Metal Ion Complexes of N,N′‑Bis(2-Pyridylmethyl)-trans-1,2-Diaminocyclohexane-N,N′‑Diacetic Acid, H2bpcd: Lanthanide(III)–bpcd2– 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)–bpcd2– complex ions, where bpcd2– 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 Na2.34[La1.22(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 bpcd2– 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–bpcd2– 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)–bpcd2– and Sm­(III)–bpcd2– 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)–bpad2– complex ions, were used to determine the relative free energies of the geometric isomers possible for eight- and nine-coordinate La­(III)–bpcd2– cationic aqua complexes in aqueous solution, i.e., [La­(bpcd)­(H2O)2]+ and La­(bpcd)­(H2O)3]+.