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
