Synthesis and properties of [Cp2Ti(Me)(H2O)][B(C 6F5)4]

Article abstract of DOI:10.1021/om100660w

Treatment of water-stable Cp₂TiMe₂ with [Ph ₃C][B(C₆F₅)₄] in CD ₂Cl₂ gives the solvent-separated ion pair [Cp ₂Ti(Me)(CD₂Cl₂)][B(C₆F ₅)₄], which reacts rapidly with water at 205 K to give the corresponding aqua complex [Cp₂Ti(Me)(H₂O)][B(C ₆F₅)₄]. The latter has been characterized by ¹H and ¹³C NMR spectroscopy at 205 K and is a rare, possibly unique, example of a complex containing aqua and methyl ligands within the coordination sphere of a titanium(IV) complex.

Full text of DOI:10.1021/om100660w

Organometallics 2010, 29, 6117–6120 6117
DOI: 10.1021/om100660w
Synthesis and Properties of [Cp2Ti(Me)(H2O)][B(C6F5)4]
ꢀ
Alexandre F. Dunlop-Briere and Michael C. Baird*
Department of Chemistry, Queen’s University, Kingston, ON K7L 3N6, Canada
Received July 7, 2010
Summary: Treatment of water-stable Cp2TiMe2 with [Ph3C]-
[B(C6F5)4] (R = CH2CHMe2, CH2CMeCH2Et), prepared
by methyl carbanion abstraction from Cp2ZrMe2 by [Ph3C]-
[B(C6F5)4] in CD2Cl2 to give inter alia the solvent-separated
ion pair [Cp2Zr(Me)(CD2Cl2)][B(C6F5)4], followed by sub-
stitution of CD2Cl2 by the alkene.3a,b Such alkene complexes
are of interest for several reasons: (a) they are alkyl alkene
complexes of a type intermediate in coordination polymer-
ization reactions but never previously observed because of
their high proclivities to migratory insertion, (b) they contain
a novel near η1 mode of alkene coordination, and (c) the
C(1)-C(2) π bonds are sufficiently weak that the dC(Me)R
groups rotate relative to the terminal dCH2 groups.3a,b
Embarking on a similar investigation of the analogous
titanium system, we have utilized the reaction of Cp2TiMe2
with [Ph3C][B(C6F5)4] in CD2Cl2 to form mixtures of what
we believe to be the solvent-separated ion pair [Cp2Ti(Me)-
(CD2Cl2)][B(C6F5)4] (I), the contact ion pair [Cp2Ti(Me)-
B(C6F5)4] (II), and, when a deficiency of [Ph3C][B(C6F5)4] is
used, the dinuclear species [Cp2Ti(Me)(μ-Me)Ti(Me)Cp2]-
[B(C6F5)4] (III).3c While III was readily identified on the basis
[B(C6F5)4] in CD2Cl2 gives the solvent-separated ion pair
[Cp2Ti(Me)(CD2Cl2)][B(C6F5)4], which reacts rapidly with
water at 205 K to give the corresponding aqua complex
[Cp2Ti(Me)(H2O)][B(C6F5)4]. The latter has been character-
ized by 1H and 13C NMR spectroscopy at 205 K and is a rare,
possibly unique, example of a complex containing aqua and
methyl ligands within the coordination sphere of a titanium(IV)
complex.
The orange compound Cp2TiMe2 exhibits properties sur-
prisingly different from those of its colorless heavy homo-
logues, Cp2ZrMe2 and Cp2HfMe2. For instance, while the
latter two compounds are thermally stable to at least 80 °C
and are readily manipulated at ambient temperature,1a,b
Cp2TiMe2 can be manipulated at ambient temperature but
is also found to decompose erratically and apparently auto-
catalytically at this temperature.1b,c-f Curiously, Cp2TiMe2
is most fragile when isolated as a solid, frequently turning
black during attempts at purification.1b,c-f It is thus best
stored as THF or toluene solutions, where dilution seems to
prevent autocatalytic decomposition processes that prob-
ably involve radical species.1g-i
Another significant difference between Cp2TiMe2, on one
hand, and Cp2ZrMe2 and Cp2HfMe2 on the other has to do
with their protolytic reactivities. Although Cp2ZrMe2 and
Cp2HfMe2 are very sensitive to water and alcohols, giving
methane and metal products containing oxo, hydroxyl, or
alkoxy ligands,2 the synthesis of Cp2TiMe2 involves meth-
ylation of Cp2TiCl2 with 2 equiv of methyl lithium followed
by hydrolysis, after which workup yields the orange pro-
duct.1b,c-i Thus Cp2TiMe2 is stable to water.
1
of its H NMR spectrum,3c such was not the case with the
two mononuclear species. Noting,3 however, that Cp chemi-
cal shifts in metallocene compounds are generally strongly
influenced by inductive effects,3d,4 we believed it reasonable
to assign the less shielded Cp resonance at δ 6.72 to the
cationic species I and the more shielded Cp resonance at
δ 6.33 to the neutral species II. Similar assignments had been
suggested for the analogous zirconium compounds [Cp2-
ZrMe(solvent)][B(C6F5)4] (solvent=CD2Cl2, C6D5Cl) and
Cp2ZrMeB(C6F5)4,3a,b and thus I was thought to contain a
(3) (a) Vatamanu, M.; Stojcevic, G.; Baird, M. C. J. Am. Chem. Soc.
2008, 130, 454. (b) Sauriol, F.; Wong, E.; Leung, A. M. H.; Elliott Donaghue,
I.; Baird, M. C.; Wondimagegn, T.; Ziegler, T. Angew. Chem., Int. Ed. 2009,
We have recently reported the syntheses and properties of
the methyl alkene complexes [Cp2Zr(Me)(CH2dCMeR)]-
ꢀ
48, 3342. (c) Sauriol, F.; Sonnenberg, J. F.; Chadder, S. J.; Dunlop-Briere,
A. F.; Baird, M. C.; Budzelaar, P. H. M. J. Am. Chem. Soc. 2010, 132, 13357.
(d) Reviewers have expressed the opinion that the arguments for the
identification of I and II are not compelling, suggesting that one of them
might be Cp2TiMeCl,3d a potential product of degradation in the solvent,
CD2Cl2, or (unspecified) dinuclear species containing bridging anions. We
find, however, that the Cp and methyl resonances of Cp2TiMeCl are observed
at δ 6.23 and 0.65, respectively, and thus this compound is clearly not present
in our reaction mixtures. A reasonable dinuclear candidate might be
[Cp2TiMe]2(μ-O), but this compound would exhibit Cp and Me resonances
at δ 5.82 and 0.52, respectively,3e-i and clearly is also not present. Other
unacceptable potential candidates are [Cp2TiCl]2(μ-O),3g for which the Cp
resonance would be observed at δ 6.30 albeit unaccompanied by a methyl
resonance, and Cp2TiMeOTiClCp2, for which the Cp resonances would be
observed at δ 5.98 and 6.14, the methyl resonance at δ 0.67.3g Thus all
suggested alternative identifications of I or II are not observed. Beachell,
H. C.; Butter, S. A. Inorg. Chem. 1965, 4, 1133. (e) Surer, H.; Claude, S.;
Jacot-Guillarmod, A. Helv. Chim. Acta 1978, 61, 2956. (f) Hughes, D. L.;
Payack, J. F.; Cai, D.; Verhoeven, T. R.; Reider, P. J. Organometallics 1996,
15, 663. (g) Huffman, M.; Payack, J. PCT Int. Appl. WO 2000024748
A1 20000504, 2000. (h) Berget, P. E.; Schore, N. E. Organometallics 2006,
25, 552. (i) Berget, P. E.; Schore, N. E. Organometallics 2006, 25, 2398.
(4) Gray, D. R.; Brubaker, C. H. Inorg. Chem. 1971, 10, 2143.
*Corresponding author. E-mail: bairdmc@chem.queensu.ca.
(1) (a) Samuel, E.; Rausch, M. D. J. Am. Chem. Soc. 1973, 95, 6263.
(b) Alt, H. G.; Di Sanzo, F. P.; Rausch, M. D.; Uden, P. C. J. Organomet.
Chem. 1976, 107, 257. (c) Clauss, K.; Bestian, H. Justus Liebigs Ann.
Chem. 1962, 654, 8. (d) Erskine, G. J.; Wilson, D. A.; McCowan, J. D.
J. Organomet. Chem. 1976, 114, 119. (e) Erskine, G. J.; Hartgerink, J.;
Weinberg, E. L.; McCowan, J. D. J. Organomet. Chem. 1979, 170, 51. (f)
Razuvaev, G. A.; Mar'in, V. P.; Andrianov, Y. A. J. Organomet. Chem. 1979,
174, 67. (g) Razuvaev, G. A.; Mar'in, V. P.; Drushkov, O. N.; Vyshinskaya,
L. I. J. Organomet. Chem. 1982, 231, 125. (h) Li, H.; Neckers, D. C. Can. J.
Chem. 2003, 81, 758. (i) Dollinger, L. M.; Ndakala, A. J.; Hashemzadeh, M.;
Wang, G.; Wang, Y.; Martinez, I.; Arcari, J. T.; Galluzzo, D. J.; Howell, A. R.;
Rheingold, A. L.; Figuero, J. S. J. Org. Chem. 1999, 64, 7074. (j) Payack,
J. F.; Hughes, D. L.; Cai, D.; Cottrell, I. F.; Verhoeven, T. R. Organic
Syntheses; Collect. 2004; Vol. 10, p 355.
(2) (a) Wailes, P. C.; Weigold, H.; Bell, P. J. Organomet. Chem. 1972,
34, 155. (b) Fronczek, F. R.; Baker, E. C.; Sharp, P. R.; Raymond, K. N.; Alt,
H. G.; Rausch, M. D. Inorg. Chem. 1976, 15, 2284, and references therein.
(c) Yakelis, N. A.; Bergman, R. G. Organometallics 2005, 24, 3579, and
references therein.
r
2010 American Chemical Society
Published on Web 10/15/2010
pubs.acs.org/Organometallics

ꢀ
Dunlop-Briere and Baird
6118 Organometallics, Vol. 29, No. 22, 2010
solvent-separated, or outer-sphere, ion pair, while II was
thought to contain a contact, or inner-sphere, ion pair in
which, presumably, the anion coordinates via one or more
fluorine atoms.3c
of these reaction mixtures at 205 K (see Figure S1 of the
Supporting Information for a typical spectrum) were found
generally to exhibit resonances of the product of methyl
abstraction by trityl ion, CMePh3 (δ 7.0-7.3, m, Ph; 2.13, s,
Me), in addition to resonances that we have previously
suggested3c are attributed to the solvent-separated ion pair
[Cp2TiMe(CD2Cl2)][B(C6F5)4] (I, 1H NMR: δ 6.72 (Cp),
1.63 (Me); 13C NMR: δ 128.1 (1JCH 176 Hz, Cp), 75.7
(1JCH 129 Hz, Me)) and the contact ion pair [Cp2TiMeB-
Unfortunately, consistent with the literature cited above,
we found that attempts to purify Cp2TiMe2 resulted fre-
quently but erratically in its decomposing to a black
material.3c The compound could therefore not be reliably
obtained free of traces of ethyl ether or water, present during
its synthesis, or of toluene, the solvent in which it was stored.
Wondering, therefore, about the possible ramifications of
having low, variable amounts of water present during the
synthesis of, for example [Cp2Ti(Me)(CD2Cl2)][B(C6F5)4],
we have investigated and report herein the chemistry of
[Cp2Ti(Me)(CD2Cl2)][B(C6F5)4] with water.
1
(C6F5)4] (II, H NMR: δ 6.33 (Cp), 1.24 (Me); 13C NMR:
δ 117.4 (1JCH 176 Hz, Cp), 62.8 (1JCH 126 Hz, Me)). If a
deficiency of [Ph3C][B(C6F5)4] was used, the dinuclear spe-
1
cies [{Cp2TiMe}2(μ-Me)][B(C6F5)4] (III, H NMR: δ 6.21
(Cp), 0.61 (Me), -1.28 (μ-Me); 13C NMR: δ 115.6 (Cp), 55.8
(1JCH 127 Hz, Me)), 44.2 (1JCH 138 Hz, μ-Me)) was also
present.3c
Experimental Section
Figure 1 illustrates a 1H NMR spectrum of a Cp2TiMe2/
[Ph3C][B(C6F5)4] reaction mixture in which the dinuclear
species [{Cp2TiMe}2(μ-Me)][B(C6F5)4] was absent. As can
be seen, complexes I and II were present in an approximate
ratio of 3:1, and addition of ∼0.3 mol of H2O per mole of I at
205 K (Figure 1b) resulted in diminution of the resonances of
I, by about 30%,6 but not those of II. New Cp and Ti-Me
resonances appeared at δ 6.44 and 1.33, respectively (ratio
10:3), in addition to a new, broader resonance integrating for
2H at δ 4.74. Reaction with additional aliquots of water
(Figure 1b-d) resulted in proportional decreases in the
resonances of I and increases in the resonances of IV, but
again in very little changes in the intensities of the resonances
of II.
All syntheses were carried out under dry, deoxygenated argon
using standard Schlenk line techniques. Argon was deoxygen-
ated by passage through a heated column of BASF copper
catalyst and then dried by passing through a column of activated
˚
4 A molecular sieves. Handling of Cp2TiMe2 was done in an
MBraun Labmaster glovebox, and the compound was stored in
toluene under argon in a freezer. NMR spectra were recorded
1
using a Bruker AV600 spectrometer, H and 13C NMR data
being referenced to TMS via the residual proton signals of the
deuterated solvent. [Ph3C][B(C6F5)4] was purchased from Asahi
Glass Company and used as obtained, and CH3Li was pur-
chased from Aldrich. Cp2TiMe2 was synthesized from Cp2TiCl2
and methyllithium, as described elsewhere,1b,c-i and was stored
in toluene solution at -30 °C. Dichloromethane-d2 was dried by
A NOESY experiment of the reaction mixture (available in
the Supporting Information as Figure S2) demonstrated that
the three resonances at 6.44, 4.74, and 1.33 all belonged to the
same molecule, while a modified HSQC experiment of a
similar reaction mixture showed that the resonance at δ 4.77
was not coupled to any carbon atoms and was therefore to be
assigned to coordinated water rather than some hydro-
carbon fragment. The Cp and methyl 13C chemical shifts
obtained from the HSQC experiment (Figure S3) were
δ 117.6 (JCH 175.7 Hz) and 61.1 (JCH 132.0 Hz), respectively.
These are very similar to the data for compounds I-III, and
˚
storage over activated 3 A molecular sieves.
Solutions for NMR studies were prepared using a 2 mL
aliquot of a toluene solution containing ∼16.5 μmol of
Cp2TiMe2 per mL of toluene. The toluene was removed rapidly
under reduced pressure at ∼50 °C, and the residue was taken
into the glovebox and dissolved in a solution containing 45 mg
(excess) of [Ph3C][B(C6F5)4] in 0.6 mL of CD2Cl2 to give a
yellow-orange solution. The mixture was then syringed into a
rubber septum sealed NMR tube (under Ar), which was taken
quickly out of the glovebox and immersed within 2-4 min in a
dry ice/acetone bath (195 K). The NMR tube was placed in the
precooled (185-205 K) probe of the NMR spectrometer, and
a 1H NMR spectrum was run. The tube was then removed from
the probe to the dry ice/acetone bath, and several aliquots of
water (molar ratio I:H2O ≈ 1:0.3-1:1.5)5a were added to the
cold solution. The tube was shaken vigorously at 195 K to
induce mixing and placed back in the probe at 205 K, and NMR
spectra were obtained at various time intervals and tempera-
tures.
1
we note that all of the methyl JCH values are typical of
literature data.7 Thus the product is identified as [Cp2Ti-
(Me)(H2O)][B(C6F5)4] (IV), which is a rare, possibly unique,
example of a complex containing aqua and methyl ligands
within the inner coordination sphere of a titanium(IV)
complex.
Results and Discussion
Reactions of Cp2TiMe2 with [Ph3C][B(C6F5)4] in CD2Cl2,
carried out at ∼293 K as described in the Experimental
Section, gave dark orange-brown solutions. NMR spectra
(5) (a) A standard solution of water in CD2Cl2 was prepared, by
dissolving 31 μL of H2O in 10 mL of CD2Cl2 at 20 °C; CH2Cl2 has
a solubility5b of 0.2-0.24 wt %, or ∼3.2 μL of H2O per mL of CD2Cl2.
After an initial 1H NMR spectrum of a Cp2TiMe2/[Ph3C][B(C6F5)4]
mixture was obtained, 0.05 mL of the standard H2O/CD2Cl2 solution
was syringed into 0.6 mL of the Cp2TiMe2/[Ph3C][B(C6F5)4] mixture in
CD2Cl2 at 195 K solution for an approximate molar ratio of Ti:H2O
≈ 4:1, making it a solution containing 0.24 μL of H2O per mL of CH2Cl2.
This is about 13 times less concentrated than the 20 °C saturated
solution concentration (3.18 μL per mL). (b) For the solubility of H2O
in CH2Cl2, see: http://www.trimen.pl/witek/ciecze/old_liquids.html,
http://macro.lsu.edu/HowTo/solvents/Dichloromethane.htm.
Interestingly, while addition of a slight excess of water
(Figure 1e) to the reaction mixture had little effect on the Cp
and methyl resonances of II and caused only small, upfield
(6) This experiment provides evidence for the stoichiometry of the
reaction with water, a result confirmed by similar experiments involving
other proportions of added water.
(7) Courtot, P.; Pichon, R.; Salaun, J. Y.; Toupet, L. Can. J. Chem.
1991, 69, 661.

Note
Organometallics, Vol. 29, No. 22, 2010 6119
Figure 1. 1H NMR spectra in the Cp and methyl regions (CD2Cl2, 205 K) of (a) a typical mixture of I and II and after the addition of
∼0.3 (b), ∼0.6 (c), ∼0.9 (d), ∼1.2 (e), and ∼1.5 (f) equiv of water per I (* = resonance of residual toluene).
shifts of those of IV, the resonance of the coordinated water
disappeared and a new, broad resonance appeared at
δ ∼3.15. On the addition of further water (Figure 1f), the
resonance at δ ∼3.15 became stronger and is tentatively
attributed to free water. That the resonance is broad while
that of the coordinated water of IV cannot be observed
suggests strong hydrogen bonding and exchange between the
two sites.
(see below) to the methyl ligand. While we were not able to
identify the titanium-containing product(s), a number of new
Cp resonances appeared in the region δ 6-7 (Figure S5), and
we note that the initially formed species would probably be
[Cp2Ti(OH)(CD2Cl2)][B(C6F5)4]. This might well convert to
the dinuclear species [Cp2Ti(μ-OH)2TiCp2][B(C6F5)4]2 and,
ultimately, perhaps, to oxide clusters of the types Cp6Ti6(μ-O)8
and Cp8Ti8(μ-O)12, which have been reported previously.8
The contrast between Cp2TiMe2 and IV with regard to
susceptibilities of their methyl ligands to undergo protono-
lysis is quite notable, especially since the positive charge of IV
should render its methyl ligand much less carbanionic in
character and thus less susceptible to protonolysis. The
proton transferred must originate in the coordinated water,
and, consistent with this hypothesis, we note that the co-
ordinated water molecules in [Cp2Ti(H2O)2]2þ are much
more acidic than is bulk water, with pKa1 ∼3.51 and pKa2
∼4.35.9
As has been mentioned above, Cp chemical shifts in
metallocene compounds are generally strongly influenced
by inductive effects,4 and we have previously utilized this
correlation to distinguish between the resonances of I and
II.3 On this basis we note that coordinated water of IV
(δCp 6.44) appears to be intermediate in donor properties
between those of the neutral CD2Cl2 (δCp 6.71) and the
[B(C6F5)4]- (δCp 6.32) and chloride (δCp 6.233c) anions.
While IV is reasonably stable for hours at 205 K, its
resonances slowly weaken and the singlet resonance of
methane appears at δ 0.16 (note the weak methane resonance
in Figure 1); the decomposition of IV is more rapid at 225 K,
with the methane resonance now at δ 0.17. Using D2O
instead of H2O, the Cp and Ti-Me resonances but not the
aqua resonance of IV were observed at 205 K, and warming
to 225 K now resulted in a quadrupolar broadened methane
resonance, which sharpened on further warming to 298 K to
a 1:1:1: triplet (JHD 1.9 Hz) at δ 0.20 (Figure S4 in the
Supporting Information). Thus, as anticipated, the decom-
position does indeed involve transfer of a proton from water
Finally, we note that the much greater reactivity of what
we believe to be the cationic, solvent-separated ion pair I
than of the contact ion pair II with water is paralleled in
the relative reactivities of these two complexes (and their
zirconocene analogues) with alkenes.3 We have noted
(8) (a) Bottomley, F.; Drummond, D. F.; Egharevba, G. O.; White,
P. S. Organometallics 1986, 4, 1620. (b) Heshmatpour, F.; Wocadlo, S.;
Massa, W.; Dehnicke, K.; Bottomley, F.; Day, R. W. Z. Naturforsch., B:
Chem. Sci. 1994, 49, 827.
(9) Toney, J. H.; Marks, T. J. J. Am. Chem. Soc. 1985, 107, 947.

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Dunlop-Briere and Baird
6120 Organometallics, Vol. 29, No. 22, 2010
previously3c that while there have been reported very few
instances where the reactivities of two analogous solvent-
separated and contact ion pair complexes can be directly
compared, it does appear from this work and elsewhere3 that
anion-cation separation is a key feature in enhancing
coordination of neutral ligands to cationic metallocene
complexes.
Acknowledgment. M.C.B. gratefully acknowledges the
Natural Science and Engineering Research Council of
Canada and Queen’s University for funding of this
research.
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.