Crystal structures of coordination polymers from CaI2 and proline

Reactions of l- and dl-proline with CaI2 yields two crystalline products. The zwitterionic proline bridges the Ca cations with its carboxylate group in different coordination modes to form one-dimensional coordination polymers.


Chemical context
The large field of crystal engineering benefits from the growing amount of structural data obtained by single-crystal diffraction. Amino acids are the building blocks of proteins and important molecules for various applications in chemistry and life sciences. Their metal complexes have, however, been investigated less often than their availability suggests. Many of these studies address the amino acids in their deprotonated form in which it mostly acts as a N,O chelating ligand. (e.g. Ito et al., 1971;Kato et al., 2008;Magill et al., 1993;Marandi & Shahbakhsh, 2007;Mathieson & Welsh, 1952;Mikhalyova et al., 2010;Oki & Yoneda, 1981). In contrast, the zwitterionic overall neutral amino acids show more analogy to carboxylates; for these, a large variety of coordination modes has been established (Batten et al., 2008). While the protonated amino group is no longer nucleophilic, it may act as a hydrogen-bond donor. The pattern formed by these interactions also depends on the chirality of the enantiopure or racemic amino acid. When both carboxylate coordination and intermolecular hydrogen bonds are taken into account, a large number of potentially competitive structures arises and subtle changes in the coordination chemistry may determine which product will be obtained. An overview of the crystal chemistry of amino acids has been published by Fleck & Petrosyan (2014). We here complete our reports concerning the reaction products from calcium halides and the amino acid proline. In this context, we encountered coordination polymers, isoreticular ISSN 2056-9890 coordination networks, and polymorphism (Lamberts et al., 2014b;Lamberts et al., 2015). The two structures reported here are coordination polymers obtained from calcium iodide and proline: the scheme shows that compounds (1) and (2) form from enantiopure l-proline and racemic proline, respectively.

Structural commentary
Compound (1) crystallizes in the chiral orthorhombic space group P2 1 2 1 2 1 with two calcium cations, five proline ligands, one coordinating water ligand, 1.7 non-coordinating water molecules and four iodide anions in the asymmetric unit; all constituents are necessarily located in general positions (Fig. 1).
The five independent proline molecules show three different coordination modes; in the following discussion, they are labelled according to their N atom. Proline 1 acts as a chelating ligand towards Ca1 and simultaneously as a bridge to Ca2 in a 2 -2 : 1 configuration. An analogous situation is found for proline 4, chelating Ca2 and bridging towards Ca1 iii [(iii) = Àx + 1, y + 1 2 , Àz + 1 2 ]. Proline 3 connects three Ca positions in a 3 -2 : 2 coordination mode. The remaining proline ligands (2 and 5) do not chelate but only bridge two cations in a syn-syn configuration. Herein, proline 2 shows a more symmetric coordination, being located approximately in the middle of Ca1 and Ca2, whereas proline 5 is strongly dislocated towards Ca1.
In view of the strongly ionic nature of an interaction between a carboxylate and a calcium dication, the 3.040 (5) Å distance between Ca1 and O9 i [(i) = Àx + 1, y À 1 2 , Àz + 1 2 ] represents an additional, energetically favourable contact which, however, is much longer than a classical coordinative bond and does not affect the topology of the compound.
We mentioned in our earlier direct comparison between coordination polymers based on Ca 2+ and Mn 2+ (Lamberts et al., 2014a) that the absence of crystal field effects is reflected in variable and often less regular coordination spheres about the alkaline earth cation. The two cations in (1)  The asymmetric unit of (1). Displacement ellipsoid are shown at the 80% probability level.

Figure 2
The polymeric chain of (1). H atoms and C atoms of the proline ring have been omitted for clarity. are provided by two oxygen atoms of the chelating part of proline 1, and five single oxygen atoms from different bridging proline molecules. Ca2 is coordinated by two chelating carboxylato groups. Only three additional CaÁ Á ÁO contacts are formed from neighbouring, bridging proline ligands, whereas the remaining coordination partner is the coordinating water molecule. Each Ca 2+ cation is coordinated by the independent syn-syn bridging proline ligands 2 and 5; they are arranged on opposite sides around Ca1 and next to each other around Ca2.
Overall, a one dimensional coordination polymer is formed (Fig. 2). The chain extends along b; its projection on the bc plane is a sinusoidal curve, with alternating Ca1 and Ca2 positions. Each chain segment is triple bridged with two very similar independent CaÁ Á ÁCa separations of 3.814 (2) and 3.832 (2) Å . The 3 -2 : 2 proline 3 coordinates within the sinusoidal plane in the concave parts, while proline 1 and the aqua ligand coordinate on the convex side. Selected distances are compiled in Table 1.
The iodide I4 shows positional disorder over two mutually exclusive sites, and three proline molecules exhibit slight disorder of carbon atoms of the five-membered proline envelopes.
Coordination polymer (2) forms under similar conditions as (1) but from racemic proline. The compound crystallizes in space group P1 with one Ca II cation, two proline ligands and two water ligands and two non-coordinating iodide anions in the asymmetric unit, all in general positions (Fig. 3).
One proline molecule chelates the calcium cation with its carboxylato group and additionally bridges towards a second calcium of the polymer chain ( 2 -2 : 1 ). The other proline molecule only bridges two adjacent calcium atoms in a synanti conformation ( 2 -1 : 1 ).
Together with the two aqua ligands, this results in a sevenfold coordination of the Ca 2+ cation. Since the inversion centres lie in between the calcium atoms, two different chain connections are obtained: one is built by two simultaneously bridging and chelating proline ligands [CaÁ Á ÁCa = 4.032 (4) Å ], the other one by two syn-anti bridging proline ligands [CaÁ Á ÁCa = 4.829 (4) Å , parallelogram-shaped motif]. Overall, a zigzag-shaped polymer chain is formed which extends along the shortest unit-cell axis a (Fig. 4). Selected distances are given in Table 2.

Supramolecular features
Since most hydrogen atoms in (1) have been constrained to calculated positions, their relevance should not be overestimated. The following points should, however, be mentioned: all hydrogen-bond donors find suitable acceptors. Most hydrogen bonds involve iodide and hence occur between different residues. However, only a few hydrogen bonds actually connect two neighbouring chains, resulting in an overall three-dimensional network (Fig. 5) Symmetry code: (i) Àx þ 1; y À 1 2 ; Àz þ 1 2 .

Figure 4
The polymeric chain of (2). H atoms have been omitted for clarity.
one of the five proline molecules contributes to an N-HÁ Á ÁO hydrogen bond along the chain [N3-H3AÁ Á ÁO2 iii ; (iii) = Àx + 1, y + 1 2 , Àz + 1 2 ]. Each of the two independent aqua ligands in (2) donates hydrogen bonds towards two iodides. The amino group associated with N2 on the one hand also forms a hydrogen bond towards iodide, on the other hand directly connects two neighbouring chains by finding a coordinating water molecule as acceptor. N1 also interacts with an iodide counter-anion. This second NH donor can, however, not be unambiguously assigned to a hydrogen-bond acceptor: Two iodide anions are situated in its vicinity and may be regarded as acceptors for a bifurcated hydrogen bond with HÁ Á ÁI distances of 3.24 (5) and 3.33 (8) Å . Overall, a two-dimensional framework is formed in the ab plane (Fig. 5). A complete overview of hydrogen-bond geometries is given in Tables 3 and 4.

Database survey
Database searches (Groom & Allen, 2014) were performed using the Cambridge Crystallographic Database (CSD, Version 5.36, including updates until November 2014). All searches were restricted to error-free entries for which 3D coordinates were available. A search for structures containing calcium and proline or derivatives in any protonation state comes up with eight hits. Six of them correspond to the aforementioned structures published by our group (Lamberts et al., 2014a(Lamberts et al., ,b, 2015. These are coordination polymers and networks based on calcium chloride and bromide with both l-proline and dl-proline. The other two structures are a molecular complex with deprotonated N,O-chelating hydroxyproline (Kim et al., 1985), and a coordination network of calcium pyroglutamate (Schmidbaur et al., 1991).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. Non-hydrogen atoms were refined with anisotropic displacement parameters where possible. H atoms connected to carbon were placed in idealized positions and treated as riding, with U iso (H) = 1.2U eq (C). In (1), significant residual density maxima indicated disorder. An alternative position for I4 was assigned and refined with an isotropic displacement parameter to a refined occupancy of 0.134 (7) (total occupancy of I4 over both positions constrained to 1). Atoms C4 and C5, C18 and C19, and C14 were also refined as split over two positions. They were given a common isotropic displacement parameter and their occupancy was refined. The occupancy of the alternative positions refined to 0.519 (12) for C4 and C5, 0.218 (12) for C18 and C19, and 0.270 (12) for C14; the occupancy sum of the alternative sites for each atom was constrained to unity. Carbon atoms connected to disordered neighbours were given two alternative geometries of calculated hydrogen positions. The occupancy of the non-coordinating water molecule associated with O13 refined to 0.707 (17); tentative refinement with full occupancy resulted in an unusually large displacement parameter. Given the limited data quality, H atoms connected to nitrogen atoms were not refined but treated as riding in idealized positions, with N-H = 0.99 Å and U iso (H) = 1.2U eq (N). The hydrogen atoms of the three water molecules were modelled as oriented towards the closest acceptor and restrained to O-H distances of 0.84 Å . Further distance restraints were applied to ensure stable refinement of a reasonable hydrogen-bond geometry.
In (2), no disorder was encountered. Hydrogen atoms attached to non-carbon atoms were located in a difference Fourier map and treated as riding, with U iso (H) = 1.2U eq (non-H). N-H distances were refined with similarity restraints whereas O-H distances were restrained to 0.84 Å . H3W was assigned a distance restraint towards a neighbouring I1 anion to ensure suitable hydrogen-bond geometry. Reflection 011 was omitted from the final refinement because it was obstructed by the beamstop.