First Anionic 1,10-Phenanthroline-2,9-dicarboxylate Containing Metal Complex Obtained from a Novel 1:1 Proton−Transfer Compound: Synthesis, Characterization, Crystal Structure, and Solution Studies

The new 1,10-phenanthroline containing 1:1 proton−transfer compound LH2, [pyda·H2]2+[phendc]2-, was synthesized
from the reaction of 2,6-pyridinediamine, pyda, and 1,10-phenanthroline-2,9-dicarboxylic acid, phendc·H2, and
characterized by elemental analysis, ES−Ms, IR, 1H, 13C NMR, and UV/vis spectroscopies. Subsequently, the first
example of [phendc]2- containing anionic complex [pyda·H]2[Co(phendc)2]·10H2O, was prepared, using the above
novel proton-transfer compound, and structurally characterized by single-crystal X-ray diffraction. The complex
crystallizes in the space group P21/n of the monoclinic system with four molecules in a unit cell of dimensions a
= 11.877(3) Å, b = 31.473(9) Å, c = 12.915(4) Å, and β = 116.223(5)°. The structure has been refined to a final
value for the crystallographic R factor of 0.0524 based on 9021 observed independent reflections. The complexation
reactions of pyda, phendc·H2, and LH2 with H+ as well as LH2 with Co(II) in aqueous solution were investigated
by potentiometric pH titrations, and the equilibrium constants for all major complexes formed are described. The
results are presented in the form of distribution diagrams revealing the concentrations of individual complex species
as a function of pH. The results revealed that, at a pH range of 5.2−6.2, the major complex species is [(pyda·H)]2[Co(phendc)2], similar to the isolated crystalline complex.