Isotope Effects on Hydride Transfer Reactions from Transition Metal Hydrides to Trityl Cation. An Inverse Isotope Effect for a Hydride Transfer

Hydride transfer from transition metal hydrides (MH) to Ph 3 C + BF 4 - gives M−FBF 3 and Ph 3 CH. Deuterium kinetic isotope effects were determined for several MH/MD pairs (CH 2 Cl 2 solution, 25 °C). For hydride transfer from Cp*(CO) 3 MoH (Cp* = η 5 -C 5 Me 5 ) to substituted trityl cations containing zero, one, two, or three p -MeO groups [Ph n ( p -MeOC 6 H 4 ) 3 - n C + BF 4 -; n = 3, 2, 1, 0], the isotope effect remains essentially constant at k MoH / k MoD = 1.7−1.9 as the rate constant decreases from k H − = 6.5 × 10 3 to 1.4 M - 1 s - 1 . For hydride transfer to Ph 3 C + BF 4 - from five metal hydrides [Cp(CO) 3 MoH, Cp*(CO) 3 WH, (indenyl)(CO) 3 WH, Cp*(CO) 3 MoH, and trans -Cp(CO) 2 (PCy 3 )MoH; Cp = η 5 -C 5 H 5 ] with second-order rate constants k H − ≥ 3.8 × 10 2 M - 1 s - 1, the kinetic isotope effects are also k MH / k MD = 1.7−1.8. For a series of five tungsten hydrides with substituted Cp ligands, the kinetic isotope effects decrease from k WH / k WD = 1.8 to 0.47 as the rate constant decreases (from k H − = 2.0 × 10 3 to 0.72 M - 1 s - 1 ). The steadily decreasing values of k MH / k MD with decreasing rate constants of hydride transfer are interpreted as indicating progressively stronger force constants of isotopically sensitive modes of the transition state, as the reaction slows down in progressing from more electron-donating Cp ligands to less electron-rich Cp ligands. The inverse isotope effect ( k WH / k WD = 0.47) found for the slowest tungsten hydride, (C 5 H 4 CO 2 Me)(CO) 3 WH, is proposed to be due to a product-like transition state for irreversible hydride transfer.