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 (H₂bpcd) and its complexation of Ga­(III) and Co­(III) are reported. H₂bpcd and the metal–bpcd^2– complexes, isolated as hexafluorophosphate salts, were characterized by elemental analysis, X-ray crystallography, IR spectroscopy, and ¹H and ¹³C NMR spectroscopy. [Ga­(bpcd)]­PF₆, [Ga­(C₂₂H₂₆N₄O₄)]­PF₆, crystallized in the orthorhombic space group Ibca, with a = 13.8975(7) Å, b = 15.0872(7) Å, c = 22.2418(10) Å, 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 ¹H and ¹³C assignments for H₂bpcd and metal–bpcd^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 (C₁ symmetry) and two trans (C₂ symmetry). NMR results indicate that the [Ga­(bpcd)]⁺, [Co­(bpcd)]⁺, and cis-O,O, cis-N_py,N_py-[Ga­(bppd)]⁺ cations, where bppd^2– 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 ligand’s 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.