LIGAND DYNAMICS IN PENTACOORDINATE COPPER(I) AND ZINC(II) COMPLEXES

Two new pentacoordinate zinc(II) and copper(I) complexes of the pentadentate ligand ((5-MeimidH)2DAP) have been prepared and studied as their BF4- salts. The primary coordination sphere of each compound is composed of a central pyridine nitrogen donor atom, two imine nitrogens, and two terminal imidazole nitrogens. [Zn(II)((5-MeimidH)2DAP)](BF4)2 has been characterized in the solid state by a single-crystal X-ray structure determination, and like its congener, [Cu(II)((5-MeimidH)2DAP)](BF4)2, it has been found to have a geometry intermediate between idealized trigonal bipyramidal and square pyramidal. A crystal structure of the Cu(I) complex is not available, but it too is almost certainly pentacoordinate like two other closely related members of this family of Cu(I) complexes for which structures are available. Thus, the [Cu(I)((5-MeimidH)2DAP)]+ cation is the newest example of Cu(I), in a growing family of compounds, with the unusual coordination number of 5. Variable-temperature H-1 NMR spectroscopy studies in nonaqueous solvents have been used to probe intramolecular ligand dynamics for the Zn(II) and Cu(I) species in solution. In these studies, other pentacoordinate members of the family, [M((imidH)2DAP)]n+ (non-methylated terminal imidazole groups) and [M((py)2DAP)]n+ (pyridine groups replacing imidazole), have also been investigated for comparison purposes. Each of these compounds possesses two chelate rings containing dimethylene groups that undergo a dynamic exchange process leading to equivalence of the methylene protons. Coalescence rate constants have been obtained in four of the six cases. The available rate constants follow the ordering [Zn(II)((5-MeimidH)2DAP)]2+ (313 s-1, 328 K) < [Zn(II)((imidH)2DAP)]2+ (345 s-1, 308 K) < [Zn(II)((py)2DAP)]2+ (313 s-1, 268 K) < [Cu(I)((py)2DAP)]+ (303 s-1, 253 K). With a coalescence temperature of ca. 203 K in CH2Cl2, the rate constant for [Cu(I)((5-MeimidH)2DAP)]+ was too large to be determined. The mechanism of exchange is believed to involve breakage of a M-N(terminal) bond, leading to optical inversion between the DELTA-(lambda,lambda) and LAMBDA-(delta,delta) isomers. With two unusually long Cu-N(imine) bond distances of up to 2.4 angstrom, the copper(I) complexes tend more toward three coordination than do their Zn(II) counterparts, and this is reflected in their greater exchange rates. Crystal data for [Zn(II)((5-MeimidH)2DAP)](BF4)2, C21H27N7B2F8Zn: a = 18.198 (4) angstrom, b = 12.043 (2) angstrom, c = 25.506 (8) angstrom, beta = 100.69 (3)-degrees, Z = 8, monoclinic, space group C2/c. A total of 1904 unique observed data were used in the solution.