Epitaxial Retrieval of a Disappearing Polymorph

Recrystallization of [PdCl2([9]­aneS2O)] ([9]­aneS2O = 1-oxa-4,7-dithiacyclononane), 1, and [PtCl2([9]­aneS2O)], 2, by diffusion of Et2O vapor into solutions of the complexes
in MeNO2 yielded three phases of 1 and two
phases of 2. The known phase of 1, herein
designated α-1, was obtained under ambient conditions.
A second phase,
designated β-1, was initially also obtained by
this method; however, following the advent of a third phase, γ-1, all subsequent efforts over a period of a year to crystallize
β-1 yielded either γ-1, obtained
by carrying out the recrystallization at elevated temperature, or
α-1, commonly found throughout the study. This
persistent absence of a phase which could initially be crystallized
with ease led us to the conclusion that β-1 was
an example of a “disappearing polymorph”. The first
phase obtained of 2, designated α-2, was obtained by recrystallization under ambient conditions and
is isomorphous and isostructural with α-1. The
second phase β-2 was obtained by slight elevation
of the recrystallization temperature and was found to be isomorphous
and isostructural with β-1. Subsequently, β-2 was used to seed the growth of the disappearing polymorph
β-1. No third phase of 2 (γ-2) has been isolated thus far.