Aliphatic carbon-carbon bond cleavage reactivity of a mononuclear Ni(II) cis-beta-keto-enolate complex in the presence of base and O2: a model reaction for acireductone dioxygenase (ARD).

The synthesis, characterization, and reactivity properties of a mononuclear Ni(II) cis-beta-keto-enolate complex, [(6-Ph2TPA)Ni(PhC(O)C(OH)C(O)Ph)]ClO4 (1) (6-Ph2TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine) are reported. Complex 1 was characterized by X-ray crystallography, elemental analysis, 1H NMR, and electronic absorption and infrared spectroscopy. Treatment of 1 with 1 equiv of Me4NOH.5H2O in the presence of O2 results in oxidative carbon-carbon bond cleavage and the formation of a new Ni(II) dicarboxylate complex, [(6-Ph2TPA)Ni(O2CPh)2(H2O)] (2). Complex 2 has been characterized by X-ray crystallography, 1H NMR, UV-vis, IR, and elemental analysis. Use of 18O2 in the reaction of 1 to produce 2 results in the incorporation of one 18O atom per carboxylate ligand in the majority of the sample. Production of CO in the oxidative conversion of 1 to 2 was detected using the palladium chloride method. This is the first functional model system of relevance to acireductone dioxygenase (ARD), a novel Ni(II)-containing enzyme that catalyzes a reaction that is a shunt out of the methionine salvage pathway in K. pneumoniae.

A deprotonated intermediate in the amide methanolysis reaction of an N4O-ligated mononuclear zinc complex.

Treatment of [(ppbpa)Zn](ClO4)2 (1(ClO4)2, ppbpa = N-((6-(pivaloylamido)-2-pyridyl)methyl)-N,N-bis((2-pyridyl)methyl)amine) with 1 equiv of Me(4)NOH.5H(2)O in methanol-acetonitrile solution results within minutes in the stoichiometric formation of a complex having a deprotonated amide, [(ppbpa-)Zn]ClO4 (3). Complex 3 has been characterized by 1H and 13C NMR, FTIR, and elemental analysis. Notably, upfield shifts of specific 1H NMR resonances of the amide-appended pyridyl moiety in 3, versus those found for 1(ClO4)2, indicate delocalization of the anionic charge within the amide-appended pyridyl donor of this complex. Heating of analytically pure 3 in methanol-acetonitrile results in amide alcoholysis. Overall, this alcoholysis reaction is second-order, with a first-order dependence on both 3 and methanol. Analysis of the rate of decay of 3 as a function of temperature yielded activation parameters consistent with an intramolecular amide cleavage process (DeltaH++ = 15.0(3) kcal/mol, DeltaS++ = -33(1) eu). A possible reaction mechanism for amide alcoholysis is presented which involves reaction of the deprotonated amide intermediate 3 with methanol to produce a Lewis activated-type structure from which amide cleavage may be initiated. Additional support for this mechanistic pathway has been obtained through examination of the analogous ethanolysis reaction and via evaluation of the effect of varying steric hindrance near the amide carbonyl unit.

Neutral acetohydroxamic acid coordination to a mononuclear Ni(II) center stabilized by an intramolecular hydrogen-bonding interaction.

Treatment of a new chelate ligand having both amide- and phenyl-appended pyridyl moieties with Ni(ClO4).6H2O and acetohydroxamic acid in methanol solution results in the production of a novel pseudo-octahedral Ni(II) complex having a neutral acetohydroxamic acid ligand stabilized by a hydrogen-bonding interaction.

Mononuclear nitrogen/sulfur-ligated cobalt(II) methoxide complexes: structural, EPR, paramagnetic 1H NMR, and electrochemical investigations.

The first examples of mononuclear nitrogen/sulfur-ligated Co(II) alkoxide complexes, species of relevance to a reactive intermediate observed for Co(II)-substituted liver alcohol dehydrogenase, have been isolated and characterized by multiple methods including X-ray crystallography, EPR, paramagnetic (1)H NMR, and cyclic voltammetry.

NMR studies of mononuclear octahedral Ni(II) complexes supported by tris((2-pyridyl)methyl)amine-type ligands.

The recent discovery of acireductone dioxygenase (ARD), a metalloenzyme containing a mononuclear octahedral Ni(II) center, necessitates the development of model systems for evaluating the role of the metal center in substrate oxidation chemistry. In this work, three Ni(II) complexes of an aryl-appended tris((2-pyridyl)methyl)amine ligand (6-Ph(2)TPA, N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), [(6-Ph(2)TPA)Ni(CH(3)CN)(CH(3)OH)](ClO(4))(2) (1), [(6-Ph(2)TPA)Ni(ONHC(O)CH(3))]ClO(4) (3), and [(6-Ph(2)TPA)Ni-Cl(CH(3)CN)]ClO(4) (4), and one Ni(II) complex of tris((2-pyridyl)methyl)amine, [(TPA)Ni(CH(3)CN)(H(2)O)](ClO(4))(2) (2), have been characterized in acetonitrile solution using conductance methods and NMR spectroscopy. In acetonitrile solution, 1-4 have monomeric cations that exhibit isotropically shifted (1)H NMR resonances. Full assignment of these resonances was achieved using one- and two-dimensional (1)H NMR techniques and (2)H NMR of analogues having deuteration of the supporting chelate ligand. COSY cross peaks were observed for pyridyl protons of the 6-Ph(2)TPA ligand in 1 and 3. This study lays the groundwork for using NMR methods to examine chemical reactions of 1 and 2 with model substrates of relevance to ARD.

First row divalent transition metal complexes of aryl-appended tris((pyridyl)methyl)amine ligands: syntheses, structures, electrochemistry, and hydroxamate binding properties.

Divalent manganese, cobalt, nickel, and zinc complexes of 6-Ph(2)TPA (N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine; [(6-Ph(2)TPA)Mn(CH(3)OH)(3)](ClO(4))(2) (1), [(6-Ph(2)TPA)Co(CH(3)CN)](ClO(4))(2) (2), [(6-Ph(2)TPA)Ni(CH(3)CN)(CH(3)OH)](ClO(4))(2) (3), [(6-Ph(2)TPA)Zn(CH(3)CN)](ClO(4))(2) (4)) and 6-(Me(2)Ph)(2)TPA (N,N-bis((6-(3,5-dimethyl)phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine; [(6-(Me(2)Ph)(2)TPA)Ni(CH(3)CN)(2)](ClO(4))(2) (5) and [(6-(Me(2)Ph)(2)TPA)Zn(CH(3)CN)](ClO(4))(2) (6)) have been prepared and characterized. X-ray crystallographic characterization of 1A.CH(3)()OH and 1B.2CH(3)()OH (differing solvates of 1), 2.2CH(3)()CN, 3.CH(3)()OH, 4.2CH(3)()CN, and 6.2.5CH(3)()CN revealed mononuclear cations with one to three coordinated solvent molecules. In 1A.CH(3)()OH and 1B.2CH(3)()OH, one phenyl-substituted pyridyl arm is not coordinated and forms a secondary hydrogen-bonding interaction with a manganese bound methanol molecule. In 2.2CH(3)()CN, 3.CH(3)()OH, 4.2CH(3)()CN, and 6.2.5CH(3)()CN, all pyridyl donors of the 6-Ph(2)TPA and 6-(Me(2)Ph)(2)TPA ligands are coordinated to the divalent metal center. In the cobalt, nickel, and zinc derivatives, CH/pi interactions are found between a bound acetonitrile molecule and the aryl appendages of the 6-Ph(2)TPA and 6-(Me(2)Ph)(2)TPA ligands. (1)H NMR spectra of 4 and 6 in CD(3)NO(2) solution indicate the presence of CH/pi interactions, as an upfield-shifted methyl resonance for a bound acetonitrile molecule is present. Examination of the cyclic voltammetry of 1-3 and 5 revealed no oxidative (M(II)/M(III)) couples. Admixture of equimolar amounts of 6-Ph(2)TPA, M(ClO(4))(2).6H(2)O, and Me(4)NOH.5H(2)O, followed by the addition of an equimolar amount of acetohydroxamic acid, yielded the acetohydroxamate complexes [((6-Ph(2)TPA)Mn)(2)(micro-ONHC(O)CH(3))(2)](ClO(4))(2) (8), [(6-Ph(2)TPA)Co(ONHC(O)CH(3))](ClO(4))(2) (9), [(6-Ph(2)TPA)Ni(ONHC(O)CH(3))](ClO(4))(2) (10), and [(6-Ph(2)TPA)Zn(ONHC(O)CH(3))](ClO(4))(2) (11), all of which were characterized by X-ray crystallography. The Mn(II) complex 8.0.75CH(3)()CN.0.75Et(2)()O exhibits a dinuclear structure with bridging hydroxamate ligands, whereas the Co(II), Ni(II), and Zn(II) derivatives all exhibit mononuclear six-coordinate structures with a chelating hydroxamate ligand.