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Dalton Transactions
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ARTICLE
Journal Name
with bidentate ligands formed stable homogeneous catalysts
with moderate hydrogenation activities.
DOI: 10.1039/C5DT03366G
4
have not been reported, and therefore attempts were made
to obtain single crystals suitable for X‐ray diffraction.
Complexes and could be crystallized by slow diffusion of
diethyl ether into a saturated solution in dichloromethane,
while crystals of complex formed by slow evaporation of a
2
4
Results and discussion
3
Synthesis of Ru‐NHC complexes. The synthetic routes to all
ruthenium catalyst precursors are summarised in Scheme 2.
The monocarbene ruthenium(II) complexes [RuCl2(p‐
solution in CDCl3. All complexes show the expected piano‐stool
structure with the η6‐arene ligand forming the seat, while the
carbene, dicarbene and chlorido ligands represent the legs
cymene)(iPr2‐imy)] ( ), [RuCl2(p‐cymene)(iPr2‐bimy)] (
1 2) and
(Figure 1). This is also observed in complex
previously reported.10 The Ru‐centroid distances of the neutral
complexes to are found to be very similar with values of
1.7066(1) Å,10 1.7144(6) Å and 1.6971(4) Å, respectively. The
1, which was
[RuCl2(p‐cymene)(Bn2‐bimy)]
(3) derived from imidazole
(imy)10 and benzimidazole (bimy)11 were prepared as
previously reported by deprotonation of the respective
1
3
azolium salts A‐C using Ag2O followed by transmetallation of
Ru‐Ccarbene distances are identical within 3
{2.083(1) Å}, {2.090(3) Å} and {2.089(3) Å}.
The structure of
resembles those of 513 and 6 14
in complexes
1
the resulting silver NHC complexes to [RuCl2(p‐cymene)]2. The
cationic dicarbene complexes [RuCl(η6‐p‐cymene)(κ2C,C‐
2
3
diNHCme)][PF6] ( ) and [RuCl(η6‐p‐cymene)(κ2C,C‐diNHCet)][PF6]
4
4
,
which
were previously reported. Compared to the neutral complexes,
the Ru‐centroid distances of 1.7391(6) Å observed for is
(
5
) were similarly obtained by using the diimidazolium
dibromides and as ligand precursors instead, followed by
anion exchange with KPF6.12 A different route to complex
and the iodido analogue of by direct deprotonation with
triethylamine has been previously reported.13 Replacement of
the diimidazolium salts with the ditopic imidazolium chloride
4
D
E
notably longer. The reason for the weaker binding of the arene
ligand is the expected reduced back‐donation from a more
Lewis acidic metal centre in the cationic complex.
5
4
The bite angle for
4 is ~83° and smaller than that found for
F
5
(~87°) due the increase in size of the metallacycles going
in the latter procedure finally afforded the CN‐chelate
from a 6‐ to a 7‐membered ring.13 For complex
6
, a bite angle
[RuCl(η6‐p‐cymene)(κ2C,N‐NHC‐Py)][PF6] ( ). 14
6
of ~84° was observed,14 which is similar to
4 as both are
containing 6‐membered metallacycles. The averaged
ruthenium‐carbene bond length of 2.044 Å in is significantly
4
R
i) 0.5 Ag2O, solvent
ii) 0.5 [RuCl2(p-cymene)]2
X
R
N
shorter than those observed for the neutral complexes. The
stronger metal‐carbene bonds can be explained by a stronger
donation from the carbenes to the ruthenium centre in the
cationic and thus more Lewis acidic complex. The ruthenium‐
chlorido ligands in all complexes are unexceptional and do not
require further comment.
Ru
N
X
CH2Cl2
- AgX
N
X
N
R
R
Salt A = iPr2imy·HCl
Salt B = iPr2bimy·HBr
1: R = Pr, X = Cl
i
i
2: R = Pr, X = Br
C
Salt = Bn2bimy·HBr
3: R = Bn, X = Br
i) Ag2O, solvent
ii) 0.5 [RuCl2(p-cymene)]2
iii) KPF6, solvent
N
N
N
PF6
2 Br
Ru
n
Cl
- AgBr, - KCl
N
N
N
N
n N
D
4
Salt , n = 1
, n = 1
5
, n = 2
E
Salt , n = 2
i) Ag2O, solvent
ii) 0.5 [RuCl2(p-cymene)]2
iii) KPF6, solvent
N
N
Figure 1. Molecular structure of 2, 3 and the cation of 4 with thermal
ellipsoids drawn at 30% probability level. Hydrogen atoms and solvent
molecules are omitted for clarity. Selected bond lengths [Å] and bond angles
[deg] for complex 2: Ru(1)‐C(7) 2.090(3), Ru(1)‐Cl(1) 2.4463(12), Ru(1)‐Cl(2)
2.4374(11), Ru(1)‐Ct1* 1.7144(6); C(7)‐Ru(1)‐Cl(1) 88.82(8), C(7)‐Ru(1)‐Cl(2)
89.59(9). Complex 3: Ru(1)‐C(7) 2.089(3), Ru(1)‐Cl(1) 2.4068(9), Ru(1)‐Cl(2)
2.4267(9), Ru(1)‐Ct1* 1.6971(4); C(7)‐Ru(1)‐Cl(1) 88.96(8), C(7)‐Ru(1)‐Cl(2)
89.75(8). Complex 4: Ru(1)‐C(4) 2.041(4), Ru(1)‐C(6) 2.047(4), Ru(1)‐Cl(1)
2.413(1), Ru(1)‐Ct1* 1.7391(6); C(4)‐Ru(1)‐C(6) 83.3(2), N(2)‐C(1)‐N(3)
109.9(4), C(4)‐Ru(1)‐Cl(1) 86.3(1), C(6)‐Ru(1)‐Cl(1) 86.3(1); *Ct1 denotes
centroid of the p‐cymene ring.
N
PF6
Cl
N
Ru
Cl
- AgBr, - KCl
N
N
F
6
Salt
Scheme 2. Synthesis of the p‐cymene Ru(II) NHC complexes 1–6.
All six complexes have been isolated as dull yellow to
brown solids and were fully characterised by multinuclear
NMR spectroscopies, ESI mass spectrometry and elemental
analysis providing data, which are generally consistent with
literature reports.
Effect of solvent on hydrogenation of LA to GVL. Complex 1
was chosen as a representative for Ru‐NHC complexes for
initial catalytic tests. The hydrogenation of LA to GVL using 0.1
mol% of the catalyst precursor at 12 bar dihydrogen pressure
2 | J. Name., 2012, 00, 1‐3
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