The
catalyzed hydrogenation of CO2 to formate via a
triphosphine-ligated Cu(I) was studied computationally at the density
functional theory level in the presence of a self-consistent reaction
field. Of the four functionals benchmarked, M06 was generally in the
best agreement with the available experimentally estimated values.
Two bases, DBU and TBD, were studied in the context of two proposed
mechanisms in the MeCN solvent. Activation of H2 was explored
by using LCu(DBU)+ to form LCuH. Dissociation of a ligand
arm results in higher barriers to form the key hydride complex, LCuH.
The preferred mechanism passes through a transition state, where the
H2 has one H atom interacting with the copper center and
the other H atom interacting with the N atom of the base, similar
to H2 insertion into a frustrated Lewis pair. There is
no significant difference between the choice of a base, DBU or TBD,
with respect to the proposed mechanisms. We propose that the experimentally
observed differences between DBU and TBD reactivities for this mechanism
are due to off-pathway changes.