Ruthenium p-cymene complexes with α-diimine ligands as catalytic precursors for the transfer hydrogenation of ethyl levulinate to γ-valerolactone

Article abstract of DOI:10.1039/c8nj03569e

The ruthenium compounds [(η⁶-p-cymene)RuCl{κ²N-(HCNR)₂}]NO₃ (R = 4-C₆H₄Me, [1]NO₃; 4-C₆H₄OH, [2]NO₃; C₆H₁₁═Cy, [3]NO₃; 4-C₆H₁₀OH, [4]NO₃; ^tBu, [5]NO₃) were prepared in high yields from [(p-cymene)RuCl₂]₂, AgNO₃ and the appropriate α-diimine. Compounds [2]PF₆ and [4]PF₆ were obtained by a straightforward reaction of [(η⁶-p-cymene)RuCl(MeCN)_0.66]PF₆, [6]PF₆, with α-diimine, whereas [4]BPh₄ was obtained by metathesis between [4]NO₃ and NaBPh₄. All the ruthenium products were characterized by analytical methods, IR, NMR and UV-Vis spectroscopy; in addition, the structure of [1]NO₃ was ascertained by an X-ray diffraction study. Compounds [1-4]NO₃, [4]PF₆ and [4]BPh₄ were investigated as catalytic precursors in the transfer hydrogenation reaction of ethyl levulinate to γ-valerolactone in isopropanol solution under microwave irradiation. [4]BPh₄ was revealed to be the best catalytic precursor, affording γ-valerolactone in 62% yield under optimized experimental conditions.

Full text of DOI:10.1039/c8nj03569e

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New J. Chem., 2018, DOI: 10.1039/C8NJ03569E.
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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Ruthenium pꢀCymene Complexes with αꢀDiimine Ligands as Catalytic
Precursors for the Transfer Hydrogenation of Ethyl Levulinate to γꢀ
Valerolactone
Lorenzo Biancalana,a,b Sara Fulignati,a,b Claudia Antonetti,a,b Stefano Zacchini,b,c Giacomo Provinciali,a
Guido Pampaloni,a,b Anna Maria Raspolli Galletti a,b,* and Fabio Marchetti a,b,*
a Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via G. Moruzzi 13, I-56124 Pisa,
Italy.
b CIRCC, via Celso Ulpiani 27, I-70126 Bari, Italy.
c Dipartimento di Chimica Industriale “Toso Montanari”, Università di Bologna, Viale Risorgimento 4,
I-40136 Bologna, Italy.
Corresponding Authors
*Eꢀmail addresses: anna.maria.raspolli.galletti@unipi.it; fabio.marchetti1974@unipi.it.
1

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Abstract
The ruthenium compounds [(η6ꢀpꢀcymene)RuCl{κ2Nꢀ(HCNR)2}]NO3 (R = 4ꢀC6H4Me, [1]NO3; 4ꢀ
C6H4OH, [2]NO3; C6H11 = Cy, [3]NO3; 4ꢀC6H10OH, [4]NO3; tBu, [5]NO3) were prepared in high yields
from [(pꢀcymene)RuCl2]2, AgNO3 and the appropriate αꢀdiimine. Compounds [2]PF6 and [4]PF6 were
obtained by straightforward reaction of [(η6ꢀpꢀcymene)RuCl(MeCN)0.66]PF6, [6]PF6, with αꢀdiimine,
whereas [4]BPh4 was afforded by metathesis between [4]NO3 and NaBPh4. All the ruthenium products
were characterized by analytical methods, IR, NMR and UVꢀVis spectroscopy; in addition, the structure
of [1]NO3 was ascertained by a Xꢀray diffraction study. Compounds [1ꢀ4]NO3, [4]PF6 and [4]BPh4 were
investigated as catalytic precursors in the transfer hydrogenation reaction of ethyl levulinate to γꢀ
valerolactone in isopropanol solution, under microwave irradiation. [4]BPh4 revealed to be the best
catalytic precursor, affording γꢀvalerolactone in 62% yield under optimized experimental conditions.
Keywords: biomass transformation, γꢀvalerolactone, ethyl levulinate, ruthenium arene complexes, αꢀ
diimine ligand, transfer hydrogenation
Introduction
Nowadays, the continuous depletion of fossil resources and the impact of environmental pollution due to
carbon emission make necessary the search for renewable resources [1]. Biomass is an attracting
alternative feedstock, being abundant, widespread, cheap and useful to produce valuable chemicals. In
particular, γꢀvalerolactone (GVL) can be accessed from lignocellulosic biomass through hydrogenation
of levulinic acid (LA) and related esters (including ethyl levulinate, EL, Scheme 1) [2], and it possesses
notable properties (high boiling and flash points, low melting point and low vapor pressure) which make
it a safe compound for industrial application and have spread its use over a range of applications, i.e. as
a green solvent, biofuel, and sustainable starting material to obtain a variety of derivatives [2b,e,h,3].
2

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Scheme 1. Structures of levulinic acid (LA), ethyl levulinate (EL) and γꢀvalerolactone (GVL).
The hydrogenation process of LA or its esters, such as EL, to GVL has aroused a considerable attention
on both academic and industrial level. Respect to LA, EL is attracting increasing interest since it can be
directly synthesized in high yields from raw biomasses working in ethanol. This approach, due to its low
cost and simple technology, is in alignment with the current tendency to sustain the development of
green chemistry industry, highlighting the possibility of biomass utilization for the oneꢀpot synthesis of
EL. Moreover, unlike LA, EL possesses relatively low boiling point, easy recovery and acidꢀfree
characteristics, all properties that make this compound an attractive substrate for the synthesis of GVL
[4].
The hydrogenation of LA and its esters to GVL has been carried out employing different hydrogen
sources: molecular hydrogen [2hꢀj,2l,5], formic acid [5a,6] and alcohols [5a,7]. Recently, these two last
alternative hydrogen sources are attracting great attention because they allow the conduction of the
reaction under more safety and economic reaction conditions, due to the absence of molecular hydrogen
as reactant. In fact, in these cases hydrogen can be generated in situ from the catalytic decomposition of
formic acid [6a] or the reduction of the substrate can proceed through a catalytic transfer hydrogenation
mechanism (TH) due to the interaction between an alcohol and the catalyst [7h,8]. This last method can
be considered a green approach because the adopted alcohol acts both as solvent and hydrogen source.
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In this regard, in this work the TH of EL to GVL has been investigated using 2ꢀpropanol as the
hydrogen source, due to the better performances of secondary alcohols than primary ones [7m]. Both
heterogeneous and homogeneous catalysts have been reported in the literature for TH of LA and/or EL
to GVL. The first ones are generally represented by supported precious metals [7jꢀm] and zirconium
compounds [7eꢀh,n]. Regarding the homogeneous catalysts, the most important examples are the
complexes of ruthenium [2c,5a,9] iridium [9a,10], palladium [11] and iron [5a,6b,7d]. Typically,
homogeneous catalysts have high catalytic efficiency with, generally, high turnover numbers. However,
up to now, only Dai et al. [7d] have investigated the synthesis of GVL from ethyl levulinate in alcoholic
medium employing homogeneous catalysts. They tested a wide range of iron complexes and founded
the best performances in the presence of iron(0)ꢀcyclopentadienoneꢀtricarbonyl system. The highest
GVL yield reported by Dai et al., equal to 95%, was ascertained working with the catalyst amount of 1
mol% and the NaHCO3 base amount of 5 mol%, both respect to the substrate, carrying out the reaction
in 2ꢀpropanol at 100 °C for 19 hours starting from a 2 M ethyl levulinate solution. Despite the good
GVL yield obtained by the authors, the reaction conditions present some drawbacks, such as the high
amount of base and the very long reaction time. In this work, for the first time, arene ruthenium(II)
complexes are investigated as catalytic precursors for the synthesis of GVL from EL through the TH
approach, highlighting the potentiality of these complexes to be employed in an integrated bioꢀrefinery
approach. Ruthenium(II) arene complexes constitute a family of widely investigated compounds for
their possible catalytic [12] and biological applications [13]. In particular, they have been intensively
studied in the last decade as catalysts for hydrogenation reactions of ketones [14], following the
breakthrough offered by the catalytic systems developed by Noyori [15], but not yet employed for TH of
EL. Moreover, for the first time, microwave heating was employed in the TH of EL to GVL in the
presence of homogeneous systems. In fact, up to now, microwaves have been employed only for the TH
of LA in the presence of heterogeneous catalysts, such as Pd/C and Ru/C [7a,16] and for the TH of
acetophenone with arene ruthenium(II) complexes [14e,17], proving that they are active also under
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microwave heating. It has to be remarked that the microwave treatment is a powerful tool favoring
process efficiency, fast temperature increase, short reaction times and energy saving [18].
Results and discussion
1) Synthesis and characterization of Ru compounds.
αꢀDiimine ligands, (R)N=CHCH=N(R) (R = 4ꢀC6H4Me, L1; 4ꢀC6H4OH, L2; C6H11 = Cy, L3; 4ꢀ
C6H10OH, L4; tBu, L5), were prepared by condensation of glyoxal with the appropriate primary amine,
according to literature procedures (see Scheme 2a and Supporting Information) [19,20]. The
unprecedented crystal structure of L2 was ascertained by a Xꢀray diffraction study: an ORTEP view is
shown in Figure 1, while relevant bonding parameters are given in Table 1. The molecular structure of
L2 resembles those previously reported for analogous αꢀdiimine molecules, concerning the bonding
parameters and the overall geometry [21]. Within the crystals, interꢀmolecular Hꢀbonds are formed
between the OH and N groups [O(1)ꢀH(1A) 0.84 Å, H(1A)···N(1)#2 2.01 Å, O(1)···N(1)#2 2.8392(17)
Å, <O(1)H(1A)N(1)#2 169.9 Å].
Figure 1. View of the structure of L2. Displacement ellipsoids are at the 50% probability level. Symmetry
transformation used to generate equivalent atoms: #1 ꢀx+2, ꢀy+1, ꢀz+1
Table 1.Selected bond distances (Å) and angles (°) for L2.
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C(1)−C(1)#1
C(2)−N(1)
1.450(3)
C(1)−N(1)
1.287(2)
1.4192(19)
C(5)−O(1)
1.3619(18)
119.54(13)
N(1)−C(1)−C(1)#1 119.93(17)
C(1)−N(1)−C(2)
Symmetry transformation used to generate equivalent atoms: #1 ꢀx+2, ꢀy+1, ꢀz+1
The air stable ruthenium complexes [(η6ꢀpꢀcymene)RuCl{κ2Nꢀ(HCNR)2}]NO3 (R = 4ꢀC6H4Me, [1]NO3;
t
4ꢀC6H4OH, [2]NO3; C6H11 = Cy, [3]NO3; 4ꢀC6H10OH, [4]NO3; Bu, [5]NO3) were synthesized upon
treatment of [(pꢀcymene)RuCl2]2 with AgNO3 in the appropriate solvent (acetonitrile or methanol) [22],
followed by addition of the αꢀdiimine L1ꢀL5 (Scheme 2b) [23]. Chloride abstraction is an essential step
to obtain [1ꢀ5]NO3, the direct interaction of [(pꢀcymene)RuCl2]2 with L1/L3 otherwise leading to a
mixture of different products (see SI for details). The synthesis of [2ꢀ4]NO3 has been already reported
[20], instead [1]NO3 (90% yield) and [5]NO3 (92% yield) are unprecedented.
Scheme 2. Synthesis of αꢀdiimine ligands L1ꢀL5 and related ruthenium complexes [1ꢀ5]NO3.
The counterion is expected to play a fundamental role in the solution chemistry, and presumably in the
catalytic behavior, of cationic Ru(II) arene complexes. In particular, it was previously demonstrated that
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the nature of the anion associated to [(arene)RuCl(αꢀdiimine)]+ [24] could be responsible for the
formation of ion pairs and higher aggregates in organic solvents [24b,25].
On account of this feature, we synthesized new salts of the complexes [2]+ and [4]+, containing anions
different from nitrate. In order to obtain [2]PF6 and [4]PF6, a twoꢀstep procedure similar to that used for
the nitrate homologues (Scheme 2) was adopted. Thus, [(pꢀcymene)RuCl2]2 was allowed to react with
NH4PF6 in acetonitrile, and the resulting solvatoꢀspecies, [6]PF6, was then isolated as an orange solid
(Scheme 3a). This material did not correspond to the expected [(η6ꢀpꢀcymene)RuCl(MeCN)2]PF6 [26],
but revealed to be a mixture of Ru compounds with overall MeCN/Cl and MeCN/pꢀcymene ratios =
1
0.66 (Cl analysis, H NMR). Accordingly, [6]PF6 is conveniently represented by the formula [(η6ꢀpꢀ
cymene)RuCl(MeCN)0.66]PF6. The use of CH2Cl2 in the workꢀup and the lability of the acetonitrile
ligands [26c] may justify this result (see Experimental).
The reactions of [6]PF6 with L2 in MeOH at ambient temperature or with L4 in refluxing MeCN
afforded [2]PF6 and [4]PF6, respectively, in 80ꢀ90% yields (Scheme 3b).
Scheme 3. Synthesis of rutheniumꢀαꢀdiimine complexes [2ꢀ4]PF6 via the acetonitrile intermediate [6]PF6.
Compound [4]BPh4 was obtained by anion metathesis between [4]NO3 and NaBPh4 in acetone at
ambient temperature (eq. 1), and then isolated in 89% yield after workꢀup.
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[4]NO3 + NaBPh4 → [4]BPh4 + NaNO3
(1)
The novel rutheniumꢀαꢀdiimine products were isolated as airꢀstable orange ([4]X, X = PF6, BPh4) or
dark brown ([1]NO3, [2]PF6) crystalline materials, whereas hygroscopic orangeꢀbrown [5]NO3 was
stored under nitrogen. The complexes were characterized by analytical (CHN analysis, conductivity)
and spectroscopic (NMR/IR/UVꢀVis) methods. Selected data of all Ru compounds [1ꢀ5]X are compiled
in Table S1, and compared to those of the related αꢀdiimines L1ꢀL5.
The solidꢀstate IR spectra of [1ꢀ5]X (X = NO3, PF6) show strong absorptions due to the vibrations of the
−
−
anion (ca. 1320 cm−1 and 830 cm−1 for NO3 and PF6 , respectively) and a medium/weak absorption in
the 1530ꢀ1630 cmꢀ1 region due to the antiꢀsymmetric stretching of the N=CC=N moiety [27]. This band
is substantially decreased in intensity, but not in wavenumber, on going from L1, L2, L5 to the
respective Ru complexes. Conversely, αꢀdiimines with cyclohexyl rings (L3, L4) undergo a marked
decrease in the wavenumber of the N=CC=N group upon coordination (ꢁν = − 85 cm−1). The UVꢀVis
spectra of [1ꢀ5]X (X = NO3, PF6, BPh4) in CH2Cl2 or MeOH display MLCT bands around 270ꢀ280 and
420ꢀ450 nm [24a]. Enhanced intensity (ε) and additional absorption at ca. 550 nm were observed for
[1]NO3 and [2]X (X = NO3, PF6), comprising an extended πꢀsystem on the αꢀdiimine.
In the NMR spectra, the HC=N unit within [1ꢀ5]+ manifests itself with a 1H signal at 8.3ꢀ8.7 ppm and a
13C signal around 164ꢀ169 ppm, the latter being significantly deshielded (ꢁδC = 4ꢀ10 ppm) on
coordination. The PF6─ ion in [2]+ and [4]+ salts gives rise to a heptet at ꢀ145 ppm and a doublet around ꢀ
70 ppm in the 31P and 19F spectra, respectively.
1
The H NMR resonances of [2]PF6 in DMSOꢀd6 closely match those of the analogous nitrate salt,
however a more evident line broadening can be observed for the latter even at high temperatures (60°C)
[20]. This outcome can be explained on the basis of ion association phenomena [25,24b], which are
─
─
1
significant in DMSO for [2]+ꢂꢂꢂNO3 but not for [2]+ꢂꢂꢂPF6 . On the other hand, the H NMR spectra of
the Nꢀcyclohexanol complex [4]+ always feature sharp resonances, regardless of the solvent used
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─
─
─
1
(DMSOꢀd6, CD3OD or CD2Cl2) and the counter anion (NO3 , PF6 or BPh4 ). Nevertheless, while H
spectra of [4]NO3 and [4]PF6 are almost identical in CD3OD, the resonances of [4]BPh4 are all
considerably highꢀfield shifted in this solvent, especially those related to the imine CH (7.63 ppm; ꢁδH =
─
−0.70 ppm vs. NO3 /PF6─ salts) and arene CH (ꢁδH ≈ −0.15 ppm) groups. In the case of [5]NO3, the 1H
resonances due to the pꢀcymene ligand are broad at 25 °C in both CDCl3 and D2O solution, while being
narrower on increasing temperature (up to 55 °C in CDCl3). This behavior could be the consequence of
hindered rotation of the tertꢀbutyl groups, limiting the free rotation of the pꢀcymene ligand around the
metalꢀcentroid axis at 25 °C [28].
The solid state structure of [1]NO3 was elucidated by singleꢀcrystal Xꢀray diffraction: an ORTEP view
of the cation is shown in Figure 2, while relevant bonding parameters are given in Table 2. Complex
[1]+ comprises the expected threeꢀleg pianoꢀstool geometry typical of other Ru(II)ꢀarene compounds
[29], and the bonding parameters around the Ru(II) centers are similar to those reported for [(pꢀ
cymene)RuCl(αꢀdiimine)]+ structures [20,24].
.
Figure 2. View of the structure of the cation [1]+ within [1]NO3. Displacement ellipsoids are at the 50% probability
level. Hydrogen atoms are omitted for clarity.
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Table 2. Selected bond distances (Å) and angles (°) for [1]+.
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Ru(1)ꢀ(η6ꢀpꢀcymene)av
Ru(1)−N(1)
2.209(10)
2.065(3)
1.288(5)
1.428(5)
1.436(6)
76.50(13)
115.7(4)
116.0(3)
Ru(1)−Cl(1)
Ru(1)−N(2)
N(2)−C(12)
N(2)−C(20)
2.3789(10)
2.064(3)
1.284(5)
1.429(5)
N(1)−C(11)
N(1)−C(13)
C(11)−C(12)
N(1)−Ru(1)−N(2)
N(1)−C(11)−C(12)
C(12)−N(2)−Ru(1)
Ru(1)−N(1)−C(11) 115.9(3)
C(11)−C(12)−N(2) 115.9(4)
Compounds [1ꢀ5]X (X = NO3, PF6) are soluble in water; the solubility was assessed in saturated D2O
1
solutions at 21°C by H NMR (Table S2). As expected, the presence of hydroxyl groups on the αꢀ
diimine ligand enhances water solubility (e.g., compare [1]NO3 with [2]NO3). However, the nature of
the anion determines a more significant effect. Indeed, the solubility of [2]+ and [4]+ increases by 5ꢀfold
−
−
and 10ꢀfold, respectively, on moving from the PF6 salt to the corresponding NO3 salt; instead [4]BPh4
is almost insoluble in water, despite carrying OH groups on the αꢀdiimine. Similarly, the substituents on
the αꢀdiimine and the anion strongly affect the solubility in organic solvents. Thus, [2]+ and [4]+, bearing
−
hydroxyl groups on the αꢀdiimine, are soluble as NO3 salts only in polar solvents (DMSO, water,
alcohols), while [1,3,5]NO3, lacking of OH groups, and also [4]BPh4, are much better soluble in
chlorinated solvents.
2) Catalytic study
The hydrogenation reactions of ethyl levulinate in isopropanol were carried out in a microwave reactor
under magnetic stirring, and the resulting solutions were analyzed by gas chromatography. A first
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selection of Ru catalytic precursors included [1ꢀ4]NO3, while [5]NO3 was excluded due to its
hygroscopicity.
It is generally assumed that the transfer hydrogenation reaction of ketones catalyzed by Ru(II) arene
compounds proceeds through the intermediate formation of rutheniumꢀhydride species, initiating the
catalytic cycle [30]. In general, a base activator is needed to achieve best performances, presumably
allowing the conversion of the catalytic precursor into the active [RuꢀH] form [31]. Nevertheless, some
efficient catalytic systems have been reported, being able to generate the key hydride intermediate under
base free conditions [32].
In the light of this preamble, in order to see whether in our case the use of a base was advantageous or
not, preliminary reactions were carried out with [3]NO3 as catalytic precursor, respectively in the
presence and in the absence of 1 mol% NaOH (Figure 3) at 140 °C for 30 min.
Figure 3. Effect of NaOH (1 mol% respect to EL) on ethyl levulinate (5 mmol) to γꢀvalerolactone conversion in 2ꢀ
propanol (10 mL), using [3]NO3 (0.1 mol% respect to EL) as catalytic precursor. MW heating, T = 140 °C, t = 30
min.
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The results indicate that NaOH supplies substantially higher conversion and selectivity (about 22% and
64%, respectively). Then, a screening was performed of the different ruthenium compounds as potential
catalytic precursors, and the results are reported in Table 3.
Table 3. Catalytic performance of [1-4]NO3 (0.1 mol% respect to EL) in the conversion of ethyl levulinate (EL, 5
mmol) to γꢀvalerolactone (GVL) in 2ꢀpropanol (10 mL). NaOH (1 mol% respect to EL); MW heating; T = 140 °C; t
= 30 min. a 0.56 mol% respect to EL of catalytic precursor.
Catalytic
Precursor
EL conversion
(%)
GVL selectivity
(%)
Run
GVL yield (%)
1
2
[1]NO3
[2]NO3
[3]NO3
[4]NO3
[1]NO3
[4]NO3
28.2
14.3
21.8
23.7
50.9
49.1
80.6
62.0
64.3
83.3
82.9
86.1
22.7
8.9
3
14.0
19.8
42.0
42.2
4
5 a
6 a
Compound [2]NO3 exhibited the lowest activity and selectivity, and the activities obtained with the
remaining catalytic precursors (runs 1, 3, 4 Table 4) under the same reaction conditions are similar,
showing EL conversion in the range 22ꢀ28%. However, [1]NO3 and [4]NO3 appeared more promising
than [3]NO3, giving higher GVL selectivity (ca. 80%), and were thus selected for further studies.
First of all, due to the low activity ascertained for every complex, the amount of catalytic precursor was
increased in order to increase EL conversion and the results reached employing an higher amount of
complexes [1]NO3 and [4]NO3 are reported in runs 5 and 6 of Table 3, respectively. The increase of the
catalytic precursor amount leads to an improvement of EL conversion, as expected, whereas GVL
selectivity remains constant, allowing us to increase GVL yields, which are in both case double respect
to the previous runs (runs 1 and 4, Table 1).
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Then, the effects of temperature and reaction time on the same catalytic precursors ([1]NO3 and [4]NO3)
were then considered (Figure 4). Preliminary runs were carried out in order to establish the lowest
temperature at which the catalytic precursors result active: negligible catalytic performances were
observed at temperatures lower than 120°C. On this basis, the range 120ꢀ160°C was chosen in order to
investigate the effect of temperature.
Figure 4. Profiles of EL (5 mmol) conversion (ꢀꢀ), GVL yield (●●) and GVL selectivity (♦♦) working with [1]NO3 or
[4]NO3 (0.56 mol% respect to EL) in 2ꢀpropanol (10 mL), respectively; NaOH (1 mol% respect to EL); MW
heating; T = a) 120 °C, b) 140 °C, c) 160 °C. Black solid line: compound [1]NO3; red dashed line: compound
[4]NO3.
For both systems, prolonging the reaction over 30 minutes at fixed temperature scarcely affected the
catalytic behavior. On the other hand, the catalytic performance strongly depends on the temperature. In
fact, working at 120 °C, [1]NO3 resulted more selective towards GVL than [4]NO3, and afforded the
highest GVL yield (Figure 4a). Probably, at low temperature the catalytic cycle is less favored
employing [4]NO3 than [1]NO3 and in the presence of the first one EL can undergo sideꢀreactions.
When the temperature was increased to 140 °C (Figure 4b), the conversion values achieved with [1]NO3
and [4]NO3 after 30 minutes were higher than those at 120 °C (51% and 49% respect to 38% and 41%,
respectively). Moreover, at 140 °C the selectivity with [4]NO3 approached that given by [1]NO3. When
the temperature was further increased to 160 °C (Figure 4c), analogous selectivity to those ascertained at
140 °C were obtained for both complexes but an improvement of EL conversion was achieved with the
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system [4]NO3 throughout the whole reaction time. In summary, raising the temperature revealed
beneficial to the catalytic performance of [4]NO3, allowing us to finally reach a GVL yield of about
50% after 1 hour at 160 °C.
Each solution recovered at the end of reaction was analyzed by means of GCꢀMS, in order to identify
possible organic byꢀproducts. All chromatograms show similar patterns, differing in the relative
amounts of the components. As a representative example, the chromatogram of the reaction carried out
at 120 °C for 30 minutes in the presence of [1]NO3 is shown in the Supporting Information, together
with the mass spectra of the detected species (Figures S25ꢀS32). The identification of the mixture
components has allowed us to suppose the reaction pathways reported in the Scheme 4.
Scheme 4. General reaction pathways identified in the temperature range 120ꢀ160°C in the presence of [1-4]NO3
as catalytic precursors.
According to Scheme 4, EL can engage several reactions in addition to the desired TH mechanism to
GVL (pathway 1). In fact, it can undergo the transꢀesterification reaction with the solvent, giving
isopropyl levulinate (compound C), identified by GCꢀMS analysis (Figure S26), which can
subsequently be converted to GVL through the TH mechanism, according to pathway 2, supposed also
in the literature [7g,h,33]. Moreover, when TH takes place, acetone deriving from the oxidation of 2ꢀ
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propanol is present in the reaction medium and it can react with EL giving compounds D and E, both
identified by GCꢀMS (Figures S29 and S30), according to pathway 3 [33].
On this basis, it is possible to rationalize the catalytic results. In fact, by the recorded chromatograms, it
is possible to observe that the formation of compound D is higher in the presence of [1]NO3 than with
[4]NO3 when the reaction was carried out at 120 °C (compare Figure S25 with Figure S31). Taking into
account that compound D derives from the reaction between EL and acetone, this last one originating
from the TH cycle, the higher amount of D ascertained with complex [1]NO3 could underline that the
TH mechanism is more active with system [1]NO3 than [4]NO3 at 120°C, leading to a higher GVL yield
(Figure 4a). On the other hand, as already observed in Figure 4, the rise of the reaction temperature from
120 to 140°C seems to promote the TH mechanism also in the presence of complex [4]NO3 and this is
confirmed by the increase of the compound D in the run catalyzed by [4]NO3 performed at 140°C
respect to 120°C (compare Figures S31 and S32).
Once identified the most promising catalytic precursor ([4]NO3) and the optimal reaction conditions (T
= 160 °C, t = 60 minutes), the possible effect of the nature of the base was investigated. More precisely,
the reaction was performed respectively in the presence of a metal hydroxide (NaOH or KOH), pyridine
(Py) or triethylamine (NEt3), which are four of the most investigated bases for the HT reaction in the
presence of homogeneous catalysts [7d,34]. The results are reported in Table 4.
Table 4. Transfer hydrogenation conversion of EL (5 mmol) to GVL in 2ꢀpropanol (10 mL) using different bases (1
mol% respect to EL). Catalytic precursor [4]NO3 (0.56 mol% respect to EL); MW heating; T = 160 °C; time = 60
min.
EL conversion
(%)
GVL selectivity
(%)
Run
Base
GVL yield (%)
7
8
NaOH
KOH
58.2
65.7
66.3
34.3
84.5
81.8
43.2
12.0
49.2
53.7
28.6
4.1
9
NEt3
10
Pyridine
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KOH performed as the best base, providing values of conversion and yield slightly higher than those
obtained with NaOH. Triethylamine determined a good conversion but a low selectivity. Instead,
pyridine was not effective in terms of both conversion and selectivity. The trends observed for ethyl
levulinate conversion (NEt3 > KOH > NaOH > pyridine) and GVL yield (KOH > NaOH > NEt3 >
pyridine), respectively, are in alignment with previous findings on the same reaction catalyzed by iron
complexes (see Introduction) [7d,35].
We finally tested [4]PF6 and [4]BPh4 as catalytic precursors, under the optimized reaction conditions
(Table 5). The role of the counter anion revealed to be crucial, and the use of [4]PF6 or [4]BPh4 in the
place of the homologous nitrate salt led to a significant increase in ethyl levulinate conversion. In
addition, [4]BPh4 maintained high GVL selectivity, providing the best GVL yield within the present
ꢀ
work (about 62%), together with a productivity towards to GVL of 110 hꢀ1 (measured as molGVL ꢂmolRu
1ꢂhꢀ1). It is noteworthy that this productivity is higher than that achieved by Dai et al. starting from the
same substrate to GVL in the presence of iron(II)ꢀcyclopentadienoneꢀtricarbonyl system under TH
mechanism, which resulted equal to 5 hꢀ1 (measured as molGVLꢂmolFeꢀ1ꢂhꢀ1). Also in the case of [4]BPh4,
the important influence of the base was verified (Table 5, runs 12 and 13).
Table 5. Effect of the anion of the catalytic precursors in the conversion of EL (5 mmol) to GVL in 2ꢀpropanol (10
mL). Catalytic precursor (0.56 mol% respect to EL); KOH (1 mol% respect to EL); MW heating; T = 160 °C; t = 60
min. a Without base.
Catalytic
Precursor
EL conversion GVL selectivity
Run
GVL yield (%)
(%)
65.7
89.2
81.4
75.2
(%)
81.8
32.7
75.8
41.2
8
[4]NO3
[4]PF6
53.7
29.2
61.7
31.0
11
12
13a
[4]BPh4
[4]BPh4
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According to spectroscopic and GCꢀMS experiments (see SI for details), the complexes undergo ligand
exchange/modification in the course of the hydrogenation reactions. On account of the important role
played by the αꢀdiimine moiety and the counterion, it is presumable that the active catalytic species are
not dissimilar from the starting Ru compounds, and that progressive thermal degradation finally leads to
inactive Ru species lacking of αꢀdiimine and pꢀcymene ligands [36].
Conclusions
Cationic ruthenium arene compounds with αꢀdiimine ligands have been synthesized, characterized and
employed for the first time as catalytic precursors in the transfer hydrogenation (from isopropanol) for
the conversion of ethyl levulinate into γꢀvalerolactone, i.e. a key reaction within the biorefinery route,
assisted by microwave irradiation. A screening of αꢀdiimines, counterions, base activators, reaction
times and temperatures was carried out, leading to the definition of optimal reaction parameters. The
results suggest that the anion and the αꢀdiimine have a strong impact on the catalytic performances.
Compound [(η6ꢀpꢀcymene)RuCl{κ2Nꢀ(4ꢀC6H10OH)N=CHCH=N(4ꢀC6H10OH)}]BPh4 was found to act
as the best catalytic precursor, which enables us to afford under sustainable conditions (only 1 mol% of
base and 60 minutes reaction time), a γꢀvalerolactone yield of 62%, highlighting, for the first time, the
promising potentialities of these complexes to be employed in an integrated bioꢀrefinery approach.
Experimental
1) General experimental details.
All reagents and solvents were obtained from Alfa Aesar, Sigma Aldrich or TCI Europe and were used
without further purifications. Ligands N,N'ꢀbis(4ꢀmetylphenyl)ethylenediimine (L1),19 N,N'ꢀbis(4ꢀ
hydroxyphenyl)ethylenediimine (L2),20 N,N'ꢀbis(cyclohexyl)ethylenediimine (L3),20,19 N,N'ꢀbis(4ꢀ
hydroxycyclohexyl)ethylenediimine (L4)20 and N,N'ꢀbis(tertꢀbutyl)ethylenediimine (L5)19 were
prepared as previously described; spectroscopic data for L1 and L5 are given in the Supporting
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Information. Needleꢀshaped Xꢀray quality crystals of L2 were obtained from a DMSO solution layered
with H2O and settled aside at 4 °C. Compounds [(η6ꢀpꢀcymene)RuCl2]2 [37] and [(η6ꢀpꢀ
cymene)RuCl{κ2Nꢀ(HCNR)2}]NO3 (R = 4ꢀC6H4OH, [2]NO3; C6H11, [3]NO3; 4ꢀC6H10OH, [4]NO3) were
prepared according to the published procedures [20]. All synthetic manipulations were performed in air
with common laboratory glassware. Once isolated, [5]NO3 was stored under N2, all the other products
being airꢀstable. NMR spectra were recorded on a Bruker Avance II DRX400 instrument equipped with
a BBFO broadband probe at 25 °C, unless otherwise specified. Chemical shifts (expressed in parts per
million) are referenced to the residual solvent peaks [38] (1H, 13C) or to external standards (19F to CFCl3,
31P to 85% H3PO4, 35Cl to 1 M NaCl in D2O). In D2O:CD3OD solutions, chemical shifts were referenced
to the residual HDO peak as in pure D2O (δH = 4.79 ppm) [38]. Spectra were assigned with the
1
1
assistance of DEPTꢀ135 spectra and Hꢀ1H (COSY), Hꢀ13C (gsꢀHSQC and gsꢀHMBC) correlation
experiments [39]. NMR signals in braces {} indicate superimpositions with other species. Infrared
spectra were recorded on a Perkin Elmer Spectrum One FTꢀIR spectrometer, equipped with a UATR
sampling accessory. UVꢀVis spectra were recorded on an Ultraspec 2100 Pro spectrophotometer with
0.1 cm quartz cuvettes. IR and UVꢀVis spectra were processed with Spectragryph software [40].
Carbon, hydrogen and nitrogen analysis was performed on a Vario MICRO cube instrument
(Elementar). The chloride content in [6]PF6 was determined with AgNO3 potentiometric titration on a
solution prepared by dissolution of the solid sample (ca. 20 mg) in acetone (1 mL) then diluted with
H2O (10 mL). The method was tested on [(η6ꢀpꢀcymene)RuCl2]2 as a reference compound (Anal. calcd.
for C20H28Cl4Ru2: Cl, 23.16. Found: 23.0; the compound was dissolved in a small volume of CH2Cl2,
diluted with EtOH and then with H2O). Melting points/decomposition temperatures were determined on
a STMP3 Stuart scientific instrument with a capillary apparatus. pH measurements were performed with
an Orion pHꢀmeter equipped with a Hamilton glass pHꢀelectrode, routinely calibrated with pH = 4.0 and
pH = 7.0 buffer solutions (SigmaꢀAldrich). Conductivity measurements were carried out at 21 °C using
an XS COND 8 instrument (cell constant = 1.0 cm−1) [41]. Molar conductivity of reference 1:1
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electrolytes. NaNO3. Λm (MeOH, c = 1.7ꢂ10ꢀ3 M): 119 Sꢂcm2ꢂmolꢀ1. NaCl. Λm (MeOH, c = 3ꢂ10ꢀ3 M) =
85 Sꢂcm2ꢂmolꢀ1. KPF6. Λm (MeOH, c = 2.7ꢂ10ꢀ3 M): 108 Sꢂcm2ꢂmolꢀ1. NaBPh4. Λm (MeOH, c = 1.1ꢂ10ꢀ3
M): 107 Sꢂcm2ꢂmolꢀ1.
2) Synthesis and characterization of Ru compounds
[(η6ꢀpꢀcymene)RuCl{κ2Nꢀ(HCN(4ꢀC6H4Me))2}]NO3, [1]NO3 (Chart 1).
Chart 1. Structure of [1]NO3 (numbering refers to carbon atoms).
A brick red suspension of [(η6ꢀpꢀcymene)RuCl2]2 (204 mg, 0.333 mmol) and AgNO3 (113 mg, 0.665
mmol) in MeCN (3 mL) was stirred at ambient temperature for 1 hour under protection from the light.
During this time, the mixture turned orange with the precipitation of a colorless solid (AgCl). Therefore,
the suspension was filtered over celite and L1 (158 mg, 0.669 mmol) was added to the orange filtrate
solution. The mixture was stirred at reflux temperature and the progress of reaction was checked by
TLC (KMnO4 stain). After 4 hours, the dark brown solution was cooled to ambient temperature and
volatiles were removed under vacuum. The residue was suspended in Et2O (20 mL) and the suspension
was filtered. The resulting dark brown solid was washed with Et2O and dried under vacuum (40 °C).
Yield: 340 mg, 90%. On the other hand, a mixture of (pꢀcymene)Ru compounds containing ca. 70% [1]+
(1H NMR) was obtained when the reaction was performed in MeOH at ambient temperature. Compound
[1]NO3 is soluble in water and chlorinated solvents, poorly soluble in Et2O and insoluble in hexane.
Crystals suitable for Xꢀray analysis were obtained from CH2Cl2 solutions of [1]NO3 layered with
heptane or Et2O and settled aside at ꢀ20 °C. Anal. calcd. for C26H30ClN3O3Ru: C, 54.88; H, 5.31; N,
7.38. Found: C, 54.69; H, 5.26; N, 7.48. Tm/°C > 200. IR (solid state): ῦ/cm─1 = 3059w, 2968w, 2925w,
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2872w, 1603w (νC=N), 1567w, 1539w, 1502m, 1471m, 1381mꢀsh, 1345s, 1330sꢀbr (νNO3), 1310s,
1216m, 1176w, 1162w, 1112w, 1093w, 1054w, 1038w, 1023m, 959w, 907w, 881m, 842wꢀsh, 818s,
773m, 732w, 711w, 676w. UVꢀVis (CH2Cl2, c = 1.0·10ꢀ3 M): λmax/nm (ε/M−1·cm−1) = 371 (1.2·104),
450sh (4.0·103), 550sh (1.1·103). Λm (c = 1.0ꢀ1.1·10ꢀ3 M) = 15 (CH2Cl2); 119 (MeOH) S·cm2·mol–1. 1H
NMR (CDCl3): δ/ppm = 8.43 (s, 2H, C8ꢀH), 7.81 (d, 3JHH = 7.9 Hz, 4H, C10ꢀH), 7.32 (d, 3JHH = 7.6 Hz,
3
3
4H, C11ꢀH), 5.18 (pseudoꢀq, JHH = 6.2 Hz, 4H, C3ꢀH + C4ꢀH), 2.74 (hept, JHH = 6.7 Hz, 1H, C6ꢀH),
3
1
2.46 (s, 6H, C13ꢀH), 2.20 (s, 3H, C1ꢀH), 1.19 (d, JHH = 6.8 Hz, 6H, C7ꢀH). No change in the H
spectrum was observed after 5 days at ambient temperature. 13C NMR (CDCl3): δ/ppm = 165.9 (C8),
150.4 (C9), 141.1 (C12), 130.2 (C11), 122.8 (C10), 110.5 (C5), 102.8 (C2), 88.3 (C3), 86.6 (C4), 31.0
(C6), 22.4 (C7), 21.5 (C13), 18.9 (C1).
[(η6ꢀpꢀcymene)RuCl{κ2Nꢀ(HCNtBu)2}]NO3, [5]NO3 (Chart 2).
Chart 2. Structure of [5]NO3 (numbering refers to carbon atoms).
The synthesis was carried out as described for [1]NO3, using [(η6ꢀpꢀcymene)RuCl2]2 (50 mg, 0.082
mmol), AgNO3 (28 mg, 0.16 mmol) and L5 (29 mg, 0.17 mmol) in MeCN (3 mL). After 3 hours, the
orange solution was cooled to ambient temperature and volatiles were removed under vacuum. The
residue was suspended in Et2O (20 mL) and the suspension was filtered. The resulting orangeꢀbrown
solid was washed with Et2O, dried under vacuum (40 °C) and stored under nitrogen (hygroscopic).
Yield: 75 mg, 92%. On the other hand, a mixture of compounds containing unreacted L5 was obtained
when the reaction was performed in MeOH at room or reflux temperature. Compound [5]NO3 is soluble
in water and chlorinated solvents, insoluble in Et2O, hexane. Anal. calcd. for C20H34ClN3O3Ru: C,
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47.94; H, 6.84; N, 8.39. Found: C, 48.02; H, 6.76; N, 8.47. IR (solid state): ῦ/cm─1 = 3600ꢀ3200wꢀbr,
2973m, 2936m, 2876w, 1870w, 1626m (νC=N), 1539w, 1478m, 1463m, 1369s, 1323sꢀbr (νNO3), 1224m,
1177s, 1090m, 1057m, 1038m, 977m, 927w, 879m, 829m, 806w, 732w, 694w, 671w. UVꢀVis (CH2Cl2,
c = 1.9·10ꢀ3 M): λmax/nm (ε/M−1·cm−1) = 278 (2.7·103), 378 (1.7·103), 431 (1.5·103). Λm (CH2Cl2, c =
1.9·10ꢀ3 M) = 8.9 S·cm2·mol–1. 1H NMR (CDCl3, 55 °C): δ/ppm = 8.72 (sꢀbr, 2H, C8ꢀH), 5.95–5.82 (mꢀ
br, 4H, C3ꢀH + C4ꢀH), 2.88 (mꢀbr, 1H, C6ꢀH), 2.30 (s, 3H, C1ꢀH), 1.68 (s, 18H, C10ꢀH), 1.26 (mꢀbr,
6H, C7ꢀH). The 1H spectrum recorded at 25 °C displays broader resonances. 13C{1H} NMR (CDCl3, 353
K): δ/ppm = 168.5 (C8), 68.6 (C9), 82* (br, C3 + C4), 32.2 (C10), 31.9 (C6), 22.5* (br, C7), 18.7* (br,
C1). 13C signals for the pꢀcymene ligand (*) were too broad to be directly observed; they were identified
through gsꢀHSQC experiment.
[(η6ꢀpꢀcymene)RuCl(MeCN)0.66]PF6, [6]PF6.
The mixture of chloridoꢀacetonitrile complexes obtained by the following procedure is treated as a
single compound with formula [(η6ꢀpꢀcymene)RuCl(MeCN)0.66]PF6 for the sake of simplicity.
A brick red suspension of [(η6ꢀpꢀcymene)RuCl2]2 (300 mg, 0.490 mmol) and NH4PF6 (160 mg, 0.982
mmol) in MeCN (4 mL) was stirred at ambient temperature for 14 hours. During this time, the mixture
turned orange with the precipitation of a colorless solid (NH4Cl). The suspension was filtered over celite
then volatiles were removed from the orange filtrate solution, affording an oily orange residue. At this
point, literature procedures26a,b prescribe a slow crystallization with Et2O to obtain [(η6ꢀpꢀ
cymene)RuCl(MeCN)2]PF6 as an orange powder. Instead, the oil was reꢀdissolved in few mL of CH2Cl2
and a foamy orange solid was readily obtained upon solvent removal under vacuum (40 °C). Yield: 430
mg, 99%. The compound is soluble in MeCN, CH2Cl2, poorly soluble in CHCl3, MeOH and insoluble in
Et2O, hexane. Anal. calcd. for C10H14ClF6PRu(CH3CN)0.66: C, 30.70; H, 3.64; N, 2.09; Cl, 8.00. Found:
C, 30.76; H, 3.55; N, 2.16; Cl, 8.0. IR (solid state): ῦ/cm─1 = 3090w, 2973w, 2944w, 2876w, 2374ꢀ
2345w (νC≡N), 2329w (νC≡N), 2301w (νC≡N), 1538w, 1504w, 1473m, 1446w, 1393m, 1328w, 1282w,
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1
1202w, 1117w, 1092w, 1059w, 1033w, 1008w, 877mꢀsh, 826vs (νPF6), 740mꢀsh, 673w. H NMR
(CD3CN): three Ru compounds (6a, 6b, 6c) in 70:15:15 molar ratio; overall MeCN/pꢀcymene ratio =
3
3
0.66. 6a. δ/ppm = 5.83, 5.58 (d, JHH = 6.1 Hz, 4H, RuꢀCH); 2.88 (hept, JHH = 6.8 Hz, 1H, CHMe2),
3
2.49 (s, 2H, RuꢀNCMe), 2.23 (s, 3H, RuꢀCMe), 1.30 (d, JHH = 6.9 Hz, 6H, CHMe2). 6b, 6c. δ/ppm =
3
6.10, 5.87, 5.55, 5.30 (d, JHH = 6.0 Hz; 4H, RuꢀCH); {2.88 (CHMe2)}, 2.27, 2.18 (s, 3H, RuꢀCMe);
2.14 (s, 4H, RuꢀNCMe), {1.30 (CHMe2)}. 1H NMR (CD2Cl2): two Ru compounds (6a’, 6b’) with 77:23
3
molar ratio + uncoordinated MeCN (δ/ppm = 1.84(br.)). 6a’. δ/ppm = 5.64, 5.46 (d, JHH = 6.0 Hz, 4H,
3
RuꢀCH); 2.78 (hept, JHH = 6.9 Hz, 1H, CHMe2), 2.55 (s, 3H, RuꢀNCMe), 2.22 (s, 3H, RuꢀCMe), 1.30
(d, 3JHH = 6.9 Hz, 6H, CHMe2). 6b’. δ/ppm = 6.12, 5.92 (d, 3JHH = 6.2 Hz, 4H, RuꢀCH); 2.86 (hept, 3JHH
= 6.8 Hz, 1H, CHMe2), 2.33 (s, 3H, RuꢀCMe), 2.12, 1.97 (s, 3H, RuꢀNCMe); 1.36ꢀ1.33 (m, 6H,
CHMe2). 13C{1H} NMR (CD2Cl2, 6a’): δ/ppm = 102.3 (CiPr), 97.6 (RuꢀCMe), 79.3, 78.5 (RuꢀCH);
31.8 (CHMe2), 22.2 (CHMe2), 19.1 (RuꢀCMe), 4.1 (RuꢀNCMe). 31P{1H} NMR (CD2Cl2): δ/ppm −144.5
(hept, 1JPF = 711 Hz, PF6 ).
─
[(η6ꢀpꢀcymene)RuCl{κ2Nꢀ(HCN(4ꢀC6H4OH))2}]PF6, [2]PF6 (Chart 3).
Chart 3. Structure of [2]PF6, (numbering refers to carbon atoms).
[(η6ꢀpꢀcymene)RuCl(MeCN)0.66]PF6 (325 mg, 0.734 mmol) and L2 (176 mg, 0.733 mmol) were
dissolved in MeOH (5 mL) with immediate darkening of the mixture. The solution was stirred at
ambient temperature for 3 hours then volatiles were removed under vacuum. The residue was suspended
in Et2O and the suspension was filtered. The black solid was washed with Et2O and dried under vacuum
(40 °C) over P2O5. Yield: 390 mg, 81%. Compound [2]PF6 is highly soluble in acetone, soluble in
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water, DMSO, methanol; insoluble in chlorinated solvents, Et2O and hexane. Anal. calcd. for
C24H26ClF6N2O2PRu: C, 43.94; H, 4.00; N, 4.27. Found: C, 44.03; H, 3.89; N, 4.40. IR (solid state):
ῦ/cm─1 = 3544w (νOꢀH), 3400ꢀ3100wꢀbr (νOꢀH), 3073w, 2967w, 2933w, 2878w, 2810w, 1709w, 1605m
(νC=N), 1591m, 1571m, 1562mꢀsh, 1505s, 1472m, 1447m, 1368w, 1271s, 1210s, 1165s, 1137w, 1105w,
1056w, 1024m, 996w, 959w, 831vsꢀbr (νPF6), 740m, 721w, 673w. UVꢀVis (MeOH, c = 1.1·10ꢀ3
M): λmax/nm (ε/M−1·cm−1) = 268 (1.1·104), 422 (1.3·104), 550ꢀ575br (2.9·103). Λm (MeOH, c = 1.1·10ꢀ3
1
M) = 71 S·cm2·mol–1. H NMR (DMSOꢀd6): δ/ppm = 10.4 (s, 2H, OH), 8.47 (s, 2H, C8ꢀH), 7.66 (d,
3JHH = 8.7 Hz, 4H, C10ꢀH), 6.98 (d, 3JHH = 8.7 Hz, 4H, C11ꢀH), 5.49 (pseudoꢀq, 3JHH = 6.4 Hz, 4H, C3ꢀ
3
3
H + C4ꢀH), 2.30 (hept, JHH = 6.9 Hz, 1H, C6ꢀH), 2.23 (s, 3H, C1ꢀH), 0.96 (d, JHH = 6.9 Hz, 6H, C7ꢀ
1
13
H). No change in the H spectrum was observed after 43 days at ambient temperature. C{1H} NMR
(DMSOꢀd6): δ/ppm = 162.4 (C8), 159.8 (C12), 143.9 (C9), 124.3 (C10), 115.8 (C11), 106.6 (C5), 105.8
19
(C2), 88.7 (C4), 86.9 (C3), 30.7 (C6), 21.5 (C7), 18.5 (C1). F{1H} NMR (DMSOꢀd6): δ/ppm = −70.2
(d, 1JFP = 711 Hz, PF6 ). 31P{1H} NMR (DMSOꢀd6): δ/ppm = −144.2 (hept, 1JPF = 711 Hz, PF6 ).
─
─
[(η6ꢀpꢀcymene)RuCl{κ2Nꢀ(HCN(4ꢀC6H10OH))2}]X, [4]X (X = PF6, BPh4) (Chart 4).
Chart 4. Structure of [4]X (X = PF6, BPh4) (numbering refers to carbon atoms).
[4]PF6. A suspension of [(η6ꢀpꢀcymene)RuCl(MeCN)0.66]PF6 (100 mg, 0.226 mmol) and L4 (57 mg,
0.23 mmol) in MeCN (5 mL) was stirred at reflux temperature for 9 hours and the progress of reaction
was monitored via 1H NMR. Therefore the red mixture was allowed to cool to ambient temperature and
was filtered over celite. Volatiles were removed under vacuum from the filtrate solution and the residue
was suspended in Et2O. The suspension was filtered and the resulting orangeꢀred solid was washed with
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Et2O and dried under vacuum (40 °C) over P2O5. Yield: 137 mg, 91%. On the other hand, a mixture of
(pꢀcymene)Ru compounds containing minor amounts of [4]+ (1H NMR) was obtained when the reaction
was performed one-pot with [(η6ꢀpꢀcymene)RuCl2]2, L4 and NH4PF6 (1:1:1 mol ratio) in MeCN at
ambient temperature. Compound [4]PF6 is soluble in acetone, H2O, DMSO and MeOH; insoluble in
chlorinated solvents, Et2O and hexane. Anal. calcd. for C24H38ClF6N2O2PRu: C, 43.14; H, 5.73; N, 4.19.
Found: C, 42.90; H, 5.82; N, 4.30. IR (solid state): ῦ/cm─1 = 3587w (νOꢀH), 3410wꢀbr (νOꢀH), 3086wꢀbr,
2964w, 2938w, 2904m, 2866w, 1640wꢀbr (νC=N), 1538w, 1508w, 1471wꢀsh, 1455m, 1406w, 1392w,
1368w, 1326w, 1284w, 1230w, 1203w, 1081m, 1056m, 964w, 905w, 877mꢀsh, 833vs (νPF6), 740w.
UVꢀVis (MeOH, c = 1.6·10ꢀ3 M): λmax/nm (ε/M−1·cm−1) = 277 (3.4·103), 372sh (2.0·103), 426 (2.9·103).
Λm (MeOH, c = 1.6·10ꢀ3 M) = 116 S·cm2·mol–1. 1H NMR (CD3OD): δ/ppm = 8.33 (s, 2H, C8ꢀH), 6.18
3
3
3
(d, JHH = 6.0 Hz, 2H, C4ꢀH), 5.81 (d, JHH = 5.9 Hz, 2H, C3ꢀH), 4.49 (t, JHH = 11.2 Hz, 2H, C9ꢀH),
3
3
3.64 (t, JHH = 10.5 Hz, 2H, C12ꢀH), 2.77 (hept, JHH = 6.7 Hz, 1H, C6ꢀH), 2.52 (d, J = 12.9 Hz, 2H,
C10ꢀH), 2.32–2.27 (m, 2H, C10’ꢀH), 2.27 (s, 3H, C1ꢀH), 2.13 (d, J = 12.0 Hz, 2H, C11’ꢀH), 2.04 (d, J =
13.4 Hz, 2H, C11ꢀH), 1.91 (q, J = 11.4 Hz, 2H, C10’ꢀH’), 1.62 (q, J = 10.1 Hz, 2H, C11’ꢀH’), 1.54 (q, J
= 10.1 Hz, 2H, C11ꢀH’), 1.37 (q, J = 11.2 Hz, 2H, C10ꢀH’), 1.16 (d, 3JHH = 6.7 Hz, 6H, C7ꢀH). 13C{1H}
NMR (CD3OD): δ/ppm = 165.3 (C8), 109.2 (C5), 108.4 (C2), 89.5 (C4), 87.3 (C3), 75.7 (C9), 70.1
19
(C12), 34.9 (C11), 34.5 (C11’), 34.0 (C10), 33.1 (C6), 32.1 (C10’), 22.7 (C7), 19.4 (C1). F{1H} NMR
−
(CD3OD): δ/ppm = −74.4 (d, 1JFP = 708 Hz, PF6 ). 31P{1H} NMR (CD3OD): δ/ppm = −146.7 (hept, 1JPF
−
= 708 Hz, PF6 ).
[4]BPh4. A solution of [4]NO3 (68 mg, 0.12 mmol) and Na[BPh4] (40 mg, 0.12 mmol) in acetone (6
mL) was stirred for 14 hour at ambient temperature, affording a redꢀorange solution and a colorless
precipitate (NaNO3). The suspension was filtered over celite and volatiles were removed under vacuum
from the filtrate solution. The residue was dissolved in CH2Cl2 (5 mL) and extracted with H2O (10 mL).
The red organic phase was separated and taken to dryness under vacuum. The resulting orange solid was
suspended in Et2O then filtered, washed with Et2O and dried under vacuum (40 °C). Yield: 87 mg, 87%.
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Compound [4]BPh4 is soluble in CH2Cl2, MeOH and MeCN, poorly soluble in CHCl3 and insoluble in
Et2O, H2O. Anal. calcd. for C48H58BClN2O2Ru: C, 68.44; H, 6.94; N, 3.32. Found: C, 68.20; H, 7.05; N,
3.42. IR (solid state): ῦ/cm─1 = 3540w (νOꢀH), 3410wꢀbr (νOꢀH), 3055w, 3036w, 3000w, 2983w, 2964w,
2934m, 2861w, 2902w, 1579w, 1538w (νC=N), 1478m, 1453m, 1426m, 1402w, 1388w, 1366m, 1328w,
1305w, 1267m, 1229w, 1201w, 1184w, 1138w, 1080s, 1054s, 1032m, 998w, 962m, 902w, 862m,
846m, 746mꢀsh, 733s (νBPh4), 704s (νBPh4). UVꢀVis (MeOH, c = 1.4·10ꢀ3 M): λmax/nm (ε/M−1·cm−1) =
265 (7.5·103), 273 (6.3·103), 277sh (3.9·103), 369sh (2.6·103), 427 (3.4·103). Λm (MeOH, c = 1.4·10ꢀ3
M) = 81 (MeOH) S·cm2·mol–1. 1H NMR (CD2Cl2): δ/ppm = 7.41–7.34 (mꢀbr, 8H, BPhmeta), 7.00 (t, 3JHH
3
= 7.3 Hz, 8H, BPhortho), 6.83 (t, 3JHH = 7.1 Hz, 4H, BPhpara), 6.57 (s, 2H, C8ꢀH), 5.52 (d, JHH = 6.2 Hz,
2H, C4ꢀH), 5.30 (d, 3JHH = 6.5 Hz, 2H, C3ꢀH), 3.97 (t, 3JHH = 11.4 Hz, 2H, C9ꢀH), 3.64 (mꢀbr, 2H, C12ꢀ
H), 2.61 (hept, 3JHH = 6.7 Hz, 1H, C6ꢀH), 2.37–2.28 (m, 2H, C10ꢀH), 2.18 (s, 3H, C1ꢀH), 2.17–2.11 (m,
2H, C11ꢀH), 2.11–2.05 (m, 2H, C11’ꢀH), 1.98–1.90 (m, 2H, C10’ꢀH), 1.75 (s, 2H, OH), 1.52–1.32 (m,
3
6H, C10’ꢀH’ + C11ꢀH’ + C11’ꢀH’), 1.28–1.13 (m, 2H, C10ꢀH’), 1.11 (d, JHH = 6.9 Hz, 6H, C7ꢀH).
13C{1H} NMR (CD2Cl2): δ/ppm = 164.5 (q, 1JCB = 49.0 Hz, BPhipso), 164.1 (C8), 136.2 (BPhortho), 126.4
(BPhmeta), 122.6 (BPhpara), 108.1 (C5), 106.8 (C2), 87.7 (C4), 86.1 (C3), 75.2 (C9), 69.3 (C12), 34.7
(C11’), 34.3 (C11), 32.8 (C10), 32.1 (C6), 31.2 (C10’), 22.6 (C7), 19.5 (C1). 1H NMR (CD3OD): δ/ppm
3
3
= 7.63 (s, 2H, C8ꢀH), 7.33–7.27 (m, 8H, BPhmeta), 6.95 (t, JHH = 7.4 Hz, 8H, BPhortho), 6.82 (t, JHH
=
7.2 Hz, 4H, BPhpara), 6.02 (d, 3JHH = 6.4 Hz, 2H, C4ꢀH), 5.65 (d, 3JHH = 6.3 Hz, 2H, C3ꢀH), 4.33 (t, 3JHH
3
= 11.5 Hz, 2H, C9ꢀH), 3.66–3.57 (m, 2H, C12ꢀH), 2.69 (hept, JHH = 6.9 Hz, 1H, C6ꢀH), 2.46–2.40 (m,
2H, C10ꢀH), 2.18 (s, 3H, C1ꢀH), 2.15–2.07 (m, 4H, C11ꢀH + C11’ꢀH), 2.06–1.99 (m, 2H, C10’ꢀH),
1.76–1.64 + 1.63–1.43 (m, 6H; C10’ꢀH’ + C11ꢀH’ + C11’ꢀH’), 1.32–1.23 (m, 2H, C10ꢀH’), 1.12 (d,
1
3JHH = 6.9 Hz, 6H, C7ꢀH). No change was observed in the H spectrum after 3 days at ambient
temperature. 13C{1H} NMR (CD3OD): δ/ppm = 165.3 (C8), 137.3 (BPhortho), 126.6 (BPhmeta), 122.9
(BPhpara), 108.9 (C5), 108.4 (C2), 89.4 (C4), 87.1 (C3), 75.6 (C9), 70.0 (C12), 34.9 (C11’), 34.5 (C11),
33.8 (C10), 33.0 (C6), 32.0 (C10’), 22.7 (C7), 19.4 (C1).
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3) Xꢀray crystallography.
Crystal data and collection details for L2 and [1]NO3 are reported in Table 6. Data were recorded on a
Bruker APEX II diffractometer equipped with a PHOTON100 detector using Mo–Kα radiation. Data
were corrected for Lorentz polarization and absorption effects (empirical absorption correction
SADABS) [42]. The structures were solved by direct methods and refined by fullꢀmatrix leastꢀsquares
based on all data using F2 [43]. Hydrogen atoms were fixed at calculated positions and refined by a
riding model, except those bonded to Oꢀatoms which have been located in the difference Fourier Map
and refined isotropically. All nonꢀhydrogen atoms were refined with anisotropic displacement
parameters.
Table 6. Crystal data and measurement details for L2 and [1]NO3.
Compound
Formula
FW
L2
[1]NO3
C14H12N2O2
240.26
C26H30ClN3O3Ru
569.05
T, K
100(2)
100(2)
0.71073
Monoclinic
P21/c
λ
, Å
0.71073
Crystal system
Space group
a, Å
Monoclinic
P21/c
4.9045(11)
11.022(2)
10.913(3)
101.589(6)
577.9(2)
2
9.8785(6)
13.9080(9)
18.1728(11)
101.9450(10)
2442.7(3)
4
b, Å
c, Å
β
, °
Cell Volume, Å3
Z
Dc, gꢁcmꢀ3
1.381
1.547
ꢀ, mm−1
0.094
0.785
F(000)
252
1168
Crystal size, mm
θ limits, °
0.19 x 0.16 x 0.14
2.654 – 26.999
6473
0.18 x 0.16 x 0.12
1.859 – 25.049
28024
Reflections collected
Independent reflections
1260 (Rint = 0.0401)
4323 (Rint = 0.0471)
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Data / restraints /parameters
Goodness of fit on F2
R1 (I > 2σ(I))
1260 / 0 / 83
4323 / 108 / 344
1.107
1.045
0.0439
0.0437
wR2 (all data)
0.1025
0.0984
Largest diff. peak and hole, e Åꢀ3
0.247 and –0.296
2.808 and –0.825
4) Catalytic study.
Catalytic conversion of ethyl levulinate (EL) to γꢀvalerolactone (GVL).
The catalytic reaction was performed in a monomodal microwave reactor CEM Discover Sꢀclass
System. In a typical run, ethyl levulinate (5 mmol), 2ꢀpropanol (10 mL) acting as hydrogen donor and
solvent, the base (0.05 mmol) and the proper amount of catalyst were charged in a 35 mL reactor vessel.
The mixture was heated at the stated temperature during the established time under magnetic stirring.
Then, the reactor was rapidly cooled to ambient temperature through an external air flow, and the
samples were collected and analyzed by gas chromatography.
Sample analysis.
EL conversion (%) and GVL yield (%), expressed respectively as [(molinEL – molfinEL)/molinEL]*100 and
(molGVL/molinEL)*100, were quantified by gas chromatography with DANI GC1000 instrument
equipped with FID detector and HPꢀPONA capillary column (50 m × 0.2 mm × 0.5 µm) 100%
dimethylpolysiloxane. Nitrogen was employed as carrier gas, with flow = 1 mL min−1. The injector and
detector temperatures were both maintained at 250 °C, and the following temperature program was
adopted for the chromatographic run: 60 °C isothermal for 2 min; 10 °C/min up to 230 °C; 230 °C
isothermal for 5 min. 1ꢀhexanol was employed as internal standard for analysis. The products present in
the reaction solution were identified by GCꢀMS (Agilent 7890Bꢀ5977A) with HPꢀ5MS capillary column
(30 m × 0.25 mm × 0.25 µm) (5%ꢀphenyl)ꢀmethylpolysiloxane. Helium was employed as carrier gas,
with flow = 1 mL min−1. The injector and detector temperatures were maintained at 250 °C and 280 °C,
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respectively, and the following temperature program was adopted for the chromatographic run: 40 °C
isothermal for 2 min; 10 °C/min up to 230 °C; 230 °C isothermal for 5 min.
Notes
The authors declare no competing financial interest.
Acknowledgement
We thank the University of Pisa (Fondi di Ateneo 2017) for financial support.
Supporting Information Available
Spectroscopic data, IR and NMR spectra of compounds, chromatograms and mass spectra, analysis of
residues. CCDC reference numbers 1830401 (L2) and 1830402 ([1]NO3) contain the supplementary
crystallographic data for the Xꢀray studies reported in this paper. These data can be obtained free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (internat.) +44ꢀ1223/336ꢀ033; eꢀmail:
deposit@ccdc.cam.ac.uk).
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