Steric factors on unsymmetrical O-hydroxyaryl N-Heterocyclic carbene ligands prevailing the stabilization of single stereoisomer of bis-ligated titanium complexes

Article abstract of DOI:10.1016/j.jorganchem.2018.02.018

Bis-ligated titanium (IV) metal complexes supported by bidentate unsymmetrical o-hydroxyaryl-substituted N-heterocyclic carbene ligands were synthesized and structurally identified. While the direct addition of the doubly deprotonated bulky imidazolidiniu

Full text of DOI:10.1016/j.jorganchem.2018.02.018

Accepted Manuscript
Steric factors on unsymmetrical O-hydroxyaryl N-Heterocyclic carbene ligands
prevailing the stabilization of single stereoisomer of bis-ligated titanium complexes
Coralie C. Quadri, Ralte Lalrempuia, Nils Åge Frøystein, Karl W. Törnroos, Erwan Le
Roux
PII:
S0022-328X(18)30107-4
10.1016/j.jorganchem.2018.02.018
JOM 20317
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 30 October 2017
Revised Date: 9 February 2018
Accepted Date: 13 February 2018
Please cite this article as: C.C. Quadri, R. Lalrempuia, Nils.Å. Frøystein, K.W. Törnroos, E. Le
Roux, Steric factors on unsymmetrical O-hydroxyaryl N-Heterocyclic carbene ligands prevailing
the stabilization of single stereoisomer of bis-ligated titanium complexes, Journal of Organometallic
Chemistry (2018), doi: 10.1016/j.jorganchem.2018.02.018.
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ACCEPTED MANUSCRIPT
Steric Factors on Unsymmetrical O-hydroxyaryl N-
Heterocyclic Carbene Ligands Prevailing the
Stabilization of Single Stereoisomer of Bis-Ligated
Titanium Complexes
Coralie C. Quadri, Ralte Lalrempuia, Nils Åge Frøystein, Karl W. Törnroos, Erwan Le Roux*
Department of Chemistry, University of Bergen, Allégaten 41, N-5007, Bergen, Norway.
Tel: +47 555 89491; E-mail: Erwan.LeRoux@uib.no
Abstract
Bis-ligated titanium(IV) metal complexes supported by bidentate unsymmetrical
o-hydroxyaryl-substituted N-heterocyclic carbene ligands were synthesized and
structurally identified. While the direct addition of the doubly deprotonated bulky
imidazolidinium chloride salts [Dipp,4-RNHC-H]Cl (with Dipp = 2,6-diisopropylphenyl, R
= H (2-hydroxyphenyl), and R = Me (2-hydroxy-4-methyl-phenyl)) with chloro-titanium
precursor favors the formation of single stereoisomer corresponding to the bis-ligated
2
titanium complexes trans-([κ -C,O]-Dipp,4-RNHC)
2
TiCl
(R = H (2-hydroxyphenyl) for 4aH,
2
and R = Me (2-hydroxy-4-methyl-phenyl) for 4aMe), the reactivity with sterically less
hindered imidazolidinium chloride salts [Mes,HNHC-H]Cl and [Dep,HNHC-H]Cl as protio-
ligands (with Mes = 2,4,6-trimethylphenyl and Dep = 2,6-diethylphenyl) did not lead to
4
single stereoisomer, but rearranged into a tetradentate salophen-like complexes cis-([κ -
O,N,N,O]-bis(imidazolidinylidene)TiCl as major isomer via presumably the NHCs
2
dimerization from bis-ligated intermediates. These results combined with topographic
steric maps as well as the buried volume descriptor (%Vbur) indicate that bidentate bulky
N-Dipp-substituted NHC ligands offer some level of steric protection preventing the
formation of other possible bis-ligated (C,O)-NHC-titanium stereoisomers and
constitutional isomers.
Keywords
Titanium; Bidentate ligand; Unsymmetrical N-heterocyclic carbene; Bis-ligated titanium
complex; Ethylene polymerization.

ACCEPTED MANUSCRIPT
1. Introduction
Since the first isolation and characterization of the N-heterocyclic carbene (NHC) by
Bertrand [1] and Arduengo [2], NHCs have gained outstanding importance as ancillary
ligands throughout most of the late transition metals and provided an useful hand of
robust and versatile organometallic compounds for homogeneous catalysis [3].
Although late transition metal complexes show strong metal carbene bonds and slow
dissociation with most of the NHC ligands, many of the early transition and f-block
metals show the opposite trend [4]. Thus, to reduce the tendency of the NHC to
dissociate from transition metal, considerable research efforts have been devoted
toward the studies of multidentate anionic carbon, nitrogen, oxygen and sulfur
functionalized-NHC ligands [3h, 5]. A series of anionic carbon (i.e. phenyl,
cyclopentadienyl, indenyl, fluorenyl), nitrogen (i.e. amido), oxygen (i.e. alkoxide,
aryloxide, enolate) and sulfur (thiolate) groups have been developed to act as tethers for
holding the NHC moiety in a close proximity to oxophilic metal centers [3f, h, 4b, d, 4f-
h]. For example, the use of multidentate oxygen-functionalized NHC ligands of group 4
has proven to be an effective synthetic method for developing robust catalysts for the
oligomerization and (co-)polymerization of olefins [6], hydroamination/cyclization of
primary aminoalkenes [7], ring-opening polymerization (ROP) of rac-lactide [8], and
selective coupling of epoxides and CO to either polycarbonates [9] or cyclic carbonates
2
[9b]. Of particular interest, the bidentate o-hydroxyaryl substituted NHCs are
structurally analogous to bidentate salicylaldimine ligands [10], and are amongst the
most commonly studied ligand classes in coordination chemistry due to their ease of
preparation, ability to stabilize various metals in different oxidation states, and
particularly valuable for the fine-tuning of electronic and steric parameters in a large
variety of catalytic organic and polymerization reactions [11]. Recent investigations on
bis-ligated bidentate alkoxy/aryloxy-functionalized NHCs of group 4, when activated
by MAO (methylaluminoxane), showed to be active in ethylene [6d, f, g, l], ethylene/1-
octene and ethylene/norbordene [6d] (co-)polymerization, and stereoselective in
propylene [6f, g] and styrene polymerization [6l] similarly to the class of salicylaldimine
group 4 catalysts (aka phenoxy-imine, FI, Chart 1) [11e]. Following these leads, we
sought to use bidentate o-hydroxyaryl substituted NHC-based ligands framework that
are analogous to FI ligands in efforts to conceive a new class of catalysts not only for
olefins polymerization, but also for the copolymerization of CO and cyclohexene oxide
2
(CHO) in order to extend our studies on active and highly selective catalysts based on
tridentate bis(aryloxy) NHC ligand sets of group 4 [9]. In addition to complex
2
developed by Grubbs, i.e. cis-([κ -C,O]-Dipp,(3-Ad,5-Me)NHC)
2
TiCl (with 3-Ad, 5-Me = (2-
2
hydroxy-3-(adamant-1-yl)-5-methylphenyl), and a Dipp moiety as N-aryl substituent,
Chart 1) [6d][6l], a similar set of unsymmetrical o-hydroxyaryl NHC ligands was
synthesized bearing sterically less-hindered substituents on the N-aryloxy moieties and
with various N-aryl substituents such as Dipp, Mes and Dep groups, as well as their
coordination behavior to titanium. Preliminary data concerning the ability of these
titanium complexes to initiate the ethylene polymerization are also presented.
2

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Chart 1
2. Results and Discussion
2.1. Synthesis of o-hydroxyaryl substituted imidazolidinium protio-ligands 3a-c
The o-hydroxyaryl substituted imidazolidinium chloride salts [Ar,4-HNHC-H]Cl 3aH (with
Ar = Dipp) and 3c (with Ar = Mes) were prepared according to the established procedure
[12], and [Dipp,4-MeNHC-H]Cl 3aMe and [Dep,4-HNHC-H]Cl 3b by adapting this literature
procedure: ethyl chlorooxoacetate was reacted in THF with the appropriate arylamine a-
c in presence of triethylamine at 0 °C to give the corresponding oxanilic acid esters 1a-c,
which were treated with the suitable aminoalcohols to lead quantitatively to oxalamides
2a-c (Scheme 1). Subsequent reduction of 2a-c with borane THF-adduct and acidification
of ethylenediamine moieties followed by their cyclization with an excess of triethyl
orthoformate affords imidazolidinium protio-ligands 3a-c in high overall yields (Scheme
1). Protio-ligand 3c was further characterized by X-ray crystallography and crystallizes
as a well-separated ion pair as found in other analogous imidazolidinium salt (Fig. S1,
1
Table S1-3) [13]. The H NMR spectra of imidazolidinium salt [Dipp,4-MeNHC-H]Cl 3aMe
and [Dep,4-HNHC-H]Cl 3b display the characteristic chemical resonances of the
CHimidazolidinium proton (singlet at δ 9.05 and 8.96 ppm, respectively) and two broad
triplets for the two geminal pairs of NCH
described as AA’XX’) of the o-hydroxyaryl unsymmetrically substituted
imidazolidinium ring (centered at δ 4.59 ppm with AB = 192 Hz and 4.64 ppm with
2
protons (apparent A
2
B
2
spin system, but better
υ
υ
AB
= 216 Hz, respectively), which are well-in-line with the reported data for 3aH and 3c (Fig.
S2-3) [12]. The 13C NMR spectra of 3aMe and 3b show typical chemical resonances at δ
156.6 and 157.2 ppm in chloroform-d, respectively, corresponding to the CHimidazolidinium
carbons (Fig. S4-5). The IR spectra of both protio-ligands also exhibit stretches at 1624
(νC=N) cm-1 characteristic of the imidazolidinium ring.
3

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Scheme 1. Synthetic route for unsymmetrical o-hydroxyaryl substituted
imidazolidinium salts 3a-c [12].
2.2. Synthesis of bis-ligated o-hydroxyaryl substituted NHC titanium complexes 4aH
and 4aMe
Following the synthetic route reported previously by Grubbs [6d], the o-
hydroxyaryl substituted imidazolidinium salts 3aH and 3aMe were doubly deprotonated
by two equivalent of potassium bis(trimethylsilyl)amide (Kbtsa) for 10 min before to be
treated with a 1M solution of TiCl
in toluene for 1 h leading to complexes 4aH and 4aMe
4
as dark-red solids (Scheme 2). Attempts using other synthetic methods for the syntheses
of NHC-group 4 complexes [8b, 9a-c], such as the direct addition of imidazolidinium
salts to Ti(OiPr) (alcohol elimination route) under different temperature conditions
4
(room temperature to reflux) and solvents (toluene, THF), which should give in
principle similar compounds, did not lead to the bis-ligated titanium complexes. It is
interesting to note that the addition of one equivalent of deprotonated imidazolidinium
salts 3aH or 3aMe to TiCl
4
mainly leads to the formation of bis-ligated NHC complexes of
1
titanium as major products. The H NMR spectra of 4aH and 4aMe show similar patterns,
i.e. the disappearance of the CHimidazolidinium proton and the appearance of a
centrosymmetric multiplet assigned to the two pairs of NCH protons on the NHC ring
2
(Fig. S6-7). The symmetry of the multiplet and the small chemical shift range covered by
the resonances suggest a strongly coupled AA’BB’ spin system where each geminal pair
of spins are magnetically non-equivalent. This pattern is consistent with a higher
symmetry of compounds 4aH/4aMe, either C2h or C2v-symmetry molecule in solution (Fig.
13
S6-7). The C NMR spectra of both complexes 4aH or 4aMe confirm the NHCcarbene ligation
to the titanium central atom with typical chemical resonances of the carbene at δ 206.7
and 206.8 ppm in benzene-d
6
, respectively, (Fig. S8-9) [4g].
4

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Scheme 2. Syntheses of bis-ligated o-hydroxyaryl NHC titanium complexes
.
Single crystals of 4aH were obtained after few days from a saturated solution of the
complex in dichloromethane at -30 °C. The molecular structure of 4aH was determined
by single-crystal X-ray diffraction that confirms the chelation of two aryloxide-NHC
ligands to titanium (Fig. 1). Compound 4aH crystallizes in the space group P2
/n,
1
1
corresponding to a C2h-symmetry molecule which is consistent with the observed H
NMR spectrum in solution. As depicted in figure 1, the structure determined for 4aH
shows that the titanium center adopts a nearly perfect octahedral geometry (Table 1). As
2
previously reported for similar bis-ligated NHC titanium complex, i.e. cis-([κ -C,O]-
Dipp,(3-Ad,5-Me)NHC)
2
TiCl
2
[6d], the two neutral NHC moieties are in trans position to each
2
other ( Ccarbene-Ti-Ccarbene = 180.00(8)°) for complex 4aH and for the complex cis-([κ -C,O]-
Dipp,(3-Ad,5-Me)NHC)
2
TiCl
2
) at 178.0(9)° occupying the axial positions (Table 1 and Table S4-
2
5). However unlike complex cis-([κ -C,O]-Dipp,(3-Ad,5-Me)NHC)
2
TiCl
2
having both two
oxygen and two chloride atoms located in cis position ( OAr-Ti-OAr = 88.35(7)° and Cl-
Ti-Cl = 96.04(3)°, respectively), complex 4aH has the two oxygen and two chloride atoms
in trans to each other ( OAr-Ti-OAr = 180.0° and Cl-Ti-Cl = 180.0°). Contrary to complex
2
cis-([κ -C,O]-Dipp,(3-Ad,5-Me)NHC)
2
TiCl
2
(which crystallizes as a C -symmetry molecule,
2
enantiomer Λ), where the cis-configuration is favored to avoid the steric repulsion
between Dipp and bulky Ad substituents [6d], complex 4aH is lacking of such bulky
substituents, and thus leading to the most thermodynamically stable trans configuration
as recently demonstrated by density functional theory calculation on bis-ligated alkoxy-
functionalized NHC-Zr systems [6f, g]. Due to the more effective way that both NHC
ligands wrap around titanium in 4aH in this trans-configuration, the overall bond
lengths (i.e. Ti-Ccarbene, and at least one of -Cl and –O bonds) are shorter, and one of the
bite angles ( OAr-Ti-Ccarbene) is larger with a less pronounced deviation from planarity
2
( OAr–CAr–N–Ccarbene) for one of the NHC ligands compared to the complex cis-([κ -
C,O]-Dipp,(3-Ad,5-Me)NHC)
2
TiCl (Table 1).
2
5

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Fig. 1 Crystal structure of compound 4aH with anisotropic displacement parameters set
at the 50% probability level. Hydrogen atoms and a co-crystallized solvent
dichloromethane molecule are omitted for clarity. Selected bond distances, bond angles
and torsions angles are listed in table 1. Symmetry code #: 1-x+1, -y+1, -z+1.
Table 1
2
Selected Bond Distances, Bond Angles and Torsion Angles for Bis-Ligated ([κ -C,O]-
Dipp,RNHC)
2
TiCl complexes.
2
trans-
cis-
a
([κ2-C,O]-Dipp,4-HNHC)2TiCl2 (4aH)
([κ2-C,O]-Dipp,(3-Ad,5-Me)NHC)2TiCl2
Bond lengths (Å)
Ti-Ccarbene
2.2597(18)/2.2597(18)
1.8578(12)/1.8579(12)
2.3269(5)/2.3269(5)
2.264(3)/2.275(3)
1.870(1)/1.842(1)
2.337(8)/2.297(6)
Ti-OAr
Ti-Cl
Bond Angles (deg)
C
carbene-Ti-Ccarbene
180.00(8)
180.0
178.0(9)
88.35(7)
O
O
Ar-Ti-OAr
Ar-Ti-Ccarbene
82.40(6)/82.40(6)
180.0
82.45(8)/80.43(8)
96.04(3)
Cl-Ti-Cl
Ar-Ti-Cl
O
90.71(4)/89.29(4)
84.73(4)/95.27(4)
169.14(5)/88.32(5)
88.15(6)/83.54(6)
C
carbene-Ti-Cl
Torsion Angles (deg)
b
O
Ar–CAr–N–Ccarbene
10.65/-10.65
1.95/-19.21
a Ref. [6d].
b OAr–CAr–N–Ccarbene: O1–C1–N1–C3/O1#–C1#–N1#–C3# for 4aH and O1–C21–N2–C1/O2–
C53–N4–C33.
Although the steric hindrance could be conceptualized for both trans and cis bis-
ligated complexes, the availability of solid-state molecular structures allows to calculate
6

ACCEPTED MANUSCRIPT
the percent buried volume (%Vbur, via SambVca 2.0 calculations) and visualize the steric
maps for further comparisons of each substituents on NHC ligands [14]. To make a best
2
comparison of both complexes, the analyses were made only by considering ([κ -C,O]-
NHC)-Ti moiety and assuming a 3.5 Å radius of the sphere around the titanium for a
Ti–Ccarbene length of 2.26 Å. Figure 2 shows minor differences between the two
unsymmetrical NHC moieties with nearly identical steric map and %Vbur (37.0 vs. 36.5);
thus, making it difficult to appreciate the steric profiles of these ligands using a value of
3.5 Å, which best defines the steric hindrance in the first coordination sphere around
the metal. The differences between those two ligands become more marked when
increasing the spherical radius around the metal to 5.0 Å allowing taking in account the
bulky groups not bound to the metal. The C2h-symmetry steric map of the NHC ligand
in 4aH, with %Vbur of 33.4, shows a rather flat steric map with two small bulges located in
the northern and western quadrants (corresponding to the iPr groups of N-Dipp
substituent), and two large hollows which can easily accommodate two Cl atoms in
trans to each other in the empty southern (SW, SE) and northern quadrants (NE). In
2
contrast, the C
2
-symmetry steric map of cis-([κ -C,O]-Dipp,(3-Ad,5-Me)NHC)
2
TiCl with %Vbur
2
of 38.0 shows a non-flat steric map particularly in the south-eastern quadrant imposed
by the upward-pointing Ad substituent and two narrow hollows (SW and NE
quadrants). From these steric maps, it is clear that the trans-configuration observed in
these bis-ligated titanium complexes (4aH and 4aMe) is largely favored by the
embedment of the second NHC ligand bearing a H as ortho substituent on the aromatic
ring, and when replaced by the more hindered Ad substituent, only the cis-
configuration with the second Ad substituent pointing downward (SW quadrant) is
allowed among other configurations.
7

ACCEPTED MANUSCRIPT
2
Fig. 2 Ball-and-stick representations and schemes (left) of bidentate ([κ -C,O]-NHC)-Ti
2
moiety: 1) from cis-([κ -C,O]-Dipp,(3-Ad,5-Me)NHC)
2
TiCl
2
[6d] and 2) from complex 4aH with
and b) 5.0
[15]. The steric maps of these complexes are oriented in a Cartesian frame with a
transverse xy-plane and a +z-axis pointing upward.
their respective steric maps (right) calculated with a sphere radius of: a) 3.5
Å
Å
Following the synthesis route described for the aforementioned complexes 4aH and 4aMe,
the doubly deprotonated o-hydroxyaryl imidazolidinium salts 3b and 3c bearing less
sterically hindered N-substituents (Dep and Mes, respectively) were treated by a
solution of TiCl consistently giving a mixture of products. Although the disappearance
4
of CHimidazolidinium and hydroxyl protons were observed for both mixtures in their
1
respective H NMR spectra, there are numerous overlapping multiple sets of signals
corresponding to one or several major species in solution along with a number of other
minor resonances, which are poorly resolved and nearly indiscernible from the baseline
(Figure S6-7). Although the elemental analyses are reasonably consistent with the
1
following formulation, i.e. bis-ligated dichlorotitanium complexes, the H NMR spectra
of the Dep- and Mes-substituted NHC titanium complexes show very different patterns
compared to the complexes 4aH and 4aMe (Fig. S10-11). The notable differences are
mainly observed in the splitting of resonances assigned to the two pairs of NCH
2
protons into several different proton signals with scalar couplings between all of them
(for N-dep: three multiplets with integral ratios of 2:4:2, and for N-Mes: four multiplets
with integral ratios of 2:2:2:2). This is in contrast to the previously observed AA’BB’ spin
system for 4aH and 4aMe. The 13C NMR spectra of 4b and 4c (Fig. S12-13) contain no
visible Ccarbene resonances in the expected range of those observed for 4aH and 4aMe
1
complexes, and the combined 2D NMR analyses (1H-1H COSY and H-13C HSQC NMR
spectra, Fig. S14-17) indicate that the major species is corresponding only to one
compound in solution for both complexes. Intrigued by the observed 1H NMR signals,
further information was obtained by simulating the NMR spectra of 4b, 4c and 4aH with
the Daisy module of the TopSpin program (Fig. S18-20). For 4c the experimental data
and simulations fit very well with an unambiguous, and close to first-order, AMRX spin
system consisting of four chemically non-equivalent protons. The corresponding
multiplets for 4b are better denoted as an AMNX spin system as two of the multiplets
almost overlap [16]. Taken together, these observations indicate that complexes 4b and
4c do not comprise a mirror plan anymore, contrary to trans-complexes 4aH and 4aMe
with their AA'BB' spin systems (Figure S6-7). All combined results imply that
complexes 4b and 4c have a low symmetry compared to 4aH/4aMe, but still possessing
two-fold symmetry, which is consistent with C
stereoisomers are structurally possible in solution (Scheme S1). Non-symmetric bis-
ligated titanium complexes of C -symmetry (including enantiomers Λ and ∆) were not
2
-symmetry molecules, meaning that four
1
considered here due to the unfavorable coordination mode for such bidentate aryl-
substituted NHC ligands (collision between aryl-N substituents and the NHC ring) and
1
1
the H NMR pattern for the NCH
2
signals should have given rise to eight different H
chemical shifts for the two NCH moieties. Attempts to grow crystals of suitable quality
2
for X-ray analysis were unsuccessful, except for Dep-substituted NHC titanium
complex which crystallized in a saturated solution of toluene at –30 °C. The crystal
structure as outlined in figure 3 reveals that out of the four expected stereoisomers for
2
the bis-ligated titanium complexes cis-([κ -C,O]-dep,4-HNHC)
2
TiCl none are formed, but
2
8

ACCEPTED MANUSCRIPT
4
an unexpected complex cis-([κ -O,N,N,O]-dep,4-Hbis(imidazolidinylidene)TiCl
2
(4b)
containing a folded tetradentate salophen-like ligand wraps around the titanium center
in fac-fac binding mode with two chloride atoms in cis position (Scheme 3).
Scheme 3. Syntheses of 4b-c and proposed intermediates (C -enantiomer Λ of bis-ligated
2
titanium complexes are not shown).
The X-ray structure of 4b shows a distorted octahedral arrangement around the
titanium center, with two oxygen atoms in trans position ( OAr-Ti-OAr angle of
160.00(14)°) and two chloride atoms and two nitrogen atoms located in cis positions to
each other with angles of 105.65(6)° and 76.48(13)°, respectively. The crystal structure of
4b shows an approximate C -symmetrical isomer belonging to a rare class of cis-α-
2
configuration [17] (versus the more stable trans- or cis-β-configurations [18], defined for
tetradentate salen-type complexes), and reveals that the stereochemistry of the titanium
center is Λ (Scheme S2). The bond lengths between Ti–O and Ti–N ca. 1.87 and 2.24 Å
respectively, fall in the range of other known Ti–O/Ti-N bond lengths in cis-α-salen-like
titanium complexes [17c, d, 17f-h], while the Ti-Cl bond lengths (ca. 2.25 Å) are shorter
than those reported for the trans- or cis-
β
-(salen)TiCl complexes (ca. 2.36 Å) [19]. Those
2
observations show that the tetradentate ligand in 4b is folded backward and is more
compact around the titanium atom most likely due to the formation of five-membered
rings (Ti-O-CAr-CAr-N), thus imposing such cis-α-configuration and short bond lengths
contrary to the six-membered rings in typical salen-type complexes. The bond length of
1.345(6) Å for the dimerized Ccarbene-Ccarbene is in the expected range for an isolated
2
double bond and comparable to the only example reported by Lappert; i.e. ([κ -N,N]-
bis(imidazolidinylidene)-Cr(CO)
4
complex with Ccarbene-Ccarbene bond length of 1.337(7) Å
[20]. In analogy with complex 4b featuring similar spectroscopic data (absence of
NHCcarbene signal and same spin systems for the NCH
2
moieties) for the Mes-substituted
4
NHC titanium complex, also probably indicate the formation of the cis-([κ -O,N,N,O]-
Mes,4-Hbis(imidazolidinylidene)TiCl
(4c) as major isomer (Scheme 3), along with two
2
other minor unidentified isomers (estimated by the integration of methyl groups of the
1
N-mesityl substituent by H NMR spectroscopy). Additional experiments in order to get
more information about the other minor isomer(s) were attempted, and it was possible to
cocrystallize 4b crystals along with few other single crystals suitable for X-ray analysis.
9

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The crystal structure of this minor isomer reveals a bis-ligated titanium octahedral
2
complex, i.e. cis-([κ -C,O]-Dep,(4-H)NHC)
2
TiCl 4b’, crystallizing as a non-crystallographic
2
C -symmetry molecule, enantiomer Λ (Scheme 3 and Scheme S1). In this case, contrary
2
2
to cis-([κ -C,O]-Dipp,(3-Ad,5-Me)NHC)
atoms are located in cis positions ( Ccarbene-Ti-Ccarbene = 83.50(7)° and Cl-Ti-Cl =
105.17(3)°, respectively), with the oxygen atoms in trans position ( OAr-Ti-OAr
2
TiCl
2
, both two neutral NHC moieties and two chloride
=
173.25(7)°) (Figure S21 and Tables S8-10). The bond lengths Ti-Ccarbene, Ti-O and Ti-Cl are
2
in the range of those aforementioned for 4aH and cis-([κ -C,O]-Dipp,(3-Ad,5-Me)NHC)
2
TiCl .
2
Unfortunately, due to the low amount of 4b’ further characterizations could not be
achieved.
Fig. 3 Crystal structure of compound 4b (left view along the vertical non-
crystallographic C
2
-axis and right along the horizontal non-crystallographic C -axis)
2
with anisotropic displacement parameters set at the 50% probability level. Hydrogen
atoms are omitted for clarity. Selected bond distances, bond angles and torsions angles
are listed in tables S6-7.
In order to rationalize these results, we envisage that the steric map and the
calculated %Vbur based on the X-ray data obtained for 4b and 4aH (i.e. by replacing the
iPr groups from the N-Dipp substituent in 4aH by Me groups) could adequately mimic
the N-dep and N-Mes substituents, respectively, and give an insight to the trend of
reactivity (Fig. 4 and Fig. S22). Interestingly, the simulated steric maps of this bidentate
NHC bearing N-Dep and pseudo N-Mes’ substituent are substantially flat for both
sphere radii of 3.5 and 5 Å in the NW and SE quadrants (with inferior %Vbur), with two
large hollows in the adjacent quadrants. Through these steric maps and the previous
ones, it can be deduced that the ortho bulky iPr groups on the N-Dipp are the steric
driving force leading mostly to trans-configuration complexes (in absence of ortho bulky
group on the aromatic ring). When those N-aryl substituents are exchanged by less
bulky groups (here Et or Me), there is a lack of steric hindrance to inhibit the formation
10

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of several species, which is leading most likely in our case to uncontrolled formation of
several bis-ligated stereoisomers (Scheme S1).
Fig. 4 Ball-and-stick representation, scheme (left) and projection (right) of the simulated
2
steric map for a bidentate ([κ -C,O]-Dep,4-HNHC)-Ti moiety with a sphere radius of a) 3.5
Å and b) 5.0 Å [15]. The steric maps of these complexes are oriented in a Cartesian frame
with a transverse xy-plane and a +z-axis pointing upward.
4
Although it is not clear whether the dimerization of NHCs observed for the cis-([κ -
O,N,N,O]-bis(imidazolidinylidene)TiCl
2
complexes is metal-assisted or runs via a
1
dissociated free-carbene, the H NMR spectrum recorded after a couple of minutes of
reaction with chloro-titanium precursor and the deprotonated 3b protio-ligand show
that several species are present in solution based on the proton signals of NCH
2
moieties. The NMR data of this mixture clearly show the formation of 4b as the
principal isomer (≈48%) along with another major isomer (≈39%) featuring a AMRX spin
system pattern (four multiplets of 2:2:2:2 ratios) assigned to a C
2
-symmetry complex,
2
which could correspond to one of bis-ligated titanium complexes cis-([κ -C,O]-dep,4-
HNHC)
2
TiCl (Scheme S23-24). Among the other minor isomers, only one is nearly
2
discernible from the baseline (≈13%) showing an AA’BB’ spin system for the NCH
2
moieties (centrosymmetric multiplets) along with another doublet of doublets signal
1
centered at 5.70 ppm (doublet of doublet assigned to Ar-H) characteristic of previous H
NMR pattern of 4aH and 4aMe suggesting the formation of a C2h-symmetry bis-ligated
2
titanium complex trans-([κ -C,O]-dep,4-HNHC)
2
TiCl . This NMR investigation confirms the
2
conclusions obtained from the simulated steric maps with the less bulky N-aryl
substituents (Dep and Mes), in which such substituents cannot hamper the formation of
other stereoisomers of C -symmetry (Scheme 3). Furthermore, the formation of several
2
isomers suggests that the coupling of Ccarbene-Ccarbene is most probably metal-assisted and
might occur through the rearrangement of those cis- and trans-bis-ligated titanium
11

ACCEPTED MANUSCRIPT
intermediates of C
2
- or C2h-symmetry toward a C -symmetric cis-carbene stereoisomer
2
4b’, reasonably assumed to favor such coupling leading to 4b (Scheme 3).
2.3. Polymerization Studies
Following previous studies on the copolymerization of epoxides and CO catalyzed
2
by group 4 metal complexes bearing benzotriazole alkoxide [21], bis(salphen) [22]
ligands, showing structural similarities with our current 6-coordinate bis-ligated
bidentate aryloxy NHC titanium complexes, and the highly selective catalysts based on
the tridentate bis-aryloxy NHC complexes of group 4 [9a, c], we were interested in
evaluating these complexes in the copolymerization of epoxides with CO .
2
Copolymerization experiments of CHO with CO
2
were performed by using 4aH, 4aMe
4b-
,
2
c and cis-([κ -C,O]-Dipp,(3-Ad,5-Me)NHC)
2
TiCl
2
compounds in the presence of most common
and[Bu N]Cl to form the putative anionic
catalysts [9]. The copolymerization of neat CHO with CO shows no polycarbonate
formation (conditions: 1 bar of CO at 60 °C, ratio CHO:Ti = 2500:1) or other side-
co-catalysts such as [PPN]Cl, [PPN]N
3
4
2
2
products such as cyclohexene carbonate or poly(cyclohexene oxide) ring-opening
polymerization of CHO. The absence of activity is probably due to the steric congestion
around the metal center and/or high stability avoiding the association of the co-catalysts
nucleophile to form the “activated” species [9a, c] and/or the epoxide.
Due to the previous studies based on (non-)metallocene and phenoxy-imine
catalysts development and their impact on the coordination polymerization of olefins
for shaping tailored microstructure of polymers or synthetizing oligomers [11e, 23],
systems based on functionalized NHC group 4 catalysts, especially the ones bearing
multidentate and asymmetric auxiliary NHC ligands, have been recently explored for
the α-olefins polymerization [6a-g, 6l, 24] and 1-hexene trimerization catalysis [6j].
More recently, a series of C
substituted NHC ligands of group 4 were found moderately active in ethylene [6d, f, g,
l], ethylene/1-octene and ethylene/norbordene [6d] (co-)polymerization and
2
-symmetric bidentate o-hydroxyaryl unsymmetrically
,
stereoselective in propylene (isotactic up to ca. 70%) [6f, g] and styrene (syndiotactic up
to ca. 99% in rrrrrr heptads) polymerization [6l]. The preliminary reported performance
2
on
C
2
-symmetric and sterically hindered cisoid catalyst (cis-([κ -C,O]-Dipp,(3-Ad,5-
Me)NHC)
2
TiCl ) in (co)polymerization show moderate and excellent stereoselectivity in
2
α-olefins polymerization [6d, l], we were interested to evaluate the performance of the
sterically less hindered 4aH and 4aMe catalysts with different spatial configuration
2
(transoid). Thus, next to the cis-([κ -C,O]-Dipp,(3-Ad,5-Me)NHC)
2
TiCl catalyst, the newly
2
synthesized compounds 4aH and 4aMe were examined for ethylene polymerization in the
presence of 500 equiv. of MAO as co-catalyst. The preliminary polymerization results
show that trans-4aH and 4aMe catalysts have a moderate overall activity: 1206 and 1453
g
PE mmolTi-1 h-1 (conditions: 40 bar of ethylene at room temperature and quenched after 5
min.), respectively, which are comparable to other group 4 systems [11e, 23]. However,
the activities of both catalysts remain slightly superior to the benchmark catalyst cis-
2
([κ -C,O]-Dipp,(3-Ad,5-Me)NHC)
2
TiCl
(1131 gPE mmolTi-1 h-1) under similar conditions [6d, l].
2
The difference in activity in this case is mostly assigned to the extremely bulky
bidentate aryloxy NHC ligand wrapping the titanium center, notably with its Ad
substituent retarding the ethylene association to the metal rather than the cis/transoid
12

ACCEPTED MANUSCRIPT
configuration itself. The reactivity in ethylene polymerization of 4b and 4c were also
explored under the same conditions, and no significant effects were observed in term of
activity for the wrapped tetradentate ligands around titanium compared to the cis- or
trans-bis-ligated titanium complexes (1113 and 1047 gPE mmolTi-1 h-1, respectively).
3. Conclusions
In summary, we have successfully synthesized a set of o-hydroxyaryl-substituted
NHC ligands and the corresponding bis(aryloxide-NHC) titanium complexes. The
bulky N-aryl substituent (such as Dipp vs. Mes and Dep) at the NHCs combined with
the other ortho-substituent on the N-aryloxy moieties appear essential for leading to a
single stereoisomer. Meaning that both steric factors have a significant effect on the
configuration (trans or cis) of the final bis-ligated titanium complex, which can be fine-
tuned by varying the size of the N-functional groups on the NHCs. Without such bulky
N-aryl (i.e. Mes and Dep) and ortho-N-aryloxy (i.e. H) substituents, several bis-ligated
stereoisomers of titanium are formed which subsequently rearrange into a tetradentate
salophen-like ligand of titanium via the coupling of NHCs. The trans-bis(aryloxide-
NHC) and cis-bis(imidazolidinylidene) complexes of titanium show moderate catalytic
activities in ethylene polymerization, and the N-substituents at the NHCs and the
cis/trans-configurations have modest effect on the activity.
4. Experimental
4.1. General procedures
All operations were performed with rigorous exclusion of air and water, using
standard Schlenk, high-vacuum and glovebox techniques (MB Braun MB200B-G; <1
ppm O
using Grubbs columns (MB Braun Solvent Purification System 800). Benzene-d
chloroform-d (99.96%) were obtained from Aldrich, dried over sodium or CaH , vacuum
2
, <1 ppm H
2
O). Dichloromethane, hexane, THF and toluene were purified by
6
and
2
transferred, degassed and filtered prior to use. All other chemicals were purchased
from Aldrich and used as received. The oxanilic acid ethyl esters (1a and 1c),
oxalamides (2aH and 2c), unsymmetrical o-hydroxyaryl substituted imidazolidinium
2
chloride salts (3aH and 3c) and (cis-[κ -C,O]-Dipp,3-Ad/5-MeNHC)
2
TiCl were prepared
2
according to the literature procedures [6d, 12]. The imidazolidinium chloride salt 3c was
crystallized from acetonitrile/hexane (1/5) at -30 °C. The NMR spectra of air and
moisture sensitive compounds were recorded using J. Young valve NMR tubes at 298 K
on Bruker BioSpin AVANCE DMX400 spectrometer (5 mm Broadband observe
1
probehead (BBO), H: 400.13 MHz; 13C: 100.62 MHz) and a Bruker BioSpin AVANCE
1
AV500 (5 mm BBO, H: 500.13 MHz; 13C: 125.77 MHz) and AV600 (5 mm triple
1
resonance inverse CryoProbe, H: 600.13 MHz; 13C: 150.91 MHz). 1H and 13C shifts are
referenced to internal solvent resonances and reported in parts per million (ppm) relative
1
to TMS. Fitting of simulated spectral regions to experimental H NMR data was
performed with the Daisy module supplied with the TopSpin package [25]. IR spectra
were recorded on a Nicolet FT-IR Protégé 460 spectrometer with a DRIFT collector. The
13

ACCEPTED MANUSCRIPT
spectra were averaged over 64 scans; the resolution was ±4 cm-1. Elemental analyses of
C, H and N were performed on an Elementar Vario EL III instrument.
4.2. Unsymmetrical o-hydroxyaryl-substituted imidazolidinium protio-ligand syntheses
4.2.1. 1-(2,6-diisopropylphenyl)-3-(2-hydroxy-4-methylphenyl)-4,5-dihydro-imidazolyl
chloride salt (3aMe)
Compound N-(2,6-diisopropylphenyl)-oxanilic acid ethyl ester 1a (1.71 g, 6.15 mmol)
and 2-amino-5-methylphenol (1 equiv., 758 mg, 6.15 mmol) were dissolved in toluene
(100 mL), and 0.86 mL of NEt (1 equiv., 6.15 mmol) was added under inert atmosphere.
3
The suspension was stirred and heated to reflux overnight. After cooling down the
solution, a precipitate was obtained which was dissolved in EtOAc. The solution was
washed with 2 M HCl (2x100 mL). The aqueous phase was then washed with EtOAc
and the combined organic phases were washed with brine, dried over MgSO and dried
4
under vacuum. The solid was crystallized from warm toluene to afford N-(2,6-
diisopropylphenyl)-N’-(2-hydroxy-4-methylphenyl)-oxalamide 2aMe (1.47 g, 67% yield).
1H NMR (400.13 MHz, chloroform-d):
δ 9.56 (s, 1H, NH), 8.81 (s, 1H, NH), 8.25 (s, 1H,
OH), 7.37 (t, J = 7.8 Hz, 1H, ArDipp-H), 7.24 (d, J = 8.2 Hz, 2H, Ar-H), 7.23 (d, J = 7.8 Hz,
1H, ArDipp-H), 6.84 (s, 1H, Ar-H), 6.74 (d, J = 8.2 Hz, 1H, Ar-H), 3.02 (sept., J = 6.9 Hz, 2H,
CH(CH
3
)
2
), 2.32 (s, 3H, Ar-CH
3
), 1.22 (d, J = 6.9 Hz, 12H, CH(CH
3
)
2
) ppm.
Under inert atmosphere, to compound 2aMe (1.47 g, 4.15 mmol) was added 33.2 mL BH
3
-
THF (8 equiv., 1M in THF) and the mixture was heated to reflux overnight. After
cooling down to room temperature, MeOH was added to the reaction mixture until gas
evolution cease and then quenched by ≈ 1.5 mL of conc. HCl. The mixture was dried,
dissolved in MeOH and these steps were repeated two more times. To this pale pink
solid was added 15 mL of triethyl orthoformate and the solution was stirred at room
temperature overnight. The solid was filtered off, washed with Et
vacuum to afford 3aMe as a white solid (1.20 g, 78% yield).1H NMR (500.13 MHz,
chloroform-d): 9.09 (s, 1H, NCHN), 7.49-7.45 (m, 2H, Ar-H), 7.24 (d, J = 7.8 Hz, 2H, Ar-
H), 6.85 (bs, 1H, Ar-H), 6.80 (bd, 1H, Ar-H), 4.86 (bm, 2H, NCH ), 4.39 (bm, 2H, NCH ),
3.00 (sept., J = 6.8 Hz, 2H, CH(CH ), 2.18 (s, 3H, Ar-CH ), 1.28 (d, J = 6.8 Hz, 6H,
CH(CH ), 1.16 (d, J = 6.8 Hz, 6H, CH(CH
) ppm. 13C NMR (125.77 MHz, chloroform-
d): 156.6 (NCHN), 149.6 (Cipso, O-Ar), 146.6 (Cq, Ar), 139.3 (Cq, Ar), 131.5 (Cq, Ar), 130.0
(Cq, Ar), 125.1 (CH, Ar), 120.5 (CH, Ar), 120.1 (CH, Ar), 119.8 (CH, Ar), 119.1 (CH, Ar),
52.5 (NCH ), 51.0 (NCH ), 28.9 (CH(CH ), 25.0 (CH(CH ), 24.3 (CH(CH ), 21.1 (Ar-
CH
) ppm. DRIFT (ν/cm-1): 2970s, 2958s, 2868s, 1624vs, 1588w, 1553w, 1526w, 1494w,
1476w, 1457m, 1420m, 1385w, 1368w, 1335w, 1311m, 1292s, 1283m, 1263s, 1185w,
2
O and dried under
δ
2
2
3
)
2
3
3
)
2
3)2
δ
2
2
3
)
2
3
)
2
3)2
3
1166w, 945w, 879w, 812m, 764w, 593w, 487w, 456w. Anal. Calcd for C22
68.98; H, 7.01; N 8.47. Found: C, 69.16; H, 7.26; N, 8.40.
H
29ClN O: C,
2
4.2.2. 1-(2,6-diethylphenyl)-3-(2-hydroxyphenyl)-4,5-dihydro-imidazolyl chloride (3b)
14

ACCEPTED MANUSCRIPT
2,6-diethylaniline (10 mL, 60.7 mmol) and triethylamine (1 equiv., 8.5 mL, 60.7 mmol)
were dissolved in dry THF (80 mL). After cooling down the solution at 0 °C, 6.8 mL of
ethyl chloroacetate (1 equiv., 60.7 mmol) was added dropwise leading to a formation of
smoke and white solid. The solution was allowed to warm up at room temperature and
stirred overnight. The solid was filtered off and the organic phase was washed with
HCl 2M (2x100 mL). The aqueous phase was then washed with EtOAc and the
combined organic phases were washed with brine, dry over MgSO and dry under
4
vacuum to afford N-(2,6-diethylphenyl)-oxanilic acid ethyl ester 1b as white solid (11.23
1
g, 75% yield). H NMR (500.13 MHz, chloroform-d):
δ
8.40 (s, 1H, NH), 7.27 (t, J = 7.6 Hz,
-CH ), 2.59
), 1.20 (t, J = 7.6 Hz, 6H,
161.0 (CO, carbimide), 155.4
(CO, ester), 141.1 5 (Cq, Ar), 130.9 (Cq, Ar), 128.6 (CH, Ar), 126.5 (CH, Ar), 63.7 (CH , O-
CH -CH ), 24.8 (CH , CH -CH ), 14.3 (CH , O-CH -CH ), 14.0 (CH , CH -CH ) ppm.
Compound 1b (10.96 g, 43.57 mmol) and 2-aminophenol (1 equiv., 4.80 mg, 43.97 mmol)
were dissolved in toluene (100 mL), and 12.3 mL of NEt (1 equiv., 87.9 mmol) was
1H, ArDep-H), 7.15 (d, J = 7.6 Hz, 2H, ArDep-H), 4.44 (q, J = 7.2 Hz, 2H, O-CH
(q, J = 7.6 Hz, 4H, CH -CH ), 1.45 (t, J = 7.2 Hz, 3H, O-CH -CH
CH -CH
) ppm. 13C NMR (125.77 MHz, chloroform-d):
2
3
2
3
2
3
2
3
δ
2
2
3
2
2
3
3
2
3
3
2
3
3
added under inert atmosphere. The suspension was stirred and heated to reflux
overnight. After cooling down the solution a precipitate was obtained, which was
dissolved in dichloromethane. The solution was washed with 2 M HCl (2x100 mL). The
aqueous phase was then washed with dichloromethane and the combined organic
phases were washed with brine, dried over MgSO and dried under vacuum. The solid
4
was crystallized from warm toluene to afford N-(2,6-diethylphenyl)-N’-(2-
1
hydroxyphenyl)-oxalamide 2b (10.06 g, 73% yield). H NMR (500.13 MHz, chloroform-
d):
δ 9.68 (s, 1H, NH), 8.88 (s, 1H, NH), 8.06 (s, 1H, OH), 7.50 (dd, J = 8.1 Hz, J = 1.4 Hz,
1H, ArDep-H), 7.30 (m, 1H, Ar-H), 7.18 (s, 1H, ArDep-H), 7.17 (s, 1H, ArDep-H), 7.15 (m, 1H,
Ar-H), 6.97 (dd, J = 8.1 Hz, J = 1.4 Hz, 1H, ArDep-H), 6.92 (m, 1H, Ar-H), 2.61 (q, J = 7.6
Hz, 4H, CH
2
-CH
3
), 1.21 (t, J = 7.6 Hz, 6H, CH
2
-CH ) ppm.
3
Following the procedure described for 3aMe, the reaction of 2b (1.92 g, 6 mmol) and 49.2
1
mL of BH
3
-THF (8 equiv., 1M in THF) yielded 3b (1.6 g, 81% yield) as a white solid. H
NMR (500.13 MHz, chloroform-d):
δ
8.96 (s, 1H, NCHN), 7.57 (d, J = 8.2 Hz, 1H, ArDep-
H), 7.41 (t, J = 7.8 Hz, 1H, Ar-H), 7.20 (d, J = 7.8 Hz, 2H, ArDep-H), 7.10 (d, J = 7.8 Hz, Ar-
H), 6.96 (t, J = 7.7 Hz, 1H, Ar-H), 6.77 (t, J = 7.7 Hz, 1H, Ar-H), 4.86 (bt, J = 10.4 Hz, 2H,
NCH
Hz, 6H, CH
(Cipso, O-Ar), 141.6 (Cq, Ar), 131.8 (Cq, Ar), 131.2 (Cq, Ar), 128.8 (CH, Ar), 127.6 (CH, Ar),
122.7 (CH, Ar), 120.3 (CH, Ar), 119.9 (CH, Ar), 118.9 (CH, Ar), 51.5 (NCH ), 51.0 (NCH ),
24.3 (CH , CH -CH ), 15.2 (CH , CH -CH
) ppm. DRIFT (ν/cm-1): 2971s, 2959s, 2869s,
2
), 4.42 (bt, J = 10.4 Hz, 2H, NCH
2
), 2.64 (q, J = 7.5 Hz, 4H, CH
2
-CH
3
), 1.22 (t, J = 7.5
2
-CH
3
) ppm. 13C NMR (125.77 MHz, chloroform-d):
δ 157.2 (NCHN), 150.0
2
2
2
2
3
3
2
3
1624vs, 1588w, 1555w, 1525w, 1494w, 1476w, 1457m, 1421m, 1385w, 1368w, 1337w,
1313m, 1292s, 1284m, 1263s, 1186w, 1166w, 945w, 880w, 812m, 763w, 593w, 487w, 455w.
Anal. Calcd for C19
H
23ClN O: C, 68.97; H, 7.01; N 8.47. Found: C, 68.79; H, 7.26; N, 8.36.
2
4.3. Preparation of bis-ligated o-hydroxyaryl substituted NHC titanium(IV) complexes
15

ACCEPTED MANUSCRIPT
4.3.1. bis(1-(2,6-diisopropylphenyl)-3-(2-hydroxyphenyl)-4,5-dihydro-imidazolyl)
titanium(IV) dichloride (4aH)
In
a
vial
1-(2,6-diisopropylphenyl)-3-(2-hydroxyphenyl)-4,5-dihydro-imidazolyl
chloride salt 3aH (588.0 mg, 1.64 mmol) and potassium bis(trimethylsilyl)amide (2
equiv., 561.35 mg, 3.27 mmol) were dissolved in THF (3 mL), and the resulting yellow
suspended mixture was stirred for 10 min at room temperature. To this mixture was
added 0.82 mL of a solution of 1M titanium tetrachloride in toluene (0.5 equiv., 0.82
mmol) in 2 mL THF. The solution rapidly turned dark red and was allowed to stir for 2
h at room temperature. Subsequent centrifugation, filtration and evaporation to dryness
afforded complex 4aH as a dark red solid in quantitative yield. Crystals of 4aH were
1
obtained from a saturated solution of dichloromethane at -30 °C for several days. H
NMR (500.13 MHz, benzene-d
4H, ArDipp-H), 7.01 (m, 2H, Ar-H), 6.75 (m, 2H, Ar-H), 6.37 (dd, J = 8.1 Hz, J = 1.4 Hz, 2H,
Ar-H), 5.26 (dd, J = 8.1 Hz, , J = 1.4 Hz, 2H, Ar-H), 3.47 (sept., J = 6.8 Hz, 4H, CH(CH ),
3.07 (m, 4H, NCH ), 2.95 (m, 4H, NCH ), 1.56 (d, J = 6.8 Hz, 12H, CH(CH ), 1.10 (d, J =
6.8 Hz, 12H, CH(CH ): 206.7 (NCN), 155.0
) ppm. 13C NMR (125.77 MHz, benzene-d
(Cipso, O-Ar), 148.0 (Cq, Ar), 137.8 (Cq, Ar), 133.6 (Cq, Ar), 129.2 (CH, Ar), 125.3 (CH, Ar),
124.0 (CH, Ar), 119.3 (CH, Ar), 118.1 (CH, Ar), 116.7 (CH, Ar), 53.9 (NCH ), 47.3 (NCH ),
28.5 (CH(CH ), 26.2 (CH(CH ), 24.8 (CH(CH
) ppm. DRIFT (ν/cm-1): 3065w, 3031w,
6
): δ 7.42 (t, J = 7.7 Hz, 2H, ArDipp-H), 7.29 (d, J = 7.7 Hz,
3)2
2
2
3)2
3
)
2
6
δ
2
2
3
)
2
3
)
2
3)2
2961s, 2925m, 2867m, 1610m, 1587m, 1495vs, 1474vs, 1429s, 1408s, 1385w, 1363w, 1317s,
1288vs, 1236m, 1123w, 1056w, 996w, 912m, 879m, 806w, 750m, 671m, 640w, 453w, 438w,
418m. Anal. Calcd for C42
57.04; H, 6.18; N 5.94.
H
50Cl
2
N
4
O
2
Ti.2CH
2
Cl : C, 56.73; H, 5.84; N 6.01. Found: C,
2
4.3.2. bis(1-(2,6-diisopropylphenyl)-3-(2-hydroxy-4-methylphenyl)-4,5-dihydro-
imidazolyl) titanium(IV) dichloride (4aMe)
Following the procedure described for 4aH, the reaction of 1-(2,6-diisopropylphenyl)-3-
(2-hydroxy-4-mehtylphenyl)-4,5-dihydro-imidazolyl chloride salt 3aMe (350.8 mg, 0.94
mmol), potassium bis(trimethylsilyl)amide (2 equiv., 375.2 mg, 1.88 mmol) and titanium
tetrachloride (0.5 equiv., 0.47 mL, 0.82 mmol) yielded majorly 4aMe. The residual
impurities were removed by stirring compound 4aMe in toluene (5 mL) at 70 °C for two
days and 4aMe was isolated after centrifugation, filtration and evaporation to dryness as
1
a brown dark solid (241 mg, 65% yield). H NMR (500.13 MHz, benzene-d
6
):
δ
7.47 (t, J =
7.8 Hz, 2H, ArDipp-H), 7.32 (d, J = 7.8 Hz, 4H, ArDipp-H), 6.78 (dd, J = 8.2 Hz, J = 1.5 Hz,
2H, Ar-H), 6.15 (d, J = 1.5 Hz, 2H, Ar-H), 5.17 (d, J = 8.2 Hz, 2H, Ar-H), 3.50 (sept., J = 6.8
Hz, 4H, CH(CH
(d, J = 6.8 Hz, 12H, CH(CH
MHz, benzene-d ): 206.8 (NCN), 153.0 (Cipso, O-Ar), 148.1 (Cq, Ar), 138.0 (Cq, Ar), 133.2
(Cq, Ar), 131.3 (Cq, Ar), 129.1 (CH, Ar), 128.5 (CH, Ar), 125.3 (CH, Ar), 124.5 (CH, Ar), 117.7
(CH, Ar), 117.5 (CH, Ar), 54.0 (NCH2), 47.1 (NCH2), 28.5 (CH(CH ), 26.3 (CH(CH ), 24.9
(Ar-CH
) ppm. DRIFT (ν/cm-1): 3065w, 3028w, 2965s, 2926m, 2869m, 1652w, 1615s, 1600s,
3
)
2
), 3.08 (m, 4H, NCH
2
), 2.87 (m, 4H, NCH
2
), 2.23 (s, 3H, Ar-CH
3
), 1.58
3
)
2
), 1.12 (d, J = 6.8 Hz, 12H, CH(CH
3)2
) ppm. 13C NMR (125.77
6
δ
3
)
2
3)2
3
1584m, 1558w, 1539vw, 1504s, 1472 m, 1456m, 1398vw, 1386vw, 1319w, 1283vs, 1238w,
1132vw, 848w, 822w, 805w, 758w, 667w, 568w, 552m, 440w, 434vw. Anal. Calcd for
16

ACCEPTED MANUSCRIPT
C
44
H
54Cl
2
N
4
O
2
Ti.1/2CH
2
Cl
2
.1/2C
7
H8: C, 65.65; H, 6.77; N, 6.38. Found: C, 65.70; H, 6.49; N
6.57.
4.4. Reactivity of less-hindered unsymmetrical o-hydroxyaryl-substituted
imidazolidinium salts with chloro-titanium precursor
4.4.1. Identification of the major component from the reaction of 3b with TiCl :
4
formation of (1,1’-(2,6-diethylphenyl)-3,3’-(2-hydroxyphenyl)bis(imidazolidin-2-
ylidene) titanium(IV) dichloride (4b) and bis(1-(2,6-diethylphenyl)-3-(2-
hydroxyphenyl)-4,5-dihydro-imidazolyl) titanium(IV) dichloride (4b’) as minor isomer
In a vial 1-(2,6-diethylphenyl)-3-(2-hydroxyphenyl)-4,5-dihydro-imidazolyl chloride
salt 2b (80.0 mg, 0.24 mmol) and potassium bis(trimethylsilyl)amide (2 equiv., 96.4 mg,
0.48 mmol) were dissolved in THF (3 mL), and the resulting yellow suspended mixture
was stirred for 10 min at –30 °C. After filtration, a solution of TiCl
4
(THF) (0.5 equiv.,
2
0.12 mmol) in 2 mL THF was slowly added to this mixture. The solution rapidly turned
dark red and was allowed to stir for 30 min at room temperature. Subsequent
centrifugation, filtration and evaporation to dryness afforded complex 4b along with
some impurities. Most of the residual impurities were removed by stirring the mixture
in hexane at room temperature overnight leading to a red dark solid. Complex 4b was
then extracted from a mixture of toluene/hexane (1:3 ratio) affording a red solid (78 mg,
90% yield). Crystals of the major isomer 4b were obtained from a saturated solution of
toluene at -30 °C for several days and crystals of the minor isomer 4b’ were co-
crystallized with 4b in a saturated solution of benzene at room temperature after three
1
days. Major isomer 4b: H NMR (500.13 MHz, benzene-d
6
):
δ
7.05-6.96 (m, 4H, Ar-H),
6.76-6.59 (m, 8H, Ar-H), 6.29 (bs, 1H, Ar-H), 6.27 (bs, 1H, Ar-H), 4.25-4.16 (m, 2H,
NCH ), 3.41-3.28 (m, 4H, NCH ), 2.71-2.65 (m, 2H, NCH ), 2.20-2.15 (m, 2H, CH -CH ),
1.82-1.69 (m, 2H, CH -CH ), 1.68-1.59 (m, 2H, CH -CH ), 1.37-1.31 (m, 2H, CH -CH ), 0.89
(t, J = 7.6 Hz, 6H, CH -CH ), 0.77 (t, J = 7.5 Hz, 6H, CH -CH
) ppm. 13C NMR (125.77
MHz, benzene-d ): 160.7 (Cipso, O-Ar), 146.9 (C , Ar), 143.7 (C , Ar), 140.3 (C , Ar), 129.8
(CH, Ar), 128.1 (CH, Ar), 127.1 (C , Ar), 126.8 (CH, Ar), 126.7 (CH, Ar), 124.7 (N C=CN ),
121.1 (CH, Ar), 120.3 (CH, Ar), 115.1 (CH, Ar), 57.0 (NCH ), 54.8 (NCH ), 24.5 (CH , CH
CH ), 22.0 (CH , CH -CH ), 15.4 (CH , CH -CH ), 14.0 (CH , CH -CH ) ppm. Minor
isomer 4b’: the other proton and carbon signals are nearly indiscernible from the
baseline due to the low concentration of 4b’. Anal. Calcd for
Ti.1/3OC .1/2CH : C, 66.67; H, 6.47; N, 7.10. Found: C, 66.23; H,
2
2
2
2
3
2
3
2
3
2
3
2
3
2
3
6
δ
q
q
q
q
2
2
2
2
2
2
-
3
2
2
3
3
2
3
3
2
3
C
38
H
42Cl
2
N
4
O
2
4
H
8
3
C
6
H
5
6.31; N 7.19.
4.4.2. Identification of the major component from the reaction of 3c and TiCl : formation
4
of bis(1-(2,4,6-trimethylphenyl)-3-(2-hydroxyphenyl)-4,5-dihydroimidazolidin-2-
ylidene) titanium(IV) dichloride (4c) and other minor isomers.
Following the procedure described for 4b, the reaction of 1-(2,4,6-trimethylphenyl)-3-(2-
hydroxyphenyl)-4,5-dihydro-imidazolyl chloride salt 3c (70 mg, 0.22 mmol) and
potassium bis(trimethylsilyl)amide (2 equiv., 88 mg, 0.44 mmol) and TiCl
4
(THF) (0.5
2
equiv., 0.11 mmol) affording a red solid (34 mg, 47% yield, composed of a mixture of 3
isomers 4c:B:C = 90:8:2 based on the integration of methyl groups of the N-mesityl
17

ACCEPTED MANUSCRIPT
1
1
substituent by H NMR spectroscopy). For 4c: H NMR (500.13 MHz, benzene-d
6
):
isomer A:
6.23 (bs, 2H, Ar-H), 6.02 (bs, 2H, Ar-H), 4.25-4.16 (m, 2H, NCH
NCH ), 3.26-3.18 (m, 2H, NCH ), 2.63-2.56 (m, 2H, NCH ), 1.91 (s, 6H, Ar-CH
6H, Ar-CH ), 1.21 (s, 6H, Ar-CH ) ppm. For isomer B 2.17 (bd, 6H, Ar-CH ), 1.93 (bd, 6H,
), 1.67 (bd, 6H, Ar-CH ) ppm. For isomer C: 2.49 (bs, 6H, Ar-CH ), 2.44 (bs, 6H,
), 2.00 (bs, 6H, Ar-CH ): 160.8
) ppm. For 4c: 13C NMR (125.77 MHz, benzene-d
, Ar), 137.2 (C , Ar), 135.7 (C , Ar), 134.6 (C , Ar), 130.2 (CH, Ar),
129.7 (CH, Ar), 129.0 (CH, Ar), 124.7 (N C=CN ), 121.2 (CH, Ar), 120.3 (CH, Ar), 115.2
(CH, Ar), 55.0 (NCH ), 54.9 (NCH ), 20.7 (Ar-CH ), 18.8 (Ar-CH ), 16.5 (Ar-CH ) ppm.
The other proton and carbon signals are nearly indiscernible from the baseline due to
δ
7.06-6.98 (m, 4H, Ar
p
-H), 6.75.6.71 (m, 2H, Ar-H), 6.68-6.32 (m, 2H, Ar-H),
), 3.45-3.38 (m, 2H,
), 1.59 (s,
2
2
2
2
3
3
3
3
Ar-CH
Ar-CH
3
3
3
3
3
6
δ
(Cipso, O-Ar), 147.3 (C
q
q
q
q
2
2
2
2
3
3
3
the low concentration of isomers B and C. Anal. Calcd for C36
67.11; H, 6.02; N, 7.28. Found: C, 67.45; H, 6.24; N 7.27.
H
38Cl
2
N
4
O
2
Ti.C
7
H8: C,
4.5. General procedure for ethylene polymerization
In the glovebox, a vial was charged with a stirring bar, Ti catalyst (5 μmol) in 5 mL of
toluene and 500 equiv. of MAO, and placed in an autoclave reactor. After addition of 40
bar of ethylene, the reaction was stirred 5 min and the pressure was released before to
quench the reaction by 1 mL of acidified MeOH. Volatiles were removed under reduced
pressure, and polymer was washed with isopropanol and dried under vacuum for 2 h at
150 °C. The polymer yield was determined gravimetrically.
4.6. X-ray crystallography and crystal structure determination
Suitable crystals for diffraction experiments were selected in a glovebox and mounted
in a minimum of Parabar 10312 oil (Hampton Research) in a nylon loop and then
mounted under a nitrogen cold stream from an Oxford Cryosystems 700 series open-
flow cryostat. Data collection was done on a Bruker AXS TXS rotating anode system
with an APEXII Pt135 CCD detector using graphite-monochromated Mo K
radiation (λ =
α
0.710ꢀ73 Å). Data collection and data processing were done using APEX2[26],
SAINT[27], and SADABS[28] version 2012/1,whereas structure solution and final model
refinement were done using SHELXS[29] version 2013/1 or SHELXT[30] version 2014/4
and SHELXL[31] version 2014/7. Crystal data: for 4aH obtained from a saturated solution
in CH
crystal habit/color: prism/dark red, monoclinic, space group P2
11.9657(7), b = 12.9163(8), c = 14.5572(9) Å, β = 99.1720(10)°, V = 2221.1(2) Å3, Z = 2,
2
Cl
2
. C42
H
50Cl
2
N
4
O
2
Ti.2(CH
2
Cl
2
), M = 931.51, crystal size: 0.125 x 0.100 x 0.050 mm3,
1
/n (No.14), a =
ρ
calc
=
1.393 g.cm-3, F(000) = 972, µ(Mo-Kα ) = 0.595 mm-1, λ = 0.71073 Å, T = 103(2) K. The
28755 reflections measured on a Bruker AXS APEXII Ultra CCD area detector system
yielded 4739 unique data (2.050 <
θ < 26.776°, Rint = 0.0485) [4739 observed reflections (I
> 2σ(I)]. Goodness-of-fit on F2 = 1.037, R1 = 0.0350, wR2 = 0.0894, R indices (all data) R1 =
0.0486, wR2 = 0.0980. Crystal data: for 4b obtained from a saturated solution in toluene
at –30 °C. C38
H
42Cl
2
N
4
O
2
Ti, M = 705.55, crystal size: 0.247 x 0.201 x 0.138 mm3, crystal
/n (No.14), a = 9.5808(7),
habit/color: flat prism/Brown-black, monoclinic, space group P2
1
b = 28.232(2), c = 16.9105(13) Å, β = 102.2050(10)°, V = 4470.6(6) Å3, Z = 4,
ρcalc = 1.048
18

ACCEPTED MANUSCRIPT
g.cm-3, F(000) = 1480, µ(Mo-Kα ) = 0.342 mm-1, λ = 0.71073 Å, T = 103(2) K. The 50718
reflections measured on a Bruker AXS APEXII Ultra CCD area detector system yielded
7904 unique data (1.443 <
θ < 25.048°, Rint = 0.0572) [7904 observed reflections (I > 2σ(I)].
Goodness-of-fit on F2 = 1.141, R1 = 0.0850, wR2 = 0.2145, R indices (all data) R1 = 0.0941,
wR2 = 0.2202. CCDC reference codes 1579164, 1579165, 1822682 and 1822683 contain the
supplementary crystallographic data for 3c, 4aH, 4b and 4b’, respectively. These data can
be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
4.7. %VBur and steric maps calculations
The %VBur values and steric maps were evaluated with the SambVca 2.0 package
[32]. The radius of the sphere around the center atom was set to: 3.5 Å or 5 Å, distance
from the center of the sphere: 2.26 Å, mesh spacing: 0.1 Å, H atoms omitted and atom
radii: Bondi radii scaled by 1.17, as recommended by Cavallo [32].
Acknowledgements
The authors acknowledge the University of Bergen, L. Meltzers Høyskolefond (K. W. T.:
bursary no. 2017/3273) and the Norwegian Research Council (FRINATEK grant no.
240333) for financial support. Dr. Marco Foscato is thanked for his fruitful discussions
on steric map calculations.
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21

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Research Highlights
•
•
•
Synthesis of unsymmetrical o-functionalized NHC ligands
Synthesis of single stereoisomer of bis-ligated NHC titanium complexes
Rearrangement bis-ligated NHC titanium intermediates into tetradentate
salophen-like titanium complexes via NHCs dimerization
•
•
Calculated percentage buried volume and steric maps of half bis-ligated (C,O)-
NHC titanium complexes using SambVca 2 web-tool
MAO-activated bis-ligated NHC titanium complexes show moderate activity for
ethylene polymerization