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) co mplexes with the 1,4,8,11-tetrakis-(2-carboxyethyl) -1,4,8,11-tetraaza cyclotetradecane ligand (TETP) are described. Crystal structures of [C e(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. Crys taldata: (1) monoclinic, C2/c, a = 29.523(4) Angstrom, b = 17.492(3) A ngstrom, c = 8.509(1) Angstrom, beta = 98.72(1)degrees, V = 4344(1) An gstrom(3), Z = 4, R(\F\) = 0.057 for 2329 data (I/sigma(I) greater tha n 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 = 9 9.10(1)degrees, V = 3069.5(7) Angstrom(3), Z = 4, R(\F\) = 0.075 for 1 147 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 macro cycle possesses a [3434] conformation; two propionate arms are extende d, 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 b elonging to four different ligands. Moreover, the four functionalized chains of the Ligand are involved in the coordination polyhedron of th e gadolinium and europium ions, but only extended propionate arms of T ETP take part in the cerium ion coordination polyhedron, one of the ox ygen atoms of asymmetric unit being bidentate between two metallic ion s. In order to better understand the reactivity and the coordination s cheme observed for these complexes, a theoretical study has been carri ed out using the molecular electrostatic potential as a tool to predic t sites of the ligands where electrophilic attack should take place. T he overall good agreement between calculated and observed structures p ermits us to explain the coordination scheme of this new class of comp lexes.