Bridge Deprotonation of nido-2,3-RR^/C₂B₄H₆ Carboranes and B₅H₉: A Kinetic Study

The heterogeneous reactions of the title compounds (R = alkyl, arylmethyl, or phenyl; R' = R or H) with suspended NaH or KH in THF to give M⁺[RR'C₂B₄H₅“] + H₂ were studied as a function of temperature, with determination of reaction rates from the rate of formation of H₂. In all cases the kinetic rate law is pseudo-first order with linear plots of -In k vs. 1 T, and the rate is independent of the amount of metal hydride present. Reactions with NaD in place of NaH give no detectable incorporation of deuterium into the carborane anion. The main trend observed is a moderate decrease in reaction rate as the sizes of R and R' are increased, the effect being greatest in the bis(chromiumbenzyl)carborane complex [(CO)₃CrPhCH₂]₂C₂B₄H₆. The data are interpreted as reflecting primarily steric inhibition of the reaction by the R groups, except in the diphenyl derivative Ph₂C₂B₄H₆ which exhibits very fast reaction rates at the higher temperatures measured (above -40 °C) and anomalously high activation energy (£_a) and low entropy of activation (AS5). In this instance, electron withdrawal by the phenyl groups via resonance and pronounced steric effects by the phenyls are proposed to account for the observed behavior.