elongated square-pyramidal geometry, with a Cu-2(mu-O)(2) core, and appears to be stabilized by an aromatic pi-pi interaction in which the pendant phenyl ring of each bridging group stacks with one of the pyridyl rings of a ligand L. In solution EPR spectroscopy shows that the dinuclear complex is cleaved to a mononuclear species.