Ligand effects in C-H and C-C bond activation by gas-phase transition metal-ligand complexes

Guided ion beam mass spectrometry has been used to examine the kinetic energy dependence of reactions of FeL(+) (L = CO and H2O) with methane and ethane. Carbon-hydrogen and carbon-carbon bond activation is observed, and there is no evidence for direct interaction of either ligand in the chemistry. Thresholds for these processes are measured and converted to the following LFe(+)-CH3 0 K band dissociation energies (BDEs): D-0[(COFe+-CH3] = 1.30 +/- 0.05 eV and D-0[(H2OFe+-CH3] = 1.95 +/- 0.10 eV. Comparison of these values to the previously determined D-0(Fe+-CH3) = 2.37 +/- 0.05 eV and to D-0(LFe(+)-D) bond energies permits a quantitative assessment of the effects of ligation on sigma-bond activation by metal complexes. Differences in the abilities of the two FeL(+) species to activate methane and ethane are compared to the behavior of the two complexes activating D-2. As in the D-2 case, sigma bond activation of these alkanes by Fe(H2O)(+) is more efficient than Fe(CO)(+) at low energies. This result is rationalized in terms of different electronic structures at the ligated metal ion centers. In addition, it is found that the selectivity of the reactions is affected by ligation, such that Fe(CO)(+) activates the C-H and C-C bonds of ethane with comparable efficiency, while Fe(H2O)(+) prefers to activate the C-H bonds.