Mononuclear Ca(II)−Bulky Aryl−Phosphate Monoanion and Dianion Complexes with Ortho-Amide Groups

Two new mononuclear Ca(II) complexes with aryl dihydrogen phosphate ligands having two strategically oriented
bulky amide groups, 2,6-(Ph3CCONH)2C6H3OPO3H2 (1), including one with a phosphate monoanion, (NMe4)[CaII{O2P(OH)OC6H3-2,6-(NHCOCPh3)2}3(N⋮CMe)3] (3), and one with a phosphate dianion, [CaII{O3POC6H3-2,6-(NHCOCPh3)2}(H2O)3(MeOH)2] (4). Both are analogues for the NH···O hydrogen bonds in the active site of Ca(II)-containing phosphotransferase. Crystallographic studies of these Ca(II) complexes revealed that the amide NHs
are directed to uncoordinated O atoms of the phosphates, and the IR and 1H NMR spectra indicate that strong
NH···O hydrogen bonds are formed only in the phosphate dianion state. The ligand exchange reaction of 3 with
a non-hydrogen-bonded phosphate ligand shows that the NH···O hydrogen bonds prevent the Ca−O bond from
dissociation. A scatter plot analysis comparing the distance of a Ca−O bond with the Ca−O−P angle, the Fourier
density analysis, and DFT calculations reveal that a partial degree of covalency in the Ca−O(phosphate) bonds is
present.