Tertiary phosphine ligand-exchange reactions involving the
M![[quadruple bond, length as m-dash]](https://www.rsc.org/images/entities/char_e003.gif)
>M quadruply bonded complexes
M2Cl4L4, where
L = PMe3, PEt3,
PBun3 or PMe2Ph
Abstract
The reactions between M
2
Cl
4
L
4
complexes and an excess of L or L′ (PMe
3
,
PEt
3
, PBu
n
3
, PMe
2
Ph or
PMePh
2
) have been studied in
[
2
H
8
]toluene by
31
P-{
1
H} NMR spectroscopy. The
substitutions proceed in a stepwise manner wherein L′ displaces L,
except for
L′ = Me
2
PCH
2
CH
2
PMe
2
(dmpe) which yields
Mo
2
Cl
4
(dmpe-P)
4
. No tertiary
phosphine in this series is capable of displacing PMe
3
from a
M
2
Cl
4
(PMe
3
)
4
complex but by
spin magnetization transfer the degenerate exchange involving
Mo
2
Cl
4
(PMe
3
)
4
and
PMe
3
(added in excess) can be detected. The complexes
Mo
2
Cl
4
(PMe
3
)
4
and
Mo
2
Cl
4
(PEt
3
)
4
in benzene
showed no PMe
3
for PEt
3
scrambling at
+50 °C over several days despite the fact that
Mo
2
Cl
4
(PMe
3
)
4
-
n
(PEt
3
)
n
, where
n = 2 or 3, are kinetically inert to ligand
redistribution. In the presence of [
2
H
5
]pyridine
Mo
2
Cl
4
(PMe
3
)
4
and
Mo
2
Cl
4
(PEt
3
)
4
underwent
tertiary phosphine scrambling at 25 °C and in neat
[
2
H
5
]pyridine
Mo
2
Cl
4
(PMe
3
)
4
revealed the
formation of an equilibrium concentration of
Mo
2
Cl
4
(PMe
3
)
3
(py)
(py = pyridine) and free PMe
3
. Under similar
conditions Mo
2
Cl
4
(PEt
3
)
4
yielded an equilibrium mixture of
Mo
2
Cl
4
(PEt
3
)
3
(py) and
Mo
2
Cl
4
(PEt
3
)
2
(py)
2
and free PEt
3
. From kinetics the
ΔH
‡
values are positive in the range
+24 to +34 kcal mol
-1
and the
ΔS
‡
values range from +12 to +28 cal
K
-1
mol
-1
. Collectively the data
reported are consistent with an interchange dissociative mechanism,
I
d
, wherein M–P bond breaking contributes
significantly to the rate-determining step with related values of
ΔH
‡
being larger for
M = W than M = Mo. The rate dependence on
the entering ligand is clearly evident from temperature-dependent
studies and leads to varying ΔS
‡
values. The I
d
mechanism is proposed to involve
pre-equilibria between M
2
Cl
4
L
4
and the
entering L′ in an axial site followed by rate-determining
M–L displacement. In neat [
2
H
5
]pyridine
this may be viewed as a solvent-assisted displacement. The present
results are discussed in terms of earlier studies from which researchers
inferred a simple dissociative process, D, involving
M–PR
3
bond rupture as the first and rate-determining
step.
-
n
(PEt
3
)
n
, where
n = 2 or 3, are kinetically inert to ligand
redistribution. In the presence of [
2
H
5
]pyridine
Mo
2
Cl
4
(PMe
3
)
4
and
Mo
2
Cl
4
(PEt
3
)
4
underwent
tertiary phosphine scrambling at 25 °C and in neat
[
2
H
5
]pyridine
Mo
2
Cl
4
(PMe
3
)
4
revealed the
formation of an equilibrium concentration of
Mo
2
Cl
4
(PMe
3
)
3
(py)
(py = pyridine) and free PMe
3
. Under similar
conditions Mo
2
Cl
4
(PEt
3
)
4
yielded an equilibrium mixture of
Mo
2
Cl
4
(PEt
3
)
3
(py) and
Mo
2
Cl
4
(PEt
3
)
2
(py)
2
and free PEt
3
. From kinetics the
ΔH
‡
values are positive in the range
+24 to +34 kcal mol
-1
and the
ΔS
‡
values range from +12 to +28 cal
K
-1
mol
-1
. Collectively the data
reported are consistent with an interchange dissociative mechanism,
I
d
, wherein M–P bond breaking contributes
significantly to the rate-determining step with related values of
ΔH
‡
being larger for
M = W than M = Mo. The rate dependence on
the entering ligand is clearly evident from temperature-dependent
studies and leads to varying ΔS
‡
values. The I
d
mechanism is proposed to involve
pre-equilibria between M
2
Cl
4
L
4
and the
entering L′ in an axial site followed by rate-determining
M–L displacement. In neat [
2
H
5
]pyridine
this may be viewed as a solvent-assisted displacement. The present
results are discussed in terms of earlier studies from which researchers
inferred a simple dissociative process, D, involving
M–PR
3
bond rupture as the first and rate-determining
step.
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![[quadruple bond, length as m-dash]](https://www.rsc.org/images/entities/h2_char_e003.gif)
>M quadruply bonded complexes
M2Cl4L4, where
L = PMe3, PEt3,
PBun3 or PMe2Ph