Stoichiometric and catalytic reactivity of the N-heterocyclic carbene ruthenium hydride complexes [Ru(NHC)(L)(CO)HCl] and [Ru(NHC)(L)(CO) H(η2-BH4)] (L = NHC, PPh3)

Article abstract of DOI:10.1039/b719373d

Thermolysis of [Ru(AsPh₃)₃(CO)H₂] with the N-aryl heterocyclic carbenes (NHCs) IMes (1,3-bis(2,4,6-trimethylphenyl) imidazol-2-ylidene), IPr (1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) or the adduct SIPr?(C₆F₅)H (SIPr = 1,3-bis(2,6- diisopropylphenyl)-4,5-dihydroimidazol-2-ylidene), followed by addition of CH₂Cl₂, affords the coordinatively unsaturated ruthenium hydride chloride complexes [Ru(NHC)₂(CO)HCl] (NHC = IMes 1, IPr 2, SIPr 3). These react with CO at room temperature to yield the corresponding 18-electron dicarbonyl complexes 4-6. Reduction of 1-3 and [Ru(IMes)(PPh ₃)(CO)HCl] (7) with NaBH₄ yields the isolable borohydride complexes [Ru(NHC)(L)(CO)H(η²-BH₄)] (8-11, L = NHC, PPh₃). Both the bis-IMes complex 8 and the IMes-PPh₃ species 11 react with CO at low temperature to give the η¹- borohydride species [Ru(IMes)(L)(CO)₂H(η¹-BH ₄)] (L = IMes 12, PPh₃), which can be spectroscopically characterised. Upon warming to room temperature, further reaction with CO takes place to afford initially [Ru(IMes)(L)(CO)₂H₂] (L = IMes, L = PPh₃14) and, ultimately, [Ru(IMes)(L)(CO)₃] (L = IMes 13, L = PPh₃15). Both 8 and 11 lose BH₃ on addition of PMe₂Ph to give [Ru(IMes)(L)(L′)(CO)H₂](L = L′ = PMe₂Ph; L = PPh₃, L′ = PMe₂Ph). Compounds 1-4 and 8-11 have been tested as catalysts for the hydrogenation of aromatic ketones in the presence of ⁱPrOH and H₂. For the reduction of acetophenone, catalytic activity varies with the NHC present, decreasing in the order IPr > IMes >> SIMes. The Royal Society of Chemistry.

Full text of DOI:10.1039/b719373d

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www.rsc.org/dalton | Dalton Transactions
Stoichiometric and catalytic reactivity of the N-heterocyclic carbene
ruthenium hydride complexes [Ru(NHC)(L)(CO)HCl] and
2
[
Ru(NHC)(L)(CO)H(g -BH )] (L = NHC, PPh )†
4
3
Victoria L. Chantler, Sarah L. Chatwin, Rodolphe F. R. Jazzar, Mary F. Mahon, Olly Saker and
Michael K. Whittlesey*
Received 17th December 2007, Accepted 21st February 2008
First published as an Advance Article on the web 26th March 2008
DOI: 10.1039/b719373d
Thermolysis of [Ru(AsPh
1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), IPr (1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene) or the adduct SIPr·(C )H (SIPr = 1,3-bis(2,6-diisopropylphenyl)-4,5-
dihydroimidazol-2-ylidene), followed by addition of CH Cl , affords the coordinatively unsaturated
ruthenium hydride chloride complexes [Ru(NHC)
(CO)HCl] (NHC = IMes 1, IPr 2, SIPr 3). These
react with CO at room temperature to yield the corresponding 18-electron dicarbonyl complexes 4–6.
3
)
3
(CO)H
2
] with the N-aryl heterocyclic carbenes (NHCs) IMes
(
6
F
5
2
2
2
Reduction of 1–3 and [Ru(IMes)(PPh
complexes [Ru(NHC)(L)(CO)H(g -BH
3
)(CO)HCl] (7) with NaBH yields the isolable borohydride
4
2
4
)] (8–11, L = NHC, PPh
3
). Both the bis-IMes complex 8 and
1
the IMes–PPh
3
species 11 react with CO at low temperature to give the g -borohydride species
1
[
Ru(IMes)(L)(CO)
2
H(g -BH
4
)] (L = IMes 12, PPh
3
), which can be spectroscopically characterised.
Upon warming to room temperature, further reaction with CO takes place to afford initially
[
Ru(IMes)(L)(CO)
2
H
2
] (L = IMes, L = PPh
3
14) and, ultimately, [Ru(IMes)(L)(CO)
3
] (L = IMes 13,
ꢀ
L = PPh
3
15). Both 8 and 11 lose BH
3
on addition of PMe
2
Ph to give [Ru(IMes)(L)(L )(CO)H
2
](L =
ꢀ
ꢀ
L = PMe
2
Ph; L = PPh
3
, L = PMe
2
Ph). Compounds 1–4 and 8–11 have been tested as catalysts for the
i
hydrogenation of aromatic ketones in the presence of PrOH and H
2
. For the reduction of acetophenone,
catalytic activity varies with the NHC present, decreasing in the order IPr > IMes >> SIMes.
Introduction
Over the last decade, N-heterocyclic carbenes (NHCs) have
become increasingly popular as alternative ligands to phosphines
1
in organometallic chemistry. While NHCs have been shown to
possess a significantly greater r-donor capability than tertiary
phosphines, a model for comparing their relative steric properties
has taken longer to emerge. Nolan, Cavallo and co-workers
have proposed the ‘percentage buried volume’ model (% Vbur),
which evaluates the volume occupied by ligand atoms in a sphere
˚
centred a defined distance (2.00 or 2.28 A) from the metal
2,3
centre. This approach allows the steric requirements of different
carbene N-substituents to be evaluated, and importantly, provides
a direct comparison between NHCs and phosphines. As shown
in Scheme 1, 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene or
IMes, which is probably the most commonly used of all the NHCs,
Scheme 1 Calculated percentage buried volumes (% Vbur) for free NHC
and PR ligands (based upon values for [NiL(CO) ] and [NiL(CO)
] (L =
NHC, PR
3
2
3
2b,3a
3
) complexes).
is significantly bulkier than PPh
similar to that of PCy
We have employed IMes for the stabilisation of a series of coor-
dinatively unsaturated five coordinate Ru(II) heteroatom hydride
3
and has a steric profile most
3
.
‘
(
umbrella’ restricting access to the vacant metal coordination site
e.g. [Ru(IMes) (CO)HF] can be left out on the bench as a solid, in
2
air, for days without any sign of decomposition) and secondly in
influencing hydrogen transfer and isomerisation processes which
occur during the formation of the [Ru(IMes)
In another case, we have shown that the presence of two IMes
ligands in the dihydride complex [Ru(IMes) (PPh )(CO)H ] help
to labilise the PPh
reactive [Ru(IMes)
C(sp )–C(sp ) bond of an ortho-methyl group of an IMes.
4,5
complexes [Ru(IMes)
The trans-IMes ligands play a dual role, firstly in acting as a bulky
2
(CO)H(X)] (X = OH, OEt, SH, SR, F).
2
(CO)H(X)] species.
Department of Chemistry, University of Bath, Claverton Down, Bath, UK
BA2 7AY. E-mail: m.k.whittlesey@bath.ac.uk
2
3
2
3
group, facilitating the formation of a highly
†
CCDC reference numbers 607121, 668486–668493 and 676387. For
crystallographic data in CIF or other electronic format see DOI:
0.1039/b719373d
2
(CO)] fragment, which is capable of cleaving a
2
3
6
1
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−
1
This paper describes investigations of the reactive bis-
1902 cm respectively. These can be compared to the CO ab-
sorption bands for the all-phosphine [Ru(PR (CO)HCl] (PR
2
2
IMes g –borohydride complex, [Ru(IMes)
2
(CO)H(g -BH
4
)] (8),
3
)
2
3
=
i
−1
t
−1
−1 13–15
and analogues containing IPr (1,3-bis(2,6-diisopropylphenyl)-
imidazol-2-ylidene) and SIPr (1,3-bis(2,6-diisopropylphenyl)-4,5-
dihydroimidazol-2-ylidene) ligands. Given that the catalytic re-
activity of a number of all-phosphine based Ru/Os borohydride
P Pr
3
, 1910 cm ; P Bu
2
Me, 1904 cm ; PCy
)(CO)HCl] (PR
1913 cm ; PCy , 1896 cm ) analogues.
3
, 1901 cm )
and carbene/phosphine [Ru(IMes)(PR
3
3
= PPh
3
,
−
1
−1 15,16
3
The molecular structures of 1–3 (Fig. 1) all displayed positional
disorder between the chloride and CO ligands, similar to that
reported previously in [Ru(IMes)
F). Metrics for these compounds, as well as those for the 18-
electron dicarbonyl derivatives 4–6 (Fig. 2) which are formed by
the addition of 1 atm CO to solutions of 1–3 are shown in Table 1.
The bond distances and angles in the two series of compounds
1–3 and 4–6 proved to be essentially independent of the NHC
7–9
complexes has been described in the literature, we have tested the
2
activity of the [Ru(NHC)
2
(CO)H(g -BH
4
)] species for the catalytic
2
(CO)H(X)] (X = OH, OEt, SH,
4,5
hydrogenation of ketones.
Results and discussion
Synthesis of [Ru(NHC)
2
(CO)HCl] and reaction with CO
3b
ligand. Interestingly, the SIPr dicarbonyl complex 6 required
crystallisation under a CO atmosphere; exposure of 6 to vacuum,
either in the solid state or upon freeze–pump–thaw degassing
in solution, resulted in CO dissociation and isolation of 3. In
contrast, the IPr complex 5 lost CO much more slowly, such that
Thermolysis (358 K, 24 h) of toluene solutions of
[
Ru(AsPh (CO)H ] containing ca. 2.5–3 equivalents of the free
carbenes IMes or IPr, followed by addition of CH Cl , gave the
corresponding 16-electron bis-NHC hydride chloride complexes
Ru(NHC)
3
)
3
2
2
2
[
2
(CO)HCl] (NHC = IMes 1, IPr 2) (Scheme 2) in
10
5
could be crystallised under an atmosphere of argon.
moderate to high yields. The analogous SIPr compound 3 was
For an additional comparison, CO was also added to the
prepared using the pentafluorobenzene adduct of the carbene,
16
mixed IMes/PPh
Ru(IMes)(PPh )(CO)
terised as shown in Fig. 3. The relevant bond lengths and angles
Table 1) show little difference to those in the bis-NHC compounds
–6.
3
compound [Ru(IMes)(PPh
3
)(CO)HCl] to give
SIPr·(C
6
F )H. These adducts, of which there are a number now
5
[
3
2
HCl] (7), which was structurally charac-
reported in the literature, are air-stable and thus more convenient
to handle than the free carbenes, although surprisingly, they
have yet to be widely utilised in the synthesis of metal NHC
(
4
11,12
complexes.
2
Formation of [Ru(NHC)(L)(CO)H(g -BH
4
)] (L = NHC, PPh
3
)
Treatment of 1–3 with ca. 4 equiv. NaBH
4
in EtOH at 298–
2
3
43 K gave the borohydride complexes [Ru(NHC) (CO)H(g -
2
BH
4
)] (NHC = IMes 8, IPr 9, SIPr 10) as off-white microcrystalline
1
solids. The room temperature H NMR spectra of all three
compounds showed two singlet resonances at ca. d −16 and d
−
7 in a 1 : 1 ratio, which were assigned to the metal hydride and
bridging Ru–H –B (trans to CO) protons respectively (Scheme 3).
b
Scheme 2 Hydride chloride complexes prepared in this study.
Upon cooling, the Ru–H –B signal sharpened, and in the case
b
of 8 at 214 K, two further resonances appeared at d 3.13 and
−5.12 in a ratio of 2 : 1 for the two equivalent terminal B–H
d
1
The H NMR spectra of 1–3 all display low frequency
hydride resonances at ca. d −25, consistent with five coordi-
nate complexes of the type [Ru(NHC) (CO)HX], which have
2
square pyramidal geometries in which the hydride is trans to
4a
the vacant coordination site. The strong electron donating
properties associated with carbene ligands are apparent from
the low frequency mCO values found for 1–3 at 1882, 1897 and
Scheme 3
◦
Table 1 Selected bond lengths [ A˚ ] and angles [ ] for compounds 1–7
1
2
3
4
5
6
7
Ru–CNHC
Ru–CO
2.075(3) 2.0872(13) 2.0899(15) 2.1158(15)
2.126(2)
2.128(2)
2.143(2)
2.146(2)
2.0974(19)
2
.1064(15)
a
a
a
1.9840(19) (trans to H) 1.975(2) (trans to H)
1
1.982(3) (trans to H) 1.973(2) (trans to H)
.890(8) (trans to Cl)
1.920(11) (trans to Cl) 1.867(7) (trans to Cl) 1.848(2) (trans to Cl)
Ru–P
Ru–Cl
2.3529(5)
2.4528(5)
a
a
a
2.384(2)
2.379(2)
170.07(8)
2.3922(19)
170.39(9)
C
C
NHC–Ru–CNHC 178.6(11) 178.78(8) 177.60(9) 168.81(6)
NHC–Ru–P
165.42(5)
98.02(10)
OC–Ru–CO
96.0(3)
92.1(7)
94.2(5)
a
Disorder prevented accurate determinations of Ru–CO and Ru–Cl distances in these compounds
2
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Fig. 1 Molecular structures of [Ru(NHC)
2
(CO)HCl] (NHC = IMes 1, IPr 2, SIPr 3). Thermal ellipsoids are shown at the 30% probability level. All
2
hydrogen atoms are omitted, with the exception of hydride ligands (when these atoms were successfully located) and sp hydrogen atoms on the NHC
ring backbone in 3. Primed labelled atoms generated via 1 − x, y, 3/2 − z, x, 2 − y, −z, and x, −y, −z, for 1, 2, and 3, respectively. Solvent omitted
throughout. Only one molecule within the asymmetric unit is shown for 1.
and bridging Ru–H
c
–B (trans to H) protons respectively (while
determined for H
of the resonance for H
an accurate T determination). These values are comparable
to those reported for [Ru(PMe )]. Similar
c
, H
b
and H
a
respectively in 8 (partial overlap
signals for Ru–H –B in 9 and 10 were observed, we were unable to
c
d
with the IMes methyl signals prevented
unequivocally assign a chemical shift for the terminal borohydride
resonances in either species (even at low temperature) due to
1
2
9
2
Ph)
2
(CO)H(g -BH
values were found for the
4
i
overlap with Pr signals). The assignment of the bridging Ru–
variable temperature behaviour and T
borohydride resonances in 9 and 10.
1
H–B hydrogen atom was made by comparison to other Group
2
8
metal [M(L)H(g -BH
4
)] complexes in the literature, in which
The mixed IMes–PPh
(d 4.16, very broad, overlapping H
(H )) B–H resonances at 298 K. T
3
complex 11 displayed separate terminal
and H
) and bridging (d −4.72
values (measured
a higher frequency B–H resonance is typically the one trans to
d
e
9,17,18
hydride.
made by 1D-ROE spectroscopy, which showed the appearance
of a clear contact between H and H , confirming their cis
orientation. T values (measured at 214 K by a standard inversion–
In the case of 8, a definitive assignment was also
c
), −6.62 (H
b
1
at 298 K, 400 MHz) were found to be very similar to those for 8,
a
b
while resolution of the two inequivalent terminal B–H groups into
1
separate signals at d 4.56 and 4.09 at 275 K allowed T values of
1
recovery pulse sequence at 400 MHz) of 130, 145 and 994 ms were
106 and 104 ms to be measured.
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Fig. 2 Molecular structures of [Ru(NHC)
hydrogen atoms are omitted for clarity with the exception of hydride ligands (when these atoms were successfully located) and sp hydrogen atoms on the
2
(CO)
2
HCl] (NHC = IMes 4, IPr 5, SIPr 6). Thermal ellipsoids are shown at the 30% probability level. All
2
NHC ring backbone in 6. Solvent omitted throughout.
1
1
1
The B{ H} NMR spectra for all four of the complexes revealed
little information with the appearance of a broad, featureless
resonance in each case between d 0.6 and d −2.8. The appearance
of two characteristic IR bands for the terminal mB–H vibrations
to extreme disorder (possibly over four positions). In the case of 9
(Fig. 4), a respectable structural model could be extracted from the
data, despite severe disorder involving the borohydride/carbonyl
ligands which seems to prevail in complexes of this type.
−
1
−
in the range 2425–2400 cm confirmed the bidentate [BH ]
4
19
coordination mode. Efforts to unequivocally characterise the
borohydride complexes by X-ray diffraction proved difficult.
Attempts to grow crystals of 8 from benzene–hexane afforded
two types of crystal, which upon analysis, showed the same unit
cell parameters. Upon solving the structure, the metal and NHC
ligands were well defined, but, alas, the electron data pertaining to
the carbonyl and borohydride moieties could not be resolved due
Stoichiometric reactions of 8 and 11
The IMes complex 8 proved to be stable at ambient temperature
in CD
the sample was subsequently heated at 343 K for 3 d. 8 also
proved to be quite stable in C solution at 298 K, but
2
Cl
2
, although the hydride chloride 1 was reformed when
6
D
6
again reacted upon heating (358 K, 20 h), this time to yield
2
606 | Dalton Trans., 2008, 2603–2614
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2
24
borodeuteride complex [Ru(IMes)
2
(CO)H(g -BD
4
)] with H
2
at
3
23 K led to slow D/H exchange into the borohydride ligand.
1
After 1 month, a H NMR spectrum showed a 0.5 : 0.5 : 1 ratio of
: H : H (H incorporation into the terminal B–D positions was
H
b
c
a
detected, but could not be accurately quantified due to the partial
overlap with the IMes resonances). The reaction was accompanied
by formation of HD gas, characterised by the appearance of 1 : 1 : 1
triplet at d 4.44.
Elimination of BH
3
(which was trapped as the phosphine
adduct ) occurred when either 8 or the IMes–PPh complex
Ph. The H NMR spectra of the two
resultant dihydride products, [Ru(IMes)(PMe Ph) (CO)H ] and
Ru(IMes)(PPh )(PMe Ph)(CO)H ], each displayed two highly
coupled hydride resonances, with JPH values characteristic of cis-
9,25
3
1
1
1 were heated with PMe
2
2
2
2
[
3
2
2
6
b,26
phosphine [Ru(NHC)(PR
tion of [Ru(IMes)(PMe Ph)
the Ru–IMes bonds breaks. This adds to the number of examples
3
)
2
(CO)H
2
] structures.
The forma-
2
2
(CO)H
2
] from 8 implies that one of
27,28
of reactions in which M–NHC bond cleavage takes place,
although there is still a widely held view that NHC ligands are
29
inert to dissociation.
Addition of 1 atm CO to 8 at room temperature afforded the
2
2 4
Fig. 3 Molecular structure of [Ru(IPr) (CO)H(g -BH )] 9. Thermal
30
known dicarbonyl dihydride complex [Ru(IMes)
2
(CO)
2
H ] in the
2
ellipsoids are shown at the 30% probability level. Hydrogen atoms, solvent
moiety and additional disordered fragment atoms omitted for clarity.
Primed labelled atoms generated via x, −y, −z.
time of mixing (Scheme 4). When the CO was introduced to
a sample of 8 at 196 K, and subsequently warmed to 273 K,
the dihydride complex was not observed, but instead, unreacted
8
was seen along with a new hydride containing species 12
(
Scheme 5). This displayed a sharp hydride signal at d −5.23 and
a much broader feature at d −2.06, which integrated in a ratio
of 1 : 4. On the basis of the integration, and by comparison to
3
1,32
1
the literature,
Ru(IMes) (CO)
12 was assigned as the g -borohydride complex
1
[
2
2
H(g -BH
4
)]. Upon cooling back down, the signal
at d −2.06 broadened further until at 190 K, it resolved into two
9
,32,33
separate signals in a 3 : 1 ratio at d 0.91 and d −10.33.
The
1
13
1
presence of two CO ligands in 12 was clear from the H– C{ H}
HMBC spectrum, which showed correlation of the Ru–H signal
1
3
to two high frequency C resonances at ca. d 206 and 197.
3 2
Fig. 4 Molecular structure of [Ru(IMes)(PPh )(CO) HCl] 7. Thermal
ellipsoids are shown at the 30% probability level. Solvent and all hydrogen
atoms omitted for clarity, with the exception of the hydride ligand.
the NHC–borane adduct IMes·BH
3
(the fate of the ruthenium
20
could not be determined). In contrast to the phosphine boro-
i
2
18
hydride complexes [M(P Pr
3
)
2
(CO)H(g -BH
4
)] (M = Ru, Os),
2
9
2
21
[
Ru(PMe
2
Ph)
2
(CO)H(g -BH
4
)] and [Ru(PMe
2
Ph)
3
H(g -BH
4
)],
8
was unaffected by dissolution in H O–saturated benzene, or
2
addition of 1–5 equiv. of 4-methylpyridine, although partial
conversion to [Ru(PPh (CO)H ] was found upon heating in
3
)
3
2
Scheme 4
22
MeOH at 323 K.
There was no evidence for any reaction between 8 and H
although in the presence of D , a 50% depletion of the Ru
1
3
2
,
When the above reaction was repeated using CO, the hydride
resonance of 12 appeared as two interpenetrating doublets with
2
hydride resonance at d −16 was observed over a period of 15
JHC values of 6.6 and 37.9 Hz. The magnitudes of these coupling
constants imply that CO being is being incorporated both cis and
trans to Ru–H. As shown in Scheme 5, this is consistent with the
1
3
d at ambient temperature; the same conversion occurred in 2 d
23
when the temperature was raised to 323 K. Treatment of the
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Scheme 5
◦
initial formation of two different intermediates, A and B, which
C(25) 169.16(7) ). Considerable asymmetry is seen around
2
result from the g -BH
4
ligand opening up either trans to hydride (to
the equatorial Ru(CO)
111.32(12), 139.56(14) and 109.12(13) . This differs slightly from
3
belt with OC–Ru–CO angles of
34
◦
give A) or trans to CO (affording B). Subsequent trapping of A
1
3
13
with CO forms the trans- CO–Ru–H isomer directly. As there is
no evidence that intermediate B is intercepted directly by CO (this
would give an unfavourable CO–trans-CO arrangement), it is likely
both [Ru(IEt
imidazol-2-ylidene) and [Ru(IMes)(PPh
tain two wide and one far more acute OC–Ru–CO angles.
Compound 11 reacted with CO in a similar manner to 8, initially
to give a dihydride species [Ru(IMes)(PPh )(CO) ] (14) but
2
Me
2
)
2
(CO)
3
] (IEt
2
Me
2
= 1,3-diethyl-4,5-dimethyl-
3
)(CO) ] (15), which con-
3
35
that B rapidly rearranges to C to avoid another unfavourable trans-
−
arrangement of strongly donating hydride and [BH
4
13
]
ligands.
3
2
35b
H
2
1
3
C is then trapped by CO to give 12 with a cis- CO–Ru–H
stereochemistry. Upon warming to room temperature, 12 loses
ultimately to afford the ruthenium(0) complex, 15. The identity
of 14 was confirmed by X-ray crystallography (Fig. 6) following
an alternative synthesis involving borohydride reduction of 7. The
bond lengths and angles in 14 show nothing unusual. NMR and
BH
Ru(IMes)
Prolonged exposure of [Ru(IMes)
temperature gave the Ru(0) tricarbonyl complex [Ru(IMes)
13) as the ultimate product of reaction. We have previ-
3
(detected as the CO adduct) from either isotopomer to give
[
2
(CO) ].
2
H
2
2
(CO)
2
H
2
] to CO at room
(CO)
IR data were consistent with the cis-Ru(CO)
2
2
H geometry (Ru–
2
3
]
H: d −6.56, d, JHP = 26.4; Ru–CO: d 203.1, d, JCP = 8.5 Hz;
−1
(
Ru–NHC: d 189.9, d, JCP = 72.5 Hz; mCO 1995, 1947 cm ). A
preliminary study on the reaction with CO at low temperature
showed it was similar to that for 8, with NMR signals indicative
5
ously reported spectroscopic characterisation of this complex,
but no solid-state structural data were reported. The struc-
ture (Fig. 5) showed a distorted trigonal bipyramidal geom-
etry at ruthenium with trans-axial NHC ligands (C(4)–Ru–
2 3
Fig. 5 Molecular structure of [Ru(IMes) (CO) ] 13. Thermal ellipsoids
are shown at the 30% probability level. All hydrogen atoms are omitted for
clarity. Selected distances ( A˚ ) and angles ( ): Ru–C(1) 1.944(3), Ru–C(2)
3 2 2
Fig. 6 Molecular structure of [Ru(IMes)(PPh )(CO) H ] 14. Ther-
mal ellipsoids are shown at the 30% probability level. Hydrogen
◦
1
.927(2), Ru–C(3) 1.900(3), Ru–C(4) 2.1093(19), Ru–C(25) 2.1125(19);
atoms (except hydrides) and additional disordered fragment atoms
omitted for clarity. Selected distances ( A˚ ) and angles ( ): Ru–C(1)
◦
C(3)–Ru–C(2) 111.32(12), C(3)–Ru–C(1) 139.56(14), C(2)–Ru–C(1)
1
8
09.12(13), C(3)–Ru–C(4) 90.21(9), C(2)–Ru–C(4) 95.74(8), C(1)–Ru–C(4)
5.97(9), C(4)–Ru–C(25) 169.16(7).
1.9202(18), Ru–C(2) 1.9315(19), Ru–C(3) 2.0874(17), Ru–P(1) 2.3228(4);
C(3)–Ru–P(1) 158.42(5), C(1)–Ru–C(2) 93.58(7).
2
608 | Dalton Trans., 2008, 2603–2614
This journal is © The Royal Society of Chemistry 2008

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1
1
1
of an g -BH
Ru–H d −5.33, d, JHP = 24.7 Hz; Ru–H–BH
BH , d 0.91, s) being observed.
4
intermediate [Ru(IMes)(PPh
3
)(CO)
2
H(g -BH
4
)] ( H:
activity is dependent on the ketone; the percentage conversion for
3
, d −10.68, s; Ru–H–
acetophenone reduction decreases in the order IPr > IMes >>
42
3
SIPr, although for p-methoxyacetophenone, an order 9 > 10 >>
is found.
8
Activity of [Ru(NHC)(L)(CO)HCl] and
2
[
Ru(NHC)(L)(CO)H(g -BH
4
)] for the catalytic hydrogenation
Conclusions
of ketones
A
series of new ruthenium borohydride complexes
In an early review of borohydride complexes, Marks and Kolb
referred to their possible role as ‘gate keepers’ to coordina-
tively unsaturated, and thus potentially catalytically active, metal
2
[
Ru(NHC)(L)(CO)H(g -BH
NHC, PPh ) have been prepared from the corresponding 16-
electron hydride chloride species [Ru(NHC)(L)(CO)HCl]. Studies
4
)] (NHC = IMes, IPr, SIPr; L =
3
19
complexes. Since then, M–BH
4
species have been used as pre-
2
on [Ru(IMes)
upon addition of either CO or PMe
being accompanied by phosphine substitution of one of the
NHC ligands. The catalytic results for ketone reduction show
that [Ru(IMes)
than [Ru(IMes)
2
(CO)H(g -BH
4
)] show that loss of BH
3
occurs
8,17,36
37
catalysts for a range of hydrogenation,
hydrogenolysis,
2
Ph, the latter reaction also
38
and cyclotrimerisation reactions, although there remains the
question in most cases of whether the borohydride ligand remains
coordinated and intact or, as seems more likely, affords a metal
2
2
(CO)H(g -BH
4
)] brings about higher conversions
17,39
hydride complex which is the catalytically active species.
2
(CO)HCl], although as the activity varies with
The catalytic activity of the borohydride complexes 8–11
in the hydrogenation of ketones was tested and compared to
that of the 16-electron hydride chloride complexes 1–3 and
both carbene and ketone, we cannot make any firm conclusions
regarding the influence of different carbene N-substituents.
[
Ru(IMes)(PPh
All reactions were carried out in PrOH under 10 atm H
studies with 1 and 2 showed that the absence of either the
alcohol solvent or H pressure gave negligible amounts of alcohol,
suggesting that both direct and transfer hydrogenation pathways
3
)(CO)HCl]. The results are summarised in Table 2.
i
since
Experimental
2
General considerations
2
All manipulations were carried out using standard Schlenk, high
vacuum and glovebox techniques. Solvents were purified using an
Innovative Technologies solvent system (thf, hexane) or under a
nitrogen atmosphere from sodium benzophenone ketyl (toluene,
4
0,41
are involved.
The data in Table 2 show that, in general,
the borohydride complexes are more active than their hydride
chloride precursors. Reactivity is restricted to aromatic ketones
(
of F > Me > OMe), with very low conversion found for the
alkyl ketone pinacolone with both the bis-IMes and mono IMes–
the presence of a p-substituent resulted in an order of reactivity
benzene) or Mg/I
thf-d (Fluorochem) were vacuum transferred from potassium.
Literature routes (or slight modifications of) were used to prepare
2
(ethanol, methanol). C
6
D
6
, toluene-d
8
and
8
PPh
3
borohydride complexes 8 and 11. There is some variation
43
44
12c
IMes, IPr and SIPr·(C
6
F )H. Proton NMR spectra were
5
seen in the activity of 8–10 as the NHC is changed, although the
recorded on Bruker Avance 300, 400 and 500 MHz NMR
spectrometers and referenced to the chemical shifts of residual
protio solvent resonances (C
6
D
5
H d 7.15, toluene-d
7
d 2.09,
1
3
1
a
thf-d d 3.58). C{ H} NMR spectra were referenced to C D
Table 2 Catalytic hydrogenation of ketones by ruthenium complexes 1–3
7
6
6
3
1
1
and 8–11
(d 128.0), toluene-d (d 21.3) and thf-d (d 67.2). P{ H} and
11 1
8
8
B{ H} NMR chemical shifts were referenced externally to 85%
b
Entry Substrate
Catalyst precursor % Conversion
1
1
1
H
3
PO
4
1
and BF
3
·OEt respectively (d 0.0). H COSY, H–X{ H}
2
3
31
(
X = C, P) HMQC and HMBC experiments were performed
1
2
3
4
5
6
7
8
9
0
Acetophenone
1
84
96
36
85
90
45
33
68
30
53
2
using standard Bruker pulse sequences. IR spectra were recorded
as Nujol mulls (unless otherwise stated) on a Nicolet Prot e´ g e´
3
8
4
60 FTIR spectrometer. Elemental analyses were performed by
9
Elemental Microanalysis Ltd, Okehampton, Devon. Mass spectra
were recorded using a micrOTOF electrospray time-of-flight (ESI-
TOF) mass spectrometer (Bruker Daltonik GmbH) coupled to an
Agilent 1200 LC system (Agilent Technologies).
10
p-Methoxyacetophenone
1
2
3
1
[Ru(IMes)(PPh
3
)-
(
CO)HCl]
1
1
1
1
1
1
1
1
1
2
1
2
3
4
5
6
7
8
9
0
8
9
44
86
76
100
49
55
48
54
14
0
Syntheses
10
11
1
[
Ru(IMes)
reaction of [Ru(AsPh
CH Cl (see synthesis of 2 and 3 below), an alternative, and higher
2
(CO)HCl] (1). While 1 was readily prepared by
)
3
(CO)H ] and IMes followed by addition of
2
p-Fluoroacetophenone
p-Methylacetophenone
Pinacolone
3
8
2
2
1
4a,5
yielding route, involved dissolution of [Ru(IMes)
2
(CO)(OH)H]
8
(494 mg, 0.65 mmol) in CH Cl (8 mL) and overnight stirring at
2
2
8
11
ambient temperature. Removal of the solvent in vacuo gave 1 as
a yellow powder, which was washed twice with hexane (498 mg,
a
i
Reaction conditions: [Ru] 0.4 mol%, ketone 0.3 M in PrOH, 10 atm H
2
,
1
b
98%). H NMR (C
6
D
6
, 298 K): d −25.39 (s, 1H, Ru-H), 2.05 (s,
3
48 K, 20 h. Conversions were determined by either NMR or GC.
1
2H, C
6
H
2
Me
3
), 2.17 (s, 12H, C Me ), 2.32 (s, 12H, C Me ),
6
H
2
3
6
H
2
3
This journal is © The Royal Society of Chemistry 2008
Dalton Trans., 2008, 2603–2614 | 2609

View Article Online
6
m-C
.18 (s, 4H, NCH), 6.78 (s, 4H, m-C
6
H
2
Me
3
), 6.82 (s, 4H,
138.0 (s, o- or p-C
194.6 (s, Ru-CO), 204.4 (s, Ru-CO). IR (cm ): 2037, 1903 (mCO).
Anal. calcd for C44
6
H
2
Me
3
), 139.8 (s, i-C
6
H
2
Me
3
), 185.7 (s, Ru-C),
1
3
1
−1
6
H
2
Me
3
). C{ H}: d 19.6 (2 s, o-Me), 22.0 (s, p-Me), 122.3
(
s, NCH), 129.8 (s, m-CH), 137.1 (s, C-i, o- or p-Me), 137.7 (s, C-i,
H
49
N
4
2
O ClRu,%: C, 65.86; H, 6.16; N, 6.98.
o- or p-Me), 138.0 (s, C-i, o- or p-Me), 195.9 (s, Ru-C), 202.8 (s,
Found,%: C, 65.45; H, 6.03; N, 6.75.
−
1
Ru-CO). IR (cm ): 1882 (mCO). Anal. calcd for C43
H
49
4
N ClORu,%:
C, 66.69; H, 6.38; N, 7.23. Found,%: C, 66.69; H, 6.41; N, 7.03.
[Ru(IPr)
was used with 2 as the ruthenium precursor. H NMR (thf-d
K): d −4.59 (s, 1H, Ru-H), 0.95 (d, JHH = 6.8 Hz, 24H, CHMe
1.12 (d, JHH = 6.8 Hz, 24H, CHMe ), 2.85 (sept, JHH = 6.8 Hz,
4H, CHMe ), 7.01 (s, 4H,
), 2.92 (sept, JHH = 6.8 Hz, 4H, CHMe
NCH), 7.07 (d, JHH = 7.6 Hz, 8H, m-CH), 7.28 (t, JHH = 7.6 Hz,
4H, p-CH). C{ H}: d 23.5 (s, CHMe ), 23.8 (s, CHMe ), 26.1
(s, CHMe ), 26.2 (s, CHMe ), 28.9 (s, CHMe ), 124.4 (s, m-C),
2
(CO)
2
HCl] (5). The same procedure as for 4 above
1
8
, 298
),
[
Ru(IPr)
2
(CO)HCl]
(2). [Ru(AsPh
3
)
3
(CO)H
2
]
(1.0
g,
2
0
.95 mmol) and IPr (0.93 g, 2.4 mmol) were dissolved in
2
toluene (5 mL) and stirred in an ampoule sealed with a J. Youngs
PTFE tap for 5 h at 358 K. After addition of 61 lL (0.95 mmol)
2
2
1
3
1
of CH
2
Cl
2
, stirring was continued at the same temperature for
2
2
a further 24 h. The solvent was removed under vacuum and the
2
2
2
product extracted into toluene (3 × 10 mL), reduced to dryness
125.9 (s, NCH), 126.0 (s, NCH), 126.1 (s, NCH), 130.2 (s, p-C),
and then washed with hexane (4 × 10 mL) to give 2 as yellow
140.0 (s, i-C), 147.1 (s, o-C), 147.2 (s, o-C), 185.9 (s, Ru-C), 193.4
1
−1
powder (400 mg, 78%). H NMR (thf-d
8
, 298 K): d −25.88 (s, 1H,
(s, Ru-CO), 203.1 (s, Ru-CO). IR (C
Anal. calcd for C56
6
D
6
, cm ): 2028, 1934 (mCO).
Ru-H), 0.89–1.00 (m, 48H, CHMe
2
), 2.88 (sept, JHH = 6.8 Hz,
), 3.02 (sept, JHH = 6.8 Hz, 4H, CHMe ), 7.00 (s,
H, NCH), 7.03–7.06 (m, 8H, m-CH), 7.29 (t, JHH = 7.6 Hz, 4H,
), 23.4 (s, CHMe ), 26.4 (s,
), 29.0 (s, CHMe ), 29.1 (s, CHMe ),
24.3 (s, m-C), 124.7 (s, NCH), 124.8 (s, NCH), 129.6 (s, p-C),
38.2 (s, i-C), 147.1 (s, o-C), 195.6 (s, Ru-C), 201.8 (s, Ru-CO).
H
73
N
4
O
2
ClRu,%: C, 69.29; H, 7.58; N, 5.77.
4
H, CHMe
2
2
Found,%: C, 68.97; H, 7.65; N, 5.60. ESI-TOF MS: [M − HCl +
+
4
H] m/z = 935.4752 (theoretical m/z = 935.4786).
1
3
1
p-CH). C{ H}: d 23.1 (s, CHMe
2
2
CHMe ), 26.5 (s, CHMe
2
2
2
2
[Ru(SIPr)
was used with 3 as the ruthenium precursor. H NMR (thf-d
K): d −5.09 (s, 1H, Ru-H), 1.08 (d, JHH = 6.8 Hz, 12H, CHMe
1.11–1.14 (m, 24H, CHMe
), 1.22 (d, JHH = 6.8 Hz, 12H, CHMe
3.23–3.25 (m, 8H, CHMe + NCH ), 3.67–3.73 (m, 8H, CHMe
NCH
4H, p-CH). C{ H}: d 24.4 (s, CHMe
s, CHMe ), 26.6 (s, CHMe ), 28.8 (s, CHMe
2
(CO) HCl] (6). The same procedure as for 4 above
2
1
1
1
8
, 298
2
),
),
+
−
1
IR (cm ): 1898 (mCO). Anal. calcd for C55
H
73
N
4
ClORu,%: C,
2
2
7
0.07; H, 7.81; N, 5.94. Found,%: C, 70.57; H, 8.06; N, 5.64.
2
2
2
+
ESI-TOF MS: [M − HCl + H] m/z = 907.4867 (theoretical
m/z = 907.4837).
2
), 7.00 (d, JHH = 7.6 Hz, 8H, m-CH), 7.15 (t, JHH = 7.6 Hz,
), 24.7 (s, CHMe ), 26.5
), 55.3 (s, NCH ),
1
3
1
2
2
(
2
2
2
2
[
Ru(SIPr)
.95 mmol) and the carbene adduct SIPr·(C
.9 mmol) were dissolved in toluene (5 mL) and stirred in
2
(CO)HCl] (3). [Ru(AsPh
3
)
3
(CO)H
2
]
(1.0 g,
124.9 (s, m-C), 125.0 (s, m-C), 125.1 (s, m-C), 129.1 (s, p-C), 129.3
(s, p-C), 129.4 (s, p-C), 140.7 (s, i-C), 147.8 (s, o-C), 147.9 (s, o-C),
0
2
6
F
5
)H (1.62 g,
−
1
193.0 (s, Ru-C), 202.3 (s, Ru-CO), 213.6 (s, Ru-CO). IR (cm ):
2032, 1937 (mCO). The lability of a CO ligand precluded accurate
an ampoule sealed with a J. Youngs PTFE tap for 5 h at
58 K. Dichloromethane (61 lL, 0.95 mmol) was added and the
solution stirred at the same temperature for an additional 48 h.
+
3
elemental analysis. ESI-TOF MS: [M − HCl + H] m/z = 939.5099
(theoretical m/z = 939.5100).
The solvent was removed in vacuo and the product washed with
1
hexane (3 × 5 mL) to give a yellow powder (450 mg, 50%). H
[Ru(IMes)(PPh
added to a C
3
)(CO)
2
HCl] (7). One atmosphere of CO was
)(CO)HCl] (60 mg,
NMR (thf-d
8
, 298 K): d −25.77 (s, 1H, Ru-H), 0.86 (d, JHH
=
=
6
D
6
sample of [Ru(IMes)(PPh
3
6
6
(
(
.8 Hz, 12H, CHMe
2
), 0.96 (br s, 12H, CHMe
), 1.06 (d, JHH = 6.8 Hz, 12H, CHMe
), 3.73 (s, 8H, NCH
m, 8H, m-CH), 7.17 (t, JHH = 7.6 Hz, 4H, p-CH). C{ H}:
), 26.8 (s, CHMe ), 28.8 (s, CHMe ), 54.8 (s,
), 124.7 (s, m-C), 128.4 (s, p-C), 148.1 (s, o- and i-C), 201.0
2
), 1.03 (d, JHH
0.082 mmol) in a resealable NMR tube fitted with a J. Youngs
PTFE valve. The sample was shaken and an immediate colour
change from yellow to pale-brown occurred. Yield: 48 mg, (77%).
.8 Hz, 12H, CHMe
2
2
), 3.37
sept, JHH = 6.8 Hz, 8H, CHMe
2
2
), 6.93–6.99
1
3
1
1
H NMR (C
6
D
6
, 298 K): d −4.25 (d, JHP = 14.7 Hz, 1H, Ru-H),
d 24.2 (s, CHMe
NCH
s, Ru-C), 224.4 (s, Ru-CO). IR (cm ): 1902 (mCO). Anal. calcd for
2
2
2
2.20 (s, 6H, p-Me), 2.26 (s, 6H, o-Me), 2.33 (s, 6H, o-Me), 6.29
(s, 2H, NCH), 6.77 (s, 2H, C Me), 6.92 (s, 2H, C Me), 6.96
2
6
H
2
6
H
2
−
1
31
1
(
(m, 9H, m- and p-Ph), 7.73 (m, 6H, o-Ph). P{ H}: d 41.0 (s).
1
3
1
C
55
H
77
N
4
ClORu·C
6
H
14,%: C, 70.92; H, 8.88; N, 5.42. Found,%:
C{ H}: d 19.4 (s, o-Me), 19.5 (s, o-Me), 21.8 (s, p-Me), 123.5 (s,
+
C, 70.67; H, 8.84; N, 5.27. ESI-TOF MS: [M − HCl + H] m/z =
NCH), 128.6 (s, m-Ph), 128.7 (s, m-Ph), 129.9 (s, p-Ph), 130.1 (s,
9
11.5127 (theoretical m/z = 911.5150).
m-C
6
H
2
Me
3
), 130.2 (s, m-C
6
H
2
Me
3
), 135.4 (s, o-Ph), 135.5 (s, o-
Me ), 137.5
Ph), 136.1 (s, o- or p-C
6
H
2
Me
3
), 136.6 (s, o- or p-C
6
H
2
3
[
Ru(IMes) (CO) HCl] (4). One atmosphere of CO was intro-
2
2
(br s, i-Ph), 139.4 (s, N-C), 139.8 (s, N-C), 184.9 (d, JCP = 96.5 Hz,
duced to a benzene (2 mL) sample of 1 (161 mg, 0.21 mmol) in
an ampoule fitted with a J. Young’s PTFE valve. The sample was
shaken and an immediate colour change from yellow to pale-straw
occurred. Removal of the solvent and washing with hexane gave 4
Ru-C), 194.5 (d, JCP = 7.3 Hz, Ru-CO), 201.8 (d, JCP = 11.6 Hz,
−
1
Ru-CO). IR (cm ): 2044, 1945 (mCO). ESI-TOF MS: [M − CO −
+
Cl + H] m/z = 697.1886 (theoretical m/z = 697.1927).
1
2
as a microcrystalline white solid. Yield: 113 mg (68%). H NMR,
[Ru(IMes)
2
(CO)H(g -BH
4
)] (8). A mixture of 1 (188 mg,
(38 mg, 1.00 mmol) were suspended
(
(
(
2
C
C
6
D
6
, 298 K): d −4.30 (s, 1H, Ru-H), 2.15 (s, 24H, o-Me), 2.22
s, 12H, p-Me), 6.09 (s, 4H, NCH), 6.71 (s, 4H, m-C Me), 6.72
Me). C{ H}: d 19.4 (s, o-Me), 19.5 (s, o-Me),
1.9 (s, p-Me), 123.7 (s, NCH), 130.0 (s, m-C Me), 130.1 (s, m-
Me), 137.0 (s, o- or p-C Me ), 137.3 (s, o- or p-C Me ),
0.25 mmol) and excess NaBH
4
6
H
2
in EtOH (5 mL) and stirred at ambient temperature for 1 h under
an atmosphere of argon. The solvent was removed in vacuo and
1
3
1
s, 4H, m-C
6
H
2
6
H
2
the product extracted twice with benzene to afford 166 mg (yield
1
6
H
2
6
H
2
3
6
H
2
3
91%) of 8. H NMR (toluene-d
8
, 228 K): d −16.28 (s, 1H, Ru-H
a
),
2
610 | Dalton Trans., 2008, 2603–2614
This journal is © The Royal Society of Chemistry 2008

View Article Online
−
6.98 (br s, 1H, B-H
Me ), 2.11 (s, 12H, o-C
.13 (br s, 2H, B-H ), 5.98 (s, 4H, NCH), 6.75 (s, 4H, m-C
.78 (s, 4H, m-C
b
), −5.12 (br s, 1H, B-H
c
), 2.06 (s, 12H, o-
23.1 Hz, 1H, Ru-H
B-H ), 2.12 (s, 6H, p-C
(s, 6H, o-C
6.74 (s, 2H, m-C
9H, PPh
d 19.7 (s, o-Me), 20.0 (s, o-Me), 21.8 (s, p-Me), 123.2 (s, NCH),
a
), −6.74 (br s, 1H, B-H
b
), −4.73 (br s, 1H,
C
6
H
2
3
6
H
2
Me
3
), 2.34 (s, 12H, p-C
6
H
H
2
Me
3
),
),
c
6
H
2
Me
3
), 2.22 (s, 6H, o-C
6
H
2
Me ), 2.35
3
3
6
d
6
2
Me
3
6
H
2
Me
3
), 4.16 (br s, 2H, B-Hd/e), 6.24 (s, 2H, NCH),
Me ), 6.83 (s, 2H, m-C Me ), 6.91–6.97 (m,
). P{ H}: d 52.0 (s). C{ H}:
1
3
1
6
H
2
Me
3
). C{ H} (298 K): d 19.1 (s, o-Me), 19.3
6
H
2
3
6
H
2
3
3
1
1
13
1
(
(
s, o-Me), 21.3 (s, p-Me), 121.6 (s, NCH), 121.9 (s, NCH), 128.8
s, m-CH), 128.9 (s, m-CH), 136.1 (2 s, C-o or p-Me), 136.9 (s, C-o
3
), 7.48–7.57 (m, 6H, PPh
3
1
1
or p-Me), 138.3 (s, i-C), 194.1 (s, Ru-C), 205.6 (s, Ru-CO). B: d
123.3 (s, NCH), 128.5 (d, JCP = 9.2 Hz, PPh
1.8 Hz, PPh ), 129.9 (s, m-CH), 130.2 (s, m-CH), 135.5 (d, JCP
11.0 Hz, PPh
3
), 129.7 (d, JCP
=
−
1
−
2.8 (br s). IR (cm ): 1900 (mCO), 2425, 2400 (mB-H). Anal. calcd for
ORu,%: C, 68.52; H, 7.09; N, 7.43. Found,%: C, 68.14;
H, 7.08; N, 7.45. The borodeuteride complex [Ru(IMes) (CO)H-
in EtOH.
3
=
C
43
H
53BN
4
3
), 136.9 (d, JCP = 11.0 Hz, PPh
3
), 137.5 (s, C-o or
2
p-Me), 137.9 (s, C-o or p-Me), 138.4 (s, C-o or p-Me), 139.1 (s,
N-C), 190.9 (d, JCP = 90.1 Hz, Ru-C), 205.6 (d, JCP = 14.7 Hz,
2
(
g -BD
4
)] was prepared by reaction of 1 with NaBD
4
1
1
−1
Ru-CO). B: d −2.79 (br s). IR (cm ): 1927 (mCO), 2422, 2399
2
[
Ru(IPr)
2
(CO)H(g -BH
4
)] (9). A sample of 2 (110 mg,
(
m
B–H). Anal. calcd for C40
H
44BN OPRu,%: C, 67.51; H, 6.23; N,
2
0
.12 mmol) and NaBH
4
(18 mg, 0.47 mmol) were suspended in
3
.94. Found,%: C, 67.70; H, 6.22; N, 3.79.
EtOH (4 mL) and stirred at 343 K for 1.5 h under argon. Removal
of the solvent under vacuum gave a colourless residue, which was
[Ru(IMes)(PMe
2
Ph)
2
(CO)H
2
]. A C
6
D
6
solution of PMe Ph
2
extracted with benzene (2 × 5 mL) to give an off-white solid
(6.8 lL, 0.047 mmol) and 8 (29 mg, 3.7 mmol) was heated
at 343 K in a J. Young’s resealable NMR tube for 48 h. The
residue remaining after removal of the solvent was washed
with hexane (2 × 2 mL) to afford the dihydride product as a
1
(
90 mg, 83%). H NMR (thf-d
8
, 298 K): d −16.19 (s, 1H, Ru-H
a
),
), 2.78–2.89
), 7.02 (s, 4H, NCH), 7.05 (d, JHH = 7.6 Hz, 8H,
−
7.20 (br s, 1H, B-H
b
), 0.92–0.97 (m, 48H, CHMe
2
(
m, 8H, CHMe
2
m-CH), 7.27 (t, JHH = 7.6 Hz, 4H, p-CH). N.B. The signals for
yellow microcrystalline solid, which was characterised by NMR
1
B-H
B-H
c
and B-H
appears as a broad singlet at d −5.18 below 274 K. C{ H}
), 23.0 (s, CHMe ), 26.5 (s, CHMe ),
), 29.0 (s, CHMe ), 124.2 (s, m-C), 124.8 (s, NCH),
24.9 (s, NCH), 129.5 (s, p-C), 139.3 (s, i-C), 146.8 (s, o-C), 146.9
d
are lost into the baseline at room temperature, but
spectroscopy. H NMR (C
6
D
6
, 298 K): d −7.97 (ddd, JHP = 32.4,
1
3
1
J
HP = 31.5, JHH = 6.0 Hz, 1H, Ru-H), −6.78 (ddd, 1H, JHP
84.5, JHP = 30.7, JHH = 6.0 Hz, Ru-H), 0.83–1.78 (m, 12H, PMe
1.89–2.47 (m, 24H, CH ), 5.95–6.41 (m, 2H, NCH), 6.43–6.91 (m,
4H, C Me ), 6.97–7.88 (m, 10H, P-C
PP = 20.6 Hz), 17.9 (d, JPP = 20.6 Hz).
=
c
(
298 K): d 22.9 (s, CHMe
2
2
2
2
),
26.6 (s, CHMe
2
2
3
3
1
1
1
6
H
2
3
6
H
5
). P{ H}: d 5.16 (d,
1
1
(
s, o-C), 195.2 (s, Ru-C), 205.4 (s, Ru-CO). B: d −0.2 (br s). IR
J
−
1
(cm ): 1917 (mCO). Anal. calcd for C55 ORu,%: C, 71.64; H,
H
77BN
4
8
.42; N, 6.08. Found,%: C, 71.53; H, 8.50; N, 5.93. ESI-TOF MS:
General procedure for catalytic ketone hydrogenation
+
[M − BH
4
+ H] m/z = 907.4842 (theoretical m/z = 907.4837).
In a typical procedure, the appropriate ketone substrate (0.3 M)
2
[
Ru(SIPr)
2
(CO)H(g -BH
4
)] (10). NaBH
4
(8 mg, 0.21 mmol)
and the catalyst precursor 1–3 or 8–10 (0.4 mol%) were suspended
i
and 3 (50 mg, 0.05 mmol) were suspended in EtOH (4 mL) and
stirred at 343 K for 1.5 h under argon. Removal of the solvent
under vacuum gave a colourless residue, which was extracted with
in degassed PrOH in a 100 mL Parr 4842 bench top autoclave,
which was subsequently charged to 10 atm H . The reaction
2
mixture was heated with stirring at 348 K for 20 h. After cooling
to room temperature, volatiles were either removed under vacuum
1
benzene (2 × 5 mL) to give an off-white solid (40 mg, 82%). H
NMR (thf-d
8
, 274 K): d −14.92 (s, 1H, Ru-H
a
), −7.07 (br s, 1H,
(for NMR analysis) or analysed directly by GC. Percentage
1
B-H
.17 (d, JHH = 6.8 Hz, 9H, CHMe
CHMe
), 1.30 (d, JHH = 6.8 Hz, 6H, CHMe
.8 Hz, 6H, CHMe
), 3.21 (quint, JHH = 6.8 Hz, 2H, CHMe
.27–3.50 (m, 14H, CHMe + NCH + 2B-H ), 3.79 (d, JHH
5.0 Hz, 1H, NCH
), 3.98 (d, JHH = 9.2 Hz, 1H, NCH
b
), −5.37 (br s, 1H, B-H
c
), 0.94 (d, JHH = 6.8 Hz, 6H, CHMe
), 1.23 (d, JHH = 6.8 Hz, 6H,
), 1.42 (d, JHH
2
),
conversions were determined by H NMR spectroscopy (CDCl
3
)
1
2
and GC (Fisons Instrument 8000 Series, 30 m HP-Innowax (aryl
ketones) or 30 m HP-5 crosslinked PH Me siloxane (pinacolone)
columns).
2
2
=
6
3
1
7
2
2
),
2
2
A
=
2
2
), 6.99–
Crystallography
1
3
1
.00 (m, 8H, m-CH), 7.14–7.18 (m, 4H, p-CH). C{ H} (298 K):
), 24.0 (s, CHMe ), 26.6 (s, CHMe ), 29.0 (s,
), 29.1 (s, CHMe ), 54.8 (s, NCH ), 124.7 (s, m-C), 124.9
s, m-C), 125.1 (s, m-C), 128.6 (s, p-C), 140.1 (s, i-C), 147.6 (s, o-
Single crystals of compounds 1–6, 7, 9, 13 and 14 were analysed
using a Nonius Kappa CCD diffractometer and Mo(Ka) radiation
d 23.9 (s, CHMe
CHMe
(
2
2
2
2
2
2
˚
(
k = 0.71073 A). Details of the data collections, solutions and
1
1
−1
refinements are given in Table 3. The structures were solved
C), 205.5 (s, Ru-C), 222.6 (s, Ru–CO). B: d 0.6 (br s). IR (cm ):
922 (mCO), 2425, 2400 (mB–H). Due to high solubility in hexane,
45
using SHELXS-97 and refined using full-matrix least squares in
1
4
5
SHELXL-97. Convergence was uneventful, with the exception
of the following noteworthy points:
the complex could not be isolated in sufficient purity to allow
accurate elemental analysis to be obtained and so only MS data
+
In 1, the asymmetric unit was seen to consist of two half
molecules of the ruthenium carbene complex plus two half
molecules of benzene each at 0.2 occupancy. Both ruthenium
centres are located on 2-fold rotation axes, as is the carbonyl
group ligated to Ru2. Moreover, the chlorides in both molecules
are disordered in a 1 : 1 ratio with the carbonyl groups. The hydride
hydrogens could not be reliably located in this structure, and
hence were omitted from the refinement. The asymmetric unit
in 2 contains 0.5 molecule of the bis-carbene complex, with Ru1,
were determined. ESI-TOF MS: [M − BH
4
+ H] m/z = 911.5148
(
theoretical m/z = 911.5150).
2
[
Ru(IMes)(PPh
3
)(CO)H(g -BH
4
)] (11). An ethanol (10 mL)
)(CO)HCl] (300 mg, 0.41 mmol)
(62 mg, 1.64 mmol) was stirred at 298 K for 1 h.
suspension of [Ru(IMes)(PPh
and NaBH
Removal of the solvent gave a grey-white residue, which was
3
4
extracted twice with benzene (2 × 10 mL) to give 11 as a white
1
solid (274 mg, 94%). H NMR (C
6
D
6
, 298 K): d −13.36 (d, JHP =
This journal is © The Royal Society of Chemistry 2008
Dalton Trans., 2008, 2603–2614 | 2611

View Article Online
the carbonyl ligand and the chloride located on a crystallographic
2
1
-fold rotation axis. These latter ligands are also disordered in a
: 1 ratio with each other. In addition, there is a solvent fragment
that approximates to 0.5 of a molecule of hexane disordered over
two sites and proximate to a crystallographic 2-fold rotation axis.
The hydrogen atoms could not be reliably added to this solvent
moiety due to the combination of disorder and symmetry, and
hence were omitted from the refinement. Remarkably, however,
the (half-occupancy) hydride was located in this structure, and
˚
refined at a distance of 1.6 A from the central metal. In 3, the
asymmetric unit was seen to contain half of a molecule, with
the central metal, the chloride ligand and the carbonyl ligand
atoms located on a crystallographic 2-fold rotation axis. The
ligands located on this symmetry element also exhibited 3 : 2 ratio
disorder with each other. In addition, the motif also contains half
of a heptane molecule. The central solvent carbon (C29) in this
solvent fragment is also located on a 2-fold rotation axis while
the remaining carbons exhibited disorder which was modelled
over two sites, using restraints on C–C bond distances and ADPs
therein. Location of the hydride ligand was precluded by disorder,
as was location of the hydrogen atoms associated with C29. As
such, these hydrogen atoms were omitted from the refinement.
Disorder (50 : 50) in 5 pertained to the chloride and carbonyl
ligands, while in 6, the carbonyl group containing C2 is simi-
larly disordered with the halide ligand. Hydride hydrogens in
˚
both structures were located and refined at 1.6 A from the
ruthenium atoms. Refinement of structure 7 brought welcome
respite from disorder. The hydride therein was located and refined
as for preceding structures. However, this brief intermission
was counterbalanced by deconvoluting the model for 9, which,
although unambiguous, was riddled with disorder. In particular,
the asymmetric unit therein comprises of one half of a molecule
of the borohydride complex, plus half a molecule of heptane.
Ru1 in the former and C29 in the latter are located on 2-
fold rotation axes implicit in the space group symmetry. The
borohydride and carbonyl ligands are almost coincident with the
2
-fold axis, but not quite, and this introduced disorder within
these two ligands as a consequence of molecular symmetry
coinciding with crystallographic ‘special positions’. The situation
was further complicated by the fact that these two ligands are also
disordered with each other, in a 30 : 20 ratio. Therefore, overall,
the three atoms in these trans ligands are each disordered over 4
positions. Distance similarity restraints were used in the final least
squares cycles, and partial atoms were treated isotropically. The
borohydride hydrogen atoms could not be located and hence were
omitted from the refinement, as was the hydride ligand. However,
the hydrogen atom attached to C29 was located and refined subject
to one C–H restraint and one H–H restraint based on ideal values.
Refinement of 13 was uneventful. In 14, however, the dihydride
complex was seen to be disordered in a 90 : 10 ratio with a
monochloride–monohydride complex. The partial chloride was
˚
refined subject to being at a distance of 2.45 A from the central
ruthenium. As a consequence of this disorder, H2 has 90%
occupancy.
Acknowledgements
We thank Dr John Lowe and Dr Claire Lowe (n e´ e Stott) for fruitful
discussions. John Lowe and Anneke Lubben are acknowledged
2
612 | Dalton Trans., 2008, 2603–2614
This journal is © The Royal Society of Chemistry 2008

View Article Online
for NMR and mass spectrometry assistance. EPSRC and the
University of Bath are thanked for financial support (SLC, RFRJ,
IMes·BEt
3
with H
3
B·THF; Y. Yamaguchi, T. Kashiwabara, K. Ogata,
Y. Miura, Y. Nakamura, K. Kobayashi and T. Ito, Chem. Commun.,
2
004, 2160.
OS) and Johnson Matthey plc for the loan of hydrated RuCl .
3
2
2
2
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9, 167.
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Inorg. Chem., 1994, 33, 3609.
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004, 23, 1629.
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1
0 The bis-IMes complex 1 was also formed by dissolving the previously
reported hydroxy hydride [Ru(IMes) (CO)H(OH)] in CH Cl at am-
(CO)HCl] with 3
2
2
2
bient temperature, or via thermolysis of [Ru(PPh
3
)
3
equiv. of IMes in toluene at 343 K for 1 h.
1
1
1 G. W. Nyce, S. Csihony, R. M. Waymouth and J. L. Hedrick, Chem.–
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1
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3
26, 2041; IMes·BH has also been reported to form upon reaction of
This journal is © The Royal Society of Chemistry 2008
Dalton Trans., 2008, 2603–2614 | 2613

View Article Online
4
4
1 We have previously shown that IMes based ruthenium hydride com-
plexes will perform both transfer and direct hydrogenation: S. Burling,
M. K. Whittlesey and J. M. J. Williams, Adv. Synth. Catal., 2005, 347,
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5
91.
44 L. Jafarpour, E. D. Stevens and S. P. Nolan, J. Organomet. Chem., 2000,
2 Recent work by Beller and co-workers has also found IPr to be the
most active of the NHCs in the Ru based transfer hydrogenation of
acetophenone: S. Enthaler, R. Jackstell, B. Hagemann, K. Junge, G.
Erre and M. Beller, J. Organomet. Chem., 2006, 691, 4652.
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2
614 | Dalton Trans., 2008, 2603–2614
This journal is © The Royal Society of Chemistry 2008