Structural Studies of the [Tris(imidazolyl)phosphine]metal Nitrate Complexes {[PimPri,But]M(NO₃)}⁺ (M = Co, Cu, Zn, Cd, Hg):  Comparison of Nitrate-Binding Modes in Synthetic Analogues of Carbonic Anhydrase

X-ray diffraction studies on a series of cationic divalent metal nitrate complexes supported by the tris(1-isopropyl-4-tert-butylimidazolyl)phosphine ligand, [[PimPri,But]M(NO3)]+ (M = Co, Cu, Zn, Cd, Hg), demonstrate that the nitrate ligand coordination mode is strongly dependent upon the metal. With the exception of that for the HgII derivative, the nitrate ligand coordination modes correlate with the activities of metal-substituted carbonic anhydrases, such that the only MII-carbonic anhydrases which exhibit significant activity, i.e., the Zn and Co species, are those for which the [[PimPri,But]M(NO3)]+ complexes possess strongly asymmetric nitrate ligands. This trend supports the notion that access to a unidentate, rather than a bidentate, bicarbonate intermediate may be a critical requirement for significant carbonic anhydrase activity. Interestingly, the nitrate coordination modes in the series of group 12 complexes, [[PimPri,But]M(NO3)]+ (M = Zn, Cd, Hg), do not exhibit a monotonic periodic trend: the bidenticity is greater for the cadmium complex than for either the zinc or mercury complexes. Since HgII-carbonic anhydrase is inactive, the correlation between nitrate coordination mode and enzyme activity is anomalous for the mercury complex. Therefore, it is suggested that the inactivity of HgII-carbonic anhydrase may be consequence of the reduced tendency of the mercury center in HgII-carbonic anhydrase to bind water.