Synthesis and Characterization of Crystalline Structures Based on Phenylboronate Ligands Bound to Alkaline Earth Cations

We describe the preparation of the first crystalline compounds based on arylboronate ligands PhB(OH)<sub>3</sub><sup>–</sup> coordinated to metal cations: [Ca(PhB(OH)<sub>3</sub>)<sub>2</sub>], [Sr(PhB(OH)<sub>3</sub>)<sub>2</sub>]·H<sub>2</sub>O, and [Ba(PhB(OH)<sub>3</sub>)<sub>2</sub>]. The calcium and strontium structures were solved using powder and single-crystal X-ray diffraction, respectively. In both cases, the structures are composed of chains of cations connected through phenylboronate ligands, which interact one with each other to form a 2D lamellar structure. The temperature and pH conditions necessary for the formation of phase-pure compounds were investigated: changes in temperature were found to mainly affect the morphology of the crystallites, whereas strong variations in pH were found to affect the formation of pure phases. All three compounds were characterized using a wide range of analytical techniques (TGA, IR, Raman, XRD, and high resolution <sup>1</sup>H, <sup>11</sup>B, and <sup>13</sup>C solid-state NMR), and the different coordination modes of phenylboronate ligands were analyzed. Two different kinds of hydroxyl groups were identified in the structures: those involved in hydrogen bonds, and those that are effectively “free” and not involved in hydrogen bonds of any significant strength. To position precisely the OH protons within the structures, an NMR-crystallography approach was used: the comparison of experimental and calculated NMR parameters (determined using the Gauge Including Projector Augmented Wave method, GIPAW) allowed the most accurate positions to be identified. In the case of the calcium compound, it was found that it is the <sup>43</sup>Ca NMR data that are critical to help identify the best model of the structure.