Dihydrogen and silane addition to base-free, monomeric bis(cyclopentadienyl)titanium oxides

Article abstract of DOI:10.1021/ic070205+

Synthesis of a family of monomeric, base-free bis(cyclopentadienyl)titanium oxide complexes, (η⁵-C₅Me₄R) ₂Ti=O (R = ⁱPr, SiMe₃, SiMe₂Ph), has been accomplished by deoxygenation of styrene oxide by the corresponding sandwich compound. One example, (η⁵-C₅Me ₄SiMe₂Ph)₂Ti=O, was characterized by X-ray diffraction. All three complexes undergo clean and facile hydrogenation at 23°C, yielding the titanocene hydroxy hydride complexes (η⁵- C₅Me₄R)₂Ti(OH)H. For (η⁵-C ₅Me₄SiMe₃)₂Ti=O, the kinetics of hydrogenation were first-order in dihydrogen and exhibited a normal, primary kinetic isotope effect of 2.7(3) at 23°C consistent with a 1,2-addition pathway. Isotope effects of the same direction but smaller magnitudes were determined for silane addition.

Full text of DOI:10.1021/ic070205+

Inorg. Chem. 2007, 46, 2359−2361
Dihydrogen and Silane Addition to Base-Free, Monomeric
Bis(cyclopentadienyl)titanium Oxides
Tamara E. Hanna, Emil Lobkovsky, and Paul J. Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell UniVersity,
Ithaca, New York 14853
Received February 4, 2007
Synthesis of a family of monomeric, base-free bis(cyclopentadienyl)-
titanium oxide complexes, ( O (R
5-C5Me4R)2Ti iPr, SiMe3,
SiMe2Ph), has been accomplished by deoxygenation of styrene
oxide by the corresponding sandwich compound. One example,
product.5a The base-stabilized zirconocene oxo also promotes
addition chemistry with alkyl halide electrophiles.5 In this
contribution, we describe the synthesis of a family of
monomeric, base-free titanocene oxide complexes and report
the clean 1,2-addition of dihydrogen and silanes across the
TidO bond.
The isolation of thermally stable bis(cyclopentadienyl)-
titanium sandwich compounds, (η5-C5Me4R)2Ti (R ) alkyl,
silyl),6 has provided a rare opportunity to study the funda-
mental reaction chemistry at a well-defined, divalent ti-
tanocene center. The introduction of sterically demanding
silyl- and alkylcyclopentadienyl substituents has resulted in
the isolation of unusual mono(dinitrogen), mono(carbonyl),7
and bis(dinitrogen) compounds.8 On the basis of these
observations, the synthesis of monomeric, base-free ti-
tanocene oxide complexes was targeted because ligand
coordination is typically required to stabilize complexes such
as [(η5-C5Me5)2TidO].9
η
d
)
(
η
5-C5Me4SiMe2Ph)2Ti
d
O, was characterized by X-ray diffraction.
All three complexes undergo clean and facile hydrogenation at 23
C, yielding the titanocene hydroxy hydride complexes
C5Me4R)2Ti(OH)H. For ( O, the kinetics of
5-C5Me4SiMe3)2Ti
hydrogenation were first-order in dihydrogen and exhibited a
normal, primary kinetic isotope effect of 2.7(3) at 23 C consistent
°
( -
η5
η
d
°
with a 1,2-addition pathway. Isotope effects of the same direction
but smaller magnitudes were determined for silane addition.
The addition of molecular hydrogen across a metal-
oxygen bond is of interest, in part, because of the potential
role of this transformation in the hydrogenation of carbon
monoxide to methanol.1 Despite the ubiquity of metal oxides,
the direct observation of the addition of dihydrogen across
metal-oxygen bonds in homogeneous compounds remains
rare. The reduction of permanganate by dihydrogen to MnO2
has been known for almost a century2 and is believed to
proceed through MnV and a [3 + 2] cycloaddition pathway.3
More recently, Mayer and co-workers discovered the ligand-
induced hydrogenation of OsO4, where kinetic, mechanistic,
and density functional theory (DFT) studies also support
[3 + 2] cycloaddition.4 In early transition metal chemistry,
hydrogenation of Cp*2Zr(dO)py (Cp* ) η5-C5Me5;
py ) pyridine) proceeds cleanly and yields [Cp*2Zr(H)](µ2-
O)[Cp*2Zr(OH)]. This product most likely arises from the
initial 1,2-addition of dihydrogen to yield a transient zir-
conocene hydroxy hydride, which undergoes subsequent
chemistry with the base-stabilized oxo to yield the observed
Ideally, preparation of the desired base-free terminal oxo
derivatives could be accomplished directly from dioxygen.
i
Exposure of a pentane solution of (η5-C5Me4 Pr)2Ti (1) to a
slight excess of dioxygen gas at -78 °C furnished a
i
diamagnetic yellow solid identified as (η5-C5Me4 Pr)2Ti(η2-
O2) (1-O2) based on NMR spectroscopy, X-ray diffraction,
and combustion analysis (eq 1).
In a benzene-d6 solution at 23 °C, 1-O2 decomposes over
the course of hours. Despite this complication, the solid-
* To whom correspondence should be addressed. E-mail: pc92@
cornell.edu.
(1) Anderson, A. B.; Nichols, J. A. J. Am. Chem. Soc. 1986, 108, 4742.
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W. T.; Mayer, J. M. J. Am. Chem. Soc. 2005, 127, 3423.
10.1021/ic070205+ CCC: $37.00
Published on Web 03/01/2007
© 2007 American Chemical Society
Inorganic Chemistry, Vol. 46, No. 7, 2007 2359

COMMUNICATION
One of the monomeric terminal oxo compounds, 3dO,
was characterized by X-ray diffraction (Figure 1) and exhibits
idealized C2 molecular symmetry, with the TidO bond
serving as the principal axis. The TidO bond length of
1.670(2) Å is statistically equivalent to the TidO distance
of 1.665(3) Å for pyridine-stabilized (η5-C5Me5)2Ti(dO)L9b
and is similar to those reported in non-cyclopentadienyl-
ligated titanium oxides.13
Attempts to prepare 1dO and 2dO by O-atom abstraction
from ethylene oxide (EO) produced a different outcome.
Treatment of 1 with 1 equiv of EO yielded 20% 1dO and
Figure 1. Molecular structure of 1-O2 (left) and 3dO (right) at 30%
probability ellipsoids. Hydrogen atoms are omitted for clarity.
i
35% (η5-C5Me4 Pr)2Ti(η2-CH2dCH2) (1-C2H4). The remain-
ing 45% was an unidentified NMR-silent product that
i
i
converted to (η5-C5Me4 Pr)2TiCl2 and (η5-C5Me4 Pr)TiCl3
upon treatment with HCl. Performing a similar experiment
with 2 yielded 50% 2)O, 30% 2-C2H4, and 20% NMR-
silent material (eq 3). Similar results were obtained when
the order of addition was reversed.
state structure was determined (Figure 1) and confirms the
side-on hapticity of the peroxide ligand. The O-O bond
length of 1.441(4) Å, coupled with the Ti-O1 and Ti-O2
distances of 1.906(3) and 1.910(3) Å, is also consistent with
a TiIV center and an [O2]2- fragment.10
Because treatment of the titanium sandwich with dioxygen
did not yield the desired terminal oxo compound, O-atom
abstraction routes were explored. The addition of 1 equiv of
styrene oxide to a pentane solution of 1 immediately released
a stoichiometric quantity of styrene and yielded a new C2V-
symmetric titanium compound identified as the monomeric
i
oxo species, (η5-C5Me4 Pr)2TidO (1dO). A similar synthetic
procedure was used for the preparation of (η5-C5Me4SiMe3)2-
TidO (2dO) and (η5-C5Me4SiMe2Ph)2TidO (3dO) (eq 2).
Both 1-C2H4 and 2-C2H4 were independently prepared by
treating 1 and 2 with ethylene, respectively. Observation of
the olefin complexes following the addition of EO to 1 and
2 suggests that, for smaller O-atom donors, epoxide coor-
dination is slow compared to ring-opening and olefin loss,
allowing ethylene capture by the unreacted sandwich.
Because the amount of the ethylene complex remains
essentially constant, the relative amount of NMR-silent
material formed from 1 and 2 likely reflects the stability of
1dO versus 2dO.
With monomeric, base-free titanocene oxo complexes in
hand, fundamental 1,2-addition chemistry of the TidO bond
was explored. The addition of dihydrogen to 1dO, 2dO,
or 3dO furnished the corresponding titanocene hydroxy
hydride compounds, (η5-C5Me4R)2Ti(OH)H (eq 4). The
observation of clean hydrogenation chemistry highlights the
importance of base-free titanium oxo compounds because
the ligand-stabilized variant, (η5-C5Me5)2Ti(dO)L, yields a
mixture of products including (η5-C5Me5)2Ti(OH)2 and (η5-
C5Me5)2Ti(OH)H upon dihydrogen addition.14
The relative stability of each oxo derivative is related to
the cyclopentadienyl substituent. The least sterically protected
member of the series, isopropyl-substituted 1dO, undergoes
decomposition (t1/2 ∼ 48 h) over the course of days at 23
i
°C in pentane. Two products, (η5-C5Me4 Pr)2Ti(OH)2 (1-
i
(OH)2) and (η5-C5Me4 Pr)4Ti4O6, were cleanly and repro-
ducibly formed in a 4:1 ratio. An unidentified NMR-silent
material, accounting for approximately 50% of the mixture,
was also obtained and was protonated with HCl to yield a
i
i
3:2 mixture of (η5-C5Me4 Pr)2TiCl2 and (η5-C5Me4 Pr)TiCl3.
The silylated compounds, 2dO and 3dO, have proven to
be more stable and were isolated in the solid state. At 23
°C, decomposition occurs over the course of weeks and also
yields (η5-C5Me4SiMe2R)4Ti4O6 and an NMR-silent material.
One of the clusters, (η5-C5Me4SiMe3)4Ti4O6, was character-
ized by X-ray diffraction11 and is structurally similar to (η5-
C5Me5)4Ti4O6.12
(7) Hanna, T. E.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2006,
128, 6018.
(8) Hanna, T. E.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
126, 14688.
(9) (a) Bottomley, F. A.; Egharevba, G. O.; Lin, J. B.; White, P. S.
Organometallics 1985, 4, 550. (b) Smith, M. R.; Matsunaga, P. T.;
Andersen, R. A. J. Am. Chem. Soc. 1993, 115, 7049. L ) 4-phenyl-
pyridine.
Both primary and secondary silanes also produced clean
1,2-addition chemistry. The addition of either PhSiH3 or
Et2SiH2 to 2dO yielded the corresponding titanocene siloxy
(10) Cramer, C. J.; Tolman, W. B.; Theopold, K. H.; Rheingold, A. L.
Proc. Natl. Acad. Sci. 2003, 100, 3635.
(11) See the Supporting Information for details.
(12) Babcock, L. M.; Day, V. W.; Klemperer, W. G. Chem. Commun. 1987,
858.
2360 Inorganic Chemistry, Vol. 46, No. 7, 2007

COMMUNICATION
hydride complex, 2-(OSi)H, as judged by NMR and IR
spectroscopies (eq 4). No reaction was observed upon
addition of the tertiary silane Et3SiH.
compared to an S-H bond. Intermolecular exchange, as
judged by isotopic exchange between 1-(OD)D16 and free
dihydrogen, occurs over the course of hours at 23 °C. The
converse experiment, dideuterium addition to 1-(OH)H, also
produced isotopic exchange with no evidence for competing
cyclometalation. Similar experiments with isotopologues of
2-(OSi)H (Si ) PhSiH2) and free silane also established that
intermolecular exchange occurs with a higher barrier because
crossover was observed only upon heating to 70 °C for
several hours.
Attempts were also made to activate C-H bonds of
saturated and unsaturated hydrocarbons with 2dO. No
changes were observed upon the addition of excesses
(typically 10 equiv) of CH4, ethylene, or benzene (neat),
despite the established propensity for these molecules to
participate in 1,2-addition chemistry.17 Warming these
mixtures simply hastened decomposition of the titanocene
oxide. This lack of reactivity contrasts the facile C-H
chemistry observed with transiently generated titanocene
alkylidene complexes18 and unsaturated imide compounds19
and is likely a result of stronger TidE π bonding for the
oxo derivative. For dihydrogen and silanes, rupture of
relatively weak H-H and Si-H bonds, coupled with the
formation of strong O-H and Si-O linkages, renders these
reactions spontaneous.
In summary, the introduction of large alkyl- and silyl-
cyclopentadienyl substituents has resulted in stabilization of
monomeric, base-free titanocene oxide complexes. These
molecules undergo facile 1,2-addition of dihydrogen and
Si-H bonds of primary and secondary silanes but are
unreactive toward C-H bonds of saturated and unsaturated
hydrocarbons because of their inability to overcome strong
TidO π bonding.
Acknowledgment. We thank the National Science Foun-
dation and the Director, Office of Basic Energy Sciences,
Chemical Sciences Division, of the U.S. Department of
Energy (Grant DE-FG02-05ER15659) for financial support.
P.J.C. is a Cottrell Scholar sponsored by the Research Corp.
and a David and Lucile Packard Fellow in science and
engineering.
The mechanism of both dihydrogen and silane addition
to 2dO was investigated in more detail. In a pentane
solution, 2dO exhibits a weak band (ꢀ ) 170 cm-1 M-1)
centered at 746 nm assigned as a ligand-to-metal charge
transfer. The low intensity of the peak is likely the result of
1
a symmetry-forbidden excitation from the A1 state to the
1A2 state. A depiction of the DFT-computed frontier molec-
ular orbitals for this compound is reported in the Supporting
Information. Monitoring the disappearance of this band as a
function of time in the presence of 370 equiv of dihydrogen
at 23 °C yielded a pseudo-first-order rate constant of 8.5(5)
× 10-3 s-1 for TidO hydrogenation. The corresponding
experiment with dideuterium gas produced a value of 3.2(2)
× 10-3 s-1 and a normal, primary kinetic isotope effect of
2.7(3) at 23 °C. Attempts to measure the pressure dependence
of dihydrogen on the observed rate constants have met with
mixed success. At high pressures (PH > 0.537 atm; 225
2
equiv), a linear correlation is observed and is reproducible.11
At lower pressures, significant deviations are observed. The
origin of this behavior is not completely understood but is
likely due to inefficient gas mixing with the pentane solution.
Kinetic isotope effects were also determined for silane
addition. Measuring the product ratio following the addition
of 10 equiv of an equimolar mixture of PhSiH3/PhSiD3 to
2dO yielded a kinetic isotope effect of 1.5(1) at 23 °C.
Control experiments were conducted and did not produce
crossover between the isotopologues of the products. A
similar value of 1.2(1) (23 °C) was measured using the same
procedure as for the addition of Et2SiH2/Et2SiD2. Kinetic
isotope effects of this direction and magnitude are compa-
rable to those measured for silane addition to the putative
(η5-C5Me5)2TidS complex14 and are significantly smaller
than that for dihydrogen/dideuterium addition, possibly as a
result of a less symmetric transition structure arising from
the addition of a relatively polar Si-H bond.
The relative rate of dihydrogen 1,2-elimination from
1-(OH)H was probed by exchange NMR spectroscopy. No
cross-peaks between the Ti-H and O-H bonds were
observed up to 90 °C (mixing time 700 ms), demonstrating
that intramolecular exchange does not occur on the NMR
time scale. Similar behavior was observed with 2-(OSi)H.
These barriers are higher than those reported for the
analogous sulfido complex (η5-C5Me5)2Ti(SH)H, where the
intramolecular process occurred on the NMR time scale,
implicating an η2-H2 intermediate.14 The higher barrier for
1,2-elimination in 1-(OH)H is consistent with a dominant
ground-state effect resulting from a stronger O-H bond as
Supporting Information Available: Experimental procedures,
kinetic data, and crystallographic data for 1-O2 and 3dO in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org. CCDC 636540 contains the supplementary data
for 2-cluster and can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif (12 Union Road, Cambridge CB2 1EZ, U.K.; tel
+44 1223 336408; fax +44 1223 336033).
IC070205+
(16) Special care must be taken when handling (η5-C5Me4R)2Ti(OD)D
compounds because proton contamination is observed upon generation
in standard flame-dried laboratory glassware. This complication can
be minimized by deuteration of the glassware prior to dideuterium
addition.
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Inorganic Chemistry, Vol. 46, No. 7, 2007 2361