Hydrido−Carbonyl Chain Clusters. Synthesis, Solid
State Structure, and Solution Behavior of the
Tetranuclear Open Cluster Anions [Re4H(μ-H)2(CO)17]-
and [Re4(μ-H)(CO)18]-
posted on 1997-09-16, 00:00authored byMirka Bergamo, Tiziana Beringhelli, Giuseppe D'Alfonso, Pierluigi Mercandelli, Massimo Moret, Angelo Sironi
The addition of
[Re2H(CO)9]- to the
electronically unsaturated complex
[Re2(μ-H)2(CO)8]
rapidly and selectively gives the anion
[Re4H(μ-H)2(CO)17]-
(2), containing an open chain
tetranuclear metal skeleton, as revealed by a single-crystal X-ray
analysis of its [NEt4]+
salt. In the solid state the three metal−metal interactions
display a staggered−eclipsed−staggered conformation, while in solution 1H and
13C NMR spectra have shown conformational freedom around the three Re−Re interactions and a dynamic
process exchanging the
two hydrides bound to the terminal
H2Re(CO)4 moiety, as well as the
carbonyls trans to
them (Ea = 48(1) kJ/mol). A
windshield-wiper motion of the
H2Re(CO)4 fragment
around
the two trans diaxial carbonyls, analogous to that
previously observed in the related anions
[Re3H(μ-H)(CO)13]-
and
[Re2H2(μ-H)(CO)8]-,
is likely responsible for this exchange. The
tetrametallic skeleton of the anion 2 in solution easily
undergoes fragmentation to trinuclear species. Under CO atmosphere the clean formation of
[ReH(CO)5] and
[Re3H(μ-H)(CO)13]- has been recognized. The anion
2 is formed (even if in lower yields) also by
reaction of
[Re2H2(μ-H)(CO)8]-
with “Re2(CO)9(THF)”, obtained by
treatment of [Re2(CO)10] with
Me3NO in THF. A 13C NMR investigation
has clarified that such
“Re2(CO)9(THF)”
reagent
is indeed a mixture of three
eq-[Re2(CO)9L] species,
containing THF, H2O, and, in a minor
amount, NMe3, as labile L ligands. The reaction of the
same eq-[Re2(CO)9L] species
with
[Re2H(CO)9]- affords in
good yields the tetranuclear cluster anion
[Re4(μ-H)(CO)18}]-
(3).
The single-crystal X-ray analysis of
[NEt4]3 has revealed also in this case a
Re4 chain, with
an all-staggered conformation, of idealized C2
symmetry. The low-temperature 13C NMR
spectrum of the carbonyls has shown a higher symmetry in solution,
suggesting conformational freedom around all of the Re−Re interactions.