Synthesis and kinetic behaviour of lanthanide(III) complexes with the mixed pendant-arm macrocyclic ligand 1,7-bis(carboxymethyl)-4,10-bis(1-methylimidazol-2-ylmethyl)-1,4,7,10-tetraazacyclododecane†

Synthesis, Kinetic Study and Molecular Orbital Investigation of Cadmium(II), Mercury(II) and Lead(II) Complexes with the Mixed Pendant‐Arm Macrocyclic Ligand 1,7‐Bis(carboxymethyl)‐4,10‐bis(1‐methylimidazol‐2‐ylmethyl)‐1,4,7,10‐tetraazacyclododecane

“…Interconversion processes between the conformational isomers may occur via (a) simple rearrangement (''rotation'') of the pendant arms (A Ǟ B or C Ǟ D, in Scheme 2), (b) macrocycle ring inversion (A Ǟ C or B Ǟ D) or (c) combinations of both types of motion, [40,53] which may afford interconversion between enantiomers. The coupling detected between the Cd or Hg atom and one imidazole carbon over the whole temperature range, which is consistent with previous results for lanthanide analogues, [36] suggests that the rearrangements occur without detachment of the pendant arms (but see below, computational results for Hg species). The above interconversion processes, particularly process (b), involve changes in the magnetic environment of the macrocycle carbon atoms, which may be characterized by a four-site line-shape analysis.…”

“…The chemical shifts of the resonances around δ ϭ 2.7 and 2.1 ppm, and the multiplicity and coupling constants, are typical of two nonequivalent ABCD systems exhibiting large geminal and axial couplings, with small equatorial couplings. Such systems parallel those observed at low temperature for the ethylenic groups of lanthanum() and lutetium() complexes with L 6 ; [36] furthermore, each ABCD system is similar to that exhibited by diamagnetic lanthanide complexes with DOTA. [38,39] By increasing the temperature the three resonances due to the protons of methylimidazole remain narrow, undergoing a low-field shift; all the other resonances broaden, collapse, and then merge to afford a spectrum which exhibits, at 358 K, narrow signals at δ ϭ 4.46 and 3.37 ppm for the CH 2 of the dangling groups and two unresolved resonances centered at δ ϭ 3.54 and 3.25 ppm, due to the ethylenic moieties of the macrocycle.…”

“…Similarities between the NMR spectroscopic data for the present cadmium, mercury and lead derivatives at the low exchange limit and the spectra of other metal complexes with L 5 and L 6 , which either possess a nearly prismatic geometry in the solid state [34,35] or have been assigned that geometry in solution, [36] suggest that the same conformation should also be assigned to the present [ML 6 ] species. This view is supported by the quantum mechanical calculations (see below).…”

“…The CH 2 bridging the macrocycle and the imidazole yields a doublet (δ ϭ 2.56 ppm; J ϭ 16 Hz) and a resonance (δ ϭ 2.28 ppm) which overlaps with those of the macrocycle. Such resonances, also seen, with minor changes, for diamagnetic lanthanide complexes with L 6 , [36] are typical of two non-equivalent AB systems. [38,39] The ethylenic moieties of the macrocycle present four triplets at δ ϭ 3.01, 2.91, 2.67 and 2.37 ppm (coupling constants ca.…”

“…The solid manganese and cadmium complexes with L 5 have nearly prismatic structures, [34,35] a geometry that is preferred in solution by some lanthanide complexes with L 6 . [36] Generally, the eight donors of the octadentate ligands L 1 ϪL 6 are also considered to be bound to the metal ion in solution, a notable exception being the seven-coordinate magnesium complex with 1,4,7,10-tetrakis(2-methoxyethyl)-1,4,7,10-tetraazacyclododecane (L 4 ), which has the oxygen of a pendant arm uncoordinated. [10] Most of the complexes undergo dynamic behaviour in solution, which has been rationalized in terms of an exchange process between two enantiomers, [29,30] at rates within the 13 C NMR timescale.…”