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Journal of Coordination Chemistry

ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20

Synthesis, X-ray crystal structure and DFT studies of two octahedral cobalt(II) complexes with N,N,Ntridentate triazine-type ligands Saied m. Soliman, Ayman El-Faham & Jörg H. Albering To cite this article: Saied m. Soliman, Ayman El-Faham & Jörg H. Albering (2017): Synthesis, Xray crystal structure and DFT studies of two octahedral cobalt(II) complexes with N,N,N-tridentate triazine-type ligands, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2017.1340646 To link to this article: http://dx.doi.org/10.1080/00958972.2017.1340646

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Date: 10 June 2017, At: 10:07

Publisher: Taylor & Francis Journal: Journal of Coordination Chemistry DOI: http://dx.doi.org/10.1080/00958972.2017.1340646

Synthesis, X-ray crystal structure and DFT studies of two octahedral cobalt(II) complexes with N,N,N-tridentate triazine-type ligands SAIED M. SOLIMAN 1a,b, AYMAN EL-FAHAMa,c and JÖRG H. ALBERINGd a

Department of Chemistry, Faculty of Science, Alexandria University, P.O. Box 426 Ibrahimia, 21321 Alexandria, Egypt b Department of Chemistry, Rabigh College of Science and Art, King Abdulaziz University, Saudi Arabia c Department of Chemistry, College of Science, King Saud University, P. O. Box 2455, Riyadh 11451, Saudi Arabia d Graz University of Technology, Mandellstr. 11/III, A-8010 Graz, Austria

Two new Co(II) complexes, [CoL2]X2·2H2O, with N,N,N-tridentate triazine type ligand (L) and X = Cl- (1) or NO3- (2) are synthesized and characterized using elemental analysis, FTIR spectra and single-crystal X-ray diffraction. Complexes 1 and 2 are crystallized in the centrosymmetric space groups P-1 and C2/c, respectively. The Co atoms are surrounded by two neutral tridentate ligands coordinated via nitrogen atoms, thus forming a distorted octahedral coordination sphere. Hirshfeld topology analyses of the molecular packing revealed that the polar Cl…H, O…H and N…H contacts are the most important intermolecular interactions while the nonpolar C…H and C…C (π-π stacking) contacts are weak and insignificant, respectively. DFT calculations indicated that the high-spin state is energetically more favored than the low-spin case. The Co(II) center transfers its spin density to the ligand donor atoms via the spin delocalization mechanism. Based on the atoms in molecules (AIM) results, all Co-N interactions have a predominant covalent character. The strength of the Co-N interactions decreases in the order Co-N(hydrazone) > Co-N(triazine) > Co-N(amine). The metal anti-bonding natural orbitals involved in the Co-N interactions have high dx2-y2, dz2 and s-or+bital characters. Both complexes showed good thermal stability up to 276 °C and 250 °C for complexes 1 and 2, respectively.

Keywords: Cobalt(II); Octahedral; Triazine; NBO; AIM

1

Corresponding author. Email: [email protected]

1. Introduction The synthesis of hybrid materials attracted great interest due to their fascinating applications in different fields such as ferroelectric transitions, conductivity, optical, catalytic and magnetic properties [1-5]. Many researchers reported the progress in this field and explored the structural aspects of novel divalent metal complexes in order to allow a rational understanding of their supramolecular structure [6-10]. The structural topology of the metal-organic hybrid compounds could be controlled by careful selection of the organic ligands and anions. The weak interactions such as hydrogen bonds and π-π stacking interactions play an important role in the construction of these molecular materials. Considering these points, special interest has been given to understand the role played by the organic groups because they not only control the distance between the layers but also are actively involved in the hydrogen bonding [11-13]. Cobalt has two important and common oxidation states, which are Co(II) and Co(III). Generally, the Co(II) salts are stable and not easily oxidized in aqueous solution to the higher oxidation state. In contrast, the oxidation of Co(II)→Co(III) is relatively easy in basic solutions or in the presence of strong ligand-field. From this point of view, the ligand properties play an important role in determining the final oxidation state of cobalt in its complexes. Many cobalt complexes were recently synthesized using multifunctional ligands [14-20]. These complexes attracted much and continuous attention from researchers due to their use in many fields such as biological applications and catalysis in organic transformations [21-28], especially due to the lower cost of cobalt when compared with its higher homologues [29]. Cobalt is a main component of vitamin B12 as it is essential for red blood cell formation and has lower toxicity than other non-essential metals. It is also important for catalyzing electron transfer, transmethylation and rearrangement reactions. Considering the interesting properties of the cobalt(II) complexes, in this work, we used the triazine-Schiff base type ligand (L, scheme 1) to synthesize two new homoleptic octahedral Co(II) complexes, [CoL2]X2·2H2O, where X = Cl- (1) or NO3- (2). The triazine-Schiff base has the advantages of several nitrogen atoms act as electron donor centers which possesses rigidity of coordination to metal ion. Also, it has NH groups and extended aromatic π-system able to enhance the supramolecular interactions via the H-bonding and π-π stacking interactions, respectively. The structural characterization of the new Co(II) complexes was performed using single-crystal X-ray diffraction, FTIR spectra, and thermal analysis. Moreover, DFT studies were

employed to understand the nature of metal-ligand interactions using atoms in molecules (AIM) and natural bond orbital (NBO) methods.

2. Experimental 2.1. Materials and physical measurements All chemicals were purchased from commercial sources and were used without purification. Infrared spectra (4000-400 cm-1) were recorded on an Alpha Bruker instrument in KBr pellets. Elemental analysis was carried out using a Perkin Elmer 2400 Elemental Analyzer; CHN mode. TGA measurements were performed using a TGA Q500 instrument by placing 2–5 mg of the sample in an open aluminum crucible at a heating rate of 10 °C min-1 within the temperature range 25 to 800 °C and under a flow of dry nitrogen at flow rate of 60 mL min-1. 1H and 13

C NMR spectra of the ligand were recorded on a 400 MHz JEOL spectrometer (JEOL Ltd.,

Tokyo, Japan) at room temperature in DMSO-d6 as solvent (figure S1, Supplementary data).

2.2. Synthesis of the triazine-Schiff base ligand (L) The ligand was prepared following the method described by El-Faham et al. [30]. IR (KBr, cm-1): 3432.8, 3387.9, 3277.1, 1611.5, 1588.2, 1573.6.

2.3. Synthesis of [CoL2]X2·2H2O; (1, 2) A solution (10 mL) of L (0.7970 g, 2 mmol) in methanol (10 mL) was added either to cobalt(II) nitrate hexahydrate or cobalt(II) chloride hexahydrate solution in 5 mL methanol at room temperature; the color of the solution became orange during the addition of L. This mixture was left for one day to evaporate slowly at room temperature. The mononuclear bis-ligand complexes, [CoL2]Cl2·2H2O; (1) and [CoL2](NO3)2·2H2O (2), were formed as orange crystals suitable for X-ray single-crystal measurements. [CoL2]Cl2·2H2O (1): Yield: 89%. IR (KBr, cm-1): 3494.2, 3447.5, 3212.0, 1601.1, 1564.3; Anal. Calcd.: C, 47.40; H, 5.86; N, 23.28 %. Found: C, 47.41; H, 5.85; N, 23.29%. [CoL2](NO3)2·2H2O (2): Yield: 86%. IR (KBr, cm-1): 3519.0, 3478.0, 3281.2, 3216.9, 1604.8, 1566.6, 1384.6, 826.3; Anal. Calcd.: C, 44.93; H, 5.56; N, 24.82%. Found: C, 44.91; H, 5.57; N, 24.83%.

2.4. X-ray measurements The crystallographic measurements of complexes 1 and 2 were made using a Bruker D8 Quest diffractometer with graphite monochromated Mo-Kα radiation at 106(2) K; (1) and 117 K; (2) and a photon detector. Absorption corrections were performed by SADABS [31a]. All nonhydrogen atoms in both compounds were localized on difference Fourier maps and refined in subsequent full-matrix least-squares calculations including anisotropic atomic displacement parameters. The hydrogen atoms bonded to carbon and nitrogen were refined with the riding model implemented in SHELX. H atoms bonded to the oxygen atoms of the solvate water molecules involved in the formation of hydrogen bridge bonds could be detected properly and were refined using a DFIX command in SHELX. The isolated non-bridging solvate water molecule was statistically oriented, thus no protons were detectable and only the position and displacement parameters of the respective oxygen atoms were refined. All calculations were performed using the Bruker APEX III program system and the SHELXTL program package [31b]. The crystallographic data, criteria for the intensity data collection, and some features of the single-crystal structure refinements are listed in table 1. Supplementary data are available from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK on request, quoting the deposition numbers CCDC–1539879 (1) and CCDC–1539880 (2). These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or Fax: (internat.) +44-1223/336-033; E-mail: [email protected]].

2.5. Computational details Density functional theory (DFT) calculations were carried out using the Gaussian 09 software package [32]. The X-ray structure coordinates were used to perform energy calculations for the high- and low-spin states of the two Co(II) complexes. For this task, we used two different DFT methods at the unrestricted level. The Becke three parameter Lee–Yang–Parr (B3LYP) [33] and the Perdew-Wang exchange as modified by Adamo and Barone combined with PW91 correlation (MPW1PW91) [34] DFT methods were used. The calculations were made using the TZVP basis set [35] for Co atom and 6-31+G(d,p) [36] basis sets for non-metal atoms. The wavefunction files were created using the B3LYP/TZVP single-point calculations at the X-ray crystal structure geometry in order to shed light on the nature and strength of the Co-N bonds in the framework of the quantum theory of atoms in molecules (QTAIM) [37] and using the Multwfn program [38].

Moreover, NBO calculations were made using the NBO 3.1 [39] program to assign the nature of orbitals included in the metal-ligand interactions. Using the same method, the natural atomic charges were obtained.

2.6. Hirshfeld surface analysis Crystal Explorer 3.1 software [40] is used to perform the Hirshfeld surface analysis of the studied complexes using the crystallographic information files (CIFs) obtained from the X-ray single crystal measurements. The Hirshfeld surfaces (HSs) and the fingerprint plots are constructed based on the electron distribution calculated as the sum of spherical atom electron densities [41-44]. In the map of dnorm, the red (negative dnorm) and the blue (positive dnorm) regions represent contacts with shorter and longer intermolecular distances than the van der Waals radii of the two elements, respectively. The white color means intermolecular distances close to van der Waals contacts with dnorm equal to zero. The 2D fingerprint map is a plot of di (distances from the surface to nearest interior) versus de (distances from the surface to nearest exterior) which recognizes the existence and amount of different types of intermolecular interactions [45, 46].

3. Results and discussion 3.1. X-ray structure description The X-ray structure analysis shows that chloride complex 1 crystallizes in the triclinic space group P-1. The asymmetric unit contains one [CoL2]2+ complex cation, two chloride anions, one solvate water molecule with detectable protons and one water molecule with disordered hydrogen atoms, which could not be localized properly in the difference Fourier maps. On the other hand, the respective nitrate compound 2 shows higher symmetry and crystallizes in the space group C2/c with a half cationic complex unit [CoL2]2+, one nitrate anion, one water molecule including hydrogen atoms and one water molecule which was refined without protons. This oxygen atom (O7) is located in a general position instead of the required special position -as it was found for the other, ordered water molecule in 2. Hence, this site was assumed to be only half-occupied. Figures 1 and 2 show the numbering and the displacement ellipsoids of the complex units, anions and solvate water molecules for compounds 1 and 2. Selected interatomic distances and angles are listed in table 2.

Both crystal structures contain monomeric cobalt complex units with two neutral tridentate ligands. The central atoms are surrounded octahedrally by the two ligands in a meridional arrangement. The ligands contain three different possible nitrogen positions suitable for coordination which are an amine-N, a hydrazine-N and a triazine-N. This leads to the formation of different enantiomers. Figure 3 shows one enantiomer in the structure of 1 and its mirror image occurring in 2. Since the absolute crystal symmetry of both structures is centrosymmetric, the unit cells contain a racemic mixture of the possible enantiomers. The Co-N bond lengths vary between 2.094 Å (Co(1)-N(10)) and 2.188 Å (Co(1)-N(9)) for compound 1 and between 2.080 Å (Co(1)-N(2) and 2.186 Å (Co(1)-N(1) for compound 2. In both complexes, the C-N(hydrazone) bonds are the shortest distances, followed by the Co-N(triazine) bonds and finally the Co-N(amine) interactions are the weakest and shows the longest bond lengths. Due to the bulky and asymmetric ligand, the octahedral coordination spheres of the central Co atom are strongly distorted. The ligand atoms in trans position should show a N-Co-N angle of 180°. In the present complexes, this is only the case for the hydrazone-N atoms in the middle of the ligand molecule. Here we found angles of 175.95° (N(10)-Co(1)-N(2)) and 178.78° (N(2)#1-Co(1)-N(2)) for compound 1 and 2, respectively. The angles for the combination N(amine)-Co-N(triazine) are 151.04° and 153.31° for compound 1 and 155.60° for compound 2, respectively, and deviate strongly from the ideal value. The other angles vary over a wide range and values of 77.46° (N(2)-Co(1)N(4)) to 110.23° (N(4)-Co(1)-N(12)) can be found in compound 1, while the ideal value for all angles would be 90°. The respective values for compound 2 are 77.51° (N(2)#1-Co(1)-N(4)#1) to 109.21° (N(4)#1-Co(1)-N(4)). The crystal structures of both compounds contain a high number of hydrogen bridge bonds. The values are summarized in table S1 (Supplementary data) while the hydrogen bond networks in compounds 1 and 2 are shown in figure S2 (Supplementary data). Those H-bridges involving N-H…Cl and O-H…Cl in compound 1 and N-H…O and O-H…O in compound 2 build up a one-dimensional infinite H-bond network, extending along the c-axes in the respective crystal structures. The protons of one solvate water molecule point towards the chloride anions, which has three additional hydrogen bridge interactions, two with the amine groups of one complex unit and one with the hydrazone proton of the neighboring complexes. Since each complex has two amine and two hydrazone groups, the formation of H-bridges leads to onedimensional infinite double strings of alternating complex and anion/solvate water parts as

shown in part A of figure S2. The situation is similar for the nitrate compound (see figure S2B). Here the NO3- groups replace the Cl- anions and hydrogen bridges are built up between the protons of the solvate water molecule, the amine protons and the hydrogen atoms of the hydrazone groups and all three oxygen atoms of the nitrate anions. Very similar double strings are formed via these hydrogen bridges. Both structures contain a number of further weak hydrogen bridges of the type C-H…acceptor (Cl or O) and these interactions occur between neighboring strings. Another structural feature can be seen in figure S2 (Supplementary data), i.e. the orientations of the morpholine rings (four per complex unit, all in the stable chair conformation) bonded to the triazine rings that are different in the chloride and nitrate compound. This might be explained by the different packing schemes in the two compounds. Both structures can be regarded as close rod packings if the one-dimensional infinite double strings are regarded as “rods”. Figure S3 (Supplementary data) shows both structures as a projection along the c-axis of the triclinic and monoclinic lattice, respectively. Compound 1 can be regarded as a monoclinic primitive packing of the “rods” (figure S3A), while 2 is close to a hexagonal-close rod-packing (figure S3B). It can also be seen from figure S3 that the dense packed complex strings mainly contain H…H and O…H contacts to another via the morpholine rings in the “outer sphere” of the complex units. The solvate water molecules for which no protons could be detected properly are easily spotted in this figure by their larger ellipsoid and their position in small voids between the complex units. There is no preferred orientation for the water molecules in such positions, thus the position of the protons is not determined by further interactions. Hence, the oxygen atoms have a higher degree of freedom for their positional parameters (indicated by their larger ellipsoids) and no sharp electron density of the water protons can be detected in difference Fourier maps. The respective oxygen atom in compound 2 showed some additional disorder which is located on the general site with eightfold multiplicity while it should be special site with a multiplicity of four.

3.2. Analysis of molecular packing The dnorm, shape index (SI) and curvedness plots of the [CoL2]2+ unit of the studied complexes are shown in figures S4 and S5 (Supplementary data). The shape index and curvedness maps showed no signs for any important π-π stacking interactions in both complexes as indicated by

the absence of any large flat green areas in the curvedness map or the red and blue triangles in the SI map. The full and decomposed fingerprint plots of the most important intermolecular contacts are given in figures S6 and S7 (Supplementary data). It is clear from the decomposed fingerprint data that the C…C interactions are almost zero. The percentage contributions of all possible intermolecular interactions are summarized in figure 4. The molecular packing of both complexes is controlled by H…H, O…H, C…H and N…H intermolecular interactions. In addition, 1 showed many Cl…H interactions. The H…H interactions are the most dominant and it was believed that this type of interaction plays an important role in the crystal stability. Many polar interactions occurred in the crystal of the studied complexes. In complex 1, two main types of C-H…O interactions occurred: a) the O5…H15A between the solvate water as H-acceptor and the C-H from the morpholine ring and b) the O1…H38B and O3…H18A which occur among the morpholine rings. Most of these contacts appeared in the dnorm map as red spots indicating that these interactions have contact distances shorter than the van der Waal radii sum of the H and O atoms and indicating their significance (figure 5). The Cl…H contacts are the second polar interactions shared significantly to connect the [CoL2]2+ units in the crystal. The shortest Cl…H contact occurred between the Cl- counter anion and the N-H groups either from the amino or hydrazone moiety. The third polar interactions are the N…H interactions occurring between the N-atom of the triazine ring and the C-H proton from one of the methyl groups in a neighboring complex unit. Those appeared as red spots of low intensity indicating that these interactions are considerably weak (figure 5). The only noted hydrophobic interactions are the C…H contacts which appeared as blue regions on the dnorm HF surface indicating that these interactions are weak (figure S8, Supplementary data). Similar results were obtained for complex 2 where both complexes showed almost the same amount of the C…H interactions. The only noted differences are: 1) the higher amount of the O…H interactions due to the replacement of the chloride by the nitrate anion; 2) two types of N…H interactions were observed, the first occurred between the N-atom from triazine ring and the C-H protons from the neighboring morpholine ring while the second N…H contact is between the N-atom from the nitrate anion and the NH proton of the hydrazone moiety (figure 6).

3.3. DFT energy analyses The X-ray structure analysis demonstrated that the studied complexes have a paramagnetic Co(II) metal ion center. As a result, the high-spin state with total multiplicity of 4 and the lowspin state with one unpaired electron and a multiplicity of 2 are the two possible electron distributions for Co(II) complexes. The results shown in table 3 indicate that the high-spin state of the studied complexes is the most favored energetically. Based on these results, the AIM and NBO analyses will be discussed.

3.4. AIM analyses In this section, the nature and the relative strength of the Co-N interactions were inspected using atoms in molecules theory (AIM). For this task, we employed the B3LYP/TZVP method to compute the topological parameters given in table 4. Based on Bader and Essen [47], the higher electron density ρ(r) at the BCP is an indication of strong interaction. From this point of view, the order Co-N(hydrazone) > Co-N(triazine) > Co-N(amine) according to the strength of interactions agree very well with the interaction energies computed using the relation Eint = V(r)/2 [48]. On the other hand, the ratio of |V(r)|/G(r) and the total energy density H(r) values are good indicators for the bonds covalent character [49]. The negative H(r) values and |V(r)|/G(r) ratio greater than 1 indicated that the covalent character is dominant for all the Co-N interactions. It is clear that the results shed light on the sensitivity of the topological parameters towards little variations in bond distances. It seems from the results shown in table 4 that the six Co-N bonding interactions are significantly different while each two similar Co-N interactions in 2 are almost equivalent, which agree well with the reported Co-N distances for both complexes. As a result, we conclude that complex 2 has more symmetric environment around the metal ion than 1, in agreement with our structural analysis.

3.5. NBO analysis NBO analysis is performed using DFT/B3LYP method and TZVP basis sets to calculate the net charges at the ligand, counter anion and Co to investigate the charge transferred among them. The natural charges at the Co atom are calculated to be 1.4100 e and 1.4229 e for complexes 1 and 2, respectively. These values indicate the presence of a significant amount of charge transferred from the ligand fragment to the central metal. As a result, the net amount of negative

charge transferred to the Co atom is 0.5900 e and 0.5771 e for complexes 1 and 2, respectively. The net charges at the two counter anions (Cl- and NO3-) are calculated to be -1.8603 e and -1.9124 e for 1 and 2, respectively. The counter anion is free uncoordinated with the Co(II) ion due to the ionic nature of the studied complexes, so there are losses of only 0.1397 e and 0.0876 e, respectively, to [CoL2] fragments. The NBO analysis offered the concept of stabilization energy due to the intramolecular charge transfer interactions among atoms in molecular system with the aid of the second order perturbation theory. The energy due to the donor (NBOi)-acceptor (NBOj) interactions is higher for the stronger interactions compared to the weaker ones. Summary of these interactions is shown in table 5. Since we deal with the studied systems based on the energy analyses given in table 3, as high-spin cases with multiplicity equal 4, the intramolecular charge transfer interactions occur among the alpha and beta orbitals of the studied systems. For simplicity, only the ligand alpha NBOs included in the Co-N interactions will be described here where both the alpha and the beta NBOs of the ligands showed almost the same features (figure 7). The LP*(6)Co is the most significant anti-bonding alpha NBO involved in the interactions with the ligand donor atoms NBOs. This metal orbital has mainly s-orbital characters as can be seen from figure 7. The stabilization energies for the donor-acceptor interactions included in the Co-N bonding are in the order Co-N(hydrazone) > Co-N(triazine) > Co-N(amino) which agree very well with the AIM analyses (table 4). Interestingly, three significant beta NBOs from the Co center are included in the interactions with the filled lone pair NBOs of the ligand groups which are LP*(3), LP*(4) and LP*(5) anti-bonding NBOs. Analysis of these metal anti-bonding NBOs indicated the high dx2-y2, dz2 and s-orbital characters of these NBOs, respectively, as shown in figure 7. The intramolecular charge transfer from donor NBO to the acceptor one affects the occupancy and the energies of the NBOs included in these interactions. Table 6 contains the occupancy and energies of the NBOs included in the ICT interactions. The occupancy of all the occupied nonbonding NBOs (LP(N)) of the ligand groups are decreased (0.0461-0.1179 e) due to the Co-N interactions. In contrast, the occupancy of the Co(II) anti-bonding NBOs are increased (0.1245-0.1842 e). Moreover, all the ligand and the Co (except LP*(3)) NBOs are stabilized due to the metal ligand interactions.

3.6. Continuous shape measure Six-coordinate metal-complexes could accommodate two possible coordination environments around the central metal ion, which are the octahedral and trigonal prism arrangement. The coordination geometry around metal in most of the metal complexes becomes distorted from these two extremes. The d7 complexes are one of those electronic systems which suffer from the distortion known by the Jahn Teller effect. One of the most suited methods used to predict the degree of distortion from an ideal polyhedron is the Continuous shape measure (CShM) [50]. This methodology is used to quantify the degree of distortion in the structure of the studied complexes, which has two similar ligands and all donor atoms are the same. The values of shape measurements range from 0 to 100, where the higher the value the more the distortion of the test structure relative to the reference one. Continuous shape measure analysis of the studied Co(II) complexes against the perfect octahedral as a reference geometry gave values of 3.67 and 2.94 for complexes 1 and 2, respectively. When compared with the trigonal prism, the values were found to be 7.80 and 9.25 for complexes 1 and 2, respectively. From this point of view the structures are better described as a distorted octahedral around the Co(II) center where 1 showed higher degree of distortion than 2.

3.7. Spin-density analysis In order to understand the mechanism by which the unpaired electrons from a paramagnetic center (Co(II)) could distribute its spin density to the neighboring atoms, specially the ligand donor atoms, we collected the spin densities of all donor atoms as well as Co(II) in table 7. There are two possible well-known mechanisms: (1) the molecular orbital of the paramagnetic center (Co(II)), which has three unpaired electrons, mainly d-orbital characters, is mixed with donor atoms orbitals from the ligand. In this case, there will be a distribution of positive spin-density throughout the molecule which is called spin delocalization mechanism; (2) the spin-polarization mechanism in which the positive spin at the Co(II) center induces some spin-density of the opposite sign at the ligand donor atoms bonded to it. In this case, a concentration of negative spin-density is favored around the ligand donor atoms. In the present cases, the spin density of the Co(II) ion is in the range of 2.6772-2.6962 e. All the donor atoms from the ligand as well as the Co(II) center have positive spin densities with a spherical spin-density shape (table 7). This illustrates that the spin-delocalization mechanism is

dominant and some of the Co(II) spin-density is delocalized to the N-atoms. Pictorial presentations of the spin density distributions of the high spin state of complexes 1 and 2 are shown in figure 8.

3.8. FTIR spectra The FTIR spectra of the studied compounds recorded as KBr discs are shown in figure S9 (Supplementary data). The infrared spectra of the free ligand showed the νasym(N-H) and νsym(N-H) of the amino group at 3432.8 and 3387.9 cm-1, respectively. These stretching modes are shifted to higher wavenumbers of 3494.2 and 3447.5 cm-1 for 1 and 3519.0 and 3478.0 cm-1 for complex 2. The increase in the vibrational wavenumbers and lowering in the intensity of absorption in the N-H stretching vibrations indicated the coordination between the Co(II) and the amino group of the ligand. On the other hand, the ν(N-H) of the hydrazone is observed at 3277.1, 3212.0 and 3281.2 cm-1 for the free ligand, 1 and 2, respectively. The shift of the ν(N-H)hydrazone in both complexes could be attributed to the involvement of this group in the H-bonding interactions with the Cl- and NO3- counter anions with different degrees. The infrared spectrum of L displays a stretching mode at 1611.5 cm-1 and the splitted band at 1588.2 and 1573.6 cm-1 which can be assigned to azomethine ν(C=N) vibrations. In the case of complexes 1 and 2, the former is shifted towards lower frequency by 7-10 cm-1, while the latter appeared as one band and also shifted towards lower frequency suggesting that the ligand is involved in coordination to the metal atom via the nitrogen atoms of the azomethine group which agrees with the reported X-ray structure. In 2, the ionic nitrate displayed two of its characteristic vibrations at 1384.6 and 826.3 cm-1 while the lowest wavenumber mode was not observed [51].

3.9. Thermal analysis The thermogravimetric analyses (TGA) of the studied complexes showed good thermal stability up to 276 °C and 250 °C for complexes 1 and 2, respectively (see figure S10, Supplementary data). Although the two complexes have the same coordination sphere, they decomposed differently after losing the solvate water slowly up to 100 °C. Complex 1 with two unequivalent sets of bonds with the two ligand units decomposed in two successive steps at 276 °C and 471 °C with plateau separating the two decomposition steps corresponding to the stepwise loss of the two ligands units, respectively. In contrast, the more symmetric complex 2 with two equivalent

ligands groups around the Co(II) center, the decomposition of the two ligands starts at 250 °C and occurs in one step with no clear break for a two-stage decomposition.

4. Conclusion Using the self-assembly process, the reaction of two Co(II) salts with a triazine-Schiff base ligand in methanol afforded two new [CoL2]X2·2H2O complexes with X = Cl- (1) or NO3- (2). Their structures were characterized using elemental analysis and FTIR spectra as well as X-ray single-crystal structure analysis. Both complexes adopt octahedral configuration around Co-center and the two N,N,N-tridentate ligands are arranged in meridional fashion. Continuous shape measurements indicated more distorted octahedral coordination geometry around Co(II) in 1 than 2. Energy analysis favored the high-spin state of both complexes over the low-spin case. AIM and NBO analyses indicated that the strength of interactions follows the order Co-N(hydrazone) > Co-N(triazine) > Co-N(amine). The alpha LP*(6) which has mainly s-orbital character and the beta LP*(3), LP*(4) and LP*(5) anti-bonding NBOs of the Co(II) which have high dx2-y2, dz2 and s-orbital characters, respectively, are the most involved in the donor-acceptor interactions with the ligand donor atoms. The occupancy of all the occupied nonbonding NBOs (LP(N)) of the ligand groups are decreased (0.0461-0.1179 e) while the Co(II) anti-bonding NBOs are increased (0.1245-0.1842 e). Most of these orbitals are stabilized due to the metalligand interactions. The Co center and N-atoms have positive spin densities with a spherical spin density shape at the metal atom, indicating that the spin-delocalization mechanism is dominant. The FTIR spectra confirmed the coordination of the Co(II) with the amino and azomethine N-atoms of the ligand groups. Both complexes showed good thermal stability till 276 °C and 250 °C for 1 and 2, respectively.

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Figure captions

Figure 1. Numbering and atomic displacement ellipsoids drawn at 50% probability level for complex 1. All H atoms except those bonded to N and O atoms are omitted for clarity. Figure 2. Numbering and atomic displacement ellipsoids drawn at 50% probability level for complex 2. All H atoms except those bonded to N and O atoms are omitted for clarity. Figure 3. Two enantiomers of the [CoL2]2+ complexes in the structures of 1 and 2. Only the C-Nbackbones of the tridentate ligands are shown for easier visualization. Figure 4. All possible intermolecular interactions in the crystal structures of complexes 1 and 2. Figure 5. The most important polar interactions in the crystal structure of 1. Figure 6. The most important polar interactions in the crystal structure of 2. Figure 7. The ligand nonbonding and metal antibonding NBOs included in the Co-N interactions of complex 1. Beta LP*(5)Co and alpha LP*(6)Co antibonding NBOs have the same features. All H atoms are omitted for clarity. Figure 8. Spin-density distribution for the high-spin state of complexes 1 and 2 calculated using B3LYP/TZVP method. H atoms and counter anions are omitted from graphs for more clarity. The blue color indicates positive spin-density. All H atoms are omitted for clarity.

Table 1. Crystal data and structure refinement for complexes 1 and 2. Compound

[CoL2]Cl2·2H2O

[CoL2](NO3)2·2H2O

Empirical formula

C38H56Cl2CoN16O6

C38H56CoN18O12

Formula weight

962.8

1015.92

Temperature

117(2) K

106(2) K

Wavelength

0.71073 Å

0.71073 Å

Crystal system

Triclinic

Monoclinic

Space group

P -1

C 2/c

Unit cell dimensions

a = 11.107(2) Å α = 82.34(4)°

a = 14.831(2) Å α = 90°

b = 11.974(2) Å β = 89.99(4)°

b = 18.722(3) Å β = 95.67(3)°

c = 16.868(3) Å γ = 76.77(3)°

c = 16.397(2) Å γ = 90°

3

Volume

2163.3(8) Å

Z

2

4530.6(12) Å3 4

3

Density (calculated)

1.475 Mg/m

1.486 Mg/m3

Absorption coefficient

0.587 mm-1

0.461 mm-1

F(000)

1006

2124 3

Crystal size

0.20 × 0.18 × 0.15 mm

0.323 × 0.128 × 0.121 mm3

Theta range for data collection

2.212 to 25.471°

2.233 to 25.344°

Index ranges

-13