Crystal structure and thermal properties of bis[μ-2-(methoxycarbonylhydrazinylidene)acetato-κ3 N 1,O:O]bis[diaqua(thiocyanato-κN)manganese(II)] tetrahydrate

The title compound was prepared by a template method starting from manganese(II) nitrate with a Schiff base ligand. The product of condensation was between methyl carbazate and glyoxylic acid, and formed in situ in aqueous solution containing ammonium thiocyanate. The manganese compound crystallized in the monoclinic space group P21/n and exists as a centrosymmetric dimer.


Chemical context
Hydrazine, dinitrogen tetrahydride (N 2 H 4 ), is the simplest diamine and parent of innumerable organic derivatives. Among them, carbazates (esters of hydrazinecarboxylic acid, NH 2 -NH-COO-R, where R = CH 3 , C 2 H 5 , CH 2 C 6 H 5 etc) are interesting as ligands in view of their variety of potential donor atoms such as oxygen and nitrogen. Interestingly, these neutral molecules can be expected to exhibit only one common coordination mode, i.e. N,O-chelating bidentate. This has been clearly observed in many metal complexes with a variety of anions such as formate (Srinivasan et al., 2011), benzoate (Kathiresan et al., 2012), thiocyanate (Srinivasan et al., 2014a,b), nitrate (Zhang et al., 2005;Srinivasan et al., 2007Srinivasan et al., ,2008 and perchlorate (Chen et al., 2016, Sitong et al., 2016. Apart from their coordination ability, alkyl carbazates can also undergo condensation reactions; the hydrazinic part of the terminal amine group can react with the carbonyl group of aldehydes or ketones to form Schiff bases. In this regard, Schiff bases and their Co III , Ni II , Pd II and Fe II complexes based on (2-phenylphosphino)benzaldehyde with ethyl carbazate (Milenković et al., 2013a(Milenković et al., ,b, 2014 have been reported. Recently, we have also reported Schiff bases generated from analogous benzyl carbazate with alkyl and heteroaryl ketones, and their metal complexes (Nithya et al., 2016(Nithya et al., , 2017a(Nithya et al., ,b, 2018a. However, no work involving Schiff base complexes of alkyl carbazates with an aldehydic, orketo acid, has been reported so far, except from our own ISSN 2056-9890 recent report of a Schiff base generated from methyl carbazate and -ketoglutaric acid, and its silver(I) complex (Parveen et al., 2018). In a continuation of our investigations, the title complex (I) was prepared by a template method starting from manganese(II) nitrate with a Schiff base ligand. The product of condensation between methyl carbazate and glyoxylic acid, formed in situ in aqueous solution containing ammonium thiocyanate.

Structural commentary 2.1. General structural details
The manganese title compound crystallizes in the monoclinic space group P2 1 /n and exists as a centrosymmetric dimer (Fig. 1). The asymmetric unit consists of an Mn atom, a tridentate Schiff base ligand, an N-bounded thiocyanate moiety, and two ligated and two solvated water molecules. The manganese atom is surrounded in a distorted octahedral geometry by symmetry-related -O-bridged carboxylate anions, one azomethine nitrogen, an N-bounded NCS anion and two ligated water molecules with an MnN 2 O 4 core. The axial sites are occupied by one of the coordinated water molecules (O2W) and the N-bonded NCS anion, whereas the -O-bridged carboxylate anions, azomethine nitrogen atom and a coordinated water molecule (O1W) occupy the equatorial positions. The two manganese atoms are connected via centrosymmetrically related -O-bridged carboxylate anions, forming a rhomboidal Mn 2 O 2 unit about an inversion centre.

Supramolecular features
The crystal structure of (I) contains both coordinated and solvated water molecules. Inter-and intra-molecular hydrogen-bonding interactions (   Molecular structure of the title complex (I), showing the atom-numbering scheme and displacement ellipsoids drawn at the 50% probability level. The molecule is located about an inversion centre and the unlabelled atoms are generated by the symmetry operation (Àx + 1, Ày, Àz + 1). molecules along the b-axis direction. These contacts combine to generate several ring motifs ( Fig. 3) viz. R 1 1 (6), R 2 3 (10) and R 4 4 (14) that stabilize the three-dimensional supramolecular network (Fig. 4).

Thermal properties
The thermal decomposition behaviour of the title complex was studied by simultaneous TG-DTG analyses recorded in a nitrogen atmosphere in the temperature range 30-800 C, as shown in Fig. 5. The TG curve displays the combined mass loss of 20.5% (calculated 21.8%) in the temperature range 30-140 C corresponding to dehydration of both the solvated and coordinated water molecules. The anhydrous compound then shows continuous decomposition between 140 and 600 C to give manganese sulfide as the end product (mass loss observed 73.6%, calculated 72.50%). The DTG curve shows a doublet (40 and 80 C) for dehydration and a multiplet (150, 164, 255 and 321 C) for the decomposition of the anhydrous compound in accordance with TG mass loss.

Database survey
There are a few structures of metal complexes in the crystallographic literature with simple hydrazones based on glyoxylic acid and salicyloyl hydrazine (Liu et al., 2010) and thiosemicarbazide (Dodoff et al., 2006;Huseynova et al., 2018). In the former salicyloyl hydrazone complex of cadmium, the Schiff base acts as a tetradentate (O,N,O,O) ligand with one of the carboxylate oxygen atoms bridging the cadmium centers, leading to a dimer, whereas in the thiosemicarbazone complexes of Zn, Pd, Pt, Co, and Ni (Milenković et al., 2013a(Milenković et al., ,b, 2014, View of hydrogen-bonding interactions (green lines) along the ac plane forming various ring motifs to further stabilize the three-dimensional network.

Figure 4
Overall packing view of the three-dimensional network for (I), viewed along the b axis, showing N-HÁ Á ÁO and O-HÁ Á ÁO hydrogen bonds and weak O-HÁ Á ÁS intermolecular interactions (green lines) and the stacking of (I) along the b axis.

Synthesis and crystallization
Elemental analyses for carbon, hydrogen and nitrogen were recorded using a Vario-ELIII elemental analyzer. The IR spectrum was recorded using a JASCO-4100 spectrophotometer and KBr pellets in the range of 4000-400.00 cm À1 . Simultaneous TG/DTG (TG/DTG) analyses were carried out using a TA instrument, SDT Q600 thermal analyzer, in a flowing nitrogen atmosphere with a heating rate of 10 C min À1 . Stoichiometric quantities of glyoxylic acid (0.184 g, 2 mmol), ethylcarbazate (0.208 g, 2 mmol) and ammonium thiocyanate (0.152 g, 2 mmol) were dissolved in 30 mL of double-distilled water. To this homogeneous solution, Mn(NO 3 ) 2 Á6H 2 O (0.287 g, 1 mmol) dissolved in 10.00 mL of double-distilled water was added dropwise, the pH of the resulting solution was noted as 3.45. The above clear solution was kept over a water-bath until the solution was reduced to ca 15 mL and allowed to stand at room temperature for slow crystallization. After two days, colourless rod-shaped crystals were obtained and filtered off, washed with ice-cold water and air dried. The product is soluble in water, methanol and ethanol and insoluble in diethyl ether. In the absence of ammonium thiocyanate, the reaction did not yield any desired product.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms attached to carbon atoms were positioned geometrically and constrained to ride on their parent atoms, with carbon-hydrogen bond lengths of 0.95 Å for alkene C-H and 0.98 Å for CH 3 groups, respectively. Methyl H atoms were allowed to rotate but not to tip to best fit the experimental electron density. U iso (H) values were set to a multiple of U eq (C) with 1.5 for CH 3 and 1.2 for C-H groups, respectively. Positions and U iso values of water and amine H atoms were freely refined.  Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.