Nikolay P. Tsvetkov

Mechanism of Heterolysis of H2 by an Unsaturated d8 Nickel Center: via Tetravalent Nickel?

Collision of H(2) with the unusual nickel complex, (PNP)Ni(+), where PNP = ((t)Bu(2)PCH(2)SiMe(2))(2)N, forms a rare dihydrogen complex of the d(8) configuration which then rearranges to heterolytically cleave the H-H bond. Experimental studies support a short H/H distance in the coordinated diatomic, and DFT calculations show that the transition state for heterolysis, in spite of the fact that this involves an amide nitrogen located trans to the H(2), has the H/H bond fully split, and has all the geometric features of Ni(IV), but this is a local maximum, not a minimum.

Ligand influence on metal aggregation: a unique bonding mode for pyridylpyrrolides.

The synthesis and characterization of a Cu(I) complex with a cis-bidentate monoanionic nitrogenous ligand, 2-pyridylpyrrolide, L, is reported. This shows binding of one base B = MeCN or CO per copper in a species LCu(B), but this readily releases the volatile ligand under vacuum with aggregation of transient LCu to a mixture of two enantiomers of a chiral trimer: a zwitterion containing inequivalent Cu(I) centers, possible via a new bonding mode of pyridylpyrrolide, and one with nitrogen lone pairs donating to two different metals. Density functional theory calculations show the energetics of both ligand binding and aggregation (including dimer and monomer alternatives), as well as the ability of this ligand to rotate away from planarity to accommodate a bridging structural role. The trimer serves as a synthon for the simple fragment LCu.

Synthesis and oxidative reactivity of 2,2'-pyridylpyrrolide complexes of Ni(II).

Synthesis and characterization of divalent nickel complexed by 2-pyridylpyrrolide bidentate ligands are reported, as possible precursors to complexes with redox active ligands. Varied substituents on the pyrrolide, two CF3 (L(2)), two (t)Bu (L(0)), and one of each type (L(1)) are employed and the resulting Ni(L(n))2 complexes show different Lewis acidity toward CO, H2O, tetrahydrofuran (THF), or MeCN, the L(2) case being the most acidic. Density functional theory calculations show that the frontier orbitals of all three Ni(L(n))2 species are localized at the pyrrolide groups of both ligands and Ni(L(n))2(+) can be detected by mass spectrometry and in cyclic voltammograms (CVs). Following cyclic voltammetry studies, which show electroactivity primarily in the oxidative direction, reactions with pyridine N-oxide or Br2 are reported. The former yield simple bis adducts, Ni(L(2))2(pyNO)2 and the latter effects electrophilic aromatic substitution of the one pyrrolide ring hydrogen for both chelates, leaving it brominated.

Unusual selectivity of a (pincer)Ni-hydride reacting with CO2.

The reaction of (PNP)NiH, where PNP is ((t)Bu2PCH2SiMe2)2-N(-1), with CO2 occurs over a period of hours at 25 degrees C to form (POP)NiH(NCO), which involves transposition of the initial amide nitrogen and one oxygen of CO2.