Synthesis and reactivity of tantalum complexes supported by bidentate X2 and tridentate LX2 ligands with two phenolates linked to pyridine, thiophene, furan, and benzene connectors: Mechanistic studies of the formation of a tantalum

Article abstract of DOI:10.1021/om8002653

Using either alkane elimination or salt metathesis methods, tantalum complexes have been prepared with new ligand systems with tridentate bis(phenolate)donor (donor = pyridine, furan, and thiophene) or bidentate bis(phenolate)benzene arrangements. The lig

Full text of DOI:10.1021/om8002653

Organometallics 2008, 27, 6123–6142
6123
Synthesis and Reactivity of Tantalum Complexes Supported by
Bidentate X2 and Tridentate LX2 Ligands with Two Phenolates
Linked to Pyridine, Thiophene, Furan, and Benzene Connectors:
Mechanistic Studies of the Formation of a Tantalum Benzylidene
and Insertion Chemistry for Tantalum-Carbon Bonds
Theodor Agapie, Michael W. Day, and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
ReceiVed March 23, 2008
Using either alkane elimination or salt metathesis methods, tantalum complexes have been prepared
with new ligand systems with tridentate bis(phenolate)donor (donor ) pyridine, furan, and thiophene) or
bidentate bis(phenolate)benzene arrangements. The ligand framework has two X-type phenolates connected
to the flat heterocyclic L-type donor at the 2,6- or 2,5- positions or to the 2,6- positions of benzene via
direct ring-ring (sp2-sp2) linkages. Solid-state structures of these complexes show that in all cases the
ligands bind in a mer fashion, but with different geometries of the LX2 frameworks. The pyridine-linked
system binds in a Cs-fashion, the furan-linked system in a C2v-fashion, and the thiophene-linked system
in a C1-fashion. A bis(phenolate)pyridine tantalum tribenzyl species (7), upon heating in the presence of
dimethylphenylphosphine, generates a stable benzylidene complex by R-hydrogen abstraction with loss
of toluene and PMe2Ph trapping. This process was found to be independent of PMe2Ph concentration
with ∆Hq ) 31.3 ( 0.6 kcal · mol-1 and ∆Sq ) 3 ( 2 cal · mol-1 · K-1, and the kinetic isotope effect
kH/kD ) 4.9 ( 0.4, consistent with a mechanism involving rate determining R-hydrogen abstraction with
loss of toluene, followed by fast phosphine coordination to the resulting benzylidene species. An X-ray
structure determination reveals that the benzylidene π-bond is oriented perpendicular to the oxygen-oxygen
vector, in accord with the prediction of DFT calculations. Tantalum alkyl complexes with the benzene-
linked bis(phenolate) ligand (Ta(CH3)2[(OC6H2-tBu2)2C6H3] (16), Ta(CH2Ph)2[(OC6H2-tBu2)2C6H3] (17),
and TaCl2CH3[(OC6H2-tBu2)2C6H4] (18)) are obtained with (to afford pincer complexes) or without
cyclometalation at the ipso-position. Deuterium labeling of the phenol hydrogens and of the linking 1,3-
benzene-diyl ring reveals an unexpected mechanism for the metalation of bis(phenol)benzene with
TaCl2(CH3)3 to generate 18. This process involves protonolysis of a methyl group, followed by C-H/
Ta-CH3 σ bond metathesis leading to cyclometalation of the linking ring, and finally protonation of the
cyclometallated group by the pendant phenol. TaCl2CH3[(OC6H2-tBu2)2C6H4] was found to undergo σ
bond metathesis at temperatures over 90 °C to give the pincer complex TaCl2[(OC6H2-tBu2)2C6H3] (19)
and methane (∆Hq ) 27.1 ( 0.9 kcal · mol-1; ∆Sq ) -2 ( 2 cal · mol-1 · K-1; kH/kD ) 1.6 ( 0.2 at 125
°C). Ta(CH3)2[(OC6H2-tBu2)2C6H3] (16) was found to react with tBuNC to insert into the Ta-CH3 bonds
and generate an imino-acyl species (23). Reaction of 16 with Ph2CO or PhCN leads to insertion into the
Ta-Ph bond to give 21 and 22. Complexes 6, 7, 10, 11-P, 12, 13, 17, 18, 19-OEt2, 21, 22, and 23 have
been structurally characterized by single crystal X-ray diffraction, and all show a mer binding mode of
the diphenolate ligands, but the ligand geometry varies leading to C2V-, pseudo-Cs-, pseudo-C2-, and
C1-symmetric structures.
pared to their metallocene counterparts, early metal systems
having ligand systems other than cyclopentadienyl remain less
explored and less well understood. In this context, the discovery
of easily prepared and tunable ligand frameworks that could
provide early transition metal complexes that are stable and well
Introduction
The organometallic chemistry of the early transition metals
evolved largely with compounds having bent metallocene
frameworks. More recently, some classes of early transition
metal complexes based on nonmetallocene frameworks have
proven to be well defined platforms for the study of elementary
organometallic transformations and as catalysts for diverse
reactions including olefin polymerizations, hydroaminations,
hydrogenations, and olefin metathesis.1-13 Nonetheless, com-
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* Corresponding author. E-mail: bercaw@caltech.edu.
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10.1021/om8002653 CCC: $40.75
2008 American Chemical Society
Publication on Web 11/07/2008

6124 Organometallics, Vol. 27, No. 23, 2008
Agapie et al.
Scheme 1a
a
R, R’ ) alkyl or aryl; linker ) thiophene, furan, pyrrole, pyridine, NHC, or phenyl; L ) C (neutral or anionic), N, O, or S.
defined, while allowing for new types of transformations with
stereocontrol, is highly desirable. Phenolates are readily tunable
ligands with respect to sterics and electronics.14 Multidentate
phenolate ligand systems, in particular, have found applications
in olefin polymerization catalysis as well as in the synthesis
and study of fundamental organometallic transformations.15-32
One drawback apparent for some of the mutidentate ligands
reported thus far is the degradation of the groups connecting
the ligating atoms.22,26,33 For example, the tripodal tetradentate
bis(phenolate) ligands described by Kol et al. have been reported
to be susceptible to ꢀ-H abstraction from the position next to
the metal-coordinated tertiary amine.22 We have recently
reported zirconium and titanium complexes supported by
tridentate LX2 ligands with two phenolates linked to furan,
thiophene, and pyridine donors.34 These complexes proved to
be thermally robust and effective precatalysts for propylene
polymerization and oligomerization.
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Herein we describe the synthesis, characterization, and
investigations of the reactivity of an analogous series of tantalum
complexes supported by this type of ligand (Scheme 1). The
ligand frameworks have two (X-type) phenolates connected to
a flat heterocyclic (L-type) donor (N-heterocyclic carbene,
pyridine, furan, or thiophene) or a benzene ring at the 2,6- or
2,5-positions via direct ring-ring (sp2-sp2) linkages. One
motivation to study this ligand family stems from their
resemblance to ubiquitous ansa metallocene frameworks that
have been used to support a variety of stoichiometric and
catalytic transformations (Scheme 1). Assuming that the ter-
phenyl (or terphenyl-like) subunit enforces a mer binding mode,
this framework could have binding geometries of C2, C2V, Cs
(two types), and C1 symmetry, depending on the direction of
twisting around the aryl-aryl linkages. This structural versatility
is of interest in the context of developing stereoselective
applications. Previous reports of boron and group 4 complexes
supported by the bis(phenolate)pyridine ligands provide ex-
amples of Cs-symmetric structures.35-37 Crystallographically
characterized complexes of FeIII, CuII, and AlIII exhibit the C2-
symmetric binding mode.38
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Synthesis and ReactiVity of Tantalum Complexes
Organometallics, Vol. 27, No. 23, 2008 6125
In the context of tridentate pincer diphenolate ligands, the
cyclometallated bis(phenolate)benzene (X3 type) version bears
resemblance to monoanionic LCL pincer ligand counterparts
(L ) phosphine, amine, thioether, or ether), an intense area of
study.39-44 Pincer-based complexes are quite versatile and have
found use in catalysis, for fundamental studies of small molecule
activation, for supporting diverse organometallic structures and
transformations, and for the design and development of novel
materials and sensors. Most of the reported systems are based
on late transition metal complexes, but early metal and lan-
thanide pincer complexes have been reported as well.43,45-57
In particular, 2,6-di(o-anisol)phenyl (OCO) systems have been
reportedtosupportyttrium,samarium,andytterbiumcomplexes.45-47
Diaminophenyl (NCN) systems have been developed for sup-
porting titanium, tantalum, lutetium, and yttrium chemistry.43,48-57
The organometallic chemistry of tantalum is distinctive for
the ubiquity of alkylidene and olefin complexes.43,48-54 In
previously reported cases, the pendant ligands are based on
neutral donors such as phosphines, amines, thioethers, ethers,
and carbenes.39-44 No multiply charged cyclometallated pincer
ligands appear to have been reported before part of the current
work was communicated;58 however, more recently, hafnium
and molybdenum complexes supported by trianionic bis(phe-
nolate)phenyl (OCO) and bis(amide)phenyl (NCN) ligands,
respectively, have been reported.59,60 The use of terphenyl
frameworks for pincer ligands has recently emerged as a route
to chiral, C2-symmetric pincer systems; systems of Cs symmetry
have been reported as well.45-47,61,62 For early metal chemistry,
in which higher oxidations states are accessible, the study of
complexes supported by robust multianionic (LX2 and X3)
ligands, which would complement the known monoanionic
(L2X) cyclometallated pincer ligands, is an underdeveloped area.
We report herein new tantalum alkyl complexes with these LX2
ligands, their preparative routes, the structures of several
representative examples, mechanistic studies of the metalation
of the bis(phenolate)benzene ligand as well as the thermal
decomposition of a tris-benzyltantalum derivative with the
bis(phenolate)pyridine ligand to a stable benzylidene complex,
and the insertion of isocyanide into tantalum-methyl and
benzophenone and benzonitrile into tantalum-aryl bonds.
Results and Discussion
Preparation of Tantalum Alkyl Complexes Supported
by Bis(phenol)pyridine (1). Bis(phenol)pyridine ((2,6-HOC6H2-
2,4-tBu2)2NC5H3, 1), thiophene ((2,5-HOC6H2-2,4-tBu2)2SC4H2,
2), furan ((2,5-HOC6H2-2,4-tBu2)2OC4H2, 3), and benzene
linkers ((2,6-HOC6H2-2,4-tBu2)2C6H4, 4) are prepared using
palladium-catalyzed aryl-aryl coupling chemistry, as described
previously.58 Tantalum metalation has been performed using
either salt metathesis or alkane eliminations routes. Common
tantalum alkyl starting materials such as Ta(CH2Ph)5,
TaCl2(CH2Ph)3, TaCl3(CH2Ph)2, TaCl2Me3, or TaCl3Me2 are
used as precursors to phenoxide supported complexes (Scheme
1).63,64 Deprotonation of 1 has been performed in situ with
KCH2Ph. The tantalum precursors are added to a slurry of
potassium bis(phenoxide)pyridine in benzene or diethyl ether
(Scheme 2). The 1H NMR spectra of the crude reaction mixtures
show nearly quantitative formation of the desired products.
Filtration to remove the KCl byproduct, followed by recrystal-
lization, provides analytically pure samples of the tantalum
complexes. The 1H NMR spectroscopic features of these
compounds are revealing. Complex 5 displays one singlet
integrating for 4 H corresponding to the benzyl CH2 resonances,
suggesting that the two benzyl groups are related by a mirror
plane (Scheme 1). Another possible explanation involves a fast
interchange process. Complex 6 shows a sharp singlet integrating
to 9 H corresponding to the three tantalum coordinated methyl
groups, in a variety of deuterated solvents. A variable temper-
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1
ature H NMR study was undertaken, and at -90 °C, the
Ta-CH3 signals give rise to two broad singlets in a two to one
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lead to three chemically equivalent methyl groups, if the
bis(phenolate)pyridine ligand is meridonally coordinated. In that
case, a fluxional process that allows for methyl exchange,
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favored structure with two equivalent methyls. This can be
accomplished by the presence of a mirror plane such as in a
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6126 Organometallics, Vol. 27, No. 23, 2008
Agapie et al.
Scheme 2
The solid-state structure of compound 6 shows mer coordina-
tion of the tridentate ligand (Figure 1). The Ta-N bond length
is in the long range compared to other reported Ta-pyridine
complexes, but the most unusual and unexpected feature is the
bending of the pyridine ring away from the Ta-N axis
(Ta-N-C9 angle ) 146.6 °). This distortion is likely a
consequence of the inherent rigidity in the terphenyl framework
that prevents all of the rings from adopting their preferred
conformations while maintaining optimal metal-ligand bond
distances and angles. The [Ta-CH3] groups are clearly different,
with one trans to the pyridine nitrogen and two roughly trans
to each other. The Ta-CH3 bond lengths are typical for tantalum
methyl complexes, and C35-Ta-C34 and C35-Ta-C36
angles are 108.42(7)° and 104.62(8)°, respectively, with atoms
C34 and C36 leaning toward the pyridine donor. With this
distortion and the long Ta-N bond, the structure could be
viewed as trigonal bipyramidal with an additional weak interac-
tion with the pyridine. It is worth noting that the two methyl
groups cis to the nitrogen donor are chemically different in the
solid state because of the tilting of the pyridine plane. The low
temperature spectrum observed for 6 could be accounted for
by fast flipping of the pyridine ring between the two sides of
Figure 1. Drawings of the structure of 6. Selected bond lengths (Å) and angles (deg): Ta-O1 1.9113(13); Ta-O2 1.9020(13); Ta-C35
2.1573(18); Ta-C34 2.1931(18); Ta-C36 2.2032(19); Ta-N 2.4428(14); Ta-N-C9 146.6; O1-Ta-O2 163.45(5); C35-Ta-C34 108.42(7);
C35-Ta-C36 104.62(8); Ta-O1-C1 138.09(11); Ta-O2-C17 141.66(11).

Synthesis and ReactiVity of Tantalum Complexes
Organometallics, Vol. 27, No. 23, 2008 6127
Scheme 3
the ONO plane. Given that this cannot be frozen even at -90
°C, the energy of the C2V transition state structure is likely not
too different from that for the ground state.
to be in close proximity to the pyridine ring indicating the
possibility of some π-π stacking interactions.65,66
Attempts to Prepare a Tantalum Benzylidene Complex:
Formation of TaF2(CH2Ph)[(OC6H2-tBu2)2NC5H3]. Given the
number of stable alkylidene complexes of tantalum, we have
pursued a commonly used route to tantalum alkylidenes that
involves deprotonation of cationic alkyl species with bases such
as Me3PCH2 or KN(SiMe3)2.67 This strategy was attempted
starting with Ta(CH2Ph)3[(OC6H2-tBu2)2NC5H3] (7). Reaction
with [Ph3C][BF4], in toluene, leads cleanly to one species
according to NMR spectroscopy. The 1H NMR spectrum of the
isolated product displays a triplet corresponding to a benzyl
group, which integrates to only 2 H (instead of expected 4 H)
relative to the phenoxide ligand tBu groups. This species also
displays a broad peak in the 19F NMR spectrum. A single-crystal
X-ray diffraction study allowed the identification of the product,
a tantalum monobenzyl difluoride 10 (Figure 3). In the solid-
state, the benzyl binds trans to the pyridine ligand, consistent
Preliminary experiments have been conducted to examine the
olefin polymerization activity using 6. An NMR scale experi-
ment was performed: Ta(CH3)3[(OC6H2-tBu2)2NC5H3] (6) was
treated with B(C6F5)3 to generate a major new species based
1
on H NMR spectroscopy. Treatment with ethylene of this in
situ generated species led to no reaction at room temperature
(1H NMR spectroscopy). Heating the mixture to 80 °C led to
the formation of ethylene oligomers as indicated by peaks in
1
the olefinic region of the H NMR spectrum.
Alkane elimination routes were found to be facile for methyl
tantalum starting materials. In those cases, reaction with
bis(phenol)pyridine is complete within hours at room temper-
aturetogiveTaCl3[(OC6H2-tBu2)2NC5H3](8)orTaCl2(CH3)[(OC6H2-
tBu2)2NC5H3] (9), when starting with TaCl3Me2 or TaCl2Me3,
respectively (Scheme 3). Tantalum pentabenzyl requires heating
at60°CforfivehoursforcompleteconversiontoTa(CH2Ph)3[(OC6H2-
tBu2)2NC5H3] (7) (Scheme 2); at room temperature, this reaction
is only 60% completed after 35 h. The lower reactivity for the
benzyl precursor could be due to the decreased basicity of benzyl
versus methyl, as well as to a more hindered metal center in
1
with the solution H and 19F NMR spectroscopic data. This
species is possibly formed by fluoride abstraction by the
electrophilic,cationictantalumspecies,followedbyafluoride-benzyl
interchange. The target benzylidene species has been prepared
by a different route (see below), and alternative procedures
involving the cationic alkyl strategy have not been further
explored.
Preparation of a Tantalum Benzylidene by Toluene
Elimination: Formation and Characterization of
Ta(CHPh)(CH2Ph)(PMe2Ph)[(OC6H2-tBu2)2NC5H3] (11-P).
Another common route to alkylidene complexes involves
intramolecular R-H abstraction, often carried out in the presence
of an additional ligand such as a phosphine.68-74 Accordingly,
1
the tantalum benzyl versus tantalum methyl species. The H
NMR spectrum of 7 shows two singlets for the benzylic protons
(Ta-CH2Ph) in a two to one ratio, consistent with a mer binding
mode. For a fac binding mode, the methylene protons trans to
oxygen donors would be diastereotopic and thus should display
two distinct signals (not observed). A single crystal X-ray
diffraction study of 7 (Figure 2) confirmed the meridional
(65) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885–3896.
(66) Hunter, C. A.; Sanders, J. K. M. J. Am. Chem. Soc. 1990, 112,
5525–5534.
1
binding mode indicated by the H NMR spectroscopic data.
Compared to the trimethyl compound 5, the tantalum tribenzyl
species shows a notable difference with respect to the binding
of the alkyl groups: the C41-Ta-C34 and C41-Ta-C48
angles are about 30° smaller (75.14(7)° and 76.32(7)°, respec-
tively). It is not clear if this orientation is adopted to avoid some
steric interactions between the pyridine ring the bulkier benzyl
groups or to facilitate some bonding interaction between the
metal center and the π system of the benzyl group: one of the
Ta-C-C(phenyl) angles is more acute than the others, at 97°
versus 119° and 120°, and the Ta-C(ipso) bond length is 2.81
Å. Furthermore, this geometry allows one of the benzyl phenyls
(67) Schrock, R. R. J. Am. Chem. Soc. 1975, 97, 6577–6578.
(68) Fellmann, J. D.; Rupprecht, G. A.; Wood, C. D.; Schrock, R. R.
J. Am. Chem. Soc. 1978, 100, 5964–5966.
(69) McLain, S. J.; Wood, C. D.; Messerle, L. W.; Schrock, R. R.;
Hollander, F. J.; Youngs, W. J.; Churchill, M. R. J. Am. Chem. Soc. 1978,
100, 5962–5964.
(70) Schrock, R. R.; Fellmann, J. D. J. Am. Chem. Soc. 1978, 100, 3359–
3370.
(71) Schrock, R. R.; Messerle, L. W.; Wood, C. D.; Guggenberger, L. J.
J. Am. Chem. Soc. 1978, 100, 3793–3800.
(72) Schrock, R. R. Acc. Chem. Res. 1979, 12, 98–104.
(73) Li, L.; Hung, M.; Xue, Z. J. Am. Chem. Soc. 1995, 117, 12746.
(74) Morton, L. A.; Chen, S. J.; Qiu, H.; Xue, Z. L. J. Am. Chem. Soc.
2007, 129, 7277–7283.

6128 Organometallics, Vol. 27, No. 23, 2008
Agapie et al.
Figure 2. Drawings of the structure of 7. Selected bond lengths (Å) and angles (deg): Ta-O1 1.8961(11); Ta-O2 1.8972(11); Ta-C41
2.3088(16); Ta-C34 2.2243(18); Ta-C48 2.2269(18); Ta-N 2.4299(13); Ta-N-C9 150.5; O1-Ta-O2 158.89(5); C41-Ta-C34 75.14(7);
C41-Ta-C48 76.32(7); Ta-O1-C1 145.20(11); Ta-O2-C17 145.45(10).
compound 7 was heated to temperatures over 90 °C in benzene-
d6 or toluene-d8 in the presence of excess PMe2Ph. Clean
1
formation of a new species (11-P) was observed by H NMR
spectroscopy (eq 1). The singlet at 9.03 ppm is particularly
diagnostic for a benzylidene species. The reaction proceeds quite
slowly at 90 °C, reaching completion after more than three days.
At 125 °C, however, the reaction is complete within hours. In
the absence of phosphine, benzylidene formation still appears
to occur, judging by the appearance of a peak at 9.20 ppm (1H
NMR spectroscopy); however, the reaction is not as clean. The
use of other bases such as quinuclidine or PMe3 has been
explored. The reaction occurs cleanly in the presence of PMe3,
presumably to the analogue of 11-P, but decomposition is
observed in the presence of quinuclidine. Even for the phos-
phines that afford clean formation of the benzylidene, prolonged
subsequent heating leads to decomposition. Preliminary studies
with 11-P indicate that it reacts with ethylene and dipheny-
lacetylene suggesting promise for further exploration. Notably,
the observed clean formation of 11-P from 7 and PMe2Ph at
such high temperatures emphasizes the chemical robustness of
this ligand framework. Even under milder conditions, other
reported multidentate biphenoxides undergo reaction with the
supporting ligands.22
troscopy, and DFT calculations. The solid-state structure of 11-P
(Figure 4) displays hexacoordination at tantalum, with meridi-
onally coordinated pyridine bisphenoxide, benzylidene trans to
pyridine, and phosphine and benzyl groups trans to each other.
Interestingly, the benzylidene is oriented such that the π-bond
roughly bisects the O2N plane. This orientation is sterically
unfavorable, pointing the phenyl group toward a phenoxide
1
t-butyl. The H coupled 13C NMR spectrum of 11-P shows a
coupling constant 1JCH ) 103 Hz for the alkylidene carbon. Both
the TaCC angle and the alkylidene 1JCH coupling constants have
been used as indicators of the degree of R-agostic interaction.
The structure and tantalum-benzylidene bonding for 11-P was
investigated using single-crystal X-ray diffraction, NMR spec-

Synthesis and ReactiVity of Tantalum Complexes
Organometallics, Vol. 27, No. 23, 2008 6129
Figure 3. Drawings of the structure of 10. Selected bond lengths (Å) and angles (deg): Ta-O1 1.8906(13); Ta-O2 1.8908(13); Ta-F1
1.8927(10); Ta-C34 2.1872(19); Ta-F2 1.9110(11); Ta-N 2.4486(14); Ta-N-C9 142.8; O1-Ta-O2 158.28(5); F1-Ta-C34 90.74(6);
F2-Ta-C34 89.21(6); Ta-O1-C1 143.78(12); Ta-O2-C17 144.38(11).
Figure 4. Drawings of the structure of 11-P. Selected bond lengths (Å) and angles (deg): Ta-O1 1.9355(18); Ta-O2 1.9095(18); Ta-C42
1.996(2); Ta-C49 2.279(2); Ta-P 2.7058(7); Ta-N 2.4827(17); O1-Ta-O2 159.17(6); Ta-C42-C43 149.7(2); Ta-O1-C1 137.10(15);
Ta-O2-C17 145.33(15); Ta-N-C9 150.0.
A large TaCC angle correlates with a small C-H coupling
constant (due to increased p-character in the CH bond) and is
indicative of an increased degree of R-agostic interaction.12,75
For example, complex Cp2Ta(CHPh)(CH2Ph), which does not
present a low-energy orbital of appropriate symmetry for an
1
R-agostic interaction, shows a coupling constant JCH ) 127
Hz and a TaCC angle of 135.2°.71 At the other end of the
spectrum lies Cp*Ta(CHPh)(CH2Ph), which has a coupling

6130 Organometallics, Vol. 27, No. 23, 2008
Agapie et al.
incoming ligand. Given that the carbene has a similar asym-
metry, only a p-orbital energetically accessible perpendicular
to the plane of the fragment, the expected geometry is the one
allowing for maximal overlap and formation of a TadC π-bond.
Mechanistic Study of the Generation of Tantalum Ben-
zylidene Complex Ta(CHPh)(CH2Ph)(PMe2Ph)[(OC6H2-
tBu2)2NC5H3] (11-P) by r-Abstraction. Several mechanisms may
be envisioned for the formation of benzylidene 11-P. One
pathway, based on literature precedents, 71,76,77 involves initial
coordination of the phosphine to give rise to a crowded species,
7-P, that brings an R-H and a neighboring benzyl group into
close proximity facilitating R-H abstraction and leads to
alkylidene formation with toluene loss. Formation of 7-P could
occur via a fast pre-equilibrium, followed by a rate determining
R-H abstraction (Scheme 4, mechanism 1). Because the major
tantalum species observed during the reaction are 7 and 11-P,
with no buildup of detectable concentrations of intermediates
such as 7-P, the observed rate constant according to mechanism
1 is expected to be first-order in the phosphine concentration.
Alternatively, 7-P could be generated in a slow step followed
by a fast R-H abstraction (Scheme 4, mechanism 2). For this
pathway, the observed rate constant again is expected to have
a first-order dependence on phosphine concentration. For
mechanism 3, R-H abstraction would occur in a rate-determining
step directly from 7, followed by fast phosphine trapping of
the alkylidene. For this pathway, the rate would be independent
of phosphine concentration.
A series of kinetics experiments was performed with varying
phosphine concentrations under pseudo-first-order conditions
(more than 20-fold excess vs 7). As can be seen in Figure 6,
the reaction rate at 115 °C was found to be cleanly first order
in 7. Provided that the rate of the reaction is predicted to be
first order in the phosphine concentration for two of the three
proposed mechanisms, kinetic measurements were performed
at 115 °C at various concentrations of phosphine. Over a
phosphine concentration range varied 15-fold, the reaction rate
was found to remain essentially constant (Figure 7) consistent
only with mechanism 3. Moreover, comparison of the rates
at 115 °C for Ta(CH2Ph)3[(OC6H2-tBu2)2NC5H3] (7) and
Ta(CD2Ph)3[(OC6H2-tBu2)2NC5H3] (7-d6) reveals a kinetic
deuterium isotope effect kH/kD ) 4.9 ( 0.4 (Figure 8) indicating
that the rate determining step involves C-H bond cleavage.
This value is thus inconsistent with mechanism 2. Rate
measurements performed over a temperature range of 37 °C
(Figure 9) provides Eyring activation parameters of ∆Hq ) 31.3
( 0.6 kcal · mol-1 and ∆Sq ) 3 ( 2 cal · mol-1 · K-1 for 7 (for
7-d6, ∆Hq ) 34.4 ( 0.6 kcal · mol-1 and ∆Sq ) 8 ( 2
cal · mol-1 · K-1). The small activation entropy is most consistent
with a unimolecular rate determining step (mechanisms 1 and
3). Thus, of the proposed pathways, only mechanism 3 is
consistent with all of the above experimental results.
Figure 5. Depiction of HOMO of the optimized gas-phase structure
of complex 11-P.
constant of JCH ) 82 Hz with a TaCC angle of 166°.76 The
1
1
value of JCH ) 103 Hz for 11-P is indicative of a modest
amount of R-agostic interaction. This correlates well with the
Ta-C42-C43 angle of 149.7°. Interestingly, the benzylidenes
prepared by Kol et al. supported by bisamino-diphenolate ligands
display significantly more acute alkylidine TaCC angles (140°
and 142°).22 However, for all those cases, the phenolates display
either chloride or hydrogen in the ortho-position to the phenolate
oxygen. Hence, the observed larger ankylidene TaCC angle in
the pyridine-diphenolate case could be due, at least in part, to
a steric interaction between the phenyl group and the bulkyl
ortho-t-butyl group of the phenolate. This possibility was
explored computationally.
DFT calculations were performed on a model of 11-P
displaying hydrogens in the positions ortho to the phenolate
oxygens (B3LYP, basis set LACVP**, model 11-Pm, Figure 1
in the Supporting Information). The unrestricted optimized
geometry of 11-Pm displays an alkylidene Ta-C-C angle of
141°, more acute than the one observed in the solid state for
the t-Bu substituted species and similar to the ones observed
for the Kol systems bearing smaller substituents. This indicates
that the intrinsic preference for R-agostic interaction is small
on the basis of the DFT calculation, almost as small as in the
metallocene systems. For the ortho substituted system of 11-P,
the t-butyl group probably pushes the phenyl group away from
the tantalum center by steric repulsion, increasing the Ta-C-C
angle and enforcing more p-character to the alkylidene CH bond.
The Ta-C π-interaction for the alkylidene moiety, the calculated
HOMO, is shown in Figure 5. This interaction is roughly
perpendicular to the Ta-O vectors and correlates with the
predicted interactions between the frontier orbitals of the
tantalum fragment and of those of the carbene (Figure 2,
Supporting Information). Thus, meridonal [TaO2N] and [TaCp2]
fragments are related electronically due to a structural asym-
metry (2-fold, rather than higher) that differentiates the nature
of the orthogonal orbitals used to interact with additional ligands.
The d-orbitals that have a contribution toward the Cp or O
ligands are involved in π-bonding with ancillary ligands and
are destabilized with respect to interaction with additional
ligands. The remaining d-orbitals, found perpedicular to the
O-O or Cp-Cp vector, are available for bonding with an
To explore the relevant electronic features of the R-abstraction
reaction, the synthesis of compounds with mixed benzyl ligand
sets was targeted. Ta(CH2Ph)2Cl[(OC6H2-tBu2)2NC5H3] (5) was
treated with Grignard reagents substituted with OMe and CF3
in the para position. Mixtures of products were obtained in both
cases. The reaction of 5 with deuterium labeled benzyl Grignard
reagent PhCD2MgBr leads to the tribenzyl species with label
incorporation at all positions, and treatment of 7 with
PhCD2MgBr also leads to [PhCH2-]/[PhCD2-] exchange. These
results suggest that exchange occurs between the benzyl
Grignard reagent and the benzyl groups of 7, precluding clean
(75) Nugent, W. A.; Mayer, J. M. Metal-Ligand Multiple Bonds; Wiley:
New York, 1988.
(76) Messerle, L. W.; Jennische, P.; Schrock, R. R.; Stucky, G. J. Am.
Chem. Soc. 1980, 102, 6744–6752.
(77) Rupprecht, G. A.; Messerle, L. W.; Fellmann, J. D.; Schrock, R. R.
J. Am. Chem. Soc. 1980, 102, 6236–6244.

Synthesis and ReactiVity of Tantalum Complexes
Organometallics, Vol. 27, No. 23, 2008 6131
Scheme 4
preparation of the desired mixed benzyl complexes to probe
the electronic effects of the R-abstraction process.
spectrum of 12 at room temperature shows only a broad peak
corresponding to the TaCH3 groups. A variable temperature 1H
NMR study revealed that at low temperatures (below -20 °C)
the broad peak decoalesces into three singlets. This behavior is
consistent with an exchange process for the methyl groups. The
low temperature static structure displays three chemically
different methyl groups.
Complex 12 was characterizated crystallographically (Figure
9) and represents the first structurally characterized example of
a thiophene tantalum complex. The Ta-S bond length (2.773(2)
Å) is among the longest reported in the Cambridge Structural
Database. Mononuclear tantalum(V) complexes displaying
Ta-S bond lengths longer than 2.7 Å involve SEt2 and
(PhSCH2)2 adducts positioned trans to multiple bound ligands
Synthesis and Characterization of a Trimethyltantalum
Complex Supported by a Bis(phenolate)thiophene Ta(CH3)3-
[(OC6H2-tBu2)2SC4H2] (12). Following the success of metallating
the bis(phenolate)pyridine ligand, five-member ring linkers have
been investigated. A solution of thiophene-2,5-diphenol (2) and
TaCl2(CH3)3 was allowed to stir at room temperature overnight.
Only unreacted diphenol is observed by 1H NMR spectroscopy.
The increased steric bulk of the larger sulfur substituent is
possibly the reason behind the reduced reactivity of the free
phenol. The salt metathesis route proved successful, however.
Phenol deprotonation with KBn or KN(SiMe3)2, followed by
reaction with TaCl2(CH3)3, leads to the clean formation of one
species assigned as Ta(CH3)3[(OC6H2-tBu2)2SC4H2] (12, eq 2)
1
1
based on H NMR spectroscopy. Surprisingly, the H NMR
Figure 7. Plot of the dependence of the observed rate constant on
the concentration of dimethylphenylphosphine for the conversion
of 7 to 11-P.
Figure 6. Kinetic plots for the conversion of 7 (blue) and 7-d6
(red) with dimethylphenylphosphine to 11-P.

6132 Organometallics, Vol. 27, No. 23, 2008
Agapie et al.
Synthesis and Characterization of Tantalum Complexes
Supported by a Bis(phenolate)furan Ligand. The bis(pheno-
late)furan ligand (3) was found to metalate well via both alkane
elimination and salt metathesis routes. A colorless benzene
solution of TaCl2(CH3)3 turns slightly orange upon addition of
furan-2,5-diphenol, and the color darkens upon stirring at room
temperature for a few hours (eq 3). The desired product,
TaCl2(CH3)[(OC6H2-tBu2)2OC4H2] (13), was isolated as an
orange powder upon precipitation from cold petroleum ether.
The rapid methane elimination with the furan-linked ligand
stands in contrast to the behavior of the thiophene linked system,
lending support to the steric argument for the lower reactivity
of the latter. For the salt metathesis route, potassium diphenolate
was used in the reaction with TaCl2(CH3)3, leading to the
isolation of Ta(CH3)3[(OC6H2-tBu2)2OC4H2] (14) as an off-white
powder (eq 4). Similar to the trimethyl compounds supported
by the pyridine- and thiophene-linked diphenolates, compound
14 displays an unexpected 1H NMR spectrum; the three tantalum
methyl groups display only one singlet at room temperature. A
variable temperature 1H NMR study revealed that lowering the
temperature leads to a broadening of the Ta-CH3 peak eventually
leading to decoalescence around -55 °C. At -90 °C, the methyl
groups display two sharp singlets in a two to one ratio. This
behavior reflects a fluxional process as described above, which
exchanges the methyl groups. The lowest energy structure is
likely that depicted in eq 4, with two equivalent methyls and
one different.
Figure 8. Eyring plots for the conversion of 7 (blue) and 7-d6 (red)
to 11-P.
withstrongtransinfluence(alkylideneandsufide,respectively).78,79
Complex 12 displays a meridional coordination mode of the
thiophene-diphenolate moiety with a few additional notable
features. The angle between the thiophene plane and the Ta-S
vector is 104°, significantly more acute than in the pyridine
linked system, possibly due to the bigger sulfur atom leading
to a longer Ta-S distance which, in turn, rotates the thiophene
ring away from tantalum. Alternatively, the involvement of the
sulfur lone pair in aromatic delocalization is diminished such
that the degree of sp2 hybridization of sulfur is smaller compared
to that of nitrogen. Consequently, the thiophene sulfur is more
likely to coordinate to tantalum more perpendicular to the ring
rather than parallel. Another difference consists in the orientation
of the tantalum methyl groups. Unlike in the bis(phenolate)py-
ridine system, the two methyl groups cis to the thiophene donor
move closer to the methyl trans to sulfur (C34-Ta-C33 and
C34-Ta-C35 are 76.2(2)° and 76.5(2) °, respectively). This
could be due to the larger size of the sulfur compared to
nitrogen, which may force the methyl groups away from the
thiophene donor. Interestingly, the three methyl groups and
tantalum are not found in the same plane, and the overall
symmetry is reduced C1. Thus, the solid-state structure is
consistent with the low temperature 1H NMR data, with all the
tantalum methyl groups chemically different. In order to
equivalence the two methyls cis to sulfur, the thiophene ring
must pivot roughly 180° about its 2,5-phenolate linkages, forcing
the sulfur closer to tantalum, a process that has a substantial
Complex 13 was examined by single crystal X-ray diffraction
(Figure 10) and confirms it as the first structurally characterized
example of a furan tantalum complex. The Ta-O bond length
(2.405(2) Å) is longer than a typical tantalum-ether bond.
Complex 13 displays a meridional coordination mode of the
furan-diphenolate moiety. As the molecule is found on a C2
crystallographic axis, the TaO vector lies in the plane of the
furan ring. Given that the twist of the two phenolate rings is
small (6° from the plane of the furan), this molecule is almost
C2V symmetric.
barrier of ca. 15 kcal · mol-1
.
(78) Wallace, K. C.; Davis, W. M.; Schrock, R. R. Inorg. Chem. 1990,
29, 1104–1106.
(79) Drew, M. G. B.; Rice, D. A.; Williams, D. M. J. Chem. Soc., Dalton
Trans. 1984, 845–848.

Synthesis and ReactiVity of Tantalum Complexes
Organometallics, Vol. 27, No. 23, 2008 6133
Figure 9. Drawings of the structure of 12. Selected bond lengths (Å) and angles (deg): Ta-O1 1.9119(38); Ta-O2 1.9136(34); Ta-C33
2.1676(66); Ta-C34 2.2164(65); Ta-C35 2.2179(62); Ta-S 2.7730(16); O2-Ta-O3 149.23(16); C34-Ta-C33 76.2(2); C34-Ta-C35
76.5(2); C16-O2-Ta 152.4(3); C1-O1-Ta 155.2(3); Ta-S-C8 103.6; Ta-S-C9 102.5.
Figure 10. Drawings of the structure of 13. Selected bond lengths (Å) and angles (deg): Ta-O1 1.8692(14); Ta-C17 2.154(2); Ta-O3
2.4052(18); O1-Ta-O1A 152.34(9); C1-O1-Ta 150.10(12).
Methyl Exchange Processes in the Tantalum Trimethyl
Species 6, 12, and 14. High temperature H NMR spectra of
state structures display mer binding of the tridentate ligand and,
accordingly, mer binding of the three methyl groups. One
possible pathway would involve twists of the trigonal faces of
the octahedral structure (Scheme 5).80,81 An alternative pathway
1
compounds 6, 12, and 14 display only one signal at high
temperature, indicative of a fast methyl exchange process. While
the precise nature of this process is not well understood, it is
probable that a significant distortion of the bis(phenolate)donor
ligand framework is required to interchange methyls. The solid
(80) Marinescu, S. C.; Agapie, T.; Day, M. W.; Bercaw, J. E.
Organometallics 2007, 26, 1178–1190.

6134 Organometallics, Vol. 27, No. 23, 2008
Agapie et al.
Scheme 5
Scheme 6
could involve decoordination of the donor on the linker to
generate a five-coordinated species, which could undergo Berry
pseudorotations. In order for the alkyl groups to exchange by
either mechanism, they need to access a trigonal face of an
octahedral (or trigonal bipyramidal) structure. In this context,
the two oxygens would have to be cis, a geometry that has not
been documented in these tantalum systems. Given the presence
of aryl-aryl linkages in the multidentate framework, facial
binding modes are expected to be strained and higher in energy
than meridional binding modes. The observed sharp singlets at
room temperature for 6 and 14 indicate, however, that these
LX2 ligands are quite flexible, being able to easily attain transient
facial geometries.
Preparation of Tantalum Complexes Supported by a
Benzene-Linked Bis(phenolate) Ligand. Two strategies have
been used for the metalation of the benzene-linked diphenol 4:
salt metathesis and alkane elimination.58 For the salt metathesis
route, diphenol 4 was deprotonated in situ with KCH2Ph and
then treated with TaCl2(CH3)3. Within hours, the reaction was
complete with the generation of one major species 16 (1H NMR).
Recrystallization from petroleum ether afforded analytically
1
via intermediate generation of a simple salt metathesis product,
complex 15, bound through the oxygens (Scheme 6). This
species then cyclometallates, with loss of methane. Utilization
of TaCl2(CH2Ph)3 instead of TaCl2(CH3)3 as a metal precursor
in this reaction leads to the formation of analogous bis(alkyl)
Ta(CH2Ph)2[(OC6H2-tBu2)2C6H3] (17). Previous reports of cy-
clometallated pincer ligand complexes of early metals rely on
using starting materials in which the hydrogen connected to the
metallated carbon had been already replaced with a transmet-
allating agent (Grignard reagents or lithium aryls). The CH (or
CC) activation route for accessing the cyclometallated pincer
frameworks is, however, common for late transition metal
systems.89 The preparations described here rely on a CH
activation at the desired position upon coordination of the ligand
to the metal center. These reactions are reminiscent of cyclo-
metalations via σ-bond metathesis observed for tantalum systems
with phenolate ligands ortho-substituted with t-butyl, i-propyl,
or phenyl groups.82,85,90-95 An interesting comparison is
clean material. The H NMR spectrum shows a singlet corre-
sponding to the Ta-CH3 groups integrating to 6H, and only
two peaks (a doublet and a triplet) for the linking benzene group
integrating to 3H. The 13C NMR spectrum of the product
presents two diagnostic peaks, at 60.6 and 198.5 ppm. A survey
of some previously reported tantalum(V) methyls with at least
two phenolate ligands indicates that the 13C NMR chemical shift
range for the methyl group is between 43 and 64 ppm.82,83 The
13C NMR chemical shift for Ta-C(ipso) is 190-210 ppm.50,84-88
Thesedatasupportthestructuralassignmentof16asTa(CH3)2[(OC6H2-
tBu2)2C6H3] (Scheme 6) Formation of 16 presumably occurs
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(94) Chamberlain, L. R.; Kerschner, J. L.; Rothwell, A. P.; Rothwell,
I. P.; Huffman, J. C. J. Am. Chem. Soc. 1987, 109, 6471–6478.

Synthesis and ReactiVity of Tantalum Complexes
Organometallics, Vol. 27, No. 23, 2008 6135
Figure 11. Drawings of the structure of 17. Selected bond lengths (Å) and angles (deg): Ta-O1 1.904(2); Ta-O2 1.903(2); Ta-C34
2.176(3); Ta-C35 2.190(4); Ta-C42 2.203(4); O1-Ta-O2 169.17(9); Ta-C35-C36 93.2(2); Ta-C42-C43 96.4(2); C17-O2-Ta 142.9(2);
C1-O1-Ta 141.6(2); C34-Ta-C35 117.86(13); C34-Ta-C42 118.24(14); Ta-C34-C9 156.5.
Figure 12. Drawings of the structure of 18. Selected bond lengths (Å) and angles (deg): Ta-O1 1.876(4); Ta-O2 1.859(4); Ta-C35
2.147(4); Ta-C18 2.7913(47); Ta-C18-C9 120°; O1-Ta-O2 152.45(15); Ta-O1-C1 151.7(3); Ta-O2-C17 149.1(4).
provided by the previously characterized Ta(CH3)3(OC6H3-2,6-
Ph2)2 and Ta(CH2Ph)3(OC6H3-2,6-Ph2)2 that lose CH4 only above
200 °C and toluene above 175 °C, respectively.85 By contrast,
the related proposed tantalum trialkyl species 15 is only a
transient in the formation of the cyclometallated product 16 at
room temperature. This suggests that the pincer nature of the
ligand, with initial chelation of the two phenolates facilitates
the cyclometalation, possibly by locking the aryl group into the
necessary orientation for σ-bond metathesis.
The structure of complex 17 was investigated by single-crystal
X-ray diffraction (Figure 11). Similarly to the pyridine-linked
systems, this molecule displays pseudo-Cs symmetry. The angle
between the Ta-C(phenyl) vector and the plane of the bridging
benzene group is, however, about 10° larger than that in the
pyridine case, and the Ta-C(benzene) distance is shorter
compared to the Ta-N(pyridine) distance, likely a consequence
of the greater strength of the normal covalent Ta-C bond Vis-
a`-Vis the dative Ta-N bond of the latter.
An alkane elimination route was explored for the metalation
of 4 with TaCl2(CH3)3 leading at room temperature to a species
that still displays signals for a TaCH3 group and the C(ipso)-H
1
in the H and 13C NMR spectra (compound 18, Scheme 6). A
single crystal X-ray diffraction study supports the spectroscopic
assignment of 18 (Figure 12). The tantalum center is six-
coordinate, taking into account the weak interaction with the
arene ipso-carbon (Ta-C bond length of 2.791(5) Å). The
methyl group is located trans to site of the arene linker, the
position with the smallest trans influence. The other bond lengths
to tantalum are typical. The Ta-C18-C8 angle of 120°
indicates a significant twist of the arene, probably to avoid a
steric interaction between tantalum and the ring as well as to
(95) Chamberlain, L.; Keddington, J.; Rothwell, I. P.; Huffman, J. C.
Organometallics 1982, 1, 1538–1540.

6136 Organometallics, Vol. 27, No. 23, 2008
Agapie et al.
Figure 14. Kinetic plots for the cyclometalation of 18 at different
temperatures (91 °C, red triangles; 101 °C, blue squares; 111 °C,
red diamonds; 125 °C, blue circles).
Figure 13. Drawings of the structure of 19-OEt2. Selected bond
lengths (Å) and angles (deg): Ta-O1 1.8828(13); Ta-O2 1.8755(13);
Ta-O3 2.2446(14); Ta-C15 2.1870(19); Ta-C15-C18 164.2;
O1-Ta-O2 168.75(6); C21-O2-Ta 145.34(13); C1-O1-Ta
142.51(13).
increase a bonding interaction between the metal center and
the π-system.
The isolation of compound 18 provides support for the
proposed route for the formation of 16 (Scheme 6) by affording
isolation of the diphenolate species without C-H activation.
In this case, the cyclometalation reaction is not as facile as for
the putative Ta(CH3)3[(OC6H2-tBu2)2C6H3] (15). Access to 18
also provides an opportunity to perform kinetic studies of the
cyclometalation reaction. To that end, the synthesis of isotopi-
cally labeled compounds was undertaken. In this context, an
unexpected mechanism for metalation of the benzene-linked
bis(phenol) with TaCl2(CH3)3 to generate 4 was uncovered.58
As reported earlier, this process involves protonolysis of a
methyl group, followed by C-H/Ta-CH3 σ bond metathesis
leading to cyclometalation of the linking benzene, and finally
protonation of the cyclometallated group by the pendant phenol.
Compound 18 does indeed undergo σ-bond metathesis at
elevated temperatures, leading to the cyclometallated product
19 (Scheme 6) along with CH4. Crystals of the cyclometallated
product, suitable for an X-ray diffraction study, were obtained
from a diethyl ether solution (Figure 13). An ether molecule
coordinates to the metal center to satisfy six-coordination. In
contrast with the structure of 18, the Ta-C(ipso) vector almost
lies in the cyclometallated benzene plane (Ta-C15-C18 angle
is 164 °) maximizing the interaction of the metal with carbon
σ-orbital.
Figure 15. Eyring plots for the cyclometalation of 18 (blue) and
18-d1 (red).
cyclometalations with methane elimination for tantalum sup-
ported by monodentate phenolates.90 Isomerization (18 to 20)
is expected to involve some degree of reorganization, possibly
via a process analogous to that shown in Scheme 5, leading to
a somewhat negative entropy of activation, especially for a
semirigid terphenyl system.80,81
To investigate further the possibility that the isomerization
from 18 to 20 is the rate determining step in this transformation,
the (incipient ipso) C-H bond was deuterated (18-d1). Kinetic
data were obtained over the same temperature range, allowing
the derivation of the activation parameters. The experimental
error does not allow a distinction between the slopes of the
cyclometalation reactions from 18 and 18-d1 (Figure 15). Thus,
the isotope effect is essentially the same (kH/kD) 1.6 ( 0.2)
over a range of 34 °C (91-125 °C). These isotope effects are
(marginally) smaller than the one observed for the intermolecular
σ-bond metathesis reactions of Cp*2Sc-CH3 and the C-H bond
of benzene (kH/kD ) 2.8(2) at 80 °C),96 and than those observed
in cyclometalations of t-butyl groups ortho to tantalum pheno-
lates (kH/kD ) 5.2(4) at 180 °C, kH/kD ) 2.3(5) at 118 °C, and
kH/kD)1.9(5) at 135 °C for different ligand sets.90,94 The
observation of an isotope effect for the present system, even
though relatively small, suggests that the rate determining step
does involve σ-bond metathesis (20 to 19).
Unimolecular conversion of 18 to 19 almost certainly requires
an isomerization to place the reactive (incipient ipso) C-H bond
and the Ta-CH3 cis as in 20 (Scheme 6). To examine the
molecularity of the process, a kinetic study was undertaken.
The reaction was performed in C6D5Br over a range of 34 °C
(91-125 °C) and found to obey kinetics first order in 18 (Figure
14) supporting the proposal of a unimolecular σ-bond metathesis.
The Eyring analysis yielded ∆Hq ) 27.1 ( 0.9 kcal · mol-1
;
(96) Thompson, M. E.; Baxter, S. M.; Bulls, A. R.; Burger, B. J.; Nolan,
M. C.; Santarsiero, B. D.; Schaefer, W. P.; Bercaw, J. E. J. Am. Chem.
Soc. 1987, 109, 203–219.
∆Sq ) -2 ( 2 cal · mol-1 · K-1 (Figure 15). The activation
enthalpy and entropy values are similar to those observed for

Synthesis and ReactiVity of Tantalum Complexes
Organometallics, Vol. 27, No. 23, 2008 6137
Scheme 7
Figure 16. Drawing of the structure of 22. Selected bond lengths
(Å) and angles (deg): Ta-O1 1.912(3); Ta-O2 1.919(4); Ta-N
2.037(4); Ta-C42 2.148(6); Ta-C43 2.163(6); O1-Ta-O2
156.34(15);Ta-N-C19114.2(4);C17-O2-Ta142.1(3);C1-O1-Ta
143.6(3); N-Ta-C42 114.7(2); N-Ta-C43 140.9(2); Ta-C18-C9
110.2.
Insertion Chemistry of Ta(CH3)2[(OC6H2-tBu2)2C6H3]
(16): Reactions with Benzophenone, Benzonitrile, and
t-Butylisonitrile. Compound 16, with two types of Ta-C single
bonds provides an interesting case for the study of insertion
chemistry with various substrates. The reaction with excess (1.5
equiv) benzophenone was performed in benzene-d6 at 60 °C.
Clean formation of a single product is observed after 12 h. The
nature of the insertion product is suggested by 13C and 1H NMR
spectroscopic data. In the 13C NMR spectrum, the peak
corresponding to the [Ta-CH3] groups of the starting material
appears at 60.6 ppm; this was replaced in the product by two
peaks with a similar chemical shift (60.8 and 62.7 ppm). If one
of these methyl groups is carbon-bound, it should shift
significantly upfield. Hence, this suggests a loss of symmetry
of the molecule, but no insertion into the Ta-CH3 bonds,
consistent with structure 21 (Scheme 7). Furthermore, the Ta-
C(ipso) peak, which has a chemical shift of 198.5 ppm in the
starting material, is absent from this region in the product.
Finally, a single crystal X-ray diffraction study confirmed the
structural assignment (Figure 4, Supporting Information), but
the quality of the data set precluded reliable determination of
structural parameters. The clean formation of only one product
suggests a significant preference for the insertion into the Ta-
aryl bond. The reasons behind this selectivity are currently not
well understood. It is also noteworthy that the excess substrate
does not insert into the Ta-CH3 bonds still available after the
first insertion into Ta-aryl.
Reaction of 16 with benzonitrile was performed under
conditions similar to the ones with benophenone. The 13C NMR
spectrum of the reaction product shows that the two [Ta-CH3]
groups become inequivalent, but their chemical shifts (57.0 and
59.1 ppm) remain in the expected region for metal-bound alkyls.
No signal is observed in the [Ta-C(phenyl)] region. The ketimide
carbon resonance appears at 169.9 ppm. These data are
consistent with insertion into the Ta-phenyl bond to generate
compound 22 (Scheme 7). Interestingly, if less than one
equivalent of PhCN is used, the reaction proceeds with the
formation of an additional product. It may be possible that the
imide functionality further reacts with another equivalent of
tantalum starting material to give a species as yet unidentified.
The solid structure of 22 was determined by a single crystal
X-ray diffraction study (Figure 16). The tantalum center is five
coordinate, with a distorted trigonal bipyramid geometry. The
Ta-N-C19 angle is very acute because of the geometric
constraints of this unusual chelating ligand. The angle between
the Ta-C19 vector and the bridging benzene ring is acute as
well, probably to accommodate the two atom bridge (CN)
between Ta and C19 resulting from the insertion of PhCN.
To investigate the scope of insertion chemistry with complex
16, the reaction with tBuNtC was also explored. It was found
that the reaction with two equivalents of isonitrile occurs fast,
reaching completion within 15 min at room temperature, to
1
cleanly generate one species. The H NMR spectrum of the
product shows only one singlet corresponding to the [Ta-CH3]
groups from the starting material. The 13C NMR spectrum
displays a peak at 179.6 ppm corresponding to Ta-C(phenyl).
Furthermore, no peaks are observed in the [Ta-CH3] region
(∼55-65 ppm). Instead, one new peak is observed in the
aliphatic region at 22.2 ppm. A new downfield peak (226.0 ppm)
indicates the formation of the iminoacyl moiety. These data are
consistent with 1,1-insertion of isonitrile into the Ta-CH3 bonds
selectively, without any observable insertion into the Ta-phenyl
bond. This assignment was confirmed by a single crystal X-ray
diffraction study (Figure 17). The solid-state structure is
essentially C2-symmetric with the phenolate aryl rings signifi-
cantly twisting away from each other. The O1-Ta-O2 angle
is 166°, similar to the cyclometallated species 19-OEt2. The
iminoacyl groups are both κ2, with the nitrogen coordinated next
to the aryl ligand. The structural parameters of the tantalum-
iminoacyl units are similar to previously reported ones.97-99
On the basis of the findings for the substrates examined thus
far, we may conclude that 1,2-inserting substrates prefer
insertion into the Ta-phenyl bond, while 1,1-inserting substrates
prefer insertion into the Ta-Me bond. While currently not
understood, the complete switch in selectivity observed for the
two types of substrates may result from the geometrical
requirements of the transition state for the insertion into the
Ta-aryl bond. The Ta-phenyl bond, rather constrained in the
chelate, may be able to better accommodate a two-atom rather
than a one-atom insertion.
Conclusions
A series of diphenols connected at the ortho positions via
semirigid, ring-ring linkages to a flat ring (pyridine, furan, or
thiophene) that presents another ligand was efficiently prepared,
and their ability to support tantalum chemistry has been
investigated. Metalations were performed using salt metathesis
(97) Chamberlain, L. R.; Durfee, L. D.; Fanwick, P. E.; Kobriger, L.;
Latesky, S. L.; McMullen, A. K.; Rothwell, I. P.; Folting, K.; Huffman,
J. C.; Streib, W. E.; Wang, R. J. Am. Chem. Soc. 1987, 109, 390–402.
(98) Durfee, L. D.; Rothwell, I. P. Chem. ReV. 1988, 88, 1059–1079.
(99) Chamberlain, L. R.; Rothwell, I. P.; Huffman, J. C. J. Chem. Soc.,
Chem. Commun. 1986, 1203–1205.

6138 Organometallics, Vol. 27, No. 23, 2008
Agapie et al.
was found to undergo 1,2-insertions with benzophenone and
benzonitrile into the Ta-(ipso)C bond and 1,1-insertion of
t-butylisonitrile into the Ta-CH3 bonds.
In the solid state, these pincer ligands were found to bind in
a meridional fashion exclusively, to generate C1, C2V, C2, and
Cs ligand geometries. The observed geometries were mostly
dependent on the size of the ring linking the two phenolates
and the nature of the donor in the ring. While with a furan linker
the three rings can accommodate an almost planar geometry
upon binding tantalum (C2V), with thiophene, having a larger
sulfur donor, the ring system twists to give a C1 structure.
Switching from a five- to a six-member ring linker, the ligand
generally assumes Cs geometries (with one exception, C2 in 23).
This trend is probably due to a smaller angle between the two
ring-ring linkages in the case of 6-membered pyridine and
benzene versus 5-membered furan and thiophene. Overall, the
present ligand frameworks have been found to be thermally
robust and to support tantalum alkyl and alkylidene motifs
lending promise to diverse, potentially useful reactions based
on these architectures.
Experimental Section
General Considerations and Instrumentation. All air- and
moisture-sensitive compounds were manipulated using standard
vacuum line, Schlenk, or cannula techniques, or in a drybox under
a nitrogen atmosphere. Solvents for air- and moisture-sensitive
reactions were dried over sodium benzophenone ketyl or by the
method of Grubbs.100 Benzene-d6 was purchased from Cambridge
Isotopes and distilled from sodium benzophenone ketyl. Chloroform-
d1 and dichloromethane-d2 were purchased from Cambridge
Isotopes and distilled from calcium hydride. Compounds 1,34 2,34
3,34 and 4,58 TaCl2(CH2Ph)3,64 TaCl3(CH2Ph)2,76 Ta(CH2Ph)5,23,64
TaCl3(CH3)2,101 and TaCl2(CH3)3,102,103 were prepared according
to literature procedures. Other materials were used as received. 1H
and 13C NMR spectra were recorded on Varian Mercury 300 or
Varian INOVA-500 spectrometers at room temperature unless
otherwise indicated. Chemical shifts are reported with respect to
internal solvent: 7.16 and 128.38 (t) ppm (C6D6); 7.27 and 77.23
Figure 17. Drawings of the structure of 23. Selected bond lengths
(Å) and angles (deg): Ta-O1 1.9517(11); Ta-O2 1.9415(11);
Ta-N1 2.1414(12); Ta-N2 2.1494(13); Ta-C36 2.1574(15);
Ta-C42 2.1576(15); Ta-C18 2.2256(14); N1-C36 1.275(2);
N2-C42 1.278(2); C(36)-Ta(1)-C(42) 103.50(6); Ta-C36-N1
72.06(9);Ta-C42-N272.39(9);O1-Ta-O2166.30(5);C17-O2-Ta
133.22(10); C1-O1-Ta 129.80(10).
1
(t) ppm (CDCl3); 5.32 and 54.00 (q) ppm (CD2Cl2); for 1H and 13
data.
C
and alkane elimination routes. Variable temperature H NMR
studies of tantalum trimethyl species indicate that, in solution,
these complexes are fluxional, leading to exchange of the methyl
groups. A tantalum tribenzyl species supported by the bis(phe-
nolate)pyridine framework undergoes R-H abstraction upon
heating to 90 °C in the presence of Lewis bases. This process
exhibiting first order rate for the disappearance of the trialkyl
species was found to be independent of phosphine concentration
and kH/kD ) 4.9 ( 0.4 (125 °C), consistent with a mechanism
involving rate determining R-H abstraction in the six-cordinate
species to give a five-coordinate benzylidene, which is trapped
by the phosphine in a fast, subsequent step.
A diphenol connected in the ortho-position via a 1,3-
benzenediyl group, related conceptually to the diphenols above,
was prepared, and its ability to support tantalum chemistry has
been investigated as well. An alkane elimination route with
Ta(CH3)3Cl2 leads to the isolation of Ta(CH3)Cl2[(OC6H2-
tBu2)2C6H4] by a mechanism implicating that the C-H/Ta-CH3
σ-bond metathesis is faster than the O-H/M-CH3 protonolysis.
High temperature kinetics and labeling studies of the cyclom-
etalationofTa(CH3)Cl2[(OC6H2-tBu2)2C6H4]togiveTaCl2[(OC6H2-
tBu2)2C6H4] suggest that σ-bond metathesis is the rate deter-
mining step, rather than isomerization to place the methyl group
cis to the C-H bond to be activated. Salt metathesis routes into
tantalum chemistry afforded access to cyclometallated species
Ta(CH3)2[(OC6H2-tBu2)2C6H3] and Ta(CH2Ph)2[(OC6H2-
tBu2)2C6H3] at room temperature. Ta(CH3)2[(OC6H2-tBu2)2C6H3]
Synthesis of 5. KBn (139 mg, 1.07 mmol, 2 equiv) was added
with the aid of C6H6 (5 mL) to a solution of 1 (260 mg, 0.534
mmol, 1 equiv) in C6H6 (10 mL). This mixture was allowed to stir
at room temperature for 2 h during which the orange KBn dissolved
and discolored to lead to the formation of a colorless emulsion of
phenolate. A C6H6 solution of TaCl3(CH2Ph)2 (250 mg, 0.534 mmol,
1 equiv) was added, and the reaction mixture was allowed to stir
at room temperature for 5 h. Salts were removed by filtration
through a bed of Celite, and volatile materials were removed under
vacuum. The resulting residue was suspended in petroleum ether,
and the mixture was cooled to -35 °C. The desired product was
collected by filtration as an orange solid and was washed with cold
petroleum ether. This procedure affords 0.460 g (97% yield) of 5.
1H NMR (500 MHz, CD2Cl2) δ: 1.40 (s, 18H, C(CH3)3), 1.60 (s,
18H, C(CH3)3), 2.39 (s, 4H, Ta(CH2)2), 6.26 (d, 4H, o-C6H3-H2),
6.60-6.67 (m, 6H, m- and p-C6H2-H3), 7.23 (d, 2H, aryl-H), 7.60
(d, 2H, aryl-H), 7.74 (d, 2H, 3,5-NC5H-H2), 7.84 (t, 1H, NC5H2-
H). 13C NMR (125 MHz, CD2Cl2) δ: 30.6 (C(CH3)3), 31.9
(C(CH3)3), 35.2 (C(CH3)3), 35.7 (C(CH3)3), 75.2 (TaCH2), 125.3,
(100) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J. Organometallics 1996, 15, 1518–1520.
(101) Fowles, G. W. A.; Rice, D. A.; Wilkins, J. D. J. Chem. Soc., Dalton
Trans. 1973, 961–965.
(102) Juvinall, G. L. J. Am. Chem. Soc. 1964, 86, 4202–4203.
(103) Schrock, R. R.; Sharp, P. R. J. Am. Chem. Soc. 1978, 100, 2389–
2399.

Synthesis and ReactiVity of Tantalum Complexes
Organometallics, Vol. 27, No. 23, 2008 6139
Table 1. Crystal and Refinement Data for Complexes 6, 7, 10, and 11-P
6
7
10
11-P
empirical formula
formula weight
T (K)
C36H52NO2Ta
711.74
100(2)
C54H64NO2Ta · 1/2C6H14
983.10
100(2)
C40H50NO2F2Ta · C6H6
873.87
100(2)
C55H68NO2PTa · 2.5(C6H6)
1182.29
100(2)
a (Å)
b (Å)
c (Å)
11.1983(3)
11.4259(3)
14.4511(4)
88.7580(10)
75.2440(10)
70.9590(10)
1686.39(8)
2
11.6884(3)
18.9587(4)
21.6301(5)
13.6879(4)
28.6266(7)
11.3658(3)
29.1034(6)
43.7491(9)
19.1359(4)
R (deg)
ꢀ (deg)
95.3950(10)
113.9359(10)
γ (deg)
volume (Å3)
Z
4771.93(19)
4
4070.57(19)
4
24364.7(9)
16
crystal system
space group
dcalc (g/cm3)
θ range (deg)
triclinic
P-1
1.402
1.89 to 47.50
3.289
SADABS
1.206
0.0392, 0.0703
monoclinic
P21/n
1.368
1.89 to 42.95
2.346
SADABS
1.216
monoclinic
P21/c
1.426
1.63 to 40.73
2.747
SADABS
1.229
orthorhombic
Fdd2
1.289
1.68 to 41.03
1.875
none
µ (mm-1
)
abs. correction
GOF
1.095
0.0415, 0.0665
b
R1,a wR2 [I > 2σ(I)]
0.0389, 0.0626
0.0398, 0.0580
b
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. wR2 ) [Σ[w(Fo - Fc2)2]/Σ[w(Fo )2]1/2
.
2
2
Table 2. Crystal and Refinement Data for Complexes 12, 13, 17, and 18
12
C35H51O2STa
716.77
13
17
18
empirical formula
formula weight
T (K)
C33H45O3Cl2Ta
809.65
100(2)
C48H57O2Ta · CH2Cl2
931.81
2(C35H47O2Cl2Ta) · C6H6
1581.26
100(2)
100(2)
100(2)
a (Å)
b (Å)
c (Å)
9.8813(16)
11.3580(18)
15.332(2)
86.956(3)
76.513(3)
80.423(3)
1649.7(5)
2
15.0003(3)
15.1231(3)
16.5600(4)
95.7320(10)
97.0480(10)
91.9980(10)
3705.40(14)
4
10.7524(5)
11.2822(5)
18.5744(8)
88.9940(10)
82.324(2)
74.9580(10)
1649.7(5)
2
11.2683(7)
17.5151(11)
18.2677(12)
88.7480(10)
88.5720(10)
89.2020(10)
3603.1(4)
2
R (deg)
ꢀ (deg)
γ (deg)
volume (Å3)
Z
crystal system
space group
dcalc (g/cm3)
θ range (deg)
triclinic
P-1
1.443
1.82 to 28.41
3.423
semiempirical from equiv.
1.276
0.0421, 0.0665
triclinic
P-1
1.451
1.35 to 50.18
3.144
none
1.000
0.0390, 0.0748
triclinic
P-1
1.435
1.87 to 27.50
2.710
semiempirical from equiv.
1.818
0.0410, 0.0908
triclinic
P-1
1.457
1.59 to 33.65
3.229
none
1.622
0.0605, 0.1014
µ (mm-1
)
abs. correction
GOF
b
R1,a wR2 [I > 2σ(I)]
b
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. wR2 ) [Σ[w(Fo - Fc2)2]/Σ[w(Fo )2]1/2
.
2
2
Table 3. Crystal and Refinement Data for Complexes 19-OEt2, 22,
and 23
mmol, 1 equiv) in C6H6 (4 mL). This mixture was allowed to stir
at room temperature for 0.5 h, which afforded a pale gray mixture
of deprotonated phenol. A C6H6 solution of TaCl2(CH3)3 (97.3 mg,
0.33 mmol, 1 equiv) was added, and the reaction mixture was
allowed to stir at room temperature for 4 h. Salts were removed by
filtration through a bed of Celite. Volatile materials were removed
19-OEt2
22
23
empirical formula
formula weight
T (K)
C38H53O3Cl2Ta C43H48NO2Ta · C5H12 C46H67N2O2Ta
809.65
863.92
860.97
100(2)
100(2)
100(2)
a (Å)
b (Å)
c (Å)
15.0003(3)
15.1231(3)
16.5600(4)
95.7320(10)°
97.0480(10)
91.9980(10)°
3705.40(14)
4
12.4786(8)
28.9867(18)
12.6931(8)
10.1483(3)
14.5504(4)
15.6639(4)
88.0490(10)
86.7020(10)
73.7530(10)
2216.53(11)
2
1
under vacuum to give the desired product as a white powder. H
NMR (500 MHz, CD2Cl2) δ: 0.40 (s, 9H, Ta(CH3)3), 1.37 (s, 18H,
C(CH3)3), 1.61 (s, 18H, C(CH3)3), 7.37 (d, 2H, aryl-H), 7.56 (d,
R (deg)
1
ꢀ (deg)
102.621(3)
2H, aryl-H), 7.70 (d, 2H, NC5H-H2), 7.92 (t, 1H, NC5H2-H). H
γ (deg)
NMR (500 MHz, CD2Cl2, -90 °C) δ: -0.01 (s, 6H, TaCH3), 0.79
(s, 3H, TaCH3). 1H NMR (500 MHz, C6D6) δ: 0.90 (s, 9H,
Ta(CH3)3), 1.33 (s, 18H, C(CH3)3), 1.73 (s, 18H, C(CH3)3), 7.03
(t, 1H, NC5H2-H), 7.24 (d, 2H, NC5H-H2), 7.34 (d, 2H, aryl-H),
7.73 (d, 2H, aryl-H). 13C NMR (125 MHz, CD2Cl2) δ: 30.2
(C(CH3)3), 31.9 (C(CH3)3), 35.0 (C(CH3)3), 35.9 (C(CH3)3), 56.1
(TaCH3), 124.8, 126.3, 127.4, 128.7, 138.9, 139.2, 144.2, 153.5,
156.9 (aryl). Anal. Calcd. for C36H52NO2Ta (%): C, 60.75; H, 7.36;
N, 1.97. Found: C, 60.77; H, 7.35; N, 1.97.
Synthesis of 7 from TaCl2(CH2Ph)3. KBn (78.5 mg, 0.604
mmol, 2 equiv) was added with the aid of C6H6 (4 mL) to a solution
of 1 (147 mg, 0.302 mmol, 1 equiv) in C6H6 (4 mL). This mixture
was allowed to stir at room temperature for 1.5 h, which afforded
a colorless mixture of deprotonated phenol. A C6H6 solution of
TaCl2(CH2Ph)3 (158.5 mg, 0.302 mmol, 1 equiv) was added, and
volume (Å3)
Z
4480.3(5)
4
crystal system
space group
dcalc (g/cm3)
θ range (deg)
triclinic
P-1
1.451
1.35 to 50.18 1.40 to 29.03
3.144
none
monoclinic
P21/c
1.281
triclinic
P-1
1.290
1.94 to 45.15
2.515
none
1.155
0.0349, 0.0749
µ (mm-1
)
2.489
Gaussian
1.439
abs. correction
GOF
1.000
b
R1,a wR2 [I > 2σ(I)] 0.0390, 0.0748 0.0556, 0.1045
b
a R1 ) Σ||Fo| - |Fc||/Σ|Fo|. wR2 ) [Σ[w(Fo - Fc2)2]/Σ[w(Fo )2]1/2
.
2
2
125.6, 127.2, 127.66, 127.68, 129.0, 131.2, 138.6, 139.0, 141.6,
145.1, 152.1, 156.1 (aryl). Anal. Calcd. for C47H57ClNO2Ta (%):
C, 63.83; H, 6.50; N, 1.58. Found: C, 63.53; H, 6.55; N, 1.58.
Synthesis of 6. KBn (85.2 mg, 0.66 mmol, 2 equiv) was added
with the aid of C6H6 (4 mL) to a solution of 1 (159.6 mg, 0.33

6140 Organometallics, Vol. 27, No. 23, 2008
Agapie et al.
the reaction mixture was allowed to stir at room temperature for
10 h. Salts were removed by filtration through a bed of Celite, and
volatile materials were removed under vacuum. The resulting
residue was suspended in petroleum ether, and the mixture was
cooled to -35 °C. The desired product was collected by filtration
and washed with cold petroleum ether. This procedure affords 168
mg (59% yield) of 7 as an orange powder.
stirring for 5 h, the mixture was allowed to cool to room
temperature, and volatile materials were removed under vacuum.
Petroleum ether was added, and the desired product was recrystal-
lized at -35 °C. Collection by filtration affords 55.4 mg (46% yield)
of 11-P. 1H NMR (500 MHz, CD2Cl2) δ: 1.10 (br s, 6H, P(CH3)2),
1.41 (s, 18H, C(CH3)3), 1.60 (s, 18H, C(CH3)3), 2.20 (s, 2H, TaCH2),
5.96 (d, 2H, aryl-H), 6.23 (t, 1H, aryl-H), 6.40 (t, 2H, aryl-H), 6.79
(t, 1H, aryl-H), 6.9-7.2 (br s and sharp d and t, 9H, aryl-H), 7.31
(t, 2H, aryl-H), 7.46 (d, 2H, aryl-H), 7.55 (d, 2H, aryl-H), 7.68 (t,
1H, aryl-H), 8.59 (s, 1H, TaCHPh). 13C NMR (125 MHz, CD2Cl2)
δ: 13.8 (br P(CH3)2), 30.6 (C(CH3)3), 32.0 (C(CH3)3), 35.0
(C(CH3)3), 35.9 (C(CH3)3), 60.6 (br TaCH2), 119.7, 124.3, 124.6,
126.3, 126.6, 126.9, 127.1, 128.0, 128.4, 128.5, 129.1 (br), 130.8,
130.9, 131.0, 137.7, 139.0, 143.6, 148.3, 153.1, 153.7, 158.7 (aryl),
243.0 (TaCHPh).
Kinetic Measurements for the Conversion of 7 to 11-P. Stock
solutions containing 6 (1.22 · 10-2 M) and PMe2Ph (4.88 · 10-2 to
73.2 · 10-2 M) were prepared in xylene-d10 and stored at -35 °C.
The NMR probe was brought to the desired temperature and
calibrated with an ethylene glycol standard. A J-Young tube charged
with the solution of 6 was utilized in these experiments. The decay
of the Ta-CH2Ph peak was integrated against residual CD2H in
the deuterated solvent. The data were plotted using Microsoft Excel.
Typical NMR spectroscopic data for the kinetic runs are presented
in Figure 16.
Synthesis of 7 from Ta(CH2Ph)5. A C6H6 solution of 1 (314.6
mg, 0.646 mmol, 1 equiv) was added to a Schlenk bomb charged
with a solution of Ta(CH2Ph)5 (411 mg, 0.646 mmol, 1 equiv) in
C6H6 (20 mL total volume). The flask was sealed, immersed in an
oil bath at 60 °C, and stirred for 5 h. Volatile materials were
removed under vacuum, and petroleum ether was added, and the
mixture was stored at -35 °C. Compound 7 was collected by
filtration and washed with cold petroleum ether (484 mg, 79%
yield). 1H NMR (500 MHz, C6D6) δ: 1.35 (s, 18H, C(CH3)3), 1.78
(s, 18H, C(CH3)3), 3.08 (s, 4H, TaCH2), 3.88 (s, 2H, TaCH2), 6.25
(t, 2H, aryl-H), 6.36 (t, 4H, aryl-H), 6.45 (d, 4H, aryl-H), 6.69 (t,
1H, aryl-H), 7.04-7.06 (m, 3H, aryl-H overlap), 7.18 (d, 2H, aryl-
H), 7.52 (t, 2H, aryl-H), 7.72 (t, 2H, aryl-H), 7.78 (t, 2H, aryl-H).
13C NMR (125 MHz, C6D6) δ: 31.1 (C(CH3)3), 32.1 (C(CH3)3),
35.0 (C(CH3)3), 36.0 (C(CH3)3), 73.5 (TaCH2), 81.6 (TaCH2), 122.9,
124.7, 125.2, 126.6, 127.2, 127.8, 128.0, 128.3, 129.2, 132.1, 138.1,
139.3, 143.1, 144.8, 152.3, 154.8, 156.9 (aryl). Anal. Calcd. for
C54H64NO2Ta (%): C, 68.99; H, 6.86; N, 1.49. Found: C, 69.10;
H, 7.38; N, 1.49.
Synthesis of 12. KBn (52.8 mg, 0.406 mmol, 2 equiv) was added
with the aid of C6H6 (2 mL) to a solution of 2 (100 mg, 0.203
mmol, 1 equiv) in C6H6 (5 mL). This mixture was allowed to stir
at room temperature for 5 h. The orange KBn discolored to give a
phosphorescent pale green mixture. TaCl2(CH3)3 (60.4 mg, 0.203
mmol, 1 equiv) in C6H6 (5 mL) was added and the reaction mixture
stirred at room temperature to give a pale yellow solution. Salts
were removed by filtration through a bed of Celite. Volatile
materials were removed under vacuum, and petroleum ether was
added to the residue. Upon cooling to -35 °C, the desired product
was collected as a very pale yellow powder (43.2 mg, 28% yield).
1H NMR (500 MHz, CD2Cl2) δ: 1.01 (v br s, 9H, TaCH3), 1.37 (s,
18H, C(CH3)3), 1.46 (s, 18H, C(CH3)3), 7.14 (s, 2H, SC4H2), 7.50
(d, 2H, aryl-H), 7.52 (d, 2H, aryl-H). 1H NMR (500 MHz, CD2Cl2,
-55 °C) δ: 0.40 (s, 3H, TaCH3), 1.06 (s, 3H, TaCH3), 1.25 (s, 3H,
TaCH3). 13C NMR (125 MHz, CD2Cl2) δ: 30.4 (C(CH3)3), 31.8
(C(CH3)3), 35.1 (C(CH3)3), 35.8 (C(CH3)3), 123.3, 124.7, 126.1,
130.7, 139.2, 141.1, 145.4, 158.1 (aryl). Anal. Calcd. for
C35H51O2STa (%): C, 59.00; H, 6.91. Found: C, 58.65; H, 7.17.
Synthesis of 13. A solution of phenol 3 (50.2 mg, 0.105 mmol,
1 equiv) in C6H6 (5 mL) was added to a solution of TaCl2(CH3)3
(31.2 mg, 0.105 mmol, 1 equiv) in C6H6 (5 mL). The reaction
mixture was allowed to stir at room temperature. Volatile materials
were removed under vacuum, and the desired product was crystal-
lized as an orange solid (53 mg, 68% yield) from petroleum ether
at -35 °C.1H NMR (500 MHz, CD2Cl2) δ: 1.42 (s, 18H, C(CH3)3),
1.75 (s, 18H, C(CH3)3), 2.11 (s, 3H, TaCH3), 7.28 (s, 2H, OC4H2),
Synthesis of 8. A solution of phenol 1 (100 mg, 0.205 mmol, 1
equiv) in C6H6 (3 mL) was added to a solution of TaCl3(CH3)2
(31.2 mg, 0.205 mmol, 1 equiv) in C6H6 (3 mL). The reaction
mixture was allowed to stir for 10 h at room temperature. Volatile
materials were removed under vacuum, and the desired product
was recrystallized from petroleum ether at -35 °C. Compound 8
was obtained as an orange powder (153 mg, 97% yield) upon
1
collection by filtration. H NMR (500 MHz, CD2Cl2) δ: 1.39 (s,
18H, C(CH3)3), 1.63 (s, 18H, C(CH3)3), 7.51 (d, 2H, aryl-H), 7.70
(d, 2H, aryl-H), 8.05 (d, 2H, NC5H-H2), 8.22 (t, 1H, NC5H2-H).
13C NMR (125 MHz, CD2Cl2) δ: 30.4 (C(CH3)3), 31.9 (C(CH3)3),
35.3 (C(CH3)3), 35.7 (C(CH3)3), 126.0, 126.5, 127.7, 128.9, 138.7,
142.1, 147.4, 151.3, 157.3 (aryl).
Synthesis of 9. A solution of phenol 1 (101 mg, 0.207 mmol, 1
equiv) in C6H6 (3 mL) was added to a solution of TaCl2(CH3)3
(61.6 mg, 0.207 mmol, 1 equiv) in C6H6 (3 mL). The reaction
mixture was allowed to stir at room temperature for 3 h. Volatile
materials were removed under vacuum to give the desired product
as a yellow powder. 1H NMR (500 MHz, CD2Cl2) δ: 1.39 (s, 18H,
C(CH3)3), 1.70 (s, 18H, C(CH3)3), 1.82 (s, 3H, TaCH3), 7.45 (d,
2H, aryl-H), 7.68 (d, 2H, aryl-H), 7.86 (d, 2H, NC5H-H2), 8.10 (t,
1H, NC5H2-H). 13C NMR (125 MHz, CD2Cl2) δ: 30.7 (C(CH3)3),
31.9 (C(CH3)3), 35.2 (C(CH3)3), 35.9 (C(CH3)3), 68.3 (TaCH3),
125.4, 126.7, 127.0, 128.2, 138.3, 141.3, 146.7, 151.8, 157.7 (aryl).
Reaction of 7 with [Ph3C][BF4]. A toluene solution of 7 (50.8
mg, 54 µmol, 1 equiv) was added to a slurry of [Ph3C][BF4] (17.8
mg, 54 µmol, 1 equiv) in toluene. The reaction mixture, initially
cloudy and orange, changes to yellow within 2 h, as the insoluble
[Ph3C][BF4] reacts and dissolves. Volatile materials were removed
under vacuum, and the residue was suspended in petroleum ether
and cooled to -35 °C. A precipitate was collected by filtration,
washed with cold petroleum ether, and dried under vacuum to give
10 as an off-white powder. 19F NMR (282 MHz, toluene) δ: 148.7.
1H NMR (300 MHz, C6D6) δ: 1.30 (s, 18H, C(CH3)3), 1.61 (s, 18H,
C(CH3)3), 3.81 (br t, JHF ) 5.3 Hz, 2H, TaCH2), 6.86-6.96 (app t,
2H, aryl-H), 7.08 (d, 2H, aryl-H), 7.28 (t, 2H, aryl-H), 7.51 (d,
2H, aryl-H), 7.68 (d, 2H, aryl-H).
1
7.64 (d, 2H, aryl-H), 7.75 (d, 2H, aryl-H). H NMR (500 MHz,
CD2Cl2, -55 °C) δ: -0.16 (s, 6H, TaCH3), 0.89 (s, 3H, TaCH3).
13C NMR (125 MHz, CD2Cl2) δ: 30.7 (C(CH3)3), 31.8 (C(CH3)3),
35.3 (C(CH3)3), 36.0 (C(CH3)3), 70.1 (TaCH3), 110.9, 121.1, 122.2,
125.2, 139.4, 147.2, 150.8, 152.6 (aryl). Anal. Calcd. for
C33H45Cl2O3Ta (%): C, 53.45; H, 6.12. Found: C, 53.50; H, 5.92.
Synthesis of 14. KBn (53.1 mg, 0.408 mmol, 2 equiv) was added
with the aid of C6H6 (4 mL) to a solution of 3 (97.3 mg, 0.204
mmol, 1 equiv) in C6H6 (4 mL). This mixture was allowed to stir
at room temperature for 0.5 h, which afforded a pale gray mixture
of deprotonated phenol. A C6H6 solution of TaCl2(CH3)3 (60.7 mg,
0.204 mmol, 1 equiv) was added, and the reaction mixture was
allowed to stir at room temperature for 1 h. Salts were removed by
filtration through a bed of Celite. Volatile materials were removed
Synthesis of 11-P. A toluene (5 mL) solution of 7 (115 mg,
0.122 mmol, 1 equiv) and PMe2Ph (84.5 mg, 0.612 mmol, 5 equiv)
was placed in a Schlenk flask fitted with a screw-in Teflon stopper.
The flask was sealed and immersed in an oil bath at 125 °C. Upon
1
under vacuum to give the desired product as a white powder. H

Synthesis and ReactiVity of Tantalum Complexes
Organometallics, Vol. 27, No. 23, 2008 6141
NMR (500 MHz, CD2Cl2) δ: 0.32 (s, 9H, Ta(CH3)3), 1.40 (s, 18H,
C(CH3)3), 1.65 (s, 18H, C(CH3)3), 6.95 (s, 2H, OC4H2), 7.54 (d,
2H, aryl-H), 7.61 (d, 2H, aryl-H). 13C NMR (125 MHz, CD2Cl2)
δ: 30.3 (C(CH3)3), 31.8 (C(CH3)3), 35.1 (C(CH3)3), 36.0 (C(CH3)3),
58.2 (TaCH3), 110.8, 121.1, 123.6, 124.9, 139.9, 144.5, 151.8, 152.2
(aryl). Anal. Calcd. for C35H51O3Ta (%): C, 59.99; H, 7.34. Found:
C, 60.32; H, 7.53.
145.3, 147.7, 157.5 (aryl). Anal. Calcd. for C35H47Cl2O2Ta (%): C,
55.93; H, 6.30. Found: C, 54.81; H, 6.06.
Preparation of 19. A C6D6 (0.6 mL) solution of 18 (15 mg)
was placed in a J-Young tube, sealed, and immersed almost
completely in an oil bath at 110 °C, behind a blast shield. After
1
10 h, the sample was cooled to room temperature. H and 13C
spectroscopic analyses show the clean formation of the species
assigned to 18. 1H NMR (500 MHz, C6D6) δ: 1.35 (s, 18H,
C(CH3)3), 1.74 (s, 18H, C(CH3)3), 7.31 (t, 1H, C6H2-H), 7.66 (d,
2H, aryl-H), 7.85 (d, 2H, aryl-H), 8.04 (d, 2H, C6H-H2). 13C NMR
(125 MHz, C6D6) δ: 31.4 (C(CH3)3), 32.2 (C(CH3)3), 35.3
(C(CH3)3), 35.9 (C(CH3)3), 124.4, 124.5, 128.3, 133.9, 135.2, 138.2,
144.2, 146.5, 154.0 (aryl), 205.4 (aryl-CTa).
Kinetic Measurements for the Conversion of 17 to 18. Stock
solutions containing 17 and Ph2CH2 as a standard were prepared
in C6D5Br and stored at -35 °C. The NMR probe was brought to
the desired temperature and calibrated with an ethylene glycol
standard. After the NMR run was complete, the temperature was
checked again. Deflections less than a degree were found indicating
temperature stability during the experiment. A J-Young tube charged
with the solution of 17 was utilized in these experiments. The decay
of the Ta-CH3 peak was integrated against the methylene
hydrogens of the standard Ph2CH2. Data were acquired for at least
three half-lives. The data were plotted using Microsoft Excel. The
standard error for each rate constant measurement was found to be
less than 1% using the regression function in Excel.
Compound 21 (Ph2CO Insertion). A toluene (5 mL) solution
containing Ph2CO (26 mg, 1.4 µmol, 1 equiv) and 16 (100 mg, 1.4
µmol, 1 equiv) was placed in a Schlenk flask with a screw-in Teflon
stopper. Upon sealing, the flask was immersed in an oil bath at 60
°C and stirred for 12 h. Volatile materials were removed under
vacuum, and the residue was triturated with petroleum ether.
Recrystallization from petroleum ether at -35 °C allows the
isolation of 21 (62.8 mg, 50%) by filtration, as a white powder. 1H
NMR (500 MHz, CD2Cl2) δ: 0.23 (s, 3H, TaCH3), 1.22 (s, 18H,
C(CH3)3), 1.31 (s, 3H, TaCH3), 1.54 (s, 18H, C(CH3)3), 6.68 (app
tt, 2H, 2 p-C6H4-H), 6.72 (app td, 2H, 2 m-C6H4-H), 6.76 (d, 2H,
aryl-H), 6.86 (app td, 2H, 2 m-C6H4-H), 6.95 (app d, 2H, 2 o-C6H4-
H), 7.03 (d, 2H, aryl-H), 7.26 (app dt, 2H, 2 o-C6H4-H), 7.28 (d,
2H, C6H-H2), 7.53 (d, 1H, C6H2-H). 13C NMR (125 MHz, CD2Cl2)
δ: 30.1 (C(CH3)3), 31.7 (C(CH3)3), 34.6 (C(CH3)3), 35.6 (C(CH3)3),
60.9 (TaCH3), 62.0 (TaCH3), 84.0 (OC), 123.2, 125.3, 126.1, 126.2,
126.4, 127.6, 127.9, 130.7, 133.7, 135.3, 135.7, 136.0, 142.6, 145.6,
151.0, 157.5 (aryl). Anal. Calcd. for C49H59O3Ta (%): C, 67.11; H,
6.78. Found: C, 68.03; H, 6.95.
Compound 22 (PhCN Insertion). A toluene (5 mL) solution
containing PhCN (30 mg, 2.9 µmol, 2 equiv) and 16 (100 mg, 1.4
µmol, 1 equiv) was placed in a Schlenk flask with a screw-in Teflon
stopper. Upon sealing, the flask was immersed in an oil bath at 60
°C and stirred for 12 h. Volatile materials were removed under
vacuum, and the residue was triturated with petroleum ether.
Recrystallization from petroleum ether at -35 °C allows the
isolation of 22 (53.3 mg, 46%) by filtration, as a white powder. 1H
NMR (500 MHz, CD2Cl2) δ: 0.23 (s, 3H, Ta(CH3)), 1.20 (s, 18H,
C(CH3)3), 1.22 (s, 3H, Ta(CH3)2), 1.52 (s, 18H, C(CH3)3), 6.98 (d,
2H, aryl-H), 7.06 (t, 2H, C6H-H2), 7.12-7.16 (d and t overlapped,
3H, aryl-H), 7.16 (d, 2H, aryl-H), 7.35 (d, 2H, aryl-H), 7.68 (t,
1H, aryl-H). 13C NMR (125 MHz, CD2Cl2) δ: 30.1 (C(CH3)3), 31.7
(C(CH3)3), 34.8 (C(CH3)3), 35.6 (C(CH3)3), 56.4 (TaCH3), 59.2
(TaCH3), 123.9, 125.0, 126.2, 126.3, 128.3, 130.5, 131.8, 132.2,
133.1, 138.2, 139.8, 141.5, 144.7, 158.5 (aryl), 169.8 (NC). Anal.
Calcd. for C43H54NO2Ta (%): C, 64.73; H, 6.82; N, 1.76. Found:
C, 63.20; H, 6.84; N, 1.73.
Preparation of Ta(CH3)2[(OC6H2-tBu2)2C6H3] (16). KBn (234
mg, 1.8 mmol, 2 equiv) was added as a solid to an Et2O/THF (17/1
mL respectively) solution of diphenol 4 (437 mg, 0.90 mmol, 1
equiv). Orange solid KBn dissolves and discolors over less than
an hour. The reaction mixture was allowed to react for 2 h, then
TaCl2Me3 (267 mg, 0.90 mmol, 1 equiv) was added. Within
minutes, the color of the reaction mixture turns brown. The mixture
was stirred for 4 h, then filtered through a bed of Celite. Volatile
materials were removed under vacuum. Petroleum ether was added
to the brown residue, and the mixture was stirred for ∼10 min.
This mixture was filtered through Celite to remove dark solids. The
pale brown filtrate was concentrated and cooled to -35 °C. The
resulting white precipitate was collected by filtration, washed with
cold petroleum ether, and dried under vacuum. This procedure
affords 339 mg (0.49 mmol, 54% yield) of 16 as a white powder.
1H NMR (500 MHz, C6D6) δ: 0.81 (s, 6H, Ta(CH3)2), 1.40 (s, 18H,
C(CH3)3), 1.73 (s, 18H, C(CH3)3), 7.48 (t, 1H, C6H2-H), 7.69 (d,
2H, aryl-H), 8.04 (d, 2H, aryl-H), 8.23 (d, 2H, C6H-H2). 13C NMR
(125 MHz, C6D6) δ: 31.3 (C(CH3)3), 32.3 (C(CH3)3), 35.3
(C(CH3)3), 35.8 (C(CH3)3), 60.6 (TaCH3), 123.8, 125.1, 131.2,
135.4, 137.4, 144.1, 144.8, 153.6 (aryl), 198.5 (aryl-CTa). Anal.
Calcd. for C36H49O2Ta (%): C, 62.24; H, 7.11. Found: C, 60.88;
H, 7.24.
Synthesis of Ta(CH2Ph)2[(OC6H2-tBu2)2C6H3] (17). KBn (35.6
mg, 0.27 mmol, 2 equiv) was added as a solid to a slurry of 4
(66.5 mg, 0.136 mmol, 1 equiv) in benzene (10 mL). The reaction
mixture was stirred for 1 h at room temperature during which the
orange color of KBn disappears. A benzene (5 mL) solution of
TaCl2(CH2Ph)3 (71.8 mg, 0.136 mmol, 1 equiv) was added to the
reaction mixture containing the deprotonated phenol. The reaction
mixture was stirred for 10 h at room temperatures, then filtered
through a bed of Celite. Volatile materials were removed under
vacuum. The residue was recrystallized from petroleum ether at
-35 °C. The desired product was collected by filtrations as a white
powder, washed with cold petroleum ether, and dried under vacuum
(62.1 mg, 73.4 µmol, 54% yield). 1H NMR (500 MHz, CD2Cl2) δ:
1.41 (s, 18H, C(CH3)3), 1.72 (s, 18H, C(CH3)3), 2.82 (s, 4H, TaCH2),
6.70-6.83 (m, 10H, C6H5), 7.38 (t, 1H, 4-C6H2-H), 7.45 (d, 2H,
aryl-H), 7.48 (d, 2H, aryl-H). 13C NMR (125 MHz, CD2Cl2) δ:
31.5 (C(CH3)3), 32.0 (C(CH3)3), 35.1 (C(CH3)3), 35.8 (C(CH3)3),
71.7 (TaCH2), 123.2, 125.6, 126.2, 127.1, 129.1, 129.3, 131.1,
131.6, 135.6, 135.9, 142.0, 144.4, 153.3 (aryl), 197.3 (aryl-CTa).
Anal. Calcd. for C48H57O2Ta (%): C, 68.07; H, 6.78. Found: C,
67.96; H, 6.72.
Preparation of TaCl2(CH3)[(OC6H2-tBu2)2C6H4] (18). An Et2O
(5 mL) solution of diphenol 4 (152 mg, 0.31 mmol, 1 equiv) was
added to an Et2O (5 mL) solution of TaCl2(CH3)3. The reaction
mixture changes gradually from colorless to yellow. The mixture
was stirred for 16 h, and then volatile materials were removed under
vacuum. The residue was suspended in petroleum ether and cooled
to -35 °C. A yellow precipitate was collected by filtration and
washed with cold petroleum ether. Drying under vacuum gives 131
1
mg (0.17 mmol, 56% yield) of 18 as a bright yellow powder. H
NMR (300 MHz, C6D6) δ: 1.31 (s, 18H, C(CH3)3), 1.74 (s, 18H,
C(CH3)3), 2.27 (s, 3H, TaCH3), 7.02 (dd, 2H, C6H2-H2), 7.17 (t,
overlap with C6D5H, 1H, C6H3-H), 7.38 (d, 2H, aryl-H), 7.52 (t,
1H, C6H3-H), 7.69 (d, 2H, aryl-H). 13C NMR (125 MHz, CD2Cl2)
δ: 31.2 (C(CH3)3), 31.9 (C(CH3)3), 35.2 (C(CH3)3), 36.0 (C(CH3)3),
69.3 (TaCH3), 112.6, 124.7, 125.0, 130.9, 133.4, 134.9, 138.4,
Compound 23 (tBuNC Insertion). A mixture of tBuNC (25
mg, 3.0 µmol, 2.7 equiv) and C6H6 (∼1 mL) was added to a solution
of 16 (72 mg, 1.1 µmol, 1 equiv) in C6H6 (5 mL) in a 20 mL vial.
Upon addition, the colorless mixture turned yellow. The reaction

6142 Organometallics, Vol. 27, No. 23, 2008
Agapie et al.
mixture was stirred for 1.5 h, and volatile materials were removed
under vacuum. H NMR spectroscopic analysis shows the clean
pically. Some details regarding refined data and cell parameters
are available in Tables 1, 2, and 3. Selected bond distances and
angles are supplied in the corresponding figures.
1
formation of 23. Recrystallization from petroleum ether at -35 °C
followed by collection by filtration leads to the isolation of 28.5
1
mg (31%) of 23 as a yellow powder. H NMR (300 MHz, C6D6)
Acknowledgment. We thank Lawrence M. Henling
(Caltech) for assistance with single crystal X-ray crystal-
lographic studies. This work has been supported by USDOE
Office of Basic Energy Sciences (Grant No. DE-FG03-
85ER13431).
δ: 0.84 (s, 18H, C(CH3)3), 1.29 (s, 18H, C(CH3)3), 1.41 (s, 18H,
C(CH3)3), 2.91 (s, 6H, TaCCH3), 7.37 (d, 2H, aryl-H), 7.58 (t, 1H,
C6H2-H), 8.05 (d, 2H, aryl-H), 8.23 (d, 2H, C6H-H2). 13C NMR
(75 MHz, C6D6) δ: 22.2 (TaCCH3), 29.5 (C(CH3)3), 30.7 (C(CH3)3),
32.4 (C(CH3)3), 35.0 (aryl-C(CH3)3), 35.4 (aryl-C(CH3)3), 62.7
(NC(CH3)3), 121.5, 125.1, 125.6, 127.7, 134.9, 135.1, 141.9, 145.6,
153.2 (aryl), 198.5 (aryl-CTa), 226.0 (TaCCH3). Anal. Calcd. for
C46H67N2O2Ta (%): C, 64.17; H, 7.84; N, 3.25. Found: C, 63.87;
H, 7.92; N, 3.21.
X-ray Crystal Data: General Procedure. Crystals were re-
moved quickly from a scintillation vial to a microscope slide coated
with Paratone N oil. Samples were selected and mounted on a glass
fiber with Paratone N oil. Data collection was carried out on a
Bruker Smart 1000 CCD diffractometer. The structures were solved
by direct methods. All non-hydrogen atoms were refined anisotro-
Supporting Information Available: Tables of bond lengths,
angles, and anisotropic displacement parameters for the presented
solid-state structures. Computed structure of 11-Pm. Frontier
orbitals of tantalocene, tantalum pyridine-bisphenolate, and carbene.
1H NMR spectra for the conversion of 7 to 11-P. Structural drawing
of 21. X-ray crystallographic data (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
OM8002653