Metal Ion Complexes of N,N'-Bis(2-Pyridylmethyl)-trans-1,2-Diaminocyclohexane-N,N'-Diacetic Acid, H₂bpcd: Lanthanide(III)-bpcd^2- Cationic Complexes

The synthesis and characterization of N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N'-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 N,N'-bis(2-pyridylmethyl)-trans-1,2-diaminocyclohexane-N,N'-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 P2₁/c space group as a cocrystallate with Na₂bpcd and NaPF₆, nominally Na_2.34[La_1.22(C₂₂H₂₆N₄O₄)₂(H₂O)₂][PF₆]₂·2H₂O, with a = 11.3343(6) Å, b = 17.7090(9) Å, c = 15.0567(8) Å, β = 110.632(3)°, and Z = 4 (Z' = 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 C₂ 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)-bpad^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]⁺.