Crystal structure and spectroscopic analysis of a new oxalate-bridged MnII compound: catena-poly[guanidinium [[aquachloridomanganese(II)]-μ2-oxalato-κ4 O 1,O 2:O 1′,O 2′] monohydrate]

A novel oxalate-bridged manganese +II compound, catena-poly[guanidinium [[aquachloridomanganese(II)]-μ2-oxalato-κ4 O 1,O 2:O 1′,O 2′] monohydrate], has been synthesized as single crystals at room temperature and characterized by X-ray diffraction, infrared and UV–Visible spectroscopic analyses, confirming the formation of a layered-type three-dimensional structure.

As part of our studies on the synthesis and the characterization of oxalatebridged compounds M-ox-M (ox = oxalate dianion and M = transition metal ion), we report the crystal structure of a new oxalate-bridged Mn II phase, {(CH 6 N 3 )[Mn(C 2 O 4 )Cl(H 2 O)]ÁH 2 O} n . In the compound, a succession of Mn II ions (situated on inversion centers) adopting a distorted octahedral coordination and bridged by oxalate ligands forms parallel zigzag chains running along the c axis. These chains are interconnected through O-HÁ Á ÁO hydrogenbonding interactions to form anionic layers parallel to (010). Individual layers are held together via strong hydrogen bonds involving the guanidinium cations (N-HÁ Á ÁO and N-HÁ Á ÁCl) and the disordered non-coordinating water molecule (O-HÁ Á ÁO and O-HÁ Á ÁCl), as well as by guanidiniumstacking. The structural data were confirmed by IR and UV-Visible spectroscopic analysis.

Chemical context
Much attention had been devoted to the coordination chemistry of oxalate (ox) anions due to the interesting structural features and physical properties they possess (Ché rif et al., 2011;Dridi et al., 2013;Decurtins et al., 1997). Oxalate anions have been demonstrated to be one of the most versatile bridging ligands for the construction of coordination polymers when combined with transition metal cations. Manganese(II) is a promising cation with possibilities of forming onedimensional oxalato-based coordination polymers, as evidenced by reports describing the structures of several topologically similar Mn II -ox-Mn II chains [see, for example, García-Couceiro et al. (2005) or Beznischenko et al. (2009)]. In those compounds, the oxalate-bridged manganese framework may be considered as a single-chain magnet based on the oxalate linker (e.g. Clemente-Leó n et al., 2011). In this work, we report the synthesis and crystal structure determination of a new oxalate-bridged coordination compound, {(CH 6 N 3 )[Mn(C 2 O 4 )Cl(H 2 O)]ÁH 2 O} n (I).

Structural commentary
The principal structural motifs of the title compound are the complex anion [MnCl(C 2 O 4 )(H 2 O)] À , the organic cation (CH 6 N 3 ) + and one disordered non-coordinating water molecule. A bond-valence-sum calculation, assuming Mn-O and Mn-Cl bonds, gives a BVS value (Brown & Altermatt, 1985) of 2.05 (7), confirming the +II oxidation state of Mn and ensuring electrical neutrality of the formed unit. The coordination environment of the Mn II ion involves two oxalate ligands exhibiting bis-chelating coordination modes, one chloride atom and one oxygen atom of the aqua ligand ( Fig. 1 (2) and ID a = 0.22 (4)%, respectively (Baur, 1974;Wildner, 1992).
The equatorial plane of the MnO 5 Cl octahedron is formed by atoms Mn1, OW1, O1, O2 and O3, with a calculated rootmean-square deviation of the fitted atoms of 0.1038 Å . The axial positions are occupied by the chloride atom [Mn1-Cl1 = 2.458 (2) Å ] and one oxygen atom from the bridging oxalato group [Mn1-O4 = 2.248 (2) Å ]. The two oxalato groups are almost perpendicular with a dihedral angle of 89.09 (6) . The oxalate ion is located on an inversion center that also relates the two Mn atoms bonded to the oxalate ion with each other. The bridged metal ions are nearly coplanar with the oxalate plane with a mean deviation of 0.0147 (8) Å .
The Mn II ion, as a d 5 high-spin system with a spherical electron distribution, has a limited number of commonly observed coordination geometries that are based on minimization of ligand-ligand repulsion. Among the Mn-O distances, the shortest are those involving an oxygen atom from the oxalate ion trans to another oxygen atom from the second oxalate ion. The range of these distances is 2.180 (1) to 2.194 (1) Å , which is in accord with those observed in other oxalate-bridged compounds such as one of the poly- et al., 2007). The Mn-O distances involving the oxygen atoms of the oxalate ion trans to the coordinating water molecule and trans to the chloride atom are slightly longer at 2.202 (2) and 2.248 (2) Å .

Figure 2
View of the structure packing showing Mn-Ox-Mn chains (highlighted by a ball-and-stick model) and layers parallel to (010) (blue planes). and aqua ligands in the coordination environment of the Mn II ion.
The geometric parameters for the guanidinium cations do not show any unusual features and are in agreement with those previously reported (Sakai et al. 2003;Vaidhyanathan et al., 2001). The bond lengths [1.318 (2)-1.329 (2) Å ] and angles [119.27 (16)-120.57 (16) ] are in the typical ranges, confirming a highly resonance-stabilized electronic structure and a completely delocalized charge between the three sp 2 nitrogen atoms. Conjugation of the nitrogen lone pairs with the empty p-orbital of the sp 2 carbon atom creates a planar cation.

Supramolecular features
Neighbouring oxalate-bridged zigzag chains are connected with each other via O-HÁ Á ÁO hydrogen bonds involving the coordinating water molecule. Its oxygen atom acts as a hydrogen-bond donor and establishes strong hydrogen bonds ( Table 1) towards one of the oxalate oxygen atoms of a neighbouring chain (Fig. 3), OW1-HW2Á Á ÁO3 v [symmetry code: (v) Àx + 2, Ày + 1, Àz], leading to the formation of anionic layers parallel to (010). A disordered non-coordinating water molecule acts as acceptor (Fig. 3) for the other hydrogen atom involving the coordinating water molecule via the hydrogen bonds OW1-HW1Á Á ÁOW2 i and OW1-HW1Á Á ÁOW2B i [symmetry code: (i) x, y À 1, z]. Both disorder components of the non-coordinating water molecules act as hydrogen-bond donors towards oxygen atom O3 (Fig. 3) via the hydrogen bonds OW2-HW3Á Á ÁO3 and OW2B-HW5Á Á ÁO3, but they form different hydrogen bonds via their second H atom, to chlorine atoms in different lattice positions via hydrogen bonds OW2-HW4Á Á ÁCl1 vi and OW2B--HW6Á Á ÁCl1 [symmetry code: (vi) Àx + 2, Ày + 2, Àz]. The combined water hydrogen bonds link the anionic layers into a 3D framework.

Figure 3
View of the hydrogen bonds developed by both coordinating (blue dashed lines) and non-coordinating (green dashed lines) water molecules.

IR and UV-Vis characterizations
The IR spectrum was recorded in the 4000-400 cm À1 region using a Perkin-Elmer spectrometer with the sample diluted in a pressed KBr pellet. The most intense IR absorption bands of (I) are given in Table 2. The spectrum (Fig. 6) displays broad and strong bands centered at 3390 and 3182 cm À1 assigned to [(O-H) + as (NH 2 )] and s (NH 2 ), respectively (Sasikala et al., 2015). The broadness of these bands is indicative of the presence of both coordinating and non-coordinating water molecules, as well as -NH 2 groups involved in an extensive hydrogen-bond framework, in agreement with the crystal structure. A weak band observed at 2352 cm À1 is attributed to an N-HÁ Á ÁO stretching mode. The characteristic vibrations of the bridging oxalato ligand are observed at 1657 cm À1 Some crystals, selected under the microscope, were dissolved in 10 cm 3 of distilled water. The solution obtained was analyzed using a UV-Visible spectrometer. The spectrum of (I) ( Table 3 and Fig. 7) shows significant transitions at 206 nm (with a shoulder at 240 nm) and 329 nm. The first band is due to the !* transition of the guanidinium system (Hoffmann et al., 2009), the second witnesses the metal-toligand charge-transfer (Sun et al., 1996)     n!* Figure 6 The IR spectrum of (I) in KBr.

Figure 7
The UV-Vis spectrum of (I) in water. The insert is an expansion of the visible region. too weak to be seen, as they are spin and Laporte forbidden, in accordance with the compound being almost colourless.

Synthesis and crystallization
Aqueous solutions of ammonium oxalate and guanidine hydrochloride were added to Mn(SO 4 )ÁH 2 O dissolved in 10 cm 3 of water in a 1:2:1 molar ratio. The resulting solution was left at room temperature and colourless crystals suitable for X-ray diffraction were obtained after two weeks of slow evaporation.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. Guanidinium hydrogen atoms were positioned geometrically as riding atoms (N-H = 0.86 Å ) using adequate HFIX instructions and refined with AFIX instructions. Hydrogen atoms of the coordinating water molecule were found in Fourier difference maps. O-H distances were restrained to a value of 0.85 (1) Å and HÁ Á ÁH distances were restrained to a value of 1.387 (1) Å .
The oxygen atom of the non-coordinating water molecule had unusually high displacement parameters, and was refined as disordered over two alternative mutually exclusive positions. The solvent molecule may be considered as being located vertically between negative-charged anionic layers formed by hydrogen-bonded polymeric chains and located horizontally between positive-charged pairs of guanidinium cations. This pseudo-channel affects its hydrogen-bonding interactions, see the discussion in the first paragraph of the Supramolecular features section and Fig. 3, which may explain the observed disorder.
The disordered oxygen atom was refined as disordered over two positions OW2 and OW2B which were restrained to have similar geometries. Their hydrogen atoms were located from the Fourier difference maps. The O-H bond lengths were restrained to a value of 0.85 (1) Å and the HÁ Á ÁH distances were restrained to a value of 1.387 (1) Å . The interatomic distances between the two pairs OW2 and HW5 and OW2B and HW3 were restrained to be equal using a SADI instruction with an effective standard deviation of 0.02. The hydrogen-bonding distance of hydrogen atom HW6 to chlorine atom Cl1 was restrained to 2.80 (1) Å . Subject to these and the above conditions, the occupancy ratio of the disordered non-coordinating water molecule refined to 0.816 (13)  Computer programs: CAD-4 EXPRESS (Duisenberg, 1992), XCAD4 (Harms & Wocadlo, 1995), SHELXS97 (Sheldrick, 2008), SHELXL2014 (Sheldrick, 2015), DIAMOND (Brandenburg, 2006), WinGX (Farrugia, 2012) and publCIF (Westrip, 2010).

Computing details
Data collection: CAD-4 EXPRESS (Duisenberg, 1992); cell refinement: CAD-4 EXPRESS (Duisenberg, 1992); data reduction: XCAD4 (Harms & Wocadlo, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: WinGX (Farrugia, 2012) and publCIF (Westrip, 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 0.39 e Å −3 Δρ min = −0.30 e Å −3 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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ. (