Preparation and reactivity study of chromium(III), iron(II), and cobalt(II) complexes of 1,3-bis(imino)benzimidazol-2-ylidene and 1,3-bis(imino)pyrimidin- 2-ylidene

Article abstract of DOI:10.1021/om300873k

Chromium(III), iron(II), and cobalt(II) complexes of bis(imino) benzimidazol-2-ylidene and bis(imino)pyrimidin-2-ylidene were successfully prepared by reaction of either the benzimidazolium or pyrimidinium salts or the corresponding copper complexes with the respective metal halide. X-ray diffraction analysis of the Cr(III) complex of the pyrimidin-2-ylidene ligand demonstrated, for the first time, the ability of bis(imino)carbene-type ligands to coordinate to metal centers in a tridentate fashion. The coordination mode of these ligands was surprisingly highly dependent on the nature of both the metal and the ligand itself. The activity of these complexes in ethylene polymerization was assessed under ambient conditions (room temperature and 1 atm of C₂H₄) using methylaluminoxane as cocatalyst. In contrast to the iron and cobalt complexes, both chromium complexes were active in ethylene polymerization.

Full text of DOI:10.1021/om300873k

Article
pubs.acs.org/Organometallics
Preparation and Reactivity Study of Chromium(III), Iron(II), and
Cobalt(II) Complexes of 1,3-Bis(imino)benzimidazol-2-ylidene and
1,3-Bis(imino)pyrimidin-2-ylidene
Jameel Al Thagfi and Gino G. Lavoie*
Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario M3J 1P3, Canada
S
* Supporting Information
ABSTRACT: Chromium(III), iron(II), and cobalt(II) complexes of bis(imino)benzimidazol-2-ylidene and bis(imino)-
pyrimidin-2-ylidene were successfully prepared by reaction of either the benzimidazolium or pyrimidinium salts or the
corresponding copper complexes with the respective metal halide. X-ray diffraction analysis of the Cr(III) complex of the
pyrimidin-2-ylidene ligand demonstrated, for the first time, the ability of bis(imino)carbene-type ligands to coordinate to metal
centers in a tridentate fashion. The coordination mode of these ligands was surprisingly highly dependent on the nature of both
the metal and the ligand itself. The activity of these complexes in ethylene polymerization was assessed under ambient conditions
(room temperature and 1 atm of C2H4) using methylaluminoxane as cocatalyst. In contrast to the iron and cobalt complexes,
both chromium complexes were active in ethylene polymerization.
INTRODUCTION
■
Iron, cobalt, and chromium complexes of tridentate 2,6-
bis(imino)pyridine ligands (A) exhibit excellent activities for
the polymerization of ethylene, attaining those observed for
group 4 metallocene catalysts.1−5 The modular nature of ligand
A allows for easy variations of the steric and electronic
properties of these post-metallocene catalytic systems. This has
led to several important structure−property relationships.1,5,6
Despite the excellent activities in ethylene polymerization,
complexes of A exhibit poor thermal stability, which results in
short lifetimes at elevated temperatures.2,5,7,8 Although
mitigated by the choice of the right ligand substitution pattern,9
the ability of these bis(imino)pyridine complexes to either
homopolymerize or copolymerize α-olefins remains extremely
phosphine counterparts,14 our group has recently prepared
transition-metal complexes of ligand C and reported their
activity in ethylene polymerization.15,16 The ligand features a
five-membered imidazol-2-ylidene ring, with angles that afford a
coordination sphere notably more opened than that of the
related six-membered pyridine ring of A and B, possibly
poor and unacceptable for commercial applications.5,8,10 Early
to middle transition-metal complexes of the related 2,6-bis(N-
heterocyclic carbene)pyridine ligand (B) have also been
reported.11−13 Although less active than complexes of A, they
are excellent catalysts for the oligomerization of ethylene, with
the best performance realized with chromium.
In light of those results and considering that transition-metal
complexes of N-heterocyclic carbenes (NHC) have shown
excellent thermal stability and enhanced performance over their
Received: September 11, 2012
Published: October 26, 2012
© 2012 American Chemical Society
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Article
facilitating coordination and insertion of α-olefins. Further-
more, the imine fragments impart π-acidic character to the
ligand, resulting in metal complexes that are more electro-
positive, and thus presumably more active, than complexes of
B.
suspension of the benzimidazolium salt 2a at low temperature
(Scheme 2). The desired copper adduct 3a was isolated in 28%
yield as an orange solid. The presence of two vibrational
stretching frequency bands at 1668 and 1653 cm−1 in the FTIR
spectrum of 3a suggests a bimetallic structure with the ligand
coordinated to each metal center in a bidentate fashion with
bridging iodides, as observed in a copper complex of C.15 Upon
We have recently reported the isolation and the catalytic
ethylene polymerization activity of chromium(III), iron(II),
and cobalt(II) complexes of acyclic 1,3-bis[1-(2,6-
dimethylphenylimino)ethyl and benzyl]imidazol-2-ylidene (C;
R = Me, Ph).16 In all complexes prepared, ligand C exclusively
coordinated to the metal in a bidentate fashion, through the
carbenoid carbon and through one of the imine substituents, as
confirmed by their solid-state structures. Upon activation with
methylaluminoxane, only chromium complexes gave poly-
ethylene at moderate activities, with the electron-poor phenyl
derivative (C; R = Ph) found to enhance the activity of the
complex over that observed for the related methyl analogue (C;
R = Me).16 This suggests that a decrease in the electron-
donating or an increase in the π-accepting capability of the
ligand may produce more active olefin polymerization catalysts.
We therefore became interested in preparing Cr, Fe, and Co
complexes of the less σ-electron donating benzimidazol-2-
ylidene ligand D,17 which would result in more electropositive
metal center. The increased steric bulk of the fused benzene
ring may also facilitate coordination of the second imine
fragment, leading to complexes that closely resemble those of
A. This could also possibly be achieved by substituting the five-
membered imidazol-2-ylidene by a six-membered pyrimidin-2-
ylidene ring. We herein report the synthesis of the
benzimidazolium and pyrimidinium salts, both precursors to
carbenes D and E, respectively. The corresponding complexes
of chromium, iron, and cobalt (and copper as transmetalating
agent) were prepared and their catalytic activities assessed.
1
coordination, the characteristic downfield H NMR resonance
for the central imidazolium proton disappeared. The
benzimidazol-2-ylidene proton resonances shifted to lower
frequencies at 8.44 (−NCCHCH−) and 7.41 ppm
(−NCCHCH−). The corresponding 13C NMR resonances
shifted upfield to 116.3 and 125.3 ppm, respectively. The iminic
carbon resonance shifted to a higher frequency by 3.4 ppm to
156.6 ppm, as was also observed in the related bis(imino)-
imidazol-2-ylidene copper(I) iodide complexes.15 Due to poor
solubility and long relaxation times, the resonance for the
carbenoid carbon (−NCN−) in 3a was not observed.
The Cr(III) complex 4a was prepared by the methodology
we recently reported for the synthesis of chromium complexes
of C (Scheme 2).16 Copper carbene complexes have been
found to be excellent transmetalating agents and have proven to
be extremely valuable when more common methods to prepare
carbene complexes are not successful (e.g., through the use of
silver adducts as transmetalating agents) or simply not possible
(e.g., isolation of the free carbene impossible due to competing
deprotonation).16,18
Reaction of compound 3a with CrCl3·3THF gave 4a as a
paramagnetic complex in 71% yield. A solution magnetic
susceptibility of 3.75 μB, measured using the Evans method,19 is
consistent with the predicted value (μeff(spin only) = 3.87 μB)
for three unpaired electrons. The FTIR stretching frequency for
the iminic group in 4a showed two bands, at 1683 and 1622
cm−1, indicative of coordination of the ligand in a bidentate
mode, with one iminic nitrogen remaining uncoordinated.
The corresponding iron (5a) and cobalt (6a) complexes
were prepared using the synthetic methodology we previously
reported for the synthesis of the corresponding imidazol-2-
ylidene complexes.16 Addition of FeCl[N(SiMe3)2] or CoCl-
[N(SiMe3)2], generated in situ by reaction of KN(SiMe3)2 with
MCl2, to a THF suspension of the benzimidazolium salt 2a at
−37 °C afforded complexes 5a and 6a in good yield (Scheme
2). Complexes 5a and 6a are paramagnetic, high-spin species, as
evidenced by their respective magnetic susceptibility of 5.86
and 4.27 μB measured using the Evans method.19 These values
are in agreement with those reported for related complexes and
are consistent with four and three unpaired electrons for the
iron and cobalt complexes, respectively.2,16,20 As previously
observed for the iron and cobalt complexes of the related
imidazol-2-ylidene ligand,16 two CN stretching bands were
observed on the FTIR spectrum for both 5a (1683 and 1629
cm−1) and 6a (1683 and 1607 cm−1), indicative of bidentate
coordination of the ligand to the metal center. Repeated
attempts to grow crystals of 4a−6a for X-ray diffraction studies
proved unsuccessful.
RESULTS AND DISCUSSION
■
Complexes of 1,3-Bis[1-(2,6-dimethylphenylimino)-
ethyl]benzimidazol-2-ylidene. Reaction of N-(2,6-
dimethylphenyl)acetimidoyl chloride with (1-(2,6-
dimethylphenylimino)ethyl)benzimidazole (1a) in toluene
afforded the desired product 2a as a white solid (Scheme 1),
Scheme 1. Synthesis of 1,3-Bis[1-(2,6-
dimethylphenylimino)ethyl]benzimidazolium Chloride (2a)
with a νCN stretching frequency of 1690 cm−1. The 1H NMR
(CDCl3) spectrum of 2a revealed the presence of both the E,Z
and E,E isomers. The central benzimidazolium proton
(−NCHN−) of the symmetric E,E isomer resonates at a
characteristic downfield chemical shift of 12.27 ppm. The
benzimidazol-2-ylidene protons were observed at 9.02
(−NCCHCH−) and 7.75 ppm (−NCCHCH−), with the
corresponding carbon resonances at 118.9 (−NCCHCH−) and
129.2 ppm (−NCCHCH−). The central azolium and iminic
carbon nuclei resonate at 126.1 and 153.2 ppm, respectively.
Copper complex 3a was prepared by adding a THF solution
of sodium hexamethyldisilazide (NaHMDS) and CuI to a
Complexes of 1,3-Bis[1-(2,6-dimethylphenylimino)-
ethyl]pyrimidin-2-ylidene. Reaction of 1,4,5,6-tetrahydro-
pyrimidine with N-(2,6-dimethylphenyl)acetimidoyl chloride in
toluene afforded (1-(2,6-dimethylphenylimino)ethyl)-
pyrimidine (1b) as a light yellow oil in 87% yield (Scheme
3). The pyrimidinium salt 2b was synthesized in benzene by the
reaction of 1b with another 1 equiv of N-(2,6-dimethylphenyl)-
acetimidoyl chloride in 57% yield. Only one band at 1628 cm−1
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Scheme 2. Synthesis of Cu(I) (3a), Cr(III) (4a), Fe(II) (5a), and Co(II) (6a) Complexes of 1,3-Bis[1-(2,6-
dimethylphenylimino)ethyl]benzimidazol-2-ylidene
Scheme 3. Synthesis of 1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]-4,5,6-trihydropyrimidinium Chloride (2b)
Scheme 4. Synthesis of Cu(I) (3b), Cr(III) (4b), Fe (5b), and Co(II) (6b) Complexes of 1,3-Bis[1-(2,6-
dimethylphenylimino)ethyl]-4,5,6-trihydropyrimidin-2-ylidene
was identified in the FTIR spectrum for the iminic groups in
2b. The central pyrimidinium proton (−NCHN−) resonance
nitrogen atoms was produced, similar to that observed in the
synthesis of the copper complex of ligand C (R = Ph).15
Complex 4b was prepared in 86% yield via transmetalation
using complex 3b and CrCl3·3THF (Scheme 4). The solution
magnetic susceptibility of 4.14 μB, determined using the Evans
method,19 is similar to that of 4a and consistent with the
predicted value for three unpaired electrons. The presence of
only one band in the FTIR spectrum for the CN bond at a
frequency (1623 cm−1) lower than that observed in 3b, a result
of π-back-donation from chromium to the CN π*
antibonding orbitals, strongly suggests coordination of both
iminic nitrogen atoms of the pyrimidin-2-ylidene ligand to the
metal.
This was confirmed by X-ray diffraction studies (Figure 1).
Suitable single crystals of 4b were grown by slow evaporation of
a saturated dichloromethane solution. The complex crystallized
in the P21/n space group and adopts a distorted-octahedral
geometry in the solid state, with C3 disordered over two
positions. To our knowledge, complex 4b is the first example of
a bis(imino)carbene ligand unambiguously shown to coor-
1
was observed in the H NMR (CDCl3) spectrum at 9.93 ppm.
The backbone methylene protons resonate in a 2:1 ratio at 4.34
(−NCH2CH2CH2N−) and 2.47 ppm (−NCH2CH2CH2N−),
with the corresponding carbon resonances at 42.2 and 18.7
ppm. The carbenoid carbon (−NCN−) and the iminic carbon
(−CN−) nuclei resonate at 154.1 and 153.7 ppm,
respectively.
The chromium(III) (4b), iron(II) (5b), and cobalt(II) (6b)
complexes of the pyrimidin-2-ylidene were prepared following
the same procedure used for the synthesis of the benzimidazol-
2-ylidene complexes (Scheme 4). The copper adduct 3b was
prepared in good yield and used as transmetalating agent in the
synthesis of compound 4b. In contrast to that observed for 3a,
the FTIR spectrum of the pyrimidin-2-ylidene copper derivative
3b shows only one band for the imine group at 1635 cm−1. The
stretching frequency, comparable to that of 2b, suggests that a
linear copper iodide complex with two uncoordinated imine
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Figure 1. ORTEP plot of 4b (50% probability level). Hydrogen atoms and dichloromethane solvent molecule are omitted for clarity. Only the major
component of the disordered structure at C3 is shown. Selected bond lengths (Å) and angles (deg): Cr1−C1 = 1.989(3), Cr1−N3 = 2.133(2), Cr1−
N4 = 2.142(2), Cr1−Cl1 = 2.2945(8), Cr1−Cl2 = 2.3293(7), Cr1−Cl3 = 2.3479(8), C1−N1 = 1.335(4), C1−N2 = 1.329(4), C5−N3 = 1.286(4),
C15−N4 = 1.289(4); N1−C1−N2 = 121.6(2), C1−N1−C5 = 114.0(2), C1−N2−C15 = 114.3(2), C1−Cr1−N3 = 75.65(10), C1−Cr1−N4 =
75.49(10), C1−Cr1−Cl1 = 96.90(8), C1−Cr1−Cl2 = 167.85(8), C1−Cr1−Cl3 = 76.91(8), Cl1−Cr1−Cl3 = 173.67(3), N3−Cr1−N4 = 150.59(9).
dinate to a metal in a tridentate fashion. The angles enforced by
the six-membered ring of the pyrimidin-2-ylidene core likely
play a dominant role in ligand E adopting such a coordination
mode. As predicted, the C1−N1−C5 (114.0(2)°) and C1−
N2−C15 (114.3(2)°) bond angles are significantly less obtuse
than the related angles in the THF-solvated [1,3-bis((2,6-
dimethylphenylimino)benzyl)imidazol-2-ylidene]CrCl3
(118.9(3), 119.6(3)°)16 but comparable to those reported for
[2,6-bis(1-(2,6-dimethylphenylimino)ethyl)pyridine]CrCl3
(113.1(3), 113.7(3)°).4 The N3−Cr1−N4 bond angle in 4b is
150.59(9)°, slightly less obtuse than the corresponding angle in
the 2,6-bis(1-(2,6-dimethylphenylimino)ethyl)pyridine (A)
analogue.4
The Cr1−C1 bond (1.989(3) Å) is considerably shorter than
the metal−carbene bonds in the CrCl3 complexes of B
(2.087(6) and 2.1206(6) Å) and C (2.048(3) Å).13,16 While
also considerably shorter than the Cr−N bond in the CrCl3
complex of B (2.049(4) Å),13 it is comparable to the Cr−
Npyridine bond in the CrCl3 complex of A (1.993(3) Å).4 Both
Cr−Nimine bonds in 4b (2.133(2) and 2.142(2) Å) are shorter
than the corresponding bond in the CrCl3 complex of imidazol-
2-ylidene C (R = Ph; 2.164(3) Å).16 They are, however,
surprisingly comparable to both Cr−Nimine bonds in the CrCl3
complex of A (2.123(3) and 2.140(3) Å)4 and to the Cr−
Ccarbene bonds in the complex of B (2.120(6) and 2.087(6)
Å).13
angles are smaller than that observed in the chromium
trichloride complex of C (76.90(11)°)16 but are in line with
those reported for the CrCl3 complex of A (75.75(11) and
77.24(11)°)4 and B (76.3(2) and 75.68(9)°).13 Coordination
of the new 1,3-bis(imino)pyrimidin-2-ylidene ligand (E) to
Cr(III) thus generates a complex that structurally closely
resembles those of the 2,6-bis(imino)pyridine (A) and of 2,6-
bis(carbene)pyridine (B) ligands, while imparting very different
electronic properties to the metal center.
The corresponding iron (5b) and cobalt (6b) complexes
were synthesized in 81 and 74% yields, respectively, from the
pyrimidinium salt 2b (Scheme 4). Similar to the case for 5a and
6a, the magnetic susceptibility of complexes 5b and 6b are 5.80
and 4.35 μB, respectively, consistent with four and three
unpaired electrons.19 In both cases, two stretching frequency
bands in the iminic region of the FTIR spectrum were observed
(5b, 1617 and 1638 cm−1; 6b, 1612 and 1635 cm−1) with
values strongly suggesting coordination of only one iminic
nitrogen atom. Despite the favorable angles of the six-
membered central ring, the greater electron density of group
8 and 9 transition metals and their lower formal +2 oxidation
state in comparison to Cr(III) seemingly preclude coordination
of the second iminic nitrogen atom. Attempts to grow crystals
of either complex to unambiguously determine the coordina-
tion mode of the ligand in 5b and 6b were not successful.
Polymerization Activity of Cr(III), Fe(II) and Co(II)
Complexes 4−6. The catalytic activity of complexes 4a−6a
and 4b−6b was evaluated in ethylene polymerization trials
performed under 1 atm of C2H4 and at room temperature over
10 min, with 1000 equiv of methylaluminoxane as cocatalyst. As
reported for the first-generation imidazol-2-ylidene systems,16
no ethylene uptake was observed with either iron or cobalt
complexes, possibly due to the tendency of late-transition-metal
complexes containing NHC ligands to undergo reductive
As expected from the symmetry of the system, both iminic
bond lengths are statistically equivalent (1.286(3) and 1.289(3)
Å). Those values are close to those reported for the
coordinated imine groups of THF-solvated [1,3-bis((2,6-
dimethylphenylimino)benzyl)imidazol-2-ylidene]CrCl3
(1.294(4) Å)16 and of [2,6-bis(1-(2,6-dimethylphenylimino)-
ethyl)pyridine]CrCl3 (1.291(4) and 1.297(4) Å).4 The C1−
Cr1−N3 (75.65(10)°) and C1−Cr1−N4 (75.49(10)°) bite
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elimination of an alkylimidazolium species.21 In contrast,
chromium complexes 4a,b both gave polyethylene in statisti-
cally equivalent rates with an average of 27 kg of PE (mol of
Cr)−1 bar−1 h−1, a value that is surprisingly very close to that
observed in the imidazol-2-ylidene chromium(III) series.16 The
observed rate is approximately 1 order of magnitude greater
than literature values for chromium(III) complexes of related
bidentate ligands22 but more than 3 orders of magnitude lower
than those reported for complexes of A and B.4,12,13 The
identical catalytic performances of both 4a and 4b are
intriguing, considering the greater electron density about the
metal center in 4b, a result of stronger σ-donating capability of
the carbene17 and of tridentate coordination of the ligand to
chromium (vide supra). Polymerization trials with 4a,b were
therefore repeated and performed over a 30 min period. A 1.5-
fold increase in polymer yield resulted (6.6 kg of PE (mol of
Cr)−1 over 30 min vs 4.5 kg of PE (mol of Cr)−1 over 10 min),
significantly less than the 3-fold increase expected for stable
catalysts. These observations indicate a short catalyst lifetime,
with rapid decomposition through reductive elimination of an
alkylbenzimidazolium or alkylpyrimidinium salt and/or through
cleavage of the carbene−imine bond, as we have observed in
related titanium complexes.23
Laboratories Ltd. Solution magnetic susceptibilities were measured
using the Evans method.19
Iron(II) chloride, cobalt(II) chloride, sodium and potassium
hexamethyldisilazides, and 1,4,5,6-tetrahydropyrimidine were pur-
chased from Sigma-Aldrich. Benzimidazole was purchased from Alfa-
Aesar. Chromium(III) chloride was purchased from Strem Chemicals.
N-(2,6-Dimethylphenyl)acetimidoyl chloride24 and CrCl3·3THF25
were synthesized using published procedures. Deuterated NMR
solvents were purchased from Cambridge Isotope Laboratories.
MAO was graciously donated by Albemarle Corp.
(1-(2,6-Dimethylphenylimino)ethyl)benzimidazole (1a). N-
(2,6-Dimethylphenyl)acetimidoyl chloride (2.26 g, 12.5 mmol) was
dissolved in dichloromethane (30 mL) and added to a dichloro-
methane solution (30 mL) of benzimidazole (2.94 g, 24.9 mmol) at
room temperature. The reaction mixture was stirred for 18 h. Water
was added to the mixture, and the organic product was extracted with
dichloromethane. The combined dichloromethane layers were dried
over Na2SO4 and subsequently filtered. The solvent was removed in
vacuo to give the product as a light yellow solid (2.83 g, 10.8 mmol,
1
3
86%). H NMR (300 MHz, CDCl3): δ 8.58 (dd, J = 3.1 Hz, 1H,
3
CNCCH), 8.41 (s, 1H, NCHN), 7.86 (dd, J = 3.3 Hz, 1H, NCCH),
7.30−7.40 (m, 2H, CNCCHCH, NCCHCH), 7.10 (d, J = 7.5 Hz,
3
2H, m-CH(2,6‑xylyl)), 6.98 (t, 3J = 7.2 Hz, 1H, p-CH(2,6‑xylyl)), 2.33 (s, 3H,
CH3(imine)), 2.10 (s, 6H, o-CH3(2,6‑xylyl)). 13C{1H} NMR (75.5 MHz,
CDCl3): δ 150.6 (CN), 145.5 (Cipso(2,6‑xylyl)), 143.9 (NCCH), 141.3
(NCN), 132.4 (NCCH), 128.3 (m-CH(2,6‑xylyl)), 126.7 (o-C(2,6‑xylyl)),
125.2 (NCCHCH), 124.4 (NCCHCH),123.7 (p-CH(2,6‑xylyl)), 120.2
(CNCCH), 116.8 (NCCH), 18.4 (o-CH3(2,6‑xylyl)). 16.7 (CH3(imine)).
1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]benzimidazolium
Chloride (2a). N-(2,6-Dimethylphenyl)acetimidoyl chloride (1.73 g,
9.55 mmol) was dissolved in 20 mL of toluene and added to a solution
of 1-(2,6-dimethylphenylimino)benzimidazole (1a; 2.51 g, 9.55 mmol)
in toluene (20 mL) at room temperature. The reaction mixture was
stirred for 24 h to give an off-white precipitate. The white solid was
washed with toluene and pentane and dried under vacuum (3.42 g,
CONCLUSIONS
■
Chromium(III), iron(II), and cobalt(II) complexes of bis-
(imino)benzimidazol-2-ylidene (D) and bis(imino)pyrimidin-
2-ylidene (E) were successfully prepared, using either the
benzimidazolium or pyrimidinium salts or the corresponding
copper complex as transmetalating agent, further demonstrating
the scope of application of this methodology. The pyrimidin-2-
ylidene ligand was unambiguously shown to coordinate to
Cr(III) in a tridentate fashion, the first demonstration of such a
binding capability for bis(imino)carbene ligands. The observed
coordination mode is possibly due to appropriate bond angles
imparted by the six-membered central ring and to a highly
electropositive Cr(III) metal center. Interestingly, spectroscopic
evidence points to coordination of the ligand to Fe(II) and
Co(II) exclusively in a bidentate fashion. In contrast to the iron
and cobalt complexes, both chromium complexes were active in
ethylene polymerization, further corroborating our previous
observations on the related imidazol-2-ylidene complexes. The
synthesis of new derivatives of these ligands aimed at improving
the performance of the corresponding transition-metal
complexes in olefin polymerization is currently underway.
1
7.70 mmol, 80%). H NMR (400 MHz, CDCl3): major isomer (E,Z
isomer) δ 9.41 (s, 1H, NCHN), 8.65 (dd, 3J = 3.4 Hz, 1H,
NCCHCH), 7.92 (dd, 3J = 3.4 Hz, 1H, NCCHCH), 7.46 (dd, 3J = 3.4
Hz, 2H, NCCHCH), 7.12 (d, 3J = 7.6 Hz, 2H, m-CH(2,6‑xylyl)), ca. 7.05
(2H, m-CH(2,6‑xylyl); obstructed by resonances from the minor isomer),
ca. 6.98 (2H, p-CH(2,6‑xylyl); overlapping magnetically inequivalent
nuclei), 2.63 (s, 3H, CH3(imine)), 2.43 (s, 3H, CH3(imine)), 2.11 (s, 6H,
o-CH3(2,6‑xylyl)), 2.08 (s, 6H, o-CH3(2,6‑xylyl)); minor isomer (E,E isomer)
δ 12.27 (s, 1H, NCHN), 9.02 (dd, 3J = 3.2 Hz, 2H, NCCHCH), 7.75
3
(dd, 3J = 3.2 Hz, 2H, NCCHCH), 7.14 (d, J = 7.6 Hz, 4H, m-
CH(2,6‑xylyl)), ca. 7.05 (2H, p-CH(2,6‑xylyl); obstructed by resonances
from the major isomer), 3.11 (s, 6H, CH3(imine)), 2.16 (s, 12H, o-
CH3(2,6‑xylyl)). 13C{1H} NMR (100 MHz, CDCl3): major isomer (E,Z
isomer) δ 151.0, 145.8, 145.2, 145.0, 141.6, 131.8, 127.9, 126.6, 126.5,
125.9, 125.4, 125.1, 124.3, 123.9, 119.2, 117.2, 29.2, 18.3, 17.9, 17.0;
minor isomer (E,E isomer): δ 153.2 (CN), 144.1 (Cipso(2,6‑xylyl)),
131.8 (NCCH), 129.2 (NCCHCH), 128.5 (m-CH(2,6‑xylyl)), 128.3 (o-
C(2,6‑xylyl)), 126.1 (NCHN), 124.9 (p-CH(2,6‑xylyl)), 118.9 (NCCHCH),
19.0 (CH3(imine)), 18.5 (o-CH3(2,6‑xylyl)). FTIR (neat): νCN 1690 cm−1.
[1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]benzimidazol-2-
ylidene]copper(I) Iodide (3a). Sodium hexamethyldisilazide (116
mg, 0.630 mmol) was dissolved in THF (4 mL) and added dropwise
to a THF (5 mL) suspension of copper(I) iodide (119 mg, 0.622
mmol) at −37 °C. The reaction mixture was slowly warmed to room
temperature and stirred for 1 h. It was then added dropwise to a THF
(5 mL) suspension of 1,3-bis[1-(2,6-dimethylphenylimino)ethyl]-
benzimidazolium chloride (2a; 277 mg, 0.622 mmol) at −37 °C.
The reaction mixture was slowly warmed to room temperature and
stirred for an additional 20 h. The volatile was removed under vacuum
EXPERIMENTAL SECTION
■
General Comments. All experiments 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 by using an MBraun Solvent
Purification System fitted with alumina columns and stored over
molecular sieves under a positive pressure of dinitrogen (pentane,
toluene, and THF) or dried by refluxing and then distilling from
sodium (toluene) under a positive pressure of dinitrogen. Deuterated
solvents were degassed using three freeze−pump−thaw cycles. C6D6
was vacuum-distilled from sodium, and CDCl3 and CD3CN were
vacuum-distilled from CaH2. NMR spectra were recorded on a Bruker
AV 400 (1H at 400 MHz, 13C at 100 MHz) or Bruker AV 300
spectrometer (1H at 300 MHz, 13C at 75.5 MHz) 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 δ 0 for tetramethylsilane.
Elemental composition was determined by Guelph Chemical
1
to give an orange solid (105 mg, 0.175 mmol, 28%). H NMR (400
MHz, CDCl3): δ 8.44 (dd, 3J = 2.9 Hz, 2H, NCCHCH), 7.41 (dd, 3J =
2.9 Hz, 2H, NCCHCH), 7.00 (d, 3J = 7.5 Hz, 4H, m-CH(2,6‑xylyl)), 6.84
3
(t, J = 7.5 Hz, 2H, p-CH(2,6‑xylyl)), 2.69 (s, 6H, CH3(imine)), 2.17 (s,
12H, o-CH3(2,6‑xylyl)). 13C{1H} NMR (100 MHz, CDCl3): δ 156.6
(CN), 145.3 (Cipso(2,6‑xylyl)), 133.8 (NCCH), 128.3 (m-CH(2,6‑xylyl)),
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1
127.1 (o-C(2,6‑xylyl)), 125.3 (NCCHCH), 123.9 (p-CH(2,6‑xylyl)), 116.3
(NCCHCH), 19.5 (CH3(imine)), 19.1 (o-CH3(2,6‑xylyl)). FTIR (neat)
νCN 1668, 1653 cm−1. Anal. Calcd for C27H28CuIN4: C, 54.14; H,
4.71; N, 9.35. Found: C, 53.97; H, 4.54; N, 9.08.
(594 mg, 1.45 mmol, 57%). H NMR (400 MHz, CDCl3): δ 9.93 (s,
1H, NCHN), 7.07 (d, 3J = 7.5 Hz, 4H, m-CH(2,6‑xylyl)), 6.98 (t, 3J = 7.5
3
Hz, 2H, p-CH(2,6‑xylyl)), 4.34 (t, J = 5.7 Hz, 4H, NCH2CH2), 2.59 (s,
6H, CH3(imine)), 2.47 (p, 3J = 5.7 Hz, 2H, NCH2CH2), 2.07 (s, 12H, o-
CH3(2,6‑xylyl)). 13C{1H} NMR (100 MHz, CDCl3): δ 154.1 (NCN),
153.7 (CN), 144.5 (Cipso(2,6‑xylyl)), 128.4 (m-CH(2,6‑xylyl)), 126.5 (o-
C(2,6‑xylyl)), 124.5 (p-CH(2,6‑xylyl)), 42.2 (NCH2CH2), 18.7 (NCH2CH2),
18.5 (o-CH3(2,6‑xylyl)), 15.9 (CH3(imine)). FTIR (neat) νCN 1628 cm−1.
[1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]-4,5,6-trihydro-
pyrimidin-2-ylidene]copper(I) Iodide (3b). Sodium hexamethyldi-
silazide (50.7 mg, 0.276 mmol) was dissolved in THF (4 mL) and
added dropwise to a THF (4 mL) suspension of copper iodide (52.1
mg, 0.274 mmol) at −37 °C. The reaction mixture was stirred for 30
min, and it was then added dropwise to a THF (5 mL) suspension of
1,3-bis[1-(2,6-dimethylphenylimino)ethyl]-4,5,6-trihydropyrimidinium
chloride (2b; 112 mg, 0.273 mmol) at −37 °C. The reaction mixture
was slowly warmed to room temperature and stirred for an additional
4 h. Pentane was added to precipitate a brown solid, and the
supernatant was removed. The volatiles were removed in vacuo to give
the product in 76% yield (118 mg, 0.208 mmol). 1H NMR (400 MHz,
[1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]benzimidazol-2-
ylidene]chromium(III) Chloride (4a). To a mixture of CrCl3·3THF
(39.4 mg, 0.105 mmol) and 1,3-bis[1-(2,6-dimethylphenylimino)-
ethyl]benzimidazol-2-ylidene copper(I) iodide (3a; 62.9 mg, 0.105
mmol) was added 10 mL of dichloromethane at room temperature.
The reaction mixture was stirred overnight and subsequently filtered.
The solvent was removed in vacuo. The solid was then dissolved in
toluene and filtered. The filtrate was collected, and the solvent was
removed in vacuo to give a blue-green solid in 71% yield (42.5 mg,
0.0750 mmol). FTIR (neat): νCN 1683, 1622 cm−1. μeff = 3.75 μB.
Anal. Calcd for C27H28CrCl3N4: C, 57.20; H, 4.98; N, 9.88. Found: C,
56.94; H, 4.98; N, 10.10.
[1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]benzimidazol-2-
ylidene]iron(II) Chloride (5a). Potassium hexamethyldisilazide (65.6
mg, 0.329 mmol) was dissolved in THF (4 mL) and added dropwise
to a THF (8 mL) suspension of iron(II) chloride (41.4 mg, 0.327
mmol) at −37 °C. The reaction mixture was slowly warmed to room
temperature and stirred for 40 min, filtered, and added dropwise to a
THF (5 mL) suspension of 1,3-bis[1-(2,6-dimethylphenylimino)-
ethyl]benzimidazolium chloride (2a; 144 mg, 0.323 mmol) at −37 °C.
The reaction mixture was slowly warmed to room temperature and
stirred for an additional 22 h. The reaction mixture was filtered, and
the solution was concentrated to about 1 mL. Pentane was added to
precipitate a yellow solid. The supernatant was removed, and the
residual solid was dried to give the product in 71% yield (122 mg,
0.228 mmol). FTIR (neat): νCN 1683, 1629 cm−1. μeff = 5.86 μB.
Anal. Calcd for C27H28FeCl2N4: C, 60.58; H, 5.27; N, 10.47. Found:
C, 60.30; H, 5.02; N, 10.15.
[1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]benzimidazol-2-
ylidene]cobalt(II) Chloride (6a). Potassium hexamethyldisilazide
(70.2 mg, 0.352 mmol) was dissolved in THF (4 mL) and added
dropwise to a THF (8 mL) suspension of cobalt(II) chloride (45.5 mg,
0.350 mmol) at −37 °C. The reaction mixture was stirred for 40 min
at room temperature. It was then filtered, and the filtrate was added
dropwise to a THF (5 mL) suspension of 1,3-bis[1-(2,6-
dimethylphenylimino)ethyl]benzimidazolium chloride (2a; 153 mg,
0.344 mmol) at −37 °C. The reaction mixture was slowly warmed to
room temperature and stirred for an additional 22 h. It was filtered,
and the solution was concentrated to about 1 mL. Pentane was added
to precipitate the product as a green solid in 62% yield (115 mg, 0.344
mmol). FTIR (neat): νCN 1683, 1607 cm−1. μeff = 4.27 μB. Anal.
Calcd for C27H28CoCl2N4: C, 60.23; H, 5.24; N, 10.41. Found: C,
60.50; H, 5.02; N, 10.18.
3
3
CD3CN): δ 7.09 (d, J = 7.7 Hz, 4H, m-CH(2,6‑xylyl)), 6.97 (t, J = 7.7
Hz, 2H, p-CH(2,6‑xylyl)), 3.69 (t, J = 6.5 Hz, 4H, NCH2CH2), 2.28 (s,
3
3
12H, o-CH3(2,6‑xylyl)), 2.27 (m, J = 6.5 Hz, 2H, NCH2CH2), 1.99 (s,
6H, CH3(imine)). 13C{1H} NMR (100 MHz, CD3CN): δ 162.3 (NCN),
146.1 (CN), 130.3 (o-C(2,6‑xylyl)), 128.9 (m-CH(2,6‑xylyl)), 128.8
(Cipso(2,6‑xylyl)), 125.2 (p-CH(2,6‑xylyl)), 46.1 (NCH2CH2), 23.3
(NCH2CH2), 20.0 (o-CH3(2,6‑xylyl)), 16.8 (CH3(imine)). FTIR (neat)
νCN 1635 cm−1. Anal. Calcd for C24H30CuIN4: C, 51.02; H, 5.35; N,
9.92. Found: C, 50.87; H, 5.16; N, 9.65.
[1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]-4,5,6-trihydro-
pyrimidin-2-ylidene]chromium(III) Chloride (4b). A dichloro-
methane solution of CrCl3·3THF (37.5 mg, 0.100 mmol) was added,
at room temperature, to a dichloromethane suspension of 1,3-bis[1-
(2,6-dimethylphenylimino)ethyl]-4,5,6-trihydropyrimidin-2-ylidene
copper(I) iodide (3b; 56.7 mg, 0.100 mmol). The reaction mixture
was stirred for 20 h and subsequently filtered. The filtrate was
collected, and the solvent was removed in vacuo to give a light blue-
green solid. Yield: 45.9 mg, 0.0861 mmol, 86%. X-ray-quality crystals
were grown from a concentrated dichloromethane solution of 4b by
slow evaporation of the solvent. FTIR (neat): νCN 1623 cm−1. μeff
=
4.14 μB. Anal. Calcd for C24H30CrCl3N4: C, 54.09; H, 5.67; N, 10.51.
Found: C, 54.57; H, 5.81; N, 9.96.
[1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]-4,5,6-trihydro-
pyrimidin-2-ylidene]iron(II) Chloride (5b). Potassium hexame-
thyldisilazide (51.6 mg, 0.259 mmol) was dissolved in THF (4 mL)
and added dropwise to a THF (4 mL) suspension of iron(II) chloride
(32.5 mg, 0.256 mmol) at −37 °C. The reaction mixture was kept at
−37 °C for 3 h and agitated occasionally. It was then filtered, and the
filtrate was added dropwise to a THF (4 mL) suspension of 1,3-bis[1-
(2,6-dimethylphenylimino)ethyl]-4,5,6-trihydropyrimidinium chloride
(2b; 101 mg, 0.245 mmol) at −37 °C. The reaction mixture was
slowly warmed to room temperature and stirred for an additional 20 h.
The mixture was then concentrated, and pentane was added to
precipitate a reddish solid. The solid was dissolved in dichloromethane
and then filtered. The volatiles were removed in vacuo to give the
product in 81% yield (123 mg, 0.245 mmol). FTIR (neat): νCN 1638,
1617 cm−1. μeff = 5.80 μB. Anal. Calcd for C24H30FeCl2N4: C, 57.50; H,
6.03; N, 11.18. Found: C, 57.21; H, 5.75; N, 10.89.
[1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]-4,5,6-trihydro-
pyrimidin-2-ylidene]cobalt(II) Chloride (6b). Potassium hexame-
thyldisilazide (46.6 mg, 0.234 mmol) was dissolved in THF (4 mL)
and added dropwise to a THF (4 mL) suspension of cobalt chloride
(30.4 mg, 0.232 mmol) at −37 °C. The reaction mixture was kept at
−37 °C for 3 h and agitated occasionally. It was then filtered, and the
filtrate was added dropwise to a THF (4 mL) suspension of 1,3-bis[1-
(2,6-dimethylphenylimino)ethyl]-4,5,6-trihydropyrimidinium chloride
(2b; 90.0 mg, 0.219 mmol) at −37 °C. The reaction mixture was
slowly warmed to room temperature and stirred for an additional 20 h.
The mixture was then concentrated, and pentane was added to
precipitate a green solid. The solid was dissolved in dichloromethane
(1-(2,6-Dimethylphenylimino)ethyl)-4,5,6-trihydropyrimi-
dine (1b). N-(2,6-Dimethylphenyl)acetimidoyl chloride (100 mg,
0.554 mmol) was dissolved in toluene (3 mL) and added to a toluene
solution (3 mL) of 1,4,5,6-tetrahydropyrimidine (90.6 mg, 1.10 mmol)
at room temperature. The reaction mixture was stirred for 16 h. The
mixture was filtered, and the solvent was removed in vacuo to give the
1
product as a light yellow oil (111 mg, 87%). H NMR (300 MHz,
CDCl3): δ 7.89 (s, 1H, NCHN), 7.03 (d, 3J = 7.4 Hz, 2H, m-
3
3
CH(2,6‑xylyl)), 6.88 (t, J = 7.4 Hz, 1H, p-CH(2,6‑xylyl)), 3.88 (t, J = 6.0
Hz, 2H, CNCH2CH2), 3.49 (t, 3J = 6.0 Hz, 2H, CNCH2CH2), 2.02
3
(s, 6H, o-CH3(2,6‑xylyl)), 1.96 (tt, J = 6.0 Hz, 2H, NCH2CH2), 1.88 (s,
3H, CH3(imine)). 13C{1H} NMR (75.5 MHz, CDCl3): δ 152.4 (CN),
146.7 (Cipso(2,6‑xylyl)), 143.8 (NCN), 127.7 (m-CH(2,6‑xylyl)), 127.2 (o-
C(2,6‑xylyl)), 122.3 (p-CH(2,6‑xylyl)), 44.3 (CNCH2CH2), 41.0
(NCH2CH2), 20.9 (NCH2CH2), 18.0 (o-CH3(2,6‑xylyl)), 14.2
(CH3(imine)).
1,3-Bis[1-(2,6-dimethylphenylimino)ethyl]-4,5,6-trihydro-
pyrimidinium Chloride (2b). N-(2,6-Dimethylphenyl)acetimidoyl
chloride (462 mg, 2.54 mmol) was dissolved in 8 mL of benzene and
added to a solution of (1-(2,6-dimethylphenylimino)ethyl)-4,5,6-
trihydropyrimidine (1b; 583 mg, 2.54 mmol) in benzene (10 mL)
at room temperature. The reaction mixture was stirred for 20 h. The
white solid was washed with toluene and pentane and dried in vacuo
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Organometallics
Article
and then filtered. The volatiles were removed under vacuum to give
the product in 74% yield (81.8 mg, 0.162 mmol). FTIR (neat): νCN
1635, 1612 cm−1. μeff = 4.35 μB. Anal. Calcd for C24H30CoCl2N4: C,
57.15; H, 6.00; N, 11.11. Found: C, 56.88; H, 5.72; N, 10.94.
General Procedure for Ethylene Polymerization. Ethylene
polymerization was performed at room temperature and at
atmospheric pressure in a 500 mL Schlenk flask containing a magnetic
stir bar. The flask was kept in an oven at 130 °C for at least 18 h prior
to use. The hot flask was brought to room temperature under dynamic
vacuum and back-filled with ethylene. Under an atmosphere of
ethylene, the flask was charged with 20 mL of dry toluene and 1000
equiv of MAO with respect to the complex (7.6 μmol). The solution
was stirred for 10−15 min before a solution of the catalyst in either
toluene or dichloromethane was introduced into the flask via a syringe.
The reaction mixture was stirred for either 10 or 30 min after the
addition of the catalyst and subsequently quenched with a 1:1 mixture
of concentrated hydrochloric acid and methanol. The resulting mixture
was filtered. Any solid collected was washed with distilled water and
dried under vacuum at approximately 50 °C for 24 h. Complexes 4a,b
each gave respectively 29 mg (3.8 kg of PE (mol of cat.)−1; 23 kg of PE
(mol of cat.)−1 h−1) and 39 mg (5.1 kg of PE (mol of cat.)−1; 31 kg of
PE (mol of cat.)−1 h−1) of polyethylene, as averages of three 10 min
runs, with a standard deviation of 20%, common for batch
polymerization experiments. In contrast, complexes 4a,b gave
respectively 45 mg (5.9 kg of PE (mol of cat.)−1; 12 kg of PE (mol
of cat.)−1 h−1) and 55 mg (7.3 kg of PE (mol of cat.)−1; 15 kg of PE
(mol of cat.)−1 h−1) of polyethylene, as averages of two 30 min runs,
with a standard deviation of 20%.
McMaster University (Hamilton, Ontario) for X-ray data
acquisition and for their assistance in refining the structure of
compound 4b. J.A.T. thanks the Government of Saudi Arabia
for a scholarship.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Tables and a CIF file giving crystallographic data for 4b
(CCDC reference number 900426), including tables of crystal
data and structure refinement, bond lengths, angles, atomic
coordinates and equivalent isotropic displacement parameters,
and anisotropic displacement parameters. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +1 416 736 2100, ext. 77728. Fax: +1 416 736 5936. E-
mail: glavoie@yorku.ca.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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(10) (a) Small, B. L.; Brookhart, M. Macromolecules 1999, 32, 2120−
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Deffieux, A. Macromol. Chem. Phys. 2001, 202, 3043−3048. (f) Bennett,
A. M. A.; Feldman, J.; Mccord, E. F. Copolymerization of Olefins
Using 2,6-Diacylpyridinebisimine Complexes as Catalysts. PCT Int.
G.G.L. gratefully acknowledges financial support from the
Ontario Ministry of Research and Innovation for an Early
Researcher Award, the Natural Sciences and Engineering
Research Council 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 startup funds. We also thank
Dr. Alan J. Lough of the University of Toronto (Toronto,
Ontario), and Drs. James F. Britten and Hilary A. Jenkins of
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