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.


Structure of α-1
Crystals of α-1 were readily obtained as orange blocks by diffusion of Et 2 O vapor into a solution of 1 in MeNO 2 at room temperature. They were also recovered from recrystallizations carried out at 30 and 35 °C.
The structure of α-1 has been previously described. 4 The complex crystallizes in monoclinic space group P2 1 /n with a = 8.4502 (4) Table A4).

Structure of β-1
Crystals of β-1 were originally obtained as orange blocks by diffusion of Et 2 O vapor into a saturated solution of 1 in MeNO 2 at room temperature and were observed alongside crystals of α-

1.
All subsequent attempts to obtain β-1 by means of standard recrystallization methods proved unsuccessful.
The complex crystallizes in monoclinic space group P2 1 /c with a = 9.3384 (3) (7) Å] in the direction away from the oxygen.
However, when considering the ideal plane formed by S4, S7 and Cl2, the metal center is found to locate 0.0197(10) Å out of the plane towards the oxygen. Similarly to α-1, the molecules pack into dimeric units related by inversion centers, in this case with the Pd centers locating much more centrally over each other (Table A7). The perpendicular separation between their PdCl 2 S 2 least-squares planes is 3.5237(10) Å. In contrast to α-1, the dimers pack into sheets in the bc plane affording alternating hydrophilic and hydrophobic layers (Fig. A4).   (4) (4) Pd1-Pd1# 3.4946 (7) # = −x,1−y, −z   (3) Å; symmetry code i: 0.5−x, −0.5+y, 1.5−z] giving rise to chains that run parallel to the b axis (Fig. A6) and can be designated as C (5) following graph set analysis 7 . This interaction lies somewhat outside the sum of the van der Waals radii [3.15 Å] 6 and so is not considered to represent a significant interaction. Displacement ellipsoids are drawn at 50 % probability. Table A8. Coordination environment and macrocycle conformation for γ-1