Prediction Of The Coordination Scheme Of Lanthanide N-Tetrasubstituted Tetraazamacrocycles: An X-Ray Crystallography And Molecular Modeling Study

The synthesis and characterization of three lanthanide (Ce, Gd, Eu) complexes with the 1,4,8,11-tetrakis-(2-carboxyethyl) -1,4,8,11-tetraazacyclotetradecane ligand (TETP) are described. Crystal structures of [Ce(H2TETP)](OH)(H2O) . 10H(2)O (1), [Gd(H3TETP)](OH)(2)(H2O)(2) . 3H(2)O (2), and [Eu(H3TETP)](OH)(2)(H2O)(2) . 3H(2)O (3) are reported. Crystaldata: (1) monoclinic, C2/c, a = 29.523(4) Angstrom, b = 17.492(3) Angstrom, c = 8.509(1) Angstrom, beta = 98.72(1)degrees, V = 4344(1) Angstrom(3), Z = 4, R(\F\) = 0.057 for 2329 data (I/sigma(I) greater than or equal to 3), and 213 parameters; (2) monoclinic, C2/c, a = 15.378(2) Angstrom, b = 14.172(2) Angstrom, c = 14.264(2) Angstrom, beta = 99.10(1)degrees, V = 3069.5(7) Angstrom(3), Z = 4, R(\F\) = 0.075 for 1147 data (I/sigma(I) greater than or equal to 3), and 213 parameters; (3) monoclinic, C2/c, a = 15.32(1) Angstrom, b = 14.19(1) Angstrom, c = 14.130(3)Angstrom, beta = 99.41(3)degrees, V = 3031(3) Angstrom(3), Z = 4, R(\F\) = 0.085 for 1133 data (I/sigma(I) greater than or equal to 3) and 98 parameters. Complexes and 98 parameters. Complexes 2 and 3 are isotypes. In the three complexes, the centrosymmetric TETP macrocycle possesses a [3434] conformation; two propionate arms are extended, and the others are folded toward two protonated nitrogen atoms. The lanthanide coordination mode with the TETP ligand does not occur via the four nitrogen atom ring but only through carboxylic oxygen atoms belonging to four different ligands. Moreover, the four functionalized chains of the Ligand are involved in the coordination polyhedron of the gadolinium and europium ions, but only extended propionate arms of TETP take part in the cerium ion coordination polyhedron, one of the oxygen atoms of asymmetric unit being bidentate between two metallic ions. In order to better understand the reactivity and the coordination scheme observed for these complexes, a theoretical study has been carried out using the molecular electrostatic potential as a tool to predict sites of the ligands where electrophilic attack should take place. The overall good agreement between calculated and observed structures permits us to explain the coordination scheme of this new class of complexes.