Ruthenium (II) thiacrown complexes as hydrogen-transfer reduction catalysts

Article abstract of DOI:10.1016/j.ica.2005.04.009

An investigation into the potential of a series of Ruthenium (II) thiacrown complexes with general formula [Ru([9]aneS₃)Cl₂(L)] and [Ru([n]aneS₄)Cl(L)]⁺, where L = DMSO or Ph₃P, n = 12, 14, or 16, as hydrogen transfer reduction catalysts is reported. As part of these studies two new complexes incorporating [Ru([12]aneS₄)] and [Ru([16]aneS₄)] metal centres have been synthesised and fully characterised. The X-ray structure of one of these complexes is also reported. The UV/Vis spectra of these complexes are dominated by π → π* transitions, with weaker d → d transitions also being apparent. Using these eight structurally related complexes, studies on the transfer hydrogenation of acetophenone using propan-2-ol as the hydride source were carried out. This work revealed that the complexes displayed catalytic activity in the presence of a base promoter. Although the activity of these complexes were considerably lower than that of structurally related mixed-donor ligand systems, there was some evidence that the flexibility of the ligand did have an effect on initial catalytic activity.

Full text of DOI:10.1016/j.ica.2005.04.009

Inorganica Chimica Acta 359 (2006) 759–765
www.elsevier.com/locate/ica
Ruthenium (II) thiacrown complexes as hydrogen-transfer
reduction catalysts
*
Naz Shan, Harry Adams, Jim A. Thomas
Department of Chemistry, University of Sheffield, Sheffield, UK
Received 9 February 2005; accepted 5 April 2005
Available online 15 June 2005
Ruthenium and Osmium Chemistry Topical Issue
Abstract
An investigation into the potential of a series of Ruthenium (II) thiacrown complexes with general formula [Ru([9]aneS3)Cl2(L)]
and [Ru([n]aneS4)Cl(L)]+, where L = DMSO or Ph3P, n = 12, 14, or 16, as hydrogen transfer reduction catalysts is reported. As part
of these studies two new complexes incorporating [Ru([12]aneS4)] and [Ru([16]aneS4)] metal centres have been synthesised and fully
characterised. The X-ray structure of one of these complexes is also reported. The UV/Vis spectra of these complexes are dominated
by p ! p* transitions, with weaker d ! d transitions also being apparent. Using these eight structurally related complexes, studies
on the transfer hydrogenation of acetophenone using propan-2-ol as the hydride source were carried out. This work revealed that
the complexes displayed catalytic activity in the presence of a base promoter. Although the activity of these complexes were con-
siderably lower than that of structurally related mixed-donor ligand systems, there was some evidence that the flexibility of the
ligand did have an effect on initial catalytic activity.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Ruthenium; Thiacrowns; Catalysis; Macrocyclic ligands
1. Introduction
Catalytic asymmetric reduction of prochiral ketones
to produce chiral secondary alcohols can be carried
out using transfer hydrogenation [6]. In view of the
low cost of the reducing agent and its operational sim-
plicity, transition-metal catalysed hydrogenation, either
with propan-2-ol or with a formic acid/triethylamine
mixture as a hydride source, has emerged as an attrac-
tive alternative to asymmetric hydrogenation with H2.
One of the most efficient systems has been reported
by the group of Noyoril [7]. Utilising a RuII catalyst,
propan-2-ol as a donor, and potassium hydroxide as a
promoter, Noyori and co-workers have demonstrated
that high yielding and highly enantioselective transfer
hydrogenation reactions of aromatic ketones can occur
at room temperature.
Historically, hydrogen-transfer reduction with
homogenous catalysts has attracted much less attention
than homogenous hydrogenation [1]. This may be in
part due to the low catalytic activity displayed by the
first generation catalysts, e.g. [RuCl2(PPh3)3] requires
high reaction temperature [2], whereas homogenous
hydrogenation catalysts like [RhCl(PPh3)3] are active
under much milder conditions [3]. However, in recent
years, major advances have been achieved in this field.
The discovery of more active catalysts [4] and more effi-
cient hydrogen donors [5] has led to the possibility of
obtaining high reaction rates under mild conditions.
More recently, Braunstein et al. [8] have reported on
the use of RuII complexes incorporating a new achiral
tridentate facially coordinating ligand, bis(2-oxazolin-
*
Corresponding author. Tel.: +44 114 222 9325; fax: +44 114 273
8673.
E-mail address: james.thomas@sheffield.ac.uk (J.A. Thomas).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.04.009

760
N. Shan et al. / Inorganica Chimica Acta 359 (2006) 759–765
Scheme 1. BraunsteinÕs RuII complexes incorporating the bopp ligand.
2-ylmethyl)phenylphosphine (bopp) in the catalytic
transfer hydrogenation of ketones – Scheme 1. Both
complexes 1 and 2 catalyse the transfer hydrogenation
of acetophenone and cyclohexanone using propan-2-ol
as the hydrogen donor. The most efficient of these base
promoted catalysts is 1, producing a 94% conversion with
a turn over frequency (TOF) per hour of 1,12,800. In
contrast, TOF for 2 is 990. This difference in activity is
attributed to the change in the ancillary ligand: it was
postulated that dissociation of a neutral ligand is neces-
sary during the catalytic cycle and that, since PPh3 is
bulkier than DMSO, its ease of dissociation is reflected
in the differences in reactivity between complexes 1 and 2.
Although they have been investigated less than li-
gands with other donor atoms, it is well established that
transition metal complexes of thioether ligands can
function as active catalysts in a considerable number
of homogenous reactions. They possess unusual r-
donor/p-acceptor properties and their trans influence is
considered to be lower than that of phosphines and
higher than that of primary amines [9]. It has been sug-
gested that this combination of properties means that
thioethers stabilise lower oxidation states of metal ions
more than amines but less than phosphines, and are thus
capable of yielding complexes with unusual reactivities.
In recent years, we have been investigating the prop-
erties of thiacrown ruthenium moieties to function as
building blocks for the construction of oligomeric sys-
tems with unusual electronic properties [10] and reactiv-
ities [11], as well as templates for the self-assembly of
novel metallomacrocycles [10c,12]. It struck us that sev-
eral of the starting materials we have employed in these
studies possess many of the structural features seen in
complexes 1 and 2. For example, readily accessible com-
plexes 3 [13] and 4 [14] (Scheme 2) are facially capped,
ruthenium (II) systems with the same combination of fa-
Scheme 3.
cially coordinated monodentate ligands in the same geo-
metric arrangement (cis-dihalides) as 1 and 2.
Given these observations, and the established interest
in thioether based catalysts outlined above, we decided to
investigate possible catalytic properties transfer hydroge-
nation of 3 and 4. These studies were also extended to
include a series of structurally related [Ru([n]aneS4)]
(n = 12, 14, 16) mono-anionic complexes – Scheme 3.
2. Experimental
Solvents were dried and purified using standard liter-
ature methods, while other commercially available
materials were used as received. Standard 1H NMR
spectra were recorded on a Bruker AM250 machine.
FAB spectra were obtained on a Kratos MS80 machine
working positive ion mode, with m-nitrobenzyl alcohol
matrix. UV/Vis spectra were recorded on a Unicam
UV2 spectrometer in twin beam mode. Spectra were re-
corded in matched quartz cells and were baseline
corrected.
Complexes 3, [13] 4, [14] 6a, [13] 5b, [15] 6b, [16] and
7b [17] were synthesised using literature procedures.
3. Catalytic experiments
Typical procedure for the catalytic transfer hydroge-
nation of acetophenone: each reaction was performed in
a four-chambered flask to allow comparison of results
under identical conditions.
Ruthenium complex (0.01 mmol) and 4 ml of de-
gassed propan-2-ol were added together and the solu-
tion was heated at 82 ꢁC for 5–10 min under nitrogen.
Acetophenone (1.201 g, 10.0 mmol) was added dropwise
to the refluxing mixture. The resulting yellow mixture
was stirred for 10 min and then a solution of NaOH,
60 eq. (0.60 mmol) in 5 ml of propan-2-ol, was added
dropwise. The yellow solution turned slight brown after
the addition of base. The extent of conversion was deter-
Scheme 2. Thiacrown complexes structurally related to 1 and 2.

N. Shan et al. / Inorganica Chimica Acta 359 (2006) 759–765
761
Table 1
Summary of crystallographic data for [5a][PF6]
mined by gas chromatography using a Zebron 1500 col-
umn. One millilitre samples were removed from the
reaction vessel using a syringe and stored in sample
tubes. These samples were then loaded onto the column
and the extent of conversion was determined.
Empirical formula
M
C26H31ClF6P2RuS4
784.21
Crystal system
Space group
Crystal dimensions (mm)
monoclinic
P21/n
0.16 · 0.10 · 0.06
8.3260(15)
28.369(5)
15.441(3)
101.622(4)
3572.4(11)
4
˚
a (A)
4. Syntheses
˚
b (A)
˚
c (A)
4.1. [RuCl([12]aneS4)(PPh3)][PF6] (5a)
b (ꢁ)
3
˚
U (A )
Z
A
suspension of [RuCl2(PPh3)3] [18] (0.356 g,
Dc (Mg/m3)
F(000)
l(Mo Ka) (mmꢀ1
Final R1 (on F)
1.458
1584
0.37 mmol) and [12]aneS4 (0.09 g, 0.37 mmol) in
degassed dichloromethane (20 ml) was stirred under
nitrogen for 1 h. Addition of a mixture of NH4PF6
(0.110 g, 2 eq.) in water (1 ml) and ethanol (20 ml) to
the resulting solution gave a yellow–brown precipitate.
The suspension was concentrated to approximately
10 ml, filtered and ethanol (20 ml) was added to the fil-
trate to give the desired product, which was isolated
by filtration, and washed with ethanol (20 ml) and
diethyl ether (20 ml) and allowed to dry in vacuo. Yel-
low powder: Yield 0.236 g (81%). UV/Vis (H2O): kmax
(e dm3 molꢀ1 cmꢀ1) = 212.00 nm (37,694), 405 nm
(710). 1H NMR (d6-acetone) dH 7.5–7.3 (m, 15H),
3.40–2.05 (m, 16H). 31P NMR dP 35.0 (s, 1P). Elemental
analysis obtained (expected RuC26H31S4ClPF6): C,
39.05% (39.82%); H, 3.74% (3.98%); Cl, 4.81%
(4.52%); S, 16.69% (16.36%). MS (FAB): m/z
(%) = 639 (100) [M+ ꢀ PF6].
)
0.884
0.0532
CCD area detector with Oxford Cryosystems low tem-
perature system and complex scattering factors were ta-
1
ken from the program package SHELXTL as
implemented on the Viglen Pentium computer. Hydro-
gen atoms were placed geometrically and refined with
a riding model and with Uiso constrained to be 1.2 times
Ueq of the carrier atom. The structures were refined
against F2 by least-squares methods using a weighting
scheme.
6. Results and discussion
6.1. Synthetic studies
4.2. [RuCl([16]aneS4)(PPh3)][PF6] (7a)
Complexes 3, 4, 6a, 5b, 6b, and 7b were synthesised
using previously reported procedures, while 5a and 7a
were synthesised for the first time. The reaction of
[12]aneS4 with [RuCl2(PPh3)3] in degassed dichloro-
methane led to the isolation of 5a as a yellow hexaflu-
orophosphate salt in good yields. An analogous
procedure with [16]aneS4 led to the isolation of 7a. Both
complexes were fully characterised by a combination of
1H NMR, 31P NMR, and mass spectroscopy, and also
elemental analysis.
A suspension of [RuCl2(PPh3)3] (0.356 g, 0.37 mmol)
and [16]aneS4 (0.11 g, 0.37 mmol) in degassed dichloro-
methane (20 ml) was stirred under nitrogen for 1 h.
Addition of a mixture of NH4PF6 (0.110 g, 2 eq.) in
water (1 ml) and ethanol (20 ml) to the resulting solution
gave a yellow–brown precipitate. The suspension was
concentrated to approximately 10 ml, filtered and etha-
nol (20 ml) was added to the filtrate to give the desired
product, which was isolated by filtration, and washed
with ethanol (20 ml) and diethyl ether (20 ml) and al-
lowed to dry in vacuo. Yellow powder: Yield 0.245 g
(79%). UV/Vis (H2O): kmax (e dm3 molꢀ1 cmꢀ1) =
215.00 nm (28,255), 350 nm (310). 1H NMR (d6-ace-
tone) dH 7.55–7.4 (m, 15H), 3.75–2.25 (m, 24H). 31P
NMR dP 35.0 (s, 1P). Elemental analysis obtained (ex-
pected RuC30H42S4ClPF6): C, 42.09% (42.72%); H,
4.85% (5.02%); Cl, 4.31% (4.02%); S, 15.54% (15.21%).
MS (FAB): m/z (%) = 695 (60) [M+ ꢀ PF6].
6.2. X-ray crystallography studies
The structure of [5a][(PF6)] was solved through X-ray
studies on yellow crystals grown by vapour diffusion of
diethyl ether into a nitromethane solution – Fig. 1.
The adoption of cis geometry for the chloride and
phosphine ligands is consistent with a stepwise chelation
of the four sulfur donor atoms, following successive
5. X-ray crystallography
1
SHELXTL version, An integrated system for solving and refining
Crystallographic data for 5a are summarised in Table
1. Data collected were measured on a Bruker Smart
crystal structures from diffraction data (Revision 5.1), Bruker AXS
Ltd.

762
N. Shan et al. / Inorganica Chimica Acta 359 (2006) 759–765
Table 3
UV/Vis absorption spectra of complexes in water
Complex
kmax (nm), emax (dm3 molꢀ1 cmꢀ1), assignment
3
206 (25,100) p ! p*, 406 (730) d ! d
213 (20,300) p ! p*, 365 (370) d ! d
212 (37,700) p ! p*, 405 (710) d ! d
215 (20,700) p ! p*, 396 (830) d ! d
213 (35,200) p ! p*, 375 (670) d ! d
218 (21,600) p ! p*, 368 (350) d ! d
202 (28,250) p ! p*, 350 (310) d ! d
215 (20,750) p ! p*, 373 (410) d ! d
4
5aa
5ba
6aa
6ba
7aa
7ba
a
Chloride salt.
folded. The ring size of [12]aneS4 is smaller compared
to [14]aneS4 and it is the reduced flexibility of the ethyl
linkages which produces greater distortion of the octa-
hedral geometry.
Fig. 1. ORTEP plot of the cation in [5a][(PF6)]. Hydrogen atoms are
removed for clarity.
Coordination of [16]aneS4 to octahedral metal ions
can lead to cis or trans geometries Although X-ray qual-
ity crystals of 7a were not obtained, NMR studies
involving comparisons to related systems confirm that
the complex is exclusively formed as the cis-isomer.
removal of ligands at the ruthenium centre. The adop-
tion of a trans geometry in octahedral complexes is
not favoured due to the small ring size of [12]aneS4.
The X-ray structure of 5a shows subtle differences to
that previously reported for 6a [13]. This is reflected in
the data within Table 2 where the bond lengths and an-
gles between the ruthenium (II) centre and the atoms
bound to it are shown.
The trans influence of the phosphine and chloride li-
gands are seen in the relative Ru–S bond lengths. The
longest Ru–S interactions occur for the sulfur trans to
the phosphine ligand even though the steric bulk of
the PPh3 would be expected to result in an increase of
Ru(1)–S(2) and Ru(1)–S(4). The Ru–S interactions are
shortest for that trans to the chloride.
6.3. Absorption spectroscopy
All UV/Vis spectra were recorded in water on chlo-
ride salts obtained from anion metathesis. The com-
plexes display similar features in their spectra – Table
3. In all cases, intense relatively high energy transitions
are observed. Each complex shows a band at around
200–220 nm with a large absorption coefficient which,
through comparison with similar structures [10], has
been assigned to a p ! p* transition on the thiacrown,
PPh3 or DMSO ligands. Weaker bands are also ob-
served at around 350–400 nm. Comparison with struc-
turally similar complexes allows these bands to be
assigned as d–d transitions [19].
The high-energy bands for all eight complexes vary
by less than 20 nm. This suggests that there is very little
energy difference between the p and p* orbitals of PPh3
and DMSO upon coordination to the different ruthe-
nium (II) centres. Likewise, the energy differences be-
tween d–d transitions of the complexes are very small,
particularly between complexes containing the same
thiacrown.
Inspection of the angles between ligands reveals dif-
ferences between the structures of 5a and 6a. The cis
octahedral geometry of 5a shows greater distortion com-
pared to 6a. The thiacrowns ligands are not symmetri-
cally coordinated; instead the carbon linkages are
Table 2
A comparison of selected bond lengths and angles for complex 5a with
those of 6a
5a
6a
˚
Bond lengths (A)
Ru(1)–Cl(1)
Ru(1)–P(2)
Ru(1)–S(1)
Ru(1)–S(2)
Ru(1)–S(3)
Ru(1)–S(4)
2.444(13)
2.325(14)
2.365(14)
2.377(13)
2.357(14)
2.287(14)
2.425(1)
2.344(2)
2.359(2)
2.383(2)
2.344(2)
2.286(2)
Bond angles (ꢁ)
P(2)–Ru(1)–Cl(1)
P(2)–Ru(1)–S(4)
P(2)–Ru(1)–S(1)
P(2)–Ru(1)–S(3)
85.32(5)
93.83(5)
95.78(5)
98.54(5)
91.5(1)
90.0(1)
89.7(1)
94.3(1)
Scheme 4.

N. Shan et al. / Inorganica Chimica Acta 359 (2006) 759–765
763
7. Catalysis studies
Catalytic studies with the complexes 3, 4, 5, 6, and 7
were performed for the transfer hydrogenation of
acetophenone by propan-2-ol – Scheme 4. Reactions
were performed in a four-chambered flask to allow com-
parison of results under identical conditions.
Preliminary studies were carried out using 3 as a cat-
alyst as this complex is most structurally analogous to
1. Initial experiments using 3 with ketone:Ru:base in
the ratio 1000:1:25 showed limited reactivity with con-
versions as low as 10% after a 24 h period. Since the
reduction of ketones with propan-2-ol is influenced by
base concentration [20], it was decided to vary its
concentration in order to determine the optimum condi-
tions – Fig. 2.
Fig. 3. Catalytic transfer hydrogenation of acetophenone using
complexes 3, 5a, 6a, and 7a.
There was found to be an upper limit of 60 eq. of
NaOH beyond which there was little further enhance-
ment observed over a 24 h period. Bubbling nitrogen
through the mixture, intended to remove the acetone
product and push the reaction equilibrium towards the
right-hand side, did increase the conversion by approx-
imately 10% and push the reaction nearer to
completion.
It is known that replacement of [9]aneS3 with [n]aneS4
(n = 12, 14, 16) thiacrowns gives ruthenium complexes
showing more distorted cis-octahedral geometries [13–
17]. In cis-complexes such as 5, 6, and 7, two sulfur
atoms of the macrocyclic ligand, one chlorine and one
sulfur or phosphorous atom from the third ligand lie
on the equatorial plane. Two remaining sulfur donor
atoms of the macrocyclic ligand occupy the axial octahe-
dral positions. To investigate whether the increased ste-
ric strain imposed by S4 macrocyclic coordination would
enhance catalytic activity, the properties of 5, 6, and 7
were investigated. Again, components were found to
give optimised results in the mixing ratio 1000:1:60
(ketone:Ru:base).
Using these conditions, a comparison of the Ph3P
coordinated systems, including 3, was first carried out
– Fig. 3. Complex 7a seems to show greatest initial cat-
alytic activity. However, given the relatively small differ-
ences in activity and the fact that error values cannot be
fully assessed, it is not possible to conclude that it is the
most reactive. Indeed, the observation of ‘‘cross-over
plots’’ suggests rates are not statistically significantly
different, and over 24 h all the complexes show essen-
tially identical activity.
To investigate the effect of auxiliary ligands on cata-
lytic activity the properties of DMSO coordinated com-
plexes 4, 5b, 6b, and 7b in the same conditions were also
investigated – Fig. 4.
Under these conditions, 4 shows comparable reactiv-
ity to that of 3: after 24 h 76% conversion was observed
compared with 80% for 3. Again, there is evidence that
the complex bearing the larger 16-membered macrocy-
cle, in this case 7b, displaying greater initial reactivity,
Fig. 2. Effect of base concentration on the transfer hydrogenation of
acetophenone using 3 as a catalyst: (m) percentage conversion in 1 h,
(n) percentage conversion in 24 h, (s) percentage conversion in
24 h + N2.
Fig. 4. Catalytic transfer hydrogenation of acetophenone using
complexes 4, 5b, 6b, and 7b.

764
N. Shan et al. / Inorganica Chimica Acta 359 (2006) 759–765
Table 4
Transfer hydrogenation of acetophenone by the ruthenium (II)
complexes 3–7
tions reach completion after 24 h at which point nitro-
gen is required to remove acetone in order to raise%
conversion further. TOF after this period is approxi-
mately the same for all eight complexes.
Catalyst
t (hꢀ1
)
Yield (%)
Turnover (hꢀ1
)
3
1
24
1
16
83
15
75
15
75
16
75
13
72
16
80
21
79
23
86
160
35
Further studies using a formic acid/triethylamine [21]
azeotrope as an alternative source of hydrogen yielded
poor results with conversions not greater than 15% over
a 24 h period.
Although no studies have been carried out to deter-
mine the mechanism for these particular catalytic pro-
cesses, it is known that transition metal complexes
prefer transfer hydrogenation via hydride formation
[4]. Thus, using complexes 3 and 4 as an example, it
may be reasonable to propose that hydrogenation takes
place via the mechanism shown in Scheme 5.
The same mechanism may be proposed for complexes
5–7. Since four coordination sites on the ruthenium cen-
tre are occupied by the sulfur macrocycle, the chloride
ligand will be absent in order to maintain a six-coordi-
nate metal centre.
4
150
31
24
1
5a
5b
6a
6b
7a
7b
150
31
24
1
160
31
24
1
130
30
24
1
160
33
24
1
210
33
24
1
230
36
24
however after 24 h all the complexes show similar activ-
ity, with substrate conversion being around 80%. Table
4 summarises the data obtained on the catalytic proper-
ties of all eight complexes.
9. Conclusions
Ruthenium (II) complexes incorporating facially
bound sulfur macrocycles have been investigated to
determine their efficiency as catalysts in the transfer
hydrogenation of acetophenone using propan-2-ol as
the source for hydrogen.
The reduction of acetophenone is influenced by the
concentration of base. An upper limit of 60 eq. of NaOH
was found beyond which little rate enhancement was ob-
served over a 24 h period. Bubbling nitrogen through the
reaction mixture raised conversion by approximately
10% so pushing reactions nearer to completion.
Ligand substitution studies found little difference in
reactivity between complexes containing PPh3 or
DMSO ligands. This suggests that dissociation of a neu-
tral ligand during the catalytic cycle is not important.
Complexes bearing [n]aneS4 (n = 12, 14, 16) did ap-
pear to display greater initial reactivity in comparison
with their [9]aneS3 counterparts. However, these slight
differences are within experimental error and after a
24 h period, all complexes produce similar percentage
conversions suggesting that that there is very little differ-
ence in overall reactivity between them.
8. Discussion
Complexes 3 and 4 are similar in structure to 1 and 2
with bopp being replaced by [9]aneS3. However, turn-
over numbers in comparison with those obtained by
Braunstein et al. [8] are much lower, suggesting that
the change in electronic properties relayed to the ruthe-
nium centre is the major factor in the reduction of the
activity of the catalyst.
Further factor in activity seems to be the flexibility of
the thiacrown ligands. Complexes 7a and 7b, which
incorporate the more flexible [16]aneS4 ligand, display
higher TON after 1 h compared with 3 and 4. All reac-
The low reactivity of these complexes may be a result
of the electronic properties relayed to the ruthenium cen-
tre by the thiacrown. Furthermore, the coordination of
sulfur macrocycles to metals is known to enhance the
acidity of C–H bonds a to sulfur. This may also compro-
mise the ability of such complexes to perform as catalysts.
Nevertheless, this work demonstrates that macrocy-
clic ligand complexes of transition metals can display
catalytic behaviour. By judiciously selecting donors
and tuning the flexibility of the coordinated macrocycle
Scheme 5. Possible catalytic mechanism for thiacrown complexes.

N. Shan et al. / Inorganica Chimica Acta 359 (2006) 759–765
765
[9] H.B. Kraatz, H. Jacobson, T. Ziegler, P.M. Boorman, Organo-
metalics 12 (1993) 76.
it may be possible to produce systems with new or novel
activity. Furthermore, by using molecular building
blocks related to the systems described here, it may be
possible to self-assemble novel multi-site and/or multi-
function catalysts.
[10] (a) S. Roche, L.J. Yellowlees, J.A. Thomas, Chem. Commun.
(1998) 1429;
´
(b) C.S. Arau´jo, M.G.B. Drew, V. Felix, L. Jack, J. Madureira,
M. Newell, S. Roche, T.M. Santos, J.A. Thomas, L. Yellowlees,
Inorg. Chem. 41 (2002) 2250;
(c) N. Shan, S. Vickers, H. Adams, M.D. Ward, J.A. Thomas,
Angew. Chem., Intl. Ed. 43 (2004) 3398.
Acknowledgement
[11] S. Roche, H. Adams, S.E. Spey, J.A. Thomas, Inorg. Chem. 39
(2000) 2385.
We gratefully acknowledge the support of The Royal
Society (J.A.T.) and EPSRC (N.S.).
[12] (a) S. Roche, C. Haslam, H. Adams, S.L. Heath, Chem. Commun.
(1998) 1681;
(b) S. Roche, H. Adams, S.E. Spey, J.A. Thomas, Inorg. Chim.
Acta 323 (2001) 157.
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