Isolation and Characterization of a Monomeric Cationic Titanium Hydride

Article abstract of DOI:10.1021/ja048952i

Methyl cations 1-Cp and 1-Cp*, stabilized by the tri-tert-butylphophinimine ligand and either C₅H₅ or C₅Me₅, were generated from the neutral dimethyl precursors and [Ph₃C]⁺[B(C₆F₅)₄]^-. Reaction of these compounds with H₂ resulted in contrasting reactions. For 1-Cp, hydrogenolysis of the Ti-CH₃ group led to rapid reduction to Ti(III) and production of a cationic Ti(III) dimer, 2, presumably formed upon loss of H₂ from a transiently generated Ti(IV) hydride. Compound 2 was characterized crystallographically and via its cleavage with donor solvents such as THF to form the monomeric [Cp(L)Ti(THF)₂]⁺[B(C₆F₅)₄]^-, 3. In contrast, 1-Cp* reacted rapidly with H₂ to form a cationic Ti(IV) hydride species, 4, which was resistant to reduction. While only moderately stable in solution under H₂, a stable, isolable THF adduct preciptitated upon addition of THF, giving 4·THF, which was fully characterized, including via X-ray crystallography. Naked hydride 4 was very reactive toward haloarene solvents such as bromobenzene, giving the cationic bromide [Cp·(L)TiBr]⁺[B(C₆F₅)₄]^-, 5, which was fully characterized as its THF adduct 5·THF. The contrasting behavior of 1-Cp and 1-Cp* is a result of the greater steric protection and electron donation provided by the Cp* ligand relative to the Cp donor. Copyright

Full text of DOI:10.1021/ja048952i

Published on Web 04/16/2004
Isolation and Characterization of a Monomeric Cationic Titanium Hydride
Kuangbiao Ma, Warren E. Piers,* Yuan Gao, and Masood Parvez
Department of Chemistry, UniVersity of Calgary, 2500 UniVersity DriVe NW, Calgary, Alberta, Canada T2N 1N4
Received February 24, 2004; E-mail: wpiers@ucalgary.ca
Organotitanium hydrides1 are a highly reactive class of com-
pounds that have been implicated in a variety of catalytic processes,
including olefin hydrogenation and hydrosilation and silane dehy-
drocoupling.2 Despite their importance, isolated, terminal titanium
hydrido compounds are restricted to a few Ti(III) examples3,4 or
highly sterically protected neutral Ti(IV) derivatives.5 Cationic
variants are even more rare,6 but presumably play a critical role in
olefin polymerization7 and oligomerization8 reactions, especially
where H2 is used as a polymer MW controlling agent. Given the
tendency of Ti(IV) hydrides to undergo reduction to Ti(III) via loss
of H2,9 it is of interest to probe the behavior of cationic titanium
hydrides in ligand environments relevant to olefin polymerization,
because reductive side reactions might be expected to impact overall
catalyst performance. Here, we report reactions of the catalysts
[Cp(tBu3PN)TiCH3]+[B(C6F5)4]- (Cp ) C5H5, 1-Cp; C5Me5,
1-Cp*) with H2 and detail the complete characterization of a
monomeric cationic organotitanium hydride.
Figure 1. Crystalmaker depiction of the molecular structure of the dication
in 2. Selected bond distances (Å): Ti(1)-N(1), 1.997(2); Ti(1)-N(2), 1.979-
(2); P(1)-N(1), 1.631(2); Ti(1)-Ti(2), 2.5966(7). Selected bond angles
(deg): N(1)-Ti(1)-N(2), 91.21(9); Ti(1)-N(1)-Ti(2), 81.21(8); Ti(1)-
N(1)-P(1), 137.83(13); Ti(2)-N(1)-P(1), 139.74(13).
Trityl borate activation of the neutral dimethyl derivatives Cp-
(tBu3PdN)Ti(CH3)210 proceeds cleanly in toluene. To solubilize the
resultant liquid clathrate-like oils, haloarene solvents such as chloro-
or bromobenzene are required. The cations 1-Cp and 1-Cp* were
characterized completely using NMR spectroscopy.11 Although
analogous cations incorporating [H3CB(C6F5)3]- counteranions via
B(C6F5)3 activation have been reported,12 and are somewhat more
well-behaved than the [B(C6F5)4]- partnered cations, their reactions
with H2 are complicated by the slower conversion of the
[H3CB(C6F5)3]- counteranions to [HB(C6F5)3]-.13 To preclude this
complication, we have focused on the [B(C6F5)4]- salts.
but X-ray crystallographic analysis revealed it to be a dicationic
TiIII-TiIII dimer (Figure 1) in which the Ti2N2 core exists in a
butterfly conformation with a short Ti-Ti distance of 2.5966(7)
Å. A related neutral dimer, also formed via loss of D2, has a Ti-
Ti distance of 2.442(1) Å.14 Dilute CD2Cl2 solutions of 2 are ESR
silent, but, upon addition of donor solvents such as THF, the dimer
is broken up into the yellow, paramagnetic d1 bis-THF Ti(III) cation
3, which was also characterized crystallographically15 and via ESR
spectroscopy (toluene, 294 K, g ) 1.9675).
The tendency of [Cp(tBu3PdN)TiH]n+ to undergo reductive loss
of H2 may be due to a combination of the lack of steric protection
and relatively poor electron donation afforded by the C5H5 ligand.
Indeed, for the cation 1-Cp*, incorporating the more highly donating
and sterically demanding Cp* donor, reaction with H2 leads rapidly
to a cationic titanium(IV) hydride (Scheme 2). The reaction must
A toluene solution of in situ generated cation 1-Cp reacts rapidly
with H2, producing a green precipitate and CH4 (Scheme 1). The
Scheme 1
Scheme 2
putative cationic hydride produced upon treatment of 1-Cp with
H2 was not observed and must be unstable toward loss of H2,
producing 2. The green microcrystalline product 2 is sparingly
soluble even in CD2Cl2, preventing the acquisition of NMR data,
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J. AM. CHEM. SOC. 2004, 126, 5668-5669
10.1021/ja048952i CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
The fact that cationic hydride 4‚THF can be obtained prepara-
tively offers an unprecedented opportunity to examine the reactivity
of such a species in detail. Studies aimed at mapping out this
reactivity, including the σ-bond metathesis chemistry mentioned
above, are ongoing. Although the long Ti-H distance suggests that
homolytic cleavage of this bond may be facile, the tendency to
undergo reduction via loss of H2 is diminished in the Cp* system
as opposed to the Cp stabilized cations, partially accounting for
the higher olefin polymerization activities observed in the former
catalyst.
Acknowledgment. Funding for this work was provided by Nova
Chemicals, Inc. of Calgary, Alberta, and NSERC of Canada through
a CRD grant to W.E.P.
Figure 2. Crystalmaker depiction of the molecular structure of the cationic
portion of 4‚THF. Selected bond distances (Å): Ti(1)-H(1), 1.84(2); Ti(1)-
N(1), 1.781(2); Ti(1)-O(1), 2.075(2); P(1)-N(1), 1.613(2). Selected bond
angles (deg): N(1)-Ti(1)-O(1), 104.33(8); N(1)-Ti(1)-H(1), 98.6(7);
O(1)-Ti(1)-H(1), 94.4(7); N(1)-Ti(1)-Cp*cent, 131.66(6); O(1)-Ti(1)-
Cp*cent, 115.78(5); Ti(1)-N(1)-P(1), 168.39(12).
Supporting Information Available: Experimental details, tables
of crystal data, atomic coordinates, bond lengths and angles, ORTEP
diagrams, and anisotropic displacement parameters for 2, 4‚THF, and
5‚THF (PDF and CIF). This material is available free of charge via
the Internet at http://pubs.acs.org.
be carried out in toluene in the absence of haloarene solvents for
hydride 4 to be generated cleanly. Toluene solutions of 4 in the
presence of H2 are spectroscopically well behaved, and a broad
signal at 7.80 ppm is tentatively assigned to the Ti-H moiety,
although this is likely an averaged position due to exchange with
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2
dissolved H2. A corresponding resonance appears in the H NMR
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1
7.25 ppm in the H NMR spectrum integrating to one proton was
2
assigned to the Ti-H moiety and confirmed by H NMR spec-
troscopy on d1-4‚THF.17 The nature of 4‚THF was also established
via an X-ray crystallographic analysis (Figure 2).
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