_P.-Powell-auth.-Principles-of-Organometallic-Chemistry-Springer-Netherlands-1988

Open-chain (acyclic) pentadienyl complexes
shows bands characteristic of a free diene unit. Moreover the 18-electron rule
predicts a structure either with one IJ 5 and one IJ 1 C 5 H 5 group or one containing
two IJ 3 -C 5 H 5 groups. When the temperature of the n.m.r. probe is lowered one
band, assigned to ry 5 -C 5 H 5 does not change. The other, however, broadens,
coalesces and finally resolves at low temperature into the pattern A 2 B 2 X expected
from an ry 1 -C,H 5 group. These changes are consistent with motion within the
molecule (called a 'fluxional' or 'dynamic' process) which is slow at low
temperature compared with the lifetime of the nuclear spin state, but which
becomes increasingly rapid as the temperature is raised. In the limit all five
protons are observed as an averaged signal.
The nature ofthe 'fluxional' process can be determined by careful examination
of the way the spectrum changes with tempera ture. In a 1, 2-shift the iron atom
moves to the adjacent position ofthe cyclopentadienyl ring. The environment of
each proton changes as indicated. Remembering that the positions A and A', B
and B' respectively are magnetically equivalent, both HA and HA change their
environments, but only one of H 8 and H 8 '. If this is the mechanism, the signal
arising from HA and HA' will collapse at a lower temperature than that from H 8 ,
H 8 '. If a 1. 3-shift operates the signal from H 8 , H 8 . will collapse at a lower
tempera ture than that from HA, HA. The observations agree with the former
postulate, indicating a 1, 2-shift mechanism.
Hx
~(
[Fp] HB'
HB x--A'
·~( A--x
H ---..:::
s--A
~ [Fp] _....-: HA' s'--s
~
Hx
A'-- B'
8' x-s·
·~::(
A' ' H ---..:::
A-- A'
H s-x
[ Fp] .--:: H X s'-A
HA A'--s
9.4 Open-chain (acydic) pentadienyl complexes
1, 4-pentadiene is deprotonated by n-butyllithium in the presence of tetrahydrofuran
to yield an orange solution ofpentadienyllithium. The potassium derivative
can be prepared from either 1, 3- or 1, 4-pentadiene in THF using LiBun /KOBut or
potassium metal in the presence of triethylamine. While the pentadienyl group
adopts a W configuration in the lithium derivative, it is U shaped in the potassium
salt.
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