'Pincer' pyridine dicarbene complexes of nickel and their derivatives. Unusual ring opening of a coordinated imidazol-2-ylidene

Article abstract of DOI:10.1039/b715769j

The reaction of NiBr₂(DME), DME = 1,2-dimethoxyethane, with the 'pincer' pyridine dicarbene ligands (C-N-C) (2) and (C-N^Me-C) (2 ^Me), (C-N-C = 2,6-bis-[(DiPP)imidazol-2-ylidene]pyridine, C-N ^Me-C = 2,6-bis-[(DiPP)imidazol-2-ylidene]-3,5-dimethylpyridine, DiPP = 2,6-diisopropylphenyl) gave the square planar complexes [Ni(C-N ^(Me)-C)Br]Br, 3?(Br)^- and 3^Me?(Br) ^- respectively. Transmetallation from [(C-N^Me-C) ₂Ag₂](Ag₆I₈), 6^Me? (Ag₆I₈)^2- to NiBr₂(DME) gave [Ni(C-N^Me-C)Br](AgI₂), 3^Me?(AgI ₂)^-. Reaction of 3?(Br)^- with KPF ₆ resulted only in exchange of the ionic bromide, however the reaction of 3?(Br)^- with AgBF₄ in MeCN or AgOTf in THF resulted in the exchange of both coordinated and ionic bromides, giving rise to the square planar 4?(BF₄)^-₂ and octahedral 5, respectively. In contrast, the reaction of 3^Me? (AgI₂)^-, with excess AgOTf resulted in an unusual 'reverse transmetallation' leading to 6^Me?(OTf)^-. The substitution of tmeda in Ni(CH₃)₂(tmeda), tmeda = N,N,N′,N′-tetramethylethylenediamine, by 2 produced the complex 7, in which ring opening of the heterocyclic imidazole ring of one of the NHC functional groups has taken place. The Royal Society of Chemistry.

Full text of DOI:10.1039/b715769j

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/dalton | Dalton Transactions
‘Pincer’ pyridine dicarbene complexes of nickel and their derivatives.
Unusual ring opening of a coordinated imidazol-2-ylidene†‡
David Pugh, Aimee Boyle and Andreas A. Danopoulos*
Received 12th October 2007, Accepted 7th December 2007
First published as an Advance Article on the web 9th January 2008
DOI: 10.1039/b715769j
The reaction of NiBr2(DME), DME = 1,2-dimethoxyethane, with the ‘pincer’ pyridine dicarbene
ligands (C-N-C) (2) and (C-NMe-C) (2Me), (C-N-C = 2,6-bis-[(DiPP)imidazol-2-ylidene]pyridine,
C-NMe-C = 2,6-bis-[(DiPP)imidazol-2-ylidene]-3,5-dimethylpyridine, DiPP = 2,6-diisopropylphenyl)
gave the square planar complexes [Ni(C-N(Me)-C)Br]Br, 3·(Br)− and 3Me·(Br)− respectively.
Transmetallation from [(C-NMe-C)2Ag2](Ag6I8), 6Me·(Ag6I8)2− to NiBr2(DME) gave [Ni(C-NMe-C)Br]-
(AgI2), 3Me·(AgI2)−. Reaction of 3·(Br)− with KPF6 resulted only in exchange of the ionic bromide,
however the reaction of 3·(Br)− with AgBF4 in MeCN or AgOTf in THF resulted in the exchange of
both coordinated and ionic bromides, giving rise to the square planar 4·(BF4)− and octahedral 5,
2
respectively. In contrast, the reaction of 3Me·(AgI2)−, with excess AgOTf resulted in an unusual ‘reverse
transmetallation’ leading to 6Me·(OTf)−. The substitution of tmeda in Ni(CH3)2(tmeda), tmeda =
N,N,Nꢀ,Nꢀ-tetramethylethylenediamine, by 2 produced the complex 7, in which ring opening of the
heterocyclic imidazole ring of one of the NHC functional groups has taken place.
Introduction
Results and discussion
The high interest in N-heterocyclic carbene (NHC) complexes of
the late transition metals and their potential in homogeneous
catalysis stems from the substitutional inertness of the M–CNHC
bond and consequent robust metal–ligand binding, leading to
well defined coordination spheres.1 Chelating NHC ligands are ex-
pected to amplify this feature. Furthermore, the chemical inertness
and stability of the coordinated NHC heterocycle should suppress
any unwanted catalyst deactivation initiated by ligand centered
reactions. A simple and popular preparation of metal–NHC
complexes involves transmetallation of an NHC ligand from silver
to a transition metal with concomitant formation of insoluble
silver halide.2 In this paper we wish to describe the synthesis and
characterisation of some new ‘pincer’ pyridine bis-NHC ligands
and their complexes with Ag and Ni, some substitution reactions
of the latter, including an unexpected ‘reverse transmetallation’ of
coordinated NHC from Ni to Ag. We also report a ring-opening
reaction of the coordinated NHC initiated by a methyl group
migration from nickel to the coordinated CNHC. The compounds
described in this paper are summarised in Schemes 1–3.
Bis-imidazolium salts and dicarbene ligands
The novel pincer dicarbene ligand 2Me was prepared by an
adaptation of the literature route to ligand 2.3 Quaternisation
of 2,6-dibromo-3,5-lutidine with 2,6-DiPP imidazole yielded
only the monosubstituted 2-(DiPP-imidazolium)-6-bromo-3,5-
lutidine. However, reaction of 2,6-diiodo-3,5-lutidine4 resulted
in good yields of the bis(imidazolium) salt 1Me·(I)− (Scheme 1).
2
Unexpectedly, double deprotonation of 1Me·(I)−2 with KN(SiMe3)2
afforded poor yields of the ligand 2Me. However, exchange of the
iodides for bromides, followed by deprotonation of 1Me·(Br)−2 gave
2Me cleanly and in good yield (ca. 77%).
Ligand 2Me was structurally characterised (Fig. 1), revealing the
non co-planar geometry of the imidazol-2-ylidene and pyridine
rings (ca. 30◦ angle between the planes of the NHC and pyridine
rings). This is in contrast with the solid state structure of 2 which
is planar.3 The mutually anti/gauche orientation of the NHC and
pyridine lone pairs is retained in 2Me. The CNHC–N bond lengths
Me
˚
[1.363(3)–1.375(3) A] in 2 are identical to those in 2. The non-
co-planar structure is also seen in the imidazolium salts 1Me·(I)−
2
and 1Me·(Br)− (see ESI‡) and is presumably due to steric reasons.
2
Nickel NHC complexes
School of Chemistry, University of Southampton, Highfield, Southampton,
UK SO17 1BJ. E-mail: ad1@soton.ac.uk
Pincer pyridine dicarbene nickel complexes were easily obtained
by the reaction of the free ligands 2 and 2Me with NiBr2(DME)
(Scheme 2).
The orange, air stable, square planar ionic complexes 3·(Br)−
and 3Me·(Br)− were isolated in good yield by slow diffusion of
Et2O into concentrated CH2Cl2 solutions. The 1H NMR spectrum
of 3Me·(Br)− has characteristic features associated with the high
† CCDC reference numbers 663954–663964. For crystallographic data in
CIF or other electronic format see DOI: 10.1039/b715769j
‡ Electronic supplementary information (ESI) available: Full details of
the X-ray crystal structures for compounds 1Me·(I−)2, 1Me·(Br−)2, 2Me
,
3·(Br−), 3·(PF6−), 3Me·(AgI2−), 3Me·(OTf), 5, 6Me·(Ag6I82−), 6Me·(OTf)2, and
7 including complete tables of crystal data, atomic coordinates, bond
lengths and angles and positional and anisotropic thermal parameters.
See DOI: 10.1039/b715769j
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 1087–1094 | 1087
©

View Article Online
Fig. 1 ORTEP diagram of compound 2Me; thermal ellipsoids at 50%
probability, hydrogen atoms have been removed for clarity. Selected bond
◦
˚
lengths (A) and angles ( ): C(13)–N(1) 1.364(4); C(13)–N(2) 1.373(3);
C(21)–N(4) 1.375(3); C(21)–N(5) 1.363(3); N(1)–C(13)–N(2) 101.7(2);
N(4)–C(21)–N(5) 101.1(2).
symmetry of the complex i.e. two doublets and one septet
assignable to the isopropyl groups of the DiPP substituents and
one sharp singlet from the methyl groups on the pyridine moiety. In
contrast, the spectrum of 3·(Br)− was uncharacteristically broad,
possibly due to dynamic processes involving exchange of the
ionic and coordinated bromide anions. Sharpening of the spectral
features was not observed even at −70 ◦C. However, the structure
of 3·(Br)− in the solid state was confirmed crystallographically and
is shown in Fig. 2.
Fig.
2
ORTEP diagram showing one of two cations of 3·(Br)−
in the asymmetric unit with virtually identical metrical data;
thermal ellipsoids at 50% probability, hydrogen atoms and one
CH2Cl2 solvent molecule have ◦been removed for clarity. Selected
˚
bond lengths (A) and angles ( ): C(13)–Ni(1) 1.920(7); N(3)–Ni(1)
1.872(5); C(21)–Ni(1) 1.914(7); Br(1)–Ni(1) 2.2813(11); C(48)–Ni(2)
1.927(8); N(8)–Ni(2) 1.873(5); C(56)–Ni(2) 1.932(8); Br(3)–Ni(2)
2.2591(11); C(13)–Ni(1)–C(21) 161.1(3); C(13)–Ni(1)–N(3) 80.3(3);
C(48)–Ni(2)–C(56) 162.7(3); C(56)–Ni(2)–N(8) 81.6(3).
The geometry around the Ni is distorted square planar, with a
CNHC–Ni–CNHC angle of 161.1(3)◦ and a pincer ligand bite angle
of ca. 81◦. The Ni lies out of the coordination plane by 0.050(3)
the range of previously observed [NiII–CNHC bond lengths [1.835–
2.019 A].5a,b An analogue of 3·(Br)− with methyl substituted NHC
˚
groups was recently reported.6 The reaction of 3·(Br)− with two
˚
˚
A. The Ni–CNHC bond lengths [1.914(7)–1.932(8) A] are within
Scheme 1 Synthesis of pincer ligands 2 and 2Me (R1 = DiPP).
Scheme 2 Metal complexes described in this paper (R1 = DiPP).
1088 | Dalton Trans., 2008, 1087–1094 This journal is
The Royal Society of Chemistry 2008
©

View Article Online
equivalents of KPF6 in THF (even in excess) yielded only complex
3·(PF6)−, in which only the ionic bromide was exchanged by
PF6−. Compound 3·(PF6)− was characterised by analytical, spec-
troscopic and diffraction methods (see ESI‡); all data revealed a 4-
coordinate square planar monocationic complex, with the metrical
data of the cation being identical to those observed in 3·(Br)−.
Reaction of 3·(Br)− with two equivalents of AgBF4 in MeCN
resulted in substitution of both coordinated and ionic bromides.
Compound 4·(BF4)−2 is a 4-coordinate square planar dication with
one MeCN completing the coordination sphere of the nickel. The
4-coordinate structure is also suggested by a poor quality single
crystal dataset; NMR and microanalytical data are also consistent
with the proposed structure.
Both ionic and coordinated bromides in 3·(Br)− can be ex-
changed for OTf− by the reaction with two equivalents of AgOTf in
THF, yielding green crystals of complex 5. Interestingly, complex
5 is paramagnetic (leff = 2.86 B.M.). It was characterised by
analytical and diffraction methods. The structure of the molecule
(Fig. 3) comprises an octahedral nickel centre which, in addition
to the tridentate ligand, contains two monodentate mutually trans
triflate ligands and one coordinated THF molecule.
Due to initial difficulties in isolating pure 2Me in synthetically
useful quantities, as described above, the silver NHC complex
6Me·(Ag6I8)2− was synthesised from the reaction of 1Me·(I)− with
2
Ag2O in dichloromethane. Based on analytical and structural data
(Fig. 4), compound 6Me·(Ag6I8)2− is dimeric with two (C-NMe-C)
ligands bridged by two Ag atoms forming a 16-membered ring
lying on an inversion centre in the unit cell. Unlike the only
previous structurally characterised example of a silver complex
of a C-N-C pincer ligand,5c 6Me·(Ag6I8)2− is not a helicate. Instead,
it can be described as a mesocate due to the absence of a ‘twist’
in the molecule. The interplanar angles between the NHC and the
pyridine rings are ca. 60◦, roughly twice that observed in the free
ligand 2Me. The (Ag6I8)2− anion contains an unusual arrangement
of Ag atoms in a planar 6-membered ring: this has only been ob-
served once before.7 There are iodides alternating above and below
the ring as well as ‘capping’ iodides; one on each face of the ring.
Fig. 4 ORTEP diagram of compound 6Me·(Ag6I8)2−; thermal ellipsoids
at 50% probability, hydrogen atoms, the (Ag6I8)2− anion and two Et2O
˚
molecules have been removed for clarity. Selected bond lengths (A) and an-
gles (◦): C(13)–Ag(1) 2.079(7); C(21)–Ag(1) 2.082(7); C(13)–Ag(1)–C(21)
177.0(3).
Transmetallation of 6Me·(Ag6I8)2− with NiBr2(DME) gave com-
plex 3Me·(AgI2)−, which is analogous to 3Me·(Br)− with (AgI2)−
rather than Br− as the non-coordinating anion. This complex was
characterised analytically, spectroscopically and crystallograph-
ically (Fig. 5). The compound contains a 4-coordinate square
planar cation analogous to that in 3·(Br)− and 3·(PF6)−. The
Ni–CNHC, Ni–Br and Ni–Npy distances are unremarkable. The
1H NMR spectrum of 3Me·(AgI2)− is in agreement with the high
symmetry structure observed in the solid state.
Fig. 3 ORTEP diagram of compound 5; thermal ellipsoids at 50% prob-
ability, hydrogen atoms, DiPP rings (bar ipso carbons), fluorines from the
OTf groups and two THF solvent mol◦ecules have been removed for clarity.
2.017(4); O(1)–Ni(1) 2.190(3); O(7)–Ni(1) 2.069(4); C(13)–Ni(1)–C(13ꢀ)
155.3(2); C(13)–Ni(1)–N(3) 77.63(11); O(1)–Ni(1)–O(1ꢀ) 179.03(15)
C(13)–Ni(1)–O(1) 87.49(12). The two halves of the molecule are related by
the symmetry operation (−x + 1, y, −z + 0.5).
˚
Selected bond lengths (A) and angles ( ): C(13)–Ni(1) 2.103(4); N(3)–Ni(1)
When 3Me·(AgI2)−, was reacted with two equivalents of AgOTf in
THF, colourless crystals were isolated after workup. The complex
obtained was characterised crystallographically: the structure
indicated that a ‘reverse NHC transmetallation’ from the cation of
3Me·(AgI2)− to silver had taken place. Product 6Me·(OTf)−2 contains
an Ag-bridged bis(ligand) dication, as seen in the previous silver
complex, with two triflate anions instead of (Ag6I8)2− (see ESI‡).
The liberated Ni(II) stayed in solution, presumably in the form of
solvated NiI2.
Complex 5 lies on a special position in the unit cell and has
crystallographically imposed C2-symmetry. The Ni–CNHC bond
˚
˚
length of 2.103(4) A is substantially (ca. 0.084 A) longer than the
previous longest Ni–CNHC bond length: interestingly, this was from
the only other known octahedral nickel carbene,5b which indicates
that the octahedral geometry contributes to the lengthening of
the Ni–CNHC bond. The pincer bite angle is ca. 77◦ and the Ni–
˚
Npy distance of 2.017(4) A is much longer than the corresponding
−
˚
length in 3·(Br) [1.872(3) A]. The dependence of the Ni geometry
The metrical data of 6Me·(Ag6I8)2− and 6Me·(OTf)− are not
2
in NHC complexes on the nature of the co-ligands has been
previously reported.5b Complex 5, by virtue of the lability of the
OTf− and THF ligands, is an attractive starting material for further
derivatisation which is currently under investigation.
quite identical; although the CNHC–Ag bond lengths are the same
(within experimental error), the CNHC–Ag–CNHC bond angle is
slightly smaller in 6Me·(OTf)− [173.42(9)◦] than in 6Me·(Ag6I8)2−
2
[177.0(3)◦].
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 1087–1094 | 1089
©

View Article Online
Fig. 6 ORTEP diagram of compound 7; thermal ellipsoids at 50%
probability, hydrogen atoms and one THF solvent mole◦cule have been
˚
removed for clarity. Selected bond lengths (A) and angles ( ): C(13)–Ni(1)
1.912(2); N(3)–Ni(1) 1.8725(19); C(21)–Ni(1) 1.930(2); C(36)–Ni(1)
1.924(2); C(21)–C(37) 1.337(3); C(23)–N(5) 1.259(3); C(13)–Ni(1)–C(21)
164.05(10); N(3)–Ni(1)–C(36) 177.77(10); N(3)–Ni(1)–C(13) 81.50(9);
N(3)–Ni(1)–C(21) 82.68(10); C(37)–C(21)–Ni(1) 132.2(2).
Fig.
5
ORTEP diagram of compound 3Me·(AgI2)−; thermal ellip-
soids at 50% probability, hydrogen atoms and the (AgI2)− anion
˚
have been removed for clarity. Selected bond lengths (A) and angles
(◦): C(13)–Ni(1) 1.911(8); N(3)–Ni(1) 1.860(6); C(21)–Ni(1) 1.895(8);
Br(1)–Ni(1) 2.2761(11); C(13)–Ni(1)–C(21) 163.3(3); C(13)–Ni(1)–N(3)
81.1(3); C(21)–Ni(1)–N(3) 82.2(3).
of a methyl group (crystallographic C37) from nickel to the CNHC
.
˚
The N(5)–C(23) bond length of 1.259(3) A is consistent with an
To prevent transmetallation, AgOTf was replaced by TlOTf
and the reaction was rerun. Compound 3Me·(Br)− was reacted
with two equivalents of TlOTf in THF, but after workup an
orange solution remained. Complex 3Me·(OTf)− was characterised
by structural (see ESI‡), spectroscopic and analytical methods.
Structural features of the cation are identical to other square
planar cations discussed above [3Me(X)−].
The occurrence of ‘reverse NHC transmetallation’ is unex-
pected: It implies some degree of (hemi)lability of the pincer ligand
in 3Me·(AgI2)− and underlines the importance of the nature of the
anions, in addition to the strengths of the M–CNHC bonds that
are being broken and formed during the reaction, in determining
the direction of the transmetallation. We have not been able to
observe this reaction with other Ni complexes, even though the
transformation described above is reproducible.
Attempts at alkylating 3·(Br)− and 3Me·(Br)− with MeLi, AlMe3
or PhMgCl gave intractable reaction mixtures. In an attempt
to prepare a methyl complex stabilised by the pincer ligand,
we reacted NiMe2(tmeda) with 2. The product 7 exhibited a
complicated 1H NMR spectrum with a broad resonance in place of
the usual sharp doublets for the iPr methyl groups, three signals for
the iPr CH groups (in a 1 : 1 : 2 ratio) and only one singlet assignable
to methyl groups attached to Ni. In addition, a singlet and several
other resonances in the olefinic region suggested possible decom-
position of the ligand system. The identity of 7 was clearly estab-
lished by single crystal X-ray diffraction (Scheme 3 and Fig. 6).
˚
imine functionality, the C(21)–C(37) bond length of 1.337(3) A is
concordant with a C C bond and is consistent with the olefinic
resonances observed in the H NMR spectrum. Assignment of
=
1
C(22) as a methylene carbon was indicated by the geometry around
the atom, along with the experimental location of the hydrogens
in the difference Fourier map. The C(13)–Ni(1) and C(21)–Ni(1)
bond lengths are different [1.912(2) A and 1.930(2) A respectively]
but, surprisingly, the Ni–CNHC distance is shorter than the Ni–Cvinyl
distance. The bite angles in the NHC-pyridine and pyridine-r-
enyl-amine chelate rings of the non-symmetric pincer ligand are
slightly different [81.50(9)◦ and 82.68(10)◦ respectively].
˚
˚
The study of complexes in which various organometallic
functionalities (for example alkyls, alkylidenes etc.) share the
same coordination sphere with a ‘spectator’ NHC ligand offers
fundamental understanding of the reactivity of NHC complexes
in homogeneous catalysis. The following transformations have
been recently observed with late transition metal complexes which
question the widely accepted view that NHCs are ideal ‘innocent’
ligands in organometallic chemistry: (i) the reductive elimination
of alkyl- or aryl-imidazolium salts from alkyl- or aryl-imidazol-
2-ylidene complexes;8 (ii) the migration of CH3 from Pd to the
CNHC of a coordinated imidazol-2-ylidene;9 (iii) the ring expansion
in metathesis catalysts;10 (iv) C–C and C–H activation of alkyl
substituents;11 (v) the ring opening of imidazolin-2-ylidenes coor-
dinated on Ni phenyl leading to Fischer type carbene complexes12
and (vi) the coupling of alkynes with imidazol-2-ylidenes on
ruthenium.13 It has been postulated that the transformations (v)
and (vi) are initiated by the migration of r-bound organometallic
to the CNHC of a coordinated NHC; this reaction was first
established in the ‘pincer’ pyridine dicarbene Pd complex (ii).9,12,13
The exact mechanism of the ring opening occurring in our
system is not clear. However, based on the direct observation of
Me migration from Pd to CNHC and the identity of 7, we propose
a plausible mechanism illustrated in Scheme 4.
Scheme 3 Reaction between 2Me and NiMe2(tmeda) (R1 = DiPP).
The structure of the molecule revealed an unexpected ring
opening of the NHC functional group, accompanied by migration
The first step is coordination of ligand 2 to Ni with displacement
of tmeda. The resultant 5-coordinate intermediate undergoes
1090 | Dalton Trans., 2008, 1087–1094
This journal is
The Royal Society of Chemistry 2008
©

View Article Online
Scheme 4 Plausible mechanism of formation for 7 (R1 = DiPP).
methyl migration to the CNHC, followed by base (tmeda) catalysed
deprotonation of the b-hydrogen bearing alkyl. In the resulting r-
vinyl group, ring-opening of the imidazol ring takes place, forming
the imine. The resulting carbanion is then reprotonated by the
ammonium cation (tmeda·H)+ at C(22) affording product 7.
2,6-Bis-[(3-(2,6-diisopropylphenyl)-imidazolium]-3,5-dimethyl-
pyridine diiodide, 1Me·(I)−
. 2,6-Diiodo-3,5-dimethylpyridine
2
(5.0 g, 13.9 mmol) and 2,6-DiPP imidazole (9.5 g, 41.7 mmol) were
◦
heated to 190 C under vacuum in a sealed ampoule and stirred
for 7 days. After this time, the cooled solid was extracted into a
minimum of dichloromethane and ether was added. The grey solid
formed was filtered, washed with Et2O and dried under Dean and
Stark conditions affording 10.98 g (97%). Crystallisation occurred
by slow diffusion of Et2O into a concentrated dichloromethane
solution. dH (400 MHz, CDCl3): 10.83 (2H, s, imidazolium CH),
9.11 (2H, s, Ar), 8.02 (1H, s, Ar), 7.56 (2H, t, J = 8.0 Hz, Ar),
7.35–7.42 (6H, m, Ar), 2.67 (6H, s, py-Me), 2.42 (4H, septet, J =
Conclusions
The synthesis of the pincer pyridine dicarbene ligands (2 and
2
Me) provided access to a class of analogous nickel complexes
and showed the influence of ligand steric and electronic factors
and the counter anion on the nature of the products obtained.
This was highlighted by the first observation of transfer of
functionalised NHC ligand from nickel to silver. A ring opening of
the coordinated imidazol-2-ylidene was also observed for the first
time when methyl co-ligands shared the same coordination sphere
with the rigid NHC pincer. A mechanism for this transformation
has been proposed and is based on migratory insertion of the
i
i
7.0 Hz, Pr CH), 1.20 and 1.27 (each 12H, d, J = 7.0 Hz, Pr
CH3) ppm. dC (100.63 MHz, CDCl3): 147.61 (CH, Ar), 145.10,
142.09 (C, Ar), 138.27, 132.15 (CH, Ar), 131.60, 129.73 (C, Ar),
i
125.10, 124.76, 124.15, 123.98 (CH, Ar), 28.82 (CH, Pr), 24.49,
24.13 (CH3, iPr), 18.23 (CH3, py-Me) ppm. Calc. for C37H47N5I2:
C 54.49; H 5.81; N 8.59. Found: C 54.58; H 5.85; N 8.51%.
coordinated methyl to coordinated CNHC
.
2,6-Bis-[(3-(2,6-diisopropylphenyl)-imidazolium]-3,5-dimethyl-
The study of complexes of 2Me with other transition metals,
especially where metallation of the pyridine ring of 2 has been
observed, and the dependence of organometallic reaction products
on the electronics of the pyridine ring is work currently in progress
and will be reported shortly.
pyridine dibromide, 1Me·(Br)−
.
Compound 1Me·(I)− (20.0 g,
2
2
24.5 mmol) was dissolved in MeOH (300 cm3) and a solution
of NaBr (100.0 g, 980 mmol) in water (100 cm3) was added. The
reaction was stirred at room temperature for 6 h after which time
the solvents were removed, the solid extracted into a minimum
amount of dichloromethane, dried (MgSO4) and filtered. Et2O
was added with vigorous stirring and the grey solid formed was
filtered, washed with Et2O and azeotropically dried, yielding 17.2 g
of the title product (97%). Crystallisation occurred from slow
diffusion of Et2O into a concentrated dichloromethane solution.
dH (400 MHz, CDCl3): 10.95 (2H, s, imidazolium CH), 9.14 (2H, s,
Ar), 7.97 (1H, s, Ar), 7.47 (2H, t, J = 6.0 Hz, Ar), 7.33 (2H, s,
Ar), 7.24 (4H, br s, Ar), 2.62 (6H, s, py-Me), 2.33 (4H, septet, J =
6.5 Hz, iPr CH), 1.18 and 1.12 (each 12H, d, J = 6.5 Hz, iPr CH3)
ppm. dC (100.63 MHz, CDCl3): 147.51 (CH, Ar), 144.79, 141.82
(C, Ar), 138.24, 131.83 (CH, Ar), 130.97, 129.53, 128.71, 127.91
(C, Ar), 124.90, 124.46, 124.01 (CH, Ar), 28.56 (CH, iPr), 24.22,
23.82 (CH3, iPr), 18.06 (CH3, py-Me) ppm. Calc. for C37H47N5Br2:
C 61.58; H 6.56; N 9.71. Found: C 61.66; H 6.65; N 9.85%.
Experimental
General methods
Elemental analyses were carried out by the London Metropolitan
University microanalytical laboratory. NMR data were recorded
on Bruker AV-300 and DPX-400 spectrometers, operating at 300
and 400 MHz (1H) respectively. The spectra were referenced
internally using the signal from the residual protio-solvent (1H),
the signals of the solvent (13C) or an external CFCl3 sample
(19F). Magnetic data were recorded at room temperature on a
Johnson Matthey balance. All manipulations were performed
under nitrogen in a Braun glove box or by using standard
Schlenk techniques, unless stated otherwise. Solvents were dried
using standard methods and distilled under nitrogen prior to use.
The light petroleum used throughout had a bp 40–60 ◦C. The
starting materials DiPP imidazole,14 2,6-diiiodolutidine4 and
NiMe2(tmeda)15 were prepared according to literature procedures.
2,6-Bis[(3-(2,6-diisopropylphenyl)-imidazol-2-ylidene]-3,5-di-
methylpyridine, 2Me
.
KN(SiMe3)2 (580 mg, 2.91 mmol) and
1Me·(Br)− (1.0 g, 1.38 mmol) were cooled to −78 ◦C and cold
2
(−78 ◦C) petrol (30 cm3) was added. The suspension was stirred at
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 1087–1094 | 1091
©

View Article Online
−78 ◦C for 15 min then warmed to room temperature and stirred
for 72 h. After this time the solution was filtered, the solid washed
with petrol (20 cm3) and concentrated in vacuo, affording 600 mg
of a pale yellow solid (77%). Crystallisation occurred by cooling
a saturated petrol solution to −35 ◦C. dH (400 MHz, C6D6): 7.73
(2H, d, J = 1.5 Hz, imidazol-2-ylidine), 7.30–7.36 (2H, m, Ar),
7.19–7.23 (5H, m, Ar), 6.62 (2H, d, J = 1.5 Hz, imidazol-2-ylidine),
septet, J = 7.0 Hz, iPr CH), 1.21 and 1.07 (each 12H, d, J = 7.0 Hz,
iPr CH3) ppm. dC (100.63 MHz, CD2Cl2): 165.22 (C, CNHC), 150.76
(C, Ar), 146.53 (CH, Ar), 144.61, 133.94 (C, Ar), 130.36 (CH, Ar),
128.32 (CH, imidazol-2-ylidene), 123.75 (CH, Ar), 116.21 (CH,
imidazol-2-ylidene), 108.32 (CH, Ar), 28.76 (CH, iPr), 24.48, 23.03
(CH3, iPr) ppm. Calc. for C35H41N5BrF6NiP: C 51.56; H 5.07; N
8.59. Found: C 51.43; H 5.16; N 8.65%.
i
2.92 (4H, septet, J = 7.0 Hz, Pr CH), 2.76 (6H, s, py-Me), 1.25
Nickel{2,6-bis-[(3-(2,6-diisopropylphenyl)-imidazol-2-ylidene]-
and 1.15 (each 12H, d, J = 7.0 Hz, iPr CH3) ppm. dC (100.63 MHz,
C6D6): 221.70 (C, CNHC), 148.81 (C, Ar), 146.66 (CH, Ar), 146.43,
138.90, 129.34 (C, Ar), 129.12 (CH, Ar), 128.57, 125.70, 125.03
(C, Ar), 123.79, 121.21, 119.83 (CH, Ar), 28.66, 24.63 (CH3, iPr),
23.88 (CH, iPr), 20.42 (CH3, py-Me) ppm. Calc. for C37H45N5: C
79.39; H 8.10; N 12.51. Found: C 79.50; H 8.13; N 12.44%.
pyridine} acetonitrile bis(tetrafluoroborate), 4·(BF4)−
. Com-
2
pound 3·(Br)− (250 mg, 0.33 mmol) and AgBF4 (136 mg,
0.70 mmol) were dissolved in MeCN (25 cm3) and stirred for 16 h.
After this time the solution was filtered through Celite, the solvent
was removed under vacuum and the solid was kept. Crystallisation
occurred from slow diffusion of Et2O into a concentrated MeCN
solution. Yield: 247 mg of green crystals (92%). dH (300 MHz,
CD3CN): 8.46 (1H, t, J = 8.5 Hz, py-4), 8.14 (2H, d, J = 2 Hz,
imidazol-2-ylidine), 7.72 (2H, d, J = 8.5 Hz, py-3,5), 7.42 (2H,
dd, J = 8.5, 7.0 Hz, Ar), 7.27–7.34 (6H, m, Ar), 2.54 (4H, septet,
Nickel{2,6-bis-[(3-(2,6-diisopropylphenyl)-imidazol-2-ylidene]-
pyridine}bromide bromide, 3·(Br)−. To
a
solution of
NiBr2(DME) (116 mg, 0.38 mmol) in THF (15 cm3) was
added a solution of 2 (200 mg, 0.38 mmol) in THF (15 cm3) at
0 ◦C. The reaction was stirred for 15 min, then warmed to room
temperature and stirred for 16 h. After this time, the solvent
was removed under vacuum and the solid product was washed
with light petroleum (20 cm3) and dried. Crystallisation occurred
from slow diffusion of ether into a concentrated dicloromethane
solution. Yield: 210 mg of an orange solid (74%). dH (300 MHz,
i
J = 7.0 Hz, Pr CH), 1.94 (3H, s, MeCN), 1.19 and 1.08 (each
i
12H, d, J = 7.0 Hz, Pr CH3) ppm. dC (100.63 MHz, CD3CN):
151.81 (C, Ar), 149.33 (CH, Ar), 145.92, 132,64 (C, Ar), 132.25,
i
128.32, 125.12, 118.67, 109.92 (CH, Ar), 28.72 (CH, Pr), 23.90,
i
23.62 (CH3, Pr) ppm. dF (282.4 MHz, CD3CN): −151.27. Calc.
for C37H44N6NiB2F8: C 55.20; H 5.51; N 10.44. Found: C 55.27;
H 5.59; N 10.51%.
i
CD2Cl2): 6.0–9.2 (13H, br m, Ar), 0.5–3.0 (38H, br m, Pr
CH/CH3 + acetone) ppm. dC (75.5 MHz, CD2Cl2): 164.62,
trans-Nickel{2,6-bis-[(3-(2,6-diisopropylphenyl)-imidazol-2-yli-
dene]pyridine}-bis-trifluoromethanesulfonate tetrahydrofuran, 5.
Compound 3·(Br)− (250 mg, 0.33 mmol) and AgOTf (171 mg,
0.66 mmol) were dissolved in THF (25 cm3) and stirred for 4 h.
After this time, the resultant green solution was filtered through
Celite, the solvent was removed under vacuum and the solid
product was dried under vacuum. Crystallisation occurred by
slow diffusion of light petroleum to a concentrated THF solution.
Yield: 300 mg of green crystals (94%). leff: 2.86 B.M. Calc. for
C41H49N5O7F6NiS2: C 51.26; H 5.14; N 7.29. Found: C 51.18; H
5.23; N 7.16%.
150.88, 144.55, 134.00, 130.09, 127.86, 123.58, 118.41, 109.83
i
(C/CH, Ar), 28.60, 24.42, 22.91 (CH/CH3, Pr) ppm. Calc. for
C35H41N5NiBr2: C 56.03; H 5.51; N 9.33. Found: C 55.97; H 5.41;
N 9.41%.
Nickel{2,6-bis-[(3-(2,6-diisopropylphenyl)-imidazol-2-ylidene]-
3,5-dimethylpyridine}bromide bromide, 3Me·(Br)−. Compound
2Me (250 mg, 0.45 mmol) and NiBr2(DME) (96 mg, 0.45 mmol)
were suspended in THF (20 cm3) and stirred for 16 h at room
temperature. After this time, solvents were removed in vacuo,
affording 326 mg of the title compound as an orange powder
(94%). dH (400 MHz, C6D6): 8.32 (1H, s, py-H), 8.28 (2H, br s,
imidazol-2-ylidene), 7.35 (2H, t, J = 7.5 Hz, Ar), 7.11 (4H, br d,
J = 7.5 Hz, Ar), 7.01 (2H, br s, imidazol-2-ylidene), 2.89 (6H, s,
py-Me), 2.46 (4H, septet, J = 6.5 Hz, iPr CH), 1.19 and 1.05 (each
12H, d, J = 6.5 Hz, iPr CH3) ppm. dC (100.6 MHz, C6D6): 182.18
(C, CNHC), 167.10 (C, Ar), 152.86 (CH, Ar), 148.58, 145.21, 134.98
(C, Ar), 130.91, 128.55, 124.38 (CH, Ar), 120.91 (C, Ar), 119.63
Bis-[2,6-bis-[(3-(2,6-diisopropylphenyl)-imidazol-2-ylidene]-3,5-
dimethylpyridine disilver (hexasilver octaiodide), 6Me·(Ag6I8)2−.
Ag2O (232 mg, 1.00 mol) and compound 1Me·(I)− (816 mg,
2
1.00 mmol) were mixed as powders, dissolved in dichloromethane
(40 cm3) and stirred for 4 h with the exclusion of light. After
this time, the solution was concentrated, filtered through Celite
and ether (200 cm3) was added to complete precipitation. The
white solid formed was filtered and dried in vacuo. Crystallisation
occurred by slow diffusion of ether into a CDCl3 solution. dH
(400 MHz, C6D6): 7.66–7.87 (2H, m, Ar), 7.39–7.48 (2H, m,
Ar), 7.18–7.32 (3H, m, Ar), 6.96–7.15 (4H, m, Ar), 2.51 (4H,
i
i
(CH, Ar), 29.42 (CH, Pr), 25.26, 23.68 (CH3, Pr), 19.50 (CH3,
py-Me) ppm. Calc. for C37H45N5NiBr2: C 57.10; H 5.83; N 9.00.
Found: C 56.96; H 5.69; N 8.82%.
Nickel{2,6-bis-[(3-(2,6-diisopropylphenyl)-imidazol-2-ylidene]-
pyridine}bromide hexafluorophosphate, 3·(PF6)−. Compound
3·(Br)− (150 mg, 0.20 mmol) and KPF6 (171 mg, 0.66 mmol) were
dissolved in THF (20 cm3) and stirred for 16 h. After this time the
solution was filtered through Celite and the solvent was removed
under vacuum. Crystallisation occurred by slow diffusion of ether
into a concentrated dichloromethane solution. Yield: 150 mg of
orange crystals, (92%). dH (400 MHz, CD2Cl2): 8.43 (1H, t, J =
8.0 Hz, py-4), 7.95 (2H, s, imidazol-2-ylidine CH), 7.69 (2H, d, J =
8.0 Hz, py-3,5), 7.36 (2H, t, J = 8.0 Hz, Ph-4), 7.13 (4H, d, J =
8.0 Hz, Ph-3,5), 7.00 (2H, s, imidazol-2-ylidine CH), 2.485 (4H,
i
br s, Pr CH), 2.40 (6H, s, py-Me), 1.24, 1.14 (each 12H, br s,
iPr CH3) ppm. dC (100.6 MHz, C6D6): 188.27 (C, CNHC), 147.51,
147.25 (C, Ar), 146.58, 146.25 (CH, Ar), 145.82, 145.08, 134.80
(C, Ar), 130.82, 130.58, 124.34, 123.71, 122.41 (CH, Ar), 28.42
(CH, iPr), 24.81, 24.49 (CH3, iPr), 19.01 (CH3, py-Me) ppm. Calc.
for C74H90N10Ag8I8: C 29.65; H 3.03; N 4.67. Found: C 29.77, H
3.10; N 4.63%.
Nickel-{[2,6-bis-[(3-(2,6-diisopropylphenyl)-imidazol-2-ylidene]-
3,5-dimethylpyridine}bromide (silver diiodide), 3Me·(AgI2)−.
Compound 6Me·(Ag6I8)2− (450 mg, 0.44 mmol) and NiBr2(DME)
1092 | Dalton Trans., 2008, 1087–1094
This journal is
The Royal Society of Chemistry 2008
©

View Article Online
◦
(136 mg, 0.44 mmol) were both dissolved in thf (10 cm3). The
solution of 6Me·(Ag6I8)2− was added to the solution of the Ni
complex via canula and stirred for 72 h. After this time, the
solution was filtered through Celite, concentrated to dryness and
crystallised by slow diffusion of PE into a concentrated THF
solution. dH (400 MHz, d8-thf): 8.41–8.45 (3H, m, Ar), 7.36–7.46
(4H, m, Ar), 7.18–7.23 (4H, m, Ar), 3.02 (6H, s, py-Me), 2.76
0.47 mmol) were dissolved in cold (−78 C) THF (20 cm3) and
stirred for 16 h at room temperature. After this time, solvents
were removed in vacuo, the solid was extracted into ether, filtered
and cooled to −35 ◦C, affording 131 mg of the title compound
as brown crystals (45%). dH (400 MHz, C6D6): 6.96–7.38 (11H,
m, Ar), 5.97 (1H, dd, J = 12.0, 10.0 Hz, imine CH), 5.59 and
5.25 (each 1H, br s, olefin CH2), 3.36 (1H, septet, J = 6.5 Hz,
i
i
i
(4H, septet, J = 6.5 Hz, Pr CH), 1.36, 1.19 (each 12H, d, J =
iPr CH), 3.08 (1H, br s, Pr CH), 2.85 (2H, br s, Pr CH), 2.36
(2H, s, CH2), 2.12 (3H, s, Ni–Me), 0.89–1.24 (24H, m, Pr CH3)
i
i
6.5 Hz, Pr CH3) ppm. dC (100.6 MHz, d8-thf): 168.60 (C, Ar),
154.23 (CH, Ar), 150.09, 146.73, 137.12 (C, Ar), 131.70, 129.69,
125.36 (CH, Ar), 122.29 (C, Ar), 121.41 (CH, Ar), 30.60 (CH,
ppm. dC (100.6 MHz, C6D6): 186.95 (C, CNHC), 168.21 (C, imine),
157.48, 148.37, 145.68, 144.67, 138.61 (C, Ar), 137.15 (CH, Ar),
136.10 (C, Ar), 134.83, 130.22 (CH, Ar), 128.70, 127.40 (C, Ar),
126.07, 125.04, 124.61, 124.14, 123.95, 123.69 (CH, Ar), 104.41
(C, olefin), 100.01 (CH2, olefin), 58.42 (CH2, CH2), 46.05 (CH,
iPr), 28.81, 28.28, 24.55, 23.93 (CH3, iPr), 15.62 (CH, iPr), −7.63
(CH3, Ni-Me) ppm. Calc. for C37H47N5Ni: C 71.62; H 7.63; N
11.29. Found: C 71.71; H 7.70; N 11.33%.
i
iPr), 26.39, 24.98 (CH3, Pr), 20.57 (CH3, py-Me) ppm. Calc. for
C37H45N5NiBrAgI2: C 41.92; H 4.28; N 6.61. Found: C 41.81, H
4.15; N 6.55%.
Nickel{2,6-bis-[(3-(2,6-diisopropylphenyl)-imidazol-2-ylidene]-
3,5-dimethylpyridine}bromide trifluoromethanesulfonate, 3Me
·
(OTf)−. Compound 3Me·(Br)− (250 mg, 0.32 mmol) and Tl(OTf)
(226 mg, 0.64 mmol) were suspended in THF (20 cm3) and stirred
for 16 h at room temperature. After this time, solvents were
removed in vacuo, affording 326 mg of the title compound as an
orange powder, (94%). dH (400 MHz, C6D6): 8.04 (2H, d, J =
2.0 Hz, imidazol-2-ylidene), 8.02 (1H, s, py-H), 7.35 (2H, t, J =
7.5 Hz, Ar), 7.09–7.16 (4H, m, Ar), 6.98 (2H, d, J = 2.0 Hz,
imidazol-2-ylidene), 2.78 (6H, s, py-Me), 2.46 (4H, septet, J =
7.0 Hz, iPr CH), 1.19 and 1.05 (each 12H, d, J = 2.0 Hz, iPr CH3)
ppm. dC (100.6 MHz, C6D6): 167.22 (C, Ar), 152.44 (CH, Ar),
148.60, 145.23, 134.97 (C, Ar), 130.98, 128.55, 124.43 (CH, Ar),
X-Ray crystallography
A summary of the crystal data, data collection and refinement
for compounds 2Me, 3·(Br)−, 5, 6Me·(Ag6I8)2−, 3Me·(OTf)− and 7 are
given in Table 1.
All data sets (except those for 2 and 7) were collected on a Enraf-
Nonius Kappa CCD area detector diffractometer with an FR591
rotating anode (Mo-Ka radiation) and an Oxford Cryosystems
low temperature device operating in x scanning mode with w
and x scans to fill the Ewald sphere. The programs used for
control and integration were Collect, Scalepack, and Denzo.16
The crystals were mounted on a glass fiber with silicon grease,
from Fomblin vacuum oil. All solutions and refinements were
performed using the WinGX package17 and all software packages
within. Refinements were carried out with all data on F2 full matrix
least-squares using SHELXL-97.18 All non-hydrogen atoms were
refined using anisotropic thermal parameters, and hydrogens were
added using a riding model.
i
120.77 (C, Ar), 119.08 (CH, Ar), 29.45 (CH, Pr), 25.24, 23.68
i
(CH3, Pr), 18.90 (CH3, py-Me) ppm. dF (282.4 MHz, C6D6):
−78.89. Calc. for C38H45N5O3F3SNiBr: C 53.86; H 5.35; N 8.26.
Found: C 53.79; H 5.31; N 8.31%.
Methyl nickel{2-[(3-(2,6-diisopropylphenyl)-imidazol-2-ylidene]-
6-[2-(2,6-diisopropylphenyl)iminomethyl vinyl amine]pyridine}, 7.
Ligand 2 (250 mg, 0.47 mmol) and NiMe2(tmeda) (96 mg,
Table 1 Crystallographic data for the compounds described in this paper
Complex
2Me
3·(Br)−
5
6Me·(Ag6I8)2−
3Me·(OTf)−
7
Chemical formula
C37H45N5
559.78
C35H41N5Br2Ni, C41H49N5O7F6S2Ni,
C74H90N10Ag8I8,
2(C4H10O)
3145.96
Monoclinic
P21/n
13.6353(4)
22.1211(6)
17.0640(5)
90
98.0900(10)
90
5095.8(3)
2
C37H45N5NiBrAgI2 C37H47N5Ni,
(C4H10O)
(CH2Cl2)
835.18
2(C4H8O)
1104.89
Monoclinic
C2/c
Formula weight
Crystal system
Space group
1060.07
Monoclinic
C2/c
694.63
Triclinic
¯
P1
Monoclinic Triclinic
¯
P1
P21/c
15.058(3)
24.920(6)
10.967(3)
90
91.360(3)
90
4114.2(14)
4
˚
a/A
12.7728(5)
13.6700(5)
25.2292(10)
87.286(2)
75.428(2)
71.776(2)
4047.2(3)
4
22.7194(19)
25.510(3)
14.1716(13)
90
125.520(4)
90
6685.2(11)
4
120(2)
41.413(2)
8.1837(4)
23.0900(11)
90
104.661(2)
90
9.907(2)
13.813(3)
15.330(3)
111.24(3)
93.82(3)
99.87(3)
1907.3(7)
2
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
1902.67(15)
8
T/K
120(2)
0.054
120(2)
120(2)
3.972
120(2)
3.735
120(2)
0.546
l/mm−1
2.616
0.415
No. of data collected
No. of unique data
Goodness of fit on F2
Rint.
22457
5879
0.976
69924
18589
1.008
0.1227
0.0900
0.1947
28442
7590
1.056
0.0715
0.0869
0.2181
78859
11729
1.166
0.0516
0.0560
0.1121
42239
8691
1.045
0.0734
0.0667
0.2119
18662
9332
1.024
0.0377
0.0498
0.1286
0.0579
Final R(|F|) for Fo > 2r(Fo) 0.0700
Final R(F2) for all data
0.2125
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 1087–1094 | 1093
©

View Article Online
CCDC reference numbers 663954–663964.
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b715769j
6 K. Inamoto, J. Kuroda, K. Hiroya, Y. Noda, W. Watanabe and T.
Sakamoto, Organometallics, 2006, 25, 3095.
7 G. K. H. Shimizu, G. D. Enright, C. I. Ratcliffe, K. F. Preston, J. L.
Reid and J. A. Ripmeester, Chem. Commun., 1999, 1485.
8 D. S. McGuinness, K. J. Cavell, B. F. Yates, B. W. Skelton and A. H.
White, J. Am. Chem. Soc., 2001, 123, 8317.
9 A. A. Danopoulos, N. Tsoureas, J. C. Green and M. B. Hursthouse,
Chem. Commun., 2003, 756.
10 B. R. Galan, M. Gembicky, P. M. Dominiak, J. B. Keister and S. T.
Diver, J. Am. Chem. Soc., 2005, 127, 15702.
11 (a) R. F. R. Jazzar, S. A. Macgregor, M. F. Mahon, S. P. Richards and
M. K. Whittlesey, J. Am. Chem. Soc., 2002, 124, 4944; (b) N. M. Scott,
R. Dorta, E. D. Stevens, A. Correa, L. Cavallo and S. P. Nolan, J. Am.
Chem. Soc., 2005, 127, 3516; (c) R. Corbera, M. Sanau and E. Peris,
Organometallics, 2006, 25, 4002.
Acknowledgements
We thank the University of Southampton for financial support
(to D. P.), M. B. Hursthouse for the provision of X-ray facilities
and W. Clegg at the EPSRC single crystal synchrotron service at
Daresbury for the data collection of complexes 2 and 7.19 We also
1
thank N. J. Wells for acquiring the low temperature H NMR
spectra.
12 A. W. Waltman, T. Ritter and R. H. Grubbs, Organometallics, 2006,
25, 4238.
13 E. Becker, V. Stingl, G. Dazinger, K. Mereiter and K. Kirchner,
Organometallics, 2007, 26, 1531.
References
14 A. L. Johnson, U. S. Patent, 1972, 3 637 731.
1 (a) W. A. Herrmann, Angew. Chem., Int. Ed., 2002, 41, 1290; (b) D.
Bourisou, O. Guerret, F. P. Gabbai and G. Bertrand, Chem. Rev., 2000,
100, 39; (c) S. P. Nolan, N-Heterocyclic Carbenes in Synthesis, Wiley-
VCH, Weinheim, 2006.
2 H. M. J. Wang and I. J. B. Lin, Organometallics, 1998, 17, 972.
3 A. A. Danopoulos, S. Winston and W. B. Motherwell, Chem. Commun.,
2002, 1376.
15 (a) W. Kaschube, K. P. Po¨rschke and G. Wilke, J. Organomet. Chem.,
1988, 355, 525; (b) E. F. Connor, T. R. Younkin, J. I. Henderson, A. W.
Waltman and R. W. Grubbs, Chem. Commun., 2003, 2272.
16 R. Hooft, COLLECT; Nonius BV, 1997–2000; Z. Otwinoski and
W. Minor, SCALEPACK, DENZO. Methods Enzymol., 1997, 276,
307.
17 L. J. Farrugia, J. Appl. Crystallogr., 1999, 32, 83.
18 G. M. Sheldrick, Program for refinement of crystal structures, University
of Go¨ttingen, Germany, 1997.
19 R. J. Cernik, W. Clegg, C. R. A. Catlow, G. Bushnell-Wye, J. V. Flaherty,
G. N. Greaves, M. Hamichi, I. Burrows, D. J. Taylor and S. J. Teat,
J. Synchrotron Radiat., 1997, 4, 279.
4 D. Pugh, Acta Crystaloogr., Sect. C, 2006, C62, o590–592.
5 (a) M. V. Baker, B. W. Skelton, A. H. White and C. C. Williams,
Organometallics, 2002, 2674; (b) S. Winston, N. Stylianides, A. A. D.
Tulloch, J. A. Wright and A. A. Danopoulos, Polyhedron, 2004, 23,
2813; (c) A. Caballero, E. Diez-Barra, F. A. Jalon, S. Merino, A. M.
Rodriguez and J. Tejeda, J. Organomet. Chem., 2001, 627, 263.
1094 | Dalton Trans., 2008, 1087–1094
This journal is
The Royal Society of Chemistry 2008
©