Zero-valent species of group 13–15 elements

Challenges and opportunities:•The paramagnetic zero-valent species, middle-size clusters, and heavier group 13 congeners are untrodden.•Investigation of the ability of the zero-valent species to transfer the elemental fragments En(0) to other substrates remains underdeveloped.•A stepwise catenation of zero-valent species aimed at a precise construction of main-group allotropes could offer an exciting new avenue for materials science. This perspective describes the development of zero-valent species En(0) of group 13–15 elements. Computational studies providing reinterpretation of bonding involved in well-known molecules, such as carbodiphosphorane, have opened up the area. Employment of Lewis bases to form donor-acceptor interactions with the elemental fragments in the zero oxidation state is the key to stabilizing such species. Because of the diversity of Lewis bases available, En(0) derivatives with a variety of bonding and structural motifs are isolable. The monoatomic E1(0) species of group 13 and 15 elements, middle-size En(0) clusters (n = 4–6) of group 13 and 14 elements, and the heavier group 13 cousins have been much less explored. The end of the article outlines one of the potential future directions of zero-valent main-group chemistry, which is an exploration of the allotrope chemistry relevant to materials science. This perspective describes the development of zero-valent species En(0) of group 13–15 elements. Computational studies providing reinterpretation of bonding involved in well-known molecules, such as carbodiphosphorane, have opened up the area. Employment of Lewis bases to form donor-acceptor interactions with the elemental fragments in the zero oxidation state is the key to stabilizing such species. Because of the diversity of Lewis bases available, En(0) derivatives with a variety of bonding and structural motifs are isolable. The monoatomic E1(0) species of group 13 and 15 elements, middle-size En(0) clusters (n = 4–6) of group 13 and 14 elements, and the heavier group 13 cousins have been much less explored. The end of the article outlines one of the potential future directions of zero-valent main-group chemistry, which is an exploration of the allotrope chemistry relevant to materials science. Because of their unfilled valence orbitals, elements (aside from noble gases) are generally unstable in the monatomic form under standard laboratory conditions and exist as diatomic to polyatomic systems. Among them, diverse bonding and structural motifs of polyatomic p-block elements allow polymorphism, leading to multiple prevalent allotropes. A variety of allotropes of carbon (diamond, graphite, fullerenes, and nanotubes), oxygen (dioxygen and ozone), and phosphorus (white, red, blue, and black phosphorus) are well known.1Sharma B.D. Allotropes and polymorphs.J. Chem. Educ. 1987; 64: 404-407Google Scholar Reflecting the bonding and electronic features, each allotrope of an element usually possesses a distinct property, which has attracted various fields of materials science. Nowadays, advances in carbon allotrope chemistry have revolutionized the field of carbon nanotechnology and electronic materials science.2Nasir S. Hussein M.Z. Zainal Z. Yusof N.A. Carbon-based nanomaterials/allotropes: a glimpse of their synthesis, properties and some applications.Materials (Basel). 2018; 11: 295Google Scholar The small clusters of group 13–15 elements have been the attention of considerable research effort because of their architectural varieties and unique electronic nature.3Mallick D. Jemmis E.D. 9.30 – Main group metal clusters.in: Reedijk J. Poeppelmeier K. Comprehensive Inorganic Chemistry. Second Edition. Elsevier, 2013: 833-867Google Scholar,4Mannix A.J. Kiraly B. Hersam M.C. Guisinger N.P. Synthesis and chemistry of elemental 2D materials.Nat. Rev. Chem. 2017; 1: 0014Google Scholar For instance, small and bare boron clusters of the most stable structures Bn (n = 3–8) bear the triangle units; the majority of quasi-planar and convex clusters Bn (n = 9–14) involve pyramidal hexagonal units; and the three-dimensional (3D) clusters Bn (n = up to 14) include trigonal, square, pentagonal, hexagonal, and heptagonal pyramidal motifs. The most stable small aluminum clusters Aln (n = up to 5) are known to exhibit flat geometry, the compact clusters Aln (n = 6–10) become three-dimensional, and the slightly bigger clusters Aln (n = 11–15) involve an inner Al atom with bulk-like coordination. Tetrahedral white phosphorous P4 features a delocalized bonding system with spherical aromatic character, which accounts for the metastability, whereas the larger aggregates Pn and solid allotropes, except for the cubic black phosphorus, are formed on the basis of the localized two-center two-electron bonds, resulting in the insulating polymers. Significantly, potential applications of atomic clusters of main-group elements in materials science are highly expected. In particular, the utility of their two-dimensional (2D) materials expands the variety of materials with diverse and tailorable nature.4Mannix A.J. Kiraly B. Hersam M.C. Guisinger N.P. Synthesis and chemistry of elemental 2D materials.Nat. Rev. Chem. 2017; 1: 0014Google Scholar For example, 2D boron sheets are known to be metallic with remarkable anisotropy behavior, a low density, a high melting point, high hardness, and high thermal conductivity, and they are promising candidates for gas sensing, fuel cells, superconductors, and hydrogen and energy storage materials. A 2D silicon material is predicted to exhibit semi-metallic nature with hosting topologically non-trivial electronic states and spin-polarized edge states, and the device applications, such as tunable transistors and photodetectors, are abided. It should be noted that an atom such as Al in small clusters can behave like a monovalent atom.3Mallick D. Jemmis E.D. 9.30 – Main group metal clusters.in: Reedijk J. Poeppelmeier K. Comprehensive Inorganic Chemistry. Second Edition. Elsevier, 2013: 833-867Google Scholar Given the fact that the oxidation state of the atom in the polyatomic elements is zero, it is fundamentally curious to unveil the nature of the small elemental clusters En(0) of the zero-oxidation state given that these can be considered smaller units of polyatomic allotropes. However, such species are most likely transient even if generated and might be detectable only spectroscopically in the gas phase.5Balasubramanian K. Spectroscopic constants and potential-energy curves of heavy p-block dimers and trimers.Chem. Rev. 1990; 90: 93-167Google Scholar Indeed, some of the small carbon allotropes are known as interstellar mediums.6Van Orden A. Saykally R.J. Small carbon clusters: spectroscopy, structure, and energetics.Chem. Rev. 1998; 98: 2313-2357Google Scholar Instability of these species in the condensed phase impedes their isolation in a laboratory, as well as a detailed investigation of their basic nature. Carbodiphosphorane C(PR3)2 C1-1, which was first prepared in 1961 by Ramirez and co-workers, involves a dicoordinate carbon atom with two C–P bonds.7Ramirez F. Desai N.B. Hansen B. McKelvie N. Hexaphenylcarbodiphosphorane, (C6H5)3PCP(C6H5)3.J. Am. Chem. Soc. 1961; 83: 3539-3540Google Scholar In 2006, Neumüller, Petz, Frenking, et al. reported the reinterpretation of bonding involved in carbodiphosphorane C(PR3)2 C1-1.8Tonner R. Öxler F. Neumüller B. Petz W. Frenking G. Carbodiphosphoranes: the chemistry of divalent carbon(0).Angew. Chem. Int. Ed. 2006; 45: 8038-8042Google Scholar The C–P bonds are formed by donor-acceptor interactions between the lone pairs of electrons of phosphines and the empty valence orbitals of carbon(0), affording a diphosphine carbon(0) complex (Figure 1A). In 2007, a new class of carbon(0) compounds, namely carbodicarbenes, was theoretically predicted,9Tonner R. Frenking G. C(NHC)2: divalent carbon(0) compounds with N-heterocyclic carbene ligands—theoretical evidence for a class of molecules with promising chemical properties.Angew. Chem. Int. Ed. 2007; 46: 8695-8698Google Scholar and shortly afterward C1-2 was indeed experimentally synthesized.10Dyker C.A. Lavallo V. Donnadieu B. Bertrand G. Synthesis of an extremely bent acyclic allene (A “carbodicarbene”): a strong donor ligand.Angew. Chem. Int. Ed. 2008; 47: 3206-3209Google Scholar,11Fürstner A. Alcarazo M. Goddard R. Lehmann C.W. Coordination chemistry of ene-1,1-diamines and a prototype “carbodicarbene”.Angew. Chem. Int. Ed. 2008; 47: 3210-3214Google Scholar The unusual bonding situation and coordination property of divalent carbon(0) molecules were comprehensively investigated by quantum-chemical calculations.12Tonner R. Frenking G. Divalent carbon(0) chemistry, part 1: parent compounds.Chem. Eur. J. 2008; 14: 3260-3272Google Scholar,13Tonner R. Frenking G. Divalent carbon(0) chemistry, part 2: protonation and complexes with main group and transition metal Lewis acids.Chem. Eur. J. 2008; 14: 3273-3289Google Scholar This concept can also be extended to not only diazoolefins C1-314Varava P. Dong Z. Scopelliti R. Fadaei-Tirani F. Severin K. Isolation and characterization of diazoolefins.Nat. Chem. 2021; 13: 1055-1060Google Scholar and C1-415Antoni P.W. Golz C. Holstein J.J. Pantazis D.A. Hansmann M.M. Isolation and reactivity of an elusive diazoalkene.Nat. Chem. 2021; 13: 587-593Google Scholar but also the tetraaminoallenes C1-5.16Alcarazo M. Synthesis, structure, and reactivity of carbodiphosphoranes, carbodicarbenes, and related species.in: Gessner V.H. Modern Ylide Chemistry. Springer, 2017: 25-50Google Scholar The nonclassical bonding situation in these zero-valent EL2 species can be rationalized by three major canonical forms, namely bent allene (L=E=L), ylidone (L→E←L), and ylidene (L=E←L) (Figure 1B). Recently, the Lewis bases have been proven to be great supporting ligands for the isolation of low-valent, low-oxidation-state element systems.17Nesterov V. Reiter D. Bag P. Frisch P. Holzner R. Porzelt A. Inoue S. NHCs in main group chemistry.Chem. Rev. 2018; 118: 9678-9842Google Scholar, 18Soleilhavoup M. Bertrand G. Cyclic (alkyl)(amino)carbenes (CAACs): stable carbenes on the rise.Acc. Chem. Res. 2015; 48: 256-266Google Scholar, 19Borthakur B. Ghosh B. Phukan A.K. The flourishing chemistry of carbene stabilized compounds of group 13 and 14 elements.Polyhedron. 2021; 197: 115049-115071Google Scholar The isolable En(0) species have been successfully developed with the use of strongly σ-donating and bulky ligands. In particular, carbenes, such as N-heterocyclic carbenes (NHCs) and cyclic (alkyl)(amino)carbenes (CAACs), play an important role in En(0) chemistry; they not only donate a lone pair of electrons to the En(0) moiety but also enable the acceptance of the π-type electrons or lone pairs from the En(0) unit at the formally unoccupied carbene carbon, leading to the superior stabilizing effect. It should be noted that because there is only one N atom next to the carbene carbon, CAACs exhibit stronger σ-donating and enhanced π-accepting properties than classical NHCs, resulting in the enhanced C(carbene)−E bonding interaction in En(0) species. The availability of the diverse Lewis bases has afforded an avenue for the chemistry of zero-valent main-group species (Figure 1C). The extant excellent reviews comprehensively cover the seminal studies of EnLm species (n = 1–3, E = groups 13–16), where the original experimental and theoretical reports (up to 2018) are referenced.20Frenking G. Tonner R. Klein S. Takagi N. Shimizu T. Krapp A. Pandey K.K. Parameswaran P. New bonding modes of carbon and heavier group 14 atoms Si–Pb.Chem. Soc. Rev. 2014; 43: 5106-5139Google Scholar, 21Frenking G. Hermann M. Andrada D.M. Holzmann N. Donor–acceptor bonding in novel low-coordinated compounds of boron and group-14 atoms C–Sn.Chem. Soc. Rev. 2016; 45: 1129-1144Google Scholar, 22Zhao L. Hermann M. Holzmann N. Frenking G. Dative bonding in main group compounds.Coord. Chem. Rev. 2017; 344: 163-204Google Scholar, 23Patel N. Sood R. Bharatam P.V. NL2+ systems as new-generation phase-transfer catalysts.Chem. 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In 2020, Ong, Frenking, and collaborators isolated a monoligated C2 adduct C2-3 supported by a bulky bis(imidazolidine-2-iminato)methylphosphine and disclosed that in contrast to cumulenes C2-1 and C2-2, two carbon atoms of the central C2 moiety in C2-3 both have carbene character.31Leung T.-F. Jiang D. Wu M.-C. Xiao D. Ching W.-M. Yap G.P.A. Yang T. Zhao L. Ong T.-G. Frenking G. Isolable dicarbon stabilized by a single phosphine ligand.Nat. Chem. 2021; 13: 89-93Google Scholar These studies demonstrate that the nature and number of the donor ligands significantly affect the bonding situation. Although the electronic structure of the ligated species En(0)Lm differs from that of the ligand-free species, the use of ligands appears to be a powerful tool in stabilizing the otherwise elusive zero-valent species. The concept of donor-acceptor bonding interaction between Lewis bases and elemental fragments is not limited to carbon but is applicable to heavier elements. In the late 20th century, the transient group-14-element carbonyl complexes En(CO)m (E = Si, Ge, Sn, Pb) were generated by the technique of laser ablation coupled with matrix isolation protocol.32Lembke R.R. Ferrante R.F. Weltner W. Carbonylsilene, diazasilene, and dicarbonylsilene molecules: electron spin resonance and optical spectra at 4 K.J. Am. Chem. Soc. 1977; 99: 416-423Google Scholar, 33Jiang L. Xu Q. Observation of the lead carbonyls, PbnCO (n=1–4): reactions of lead atoms and small clusters with carbon monoxide in solid argon.J. Chem. Phys. 2004; 122034505Google Scholar, 34Zhou M. Jiang L. Xu Q. Reactions of silicon atoms and small clusters with CO: experimental and theoretical characterization of SinCO (n=1–5), Si2(CO)2, c-Si2(μ-O)(μ-CSi), and c-Si2(μ-O)(μ-CCO) in solid argon.J. Chem. Phys. 2004; 121: 10474-10482Google Scholar, 35Zhou M. Jiang L. Xu Q. 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Ed. 2022; 61e202114073https://doi.org/10.1002/anie.202114073Google Scholar have also been developed.25Yao S. Xiong Y. Driess M. A new area in main-group chemistry: zerovalent monoatomic silicon compounds and their analogues.Acc. Chem. Res. 2017; 50: 2026-2037Google Scholar,26Majhi P.K. Sasamori T. Tetrylones: an intriguing class of monoatomic zero-valent group 14 compounds.Chem. Eur. J. 2018; 24: 9441-9455Google Scholar The redox non-innocent ligands, such as iminocarbene (Ge1-7), diiminocarbene (Ge1-8), and diiminopyridines (Ge1-9 and Sn1-3), are also applicable for the stabilization of Ge(0) and Sn(0) species.25Yao S. Xiong Y. Driess M. A new area in main-group chemistry: zerovalent monoatomic silicon compounds and their analogues.Acc. Chem. Res. 2017; 50: 2026-2037Google Scholar,26Majhi P.K. Sasamori T. Tetrylones: an intriguing class of monoatomic zero-valent group 14 compounds.Chem. Eur. J. 2018; 24: 9441-9455Google Scholar,47Nguyen M.T. Gusev D. Dmitrienko A. Gabidullin B.M. 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It is salient to note that Wiberg et al. already described Sn1-1 as a stannylene (R2Sn:) adduct of a distannavinylidene (:Sn=SnR2).25Yao S. Xiong Y. Driess M. A new area in main-group chemistry: zerovalent monoatomic silicon compounds and their analogues.Acc. Chem. Res. 2017; 50: 2026-2037Google Scholar,26Majhi P.K. Sasamori T. Tetrylones: an intriguing class of monoatomic zero-valent group 14 compounds.Chem. Eur. J. 2018; 24: 9441-9455Google Scholar The Lewis base-coordinated silicon and germanium analogs, namely disilavinylidenes Si1-11 and Si1-1248Ghana P. Arz M.I. Das U. Schnakenburg G. Filippou A.C. Si=Si double bonds: synthesis of an NHC-stabilized disilavinylidene.Angew. Chem. Int. Ed. 2015; 54: 9980-9985Google Scholar,49Kobayashi R. Ishida S. Iwamoto T. Synthesis of an NHC-coordinated dialkyldisilavinylidene and its oxidation providing a silicon analog of an acetolactone.Organometallics. 2021; 40: 843-847Google Scholar and digermavinylidene Ge1-10,50Krebs K.M. Hanselmann D. 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In sharp contrast, a base-free digermavinylidene Ge1-12 reported by Aldridge et al. features the monocoordinate Ge center with a six-electron valence shell.17Nesterov V. Reiter D. Bag P. Frisch P. Holzner R. Porzelt A. Inoue S. NHCs in main group chemistry.Chem. Rev. 2018; 118: 9678-9842Google Scholar The Ge(0) center is stabilized by weak π interactions with the flanking arene moieties of the substituents, indicating the potential strategy to design other E(0) species with the six-electron system. The seminal finding of a diatomic silicon(0) compound Si2-1 by Robinson, Schleyer, and co-workers opened the way to isolable E2(0) species.17Nesterov V. Reiter D. Bag P. Frisch P. Holzner R. Porzelt A. Inoue S. NHCs in main group chemistry.Chem. Rev. 2018; 118: 9678-9842Google Scholar Each Si atom is supported by an IDip ligand, and the singlet Si2(0) fragment comprises a Si=Si double bond and a lone pair of electrons on each Si atom. The heavier analogs of Ge Ge2-1 and Sn Sn2-1, as well as CyCAAC-supported disilicon Si2-2 and N-heterocyclic silylene (NHSi)-stabilized digermanium Ge2-2, feature a similar bonding interaction between the core E2(0) moiety and the ligands.17Nesterov V. Reiter D. Bag P. Frisch P. Holzner R. Porzelt A. Inoue S. NHCs in main group chemistry.Chem. Rev. 2018; 118: 9678-9842Google Scholar,53Shan Y.-L. Yim W.-L. So C.-W. An N-heterocyclic silylene-stabilized digermanium(0) complex.Angew. Chem. Int. Ed. 2014; 53: 13155-13158Google Scholar In 2019, the Iwamoto group reported the tetrasilabicyclo[1.1.0]but-1(3)-ene Si2-3, in which the bridgehead Si=Si moiety comprises a σ bond with an inverted geometry and a π-bond.54Iwamoto T. Abe T. Sugimoto K. Hashizume D. Matsui H. Kishi R. Nakano M. Ishida S. A Tetrasilicon analogue of bicyclo[1.1.0]but-1(3)-ene containing a Si=Si double bond with an inverted geometry.Angew. Chem. Int. 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