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

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− [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‑diisopropylphenyl)­ 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. The selectivity Mg–Mg covalent bond, whereas the lowest magnesium22, as well as mixtures of Mg and with which magnesium(i) dimers reduce unoccupied molecular orbital (LUMO) MgX2 (X = Cl or Br), which, at equilibrium, organic substrates enables their effective largely takes the form of a π‑type bonding include minute amounts of monovalent use in various high-yielding reductive 7 • orbital between the Mg atoms . Given magnesium halides (MgX or Mg2X2) in bond coupling and cleavage processes. For the very high polarity of the Mgδ+–Nδ− solution23. A further example is magnesium example, several magnesium(i)-induced Ar bonds, dimers of [Mg2( nacnac)2] can anthracene (for example, [Mg(C10H8) reductive C–C and N–N bond-forming 2+ 3 be viewed as Mg2 dications that are (THF)3], THF = tetrahydrofuran) , which is reactions are now known for nitriles, stabilized by nacnac− ligands. Indeed, the a poorly soluble but very reactive source of isonitriles, isocyanates and alkyl low-coordinate, electrophilic nature of ‘elemental’ magnesium. It is often difficult azides15,26,29. The products of these reactions the metal centres is central to the reducing to control the selectivity of organic reactions cannot typically be accessed using other capabilities of the dimers, the reactions that involve these magnesium compounds, s‑block-based reductants, but related of which proceed by an inner-sphere one reason that SET reagents, such as SmI2, products can sometimes be prepared using mechanism whereby substrates bind to have remained so popular2. Although samarium(ii) compounds or low-valent Ar the metal sites before delivery of the two [Mg2( nacnac)2] compounds have found first-row transition metals. A prominent electrons from the Mg–Mg bond18. Electron good use in inorganic chemistry, fewer family of examples are the remarkably transfer is further enhanced by the large applications are known in organic chemistry. stable hexazenediide complexes, such as and diffuse nature of the metal-centred However, this is about to change, as these 4, with the only other complexes of this HOMO, the electrons in which are only highly selective and controllable reducing dianionic ligand being those of transition weakly associated with the Mg atoms. agents are rapidly being added to the metals30. Relative to reductive couplings,

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a Ar Ar b ligand can afford a trinuclear complex, N N Ar which is the first cyclopropanetriolate OEt2 2 M N 2 Mg Mg Mg complex of any metal. Experiments I Toluene N N N Ar − 2 MI indicate that the reactions proceed via Ar Ar − 2 OEt2 the magnesium hydrides [Mg(nacnac) > 80% yields M = Na or K (μ‑H)] , which, according to computational > 10 g scale 2 studies, hydromagnesiate CO to give the Dip [Mg2( nacnac)2] N 2 2 Dep Increasing Increasing η -formyls [Mg(nacnac)(η -OCH)]; these [Mg2( nacnac)2] C Mes steric bulk reactivity [Mg2( nacnac)2] Mg then undergo a series of further reactions and rearrangements to eventually give Ar Nature Reviews | Chemistry the isolated di- or trinuclear complexes Figure 2 | Dimagnesium(i) complexes of the form [Mg2( nacnac)2]. Although stable in isolation, the complexes are strong and selective reductants, with the least sterically bulky examples being the most of the C–C coupled ligands. Overall, such reactive. a | The metal–metal bonded complexes are readily prepared by reductive coupling of the work highlights the potential utility of Ar Ar − corresponding [Mg( nacnac)I(OEt2)] species (where nacnac is a β‑diketiminato ligand). The most magnesium(i) dimers for the generation common examples feature, in order of decreasing steric bulk, 2,6‑diisopropylphenyl (Dip), 2,6‑diethyl­ of higher alcohols from synthesis gas phenyl (Dep) or 2,4,6‑trimethylphenyl (Mes) substituents at the nitrogen atoms. Complexes with the (a mixture of H and CO). least bulky substituents are the most reactive as their highest occupied molecular orbitals are more 2 exposed to potential substrates7,15,16. b | The X‑ray crystal structure of [Mg (Dipnacnac) ], with the 2 2 Use in inorganic synthesis hydrogen atoms omitted for clarity, shows the perpendicular orientation of the two MgN planes. 2 More so than in organic chemistry, magnesium(i) dimers have been widely

reductive bond cleavages that are effected reductive disproportionation of CO2 used as selective reductants in inorganic by magnesium(i) are less prevalent. Despite affords CO and the carbonato complex and/or organometallic syntheses12,13. These Dip this, cleavage of the C–N single bonds in [Mg2( nacnac)2(μ-CO3)] (7) as the major reagents have proved to be particularly alkyl-substituted isocyanides has been (kinetic) products. The thermodynamic effective in the synthesis of low-valent Dip (REF. 29) found to give [Mg( nacnac)(CN)]3 ; product is thought to be the oxalato main group compounds, the chemistry Dip this is similar to the chemistry that is [Mg2( nacnac)2(μ-C2O4)] (8), which also of which has flourished over the past two observed when samarium(ii) compounds forms in small amounts, in this case by decades. These highly reactive species are 31 27 are used as reductants . More interestingly, reductive coupling of CO2 molecules . not only fascinating from a fundamental magnesium(i) dimers smoothly activate The reductive disproportionation point of view, but have more recently begun strong C–F bonds in partially fluorinated reaction was initially believed to proceed to emerge as powerful reagents for the and perfluorinated arenes — in the absence via the magnesium oxo intermediate activation of small molecules and catalysis Dip of catalysts — to give aryl- and fluoro- [Mg2( nacnac)2(μ‑O)], followed by — processes that normally necessitate 32 35–37 magnesium(ii) products , such as 5 and reaction with a second molecule of CO2 the use of late transition metals . Such 6. Quenching of the arylmagnesium(ii) to give 7. However, a later computational chemistry has led to the realization that product with an electrophile represents a study pointed to the formation of 7 cheap, benign low-valent main group transition metal-free route to functionalized occurring through a more concerted compounds could eventually replace toxic fluoroarenes. Overall, these C–F activation mechanism33. By analogy to the formation precious metal complexes in some areas of

reactions can be likened to the formation of oxalato 8, SO2 can also be reductively synthesis and catalysis. Although developing Dip of Grignard reagents. The possible coupled to give [Mg2( nacnac)2(μ-S2O4)] these low-valent compounds is a major focus involvement of the magnesium(i) radical — the first known magnesium dithionite of main group chemists today, the nascent (Arnacnac)Mg• in the activation of C–F complex — in a clean reaction in which field has been hindered to some degree by bonds has been probed by conducting a reductive disproportionation is not a the difficulty that is associated with cleanly reaction between the symmetrical dimers competing process11. accessing low-valent p‑block species using Dip Mes [Mg2( nacnac)2] and [Mg2( nacnac)2] In a related exploration of the viability conventional alkali metal-based reductants. (Mes = 2,4,6‑trimethylphenyl). Although of using magnesium(i) dimers to convert It is in this regard that magnesium(i) dimers this does slowly lead to the unsymmetrical feedstock gases into value-added products, have proved to be highly effective, with their Dip redistribution product, [Mg2( nacnac) two such dimers were treated at room readily controllable reducing characteristics (Mesnacnac)], it is not apparent whether temperature with a molecular source of affording chemists access to various

the reaction proceeds through Mg−Mg H2 (that is, 1,3‑cyclohexadiene) under an compound types that cannot otherwise be bond cleavage (and magnesium(i) radical atmosphere of CO. Remarkable Fischer– prepared (or can only be obtained in low formation) or ligand exchange. Tropsch-like C–C couplings were observed, yields). A summary of this chemistry and which resulted in the formation of alkoxo an overview of the use of magnesium(i) Reductive disproportionations and complexes such as magnesium ethylene diolate compounds to access related low-valent Dip 2 1 (REF. 16) related reactions. Much like the [Mg2( nacnac)2(μ-κ :κ -O2C2H2)] . first-row transition metal compounds are expensive, paramagnetic and radioactive This complex is also accessible directly from provided in the following; this section is (REF. 34) samarium(ii) (and uranium(iii)) systems, [Mg(nacnac)(μ‑H)]2 , as is cyclopro- organized according to the group of the Dep 3 dimagnesium(i) complexes selectively panetriolate [Mg3( nacnac)3(μ -O3C3H3)] periodic table from which the reduced

reduce small molecules such as CO2 and (9, Dep = 2,6‑diethylphenyl). It seems that element originates. Dip − SO2. The potentially value-added products the bulky nacnac ligand only permits that result are not typically accessible formation of a dinuclear species, whereas use Group 2. There has been only one example using s‑block reductants. In one example, of the smaller 2,6‑diethylphenyl-substituted of a low-oxidation-state s‑block compound

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PERSPECTIVES

Ar Group 13. The syntheses of several Table 1 | Selected reductions of unsaturated substrates with [Mg2( nacnac)2] important E–E-bonded group 13 species Precursor(s) Magnesium(i) dimer Product(s) Refs (where E denotes a group 13 element) rely on Dip Benzophenone, DMAP [Mg2( nacnac)2] Ph 24 the use of magnesium(i) reductants. In the Dip O N Ph 1 Mg case of boron, reduction of the platinum(ii) N boryl complex [PtBr(PEt ) (B Dur Br)] Dip DMAP 3 2 2 2 (Dur = 2,3,5,6‑tetramethylphenyl) afforded Dip Cyclooctatetraene, THF [Mg2( nacnac)2] Dip 26 the platinum(0) product [Pt(PEt ) (B Dur )] N 3 2 2 2 Mg (11), the first diborene complex39. Analogous N THF 2 Dip Dip to the complexation of electron-rich metals THF Mg N with olefins, experimental and computational Dip N evidence has revealed that electron density is donated from Pt to an empty π–bonding Hexaazatrinaphthylene [Mg (Dipnacnac) ] 28 orbital on the orthogonal diborene fragment 2 2 of 11. This leads to a strengthening of the N Dip Dip N B=B bond upon coordination; thus, this N N N Mg Mg N system is a rare exception of the Dewar– Dip Dip Chatt–Duncanson bonding model. 3 N 3− N The reduction of a series of hydrido­ N N aluminium(iii) complexes has afforded Mg Dip Dip an array of dimeric hydridoaluminium(ii) N N compounds that feature Al–Al covalent bonds. For example, N‑heterocyclic carbenes (NHCs), in particular IPr Mes 1‑Adamantyl azide [Mg2( nacnac)2] Ad 15 (:C{N(Dip)C(H)}2), can form alane Mes N N N adducts of the type [AlH3(IPr)], which Mg Mes N N Mes upon reduction with [Mg2( nacnac)2] 4 Mes N N Mg give the thermally stable dinuclear species N N 40 N Mes [Al2H4(IPr)2] in good yield . [Al2H4(IPr)2] Ad can be considered an NHC adduct of

Dip Al2H4 (dialane(4)), with the parent Hexafluorobenzene, [Mg2( nacnac)2] Dip THF 32 THF N compound only being stable at very low 5 Mg 41 N temperatures (~5 K) . Related amidinato­ Dip C6F5 dihydridoaluminium(iii) complexes of − THF the form [Al(amid)H2] (where amid is a Dip Dip F bulky amidinato ligand) also react with N N Mes Dip 6 Mg Mg [Mg2( nacnac)2] (or [Mg2( nacnac)2]), N N F in this case giving the aluminium(ii) Dip Dip THF product [Al2H2(amid)2]. Interestingly, the by‑product of this reduction, namely Dip Carbon dioxide [Mg ( nacnac) ] Dip 27 Dip 2 2 O O Dip [{Mg( nacnac)(μ‑H)}2], could be reduced N N Mg Mg back to the magnesium(i) starting material 7 N O N by treatment with potassium metal40. In Dip Dip related chemistry, reduction of a magnesium − 2− Dip Dip O O salt of [AlH4] (alanate) yielded [Al2H6] N N (dialanate), in what was the first preparation 8 Mg Mg N N O O of this dianionic moiety; in this case, Dip Dip 2− [Al2H6] exists as a contact ion complex Dep with two dimagnesium hydride cations 1,3‑Cyclohexadiene, [Mg2( nacnac)2] 16 O O Dep 12 42 carbon monoxide Dep O Dep [Mg4( nacnac)4H2(Al2H6)] ( ) . Dialanate N Mg Mg is a valence isoelectronic analogue of ethane 9 Mg N Dep Dep N and was computationally predicted to be an N N N Dep Dep accessible synthetic target the year before it was prepared43. The only other example of Ad, 1‑adamantyl; Dep, 2,6‑diethylphenyl; Dip, 2,6‑diisopropylphenyl; DMAP, 4‑(dimethylamino)pyridine; magnesium(i) dimers being used in group Mes, 2,4,6‑trimethylphenyl; THF, tetrahydrofuran. 13 chemistry involved treatment of H3NBH3 Dip with [Mg2( nacnac)2]; this resulted in the being accessed using a magnesium(i) dimer: (X = Br or I) precursors can be converted, reductive dehydrogenation of ammonia Mes using [Mg2( nacnac)2] as the reducing in moderate yields, into the corresponding borane, leading to the formation of the agent, the (diiminophosphinato)halido- magnesium(i) dimer, [Mg2{Ph2P(NDip)2}2] magnesium(ii) amidoborane complex TABLE 2 38 Dip (REF. 44) magnesium(ii) [Mg{Ph2P(NDip)2}(X)] (10, ) . [Mg2( nacnac)2(H2NBH3)2] .

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Group 14. The application of Various ligand-stabilized group 14 analogues of diaminoalkynes53. In a counter-

magnesium(i) reductants has been most element(i) dimers [E2L2], as well as their intuitive result, germanium(i) dimers that

successful in the synthesis of group 14 NHC adducts [E2L2(NHC)n] (n = 1 or 2), bear slightly less bulky amido ligands can compounds, with many interesting and have also been prepared by reduction of have very long Ge–Ge single bonds, as is the useful compound types having been made di- or tetravalent halido precursors such case with 17 (REF. 54). Such systems can be − available. This is perhaps unsurprising as [ELX] or [ELX3]. Although L can be a viewed as 1,2‑diamino­digermylenes, which given that magnesium metal itself is bidentate ligand, such as amidinato (as in are devoid of Ge–Ge multiple bonding often used to prepare low-valent group 15)49 or β-diketiminato50, it is more often because the smaller amido ligands allow 45 14 systems . One of the first indications the case that it takes the form of a very the Si2C2N2Ge2 fragments to be planar. This of the effectiveness of magnesium(i) bulky monodentate donor such as a boryl51, isomer is conducive to N→Ge π bonding reagents came in 2009 with the reduction terphenyl52 or amido (as in 16 and 17)53,54. involving the N‑centred lone pair and the − of the NHC complex [Ge(IPr)Cl2], which When L is monodentate, the corresponding empty Ge‑centred 4p‑type orbitals. Both 0 0 afforded the Ge =Ge bonded dimer two-coordinate element(i) dimers [E2L2] can forms of these dimers have been calculated 46 [Ge2(IPr)2] (13) ; the tin(0) analogue (14) have varying bond orders between the central to have very narrow HOMO–LUMO of this complex is also accessible47. Both atoms, depending on the metal involved gaps, a factor that contributes to their high dimers can be considered as NHC adducts and the steric characteristics of the ligand. reactivity. For example, these germanium

of element fragments (E2), and like their For example, digermanium(i) complexes dimers can activate strong bonds in small (REF. 48) (REFS 53–55) silicon(0) analogue [Si2(IPr)2] , are of extremely bulky amido ligands (such as molecules such as H2 , CO2 sometimes viewed as ‘allotropes’ that have 16) feature short Ge–Ge multiple bonds (REF. 56) and olefins57 — substrates that potential as soluble sources of the elements (~2.36 Å), such that they can be considered are traditionally processed by low-valent for synthetic chemistry. diaminodigermynes — the germanium transition metal complexes. For example,

Ar Table 2 | Examples of low-oxidation-state group 2, 13 and 14 complexes prepared using [Mg2( nacnac)2] as the reducing agent Precursor Magnesium(i) By-product Product Refs dimer

Mes Mes [Mg{Ph2P(NDip)2}(X)] [Mg2( nacnac)2] [Mg2( nacnac)2(X)2] Dip 38 Dip Ph X = Br or I N N 10 P Mg Mg P Ph N Ph Ph N Dip Dip

Mes Mes [PtBr(PEt3)2(B2Dur2Br)] [Mg2( nacnac)2] [Mg2( nacnac)2Br2] Dur 39 Et3P B 11 Pt Et3P B Dur

Dep Dep [Mg2( nacnac)2(AlH4)2(NMe3)2] [Mg2( nacnac)2] 2NMe3 42

N N N N Dep Dep Dep Dep Mg Mg H H 12 H H Al Al H H H H Mg Mg Dep Dep Dep Dep N N N N

Dip Dip [E(IPr)Cl2] [Mg2( nacnac)2] [Mg2( nacnac)2Cl2] 46,47 N N E = Ge or Sn Dip Dip 13: E = Ge E E 14: E = Sn

Dip N N Dip

Dip Dip [E{ArʹC(NDip)2}Cln] [Mg2( nacnac)2] [Mg2( nacnac)2Cl2] Dip 49 N E = Si, Ge or Sn Dip Ar ′ E 15 N N n = 1 or 3 E Ar Dip ′ N Dip

Mes Mes i [Ge{NAr(SiR3)}Cl] [Mg2( nacnac)2] [Mg2( nacnac)2Cl2] Si( Pr)3 SiMe3 53,54 Ar† Ar* Ar = Ar* and R = Me; or N N Ge Ge Ge Ge Ar = Ar† and R = iPr 16 17 N † N Ar Ar* i ( Pr)3Si Me3Si

t † i Arʹ = 4-( Bu)C6H4; Ar* = 2,6-[C(H)Ph2]2-4‑MeC6H2; Ar = 2,6-[C(H)Ph2]2-4- PrC6H2; Dep, 2,6‑diethylphenyl; Dip, 2,6‑diisopropylphenyl; Dur, 2,3,5,6‑tetramethylphenyl; Mes, 2,4,6‑trimethylphenyl.

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• 16 and 17 split H2 at temperatures as low as [GeL] that dimerize to give the respective oxidation states. In particular, the dimers (REF. 54) −10 °C, both in solution and the solid state, diamagnetic products [Ge2L2] . can reduce dihalido­silicon(iv) precursors to quantitatively afford the diamidodiger- Only one example of such a germanium(i) R2SiX2 to the stable corresponding − mene [Ge2L2H2] (where L = [N(2,6-{C(H) radical, which was generated on reduction silicon(ii) products SiR2. The two reported i i − 53,54 Ph2}2-4- PrC6H2)(Si Pr3)] ) . The formal of an extremely bulky (β‑diketiminato) examples of such silylenes include an Ge=Ge bond in this system is weak, such chlorogermanium(II) complex with aromatic carbocyclic silylene62 and Mes that in solution the dimer can exist in [Mg2( nacnac)2], has been stabilized and a rare example of an acyclic silylene, equilibrium with the two-coordinate hydri- characterized (18, TABLE 3)61. The steric 19, which incorporates two bulky dogermylene monomer GeLH (REF. 53). bulk of the chelating ligand hinders arylthiolato ligands63. Furthermore, several These unprecedented species, as well as their dimerization of the mononuclear species, mixed-valent tin cluster compounds have tin(ii) analogues, have been shown to be which has a half-life of several hours in been prepared by reducing oxidized tin powerful reagents for the hydrometallation solution but is indefinitely stable in the precursors with magnesium(i) systems. 58 of substrates, such as unactivated alkenes , solid state. Extensive experimental and The products include [Sn10(Trip)8] and are highly efficient catalysts for the computational electron paramagnetic (Trip = 2,4,6‑triisopropylphenyl)64, 59 (REF. 65) hydroboration of aldehydes, ketones and resonance and electron nuclear double [Sn4{CH(SiMe3)2}4(μ-CMe2)2] and (REF. 60) CO2 . resonance studies on 18 revealed that its the remarkable heterometallic complex 66 The reduction of halidogermanium(ii) unpaired spin density is predominantly [Pt7Sn8Cl4(PCy3)6] (20, Cy = cyclohexyl) . complexes to give species such as 16 and situated on the germanium atom. The latter features an interstitial Pt atom 17 has been postulated to proceed through Magnesium(i) dimers can also be used at the centre of a rhombic dodecahedron intermediary germanium(i) radicals to prepare group 14 systems in higher with 14 metal vertices, with the average

Ar Table 3 | Examples of low-oxidation-state group 14, 15 and d‑block complexes prepared using [Mg2( nacnac)2] as the reducing agent Precursor Magnesium(i) dimer By-product Product Refs t Mes Mes Dip [GeCl{HC(C( Bu)NDip)2}] [Mg2( nacnac)2] [Mg2( nacnac)2Cl2] tBu 61 N 18 Ge N tBu Dip

[(TerS) SiBr ] [Mg (Mesnacnac) ] [Mg (Mesnacnac) Br ] 63 2 2 2 2 2 2 2 Ter S S Ter 19 Si

Mes Mes [Pt(PCy3)2SnCl2] [Mg2( nacnac)2] [Mg2( nacnac)2Cl2] 66

20 Pt Pt(PCy3) SnCl Sn

Ar Ar [Sb2R4] [Mg2( nacnac)2] [Mg( nacnac)R] 67

R = Me or Et Ar = Dip or Mes N N Ar Ar Mg Sb Sb Ar Sb Ar N N 21 Mg Sb Sb Mg N N Sb Ar Sb Sb Ar Mg Ar Ar N N

Dip Dip [Fe{R2NC(NDip)2}Br] [Mg2( nacnac)2] [Mg2( nacnac)2Br2] Dip Dip 69 R = 2,6‑dimethylpiperidinyl N Fe N 22 N N N Fe N Dip Dip

i Mes Mes [M{NAr(Si( Pr)3)}Br] [Mg2( nacnac)2] [Mg2( nacnac)2Br2] Mes Ar 71,73 † N M = Mn and Ar = Ar†; or 23: M = Mn; Ar = Ar Mg M N 26: M = Zn; Ar = Ar* N i M = Zn and Ar = Ar* Mes Si( Pr)3

Mes Mes [M{NAr(SiMe3)}X] [Mg2( nacnac)2] [Mg2( nacnac)2X2] Me3Si Ar 71,73 24: M = Mn; Ar = Ar* * † N M M N M = Mn, Ar = Ar and X = Br; or 25: M = Zn, Cd or Hg; Ar = Ar Ar SiMe M = Zn, Cd or Hg, Ar = Ar† 3 and X = Br or I

† i Ar* = 2,6-[C(H)Ph2]2-4‑MeC6H2; Ar = 2,6-[C(H)Ph2]2-4- PrC6H2; Cy, cyclohexyl; Dip, 2,6‑diisopropylphenyl; Mes, 2,4,6‑trimethylphenyl; Ter, 2,6-(Mes)2C6H3.

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− † i − oxidation state of tin being 0.5. Formed complex [MnLBr] (L = [N(Ar )(Si Pr3)] , and inorganic synthesis. This has been † i by reduction of the metal-only Lewis pair Ar = 2,6-[C(H)Ph2]2-4- PrC6H2) with a amply demonstrated over the past decade,

[Pt(Cy3P)2SnCl2], the large polyhedron magnesium(i) dimer does not, surprisingly, with β‑diketiminato-coordinated examples,

represents a small-molecule model for Sn–Pt lead to the dimanganese(i) species [Mn2L2] in particular, being exploited by an intermetallics, which are important to several through a typical one-electron reduction increasing number of researchers from heterogeneous catalytic processes. of the manganese(ii) centre71. Instead, a around the globe. Such magnesium(i) two-electron reduction of the substrate dimers have allowed access to numerous Group 15. Although the reduction of occurs to afford [Mg(Mesnacnac)MnL] (23), novel compound types that are not group 15 complexes using magnesium(i) which features a high-spin two-coordinate accessible, or can only be prepared in compounds has not been widely studied, Mn centre bonded to Mg. It is the steric much lower yields, when using more recent work has highlighted the value of bulk of the amido ligand that is believed to established reducing agents. Furthermore, these reagents in pnictogen chemistry. prevent the targeted dimer from forming, many of the products of magnesium(i)

Reduction of distibanes [Sb2R4] (R = Me and indeed a dimanganese(i) product reductions have gone on to find unique or Et) with magnesium(i) dimers involves (24) does form when the manganese(ii) synthetic applications in their own right, a reductive cleavage of Sb−C bonds to afford precursor bears a slightly smaller amido situation which only strengthens the case the magnesium-substituted polystibides ligand. The manganese centres in 24 are for magnesium(i) dimers to be viewed as Ar [Mg4( nacnac)4(Sb8)] (21, Ar = Mes or high-spin and antiferromagnetically an important new class of quasi-universal Dip)67, each of which can be viewed as a coupled. Known reactions of 23 include reducing agent. 4− Ar + [Sb8] core ligated to four [Mg( nacnac)] its use as an inorganic ‘Grignard reagent’ So what does the future hold for the cations. Thus, the magnesium(i) dimer for the transfer of its manganese amide present class of compounds? Although it reacts with the antimony centres and fragment to other metal centres, yielding in seems clear that currently available three- fragments into magnesium(ii) monomers one case an unsymmetrically substituted, coordinate magnesium(i) dimers will that are bound to the reduced metal two-coordinate dimanganese(i) complex continue to find more applications, − * − centres, a reaction scheme that has [Mn2LLʹ] (Lʹ = [N(Ar )(SiMe3)] , certain situations will call for species with

previously been demonstrated for some Ar* = 2,6-[C(H)Ph2]2-4‑MeC6H2). Moreover, different steric and electronic properties. reductions of transition metal halides (see the electron-rich Mn–Mg bond of 23 is The reactivity scope of dimagnesium(i) below). A subsequent report described reactive towards small-molecule substrates. compounds will be broadened by

the related polystibide [Mg4(guan)4(Sb4)] For example, treatment of 23 with N2O the emergence of promising new − i − Mes (guan = [ Pr2NC(NDip)2] ), the synthesis of afforded [MnL(μ‑O)Mg(THF)( nacnac)], complexes such as the recently reported

which involved treating the bis(guanidinato) the first example of a two-coordinate dimagnesium(i) compound [Mg2{N(Ar*) 71 dimagnesium(i) reductant [Mg2(guan)2] oxomanganese complex . (SiMe3)}2], which features extremely bulky with the tetraantimony(i) butterfly complex The chemistry described above monodentate amido ligands8. Examples (REF. 68) [Sb4(C5Me5)4] . also largely translates to the group 12 in this class of compounds are simple to metals72,73. Most importantly, reductions prepare and thermally very stable. These d‑Block. Magnesium(i) dimers have also of amidohalidometal(ii) precursors with a two-coordinate complexes, given their demonstrated their value in the synthesis magnesium(i) dimer give the homologous low coordination number and the higher of several d‑block metal complexes that series of two-coordinate group 12 metal– electrophilicity of their metal centres, are are not attainable using other reducing metal bonded dimers (25) when the likely to be significantly more reactive than agents. A case in point here is the diiron(i) bulky monodentate amido ligand bears their β‑diketiminato-ligated counterparts. complex 22, which was prepared by a trimethylsilyl substituent. However, if A more challenging (although no less reduction of a bromidoguanidinatoiron(ii) the bulkier triisopropylsilyl-substituted appealing) goal is to use magnesium(i) precursor69. The diiron(i) system exhibits amido is used, reduction of the amido- dimers in catalytic transformations that the shortest Fe–Fe bond (~2.13 Å) yet bromidozinc(ii) precursor instead affords normally require toxic and expensive reported, an unusual interaction given 26, the first example of a complex with an low-valent, late transition metal complexes. that it involves significant multiple bond unsupported Zn–Mg bond. The product, Effecting such transformations would character between two high-spin iron like its manganese counterpart 23, acts as probably necessitate the oxidative addition centres. This highly reactive molecule an inorganic ‘Grignard reagent’ in reactions or reductive elimination of the Mg–Mg is emerging as a promising reagent in with metal halides to give unusual products bond to substrates under mild conditions, several areas, including in small-molecule that include an unprecedented, near linear, it is not beyond the realm of possibility. activations30. A related complex, the two-coordinate mixed-valent trizinc complex Indeed, it is no less likely than the prospect Mn–Mn‑bonded bis(amidinato) [LʹʹZnZnZnLʹʹ] and its heavier analogues of ‘bottleable’ magnesium(i) dimers seemed − * i − dimanganese(i) complex [Mn2(Piso)2] [LʹʹZnMZnLʹʹ] (Lʹʹ = [N(Ar )(Si Pr3)] , to many before 2007. Whatever the case, the − t − 73 (Piso = [(Bu)C(NDip)2] ), was prepared M = Cd or Hg) . future looks promising for the chemistry of using a magnesium(i) reductant69, as magnesium(i) dimers and related low-valent was the related manganese(ii) hydride Conclusions and outlook s‑block systems. (REF. 70) [Mn2(Piso)2(μ‑H)2] . Interestingly, With the isolation of the first stable Cameron Jones is at the School of Chemistry, the latter complex acts as a ‘masked’ source magnesium(i) dimers in 2007 came the Monash University, PO Box 23, Melbourne, of manganese(i) in reactions with small realization that such systems have unique VIC 3800, Australia.

molecules such as N2O and O2. properties that make them attractive as [email protected] Treatment of the extremely bulky soluble, selective, stoichiometric and safe doi:10.1038/s41570-017-0059 monodentate amidohalidomanganese(ii) reducing agents in many areas of organic Published online 19 Jul 2017

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1. Connelly, N. G. & Geiger, W. E. Chemical redox agents 25. Gomberg, M. & Bachmann, W. E. The reducing action 48. Wang, Y. Z. et al. A stable silicon(0) compound with a for organometallic chemistry. Chem. Rev. 96, 877–910 of a mixture of magnesium iodide (or bromide) and Si=Si double bond. Science 321, 1069–1071 (2008). (1996). magnesium on aromatic ketones. Probable formation 49. Jones, C., Bonhady, S. J., Holzmann, N., Frenking, G. 2. Szostak, M., Spain, M. & Procter, D. J. Recent of magnesium subiodide (or subbromide). J. Am. & Stasch, A. The preparation, characterization and advances in the chemoselective reduction of functional Chem. Soc. 49, 236–257 (1927). theoretical analysis of group 14 element(i) dimers: a groups mediated by samarium(ii) iodide: a single 26. Bonyhady, S. J., Green, S. P., Jones, C., Nembenna, S. case study of magnesium(i) compounds as reducing electron transfer approach. Chem. Soc. Rev. 42, & Stasch, A. A dimeric magnesium(i) compound as a agents in inorganic synthesis. Inorg. Chem. 50, 9155–9183 (2013). facile two-center/two-electron reductant. Angew. 12315–12325 (2011). 3. Cintas, P. in Activated metals in organic synthesis Chem. Int. Ed. 48, 2973–2977 (2009). 50. Choong, S. L., Schenk, C., Stasch, A., Dange, D. & (CRC,1993). 27. Lalrempuia, R., Stasch, A. & Jones, C. The reductive Jones, C. Contrasting reductions of group 14 metal(ii)

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69. Fohlmeister, L. et al. Low-coordinate iron(i) and manganese(i) dimers: kinetic stabilization of an exceptionally short Fe–Fe multiple bond. Angew. Chem. Int. Ed. 51, 8294–8298 (2012). 70. Fohlmeister, L. & Jones, C. Stabilisation of carbonyl free amidinato-manganese(ii) hydride complexes: “masked” sources of manganese(i) in organometallic synthesis. Dalton Trans. 45, 1436–1442 (2016). 71. Hoyer, C. E. et al. A two-coordinate manganese(0) complex with an unsupported Mn–Mg bond: allowing access to low coordinate homo- and hetero-bimetallic compounds. J. Am. Chem. Soc. 136, 5283–5286 (2014). 72. Stasch, A. Synthesis, structure, and reactivity of a dimeric zinc(i) compound stabilized by a sterically demanding diiminophosphinate ligand. Chem. Eur. J. 18, 15105–15112 (2012). 73. Hicks, J., Underhill, E. J., Kefalidis, C. E., Maron, L. & Jones, C. A mixed-valence tri-zinc complex, LZnZnZnL (L = bulky amide), bearing a linear chain of two- coordinate zinc atoms. Angew. Chem. Int. Ed. 54, 10000–10004 (2015).

Acknowledgements The author thanks the Australian Research Council and the US Air Force Asian Office of Aerospace Research and Development for funding.

Competing interests statement The author declares no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

How to cite this article Jones, C. Dimeric magnesium(i) β‑diketiminates: a new class of quasi-universal reducing agent. Nat. Rev. Chem. 1, 0059 (2017).

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