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 (H 2 bpcd) 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 PF 6 - salts. These salts were characterized by elemental analysis, X-ray crystallography, IR, and 1 H and 13 C NMR spectroscopy. Binuclear [La 2 (bpcd) 2 (H 2 O) 2 ] 2+ crystallized from an aqueous solution in the monoclinic P2 1 /c space group as a cocrystallate with Na 2 bpcd and NaPF 6 , nominally Na 2.34 [La 1.22 (C 22 H 26 N 4 O 4 ) 2 (H 2 O) 2 ][PF 6 ] 2 ·2H 2 O, 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 N 4 O 4 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 1 H and 13 C assignments for H 2 bpcd and the metal-bpcd 2- complexes were made on the basis of 2D COSY and HSQC experiments, which established 1 H- 1 H and 1 H- 13 C 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 C 2 symmetry. The 1 H NMR spectrum of [Nd(bpcd)]PF 6 in D 2 O 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)(H 2 O) 2 ] + and La(bpcd)(H 2 O) 3 ] + .