Polymorphism of an organo-phosphorus compound

MICROCHEMICAL

JOURNAL

Polymorphism

13, 66-74 ( 1968)

of an Organo-Phosphorus RICHARD

American

Cyanamid Bound

E. STEVENS

Company, Organic Chemicals Brook, New Jersey 08805

Received

Compound

Division,

June 27, 1967

A novel example of polymorphism is afforded by triphenylphosphonium 3,3,4,4-tetrafluoro-2,5-dioxocyclopentyhde (I), the condensation product of 1,2-dichlorohexafluorocyclopentene with triphenylphosphine. A white crystalline compound is produced whose chemical identity has been verified by elemental analyses, NMR and infrared spectrosc0py.l The unique resonance contribution of delocalized electrons in the carbocyclic ring is believed to contribute substantially to ‘the planarity of the molecule. Localization of opposing charges on adjacent atoms in the structure, in the manner of a zwitterion, may account for the salt-like character of the compound and its remarkable thermal stability. Heating to 300°C results in only a slight decomposition; the bulk of the sample can be recovered unchanged on a single recrystallization from methanol after heat treatment. The ability of a compound to exist in at least two distinctly different crystalline phases is actually a common occurrence in organic chemistry. Deffet (1) has catalogued over 1100 examples of compounds that show polymorphism. Many of these show at least three different structures; some as many as seven, However, unlike most polymorphic compounds, crystals of (I) show two phases stable below the melting point, and two quite different phases obtained on cooling the melt. Long, silky needles are produced by recrystallization of (I) from methanol. This gives Phase 1. Gradual heating on a Koffler micro hot stage (heating rate: 4”/minute) reveals a sluggish phase transition between 164-168°C. A partial meltback is followed by the advance of angular prismatic crystals with high order polarization colors (Phase 2). This phase melts sharply between 178-179°C to a clear liquid. Rapid chilling of the melt in an ice bath produces a translucent, viscous liquid in which spherulites nucleate and grow (Phase 3). 1 Stockel, R. F., personal communication. 66

ORGANO-PHOSPHORUS

FIG. 1. A possible 2,5-dioxocyclopentylide.

structure

for

(I)

67

POLYMORPHS

triphenylphosphonium

3,3,4,4-tetrafluoro-

Slow heating accelerates the growth and development of the spherulitic phase. At 124°C the spherulites transform into Phase 4, a massive, polygonal granular structure with radial cracks and tear-shaped shrinkage voids. Phase 4 is also prepared by slow cooling of the melt obtained from Phase 2. Phase 4 melts sharply between 181-182°C and forms a clear liquid identical with that from Phase 2. Dispersion curves obtained by plotting refractive index of the melt as a function of temperature show that the two liquids have the same refractive indices at a given temperature, as well as the same rate of change with temperature (dn/dT = 0.00055). All attempts to recover Phases 1 and 2 from the melt have been unsuccessful. Rapid chilling to 0°C yields a glassy supercooled liquid, the precursor of Phase 3. Cooling rapidly from the melting point to room temperature (24°C) produces Phase 3 at once. Very slow cooling to temperatures just below the melting point yields crystals of Phase 4. Photomicrographs of the various crystal phases are included (Figs. 2-7). TABLE

1

TR~NSI?.ION TEMPERATURES FOR POLVMORPHS OF (I) Polymorphic 1+2

2 3-4 4

Form

Transition

Temperature

164-168 178-179 melts 124.0-124.5 181-182 melts

( “C)

68

FIG. 2.

STEVENS

Crystals

of the parent

compound

(I),

from

methanol;

(X 90).

ORGANO-PHOSPHORUS

FIG. 3.

Phase 2 nucleating

69

POLYMORPHS

from

molten

droplets;

(X 90).

70

STEVENS

FIG. 4.

Phase 3 spherulites

nucleating

from

the melt;

(X 90).

DRGANO-PHOSPHORUS

FIG. 5.

Fully-developed

spherulites

71

POLYMORPHS

of Phase 3 material;

( x 90).

72

STEVENS

FIG. 6.

Phase 4 crystals,

under

crossed polars;

( X 90).

ORGANO-PHOSPHORUS

FIG. 7. Characteristic

shrinkage

cracks

POLYMORPHS

and voids

in Phase 4 material;

73

(X 90).

74

STEVENS

Table 1 lists transition temperatures for the polymorphs of (I). The bulk density of (I) was determined by weighing powdered solid in a capillary tube of known dimensions. From the calculated volume the bulk density was found to be 0.968 g/cc at 24°C. This compares favorably with the density determined by flotation using benzene-carbon tetrachloride mixtures. These results may indicate the advisability of reexamining reported melting points in related compounds, particularly those containing the triphenylphosphine moiety. ACKNOWLEDGMENTS The author is indebted to Dr. R. F. Stockel for a sample of the compound, and to Dr. J. Gove for NMR spectra, Miss M. Landon for infrared spectra, and Mr. J. Kobliska for microchemical analyses. REFERENCE

1. DEFFET, L., “Repertoire Desoer,

LiBge,

1942.

des Composes

Organiques

Polymorphes.”

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