Photochemical C−H Activation and Ligand Exchange Reactions of CpRe(PPh₃)₂H₂. Phosphine Dissociation Is Not Involved

The dihydride CpRe(PPh 3 ) 2 H 2 ( 1 ) catalyzes H/D exchange between C 6 D 6 and other arenes or alkanes. Compound 1 also undergoes photochemical phosphine substitution with PMe 3 to give CpRe(PPh 3 )(PMe 3 )H 2 and then CpRe(PMe 3 ) 2 H 2 . Mechanistic studies of these reactions are inconsistent with [CpRe(PPh 3 )H 2 ] as an intermediate. An alternative mechanism is presented proposing that the active species for H/D exchange is the 14-electron cyclic allyl intermediate [(η 3 -C 5 H 7 )Re(PPh 3 ) 2 ] ( E ), in which both hydrides have migrated from the rhenium to the cyclopentadienyl ligand. This intermediate accounts for the fact that (1) deuterium does not exchange into the hydride ligands of complex 1 during the H/D exchange catalysis and (2) phosphine substitution occurs by an associative pathway. The precursor to intermediate E, [(η 4 -C 5 H 6 )Re(PPh 3 ) 2 H] ( D ), can undergo reversible orthometalation, allowing H/D exchange between the hydride ligands and the ortho phosphine positions. Evidence is presented to support this new mechanism as well as to rule out other feasible mechanisms.