On the mechanism of (PCP)Ir-catalyzed acceptorless dehydrogenation of alkanes: A combined computational and experimental study

Pincer complexes of the type (RpCp)IrH2, where ((PCP)-P-R)Ir is [eta(3)-2,6-(R2PCH2)(2)C6H3]Ir, are the most effective catalysts reported to date for the "acceptorless" dehydrogenation of alkanes to yield alkenes and free H-2 We calculate (DFT/B3LYP) that associative (A) reactions of ((PCP)-P-Me)IrH2 with model linear (propane, n-PrH) and cyclic (cyclohexane, CyH) alkanes may proceed via classical Ir(V) and nonclassical Ir(III)(eta(2)-H-2) intermediates. A dissociative (D) pathway proceeds via initial loss of H-2, followed by C-H addition to ((PCP)-P-Me)Ir. Although a slightly higher energy barrier (DeltaE(double dagger)) is computed for the D pathway, the calculated free-energy barrier (DeltaG(double dagger)) for the D pathway is significantly lower than that of the A pathway Under standard thermodynamic conditions (STP), C-H addition via the D pathway has DeltaG(odouble dagger) = 36.3 kcal/mol for CyH (35.1 kcal/mol for n-PrH). However, acceptorless dehydrogenation of alkanes is thermodynamically impossible at STP. At conditions under which acceptorless dehydrogenation is thermodynamically possible (for example, T = 150 degreesC and P-H2 = 1.0 x 10(-7) atm), DeltaG(double dagger) for C-H addition to ((PCP)-P-Me)Ir (plus a molecule of free H-2) is very low (17.5 kcal/mol for CyH, 16.7 kcal/mol for n-PrH). Under these conditions, the rate-determining step for the D pathway is the loss of H-2 from ((PCP)-P-Me)IrH2 with DeltaG(D)(double dagger) approximate to DeltaH(D)(double dagger) = 27.2 kcal/mol. For CyH, the calculated DeltaG(odouble dagger) for C-H addition to ((PCP)-P-Me)IrH2 on the A pathway is 35.2 kcal/mol (32 7 kcal/mol for n-PrH). At catalytic conditions, the calculated free energies of C-H addition are 31.3 and 33.7 kcal/mol for CyH and n-PrH addition, respectively. Elimination of H-2 from the resulting "seven-coordinate" Ir-species must proceed with an activation enthalpy at least as large as the enthalpy change of the elimination step itself (DeltaH approximate to 11-13 kcal/mol), and with a small entropy of activation. The free energy of activation for H-2 elimination (DeltaG(A)(double dagger)) is hence found to be greater than ca. 36 kcal/mol for both CyH and n-PrH under catalytic conditions. The overall free-energy barrier of the A pathway is calculated to be higher than that of the D pathway by ca 9 kcal/mol. Reversible C-H(D) addition to ((PCP)-P-R)IrH2 is predicted to lead to H/D exchange, because the barriers for hydride scrambling are extremely low in the "seven-coordinate" polyhydrides. In agreement with calculation, H/D exchange is observed experimentally for several deuteriohydrocarbons with the following order of rates: C6D6 > mesitylene-d(12) > n-decane-d(22) >> cyclohexane-d(12) Because H/D exchange in cyclohexane-d(12) solution is not observed even after 1 week at 180 degreesC, we estimate that the experimental barrier to cyclohexane C-D addition is greater than 36.4 kcal/mol. This value is considerably greater than the experimental barrier for the full catalytic dehydrogenation cycle for cycloalkanes (ca. 31 kcal/mol). Thus, the experimental evidence, in agreement with calculation, strongly indicates that the A pathway is not kinetically viable as a segment of the "acceptorless" dehydrogenation cycle.