Two concomitant polymorphs that interconvert via crystal-to-crystal phase transitions, and single crystals obtained by heteromolecular seeding

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Abstract

The organometallic salt [(η⁵-C₅H₅)₂Fe][AsF₆] crystallises as a mixture of two concomitant polymorphs, a trigonal and a monoclinic form, which interconvert via fully reversible solid-to-solid phase transitions. Single crystals of the two forms have been obtained by heteromolecular seeding.

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The existence of more than one crystal structure for the same compound (i.e. polymorphism¹) is a well-established phenomenon. On the other hand, the observation of concomitant polymorphs,²i.e. the simultaneous crystallisation of different forms of the same species, has only recently begun to be investigated in a systematic manner. Polymorphism is full of practical (and economical) implications, since polymorphic modifications of the same substance (resulting from different arrangements of the molecules in the solid state) may differ markedly in chemical and physical properties.³

We are investigating polymorphism and phase transition behaviour in a systematic manner, with a focus on organometallic systems.⁴ In particular, we are interested in obtaining new crystal forms by non-solution methods (e.g. grinding,⁵ seeding,⁶ desolvation⁷) and in controlling the interconversion between polymorphs⁸ and between pseudo-polymorphs.^5,7 One way to control the outcome of a crystallisation process, for instance, is that of providing nucleation ‘seeds’ that may act as templating units during crystal growth. Seeding is a common laboratory practice and has a particular relevance in the large scale industrial production of desired polymorphic forms of substances such as drugs^3,9 and pigments.¹⁰

In this paper we report that precipitation of [(η⁵-C₅H₅)₂Fe]⁺ as its [AsF₆]⁻ salt† generates two concomitant polymorphs: a trigonal phase (Fe-T) and a monoclinic phase (Fe-M) as shown by single crystal X-ray diffraction.‡ On the other hand, crystallisation of [(η⁵-C₅H₅)₂Co]⁺[AsF₆]⁻ only leads to a trigonal form (Co-T) isomorphous with Fe-T (for crystal data see Table 1). The monoclinic phase Fe-M is isomorphous with the room temperature, monoclinic phase of the pair [(η⁵-C₅H₅)₂Fe][PF₆] and [(η⁵-C₅H₅)₂Co][PF₆], which has been shown to form molecular alloys by co-crystallisation.¹³

Table 1 Crystallographic data for polymorphs of [(η⁵-C₅H₅)₂Fe][AsF₆]^a

Parameter	Fe-T	Co-T	Fe-M
a Click b107489j.txt for full crystallographic data (CCDC 169148–169150).
Formula	C10H10AsF6Fe	C10H10AsCoF6	C10H10AsF6Fe
M	374.95	378.03	374.95
Crystal system	Trigonal	Trigonal	Monoclinic
Space group	P3121	P3121	P21/c
a/Å	9.136(5)	9.111(2)	13.506(2)
b/Å	9.136(4)	9.111(2)	9.605(2)
c/Å	12.467(8)	12.388(2)	9.572(2)
β/°	90	90	93.01(2)
V/Å^3	901(1)	891(1)	1240(1)
Z	3	3	4
T/K	293(2)	293(2)	293(2)
μ/mm^−1	4.042	4.265	4.084
θ Range/°	3–25	3–30	3–30
Independent reflections	1025	1612	3600
R 1 [I⊕>⊕2σ(I)]	0.0899	0.0402	0.0821
wR 2 (all data, F^2)	0.1170	0.1030	0.1009

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The relationship between crystalline Fe-T and Fe-M is shown in Fig. 1. It is worth noting that, besides the differences in packing arrangements of the forms, the cyclopentadienyl ligands are eclipsed in Fe-T, whereas they are all staggered in Fe-M.

Fig. 1 Relationship between the ion arrangements in trigonal (a) and monoclinic (b) [(η⁵-C₅H₅)₂Fe][AsF₆] at room temperature. Note how the C₅H₅ rings belonging to the cations are eclipsed in Fe-T (a), whereas they are staggered in Fe-M (b).

Powder samples of the mixture Fe-T/Fe-M and of Co-T were subjected to full cycles of differential scanning calorimetry (DSC) measurements, both in the cooling and in the heating regimes.§ The single peak observed for [(η⁵-C₅H₅)₂Co]⁺[AsF₆]⁻ at 324(1) K (Fig. 2, heating cycle) shows the existence of a phase transition between the Co-T phase and a high temperature cubic (Co-C) phase (see below). For the iron complex two peaks are observed (Fig. 2, heating cycle) at 316(1) and 327(1) K, respectively, corresponding to the Fe-T⊕→⊕Fe-M and the Fe-M⊕→⊕Fe-C phase transitions, respectively. It is noteworthy that the interval of thermal stability (in the heating cycle) of Fe-M is only 11 K. On the other hand, the cooling cycle shows that whereas the Fe-C⊕→⊕Fe-M transition occurs at ca. 322 K, the Fe-M⊕→⊕Fe-T peak is observed in the range 275–265 K, i.e. with a much larger thermal hysteresis (35–45 K). This difference could explain why the trigonal and the monoclinic forms can be crystallised concomitantly at room temperature. All of these processes are fully reversible.

Fig. 2 DSC thermograms (heating cycle) showing the trigonal⊕→⊕cubic phase transition for [(η⁵-C₅H₅)₂Co][AsF₆] (top curve) and the trigonal⊕→⊕monoclinic and monoclinic⊕→⊕cubic phase transitions for [(η⁵-C₅H₅)₂Fe][AsF₆] (bottom curve).

In order to drive the crystallisation process towards the formation of the separate polymorphs, we have resorted to heteromolecular seeding. To this end, crystals of trigonal [(η⁵-C₅H₅)₂Co][AsF₆] have been used to grow the trigonal form of [(η⁵-C₅H₅)₂Fe][AsF₆], and crystals of monoclinic [(η⁵-C₅H₅)₂Co][PF₆] have been used to obtain the monoclinic form of [(η⁵-C₅H₅)₂Fe][AsF₆] (see Scheme 1). The seeding was successful, even though a small amount of the alternative phase could always be detected in the powder diffractograms and in the DSC scans. What is more, the two seeding processes yielded good quality single crystals of Fe-T and Fe-M, which made possible the investigation of the phase transitional behaviour directly on the diffractometer.‡

The results of the separate experiments carried out on Fe-T and Fe-M are summarised in Scheme 2.

Scheme 1 Graphical representation of the crystallisation and seeding processes of [(η⁵-C₅H₅)₂Fe]⁺[AsF₆]⁻.

Scheme 2 Schematic representation of the X-ray diffraction experiments on single crystals of Fe-T and Fe-M. The two frames indicate the starting points of the heating/cooling cycles on Fe-T (left) and Fe-M (right), respectively.

A single crystal of Fe-T converted into the monoclinic Fe-M phase at ca. 313 K; the monoclinic phase was then converted into the cubic phase (Fe-C, isomorphous with Co-C) at ca. 328 K. Upon cooling, the cubic form reverted to the monoclinic one at room temperature.

A single crystal of Fe-M was first cooled to 253 K: the phase transition to the monoclinic phase was observed at ca. 273 K. The same crystal was then converted into the trigonal form at 313 K and subsequently to the cubic form at 328 K, then cooled again to room temperature to show the transition to the monoclinic form.

A similar experiment was carried out on a single crystal of [(η⁵-C₅H₅)₂Co][AsF₆]. A phase transition to the cubic system (Co-T⊕→⊕Co-C) was observed at ca. 333 K, and cell parameters for the cubic phase were measured at 353 K.¶

All phase transitions were followed by cell parameter determination. Separate single crystals of Co-T and Fe-T were also subjected to cooling down to 100 K on the diffractometer. Shrinking in the values of the cell parameters and the cell volume was observed for both solids, but no further phase transitions were detected.

In summary, we have discovered the existence of concomitant polymorphs of the salt [(η⁵-C₅H₅)₂Fe]⁺[AsF₆]⁻, which can be separated via heteromolecular seeding. The two forms, Fe-T and Fe-M, reversibly interconvert. The crystals are sufficiently robust to undergo a full cycle of four phase transitions directly on the diffractometer (Fe-T⊕→⊕Fe-M⊕→⊕Fe-C⊕→⊕Fe-M⊕→⊕Fe-T), a rather uncommon situation that permitted a whole rationalization of the phase transitional behaviour shown by DSC.

We believe that studies of polymorphism allow us to learn how to control crystal nucleation and growth as well as the physical properties of crystalline materials. These are all relevant aspects of materials chemistry and crystal engineering.¹⁴

Acknowledgements

We thank M.U.R.S.T. (project Solid Supermolecules 2000–2001) and the Universities of Bologna (project Innovative Materials) and Sassari for financial support.

References

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Footnotes

† The salts were prepared following the same general procedure utilized for the salts [(η⁵-C₅H₅)₂M][PF₆] (M⊕=⊕Co, Fe) (see ref. 8), with K[AsF₆] replacing K[PF₆]. Correspondence between the structures determined by single crystal X-ray diffraction and the bulk material was confirmed by powder diffraction measurements. Powder diffractograms were collected on a Philips PW-1100 automated diffractometer (Cu-Kα radiation, graphite monochromator).

‡ X-Ray diffraction data for Co-T, Fe-T and Fe-M were collected at room temperature using a Nonius CAD-4 diffractometer equipped with an Oxford Cryostream liquid-N₂ device, which was also used to heat the crystals; Mo-Kα radiation (λ⊕=⊕0.71073 Å), graphite monochromator. The SHELXL97¹¹ package was used for structure solution and refinement based on F². All non-H atoms were refined anisotropically, except for C atoms in Fe-M. H_(CH) atoms were added at calculated positions. SCHAKAL99¹² was used for the graphical representations.Single crystals of both Fe-T and Fe-M are deep blue in colour and do not possess different morphologies, i.e. it is not possible to separate the two polymorphs by simple visual inspection under the microscope.

§ DSC thermograms were measured on a Perkin-Elmer DSC-7 apparatus in sealed Al pans, at a scanning rate of 5.0 °C min⁻¹.

¶ Cell parameters for Fe-C: a⊕=⊕6.820(1) Å, V⊕=⊕318(1) Å³, T⊕=⊕323 K; for Co-C: a⊕=⊕6.798(1) Å, V⊕=⊕314(1) Å³, T⊕=⊕353 K.

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