LOW-POTENTIAL NICKEL(III,II) COMPLEXES - NEW SYSTEMS BASED ON TETRADENTATE AMIDATE THIOLATE LIGANDS AND THE INFLUENCE OF LIGAND STRUCTURE ON POTENTIALS IN RELATION TO THE NICKEL SITE IN [NIFE]-HYDROGENASES

In a continuing study of Ni(II,III) complexes with low redox potentials that are of possible relevance to the nickel site in [NiFe]-hydrogenases. Ni(II) complexes of the tetraanions of the amide-thiol ligands N,N'-ethylenebis(2-mercaptoacetamide) (H4ema), N,N'-1,2-phenylenebis(2-mercaptoacetamide) (H4phma), and N,N'-ethylenebis(2-mercaptoisobutyramide) (H4emi) have been prepared. The ligands were isolated as their S-acetyl derivatives; their reaction with Ni(II) under basic conditions afforded the red diamagnetic complexes [Ni(ema)]2- (5), [Ni(phma)]2- (6), and [Ni(emi)]2- (7), which were isolated as Et4N+ salts. The tetradentate nature of the ligands was demonstrated by the X-ray structure determination of (Et4N)2[Ni(ema)].2H2O, which crystallizes in orthorhombic space group Pmma with a = 28.973 (8) angstrom, b = 12.239 (4) angstrom, c = 8.389 (2) angstrom, and Z = 4. The anion is planar with Ni-S = 2.179 (1) angstrom and Ni-N = 1.857 (3) angstrom. The complexes undergo reversible one-electron oxidations (E1/2 vs SCE, cyclic voltammetry) in DMF solutions: [Ni(ema)]-,2-, -0.34 V; [Ni(phma)]-,2-, -0.24 V; [Ni(emi)]-,2-, -0.42 V. The EPR spectra of the products demonstrate metal-centered oxidation to form Ni(III) species; of these, [Ni(emi)]- is the most stable, but it could not be isolated in pure condition. These potentials are among the lowest known for the Ni(III,II) couple. The factors affording low potentials of this couple in complexes prepared in this and other laboratories are summarized. Chief among these are anionic polarizable ligands and 2- net charge of the Ni(II) complexes. Electronic structural calculations were made for the low-potential complexes [Ni(pdtc)2]-,2- (pdtc = pyridine-2, 6-bis(thiocarboxylate)(2-)) and [Ni(nbdt)2]-1-,2- (nbdt = norbornane-2, 3-dithiolate(2-)); for the respective Ni(III) species the ground states sigma-*(d(z)2) and pi-*(d(xz) or d(yz)) are indicated. When nominally compared, the [Ni(emi)]-,2- potential is close to that (-0.39 V) for reduction of the Ni-A state in Desulfovibrio gigas hydrogenase. However, when the effects of solvent and pH on redox potentials are considered, it is concluded that, among synthetic species, the potential of the [Ni(nbdt)2]-,2- couple is closest to that of the enzyme site. Cysteinate is likely the most effective native ligand in stabilizing Ni(III), but not biological modes of metal ligation may have been discovered.