Living polymerization of α-olefins: Catalyst precursor deactivation via the unexpected cleavage of a B-C6F5 bond

Article abstract of DOI:10.1021/om961034b

The titanium diamide complex [ArN(CH₂)₃-NAr]TiMe₂ (1a; Ar = 2,6-ⁱPr₂C₆H₃) reacts with B(C₆F₅)₃ in pentane to give an insoluble yellow-orange solid (2a). Complex 2a is a catalyst for the living polymerization of 1-hexene. Pentane suspensions of 2a slowly evolve methane over the course of several hours to give the new pentane-soluble derivative [ArN(CH₂)₃NAr]Ti[CH₂B-(C₆F ₅)₂](C₆F₅) (3a), which was structurally characterized. Addition of a B-C₆F₅ group across an intermediate Ti=CH₂ unit is proposed. Compound 3a is inactive for the polymerization of α-olefins.

Full text of DOI:10.1021/om961034b

1810
Organometallics 1997, 16, 1810-1812
Livin g P olym er iza tion of r-Olefin s: Ca ta lyst P r ecu r sor
Dea ctiva tion via th e Un exp ected Clea va ge of a B-C6F 5
Bon d
J ohn D. Scollard, David H. McConville,* and Steven J . Rettig†
Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, British Columbia, Canada V6T 1Z1
Received December 9, 1996X
Summary: The titanium diamide complex [ArN(CH2)3-
NAr]TiMe2 (1a ; Ar ) 2,6-iPr2C6H3) reacts with B(C6F5)3
in pentane to give an insoluble yellow-orange solid (2a ).
Complex 2a is a catalyst for the living polymerization
of 1-hexene. Pentane suspensions of 2a slowly evolve
methane over the course of several hours to give the new
pentane-soluble derivative [ArN(CH2)3NAr]Ti[CH2B-
(C6F5)2](C6F5) (3a ), which was structurally character-
ized. Addition of a B-C6F5 group across an interme-
diate TidCH2 unit is proposed. Compound 3a is
inactive for the polymerization of R-olefins.
or [Ar′N(CH2)3NAr′]TiMe2 (1b; Ar′ ) 2,6-Me2C6H3)14
15
and B(C6F5)3 (eq 1).
Herein we report that an inactive compound is formed
quantitatively in the absence of monomer, representing
a formal deactivation of these living systems.
In tr od u ction
The design and synthesis of single-site olefin polym-
erization catalysts has developed rapidly in the last few
decades, with the group 4 metallocene class of com-
pounds receiving the most attention.1,2 Studies in this
area have been concerned with the way in which the
catalytic activity, comonomer incorporation, and ste-
reoregularity can be altered with changes to the Cp
ligand(s). There is, however, a growing interest in new
non-Cp ligand environments as possible alternatives for
olefin polymerization catalysts; in particular, amide
ligands3-9 have recently received considerable attention.
Resu lts a n d Discu ssion
The addition of a pentane solution of compound 1a
or 1b to a pentane solution of B(C6F5)3 at 23 °C yields
an insoluble yellow-orange solid (2a ,b) in nearly quan-
titative yield. Although it has so far proven very
difficult to spectroscopically identify compounds 2a ,b,
their reactivity is consistent with a cationic4,16 complex
of the type {[RN(CH2)3NR]TiMe}+[MeB(C6F5)3]- (R )
Ar, Ar′). For example, compounds 2a ,b react rapidly
with CD2Cl2 to give the known dichloride derivatives
[RN(CH2)3NR]TiCl2 (R ) Ar, Ar′).14 It should be noted
that the dimethyl complexes 1a ,b are stable in CD2Cl2.
Deep red insoluble oils are formed when compounds
2a ,b are “dissolved” in aromatic solvents or when 1a ,b
and B(C6F5)3 are mixed in these solvents. The oils
generated from toluene as well as compounds 2a ,b
(isolated solids) are catalysts for the living polymeriza-
tion of 1-hexene.17
Although pentane suspensions of compound 2b are
stable for days, complex 2a slowly evolves methane
(confirmed by NMR spectroscopy) over the course of
several hours to give a new pentane-soluble derivative
(3a ) in quantitative yield by NMR spectroscopy (Scheme
1). Piers et al. have reported a similar reaction between
Cp2ZrMe2 and HB(C6F5)2, which evolves methane to give
the complex [Cp2Zr](µ-CH2)(µ-H)[B(C6F5)2].18 Com-
pound 3a can be isolated in high yield from pentane at
-30 °C and is soluble in most organic solvents with no
decomposition.
As part of our program to investigate the chemistry
of early-transition-metal amide derivatives,10-12 we
recently reported that chelating diamide complexes of
titanium serve as precursors for the living13 polymeri-
zation of R-olefins. These living systems are derived
from equimolar amounts of the titanium dimethyl
complex [ArN(CH2)3NAr]TiMe2 (1a ; Ar ) 2,6-iPr2C6H3)
† Professional Officer: UBC Crystallographic Service.
X Abstract published in Advance ACS Abstracts, March 15, 1997.
(1) Brintzinger, H. H.; Fischer, D.; Mu¨lhaupt, R.; Rieger, B.;
Waymouth, R. M. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143.
(2) Mohring, P. C.; Coville, N. J . J . Organomet. Chem. 1994, 479, 1.
(3) Cloke, F. G. N.; Geldbach, T. J .; Hitchcock, P. B.; Love, J . B. J .
Organomet. Chem. 1996, 506, 343.
(4) Warren, T. H.; Schrock, R. R.; Davis, W. M. Organometallics
1996, 15, 562.
(5) Horton, A. D.; de With, J .; van der Linden, A. J .; van de Weg, H.
Organometallics 1996, 15, 2672.
(6) Aoyagi, K.; Gantzel, P. K.; Kalai, K.; Tilley, T. D. Organometallics
1996, 15, 923.
(7) Clark, H. C. S.; Cloke, F. G. N.; Hitchcock, P. B.; Love, J . B.;
Wainwright, A. P. J . Organomet. Chem. 1995, 501, 333.
(8) Scoles, L.; Minhas, R.; Duchateau, R.; J ubb, Y.; Gambarotta, S.
Organometallics 1994, 13, 4978.
(9) Herrmann, W. A.; Denk, M.; Albach, R. W.; Behm, J .; Herdtweck,
E. Chem. Ber. 1991, 124, 683.
(10) Scollard, J . D.; McConville, D. H.; Vittal, J . J . Organometallics
1995, 14, 5478.
(11) Gue´rin, F.; McConville, D. H.; Vittal, J . J . Organometallics
1995, 14, 3154.
(14) Scollard, J . D.; McConville, D. H.; Vittal, J . J . Macromolecules
1996, 29, 5241.
(15) Massey, A. G.; Park, A. J . J . Organomet. Chem. 1964, 2, 245.
(16) Yang, X.; Stern, C. L.; Marks, T. J . J . Am. Chem. Soc. 1994,
116, 10015.
(12) Gue´rin, F.; McConville, D. H.; Vittal, J . J . Organometallics
1996, 15, 5586.
(13) Scollard, J . D.; McConville, D. H. J . Am. Chem. Soc. 1996, 118,
10008.
(17) Scollard, J . D.; McConville, D. H. Unpublished results.
(18) Spence, R. E. v. H.; Parks, D. J .; Piers, W. E.; MacDonald, M.;
Zaworotko, M. J .; Rettig, S. J . Angew. Chem., Int. Ed. Engl. 1995, 34,
1230.
S0276-7333(96)01034-5 CCC: $14.00 © 1997 American Chemical Society

Notes
Organometallics, Vol. 16, No. 8, 1997 1811
a
Ta ble 1. Cr ysta llogr a p h ic Da ta for 3a ‚CH2Cl2
Sch em e 1
formula
fw
C46H42BF15N2Ti‚CH2Cl2
1051.47
yellow, needle
0.08 × 0.25 × 0.60
monoclinic
P21/n (No. 14)
20.292(2)
11.253(2)
23.459(2)
114.967(7)
4856(1)
color, habit
cryst size, mm
cryst syst
space group
a, Å
b, Å
c, Å
â, deg
V, Å3
Z
4
T, °C
21
1.438
Dc, g/cm3
The proton NMR spectrum of 3a reveals that the
complex has Cs symmetry coupled with restricted rota-
tion11 about the N-Cipso bonds of the ligand. A broad
singlet at 3.92 ppm is assigned to the methylene
protons, which are split into a doublet (1J CH ) 114 Hz)
in the isotopically labeled complex [ArN(CH2)3NAr]Ti-
[13CH2B(C6F5)2](C6F5) (13C-3a) prepared from [ArN(CH2)3-
NAr]Ti(13CH3)2 (13C2-1a ). For comparison, the methyl-
ene protons in the complex Cp2Zr(µ-CH2)(µ-H)B(C6F5)2
appear at 5.30 ppm (no carbon NMR data are reported
for this compound).18 The carbon NMR spectrum of
compound 3a shows a broad resonance at 109.5 ppm
for the methylene carbon bridging the titanium and
boron. The 19F NMR spectrum of 3a shows six different
fluorine resonances (two ortho, two meta, and two para)
consistent with migration19 of one of the perfluorophenyl
groups to titanium. The 11B NMR spectrum of 3a shows
a peak at 79.4 ppm, which compares well with [η5-
C5H4(CH2)3B(C6F5)2]2ZrCl2 (73.9 ppm)20 and is typical
for three-coordinate boron.21
The solid-state structure of 3a was determined by
X-ray crystallography (Table 1). The molecular struc-
ture of complex 3a can be found in Figure 1, and
relevant bond distances and angles in Table 2. The
structure is best described as a distorted tetrahedron
with the boromethyl group occupying one site. The
titanium-amide and titanium-carbon bond lengths are
comparable to the distances in the dimethyl complex
[Ar′N(CH2)3NAr′]TiMe2.14 The Ti(1)-C(34)-B(1) angle
is more obtuse than is expected for an sp3-hybridized
carbon, likely due to steric interactions between B(C6F5)2
and the pentafluorophenyl group bound to titanium. A
close contact exists between the ortho fluorine F(1) and
the boron atom (B(1)-F(1) ) 2.94 Å); however, the boron
center is trigonal-planar, as evidenced by the sum of
the angles about boron (360.0°).
F(000)
2144
33.36
0.70-1.00
ω-2θ
0.89 + 0.20 tan θ
16 (up to 8 rescans)
+h,+k,(l
155
1.8
10 798
10 503
0.037
3442
637
0.041
0.034
1.64
0.002
µ(Cu KR), cm-1
transmissn factors
scan type
scan range (in ω), deg
scan speed, deg/min
data collected
2θmax, deg
cryst decay, %
total no. of rflns
no. of unique rflns
Rmerge
no. of rflns with I g 3σ(I)
no. of variables
R
Rw
GOF
max ∆/σ
residual density, e/Å3
-0.19, 0.20
a
Conditions and definitions: Rigaku AFC6S diffractometer,
takeoff angle 6.0°, aperture 6.0 × 6.0 mm at a distance of 285 mm
from the crystal, stationary background counts at each end of the
scan (scan/background time ratio 2:1), Cu KR radiation (λ )
1.541 78 Å), graphite monochromator, σ2(F2) ) [S2(C + 4B)]/(Lp)2
(S ) scan speed, C ) scan count, B ) normalized background
count), function minimized ∑w(|Fo| - |Fc|)2, where w ) 4Fo2/σ2(Fo2),
R ) ∑||Fo| - |Fc||/∑|Fo|, Rw ) (∑w(|Fo| - |Fc|)2/∑w|Fo| )1/2, and GOF
2
) [∑w(|Fo| - |Fc|)2/(m - n)]1/2. Values given for R, Rw, and GOF
are based on those reflections with I g 3σ(I).
Complex 3a is significant because of its inactivity as
an R-olefin polymerization catalyst. In the absence of
R-olefin, we propose that the methylborate adduct 2a
loses methane to give an intermediate 12-electron
titanium methylidene complex (e.g., [ArN(CH2)3NAr]-
TidCH2) which may be stabilized by the released
borane. Notably, Gambarotta et al. have reported that
the cyclohexylamide complex (Cy2N)2TiMe2 slowly evolves
methane in the absence of a Lewis acid to give the
methylene-bridged dimer [(Cy2N)2Ti]2(µ-CH2)2,8 sug-
gesting that this may be a common mode of decomposi-
tion for four-coordinate amide complexes bearing methyl
groups at titanium. In our system this elimination
F igu r e 1. Molecular structure of 3a (33% probability
thermal ellipsoids are shown for the non-hydrogen atoms).
seems to be accelerated by the presence of a Lewis acid;
hence, we are exploring alternative routes to diamide-
stabilized alkylidene species. Addition of the B-C6F5
group across the proposed TidCH2 unit yields the final
product 3a . Deactivation of metallocene/MAO (MAO )
(19) Go´mez, R.; Green, M. L. H.; Haggitt, J . L. J . Chem. Soc., Chem.
Commun. 1994, 2607.
(20) Spence, R. E. v. H.; Piers, W. E. Organometallics 1995, 14, 4617.
(21) Kidd, R. G. In NMR of Newly Accessible Nuclei; Laszlo, P., Ed.;
Academic Press: New York, 1983; Vol. 2.

1812 Organometallics, Vol. 16, No. 8, 1997
Notes
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
[Ar N(CH2)3NAr ]Ti[CH2B(C6F 5)2](C6F 5) (3a ). B-
(C6F5)3 (76 mg, 0.15 mmol) in pentane (5 mL) was added
dropwise at 23 °C to [ArN(CH2)3NAr]TiMe2 (1a ; 70 mg,
0.15 mmol) dissolved in pentane (5 mL) to give a yellow-
orange precipitate. After ∼2 h the solution turned
homogeneous. Stirring was continued for 16 h. The
clear solution was passed through a plug of Celite,
concentrated to ∼2 mL, and cooled to -30 °C. 3a (120
mg, 0.12 mmol, 83%) was obtained as an orange crystal-
line solid. 1H NMR: δ 7.12 (m, 3H, Ar), 7.01 (m, 3H,
Ar), 4.58 (m, 2H, NCH2), 3.92 (br s, 2H, TiCH2), 3.31
(sept, 2H, CHMe2), 3.28 (sept, 2H, CHMe2), 3.20 (m, 2H,
NCH2), 1.42 (m, 1H, NCH2CH2), 2.29 (m, 1H, NCH2CH2),
1.30 (d, 6H, CHMe2), 1.04 (d, 6H, CHMe2), 0.95 (d, 6H,
CHMe2), 0.78 (d, 6H, CHMe2). 13C{1H} NMR: δ 145.4,
144.0, 141.4, 129.4, 125.0, 109.5 (TiCH2), 65.0, 33.4,
29.5, 28.2, 27.0, 26.2, 22.6, 22.4 (the carbon resonances
for the perfluorophenyl groups appear as multiplets
from 150-135 ppm). 19F{1H} NMR: δ -119.4 (d, 2F,
Fo), -132.3 (d, 4F, Fo), -154.0 (t, 2F, Fp), -155.1 (t, 1F,
Fp), -163.2 (m, 2F, Fm), -164.2 (m, 4F, Fm). 11B{1H}
NMR: δ 79.4. Anal. Calcd for C33H53ClN2Ti: C, 57.16;
H, 4.38; N, 2.90. Found: C, 57.49; H, 4.81; N, 2.69.
X-r a y Cr ysta llogr a p h ic An a lysis of 3a ‚CH2Cl2.
Crystallographic data appear in Table 1. The final unit
cell parameters were obtained by least squares on the
setting angles for 25 reflections with 2θ ) 31.5-45.6°.
The intensities of three standard reflections, measured
every 200 reflections, decayed linearly by 1.8%. The
data were processed24 and corrected for Lorentz and
polarization effects, decay, and absorption (empirical,
based on azimuthal scans). The structure was solved
by direct methods. The dichloromethane molecule was
severely disordered. The 6 largest peaks in the solvent
region were refined as chlorine and the next 4 peaks as
carbon. The occupancy factors and isotropic thermal
parameters were refined for all 10 solvent peaks. All
other non-hydrogen atoms were refined with anisotropic
thermal parameters. Hydrogen atoms (excluding those
associated with the solvent) were fixed in idealized
(d eg) for 3a ‚CH2Cl2
Bond Distances
Ti(1)-N(1)
Ti(1)-C(28)
C(34)-B(1)
C(41)-B(1)
1.846(4)
2.191(4)
1.503(6)
1.584(7)
Ti(1)-N(2)
Ti(1)-C(34)
C(35)-B(1)
B(1)-F(1)a
1.849(3)
2.111(4)
1.567(7)
2.94
Bond Angles
N(1)-Ti(1)-N(2)
N(1)-Ti(1)-C(34)
N(2)-Ti(1)-C(34)
Ti(1)-N(1)-C(1)
C(1)-N(1)-C(4)
Ti(1)-N(2)-C(16)
100.8(2) N(1)-Ti(1)-C(28)
102.8(2)
104.8(2)
113.5(2) N(2)-Ti(1)-C(28)
111.2(2) C(28)-Ti(1)-C(34) 121.5(2)
119.1(3) Ti(1)-N(1)-C(4)
116.3(4) Ti(1)-N(2)-C(3)
122.7(3) C(3)-N(2)-C(16)
124.5(3)
122.8(3)
114.6(3)
Ti(1)-C(28)-C(29) 120.9(3) Ti(1)-C(28)-C(33) 127.7(4)
Ti(1)-C(34)-B(1)
C(34)-B(1)-C(41)
125.1(3) C(34)-B(1)-C(35)
122.0(5) C(35)-B(1)-C(41)
120.9(5)
117.1(4)
a
Close contact.
methylaluminoxane) systems concomitant with meth-
ane evolution has been observed.22 Perhaps the forma-
tion of complex 3a can be viewed as a model for this
process.
Con clu sion
A living R-olefin polymerization system is generated
from the titanium dimethyl complexes 1a,b and B(C6F5)3
(2a ,b). In the absence of monomer, compound 2b is
stable; however, the bulkier complex 2a deactivates to
give the methylene derivative 3a . In principle, migra-
tion of the pentafluorophenyl group back to boron could
generate a single-component catalyst. The titanium is
clearly more electrophilic than boron in this instance,
precluding formation of this species. We are currently
exploring the interaction of other group 13 Lewis acids,
for example AlClMe2, in the hope of generating chelating
diamide analogues of Tebbe’s reagent [Cp2Ti](µ-CH2)-
(µ-Cl)[AlMe2].23
Exp er im en ta l Section
Gen er a l Deta ils. All experiments were performed
under an atmosphere of dry dinitrogen using standard
Schlenk techniques or in an Innovative Technology Inc.
glovebox. Pentane was treated with 5% HNO3 in H2-
SO4 to remove any olefins prior to being distilled under
argon from sodium/benzophenone ketyl. The com-
pounds [ArN(CH2)3NAr]TiMe2 (1a ; Ar ) 2,6-iPr2C6H3)
and [Ar′N(CH2)3NAr′]TiMe2 (1b; Ar′ ) 2,6-Me2C6H3)14
positions (C-H ) 0.98 Å, BH ) 1.2Bbonded atom).
A
secondary extinction correction (Zachariasen type, iso-
tropic) was applied, the final value of the extinction
coefficient being [2.6(3)] × 10-7. Neutral atom scatter-
ing factors and anomalous dispersion corrections for all
atoms were taken from ref 25. Selected bond lengths
and angles for 3a appear in Table 2.
15
and B(C6F5)3 were prepared according to literature
Ack n ow led gm en t . Funding from the NSERC
(Canada) in the form of a Research Grant to D.H.M. and
Union Carbide Canada is gratefully acknowledged.
procedures. [ArN(CH2)3NAr]Ti(13CH3)2 (13C2-1a ) was
prepared from 2 equiv of 13CH3MgI and [ArN(CH2)3NAr]-
TiCl2 in ether.14 Proton (300 MHz), carbon (75.5 MHz),
boron (96.2 MHz), and fluorine (282.2 MHz) NMR
spectra were recorded in d8-toluene at approximately
23 °C on a Varian XL-300 spectrometer. The proton
chemical shifts were referenced to internal C6D5CD2H
(δ 2.09 ppm) and the carbon resonances to Cipso-C6D5-
CD3 (δ 137.5 ppm). Fluorine and boron chemical shifts
were referenced externally to CFCl3 (δ 0.0 ppm) and
BF3‚Et2O (δ 0.0 ppm), respectively. Elemental analyses
were performed by Guelph Chemical Laboratories Ltd.,
Guelph, Ontario, Canada.
Su p p or tin g In for m a tion Ava ila ble: Text giving experi-
mental details and tables giving final atomic coordinates and
equivalent isotropic thermal parameters, all bond lengths and
angles, hydrogen atom parameters, and anisotropic thermal
parameters for 3a (20 pages). Ordering information is given
on any current masthead page.
OM961034B
(24) teXsan: Crystal Structure Analysis Package; Molecular Struc-
ture Corp.: The Woodlands, TX, 1995.
(25) (a) International Tables for X-Ray Crystallography; Kynoch
Press: Birmingham, U.K. (present distributor Kluwer Academic:
Boston, MA), 1974; Vol. IV, pp 99-102. (b) International Tables for
Crystallography; Kluwer Academic: Boston, MA, 1992; Vol. C, pp 200-
206.
(22) Kaminsky, W. In Education in Advanced Chemistry; Marciniec,
B., Ed.; Poznan: Wroclaw, Poland, 1996; Vol. 2.
(23) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J . Am. Chem. Soc.
1978, 100, 3611.