Dinuclear N-heterocyclic carbene complexes of silver(I), derived from imidazolium-linked cyclophanes

Article abstract of DOI:10.1039/b408741k

The synthesis of four new silver complexes of bidentate N-heterocyclic carbenes, derived from imidazolium-linked cyclophanes, has been achieved via a simple complexation reaction of the imidazolium-linked cyclophanes with the basic metal source Ag₂O. The cyclophane structures contain two imidazolyl links between ortho-, meta- and mixed ortho/meta-substituled aromatic rings. The new silver carbene systems are thermally stable and have been characterised by NMR spectroscopy and X-ray crystallography. Three of the complexes have a dimeric structure of the form [L₂Ag ₂]²⁺ in the solid state that is rigid on the NMR timescale in solution. The fourth complex has a neutral structure of the form [L(AgBr)₂], the NMR studies suggesting some lability of the L-Ag bonding in solution.

Full text of DOI:10.1039/b408741k

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F U L L P A P E R
Dinuclear N-heterocyclic carbene complexes of silver(I), derived
from imidazolium-linked cyclophanes
Murray V. Baker,* David H. Brown, Rosenani A. Haque, Brian W. Skelton and Allan H. White
Chemistry M313, The University of Western Australia, 35 Stirling Highway, Crawley,
WA 6009, Australia. E-mail: mvb@chem.uwa.edu.au
Received 9th June 2004, Accepted 28th September 2004
First published as an Advance Article on the web 11th October 2004
The synthesis of four new silver complexes of bidentate N-heterocyclic carbenes, derived from imidazolium-linked
cyclophanes, has been achieved via a simple complexation reaction of the imidazolium-linked cyclophanes with
the basic metal source Ag2O. The cyclophane structures contain two imidazolyl links between ortho-, meta- and
mixed ortho/meta-substituted aromatic rings. The new silver carbene systems are thermally stable and have been
characterised by NMR spectroscopy and X-ray crystallography. Three of the complexes have a dimeric structure of
the form [L2Ag2]2+ in the solid state that is rigid on the NMR timescale in solution. The fourth complex has a neutral
structure of the form [L(AgBr)2], the NMR studies suggesting some lability of the L–Ag bonding in solution.
The palladium complexes were found to be exceptionally
Introduction
active and robust catalysts for Heck and Suzuki couplings, and
N-Heterocyclic carbenes (NHCs), commonly derived from
also displayed remarkable air- and thermal-stability.10 It was
imidazolium salts, have attracted widespread attention as ligands
postulated that the stability and exceptional catalytic activity
for transition metal complexes.1,2 Since the synthesis of Hg and
of the complexes was due to the rigidity of the cyclophane
Cr complexes of NHC by Wanzlick and Öfele in 1968,3,4 and as
skeleton and the free access to the metal centre provided by the
a direct result of more recent reports of their catalytic activity,
geometry of the cyclophane–metal binding. However, although
numerous examples of NHC-transition metal complexes have
numerous examples of NHC–transition metal complexes exist,
appeared in the literature.5–14 Their application as catalysts
reports of NHC–metal complexes derived from imidazolium-
covers a broad range of reactions, most notably carbon–carbon
based cyclophanes are rare. Outside our own work, Youngs and
bond-forming reactions such as the Heck and Suzuki coupling
coworkers have reported several silver complexes derived from
reactions,15–19 and olefin metathesis.20 NHC metal complexes
imidazolium-linked cyclophanes,14,34,38 including the dinuclear
are seen as attractive replacements for the well-established
cyclophane–silver complex 3 obtained from the reaction of an
industrially-important catalysts made from metal–phosphine
imidazolium-linked ‘pyridinophane’ with Ag2O,38 and Cavell
complexes.21–24 More recently, NHC metal complexes of silver25
and coworkers reported a catalytically-active palladium(II)
and gold26,27 have shown interesting biological activity as anti-
complex derived from the meta-cyclophane III·2Br.39
microbial and anti-mitochondrial agents respectively.
In this paper we report the synthesis of silver complexes
Silver complexes of NHCs have been of significant
derived from imidazolium-linked cyclophanes having ortho-
interest, particularly because of their ability to act as effective
(I), meta- (II and III) and mixed ortho/meta- (IV) substitution
carbene transfer agents. Wang and Lin28 first reported the use of
patterns. These new complexes are of interest as potential
NHC–silver complexes to prepare NHC–gold and –palladium
precursors for NHC complexes of other transition metals. The
complexes via carbene transfer. This transmetallation route may
cyclophane systems impose interesting constraints on the struc-
provide a convenient way of preparing carbene complexes that
tures and geometries of the silver complexes, which have been
are hard to obtain by conventional methods. Since the report
characterised by spectroscopic and crystallographic studies.
by Wang and Lin, the use of NHC–silver complexes instead of
free NHCs is becoming increasingly popular. Carbene transfer,
using NHC–silver complexes, has been used to prepare NHC
Results and discussion
complexes of rhodium(I),29,30 iridium,29 palladium,30 gold28 and
Synthesis of cyclophanes
copper(I).6 NHC–Silver complexes are easily prepared by a one-
pot reaction of an imidazolium salt with Ag2O, which acts as
the base and metal source. The resulting complexes are consider-
ably more tolerant to air and moisture compared to free NHCs.
Depending on the reaction conditions (ligands, the ratio of
ligands and Ag2O and the solvents used), NHC–silver complexes
with a variety of interesting structural geometries have been
isolated, including mononuclear and dinuclear complexes.8,31–35
We have recently reported a series of NHC–palladium(II),
–nickel(II) and –gold(I) complexes where the imidazolium-based
carbenes were part of a cyclophane structure (e.g. 1 and 2).10,36,37
The cyclophane salts I·2Br, II·2Br and III·2Br were prepared
following published procedures, by the reaction of a bis(imida-
zolylmethyl)arene with a bis(bromomethyl)arene.10,39 Solubil-
ity issues frequently make these cyclophanes difficult to work
with, and in the ortho/meta-cyclophane IV·2Br (Scheme 1),
heptyl chains were incorporated to increase solubility in
organic solvents. During one step in the synthesis of IV·2Br,
1,2-diheptylbenzene was di-bromomethylated with HBr and
paraformaldehyde in a mixture of acetic and propionic acids.
A method for di-bromomethylation of 1,2-dialkylbenzenes has
3 7 5 6
D a l t o n T r a n s . , 2 0 0 4 , 3 7 5 6 – 3 7 6 4
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

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been reported by Garg and Lee,40 but for systems containing
long alkyl chains the method is slow (reaction times longer
than 10 days). We noted that if only acetic acid was used as
the solvent, as in the method of Garg and Lee, the reaction
mixture was biphasic, a consequence of phase separation of
the non-polar dialkylbenzene and the highly polar HBr–acetic
acid mixture. The use of propionic acid as a co-solvent renders
the reaction mixture monophasic and as a result the reaction is
complete after 24 h.
grey powder in 43% yield, after recrystallisation from dmf. This
complex was insoluble in most common organic solvents and
only sparingly soluble in dmf and dmso.
Syntheses of complexes 4–7 were conducted with the
exclusion of light, to prevent formation of black precipitates.
After their synthesis, however, the complexes are generally stable
to moisture, light and heat. Solid samples of 4–7 exposed to the
laboratory atmosphere and ambient light for 8 days show no
sign of decomposition (as indicated by their NMR spectra in
(CD3)2SO solution). Similarly, solutions of 4–7 in (CD3)2SO
did not exhibit decomposition after 8 days of exposure to
light and moisture, or heating at 100 °C in air. In the case of
6·2BPh4, despite excellent stability in dmso solutions (very
slow decomposition occurred during prolonged heating), some
decomposition (darkening) occurred during recrystallisation
from CH3CN if the solutions were exposed to light. Cooling
and evaporation of solutions of 6·2BPh4 to afford crystals were
performed in darkness.
The structures of 4–7 were established by a combination of
NMR and structural studies (see below) and elemental analysis.
Elemental analysis showed that the complex assigned as 7 has
the stoichiometry [(cyclophane)(AgBr)2], and the results of
NMR studies were consistent with that stoichiometry and
the monomeric structure 7. However, a single-crystal X-ray
study performed on a crystal grown from a dmso solution of
7 revealed the more complex dimeric structure 8, containing an
(AgBr)2 core. The (AgX)2 core (X = halide) is commonly seen
in X-ray studies of NHC–silver complexes33 and in this case
presumably arose from residual AgBr or AgBr formed from
minor decomposition of 7.
Synthesis of the silver complexes
The silver complexes 4–7 were prepared by the reaction of Ag2O
with the corresponding cyclophane salt with the exclusion of
light, based on procedures developed by Wang and Lin.28
Reaction of I·2PF6 with 2 equivalents of Ag2O in hot
acetonitrile afforded 4·2PF6, which was isolated as colourless
crystals in 43% yield after recrystallisation. In this synthesis,
AgPF6 was a contaminant that proved difficult to remove by
recrystallisation from organic solvents. Thus, during the workup,
the reaction mixture was poured into water; 4·2PF6 precipitated
and was easily isolated by filtration while the AgPF6 byproduct
remained dissolved. We attempted to synthesize the dibromide
salt 5·2Br by the reaction of III·2Br with 2 equivalents of Ag2O
in dmf. We were unable to obtain analytically pure samples
of this material, presumably because of contamination of
the product with silver bromide species. Interestingly, during
attempts to purify 5·2Br by recrystallisation from dmf,
5·(Ag2Br4) was obtained (see Structural studies section).
Analytically pure samples of the complex 5 were obtained as
the hexafluorophosphate salt 5·2PF6, which was prepared by
salt metathesis. Complex 6 was prepared in a similar manner
to 5, by reaction of the bromide salt of the corresponding
cyclophane (IV·2Br) with 2 equivalents of Ag2O in methanol.
In this experiment, precipitated AgBr was removed by filtration
and addition of NaBPh4 induced precipitation of 6 as the salt
6·2BPh4. The binuclear complexes 4–6 are soluble in dmso and
CH3CN.
The synthesis of 7 was similar to those of complexes 4–6,
except that in this case II·2Br was allowed to react with only one
equivalent of Ag2O, in dmso. This reaction was much slower
than those leading to 4–6, taking 4 days, as monitored by NMR.
Presumably this slowness is a consequence of the conformation
of II; the mesitylene units are mutually syn and hinder access to
the imidazolium-H2 protons.41 The complex 7 was obtained as a
Scheme 1 Reagents and conditions: (i) paraformaldehyde, HBr, acetic acid, propionic acid; (ii) imidazole, KOH, thf; (iii) 1,3-bis(bromomethyl)-2,4,6-
trimethylbenzene, acetone.
D a l t o n T r a n s . , 2 0 0 4 , 3 7 5 6 – 3 7 6 4
3 7 5 7

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Structural studies
This symmetry is realized, although not with crystallographic
exactitude, in 4, in which the aromatic C6 ‘spacer’ groups bind
ortho to the imidazole rings and incline away (“exo”) from the
cation core. In the cations of 5, 6 and 3, in which a C6 or C5N
spacer is linked meta (the N being between the imidazolyl units in
the case of the C5N spacer), the symmetry is diminished, one of
the spacer rings inclined “endo”. The possibility of both spacer
rings being inclined “endo” is not realizable in the context of the
centrosymmetric bis(ligand) binuclear array [Ag2L2]2+, because
of potential steric interference.
Steric interactions within the cyclophane units in 6 and 8
may force the mesitylene ring into the “endo” inclination; in all
structurally characterised imidazolium-linked ortho/meta- and
meta-cyclophanes that contain a mesitylene group, that group is
in the “endo” inclination.41,43
The results of the ‘low’-temperature single crystal X-ray studies
of 4·2PF6·2CH3CN, 5·(Ag2Br4)·4dmf, 6·2BPh4 and 8·2CH3CN
are consistent in terms of stoichiometry and connectivity with
the formulations given above. All may be regarded as agglo-
merations of binuclear cations, in which the two silver atoms are
held in proximity by a pair of bis(carbenoid) ligands in the case
of 4·2PF6, 5·(Ag2Br4) and 6·2BPh4, or a carbenoid ligand and
anion in the case of 8, and an appropriate array of counterions,
more or less intimately associated in some cases, and, in some
cases, solvent. For 4·2PF6, 5·(Ag2Br4) and 6·2BPh4, the cation is
disposed about a crystallographic inversion centre, one half of
the formula unit making up the asymmetric unit of the structure.
For 8, the totality of the agglomeration of a pair of cations
intimately linked by the anion, together with the solvent, make
up the asymmetric unit.
Intra-cation AgAg in 4 is only 2.9620(6) Å; the reduction
of the C–Ag–C angle to 176.3(1) suggesting some degree of
argentophilic interaction; a similar metal–metal interaction is
seen in the Au analogue of 4.37 In 4 there are ‘semi-inclusive’
approaches of the PF6 anions, but toward the core of the cation,
The geometries of the cations of 4, 5 and 6 and the previously
studied 3,42 also centrosymmetric, are given in Table 1, with
their depiction in Fig. 1. The cations of 4 and 5 are potentially
quite symmetrical with maximal putative symmetry mmm.
Fig. 1 The centrosymmetric cations of (a) 4·2PF6, with associated anions, (b) 5·(Ag2Br4), (c) 6·2BPh4 and (d) 3 (from ref. 42).
3 7 5 8
D a l t o n T r a n s . , 2 0 0 4 , 3 7 5 6 – 3 7 6 4

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Table 1 Selected cation descriptors
Cation
4
5
342
6
8
Distances (Å)
Ag–C(22)
Ag–C(42)
AgAg
2.095(3)
2.093(3)
2.9620(6)
2.095(4)
2.089(4)
5.0267(5)
2.10(1)
2.08(1)
4.488(2)
2.088(4)
2.087(4)
3.2908(4)
2.14(2); 2.15(2)
2.10(2); 2.07(2)
2.996(2); 3.003(3)
Angles (°)
C–Ag–C
176.3(1)
174.8(1)
171.3(3)
164.7(1)
—
Plane parameters: out of plane deviations (d/Å) and interplanar dihedral angles (h, /°)a
h2/4
12.5(1)
0.300(5)
0.049(5)
−40.29(9)
−42.02(8)
0.3(1)
10.2(4)
0.02(2)
0.11(2)
−59.8(3)
88.5(3)
48.0(2)
0.383(7)
0.461(8)
35.7(1)
49.6(9); 47.4(8)
0.08(4); 0.19(3)
0.09(4); 0.17(3)
20.8(5); 21.3(5)
21.8(5); 20.8(5)
dAg(C3N2(2))
0.055(7)
0.050(6)
−71.8(1)
41.5(1)
dAg(C3N2(4))
1
3
−35.7(1)
a h are the angles between the pair of imidazole planes;  are the angles between the C5N/C6 planes and the central carbenoid C4 planes (a negative sign
indicates an “exo” disposition); for 8 the central plane is defined by Ag2C2Br2.
Table 2 Geometries of the [BrAg(l-Br)2Br]2− moieties
8 (n = 1, 2)
rather than within the ligand “cups” (Fig. 1(a)). The acetonitrile
molecule of the asymmetric unit displays no unusually close
interactions.
5·(Ag2Br4)
In 5·(Ag2Br4), the intra-cation AgAg distance is now
extended by the larger spacers. Interestingly, the anion is the
rather uncommon [Ag2Br4]2−, potentially also mmm, here a
centrosymmetric [BrAg(l-Br)2AgBr]2− moiety. The anion is
found packing between successive cations, with successive
carbene/anion planes quasi-parallel (Fig. 2). The cation/anion
proximity is not as might be imagined, by way of silver (cation)–
halide (anion) interactions, but, rather, by silver (cation)–silver
(anion) interactions, at 2.8999(4) Å and with C–Ag–C now
diminished to 174.8(1)°, the silver bowing outwards from the
cation core in consequence of this interaction (Fig. 2). There
is a pair of crystallographically independent dmf solvent
molecules in the asymmetric unit, one ordered, one disordered.
The hydrogen of the ordered molecule contacts the bridging
bromine of the anion at 2.8 Å (est.). The symmetry of the anion
is diminished, perhaps because of these interactions (Table 2).
Distances (Å)
Ag(n)–Br(n)
Ag(n)–Br(n)
Ag(n)Ag(n)
Br(n)Br(n)
Ag(n)–Br(t)
2.624(3), 2.617(3)
2.709(3), 2.705(3)
3.011(2)
4.396(3)
2.527(3), 2.529(3)
2.6525(6)
2.6527(6)
3.5981(5)
3.8986(8)
2.5154(6)
Angles (°)
Br(n)–Ag(n)–Br(n)
Ag(n)–Br(n)–Ag(n)
Br(n)–Ag(n)–Br(t)
Br(n)–Ag(n)–Br(t)
111.04(9), 111.36(9)
68.78(8), 68.82(8)
132.4(1), 131.7(1)
116.6(1), 117.0(1)
94.59(2)
85.41(2)
135.22(2)
123.82(2)
In 8: Br(11)–Ag(11,12); Br(21)–Ag(21,22) are 2.634(3), 2.450(3);
2.623(3), 2.449(3) Å, the angles at Br(11,21) being 72.11(9), 72.52(8)°.
Br(12)–Ag(11), Br(22)–Ag(21) are 2.821(3), 2.837(3) Å, with Br(12)–
Ag(11)–Br(11),C(122) 96.54(9), 109.9(6), Br(22)–Ag(21)–Br(21), C(222)
99.14(9), 107.8(6) and Ag(1)–Br(12)–Ag(11), Ag(2)–Br(22)–Ag(21)
125.5(1), 126.7(1)°. Br(11)–Ag(12)–C(142), Br(21)–Ag(22)–C(242) are
175.5(6), 176.5(6)°. Br(11)–Ag(11)–C(122) 153.5(6); Br(21)–Ag(21)–
C(222) 153.1(6).
with regions accommodating hydrocarbon ‘tails’ and other
regions where parallel stacking of aromatic planes clearly
predominates (Fig. 3).
The [Ag2L2]2+ cation with the full complement of mesityl
substituents, all meta, has not been obtained, and may be
Fig. 2 The cation of 5·(Ag2Br4), showing associated solvent and anion
interactions.
In 6·2BPh4, accommodation of the “endo” mesityl group is
achieved with significant deformation of the mesitylene moiety
(Fig 1(c)). The methyl group lying between the imidazolyl units
lies 0.374(7) Å out of the plane of the arene ring, presumably
in response to steric repulsion by the imidazolyl rings, this in
turn counterpoised by contact between the periphery of the
mesityl group and the inversion related benzylic methylenes
(H(161a)H(3a) (1 − x, 1 − y, 1 − z) 2.33 Å (est.)). In this
compound the silver atoms lie well out of the imidazolyl planes
(Table 1). The crystal packing in 6·2BPh4 is of some interest,
Fig. 3 Unit cell contents of 6·2BPh4 projected down b.
D a l t o n T r a n s . , 2 0 0 4 , 3 7 5 6 – 3 7 6 4
3 7 5 9

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Table 3 1H NMR data for complexes 4–7 and 9a
Complex
Benzylics
Imidazolyl H4/H5
Aromatics
Other
4
5
5.28, 6.14
6.27 (8H, s)
7.31 (8H, s)
7.59–7.62, 7.78–7.81
(16H, AAXX)
5.67 (4H, s, H2);
(16H, AX, 2JHH 14 Hz)
5.17, 5.22
(16H, AB, 2JHH 16 Hz)
7.07 (4H, t, 3JHH 7 Hz, H5);
7.18 (8H, d, 3JHH 7 Hz, H4/H6)
6.29 (2H, s, H5 on meta-arene);
7.78 (4H, s, H3/6 on ortho-arene)b
6
5.04, 5.18
6.99 (4H, d, 3JHH 1 Hz, H5);
7.61 (4H, m, H4)
0.47 (6H, s, 2 × ArCH3);
0.89 (12H, t, 3JHH = 7.0 Hz,
4 × –CH2CH3);
(8H, AB, 2JHH 13.7 Hz, ortho);
5.22, 5.31
(8H, AB, 2JHH 16.3 Hz, meta)
1.28–1.36 and 1.36–1.49
(32H, 2 × m, 16 × CH2);
1.72 (8H, m, 4 × CH2);
2.24 (12H, s, 4 × ArCH3);
2.77 (8H, m, 4 × CH2)
1.78 (6H, s, 2 × CH3);
2.15 (12H, s, 4 × CH3)
2.27 (6H, s, 2 × CH3);
2.03 (12H, s, 4 × CH3)
7
5.14, 5.20
7.60 (4H, s)
7.62 (4H, s)
6.81 (2H, s, H5)
6.96 (2H, s, H5)
(8H, AB 2JHH 14 Hz)
5.14, 5.28
9c
(8H, AB 2JHH 14.5 Hz)
b
−
a Recorded at 500.13 MHz and ambient temperature from solutions in (CD3)2SO. Signals for BPh4 : 6.78 (8H, m); 6.91 (16H, m); 7.17 (16H, m).
c Tentative assignment. Data obtained from a solution containing 7 and 9 (4:1 ratio respectively).
inaccessible. Rather, the intriguing artefact 8 (Fig. 4), which
may be considered as a combination of cationic [LAg2Br]+ and
anionic [BrAg(l-Br)2AgBr]2−, has been obtained. Here, both of
the mesityl spacers of the single ligand of the cation are “endo”.
The ‘half-arrays’ are distorted by excessive inclinations of the
carbene rings concomitant with the bridging of the pairs of silver
atoms, now close for a spacer of this type (2.996(2), 3.003(2) Å),
by the bromines. The putative symmetry of the cation, of which
there are two independent examples in the asymmetric unit is
degraded by the perturbation brought about by the interaction
of one of the cation silver atoms with the anion. This time, the
cation/anion interaction is not via a silversilver contact, but by
interaction with the terminal bromine, an interaction replicated
at the other terminal bromine in a quasi-centrosymmetric
manner by the other cation, the two approaches to the anion
being from either side. In consequence, the distances to either
side of Br(n1) are dissimilar, as also the angles enclosed at
Ag(n1), cf. Ag(n2), those at the latter approaching linearity. The
Br2Ag2C2 arrays are quite closely coplanar (v2 1049, 872) with
dihedral angles to the anionic Ag2Br2 plane (v2 1) of 60.65(7),
59.77(7)°. Methyl groups n121 do not deviate significantly from
their parent arene rings (cf. 6·2BPh4). In 8, the mesitylene unit
can lie “flatter” (closer to parallel to the C(142)–Ag(11)–Ag(12)
plane) since there is no steric repulsion from an opposing
cyclophane unit to prevent this alignment.
As noted above, examples of the [BrAg(l-Br)2AgBr]2− anion
are limited.44–46 The two examples here (Table 2) are essentially
planar, with Ag–Br (bridging) longer than Ag–Br (terminal),
and that in 5·(Ag2Br4) approaching nearly mmm symmetry. The
two examples differ considerably in that in 8 interacts strongly
with the cation through contacts between the terminal bromines
and cationic metal, while that in 5·(Ag2Br4) as shown above has
metalmetal interactions. The geometry of the anion in 8 is
highly distorted from mmm, although the anioncation inter-
action has little impact on the Ag–Br (terminal) distance.
NMR studies
1
The H and 13C NMR spectral data of complexes 4–7 are
1
summarised in Tables 3 and 4. In the H NMR spectrum,
each complex exhibits a set of sharp doublets for the benzylic
protons, being an AX pattern for 4 and AB patterns for 5–7,
and suggesting that each complex has a rigid conformation in
solution.10 The 1H NMR data for 4 and 5 are similar to those of
the Au(I) analogues.37 The 13C NMR spectrum for each complex
exhibits a carbene carbon signal at ca. d180. For 4–6 this signal
appears as two doublets with Ag–C coupling constants of
ca. 180 and 210 Hz. These doublets are attributed to carbene
carbons bound to 107Ag and 109Ag, respectively, and the splitting
patterns are consistent with literature reports for carbene–silver
coupling constants being in the range of 180–189 Hz for 107Ag
and 204–220 Hz for 109Ag.28,47,48 In the case of the neutral
complex 7 the carbene carbon signal shows no Ag–C coupling.
Coupling between Ag and C is usually absent from the 13C NMR
spectra of Ag–NHC systems containing halides,29–31,49,50 a result
which is often attributed28 to the existence of equilibria involving
NHC–Ag–X, [NHC–Ag–NHC]+ and [AgX2]− (e.g., Scheme 2,
see below).
1
In the H NMR spectra of complexes containing a meta-
substituted arene unit, the signal due to the arene’s substituent
lying between the imidazolyl groups shows an upfield chemical
shift, consistent with shielding of the substituent by the
magnetically anisotropic imidazolyl units. For example, in the
spectrum of 5, the proton between the imidazolyl units (arene
H2) resonates at d 5.67, unusually upfield for an aryl proton
and significantly upfield of H4–H6 (d 7.07–7.18). A similarly
pronounced effect is seen in the chemical shifts of the mesitylene
methyl groups in 6—the methyl group lying between the
imidazolyl groups resonates at d 0.47, while the others resonate
at d 2.24. For 7, the difference between the chemical shifts of
Fig. 4 The aggregate 8.
3 7 6 0
D a l t o n T r a n s . , 2 0 0 4 , 3 7 5 6 – 3 7 6 4

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Table 4 13C NMR data for complexes 4–7 and 9a
Imidazolyl
C4/C5
Complex
Carbene
Benzylics
Aromatics
Other
4
5
6
183.9 (d, Ag–C, 1JC–Ag
=
53.5
53.3
120.4
120.5
129.9 (CH); 133.7 (CH);
135.3 (C)
210 Hz, 1JC–Ag = 181 Hz)
181.1 (d, Ag–C, 1JC–Ag
208 Hz, 1JC–Ag = 180 Hz)
=
123.0 (CH); 126.0 (CH);
128.4 (CH); 138.2 (C)
128.2 (CH, meta-Ar);
130.4 (C, ortho-Ar, C1/2);
133.8 (C, meta-Ar, C4/6);
134.8 (CH, ortho-Ar, C3/6);
136.0 (C, meta-Ar, C1/3);
139.1 (C, meta-Ar, C2);
142.3 (C, ortho-Ar, C4/5)b
131.3 (CH); 131.7 (C);
137.0 (C); 137.1 (C)
180.0 (d, Ag–C, 1JC–Ag
=
49.8 (meta);
52.3 (ortho)
117.4 (C5);
127.5 (C4)
13.9 (–CH2CH3); 18.9 (C2-ArCH3);
20.2 (C4/6-ArCH3);
22.1, 28.5, 29.2,
212 Hz,1JC–Ag = 182 Hz)
30.5, 31.3, 31.6 (all CH2)
7
9
178.3
48.4
48.2
122.59
122.60
16.4 (C2-ArCH3);
19.9 (C4/6-ArCH3);
17.3 (C2-ArCH3);
19.1 (C4/6-ArCH3)
not detected
131.0 (CH); 130.8 (C);
136.8 (C); 137.6 (C)
−
a Recorded at 125.8 MHz and ambient temperature from solutions in (CD3)2SO. b Signals for BPh4 : 121.5 (CH), 125.2 (CH), 135.9 (CH), 163.3 (C).
Scheme 2
complexes as carbene transfer agents is underway and will be
reported in due course.
the mesitylene methyl groups is much less significant, d 1.78 vs.
d 2.15. The observations for 6 vs. 7 are consistent with features
revealed in the solid state, in particular, the extent to which the
“inner” methyl group in 6·2BPh4 is pushed against the imidazo-
lyl rings by steric repulsion between the mesitylene unit and the
benzylic protons of the opposing cyclophane unit, interactions
that are absent in both 7 (in solution) and 8 (in the solid state).
Variable-temperature 1H NMR studies of solutions prepared
from 7 revealed the occurrence of an equilibrium involving 7
and an additional species, tentatively assigned as 9 (Scheme 2).
Electrospray mass spectra of these solutions showed signals at
m/z = 531 (107AgC28H32N4) and 533 (109AgC28H32N4) as expected
for 9. Consistent with the assignment of 9 as a product of
dissociation of 7 into two species (9 + [AgBr2]−, Scheme 2), the
relative proportion of 9 increased when the sample temperature
was raised (but 9 was still the minor component) or when the
sample was diluted. Exchange between 7 and 9 was slow on the
NMR timescale even at 80 °C, and so interconversion of 7 and
9 cannot directly account for the absence of Ag–C coupling in
the carbene signal of 7. However, exchange of Ag between 7
and [AgBr2]− via a mechanism similar to that suggested by Wang
and Lin28 would account for the absence of coupling. The low
intensity of the signal due to the carbene carbon of 7 prevented
exploration of this problem by variable-temperature 13C NMR.
In concentrated solutions, 9 was present in only trace amounts
and was easily overlooked.
Experimental
Nuclear magnetic resonance spectra were recorded using
Bruker Avance 500 (500.1 MHz for 1H and 125.8 MHz for 13C)
and Bruker ARX300 (300.1 MHz for 1H and 75.5 MHz for 13C)
spectrometers at ambient temperature. Chemical shifts were
referenced to solvent resonances. Assignments of 13C NMR
spectra were made with the aid of DEPT spectra. The complete
1
assignment of the signals in the H and 13C NMR spectra for
6 was achieved by using a combination of 1D and 2D NMR
techniques(HMBC,fortwo-andthree-bond1H–13Ccorrelations;
HSQC, for one-bond 1H–13C correlations). Microanalyses
were performed by the Microanalytical Laboratory at the
Research School of Chemistry, Australian National University,
Canberra. Synthesis and isolation of the cyclophane salts and
their corresponding silver complexes were performed in air. All
solvents were distilled prior to use, except for dimethyl sulfoxide
(Ajax, AR grade), which was used as received. Diethyl ether was
dried over sodium and distilled from sodium benzophenone ketyl
under nitrogen, N,N-dimethylformamide and dimethyl sulfoxide
were dried over 4 Å molecular sieves and acetonitrile was dried
over 3 Å molecular sieves before use. 1,2-Diheptylbenzene was
prepared from 1-bromoheptane and 1,2-dichlorobenzene based
on a variation of the procedure by Hanack et al.51
Conclusion
Synthesis of the cyclophane salts
The synthesis of four new silver complexes of NHC–
cyclophanes (4–7) has been achieved via a direct method of
complexation involving a mixture of the imidazolium-linked
cyclophanes and the basic metal source Ag2O. The new silver
carbene systems are thermally stable and have been characterised
by NMR spectroscopy and X-ray crystallography. Three of the
complexes (4–6) have a dimeric structure of the form [L2Ag2]2+
in the solid state that is rigid on the NMR timescale in solution.
The fourth complex (7) has a neutral structure of the form
[L(AgBr)2], and NMR studies suggest some lability of the L–Ag
bonding in solution. Investigation of the use of these new silver
The cyclophane salts I·2PF6, II·2Br and III·2Br were prepared
by procedures based on the methods of Baker et al.10 and
Magill et al.39
1,2-Bis(bromomethyl)-4,5-diheptylbenzene.
A mixture of
1,2-diheptylbenzene (4.5 g, 16 mmol), paraformaldehyde
(1.97 g, 66 mmol), hydrobromic acid in acetic acid (5.7 M,
11.5 cm3, 65 mmol), extra acetic acid (11 cm3) and propionic
acid (11 cm3) was heated at ca. 140 °C for 24 h. The mixture
was cooled and then poured into a mixture of light petroleum
(bp 64–80 °C) (200 cm3), ice and water (400 cm3). The phases
D a l t o n T r a n s . , 2 0 0 4 , 3 7 5 6 – 3 7 6 4
3 7 6 1

View Article Online
were separated and the aqueous phase was extracted with light
petroleum (2 × 200 cm3). The organic phases were combined,
washed with water (200 cm3) and saturated NaHCO3 solution
(2 × 200 cm3), dried (CaCl2) and then filtered through a plug
of silica. The filtrate was concentrated in vacuo to yield a
yellow oil (6.0 g). The oil (min. 70% pure by NMR) could
not be purified by flash chromatography, and was used with-
out further purification. dH (CDCl3, 300.1 MHz) (0.89, 6H, t,
H4), 131.0 (C, ortho-Ar para to C7H15), 132.3 (C, Ar, Ar para to
ArCH3), 133.5 (CH, meta-Ar), 135.2 (br m, CD due to exchange,
imidazolium C2), 135.8 (CH, Ar ortho to C7H15), 139.2 (C, Ar,
para to ArH), 140.9 (C, Ar adjacent to ArCH3) and 145.4 (C,
Ar, ortho to C7H15).
Synthesis of the silver complexes
4·2PF6. Ag2O (356 mg, 1.5 mmol) was added to a solution
of the cyclophane salt I·2PF6 (325 mg, 0.51 mmol) in
acetonitrile (50 cm3). The mixture was heated at 50 °C for
24 h under the exclusion of light. A clear solution with
some black suspension was obtained. The black suspension
was filtered off, the filtrate was poured into water (50 cm3),
and the resulting white powder was collected by filtration.
Recrystallisation of the powder from acetonitrile yielded
colourless crystals (139 mg, 43%) (Found: C, 44.77; H, 3.79;
N, 10.56. C44H40Ag2F12N8P2·CH3CN requires C, 45.01; H, 3.53;
N, 10.27%). Crystals suitable for X-ray diffraction studies were
grown by layering a concentrated solution of the complex in
acetonitrile with neat diethyl ether.
3JCH ,CH = 7.0 Hz, 2 × –CH2CH3), 1.20–1.42 and 1.50–1.61 (20H,
2
2 × m, 32 × –CH2(CH2)5CH3), 2.56 (4H, t, 3JCH ,CH = 7.9 Hz, 4 ×
2
–CH2CH2(CH2)4CH3), 4.64 (4H, s, 2 × –CH2Br)2and 7.12 (2H,
s, 2 × ArH); dC (CDCl3, 75.5 MHz) 14.1 (CH3), 22.7, 29.1, 29.7,
30.6, 31.0, 31.2, 31.6, 31.8, 32.3 (CH2), 131.8 (Ar CH), 133.6 and
142.3 (Ar C); m/z (EI) 458.1196 (M) (requires 458.1184).
1,2-Bis(imidazol-1-yl)-4,5-diheptylbenzene. A solution of the
crude 1,2-bis(bromomethyl)-4,5-diheptylbenzene (6.0 g) and
imidazole (9.6 g, 140 mmol) in thf (200 cm3) was heated at
reflux for 2 h. Powdered potassium hydroxide (2.6 g, 39 mmol)
was added to the solution and refluxing was continued over-
night. The mixture was cooled and then concentrated in vacuo.
The residue was mixed with water (200 cm3) and then warmed
on a steam bath for 10 min. The resulting oil was separated and
dissolved in diethyl ether (100 cm3). The ethereal phase was
washed with water (100 cm3) and then concentrated in vacuo. The
residue was purified by rapid silica filtration (dry-packing onto
silica, washing successively with dichloromethane and diethyl
ether, then elution of the desired compound with methanol).
The residue was then recrystallised from wet light petroleum
(50 cm3) to afford the product as a colourless powder (2.3 g)
(Found: C, 76.67; H, 10.11; N, 12.42. C28H42N4.·1/3H2O requires
C, 76.32; H, 9.76; N, 12.71%); dH (CDCl3, 300.1 MHz) 0.88 (6H,
5·2PF6. A solution of the cyclophane salt III·2Br (90 mg,
0.18 mmol) and Ag2O (85 mg, 0.37 mmol) in dmf (15 cm3) was
heated at 90 °C for 24 h under the exclusion of light. A clear
solution with black suspension was obtained. The mixture was
filtered and the solvent was removed under reduced pressure.
The off-white residue was dissolved in water (5 cm3) and added
to an aqueous solution of KPF6 (67 mg, 0.36 mmol, 5 cm3 H2O).
The white precipitate that formed immediately was collected and
recrystallised from acetonitrile to yield the product as colour-
less crystals (41 mg, 39%) (Found: C, 44.14; H 3.63; N 9.33.
C44H40N8Ag2P2F12 requires C, 44.54; H, 3.40; N, 9.44%).
3
t, JCH ,CH = 6.7 Hz, 2 × –CH2CH3), 1.25–1.36 (16H, m, 2 ×
2
–CH2CH23(CH2)4CH3), 1.51 (4H, m, 2 × –CH2CH2(CH2)4CH3),
2.56 (4H, m, 2 × –CH2CH2(CH2)4CH3), 4.97 (4H, s, 2 × benzylic
CH2), 6.76 (2H, s, 2 × imidazolyl CH), 6.88 (2H, s, 2 × ArH),
7.08 (2H, s, 2 × imidazolyl CH) and 7.51 (2H, s, 2 × imidazolyl
CH); dC (CDCl3, 75.5 MHz) 14.1 (CH3), 22.6, 29.1, 29.6, 31.1,
31.7, 32.3, (CH2), 48.1 (benzylic CH2), 119.1, 129.5, 137.1
(imidazolyl CH), 130.8 (Ar CH), 130.7 and 142.1 (Ar C); m/z
(EI) 434.3398 (M) (requires 434.3409).
6·2BPh4. A mixture of Ag2O (20 mg, 86 lmol) and the cyclo-
phane salt IV·2Br (38 mg, 51 lmol) in methanol (20 cm3) was
stirred, in darkness, for ca. 2.5 h, then filtered through a short
plug of Celite. To the filtrate was added a solution of NaBPh4
(77 mg, 0.2 mmol) in methanol (5 cm3). The resulting (48 mg)
solid was collected and recrystallised from acetonitrile to yield
the product as colourless crystals (24 mg, 47%) (Found: C,
74.24; H, 7.21; N, 5.35. C126H148Ag2B2N8·H2O requires C, 74.55;
H, 7.45; N, 5.52%). Crystals suitable for X-ray diffraction stud-
ies were grown by slowly cooling a solution of the complex in
acetonitrile.
The ortho/meta-cyclophane IV·2Br. Solutions of 1,2-
bis(imidazol-1-yl)-4,5-diheptylbenzene (0.98 g, 2.2 mmol) in
acetone (30 cm3) and 1,3-bis(bromomethyl)-2,4,6-trimethyl-
benzene (0.72 g, 2.3 mmol) in acetone (30 cm3) were added
portionwise, simultaneously, to acetone (75 cm3) at reflux over
the course of 2 h. The resulting mixture was heated at reflux
for a further 2 h and was then allowed to stand overnight. The
precipitate was collected, washed with cold acetone and dried to
yield a white powder (1.5 g). The powder was recrystallised from
acetone–water to give IV·2Br as a colourless solid (0.90 g, 54%)
(Found: C, 61.41; H, 7.84; N, 7.18. C39H56N4Br2·H2O requires C,
61.74; H, 7.71; N, 7.38%); dH (CD3OD, 500.1 MHz) 0.91 (12H,
7. A mixture of the cyclophane salt II·2Br (267 mg, 0.46 mmol)
and Ag2O (105 mg, 0.46 mmol) in dmso (15 cm3) was heated at
80 °C for 4 days under the exclusion of light. A clear solution
with black suspension was obtained. The mixture was filtered
and concentrated under reduced pressure. Recrystallisation
of the residue from dmf yielded a grey powder (75 mg, 43%)
(Found: C, 41.97; H, 3.91; N, 7.03. C28H32N4Ag2Br2 requires C,
42.03; H, 4.03; N, 7.00%). Attempts to grow crystals suitable for
X-ray diffraction studies from a concentrated solution of 7 in
d6-dmso–acetonitrile yielded crystals of 8.
3
t, JCH ,CH = 6.9 Hz, 2 × –CH2CH3), 1.29–1.37 and 1.37–1.48
2
(19H, 2 ×3 m, 2 × –CH2CH2(CH2)4CH3 + ArCH3 para to ArH),
1.65 (4H, m, 2 × –CH2CH2(CH2)4CH3), 2.56 (6H, s, 2 × ArCH3
ortho to ArH), 2.75 (4H, m, 2 × –CH2CH2(CH2)4CH3), 5.21
Structure determinations
2
(2H, br d, JCHH,CHH = 14.8 Hz, 2 × ortho-benzylic CHH), 5.51
Full spheres of ‘low’-temperature CCD area-detector diffracto-
meter data were measured (Bruker AXS instrument, x-scans;
monochromatic Mo-Ka radiation, k = 0.71073 Å, T ca. 153 K)
yielding Nt(otal) reflections, these merging to N unique (Rint cited)
after ‘empirical’/multiscan absorption correction (proprietary
software), No with F > 4r(F) considered ‘observed’ and used
in the full matrix least squares refinements, refining anisotropic
displacement parameter forms for the non-hydrogen atoms,
(x,y,z,Uiso)H being constrained at estimates. Conventional
residuals R, Rw (weights: (r2(F) + 0.000 nwF2)−1) are quoted
at convergence; neutral atom complex scattering factors were
employed within the context of the Xtal 3.7 program system.52
Pertinent results are given below and in the Tables and Figures,
(2H, d, 2JCHH,CHH = 15.2 Hz, 2 × meta-benzylic CHH), 5.60 (2H,
2JCHH,CHH = 15.2 Hz, 2 × meta-benzylic CHH), 5.95 (2H, br d,
2JCHH,CHH = 14.8 Hz, 2 × ortho-benzylic CHH), 7.30 (1H, s, ArH
ortho to ArCH3), 7.67 (2H, s, 2 × ArH ortho to –C7H15), 7.68
3
(2H, d, JH4,H5 = 2 Hz, 2 × imidazolium H5, near ortho-arene)
3
and 7.87 (4H, d, JH4,H5 = 2 Hz, 2 × imidazolium H4) [Note:
1
imidazolium H2 not seen in H NMR spectrum due to rapid
H–D exchange]; dC (CD3OD, 125.8 MHz) 14.4 (–CH2CH3), 15.7
(ArCH3 para to ArH), 20.1 (ArCH3 ortho to ArH), 23.7 (CH2),
30.3 (CH2), 30.8 (CH2) 32.5 (CH2), 33.0 (CH2), 33.3 (CH2), 49.6
(meta-benzylic CH2), 50.5 (ortho-benzylic CH2), 122.2 (br, CH,
imidazolium H5, near ortho-arene), 126.3 (CH, imidazolium
3 7 6 2
D a l t o n T r a n s . , 2 0 0 4 , 3 7 5 6 – 3 7 6 4

View Article Online
6 A. J. Arduengo III, H. V. R. Dias, J. C. Calabrese and F. Davidson,
Organometallics, 1993, 12, 3405.
7 J. C. C. Chen and I. J. B. Lin, J. Chem. Soc., Dalton Trans., 2000,
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8 J. Huang, E. D. Stevens and S. P. Nolan, Organometallics, 2000, 19,
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10 M. V. Baker, B. W. Skelton, A. H. White and C. C. Williams,
J. Chem. Soc., Dalton Trans., 2001, 111.
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J. Organomet. Chem., 1998, 554, 175.
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13 W. A. Herrmann and J. Schwarz, Organometallics, 1999, 18,
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14 J. C. Garrison, R. S. Simons, W. G. Kofron, C. A. Tessier and W. J.
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the latter showing 50% probability amplitude displacement
envelopes for the non-hydrogen atoms, hydrogen atoms, where
shown, having arbitrary radii of 0.1 Å. Individual divergences in
procedure are recorded as ‘variata’.
Crystal/refinement data
4·2PF6·2MeCN ≡ C48H48Ag2F12N10P2. M = 1268.6, mono-
clinic, space group P21/c (C52h, no. 14), a = 13.763(3),
b = 8.126(2), c = 22.858(5) Å, b = 104.517(6)°, V = 2475 Å3.
Dc (Z = 2 f.u.) = 1.702 g cm−3. lMo = 0.95 mm−1; specimen:
0.32 × 0.06 × 0.04 mm; ‘T’min/max = 0.83. 2hmax = 65°; Nt = 38388,
N = 8981 (Rint = 0.073), No = 5643; R = 0.042, Rw = 0.038
(nw = 3).
5·Ag2Br4·4dmf ≡ C54H68Ag4Br4N12O4. M = 1724.3, triclinic,
space group P1, (C1i, no. 2), a = 11.333(1), b = 11.578(1),
c = 13.263(2) Å, a = 76.474(3), b = 83.856(3), c = 64.354(3)°,
V = 1525 Å3. Dc (Z = 1 f.u.) = 1.877 g cm−3. lMo = 3.9 mm−1;
specimen: 0.32 × 0.15 × 0.04 mm; ‘T’min/max = 0.68. 2hmax = 75°;
Nt = 27644, N = 13259 (Rint = 0.036), No = 9188; R = 0.041,
Rw = 0.050 (nw = 7).
15 D. S. McGuiness, M. J. Green, K. J. Cavell, B. W. Skelton and
A. H. White, J. Organomet. Chem., 1998, 565, 165.
16 D. S. Clyne, J. Jin, E. Genest, J. C. Galluci and T. V. R. Babu, Org.
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17 K. Köhler, R. G. Heidenreich, J. G. E. Krauter and J. Pietsch, Chem.
Eur. J., 2002, 8, 622.
18 D. S. McGuiness and K. J. Cavell, Organometallics, 2000, 19,
741.
19 W. A. Herrmann, C.-P. Reisinger and M. J. Spiegler, J. Organomet.
Chem., 1998, 557, 93.
20 T. Weskamp, W. C. Schattenmann, M. Spiegler and W. A. Herrman,
Angew. Chem., Int. Ed. Engl., 1998, 37, 2490.
21 R. H. Grubbs and S. Chang, Tetrahedron, 1998, 54, 4413.
22 R. H. Grubbs, S. J. Miller and G. C. Fu, Acc. Chem. Res., 1995, 28,
446.
Variata. The carbon atoms of one of the two crystallo-
graphically independent dmf molecules (#2) were modelled as
disordered over a pair of sites, occupancies refining to 0.796(5)
and complement (isotropic displacement parameter forms for
the latter).
23 K. Burgess and M. J. Ohlmeyer, Chem. Rev., 1991, 91, 1179.
24 E. Negishi, Y. Zhang, I. Shimogama and G. Wu, J. Am. Chem. Soc.,
1989, 111, 8018.
6·2BPh4 ≡ C126H148Ag2B2N8. M = 2012.2, monoclinic, space
group P21/c, a = 16.404(2), b = 12.003(1), c = 28.387(3) Å,
b = 101.458(2)°, V = 5478 Å3. Dc (Z = 2 f.u.) = 1.220 g
cm−3. lMo = 0.41 mm−1; specimen: 0.32 × 0.26 × 0.14 mm;
25 A. Melaiye, R. S. Simons, A. Milstead, F. Pingitore, C. Wesdemiotis,
C. A. Tessier and W. J. Youngs, J. Med. Chem., 2004, 47, 973.
26 P. J. Barnard, A. Y. Y. Ho, M. V. Baker, D. A. Day and S. J.
Berners-Price, Proceedings of Gold 2003, International Conference
on the Science, Technology and Industrial Applications of Gold,
Vancouver, September 2003. Paper s36a1258p883; available
on the web at http://www.gold.org/discover/sci-indu/gold2003/
pdf/s36a1258p883.pdf.
27 P. J. Barnard, M. V. Baker, S. J. Berners-Price and D. A. Day,
J. Inorg. Biochem., 2004, 98, 1642.
28 H. M. J. Wang and I. J. B. Lin, Organometallics, 1998, 17, 972.
29 A. R. Chianese, X. Li, M. C. Janzen, J. W. Faller and R. H.
Crabtree, Organometallics, 2003, 22, 1663.
30 R. S. Simons, P. Custer, C. A. Tessier and W. J. Youngs, Organo-
metallics, 2003, 22, 1979.
31 K. M. Lee, H. M. J. Wang and I. J. B. Lin, J. Chem. Soc., Dalton
Trans., 2002, 2852.
32 W. Chen, B. Wu and K. Matsumoto, J. Organomet. Chem., 2002,
654, 233.
‘T’min/max = 0.79. 2hmax = 70°; Nt = 108435, N = 23343 (Rint
0.049), No = 16380; R = 0.080, Rw = 0.19 (nw = 2.6).
=
Variata. The peripheral portion of one of the hydrocarbon
‘tails’ of the ligand was modelled as disordered over two sets
of sites, occupancies refining to 0.626(6) and complement
(isotropic displacement parameter forms for the latter).
8·2MeCN ≡ C60H70Ag6Br6N10. M = 2057.9, monoclinic,
space group P21 (C22, no. 4), a = 8.997(2), b = 34.806(7), c =
10.327(2) Å, b = 91.713(3)°, V = 3232 Å3. Dc (Z = 2 f.u.) = 2.114 g
cm−3. lMo = 5.5 mm−1; specimen: 0.16 × 0.09 × 0.07 mm;
‘T’min/max = 0.72. 2hmax = 52.5°; Nt = 30449, N = 6687 (Rint
0.047), No = 5265; R = 0.052, Rw = 0.072 (nw = 60).
=
33 W. Chen and F. Liu, J. Organomet. Chem., 2003, 673, 5.
34 J. C. Garrison, R. S. Simons, C. A. Tessier and W. J. Youngs, J.
Organomet. Chem., 2003, 673, 1.
35 P. L. Arnold, A. C. Scarisback, A. J. Blake and C. Wilson, Chem.
Commun., 2001, 2340.
36 M. V. Baker, B. W. Skelton, A. H. White and C. C. Williams,
Organometallics, 2002, 21, 2674.
Variata. ‘Friedel’ data were retained distinct, xabs refining
to 0.07(2); weak and limited data would support meaningful
anisotropic displacement parameter form refinement for Ag,
Br only.
37 P. J. Barnard, M. V. Baker, S. J. Berners-Price, B. W. Skelton and
A. H. White, Dalton Trans., 2004, 1038.
CCDC reference numbers 241195–241198.
See http://www.rsc.org/suppdata/dt/b4/b408741k/ for crystal-
lographic data in CIF or other electronic format.
38 J. C. Garrison, R. S. Simons, J. M. Talley, C. Wesdemiotis, C. A.
Tessier and W. J. Youngs, Organometallics, 2001, 20, 1276.
39 A. M. Magill, D. S. McGuiness, K. J. Cavell, G. J. P. Britovsek,
V. C. Gibson, A. J. P. White, D. J. Williams, A. H. White and B. W.
Skelton, J. Organomet. Chem., 2001, 617–618, 546.
40 N. Garg and T. R. Lee, Synlett, 1998, 310.
41 M. V. Baker, M. J. Bosnich, D. H. Brown, L. T. Byrne, B. W.
Skelton, A. H. White and C. C. Williams, J. Org. Chem., 2004,
69, 7640.
Acknowledgements
We thank the Australian Research Council for financial support
(to M. V. B. and A. H. W.) and Universiti Sains Malaysia for a
postgraduate research scholarship (to R. A. H.).
42 J. C. Garrison, R. S. Simons, J. M. Talley, C. Wesdemiotis, C. A.
Tessier and W. J. Youngs, Organometallics, 2001, 20, 1276.
43 C. C. Williams, Ph. D. Thesis, The University of Western Australia,
2002.
44 G. Helgesson and S. Jagner, J. Chem. Soc., Dalton Trans., 1988,
2117.
45 G. Helgesson and S. Jagner, J. Chem. Soc., Dalton Trans., 1990,
2413.
46 B. Neumüller, S. Chitsaz and K. Dehnicke, Z. Anorg. Allg. Chem.,
2002, 628, 523.
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D a l t o n T r a n s . , 2 0 0 4 , 3 7 5 6 – 3 7 6 4
3 7 6 3

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