DFT/ECP study of C-H activation by (PCP)Ir and (PCP)Ir(H)(2) (PCP = eta(3)-1,3-C6H3(CH2PR2)(2)). Enthalpies and free energies of associative and dissociative pathways
K. Krogh-jespersen et al., DFT/ECP study of C-H activation by (PCP)Ir and (PCP)Ir(H)(2) (PCP = eta(3)-1,3-C6H3(CH2PR2)(2)). Enthalpies and free energies of associative and dissociative pathways, J CHEM INF, 41(1), 2001, pp. 56-63
Citations number
59
Categorie Soggetti
Chemistry
Journal title
JOURNAL OF CHEMICAL INFORMATION AND COMPUTER SCIENCES
(PCP)Ir(H)(2) (PCP = eta (3)-1,3-C6H3(CH2PR2)(2)) complexes are highly effe
ctive catalysts for the dehydrogenation of alkanes; in particular, they are
the first efficient molecular catalysts for alkane dehydrogenation that do
not require a sacrificial hydrogen acceptor. Using density functional theo
ry/effective core potential methods, we have examined C-H bond cleavage in
alkanes and arenes by both (PCP)Ir and (PCP)Ir(H)(2). C-H addition to the d
ihydride is accompanied by loss of H-2; both associative and dissociative p
athways for this exchange reaction have been examined. The energetic barrie
r (DeltaE double dagger) for associative displacement of H-2 by benzene is
much lower than the barrier for a dissociative pathway involving initial lo
ss of H-2; however, the pathways have very comparable free energy barriers
(DeltaG double dagger). Extrapolation to the higher temperatures, bulkier p
hosphine ligands, and the alkane substrates used experimentally leads to th
e conclusion that the pathway for the "acceptorless" dehydrogenation of alk
anes is dissociative. For hydrocarbon/hydrocarbon exchanges, which are requ
ired for transfer-dehydrogenation, dissociative pathways are calculated to
be much more favorable than associative pathways. We emphasize that it is t
he free energy, not just the internal energy or enthalpy, that must be cons
idered for elementary steps that show changes in molecularity.