Crystal structure of a polymeric calcium levulinate dihydrate: catena-poly[[diaquacalcium]-bis(μ2-4-oxobutanoato)]

The crystal structure of calcium levulinate dihydrate forms a one-dimensional coordination polymer based on a CaO8 complex unit which lies on a twofold rotation axis. This unit comprises two monodentate water O-atom donors and six carboxylate O-atom donors, two of which are also bridging, from the two bidentate chelate levulinate ligands. The complex chains are stabilized by intra- and intermolecular water O—H⋯O hydrogen bonds, forming an overall three-dimensional structure.

In the title calcium levulinate complex, [Ca(C 5 H 7 O 3 ) 2 (H 2 O) 2 ] n , the Ca 2+ ion lies on a twofold rotation axis and is octacoordinated by two aqua ligands and six O atoms from four symmetry-related carboxylate ligands, giving a distorted square-antiprismatic coordination stereochemistry [Ca-O bond-length range = 2.355 (1)-2.599 (1) Å ]. The levulinate ligands act both in a bidentate carboxyl O,O 0 -chelate mode and in a bridging mode through one carboxyl O atom with an inversion-related Ca 2+ atom, giving a CaÁ Á ÁCa separation of 4.0326 (7) Å . A coordination polymeric chain structure is generated, extending along the c-axial direction. The coordinating water molecules act as double donors and participate in intra-chain O-HÁ Á ÁO hydrogen bonds with carboxyl O atoms, and in inter-chain O-HÁ Á ÁO hydrogen bonds with carbonyl O atoms, thus forming an overall three-dimensional structure.

Chemical context
Levulinic acid (4-oxopentanoic acid) is a biomass-derived keto acid and is a potential precursor for renewable fuels as well as polymeric materials (Mukherjee et al., 2015). A number of metal salts of levulinic acid have been prepared for a variety of applications and the calcium salt with formula Ca(C 5 H 7 O 3 ) 2 Á2H 2 O is the most widely studied levulinate, as it has been used for over 80 years as a calcium supplement (Proskouriakoff, 1933). The revived interest in calcium levulinate is due to a recent discovery that pyrolysis of this readily accessible renewable biomass-based calcium salt can be used to produce biofuels via a ketonic decarboxylation process with recycling of calcium as CaCO 3 (Schwartz et al., 2010;Case et al., 2012). In addition, we have recently shown that acidcatalyzed hydrothermal degradation of cellulose and neutralization of the filtrate with calcium hydroxide can be used to prepare a mixture of calcium levulinate and calcium formate and the pyrolysis of this mixture at 623 K can be used to produce -valerolactone (Amarasekara et al., 2015). Recently, Bryce and co-workers published the solid-state 13 C NMR spectrum of calcium levulinate in which they identified only one type of a levulinate anion (Widdifield et al., 2014). However, there are no reports on X-ray crystallographic studies on this well known calcium carboxylate. Our interest in thermal properties and biofuel applications of calcium levulinate has led us to study the structure of this salt and in this communication we report the crystal structure of calcium levulinate dihydrate, [Ca(C 5 H 7 O 3 ) 2 (H 2 O) 2 ] n .

Structural commentary
The calcium levulinate structure contains one Ca 2+ cation, two levulinate anions and two water molecules per formula unit, with the Ca 2+ cation situated on a twofold rotation axis (Fig. 1). The cation is octacoordinated and exhibits a distorted square antiprismatic stereochemistry with Ca-O bond lengths in the range of 2.355 (1)-2.599 (1) Å (Table 1). The levulinate carboxyl O atoms (O1 and O2) coordinate to Ca 2+ cations in two coordination modes, a bidentate O,O 0 -chelate mode and a bridging mode through O1 i with an inversion-related Ca 2+ centre, giving a Ca1Á Á ÁCa1 i or Ca1Á Á ÁCa1 v separation of 4.0326 (7) Å [for symmetry code (i) see Table 1; symmetry code (v): Àx + 1, Ày, Àz]. Furthermore, due to this type of coordination environment, the two levulinate anions are almost perpendicular to each other, with an O2-Ca1-O2 iii angle = 75.78 (5) [for code (iii), see Table 1]. The extended one-dimensional coordination polymeric chain generated lies parallel to the c axis ( Fig. 2) and within each chain, the coordinating water molecules form intra-chain O4-H4BÁ Á ÁO2 v carboxyl hydrogen-bonds (Table 2).

Database survey
The Cu 2+ levulinate structures represent examples of a very small number of metal levulinates in the crystallographic literature (Zubkowski et al., 1997). Only one of these involves the levulinate ligand alone: a polymeric structure formed through carboxyl O-linked tetracarboxylate-bridged dimers, in which the copper atoms have nearly square-pyramidal coordination geometry. In the same report are the structures of three additional Cu 2+ complexes with levulinate as well as other ligands: pyridine, 2,2 0 -bipyridine and triphenylphosphine. The crystal structures of two polymorphic forms of the analogous calcium acetate monohydrate salt are also known (Klop et al., 1984;Van der Sluis et al., 1987).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The C-bound H atoms were placed in calculated positions and allowed to ride on their carrier atoms: C-H = 0.93-0.97 Å with U iso (H) = 1.5U eq (C) for methyl H atoms and 1.2U eq (C) for other H atoms. The water H atoms were found using a Fourier map and were also allowed to ride in the refinement, O-H = 0.90 Å and with U iso (H) = 1.5U eq (O).

Figure 3
The three-dimensional hydrogen-bonded structure in the unit cell viewed along the c axis. Hydrogen-bonding interactions are shown as dashed lines.

catena-Poly[[diaquacalcium]-bis(µ 2 -4-oxobutanoato)]
Crystal data where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.53 e Å −3 Δρ min = −0.54 e Å −3 Special details Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s 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 > σ(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.