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

Abstract

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 (1)H and (13)C 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 (1)H and (13)C 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-Npy,Npy-[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.

Metal Ion Complexes of N,N′-Bis(2-Pyridylmethyl)-trans-1,2-Diaminocyclohexane-N,N′-Diacetic Acid, H2bpcd: Lanthanide(III)–bpcd2– Cationic Complexes

“…NMR data for the [Ln­(bpcd)]­PF 6 salts are given in the Experimental Section. There are 11 resonances in the 1 H NMR spectrum of H 2 bpcd and 11 of 22 possible resonances in its 13 C NMR spectrum . All of the resonances observed for H 2 bpcd as well as the La­(III)–bpcd 2– and Sm­(III)–bpcd 2– complex ions show the expected splitting patterns, intensities, and chemical shifts characteristic of a diamine with a cyclohexyl backbone bearing acetate and 2-pyridyl-methyl functionalities.…”

“…H 2 bpcd was synthesized by a facile, two-step procedure developed for the synthesis of H 2 bppd using simple starting reagents . Typical yields for this synthetic route are ∼60% after removal of sodium halide byproducts by ionic exchange chromatography . The Ln­(III)–bpcd 2– cationic complexes were prepared from an anhydrous methanol solution of a Ln­(III) nitrate or chloride by neutralization of H 2 bpcd·2HCl with 4 equiv of sodium acetate.…”

“…The synthesis of the 2-pyridylmethyl-substituted parent diamine bpmdac, N , N ′-bis­(2-pyridylmethyl)-1,2-diaminocyclohexane, and its subsequent elaboration to the diacetic acid H 2 bpcd, N , N ′-bis­(2-pyridylmethyl)-1,2-diaminocyclohexane- N , N ′-diacetic acid, were achieved using the procedures reported previously for the 1,3-diaminopropane analogues bpmdap and H 2 bppd. , …”

“…La’s relatively large size and preference for higher coordination numbers, satisfied in the present case by coordinated water, requires the acronyms trans -O,O, trans -N py ,N py and cis -O,O, cis -N py ,N py previously used to differentiate among the three possible geometric isomers for octahedral [Co­(bp a d)] + and [Ga­(bp a d)] + cationic complexes to be expanded. , The trans -O ac ,O ac , trans -N py ,N py isomers for the eight- and nine-coordinate [La­(bpcd)­(H 2 O) 2 ·H 2 O] + and [La­(bpcd)­(H 2 O) 3 ] + complexes, which are constitutional isomers, can be distinguished by the orientation of the torsional angle defined by the C b –H b and N–C ac bonds, i.e., the trans- and gauche- C b H b ,NC ac torsion angles. The trans -O ac ,O ac , trans -N py ,N py / trans- C b H b ,NC ac isomers for eight- and nine-coordinate La­(III)–bpcd 2– cationic complexes are shown in Figure .…”

“…The synthesis and characterization of selected Ln­(III)–bpcd 2– cationic complexes, i.e., La­(III), Nd­(III), and Sm­(III), and an X-ray structure for the binuclear [La 2 (bpcd) 2 (H 2 O) 2 ] 2+ cationic complex are reported. The objectives of the investigation were (1) to design and synthesize a hydrophilic ligand as a potential candidate for use as a selective reagent for trivalent lanthanide–actinide separations; (2) to obtain structural information for a trivalent Ln complex of a bis­(2-pyridylmethyl)-substituted diaminodiacetic acid; and (3) to gain insight into the structure and relative stability of the [Ln­(bpcd)] + cationic species present in solution using computational methodology established previously. , …”