Titanium bis(alkylamido)phenylborane complexes

Article abstract of DOI:10.1016/S0020-1693(02)01352-X

The bis(tert-butylamido)phenylborane ligand [^tBuN-B^Ph-N^tBu], may be delivered to titanium from its dilithio salt. Complexes containing a single chelating boradiamido ligand, [^tBuN-B^Ph-N^tBu]TiX₂, (X=NMe₂ (1), Cl (2), Bn (3)), have been prepared and characterized by ¹H and ¹¹B NMR, and single crystal X-ray structure analysis (2,3).

Full text of DOI:10.1016/S0020-1693(02)01352-X

Inorganica Chimica Acta 345 (2003) 235ꢁ240
/
www.elsevier.com/locate/ica
Titanium bis(alkylamido)phenylborane complexes
David R. Manke, Daniel G. Nocera *
Department of Chemistry, 6-335, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139-4307, USA
Received 5 June 2002; accepted 13 September 2002
Dedicated in honor of Professor Richard R. Schrock
Abstract
The bis(tert-butylamido)phenylborane ligand [tBuNÃ
/
BPh
Ã
/
NtBu], may be delivered to titanium from its dilithio salt. Complexes
NtBu]TiX2, (Xꢀ
NMe2 (1), Cl (2), Bn (3)), have been prepared and
containing a single chelating boradiamido ligand, [tBuNÃ
/
BPh
Ã
/
/
1
characterized by H and 11B NMR, and single crystal X-ray structure analysis (2,3).
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Titanium complexes; Olefin polymerization; Borylamides; Suitemate
1. Introduction
polymerization [28,29]. Along these lines, the simplest
structural ligand construct is offered by bis(amido)bor-
anes, which situate a single boron atom between two
amides.
Non-metallocene early transition metal complexes
provide entry to catalysts capable of effecting Zieglerꢁ
/
Natta polymerizations [1]. A successful design approach
to such catalyst precursors involves the replacement of
cyclopentadienyl ligands on titanium and zirconium
The donorꢁ
/
acceptorꢁ
/
donor (NÃ
/
BÃ
/
N) motif of bis-
(amido)borane ligands coincides with our recent inter-
ests in developing new reaction chemistry by controlling
p-bonding within bidentate ligand frameworks. Our
most recent studies have examined late transition metals
dialkyls with chelating dianionic amido ligands [2ꢁ
these diamido-based pre-catalysts exhibit novel reactiv-
ity including the living polymerization of a-olefins [21ꢁ
/20];
/
bearing ligands with the inverse construct, an acceptorꢁ
donorꢁacceptor (PÃNÃP) ligand motif [30ꢁ33]. The
complementary donorꢁacceptorꢁdonor construct of
/
25]. Catalyst activity is profoundly affected by the
nature of the ancillary groups on the amide nitrogen
owing to perturbations of the ligand’s electronic and
steric properties.
One line of inquiry that has begun to be investigated is
the direct attachment of Group 13 elements to the amide
/
/
/
/
/
/
bis(amido)borane provides us the opportunity to de-
velop new reaction chemistry for early transition metals.
Accordingly, we have explored the chemistry of bis(alk-
ylamido)borane ligands with zirconium, hafnium and
vanadium [34]. Our interests to elaborate further the
coordination chemistry of bis(alkylamido)boranes and
to participate in this festschrift issue in honor of our
special colleague and friend led us to target titanium
organometallic complexes of the ligand. Prior to the
studies reported here, the only titanium compounds
containing a bis(alkylamido)borane ligand were the bis-
nitrogens. Schrock has utilized the NÃB p-bonding
/
character of borylamides to force the aryl groups on
the terminal borane to assume a perpendicular orienta-
tion to the plane of the diamido ligand, thus signifi-
cantly increasing the steric congestion about the metal
center [26,27]. Alternatively, Patton has incorporated
BÃ
/
B and In into the diamide backbone to examine the
effect of an electrophilic bridge on the activity of olefin
chelate, Ti[tBuNÃ
/
BPh
Ã
/
NtBu]2 where tBuNÃ
/
BPh
Ã
/
NtBuꢀ
/bis(tert-butylamido)phenylborane [35] and a
binuclear titanium center bridged by bis(methylam-
ido)phenylborane [36]. We now report the synthesis,
spectroscopic characterization, and X-ray crystallo-
* Corresponding author. Tel.: ꢂ
7670.
E-mail address: nocera@mit.edu (D.G. Nocera).
/
1-617-253 5537; fax: ꢂ1-617-253
/
0020-1693/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 0 2 ) 0 1 3 5 2 - X

236
D.R. Manke, D.G. Nocera / Inorganica Chimica Acta 345 (2003) 235ꢁ240
/
graphic analysis of a family of titanium complexes
BPh NtBu ligand.
2.4. [tBuNÃ
/
BPh
Ã
/
NtBu]TiBn2 (3)
t
containing a single chelating BuNÃ
/
Ã
/
[tBuNÃBPh
/
Ã
/
NtBu]TiCl2 (100 mg) was dissolved in 10
ml of C6H5CH3. The solution was immediately frozen
and then slowly warmed; 0.280 ml of BnMgCl (2.06 M
in THF) was added dropwise to the partially thawed
solution. The mixture was allowed to warm to r.t. After
stirring overnight, solvent was removed in vacuo to yield
an orange solid. Pentane (10 ml) was added, and the
mixture was filtered through Celite to remove LiCl.
Solvent was removed in vacuo to yield 121.4 mg of
2. Experimental
2.1. General procedures
All synthetic manipulations were carried out using
modified Schlenk techniques under an atmosphere of N2
or within the confines of a Vacuum Atmosphere HE-
553-2 glove box. Solvents for synthesis were of reagent
grade or better and were dried according to standard
methods [37]. Bis(tert-butylamino)phenylborane [38],
N,N?-dilithiobis(tert-butylamino)phenylborane (PhB-
(tBuNLi)2) [35] and dichlorobis(dimethylamido)titaniu-
m(IV) [39] were prepared by literature methods. All
other materials were used as received.
1
bright orange crystalline solid (92.4% yield). H NMR
(300 MHz, C6D6, 25 8C): d 1.166 (s, 18H), 2.149 (s, 4H),
6.8ꢁ
7.5 (m, 15H). 11B NMR (96.205 MHz, C6D6,
/
25 8C): d 30.659. Anal. Calc. for C28H37BN2Ti: C,
73.06; H, 8.10; N, 6.09. Found: C, 72.84; H, 8.03; N,
5.96%.
2.5. Reaction of 3 with [Ph3C][B(C6F5)4] and 1-hexene
2.2. [tBuNÃ
/
BPh NtBu]Ti(NMe2)2 (1)
Ã
/
Separate solutions of C6D5Cl (0.3 ml) containing 10
mg of 3 and 20 mg of [Ph3C][B(C6F5)4] were frozen
immediately upon dissolution of the reactants. Upon
thawing, the two solutions were combined and stirred at
Pentane (20 ml) solutions containing 1.75 g of
TiCl2(NMe2)2 and 2.06 g of PhB(tBuNLi)2 were frozen
immediately upon the dissolution of the reactants. Upon
thawing, the PhB(tBuNLi)2 solution was added drop-
wise over 7 min to a partially thawed TiCl2(NMe2)2
solution. The mixture was allowed to slowly warm to
room temperature (r.t.). After stirring overnight, the
solution was filtered through Celite to remove LiCl and
a green titanium byproduct resulting from reduction of
the Ti(IV) starting material. Solvent removal by vacuum
evaporation afforded a red liquid, which was refiltered
through fresh Celite. The filtrate was placed under
vacuum for 12 h after which 1.365 g of an orange solid
was collected (45.4% yield). 1H NMR (300 MHz, C6D6,
ꢃ
/
30 8C for 5 min. 1-Hexene (182 mg, 200 equiv.) was
then added and the solution was allowed to react at ꢃ
/
30 8C over the course of 3 h. Poly(1-hexene) was not
1
observed by H and 13C NMR. The reaction was also
performed at 0 and 25 8C; these too showed no poly(1-
hexene) production.
2.6. Physical methods
1H NMR spectra were recorded on solutions at 25 8C
within the magnetic fields of a Varian Unity 300 or
Mercury 300 spectrometers, which were located in the
Department of Chemistry Instrumentation Facility
(DCIF) at MIT. Chemical shifts are reported using the
standard d notation in parts-per-million; positive che-
mical shifts are to higher frequency from the given
reference. 11B NMR spectra were collected at the DCIF
on a Varian Unity 300 spectrometer and referenced to
25 8C): d 1.286 (s, 18H), 3.190 (s, 12H), 7.1ꢁ7.6 (m, 5H).
/
11B NMR (96.205 MHz, C6D6, 25 8C): d 29.961. Anal.
Calc. for C18H35BN4Ti: C, 59.04; H, 9.63; N, 15.30.
Found: C, 58.88; H, 9.53; N, 15.34%.
2.3. [tBuNÃ
/
BPh NtBu]TiCl2 (2)
Ã
/
an external BF3×/OEt standard. Elemental analyses were
A
15-ml hexanes/chlorotrimethylsilane (TMSCl)
BPh
performed at H. Kolbe Mikroanalytisches Laborator-
ium.
(50:50) solution containing 560 mg of [tBuNÃ
/
Ã
/
NtBu]Ti(NMe2)2 was allowed to stir overnight. A bright
orange precipitate was collected by filtration and the
filtrate was concentrated to promote precipitate forma-
tion. The additional product was collected, combined
with the first batch and the resulting total quantity of
solid was washed with cold hexanes; 451 mg (90.5%
yield) of the fluffy bright orange product was obtained.
1H NMR (300 MHz, C6D6, 25 8C): d 1.181 (s, 18H),
X-ray diffraction experiments were performed on
single crystals grown from concd. C5H12 or hexanes
solutions at ꢃ35 8C. Crystals were removed from the
/
supernatant liquid and transferred onto a microscope
slide coated with Paratone N oil. Selected crystals were
affixed to a glass fiber in wax and Paratone N oil and
cooled to ꢃ
shining Mo Ka (lꢀ
mounted onto Siemens’ three-circle goniometer
/
90 8C. Data collection was performed by
˚
0.71073 A) radiation onto crystals
/
7.0ꢁ
/
7.2 (m, 5H). 11B NMR (96.205 MHz, C6D6, 25 8C):
a
d 30.485. Anal. Calc. for C14H23BCl2N2Ti: C, 48.19; H,
6.64; N, 8.03. Found: C, 48.26; H, 6.55; N, 7.90%.
equipped with a CCD detector. The data were processed
and refined by using the program SAINT supplied by

D.R. Manke, D.G. Nocera / Inorganica Chimica Acta 345 (2003) 235ꢁ
/240
237
Siemens Industrial Automation, Inc. The structures
were solved by direct methods (G.M. Sheldrick, and
Siemens Industrial Automation, Inc., SHELXTL-v6.10
(2000)) in conjunction with standard difference Fourier
techniques. All non-hydrogen atoms were refined aniso-
tropically. Hydrogen atoms were placed in calculated
positions. Some details regarding the refined data and
cell parameters are contained in Table 1. Selected bond
distances and angles are supplied in Table 2.
Table 2
Selected bond lengths (A) and angles (8) for [ BuNÃ
(2) and [tBuNÃBPh NtBu]TiBn2 (3)
t
BPh
Ã
/
NtBu]TiCl2
˚
/
/
Ã
/
Compound 2
Bond lengths
Ti1Ã
Ti1Ã
Ti1Ã
Ti1Ã
/
Cl1
2.5771(9)
2.4482(9)
2.4476(9)
2.5565(8)
Ti1Ã
Ti1Ã
N1Ã
N2Ã
/
N1
1.852(2)
1.850(2)
1.471(4)
1.464(4)
/
Cl2
Cl1Aa
Cl2Ab
/N2
/
/B1
/
/B1
Bond angles
N1ÃTi1ÃN2
N1ÃB1ÃN2
Ti1ÃN1Ã
Ti1ÃN2Ã
Cl1ÃTi1Ã
/
/
80.67(10)
109.5(2)
Cl1Ã
Cl1Ã
Cl2Ã
Cl2Ã
Cl1AÃ
/
Ti1Ã
Ti1Ã
Ti1Ã
Ti1Ã
/
Cl1A
Cl2A
Cl1A
Cl2A
77.43(3)
81.45(3)
163.71(3)
77.92(3)
89.21(3)
/
/
/
/
3. Results and discussion
/
/
B1
B1
84.80(17)
85.05(16)
90.67(3)
/
/
/
/
/
/
Synthesis of titanium complexes containing a single
bis(alkylamido)phenylborane ligand proved to be ini-
tially troubling. Attempts at transamination of
Ti(NMe2)4 with the diamine of the ligand were unsuc-
cessful as was the reaction of PhB(tBuNLi)2 with TiCl4.
The latter reaction produces significant amounts of
reduced titanium product and only small amounts of
1, which were inseparable from the other titanium
/
/Cl2
/
Ti1ÃCl2A
/
Compound 3
Bond lengths
Ti1Ã
Ti1Ã
Ti1Ã
Ti1Ã
/
N1
1.8529(18)
1.8578(17)
2.155(2)
Ti1Ã
N1Ã
N2Ã
/
C23
2.458(2)
1.467(3)
1.455(3)
/N2
/B1
/C15
/B1
/C22
2.158(2)
Bond angles
N1ÃTi1ÃN2
N1ÃB1ÃN2
Ti1ÃN1Ã
Ti1ÃN2Ã
byproducts. The initial target of a single chelate complex
t
/
/
79.97(8)
109.34(19)
85.22(13)
85.36(13)
Ti1Ã
Ti1Ã
C15Ã
/
C15Ã
C22Ã
Ti1Ã
/
C16
C23
C22
115.38(14)
83.47(14)
127.91(9)
of BuNÃ
/
BPh NtBu was eventually obtained with the
Ã
/
/
/
/
/
/
/
B1
B1
/
/
methods outlined in Scheme 1. Although the reaction of
TiCl2(NMe2)2 with PhB(tBuNLi)2 still shows significant
reduction of the Ti(IV) reactant, 1 may be isolated in
45% yield. This orange compound proved to be a
convenient synthon for the dichloride analog, 2, which
may be cleanly converted to bisbenzyl complex 3 by
/
/
Symmetry transformations used to generate equivalent atoms: aꢃ
xꢂ1, ꢃy, ꢃzꢂ xꢂ1, ꢃyꢂ1, ꢃzꢂ1.
1; bꢃ
/
/
/
/
/
/
/
/
/
/
/
using conventional Grignard chemistry.
1
The H NMR spectra of 1ꢁ/3 exhibit a pronounced
singlet resonance in the region of 1.1ꢁ
/
1.3 ppm that we
1
ascribe to the tert-butyl groups of the diamide. All H
spectra also show multiplets at approximately 7 ppm
Table 1
Crystallographic data for [tBuNÃ
/
BPh
Ã
/
NtBu]TiCl2 (2) and [tBuNÃ
/
BPh NtBu]TiBn2 (3)
Ã
/
2
3
Formula
Formula weight
Space group
C14H23BCl2N2Ti C28H37BN2Ti
460.31
P21/c
348.95
P21/c
˚
a (A)
12.895(2)
6.3282(10)
20.922(3)
90
10.3234(8)
23.1906(19)
11.7952(10)
90
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
98.471(2)
90
113.054(1)
90
3
˚
V (A )
Z
1688.6(5)
4
2598.3(4)
4
Cryst. desc.
Dcalc (Mg mꢃ3
orange rod
1.373
0.814
orange block
1.177
0.347
Scheme 1. (a) Toluene, frozen; (b) TMSCl hexanes; (c) BnMgCl,
toluene, frozen.
)
Absorption coefficient (mmꢃ1
F(000)
)
728
984
corresponding to the phenyl protons, although this
signal overlaps with that of benzene, thereby hindering
any meaningful analysis of coupling to the other aryl
protons and to boron . Sharp singlet resonances for the
R1
wR2
0.0392
0.0965
1.153
0.0362
0.0914
1.037
Goodness-of-fit

238
D.R. Manke, D.G. Nocera / Inorganica Chimica Acta 345 (2003) 235ꢁ240
/
Fig. 1. Solid state structure of {[tBuNÃ
/
BPh NtBu]TiCl2}n (2), with thermal ellipsoids shown at the 50% probability level.
Ã
/
dimethylamido protons of 1 and the methylene protons
of the benzyl group of 3 appear at 3.19 and 2.149 ppm,
NtBu]Ti subunits; hence the polymer chains of Fig. 1 are
formed. A structural benchmark for 2 is provided by
Ti[Me2Si(tBuN)2]Cl2 in which the replacement of the
BPh bridgehead of 2 by a SiMe2 group gives rise to a
respectively. The 11B NMR spectra of 1ꢁ
/3 show a single
resonance in the region of 29.9ꢁ/30.6 ppm.
Single-crystal X-ray structure analysis of 2 establishes
that the dichloride compound is a polymer in the solid
state (Fig. 1). Polymeric chains are propagated by
more acute TiÃ
ingly, this bis(amido)silane compound is a monomer in
the solid state.
/
NÃC angle of 136.1(5)8 [41]. Interest-
/
chlorine atoms, which bridge adjacent [tBuNÃ
/
BPh
Ã
/
NtBu]Ti subunits. The chelating diamide and four
chlorines define a pseudo-octahedral geometry about
the titanium. The coordination geometry resulting from
tBuNÃ
NÃBÃ
80.67(10)8. The NÃ
/
BPh
N angle of 109.5(2)8 and a bite angle of
B distances of 1.464(4) and 1.471(4)
Ã
/
NtBu chelation in 2 is characterized by an
/
/
/
˚
A for 2 are similar to that of borazine, indicating
significant double bond character [40]. This distance is
consistent with that observed by Schrock for terminal
borylamido complexes of Group 4 metals [26,27]. A
trans-influence of the diamido ligand is demonstrated by
˚
Cl bond lengths, which are approximately 0.1 A
the TiÃ
/
longer when trans to a diamido nitrogen as opposed to
those trans to another chlorine. Comparison of the
trans-influence observed here with other amido ligands
is precluded by a dearth of structurally similar com-
plexes. The tert-butyl groups on the amide are directed
away from the metal center as indicated by TiÃ
/
NÃ
/
C
angles of 147.12(18) and 146.68(19)8. The reduced steric
congestion at Ti, resulting from the retracted tert-butyl
Fig. 2. Solid state structure of [tBuNÃ
thermal ellipsoids shown at the 50% probability level.
/
BPh NtBu]TiBn2 (3) with
Ã
/
groups, is manifested in the catenation of [tBuNÃ
/
BPh
Ã
/

D.R. Manke, D.G. Nocera / Inorganica Chimica Acta 345 (2003) 235ꢁ
/240
239
A monomeric complex is formed when the chlorides
of 2 are replaced with more bulky benzyl groups.
thank Ms. Parisa Mehrkhodavandi, Dr. Yi-Chou Tsai
and Mr. Zhi-Heng Loh for helpful discussions.
Dibenzyl complex 3 exhibits the pseudo-tetrahedral
t
coordination geometry shown in Fig. 2. The BuNÃ
/
BPh NtBu ligand metrics are similar to those observed
for 2. The notable aspect of this structure is the
Ã
/
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˚
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Cipso distance is 2.458(2) A and a TiÃ
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CÃ
/
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/
Cipso)ꢀ
/
˚
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/
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/
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/
C(24) and TiÃ
/
C(28) distances
˚
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/
benzyl bond. Coordination of the aryl ring of the
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contention is in accordance with the inability of
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