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(NCMe)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.