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

A higher yield synthesis of <i>N,N</i>′-bis­(2-pyridylmethyl)-1,3-diaminopropane-<i>N,N</i>′-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–bppd^2– 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 <i>P</i>2₁/<i>c</i>, with <i>a</i> = 9.6134(2) Å, <i>b</i> = 20.2505(4) Å, <i>c</i> = 11.6483(3) Å, β = 97.520(1)^o, and <i>Z</i> = 4. Ga is coordinated in a distorted octahedral geometry provided by a N₄O₂ donor atom set with cis-monodentate acetate groups and <i>cis</i>-2-pyridylmethyl N atoms. Quantum mechanical calculations were performed for the three possible geometric isomers of a pseudo-octahedral metal–bppd^2– complex with five different metal ions. The results indicate, that in aqueous solution, the stability of the <i>trans</i>-O,O isomer is similar to that of the <i>cis</i>-O,O; <i>cis</i>-N_py,N_py isomer but is greater than that of the <i>trans</i>-N_py,N_py isomer. Calculations for a six-coordinate La­(III)-bppd^2– complex converge to a structure with a very large N_py–La–N_py 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 bppd^2– 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–bppd^2– complexes were made on the basis of 2D COSY, NOESY, and ¹H–¹³C HSQC experiments, which were used to differentiate among the cis (<i>C</i>₁ symmetry) and the two trans (<i>C</i>₂ symmetry) isomers

Metal Ion Complexes of <i>N,N</i>′‑Bis(2-Pyridylmethyl)-<i>trans</i>-1,2-Diaminocyclohexane-<i>N,N</i>′‑Diacetic Acid, H₂bpcd: Lanthanide(III)–bpcd^2– Cationic Complexes

The synthesis and characterization of <i>N,N</i>′-bis­(2-pyridylmethyl)-<i>trans</i>-1,2-diaminocyclohexane-<i>N,N</i>′-diacetic acid (H₂bpcd) cationic complexes of La­(III), Nd­(III), and Sm­(III) are reported. The Ln­(III)–bpcd^2– complex ions, where bpcd^2– stands for <i>N,N</i>′-bis­(2-pyridylmethyl)-<i>trans</i>-1,2-diaminocyclohexane-<i>N,N</i>′-diacetate, were isolated as PF₆^– salts. These salts were characterized by elemental analysis, X-ray crystallography, IR, and ¹H and ¹³C NMR spectroscopy. Binuclear [La₂(bpcd)₂(H₂O)₂]²⁺ crystallized from an aqueous solution in the monoclinic <i>P</i>2₁/<i>c</i> space group as a cocrystallate with Na₂bpcd and NaPF₆, nominally Na_2.34[La_1.22(C₂₂H₂₆N₄O₄)₂(H₂O)₂]­[PF₆]₂·2H₂O, with <i>a</i> = 11.3343(6) Å, <i>b</i> = 17.7090(9) Å, <i>c</i> = 15.0567(8) Å, β = 110.632(3)°, and <i>Z</i> = 4 (<i>Z</i>′ = 2). La is eight-coordinate with distorted dodecahedral coordination geometry provided by a N₄O₄ donor atom set. In addition to four N atoms from the bpcd^2– ligand, La’s coordination sphere includes O atoms from a water molecule and three acetate groups (one O atom from singly bound acetate and two O atoms from acetate groups that bridge the La centers). The ¹H and ¹³C assignments for H₂bpcd and the metal–bpcd^2– complexes were made on the basis of 2D COSY and HSQC experiments, which established ¹H–¹H and ¹H–¹³C correlations. The NMR spectral data were used to establish the symmetry of the cationic complexes present in aqueous solution. The data indicate that the La­(III)–bpcd^2– and Sm­(III)–bpcd^2– complexes are present in solution as a single species with <i>C</i>₂ symmetry. The ¹H NMR spectrum of [Nd­(bpcd)]­PF₆ in D₂O consists of eight considerably line-broadened, paramagnetic-shifted singlets. The ab initio quantum mechanical calculations at the PCM/MP2/SDD//HF/SDD level, which were established previously for determining isomerization energies for octahedral M­(III)–bp<i>a</i>d^2– complex ions, were used to determine the relative free energies of the geometric isomers possible for eight- and nine-coordinate La­(III)–bpcd^2– cationic aqua complexes in aqueous solution, i.e., [La­(bpcd)­(H₂O)₂]⁺ and La­(bpcd)­(H₂O)₃]⁺

Metal Ion Complexes of <i>N,N</i>′‑Bis(2-Pyridylmethyl)-<i>trans</i>-1,2-Diaminocyclohexane-<i>N,N</i>′‑Diacetic Acid, H₂bpcd: Lanthanide(III)–bpcd^2– Cationic Complexes

The synthesis and characterization of <i>N,N</i>′-bis­(2-pyridylmethyl)-<i>trans</i>-1,2-diaminocyclohexane-<i>N,N</i>′-diacetic acid (H₂bpcd) cationic complexes of La­(III), Nd­(III), and Sm­(III) are reported. The Ln­(III)–bpcd^2– complex ions, where bpcd^2– stands for <i>N,N</i>′-bis­(2-pyridylmethyl)-<i>trans</i>-1,2-diaminocyclohexane-<i>N,N</i>′-diacetate, were isolated as PF₆^– salts. These salts were characterized by elemental analysis, X-ray crystallography, IR, and ¹H and ¹³C NMR spectroscopy. Binuclear [La₂(bpcd)₂(H₂O)₂]²⁺ crystallized from an aqueous solution in the monoclinic <i>P</i>2₁/<i>c</i> space group as a cocrystallate with Na₂bpcd and NaPF₆, nominally Na_2.34[La_1.22(C₂₂H₂₆N₄O₄)₂(H₂O)₂]­[PF₆]₂·2H₂O, with <i>a</i> = 11.3343(6) Å, <i>b</i> = 17.7090(9) Å, <i>c</i> = 15.0567(8) Å, β = 110.632(3)°, and <i>Z</i> = 4 (<i>Z</i>′ = 2). La is eight-coordinate with distorted dodecahedral coordination geometry provided by a N₄O₄ donor atom set. In addition to four N atoms from the bpcd^2– ligand, La’s coordination sphere includes O atoms from a water molecule and three acetate groups (one O atom from singly bound acetate and two O atoms from acetate groups that bridge the La centers). The ¹H and ¹³C assignments for H₂bpcd and the metal–bpcd^2– complexes were made on the basis of 2D COSY and HSQC experiments, which established ¹H–¹H and ¹H–¹³C correlations. The NMR spectral data were used to establish the symmetry of the cationic complexes present in aqueous solution. The data indicate that the La­(III)–bpcd^2– and Sm­(III)–bpcd^2– complexes are present in solution as a single species with <i>C</i>₂ symmetry. The ¹H NMR spectrum of [Nd­(bpcd)]­PF₆ in D₂O consists of eight considerably line-broadened, paramagnetic-shifted singlets. The ab initio quantum mechanical calculations at the PCM/MP2/SDD//HF/SDD level, which were established previously for determining isomerization energies for octahedral M­(III)–bp<i>a</i>d^2– complex ions, were used to determine the relative free energies of the geometric isomers possible for eight- and nine-coordinate La­(III)–bpcd^2– cationic aqua complexes in aqueous solution, i.e., [La­(bpcd)­(H₂O)₂]⁺ and La­(bpcd)­(H₂O)₃]⁺

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

The synthesis of <i>N,N</i>′-bis­(2-pyridylmethyl)-<i>trans</i>-1,2-diaminocyclohexane-<i>N,N</i>′-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 <i>Ibca</i>, with <i>a</i> = 13.8975(7) Å, <i>b</i> = 15.0872(7) Å, <i>c</i> = 22.2418(10) Å, and <i>Z</i> = 8. Ga is coordinated in a distorted octahedral geometry provided by a N₄O₂ donor atom set with <i>trans</i>-monodentate acetate groups and <i>cis</i>-2-pyridylmethyl N atoms, i.e., the <i>trans</i>-O,O isomer. The diamagnetic [Co­(bpcd)]­PF₆, [Co­(C₂₂H₂₆N₄O₄)]­PF₆, also crystallized from solution in the <i>Ibca</i> space group as the <i>trans</i>-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 (<i>C</i>₁ symmetry) and two trans (<i>C</i>₂ symmetry). NMR results indicate that the [Ga­(bpcd)]⁺, [Co­(bpcd)]⁺, and <i>cis</i>-O,O, <i>cis</i>-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