Unifying the mechanisms for alkane dehydrogenation and alkene H/D exchange with [IrH2(O2CCF3)(PAr3)2]: the key role of CF3CO2 in the “sticky” alkane route

Abstract

To understand photochemical and thermal alkane activation with IrH₂(O₂CCF₃)(PAr₃)₂ (Ar =  p-FC₆H₄), H/D isotope scrambling between alkenes and IrD₂(O₂CCF₃)(PAr₃)₂ was studied. No unique interpretation of the experimental data was possible, so DFT(B3PW91) calculations on the exchange process in Ir(H)₂(O₂CCF₃)(PH₃)₂(C₂H₄) were carried out to distinguish between the possibilities allowed by experiment. Of several possible mechanisms for H/D scrambling, one was strongly preferred and is therefore proposed here. It involves the insertion of the olefin to give an alkyl hydride that reductively eliminates to lead to a transition state that contains an η³-bound alkane. This transition state, which achieves a 1,1′ geminal H/D exchange, is significantly lower in energy than a dihydrido carbene, located as a secondary minimum, eliminating the alternative carbene mechanism. The unexpectedly large binding energy (BDE) of the alkane (“sticky alkane”) to the Ir(O₂CCF₃)(PH₃)₂ fragment (BDE = 11.9 kcal mol⁻¹) in this transition state is ascribed in part to the presence of a weakly σ- and π-donating (CF₃CO₂) group trans to the alkane binding site. The H/D exchange selectivity observed requires that 1,1′-shifts (i.e., M moving to a geminal C–H bond), but not 1,3-shifts, be allowed in the alkane complex. In a key finding, a 1,3-shift in which the metal moves down the alkane chain is indeed found to have a much higher activation energy than the 1,1′-process and is therefore slow in our system. A 1,2-shift has not been considered since it would involve a strong steric hindrance at a tertiary carbon in this system. The mechanism ia an alkane path provides an insight into the closely related photochemical and catalytic thermal alkane dehydrogenation processes mediated by IrH₂(O₂CCF₃)(PAr₃)₂; the thermal route requires ^tBuCHCH₂ as the hydrogen acceptor. These two alkane reactions are intimately related mechanistically to the isotope exchange because they are proposed to have the same intermediates, in particular the sticky alkane complex. Remarkably, the rate determining step of the thermal (150 °C) alkane dehydrogenation process is predicted to be substitution of the hydrogen acceptor-derived alkane by the alkane substrate.