Main-group metal complexes of α -diimine ligands: structure, bonding and reactivity

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Introduction
α-Diimine ligands have been widely used for coordination with metals, including most main-group, transition, and f-block metals.These N,N'-donor ligands have a stable N=C−C=N backbone, such as 1,4-diazabutadiene (dad or dab), bis(iminoacenaphthene) (bian), o-benzoquinonediimine (dianion of 1,2phenylenediamine), imino-pyridine, and 2,2'-bipyridine, etc., which can chelate to a metal centre to form a five-membered metallocycle.While the complexes of all these ligands display rich chemistry and applications in many areas, herein only those with the dad (and in some cases bian) species are discussed.
The most notable application of dad (and bian) species is perhaps their use as ligands for late transition metal (Pd II and Ni II ) catalysts for olefin polymerization, as discovered by Brookhart and coworkers in 1995. 1 Owing to the easily modified steric and electronic effects of dad species available by attaching variable substituents at the carbon and nitrogen atoms, it is convenient to control the properties at the metal centre, thereby changing the polymerization activity of the catalyst.As a result, much research has been done in olefin polymerization catalysed by late transition metal α-diimine systems, which has been well-documented in a couple of excellent reviews [2][3][4][5] and will therefore not be included in this article.
Besides the steric and electronic properties, the dad ligands possess another favourable characteristic, that is, they are redox active and have adjustable valence states.These species have a low-lying LUMO orbital and can readily accept one or two electrons upon reduction, thereby converting into the radical-anionic (L •− ) or dianionic (enediamido, L 2− ) form, respectively (Scheme 1).This non-innocence of α-diimine ligands, on the one hand, causes problems in assigning the oxidation levels of both metal and ligand; but on the other hand, it brings about rich electronic structures and properties of the ligand and complex.Firstly, the dad ligands in their three redox levels can effectively stabilize a series of metal centres in different oxidation states, including low-valent metals and those in metal-metal-bonded compounds.Secondly, it is possible to form electron-transfer complexes with novel optical and magnetic properties.Thirdly, these ligands can serve as electron reservoirs and donate/accept electrons when needed, thus making them capable of participating in reactions toward other molecules together with metals.Therefore, such ligands are of great interest in the areas of low-valent complexes and small molecule activation.
][8][9][10] Due to the existence of both transition metal and αdiimine ligands that have many possible valence states, the clear assignment of the electronic structure of such complexes is highly challenging.Nevertheless, by a combination of several experimental techniques, including EPR, magnetic susceptibility, Mössbauer spectroscopy, X-ray diffraction, etc., as well as theoretical calculations, the electronic structures of a large number of homo-and heteroleptic complexes containing α-diimine ligands have been thoroughly Please do not adjust margins Please do not adjust margins elucidated. 7,11 n the other hand, complexes of s-, p-, and fblock metals with the dianionic enediamido form (L 2− ) of dad (and related diimine) ligands have been summarized recently by Cui et al. 12 Very recently, dad (in the reduced forms) complexes of earlier transition metals and f-block metals (groups 3, 5, 6) have been reviewed. 13An earlier perspective article summarized the coordination chemistry of bian ligands with sand p-block elements, 14 while a more recent review presented the synthesis and catalytic properties of p-block complexes with bian derivatives. 15These specific areas are largely excluded here with only small portions being covered in this current article.
Herein, we intend to focus on the use of dad (and bian) ligands in the stabilization of metal-metal-bonded compounds, in particular those of main-group metals, as well as on the small molecule activation by these (low-valent) metal coordination species where the non-innocence of the ligands plays a key role.Recent progress in the study of heavier maingroup elements not only reveals new structural and bonding features, but also demonstrates that these species can display similar chemistry to the transition metals with respect to their ability to catalyse many organic reactions and to promote chemical transformations. 16These features, together with the advantages of α-diimine ligands, result in unique fundamental and practical advances in this field.

Non-innocence of α-diimine ligands
As mentioned above, when the LUMO of the neutral α-diimine is filled with one or two electrons, the ligand is reduced to the radical monoanion (L •− ) or dianion (L 2− ), respectively.This can be achieved by a variety of reducing agents, especially metals such as Li, Na, K, Mg, Al, Zn, Ga, etc., accompanied by the formation of the corresponding metal complexes/salts.On populating electron(s), the C−C and C−N bond lengths of the initial N=C−C=N backbone of the dad molecule gradually change.The neutral ligands display a typical C=N double bond (ca.1.29 Å) and a C−C single bond length (ca.1.50 Å).In the monoanion (L •− ), the single electron is delocalized over the central C2N2 backbone, and the CN distance is elongated while the CC distance is shortened, resulting in averaged bond lengths (N C C N) in between a single and a double bond (both can vary over a relatively wide range).When the ligand acquires two electrons to become dianionic (L 2− ), the CN distance is further lengthened and the CC length shortened, consequently displaying a "long-short-long" (N−C=C−N) 2− mode.Besides the internal bonds within the ligand, the metal−ligand bond lengths are also dependent on the redox form of the ligands, as the M−N bond with dianionic ligand is covalent in character and normally is shorter than the dative coordination bond with the neutral ligand.Such features are well-established in the literature for dad complexes, which can be conveniently judged from the respective crystal structures. 4,5Therefore, single-crystal X-ray diffraction is a reliable and most important method in the assignment of the redox levels of α-diimine ligands (and subsequently the oxidation state of metal centre).Nevertheless, other techniques such as NMR and EPR spectroscopy, magnetic susceptibility measurements, UV-Vis spectra, and DFT computations are also necessary to provide complementary evidence for the elucidation of the electronic structures of such complexes.For example, EPR spectroscopy and magnetic studies can help to establish the paramagnetic nature of the species and to localize the radicals as well as their interactions, while theoretical studies may give some clue about the bond order, charge distribution, spin status, etc.

Various complexes of α-diimine ligands
All of the three states of the α-diimine ligand can coordinate to various metal centres.As mentioned above, the neutral ligands normally coordinate in the N,N'-chelating fashion (Scheme 2) to metal ions in the common oxidation states (e.g.Ni II and Pd II ) to yield typical Werner-type complexes, such as LMX2. 4,5 n a few cases, neutral dad ligands can also bond to low-valent metal centres, such as in Ni(0) complexes with alkenes, L'Ni(C2H4)2 17 and LNi(alkene), 18 which may be facilitated by back bonding to the alkene thereby depleting the electron density at nickel(0).For the monoanionic and dianionic dad ligands, there are several coordination modes, including the normally seen σ 2 -, σ 2 ,π-and μ-(σ 2 )(σ 2 ,π) in which the two N atoms are arranged in a cis (chelating) conformation, and the uncommon bridging modes, either cisμ-σ 1 ,σ 1 - [19][20][21] or trans-μ-σ 1 ,σ 1 -coordination [22][23][24] where the two N atoms adopt a cisor trans orientation, respectively, and each of them coordinates to one metal centre (Scheme 2).These two reduced forms of dad ligands are found to be able to stabilize low-valent metal centres.For example, the Cr-Cr quintuply bonded compound [LCrCrL] with Cr(I) centres is stabilized by the radical monoanionic form of the α-diimine ligand dpp-H dad (dpp-NCHCHN-dpp) in the bridging cis-μ-σ 1 ,σ 1mode. 21 Coordination modes of α-diimine (dad) ligands in the three oxidation levels.
In this part, representative complexes of α-diimine ligands, particularly in the radical anionic and dianionic form, will be introduced.These include the main groups 1 (Li, Na, K), 2 (Be to Ba), 12 (Zn), 13 (Al, Ga, In), 14 (Ge, Sn), and 15 (As, Sb, Bi) metals.It is worth noting that most of these complexes are comprised of dianionic (enediamido) ligands.Moreover, besides the conventional solution-based synthesis, recently bian ligands and their complexes have also been synthesized 6][27] The structure and bonding of selected complexes are presented herein, while their reactivity will be discussed in Part 4 with emphasis on the effects of ligand non-innocence.

Complexes of groups 1 and 2 metals
Group 1 metals are most commonly used as reducing agents to generate the anionic α-diimine ligands.The reduction of αdiimines by the alkali metals Li, Na or K gives the monoanionic or dianionic form as metal complexes (or salts).A couple of alkali metal complexes have been structurally characterized, which generally act as intermediate reagents in the synthesis of organometallic compounds through metathesis with other metals.][30] The Yang group synthesized a series of sodium complexes with the doubly reduced, enediamido ligands by reduction of the neutral ligands with Na metal. 31In the dimeric complexes 3-5, the two Na + ions bonding to a same dad 2− ligand show different coordination modes {μ-(σ 2 )(σ 2 ,π)}, i.e. one of them is chelated by the two N donors (σ 2 -), while the other is bonded by the N−C=C−N moiety (σ 2 ,π-) of the same ligand (and solvent molecules), the latter of which deviates significantly from the ligand plane.When the non-coordinating solvent toluene was used, a polymeric compound (6) was obtained, in which the Na + ions are not solvated but interact with aryl rings and bridge two ligands to form an infinite structure (Fig. 1).In these sodium complexes, the C2N2 moiety displays obvious "long-short-long" enediamido character (C-N bond lengths are in the range 1.402~1.430Å and C-C bonds in the range 1.356~1.366Å).
Fig. 1 Alkali metal complexes of mono-and dianionic dad ligands.
3][34] These ligands combine both the α-diimine and robust naphthalene π systems in a single conjugated system, thus exhibiting specific redox properties. 35In 2003, Fedushkin et al. reported that the dpp-substituted ligand 1,2-bis[(2,6-diisopropylphenyl)imino] acenaphthene (dpp-bian) can accept up to four electrons in a step-by-step reduction by Na metal, forming the mono-, di-, tri-, and tetra-anions (complexes 7-10), where the first two electrons are placed on the α-diimine backbone (to yield 7 and 8; Fig. 2), while the other two electrons are mainly delocalized over the naphthalene part. 36The Fedushkin group has extensively studied the complexes of this ligand with main group and lanthanide metals, some of which will be discussed later in this review.The chemistry of group 2 complexes with α-diimine ligands has been well developed.These alkaline earth metal complexes can be obtained either by salt metathesis of the above alkali metal complexes with MgX2 (M = Mg, Ca, Sr, Ba), or by direct reduction of the neutral dad ligands with alkaline earth metals.A series of magnesium complexes bearing either monoanionic or dianionic dad ligands, including [L 2− Mg II (thf)n] (11, 12), [Mg II (L •− )2] (13, 14), and [K]2[Mg II (L 2− )2] (15) (Fig. 3), have been synthesized through the reduction of dad ligands by K metal followed by reaction with MgCl2.Notably, the complexes [Mg II (L •− )2] display typical diradical character as proved by EPR spectroscopy, and the C−C (L iPr , 1.403(5) Å; L Mes ,  1.460(3) Å) and C−N (L iPr , 1.358(4), 1.367(4) Å; L Mes , 1.344(3), 1.342(3) Å) bond lengths of the α-diimine backbone are consistent with averaged single and double bonds of the radical monoanionic ligands. 37Similarly, calcium complexes 16-20 have also been obtained from the reduced ligands (by Li, Na, or K) and CaCl2, 38 while analogous complexes 21-24 of heavier group 2 congeners have been accessed by both salt metathesis and direct metalation of different dad ligands with metallic Ca, Sr, and Ba. 39These complexes display mono-or binuclear structures with two reduced forms of the dad ligands, and (solvated) alkali metal ions are frequently incorporated to balance the charge.
Alkaline earth metal complexes of the bian ligands have also been studied by the Fedushkin group.Reaction of the neutral ligands with the corresponding metals afforded the mononuclear Mg(II) and Ca(II) complexes with the dianionic ligand, [L 2− M(thf)n] (25, 26; Fig. 3), 40 in which the Mg or Ca centre is coordinated by one σ-N,N' ligand and solvent (thf) Please do not adjust margins Please do not adjust margins molecules.These complexes exhibit versatile reactivities towards a series of substrates, which will be presented later.
Although the oxidation state of alkaline earth metals in their complexes and reagents is dominated by +2, as has been found in above complexes with dad ligands, the synthesis of the first stable molecular Mg(I) compounds containing an Mg−Mg bond ligated by β-diketiminate or guanidinate ligands 41 has expanded this chemistry to the subvalent Ae(I) (Ae = alkaline earth metals) species.Yang et al. reported that the dpp-substituted enediamido ligand can also effectively stabilize the Mg−Mg bond. 42The centrosymmetric dimer [K(thf)3]2[L 2− Mg I −Mg I L 2− ] (27, Fig. 3) was prepared by a one-pot reduction of the ligand dpp-dad and MgCl2 with excess potassium metal in tetrahydrofuran, in which the initial Mg II ion is reduced to Mg I , while the neutral ligand is reduced to the dianion L 2− .The compound is air-and moisture-sensitive but thermally stable in solution and the solid state.][45][46][47] Computations revealed that in these Mg I species, the metal−metal bond is formed mainly by the 3s orbitals of magnesium.These species display excellent reactivity and some of their reactions will be discussed in this article (vide infra).
Inspired by these results, attempts have been made to synthesize metal−metal-bonded species of other group 2 metals by using different types of ligands.However, up to now, no analogous dimetallic compounds with a metal−metal bond could be isolated other than for Mg, although theoretical calculations predict that some of these low-valent alkaline metal compounds should be stable. 48,49 n the case of αdiimine ligands, computations also indicated that the [LMML] dimer (M = Be to Ba) can be stabilized by the dianionic dad ligand, 50 but attempts to synthesize the possible dimeric calcium(I) compound yielded the Ca(II) complexes (16-20) as mentioned above.

Complexes of group 12 metals (Zn)
Having the 3d 10 4s 2 valence electron configuration, zinc displays very similar coordination properties to group 2 metals.In recent years, one of the most important findings in zinc chemistry is perhaps the synthesis of the first Zn-Zn-bonded compound Cp*Zn-ZnCp* (Zn-Zn: 2.305(3) Å) in 2004, 57 which was followed by a number of dimeric zinc(I) species ligated by a wide variety of ligands, including α-diimines dad and bian, thus indicating that the Zn-Zn bond has greater tolerance of the supporting ligands than does the Mg-Mg bond.][60] The dpp-dad stabilized Zn-Zn-bonded compounds [M(thf)2]2[L 2− Zn I −Zn I L 2− ] (31, M = Na, Zn-Zn: 2.393(1) Å; 32, M = K, Zn-Zn: 2.399(1) Å) were synthesized by Yang et al. via the reduction of the dihalide L 0 ZnCl2 by Na or K metal. 61,62 ndeed, the synthesis and structure of the aforementioned Mg-Mgbonded compound 27 44 greatly resembles those of the Zn-Zn analogues, except that the former is prepared from a one-pot reaction of MgCl2, L, and alkali metal (K), while in the latter case, the LZnCl2 precursor was isolated first and then reduced by Na or K.In both cases, the initial divalent metal ion is reduced to the subvalent M(I), while the neutral ligand is doubly reduced to the dianion.The structures of the two compounds are centrosymmetric, with M(I) centres being three-coordinated by the N donors of the ligand and another M atom.Interestingly, there are two solvated alkali metal ions, which reside over the central N-C=C-N ligand backbone and interact with the moiety in η 4 -C2N2 fashion.Such an "alkali metal capped" feature is found to be characteristic for the enediamido ligands as required for charge balance.On the other hand, it may provide additional stabilization for the molecule from both an electronic and a steric viewpoint.
The effects of substituents of the dad ligand on the formation of Zn-Zn bond have been further explored by the Yang group.It has been found that 2,6-diisopropylphenyl (dpp) groups are essential for the stabilization of the low-valent complex, as other substituents such as mesityl or 2,6-diethylor 2,6-dimethylphenyl led to mononuclear Zn II complexes (33-36; Fig. 5) rather than the dimeric Zn I species. 63Moreover, attempts to synthesize the Zn-Zn bond bearing the radical monoanion of the dad ligand starting from the L 0 ZnCl2 precursor have proven to be unsuccessful (complex 37) despite attempts employing various amounts of the reducing agent (Na or K).Notably, complex 37 contains a ligand with a central C2N2 core in the trans conformation, acting as a μ-σ 1 ,σ 1 bridge between two zinc atoms (Scheme 2).Instead, through first reduction of the ligand to the dianion and then metathesis with ZnCl2, the desired overall neutral compound [L •− Zn-ZnL •− ] (38) was eventually obtained, in which the ligand noninnocence played a key role, as the initial dianionic ligand acted as the electron donor to reduce the Zn II to Zn I .
A similar Zn-Zn-bonded compound with the radical anionic bian ligand, [(dpp-bian) •− Zn-Zn(dpp-bian) •− ] (39, Fig. 5), has been synthesized by Fedushkin et al. either from the reductive salt elimination of (dpp-bian)ZnI or by reaction of the reduced dianionic ligand with ZnCl2. 64The Zn-Zn bond length (2.3321(2) Å) is comparable to that of the dad analogue Please do not adjust margins Please do not adjust margins (2.340(2) Å).However, the dianion-ligated dizinc(I) species was not reported, nor the Mg-Mg bond with either monoanionic or dianionic bian ligands.These results suggest that there are both similarities and differences between the dad and bian ligands, which are clearly reflected in the reactivity studies (vide infra).

Complexes of group 13 metals (Al, Ga, In)
Organoaluminium compounds are widely used in organic synthesis and catalytic reactions.6][67] These compounds are ligated by Cp derivatives, 68 β-diketiminate (nacnac), 69 alkyls and silyls, 70,71 and α-diimines, etc.Such low-valent species display novel bonding and reactivities and have been extensively studied.In this section, the complexes of lowvalent group 13 metals Al and Ga with reduced dad and bian ligands are summarized, while their reactivity will be described later in Part 4.
The development of aluminium and gallium complexes bearing dad or bian ligands largely parallels each other.In both cases, the Al-Al-bonded dialumane species have been prepared.The centrosymmetric dimer [L 2− (thf)Al II -Al II (thf)L 2− ] (L = dpp-dad, 40, Al-Al bond length: 2.658(2) Å; Fig. 6) is the product of the reduction of the mixture [Na2L] (prepared in situ) and AlCl3 by sodium metal in thf. 72The Al atom is fourfold coordinated with the two N donors of a dad ligand, one thf molecule and the other Al atom.
Notably, further reduced carbene-like Al I species [LAl:] − have not yet been isolated for either dad or bian ligands.In contrast, a couple of either neutral (e.g.7][78][79][80] Nevertheless, the Ga I analogues of both dad and bian derivatives have been isolated (see below).This may be due to the reason that the dad and bian Al I species are too reactive to be captured, and transform further during the small molecule reactions as described later in this article.Gallium complexes of α-diimine ligands have also been synthesized with the gallium centre in the formal oxidation states of +3, +2 and +1, with the ligands as both radical monoanion and dianion forms.Examples include the paramagnetic gallium(III) complex 45 that contains a dianion and a radical anion (to yield an overall neutral complex), 81 the Please do not adjust margins Please do not adjust margins diamagnetic gallium(II) complex 46, 82 and the anionic gallium(I) heterocycle 47. 83 Jones et al. synthesized the anionic Ga I heterocycle and obtained the Zn-Ga-bond compounds 48 and 49 (Fig. 7) by reactions with two N,N-chelated zinc chloride complexes. 84In these complexes, the [LGa:] − species can be viewed as a Lewis base donor that coordinates to the zinc(II) centre.Fedushkin et al. synthesized the homonuclear Ga-Gabonded [(dpp-bian)Ga-Ga(dpp-bian)] ( 50) and heteronuclear Ga-Zn-bonded [(dpp-bian)Zn-Ga(dpp-bian)] ( 51) compounds with the dpp-bian ligand.The former was obtained either by reaction of the trianionic ligand K3(dpp-bian) with GaCl3 at room temperature in Et2O/thf, 85 or by direct reaction of the ligand with metallic gallium in toluene at reflux. 86The latter [GaZn] compound was synthesized starting with the dimeric zinc iodide, [(dpp-bian)ZnI]2, which was prepared from the neutral ligand with zinc metal and iodine (I2), through reaction with GaCl3 and the tetraanion K4(dpp-bian). 85In the digallium(II) compound, both ligands are in the dianionic form, while in the heteronuclear Zn-Ga species there is a radical anionic and a dianionic ligand present.
Moreover, the digallane 50 was further reduced by groups 1 and 2 metals, which afforded molecular compounds containing Ga-M bonds, [(dpp-bian)Ga-M(Solv)n] (M = Li, Na, K; Solv = solvent molecules; 52-54) and [(dpp-bian)Ga]2-M'(Solv)n (M' = Mg, Ca, Sr, Ba; 55-58) (Fig. 8). 44The reduction generated the formal oxidation state of Ga: I , which bonds to the alkali metal or alkaline earth metal ions through lone pair donation.Similarly, the digallane and carbene-like gallylene species of the analogous dad ligand have been reported by the Yang group (Fig. 9).The precursor [(L) •− GaCl2] (59), which contains a Ga III centre and a radical-anionic ligand, was prepared by the reaction of anhydrous GaCl3 with the monoanion [NaL] (in 1:1 ratio). 87The digallane 60 was obtained from reduction of the dichloride precursor 59 by 2 equivalents of Na, which reduced the Ga III centre to Ga II and the ligand L •− to L 2− . 88Then, further reduction of the [(L) •− GaCl2] complex by 3 equivalents of alkali metal (Na, Li, K, and KC8) afforded a series of Ga-M complexes 61-63. 87During the latter reaction process, the trivalent Ga III was reduced to the monovalent Ga I , while the radical anion of the α-diimine ligand acquired one electron to form the dianion.In these alkali metal adducts of the [LGa:] − moiety, Ga-M bonds are also formed by donation of the lone-pair electrons on Ga to the alkali metal ions.
Going down the group 13 elements to indium, related studies on its α-diimine complexes are much rarer.While both of the four-(with amidinate or guanidinate backbone) and sixmembered (with β-diketiminate ligand) heterocycles of indium(I) are known, the (anionic) five-membered analogues (with α-diimine) are yet to be synthesized although they were theoretically predicted to be similar to their Al and Ga congeners. 65Only a few examples of indium α-diimine halides have emerged recently, including investigations of the synthesis, optical and electrochemical properties of a series of indium(III) complexes (64-68) which consist of neutral Ar-bian ligands coordinating to indium(III) centre via dative bonds (Fig. 10). 27,89

Complexes of group 15 metals (As, Sb, Bi)
A few group 15 element complexes with bian ligands have been reported.The arsenium salt [(dpp-bian)As][SnCl5•thf] (85) was synthesized from the reaction of AsCl3 with SnCl2 in thf and then treatment with dpp-bian. 101In the complex, the arsenic is in the +3 oxidation state, whilst the AsN2C2 ring is planar.The C-N and As-N bond distances compare well to those in previously reported cyclic arsenium cations. 102he mononuclear antimony(III) compound [(dppbian)SbCl3] (86) was isolated from the reaction of SbCl3 with the dpp-bian ligand. 98The Sb atom is penta-coordinated with a distorted square-pyramidal geometry, where the antimonybased lone pair is located at the equatorial position.Similarly, reactions of BiCl3 with the corresponding Ar-bian ligands afforded the compounds [(dpp-bian)BiCl3] (87) and [(mesbian)BiCl3] (88). 98Different from 86, complexes 87 and 88 are unsymmetrically μ2-chloride-bridged dimers, with Bi-Cl Please do not adjust margins Please do not adjust margins distances of 2.6976(11) and 3.0488(18) Å.However, the reaction of dpp-bian or mes-bian with AsCl3 was unsuccessful, which may be due to the weaker Lewis acidity of the arsenic(III) halides in comparison to their heavier congeners.

Reactivity of α-diimine-ligated complexes
Small molecules, which are ubiquitous in nature or are produced in industrial processes, are useful stock materials for further transformation into value-added products, in which the activation of chemical bonds is a necessary step.Traditionally, the activation and transformation of small molecules are mostly mediated by transition metals, but the use of maingroup metals has increased in recent years because the heavier main-group elements can exhibit transition metal-like behaviour when forming suitable complexes. 16For example, the low-valent main-group species, in particular metal-metalbonded compounds, have been found to be able to activate a variety of inorganic and organic small molecules. 103,104  this part, we will collect some representative examples of the studies on small molecule reactivity of selected maingroup metal-metal-bonded compounds, with emphasis on the work of the Fedushkin and Yang groups on the reactivity of group 13 metal (Al and Ga) α-diimine complexes towards various small substrates.In addition, some related studies on small molecule activation properties of magnesium and zinc compounds with α-diimine ligands will be included.
For the metal-metal-bonded compounds, the activation of small molecule can occur cooperatively between the metal centers, 105 behaviour which is also observed in biological processes. 106On the other hand, the non-innocent α-diimine ligands can endow their complexes with unique reactivities that cannot be realized without these ligands, because such ligands can actively participate in small molecule activation processes through several routes.Firstly, it has been proven that the reduced forms of the α-diimine ligands can support low-valent metal centres, thus evoking special reactivity of the metals.Secondly, they can act as electron reservoirs and participate in the electron transfer processes either alone or cooperatively with the metal centre.Thirdly, these ligands possess the ability to undergo transformations themselves or to bond with the substrates, thus providing a possible method to form new bonds.Therefore, the α-diimine-ligated metalmetal-bonded compounds have exhibited unusual reactivities.
As typical examples, the main-group metal complexes of dad and bian ligands (mostly in the enediamido form) display surprisingly diverse reactions with a wide variety of substrates.They can act as reducing agents and promote oxidative additions, 107 cycloaddition or reductive coupling of unsaturated molecules such as alkynes, [108][109][110][111] isocyanates, 112 isothiocyanates, 113 alkenes, 114 SO2, 115 azabenzene derivatives, 72 nitriles and isocyanides, 42,116 and so on. 117erein, some examples will be given and the non-innocent behaviour of the α-diimine ligands will be discussed.

Reactivity of magnesium complexes
Fedushkin et al. prepared the magnesium complex [(dpp-bian) Mg(thf)3] (25) 40 by the reduction of dpp-bian with metallic magnesium in thf and studied the reactivity of this complex (Fig. 13).In the reactions with Ph2CO, anthracenone, 118,119 iodine, 119 and RS-SR, 120 the dianionic ligand provided one electron and became the radical-anionic form (in products 89-95).When complex 25 was reacted with ethyl halides EtX (X = Cl, Br, I) in thf, oxidative addition occurred via a single-electron transfer process, wherein the ethyl group is bonded to the carbon atom of the ligand, and an X − ion is coordinated to the Please do not adjust margins Please do not adjust margins magnesium atom (96-98). 121Moreover, reactions with PhCCH, enolisable ketones, 118 and Ph2CHCN 122 proceeded via protonation of the ligand at the basic nitrogen atom, leading to the transfer of protons to yield products 89-93.
Yang et al. reported that the Mg−Mg bond compound 27 exhibits good reactivity toward a series of nitriles (Fig. 14). 42eaction of the Mg(I) dimer with 2.0 equiv. of Me3SiCN proceeded through reductive Si−C(CN) bond cleavage to generate the cyanide ion (CN − ), which acts as a bridge to link the tetrameric complex 104.When increasing the molar amount of Me3SiCN or tBuCN to 3 equivalents, both Si−C (or C−C) bond cleavage and terminal coordination of the nitriles occurred, leading to the tetramers 105 and 106.Moreover, the Mg(I) compound can promote the reductive deprotonation of the α-H (through release of H2) of isobutyronitrile (iPrCN) or cyclohexyl nitrile (CyCN) to form the dinuclear products (107, 108) with two bridging ketimine (R'C=C=N) ligands.In these reactions, the Mg(I) centres act as electron donors and are oxidized to Mg(II), while the dad ligand remains in the dianionic form during the reaction processes.

Reactivity of zinc complexes
In addition to the Zn−Zn-bonded species 31 and 32 of the dianionic ligand (L 2− ) and 38 of the radical monoanion (L •− ), Yang et al. reported three related Zn(I) compounds 109-111, in which the initially η 4 (C2N2)-bonded alkali metal ions (Na + or K + ) are captured by crown ethers (Fig. 15). 123Removal of the alkali metal ions from the ligand central backbone results in elongated Zn−Zn bonds.Then, the reactions of all the three types of Zn−Zn-bonded compounds with phenylacetylene (PhCCH) were carried out, which proceeded through different redox processes and gave three types of alkynylzinc products.
For the monoanion-ligated precursor [L •− Zn−ZnL •− ] (38), reactions with different amounts of alkyne afforded two products, the bis(alkynyl)-bridged dinuclear complex [L •− Zn(μ-CCPh)2ZnL •− ] (112) and the mononuclear complex [L 0 Zn(CCPh)2] (113).In both cases, the Zn(I) centres are oxidized to Zn(II), while the initial radical monoanionic ligands either remain intact (in 112) or, rather surprisingly, are oxidized to the neutral form L 0 (in 113).In the latter case, both of the Zn(I) centres and the monoanionic ligands participate in the reductive dehydrogenation of phenylacetylene (to form H2), leading to the simultaneous activation (deprotonation) of 4 equivalents of alkyne molecules.On the other hand, the (15-crown-5)-containing 110 also reacted with phenylacetylene through a disproportionation of the Zn 2+ units to give the homoleptic tetraphenylethynyl zincate 114, in which the dad ligands are dissociated as the free neutral form and the Zn(II) centre is coordinated solely by (four) alkynyl anions (Fig. 15).Also, the bian analogue 115 has been isolated from the reaction of [(dpp-bian) •− Zn−Zn(dppbian) •− ] (39) with PhCCH. 124In these reactions, different redox processes are observed, in which the non-innocent dad ligand plays important roles in receiving or donating electrons.

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Reactivity of aluminium and gallium complexes
The low-valent complexes of aluminium and gallium in the formal oxidation states of +2 and +1 display very rich reactivities toward inorganic and organic small molecules.In this section, the recent progress in the reactions of dialumanes (Al II ) and digallanes (Ga II ) bearing dad and bian ligands, respectively, as well as of the corresponding Al I and Ga I species, with various substrates will be summarized.
The dialumane of dpp-dad, [L(thf)Al−Al(thf)L] (40), which contains dianionic ligands and Al(II) ions, has been shown to be an excellent electron donor that is capable of providing multiple electrons to substrates.A good example of metalligand cooperative reduction of small molecules is the reaction of 40 with azobenzene derivatives to give bis(imido)-bridged dinuclear complexes 116-118 (Fig. 16).During the reaction, the dialumane behaves as a four-electron reductant, leading to complete reductive cleavage of the N=N double bond, while the initially dianionic ligands are oxidized into monoanions and the Al(II) centers to Al(III).Notably, when the asymmetric azobenzenes (4-methoxyazobenzene and 4dimethylaminoazobenzene) were used, asymmetric products were obtained, thus proving the cooperation of the metal−metal bond in the reaction process.Overall, all components, the two metal centres and two ligands, simultaneously participated in the reactions, resulting in the multi-electron transfer products.Another characteristic metal-ligand cooperation in such systems is exemplified by the cycloaddition reactions of the αdiimine-ligated Ga(II) and Al(II) species with a variety of unsaturated bonds, such as alkynes (C≡C) and cumulated C=C and C=E bonds (vide infra).
In 2010, Fedushkin et al. reported the reversible cycloaddition of alkynes to the Ga-N-C fragments of the digallane [(dpp-bian)Ga−Ga(dpp-bian)] (50). 109The triple bond of an alkyne can add to each metal-ligand moiety by forming both carbon-carbon and carbon-gallium bonds at room temperature, while at elevated temperatures the alkyne molecules can dissociate (Fig. 17).In the cycloaddition products (119−122) of different alkyne substrates, regioselectivity was observed in that a bulkier substituent at the triple C≡C bond (e.g.Ph group) is oriented away from the metal, but the smaller ester group is oriented toward the metal.
Please do not adjust margins Please do not adjust margins In 2012, the same group reported analogous cycloadditions of alkynes to the dialumane [(dpp-bian)Al−Al(dpp-bian)] (44) to give the products 123−127 (Fig. 17). 74In contrast to the digallane species, the dialumane is reactive also toward the internal alkyne MeCCPh, where one or two alkynes can add to the Al-N-C moieties of dialumane.Notably, the cyclo-adduct of phenylacetylene can undergo rearrangement at elevated temperatures to form a bis alken-1,2-diyl-bridged dinuclear complex 125 upon Al−Al bond cleavage and transformation of the ligand to the radical-anionic form.
The reactivity of the dad dialumane [L(thf)Al−Al(thf)L] (40) toward alkynes has also been explored by Yang et al. (Fig. 18). 111It is noted that this dialumane exhibits different reaction types from the bian analogue.Although 40 is also active to internal alkynes, the reaction with diphenylacetylene affords a unique dinuclear product 128, wherein the C≡C triple bond inserts into the Al−Al bond, resulting in the cleavage of Al II −Al II with accompanying electron transfer from metal to substrate to form Al III and reduction of the alkyne to a dianion (while the dianionic ligands remain unchanged).However, with phenylacetylene (or 4-ethynyltoluene) the cycloaddition of the alkyne to the AlN2C2 ring, as in the above cases, has occurred, yielding the bis(bicyclic) products 129 and 130 where the Al II −Al II core is retained.Thus, it is clearly seen that the dialumane 40 can behave as a multicentre electron donor, in which different redox centres (L 2− or Al II ) can be involved in the electron transfer processes.On the other hand, the Ga II species of dad does not react with alkynes, which is also different from the bian analogue.The reactions of dialumane 40 with alkenes (C=C double bond) have also been studied, 114 which represent the first examples of reactions of an Al−Al bonded compound with alkenes (Fig. 19).The insertion of styrene or stilbene into the Al−Al bond occurs upon electron transfer from Al(II) to the alkene similar to the case of diphenylacetylene insertion, yielding two Al(III) centres bridged by the reduced alkene (131, 132).In comparison, in the presence of metallic sodium, the reactions of 40 with butadienes yielded aluminium cyclopentenes as [1+4] cycloaddition products 133, 134, as well as the [2+4] cycloaddition product 135.In these two reactions, it is proposed that the dialumane was actually reduced to the Al(I) species, as either the dinuclear "dialumene" [Al=Al] 2− or the mononuclear carbene analogue [LAl:] − by the Na metal, as demonstrated by theoretical calculations, although these two species have not yet been isolated.In the products, the diene exists as the enediyl form, while the aluminium displays the formal oxidation state of +3.Fedushkin et al. studied the reactions of bian-ligated digallane 50 with various substrates, such as acenaphthenequinone (AcQ), sulfur dioxide, and azobenzene, 115 which yielded the products 136−140 (Fig. 20).Owing to the existence of two types of redox-active centres in the molecule, either metal-or ligand-centred (or both) redox processes may occur, leading to diverse reactivities.For example, reactions with different equivalents of AcQ and SO2 undergo different redox processes.First, with 1 equiv. of AcQ, the dianionic ligands donate electrons and become monoanionic, while the Ga II centres and Ga−Ga bond remain intact (in 136).However, with increasing amounts of substrate (2 equiv.), the Ga II centres begin to participate in the electron transfer, resulting in metal−metal bond cleavage to give Ga III (137).Similarly, in the reactions with sulfur dioxide, addition of 2 or 4 equiv. of the substrate gives different products (138 and 139) in which only the ligand or cooperative metal-ligand are involved, respectively, in the reduction process.In the case of azobenzene (140), both metal and ligand redox centres are involved in the four-electron reductive cleavage of N=N double bond as in the case of dad dialumane. 118lease do not adjust margins Please do not adjust margins The Yang and Fedushkin groups further studied the redox activity and reactions of both low-valent gallium and aluminium compounds (in the oxidation states of +2 and +1) towards a wide variety of small molecules, including isothiocyanates, 113 isocyanates, 112 carbodiimides, 125,126 nitriles, 115 isocyanides, 116 and some nitrogen-rich compounds (organic azides, azo, diazo-compounds), 127 where many reaction pathways have been observed, yielding novel products.Both similarities and differences between the dadand bian-ligated complexes are observed and discussed.
Please do not adjust margins Please do not adjust margins Herein, we only discuss the reactions of dad and bian digallanes with isothiocyanates as an example to demonstrate the rich reaction pathways possible (Fig. 21).Three types of reactions are observed for the reactions of the digallanes with isothiocyanates, including: (i) [2+4] cycloaddition with retention of the Ga−Ga bond (141−144), in which the C=S double bond of RN=C=S adds to the metal-ligand moiety (as in the case of alkyne cycloaddition), forming both Ga−S and new C−C bonds.Notably, this reaction is reversible for the bianligated digallane but not for the dad analogue, thus demonstrating the difference of the two systems with different ligands; (ii) complete cleavage of the C=S bond in the presence of Na metal, yielding the disulfide-bridged dinuclear compounds (145−147); and (iii) both cycloaddition and reductive cleavage of the C=S bond can occur (products 148 and 149).These compounds are the first examples of the cycloaddition of isothiocyanates to the dianionic α-diimine ligands.In some of the reactions, the ligands are found to be active, transferring electrons to the substrate together with metals.For example, in the formation of the disulfide product 146, both of the Ga(II) ions and dianionic ligand behave as electron donors, leading to the reductive cleavage of the C=S double bond to yield S 2-ions that bridge the two Ga(III) centres, while the bian ligand is oxidized to the monoanion.Moreover, the two dialumanes (40 and 44) have been shown to be able to activate different kinds of nitrogen-rich substrates (Fig. 22), yielding the diimido-bridged dinuclear complexes (150-153) through bond cleavage of azides or azobenzene. 127They can also doubly reduce the diazomethane derivatives to form the terminal N-bridged complexes 154 and 155.In all these cases, the dialumanes act as four-electron donors through the cooperation of metal and ligand, leading to either N=N double bond cleavage or reduction of the substrates, while the metal and ligand are oxidized from Al(II) to Al(III) and L 2-to the radical anion L •-, respectively.
The dad ligated dialumane 40 was found to exhibit novel reactivity toward isocyanides, and both linear-and cyclotrimerization of tert-butylisocyanide has been observed (156-159, Fig. 23), wherein the latter resulted in a unique threemembered ring system, namely the aromatic tris(tertbutylimino)deltate dianion 159. 116Moreover, 40 can effectively activate pyridines, resulting in an organometallic metallo-macrocycle (161) with six Al atoms bridged by six pyridine-4-yl carbanions formed by the reductive dehydrogenation of pyridine (Fig. 24). 128hese studies clearly demonstrate the excellent ability of the complexes with α-diimine ligands (mostly in the reduced dianionic or monoanionic form) and low-valent metal centres to activate a wide variety of small molecules.In many cases, the redox non-innocent ligand actively participates in the reaction process, playing an important role in the transformation of the substrates and endowing the metal complex with specific reactivity.

Fig. 3
Fig.3Alkaline earth metal complexes of mono-and dianionic dad and bian ligands.

Fig. 19
Fig. 19 Reactions of 40 with alkenes to afford insertion or cycloaddition products.