Metal Ion Complexes of N,N′‑Bis(2-Pyridylmethyl)-1,3-Diaminopropane-N,N′‑Diacetic Acid, H2bppd

A higher yield synthesis of N,N′-bis­(2-pyridylmethyl)-1,3-diaminopropane-N,N′-diacetic acid (H2bppd) and its complexation of trivalent metal ions (Al­(III), Ga­(III), In­(III)) and selected lanthanides (Ln­(III)) are reported. H2bppd and the metal–bppd2– complexes, isolated as hexafluorophosphate salts, were characterized by elemental analysis, mass spectrometry, IR, and 1H and 13C NMR spectroscopy. [Ga­(bppd)]­PF6, [Ga­(C19H22N4O4)]­PF6, was crystallized as colorless needles by slow evaporation from anhydrous methanol; its molecular structure was solved by direct X-ray crystallography methods. The compound crystallized in the monoclinic space group P21/c, with a = 9.6134(2) Å, b = 20.2505(4) Å, c = 11.6483(3) Å, β = 97.520(1)o, and Z = 4. Ga is coordinated in a distorted octahedral geometry provided by a N4O2 donor atom set with cis-monodentate acetate groups and cis-2-pyridylmethyl N atoms. Quantum mechanical calculations were performed for the three possible geometric isomers of a pseudo-octahedral metal–bppd2– complex with five different metal ions. The results indicate, that in aqueous solution, the stability of the trans-O,O isomer is similar to that of the cis-O,O; cis-Npy,Npy isomer but is greater than that of the trans-Npy,Npy isomer. Calculations for a six-coordinate La­(III)-bppd2– complex converge to a structure with a very large Npy–La–Npy bond angle (146.4°), a high metal charge (2.28 au), and a high solvation free energy (−79.4 kcal/mol). The most stable geometric arrangement for bppd2– around the larger La­(III) is best described as an open nestlike structure with space available for additional ligands. IR spectroscopy was used to investigate the nature of the H2bppd–metal complexes isolated in the solid state and the binding modes of the carboxylate functionalities. The spectra indicate that fully deprotonated [M­(bppd)]+ complexes as well as partially protonated complexes [M­(Hbppd)­Cl]+ were isolated. The 1H and 13C assignments for H2bppd and metal–bppd2– complexes were made on the basis of 2D COSY, NOESY, and 1H–13C HSQC experiments, which were used to differentiate among the cis (C1 symmetry) and the two trans (C2 symmetry) isomers.