Chemistry Reference
In-Depth Information
Table 6.35
Simple aryloxides of osmium
Bond length ( A)
Bond angle (
Ž
)
Compound
Aryloxide
M-O
M-O-Ar
Ref.
[Os(OAr)(H)(CO)
PPr
i
3
2
]
chelated aryloxide, H
trans
to bound Cl,
trans
P
OC
6
Cl
5
2.124 (3)
123
i
[Os(porphyrin)
OAr
2
]
OC
6
H
5
1.938 (2)
128
ii
i
M.A. Tena, O. Nurnberg, and H. Werner,
Chem. Ber.
,
126
, 1597 (1993).
ii
Ming-Chu Cheng, Chih-Chieh Wang, and Yu Wang,
Inorg. Chem.
,
31
, 5220 (1992).
tantalum species [Ta(OAr)
2
Cl
2
(L)
2
](LD various P and N donor ligands) are produced
by sodium amalgam reduction of the trichlorides.
400
Two general structural motifs are
found for the isolated hydrides of tantalum. The seven-coordinate compounds adopt
pentagonal bipyramidal geometry with
trans
, axial aryloxide ligands. Spectroscopic
studies show that the molecules are stereochemically rigid.
401
There is no evidence
that the colourless trihydride compounds exist as
2
-H
2
species. With very bulky
aryloxides or with three aryloxide ligands, six-coordinate hydrides are produced. There
is a dramatic bending of the two hydride ligands towards the donor phosphine ligand.
This can be accounted for by an increase in
-bonding between the Cl or OAr ligand
trans
to P upon bending the hydride ligands.
398
It can be seen that during the hydrogenolysis of alkyls containing 2,6-diphenylphen-
oxide ligands, the intramolecular hydrogenation of the
ortho
-phenyl rings takes place.
31
These hydride species of tantalum and in particular niobium will carry out the catalytic
hydrogenation of simple arenes as well as aryl-phosphines.
408
,
409
Polynuclear aromatic
hydrocarbons are hydrogenated faster than simple benzenes.
410
The catalysts exhibit a
high degree of regioselectivity and have been shown to carry out the
all
-
cis
hydro-
genation of many arenes.
411
The most active catalysts for arene hydrogenation are the
solutions generated by hydrogenolysis of either the isolated alkyls [Nb(OAr)
2
R
3
]or
the mixture [Nb(OAr)
2
Cl
3
/3
n
-BuLi]. This reactivity has also been developed into a
process for the bulk synthesis of cyclohexylphosphines from their aryl counterparts.
408
Studies have shown the reaction proceeds in a predominantly
all
-
cis
fashion with no
intermediate cyclohexadienyl or cyclohexenylphosphines being detected by
31
PNMR
spectroscopy.
The two-electron reduction of mixed chloro, aryloxides can lead to cyclometallated
derivatives
via
CH bond activation by low valent intermediates (Section 5.1). However,
reduction in the presence of unsaturated organic substrates can lead to interesting
compounds. The presence of the
-donating aryloxide ligands results in highly reducing
d
2
-metal fragments that can strongly bind and activate the organic substrate.
412 - 416
Reduction in the presence of alkynes can lead to mono-alkyne complexes,
tantalacyclocyclopentadiene rings or
6
-arene derivatives, hence mapping out the
overall cyclotrimerization process.
417
The alkyne complex [Ta(OC
6
H
3
Pr
i
2
-2,6)
3
(
2
-
PhC
CPh)]
418
has tantalacyclopropene character and reacts with ketones to produce
2-oxa-tantalacyclopent-4-ene rings. With aldehydes, coupling is followed by a hydride
transfer to a second aldehyde to produce an alkoxide species,
e.g.
[Ta(OC
6
H
3
Pr
i
2
-
2,6)
3
(OCH
2
Ph)(CPh
D
CPhCPh
D
O)] from benzaldehyde.
419
Addition of nitriles to
the alkyne complex produces adducts which thermolyse to produce metallacyclic
