| Nickel ions having sulfur atoms are often found at the active centers of nickel-containing oxidation/reduction metalloenzymes, such as [Ni-Fe] hydrogenase, acetyl coenzyme A synthetase/carbon monoxidedehydrogenase (ACS/CODH), and Ni superoxide dismutase (Ni-SOD), which have currently been interested by not only bio-inorganic chemists but also industrial engineers, because they show unique functions such as hydrogen activation, carbon fixation, and disproportionation of active oxygen, respectively, in environment-friendly biological systems. In order to understand the structure/function relationship in Ni-containing metalloenzymes having sulfur atoms at the active centers, five nickel(â…¡) complexes with N2S2 macrocyclic ligands (1,4-dithia-8,11 -diazacyclotetradecane ([14]aneN2S2), 8-(2-pyridylmethyl)1,4-dithia -8,11-diazacyclotetradecane (P[14]aneN2S2) and 8,11-bis(2-pyridylmethyl) -1,4-dithia-8,11-diazacyclotetradecane (BP[14]aneN2S2)),[Ni[14]aneN2S2](ClO4)2 (1), [Ni[14]aneN2S2Cl]2Cl2 (2),[NiP[14]aneN2S2Br](ClO4) (3), [NiP[14]aneN2S2(H2O)](ClO4)2 (4), and[NiBP[14]aneN2S2](ClO4)2 (5) have been synthesized. Two nickel(â…¡) complexes with NSSN-type macrocyclic ligands (1,5-dithia-8,12-diazacyclotetradecane ([14]aneNSSN) and 8-(2-pyridylmethyl)-1,5-dithia-8,12-diazacyclotetradecane (P[14]aneNSSN)),[Ni[14]aneNSSN](ClO4)2 (6) and [NiP[14]aneNSSNBr](ClO4) (7) have beensynthesized. These complexes have been characterized by electronicabsorption spectroscopy, cyclic voltammetry (CV), X-ray structure analysis,and EPR spectroscopy. Absorption spectra of the complex 1 and complex 6showed the spectra typical of 4- and 6-coordinate Ni(â…¡) complexes innon-coordinating and coordinating solvents, respectively, while those of othercomplexes 2, 3, 4, 5 and 7 demonstrated to be mainly 6-coordinate in bothtypes of solvents. The crystal structure of complex 2 reveals a di-μ-chlorodimer composed of two Ni(â…¡) complexes of [14]aneN2S2, each of whichforms an octahedral geometry. The complex 3 also has octahedral structurecoordinated with P[14]aneN2S2 and Br-. The complex 5 has an octahedralgeometry surrounded with two imino nitrogen, two sulfur, and two pyridylnitrogen atoms of BP[14]aneN2S2, in which two pyridyl nitrogens of the sidechains bind to the metal ion in a cis-form. The complex 7 also has octahedralstructure coordinated with P[14]aneNSSN and Br-. The cyclic voltammetrymeasurements of 1, 3, 4 and 5 in MeOH have exhibited reversiblevoltammograms which are assignable to the Ni2+/+ couples at E1/2 = -0.76,-0.96, -0.92, and -0.96 V (vs. SCE), respectively. The reductions of complexes1, 3, 4, and 5 with NaBH4 at -20℃, as studied by ESR, have given the Ni(â… ) complexes with a dx2-y2 ground state for 1 and 5 and a dz2 ground state for 3 and 4, respectively, in which those of 3 and 4 have demonstrated a remarkably large hyperfine splitting (A3 = 50G) in the gz region, suggesting the presence of an axially-bound hydride ion (I= 1/2) on the dz2 orbital of the Ni(â… ) ion. However, ESR spectra of reduction products of complexes 1, 3, 4, and 5 with Na2S2O4 have all exhibited a dx2-y2 ground state.
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