Oxidative Hydrogen Migration:
Characterization of a Novel Mechanism for C-H Activation

Activation of carbon-hydrogen bonds by organometallic complexes is a topic that that has attracted significant attention in both academic and industrial communities. While normally a very inert bond, the breaking of the C-H bond can be catalyzed by metals spanning the range of the periodic table.

The two mechanisms normally considered for activation are oxidative addition / reductive elimination (OA/RE) and sigma-bond metathesis (SBM), illustrated right. The OA/RE mechanism requires an accessible filled non-bonding d-orbital, in order to promote the metal from an oxidation state of Mn to Mn+2. OA/RE is a stepwise mechanism, where the Mn+2 complex is an intermediate.  SBM, on the other hand, is a concerted mechanism, formally classified as a 2+2 addition. The SBM mechanism makes use of a metal d-orbital to circumvent the forbidden character of a 2+2 addition, but since the d-orbital is part of the bonding even d0 metals can access the SBM mechanism. Indeed, the SMB mechanism appears to be particularly prevalent amongst electron poor d0 metals.

During our work on C-H activation we discovered a novel mechanism, where activation of the C-H bond occurs in a concerted fashion best described as an Oxidative Hydrogen Migration (OHM). The character of the transition structure is reminiscent of an OA, but the absence of a stable intermediate infers that this is not an OA/RE mechanism. Neither is it a SBM mechanism, however, as the OHM transition state features a fully formed bond between the metal and the migrating hydrogen.

In this work we are defining the characteristics of the OHM transition state, and what separates it from the OA/RE and SMB mechanisms. We show that the OHM mechanism is indeed related to OA/RE, with the energy of the OA/RE intermediate and the OHM transition state related to the accessibility of the Mn+2 state. We are currently exploring what factors causes a system to switch from an OA/RE to an OHM mechanism, including ligand environment, choice of metal and nature of reactants.

Personnel: Dr. Jonas Oxgaard