| A series of [RhClL2]2 catalysts (L = tricyclopropylphosphine, tricyclobutylphosphine, tricyclopentylphosphine, tricyclohexylphosphine) have been synthesized and their activity as catalyst for alkane transfer-dehydrogenation has been investigated. [RhCl(PcyBu3) 2]2 (cyBu = cyclobutyl) is found to exhibit the highest catalytic activity under argon atmosphere at 90°C. The proposed mechanism involves rate-determining direct oxidative addition of C-H bonds to the dimer [RhCl(PcyBu 3)2]2 followed by cleavage of the μ-Cl bridge bond and the β-hydrogen elimination. Complex [(η2-4-butenyl)P cy Bu2]Rh(PcyBu 3)Cl was detected as the decomposition product in both the stoichiometric reaction between [RhCl(PcyBu3) 2]2 and cyclooctane and the catalytic transfer-dehydrogenation. This product presumably arises from opening of one of the cyclobutyl ring. It reacted with CO very quickly to give a mixture of Rh(Pcy Bu3)2Cl(CO), [(4-butenyl)P cy Bu2]Rh(PcyBu 3)Cl(CO) and Rh[(4-butenyl)PcyBu 2]2Cl(CO).; The kinetics of RhL*2Cl(N2) (L* = P iPr3) decomposition in cyclooctane solvent have been measured. An Eyring plot of the decomposition of RhL*2Cl(N 2) yields values for ΔH≠ and ΔS ≠ of (27.4 ± 0.6) kcal/mol and (3.5 ± 1.70) eu, respectively. A lower limit to the Rh-N2 bond dissociation enthalpy (BDE) was estimated to be 24 kcal/mol. Based on this, lower limits of other Rh-L ′BDE's were determined (L′, BDE in kcal/mol): C2H4, 32.7; PhC ≡ CPh, 30.4; t BuNC, 50.3; CO, 56.1. Lower limits for the bridge strength in [RhL*2Cl]2 and the average Rh-H BDE in H2RhL* 2Cl were determined to be 33.6 kcal/mol and 72.3 kcal/mol, respectively.; The iridium “pincer” complexes, IrHCl[η3-C 6 H3-2,6-(CH2PiPr 2)2] and IrH4[η3-C6H 3 -2,6-(CH2PiPr2) 2], have been synthesized and characterized. IrH4[η 3-C6H3-2,6-(CH2Pi Pr2)2] catalyzes the efficient dehydrogenation of alkanes under reflux in the absence of a sacrificial hydrogen acceptor to give the corresponding alkenes and dihydrogen. A mixture of cis- and trans-diethylcyclohexane were formed as secondary products in the thermo-dehydrogenation nation of cyclodecane. The mechanism for their formation is proposed to involve a Cope rearrangement of 1,5-cyclodecadiene.; IrH4[η3-C6H3-2,6-(CH 2PiPr2)2] catalyzes the transfer-dehydrogenation of linear alkane (n-octane) to yield the 1-alkene (alpha-olefin) as the major kinetic product, using various sacrificial acceptors. The yield of alpha-olefin was limited by subsequent isomerization which leads ultimately to the formation of internal olefins. A possible mechanism was proposed for the catalysis of both transfer-dehydrogenation as well as isomerizatio... |
