FIXATION OF ATMOSPHERIC CO2 BY A SERIES OF HYDROXO COMPLEXES OF DIVALENT METAL-IONS AND THE IMPLICATION FOR THE CATALYTIC ROLE OF METAL-ION IN CARBONIC-ANHYDRASE - SYNTHESIS, CHARACTERIZATION, AND MOLECULAR-STRUCTURE OF [LM(OH)]N (N = 1 OR 2) AND LM(MU-CO3)ML (M(II) = MN, FE, CO, NI, CU, ZN L = HB(3,5-IPR2PZ)3)

By using the hindered tris(pyrazolyl)borate ligand HB(3,5-iPr2pz)3, (hydrotris(3,5-diisopropyl-1-pyrazolyl)-borate), a series of hydroxo complexes of first-row divalent metal ions (Mn (1), Fe (2), Co (3), Ni (4), Cu (5), Zn (6)) was synthesized. X-ray crystallography was applied to 1-5, establishing that all these hydroxo complexes have a dinuclear structure solely bridged with a bis(hydroxo) unit. The structure of 6 was characterized by spectroscopy, which indicates that 6 is monomeric. All these hydroxo complexes were found to react with CO2, even atmospheric CO2, to afford mu-carbonato dinuclear complexes of Mn (7), Fe (8), Co (9), Ni (10), Cu (11), and Zn (12). The molecular structures of the complexes 8-12 were determined. A variety of coordination modes of the carbonate group was seen. In 10 and 11, the carbonate group is bound to both metal centers bidentately in a symmetric fashion, while in 8 and 9, the carbonate coordination modes are described as an unsymmetric bidentate. The carbonate group in 12 is coordinated to one zinc ion bidentately, but it is bound to the other zinc ion unidentately. From IR data, the coordination mode of the carbonate group in 7 was suggested to be similar to those found in 8 and 9. Thus, the order of the coordination distortions of the carbonate groups in this series of mu-carbonato dinuclear complexes is as follows: Zn > Mn almost-equal-to Fe almost-equal-to Co > Ni almost-equal-to Cu. On the other hand, the reactivities of the hydroxo complexes toward CO2 fixation were found to be ordered Zn > Cu > Ni almost-equal-to Co > Mn > Fe. It is noteworthy that the order of the CO2 fixation capabilities of the hydroxo complexes does not fit with the order of activities known for metal-substituted carbonic anhydrases. The order of activities for CO2 hydration by the carbonic anhydrases is Zn > Co >> Ni almost-equal-to Mn > Cu almost-equal-to 0. Thus, the order is correlated mostly with the coordination distortions of the carbonate group in the mu-carbonate complexes but not the reactivities of the hydroxo complexes toward CO2.