Cationic hydrogen complexes of Rhenium .2. synthesis, reactivity, and competition studies

Cationic rhenium dihydrogen complexes, [Re(H-2)(PR3)(2)(CO)(3)]B(Ar')(4)(PR3=PCy3, PiPr(3), PiPrPh(2), PPh3; Ar'=3,5-((CF3)(2)C6H3), have been prepared by the protonation of ReX(PR3)(2)(CO)(3) (X=H, CH3) with [H(Et2O)(2)]B(Ar')(4) under a hydrogen atmosphere. Deuterium is incorporated into the H-2 ligand when placed under a D-2 atmosphere and large J(HD) values (30-33 Hz) are consistent with a dihydrogen formulation. Relaxation data indicate very short T-1min for these complexes. These complexes are susceptible to heterolytic cleavage of dihydrogen, and the reactivity with several bases has been investigated. Under vacuum or argon atmosphere the complexes readily lose hydrogen to form 16-electron complexes. In the solid state, [Re(PCy3)(2)(CO)(3)]B(Ar')(4) exhibits an agostic interaction to a beta C-H bond of the phosphine ligand. Variable-temperature P-31{H-1} NMR spectra of [Re(PCy3)(2)(CO)(3)]B(Ar')(4) indicate a dynamic process involving hindered rotation about the Re-P bond. Competition studies have been conducted, and the hydrogen binding affinity is higher for [Re(H-2)(PCy3)(2)(CNtBu)(3)]B(Ar')(4), (5) than for [Re(H-2)(PCy3)(2)(CO)(3)]B(Ar')(4) (2a). Similar experiments also find that 5 also binds hydrogen preferentially over W(H-2(PCy3)(2)(CO)(3).