Novel ruthenium-terpyridyl complex for direct oxidation of amines to nitriles

Article abstract of DOI:10.1039/c3cy00209h

High catalytic activity and selectivity has been demonstrated for the oxidation of both aliphatic and aromatic amines to nitriles under benign conditions with dioxygen or air using the Ru₂Cl₄(az-tpy) ₂ complex. The conversion was found to be strongly influenced by the alkyl chain length of the reactant with shorter chain amines found to have lower conversions than those with longer chains. Importantly, by using the ruthenium terpyridine complex functionalized with azulenyl moiety at the 4 position of central pyridine core provided a much higher reactivity catalyst compared with a series of ruthenium terpyridine-based ligand complexes reported. Mechanistic studies using deuterated benzylamine demonstrated the importance of RuOH in this reaction.

Full text of DOI:10.1039/c3cy00209h

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Novel ruthenium–terpyridyl complex for direct
oxidation of amines to nitriles†
Cite this: DOI: 10.1039/c3cy00209h
Liliana Cristian,ab Simona Nica,b Octavian D. Pavel,a Constantin Mihailciuc,a
Valer Almasan,c Simona M. Coman,a Christopher Hardacre*d and
Vasile I. Parvulescu*a
High catalytic activity and selectivity has been demonstrated for the oxidation of both aliphatic and
aromatic amines to nitriles under benign conditions with dioxygen or air using the Ru2Cl4(az-tpy)2
complex. The conversion was found to be strongly influenced by the alkyl chain length of the reactant
with shorter chain amines found to have lower conversions than those with longer chains. Importantly,
by using the ruthenium terpyridine complex functionalized with azulenyl moiety at the 4 position of
central pyridine core provided a much higher reactivity catalyst compared with a series of ruthenium
terpyridine-based ligand complexes reported. Mechanistic studies using deuterated benzylamine
demonstrated the importance of RuOH in this reaction.
Received 29th March 2013,
Accepted 11th July 2013
DOI: 10.1039/c3cy00209h
www.rsc.org/catalysis
have been described in literature. For example, a range of
amines can be oxidized to the corresponding nitriles using
Introduction
copper catalysts15 or manganese catalysts,16 whereas iron(II)
and iron(III) were reported as being very efficient in the oxidative
a-cyanation of tertiary amines with trimethylsilyl cyanide in the
presence of tert-butylhydroperoxide.17 An efficient aerobic oxida-
tive cyanation of tertiary amines with sodium cyanide has been
studied using vanadium based catalysts.18 Hence, the carbon–
carbon bond formation at the a position of the tertiary amines is
an alternative route for the preparation of nitriles. Other reports
recommended complex oxidation systems.19
Over the last few years, ruthenium compounds have been
reported to be effective catalysts for the oxidation of amines to
nitriles leading to remarkable results with iodosylbenzene20
and persulfate ions21 as oxidants especially in the presence of
non benign oxidising agents such as hydrogen peroxide22 and
dioxygen.23 Moreover, the ruthenium catalysts were also effec-
tive for the aerobic oxidation of several amines without the
need of other toxic reagents both under homogenous24,25 and
heterogenous26,27 catalytic conditions. Whilst ruthenium-based
compounds have been shown to oxidise amines to nitriles
under mild conditions, in particular using benzyl amine and
activated aromatic amines, the reactions showed low selectivity.
Better results were obtained for a ruthenium dioxo porphyrin
complex which catalytically dehydrogenates benzyl amine and
n-butylamine in the presence of air.28 However, the direct
oxidation of aliphatic amines to selectively produce alkyl
nitriles remains a challenging target for this catalytic reac-
tion.29 The efficiency of the ruthenium-catalysts was improved
Nitriles represent one of the most important class of organic
compounds used as building blocks in pharmaceuticals, agro-
chemicals, herbicides, natural products, polymers and dyes.1
They are also versatile intermediates in synthetic organic
chemistry due to their easy conversion into amines, amides,
carboxylic acids, aldehydes, esters, heterocycles, etc.2 Com-
monly nitriles are synthesized via the dehydration of amides
4
and aldoximes,3 condensation of carboxylic acids with NH3
and conversion of aldehydes,5 alcohols6 and esters.7 The draw-
backs of these methods include the use of harsh reaction
conditions, toxic iodine derivatives as oxidants and polluting
reagents as well as the formation of side-products.8–14
Increasingly, attention has been directed at finding catalysts
which are able to oxidise organic substrates into nitriles with
high conversion and selectivity both from economic and environ-
mental perspective. In this regard, many metal based catalysts
a University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry,
Biochemistry and Catalysis, Bd. Regina Elisabeta 4-12, Bucharest 030016,
Romania. E-mail: vasile.parvulescu@g.unibuc.ro
b Institute of Organic Chemistry ‘‘C. D. Nenitzescu’’, Splaiul Independentei 202B,
Bucharest 060023, Romania
c National Institute for Isotopic and Molecular Technologies, str. Donath,
nr. 65-103, 400293 Cluj-Napoca, Romania
d School of Chemistry and Chemical Engineering, Queen’s University, Belfast,
BT9 5AG, UK
† Electronic supplementary information (ESI) available: 13C NMR and UV-vis
spectra of the terpyridyl ligand and the UV-vis spectrum of the ruthenium
complex. See DOI: 10.1039/c3cy00209h
c
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in multicomponent coupled systems involving an electron-rich tetrafluoroborate, TBABF4 (Sigma, A. R. degree) was used as the
quinone and a co-catalyst of the Co(salen) type complex.30 The supporting electrolyte, in dimethylformamide as the solvent
disadvantage of these systems is the formation of significant (Sigma). Both the Ru(III) complexes as well as one of the two
amounts of by-products, the limited range of amines which can different organic ligands involved in the complex were examined
be transformed and the severe deactivation of the catalysts. using voltammetry. In each case, a 0.5 mM of the analyte
Therefore, whilst it is true that much has been achieved in this solution was degassed for 5 min using extra-pure argon gas
field, significant improvements are needed to make this route prior to each measurement and an inert atmosphere was main-
commercially viable for nitrile production.
tained by purging argon at a low pressure above the solution
Terpyridine metal complexes have been extensively studied during each measurement. All the peak potentials were mea-
in view of their electronic and redox properties and in connec- sured relative to the Ag wire pseudo-reference electrode. The
tion with applications in photocatalysis or energy conver- DPV-traces were baseline-corrected using the moving average
sion.31,32 Recently, a ruthenium bearing terpyridine-ligand application included in GPES version 4.9 software.
has also been reported as catalyst for aerobic oxidative dehydro-
All reagents used were obtained from Sigma-Aldrich and
genation of benzylamines.33 However, the reported catalytic used without further purification.
reaction only involved aromatic amines as substrate and
General procedure for the amine oxidation to nitriles
requires the presence of a base to form the corresponding
nitriles.
The oxidations were carried out in an autoclave, as follows:
We report, herein, a green and effective oxidation of both amine (25 mg, 0.14 mmol) was added to a suspension of
aliphatic and aromatic amines to nitriles, with molecular Ru2Cl4(az-tpy)2 (10 mg, 0.01 mmol) (Ru: 0.6 mol%) in methanol
dioxygen using a ruthenium terpyridine complex, functiona- (5 mL) under dioxygen (Linde) (5 atm) or air (Linde) (25 atm)
lized with azulenyl moiety at the 4 position of central pyridine pressure. All amines (ethylamine, n-butylamine, iso-butylamine,
core. In order to form the active conformation, Kanbara et al.34 n-hexylamine, n-octylamine, n-dodecylamine, n-stearylamine,
complexed the terpyridine ruthenium with a second ligand. benzylamine, 2-methoxy benzylamine, 4-methyl benzylamine,
Based on this state of the art, one of the objectives of this study 4-chloro benzylamine) were purchased from Sigma-Aldrich and
was to achieve a ligand that can activate ruthenium for this were of >99%, purity. The mixture was stirred (600 rpm) between
reaction without the necessity to add a co-ligand. In this scope room temperature–70 1C. It was observed that, after 20 min, the
we prepared and investigated 40 modified 2,20:60,200-terpyridine dioxygen pressure in the autoclave began to decrease, indicating
with a donor electron group like azulenyl or substituted azulenyl the start of the reaction. Reactants and products were analysed
fragments. The resulted complexes were compared with simple by GC-MS, after filtration and centrifugation, using a Trace GC
unsubstituted terpyridine complexes.
2000 system with MS detector (Thermo Electron Scientific
Corporation, USA) incorporating a TR-WAX capillary column.
The injection chamber was set up at 200 1C and the temperature
in the detector cell was 270 1C. Deuterated benzylamine was
Experimental
Elemental analyses were performed on a Perkin Elmer CHN prepared by isotopic exchange using 0.5 moles (50 mL) benzyla-
240B. UV-Vis spectra were recorded on a Varian Cary 100 mine dissolved in 5 moles (100 mL) deuterated water. The
spectrophotometer. DRIFT spectra were collected using a NICO- solution was acidified using conc. HCl (37%, Sigma-Aldrich)
LET 4700 spectrometer from the accumulation of 200 scans in and refluxed for 2 h. After cooling, the deuterated benzylamine
the range of 500–4000 cmꢀ1 with a resolution of 4 cmꢀ1. NMR was extracted from the aqueous solution using benzene before
spectra were measured on a Bruker Avance DRX4 (1H: 400 MHz, being removed by distillation. The same procedure was applied
13C: 100 MHz) spectrometer. Mass spectra were recorded on a again to the isolated product to maximise the isotopic exchange.
Varian 1200L QuadrupoleMS/MS spectrometer by direct injec- Infrared analysis of the product showed that the degree of
tion in EI. Microwave irradiation was carried out using a exchange of the amino group was B85%. No deuteration of
Biotage Initiator 2.0 EXP – ED instrument. Electrochemical C–H bonds was determined. The oxidations using isotopically-
studies, consisting of cyclic voltammetry (CV) and differential substituted benzylamine were carried out in dioxygen at 60 1C.
pulse voltammetry (DPV) techniques, were performed using an
Dinuclear ruthenium(II) complex with 40-azulenyl-2,20:60,200-
terpyridine
electrochemical workstation (Autolab 12, Eco-Chemie, Utrecht,
Netherlands) coupled to a PC running the GPES software to
allow experimental control and data acquisition. A Pt (Metrohm, A mixture of RuCl3 (Sigma-Aldrich) (103.5 mg, 0.5 mmol) and
3 mm in diameter) working electrode, Ag (wire)/AgCl as the hydrazine hydrate (Sigma-Aldrich) (25 mg, 0.5 mmol) in methanol
reference electrode and a platinum disk as counter electrode (5 mL) was stirred under inert atmosphere, at room temperature
(Metrohm, 3 mm in diameter) were used in a three-electrode for 10 min. The methanolic solution was poured onto a suspen-
configuration. The surface of both platinum electrodes was sion of 40-azulenyl-2,20:60,200-terpyridine ligand, L,32 (179.5 mg,
polished with an alumina (0.05 mm) slurry on a polishing pad, 0.5 mmol) in methanol (10 mL) and heated in a microwave system
washed with distilled water and sonicated for 3 min in doubly at 100 1C for 15 min to yield a deep red solution. The methanol
distilled water then washed again with doubly distilled water. An was removed under vacuum and the obtained solid was washed
electrolyte solution containing 0.1 M tetra(n-butyl)-ammonium with dichloromethane to remove traces of unreacted ligand
c
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resulting in 504 mg of dark red crystalline powder (95% yield). 30 0 0,50 0 0-H), 9.04 (d, J = 8.9 Hz, 2H, 3,300-H), 9.35 (s, 2H, 30,50-H)
UV-Vis (MeOH) [lmax/nm (loge)]: 273 (4.98), 288 (5.0), 307 (5.01), ppm. 13C NMR (400 MHz, DMSO-d6): dC 157.2 (Cq), 153.7 (Cq),
380 (4.42), 513 (4.60); 1H NMR (400 MHz, DMSO-d6): dH7.34 148.1 (Cq), 140.35 (C4, C400), 132.2 (Cq), 130.6 (Cq), 129.03
(t, J = 6.8; 6.4 Hz, 2H, 5,500-H), 7.53 (t, J = 9.6 Hz, 1H, 50 0 0-H), (C30 0 0,C50 0 0), 127.56 (C20 0 0, C600 0), 127.13 (C5, C500), 126.02
7.63 (t, J = 9.6 Hz, 1H, 70 0 0-H), 7.65 (d, J = 6 Hz, 2H, 6,600-H), 7.76 (C6, C600), 122.48 (C3, C300), 118.07 (C30, C50), 21.01 (C7) ppm.
(d, J = 3.6 Hz, 1H, 30 00-H), 7.98 (t, J = 9.6 Hz, 1H, 60 00-H), 8.06 Anal. (%). Found (calc. for C44H34Cl4N6Ru2): C 53.64 (53.34);
(t, J = 7.6 Hz, 2H, 4,400-H), 8.7 (d, J = 3.2 Hz, 1H, 20 00-H), 8.71 H 3.54 (3.44); Cl 14.49 (14.34); N 8.35 (8.46). (ESI): 990 (M)
(d, J = 9.6 Hz, 1H, 400 0-H), 9.10 (d, J = 8.4 Hz, 2H, 3,300-H), 9.22 (m/z = 374, 788).
(d, J = 9.6 Hz, 1H, 80 0 0-H), 9.38 (s, 2H, 30,50-H) ppm. 13C NMR
(400 MHz, DMSO-d6): dC 158.2 (Cq), 154.7 (Cq), 152.1 (Cq), 144.2
Dinuclear ruthenium(II) complex with 2,20200-terpyridine
(Cq), 143.2 (C80 0 0), 139.8 (C4, C400), 138.5 (C600 0), 138.1 (C20 0 0), The dinuclear ruthenium(II) complex with 2,20200-terpyridine
136.6 (Cq), 135.8 (C400 0), 127.6 (C5, C500), 126.2 (C70 0 0), 126.05 was prepared following a procedure reported elsewhere.35
(C500 0), 125.5 (C6, C600), 124.8 (C3, C300), 123.4 (C30, C50), 118.7 Accordingly, RuCl3 (Sigma-Aldrich) (103.5 mg, 0.5 mmol) was
(C300 0) ppm. Anal. (%). Found (calc. for C50H34Cl4N6Ru2): C 56.58 reacted with boiling 6 M hydrochloric acid (Sigma-Aldrich) for
(56.51); H 3.27 (3.22); Cl 13.39 (13.34); N 7.98 (7.91). MS (ESI): 24 h. The resulting pentachloromono-aquoruthenate(III) was
1062 (M) (m/z = 410, 860).
separated as the potassium salt and recrystallized from 6 M
hydrochloric acid. Thereafter, to an acidic solution of the
soluble potassium salt was added a quantity of ligand only
slightly greater than the stoichiometric equivalent for the
Dinuclear ruthenium(II) complex with 40-(40 00,60 0 0,80 0 00
trimethyl-azulenyl)-2,20:60,200-terpyridine
-
A mixture of RuCl3 (Sigma-Aldrich) (103.5 mg, 0.5 mmol) and formation of the 1 : 1 (metal–ligand) complex, and an excess
hydrazine hydrate (Sigma-Aldrich) (25 mg, 0.5 mmol) in methanol of reducing agent such as or hydroxylammonium chloride
(5 mL) was stirred under inert atmosphere, at room temperature (Sigma-Aldrich). The solution was then heated to 50–60 1C for
for 10 min. The methanolic solution was poured onto a sus- several hours. Whilst at temperature, the pH of the solution was
pension of 40-(40 00,60 0 0,80 0 00-trimethyl-azulenyl)-2,20:60,200-terpyri- slowly and progressively raised with small additions of a dilute
dine (200 mg, 0.5 mmol) in methanol (10 mL) and heated in a sodium hydroxide solution until neutralization was reached. In
microwave system at 100 1C for 15 min to yield a brown-red this way the ruthenium present was quantitatively transformed
solution. The methanol was removed under vacuum and the into Ru2Cl4(tpy)2, which was isolated from thoroughly cooled
obtained solid was washed with dichloromethane to remove solutions (92% yield). 1H NMR (400 MHz, DMSO-d6): dH 7.54
traces of unreacted ligand resulting in 257 mg of dark red- (d, J = 6 Hz, 2H, 6,600-H), 7.6 (t, J = 6.8 Hz, 2H, 5,500-H), 8.43
brown crystalline powder (45% yield). UV-Vis (MeOH) [lmax/nm (t, J = 7.6 Hz, 2H, 4,400-H), 8.84 (t, J = 8.4 Hz, 2H, 3,300-H), 8.95
(log e)]: 271 (4.43), 308 (4.59), 477 (3.91). 1H NMR (400 MHz, (t, J = 7.4 Hz, 1H, 40-H), 9.23 (d, J = 8.0 Hz, 2H, 30,50-H). 13C NMR
DMSO-d6): dH 2.72 (s, 3H, Me(600 0)), 2.89 (s, 3H, Me(400 0)), 2.99 (400 MHz, DMSO-d6): dC 120.8 (C3, 300), 121.0 (C5, 500), 123.5
(s, 3H, Me(800 0)), 7.33 (t, J = 5.6 Hz, 2H, 5,500-H), 7.39 (s, 1H, (C30, 50), 136.6 (C4, 400), 137.6 (C40), 148.9 (C6, 600), 155.2
50 0 0-H), 7.53 (d, J = 5.6 Hz, 2H, 6,600-H), 7.62 (d, J = 4.0 Hz, 1H, (Cq, Cq), 156.1 (Cq, Cq). Found (calc. for C30H22Cl4N6Ru2):
30 0 0-H), 8.02 (t, J = 7.8 Hz, 2H, 4,400-H), 8.08 (s, 1H, 70 00-H), 8.12 C 44.52 (44.41); H 2.75 (2.72); Cl 17.49 (17.51); N 10.35 (10.37).
(d, J = 4.0 Hz, 1H, 20 00-H), 9.01 (d, J = 8.0 Hz, 2H, 3,300-H), 9.15 (ESI): 810 (M) (m/z = 284, 608).
(s, 2H, 30,50-H) ppm. Anal. (%). Found (calc. for C56H46Cl4-
N6Ru2): C 59.68 (60.10); H 3.61 (3.41); Cl 12.02 (12.10); N 6.59
(6.84). (ESI): 1104 (M) (m/z = 452, 902).
Results and discussion
The s-donor character of the cyclometalated ligand was pre-
Dinuclear ruthenium(II) complex with 40-(4-methylphenyl)-
2,20:60,200-terpyridine
viously reported to assist the ruthenium-promoted aerobic
oxidation.34a On this basis, polypyridyl complexes containing
A mixture of RuCl3 (Sigma-Aldrich) (103.5 mg, 0.5 mmol) and the Ru(IV)QO group have been observed to act as stoichiometric
hydrazine hydrate (Sigma-Aldrich) (25 mg, 0.5 mmol) in metha- or catalytic oxidants for a variety of organic and inorganic
nol (5 mL) was stirred under inert atmosphere, at room substrates.36 However, to achieve an effective oxidation catalyst
temperature for 10 min. The methanolic solution was poured therein, another key feature was reported to be the presence of a
onto a suspension of 40-(4-methylphenyl)-2,20:60,200-terpyridine Cl ligand that easily dissociate allowing the substrate to coordi-
(161.5 mg, 0.5 mmol) in methanol (10 mL) and heated in a nate to the vacant position. Lowering the redox potential neces-
microwave system at 100 1C for 15 min to yield a brown-red sary to enable the aerobic oxidation of the ruthenium center was
solution. The methanol was removed under vacuum and the achieved by introducing a second ligand.37
obtained solid was washed with dichloromethane to remove
Complexes with 2,20200-terpyridine presented no activity in
traces of unreacted ligand resulting in 257 mg of dark red- oxygen or aerobic oxidation of benzylamine that demonstrates
brown crystalline powder (56% yield). 1H NMR (400 MHz, the structure of this complex is not proper for this reaction. In
DMSO-d6): dH 2.51(s, 3H, 7-H), 7.49 (d, J = 6 Hz, 2H, 6,600-H), addition, the complex decomposed. This in line with previous
8.02 (t, J = 7.1 Hz, 2H, 5,500-H), 8.12 (t, J = 7.6 Hz, 2H, 4,400-H), reports showing that higher oxidation states of aquo or hydroxo
8.41 (d, J = 8.4 Hz, 2H, 20 0 0,600 0-H), 8.63 (d, J = 7.8 Hz, 2H, polypyridyl complexes of ruthenium are relatively unstable
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under certain reaction conditions.36 Under the same experi- selective oxidative conversion of amines to nitriles. For example,
mental conditions ruthenium(II) complexes with 40-(4-methyl- the best ruthenium-complex used for this catalytic reaction, a
phenyl)-2,20:60,200-terpyridine and substituted azulenyl fragment ruthenium–porphyrin catalyst, is able to convert both aliphatic
with electron donating groups like 40-(4,6,8-trimethyl-azulenyl)- and aromatic amines to the corresponding nitriles but only at
2,20:60,200-terpyridine led to poor conversions of 34 and 30%, high temperature (50 1C) with the best results reported for
respectively. This observation persuaded us to explore the 40 activated benzylamines.27 In comparison, multicomponent
modification of 2,20:60,200-terpyridine with a another donor elec- coupled systems involving ruthenium and a range of co-catalysts
tron group like azulenyl fragment.
with additional electron-rich components were less effective in
The Ru2Cl4(az-tpy)2 complex showed high catalytic activity nitrile formation with a maximum selectivity of 88% achieved for
for the oxidation of amines to nitrile with dioxygen (Scheme 1). benzonitrile.28 More recently, better results were reported for
The results are listed in Table 1. With low ruthenium catalyst ruthenium-catalyzed amine oxidation using cobalt oxides as the
loadings (Ru: 0.6 mol%), high reaction yields in short reaction support.25 However, this cobalt-supported ruthenium catalyst only
times (2–5 h) were achieved for the oxidation of both aliphatic proved to be efficient for the oxidation of activated benzylamines
and aromatic amines with –CH2NH2 functional groups. The again showing the importance of the Ru2Cl4(az-tpy)2 com-
catalytic oxidation of primary amines is temperature depen- plex described, herein, which efficiently catalyzes the oxidation
dent, for example, oxidation of benzylamines to benzonitriles of both aliphatic and aromatic amines with high selectivity
in methanol at room temperature reached 25% yield after 5 h in the corresponding nitriles. It is worth emphasising the
whilst at 60–70 1C quantitative conversion occurred within 2 h. fact that, the Ru2Cl4(az-tpy)2 complex was also capable of
It should be noted that no benzonitrile was formed under selectively oxidizing ethylamine and bulky iso-butylamine.
identical conditions in the absence of the ruthenium catalyst. This is unusual for the ruthenium-catalyzed oxidation of amines
Oxidation of aliphatic amines with long alkyl chains required to nitriles.
shorter reaction times. For example, n-dodecylamine and n-
Repeating these experiments in air (10 atm, 75 1C) compared
stearylamine were converted into the corresponding nitriles with pure O2 led to lower conversions (Table 2). For example the
within 5 h, while after 2.5 h the conversions were 62% and 58%, total conversion of benzylamine to benzonitrile was achieved in
respectively. In contrast, after 5 h, only 8% conversion 5 h in air compared with 2 h under O2. Under, these conditions
was found for ethylamine. For butylamines an effect of the ethylamine and iso-butylamine gave no reaction. With both air
steric environment has been shown. Thus, the conversion of and O2 100% selectivity was observed in all cases.
n-butylamine after 5 h was 15%, while for the iso-isomer 24 h
In all these examples, after reaction, the catalyst was found
were necessary to reach 18%. The oxidation of n-hexylamine to be recyclable at least three times. For benzylamine the
and n-octylamine occurred with a higher reaction rate than the oxidation occurred with no change in conversion in the first
oxidation of n-butylamine but with the same selectivity.
two cycles and decreased to 96% after the third cycle.
In all the cases the selectivity was 100% to the corresponding
As it has been mentioned in the experimental part the
nitrile. Compared with previous reported results using ruthenium catalyst showing the highest activity, i.e. the di-nuclear species
catalysts, this represents a significant progress concerning the of type Ru2Cl4(az-tpy)2 resulted from the stoichiometric reac-
tion of 40-azulenyl-2,20:60,200-terpyridine (az-tpy) with ruthenium
chloride and hydrazine hydrate in methanol under microwave
irradiation for 15 min (Scheme 2).
The diamagnetic nature of ruthenium(II) ions allowed the
characterisation of the complex by NMR spectroscopy. The
1H-NMR spectrum showed significant differences when com-
pared with the spectrum of the free ligand (Fig. S1, ESI†). The
shift in the signals are mainly due to the influence of the metal
electrons on the magnetic field of the organic framework with
the terpyridine moieties locked in the cis position. The nitrogen
ligation of the ligand is also confirmed by upfield 13C reso-
nances of the adjacent groups comparing the free ligand and
the metal complex (Fig. S2, ESI†). Furthermore, the UV-Vis
Scheme 1 Direct oxidation of amines with oxygen (R = alkyl, aryl).
Table 1 Ruthenium(II) catalysed oxidation of amines with oxygen (60 1C)
Time Conversion
Selectivity
(%)
Substrate
(h)
(%)
Product
n-Dodecyl amine
n-Stearylamine
Benzylaminea
Ethylamine
n-Butylamine
iso-Butylamine
n-Hexylamine
n-Octylamine
2-Methoxy
benzylamine
4-Methyl
benzylamine
4-Chloro
5
5
2
5
5
24
5
5
100
100
100
8
15
18
58
67
34
n-Dodecane nitrile 100
n-Stearylo nitrile 100
Benzonitrile
Acetonitrile
n-Butanenitrile
100
100
100
iso-Butyronitrile 100
Table 2 Ruthenium(II) catalyzed oxidation of amines with air
n-Hexanenitrile
n-Octanenitrile
2-Methoxy
100
100
100
Time Conversion
Selectivity
(%)
5
Substrate
(h)
(%)
Product
benzonitrile
4-Methyl
benzonitrile
4-Chloro
5
5
100
66
100
100
n-Dodecyl amine
n-Stearylamine
Benzylamine
Ethylamine
iso-Butylamine
5
5
5
5
5
50
30
100
0
n-Dodecane nitrile 100
n-Stearylo nitrile
Benzonitrile
—
100
100
—
benzylamine
a
benzonitrile
0
—
—
Room temperature, after 5 h, 25% benzonitrile formation.
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Scheme 2 Schematic representation for formation of dinuclear ruthenium(II)
complex with 40-azulenyl-2,20:60,200-terpyridine.
spectrum of the complex is characterized by the intense absorp-
tion bands attributed to the p–p* transitions associated with
the aromatic rings of the ligands. The metal to ligand charge
transfer (MLCT) transition of the ruthenium complex was
observed at around 513 nm (Fig. S3, ESI†).
The dinuclear nature of the catalyst was assigned by mass-
spectrometry and further confirmed by electrochemical inves-
tigations both by cyclic voltammetry (CV) and differential pulse
voltammetry (DPV). The CV of the dinuclear complex exhibits
two relatively well-defined coupled peaks, confirmed also by
DPV measurements. The cyclic voltammograms of Ru2Cl4-
(az-tpy)2 solution (0.05 mM) at different scan rates, are pre-
sented in Fig. 1. As expected, these show two anodic peaks on
the forward scan and two cathodic peaks on the reverse scan,
thus confirming the proposed structure. The first anodic couple
appears at a half-peak potential E1/2,1 = 0.166 V while, the
second at E1/2,2 = 0.454 V, each showing a very small variation
with the variation of scan rate between 50 mV sꢀ1 to 500 mV sꢀ1
of B8 mV and 4 mV, respectively. The peak-to-peak separations
are DEp,1 = 70 mV and DEp,2 = 65 mV demonstrating that they
are almost reversible. These results are in good agreement with
Fig. 2 DPV-traces, after baseline correction procedure, of 0.5 mM Ru-az-tpy
(curve a) and 0.5 mM az-tpy (curve b), with SP = 10 mV and MA = 50 mV, in the
potential range 0.0–1.0 V.
Small anodic anodic peaks, shoulder-like, were also
observed in DPV experiments (Fig. 2). The DPV experiment
(Fig. 2, curve a) exhibits two other small anodic peaks at more
positive potentials. The first one, shifted in the anodic direc-
tion, at 0.624 V is assigned to the anodic potential of the ligand.
The second, at 0.896 V, is attributed to a supplementary
oxidation of the Ru3+Ru3+ to Ru3+Ru4+ and occurs at a very
low anodic current. The ligand is, however, stabilized by the
ruthenium coordination and is harder to be oxidized as com-
pared with the free ligand in solution. The DPV-trace of the
ligand indicates an anodic peak occurring at 0.564 V (Fig. 3,
curve b).
the anodic peak potentials determined from DPV (Epa,1
=
0.141 V and Epa,2 = 0.433 V). This behaviour is consistent
with two single electron reactions according to the following
equations:
A key feature of the ruthenium catalyst is the presence of
labile bonds that makes the attack of the substrate at the metal
centres possible. Therefore, the presence of halides (in our case
chloride) as co-ligands in the coordination sphere of the
Ru2+Ru2+ - Ru2+Ru3+ + eꢀ
Ru2+Ru3+ - Ru3+Ru3+ + eꢀ
Fig. 1 Cyclic voltammograms of a solution of 0.5 mM Ru2Cl4(az-tpy)2 at different
scan rates (50 – black, 100 – red, 200 – blue, 300 – magenta, 500 – navy mV sꢀ1),
over the potential range 0.0–1.0 V.
Fig. 3 DRIFT spectra of the fresh (a) and spent ruthenium–terpyridyl complexes
following the reaction with D2N–C6H5 (b).
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ruthenium centers contributes to an increased catalytic activity
of the dinuclear ruthenium(II) compound described, herein.
Aiki et al. suggested that the redox behavior of the ruthenium(II)
complex is the reason for its catalytic effectiveness with a low
redox potential indicating a higher catalytic activity towards
oxidation reactions.33 Our data confirms this proposal. The
insertion of the azulenyl fragment into the terpyridine scaffold
led to an increased catalytic activity for the direct oxidation
of amines with molecular oxygen. Unsubstituted terpyridine–
ruthenium(II) complexes were reported to smoothly oxidize
reactive benzylamines, depending on the used co-ligand, with
no information for alkyl amines or benzylamine.33 The data
presented in this study demonstrates that, by using the azulenyl
substitution of the terpyridine unit, very good conversions and
selectivities can be achieved. Both aromatic and aliphatic sub-
strates can be oxidized under benign, environmentally friendly
conditions. Brandt and Wright38 reported that terpyridine acts as
a diacid base while Nakamoto39 concluded that in basic solution
and in organic solvents, the terpyridine molecule is trans–trans,
in acidic solution the cis–cis form predominates, and at inter-
mediate pH values the cis–trans form is present. The central ring
in terpyridine acquires increased aromatic character because of
the pyridine rings at the 2- and 6-positions.40 Further modifica-
tion of terpyridine with azulene may enhance the basicity of the
central atom of pyridine.
The catalytic behaviour was confirmed by both the electro-
chemical measurements and by substituting azulenyl fragment
with both donor and acceptor groups. Thus, after the substitu-
tion of the azulenyl fragment with electron donating groups
like for example 4,6,8-trimethyl-azulene, the catalytic activity of
the ruthenium(II) complex dramatically dropped and the oxida-
tion of benzyl amine to benzonitrile took place with a maxi-
mum conversion of B30%. An explanation of this behaviour is
likely to be associated with the steric hindrance induced by
the methyl groups which will limit the access of the substrate
to the ruthenium centres. This assessment is also supported by
the electrochemical investigations where the oxidation of the
ruthenium centres occurs at a more positive potential (0.876 V).
However, further work on substituted azulenyl moiety in the
terpyridine–ruthenium(II) complexes is underway to elucidate
these effects.
Fig. 4 IR spectrum of D2N–C6H5 (black) and product oxidation – benzonitrile
(red).
deuterated benzylamine. Moreover, two new bands at 1642 and
1576 cmꢀ1 are observed in the spectrum of the mixture corre-
sponding to the formation of benzonitrile. No other exchange
was observed between C6H5–CH2–ND2 and C6H5–CH2–NH2 in
the presence of water or generated D2O. Oxidation appeared to
be faster than additional isotopic exchange. 100% conversion
was observed for C6H5–CH2–NH2 compared with 44% for C6H5–
CH2–ND2 after 2 h was observed indicating that N–H cleavage is
involved in the rate determining step.
Fig. 5 provides DRIFTs evidence of the evolution of the
kinetics of the reaction. The decrease of the intensity of the
NH band mapped the increase of the intensity of the CN band.
Based on the experimental evidence from this study, and in
full agreement with previous reports,24 a reaction mechanism
has been proposed (Scheme 3). In the first step, Ru exchanges
chlorine with a hydroxyl ligand43 which further reacts with the
amine. This ruthenium amine intermediate formed subsequently
undergoes a b-hydrogen elimination to yield the corresponding
imine and ruthenium hydride. A rapid dehydrogenation of the
imine to form the nitrile occurs, while the catalyst is reactivated
via the oxidation of the formed hydride by dioxygen. Thus, the
water which initiates the catalytic reaction comes from the used
solvent (methanol). To confirm that the effect of water in step (I) is
In order to investigate the possible reaction mechanism,
experiments with deuterated benzylamine (D2N–CH2C6H5)
were performed. Whilst the reaction was similar, the DRIFT
spectra showed significant changes in the state of the catalyst.
Fig. 3 shows spectra of fresh (a) and spent ruthenium–terpyridyl
complexes (b) in the reaction with D2N–CH2C6H5. The new
bands in the spectra of deuterated ruthenium–terpyridyl
complex at 2780 cmꢀ1 band41 and 620 cmꢀ1 (uncoupled
gOD)42 suggest (i) the formation of DHO (D2O) in step (II), and
then (ii) the further exchange of the catalyst OH group with
deuterium in step (I) during recycle. These results are confirmed
by a comparison of IR spectra of the pure D2N–CH2C6H5 and
reaction mixture (Fig. 4). This comparison confirms the forma-
tion of benzonitrile and the disappearance of the specific N–D
stretching vibration recorded at 2463 cmꢀ1 in the IR spectrum of
Fig. 5 DRIFTs evidence of the evolution of the kinetics of the reaction.
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Paper
(ID POSDRU/89/1.5/S/58852) and FP-7 SYNFLOW and CNCS
113EU/2011 projects are gratefully acknowledged for the
financial support.
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