Syntheses , Structures , and Catalytic Hydrocarbon Oxidation Properties of N-Heterocycle-Sulfonated Schiff Base Copper ( II ) Complexes

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 (2–5) 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 (2–5) 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.

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  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 (H 2 O 2 )  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.

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).
The IR spectrum of the Schiff base H 2 L (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 (2-5), 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.

Description of Crystal
The asymmetric unit of H 2 L (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. 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 0.25-0.44range, Table 1) and the L 2− ligand acts as an O,N,O-chelator by means of the phenolic O-, the imine N-, and   In the Organic Moiety H 2 L (or L  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 (τ 5 parameter in the 0.25-0.44range, Table 1) and the L 2− 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 N 3 O 2 metal environment, or by a solvent and one more organic moiety (2, 4, and 5) and forming N 2 O 3 settings (Table 1).Probably as a result of the chelating mode of bipyridine, compound 3 is the only one in which the L 2− ligand occupies both equatorial (O phenoxido and N imino atoms) and apical (O sulfonato ) 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 metaloxygen distances for the axial sites (i.e., Cu−Ocoligand lengths, Table 1) and is due to the Jahn-Teller effect.

Oxidation of Hydrocarbons
The catalytic properties of the copper(II) complexes (2-5) 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).The Cu-N imino 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-O phenoxido distances (1.8770( 16)-1.900(3)Å) are considerably shorter than the Cu−O sulfonato 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−O coligand lengths, Table 1) and is due to the Jahn-Teller effect.

Oxidation of Hydrocarbons
The catalytic properties of the copper(II) complexes (2)(3)(4)(5) 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 2-5 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 (2-5).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.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 2-5 in the model system is shown in Figure 3.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 2-5 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 (2-5).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.Blank experiments confirmed that no cyclooctanol or cyclooctanone are formed appreciably in the absence of the complexes (2-5).The Schiff base H 2 L (1) is inactive (Table S2, Supplementary Materials) toward the cyclooctane oxidation, whereas CuCl 2 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 HNO 3 as a basic and an acid promoter, respectively.
For all the complexes 2-5, the presence of such additives resulted, in general, in an enhancement of the maximum yield and TON values, mainly in the case of HNO 3 (Tables 2-5).Changes in selectivity were also observed.
All the catalysts 2-5 exhibit comparable overall yields (Tables 2-5) 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).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 (Tables 2 and 3).
Table 3.Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 5 a .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 (Tables 4 and 5).

No Additive py HNO3
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 (Tables 2  and 3).
The application of HNO 3 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).
Inorganics 2019, 7, x FOR PEER REVIEW 10 of 19 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 2-5 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].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 HNO 3 , 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 HNO 3 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 2-5 using the HNO 3 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 Ph 2 NH, an oxygen radical trap [37], and the effect of the addition of CBrCl 3 , a carbon radical trap [37], on the cyclooctane oxidation.In agreement with such mechanistic considerations, the good catalytic activity of 2-5 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 agreement with such mechanistic considerations, the good catalytic activity of 2-5 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 PPh 3 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 H 2 O 2 , 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. a Reaction conditions: substrate (0.25 M), complex 3 (10 ) 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 PPh 3 (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).
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).

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 1 H 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).

Synthesis of H 2 L (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 C 13 H 11 NO

Synthesis of [Cu(L)(py)(EtOH)] (2)
To a hot and stirred ethanol suspension (10 mL) of H 2 L (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 CuCl 2 •2H 2 O (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%

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).

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 (2-5) 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 H 2 O 2 with organic compounds at elevated temperatures may be explosive!)The initial concentrations of hydrocarbon, catalyst, H 2 O 2 , 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 PPh 3 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 PPh 3 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 (Ph 2 NH) and the carbon radical trap [37] bromotrichloromethane (CBrCl 3 ) were applied in a stoichiometric amount relatively to the oxidant and substrate, respectively.

Conclusions
By taking advantage of the chelating capacity of the acyclic Schiff base o-[(o-hydroxyphenyl)methylideneamino]benzenesulfonic acid H 2 L (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 4), and [Cu 2 (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 (2-5) 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 HNO 3 (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 3-5) 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 H 2 O 2 decomposition, the influence of the O 2 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 http://www.mdpi.com/2304-6740/7/2/17/s1,H-Bonded networks in 1-5, 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 1-5, Table S2: Tests concerning the oxidation of cyclooctane in the presence of H 2 L (1) and in the absence of any metal catalyst (blank tests), Table S3: Results obtained before the reduction with PPh 3 of the aliquots of the oxidation of cyclooctane catalyzed by 3, Table S4: Crystallographic data for 1-5, 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 1-5, 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: deposit@ccdc.cam.ac.uk.

Figure 1 .
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 .
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.

Table 1 .
Comparison of some selected features of Schiff base 1 and complexes 2-5.Distances in (Å) and angles in ( • ).

Inorganics 2019, 7 ,
x FOR PEER REVIEW 5 of 19 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.

19 Scheme 2 .
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 2-5, in the presence or absence of any additive, under typical mild reaction conditions of this work.

Figure 3 .
Figure 3. Accumulation of oxygenates with time in the cyclooctane (initial concentration 0.25 M) oxidation with H 2 O 2 (1.2 M) catalyzed by 2-5 in acetonitrile at 60 • C.

a
Reaction conditions: cyclooctane (0.25 M), complex 4 (10 −3 M per copper), additive (0.072 M), H 2 O 2 (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 PPh 3 (for this method, see references [61-65]).c Cyclooctanone/cyclooctanol ratio.d Turnover number of the catalyst (sum of moles of all products per mole of 4).Inorganics 2019, 7, x FOR PEER REVIEW 8 of 19

Scheme 3 .
Scheme 3. Radical mechanism of the peroxidative oxidation of hydrocarbons with H2O2.

Scheme 3 .
Scheme 3. Radical mechanism of the peroxidative oxidation of hydrocarbons with H 2 O 2 .

Table 1 .
Comparison of some selected features of Schiff base 1 and complexes 2-5.Distances in (Å) and angles in (°).

Table 2 .
Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 3 a .

Table 3 .
Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 5 a .

Table 4 .
Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 2 a .

Table 5 .
Effect of the presence of an additive in the oxidation of cyclooctane catalyzed by 4 a .

Table 6 .
Scope of the peroxidative hydrocarbon oxidation employing catalyst 3 a .