Organometallic and coordination chemistry of the s-block metals

The thought of s-block elements puts a smile on the face of most chemists. These metals at the start of the periodic table fill up their s-orbitals with loosely bound valence electrons and are well-known for their extreme reactivity. This is often demonstrated by the increasingly spectacular reactions of Li, Na and K with water which can even be topped by the queen and king of the alkali metals, Rb and Cs. Main group 2 metals may be less reactive in water but their organometallic compounds certainly show a complementary potential.

Although fascinating, the extreme reactivity of especially the alkali metals has been a barrier to the development of their organometallic chemistry. All organometallic chemists are fully aware of Frankland's early discovery of diethyl zinc in 1849 which at that time was thought to be an Et˙ radical. With its typical “organometallic behaviour”, i.e. spontaneous ignition and vigorous burning in air, it is the prototypical organometallic compound. In contrast, only the minority of chemists is aware that two years prior to his Et₂Zn report, Frankland attempted to produce ethyl “radicals” using K as a metal. As the reactions were found to be rather violent and did not produce the products he expected, the more well-behaving Zn seemed the better choice of metal.¹

After Frankland's discoveries, it took only half a decade before “wild horse” alkali metals could be broken. Important pioneers, among which Wanklyn, Buckton, Wurtz, Fittig, Schorlemmer, Schorigin, mainly isolated mixed-metal “ate” complexes but their significant contributions set the stage for Wilhelm Schlenk who in the early 1900s demonstrated for the first time the isolation of pure Li and Na compounds. The birth of n-BuLi, arguably the most produced organometallic reagent of all time, would never have been possible without these pioneering investigations. Crucial to this chemistry is Schlenk's development of special glassware (tubes, flasks, lines) which is still today heavily used for working with organometallic compounds under inert conditions. Progress in alkali metal chemistry went hand in hand with the discovery of the famous Grignard reagents: RMgX (X = Cl, Br, I). Although their formation is based on the same classic oxidative addition route as had been used for the preparation of Et₂Zn (EtI + Zn → EtZnI followed by ligand exchange to give 0.5 Et₂Zn + 0.5 ZnI₂), Grignard reagents are clearly much less reactive in air while maintaining their potency in chemical reactions. This paved the way to broad and ever increasing applications in synthetic organic chemistry and culminated in an early Nobel prize for chemistry in 1912.

During the 1900s, in particular organolithium chemistry developed into a major discipline within the larger field of organometallic chemistry. This was not only driven by application but also by advances in technology. The growth of single crystal X-ray diffraction from a peculiarity to a common technique certainly promoted the 1980s research boom which led to the determination of the often complicated and intriguing structures of these reagents. Although the structures of the ubiquitous magnesium reagents experienced a similar attention, the rest of the alkaline earth metals have remained “sleeping beauties” that nearly 100 years after Grignard's famous discoveries are starting to be awakened. While the notorious health risks associated with Be may explain the unpopularity of the lightest group member, it is the especially high reactivity and difficult behaviour (low solubility, fast ligand exchange) that has impeded the organometallic chemistry of the heavier group members Ca, Sr and Ba. The turn of the century saw, particularly for the organometallic chemistry of these elements, an enormous imperative mainly fed by the recognition that their complexes were found to be great catalysts for polymerization or other transformations and partially can substitute lanthanide or transition metal catalysts.

Recent years have seen a renaissance in the field of early main group metal chemistry.² Breakthroughs were achieved in the isolation of complexes with metals in a low oxidation state (Mg^I, Ca^I),³ “heavy” Grignard reagents have been isolated and characterized (RCaI),⁴ the chemistry of strong mixed-metal bases⁵ and TURBO Grignards⁶ was further developed, and highly reactive heavier alkaline-earth metal benzyl, allyl and hydride reagents have been isolated.⁷ Apart from applications in classical synthetic chemistry, early main group metals are increasingly topical in areas varying from catalysis⁸ to new materials⁹ and hydrogen storage.¹⁰ This themed issue on “Organometallic and Coordination Chemistry of the s-Block Metals” could not have been timelier. Many of the contributions witness the great diversity of this metal block: a comprehensive review on the coordination chemistry of s-block metals with PC(H)P-bridged chalcogen-centred methanide ligands (Chivers), heavy alkali metal alkoxides (Klett), facile methods for synthesis of alkali metal amides (Westerhausen), water-stable potassium compounds (Stalke), COT ligands (Etienne), magnesium hydride reactivity (Parkin), arylcalcium reagents (Westerhausen, Zhang), bulky amide ligands (Mills), arsino-amides (P. Roesky), crystal structure determination of a classic reagent: Me₂Mg (Anwander). Remarkable are the contributions dealing with Be chemistry, showing increased interest in this unusual element: while two are of a theoretical nature, dealing with aromaticity of (BeO)₃ rings (Dutton & Wilson) or the hydrogen storage properties of BeO (Alkorta & Yanez), two contributions stem from more daring chemists reporting facile synthetic routes to BeX₂ (X = Cl, Br, I: Buchner) or a cationic Be complex (Schulz). Equally remarkable is the number of contributions in group 2 metal catalysis: polymerization (Cui, Zhang), silane–amine coupling (Trifonov), hydroboration or cyanosilylation (Panda), pyridine hydroboration (Okuda), clearly demonstrating growing interest in catalytic applications.

This fine selection of reports in early main group metal chemistry is testimony to the increasing interest in this historically established field and will certainly stimulate its further growth. Given the many breakthroughs in the last decades, the future of s-block chemistry is bright. We'd like to finish by thanking the contributors to this collection, the diligent reviewers who evaluated the manuscripts and hope that all those readers working in this exciting area will find a smile on their face as they peruse these articles.

References

1. First steps in the organometallic chemistry of Zn and the early main group metals have been expertly reviewed: (a) D. Seyferth, Organometallics, 2001, 20, 2940–2955; (b) D. Seyferth, Organometallics, 2006, 25, 2–24; (c) D. Seyferth, Organometallics, 2009, 28, 2–33.
2. (a) U. Wietelmann and J. Klett, Z. Anorg. Allg. Chem., 2018, 644, 194–204; (b) Alkaline-Earth Metal Compounds: Oddities and Applications, Topics in Organometallic Chemistry 45, ed. S. Harder, Springer, Berlin, 2013.
3. (a) S. P. Green, C. Jones and A. Stasch, Science, 2007, 318, 1754–1757; (b) S. Krieck, H. Görls, L. Yu, M. Reiher and M. Westerhausen, J. Am. Chem. Soc., 2009, 131, 2977–2985.
4. M. Westerhausen, A. Koch, H. Görls and S. Krieck, Chem. – Eur. J., 2017, 23, 1456–1483.
5. R. E. Mulvey and S. D. Robertson, Angew. Chem., Int. Ed., 2013, 52, 11470–11487.
6. P. Knochel, W. Dohle, N. Gommermann, F. F. Kneisel, F. Kopp, T. Korn, I. Sapountzis and V. A. Vu, Angew. Chem., Int. Ed., 2003, 42, 4302–4320.
7. (a) S. Harder, F. Feil and K. Knoll, Angew. Chem., Int. Ed., 2001, 40, 4261–4264; (b) P. Jochmann, T. S. Dols, T. P. Spaniol, L. Perrin, L. Maron and J. Okuda, Angew. Chem., Int. Ed., 2009, 48, 5715–5719; (c) S. Harder and J. Brettar, Angew. Chem., Int. Ed., 2006, 45, 3474–3478.
8. (a) S. Harder, Chem. Rev., 2010, 110, 3852–3876; (b) M. S. Hill, D. J. Liptrot and C. Weetman, Chem. Soc. Rev., 2016, 45, 972–988.
9. X. Xu, C. Niu, M. Duan, X. Wang, L. Huang, J. Wang, L. Pu, W. Ren, C. Shi, J. Meng, B. Song and L. Mai, Nat. Commun., 2017, 8, 460.
10. S. Harder, Chem. Commun., 2012, 48, 11165–11177.

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