Phosphine-free pincer-ruthenium catalyzed biofuel production: High rates, yields and turnovers of solventless alcohol alkylation

Article abstract of DOI:10.1039/d0cy01679a

Phosphine-free pincer-ruthenium carbonyl complexes based on bis(imino)pyridine and 2,6-bis(benzimidazole-2-yl) pyridine ligands have been synthesized. For the β-alkylation of 1-phenyl ethanol with benzyl alcohol at 140 °C under solvent-free conditions, (Cy2NNN)RuCl2(CO) (0.00025 mol%) in combination with NaOH (2.5 mol%) was highly efficient (ca. 93% yield, 372?000 TON at 12?000 TO h-1). These are the highest reported values hitherto for a ruthenium based catalyst. The β-alkylation of various alcohol combinations was accomplished with ease which culminated to give 380?000 TON at 19?000 TO h-1 for the β-alkylation of 1-phenyl ethanol with 3-methoxy benzyl alcohol. DFT studies were complementary to mechanistic studies and indicate the β-hydride elimination step involving the extrusion of acetophenone to be the overall RDS. While the hydrogenation step is favored for the formation of α-alkylated ketone, the alcoholysis step is preferred for the formation of β-alkylated alcohol. The studies were extended for the upgradation of ethanol to biofuels. Among the pincer-ruthenium complexes based on bis(imino)pyridine, (Cy2NNN)RuCl2(CO) provided high productivity (335 TON at 170 TO h-1). Sterically more open pincer-ruthenium complexes such as (Bim2NNN)RuCl2(CO) based on the 2,6-bis(benzimidazole-2-yl) pyridine ligand demonstrated better reactivity and gave not only good ethanol conversion (ca. 58%) but also high turnovers (ca. 2100) with a good rate (ca. 710 TO h-1). Kinetic studies indicate first order dependence on concentration of both the catalyst and ethanol. Phosphine-free catalytic systems operating with unprecedented activity at a very low base loading to couple lower alcohols to higher alcohols of fuel and pharmaceutical importance are the salient features of this report. This journal is

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ARTICLE
PhosphineFree PincerRuthenium Catalyzed Biofuel Production:
High Rate, Yields and Turnovers of Solventless Alcohol Alkylation
Kanu Das,a Eileen Yasmin,a Babulal Das,a Hemant Kumar Srivastava,*b Akshai Kumar*a,c
Received 00th January 20xx,
Accepted 00th January 20xx
Phosphinefree pincerruthenium carbonyl complexes based on bis(imino)pyridine and 2,6bis(benzimidazole2yl)
pyridine ligands have been synthesized. For the alkylation of 1phenyl ethanol with benzyl alcohol at 140 C under
solventfree conditions, (Cy2NNN)RuCl2(CO) (0.00025 mol %) in combination with NaOH (2.5 mol %) was highly efficient (ca.
93% yield, 372000 TONs at 12000 TOh1). These are the highest reported values hitherto for a ruthenium based catalyst. The
alkylation of various alcohol combinations were accomplished with ease which culminated to give 380000 TONs at 19000
TOh1 for the alkylation of 1phenyl ethanol with 3methoxy benzyl alcohol. DFT studies were complementary to
mechanistic studies and indicate the hydride elimination step involving the extrusion of acetophenone to be the overall
RDS. While the hydrogenation step is favored for the formation of alkylated ketone, the alcoholysis step is preferred for
the formation of alkylated alcohol. The studies were extended for the upgradation of ethanol to biofuels. Among the
pincerruthenium complex based on bis(imino)pyridine, (Cy2NNN)RuCl2(CO) provided high productivity (335 TONs at 170
DOI: 10.1039/x0xx00000x
TOh1). Sterically more open pincerruthenium complexes such as
(
Bim2NNN)RuCl2(CO) based on
2,6bis(benzimidazole2yl) pyridine ligand, demonstrated better reactivity and gave not only good ethanol conversion (ca.
58%) but also high turnovers (ca. 2100) with a good rate (ca. 710 TOh1). Kinetic studies indicate first order dependence on
concentration of both catalyst and ethanol. Phosphinefree catalytic systems operating with unprecedented activity at a
very low base loading to couple lower alcohols to higher alcohols of fuel and pharmaceutical importance are the salient
features of this report.
equivalent amount of waste.4b, c On the other hand, transition
metal catalyzed alkylation has enjoyed great success. The
reports include several homogeneous catalysts derived not only
from precious metals such as palladium,5 iridium,6 rhodium7
and ruthenium6b, d, j, 8 but also from cheap transition metals such
as copper,9 cobalt,10 manganese11 and nickel.12
Ruthenium complexes have performed exceedingly well in
catalyzing the alkylation of alcohols. Cho reported the
RuCl2(PPh3)3 catalyzed alkylation of secondary alcohols.8f The
Lau group have accomplished alcohol alkylation using
ruthenium complexes based on cyclopentadienyl (Cp),
hydrotris(pyrazolyl)borato (Tp), and bipyridine (Bipy) ligands.6b
They have attributed the catalytic activity to the formation of a
Ruhydrido species.6b Crabtree has demonstrated the excellent
catalytic activity of Cpruthenium complexes with chelating
Introduction
Carboncarbon bond forming reactions are pivotal to the
synthesis of several valuable precursors to fuel, fine chemicals,
agrochemicals, pharmaceutical and natural products.1 Well
explored approaches towards greener and sustainable CC
bond forming reactions involve catalytic CX (X = H, Cl, Br, I)
activation and subsequent cross coupling either involving
radical mechanisms2 or purely organometallic mechanisms.3
Recently, emphasis has shifted to the use of alcohols as
alkylating agents with water as the sole byproduct. The
reactivity is based on the concept of hydrogenborrowing that
involves tandem catalytic dehydrogenation, aldol condensation
and catalytic hydrogenation that leads to a net alkylation of
alcohol.
Nheterocyclic
carbene
(NHC)
ligands
towards
The alkylation of alcohols under metalfree conditions,4 are
limited by stoichiometric amounts of base that generates
alkylation of alcohols.6d Musa, Ackermann and Gelman have
a. Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati –
781039, Assam, India. Email: akshaikumar@iitg.ac.in
b. Department of Medicinal Chemistry, National Institute of Pharmaceutical
Education and Research Guwahati, Guwahati – 781101, Assam, India.
Email: hemantkrsri@gmail.com
c. Centre for Nanotechnology, Indian Institute of Technology Guwahati, Guwahati –
781039, Assam, India.
Electronic Supplementary Information (ESI) available: X-ray crystallographic
parameters, NMR spectra and computational data (PDF) and X-ray crystallo-graphic
data (CIF). CCDC 1986710 and 1986711. For ESI and crystallographic data in CIF or
other electronic format see DOI:
Figure 1. Efficient ruthenium catalysts for alkylation of alcohols.
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investigated the activity of pincerruthenium complexes based when compared with the best of the reporteVdiewruArtthicleenOinulimne
DOI: 10.1039/D0CY01679A
on triptycene framework in catalyzing the alkylation of catalysts (Figure 1). The complexes have been further utilized to
alcohols.6j
upgrade ethanol to butanol with good efficiency leading to
The Kundu group designed bifunctional Ru(II) catalysts conditions that give high conversions (ca. 58%) and rate (ca. 710
based on 2(2pyridyl2ol)1,10phenanthroline8e and TOh1).
6,6dihydroxy2,2bipyridine8d ligands for efficient catalytic
alkylation of alcohols. Apurba et.al. have utilized
a
Results and Discussions
Synthesis and Characterization of PincerRuthenium Complexes
Based on Bis(imino)pyridine and Based on 2,6Bis(benzimidazole-
2yl) Pyridine Ligands.
phosphinepyridone pincerruthenium(II) complex for the
catalytic synthesis of alkylated ketones via the cross coupling
of alcohols.8b Yu has studied the use of Ru(III) pincer catalysts
containing unsymmetrical pyridyl based Nheterocyclic ligands
for the alcohol alkylation.8c The Chen group have obtained
good TONs of alkylated ketones and alkylated alcohols by
the use of a Ru(II) bifunctional catalyst that consists of
The ligands (3af) were synthesized using a protocol that was
recently reported by us14a,c and others.14b Treatment of 3af
(Scheme 1) with dichloro(pcymene)ruthenium(II) dimer in
refluxing THF followed by introduction of an atmosphere of CO
provided the corresponding pincerruthenium(II) carbonyl
complexes (2af).
6hydroxy2,2bipyridine ligands with
a uncoordinated
pyridyl group.13 Chen subsequently improved the catalytic
efficiency by employing a ruthenium pyridonate complex that
contains a thiazolyl pendant group.8a
The newly synthesized pincerruthenium complexes (2ad)
were fully characterized by 1H NMR, 13C NMR, IR and HRMS(ESI)
analysis. While the carbonyl carbon of 2a appeared at 207.2
ppm, the corresponding carbonyl signals for 2b and 2c were
found around 205 ppm. The complexes 2d, 2e and 2f were
highly insoluble and NMR analysis was not possible. The IR
studies of 2a, 2b, 2c, 2d, 2e and 2f also revealed the presence
of carbonyl group and the prominent CO stretching frequency
was observed at 1934 cm1, 1944 cm1, 1952 cm1, 1954 cm1,
1965 cm1 and 1939 cm1 respectively. The HRMS(ESI) analysis
of the carbonyl complexes 2af revealed m/z values that
correspond to ions formed by loss of chloride ions
[(R2NNN)RuCl(CO)]+.
Figure 1 depicts the reported ruthenium catalysts that
demonstrate exceptional activity towards alkylation of
alcohols. Notably, the successful catalysts are based on
bidentate ligands and most of them have PPh3 ancillary ligands.
Following up on our recent success in the synthesis14a,c of
pincerruthenium complexes of the type (1ae) and their
application in efficiently catalyzing the Nalkylation15 and
glycerol dehydrogenation,14c we envisaged their utility in
accomplishing
related
tandem
reactions
involving
dehydrogenation.
In the current study, we report the synthesis of
pincerruthenium
bis(imino)pyridine
carbonyl
(2ad)
complexes
and
based
based
on
on
Attempts made to grow single crystals by slow diffusion of
hexane into the dichloromethane solution of these complexes
gave good quality crystals with 2b and 2c which were
characterized by Xray diffraction studies (Figure 3).‡ Both
complexes were found to adopt a slightly distorted octahedral
2,6bis(benzimidazole2yl) pyridine ligands (2ef) (Figure 2).
The activities of these phosphinefree complexes have been
compared with our previous reported catalysts (1ae)14a,c
towards catalytic alkylation of secondary alcohols. Among
the considered catalysts, (Cy2NNN)RuCl2(CO) (2b) provides
380000 TONs at 19000 TOh1 at 140 C that is unprecedented
Scheme 1. General synthetic route to (R2NNN)Ru(CO)Cl2(2af).
Figure 2. Pincerruthenium complexes investigated in the current study.
2 | J. Name., 2012, 00, 1-3
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provided good yields with good selectivity towards the
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DOI: 10.1039/D0CY01679A
alkylated alcohol 7 (entries 14, Table S2a). Interestingly, use
of sodium (5 mol %) to generate the base insitu15a (prior to
addition of 2b) also resulted in good yields of 7 (entry 5, Table
S2a). Relatively poor reactivity was observed upon use of K2CO3
and Na2CO3 (entries 6 and 7, Table S2a). The total yield of
products obtained with 2.5 mol % base loading was better than
the yield obtained with a base loading of either 5 mol % or 10
mol % (Entry 8 vs. entries 2 & 9, Table S2a; entry 2 vs. entries 1
& 3, Table 1). The total yield of products increased upon
lowering the loading of 2b to 0.025 mol % (entry 4, Table 1,
entry 13, Table S2a). Notably, the alkylation did not proceed
either in the absence of catalyst or in the absence of base
(entries 14 and 15, Table S2a). The rest of the catalytic reactions
were performed with 2.5 mol % NaOH as the total yield of
products was better with selectivity towards 7 comparable to
that obtained with KOtBu. Upon further lowering the 2b loading,
the total yields were comparable (entries 5 and 6, Table 1). The
apparent decrease in selectivity towards 7 at lower catalyst
loadings (Entries 36, Table 1) is attributable to secondary
dehydrogenation of 7 to 6 (vide infra). Gratifyingly, the
turnovers (ca. 372000) observed with 0.00025 mol % of 2b is
the highest reported yet with any ruthenium based catalytic
systems. Among the other variants of the carbonyl complexes,
while 2a (entry 11, Table 1) and 2c (entry 12, Table 1) gave
slightly lower yields in comparison to 2b (entry 6, Table 1), the
activity of 2d (entry 13, Table 1) was considerably lower.
Similarly, among the corresponding PPh3 analogues (1ad), the
complex 1d (entry 10, Table 1) performed poorly in comparison
with (1ac) (entries 79, Table 1).
Figure 3. ORTEP plot of 2b and 2c drawn at 50% probability. The H atoms have been
omitted for the sake of clarity.
geometry with the pincer ligand occupying meridional
configuration. In stark contrast to the structures of their
phosphine analogues (1b and 1c) which had the PPh3 molecule
trans to one of the chlorides,14 the carbonyl ligand in both 2b
and 2c are trans to pyridyl N and cis to both the chlorides. The
crystallographic data and the selected bond parameters around
the metal centre for both the complexes are provided in Table
S1a and Table S1b respectively.
Catalytic Activity of 2ad Towards Alkylation of Secondary
Alcohols.
The initial optimization of the catalytic conditions was
performed with 2b (0.05 mol %) for the alkylation of
1phenyl ethanol 5 with benzyl alcohol 4 in the presence of
various base (Table 1, S2a) at 140 C under solventfree
conditions. In the 2b (0.05 mol %) catalyzed alkylation of 5
with 4, bases (5 mol %) such as NaOtBu, KOtBu, NaOH and KOH
Table 1. Catalytic Alkylation of 5 with 4 Under Varying Conditions.a
Entry
Catalyst
(Y Mol %)b
Base
(X Mol %)b
% Yieldc
Total Yield
(%)
Total TON
7 Selectivityd (%)
4
5
6
7
1
2
3
4
5
6
7
8
9
10
11
12
13
2b (0.05)
2b (0.05)
2b (0.05)
NaOH, 5
NaOH, 2.5
NaOH, 10
NaOH, 2.5
NaOH, 2.5
NaOH, 2.5
NaOH, 2.5
NaOH, 2.5
NaOH, 2.5
NaOH, 2.5
NaOH, 2.5
NaOH, 2.5
NaOH, 2.5
0
0
0
0
0
0
0
0
0
0
0
0
0
10
7
10
14
16
1
2
1
2
2
12
7
63
79
65
70
63
73
78
81
75
65
78
78
57
85
93
85
97
99
93
87
89
86
80
89
89
62
1700
1860
1700
84
91
88
84
79
79
92
92
89
81
88
88
95
10
13
16
19
7
7
9
15
10
10
3
2b (0.025)
3880
2b (0.0025)
2b (0.00025)
1a (0.00025)
1b (0.00025)
1c (0.00025)
1d (0.00025)
2a (0.00025)
2c (0.00025)
2d (0.00025)
39600
372000
348000
356000
344000
320000
356000
356000
248000
1
1
2
aReaction conditions: 4.14 mmol of 4, 4.14 mmol of 5, X mol % of base and Y mol % of pincer-ruthenium. bMol % of base and catalyst is with respect to total alcohol
content (4+5). cYield is determined from 1H NMR using toluene as external standard. dSelectivity = Yield of 7/Total Yield.
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Figure 4. Time course of the reaction of benzyl alcohol (4) with 1phenyl ethanol (5) at 140 C catalyzed by (a) 0.025 mol % 2b, (b) 0.0025 mol % 2b. Also see Figure S1c.
Figure 4 depicts the time course of the alkylation of 5 with 4 electrondonating groups. This fortifies the fact that the overall
at different loadings of 2b. At a higher loading (0.025 mol %) of RDS is present in the dehydrogenation segment (Scheme 3 and
2b, the absence of an initial buildup of 6 (Figure 4a) indicates mechanistic studies vide infra). The alkylation of 1phenyl
that, the formation of 7 via 6 is very rapid (Scheme 3 and ethanol with aliphatic alcohols (7n, 7o, 7p and 7s; Table 2) and
mechanistic studies vide infra). The observed growth of 6 in primary alcohols containing heterocyclic groups (7d, 7e, 7f, 7l
minor amounts actually occurs through
a secondary and 7m; Table 2) proceeded with lower efficiency.
dehydrogenation of 7 catalyzed by 2b at a stage when its activity Mechanism of Alkylation of Alcohols Catalyzed by 1 and 2.
eventually levels off (Figure 4a). Evidence to this understanding Recently, we have shown that benzaldehyde (4) is formed in
could be obtained by slowing down the alkylation (Figure 4b the 1a catalyzed dehydrogenation of benzyl alcohol (4) along
and S1c). Accordingly, in the reaction carried out with 0.0025 with the liberation of hydrogen (eq. 1, Scheme 2).15a The
mol % 2b and 0.00025 mol % 2b, the formation of 6 was catalytic intermediates were also identified (Scheme 3a).15a
observed only after 3 h (Figure 4b) and 6 h (Figure S1c) of Similarly, we could expect the formation of acetophenone (5)
reaction respectively. Similar results were observed by starting from 1-phenyl ethanol (5). Several studies including
Guelcemal in Ir(I) catalyzed reactions.6h This points to the fact those of Kundu8d, e and Ding10a have shown that benzaldehyde
that in the 2b catalyzed alkylation of 5 with 4, the reacts with acetophenone in the presence of base to give the
hydrogenation and/or alcoholysis step leading to both 6 and 7 ,unsaturated ketone (6) (eq. 2, Scheme 2). The fact that we
are very fast compared to dehydrogenation of 4 and 5 (Scheme do not observe 6 indicates its fast transformation to product 7
3 and mechanistic studies vide infra). This implies that the via 6 which is catalyzed by the NNN pincer ruthenium catalysts
overall ratedetermining step (RDS) is in the dehydrogenation (1a-f and 2a-f). These observations form the basis of our
segment (Scheme 3, Figure 5 and mechanistic studies vide proposed mechanism (Scheme 3) using 2c as a model catalyst.
infra).6h Nevertheless, the initial rate of catalysis was very high The first step of the alkylation involves dissociation of either
(10972 TOh1 and 12000 TOh1) at 0.0025 mol % and 0.00025 PPh315a from 1c or CO15b-f from 2c to generate the 16electron
mol % loading of 2b respectively (Figure 4b and S1c). fivecoordinate dichloride species. In the presence of benzyl
Interestingly, according to the initial rate method, the plot of alcohol (4) and 1phenyl ethanol (5), saltmetathesis of
initial rate vs. [2b] was found to be linear indicative of the first 16electron fivecoordinate dichloride species with KOtBu
order dependence of the rate of the reaction on the catalyst results in the formation of 8 and 9 respectively.15a A hydride
concentration (Figure S1d).
elimination to give either benzaldehyde (4) from 8 or
The most efficient catalyst 2b was selected for alkylation of acetophenone (5) from 9 results in formation of RuH species
substituted 1phenyl ethanols with various benzyl alcohols 12 via TSs 10 or 11. This is followed by a bond metathesis
using 2.5 mol % NaOH at 140 C (Table 2). Electrondonating between RuH of 12 and OH of 4/5 as in TS 13/14 that results
groups such as methyl and methoxy either on benzyl alcohol or in liberation of H2 along with the regeneration of the active
on 1phenyl ethanol provided better yields of 7 (7b, 7c, 7i, 7k species 8/9 (Scheme 3a). The formation of 4 and H2 in these
and 7q; Table 2). On the other hand, substrates with reactions have been recently reported by us.15a
electronwithdrawing groups gave poor results (7a, 7g, 7j and The benzaldehyde (4) and acetophenone (5) formed in the
7r; Table 2). These observations indicate that the CH bond dehydrogenation segment react with each other in the
breaking step (hydride elimination) is hampered by presence of base to form the ,unsaturated ketone (6) (eq.
electronwithdrawing groups while being accelerated by 2, Scheme 2). The CC double bond in molecule 6 undergoes
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Table 2. Alkylation of Substituted 1Phenyl Ethanol with Various Benzyl Alcohols Catalyzed by 0.00025 Mol % of 2b.a
Entry
1
Product
7 Yieldb
[7 Selectivity]c
73 % [79:21]
Total TON
(6 + 7)
372000
Entry
12
Product
7 Yieldb
Total TON
(6 + 7)
316000
[7 Selectivity]c
70 % [89:11]
2
3
4
5
6
7
8
9
63 % [93:7]
85 % [94:6]
74 % [83:17]
368000
360000
356000
13
14
15
16
17
18
19
20
28 % [80:20]
21 %d [100: 0]
53 %d [99: 1]
30 %d [99: 1]
32 %d [91: 9]
73 % [88:12]
10 % [83:17]
19 % [99:1]
140000
840d
2120d
1200d
1400d
332000
40000
76000
38 % [93:7]
46 %d [88:12]
164000
2080d
6 % [55:45]
30 %d [83:17]
44000
1440d
3 % [75:25]
63 %d [97:3]
16000
2600d
50 % [91:9]
220000
248000
50 % [81:19]
10
11
80 % [84:16]
380000
21
22
77 % [95:5]
324000
144000
37 % [90:10]
48 %d [87:13]
160000
2200d
36 %
2 Diastereomers
[95:5]
aReaction conditions: 4.14 mmol of 4, 4.14 mmol of 5, 0.207 mmol of NaOH and 20 µmol of 2b. bYield is determined from 1H NMR using toluene as external standard.
cSelectivity = Yield of 7/Total Yield. dReaction performed with 0.025 mol % of 2b.
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an insertion into the RuH bond in 12 to generate the species
16 via TS 15 (Scheme 3b). Only the H transfer to the  carbon
is considered owing to steric reasons. The product 6 formation
step along with the generation of catalytically active species
12/8/9 could occur either by a bond metathesis between
RuC of 16 and OH of 4/5 via TS 18/19 (alcoholysis) or by
bond metathesis between RuC of 16 and HH while going
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through TS 17 (hydrogenation) (Scheme 3b).
A similar
hydrogenation and/or alcoholysis path can account for the
transformation of 6 to 7 (Scheme 3c). Kinetic studies (Figure 4,
vide supra) is indicative of a facile transformation of 6 to 7.
Furthermore, kinetic studies performed on the transformation
of pincerruthenium catalyzed upgradation of ethanol to
butanol provide evidence for the proposed mechanism (Figure
6).
Scheme 2. Evidence for the intermediates involved.
Quantum Mechanical Calculations on the NNN PincerRuthenium
(1,2) Catalyzed Alkylation of Alcohols.
Additional mechanistic information was obtained from DFT
studies by modelling the various transition states (TSs) and
intermediates involved in the pincerruthenium (1, 2) catalyzed
alkylation of 5 with 4 (Figure 5). The formation of
benzaldehyde (4) and the RuH species 12 via a hydride
elimination from 8 through TS 10 (ΔG⧧140 = 14.94 kcal/mol) is a
thermodynamically downhill process (ΔG140 = 4.56 kcal/mol)
(Blue lines, Figure 5a). Interestingly the corresponding
formation of acetophenone (5) is also downhill (ΔG140 = 6.59
kcal/mol) but with a higher barrier (TS:11, ΔG⧧
= 21.71
140
kcal/mol) (Magenta lines, Figure 5a). The steps involving the
bond metathesis of RuH in 12 with the OH of 4 (Blue lines,
Figure 5a) and with the OH of 5 (Magenta lines, Figure 5a) are
uphill (ΔG140 = 3.03 and 2.08 kcal/mol respectively) with almost
similar barriers (TS:13, ΔG⧧140 = 21.55 kcal/mol and TS:14, ΔG⧧
140
= 20.97 kcal/mol). For the dehydrogenation segment, the
hydride elimination step involving the extrusion of
acetophenone (5) is the rate determining step (RDS).
The insertion of the double bond in the ,unsaturated
ketone (6) into the RuH in 12 to give species 16 via TS 15 is
slightly uphill (ΔG140 = 1.60 kcal/mol) with a barrier of 12.23
kcal/mol. In comparison to the alcoholysis step (brown lines,
Figure 5b), the hydrogenation step (purple lines, Figure 5b) is
more favorable as it is not only more downhill by 3.5 kcal/mol
but also has lower barrier (TS:17, ΔG⧧140 = 16.54 kcal/mol). Thus,
in the
Scheme 3. Plausible mechanism involved in the 2 catalyzed Alkylation of 1phenyl
ethanol with benzyl alcohols.
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transformation of 6 to 6, the hydrogenationVisewteAprticliesOntlhinee
DOI: 10.1039/D0CY01679A
preferred pathway and it is the RDS.
The first step in the conversion of 6 to 7 that involves the
insertion of the carbonyl group of 6 into RuH in 12 (Figure 5c)
is a thermodynamically downhill process (ΔG140 =  21.12
kcal/mol) and proceeds with a low barrier (TS:20, ΔG⧧140 = 5.15
kcal/mol). The alcoholysis step (brown lines, Figure 5c) is more
favored than the hydrogenation step (purple lines, Figure 5c)
owing to its lower barrier (TS:23, ΔG⧧ = 4.70 kcal/mol). The
140
insertion of 6 into the RuH bond in 12 is the RDS for the
conversion of 6 to 7. For the 1 and 2 catalyzed alkylation, the
hydride elimination step involving the extrusion of 5 is the
overall RDS. This satisfactorily explains the absence of 6 during
the initial time in the alkylation (Figure 4b and S1c).
Application of NNNPincer Catalyzed Alkylation of Alcohols
towards Upgradation of Ethanol.
The fast depleting fossil fuels in combination with the
everincreasing energy demand has triggered an intense
pursuit for alternative energy sources.16 In comparison to
conventional gasoline, ethanol is widely accepted to be a
sustainable fuel.17 However, use of ethanol has several
drawbacks, as it has poor energy density (70% with respect to
gasoline),16b, 17a, 18 it is corrosive in nature towards engines19
and has higher water absorptivity.18a, 20 These limitations can be
circumvented by the use of butanol, which not only has a higher
energy density (86%), but also is noncorrosive while being
21
immiscible in water.19,
Not surprisingly, the Guerbet
reaction22 that involves the transformation of bioethanol to
butanol has been widely explored.23 Heterogeneous catalysts
often require harsh conditions and suffer from selectivity
issues.24 However, these methods that upgrade ethanol to
either butanol or higher analogues with higher energy
density18b are a subject of great interest.
The last decade has witnessed development of several
homogenous catalysts for Guerbet reaction. In 2009, Ishii
reported that [Ir(COD)(acac)]/dppp complex catalyzes the
ethanol to butanol transformation in presence of
1,5cyclooctadiene.25 Notably, the higher butanol selectivity
(up to 67%) could be obtained by use of [Ir(COD)(acac)]/dppb in
the presence of sodium ethoxide. In 2015, Jones group have
reported a bifunctional catalytic system comprising of iridium
and a base comprising of a nickel/copper complex for the
upgradation of ethanol.26b Wass and coworkers have reported
that the use of trans[RuCl2(dppm)2] in presence sodium
ethoxide (5 mol %), resulted in 1300 TONs with 91% selectivity
towards the nbutanol formation.23b The same group has
demonstrated ruthenium 6complex with varying bidentate
ligands to obtain high selectivity (99%) at 98 TONs. At the
highest productivity (314 TON) of nbutanol, the selectivity
drops to 93%.23c
Szymczak reported a NNNRu pincer catalyst for butanol
formation at an ethanol conversion of 53% with 78% selectivity
towards nbutanol (TON 530).23a Milstein and coworkers
investigated PNP-Ru complexes for the Guerbet reaction, which
converted 73.4% ethanol (ca. 18000 TONs) to the product.26
Recently, Liu developed a highly efficient (114120 TONs) PNP
Figure 5. Relative free energies (140 °C) of intermediates involved in pincer-ruthenium
catalyzed (a) dehydrogenation of 4/5 (b) transformation of 6 to 6 and (c) conversion of
6 to 7.
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manganese pincer to upgrade ethanol to higher alcohol in the
Table 3. The Upgradation of Ethanol (4v) to nButanol (4p) Catalyzed by Phosphine-Free
Pincer-Ruthenium Complexes.a
View Article Online
presence of NaOEt.27 Using the same coDmOpI:le10x.1b0u3t9/aDt0CaYh0i1g6h79eAr
NaOEt loading, the Jones group demonstrated a drop in
catalytic activity (145 TONs).27 In this context, we wished to
investigate the catalysts (1af and 2af) towards upgradation
of ethanol to nbutanol.
The lower reactivity (Table 2) of aliphatic alcohols towards
catalytic alkylation prompted us to revisit optimization
conditions for the upgradation of ethanol using 2b (Table S2c)
at 140 C. In the 2b (0.05 mol %) catalyzed reactions, very high
turnovers (ca. 335, entry 14, Table S2c) were obtained with 90%
selectivity towards nbutanol after 24 h using a protocol
following our recent approach15a of generating the base insitu,
where 10 mol % Na was used to generate 10 mol % NaOEt (prior
to 2b addition). Upon continuing the reaction to 48 h, the
productivity was further enhanced with no loss of selectivity
(entry 15, Table S2c). This condition was used for further
optimization with various catalysts (Table 3, Figure S57S68).
Entry
1
Catalyst
Time
(h)
% Butanol
Selectivityb
80
Total
TONsb
60
0.5
N
PPh3
(1a)
Ru Cl
N
Cl
72
70
355
N
2
3
4
0.5
72
90
75
90
75
95
90
N
PPh3
N
Ru Cl
(1b)
Cl
450
160
395
170
N
0.5
72
N
PPh3
N
Ru Cl
(1c)
Cl
Table 4. The Upgradation of Ethanol (4v) to nButanol (4p) Catalyzed by Varying
Amounts of 2e. a
N
0.5
N
PPh3
N
Ru Cl
(1d)
Cl
N
72
75
495
5
6
7
0.5
72
80
70
60
HN
NH
N
G
Cl
Entry
1
Catalyst
0.025
Time (h)
0.5
Butanol
Selectivity[b]
98%
Total
TONs[b]
336
N
Ru
N
G
1115
Cl
(1e) G = PPh3
0.5
72
65
60
420
620
N
N
N
G
Cl
72
0.5
72
70%
90%
65%
85%
80%
95%
2100
165
1670
355
N
Ru
N
G
Cl
2
3
4
0.035
0.05
(1f) G = PPh3
0.5
72
75
65
150
215
N
0.5
72
Cl
(2a)
N
Ru CO
975
Cl
N
0.075
0.5
255
8
9
0.5
72
90
80
90
75
95
135
335
90
72
0.5
72
70%
98%
75%
737
223
582
N
Cl
5
0.1
N
Ru CO
(2b)
Cl
N
aReaction conditions: 8.56 mmol of ethanol, 0.86 mmol of base and 0.05 mol % of
2e. bSelectivity and TON were calculated by GC analysis using toluene as an internal
standard.
0.5
72
N
Cl
N
Ru CO
(2c)
Cl
520
105
N
The ethanol upgradation was first tested for a series of NNN
pincerruthenium complexes containing PPh3 ancillary ligands
(entry 16, Table 3, Figure S57S62). Among (1a1f), while the
catalyst 1e exhibited the highest (3.1 folds) productivity (1115
TONs after 72 h, entry 5, Table 3), the Nmethyl analogue 1f
demonstrated the highest rate (ca. 840 TOh-1). The
phosphinefree complexes (2af) were also tested as catalysts
for the Guerbet reaction (entry 712, Table 3, Figure S63S68).
In comparison to 1a and 1b, their phosphinefree counterparts
2a and 2b exhibited a better rate towards the ethanol
upgradation reaction (entries 7 and 8, Table 3). The rate of
ethanol upgradation catalyzed by 2c and 2d were comparable
(entries 9 and 10, Table 3) and about 1.31.5 times slower that
2a and 2b. The rate of 2f catalyzed (entry 12, Table 3) Guerbet
10
0.5
N
Cl
Ru CO
N
(2d)
Cl
N
72
75
340
11
12
0.5
72
85
80
75
355
975
180
HN
NH
N
Cl
Cl
N
Ru
N
Cl
CO
(2e)
0.5
N
N
N
N
Ru
N
Cl
CO
72
75
385
(2f)
a Reaction conditions: 8.56 mmol of ethanol, 0.86 mmol of base and 0.05 mol % of
2. bCalculated by GC analysis using toluene as an internal standard.
8 | J. Name., 2012, 00, 1-3
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reaction was two folds faster than that catalyzed by 2c and 2d. (Figure 6b) was linear. This points to the first order dependence
View Article Online
DOI: 10.1039/D0CY01679A
Among all the catalysts screened, the catalyst 2e demonstrated of rate on concentration of both 2e and ethanol. A similar first
not only very good rate (ca. 710 TOh-1) but also high productivity order dependence on [2b] was observed for the alkylation of
(975 TONs at 49% ethanol conversion) (entry 11, Table 3). 5 (Figure S1d). Not surprisingly these observations indicate the
Hence, further exploration involving the variation of catalyst involvement of a mechanistic path as depicted in Scheme 3 with
loading for Guerbet reaction was carried out using the a rate = (k2)[(iPr2NNN)RuClH][EtOH] (equation (1), Scheme S1).
phosphinefree catalyst 2e (Table 4).
Conclusions
We report here the synthesis and characterization of a series of
phosphinefree NNN pincerruthenium carbonyl complexes
based on bis(imino)pyridine and 2,6bis(benzimidazole2yl)
pyridine ligands. These complexes have been utilized with great
success to execute the catalytic alkylation of alcohols under
solventfree conditions. For the alkylation of 1phenyl
ethanol with benzyl alcohol at 140 C, (Cy2NNN)RuCl2(CO)
(0.00025 mol %) in combination with NaOH (2.5 mol %) was
found to be the most efficient among the considered catalysts.
Gratifyingly, at such a low loading of the catalyst and the base,
the alkylated product was obtained in high yield (ca. 93%,
372000 TONs at 12000 TOh1) after 20 h. A variety of alcohol
combinations could be alkylated by employing this catalytic
system. Unprecedented activities (380000 TONs at 19000
TOh1) were observed for the alkylation of 1phenyl ethanol
with 3methoxy benzyl alcohol. Hitherto, this is the highest
reported rate and turnovers for a ruthenium based catalyst.
Mechanistic studies provide key information about the
presence of the overall rate determining step (RDS) in the
dehydrogenation segment. Complementary evidence was
obtained from DFT studies that indicate the hydride
elimination step involving the extrusion of acetophenone is the
overall RDS. While the hydrogenation step is favored for the
formation of alkylated ketone, the alcoholysis step is
preferred for the formation of alkylated product.
The catalytic alkylation protocol was applied to investigate
the upgradation of ethanol to biofuels such as butanol and
higher alcohols. Use of (Cy2NNN)RuCl2(CO) resulted in high
productivity (335 TONs at 170 TOh1) among the
pincerruthenium
bis(imino)pyridine ligands. By employing sterically more open
pincerruthenium carbonyl complexes such as
Bim2NNN)RuCl2(CO) and MeBim2NNN)RuCl2(CO) based on
carbonyl
complexes
based
on
(
(
2,6bis(benzimidazole2yl) pyridine ligands, the catalytic
activity was greatly enhanced. It is gratifying to note that we
have arrived at conditions that give not only good ethanol
conversion (ca. 58%) but also high turnovers (ca. 2100) with a
good rate (ca. 710 TOh1) in the (Bim2NNN)RuCl2(CO) catalyzed
upgradation of ethanol. Kinetic studies on the ethanol
upgradation catalyzed by (Bim2NNN)RuCl2(CO) exhibit linear
dependence of rate on the concentration of both catalyst and
ethanol which is in strong agreement with the DFT studies.
Figure 6. Plot depicting the (a) initial rate vs. [2e] and (b) initial rate vs. [Ethanol].
The highest rate (ca. 710 TOh-1) was obtained in 0.05 mol % 2e
catalyzed reaction (entry 3, Table 4). While the highest
turnovers (ca. 2100) were obtained at 0.025 mol % loading of
2e (entry 1, Table 4), the highest conversion (ca. 58%) of ethanol
to products was obtained upon use of either 0.035 mol % (entry
2, Table 4) or 0.1 mol % (entry 5, Table 4) of 2e. Kinetic
experiments were carried out on the Guerbet reaction
catalyzed by 2e at 140 C at varying catalyst loading (Figure
S69S73) and varying ethanol concentration (Figure S74S77).
With the aid of initial rate method, it was observed that the plot
of initial rate vs. [2e] (Figure 6a) and initial rate vs. [ethanol]
Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
A.K. is grateful to Science and Engineering Research Board,
Department of Science and Technology (Grant DSTSERB
CRG/2018/000607), Scheme for Transformational and
Advanced Research in Sciences, Ministry of Human Resource
Development,ꢀimplemented by Indian Institute of Science,
Bangalore (Grant STARS/APR2019/CS/629/FS). H.K.S thanks the
Science and Engineering Research Board, Department of
Science and Technology (Grant DSTSERB SB/S2/RJN/004-
2015). K.D and E.Y. thank IITG for research fellowship. We
acknowledge
the
DSTFIST
program,
ParamIshan
supercomputing facility at IITG, Department of Chemistry at
IITG and CIFIITG for various computing and instrumental
facilities.
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