protons), 8.2 (d, 2H, imidazol-2-ylidene), 8.6 (d, 2H, pyridine ring protons).
13C{1H}, 24.0 and 24.4 [(CH3)2CH], 28.6 [(CH3)2CH], 111.7, 116.2, 122.8,
123.8, 129.2, 138.7, 140.7, 146.2, and 152.6 (aromatic, pyridine and
carbene ring carbons), 219 (carbene C).
For L2: NMR (C6D6), 1H, d 1.3 [s, 18H, (CH3)3C], 2.2 (s, 12H, o-CH3),
6.4 (d, 2H, imidazol-2-ylidene), 7.2–7.3 (m, 5H, aromatic and pyridine ring
protons), 8.2 (d, 2H, imidazol-2-ylidene), 8.6 (d, 2H, pyridine ring protons).
13C{1H}, 18.6 [(CH3)3C], 31.6 (o-CH3), 34.5 [(CH3)3C], 111.2, 116.6,
121.6, 125,6, 135.2, 139.0, 140.8, 151.0, 152.8, (all aromatic carbons), 220
(carbene C).
For 1: NMR (CD2Cl2), 1H 0.6, 0.7, 0.9 and 1.0 [all doublets, 24H,
(CH3)2CH ] 2.3 and 2.7 [4H, septet, (CH3)2CH )], 6.8–9.2 (multiplets, 28H,
aromatic). 31P{1H}: d 10.29.
‡
Crystal data for L1: C42.5 H53 N5 O1, clear needles (0.01 3 0.005 3
0.005 mm), Mw 649.9, Orthorhombic, Fdd2 (No. 43), a = 29.947(2) Å, b
= 48.760(4) Å, c = 10.9022(9) Å, a = ß = g = 90°, V = 15920(2) Å3, Z
= 16, Dc = 1.085 mg m23, m = 0.066 mm21, l = 0.6875 Å, T = 150 K,
Total reflections = 17293, unique reflections = 7126 (Rint = 0.0469), Final
R indices [I > 2s(I)] R = 0.1115, Rw = 0.3259 (all data). These were
collected at the Daresbury SRS on station 9.8 due to the extremely small size
of the crystals.
Crystal data for 1: C108 H116 Cl8 N10 P2 Ru2, orange plates (0.06 3 0.04
3 0.01 mm), Mw 2101.79, Orthorhombic, Pca21 (No. 29), a = 17.9597(5)
Å, b = 19.2091(6) Å, c = 28.7358(10) Å, a = ß = g = 90°, V = 9913.6(5)
Å3, Z = 4, Dc = 1.408 mg m23, m = 0.607 mm21, l = 0.71073 Å, T = 150
K, Total reflections = 23471, unique reflections = 9624, Final R indices [I
> 2s(I)] R = 0.0455, Rw = 0.0986 (all data). This compound was refined
as racemic twin [flack parameter 0.48(4)]. CCDC 182353 and 182354. See
.cif or other electronic format.
Fig. 2 Molecular structure of 1. Thermal ellipsoids shown at 50%
probability. The asymmetric unit contains two molecules and two molecules
of dichloromethane. Selected bond lengths (Å): C(13)–Ru(1) 2.025(8),
C(21)–Ru(1) 2.028(8), N(3)–Ru(1) 1.971(7), Cl(1)–Ru(1) 2.424(2), Cl(2)–
Ru(1) 2.478(2), Ru(1)–P(1) 2.318(2).
Table 1 Selected results of transfer hydrogenation catalysed by 1a
Temper-
ature/°C
Catalyst/
mol %
1 (a) For recent reviews see: D. Bourisou, O. Guerret, F. P. Gabbai and G.
Bertrand, Chem. Rev., 2000, 100, 39; (b) W. A. Herrmann and C.
Köcher, Angew. Chem., Int. Ed. Engl., 1997, 36, 2162.
2 J. C. Green, R. G. Scurr, P. L. Arnold and F. G. N. Cloke, Chem.
Commun., 1997, 1963.
3 (a) A. A. D. Tulloch, A. A. Danopoulos, R. P. Tooze, S. M. Cafferkey,
S. Kleinhenz and M. B. Hursthouse, Chem. Commun., 2000, 1247; (b)
A. A. D. Tulloch, A. A. Danopoulos, S. Winston, S. Kleinhenz and G.
Eastham, J. Chem.Soc., Dalton Trans., 2000, 4499; (c) A. A. D. Tulloch,
A. A. Danopoulos, G. J. Tizzard, S. J. Coles, M. B. Hursthouse R. S.
Hay-Motherwell and W. B. Motherwell, Chem. Commun., 2001,
1270.
Substrate
Time/h
TON
Cyclohexanone
Cyclohexanone
Cyclohexanone
Acetophenone
Acetophenone
Benzylidene aniline
25
55
55
55
80
55
12
12
20
20
12
20
0.1
150
1900
8800
2500
4000
4200
0.050
0.010
0.015
0.015
0.015
a Amounts: 5 3 1023 M catalyst, 50 mmol substrate, 0.5 g (4.5 mmol)
ButOK, in PriOH. Yields determined by GC of the isolated product
mixture.
4 (a) M. F. Lappert, J. Organomet. Chem., 1993, 451, 389; (b) J. C. C.
Chen and I. J. B. Lin, Organometallics, 2000, 19, 5113; (c) D. S.
McGuiness and K. S. Cavell, Organometallics, 2000, 741; (d) E. Peris,
J. A. Loch, J. Mata and R. H. Crabtree, Chem. Commun., 2001, 201; (e)
P. L. Arnold, A. C. Scarisbrick, A. J. Blake and C. Wilson, Chem.
Commun., 2001, 2340; (f) W. A. Herrmann, C. Kocher, L. J. Goossen
and G. R. J. Artus, Chem. Eur. J., 1996, 2, 1627; (g) A. Fürstner, H.
Krause, L. Ackermann and C. W. Lehmann, Chem. Commun., 2001,
2240; (h) C. Yang, H. M. Lee and S. P. Nolan, Org. Lett., 2001, 3,
1511.
5 A. A. Danopoulos, S. Winston, T. Gelbrich, M. B. Hursthouse and R. P.
Tooze, Chem. Commun., 2002, 482.
6 (a) M. F. Lapppert, J. Organomet. Chem., 1988, 358, 185; (b) H. J.
Schanz, L. Jafarpour, E. D. Stevens and S. P. Nolan, Organometallics,
1999, 18, 5187.
7 (a) J. A. Loch, M. Albrecht, E. Peris, J. Mata, J. W. Faller and R. H.
Crabtree, Organometallics, 2002, 21, 700; (b) D. J. Nielsen, K. J.
Cavell, B. W. Skelton and A. H. White, Inorg. Chim. Acta, 2002, 327,
116.
8 (a) N. Rahmouni, J. A. Osborn, A. de Cian, J. Fischer and A. Ezzamarty,
Organometallics, 1998, 17, 2470; (b) M. Albrecht and G. van Koten,
Angew. Chem., Int. Ed., 2001, 40, 3750.
9 R. Noyori and T. Ohkuma, Angew. Chem., Int. Ed., 1998, 37, 1703.
10 P. Dani, T. Karlen, R. A. Gossage S. Gladiali and G. van Koten, Angew.
Chem., Int. Ed., 2000, 39, 743.
11 Transfer hydrogenation by iridium NHC complexes has been recently
described: (a) A. C. Hillier, H. M. Lee, E. D. Stevens and S. P. Nolan,
Organometallics, 2001, 20, 4246; (b) M. Albrecht, R. H. Crabtree, J.
Mata and E. Peris, Chem. Commun., 2002, 32.
not yet been optimised, it is obvious that the activity is
comparable to that observed with phosphine and amine
arylpincer ruthenium complexes,10 but lower than the ami-
noalcohol based systems of Noyori.9 Furthermore, these data
show the viability of ruthenium transfer hydrogenation catalysts
stabilised by NHCs operating in protic solvents.11 The stability
provided by the chelate ligand, the availability of two cis
reactive sites and the potential for electronic/coordinative
unsaturation are possible reasons for the activity observed.
Further mechanistic studies on this reaction are in progress.
In summary, the isolation of free ‘pincer’ type bis carbene
ligands has opened new and easy synthetic routes to a variety of
complexes with wide scope for functionalisation and catalyst
optimisation and tuning. The synthesis of other ‘pincer’
complexes with catalytically important metals using this
methodology is under way.
We thank Professor M. B. Hursthouse for provision of X-ray
facilities and Mr Nikolaos Tsoureas for help in data collection
and solving the crystal structures reported here.
Notes and references
†
Spectroscopic data for L1: NMR (C6D6), 1H, d 1.2 and 1.4 [two
doublets, 24H, diastereotopic (CH3)CH], 3.0 [septet, 4H, (CH3)CH ], 6.7 (d,
2H, imidazol-2-ylidene), 7.2–7.5 (m, 7H, aromatic and pyridine ring
CHEM. COMMUN., 2002, 1376–1377
1377