Recent reports of 1,2-addition of C-H bonds across Ru-X (X ) amido, hydroxo) bonds of TpRu-(PMe3)X fragments {Tp ) hydridotris(pyrazolyl)borate} suggest opportunities for the development of new catalytic cycles for hydrocarbon functionalization. In order to enhance understanding of these transformations, computational examinations of the efficacy of model d 6 transition metal complexes of the form [(Tab)M-(PH3)2X] q (Tab ) tris-azo-borate; X ) OH, NH2; q ) -1 to +2; M ) Tc I , Re I , Ru II , Co III , Ir III , Ni IV , Pt IV ) for the activation of benzene C-H bonds, as well as the potential for their incorporation into catalytic functionalization cycles, are presented. For the benzene C-H activation reaction steps, kite-shaped transition states were located and found to have relatively little metal-hydrogen interaction. The C-H activation process is best described as a metal-mediated proton transfer in which the metal center and ligand X function as an activating electrophile and intramolecular base, respectively. While the metal plays a primary role in controlling the kinetics and thermodynamics of the reaction coordinate for C-H activation/ functionalization, the ligand X also influences the energetics. On the basis of three thermodynamic criteria characterizing salient energetic aspects of the proposed catalytic cycle and the detailed computational studies reported herein, late transition metal complexes (e.g., Pt, Co, etc.) in the d 6 electron configuration {especially the TabCo(PH3)2(OH) + complex and related Co(III) systems} are predicted to be the most promising for further catalyst investigation.

No abstract

The correlation consistent composite approach ͑ccCA͒ was applied to the prediction of reaction barrier heights ͑i.e., transition state energy relative to reactants and products͒ for a standard benchmark set of reactions comprised of both hydrogen transfer reactions and nonhydrogen transfer reactions ͑i.e., heavy-atom transfer, S N 2, and unimolecular reactions͒. The ccCA method was compared against G3B for the same set of reactions. Error metrics indicate that ccCA achieves "chemical accuracy" with a mean unsigned error ͑MUE͒ of 0.89 kcal/ mol with respect to the benchmark data for barrier heights; G3B has a mean unsigned error of 1.94 kcal/ mol. Further, the greater accuracy of ccCA for predicted reaction barriers is compared to other benchmarked literature methods, including density functional ͑BB1K, MUE= 1.16 kcal/ mol͒ and wavefunction-based ͓QCISD͑T͒, MUE= 1.10 kcal/ mol͔ methods.

Phosphorus-carbon bond formation from discrete transition metal complexes have been investigated through a combination of synthetic, spectroscopic, crystallographic, and computational methods. Reactive intermediates of the type (diphosphine)Pd(aryl)(P(O)(OEt) 2 ) have been prepared, characterized, and studied as possible intermediates in metal-mediated coupling reactions. Several of the reactive intermediates were characterized crystallographically, and a discussion of the solid state structures is presented. In contrast to other carbon-heteroelement bond forming reactions, palladium complexes containing electrondonating substituents on the aromatic fragment exhibited faster rates of reductive elimination. Large bite angle diphosphine ligands induced rapid rates of elimination, while bipyridine and small bite angle diphosphine ligands resulted in much slower rates of elimination. An investigation of the effect of typical impurities on the elimination reaction was carried out. While excess diphosphine, pyridine, and acetonitrile had little effect on the observed rate, the addition of water slowed the phosphorus-carbon bond forming reaction. Coordination of water to the complex was observed spectroscopically and crystallographically. Computational studies were utilized to probe the reaction pathways for P-C bond formation via Pd catalysis.