Influence of arene and alkane coordination on reversible hydrogen elimination from IrXH 2(H 2)(PPr i3) 2 (X = Cl,Br,I)

Abstract

The equilibrium positions between IrXH 2(H 2)(PPr i 3) 2 (X=Cl ( 1), Br ( 2), I ( 3)) and the corresponding five-coordinate complexes, IrXH 2(PPr i 3) 2 (X = Cl ( 4), Br ( 5), 1 ( 6)) have been found to be highly solvent dependent. Values of ΔH and ΔS have been determined from van't Hoff plots of the equilibrium constants calculated from 1H NMR spectroscopic data obtained in variable temperature studies of equilibrated solutions of complexes 1, 2 and 3 in toluene-d 8, methylcyclohexane-d 14 and n-hexane-d 14. In the alkane solvents, the values of ΔH = 12 kcal mol −1 and Δ S of 33–37 e.u. were determined for the loss of H 2 from 1 and 2. The finding that ΔH = 8.0–8.6 kcal mol −1 and ΔS = 15–19 e.u. in toluene solution suggests that while simple dissociation of H 2 occurs in alkane solution, solvent coordination to the resulting five-coordinate complex occurs in toluene. The significantly lower values (10.2 and 10.5 kcal mol −1 of Δ H cletetntined for complex 3 in alkane solutions suggest the establishment of a coordinative interaction with a strength of at least 2 kcal mol −1. The significantly lower (23–24 e.u.) values Δ S determined for 3 in alkane solutions indicate that the 3 6 equilibrium is influenced by alkane coordination rather than agostic interactions with the phosphine isopropyl groups.