SYNTHETIC AND STRUCTURAL STUDIES ON COPPER(II) COMPLEXES CONTAINING COENZYME PQQ AND TERPYRIDINE

The mononuclear and dinuclear ternary Cu(II) complexes of coenzyme pyr roloquinoline quinone (PQQ) and 2,2': 6',2''-terpyridine (terpy) have been isolated from aqueous solutions containing PQQ and [Cu(terpy)]2complex at pH 3 and 7, respectively, regardless of the molar ratios of the starting materials. The structure of the dinuclear Cu(II) complex has been determined by X-ray diffraction method. Crystallographic dat a are as follows: [Cu2(PQQ) (terpy)2].12H2O-CH3CN, triclinic, P1BAR, a = 22.466(3) angstrom, b = 10.862(5) angstrom, c = 10.890(2) angstrom, alpha = 94.93(3)degrees, beta = 97.36(2)degrees, gamma = 90.31(2)degr ees, and Z = 2. The molecular structure consists of two six-coordinate Cu(II) ions bridged by PQQ with two equatorially coordinating terpy l igands. Each of the Cu(II) ions occupies the pyridine and the pyrrole moieties of PQQ, which has the tetraanionic form with three carboxylat es and the deprotonated pyrrole group. On account of the binding of PQ Q to Cu(II), the ring system is twisted. On the other hand, PQQ in the mononuclear complex ([Cu(PQQ)(terpy)]) obtained from acidic solution seems to be predominantly coordinated to Cu(II) with the pyridine moie ty, as judged from the pH values of the complex syntheses and a compar ison of the diffuse reflectance spectra of two ternary Cu(II) complexe s. In neutral solution, the second [Cu(terpy)]2+ complex would attach the pyrrole moiety of PQQ in the mononuclear complex, which results in the formation of the dinuclear Cu(II) complex containing PQQ with the deprotonated pyrrole moiety.