Synthesis of a base-free titanium imido and a transient alkylidene from a titanocene dinitrogen complex. Studies on Ti=NR hydrogenation, nitrene group transfer, and comparison of 1,2-addition rates

Article abstract of DOI:10.1021/om049817h

The synthesis and reactivity of the end-on bound dinitrogen complex [(η⁵-C₅H₃-1,3-(SiMe₃₎₂₎₂Ti] ₂(μ₂,η¹,η¹-N ₂) is described. The solid state structure of the dinitrogen compound reveals a weakly activated end-on bound N₂ ligand with an N-N bond length of 1.164(5) A. Displacement of the N₂ ligand by organic azides has been used to prepare monomeric, base-free titanocene imido complexes, (η⁵-C₅H₃-1,3-(SiMe₃₎₂₎₂Ti=NR (R = SiMe₃, 2,4,6-Me₃-C₆H₂). While unreactive toward C-H bonds, the Ti-N linkage is readily hydrogenated and participates in group transfer reactions with unsaturated organic molecules such as carbon monoxide and benzophenone. Reaction of the N₂ complex with Ph₂CN₂ allowed isolation of (η5-C₅H ₃-1,3-(SiMe₃₎₂₎₂Ti(N₂CPh₂), which exists as a mixture of interconverting η² and n¹ isomers in solution. The diazoalkane complex also participates in imido-like reactivity, producing (η⁵-C ₅H₃-1,3-(SiMe₃₎₂₎₂Ti(NHN=CPh₂)H upon addition of H₂. Changing the diazoalkane to Me₃SiCHN ₂ resulted in isolation of the double cyclometalated titanocene (η⁵-C₅H₃-η¹-SiM ₂CH₂-3-SiMe₃₎₂Ti, arising from facile intramolecular C-H activation of the cyclopentadienyl substituent by a transient titanocene alkylidene.

Full text of DOI:10.1021/om049817h

3448
Organometallics 2004, 23, 3448-3458
Syn th esis of a Ba se-F r ee Tita n iu m Im id o a n d a
Tr a n sien t Alk ylid en e fr om a Tita n ocen e Din itr ogen
Com p lex. Stu d ies on TidNR Hyd r ogen a tion , Nitr en e
Gr ou p Tr a n sfer , a n d Com p a r ison of 1,2-Ad d ition Ra tes
Tamara E. Hanna, Ivan Keresztes, Emil Lobkovsky,
Wesley H. Bernskoetter, and Paul J . Chirik*
Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University,
Ithaca, New York 14853
Received March 12, 2004
The synthesis and reactivity of the end-on bound dinitrogen complex [(η5-C5H3-1,3-
(SiMe3)2)2Ti]2(µ2,η1,η1-N2) is described. The solid state structure of the dinitrogen compound
reveals a weakly activated end-on bound N2 ligand with an N-N bond length of 1.164(5) Å.
Displacement of the N2 ligand by organic azides has been used to prepare monomeric, base-
free titanocene imido complexes, (η5-C5H3-1,3-(SiMe3)2)2TidNR (R ) SiMe3, 2,4,6-Me3-C6H2).
While unreactive toward C-H bonds, the Ti-N linkage is readily hydrogenated and
participates in group transfer reactions with unsaturated organic molecules such as carbon
monoxide and benzophenone. Reaction of the N2 complex with Ph2CN2 allowed isolation of
(η5-C5H3-1,3-(SiMe3)2)2Ti(N2CPh2), which exists as a mixture of interconverting η2 and η1
isomers in solution. The diazoalkane complex also participates in “imido-like” reactivity,
producing (η5-C5H3-1,3-(SiMe3)2)2Ti(NHNdCPh2)H upon addition of H2. Changing the
diazoalkane to Me3SiCHN2 resulted in isolation of the double cyclometalated titanocene (η5-
C5H3-η1-1-SiMe2CH2-3-SiMe3)2Ti, arising from facile intramolecular C-H activation of the
cyclopentadienyl substituent by a transient titanocene alkylidene.
In tr od u ction
elimination from zirconocene alkyl hydride complexes
has allowed access to interesting zirconium dinitrogen
complexes. One example is the side-on bound zir-
conocene dinitrogen complex [(η5-C5H3-1,3-(SiMe3)2)2-
Zr]2(µ2,η2,η2-N2), which possesses a strongly activated
N2 ligand with an N-N bond length of 1.46 Å.8 Most
recently, we have discovered that the side-on bound
dinitrogen complex [(η5-C5Me4H)2Zr]2(µ2,η2,η2-N2) reacts
rapidly with 1 atm of dihydrogen at ambient tempera-
ture to afford the zirconium hydrazido complex [(η5-C5-
Me4H)2ZrH]2(µ2,η2,η2-N2H2). Mild thermolysis in the
presence of excess H2 affords small quantities of free
ammonia.10
With these results in hand, we became interested in
studying the corresponding titanocene dinitrogen chem-
istry. Here we describe the synthesis and characteriza-
tion of a weakly activated titanocene dinitrogen complex
that can be used as a synthon for monomeric, base-free
titanium imido, diazoalkane, and transient alkylidene
complexes. The reactivity of the imido and diazoalkane
complexes with dihydrogen has been explored, and the
rates of intramolecular C-H activation by the imido and
alkylidene complexes have been compared by both
experimental and computational studies.
Recently there has been renewed interest in the
coordination chemistry of dinitrogen, fueled by new
examples of transition metal,1 lanthanide,2 and actinide3
N2 complexes. Coupled with these developments is the
discovery of new stoichiometric4 and even catalytic5
reactions that functionalize N2. Group 4 transition metal
complexes have played a prominent role in this renais-
sance with discoveries such as the partial hydrogenation
of N2 with a zirconium phosphine-amide complex6 and
the synthesis of N-heterocycles from in situ-generated
titanium dinitrogen species.7
Our laboratory has focused on the synthesis and
reactivity of bis-cyclopentadienyl zirconium dinitrogen
complexes. Elucidation of both cyclopentadienyl8 and
alkyl substituent9 effects on the rate of alkane reductive
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Resu lts a n d Discu ssion
Syn th esis a n d Ch a r a cter iza tion of [(η5-C5H3-1,3-
(SiMe3)2)2Ti]2(µ2,η1, η1-N2). The strongly activated N2
(9) Pool, J . A.; Lobkovsky, E.; Chirik, P. J . Organometallics 2003,
22, 2797.
(10) Pool, J . A.; Lobkovsky, E.; Chirik, P. J . Nature 2004, 427, 527.
10.1021/om049817h CCC: $27.50 © 2004 American Chemical Society
Publication on Web 06/08/2004

Base-Free Titanium Imido
Organometallics, Vol. 23, No. 14, 2004 3449
ligand in [(η5-C5H3-1,3-(SiMe3)2)2Zr]2(µ2,η2,η2-N2)prompted
us to explore the synthesis of the corresponding tita-
nium complex. Specifically, we were interested in the
effect of the smaller, less reducing first-row transition
metal on the activation and hapticity of the N2 ligand.
Typically titanocene dinitrogen complexes such as [(η5-
C5Me5)2Ti]2(µ2,η1,η1-N2)11 and [(η5-C5Me4H)2Ti]2(µ2,η1,η1-
N2)12 contain end-on bound dinitrogen ligands with
relatively short (∼1.1 Å) N-N bond lengths.13 However,
examples of complexes with strongly activated side-on
coordinated N2 ligands have been reported.14
Alkali metal reduction of a suitable Ti(III) or Ti(IV)
halide precursor is an effective means of preparing the
corresponding dinitrogen complex.15 Because (η5-C5H3-
1,3-(SiMe3)2)2TiCl16 is readily synthesized, we chose this
route for the preparation of the desired dinitrogen
compound.17 Treatment of the titanocene monochloride
with 5 equiv of sodium amalgam under 1 atm of
dinitrogen afforded a royal blue crystalline solid identi-
fied as [(η5-C5H3-1,3-(SiMe3)2)2Ti]2(µ2,η1,η1-N2) (1) (eq 1).
Although 1 is paramagnetic, a single broad resonance
(∆ν1/2 ) 260 Hz) is observed at 2.7 ppm, assigned to the
equivalent protons of the [SiMe3] groups. The solution
magnetic moment of 1 was determined in benzene-d6
by Evans’ method18 and found to be 2.38 µB per titanium
center. This value is significantly lower than the spin-
only value of 2.83 µB for two unpaired electrons and is
suggestive of exchange coupling of the two titanium
centers across the N2 ligand. The observed magnetism
is in good agreement with the solution (2.28 µB)19 and
solid state (2.38 µB)11 magnetic moments reported for
[(η5-C5Me5)2Ti]2(µ2,η1,η1-N2).
F igu r e 1. Molecular structure of 1 with 30% probability
ellipsoids. Hydrogen atoms omitted for clarity.
Ta ble 1. N-N Bon d Dista n ces in Tita n ocen e
Din itr ogen Com p lexes
compound
d(N-N) (Å)
ref
1
1.164(5)
1.160(14)
1.170(4)
1.191(8)
1.162(12)
this work
[(η5-C5Me5)2Ti]2(µ2,η1,η1-N2)
11
12
11
20
[(η5-C5Me4H)2Ti]2(µ2,η1,η1-N2)
[(η5-C5H5)2Ti(PMe3)]2(µ2,η1,η1-N2)
[(η5-C5H5)2Ti(4-Me-C6H4)]2(µ2,η1,η1-N2)
with the Ti(1)-N(1)-N(2) bond angle of 176(3)°. The
titanium centers are canted with respect to each other,
forming a 9.6° dihedral angle between the planes
defined by the metallocene wedges. The [SiMe3] sub-
stituents on the cyclopentadienyl ligands adopt a fa-
miliar orientation where the [SiMe3] groups gear in an
alternating “forward-back” arrangement with respect to
the dinitrogen ligand. Similar conformations are ob-
served in the related zirconium,8 thulium,2c and dyspros-
ium2d dinitrogen complexes.
The weak activation of the N2 ligand suggested that
1 may serve as a synthon for other interesting ti-
tanocene derivatives by displacement of weakly coordi-
nated dinitrogen. A similar approach has been used in
the synthesis of a guanidinate-supported titanium imido
complex15b as well as in the preparation of chalcogenido
titanium amidinate species.15a Because metal imido and
hydrazido complexes may be important intermediates
in the reduction of dinitrogen to ammonia,10,22 we
targeted the preparation of compounds with well-
defined titanium-nitrogen bonds. Addition of an excess
of commercially available Me3SiN3 to 1 resulted in
effervescence and liberation of 3 equiv of N2, allowing
isolation of red crystals identified as (η5-C5H3-1,3-
(SiMe3)2)2TidNSiMe3 (2). Using an identical procedure,
(η5-C5H3-1,3-(SiMe3)2)2TidNMes (Mes ) 2,4,6-Me3-C6H2,
3) was also prepared (eq 2).
Single crystals suitable for X-ray diffraction were
obtained from slow evaporation of a concentrated pen-
tane solution of 1 under an atmosphere of N2. The solid
state structure of 1 is presented in Figure 1 and
confirms the identity of the molecule as the end-on
bound dinitrogen complex. The N-N bond length of
1.164(5) Å is consistent with a weakly activated N2
ligand and is in good agreement with other structurally
characterized titanocene dinitrogen complexes (Table
1).20,21 The Ti(1)-N(1)-N(2)-Ti(2) core is nearly linear
(11) Sanner, R. D.; Duggan, D. M.; McKenzie, T. C.; Marsh, R. E.;
Bercaw, J . E. J . Am. Chem. Soc. 1976, 98, 8358.
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1996, 15, 4977.
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379.
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Am. Chem. Soc. 1991, 113, 8986.
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893. (b) Mullins, S. M.; Duncan, A. P.; Bergman, R. G.; Arnold, J . Inorg.
Chem. 2001, 40, 6952.
(16) Okuda, J . J . Organomet. Chem. 1988, 356, C43.
(17) We have also discovered that bimolecular reductive elimination
of dihydrogen from (η5-C5H3-1,3-(SiMe3)2)2TiH is an effective means
for preparing 1. Hanna, T. E.; Chirik, P. J ., unpublished results.
(18) Sur, S. K. J . Magn. Reson. 1989, 82, 169.
1
Both titanocene imido complexes display H and 13C
NMR spectra consistent with C2v symmetric molecules
in solution. The solid state structure of 2 was deter-
(19) Bercaw, J . E. J . Am. Chem. Soc. 1974, 96, 5087.

3450 Organometallics, Vol. 23, No. 14, 2004
Hanna et al.
Sch em e 1
F igu r e 2. Molecular structure of 2 with 30% probability
ellipsoids. Hydrogen atoms omitted for clarity.
These observations are consistent with a dynamic
process of amido group oscillation and rotation about
the Ti-N bond as described previously for analogous
hafnocenes.29 Cooling a toluene-d8 solution of 4 to -70
°C resulted in broadening of the cyclopentadienyl hy-
drogens, but complete desymmetrization of the ligand
environment was not observed, suggesting the barrier
for oscillation and rotation is relatively low (<10 kcal/
mol). The reactivity of 2 and 3 with dihydrogen parallels
the hydrogenation chemistry for (η5-C5Me5)2TidNPh30
and (η5-C5Me5)2Ti(S)(py)31 (py ) pyridine), both of which
readily add H2 at ambient temperature.
mined by X-ray diffraction (Figure 2) and reveals a
monomeric, base-free titanocene imido complex. In
agreement with the solution NMR data, a near linear
titanium imido fragment is observed with a Ti(1)-N(1)-
Si(5) bond angle of 175.8(2)°. This value, in addition to
the titanium nitrogen bond length of 1.722(4) Å, is
similar to those found in (η5-C5Me5)2TidNPh23 as well
as other non-cyclopentadienyl-ligated titanium imido
compounds.24
Both 2 and 3 are thermally robust molecules in
benzene-d6, producing no reaction or decomposition after
heating to 110 °C for several days. Neither intramo-
lecular activation of the [SiMe3] substituents nor inter-
molecular benzene activation was observed. This be-
havior contrasts the high reactivity of the transient
titanium imido complexes [(tBu3SiNH)2TidNSitBu3]25
and [(tBu3SiO)2TidNSitBu3],26 as well as a zirconocene
imido27 that activates carbon-hydrogen bonds. This
lack of reactivity for C-H bond activation is reminiscent
of titanocene imido base adducts28 and other isolable
non-cyclopentadienyl-supported imido complexes.24
More relevant to dinitrogen hydrogenation is the
reactivity of 2 and 3 with H2. Exposure of benzene-d6
solutions of 2 and 3 to 1 atm of dihydrogen resulted in
immediate hydrogenation to the titanocene amido hy-
dride complexes (η5-C5H3-1,3-(SiMe3)2)2Ti(NRH)H (R )
SiMe3 (4), 2,4,6-Me3-C6H2 (5)), arising from facile 1,2-
addition of H2 (eq 3). Both complexes are Cs symmetric
in solution, as judged by equivalent cyclopentadienyl
ligands in both 1H and 13C NMR spectra. Titanium
hydrides are observed at 2.05 ppm, and N-H reso-
nances appear at 7.05 ppm for 4 and 7.90 ppm for 5.
Preparation of the corresponding deuterated ti-
tanocene amido deuteride complexes, (η5-C5H3-1,3-
(SiMe3)2)2Ti(NRD)D (R ) SiMe3 (4-d 2), 2,4,6-Me3-C6H2
(5-d 2)), was accomplished by addition of 1 atm of
deuterium to 2 and 3. Addition of H2 to either 4-d 2 or
5-d 2 resulted in isotopic exchange over the course of
hours at ambient temperature in both the titanium-
deuteride and N-D positions. Preferential (∼2:1) H/D
exchange is observed in the titanium-deuteride over the
N-D position, indicating that σ-bond metathesis of the
Ti-D bond with free H2 is competitive with 1,2-addition/
elimination (Scheme 1).25
Rea ction of 1 w ith Dia zoa lk a n es. In addition to
titanocene imido complexes, we have also targeted the
synthesis of titanium alkylidenes using the dinitrogen
leaving group strategy. Interest in these molecules dates
to the aluminum-bridged methylene complexes of
Tebbe,32 which have evolved into useful “Wittig-type”
reagents for organic synthesis.33 Since that time several
(20) Berry, D. H.; Procopio, L. J .; Carroll, P. J . Organometallics 1998,
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(23) (η5-C5Me5)2TidNPh: Ti-N ) 1.765(3) Å, Ti-N-C ) 176.8(3)°.
Smith, M. R.; Ball, G. E.; Andersen, R. A., unpublished results.
(24) Mountford, P. Chem. Commun. 1997, 2127.
(25) Cummins, C. C.; Schaller, C. P.; Van Duyne, G. D.; Wolczanski,
P. T.; Chan, A. W. E.; Hoffmann, R. J . Am. Chem. Soc. 1991, 113, 2985.
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Organometallics 1988, 7, 1309.
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1998, 120, 13405.
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Trans. 1997, 293.
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Kubinec, M. G. J . Am. Chem. Soc. 1997, 119, 4543.
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1978, 100, 3611.

Base-Free Titanium Imido
Organometallics, Vol. 23, No. 14, 2004 3451
titanium alkylidene complexes have been reported,34
most of which have been prepared by R-hydrogen
abstraction.35 One notable exception is the extensive
work of Andersen and Bergman, who studied the
reaction between (η5-C5Me5)2Ti(η2-CH2dCH2) and dia-
zoalkanes to yield highly reactive alkylidene species.36-38
Addition of Ph2CN2 to a pentane solution of 1 resulted
in isolation of a dark red solid identified as (η5-C5H3-
1,3-(SiMe3)2)2Ti(N2CPh2) (6) (eq 4). At ambient temper-
ature in benzene-d6, the 1H NMR spectrum of 6 appears
broad and featureless and is suggestive of dynamic
behavior. Cooling a toluene-d8 solution of 1 to temper-
atures below -20 °C produced an NMR spectrum
consistent with two isomeric titanium diazoalkane
products. Assignment of the resonances for both the
major and minor product was accomplished by a DQ-
COSY experiment, the results of which are reported in
the Experimental Section and the Supporting Informa-
tion.
of the cyclopentadienyl hydrogens (Cp1a) of the η1
product with the corresponding hydrogen (Cp2a) in the
η2 isomer. Similar results are obtained for the other
cyclopentadienyl hydrogens, the trimethylsilyl reso-
nances, and phenyl rings (Figure 3). These data estab-
lish rapid interconversion between the η2 and η1 isomers
on the time scale of the EXSY experiment. Rate con-
stants as a function of temperature for this exchange
process were measured by variable-temperature 1H
NMR spectroscopy by recording spectra in 10 deg
increments from -20 to 70 °C in toluene-d8. Line shape
analysis applying a two-site model corrected for the
relative populations produced an excellent fit to the
Eyring equation and yielded activation parameters of
∆Hq ) 16.9(3) kcal/mol and ∆Sq ) 8(2) eu, consistent
with a rate-determining dissociation of the â-nitrogen
for the η2 f η1 interconversion.
Several examples of titanocene diazoalkane complexes
have been reported previously. Reaction of diethyl
diazomalonate with (η5-C5H5)2Ti(CO)2 resulted in expul-
sion of carbon monoxide and afforded the diazoalkane
complex where the diazo ligand is η3-N,N,O bound to
the titanium.39 The titanocene bis-phosphine complex
(η5-C5H5)2Ti(PMe3)2 reacts with N2CPh2 in a similar
fashion, liberating 1 equiv of PMe3 and yielding (η5-
C5H5)2Ti(η1-N2CPh2)PMe3.40 More recently, a series of
permethyltitanocene diazoalkane complexes have been
synthesized by addition of N2CR2 to (η5-C5Me5)2Ti(η2-
CH2dCH2).36,37 Formation of the diazoalkane complex
is believed to proceed by insertion of the diazoalkane
into a titanium-carbon bond to form an azametallacy-
clobutane, which undergoes loss of olefin to yield the
observed product.37 This mechanism is supported by
similar observations with zirconium41 and the reluc-
tance of the ethylene complex to lose olefin to form the
titanocene.42
The major product (∼70%) is Cs symmetric, displaying
two [SiMe3] groups, three cyclopentadienyl resonances,
and six distinct phenyl peaks, consistent with the η2-
diazoalkane complex. The minor product, comprising
approximately 30% of the product mixture at this
temperature, is assigned as the η1 diazoalkane adduct
based on the observation of a C2v symmetric molecule
with one [SiMe3] resonance, two cyclopentadienyl peaks,
and three phenyl resonances. Warming the solution
above -20 °C resulted in broadening of all of the
resonances for both compounds at most intermediate
temperatures. Unfortunately peak overlap and signifi-
cant line broadening prevented accurate quantitation
of both isomers by integration. Above 70 °C, only one
set of NMR signals is observed, consistent with either
complete conversion to the η1 product or rapid inter-
conversion between the η2 and η1 isomers.
For 6, the mechanism of formation most likely pro-
ceeds by displacement of coordinated dinitrogen by
diazoalkane. Dissociation of N2 from 1 is observed when
the molecule is exposed to vacuum,43 supporting the
notion that dinitrogen can be easily displaced by more
potent ligands. Although several examples of titanocene
diazoalkane complexes are known, to our knowledge
observation of the dynamic η2, η1 interconversion of the
diazoalkane ligand in 6 appears to be unique. However,
this dynamic behavior does not translate into ability to
lose N2 and form an alkylidene complex. Attempts to
induce alkylidene formation by thermolysis of 6 at 95
°C in benzene-d6 for 2 days produced no reaction, similar
to thermal stability reported for (η5-C5Me5)2Ti(η2-N2-
CPh2), which resists loss of N2 up to temperatures of
135 °C.37 Attempts to induce alkylidene formation by
addition of Sm(OTf)3 have also been unsuccessful.44
Likewise, addition of the benzene-soluble Lewis acid
B(C6F5)3 produced no reaction.
The solution dynamics of the mixture of diazoalkane
products was studied using EXSY NMR spectroscopy
at -20 °C with a mixing time of 0.2 s. The EXSY
spectrum (Figure 3) establishes exchange between one
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(39) Gambarotta, S.; Floriani, C.; Chiesi-Villa, A.; Guastini, C. J .
Am. Chem. Soc. 1982, 104, 1918.
(40) Kool, L. B.; Rausch, M. D.; Alt, H. G.; Herberhold, M.; Hill, A.
F.; Thewalt, U.; Wolf, B. Chem. Commun. 1986, 408.
(41) Vaughan, G. A.; Hillhouse, G. L.; Rheingold, A. L. J . Am. Chem.
Soc. 1990, 112, 7994.
(42) Cohen, S. A.; Auburn, P. R.; Bercaw, J . E. J . Am. Chem. Soc.
1983, 105, 1136.
(43) Hanna, T. E.; Chirik, P. J ., unpublished results.
(44) Mindiola, D. J .; Hillhouse, G. L. J . Am. Chem. Soc. 2002, 124,
9976.
(37) Polse, J . L.; Kaplan, A. W.; Andersen, R. A.; Bergman, R. G. J .
Am. Chem. Soc. 1998, 120, 6316.
(38) Kaplan, A. W.; Polse, J . L.; Ball, G. E.; Andersen, R. A.;
Bergman, R. G. J . Am. Chem. Soc. 1998, 120, 11649.

3452 Organometallics, Vol. 23, No. 14, 2004
Hanna et al.
F igu r e 3. EXSY NMR spectrum of 6 in toluene-d8 at -20 °C.
The η1 titanocene diazoalkane complex can be trapped
by the addition of π-acid or σ-donating ligands. Exposure
of a pentane solution of 6 to 1 atm of CO yielded (η5-
C5H3-1,3-(SiMe3)2)2Ti(η1-N2CPh2)(CO) (7) (eq 5). The
solution infrared spectrum of 7 in benzene-d6 displays
a strong CO stretch centered at 2044 cm-1. This value
is shifted significantly from the stretches observed at
1881 and 1960 cm-1 for (η5-C5H3-1,3-(SiMe3)2)2Ti(CO)2.45
The difference in CO stretching frequencies is attributed
to a greater degree of π-back-bonding from the formally
Ti(II), d2 center in the dicarbonyl complex as compared
to the Ti(IV) center in 7. However, the CO stretching
pyridine (DMAP) to 6 to afford (η5-C5H3-1,3-(SiMe3)2)2-
Ti(η1-N2CPh2)(DMAP).
Previous work with permethyltitanocene diazoalkane
complexes has shown that coordination of a Lewis base
t
such as pyridine, DMAP, or BuNtC induces a change
in the hapticity of the diazoalkane ligand.37,38 However,
addition of CO to (η5-C5Me5)2Ti(η2-N2CHTol) (Tol )
p-CH3-C6H4) induces N-N bond cleavage to form the
isocyanate complex (η5-C5Me5)2Ti(NdCHTol)(NCO).38
For 6, only the carbonyl adduct 7 is observed upon
reaction with CO. This difference in reactivity is most
likely due to the reduced nucleophilicity of the nitrogen
atom bound to the titanium in 7 arising from the
relative electron-withdrawing nature of the silylated
cyclopentadienyl ligands. For the putative (η5-C5Me5)2-
frequency of 7 is reduced compared to free CO (νCO
)
2143 cm-1). Reduction of the CO bond order can be
rationalized by π-back-bonding interaction arising from
a significant contribution from a resonance structure
containing a formal titanium carbon double bond (struc-
ture B, eq 5). The η1 diazoalkane adduct can also be
trapped by addition of 1 equiv of (dimethylamino)-
(45) Lappert, M. F.; Leung, W. P.; Bartlett, R. A.; Power, P.
Polyhedron 1989, 8, 1883.

Base-Free Titanium Imido
Organometallics, Vol. 23, No. 14, 2004 3453
Sch em e 2
Ti(η1-N2CHTol)CO, the more electron-donating [C5Me5]
ligand engenders a more electron-rich metal center and
hence a more nucleophilic nitrogen atom, which then
can attack coordinated carbon monoxide, inducing N-N
bond cleavage.
assign the low-temperature structure of 8 as the η1 or
η2 adduct. It should be noted that silane addition to (η5-
C5Me5)2Ti(η2-N2CHTol) produced a η2-coordinated dia-
zoalkane ligand in the solid state,38 although observa-
tion of the η1 isomer for 6 suggests that dissociation of
the â-nitrogen from the titanium is plausible. The clean
hydrogenation chemistry associated with 6 and 7 ap-
pears to be unique since addition of H2 to (η5-C5Me5)2-
Ti(η2-N2CHTol) resulted in decomposition.38
The facile interconversion between the η2 and η1
structures prompted us to study the reaction of 6 with
dihydrogen. Addition of 1 atm of H2 to 6 resulted in
rapid hydrogenation of the diazoalkane ligand, forming
(η5-C5H3-1,3-(SiMe3)2)2Ti(NHNdCPh2)H (8) (eq 6). Evi-
dence for N-H bond formation was provided by benzene-
d6 solution 1H NMR and IR spectroscopies, which
displayed a singlet at 9.95 ppm and a broad absorption
centered at 3243 cm-1, respectively. The 1H NMR
resonance disappeared upon preparation of the corre-
sponding deuterium-labeled compound (η5-C5H3-1,3-
(SiMe3)2)2Ti(NDNdCPh2)D (8-d 2), and the IR stretch
shifted appropriately to 2298 cm-1. In addition, a
titanium hydride resonance was observed at 1.97 ppm
in the 1H NMR spectrum, which disappears upon
deuteration. Similarly, addition of H2 to 7 resulted in
loss of carbon monoxide and formation of 8.
Reaction of the dinitrogen complex 1 with the less
hindered diazoalkane Me3SiCHN2 resulted in loss of N2
and formation of the double-cyclometalated titanocene
complex (η5-C5H3-η1-1-SiMe2CH2-3-SiMe3)2Ti (9) and
SiMe4 (eq 7). Monitoring the reaction in situ by 1H NMR
spectroscopy allowed detection of the 2 equiv of SiMe4
formed. Although two different isomers of 9 are possible,
1
only one is observed by H and 13C NMR spectroscopy.
In benzene-d6 solution, a molecule of C2 rather than Cs
symmetry is observed, consistent with cyclometalation
occurring from [SiMe3] groups on opposite cyclopenta-
dienyl rings.
Formation of 9 most likely proceeds by formation of
the transient alkylidene complex (η5-C5H3-1,3-(SiMe3)2)2-
TidCHSiMe3, followed by 1,2-addition of a [SiMe3] C-H
bond to form the unobserved titanocene cyclo-
metalated alkyl (η5-C5H3-1,3-(SiMe3)2)(η5-C5H3-η1-1-
SiMe2CH2-3-SiMe3)TiCH2SiMe3 (Scheme 2). Attempts
to trap the intermediate alkylidene complex by perform-
ing the reaction in the presence of PMe3 yielded only 9.
Similar intramolecular C-H bond activations have
been observed with transient titanocene alkylidene
complexes.37,46,47 Cyclometalation of the second [SiMe3]
group can occur either by σ-bond metathesis or by R-hy-
drogen abstraction. Previous work with (η5-C5H4R)(η5-
C5H4-η1-CMe2CH2)TiCH2CMe3 supports the R-hydrogen
abstraction pathway.46 It should also be noted that
thermolysis of 1 does not produce 9.43
At temperatures above 0 °C, 1H NMR spectra of 8 are
consistent with a molecule of Cs symmetry with equiva-
lent cyclopentadienyl ligands in analogy with the spec-
tral properties of the titanocene amido hydride, 4.
Cooling toluene-d8 solutions of 8 to -70 °C produced a
C1 symmetric molecule with four inequivalent SiMe3
resonances and six cyclopentadienyl peaks. A molecule
of this symmetry is consistent with restricted rotation
about the titanium nitrogen bond arising either from a
hindered amido-type ligand29 or from coordination of the
â-nitrogen to the titanium center forming the η2 adduct.
Another possibility is freezing out the rotation of steri-
cally demanding cyclopentadienyl ligands as observed
for the corresponding titanocene dichloride complex.16
In an attempt to differentiate between these possibili-
ties, both COSY and ROESY NMR experiments were
performed. From these data, assignment of all of the
hydrogens in the molecule was possible, although no
definitive through-space interactions were evident to
(46) van der Heijden, H.; Hessen, B. Inorg. Chem. Acta 2003, 345,
27.
(47) Lee, H.; Bonanno, J . B.; Hascall, T.; Cordaro, J .; Hahn, J . M.;
Parkin, G. J . Chem. Soc., Dalton Trans. 1999, 1365.

3454 Organometallics, Vol. 23, No. 14, 2004
Hanna et al.
F igu r e 4. DFT geometry optimized structures for 2 (left)
and (η5-C5H3-1,3-(SiMe3)2)2TidCHSiMe3 (right)(ADF 2003.01,
TZ2P, ZORA).
The increased reactivity associated with the putative
diazoalkane complex (η5-C5H3-1,3-(SiMe3)2)2Ti(N2CHSi-
Me3) parallels the chemistry of (η5-C5Me5)2Ti(N2CH-
SiMe3), which undergoes facile loss of N2 at 25 °C.30,31
However, generation of permethyltitanocene alkylidene
resulted in the fulvalene complex (η5-C5Me5)(η5-C5Me4-
η1-CH2)Ti(CH2SiMe3), whereas in the present case
double cyclometalation of the [SiMe3] substituents is
observed. This difference is attributed to the increased
steric congestion associated with (η5-C5H3-1,3-(SiMe3)2)-
(η5-C5H3-η1-1-SiMe2CH2-3-(SiMe3))Ti(CH2SiMe3), which
increases the rate of R-hydrogen abstraction. This
coupled with the propensity of [SiMe3] groups to par-
ticipate in cyclometalation reactions8 results in forma-
tion of the double-cyclometalated titanocene.
Com p a r ison of Im id o ver su s Alk ylid en e Cyclo-
m eta la tion Ra tes. Although 2 and the transient akyl-
idene (η5-C5H3-1,3-(SiMe3)2)2TidCHSiMe3 are isolec-
tronic and sterically similar, their ability to promote
intramolecular C-H activation varies dramatically.
Whereas 2 (and 3) resist cyclometalation with prolonged
heating at 110 °C, the alkylidene undergoes facile C-H
activation even at low temperatures (-50 °C). The origin
of this reactivity difference was explored computation-
ally with the ADF program suite.48 Geometry optimiza-
tion of 2 reproduced the experimentally determined
solid state structure within the 3σ criterion. The coor-
dinates were used as a starting point for the geometry
optimization of the unobserved alkylidene, (η5-C5H3-1,3-
(SiMe3)2)2TidCHSiMe3. The optimized structures are
presented in Figure 4 and are topologically similar.
The difference in reactivity between the two com-
pounds can be traced to the frontier molecular orbitals
(Figure 5) of both titanocenes, which are in agreement
with seminal predictions made by Lauher and Hoff-
mann.49 For the titanium imido 2, the HOMO-1 is
principally the titanium-nitrogen π bond, while the
HOMO is mostly comprised of a nonbonding nitrogen p
orbital orthogonal to the plane of the metallocene wedge.
In contrast, the HOMO for (η5-C5H3-1,3-(SiMe3)2)2Tid
CHSiMe3 is primarily the titanium-carbon π bond and
is thus more energetically accessible for 1,2-addition.
It should be noted that the LUMO for both molecules
is a titanium-based 1a1 orbital49 and is available to
accommodate the incoming C-H bond.50
F igu r e 5. HOMO-1, HOMO, and LUMO for 2 and (η5-
C5H3-1,3-(SiMe3)2)2TidCHSiMe3.
complexes 2 and 3 prompted us to investigate their
ability to participate in nitrene group transfer reactions.
While there have been reports of nitrene group transfer
to carbon monoxide to yield isocyanate with well-defined
imido complexes of iron(III),51 cobalt(III),52 and nickel-
(II),53 to our knowledge such a transformation has not
been observed with titanium.54 Exposure of 2 to 1 atm
of carbon monoxide produced a rapid reaction, affording
both Me3SiNCO and (η5-C5H3-1,3-(SiMe3)2)2Ti(CO)2 in
near quantitative yield (eq 8).
The clean stoichiometric reaction of 2 with CO
prompted us to investigate the possibility of catalytic
nitrene group transfer. Previous studies have shown
that titanocene dicarbonyl complexes are useful sources
of Ti(II),37 suggesting that regeneration of 2 by addition
of Me3SiN3 to (η5-C5H3-1,3-(SiMe3)2)2Ti(CO)2 may be
feasible. However, addition of 3 equiv of Me3SiN3 to (η5-
C5H3-1,3-(SiMe3)2)2Ti(CO)2 under an atmosphere of
dinitrogen produced no reaction. Performing the same
procedure under vacuum initiated a transformation
yielding the isocyanate along with an unidentified
mixture of titanocene products.
This behavior prompted us to study the reactivity of
2 in more detail. Reaction of 2 with 1 equiv of Me3-
SiNCO produced an immediate reaction, allowing isola-
tion of (η5-C5H3-1,3-(SiMe3)2)2Ti(η2-N(SiMe3)CdNSiMe3O)
(50) Cundari, T. R.; Klinckman, T. R.; Wolczanski, P. T. J . Am.
Chem. Soc. 2002, 124, 1481.
(51) Brown, S. D.; Betley, T. A.; Peters, J . C. J . Am. Chem. Soc.
2003, 125, 322.
(52) J enkins, D. M.; Betley, T. A.; Peters, J . C. J . Am. Chem. Soc.
2002, 124, 11238.
Nitr en e Gr ou p Tr a n sfer Rea ction s. The synthesis
and isolation of the monomeric, base-free titanium imido
(48) Complete computational details are provided in the Experi-
mental Section and results of these calculations in the Supporting
Information.
(53) Mindiola, D. J .; Hillhouse, G. L. Chem. Commun. 2002, 1840.
(54) Reactions of a zirconocene imido with metal carbonyl com-
pounds yielded the products of imido-oxo exchange: Lee, S. Y.;
Bergman, R. G. J . Am. Chem. Soc. 1995, 117, 5877.
(49) Lauher, J . W.; Hoffmann, R. J . Am. Chem. Soc. 1976, 98, 1729.

Base-Free Titanium Imido
Organometallics, Vol. 23, No. 14, 2004 3455
(10), arising from nucleophilic attack of the imido
nitrogen on the coordinated isocyanate (eq 9). Both H
Both elemental analysis and in situ monitoring of the
group transfer reaction are consistent with an empirical
formula of [(η5-C5H3-1,3-(SiMe3)2)2TiO]n. Determination
of an accurate solution molecular weight of 11 has been
precluded by the low solubility of the compound in inert
organic solvents such as benzene, toluene, or alkanes.
Attempts to dissolve 11 in more polar solvents such as
chloroform, dichloromethane, or pyridine resulted in
decomposition. Preparation of soluble derivatives by
reaction with PMe3, CO, MeI, H2, and alkynes has been
unsuccessful, producing no reaction even at elevated
temperatures (50-80 °C).
1
and 13C NMR spectroscopy are consistent with a Cs
symmetric molecule, demonstrating exclusive formation
of the N,O- rather than the N,N-bound isomer. Similar
reactivity has been reported by Mountford, who ob-
served that a non-metallocene titanium imido complex
reacts with isocyanate to yield the N,N-bound titana-
cycle.55 Bergman has implicated an N,O-bound complex
from reaction of a permethylzirconocene imido complex
with tert-butylisocyanate.56
The structure and thermal stability of 11 contrasts
the analogous chemistry observed for the permethyti-
tanocene system. Treatment of (η5-C5Me5)2Ti(η2-CH2d
CH2) with N2O in the presence of pyridine affords the
titanocene oxo pyridine complex (η5-C5Me5)2Ti(O)(py).58
Heating the compound resulted in a complex reaction
forming [(η5-C5Me5)4Ti4(O)6], presumably arising from
decomposition of (η5-C5Me5)2Ti(O).
Similar results were obtained upon treatment of 2
with limited amounts of carbon monoxide. While addi-
tion of 1 equiv of CO produced no reaction, reaction of
2 with 3 equiv of carbon monoxide yielded 10 and (η5-
C5H3-1,3-(SiMe3)2)2Ti(CO)2 (eq 9). The resulting titana-
cycle, 10, is quite stable, producing no reaction upon
exposure to 1 atm of carbon monoxide or dihydrogen.
As a result, one limitation in developing a catalytic
group transfer process is the formation of 10 from low
concentrations of carbon monoxide. If high concentra-
tions are used, then the resulting dicarbonyl complex
does not undergo loss of CO. Attempts to observe a well-
defined isocyanate complex either during group transfer
or by reaction of Me3SiNCO with 1 have been unsuc-
cessful.
Nitrene group transfer was also studied with unsat-
urated carbonyl compounds. Reaction of 2 with 1 equiv
of benzophenone produced the imine Ph2CdNSiMe3,
along with an orange solid identified as the titanocene
oxo complex [(η5-C5H3-1,3-(SiMe3)2)2TiO]n (11) (eq 10).
At temperatures below 25 °C, the 1H and 13C NMR
spectra of 11 in toluene-d8 display resonances consistent
with a Cs symmetric molecule with inequivalent cyclo-
pentadienyl rings. Four cyclopentadienyl hydrogens and
overlapping [SiMe3] resonances are observed by 1H
NMR spectroscopy at these temperatures. The in-
equivalent [SiMe3] resonances can be resolved by 13C
NMR spectroscopy. Warming the sample produced
dynamic behavior, with a C2v symmetric molecule being
observed at temperatures above 45 °C. One structure
consistent with the low-temperature data is a buckled,
dimeric titanocene oxo that undergoes rapid inversion
upon warming. It should be noted, however, that several
dimeric titanium oxo complexes with non-cyclopenta-
dienyl ligands have been characterized by X-ray dif-
fraction and shown to contain planar TiO2Ti cores,57
suggesting that other processes, such as hindered cy-
clopentadienyl rotation, may be the origin of the ob-
served variable-temperature behavior.
The reaction of 2 with phosphines was examined in
an attempt to develop a titanium-promoted Staudinger
reaction. Addition of 1 equiv of PMe3 to a benzene-d6
solution of 2 afforded the phosphine adduct (η5-C5H3-
1,3-(SiMe3)2)2Ti(NSiMe3)PMe3 (12) (eq 11). Attempts to
induce nitrene group transfer by addition of excess
phosphine produced no additional reaction. However,
monitoring a toluene-d8 solution by variable-tempera-
ture 1H and 31P NMR spectroscopy revealed dynamic
phosphine coordination and dissociation. At tempera-
tures below -20 °C, a single PMe3 doublet, inequivalent
[SiMe3] groups, and three cyclopentadienyl resonances
1
are observed in the H NMR spectrum, consistent with
a static, Cs symmetric phosphine adduct. Warming the
sample slightly to 0 °C produced two broad cyclopen-
tadienyl peaks in addition to a broadened PMe3 reso-
nance. Further heating to temperatures above 70 °C
produced sharp resonances associated with a molecule
of C2v symmetry, consistent with rapid dissociation and
recoordination of the phosphine ligand.
Activation parameters for the dynamic behavior of 12
were determined by line shape analysis over a 60 deg
temperature range. The Eyring plot from the data
obtained from modeling the cyclopentadienyl hydrogens
is shown in Figure 6. The entropy of activation was
determined to be 1(1) eu, consistent with phosphine
dissociation as the rate-determining step. As a result,
the enthalpy of activation of 14.2(5) kcal/mol may be
used as a measure of the titanium-phosphorus bond
strength.
(55) Blake, A. J .; McInnes, J . M.; Mountford, P.; Nikonov, G. I.;
Swallow, D.; Watkin, D. J . J . Chem. Soc., Dalton Trans. 1999, 379.
(56) Walsh, P. J .; Hollander, F. J .; Bergman, R. G. Organometallics
1993, 12, 3705.
(57) Guiducci, A. E.; Cowley, A. R.; Skinner, M. E. G.; Mountford,
P. J . Chem. Soc., Dalton Trans. 2001, 1392.
(58) Smith, M. R.; Matsunaga, P. T.; Andersen, R. A. J . Am. Chem.
Soc. 1993, 115, 7049.

3456 Organometallics, Vol. 23, No. 14, 2004
Hanna et al.
F igu r e 6. Eyring plot for the dynamic behavior of 12 from modeling the line shape of the cyclopentadienyl hydrogens.
61
62
Both Ph2CN2 and MesN3 were prepared according to
Con clu d in g Rem a r k s
literature procedures. Preparations of Cp′′2TiCl,16 Cp′′2TiCl2,63
and Cp′′2TiMe2 were accomplished according to literature
63
Synthesis of the titanocene dinitrogen complex 1 was
accomplished by reduction of the corresponding mono-
chloride species. Unlike its zirconium congener, 1
contains a weakly activated, end-on bound dinitrogen
ligand, providing a convenient source of low-valent
titanium. Displacement of the N2 ligand has been used
to prepare titanocene imido, diazoalkane, and transient
alkylidene species. The imido complex, while resistant
to cyclometalation, displays facile reactivity with dihy-
drogen and participates in nitrene group transfer reac-
tions with carbon monoxide and benzophenone. The
titanocene diazoalkane complex also undergoes rapid
hydrogenolysis and η2, η1 interconversion in solution.
Computational studies complemented the experimental
work and demonstrated that the rapid cyclometalation
observed for the putative titanocene alkylidene arises
from an energetically accessible titanium-carbon π
bond as compared to the imido complex.
procedures.
1H spectra were recorded on Varian Mercury 300 and Inova
400 and 500 spectrometers operating at 299.763, 399.780, and
499.92 MHz, respectively. 13C NMR spectra were recorded on
the spectrometers operating at 75.383, 100.54, and 125.716
MHz, respectively. All chemical shifts are reported relative to
1
SiMe4 using the H (residual) chemical shift of the solvent as
a secondary standard. 31P NMR spectra were recorded on a
Varian Inova 500 spectrometer operating at 202.33 MHz, and
all spectra were referenced externally to neat H3PO4. Tem-
peratures were calibrated using methanol and ethylene glycol
standards. Line shape analyses were performed using D NMR
as implemented by Spinworks 2.2.64
Gradient-selected DQCOSY was acquired with a sweep
width of 1.8 kHz in phase-sensitive mode. A total of 64 complex
points were collected in the indirectly detected dimension with
2 scans and 1k points per increment. The resulting matrix was
zero filled to 2k × 2k complex data points, and 90 shifted
squared sinusoidal window functions were applied in both
dimensions prior to Fourier transform. EXSY spectra were
acquired with a sweep width of 1.8 kHz and a mixing time of
0.2 s in phase-sensitive mode. A total of 64 complex points were
collected in the indirectly detected dimension with 16 scans
and 1k points per increment. The resulting matrix was zero
filled to 2k × 2k complex data points, and 90 shifted squared
sinusoidal window functions were applied in both dimensions
prior to Fourier transform.
Magnetic moments were measured at 22 °C by the method
originally described by Evans18 with stock and experimental
solutions containing a known amount of a ferrocene standard.
Single crystals suitable for X-ray diffraction were coated with
polyisobutylene oil in a drybox and were quickly transferred
to the goniometer head of a Siemens SMART CCD area
detector system equipped with a molybdenum X-ray tube (λ
) 0.71073 Å). Preliminary data revealed the crystal system.
A hemisphere routine was used for data collection and deter-
mination of lattice constants. The space group was identified,
and the data were processed using the Bruker SAINT program
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All air- and moisture-sensitive
manipulations were carried out using standard high-vacuum
line, Schlenk, or cannula techniques or in an M. Braun inert
atmosphere drybox containing an atmosphere of purified
nitrogen. The M. Braun drybox was equipped with a cold well
designed for freezing samples in liquid nitrogen. Solvents for
air- and moisture-sensitive manipulations were dried and
deoxygenated using literature procedures.59 Benzene-d6 and
toluene-d8 were purchased from Cambridge Isotope Labora-
tories and were distilled from sodium metal under an atmo-
sphere of argon. The toluene-d8 was further dried over
“titanocene”.60 Hydrogen and argon gas were purchased from
Airgas Incorporated and passed through a column containing
manganese oxide supported on vermiculite and 4 Å molecular
sieves before admission to the high-vacuum line. Carbon
monoxide was purchased from Aldrich and passed through a
liquid-nitrogen-cooled trap immediately before use. Me3SiN3
was purchased from Acros and was used as received. Tri-
methylsilylisocyanate was purchased from Aldrich and was
degassed and filtered through activated alumina before use.
(60) Marvich, R. H.; Brintzinger, H. H. J . Am. Chem. Soc. 1971,
93, 2046.
(61) Miller, J . B. J . Org. Chem. 1959, 24, 560.
(62) Murata, S.; Abe, S.; Tomioka, H. J . Org. Chem. 1997, 62, 3055.
(63) Winter, C. H.; Zhou, X. X.; Heeg, M. J . Inorg. Chem. 1992, 31,
1808.
(59) Pangborn, A.; Giardello, M.; Grubbs, R. H.; Rosen, R. K.;
Timmers, F. J . Organometallics 1996, 15, 1518.
(64) Marat, K. Spinworks; University of Manitoba, 2004.

Base-Free Titanium Imido
Organometallics, Vol. 23, No. 14, 2004 3457
and corrected for absorption using SADABS. The structures
were solved using direct methods (SHELXS) completed by
subsequent Fourier synthesis and refined by full-matrix least-
squares procedures. Elemental analyses were performed at
Robertson Microlit Laboratories, Inc., in Madison, NJ .
All calculations were performed with the Amsterdam Den-
sity Functional Theory (ADF2003.01) suite of programs.65-67
The calculations included scalar relativistic effects (ZORA)68
for all atoms. The Vosko, Wilk, and Nusair (VWM) local
density approximation,69 Becke’s exchange,70 and Perdew’s
correlation71 were used. The cores of the atoms were frozen
up to 1s for C and N and 2s for Si and Ti. Uncontracted Slater-
type orbitals (STOs) of triple-ú quality with two polarizations
were employed. This basis set is denoted TZ2P in the ADF
program. Each geometry optimization was carried out without
symmetry constraints. Input files and computational results
are contained in the Supporting Information.
Syn th esis a n d Ch a r a cter iza tion of [(η5-C5H3-1,3-
(SiMe3)2)2Ti]2(µ2,η1,η1-N2) (1). A 250 mL round-bottom flask
was charged with 92.2 g of 0.5% sodium amalgam and
approximately 100 mL of toluene. With vigorous stirring, 2.00
g (3.99 mmol) of (η5-C5H3-1,3-(SiMe3)2)2TiCl was added, and
the resulting reaction mixture was stirred for 3 days at
ambient temperature. The blue solution was filtered through
a pad of Celite, and the toluene was removed in vacuo, leaving
a blue solid. Recrystallization from pentane at -35 °C afforded
1.50 g (78%) of royal blue crystals identified as 1. Anal. Calcd
for C44H84N2Si8Ti2: C, 54.96; H, 8.81; N, 2.91. Found: C, 54.67;
mmol) of 2 and approximately 0.5 mL of benzene-d6. On the
high-vacuum line the tube was degassed and 1 atm of
hydrogen was admitted at -196 °C. The tube was thawed and
shaken, resulting in an immediate color change from red to
1
yellow. H NMR (benzene-d6): δ 0.19 (s, 9H, SiMe3), 0.32 (s,
18H, SiMe3), 0.35 (s, 18H, SiMe3), 2.05 (s, 1H, Ti-H), 5.30 (s,
2H, Cp), 5.35 (s, 2H, Cp), 6.16 (s, 2H, Cp), 7.05 (s, 1H, NH).
13C NMR (benzene-d6): δ 0.67 (SiMe3), 1.29 (SiMe3), 4.51
(SiMe3), 112.78, 113.71, 118.47, 119.59, 122.39 (Cp).
P r ep a r a t ion of (η5-C5H3-1,3-(SiMe3)2)2Ti(N(H)Mes)H
(5). This molecule was prepared in a manner identical to 4
with 0.015 g (0.025 mmol) of 3, producing a light red solution
upon hydrogenation. 1H NMR (benzene-d6): δ 0.20 (s, 18H,
SiMe3), 0.31 (s, 18H, SiMe3), 2.05 (s, 1H, Ti-H), 2.14 (s, 3H,
o-CH3), 2.18 (s, 3H, o-CH3), 2.36 (s, 3H, p-CH3), 5.21 (s, 2H,
Cp), 5.52 (s, 2H, Cp), 5.97 (s, 2H, Cp), 6.81 (s, 1H, m-CH), 6.87
(s, 1H, m-CH), 7.90 (s, 1H, NH). 13C NMR (benzene-d6): δ 0.39
(SiMe3), 0.46 (SiMe3), 19.36, 20.79, 21.05 (CH3), 95.08, 112.57,
120.61, 123.71, 128.29, 128.50, 128.57, 129.34, 130.83, 154.53
(Cp/Mes). One peak not located. IR (pentane): ν, 3339 cm-1
(NH).
P r ep a r a t ion of (η5-C5H 3-1,3-(SiMe3)2)2Ti(N2CP h 2) (6).
This molecule was prepared in a manner identical to 2 using
0.200 g (0.208 mmol) of 1 and 0.081 g (0.416 mmol) of a
magenta pentane solution of Ph2CN2, yielding 0.150 g (55%)
of red crystals identified as 6. Anal. Calcd for C35H52N2Si4Ti:
C, 63.60; H, 7.93; N, 4.24. Found: C, 63.34; H, 7.69; N, 4.04.
1H NMR (toluene-d8, -20 °C (major product)): δ 0.04 (s, 18H,
SiMe3), 0.26 (s, 18H, SiMe3), 6.07 (s, 2H, Cp), 6.44 (s, 2H, Cp),
6.96 (m, 1H, p-Ph), 6.98 (m, 1H, p-Ph), 7.14 (m, 2H, m-Ph),
7.20 (m, 2H, m-Ph), 7.54 (s, 2H, Cp), 7.58 (d, 7.38 Hz, 2H,
o-Ph), 7.83 (d, 7.91 Hz, 2H, o-Ph). 1H NMR (toluene-d8, -20
°C (minor product)): δ 0.04 (s, 36H, SiMe3), 6.08 (s, 4H, Cp),
7.09 (m, 4H, o-Ph), 8.86 (s, 2H, Cp). Meta and phenyl peaks
not located. 1H NMR (toluene-d8, 70 °C): δ 0.07 (s, 36H, SiMe3),
6.27 (s, 4H, Cp), 6.94 (m, 2H, Ph), 7.10 (m, 4H, Ph), 7.35 (m,
4H, Ph), 7.83 (s, 2H, Cp). 13C NMR (toluene-d8, -20 °C): δ
-0.20 (SiMe3), -0.02 (SiMe3), 121.20, 122.10, 124.38, 125.53,
125.65, 125.83, 128.47, 132.13, 133.24, 133.47, 137.80, 139.85
(Cp/Ph). Two peaks not located.
P r ep a r a tion of (η5-C5H3-1,3-(SiMe3)2)2Ti(η1-N2CP h 2)-
(CO) (7). A thick-walled vessel was charged with 0.100 g
(0.151 mmol) of 6 and approximately 10 mL of pentane. On
the high-vacuum line, the vessel was degassed and 1 atm of
carbon monoxide was added at -196 °C. The vessel was
thawed and stirred for 30 min, after which time the solvent
was removed in vacuo. The vessel was transferred into the
drybox, and the resulting solid was recrystallized from pentane
at -35 °C, affording 0.065 g (63%) of dark red crystals
identified as 7. Anal. Calcd for C36H52N2OSi4Ti: C, 62.75; H,
7.61; N, 4.06. Found: C, 62.30; H, 7.59; N, 3.54. 1H NMR
(benzene-d6): δ 0.16 (s, 18H, SiMe3), 0.26 (s, 18H, SiMe3), 5.50
(s, 2H, Cp), 6.03 (s, 2H, Cp), 6.05 (s, 2H, Cp), 7.04 (m, 2H,
Ph), 7.25 (m, 4H, Ph), 7.58 (d, 8 Hz, 4H, Ph). 13C NMR
(benzene-d6): δ -0.07 (SiMe3), 0.07 (SiMe3), 105.78, 111.46,
113.92, 116.68, 119.42, 120.99, 125.44, 127.21, 128.61, 129.40,
129.72, 130.99 (Cp/Ph), 226.64 (CO). One peak not located. IR
(benzene-d6): ν, 2044 cm-1 (CO).
H, 8.61; N, 2.62. Magnetic susceptibility (benzene-d6): µeff
2.38 µB. H NMR (benzene-d6): δ 2.7 (∆ν1/2 ) 260 Hz).
)
1
P r ep a r a tion of (η5-C5H3-1,3-(SiMe3)2)2TidNSiMe3 (2). A
scintillation vial was charged with 0.305 g (0.317 mmol) of 1
and approximately 5 mL of pentane. The blue solution was
chilled in the drybox freezer set at -35 °C for 20 min. To the
warming solution, an excess of Me3SiN3 was added, producing
vigorous bubbling and a red solution. The reaction mixture
was allowed to warm to ambient temperature and stirred for
an additional 20 min. The red solution was filtered through a
glass frit, and the solvent was removed in vacuo to yield a red
solid. Recrystallization from pentane at -35 °C yielded 0.200
g (53%) of analytically pure dichroic (red-yellow) crystals. Anal.
Calcd for C25H51NSi5Ti: C, 54.20; H, 9.28; N, 2.53. Found: C,
1
54.10; H, 8.95; N, 2.29. H NMR (benzene-d6): δ 0.23 (s, 36H,
SiMe3), 0.28 (s, 9H, SiMe3), 6.65 (s, 4H, Cp), 8.17 (s, 2H, Cp).
13C NMR (benzene-d6): δ 0.38 (SiMe3), 3.17 (SiMe3), 126.12,
132.77, 136.67 (Cp).
P r ep a r a tion of (η5-C5H3-1,3-(SiMe3)2)2TidNMes (3). This
molecule was prepared in a manner identical to 2 with 0.102
g (0.106 mmol) of 1 and 0.034 g (0.212 mmol) of MesN3,
yielding 0.120 g (94%) of red-brown crystals identified as 3.
Anal. Calcd for C31H53NSi4Ti: C, 62.06; H, 8.90; N, 2.33.
Found: C, 61.97; H, 8.88; N, 2.50. 1H NMR (benzene-d6): δ
0.18 (s, 36H, SiMe3), 2.17 (s, 3H, p-CH3), 2.28 (s, 6H, o-CH3),
6.55 (s, 4H, Cp), 6.73 (s, 2H, Cp), 7.86 (s, 2H, m-CH). 13C NMR
(benzene-d6): δ 0.06 (SiMe3), 20.61 (o-CH3), 20.90 (p-CH3),
126.25, 127.66, 128.64, 133.15, 134.31, 161.40 (Cp/Mes). Three
peaks not located.
P r ep a r a tion of (η5-C5H3-1,3-(SiMe3)2)2Ti(N(H)SiMe3)H
(4). A J . Young NMR tube was charged with 0.015 g (0.027
P r ep a r a tion of (η5-C5H3-1,3-(SiMe3)2)2Ti(η1-N2CP h 2)-
(DMAP ). A J . Young NMR tube was charged with 0.040 g
(0.061 mmol) of 6, 0.008 g (0.07 mmol) of (dimethylamino)-
pyridine (DMAP), and approximately 0.5 mL of benzene-d6.
The tube was shaken, producing a light red solution. 1H NMR
(benzene-d6): δ 0.25 (s, 18H, SiMe3), 0.30 (s, 18H, SiMe3), 2.01
(s, 6H, NMe2), 6.02 (d, 7 Hz, 2H, Ph), 6.28 (s, 2H, Cp), 6.47 (s,
2H, Cp), 6.52 (s, 2H, Cp), 7.09 (t, 7 Hz, 2H, DMAP), 7.32 (t, 8
Hz, 4H, Ph), 7.79 (d, 7 Hz, 4H, Ph), 8.42 (d, 7 Hz, 2H, DMAP).
13C NMR (benzene-d6): δ 0.59 (SiMe3), 0.68 (SiMe3), 38.03
(NMe2), 105.67, 118.42, 123.65, 125.511, 128.21, 128.50, 153.91,
153.99 (Cp/Ph/DMAP). Three Ph/ DMAP peaks not located.
(65) Velde, G. T.; Bickelhaupt, F. M.; Baerends, E. J .; Guerra, C.
F.; van Gisbergen, S. J . A.; Snijders, J . G.; Ziegler, T. J . Comput. Chem.
2001, 22, 931.
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istry; Vrije Universiteit: Amsterdam, The Netherlands, http://ww-
w.scm.com/.
(68) van Lenthe, E.; Ehlers, A.; Baerends, E. J . J . Chem. Phys. 1999,
110, 8943.
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3458 Organometallics, Vol. 23, No. 14, 2004
Hanna et al.
P r ep a r a tion of (η5-C5H3-1,3-(SiMe3)2)2Ti(NHNdCP h 2)H
(8). A thick-walled glass vessel was charged with 0.081 g (0.123
mmol) of 6 and approximately 10 mL of pentane. On the high-
vacuum line, the vessel was degassed using standard freeze-
pump-thaw techniques, and 1 atm of hydrogen was admitted
at -196 °C. The vessel was thawed and stirred for 2 h,
whereupon an orange solid precipitated from the red solution.
The solvent was removed in vacuo, and the reaction was
brought into the drybox. Recrystallization from pentane at -35
°C yielded 0.043 g (52%) of an orange solid identified as 8.
Anal. Calcd for C35H54N2Si4Ti: C, 63.40; H, 8.21; N, 4.22.
Found: C, 63.10; H, 8.05; N, 3.80. 1H NMR (benzene-d6): δ
0.22 (s, 18H, SiMe3), 0.35 (s, 18H, SiMe3), 1.97 (s, 1H, Ti-H),
4.83 (s, 2H, Cp), 5.75 (s, 2H, Cp), 6.35 (s, 2H, Cp), 7.11 (m,
4H, Ph), 7.21 (m, 2H, Ph), 7.77 (d, 7.6 Hz, 4H, Ph), 9.95 (s,
1H, NH). 13C NMR (benzene-d6): δ 0.64 (SiMe3), 0.84 (SiMe3),
110.16, 114.06, 119.41, 122.85, 127.41, 128.40, 128.66, 129.20,
130.47, 134.72, 139.78, 140.98 (Cp/Ph). Two Ph peaks were not
located. IR (benzene-d6): ν, 3243 cm-1 (NH), 2298 cm-1 (ND).
P r ep a r a tion of (η5-C5H3-η1-1-SiMe2CH2-3-SiMe3)2Ti (9).
This molecule was prepared in a manner identical to 2 using
0.125 g (0.130 mmol) of 1 and 0.105 g (0.273 mmol) of a 2.0 M
solution of Me3CHN2 in hexanes, yielding 0.094 g (77%) of 9
and SiMe4. The compound could be isolated only as a thick
Cp), 6.55 (s, 2H, Cp), 7.04 (s, 2H, Cp). 13C NMR (benzene-d6):
δ 0.38 (SiMe3), 0.78 (SiMe3), 2.55 (SiMe3), 4.37 (SiMe3), 116.90,
125.31, 125.75, 130.55, 142.60 (Cp). One peak not located. IR
(benzene-d6): ν, 2135, 2210 cm-1
.
P r ep a r a tion of [(η5-C5H3-1,3-(SiMe3)2)2TiO]n (11). A
scintillation vial was charged with 0.132 g (0.238 mmol) of 2
and approximately 10 mL of pentane. A pentane solution
containing 0.044 g (0.238 mmol) of benzophenone was added
to the vial. The reaction was stirred at ambient temperature,
and after 5 min, an orange precipitate was observed. The
reaction mixture was stirred for an additional hour, and the
orange precipitate was collected by filtration. Analysis of the
filtrate by 1H NMR and IR spectroscopy revealed formation
of N-trimethylsilyl(diphenylmethylene)amine by comparison
to literature values.73 The orange solid was dried in vacuo and
yielded 0.095 g (83%) of analytically pure 11. Anal. Calcd for
1
C
22H42OSi4Ti: C, 54.73; H, 8.77. Found: C, 54.46; H, 8.53. H
NMR (benzene-d6, 20 °C): δ 0.43 (s, 36H, SiMe3), 6.41 (s, 2H,
Cp), 6.84 (s, 2H, Cp), 7.10 (s, 1H, Cp), 7.27 (s, 1H, Cp). 1H
NMR (benzene-d6, 75 °C): δ 0.40 (s, 36H, SiMe3), 6.56 (s, 4H,
Cp), 7.13 (s, 2H, Cp). 13C NMR (benzene-d6): δ 0.99 (SiMe3),
1.03 (SiMe3), 110.10, 122.69, 126.54, 134.09, 134.88, 136.71
(Cp).
P r epar ation of (η5-C5H3-1,3-(SiMe3)2)2Ti(NSiMe3)(P Me3)
(12). A J . Young tube was charged with 0.030 g (0.054 mmol)
of 2 and approximately 0.5 mL of toluene-d8. On the high-
vacuum line, the tube was frozen in liquid nitrogen and
degassed. At -196 °C, 32 Torr of PMe3 (0.054 mmol) was added
by a 31.6 mL calibrated gas bulb. The tube was thawed and
1
red oil, precluding elemental analysis. H NMR (benzene-d6):
δ -2.09 (d, 16.4 Hz, 2H, TiCH2), -1.14 (d, 16.4 Hz, 2H, TiCH2),
0.06 (s, 18H, SiMe3), 0.39 (s, 6H, SiMe2), 0.51 (s, 6H, SiMe2),
5.43 (s, 2H, Cp), 5.63 (s, 2H, Cp), 7.85 (s, 2H, Cp). 13C NMR
(benzene-d6): δ -2.17 (SiMe2), -0.13 (SiMe2), 2.57 (SiMe3),
32.83 (CH2), 117.84, 118.13, 123.16, 134.04 (Cp).
1
shaken, forming a lighter red solution. H NMR (toluene-d8,
Nitr en e Gr ou p Tr a n sfer to Ca r bon Mon oxid e. A J .
Young tube was charged with 0.040 g (0.075 mmol) of 2 and
approximately 0.5 mL of benzene-d6. On the high-vacuum line,
the tube was degassed and 1 atm of carbon monoxide was
added. The tube was thawed and shaken, and the products
were identified as (η5-C5H3-1,3-(SiMe3)2)2Ti(CO)2 and Me3-
SiNCO on the basis of comparison to literature data.72 (η5-
C5H3-1,3-(SiMe3)2)2Ti(CO)2: 1H NMR (benzene-d6): δ 0.21 (s,
36H, SiMe3), 5.14 (s, 4H, Cp), 5.37 (s, 2H, Cp). 13C NMR
(benzene-d6): δ -0.04 (SiMe3), 99.31, 104.69, 109.24 (Cp),
260.36 (CO). IR (benzene-d6): ν, 1881, 1960 cm-1 (CO).
P r ep a r a tion of (η5-C5H3-1,3-(SiMe3)2)2Ti(η2-N(SiMe3)Cd
NSiMe3O) (10). A scintillation vial was charged with 0.075 g
(0.135 mmol) of 2 and approximately 10 mL of pentane. To
the stirring red solution was added 18 µL (0.135 mmol) of
trimethylsilylisocyanate. The reaction mixture was stirred for
30 min, the resulting red solution was filtered through a glass
frit, and the solvent was removed in vacuo, yielding a red solid.
Recrystallization from pentane at -35 °C yielded 0.042 g (47%)
of analytically pure red crystals. Anal. Calcd for C29H60N2OSi6-
Ti: C, 52.05; H, 9.04; N, 4.18. Found: C, 51.90; H, 9.19; N,
3.83. 1H NMR (benzene-d6): δ 0.33 (s, 9H, SiMe3), 0.36 (s, 9H,
SiMe3), 0.36 (s, 18H, SiMe3), 0.37 (s, 18H, SiMe3), 6.24 (s, 2H,
-40 °C): δ 0.16 (s, 9H, SiMe3), 0.38 (s, 18H, SiMe3), 0.42 (s,
18H, SiMe3), 1.14 (d, 10 Hz, 9H, PMe3), 6.04 (s, 2H, Cp), 6.09
(s, 2H, Cp), 6.64 (s, 2H, Cp). 1H NMR (toluene-d8, 95 °C): δ
0.16 (s, 9H, SiMe3), 0.21 (s, 36H, SiMe3), 0.86 (s, 9H, PMe3),
6.59 (s, 4H, Cp), 7.77 (s, 2H, Cp). 31P NMR (toluene-d8, 20 °C):
δ -7.1 (∆ν1/2 ) 268).
Ack n ow led gm en t. We would like to thank Cornell
University, the donors of the Petroleum Research Fund,
and the National Science Foundation (CAREER award
to P.J .C.) for financial support. We would also like to
acknowledge Professors Bob Bergman and Dick Ander-
son (UC Berkeley) for sharing their results prior to
publication.
Su p p or tin g In for m a tion Ava ila ble: Selected NMR spec-
tra, computational details, and crystallographic data for 1 and
2 including full atom-labeling schemes, bond distances, and
bond angles. This material is available free of charge via the
Internet at http://pubs.acs.org.
OM049817H
(72) Veszpremi, T. J .; Nagy, J .; Barta, I. A.; Zsombok, G. J .
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