Enabling Molecular-Level Computational Description of Redox and Proton-Coupled Electron Transfer Reactions of Samarium Diiodide

Samarium diiodide (SmI2, Kagan’s reagent)
is
a one-electron reductant with applications ranging from organic synthesis
to nitrogen fixation. Highly inaccurate relative energies of redox
and proton-coupled electron transfer (PCET) reactions of Kagan’s
reagent are predicted by pure and hybrid density functional approximations
(DFAs) when only scalar relativistic effects are accounted for. Calculations
including spin–orbit coupling (SOC) show that the SOC-induced
differential stabilization of the Sm(III) versus the Sm(II) ground
state is little affected by ligands and solvent, and a standard SOC
correction derived from atomic energy levels is thus included in the
reported relative energies. With this correction, selected meta-GGA
and hybrid meta-GGA functionals predict Sm(III)/Sm(II) reduction free
energies to within 5 kcal/mol of the experiment. Considerable discrepancies
remain, however, in particular for the PCET-relevant O–H bond
dissociation free energies, for which no regular DFA is within 10
kcal/mol of the experiment or CCSD(T). The main cause behind these
discrepancies is the delocalization error, which leads to excess ligand-to-metal
electron donation and destabilizes Sm(III) versus Sm(II). Fortunately,
static correlation is unimportant for the present systems, and the
error may be reduced by including information from virtual orbitals
via perturbation theory. Contemporary, parametrized double-hybrid
methods offer promise as companions to experimental campaigns in
the further development of the chemistry of Kagan’s reagent.