Kinetics and Thermodynamics of H-/H•/H+ Transfer from a Rhodium(III) Hydride

The thermodynamics and kinetics of all three cleavage modes for Rh-H, the transfer of H-, H+, or H center dot, have been studied for the Rh(III) hydride complex Cp*Rh(2-(2-pyridyl)phenyl)H (1a). The thermodynamic hydricity, Delta G(H-)degrees, for la has been measured (49.5(1) kcal/mol) by heterolytic cleavage of H-2 with Et3N in CH3CN. The transfer of H- from 1a to 1-(1-phenylethylidene)pyrrolidinium is remarkably fast (k(H-) = 3.5(1) x 10(5) M-1 s(-1)), making la a very efficient catalyst for the ionic hydrogenation of iminium cations. The pK(a) of 1a in CH3CN has been measured as 30.3(2) with (tert-butylimino)tris(pyrrolidino)phosphorane (12), and the rate constant for H+ transfer from la to 12 has been estimated (k(H+) = 5(1) x 10 M-1 s(-1)) from the half-life of the equilibration. Thus, la is a poor H+ donor both thermodynamically and kinetically. However, 1a transfers H center dot to TEMPO smoothly, forming a stable Rh(II) radical Cp*Rh(2-(2-pyridyl)phenyl)center dot (14a) that can activate H-2 at room temperature and 1 atm. The metalloradical 14a has a g value of 2.0704 and undergoes reversible one-electron reduction at -1.85 V vs Fc(+)/Fc in benzonitrile, implying a bond-dissociation enthalpy for the Rh-H bond of 1a of 58.2(3) kcal/mol-among the weakest Rh(III)-H bonds reported. The transfer of H center dot from 1a to Ar3C center dot (Ar = p-(BuC6H4)-Bu-t) is fast, with k(H center dot) = 1.17(3) x 10(3) M-1 s(-1). Thus, 1a is a good H- and H center dot donor but a poor H+ donor, a combination that reflects the high energy of the Rh(I) anion [Cp*Rh(2-(2-pyridyl)phenyl)](-).