Zirconium complexes of the tridentate bis(aryloxide)-N-heterocyclic-carbene ligand: Chloride and alkyl functionalized derivatives

Article abstract of DOI:10.1016/j.jorganchem.2006.03.044

The disodium salt of a bis(aryloxide)-N-heterocyclic-carbene dianionic ligand, Na₂[L], was prepared by reaction of 1,3-bis(4,6-di-tert-butyl-2-hydroxybenzyl)imidazolium bromide [H₃L]Br with 3 equiv. of NaN(SiMe₃)₂

Full text of DOI:10.1016/j.jorganchem.2006.03.044

Journal of Organometallic Chemistry 692 (2007) 234–242
www.elsevier.com/locate/jorganchem
Zirconium complexes of the tridentate
bis(aryloxide)-N-heterocyclic-carbene ligand: Chloride and alkyl
functionalized derivatives
*
Dao Zhang, Hidenori Aihara, Takahito Watanabe, Tsukasa Matsuo, Hiroyuki Kawaguchi
Coordination Chemistry Laboratories, Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan
Received 23 February 2006; accepted 20 March 2006
Available online 30 August 2006
Abstract
The disodium salt of a bis(aryloxide)-N-heterocyclic-carbene dianionic ligand, Na2[L], was prepared by reaction of 1,3-bis(4,6-di-tert-
butyl-2-hydroxybenzyl)imidazolium bromide [H3L]Br with 3 equiv. of NaN(SiMe3)2. Reaction of ZrCl4(thf)2 with 1 equiv. of Na2[L]
gave a mixture of [L]ZrCl2(thf) (1) and [L]2Zr (2). When the amount of Na2[L] was increased to 2 equiv., the bis(bis(aryloxide)-N-het-
erocyclic-carbene) complex 2 was obtained in good yield. The dichloro complex 1 is a precursor to organometallic derivatives, and treat-
ment with PhCH2MgCl or Me3SiCH2Li yielded [L]ZrR2 [R = CH2Ph (3), CH2SiMe3 (4)]. The disodium salt of the ligand Na2[L] is
unstable and undergoes 1,2-benzyl migration, whereas zirconium complexes of the [L]2ꢀ ligand are found to be thermally stable in solid
and solution. The X-ray crystal structures of 1, 2, and 3 are described.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Zirconium; Carbene; Aryloxide; Multidentate ligand
1. Introduction
of dissociation of the NHC ligand from the electron defi-
cient metal center, which makes it difficult to study the
chemistry of NHCs in early transition metals and f-elements
[7]. A potential means of directing metal–NHC interactions
is the covalent tethering of the anionic functional groups to
the NHC ligand system, where the NHC moiety is held in
proximity of the metal center by a covalent tether and
should affect reactivity in a specific way. Recent examples
include amide-, alkoxide-, and cyclopentadienyl-functional-
ized NHC ligands [7–10]. As part of an ongoing project to
study the chemistry of aryloxide-based multidentate ligands
[11], we have successfully used the tridentate dianionic
bis(aryloxide)–NHC ligand ([L]2ꢀ) as an ancillary ligand
to access NHC complexes of titanium. These compounds
are found to be highly active procatalysts for polymeriza-
tion of ethylene [12]. We therefore wish to extend this chem-
istry to early transition metals and synthesize a range of
[L]2ꢀ supported zirconium complexes.
The chemistry and application of N-heterocyclic carb-
enes (NHCs) have been extensively explored [1]. They can
bind as two-electron donors to a wide range of transition
metal, main group, and f-block derivatives [2]. Especially,
NHCs have been found extensive use as ancillary ligands
in late transition metal complexes, in which they have
shown enhanced catalytic activity compared to their phos-
phine analogues [1,3]. These improved catalytic systems
have been attributed to the robust nature of late transition
metal–NHC bond and their excellent r-donating properties
[4]. In contrast, NHC complexes of early transition metals
are considerably less developed despite the great potential
of this class of molecules [5,6]. This is mainly due to the ease
*
Corresponding author.
E-mail address: hkawa@ims.ac.jp (H. Kawaguchi).
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.03.044

D. Zhang et al. / Journal of Organometallic Chemistry 692 (2007) 234–242
235
2. Results and discussion
2.1. Synthesis of [H3L]Br
The proligand, 1,3-bis(4,6-di-tert-butyl-2-hydroxybenzyl)-
imidazolium bromide H3[L]Br, was prepared by stepwise
alkylation on the nitrogen atoms of imidazole using
2-bromomethyl-4,6-di-tert-butylphenol (Scheme 1). Reac-
tion of imidazole with 2-bromomethyl-phenol in the
presence of NaHCO3 in refluxing THF gave N-(2-hydrox-
ybenzyl)imidazole as a colorless solid in 96% yield. Subse-
quent treatment of it with 2-bromomethyl-phenol in THF
yielded H3[L]Br as colorless microcrystals in 63%. The
Fig. 1. The molecular structure of H3[L]Br. tBu groups and hydrogen
atoms except for the hydroxyl and imidazolium protons are omitted for
clarity.
1
most prominent features of the H and 13C NMR spectra
of H3[L]Br are the resonances for the imidazolium proton
at 9.51 ppm and the corresponding imidazolium carbon
at 139.1 ppm.
We examined deprotonation of H3[L]Br to prepare
alkali metal salts of the ligand for use in subsequent salt
metathesis with metal halides. The sodium salt Na2[L]
was readily prepared by the reaction of H3[L]Br with 3
equiv. of NaN(SiMe3)2 in THF at ꢀ78 ꢁC. However,
attempts to isolate Na2[L] met with difficulties because of
its thermal instability. The facile 1,2-benzyl migration took
place at ambient temperature, giving a 2-alkylated imidaz-
ole derivative Na2[L*] (Scheme 1). This type of 1,2-migra-
tion is a common reaction for singlet carbenes [9,14]. The
X-ray diffraction analysis shows it to be the dimeric
complex {Na2[L*]}2 (Fig. 2). The dimer resides on a
crystallographic C2-axis; its core consists of two four-
membered Na2O2 rings. Hydrolysis of Na2[L*] afforded
1,2-bis(2-hydroxybenzyl)imidazole H2[L*], which was char-
acterized by NMR spectroscopy and X-ray crystallography
(Fig. 3).
The molecular structure of H3[L]Br was determined by
X-ray analysis (Fig. 1). Recrystallization from DME gave
single crystals of H3[L]Br Æ DME. Hydrogen atoms
attached to imidazolium carbon and oxygen atoms were
located from the different Fourier map and were refined
isotropically. The molecule adopts an S-shaped conforma-
tion. Of interest is the incorporation of a DME molecule of
crystallization held in place through a hydrogen bond
between the imidazolium proton [H(3)] and one of the oxy-
˚
gen atoms of DME [O(3)] (C–Hꢁ ꢁ ꢁO, 2.62 A, 136ꢁ). Each
H3[L] unit is connected to another unit through hydrogen
bonds between bromide ions and hydroxyl groups [O(1)–
H(1)ꢁ ꢁ ꢁBr, 2.39 A, 171ꢁ; O(2)–H(2)ꢁ ꢁ ꢁBr*, 2.50 A, 165ꢁ]
˚
˚
[13], giving a dimeric structure in the solid state.
As shown later, the proligand H3[L]Br itself can be used
directly in reaction with transition metal complexes, but it
is clearly advantageous to have in hand sodium derivatives,
too, for use via salt elimination reactions. Fortunately, the
sodium salt Na2[L] obtained from the reaction between
H3[L]Br and NaN(SiMe3)2 is sufficient clean to be used
without additional purification.
t
Fig. 2. The molecular structure of Na2[L*]. Bu groups, carbon atoms of
Scheme 1.
THF, and hydrogen atoms are omitted for clarity.

236
D. Zhang et al. / Journal of Organometallic Chemistry 692 (2007) 234–242
afforded the mono-ligand complex as the sole isolable
product in 74% yield [12]. The large ionic radius of zirco-
nium relative to titanium may enable facile formation of
the bis-ligand complex 2. Complexes 1 and 2 were charac-
terized by NMR data and X-ray analysis.
The molecular structure of 1 is shown in Fig. 4, together
with relevant bond distances and angles. The structure is
similar to the titanium derivative, [L]TiCl2(thf) [12]. Com-
pound 1 displays a distorted octahedral geometry at
zirconium, in which the aryloxide units of the meridionally
coordinated [L]2ꢀ ligand occupy the axial positions. These
are considerably disordered from idealized geometry by
approximately 17ꢁ [O(1)–Zr–O(2), 162.5(3)ꢁ]. The bis-
(aryloxide)–NHC ligand assumes an S-shaped conforma-
tion with the dihedral angle of 33.3(4)ꢁ between
ZrC(8)N(1)N(2) and ZrC(8)O(1)O(2) planes, a well-estab-
lished structural feature of this ligand set. The two chloride
atoms adopt a mutually cis disposition, and a THF mole-
cule is trans to one of the chloride ligands. The metal–car-
Fig. 3. The molecular structure of H2[L*]. Hydrogen atoms except for the
hydroxyl protons are omitted for clarity.
2.2. Synthesis of Zr(L)Cl2(thf) (1) and Zr(L)2 (2)
Treatment of ZrCl4(thf)2 with 1 equiv. of freshly pre-
pared Na2[L] in THF at ꢀ78 ꢁC afforded the mono-ligand
complex [L]ZrCl2(thf) (1) as well as the bis-ligand complex
[L]2Zr (2) (Scheme 2). Different solubility properties of 1
and 2 allowed us to separate them. Recrystallization of
the products from toluene/hexane resulted in colorless
crystals of 1 in 20% yield, and 2 could be crystallized as
light yellow crystals from THF/hexane in 38% yield. The
latter could also be synthesized in 84% yield by the reaction
of ZrCl4(thf)2 with 2 equiv. of Na2[L] in THF, while
attempts to prepare the desirable mono-ligand complex 1
in high yield have been unsuccessful. The isolated com-
plexes 1 and 2 are stable in solution, and 1 is not found
to generate 2 via ligand redistribution on prolong standing
in CDCl3 according to NMR spectroscopy. Previously, we
found that the analogous (1:1) TiCl4(thf)2/Na2[L] reaction
˚
bene distance of 1 [2.35(1) A] is somewhat shorter than
those of the known Zr(IV)–NHC complexes [2.391(5)–
˚
2.456(3) A] [8e,15]. This decrease reflects geometrical
constrains arising from strain in the tridentate bis-
(aryloxide)–NHC system. The two Zr–Cl bonds are alike
˚
[2.450(3) and 2.445(3) A]. It should be noted that the
NHC moiety of the [L]2ꢀ ligand has trans influence compa-
rable to that of THF, although the strong donating prop-
erties of NHCs are generally emphasized.
The temperature-dependent NMR spectroscopic fea-
1
tures of 1 are complex. The room temperature H NMR
spectrum of 1 in CDCl3 shows two broad signals of the
N–CH2 protons at 3.73 and 6.52 ppm. Below 273 K, the
N–CH2 protons give rise to one set of mutually coupled
doublets at 4.43 and 6.39 ppm as well as two sets of mutu-
ally coupled doublets at 4.4, 4.6, 5.2, and 6.3 ppm. As the
temperature is lowered to 223 K, the signals of the 1H
NMR spectra gradually sharpen and the latter sets of sig-
nals increase in intensity relative to the other.
The two sets of mutually coupled doublets at 4.4, 4.6,
5.2, and 6.3 ppm at low temperature can be assigned to a
molecular structure with C1 symmetry found in the solid
Fig. 4. The molecular structure of 1. tBu groups and hydrogen atoms are
˚
omitted for clarity. Selected bond distances (A) and angles (ꢁ): Zr–C(8)
2.35(1), Zr–O(1) 1.995(8), Zr–O(2) 1.986(7), Zr–O(3) 2.262(8), Zr–Cl(1)
2.450(3), Zr–Cl(2) 2.445(3); O(1)–Zr–O(2) 162.5(3), Cl(1)–Zr–C(8)
169.5(3), Cl(1)–Zr–O(3) 85.3(2), Cl(1)–Zr–Cl(2) 96.8(1).
Scheme 2.

D. Zhang et al. / Journal of Organometallic Chemistry 692 (2007) 234–242
237
1
state (isomer A, in Scheme 3), as its H NMR spectrum
shows well-defined peaks in an overall pattern that resem-
bles closely that of the titanium derivative [L]TiCl2(thf)
[12]. The other pair of doublets of the N–CH2 protons evi-
dently belongs to another single isomer with higher symme-
try (B). Below room temperature, the THF protons appear
as four distinct signals, although no fine structure is
observed, suggesting that the THF is coordinated to the
A and B, the concentration of each species was measured
at separate temperatures, for which reliable measurements
could be made from integration of the H NMR spectra.
1
Equilibrium constants were calculated according to
K = [A]/[B] and plotted as a function of temperature. The
thermodynamic parameters obtained by the van’t Hoff plot
show that the isomer A is preferred over the isomer B on
enthalpic terms (DHꢂ = ꢀ8.0 kJ molꢀ1), but the equilib-
rium is disfavored entropically (DSꢂ = ꢀ31 J molꢀ1 Kꢀ1).
There may be steric interactions between the tert-butyl
groups and the THF molecule cis to the NHC carbene car-
bon in a structure for A, resulting in lowered mobility of
the tert-butyl groups or THF.
The molecular structure of the bis-ligand complex 2 is
depicted in Fig. 5, together with relevant bond distances
and angles. The zirconium center shows a distorted octahe-
dral geometry and features two meridionally coordinated
tridentate [L]2ꢀ ligands, each with mutually trans NHC
carbene donors [C(8)–Zr–C(41), 160.35(7)ꢁ]. The molecule
has approximate C2 symmetry. The ligands adopt an S-
shaped conformation again, and the twisted angles of the
1
metal in each isomer A and B. The H NMR spectrum of
the isomer B is consistent with the molecule possessing
effective C2 symmetry, in which the THF molecule is posi-
tioned trans to the NHC carbene carbon with a twofold
axis passing through the Zr metal center, the THF oxygen,
and the carbene of the S-shaped ligand. Although we have
yet to find that the tridentate bis(aryloxide)–NHC ligand
adopts a U-shaped conformation, the current results do
not conclusively rule out an alternative isomer with Cs sym-
metry (B0).
1
It is apparent from the H NMR spectral data that the
two isomers A and B are in equilibrium (Scheme 3). This
is also supported by variable temperature 13C NMR spec-
troscopy. For example, the NHC carbene carbon is
observed as one weak signal (183.5 ppm) at room temper-
ature and gives rise to two signals (182.5 and 183.0 ppm)
at lower temperature (243 K). The broadened resonances
in the NMR spectra for this system arise, because the equi-
librium process that interconverts these species occurs at a
rate that is comparable to the NMR time scale. We suggest
that the dissociation/re-coordination of THF occurs in
solution. This could give a five-coordinate intermediate
through which one chloride and the THF group could
exchange positions. To evaluate the equilibrium between
ligands
[ZrC(8)N(1)N(2)/ZrC(8)O(1)O(2),
32.48(8)ꢁ;
ZrC(41)N(3)N(4)/ZrC(41)O(3)O(4), 33.51(9)ꢁ] are similar
to that of 1. The dihedral angle between two NHC rings
˚
is 18.1(1)ꢁ. The Zr–C(carbene) [2.383(3) and 2.380(3) A]
˚
and Zr–O(aryloxide) distances [average 2.075 A] are com-
parable to the corresponding distances found in 1. The
NMR data of 2 are consistent with the solid state structure,
and signals for only a single D2 symmetric bis(bis(arylox-
ide)–NHC) ligand environment are seen, although the
slight broadening of signals assigned to the N–CH2 protons
at 4.41 and 6.08 ppm indicates some fluxional process at
room temperature. The tert-butyl groups appear as two
singlets at 0.87 and 1.23 ppm, and the NHC ring protons
are observed as a singlet at 7.05 ppm. In the room temper-
ature 13C NMR spectrum, the NHC carbene carbon is
observed at 185.5 ppm.
Fig. 5. The molecular structure of 2. tBu groups and hydrogen atoms are
˚
omitted for clarity. Selected bond distances (A) and angles (ꢁ): Zr–C(8)
2.383(3), Zr–C(41) 2.380(3), Zr–O(1) 2.027(1), Zr–O(2) 2.039(2), Zr–O(3)
2.018(2), Zr–O(4) 2.034(2); C(8)–Zr–C(41) 160.35(7), O(1)–Zr–O(2)
161.73(7), O(3)–Zr–O(4) 160.93(8).
Scheme 3.

238
D. Zhang et al. / Journal of Organometallic Chemistry 692 (2007) 234–242
2.3. Synthesis of [L]Zr(CH2Ph)2 (3) and
[L]Zr(CH2SiMe3)2 (4)
able-temperature NMR studies to explore the dynamic
behavior of 3 in C7D8. As an NMR sample is cooled, the
N–CH2 and Zr–CH2 protons give two sets of mutually
1
The successful isolation of 1 prompted us to examine
substitution reactions of the chloride ligands (Scheme 2).
Addition of 2 equiv. of PhCH2MgCl to 1 in Et2O at
ꢀ78 ꢁC followed by standard workup afforded [L]Zr-
(CH2Ph)2 (3) as light yellow crystals in 48% yield. The
analogous reaction using Me3SiCH2Li produced
[L]Zr(CH2SiMe3)2 (4) in 33%. We also examined the pro-
tonolysis of Zr(CH2Ph)4 with [H3L]Br in toluene, but the
reaction did not lead to the expected product [L]ZrBr-
(CH2Ph). Instead, the dibenzyl complex 3 was obtained
in 20% isolated yield. Ligand redistribution processes
would complicate the reaction pathways. The formulas of
alkyl derivatives 3 and 4 were inferred by NMR spectros-
copy and elemental analysis, and the molecular structure
of 3 was determined by X-ray crystallography.
coupled doublets. The H NMR line shape analysis for
these signals gives the following activation parameters for
the atropisomerization: DHꢀ = 62.1 kJ molꢀ1; DSꢀ = ꢀ8.2
J molꢀ1 Kꢀ1, which are comparable to those of isostruc-
tural
Ti(L)(CH2Ph)2
[DHꢀ = 64.8 kJ molꢀ1
;
DSꢀ =
ꢀ6.2 J molꢀ1 Kꢀ1] [12]. The small magnitude of DSꢀ is in
accord with a nondissociative rearrangement [17]. For 4,
the room temperature 1H NMR spectrum exhibits two sets
of mutually coupled doublets assignable to the N–CH2 and
Zr–CH2 protons. The encumbering alkyl groups may ham-
per atropisomerization of 4.
3. Conclusion
This investigation into zirconium N-heterocyclic carbene
chemistry has focused on the chelating bis(aryloxide)–
NHC ligand [L]2ꢀ, which is readily prepared in high yield.
Reaction of ZrCl4(thf)2 with Na2[L] resulted in a mixture
of [L]ZrCl2(thf) (1) and [L]2Zr (2). Their X-ray structures
showed the meridional coordination manner of the triden-
tate [L]2ꢀ ligand with an S-shaped conformation. The com-
plex 1 proved to be a versatile precursor to organometallic
complexes via chloride displacement reactions with
PhCH2MgCl and Me3SiCH2Li, giving the corresponding
alkyl derivatives [L]ZrR2 (R = CH2Ph (3), CH2SiMe3
(4)). During the alkylating reactions, the S-shaped confor-
mation of the [L]2ꢀ ligand is retained. Coupled with the
NMR data, the short Zr–NHC distances of 1, 2 and 3 sug-
gest that the NHC carbene carbon is strongly bound to the
zirconium metal. Compounds 1, 3, and 4 should provide
excellent starting materials for investigation of the reactiv-
ity of this class of group 4 metal complexes. We expect it to
demonstrate a rich and varied chemistry.
The molecular structure of 3 is shown in Fig. 6, together
with relevant bond distances and angles. The structure of 3
is similar to that of the titanium derivative [L]Ti(CH2Ph)2
[4]. The geometry at zirconium is distorted trigonal bipyra-
midal with a meridional coordination of the tridentate
[L]2ꢀ ligand. The two aryloxide donors [O(1) and O(2)]
occupy the axial positions with the O–Zr–O angle of
160.84(9)ꢁ. The twist angle of the S-shaped ligand in 3
[40.1(1)ꢁ, ZrC(8)N(1)N(2)/ZrC(8)O(1)O(2)] is larger than
˚
those of 1 and 2. The Zr–C(carbene) distance [2.309(3) A]
is shortened relative to those of the six-coordinated NHC
complexes 1 and 2. The Zr–Ca–Cipso angles of 109.1(2)ꢁ
and 118.9(2)ꢁ indicate g1 binding of the benzyl groups,
and the Zr–C(benzyl) distances of 2.264(4) and
˚
2.247(3) A are normal [16].
The NMR data of 3 and 4 show C2 symmetry in solu-
tion, and the structure of 3 is more fluxional in solution
than that of 4. The room temperature 13C NMR spectra
of 3 and 4 exhibit a NHC carbene signal at 187.5 and
1
185.8 ppm. In the room temperature H NMR spectrum
4. Experimental
of 3 in C6D6, two signals of tert-butyl groups appear at
1.36 and 1.87 ppm. The N–CH2 and Zr–CH2 protons are
observed as four broad signals at 2.81, 3.26, 3.53 and
3.71 ppm due to a fluxional process. We performed vari-
4.1. General methods
All manipulations of air- and/or moisture-sensitive com-
pounds were performed under argon atmosphere using
standard Schlenk-line techniques and
a
glove-box
(MBraun Labmaster 130) filled with dinitrogen (<1 ppm
H2O/O2). All dried, oxygen-free solvents and chemicals
commercially available were used as received without fur-
ther purification. Deuterated benzene (C6D6) was dried
and degassed over a potassium mirror in vacuo prior to
use. Deuterated chloroform (CDCl3) was distilled from cal-
cium hydride prior to use. NMR measurements were car-
ried out on JEOL Lambda-500, JNM-GX500, and LA-
400 spectrometers. Chemical shifts (d) for 1H NMR spectra
were referenced to residual protic solvents peaks (residual
t
Fig. 6. The molecular structure of 3. Bu groups and hydrogen atoms are
˚
omitted for clarity. Selected bond distances (A) and angles (ꢁ): Zr–C(8)
2.309(3), Zr–C(34) 2.264(4), Zr–C(41) 2.247(3), Zr–O(1) 2.011(2), Zr–O(2)
1.992(2); O(1)–Zr–O(2) 160.84(9), C(8)–Zr–C(34) 125.3(1), C(8)–Zr–C(41)
126.1(1), C(34)–Zr–C(41) 108.6(1), Zr–C(34)–C(35) 109.1(2), Zr–C(41)–
C(42) 118.9(2).
1
C6D5H in C6D6, H(d) = 7.15; residual CHCl3 in CDCl3,
1H(d) = 7.24). Elemental analyses (C, H, and N) were mea-
sured using YANACO MT-6 and MSU-32 microanalyzers.

D. Zhang et al. / Journal of Organometallic Chemistry 692 (2007) 234–242
t
239
t
4.2. Synthesis
tBu), 1.42 (s, 9H, Bu), 1.43 (s, 9H, Bu), 4.01 (s, 2H,
CH2), 5.16 (s, 2H, CH2), 6.60 (d, J = 2.4 Hz, 1H, ArH),
6.73 (d, J = 1.5 Hz, 1H, CH), 6.83 (d, J = 2.4 Hz, 1H,
ArH), 6.91 (d, J = 1.5 Hz, 1H, CH), 7.18 (d, J = 2.4 Hz,
1H, ArH), 7.23 (d, J = 2.4 Hz, 1H, ArH). The proton sig-
nals of the hydroxyl groups were not observed due to
broadening.
4.2.1. N-(4,6-Di-tert-butyl-2-hydroxybenzyl)imidazole
A flask was charged with imidazole (4.8 g, 70 mmol),
anhydrous sodium bicarbonate (NaHCO3, 6.2 g, 74 mmol),
and THF (50 mL). A solution of 2-bromomethyl-4,6-di-
tert-butylphenol (20 g, 67 mmol) in THF (100 mL) was
slowly added to the mixture at reflux temperature. After
stirring for 12 h, the mixture was poured into water, and
the product was extracted with Et2O. The combined
organic layers were dried over Na2CO3 and then volatiles
were removed in vauo to give an off-white solid. Crystalli-
zation of the crude product from Et2O/hexane gave a col-
orless crystalline solid of N-(4,6-di-tert-butyl-2-hydroxy-
benzyl)imidazole (18.3 g, 96%).
4.2.4. Reaction of ZrCl4(thf)2 with Na2[L]
A THF solution of NaN(SiMe3)2 (1.0 M, 9.9 mL,
9.9 mmol) was added to a suspension of [H3L]Br (1.93 g,
3.30 mmol) in THF (50 mL) at ꢀ78 ꢁC. The ligand solution
was cannulated into a THF (50 mL) solution of ZrCl4(thf)2
(1.25 g, 3.30 mmol) keeping the temperature below ꢀ78 ꢁC.
The mixture was allowed to warm to ambient temperature
and stirred for further 6 h. After removal of volatiles in
vacuo, the residue was extracted with toluene (50 mL)
and centrifuged to remove insoluble materials. The result-
ing supernatant was concentrated by evaporation. Then
the solution (10 mL) was layered with hexane (30 mL)
and cooled to ꢀ30 ꢁC to give [L]ZrCl2(thf) (1) as colorless
plates (0.49 g, 20%). The mother liquid was evaporated to
dryness, and the light yellow residue was extracted with
hexane (30 mL · 3). The combined extracts were evapo-
rated to dryness to leave a yellow residue, and recrystalliza-
tion from THF/hexane afforded 1.37 g of [L]2Zr (2) as light
yellow crystals in 38% yield. Crystals of 1 were contami-
nated with a small amount of 2, which we could not sepa-
rate by any method. Consequently we were unable accurate
CNH microanalysis that could be unambiguously
interpreted.
Elemental analysis. Calc. for C18H28N2O: C, 75.48; H,
9.15; N, 9.78. Found: C, 74.48; H, 9.07; N, 9.69%. 1H
t
NMR (CDCl3, 298 K): d = 1.30 (s, 9H, Bu), 1.66 (s, 9H,
tBu), 2.56 (s, 2H, CH2), 6.35 (s, 1H), 6.63 (s, 1H), 6.71 (s,
1H), 6.35 (s, 1H), 6.91 (d, J = 2.4 Hz, 1H, ArH), 7.58 (d,
J = 2.4 Hz, 1H, ArH).
4.2.2. H3[L]Br
A flask was charged with N-(4,6-di-tert-butyl-2-hydrox-
ybenzyl)imidazole (10.0 g, 35 mmol) and THF (25 mL). A
solution of 2-bromomethyl-4,6-di-tert-butylphenol (9.0 g,
30 mmol) in THF (50 mL) was slowly added at reflux tem-
perature. After stirring for 12 h, the solution was evapo-
rated to dryness to give the crude product. The product
was washed with toluene and hexane, and dried in vacuo,
which yielded colorless microcrystals of [H3L]Br (11.0 g,
63%).
Complex 1: 1H NMR (C6D6, 298 K): d = 1.25 (br.s, 4H,
THF), 1.40 (s, 18H, tBu), 1.82 (s, 18H, tBu), 3.73 (br.s, 2H,
CH2), 4.39 (br.s, 4H, THF), 5.69 (s, 2H, CH), 6.52 (br.s,
2H, CH2), 6.98 (s, 2H, ArH), 7.62 (s, 2H, ArH); 13C{1H}
NMR (CDCl3, 298 K): d = 30.8 (CCH3), 31.6 (CCH3),
34.2 (CCH3), 35.4 (CCH3), 54.2 (CH2), 120.1 (NCH),
124.9 (CH), 125.3 (CH), 141.5 (C), 159.2 (C), 183.5 (C, car-
bene). Several signals are broadened and/or overlapped in
aromatic region, and signals of THF are broadened at
room temperature.
Elemental analysis. Calc. for C33H49N2O2Br: C, 67.68;
1
H, 8.43; N, 4.78. Found: C, 67.39; H, 8.41; N, 4.74%. H
t
NMR (CDCl3, 298 K): d = 1.25 (s, 18H, Bu), 1.38 (s,
t
18H, Bu), 5.53 (s, 4H, CH2), 6.95 (br.s, 2H, OH), 7.07
(d, J = 2.4 Hz, 2H, ArH), 7.12 (d, J = 1.2 Hz, 2H, CH),
7.32 (d, J = 2.4 Hz, 2H, ArH), 9.51 (br.s, 1H, CH);
13C{1H} NMR (CDCl3, 298 K): d = 30.3 (CCH3), 31.7
(CCH3), 34.5 (CCH3), 35.1 (CCH3), 51.3 (CH2), 122.3
(C), 121.4 (CH), 125.3 (CH), 125.7 (CH), 136.0 (C), 139.1
(CH, imidazolium C2), 143.4 (C), 151.4 (C).
Complex 2: Elemental analysis. Calc. for C66H94N4-
O4Zr: C, 72.15; H, 8.62; N, 5.10. Found: C, 71.78; H,
8.23; N, 4.17%. 1H NMR (CDCl3, 298 K): d = 0.87 (s,
4.2.3. 1,2-Bis(4,6-di-tert-butyl-2-hydroxybenzyl)imidazole
(H2[L*])
t
t
36H, Bu), 1.23 (s, 36H, Bu), 4.41 (br.s, 4H, CH2), 6.08
(br.s, 4H, CH2), 6.96 (s, 4H, ArH), 7.05 (s, 4H, CH),
7.08 (s, 4H, ArH); 13C{1H} NMR (CDCl3, 298 K):
d = 30.4 (CCH3), 31.8 (CCH3), 34.1 (CCH3), 34.9
(CCH3), 53.7 (CH2), 119.5 (NCH), 123.1 (CH), 123.7
(CH), 124.4(C), 136.2 (C), 138.1 (C), 160.9 (C), 185.5 (C,
carbene).
A THF solution of NaN(SiMe3)2 (1.0 M, 15.4 mL,
15.4 mmol) was added to a suspension of [H3L]Br (3.0 g,
5.12 mmol) in THF (50 mL) at ꢀ78 ꢁC. The mixture was
allowed to warm up to room temperature and stirred for
further 3 h. Then the mixture was poured into water and
extracted with Et2O. The combined organic layers were
evaporated to dryness, and the residue was crystallized
from Et2O/hexane to give 1,2-disubstituted imidazole
(H2L*) as colorless crystals.
4.2.5. Analysis of the equilibrium of 1 in solution
Complex 1 was dissolved in CDCl3, and the tube was
1
Elemental analysis. Calc. for C33H48N2O2: C, 78.53; H,
9.59; N, 5.55. Found: C, 78.41; H, 9.67; N, 5.55%. 1H
sealed under an argon atmosphere. H NMR spectra were
taken at 9 temperatures ranging from 223 to 263 K. The K
t
NMR (CDCl3, 298 K): d = 1.15 (s, 9H, Bu), 1.21 (s, 9H,
values obtained at given temperature are as follows: 223 K,

240
D. Zhang et al. / Journal of Organometallic Chemistry 692 (2007) 234–242
1.788; 228 K, 1.622; 233 K, 1.480; 238 K, 1.388; 243 K,
1.282; 248 K, 1.176; 253 K, 1.080; 258 K, 0.990; 263 K,
0.930. Thermodynamic parameters were determined from
van’t Hoff plots of the above data.
Line shape analysis of the methylene region was carried
out at each temperature using a GNMR program [18]. The
k1 values obtained at given temperature are as follows:
298 K, 32; 313 K, 100; 323 K, 240; 333 K, 410; 343 K,
900; 353 K, 1900; 363 K, 3500; 373 K, 6000. Activation
parameters were determined from Eyring plots of the
above data.
4.2.6. [L]2Zr (2)
A THF solution of NaN(SiMe3)2 (1.0 M, 3.3 mL,
3.3 mmol) was added to a suspension of [H3L]Br (0.59 g,
1.00 mmol) in THF (20 mL) at ꢀ78 ꢁC. The ligand solution
was cannulated into a THF (20 mL) solution of ZrCl4(thf)2
(0.19 g, 0.50 mmol) keeping the temperature below ꢀ78 ꢁC.
The mixture was allowed to warm to ambient temperature
and stirred for further 12 h. After removal of volatiles in
vacuo, the residue was extracted from hexane (50 mL)
and centrifuged to remove insoluble materials. The extract
was evaporated to dryness. Recrystallization from THF/
hexane gave 2 as light yellow crystals (0.92 g, 84%).
4.2.10. [L]Zr(CH2SiMe3)2 (4)
A pentane solution of Me3SiCH2Li (1.0 M, 1.1 mL,
1.1 mmol) was added to 1 (0.40 g, 0.54 mmol) in Et2O
(40 mL) at ꢀ78 ꢁC. The mixture was allowed to warm up
to ambient temperature. After stirring for 10 h, removal
of the solvent in vacuo left a light yellow residue. Extrac-
tion with toluene (10 mL) and centrifugation gave a yellow
solution. The yellow extract was evaporated to dryness.
Recrystallization from toluene/hexane afforded a light yel-
low crystalline solid of 4 (136 mg, 33%).
4.2.7. [L]Zr(CH2Ph)2 (3): reaction of 1 and PhCH2MgCl
An Et2O solution of PhCH2MgCl (1.0 M, 1.1 mL,
1.1 mmol) was added to 1 (0.40 g, 0.54 mmol) in Et2O
(40 mL) at ꢀ78 ꢁC. The mixture was allowed to warm up
to ambient temperature and stirred for further 10 h. After
removal of the solvent under reduced pressure, the light
yellow residue was extracted with Et2O (30 mL) and centri-
fuged to remove insoluble material. The supernatant was
evaporated to dryness, and then the residue was crystal-
lized from Et2O/hexane to afford 3 as light yellow crystals
(198 mg, 48%).
Elemental analysis. Calc. for C41H69N2O2Si2Zr: C,
64.00; H, 9.04; N, 3.64. Found: C, 63.82; H, 8.62; N,
1
3.35%. H NMR (C6D6, 298 K): d = 0.20 (s, 18H, SiMe3),
t
0.99 (d, J = 11 Hz, 2H, CH2), 1.37 (s, 18H, Bu), 1.48 (d,
J = 11 Hz, 2H, CH2), 1.85 (s, 18H, tBu), 3.83 (d,
J = 14 Hz, 2H, CH2), 5.24 (d, J = 14 Hz, 2H, CH2), 5.55
(s, 2H, CH), 7.00 (s, 2H, ArH), 7.63 (s, 2H, ArH);
13C{1H} NMR (C6D6, 298 K): d = 4.4 (SiMe3), 31.6
(CCH3), 32.7 (CCH3), 35.1 (CCH3), 36.7 (CCH3), 54.0
(CH2), 60.0 (CH2), 119.0 (NCH), 124.65 (CH), 124.73
(CH), 125.8 (C), 138.5 (C), 140.5 (C), 160.4 (C), 185.8 (C,
carbene).
Elemental analysis. Calc. for C47H61N2O2Zr: C, 72.63;
1
H, 7.91; N, 3.60. Found: C, 72.05; H, 8.05; N, 3.62%. H
NMR (C6D6, 298 K): d = 1.36 (s, 18H, tBu), 1.87 (s,
4.3. Crystallography
t
18H, Bu), 2.81 (br.s, 2H, CH2), 3.26 (br.s, 2H, CH2),
3.53 (br.s, 2H, CH2), 3.71 (br.s, 2H, CH2), 5.53 (s, 2H,
CH), 6.74 (t, J = 8 Hz, 2H, benzyl(para)), 6.88 (d,
J = 3 Hz, 2H, ArH), 6.89 (dd, J = 8 Hz, 4H, benzyl(meta)),
7.16 (d, J = 8 Hz, 4H, benzyl(ortho)), 7.62 (d, J = 3 Hz,
2H, ArH); 13C{1H} NMR (C6D6, 298 K): d = 32.0
(CCH3), 32.9 (CCH3), 35.3 (CCH3), 36.9 (CCH3), 53.7
(CH2), 67.5 (CH2), 119.5 (NCH), 122.1(CH), 125.1 (CH),
125.3 (CH), 126.9 (C), 127.8 (CH), 129.2(CH), 138.5 (C),
141.0 (C), 143.9 (C), 160.1 (C), 187.5 (C, carbene).
Crystallographic data are summarized in Table 1. Single
crystals for X-ray diffraction were grown from DME for
[H3L]Br (colorless crystals), Et2O/THF for Na2[L*] (color-
less crystals), Et2O/hexane for H2[L*] (colorless crystals)
and 3 (light yellow crystals), toluene/hexane for 1 (colorless
crystals), THF/hexane for 2 (colorless crystals). Crystals
were covered in oil, and suitable single crystals were
selected under a microscope and mounted on a Rigaku
Mercury CCD (for H3[L]Br, Na2[L*], H2[L*], and 2) or Sat-
urn CCD (for 1 and 3) diffractometer equipped with a Rig-
aku GNNP low-temperature device. Data were collected at
ꢀ100(1) ꢁC under a cold nitrogen stream using graphite-
4.2.8. [L]Zr(CH2Ph)2 (3): reaction of Zr(CH2Ph)4 with
H3[L]Br
˚
A suspension of H3[L]Br (950 mg, 1.62 mmol) in toluene
(65 mL) was added to Zr(CH2Ph)4 (746 mg, 1.64 mmol) in
toluene (20 mL) at 0 ꢁC. The mixture was stirred for fur-
ther 4 h at ambient temperature. Volatiles were removed
in vacuo to leave a light yellow residue. The residue was
washed with Et2O/hexane (30 mL/20 mL) and dried in
vacuo, yielding a yellow solid of 3 (256 mg, 20%).
monochromated Mo Ka radiation (k = 0.71070 A). The
intensity images were obtained with x scans of 0.5ꢁ interval
per flame. The flame data processed using the CrystalClear
(Rigaku) program, and the reflection data were corrected
for absorption with a REQAB program. The structures for
Na2[L*], H2[L*], 1 and 2 were solved by direct method,
and the structures of H3[L]Br and 3 were solved by Patter-
son method. These structures were refined on F2 by the full-
matrix least-squares method using CrystalStructure (Rig-
aku) software package [19]. For Na2[L*] and H2[L*],
unique reflections at 2h < 45ꢁ and 50ꢁ were used for refine-
ment due to the low quality of crystals. The hydrogen
4.2.9. Analysis of the fluxionality of 3 in solution
Complex 3 was dissolved in toluene-d8, and the tube was
1
sealed under an argon atmosphere. H NMR spectra were
taken at eight temperatures ranging from 25 to 100 ꢁC.

D. Zhang et al. / Journal of Organometallic Chemistry 692 (2007) 234–242
241
Table 1
Crystallographic data for H3[L]Br, Na2[L*], H2[L*], 1, 2, and 3
Compound
H3[L]Br Æ DME Na2[L*]2 Æ Et2O
H2[L]*
1 Æ toluene
2 Æ 6 THF
3 Æ 0.5 hexane
Formula
C
37H59N2O4Br C94H150N4O11Na4 C33H48N2O2
C44H62N2O3Cl2Zr C90H140N4O10Zr C50H67N2O2Zr
Molecular weight
Space group
a (A)
675.79
1604.20
504.75
829.11
1529.34
819.31
P21/c (No. 14)
16.240(5)
11.561(3)
24.512(8)
90
104.961(4)
90
4446(2)
4
1.224
2.869
35532
10498
541
ꢀ
P21/c (No. 14) C2/c (No. 15)
P21/c (No. 14) P21/c (No. 14)
P1 ðNo: 2Þ
˚
12.316(4)
18.308(5)
17.085(5)
90
96.799(4)
90
3824(1)
4
1.173
11.149
30730
8499
28.38(2)
16.14(1)
21.48(1)
90
95.848(8)
90
13.82(1)
11.66(1)
19.88(2)
90
103.41(1)
90
3115(4)
4
1.076
0.659
25215
7112
17.241(8)
13.813(6)
19.176(8)
90
103.027(9)
90
4449(3)
4
1.238
4.049
17784
8917
13.4025(4)
13.5072(1)
26.8860(9)
76.450(7)
89.583(9)
68.959(6)
4400.3(2)
2
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
g (ꢁ)
3
˚
V (A )
9787(10)
4
Z
Dcalc (g cmꢀ3
)
1.089
0.846
39071
11197
555
1.154
1.815
35776
19338
l(Mo Ka) (cmꢀ1
)
Collected reflections
Unique reflections
Parameters
457
348
488
1020
R1 [I > 2s(I)]
wR2 (all)
Goodness-of-fit
Largest residual peak and hole (e A
0.054
0.171
1.001
0.094
0.287
1.013
0.095
0.261
0.974
0.083
0.199
0.990
0.072
0.163
1.069
0.075
0.088
0.985
ꢀ3
˚
)
1.51 and ꢀ0.61 2.66 and ꢀ1.16
0.99 and ꢀ0.79 6.96 and ꢀ4.31
3.12 and ꢀ2.20 1.31 and ꢀ0.85
(b) W.A. Herrmann, C. Ko¨cher, Angew. Chem., Int. Ed. Engl. 36
(1997) 1262.
[3] (a) Melanie S. Sanford, Jennifer A. Love, Robert H. Grubbs, J. Am.
Chem. Soc. 123 (2001) 6543;
atoms of the hydroxyl groups and the imidazolium protons
for H3[L]Br and the hydroxyl groups for H2[L*] were
located and were refined isotropically. Anisotropic refine-
ment was applied to all non-hydrogen atoms except for dis-
ordered atoms and crystal solvent molecules. The other
hydrogen atoms were put at calculated positions with C–
(b) T. Weskamp, F.J. Kohl, W. Hieringer, D. Gleich, W.A.
Herrmann, Angew. Chem., Int. Ed. 38 (1999) 2416;
(c) D.S. McGuinness, K.J. Cavell, B.W. Skelton, A.H. White,
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(d) V. Ce´sar, S. Bellemin-Laponnaz, L.H. Gade, Chem. Soc. Rev. 33
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˚
H distances of 0.97 A.
5. Supplementary data
[4] (a) J.C. Green, R.G. Scurr, P.L. Arnold, F.G.N. Cloke, Chem.
Commun. (1997) 1963;
(b) J. Huang, H.J. Schanz, E.D. Stevens, S.P. Nolan, Organometallics
18 (1999) 5375;
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Soc. 125 (2003) 10490.
Crystallographic data for the structural analyses have
been deposited with the Cambridge Crystallographic Data
Center, CCDC Nos. 299115, 299116, 299117, 299118,
299119, and 299120 for compounds H3[L]Br, Na2[L*],
H2[L*], 1, 2, and 3, respectively. Copies of this information
may be obtained free of charged from The Director,
CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
+44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk).
¨
[5] (a) W.A. Herrmann, K. Ofele, M. Elison, F.E. Kuhn, P.W. Roesky,
¨
J. Organomet. Chem. 480 (1994) C7;
(b) M. Niehues, G. Erker, G. Kehr, P. Schwab, R. Fro¨hlich, O.
Blacque, H. Berke, Organometallics 21 (2002) 2905;
(c) N. Kuhn, T. Kratz, D. Bla¨ser, R. Boese, Inorg. Chim. Acta 238
(1995) 179;
(d) H. Nakai, X. Hu, L.N. Zakharov, A.L. Rheingold, K. Meyer,
Inorg. Chem. 43 (2004) 855;
Acknowledgements
(e) D. Pugh, J.A. Wright, S. Freeman, A.A. Danopoulos, Dalton
Trans. (2006) 775.
[6] D.S. McGuinness, V.C. Gibson, J.W. Steed, Organometallics 23
(2004) 6288.
[7] (a) P.L. Arnold, S.A. Mungur, A.J. Blake, C. Wilson, Angew.
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[8] (a) S.A. Mungur, S.T. Liddle, C. Wilson, M.J. Sarsfield, P.L. Arnold,
Chem. Commun. (2004) 2738;
Institute for Molecular Science (IMS) is thanked for
financial support. This work was also supported by a
Grant-in-Aid for Scientific Research [Nos. 14703008,
16655025, 17350031, and 14078101 (Priority Areas ‘‘Reac-
tion Control of Dynamic Complexes’’)] from Ministry of
Education, Culture, Sports, Science, and Technology,
Japan.
(b) P.L. Arnold, S.T. Liddle, Chem. Commun. (2005) 5638;
(c) S.T. Liddle, P.L. Arnold, Organometallics 24 (2005) 2597;
(d) P.L. Arnold, S.T. Liddle, Organometallics 25 (2006) 1845;
(e) L.P. Spencer, S. Winston, M.D. Fryzuk, Organometallic 23
(2004) 3372;
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