Novel zwitterionic complexes of Ti(IV) via reaction of Lewis acids M(C6F5)3 (M = B, Al) with a titanium diene complex (η5-C5Me4SiMe2N(t)Bu)Ti(1,3-pentadiene)

Article abstract of DOI:10.1039/a809438a

The reaction of (η⁵-C₅Me₄SiMe₂N(t)Bu)Ti(1,3-pentadiene) with B(C₆F₅)₃ Or Al(C₆F₅)₃ in 1:1 mole ratio in hexane solution at room temperature results in the formation of titanium borate or aluminate zwitterions which both feature stabilisation of the Ti(IV) centre by two agostic Ti···H-C interactions.

Full text of DOI:10.1039/a809438a

Novel zwitterionic complexes of Ti(iv) via reaction of Lewis acids
M(C6F5)3 (M = B, Al) with a titanium diene complex
5
(h -C5Me4SiMe2NtBu)Ti(1,3-pentadiene)
Alan H. Cowley,* Gregory S. Hair, Brian G. McBurnett and Richard A. Jones*
Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712, USA.
E-mail: cowley@mail.utexas.edu
Received (in Columbia, MO, USA) 2nd December 1998, Accepted 21st January 1999
5
The reaction of (h -C5Me4SiMe2NtBu)Ti(1,3-pentadiene)
with B(C6F5)3 or Al(C6F5)3 in 1:1 mole ratio in hexane
solution at room temperature results in the formation of
titanium borate or aluminate zwitterions which both feature
stabilisation of the Ti(iv) centre by two agostic Ti…H–C
interactions.
It has been demonstrated recently that Cp2Zr(butadiene) reacts
readily with B(C6F5)3 to afford a zirconium borate zwitterion
(1) that is an active catalyst for a-olefin polymerization.1 An
important structural facet of this zwitterion is the presence of a
weak dative interaction between an ortho-fluorine of one of the
BC6F5 groups and an otherwise vacant coordination site on
zirconium.2 We report here (i) the first structurally characterised
example of a new type of titanium borate zwitterion that does
not involve intramolecular F?metal stabilisation,† and (ii) the
first example of an analogous titanium aluminate zwitterion. In
addition, both complexes feature stabilisation of the Ti(iv)
metal centre by two agostic C–H…Ti interactions.
It has been reported3 that, when treated with B(C6F5)3, the
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bridged monocyclopentadienyl titanium diene complex (h -
C5Me4SiMe2NtBu)Ti(1,3-pentadiene) 2 forms an active olefin
polymerization catalyst. We have found that the reaction of 2
with an equimolar quantity of B(C6F5)3 in hexane at 25 °C
affords a virtually quantitative yield of a dark green crystalline
Fig. 1 X-Ray crystal structure of 3. Selected bond distances (Å) and angles
(°) for 3 (the corresponding values for 4 are shown in parentheses with Al(1)
replacing B(1)): Ti(1)–C(1) 2.360(2) (2.355(2)), Ti(1)–C(2) 2.282(2)
(2.261(3)), Ti(1)–C(3) 2.327(2) (2.344(3)), Ti(1)–C(4) 2.233(2) (2.271(3)),
Ti(1)–H(1A) 2.20(2) (2.27(3)), Ti(1)–H(1B) 2.27(2) (2.45(3)), C(1)–B(1)
1.706(3) (2.052(3)), C(1)–C(2) 1.494(3) (1.479(5)), C(2)–C(3) 1.382(3)
(1.386(5)), C(3)–C(4) 1.413(3) (1.416(5)), C(4)–C(5) 1.505(3) (1.512(5)),
B(1)–C(1)–C(2) 116.8(2) (108.3(2)), C(1)–C(2)–C(3) 125.8(2) (126.6(3)),
C(2)–C(3)–C(4) 125.8(2) (125.4(3)), C(3)–C(4)–C(5) 121.9(2) (120.3(3)).
Thermal ellipsoids are drawn at the 30% level.
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solid (mp 156–157 °C) with an empirical composition (h -
C5Me4SiMe2NtBu)Ti(1,3-pentadiene)·B(C6F5)3 3.‡ An X-ray
diffraction study of 3 (Fig. 1)§ revealed a zwitterionic structure
in which the B(C6F5)3 group is bound to the terminal –CH2
carbon atom (C(1)) of the original pentadiene ligand. The C(2)–
C(3) and C(3)–C(4) bond distances and the C(2)–C(3)–C(4)
bond angle of the coordinated diene are similar to those reported
by Erker et al.1 for 1 hence the C(2)–C(3)–C(4) moiety can be
tBu and MeCHCHCHCH2 signals overlap at d 21.05. Sec-
ondly, the carbon attached to boron, C(1), is not observed in the
13C NMR spectrum, and finally there is a ≈ 1.0 ppm difference
3
regarded as being h -attached to the metal. However, the metal–
1
C(2)–C(1) bond angle in 3 is considerably more acute
(74.08(10)°) than the corresponding bond angle in 1 such that
in the H chemical shifts for the C(1) methylene protons. The
primary cause of this difference in chemical shifts is probably
the existence of the chiral centre at titanium. However, other
possible contributors include (i) the influence of the ring current
from the tetramethylcyclopentadienyl group and (ii) only one H
atom may be agostic in solution.
the Ti–C(1)–B fragment is almost linear (Ti–C(1)–B
=
174.1(1)°), the Ti–C(1) distance is short (2.360(2) Å), and the
C(1)–B distance is longer in 3 (1.705(3) Å) than in 1 (1.633(9)
Å). Collectively, the foregoing structural features strongly
suggested the presence of agostic C–H…Ti interactions for the
CH2 hydrogens of C(1). This surmise was confirmed in the later
stages of refinement of the X-ray structure. Both hydrogen
atoms attached to C(1) were located and refined with isotropic
thermal parameters and show relatively short Ti…H distances
(Ti–H(1A) = 2.20(2); Ti–H(1B) = 2.27(2) Å). Further support
for the existence of the proposed agostic interactions stemmed
from the observation of a C–H stretching frequency at 2669
cm21 in the IR spectrum (Nujol mull). 1H, 13C, and 11B NMR
spectroscopic data¶ are also consistent with the X-ray analysis,
thus implying that the solid state structure persists in solution.
The 11B chemical shift for 2 (d 28.4) falls in the region
observed for four-coordinate boron.4 Three additional points are
worth noting regarding the NMR data. Firstly, a two-dimen-
sional 1H–13C study (at 500 and 125 MHz) revealed that the N–
The reaction of 2 with Al(C6F5)3 in hexane solution at 25 °C
afforded high yields of 4, the aluminium analogue of 3.‡ To our
knowledge, this represents the first example of a titanium
aluminate zwitterion of this type. Structural authentication was
provided by an X-ray crystallographic study§ which demon-
strated that the geometry of 4 is very similar to that of 3 (Fig. 2).
Two agostic Ti–H–C interactions are present with Ti–H(1A)
and Ti–H(1B) distances of 2.27(3) and 2.45(3) Å, respectively.
As in the case of 3, NMR spectroscopic data indicate that the
solution and solid state structures are very similar. The only
significant difference in the 1H and 13C NMR spectra for 3 and
4 is that in the latter there is no overlap of the N–tBu and diene
methyl 1H resonances.
We are grateful to the National Science Foundation and the
Robert A. Welch Foundation for financial support.
Chem. Commun., 1999, 437–438
437

Å3, Z = 4, m = 3.36 cm21, Dc = 1.472 Mg m23, F(000) = 2016, l =
0.71073 Å, T = 183 K. Of a total of 12 132 collected reflections, 10 205
were unique. The structure was solved by direct methods. The hydrogens on
the coordinated diene were located; the remaining hydrogens were placed in
calculated positions (C–H, 0.96 Å). Refinement was by full matrix least
squares on F2 with anisotropic displacement parameters for the non-located
H atoms. Final R indices [I > 2s(I)], R1 = 0.0536, wR2 = 0.1116. CCDC
182/1160. See http://www.rsc.org/suppdata/cc/1999/437/ for crystallo-
graphic files in .cif format.
¶ NMR data for 3: 1H NMR (500 MHz, C6D6) d 20.37 (s, CH2-diene, 1H),
0.35 (br s, SiCH3, 3H), 0.58 (s, SiMe, 3H), 0.88 (br s, CH2-diene, 1H), 1.05
(s, NtBu + CH3-diene, 12H), 1.22 (s, CpMe, 3H), 1.28 (s, CpMe, 3H), 1.29
(s, CpMe, 3H), 1.85 (s, CpMe, 3H), 3.26 (m, CH-diene, 1H), 3.90 (m, CH-
diene, 1H), 4.91 (m, CH-diene, 1H). 13C{1H} NMR (125 MHz, C6D6) d
6.42 (s, SiCH3), 6.81 (s, SiCH3), 10.83 (s, CpCH3), 12.19 (s, CpCH3), 14.37
(s, CpCH3), 14.46 (s, CpCH3), 18.74 (s, CH3(diene)), 33.79 (s, CH3-tBu),
61.42 (s, C-tBu), 90.32 (s, CH-diene), 110.52 (s, C(Cp)), 112.04 (s, CH-
diene), 127.45 (s, CpMe(Cp)), 130.56 (s, CH-diene), 132.75 (s, CMe(Cp)),
134.85 (s, CMe(Cp)), 137.88 (s, CMe(Cp)), 136.50 (m, C(C6F5)), 138.50
(m, CF), 147.69 (m, CF), 149.55 (m, CF). 19F NMR (282 MHz, C6D6) d
2129.0 (m, o-F, 6F), 2157.9 (m, p-F, 3F), 2163.1 (m, m-F, 6F). 11B NMR
(96 MHz, C6D6) d 28.4.
Fig. 2 Comparison of the ball-and-stick structures of 3 and 4.
Notes and references
† A referee has suggested that the structural differences between the title
compounds and the zirconium borate zwitterion (1) may be due to the
greater electron deficiency at titanium.
NMR data for 4: 1H NMR (500 MHz, C6D6) d 20.09 (d, CH2-diene, 1H,
3J = 3.8), 0.33 (s, SiCH3, 3H), 0.54 (s, SiCH3, 3H), 0.72 (d, CH2-diene, 1H,
3J = 15.5), 1.05 (s, NtBu, 9H), 1.12 (d, CH3-diene, 3H, 3J = 5.8), 1.15 (s,
CpCH3, 3H), 1.18 (s, CpCH3, 3H), 1.31 (s, CpCH3, 3H), 1.73 (s, CpCH3,
3H), 3.02 (qd, CH-diene, 1H), 4.14 (m, CH-diene, 1H), 4.69 (dd, CH-diene,
1H, 3J = 9.93, 13.75). 13C{1H} NMR (125 MHz, C6D6) d 6.26 (s, SiCH3),
7.36 (s, SiCH3), 10.71 (s, CpCH3), 11.84 (s, CpCH3), 14.02 (s, CpCH3),
‡ Experimental procedures: 3: all manipulations were performed under an
atmosphere of dry nitrogen or under vacuum using a Vacuum Atmospheres
drybox or standard Schlenk techniques. All solvents were dried prior to use.
A solution of B(C6F5)3 (1.06 g, 2.07 mmol) in hexane (75 ml) was added to
a solution of 2 (0.754 g, 2.06 mmol) in hexane at 25 °C with stirring. A dark
green precipitate of 3 formed immediately and was collected and dried
under vacuum. X-Ray quality crystals of 3 were grown by slow cooling of
concentrated hexane solutions from 50 °C to room temperature. Isolated
yield: 1.43 g, 80%; 1H NMR assay indicated that the reaction is essentially
quantitative. (Found: C, 52.02; H, 4.19; Ti, 5.45. Calc. for
C38H35BF15NSiTi: C, 52.02; H, 4.02; Ti 5.46%). Compound 4 was prepared
in a similar fashion to that described for 3.
14.98 (s, CpCH3), 18.87 (s, CH3-diene), 33.93 (s, CH3 Bu), 60.91 (s, C-tBu),
t
89.83 (s, CH-diene), 109.36 (s, C(Cp)), 112.44 (s, CH-diene), 127.45 (s,
CH-diene), 130.75 (s, CMe(Cp)), 133.12 (s, CMe(Cp)), 135.49 (s,
CMe(CP)), 136.20 (m, CF), 138.25 (s, CMe(Cp)), 149.47 (m, CF). 19F
NMR (282 MHz, C6D6) d 2121.9 (m, o-F, 6F), 2153.9 (m, p-F, 3F),
2161.9 (m, m-F, 6F).
1 (a) B. Temme, G. Erker, J. Karl, H. Luftmann, R. Fro¨hlich and S. Kotila,
Angew. Chem., Int. Ed. Engl., 1995, 34, 1755; (b) B. Temme, J. Karl and
G. Erker, Chem. Eur. J., 1996, 2, 919; (c) J. Karl, G. Erker and R.
Fro¨hlich, J. Am. Chem. Soc., 1997, 119, 11165.
2 For other examples of a zirconium borate zwitterion featuring Zr…F
stabilisation, see J. Ruwwe, G. Erker and R. Fro¨hlich, Angew. Chem., Int.
Ed. Engl., 1996, 35, 80; Y. Sun, R. E. v. H. Spence, W. E. Piers, M.
Parvez and G. P. A. Yap, J. Am. Chem. Soc., 1997, 119, 5132.
3 D. D. Devore, F. J. Timmers, D. L. Hasha, R. K. Rosen, T. J. Marks, P. A.
Deck and C. L. Stern, Organometallics, 1995, 14, 3132.
¯
§ Crystal data: 3, C38H35BF15NSiTi, M = 877.47, triclinic, space group P1,
a = 10.243(1), b = 11.376(1), c = 16.419(2) Å, a = 78.67(1), b =
79.45(1), g = 84.85(1)°, U = 1841.3(3) Å3, Z = 2, m = 3.72 cm21, Dc
=
1.583 Mg m23, F(000) = 892, l = 0.71073 Å, T = 183 K. Data for 3 and
4 were collected on a Siemens P4 diffractometer. Of a total of 9281
collected reflections, 8284 were unique. The structure was solved by direct
methods. The hydrogens on the coordinated diene were located; the
remaining hydrogens were placed in calculated positions (C–H, 0.96 Å).
Refinement was by full matrix least squares on F2 with anisotropic
displacement parameters for the non-located H atoms. Final R indices [I >
2s(I)], R1 = 0.0410, wR2 = 0.1092.
4 R. G. Kidd, in NMR of Newly Accessible Nuclei, vol. 2, ed. P. Laszlo,
Academic Press, New York, 1983.
4, C45H43AlF15NSiTi, M = 985.77, monoclinic, space group P21/n, a =
10.879(3), b = 38.665(6), c = 11.069(2) Å, b = 107.19(2)°, U = 4448(1)
Communication 8/09438A
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Chem. Commun., 1999, 437–438