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•, have been studied for the Rh­(III) hydride complex Cp*Rh­(2-(2-pyridyl)­phenyl)­H (1a). The thermodynamic hydricity, ΔG°_H^– , for 1a has been measured (49.5(1) kcal/mol) by heterolytic cleavage of H₂ with Et₃N in CH₃CN. The transfer of H^– from 1a to 1-(1-phenylethylidene)­pyrrolidinium is remarkably fast (k _H^– = 3.5(1) × 10⁵ M^–1 s^–1), making 1a 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 1a to 12 has been estimated (k _H⁺ = 5(1) × 10^–4 M^–1 s^–1) from the half-life of the equilibration. Thus, 1a is a poor H⁺ donor both thermodynamically and kinetically. However, 1a transfers H• to TEMPO smoothly, forming a stable Rh­(II) radical Cp*Rh­(2-(2-pyridyl)­phenyl)• (14a) that can activate H₂ 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• from 1a to Ar₃C• (Ar = p-^tBuC₆H₄) is fast, with k _H• = 1.17(3) × 10³ M^–1 s^–1. Thus, 1a is a good H^– and H• donor but a poor H⁺ donor, a combination that reflects the high energy of the Rh­(I) anion [Cp*Rh­(2-(2-pyridyl)­phenyl)]⁻.