Nickel redox chemistry

Bagyinka, C., Whitehead, J. P., and Maroney, M. J., 1993, An X-ray absorption spectroscopic study of nickel redox chemistry in hydrogenase, J. Am. Chem. Soc. 115 3576n3585. 

Nickel is found in thiolate/sulflde environment in the [NiFe]-hydrogenases and in CODH/ACS.33 In addition, either a mononuclear Ni-thiolate site or a dinuclear cysteine-S bridged structure are assumed plausible for the new class of Ni-containing superoxide dismutases, NiSOD (A).34 [NiFe]-hydrogenase catalyzes the two-electron redox chemistry of dihydrogen. Several crystal structures of [NiFe]-hydrogenases have demonstrated that the active site of the enzyme consists of a heterodinuclear Ni—Fe unit bound to thiolate sulfurs of cysteine residues with a Ni—Fe distance below 3 A (4) 35-39 This heterodinuclear active site has been the target of extensive model studies, which are summarized in Section 6.3.4.12.5. 

M(iii) and Cn(m) complexes. In early classic studies the redox chemistry of tetraaza macrocyclic complexes of Ni(n) and Cu(n) (of the Curtis and reduced Curtis type) was investigated in acetonitrile (Olson Vasilevskis, 1969 1971). These authors were the first to report the electrochemical generation of Ni(m) and Cu(m) complexes of such N4-cyclic ligands. Since this time, a considerable number of related studies, involving both nickel and copper macrocyclic species, have been reported. 

Nickel (continued) polysulfide complex, 31 98 half chair conformation, 31 115 porphyrin complexes, 32 13 in proteins, 47 284-285 quadruply bridged dimers, 40 211-214 steric structures, 40 190-194 quaternary phosphonium salts of, 6 31-32 redox chemistry probes of structure, 32 243-245 steric and electronic requirements, 32 242-243... 

Many new Ni(II) complexes of aza-type macrocycles have been synthesized, and their redox chemistries have been explored. In particular, complexes of macropolycyclic ligands and bismacrocyclic ligands have been prepared. Complexes with uncommon oxidation states of nickel (Nim and Ni1 complexes) have also been synthesized by employing a specially designed macrocycle, and their characteristic spectroscopic properties and X-ray structures reported. These nickel(II) complexes... 

The starting point for most of the redox chemistry considered in this review is the nickel(II) ion. The nickel(II) ion has a d8 electronic configuration and, with weak-field ligands such as H20, it forms a six-coordinate ion with approximately octahedral symmetry and a paramagnetic (two unpaired electrons) 3A2 ground state. The characteristic solution chemistry of six-coordinate nickel(II) is well documented and, in particular, the substitution behavior has been extensively studied and is the subject of recent reviews (11, 12). It is a labile ion with solvent exchange rates around 104 sec-1 at 25°C and activation parameters are consistent with dissociatively activated interchange behavior (13). 

Complexes with macrocyclic ligands have a rich chemistry for both higher and lower oxidation states of nickel (62-66). The smallest members of the series, the triaza macrocycles, show no redox chemistry for 1 1 complexes with nickel(II), but there are recent reports of stable bis complexes which will be discussed later in this section. 

---