Titanium(iv) imido complexes of imine imidazol-2-imine ligands

Article abstract of DOI:10.1039/c2dt31004j

Free imine imidazol-2-imine ligands with two different substitution patterns have been isolated for the first time and they were found to exist as an equilibrium mixture of geometric and mesomeric isomers. The relative ratios of these isomers are dependen

Full text of DOI:10.1039/c2dt31004j

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PAPER
Titanium(IV) imido complexes of imine imidazol-2-imine ligands†
Sarim Dastgira,b and Gino G. Lavoie*a
Received 27th March 2012, Accepted 30th May 2012
DOI: 10.1039/c2dt31004j
Free imine imidazol-2-imine ligands with two different substitution patterns have been isolated for the
first time and they were found to exist as an equilibrium mixture of geometric and mesomeric isomers.
The relative ratios of these isomers are dependent on both the nature of the substituents and of the solvent.
The synthesis of the titanium(IV) alkyl and arylimido complexes of these ligands was unexpectedly found
to be very selective and was successfully achieved only with the lesser sterically-demanding 2,4,6-
trimethylphenyl derivative IMesN^Imine 2a. The solid-state structure of the alkylimido complex further
confirms the zwitterionic character of the ligand. The isolated titanium imido complexes were found to be
active catalysts for the polymerisation of ethylene.
Introduction
Transition metal complexes containing metal nitrogen multiple
bonds play critical roles in a number of biological, industrial and
catalytic processes.1–8 For example, metal imido (MvNR) com-
plexes based on group 4 transition metals undergo a wide variety
of organic transformations such as [2 + 2] cycloaddition,3,9
metathesis,9–11 insertion,3,12,13 C–H bond activation3,14 and
1,2-addition reactions such as hydroamination.9,14–18 In all these
cases, the polarisable metal(IV)–imido bond is responsible for
enhanced reactivity that can be controlled through the coordi-
nation geometry and electronics of the metal centre. In addition,
imido groups have been also used as robust ancillary ligands in
ring-opening metathesis polymerisation11 and Ziegler–Natta
olefin polymerisation.3,19
Scheme 1 Mesomeric structures for imidazol-2-imine.
Since the discovery of a stable N-heterocyclic carbene (NHC)
by Arduengo et al.,20 the chemistry of these potent ligands has
witnessed an extensive growth due to their similarities to tertiary
phosphines in terms of thermodynamics and coordination
chemistry.21–24 More recently, these carbenes have been used in
reactions with organic azides to produce imidazol-2-imines.25,26
Deprotonation of these cyclic guanidine analogues yields a new
class of highly basic monoanionic ligands that arises from the
ability of the 5-membered imidazole ring to stabilise the result-
ing positive charge (Scheme 1).27,28 The ligand can thus be con-
Scheme 2 Isomeric structures for imine imidazol-2-imine ligands.
amphielectronic nature, formally donating as many as six
(2σ and 4π) electrons to the metal centre (Scheme 1).29,30
sidered
isoelectronic
with
phosphinimides
(R3PN−),
We have recently exploited the concept and developed a new
family of ligands based on the imine imidazol-2-imine scaffold
A (Scheme 2). Although the ligand is overall neutral, both exo-
cyclic nitrogen atoms are electron-rich, as illustrated by the
mesomeric structures B and C. The ligand scaffold is analogous
to amidines, where one of the nitrogen atoms has been substi-
tuted with an imidazol-2-ylidene fragment allowing for further
tailoring of the ligand electronics. Moreover, this fragment effec-
tively shifts the steric bulk from the first to the second coordi-
nation sphere, leading to a more open metal centre that is still
protected from bimolecular decompositions. This shift emulates
the steric feature typically offered by cyclopentadienyl, bulky
cyclopentadienyls (C5R5−), bulky alkoxides (R3CO−), siloxides
(R3SiO−), and even with imido ligands (RN−2) thanks to its
aDepartment of Chemistry, York University, 4700 Keele Street, Toronto,
Ontario M3J 1P3, Canada. Fax: +1 416 736 5936;
Tel: +1 416 736 2100, ext. 77728
bDepartment of Chemistry, College of Science, Sultan Qaboos
University, P.O. Box 36, Muscat 123, Sultanate of Oman
†Electronic supplementary information (ESI) available: NMR spectra
for 2a and 2b. Crystallographic data for 4a, CCDC reference number
842511. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt31004j
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further supported by variations in relative ratios of isomers
observed during variable-temperature NMR experiments.
The major isomer of 2a shows characteristic resonances in
benzene-d6 for the mesityl para- and ortho-CH3 protons in a
1 : 2 ratio at δ 2.08 and 2.25, whereas the other two minor
isomers show two sets of resonances at δ 2.12 and 1.91, and at δ
2.25 and 2.11. The resonances arising from the iminic methyl
protons for the major isomer were observed at δ 1.54, while the
corresponding resonances for the two minor isomers were
observed at δ 1.80 and 1.91, as unresolved resonances. The
benzylic protons of the 2,6-dimethylphenyl ring appeared as a
singlet at δ 1.77 for the major isomer and as broad unresolved
singlets with the expected integration at δ 2.03 and 1.77 for the
minor ones. The characteristic higher frequency resonances for
the imidazole backbone (–NCHCHN–) protons were observed at
δ 5.81 and at δ 5.70 and 5.64 for the major and minor isomers,
respectively.
Scheme 3 Synthesis of imine imidazol-2-imine ligand precursors.
alkoxide, siloxide, amide and phosphinimide ligand systems and
has led to enhanced catalytic activities in reactions such as olefin
polymerisation mediated by early transition metals.30,31
Considering the importance of metal imido (MvNR) com-
plexes and the critical role of ancillary ligands in controlling
their reactivity, we decided to use our recently reported N-(1-
(2,6-dimethylphenylimino)ethyl)-1,3-bis(aryl)imidazol-2-imine
hydrochloride salts, wherein aryl = 2,4,6-trimethylphenyl and
2,6-diisopropylphenyl, abbreviated as IMesN^Imine·HCl (1a)
and IPrN^Imine·HCl (1b), respectively, as ligand precursors to
prepare titanium(IV) imido complexes. We herein describe the
synthesis and characterisation of the ligand itself and the effect
of the substitution pattern on the coordination to titanium.
1
Unlike the H NMR spectrum in dichloromethane-d2 for 2a,
that for the bulkier 2b shows the presence of only two isomers
1
in a 12 : 1 ratio. Interestingly, the H NMR spectrum in toluene-
d8 for 2b is further simplified and shows the presence of only
one isomer (BZ). A typical AX2 spectrum for the methyl protons
of the isopropyl groups was observed as a set of two doublets at
δ 1.29 and 1.16 each integrating to 12H, whereas the correspond-
ing septet for the methine protons was observed at δ 3.19 with
vicinal coupling of 6.9 Hz. The resonance for the iminic methyl
protons was observed at δ 1.40, while those for the benzylic
protons of the 2,6-dimethylphenyl ring appeared as a singlet at δ
1.62. The characteristic downfield resonance for the imidazolium
backbone (–NCHCHN–) was observed at δ 6.01.
Results and discussion
Synthesis and characterisation of IMesN^Imine (2a) and
IPrN^Imine (2b)
In the 13C{1H} NMR spectrum for 2a (benzene-d6), the exo-
cyclic iminic carbon resonate at the characteristic high frequency
shifts at δ 158.9 (major isomer) and 152.9 (minor isomer)
respectively, whereas the corresponding resonance for the single
isomer of 2b (toluene-d8) was observed at δ 157.5. The 13C{1H}
NMR resonance for the exocyclic iminic carbon for the third
isomer of 2a was not observed presumably due to the combi-
nation of its long relaxation time, quaternary nature and low rela-
tive contribution (6%) in the equilibrium mixture. The central
imidazol-2-imine carbon for 2a (major isomer) and 2b was
observed at δ 148.7 and 148.6, respectively. The corresponding
resonances for the minor isomer of 2a appeared at δ 143.1 and
151.9. Full 1H and 13C{1H} assignments for 2a and 2b are
detailed in the Experimental section.
The protonated ligands IMesN^Imine·HCl (1a) and IPrN^
Imine·HCl (1b) were obtained as air-stable white solids by reaction
of the corresponding imidazol-2-imine and imidoyl chloride in
toluene (125 °C) for 12 h.32 Deprotonation of the amidinium
salts 1a and 1b with NaOtBu in THF cleanly produces the imine
imidazol-2-imine products as moisture-sensitive white solids in
74% (2a) and 90% (2b) yields, respectively (Scheme 3). While
1
1a exists as one single species in solution,32 the H NMR spec-
trum of IMesN^Imine (2a) shows evidence of three isomers in
relative ratios that are highly solvent-dependent. As such, the
three isomers are present in 12 : 4 : 1 and 4 : 3 : 1 ratios in
benzene-d6 and in dichloromethane-d2, respectively. Considering
that the X-ray structures of 1a and 1b, and of the corresponding
titanium and palladium complexes show little double bond char-
acter between the azole ring central carbon atom and the exocyc-
lic nitrogen,32 the three isomers are believed to be E- and Z-
conformers of the zwitterionic structures B and C, as defined in
Scheme 2. A series of NMR correlation experiments using the
well-resolved and unambiguously assigned 13C NMR resonances
for 2a (benzene-d6 and dichloromethane-d2) and 2b (toluene-d8)
allowed us to identify BZ as the major component of the equili-
brium mixture. The exact nature of the minor component could
not however be unequivocally assigned to any of the three
remaining zwitterionic structures (BE, CE and CZ) due to limit-
ations of NMR spectroscopy experiments. Exchange peaks in
the NOESY spectrum suggest that all species are in equilibrium
with each other, albeit slow on the NMR timescale as evidenced
by the absence of line broadening. The dynamic equilibrium is
Synthesis, characterisation and catalytic activity of Ti(IV) imido
complexes
The preparation of the five coordinated titanium(IV) imido com-
plexes of ligand 2a and 2b was first attempted by reaction of
Ti(IMesN^Imine)Cl4 (3a)32 and Ti(IPrN^Imine)Cl4 (3b)32 with
an excess of tBuNH2 (Scheme 4). In a typical experiment, a cold
(CH2Cl2 or toluene) solution of 3a or 3b was treated with 3, 4 or
t
6 equivalents of BuNH2 under conditions similar to those used
for the synthesis of analogous complexes.33–35 Upon workup,
the sample showed complete decomposition of the starting com-
plexes 3a or 3b with the formation of the protonated ligand (1a
or 1b) and other unidentified species. After the initial
9652 | Dalton Trans., 2012, 41, 9651–9658
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Fig. 1 ORTEP drawing (30% probability level) of the molecular struc-
ture of 4a. Hydrogen atoms and dichloromethane molecule omitted for
clarity.
Scheme 4 Synthesis of titanium(IV) tert-butylimido complexes.
1
The H NMR spectrum (chloroform-d) of 4a shows singlet
resonances at δ 0.63 and 1.26 for the tert-butyl and for the
iminic methyl protons, respectively. A prominent high-frequency
chemical shift for the imidazole backbone (–NCHCHN–)
protons was observed at the δ 6.88 and found to be consistent
with the coordination of the ligand, when compared with the cor-
responding resonances observed for 2a. The ortho-CH3 and the
meta-CH protons of the mesityl ring appeared as broad reson-
ances at δ 2.26 and 6.94, respectively, indicating a restricted
rotation around N–Cipso bond. This further translated in broad
resonances in the 13C{1H} spectrum for the ortho-CH3 and the
meta-CH of the mesityl ring at δ 19.3 and 130.4, respectively.
As expected, a high-frequency chemical shift at δ 168.3 was
observed for the endocyclic iminic carbon atom, in agreement
Scheme 5 Synthesis of titanium(IV) arylimido complexes.
1
with coordination of the ligand in a bidentate fashion. Full H
and 13C{1H} NMR assignments are detailed in the Experimental
section.
disappointing results, simple ligand displacement on the readily
prepared [Ti(NtBu)Cl2(NHMe2)2] and [Ti(NtBu)Cl2(py)n] (n = 2
or 3) complexes was investigated. To our surprise, under varying
conditions, the reaction of either 2a or 2b with [Ti(NtBu)-
Cl2(NHMe2)2] only resulted in the protonation and decompo-
sition of the ligand with the formation of unidentified species.
The reaction of ligand 2a with [Ti(NtBu)Cl2(py)n] (n = 2 or 3)
results in the formation of the desired product 4a in 30% yield
by NMR with some other unidentified products. All our attempts
to increase the yield or to isolate the required product from the
reaction mixture failed. Substitution of pyridine for TMEDA in
the imido metal precursor allowed for the isolation of 4a in spec-
troscopically-pure form after solvent evaporation and recrystalli-
sation from dichloromethane/pentane (Scheme 5). Unfortunately,
all our attempts to react ligand 2b in similar ways to form the
corresponding titanium imido complex failed to show any ligand
displacement reaction with either [Ti(NtBu)Cl2(py)n] or [Ti-
(NtBu)Cl2(TMEDA)] and only led to slight decomposition of
the ligand and of the imido precursors. This selectivity in the
ligand coordination is presumably due to the interaction of the
steric bulk of the ligand in the second coordination sphere with
the sterically demanding tert-butylimido group.
The coordination environment around the titanium(IV) centre
for 4a was elucidated by X-ray diffraction studies on single crys-
tals grown at room temperature by layering a saturated dichloro-
methane solution with pentane. The solid-state structure (Fig. 1)
confirms the bidentate coordination mode of the ligand to form a
distorted square-pyramidal geometry metal complex, with the
imido group formally occupying the apical position and the
chloride ligands cis with respect to each other. The metal centre
is displaced from the basal plane formed by Cl1, Cl2, N3 and
N4 by 0.666 Å. Selected bond lengths and angles for 4a are
summarised in Table 1, along with the corresponding values for
1a and 3a.18 The bond angles around the titanium atom are in
the range of 50.35 to 99.05°, with the smallest angle attributed
to the amidinate N3–Ti–N4 bite angle and the largest one to the
Cl1–Ti–Cl2 angle. The lengthening of the C4–N3 (1.363(4) Å)
and the shortening of C4–N4 (1.299(5) Å), along with the
reduction of the N1–C1–N2 bond angle to 106.4° compared to
that observed for 2a, are due to the delocalisation of the electron
density over the C1–N3–C4–N4 π-system. Similar trends were
observed in the solid-state structure of 3a, which also shows
bidentate coordination of the ligand.32 These changes in bond
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Table 1 Comparison of selected bond lengths (Å) and bond angles (deg) for 4a with those reported for 1a and 3a
IMesN^Imine·HCl (1a)a
Ti(IMesN^Imine)Cl4 (3a)a
Ti(IMesN^Imine)2(NtBu)Cl2 (4a)
Bond lengths (Å)
C1–N1, C1–N2
N1–C2, N2–C3
C2–C3
N1–Cipso, N2–Cipso
C1–N3
1.354(4), 1.356(4)
1.396(4), 1.397(4)
1.336(5)
1.448(4), 1.450(3)
1.365(5), 1.349(5)
1.386(5), 1.400(5)
1.341(6)
1.455(5), 1.449(5)
1.354(5)
1.359(4), 1.361(4)
1.391(5), 1.393(5)
1.339(5)
1.447(5), 1.448(4)
1.340(4)
1.338(4)
C4–N3, C4–N4
Ti–N5
Ti–N3, Ti–N4
Ti–Cl(trans to N3)
Ti–Cl(trans to N4)
Bond angles (deg)
N1–C1–N2
N1,2–C1–N3
C1–N3–C4
N3–C4–N4
1.311(4), 1.330(4)
1.361(5), 1.312(5)
—
2.253(3), 2.106(3)
2.2480(13)
1.363(4), 1.299(5)
1.678(3)
2.267(3), 2.172(3)
2.3375(12)
—
—
—
—
2.2913(13)
2.3371(12)
109.0(3)
123.3(3), 129.6(3)
122.7(3)
118.8(3)
125.3(3)
—
106.5(3)
127.2(4), 126.2(4)
122.3(4)
110.7(4)
122.3(3)
—
106.4(3)
123.5(3), 130.0(3)
125.5(3)
111.3(3)
121.3(3)
C4–N4–C6
C32–N5–Ti
160.6(3)
a Ref. 32.
lengths and the 52.02° angle formed between the imidazole ring
and the diazametallacyclobutene further support zwitterionic
character of the ligand through enhanced delocalisation of the
π-electrons over the entire imine imidazol-2-imine framework.
In contrast to compound 3a, the ability of the apical imido
ligand to act as a 2σ,4π-donor ligand to the highly electrophilic
d0 titanium(IV) centre in 4a likely leads to a decrease in
π-donation from the basal chlorine and nitrogen donors. This
results into a marked elongation of the corresponding Ti–Cl and
Ti–N bonds (Table 1). Such electronic effects are further sup-
ported by the short Ti–N5 bond length (1.678(3) Å) for 4a,
which is indicative of significant TiuN triple bond character,
and are in agreement with theoretical models and symmetry
arguments presented in previous studies.1,36,37 The Ti–N5 bond
is shorter than the corresponding bond in either Heyduk’s (tmp-
BIAN)Ti(vNtBu)Cl2 (1.690(4) Å)35 or Mountford’s (py)3Ti-
(vNtBu)Cl2 (1.705(3) Å)34 complexes but is comparable to that
observed in Winter’s (TMEDA)Ti(vNtBu)Cl2 (1.662(4) Å)38
complex. The slightly non-linear Ti–N5–C32 bond angle of
160.6(3)°, in comparison to the corresponding bond angle of
164.1(4)° observed in both [(tmp-BIAN)Ti(vNtBu)Cl2]20 and
[(TMEDA)Ti(vNtBu)Cl2],23 is probably due to the large repul-
sion between the tert-butyl imido group and the steric bulk in
the second coordination sphere from the mesityl and xylyl sub-
stituents on the bidentate ligand.39,40 This repulsion results in the
tert-butyl group tilting towards the chloride ligands, with C34
and C35 approximately eclipsing Cl1 and Cl2, respectively,
when the molecule is viewed along the Ti1–N5 bond. The
angles between the basal plane and the best plane formed by
either the imidazole ring or the 2,6-dimethylphenyl group are
comparable at 71.8° and 73.2°, respectively. The average angle
between all four apical planes containing the titanium imido
(TivN3) bond and the basal plane is 87.9°. All the remaining
bond lengths and angles are unexceptional and lie within the
expected range.
thermodynamically favourable due to a greater reactivity of the
tert-butylimido group compared to that of the resulting arylimido
complexes.34,35,41–45 To synthesise the corresponding aryl imido
complex 5a, we therefore attempted the reactions of 4a with
aniline, 2,4,6-trimethylaniline and 2-tert-butylaniline under
various conditions (Scheme 5). To our surprise, the tert-butyl-
imido/arylamine exchange did not readily proceed under reported
conditions34,35 and further attempts to promote the exchange by
prolonged heating in different solvent systems only resulted in
the formation of the protonated imidazol-2-imine with some un-
identified species. Similar to our approach to prepare 4a, reaction
of 2a with the TMEDA adduct of Ti(vN-2-tBuPh)Cl2 in
46
benzene at room temperature produced the targeted titanium
complex Ti(IMesN^Imine)Ti(vN-2-tBuPh)Cl2 (5a) as a faint
orange solid in 78% yield. Our attempts to synthesise the analo-
gous Ti(IPrN^Imine)Ti(vN-2-tBuPh)Cl2 (5b) in a similar way
were not successful, again suggesting a greater repulsion
between the imido substituents and the more sterically demand-
ing bis-1,3-(2,6-diisopropylphenyl)imizol-2-imine moiety.
The substituents on imido ligands have been shown to have a
significant impact on the polymerisation activities of related tita-
nium dichloride complexes of neutral four- and six-electron
donor ligands, with some complexes being completely inactive
and others producing polyethylene at a rate of 10 000 kg mol−1
h−1 bar−1 46,47
The catalytic activities for compounds 4a and 5a
.
towards ethylene polymerisation in toluene were thus evaluated
at atmospheric pressure and room temperature, with 1000
equivalents of methylaluminoxane. While complexes 4a and 5a
showed relatively low activities for ethylene polymerisation,
with respective rates of 6.5 and 7.8 kg PE mol−1 h−1 bar−1
,
our initial results demonstrate the impact of the imido
substituents on the performance of the catalysts, with an
increase in catalyst activities observed upon replacing the
electron-rich tert-butyl group with the electron-poor and
sterically-demanding 2-tert-butylphenyl group. In both cases, the
formation of soluble waxes or low molecular weight oligomers
was not observed.
Previous studies using early and late transition metals
have shown that tert-butylimido/arylamine exchange is
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Synthesis of ligand precursors
Summary
We have isolated and characterised a family of free imine imida-
zol-2-imine ligands. An interesting solvent dependent equili-
brium between the different geometric and mesomeric structures
for the ligands was observed by NMR spectrometry. The syn-
thesis of titanium(IV) imido complexes was unexpectedly found
to be very selective and was successfully achieved only with the
lesser sterically-demanding 2,4,6-trimethylphenyl derivative
IMesN^Imine 2a.
N-(1-(2,6-Dimethylphenylimino)ethyl)-1,3-bis(2,4,6-trimethyl
phenyl)imidazol-2-imine; IMesN^Imine (2a). To a suspension
of IMesN^Imine·HCl (4.30 g, 8.61 mmol) in THF (40 mL) at
−78 °C was added a cold THF (25 mL) solution of sodium tert-
butoxide (868 mg, 9.03 mmol). The reaction mixture was stirred
for 30 min at this temperature, and then slowly warmed to room
temperature and stirred for an additional 4 h. During this time,
the color of the reaction mixture changed to pale yellow. The
mixture was subsequently filtered through a plug of Celite. Vola-
tiles were removed at reduced pressure and the resulting solid
was extracted with toluene (30 mL). The desired product was
recovered by removing the solvent in vacuo. Yield: 2.94 g
(74%).
Experimental
General comments
1H NMR (600 MHz, C6D6): Major isomer (72%): δ 7.03 (d,
2H, 3J = 7.4 Hz, m-CH(2,6-xylyl)), 7.03 (t, 1H, 3J = 7.4 Hz,
p-CH(2,6-xylyl)), 6.69 (s, 4H, m-CH(mesityl)), 5.81 (s, 2H,
–NCHCHN–), 2.25 (s, 12H, o-CH3(mesityl)), 2.08 (s, 6H, p-
CH3(mesityl)), 1.77 (s, 6H, o-CH3(2,6-xylyl)), 1.54 (s, 3H,
All manipulations of air and/or moisture sensitive materials were
carried out under an inert atmosphere of dinitrogen using stan-
dard Schlenk vessel techniques, or in an inert-atmosphere glove-
box containing dinitrogen. Dinitrogen used for the Schlenk
manifolds was further purified by passage through columns
filled with molecular sieves (4 Å) and manganese(II) oxide sus-
pended on vermiculite. Solvents used in the preparation of air
and/or moisture sensitive compounds were dried using an
MBraun Solvent Purification System fitted with alumina
columns. Solvents were deoxygenated by bubbling dry dinitro-
gen through the dried solvents for twenty minutes before use.
Solvents and solutions were transferred through stainless steel
cannulae using a positive pressure of dinitrogen. Deuterated sol-
vents were degassed using three freeze–pump–thaw cycles and
were vacuum distilled from sodium (C6D6 and toluene-d8) or
CaH2 (CDCl3, CD2Cl2 and CD3CN) and stored in an inert-
atmosphere glovebox. Filtrations of air and/or moisture-sensitive
compounds were achieved by using modified stainless steel can-
nulae fitted with glass fibre filter discs at one end. All glassware
and cannulae were dried at 120 °C for 24 h before use. NMR
spectra were recorded on a Bruker DRX 600 (1H at 600 MHz,
13C at 150 MHz), Bruker AV 400 (1H at 400 MHz, 13C at
100 MHz) or Bruker AV 300 (1H at 300 MHz, 13C at 75 MHz)
spectrometer 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. Micro-
analyses were performed either by ANALEST of the Department
of Chemistry, University of Toronto or by Guelph Chemical Lab-
oratories, Guelph, Ontario, Canada, N1G 5G5. Exact masses
were determined by the AIMS Laboratory of the Department of
Chemistry, University of Toronto or by the microanalytical lab-
oratory of the Department of Chemistry, McMaster University.
All reagents were purchased from Aldrich or Alfa Aesar with
the exception of TiCl4, which was purchased from BDH. They
were used as received with the exception of NaOtBu, which was
sublimed and kept in an inert-atmosphere glovebox. Methyl-
aluminoxane (MAO) was graciously donated by Albemarle Corp.
IMes·HCl,48 IPr·HCl,48 IMes,49 IPr,49 imidazol-2-imines,28
3
CH3(imine)); Minor isomer 1 (22%): δ 7.03 (d, 2H, J = 7.3 Hz,
m-CH(2,6-xylyl)), 7.03 (t, 1H, 3J = 7.3 Hz, p-CH(2,6-xylyl)), 6.76 (s,
4H, m-CH(mesityl)), 5.64 (s, 2H, –NCHCHN–), 2.12 (s, 6H,
p-CH3(mesityl)), 2.03 (s, 6H, o-CH3(2,6-xylyl)), 1.91 (s, 12H, o-
CH3(mesityl)), 1.80 (s, 3H, CH3(imine)); Minor isomer 2 (6%): δ
7.12 (t, 2H, 3J = 7.6 Hz, m-CH(2,6-xylyl)), 7.10 (d, 1H, 3J =
7.6 Hz, p-CH(2,6-xylyl)), 6.76 (s, 4H, m-CH(mesityl)), 5.70 (s, 2H,
–NCHCHN–), 2.25 (s, 6H, p-CH3(mesityl)), 2.11 (s, 12H, o-
CH3(mesityl)), 1.91 (s, 3H, CH3(imine)), 1.77 (s, 6H, o-CH3(2,6-xylyl)).
1H NMR (400 MHz, CD2Cl2): Major isomer (49%): δ 6.89
(s, 4H, m-CH(mesityl)), 6.73–6.69 (br m, 2H, m-CH(2,6-xylyl)), 6.54
3
(t, 1H, J = 7.4 Hz, p-CH(2,6-xylyl)), 6.46 (s, 2H, –NCHCHN–),
2.26 (s, 6H, p-CH3(mesityl)), 2.17 (s, 12H, o-CH3(mesityl)), 1.42 (s,
6H, o-CH3(2,6-xylyl)), 1.22 (s, 3H, CH3(imine)); Minor isomer 1
(38%): δ 6.91 (s, 4H, m-CH(mesityl)), 6.73–6.69 (br m, 2H,
m-CH(2,6-xylyl)), 6.60–6.57 (br m, 1H, m-CH(2,6-xylyl)), 6.41 (s,
2H, –NCHCHN–), 2.30 (s, 6H, p-CH3(mesityl)), 1.91 (s, 12H,
o-CH3(mesityl)), 1.53 (s, 6H, o-CH3(2,6-xylyl)), 1.50 (s, 3H,
CH3(imine)); Minor isomer 2 (13%): δ 6.93 (s, 4H, m-CH(mesityl)),
6.73–6.69 (br m, 2H, m-CH(2,6-xylyl)), 6.60–6.57 (br m,
1H, m-CH(2,6-xylyl)), 6.17 (s, 2H, –NCHCHN–), 2.26 (s,
6H, p-CH3(mesityl)), 2.13 (s, 12H, o-CH3(mesityl)), 1.91 (s, 6H,
o-CH3(2,6-xylyl)), 1.53 (s, 3H, CH3(imine)).
13C{1H} NMR (160 MHz, C6D6): Major isomer (Z-isomer):
δ 158.9 (s, –NC(imine)N–), 150.6 (s, C(2,6-xylyl)), 148.7 (s,
–NC(imidazole)N–), 138.1 (s, p-CH(2,6-xylyl)), 136.2 (s, C(mesityl)),
134.6 (s, o-C(mesityl)), 129.4 (s, m-CH(mesityl)), 128.8 (s, o-
C
(2,6-xylyl)), 120.6 (s, m-C(mesityl) + p-CH(2,6-xylyl)), 115.6 (s,
–NCHCHN–), 20.9 (s, p-CH3(mesityl) + CH3(imine)), 18.2 (s, o-
CH3(mesityl) + o-CH3(2,6-xylyl)); Minor isomer 1: δ 152.9 (s,
–NC(imine)N–), 151.6 (s, C(2,6-xylyl)), 143.1 (s, –NC(imidazole)N–),
138.6 (s, p-CH(2,6-xylyl)), 136.2 (s, C(mesityl)), 133.2 (s, o-
C
(mesityl)), 129.5 (s, m-CH(mesityl)), 127.1 (s, o-C(2,6-xylyl)), 120.8
(s, m-CH(2,6-xylyl) + p-CH(2,6-xylyl)), 114.7 (s, –NCHCHN–), 23.9
(s, CH3(imine)), 21.0 (s, p-CH3(mesityl)), 18.8 (s, o-CH3(2,6-xylyl)),
(1a),32
IPrN^Imine·HCl
(1b),32
18.1 (s, o-CH3(mesityl)); Minor isomer 2:
δ 151.9 (s,
IMesN^Imine·HCl
TiCl4(THF)2,50 Ti(NtBu)Cl2(TMEDA),38 Ti(N-2-tBuPh)Cl2
(TMEDA)46 and N-(2,6-Dimethylphenyl)acetimidoyl chloride51
were prepared using published procedures.
–NC(imidazole)N–), 143.1 (s, C(mesityl)), 151.9 (s, C(2,6-xylyl)),
129.5 (s, m-CH(mesityl)), 128.3 (s, o-C(mesityl)), 127.8 (s,
p-CH(mesityl)), 120.8 (s, m-CH(2,6-xylyl) + p-CH(2,6-xylyl)), 112.5
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(br s, –NCHCHN–), 18.6 (s, p-CH3(mesityl)), 18.2 (s, o-CH3(mesityl)
o-CH3(2,6-xylyl)), 18.1 (s, CH3(imine)).
+
(s, CH3(imine)), 18.1 (s, o-CH3(2,6-xylyl)); Minor isomer: δ
153.9 (s, –NC(imine)N–), 145.6 (s, –NC(imidazole)N–), 131.1
(s, p-CH(2,6-diisopropylphenyl)), 127.3 (s, m-CH(2,6-xylyl)), 124.7
Anal. Calcd for C31H36N4 (%): C, 80.13; H, 7.81; N, 12.06;
Found (%): C, 79.86; H, 7.97; N, 11.88. HRMS (ESI+, CH3CN):
Calculated for C31H36N4, m/z = 464.2948 [M]+; Found:
464.2940 [M]+; FTIR (thin film): νCvN 1616 cm−1, νCvC
(s,
p-CH(2,6-diisopropylphenyl)
)
120.6
(s,
p-CH(2,6-xylyl)),
113.9 (s, –NCHCHN–), 28.9 (s, CH(2,6-diisopropylphenyl)),
25.5 (s, CH3(2,6-diisopropylphenyl)), 23.7 (s, CH3(imine)), 22.4
(s, CH3(2,6-diisopropylphenyl)), 17.3 (s, o-CH3(2,6-xylyl)).
1520 cm−1
.
Anal. Calcd for C37H48N4 (%): C, 80.98; H, 8.82; N, 10.21;
Found (%): C, 81.13; H, 8.63; N, 10.28; HRMS (ESI+, CH3CN):
Calculated for C37H48N4, m/z = 548.3872 [M]+; Found:
548.3879 [M]+; FTIR (thin film): νCvN 1610 cm−1, νCvC
N-(1-(2,6-Dimethylphenylimino)ethyl)-1,3-bis(2,6-diisopropyl
phenyl)imidazol-2-imine; IPrN^Imine (2b). To a suspension of
IPrN^Imine·HCl (6.11 g, 10.5 mmol) in THF (40 mL) at −78 °C
was added a cold THF (25 mL) solution of sodium tert-butoxide
(1.06 g, 11.0 mmol). The reaction mixture was stirred for 30 min
at this temperature, and then slowly warmed to room temperature
and stirred for an additional 4 h. During this time, the color of
the reaction mixture changed to pale yellow. The mixture was
subsequently filtered through a plug of Celite. Volatiles were
removed at reduced pressure and the resulting solid was extracted
with a toluene–pentane (3 : 1) mixture (3 × 30 mL). The desired
product was recovered by removing the solvent in vacuo. Yield:
5.16 g (90%).
1519 cm−1
.
(N-(1-(2,6-Dimethylphenylimino)ethyl)-1,3-bis(2,4,6-trimethyl
phenyl)imidazol-2-imine)-tert-butylimidodichlorotitanium(IV); Ti-
(IMesN^Imine)(NtBu)Cl2 (4a). To a solution of Ti(NtBu)-
Cl2(TMEDA) (61.1 mg, 0.199 mmol) in benzene (2 mL) was
added dropwise a benzene (5 mL) solution of IMesN^Imine
(102 mg, 0.219 mmol) at room temperature. The reaction
mixture changed to a bright orange-red color from the original
lemon yellow. The reaction mixture was stirred for 12 h at this
temperature and was subsequently filtered through a plug of
Celite. Volatiles were removed at reduced pressure and the crude
was redissolved in minimum amount of methylene chloride and
layered with pentane. Yellow needles formed overnight and were
collected, washed with pentane and dried in vacuo. Yield:
107 mg (82%). Single crystals suitable for X-ray diffraction
study were grown by layering a saturated methylene chloride
with pentane at room temperature.
1H NMR (400 MHz, toluene-d8): δ 7.16 (t, 2H, J = 7.7 Hz,
p-CH(2,6-diisopropylphenyl)), 7.05 (d, 4H,
J = 7.8 Hz, m-
CH(2,6-diisopropylphenyl)), 6.92 (d, 2H, J = 7.4 Hz, m-CH(2,6-xylyl)),
6.75 (t, 1H, J = 7.4 Hz, p-CH(2,6-xylyl)), 6.01 (s, 2H,
–NCHCHN–), 3.19 (sept, 4H, J = 6.9 Hz, CH(2,6-diisopropylphenyl)),
1.62 (s, 6H, o-CH3(2,6-xylyl)), 1.40 (s, 3H, CH3(imine)), 1.29 (d,
12H, J = 6.9 Hz, CH3(2,6-diisopropylphenyl)), 1.16 (d, 12H, J =
6.9 Hz, CH3(2,6-diisopropylphenyl)).
1H NMR (400 MHz, CD2Cl2): Major isomer (92%): δ 7.33 (t,
2H, J = 7.7 Hz, p-CH(2,6-diisopropylphenyl)), 7.18 (d, 4H, J =
7.7 Hz, m-CH(2,6-diisopropylphenyl)), 6.69 (d, 2H, J = 7.4 Hz, m-
CH(2,6-xylyl)), 6.52 (t, 1H, J = 7.4 Hz, p-CH(2,6-xylyl)), 6.46 (s, 2H,
–NCHCHN–), 3.04 (sept, 4H, J = 6.9 Hz, CH(2,6-diisopropylphenyl)),
1.32 (s, 6H, o-CH3(2,6-xylyl)), 1.25 (s, 3H, CH3(imine)), 1.17 (d,
12H, J = 7.0 Hz, CH3(2,6-diisopropylphenyl)), 1.14 (d, 12H, J =
7.0 Hz, CH3(2,6-diisopropylphenyl)); Minor isomer (8%): δ 7.40 (t,
2H, J = 7.8 Hz, p-CH(2,6-diisopropylphenyl)), 7.22 (d, 4H, J = 7.8 Hz,
1H NMR (400 MHz, CDCl3): δ 6.94 (br s, 4H, m-CH(mesityl)),
6.94–6.91 (br m, 3H, m-CH(2,6-xylyl) + p-CH(2,6-xylyl)), 6.88 (s,
2H, –NCHCHN–), 2.32 (s, 6H, p-CH3(mesityl)), 2.26 (br s, 12H,
o-CH3(mesityl)), 2.03 (s, 6H, o-CH3(2,6-xylyl)), 1.26 (s, 3H,
CH3(imine)), 0.63 (s, 9H, tert-butyl-CH3).
13C{1H} NMR (100 MHz, CDCl3): δ 168.3 (s, –NC(imine)N–),
146.8 (s, C(2,6-xylyl)), 144.9 (s, –NC(imidazole)N–), 140.2 (s,
p-C(mesityl)), 134.1 (br s, C(mesityl)), 131.6 (s, o-C(mesityl)),
131.1(s, o-C(2,6-xylyl)), 130.4 (br s, m-CH(mesityl)), 128.1 (s, m-
CH(2,6-xylyl)), 125.1 (s, p-CH(2,6-xylyl)), 119.1 (s, –NCHCHN–),
71.5 (s, C(tert-butyl)), 30.4 (s, CH3(tert-butyl)), 21.1 (s, p-
CH3(mesityl)), 19.3 (br s, o-CH3(mesityl)), 18.6 (s, o-CH3(2,6-xylyl)),
18.1 (s, CH3(imine)).
m-CH(2,6-diisopropylphenyl)), 6.59–6.55 (d+ br, 3H, m-CH(2,6-xylyl)
+
p-CH(2,6-xylyl)), 6.25 (s, 2H, –NCHCHN–), 2.69 (sept, 4H, J =
6.9 Hz, CH(2,6-diisopropylphenyl)), 1.35 (s, 6H, o-CH3(2,6-xylyl)), 1.19
(s, 3H, CH3(imine)), 1.06 (d, 12H, J = 6.9 Hz, CH3(2,6-diisopropylphenyl)),
1.02 (d, 12H, J = 7.0 Hz, CH
).
Anal. Calcd for C35H45Cl2N5Ti (%): C, 64.22; H, 6.93;
N, 10.70; Found (%): C, 64.39; H, 6.98; N, 10.58.
3(2,6-diisopropylphenyl)
13C{1H} NMR (100 MHz, toluene-d8):
δ 157.5 (s,
–NC(imine)N–), 150.6 (s, C(2,6-xylyl)), 148.6 (s, –NC(imidazole)N–),
146.8 (s, o-C(2,6-diisopropylphenyl)), 135.1 (s, C(2,6-diisopropylphenyl)),
129.6 (s, p-CH(2,6-diisopropylphenyl)), 128.5 (s, m-C(2,6-xylyl)), 124.2
(s, m-CH(2,6-diisopropylphenyl)), 121.0 (s, p-CH(2,6-xylyl)), 116.7
(s, –NCHCHN–), 29.2 (s, CH(2,6-diisopropylphenyl)), 24.6 (s,
CH3(2,6-diisopropylphenyl)), 23.5 (s, CH3(2,6-diisopropylphenyl)), 21.5 (s,
CH3(imine)), 18.5 (s, o-CH3(2,6-xylyl)).
(N-(1-(2,6-Dimethylphenylimino)ethyl)-1,3-bis(2,4,6-trimethyl
phenyl)imidazol-2-imine)-(2-tert-butylphenylimido)dichloro
titanium(IV); Ti(IMesN^Imine)(N-2-tBuPh)Cl2 (5a)
To
a
solution of Ti(N-2-tBuPh)Cl2(TMEDA) (191 mg,
13C{1H} NMR (100 MHz, CD2Cl2): Major isomer: δ 157.5
(s, –NC(imine)N–), 150.8 (s, C(2,6-xylyl)), 148.6 (s,
–NC(imidazole)N–), 147.1 (s, o-C(2,6-diisopropylphenyl)), 135.1 (s,
0.502 mmol) in benzene (10 mL) was added dropwise a benzene
(10 mL) solution of IMesN^Imine (245 mg, 0.527 mmol) at
room temperature. The color of the reaction mixture turned from
brown to bright orange-yellow. The reaction mixture was stirred
for 12 h at this temperature. The precipitated off-white was
filtered and washed with benzene (5 mL), pentane (5 mL) and
dried under reduced pressure. The crude was redissolved in
methylene chloride (2 mL) and layered with pentane. The
C
(2,6-diisopropylphenyl)),
128.9 (s, o-C(2,6-xylyl)), 127.1 (s, m-CH(2,6-xylyl)), 124.3 (s,
m-CH(2,6-diisopropylphenyl) 120.3 (s, p-CH(2,6-xylyl)), 117.3
129.6
(s,
p-CH(2,6-diisopropylphenyl)),
)
(s, –NCHCHN–), 28.9 (s, CH(2,6-diisopropylphenyl)), 24.7
(s, CH3(2,6-diisopropylphenyl)), 23.3 (s, CH3(2,6-diisopropylphenyl)), 20.9
9656 | Dalton Trans., 2012, 41, 9651–9658
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resulting pale orangish off-white solid was collected, washed
with pentane and dried in vacuo. Yield: 285 mg (78%).
Acknowledgements
1H NMR (400 MHz, CD2Cl2): δ 6.94 (br s, 4H, m-CH(mesityl)),
6.93 (s, 2H, –NCHCHN–), 6.83 (br s, 3H, m-CH(2,6-xylyl) + p-
CH(2,6-xylyl)), 6.81 (d, 1H, J = 7.8 Hz, o-CH(2-t-BuPh)), 6.61
(t, 1H, J = 7.4 Hz, m-CH(2-t-BuPh)), 6.49 (t, 1H, J = 7.4 Hz,
p-CH(2-t-BuPh)), 5.93 (d, 1H, J = 7.7 Hz, m-CH(2-t-BuPh)),
2.25 (br s, 18H, o-CH3(mesityl) + p-CH3(mesityl)), 1.78 (s, 6H,
o-CH3(2,6-xylyl)), 1.33 (s, 3H, CH3(imine)), 1.22 (s, 9H, t-butyl CH3).
GGL 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 Dis-
covery Grant and a Research and Tools Instrumentation Grant,
the Canadian Foundation for Innovation and the Ontario
Research Fund for infrastructure funding, and York University
for start-up funds.
13C{1H} NMR (100.6 MHz, CD2Cl2):
δ
170.4 (s,
–NC(imine)N–), 161.2 (s, C(2-t-BuPh)), 146.9 (s, –NC(imidazole)N–),
143.6 (s, C(2,6-xylyl)), 140.8 (s, o-C(mesityl)), 140.5 (s, o-C2(2-t-BuPh)),
132.2 (s, o-C(2,6-xylyl)), 131.6 (s, m-C3(2-t-BuPh)), 130.8 (s, p-C(mesityl)),
130. 8 (m-CH(mesityl)), 130.2 (br s, C(mesityl)), 128.4 (s, m-
CH(2,6-xylyl)), 125.6 (s, p-CH(2,6-xylyl)), 125.2 (s, m-C5(2-t-BuPh)),
124.7 (s, o-C6(2-t-BuPh)), 124.7 (s, p-C4(2-t-BuPh)), 120.3 (s,
–NCHCHN–), 35.3 (s, C(tert-butyl)), 30.4 (s, CH3(tert-butyl)), 21.2
(s, p-CH3(mesityl)), 19.6 (br s, o-CH3(mesityl)), 18.7 (s, CH3(imine)),
18.6 (s, o-CH3(2,6-xylyl)).
Notes and references
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