Zn(II)-to-Cu(II) Transmetalation in an Amide Functionalized Complex and Catalytic Applications in Styrene Oxidation and Nitroaldol Coupling

The mononuclear zinc(II) complex cis-[ZnL2(H2O)2] (1; L = 4-(pyridin-3-ylcarbamoyl)benzoate) was synthesized and characterized. By soaking crystals of 1 in a mixture of DMF-H2O solution containing a slight excess of Cu(NO3)2 × 3H2O a transmetalation reaction occurred affording the related copper(II) complex trans-[CuL2(H2O)2] (2). The structures of the compounds were authenticated by single crystal X-ray diffraction revealing, apart from a change in the isomerism, an alteration in the relative orientation of the chelating carboxylate groups and of the pyridine moieties. H-bond interactions stabilize both geometries and expand them into two-dimensional (2D) networks. The transmetalation was confirmed by SEM–EDS analysis. Moreover, the thermodynamic feasibility of the transmetalation is demonstrated by density-functional theory (DFT) studies. The catalytic activities of 1 and 2 for the oxidation of styrene and for the nitroaldol (Henry) C-C coupling reaction were investigated. The copper(II) compound 2 acts as heterogeneous catalyst for the microwave-assisted oxidation of styrene with aqueous hydrogen peroxide, yielding selectively (>99%) benzaldehyde up to 66% of conversion and with a turnover frequency (TOF) of 132 h−1. The zinc(II) complex 1 is the most active catalyst (up to 87% yield) towards the nitroaldol (Henry) coupling reaction between benzaldehyde and nitro-methane or -ethane to afford the corresponding β-nitro alcohols. The reaction of benzaldehyde with nitroethane in the presence of 1 produced 2-nitro-1-phenylpropanol in the syn and the anti diastereoisomeric forms, with a considerable higher selectivity towards the former (66:34).


Introduction
Transmetalation is commonly associated with an organometallic compound and involves a transfer of organic groups or ligands from one metal to another [1][2][3]. It thus provides a common and convenient method for preparing various organo-transition metal complexes, which are otherwise difficult to synthesize via direct metal-ligand self-assembly. Such phenomenon is well documented and has expanded to other types of materials [4][5][6][7][8][9][10][11]. The process usually proceeds through either "direct exchange", where the incoming metal does not alter the topology of the frameworks or through "disruptive exchange" in which the geometry of the transmetalation complex differs from the original structure as a result of the metal exchange.
Several metal-organic frameworks (MOFs) have been employed in transmetalation to synthesize novel materials which would otherwise be difficult to obtain [12][13][14][15]. The role of ligands in the process cannot be ruled out.
Previously, the presence of a carboxylate and a pyridyl moieties in the ligand framework of (4-(pyridin-3-ylcarbamoyl)benzoate) was used for the synthesis of MOFs or coordination polymers (CPs) by Luo et al. [16]. The presence of a carboxylate and pyridyl moieties was used for the synthesis of MOFs or coordination polymers (CPs) but, in the present study, we failed to generate such types of compounds through hydro/solvothermal reaction using a zinc salt. Instead, unexpectedly a mononuclear molecular zinc(II) complex was obtained, being susceptible to undergo transmetalation with copper(II) (see below).
In pursuing our interest on the synthesis of metal-based complexes for application in catalysis, we have been using amide functionalized ligands primarily because the amide backbone in the ligand framework can provide a basic environment and facilitate the catalytic reaction [17][18][19][20]. Furthermore, the presence of multiple coordinating sites in the ligand skeleton can provide favorable stretched frameworks having a large surface area, also advantageous for catalysis.
Concerning a catalytic application, the preparation of benzaldehyde is an important process as this compound is a valuable chemical for perfumery, dyestuff, pharmaceutical and agro-chemical industries [21][22][23][24][25]. Although it can be obtained by oxidation of benzyl alcohol, traditionally benzaldehyde is produced by catalytic partial oxidation of toluene or by hydrolysis of benzylidene chloride [26,27]. Both processes present low yields, harsh conditions and a significant generation of waste. Therefore, there is a need to find more environmentally friendly processes using green oxidation agents (e.g., hydrogen peroxide) and heterogeneous catalyst systems.
On the other hand, the nitroaldol (Henry) reaction is also relevant, being widely used to build C-C bonds by the formation of β-nitro alcohols (from the Csp 3 -Csp 2 coupling reaction between a nitroalkane and a carbonyl compound), which are important synthetic precursors. This reaction can be carried out with homogeneous and heterogeneous catalysts, as well as with alkali metal hydroxides, alkoxides or amines [28][29][30][31]. We have already reported a variety of homogeneous and heterogeneous catalysts for this kind of reaction [17,18,[32][33][34][35]. Despite several progresses, good enantio-and diastereoselectivity is rarely achieved. Thus, also for this reaction, a need for improved catalytic systems remains.
Herein, we present the synthesis and structural analysis of the new mononuclear complex of zinc(II) cis-[ZnL 2 (H 2 O) 2 ] (1, L = 4-(pyridin-3-ylcarbamoyl)benzoate) and of the derived copper(II) compound trans-[CuL 2 (H 2 O) 2 ] (2). 1 was synthesized by hydrothermal reaction, whereas its post-synthetic modification led to the formation of compound 2 through transmetalation, which is irreversible. Complexes 1 and 2 are shown to catalyze the oxidation of styrene towards benzaldehyde and the Henry reaction of an aldehyde with a nitro alkane to form a β-nitro alcohol. The mononuclear Cu(II) complex 2 acts as a heterogeneous catalyst for the microwave-assisted selective oxidation of styrene to benzaldehyde, whereas the Zn(II) complex 1 converts successfully benzaldehyde into β-nitro alcohols.

Materials and Methods
Solvents were dried and distilled before their use. Reagents were obtained from commercial sources and used without further purification. Methyl-4-(chlorocarbonyl)benzoate was prepared following a procedure reported earlier [36]. FT-IR spectra were recorded on a Bruker Vertex 70 (Bruker Corporation, Ettlingen, Germany) instrument in KBr pellets. 1 H (300 MHz) and 13 C (75.45 MHz) NMR spectra were obtained at room temperature (RT) on a Bruker Avance II + 300 (UltraShield™Magnet, Rheinstetten, Germany) spectrometer using tetramethylsilane [Si(CH 3 ) 4 ] as an internal reference. Carbon, hydrogen, and nitrogen elemental analyses were carried out by the Microanalytical Service of the Instituto Superior Técnico, Lisboa, Portugal. Powder X-ray diffraction (PXRD) was conducted in a D8 Advance Bruker AXS (Bragg Brentano geometry, Bruker, Madison, WI, USA) theta-2-theta diffractometer, with copper radiation (Cu Kα, λ = 1.5406 Å) and a secondary monochromator, operated at 40 kV and 40 mA. Flat plate configuration was used, and the typical data collection range was between 5 • and 40 • . Thermal properties were performed using a PerkinElmer Instrument system Molecules 2020, 25, 2644 3 of 14 (STA6000, Perkin-Elmer, Boston, MA, USA) at a heating rate of 10 • C min −1 under a dinitrogen atmosphere flow rate of 30 mL/min. The synthesized compounds were characterized in terms of structural and optical properties by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) using a scanning electron microscope JEOL 7001 F (JEOL, Akishima, Tokyo, Japan) with Oxford light elements EDX detector.

Crystal Structure Determination
Single crystals of 1 and 2 were leapt in cryo-oil, mounted in a nylon loop and measured at room temperature. Intensity data were collected using a Bruker APEX-II PHOTON 100 diffractometer (Bruker, Madison, WI, USA) with graphite monochromated Mo-Kα (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 SMART [37] software and refined using Bruker SAINT [37] on all the observed reflections. Absorption corrections were applied using SADABS [38]. Structures were solved by direct methods by using the SIR97 program and refined with SHELX-2014/7 [39,40]. Calculations were performed using the WinGX System-Version 2014.1 [41]. The hydrogen atoms were inserted in calculated positions and included in the refinement using the riding-model approximation; Uiso (H) were defined as 1.2 Ueq of the parent carbon atoms for phenyl residues. The water hydrogen atoms were found in the difference Fourier map and were included in the refinement using the riding-model approximation with the Uiso(H) defined as 1.5 Ueq of the parent O-atom. PLATON/SQUEEZE [42] was used to correct the data. Crystallographic data are summarized in Table 1 and selected hydrogen bonding interactions are presented in Table S1. CCDC 2000481-2000482 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Computational Details
The full geometry optimization of all structures was carried out at the DFT level of theory by using the M06 functional [43] with the help of the Gaussian 09 program package [44]. No symmetry operations were applied. The standard basis set 6-31G* was applied for all atoms. Solvent effects were taken into account in single-point calculations based on gas phase geometry using the "density-based" solvation model (SMD) solvation model with water and DMF selected as solvents [45]. The 6-311+G** and Def2-TZVP basis sets were applied in these calculations for non-metal and metal atoms, respectively. The solvent effect for the 1:1 mixture of water and DMF was approximated as the average value obtained for pure water and DMF solvents. The optimization of complex 1 was additionally performed at the SMD(H 2 O)-B3LYP/6-31+G* and SMD(H 2 O)-ωB97XD/Def2-TZVP levels of theory. The Cartesian atomic coordinates of the calculated equilibrium structures are presented in Table S2. The Hessian matrix was calculated analytically for the optimized structures to prove the location of correct minima (no imaginary frequencies) and to estimate the thermodynamic parameters, with the latter calculated at 25 • C. The Gibbs free energies in solution, G s , discussed in the text were obtained as G s = E s (6-311 + G**/Def2-TZVP) -E g (6-31G*) + G g (6-31G*), where E s and E g are total energies in solution and gas phase and G g is gas phase Gibbs free energy obtained using the indicated basis set.

Catalytic Studies
Typical oxidation of styrene was performed under microwave irradiation (10 W) in a focused microwave Anton Paar Monowave 300 (Anton Paar GmbH, Graz, Austria) discover reactor fitted with a rotational system and an IR temperature detector. A 10 mL capacity cylindrical Pyrex tube with a 13 mm internal diameter was used: catalyst (10 µmol), the substrate (1.0 mmol), hydrogen peroxide (30% w/w aq. sol., 2.0 mmol), 1.5 mL of acetonitrile and 100 µL of chlorobenzene as internal standard were stirred at 80 • C for the desired reaction time. During the reaction, small aliquots were taken, cooled down, centrifuged (to separate the catalyst from the sample), and analyzed by GC-MS using the internal standard method for quantification. GC-MS analyses were conducted at a Perkin-Elmer Clarus 600 C (Shelton, CT, USA) apparatus, using He as the carrier gas, with an ionization voltage of 70 eV and an SGE BPX5 column (30 m × 0.25 mm × 0.25 µm). The comparison of the products retention times with those of known reference compounds enabled their identification. Moreover, their mass spectra fragmentation patterns were compared with those obtained from the National Institute of Standards and Technology (NIST) spectral library of the computer software of the spectrometer. The values of conversion considered result from two concordant assays. Blank reactions were performed for the best conditions in the absence of complexes 1 or 2 and confirmed that almost no oxidation occurs. In some runs under the optimized conditions, 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO, 1.0 mmol) was added to the initial reaction mixture as a radical scavenger.
The recyclability of complex 2 was examined, by recovering it by centrifugation after the reaction being cooled down, washing with ethanol, and drying overnight. A new cycle was run with the recovered catalyst by addition of new typical portions of all other reagents. After completion of the reaction, the products were analyzed as above-mentioned.
The experiments for the Henry reaction were carried out in a glass reactor. A mixture of the nitroalkane (1.5 mmol), benzaldehyde (0.5 mmol), catalyst 1 (50 µmol) and 3 mL of solvent (deionized water, methanol, dichloromethane, dichloroethane, THF, or acetonitrile) was stirred up to 24 h at room temperature to 60 • C. After the desired reaction time, the solution was extracted with diethyl ether (3 × 5 mL), the combined extracts dried over anhydrous Na 2 SO 4 , and then the mixture was filtered off. The diethyl ether was removed in vacuo and the organic residue was analyzed by 1 H NMR spectrometry, in CDCl 3 , for quantification of the formed β-nitro alcohols (as no other products were detected). The anti:syn diastereoisomer ratio of 1-phenyl-2-nitropropanol was determined taking into account the vicinal coupling constants values between the α-O-C-H and the α-N-C-H protons for the isomers: anti, J = 7-9 Hz; syn, J = 3.2-4 Hz) [46,47].

Syntheses and Characterization
The pro-ligand HL was prepared according to the reported modified procedure [16]. Compound 1 was synthesized by the hydrothermal reaction between Zn(NO 3 ) 2 .6H 2 O and HL in a DMF:H 2 O solvent mixture (Scheme 1 and experimental section for synthetic details). The transmetalation reaction to afford 2 was carried out by soaking crystals of 1 in a DMF:H 2 O (1:1, v/v) solvent mixture containing a slight excess of aqueous Cu(NO 3 ) 2 × 3H 2 O and leaving it undisturbed at room temperature for two days (Figure 1). Both compounds were characterized by elemental analysis, IR, SEM-EDS and single crystal X-ray diffraction analyses. It is worthy to mention here that we failed the attempts to achieve the direct synthesis of the transmetalated Cu(II) complex 2 by deprotonating the pro-ligand HL with a base, such as KOH, NaOH or NH 4   The FT-IR spectrum of HL displays a peak at ca. 1705 cm −1 assigned to the asymmetric stretching vibration of the carboxyl group which, upon coordination to the metal cations shifts to the lower frequency values of 1648 cm −1 (in 1) and 1653 cm −1 (in 2) ( Figure S1).

Crystal Structure Analysis
The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis and are presented in Figure 2. Crystallographic data of the compounds 1 and 2 are presented in Table  1 and hydrogen bond contacts are detailed in Tables S1 (supplementary materials).
The asymmetric units of the compounds include one metal ion (Zn 2+ in 1 or Cu 2+ in 2), one deprotonated L − ligand binding the cations through chelating (in 1) or monodentate (in 2) carboxylate moieties, and one water molecule ( Figure 2). Symmetry expansion reveals mononuclear complexes where the water ligands are in mutual cis (in 1) or trans (in 2) position. In view of the constraints derived from the chelating mode of L − in 1 the six-coordinate geometry is better described as trigonal prismatic with the angle between the least-square planes of the carboxylate groups being of 78.06°. In 2, the environment around the metal cation can be envisaged as square planar but the long-range interaction between Cu1 and O2 or O2′-carboxylate atoms (Cu1⋅⋅⋅O2/O2′ 2.606 Å), being shorter than the sum of the van der Waals radii of Cu (1.  The FT-IR spectrum of HL displays a peak at ca. 1705 cm −1 assigned to the asymmetric stretching vibration of the carboxyl group which, upon coordination to the metal cations shifts to the lower frequency values of 1648 cm −1 (in 1) and 1653 cm −1 (in 2) ( Figure S1).

Crystal Structure Analysis
The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis and are presented in Figure 2. Crystallographic data of the compounds 1 and 2 are presented in Table  1 and hydrogen bond contacts are detailed in Tables S1 (supplementary materials).
The asymmetric units of the compounds include one metal ion (Zn 2+ in 1 or Cu 2+ in 2), one deprotonated L − ligand binding the cations through chelating (in 1) or monodentate (in 2) carboxylate moieties, and one water molecule (Figure 2). Symmetry expansion reveals mononuclear complexes where the water ligands are in mutual cis (in 1) or trans (in 2) position. In view of the constraints derived from the chelating mode of L − in 1 the six-coordinate geometry is better described as trigonal prismatic with the angle between the least-square planes of the carboxylate groups being of 78.06°. In 2, the environment around the metal cation can be envisaged as square planar but the long-range interaction between Cu1 and O2 or O2′-carboxylate atoms (Cu1⋅⋅⋅O2/O2′ 2.606 Å), being shorter than the sum of the van der Waals radii of Cu (1.  The FT-IR spectrum of HL displays a peak at ca. 1705 cm −1 assigned to the asymmetric stretching vibration of the carboxyl group which, upon coordination to the metal cations shifts to the lower frequency values of 1648 cm −1 (in 1) and 1653 cm −1 (in 2) ( Figure S1).

Crystal Structure Analysis
The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analysis and are presented in Figure 2. Crystallographic data of the compounds 1 and 2 are presented in Table 1 and hydrogen bond contacts are detailed in Table S1 (Supplementary Materials). Both compounds are involved in intermolecular H-interaction (Table S1)   complexes where the water ligands are in mutual cis (in 1) or trans (in 2) position. In view of the constraints derived from the chelating mode of L − in 1 the six-coordinate geometry is better described as trigonal prismatic with the angle between the least-square planes of the carboxylate groups being of 78.06 • . In 2, the environment around the metal cation can be envisaged as square planar but the long-range interaction between Cu1 and O2 or O2 -carboxylate atoms (Cu1···O2/O2 2.606 Å), being shorter than the sum of the van der Waals radii of Cu (1.40 Å) and O (1.52 Å), can be considered as giving rise to a highly distorted octahedral geometry. The Zn-O and Cu-O bond distances are in the range of 2.0197(16)-2.2020(15) Å and 1.955(4)-1.968(3) Å, respectively. The N2 and N2 atoms are uncoordinated, displaying the syn orientation in 1 and the anti in 2 (Figure 2).
Both compounds are involved in intermolecular H-interaction (Table S1)   Both compounds are involved in intermolecular H-interaction (Table S1)

DFT Calculations
In order to understand the thermodynamic feasibility of the conversion of 1 to 2 through transmetalation, DFT calculations were carried out. The DFT calculations with various functionals and basis sets (gas-M06/6-31G*, SMD(H 2 O)-B3LYP/6-31+G*, and SMD(H 2 O)-ωB97XD/def2-TZVP) indicated that in both gas and liquid phases the structure of the Zn complex 1 corresponds to a distorted tetrahedral coordination sphere with both 4-(pyridin-3-ylcarbamoyl)benzoate ligands being monodentate (the observed long range Zn-O interaction of the chelating carboxylate groups is discarded) (Figure 4). Therefore, the structure of complex 1 in the solid state is controlled by crystal packing effects, first of all by multiple intermolecular interactions described above.
Molecules 2020, 24, x FOR PEER REVIEW 8 of 14 indicated that in both gas and liquid phases the structure of the Zn complex 1 corresponds to a distorted tetrahedral coordination sphere with both 4-(pyridin-3-ylcarbamoyl)benzoate ligands being monodentate (the observed long range Zn-O interaction of the chelating carboxylate groups is discarded) (Figure 4). Therefore, the structure of complex 1 in the solid state is controlled by crystal packing effects, first of all by multiple intermolecular interactions described above. In accord with the calculations, the experimentally isolated trans-isomer of 2 is more stable than the corresponding cis-isomer in all water, DMF, and 1:1 water/DMF mixture by 1. (1)

Conversion of 1 into 2
The conversion of 1 into 2 was evidenced by the color change from colorless to blue ( Figure 5). During such conversion process, no loss in crystal quality was observed as evidenced by SEM analysis (inset of Figure 6) as well as by X-ray (see above). The complete conversion of 1 to 2 was validated by SEM-EDS method ( Figure 6). Figure 7 presents the PXRD patterns of 1 and 2 both calculated from their crystal structures and experimentally obtained from the bulk materials, with undeniable resemblance. In accord with the calculations, the experimentally isolated trans-isomer of 2 is more stable than the corresponding cis-isomer in all water, DMF, and 1:1 water/DMF mixture by 1.6, 2.  (1)

Conversion of 1 into 2
The conversion of 1 into 2 was evidenced by the color change from colorless to blue ( Figure 5). During such conversion process, no loss in crystal quality was observed as evidenced by SEM analysis (inset of Figure 6) as well as by X-ray (see above). The complete conversion of 1 to 2 was validated by SEM-EDS method ( Figure 6). Figure 7 presents the PXRD patterns of 1 and 2 both calculated from their crystal structures and experimentally obtained from the bulk materials, with undeniable resemblance.     The recurrent attempted synthesis and isolation of 2 by hydrothermal reaction between HL and Cu(NO3)2 × 3H2O were unsuccessful. Moreover, by soaking crystals of 2 in a DMF:H2O (1:1, v/v) solvent mixture and subsequent addition of an aqueous solution containing a molar excess of     The recurrent attempted synthesis and isolation of 2 by hydrothermal reaction between HL and Cu(NO 3 ) 2 × 3H 2 O were unsuccessful. Moreover, by soaking crystals of 2 in a DMF:H 2 O (1:1, v/v) solvent mixture and subsequent addition of an aqueous solution containing a molar excess of Zn(NO 3 ) 2 × 6H 2 O, no transmetalation reaction took place even after more than a month, showing the irreversibility of the process on account of the stronger coordination of the ligand to Cu(II) in comparison with Zn(II). Furthermore, to know if Zn(II) in 1 could be replaced by any other transition metal ion, apart from Cu(II), analogous procedures to that mentioned above for the 1 to 2 conversion were followed using salts of Mn 2+ , Co 2+ , Ni 2+ , Pb 2+ and Cd 2+ , but no transmetalation was observed on the basis of EDS analyses ( Figure S2), thus, disclosing a curious selectivity towards Cu(II).

Catalytic Oxidation of Styrene to Benzaldehyde
Both complexes 1 and 2 were tested for the peroxidative oxidation of styrene. Complex 1 was not active as a catalyst, since the styrene conversion obtained in its presence was similar to the one obtained in blank tests (without any metal catalyst). On the contrary, the copper complex 2 was successfully tested as a heterogeneous catalyst for the oxidation of styrene by aqueous hydrogen peroxide, in acetonitrile, yielding selectively (>99%) benzaldehyde (Scheme 2a) under optimized reaction conditions (microwave irradiation, 80 • C, 30 min., 600 rpm, 10 W, n oxidant /n substrate = 2) up to 66% of conversion and a turnover frequency (moles of product per mole of catalyst per unit of time, TOF) of 132 h −1 . metal ion, apart from Cu(II), analogous procedures to that mentioned above for the 1 to 2 conversion were followed using salts of Mn 2+ , Co 2+ , Ni 2+ , Pb 2+ and Cd 2+ , but no transmetalation was observed on the basis of EDS analyses ( Figure S2), thus, disclosing a curious selectivity towards Cu(II).

Catalytic Oxidation of Styrene to Benzaldehyde
Both complexes 1 and 2 were tested for the peroxidative oxidation of styrene. Complex 1 was not active as a catalyst, since the styrene conversion obtained in its presence was similar to the one obtained in blank tests (without any metal catalyst). On the contrary, the copper complex 2 was successfully tested as a heterogeneous catalyst for the oxidation of styrene by aqueous hydrogen peroxide, in acetonitrile, yielding selectively (>99%) benzaldehyde (Scheme 2a) under optimized reaction conditions (microwave irradiation, 80 °C, 30 min., 600 rpm, 10 W, noxidant/nsubstrate = 2) up to 66% of conversion and a turnover frequency (moles of product per mole of catalyst per unit of time, TOF) of 132 h −1 . When a decrease in the selectivity for benzaldehyde was found (higher oxidant to styrene molar ratios, noxidant/nsubstrate ≥ 4), although being the major product, the formation of 1-phenylethane-1,2-diol (up to 23%) was detected. This can suggest that the mechanism for styrene oxidation is initiated by the Cu-assisted formation of oxygen-centered radicals (hydroxyl and peroxyl, similar to Fenton chemistry) which oxidize styrene, what is also suggested by the reaction inhibition upon addition of the radical scavenger 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to the reaction mixture. Moreover, the epoxide product was not detected, neither acetophenone nor phenylacetaldehyde (from isomerization of the epoxide).
Besides, an increase in the microwave irradiation time led to the formation of benzoic acid by overoxidation of benzaldehyde. In fact, if the desired product would be benzoic acid, full conversion of styrene could be obtained performing the reaction under MW irradiation during 2 h at 80 °C with an increased amount of hydrogen peroxide. Higher temperatures than 80 °C were not tested due to safety reasons.
The recyclability of complex 2 was examined by recovering it through centrifugation (see Experimental). Although a slight color change was detected for the recovered catalyst, a new cycle was run with it by addition of new typical portions of all other reagents. Unfortunately, the catalytic activity of recovered 2 for the second cycle was significantly lower (up to 17% conversion) what prevented its reuse.

Catalytic Benzaldehyde Nitroaldol C-C Coupling
Complex 1 showed a good activity as a heterogeneous catalyst for the Henry coupling reaction of benzaldehyde with nitroalkanes yielding β-nitro alcohols (Scheme 2b) up to 87% conversion under optimized reaction conditions (24 h at 60 °C in MeOH, benzaldehyde: nitroalkane 1:3 and 10 mol% of 1 vs. benzaldehyde). Moreover, acceptable values of syn:anti selectivity for the diastereoisomers of When a decrease in the selectivity for benzaldehyde was found (higher oxidant to styrene molar ratios, n oxidant /n substrate ≥ 4), although being the major product, the formation of 1-phenylethane-1,2-diol (up to 23%) was detected. This can suggest that the mechanism for styrene oxidation is initiated by the Cu-assisted formation of oxygen-centered radicals (hydroxyl and peroxyl, similar to Fenton chemistry) which oxidize styrene, what is also suggested by the reaction inhibition upon addition of the radical scavenger 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) to the reaction mixture. Moreover, the epoxide product was not detected, neither acetophenone nor phenylacetaldehyde (from isomerization of the epoxide).
Besides, an increase in the microwave irradiation time led to the formation of benzoic acid by overoxidation of benzaldehyde. In fact, if the desired product would be benzoic acid, full conversion of styrene could be obtained performing the reaction under MW irradiation during 2 h at 80 • C with an increased amount of hydrogen peroxide. Higher temperatures than 80 • C were not tested due to safety reasons.
The recyclability of complex 2 was examined by recovering it through centrifugation (see Experimental). Although a slight color change was detected for the recovered catalyst, a new cycle was run with it by addition of new typical portions of all other reagents. Unfortunately, the catalytic activity of recovered 2 for the second cycle was significantly lower (up to 17% conversion) what prevented its reuse.

Catalytic Benzaldehyde Nitroaldol C-C Coupling
Complex 1 showed a good activity as a heterogeneous catalyst for the Henry coupling reaction of benzaldehyde with nitroalkanes yielding β-nitro alcohols (Scheme 2b) up to 87% conversion under optimized reaction conditions (24 h at 60 • C in MeOH, benzaldehyde: nitroalkane 1:3 and 10 mol% of 1 vs. benzaldehyde). Moreover, acceptable values of syn:anti selectivity for the diastereoisomers of 1-phenyl-2-nitropropanol, formed by the nitroaldol coupling using nitroethane, were achieved (up to 66:34 syn:anti), with superior selectivity for the syn isomer. The efficiency of the nitroaldol coupling was shown to be dependent on the solvent: protic solvents (e.g., water or methanol) led to higher yields of β-nitro alcohols than aprotic ones (e.g., acetonitrile or THF) (see Figure 8 for 2-nitro-1-phenylethanol). With the Cu(II) complex 2 as a catalyst, only 32 to 68% benzaldehyde conversions were achieved after 24 h at 60 • C. Following previous studies [18,[48][49][50][51], the reaction mechanism should involve metal-assisted (upon coordination) deprotonation of nitroalkane to produce the nitronate species and activation of benzaldehyde for its electrophilic attack to the nitronate. Therefore, the Zn(II) complex 1 acts more efficiently as a Lewis acid for the aforementioned activation process than its related Cu(II) 2. phenylethanol). With the Cu(II) complex 2 as a catalyst, only 32 to 68% benzaldehyde conversions were achieved after 24 h at 60 °C. Following previous studies [18,[48][49][50][51], the reaction mechanism should involve metal-assisted (upon coordination) deprotonation of nitroalkane to produce the nitronate species and activation of benzaldehyde for its electrophilic attack to the nitronate. Therefore, the Zn(II) complex 1 acts more efficiently as a Lewis acid for the aforementioned activation process than its related Cu(II) 2. Under solvent-free conditions, a significant decrease on the β-nitro alcohol production was observed (19% after 24 h at 60 °C, with benzaldehyde: nitroalkene 1:3 and 10 mol% of 1 vs. benzaldehyde). Besides, the reaction does not show any progression in the absence of 1. The recyclability of complex 1 was impaired by its decomposition during its recovering from the reaction mixture.

Conclusion
The mononuclear Zn(II) complex cis-[ZnL2(H2O)2] (1) was synthesized through hydrothermal reaction using 4-(pyridin-3-ylcarbamoyl)benzoic acid (HL) as the ligand source, and can be irreversibly transmetalated to the Cu(II) analogue trans-[CuL2(H2O)2] (2). The thermodynamic feasibility of transmetalation is evidenced by DFT studies. The catalytic activity of 1 and 2 were tested for oxidation and for CC-coupling reactions. Compound 2 with the redox active Cu(II) site acts as heterogeneous catalyst for the microwave-assisted selective oxidation of styrene to benzaldehyde by aqueous hydrogen peroxide. In contrast, compound 1 with the strong Lewis acid Zn(II) site was found to be an active catalyst towards the nitroaldol (Henry) reaction between benzaldehyde and nitroalkanes to afford the corresponding β-nitro alcohols. Further, the reaction of benzaldehyde with nitroethane in the presence of 1 produced 2-nitro-1-phenylpropanol in the syn and the anti diastereoisomeric forms, with a considerable higher selectivity towards the syn isomer.
Supplementary Materials: The following are available online, Figure S1. IR spectra of HL, 1 and 2, Figure S2. Under solvent-free conditions, a significant decrease on the β-nitro alcohol production was observed (19% after 24 h at 60 • C, with benzaldehyde: nitroalkene 1:3 and 10 mol% of 1 vs. benzaldehyde). Besides, the reaction does not show any progression in the absence of 1. The recyclability of complex 1 was impaired by its decomposition during its recovering from the reaction mixture.

Conclusions
The mononuclear Zn(II) complex cis-[ZnL 2 (H 2 O) 2 ] (1) was synthesized through hydrothermal reaction using 4-(pyridin-3-ylcarbamoyl)benzoic acid (HL) as the ligand source, and can be irreversibly transmetalated to the Cu(II) analogue trans-[CuL 2 (H 2 O) 2 ] (2). The thermodynamic feasibility of transmetalation is evidenced by DFT studies. The catalytic activity of 1 and 2 were tested for oxidation and for CC-coupling reactions. Compound 2 with the redox active Cu(II) site acts as heterogeneous catalyst for the microwave-assisted selective oxidation of styrene to benzaldehyde by aqueous hydrogen peroxide. In contrast, compound 1 with the strong Lewis acid Zn(II) site was found to be an active catalyst towards the nitroaldol (Henry) reaction between benzaldehyde and nitroalkanes to afford the corresponding β-nitro alcohols. Further, the reaction of benzaldehyde with nitroethane in the presence of 1 produced 2-nitro-1-phenylpropanol in the syn and the anti diastereoisomeric forms, with a considerable higher selectivity towards the syn isomer.