Diverse Functionality of Molecular Germanium: Emerging Opportunities as Catalysts

Fundamental research on germanium as the central element in compounds for bond activation chemistry and catalysis has achieved significant feats over the last two decades. Designing strategies for small molecule activations and the ultimate catalysts established capitalize on the orbital modalities of germanium, apparently imitating the transition-metal frontier orbitals. There is a growing body of examples in contemporary research implicating the tunability of the frontier orbitals through avant-garde approaches such as geometric constrained empowered reactivity, bimetallic orbital complementarity, cooperative reactivity, etc. The goal of this Perspective is to provide readers with an overview of the emerging opportunities in the field of germanium-based catalysis by perceiving the underlying key principles. This will help to convert the discrete set of findings into a more systematic vision for catalyst designs. Critical exposition on the germanium's frontier orbitals participations evokes the key challenges involved in innovative catalyst designs, wherein viewpoints are provided. We close by addressing the forward-looking directions for germanium-based catalytic manifold development. We hope that this Perspective will be motivational for applied research on germanium as a constituent of pragmatic catalysts.

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.

Proton to hydride umpolung at a phosphonium center via electron relay: a new strategy for main-group based water reduction

Generation of dihydrogen from water splitting, also known as water reduction, is a key process to access a sustainable hydrogen economy for energy production and usage. The key step is the selective reduction of a protic hydrogen to an accessible and reactive hydride, which has proven difficult at a p-block element. Although frustrated Lewis pair (FLP) chemistry is well known for water activation by heterolytic H-OH bond cleavage, to the best of our knowledge, there has been only one case showing water reduction by metal-free FLP systems to date, in which silylene (SiII) was used as the Lewis base. This work reports the molecular design and synthesis of an ortho-phenylene linked bisborane-functionalized phosphine, which reacts with water stoichiometrically to generate H2 and phosphine oxide quantitatively under ambient conditions. Computational investigations revealed an unprecedented multi-centered electron relay mechanism offered by the molecular framework, shuttling a pair of electrons from hydroxide (OH-) in water to the separated proton through a borane-phosphonium-borane path. This simple molecular design and its water reduction mechanism opens new avenues for this main-group chemistry in their growing roles in chemical transformations.

Reversible and Irreversible [2+2] Cycloaddition Reactions of Heteroallenes to a Gallaphosphene

[2+2] Cycloaddition reactions of gallaphosphene L(Cl)GaPGaL 1 (L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ) with carbodiimides [C(NR)2 ; R=i-Pr, Cy] and isocyanates [RNCO; R=Et, i-Pr, Cy] yielded four-membered metallaheterocycles LGa(Cl)P[μ-C(X)NR]GaL (X=NR, R=i-Pr 2, Cy 3; X=O, R=Et 4, i-Pr 5, Cy 6). Compounds 4-6 reversibly react with CO2 via [2+2] cycloaddition at ambient temperature to the six-membered metallaheterocycles LGa(Cl)P[μ-C(O)O]-μ-C(O)N(R)GaL (R=Et 7, i-Pr 8, Cy 9). Compounds 2-9 were characterized by IR and heteronuclear (1 H, 13 C{1 H}, 31 P{1 H}) NMR spectroscopy and elemental analysis, while quantum chemical calculations provided a deeper understanding on the energetics of the reactions.

Recent advances of group 14 dimetallenes and dimetallynes in bond activation and catalysis

Since the first heavy alkene analogues of germanium and tin were isolated in 1976, followed by West's disilene in 1981, the chemistry of stable group 14 dimetallenes and dimetallynes has advanced immensely. Recent developments in this field veered the focus from the isolation of novel bonding motifs to mimicking transition metals in their ability to activate small molecules and perform catalysis. The potential of these homonuclear multiply bonded compounds has been demonstrated numerous times in the activation of H2, NH3, CO2 and other small molecules. Hereby, the strong relationship between structure and reactivity warrants close attention towards rational ligand design. This minireview provides an overview on recent developments in regard to bond activation with group 14 dimetallenes and dimetallynes with the perspective of potential catalytic applications of these compounds.

Facile activation of alkynes with a boraguanidinato-stabilized germylene: a combined experimental and theoretical study

A boraguanidinato-stabilized germylene, [(i-Pr)₂NB(N-2,6-Me₂C₆H₃)₂]Ge, reacts with alkynes RC[triple bond, length as m-dash]CR selectively in a 2 : 1 molar ratio to afford 3,4-R,R'-1,2-digermacyclobut-3-enes 1a-e as the products of formal [2 + 2 + 2] cyclization [R/R' = Me/Me (1a), Ph/Ph (1b), Ph/H (1c), t-Bu/H (1d) and Cy/H (1e)]. Ferrocenyl-substituted alkynes react similarly, yielding the corresponding ferrocenylated 3,4-R,R'-1,2-digermacyclobut-3-enes 2a-d [where R/R' = Fc/H (2a), Fc/Me (2b), Fc/Ph (2c), and Fc/Fc (2d); Fc = ferrocenyl]. By contrast, only one of the triple bonds available in conjugated diynes RC[triple bond, length as m-dash]CC[triple bond, length as m-dash]CR is activated with the germylene, while the second one remains intact even in the presence of an excess of the germylene. The exclusive formation of 3,4-R,(C[triple bond, length as m-dash]CR)-1,2-digermacyclobut-3-enes 3a-c [R = Ph (3a), t-Bu(3b), and Fc (3c)] was ascribed to a steric repulsion around the second triple bond. On the other hand, the reaction of the germylene with more flexible dialkyne fc(C[triple bond, length as m-dash]CPh)₂ (fc = ferrocene-1,1'-diyl) proceeded in the expected manner, producing compound 4, where both triple bonds are transformed into 1,2-digermacyclobut-3-ene rings by reaction with four equivalents of the germylene. All compounds were characterized by multinuclear NMR spectroscopy, Raman and IR spectroscopy, and in the case of 1a-c, 2a, 2c, 3a, 3b and 4, also by single-crystal X-ray diffraction analysis. The ferrocenyl substituted compounds were studied by cyclic voltammetry (CV). Finally, the plausible reaction pathway was studied for a model reaction of [(i-Pr)₂NB(N-2,6-Me₂C₆H₃)₂]Ge with MeC[triple bond, length as m-dash]CMe using DFT computations.

Addition of Isocyanides to Tetramesityldigermene: A Comparison of the Reactivity between Surface and Molecular Digermenes

The reaction of benzyl isocyanide, tert-butyl isocyanide, and 2,6-dimethylphenyl isocyanide with tetramesityldigermene (Mes₂ Ge=GeMes₂ ) was examined. Whereas the addition of benzyl isocyanide gave the C-NC activation product, Mes₂ Ge(CH₂ Ph)Ge(CN)Mes₂ , tert-butyl isocyanide, and 2,6-dimethylphenyl isocyanide did not give stable adducts, rather the rate of conversion of the digermene to the corresponding cyclotrigermane was accelerated. A comparison between the reactivity of the isocyanides with Mes₂ Ge=GeMes₂ and the Ge(100)-2×1 surface was made and some insights into the surface chemistry are offered.