Synthesis and reactivity of niobium complexes having a tripodal triaryloxide ligand in bidentate, tridentate, and tetradentate coordination modes

Article abstract of DOI:10.1039/c0dt01022g

The synthesis and reactivity of niobium complexes incorporating a tripodal triphenol (tris(3,5-tert-butyl-2-hydroxylphenyl)methane = H₃[O ₃]) have been investigated. Addition of one equivalent of NbCl ₅ in CH₃CN to H₃[O₃] in toluene led to partial HCl elimination, giving [H(O₃)]NbCl₃(CH ₃CN) (1) with a bidendtate bis(aryloxide) ligand and a pendant phenol arm. Treatment of 1 with THF afforded [H(O₃)]NbCl₃(THF) (2). Deprotonation of 1 with NEt₃ in toluene promoted coordination of the pendant phenol group to generate (Et₃NH)[(syn-O ₃)NbCl₃] (3-syn). Prolonged heating of 3-syn resulted in clean conversion to the anti isomer (3-anti). Attempted deprotonation of 2 with PhCH₂MgCl provided [H(O₃)]Nb(CH₂Ph)₃ (4), in which alkylation took place at the metal center but the pendant phenol arm remained intact. When 3-syn was treated with PhCH₂MgCl, [O ₃C]Nb(CH₂Ph) (5) was produced via C-H activation of the methine C-H bond. The analogous reaction with 3-anti provided a benzylidene complex [anti-O₃]Nb(CHPh)(THF) (6). During the course of the reaction, the anti ligand conformation is retained. Upon heating, 4 underwent methine C-H and phenol O-H activation, yielding the metalatrane 5. Complexes 1, 3-syn, 3-anti, 4, and 5 were characterized by X-ray diffraction.

Full text of DOI:10.1039/c0dt01022g

View Article Online / Journal Homepage / Table of Contents for this issue
This article is published as part of the Dalton Transactions themed issue entitled:
New Talent Asia
Highlighting the excellent work being carried out by younger members
of the inorganic academic community in Asia
Guest Editor Masahiro Yamashita
Tohoku University, Japan
Published in issue 10, 2011 of Dalton Transactions
Image reproduced with permission of Kenneth Kam-Wing Lo
Articles in the issue include:
PERSPECTIVES:
Pyrazolin-4-ylidenes: a new class of intriguing ligands
Yuan Han and Han Vinh Huynh, Dalton Trans., 2011, DOI: 10.1039/C0DT01037E
Solvent induced molecular magnetic changes observed in single-crystal-to-single-crystal
transformation
Zheng-Ming Hao and Xian-Ming Zhang, Dalton Trans., 2011, DOI: 10.1039/C0DT00979B,
ARTICLES:
Negative thermal expansion emerging upon structural phase transition in ZrV2O7 and HfV2O7
Yasuhisa Yamamura, Aruto Horikoshi, Syuma Yasuzuka, Hideki Saitoh and Kazuya Saito
Dalton Trans., 2011, DOI: 10.1039/C0DT01087A
Preparation of surface molecularly imprinted Ru-complex catalysts for asymmetric transfer
hydrogenation in water media
Zhihuan Weng, Satoshi Muratsugu, Nozomu Ishiguro, Shin-ichi Ohkoshi and Mizuki Tada
Dalton Trans., 2011, DOI: 10.1039/C0DT00950D
Visit the Dalton Transactions website for more cutting-edge inorganic and organometallic research
www.rsc.org/dalton

Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 2375
www.rsc.org/dalton
PAPER
Synthesis and reactivity of niobium complexes having a tripodal triaryloxide
ligand in bidentate, tridentate, and tetradentate coordination modes†
Fumio Akagi, Yutaka Ishida, Tsukasa Matsuo and Hiroyuki Kawaguchi*
Received 16th August 2010, Accepted 3rd December 2010
DOI: 10.1039/c0dt01022g
The synthesis and reactivity of niobium complexes incorporating a tripodal triphenol
(tris(3,5-tert-butyl-2-hydroxylphenyl)methane = H3[O3]) have been investigated. Addition of one
equivalent of NbCl5 in CH3CN to H3[O3] in toluene led to partial HCl elimination, giving
[H(O3)]NbCl3(CH3CN) (1) with a bidendtate bis(aryloxide) ligand and a pendant phenol arm.
Treatment of 1 with THF afforded [H(O3)]NbCl3(THF) (2). Deprotonation of 1 with NEt3 in toluene
promoted coordination of the pendant phenol group to generate (Et3NH)[(syn-O3)NbCl3] (3-syn).
Prolonged heating of 3-syn resulted in clean conversion to the anti isomer (3-anti). Attempted
deprotonation of 2 with PhCH2MgCl provided [H(O3)]Nb(CH2Ph)3 (4), in which alkylation took place
at the metal center but the pendant phenol arm remained intact. When 3-syn was treated with
PhCH2MgCl, [O3C]Nb(CH2Ph) (5) was produced via C–H activation of the methine C–H bond. The
analogous reaction with 3-anti provided a benzylidene complex [anti-O3]Nb(CHPh)(THF) (6). During
the course of the reaction, the anti ligand conformation is retained. Upon heating, 4 underwent methine
C–H and phenol O–H activation, yielding the metalatrane 5. Complexes 1, 3-syn, 3-anti, 4, and 5 were
characterized by X-ray diffraction.
of metal complexes, we have drawn our attention to a tripodal
Introduction
triaryloxide, in which three aryloxide units are joined to one
methine carbon ([O3]3-, Scheme 1). Unlike linear-type tridentate
The use of multidentate ligands in coordination chemistry and
homogeneous catalysis is one of the versatile strategies in ligand
ligands adopting facial and meridional coordination modes, the
design for the discovery of metal complexes with novel proper-
ties. Multidentate ligands with various combinations of donor
tripodal triaryloxide ligand is geometrically predisposed to facial
coordination. The tridentate [O3] ligand binds to the metal in two
fashions, in which the C–H bond of the bridging methine has a
different orientation (syn and anti forms, Scheme 2). In addition,
functions and ligand structures (linear, cyclic, etc.) have been
extensively investigated.1–5 Aryloxides can serve as a useful scaffold
for the construction of multidentate ligands owing to the avail-
the [O3] ligand could be used as a precursor for introducing
ability of two substitutes at the ortho positions on the aryloxide
a tetradentate [O3C] ligand via C–H activation of the methine
ring.6,7 This dual substitute pattern allows for the integration of
carbon.
aryloxide units into the multidentate ligand framework as well
as the tunability of steric and electronic factors. As a result,
aryloxide-based multidentate ligands have been found to support a
number of catalytic and stoichiometric transformations, e.g., olefin
polymerization,8–10 aerial oxidation of alcohols,11 and activation of
small molecules.12–14
We are interested in the coordination chemistry of aryloxide-
based multidentate ligands.15 These include triaryloxides ([lin-
O3]3-)16–19 and mixed-donor ligands containing two aryloxides
and one neutral donor.20,21 In these tridentate ligands, three
donor functions are linearly arranged in the ligand framework.
Scheme 1 Linear and tripodal triaryloxide ligands.
Interested by the effect of ligand structure on chemical properties
Previously, we reported the synthesis and structures of titanium
and zirconium complexes having [syn-O3]3-, [anti-O3]3- and [O3C]4-
ligands.22 We also employed the [anti-O3] ligand to prepare a
Department of Chemistry, Tokyo Institute of Technology, Ookayama,
Meguro-ku, Tokyo, 152-8551, Japan. E-mail: hkawa@chem.titech.ac.jp
diniobium hydride complex capable of cleaving dinitrogen.23 In
† CCDC reference numbers 655244 and 789692–789695. For crys-
tallographic data in CIF or other electronic format see DOI:
10.1039/c0dt01022g
this report, we outline some chemistry of niobium complexes
bearing the tripodal triaryloxide. The study details synthetic
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2375–2382 | 2375
©

Table 1 1H and 13C NMR data for the bridging methine
1H (d)
13C (d)
1JCH (Hz)
1
7.83
7.80
7.45
5.91
5.87
6.18
5.94
39.0
38.9
36.8
65.8
52.1
62.0
42.5
124.5
—
91.8
119.8
117.7
124.0
126.8
a
2
3-syn
3-anti
4
6
H3[O3]
a The resonances for the ligand backbone are broadened due to a fluxional
process in solution.
Scheme 2 Coordination modes of [O3] and [O3C] ligands.
protocols for complexes employed in studies on reactivity. Some
of the results included have been previously communicated.23
Results and discussion
There are various synthetic routes to aryloxide complexes. The
choice of the synthetic method depends on metal precursors as
well as the nature of aryloxide ligands. Previously, Scott and
Dinger reported that the lithium and sodium salts of [O3]3- reacted
with metal halides such as TiCl4, NbCl5, and TaCl5, resulting in
formation of multiple and/or decomposition products.24 Nielson
et al. prepared a tantalum complex [(syn-O3)TaCl3]- by the
reaction of TaCl5 with H3[O3] in the presence of NEt3, but it
was obtained as a small quantity of crystals.25 These two methods
appear not to be a straightforward route for installing the tripodal
[O3] ligand on early transition metals.
Given the difficulty of isolating chloride complexes of the
type [O3]MCl2 in these reactions, we turned to an alternative
procedure based on earlier investigations on the preparation of
linear-triaryloxide [lin-O3] complexes. We have shown that HCl
elimination reactions between MCl5 (M = Ta, Nb) and the protio
ligands are a convenient entry to linear-triaryloxide derivatives
of group 5 metals.26 A similar approach was examined with
NbCl5 and H3[O3] (Scheme 3). The reaction proceeded cleanly in
CH3CN to provide [H(O3)]NbCl3(CH3CN) (1). After evaporating
the solvent and washing the residue with hexane, 1 was isolated as
a brown powder in 84% yield. The 1H NMR spectrum of 1 displays
resonances for two types of aryl rings in a 2 : 1 ratio. The presence
of a hydroxyl group and a coordinated CH3CN molecule is
characterized by two singlets at d 5.62 and 1.23, respectively. In the
IR spectrum, the n(OH) and n(CN) absorptions are observed at
3543 and 2290 cm-1. These spectroscopic data are consistent with
partial HCl elimination to generate two coordinated aryloxide
groups and the remaining ligand arm dangling as the free phenol.
Diagnostic spectroscopic features for [O3] derivatives include
NMR resonances for the bridging methine, which provide a
criterion for determining coordination modes of the [O3] ligand
Scheme 3 Synthesis of 1, 2, 3-syn and 3-anti. Reagents and conditions:
(i) NbCl5, CH3CN, toluene, rt, 3 h; (ii) THF, rt; (iii) 1, NEt3, toluene, rt,
3 h; (iv) toluene, 80 ◦C, 3 d.
(Table 1).22,24,27 The 1H and 13C NMR methine signals of 1 resonate
at d 7.83 and 39.0, which are shifted relative to the corresponding
values for free H3[O3] (1H, d 5.94; 13C, d 42.5).
The molecular structure of 1 is presented in Fig. 1, which is
closely related to that of [H(lin-O3)]NbCl3(CH3CN).26 Complex
1 contains a distorted octahedral Nb(V) center with a bidentate
H[O3] dianionic ligand, three meridionally arranged chloride
ligands and a CH3CN ligand trans to one of the chelating
aryloxide oxygen atoms [O(1)]. The remaining phenol group is
˚
not coordinated to niobium [Nb ◊ ◊ ◊ O(2) = 4.63 A], while this
hydroxy group could form an intramolecular hydrogen bond to
˚
˚
one chloride ligand [Cl(3) ◊ ◊ ◊ O(2) = 3.29 A, Cl(3) ◊ ◊ ◊ H(2) = 2.50 A,
Cl(3) ◊ ◊ ◊ H(2)–O(2) = 156◦].28,29 This intramolecular hydrogen
bonding might causes a slight elongation in the Nb–Cl(3) distance
˚
[2.4061(13) A] compared to the Nb–Cl(1) and Nb–Cl(2) distances
˚
˚
[2.376(2), 2.3495(13) A]. The Nb–O distances (average 1.865 A)
are typical of those observed for related niobium(V) aryloxide
30–34
˚
˚
complexes (1.82–1.92 A).
The Nb–N distance [2.268(5) A]
35
˚
is slightly longer than that of NbCl5(CH3CN) [2.268(6) A].
The molecule possesses C1 symmetry in the solid state, while Cs
symmetry is observed in solution due to dissociation/association
of CH3CN on the NMR time scale. Upon dissolution in THF, the
2376 | Dalton Trans., 2011, 40, 2375–2382
This journal is
The Royal Society of Chemistry 2011
©

Fig. 2 Molecular structure of the anion part of 3-syn with thermal
ellipsoids drawn at the 50% probability level. One of the independent
molecules in the asymmetric unit is presented. All hydrogen atoms and
tert-butyl methyl groups are omitted for clarity. Selected bond distances
(A) and angles ( ): Nb(1)–Cl(1) 2.4346(13), Nb(1)–Cl(2) 2.4496(12),
Nb(1)–Cl(3) 2.4215(14), Nb(1)–O(1) 1.943(3), Nb(1)–O(2) 1.919(3),
Nb(1)–O(3) 1.939(3), Cl(1)–Nb(1)–Cl(2) 85.02(5), Cl(1)–Nb(1)–Cl(3)
85.85(5), Cl(2)–Nb(1)–Cl(3) 88.93(5), Cl(1)–Nb(1)–O(1) 168.91(9),
Cl(2)–Nb(1)–O(2) 175.60(9), Cl(3)–Nb(1)–O(3) 174.25(7), O(1)–
Nb(1)–O(2) 96.86(11), O(1)–Nb(1)–O(3) 96.50(11), O(2)–Nb(1)–O(3)
94.27(11), Nb(1)–O(1)–C(3) 147.7(3), Nb(1)–O(2)–C(9) 147.6(3),
Nb(1)–O(3)–C(15) 147.3(2).
Fig.
1 Molecular structure of 1 with thermal ellipsoids drawn
◦
˚
at the 50% probability level. All hydrogen atoms and tert-butyl
methyl groups are omitted for clarity. Selected bond distances (A)
˚
and angles (◦): Nb–Cl(1) 2.376(2), Nb–Cl(2) 2.3495(13), Nb–Cl(3)
2.4061(13), Nb–O(1) 1.852(3), Nb–O(3) 1.877(4), Nb–N 2.268(5),
N–C(44) 1.136(7), Cl(1)–Nb–O(3) 171.33(10), Cl(2)–Nb–Cl(3) 164.51(6),
O(1)–Nb–O(3) 96.56(14), O(1)–Nb–N 177.03(18), Nb–O(1)–C(3)
150.0(4), Nb–O(3)–C(39) 160.4(3), Nb–N–C(44) 172.2(4), N–C(44)–C(45)
176.3(7).
to 88.93(5)◦, respectively. Notably, the syn conformation adopted
by the [O3] ligand positions the methine H(1) proton proximal
labile CH3CN ligand is readily substituted to give a THF adduct,
[H(O3)]NbCl3(THF) (2).
˚
to niobium. The two atoms are only 2.15 A apart, a distance
Although installation of the [O3] ligand on the metal is
incomplete, clean formation of 1 and 2 prompts us to investigate
within the range previously observed for agostic interaction.36–40
Therefore, there may exist some transannular interaction of the
methine proton with the metal center. This is in agreement with
the low JCH value for the methine carbon.36–41 The Nb–O distances
3
deprotonation of the pendant phenol arm, resulting in a k -O3
coordination mode. Treatment of 1 with Et3N could effect depro-
tonation of the remaining hydroxyl group, giving (Et3NH)[(syn-
O3)NbCl3] (3-syn) in 86% isolated yield. The 1H NMR spectrum
suggests a C3 symmetry of the ligand backbone, possessing
two tert-butyl singlets and two aromatic doublets. A tridentate
interaction of the ancillary ligand is inferred from the loss of the
hydroxyl resonance. In addition, the resonance due to the bridging
methine proton at d 7.54 is slightly shifted upfield relative to that
seen for 1 (Table 1). In the gated decoupled 13C NMR spectrum,
the bridging methine of the [syn-O3] ligand appears as a doublet
˚
(average 1.934 A) in 3-syn are considerably longer than those of
1. This elongation is presumably a consequence of some strain
imposed by encapsulation of the methine proton within the [syn-
˚
O3]Nb cage. The Nb–Cl distances of 3-syn (average 2.435 A) are
˚
longer than those of 1 (average 2.377 A) due to the increased
number of coordinated strong aryloxide donors.
We previously reported that syn/anti isomerization in
[O3]Ti(NEt2) was thermally induced.21 Likewise, upon heating
a toluene solution at 80 ◦C for 3 days, 3-syn underwent clean
conversion to the anti isomer (Et3NH)[(anti-O3)NbCl3] (3-anti).
1
1
at d 36.8 with JCH = 91.8 Hz. This JCH coupling constant is
appreciably lower than those of H3[O3] (126.8 Hz) and 1 (124.5
Hz).
1
The H and 13C NMR spectra are consistent with the complex
3-anti retaining C3 symmetry in solution. Isomerization of 3-syn
to 3-anti is confirmed by an upfield shift of the methine proton
at d 5.91 along with a downfield shift of the methine carbon
at d 65.8 with the normal JCH value of 119.8 Hz (Table 1). To
further investigate the structure of 3-anti, we turned to X-ray
diffraction (Fig. 3). While the coordination geometry of niobium
is similar to that of 3-syn, a prominent difference pertains to
the conformation of the triaryloxide ligand. The conformational
constraint present in 3-syn is relieved by adopting an anti from,
in which the methine proton is pointed away from the metal
center, leading to a shortening of the Nb–O distances in 3-anti
Brown crystals of 3-syn suitable for X-ray diffraction were
obtained from a saturated toluene solution. The crystals of 3-syn
contain two independent molecules in the asymmetric unit. Since
bond distances and angles for the two independent molecules are
effectively identical, discussion will focus on only that shown in
Fig. 2. The structure of 3-syn closely resembles that of the related
tantalum complex.25 The anion is a distorted octahedral geometry,
in which the [O3] ligand is bound in a facial manner [average
O–Nb–O = 95.9◦]. The remainder of the coordination sphere is
occupied by three chloride ligands with O–Nb–Cltrans and Cl–Nb–
Cl angles ranging from 168.91(9) to 175.60(9)◦ and from 85.02(5)
˚
by ca. 0.05 A relative to 3-syn. The two isomers 3-syn and 3-anti
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2375–2382 | 2377
©

Fig. 3 Molecular structure of the anion part of 3-anti with thermal
ellipsoids drawn at the 50% probability level. All hydrogen atoms
and tert-butyl methyl groups are omitted for clarity. Selected
bond distances (A) and angles ( ): Nb–Cl(1) 2.4571(11), Nb–Cl(2)
2.4630(10), Nb–Cl(3) 2.4200(10), Nb–O(1) 1.882(2), Nb–O(2) 1.881(2),
Nb(1)–O(3) 1.883(3), Cl(1)–Nb–Cl(2) 87.64(4), Cl(1)–Nb–Cl(3) 86.20(4),
Cl(2)–Nb–Cl(3) 85.77(3), Cl(1)–Nb–O(2) 177.75(7), Cl(2)–Nb–O(3)
◦
Scheme 4 Synthesis of 4 and 5. Reagents and conditions: (i) PhCH2MgCl
(in Et2O), toluene, -98 ◦C to rt, 5 h; (ii) toluene, 50 ◦C, 5 d; (iii) PhCH2MgCl
(in Et2O), toluene, -70 ◦C to rt, 2 h.
˚
176.85(6),
Cl(3)–Nb–O(1)
173.16(6),
O(1)–Nb–O(2)
89.97(9),
O(1)–Nb–O(3) 94.15(9), O(2)–Nb–O(3) 90.98(9), Nb–O(1)–C(3)
155.15(17), Nb–O(2)–C(9) 150.75(19), Nb–O(3)–C(15) 155.00(18).
display approximately C3 symmetry in the solid state, where the
aryl rings of the [O3] ligand are twisted in a propeller like fashion
with average O–Nb–C(methine)–Cipso torsion angles q = 6.74◦ and
4.92◦, respectively.
The synthesis of the desired niobium complexes having the
k -[O3] ligand has been achieved by deprotonation of 1 and 2
3
with Et3N, resulting in formation of 3-syn and 3-anti as Et3NHCl
adducts. To avoid generation of ate complexes, we examined the
reaction of 2 with Grignard reagents. Attempted deprotonation of
2 with PhCH2MgCl in Et2O led to the formation of an unexpected
product [H(O3)]Nb(CH2Ph)3 (4) (Scheme 4), in which alkylation at
the metal center took place and the pendant phenol arm remained
intact. When the amount of PhCH2MgCl was increased to three
equivalents, the tribenzyl complex 4 was isolated in 77% yield.
1
The H NMR spectrum shows two sets of benzyl resonances in
Fig. 4 Molecular structure of 4 with thermal ellipsoids drawn at
the 50% probability level. All hydrogen atoms and tert-butyl methyl
groups are omitted for clarity. The pendant phenol oxygen O(3) was
found to be disordered over two positions with occupancy factors
a 2 : 1 ratio as well as four tert-butyl resonances in a 2 : 2 : 1 : 1
ratio, suggesting a Cs-symmetric molecular structure in solution.
A bidentate interaction of the ancillary ligand is further inferred
on the basis of the low symmetry along with the resonance due
to the hydroxyl group at d 5.44. In the IR spectrum, the n(OH)
absorption is observed at 3448 cm-1.
The X-ray structure of 4 has been determined (Fig. 4). The
coordination geometry of the metal center is square pyramidal.
The basal plane is comprised of two oxygen atoms of the chelating
H[O3] ligand and two benzyl ligands [C(44), C(58)], while the
apical position is occupied by the benzyl ligand [C(51)]. The
boat conformation of the eight-membered NbO2C5 chelating
ring brings the methine proton to the vacant site trans to the
◦
˚
of 0.70 : 0.30. Selected bond distances (A) and angles ( ): Nb–O(1)
1.915(2), Nb–O(2) 1.9449(17), Nb–C(44) 2.229(3), Nb–C(51) 2.193(3),
Nb–C(58) 2.216(4), O(1)–Nb–O(2) 86.39(8), O(1)–Nb–C(51) 98.66(11),
O(1)–Nb–C(58) 150.65(10), O(2)–Nb–C(44) 134.85(9), O(2)–Nb–C(51)
105.95(10), C(44)–Nb–C(58) 80.92(10), Nb–O(1)–C(3) 141.91(16),
Nb–O(2)–C(17) 140.51(17), Nb–C(44)–C(45) 93.17(17), Nb–C(51)–C(52)
117.0(2), Nb–C(58)–C(59) 126.62(19).
˚
O distances [average 1.930 A] is elongated relative to those of
the chloride complex ligand 1, reflecting the increase in electron
density at niobium arising from the presence of the alkyl groups
compared with chlorides. Two of the benzyl ligands◦show normal
˚
apical benzyl ligand [H(1) ◊ ◊ ◊ Nb = 2.27 A, C(1)–H(1) ◊ ◊ ◊ Nb =
149◦], indicating the possibility of some agostic interaction.36–40
However, this interaction is not retained in solution, as evidenced
by a JCH value of the methine carbon (117.7 Hz). The Nb–
h -bonding to niobium [Nb–C(51)–C(52) = 117.0(2) , Nb–C(58)–
1
C(59) = 126.62(19)◦], while the third is distorted such that the Nb–
C(44)–C(45) angle is acute [93.17(17)◦] and the Nb–Cipso carbon
2378 | Dalton Trans., 2011, 40, 2375–2382
This journal is
The Royal Society of Chemistry 2011
©

100.16(11)◦]. The overall geometry at niobium is best described as
square pyramidal, with one aryloxide [O(2)] group of the [O3C]
ligand occupying the apical position. The coordination sp◦here is
2
˚
[Nb–C(45) = 2.738(3) A] is short. This h -benzyl bonding mode
results from a weak Nb ◊ ◊ ◊ Ph p interaction and is characteristic
of electron-deficient early transition metal benzyl complexes.7,42–45
The phenyl ring of the distorted benzyl ligand points towards
the apical benzyl ligand and away from the pendant phenol
arm. The apical benzyl group is slightly closer to niobium
2
completed by a h -benzyl ligand [Nb–C(20)–C(21) = 83.2(3) , Nb–
˚
Cipso(21) = 2.515(4) A], which is located trans to the C(1) atom and
has its phenyl ring pointing toward the apical aryloxide ligand
[O(2)]. The Nb–O distances fall in the range of the corresponding
bonds found in 3-syn and 3-anti.
˚
[Nb–C(51) = 2.193(3) A] than the corresponding basal ligands
[2.229(3), 2.216(4) A]. Steric congestion around the hydroxyl
˚
group is probably ascribed as the cause of alkylation of the
metal center in preference to deprotonation of the hydroxyl group.
The structure of 4 is reminiscent of [H2(O3N)]Ta(CH2CMe3)4
[H3(O3N) = tris(3,5-methyl-2-hydroxylphenyl)amine], in which the
H2[O3N] ligand is coordinated to the metal with one phenoxide
group and two dangling phenol arms.46
The [O3C] complex 5 was also prepared in 20% yield by alky-
lation of 3-syn with four equivalents of PhCH2MgCl. In contrast
to thermally induced C–H activation of 4, the reaction is found to
proceed without heating the solution to elevated temperature. A
possible explanation is that the Nb ◊ ◊ ◊ H–C(methine) interaction
preorganizes the reactive site and locks the metal center and
the methine carbon in a confined arrangement. Cyclometalation
via s-bond metathesis is relatively common in early transition
metal complexes with 2,6-disubstituted aryloxide ligands.47–49
For example, thermolysis of M(OAr¢)2(CH2Ph) (M = Ti, Zr;
OAr¢ = 2,6-di-tert-butylphenoxide) produced a metalated complex
To promote coordination of the pendant phenol arm, we carried
◦
out thermolysis of 4. Prolonged heating of 4 in toluene at 50 C
for 5 days resulted in the formation of [O3C]Nb(CH2Ph) (5) in
43% isolated yield. The 1H NMR spectrum of 5 reveals a pseudo
C3 symmetric species in solution with one set of resonances
due to aryloxide groups. There are no resonances due to the
methine proton and the hydroxyl group. The molecular structure
of 5 is shown in Fig. 5. The methine C–H bond of the ligand
backbone is activated along with deprotonation of the pendant
phenol arm, leading to tetradentate binding of the [O3C] ligand
to the metal center. The O(1)–Nb–O(3) bite angle of 140.56(12)◦
is significantly wider than the other O–Nb–O angles [105.88(11),
t
M(OC6H3 BuCMe2CH2)(OAr¢)(CH2Ph) along with elimination of
toluene. This type of reaction has been successfully used as a
method for installation of cyclometalated [OCO] pincer ligands
on early transition metals.7,50–52 The [syn-O3] complex could serve
as a metalated [O3C] precursor.
Utilization of 3-anti as a starting material instead of 3-syn in
alkylation led to the formation of a benzylidene complex [anti-
O3]Nb(CHPh)(THF) (6) in 69% (Scheme 5). The presence of the
benzylidene ligand is manifested by 1H and 13C NMR resonances
1
at d 10.25 and 245.0, respectively.7,53,54 The H NMR spectrum
exhibits two sets of resonances due to aryloxide groups in a 2 : 1
ratio, consistent with Cs symmetry in solution on NMR time scale.
In addition, the anti form of the [O3] ligand is retained as evidenced
by the methine proton (d 6.18) and carbon resonances (d 62.0) with
1JCH = 124 Hz (Table 1).
Scheme 5 Synthesis of 6. Reagents and conditions: (i) PhCH2MgCl (in
Et2O), toluene, -98 ◦C to rt, 1 h; (ii) THF, rt, 3 h.
The mechanism of the formation of 5 remains unclear. However,
based on literature precedents, two plausible pathways can be
envisioned for alkylation of 3-syn.55–61 On the assumption that
a dibenzyl complex [syn-O3]Nb(CH2Ph)2 is formed in the first
step, one pathway involves a-abstraction to form a benzylidene
intermediate, followed by C–H activation of the ligand backbone
methine to give 5. The other pathway involves a direct s-bond
metathesis. Isolation of 6 provides support for the formation of
the [syn-O3] benzylidene analogue as an intermediate, while the
latter pathway reminds one of conversion of [syn-O3]Zr(CH2Ph)
to [O3C]Zr(THF)3 via C–H activation.22
Fig. 5 Molecular structure of 5 with thermal ellipsoids drawn at the 50%
probability level. All hydrogen atoms and tert-butyl methyl groups are
◦
˚
omitted for clarity. Selected bond distances (A) and angles ( ): Nb–O(1)
1.921(3), Nb–O(2) 1.940(3), Nb–O(3) 1.898(3), Nb–C(1) 2.243(4),
Nb–C(20) 2.227(4), O(1)–Nb–O(2) 105.88(11), O(1)–Nb–O(3) 140.56(12),
O(2)–Nb–O(3) 100.16(11), O(1)–Nb–C(1) 76.01(12), O(2)–Nb–C(1)
87.42(12), O(3)–Nb–C(1) 76.12(12), C(1)–Nb–C(20) 144.01(14),
Nb–O(1)–C(3) 125.5(2), Nb–O(2)–C(9) 112.9(3), Nb–O(3)–C(15)
125.5(2), Nb–C(20)–C(21) 83.2(3).
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2375–2382 | 2379
©

t
t
d6, rt) ppm: 1.21 (s, 18H, Bu), 1.22 (s, 9H, Bu), 1.54 (br, 9H,
tBu), 1.56 (s, 18H, tBu), 4.25 (br, 4H, CH2O), 5.51 (br, 1H, OH),
7.39 (br, 1H, Ar), 7.42 (d, 2H, J = 2.2 Hz, Ar), 7.50 (d, 1H, J =
Conclusion
We have demonstrated that the triaryloxide trianion [O3]3- can be
introduced to the niobium metal center by a two-step procedure.
The first step is the partial HCl elimination between NbCl5 and
H3[O3], giving 1 with a bidentate bis(aryloxide) ligand and a
pendant phenol arm. In the second step, deprotonation of the
2.2 Hz, Ar), 7.67 (d, 2H, J = 2.2 Hz, Ar), 7.80 (br, 1H, CH). 13
NMR d (100 MHz, benzene-d6, rt) ppm: 25.1 (thf), 30.1, 31.3,
31.8, 32.2, 34.6, 34.8, 35.4, 36.2 (tBu), 38.9 (CH), 78.4 (thf), 123.1,
123.7, 124.4, 137.9, 141.8, 151.8, 160.0 (Ar). IR (cm-1): 3547 (s,
C
3
pendant phenol group with NEt3 led to a tridentate (k -O3)
n
OH). Anal. Calc. for C47H70O4Cl3Nb: C, 62.84; H, 7.85. Found: C,
coordination mode. The tridentate [O3] ligand can coordinate to
niobium in two forms, which differ mainly by the orientation of the
methine C–H bond. The close proximity of the methine proton to
the metal center in the syn form ([syn-O3]3-) facilitates C–H bond
63.18; H, 8.22.
Synthesis of [Et3NH][(syn-O3)NbCl3] (3-syn). Addition of
Et3N (0.16 mL, 1.15 mmol) to 1 (835 mg, 0.963 mmol) in toluene
(50 mL) gave a red solution. The mixture was stirred for 3 h at
room temperature. The solvent was evaporated and the residue
was washed with hexane, giving 3-syn as a brown powder in 86%
activation, resulting in a tetradentate (k -O3C) ligand [O3C]4-.
4
This implies that the [O3] ligand in the syn form can provide a
direct synthetic access to [O3C]-type metalatranes.22 Compounds
3-syn, 3-anti, and 5 should provide excellent starting materials for
investigation of the reactivity of this class of niobium complex. We
expect it to demonstrate a rich chemistry.
1
(770 mg, 0.830 mmol). H NMR d (400 MHz, benzene-d6, rt)
t
ppm: 0.75 (t, 9H, J = 7.0 Hz, CH3), 1.35 (s, 27H, Bu), 1.74 (s,
27H, tBu), 2.36 (q, 6 H, J = 7.0 Hz, CH2), 7.27 (d, 3H, J = 2.5 Hz,
ArH), 7.54 (s, 1H, CH), 8.11 (d, 3H, J = 2.5 Hz, ArH), 8.70 (s,
1H, NH). 13C NMR d (100 MHz, benzene-d6, rt) ppm: 9.2 (CH3),
Experimental
31.5, 31.6, 34.6, 35.8 (tBu), 36.8 (d, JCH = 91.8 Hz, CH), 46.8
1
General experimental details
(CH2), 122.2, 122.4, 139.6, 145.0, 148.0, 159.6 (Ar). Anal. Calc.
for C49H77O3NCl3Nb: C, 63.46; H, 8.37; N, 1.51. Found: C, 63.44;
H, 8.49; N, 1.49.
All manipulations were carried out using standard Schlenk or
glovebox techniques under an argon atmosphere. Anhydrous
solvents were purchased from Kanto Chemical Co. Hexane and
toluene were further dried by passage through two columns of
activated alumina and a Q-5 column, while THF, Et2O, and
CH3CN were dried by passage through two columns of activated
alumina. Benzene-d6 was dried and degassed over a potassium
mirror, and vacuum transferred prior to use. Chloroform-d1 was
dried and degassed over CaH2, and vacuum transferred prior to
use. H3[O3] was prepared by a literature procedure.62 NMR spectra
were recorded on JEOL LA-400 or LA-500 spectrometer. 1H and
13C NMR were reported with reference to solvent resonances
(residual C6D5H in C6D6, d 7.15 and d 128.0, residual CHCl3
in CDCl3, d 7.24 and d 77.0). Elemental analyses were measured
using Yanaco MT-6 and MSU-32 microanalyzers.
Synthesis of [Et3NH][(anti-O3)NbCl3] (3-anti).
Method A. The mixture of 1 (2.72 g, 3.14 mmol) and Et3N
(0.55 mL, 3.95 mmol) in toluene (80 mL) was stirred for 3 days at
80 C. After evaporation of the solvent, the residue was washed
◦
with hexane to give 3-anti as a red powder in 92% (2.42 g,
2.61 mmol),
Method B. A toluene (20 mL) solution of 3-syn (538 mg,
0.580 mmol) was stirred for 3 days at 80 ◦C. The same purification
steps as in method A gave 506 mg of 3-anti as a brown powder
(94%).
1H NMR d (400 MHz, benzene-d6, rt) ppm: d 0.67 (t, 9H, J =
7.0 Hz, CH3), 1.19 (s, 27H, tBu), 1.79 (s, 27H, tBu), 2.42 (dq, 6 H,
J = 5.5, 7.0 Hz, CH2), 5.91 (s, 1H, CH), 7.44 (d, 3H, J = 2.2 Hz,
ArH), 7.54 (d, 3H, J = 2.2 Hz, ArH), 7.84 (br, 1H, NH). 13C NMR
d (100 MHz, benzene-d6, rt) ppm: d 8.5 (CH3), 31.6, 32.7, 34.4,
36.1 (tBu), 46.6 (CH2), 65.8 (d, 1JCH = 119.8 Hz, CH), 124.1, 128.4,
139.4, 142.3, 144.9, 161.1 (Ar). Anal. Calc. for C49H77O3NCl3Nb:
C, 63.46; H, 8.37; N, 1.51. Found: C, 63.52; H, 8.48; N, 1.50.
Synthetic procedures
Synthesis of [H(O3)]NbCl3(CH3CN) (1). A solution of NbCl5
(1.31 g, 4.85 mmol) in CH3CN (40 ml) was added to a suspension of
H3[O3] (3.03 g, 4.82 mmol) in toluene (70 ml) at room temperature.
The mixture was stirred for 3 h to give a brown homogeneous
solution. The volatiles were then removed in vacuo. The residue
was washed with hexane to yield 1 as a brown powder in 84%
(3.55 g). 1H NMR d (400 MHz, benzene-d6, rt) ppm: 0.35 (s, 3H,
CH3CN), 1.23 (s, 18H, Bu), 1.25 (s, 9H, Bu), 1.52 (s, 9H, Bu),
1.58 (br, 18H, tBu), 5.62 (s, 1H, OH), 7.43 (d, 3H, J = 2.2 Hz, Ar),
7.52 (d, 1H, J = 2.2 Hz, Ar), 7.72 (d, 2H, J = 2.2 Hz, Ar), 7.83 (s,
1H, CH). 13C d (100 MHz, benzene-d6, rt) ppm: 0.11 (CH3), 30.1,
31.3, 31.9, 32.0, 34.7, 34.9, 35.4, 36.0 (tBu), 39.0 (d, 1JCH = 124.5
Hz, CH), 122.1, 123.1, 123.2, 124.2, 125.0, 140.8, 141.7, 148.4,
151.8 (Ar). Anal. Calc. for C45H65O3NCl3Nb: C, 62.32; H, 7.55;
N, 1.62. Found: C, 62.81; H, 7.89; N, 1.57.
Synthesis of [H(O3)]Nb(CH2Ph)3 (4). To a solution of 2
(957 mg, 1.06 mmol) in toluene (40 mL) was added a 1.0 M solut◦ion
of PhCH2MgCl (3.3 ml, 3.3 mmol) in Et2O dropwise at -98 C.
The mixture was warmed to room temperature and stirred for 5 h.
The resulting red slurry was centrifuged to remove an insoluble
material, and the supernatant solution was evaporated to dryness.
Extraction of the residue with hexane followed by centrifugation
gave a red solution. Concentration of the solution yielded an
t
t
t
1
orange powder of 4 in 77% (813 mg). H NMR d (400 MHz,
benzene-d6, rt) ppm: 1.23 (s, 18H, tBu), 1.28 (s, 9H, tBu), 1.29 (s,
18H, tBu), 1.66 (s, 9H, tBu), 2.74 (s, 2H, CH2), 3.58 (br, 4H, CH2),
5.44 (s, 1H, OH), 5.87 (s, 1H, CH), 6.91 (t, 6H, J = 6.8 Hz, Ar),
7.03–7.13 (m, 9H, Ar), 7.38 (d, 2H, J = 2.4 Hz, Ar), 7.60 (d, 2H, J =
2.4 Hz, Ar), 7.67 (d, 1H, J = 2.4 Hz, Ar), 7.78 (d, 1H, J = 2.4 Hz,
Ar). 13C NMR d (100 MHz, benzene-d6, rt) ppm: 30.3, 30.5, 31.5,
Synthesis of [H(O3)]NbCl3(THF) (2). Complex 1 (1.75 g,
2.02 mmol) was dissolved in THF (15 ml), and then the volatiles
were removed in vacuo. Washing the residue with hexane gave 2 as
a brown powder in 91% (1.64 g). 1H NMR d (400 MHz, benzene-
31.9, 34.6, 34.7, 35.6, 35.7 (tBu), 52.1 (d, JCH = 117.7 Hz, CH),
1
2380 | Dalton Trans., 2011, 40, 2375–2382
This journal is
The Royal Society of Chemistry 2011
©

87.2 (CH2), 123.3, 124.0, 125.1, 125.9, 126.2, 127.2, 128.9, 129.0,
129.1, 131.6, 137.8, 137.9, 141.9, 142.9, 145.1, 152.6, 158.9 (Ar).
IR (cm-1): 3448 (s, nOH). Anal. Calc. for C64H82O3Nb: C, 77.47; H,
8.33. Found: C, 77.23; H, 8.50.
Fourier maps. The structures were refined on F2 by the full-
matrix least-squares method. For 3-syn, four tert-butyl groups
and one benzene solvent molecule were disordered. For 3-anti,
one ethyl group and a toluene solvent molecule were disordered.
For 4, one tert-butyl group was disordered. For 5, two tert-butyl
groups were disordered. These disordered carbon atoms were
refined isotropically. For 4, the pendant phenol was disordered,
and its oxygen atoms were refined isotropically. The remaining
non-hydrogen atoms were refined anisotropically. All hydrogen
atoms were put at calculated positions, while no hydrogen atom
was put on disordered carbon atoms except for ethyl groups of
3-anti.
Synthesis of [O3C]Nb(CH2Ph) (5). To a solution of 3-syn
(1.83 g, 1.97 mmol) in toluene (50 mL) was added a 1.0 M
solution of PhCH2MgCl (6.0 mL, 6.0 mmol) in Et2O dropwise
◦
at -70 C. The mixture was warmed to room temperature. After
stirring for 2 h, the resulting brown slurry was centrifuged to
remove an insoluble material. The supernatant solution was
evaporated to dryness. Extraction of the residue with hexane
followed by centrifugation gave a brown solution. The solution
was concentrated to yield 5 as an orange powder in 20% (316 mg,
0.391 mmol). 1H NMR d (400 MHz, benzene-d6, rt) ppm: 1.35 (s,
27H, tBu), 1.37 (s, 27H, tBu), 3.12 (s, 2H, CH2), 6.97 (t, J = 7.3 Hz,
1H, ArH), 7.18 (t, J = 7.3 Hz, 2H, ArH), 7.24 (d, J = 2.2 Hz, 3H,
ArH), 7.29 (d, J = 7.3 Hz, 2H, ArH), 7.88 (d, J = 2.2 Hz, 3H, ArH).
13C NMR d (100 MHz, benzene-d6, rt) ppm: 30.2, 31.9, 34.7, 34.9
(tBu), 68.5 (CH2), 121.4, 125.9, 130.4, 131.1, 133.3, 134.3, 134.5,
143.8, 144.8, 162.6 (Ar). The 13C NMR signal assignable to the
Nb-bound carbon of the [O3C] ligand was unobservable, likely
because of line broadening resulting from the quadrupolar 93Nb
nucleus. Anal. Calc. for C50H67O3Nb: C, 74.23; H, 8.35. Found: C,
74.29; H, 8.60.
Crystal data for 1. C45H65O3NCl3Nb, M = 867.28, monoclinic,
space group P21/c (#14), a = 12.485(5), b = 20.674(8), c =
◦
3
˚
˚
18.359(7) A, b = 100.406(5) , V = 4661(3) A , T = 173 K, Z =
4, m(Mo-Ka) = 4.665 cm-1, 34 400 reflections measured, 9471
independent reflections (Rint = 0.0983). The final R1 values were
0.0789 (I > 2s(I)). The final wR(F2) values were 0.1322 (all data).
The goodness of fit on F2 was 1.116.
Crystal data for 3-syn. C49H77O3NCl3Nb·1.75(C6H6), M =
¯
1064.12, triclinic, space group P1(#2), a = 13.987(5), b = 20.447(7),
◦
˚
c = 21.538(8) A, a = 94.951(6), b = 99.947(5), g = 97.388(6) , V =
3
-1
˚
5980(4) A , T = 173 K, Z = 4, m(Mo-Ka) = 1.182 cm , 45 362
reflections measured, 24 591 independent reflections (Rint = 0.045).
The final R1 values were 0.0658 (I > 2s(I)). The final wR(F2)
values were 0.1520 (all data). The goodness of fit on F2 was 1.078.
The compound crystallized with 1.75 equivalents of benzene per
Nb complex.
Synthesis of [anti-O3]Nb(CHPh)(THF) (6). To a toluene
(30 mL) solution of 3-anti (922 mg, 0.994 mmol) was added a 1.0 M
solution of PhCH2MgCl (3.0 mL, 3.0 mmol) in Et2O dropwise
at -98 ◦C. The mixture was warmed to room temperature and
was stirred for 1 h. After an insoluble material was removed by
centrifugation, the resulting brown solution was evaporated to
dryness. The residue was washed with cold hexane and dissolved
in 20 mL of THF. After stirring for 3 h, the solvent was removed.
Recrystallization from hexane afforded 5 as brown crystals in 69%
Crystal data for 3-anti. C49H77O3NCl3Nb·C7H8, M = 1019.56,
monoclinic, space group C2/c (#15), a = 47.957(11), b = 13.286(3),
◦
3
˚
˚
c = 19.889(5) A, b = 116.905(3) , V = 11300(4) A , T = 173 K, Z =
8, m(Mo-Ka) = 3.946 cm-1, 43 895 reflections measured, 12 900
independent reflections (Rint = 0.047). The final R1 values were
0.0611 (I > 2s(I)). The final wR(F2) values were 0.1508 (all data).
The goodness of fit on F2 was 1.083. The compound crystallized
with 1 equivalent of toluene per Nb complex.
1
(605 mg). H NMR d (400 MHz, benzene-d6, rt) ppm: 1.12 (br,
22H, tBu + THF), 1.28 (s, 9H, tBu), 1.54 (s, 9H, tBu), 1.58 (s, 18H,
tBu), 3.43 (br, 4H, THF), 6.18 (s, 1H, CH), 6.78 (t, J = 7.0 Hz, 1H,
Ph), 7.26–7.32 (m, 5H, Ph + ArH), 7.35 (d, J = 2.4 Hz, 2H, ArH),
7.44 (d, J = 2.4 Hz, 1H, ArH), 7.47 (d, J = 2.4 Hz, 2H, ArH),
10.25 (s, 1H, CH). 13C NMR d (100 MHz, benzene-d6, rt) ppm:
26.0 (THF), 30.4, 30.7, 31.4, 32.0, 34.3, 34.4, 35.6, 35.9 (tBu), 62.0
Crystal data for 4. C64H83O3Nb·C7H8, M = 1085.40, mono-
clinic, space group P21/c (#14), a = 22.812(6), b = 14.173(4), c =
◦
3
˚
˚
20.084(5) A, b = 106.140(4) , V = 6238(3) A , T = 173 K, Z =
4, m(Mo-Ka) = 2.373 cm-1, 48519 reflections measured, 14088
independent reflections (Rint = 0.037). The final R1 values were
0.0604 (I > 2s(I)). The final wR(F2) values were 0.1521 (all data).
The goodness of fit on F2 was 1.093. The compound crystallized
with 1 equivalent of toluene per Nb complex.
1
(d, JCH = 124 Hz, CH), 77.1 (THF), 123.2, 123.8, 125.0, 126.8,
127.1, 129.6, 129.9, 137.4, 138.2, 142.0, 143.9, 155.5, 163.4 (Ar
+ Ph), 245.0 (br, CH). Anal. Calc. for C54H75O4Nb: C, 73.61; H,
8.58. Found: C, 74.05; H, 9.27.
X-Ray crystallographic studies†
Crystal data for 5. C50H67O3Nb, M = 808.98, monoclinic,
space group P21/c (#14), a = 14.679(5), b = 11.916(4), c =
27.664(9) A, b = 110.455(4) , V = 4534(2) A , T = 173 K, Z =
4, m(Mo-Ka) = 3.038 cm-1, 35 050 reflections measured, 10 345
independent reflections (Rint = 0.041). The final R1 values were
0.0684 (I > 2s(I)). The final wR(F2) values were 0.1779 (all data).
The goodness of fit on F2 was 1.140.
Single crystals immersed in mineral oil on nylon loops and
transferred to a Rigaku Mercury CCD system equipped with a
Rigaku GNNP low-temperature device. Data were collected under
a cold nitrogen stream at 173 K using graphite-monochromated
◦
3
˚
˚
˚
Mo-Ka radiation (l = 0.71070 A). Equivalent reflections were
merged, and the images were processed with the CrystalClear
(Rigaku) program. Corrections for Lorentz-polarization effects
and absorption were performed.
Calculations were performed with the CrystalStructure
(Rigaku) software package. All structures were solved by direct
methods, and the remaining heavy atoms were found in subsequent
Acknowledgements
This work was financially supported by Grants-in-Aid for
Scientific Research (Nos. 22750049, 20262850, 22105008 and
This journal is
The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 2375–2382 | 2381
©

8GS0207A) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
33 D. Homden, C. Redshaw, J. A. Wright, D. L. Hughes and M. R. J.
Elsegood, Inorg. Chem., 2008, 47, 5799–5814.
34 L. Higham, M. Thornton-Pett and M. Bochmann, Polyhedron, 1998,
17, 3047–3051.
35 G. R. Willey, T. J. Woodman and M. G. B. Drew, Polyhedron, 1997, 16,
Notes and references
351–353.
1 W. A. Chomitz and J. Arnold, Chem.–Eur. J., 2009, 15, 2020–2030.
2 L. H. Gade, Chem. Commun., 2000, 173–181.
3 E. Y. Tshuva, I. Goldberg, M. Kol and Z. Goldschmidt,
Organometallics, 2001, 20, 3017–3028.
4 M. D. Fryzuk, S. A. Johnson and S. J. Rettig, J. Am. Chem. Soc., 1998,
120, 11024–11025.
36 M. Brookhart, M. L. H. Green and L. L. Wong, Prog. Inorg. Chem.,
1980, 36, 1–124.
37 M. Brookhart, M. L. H. Green and G. Parkin, Proc. Natl. Acad. Sci.
U. S. A., 2007, 104, 6908–6914.
38 W. Scherer and G. S. McGrady, Angew. Chem., Int. Ed., 2004, 43,
1782–1806.
5 I. Kuzu, I. Krummenacher, J. Meyer, F. Armbruster and F. Breher,
Dalton Trans., 2008, 5836–5865.
6 H. Kawaguchi and T. Matsuo, J. Organomet. Chem., 2004, 689, 4228–
4243.
7 T. Agapie, M. W. Day and J. E. Bercaw, Organometallics, 2008, 27,
6123–6142.
8 C. Capacchione, A. Proto, H. Ebeling, R Mu¨lhaupt, K. Mo¨ller, T. P.
Spaniol and J. Okuda, J. Am. Chem. Soc., 2003, 125, 4964–4965.
9 S. Gendler, A. L. Zelikoff, J. Kopilov, I. Goldberg and M. Kol, J. Am.
Chem. Soc., 2008, 130, 2144–2145.
10 F. Bonnet, A. R. Cowley and P. Mountford, Inorg. Chem., 2005, 44,
9046–9055.
39 A. D. Poole, D. N. Williams, A. M. Kenwright, V. C. Gibson, W. Clegg,
D. C. R. Hockless and P. A. O’Neil, Organometallics, 1993, 12, 2549–
2555.
40 M. Etienne, R. Mathieu and B. Donnadieu, J. Am. Chem. Soc., 1997,
119, 3218–3228.
41 R. R. Schrock and J. D. Fellmann, J. Am. Chem. Soc., 1978, 100, 3359–
3370.
42 M. E. G. Skinner, T. Toupance, D. A. Cowhig, B. R. Tyrrell and P.
Mountford, Organometallics, 2005, 24, 5586–5603.
43 E. Y. Tshuva, S. Groysman, I. Goldberg, M. Kol and Z. Goldschmidt,
Organometallics, 2002, 21, 662–670.
44 S. L. Latesky, A. K. McMullen, G. P. Niccolai, I. P. Rothwel and J. C.
Huffmann, Organometallics, 1985, 4, 902–908.
45 L. S. Natrajan, C. Wilson, J. Okuda and P. L. Arnold, Eur. J. Inorg.
Chem., 2004, 3724–3732.
46 S. Groysman, I. Goldberg, M. Kol, E. Genizi and Z. Goldschmidt, Adv.
Synth. Catal., 2005, 347, 409–415.
47 R. W. Chesnut, G. G. Jacob, J. S. Yu, P. E. Fanwick and I. P. Rothwell,
Organometallics, 1991, 10, 321–328.
48 I. P. Rothwell, Acc. Chem. Res., 1988, 21, 153–159.
49 J. S. Vilardo, M. A. Lockwood, L. G. Hanson, J. R. Clark, B. C. Parkin,
P. E. Fanwick and I. P. Rothwell, J. Chem. Soc., Dalton Trans., 1997,
3353–3362.
11 P. Chaudhuri, M. Hess, T. Weyhermu¨ller and K. Wieghardt, Angew.
Chem., Int. Ed., 1999, 38, 1095–1098.
12 A. Caselli, E. Solari, R. Scopelliti, C. Floriani, N. Re, C. Rizzoli and
A. Chiesi-Villa, J. Am. Chem. Soc., 2000, 122, 3652–3670.
13 A. Caselli, E. Solari, R. Scopelliti and C. Floriani, J. Am. Chem. Soc.,
2000, 122, 538–539.
14 I. Korobkov, S. Gambarotta and G. P. A. Yap, Angew. Chem., Int. Ed.,
2003, 42, 4958–4961.
15 H. Kawaguchi and T. Matsuo, Chem. Lett., 2004, 33, 640–645.
16 T. Watanabe, Y. Ishida, T. Matsuo and H. Kawaguchi, J. Am. Chem.
Soc., 2009, 131, 3474–3475.
17 H. Kawaguchi and T. Matsuo, J. Am. Chem. Soc., 2003, 125, 14254–
50 S. Sarkar, A. R. Carlson, M. K. Veige, J. M. Falkowski, K. A. Abboud
and A. S. Veige, J. Am. Chem. Soc., 2008, 130, 1116–1117.
51 M. O’Reilly, J. M. Falkowski, V. Ramachandran, M. Pati, K. A.
Abboud, N. S. Dalal, T. G. Gray and A. S. Veige, Inorg. Chem., 2009,
48, 10901–10903.
14255.
18 H. Kawaguchi and T. Matsuo, Angew. Chem., Int. Ed., 2002, 41, 2792–
2794.
19 T. Matsuo, H. Kawaguchi and M. Sakai, J. Chem. Soc., Dalton Trans.,
2002, 2536–2540.
52 T. Watanabe, T. Matsuo and H. Kawaguchi, Inorg. Chem., 2006, 45,
20 H. Aihara, T. Matsuo and H. Kawaguchi, Chem. Commun., 2003, 2204–
2205.
6580–6582.
53 M C. W. Chan, J. M. Cole, V. C. Gibson, J. A. K. Howard, C. Lehmann,
A. D. Poole and U. Siemeling, J. Chem. Soc., Dalton Trans., 1998, 103–
111.
21 T. Matsuo and H. Kawaguchi, J. Am. Chem. Soc., 2005, 127, 17198–
17199.
22 F. Akagi, T. Matsuo and H. Kawaguchi, J. Am. Chem. Soc., 2005, 127,
11936–11937.
54 S. Groysman, I. Goldberg, M. Kol, E. Genizi and Z. Goldschmidt,
Organometallics, 2004, 23, 1880–1890.
55 R. R. Schrock, L. W. Messerle, C. D. Wood and L. J. Guggenberger, J.
Am. Chem. Soc., 1978, 100, 3793–3800.
23 F. Akagi, T. Matsuo and H. Kawaguchi, Angew. Chem., Int. Ed., 2007,
46, 8778–8781.
24 M. B. Dinger and M. J. Scott, Inorg. Chem., 2000, 39, 1238–1254.
25 A. B. Chaplin, J. A. Harrison, A. J. Nielson, C. Shen and J. M. Waters,
Dalton Trans., 2004, 2643–2648.
26 T. Matsuo and H. Kawaguchi, Inorg. Chem., 2002, 41, 6090–6098.
27 M. B. Dinger and M. J. Scott, Inorg. Chem., 2001, 40, 856–864.
28 G. Aullo´n, D. Bellamy, A. G. Orpen, L. Brammer and E. A. Bruton,
Chem. Commun., 1998, 653–654.
56 L. W. Messerle, P. Jennische, R. R. Schrock and G. Stucky, J. Am.
Chem. Soc., 1980, 102, 6744–6752.
57 G. A. Rupprecht, L. W. Messerle, J. D. Fellmann and R. R. Schrock, J.
Am. Chem. Soc., 1980, 102, 6236–6244.
58 H. C. L. Abbenhuis, R. Van Belzen, D. M. Grove, A. J. A. Klomp, G.
P. M. Van Mier, A. L. Spek and G. Van Koten, Organometallics, 1993,
12, 210–219.
29 S. Milione, C. Capacchione, C. Cuomo, M. Strianese, V. Bertolasi and
A. Grassi, Inorg. Chem., 2009, 48, 9510–9518.
30 R. W. Chesnut, P. E. Fanwick and I. P. Rothwell, Inorg. Chem., 1988,
27, 752–754.
59 L. R. Chamberlain, I. P. Rothwell and J. C. Huffman, J. Am. Chem.
Soc., 1986, 108, 1502–1509.
60 L. P. Spencer, C. Beddie, M. B. Hall and M. D. Fryzuk, J. Am. Chem.
Soc., 2006, 128, 12531–12543.
31 J. R. Clark, A. L. Pulvirenti, P. E. Fanwick, M. Sigalas, O. Eisenstein
and I. P. Rothwell, Inorg. Chem., 1997, 36, 3623–3631.
32 D. R. Mulford, P. E. Fanwick and I. P. Rothwell, Polyhedron, 2000, 19,
35–42.
61 B. Qian, W. J. Scanlon IV, M. R. Smith III and D. H. Motry,
Organometallics, 1999, 18, 1693–1698.
62 M. B. Dinger and M. J. Scott, Eur. J. Org. Chem., 2000, 2467–
2478.
2382 | Dalton Trans., 2011, 40, 2375–2382
This journal is
The Royal Society of Chemistry 2011
©