Intramolecular activation of aromatic C-H bonds at tantalum(v) metal centers: Evaluating cyclometallation 'resistant' and 'immune' aryloxide ligation

Article abstract of DOI:10.1039/a702325a

The trichloride compounds [Ta(OC₆HPh₂-2,6-R₂-3,5)Cl₃] (1: R = H a, Ph b, Me c, Prⁱ d or Bu^t e) have been obtained by treating [Ta₂Cl₁₀] with the corresponding 3,5-disubstituted-2,6-diphenylphenols Ia-Ie. The solid-state structures of 1c and 1d show a square-pyramidal structure with an axial aryloxide ligand. The reaction of 1 with LiCH₂SiMe₃ (3 equivalents) led to the isolation of the tris(alkyls) [Ta(OC₆HPh₂-2,6-R₂-3,5)₂(CH ₂SiMe₃)₃] (4a-4d) except in the case of the 3,5-di-tert-butyl derivative 1e which generated the alkylidene compound [Ta(OC₆H₃Ph₂-2,6-Bu^t-3,5) ₂(=CHSiMe₃)(CH₂SiMe₃)] 6e. The alkylidenes 6a-6d can be produced by photolysis of the corresponding tris(alkyls) 4a-4d. The alkylidenes 6a-6d undergo intramolecular cyclometallation of the aryloxide ligand (addition of an aromatic C-H bond to the tantalum alkylidene) at a rate which is extremely dependent on the meta substituents on the phenoxide nucleus. Kinetic studies show that conversion of 6a-6d into mono-metallated 7a-7d is first order with the phenyl, methyl and isopropyl substituents slowing the ring closure down by factors of 20, 90 and 360 respectively. The tert-butyl substituent completely shuts down cyclometallation of the adjacent phenyl ring. It is argued that bulky substituents inhibit rotation of the ortho-phenyl ring into a conformation necessary for C-H bond activation. Structural analysis of the torsion angles between ortho-phenyl and phenoxy rings has been carried out. The use of ¹H NMR chemical shifts has been demonstrated to be a valuable tool to probe the average conformations adopted in solution.

Full text of DOI:10.1039/a702325a

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Intramolecular activation of aromatic C᎐H bonds at tantalum(V) metal
centers: evaluating cyclometallation ‘resistant’ and ‘immune’ aryloxide
ligation†
Jonathan S. Vilardo, Mark A. Lockwood, Linda G. Hanson, Janet R. Clark,
Bernardeta C. Parkin, Phillip E. Fanwick and Ian P. Rothwell*
Department of Chemistry, 1393 Brown Building, Purdue University, West Lafayette, IN 47907-1393,
USA
The trichloride compounds [Ta(OC6HPh2-2,6-R2-3,5)Cl3] (1: R = H a, Ph b, Me c, Pri d or But e) have been
obtained by treating [Ta2Cl10] with the corresponding 3,5-disubstituted-2,6-diphenylphenols Ia–Ie. The solid-state
structures of 1c and 1d show a square-pyramidal structure with an axial aryloxide ligand. The reaction of 1 with
LiCH2SiMe3 (3 equivalents) led to the isolation of the tris(alkyls) [Ta(OC6HPh2-2,6-R2-3,5)2(CH2SiMe3)3] (4a–4d)
except in the case of the 3,5-di-tert-butyl derivative 1e which generated the alkylidene compound [Ta(OC6H3Ph2-
2,6-But-3,5) (᎐CHSiMe )(CH SiMe )] 6e. The alkylidenes 6a–6d can be produced by photolysis of the corre-
᎐
2
3
2
3
sponding tris(alkyls) 4a–4d. The alkylidenes 6a–6d undergo intramolecular cyclometallation of the aryloxide
ligand (addition of an aromatic C᎐H bond to the tantalum alkylidene) at a rate which is extremely dependent on
the meta substituents on the phenoxide nucleus. Kinetic studies show that conversion of 6a–6d into mono-
metallated 7a–7d is first order with the phenyl, methyl and isopropyl substituents slowing the ring closure down
by factors of 20, 90 and 360 respectively. The tert-butyl substituent completely shuts down cyclometallation of
the adjacent phenyl ring. It is argued that bulky substituents inhibit rotation of the ortho-phenyl ring into a
conformation necessary for C᎐H bond activation. Structural analysis of the torsion angles between ortho-phenyl
1
and phenoxy rings has been carried out. The use of H NMR chemical shifts has been demonstrated to be a
valuable tool to probe the average conformations adopted in solution.
The use of sterically demanding ligation has made impacts in
many areas of inorganic and organometallic chemistry.1
A
recurring problem with the design of bulky ligands to stabilize
highly reactive species is the sometimes facile, intramolecular
activation of C᎐H bonds within the ligand itself.2 In the case of
aryloxide ligation, cyclometallation reactions have led not only
to the isolation of a variety of oxa–metallacycle rings3,4 but
also in some cases to the overall dehydrogenation of substituent
alkyl groups.5 Having spent considerable effort to understand
the mechanistic pathways whereby cyclometallation of aryl-
oxide ligands occurred, we set out to design a new generation of
more metallation resistant, bulky aryloxide ligands.
The cyclometallation of 2,6-diphenylphenoxide ligands (I)
occurs readily at electrophilic main-group-metal centers6 as
well as with a variety of high- and low-valent d-block metal
systems.7 Recognizing that rotation of the ortho-phenyl ring
almost coplanar with the central phenoxide core is necessary
for C᎐H bond activation led to the simple notion that the intro-
duction of meta substituents should impede the cyclometal-
lation.8 This paper details experiments in which intramolecular
activation of aromatic C᎐H bonds of 2,6-diphenylphenoxides
(meta-Ph Ib, Me Ic, Pri Id or But Ie) by tantalum alkylidene
functional groups is indeed controlled by the nature of
meta substituents. These experiments show that the ligand
2,6-diphenyl-3,5-di-tert-butylphenoxide (first synthesized by
Barton and co-workers)9 is inert towards cyclometallation in
this system and leads to a thermally stable tantalum alkylidene
derivative.10,11
Scheme 1 R = H a, Ph b, Me c, Pri d or But e
Results and Discussion
phenols (HOC6HPh2-2,6-R2-3,5; R = H Ia, Ph Ib, Me Ic, Pri
Id or But Ie) in hydrocarbon solvents leads to formation
of the bright yellow bis(aryloxides) 1 in good yield (Scheme 1).
The solid-state structures of 1c and 1e show a square-pyramidal
geometry for the mononuclear compounds with an axial
aryloxide ligand (Table 1, Figs. 1 and 2). These molecules are
Synthesis and characterization of compounds
Thereaction of [Ta2Cl10]with the3,5-disubstituted-2,6-diphenyl-
† Dedicated to the Memory of Professor Sir Geoffrey Wilkinson.
J. Chem. Soc., Dalton Trans., 1997, Pages 3353–3362
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Table 1 Selected bond distances (Å) and angles (Њ) for [Ta(OC6HPh2-
2,6-R2-3,5)2Cl3] (R = Me 1c or But 1e)
Table 2 Selected bond distances (Å) and angles (Њ) for [Ta(OC6HPh2-
2,6-Me2-3,5)2(CH2SiMe3)3] 4c
1c
1e
Molecule 1
Molecule 2
Ta᎐O(1)
Ta᎐O(2)
Ta᎐Cl(1)
Ta᎐Cl(2)
Ta᎐Cl(3)
1.803(3)
1.862(3)
2.358(1)
2.308(1)
2.347(1)
1.810(2)
1.883(2)
2.3348(9)
2.3092(9)
2.3430(9)
Ta(1)᎐O(10)
Ta(1)᎐O(12)
Ta(1)᎐C(150)
Ta(1)᎐C(160)
Ta(1)᎐C(170)
1.89(1)
1.90(1)
2.10(2)
2.16(2)
2.13(1)
Ta(2)᎐O(20)
Ta(2)᎐O(22)
Ta(2)᎐C(250)
Ta(2)᎐C(260)
Ta(2)᎐C(270)
1.93(1)
1.91(1)
2.17(2)
2.16(1)
2.10(2)
Cl(1)᎐Ta᎐Cl(2)
Cl(1)᎐Ta᎐Cl(3)
Cl(1)᎐Ta᎐O(1)
Cl(1)᎐Ta᎐O(2)
Cl(2)᎐Ta᎐Cl(3)
Cl(2)᎐Ta᎐O(1)
Cl(2)᎐Ta᎐O(2)
Cl(3)᎐Ta᎐O(1)
Cl(3)᎐Ta᎐O(2)
O(1)᎐Ta᎐O(2)
Ta᎐O(1)᎐C(11)
Ta᎐O(2)᎐C(21)
158.09(5)
85.25(5)
100.66(9)
85.08(10)
86.53(5)
100.99(9)
92.1(1)
103.31(9)
150.37(9)
106.0(1)
168.4(3)
151.1(3)
153.63(4)
85.08(4)
102.40(7)
84.31(7)
87.06(4)
103.86(7)
90.98(7)
102.15(7)
151.81(7)
105.61(9)
178.3(2)
140.9(2)
O(10)᎐Ta(1)᎐O(12)
O(10)᎐Ta(1)᎐C(150)
O(10)᎐Ta(1)᎐C(160)
O(10)᎐Ta(1)᎐C(170)
O(12)᎐Ta(1)᎐C(150)
O(12)᎐Ta(1)᎐C(160)
O(12)᎐Ta(1)᎐C(170)
C(150)᎐Ta(1)᎐C(160) 125.7(6)
C(150)᎐Ta(1)᎐C(170) 120.1(6)
C(160)᎐Ta(1)᎐C(170) 114.2(6)
174.8(5)
93.7(5)
88.3(5)
91.2(5)
88.0(5)
86.7(5)
92.3(5)
O(20)᎐Ta(2)᎐O(22)
O(20)᎐Ta(2)᎐C(250)
O(20)᎐Ta(2)᎐C(260)
O(20)᎐Ta(2)᎐C(270)
O(22)᎐Ta(2)᎐C(250)
O(22)᎐Ta(2)᎐C(260)
O(22)᎐Ta(2)᎐C(270)
C(250)᎐Ta(2)᎐C(260) 120.1(6)
C(250)᎐Ta(2)᎐C(270) 118.8(6)
C(260)᎐Ta(2)᎐C(270) 121.1(6)
174.0(4)
88.6(5)
92.3(5)
89.6(5)
85.4(5)
91.3(5)
92.6(6)
Ta(1)᎐O(10)᎐C(101)
Ta(1)᎐O(12)᎐C(121)
177.5(11)
165.4(11)
Ta(2)᎐O(20)᎐C(201)
Ta(2)᎐O(22)᎐C(221)
174.5(11)
164.3(12)
Ta(1)᎐C(150)᎐Si(15) 130.7(9)
Ta(1)᎐C(160)᎐Si(16) 139.9(9)
Ta(1)᎐C(170)᎐Si(17) 130.4(9)
Ta(2)᎐C(250)᎐Si(25) 132.6(9)
Ta(2)᎐C(260)᎐Si(26) 126.4(8)
Ta(2)᎐C(270)᎐Si(27) 127.7(8)
Table 3 Selected bond distances (Å) and angles (Њ) for [Ta(OC6HPh2-
2,6-Pri2-3,5)2(CH2SiMe3)3] 4d
Ta᎐O(10)
Ta᎐O(12)
Ta᎐C(150)
1.922(3)
1.908(3)
2.180(5)
Ta᎐C(160)
Ta᎐C(170)
2.130(5)
2.121(5)
O(10)᎐Ta᎐O(12)
O(10)᎐Ta᎐C(150)
O(10)᎐Ta᎐C(160)
O(10)᎐Ta᎐C(170)
O(12)᎐Ta᎐C(150)
O(12)᎐Ta᎐C(160)
O(12)᎐Ta᎐C(170)
168.9(1)
85.6(2)
93.0(2)
92.2(2)
84.7(2)
89.1(2)
96.3(2)
C(150)᎐Ta᎐C(170) 110.3(2)
C(160)᎐Ta᎐C(170) 119.1(2)
Ta᎐O(10)᎐C(101)
Ta᎐O(12)᎐C(121)
Ta᎐C(150)᎐Si(15)
Ta᎐C(160)᎐Si(16)
Ta᎐C(170)᎐Si(17)
151.7(3)
150.3(3)
142.5(3)
128.3(3)
132.4(3)
C(150)᎐Ta᎐C(160) 130.6(2)
Table 4 Selected bond distances (Å) and angles (Њ) for [Ta(OC6H3Ph2-
2,6)3(CH2SiMe3)2] 5a
Fig.
1
Molecular structure of [Ta(OC6HPh2-2,6-Me2-3,5)2Cl3] 1c
Ta᎐O(3)
Ta᎐O(5)
Ta᎐C(21)
1.930(3)
1.912(3)
2.122(5)
Ta᎐O(4)
Ta᎐C(11)
1.918(3)
2.090(6)
showing the atomic numbering scheme
O(3)᎐Ta᎐O(4)
O(3)᎐Ta᎐C(11)
O(4)᎐Ta᎐O(5)
O(4)᎐Ta᎐C(21)
O(5)᎐Ta᎐C(21)
Ta᎐O(4)᎐C(41)
Ta᎐C(11)᎐Si(1)
87.7(1)
91.6(2)
89.2(1)
131.6(2)
91.5(1)
152.4(3)
139.2(3)
O(3)᎐Ta᎐O(5)
O(3)᎐Ta᎐C(21)
O(4)᎐Ta᎐C(11)
O(5)᎐Ta᎐C(11)
Ta᎐O(3)᎐C(31)
Ta᎐O(5)᎐C(51)
Ta᎐C(21)᎐Si(2)
174.3(1)
87.1(2)
117.6(2)
94.1(2)
152.8(3)
159.1(3)
128.5(3)
by treating 1a with only 2 equivalents of either ClMgCH2SiMe3
or LiCH2SiMe3. The meta-phenyl, -methyl and -isopropyl sub-
stituted trichlorides 1b–1d react similarly with LiCH2SiMe3 to
produce 4b–4d (Scheme 2). The solid-state structures of 4c, 4d
and 5a show trigonal-bipyramidal geometries with axial aryl-
oxide groups (Tables 2–4; Figs. 3–5).
Fig.
2
Molecular structure of [Ta(OC6HPh2-2,6-But2-3,5)2Cl3] 1e
1
showing the atomic numbering scheme
The H NMR spectra of the compounds 4a–4d are highly
informative. In 4a the methylene protons Ta᎐CH2SiMe3 appear
as a sharp singlet at δ Ϫ0.05. As the meta substituent is changed
this signal moves progressively upfield and broadens, appearing
in the ambient spectrum of the meta-isopropyl derivative as a
hump above the baseline at δ Ϫ0.59 (Table 5). An important
characteristic of terminal, non-metallated ortho-aryl phenoxide
ligands is the diamagnetic shielding of protons of ligands that
are in close proximity to the central co-ordination sphere.15
In the solid-state structures of 4c and 4d (Figs. 3 and 4) the
orientation of the two axial aryloxide ligands is such that
isostructural with previously reported [Ta(OC6H3But2-2,6)2-
Cl3]12 and [Nb(OC6HPh4-2,3,5,6)2Cl3]13 and contrast with the
chloro-bridged, dimeric [M(OC6H3Pri2-2,6)2Cl3]2 (M = Nb or
Ta).14 The tris(2,6-diphenylphenoxide) [Ta(OC6H3Ph2-2,6)3Cl2]
2a has also been previously reported by reaction of 1a with 1
equivalent of LiOC6H3Ph2-2,6 (Scheme 1).15 The reaction of
trichloride 1a and dichloride 2a with LiCH2SiMe3 leads to the
formation of the corresponding alkyls 4a and 5a (Scheme 2).
The intermediate monochloride 3a was isolated in good yield
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J. Chem. Soc., Dalton Trans., 1997, Pages 3353–3362

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Table 5 Selected NMR spectroscopic data (C6D6)
4
δ(TaCH2SiMe3)
6
I
Compound
δ(OH)
25 ЊC
Ϫ80 ЊC
δ(Ta᎐CHSiMe3) δ(TaCH2SiMe3) δ(Ta᎐CHSiMe3) 1J(13C᎐1H)/Hz
᎐ ᎐
a
b
c
d
e
5.39
5.24
4.72
4.58
4.33
Ϫ0.15
Ϫ0.32
Ϫ0.49
Ϫ0.59
—
0.96, Ϫ0.59
0.77, Ϫ0.82
0.69, Ϫ1.02
0.60, Ϫ1.20
—
8.56
7.85
*
*
6.55
Ϫ0.05
Ϫ0.49
Ϫ0.83
Ϫ0.87
Ϫ1.19
237.0
116.4
113.5
113.6
111.1
113.1
234.4
229.2
227.8
227.3
* Obscured by aromatic signals.
Fig. 3 Molecular structure of [Ta(OC6HPh2-2,6-Me2-3,5)2(CH2Si-
Me3)3] 4c showing the atomic numbering scheme
Scheme 2 (i) 2 LiCH2SiMe3; (ii) LiCH2SiMe3; (iii) 3 LiCH2SiMe3
the alkyl ligands occupy two distinct environments. Two of the
CH2SiMe3 ligands are directly below the aryloxide phenyl rings
while the third alkyl in the trigonal plane lies away from the
aryloxide ligands. Upon cooling to sub-ambient temperatures
the spectra of 4a–4d show collapse and separation of the
Ta᎐CH2 signal into two resonances in the ratio 1:2, with the
larger resonance being upfield (Table 5). The position of
the low field resonance is typical of Ta᎐CH2SiMe3 groups in the
absence of any unusual ligand effects. The dramatically upfield
shifted resonance can be ascribed to the two alkyl groups below
the aryloxide phenyl rings. The chemical shift difference
between these two signals attests to the diamagnetic shielding
caused by these aryloxide ligands. Based upon these data we
conclude that restricted rotation of the aryloxide ligands takes
place on the NMR time-scale. The coalescence temperature for
the exchange is raised as the bulk of the meta substituent
increases. This can be rationalized by arguing that substitution
at the meta position decreases the conformational flexibility of
the ligand and leads to an increase in effective steric bulk (see
below).
Fig.
4
Molecular structure of [Ta(OC6HPh2-2,6-Pri2-3,5)2(CH2Si-
Me3)3] 4d showing the atomic numbering scheme
Treatment
of
meta-tert-butyl-substituted
1e
with
LiCH2SiMe3 (3 equivalents) led to the formation of the
alkylidene complex 6e along with 1 equivalent of SiMe4
(Scheme 3). This reactivity is identical to that documented for
the corresponding 2,6-di-tert-butylphenoxide complex.11 It
has been well documented that bulky ancillary ligation can
expedite α-hydrogen abstraction processes.10 This result, there-
fore, indicates that the tert-butyl substituent has dramatically
increased the steric bulk of the ligand over its isopropyl
counterpart. The analogous alkylidene compounds 6a–6d
can, however, be generated by photolysis of tris(alkyls) 4a–4d
in hydrocarbon solution (Scheme 3). Monitoring the reaction
1
in C6D6 solution by H NMR spectroscopy shows the photo-
reaction is highly efficient at producing the alkylidene and
J. Chem. Soc., Dalton Trans., 1997, Pages 3353–3362
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Scheme 4
Fig. 5 Molecular structure of [Ta(OC6H3Ph2-2,6)3(CH2SiMe3)2] 5a
showing the atomic numbering scheme
within the d0-alkyl precursor to generate an incipient alkyl
radical.11
A surprising observation is that photolysis of the tris(aryl-
oxide) 5a leads to the alkylidene 6a with elimination (1H NMR
spectroscopy) of 1 equivalent of 2,6-diphenylphenol (Scheme
3). The mixture of 6a and 2,6-diphenylphenol does not ther-
mally convert back to the tris(aryloxide) 5a. This represents a
rare example of the formation of an early d-block metal alkyl-
idene by α-hydrogenation abstraction by a heteroatom-bound
leaving group. This result implies that photolysis of 5a leads to
selective breaking of a tantalum–phenoxide bond (presumably
the equatorial ligand, Fig. 5) over a tantalum–alkyl bond lead-
ing either to an incipient or solvent trapped 2,6-diphenyl-
phenoxide radical which abstracts the α-hydrogen atom leading
to 6a.
The alkylidene compounds 6a–6e exhibit dramatically differ-
ent thermal stabilities in benzene and toluene solution. The
simple 2,6-diphenylphenoxide 6a converts over hours in C6D6
or C6D5CD3 solution at ambient temperatures to produce the
cyclometallation product 7a. Analogous monocyclometallated
compounds 7b–7d can also be formed from the meta-phenyl,
-methyl and -isopropyl derivatives 6b–6d (Scheme 4). In this
case, however, ring closure occurs very slowly at ambient tem-
peratures but heating 6b–6d in C6D6 or C6D5CD3 solution
shows the clean formation of 7b–7d at progressively slower
rates as the bulk of the meta substituent increases (see mech-
anistic study below). In contrast, tert-butyl-substituted 6e fails
to form any cyclometallated derivative upon extended therm-
olysis. Solutions of 6e in C6D5CD3 show little change (1H
NMR) upon heating at 105 ЊC for weeks. This is a remarkable
thermal stability for an early-transition-metal alkylidene
compound.
Monocyclometallated compounds can also be generated by
thermolysis of the alkyls 4a–4d and 5a. The tris(alkyls) 4a–4d
produce the metallated compounds 7a–7d that were also
obtained via photogenerated 6a–6d. These reactions require
high temperatures and were found to also produce secondary
thermal products, which were not isolated, but are presumably
bis-cyclometallated derivatives as previously reported for
related compounds.3,7a In contrast, bis(alkyl) 5a generated the
complex 8a and 1 equivalent of SiMe4. Hence, different mono-
cyclometallated complexes are generated via photochemical or
thermal pathways for 5a (Scheme 5).
Scheme 3
1
equivalent of SiMe4. Previous studies by our group
have shown that the generation of the tantalum alkylidene
function by photochemical α-hydrogen abstraction occurs
by excitation into a ligand-to-metal charge-transfer band
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Table 6 Values of dihedral angle (τ) for selected 2,6-diphenyl-
phenoxides containing 3,5-substituents (R)
R
Compound
τ
Ref.
Ph
1b HOC6HPh4-2,3,5,6
69, 75
*
[Nb(OC6HPh4-2,3,5,6)2Cl3]
[W(OC6HPh4-2,3,5,6)2(NPh)2]
60, 61
60, 58, 56, 55
13
8(a)
Me
Ic HOC6HPh2-2,6-Me2-3,5
1c [Ta(OC6HPh2-2,6-Me2-3,5)2Cl3]
4c [Ta(OC6HPh2-2,6-Me2-3,5)2-
(CH2SiMe3)3]
67, 89
61, 72, 74, 88
66, 71, 79, 80
*
*
*
*
Pri
4d [Ta(OC6HPh2-2,6-Pri2-3,5)2-
(CH2SiMe3)3]
70, 78, 82, 83
*
*
But
Ie HOC6HPh2-2,6-But2-3,5
87, 89
76, 81, 81, 84
1e [Ta(OC6HPh2-2,6-But2-3,5)2Cl3]
* This work.
Fig. 6 Molecular structures of HOC6HPh4-2,3,5,6 Ib, HOC6H3Ph2-
2,6-Me2-3,5 Ic and HOC6H3Ph2-2,6-But2-3,5 Ie
Scheme 5
greater than 75Њ. Introduction of meta substituents leads to
a distinct increase in the values of τ (Table 6) with the tert-
butyl substituent forcing the ortho-phenyl ring very close to
perpendicular to the phenoxide ring.
Structural analysis and spectroscopic probes of conformational
flexibility
The parent aryl alcohols 2-phenylphenol16 and 2,6-diphenyl-
phenol17 have previously been subjected to structural study.
Early interest in the ortho-arylphenols stemmed from the
spectroscopic detection of O᎐H π hydrogen bonding.18 We have
subjected the ligand precursors 2,3,5,6-tetraphenylphenol Ib,
2,6-diphenyl-3,5-dimethylphenol Ic and 2,6-diphenyl-3,5-di-
tert-butylphenol Ie to single crystal X-ray diffraction analysis
(Fig. 6). All three molecules are found to be monomeric in the
solid state with the hydroxyl proton bent towards one of the
ortho-phenyl rings. For the purposes of this study the most
important feature of these structures is not the parameters for
the π interaction, but the ground-state conformation of the aryl
rings. It can be seen (Table 6) that the introduction of meta
substituents leads to larger dihedral angles (τ) between the
ortho-phenyl and phenoxy rings. There is now an extensive
amount of literature on metal derivatives of the 2,6-diphenyl-
phenoxide ligand. The data derived from the Cambridge Struc-
tural Database combined with the parameters obtained in this
study for 5a can be analyzed as shown in Fig. 7. It can be seen
that the value of τ for this ligand is clustered between 40–55Њ
with an average value of 49Њ. In only a few cases is the angle
Analysis of the NMR spectroscopic data for the organo-
metallic compounds obtained in this study shows a correlation
between the chemical shift of alkyl and alkylidene α-protons
and the nature of the meta substituents. The size of these shifts
(Table 5) cannot be accounted for by electronic effects transmit-
ted through the phenoxide backbone. The upfield shifting of
alkylidene α-CH resonances has been shown to take place when
an agostic interaction is present.19 The values of the 1J(13C᎐1H)
coupling constant show little evidence for such an interaction
(Table 5).20 Instead we ascribe the progressive upfield shifting
of these resonances to the average conformation adopted by
the ortho-phenyl rings in solution. As the bulk of the meta
substituent increases it forces the phenyl rings perpendicular
and restricts their conformational flexibility. This leads to
greater diamagnetic shielding of protons in close proximity to
the metal and provides a spectroscopic tool for judging the
steric impact of the substituents. Furthermore, the conform-
ational rigidity of the 3,5-di-tert-butylphenoxide ligand results
in it functioning as a more bulky ligand than the other
analogues.
J. Chem. Soc., Dalton Trans., 1997, Pages 3353–3362
3357

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Fig. 9 First-order plots of the disappearance of photogenerated 6a
and 6c at 58 ЊC in C6D6 solvent
Fig. 7 Distribution of torsion angles (τ) for structurally characterized
2,6-diphenylphenoxide derivatives of transition metals. Data obtained
from the Cambridge Crystallographic Database
Fig. 10 First-order plots of the disappearance of photogenerated 6c
and 6d at 97 ЊC in C6D5CD3 solvent
Fig. 8 First-order plots of the disappearance of photogenerated 6a
and 6b at 20 ЊC in C6D6 solvent
Scheme 6
Fig. 11 Plots of ln(k/T) vs. 1/T for the disappearance of photo-
generated 6a and 6c (cal = 4.184 J, e.u. = 4.184 J KϪ1 molϪ1
)
Mechanistic considerations
The conversion of the tantalum alkylidene compounds 6a–6d
into the monocyclometallated derivatives 7a–7d represents the
intramolecular addition of an aromatic C᎐H bond across the
tantalum᎐alkylidene double bond.21 The intramolecular acti-
vation of aliphatic C᎐H bonds by tantalum alkylidenes and by
other metal alkylidene and alkylidyne bonds is well prece-
dented.22 More recently the intermolecular activation of arene
C᎐H bonds by early d-block metal alkylidenes has been
reported.23 There are also similarities with the activation of
C᎐H bonds by early d-block metal imido functional groups.24
Based upon previous mechanistic work, it is reasonable to pro-
pose a transition state for the reaction involving a four-center,
four-electron transition state in which the hydrogen of the arene
C᎐H bond is transferred to the alkylidene α-carbon simul-
taneous with the formation of the new Ta᎐C (aryl) bond
(Scheme 6). The transition state requires the ortho-phenyl ring
to rotate almost coplanar with the central phenoxy ring in order
for the C᎐H bond to be brought into close proximity to the
tantalum alkylidene group. Kinetic studies of the reactions in
C6D6 or C6D5CD3 solution show clean first-order decay of 6a–
1
6d when monitored by H NMR spectroscopy. These studies
show the meta-phenyl substituent inhibits ring closure by a fac-
tor of 22 at 20 ЊC (Fig. 8) while the meta-methyl compound
cyclometallates 90 times slower than the parent molecule 6a at
58 ЊC (Fig. 9). At 97 ЊC the isopropyl compound reacts four
times slower than the methyl derivative (Fig. 10). These data
(with the caveat that they were obtained at different tem-
peratures, see below) leads us to specify that meta-phenyl,
-methyl and -isopropyl substituents retard cyclometallation
of adjacent phenyl rings in 6 by a factor of 20, 90 and 360
respectively in this temperature regime.
Activation parameters obtained for the cyclometallation of
the unsubstituted and meta-methyl compounds (6a and 6c,
Fig. 11, Table 7) show both reactions have a moderately large
negative entropy of activation. The intramolecular activation
of C᎐H bonds by σ-bond metathesis25,26 or by addition to
3358
J. Chem. Soc., Dalton Trans., 1997, Pages 3353–3362

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2,6-Diphenyl-3,5-dimethylphenol Ic
Table 7 First-order rate constants measured for the conversion of 6 to
7
A 250 cm3 round bottom flask was charged with 2,6-diphenyl-
3,5-dimethylcyclohex-2-enone (4.3 g, 39 mmol) and 10% pal-
ladium on activated carbon (0.5 g). The reaction mixture was
heated until the evolution of hydrogen gas ceased. The cooled
solid was extracted with methylene chloride, filtered and the
solvent removed under reduced pressure to yield 3.6 g of 2,6-
diphenyl-3,5-dimethylphenol (13 mmol, 87%) as a white solid.
High resolution mass spectrum: Found: 274.1359. Calc. for
C20H18O: 274.1358. 1H NMR (CDCl3 30 ЊC): δH 7.2–7.7 (m, 10
H), 6.88 (s, 1 H), 4.78 (s, 1 H), 2.16 (s, 6 H). 13C NMR (CDCl3,
30 ЊC): δ 149.9, 136.2, 130.2, 128.8, 128.3, 127.5, 125.8, 123.3,
20.3. M.p. = 197–199 ЊC.
Reaction
T/ЊC
104 k/sϪ1
6a → 4a
6a → 7a
6a → 7a
6a → 7a
6a → 7a
6b → 7b
6c → 7c
6c → 7c
6c → 7c
6c → 7c
6c → 7c
6d → 7d
20
40
45
50
58
20
58
76
85
97
103
97
2.0
13
20
25
36
0.083
0.40
3.0
6.1
9.2
21
2.2
2,6-Diphenyl-3,5-diisopropylcyclohex-2-enone
To a 250 cm3 round bottom flask containing sodium (1.0 g,
43 mmol) dissolved in methanol (50 cm3) was added 1,3-
diphenylacetone (5.0 g, 36 mmol) and 2,6-dimethyl-4-hepten-3-
one (8.3 g, 40 mmol). The solution was stirred at room temp-
erature for 1 h, a further 4.5 g of sodium (200 mmol) in
methanol (50 cm3) were added and the solution refluxed for
12 h. The solid that formed upon cooling in an ice bath was
collected by filtration, washed with ice-cold methanol and dried
in vacuo for 2 h to yield 6.5 g of 2,6-diphenyl-3,5-diisopropyl-
cyclohex-2-enone (56% yield) as a white solid. 1H NMR
(CDCl3, 30 ЊC): δ 7.0–7.4 (m, 10 H, aromatics), 3.58 (d, 1 H),
2.77 (spt, 1 H), 2.49 (m, 1 H), 2.31 (m, 2 H), 1.63 (spt of d, 1 H),
0.8–1.1 (overlapping doublets, 12 H). 13C NMR (CDCl3, 30 ЊC):
unsaturated functional groups24 typically display such negative
values of ∆S‡ (highly ordered transition state). The enthalpic
increase observed on introducing the meta-methyl substituent
accounts for the rate drop of approximately 90 at 58 ЊC. This
increase in ∆H ‡ can be ascribed to the extra energy needed to
rotate the phenyl ring approximately coplanar with the phen-
oxide ring due to clash of the ortho-proton with the methyl
substituent. It should, however, be pointed out that the non-
parallel nature of the lines in Fig. 11 means that the relative
rates of cyclometallation are temperature dependent. Further-
more the activation parameters imply an isokinetic temperature
of 800 K, i.e. above this temperature the meta-methyl com-
pound will cyclometallate faster than the unsubstituted
derivative.
δ 198.9 (C᎐O), 164.0, 139.0, 136.4, 136.1, 129.6, 128.8, 128.2,
᎐
127.8, 126.8, 126.4, 57.7, 45.8, 32.4, 27.9, 23.1, 20.8, 20.4, 20.2,
16.0. M.p. = 124–125 ЊC.
Experimental
2,6-Diphenyl-3,5-diisopropylphenol Id
All operations were carried out under a dry nitrogen atmos-
phere in a Vacuum Atmospheres Dri-Lab or by standard
Schlenk techniques. The hydrocarbon solvents were distilled
from sodium benzophenone and stored under nitrogen until
use. 2,6-Diphenylphenol Ia is commercially available (Aldrich),
2,3,5,6-tetraphenylphenol Ib,25 2,6-diphenyl-3,5-di-tert-butyl-
phenol Ie,9 2,6-dimethyl-4-hepten-3-one,26 [Ta(OC6H3Ph2-
2,6)2Cl3]15 1a and [Ta(OC6H3Ph2-2,6)3Cl2]15 2a were made by
previously reported procedures. Literature preparation of 2,6-
diphenyl-3,5-dimethylphenol has previously been reported in
a preliminary fashion,27 additional details are included here.
Photolysis experiments were carried out using an Ace Hanovia
An identical procedure to that used for Ic utilizing 2,6-diphenyl-
3,5-diisopropylcyclohex-2-enone yielded 2,6-diphenyl-3,5-di-
1
isopropylphenol as white crystals. H NMR (CDCl3, 30 ЊC):
δ 7.3–7.5 (m, 10 H, aromatics), 6.98 (s, 1 H, para-H), 4.58 (s, 1 H,
OH), 2.80 (spt, 2 H), 1.17 (d, 12 H). 13C NMR (CDCl3, 30 ЊC):
δ 149.2, 147.2, 136.4, 130.5, 128.7, 127.5, 124.6, 113.7, 30.1, 24.1.
[Ta(OC6HPh4-2,3,5,6)2Cl3] 1b
A two-neck, 250 cm3 flask equipped with a nitrogen adaptor
and reflux condenser was filled with [TaCl5] (9.0 g, 26.0 mmol)
suspended in benzene (100 cm3). While being stirred, 2,3,5,6-
tetraphenylphenol (21.8 g, 55.0 mmol) was slowly added por-
tionwise to the pale yellow solution under a nitrogen flush. The
orange-yellow solution was periodically placed under a vacuum
to remove HCl, then refluxed for 2 h, cooled and the solvent
removed in vacuo to give a yellow powder. The crude solid was
washed with a small portion of pentane and dried to yield 25.0
g of [Ta(OC6HPh4-2,3,5,6)2Cl3] (88% yield) (Found: C, 66.12;
H, 4.06; Cl, 10.03. Calc. for C60H42Cl3O2Ta: C, 66.59; H, 3.91;
Cl, 9.83%).
1
450 W Hg lamp cooled by a water condenser. The H and 13C
NMR spectra were recorded on a Varian Associates Gemini-
200 spectrometer and referenced to protio impurities of com-
mercial C6D6 or C6D5CD3. Variable-temperature 1H NMR
spectra were recorded on a broad band Gemini XL 200A spec-
trometer and referenced to the protio impurities in C6D5CD3.
The X-ray diffraction studies were carried out ‘in house’ at
Purdue University.
2,6-Diphenyl-3,5-dimethylcyclohex-2-enone
[Ta(OC6HPh2-2,6-Me2-3,5)2Cl3] 1c
A 100 cm3 round bottom flask was charged with pent-3-en-2-
one (65% pure, 5.0 g, 39 mmol), 1,3-diphenylpropan-2-one
(10.7 g, 51 mmol), dry methanol (75 cm3) and NaOMe (5.4 g,
0.11 mol). Large white crystals formed from the dark orange
solution upon standing overnight at room temperature. The
supernatant was decanted, the solid washed with a small por-
tion of ice-cold methanol and dried in vacuo to yield 2,6-
diphenyl-3,5-dimethylcyclohex-2-enone (4.3 g, 16 mmol, 41%)
as a white solid. High resolution mass spectrum: Found:
276.1516. Calc. for C20H20O: 276.1514. 1H NMR (CDCl3,
30 ЊC): δ 7.1–7.6 (m, 10 H, aromatics), 3.40 (d, 1 H), 2.5 (m, 3
H), 1.93 (s, 3 H), 1.01 (d, 3 H). 13C NMR (CDCl3, 30 ЊC):
δ 197.6, 155.2, 138.7, 137.3, 135.7, 128.95, 128.9, 128.0, 127.6,
126.7, 126.4, 60.8, 40.4, 35.0, 22.5, 20.5. M.p. = 129–130 ЊC.
A 250 cm3 two-neck flask was charged with [TaCl5] (2.77 g, 8.0
mmol) in benzene (100 cm3) under a nitrogen flush. Solid 2,6-
diphenyl-3,5-dimethylphenol (4.4 g, 16.0 mmol) was added in
portions over a 30 min period. Stirring was continued for 2 h at
room temperature and the solvent was then removed in vacuo.
The yellow solid was redissolved in a minimum amount of hot
toluene–benzene and allowed to stand overnight. The orange
crystals which formed were washed with a small amount of
hexane and dried in vacuo to yield 1.8 g (3.0 mmol, 38%) of
[Ta(OC6HPh2-2,6-Me2-3,5)Cl4] (Found: C, 40.74; H, 2.88; Cl,
22.77. Calc. for C20H17Cl4OTa: C, 40.30; H, 2.88; Cl, 23.78%).
The supernatant was layered with hexane to induce the form-
ation of yellow crystals of [Ta(OC6HPh2-2,6-Me2-3,5)2Cl3]
J. Chem. Soc., Dalton Trans., 1997, Pages 3353–3362
3359

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which were washed with a small portion of pentane and dried in
vacuo. Yield: 2.4 g (2.9 mmol, 36%) (Found: C, 59.40; H, 4.31;
Cl, 11.68. Calc. for C43H37Cl3OTa 1cؒ0.5C6H6: C, 59.16; H,
mmol). The reaction was stirred for 6 h, filtered to remove the
lithium chloride and dried in vacuo to give an off-white solid.
Recrystallization from toluene afforded colorless crystals of 4a
which were washed with hexane and dried in vacuo. Yield: 1.02
g (86%) (Found: C, 61.93; H, 6.35. Calc. for C48H59O2Si3Ta:
1
4.27; Cl, 12.18%). H NMR (C6D6, 30 ЊC): δ 7.0–7.3 (m, 20 H,
aromatics), 6.72 (s, 2 H, para-H), 1.93 (s, 12 H, Me). 13C NMR
(C6D6, 30 ЊC): δ 159.1 (TaOC), 136.5, 131.3, 130.7, 130.0, 129.1,
129.0, 128.1, 20.4 (Me).
1
C, 61.78; H, 6.37%). H NMR (C6D6, 30 ЊC): δ 6.78–7.58 (m,
26 H, aromatics), 0.04 (s, 27 H, CH2SiMe3), Ϫ0.05 (s, 6 H,
CH2SiMe3). 13C NMR (C6D6, 30 ЊC): δ 157.7 (TaOC), 122.3–
141.6 (aromatics), 72.2 (TaCH2SiMe3), 3.84 (SiMe3).
[Ta(OC6HPh2-2,6-Pri2-3,5)2Cl3] 1d
A 250 cm3 two-neck flask was charged with [TaCl5] (1.20 g, 3.5
mmol) in benzene (100 cm3) under a nitrogen flush. With stir-
ring, 3,5-diisopropyl-2,6-diphenylphenol (2.45 g, 7.1 mmol) was
slowly added portionwise to the pale yellow solution under a
nitrogen flush. The deep yellow-green solution was periodically
placed under vacuum to remove HCl, then refluxed for 2 h,
cooled and the solvent removed in vacuo to give a yellow
powder. The crude solid was washed with a small portion of
[Ta(OC6HPh4-2,3,5,6)2(CH2SiMe3)3] 4b
To a suspension of [Ta(OC6HPh4-2,3,5,6)2Cl3] 1b (5.0 g, 4.62
mmol) in benzene (20 cm3) was added LiCH2SiMe3 (1.35 g, 14.3
mmol). The reaction was stirred for 6 h, filtered to remove the
lithium chloride salts and dried in vacuo to give 4b as a light
yellow solid which was washed with hexane and dried in vacuo.
1H NMR (C6D6, 30 ЊC): δ 6.89–7.41 (m, 42 H, aromatics), 0.15
(s, 27 H, CH2SiMe3), Ϫ0.24 (br s, 6 H, CH2SiMe3). 13C NMR
(C6D6, 30 ЊC): δ 158.3 (TaOC), 143.3–125.8 (aromatics), 72.3
(br, TaCH2SiMe3), 3.90 (SiMe3).
pentane and dried in vacuo for
2 h to yield 1.8 g
of [Ta(OC6HPh2-2,6-Pri2-3,5)2Cl3] (55% yield). 1H NMR (C6D6,
30 ЊC): δ 7.0–7.5 (m, 22 H, aromatics), 2.85 (spt, 4 H), 1.10 (d,
24 H). 13C NMR (C6D6, 30 ЊC): δ 158.9 (TaOC), 147.9, 136.3,
131.0, 130.2, 129.3, 128.2, 127.8, 119.5, 30.3, 24.2.
[Ta(OC6HPh2-2,6-Me2-3,5)2(CH2SiMe3)3] 4c
A benzene solution (50 cm3) containing [Ta(OC6HMe2-3,5-Ph2-
2,6)2Cl3] 1c (1.0 g, 1.2 mmol) was treated with LiCH2SiMe3 (0.4
g, 4.2 mmol) and the resulting mixture was stirred at room
temperature for 2 h during which time a fine white precipitate
of LiCl formed. The solution was filtered and the solvent
removed in vacuo. The crude white solid was dissolved in a
minimum amount of pentane, cooled slowly to Ϫ20 ЊC and the
resulting white crystals of 4c dried in vacuo. Yield: 1.0 g (85%)
suitable for X-ray diffraction (Found: C, 62.78; H, 7.09. Calc.
for C52H67O2Si3Ta: C, 63.13; H, 6.83%). 1H NMR (C6D6,
30 ЊC): δ 7.00–7.35 (m, 20 H, aromatics), 6.74 (s, 2 H, para-H),
1.87 (s, 12 H, meta-Me), 0.10 (s, 27 H, CH2SiMe3), Ϫ0.43 (s,
6 H, CH2SiMe3). 13C NMR (C6D6, 30 ЊC): δ 158.1 (TaOC), 70.0
(TaCH2SiMe3), 4.0 (CH2SiMe3), 140.1, 137.2, 131.9, 131.6,
129.2, 127.3, 126.0.
[Ta(OC6HPh2-2,6-But2-3,5)2Cl3] 1e
A 250 cm3 two-neck flask was charged with [TaCl5] (2.40 g, 7.0
mmol) in benzene (50 cm3) under a nitrogen flush and 2,6-
diphenyl-3,5-di-tert-butylphenol (6.0 g, 17 mmol) were then
added portionwise over a 20 min period. After the addition was
complete, the solution was refluxed for 6 h. After removing the
solvent in vacuo, the crude solid was redissolved in a minimum
amount of hot hexane and slowly cooled to produce large
yellow crystals of [Ta(OC6HPh2-2,6-But2-3,5)2Cl3]. These crys-
tals were washed with a small amount of hexane and dried in
vacuo. Yield: 1.5 g (21%) (Found: C, 62.81; H, 6.15; Cl, 10.03.
1
Calc. for C52H58Cl3O2Ta: C, 63.43; H, 5.90; Cl, 10.21%). H
NMR (C6D6, 30 ЊC): δ 7.68 (s, 2 H, para-H), 7.16–7.25 (m, 20
H, aromatics), 1.17 (s, 36 H, But). 13C NMR (C6D6, 30 ЊC):
δ 162.5 (TaOC), 148.4, 138.6, 132.8, 131.3, 129.3, 128.2, 123.6,
38.1 (CMe3), 33.5 (CMe3).
[Ta(OC6HPh2-2,6-Pri2-3,5)2(CH2SiMe3)3] 4d
A benzene solution (50 cm3) containing [Ta(OC6HPh2-2,6-Pri2-
3,5)2Cl3] 1d (0.60 g, 0.60 mmol) was treated with LiCH2SiMe3
(0.25 g, 2.0 mmol). After 2 h the solution was filtered and the
solvent removed in vacuo. The crude white solid was dissolved
in toluene and layered with pentane to induce the formation of
colorless blocks of [Ta(OC6HPh2-2,6-Pri2-3,5)2(CH2SiMe3)3] 4d
(yield = 0.44 g, 70%) that were suitable for an X-ray diffraction
[Ta(OC6H3Ph2-2,6)2Cl(CH2SiMe3)2] 3a
Method (a). To a suspension of [Ta(OC6H3Ph2-2,6)2Cl3] 1a
(10 g, 12.85 mmol) in benzene (50 cm3) was added LiCH2SiMe3
(2.54 g, 26.98 mmol). The reaction was stirred for 30 min,
filtered to remove the lithium chloride salts and dried in vacuo
to give 3a an off-white solid which was washed with hexane
and dried in vacuo. Yield: 3.50 g (31%).
1
study. H NMR (C6D6, 30 ЊC): δ 7.0–7.5 (m, 22 H, aromatics),
2.64 (spt, 4 H), 1.14 (d, 24 H), 0.09 (s, 27 H), Ϫ0.50 (br s, 6 H,
TaCH2SiMe3). 13C NMR (C6D6, 30 ЊC): δ 157.7 (TaOC), 78.5
(TaCH2SiMe3), 4.1 (CH2SiMe3), 148.5, 139.7, 131.9, 130.4,
130.0, 128.9, 128.3, 116.1, 30.3, 24.4.
Method (b). To a suspension of [Ta(OC6H3Ph2-2,6)2Cl3] 1a
(34.72 g, 44.63 mmol) in benzene (100 cm3) was added slowly
dropwise at 0 ЊC a solution of ClMgCH2SiMe3 (147 mmol) in
diethyl ether (150 cm3). Upon complete addition, the mixture
was gradually warmed to room temperature and stirred for
16 h, filtered to remove the lithium chloride salts and dried in
vacuo to give an off-white solid. Recrystallization of the result-
ing crude solid from toluene afforded colorless crystals of 3a
which were washed with hexane and dried in vacuo. Yield:
35.5 g (91%) (Found: C, 60.00; H, 5.47; Cl, 4.19. Calc. for
C44H48ClO2Si2Ta: C, 59.96; H, 5.49; Cl, 4.02%). 1H NMR
(C6D6, 30 ЊC): δ 6.82–7.50 (m, 26 H, aromatics), 0.42 (s, 4 H,
CH2SiMe3), Ϫ0.04 (s, 18 H, SiMe3). 13C NMR (C6D6, 30 ЊC):
δ 157.4 (TaOC), 123.5–140.6 (aromatics), 83.9 (TaCH2SiMe3),
2.61 (SiMe3).
[Ta(OC6H3Ph2-2,6)3(CH2SiMe3)2] 5a
To a suspension of [Ta(OC6H3Ph2-2,6)3Cl2] 2a (1.12 g, 1.13
mmol) in benzene (10 cm3) was added dropwise a solution of
LiCH2SiMe3 (0.26 g, 2.76 mmol) in benzene (8 cm3). The reac-
tion was stirred for 18 h, filtered to remove the lithium chloride
solids and dried in vacuo to give an off-white solid. Recrystal-
lization from toluene afforded white crystals which were washed
1
with hexane and dried in vacuo. Yield 0.94 g (76%). H NMR
(C6D6, 30 ЊC): δ 6.35–7.50 (m, 39 H, aromatics), 0.82 (s, 4 H,
CH2SiMe3), Ϫ0.15(s, 18H, CH2SiMe3). 13C NMR (C6D6, 30 ЊC):
δ 157.8 (TaOC), 122.2–142.2 (aromatics), 82.8 (TaCH2SiMe3),
3.37 (SiMe3).
[Ta(OC6H3Ph2-2,6)2(CH2SiMe3)3] 4a
[Ta(OC H Ph -2,6) (᎐CHSiMe )(CH SiMe )] 6a
᎐
6
3
2
2
3
2
3
To a suspension of [Ta(OC6H3Ph2-2,6)2Cl3] 1a (0.99 g, 1.27
mmol) in benzene (10 cm3) was added LiCH2SiMe3 (0.37 g, 3.92
A saturated C6D6 solution of [Ta(OC6H3Ph2-2,6)2(CH2SiMe3)3]
3360
J. Chem. Soc., Dalton Trans., 1997, Pages 3353–3362

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4a in a 5 mm NMR tube was placed 5 cm away from a 450 W
Ace Hanovia medium-pressure Hg lamp housed in a water-
cooled quartz jacket. The tube was cooled in water while being
photolyzed for 45 min to produce a solution and the product
characterized using NMR spectroscopy. 1H NMR (C6D6,
[Ta(OC6HPh2-2,6-R2-3,5)(OC6HPh-2-R2-3,5-ç1-C6H4)-
(CH2SiMe3)2] 7 (R = Ph b, Me c or Pri d)
These compounds were obtained by a procedure identical to
that used for 7a.
30 ЊC): δ 8.56 (s, 1 H, Ta᎐CH), 6.86–7.45 (m, 26 H, aromatics),
᎐
[Ta(OC6HPh4-2,3,5,6)(OC6HPh3-ç1-C6H4)(CH2SiMe3)2] 7b.
1H NMR (C6D6, 30 ЊC): δ 8.18 (d, 1 H), 6.70–7.60 (m, 40 H,
0.07 (s, 9 H, CH2SiMe3), 0.06 (s, 9 H, CH2SiMe3), Ϫ0.05 (s, 2 H,
CH2SiMe3). 13C NMR (C D , 30 ЊC): δ 237.0 [Ta᎐CHSiMe ,
᎐
6
6
3
2
aromatics), 0.77 (d, 2 H), Ϫ0.58 [d, 2 H, J(1H᎐1H) = 11.9 Hz,
1J(13C᎐1H) = 116.4 Hz], 157.3 (TaOC), 52.6 (TaCH2SiMe3), 2.06
(SiMe3), 2.00 (SiMe3).
CH2SiMe3], Ϫ0.14 (s, 18 H, CH2SiMe3). 13C NMR (C6D6,
30 ЊC):
δ
207.2 (TaC ipso), 76.0 (TaCH2SiMe3), 2.1
(CH2SiMe3).
[Ta(OC HPh -2,6-R -3,5) (᎐CHSiMe )(CH SiMe )] 6 (R = Ph
᎐
6
2
2
2
3
2
3
b, Me c or Pri d)
[Ta(OC6HPh2-2,6-Me2-3,5)(OC6HPh-2-Me2-3,5-ç1-C6H4)-
(CH2SiMe3)2] 7c. H NMR (C6D6, 30 ЊC): δ 8.03 (d, 1 H), 7.67
(d, 2 H), 6.60–7.40 (m, 20 H, aromatics), 2.57 (s, 3 H), 1.90 (s,
6 H), 1.80 (s, 3 H, meta-Me), 0.77 (d, 2 H), 0.54 (d, 2 H), Ϫ0.81
[d, 2 H, 2J(1H᎐1H) = 11.9 Hz, CH2SiMe3], Ϫ0.18 (s, 18 H,
CH2SiMe3). 13C NMR (C6D6, 30 ЊC): δ 205.5 (TaC ipso), 75.0
(TaCH2SiMe3), 21.3, 20.8, 20.7 (meta-Me), 2.1 (CH2SiMe3).
1
These compounds were obtained by a procedure identical to
that used for 6a.
[Ta(OC HPh -2,3,5,6) (᎐CHSiMe )(CH SiMe )] 6b. 1H
᎐
6
4
2
3
2
3
NMR (C D , 30 ЊC): δ 7.85 (s, 1 H, Ta᎐CH), 6.91–7.36 (m,
᎐
6
6
42 H, aromatics), 0.12 (s, 9 H, CH2SiMe3), 0.07 (s, 9 H,
CH2SiMe3), Ϫ0.49 (s, 2 H, CH2SiMe3). 13C NMR (C6D6,
30 ЊC): δ 234.4 [Ta᎐CHSiMe , 1J(13C᎐1H) = 113.5 Hz], 158.7
[Ta(OC6HPh2-2,6-Pri2-3,5)(OC6HPh-2-Pri2-3,5-Ph-ç1-C6H4)-
(CH2SiMe3)2] 7d. H NMR (C6D6, 30 ЊC): δ 8.16 (d, 1 H), 7.76
᎐
3
1
(TaOC), 51.7 (TaCHSiMe3), 3.68 (SiMe3), 2.52 (SiMe3).
(d, 1 H), 6.80–7.50 (m, 20 H, aromatics), 3.85 (spt, 6 H), 2.86
1
[Ta(OC HPh -2,6-Me -3,5) (᎐CHSiMe )(CH SiMe )] 6c. H
(spt, 12 H), 2.64 (spt, 6 H, meta-Pri), 1.46 (d, 6 H), 1.10–1.25
᎐
6
2
2
2
3
2
3
NMR (C D , 30 ЊC): δ 6.90–7.45 (m, 21 H, Ta᎐CH plus aro-
matics), 6.66 (s, 2 H, para-H), 2.01 (s, 12 H, meta-Me), 0.10
2
(m, 18 H), 0.60 (d, 2 H), Ϫ0.88 [d, 2 H, J(1H᎐1H) = 12.0 Hz,
᎐
6
6
CH2SiMe3], Ϫ0.11 (s, 18 H, CH2SiMe3). 13C NMR (C6D6,
30 ЊC): δ 206.2 (TaC ipso), 74.4 (TaCH2SiMe3), 30.6, 25.9, 24.3
(meta-Pri), 2.2 (CH2SiMe3).
(s, 9 H, CH2SiMe3), 0.02 (s, 9 H, CH2SiMe3), Ϫ0.83 (s, 2 H,
CH2SiMe3). 13C NMR (C D , 30 ЊC): δ 229.2 [Ta᎐CHSiMe ,
᎐
6
6
3
1J(13C᎐1H) = 113.6 Hz], 158.1 (TaOC), 48.3 (TaCH2SiMe3), 22.5
(meta-Me), 3.36 (SiMe3), 2.63 (SiMe3).
[Ta(OC6H3Ph2-2,6)2(OC6H3Ph-ç1-C6H4)(CH2SiMe3)] 8a
1
[Ta(OC HPh -2,6-Pri -3,5) (᎐CHSiMe )(CH SiMe )] 6d. H
The formation of the monocyclometallated compound 8a
was accomplished by thermolysis of C6D6 solutions of
[Ta(OC6H3Ph2-2,6)3(CH2SiMe3)2] 5a in an oil bath at elevated
temperatures (e.g. 90 ЊC). 1H NMR (C6D6, 30 ЊC): δ 8.67
(d, 1 H, o-H on metallated phenyl ring), 8.07 (d, 1 H, m-H on
metallated phenyl ring), 7.77 (d, 1 H, m-H on phenoxy ring),
7.70–6.32 (m, 29 H, aromatics), 0.60 (d, 1 H, CH2SiMe3), Ϫ0.03
(d, 1 H, CH2SiMe3), Ϫ0.07 (s, 9 H, SiMe3). 13C NMR (C6D6,
30 ЊC): δ 201.4 (TaC ipso), 81.8 (TaCH2), 1.50 (SiMe3).
᎐
6
2
2
2
3
2
3
NMR (C6D6, 30 ЊC): δ 6.9–7.5 (m, 22 H, aromatics including
alkylidene proton), 3.04 (spt, 4 H), 1.1–1.3 (m, 24 H), 0.22 (s, 9
H, SiMe3), 0.03 (s, 9 H, SiMe3), Ϫ0.87 (br s, 2 H, TaCH2SiMe3).
13C NMR (C D , 30 ЊC): δ 227.8 [Ta᎐CHSiMe , J(13C᎐1H) =
1
᎐
6
6
3
111.1 Hz], 157.0 (TaOC), 47.6 (TaCH2SiMe3), 4.0 (SiMe3), 3.4
(SiMe3), 147.2, 138.1, 130.6, 129.0, 127.8, 127.3, 116.0, 30.6,
24.4.
[Ta(OC HPh -2,6-But -3,5) (᎐CHSiMe )(CH SiMe )] 6e
᎐
6
2
2
2
3
2
3
A benzene solution (50 cm3) containing [Ta(OCHPh2-2,6-But2-
3,5)2Cl3] (0.4 g, 0.4 mmol) was treated with 3 equivalents
of LiCH2SiMe3 (0.11 g, 1.2 mmol) and stirred at room temp-
erature for 2 h during which time a fine white precipitate of LiCl
formed. The solution was filtered to remove the LiCl and the
solvent removed in vacuo. The extremely soluble yellow product
Crystallography
Crystal data for Ib. C30H22O, M = 398.51, monoclinic, space
¯
group P1 (no. 2), a = 6.1569(8), b = 8.4102(5), c = 10.6947(13)
Å, α = 99.764(7), β = 92.094(11), γ = 91.153(7)Њ, T = 296 K,
Z = 1, U = 545.20(19) Å3, µ = 0.519 mmϪ1, number of reflec-
tions measured 2379, R = 0.057, RЈ = 0.143.
1
6e (3.0 g, 75% yield) was spectroscopically characterized. H
NMR (C6D6, 30 ЊC): δ 7.63 (s, 2 H, para-H), 6.8–7.4 (m, 20 H,
Crystal data for Ic. C20H18O, M = 274.37, monoclinic, space
group P21/n (no. 14), a = 14.6884(11), b = 11.8059(14), c =
18.661(2) Å, β = 104.321(7)Њ, T = 296 K, Z = 8, U = 3135.5(10)
Å3, µ = 0.065 mmϪ1, number of reflections measured 5869,
R = 0.048, RЈ = 0.122.
aromatics), 6.55 (s, 1 H, Ta᎐CH), 1.33 (s, 9 H, ᎐CHSiMe ),
᎐
᎐
3
1.28 (s, 36 H, But), 0.09 (s, 9 H, CH2SiMe3), Ϫ1.19 (s, 2 H,
CH2SiMe3). 13C NMR (C D , 30 ЊC): δ 227.3 [Ta᎐CHSiMe ,
᎐
6
6
3
1J(13C᎐1H) = 113.1 Hz], 159.9 (TaOC), 47.8 (TaCH2SiMe3), 37.8
(CMe), 33.5 (CMe3), 3.9 (SiMe3), 2.6 (SiMe3), 140.1, 134.0,
132.2, 130.2, 119.8, 119.6.
Crystal data for Ie. C20H30O, M = 358.53, monoclinic, space
group P21/c (no. 14), a = 5.9464(7), b = 18.893(3), c =
19.0368(15) Å, β = 97.113(8)Њ, T = 295 K, Z = 4, U = 2122.2(7)
Å3, µ = 0.472 mmϪ1, number of reflections measured 4449,
R = 0.059, RЈ = 0.145.
[Ta(OC6H3Ph2-2,6)(OC6H3Ph-ç1-C6H4)(CH2SiMe3)2] 7a
The formation of the monocyclometallated compound 7a
was accomplished by thermolysis of C6D6 solutions of
[Ta(OC6H3Ph2-2,6)2(CH2SiMe3)3] 4a in an oil bath at elevated
temperatures (e.g. 90 ЊC). The kinetics of conversion of
Crystal data for 1cؒC6H6. C46H40Cl3O2Ta, M = 912.14,
¯
triclinic, space group P1 (no. 2), a = 9.871(8), b = 11.618(7),
[Ta(OC H Ph -2,6) (᎐CHSiMe )(CH SiMe )] 6a into 7a was
᎐
6
3
2
2
3
2
3
1
c = 19.714(2) Å, α = 87.21(3), β = 89.74(3), γ = 64.90(5)Њ,
T = 296 K, Z = 2, U = 2044(2) Å3, µ = 2.889 mmϪ1, number of
reflections measured 8528, R = 0.039, RЈ = 0.047.
monitored in C6D6 solution by H NMR spectroscopy. The
product was not isolated in either case and was characterized by
spectroscopic methods. 1H NMR (C6D6, 30 ЊC): δ 8.25 (d, 1 H),
7.99 (d, 1 H), 7.89 (d, 1 H), 7.69–6.83 (m, 19 H, aromatics), 0.90
(d, 2 H), Ϫ0.31 [d, 2 H, 2J(1H᎐1H) = 11.8 Hz, CH2SiMe3], Ϫ0.14
(s, 18 H, CH2SiMe3). 13C NMR (C6D6, 30 ЊC): δ 203.8 (TaC
ipso), 76.5 (TaCH2SiMe3), 2.0 (CH2SiMe3).
Crystal data for 1eؒ0.5C6H6. C55H61Cl3O2Ta, M = 1041.41,
¯
triclinic, space group P1 (no. 2), a = 10.376(3), b = 13.1075(18),
c = 19.798(3) Å, α = 72.391(12), β = 89.39(2), γ = 79.232(19)Њ,
J. Chem. Soc., Dalton Trans., 1997, Pages 3353–3362
3361

View Article Online
T = 297 K, Z = 2, U = 2518.3(12) Å3, µ = 2.354 mmϪ1, number
of reflections measured 7103, R = 0.024, RЈ = 0.032.
J.-P. Finet, D. J. Lester, W. B. Motherwell, M. T. B. Papoula and
S. P. Stanforth, J. Chem. Soc., Perkin Trans. 1, 1985, 2657.
10 R. R. Schrock, Polyhedron, 1995, 14, 3177.
11 L. R. Chamberlain, I. P. Rothwell, K. Folting and J. C. Huffman,
J. Chem. Soc., Dalton Trans., 1987, 155; L. R. Chamberlain and
I. P. Rothwell, J. Chem. Soc., Dalton Trans., 1987, 163.
12 L. R. Chamberlain, I. P. Rothwell and J. C. Huffman, Inorg. Chem.,
1984, 23, 2575.
Crystal data for 4c. C52H67O2Si3Ta, M = 989.32, monoclinic,
space group P21/c (no. 14), a = 23.354(4), b = 19.323(3),
c = 23.101(4) Å, β = 102.474(15)Њ, T = 296 K, Z = 8, U =
10 178(5) Å3, µ = 4.925 mmϪ1, number of reflections measured
13 201, R = 0.063, RЈ = 0.129.
13 M. A. Lockwood, M. C. Potyen, B. D. Steffey, P. E. Fanwick and
I. P. Rothwell, Polyhedron, 1995, 14, 3293.
14 J. R. Clark, A. L. Pulvirenti, P. E. Fanwick, M. Sigalis, O. Eisenstein
and I. P. Rothwell, Inorg. Chem., in the press.
15 R. W. Chesnut, L. D. Durfee, P. E. Fanwick, I. P. Rothwell,
K. Folting and J. C. Huffman, Polyhedron, 1987, 6, 2019.
16 M. Perrin, K. Bekkouch and A. Thozet, Acta Crystallogr., Sect. C,
1987, 43, 2357.
17 K. Nakatsu, H. Yoshioka, K. Kunimoto, T. Kingasa and S. Ueji,
Acta Crystallogr., Sect. B, 1978, 34, 980.
18 M. Oki and H. Iwamura, J. Am. Chem. Soc., 1967, 89, 576;
F. H. Allen, J. A. K. Howard, V. J. Hoy, G. R. Desiraju, D. S. Reddy
and C. C. Wilson, J. Am. Chem. Soc., 1996, 118, 4081 and refs.
therein.
Crystal data for 4d. C60H83O2Si3Ta, M = 1101.54, monoclinic,
space group P21/c (no. 14), a = 12.0448(12), b = 14.399(2),
c = 34.848(4) Å, β = 96.723(9)Њ, T = 296 K, Z = 4, U = 6002(2)
Å3, µ = 1.908 mmϪ1, number of reflections measured 7973,
R = 0.030, RЈ = 0.066.
Crystal data for 5a. C62H61O3Si2Ta, M = 1091.30, triclinic,
¯
space group P1 (no. 2), a = 13.5220(19), b = 13.6243(17),
c = 14.466(3) Å, α = 89.837(14), β = 82.742(14), γ = 81.403(11)Њ,
T = 295 K, Z = 2, U = 2613.7(11) Å3, µ = 4.648 mmϪ1, number
of reflections 8672, R = 0.035, RЈ = 0.041.
19 M. Brookhart and M. L. H. Green, J. Organomet. Chem., 1983, 250,
395; M. Brookhart, M. L. H. Green and L. L. Wong, Prog. Inorg.
Chem., 1988, 36, 1.
CCDC reference number 186/611.
20 R. R. Schrock, R. T. DePue, J. Feldman, K. B. Yap, D. C. Yang,
W. M. Davis, L. Y. Park, M. DiMare, M. Schofield, J. Anhaus,
E. Walborsky, E. Evitt, C. Kruger and P. Betz, Organometallics,
1990, 9, 2262.
21 I. P. Rothwell, in Homogeneous Alkane Activation, ed. C. G. Hill,
Wiley, New York, 1989, pp. 151–195.
Acknowledgements
We thank the National Science Foundation (Grant CHE-
9700269) for financial support of this research.
22 C. McDade, J. C. Green and J. E. Bercaw, Organometallics, 1982, 1,
1629; A. R. Bulls, W. P. Schaefer, M. Serfas and J. E. Bercaw,
Organometallics, 1987, 6, 1219; L. R. Chamberlain, I. P. Rothwell
and J. C. Huffmann, J. Am. Chem. Soc., 1986, 108, 1502;
J.-L. Couturier, C. Paillet, M. Leconte, J. M. Basset and K. Weiss,
Angew. Chem., Int. Ed. Engl., 1992, 31, 628; K. C. Wallace,
A. H. Liu, J. C. Dewan and R. R. Schrock, J. Am. Chem. Soc., 1988,
110, 4964; J. A. van Doorn, H. van der Heijden and A. G. Orpen,
Organometallics, 1994, 13, 4271.
23 H. van der Heijden and B. J. Hessen, J. Chem. Soc., Chem.
Commun., 1995, 145; M. P. Coles, V. C. Gibson, W. Clegg,
M. R. J. Elsegood and P. A. Porrelli, J. Chem. Soc., Chem. Commun.,
1995, 145.
24 C. P. Schaller, C. C. Cummins and P. T. Wolczanski, J. Am. Chem.
Soc., 1996, 118, 591.
25 P. Yates and J. R. Hyre, J. Org. Chem., 1962, 27, 4101; A. S. Hay and
R. F. Clark, Macromolecules, 1970, 3, 533; H. Yang and A. S. Hay,
Synthesis, 1992, 467; D. E. Dana and A. S. Hay, Synthesis, 1982,
164.
References
1 F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry,
Wiley-Interscience, New York, 5th edn., 1988.
2 Activation and Functionalization of Alkanes, ed. C. Hill, John
Wiley and Sons, New York, 1989; Selective Hydrocarbon Activation,
eds. J. A. Davis, P. L. Watson, J. F. Liebman and A. Greenberg, VCH
Publishers, Toledo, OH, 1989; A. E. Shilov, Activation of Saturated
Hydrocarbons by Transition Metal Complexes, Reidel, Boston, 1984.
3 I. P. Rothwell, Acc. Chem. Res., 1988, 21, 153.
4 D. Rabinovich, R. Zelman and G. Parkin, J. Am. Chem. Soc., 1990,
112, 9632; 1992, 114, 4611.
5 J. S. Yu, L. Felter, M. C. Potyen, J. R. Clark, V. M. Visciglio,
P. E. Fanwick and I. P. Rothwell, Organometallics, 1996, 15, 4443.
6 G. D. Smith, V. M. Visciglio, P. E. Fanwick and I. P. Rothwell,
Organometallics, 1992, 11, 1064.
7 (a) R. W. Chesnut, G. J. Gayatry, J. S. Yu, P. E. Fanwick and
I. P. Rothwell, Organometallics, 1991, 10, 321; (b) F. Lefebvre,
M. Leconte, S. Pagano, A. Mutch and J.-M. Basset, Polyhedron,
1995, 14, 3209.
26 D. H. Grayson and M. J. R. Tuite, J. Chem. Soc., Perkin Trans. 1,
1986, 2137.
8 (a) M. A. Lockwood, P. E. Fanwick, O. Eisenstein and I. P.
Rothwell, J. Am. Chem. Soc., 1996, 118, 2762; (b) M. A. Lockwood,
P. E. Fanwick and I. P. Rothwell, Polyhedron, 1995, 14, 3363.
9 D. H. R. Barton, D. M. X. Donnelly, P. J. Guiry and
J. H. Reibenspies, J. Chem. Soc., Chem. Commun., 1990, 1110;
D. H. R. Barton, N. Y. Bhatnagar, J.-C. Blazejewski, B. Charpiot,
27 A. Galan, A. J. Sutherland, P. Ballester and J. Rebek jun.,
Tetrahedron Lett., 1994, 35, 5359.
Received 4th April 1997; Paper 7/02325A
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J. Chem. Soc., Dalton Trans., 1997, Pages 3353–3362