Structure of the decamethyl titanocene cation, a metallocene with two agostic C-H bonds, and its interaction with fluorocarbons

Article abstract of DOI:10.1021/ja027617w

The tetraphenylborate salt of the decamethyl titanocene cation, [Cp*₂Ti][BPh₄] (1, Cp* = C₅Me₅), was prepared by reaction of Cp*₂TiH with [Cp₂Fe][BPh₄] and by reaction of Cp*₂TiMe with [PhNMe₂H][BPh₄]. The crystal structure of 1 shows that the Cp*₂Ti cation has a bent metallocene structure with agostic interactions with the metal center of two adjacent methyl groups on one of the Cp* ligands. Compound 1 reacts readily with THF to give the adduct [Cp*₂Ti(THF)][BPh₄] (2). In fluorobenzene, 1 forms the η¹-fluorobenzene adduct [Cp*₂Ti(η¹-FC₆H₅)][BPh₄] (3), which was structurally characterized. In contrast to the thermal stability of 3, addition of α,α,α-trifluorotoluene to either 1 or 2 results in C-F activation to give Cp*₂TiF₂ and PhCF₂CF₂Ph as the main products. This reactivity toward benzylic C-F bonds is also reflected in the reactivity toward the fluorinated borate anions [B(C₆F₅)₄]- and {B(3,5-(CF₃)₂C₆H₃]₄}-: reaction of Cp*₂TiMe with their [PhNMe₂H]+ salts results in a stable complex for the former anion, whereas rapid C-F activation is observed for the latter. Copyright

Full text of DOI:10.1021/ja027617w

Published on Web 10/12/2002
Structure of the Decamethyl Titanocene Cation, a Metallocene with Two
Agostic C-H Bonds, and Its Interaction with Fluorocarbons†
Marco W. Bouwkamp, Jeanette de Wolf, Isabel del Hierro Morales, Jeroen Gercama, Auke Meetsma,
Sergei I. Troyanov,‡ Bart Hessen,* and Jan H. Teuben
Center for Catalytic Olefin Polymerization, Stratingh Institute for Chemistry and Chemical Engineering,
UniVersity of Groningen, Nijenborgh 4, 9747 AG, Groningen, The Netherlands
Received July 9, 2002
Cationic transition-metal complexes find wide application as
catalysts for a range of chemical transformations and in particular
in the catalytic polymerization of olefins.1 The interaction of the
electron-deficient, cationic metal center with the complementary
anion can greatly affect catalyst performance, and to minimize
interference with the catalytically active species, anions are used
with very weakly coordinating properties.2 These counterions
frequently employ fluoroorganic moieties to reduce Coulombic
interactions by dissipating the negative charge and to minimize the
nucleophilicity. Nevertheless, they can sometimes participate in
catalyst deactivation processes.3 We are studying the various aspects
of anion coordination and activation by investigating the complex-
ation and reactivity behavior of some simple cationic organo
transition-metal complexes toward fluorinated hydrocarbons and
Figure 1. Structure of the cation of 1. Thermal ellipsoids are at 50% level.
fluorinated anions. Here we describe the salt of the “naked” Ti(III)
permethyl titanocene cation [Cp*2Ti][BPh4] (1) (Cp* ) η5-C5Me5),
with an unprecedented structure, and its reactivity with fluorinated
aromatic substrates. Results include the first structural characteriza-
tion of a transition-metal η1-fluorobenzene adduct.
Selected bond lengths: Ti(1)-C(111) ) 2.352(3) Å, Ti(1)-C(112) )
2.471(3) Å, Ti(1)-C(113) ) 2.353(3) Å, Ti(1)-C(114) ) 2.159(3) Å,
Ti(1)-C(115) ) 2.167(3) Å.
Scheme 1
The compound [Cp*2Ti][BPh4] (1) was synthesized via two
routes (Scheme 1). One is the reaction of Cp*2TiH 4 with [Cp2Fe]-
[BPh4] in toluene, yielding 1, ferrocene, and 0.5 equiv of H2. The
other is the reaction of Cp*2TiMe5 with [PhNMe2H][BPh4] in
toluene, yielding 1, free N,N-dimethylaniline, and 1 equiv of CH4.
Compound 1 cleanly reacts with THF to give the mono-THF adduct
[Cp*2Ti(THF)][BPh4] (2, Scheme 1),6 which was structurally
characterized.7 A single-crystal structure determination8 of 1 showed
that it consists of discrete [Cp*2Ti]+ and [BPh4]- ions without direct
interionic contacts.9 The cation (Figure 1) contains one normal η5-
Cp* ligand and one that has two methyl C-H‚‚‚Ti agostic
interactions, one each on two adjacent methyl groups: Ti‚‚‚H(119′′)
) 2.16(3) Å, Ti‚‚‚H(120′) ) 2.20(3) Å. The agostic Cp* ligand is
slipped back, resulting in Ti-C(114/115) distances that are much
shorter than the other Ti-C(ring) distances. Nevertheless, the
intraligand ring C-C and C(ring)-C(Me) distances are all normal
for a Cp* ligand. The agostic methyl groups are bent down by 24°
toward the metal center out of the Cp-plane. It is interesting to
compare the structure of 1 with that of a neutral Ti complex with
a tetramethyl-phenyl-substituted cyclopentadienyl ligand that has
been doubly deprotonated on adjacent methyl groups, (η5-C5Me4Ph)-
[η4:η3-C5Me2Ph(CH2)2]Ti.10 In this compound, there is a clear
elongation in the ring C-C distances connecting the “diene” and
“allyl” moieties of the ligand, the C-CH2 distances of 1.439(4)
and 1.447(4) Å are shorter than the C-CH3 distances to the agostic
methyl groups in 1 (1.486(5) and 1.502(5) Å), and the Ti-CH2
distances of 2.252(3) and 2.316(4) Å are much shorter than the
Ti...CH3 distances in 1 of 2.685(5) and 2.652(4) Å. The cation in 1
adopts a bent rather than a linear metallocene geometry, in contrast
to neutral titanocenes.11 Calculations on the Cp2M+ (M ) Sc, La)
system indicated that these cationic metallocenes prefer a bent
structure.12 The bonding of the doubly agostic Cp* ligand in 1 thus
far appears to be quite unique. Although 1 is highly reactive (vide
infra), it is apparently reluctant to form dinitrogen complexes, unlike
the neutral Ti(II) Cp*2Ti13 and Ti(III) Cp2Ti(aryl)14 species. The
synthesis of 1, as described above, is performed under a nitrogen
atmosphere, and cooling powdered samples under nitrogen to -196
°C does not lead to a visible color change that could indicate N2
complexation.
When dissolved or generated in fluorobenzene, 1 forms a green
fluorobenzene adduct [Cp*2Ti(η1-FC6H5)][BPh4] (3, Scheme 1) that
was structurally characterized.15 Its structure (Figure 2) reveals a
normal bent metallocene geometry and η1-coordination of the
fluorobenzene to the metal via the fluorine atom. The Ti-F bond
of 2.151(2) Å is relatively short compared to that in the zwitterionic
* To whom correspondence should be addressed. E-mail: B.Hessen@chem.rug.nl.
† Netherlands Institute for Catalysis Research (NIOK) publication no. RUG-
02-04-03.
‡ Permanent address: Moscow State University, Department of Chemistry,
Leninskie Gory, 119899 Moscow, Russia.
9
12956
J. AM. CHEM. SOC. 2002, 124, 12956-12957
10.1021/ja027617w CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Acknowledgment. We thank A. P. Jekel for recording the GC-
MS spectra. This research was supported by the Council for
Chemical Sciences of the Netherlands Foundation for Scientific
Research (NWO-CW).
Supporting Information Available: Synthesis and characterization
data of the complexes described (PDF). Crystallographic data for 1, 2,
and 3 (CIF). This material is available free of charge via the Internet
at http://pubs.acs.org.
References
(1) For reviews on single-site olefin polymerization catalysis: (a) Brintzinger,
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(2) For a review on activators and weakly coordinating anions in olefin
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Figure 2. Structure of the cation of 3. Thermal ellipsoids are at 50% level.
Selected bond lengths and angles: Ti(1)-F(1) ) 2.151(2) Å, F(1)-C(21)
) 1.402(3) Å, Ti(1)-F(1)-C(21) ) 168.2(2)°.
Ti(III) complex Cp*[C5Me4CH2B(C6F5)3]Ti with intramolecular
C-F bond coordination (2.406 Å).16 The F-C distance of the
coordinated fluorobenzene of 1.402(3) Å is elongated relative to
the C-F distance in solid fluorobenzene, 1.364(2) Å.17 The Ti-
F-C(21) angle of 168.2(2)° is very obtuse compared to the
M-X-C angles in other η1-halobenzene transition-metal com-
plexes (X ) Br, I; 101.8-116.4°).18 The fluorobenzene in 3 appears
to be relatively weakly bound and is readily displaced by more
strongly coordinating ligands such as diethyl ether or THF.
The fluorobenzene adduct 3 is stable at ambient temperature in
fluorobenzene solution for more than 5 days. In contrast, addition
of R,R,R-trifluorotoluene to such a solution results in a rapid
reaction, yielding Cp*2TiF2 and 1,2-diphenyl-1,1,2,2-tetrafluoro-
ethane as main products (together with products resulting from
concomitant anion degradation). Thus, it appears that benzylic
fluorides are much more rapidly activated by the Cp*2Ti cation
than aryl fluorides. This has implications for the compatibility of
this cation with fluorinated borate anions, as suggested by the
following observations. Reaction of Cp*2TiMe with [PhNMe2H]-
[B(C6F5)4] in fluorobenzene yields (after evaporation of the solvent,
and washing with pentane) a brown solid that dissolves in THF-d8
to give [Cp*2Ti(THF-d8)][B(C6F5)4]. No evidence was found for
coordination of fluorobenzene to the metal center in [Cp*2Ti]-
[B(C6F5)4], suggesting that in this compound the anion coordinates
to the Ti center through one or two of its fluorides, without
subsequent C-F activation. In contrast, reaction of Cp*2TiMe with
[PhNMe2H][B{3,5-(CF3)2C6H3}4] in fluorobenzene yields Cp*2TiF2
as the organometallic product, similar to the reaction of 1 with
R,R,R-trifluorotoluene. When the reaction was performed in neat
THF-d8, clean conversion to the THF adduct [Cp*2Ti(THF-d8)]-
[B{3,5-(CF3)2C6H3}4] was observed.
(3) (a) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116,
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(7) Details on the structure determination of 2 can be found in the Supporting
Information.
(8) Crystal data for 1. C20H30Ti. C24H20B, Mr ) 637.56, triclinic, P-1, a )
13.3636(7) Å, b ) 15.0600(7) Å, c ) 17.7873(9) Å, R ) 97.131(1)°, â
) 90.604(1)°, γ ) 91.970(1)°, V ) 3549.6(3) Å3, Z ) 4, Dc ) 1.193 g
cm-3, T ) 100(1) K, µ(Mo KR) ) 0.71073 Å. All H-atoms were located
and refined freely; wR(F2) ) 0.1265 for 12451 reflections and 1229
parameters, R(F) ) 0.0532 for 7628 reflections with Fo g 4.0 σ(Fo). The
unit cell contains two independent formula units that do not differ
significantly in structure. Geometrical data given in the text pertain only
to one of these units.
(9) This is unlike, e.g., the permethylsamarocene cation [Cp*2Sm][BPh4],
where the anion coordinates to the metal in η2:η2 fashion via the phenyl
groups: Evans, W. J.; Seibel, C. A.; Ziller, J. W. J. Am. Chem. Soc. 1998,
120, 6745.
(10) Kupfer, V.; Thewalt, U.; Tisˇlerova´, I.; Sˇteˇpnicˇka, P.; Gyepes, R.; Kubisˇta,
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(15) Crystal data for 3. C26H35FTi‚C24H20B‚C6H5F, Mr ) 829.79, triclinic, P-1,
a ) 11.992(1) Å, b ) 13.649(2) Å, c ) 14.883(1) Å, R ) 90.95(1)°, â
) 109.499(8)°, γ ) 90.865(9)°, V ) 2295.5(4) Å3, Z ) 2, Dc ) 1.200 g
cm-3, T ) 130(1) K, µ(Mo KR) ) 0.71073 Å; wR(F2) ) 0.1407 for
9284 reflections and 568 parameters, R(F) ) 0.0536 for 8148 reflections
with Fo g 4.0 σ(Fo).
In conclusion, we have prepared a salt of the permethyl titanocene
cation that exhibits unique structural features, containing a Cp*
ligand with two C-H‚‚‚Ti agostic interactions. This cation revers-
ibly coordinates the fluorine atom of fluorobenzene, but readily
cleaves benzylic C-F bonds. Presently we are investigating the
effect of the metal electronic configuration on the coordination and
reactivity behavior of metallocene cations, aiming to increase the
understanding of the properties of highly reactive electrophilic
transition-metal species that are increasingly used in catalysis, and
to provide more insight into possible catalyst deactivation pathways.
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