Why Do Five Ga+ Cations Form a Ligand-Stabilized [Ga5 ]5+ Pentagon and How Does a 5:1 Salt Pack in the Solid State?

On the Synthesis and Structure of 'Naked' Ga(I) and In(I) Salts and the Surprising Stability of Simple Ga(I) and In(I) Salts in the Coordinating Solvents Ether and Acetonitrile

In this work, we present the solid-state structures of solvent-free Ga[pf] and In[pf] salts ([pf]-=[Al(ORF)4]-; RF=C(CF3)3), which are very rare examples of salts with truly 'naked' metal cations. Both salts may serve as starting materials for subvalent gallium and indium chemistry with very weakly coordinating ligands providing the freedom of choice for solvents and ligands for the future. On the other hand, we report and rationalize the formation and isolation of [M(OEt2)2][pf] and [M(MeCN)2][pf] (M=Ga, In), underlining the surprising stability of these subvalent group 13 M+ ions against disproportionation. Unexpectedly, dicoordinate and carbene analogous [M(L)2]+ ions with the [pf]- counterion are stable in L=acetonitrile and diethyl ether at room temperature, opening up possible applications for example in organic synthesis and catalysis.

Structures, Bonding Analyses and Reactivity of a Dicationic Digallene and Diindene Mimicking trans-bent Ditetrylenes

The reaction of bisdicyclohexylphosphinoethane (dcpe) and the subvalent MI sources [MI (PhF)2 ][pf] (M=Ga+ , In+ ; [pf]- =[Al(ORF )4 ]- ; RF =C(CF3 )3 ) yielded the salts [{M(dcpe)}2 ][pf]2 , containing the first dicationic, trans-bent digallene and diindene structures reported so far. The non-classical MI ⇆MI double bonds are surprisingly short and display a ditetrylene-like structure. The bonding situation was extensively analyzed by quantum chemical calculations, QTAIM (Quantum Theory of Atoms in Molecules) and EDA-NOCV (Energy Decomposition Analysis with the combination of Natural Orbitals for Chemical Valence) analyses and is compared to that in the isoelectronic and isostructural, but neutral digermenes and distannenes. The dissolved [{Ga(dcpe)}2 ]2+ ([pf]- )2 readily reacts with 1-hexene, cyclooctyne, diphenyldisulfide, diphenylphosphine and under mild conditions at room temperature. This reactivity is analyzed and rationalized.

Activation of the GaI Cation for Bond Activation: from Oxidative Additions into C-Cl and H-P Bonds to Reversible Insertion into P4

Although the discovery of the GaI complex salt [Ga(PhF)2-3 ][Al(ORF )4 ] (RF =C(CF3 )3 , PhF=C6 H5 F) invoked the preparation of a diverse library of cationic Ga(I) coordination complexes and clusters, studies on small molecule activation with low-valent GaI cations are scarce. Herein, a first experimental study on the reactivity of a monomeric Ga(I) cation activated with a pyridine-diimine pincer ligand (in [Ga(PDIdipp )][Al(ORF )4 ]) towards small-molecules is reported. First controlled oxidative additions of the GaI cation into C-Cl, H-P and P-P bonds are presented. Moreover, the [4+1]cycloaddition to butadienes was achieved. Intriguingly, the isolated, blue insertion product into the P-P bond of P4 allows for the quantitative release of the P4 molecule upon reaction with AlEt3 and butadienes. Reversible P4 insertion of main-group metals has previously been reported for Ge and Sn, respectively. The experimental study is supported by high-level computational analysis of the in-part reversible oxidative additions at the DLPNO-CCSD(T)/def2-TZVPP//PBEh-3c/def2-mSVP level of theory with COSMO-RS solvation in 1,2-difluorobenzene.

Oxychloridoselenites(IV) with cubane-derived anions and stepwise chlorine-to-oxygen exchange

The novel oxychloridoselenites(IV) [BMIm][Se₃Cl₁₃] (1), [BMIm][Se₄Cl₁₅O] (2), [BMIm]₂[Se₄Cl₁₄O₂] (3), [BMPyr]₂[Se₄Cl₁₄O₂] (4), [BMPyr]₂[Se₆Cl₁₈O₄] (5), [BMIm]₂[SeCl₄O] (6), [BMPyr]₂[Se₂Cl₆O₂] (7), and [BMPyr]₂[Se₆Cl₁₄O₆] (8) are prepared by ionic-liquid-based synthesis. Accordingly, SeCl₄, SeO₂ (1-6), and/or SeOCl₂ (7,8) as the starting materials are reacted in [BMIm]Cl or [BMPyr]Cl as ionic liquid (BMIm: 1-butyl-3-methylimidazolium, BMPyr: 1-butyl-1-methylpyrrolidinium; partially with AlCl₃ in addition). Generally, the composition and structure of title compounds can be derived from the tetrameric, heterocubane-type (SeCl₄)₄ as the initial building unit. Thus, chlorine is successively exchanged by oxygen from 1 to 8. Moreover, the four edge-sharing (SeCl₆) octahedra in (SeCl₄)₄ are increasingly dismantled, ending with a [SeCl₄O]^2- anion as a single pseudo-octahedron in 6. Based on the weakly coordinating ionic liquid, it is possible to selectively obtain the different species via synthesis near room temperature (20-80 °C). The oxychloridoselenite anions [Se₄Cl₁₅O]^-, [Se₄Cl₁₄O₂]^2-, [Se₆Cl₁₈O₄]^2-, and [Se₆Cl₁₄O₆]^2- are obtained for the first time. The title compounds are characterized by X-ray structure analysis based on single crystals and powders as well as by infrared spectroscopy and thermal analysis.

Reactions of noble-metal oxides in ionic liquids near room temperature

The reaction of Ag₂O, Au₂O₃, and HgO with CuCl, CuI, AgCl, AgI, AuCl, and AuI in ionic liquids ([EMIm]Cl, [BMIm]Cl) near room temperature (20-80 °C) is evaluated and results in the new compounds (C₈H₁₄N₂)CuCl, (C₈H₁₄N₂)AgI, (C₆H₁₀N₂)AuCl, [(C₈H₁₄N₂)₂Hg][CuCl₃], [(C₈H₁₄N₂)₂Hg][AgCl₃], and [EMIm][Ag₂I₂Cl]. Thereof, (C₈H₁₄N₂)CuCl, (C₈H₁₄N₂)AgI, (C₆H₁₀N₂)AuCl, [(C₈H₁₄N₂)₂Hg][CuCl₃], and [(C₈H₁₄N₂)₂Hg][AgCl₃] are NHC complexes (NHC: N-heterocyclic carbene) with M-C bonds (M: Cu, Ag, Au, Hg). Whereas (C₈H₁₄N₂)CuCl and (C₈H₁₄N₂)AgI crystallize as single molecules, (C₆H₁₀N₂)AuCl is dimerized via aurophilic interactions. [(C₈H₁₄N₂)₂Hg][CuCl₃] and [(C₈H₁₄N₂)₂Hg][AgCl₃] exhibit Hg atoms with two Hg-C bonds. Moreover, (C₈H₁₄N₂)AgI shows intense green fluorescence at room temperature with a quantum yield of 44%, whereas all other compounds do not show any emission at room temperature. Finally, [EMIm][Ag₂I₂Cl] is not an NHC compound but contains _∞ ¹[AgI_1/2I_2/4Cl_1/2]^- chains with infinite d¹⁰-d¹⁰ interaction of the silver atoms. The title compounds are characterized by single-crystal structure analysis, infrared spectroscopy, thermogravimetry, and fluorescence spectroscopy.

Direct Comparison of Subvalent, Polycationic Group 13 Cluster Compounds: Lessons learned on Isoelectronic DMPE Substituted Gallium and Indium Tetracation Salts

The tetracationic, univalent cluster compounds [{M(dmpe)}₄ ]⁴⁺ (M=Ga, In; dmpe=bis(dimethylphosphino)ethane) were synthesized as their pf salts ([pf]^- =[Al(OR^F )₄ ]^- ; R^F =C(CF₃ )₃ ). The four-membered ring in [{M(dmpe)}₄ ]⁴⁺ is slightly puckered for M=Ga and almost square planar for M=In. Yet, although structurally similar, only the gallium cluster is prevalent in solution, while the indium cluster forms temperature dependent equilibria that include even the monomeric cation [In(dmpe)]⁺ . This system is the first report of one and the same ligand inducing formation of isoelectronic and isostructural gallium/indium cluster cations. The system allows to study systematically analogies and differences with thermodynamic considerations and bonding analyses, but also to outline perspectives for bond activation using cationic, subvalent group 13 clusters.

Synthesis of a low-valent Al4+ cluster cation salt

Low-valent aluminium compounds are very reactive main-group species and have therefore been widely investigated. Since the isolation of a stable molecular Al(I) compound in 1991, [(AlCp*)₄] (Cp* = [C₅Me₅]^-), a variety of highly reactive neutral or anionic low-valent aluminium complexes have been developed. By contrast, their cationic counterparts have remained difficult to access. Here, we report the synthesis of [Al(AlCp*)₃]⁺[Al(OR^F)₄]^- (R^F = C(CF₃)₃) through a simple metathesis reaction between [(AlCp*)₄] and Li[Al(OR^F)₄]. Unexpectedly, the [Al(AlCp*)₃]⁺ salt forms a dimer in the solid state and concentrated solutions. Addition of Lewis bases results in monomerization and coordination to the unique formal Al⁺ atom, giving [(L)_xAl(AlCp*)₃]⁺ salts where L is hexaphenylcarbodiphosphorane (x = 1), tetramethylethylenediamine (x = 1) or 4-dimethylaminopyridine (x = 3). The Al⁺-Al_Cp* bonds in the resulting [(L)_xAl(AlCp*)₃]⁺ cluster cations can be finely tuned between very strong (with no ligand L) to very weak and approaching isolated [Al(L)₃]⁺ ions (when L is dimethylaminopyridine).

A Planar Five‐Membered Aromatic Ring Stabilized by Only Two π‐Electrons

Many chemicals known today are partially or fully aromatic, since a ring framework experiences additional stabilization through the delocalization of π‐electrons. While aromatic rings with equal numbers of π‐electrons and ring atoms such as benzene are particularly stable, those with the minimally required two π‐electrons are very rare and yet remain limited to three‐ and four‐membered rings if not stabilized in the coordination sphere of heavy metals. Here we report the facile synthesis of a dipotassium cyclopentagallene, a unique example of a five‐membered aromatic ring stabilized by only two π‐electrons. Single‐crystal X‐ray diffraction revealed a planar Ga₅ ring with almost equal gallium–gallium bond lengths, which together with computational and spectroscopic data confirm its aromatic character. Our results prove that aromatic stabilization goes far beyond what has previously been assumed as minimum π‐electron count in a five‐atom ring fragment.

Room-Temperature Synthesis of [BMIm][Sn5O2Cl7] with ∞1(Sn2OCl2) Strands in a Saline [BMIm][SnCl3] Matrix

The novel tin(II) oxychloride [BMIm][Sn₅O₂Cl₇] (BMIm = 1-butyl-3-methylimidazolium) is obtained by the room-temperature reaction (25 °C) of black SnO and SnCl₂ in [BMIm]Cl/SnCl₂ as an ionic liquid. The title compound can be described as composed of noncharged, infinite _∞¹(Sn₂OCl₂) strands that are embedded in a saline matrix of [BMIm]⁺ and [SnCl₃]^-. The _∞¹(Sn₂OCl₂) strands consist of a backbone of edge-sharing OSn_4/2 tetrahedra, which represent one-dimensional (1D) strands cut out of the layer-type structure of SnO. In [BMIm][Sn₅O₂Cl₇], the _∞¹(Sn₂OCl₂) strands, which mimic a 1D semiconductor, are terminated by chlorine atoms, whereas they are interconnected by oxygen atoms in the 2D semiconductor SnO. The view of the noncharged _∞¹(Sn₂OCl₂) strands in a saline [BMIm][SnCl₃] matrix is validated by dissolution experiments. Thus, electron microscopy and Raman spectroscopy show a deconstruction of [BMIm][Sn₅O₂Cl₇] single crystals after treatment with chloroform with a dissolution of [BMIm][SnCl₃], the formation of SnCl₂ needles, and tin oxide as a solid remain.

A Brønsted Acidic Gallium Hydride: Facile Interconversion of NNNN-Macrocycle Supported [GaI]+ and [GaIIIH]2

Protonolysis of [Cp*M] (M=Ga, In, Tl) with [(Me₄TACD)H][BAr₄^Me] (Me₄TACD=N,N′,N′′,N′′′‐tetramethyl‐1,4,7,10‐tetraazacyclododecane; [BAr₄^Me]⁻=[B{C₆H₃‐3,5‐(CH₃)₂}₄]⁻) provided monovalent salts [(Me₄TACD)M][BAr₄^Me], whereas [Cp*Al]₄ yielded trivalent [(Me₄TACD)AlH][BAr₄^Me]₂. Protonation of [(Me₄TACD)Ga][BAr₄^Me] with [Et₃NH][BAr₄^Me] gave an unusually acidic (pK_a(CH₃CN)=24.5) gallium(III) hydride dication [(Me₄TACD)GaH][BAr₄^Me]₂. Deprotonation with IMe₄ (1,3,4,5‐tetramethyl‐imidazol‐ylidene) returned [(Me₄TACD)Ga][BAr₄^Me]. These reversible processes occur with formal two‐electron oxidation and reduction of gallium. DFT calculations suggest that gallium(I) protonation is facilitated by strong coordination of the tetradentate ligand, which raises the HOMO energy. High nuclear charge of [(Me₄TACD)GaH]²⁺ facilitates hydride‐to‐metal charge transfer during deprotonation. Attempts to prepare a gallium(III) dihydride cation resulted in spontaneous dehydrogenation to [(Me₄TACD)Ga]⁺.

Oxidative addition, reduction and reductive coupling: the versatile reactivity of subvalent gallium cations

Inspired by the successful oxidative addition of a P-H bond to univalent Ga[Al(OR^F)₄] that gives the unprecedented dicationic gallium hydride complex [H-Ga(PPh₃)₃][Al(OR^F)₄]₂ (OR^F = OC(CF₃)₃), the oxidative addition of E-Cl containing substrates was investigated. The reductive coupling of three PPh₂Cl to the catenated phosphorus cation [P₃Ph₆]⁺ hinted towards a formal two-electron-three-halide reduction (2e^--3X^- reduction). Similarly, from SbCl₃, a cationic formal Sb^I compound and from RhCl₃, [Rh^I(HMB)(COD)]⁺ and [Rh^I(COD)₂]⁺ (HMB = C₆Me₆, COD = 1,5-cyclooctadiene) are formed as [Al(OR^F)₄]^- salts when reacted with Ga⁺. Thus, Ga[Al(OR^F)₄] allows for a one-pot 2e^--3X^- reduction with the concomitant introduction of a weakly coordinating anion (WCA).

Why Do Five Ga+ Cations Form a Ligand-Stabilized [Ga5 ]5+ Pentagon and How Does a 5:1 Salt Pack in the Solid State?

The reaction of the Ga⁺ source [Ga(PhF)₂ ]⁺ [Al(OR^F )₄ ]^- with the neutral σ-donor ligand dmap (4-Me₂ N-C₆ H₄ N) produces the unexpectedly large and fivefold positively charged cluster cation salt [Ga₅ (dmap)₁₀ ]⁵⁺ ([Al(OR^F )₄ ]^- )₅ . It includes a regular and planar Ga₅ pentagon with strong metal-metal bonding. Additionally, the compound represents the first salt in which an ionic 1:5 packing is realized. We discuss the nature of this structure which results from the conversion of the non-bonding 4s² lone-pair orbitals into fully Ga-Ga-bonding orbitals and the solid-state arrangement of the ions constituting the lattice as an almost orthohexagonal AX₅ lattice, possibly the aristotype of any 5:1 salt.