A Cu(II) MOF with a flexible bifunctionalised terpyridine as an efficient catalyst for the single-pot hydrocarboxylation of cyclohexane to carboxylic acid in water/ionic liquid medium

Article abstract of DOI:10.1039/c6dt01852a

The copper(ii) metal-organic framework (MOF) [Cu(η-1κN,N′,N′′:2κO,O′-L)(NO₃)]_n (1) [L = 4-((4-([2,2′:6′,2′′-terpyridin]-4′-yl)benzyl)oxy)benzoate] has been synthesized from a flexible bifunctionalised terpyridine species (HL). It was characterized by elemental, FT-IR, powder and single crystal X-ray diffraction analyses. Single crystal X-ray crystallography of 1 shows a 1D polymeric architecture, whose topological analysis illustrates a uninodal (2)-connected net having topological type 2C1. 1 shows good catalytic activity and selectivity towards the hydrocarboxylation of cyclohexane to cyclohexanecarboxylic acid in water/acetonitrile or water/ionic liquid [BMPyr][NTf₂] [BMPyr = 1-butyl-1 methylpyrrolidinium; NTf₂ = bis(trifluoromethanesulfonyl)imide] medium. It can be recycled and reused without any significant loss of catalytic efficiency. This study provides the first example of an efficient alkane hydrocarboxylation to carboxylic acid, in an ionic liquid and under mild conditions.

Full text of DOI:10.1039/c6dt01852a

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A Cu(II) MOF with a flexible bifunctionalised terpyridine as an efficient
catalyst for the single-pot hydrocarboxylation of cyclohexane to carboxylic
acid in water/ionic liquid medium
Anup Paul,*,† Ana P. C. Ribeiro,*,† Anirban Karmakar,† M. Fátima C. Guedes da Silva,*,†
Armando J. L. Pombeiro*,†
†Centro de Química Estrutural, Complexo I, Instituto Superior Técnico, Universidade de
Lisboa, Av. Rovisco Pais, 1049ꢀ001 Lisboa. Portugal.
Abstract:
The copper(II) metalꢀorganic framework (MOF) [Cu(ηꢀ1κN,N’,N’’:2κO,O’ꢀL)(NO3)]n
(1) [L = 4ꢀ((4ꢀ([2,2':6',2''ꢀterpyridin]ꢀ4'ꢀyl)benzyl)oxy)benzoate] has been synthesized from a
flexible bifunctionalised terpyridine species (HL). It was characterized by elemental, FTꢀIR,
powder and single crystal Xꢀray diffraction analyses. Single crystal Xꢀray crystallography of 1
shows a 1D polymeric architecture, whose topological analysis illustrates a uninodal (2)ꢀ
connected net having topological type 2C1. 1 shows a good catalytic activity and selectivity for
the hydrocarboxylation of cyclohexane to cyclohexanecarboxylic acid in water/acetonitrile or
water/ionic liquid [BMPyr][NTf2] [BMPyr = 1ꢀbutylꢀ1ꢀmethylpyrrolidinium; NTf2
=
bis(trifluoromethanesulfonyl)imide] medium. It can be recycled and reused without any
significant loss of catalytic efficiency. This study provides the first example of an efficient
alkane hydrocarboxylation to carboxylic acid, in an ionic liquid and under mild conditions.
*Corresponding
authors,
E-mail:
kanupual@gmail.com,
ana.paula.ribeiro@ist.utl.pt,
fatima.guedes@tecnico.ulisboa.pt, pombeiro@tecnico.ulisboa.pt.
Introduction
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Metalꢀorganic frameworks (MOFs) as a class of wellꢀordered porous crystalline materials have
captivated an enormous interest in recognition to their structural (oneꢀ, twoꢀ or threeꢀ
dimensional infinite networks), functional diversity and tailorability as well as high porosity
and large surface area.1 Their potential applications in storage of fuel and environmental
gases,2,3 molecular separation,4 luminescence,5 conductive materials,6 drug delivery,7 sensing8
and in catalysis9ꢀ13 have spurred a high attention. The MOF structures and properties are
largely dependent on the organic linkers which play a pivotal role.14 Rigid benzene diꢀ, triꢀ, and
tetraꢀcarboxylic acids and azolateꢀbased ligands, as well as their derivatives, have been
extensively employed to construct robust and highly porous coordination frameworks with
versatile structures.14 Further, bifunctionalized terpyridine based ligands and derived
complexes have been explored for their tunable photophysical and electrochemical
properties.15,16 Among the various terpyridine based bifunctionalised ligands, rigid terpyridines
bearing the carboxylate function, such as 4´ꢀ(4ꢀcarboxyphenyl)ꢀ2,2´:6´,2´´ꢀterpyridine15 and 4′ꢀ
(4ꢀcarboxyphenyl)ꢀ2,2′:6′,2″ꢀterpyridine,16 have been reported.
However, in contrast to the rigid bifunctionalized chelating terpyridine ligands, the
flexible analogues which may generate unprecedented coordination frameworks, have rarely
been explored.17 Thus, in our continuing effort on the synthesis of multidimensional MOFs,
we have designed and synthesized a new flexible terpyridine carboxylic acid derivative, 4ꢀ((4ꢀ
([2,2':6',2''ꢀterpyridin]ꢀ4'ꢀyl)benzyl)oxy)benzoic acid (HL, Fig. 1), considering the following
characteristics: (i) the terpyridine moiety in the ligand framework possesses a strong chelating
coordination ability; (ii) the flexible methylene moiety in the ligand backbone can rotate freely
to produce different conformations; (iii) the introduction of a carboxylate as an Oꢀdonor site
2

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with possible bridging/chelating modes can lead to the construction of MOFs with diverse
structures and topologies.15,16
Further, the role of MOFs in catalysis in various organic transformations is gaining relevance.9ꢀ
13
A field of potential high catalytic significance concerns the functionalization of saturated
hydrocarbons (alkanes), major constituents of natural gas and oil, into high value oxygenated
species.18 But transformations of alkanes often require harsh conditions and the use of strongly
acidic media,19 and the development of acidꢀsolventꢀfree and/or aqueous medium alternatives20
is of major significance in areas of green and sustainable Chemistry. The use of ionic liquids
(ILs) is a green alternative in a wide range of areas, including industrially important processes.
Such applications can be found in chemical synthesis, catalysis, separation and extraction as an
environmentally benign alternative to organic solvents21ꢀ22; energy storage and conversion
devices.21k The ILs are very appealing mainly due to their nonꢀmeasurable vapour pressure
under ambient conditions and their ability to dissolve transitional metal complexes for catalytic
purposes.22 In terms of yield and selectivity, important improvements on the catalytic
performances have been observed in some cases when using an IL medium.23b,d, 22fꢀi They can
act a solvent, a catalyst, a coꢀcatalyst, a support or a ligand,21l therefore influencing markedly
the catalytic reaction pathway.21 In view of the particular nature of an IL, its use can lead to a
biphasic system and to the immobilization of the catalyst, facilitating its separation and
recycling. In spite of that, the use of greener ILs than conventional organic solvents is still
understudied.23ꢀ24 And, in view of the environmentally benign properties of ILs,23 this use can
provide an alternative for developing
a
greener method namely in alkane
hydrocarboxylation,21ꢀ22 a type of reaction for which ILs have not yet been applied.
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To our knowledge, bifunctionalized chelating 2,2′:6′,2″ꢀterpyridine flexible ligand
metal complexes have never been reported for such catalytic studies. Herein we report the
synthesis and full characterization of a new Cu(II) MOF with a flexible terpyridine ligand with
a coordinating substituent, and its catalytic application in the hydrocarboxylation of
cyclohexane to cyclohexanecarboxylic acid in aqueous acetonitrile and in aqueous IL medium.
Moreover, the current study appears to provide the first example of an efficient alkane
functionalization in an IL medium using a MOF, as well as of hydrocarboxylation of an alkane
in an IL medium. It discloses important advantages of the use of an IL over that of an organic
solvent (higher yield, selectivity and the possibility of recycling the catalyst).
Fig. 1. 4ꢀ((4ꢀ([2,2':6',2''ꢀterpyridin]ꢀ4'ꢀyl)benzyl)oxy)benzoic acid (HL).
Experimental Section
Materials and methods
Solvents were dried and distilled prior to their use following standard procedures.25 Water used
in all the syntheses and catalytic studies were double distilled. [BMPyr][NTf2] were prepared
according to published procedures as reported earlier.26 The reagents used for the syntheses of
the terpyridine substituted compound and Cu(NO3)2.2.5H2O were purchased from Sigma
Aldrich Chemical Co. and used as received. IR spectra were recorded on a Bruker Alpha
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1
Fourier transform infrared (FTꢀIR) spectrometer. H (300 MHz) and 13C (75.45 MHz) NMR
spectra were obtained at room temperature (RT) on a Bruker 300 MHz spectrometer using
tetramethylsilane [Si(CH3)4] as an internal reference. Elemental analyses were carried out by
the Microanalytical Service of the Instituto Superior Técnico. Electrospray mass spectra (ESIꢀ
MS) were run with an ionꢀtrap instrument (Varian 500ꢀMS LC Ion Trap Mass Spectrometer)
equipped with an electrospray ion source. GC analyses were performed on a Fisons
Instruments GC 8000 series gas chromatograph with a DB WAX (J&W) capillary column (P/N
122–7032; 30 m×0.25 mm×0.25 ꢁm; helium was the carrier gas) and the JascoꢀBorwin v.1.50
software. In some cases, hydrocarboxylation products were also identified by GCꢀMS
techniques using a PerkinꢀElmer Clarus 600 gas chromatograph, equipped with two capillary
columns (SGE BPX5; 30 m 0.32 mm 25 mm), one having an EIꢀMS (electron impact) detector
and the other one with a FID detector, were used for analysing the reaction mixtures. Helium
was used as the carrier gas.
Synthesis and characterization
Synthesis of 4-((4-([2,2':6',2''-terpyridin]-4'-yl)benzyl)oxy)benzoic acid (HL)
The synthesis of the target compound, HL, involves multiple steps. The intermediate
precursors were synthesized according to a published procedure,27 described in the supporting
information (ESI).
In the subsequent step, a mixture of methylꢀ4ꢀhydroxybenzoate (152 mg; 1 mmol) and K2CO3
(552 mg; 4 mmol) in 30 mL of acetonitrile was stirred for 1 h at room temperature. 4'ꢀ[4ꢀ
(bromomethyl)phenyl]ꢀ2,2':6',2"ꢀterpyridine (BMPT) (402 mg; 1 mmol) was then added and
the mixture was left to reflux for 24 h. The warm solution was filtered on the following day
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and the solvent was removed to dryness in a rotary evaporator. The pale yellow solid thus
obtained was suspended in water and extracted with CH2Cl2. The organic phase was dried over
anhydrous Na2SO4, filtered and the solvent was evaporated to dryness to afford 4'ꢀ[methylꢀ4ꢀ
(benzyloxy)benzoate]ꢀ2,2':6',2"ꢀterpyridine (MBTP) as a pale yellow solid. Yield: 82%. Anal.
Calcd for C30H23N3O3: C, 76.09; H, 4.90; N, 8.87. Found: C, 75.91; H, 4.92; N, 8.83. IR
1
(KBr, cmꢀ1): 1750 (s), 1530 (m), 1380 (s), 1130 (m), 1092 (w). HꢀNMR (δ, ppm, 300 MHz;
DMSOꢀd6): 8.74 (m, 4H, ArꢀH), 8.79 (d, 2H, ArꢀH), 8.34 (d, 2H, ArꢀH), 7.96 (d, 2H, ArꢀH),
7.88 (m, 4H, ArꢀH), 7.63 (d, 2H, ArꢀH), 7.39 (d, 2H, ArꢀH), 5.21 (s, 2H, ꢀOCH2), 3.97 (s, 3H, ꢀ
13
OCH3). C NMR (δ, ppm, 75.45 MHz ; DMSOꢀd6): 167.4, 156.6, 154.7, 153.9, 148.5, 145.4,
136.8, 134.1, 131.3, 128.1, 127.4, 125.8, 124.0, 123.7, 120.1, 119.3, 67.9, 53.1.
MBTP was then hydrolyzed, where its (227 mg; 0.48 mmol) solution in 10 mL of THF was
warmed up while stirring to complete dissolution. To this 10 mL of an aqueous solution of
KOH (420 mg; 7.4 mmol) was added and the final mixture was left to reflux for 24 h. This was
then filtered and the volume of the solvent was reduced to ca. 10 mL, whereupon conc. HCl
was added until pH~1. The desired product (HL) thus precipitated upon addition of conc. HCl
was collected by filtration, washed several times with water until neutral and dried. Yield:
65%. Anal. Calcd for C29H21N3O3: C, 75.80; H, 4.61; N, 9.14. Found: C, 75.59; H, 4.59; N,
1
9.17. IR (KBr, cmꢀ1): 1703, ν(COOH)asym. HꢀNMR (δ, ppm, 300MHz; DMSOꢀd6): 12.66 (s,
1H, ꢀCOOH), 8.77 (m, 4H, ArꢀH), 8.69 (d, 2H, ArꢀH), 8.34 (d, 2H, ArꢀH), 7.97 (d, 2H, ArꢀH),
7.92 (m, 4H, ArꢀH), 7.63 (d, 2H, ArꢀH), 7.40 (d, 2H, ArꢀH), 5.32 (s, 2H, ꢀOCH2). 13C NMR (δ,
ppm, 75.45 MHz; DMSOꢀd6): 166.8, 158.9, 155.6, 154.8, 149.6, 149.3, 138.5, 137.3, 133.3,
128.9, 127.6, 125.2, 123.0, 121.8, 119.8, 118.7, 69.7. ESIꢀMS: m/z [(M + H)]+, calcd. 459.16,
found 460.43, [(M + Na)]+, calcd. 482.16, found 482.1.
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Synthesis of [Cu(η-1κN,N’,N’’:2κO,O’-L)(NO3)]n (1)
A mixture of Cu(NO3)2.2.5H2O (10 mg; 0.0217 mmol) and HL (20 mg; 0.0217 mmol) in 1 mL
of DMF was sealed in a 8 mL glass vessel and heated at 75 ˚C for 24 h. Green crystals of 1
were obtained upon gradual cooling to room temperature, which were washed with deionized
water, DMF and then dried in air. Yield: 60% (based on Cu). Anal. Calcd for C32H27CuN5O7:
C, 58.49; H, 4.14; N, 10.66. Found: C, 58.53; H, 4.12; N, 10.64. IR (KBr, cmꢀ1): 3447 (bs),
1604 (m), 1384 (s), 1261 (w), 1161 (w), 1020 (w).
Crystal structure determination
A single crystal of 1 was leaped in cryoꢀoil, mounted in a nylon loop and measured at room
temperature. Intensity data were collected using a Bruker APEXꢀII PHOTON 100
diffractometer with graphite monochromated MoꢀKα (λ = 0.71069) radiation. Data were
collected using phi and omega scans of 0.5° per frame and a full sphere of data was obtained.
Cell parameters were retrieved using Bruker SMART28a software and refined using Bruker
SAINT28a on all the observed reflections. Absorption corrections were applied using
SADABS.28a Structures were solved by direct methods by using the SHELXSꢀ2014/6
package28b and refined with SHELXLꢀ2014/6.28b Calculations were performed using the
WinGX System–Version 1.80.03.28c The hydrogen atoms were inserted in calculated positions
and included in the refinement using the ridingꢀmodel approximation; Uiso (H) were defined as
1.2 Ueq of the parent carbon atoms for phenyl and methylene residues.
PLATON/SQUEEZE28d was used to correct the data since in compound 1 a potential volume
of 996 Å3 was found with 308 electrons per unit cell worth of scattering. The electron count
suggests the presence of one dimethylformamide molecule per asymmetric unit. These were
removed from the model but included in the empirical formula. The elemental analyses also
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support this result. Least square refinement with anisotropic thermal motion parameters for all
the nonꢀhydrogen atoms and isotropic ones for the remaining atoms were employed.
Crystallographic data are summarized in Table 1 and selected bond distances, angles and
hydrogen bonding interactions are presented in Tables S1ꢀS2.
Carboxylation of cyclohexane to carboxylic acid
In a typical experiment the reaction mixtures were prepared as follows: to 2.0–16.0 ꢁmol of
complex 1 contained in a 13.0ꢂmL stainless steel autoclave, equipped with a Teflonꢀcoated
magnetic stirring bar, were added 1.50 mmol of K2S2O8, 3.0 mL of H2O, 3 mL of MeCN or
[BMPyr][NTf2] (typical total solvent volume was 6.0 mL) and 1.00 mmol of cyclohexane.
Then the autoclave was closed and flushed with N2 three times for removing the air and
pressurized with 5–40 atm (typically 20 atm) of CO. The reaction mixture was stirred for 3–9 h
(typically 6 h) at 30–80ꢂ°C (typically 60ꢂ°C) using a magnetic stirrer and an oil bath, where
upon it was cooled in an ice bath, degassed, opened and the contents were transferred to a
Schlenk flask. Diethyl ether (9.0–11.0 mL) and 90 ꢁL of cycloheptanone (GC internal
standard) were added. The obtained mixture was vigorously stirred for 10 min, and the organic
layer was analysed typically by GC.
Additional experiments were performed under typical reaction conditions in the presence of the
carbonꢀcentred radical trap CBrCl3 (used in an equimolar amount relatively to cyclohexane),
revealing the decrease of 87ꢀ95% of carboxylic acid formation, either in the presence or in the
absence of the metal promoter.
Results and Discussion
Syntheses and Characterisation
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As depicted in Scheme S1 (ESI), 2ꢀacetylpyridine and 4ꢀmethylbenzaldehyde were used as the
starting materials for the synthesis of 4'ꢀ(4ꢀ(bromomethyl)phenyl)ꢀ2,2':6',2''ꢀterpyridine
(BMPT). Its coupling with methyꢀ4ꢀhydroxybenzoate yielded the key building block 4ꢀmethyl
4ꢀ((4ꢀ([2,2':6',2''ꢀterpyridin]ꢀ4'ꢀyl)benzyl)oxy)benzoate (MBTP) which upon hydrolysis yielded
the desired 4ꢀ((4ꢀ([2,2':6',2''ꢀterpyridin]ꢀ4'ꢀyl)benzyl)oxy)benzoic acid (HL, Scheme 1).
Solvothermal reaction of HL with Cu(NO3)2.2.5H2O in N,Nꢀdimethylformamide affords the
MOF [Cu(ηꢀ1κN,N’,N’’:2κO,O’ꢀL)(NO3)]n (1) (Scheme 1).
Both HL and 1 were characterized by elemental analyses, IR and multinuclear (1H and
1
13C in case of HL) NMR techniques. The H NMR spectrum of HL exhibits characteristic
resonances due to the –COOH and –CH2 protons at δ 12.51 and 5.32, respectively (Fig. S1).
The phenyl ring protons resonate in the range of δ 8.84ꢀ7.14. In its 13C NMR spectrum, the
signals due to ꢀCOOH and –CH2 appear at δ 164.30 and 30.91, respectively, along with other
resonances at their usual positions (Fig. S2). The FTꢀIR spectrum of the free proꢀligand HL
displays a sharp and strong band at 1703 cmꢀ1 attributable to [ν(OCO)] stretching vibrations
which upon coordination to the metal centre shifts to a lower frequency at 1603 cmꢀ1,
suggesting the formation of 1 through deprotonation of the carboxylic acid group (Figs. S3ꢀ
S4).13 Additionally, the appearance of a sharp and strong band at 1384 cmꢀ1 in the IR spectrum
of the complex 1 suggests the presence of a coordinated nitrate to the metal (Fig. S4).29 The
crystal structure of 1 was obtained by single crystal Xꢀray crystallography analysis (discussed
below). Further, to substantiate that the crystal structure is representative of the synthesized
bulk materials, powderꢀXRD assessments were performed. The experimental and computerꢀ
simulated powderꢀXRD patterns of 1 clearly show that the synthesized bulk materials are of a
similar nature (Fig. S5, ESI).
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O
O
O
OH
Br
O
O
H O
THF:
2
nzoate
ydroxybe
ethylꢀ4ꢀh
M
reflux 24 h
reflux 24 h
K2CO3, MeCN,
N
N
N
N
N
N
N
N
N
BMPT
MBTP
HL
0
O2NO
O
N
N
Cu
O
O
Cu
O
Cu
N
N
O
O
N
N
1
Scheme 1: Synthesis of HL and derived complex 1.
Crystal Structure Analysis of 1
Complex 1 crystallizes in the monoclinic C2/c space group, its asymmetric unit containing one
Cu2+ ion, one Lꢀ ligand and one coordinated nitrate anion as depicted in Fig. 2A. The
compound features a oneꢀdimensional coordination polymeric chain with the organic ligand L−
chelating one copper cation with the three terpyridine Nꢀatoms, and bridging towards a
symmetry–generated metal centre, binding it with carboxylate Oꢀatoms (Fig. 2B). The Cu1
centres present a highly distorted octahedral environment (quadratic elongation of 1.095 and
angle variance of 208.12º2),30 the four equatorial sites being occupied by one carboxylate
oxygen atom of a Lꢀ unit [Cu1–O2, 1.918(4) Å] and three Nꢀatoms from terpyridyl unit [Cu1–
N1, 1.922(5) Å, Cu1–N2 2.037(6) Å and Cu1–N3, 2.031(6) Å]. The axial sites are engaged
with a nitrate anion [Cu1–O4, 2.282(6) Å] and the remaining Oꢀcarboxylate atom [Cu1–O3
2.690(7) Å]. The longer axial CuꢀO bond length relative to the equatorial one can be due to
Jahn–Teller distortion.31
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In this framework the organic ligand 4ꢀ((4ꢀ([2, 2’:6’, 2’’ꢀterpyridin]ꢀ4'ꢀyl)benzyl)oxy)benzoate
(Lꢀ) is not planar. While the carboxylateꢀcontaining phenyl ring is almost perpendicular to the
plane defined by the terpyridyl unit (the respective leastꢀsquare planes make an angle of
88.01o), the other phenyl ring is markedly twisted (dihedral angle of 44.07o between that ring
and the terpyridyl unit). The structure expands to the third dimension by means of contact
interactions involving the aromatic Hꢀatoms as donors and the nitrate or carboxylate Oꢀatoms
as acceptors (Fig. 2C and 2D, Table S2).
(A)
(B)
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(C)
(D)
Fig. 2. Fragments of (A) the molecular structure of 1 with partial atom labelling scheme, (B)
the 1D chain, (C) the packing diagram and, (D) the hydrogen bonded assembly (generic
views).
To improve the description of the crystal structure of 1, we performed topological analysis by
reducing their multidimensional structures to simple nodeꢀandꢀlinker nets. We have carried out
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these analyses by using TOPOS 4.0.32 The 1D chain of the compound represents uninodal (2)ꢀ
connected nets having topological type 2C1, which results from the metal coordination to two
Lꢀ ligands in line connected with two Cu centres (Fig. 3).
Fig. 3. Nodeꢀandꢀlinkerꢀtype description of the 1D coordination polymer 1. The metal nodes
are represented in green and the linkers in pink color.
Catalytic activity of MOF 1 for the hydrocarboxylation of cyclohexane to
cyclohexanecarboxylic acid
The single pot, lowꢀtemperature hydrocarboxylation reaction of the cyclohexane with carbon
monoxide, water and potassium peroxodisulfate in water/acetonitrile or water/ionic liquid
medium was studied. The IL chosen for this work is 1ꢀbutylꢀ1ꢀmethylpyrrolidinium
bis(trifluoromethanesulfonyl)imide, [BMPyr][NTf2] (Scheme 2), in view of its stability
towards CO.33 By this simple method, the cyclohexane is transformed into the corresponding
cyclohexanecarboxylic acid (Scheme 3). The method has the advantage of using an aqueous
medium and, for the first time, an IL, under mild conditions, without the use of an acid, such as
trifluoroacetic acid. To our knowledge, this is the first report of alkane hydrocarboxylation at
an IL.
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Scheme
2:
1ꢀbutylꢀ1ꢀmethylpyrrolidinium
bis(trifluoromethanesulfonyl)
imide,
[BMPyr][NTf2].
The hydrocarboxylation reaction of cyclohexane (Scheme 3) was performed in a stainless steel
autoclave by allowing to react at typical temperatures of 30–60 ºC and in two different solvent
mixtures (H2O/MeCN and H2O/[BMPyr][NTf2]). Tests were performed in metalꢀfree and in
the presence of the metalꢀpromoter Cu(II) MOF 1. The effects of various reaction parameters
were studied, aiming at the optimization of the cyclohexane hydrocarboxylation process. The
choice of cyclohexane as substrate is due to the fact that it only forms one acid product,
cyclohexanecarboxylic acid, and also on account of the industrial significance of this acid,34
mainly as an intermediate in the tolueneꢀbased caprolactam process. The hydrocarboxylation
reaction of Scheme 3 in H2O/MeCN (but not in an IL) was previously reported by our group.23
It concerns an efficient and clean reaction for the synthesis of carboxylic acids from
alkanes.23,24
Blank tests were performed to assess if the hydrocarboxylation reaction proceeds in the
absence of metal (Table 2, entries 1, 16 and 17), or in the presence of only one of the solvents
(Table 2, entries 7, 8, 14 and 16) or as a solventꢀfree reaction (Table 2, entry 18). The case of
the absence of metal catalyst is discussed below, whereas in the other cases no
cyclohexanecarboxylic acid was detected.
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Scheme 3: Hydrocarboxylation of cyclohexane to cyclohexanecarboxylic acid in H2O/MeCN
or H2O/IL medium.
Table 2: Influence of different reactional parameters on the hydrocarboxylation of
cyclohexane.[a]
Yields of products [%][b]
TON[c]
Promoter
amount
[ꢁmol]
H2O/MeCN
[mL/mL]
P
Time
[h]
T
ºC
Entry
Carboxylic
Cycloꢀ
Cycloꢀ
[atm]
hexanone
hexanol
acid
11.5
33.4
20.3
15.1
22.4
14.7
0.0
1
2
No metal 60
3/3
3/3
3/3
3/3
3/3
3/3
0/6
6/0
3/3
3/3
3/3
3/3
3/3
0/6
3/3
0/6
20
20
5
6
6
6
6
6
6
6
6
6
6
3
9
6
6
6
6
0.9
1.3
3.1
1.3
0.9
0.5
0.0
0.0
0.0
0.2
0.7
0.2
1.7
0.0
0.2
0.0
0.2
0.9
2.0
1.2
0.1
0.2
0.0
0.0
0.0
0.1
0.4
0.1
1.3
0.0
0.0
0.0
0.0
44.5
31.8
22.0
14.6
77.0
0.0
8
8
60
60
60
60
60
60
60
30
80
60
60
60
60
60
3
4
8
40
20
20
20
20
20
20
20
20
20
20
20
20
5
16
2
6
7
8
8
8
0.0
0.0
9
8
1.3
1.6
10
11
12
13[d]
14[e]
15[e]
16[e]
8
12.3
29.3
34.6
14.8
0.0
15.8
38.0
43.6
17.8
0.0
8
8
8
8
8
35.9
0.0
45.1
0.0
No metal 60
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17[e]
18
No metal 60
60
3/3
0/0
20
20
6
6
15.0
0.0
0.2
0.0
0.1
0.0
0.0
0.0
8
[a]Typical (unless stated otherwise) reaction conditions: cyclohexane (1.00 mmol), p(CO) = 20 atm (5.32 mmol),
K2S2O8 (1.50 mmol), H2O (3.0 mL)/MeCN (3.0 mL), 60ꢂ°C, 6 h in an autoclave (13.0 mL capacity).[b] Product
yield [%] (moles of product/100 mol of cyclohexane).[c] Turnover number (TON) = mol of product per mol of
catalyst. [d] Using Cu(NO3)2·2.5 H2O as catalyst.[e] Ionic Liquid ([BMPyr][NTf2]) instead of MeCN.
Metal-Free vs. Metal-Promoted Carboxylation of Cyclohexane
Contrary to the traditional alkane carboxylations in trifluoroacetic acid,35 the
hydrocarboxylation of cyclohexane by CO in the H2O/MeCN/K2S2O8 and in the
H2O/IL/K2S2O8 systems proceeds even in the absence of a metal catalyst, leading to the
formation of cyclohexanecarboxylic acid (C6H11COOH) in ca. 12% yield based on
cyclohexane, for the H2O/MeCN system (after 6 h reaction, Table 2, entry 1) and ca. 15%
yield, for the H2O/IL system (after 6 h reaction, Table 2, entry 17). This is due to the active
ꢀ•
radical sulphate SO4 (an efficient single electronꢀtransfer oxidant),23a,b generated upon
thermolysis of peroxodisulfate. In a previously reported work from our group,23b it was
confirmed that this reaction is not due to the presence of the metal of the reactor wall, by
performing it in a Teflon coated reactor.
In the presence of 1, the cyclohexanecarboxylic acid yield increases up to 33.4 % in the
H2O/MeCN/K2S2O8 system (Table 2, entry 2) and 35.9 % in the H2O/IL/K2S2O8 system (Table
2, entry 15) after 1 h reaction time. When the same procedure is applied with Cu(NO3)2·2.5
H2O, the copper salt precursor of complex 1, the yield (Table 2, entry 13) does not surpass ca.
3% above the metalꢀfree conditions (Table 2, entry 1). Further, it is noteworthy to state that
formation of cyclohexanol and cyclohexanone are also detected as byꢀproducts, formed upon
oxidation of cyclohexane, but only in very low yields. This is an indication that our process is
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quite selective for the production of carboxylic acid, since all of the formed byproducts have a
yield below ca. 3.2%.
In terms of the same quantity of copper atoms available, a copper complex with a mono
deprotonated form of triethanolamine and azide group23b leads to a slightly lower yield for the
carboxylic acid, with twice the amount of metal used in this study, and shows a lower
selectivity when compared to our current system.
Effect of Solvent Composition and catalyst recycling
The solvent composition is an important reaction parameter in the hydrocarboxylation of
cycloalkanes, which does not proceed in either only water (Table 2, entry 8), only acetonitrile
(Table 2, entry 7), or in the absence of both of them (i.e., only in cyclohexane) (Table 2, entry
18). In the case of using the IL, the reaction also does not proceed in pure IL (Table 2, entry
14), regardless of the presence or absence of the metal catalyst (Table 2, entry 14 or 16). The
mixture H2O/IL can improve the interaction of CO with the IL, whereas water is necessary not
only to solubilize the peroxodisulfate oxidant, but also to provide the main source of the
hydroxyl group for the carboxylic acid product.23b,c The important role of water was proven
previoulsy23b,c by mechanistic studies using labelled elements. However, the mixture of H2O
with an IL, [BMPyr][NTf2], has never been reported before for the hydrocarboxylation of
alkanes. Although some direct alkane transformations to carboxylic acids in water had been
previously reported,36 the yields of carboxylic acids based on alkanes did not exceed 0.4%.
Only studies23b,c where the catalyst used was a dicopper(II) or a tetracopper(II) complex in
mixed H2O/MeCN solvent led to higher yields of cyclohexanecarboxylic acid (39 and 72 %,
respectively) , and no IL medium was attempted.
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The catalyst recyclability was investigated with the IL medium, for up to six consecutive
cycles (Fig. 4). Each new cycle was initiated upon addition of new portions of all reagents,
except the catalyst 1. After completion of each run, the products were analysed as previously
and the IL with the catalyst was recovered by drying in vacuum overnight at 70 ºC. The
activity and selectivity of the catalyst 1 is maintained through four consecutive cycles,
decreasing 16% in the 5th and 35% in the 6th cycle, indicating gradual loss of catalytic activity
of the catalyst only after the 4th cycle (Fig. S5). PowderꢀXRD recorded on the extracted
catalyst after the 4th recycling run (Fig. S5) confirms the preservation of its structural integrity.
Moreover only, ca. 0.01% of the Cu used in the reaction was detected by ICP in the extracted
solution sample after the 4th run, eliminating the possibility of any appreciable leaching. Hence,
the possibility of effective catalyst recycling is a relevant feature of this system.
Fig. 4. Effect of the catalyst 1 recycling on overall product yield, in the hydrocarboxylation of
cyclohexane in IL ([BMPyr][NTf2]) medium. Reaction conditions: cyclohexane (1.00
mmol), p(CO) = 20 atm (5.32 mmol), K2S2O8 (1.50 mmol), H2O (3.0 mL)/IL (3.0 mL), 60ꢂ°C,
6 h in an autoclave (13.0 mL capacity).
Effect of Temperature and Time
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For the hydrocarboxylation of cyclohexane three temperatures (30, 60 and 80 ºC) were tested
(Table 2, entries 2, 9 and 10). At 30 ºC, the process is rather slow and after 6 h the yield is
1.3% of cyclohexanecarboxylic acid (Table 2, entry 9). The reaction is more efficient upon
increasing the temperature from 30 to 60ꢂ°C leading to a yield growth from ca. 1.3 to 33.4%
(Table 2, entries 2 and 9). When the temperature was further increased from 60 to 80ꢂ°C, we
observed a yield drop (Table 2, entries 2 and 10), possibly due to overoxidation. The yields of
the cyclohexanone and cyclohexanol byꢀproducts are not significantly influenced by the
temperature (in the 30–80ꢂ°C range). Upon increasing the reactional time to 9 h (Table 2, entry
12), the cyclohexanecarboxylic acid yield slightly increased to 34.6 %.
From the selectivity viewpoint, it is worthwhile to mention that even at prolonged reaction
times the hydrocarboxylation of cyclohexane usually proceeds with practically no formation
(or in rather low yield) of cyclohexanone and cyclohexanol byꢀproducts. Moreover, neither
linear nor cyclic dicarboxylic acids, eventually formed as a result of ring opening or
carboxylation at two positions, have been detected.
Effect of the Copper Promoter Amount
Although the hydrocarboxylation of cyclohexane proceeds even in the absence of any metal
promoter (Table 2, entry 1), the amount of copper(II) complex 1 has an important effect.
Hence, even a very small quantity of 1 (i.e., 2 ꢁmol corresponding to a C6H12/1 molar ratio of
500) is sufficient to reach a substantial yield of C6H11COOH (14.7%) with a turnover number
(TON) of 77 (Table 2, entry 6), in H2O/MeCN. However in this medium and with the same
reactional time, the highest yield (33.4%) is obtained when using 8 ꢁmol of 1 (C6H12/1 = 125),
with the TON value of 45 (Table 2, entry 2). A further enhancement of the copper promoter
amount leads to a yield drop (Table 2, entry 5). In the case of H2O/IL, a similar behaviour is
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observed as that in H2O/MeCN, and e.g. the maximum yield is obtained using the same low
amount of catalyst (35.9%, entry 15, Table 2, for 8 ꢁmol of 1 i.e. C6H12/1 = 125). In a previous
work23b featuring a dicopper and a tetracopper complex produced a higher yield of 39% and
72%, respectively, in H2O/MeCN, but neither recycling studies were performed, nor was an IL
medium tested.
Effect of CO Pressure
As a carboxylating agent, CO should be introduced into the system in some excess relative to
the cycloalkane to achieve the best yields of carboxylic acids.23b,c For complex 1, an increase
of the CO pressure from 5 to 20 atm leads to a C6H11COOH yield growth (Table 2, entries 3
and 2) that reaches the maximum value of 33.4% at 20 atm of CO. A further increase of the CO
pressure results in a yield decrease (Table 2, entry 4), eventually on account of carbonylation
of the metal catalyst.
Mechanism of the catalytic reaction
In order to get an insight into the mechanism of the catalytic reaction, the oxidation of
cyclohexane was performed in the presence of a radical trap (CBrCl3 or Ph2NH for Cꢀ or Oꢀ
centred radicals, respectively).20a,b,22b,eꢀm,24b The drastic decrease in the yield (by ca. 95−87%)
suggests that the reaction proceeds mainly via a radical mechanism similar to that reported for
H2O/MeCN23b,c (Scheme 4).
It involves the formation of a free cyclohexyl radical, which is generated by Hꢀatom
ꢀ•
abstraction from C6H12 (reaction 1) by the sulfate radical SO4 obtained from the thermolytic
decomposition of K2S2O8.23c,35e,36b The cyclohexyl radical reacts with carbon monoxide
resulting in the acyl radical C6H11CO• (reaction 2) that then is oxidized by S2O82ꢀ generating
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ꢀ
the acyl sulfate C6H11C(O)OSO3 (reaction 3). This species is hydrolyzed by water (reaction 4),
thus forming the cyclohexanecarboxylic acid.23b,c
In the copperꢀpromoted process, an alternative route (reaction 5) can occur. In this process the
copper(II) MOF 1 can behave as an oxidant of the acyl radical. This route involves the
Cu(II)/Cu(I) redox couple and requires K2S2O8 for regeneration (reaction 5’) of the Cu(II)
form.23c,35e,36b,37 The effectiveness of copper(II) for the oxidation of carbonꢀcentred radicals has
been reported.20a,b,22bꢀm,24b,36d The formed acyl cation C6H11CO+ suffers hydrolysis leading to
the C6H11COOH product (reaction 6).
Scheme 4. Proposed simplified mechanism for the hydrocarboxylation of cyclohexane.23b,c
Conclusion
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We have synthesised and characterized a new Cu(II) metal organic framework (MOF 1), using
a flexible bifunctionalised terpyridine ligand (HL). The single crystal Xꢀray diffraction
analysis of 1 shows a 1D polymer whose topological analysis revealed uninodal (2)ꢀconnected
nets of type 2C1.
Compound 1 can effectively catalyze the hydrocarboxylation of cyclohexane not only in
aqueous MeCN but also in an aqueous ionic liquid (IL) medium, producing the corresponding
cyclohexanecarboxylic acid as the main product. To our knowledge, the present study provides
the first example of alkane hydrocarboxylation in an ionic liquid medium. The use of the ionic
liquid [BMPyr][NTf2] instead of a typical organic solvent (MeCN) is shown to present
important advantages, allowing an effective catalyst recycling, without loss of activity, and
leading to an improvement in the yield of the hydrocarboxylation product and in the selectivity
towards this product.
The application of water as not only a solvent component but also as a relevant reagent
(providing the main source of the OH group in the carboxylic acid product) is an interesting
feature of this method which, in a single pot and under mild conditions, converts an alkane into
a valuable product with a good yield and high selectivity, operating with a low amount of
catalyst and with the possibility of its recycling. The reaction has been optimized by studying
the effects of various experimental factors and tests with radical traps indicate that it occurs via
a free radical mechanism. The study shows that an ionic liquid can be used in
hydrocarboxylation of an alkane with advantages relative to the conventional organic solvents,
and it deserves to be further extended to a variety of alkanes, oxidants and reaction conditions.
ACKNOWLEDGMENTS
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Financial support from the Fundação para a Ciência e a Tecnologia (FCT), Portugal, for
fellowships to A. P., A. P. C. R and A. K. (SFRH/BPD/88450/2012, SFRH/BPD/90883/2012
and SFRH/BPD/76192/2011), and for the UID/QUI /00100/2013, PTDC/QEQꢀQIN/3967/2014
projects are gratefully acknowledged. The authors also acknowledge the Portuguese NMR
Network (ISTꢀUL Centre) for access to the NMR facility, and the IST Node of the Portuguese
Network of massꢀspectrometry (Dr. Conceição Oliveira) for the ESIꢀMS measurements.
Supporting Information
Electronic Supplementary Information (ESI) available: Experimental procedures, Figs. S1ꢀ
1
S5 containing FTꢀIR, H and 13CꢀNMR and Tables S1ꢀS2. CCDC 1471928 for ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/c000000x/
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