Showing papers by "Christopher C. Cummins published in 2005"

Mechanism of white phosphorus activation by three-coordinate molybdenum(III) complexes: a thermochemical, kinetic, and quantum chemical investigation.

Abstract: White phosphorus (P4) reacts with three-coordinate molybdenum(III) trisamides or molybdaziridine hydride complexes to produce either bridging or terminal phosphide (P3-) species, depending upon the ancillary ligand steric demands. Thermochemical measurements have been made that place the Mo⋮P triple bond dissociation enthalpy at 92.2 kcal·mol-1. Thermochemical measurements together with computational analysis rule out simple P-atom abstraction from P4 as a step in the phosphorus activation mechanism. Kinetic measurements made by the stopped-flow method show that the reaction between the monomeric molybdenum complexes and P4 is first-order both in metal complex and in P4. cyclo-P3 complexes can be obtained when ancillary ligand steric demands are small, but kinetic measurements rule them out as monometallic intermediates in the P4 activation mechanism. Also studied by calorimetric, kinetic, and in one case variable-temperature NMR methods is the process of μ-phosphide bridge formation. Post-rate-determinin...

Solution calorimetric and stopped-flow kinetic studies of the reaction of *Cr(CO)3C5Me5 with PhSe-SePh and PhTe-TePh. Experimental and theoretical estimates of the Se-Se, Te-Te, H-Se, and H-Te bond strengths.

Abstract: The kinetics of the oxidative addition of PhSeSePh and PhTeTePh to the stable 17-electron complex •Cr(CO)3C5Me5 have been studied utilizing stopped-flow techniques. The rates of reaction are first-order in each reactant, and the enthalpy of activation decreases in going from Se (ΔH‡ = 7.0 ± 0.5 kcal/mol, ΔS‡ = −22 ± 3 eu) to Te (ΔH‡ = 4.0 ± 0.5 kcal/mol, ΔS‡ = −26 ± 3 eu). The kinetics of the oxidative addition of PhSeH and •Cr(CO)3C5Me5 show a change in mechanism in going from low (overall third-order) to high (overall second-order) temperatures. The enthalpies of the oxidative addition of PhE−EPh to •Cr(CO)3C5Me5 in toluene solution have been measured and found to be −29.6, −30.8, and −28.9 kcal/mol for S, Se, and Te, respectively. These data are combined with enthalpies of activation from kinetic studies to yield estimates for the solution-phase PhE−EPh bond strengths of 46, 41, and 33 kcal/mol for E = S, Se, and Te, respectively. The corresponding Cr−EPh bond strengths are 38, 36, and 31 kcal/mol. Two...

Heterobimetallic reductive cross-coupling of benzonitrile with carbon dioxide, pyridine, and benzophenone.

Abstract: Described herein are heterobimetallic radical cross-coupling reactions between the benzonitrile adduct of the molybdenum(III) complex Mo(N[t-Bu]Ar)3 (Ar = 3,5-C6H3Me2) and titanium(III) complexes with carbon dioxide, pyridine, and benzophenone. The titanium(III) system employed was either Ti(N[t-Bu]Ar)3 (Ar = 3,5-C6H3Me2) or Ti(N[t-Bu]Ph)3. Crystal structure studies are described for the Mo/PhCN/CO2/Ti coupled system and for an analogue of the Mo/PhCN/Ph2CO/Ti coupled system in which PhCN is replaced with 2,6-Me2C6H3CN. In the case of the couplings involving pyridine and benzophenone, C-C bond formation takes place with dearomatization, with the new C-C bond being formed between the nitrile carbon of PhCN and the para carbon of pyridine or one of the benzophenone phenyl groups. Of the radical metal complex/substrate adducts invoked in this work, that between titanium(III) and CO2 is the only one not directly observable. In all cases, the selective cross-coupling reactions are interpreted as arising by heterodimerization of titanium(III) substrate complexes (substrate = CO2, py, or Ph2CO) with the persistent molybdenum-PhCN radical adduct. All of the heterobimetallic coupling products are diamagnetic, and the metal ions Ti and Mo in them both are assigned to the formal 4+ oxidation state.

Portability of the RNNMo(3+) core : Application to the synthesis of dinitrogen-derived trialkoxymolybdenum organodiazenido complexes

Abstract: An alcoholysis strategy has been employed in the synthesis of new oligo- or polymeric trialkoxymolybdenum diazenido complexes [R1NNMo(O-1-Ad)3]x (R1 = Me or C(O)OMe) as well as their mononuclear THF solvates R1NNMo(O-1-Ad)3(THF). Dinitrogen-derived diazenido precursor complexes R1NNMo(N[R]Ar)3 (R = i-Pr, Ar = 3,5-C6H3Me2) react smoothly with 3 equiv. 1-adamantanol to liberate 3 equiv. HN(R)Ar without disruption of the diazenidomolybdenum(3+) core. X-ray structural investigations were carried out for diazenido complex MeNNMo(O-1-Ad)3(THF) and for the corresponding cationic dimethyldiazenido complex [Me2NNMo(O-1-Ad)3(THF)]+, obtained as its triflate salt. The latter cation was found upon cobaltocene reduction to undergo N—N bond cleavage, producing nitride NMo(O-1-Ad)3 (synthesized independently) and dimethylamine. Key words: molybdenum, dinitrogen, bond cleavage, hydrazido, diazenido.