Dalton
Transactions
COMMUNICATION
Hydrogenation of imines catalysed by ruthenium(II)
complexes based on lutidine-derived CNC pincer
ligands†
Cite this: Dalton Trans., 2013, 42, 351
Received 20th August 2012,
Accepted 26th September 2012
Martín Hernández-Juárez,a Mónica Vaquero,b Eleuterio Álvarez,b Verónica Salazar*a
and Andrés Suárez*b
DOI: 10.1039/c2dt31907a
The preparation of new Ru(II) complexes incorporating fac-coordi- ligands are scarce, and only derivatives of type Ru(CNC)(CO)
nated lutidine-derived CNC ligands is reported. These derivatives ClH based on meridionally coordinated CNC ligands with 2,6-
are selectively deprotonated by tBuOK at one of the methylene diisopropylphenyl and mesityl wingtips have been reported.4
arms of the pincer, leading to catalytically active species in the
hydrogenation of imines.
In this communication, we present the synthesis and struc-
tural characterisation of new Ru complexes 3 containing
fac-coordinated bis-N-heterocyclic carbene CNC ligands. Further-
more, application of these complexes in the hydrogenation of
various imines is reported.
Lutidine-derived pincer complexes have become a prominent
class of derivatives in organometallic chemistry.1 In these com-
plexes, pyridine dearomatisation occurs upon deprotonation
of the acidic –CH2– arms, leading to species that are capable
of bond activation by a metal–ligand cooperative mechanism.
With respect to the flanking donor groups of the pincer, atten-
tion has been largely paid to phosphorous derivatives of type
PNX (P = phosphine, X = phosphine or a hemilabile N-donor
ligand). Of particular importance, group 8 (Ru, Fe) catalysts
based on PNX ligands or their deprotonated analogues have
provided good levels of activity in the hydrogenation of a
variety of polar functionalities, including ketones, esters,
amides, ureas, formates, carbamates, and organic carbonates.2
In addition, replacement of P-donors in PNX–Ru complexes by
more electron-donating N-heterocyclic carbene (NHC) ligands
has recently been reported. Thus, Ru pincer complexes incor-
porating CNN ligands with a hemilabile amine or pyridine
fragment have been described.3,4 Some of these derivatives are
active catalysts in the hydrogenation of non-activated esters, in
some cases outperforming their phosphine counterparts.3
Alternatively, examples of ruthenium complexes of CNC
Synthesis of new bis-imidazolium salts 1 has been effected
by refluxing acetonitrile or THF solutions of the corresponding
2,6-bis(halomethyl)pyridine and 1-substituted 1H-imidazole in
a 1 : 2 ratio.5 Initial experiments directed to the synthesis of
ruthenium complexes incorporating CNC ligands derived from
1 were performed by the reaction of the imidazolium salt 1a
(Br) with different Ru precursors (RuHCl(PPh3)3, RuCl2(PPh3)3,
RuHCl(CO)(PPh3)3, RuH2(CO)(PPh3)3) in the presence of a
base. This approach, however, leads to an inseparable mixture
of products, and an alternative procedure based on N-hetero-
cyclic carbene transfer with Ag–NHC complexes was con-
sidered.6 The reaction of bis-imidazolium salts 1 with 1 equiv.
of Ag2O in CH2Cl2 at room temperature results in the clean for-
mation of bimetallic silver complexes 2 (Scheme 1).5 These
derivatives were found to be adequate for CNC ligand transfer
to RuHCl(CO)(PPh3)3. Thus, complexes 3a(Cl) and 3b(Cl) were
conveniently prepared from the appropriate silver reagent 2
and RuHCl(CO)(PPh3)3 in THF at 55 °C. Similarly, complexes
3a(BF4) and 3c(Br) were synthesised by the reaction of the cor-
responding bromide derivatives 2a(Br) and 2c(Br) with RuHCl
(CO)(PPh3)3 followed by treatment with NaBF4 and NaBr,
respectively. Finally, synthesis of 3,5-xilyl-substituted 3d(Cl)
was more conveniently carried out in CH2Cl2 at room
temperature.
aCentro de Investigaciones Químicas, Universidad Autónoma del Estado de Hidalgo,
Carretera Pachuca-Tulancingo Km 4.5, 42184, Mineral de la Reforma, Hidalgo,
Mexico. E-mail: salazar@uaeh.edu.mx; Fax: +52 771 717200x6502;
Tel: +52 771 1550933
bInstituto de Investigaciones Químicas (IIQ), Consejo Superior de Investigaciones
Científicas and Universidad de Sevilla, Avda Américo Vespucio No. 49, 41092,
Complexes 3 have been fully characterized, and their NMR
data reveal very similar features for all complexes of the series.
For example, the 31P{1H} NMR spectrum of 3a(Cl) shows a
singlet at 42.4 ppm. Furthermore, 1H and 13C{1H} NMR
spectra reflect the non-equivalence of the two halves of the
CNC ligand. In the 1H NMR spectrum of 3a(Cl), the hydrido
ligand gives rise to a doublet at −7.38 ppm (JHP = 30.4 Hz),
Sevilla, Spain. E-mail: andres.suarez@iiq.csic.es; Fax: +34954460565;
Tel: +34 954489556
†Electronic supplementary information (ESI) available: Representative experi-
mental procedures, compound characterisation, crystallographic information for
3a(BF4). CCDC 894892. For ESI and crystallographic data in CIF or other elec-
tronic format see DOI: 10.1039/c2dt31907a
This journal is © The Royal Society of Chemistry 2013
Dalton Trans., 2013, 42, 351–354 | 351