N-Heterocyclic imine-based bis-gallium(I) carbene analogs featuring a four-membered Ga2N2 ring

A combination of Ga(I) centers as important building blocks and scaffolds containing N-heterocyclic imines gives new insights into low-valent Ga chemistry. In this study, a mixture of LDipNLi (LDip = 1,3-bis(2,6-diisopropylphenyl)-imidazolin-2-ylidene), tBuOK, and Cp*Ga (Cp* = pentamethylcyclopentadienyl) in toluene afforded [LDipN-Ga]2 (1) via salt metathesis. X-ray structure analysis of 1 revealed a four-membered Ga2N2 ring, and DFT studies indicated the presence of a lone pair at each Ga center. In addition, compound 1 demonstrated diverse reactivities towards methyl trifluoromethanesulfonate, diphenyl disulfide, 9,10-phenanthrenequinone, and ECl2 (E = Ge or Sn).

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Partnering a Three-Coordinate Gallium Cation with a Hydroborate Counter-Ion for the Catalytic Hydrosilylation of CO2

A novel β-diketiminate stabilized gallium hydride, (^Dipp L)Ga(Ad)H (where (^Dipp L)={HC(MeCDippN)₂ }, Dipp=2,6-diisopropylphenyl and Ad=1-adamantyl), has been synthesized and shown to undergo insertion of carbon dioxide into the Ga-H bond under mild conditions. In this case, treatment of the resulting κ¹ -formate complex with triethylsilane does not lead to regeneration of the hydride precursor. However, when combined with B(C₆ F₅ )₃ , (^Dipp L)Ga(Ad)H catalyses the reductive hydrosilylation of CO₂ . Under stoichiometric conditions, the addition of one equivalent of B(C₆ F₅ )₃ to (^Dipp L)Ga(Ad)H leads to the formation of a 3-coordinate cationic gallane complex, partnered with a hydroborate anion, [(^Dipp L)Ga(Ad)][HB(C₆ F₅ )₃ ]. This complex rapidly hydrometallates carbon dioxide and catalyses the selective reduction of CO₂ to the formaldehyde oxidation level at 60 °C in the presence of Et₃ SiH (yielding H₂ C(OSiEt₃ )₂ ). When catalysis is undertaken in the presence of excess B(C₆ F₅ )₃ , appreciable enhancement of activity is observed, with a corresponding reduction in selectivity: the product distribution includes H₂ C(OSiEt₃ )₂ , CH₄ and O(SiEt₃ )₂ . While this system represents proof-of-concept in CO₂ hydrosilylation by a gallium hydride system, the TOF values obtained are relatively modest (max. 10 h^-1 ). This is attributed to the strength of binding of the formatoborate anion to the gallium centre in the catalytic intermediate (^Dipp L)Ga(Ad){OC(H)OB(C₆ F₅ )₃ }, and the correspondingly slow rate of the turnover-limiting hydrosilylation step. In turn, this strength of binding can be related to the relatively high Lewis acidity measured for the [(^Dipp L)Ga(Ad)]⁺ cation (AN=69.8).

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

Herein we present an overview of the last 10 years for aluminum(i) and gallium(i) stabilized by β-diketiminate ligands that undergo a series of oxidative addition reactions with molecules containing single and multiple bonds.

Salt metathesis as an alternative approach to access aluminium(i) and gallium(i) β-diketiminates

Aluminium(i) and gallium(i) β-diketiminates are accessed by a new route that provides better overall yields. In the case of aluminium it is also much faster, but some molecules turn into a dead end and merge into a dinuclear aluminium(iii) hydride.

Facile oxidative addition of O2 and S8 by an indium bis(carbene) analogue

The oxidative addition of oxygen towards an indium bis(carbenoid) yields an almost planar In₄O₂ ring, an isovalence-electronic analogue of dioxane, in which the indium is in the formal oxidation state +ii. The weaker oxidant sulfur, however, yields an eight-membered ring in a deck-chair conformation containing four indium(iii) centres and four sulfides in an alternating fashion. According to DFT calculations, this reactivity difference originates from kinetic rather than thermodynamic reasons. When the reaction with S₈ is performed with the in situ generated bis(carbenoid), a β-diketimine (NacNac) incorporating an S₄-chain bridging the two γ-carbon atoms of each NacNac unit is derived.

Reactivity patterns for the activation of CO2 and CS2 with alumoxane and aluminum hydrides

Carbon dioxide is readily fixed when reacting with either alumoxane dihydride [{MeLAl(H)}2(μ-O)] (1) or aluminum dihydride [MeLAlH2] (2) (MeL = HC[(CMe)N(2,4,6-Me3C6H2)]2-) to produce bimetallic aluminum formates [(MeLAl)2(μ-OCHO)2(μ-O)] (3) and [(MeLAl)2(μ-OCHO)2(μ-H)2] (5), respectively. Furthermore, [(MeLAl)2(μ-OCHO)2(μ-OH)2] (4) is easily obtained upon the reaction of 3 or 5 with H2O. The stability of the unusual dialuminum diformate dihydride core observed in 5 stems from the proximity of the Al centers allowing the formation of two Al-HAl bridges and precluding further hydride transfer to the HCO2 moieties. Contrary to this behavior, 1 and 2 react with CS2 giving cyclic alumoxane and aluminum sulfides [(MeLAl)2(μ-S)(μ-O)] (6) and [{MeLAl(μ-S)}2] (7), respectively. The molecular structures of 3-7 were characterized by IR, Raman, solution or solid-state (MAS) NMR spectroscopy and mass spectrometry and for 4-7 were characterized by X-ray diffraction studies. NMR kinetic studies and DFT calculations suggest that the mechanisms for the formation of 6 and 7 involve the transfer of a hydride group forming transient aluminum thioformate intermediates which proceed to form Al-S-Al moieties through the cleavage of C-S bonds and insertion of a sulfur atom, followed by the elimination of thioformaldehyde.

Self-Assembly of Aluminum- and Gallium-Based meso-Metallaporphyrins

The molecular meso-metallaporphyrin has been obtained from the reaction of AlMe₃ with the bulky 4,5-(Ph₂(HO)C)₂-1,2,3-triazole (1). The presence of Al-Me groups coordinated to the triazole rings creates three different stereoisomers that were identified by single-crystal X-ray diffraction. Further studies revealed that, for steric reasons, only one of the two main stereoisomers is active in the polymerization of ε-caprolactone. When GaMe₃ is used instead of AlMe₃, a trimetallic species is formed instead of the meso-metallaporphyrin pointing to a metal-directed self-assembly. On the other hand, the reaction of the monolithium salt [{Li(THF)₂}{κ²- N, N'-4,5-(Ph₂(HO)C)-1,2,3-triazole}]₂ (2; THF = tetrahydrofuran) with MCl₃ (M = Al, Ga) yields meso-metallaporphyrin species with a lithium atom in the center of the metallacycle. While the gallium derivative is rather stable in solution, the aluminum analogue decomposes rapidly. In the solid state, continuous cationic columns running throughout the whole crystal are formed from alternating Li⊂[M]₄ (M = Al, Ga) meso-metallaporphyrin and [Li(THF)₄]⁺ cations. Density functional theory calculations determined that the weak Cl···H, H···H, N···H, and Cl···O interactions with a total interaction energy of -38.6 kcal·mol^-1 are responsible for this unusual packing.

En Route to Bis-Carbene Analogues of the Heavier Group 13 Elements: Consideration of Bridging Group and Metal(I) Source

With the aim of exploring the effect of steric constraints imposed on the metal-metal interaction of bis-carbene analogues of thallium by the linking scaffold, seven dinuclear thallium diyls with a series of rigid, semirigid, and flexible bridging scaffolds were synthesized. The solid-state molecular structures were determined for four of these compounds by single-crystal XRD and compared with the results of DFT calculations, which were performed for all substances. These compounds serve as models to investigate the metal-metal distance in the absence of co-coordinated molecules (additional ligands, solvent molecules). In addition, the effect of the metal(I) precursor, and more specifically the counterion, on the synthetic access to bis-carbene analogues of indium and thallium was investigated. For indium, only InI yields the desired dinuclear indium diyl. With InBr no reaction was observed, and using InCl gave rise to a mononuclear indium(III) compound. For thallium, both TlI and TlBr allow access to the related bis-carbene analogue, although the yield with the latter is significantly lower. In contrast, no reactions were observed with TlCl and TlBF₄ .

A General Route to Metal-Substituted Dipnictenes of the Type [L(X)M]2 E2

Two equivalents of LGa (L=HC[C(Me)N(2,6-iPr₂ C₆ H₃ )]₂ ) reacted with PX₃ (X=Cl, Br) with insertion into two P-X bonds and formation of [L(X)Ga]₂ PX (X=Cl 1, Br 2), whereas the analogous reaction with AsCl₃ occurred with twofold insertion and subsequent elimination of LGaCl₂ and formation of the Ga-substituted diarsene [L(Cl)Ga]₂ As₂ (3). Analogous findings were observed in the reactions with Me₂ NAsCl₂ , yielding the unsymmetrically-substituted diarsene [L(Cl)Ga]As=As[Ga(NMe₂ )L] (4). The reaction of As(NMe₂ )₃ with LGa gave [L(Me₂ N)Ga]₂ As₂ (5) after heating at 165 °C for five days, whereas the reaction with LAl gave [L(Me₂ N)Al]₂ As₂ (6) after heating at only 80 °C for one day. Finally, two equivalents of LGa reacted with Bi(NEt₂ )₃ to give [L(Et₂ N)Ga]₂ Bi₂ (7). Complexes 1-7 were characterized by NMR spectroscopy (¹ H, ¹³ C, ³¹ P), elemental analysis, and single-crystal X-ray diffraction (except for 1 and 5). The bonding situations in 4, 6, and 7 were analyzed by quantum chemical calculations.

Gallium Hydrides with a Radical-Anionic Ligand

The reaction of Cl₂GaH with a sodium salt of the dpp-Bian radical-anion (dpp-Bian^•-)Na (dpp-Bian = 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene) affords paramagnetic gallane (dpp-Bian^•-)Ga(Cl)H (1). Oxidation of (dpp-Bian^2-)Ga-Ga(dpp-Bian^2-) (2) with N₂O results in the dimeric oxide (dpp-Bian^•-)Ga(μ²-O)₂Ga(dpp-Bian^•-) (3). A treatment of the oxide 3 with phenylsilane affords paramagnetic gallium hydrides (dpp-Bian^•-)GaH₂ (4) and (dpp-Bian^•-)Ga{OSi(Ph)H₂}H (5) depending on the reagent's stoichiometry. The reaction of digallane 2 with benzaldehyde produces pinacolate (dpp-Bian^•-)Ga(O₂C₂H₂Ph₂) (6). In the presence of PhSiH₃, the reaction between digallane 2 and benzaldehyde (2: PhSiH₃: PhC(H)O = 1:4:4) affords compound 4. The newly prepared complexes 1, 3-6 consist of a spin-labeled diimine ligand-dpp-Bian radical-anion. The presence of the ligand-localized unpaired electron allows the use of the ESR spectroscopy for characterization of the gallium hydrides reported. The molecular structures of compounds 1, 3-6 have been determined by the single-crystal X-ray analysis.

Correction: Ga[OSi(OtBu)3]3·THF, a thermolytic molecular precursor for high surface area gallium-containing silica materials of controlled dispersion and stoichiometry

Correction for 'Ga[OSi(O^tBu)₃]₃·THF, a thermolytic molecular precursor for high surface area gallium-containing silica materials of controlled dispersion and stoichiometry' by James P. Dombrowski et al., Dalton Trans., 2016, 45, 11025-11034.

On the non-innocence of “Nacnacs”: ligand-based reactivity in β-diketiminate supported coordination compounds

While β-diketiminate (BDI or ‘nacnac’) ligands have been widely adopted to stabilize a wide range of metal ions in multiple oxidation states and coordination numbers, in several occurrences these ligands do not behave as spectators and participate in reactivity.