Perspectives
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PERSPECTIVES ligand, FIG. 2)7 were reported. Since that time, various related dimers bearing mono-8,9, Dimeric magnesium(I) di-10 and terdentate11 anionic ligands have been prepared and their chemistry β‑diketiminates: a new class of explored in detail. From these studies, it has become evident that the unique properties quasi‑universal reducing agent of magnesium(i) dimers — especially those featuring β-diketiminato (Arnacnac−) ligands — make these systems among the Cameron Jones most user-friendly and widely applicable 12–14 Abstract | Since the first report of their isolation in 2007, magnesium(i) dimers have reductants . Indeed, many chemists now routinely use [Mg (Arnacnac) ] reagents to transitioned from being chemical curiosities to versatile reducing agents that are 2 2 access synthetic targets that are not easily used by an ever-increasing number of synthetic chemists. Magnesium(i) dimers prepared using other reductants. have a unique combination of advantageous properties that sees them used in the The aim of this Perspective is to syntheses of new, and often applicable, compound types that are impossible or give the reader a general overview of Ar difficult to access using conventional reductants. This Perspective describes the [Mg2( nacnac)2] complexes — in terms of synthesis and properties of these dimers, and provides notable examples of their both their properties and their chemistry — such that these reagents may be successfully application in organic and inorganic synthesis. Magnesium(i) dimers, especially applied in the reader’s own syntheses. complexes of β-diketiminates, may now be viewed as widely applicable, Summaries of the reactivity of magnesium(i) quasi-universal reducing agents with a promising future in synthetic chemistry. It dimers towards both organic and inorganic is hoped that the reader will develop a familiarity with these reagents, such that substrates are provided. Special attention the complexes can be successfully used in many synthetic programmes. is given to reactions that afford compound types that are not accessible when using other reducing agents. These new products Innumerable chemical transformations their insolubility in common solvents. might take the form of novel metal–metal make use of reducing agents to transfer one This lack of solubility typically results in bonded systems and/or compounds that or more electrons to a substrate. As a result, electron transfer from the reductant to a have interesting further reactivity. recent decades have seen numerous reducing substrate at a solid/solution interface with agents being developed for the synthesis little control of selectivity, which often leads Preparation and properties of organic, organometallic and inorganic to over-reduction of the substrate to give For a reducing agent to be of universal products1–3. When a procedure requires a complex product mixtures. Such a problem appeal, it must be easy to prepare and strong reductant — for example, one that can be overcome, to some extent, by using manipulate. The metal–metal bonded Ar oxidizes at potentials lower than −1.5 V soluble reducing agents that can stoichio- species [Mg2( nacnac)2] meet these criteria: relative to the saturated calomel electrode metrically deliver electrons to the substrate these species can be synthesized at room (SCE)1 — synthetic chemists often make use in a more controlled manner; however, temperature, in high yields (crude yields are of electropositive metals and their complexes these reductants can also be very harsh typically >80%) and on multigram scales (up because they offer a range of useful (for example, Na(C10H8)), and often lead to 10 g) by simple alkali metal reductions of (FIG. 1) properties and reducing strengths . to by-products (for example, C10H8) that magnesium(ii) precursors, which themselves Some commonly used reductants include are difficult to separate from the targeted are trivial to prepare7,15,16 (FIG. 2). In turn, elemental alkali and alkaline earth metals, product(s)1. Furthermore, alkali metal-based Arnacnac− ligands bearing a variety of N-aryl graphite intercalated potassium (KC8), alkali reducing agents are especially difficult to substituents can be prepared, such that the metal naphthalenides (M(C10H8), M = Na or prepare and store, and their handling may reactivity of their respective magnesium(i) K), SmI2 and the decamethylmetallocenes present a considerable fire hazard. compounds can be readily tuned. For kinetic 1–6 [M(C5Me5)2] (M = Co or Sm) . Although Considering the disadvantages of many reasons, the reactivity of the magnesium(i) the synthetic utility of these reagents is reducing agents, and the difficulties faced in system typically increases with decreasing not in question, they have drawbacks that selecting such reagents for a given synthetic steric bulk of the N-aryl substituent12,13. The limit their applicability, such that selecting task, it would be desirable to have a class of compounds are stable in isolation, and all a reducing agent becomes a trial-and-error universal reducing agent that can be reliably known magnesium(i) dimers are crystalline process. For example, reactions that use used with a wide range of both organic solids that typically do not decompose alkali metals or KC8 can be difficult to and inorganic substrates. In 2007, the first below 200 °C. The solids are only moderately control on account of the strongly cathodic stable magnesium(i) dimers [Mg2L2] (where air and moisture sensitive, present no fire redox potentials of these reductants and L− is a bulky guanidinato or β-diketiminato hazard, have no known toxicity and can be NATURE REVIEWS | CHEMISTRY VOLUME 1 | ARTICLE NUMBER 0059 | 1 ©2017 Mac millan Publishers Li mited, part of Spri nger Nature. All ri ghts reserved. PERSPECTIVES − [C10H8] Mg chemist’s toolkit. The organic reactions in K Na Sm(C5Me5)2 Co(C5Me5)2 SmI2 which they have been used are summarized in the following. More E º (V) reducing versus SCE Substrate reductions. Reactions of –3.0 –2.5 –2.0 –1.5 –1.0 magnesium(i) dimers with various Figure 1 | Oxidation potentials for reductants that are commonly usedNature in Reviewsorganic |and Chemistry organo- unsaturated substrates can lead to the latter metallic synthesis. Values are approximate, and in some cases are converted from potentials that being reduced by one, two or three electrons, 1,4,6 have been reported against other reference electrodes or chemical references . SCE, saturated with the products typically forming in high calomel electrode. yields. The only reported one-electron reduction is that of benzophenone, which readily manipulated using standard air-free Experimental charge density studies have enabled the first isolation and structural techniques. Furthermore, their versatility shown that there is a local maximum in the characterization of magnesium ketyl is enhanced by their solubility and stability electron density between the Mg centres radical 1 (TABLE 1) when carried out in the in most common aprotic organic solvents, — a rare case of a so-called non-nuclear presence of 4-(dimethylamino)pyridine24. including toluene, benzene and diethyl ether. attractor19,20. Overall, the dimers can be Such ketyl radicals were proposed as In addition to being easy to prepare thought of as ‘molecular bottles’ in which intermediates in the pinacol coupling of and handle, another attraction of two electrons are stored. Unfortunately, benzophenone as early as 1927 (REF. 25). Ar [Mg2( nacnac)2] complexes is that they all experimental attempts to measure the More common are two-electron reductions, can each act as a soluble, stoichiometric redox potentials of magnesium(i) dimers which proceed by insertion of the substrate source of electrons that can be delivered have so far been unsuccessful. Although into the Mg−Mg bond to afford novel with control to a substrate. In many they will certainly be less reducing than the diamagnetic, dimagnesiated products. cases, the dimers deliver electrons and alkali metals from which they are made, Substrates that have been doubly reduced by convert to poorly soluble by-products given the reported potentials for the Mg2+/0 magnesium(i) dimers include azobenzene, that are readily removed by filtration. For and Mg2+/+ couples (−2.61 V and −2.29 V anthracene, cyclooctatetraene (for example, example, when using a magnesium(i) versus SCE, respectively)6, it is likely to give 2), carbodiimides, ketenimines dimer to reduce a transition metal that magnesium(i) dimers have similar and dioxygen15,24,26,27. The three-electron Ar halide, [Mg2( nacnac)2] is converted to a reducing capabilities, such that they can be reduction of polyaromatic hexaazatri- halido-bridged magnesium(ii) complex12,13 considered strong reductants1. naphthylene (HAN) yields 3, which can Ar 3− of the form [Mg2( nacnac)2(μ-X)2]; this is be described as [HAN] coordinated easily separated from the reduced reaction Use in organic synthesis to three [Mg(Dipnacnac)]+ fragments products and can be reduced to regenerate Magnesium, both in elemental and (Dip = 2,6-di isopropylphenyl)28. The ground the magnesium(i) dimer for further use. organometallic forms, has long been used state of the trianionic ligand was found to Crystallographic studies reveal as a reducing agent in organic synthesis. Its be a doublet (S = ½) with some triradicaloid Ar that [Mg2( nacnac)2] complexes use dates back to the beginning of the 20th character. The high-yielding formation, ease have unsupported Mg–Mg bonds century, when it was found that magnesium of preparation and selective formation of (rMg– Mg = 2.8–2.9 Å), with each Mg metal reacts with organic halides to form all of these reduced systems lends them to, centre being chelated by a κ2-Arnacnac− Grignard reagents, compounds that are for example, reaction with electrophiles to ligand12,13,17. Computational studies show thought to act as single electron transfer afford functionalized products. that the highest occupied molecular (SET) reagents in some cases21. Other orbital (HOMO) of the compounds magnesium based reductants that have been Reductive coupling, bond cleavage and largely comprises the high s-character applied to organic synthesis include Reike C−X bond activation.