The first structural characterization and determination of the isomerization activation parameters of a chiral phosphatitanocene

Article abstract of DOI:10.1039/b208945a

The activation parameters (ΔG?₂₉₈ = 11.5 (±1.0) kcal mol^-1, ΔH? = 16.3 (±3.0) kcal mol^-1, ΔS? = 16 (±11) cal mol^-1 K^-1) have been determined for the rac to meso isomerization of a phosphametallocene,

Full text of DOI:10.1039/b208945a

The first structural characterization and determination of the
isomerization activation parameters of a chiral phosphatitanocene†
T. Keith Hollis,* Yi Joon Ahn and Fook S. Tham
Department of Chemistry, University of California, Riverside, CA, USA. E-mail: keith.hollis@ucr.edu;
Fax: +909-787-4713; Tel: +909-787-3024
Received (in Cambridge, UK) 13th September 2002, Accepted 24th October 2002
First published as an Advance Article on the web 11th November 2002
The activation parameters (DG‡
= 11.5 ( 1.0) kcal
298
mol21, DH‡ = 16.3 ( 3.0) kcal mol21, DS‡ = 16 ( 11) cal
mol21 K21) have been determined for the rac to meso
isomerization of a phosphametallocene, bis(3,4-dimethyl-
2-phenylphospholyl)titanium dichloride, 2, which has been
structurally characterized.
We recently reported the facile isomerization of a phosphazirco-
nocene.1 Fu and coworkers subsequently reported that a
phosphahafnocene undergoes isomerization at a rate faster than
the analogous phosphazirconocene.2 Heterometallocenes have
been a focus of recent interest, and several groups world-wide
have described several successful chiral ligands for asymmetric
catalysis based on them.3 A better understanding of isomeriza-
tion is critical for full implementation of their methodologies
based on such ligands. Further, rac to meso interconversion of
conformation in metallocenes has been reported to control
tacticity of stereoblock copolymers of elastomeric poly-
propylene.4 Therefore, determination of the rates of isomeriza-
tion and activation parameters in phosphametallocene deriva-
tives is important in predicting the applicability of these systems
to stereoselective synthesis. Presented here is the synthesis and
structural characterization of a phosphatitanocene, bis(3,4-
dimethyl-2-phenylphospholyl) titanium dichloride, and the
determination of the activation parameters for interconversion
of the rac and meso isomers, which is to our knowledge the first
report of such data.
Phosphatitanocenes were first reported by Nief and Mathey
in 1988.5 Their methodology was modified for the preparation
of 2 as outlined in eqn. (1). The readily-available phosphole 1
was heated in the presence of KOt-Bu resulting in a 1,5 phenyl
shift and deprotonation yielding the 3,4-dimethyl-2-phenyl-
phospholyl anion, which was treated with Me3SnCl. The purity
of the Sn intermediate is critical to a successful synthesis.
Subsequent reaction with TiCl4 produced the desired complex 2
in excellent yield ranging from 63% to 75%.‡ The room
temperature 31P NMR spectrum of 2 in C6D6 shows a broad
signal centered at 101 ppm. This observation is in stark contrast
to the 31P NMR spectrum of the Zr analog of 2, which exhibits
a sharp line for each isomer.1 Cooling a solution of 2 does allow
the observation of discrete 31P NMR signals for each isomer
(vide infra).
Fig. 1 ORTEP® plot at 30% thermal ellipsoids of the molecular structure of
2. Selected structural data: Ti–Cl1, Cl2 = 2.3109(5), 2.3427(5) Å, Ti–Ctr1,
Ctr2 = 2.15, 2.19 Å, Ti–P1a, P1b = 2.6104(5), 2.6425(6) Å, Ctr–Ti–Ctr =
134.7°, Cl–Ti–Cl = 94.6°, P–Ctr–Ctr–P = 22.8°.
indicate that the P-atoms are equivalent in solution. In the
crystal, both P-atoms are on the same side of the Cl–Ti–Cl
bisector. This conformation may be due to p-stacking of the
numbered phenyl ring (Fig. 1).7 The stacking is derived from a
(21 2 x, 2y, 1 2 z) translation of the molecule through a center
of inversion. This translation results in the para-C of each
phenyl ring lying over the centroid of the other (angle = 90.2°)
at a distance of 3.630 Å in a classic example of a p–s
attraction.7b,8 Solid state 31P NMR data on the crystals indicate
that the unique environments are detectable and only the rac
isomer is present.9
The facile isomerization of 2 was convenient for the analysis
of the isomerization and determination of the activation
parameters by dynamic 31P NMR analysis.10 Illustrated in Fig.
2 is a stack plot of the 31P NMR spectra of 2 from 220 °C to +70
°C. The spectroscopic changes are fully reversible over the
entire temperature range examined. Fitting the NMR data for an
unequally populated two-state exchanging system allowed the
determination of the activation parameters for the system.11 At
298 K the calculated values for the activation parameters are
(1)
Crystals of phosphatitanocene 2 were obtained from Et2O.
X-Ray analysis revealed that the compound had crystallized in
the rac isomer as illustrated in Fig. 1.§ Key structural details of
the molecule are summarized in Fig. 1 and are typical of
phosphatitanocenes.6 In the crystal structure the molecule lacks
pseudo-C2 symmetry despite the fact that the 31P NMR spectra
† Electronic supplementary information (ESI) available: X-ray structure
determination. See http://www.rsc.org/suppdata/cc/b2/b208945a/
Fig. 2 Variable temperature 31P NMR of phosphatitanocene dichloride 2 in
toluene. Temperature is indicated in Celsius.
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CHEM. COMMUN., 2002, 2996–2997
This journal is © The Royal Society of Chemistry 2002

g, 5.54 mmol) in THF (10 mL) was added. The reaction was stirred for 15
min and the solvent was removed under vacuum. The tin intermediate was
extracted with hexanes (30 mL), filtered through Celite and concentrated
1
yielding a pale yellow oil that slowly solidified (1.69 g, 87%). H NMR
(300.053 MHz, C6D6): d 7.4–7.0 (m, 5H, Ph), 6.56 (d, 2JPH = 41.1 Hz, 1H,
PCH), 2.14–2.05 (m, 6H, CH3-C), 20.15 (d, 3JPH = 1.8 Hz, 9H, Sn(CH3)3).
31P {1H} NMR (121.469 MHz, C6D6): d 252.69.
3,4-Dimethyl-2-phenyl-1-trimethylstannylphosphole (1.69 g, 4.8 mmol)
in hexanes (10 mL) was added rapidly to TiCl4·2THF ( 0.78 g, 2.35 mmol)
in CH2Cl2 (20 mL) at room temperature. After 4 h stirring, the resulting dark
green solution was concentrated and washed with hexanes (100 mL), then
dried under vacuum (0.87 g, 75%). Single crystals suitable for X-ray
analysis were prepared by recrystallization from ether at room temperature.
1H NMR (300.053 MHz, C6D6): d 7.4–7.0 (m, 10H, Ph), 5.93 (m, 2H, PCH),
2.27 (s, 6H, CH3), 2.09 (s, 6H, CH3). 31P {1H} NMR (121.469 MHz, C6D6):
d 100.92 (b). 13C {1H} NMR (75.456 MHz, CD2Cl2): d 149.73 (b), 148.96,
137.14 (t), 133.01 (m), 131.09 (t), 128.84, 128.62, 19.54, 16.84.
¯
§ Crystal data for C24H24Cl2P2Ti, 2: M = 493.17, triclinic, P1, a =
Scheme 1 Possible mechanism of isomerization of phosphametallocenes
from (S,S) to meso. Potential coordinated solvents or other ligands omitted
for clarity.
8.7827(8), b = 8.8656(8), c = 15.7127(14) Å, a = 96.547(2)°, b =
106.049(2)°, g = 102.179(2)°, U = 1129.80(18) Å3, Z = 2, T = 223(2) K,
dcalcd = 1.450 Mg m23, F(000) = 508, m(Mo-Ka) = 0.623 mm21, l(Mo-
Ka) = 0.71073 Å, 5584 independent reflections measured, R1 = 0.0330 (I
> 2.00s(I)), wR(F2) = 0.0950 (all data). CCDC 193948. See http:/
/www.rsc.org/suppdata/cc/b2/b208945a/ for crystallographic files in .cif or
other electronic format.
DG‡
= 11.5 (±1.0) kcal mol21, DH‡ = 16.3 (±3.0) kcal
298
mol21, and DS‡ = 16 (±11) cal mol21 K21. The rate constant
varied from 1500 s21 at 0 °C to 9.9 3 105 s21 at +70 °C. At 220
°C Keq = 3.7 indicating DG° = 0.66 kcal mol21
.
1 T. K. Hollis, L.-S. Wang and F. Tham, J. Am. Chem. Soc., 2000, 122,
11737.
2 S. Bellemin-Laponnaz, M. M. C. Lo, T. H. Peterson, J. M. Allen and G.
C. Fu, Organometallics, 2001, 20, 3453.
The exact nature of the chemical change that is taking place
is still under investigation, but the observed data are consistent
with P-inversion as the rate determining step.12,13 Our working
hypothesis for the mechanism of isomerization is outlined in
3 For a recent review see: C. Ganter, J. Chem. Soc.,Dalton Trans., 2001,
3541. For a seminal contribution see: S. Qiao and G. C. Fu, J. Org.
Chem., 1998, 63, 4168. For references to azametallocenes see: M. M. C.
Lo and G. C. Fu, J. Am. Chem. Soc., 2002, 124, 4572; M. M. C. Lo and
G. C. Fu, Tetrahedron, 2001, 57, 2621.
4 G. W. Coates and R. M. Waymouth, Science (Washington, D.C.), 1995,
267, 217; M. Dankova, R. L. Kravchenko, A. P. Cole and R. M.
Waymouth, Macromolecules, 2002, 35, 2882; S. Lin, C. D. Tagge, R.
M. Waymouth, M. Nele, S. Collins and J. C. Pinto, J. Am. Chem. Soc.,
2000, 122, 11275. For discussions of the mechanism, see: V. Busico, R.
Cipullo, W. P. Kretschmer, G. Talarico, M. Vacatello and V. V. Castelli,
Angew. Chem., Int. Ed., 2002, 41, 505.
5 F. Nief and F. Mathey, J. Chem. Soc., Chem. Commun., 1988, 770.
6 F. Nief, L. Ricard and F. Mathey, Organometallics, 1989, 8, 1473.
7 (a) For a recent example, see: V. R. Vangala, A. Nangia and V. M.
Lynch, Chem. Commun., 2002, 1304; K. B. Shiu, S. W. Jean, Y. Wang
and G. H. Lee, J. Organomet. Chem., 2002, 650, 268; (b) For reviews,
see: C. Janiak, J. Chem. Soc., Dalton Trans., 2000, 3885.
8 C. A. Hunter and J. K. M. Sanders, J. Am. Chem. Soc., 1990, 112, 5525;
C. A. Hunter, Chem. Soc. Rev., 1994, 23, 101.
9 L. Mueller, T. K. Hollis, Y. J. Ahn unpublished work. Solid state 31P
NMR of a rapidly concentrated solution indicated the presence of both
isomers.
10 Analysis performed using DNMR5: D. S. Stephenson and G. Binsch,
Program No. 365 QCPE, Indiana University, Bloomington, IN,
47405.
11 For references to the theory of two-state unequal populations, see: H.
Shanan-Atidi and K. H. Bar-Eli, J. Phys. Chem., 1970, 74, 961; L. H.
Piette and W. A. Anderson, J. Chem. Phys., 1959, 30, 899. For recent
examples of analysis of metallocenes by NMR, see: M. G. Klimpel, W.
A. Herrmann and R. Anwander, Organometallics, 2000, 19, 4666; L.
Jia, X. M. Yang, C. L. Stern and T. J. Marks, Organometallics, 1997, 16,
842.
5
Scheme 1. It is proposed that the h complex undergoes ring
slippage14 to an h complex, converting the planar chirality to a
1
center of chirality at P. The P-atom undergoes inversion
5
followed by ring slippage back to an h complex. Thus one
absolute configuration of the chiral plane is converted to the
other. This process interconverts one of the racemic isomers to
the meso or vice versa. It should be emphasized that coor-
dinately-unsaturated ring-slipped intermediates may have addi-
tional ligands such as coordinated solvent. In toluene the
observed DS‡ is positive with a large error. Coordination of
solvent by the intermediates should decrease entropy. However,
decomplexation of the ‘diene’ and the rearrangements involved
in inversion, including significant solvent reorganization and
loss of coordinated solvent at the transition state, increase
entropy. Strong inferences based on DS‡ require additional
results in more-coordinating solvents. Of course, this low
energy inversion is not available to cyclopentadienyl com-
plexes, but it may be available to heterocyclopentadienyl or
‘hetero-benezene’ ligands that have an atom with a lone pair or
an appropriate vacant orbital.
In conclusion, we have presented the first structural charac-
terization of a chiral phosphatitanocene complex. The activation
parameters for the rac/meso isomerization have been deter-
mined, which are relevant to the applications of hetero-
metallocenes for stereoselective catalysis. Experiments are
underway to distinguish between the ring slip and P-inversion
steps in the isomerization.
Acknowledgement is made to the Donors of the Petroleum
Research Fund, administered by the American Chemical
Society for partial support of this research. We thank the
Department of Education for a G.A.A.N.N. Fellowship for
Y.J.A. Acknowledgement is made to Dr Dan Borchardt of the
UCR-Analytical Chemistry Infrastructure Facility for assis-
tance with NMR Spectroscopy.
12 For reference to the decreased inversion barriers in phospholes, see: W.
Egan, R. Tang, G. Zon and K. Mislow, J. Am. Chem. Soc., 1971, 93,
6205; W. Egan, R. Tang, G. Zon and K. Mislow, J. Am. Chem. Soc.,
1970, 92, 1442.
13 For references to the low inversion barriers in phosphido complexes,
see: D. K. Wicht, I. Kovacik, D. S. Glueck, L. M. Liable-Sands, C. D.
Incarvito and A. L. Rheingold, Organometallics, 1999, 18, 5141; D. K.
Wicht, D. S. Glueck, L. M. Liable-Sands and A. L. Rheingold,
Organometallics, 1999, 18, 5130; M. A. Zhuravel, D. S. Glueck, L. N.
Zakharov and A. L. Rheingold, Organometallics, 2002, 21, 3208.
14 J. M. O’Connor and C. P. Casey, Chem. Rev., 1987, 87, 307. For
Notes and references
‡
rac- and meso-((3,4-(CH3)2-2-Ph)C4HP)2TiCl2. 3,4-Dimethyl-1-
phenylphosphole (1.04 g, 5.54 mmol), t-BuOK (0.62 g, 5.54 mmol) and
THF (15 mL) were heated at 140 °C for 20 h in a sealed tube. After cooling
the yellow solution to room temperature all volatiles were removed under
vacuum. THF (20 mL) was added and removed under vacuum. The yellow
solid was dissolved in THF (10 mL), a solution of trimethyltin chloride (1.1
1
examples of h phospholyl complexes, see: F. Mathey, Coord. Chem.
Rev., 1994, 137, 1. For an example of a dynamic diphospholyl ring
system, see: F. G. N. Cloke, K. R. Flower, C. Jones, R. M. Matos and J.
F. Nixon, J. Organomet. Chem., 1995, 487, C21.
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