Chemistry Reference
In-Depth Information
dimeric with chloro bridges.
398
Although the tribromide [Ta(OC
6
HPh
4
-2,3,5,6)
2
Br
3
]
is also square pyramidal, the iodide [Nb(OC
6
HPh
4
-2,3,5,6)
2
I
3
] is trigonal bipyramidal
with axial OAr. There is an extensive series of six-coordinate adducts with O,
399
N,
400
and P
398
donor ligands. These d
0
-adducts are distorted from strict octahedral and
this distortion has been analysed.
398
A more pronounced distortion is present in the
dihydrides [Ta(OAr)
2
Cl(H)
2
(PMePh
2
)] and [Ta(OAr)
3
(H)
2
(PMe
2
Ph)] (see below).
401
Treatment of the dimeric [TaCl
3
(THF)
2
]
2
(
-N
2
) with six equivalents of [LiOC
6
H
3
Pr
i
2
-
2,6] was found to lead to the compound [(THF)(ArO)
3
Ta(NN)Ta(OAr)
3
(THF)].
402
The
structure contains a hydrazido(4-) ligand with an N-N distance of 1.32 (1) A.
The alkylation of chloro, aryloxides of niobium and tantalum with Grignard or
lithium reagents leads to a wide range of mixed alkyl, aryloxides. Alkylation of the
metal - chloride bonds occurs first, but subsequent displacement of aryloxide groups can
occur with excess alkylating agent.
403
With very bulky aryloxide ligands, alkylation
can lead to alkylidene derivatives
via
an
˛
-hydrogen abstraction process. It is also
possible to photochemically generate corresponding alkylidene species. Mechanistic
studies show that irradiation into alkyl-to-metal charge transfer bands leads to transient
alkyl radicals that abstract the adjacent
˛
-hydrogen.
203
A series of tris-aryloxide neopentylidene compounds [Ta(DCHCMe
3
)(OAr)
3
(THF)]
(OAr D OC
6
H
3
Pr
i
2
-2,6, OC
6
H
3
Me
2
-2,6) have been obtained by treating
[Ta(
D
CHCMe
3
)Cl
3
(THF)
2
] with LiOAr.
404
They react with a variety of olefins to produce
isolable tantalacyclobutane derivatives,
e.g.
styrene produces [Ta
OC
6
H
3
Pr
i
2
-
2,6
3
fCH
Ph
CH
Bu
t
CH
2
g] whose structure shows the Bu
t
substituent at the
ˇ
-position.
An equilibrium between the tantalacyclobutane and alkylidene/THF adduct was observed
accounting for both formation of new alkylidenes (
e.g.
[Ta(
D
CHSiMe
3
)(OAr)
3
(THF)]
from CH
2
DCHSiMe
3
) and tantalacyclobutane isomerization. Addition of ethylene
generates the simplest tantalacyclobutane [Ta(OC
6
H
3
Pr
i
2
-2,6)
3
(CH
2
CH
2
CH
2
)] which
forms a pyridine adduct without fragmenting. The compound [Ta
OC
6
H
3
Pr
i
2
-
2,6
3
fCHBu
t
CH
C
5
H
8
CH
g] (structurally characterized) formed from norbornene acts as
a living polymerization catalyst for the formation of polynorbornene.
404
Kinetic studies
show that the rate determining step is ring opening of the metallacycle.
Treatment of the compound [(Me
3
SiCH
2
)
2
Ta(
-CSiMe
3
)
2
Ta(CH
2
SiMe
3
)
2
] with
bulky phenols leads to a series of substitution products in which the central 1,3-
dimetallacyclobutadiene core remains intact.
136
The relative rates of substitution of the
first alkyl group by a series of phenols have been obtained by competition reactions.
In the case of 2,6-diarylphenols the relative rate of substitution decreases as the bulk
of the
meta
-substituent increases.
405
This is argued to be a consequence of a decrease
in the conformational flexibility of the
ortho
-phenyl rings hence leading to a bulkier
ligand.
Mixed hydrido, aryloxides of niobium and tantalum are important reagents. The
addition of bulky phenols to [Cp
Ł
Ta(PMe
3
)(H)
2
(
2
-CH
2
PMe
2
)] has been shown to
lead to [Cp
Ł
Ta(H)
2
(OAr)
2
] derivatives.
153
Treatment of [Ta(OAr)(R)(Cl)(
6
-C
6
Me
6
)]
precursors with [LiBEt
3
H] leads to [Ta(OAr)(R)(H)(
6
-C
6
Me
6
)] species.
406
The
hydrogenolysis of mixed alkyl, aryloxides of tantalum in the presence of phosphine
donor ligands can produce the corresponding hydrides (Scheme 6.8).
401
It is also
possible to generate hydride compounds by addition of [Bu
3
SnH] to chloro, aryloxides
of tantalum (Scheme 6.8). In the case of niobium this reaction does not lead to stable
hydrides but to the d
1
-species
all
-
trans
-[Nb(OAr)
2
Cl
2
(PR
3
)
2
].
407
The corresponding
