Crystal structure, solvothermal synthesis, thermogravimetric studies and DFT calculations of a five-coordinate cobalt(II) compound based on the N,N-bis(2-hydroxyethyl)glycine anion

The molecular structure of the title compound confirms the findings of FTIR, elemental analysis, ESI–MS analysis and TG analysis. By using the density functional theory (DFT) (B3LYP) method with 6-31G(d) basis set, the structure has been calculated and optimized. In the crystal, two types of O—H⋯O hydrogen bonds connect the molecules, forming a two-dimensional network parallel to (001).


Chemical context
In recent years, coordination compounds have attracted a great deal of interest for their structural aesthetics and potential functional applications (Fujita et al., 2004). The design of molecular structures is highly influenced by factors such as the coordination nature of the metal ion, the coordinating ability and functionality of the organic ligands and the reaction conditions (Zhang et al., 2015;Yin et al., 2015). Hence, the prediction of crystal structure is largely considered to be serendipitous except for simple compounds such as mononuclear molecules. The 3d 7 configuration of Co II is particularly suited for the construction of metal-organic compounds (Kurmoo, 2009). One of the interesting structural aspects of studying cobalt compared to nickel, iron or manganese is the range of coordination geometries -octahedral, tetrahedral, square-pyramidal, trigonal-bipyramidal and square-planar -which are all stable (Kurmoo, 2009). There are several coordination modes for the cobalt ion. The common mode is six-coordinate (Bryant et al., 2015;Artetxe et al., 2015), and only relatively few four-coordinate (Gupta et al., 2015) and five-coordinate (Lee et al., 2015) cobalt complexes have been recorded. Generally, five-coordinate compounds have two classical configurations, trigonal-bipyramidal and square-pyramidal, and the extent of each geometry each can be determined by the value (Addison et al., 1984).
The carboxylate unit is widely used in the synthesis of coordination compounds and is part of commonly used ligands. It is a good bridging group, favouring the formation of ISSN 2056-9890 products (Zhou et al., 2009). Very recently, we have been investigating Co II compounds constructed from ligands containing carboxylate and hydroxyl groups, which usually form multinuclear and/or polymeric structures and show interesting magnetic behavior (Zhou et al., 2009;Zeng et al., 2010). Similarly, herein, we chose N,N-bis(2-hydroxyethyl)glycine (bicH 3 ) containing two hydroxyl oxygen atoms, one carboxylate oxygen atom and one nitrogen atom, which can potentially coordinate to a metal ion as a tetradentate ligand (He et al., 1999). BicH 3 contains the properties of both amino acid and amino alcohol as a result of the N-substituted amino, carboxyl, and two hydroxyl groups in the molecule. To the best of our knowledge, the crystal structures of metal-organic compounds with the bicH 3 ligand have not been very well explored to date. Potential coordination modes for bicH 2 À , bicH 2À and bic 3À are shown in Fig. 1

Structural commentary
The molecular structure of the title compound is shown in Fig. 2. The Co II ion is five-coordinated by four atoms from the bicH 2 À ligand (one carboxylato oxygen atom, two hydroxyl oxygen atoms, one nitrogen atom) and one terminal chlorine atom in a slightly distorted trigonal-bipyramidal environment ( = 0.94, = | À |/60, and being the two largest angles around the central atom; values for in perfect coordination geometries are 1.0 for trigonal-bipyramid and 0.0 for squarepyramidal). In a similar reported compound which was formed by bicH 2 2À and a Cu II ion, a five-coordinate mode was observed (He et al., 1999); the difference is that one nitrogen atom of benzimidazole or isoquinoline has replaced the terminal chloride ion in compound 1. In 1, the bond lengths around the Co II ion are Co1-N1 = 2.1626 (15), Co1-O1 = 2.0482 (13), Co1-O2 = 2.0463 (14), Co1-O3 = 2.0095 (14) and Co1-Cl1 =2.2701 (6) Å . The length of the Co-O(carboxylate) bond is shorter than that of Co-O(hydroxyl), which may be due to the difference between the electron density of carboxylate oxygen atoms and that of hydroxyl oxygen atoms (He et al., 1999). According to the total valencecharge balance and the bond lengths, we can conclude that cobalt is in oxidation state +II.

Figure 2
The molecular structure of the title compound, showing the atom labeling. Displacement ellipsoids are drawn at the 30% probability level. Table 1 Hydrogen-bond geometry (Å , ).

Database survey
A search of the Cambridge Structural Database (CSD; Groom et al., 2016) was carried out for structures containing the bicH 3 ligand. This revealed bicH 3 coordinating to a lanthanide metal (Inomata et al., 2001), Cd and Na (Katsoulakou et al., 2011), Cu, Ni and Zn (Thakuria & Das, 2007;Liu et al., 2013;Lo & Ng, 2010), Re, Mn and Fe (Kirillov et al., 2005;Sun et al., 1997;Graham et al., 2009). A related structure with copper and bromide (Yamaguchi et al., 1991) shows a very similar mononuclear crystal structure to the title compound. There are only a small number of reports for the ligand coordinating to Co (Funes et al., 2015;Zhao & Liu, 2010;Liu et al., 2015).

Synthesis and crystallization
The ligand bicH 3 (0.5 mmol) in a ethanol solution (2 mL) was added to a ethanol solution (5 mL) of CoCl 2 Á6H 2 O (1 mmol). 0.02 mL of triethylamine was added dropwise to the mixed solution and stirred for 15 min at room temperature. The reactants were sealed in a 12 mL Teflon-lined autoclave, heated at 413 K for three days and then cooled to room temperature at a rate of 10 K h À1 . Purple single crystals (Fig. 4

ESI-MS spectroscopic analysis
The ESI mass spectra were recorded using an LCQ-FLEET mass spectrometer (Thermo). To give further evidence for the inner structure of compound 1, characterization of the molecule in solution was accomplished by ESI-MS experiments.
For the methanol solution of 1, the ESI mass spectrum (Fig. 5) exhibits the main ion peak observed at an m/z of 254.93, which can be assigned as [Co(C 6 H 11 NO 4 )Cl] + (fit: 254.970860). The observed m/z clearly matches the assigned formula as well as the simulated spectra. This suggests that compound 1 produced in solution was stable during the ionization process. ESI-MS can also be used to examine a series of inner-bridge replacement reactions for multinuclear Co compounds Hu et al., 2013), which is an important complement to ligand exchange, ion exchange, template exchange and supramolecular transformations (Chakrabarty et al., 2011;Miras et al., 2009). The study of the chemistry of coordination compounds by mass spectroscopy is an excellent tool to demonstrate the stability and existence of multinuclear moleculesin solution. Part of the crystal structure showing the two different O-HÁ Á ÁO hydrogen bonds as distinct colors, blue for O1-HÁ Á ÁO4 ii bonds and green for O2-H2Á Á ÁO4 i bonds (symmetry codes as in Table 1).

Figure 4
The optical microscope image of single crystals of compound 1.

Figure 5
ESI mass spectrum of compound 1.

TG analysis
The TG analysis was performed on Pyris Diamond TG/DTA. The appearance of the flexible polydentate ligands inspired us to investigate the thermal stability of the network. The crushed single-crystal samples were heated to 1073 K in an N 2 atmosphere at a heating rate of 5 K min À1 (Fig. 6). The TGA curve for 1 shows that the framework begins to decompose at 413 K, and the 21.6% remaining weight is assuming to the mass loss percentage of cobalt (cal. 22.9%). One similar compound, [CuCd(bicH 2 (NO 3 )Cl 2 (H 2 O)]ÁH 2 O, constructed with bicH 2 is quite unstable and begins to lose lattice water at 393 K (Liu et al., 2013). Other compounds synthesized using the same ligand also show mass loss below 413 K (Inomata et al., 2001), owing to the loss of coordinating water. However, another reported complex with five-coordinate cobalt begins to disintegrate at 669 K (Lee et al., 2015). In summary, the crystals synthesized using bicH 2 2À tend to decompose at a relatively low temperature.

DFT calculations
All the calculations were performed by using the GAUS-SIAN09 program package (Frisch et al., 2009). The molecular structure of the title compound in the ground state was optimized in vacuo without considering the solvent, and a basis set (b1) consisting of a standard LanL2DZ basis set (Dunning & Hay, 1976; for Co, while the other atoms, C, H, N, O were described by a standard 6-31G(d) set. To investigate the energy differences between the high-spin and low-spin states of the title compound, the ÁE of these two energy states was evaluated using the B3LYP/ b1 method (Carabineiro et al., 2008;Saraçog lu & Cukurovali, 2016). Vibrational frequencies were calculated for all stationary points to verify that each was a minimum (NIMAG = 0) on the potential energy surface.
To gain an insight of the electronic structures, bonding properties and relative stability of the two different spin ground states (S = 3/2 and 1/2) for compound 1, the calculations in the DFT method were investigated. The optimized geometries calculated for compound 1 are presented in Fig. 7. The structure of 1 presented an almost perfect trigonalbipyramidal geometry by means of the X-ray diffraction. The chlorine and nitrogen atoms occupy the axial positions, while the equatorial plane is occupied by three oxygen atoms. According to the energies for the two calculated structures (see Supporting information), the X-ray structure determined for complex 1 should correspond to the complex with highspin ground state. The result can also be indicated by the mean () and maximum (Á) absolute deviations obtained for the coordination distances (Co-X). For the high-spin form of complex 1, values of = 0.104 Å and Á = 0.148 Å indicate a reasonable agreement. As for the high-spin ground state and the experimental value, the biggest difference for the bond lengths is found to be 0.148 Å for Co-N, similar to what has been observed in related Co II compounds (Carabineiro et al., 2008). The results of the schematic representation of both ground states supported its coordination behavior and the value of ÁE is 13.4 kcal mol À1 , which shows that compound 1 can well exist stably. The cartesian coordinates for the two calculated structures are given in the Supporting information.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2 The TG curve of compound 1.