Zinc, Cadmium, and Mercury Complexes of a Pyridyloxy-Substituted Cyclotriphosphazene: Syntheses, Structures, and Fluxional Behavior

The synthesis and characterization of the fluxional, d(10) cyclotriphosphazene complexes, [MLCl2] (M = Zn, Cd, and Hg; L = spiro-[(1,1'-biphenyl)-2,2'-dioxy]tetrakis-(4-methyl-2-pyridyloxy)cyclotriphosphazene), are described. Single-crystal X-ray structures show that the zinc complex has crystallized into two crystal forms: one as a tetrahedral species, with a N2Cl2 donor set in which a geminal pair of the pendant pyridyloxy nitrogen atoms binds to the zinc, and the other as a trigonal-bipyramidal (tbp) one, with an N3Cl2 donor set. The third nitrogen atom comes from the phosphazene ring and the two pyridyl ligands are non-geminal. The asymmetric unit of the cadmium complex contains three structurally distinct molecules. One molecule has a tbp structure similar to that of the zinc complex. The second molecule has a six-coordinate, distorted octahedral geometry around the cadmium center with a N4Cl2 donor set, with three of the nitrogen donor atoms coming from the pendant pyridyloxy arms. The third site contains a tbp complex and a distorted octahedral species with a relative occupancy of 3:1. The identification of these three different forms in the one crystal suggests that the energy difference between the tbp and distorted octahedral isomers is not large. Quantitative analysis of the H-1 NMR and variable-temperature P-31 NMR spectra of the zinc, cadmium, and mercury complexes in a CD2Cl2 solution, coupled with the X-ray structural results, shows that an associative fluxional mechanism (Delta S-double dagger < -65 J mol(-1) K-1) is operating.