Cobalt as a Probe for Zinc in Metalloenzyme Model Compounds? A Comparison of Spectroscopic Features and Coordination Geometry of Four- and Five-Coordinate Complexes. Crystal and Molecular Structures of [Co(eta(3)-Tp(Ph))(eta(2)-Tp(Ph))], [(eta(3)-Tp(Ph))Zn(anthranilate)], and [(eta(3)-Tp(Ph))M(eta(2)-acac)] (Tp(Ph) = Hydrotris(3-phenylpyrazol-1-yl)borate, acac = Pentane-2,4-dionate, and M = Zn, Co).

The reaction of cobalt(II) perchlorate with 1 equiv each of potassium hydroxide and potassium hydrotris(3-phenylpyrazolyl)borate (KTp(Ph)) leads to the formation of the blood-red bis(ligand) complex [Co(eta(3)-Tp(Ph))(eta(2)-Tp(Ph))]. This compound crystallizes in the monoclinic space group P2(1)/n, with a = 10.690(5) Å, b = 34.243(15) Å, c = 12.923(4) Å, beta = 96.17(3) degrees, Z = 4, V = 4703(3) Å(3), and R = 0.040. The molecular structure contains a square CoN(5) pyramid with an agostic BH.Co interaction of 2.17(2) Å. One Tp(Ph) ligand acts tridentate; the other, bidentate. The reaction of KTp(Ph) with zinc and cobalt halides yields a series of heteroleptic halogeno complexes [(Tp(Ph))MX]. The zinc species are all tetrahedral, while in the case of cobalt(II) the UV-vis spectra indicate an equilibrium of tetra- and pentacoordinated species depending on the anion size and the donor properties of the solvent. Metathesis reactions with carboxylates yield mononuclear complexes [(Tp(Ph))M(O(2)CR)]; M = Zn(II), Co(II); RCO(2)(-) = acetate, benzoate, 4-fluorobenzoate, and 4-nitrobenzoate. The infrared bands nu(as)(CO(2)) and nu(sym)(CO(2)) indicate monodentate carboxylate ligands in the zinc complexes in the solid state and in solution. The cobalt complexes [(Tp(Ph))Co(carboxylate)] are dark blue in dichloromethane and in the solid state when grown from dichloromethane solution. Tetrahydrofuran solutions and crystals grown from tetrahydrofuran are pink, purple, or reddish violet. The 2-aminobenzoate (anthranilate) complexes [(Th(Ph))M(anthranilate)], M = Zn(II), Co(II), have been prepared. [(eta(3)-Tp(Ph))Zn(anthranilate)] crystallizes in the monoclinic space group P2(1)/n, with a = 13.205(4) Å, b = 15.643(3) Å, c = 15.116(3) Å, beta = 98.86(2) degrees, Z = 4, V = 3085(1) Å(3), and R = 0.034. Anthranilate acts as a chelating oxygen ligand with Zn-O distances of 1.932(2) and 2.460(2) Å. The amino group is not involved in metal coordination. The reaction of the chloro complexes [(Tp(Ph))MCl] with sodium or potassium acetylacetonate leads to isotypic zinc and cobalt complexes [(eta(3)-Tp(Ph))M(eta(2)-acac)]. Both crystallize in the monoclinic space group P2(1)/c, with Z = 4, the zinc compound with a = 16.334(3) Å, b = 11.545(2) Å, c = 15.816(3) Å, beta = 94.77(1) degrees, V = 2972(1) Å(3), and R = 0.039 and the cobalt compound with a = 16.358(7) Å, b = 11.519(2) Å, c = 15.847(6) Å, beta = 95.30(3) degrees, V = 2973(2) Å(3), and R = 0.035. The structures are best described as slightly distorted trigonal bipyramids with the two axial positions occupied by one of the acetylacetonate O and one of the tripodal N donor atoms. Ligand profiles of Tp(Ph) and related ligands have been calculated to visualize the angular encumbrance of the ligands.