Low-Temperature X-Ray Structure Techniques for the Characterization of Thermolabile Molecules

Chem. Rev. 1988, 88. 81-92 81 Low-Temperature X-ray Structure Techniques for the Characterization of Thermolabile Molecules MICHAEL MITH' and WALTER FRANK Unh-Wiit des Saadandas, Anwganische chemic. 116600 SaarbriMen, Federal Republic of Germany ReceW April 3, 1987 (Revfsed Manuscript ReceivedJW 23, 1987) Contents I. Introduction 81 A. General Aspects 81 B. Brief Comparison of the Methods for me 82 Characterization of Unstable Intermediates 11. Methods and Devices 82 A. General Remarks 82 B. Main Elements of Gas Stream X-ray 83 Diffraction Cooling Devices 1. Basic Principles 83 2. Gas Supply UnR 83 3. Delivery Nozzle 84 4. Transfer Line 84 Michael Veith was born in 1944 in Goerliiz, Germany. He studied Chemistry at the University of Munich, where he received his Di- 5. Refill System 84 plomChemiker degree in 1989. Continuing work with Prof. N. 8. Automatic Temperature Control 85 Wiberg he received his doctoral rhgree in 1971. He then moved C. Growih and Handling of Thermolabile Single 85 to the University of Karlsruhe. where he started his postdoctoral Crystals work with Prof. H. Baernighausen. In 1977 he completed his Habilitation in inorganic chemistry. Until the end of 1978 he was 1. Crystal Growth in a Capillary 85 Privat-Dozent at the University of Karsruhe. In 1979 he moved 2. Crystal Growth in One Place and 86 to the Technische Universitaet Braunschweig as a professor of Transfer to the Diffraction Device under inorganic chemistry. In 1984 he received offers for a full pro- Protective Conditions Elsewhere fessorship at the University of Oldenburg, the University of He!- 111. Use of Three-Dimensional Structures of 87 delberg. and the University of ttm Saarbnd (Saarbruecken). Since 1984 he has been in Saarbruecken. He has received several Unstable Inorganic Molecules and Solids in the awards. His main interests are in the fieid of synthetic and Evaluation of Their Reactivity structural chemistry. focusing on molecular compounds containing A. Correlation of Instability and Molecular 87 metallic elements. Structure B. Reaction Intermediates 89 C. Unstable Phases in Solid-state Chemistry 90 IV. Conclusion and Outlook 90 V. Acknowledgments 90 VI. References 90 I. Introductlon A. General Aspects The chemist is currently confronted with compounds that are unstable for either kinetic or thermodynamic reasons. If these compounds are produced during the course of a reaction sequence, they are designated as Walter Frank was born in 1957 in Munster. West Germany. He reaction intermediates. As well as chemical procedures, received hb DiplomChemiker degree in 1982 and his Cf. rer. nat. a variety of physicochemical methods has been used after a twc-year scholarship of the Stiftung Stipendien-Fonds der (see next chapter) to elucidate the composition and Chs"n Induslrk at the Technische Universitaet Braunschweg three-dimensional structure of the intermediate. The with Prof. M. Veith. In 1984 he accepted a postdoctoral position at the University of the Saarland. He is currently working toward most complete and exact results are obtained by dif- his Habilitation. His research interests are in the field of complexes fraction methods, followed by three-dimensional of heavy metals. especialty concerning preparative and structural structure determinations. With X-ray diffraction on aspects at the borderline between molecular and solid-state single crystals, the sample has to be cooled because of chemistry. its instability, and the crystal growth and transfer must be performed under low-temperature conditions. monograph, by Rudman.' It appeared 10 years ago and In this review the special techniques for the synthesis referred to the different procedures of cooling. Solid- and handling of single crystals of unstable compounds state and crystallographic problems also are widely are described. As far as we know, there is only one discussed in this book. We intend here to focus on 0009-2665/88/078~0081$06.50/0 0 1988 Amerlcan Chemical Society 82 Chemical Reviews, 1988, Vol. 88, No. 1 Veith and Frank chemical problems on a molecular scale. To illustrate fruitfu1.l' A complete three-dimensional arrangement the capacity of the techniques described, a chapter has of the atoms within a molecule can, however, rarely be been included that is dedicated to the use of low-tem- deduced from the spectra. perature crystallographic techniques in establishing Solid-state NMR,12 ESCA,13 and Mossbauer spec- reaction mechanisms (chapter 111). The examples troscopy14 are used in the solid state but X-ray dif- chosen are arbitrary and far from complete, but serve fraction on single crystals or crystal powders15J6is the to demonstrate the different principles. most widely used method to characterize metastable compounds or phases. In this last procedure the com- 6. Brief Comparison of the Methods for the pound or phase under investigation has to be quenched Characterization of Unstable Intermediates to low temperatures in order to stabilize the phase for the diffraction experiment. Unstable intermediates are an important part of It is astonishing that this quenching method coupled general chemistry, from both a theoretical and a prac- with diffraction techniques has not been used more tical point of view, and scientific journals exist devoted extensively for the determination of transient species entirely to this special topic.2 To avoid confusion the in solutions or in the gas phase. There are, nevertheless, term unstable intermediate or transient species as used some cases in which this procedure may be employed in this review should be briefly characterized. This is quite easily. The central and most difficult point in not an unstable configuration of reacting atoms or these methods is the crystallization of the unstable in- molecules to which corresponds a maximum of potential termediate. The difficulties may have several causes: energy in a reaction path. Such a "complex" cannot be (a) the crystallization of the intermediate may be hin- frozen out and thus cannot be stabilized by altering a dered, (b) different solvents or temperatures may have thermodynamic parameter such as the temperature. to be tested, (c) products of side reactions may crys- We define an intermediate in a given reaction as an tallize, (d) the substrates or products may crystallize atomic arrangement with a local minimum of potential more quickly than the intermediate, and (e) inclusion energy but a higher energy than the reaction products. of solvents in the crystal may occur. Despite these When the composition and structure of a transient problems the isolation can be attempted. species are known, the most promising reactants may If the transient molecule can be stabilized in the gas be selected, e.g., by mere steric considerations. Instead phase under reduced pressure, its transformation to the of 10-20 reaction partners, a much lower number has crystalline state can be performed via sublimation on to be tested as exemplified in the reaction scheme (2) to a cold surface as exemplified in chapter 1I.C. compared to the reaction scheme (1). Crystallization of unstable compounds in solution, inter med late ,&~/o,, which is described in detail in chapter II.C, can be substrate - of unknown - (1) Structure achieved if the energy of activation between the intermediate and the substrate (or substrates) is small enough to be attained at low temperatures, either by D,r,"err (2) . .._. jfewproductsl the environment or by a destinative radiation of the starting material. The equilibrium of a bimolecular The advantage of the second scheme is that time is primary process can often be shifted to the one-to-one saved, leading to a better utilization of the synthetic intermediate by decreasing temperature (see example procedure. There is, nevertheless, one difficulty: how in chapter 111). can the structure of the intermediate be characterized correctly? ZZ. Methods and Devices The physicochemical methods used for the determination of the structures of unstable molecules depend A. General Remarks on the state of the matter. If the intermediate emerges from a substrate in the gas phase at reduced pre~sure,~ Various procedures for cooling the specimen in X-ray electron diffra~tion,~photoelectron spectroscopy,5 UV diffraction studies have been proposed and were sum- spectroscopy,6 microwave spectro~copy,~or similar marized by StewartI7 in a paper dealing with poly- techniques can be used to characterize the compound. crystalline material. Rudman18 later reviewed the ap- Easily the richest information can be derived from the plication of low-temperature techniques in light mi- diffraction experiment, as in this case the three-di- croscopy, IR, NMR, and ESR spectroscopy, and neu- mensional structure of the molecule can be established tron diffraction to low-temperature X-ray diffraction. from a radial distribution fun~tion.~If very small Robertsonlg groups the numerous methods into three molecules are produced, matrix techniques can be used. categories (gas cooling, liquid cooling, and conduction) In this procedure, the intermediate is frozen out at low and points out that several special requirements of temperatures in a noble gas environment,8 whereby single-crystalX-ray diffraction make gas stream cooling infrared spectroscopy and Raman spectroscopy are the most suitable technique for maintaining low tem- generally used to characterize it. High resolution is then peratures. Early examples were reported by Cioffi and needed to evaluate the stru~ture.~ESR, SIMS, and Taylor20and by Twenty-five years

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