Metal Ion Complexes of N,N′-Bis(2-Pyridylmethyl)-trans-1,2-Diaminocyclohexane-N,N′-Diacetic Acid, H2bpcd: Cis/Trans Isomerization Equilibria

The synthesis of N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N'-diacetic acid (H(2)bpcd) and its complexation of Ga(III) and Co(III) are reported. H(2)bpcd and the metal-bpcd(2-) complexes, isolated as hexafluorophosphate salts, were characterized by elemental analysis, X-ray crystallography, IR spectroscopy, and H-1 and C-13 NMR spectroscopy. [Ga(bpcd)]PF6, [Ga(C22H26N4O4)]PF6, crystallized in the orthorhombic space group Ibca, with a = 13.8975(7) angstrom, b = 15.0872(7) angstrom, c = 22.2418(10) angstrom, and Z = 8. Ga is coordinated in a distorted octahedral geometry provided by a N4O2 donor atom set with trans-monodentate acetate groups and cis-2-pyridylmethyl N atoms, i.e., the trans-O,O isomer. The diamagnetic [Co(bpcd)]PF6, [Co(C22H26N4O4)]PF6, also crystallized from solution in the Ibca space group as the trans-O,O isomer. The H-1 and C-13 assignments for H(2)bpcd and metalbpcd(2) complexes were made on the basis of 2D COSY and HSQC experiments, which were used to differentiate among three possible isomers, i.e., one cis (C-1 symmetry) and two trans (C2 symmetry). NMR results indicate that the [Ga(bpcd)](+), [Co(bpcd)](+), and cis-O,O, cis-N-py,Npy-[Ga(bppd)](+) cations, where bppd2 stands for bis(2-pyridylmethyl)-1,3-diaminopropane diacetate, are present in solution as isomers with the same symmetry as observed in the solid state. The crystallographic data and the dramatic shift that occurs in the position of the cis/trans isomerization equilibria for the [Ga(bpad)](+) cations simply by increasing the number of bridging CH2 groups in the ligands diamine backbone represent a unique opportunity to assess the accuracy of modern computational methods. The performance of several local density functionals using a pseudopotential-based SDD basis set was compared with the more rigorous HF and MP2 ab initio calculations. The SVWN5 and SV5LYP functionals provide significantly better Ga-O and Ga-N distances than the HF method or the nonlocal BLYP functional. However, to provide proper isomerization energies the pseudopotential-DFT calculations must be augmented by MP2 single-point energies and calculations of solvation free energies.