Communications to the Editor
J. Am. Chem. Soc., Vol. 123, No. 37, 2001 9213
and 0.83 Å shorter than in 1. This distance is equal to the sum of
covalent radii15 for Fe and Ti and is well within the range reported
by Selegue et al. and Gade and co-workers for Fe-Ti single bonds
in (Cp)Fe(CO)2--coordinated titanium complexes.16
Such close approach of Ti to Fe results in the distortion of the
complex compared to 1 and 2. The dihedral angle between the
Cp rings has increased to 12.87° and, unlike in 1 and 2, Ti is
found out of the plane formed by Fe, N1, and N2. The geometry
around Ti is best described as trigonal bipyramidal with N1, N2,
and Cl residing in the equatorial plane while Fe and Cl* are in
the axial positions.17 The Ti-Cl bond (2.505(2) Å) for the chloride
trans to Fe is longer than the Ti-Cl* bond (2.377(2) Å)).
An Fe-M dative bond in ferrocene-containing complexes was
first reported by Seyferth and co-workers in 1983 for the
palladium complex [(C5H4S)2Fe]Pd(PPh3),18 where a weak Fe-
Pd dative bond stabilizes an otherwise three-coordinate Pd center.
Despite the fact that a number of related systems containing late
metals have been reported since,19 to the best of our knowledge
(a) 2 and 5 (and possibly 3 and 4) represent the first examples of
a ferrocene group stabilizing an electron-deficient early transition
metal center through an Fe-M dative bond and (b) 5 contains
the shortest Fe-M bond for a ferrocene-containing bimetallic
compound.
Figure 2. ORTEP plot of 5 showing side and top views of the dication.
Hydrogen atoms and silicon-bound methyls have been omitted.
Scheme 1
The Fe-Ti dative interaction in the cationic Ti complexes
exemplifies the Lewis-basic behavior of ferrocene. Although not
a strong base in solution, gas-phase measurements show that its
proton affinity lies between that of NH3 and MeNH2.20 In solution,
protonation of ferrocene has been shown to occur at iron;21
furthermore, in many cases electrophilic substitution reactions
proceed through initial formation of an Fe-E dative bond,
followed by transfer of the electrophile to the Cp.22 Understanding
the interaction between Fe and strong Lewis acids has, therefore,
attracted much interest. Compounds 2 and 5 may serve as
constrained models for such interactions.
suitable for X-ray diffraction have not yet been obtained and the
exact coordination geometry at Ti remains uncertain.
Using 0.5 equiv of TB in chlorobenzene-d5 resulted in
formation of the methyl-bridged, dinuclear 4. Its 1H NMR
spectrum shows a silyl resonance at 0.34 ppm, a TiMe resonance
at 0.87 ppm, and two ferrocenyl pseudo-triplets at 4.40 and 3.02
ppm. The presence of a single TiMe resonance integrating to 9
H indicates rapid exchange between bridging/terminal methyl
groups. The compound can be thought of as a LTiMe+ cation
coordinated by a neutral LTiMe2 molecule.11
Work in progress aims to more fully understand the nature of
the Fe-Ti interaction by using spectroscopic and electrochemical
techniques, and to extend this chemistry to other metals.
The polymerization activity of 2 and 3 was tested with
1-hexene. Both compounds are active at producing short-chain
oligomers of 5-6 monomer units. As expected, 3 is a more active
catalyst than 2, producing 102 g (oligomer)/(mmol catalyst)‚(h).
Acknowledgment. We thank the Department of Energy for financial
support.
1
The H NMR spectrum of the oligomer shows a characteristic
olefinic resonance at 5.3-5.4 ppm, indicating an internal double
bond. Therefore, we postulate that â-H elimination is the chain-
termination mechanism and that a new chain can then be initiated
by the resulting Ti-hydride cation.
Supporting Information Available: Details of synthetic work,
characterization data, polymerization tests, and tables of crystallographic
data for 2 and 5 (PDF). This material is available free of charge via the
Despite the fact that 3 can be generated in CD2Cl2 and is stable
in this solvent for several hours, crystallization from this solvent
afforded dark-purple crystals shown by elemental analysis to be
[LTiCl][B(C6F5)4 ] (5), i.e., the result of CH2Cl2 activation by 3
(eq 2). The X-ray crystal structure of 5 revealed its dimeric nature
(Figure 2).12 The dicationic dimer resides on a crystallographic
inversion center and contains a planar Ti2Cl2 core. Analogous
transformations leading to a dimeric dication have been reported
for Zr13 and very recently for Ti.14 In our system, 5 is the major
product and was isolated in 69% yield. The most notable feature
of the solid-state structure is an extremely short Fe-Ti distance
(2.49 Å), which is 0.58 Å shorter than the Fe-Ti distance in 2
JA0161857
(15) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell
University Press: Ithaca, NY, 1960.
(16) (a) Sartain, W. J.; Selegue, J. P. J. Am. Chem. Soc. 1985, 107, 5818.
(b) Sartain, W. J.; Selegue, J. P. Organometallics 1987, 6, 1812. (c) Friedrich,
S.; Memmler, H.; Gade, L. H.; Li, W.-S.; Scowen, I. J.; McPartlin, M.;
Housecroft, C. E. Inorg. Chem. 1996, 35, 2433. (d) Gade, L. H.; Schubart,
M.; Findeis, B.; Fabre, S.; Bezougli, I.; Lutz, M.; Scowen, I. J.; McPartlin,
M. Inorg. Chem. 1999, 38, 5282.
(17) Ti complexes with phenylenediamide ligands (ref 8a) undergo what
may be a related distortion, presumably to facilitate a donor interaction between
the phenyl π-system and the metal.
(18) Seyferth, D.; Hames, B. W.; Rucker, T. G.; Cowie, M.; Dickson, R.
S. Organometallics 1983, 2, 472.
(19) (a) Akabori, S.; Kumagi, T.; Shirahige, T.; Sato, S.; Kawazoe, K.;
Tamura, C.; Sato, M. Organometallics 1987, 6, 526. (b) Sato, M.; Sekino,
M.; Akabori, S. J. Organomet. Chem. 1988, 344, C31. (c) Sato, M.; Sekino,
M.; Katada, M.; Akabori, S. J. Organomet. Chem. 1989, 377, 327. (d) Sato,
M.; Suzuki, K.; Asano, H.; Sekino, M.; Kawata, Y.; Habata, Y.; Akabori, S.
J. Organomet. Chem. 1994, 470, 263.
(11) For recent examples see: Zhang, S.; Piers, W. E. Organometallics
2001, 20, 2088 and theferences therein.
(12) Crystal data for 5: C40H26N2Si2FeTiF20BCl, FW ) 1120.81, mono-
clinic, space group P21/c, Z ) 4, a ) 13.711(1) Å, b ) 12.699(1) Å, c )
24.453(2) Å, â ) 94.329(1)o, V ) 4245.5(5) Å3, Fcalcd ) 1.753 g/cm3, µ )
7.72 cm-1, R1(I>3.00σ(I)) ) 0.047, wR2(all data) ) 0.084 for 3353
observations and 493 parameters.
(13) Gomez, R.; Green, M. L. H.; Haggit, J. L. J. Chem. Soc., Dalton Trans.
1996, 939.
(14) Yue, N. L. S.; Stephan, D. W. Organometallics 2001, 20, 2303.
(20) (a) Foster, M. S.; Beachamp, J. L. J. Am. Chem. Soc. 1975, 97, 4814.
(b) Meot-Ner, M. J. Am. Chem. Soc. 1989, 111, 2830.
(21) Bitterwolf, T. E.; Ling, A. C. J. Organomet. Chem. 1972, 40, 197.
(22) Cunningham, A. F., Jr. Organometallics 1997, 16, 1114.