Description: This article discusses research on cobalt-dinitrogen complexes with weakened N-N bonds. Abstracts: Reported N₂ complexes of cobalt do not have substantial weakening of the N-N bond. Using diketiminate ligands to enforce three-coordinate geometries, we have synthesized several novel CoNNCo complexes. In formally univalent complexes, cobalt is poorer than iron at weakening the N-N bond, but in formally zerovalent complexes, cobalt and iron give similar N-N weakening. The weakening is due to cobalt-to-N₂ π-backbonding, and potassium cations pull more electron density into N₂. These results show that the low coordination number of a trigonal-planar geometry is impetus enough to make even the electronegative cobalt weaken the N-N bond of N₂.

Description: This article discusses combined experimental and computational studies on the nature of aromatic C-H activation by octahedral ruthenium(II) complexes. Abstract: Octahedral ruthenium complexes of the type TpRu(L)(NCMe)R [Tp = hydridotris(pyrazolyl)borate; R = alkyl or aryl; L = CO or PMe3] have been shown previously to initiate the C-H activation of aromatic substrates. In order to probe the nature of the C-H activation step, reaction rates have been theoretically obtained for the conversion of TpRu(L)(ƞ2-C,C-C6H5X)Me to TpRu(L)(ρ-C6H4X) and CH4 where X is varied among Br, Cl, CN, F, H, NH2, NO2, and OMe. A linear Hammett correlation is calculated with a positive ρ value of 2.6 for L = CO and 3.2 for L = PMe3. Calculated kinetic data for the aromatic C-H activations indicate that an electrophilic aromatic substitution mechanism is unlikely. While experiments cannot fully replicate the entire range of calculated Hammett plots, reactivity trends are consistent with the calculations that suggest activation barriers to overall metal-mediated arene C-H bond cleavage are reduced by the presence of electron-withdrawing groups in the position para to the site of activation. Previous mechanistic studies, as well as the structure and imaginary vibrational modes of the present transition states, validate that the C-H ...

Description: This article comments on the prediction of gas chromatographic retention behavior with mixed liquid phases.

Description: This article discusses comparative reactivity of TpRu(L)(NCMe)Ph (L = CO or PMe3). Abstract: Complexes of the type TpRu(L)(NCMe)R [L = CO or PMe3; R = Ph or Me; Tp = hydridotris-(pyrazolyl)borate] initiate C-H activation of benzene. Kinetic studies, isotopic labeling, and other experimental evidence suggest that the mechanism of benzene C-H activation involves reversible dissociation of acetonitrile, reversible benzene coordination, and rate-determining C-H activation of coordinated benzene. TpRu(PMe3)(NCMe)Ph initiates C-D activation of C6D6 at rates that are approximately 2-3 times more rapid than that for TpRu(CO)(NCMe)Ph (depending on substrate concentration); however, the catalytic hydrophenylation of ethylene using TpRu(PMe3)(NCMe)Ph is substantially less efficient than catalysis with TpRu(CO)(NCMe)Ph. For TpRu(PMe3)(NCMe)Ph, C-H activation of ethylene, to ultimately produce TpRu-(PMe3)(ɳ3-C4H7), is found to kinetically compete with catalytic ethylene hydrophenylation. In THF solutions containing ethylene, TpRu(PMe3)(NCMe)Ph and TpRu(CO)(NCMe)Ph separately convert to TpRu(L)(ɳ3-C4H7)(L = PMe3 or CO, respectively) via initial Ru-mediated ethylene C-H activation. Heating mesitylene solutions of TpRu(L)(ɳ3-C4H7) under ethylene pressure results in the catalytic production of butenes (i.e., ethylene hydrovinylation) and hexenes.

Description: This article discusses computational s-block thermochemistry with the correlation consistent composite approach. Abstract: The correlation consistent composite approach (ccCA) is a model chemistry that has been shown to accurately compute gas-phase enthalpies of formation for alkali and alkaline earth metal oxides and hydroxides (Ho, D.S.; DeYonker, N.J.; Wilson, A.K.; Cundari, T.R., J. Phys. Chem. A 2006, 110, 9767). The ccCA results contrast to more widely used model chemistries where calculated enthalpies of formation for such species can be in error by up to 90 kcal molˉ¹. In this study, the authors have applied ccCA to a more general set of 42 s-block molecules and compared the ccCA ∆Hf values to values obtained using the G3 and G3B model chemistries. Included in this training set are water complexes such as Na(H₂O)n⁺ where n = 1 - 4, dimers and trimers of ionic compounds such as (LiCl)₂ and (LiCl)₃, and the largest ccCA computation to date: Be-(acac)₂, BeC₁₀H₁₄O₄. Problems with the G3 model chemistries seem to be isolated to metal-oxygen bonded systems and Be-containing systems, as G3 and G3B still perform quite well with a 2.7 and 2.6 kcal mol⁻¹ mean absolute deviation (MAD), respectively, for gas-phase enthalpies of formation. The MAD ...