Hydrogenation and hydrogenolysis of furfural and furfuryl alcohol catalyzed by ruthenium(II) bis(diimine) complexes

Article abstract of DOI:10.1002/aoc.2819

The catalytic activity of ruthenium(II) bis(diimine) complexes cis-[Ru(6,6′-Cl₂bpy)₂(OH₂) ₂](Z)₂ (1, Z = CF₃SO₃; 2, Z = (3,5-(CF₃)₂C₆H₃)₄B, i.e. BArF) and cis-[Ru(4,4′-Cl₂bpy)₂(OH₂) ₂](Z)₂ (3, Z = CF₃SO₃; 4, Z = BArF) for the hydrogenation and/or the hydrogenolysis of furfural (FFR) and furfuryl alcohol (FFA) was investigated. The molecular structures of cis-[Ru(4,4′- Cl₂bpy)₂(CH₃CN)₂](CF ₃SO₃)₂ (3′) and dimeric cis-[(Ru(4,4′-Cl₂bpy)₂Cl)₂](BArF) ₂ (5) were characterized by X-ray crystallography. The structures are consistent with the anticipated reduction in steric hindrance about the ruthenium centers in comparison with corresponding complexes containing 6,6′-Cl₂bpy ligands. While compounds 1-4 are all active and highly selective catalysts for the hydrogenation of FFR to FFA under modest reaction conditions, 3 and 4 showed decreased activity. This is best explained in terms of reduced Lewis acidity of the Ru²⁺ centers and reduced steric hindrance about the metal centers of catalysts 3 and 4. cis-[Ru(6,6′-Cl₂bpy)₂(OH₂) ₂](BArF)₂ (2) also displayed high catalytic efficiency for the hydrogenation of FFA to tetrahydrofurfuryl alcohol. Presumably, this is because coordination of C=C bonds of FFA to the ruthenium center is poorly inhibited by non-coordinating BArF counterions. Interestingly, cis-[Ru(6,6′-Cl₂bpy)₂(OH₂) ₂](CF₃SO₃)₂ (1) showed some catalytic activity in ethanol for the hydrogenolysis of FFA to 2-methylfuran, albeit with fairly modest selectivity. Nonetheless, these results indicate that ruthenium(II) bis(diimine) complexes need to be further explored as catalysts for the hydrogenolysis of C-O bonds of FFR, FFA, and related compounds. Copyright

Full text of DOI:10.1002/aoc.2819

Full Paper
Received: 1 June 2011
Revised: 23 November 2011
Accepted: 14 December 2011
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.2819
Hydrogenation and hydrogenolysis of furfural
and furfuryl alcohol catalyzed by ruthenium(II)
bis(diimine) complexes
Anitha S. Gowda, Sean Parkin and Folami T. Ladipo*
The catalytic activity of ruthenium(II) bis(diimine) complexes cis-[Ru(6,6′-Cl2bpy)2(OH2)2](Z)2 (1, Z = CF3SO3; 2, Z = (3,5-(CF3)2C6H3)4B,
i.e. BArF) and cis-[Ru(4,4′-Cl2bpy)2(OH2)2](Z)2 (3, Z = CF3SO3; 4, Z = BArF) for the hydrogenation and/or the hydrogenolysis of furfural
(FFR) and furfuryl alcohol (FFA) was investigated. The molecular structures of cis-[Ru(4,4′-Cl2bpy)2(CH3CN)2](CF3SO3)2 (3′) and dimeric
cis-[(Ru(4,4′-Cl2bpy)2Cl)2](BArF)2 (5) were characterized by X-ray crystallography. The structures are consistent with the anticipated
reduction in steric hindrance about the ruthenium centers in comparison with corresponding complexes containing 6,6′-Cl2bpy
ligands. While compounds 1–4 are all active and highly selective catalysts for the hydrogenation of FFR to FFA under modest reaction
conditions, 3 and 4 showed decreased activity. This is best explained in terms of reduced Lewis acidity of the Ru2+ centers and
reduced steric hindrance about the metal centers of catalysts 3 and 4. cis-[Ru(6,6′-Cl2bpy)2(OH2)2](BArF)2 (2) also displayed high
catalytic efficiency for the hydrogenation of FFA to tetrahydrofurfuryl alcohol. Presumably, this is because coordination of C=C bonds
of FFA to the ruthenium center is poorly inhibited by non-coordinating BArF counterions. Interestingly, cis-[Ru(6,6′-Cl2bpy)2(OH2)2]
(CF3SO3)2 (1) showed some catalytic activity in ethanol for the hydrogenolysis of FFA to 2-methylfuran, albeit with fairly modest
selectivity. Nonetheless, these results indicate that ruthenium(II) bis(diimine) complexes need to be further explored as catalysts
for the hydrogenolysis of C-O bonds of FFR, FFA, and related compounds. Copyright © 2012 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: catalytic hydrogenation; carbon–oxygen bond hydrogenolysis; ruthenium bis(diimine) complexes; furfuryl alcohol;
tetrahydrofurfuryl alcohol
While copper chromite catalysts are most often employed in
Introduction
industry,[1,6] disposal of deactivated copper chromite catalysts in
Continuing depletion of nonrenewable fossil resources has spurred
intense demand for a shift of feedstock for energy and chemical pro-
duction towards renewable biomass resources.[1–5] However, since
biomass-derived raw materials usually have a high oxygen content,
efficient technologies are needed for selectively tailoring their oxy-
gen content and functionality so that carbohydrate biomass, the
world’s most abundant renewable carbon resource, may be effectively
utilized. In this context, furan derivatives, such as furfural (FFR) and
5-hydroxymethylfurfural are highly attractive as renewable chemical
platforms for the production of fuels and chemical intermediates.[1,5,6]
FFR, produced by acidic degradation of hemicellulose,[7] is a versatile
industrial chemical[6] and its catalytic hydrogenation/hydrogenolysis
is often used to produce furfuryl alcohol (FFA),[8,9] tetrahydrofurfuryl
alcohol (THFA),[10] and 2-methylfuran, (2-MF)[8,9,11] all of which are
important intermediates in the chemical industry.[8,11,12] Only a few
catalysts have been reported to be selective for hydrogenation of
FFR to FFA[9,13] or for hydrogenolysis of FFR or FFA to 2-MF.[12,14,15]
Depending on the catalyst used, vapor phase hydrogenation of
FFR yields not only the desired products but also a variety of by-
products, including furan, tetrahydrofuran, and even ring-opening
products, such as pentanols and pentanediols (equation (1)):[16]
landfill sites is restricted by new environmental regulations
because of the high toxicity of Cr. Hence there is strong incentive
for the development of new Cr-free catalysts that exhibit high
selectivity for FFA and 2-MF formation. While carbon-supported
copper (Cu/C)[9,17] and ruthenium (Ru/C)[18] catalysts, and amor-
phous alloy catalysts, such as Ni–Cu or Fe–Cu alloy catalysts,[19,20]
have been developed as possible replacements for Cu–Cr catalysts,
most are unsuitable for industrial application owing to severe deac-
tivation phenomena.[16,19–21] Moreover, given the heterogeneous
nature of the catalysts, the potential for achieving improvements
in selectivity for the desired products by concentrating on the com-
position of catalysts and the operating conditions for FFR hydroge-
nation appears somewhat limited. In addition, experimental data
on the reaction pathway(s) of vapor phase hydrogenation of FFR
are scarce. Thus there is a current need for the development of sol-
uble catalysts for FFR hydrogenation or hydrogenolysis in order to
gain a better understanding of the reaction mechanism(s) and the
origin of by-products, as well as to ultimately obtain catalysts that
display high activity and product selectivity. In this context, homo-
geneous ruthenium-based catalysts, such as ruthenium–carbene
*
Correspondence to: F. T. Ladipo, Department of Chemistry, University of
Kentucky, Lexington, KY 40506-0055 USA. E-mail: fladip0@uky.edu
Department of Chemistry, University of Kentucky, Lexington, KY, 40506-0055, USA
Appl. Organometal. Chem. 2012, 26, 86–93
Copyright © 2012 John Wiley & Sons, Ltd.

Ru(II)-catalyzed hydrogenation and hydrogenolysis of furfural
complexes[22,23] and RuCl2(PPh3)3,[24] have recently been reported
to be active for FFR hydrogenation.
donating 4,4′-Cl2bpy ligand should generate a less electrophilic
ruthenium center than 6,6′-Cl2bpy ligand.[37] Thus cis-[Ru(4,4′-
Cl2bpy)2(OH2)2](BArF)2 (4) was instead obtained in 81% yield via
metathetical reaction of 3 with NaBArF in water at reflux for 2 h
(Scheme 1). Compounds 1–5 dissolve well in polar organic sol-
vents, such as ethanol, DMSO, acetonitrile (MeCN), sulfolane, and
N-methylpyrrolidone (NMP). However, 2, 4 and 5 were only spar-
ingly soluble in CHCl3 and insoluble in water, while 3 was insoluble
in CHCl3 but partially soluble in water.
The potential of cis-[Ru(6,6′-Cl2bpy)2(OH2)2](CF3SO3)2 (1)[25] as a
promising catalyst for the hydrogenation of carbonyl compounds
and alkenes has previously been recognized.[2,26,27] Based on isoto-
pic labeling studies with D2O, which showed deuterium incorpora-
tion into both H2 gas and the product alcohols, Lau and Cheng[25,28]
proposed a hydrogenation mechanism wherein a metal hydride
species cis-[Ru(6,6′-Cl2bpy)2H(Z)]+/0 (II, Z = H2O, substrate, solvent,
CF3SOꢀ3 ) is generated via heterolytic activation of H2 by a transiently
formed dihydrogen complex cis-[Ru(6,6′-Cl2bpy)2(Z2-H2)Z]2+/+ (I).
The hydride transfer step was postulated to occur via coordination
of the carbonyl or alkene substrate cis to a hydride ligand followed
by insertion into a Ru-H bond; product release then occurs through
protonation of the resulting Ru-O or Ru-C bond. However, an alter-
native hydrogenation pathway involving direct transfer of a hydride
ligand from the ruthenium center to a protonated substrate
without binding of the substrate to ruthenium is also possible, as
has been observed in the reaction between the related complex
[Ru(bpy)(terpy)H]+ (terpy = terpyridine) and CO2.[29,30] Although
there is currently no direct experimental evidence for either species
I or II, the proposed mechanism seems feasible, particularly since
alternatives that involve oxidative addition of H2 to give a Ru4+
species appear more unlikely.
The formulation and structure of compounds 2–4 were estab-
lished by microanalysis and solution (1H, 13C, and 19F) NMR data.
Since the compounds readily exchanged their H2O ligands in
acetonitrile-d3, NMR data were recorded for the resultant bis
(acetonitrile) complexes (after letting the solutions sit for 1–4 h).
Both the 1H and 13C NMR data for each compound displayed sharp
resonances that are consistent with the expected C2 symmetry. For
1
example, the H NMR spectrum of each compound displayed six
aromatic resonances, which integrate as two protons each, for the
disubstituted 2,2′-bipyridine ligands (analogous to data previously
reported for cis-[Ru(6,6′-Cl2bpy)2(OH2)2](CF3SO3)2 (1)).[28,32] Consis-
tent with weak or non-coordination of the counterions, 3 and 4
each displayed a single 19F NMR signal at d-78.0 (CF3SOꢀ3 )[38] and
ꢀ62.0 (BArFꢀ)[39] ppm, respectively. As shown in Fig. 1, X-ray analy-
sis of single crystals of cis-[Ru(4,4′-Cl2bpy)2(CH3CN)2](CF3SO3)2 (3′)
confirmed the structure assigned by spectroscopy; the compound
adopts a distorted octahedral structure with the two bidentate
4,4′-dichloro-2,2′-bipyridine ligands coordinated cis at the ruthenium
center. The distortion from idealized octahedral geometry arises
from the acute bite angle of the bipyridine ligands [e.g. N(1)-Ru-
N(2) = ~79ꢁ]. All of the Ru-N bond distances are within the
In this study, we have explored the potential of ruthenium(II)
bis(diimine) complexes, including some which can potentially
generate more electron-rich ruthenium hydride species than
obtained from 1, as catalysts for the hydrogenation and/or
hydrogenolysis of FFR and FFA. We have also examined the effect
of the nature of the counterion on reactivity of the complexes.
Specifically, we compared the effect of weakly coordinating [B
(3,5-(CF3)2C6H3)4]ꢀ (BArF) anion versus CF3SOꢀ3 anion. It is now
well recognized that the rate or even probability of a reaction
at a cationic metal center may be attenuated by coordination
of counterion.[31] Herein we demonstrate that ruthenium(II) bis
(diimine) complexes are effective homogeneous catalysts for
the chemoselective hydrogenation of FFR and FFA to furfuryl al-
cohol and THFA, respectively. In addition, we show that the com-
plexes display good promise as catalysts for the hydrogenolysis of
FFR and FFA to 2-MF.
expected range and comparable to those (2.04–2.07 Å) observed
[40]
for related complexes cis-[Ru(bpy)2(CH3CN)2](ClO4)2
and cis-
[Ru(6,6′-Cl2bpy)2(k2-acac)2](ClO4).[41] Crystallographic data and
selected metrical parameters for compounds 3′ and cis-[(Ru
(4,4′-Cl2bpy)2Cl)2](BArF)2 (5) are given in Tables 1 and 2, respec-
tively. The structure of 5 was unequivocally established by X-ray
crystallography (Fig. 2) and its 1H NMR data in CDCl3 (which
displayed six equally intense aromatic resonances for the 4,4′-
Cl2bpy ligands, consistent with D2 symmetry) showed that the
solid-state structure of 5 is maintained in CDCl3 solution (see
supporting information). As shown in Fig. 2, a distorted octahe-
dral environment exists about each Ru2+ center in 5, owing to
the acute bite angles resulting from the 4,4′-Cl2bpy ligands
[e.g. N(1)-Ru-N(2) = ~79ꢁ] and the bridging chlorides [Cl(1)-Ru-
Cl(2) = ~84ꢁ). All of the Ru-N and Ru-Cl bond distances (2.02–2.07 Å
and ~2.43 Å, respectively) are within the expected ranges
and comparable to those observed for the related complex
[Ru(phen)(CO)Cl2]2.[42]
Results and Discussion
Synthesis and Characterization of Ruthenium(II) Bis(diimine)
Complexes
Ruthenium(II) bis(diimine) complexes cis-[Ru(6,6′-Cl2bpy)2(OH2)2]
(BArF)2 (2, BArF = (3,5-(CF3)2C6H3)4B) and cis-[Ru(4,4′-Cl2bpy)2
(OH2)2](CF3SO3)2 (3) were prepared in excellent yield via modifi-
cation of the method reported by Che and Leung for the prepa-
ration of cis-[Ru(6,6′-Cl2bpy)2(OH2)2](CF3SO3)2 (1),[32] by metathesis
of the corresponding ruthenium dichloride with two equivalents of
NaBArF[33,34] or Ag(CF3SO3) in ethanol/water, respectively (Scheme 1).
In contrast, similar reaction between cis-[Ru(4,4′-Cl2bpy)2Cl2].2H2O[35]
and NaBArF (2 equiv.) in 4:1 ethanol–water at reflux for 2 h produced
a red solution which after work-up furnished dimeric cis-[(Ru(4,4′-
Cl2bpy)2Cl)2](BArF)2 (5, Scheme 1; similar preparation of the related
compound cis-[Ru(Cl2bpy)2Cl]2(PF6)2 has previously been reported).[36]
Apparently, dimerization is more facile for the initially formed cis-
[Ru(4,4′-Cl2bpy)2Cl(Z)]+ species than for cis-[Ru(6,6′-Cl2bpy)2Cl(Z)]+
species (Z = H2O, EtOH). This is presumably due to reduced steric bulk
of 4,4′-Cl2bpy compared with 6,6′-Cl2bpy since the more electron-
Hydrogenation/Hydrogenolysis Activity Studies
As shown in Table 3 (entries 1–6), compounds 1–4 are all active
catalysts for the hydrogenation of FFR in ethanol, exhibiting
essentially 100% conversion of FFR and near-complete selectivity
for FFA formation at modest temperatures. cis-[Ru(6,6′-Cl2bpy)2
(OH2)2](CF3SO3)2 (1) displayed better selectivity than cis-[Ru(6,6′-
Cl2bpy)2(OH2)2](BArF)2 (2) for C=O bond (versus C=C bond) hydro-
genation. For example, whereas the hydrogenation of FFR in the
presence of 1 mol% of catalyst 1 proceeded at 100ꢁC/51 atm H2
pressure for 2 h to furnish FFA in near-quantitative yield (Table 3,
entry 1), FFR hydrogenation proceeded in the presence of catalyst
2 (0.40 mol%) at lower temperature and pressure (85ꢁC/10.2 atm H2)
Appl. Organometal. Chem. 2012, 26, 86–93
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

A. S. Gowda et al.
N
Cl
N
cis-[Ru(6,6’-Cl2bpy)2Cl2]•2H2O
OH2
70˚ C, 0.5 h
[BArF]2
Cl
Ru
Cl
+
OH2
Cl
4:1 EtOH/H2O
2 NaBArF
N
(2)
N
cis-[Ru(4,4’-Cl2bpy)2(OH2)2][OTf]2 (3)
2 AgOTf
2 NaBArF
1:9 EtOH:H2O,
reflux, 2h
H2O, reflux, 2h
cis-[Ru(4,4’-Cl2bpy)2Cl2]•2H2O
cis-[Ru(4,4’-Cl2bpy)2(OH2)2][BArF]2 (4)
Cl
Cl
2 NaBArF
Cl
Cl
N
Ru
N
N
4:1 EtOH:H2O,
reflux, 2h
Cl
N
N
N
[BArF]2
Ru
N
N
Cl
Cl
Cl
Cl
Cl
(5)
Scheme 1. Synthesis of ruthenium(II) bis(diimine) complexes 1–5
Figure 1. Molecular structure of 3′
to give 93% yield of FFA and 7% yield of THFA; Table 3, entry 3).
The yield of THFA increased when the reaction was conducted
under similar conditions for a longer duration (cf. entry 3 vs. 4 in
Table 3), suggesting that THFA formation is preceded by FFA
formation (i.e. C=O bond hydrogenation is more facile than C=C
bond hydrogenation under the reaction conditions). Consistent
wileyonlinelibrary.com/journal/aoc
Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 86–93

Ru(II)-catalyzed hydrogenation and hydrogenolysis of furfural
Table 1. Crystallographic and refinement data for 3′ and 5
with this suggestion, the hydrogenation of FFA in the presence of
0.40 mol% of catalyst 2 at 130ꢁC in ethanol for 4 h gave THFA in
quantitative yield (Table 3, entry 7). The propensity displayed by
2 for hydrogenation of the C=C bonds of FFA is likely due to the
non-coordinating nature of the BArF counterion.[34] As discussed
before, one of the proposed pathways for carbonyl or alkene
hydrogenation by catalyst 1 involves substrate coordination cis to
a hydride ligand followed by insertion into a Ru-H bond.
Evidently, coordination of a furan ring C=C bond to the electrophilic
Ru2+ center of a metal hydride species generated from catalyst 2 is
competitive with coordination of H2O and/or EtOH. We presume
that 1 did not similarly catalyze the hydrogenation of FFA to give
THFA (cf. Table 3, entry 7 vs. 9) because coordination of triflate
prevails against coordination of a furan ring C=C bond.
Compound
3′
5
Empirical formula
Formula weight
Crystal system
Space group
a (Å)
C26H18Cl4F6N6O6RuS2 C107H51B2Cl19F48N8Ru2
931.45
3257.87
Monoclinic
C 2/c
Triclinic
P ꢀ1
18.9102(1)
10.8163(1)
18.8782(2)
90
13.5881(2)
17.1304(3)
27.0993(5)
82.9755(8)
89.5023(7)
88.0876(7)
6256.99(18)
2
b (Å)
c (Å)
a (deg.)
b (deg.)
118.5306(4)
90
g (deg.)
Volume (Å3)
3392.41(5)
4
Even though cis-[Ru(4,4′-Cl2bpy)2(OH2)2](CF3SO3)2 (3) and cis-
[Ru(4,4′-Cl2bpy)2(OH2)2](BArF)2 (4) both catalyzed FFR hydrogena-
tion to produce FFA in near-quantitative yield for 100% FFR
conversion, each catalyst is significantly less active than the
corresponding 6,6′-Cl2bpy-based derivative 1 and 2, respectively
(cf. Table 3, entries 1 and 3 vs. entries 5 and 6). Moreover, unlike
2, which catalyzed FFA hydrogenation to produce THFA in near-
quantitative yield (Table 3, entry 7), no reaction was observed
when FFA was heated in the 130–150ꢁ C range in the presence
of H2 (51 atm) and 1 mol% of catalyst 4 for up to 23 h. The
decreased activity of catalysts 3 and 4 most probably derives from
a combination of reduced Lewis-acidic character of the Ru2+ cen-
ters and reduced steric hindrance about the metal centers.
First, ligation of Ru2+ by more electron-donating 4,4′-Cl2by
ligands would likely lower the equilibrium concentration of the
dihydrogen complexes cis-[Ru(4,4′-Cl2bpy)2(Z2-H2)Z]2+/+ (Z= H2O,
substrate, solvent, or CF3SO3ꢀ) generated from 3 and 4, and would
undoubtedly increase the pKa values (i.e. decrease acidity) of the
dihydrogen complexes.[43] Second, triflate would almost certainly
compete more effectively with the C=O group of FFR for coordina-
tion to Ru2+ in the metal hydride species cis-[Ru(4,4′-Cl2bpy)2H
(Z)]+/0 (Z= H2O, substrate, solvent, CF3SOꢀ3 ) than in cis-[Ru(6,6′-
Cl2bpy)2H(Z)]+/0 (II) since 4,4′-Cl2bpy is sterically less demanding
than 6,6′-Cl2bpy; such behavior could possibly decrease the cata-
lytic activity of 3 relative to 1. Third, given the non-coordinating
nature of BArF counterion, dimerization of cis-[Ru(4,4′-Cl2bpy)2H
(Z)]+/0 species (Z= H2O, substrate, solvent) generated from cis-
[Ru(4,4′-Cl2bpy)2(OH2)2](BArF)2 (4) would likely be more facile than
dimerization of cis-[Ru(6,6′-Cl2bpy)2H(Z)]+/0 species generated from
cis-[Ru(6,6′-Cl2bpy)2(OH2)2](BArF)2 (2). Consistent with this sugges-
tion, we noted earlier that cis-[Ru(4,4′-Cl2bpy)2Cl(Z)]+ species
(Z= H2O, EtOH) readily dimerized to furnish cis-[(Ru(4,4′-Cl2bpy)
2Cl)2](BArF)2 (5) while cis-[Ru(6,6′-Cl2bpy)2Cl(Z)]+ species reacted
further with NaBArF in ethanol/water to give 2 (see above).
Dimerization of ruthenium hydride species would effectively
reduce the concentration of active catalyst in solution, which
would account for decrease activity of 4 relative to 2. Also, more
facile dimerization of the ruthenium hydride species generated
from 4 (versus 2) would account for why 4 did not catalyze the
hydrogenation of FFA to THFA when 2 did. Presumably, dimeriza-
tion of the cis-[Ru(4,4′-Cl2bpy)2H(Z)]+ species prevails against
coordination of FFR ring C=C bond under our reaction conditions.
Given that hydrogenolysis of FFR (a trace of 2-MF) was observed
in the reaction of FFR with H2 (51 atm) catalyzed by cis-[Ru(6,6′-
Cl2bpy)2(OH2)2](CF3SO3)2 (1) at 100ꢁC, we have briefly investigated
the hydrogenolysis of FFR (and FFA) in the 125–130ꢁC range in
ethanol (the decomposition temperature for cis-[Ru(6,6′-Cl2bpy)2
(OH2)2](CF3SO3)2 (1) has been reported to be 135˚ C),[44] as well as
Z
Dc (Mg mꢀ3
m (mm-1)
F (000)
)
1.824
1.729
0.984
0.769
1 848
3 204
Crystal size (mm)
0.20 ꢂ 0.20 ꢂ 0.20
0.18 ꢂ 0.12 ꢂ 0.06
131 393
22 032
Reflections collected 47 530
Unique reflections
GOF on F2
3 899
1.192
1.047
R indices [I > 2s(I)]
R1, wR2
0.0460, 0.1173
0.0632, 0.1558
R indices (all data)
R1, wR2
0.0524, 0.1235
828086
0.1179, 0.1823
828087
CCDC no.
Table 2. Selected bond lengths (Å) and angles (ꢁ) for 3′ and 5
3′
5
Ru(1)-N(1S)
2.035(3)
Ru(1)-N(3)
Ru(1)-N(2)
Ru(1)-N(4)
Ru(1)-N(1)
Ru(1)-Cl(1)
Ru(1)-Cl(2)
Ru(2)-N(6)
Ru(2)-N(7)
Ru(2)-N(5)
Ru(2)-N(8)
Ru(2)-Cl(1)
Ru(2)-Cl(2)
2.035(5)
Ru(1)-N(1)
2.054(2)
2.042(5)
Ru(1)-N(2)
2.067(2)
2.043(4)
N(1S)-Ru(1)-N(1)
N(1S)-Ru(1)-N(2)
N(1)-Ru(1)-N(2)
171.68(9)
92.83(10)
78.91(9)
2.052(5)
2.4325(15)
2.4384(15)
2.023(5)
N(1S)#1-Ru(1)-N(1S)
N(1S)-Ru(1)-N(1)#1
N(1)#1-Ru(1)-N(1)
N(1S)#1-Ru(1)-N(2)
N(1)#1-Ru(1)-N(2)
N(2)-Ru(1)-N(2)#1
N(1S)#1-Ru(1)-N(1)#1
N(1S)#1-Ru(1)-N(1)
N(1S)#1-Ru(1)-N(2)#1
N(1S)-Ru(1)-N(2)#1
N(1)#1-Ru(1)-N(2)#1
N(1)-Ru(1)-N(2)#1
N(2)-Ru(1)-N(2)#1
94.55(14)
87.18(10)
92.27(13)
94.80(10)
93.25(9)
2.030(5)
2.040(5)
2.050(5)
2.4297(15)
2.4377(15)
100.65(19)
79.31(18)
93.28(18)
97.80(18)
79.58(18)
171.74(19)
171.01(14)
84.26(5)
168.76(14)
171.67(9)
87.18(10)
92.82(10)
94.80(10)
78.91(9)
N(3)-Ru(1)-N(2)
N(3)-Ru(1)-N(4)
N(2)-Ru(1)-N(4)
N(3)-Ru(1)-N(1)
N(2)-Ru(1)-N(1)
N(4)-Ru(1)-N(1)
N(3)-Ru(1)-Cl(1)
Cl(1)-Ru(1)-Cl(2)
Cl(1)-Ru(2)-Cl(2)
Ru(2)-Cl(1)-Ru(1)
Ru(2)-Cl(2)-Ru(1)
N(7)-Ru(2)-N(5)
N(6)-Ru(2)-N(8)
N(7)-Ru(2)-N(8)
N(5)-Ru(2)-N(8)
93.25(9)
168.76(14)
84.33(5)
95.81(5)
95.45(5)
95.12(18)
96.37(19)
79.49(19)
172.69(19)
Appl. Organometal. Chem. 2012, 26, 86–93
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

A. S. Gowda et al.
Figure 2. Molecular structure of 5
Table 3. Ru(II) bis(diimine) catalyzed hydrogenation/hydrogenolysis of FFR and FFA
Entry
Cat.
Substrate
Solventf
T
t
(h)
Conv.
(%)g,h
Selectivity (%)g,i
(ꢁC)
FFA
THFA
2-MF
1a
1c
1d
2e
2e
3c
4
2e
1c
1c
1c
1c
FFR
FFR
FFR
FFR
FFR
FFR
FFA
FFR
FFA
FFR
FFR
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
NMP
100
100
85
2
4
99
100
100
100
100
100
100
100
90
>99
96
—
—
Trace
Trace
2a
3b
4b
5a
2
93
7
90
9
74
26
110
125
130
130
130
130
125
18
23
4
98
<1
<1
>99
Trace
5
6a
99
7a
—
8a
4
17
20
25
9a
4
—
10a
11a
4
98
99
Sulfolane
6
100
100
aH2 pressure = 51 atm.
bH2 pressure = 10.2 atm.
c1.0 mol% catalyst.
d0.28 mol% catalyst.
e0.40 mol% catalyst.
f10 ml solvent.
gAverage of ≥3 experiments.
hDetermined by GC analysis using n-octane as internal standard.
iProduct selectivity = (mol product formed/mol substrate consumed) ꢂ 100.
in solvents that would preclude formation of acetal and/or ether
by-product(s). Reaction of FFR with H2 (51 atm) in the presence of
1 mol% of 1 at 130ꢁC in ethanol for 4 h proceeded to 100% FFR
conversion and furnished a mixture of products (Table 3, entry 8),
including FFA (17% yield), 2-MF (20% yield), and 2-(diethoxymethyl)
furan (furfural diethyl acetal, 14% yield). (Typically, four to five
wileyonlinelibrary.com/journal/aoc
Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 86–93

Ru(II)-catalyzed hydrogenation and hydrogenolysis of furfural
2+/+
H2
Z
OH2
(OTf)2
[Ru]
[Ru]
OH
H
OH2
O
H
(1)
H2
OH2
O
+/0
O
Z
+
[Ru]
H
[Ru] = Ru(6,6'-Cl2bpy)2; Z = H2O, FFA, solvent, OTf-
Scheme 2. A plausible mechanism for FFA hydrogenolysis
additional products are also formed. While the products have not
yet been fully identified, they appear to derive from reactions
between ethanol and substrate followed by different extents of
furan ring hydrogenation and or hydrogenolysis. Similar results
were obtained using isopropyl alcohol as solvent.) Analogous
reaction of FFA with H2 in ethanol catalyzed by 1 mol% of 1 also
produced a mixture of products (Table 3, entry 9), which included
2-MF and THFA in 25% and 5% yield, respectively. Initial attempts
to improve the efficiency of FFR hydrogenolysis by using catalyst
1 in the 125–130ꢁC range in polar organic solvents, such as NMP
or sulfolane (thus precluding formation of acetal and ether by-
products) led to exclusive production of FFA (Table 3, entries
10 and 11).
A plausible mechanism for catalytic formation of 2-MF via FFR
hydrogenolysis, which involves protonation of FFA followed by
hydride attack on the electrophilic carbon of protonated FFA, is
depicted in Scheme 2. A similar acid-assisted nucleophilic substitu-
tion (SN2) mechanism has been proposed to explain Pd-catalyzed
hydrogenolysis of benzyl alcohol in acidic medium.[45] In connec-
tion with the hydrogenolysis of FFA (and FFR) occurring in ethanol
but not in NMP or sulfolane, we note that a transiently formed
dihydrogen species cis-[Ru(6,6′-Cl2bpy)2(Z2-H2)Z]2+/+ (I) is the only
acid present under our reaction conditions; hence the rate (hence
the Ru2+ centers and reduced steric hindrance about the metal
centers, catalysts 3 and 4 (containing 4,4′-Cl2bpy ligands) are less
active than 6,6′-Cl2bpy-containing catalysts 1 and 2, respectively.
cis-[Ru(6,6′-Cl2bpy)2(OH2)2](BArF)2 (2) is also an efficient catalyst
for the hydrogenation of FFA to THFA. Presumably, this is because
coordination of C=C bonds of FFA to the ruthenium center is not
inhibited by non-coordinating BArF counterions. Our initial stud-
ies demonstrate that cis-[Ru(6,6′-Cl2bpy)2(OH2)2](CF3SO3)2 (1) is
an active catalyst in ethanol for the hydrogenolysis of FFA
to 2-MF, albeit the selectivity is poor. However, given the low con-
centration of acid under the reaction conditions and acid-catalyzed
side reactions involving ethanol, we are currently investigating the
effects of added acid and polar aprotic solvent media on the reac-
tion. In contrast to 1, cis-[Ru(4,4′-Cl2bpy)2(OH2)2](CF3SO3)2 (3) did
not catalyze hydrogenolysis of FFA (or FFR) in ethanol under similar
conditions. Since the reduced steric hindrance about the electro-
philic Ru2+ center of 3 likely increases the propensity for dimeriza-
tion of generated metal hydride species (thereby removing active
catalyst from solution), we are currently exploring approaches to
stabilizing complexes of type 3 (and the resultant hydrides) to
dimerization.
efficacy) of FFA hydrogenolysis is probably slowed by the rather Experimental
low concentration of acid. Furthermore, while protonated FFA
Materials and Instruments
and protonated ethanol possess similar acid strengths (pKa ffi ꢀ2),
protonated NMP is a significantly weaker acid (pKa ffi ꢀ0.5).[46] Thus
the concentration of protonated FFA would be negligible in NMP
solution due to the solvent leveling effect, causing the hydrogenolysis
reaction to be much less effective.
All experiments were performed under dry nitrogen atmosphere
using standard Schlenk techniques (unless stated otherwise). All
solvents were dried and distilled by standard methods[47] prior to
use and stored in a glovebox over 4A molecular sieves that had
been dried in a vacuum oven at 150ꢁC for at least 48 h. Unless
otherwise stated, all reagents were purchased from Sigma-Aldrich
Chemical Co. and used as received. FFR and FFA were distilled
under vacuum prior to use. RuCl3.3H2O was purchased from
Pressure Chemical Co. and hydrogen gas (99.9% purity) was
purchased from Scott-Gross Co. The compounds NaBArF,[33,34] cis-
[Ru(6,6′-Cl2bpy)2Cl2].2H2O,[32] cis-[Ru(4,4′-Cl2bpy)2Cl2].2H2O,[35] and
cis-[Ru(6,6′-Cl2bpy)2(OH2)2](CF3SO3)2 (1)[32] were prepared by the
literature methods (or modification thereof).
Conclusions
Ruthenium(II) bis(diimine) complexes cis-[Ru(6,6′-Cl2bpy)2(OH2)2]
(Z)2 (1, Z = CF3SO3; 2, Z = BArF) and cis-[Ru(4,4′-Cl2bpy)2(OH2)2]
(Z)2 (3, Z = CF3SO3; 4, Z = BArF) are all active and highly selective
catalysts for the hydrogenation of FFR to FFA under modest reac-
tion conditions. Due to a combination of reduced Lewis acidity of
Appl. Organometal. Chem. 2012, 26, 86–93
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

A. S. Gowda et al.
1H, 13C, 19F, and 31P NMR spectra were recorded on a Varian
VXR-400 spectrometer at room temperature unless otherwise
stated. All chemical shifts are reported in units of d (downfield
from tetramethylsilane) and 1H and 13C chemical shifts were
referenced to residual solvent peaks. 19F NMR chemical shifts
were referenced to CFCl3 internal standard and 31P NMR chemical
shifts were referenced to 85% H3PO4 internal standard. Analyses
by gas chromatography (GC) were performed on a Shimadzu
GC-17A instrument with flame ionization detection (FID), a
60 m ꢂ 0.32 mm (0.25 mm film thickness) Agilent JW Scientific
DB-5 GC column, and helium as carrier gas. An injection tem-
perature of 140ꢁC was employed, which was found to be
sufficiently low to avoid the occurrence of secondary reac-
tions in the injection port. GC-MS analyses were performed
on an Agilent Technologies 6890/5973N inert gas GC/mass
selective detection system at an ionizing potential of 70 eV.
Elemental analysis for C, H, and N was performed by Robertson
Microlit Laboratories (Ledgewood, NJ, USA).
recrystallization of 3 from acetonitrile and diethyl ether at
ambient temperature.
Synthesis of cis-[Ru(4,4′-Cl2bpy)2(OH2)2](BArF)2 (4, BArF = (3,5-
(CF3)2C6H3)4B)
A mixture of cis-[Ru(4,4′-Cl2bpy)2(OH2)2](CF3SO3)2 (3, 0.100 g,
0.113 mmol) and NaBArF (0.202 g, 0.228 mmol) in water (50 ml)
was heated at reflux for 2 h. The reaction mixture was cooled to
room temperature and then filtered. The dark-red precipitate was
washed three times with water (3ꢂ 20 ml). The product was then
dried under vacuum overnight. Yield= 0.210 g, 81%. 1H NMR
(CD3CN, recorded after letting the solution sit for 1 h; see Discussion
section): d 9.16 (d, 2H, 3J = 6.4 Hz, 4,4′-Cl2bpy), 8.60 (d, 2 H, 4J= 2.0
Hz, 4,4′-Cl2bpy), 8.47 (d, 2 H, 4J = 2.4 Hz, 4,4′-Cl2bpy), 7.91 (dd, 2 H,
3J = 6.0 Hz, 4J= 2.0 Hz, 4,4′-Cl2bpy), 7.70 (m, 16 H, BArF) 7.66
3
(m, 8 H, BArF), 7.56 (d, 2 H, J = 6.8 Hz, 4,4′-Cl2bpy), 7.34 (dd, 2 H,
3J = 6.0 Hz, 4J = 2.4 Hz, 4,4′-Cl2bpy). 13C NMR (CD3CN): d 162.7
(q, 1JBC = 50 Hz, Cipso, BArF), 159.3, 158.6, 155.4, 154.5, 147.6, 147.1,
135.7 (Co, BArF), 129.9 (m, Cm, BArF), 129.1, 128.2, 126.1, 125.7,
1
125.5 (q, JCF = 272 Hz, CF3), 118.8 (Cp, BArF). 19F NMR (CD3CN): d
Synthesis of cis-[Ru(6,6′-Cl2bpy)2(OH2)2](BArF)2 (2, BArF = (3,5-
ꢀ62.0 (BArF). Anal. Calcd. for (4.2H2O) C84H44B2Cl4F48N4O4Ru: C,
(CF3)2C6H3)4B)
42.94; H, 1.89; N, 2.38. Found: C, 42.33; H, 1.97; N, 2.27.
A mixture of cis-[Ru(6,6′-Cl2bpy)2Cl2].2H2O (0.100 g, 0.152 mmol)
and NaBArF (0.267 g, 0.304 mmol) in 4:1 ethanol–water (40 ml)
was heated at 70ꢁC for 0.5 h. The resulting red solution was then
cooled to ambient temperature and the solvent was removed
under vacuum. The red residue was washed three times with
an excess of water (~ 20 ml) and then dried under vacuum to
give a dark-red sticky powder. Yield = 0.34 g, 97%. 1H NMR
(CD3CN, recorded after letting the solution sit for 1 h; see Discus-
Typical Procedure for Hydrogenation/Hydrogenolysis
Activity Tests
Hydrogenation/hydrogenolysis activity tests were performed in a
125 ml stainless steel Parr reactor. In a typical reaction, solvent
(10 ml), n-octane (20 ml, internal standard), substrate (1.21 mmol),
catalyst (1 mol%), and a stirrer bar were charged into the reactor.
After sealing the reactor, the air content was purged by flushing
thrice with 51 atm hydrogen. The reactor was pressurized with
hydrogen, immersed in a preheated silicone oil bath, and magnet-
ically stirred (reaction conditions are described for each result and
the stirring rate was ~450 rpm). The pressure inside the chamber
was maintained at the specified pressure throughout the course
of the reaction. After the appropriate reaction time, the reactor
was cooled to room temperature, vented, and the products were
analyzed quantitatively by GC-FID (and further identified by
comparison of GC-MS data with corresponding data for authentic
samples).
3
4
sion section): d 8.38 (dd, 2 H, J = 8.0 Hz, J = 1.2 Hz, 6,6′-Cl2bpy),
8.32 (dd, 2 H, 3J = 8.0 Hz, 4J = 1.2 Hz, 6,6′-Cl2bpy), 8.16 (t, 2 H,
3
3J = 8.0 Hz, 6,6′-Cl2bpy), 8.08 (t, 2 H, J = 8.0 Hz, 6,6′-Cl2bpy), 7.79
3
4
(dd 2 H, J = 8.0 Hz, J = 1.2 Hz, 6,6′-Cl2bpy), 7.64–7.72 (m, 24 H,
3
4
BArF), 7.58 (dd, 2 H, J = 8.0 Hz, J = 1.2 Hz, 6,6′-Cl2bpy). 13C NMR
1
(CD3CN): d 162.7 (q, JBC = 50 Hz, Cipso, BArF), 161.1, 160.3, 160.1,
159.8, 141.9, 141.8, 135.7 (Co, BArF), 129.9 (m, Cm, BArF), 129.0,
1
128.6, 125.5 (q, JCF = 272 Hz, CF3), 124.3, 123.9, 118.8 (Cp, BArF).
A sample of 2 isolated as 2.2H2O was characterized by microanal-
ysis. Anal. Calcd for C84H44B2Cl4F48N4O4Ru: C, 42.94; H, 1.89; N,
2.38. Found: C, 42.51; H, 1.56; N, 2.53.
Acknowledgments
Synthesis of cis-[Ru(4,4′-Cl2bpy)2(OH2)2](CF3SO3)2 (3)
Thanks are expressed to the Kentucky Science and Engineering
Foundation (grant number KSEF-2428-RDE-014) for financial sup-
port of this work. NMR instruments utilized in this research were
funded in part by the CRIF program of the US National Science
Foundation (grant number CHE-9974810).
A mixture of cis-[Ru(4,4′-Cl2bpy)2Cl2].2H2O (0.480 g, 0.729 mmol) and
Ag[CF3SO3] (0.412 g, 1.603 mmol) in 1:9 ethanol–water (80 ml) was
heated at reflux for 2 h. The resulting red solution was cooled to
ambient temperature and then filtered to remove AgCl. The filtrate
was concentrated to dryness under reduced pressure and the red
1
residue was dried under vacuum for 12 h. Yield = 0.64 g, 99%. H
References
NMR (CD3CN, recorded after letting the solution sit for 4 h; see
Discussion section): d 9.18 (d, 2 H, 3J= 6.4 Hz, 4,4′-Cl2bpy), 8.61
(d, 2 H, 4J= 2.0 Hz, 4,4′-Cl2bpy), 8.48 (d, 2 H, 4J= 2.0 Hz, 4,4′-Cl2bpy),
7.93 (dd, 2 H, 3J= 6.4 Hz, 4J= 2.0 Hz, 4,4′-Cl2bpy), 7.57 (d, 2 H,
[1] A. Corma, S. Iborra, A. Velty, Chem. Rev. 2007, 107, 2411.
[2] M. Schlaf, Dalton Trans. 2006, 4645.
[3] J. N. Chheda, Y. Roman-Leshkov, J. A. Dumesic, Green Chem. 2007,
9, 342.
[4] Y.-C. Lin, G. W. Huber, Energy Environ. Sci. 2009, 2, 68.
[5] J. B. Binder, R. T. Raines, J. Am. Chem. Soc. 2009, 131, 1979.
[6] J. Q. Li in The Chemistry and Technology of Furfural and its Many
By-Products, Vol. 81Ed. (Ed.: K. J. Zeitsch), Elsevier, Amsterdam, 2001.
[7] J. N. Chheda, G. W. Huber, J. A. Dumesic, Angew. Chem. Int. Ed. 2007,
46, 7164.
3
4
3J= 6.4 Hz, 4,4′-Cl2bpy), 7.36 (dd, 2 H, J= 6.4 Hz, J= 2.0 Hz, 4,4′-
Cl2bpy). 13C NMR (CD3CN): d 159.3, 158.5, 155.4, 154.5, 147.5, 147.0,
129.2, 128.2, 126.1, 125.7. 19F NMR (CD3CN): d ꢀ78.0 (CF3SO-3). Anal.
Calcd for (3.2H2O) C22H20Cl4F6N4O10RuS2: C, 28.68; H, 2.19; N, 6.08.
Found: C, 29.06; H, 1.80; N, 6.27.
Single crystals of cis-[Ru(4,4′-Cl2bpy)2(CH3CN)2](CF3SO3)2 (3′)
suitable for X-ray diffraction study were obtained via slow
[8] J. Kijenski, P. Winiarek, T. Paryjczak, A. Lewicki, A. Mikolajska, Appl.
Cat. A 2002, 233, 171.
wileyonlinelibrary.com/journal/aoc
Copyright © 2012 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2012, 26, 86–93

Ru(II)-catalyzed hydrogenation and hydrogenolysis of furfural
[9] R. S. Rao, R. T. K. Baker, M. A. Vannice, Cat. Lett. 1999, 60, 51.
[10] N. Merat, C. Godawa, A. Gaset, J. Chem. Technol. Biotechnol. 1990,
48, 145.
[11] Y.-L. Zhu, H.-W. Xiang, Y.-W. Li, H. Jiao, G.-S. Wu, B. Zhong, G.-Q. Guo,
New J. Chem. 2003, 27, 208.
[28] C. P. Lau, L. Cheng, Inorg. Chim. Acta 1992, 195, 133.
[29] C. Creutz, M. H. Chou, J. Am. Chem. Soc. 2007, 129, 10108.
[30] H. Konno, A. Kobayashi, K. Sakamoto, F. Fagalde, N. E. Katz, H. Saitoh,
O. Ishitani, Inorg. Chim. Acta 2000, 299, 155.
[31] S. H. Strauss, Chem. Rev. 1993, 93, 927.
[12] H.-Y. Zheng, J. Yang, Y.-L. Zhu, G.-W. Zhao, React. Kinet. Catal. Lett.
[32] C. M. Che, W. H. Leung, Chem. Commun. 1987, 1376.
[33] S. R. Bahr, P. Boudjouk, J. Org. Chem. 1992, 57, 5545.
[34] M. Brookhart, B. Grant, A. F. Volpe Jr., Organometallics 1992, 11, 3920.
[35] P. G. Hoertz Jr, PhD thesis, Johns Hopkins University, 2004.
[36] E. C. Johnson, B. P. Sullivan, D. J. Salmon, S. A. Adeyemi, T. J. Meyer,
Inorg. Chem. 1978, 17, 2211.
2004, 82, 263.
[13] H. Li, H. Luo, L. Zhuang, W. Dai, M. Qiao, J. Mol. Catal. A: Chem. 2003,
203, 267.
[14] V. Vetere, A. B. Merlo, J. F. Ruggera, M. L. Casella, J. Braz. Chem. Soc.
2010, 21, 914.
[15] J. Lessard, J.-F. Morin, J.-F. Wehrung, D. Magnin, E. Chornet, Top.
[37] Y. Takahata, C. E. Wulfman, D. P. Chong, J. Mol. Struct. (THEOCHEM)
Catal. 2010, 53, 1231.
2008, 863, 33.
[16] H.-Y. Zheng, Y.-L. Zhu, B.-T. Teng, Z.-Q. Bai, C.-H. Zhang, H.-W. Xiang,
Y.-W. Li, J. Mol. Catal. A: Chem. 2006, 246, 18.
[38] D. Souverain, A. Leborgne, G. Sauvet, P. Sigwalt, Eur. Polym. J. 1980,
16, 861.
[17] G. Seo, H. Chon, J. Catal. 1981, 67, 424.
[39] M. Kira, T. Hino, H. Sakurai, J. Am. Chem. Soc. 1992, 114, 6697.
[40] S. K. Chattopadhyay, K. Mitra, S. Biswas, S. Naskar, D. Mishra, B. Adhikary,
R. G. Harrison, J. F. Cannon, Transition Met. Chem. 2004, 29, 1.
[41] P. N. Liu, Z. Y. Zhou, C. P. Lau, Chem. Eur. J. 2007, 13, 8610.
[42] G. B. Deacon, C. M. Kepert, N. Sahely, B. W. Skelton, L. Spiccia,
N. C. Thomas, A. H. White, Dalton Trans. 1999, 275.
[43] G. J. Kubas, Proc. Natl Acad. Sci. USA 2007, 104, 6901.
[44] D. Taher, M. E. Thibault, D. Di Mondo, M. Jennings, M. Schlaf, Chem.
Eur. J. 2009, 15, 10132.
[18] D. C. Elliott, T. R. Hart, Energy Fuel 2009, 23, 631.
[19] R. M. Lukes, C. L. Wilson, J. Am. Chem. Soc. 1951, 73, 4790.
[20] H. P. Thomas, C. L. Wilson, J. Am. Chem. Soc. 1951, 73, 4803.
[21] C. L. Wilson, J. Chem. Soc. 1945, 52.
[22] Z. Strassberger, M. Mooijman, E. Ruijter, A. H. Alberts, C. de Graaff,
R. V. A. Orru, G. Rothenberg, Appl. Organomet. Chem. 2010, 24, 142.
[23] Z. Strassberger, M. Mooijman, E. Ruijter, A. H. Alberts, A. G. Maldonado,
R. V. A. Orru, G. Rothenberg, Adv. Synth. Catal. 2010, 352, 2201.
[24] F. Huang, W. Li, Q. Lu, X. Zhu, Chem. Eng. Technol. 2010, 33, 2082.
[25] C. P. Lau, L. Cheng, J. Mol. Catal. 1993, 84, 39.
[26] M. Schlaf, M. E. Thibault, D. Di Mondo, D. Taher, E. Karimi, D. Ashok,
Int. J. Chem. React. Eng. 2009, 7, Article A34.
[45] A. P. G. Kieboom, J. F. De Kreuk, H. Van Bekkum, J. Catal. 1971, 20, 58.
[46] pKa values from J. March, Advanced Organic Chemistry, 6th edn,
Wiley-Interscience, New York, 2006.
[47] W. L. F. Armarego, D. D. Perrin, Purification of Laboratory Chemicals,
[27] Z. Xie, M. Schlaf, J. Mol. Catal. A: Chem. 2005, 229, 151.
4th edn, Elsevier Butterworth-Heinemann, Oxford, England, 1997.
Appl. Organometal. Chem. 2012, 26, 86–93
Copyright © 2012 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc