Calcium complexation in aqueous solution: An x-ray and ab initio approach

Understanding the aqueous chemistry of cations in solution is central to modeling ion transport, complexation, and reactivity. Calcium, for example, plays a critical role in biological and environmental systems; however, a fundamental understanding of its coordination in dilute, aqueous solution is lacking. There is still debate regarding the number of water molecules in its first solvation shell. Moreover, studies of Ca2+ coordination in aqueous complexes with ligands other than water are rare. Herein, we apply Ca K-edge x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structure (EXAFS) spectroscopy combined with ab initio molecular dynamics and time-dependent density functional theory to investigate the coordination environment of aqueous Ca2+. On the one hand, EXAFS spectroscopy is sensitive to bond distances; however, the determination of coordination numbers can be imprecise. XANES spectroscopy, on the other hand, is sensitive to molecular symmetry and therefore to coordination number. We first confirmed that calcium is coordinated, on average, by seven water molecules in dilute aqueous solution. We then extended this approach to examine the coordination environment of aqueous Ca2+ ethylenediaminetetraacetic acid (EDTA) complexes. Notably, a water molecule was present in the first coordination shell of Ca2+, in addition to four O atoms and two N atoms from EDTA4-, such that Ca2+ achieved sevenfold coordination. We conclude that Ca2+ tends to adopt low-symmetry, 7-coordinate complexes in aqueous solution, even in the presence of a hexadentate chelator. Our work lays the groundwork needed to understand Ca coordination in numerous biological and environmental systems.