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

The synthesis and characterization of the fluxional,
d10 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 1H NMR and variable-temperature 31P 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 (ΔS⧧ < −65 J mol–1 K–1) is operating.