New stable aryl-substituted acyclic imino-N-heterocyclic carbene: Synthesis, characterisation and coordination to early transition metals

Article abstract of DOI:10.1039/c1dt11565k

The synthesis of the bulky 1-(1-arylimino-2,2-dimethylpropyl)-3-(aryl) imidazolium salt from the corresponding imidazole and the activated imidoyl chloride is presented. The absence of acidic protons adjacent to the iminic carbon allowed for the first iso

Full text of DOI:10.1039/c1dt11565k

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PAPER
New stable aryl-substituted acyclic imino-N-heterocyclic carbene: synthesis,
characterisation and coordination to early transition metals†
Timothy G. Larocque, Anna C. Badaj, Sarim Dastgir‡ and Gino G. Lavoie*
Received 18th August 2011, Accepted 5th September 2011
DOI: 10.1039/c1dt11565k
The synthesis of the bulky 1-(1-arylimino-2,2-dimethylpropyl)-3-(aryl)imidazolium salt from the
corresponding imidazole and the activated imidoyl chloride is presented. The absence of acidic protons
adjacent to the iminic carbon allowed for the first isolation of an imino-N-heterocyclic carbene of this
ligand class. The free carbene was isolated, structurally characterised, and coordinated to titanium,
zirconium, hafnium and chromium. The resulting metal halide complexes were fully characterised and
were tested at room temperature and atmospheric pressure for their activity as ethylene polymerisation
catalysts. The Zr(IV) complex was found to be the most active with a productivity of 140 kg PE mol M-1
h-1.
ring is replaced by an alkyl group have been reported by other
groups and used to prepare late transition metal complexes for
studying various catalytic reactions, including Suzuki–Miyaura
Introduction
Since their first isolation by Arduengo, N-heterocyclic car-
cross-coupling and cyclopropanation.14 However, considering the
significance of steric bulk in the a-diimine system to achieve high-
molecular weight polymers,9 we decided to investigate the effect
of having two aryl rings close to the coordination site in ligand
scaffold A.
benes (NHCs) have played an increasingly important role in
organometallic chemistry and catalysis.1 These carbenes, which
are excellent s-donors and poor p-acceptors, impart excellent
thermodynamic stability to transition metal complexes.2 In many
cases, replacement of phosphine ligands with NHCs has resulted
in enhanced thermal stability and catalytic activities.3 This obser-
vation has led to widespread applications of NHCs in transition
metal complexes mediated organic transformations. However,
despite the large number of various catalysts that have been
developed for the polymerisation of olefins,4–7 very little work has
been reported on the use of NHCs in such catalysts.8
We recently became interested in preparing and studying NHC
analogues of the bulky a-diimines and 2,6-diiminopyridines to
address the poor thermal stability of these systems.9,10 Although
these late transition metal complexes produce high molecular
weight polyethylene in good rates, their performance decreases
◦
Herein, we report the synthesis and structural characterisation
of the bulky 1-(1-(2,6-dimethylphenylimino)-2,2-dimethylpropyl)-
3-(2,4,6-trimethylphenyl)imidazol-2-ylidene carbene (2), CŸImine
(Scheme 1). Considering the ubiquity of early transition metal
catalysts in olefin polymerization,4,7,15–17 our initial coordination
studies with free carbene 2 focused on Group 4 and 6 metals in
their common oxidation states. As such, we describe the synthesis
and isolation of the corresponding titanium, zirconium, hafnium
and chromium metal halides. Differences between X-ray structures
of these complexes and that of the free carbene are presented.
These new complexes will serve in future studies as precursors for
the preparation of metal complexes where the chloride atoms are
partially replaced by other ancillary ligands (e.g., alkoxo, aryloxo,
amido and imido ligands) to generate five- and six-coordinate
metal dichloride complexes that are commonly used as olefin
oligomerisation and polymerisation catalysts.4,6,7,15,16,18
dramatically at temperatures greater than 50 C due to thermal
decomposition.11 Introduction of the NHC fragment in the ligand
scaffold thus offers the potential to mitigate this important
shortcoming of these systems.
We have previously described the synthesis and full character-
isation of the imidazolium salts A and B, and the corresponding
Ag(I) and Cu(I) complexes.12,13 Salts similar to A where either aryl
Department of Chemistry, York University, 4700 Keele Street, Toronto,
Ontario, M3J 1P3, Canada. E-mail: glavoie@yorku.ca; Fax: +1 416-736-
5936; Tel: +1 416-736-2100 Ext. 77728
† Electronic supplementary information (ESI) available: Crystallographic
data. CCDC reference numbers 808924, 808925, 840138, 808926 and
840139 for 2, 3a, 3b, 4a and 6b, respectively. For ESI and crystallographic
data in CIF or other electronic format, see DOI: 10.1039/c1dt11565k
‡ Present address: Department of Chemistry, College of Science, Sultan
Qaboos University, P.O. Box 36, Muscat 123, Sultanate of Oman.
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Scheme 1 Synthesis of Group 4 transition metal complexes from free carbene 2.
Results and discussion
Isolation of free carbene
Following our initial report of the 1-(1-(2,6-dimethylphenyl-
imino)ethyl)-3-(2,4,6-trimethylphenyl)imidazolium salt (A), we
became interested in expanding the family of ligands to the tert-
butyl analogue (R1 = t-Bu), in part for the absence of acidic
protons adjacent to the iminic carbon atom. The imidazolium
salt, 1, was prepared in good yield by reacting 1-mesitylimidazole
with the 2,6-dimethylphenylnitrilium salt generated in situ from
N-(2,6-dimethylphenyl)pivalimidoyl chloride and sodium tetraflu-
oroborate in acetonitrile at room temperature. Activation of the
bulky imidoyl chloride was in fact critical. All attempts to react
1-mesitylimidazole with the imidoyl chloride in the absence of
sodium tetrafluoroborate, as reported for the synthesis of the
related methyl analogue (A, R1 = Me),12 failed to give the desired
Fig. 1 ORTEP plot of 2 (50% probability level). All hydrogen atoms,
except for those on the azole ring, omitted for clarity.
1
Coordination of free carbene to titanium
product. The H NMR spectrum of 1 in CDCl3 showed a single
geometric isomer, unambiguously assigned to the E conformer by
a series of 1D NOESY NMR experiments. Slow isomerisation to
a 5 : 2 thermodynamic equilibrium mixture of E and Z isomers
was observed upon the solution standing at room temperature for
6 days.
Addition of one equivalent of 2 to TiCl4(THF)2 in THF afforded
complex 3a as a spectroscopically-pure yellow powder in excellent
yield (91%). The NMR spectrum is consistent with the desired
product. A decrease in the C N stretching frequency (nC
N
1609 cm-1) compared to the free ligand (nC
1662 cm-1) is
N
Reaction of the imidazolium salt 1 with sodium bis(trimethyl-
silyl)amide cleanly gave the free carbene 2, CŸimine, in 91%
yield (Scheme 1). This is in contrast to our previous attempts
to deprotonate the 1-(1-(2,6-dimethylphenylimino)ethyl)-3-(2,4,6-
trimethylphenyl)imidazolium (A, R1 = Me) salt that only led to a
mixture of unidentified compounds, presumably due to competing
deprotonation of the iminic methyl protons with subsequent
decomposition, similar to that proposed for the tautomerization
of similar compounds containing acidic b-protons.19 To our
knowledge, compound 2 is the first carbene of the general 1-
(iminoalkyl)imidazol-2-ylidene formula ever isolated and struc-
observed, suggesting coordination of the imine nitrogen to the
metal centre. Compound 3a crystallises in the P21/n space group,
with the ligand coordinated in a bidentate fashion through the
central imidazol-2-ylidene carbon and the imine nitrogen, forming
a distorted octahedral complex (Fig. 2). The observed increase
in the imine bond length upon coordination from 1.266(3) to
˚
1.287(6) A is in agreement with the corresponding decrease in
IR stretching frequency. The equatorial plane formed by Ti1, C1,
N3, Cl1 and Cl4 atoms is almost perfectly orthogonal to the plane
formed by Ti1, Cl2 and Cl3, and by those formed by the xylyl and
mesityl rings, with respective angles of 89.47◦, 88.77◦ and 89.09◦.
1
turally characterized. The H and 13C NMR spectra of 2 are in
1
agreement with the desired product. As expected, the H NMR
resonance for the central imidazolium proton of 1 vanished and a
new characteristic high-frequency resonance was observed in the
13C NMR spectrum at 218.3 ppm, indicative of the free carbene.20
1D NOESY NMR spectroscopy showed that 2 exists as the Z
conformer in solution, with no observed isomerisation to the E
isomer at room temperature over 18 h. Single crystal X-ray diffrac-
tion analysis confirmed generation of the free carbene 2 (Fig. 1).
The imine group also adopts the Z-conformation in the solid
state, with a 55.57◦ angle between the mean planes formed by N3,
C4, C5 and C6 and by the imidazol-2-ylidene ring, with the 2,4,6-
trimethylphenyl ring twisted off by 82.95◦ with respect to the azole
ring.
Fig. 2 ORTEP plot of 3a (50% probability level). Hydrogen atoms and
dichloromethane omitted for clarity.
12706 | Dalton Trans., 2011, 40, 12705–12712
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Table 1 Selected bond lengths and angles for compounds 2, 3a, 3b, 4a and 6b
2
3a
3b
4a
6b
˚
Bond length (A)
M–C1
M–N3
M–Cl1
M–Cl2
M–Cl3
M–Cl4
N1–C1
N2–C1
N1–C4
N3–C4
C2–C3
—
—
—
—
—
2.167(5)
2.358(4)
2.2370(16)
2.2930(18)
2.2775(18)
2.2808(16)
1.373(6)
1.344(6)
1.448(6)
1.287(6)
1.329(8)
2.178(3)
2.282(2)
2.3139(9)
2.4202(9)
2.3595(9)
—
1.374(3)
1.336(3)
1.431(3)
1.280(3)
1.347(4)
2.297(17)
2.479(14)
2.385(5)
2.419(5)
2.401(5)
—
1.42(2)
1.38(2)
1.35(2)
1.38(2)
1.19(3)
2.041(4)
2.156(3)
2.2954(13)
2.3596(12)
2.2985(12)
—
1.380(5)
1.345(5)
1.424(5)
1.287(5)
1.340(6)
1.380(3)
1.354(3)
1.444(3)
1.266(3)
1.338(3)
Bond angle (deg)
C1–M–N3
—
70.42(16)
112.3(4)
104.7(4)
97.18(14)
83.34(15)
82.16(15)
92.49(6)
158.65(14)
165.21(7)
71.37(9)
113.3(2)
104.9(2)
97.65(8)
81.58(7)
98.61(8)
92.96(3)
—
67.0(5)
75.50(14)
112.3(3)
105.0(3)
97.53(11)
82.82(11)
94.86(11)
91.48(5)
—
N1–C4–N3
N1–C1–N2
C1–M–Cl1
C1–M–Cl2
C1–M–Cl3
Cl1–M–Cl3
C1–M–Cl4
Cl2–M–Cl3
122.38(18)
101.48(19)
112.8(15)
105.1(14)
100.1(4)
82.85(13)
155.0(4)
104.9(2)
—
—
—
—
—
—
—
171.63(4)
96.48(12)
175.77(5)
Other atoms such as N1, N2, C2, C3, C4, C5, C6, and C14 also
lie on the equatorial plane defined above. The Cl2–Ti1–Cl3 bond
angle is 165.21(7)◦ with both chlorine atoms bent towards C1
at an average angle of 82.75◦ and with chloride–carbenic carbon
˚
distances of 2.921 and 2.967 A, considerably shorter than the
21
˚
sum of the van der Waals radii for both atoms (3.45 A). This
could possibly arise from intermolecular interactions between the
formally vacant p orbital on C1 and the lone pairs of the adjacent
chlorides.22 Alternatively, the bending of Cl2 and Cl3 towards the
carbene centre may be the result of repulsions between lone pairs
on the other two adjacent chlorine atoms, as proposed by Arnold
for a comparable highly congested d0 system.23 The trans effect
of the strong carbene s-donor is manifested by the longer Ti1–
˚
˚
Cl4 (2.2808(16) A), compared to Ti1–Cl1 (2.2370(16) A). Selected
bond lengths and angles for 3a and other compounds are listed in
Table 1.
Fig. 3 ORTEP plot of 3b (50% probability level). Hydrogen atoms
omitted for clarity.
The corresponding Ti(III) complex 3b was prepared by a
synthetic route similar to that used for 3a and was isolated
as a purple solid. The solution magnetic susceptibility of 3b
was determined to be meff = 1.65 mB using the Evans method,24
consistent with the predicted value (meff(spin only) = 1.73) for
one unpaired electron. X-Ray quality crystals were obtained from
vapour diffusion of pentane into a saturated THF solution (Fig.
3). TiCl3(CŸImine) (3b) crystallised as a THF adduct in the I41/a
81.58(7)◦ and comparable to the corresponding values observed in
3a, both Cl2 and Cl3 point directly towards N3, further supporting
that the bending of the trans chlorine atoms in 3a towards C1
may be strictly due to steric reasons and not electronic ones, as
also previously proposed by Arnold.23 As expected, all metal–
chloride bonds in 3b are longer than in the corresponding more
electropositive and less sterically-congested Ti(IV) complex 3a.
Other bond lengths and angles are within the normal range.
˚
space group with an elongated C4–N3 bond length of 1.280(3) A,
in agreement with the corresponding C N stretching frequency of
1607 cm-1. The complex is a highly distorted octahedral geometry.
Coordination of free carbene to zirconium and hafnium
˚
The Ti1–C1 bond length of 2.178(3) A is within expected values
and comparable to that observed in 3a. Interestingly, the vector
formed by that bond is 11.5◦ off the plane of the imidazole ring.
Moreover, the N1–C4–N3–C6 torsion angle is 163.95◦, about 16◦
off the expected 180◦ value. The added bulk coming from the
coordinated THF is likely contributing to these peculiar values. We
believe that this increased steric bulk significantly influences other
atom positions. In fact, the C1–Ti1–Cl3 bond angle is obtuse with
a value of 98.61(8)◦. Although the bond angle for C1–Ti1–Cl2 is
The zirconium (4a) and hafnium (5a) complexes were prepared
in an analogous manner and isolated as white powders in yields
of 67% and 73%, respectively. The NMR spectroscopic data for
both complexes were consistent with the desired products and
are similar to those for the titanium homologue 3a. The C
N
stretching frequency for 4a and 5a was observed at 1606 cm-1
and 1604 cm-1, respectively, indicating a bidentate coordination
motif. Coordination of the ligand through the carbene and the
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imine nitrogen was further corroborated for 4a through its X-ray
structure (Fig. 4).
Fig. 5 ORTEP plot of 6b (50% probability level). Hydrogen atoms and
Fig. 4 ORTEP plot of 4a (50% probability level). Hydrogen atoms
dichloromethane omitted for clarity.
omitted for clarity.
25
˚
˚
radii of chromium (1.39 A) and titanium (1.60 A). This is in
contrast to the observed values for the Cr–Cl and Cr–O bonds that
Compound 4a exhibits a distorted octahedral coordination
geometry. In contrast to 3a, it crystallises in the Pnma space group,
with the molecule lying on the crystallographic mirror plane or-
thogonal to the xylyl and to the mesityl rings, and passing through
the imidazol-2-ylidene ring, as well as through C4, C5, C17, N3,
Zr1, Cl1 and Cl3. All bond lengths involving the Zr(IV) metal
˚
are on average only 0.06 A shorter than the corresponding bonds in
3b, presumably because of greater repulsive interactions between
the p-electrons on the main elements and those in the metal dp-
orbitals. Interestingly, as observed for 3b, the vector formed by
Cr1 and C1 is also off the plane of the imidazole ring by 10.7◦ and
the N1–C4–N3–C6 torsion angle is 163.67◦. Moreover, Cl2 and
Cl3 are also leaning more towards N3 than C1. All other bond
lengths and angles are within expected range and comparable to
those observed in the analogous Ti(III) complex 3b.
˚
centre are on average 0.13 A longer than those observed in the
corresponding Ti(IV) complex 3a, in agreement with the difference
in covalent radii of both metals reported by Alvarez and Pyykko¨.25
˚
The Zr1–C1 and Zr1–N3 bond lengths are 2.319(7) A and
˚
2.483(5) A, respectively, with the zirconium–carbene bond length
similar to previously reported zirconium NHC complexes.20,26 As
Ethylene polymerisation catalysis
expected, the metal–chlorine bond trans to the strong carbene s-
˚
donor is slightly longer (2.407(2) A) than the one trans to the imine
The catalytic activities of compounds 3–6 towards ethylene poly-
merisation in toluene were studied at atmospheric pressure and
room temperature in the presence of 1000 equivalents methylalu-
minoxane (MAO) as cocatalyst. ZrCl4(CŸimine) (4a) was found to
be the most active of all three complexes tested with a productivity
of 140 kg PE mol M-1 h-1, followed by the Ti(IV) homologue
3a at 40 kg PE mol M-1 h-1. Those productivities are three
and two times greater than those observed in respective control
experiments using ZrCl4(THF)2 and TiCl4(THF)2. Furthermore,
they are only one to two orders of magnitude lower than those
observed using zirconocene dichloride as a benchmark catalyst.
Reaction with the other complexes reported herein did not lead to
any significant amount of solid polymer. In all cases, no soluble
waxes or low molecular weight oligomers were generated. The
maximum melting endotherms for the polyethylene produced by
3a and 4a were determined by differential scanning calorimetry
and found to be 135.9 and 134.0 ◦C, respectively, indicative of
linear polyethylene.
These preliminary results are promising and suggest that further
development of the catalysts should focus on both titanium and
zirconium. These systems are ideally suited for further tailoring of
the activity by incorporation of alkoxide (RO-), aryloxide (ArO-),
amide (R2N-) or even imido (RN2-) ligands to form five- and
six-coordinate metal dihalide complexes. Such strategies of fine-
tuning the performance of the catalyst by incorporating one of
these ancillary ligands while retaining two chlorides for subsequent
activation has been used to develop other highly active catalytic
systems.27
˚
(2.384(2) A). Cl2 and its crystallographically-equivalent Cl2a are
bent towards C1 at an 82.85(13)◦ angle, similar to that observed for
˚
3a, with a carbon ◊ ◊ ◊ chlorine distance of 3.136 A, possibly simply
due to sterics considerations based on our previous observations
for both Ti(III) (3a) and Ti(IV) (3b) compounds.
Coordination of free carbene to chromium
The Cr(III) (6b) and Cr(II) (6c) complexes were prepared from the
corresponding THF adduct metal halide precursor in 89% yield.
In both cases, the complexes were purified by recrystallisation
from THF under N2 at -35 ◦C. Complexes 6b and 6c are both
paramagnetic with a solution magnetic susceptibility of 3.81 and
2.90 mB, respectively, consistent with the predicted values for
systems with three (meff(spin only) = 3.87 mB) and two (2.83 mB)
unpaired electrons.
X-Ray quality crystals were obtained for the trivalent chromium
complex 6b (Fig. 5). The complex crystallised in the I41/a space
group as a THF adduct and is isostructural with the analogous
Ti(III) complex 3b. The nC stretching frequency of 1604 cm-1
N
is in good agreement with the observed C4–N3 bond length of
˚
1.287(5) A and comparable to the values observed for 3b. All other
C–C and C–N bond lengths of the CŸImine ligand in 6b are within
experimental error of those observed in 3b. In contrast, bond
lengths between the metal centre and all six coordinating atoms are
shorter than in the corresponding Ti(III) complex. The observed
Cr1–C1 and Cr1–N3 bond lengths are in excellent agreement with
the expected values based on the difference between the covalent
12708 | Dalton Trans., 2011, 40, 12705–12712
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in vacuo and the crude was redissolved in CH2Cl2 (25 mL),
filtered and the product was precipitated with pentane (60 mL).
The white solid was filtered and washed with pentane to yield a
spectroscopically-pure product (5.38 g, 72%). 1H NMR (400 MHz,
CDCl3) (major isomer E): d 8.39 (s, 1H, NCHN(mesityl)), 7.70 (s,
1H, NCHCN(mesityl)), 7.25 (s, 1H, NCCHN(mesityl)), 6.93–6.85 (m,
3H, m-CH(2,6-xylyl) + p-CH(2,6-xylyl)), 6.91 (s, 2H, m-CH(mesityl)), 2.28
(s, 3H, p-CH3(mesityl)), 2.09 (s, 6H, o-CH3(2,6-xylyl)), 1.65 (s, 6H, o-
Conclusions
We have synthesised, isolated and characterised the first stable
aryl-substituted acyclic imino-N-heterocyclic carbene and inves-
tigated its coordination to Group 4 and 6 transition metals. The
catalytic activities in ethylene polymerisation of all six complexes
were explored. The zirconium and titanium complexes exhibit
activities that warrant further tailoring of their coordination
sphere. Current efforts aim at derivatising these first-generation
catalysts to improve their activity towards ethylene and to explore
polymerisation and copolymerisation of a-olefins. As such, the
synthesis of metal imido and metal bisalkoxide complexes is
underway.
1
CH3(mesityl)), 1.53 (s, 9H, C(CH3)3(imine)). 13C{ H} NMR (100 MHz,
CDCl3): d 154.6 (C N), 143.1 (C(2,6-xylyl), 141.7 (p-C(mesityl)), 134.8
(NCN(mesityl)), 134.2 (o-C(mesityl)), 129.9 (C(mesityl)), 129.8 (m-CH(mesityl)),
128.6 (m-CH(2,6-xylyl)), 125.7 (o-C(2,6-xylyl)), 125.2 (p-CH(2,6-xylyl)), 124.2
(NCCN(mesityl)), 122.5 (NCCN(mesityl)), 40.5 (C(CH3)3(imine)), 28.3
(C(CH3)3(imine)), 21.2 (p-CH3(mesityl)), 18.1 (o-CH3(2,6-xylyl)), 16.7 (o-
CH3(mesityl)). FTIR (cast film) nC 1696, 1673 cm-1.
Experimental
N
1-(1-(2,6-Dimethylphenylimino)-2,2-dimethylpropyl)-3-(2,4,6-
trimethylphenyl)imidazol-2-ylidene (2), CŸImine. A cooled solu-
tion of NaN[Si(CH3)3]2 (1.12 g, 6.11 mmol) in THF (20 mL) was
added slowly to a THF (20 mL) solution of CŸImine·HBF4 (1)
(2.37 g, 5.13 mmol) at -78 ◦C. The light brown solution was
allowed to stir at -78 ◦C for 30 min before the flask was removed
from the bath and allowed to stir at room temperature for an
additional 1.5 h. Volatiles were removed under reduced pressure
and the light brown crude was redissolved in pentane, filtered,
and dried in vacuo to yield the spectroscopically-pure product
(1.75 g, 91%). Crystals suitable for X-ray diffraction studies were
General comments
All manipulations were performed under a dinitrogen atmosphere
in a drybox or using standard Schlenk techniques. Solvents used
in the preparation of air and/or moisture sensitive compounds
were dried using an MBraun Solvent Purification System fitted
with alumina columns and stored over molecular sieves under a
positive pressure of dinitrogen. Deuterated solvents were degassed
using three freeze–pump–thaw cycles. C6D6 and CDCl3 were
vacuum distilled from sodium and CaH2, respectively, and stored
under dinitrogen. NMR spectra were recorded on a Bruker
DRX 600 (1H at 600 MHz, 13C at 150.9 MHz), Bruker AV
400 (1H at 400 MHz, 13C at 100 MHz) or Bruker AV 300 (1H
at 300 MHz, 13C at 75.5 MHz) spectrometer and are at room
temperature unless otherwise stated. The spectra were referenced
internally relative to the residual protio-solvent (1H) and solvent
(13C) resonances and chemical shifts were reported with respect
to d = 0 for tetramethylsilane. A TA Model 2010 differential
scanning calorimeter was used to measure the melting endotherm.
The sample was first heated to 160 ◦C, quenched with liquid
◦
grown from a saturated pentane solution at -35 C.1H NMR
(400 MHz, C6D6) d 6.88 (d, 3J = 7.3 Hz, 2H, m-CH(2,6-xylyl)),
3
6.81 (t, J = 7.3 Hz, 1H, p-CH(2,6-xylyl)), 6.70 (s, 2H, m-CH(mesityl)),
6.40 (s, 1H, NCHCN(mesityl)), 5.98 (s, 1H, NCCHN(mesityl)), 2.21
(s, 6H, o-CH3(2,6-xylyl)), 2.09 (s, 3H, p-CH3(mesityl)), 1.87 (s, 6H, o-
1
CH3(mesityl)), 1.63 (s, 9H, C(CH3)3(imine)). 13C{ H} NMR (100 MHz,
C6D6): d 218.3 (NCN(mesityl)), 162.5 (C N), 146.7 (C(2,6-xylyl), 138.5
(C(mesityl)), 137.5 (p-C(mesityl)), 135.3 (o-C(mesityl)), 129.0 (m-CH(mesityl)),
127.9 (m-CH(2,6-xylyl)), 126.5 (o-C(2,6-xylyl)), 123.3 (p-CH(2,6-xylyl)), 119.0
(NCCN(mesityl)), 118.9 (NCCN(mesityl)), 41.0 (C(CH3)3(imine)), 29.3
(C(CH3)3(imine)), 21.0 (p-CH3(mesityl)), 18.8 (o-CH3(2,6-xylyl)), 17.7 (o-
nitrogen, and then reheated to 160 ◦C at a rate of 20 C min-1.
◦
Elemental compositions and exact masses were determined by
either ANALEST Laboratory of the University of Toronto or by
Guelph Chemical Laboratories Inc. located in Guelph, Ontario.
All metal precursors were purchased from either BDH or Sigma-
Aldrich. N-(2,6-Dimethylphenyl)acetamide was purchased from
Sigma-Aldrich or Alfa Aesar and used without further purifi-
cation. N-(2,6-Dimethylphenyl)pivalimidoyl chloride,28 1-(2,4,6-
CH3(mesityl)). FTIR (cast film) nC
1662 cm-1. Anal. Calcd. for
N
C25H31N3 (%): C, 80.39; H, 8.37; N, 11.25. Found (%): C, 80.15;
H, 8.70; N, 10.91.
TiCl4(CŸImine) (3a). A vial was charged with 2 (243 mg,
0.650 mmol) and was dissolved in a minimal amount of THF
(5 mL) and was added to a THF solution (2 mL) of TiCl4(THF)2
(212 mg, 0.634 mmol). The solution immediately darkened to a
dark yellowish-brown and was allowed to stir at room temperature
for 2.5 h. Volatiles were removed under reduced pressure to yield a
bright yellow solid that was washed with pentane (15 mL) to yield
a spectroscopically pure powder (325 mg, 91%). Crystals suitable
for X-ray diffraction study were grown at -35 ◦C under nitrogen by
slow liquid diffusion of pentane into a saturated CH2Cl2 solution.
1H NMR (400 MHz, CDCl3): d 7.93 (s, 1H, NCHCN(mesityl)),
7.06 (s, 1H, NCCHN(mesityl)), 7.03–6.99 (m, 3H, p-CH(2,6-xylyl) + m-
CH(2,6-xylyl)), 6.96 (s, 2H, m-CH(mesityl)), 2.50 (s, 6H, o-CH3(2,6-xylyl)),
2.31 (s, 3H, p-CH3(mesityl)), 2.28 (s, 6H, o-CH3(mesityl)), 1.45 (s,
trimethylphenyl)imidazole,29 TiCl4(THF)2,30 ZrCl4(THF)2 and
30
HfCl4(THF)230 were prepared using published procedures. Deuter-
ated NMR solvents were purchased from Cambridge Iso-
tope Laboratories. MAO was graciously donated by Albemarle
Corp.
Preparations
1-(1-(2,6-Dimethylphenylimino)-2,2-dimethylpropyl)-3-(2,4,6-
trimethylphenyl)imidazolium tetrafluoroborate (1), CŸImine·HBF4.
Solid NaBF4 (1.78 g, 16.2 mmol) was added to a solution of N-
(2,6-dimethylphenyl)pivalimidoyl chloride (3.63 g, 16.2 mmol) in
acetonitrile (70 mL). The turbid solution was allowed to stir for
48 h at ambient temperature. Solid 1-mesitylimidazole (3.03 g,
16.3 mmol) was added to the solution and the brown solution was
allowed to stir for an additional 24 h. Volatiles were removed
1
9H, (CH3)3C(imine)); 13C{ H} NMR (100 MHz, CDCl3): d 197.4
(NCN), 165.3 (C N), 146.9 (C(2,6-xylyl)), 140.2 (p-C(mesityl)), 135.7
(o-C(mesityl)), 134.8 (C(mesityl)), 131.1 (o-C(2,6-xylyl)), 129.3 (m-CH(mesityl)),
128.3 (m-CH(2,6-xylyl)), 126.7 (p-CH(2,6-xylyl)), 122.9 (NCCN(mesityl)),
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Dalton Trans., 2011, 40, 12705–12712 | 12709
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120.0 (NCCN(mesityl)), 40.9 (C(imine)), 30.6 ((CH3)3C(imine)), 21.4
(o-CH3(2,6-xylyl)), 21.3 (p-CH3(mesityl)), 18.9 (o-CH3(mesityl)) FTIR (cast
HfCl4(CŸImine) (5a). Compound 5a was isolated as a white
solid according to the procedure described for 3a, using
HfCl4(THF)2 as metal precursor and toluene as solvent. The
complex was further purified by recrystallisation at -35 ◦C under
nitrogen by slow liquid diffusion of pentane into a saturated
CH2Cl2 solution. Yield: 73%. 1H NMR (400 MHz, CDCl3): d 8.01
(s, 1H, NCHCN(mesityl)), 7.11 (s, 1H, NCCHN(mesityl)), 7.07–7.03 (m,
3H, p-CH(2,6-xylyl) + m-CH(2,6-xylyl)), 6.98 (s, 2H, m-CH(mesityl)), 2.49
(s, 6H, o-CH3(2,6-xylyl)), 2.32 (s, 3H, p-CH3(mesityl)), 2.23 (s, 6H, o-
film) nC 1609 cm-1. Anal. Calcd. for C25H31Cl4N3Ti·0.5 CH2Cl2
N
(%): C, 53.02; H, 5.52; N, 7.40. Found (%): C, 52.68; H, 5.93; N,
7.31.
TiCl3(CŸImine) (3b). Compound 3b was isolated as a pur-
ple solid according to the procedure described for 3a, using
TiCl3(THF)3 as metal precursor and toluene as solvent. Yield:
97%. Crystals suitable for X-ray diffraction study were grown at
room temperature under nitrogen by slow vapour diffusion of
1
CH3(mesityl)), 1.48 (s, 9H, (CH3)3C(imine)); 13C{ H} NMR (100 MHz,
CDCl3): d 201.6 (NCN), 168.5 (C N), 143.7 (C(2,6-xylyl)), 140.5
(p-C(mesityl)), 135.5 (o-C(mesityl)), 134.3 (C(mesityl)), 131.5 (o-CH(2,6-xylyl)),
129.4 (m-CH(mesityl)), 128.7 (m-C(2,6-xylyl)), 127.2 (p-CH(2,6-xylyl)),
123.5 (NCCN(mesityl)), 121.5 (NCCN(mesityl)), 41.4 (C(imine)), 30.9
((CH3)3C(imine)), 21.3 (p-CH3(mesityl)), 21.1 (o-CH3(2,6-xylyl)), 18.6 (o-
pentane into a saturated CH2Cl2 solution. FTIR (cast film) nC
N
1607 cm-1. Anal. Calcd. for C25H31Cl3N3Ti·1.0 THF (%): C, 58.06;
H, 6.55; N, 7.00. Found (%): C, 57.77; H, 6.30; N, 6.81.
CH3(mesityl)). FTIR (cast film) nC
1604 cm-1. Anal. Calcd. for
ZrCl4(CŸImine) (4a). Compound 4a was isolated as a white
solid according to the procedure described for 3a, using
ZrCl4(THF)2 as metal precursor and toluene as solvent. Yield:
67%. Crystals suitable for X-ray diffraction study were grown
at -35 ◦C under nitrogen by slow liquid diffusion of pentane
N
C25H31Cl4N3Hf (%): C, 43.28; H, 4.50; N, 6.06. Found (%): C,
42.94; H, 4.71; N, 5.87.
CrCl3(CŸImine)(THF) (6b). Compound 6b was isolated as a
deep dark blue solid according to the procedure described for 3a,
using CrCl3(THF)3 as metal precursor and THF as solvent. Yield:
89%. Crystals suitable for X-ray diffraction study were grown at
room temperature under nitrogen by slow vapour diffusion of
1
into a saturated THF solution. H NMR (400 MHz, CDCl3):
d 7.98 (s, 1H, NCHCN(mesityl)), 7.12 (s, 1H, NCCHN(mesityl)),
7.06–7.04 (m, 3H, p-CH(2,6-xylyl) + m-CH(2,6-xylyl)), 7.00 (s, 2H, m-
CH(mesityl)), 2.50 (s, 6H, o-CH3(2,6-xylyl)), 2.33 (s, 3H, p-CH3(mesityl)),
1
2.25 (s, 6H, o-CH3(mesityl)), 1.49 (s, 9H, (CH3)3C(imine)); 13C{ H}
pentane into a saturated CH2Cl2 solution. FTIR (cast film) nC
N
1604 cm-1. Anal. Calcd. for C25H31Cl3N3Cr·1.0 THF (%): C, 57.67;
NMR (100 MHz, CDCl3): d 195.3 (NCN), 167.6 (C N), 144.0
(C(2,6-xylyl)), 140.6 (p-C(mesityl)), 135.5 (C(mesityl)), 134.4 (o-C(mesityl)),
131.2 (o-CH(2,6-xylyl)), 129.5 (m-CH(mesityl)), 128.7 (m-C(2,6-xylyl)),
127.1 (p-CH(2,6-xylyl)), 123.6 (NCCN(mesityl)), 121.0 (NCCN(mesityl)),
41.6 (C(imine)), 30.9 ((CH3)3C(imine)), 22.6 (p-CH3(mesityl)), 21.0 (o-
H, 6.51; N, 6.96. Found (%): C, 57.39; H, 6.41; N, 7.20.
CrCl2(CŸImine)(THF) (6c). Compound 6c was isolated as a
blue solid according to the procedure described for 3a, using
CrCl2(THF)3 as metal precursor and THF as solvent. The complex
was further purified by recrystallisation at -35 ◦C under nitrogen
by slow liquid diffusion of pentane into a saturated CH2Cl2
CH3(2,6-xylyl)), 18.6 (o-CH3(mesityl)). FTIR (cast film) nC 1606 cm-1.
N
Anal. Calcd. for C25H31Cl4N3Zr (%): C, 49.50; H, 5.15; N, 6.93.
Found (%): C, 47.35; H, 4.91; N, 6.30 (repeated microcombustion
analyses of spectroscopically-pure material consistently gave low
solution. Yield: 89%. FTIR (cast film) nC
1604 cm-1. Anal.
N
R
carbon values). HRMS (AccuTOFTM-DARTꢀ): Calculated for
Calcd. for C25H31Cl2N3Cr·1.0 THF (%): C, 61.26; H, 6.91; N, 7.39.
C25H31Cl4N3Zr·C12H14N2, m/z = 789.1476; Found: 789.1483.
Found (%): C, 60.89; H, 7.11; N, 7.58.
Table 2 Crystal data and structure refinement details for 2, 3a, 3b, 4a, and 6b
2
3a
3b
4a
6b
Empirical formula
Formula weight/g mol-1
Crystal size/mm
q range/deg
C25H31N3
373.53
C25H31Cl4N3Ti
648.15
C29H39Cl3N3OTi
479.91
C25H31Cl4N3Zr
606.55
C30H41Cl5N3OCr
610.57
0.34 ¥ 0.34 ¥ 0.20
2.59–27.50
11; -14, 15; 15
Triclinic
0.15 ¥ 0.06 ¥ 0.04
2.60–25.07
11; 20; 19
Monoclinic
P21/n
9.7534(5)
18.8425(11)
16.5548(7)
90
93.221(3)
90
4
1.417
1336
0.34 ¥ 0.24 ¥ 0.20
2.61–27.50
-31, 32; 45; 14
Orthorhombic
I41/a
34.9199(4)
34.9199(4)
11.2791(2)
90
90
90
16
1.159
0.20 ¥ 0.18 ¥ 0.09
2.59–25.08
30; 12; 19
Orthorhombic
Pnma
25.3593(6)
10.6747(9)
16.5496(12)
90
90
90
4
0.899
1240
0.08 ¥ 0.10 ¥ 0.35
1.89–26.42
-31, 43; -43, 33; 14
Tetragonal
I41/a
34.553(6)
34.553(6)
11.316(2)
90
90
90
18
1.351
h; k; l range
Crystal system
¯
Space group
P1
˚
a/A
8.5723(3)
11.8829(7)
12.3644(8)
70.656(2)
88.378(3)
69.372(3)
2
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
Z
Dc/Mg m-3
1.121
404
F(000)
5040
5712
No. of reflns collected/unique
Rint
No. data/restraints/parameters
Final R indices [F2 < 2s(F2)]: R1, wR2
R indices (all data): R1, wR2
Goodness-of-fit on F2
10 345/4990
0.0480
4990/0/261
0.0699, 0.1747
0.1269, 0.2146
1.049
20 804/5388
0.0855
5388/18/337
0.0674, 0.1630
0.1172, 0.1886
1.042
51 252/7841
0.0670
7841/0/343
0.0615, 0.1605
0.0899, 0.1788
1.115
4139/4139
0.0898
4139/0/180
0.2033, 0.5724
0.2371, 0.5922
1.126
69 443/6938
0.1289
6938/10/393
0.0595, 0.1178
0.0942, 0.1291
1.372
12710 | Dalton Trans., 2011, 40, 12705–12712
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X-Ray crystallography
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X-Ray crystallographic data for 3a, 3b, 4a and 5a were collected
at the University of Toronto on a Bruker-Nonius Kappa-CCD
diffractometer using monochromated Mo-Ka radiation (l =
˚
0.71073 A) at 150 K and were measured using a combination
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˚
Ka radiation (l = 0.71073 A) at 100 K and were measured
using f and w scans. Unit cell parameters were determined
using at least 50 frames from three different orientations. Data
were processed using SAINT, and corrected for absorption with
accurate face-indexing as well as redundant data (SADABS), and
solved using direct methods and the SHELX program suite. The
structures were solved and refined using SHELXTL V6.133 for
full-matrix least-squares refinement based on F2. All H atoms
were included in calculated positions and allowed to refine in
riding-motion approximation with Uiso-tied to the carrier atom.
Details of the X-ray crystal data and structure refinement details
are listed in Table 2. Other detailed crystallographic data (Tables
of atomic coordinates with isotropic and anisotropic displacement
parameters, bond lengths and angles) are provided in the ESI.†
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Acknowledgements
GGL gratefully acknowledges financial support from the Ontario
Ministry of Research and Innovation for an Early Researcher
Award, the Natural Sciences and Engineering Research Council
of Canada for a Discovery Grant and a Research and Tools
Instrumentation Grant, the Canadian Foundation for Innovation
and the Ontario Research Fund for infrastructure funding, and
York University for start-up funds. The authors also wish to thank
Dr Alan J. Lough of the University of Toronto (Toronto, Ontario)
and Dr James F. Britten and Dr Hilary A. Jenkins of McMaster
University (Hamilton, Ontario) for X-ray data acquisition and for
either solving and refining the structures, or for their invaluable
assistance in doing so.
15 T. Agapie, Coord. Chem. Rev., 2011, 255, 861–880.
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