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

To understand photochemical and thermal alkane activation with IrH2(O2CCF3) (PAr3)(2) (Ar = p-FC6H4), H/D isotope scrambling between alkenes and IrD2(O 2CCF3)(PAr3)(2) was studied. No unique interpretation of the experimental d ata 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 po ssibilities allowed by experiment. Of several possible mechanisms for H/D s crambling, one was strongly preferred and is therefore proposed here. It in volves the insertion of the olefin to give an alkyl hydride that reductivel y eliminates to lead to a transition state that contains an eta (3)-bound a lkane. This transition state, which achieves a 1,1' geminal H/D exchange, i s significantly lower in energy than a dihydrido carbene, located as a seco ndary minimum, eliminating the alternative carbene mechanism, The unexpecte dly large binding energy (BDE) of the alkane ('sticky alkane") to the Ir(O2 CCF3)(PH3)(2) fragment (BDE = 11.9 kcal mol(-1)) in this transition state i s ascribed in part to the presence of a weakly sigma- and pi -donating (CF3 CO2) group trans to the alkane binding site. The H/D exchange selectivity o bserved requires that 1,1'-shifts (i.e., M moving to a geminal C-H bond), b ut 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 h ave 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 invo lve a strong steric hindrance at a tertiary carbon in this system. The mech anism via an alkane path provides an insight into the closely related photo chemical and catalytic thermal alkane dehydrogenation processes mediated by IrH2(O2CCF3)(PAr3)(2); the thermal route requires (BuCH)-Bu-t=CH2 as the h ydrogen acceptor. These two alkane reactions are intimately related mechani stically to the isotope exchange because they are proposed to have the same intermediates, in particular the sticky alkane complex. Remarkably, the ra te determining step of the thermal (150 degreesC) alkane dehydrogenation pr ocess is predicted to be substitution of the hydrogen acceptor-derived alka ne by the alkane substrate.