Modeling the OEC with two new biomimetic models: preparations, structural characterization, and water photolysis studies of a Ba–Mn box type complex and a Mn4N6 Planar-diamond cluster

catalysts Article Modeling the OEC with Two New Biomimetic Models: Preparations, Structural Characterization, and Water Photolysis Studies of a Ba–Mn Box Type Complex and a Mn4N6Planar-Diamond Cluster Lara Rouco 1, M. Isabel Fernández-García1, Rosa Pedrido 2, Luis M. Botana 3, David Esteban-Gómez 4, Carlos Platas-Iglesias 4and Marcelino Maneiro 1,* 1Departamento de Química Inorgánica, Facultade de Ciencias, Universidade de Santiago de Compostela, 27002 Lugo, Spain; [email protected] (L.R.); misabel.fernandez.gar[email protected] (M.I.F.-G.) 2Departamento de Química Inorgánica, Facultade de Química, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain; [email protected] 3Departament of Pharmacology, Faculty of Veterinary, University of Santiago de Compostela, 27002 Lugo, Spain; [email protected] 4 Centro de Investigacións Científicas Avanzadas (CICA) and Departamento de Química, Campus Zapateira, Universidade da Coruña, 15008 A Coruña, Spain; [email protected] (D.E.-G.); [email protected] (C.P.-I.) *Correspondence: marcelino.maneir[email protected]; Tel.: +34-982-824-106 Received: 28 July 2018; Accepted: 3 September 2018; Published: 7 September 2018   Abstract: The oxygen-evolving complex (OEC) is the native enzyme that catalyzes the oxidation of water in natural photosynthesis. Two new classes of manganese cluster complexes of formula Ba2Mn2L12(H3L1)2(CH3OH)41and Mn4L26Cl22were prepared (H4L1=N,N0-(ethane-1,2-diyl)bis (2-hydroxybenzamide); L 2 = methyl picolinimidate) and characterized by standard techniques including microanalysis, IR spectroscopy, ESI spectrometry, and magnetic susceptibility measurements. X-ray diffraction studies of these complexes revealed (i) a box-type structure for 1 formed by two redox-active manganese(III) ions and two barium(II) ions connected by two bridging bisamido-bisphenoxy ligand molecules; and (ii) a planar-diamond array for Mn 4 N 6 cluster 2 where the picolinimidates act as chelating ligands through the two nitrogen atoms. The ability of 1 and 2 to split water has been studied by means of water photolysis experiments. In these experiments, the oxygen evolution was measured in aqueous media in the presence of p-benzoquinone (acting as the hydrogen acceptor), the reduction of which was followed by UV-spectroscopy. The relevant photolytic activity found for 1 is in contrast to the inactivity of 2 in the photolytic experiments. This different behavior is discussed on the basis of the structure of the biomimetic models and the proposed reaction mechanism for this process supported by DFT calculations. Keywords: artificial photosynthesis; photocatalyst; water splitting; manganese 1. Introduction The oxygen-evolving complex (OEC) is the catalytic site that catalyzes, in natural photosynthesis, the oxidation of water into dioxygen, protons, and electrons [ 1 – 3 ]. The native Mn 4 CaO 5 complex, located at the donor site of photosystem II (PSII), is responsible for both the atmospheric oxygen that we breathe, and also the conversion of solar energy into chemical energy. In this process, water splitting is the key step that drives the chain of electron-transduction reactions to form the energy transporting molecule NADPH, which holds the electrons in this chain [ 4 , 5 ]. The natural catalyst contains four Catalysts 2018,8, 382; doi:10.3390/catal8090382 www.mdpi.com/journal/catalysts Catalysts 2018,8, 382 2 of 17 manganese atoms and one calcium atom, coordinated by four water molecules, one imidazole, and six carboxylate groups. In addition, two chloride ions are also in the vicinity of the Mn 4 Ca cluster [ 1 , 6 ]. Much of what we know about this catalytic complex was learned through structural and functional artificial models of the OEC [ 7 – 10 ]. Structural understanding of this catalytic site has been achieved thanks to multiple techniques, such as EPR [ 11 – 13 ], XAS [ 14 – 16 ] or XRD [ 17 , 18 ], often based on the information obtained through mimetic models. The understanding of the mechanism of water oxidation is also better understood on the basis of data obtained from bioinorganic modeling of the OEC [19–21]. We have previously reported that a number of manganese model compounds are catalysts for water photolysis [ 22 , 23 ]. In this sense, manganese-Schiff base µ -aqua dimers were found to be active systems under light irradiation and in the presence of p-benzoquinone, which acts as a hydrogen acceptor. The ONNO set of the tetradentate-Schiff base (using the iminic nitrogen and phenoxy oxygen atoms) provides a strong chelating effect which confers an increased robustness to this type of complex in comparison to other systems [ 24 , 25 ]. We propose a mechanism for this catalysis involving successive hydrogen abstractions from water molecules bound to the metal ions by optically excited p-benzoquinone. Photogenerated dioxygen is evolved in the overall process, while p-benzoquinone is reduced to hydroquinone (Scheme 1). Catalysts 2018, 8, x FOR PEER REVIEW 2 of 18 transporting molecule NADPH, which holds the electrons in this chain [4,5]. The natural catalyst contains four manganese atoms and one calcium atom, coordinated by four water molecules, one imidazole, and six carboxylate groups. In addition, two chloride ions are also in the vicinity of the Mn4Ca cluster [1,6]. Much of what we know about this catalytic complex was learned through structural and functional artificial models of the OEC [7–10]. Structural understanding of this catalytic site has been achieved thanks to multiple techniques, such as EPR [11–13], XAS [14–16] or XRD [17,18], often based on the information obtained through mimetic models. The understanding of the mechanism of water oxidation is also better understood on the basis of data obtained from bioinorganic modeling of the OEC [19–21]. We have previously reported that a number of manganese model compounds are catalysts for water photolysis [22,23]. In this sense, manganese-Schiff base µ-aqua dimers were found to be active systems under light irradiation and in the presence of p-benzoquinone, which acts as a hydrogen acceptor. The ONNO set of the tetradentate-Schiff base (using the iminic nitrogen and phenoxy oxygen atoms) provides a strong chelating effect which confers an increased robustness to this type of complex in comparison to other systems [24,25]. We propose a mechanism for this catalysis involving successive hydrogen abstractions from water molecules bound to the metal ions by optically excited p-benzoquinone. Photogenerated dioxygen is evolved in the overall process, while p-benzoquinone is reduced to hydroquinone (Scheme 1). Scheme 1. Photogeneration of dioxygen from water molecules bound to the metal ion in manganese-Schiff base µ-aqua dimers. In our search for more efficient catalysts, we are looking for systems with higher nuclearity, different topologies, and other donor atoms beyond the iminic nitrogen or the phenoxy oxygen atoms. We are also interested in checking the hypothesis of the ease of coordination of the water molecules to achieve active catalysts. In order to address some of these issues, in the work described here, we report on two new potential biomimetic models of the OEC using other type of organic ligands, such as the bisamido-bisphenoxy H4L1 or the methyl-picolinimidate L2 (see Scheme 2). H4L1 contains six potential donor atoms: two amide nitrogen, two phenoxy, and two amide oxygen atoms. The latter donor atoms point outward with respect to the ligand cavity which may facilitate the extension of the structure dimensionality. In addition, the synthetic procedure used to obtain the manganese complex incorporates an alkaline earth metal (i.e., barium) which may be important in stabilizing the structure taking into account the presence of other alkaline earth metal ion, calcium, in the natural OEC. In the case of the L2 ligand, its structure may favor the formation of high-nuclearity clusters. (a) (b) Scheme 2. (a) Structure of H4L1. (b) Methanolysis process to explain the formation of the L2 ligand from the initial 2-cyanopyridine. Scheme 1. Photogeneration of dioxygen from water molecules bound to the metal ion in manganese-Schiff base µ-aqua dimers. In our search for more efficient catalysts, we are looking for systems with higher nuclearity, different topologies, and other donor atoms beyond the iminic nitrogen or the phenoxy oxygen atoms. We are also interested in checking the hypothesis of the ease of coordination of the water molecules to achieve active catalysts. In order to address some of these issues, in the work described here, we report on two new potential biomimetic models of the OEC using other type of organic ligands, such as the bisamido-bisphenoxy H 4 L 1 or the methyl-picolinimidate L 2 (see Scheme 2). H 4 L 1 contains six potential donor atoms: two amide nitrogen, two phenoxy, and two amide oxygen atoms. The latter donor atoms point outward with respect to the ligand cavity which may facilitate the extension of the structure dimensionality. In addition, the synthetic procedure used to obtain the manganese complex incorporates an alkaline earth metal (i.e., barium) which may be important in stabilizing the structure taking into account the presence of other alkaline earth metal ion, calcium, in the natural OEC. In the case of the L2ligand, its structure may favor the formation of high-nuclearity clusters. Catalysts 2018, 8, x FOR PEER REVIEW 2 of 18 transporting molecule NADPH, which holds the electrons in this chain [4,5]. The natural catalyst contains four manganese atoms and one calcium atom, coordinated by four water molecules, one imidazole, and six carboxylate groups. In addition, two chloride ions are also in the vicinity of the Mn4Ca cluster [1,6]. Much of what we know about this catalytic complex was learned through structural and functional artificial models of the OEC [7–10]. Structural understanding of this catalytic site has been achieved thanks to multiple techniques, such as EPR [11–13], XAS [14–16] or XRD [17,18], often based on the information obtained through mimetic models. The understanding of the mechanism of water oxidation is also better understood on the basis of data obtained from bioinorganic modeling of the OEC [19–21]. We have previously reported that a number of manganese model compounds are catalysts for water photolysis [22,23]. In this sense, manganese-Schiff base µ-aqua dimers were found to be active systems under light irradiation and in the presence of p-benzoquinone, which acts as a hydrogen acceptor. The ONNO set of the tetradentate-Schiff base (using the iminic nitrogen and phenoxy oxygen atoms) provides a strong chelating effect which confers an increased robustness to this type of complex in comparison to other systems [24,25]. We propose a mechanism for this catalysis involving successive hydrogen abstractions from water molecules bound to the metal ions by optically excited p-benzoquinone. Photogenerated dioxygen is evolved in the overall process, while p-benzoquinone is reduced to hydroquinone (Scheme 1). Scheme 1. Photogeneration of dioxygen from water molecules bound to the metal ion in manganese-Schiff base µ-aqua dimers. In our search for more efficient catalysts, we are looking for systems with higher nuclearity, different topologies, and other donor atoms beyond the iminic nitrogen or the phenoxy oxygen atoms. We are also interested in checking the hypothesis of the ease of coordination of the water molecules to achieve active catalysts. In order to address some of these issues, in the work described here, we report on two new potential biomimetic models of the OEC using other type of organic ligands, such as the bisamido-bisphenoxy H4L1 or the methyl-picolinimidate L2 (see Scheme 2). H4L1 contains six potential donor atoms: two amide nitrogen, two phenoxy, and two amide oxygen atoms. The latter donor atoms point outward with respect to the ligand cavity which may facilitate the extension of the structure dimensionality. In addition, the synthetic procedure used to obtain the manganese complex incorporates an alkaline earth metal (i.e., barium) which may be important in stabilizing the structure taking into account the presence of other alkaline earth metal ion, calcium, in the natural OEC. In the case of the L2 ligand, its structure may favor the formation of high-nuclearity clusters. (a) (b) Scheme 2. (a) Structure of H4L1. (b) Methanolysis process to explain the formation of the L2 ligand from the initial 2-cyanopyridine. Scheme 2. ( a ) Structure of H 4 L 1 . ( b ) Methanolysis process to explain the formation of the L 2 ligand from the initial 2-cyanopyridine. Catalysts 2018,8, 382 3 of 17 2. Results 2.1. Preparation and Characterization of Biomimetic Model 1 The multidentate bisamido-bisphenoxy H 4 L 1 readily reacts with manganese(II) acetate in the presence of barium hydroxide and air, as detailed in the experimental section, to give biomimetic model 1 (Ba 2 Mn 2 L 12 (H 3 L 1 ) 2 (CH 3 OH) 4 ). The alkaline earth-metal hydroxide provides the basic conditions required to achieve tetradeprotonation of the ligand and oxidation of manganese(II) to manganese(III) in the presence of oxygen. Elemental analysis of the complex indicated a stoichiometry Ba 2 Mn 2 L 12 (H 3 L 1 ) 2 (CH 3 OH) 4 , which is consistent with the formation of neutral species, where L identifies the ligand in its tetra-anionic form, and H 3 L identifies the ligand in its monoanionic form. The analytical, magnetic, spectroscopic, and mass spectrometry data for model 1 are given in the Materials and Methods section. Complex 1 seems to be stable in air as well as thermally stable, melting above 300 ◦ C without decomposition. It is sparingly soluble in water, partially soluble in common organic solvents such as methanol, and very soluble in polar coordinating solvents such as DMF and DMSO. The formulation of model 1 is in agreement with the molar conductivity measured in 10 −3 M DMF solution, which is 23 µS cm−1, typical of non-electrolyte complexes [26]. The value for the magnetic moment at room temperature, 4.8 B.M., is very close to the spin-only value of 4.89 B.M. expected for a high-spin magnetically diluted d 4 manganese(III) ion. The ESI (electrospray ionization) mass spectrum (Figure 1) registered in methanol shows a peak corresponding to the fragment [MnL + H + ] + , indicating coordination of the manganese ion with the deprotonated ligand. Other minor signals, assigned to [MnL(H 3 L) + H + ] + , [Mn 2 L(H 3 L) + H + ] + , [MnL(H 3 L)Ba + H + ] + and [MnL(H 3 L)Ba 2 + H + ] + also confirm the formation of the polynuclear complex including manganese, barium, and the ligand both in its tetra-anionic (L −4 ) and monoanionic (H 3 L − ) forms. IR spectroscopy also suggests the formation of model 1 , exhibiting the set of bands attributable to the tetra-anionic coordinated ligand, but also the corresponding bands that can be assigned to the monoanionic form of the ligand. Thus, the strong band at 1605 cm −1 , characteristic of the ν (amide)I (C=O) stretching mode, is shifted 38 cm −1 to lower wavenumbers with respect to the free ligand (1643 cm −1 ), and the band 1540 of the ν (amide)II stretching mode, is shifted 12 cm −1 with respect to the free ligand (1552 cm −1 ). These data suggest coordination of the L −4 ligand through the amide nitrogen atoms, but additional bands at 1641 cm −1 and 1551 cm −1 can be assigned to the H 3 L − ligand which does not coordinate through these amide nitrogen atoms. The band attributed to the ν (C–O) mode is also shifted 11 cm −1 to lower frequencies with respect to the free ligand, indicating coordination through the hydroxyl groups. A strong band centered at ca 3400 cm −1 can be assigned to the ν(O–H) of methanol molecules. Catalysts 2018, 8, x FOR PEER REVIEW 3 of 18 2. Results 2.1. Preparation and Characterization of Biomimetic Model 1 The multidentate bisamido-bisphenoxy H4L1 readily reacts with manganese(II) acetate in the presence of barium hydroxide and air, as detailed in the experimental section, to give biomimetic model 1 (Ba2Mn2L12(H3L1)2(CH3OH)4). The alkaline earth-metal hydroxide provides the basic conditions required to achieve tetradeprotonation of the ligand and oxidation of manganese(II) to manganese(III) in the presence of oxygen. Elemental analysis of the complex indicated a stoichiometry Ba2Mn2L12(H3L1)2(CH3OH)4, which is consistent with the formation of neutral species, where L identifies the ligand in its tetra-anionic form, and H3L identifies the ligand in its monoanionic form. The analytical, magnetic, spectroscopic, and mass spectrometry data for model 1 are given in the Materials and Methods section. Complex 1 seems to be stable in air as well as thermally stable, melting above 300 °C without decomposition. It is sparingly soluble in water, partially soluble in common organic solvents such as methanol, and very soluble in polar coordinating solvents such as DMF and DMSO. The formulation of model 1 is in agreement with the molar conductivity measured in 10−3 M DMF solution, which is 23 µS cm−1, typical of non-electrolyte complexes [26]. The value for the magnetic moment at room temperature, 4.8 B.M., is very close to the spin-only value of 4.89 B.M. expected for a high-spin magnetically diluted d4 manganese(III) ion. The ESI (electrospray ionization) mass spectrum (Figure 1) registered in methanol shows a peak corresponding to the fragment [MnL + H+]+, indicating coordination of the manganese ion with the deprotonated ligand. Other minor signals, assigned to [MnL(H3L) + H+]+, [Mn2L(H3L) + H+]+, [MnL(H3L)Ba + H+]+ and [MnL(H3L)Ba2 + H+]+ also confirm the formation of the polynuclear complex including manganese, barium, and the ligand both in its tetra-anionic (L−4) and monoanionic (H3L−) forms. IR spectroscopy also suggests the formation of model 1, exhibiting the set of bands attributable to the tetra-anionic coordinated ligand, but also the corresponding bands that can be assigned to the monoanionic form of the ligand. Thus, the strong band at 1605 cm−1, characteristic of the ν(amide)I (C=O) stretching mode, is shifted 38 cm−1 to lower wavenumbers with respect to the free ligand (1643 cm−1), and the band 1540 of the ν(amide)II stretching mode, is shifted 12 cm−1 with respect to the free ligand (1552 cm−1). These data suggest coordination of the L−4 ligand through the amide nitrogen atoms, but additional bands at 1641 cm−1 and 1551 cm−1 can be assigned to the H3L− ligand which does not coordinate through these amide nitrogen atoms. The band attributed to the ν(C–O) mode is also shifted 11 cm−1 to lower frequencies with respect to the free ligand, indicating coordination through the hydroxyl groups. A strong band centered at ca 3400 cm−1 can be assigned to the ν(O–H) of methanol molecules. Figure 1. Electrospray ionization (ESI) mass spectrum for complex 1. The electronic absorption spectrum of 1 shows two broad bands: a broad shoulder obtained at 510 nm (ε = 700 M−1 cm−1), attributable to a d–d transition, and a broad band at 484 nm (ε = 3100 M−1 Figure 1. Electrospray ionization (ESI) mass spectrum for complex 1. Catalysts 2018,8, 382 4 of 17 The electronic absorption spectrum of 1 shows two broad bands: a broad shoulder obtained at 510 nm ( ε = 700 M −1 cm −1 ), attributable to a d–d transition, and a broad band at 484 nm (ε= 3100 M−1cm−1) , assigned to the phenolate → manganese(III) charge transfer. The energy and intensity of these two bands are in agreement with those reported for related manganese(III) complexes [ 27 , 28 ]. The paramagnetic 1 H NMR spectrum (Figure S1 contains an up-field proton resonance outside the diamagnetic region at –24.17 ppm (Figure S1) due to the isotropic shifting of the ligand protons for high-spin manganese(III) complexes in an octahedral field. This signal corresponds to the protons in ortho positions relative to the hydroxyl groups [ 22 , 23 , 29 ] and serves to substantiate the formation of a manganese(III) complex. Single crystals of complex 1 suitable for X-ray diffraction studies were obtained as described in the Materials and Methods section. The main crystal data and structure refinement details are shown in Tables 1and 2; detailed crystallographic data is shown in Tables S1–S5 of the Supplementary Materials. Different drawings showing the crystal structure are shown in Figures 2and 3. Table 1. Crystal data and structure refinement parameters for compounds 1and 2. Compound 1 2 Empirical formula C34H35BaMnN4O10 C21H24ClMn2N6O3 Formula weight 851.93 553.79 Temperature (K) 100(2) 293(2) Wavelength (Å) 0.71073 0.71069 Crystal system Monoclinic Monoclinic Space group P21/c P21/n a(Å) 12.245(2) 11.953(5) b(Å) 17.345(3) 11.256(5) c(Å) 18.041(4) 17.889(5) α(◦) 90 90 β(◦) 106.38(3) 99.051(5) γ(◦) 90 90 Volume (Å3)3676.2(13) 2376.9(16) Z 4 4 Dcalcd. (g cm−3)1.525 1.548 µ(mm−1)1.467 1.21 F (000) 1712 1132 θmin/max (◦) 2.62/21.14 1.92/24.73 Goodness-of-fit on F21.005 1.067 Total data 27,410 4038 Unique data 6303 4038 Data/restrains/parameters 6303/3/440 4038/0/299 Final Rindices (I> 2σ(I)) R1= 0.0521; wR2= 0.1220 R1= 0.0894; wR2= 0.2658 R indices (all data) R1= 0.0941; wR2=0.1356 R1= 0.1223; wR2=0.2812 Catalysts 2018, 8, x FOR PEER REVIEW 5 of 18 Figure 2. ORTEP view for the BaMnL1(H3L1)(CH3OH)2 asymmetric unit for compound 1, with atoms showing the atomic numbering scheme. The asymmetric unit comprises a barium ion, a manganese ion, a tetraanionic (L1)4− ligand, a monoanionic (H3L1)− entity, and a methanol solvent molecule. The two manganese ions of 1 have similar coordination environments. (a) (b) (c) Figure 3. (a) Stick diagram of supramolecular box 1 (barium cation in green, manganese ion in purple, oxygen in red, nitrogen in blue, and carbon in grey); (b) coordination environment around the manganese center in 1, showing the square-pyramidal geometry for this ion; (c) coordination environment around the barium center in 1. The geometry around the manganese(III) ion is a five-coordinated distorted square-pyramidal geometry (Figure 3b). The coordination sphere around each manganese center comprises the planar tetraanionic bisamido-bisphenoxy L4− ligand, which is tightly bound to the metal ion through the inner N2O2 compartment by the Namide and Ophenol atoms. The Mn-Namide (1.938 Å and 1.944 Å) and Mn-Ophenol (1.877 Å and 1.883 Å) bond lengths are in the range expected for the tetradeprotonation of the ligand [23]. The fifth coordination position is completed with a phenolic oxygen atom (O571) Figure 2. ORTEP view for the BaMnL 1 (H 3 L 1 )(CH 3 OH) 2 asymmetric unit for compound 1 , with atoms showing the atomic numbering scheme. Catalysts 2018,8, 382 5 of 17 Table 2. Selected bond lengths (Å) and angles (°) for 1. Mn(2)–N(1) 1.938(5) Ba(1)–O(411) 2.636(5) Mn(2)–N(4) 1.944(6) Ba(1)–O(511) 2.648(5) Mn(2)–O(471) 1.877(5) Ba(1)–O(811) 2.658(5) Mn(2)–O(171) 1.883(5) Ba(1)–O(171) 2.754(5) Mn(2)–O(571) 2.125(5) Ba(1)–O(471) 2.816(5) Ba(1)–Mn(2) 3.5510(12) Ba(1)–O(571) 3.074(5) Ba(1)–O(200) Ba(1)–O(100) O(471)–Mn(2)–O(171) 88.6(2) O(411)–Ba(1)–O(511) 120.81(16) O(471)–Mn(2)–N(1) 173.4(2) O(411)–Ba(1)–O(811) 75.91(14) O(171)—Mn(2)–N(1) 92.4(2) O(511)–Ba(1)–O(811) 91.95(14) O(471)—Mn(2)–N(4) 92.7(2) O(411)–Ba(1)–O(171) 97.69(15) O(171)—Mn(2)–N(4) 164.2(2) O(511)–Ba(1)–O(171) 140.07(14) N(1)–Mn(2)–N(4) 84.6(2) O(811)–Ba(1)–O(171) 87.29(14) O(471)–Mn(2)–O(571) 84.4(2) O(411)–Ba(1)–O(471) 139.51(16) O(171)–Mn(2)–O(571) 92.40(19) O(511)–Ba(1)–O(471) 94.13(15) N(1)–Mn(2)–O(571) 102.1(2) O(811)–Ba(1)–O(471) 126.37(14) N(4)–Mn(2)–O(571) 103.4(2) O(171)–Ba(1)–O(471) 56.24(14) O(411)–Ba(1)–O(571) 142.68(14) O(511)–Ba(1)–O(571) 82.08(13) O(811)–Ba(1)–O(571) 74.13(13) O(171)–Ba(1)–O(571) 59.34(13) O(471)–Ba(1)–O(571) 54.23(13) Catalysts 2018, 8, x FOR PEER REVIEW 5 of 18 Figure 2. ORTEP view for the BaMnL1(H3L1)(CH3OH)2 asymmetric unit for compound 1, with atoms showing the atomic numbering scheme. The asymmetric unit comprises a barium ion, a manganese ion, a tetraanionic (L1)4− ligand, a monoanionic (H3L1)− entity, and a methanol solvent molecule. The two manganese ions of 1 have similar coordination environments. (a) (b) (c) Figure 3. (a) Stick diagram of supramolecular box 1 (barium cation in green, manganese ion in purple, oxygen in red, nitrogen in blue, and carbon in grey); (b) coordination environment around the manganese center in 1, showing the square-pyramidal geometry for this ion; (c) coordination environment around the barium center in 1. The geometry around the manganese(III) ion is a five-coordinated distorted square-pyramidal geometry (Figure 3b). The coordination sphere around each manganese center comprises the planar tetraanionic bisamido-bisphenoxy L4− ligand, which is tightly bound to the metal ion through the inner N2O2 compartment by the Namide and Ophenol atoms. The Mn-Namide (1.938 Å and 1.944 Å) and Mn-Ophenol (1.877 Å and 1.883 Å) bond lengths are in the range expected for the tetradeprotonation of the ligand [23]. The fifth coordination position is completed with a phenolic oxygen atom (O571) Figure 3. ( a ) Stick diagram of supramolecular box 1 (barium cation in green, manganese ion in purple, oxygen in red, nitrogen in blue, and carbon in grey); ( b ) coordination environment around the manganese center in 1 , showing the square-pyramidal geometry for this ion; ( c ) coordination environment around the barium center in 1. The asymmetric unit comprises a barium ion, a manganese ion, a tetraanionic (L 1 ) 4− ligand, a monoanionic (H 3 L 1 ) − entity, and a methanol solvent molecule. The two manganese ions of 1 have similar coordination environments. Catalysts 2018,8, 382 6 of 17 The geometry around the manganese(III) ion is a five-coordinated distorted square-pyramidal geometry (Figure 3b). The coordination sphere around each manganese center comprises the planar tetraanionic bisamido-bisphenoxy L 4− ligand, which is tightly bound to the metal ion through the inner N 2 O 2 compartment by the N amide and O phenol atoms. The Mn-N amide (1.938 Å and 1.944 Å) and Mn-O phenol (1.877 Å and 1.883 Å) bond lengths are in the range expected for the tetradeprotonation of the ligand [ 23 ]. The fifth coordination position is completed with a phenolic oxygen atom (O571) from a monoanionic (H 3 L 1 ) ligand. Thus, two different coordination behaviors are found in 1 for the parent bisamido-bisphenoxy ligand: the inner compartment of the tetra-anionic (L 1 ) 4− forms three chelate rings (two six-membered and one five-membered) once the manganese ion is coordinated, while the monoanionic (H 3 L 1 ) − ligand coordinates to one position of the manganese ion coordination environment, acting as a bridging ligand between the manganese ion and the farthest barium ion. The axial Mn–O571 length of 2.125 Å is longer than the other Mn–O phenol lengths due to the Jahn–Teller effect derived from the Mn(III) d 4 high-spin configuration. The angles between the O–(or N)–Mn–O571, representing O or N donor atoms at the equatorial positions occupied by the inner N 2 O 2 compartment of (L 1 ) 4− , range from 84.4 ° to 103.4 ° , revealing certain distortions of the square-pyramidal geometry around each Mn(III) ion. Moreover, the amide atoms of (H 3 L 1 ) − and (L 1 ) 4− , which point towards the outside of the inner cavity, play crucial roles in to extending the dimensionality of the structure, since they are bound to the barium ions. The two barium atoms of 1 have similar coordination environments. The barium atom is bound to eight oxygen atoms (Figure 3c): two phenoxy oxygen atoms (O171 and O471) from the tetra-anionic L 4− ligand, also bound to the manganese ion; two methanolic oxygen atoms (O100 and O200); and three amidic oxygen atoms (O411, O511, and O811) from two different neighboring ligands, (L 1 ) 4− and (H 3 L 1 ) − . Therefore, each barium atom is coordinated with four different residues, which results in Ba–O distances ranging from 2.636 to 3.073 Å. The relatively broad range of Ba–O distances is also associated with increased structural flexibility and an increased capacity for close contacts between the barium ion and aromatic rings [ 30 ]. Nevertheless, these distances are also in the expected range for Ba–O bonds [31]. The barium atoms and the bridging (H 3 L 1 ) − ligands play crucial roles in the assembly of the final supramolecular structure of 1 , which can be also visualized as a Ba 2 Mn 2 L 12 (H 3 L 1 ) 2 (CH 3 OH) 4 supramolecular box (Figure 3a). The size of the cavity inside the box is larger than those displayed by supramolecular boxes induced by alkali metal ions [ 23 ]. Thus, the Ba–Ba distance in 1 is 8.339 Å, while the M–M distances for alkali metal ions range from 4.47 to 4.87 Å for potassium, rubidium, and cesium supramolecular boxes. The distance between the manganese ions is 7.886 Å, which is long enough to prevent intermetallic interactions to be established, in agreement with the observed spin-only magnetic moment of 4.8 B.M. 2.2. Preparation and Characterization of Biomimetic Model 2 The reaction of 2-cyanopyridine and Mn(II) in methanol solution leads to the formation of biomimetic model 2 (Mn 4 L 26 Cl 2 ) containing O-methyl picolinimidate L 2 as the chelate ligand. The methanolysis of the initial 2-cyanopyridine takes place upon coordination with the Mn(II) ion as a chelating bidentate ligand through the two nitrogen atoms of the pyridine ring and the carbonitrile group. As observed previously, the coordination of 2-cyanopyridine to some divalent metal ions activates the CN triple bond and makes it much more amenable toward nucleophilic attack by CH 3 OH molecules [ 32 – 34 ]. The proposed stoichiometry for complex 2 , Mn 4 L 26 Cl 2 , in which six O-methyl picolinimidate ligands are in a monoanionic mode (L 2 ) − , was confirmed by analytical and spectroscopic data. Moreover, recrystallization from the mother liquors afforded X-ray quality crystals for 2 . Complex 2 melts above 300 ◦ C without decomposition. It is insoluble in water and sparingly soluble in common organic solvents such as methanol, but soluble in polar coordinating solvents such as DMF and DMSO. The molar conductivity measurement in 10 −3 M DMF solution of 18 µ S cm −1 is also consistent with the formation of the neutral Mn 4 L 26 Cl 2 species [ 26 ]. The value of the room Catalysts 2018,8, 382 7 of 17 temperature magnetic moment with diamagnetism corrections is 5.6 B.M., which is compatible with high-spin magnetically diluted d 5 manganese(II) ions. This value does not allow for differentiation between octahedral and bipyramid trigonal coordination modes, since the ligand field theory raises the same number of unpaired electrons in both types of geometries. The ESI-MS of the CH 2 Cl 2 solution of 2 gives peaks at m/z1108.4 and 1129.5, which corresponds to [ 2 + H] + and [ 2 + Na] + (positive mode), suggesting the stability of this biomimetic model in solution (Figure S2). The IR spectrum for 2 also confirms the methanolysis reaction of the 2-cyanopyridine to give the O-methyl picolinimidate ligand. Thus, the spectrum (Figure S3) has a sharp band with a medium intensity at 3237 cm −1 , characteristic of the N–H vibration of O-methyl picolinimidate [ 35 ]. The C–H stretching vibrations of the methyl groups of the carboxamide appear at 2981 and 2940 cm −1 , while the absence of the ν (C ≡ N) band (which should have appeared at about 2240 cm −1 ) is indicative that the nitrile group has been converted to a carboxamide one. An additional strong band at 1659 cm −1 is also assigned to ν (C=NH) of the carboxamide group. The C–H stretching vibrations of the pyridine rings appear at 3072 cm −1 . Different medium and strong bands observed in the range 1631–1591 cm −1 are assigned to C=N, C=C, and C–C stretching vibrations [ 36 ]. The absorption band at 1379 cm −1 is assigned to the ν (=C–O–) stretching vibration which mixes with δ (NH) of the imino ether group. The νas (C–O–C) and νs (C–O–C) absorption bands appear at 1138 and 965 cm −1 , respectively. The absorption band observed at 1206 cm −1 is assigned to δ (O–CH 3 ). The medium absorption band observed at the far-infrared spectrum region at 303 cm −1 is assigned to Mn–Cl stretching vibrations [37], indicating the coordination of the chloride to the manganese ions. Single crystals of complex 2 , suitable for X-ray diffraction studies, were obtained by slow evaporation of the mother liquors at room temperature. The main crystal data and structure refinement details are collected in Tables 1and 3; detailed crystallographic data is collected in Tables S6–S10. Figures 4and 5show different views of the structure of 2 , which displays a planar-diamond core of the tetrameric cluster. The creation of bioinspired catalysts to reproduce the basic chemistry of the natural OEC has aroused great interest in the preparation of tetranuclear manganese clusters [3,7,9,10,13,15,38–43]. Table 3. Selected bond lengths (Å) and angles (°) for 2. Mn(1)–N(28) 2.048(8) Mn(1)–Mn(2)#1 3.206(2) Mn(1)–N(8) 2.051(8) Mn(2)–N(28)#1 1.939(8) Mn(1)–N(18)#1 2.120(7) Mn(2)–N(8) 1.971(8) Mn(1)–N(1) 2.123(9) Mn(2)–N(11) 2.081(11) Mn(1)–N(18) 2.141(8) Mn(2)–Cl(31) 2.305(4) Mn(1)–N(21) 2.143(10) Mn(2)–N(18) 2.353(8) Mn(1)–Mn(2) 3.203(2) Mn(2)–Mn(1)#1 3.206(2) N(28)–Mn(1)–N(8) 175.2(3) N(28)#1–Mn(2)–N(8) 124.9(4) N(28)–Mn(1)–N(18)#1 80.3(3) N(28)#1–Mn(2)–N(11) 118.5(4) N(8)–Mn(1)–N(18)#1 102.2(3) N(8)–Mn(2)–N(11) 101.3(4) N(28)–Mn(1)–N(1) 98.5(3) N(28)#1–Mn(2)–Cl(31) 101.6(2) N(8)–Mn(1)–N(1) 77.1(3) N(8)–Mn(2)–Cl(31) 102.9(2) N(18)#1–Mn(1)–N(1) 98.9(3) N(11)–Mn(2)–Cl(31) 105.2(3) N(28)–Mn(1)–N(18) 102.7(3) N(28)#1–Mn(2)–N(18) 76.8(3) N(8)–Mn(1)–N(18) 81.9(3) N(8)–Mn(2)–N(18) 78.4(3) N(18)#1–Mn(1)–N(18) 78.2(3) N(11)–Mn(2)–N(18) 75.4(4) N(1)–Mn(1)–N(18) 157.7(3) Cl(31)–Mn(2)–N(18) 178.4(2) N(28)–Mn(1)–N(21) 77.9(4) N(8)–Mn(1)–N(21) 100.4(4) N(18)#1–Mn(1)–N(21) 155.7(4) N(1)–Mn(1)–N(21) 94.8(4) N(18)–Mn(1)–N(21) 96.2(4) Catalysts 2018,8, 382 8 of 17 Catalysts 2018, 8, x FOR PEER REVIEW 8 of 18 Figure 4. ORTEP view for the asymmetric unit for 2 with atoms showing the atomic numbering scheme. The biomimetic model Mn4L26Cl2 (2) contains six O-methyl picolinimidate and two chloride ligands. The structure of 2 also reveals the decomposition of the 2-cyanopyridine via the addition of methanol across the C≡N triple bond to form a chelating ligand O-methyl picolinimidate (L2)−. The coordination numbers are six and five for Mn1 and Mn2, respectively. The metal coordination geometry is described as distorted octahedral for Mn1 and distorted trigonal bipyramidal for Mn2 [44]. Analysis of the shape determining angles for Mn2, using the approach of Reedijk and coworkers [45], yielded τ [(α-β)/60, being with α and β being the two greatest valence angles of the coordination center] having a value of 0.9 for Mn2 (τ = 0.0 and 1.0 for square-pyramidal and trigonal bipyramidal geometries respectively). Thus, if we apply the same approach for the five-coordinated manganese ion in 1, we obtain a value of 0.15 for τ, corresponding to the described square-pyramidal geometry. Although M(II) metal complexes tend to stabilize in octahedral geometries, which is the case for Mn1 in 2, the symmetrical high-spin configuration of the Mn(II) ion provides no crystal field stabilization energy (CFSE), and the stability constants of its high-spin complexes are consequently lower than those of corresponding complexes of neighboring M(II) ions. This may be one of the reasons for the occurrence of different geometries such as the trigonal bipyramidal displayed by Mn2 in 2. Each manganese atom in 2 is coordinated to three or four different O-methyl picolinimidate ligands, depending on whether the ion is trigonal bipyramidal or octahedral. In the case of Mn1, which has a octahedral geometry, two chelating (L2)− are bound via the pyridyl nitrogen donor (Mn1–N1 = 2.123(9) Å and Mn1–N21 = 2.143(10)) and the imine nitrogen atoms (Mn1–N8 = 2.051(8) Å and Mn1–N28 = 2.048 Å), two additional monodentate (L2)− ligands are also bound through their imine nitrogen atoms (Mn1–N18 = 2.141(8) and 2.120(7) Å). For Mn2, three (L2)− are bound; one of them behaves as the chelating ligand through the pyridyl and the imine nitrogen atoms (Mn2–N11 = 2.081(11) Å and Mn2–N18 = 2.353(8) Å), while two (L2)− act as monodentates via the imine nitrogen atoms (Mn2–N28 = 1.939(8) and Mn2–N8 = 1.971(8)). The fifth coordination position for Mn2 is completed with a chloride ion. Accordingly, each one of the six O-methyl picolinimidate ligands chelates a manganese ion but also bridges two manganese centers via the imine nitrogen atom. Table 3. Selected bond lengths (Å) and angles (˚) for 2. Mn(1)–N(28) 2.048(8) Mn(1)–Mn(2)#1 3.206(2) Mn(1)–N(8) 2.051(8) Mn(2)–N(28)#1 1.939(8) Mn(1)–N(18)#1 2.120(7) Mn(2)–N(8) 1.971(8) Mn(1)–N(1) 2.123(9) Mn(2)–N(11) 2.081(11) Mn(1)–N(18) 2.141(8) Mn(2)–Cl(31) 2.305(4) Mn(1)–N(21) 2.143(10) Mn(2)–N(18) 2.353(8) Figure 4. ORTEP view for the asymmetric unit for 2 with atoms showing the atomic numbering scheme. Catalysts 2018, 8, x FOR PEER REVIEW 9 of 18 Mn(1)–Mn(2) 3.203(2) Mn(2)–Mn(1)#1 3.206(2) N(28)–Mn(1)–N(8) 175.2(3) N(28)#1–Mn(2)–N(8) 124.9(4) N(28)–Mn(1)–N(18)#1 80.3(3) N(28)#1–Mn(2)–N(11) 118.5(4) N(8)–Mn(1)–N(18)#1 102.2(3) N(8)–Mn(2)–N(11) 101.3(4) N(28)–Mn(1)–N(1) 98.5(3) N(28)#1–Mn(2)–Cl(31) 101.6(2) N(8)–Mn(1)–N(1) 77.1(3) N(8)–Mn(2)–Cl(31) 102.9(2) N(18)#1–Mn(1)–N(1) 98.9(3) N(11)–Mn(2)–Cl(31) 105.2(3) N(28)–Mn(1)–N(18) 102.7(3) N(28)#1–Mn(2)–N(18) 76.8(3) N(8)–Mn(1)–N(18) 81.9(3) N(8)–Mn(2)–N(18) 78.4(3) N(18)#1–Mn(1)–N(18) 78.2(3) N(11)–Mn(2)–N(18) 75.4(4) N(1)–Mn(1)–N(18) 157.7(3) Cl(31)–Mn(2)–N(18) 178.4(2) N(28)–Mn(1)–N(21) 77.9(4) N(8)–Mn(1)–N(21) 100.4(4) N(18)#1–Mn(1)–N(21) 155.7(4) N(1)–Mn(1)–N(21) 94.8(4) N(18)–Mn(1)–N(21) 96.2(4) (a) (b) Figure 5. (a) Stick diagram of planar-diamond cluster 2. (b) Planar-diamond cluster core showing the atomic numbering scheme. 2.3. Photolytic Studies The photolytic experiments were carried out in the presence of p-benzoquinone, a water-soluble hydrogen atom acceptor [46]. The experimental details are given in Section 4.5, and the experimental setup was improved by us [22,23] with respect to previous experiments [47,48] in order to obtain a better sealing of the system. The magnetic stirrer used in the original experimental setup had a detrimental effect on the reproducibility of the dissolved oxygen measurements, while now the use of a methacrylate bath allows magnetic stirring, so the setup is more airtight. The photolytic activity of the biomimetic models was followed in two ways: quantitative oxygen evolution and variation of the electronic spectrum of the BQ during photolysis. It was noted earlier that excited-state benzoquinone abstracts a hydrogen atom directly from water [49] at pH 7 and is greater in aqueous solutions. The final products were hydroquinone and 2-hydroxybenzoquinone. Figure 5. ( a ) Stick diagram of planar-diamond cluster 2. ( b ) Planar-diamond cluster core showing the atomic numbering scheme. The biomimetic model Mn 4 L 26 Cl 2 ( 2 ) contains six O-methyl picolinimidate and two chloride ligands. The structure of 2 also reveals the decomposition of the 2-cyanopyridine via the addition of methanol across the C≡N triple bond to form a chelating ligand O-methyl picolinimidate (L2)−. The coordination numbers are six and five for Mn1 and Mn2, respectively. The metal coordination geometry is described as distorted octahedral for Mn1 and distorted trigonal bipyramidal for Mn2 [ 44 ]. Analysis of the shape determining angles for Mn2, using the approach of Reedijk and coworkers [ 45 ], yielded τ [( α - β )/60, being with α and β being the two greatest valence angles of the coordination center] having a value of 0.9 for Mn2 ( τ = 0.0 and 1.0 for square-pyramidal and trigonal bipyramidal geometries respectively). Thus, if we apply the same approach for the five-coordinated manganese ion in 1 , we obtain a value of 0.15 for τ , corresponding to the described square-pyramidal geometry. Although M(II) metal complexes tend to stabilize in octahedral geometries, which is the case for Mn1 in 2 , the symmetrical high-spin configuration of the Mn(II) ion provides no crystal field stabilization energy (CFSE), and the stability constants of its high-spin complexes are consequently lower than those of corresponding complexes of neighboring M(II) ions. This may be one of the reasons for the occurrence of different geometries such as the trigonal bipyramidal displayed by Mn2 in 2. Each manganese atom in 2 is coordinated to three or four different O-methyl picolinimidate ligands, depending on whether the ion is trigonal bipyramidal or octahedral. In the case of Mn1, which has a octahedral geometry, two chelating (L 2 ) − are bound via the pyridyl nitrogen donor (Mn1–N1 = 2.123(9) Å and Mn1–N21 = 2.143(10)) and the imine nitrogen atoms (Mn1–N8 = 2.051(8) Å Catalysts 2018,8, 382 9 of 17 and Mn1–N28 = 2.048 Å), two additional monodentate (L 2 ) − ligands are also bound through their imine nitrogen atoms ( Mn1–N18 = 2.141(8) and 2.120(7) Å). For Mn2, three (L 2 ) − are bound; one of them behaves as the chelating ligand through the pyridyl and the imine nitrogen atoms ( Mn2–N11 = 2.081(11) Å and Mn2–N18 = 2.353(8) Å), while two (L 2 ) − act as monodentates via the imine nitrogen atoms (Mn2–N28 = 1.939(8) and Mn2–N8 = 1.971(8)). The fifth coordination position for Mn2 is completed with a chloride ion. Accordingly, each one of the six O-methyl picolinimidate ligands chelates a manganese ion but also bridges two manganese centers via the imine nitrogen atom. 2.3. Photolytic Studies The photolytic experiments were carried out in the presence of p-benzoquinone, a water-soluble hydrogen atom acceptor [ 46 ]. The experimental details are given in Section 4.5, and the experimental setup was improved by us [ 22 , 23 ] with respect to previous experiments [ 47 , 48 ] in order to obtain a better sealing of the system. The magnetic stirrer used in the original experimental setup had a detrimental effect on the reproducibility of the dissolved oxygen measurements, while now the use of a methacrylate bath allows magnetic stirring, so the setup is more airtight. The photolytic activity of the biomimetic models was followed in two ways: quantitative oxygen evolution and variation of the electronic spectrum of the BQ during photolysis. It was noted earlier that excited-state benzoquinone abstracts a hydrogen atom directly from water [ 49 ] at pH 7 and is greater in aqueous solutions. The final products were hydroquinone and 2-hydroxybenzoquinone. The concentrations of O 2 in the solutions during the experiments increased linearly from about 2% dissolved oxygen to 14.4% for photolysis catalyzed by biomimetic model 1 , whereas the concentrations of O 2 remained almost constant for hours in the case of photolytic experiments using biomimetic model 2 (Figure 6). The ability of 1 to split water was also been tested by the reduction of p-benzoquinone into hydroquinone, which was determined by spectrophotometric monitoring of the reaction. Benzoquinone in water has major absorption at 246 nm ( ε = 2.2 × 10 4 M –1 cm –1 ), which decreased during the experiments, whilst a characteristic hydroquinone peak at 290 nm developed (Figure 7). Catalysts 2018, 8, x FOR PEER REVIEW 10 of 18 The concentrations of O2 in the solutions during the experiments increased linearly from about 2% dissolved oxygen to 14.4% for photolysis catalyzed by biomimetic model 1, whereas the concentrations of O2 remained almost constant for hours in the case of photolytic experiments using biomimetic model 2 (Figure 6). The ability of 1 to split water was also been tested by the reduction of p-benzoquinone into hydroquinone, which was determined by spectrophotometric monitoring of the reaction. Benzoquinone in water has major absorption at 246 nm (ε = 2.2 × 104 M–1 cm–1), which decreased during the experiments, whilst a characteristic hydroquinone peak at 290 nm developed (Figure 7). 0 5 10 15 20 25 0 4 8 12 16 % sat O2 time (h) Figure 6. Plot of the percentage of O2 dissolved in solution vs time for complexes 1 () and 2 (). Figure 7. Ultraviolet spectra variation for p-benzoquinone during the water photolysis experiments using biocatalyst 1, showing the decrease in the 246 nm band (disappearance of benzoquinone) and increase in the 290 nm band (formation of hydroquinone). Grey color corresponds to spectrum at time zero, and pink color corresponds to spectrum at 24 h. The photolytic experiment for 2 showed similar behavior to an aqueous solution containing only benzoquinone (without any complex)—a slow decrease in the amount of benzoquinone, followed by the formation of a mixture of hydroquinone and 2-hydroxy-p-benzoquinone without the generation of molecular oxygen [49]. The fact that 2-hydroxy-p-benzoquinone was not observed in the UV-VIS spectrum for the present studies with 1 indicates that it is probably stabilized by the manganese complex, presumably due to the approaching of the quinones to the complex. In this sense, it is worth noting that no dioxygen evolves when the sterically hindered Figure 6. Plot of the percentage of O2dissolved in solution vs time for complexes 1 (N) and 2 ( ). The photolytic experiment for 2 showed similar behavior to an aqueous solution containing only benzoquinone (without any complex)—a slow decrease in the amount of benzoquinone, followed by the formation of a mixture of hydroquinone and 2-hydroxy-p-benzoquinone without the generation of molecular oxygen [ 49 ]. The fact that 2-hydroxy-p-benzoquinone was not observed in the UV-VIS spectrum for the present studies with 1 indicates that it is probably stabilized by the manganese complex, presumably due to the approaching of the quinones to the complex. In this sense, it is worth noting that no dioxygen evolves when the sterically hindered 2,5-tert-butyl-p-benzoquinone is used rather than p-benzoquinone, showing a steric requirement in the hydrogen abstraction process. Catalysts 2018,8, 382 16 of 17 29. Bonadies, J.A.; Maroney, M.L.; Pecoraro, V.L. Structurally diverse manganese (III) Schiff base complexes: Solution speciation via paramagnetic 1 H NMR spectroscopy and electrochemistry. Inorg. Chem. 1989 ,28, 2044–2051. [CrossRef] 30. Zuniga, M.F.; Deacon, G.B.; Ruhlandt-Senge, K. Developments in heterobimetallic s-block systems: Synthesis and structural survey of molecular M/Ae (M = Li, Na, K, Cs; AE = Ca, Sr) aryloxo complexes. Inorg. Chem. 2008,47, 4669–4681. [CrossRef] [PubMed] 31. Costes, J.P.; Laurent, J.P.; Chabert, P.; Commenges, G.; Dahan, F. Solid state and solution study of trinuclear (Ni, Ba, Ni) complexes: (L 12 Ni) 2 Ba(ClO 4 ) 2· 2H 2 O ( 1 ) and (L 22 Ni) 2 Ba(ClO 4 ) 2· 2H 2 O ( 2 ) (L 1 = 3-Methoxysalicylaldiminato and L 2 = 3-(2-Methoxyethoxy)salicylaldiminato). Crystal and Molecular Structure of 2. Inorg. Chem. 1997,36, 656–660. [CrossRef] 32. Jamnicky, M.; Segla, P.; Koman, M. Methanolysis of pyridine-2-carbonitrile in the coordination sphere of copper(II), cobalt(II) and nickel(II). The structure of [Ni(O-methylpyridine-2-carboximidate) 3 ]Br 2· 4H 2 O. Polyhedron 1995,14, 1837–1847. [CrossRef] 33. Garduno, J.A.; García, J.J. Synthesis of annidines and benzoxazoles from activated nitriles with Ni(0) catalysts. ACS Catal. 2015,5, 3470–3477. [CrossRef] 34. Amami, V.; Ahmadi, R.; Naseh, M.; Ebadi, A. Synthesis, spectroscopic characterization, crystal structure and thermal analyses of two zinc(II) complexes with methanolysis of 2-pyridinecarbonitrile as a chelating ligand. J. Iran. Chem. Soc. 2017,14, 635–642. [CrossRef] 35. Devi, S.P.; Devi, R.B.; Devi, N.S.; Singh, L.J.; Singh, R.H. Structural and spectroscopic investigations on bis(1-amidino-O-2-alkoxyethylurea) copper (II) perchlorate complexes (alkoxy = methoxy, ethoxy or butoxy). Polyhedron 2012,47, 1–8. [CrossRef] 36. Amani, V.; Safari, N.; Khavasi, H.R. Synthesis, characterization and crystal structure determination of iron (III) hetero-ligand complexes containing 2,2 0 -bipyridine, 5,5 0 -dimethyl-2,2 0 -bipyridine and chloride,[Fe(bipy)Cl4][bipy·H] and [Fe(dmbipy)2Cl2][FeCl4]. Polyhedron 2007,26, 4257–4262. [CrossRef] 37. Paulat, F.; Praneeth, V.K.K.; Näther, C.; Lehnert, N. Quantum chemistry-based analysis of the vibrational spectra of five-coordinate metalloporphyrins [M(TPP)Cl]. Inorg. Chem. 2006 ,45, 2835–2856. [CrossRef] [PubMed] 38. Yang, C.I.; Lee, G.H.; Wur, C.S.; Lin, J.G.; Tsai, H.L. Syntheses, structures and single-molecule magnetic behaviors of two dicubane Mn4complexes. Polyhedron 2005,24, 2215–2221. [CrossRef] 39. Langley, S.K.; Chilton, N.F.; Massi, M.; Moubaraki, B.; Berry, K.J.; Murray, K.S. Synthesis and characterization of homo- and heterovalent tetrahexa- hepta- and decanuclear manganese clusters using pyridyl functionalized β -diketone, carboxylate and triethanolamine ligands. Dalton Trans. 2010 ,39, 7236–7249. [CrossRef] [PubMed] 40. Wang, M.; Ma, C.B.; Yuan, D.Q.; Wang, H.S.; Chen, C.N.; Liu, Q.T. Synthesis and characterization of a family of penta- and tetra-manganese(III) complexes derived from an assembly system containing tert-butylphosphonic acid. Inorg. Chem. 2008,47, 5580–5590. [CrossRef] [PubMed] 41. Wemple, M.W.; Tsai, H.-L.; Wang, S.; Claude, J.P.; Streib, W.E.; Huffman, J.C.; Hendrickson, D.N.; Christou, G. Tetranuclear and octanuclear manganese carboxylate clusters: Preparation and reactivity of (NBu n4 )[Mn 4 O 2 (O 2 CPh)9(H 2 O)] and synthesis of (NBu n4 )2[Mn 8 O 4 (O 2 CPh) 12 (Et 2 mal) 2 (H 2 O) 2 ] with a “linked-butterfly” structure. Inorg. Chem. 1996,35, 6437–6449. [CrossRef] [PubMed] 42. Ruettinger, W.; Yagi, M.; Wolf, K.; Bernasek, S.; Dismukes, G.C. O 2 Evolution from the manganese–oxo cubane core Mn 4 O 46+ : A molecular mimic of the photosynthetic water oxidation enzyme? J. Am. Chem. Soc. 2000,122, 10353–10357. [CrossRef] 43. Hendrickson, D.N.; Christou, G.; Schmitt, E.A.; Libby, E.; Bashkin, J.S.; Wang, S.; Tsai, H.L.; Vincent, J.B.; Boyd, P.D.W. Photosynthetic water oxidation center: Spin frustration in distorted cubane Mn IV Mn III3 model complexes. J. Am. Chem. Soc. 1992,114, 2455–2471. [CrossRef] 44. Pedrido, R.; Bermejo, M.R.; García-Deibe, A.M.; González-Noya, A.M.; Maneiro, M.; Vázquez, M. Metal complexes of a novel achiral symmetric pentadentate ligand–Crystal structures of monohelical zinc(II) and cadmium(II) complexes. Eur. J. Inorg. Chem. 2003, 3193–3200. [CrossRef] 45. Addison, A.; Nageswara, R.T.; Reedijk, J.; Van Rijn, J.; Verschoor, G.C. Synthesis, structure, and spectroscopic properties of copper(II) compounds containing nitrogen–sulphur donor ligands; the crystal and molecular structure of aqua[1,7-bis(N-methylbenzimidazol-2 0 -yl)-2,6-dithiaheptane]copper(II) perchlorate. J. Chem. Soc. Dalton Trans. 1984, 1349–1356. [CrossRef] Catalysts 2018,8, 382 17 of 17 46. Awad, M.K.; Anderson, A.B. Photoactivation of H 2 O by p-benzoquinone and the role of Mn III complexes in O2evolution: Molecular orbital theory. J. Am. Chem. Soc. 1989,111, 802–806. [CrossRef] 47. Ashmawy, F.M.; McAuliffe, C.A.; Parish, R.V.; Tames, J. Water photolysis. Part 1. The photolysis of coordinated water in [{MnL(H 2 O)} 2 ][ClO 4 ] 2 (L = dianion of tetradentate O 2 N 2 -donor Schiff bases). A model for the manganese site in photosystem II of green plant photosynthesis. J. Chem. Soc. Dalton Trans. 1985, 1391–1397. [CrossRef] 48. Aurangzeb, N.; Hulme, C.E.; McAuliffe, C.A.; Pritchard, R.G.; Watkinson, M.; Bermejo, M.R.; Garcia-Deibe, A.; Sanmartin, J.; Sousa, A. Crystallographic characterization of a possible model for photosystem II. J. Chem. Soc. Chem. Commun. 1994, 1153–1155. [CrossRef] 49. Ononye, A.I.; McIntosh, A.R.; Bolton, J.R. Mechanism of the photochemistry of p-benzoquinone in aqueous solutions. 1. Spin trapping and flash photolysis electron paramagnetic resonance studies. J. Phys. Chem. 1986,90, 6266–6270. [CrossRef] 50. Taguchi, T.; Stone, K.L.; Gupta, R.; Kaiser-Lassalle, B.; Yano, J.; Hendrich, M.P.; Borovik, A.S. Preparation and properties of an Mn IV -hydroxide complex: Proton and electron transfer at a mononuclear manganese site and its relationship to the Oxygen Evolving Complex within Photosystem II. Chem. Sci. 2014 ,5, 3064–3071. [CrossRef] [PubMed] 51. Gupta, R.; Taguchi, T.; Lassalle-Kaiser, B.; Bominaar, E.L.; Yano, J.; Hendrich, M.P.; Borovik, A.S. High-spin Mn-oxo complexes and their relevance to the oxygen-evolving complex within photosystem II. Proc. Natl. Acad. Sci. USA 2015,112, 5319–5324. [CrossRef] [PubMed] 52. Bruker. SAINT, Siemens Area Detector Integration Software; Bruker AXS Inc.: Madison, WI, USA, 2003. 53. Sheldrick, G.M. SADABS, Program for Scaling and Correction of Area Detector Data; University of Göttingen: Göttingen, Germany, 1996. 54. Altomare, A.; Burla, M.C.; Camalli, M.; Cascarano, G.L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A.G.G.; Polidori, G.; Spagna, R. SIR97: A new tool for crystal structure determination and refinement. J. Appl. Crystallogr. 1999,32, 115–119. [CrossRef] 55. Sheldrick, G.M. A short history of SHELX. Acta Cryst. 2008,A64, 112–122. [CrossRef] [PubMed] 56. Farrugia, L.J. ORTEP-3 for Windows—A version of ORTEP-III with a Graphical User Interface (GUI). J. Appl. Cryst. 1997,30, 565. [CrossRef] 57. Macrae, C.F.; Bruno, I.J.; Chisholm, J.A.; Edgington, P.R.; McCabe, P.; Pidcock, E.; Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P.A. MERCURY CSD 2.0—New features for the visualization and investigation of crystal structures. J. Appl. Crystallogr. 2008,41, 466–470. [CrossRef] 58. Tao, J.M.; Perdew, J.P.; Staroverov, V.N.; Scuseria, G.E. Climbing the density functional ladder: Nonempirical meta-generalizated gradient approximation designed for molecules and solids. Phys. Rev. Lett. 2003,91, 146401–146405. [CrossRef] [PubMed] 59. Schaefer, A.; Huber, C.; Ahlrichs, R. Fully optimized contracted Gaussian basis sets of triple zeta valence quality for atoms Li to Kr. J. Chem. Phys. 1994,100, 5829–5835. [CrossRef] 60. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A.; et al. Gaussian 09, Revision, A.01; Gaussian, Inc.: Wallingford, CT, USA, 2009. 61. Tomasi, J.; Mennucci, B.; Cammi, R. Quantum mechanical continuum solvation models. Chem. Rev. 2005,105, 2999–3094. [CrossRef] [PubMed] © 2018 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).