Coordination and reactivity study of group 4 and 10 transition metal complexes of N-imidazol-2-ylidene-N′-p-tolylureate and thioureate ligands

Article abstract of DOI:10.1021/om400220v

The synthesis and isolation of N-imidazol-2-ylidene-N′-p-tolylureate and thioureate proligands is described. Titanium(IV), nickel(II), and palladium(II) complexes containing one or two equivalents of the ligand were prepared by transmetalation of the ureate or thioureate salts. The X-ray structure of titanium complexes corroborated our prediction based on spectroscopic data and showed the ureate binds in a bidentate fashion through the oxygen and nitrogen atoms of the isocyanate building block, leaving the imidazol-2-imine fragment uncoordinated. Carbon-nitrogen bond lengths indicate substantial electron delocalization from the imidazole ring to the ureate group. In contrast, the ligand adopts a different coordination mode and binds to nickel and palladium through the N_{imidazol-2-ylidene} and N _p-tolyl atoms. Surprisingly, despite the softer nature of sulfur, coordination of the thioureate was exclusively through the sulfur and nitrogen atoms of the isothiocyanate building block with all three metals studied. All complexes were tested for ethylene polymerization at ambient conditions using methylaluminoxane as cocatalyst. Titanium(IV) complexes were found to be the most active, with activities up to 60 kg PE mol^-1 catalyst h ^-1.

Full text of DOI:10.1021/om400220v

Article
pubs.acs.org/Organometallics
Coordination and Reactivity Study of Group 4 and 10 Transition
Metal Complexes of N‑Imidazol-2-ylidene‑N′‑p‑tolylureate and
Thioureate Ligands
Michael B. Harkness, Edwin Alvarado, Anna C. Badaj, Barbara C. Skrela, Louie Fan, and Gino G. Lavoie*
Department of Chemistry, York University, 4700 Keele Street, Toronto, Ontario, M3J 1P3, Canada
S
* Supporting Information
ABSTRACT: The synthesis and isolation of N-imidazol-2-
ylidene-N′-p-tolylureate and thioureate proligands is described.
Titanium(IV), nickel(II), and palladium(II) complexes con-
taining one or two equivalents of the ligand were prepared by
transmetalation of the ureate or thioureate salts. The X-ray
structure of titanium complexes corroborated our prediction
based on spectroscopic data and showed the ureate binds in a bidentate fashion through the oxygen and nitrogen atoms of the
isocyanate building block, leaving the imidazol-2-imine fragment uncoordinated. Carbon−nitrogen bond lengths indicate
substantial electron delocalization from the imidazole ring to the ureate group. In contrast, the ligand adopts a different
coordination mode and binds to nickel and palladium through the Nimidazol‑2‑ylidene and Np‑tolyl atoms. Surprisingly, despite the
softer nature of sulfur, coordination of the thioureate was exclusively through the sulfur and nitrogen atoms of the isothiocyanate
building block with all three metals studied. All complexes were tested for ethylene polymerization at ambient conditions using
methylaluminoxane as cocatalyst. Titanium(IV) complexes were found to be the most active, with activities up to 60 kg PE mol−1
catalyst h−1.
complexes, an attribute that has been successfully employed in
a number of catalytic transformations,12 including olefin
polymerization.13 In addition to their role as ligands, the
reactivity of these carbenes with organic azides has also been
exploited, producing imidazol-2-iminates as a new class of
monoanionic ligands (Figure 1).14−16 The formation of a stable
INTRODUCTION
■
The utility of titanium halides and alkylaluminum reagents in
the polymerization of ethylene and propylene, first recognized
by Ziegler and Natta,1 has led to the development of several
new homogeneous single-site olefin polymerization catalysts
over the past decades.2 Studies of well-defined group 4
metallocenes have in fact provided an excellent mechanistic
understanding of the critical elementary steps involved in the
formation of polyolefins.3 Further variations on the ancillary
ligands have resulted in several catalysts of group 4 transition
metals with enhanced activity and exceptional control over the
microstructure of simple polyolefins, such as polypropylene and
polystyrene.4,5
Figure 1. Mesomeric structures for imidazol-2-imine.
The report by Brookhart that cationic α-diimine nickel and
palladium complexes also gave high molecular weight
polyolefins, including functionalized ones such as ethylene−
acrylate copolymers, spurred a renewed interest in the field.6
Neutral group 10 catalysts bearing salicylaldiminate ligands
reported shortly thereafter also showed functional group
tolerance but resulted in lower polymerization rates.7 The
versatility of these monoanionic salicylaldiminate ligands, which
form six-membered chelates, was also demonstrated through
their coordination to early transition metals, giving some of the
most active polyolefin catalysts to date.8,9 Our group became
interested in developing a new class of bidentate monoanionic
ligands that form smaller chelates with metals and exploring the
activity of these new complexes in olefin polymerization.
Since the isolation of the first stable N-heterocyclic carbene
(NHC),10 several new classes of carbenes have been studied.11
Upon coordination, these carbenes form robust transition metal
imidazolium center leads to a significant shift of the electron
density to the exocyclic nitrogen, through the formation of the
formally zwitterionic structure, with steric and electronic
parameters easily tailored through the substituents on the
azole ring. While analogous to phenoxides, the bond angles
about the azole five-membered ring confer a more opened
coordination sphere by positioning the 1,3-substituents farther
away from the metal center, critical for the polymerization of α-
olefins.
Our group has recently prepared a new family of neutral
bidentate ligands that incorporate this imidazol-2-imine frag-
ment in their scaffold, and we explored their coordination to
titanium and palladium.17 Solid-state structures of the imine
Received: March 15, 2013
© XXXX American Chemical Society
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imidazol-2-imine titanium and palladium complexes showed
that the denticity of the ligand is highly dependent on the metal
used (Figure 2). While the ligand coordinates to titanium in the
The corresponding thiourea proligand analogue 2b was
similarly prepared by reacting imidazol-2-imine 1 with
isothiocyanate (Scheme 1). The H NMR (C6D6) resonances
1
for the backbone imidazole protons and the ortho and meta
protons of the p-tolyl group are observed at δ 5.89, 6.90, and
6.78, respectively, downfield from those of the starting
materials. The FTIR spectrum of 2b revealed strong absorption
bands at 1400−1600 cm−1 assigned to the NCS group. The
identification of the vibration modes responsible for these
absorptions is however difficult due to mixing of bond
deformation, rocking, bending, and stretching, characteristics
of both organic thioureas and inorganic thioureate metal
complexes.20 Furthermore, absorption bands that are princi-
pally due to stretching of the CS double bonds are expected
at approximately 700 cm−1 and predicted to be weak. This was
confirmed by DFT frequency calculations (B3LYP/LanL2DZ)
on geometry-optimized thioureate metal complexes.21 Thus,
while infrared spectroscopy is an excellent diagnostic tool to
predict the coordination mode of ureates in metal complexes, a
similar use in thioureate chemistry remains highly uncertain
and questionable and, therefore, will not be further discussed in
this study.
Figure 2. Titanium(IV) and palladium(II) imidazol-2-imine com-
plexes.17
expected chelating form, coordination to palladium was
exclusively through the distal imine nitrogen, possibly a result
of its enhanced basicity due to electron delocalization from the
imidazol-2-imine fragment over the conjugated system.17
To enforce coordination of the ligand in the desired
bidentate fashion, we prepared anionic N-(imidazol-2-yli-
dene)-N′-(p-tolyl)ureate (A) and thioureate (B) ligands
(Figure 3). Herein, we report for the first time the synthesis
Synthesis and Characterization of N-Imidazol-2-
ylidene-N′-p-tolylureate Titanium(IV) Complexes. The
(cyclopentadienyl)(ureate)titanium(IV) dichloride complex
(3a) was prepared by reaction of CpTiCl3 with the lithiated
salt of 2a (Scheme 2). Complex 3a was isolated in 70% yield as
a bright reddish-orange crystalline solid. The 1H NMR
spectrum (C6D6) of 3a revealed a downfield shift relative to
that observed in 2a for the ortho and meta protons of the tolyl
group to δ 7.39 and 6.94, respectively, and an upfield shift for
the p-CH3 protons to δ 2.09. The imidazolium backbone
protons resonate at δ 5.74, only slightly more upfield from that
in 2a, suggesting coordination of the ligand through the oxygen
and p-tolyl nitrogen atoms. This is further supported by the
small change in the 13C NMR chemical shift for the central
imidazole carbon observed (δ 148.7 for 3a vs 149.8 for 2a) and
the more significant change in that of the ipso-carbon of the p-
tolyl ring (δ 146.2 for 3a vs 139.4 for 2a). The carbonyl carbon
nucleus resonates at δ 162.0, compared to δ 158.0 for 2a.
Furthermore, the decrease in the CO stretching frequency
(νCO 1524 cm−1) compared to that of the starting urea (νCO
1642 cm−1) strongly supports coordination through the oxygen
atom.
X-ray quality crystals of complex 3a were grown at −35 °C
by slow liquid diffusion of pentane into a concentrated
dichloromethane solution. Complex 3a crystallized in the
P21/n space group (Figure 4). Selected bond lengths and angles
are provided in Table 1. The solid-state structure of the four-
legged piano-stool titanium complex 3a confirmed the
Np‑tolyl∧O coordination mode of the ligand, forming a four-
membered metallacycle with a bite angle of 64.04(8)°. The
cyclopentandienyl (Cp) ligand is bound to the metal center in
Figure 3. Ureate (A) and thioureate (B) ligands.
of both ligands, their coordination to group 4 and 10 transition
metals, the effect of the acyl and thioacyl groups on the
coordination mode of the ligand, and the activity of these new
complexes in ethylene polymerization.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Urea and
Thiourea Proligands. The urea 2a was prepared in
quantitative yield from reaction of 1,3-dimesitylimidazol-2-
imine (IMesNH) (1)18 with p-tolyl isocyanate and isolated
as a crystalline solid (Scheme 1). The imidazolium backbone
1
protons of 2a resonate as a singlet in the H NMR spectrum
(C6D6) at δ 5.83, a downfield shift from that of the imidazol-2-
imine 1 (δ 5.70). The o- and m-CH protons of the p-tolyl group
also experience a downfield shift from δ 6.54 and 6.65 in p-tolyl
isocyanate to δ 7.02 and 6.71 in 2a. The acyl stretching
frequency of 2a at 1642 cm−1 is substantially lower than that of
the starting isocyanate (2274 cm−1), indicating a marked
decrease in C−O bond strength upon formation of the urea.
This observed stretching frequency is also slightly lower than
that commonly observed in organic amides,19 a manifestation
of the electron delocalization from the azole ring to the acyl
group, resulting in further decreasing the C−O bond order.
Scheme 1. Synthesis of Urea (2a) and Thiourea (2b) Proligands
B
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Scheme 2. Synthesis of Mono(ureate) (3a) and Bis(ureate) (4a) Titanium(IV) Complexes
Figure 4. ORTEP diagram of complex 3a (40% probability level). Hydrogen atoms are omitted for clarity.
1
the expected η5-hapticity, with the titanium−carbon bonds
ranging from 2.343(3) to 2.379(3) Å. The plane formed by the
Cp ring is tilted slightly away from the chlorine atoms at a 3.4°
angle from the basal plane formed by Cl1, Cl2, O1, and N1.
Significant electron donation from the azole ring is evidenced
by the unusually long N2−C9 (1.339(3) Å) and short N2−C1
(1.323(3) Å) bonds. Furthermore, a C1−N2−C9−N3 torsion
angle of 63.9° clearly indicates significant single-bond character
for N2−C9, as illustrated in the zwitterionic structure in Figure
1. Both C1−O1 and C1−N1 bond lengths are intermediate
between that of a carbon−oxygen and carbon−nitrogen single
bond and double bond, indicating extensive electron
delocalization over O1−C1−N1.22,23 To our knowledge, 3a is
the first reported ureate(−1) half-sandwich titanium dichloride
complex. A closely related cyclopentadienyl amidate titanium
dichloride complex24 displays longer ureate CO and CN
bond lengths (1.346(6) and 1.388(8) Å, respectively), while the
exocyclic iminic bond C−Np‑tolyl is significantly shorter
(1.283(7) Å).
complex 4a (Scheme 2). The H NMR (C6D6) spectrum
shows only one set of resonances, indicating the selective
formation of only one highly symmetric isomer. Similar to that
observed for 3a, the most significant changes in chemical shifts
are observed for the aromatic protons of the p-tolyl group (δ
7.36 and 6.82 for 4a vs δ 7.02 and 6.71 for 2a), for the
corresponding ipso-carbon (δ 145.3 for 4a vs δ 139.4 for 2a),
and for the carbon nucleus of the carbonyl group (δ 167.2 for
4a vs δ 158.0 for 2a). In contrast, the imidazole ring backbone
protons and the central carbon nuclei remain fairly unchanged
at δ 5.72 (δ 5.83 for 2a) and 149.1 (δ 149.8 for 2a),
respectively. These spectroscopic data strongly suggest
Np‑tolyl∧O coordination of the ligand, as observed for 3a. This
coordination motif was further substantiated by the large
decrease in the CO stretching frequency (νCO 1506 cm−1)
from that of the free ligand 2a (νCO 1642 cm−1).
X-ray quality crystals of complex 4a were grown at −35 °C
by slow liquid diffusion of pentane into a concentrated
dichloromethane solution. Complex 4a crystallized in the
P21/c space group (Figure 5). Selected bond lengths and angles
are provided in Table 1. The solid-state structure is in
Reaction of two equivalents of 2a with TiCl4(THF)2 in
toluene afforded the expected bis(ureate) titanium(IV)
C
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(and, consequently, the cis arrangement of the nitrogen atoms)
by minimizing the overall steric congestion of these complexes.9
The mesityl groups generate a pseudo-C2-symmetric reaction
pocket, with potential face selectivity for prochiral α-olefins and
formation of stereoregular poly(α-olefins), while providing
enough steric bulk in the secondary coordination sphere to
prevent bimolecular decomposition. As observed in complex
3a, the longer N2−C9/N6−C38 bonds (1.345(3) Å)
compared to N1−C1/N5−C30 (1.336(3) Å) and N2−C1/
N6−C30 (1.322(3) Å) bond lengths strongly suggest extensive
electron delocalization from the imidazol-2-ylidene. This is
further supported by the C1−N2−C9−N3 (55.9°) and O1−
C1−N2−C9 (10.8°) torsion angles, each indicating significant
single- and double-bond character for N2−C9 and N2−C1,
respectively. The values for both C1−O1 and C1−N1 bond
lengths also indicate significant electron delocalization over
O1−C1−N1.
Synthesis and Characterization of N-Imidazol-2-
ylidene-N′-p-tolylthioureate Titanium(IV) Complexes.
Titanium complexes of the related thioureate ligand were also
prepared to determine the impact of the softer sulfur on the
coordination mode to the metal and on the activity of the
complexes. Thioureate complexes of early transition metals
have previously been prepared by migratory insertion of a metal
imide27 or iminate16 into an isothiocyanate, resulting in the
formation of N∧N metal chelates. However, our attempts to
adopt this procedure using (1,3-bis(mesityl)imidazol-2-
iminato)titanium(IV) trichloride led only to the formation of
unidentifiable products.15
Table 1. Selected Bond Lengths and Bond Angles for
Compounds 3a, 4a, 5a, and 7b
a
b
3a
4a
5a
7b
Bond Lengths (Å)
c
C1−E1
1.312(3)
1.329(3)
1.323(3)
1.339(3)
1.328(3)
1.336(3)
1.322(3)
1.345(3)
1.211(7)
1.369(8)
1.431(7)
1.328(7)
1.440(13)
1.305(11)
1.760(2)
1.325(3)
1.324(3)
1.326(3)
C1−N1
C1−N2
C9−N2
C30−C31
C31−C32
c
M−E1
2.056(2)
2.030(2)
1.967(1)
2.317(1)
2.063(2)
2.210(6)
M−Cl1
M−N1
M−N2
2.090(5)
2.096(4)
2.112(7)
2.104(7)
1.900(17)
M−C30
M−C32
Bond Angles (deg)
c
E1−M−N1
64.04(8)
110.2(2)
65.16(6)
109.0(2)
74.46(5)
c
E1−C1−N1
107.43(16)
N1−C1−N2
N1−M−N2
C30−M−C32
104.3(5)
63.75(18)
69.8(3)
a
Average bond lengths and angles for both ureate ligands that are not
related by any symmetry operation. Average bond lengths and angles
b
of both independent molecules of 7b found in the asymmetric unit
c
cell. E1 = O1 in 3a, 4a, and 5a and E1 = S1 in 7b.
Similar to the ureate titanium(IV) complexes 3a and 4a, the
thioureate complexes 3b and 4b were thus prepared by addition
of the thioureate salt, generated in situ by deprotonation of 2b
with sodium hexamethyldisilazide, to the appropriate metal
precursor (Scheme 3). Complex 3b was isolated in 65% yield as
a bright reddish-orange crystalline solid. The protons of the
cyclopentadienyl ring resonate at δ 6.18 in benzene-d6, a
downfield shift from that of CpTiCl3 (δ 5.93). Surprisingly, the
1H NMR spectrum is consistent with coordination of the soft
sulfur to the hard titanium(IV) metal center, resulting in an
Np‑tolyl∧S coordination mode. As observed in the ureate series,
the o- and m-CH protons of the p-tolyl group both experienced
a downfield shift to δ 7.12 and 6.98, respectively, from δ 6.90
and 6.78 for 2b. The upfield shift for the backbone imidazolium
protons to 5.67 in complex 3b, as observed in 3a and 4a, both
structurally characterized, suggests an uncoordinated imidazol-
2-imine fragment.
The predicted Np‑tolyl∧S binding motif is supported by 13C
NMR data. The resonance for the ipso-carbon of the p-tolyl ring
is shifted by approximately 9 ppm from that observed of 2b.
This change in chemical shift is consistent with the p-tolyl
nitrogen atom coordinated to titanium.16 Coordination through
the sulfur atom is substantiated by the thioacyl carbon
resonance at δ 169.5, significantly lower than that observed
for 2b (δ 181.0). Coordination of the ligand in an
Figure 5. ORTEP diagram of complex 4a (40% probability level).
Hydrogen atoms and one molecule of dichloromethane are omitted
for clarity.
agreement with the high symmetry predicted by NMR
spectroscopy. The solid-state structure also confirms the
Np‑tolyl∧O coordination mode of the ligand. As observed in
other bis(ureate) complexes25 and in the well-studied
salicylaldiminato complexes,26 complex 4a exhibits a distorted
octahedral geometry with the oxygen atoms trans to each other
and a cis arrangement for the chloride atoms, ideal for use as
olefin polymerization catalysts.9,16 The shorter Ti−O bonds
compared to Ti−N bonds,22,23 as observed in 4a (Table 1), are
believed to favor the trans arrangement of the oxygen atoms
∧
Np‑tolyl Niminate mode would have resulted in the thioacyl
carbon nucleus resonating at a higher frequency, around 190
ppm.16 The chemical shift of the thioacyl carbon is furthermore
within the range reported for transition metal complexes for
which the thioureate ligand coordinates in an N∧S fashion.28,29
Multiple attempts to generate single crystals of 3b suitable for
X-ray diffraction studies were unsuccessful. However, the
related fluorinated [N-(1,3-dimesitylimidazol-2-ylidene)-N′-(4-
trifluoromethylphenyl)thioureato]cyclopentadienyl titanium di-
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Scheme 3. Synthesis of Mono(ureate) (3b) and Bis(thioureate) (4b) Titanium(IV) Complexes
Scheme 4. Synthesis of Palladium(II) (5a) and Nickel(II) (6a) Ureate Complexes
Figure 6. ORTEP diagram of complex 5a (40% probability level). Hydrogen atoms and minor allyl configuration are omitted for clarity.
chloride (3b′) was successfully isolated, and the ligand was
found to be bound to the metal center in an Np‑tolyl∧S
coordination mode (see Supporting Information). Considering
the similarities in NMR spectroscopic features between both
compounds, we conclude that the thioureate ligand in 3b also
adopts an Np‑tolyl∧S coordination mode.
The related bis(thioureate) titanium complex 4b was
prepared by addition of two equivalents of the thioureate salt
to TiCl4(THF)2 and isolated in 90% yield as a red solid. As
observed for the related bis(ureate) complex 4a, both ligands
are magnetically equivalent, with only one set of resonances
observed in the NMR spectra, indicating a highly symmetric
molecule. The key 1H and 13C NMR spectroscopic data suggest
an Np‑tolyl∧S binding motif, with an uncoordinated imidazo-2-
ylidene-substituted nitrogen atom as observed in 4a. Consid-
ering the hardness of Ti(IV) and the softness of sulfur, we were
quite surprised to conclude a coordination of the thioureate
ligand through sulfur. However, out of all the thioureate
transition metal complexes structurally characterized and
registered with the Cambridge Structural Database,28−30 only
two cases with an N∧N coordination mode were found.16,27
Our system thus provides further evidence of the shortcomings
E
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Scheme 5. Synthesis of Palladium(II) (5b) and nickel(II) (6b) Thioureate Complexes
of the concept of hard and soft acids and bases, recently
documented by Mayr.31 Multiple attempts to grow single
crystals for X-ray diffraction studies using several solvent
combinations were not successful.
ureate is bound in an Np‑tolyl∧O fashion, and is in agreement
with the expected values for carbon−oxygen double bonds.23
The observed C1−O1 bond length is also consistent with the
marginally unchanged νCO stretching frequency upon coordi-
nation. The imidazole ring also displays less zwitterionic
character, as evidenced by the shorter C9−N2 bond compared
to those in 3a and 4a and by the substantially smaller C1−N2−
C9−N3 torsion angle (36.7° vs 55.9° and 63.9° for 3a and 4a,
respectively). As a result of the N∧N coordination mode, the
C1−N1 and C1−N2 bonds are also significantly longer than
the corresponding bonds in 3a and 4a. Surprisingly, both Pd1−
N1 and Pd1−N2 bond lengths are statistically equivalent
despite N1 and N2 being formally anionic and neutral,
respectively.
Synthesis and Characterization of Palladium(II) and
Nickel(II) Allyl Ureate Complexes. Our coordination study
of the ureate and thioureate ligands to titanium demonstrated
∧
the challenge in enforcing the desired Nimidazol‑2‑ylidene Np‑tolyl
binding mode with this new class of ligands. We thus became
interested in exploring coordination of both ligands to
nickel(II) and palladium(II) and assessing the effect of using
late transition metals on the binding mode selectivity.
The palladium(II) allyl imidazol-2-imine ureate complex
(5a) was synthesized according to Scheme 4. A downfield shift
in the resonances for the ortho- and meta-CH p-tolyl protons to
δ 7.66 and 7.01, respectively, from that of 2a was observed in
The related nickel(II) complex (6a) was prepared under
similar conditions by reaction of the ureate salt with
nickel(methylallyl) chloride dimer (Scheme 4). The diamag-
netic nature of the complex indicates that a square planar
geometry is also adopted. As seen for 5a, the ortho and meta
protons of the p-tolyl ring experience a significant downfield
shift to δ 7.53 and 6.95, respectively, from that for 2a, indicating
coordination of the Np‑tolyl atom. The 13C nuclei for the central
azole carbon, the ipso-carbon of the p-tolyl ring, and the acyl
carbon all resonate at frequencies comparable to those of the
palladium analogue 5a. More importantly, the νCO stretching
frequency (νCO 1645 cm−1) marginally differs from that of
compound 5a (1636 cm−1) and of the proligand 2a (1642
cm−1), but it is clearly larger than that of 3a (1524 cm−1) and
4a (1506 cm−1), for which the ligand was unambiguously
shown to bind through the acyl oxygen atom (vide supra). The
similarities in NMR and FTIR spectroscopic data for both 5a
1
the H NMR (C6D6) spectrum of 5a, similar to that noted for
3a and 4a. The ipso-carbon of the p-tolyl ring resonates at δ
147.4, an upfield shift from that of 2a, further suggesting
1
coordination of Np‑tolyl to the metal center. The H and 13C
resonances for the imidazole ring only marginally change from
those observed in the ligand precursor 2a, with the imidazole
backbone protons and the central carbon nucleus resonating at
δ 5.80 (compared to δ 5.83 for 2a) and 148.7 (compared to δ
149.8 for 2a), respectively. In contrast, the acyl carbon
resonance shifted upfield by 6.3 ppm to δ 164.3. The small
decrease in the CO stretching frequency (1636 cm−1) from
that of the free ligand 2a (νCO 1642 cm−1) is however
inconsistent with an Np‑tolyl∧O binding motif, which should have
resulted in a much larger decrease (>100 cm−1) in stretching
frequency (vide supra). These spectroscopic data thus suggest
∧
∧
the formation of an Nimidazol‑2‑ylidene Np‑tolyl chelate upon
and 6a thus strongly suggest an Nimidazol‑2‑ylidene Np‑tolyl
coordination of the ligand despite the absence of any significant
changes in the 1H and 13C chemical shifts for both the
imidazole backbone protons and the central carbon.
coordination mode to nickel.
Synthesis and Characterization of Palladium(II) and
Nickel(II) Allyl Thioureate Complexes. Considering the
change in coordination mode of the ureate from Np‑tolyl∧O on
X-ray quality crystals of 5a were grown at room temperature
by slow vapor diffusion of pentane into a saturated THF
solution. Compound 5a crystallized in a monoclinic crystal
system in the P21/n space group, with the central carbon atom
of the allyl group disordered over two positions (Figure 6). The
complex adopts a square planar geometry with the metal
positioned on the plane formed by N1, N2, C30, and C31.
Selected bond lengths and angles are provided in Table 1. The
solid-state structure of 5a confirmed our prediction based on
NMR and IR spectroscropic data, with the ureate ligand bound
to the metal center through both nitrogen atoms, forming a
four-membered metallacycle with a N−Pd−N bite angle of
63.75(18)°. The C1−O1 bond length is significantly shorter
than those observed in compounds 3a and 4a, in which the
∧
titanium to Nimidazol‑2‑ylidene Np‑tolyl for both nickel and
palladium, we were interested in determining whether the
thioureate ligand would also preferentially adopt the
∧
Nimidazol‑2‑ylidene Np‑tolyl coordination mode with nickel and
palladium.
The palladium(II) (5b) and nickel(II) (6b) thioureate
complexes were prepared in excellent yields (85%) by reacting
the thioureate salt of 2b with the corresponding metal allyl
1
(Scheme 5). The H NMR (C6D6) spectra of both complexes
showed striking similarities to those for titanium complexes 3b
and 4b. First, the ortho and meta protons of the p-tolyl group
and the backbone protons of the azole ring all resonated at a
higher frequency upon coordination. Second, the resonance for
F
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the imine carbon for 5b and 6b appears at chemical shifts (δ
146.3 and 146.6, respectively) similar to those for the titanium
complexes 3b and 4b (δ 147.3 and 147.2, respectively). The
thioacyl carbon for 5b and 6b resonates at δ 176.8 and 175.7,
respectively, upfield from that observed in 2b, consistent with
sulfur coordination.16,28,29 These NMR spectroscopic data
strongly suggest coordination of the Np‑tolyl atom and of sulfur.
Crystals of 6b were grown at −35 °C by liquid diffusion of
pentane into a saturated THF solution. While the quality of the
diffraction data acquired on the nonmerohedral twinned crystal
prevented satisfactory refinement, the atoms’ connectivity in 6b
could nonetheless be established, unequivocally confirming the
Np‑tolyl∧S coordination mode for 6b (Figure 7).
complexes might be poor catalysts for olefin polymerization at
1 atm of ethylene and at room temperature due to poor
initiation kinetics. Indeed, nickel and palladium allyl complexes
all require elevated temperatures and pressures and long
reaction times to produce polymer in moderate yields, if at all.32
We thus became interested in preparing the nickel phenyl
complexes, by adaptation of a published procedure developed
for a monoanionic chelating NHC ligand.33 Attempts to
prepare the ureate complex were unsuccessful. In contrast, the
salt of 2b reacted cleanly with Ni(COD)2 and chlorobenzene.
To our surprise, the bis(thioureate)nickel(II) complex 7b was
isolated as the main reaction product in 61% yield (Scheme 6).
A related bis(α-iminocarboxamide)nickel(II) complex was
reported by Bazan, albeit as a minor product from the reaction
of a potassium amidate with Ni(COD)2 and benzoyl chloride.34
NMR spectroscopy data of 7b are consistent with the selective
generation of only one isomer, with the ligand bound to the
metal in an Np‑tolyl∧S coordination mode.
X-ray quality crystals of 7b were grown at room temperature
by slow vapor diffusion of ether into a saturated THF solution.
Compound 7b crystallized in a triclinic crystal system in the P1
space group, with nickel located at an inversion center (Figure
8). The complex adopts a square planar geometry with the
metal positioned on the plane formed by S1, C1, N1 and S1′,
C1′, N1′. Selected bond lengths and angles are provided in
Table 1. The solid-state structure of 7b confirmed our
prediction based on spectroscopic data, with the thioureate
ligand bound to the metal center through the sulfur atom,
forming a four-membered metallacycle with an S1−Ni1−N1
bite angle of 74.46(5)°. The imidazole ring displays significant
zwitterionic character, as evidenced by the large and statistically
equivalent N2−C9 and N2−C1 bond lengths and by the C1−
N2−C9−N3 torsion angle (74.3°), which deviates greatly from
the expected value (0°). This torsion angle is also significantly
larger than those observed for 3a (55.9°) and 4a (63.9°).
In an attempt to generate a mono(thioureate)nickel complex,
complex 7b was treated with methanol and HCl. Surprisingly,
complex 7b was found to be very stable to protonolysis.
Addition of a large excess of methanol to a solution of 7b in air
led to no reaction. Similarly, attempts to prepare a thioureate
nickel chloride complex by addition of one or two equivalents
of ethereal HCl to a THF solution of 7b led to no reaction. In
contrast, addition of a larger excess of HCl led to uncontrolled
decomposition of the complex.
̅
Figure 7. ORTEP diagram of complex 6b (40% probability level).
Hydrogen atoms, a solvent molecule of THF, and pentane are omitted
for clarity, and only one of two symmetrically independent molecules
in the asymmetric unit cell is shown.
The nickel (5a,b) and palladium (6a,b) complexes gave
valuable insight into the coordination mode of the ureate and
thioureate ligands. We however suspected that these allyl
Scheme 6. Synthesis of Bis(thioureato) nickel(II) Complex (7b)
G
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Figure 8. ORTEP diagram of complex 7b (40% probability level). Hydrogen atoms and a solvent molecule of diethyl ether are omitted for clarity.
Only one of two symmetrically independent molecules in the asymmetric unit cell is shown.
Ethylene Polymerization. The catalytic activity of all
complexes was evaluated for ethylene polymerization (1 atm
C2H4, room temperature) with 1000 equivalents of methyl-
aluminoxane (MAO) as cocatalyst. Complexes 3a, 3b, and 4a
all gave polyethylene in comparable productivities of 60 kg
mol−1 catalyst over 10 min. Complex 4b gave only trace
amounts of polymer. The maximum melting endotherm for the
polymer produced by these complexes was determined by
differential scanning calorimetry and was found to be in the
range 134−136 °C, indicating the production of linear
polyethylene. Related group 4 complexes of mono- and
bis(amidinato)35 and bis(guanidinato)36 also exhibited rela-
tively low activity.5,37 Neither the palladium(II) (5a and 5b)
nor the nickel(II) (6a and 6b) allyllic complexes showed any
activity under the exact conditions. Prolonged reaction times
(19 h) led to the formation of black particulates, indicative of
catalyst decomposition, and gave very little polyethylene, with a
maximum productivity of 2 kg mol−1 catalyst. The inactivity of
the late transition metal complexes 5 and 6 is likely a result of
the unfavorable formation of the η1-allyl species to create an
open coordination site for binding of the monomer,32 and of
the poor thermal stability of the catalytic species. Unfortu-
nately, decomposition of complexes 5 and 6 even at room
temperature precludes the use of elevated temperatures to
improve initiation kinetics. The bis(thioureato) nickel(II)
complex 7b also showed no activity, likely due to the inability
of MAO to generate the cationic nickel methyl initiator. In all
the trials, no wax or oligomer was detected in the reaction
medium.
metals in a bidentate fashion. Interestingly, the binding mode
greatly depends on both the metal and the ligand used. The
ureate ligand coordinates to titanium in an Np‑tolyl∧O binding
∧
mode, while an Nimidazol‑2‑ylidene Np‑tolyl chelate is produced upon
coordination to nickel and palladium. In contrast, the
thioureate ligand exclusively coordinates to both early and
late transition metals in an Np‑tolyl∧S binding mode. The nature
of the chelate was determined by a combination of NMR and
FTIR spectroscopy and, in many cases, confirmed by solid-state
structures. The titanium complexes were active in the
polymerization of ethylene, albeit at rates considerably lower
than those reported for metallocenes and salicylaldiminate-
based group 4 metal complexes.9
EXPERIMENTAL SECTION
■
All manipulations involving air-sensitive materials were carried out in
oven-dried glassware and performed under nitrogen using standard
Schlenk line techniques or in a nitrogen atmosphere glovebox. All
glassware and cannulae were dried overnight at 160 °C for at least 12 h
prior to use. 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 dinitrogen. Cyclopentadienyltitanium(IV) trichloride, allylpalla-
dium chloride dimer, and methallylnickel chloride dimer were
purchased from Strem and used without further purification. Sodium
bis(trimethylsilyl)amide, p-tolyl isocyanate, and p-tolyl isothiocyanate
were purchased from Sigma-Aldrich and used as received. Compounds
38
118 and TiCl4(THF)2 were prepared using published procedures.
Methylaluminoxane was graciously donated by Albermarle Corp.
Deuterated NMR solvents were purchased from Cambridge Isotope
Laboratories and 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 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
CONCLUSIONS
■
In contrast to our first-generation neutral imine imidazol-2-
imine ligands, the new homologous monoanionic ureate and
thioureate ligands coordinate to both early and late transition
H
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solvent (13C) resonances, and chemical shifts are reported with respect
to δ = 0 for tetramethylsilane. J-coupling constants are reported in
hertz (Hz). The multiplicity of signals is reported as singlet (s),
doublet (d), triplet (t), quartet (q), multiplet (m), broad (br), or a
[N-(1,3-Dimesitylimidazol-2-ylidene)-N′-(p-tolyl)thioureato]-
cyclopentadienyl titanium Dichloride, CpTiCl2[IMesN(CS)N-p-
tolyl] (3b). To a solution of 2b (105 mg, 0.220 mmol) in 2 mL of
toluene was added bis(trimethylsilyl)amide (45.2 mg g, 0.250 mmol)
in 1 mL of toluene. The solution was stirred at room temperature for 1
h before a 2 mL toluene solution of cyclopentadienyltitanium(IV)
trichloride (50.4 mg, 0.220 mmol) was added. The reaction mixture
was stirred for 1 h and filtered, and the volatiles were removed under
reduced pressure. The recovered brownish-red solid was purified by
slow diffusion using dichloromethane and n-pentane to afford a
spectroscopically pure red solid (93.1 mg, 0.140 mmol, 65%). 1H
1
combination of any of these. Where 13C spectra are reported, the H
1
and 13C assignments were confirmed by two-dimensional H−1H and
13C−1H correlation NMR experiments. The proton or carbon
attributed to the resonance is sometimes italicized for clarity. FTIR
spectra were recorded on a Thermo Scientific Nicolet 6700 FT-IR
spectrometer. Elemental composition was determined by Guelph
Chemical Laboratories Incorporated.
3
NMR (400 MHz, C6D6): δ 7.12 (d, JHH = 8.3 Hz, 2H, o-CH(p‑tolyl)),
N-(1,3-Dimesitylimidazol-2-ylidene)-N′-(p-tolyl)urea, IMesN-
(CO)N(H)-p-tolyl (2a). To a toluene solution of 1,3-dimesitylimida-
zol-2-imine (1) (300 mg, 0.940 mmol) was added a 5% excess of p-
tolyl isocyanate (124 μL, 0.990 mmol), and the reaction mixture was
stirred at room temperature for 12 h. The solvent was evaporated in
vacuo, and the recovered beige solid was purified by an n-pentane
wash to afford a spectroscopically pure light beige solid (404 mg, 0.890
3
6.98 (d, JHH = 8.3 Hz, 2H, m-CH(p‑tolyl)), 6.69 (s, 4H, m-CH(mesityl)),
6.18 (s, 5H, C5H5), 5.67 (s, 2H, NCHNCHN), 2.12 (s, 3H, p-
CH3(p‑tolyl)), 2.05 (s, 6H, p-CH3(mesityl)), 2.02 (s, 12H, o-CH3(mesityl)).
13C{1H} NMR (100 MHz, C6D6): δ 169.5 (CS), 147.5
(Cipso(p‑tolyl)), 147.3 (CN), 139.9 (p-CCH3(mesityl)), 135.8 (o-
CCH3(mesityl)), 133.6 (p-CCH3(p‑tolyl)), 131.6 (Cipso(mesityl)), 129.7 (m-
CH(mesityl)), 128.7 (m-CH(p‑tolyl)), 126.6 (o-CH(p‑tolyl)), 119.7 (C5H5),
117.1 (NCHCHN), 21.1 (p-CCH3(p‑tolyl)), 21.0 (p-CCH3(mesityl)), 18.4
(o-CCH3(mesityl)). Anal. Calcd for C34H36Cl2N4STi: C, 62.68; H, 5.57;
N, 8.60. Found: C, 62.42; H, 5.30; N, 8.37.
1
3
mmol, 95%). H NMR (300 MHz, C6D6): δ 7.02 (d, JHH = 8.2 Hz,
2H, o-CH(p‑tolyl)), 6.73−6.70 (m, 6H, m-CH(mesityl) + m-CH(p‑tolyl)),
6.36 (br s, 1H, NH), 5.83 (s, 2H, NCHCHN), 2.27 (s, 12H, o-
CH3(mesityl)), 2.06 (s, 6H, p-CH3(mesityl)), 1.94 (s, 3H, CH3(p‑tolyl)).
13C{1H} NMR (100 MHz, C6D6): δ 158.0 (CO), 149.8 (CN),
Bis[N-(1,3-dimesitylimidazol-2-ylidene)-N′-(p-tolyl)ureato]ti-
tanium Dichloride, TiCl2[IMesN(CO)N-p-tolyl]2 (4a). To a
solution of 2a (104 mg, 0.230 mmol) in 2 mL of toluene was slowly
added n-butyllithium (100 μL, 0.250 mmol). The solution was stirred
at room temperature for 1 h before a 2 mL toluene solution of
TiCl4·2THF (38.2, 0.120 mmol) was added. The yellow solution
gradually turned deep red. The reaction mixture was stirred for 2 h and
filtered, and the volatiles were removed under reduced pressure. The
recovered red solid was purified by crystallization using dichloro-
methane and n-pentane to afford a spectroscopically pure red solid
139.4 (Cipso(p‑tolyl)), 138.3 (p-CCH3(mesityl)), 135.9 (Cipso(mesityl)), 134.0
(o-CCH3(mesityl)), 129.4 (m-CH(mesityl)), 129.4 (p-CCH3(p‑tolyl)), 129.0
(m-CH(p‑tolyl)), 118.2 (o-CH(p‑tolyl)), 115.4 (NCHCHN), 21.0 (p-
CCH3(mesityl)), 20.6 (p-CCH3(p‑tolyl)), 18.2 (o-CCH3(mesityl)). FTIR (cast
film): νCO 1642 cm−1. Anal. Calcd for C29H31N4O: C, 76.96; H, 7.13;
N, 12.38. Found: C, 77.23; H, 6.89; N, 12.15.
N-(1,3-Dimesitylimidazol-2-ylidene)-N′-(p-tolyl)thiourea,
IMesN(CS)N(H)-p-tolyl (2b). To a dichloromethane solution of 1,3-
dimesitylimidazol-2-imine (1) (170 mg, 0.540 mmol) was added a 5%
excess of p-tolyl isothiocyanate (78.8 μL, 0.500 mmol), and the
reaction mixture was stirred at room temperature for 4 h. The solvent
was evaporated in vacuo, and the recovered beige solid was purified by
an n-pentane wash to afford a spectroscopically pure light beige solid
(250 mg, 0.520 mmol, 97%). 1H NMR (400 MHz, C6D6): δ 7.37 (br s,
1H, NH), 6.90 (d, 3JHH = 8.2 Hz, 2H, o-CH(p‑tolyl)), 6.78 (d, 3JHH = 8.2
Hz, 2H, m-CH(p‑tolyl)), 6.67 (s, 4H, m-CH(mesityl)), 5.89 (s, 2H,
NCHCHN), 2.36 (s, 12H, o-CH3(mesityl)), 2.02 (s, 3H, p-CH3(p‑tolyl)),
1.96 (s, 6H, p-CH3(mesityl).). 13C{1H} NMR (100 MHz, C6D6): δ 181.0
(CS), 151.4 (CN), 139.2 (p-CCH3(mesityl)), 138.9 (Cipso(p‑tolyl)),
135.9 (o-CCH3(mesityl)), 132.5 (Cipso(mesityl)), 130.5 (p-CCH3(p‑tolyl)),
129.6 (m-CH(mesityl)), 128.7 (m-CH(p‑tolyl)), 119.9 (o-CH(p‑tolyl)), 117.1
(NCHCHN), 20.9 (p-CCH3(mesityl)), 20.7 (p-CCH3(p‑tolyl)), 18.8 (o-
CCH3(mesityl)). Anal. Calcd for C29H32N4S: C, 74.32; H, 6.88; N, 11.95.
Found: C, 74.11; H, 6.75; N, 12.16.
1
(160 mg, 0.160 mmol, 68%). H NMR (400 MHz, C6D6): δ 7.36 (d,
3
3JHH = 8.3 Hz, 2H, o-CH(p‑tolyl)), 6.82 (d, JHH = 8.3 Hz, 2H, m-
CH(p‑tolyl)), 6.6 (s, 4H, m-CH(mesityl)), 5.72 (s, 2H, NCHCHN), 2.17 (s,
12H, o-CH3(mesityl)), 2.08 (s, 3H, p-CH3(p‑tolyl)), 2.04 (s, 3H, p-
CH3(mesityl)). 13C{1H} NMR (100 MHz, C6D6): δ 167.2 (CO),
149.1 (CN), 145.3 (Cipso(p‑tolyl)), 139.6 (p-CCH3(mesityl)), 135.5
(Cipso(mesityl)), 131.8 (o-CCH3(mesityl)), 131.0 (p-CCH3(p‑tolyl)), 129.8 (m-
CH(mesityl)), 128.1 (m-CH(p‑tolyl)), 123.0 (o-CH(p‑tolyl)), 116.8
(NCHCHN), 21.1 (p-CCH3(mesityl)), 21.0 (p-CCH3(p‑tolyl)), 18.1 (o-
CCH3(mesityl)). FTIR (cast film): νCO 1506 cm−1. Anal. Calcd for
C58H62N8OTiCl2: C, 68.17; H, 6.12; N, 10.96. Found: C, 68.10; H,
5.93; N, 11.15.
Bis[N-(1,3-dimesitylimidazol-2-ylidene)-N′-(p-tolyl)-
thioureato]titanium Dichloride, TiCl2[IMesN(CS)N-p-tolyl]2
(4b). To a solution of 2b (95.0 mg, 0.200 mmol) in 2 mL of toluene
was added sodium bis(trimethylsilyl)amide (40.9 mg, 0.220 mmol) in
2 mL of toluene. The solution was stirred at room temperature for 1 h
before a 2 mL toluene solution of TiCl4·2THF (33.8, 0.110 mmol)
was added. The reaction mixture was stirred for 1 h and filtered, and
the volatiles were removed under reduced pressure. The recovered red
solid was purified by slow diffusion using dichloromethane and n-
pentane to afford a spectroscopically pure red solid (104.6 mg, 0.090
[N-(1,3-Dimesitylimidazol-2-ylidene)-N′-(p-tolyl)ureato]-
cyclopentadienyl titanium Dichloride, CpTiCl2[IMesN(CO)N-p-
tolyl] (3a). To a solution of 2a (103 mg, 0.230 mmol) in 2 mL of
toluene was slowly added n-butyllithium (100 μL, 0.250 mmol). The
solution was stirred at room temperature for 1 h before a 2 mL toluene
solution of cyclopentadienyltitanium(IV) trichloride (76.4 mg, 0.230
mmol) was added. A color change to an orange solution was observed.
The reaction mixture was stirred for 2 h and filtered, and the volatiles
were removed under reduced pressure. The recovered brownish-red
solid was purified by crystallization using dichloromethane and n-
pentane to afford a spectroscopically pure orange solid (102 mg, 0.160
1
3
mmol, 90%). H NMR (400 MHz, C6D6): δ 6.86 (d, JHH = 7.5 Hz,
3
2H, o-CH(p‑tolyl)), 6.81 (d, JHH = 7.5 Hz, 2H, m-CH(p‑tolyl)), 6.71 (s,
4H, m-CH(mesityl)), 5.64 (s, 2H, NCHNCHN), 2.18 (s, 3H, p-
CH3(p‑tolyl)), 2.15 (s, 6H, p-CH3(mesityl)), 1.94 (s, 12H, o-CH3(mesityl)).
13C{1H} NMR (100 MHz, C6D6): δ 174.3 (CS), 148.3
1
3
mmol, 70%). H NMR (400 MHz, C6D6): δ 7.39 (d, JHH = 8.3 Hz,
3
2H, o-CH(p‑tolyl)), 6.94 (d, JHH = 8.2 Hz, 2H, m-CH(p‑tolyl)), 6.65 (s,
(Cipso(p‑tolyl)), 147.2 (CN), 139.4 (p-CCH3(mesityl)), 135.5 (o-
CCH3(mesityl)), 132.2 (p-CCH3(p‑tolyl)), 131.5 (Cipso(mesityl)), 129.7 (m-
CH(mesityl)), 125.2 (o-CH(p‑tolyl)), 116.9 (NCHCHN), 21.3 (p-
CCH3(p‑tolyl)), 21.2 (p-CCH3(mesityl)), 18.3 (o-CCH3(mesityl)). Anal.
Calcd for C58H62Cl2N8S2Ti: C, 66.09; H, 5.93; N, 10.63. Found: C,
65.78; H, 5.75; N, 10.39.
4H, m-CH(mesityl)), 6.18 (s, 5H, C5H5), 5.74 (s, 2H, NCHCHN), 2.12
(s, 12H, o-CH3(mesityl)), 2.09 (s, 3H, p-CH3(p‑tolyl)), 2.00 (s, 6H, p-
CH3(mesityl)). 13C{1H} NMR (100 MHz, C6D6): δ 162.0 (CO),
148.7 (CN), 146.2 (Cipso(p‑tolyl)), 139.7 (p-CCH3(mesityl)), 135.9 (o-
CCH3(mesityl)), 132.2 (Cipso(mesityl)), 131.8 (p-CCH3(p‑tolyl)), 129.7 (m-
CH(mesityl)), 128.7 (m-CH(p‑tolyl)), 123.8 (o-CH(p‑tolyl)), 120.2 (C5H5),
117.1 (NCHCHN), 21.0 (p-CCH3(mesityl)), 20.9 (p-CCH3(p‑tolyl)), 18.0
(o-CCH3(mesityl)). FTIR (cast film): νCO 1524 cm−1. Anal. Calcd for
C34H36N4OTiCl2: C, 64.26; H, 5.71; N, 8.82. Found: C, 64.01; H,
5.55; N, 8.63.
[N-(1,3-Dimesitylimidazol-2-ylidene)-N′-(p-tolyl)ureato]pal-
ladium Allyl, Pd(η3-C3H5)[IMesN(CO)N-p-tolyl] (5a). To a solution
of 2a (112 mg, 0.250 mmol) in 2 mL of toluene was slowly added n-
butyllithium (109 μL, 0.270 mmol). The solution was stirred at room
temperature for 2 h before being added to a 3 mL toluene suspension
I
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of palladium allyl chloride dimer (45.4 mg, 0.124 mmol). The reaction
mixture was stirred for 2 h, filtered, concentrated to approximately 2
mL, and precipitated using 5 mL of n-pentane. The supernatant was
decanted, and the remaining solid was dried under reduced pressure.
The recovered yellow powder was purified by crystallization using
THF and n-pentane to afford a spectroscopically pure yellow solid
CCH3(p‑tolyl)), 122.8 (o-CHp‑tolyl), 116.8 (CH2C(CH3)CH2), 116.6
(NCHCHN), 47.7 (CH2C(CH3)CH2), 46.3 (CH2C(CH3)CH2), 22.9
(CH2C(CH3)CH2), 20.9 (p-CCH3(mesityl + p-CCH3(p‑tolyl)), 18.4 (o-
CCH3(mesityl)). FTIR (cast film): νCO 1645 cm−1. Anal. Calcd for
C33H38N4ONi: C, 70.10; H, 6.77; N, 9.91. Found: C, 69.86; H, 6.52;
N, 10.18.
1
(120 mg, 0.200 mmol, 81%). H NMR (400 MHz, C6D6): δ 7.66 (d,
[N-(1,3-Dimesitylimidazol-2-ylidene)-N′-(p-tolyl)thioureato]
nickel Methylallyl, Ni(η3-C3H4CH3)[IMesN(CS)N-p-tolyl] (6b). To
a solution of 2b (75.0 mg, 0.200 mmol) in 2 mL of THF was added
sodium bis(trimethylsilyl)amide (32.3 mg, 0.210 mmol) in 2 mL of
THF. The reaction mixture was stirred for 1 h before being added to a
2 mL THF solution of nickel methallyl chloride dimer (23.9 mg, 0.10
mmol). The reaction mixture was stirred for 2 h and filtered, and the
volatiles were removed under reduced pressure. The recovered brown
solid was purified by crystallization using THF and n-pentane to afford
a spectroscopically pure orange solid (79.5 mg, 0.106 mmol, 85%). 1H
3
3JHH = 6.6 Hz, 2H, o-CH(p‑tolyl)), 7.01 (d, JHH = 7.6 Hz, 2H, m-
CH(p‑tolyl)), 6.68 (s, 4H, m-CH(mesityl)), 5.80 (s, 2H, NCHCHN), 4.38
3
(m, 1H, CH2CHCH2(allyl)), 3.53 (d, JHH = 6.9 Hz, 1H,
3
CH2CHCH2(allyl)), 2.91 (d, JHH = 5.0 Hz, 1H, CH2CHCH2(allyl)),
3
2.30 (d, JHH = 7.0 Hz, 1H, CH2CHCH2(allyl)), 2.22 (s, 6H, o-
CH3 (mesityl)), 2.21 (s, 6H, o-CH3(mesityl)), 2.15 (s, 3H, p-CH3(p‑tolyl)),
3
2.06 (s, 6H, p-CH3(mesityl)), 1.84 (d, JHH = 6.2 Hz, 1H,
CH2CHCH2(allyl)). 13C{1H} NMR (100 MHz, CDCl3): δ 164.3
(CO), 148.7 (CN), 147.4 (Cipso(p‑tolyl)), 138.4 (p-CCH3(mesityl)),
135.4 (o-CCH3(mesityl)), 133.2 (Cipso(mesityl)), 129.3 (m-CH(mesityl)),
128.8 (m-CH(p‑tolyl)), 127.3 (p-CCH3(p‑tolyl)), 122.6 (o-CH(p‑tolyl)),
116.3 (NCHCHN), 107.4 (CH2CHCH2), 53.1 (CH2CHCH2), 51.6
(CH2CHCH2), 20.9 (p-CCH3(mesityl), 20.9 (p-CCH3(p‑tolyl)), 18.6 (o-
CCH3(mesityl)). FTIR (cast film): νCO 1636 cm−1. Anal. Calcd for
C32H36N4OPd: C, 64.16; H, 6.06; N, 9.35. Found: C, 64.10; H, 5.87;
N, 9.12.
3
NMR (400 MHz, C6D6): δ 7.28 (d, JHH = 8.3 Hz, 2H, o-CH(p‑tolyl)),
3
7.00 (d, JHH = 8.1 Hz, 2H, m-CH(p‑tolyl)), 6.70 (s, 2H, m-CH(m3esityl)),
6.63 (s, 2H, m-CH(mesityl)), 5.74 (s, 2H, NCHCHN), 2.73 (d, JHH
=
3
2.8, 1H, CH2C(CH3)CH2(allyl)), 2.41 (d, JHH = 2.6 Hz, 1H,
CH2C(CH3)CH2(allyl)), 2.29 (s, 6H, o-CH3(mesityl)), 2.20 (s, 6H, o-
CH3(mesityl)), 2.18 (s, 3H, p-CH3(p‑tolyl)), 1.99 (s, 6H, p-CH3(mesityl)),
1.70 (s, 1H, CH2C(CH3)CH2(allyl)), 1.69 (s, 3H, CH2C(CH3)-
CH2(allyl)), 1.51 (s, 1H, CH2C(CH3)CH2(allyl)). 13C{1H} NMR (100
MHz, C6D6): δ 175.7 (CS), 147.6 (Cipso(p‑tolyl)), 146.6 (CN),
139.1 (p-CCH3(mesityl)), 136.0 (o-CCH3(mesityl)), 135.9 (o-CCH3(mesityl)),
132.6 (Cipso(mesityl)), 130.2 (p-CCH3(p‑tolyl)), 129.7 (m-CH(mesityl)), 129.6
(m-CH(mesityl)), 123.6 (o-CH(p‑tolyl)), 117.2 (CH2C(CH3)CH2(allyl)),
116.5 (NCHCHN), 53.1 (CH2C(CH3)CH2(allyl)), 44.9 (CH2C(CH3)
CH2(allyl)), 23.5 (CH2C(CH3)CH2(allyl)), 21.1 (p-CCH3(mesityl)), 21.3 (p-
CCH3(mesityl)), 18.7 (o-CCH3(mesityl)), 18.6 (o-CCH3(mesityl)). Anal.
Calcd for C33H38N4SNi: C, 68.17; H, 6.59; N, 9.64. Found: C,
67.89; H, 6.32; N, 9.78.
[N-(1,3-Dimesitylimidazol-2-ylidene)-N′-(p-tolyl)thioureato]-
palladium Allyl, Pd(η3-C3H5)[IMesN(CS)N-p-tolyl] (5b). To a
solution of 2b (50.0 mg, 0.110 mmol) in 2 mL of THF was added
sodium bis(trimethylsilyl)amide (21.0 mg, 0.110 mmol) in 2 mL of
THF. The reaction mixture was stirred for 1 h before being added to a
2 mL THF solution of palladium allyl chloride dimer (19.4 mg, 0.050
mmol). The reaction mixture was stirred for 2 h and filtered, and the
volatiles were removed under reduced pressure. The recovered brown
solid was purified by slow diffusion using THF and n-pentane to afford
a spectroscopically pure red solid (57 mg, 0.094 mmol, 85%). 1H
3
NMR (400 MHz, C6D6): δ 7.52 (d, JHH = 8.1 Hz, 2H, o-CH(p‑tolyl)),
Bis[N-(1,3-dimesitylimidazol-2-ylidene)-N′-(p-tolyl)-
thioureato]nickel, bis[IMesN(CS)N-p-tolyl]Ni (7b). Compound 2b
(51.0 mg, 0.110 mmol) and sodium bis(trimethylsilyl)amide (40.0 mg,
0.210 mmol) were added to a Schlenk tube and dissolved in THF. The
resulting solution was stirred for 1 h. In a separate Schlenk tube, a
solution of bis(cyclooctadiene)nickel(0) (14.8 mg, 0.060 mmol) in
THF was prepared and cooled to −78 °C, and an excess of
chlorobenzene (119 mg, 1.06 mmol) subsequently added. After 10
min, the solution containing the ureate salt was added to the solution
of the nickel precursor and stirred for 18 h. The reaction mixture was
filtered, and the volatiles were removed in vacuo. The recovered dark
green solid was purified by crystallization using THF and diethyl ether
to give a spectroscopically pure green solid (64 mg, 0.07 mmol, 61%
3
7.01 (d, JHH = 8.1 Hz, 2H, m-CH(p‑tolyl)), 6.67 (s, 2H, m-CH(mesityl)),
6.63 (s, 2H, m-CH(mesityl)), 5.78 (s, 2H, NCHCHN), 4.24 (m, 1H,
CH2CHCH2(allyl)), 3.50 (d, 3JHH = 6.8 Hz, 1H, CH2CHCH2(allyl)), 3.24
3
(d, JHH = 7.8 Hz, 1H, CH2CHCH2(allyl)), 2.31 (s, 6H, o-CH3(mesityl)),
3
2.26 (s, 6H, o-CH3(mesityl)), 2.20 (s, 3H, p-CH3(p‑tolyl)), 2.08 (d, JHH
=
4.5 Hz, 1H, CH2CHCH2(allyl)), 1.93 (s, 6H, p-CH3(mesityl)). 13C{1H}
NMR (100 MHz, C6D6): δ 176.8 (CS), 146.4 (Cipso(p‑tolyl)), 146.3
(CN), 138.8 (p-CCH3(mesityl)), 135.7 (o-CCH3(mesityl)), 135.2 (o-
CCH3(mesityl)), 132.4 (Cipso(mesityl)), 131.7 (p-CCH3(p‑tolyl)), 129.4 (m-
CH(mesityl)), 124.1 (o-CH(p‑tolyl)), 116.1 (NCHCHN), 67.6
(CH2CCH2), 59.2 (CH2CCH2), 48.7 (CH2CCH2), 20.8 (p-
CCH3(mesityl)) + p-CCH3(p‑tolyl)), 18.5 (o-CCH3(mesityl)), 18.4 (o-
CCH3(mesityl)). Anal. Calcd for C32H36N4SPd: C, 62.48; H, 5.90; N,
9.11. Found: C, 62.75; H, 5.72; N, 8.86.
1
3
yield). H NMR (400 MHz, C6D6): δ 6.93 (d, JHH = 8.3 Hz, 2H, o-
3
CH(p‑tolyl)), 6.75 (d, JHH = 8.1 Hz, 2H, m-CH(p‑tolyl)), 6.72 (s, 4H, m-
[N-(1,3-Dimesitylimidazol-2-ylidene)-N′-(p-tolyl)ureato]nick-
el Methylallyl, Ni(η3-C3H4CH3)[IMesN(CO)N-p-tolyl] (6a). To a
solution of 2a (84.8 mg, 0.190 mmol) in 2 mL of toluene was slowly
added n-butyllithium (82.5 μL, 0.206 mmol). The solution was stirred
at room temperature for 1 h before being added to a 3 mL toluene
suspension of nickel methallyl chloride dimer (28.0 mg, 0.090 mmol).
A color change from a dark orange to a reddish-orange solution was
observed. The reaction mixture was stirred for 2 h, filtered,
concentrated to approximately 2 mL, and precipitated using 5 mL of
n-pentane. The supernatant was decanted, and the remaining solid was
dried under reduced pressure. The recovered orange powder was
CH(mesityl)), 5.63 (s, 2H, NCHCHN), 2.14 (s, 6H, p-CH3(mesityl)), 2.04
(s, 15H, o-CH3(mesityl) + p-CH3(p‑tolyl)). 13C{1H} NMR (100 MHz,
C6D6): δ 173.3 (CS), 145.0 (Cipso(p‑tolyl)), 143.6 (CN), 138.5 (p-
CCH3(mesityl)), 135.7 (o-CCH3(mesityl)), 132.3 (Cipso(mesityl)), 129.9 (p-
CCH3(p‑tolyl)), 129.2 (m-CH(mesityl)), 124.7 (o-CH(p‑tolyl)), 115.6
(NCHCHN), 20.9 (p-CCH3(mesityl)), 20.6 (p-CCH3(p‑tolyl)), 18.1 (o-
CCH3(mesityl)). Anal. Calcd for C58H64N8S2Ni: C, 69.94; H, 6.48; N,
11.25. Found: C, 70.18 ; H, 6.63; N, 10.98.
General Procedure for Ethylene Polymerization. Polymerization
trials were performed under 1 atm of ethylene at room temperature in
a Schlenk-type flask containing a magnetic stir bar. The flask was
conditioned at 160 °C for at least 12 h prior to use. The flask was
evacuated, backfilled with ethylene, and charged with toluene (20 mL)
and methylaluminoxane (2 M Al in toluene; 1000 equiv of Al). After
vigorously stirring for 15 min under 1 atm of C2H4, the catalyst (9
μmol) dissolved in 1 mL of toluene was added. The reaction mixture
was vigorously stirred for an additional 10 min. The reaction mixture
was quenched with a 50:50 mixture of methanol/HCl. The resulting
mixture was filtered, and any solid collected was washed with distilled
water and dried for 24 h in a vacuum oven. The polymer yields were
determined by gravimetry.
1
spectroscopically pure (98.7 mg, 0.180 mmol, 93%). H NMR (400
MHz, C6D6): δ 7.53 (d, 3JHH = 8.0 Hz, 2H, o-CH(p‑tolyl)), 6.95 (d, 3JHH
= 8.0 Hz, 2H, m-CH(3p‑tolyl)), 6.70 (s, 4H, m-CH(mesityl)), 5.77 (s, 2H,
NCHCHN), 2.82 (d, JHH = 2.5 Hz, 1H, CH2C(CH3)CH2(allyl)), 2.24
(s, 12H, o-CH3 (mesityl)), 2.10 (s, 3H, p-CH3(p‑tolyl)), 2.05 (s, 6H, p-
3
CH3(mesityl)), 1.90 (d, JHH = 3.2 Hz, 1H, CH2C(CH3)CH2(allyl)), 1.72
(s, 1H, CH2C(CH3)CH2(allyl)), 1.68 (s, 3H, CH2C(CH3)CH2(allyl)),
1.10 (s, 1H, CH2CHCH2(allyl)). 13C{1H} NMR (100 MHz, CDCl3): δ
163.1 (CO), 149.5 (CN), 147.6 (Cipso(p‑tolyl)), 138.5 (p-
CCH3(mesityl)), 135.3 (o-CCH3(mesity)l), 135.2 (o-CCH3(mesityl)), 133.2
(Cipso(mesityl)), 129.7 (m-CH(mesityl)), 128.6 (m-CH(p‑tolyl)), 127.0 (p-
J
dx.doi.org/10.1021/om400220v | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Table 2. Crystal Data and Structure Refinement Details for 3a, 4a, 5a, and 7b
3a
4a
5a
7b
empirical formula
fw/g mol−1
cryst size/mm
θ range/deg
h; k; l range
cryst syst
space group
a/Å
C34H36Cl2N4OTi
635.47
C59H61Cl4N8O2Ti
1103.86
C32H36N4OPd
599.05
C62H72N8NiOS2
1068.11
0.26 × 0.15 × 0.12
2.60−27.52
13; 20; −24, 25
monoclinic
P21/n
0.35 × 0.35 × 0.12
2.62−27.62
20; 18; −25, 33
monoclinic
P21/c
0.16 × 0.12 × 0.10
2.73−27.49
11; 18; 30
monoclinic
P21/n
0.22 × 0.20 × 0.13
1.45−27.55
14; 19; 23
triclinic
P1
̅
10.7126(4)
15.6730(5)
19.6255(7)
90
15.8799(2)
14.5184(3)
25.9490(6)
90
8.82619(3)
14.1257(4)
23.4966(9)
90
11.5188(14)
15.159(2)
17.772(2)
96.840(3)
93.540(3)
110.670
b/Å
c/Å
α/deg
β/deg
98.552(2)
90
102.0350(10)
90
93.891(2)
90
γ/deg
Z
4
4
4
2
D
calc/Mg m−3
1.295
1.253
1.361
1.238
F(000)
1328
2308
1240
1136
no. of reflns collected/unique
Rint
22 519/7427
0.0624
48 686/13 332
0.0769
26 613/6681
0.1013
49 175/13 097
0.0403
no. data/restraints/params
final R indices [F2 < 2σ(F2)]: R1, wR2
R indices (all data): R1, wR2
goodness-of-fit on F2
7427/0/386
0.0541, 0.1129
0.1203, 0.1396
1.017
13 332/0/680
0.0566, 0.1299
0.1356, 0.1652
1.019
6681/3/354
0.0703, 0.1665
0.1652, 0.2185
1.021
13 097/0/686
0.0476, 0.1217
0.0695, 0.1337
1.030
X-ray Crystallography. Selected details of the X-ray crystal data and
structure refinement for 3a, 4a, 5a, and 7b 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 as Supporting Information. Crystallographic data
for 3a, 4a, and 5a were collected at the University of Toronto on a
Bruker-Nonius Kappa-CCD diffractometer using monochromated Mo
Kα radiation (λ = 0.71073 Å) at 150 K and were measured using a
combination of ϕ scans and ω scans with κ offsets, to fill the Ewald
sphere. Intensity data were processed using the Denzo-SMN
package.39 Absorption corrections were carried out using SORTAV.40
Data for 3b′ and 7b were collected on a Bruker Kappa APEX-DUO
diffractometer using a copper ImuS (microsource) tube with multi
layer optics and a monochromated Mo-Kα radiation (Bruker
Triumph), respectively. Data were measured using a combination of
ϕ scans and ω scans, and were processed using APEX2 and SAINT.41
Absorption corrections were carried out using SADABS.41 Structure
3b′ was solved and refined using OLEX2 (v. 1.2)42 with SHELXS-97
for full-matrix least-squares refinement that was based on F2.43
Compound 3a, 4a, 5a and 7b was solved using Superflip44 and refined
using WinGX45 with SHELXS-97 for full-matrix least-squares
refinement that was based on F2.43 All H atoms were included in
calculated positions and allowed to refine in riding-motion
approximation with Uiso tied to the carrier atom.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
G.G.L. gratefully acknowledges financial support from the
Ontario Ministry of Research and Innovation for an Early
Researcher Award, GreenCentre Canada for a Proof-of-
Principle Grant, the Natural Sciences and Engineering Research
Council for a Discovery Grant and a Research and Tools
Instrumentation Grant, the Canadian Foundation for Innova-
tion and the Ontario Research Fund for infrastructure Funding,
and York University for start-up funds. The authors wish to
thank Dr. Alan J. Lough of the University of Toronto (Toronto,
Ontario) and Dr. James F. Britten of McMaster University
(Hamilton, Ontario) for X-ray data acquisition and for
assistance in solving and refining the solid-state structures.
The authors also wish to thank Jameel Al Thagfi of York
University for performing DFT calculations on ureate and
thioureate complexes and generating their simulated IR spectra.
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ASSOCIATED CONTENT
■
S
* Supporting Information
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reference numbers 928501−928505). Crystallographic data in
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K
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