Crystal structures of coordination polymers from CaI2 and proline

Area of Science:

• Coordination Chemistry
• Materials Science
• Crystallography

Background:

• Calcium halides are known to react with amino acids to form coordination compounds.
• Proline, a cyclic amino acid, presents unique structural possibilities in coordination chemistry.
• Understanding the influence of stereochemistry (enantiopure vs. racemic) on solid-state structures is crucial.

Purpose of the Study:

• To investigate the reaction products of calcium iodide (CaI2) with both enantiopure l-proline and racemic dl-proline.
• To characterize the crystal structures and coordination behavior of the resulting solid-state materials.
• To explore the diversity in carboxylate coordination geometry within these calcium-proline complexes.

Main Methods:

• Synthesis of solid products from reactions of CaI2 with l-proline and dl-proline.
• Single-crystal X-ray diffraction analysis to determine the crystallographic structures of the obtained compounds.
• Characterization of coordination environments around calcium cations and the role of proline molecules.

Main Results:

• Reaction with l-proline yielded a 1D coordination polymer, catena-poly[[aqua-μ3-l-proline-tetra-μ2-l-proline-dicalcium] tetra-iodide 1.7-hydrate] ({[Ca2(C5H9NO2)5(H2O)]I4·1.7H2O}n), featuring two independent Ca(2+) cations with seven- and eightfold coordination.
• Reaction with dl-proline produced a different 1D polymer, catena-poly[[di-aquadi-μ2-dl-proline-calcium] diiodide] ({[Ca(C5H9NO2)2(H2O)2]I2}n), with a centrosymmetric structure built around a single Ca(2+) cation.
• In both structures, iodide ions remained non-coordinating, and extensive hydrogen bonding was observed involving water molecules and proline functional groups.

Conclusions:

• The stereochemistry of proline significantly influences the resulting coordination polymer structure when reacted with CaI2.
• The study reveals diverse coordination modes for calcium ions and varied carboxylate coordination geometries, adding to the known structures of calcium halide-amino acid complexes.
• The findings underscore the complexity and potential for structural diversity in metal-organic materials derived from simple building blocks.