The template synthesis and X-ray characterization of pyrrole-derived hexadentate uranyl(VI) Schiff-base macrocyclic complexes

The template condensation of 3,4-diethyl-2,5-dicarbaldehyde with ethylenediamine and 1,3-propanediamine in the presence of UO2(NO3)(2) gave neutral complexes of formulae UO2(C24H32N6) ([UO2(bi-pyen)], 2) and UO2(C26H36N6) ([UO2(bi-pytmd)], 3), respectively. X-Ray diffraction quality single crystals were obtained from chloroform/methanol. In the case of 2, these were monoclinic, space group C2/c, with a = 30.549(5), b = 8.251(1), c = 21.969(4) Angstrom, beta = 114.47(1)degrees, V = 5040(1) Angstrom(3), Z = 8, rho(calc) = 1.78 g cm(-3). Crystals of 3 were also monoclinic, space group C2/c, with a = 26.946(7), b = 9.436(2), c = 10.862(2) Angstrom, beta = 105.04(2)degrees, V = 2667(1) Angstrom(3), Z = 4, rho(calc) = 1.75 g cm(-3). In the X-ray structure of 3, the complex lies on a crystallographic twofold axis. For 2, the final R = 0.0335, wR = 0.0371 for 297 parameters and 4475 reflections [F greater than or equal to 4(sigma(F-0))], while for 3, the final R = 0.0223, wR = 0.0265 for 213 parameters and 2810 reflections [F greater than or equal to 4(sigma(F-0))]. In each complex, the urany(VI) cation is bound to all six nitrogen atoms such that the metal center, with its two apical oxygen atoms, lies in a distorted hexagonal bipyramidal environment. The change in overall macrocycle size that results in going from 2 to 3 does not affect the U-N bond lengths. Rather, the principal difference between 2 and 3 is the extent of the twist between the two nearly planar halves of the macrocycle. In 2, the imine-pyrrole-imine dihedral angle is 17.5(1)degrees while in 3 it is 35.7(1)degrees. On the other hand, it is near 0 degrees in an earlier reported complex, 1, derived from o-phenylenediamine [J.L. Sessler, T.D. Mody and V. Lynch, Inorg. Chem., 31 (1992) 531]. Complex 1 displays two reversible oxidations at +0.82 V and +1.09 V versus F-c/F-c(+) in CH2Cl2 containing TBAPF(6), while complexes 2 and 3 display irreversible oxidation waves at +1.22 V, +1.46 V and +1.22 V and +1.47 V versus F-c/F-c(+), respectively. Complexes 1, 2, and 3 also display quasi-reversible uranyl-centered reductions at -0.85 V, -1.02 V, and -0.96 V versus F-c/f(c)(+) respectively.