Hydrogen Bonds Dictate the Coordination Geometry of Copper: Characterization of a Square-Planar Copper(I) Complex

6,6-Bis(2,4,6-trimethylanilido)terpyridine (H(2)Tpy(NMes)) was prepared as a rigid, tridentate pincer ligand containing pendent anilines as hydrogen bond donor groups in the secondary coordination sphere. The coordination geometry of (H(2)Tpy(NMes))copper(I)-halide (Cl, Br and I) complexes is dictated by the strength of the NH-halide hydrogen bond. The (CuCl)-Cl-I and (CuCl)-Cl-II complexes are nearly isostructural, the former presenting a highly unusual square-planar geometry about Cu-I. The geometric constraints provided by secondary interactions are reminiscent of blue copper proteins where a constrained geometry, or entatic state, allows for extremely rapid Cu-I/Cu-II electron-transfer self-exchange rates. Cu(H(2)Tpy(NMes))Cl shows similar fast electron transfer (approximate to 10(5)m(-1)s(-1)) which is the same order of magnitude as biological systems.