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Inorganics 2019, 7(2), 17; https://doi.org/10.3390/inorganics7020017

Article
Syntheses, Structures, and Catalytic Hydrocarbon Oxidation Properties of N-Heterocycle-Sulfonated Schiff Base Copper(II) Complexes
Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal
*
Authors to whom correspondence should be addressed.
Received: 16 October 2018 / Accepted: 31 January 2019 / Published: 6 February 2019

Abstract

:
Reaction of the o-[(o-hydroxyphenyl)methylideneamino]benzenesulfonic acid (H2L) (1) with CuCl2·2H2O in the presence of pyridine (py) leads to [Cu(L)(py)(EtOH)] (2) which, upon further reaction with 2,2’-bipyridine (bipy), pyrazine (pyr), or piperazine (pip), forms [Cu(L)(bipy)]·MeOH (3), [Cu2(L)2(μ-pyr)(MeOH)2] (4), or [Cu2(L)2(μ-pip)(MeOH)2] (5), respectively. The Schiff base (1) and the metal complexes (25) are stabilized by a number of non-covalent interactions to form interesting H-bonded multidimensional polymeric networks (except 3), such as zigzag 1D chain (in 1), linear 1D chain (in 2), hacksaw double chain 1D (in 4) and 2D motifs (in 5). These copper(II) complexes (25) catalyze the peroxidative oxidation of cyclic hydrocarbons (cyclooctane, cyclohexane, and cyclohexene) to the corresponding products (alcohol and ketone from alkane; alcohols, ketone, and epoxide from alkene), under mild conditions. For the oxidation of cyclooctane with hydrogen peroxide as oxidant, used as a model reaction, the best yields were generally achieved for complex 3 in the absence of any promoter (20%) or in the presence of py or HNO3 (26% or 30%, respectively), whereas 2 displayed the highest catalytic activity in the presence of HNO3 (35%). While the catalytic reactions were significantly faster with py, the best product yields were achieved with the acidic additive.
Keywords:
sulfonated Schiff base; copper(II) compounds; pyridine; 2,2’-bipyridine; pyrazine; piperazine; hydrocarbon oxidation

1. Introduction

Over the past decades, there has been a marked development in the coordination chemistry of Schiff base metal complexes [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29], encouraged by the variety of their solid-state structures [1,2,3], as well as magnetic [4,5,6,7,8,9], fluorescence [10,11,12], and catalytic [1,13,14,15] properties. Though a large number of Schiff bases of various types, including those containing the carboxylic acid group, has been reported [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18], sulfonic acid containing Schiff bases are rare and only a few metal complexes have been investigated [19,20,21,22,23,24,25,26,27,28,29]. Moreover, intra- and intermolecular interactions via sulfonate oxygens could generate interesting multinuclear copper complexes [19,20,22,28] which may be useful for further catalytic application.
In fact, sulfonated Schiff bases display versatile complex formation abilities leading, for example, to mono-, di-, or polymeric structures [19,20,21,22,23,24,25,26,27,28,29]. In an earlier study, we reported that the Schiff base 2-[(2-hydroxyphenyl)methylideneamino]benzenesulfonic acid (H2L) can form solvatomorphs which have identical basic structures but with different non-coordinated solvents [22]. We also obtained a bis(μ4-(ae)-cyclohexane-1,4-dicarboxylato-O,O′,O″,O′′′′)-tetracopper [19,23], two carboxylate-sulfonated copper polymers [28], and three pseudohalide-bridged copper complexes [20] containing 2-(2-pyridylmethyleneamino)benzenesulfonate. The tetranuclear [19,23] and polymeric copper complexes were efficient catalysts [28] for the cyclohexane oxidation in both conventional (CH3CN/H2O) [19,28] and non-conventional (ionic liquid) solvents [23,28], whereas the pseudohalide complexes [20] were found to catalyze the Henry reaction in aqueous medium. Recently, we reported another new sulfonated Schiff base, 2-[(2-hydroxy-3-methoxyphenyl)methylideneamino]benzenesulfonic acid, which was utilized to synthesize a few mono- and dicopper complexes [24,27]. Interestingly, the dimeric complexes were found to be highly efficient catalysts for the microwave-assisted oxidation of 1-phenylethanol under mild conditions and in the absence of any additive [24].
The curious molecular structures and catalytic properties of such compounds [19,20,21,22,23,24,25,26,27,28,29] inspired us (i) to synthesize other copper complexes of the sulfonated Schiff base ligand L2− and (ii) to apply them in alkane oxidation studies.
Saturated hydrocarbons are rarely used as starting materials in the chemical industry due to their low reactivity, despite being the most abundant and least expensive potential carbon sources for the organic synthesis of functionalized valuable products [30,31,32,33,34,35,36,37,38,39,40,41,42,43]. Peroxidative oxidation [30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67] of alkanes is a promising approach for the synthesis of the corresponding alcohols and ketones. In particular, oxidation with environmentally friendly oxidants, such as hydrogen peroxide (H2O2) [30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67] or dioxygen [68,69,70,71], is a topic of great interest, and the use of copper complexes as catalysts [19,28,44,45,47,48] is particularly promising. However, the catalytic efficiency has still to be improved, which accounts for another aim of the current study.
Moreover, in view of the multi-copper nature of particulate methane monooxygenase (pMMO), an enzyme that catalyzes the oxidation of alkanes to alcohols, particular attention [47,48,49,50,72,73,74,75] should be paid to multinuclear copper catalysts, a topic which also concerns this work.
Hence, due to the above considerations, and also inspired by our previous successful catalytic application of a tetracopper(II) complex containing a mixed ligand (carboxylate and sulfonate) in the peroxidative oxidation of cyclohexane [19] and by the fact that N-heterocyclic bases can promote the reaction [30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67], we anticipated that copper(II) complexes bearing a mixed ligand system of sulfonate and a N-heterocyclic base could be also particularly active for alkane oxidation. Thus, in the present study, we investigate the reactions of the sulfonated Schiff base 2-[(2-hydroxyphenyl)methylideneamino]benzenesulfonic acid (H2L, 1) [21,22] with CuCl2·2H2O and a N-heterocyclic base, such as pyridine (py), 2,2’-bipyrdine (bipy), pyrazine (pyr), or piperazine (pip) (Scheme 1). Herein, we report the syntheses, structures, and hydrocarbon oxidation properties of the two new mononuclear complexes [Cu(L)(py)(EtOH)] (2) and [Cu(L)(bipy)]·MeOH (3) and the two new dinuclear compounds [Cu2(L)2(μ-pyr)(MeOH)2] (4) and [Cu2(L)2(μ-pip)(MeOH)2] (5) with such ligands.

2. Results and Discussion

2.1. Syntheses and Characterization

The [1+1] condensation of salicylaldehyde and 2-aminobenzenesulfonic acid in aqueous methanol (1:2) leads to the formation of the Schiff base H2L (1) [21,22] which, upon reaction with CuCl2·2H2O in ethanol and in the presence of pyridine (py), produces the mononuclear copper(II) complex [Cu(L)(py)(EtOH)] (2) (Scheme 1). Furthermore, the replacement of the pyridine moiety of 2 with 2,2’-bipyridine (bipy), pyrazine (pyr), and piperazine (pip) in methanol produces the corresponding complexes [Cu(L)(bipy)]·MeOH (3), [Cu2(L)2(μ-pyr)(MeOH)2] (4), and [Cu2(L)2(μ-pip)(MeOH)2] (5), which were characterized by elemental microanalysis, IR spectroscopy, and single crystal X-ray diffraction study. The Schiff base H2L (1) was characterized by both NMR and single crystal X-ray diffraction analyses. All the compounds including the Schiff base were isolated in very good yields (92–78%).
The IR spectrum of the Schiff base H2L (1) exhibits the expected bands at 1638 cm−1 and 1376 cm−1, which are indicative of the C=N bond and the sulfonate group, respectively. In the IR spectra of the metal complexes (25), the ν(C=N) bands are observed in the range of 1606–1613 cm−1, whereas the sulfonate groups are evidenced by the medium intense bands at the 1382–1386 cm−1 range.

2.2. Description of Crystal Structures of 15

2.2.1. Crystal Structure of H2L (1)

The asymmetric unit of H2L (1) (Figure 1) contains two symmetry-independent molecules of the Schiff base with relative positions slightly shifted from perpendicular as evidenced by the angle of 80.46° between the least-square planes of the two units. The two molecules superimpose quite well only with a slight discrepancy in the angle between the least-square planes of the aromatic rings, which assume values of 8.41° for one molecule and 10.98° for the other (Table 1). The azomethine linkages in 1 are evident from the N–C bond lengths (1.299(4) and 1.294(4) Å (Table 1). The molecules are stabilized by intramolecular hydrogen bonds involving the imino nitrogen atoms (as donors) and both the phenolic and sulfonate oxygen atoms (as acceptors) (Figure 1; Table S1, Supplementary Materials). The crystal structure is stabilized by intermolecular H-contacts involving the phenolic oxygen atoms (as donor) of one molecule and the sulfonate oxygen atoms (as acceptor) of another molecule (Figure 1).

2.2.2. Crystal Structures of 25

The crystal structures of [Cu(L)(py)(EtOH)] (2), [Cu(L)(bipy)]·MeOH (3), [Cu2(L)2(μ-pyr)(MeOH)2] (4), and [Cu2(L)2(μ-pip)(MeOH)2] (5) are shown in Figure 2. The single crystal X-ray diffraction analyses show that 2 and 3 are mononuclear copper(II) complexes, whereas 4 and 5 are dicopper(II) compounds bridged by pyrazine and piperazine, respectively. In all cases, the metal center adopts distorted square pyramid geometries (τ5 parameter in the 0.25–0.44 range, Table 1) and the L2− ligand acts as an O,N,O-chelator by means of the phenolic O-, the imine N-, and one sulfonate O-atoms. The coordination sphere of copper is then fulfilled either by another chelating molecule (3) leading to a N3O2 metal environment, or by a solvent and one more organic moiety (2, 4, and 5) and forming N2O3 settings (Table 1). Probably as a result of the chelating mode of bipyridine, compound 3 is the only one in which the L2− ligand occupies both equatorial (Ophenoxido and Nimino atoms) and apical (Osulfonato) sites, thus contrasting with the other complexes (2, 4, and 5) in which it occupies three of the equatorial positions.
The Cu–Nimino bond distances in the structures of all complexes lie in the 1.975(3)–1.9837(19) Å range (Table 1) and are slightly shorter than those involving the metal and the other N-ligands (2.004(3)–2.0709(17) Å). Concerning the copper–oxygen lengths, the Cu–Ophenoxido distances (1.8770(16)–1.900(3) Å) are considerably shorter than the Cu−Osulfonato ones whose minimum value is 1.9733(16) Å. In this respect, the longest value of 2.3789(18) Å found for 3 relates with the metal–oxygen distances for the axial sites (i.e., Cu−Ocoligand lengths, Table 1) and is due to the Jahn–Teller effect.

2.2.3. Oxidation of Hydrocarbons

The catalytic properties of the copper(II) complexes (25) in the peroxidative oxidation of hydrocarbons (cyclooctane, cyclohexane, and cyclohexene) to the corresponding alcohols, ketone, and epoxide, under mild conditions, were evaluated. Inspired by our previous findings [19,53], we chose as a model system the reaction of peroxidative oxidation of cyclooctane using hydrogen peroxide as oxidant. The catalytic procedure ran at 60 °C in acetonitrile and the reactions were monitored by GC with a typical catalyst loading (per copper) of approximately 1 × 10−3 M (Scheme 2).
Among all the catalysts studied in this reaction, the mononuclear complex 3 was the most effective one. In fact, after 1 h of reaction time and in the absence of any additive, it led to an overall turnover number (TON) up to 50 moles of products per mole of catalyst (Table 2, entry 11) and an overall yield of approximately 20% based on cyclooctane. Compound 5 exhibited a lower activity, followed by compounds 2 and 4 that were the least active ones under these conditions (Table 3, entry 15; Table 4, entry 14; Table 5, entry 14; respectively). The yield accumulation of the oxygenates (cyclooctanol and cyclooctanone) over the reaction time of 25 in the model system is shown in Figure 3.
Blank experiments confirmed that no cyclooctanol or cyclooctanone are formed appreciably in the absence of the complexes (25). The Schiff base H2L (1) is inactive (Table S2, Supplementary Materials) toward the cyclooctane oxidation, whereas CuCl2 produces insignificant amounts of products, that is, <3% of total yield can be detected.
The peroxidative oxidation of hydrocarbons catalyzed, for example, by some copper complexes can proceed more efficiently in the presence of a suitable additive [53], and thus we tested pyridine (py) and HNO3 as a basic and an acid promoter, respectively.
For all the complexes 25, the presence of such additives resulted, in general, in an enhancement of the maximum yield and TON values, mainly in the case of HNO3 (Table 2, Table 3, Table 4 and Table 5). Changes in selectivity were also observed.
All the catalysts 25 exhibit comparable overall yields (Table 2, Table 3, Table 4 and Table 5) in the presence of pyridine after a sufficiently long reaction time (e.g., 24 h). However, the differences in reaction rates are clearly observed in the first 15 min-period (Figure 4).
In fact, for example, compound 2 in comparison with 4 leads to a higher initial rate of cyclooctane oxidation (Figure 4) and a slightly lower ketone:alcohol selectivity, but a convergence of their behaviors occurs over time in the presence of pyridine, suggesting the eventual conversion of 4 into 2, upon reaction with this base (Table 4 and Table 5).
Compound 5 also leads to lower initial rate than 2, but displays a higher selectivity toward the alcohol, comparable to that of 3.
The use of compound 3 (Table 2) results in the highest rate (Figure 4), the highest product yield, and the highest selectivity (comparable to 5) toward the formation of the alcohol product.
In all the cases, at the maximum product yield, a higher selectivity toward the alcohol is observed, but it decreases over time and eventually reverses. For example, when the maximum yields are accomplished in the reactions with 3 and 5, the cyclooctanol:cyclooctanone molar ratio is approximately 90:10, but at 24 h of reaction time the inversion of the ketone:alcohol ratio has already occurred (Table 2 and Table 3).
The application of HNO3 as a promoter, instead of pyridine, in the peroxidative oxidation of cyclooctane resulted in a considerable yield promotion (by approximately 10% for reactions using 2 and 5 and by approximately 5% in the case of 3 and 4) in comparison to the best yields in the reactions in the presence of pyridine. In acidic conditions, the best yield (35%) is obtained with the mononuclear 2 (Figure 5) (curiously, in the presence of pyridine, the best catalyst, 3, is also mononuclear).
The drawback of the acidic additive is the slower reactivity when compared to the much faster reactions in the presence of pyridine. In fact, the maximum yield of the cyclooctane oxidation was achieved within a few minutes when pyridine was used as a promoter, whereas when this was replaced by HNO3, the reaction time to achieve the maximum yield increased to 4 h, in the cases of compounds 2, 4, and 5, and to 16 h for 3 (Figure 5).
Contrary to what happens in the reactions in the presence of pyridine, those with HNO3 do not follow a clear pattern (Figure 5).
The cyclooctanol/cyclooctanone ratio in acidic conditions, when the maximum yield is achieved, is better (for 2, 4, and 5) or comparable (in the case of 3) to that observed when pyridine is used as additive. As in the case of the basic promoter, the highest yield corresponds to a marked alcohol predominance over the ketone (approximately 98:2 ratio for 2, 4, and 5) but over the reaction time this selectivity decreases (as the overall yield does) although without reversing, thus also attesting to the better selectivity toward the alcohol for compounds 25 using the HNO3 promoter.
In order to get an insight into the reaction mechanism, we selected the overall best (with and without additives) catalyst, 3, and studied the effect of the addition of Ph2NH, an oxygen radical trap [37], and the effect of the addition of CBrCl3, a carbon radical trap [37], on the cyclooctane oxidation.
The addition of either Ph2NH or CBrCl3, in a stoichometric amount relative to H2O2 or cyclooctane, respectively, leads to approximately 5–8%, 29–32%, and 54% suppression of the products formation for 4, 9, and 120 min reaction time, respectively (Tables S4 and S5, Supplementary Materials, entries 2–4). Thus, the cyclooctane oxidation reaction appears to proceed mainly via a non-free radical pathway for short reaction times, although over time a free radical mechanism becomes more pronounced.
This is also consistent with the high selectivity toward the alcohol at short reaction times, which decreases during the reaction. For shorter reaction times, such a behavior supports the predominance of a non-free radical pathway conceivably associated with a metal-centered oxidant instead of a free HO radical [58]. Various mechanistic possibilities can be postulated [47,48,57,58].
For the free radical mechanism, a plausible pathway can be suggested [31,32,44,46,57,58] to involve the metal-assisted decomposition of H2O2 into the hydroxyl radical HO which then abstracts H from RH to give the alkyl radical R. The formation of HO from H2O2 involves proton-transfer steps among H2O2, hydroperoxo-, peroxo-, and oxo-metal species [34,47,48,49,50].
As presented in Scheme 3, R either reacts with molecular oxygen producing ROO (which upon H-abstraction, for example, from H2O2 or the derived H2O radical, forms alkylhydroperoxide ROOH) or reacts with a possible metal-hydroperoxo intermediate species [Cu]–OOH producing ROOH. This organoperoxide then undergoes metal-promoted decomposition to form O-centered organo radicals, RO (upon O–O bond cleavage), and ROO (upon O–H bond rupture). While the H-abstraction from RH by RO leads to the formation of ROH, the decomposition of ROO produces ROH and the corresponding ketone R(–H)=O.
In agreement with such mechanistic considerations, the good catalytic activity of 25 can be associated with the hydrophilicity of the sulfonate groups. This can activate the water molecule toward its important role as a promoter/catalyst for proton-shift steps involved in the formation of hydroxyl radicals from hydrogen peroxide [50], with a key role in the mechanism of the alkane oxidation.
The participation of the alkylhydroperoxide ROOH species was proved [59,60,61,62,63,64,65] by the increase in the amount of ROH and consequent decrease of R(–H)=O when the final reaction solution was treated with PPh3 prior to the GC analysis (Table S3, Supplementary Materials).
Moreover, the promoting effect of pyridine can result from the assistance on the proton-transfer steps involved in the formation of the hydroxyl radical [50,59,60,61,62,63] from H2O2, whereas the role of the acid additive can be associated with the activation of the metal center by further unsaturation upon ligand protonation, the enhancement of oxidative properties of metal complexes, the stabilization of oxidants, and the promotion of peroxo (or hydroperoxo)-complex formation as indicated in previous cases [37,48,76,77].
For further screening of the hydrocarbon substrates and the scope of our system, the most promising catalyst 3 without any additive was used. Cyclohexane and cyclohexene were successfully transformed employing our catalytic system, thus showing its versatility. The obtained results are summarized in Table 6.
In the peroxidative oxidation of cyclohexane, compound 3 leads to a 20% yield, based on the substrate, after 2 h of reaction time with a ketone/alcohol ratio of approximately 20:80 (Table 6, entry 1). As it was observed when cyclooctane was used without any additive, longer reaction times resulted in yield preservation, but a loss on the selectivity until approximately 30:70, after 32 h of reaction time (Table 6).
Overall product yield decreased in the peroxidative oxidation of the alkene (15% based on cyclohexene, after 2 h of reaction time). At 2 h of reaction time, the ratios of cyclohexene oxide:2-cyclohexen-1-one:2-cyclohexen-1-ol:cis-1,2-cyclohexanediol:trans-1,2-cyclohexanediol were 7:36:44:5:8 (Table 6, entry 6). As in the previous cases, the reaction followed the same trend, not only in terms of yield but also in selectivity, over the reaction time.

3. Materials and Methods

All the reagents and solvents were purchased from commercial sources and used as received. The water used for all preparations and analyses was double-distilled and deionized. Elemental analyses were performed by the Microanalytical Service of the Instituto Superior Técnico (Lisbon, Portugal). FT-IR spectra were recorded in the 400–4000 cm−1 region on a Bruker Vertex 70 spectrophotometer (Bruker Optik GmbH, Ettlingen, Germany) with samples as KBr discs. The 1H NMR spectra were recorded at room temperature on a Bruker Avance II + 300 (UltraShield™ Magnet) spectrometer (Bruker BioSpin AG, Fällanden, Switzerland). Chromatographic measurements were carried out in a Perkin-Elmer Clarus 500 gas chromatograph (PerkinElmer Inc., Shelton, CT, USA) with a BP-20 capillary column (SGE). The parameters of the column are 30 m × 0.32 mm × 25 μm and Helium was used as the carrier gas (1 mL per minute constant flow).

3.1. Synthesis of H2L (1)

To a hot and stirred water solution (10 mL) of 2-aminobenzenesulfonic acid (0.692 g, 4.0 mmol) was added dropwise a methanol solution (20 mL) of salicylaldehyde (0.488 g, 4.0 mmol). The resulting yellow solution was filtered and kept at room temperature overnight. After one day, yellow crystals suitable for X-ray diffraction analysis formed and the crystals were collected by filtration and washed with methanol. Yield 1.022 g (92%). Anal. calcd. for C13H11NO4S (277.29): C 56.31, H 4.00, N 5.05; found: C 56.21, H 4.03, N 5.08. FT-IR (cm−1, KBr): ν(OH), 3445br; ν(N–H), 2919br; ν(C=N), 1638s; ν(C–O), 1233s; ν(sulfonate), 1376s, 1171s, 763m. 1H NMR (300 MHz, DMSO-d6) δ values (ppm): 10.69 (br, N–H); 10.26 (s, CH=N); 6.91–7.92 (m, 8-Ar-H). 13C NMR (75.468 MHz) in DMSO-d6, internal TMS, δ (ppm): 116.8 (Ar-H), 120.1 (Ar-H), 127.6 (Ar-C), 129.0 (Ar-C), 130.8 (Ar-C), 135.4 (Ar-C), 136.5 (Ar-C), 137.4 (Ar-C), 139.4 (Ar-C), 140.1 (Ar-C–SO3), 162.2 (Ar-C–OH), 164.1 (N=C).

3.2. Synthesis of [Cu(L)(py)(EtOH)] (2)

To a hot and stirred ethanol suspension (10 mL) of H2L (1) (0.277 g, 1.0 mmol) was added dropwise an ethanol solution (5 mL) of pyridine (0.316 g, 4.0 mmol) affording a clear orange solution. Then, an ethanol solution (5 mL) of CuCl2·2H2O (0.170 g, 1.0 mmol) was added dropwise to obtain a dark green solution. The solution was filtered and kept at room temperature. After two days, green crystals suitable for X-ray diffraction analysis formed and the crystals were collected by filtration and washed with ethanol. Yield: 0.362 g (78%). Anal. calcd. for C20H20CuN2O5S (463.98): C 51.77, H 4.34, N 6.04; found C 51.75, H 4.26, N 6.07. FT-IR (cm−1, KBr): ν(C=N), 1606; ν(C–O), 1283s; ν(sulfonate), 1383m, 1174s, 764m.

3.3. Synthesis of [Cu(L)(bipy)]·MeOH (3)

To a methanol solution (40 mL) of 2 (0.232 g, 0.5 mmol) was added dropwise a methanol solution (10 mL) of 2,2’-bipyridine (0.156 g, 1.0 mmol) affording a dark green solution. After a few hours, a dark green crystalline compound suitable for X-ray diffraction analysis formed and was collected by filtration and washed with methanol. Yield: 0.235 g (89%). Anal. calcd. for C24H21CuN3O5S (527.04): C 54.69, H 4.02, N 7.97; found C 54.80, H 4.06, N 7.91. FT-IR (cm−1, KBr): ν(C=N), 1611; ν(C–O), 1294m; ν(sulfonate), 1386m, 1181s, 769m.

3.4. Synthesis of [Cu2(L)2(μ-pyr)(MeOH)2] (4)

To a methanol solution (15 mL) of 2 (0.232 g, 0.5 mmol) was added dropwise a methanol solution (5 mL) of pyrazine (0.080 g, 1.0 mmol) affording a dark green solution. After one day, a dark green crystalline compound suitable for X-ray diffraction analysis formed and was collected by filtration and washed with methanol. Yield: 0.177g (86%). Anal. calcd. for C32H30Cu2N4O10S2 (821.80): C 46.77, H 3.68, N 6.82; found C 46.87, H 3.76, N 6.75. FT-IR (cm−1, KBr): ν(C=N), 1613s; ν(C–O), 1264m; ν(sulfonate), 1382m, 1160s, 770m.

3.5. Synthesis of [Cu2(L)2(μ-pip)(MeOH)2] (5)

To a methanol solution (40 mL) of 2 (0.232 g, 0.5 mmol) was added dropwise a methanol solution (10 mL) of piperazine (0.086 g, 1.0 mmol) affording a dark green solution. After a few hours, a dark green crystalline compound suitable for X-ray diffraction analysis formed and was collected by filtration and washed with methanol. Yield: 0.176 g (85%). Anal. calcd. for C32H36Cu2N4O10S2 (827.85): C 46.43, H 4.38, N 6.77; found C 46.55, H 4.28, N 6.73. FT-IR (cm−1, KBr): ν(C=N), 1612s; ν(C–O), 1248m; ν(sulfonate), 1385m, 1176s, 756m.

3.6. Crystal Structure Determinations

X-ray quality crystals of all compounds were immersed in cryo-oil, mounted in a Nylon loop, and measured at a temperature of 150 K (1, 2, and 5) or 296 K (3 and 4). Intensity data were collected using a Bruker AXS-KAPPA APEX II or a Bruker APEX-II PHOTON 100 diffractometer (Bruker AXS GmbH, Karlsruhe, Germany) with graphite monochromated Mo-Kα (λ 0.71073) radiation. Data were collected using omega scans of 0.5° per frame and a full sphere of data was obtained. Cell parameters were retrieved using Bruker SMART software and refined using Bruker SAINT [78] on all the observed reflections. Absorption corrections were applied using SADABS [78]. Structures were solved by direct methods using the SHELXS-97 package [79,80] and refined with SHELXL-97 [79,80]. Calculations were performed using the WinGX System, Version 1.80.03 [81]. The hydrogen atoms attached to carbon atoms and to nitrogen atoms were inserted at geometrically calculated positions and included in the refinement using the riding-model approximation; Uiso(H) were defined as 1.2 Ueq of the parent nitrogen atoms or the carbon atoms for phenyl and methylene residues and 1.5 Ueq of the parent carbon atoms for the methyl groups. The hydrogen atoms of the hydroxide (in all structures but 5) were located from the final difference Fourier map, and the isotropic thermal parameters were set at 1.5 times the average thermal parameters of the belonging oxygen atoms. Least-square refinements with anisotropic thermal motion parameters for all the non-hydrogen atoms and isotropic for the remaining atoms were employed. Crystallographic data are summarized in Table S4 (Supplementary Materials).

3.7. Hydrocarbon Oxidation Studies

The catalytic oxidation reactions of hydrocarbons were carried out in MeCN solvent (total volume of 10.0 mL) in thermostated Pyrex round-bottom vessels and in open atmosphere, at 60 °C. The catalyst was used and introduced into the reaction mixture in the form of a stock solution in acetonitrile prepared by dissolving the catalyst (25) in acetonitrile. The substrate and promoter (if any) were then added in this order and the reaction started with the addition of 50% aqueous hydrogen peroxide in a portion. (CAUTION: the combination of air or molecular oxygen and H2O2 with organic compounds at elevated temperatures may be explosive!) The initial concentrations of hydrocarbon, catalyst, H2O2, and promoter (if used) were 0.25 M, 10−3 M per copper, 1.2 M, and 0.072 M, respectively. Solutions were analyzed by GC after the addition of nitromethane, as a standard compound, and the attribution of peaks was made by comparison with chromatograms of authentic samples.
GC analyses in the presence and in the absence of PPh3 were carried out, and it was found that the oxygenation of cycloalkanes resulted in the formation of the corresponding cycloalkyl hydroperoxides as the main primary products as expected, according to the method developed by Shul’pin [59,60,61,62,63,64,65]. For a precise determination of the product concentrations, only data obtained after the reduction of the reaction sample with PPh3 were typically used, taking into account that the original reaction mixture contained the three products: cycloalkxyl hydroperoxide (as the primary product), ketone, and alcohol. When used, the oxygen radical trap [37] diphenylamine (Ph2NH) and the carbon radical trap [37] bromotrichloromethane (CBrCl3) were applied in a stoichiometric amount relatively to the oxidant and substrate, respectively.

4. Conclusions

By taking advantage of the chelating capacity of the acyclic Schiff base o-[(o-hydroxyphenyl)methylideneamino]benzenesulfonic acid H2L (1), as well as of the H-bonding formation ability of the non-coordinating sulfonate group and in the presence of an N-heterocyclic base, namely, pyridine (py), 2,2’-bipyridine (bipy), pyrazine (pyr), or piperazine (pip), four new copper(II) complexes, that is, [Cu(L)(py)(EtOH)] (2), [Cu(L)(bipy)]·MeOH (3), [Cu2(L)2(μ-pyr)(MeOH)2] (4), and [Cu2(L)2(μ-pip)(MeOH)2] (5) were synthesized. The crystal lattices of all compounds, with the exception of 3, were stabilized by a number of non-covalent H-bonding interactions and generated interesting H-bonded polymeric networks, namely, a zigzag 1D chain (in 1), a linear 1D (in 2), a hacksaw double chain 1D (in 4), and a 2D motif (in 5). Double coordination of 2,2-bipyridine diminished the possibility of solvent coordination and no dimensionality was formed in the crystal lattice of 3.
These copper(II) complexes (25) catalyzed the peroxidative hydrocarbon (cycloalkane and cycloalkene) oxidation under mild conditions either in the absence or presence of an additive. As a model system, we used cyclooctane and hydrogen peroxide as substrate and oxidant, respectively. The presence of a basic or an acid promoter usually enhanced the catalytic activity.
The best activity was exhibited, in general, by the mononuclear compounds, where 3 was the most effective one, either without any promoter (20% max. yield) or in the presence of pyridine (26% max. yield), whereas 2 displayed the highest activity in the presence of HNO3 (35% max. yield). For each complex, although the reactions were significantly faster with pyridine, the best product yields were achieved with the acid additive. Other substrates, namely, cyclohexane and cyclohexene, were also oxidized catalytically with complex 3 into the oxidized products in overall 20% and 15% yields, respectively, without any additive, attesting to the versatility of our catalytic system.
Our studies with diphenylamine and bromotrichloromethane showed that the cyclooctane oxidation reaction appears to proceed mainly via a non-free radical pathway for short reaction times, although over time a free radical mechanism involving oxygen- and carbon-centered radicals becomes relevant. Such a curious behavior does not seem to have been recognized earlier and deserves to be tested not only in already known catalytic systems but also in novel ones.
The synthetic methodologies based on the addition (for the synthesis of 2) or substitution of pyridine for a N-heterocyclic base (2,2’-bipyridine, pyrazine, or piperazine for the synthesis of 35) to produce mono- and dinuclear copper systems in alcoholic medium provide easy synthetic procedures in comparison with other approaches for the syntheses of copper(II) complexes which are active for catalytic alkane oxidation [44,45,47,48,51,52].
In summary, a simple protocol for the synthesis of four effective catalysts is presented. However, detailed mechanistic studies—which include the kinetics of H2O2 decomposition, the influence of the O2 atmosphere in the catalytic activity, electron paramagnetic resonance (EPR) measurements, the use of dimethylsulfoxide as selective hydroxyl radical scavenger, and (Density Functional Theory) DFT calculations to disclose the active species and understand the mechanism—are under progress and should be the subject of a further publication.

Supplementary Materials

The following are available online at https://www.mdpi.com/2304-6740/7/2/17/s1, H-Bonded networks in 15, Figure S1: Idealized capped stick presentation of the H-bonds in 1 with partially atom labelling scheme, Figure S2: Idealized capped stick presentation of the H-bonded zigzag 1D chain in 1, Figure S3: Idealized capped stick presentations of the H-bonded linear 1D in 2, double chain 1D in 4, and 2D motif in 5, Table S1: Geometries (distances in (Å) and angles in (°)) of the H-bonds in 15, Table S2: Tests concerning the oxidation of cyclooctane in the presence of H2L (1) and in the absence of any metal catalyst (blank tests), Table S3: Results obtained before the reduction with PPh3 of the aliquots of the oxidation of cyclooctane catalyzed by 3, Table S4: Crystallographic data for 15, Table S5: Effect of the presence of diphenylamine on the oxidation of cyclooctane catalyzed by 3, Table S6: Effect of the presence of bromotrichloromethane on the oxidation of cyclooctane catalyzed by 3. CCDC 958691–958695 for 15, respectively, contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre at http://www.ccdc.cam.ac.uk/data_request/cif or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: [email protected].

Author Contributions

Conceptualization, S.H., M.F.C.G.d.S. and A.J.L.P.; Methodology, S.H., B.G.M.R. and A.K.; Software, S.H., B.G.M.R. and M.F.C.G.d.S.; Validation, S.H., B.G.M.R. and A.K.; Formal Analysis, S.H. and B.G.M.R.; Investigation, S.H. and B.G.M.R.; Resources, S.H., B.G.M.R. and A.K.; Data Curation, S.H. and B.G.M.R.; Writing—Original Draft Preparation, S.H. and B.G.M.R.; Writing—Review&Editing, S.H., M.F.C.G.d.S. and A.J.L.P.; Visualization, S.H.; Supervision, S.H., M.F.C.G.d.S. and A.J.L.P.; Project Administration, S.H. and A.J.L.P.; Funding Acquisition, S.H., B.G.M.R., A.K. and A.J.L.P.

Funding

The work was partially supported by the Fundação para a Ciência e a Tecnologia (FCT), Portugal, and its PPCDT program (FEDER funded) through the research projects PTDC/QEQ-QIN/3967/2014 and UID/QUI/00100/2013. S.H and A.K. acknowledge FCT for the postdoctoral grants SFRH/BPD/78264/2011 and SFRH/BPD/76192/2011. S.H. also acknowledges FCT and IST (Instituto Superior Tecnico, Lisbon, Portugal) for his working contract (Contract No. IST-ID/103/2018). B.G.M.R. is grateful to the Group V of Centro de Química Estrutural and the FCT, for the fellowships BL/CQE-2012-026, BL/CQE-2013-010 and the PhD fellowship under the CATSUS PhD program (SFRH/BD/52370/2013).

Acknowledgments

The authors acknowledge the Portuguese NMR Network (IST-UTL Center, Lisbon, Portugal) for providing access to the NMR facility.

Conflicts of Interest

The authors declare no conflict of interest.

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Scheme 1. Syntheses of 25 from 1.
Scheme 1. Syntheses of 25 from 1.
Inorganics 07 00017 sch001
Figure 1. Ball and stick presentation of the crystal structure of 1. All H-atoms except those participating in H-bonding are omitted for clarity. Symmetry: −x, 1−y, −z.
Figure 1. Ball and stick presentation of the crystal structure of 1. All H-atoms except those participating in H-bonding are omitted for clarity. Symmetry: −x, 1−y, −z.
Inorganics 07 00017 g001
Figure 2. Idealized ball and stick presentation of the crystal structures of 25. All H-atoms except those participating in H-bonding are omitted for clarity. Symmetry: x, y, −1+z (in 3), 1−x, −y, 1−z (in 4), and 2−x, −y, −z (in 5).
Figure 2. Idealized ball and stick presentation of the crystal structures of 25. All H-atoms except those participating in H-bonding are omitted for clarity. Symmetry: x, y, −1+z (in 3), 1−x, −y, 1−z (in 4), and 2−x, −y, −z (in 5).
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Scheme 2. Peroxidative oxidation of hydrocarbons [(a) cyclohexane (n = 1) or cyclooctane (n = 3) and (b) cyclohexene] to the corresponding products, with aqueous H2O2, catalyzed by the copper(II) complexes 25, in the presence or absence of any additive, under typical mild reaction conditions of this work.
Scheme 2. Peroxidative oxidation of hydrocarbons [(a) cyclohexane (n = 1) or cyclooctane (n = 3) and (b) cyclohexene] to the corresponding products, with aqueous H2O2, catalyzed by the copper(II) complexes 25, in the presence or absence of any additive, under typical mild reaction conditions of this work.
Inorganics 07 00017 sch002
Figure 3. Accumulation of oxygenates with time in the cyclooctane (initial concentration 0.25 M) oxidation with H2O2 (1.2 M) catalyzed by 25 in acetonitrile at 60 °C.
Figure 3. Accumulation of oxygenates with time in the cyclooctane (initial concentration 0.25 M) oxidation with H2O2 (1.2 M) catalyzed by 25 in acetonitrile at 60 °C.
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Figure 4. Comparison of the accumulation of oxygenates with time (0–15 min) in the cyclooctane (initial concentration 0.25 M) oxidation with H2O2 (1.2 M), in the presence of pyridine (0.072 M), catalyzed by 25 (10−3 M) in acetonitrile at 60 °C.
Figure 4. Comparison of the accumulation of oxygenates with time (0–15 min) in the cyclooctane (initial concentration 0.25 M) oxidation with H2O2 (1.2 M), in the presence of pyridine (0.072 M), catalyzed by 25 (10−3 M) in acetonitrile at 60 °C.
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Figure 5. Comparison of the accumulation of oxygenates with time (0–32 h) in the cyclooctane (initial concentration 0.25 M) oxidation with H2O2 (1.2 M) in the presence of HNO3 (0.072 M), catalyzed by 25 (10−3 M) in acetonitrile at 60 °C.
Figure 5. Comparison of the accumulation of oxygenates with time (0–32 h) in the cyclooctane (initial concentration 0.25 M) oxidation with H2O2 (1.2 M) in the presence of HNO3 (0.072 M), catalyzed by 25 (10−3 M) in acetonitrile at 60 °C.
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Scheme 3. Radical mechanism of the peroxidative oxidation of hydrocarbons with H2O2.
Scheme 3. Radical mechanism of the peroxidative oxidation of hydrocarbons with H2O2.
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Table 1. Comparison of some selected features of Schiff base 1 and complexes 25. Distances in (Å) and angles in (°).
Table 1. Comparison of some selected features of Schiff base 1 and complexes 25. Distances in (Å) and angles in (°).
12345
In the Organic Moiety H2L (or L2−)
C−Nimino1.299(4)
1.294(4)
1.303(4)1.292(3)1.301(3)1.309(5)
Caromatic−Nimino1.433(4)
1.438(4)
1.429(4)1.434(3)1.426(3)1.429(5)
Coplanarity of aromatic rings (°)8.41
10.98
58.1962.0151.0451.81
Surrounding the Copper Atom
Cu−Nimino-1.976(2)1.9837(19)1.9769(17)1.975(3)
Cu−Osulfonato-2.014(2)2.3789(18)1.9733(16)2.019(3)
Cu−Ophenoxido-1.898(2)1.8942(18)1.8770(16)1.900(3)
Cu−Ncoligand-2.015(3)2.009(2)
2.061(2)
2.0709(17)2.004(3)
Cu−Ocoligand-2.290(2)-2.3760(18)2.303(3)
∠N−Cu−N-177.98(10)79.71(8)
102.71(8)
170.85(8)
174.06(7)177.57(14)
∠Osulfonato−Cu−
Ophenoxido
-157.30(9)113.95(8)158.97(8)151.07(14)
Cu coordination environment-N2O3N3O2N2O3N2O3
τ5-0.350.270.250.44
Cu···Cu (minimum)-7.5418.1106.9196.782
Table 2. Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 3 a.
Table 2. Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 3 a.
No AdditivepyHNO3
EntryTime (min)Yield b (%)Selectivity cTON dYield b (%)Selectivity cTON dYield b (%)Selectivity cTON d
100-00-00-0
21 30:1008
33 91:9923
4431:998144:9635131:9931
55.5 237:9358
67 2611:8965
7983:97202614:8665
810 2621:7965142:9835
915145:95352624:7665
1030186:94452628:7265173:9742
11602010:90502627:7365
12902017:83502629:7165195:9547
132 × 602020:80502634:6665207:9350
144 × 602023:77502637:63652210:9056
155.5 × 60 2310:9058
168 × 602027:73512639:61652511:8963
1710 × 60 2711:8968
1816 × 60 3013:8774
1921.5 × 60 2620:8065
2024 × 602031:69502653:47652520:8062
2129.5 × 60 2023:7750
2232 × 602036:64512654:46651825:7546
a Reaction conditions: cyclooctane (0.25 M), complex 3 (10−3 M), additive (0.072 M), H2O2 (1.2 M) in acetonitrile at 60 °C; total volume of reaction mixture is 10 mL. b Amounts of cyclooctanone and cyclooctanol were determined after reduction of the aliquots with solid PPh3 (for this method, see references [61,62,63,64,65]). c Cyclooctanone/cyclooctanol ratio. d Turnover number of the catalyst (sum of moles of all products per mole of 3).
Table 3. Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 5 a.
Table 3. Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 5 a.
No AdditivepyHNO3
EntryTime(min)Yield b (%)Selectivity cTON dYield b (%)Selectivity cTON dYield b (%)Selectivity cTON d
100-00-00-0
21 10:1003
33 53:9713
44 74:9617
56 135:9533
68 187:9345
710 238:9258
815 2316:845830:1008
93062:98152321:795860:10015
106073:97182326:745881:9819
1190 2327:7358101:9825
122 × 6093:97232329:7158122:9831
133 × 60 19 47
144 × 60119:91282334:6658303:9776
158 × 601323:77322340:60582623:7764
1610 × 601328:7231
1724 × 601334:66322356:44581534:6637
1832 × 601339:61312361:39581339:6132
a Reaction conditions: cyclooctane (0.25 M), complex 5 (10−3 M per copper), additive (0.072 M), H2O2 (1.2 M) in acetonitrile at 60 °C; total volume of reaction mixture is 10 mL. b Amounts of cyclooctanone and cyclooctanol were determined after reduction of the aliquots with solid PPh3 (for this method, see references [61,62,63,64,65]). c Cyclooctanone/cyclooctanol ratio. d Turnover number of the catalyst (sum of moles of all products per mole of 5).
Table 4. Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 2 a.
Table 4. Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 2 a.
No AdditivepyHNO3
EntryTime(h)Yield b (%)Selectivity cTON dYield b (%)Selectivity cTON dYield b (%)Selectivity cTON d
100-00-00-0
21 30:1008
33 93:9723
44 1512:8838
55.5 1920:8048
67 2122:7853
79 2325:7558
810 2426:7460
915 2427:736030:1007.5
1030 2429:716080:10020
1160 2430:7060160:10040
1290 221:9855
132 × 6060:100162432:6860312:9878
144 × 6082:98202434:6660352:9888
158 × 6084:96212439:6160265:9565
1624 × 60832:68212449:51602038:6250
1732 × 60836:64212452:48601733:6743
a Reaction conditions: cyclooctane (0.25 M), complex 2 (10−3 M), additive (0.072 M), H2O2 (1.2 M) in acetonitrile at 60 °C; total volume of reaction mixture is 10 mL. b Amounts of cyclooctanone and cyclooctanol were determined after reduction of the aliquots with solid PPh3 (for this method, see references [61,62,63,64,65]). c Cyclooctanone/cyclooctanol ratio. d Turnover number of the catalyst (sum of moles of all products per mole of 2).
Table 5. Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 4 a.
Table 5. Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 4 a.
No AdditivepyHNO3
EntryTime(min)Yield b (%)Selectivity cTON dYield b (%)Selectivity cTON dYield b (%)Selectivity cTON d
100-00-00-0
21 10:1003
33 50:10013
44 91:9923
55.5 138:9233
67 1712:8843
79 2015:8550
810 2117:8353
915 2319:8158
1030 2326:7458
1160 2328:725870:10019
1290 2330:7058
132 × 6040:100112331:6958142:9836
144 × 6072:98172336:6458272:9868
158 × 6073:97182343:5758234:9657
1610 × 60 246:9459
1724 × 60726:74172353:47581332:6832
1832 × 60733:67182353:47581437:6335
a Reaction conditions: cyclooctane (0.25 M), complex 4 (10−3 M per copper), additive (0.072 M), H2O2 (1.2 M) in acetonitrile at 60 °C; total volume of reaction mixture is 10 mL. b Amounts of cyclooctanone and cyclooctanol were determined after reduction of the aliquots with solid PPh3 (for this method, see references [61,62,63,64,65]). c Cyclooctanone/cyclooctanol ratio. d Turnover number of the catalyst (sum of moles of all products per mole of 4).
Table 6. Scope of the peroxidative hydrocarbon oxidation employing catalyst 3 a.
Table 6. Scope of the peroxidative hydrocarbon oxidation employing catalyst 3 a.
EntryTime (h)SubstrateYield b, %Selectivity cTON d
12Cyclohexane2022:7850
242024:7650
382027:7350
4242032:6850
5322033:6750
62Cyclohexene158:36:44:4:850
74157:40:39:5:950
88156:45:36:5:850
924151:49:34:8:850
1032150:48:32:10:950
a Reaction conditions: substrate (0.25 M), complex 3 (10−3 M), H2O2 (1.2 M) in acetonitrile at 60 °C; total volume of reaction mixture is 10 mL. b Amounts of oxygenate products were determined after reduction of the aliquots with solid PPh3 (for this method, see references [61,62,63,64,65]). c Cyclohexanone:cyclohexanol and cyclohexene oxide:2-cyclohexen-1-one:2-cyclohexen-1-ol:cis-1,2-cyclohexanediol:trans-1,2-cyclohexanediol ratio (for reactions with cyclohexane and cyclohexene, respectively). d Turnover number of the catalyst (sum of moles of all products per mole of 3).

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