OXIDATION OF NICKEL THIOLATE LIGANDS BY DIOXYGEN

The reactions of Ni complexes of the tridentate dithiolate ligands RN(CH2CH2S-)2 and S(CH2CH2S-)2 with O2 leading to the formation of complexes containing one S-bound sulfinate ligand are described. Reaction of the ligands with Ni(OAC)2 yields dinuclear [Ni(L)12 complexes. Reaction of these dimers with Et4N(CN) yields Et4N+ salts of the four-coordinate complex anions, [Ni(L)CN]-. For two of the ligands in the series presented, N,N-bis(2-mercaptoethyl)-2-(methylthio)ethylamine (2) and bis(2-mercaptoethyl) sulfide (6), crystals of the compounds have been obtained and structurally characterized by single crystal X-ray diffraction. Et4N[Ni(2)CN] crystallizes in monoclinic space group P2(1)/c with cell dimensions a = 12.803(8) angstrom, b = 12.713(9) angstrom, c = 14.788(12) angstrom, beta = 119.31(6)-degrees, V = 2098.8 angstrom3, and Z = 4. The structure was refined to R = 0.087 and R(w) = 0.106. The anion consists of an essentially planar Ni(II) complex with two trans-oriented thiolate ligands, one tertiary amine N-donor atom, and one cyanide ligand. Et4N[Ni(6)CN] crystallizes in orthorhombic space group P2(1)2(1)2(1) with cell dimensions a = 8.871(2) angstrom, b = 11.505(2) angstrom, c = 17.767(2) angstrom, v = 1813.3 A3, and Z = 4. The structure was refined to R = 0.044 and R(w) = 0.064. The anion consists of a highly distorted planar Ni(II) complex with two trans-oriented thiolate ligands, one thioether S-donor atom, and one cyanide ligand, where the thioether S-donor atom lies significantly out of the plane described by Ni and the remaining donor atoms. The anion exhibits two conformations in the crystal involving placement of the thioether S-donor atom above or below this plane. The [Ni(L)CN]- anions are air sensitive and react with O2 to form planar Ni(II) complex anions with one thiolate and one sulfinate ligand, [Ni(L-O2)CN]-. Et4N[Ni(2-02)CN] crystallizes in monoclinic space group P2(1)/c with cell dimensions a = 12.784(4) angstrom, b = 12.757(2) angstrom, c = 14.847(4) angstrom, beta = 115.00(3)-degrees, V = 2194.5 angstrom3, and Z = 4. The structure was refined to R = 0.033 and R(w) = 0.042. Oxygen uptake measurements reveal that the oxidation processes have a stoichiometry Of 1:1 02:Ni. The oxidations were followed spectrophotometrically. Kinetic analysis of the data reveals that the reactions obey the following rate law: d[[Ni(L-O2)CN]-]/dt = k[[Ni(L)CN]-][O2]. Measurements of the rate of oxidation at 30-degrees-C under constant [O2] = 1 atm yield pseudo-first-order rate constants, k(obs) = (0.72(2)-l.60(l)) x 10(-4) s-1 for the amine ligands. Second-order rate constants were determined to be (1.4-3.1) X 10(-2) M-1 s-1 at 30-degrees-C in DMF. The reaction of [Ni(6)CN]- is 2-5 times slower than those of the tertiary amine ligands. The dependence of the reactions on the ligand donor atom sets, the N-substituents, the solvent, and the addition of radical and O-1(2) scavengers is discussed. Reactions conducted with isotopically labeled O2 gas mixtures reveal that the sulfinate oxygen atoms in the products derive principally from a single O2 molecule. A mechanism involving the formation of thiadioxirane intermediates is proposed, with the rate-determining step being the cleavage of the O-O bond. The possible relevance of the chemistry involved to the biological oxidation of cysteine by metalloenzymes is also discussed.