Intramolecular C-H activation of a bisphenolate(benzene)-ligated titanium dibenzyl complex. competing pathways involving α-hydrogen abstraction and σ-bond metathesis

Article abstract of DOI:10.1021/om100253u

A titanium dibenzyl complex featuring a ligand with two phenolates linked by a benzene-1,3-diyl group was found to undergo thermal decomposition to give toluene and a cyclometalated dimeric complex. The thermal decomposition followed first-order kinetics

Full text of DOI:10.1021/om100253u

5026 Organometallics 2010, 29, 5026–5032
DOI: 10.1021/om100253u
Intramolecular C-H Activation of a Bisphenolate(benzene)-Ligated
Titanium Dibenzyl Complex. Competing Pathways Involving r-Hydrogen
Abstraction and σ-Bond Metathesis†
Suzanne R. Golisz, Jay A. Labinger, and John E. Bercaw*
Arnold and Mabel Beckman Laboratories of Chemical Synthesis, California Institute of Technology,
Pasadena, California 91125
Received March 31, 2010
A titanium dibenzyl complex featuring a ligand with two phenolates linked by a benzene-1,3-diyl group
was found to undergo thermal decomposition to give toluene and a cyclometalated dimeric complex.
The thermal decomposition followed first-order kinetics and was studied at a number of temperatures
to determine the activation parameters (ΔHq=27.2(5) kcal/mol and ΔSq=-6.2(14) cal/(mol K)).
Deuterated isotopologues were synthesized to measure the kinetic isotope effects. The complexes with
deuterium in the benzyl methylene positions decomposed more slowly than the protio analogues.
Isotopologues of toluene with multiple deuteration positions were observed in the product mixtures.
These data are consistent with competing R-abstraction and σ-bond metathesis.
Scheme 1
Introduction
Metal-benzyl linkages are common in the field of olefin
polymerization, as they can be activated by methylaluminox-
ane (MAO) or stoichiometric activators (e.g. [Ph3C][BF4]).
Tetrabenzyltitanium has been known to polymerize ethylene
and R-olefins since 1968.1 More recently, metal-benzyl link-
ages have received attention in the field of C-H bond
activation.2-12 Fundamental transformations such as C-H
bond activation are usually invoked as elementary steps of
higher order catalytic reactions such as polymerization, dehy-
drogenation, and alkane functionalization.13
We have recently reported group 4 dibenzyl complexes
having two phenolates linked by 2,6-pyridyl, 2,5-pyrazolyl,
† Part of the Dietmar Seyferth Festschrift.
*To whom correspondence should be addressed. E-mail: bercaw@
caltech.edu.
(1) Giannini, U.; Zucchini, U. Chem. Commun. 1968, 940.
(2) Cleary, B. P.; Eisenberg, R. Organometallics 1992, 11, 2335–2337.
(3) Cleary, B. P.; Eisenberg, R. J. Am. Chem. Soc. 1995, 117, 3510–
3521.
(4) Kataoka, Y.; Imanishi, M.; Yamagata, T.; Tani, K. Organo-
metallics 1999, 18, 3563–3565.
2,5-furanyl, 2,5-thiophenyl, or benzene-1,3-diyl groups that
are active for olefin polymerization.14,15 Attempts to activate
the titanium dibenzyl complex having the bisphenolate-
(benzene-1,3-diyl) ligand by treatment with substoichio-
metric amounts of reagents that remove one benzide group
gave instead a cyclometalated dimeric product bridged by
two phenolate oxygens where activation of the 2-C-H bond
of the benzene-1,3-diyl linker resulted in cyclometalation
and loss of toluene. This same product could also be
obtained by simply heating the neutral dibenzyl complex.
Chromium,16 tantalum,17,18 and molybdenum19 complexes
having similar ligands show an analogous propensity for
(5) Kataoka, Y.; Shizuma, K.; Yamagata, T.; Tani, K. Chem. Lett.
2001, 300–301.
€
(6) Sjovall, S.; Andersson, C.; Wendt, O. F. Organometallics 2001, 20,
4919–4926.
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18, 2198–2205.
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(8) Sjovall, S.; Kloo, L.; Nikitidis, A.; Andersson, C. Organometallics
1998, 17, 579–583.
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(9) Sjovall, S.; Svensson, P. H.; Andersson, C. Organometallics 1999,
18, 5412–5415.
(10) Tejel, C.; Bravi, R.; Ciriano, M. A.; Oro, L. A.; Bordonaba, M.;
Graiff, C.; Tiripicchio, A.; Burini, A. Organometallics 2000, 19, 3115–
3119.
ꢀ
(11) Tejel, C.; Ciriano, M. A.; Jimenez, S.; Oro, L. A.; Graiff, C.;
Tiripicchio, A. Organometallics 2005, 24, 1105–1111.
(12) Zhu, Y. J.; Fan, L.; Chen, C. H.; Finnell, S. R.; Foxman, B. M.;
Ozerov, O. V. Organometallics 2007, 26, 6701–6703.
(14) Agapie, T.; Henling, L. M.; DiPasquale, A. G.; Rheingold, A. L.;
Bercaw, J. E. Organometallics 2008, 27, 6245–6256.
(13) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507–514.
(15) Golisz, S. R.; Bercaw, J. E. Macromolecules 2009, 42, 8751–8762.
r
pubs.acs.org/Organometallics
Published on Web 07/02/2010
2010 American Chemical Society

Article
Organometallics, Vol. 29, No. 21, 2010 5027
Figure 1. Solid-state structure of {Ti(CH2C6H5)[(OC6H2-2-CMe3-4-CH3)C6H3(μ2-OC6H2-2-CMe3-4-CH3]}2 (2): (a) side view; (b)
skeletal view with ligand framework atoms (only) and residual benzyl ligand for each titanium. Hydrogens have been omitted for
˚
clarity. Selected bond lengths (A) and angles (deg) for 2: Ti1-O2 = 2.038(3), Ti1-O4 = 2.042(3), Ti2-O2 = 2.049(3), Ti2-O4 =
2.025(2); O2-Ti1-O4 = 70.20(11), O2-Ti2-O4 = 70.34(11), Ti1-C29-C30 = 105.1(2), Ti2-C64-C65 = 106.2(2).
C-H activation, although the resulting products remain
monomeric. The mechanism(s) for thermal loss of toluene
from the [bisphenolate(benzene-1,3-diyl)]titanium dibenzyl
complex has been examined, and an interesting and unusual
process involving parallel pathways has been identified.
Table 1. Crystal and Refinement Data for {Ti(CH2C6H5)-
[(OC6H2-2-CMe3-4-CH3)C6H3(μ2-OC6H2-2-CMe3-4-CH3]}2 (2)
empirical formula
formula wt
C70H76O4Ti2
1077.11
cryst size (mm3)
temp (K)
0.15 ꢀ 0.12 ꢀ 0.01
100(2)
14.018(8)
˚
a (A)
Results
˚
b (A)
14.480(8)
14.732(10)
˚
c (A)
Synthesis of Ti(CH2C6H5)2[(OC6H2-2-CMe3-4-CH3)2-
C6H3] (1) and Its Conversion to Dimer 2. As described in an
earlier publication,15 a bisphenolate(benzene-1,3-diyl) ligand
featuring tert-butyl groups in the ortho positions and methyl
groups in the para positions has been prepared and metalated
using titanium tetrabenzyl (TiBn4) via toluene elimination to
give a ligated dibenzyl complex (1). Heating a solution of 1 in
an aromatic solvent facilitates C-H bond activation to give
toluene and a cyclometalated complex that readily dimerizes
(2; see Scheme 1). The dimer was asymmetric, showing two
peaks for the tert-butyl groups, two peaks for the methyl
R (deg)
β (deg)
γ (deg)
V (A )
Z
99.37(3)
104.86(4)
92.77(3)
2838.9(3)
2
triclinic
P1
1.260
1.79-23.31
0.331
semiempirical from equivalents
1.22
0.061, 0.064
P
3
˚
cryst syst
space group
dcalcd (Mg/m3)
θ range (deg)
μ (mm-1
abs cor
GOF
)
R1,a wR2b (I > 2σ(I))
1
P
P
P
a R1 = ||Fo|- |Fc||/ |Fo|. b wR2 = [ [w(Fo2 - Fc2)2]/ [w(Fo2)2]]1/2
.
groups, and two doublets for the benzyl protons in the H
NMR spectrum. This evidence alone does not prove dimeric
speciation in solution, because a monomeric complex featur-
ing a C2-symmetric ligand with a single benzyl group on
Table 2. Rate Data and Activation Parameters for the
Formation of 2a
1
the same metal would exhibit identical H NMR coupling;
T (K)
kobs (s-1
)
however, the monomeric cyclometalated complex is coordi-
natively unsaturated and sterically unencumbered due to the
tridentate ligand, thus making dimerization favorable. A single-
crystal X-ray structure determination of 2 (Figure 1, Table 1)
reveals that each titanium is five-coordinate, bound by the
two phenolates of the bisphenolate(benzene-1,3-diyl) ligand,
one bridging phenolate oxygen from the ligand on the other
titanium, one benzyl group positioned away from the dimer
core and bent toward the linker of the ligand, and one phenyl
linkage from the now metalated ligand. The reaction forming
the dimer is very clean, with no detectable amounts of other
368
378
388
398
[2.27(5)] ꢀ 10-5
[6.62(3)] ꢀ 10-5
[1.7(1)] ꢀ 10-4
[4.0(5)] ꢀ 10-4
a ΔHq = 27.2(5) kcal/mol. ΔSq = -6.2(14) cal/(mol K).
decomposition or side products, and is thus amenable to
mechanistic study.
Determination of the Activation Parameters for Conversion
of 1 to 2. The transformation of 1 to 2 in o-xylene-d10 was
monitored over a temperature range of 30 K (368-398 K),
with first-order behavior observed over 2 half-lives for all
temperatures. The rates for the temperature study are re-
ported in Table 2, and the Eyring plot is shown in Figure S1
(Supporting Information). The activation parameters so
derived (ΔHq = 27.2(5) kcal/mol and ΔSq = -6.2(14) cal/
(mol K)) are similar to those observed for the thermal
decomposition of Cp*2Ti(CH3)2 (ΔHq = 27.6(3) kcal/mol
(16) O’Reilly, M.; Falkowski, J. M.; Ramachandran, V.; Pati, M.;
Abboud, K. A.; Dalal, N. S.; Gray, T. G.; Veige, A. S. Inorg. Chem. 2009,
48, 10901–10903.
(17) Agapie, T.; Bercaw, J. E. Organometallics 2007, 26, 2957–2959.
(18) Agapie, T.; Day, M. W.; Bercaw, J. E. Organometallics 2008, 27,
6123–6142.
(19) Sarkar, S.; Carlson, A. R.; Veige, M. K.; Falkowski, J. M.;
Abboud, K. A.; Veige, A. S. J. Am. Chem. Soc. 2008, 130, 1116–1117.

5028 Organometallics, Vol. 29, No. 21, 2010
Golisz et al.
Figure 2. Isotopologues of 1.
Scheme 2
and ΔSq = -2.9(7) cal/(mol K)),20 (η5-C5H4CMe2Ph)TiBn3
(ΔHq=24(2) kcal/mol and ΔSq=-5(5) cal/(mol K)),21 and
Cp*2HfBn2 (ΔHq=34(1) kcal/mol and ΔSq=1(3) cal/(mol
K)),22 where R-hydrogen abstraction has been proposed as
the rate-determining step. The other commonly proposed
mechanism for transformations of early-transition-metal alkyls
is σ-bond metathesis. The reaction between Cp*2ScCH3 and
4-methylstyrene in hydrocarbon solvent at elevated tempera-
ture results in Cp*2ScCHdCH(p-C6H4CH3) and methane
with ΔHq=12 kcal/mol and ΔSq=-36 cal/(mol K) via a
σ-bond metathesis process.23
Kinetic Isotope Effects Associated with Conversion of 1 to 2.
A number of complexes suitable for kinetic isotope experi-
mentation have been prepared by selectively incorporat-
ing deuterium in the ligand or the benzyl groups (Figure 2).
The synthesis of an analogous version of the ligand having
a partially deuterated benzene-1,3-diyl linker has been
reported.17,18 As such, it was straightforward to synthesize
the ligand-deuterated titanium dibenzyl complex 1-d3 by
using perdeuterated aniline to generate the partially deuter-
ated 1,3-dibromobenzene-d3 prior to the coupling with the
two phenolates. The procedure used for the synthesis of
benzyl groups deuterated at either the methylene or phenyl
positions is shown in Scheme 2. Benzoic acid (or benzoic
acid-d5) was reduced with LiAlD4 (or LiAlH4) to give the
partially deuterated benzyl alcohol. The benzyl alcohol was
converted to the benzyl chloride with SOCl2 in the presence
Figure 3. Plot of the rates of the thermolysis of 1 and its iso-
topologues in o-xylene-d10 at 388 K.
of benzotriazole. Addition of magnesium turnings to freshly
distilled benzyl chloride in dry diethyl ether afforded the
benzyl magnesium chloride Grignard reagent, which was
readily metalated with titanium tetrachloride. The deuter-
ated tetrabenzyltitanium complexes were crystallized from
petroleum ether. Mixing the protio ligand with the deute-
rated tetrabenzyltitanium complexes gave the isotopologues
1-d4, 1-d10, and 1-d14, shown in Figure 2. The reaction rates of
the isotopologues were monitored at 388 K in o-xylene-d10
and showed first-order behavior over at least 2 half-lives
(Figure 3 and Table 3).
(20) McDade, C.; Green, J. C.; Bercaw, J. E. Organometallics 1982,
1, 1629–1634.
(21) Deckers, P. J. W.; Hessen, B. Organometallics 2002, 21, 5564–
5575.
(22) Bulls, A. R.; Schaefer, W. P.; Serfas, M.; Bercaw, J. E. Organo-
metallics 1987, 6, 1219–1226.
(23) 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.

Article
Organometallics, Vol. 29, No. 21, 2010 5029
Table 3. Rates of the Thermolysis of 1 and Its Isotopologues in o-Xylene-d10 at 388 K
H/D labeling of the evolved toluenea H/D labeling of methylene for the residual benzyl of 2b
isotopologue
kobs (s-1
)
amt (%)
species
amt (%)
species
1
1-d3
[1.7(1)] ꢀ 10-4
100
56
44
84
16
100
78
C6H5CH3
C6H5CH3
C6H5CH2D
C6H5CHD2
C6H5CD3
C6D5CH3
C6D5CHD2
C6D5CD3
100
54
46
[Ti-CH2-Ph]
[Ti-CHD-Ph]
[Ti-CH2-Ph]
[Ti-CD2-Ph]
[Ti-CHD-Ph]
[Ti-CH2-Ph]
[Ti-CD2-Ph]
[Ti-CHD-Ph]
[1.55(2)] ꢀ 10-4
1-d4
[9.5(7)] ꢀ 10-5
15
100
1-d10
1-d14
[1.58(2)] ꢀ 10-4
[7.8(2)] ꢀ 10-5
22
13
a Approximate ratios of labeled toluenes were determined by 1H NMR analysis of the product toluene methyl resonance for 1, 1-d3, and 1-d10 and
2H NMR analysis of the product toluene methyl resonance for 1-d4 and 1-d14 b Approximate amounts of labeled methylene groups were determined
.
by 1H NMR analysis of the product methylene resonance.
was not resolved; however, this H-D coupling is not always
resolved, e.g. for [Ti-CHD-Ph] in the monomeric tucked-in
cyclopentadienyl complex (η5:η1-C5H4CMe2C6D4)Ti(CH2Ph)-
(CHDPh).21
Discussion
The enthalpy and entropy of activation are similar to those
for systems which are known to undergo C-H activation via
rate-determining R-abstraction,20-22 and the kinetic isotope
effect (KIE) for each isotopologue also implied R-abstrac-
tion was the rate-determining step;the rates for 1-d4 and
1-d14 which have deuterium in the benzyl methylene posi-
tions are approximately half of those for 1, 1-d3, and 1-d10,
which have protons in those positions. On the other hand,
the toluene distribution (Table 3) cannot be explained by
simple R-abstraction. We considered the possibility that the R-
abstraction step is reversible, such that the eliminated toluene
can reinsert into the benzylidene intermediate, thus accounting
for the isotope distribution. However, when 1 was heated in the
presence of toluene-d8, only C6H5CH3 and toluene-d8 were
observed. An alternate explanation for the toluene distribution
is a bimetallic process. When equimolar amounts of 1 and 1-d14
were heated together, neither C6H5CH2D nor C6D5CH2D were
observed. Therefore, the mechanism most consistent with the
data is two competing pathways: R-abstraction and σ-bond
metathesis (Scheme 3).25
Figure 4. 1H NMR (C6D6) spectrum of the methylene of the
residual benzyl of 2, otained from 1-d3.
The rates of 1, 1-d3, and 1-d10 were approximately the same,
while the rates for 1-d4 and 1-d14 were similar to each other but
slower than for the other three complexes, suggesting that rate-
limiting R-H abstraction from one benzyl group, generating
1 equiv of toluene and a titanium benzylidene intermediate, is
an important step in the overall conversion. However, the most
interesting and revealing outcomes are the isotopic composi-
tions of the toluenes liberated from some of the isotopologues
(Table 3) which imply that R-abstraction is not the only pro-
cess. For complexes 1 and 1-d10 only protium is observed in
the methyl group of the toluene. However, for 1-d3, a roughly
equimolar mixture of C6H5CH3 and C6H5CH2D was found,
and for 1-d4 (or 1-d14), two isotopologues of toluene, a major
component, C6H5CHD2 (or C6D5CHD2), and a minor com-
ponent, C6H5CD3 (orC6D5CD3), were observed. Concomitant
with the observed toluene isotopologue ratios, two isotopo-
logues were observed in the benzyl methylene groups of the
products of 1-d3, 1-d4, and 1-d14, whose ratios were roughly
complementary with those for the toluene (Table 3). In the
1H NMR spectrum for 2 (obtained from thermolysis of 1 or
1-d10), the [Ti-CH2-Ph] group of the benzyl appeared as two
doublets at 2.97 and 4.02 ppm with a coupling constant of
10 Hz. The products of thermolysis of 1-d3, 1-d4, and 1-d14 con-
tain, in addition to these two doublets attributable to [Ti-
CH2-Ph], two additional singlets at 2.94 and 3.97 ppm due to
[Ti-CHD-Ph], shifted slightly upfield due to an isotopic
chemical shift (Figure 4).24 The H-D coupling for the latter
The R-abstraction process involves one benzyl group ab-
stracting a methylene hydrogen from the other benzyl group
to form toluene and a benzylidene intermediate. The C-H
bond of the benzene-1,3-diyl linker subsequently undergoes
1,2-migratory insertion to the Ti-benzylidene to form the four-
coordinate cyclometalated species. The σ-bond metathesis
alternative involves the C-H bond of the benzene-1,3-diyl
linker approaching the Ti-benzyl linkage to form a four-center
transition state. Concerted release of toluene and formation of
the new Ti-C bond ensues. In the case of 1, the two mechan-
isms cannot be differentiated, because the toluene product
would be the same under either mechanism. For the 1-d10
1
example only C6D5CH3 was observed in the H NMR spec-
trum, again consistent with either mechanism. In the case of
1-d3, R-abstraction leads to C6H5CH3 while σ-bond metathesis
leads to C6H5CH2D. Comparing the amounts of toluene allows
for the determination of the relative rates of protio R-abstrac-
tion (kRH) versus deuterio σ-bond metathesis (kσD). The iso-
topologues which have deuterium in the methylene position of
(24) Friebolin, H. Basic One- and Two- Dimensional NMR Spectro-
scopy, 4th ed.; Wiley-VCH: Weinheim, Germany, 2004.
(25) The involvement of the oxygens as a shuttle for the hydrogen
atoms cannot be excluded.

5030 Organometallics, Vol. 29, No. 21, 2010
Golisz et al.
Scheme 3
Table 4. Rates and Kinetic Isotope Effects for both r-Abstraction
and σ-Bond Metathesis at 388 K
R-abstraction
σ-bond metathesis
kRH = [8.75(2)] ꢀ 10-5
kRD = [1.60(5)] ꢀ 10-5
KIE = 5.5(3)
kσH = [7.04(5)] ꢀ 10-5
kσD = [6.74(2)] ꢀ 10-5
KIE = 1.0(1)
at 413 K).22 However, in related systems which undergo
R-abstraction (albeit different due to the heteroatom),
(tBu3SiO)2Ti(Bn)(tBu3SiNH)27 and (tBu3SiNH)3Zr(Bn)28 re-
lease toluene to form metal-imido species with KIEs of 5.6(2)
at 363 K and 7.1(6) at 370 K, respectively. These data show that
the value of the KIE is more a result of the geometry of the
transition state rather than the mode of the chemical transfor-
mation. In the present case, we can interpret the KIEs to mean
that the transition state for the R-abstraction process is highly
symmetrical with a large amount of bond breaking. The
transition state for the σ-bond metathesis process is likely
unsymmetrical, with the bonds between the C-H of the linker
mostly intact. Furthermore, a large amount of rearrangement
must occur so that the complex aligns properly for the σ-bond
metathesis transition state. This process does not influence the
value of the KIE, as no bonds have been broken. Instead, the
rearrangement manifests as the moderately negative ΔSq.
Figure 5. System of equations used to determine the rates.
the benzyl groups (1-d4 and 1-d14) lead to C6H5CD3 or
C6D5CD3 when the reaction proceeds via R-abstraction and
C6H5CHD2 or C6D5CHD2 when the reaction proceeds via
σ-bond metathesis. Comparing the toluene ratios using H
NMR leads to the relative rates of deuterio R-abstraction (kRD
2
)
versus protio σ-bond metathesis (kσH). A system of equations
can be used to determine the individual rates from the kobs and
toluene ratios (see Figure 5 and the Supporting Information).
From these equations, the rates and KIE values for each
process were determined (see Table 4).
The data show that the kinetic isotope effect is more pro-
nounced for the R-abstraction mechanism. At first, this may
seem counterintuitive because σ-bond metathesis processes
generally have large KIE values (the elimination of CH4
from Cp2Zr(Me)(SCH2R) has a KIE of 5.2(0.2) at 353 K26
and the reaction of (Cp*-d15)2ScCH3 with C6Y6 (Y=H, D) at
353 K has a KIE of 2.8(2)),23 and R-abstraction processes
usually have smaller KIE (the thermal decomposition of (η5-
C5H4CMe2Ph)TiBn3 has a KIE of 1.23(0.05) at 338 K21 and
the thermal decomposition of Cp*2HfBn2 has KIE of 3.1(2)
Conclusion
The thermal decomposition of 1 generates the cyclometa-
lated dimer 2 and toluene. Both the activation parameters
and kinetic isotope effects are consistent with an R-abstrac-
tion mechanism. However, the toluene isotopologues and
the isotope distribution in 2 suggest that σ-bond metathesis is
competitive with R-abstraction. The KIEs suggest a transi-
tion state for the R-abstraction process that is late, while that
(27) Bennett, J. L.; Wolczanski, P. T. J. Am. Chem. Soc. 1997, 119,
10696–10719.
(28) Schaller, C. P.; Cummins, C. C.; Wolczanski, P. T. J. Am. Chem.
(26) Buchwald, S. L.; Nielsen, R. B. J. Am. Chem. Soc. 1988, 110,
3171–3175.
Soc. 1996, 118, 591–611.

Article
Organometallics, Vol. 29, No. 21, 2010 5031
for σ-bond metathesis is very early. The finding that 1 is active
while 2 is inactive illustrates the potential complication of a
proximal C-H bond in a catalyst for olefin polymerization.
98% yield. 1H NMR (500 MHz, CDCl3): δ 1.85 (t, 1H, 3JH-H
=
3
5.6 Hz, OH), 4.68 (d, 2H, JH-H = 5.4 Hz, CH2). 13C NMR
(125 MHz, CDCl3): δ 65.48, 126.75 (t, 1JC-D=24 Hz), 127.33
1
1
(t, JC-D=24 Hz), 128.24 (t, JC-D=24 Hz), 140.88. HRMS
(EIþ, m/z): obsd Mþ 113.0892, calcd for C7H3OD5 113.0889.
Benzyl Chloride-d2. This compound was synthesized from a
modified literature preparation33 to give a yellow oil. The crude
mixture was distilled under vacuum to give a colorless oil: 0.6 g,
46% yield. 1H NMR (500 MHz, CDCl3): δ 7.37 (m, 5H, C6H5).
13C NMR (125 MHz, CDCl3): δ (CD2 in baseline) 128.40,
128.57, 128.73, 137.36. HRMS (EIþ, m/z): obsd Mþ 128.0379,
calcd for C7H5ClD2 128.0362.
Experimental Section
General Considerations. All air- and moisture-sensitive com-
pounds were manipulated under argon or nitrogen using stan-
dard glovebox, Schlenk, and high-vacuum-line techniques.29
Argon was purified and dried by passage through columns of
˚
MnO on vermiculite and activated 4 A molecular sieves. Solvents
were dried over sodium benzophenone ketyl or titanocene.30 All
organic chemicals were purchased and used as received from
Aldrich. The metal precursor TiCl4 was purchased from Strem
and used as received. All deuterated compounds were purchased
from Cambridge Isotopes. NMR spectra of ligands and metal
complexes were recorded on a Varian Unity Inova 500 (499.852
MHz for 1H) spectrometer. HRMS was obtained from the
California Institute of Technology Mass Spectrometry Facility.
X-ray-quality crystals were mounted on a glass fiber with Para-
tone-N oil. Data were collected on a Bruker KAPPA APEX II
instrument. Structures were determined using direct methods or,
in some cases, Patterson maps with standard Fourier techniques
using the Bruker AXS software package. Tetrabenzyltitanium31
was prepared according to a literature procedure. 1,3-Dibromo-
Benzyl Chloride-d5. This compound was synthesized from a
modified literature preparation33 to give a brown oil. The crude
mixture was distilled under vacuum to give a colorless oil: 0.9 g,
1
84% yield. H NMR (500 MHz, CDCl3): δ 4.61 (s, 2H, CH2).
=
1
13C NMR (125 MHz, CDCl3): δ 46.41, 128.36 (t, JC-D
1
24 Hz), 128.44 (t, JC-D=24 Hz), 137.53. HRMS (EIþ, m/z):
obsd Mþ 131.0554, calcd for C7H2ClD5 131.0550.
General Procedure for the Synthesis of the Isotopologues of
Tetrabenzyltitanium. In the glovebox, benzyl chloride (4.6 mmol)
was dissolved in diethyl ether. Magnesium turnings (6.9 mmol) were
added to the solution. The reaction was monitored by GC/MS.
The resulting Grignard reagent was filtered. Titanium tetrachloride
(1.1 mmol) was added to the Grignard solution. The reaction was
stirred at room temperature overnight. The crude mixture was
filtered through Celite and washed with diethyl ether. The diethyl
ether was removed in vacuo. The dark brown sludge was suspended
in petroleum ether and filtered to give a dark red solution. Storing
the dark red solution in the freezer at -35 °C overnight gave dark
red crystals of the desired tetrabenzyltitanium isotopologue.
Tetrabenzyltitanium-d28: 474 mg, 26% yield. 13C NMR (125
17
benzene-d3 and the bisphenolate ligand15 were prepared as
reported previously. All errors are standard deviations.
Ligand-d3. The protected 2-tert-butyl-4-methyl-5-bromo-
phenol15 (534 mg, 1.9 mmol) was dissolved in THF (30 mL) in a
glass vessel with a Kontes valve and cooled to just above its freezing
point (-100 °C). tert-Butyllithium (3.9 mmol) was added drop-
wise, and the reaction mixture was stirred for 30 min to give a
cloudy, light brown mixture. Zinc chloride (177 mg, 1.3 mmol) was
added to the reaction mixture, which was stirred for 30 min to give
a transparent orange solution. 1,3-Dibromobenzene-d3 (200 mg,
0.8 mmol) and tetrakis(triphenylphosphine)palladium (22 mg,
0.02 mmol) were added to the reaction mixture, which was sealed
and placed in an oil bath. The mixture was heated to 70 °C over-
night. The reaction mixture was quenched with H2O and concen-
trated under reduced pressure. The remaining aqueous sludge was
extracted in CH2Cl2, dried over MgSO4, filtered, and dried by
rotary evaporation. The crude product was suspended in acidified
methanol and heated to reflux to remove both impurities and the
protecting group. After it was cooled to room temperature, rotary
evaporation of the neutralized suspension gave the desired pro-
duct. Further purification was achieved by column chromato-
graphy (hexanes/CH2Cl2, 9:1) to give a pale yellow crystalline
MHz, C6D6): δ 96.93 (qn, 1JC-D=20 Hz), 124.59 (t, 1JC-D
=
24 Hz), 129.29 (t, JC-D=24 Hz), 129.56 (t, JC-D=24 Hz),
142.38.
1
1
1
Tetrabenzyltitanium-d20: 250 mg, 30% yield. H NMR (500
MHz, C6D6): δ 2.81 (s, 8H, CH2C6D5). 13C NMR (125 MHz,
C6D6): δ 98.46, 124.56 (t, 1JC-D=24 Hz), 129.22 (t, 1JC-D=24
Hz), 129.53 (t, 1JC-D=24 Hz), 142.51.
1
Tetrabenzyltitanium-d8: 132 mg, 29% yield. H NMR (500
MHz, C6D6): δ 6.60 (dd, 8H, J=1 Hz, J=8 Hz, C6H5), 6.95
(tt, 4H, J=1 Hz, J=8 Hz, C6H5), 7.09 (tt, 8H, J=1 Hz, J=8 Hz,
=
1
C6H5). 13C NMR (125 MHz, C6D6): δ 97.11 (qn, JC-D
20 Hz), 125.10, 129.72, 130.09, 142.59.
General Procedure for the Synthesis of Ligated Titanium
Dibenzyl Complexes. Tetrabenzyltitanium (0.357 mmol) was
dissolved in toluene (10 mL). The ligand (0.357 mmol) predis-
solved in the same solvent (10 mL) was added to the metal. The
reaction mixture was stirred for 1 h. The solvent was removed in
vacuo. Recrystallization was achieved by dissolving in pentane
or petroleum ether, filtering, and cooling to -35 °C for a few
days.
1
solid: 195 mg, 57% yield. H NMR (500 MHz, CDCl3): δ 1.47
(s, 18H, C(CH3)3), 2.35 (s, 6H, CH3), 5.33 (s, 2H, OH), 6.97 (s, 2H,
C6H2), 7.15 (s, 2H, C6H2), 7.53 (s, 1H, C6D3H). 13C NMR (125
MHz, CDCl3): δ 21.02, 29.91, 35.08, 127.82, 128.21, 128.24,
128.53, 128.89, 129.20, 136.45, 138.94, 139.02, 148.91. HRMS
(FABþ, m/z): obsd Mþ 405.2763, calcd for C28H31O2D3 405.2747.
Benzyl Alcohol-d2. This compound was synthesized from a
modified literature preparation32 to give a colorless oil: 1.1 g,
1: dark red crystals; 220 mg, 55% yield. 1H NMR (500 MHz,
C6D6): δ 1.65 (s, 18H, C(CH3)3), 2.02 (s, 2H, CH2C6H5), 2.26
(s, 6H, CH3), 3.49 (s, 2H, CH2C6H5), 6.65 (s, 1H, C6H4), 6.67
(d, 2H, J=7 Hz, CH2C6H5), 6.75 (t, 1H, J=7.5 Hz, CH2C6H5),
6.90 (m, 3H, J = 7.5 Hz, CH2C6H5), 7.03 (d, 2H, J = 8 Hz,
CH2C6H5), 7.06 (bs, 2H, C6H2), 7.10 (m, 5H, J=8.5 Hz, C6H4
and CH2C6H5), 7.28 (d, 2H, J=2 Hz, C6H2). 13C NMR (125
MHz, C6D6): δ 21.54, 30.71, 35.91, 94.85, 96.99, 124.14, 124.34,
127.21, 127.92, 128.01, 128.09, 128.14, 128.28, 128.84, 128.91,
131.83, 131.99, 135.84, 138.27, 142.30, 145.07, 146.41, 161.74.
1-d3: dark red crystals; 81 mg, 47% yield. 1H NMR (500 MHz,
C6D6): δ 1.65 (s, 18H, C(CH3)3), 2.01 (s, 2H, CH2C6H5), 2.25
(s, 6H, CH3), 3.49 (s, 2H, CH2C6H5), 6.67 (d, 2H, J=7 Hz,
CH2C6H5), 6.75 (t, 1H, J=7.5 Hz, CH2C6H5), 6.90 (m, 3H, J=
7.5 Hz, CH2C6H5), 7.03 (d, 2H, J=7 Hz, CH2C6H5), 7.06 (bs,
2H, C6H2), 7.08 (s, 1H, C6D3H), 7.10 (d, 2H, J = 7 Hz,
1
84% yield. H NMR (500 MHz, CDCl3): δ 1.74 (s, 1H, OH),
7.31 (m, 1H, p-C6H5), 7.38 (d, 4H, 3JH-H=4.4 Hz, C6H5). 13
C
NMR (125 MHz, CDCl3): δ 64.91, 127.24, 127.89, 128.77,
140.96. HRMS (EIþ, m/z): obsd Mþ 110.0700, calcd for
C7H6OD2 110.0701.
Benzyl Alcohol-d5. This compound was synthesized from a
modified literature preparation32 to give a colorless oil: 0.9 g,
(29) Burger, B. J.; Bercaw, J. E. ACS Symp. Ser. 1987, 357, 79–98.
(30) Marvich, R. H.; Brintzinger, H. H. J. Am. Chem. Soc. 1971, 93,
2046–2048.
(31) Zucchini, U.; Giannini, U.; Albizzat, E.; Dangelo, R. J. Chem.
Soc., Chem. Commun. 1969, 1174–1175.
(32) Bialecki, J. B.; Ruzicka, J.; Attygalle, A. B. J. Labelled Compd.
Radiopharm. 2007, 50, 711–715.
(33) Chaudhari, S. S.; Akamanchi, K. G. Synlett 1999, 1763–1765.

5032 Organometallics, Vol. 29, No. 21, 2010
Golisz et al.
CH2C6H5), 7.27 (d, 2H, J=2 Hz, C6H2). 13C NMR (125 MHz,
C6D6): δ 21.53, 30.69, 35.91, 94.86, 96.99, 124.14, 124.34,
127.91, 128.00, 128.09, 128.27, 128.84, 128.91, 128.92, 131.85,
135.81, 135.84, 138.28, 142.11, 142.18, 145.09, 146.45, 161.75.
1-d4: dark red crystals; 40 mg, 24% yield. 1H NMR (500 MHz,
C6D6): δ 1.63 (s, 18H, C(CH3)3), 2.22 (s, 6H, CH3), 6.57 (t, 1H,
J=1.5 Hz, C6H4), 6.63 (dd, 2H, J=8 Hz, J=1.5 Hz, C6H5), 6.72
(tt, 1H, J=7 Hz, J=1.5 Hz, C6H5), 6.86 (m, 3H, C6H5), 6.99 (d,
1H, J=1 Hz, C6H5), 7.01 (d, 1H, J=1.5 Hz, C6H5), 7.03 (d, 2H,
J=2 Hz, C6H2), 7.05 (d, 2H, J=1 Hz, C6H5), 7.09 (s, 1H, C6H4),
J. Young NMR tube. An initial 1H NMR spectrum was acquired at
room temperature. The tube was submerged in an oil bath for
regular time intervals where the oil level reached the bottom of the
Teflon cap. 1H NMR spectra were recorded at room temperature
for each regular time interval. The disappearance of the tert-butyl
peak was monitored against the ferrocene.
2: dark brown solution in benzene. 1H NMR (500 MHz,
C6D6): δ 1.29 (s, 18H, C(CH3)3), 1.88 (s, 18H, C(CH3)3), 2.21
(s, 6H, CH3), 2.27 (s, 6H, CH3), 2.98 (d, 4H, CH2, J=9.7 Hz),
4.02 (d, 4H, CH2, J=9.7 Hz), 6.36 (d, 2H, J=7.4 Hz), 6.42 (m,
6H), 6.53 (t, 4H, J=7.5 Hz), 6.61 (t, 2H, J=7.7 Hz), 6.69 (s, 2H),
7.05 (d, 4H, J=7.1 Hz), 7.24 (s, 2H), 7.27 (s, 2H). 13C NMR (126
MHz, C6D6): δ 21.44, 21.71, 30.62, 33.02, 35.23, 35.83, 98.39,
105.39, 124.26, 125.21, 126.13, 126.55, 126.72, 129.00, 130.06,
130.24, 131.59, 132.94, 133.85, 135.55, 136.33, 136.42, 136.78,
138.13, 140.84, 155.17, 158.58.
7.10 (d, 2H, J=3 Hz, C6H4), 7.24 (d, 2H, J=2 Hz, C6H2). 13
C
NMR (125 Hz, C6D6): δ 21.53, 30.74, 35.93, 105.73, 124.14,
124.34, 126.92, 127.98, 128.03, 128.10, 128.15, 128.33, 128.49,
128.83, 128.92, 131.82, 131.99, 135.83, 138.25, 142.34, 144.84,
146.23, 161.71.
1
1-d10: dark red crystals; 61 mg, 41% yield. H NMR (500
MHz, C6D6): δ 1.63 (s, 18H, C(CH3)3), 2.00 (s, 2H, CH2C6D5),
2.23 (s, 6H, CH3), 3.46 (s, 2H, CH2C6D5), 6.60 (t, 1H, J=1.5 Hz,
C6H4), 7.03 (d, 2H, J=2 Hz, C6H2), 7.07 (t, 1H, J=2.5 Hz,
C6H4), 7.08 (d, 2H, J=2 Hz, C6H4), 7.24 (d, 2H, J=2 Hz, C6H2).
13C NMR (126 MHz, C6D6): δ 21.54, 30.72, 35.92, 94.68, 96.89,
123.72, 124.66, 127.09, 127.69, 127.88, 128.02, 128.09, 128.12,
129.23, 129.53, 131.81, 131.99, 135.82, 138.26, 142.32, 144.76,
146.12, 161.73.
Acknowledgment. S.R.G. thanks Scott Virgil of Cal-
tech for synthetic advice. This work was supported by
the USDOE Office of Basic Energy Sciences (Grant No.
DE-FG03-85ER13431). We thank Lawrence M. Henling
and Michael W. Day of Caltech for mounting and solving
the crystal structure. The Bruker KAPPA APEXII X-ray
diffractometer was purchased via an NSF CRIF:MU
award to the California Institute of Technology (No.
CHE-0639094).
1
1-d14: dark red crystals; 36 mg, 25% yield. H NMR (500
MHz, C6D6): δ 1.64 (s, 18H, C(CH3)3), 2.23 (s, 6H, CH3), 6.55 (t,
1H, J=1.5 Hz, C6H4), 7.03 (d, 2H, J=2 Hz, C6H2), 7.07 (t, 1H,
J=2.5 Hz, C6H4), 7.08 (d, 2H, J=2 Hz, C6H4), 7.25 (d, 2H, J=
2 Hz, C6H2). 13C NMR (126 MHz, C6D6): δ 21.54, 30.75, 35.93,
93.21 (m, 1C, CD2), 95.70 (m, 1C, CD2), 123.72, 126.76, 127.38,
127.56, 127.74, 127.87, 128.03 128.10, 128.15, 128.68, 131.79,
131.99, 135.80, 138.22, 142.35, 144.52, 145.92, 161.69.
General Procedure for Monitoring Decomposition. [Bisphenolate-
(benzene-1,3-diyl)]titanium dibenzyl complexes (20-25 mg) were
dissolved in o-xylene-d10 (0.5 mL, bp 143-145 °C) with a small
amount of ferrocene (3-5 mg) as the internal standard in a sealed
Supporting Information Available: Tables, text, a figure, and a
CIF file giving rate data, an Eyring plot, rate equations, and
crystal data. This material is available free of charge via the
Internet at http://pubs.acs.org. CCDC 726163 (2) also contains
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from the Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.