Reactions = Si (2a), Ge (3a), and Sn
(4a)) with a Lewis acid (BF3·OEt2
or Me3SiOSO2CF3 (TMSOTf))
have been examined. The
silyl complex 2a reacts with
BF3·OEt2 to give a stable cationic
phosphenium complex OMe anion abstraction from phosphorus. This
reaction is in contrast to that of the corresponding alkyl complex
reported earlier showing
that the phosphenium complex once formed immediately undergoes
migratory insertion of
the phosphenium ligand into the Fe−C bond. The X-ray analysis that there is considerable double-bond character
in
the Fe−P bond. The rotation barrier around the
Fe−P(phosphenium) bond is estimated
(ΔG⧧ (248 K) = 12.7 kcal/mol) from a
variable-temperature 1H NMR study. The
germyl
complex 3a exhibits the same reactivity as that of
2a. In contrast, the stannyl complex
4a
reacts with TMSOTf to give a stannylene (4e). X-ray analysis reveals that
4e is regarded as a stannylene iron complex
doubly-base-stabilized by an oxygen of OTf- and one
nitrogen of ligand. In solution, there is an equilibrium between a
base-stabilized and a base-free
stannylene form. The activation parameter
(ΔG⧧ (188 K) = 9.0 kcal/mol) is estimated
for
the methyl group exchange in the stannylene ligand. The reaction
of TMSOTf verifies that a cationic phosphenium iron
complex is formed first and that then an alkyl migration from the Sn to
the phosphenium
phosphorus takes place
