The use of bulky ligands for the stabilization of group 15 radicals and cations
Abstract
A variety of bulky ligands have been utilized for the stabilization of group
15 radicals and cations. The molecular structures of the stable phosphinyl and
arsinyl radicals, •
PnR2 (Pn = P, As; R = CH(SiMe3)2), have been determined by
gas-phase electron diffraction (GED) in conjunction with ab initio molecular
orbital calculations. The X-ray crystal structures of the corresponding dipnictines,
the "dimers", R2PnPnR2, and the corresponding chloro derivatives R2PnCl have
also been determined. Collectively, these gas-phase and solid-state structural
investigations demonstrate that large distortions of the ligands attached to
phosphorous and arsenic occur when the pnictinyl radicals unite to form the
corresponding dipnictine dimers. Theoretical studies indicate that in the process
of pnictinyl radical dimerization, the molecules accumulate substantial amounts of
potential energy and are thus primed to spring apart upon release from the solid
state by melting, dissolution, or evaporation.
The first example of group 15-containing “constrained geometry”
complexes, [Me2Si(η-C5Me4)(N-t-Bu)]PnCl (Pn = P, As, Sb), have been prepared
and isolated. In the case of the chlorophosphine, [Me2Si(η-C5Me4)(N-t-Bu)]PCl,
an unprecedented shift of the µ-SiMe2 group from the ipso to the α-position on the
Me4C5 ring occurs. Treatment of [Me2Si(η-C5Me4)(N-t-Bu)]PnCl (Pn = P, Sb)
with AlCl3 results in chloride ion abstraction and formation of the pnictocenium
salt, {[Me2Si(η-C5Me4)(N-t-Bu)]Pn}{AlCl4}. The structures of these salts were
established by X-ray crystallography.
The chloropnictines (η-C5Me5)2PnCl (Pn = As, Sb, Bi) have been
synthesized and structurally characterized, and exhibit the expected periodic trend
toward increased multihapto bonding with increasing atomic number. The
arsenocenium and stibocenium tetrachloroaluminates salts were prepared by
chloride anion abstraction from the precursor chlorides and the structures of the
cations in these salts were found to be different than those in previously reported
examples.
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