The unique β -diketiminate ligand in aluminum( I ) and gallium( I ) chemistry

Over the past few decades, β -diketiminate ligands have been widely used in coordination chemistry and are capable of stabilizing various metal complexes in multiple oxidation states. Recently, the chemistry of aluminum and gallium in their +1 oxidation state has rapidly emerged. NacNacM( I ) (M = Al, Ga; NacNac = β -diketiminate ligand) shows a two coordinate metal center comparable with singlet carbene-like species. The metal center also possesses a formally vacant p-orbital. In this article we present an overview of the last 10 years for aluminum( I ) and gallium( I ) stabilized by β -diketiminate ligands that have been widely explored in bond breaking and forming species.


Introduction
Complexes containing a low-oxidation metal center is a key topic in modern organometallic chemistry as it leads to the development of new systems. A number of transition metal complexes were synthesized in a broad range of oxidation states to activate small molecules or used as catalysts in organic reactions. 1 It was not until 1991 that the first structurally characterized molecular aluminum(I) compound was reported. 2 Since then, the chemistry of Group 13 metals in the +1 oxidation state have played a great part in the development of p-block chemistry. The synthesis of compounds with aluminum and gallium in the +1 oxidation state is experimentally challenging and consistently hampered by their high reactivity and pronounced tendency towards disproportionation. 3 The lone pair of electrons in MR (R = β-diketiminate) (M = Al, Ga) compounds can act as a basic site towards a range of Lewis acids, forming a σ donating bond. 4 Donor-acceptor bonds have been observed in a range of main group elements, transition metals, and lanthanide and actinide metal complexes. The general M I R unit (M = Al, Ga) can be considered isolobal with singlet carbenes, CO, and CNR.
β-Diketiminate ligands have found widespread application as supporting ligands in metal-mediated catalysis. 5 The stoichiometric transformations of NacNacAl(I) and NacNacGa(I) have also been explored widely owing to the lone pair of electrons and a formally vacant p-orbital on aluminum affording high electrophilic and nucleophilic reactivity. 6 A comparison between main group elements and transition metals was drawn when main group species were found to have reactivity towards small molecules under ambient conditions. This was rationalized by main group species possessing donor/acceptor frontier orbitals which are separated by modest energy gaps, thus drawing comparisons with open-shell transition metal species. 7 In 2000, Roesky et al. chose the β-diketiminate ligand to synthesize a more kinetically stable monomeric aluminum(I) compound Al{HC[C(Me)NDipp] 2 } (Dipp = 2,6-i Pr 2 C 6 H 3 ) (1). 8a This was the first stable dicoordinate aluminum(I) compound to be prepared and structurally characterized in the solidstate. Later, Cui et al. also reported a β-diketiminate ligand stabilized aluminum(I) compound Al{HC[C( t Bu)NDipp] 2 }. 8b Computational studies of β-diketiminate stabilized heavier group metal complexes have shown that their metal lone pairs are associated with the HOMO−2. 9 As a result, they are good σ donor ligands and poor π acceptors like N-heterocyclic carbenes (NHCs), and thus have the potential to display carbenelike chemistry. Inoue et al. reported the first neutral Al(I) compound containing an AlvAl double bond, which was achieved through the reductive dimerization of the corresponding N-heterocyclic carbene (NHC)-stabilized silyl substituted aluminum(III) dihalide (I, Fig. 1), and its reactivity toward the fixation and selective reduction of CO 2 , both of which can be accessed in a stoichiometric and catalytic fashion. 10 In 2018, Aldridge and Goicoechea et al. reported the first isolation of a nucleophilic aluminyl anion [(NON)Al] − by employing a chelating ligand (NON = 4,5-bis(2,6-diisopropylanilido)-2,7-di-tertbutyl-9,9-dimethylxanthene) (II, Fig. 1), which acts as an unprecedented aluminum(I) nucleophile (e.g., in reactions with t Bu 3 PAuI), and which effects the formal oxidative addition of the C-C bond in benzene. 11 After that, Coles and co-workers also synthesized a two-coordinate N-heterocyclic aluminyl anion K[Al(NON Dipp )] (NON Dipp = [O(SiMe 2 NDipp) 2 ] 2− , Dipp = 2,6-i Pr 2 C 6 H 3 , III, Fig. 1), which is able to undergo further reactions such as activation of elemental selenium to form an aluminum complex containing an aluminum-selenium multiple bond and with 1,3,5,7-cyclooctatetraene (COT) to give the first aluminum complex containing a reduced COT-ligand with a strong aromatic character, respectively. 12 Both aluminyl anion complexes reacted with two abundant greenhouse gases (CO 2 and N 2 O) via cycloaddition to generate a monoalumoxane anion. 13 The first isolable example of a room temperature stable monomeric cyclopentadienylaluminum(I) derivative was reported by Braunschweig and co-workers, which was supported by a bulky 1,2,4-tri-tert-butylcyclopentadienyl (Cp 3t ) ligand (IV, Fig. 1). 14a The same group also reported the first example of a monomeric Lewis base stabilized Al(I) hydride that can be isolated and handled under ambient conditions (V, Fig. 1). 14b Very recently, Yamashita's group reported an alkylsubstituted aluminum anion that exhibits very strong basicity and nucleophilicity (VI, Fig. 1). 14c These species have been observed to form both covalent and donor-acceptor bonds, revealing both the reducing and nucleophilic properties of these novel complexes.
Gallium(I) compounds are often driven by the thermodynamic preference for the metal center to exist in the +3 oxi-dation state. Having said this, much of the reported chemistry of monomeric gallium(I) compounds is derived from the significant basicity of the metal through its lone pair of electrons. Gallium(I) compounds are generally more stable towards disproportionation than the corresponding aluminum(I) compounds. So far, several gallium(I) N-heterocycles have been reported. 15,16 Bi-and tridentate ligand systems have been used in the preparation of a variety of neutral and anionic gallium(I) heterocycles (e.g., five-membered anionic complexes VII-VIII, a guanidinate complex IX and a monomeric tris( pyrazolylborate) complex X, see Fig. 2). 16 These compounds have been prepared either by salt-metathesis reactions between alkali metal salts of the ligands and "GaI" or by alkali metal reduction of Ga II or Ga III precursors. The monomeric example of Ga I amide (XI, Fig. 2) can be considered to be having a quasi one-coordinate metal center, which also exhibits weak intramolecular arene interactions in the solid state. 16m Recently, a pincer-type gallylene ligand has been successfully synthesized utilizing bis ( phosphino)-terpyridine as an efficient scaffold for the Ir-Ga I bond, which enabled various reactions at the Ir center by keeping the gallylene ligand intact. 17 The β-diketiminate ligands typically provide monoanionic, bidentate support for metal complexes and offer a much higher degree of steric control through the choice of N-substituents. By tuning the steric and electronic properties of the supporting β-diketiminate ligands, the reactivity of the compounds can be significantly improved. 18 Thus, complexes with a low-valent metal could be stabilized by the employment of sterically encumbering β-diketiminate ligands. It is interesting to note that with a redox-inactive metal bound and appropriate substituents, β-diketiminate ligands become redox-active ligands. 19 Herein we present an overview that is of relevance to the corresponding bond  activation by aluminum(I) and gallium(I) with β-diketiminate ligands.

Chemistry of Dipp NacNacAl(I) and
Dipp NacNacGa(I) The monomeric aluminum carbenoid Dipp NacNacAl(I) (1) was prepared through reduction of the corresponding Dipp NacNacAlI 2 with potassium (Scheme 1). 8a At the same time, Power and co-workers reported a β-diketiminate stabilized Ga(I) monomer Dipp NacNacGa(I) (2) (Scheme 1). 20 It was obtained by the reaction of [Li{HC(CMeNDipp) 2 }] with "GaI". The remarkable thermal stability of the compounds toward disproportionation reaction (decomp. >150°C) can be attributed to the steric bulk of the β-diketiminate ligand, which provides kinetic protection to the metal center. X-ray crystal structure analysis showed that compound 2 is monomeric and isostructural with its aluminum counterpart. With a singlet lone pair and formally empty p-orbital on the metal, the neutral heterocycles Dipp NacNacAl(I) and Dipp NacNacGa(I) have the potential to exhibit both nucleophilic and electrophilic characteristics.

Small-molecule activation
CO and CO 2 activation by transition-metal complexes has been studied extensively for many years. However, the activation of CO and CO 2 with Group 13 metal elements and their compounds has been explored scarcely. 21 In 2018, Crimmin et al. reported carbon chain growth from C1 to C3 and to C4 by sequential reactions of CO and CO 2 with a transition metal carbonyl complex in the presence of an aluminum(I) complex (Scheme 2). 22 Warming a frozen suspension of [W(CO) 6 ] with 2 equiv. of 1 under 1 atm of CO from −78°C to r.t. in a benzene-d 6 solvent results in the formation of the C3 homologated product 3. Heating the isolated and purified sample of 3 under one atmosphere of CO leads to the formation of the chain growth product 4. Further chain growth of C3 to a C4 fragment could be achieved upon the reaction of 3 with one atmosphere of CO 2 . Although the reaction of CO 2 with 3 at 25°C initially produces 5, when the sample 3 is heated at 100°C for 48 h, it completely converts to 6. No reaction occurs between 1 and CO in the absence of [W(CO) 6 ]. The gallium products of these reactions are not reported.
The [2.2.1] metallobicyclic compound 7 was synthesized by the cycloaddition of complex (1) with low-valent aluminum and 1,3-cyclohexadiene. The exposure of a C 6 D 6 solution of 7 to one atmosphere of CO generated the insertion product 8. As shown in Scheme 3 the reaction mixture is reversible, when 8 is heated for longer time. Compound 9 was also studied in the reaction with CO, and the insertion of CO into the Al-C bond was observed. However, compound 10 decomposes at 25°C within 12 h into an intractable mixture of products. 23 Crimmin and co-workers documented the first reversible addition of ethylene to aluminum(I) 1. The monomeric molecular aluminum(I) complex reacted with a series of terminal and strained alkenes including norbornene, 24a ethylene, propylene, hex-1-ene, 3,3-dimethyl-1-butene, allylbenzene and 4-allylanisole. Remarkably all these reactions are reversible under mild conditions (Scheme 4). 24b

Cleavage of the M-X single bond
Aluminum(I) 1 has been developed to act as a synthon for the preparation of aluminum-metal bonded compounds via oxidative insertion of the Al center into metal-halogen linkages. Jones et al. reported the first example of molecular complexes containing an unsupported Be-Al bond. The Be-Al bonded complexes 17 and 18 were obtained as yellow crystalline solids from the reaction of Dipp NacNacAl(I) (1) with [BeX 2 (tmeda)] (X = Br or I, tmeda = tetramethylethylenediamine) in 1 : 1 stoichiometry (Scheme 5). The Be-Al bond distances in 17 and 18 are 2.474(1) Å and 2.432(6) Å, respectively. They are significantly longer than the sum of single bond covalent radii of the elements (2.28 Å). DFT calculations reveal that the compounds with metal-metal bonds have a high s-character. This is consistent with similar Pauling electronegativities between Al and Be. The isostructural Mg-Al (19) and Zn-Al (20) analogues of these complexes have been isolated in the 1 : 1 reaction of Dipp NacNacAl(I) (1) with [MgI 2 (tmeda)] and [ZnBr 2 (tmeda)], respectively (Scheme 5). 25 The composition of compound 20 was confirmed by means of single-crystal X-ray structural analysis (Fig. 3). Compound 20 has distorted tetrahedral Al and Zn centers. The Zn-Al bond distance is 2.471(1) Å, which is longer than the sum of covalent radii of the elements (2.44 Å). Dipp NacNacGa(I) (2) shows no reactivity towards [BeX 2 (tmeda)], even at elevated temperature. Roesky et al. reported unsymmetrical dialumanes by the disproportionation of Dipp NacNacAl(I) (1) with ( Me2 cAAC)AlX 3 (X = Cl, I) adducts (Scheme 5). The Al-Al bond lengths in compound 21 (2.6327(11) Å) and compound 22 (2.5953(16) Å) are slightly shorter than those of symmetric Al-Al bond lengths, owing to the relaxation of the electrostatic repulsion between the Al atoms. 26 The reactions of Dipp NacNacAl(I) (1) with AgX (X = OCN, SCN) resulted in compounds 23 and 24 containing two pseudohalide groups coordinated to the aluminum(III) center. 27 The reactions proceed via oxidative addition of the pseudohalides and elimination of the silver metal.
Harder et al. reported a combined attack of [( Dipp NacNac) Ca + ·(C 6 H 6 )][B(C 6 F 5 ) 4 − ] and Dipp NacNacAl(I) (1), which led to the complete dearomatization of benzene to give C 6 H 6 2− that chelates to the Al(III) center (Scheme 6). The molecular structure of 25, however, showed a heterobimetallic complex in which the C 6 H 6 2− fragment is bridging to Ca and to the Al center. 28a Crimmin et al. . 28c However, the reaction of the calcium hydride complex with 1 in benzene followed a different course leading to benzene C-H alumination. The cleavage of the sp 2 C-H bond in unactivated arenes (benzene, toluene and xylene) by the low valent Al(I) complex 1 at room temperature has been achieved by the catalytic presence of [( Dipp NacNac)CaH] 2 (Scheme 8). The possible pathway is the combined action of nucleophilic Al and arene activation by π-coordination to a Lewis acidic Ca center, which is parallel to the first aluminyl anions. 11c Schulz et al. reported that a solution of Dipp NacNacAl(I) (1) reacts with E 2 Et 4 (E = Sb, Bi) in toluene to insert Dipp NacNacAl(I) (1) into the E-E bond with the formation of Dipp NacNacAl(EEt 2 ) 2 Scheme 6 Activation of benzene using Al(I) and Ca 2+ .  (E = Sb 32, Bi 33) (Scheme 9). Orange crystals of compound 33 were isolated from hexane at −30°C, and slowly decompose in solution with the formation of BiEt 3 and elemental Bi. The analogous reaction of Dipp NacNacGa(I) (2) with E 2 Et 4 is fully reversible and temperature dependent. Analytically pure compounds 34 and 35 were isolated from the 1 : 1 mixture of Dipp NacNacGa(I) (2) and E 2 Et 4 , respectively (Scheme 9). By changing the molar ratio of Dipp NacNacGa(I) (2) and Bi 2 Et 4 into a 1 : 2 ratio, 35 can be isolated in good yield. 29  Heavy-metal complexes containing gallium-lead and gallium-mercury bonds were derived from the oxidative addition of Dipp NacNacGa(I) (2) with the corresponding metal precursors. The reaction of Me 3 PbCl with Dipp NacNacGa(I) (2) in THF at ambient temperatures afforded compound [{( Dipp NacNac)Ga(Cl)}PbMe 3 ] (40) in high yield. In addition, the reaction between [Pb(OSO 2 CF 3 ) 2 ] and Dipp NacNacGa(I) (2) (two equiv.) leads to the complex 41 containing a Ga-Pb II -Ga linkage (Fig. 5). When two equiv. of Dipp NacNacGa(I) (2) were treated with [Pb(OSO 2 CF 3 ) 2 ·2H 2 O] in THF, deep red crystals of 42 were formed in very poor yield (Scheme 10). The structure of the compound consists of a bent Ga-Pb-Ga backbone with a bridging triflate group between the Ga-Pb bond and a

Cleavage of E′-E single and E′vE double bonds
To stabilize Ga-coordinated dipnictenes of the type [ Dipp NacNac (X)Ga] 2 E 2 (E = P-Bi), the reactions of Dipp NacNacGa(I) (2) with phosphorus, arsenic, and bismuth halides and amides were studied. 42 Two equiv. of Dipp NacNacGa(I) (2) reacted with PX 3 (X = Cl, Br) in toluene at ambient temperature with the insertion of Dipp NacNacGa(I) (2) into two P-X bonds, which resulted in [ Dipp NacNac(X)Ga] 2 PX (X = Cl 75, Br 76) (Scheme 19). Similar twofold insertion reactions into AsCl 3 and the subsequent elimination of Dipp NacNacGaCl 2 resulted in the formation of the stable Ga-coordinated diarsene species, which was isolated as a green crystalline solid 77 (Scheme 20). The analogous reaction with Me 2 NAsCl 2 yielded unsymmetrically-substituted diarsene [ Dipp NacNac(Cl)Ga]AsvAs[Ga (NMe 2 ) Dipp NacNac] (78) (Scheme 20). In contrast, the reaction of Dipp NacNacGa(I) (2) with As(NMe 2 ) 3 required much harsher reaction conditions. A mixture of Dipp NacNacGa(I) (2) and As (NMe 2 ) 3 heated at 165°C for 5 days resulted in compound 79 (Scheme 19). Its analogous reaction with Dipp NacNacAl(I) (1) yielded [ Dipp NacNac(Me 2 N)Al] 2 As 2 (80) after heating at 80°C for one day (Scheme 20). Finally, the reaction of Dipp NacNacGa(I) (2) with Bi(NEt 2 ) 3 also occurred with the insertion and elimination of Dipp NacNacGa(NEt 2 ) 2 and resulted in the corresponding Ga-substituted dibismuthene [ Dipp NacNac(Et 2 N)Ga] 2 Bi 2 (81) (Scheme 19). The reaction of 2 with elemental tellurium yielded the Te-bridged compound [ Dipp NacNacGa-μ-Te] 2 (82). Moreover, the cleavage of the Te-Te and Te-C bonds upon reactions of 2 with Ph 2 Te 2 and i Pr 2 Te resulted in the formation of Dipp NacNacGa(TePh) 2 (83) and Dipp NacNacGa( i Pr)Te( i Pr) (84), respectively (Scheme 20). 34 Nikonov and Crimmin groups separately reported the reactions of the monomeric Al(I) complex with various fluoroalkenes and fluoroarenes, resulting in the breaking of strong sp 2 and sp 3 C-F bonds. Aluminum(I) compound 1 undergoes a facile oxidative addition with aryl C-F bonds. 43 The reaction of 1 with an excess of hexafluorobenzene or pentafluorobenzene resulted in compounds 85 and 86, respectively (Scheme 21). A further decrease in the number of fluorine atoms in the starting arene necessitates an increase in the reaction temperature to cleave the C-F bond. The cleaving ability decreases in the order o-> p-> m-. The addition of 1-fluorohexane or fluorocyclohexane to 1 at room temperature yielded the corresponding aluminum alkyl 92 and 93, respectively (Scheme 22). 43a The reaction with (E)-1,3,3,3-tetrafluoro-propene(HFO-1234ze) resulted in the immediate formation of a 4 : 1 mixture of 94-E and 94-Z (Scheme 23). The addition of hexafluoropropene to 1 gave two products which were separated by fractional crystallization from hexane. 95 is formed from the internal sp 2 C-F bond cleavage, while 96 is the result of breaking the terminal sp 2 C-F bond trans to the CF 3 group. The reaction of 1 with 3,3,3-trifluoropropene yielded 97 by the formation of a metallocyclopropane intermediate followed by β-fluoride elimination (Scheme 23). 24a Streubel and co-workers described the reaction of monovalent compounds Dipp NacNacM (M = Al, Ga) with imidazole-2thione based tricyclic 1,4-diphospinine, which produced the corresponding 7-metalla-1,4-diphosphanorbornadiene (98, 99) (Scheme 24). 44a Previously Nikonov et al. dative cleavage of the CvS bond at the metal center, 44b while 98 and 99 undergo the [4 + 1] cycloaddition reaction, which is both kinetically and thermodynamically favorable. The aluminum(I) compound Dipp NacNacAl(I) (1) reacted with diethyl sulfide at 50°C, which resulted in the oxidative addition of the C(sp 3 )-S bond. This is the first example of C(sp 3 )-S bond activation by a main-group element. 45 The groups of Nikonov, Crimmin, and Kinjo independently reported the reactions of monomeric Al(I) compound 1 towards C-O bonds. The oxidative addition reaction of tetrahydrofuran with Dipp NacNacAl(I) (1) smoothly occurred at room temperature to give complex 101, 43b while the reaction between 1 and benzofuran upon heating at 80°C slowly converted it to product 102. 43a The reaction of 1 with an equiv. amount of L′ 2 PhB (103) (L′ = oxazol-2-ylidene) in toluene instantly occurred with the insertion of Dipp NacNacAl(I) (1) into the C-O bond, affording complex 104 involving an Al, N, and O mixed heterocyclic carbene or anionic (amino)(boryl) carbene derivative (Scheme 25). 46 Treatment of Dipp NacNacAl(I) (1) with thiourea resulted in the first carbene-stabilized terminal aluminum sulfide complexes 105 and 106 by the oxidative cleavage of the CvS bond. In contrast, the mixing of compound 1 and triphenyl-phosphine sulfide in a 1 : 1 ratio afforded a mixture of terminal sulfide Dipp NacNacAlvS(SvPPh 3 ), unreacted 1, and free triphenylphosphine. The existence of the Al-S double bond in 105 and 106 was supported by DFT calculations. Complex 105 undergoes facile cycloaddition with phenyl isothiocyanate to form compound 107 along with zwitterion 108 obtained from the coupling between the liberated carbene and PhNvCvS (Scheme 26). 44b To investigate the oxidative cleavage of the unsaturated bond of the CvN unit, the reaction of Dipp NacNacAl(I) (1) with cyclic guanidine was accomplished and showed the unprecedented cleavage of the C-N multiple bond to give the carbene-ligated amido complex Dipp NacNacAl (NHTol)(SIMe) (SIMe = C{N(Me)CH 2 } 2 ) (109). The splitting of the CvN bond in 109 is the first example of the oxidative addition of the CvN double bond to any metal center. 47 The DFT study supported that the production of 109 occurs via an intermediate of aluminum imide as a result of the oxidative cleavage of TolNvSIMe (SIMe = C{N(Me)CH 2 } 2 ) by 1. The reactions of phosphine oxides with 1 occurred readily with the formation of hydroxyl derivatives Dipp NacNacAl(OH)(OvPR 3 ) (R = Ph 110, Et 111). The CvO bond (179 kcal mol −1 ) is much stronger when compared with the PvO bond (110 kcal mol −1 ). Therefore, the reaction of cyclic urea 1,3-dimethyl-2-imidazolidinone with 1 resulted in an unexpected aluminum hydride Dipp NacNacAlH(OvSIMe) (SIMe = C{N(Me)CH 2 } 2 ) 112, with the deprotonation of the weakly acidic methyl group in the backbone of the Dipp NacNac ligand. 48a In contrast, the reactivity of 1 towards benzophenone afforded a ketylate species NacNacAl (η 2 (C,O)-OCPh 2 ) (113) (Scheme 27). The latter compound undergoes easy cyclization reaction with an unsaturated substrate. 48b Recently (Scheme 28). The compounds containing the AlCP ring can be used as synthons to prepare a series of unprecedented Al-and P-containing heterocyclic frameworks. 49 The Ga(I) compound easily undergoes cyclization with methacrolein at room temperature within 10 min to give gallium enolate 116. Unlike the aluminum congener 1, the gallium compound 2 does not cleave the PvS bond of Et 3 PvS, even upon heating to 80°C. With Ph 3 PvS, however, a slow reaction occurs upon heating to 80°C to obtain the sulfide ( Dipp NacNacGa-S) 2 (117). Nevertheless, compound 2 readily reacts with two equiv. of PhNCS to give product 118 via CvS bond cleavage and cyclization, and also the dimer ( Dipp NacNacGa) 2 (μ-S)(μ-CNPh) (119) at a ratio of 5 : 1 (3,5-Me 2 C 6 H 3 )NCO and PhNCO, respectively, reacts with 2, readily to form the coupling products 120 and 121. 1,3-Di-p-tolylcarbodiimide and Dipp NacNacGa(I) formed the coupling product 122 (Scheme 29). 50 Dipp NacNacAl(I) (1) reacted with diphenyl disulfide to afford the symmetrically substituted bis( phenyl sulfide) aluminum complex 123 via the cleavage of the S-S bond in diphenyl disulfide. In a similar fashion, the reaction of 1 with bulky tetraphenyl diphosphine resulted in the cleavage of the P-P bond over the course of 3 d at 70°C to produce the novel aluminum bis(diphenyl phosphido) complex (124) (138)

Conclusions and perspectives
In conclusion, we report monomeric aluminum and gallium carbenoid complexes supported by β-diketiminate ligands which possess a lone pair of electrons and a formally vacant p orbital. These features afford high electrophilic and nucleophilic reactivity that could be used in bond activation and cleavage upon reactions with small molecules. The aluminum and gallium carbenoid complexes undergo a series of oxidative addition reactions with σ H-X and E′-E bonds where E is an element from groups 13 to 16. These compounds also demonstrate the oxidative cleavage of multiple bonds and enthalpically strong bonds (M-X, E′vE). An extension of the synthetic approach is presented in this review for the synthesis of Group 13 metalloid complexes that consist of an unsupported M′-M (M′ = Al, Ga) bond. The oxidative cleavage of multiple bonds shows new and unusual reactivities. These reactivities have opened up a new realm in aluminum and gallium chemistry that could lead to many new products. Nevertheless, the compounds with low valent aluminum and gallium are still showing some limitations in catalytic applications. However, dialumene (I) promoted both the catalytic and stoichiometric reduction of CO 2 to value added C1 products, which is the first example of catalysis using a homonuclear main-group multiple bond. It is important to explore reductive elimination of C-C bonds, which leads to reversible bond activation paving the way towards catalytic applications. Recent developments in the isolation of aluminyl complexes (II) will likely extend the low oxidation state Al chemistry that is used to activate the C-C bonds. We look forward to conducting further studies of the Al(I) and Ga(I) complexes which will result in more unprecedented bond cleavage reactions and important future applications in catalysis.

Conflicts of interest
There are no conflicts to declare.