Are N-Heterocyclic Carbenes
“Better”
Ligands than Phosphines in Main Group Chemistry? A Theoretical Case
Study of Ligand-Stabilized E<sub>2</sub> Molecules, L-E-E-L (L = NHC,
phosphine; E = C, Si, Ge, Sn, Pb, N, P, As, Sb, Bi)
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Abstract
A theoretical examination of the L-E-E-L class of molecules
has
been carried out (E = group 14, group 15 element; L = N-heterocyclic
carbene, phosphine), for which Si, Ge, P, and As-NHC complexes have
recently been synthesized. The focus of this study is to predict whether
it is possible to stabilize the elusive E<sub>2</sub> molecule via
formation of L-E-E-L beyond the few known examples, and if the ligand
set for this class of compounds can be extended from the NHC to the
phosphine class of ligands. It is predicted that thermodynamically
stable L-E-E-L complexes are possible for all group 14 and 15 elements,
with the exception of nitrogen. The unknown ligand-stabilized Sn<sub>2</sub> and Pb<sub>2</sub> complexes may be considered attractive
synthetic targets. In all cases the NHC complexes are more stable
than the phosphines, however several of the phosphine derivatives
may be isolable. The root of the extra stability conferred by the
NHC ligands over the phosphines is determined to be a combination
of the NHCs greater donating ability, and for the group 15 complexes,
superior π acceptor capability from the E-E core. This later
factor is the opposite as to what is normally observed in transition
metal chemistry when comparing NHC and phosphine ligands, and may
be an important consideration in the ongoing “renaissance”
of low-valent main group compounds supported by ligands