No abstract

The thermodynamic hydricity of a metal hydride can vary considerably between solvents. This parameter can be used to determine the favourability of a hydride-transfer reaction, such as the reaction between a metal hydride and CO2 to produce formate. Because the hydricities of these species do not vary consistently between solvents, reactions that are thermodynamically unfavourable in one solvent can be favourable in others. The hydricity of a water-soluble, bis-phosphine nickel hydride complex was compared to the hydricity of formate in water and in acetonitrile. Formate is a better hydride donor than [HNi(dmpe)2](+) by 7 kcal mol(-1) in acetonitrile, and no hydride transfer from [HNi(dmpe)2](+) to CO2 occurs in this solvent. The hydricity of [HNi(dmpe)2](+) is greatly improved in water relative to acetonitrile, in that reduction of CO2 to formate by [HNi(dmpe)2](+) was found to be thermodynamically downhill by 8 kcal mol(-1). Catalysis for the hydrogenation of CO2 was pursued, but the regeneration of [HNi(dmpe)2] under catalytic conditions was unfavourable. However, the present results demonstrate that the solvent dependence of thermodynamic parameters such as hydricity and acidity can be exploited in order to produce systems with balanced or favourable overall thermodynamics. This approach should be advantageous for the design of future water-soluble catalysts.

Nickel complexes were prepared with diphosphine ligands that contain pendant amines, and these complexes catalytically oxidize primary and secondary alcohols to their respective aldehydes and ketones. Kinetic and mechanistic studies of these prospective electrocatalysts were performed to understand what influences the catalytic activity. For the oxidation of diphenylmethanol, the catalytic rates were determined to be dependent on the concentration of both the catalyst and the alcohol and independent of the concentration of base and oxidant. The incorporation of pendant amines to the phosphine ligand results in substantial increases in the rate of alcohol oxidation with more electron-donating substituents on the pendant amine exhibiting the fastest rates.

The neopentylidene-neopentyl complex (PNP)TiCHtBu(CH2 tBu) (1; (PNP− = N[2-P(CHMe2)2-4-methylphenyl]2) extrudes neopentane in neat fluorobenzene under mild conditions (25 °C) to generate the transient titanium alkylidyne (PNP)TiCtBu (A), which subsequently undergoes regioselective 1,2-CH bond addition of a fluorobenzene across the TiC linkage to generate (PNP)TiCHtBu(o-FC6H4) (2). Kinetic and mechanistic studies suggest that the C−H activation process is pseudo-first-order in titanium, with the α-hydrogen abstraction being the rate-determining step and the post-rate-determining step being the C−H bond activation of fluorobenzene. At 100 °C complex 2 does not equilibrate back to A and the preference for C−H activation in benzene versus fluorobenzene is 2:3, respectively. Compound 1 also reacts readily, and in most cases cleanly, with a series of hydrofluoroarenes (HArF), to form a family of alkylidene-arylfluoride derivatives of the type (PNP)TiCHtBu(ArF). Thermolysis of the latter compounds generates the titanium alkylidene-fluoride (PNP)TiCHtBu(F) (14) by a β-fluoride elimination, concurrent with formation of o-benzyne. β-Fluoride elimination to yield 14 occurs from 2 under elevated temperatures with k average = 4.96(16) × 10−5 s−1 and with activation parameters ΔH ⧧ = 29(1) kcal/mol and ΔS ⧧ = −3(4) cal/mol·K. It was found that β-fluoride elimination is accelerated when electron-rich groups are adjacent to the fluoride group, thus implying that a positive charge buildup at the arylfluoride ring occurs in the activated complex of 2. The alkylidene derivative (PNP)TiCHSiMe3(CH2SiMe3) (15) also undergoes α-hydrogen abstraction to form the putative (PNP)Ti’CSiMe3 (B) at higher temperatures (>70 °C) and dehydrofluorinates the same series of HArF when the reaction mixture is thermolyzed at >100 °C over 72 h to produce o-benzyne products and the fluoride analogue (PNP)TiCHSiMe3(F) (26). Only in the case of the substrate 1,2-F2C6H4 can the kinetic C−H activation product (PNP)TiCHSiMe3(o,m-F2C6H3) be isolated and crystallographically characterized. 1-Fluorohexane and fluorocyclohexane can also be dehydrofluorinated by intermediates A and B. No intermediates are observed, but in the case of 1-fluorohexane, the terminal olefin is spectroscopically identified. The dehydrofluorination of HArF and hydrofluoroalkanes (HAlF) can be made cyclic via the quantitative conversion of the alkylidene-fluorides to 1 and 15, by means of transmetalation with LiCH2XMe3 (X = C and Si), and the reactivity of 1 with halobenzenes is also presented and discussed.