Group 13 five- and six-membered rings with rare 2π aromaticity

According to Hückel's rule, cyclic species are aromatic if they have 4n + 2 (n = 0, 1, 2, etc.) π electrons. However, large aromatic rings (atom number > 4) with minimum 2π electrons (i.e., n = 0) are rather rare because of the structural instability stemming from the deficient π electrons compared to the ring size. To date, the largest 2π-aromatic ring is a five-membered Ga5 core reported in a recent experiment. Herein, density functional theory calculations predicted seven inverse-sandwich Na2(MH)5 and half-sandwich Ca(MH)5 or Ca(MH)6 (M = Al, Ga or In) structures. They all have planar central five- or six-membered Al/Ga/In rings, rather negative binding energies and large HOMO-LUMO gaps. Their dianionic metal rings exhibit obvious aromatic characters and appreciable diatropic ring currents due to the good delocalization of 2π electrons donated by the Na/Ca metals. Interestingly, they also have novel collective bonding with the Na/Ca atoms interacting with both the metal ring and surrounding H atoms. These metalloaromatic rings not only greatly enrich the precious family of 2π aromatics, but also increase the maximum ring atom number from five to six, thus paving the way for Hückel-type 2π-aromatic large rings.

Aluminum-Boron Bond Formation by Boron Ester Oxidative Addition at an Alumanyl Anion

The potassium diamidoalumanyl, [K{Al(SiNDipp)}]2 (SiNDipp = {CH2SiMe2NDipp}2), reacts with the terminal B-O bonds of pinacolato boron esters, ROBpin (R = Me, i-Pr), and B(OMe)3 to provide potsassium (alkoxy)borylaluminate derivatives, [K{Al(SiNDipp)(OR)(Bpin)}]n (R = Me, n = 2; R = i-Pr, n = ∞) and [K{Al(SiNDipp)(OMe)(B(OMe)2)}]∞, comprising Al-B σ bonds. An initial assay of the reactivity of these species with the heteroallene molecules, N,N'-diisopropylcarbodiimide and CO2, highlights the kinetic inaccessibility of their Al-B bonds; only decomposition at high temperature is observed with the carbodiimide, whereas CO2 preferentially inserts into the Al-O bond of [K{Al(SiNDipp)(OMe)(Bpin)}]2 to provide a dimeric methyl carbonate species. Treatment of the acyclic dimethoxyboryl species, however, successfully liberates a terminal alumaboronic ester featuring trigonal N2Al-BO2 coordination environments at both boron and aluminum.

Synthesis of Homo-Metallic Heavier Analogues of Cyclobutene and the Cyclobutadiene Dianion

The reduction of the boryl-substituted SnII bromide {(HCDippN)2 B}Sn(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetratin tetraboryl system K2 [Sn4 {B(NDippCH)2 }4 ], a homo-metallic heavier analogue of the cyclobutadiene dianion. This system is non-aromatic as determined by Nucleus Independent Chemical Shift Calculations (NICS(0)=-0.28, NICS(1)=-3.17), with the primary contributing resonance structures shown by Natural Resonance Theory (NRT) to involve a Sn=Sn double bond and 1,2-localized negative charges. Abstraction of the K+ cations or oxidation leads to contraction or cleavage of the Sn4 unit, respectively, while protonation generates the neutral dihydride 1,2-Sn4 {B(NDippCH)2 }4 H2 (a heavier homologue of cyclobutene) in a manner consistent with the predicted charge distribution in the [Sn4 {B(NDippCH)2 }4 ]2- dianion.

An indium(I) tetramer bound by anionic N-heterocyclic olefins: ambiphilic reactivity, transmetallation and a rare indium-imide

An organometallic tetrahedron-shaped indium(I) tetramer [(^MeIPrCH)In]₄ (^MeIPrCH = [(MeCNDipp)₂CCH]^-; Dipp = 2,6-ⁱPr₂C₆H₃) supported by anionic N-heterocyclic olefin (aNHO) ligands is reported. The monomeric unit of this species exhibits both Lewis acidic and basic character at indium, while the steric profile of the aNHO ligand enables isolation of a rare monomeric imide, RInNR'.

Disproportionation and Ligand Lability in Low Oxidation State Boryl-Tin Chemistry

Boryltin compounds featuring the metal in the+1 or 0 oxidation states can be synthesized from the carbene-stabilized tin(II) bromide (boryl)Sn(NHC)Br (boryl={B(NDippCH)2 }; NHC=C{(Ni PrCMe)2 }) by the use of strong reducing agents. The formation of the mono-carbene stabilized distannyne and donor-free distannide systems (boryl)SnSn(IPrMe)(boryl) (2) and K2 [Sn2 (boryl)2 ] (3), using Mg(I) and K reducing agents mirrors related germanium chemistry. In contrast to their lighter congeners, however, systems of the type [Sn(boryl)]n are unstable with respect to disproportionation. Carbene abstraction from 2 using BPh3 , and two-electron oxidation of 3 both result in the formation of a 2 : 1 mixture of the Sn(II) compound Sn(boryl)2 , and the hexatin cluster, Sn6 (boryl)4 (4). A viable mechanism for this rearrangement is shown by quantum chemical studies to involve a vinylidene intermediate (analogous to the isolable germanium compound, (boryl)2 Ge=Ge), which undergoes facile atom transfer to generate Sn(boryl)2 and trinuclear [Sn3 (boryl)2 ]. The latter then dimerizes to give the observed hexametallic product 4, with independent studies showing that similar trigermanium species aggregate in analogous fashion.

Platonic and Archimedean solids in discrete metal-containing clusters

Metal-containing clusters have attracted increasing attention over the past 2-3 decades. This intense interest can be attributed to the fact that these discrete metal aggregates, whose atomically precise structures are resolved by single-crystal X-ray diffraction (SCXRD), often possess intriguing geometrical features (high symmetry, aesthetically pleasing shapes and architectures) and fascinating physical properties, providing invaluable opportunities for the intersection of different disciplines including chemistry, physics, mathematical geometry and materials science. In this review, we attempt to reinterpret and connect these fascinating clusters from the perspective of Platonic and Archimedean solid characteristics, focusing on highly symmetrical and complex metal-containing (metal = Al, Ti, V, Mo, W, U, Mn, Fe, Co, Ni, Pd, Pt, Cu, Ag, Au, lanthanoids (Ln), and actinoids) high-nuclearity clusters, including metal-oxo/hydroxide/chalcogenide clusters and metal clusters (with metal-metal binding) protected by surface organic ligands, such as thiolate, phosphine, alkynyl, carbonyl and nitrogen/oxygen donor ligands. Furthermore, we present the symmetrical beauty of metal cluster structures and the geometrical similarity of different types of clusters and provide a large number of examples to show how to accurately describe the metal clusters from the perspective of highly symmetrical polyhedra. Finally, knowledge and further insights into the design and synthesis of unknown metal clusters are put forward by summarizing these "star" molecules.

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.

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.

Synthesis and computational aspects of Al(II)-Al(II) and Ga(II)-Ga(II) dihalides based on an amidinate scaffold

Amidinate compounds with stabilized aluminium(II) and gallium(II) elements of composition L₂M₂X₂ (3 and 4) have been prepared from their LMX₂ (1 and 2) precursor, where M = Al (1 and 4) and Ga (2 and 3); L = PhC(NⁱPr₂C₆H₃)₂ (1 and 4) and PhC(N^tBu)₂ (2 and 3); and X is I (1 and 4) and Cl (2 and 3) and insights into their bonding are gained. The M-M bond lengths are reported along with the single-crystal X-ray structures of 1-4.

Sequential Reduction of Borylstibane to an Electronically Nonsymmetric Diboryldistibene Radical Anion

Understanding the formation of metal-metal bonds and their electronic structures is still a scientific task. We herein report on the stepwise synthesis of boryl-substituted antimony compounds in which the antimony atoms adopt four different oxidation states (+III, +II, +I, +I/0). Sb-C bond homolysis of Cp*[(HCNDip)₂B]SbCl (1; Cp* = C₅Me₅; Dip = 2,6-iPr₂C₆H₃) gave diboryldichlorodistibane [(HCNDip)₂BSbCl]₂ (2), which reacted with KC₈ to form diboryldistibene [(HCNDip)₂BSb]₂ (3) and traces of cyclotetrastibane [(HCNDip)₂B]₃Sb₄Cl (5). One-electron reduction of 3 yielded the potassium salt of the diboryldistibene radical anion [(HCNDip)₂BSb]₂̇^-, [K(18-c-6)(OEt₂)][{(HCNDip)₂BSb}₂] (4), which exhibits an unprecedented inequivalent spin localization on the Sb-Sb bond and hence an unsymmetric electronic structure. Compounds 1-4 were characterized by heteronuclear nuclear magnetic resonance (NMR) (¹H, ¹³C, ¹¹B), infrared (IR), ultraviolet-visible (UV-vis) spectroscopy (3, 4), and single crystal X-ray diffraction (sc-XRD, 1-5), while the bonding nature of 3 and 4 was analyzed by quantum chemical calculations. EPR spectroscopy resolves the dissimilar Sb hyperfine tensors of 4, reflecting the inequivalent spin distribution, setting 4 uniquely apart from all previously characterized dipnictene radical anions.

Structural Chemistry of Giant Metal Based Supramolecules

The review presents a bird-eye view on the state of research in the field of giant nonbiological discrete metal complexes and ions of nanometer size, which are structurally characterized by means of single-crystal X-ray diffraction, using the crystal structure as a common key feature. The discussion is focused on the main structural features of the metal clusters, the clusters containing compact metal oxide/hydroxide/chalcogenide core, ligand-based metal-organic cages, and supramolecules as well as on the aspects related to the packing of the molecules or ions in the crystal and the methodological aspects of the single-crystal neutron and X-ray diffraction of these compounds.

Heavier element-containing aromatics of [4n+2]-electron systems

While the implication of the aromaticity concept has been dramatically expanded to date since its emergence in 1865, the classical [4n+2]/4n-electron counting protocol still plays an essential role in evaluating the aromatic nature of compounds. Over the last few decades, a variety of heavier heterocycles featuring the formal [4n+2] π-electron arrangements have been developed, which allows for assessing their aromatic nature. In this review, we present recent developments of the [4n+2]-electron systems of heavier heterocycles involving group 13-15 elements. The synthesis, spectroscopic data, structural parameters, computational data, and reactivity are introduced.

Boranyl-Functionalized [Ge9 ] Clusters: Providing the Idea of Intramolecular Ge/B Frustrated Lewis Pairs

The unique three-dimensional structure of spherical, homoatomic nine-atom germanium clusters opens various possibilities for the spatial arrangement of functional groups. Ligands comprising lone pairs have recently been introduced in the cluster sphere, and we now report the addition of a boranyl group to the cluster featuring a Ge-B exo-cluster bond. The reaction of the twofold-silylated cluster [Ge9 {Si(TMS)3 }2 ]2- (TMS=trimethylsilyl) with 2-chloro-1,3,2-diazaborolidines DABR -Cl leads to the first boranyl-functionalized [Ge9 ] clusters [Ge9 {Si(TMS)3 }2 DABR ]- (R=methyl (1 a), iso-propyl (2 a), ortho-tolyl (3 a)). The anions 2 a and 3 a were structurally characterized as [NHCDipp Cu]+ complexes (NHCDipp =1,3-di(2,6-diisopropylphenyl)imidazolylidine) through single crystal X-ray structure determination. Quantum-chemical calculations manifest the frustrated Lewis pair (FLP) character of the boranyl-functionalized cluster [Ge9 {Si(TMS)3 }2 BCy2 ]- (4 a).

Amidinate based indium(III) monohalides and β-diketiminate stabilized In(II)-In(II) bond: synthesis, crystal structure, and computational study

Synthesis and bonding aspects of mononuclear bis-amidinate indium(iii) monohalides L2InX (1-3), where L = PhC(NtBu)2; X is F (1), Br (2) or I (3) and β-diketiminate (NacNac) stabilized In(ii) dimer (MesNacNac)2In2Br2 (4) with In-In bond are reported along with the single-crystal X-ray structures of 2-4.

Synthesis and Characterization of a Magnesium Boryl and a Beryllium-Substituted Diazaborole

Reaction of a lithium boryl, [(THF)₂ Li{B(DAB)}] (DAB=[(DipNCH)₂ ]^2- , Dip=2,6-diisopropylphenyl), with a dinuclear magnesium(I) compound [{(^Mes Nacnac)Mg}₂ ] (^Mes Nacnac=[HC(MeCNMes)₂ ]^- , Mes=mesityl) unexpectedly afforded a rare example of a terminal magnesium boryl species, [(^Mes Nacnac)(THF)Mg{B(DAB)}]. Attempts to prepare the magnesium boryl via a salt metathesis reaction between the lithium boryl and a β-diketiminato magnesium iodide compound, instead led to an intractable mixture of products. Similarly, reaction of the lithium boryl with a β-diketiminato beryllium bromide precursor, [(^Dep Nacnac)BeBr] (Dep=2,6-diethylphenyl) did not give a beryllium boryl, but instead afforded an unprecedented example of a beryllium substituted diazaborole heterocycle, [{(^Dep Nacnac)Be(4-DAB^-H )}BBr]. For sake of comparison, the same group 2 halide precursor compounds were treated with a potassium gallyl analogue of the lithium boryl, viz. [(tmeda)K{:Ga(DAB)}] (tmeda=N,N,N',N'-tetramethylethylenediamine), but no reactions were observed.

Synthetic, structural and reaction chemistry of N-heterocyclic germylene and stannylene compounds featuring N-boryl substituents

This study details the syntheses of N-heterocyclic germylenes and stannylenes featuring diazaborolyl groups, {(HCDippN)₂B} (Dipp = 2,6-ⁱPr₂C₆H₃), as both of the N-bound substituents, with a view to generating electron rich and sterically protected metal centres. The energies of their key frontier orbitals - the group 14-centred lone pair and orthogonal p_π-orbital (typically the HOMO-2 and LUMO) have been probed by DFT calculations and compared with a related acyclic analogue, revealing (in the case of the stannylenes) a correlation with the measured ¹¹⁹Sn chemical shifts. The reactivity of the germylene systems towards oxygen atom transfer agents has been examined, with 2 : 1 reaction stoichiometries being observed for both Me₃NO and pyridine N-oxide, leading to the formation of products thought to be derived from the activation of C-H bonds by a transient first-formed germanone.

Borylated N-Heterocyclic Carbenes: Rearrangement and Chemical Trapping

This study details attempts to access N-heterocyclic carbenes (NHCs) featuring the diazaborolyl group, {(HCNDipp)₂ B}, as one or both of the N-bound substituents, to generate strongly electron-donating and sterically imposing new carbene ligands. Attempts to isolate N-heterocyclic carbenes based around imidazolylidene or related heterocycles, are characterized by facile N-to-C migration of the boryl substituent. In the cases of imidazolium precursors bearing one N-bound diazaborolyl group and one methyl substituent, deprotonation leads to the generation of the target carbenes, which can be characterized in situ by NMR measurements, and trapped by reactions with metal fragments and elemental selenium. The half-lives of the free carbenes at room temperature range from 4-50 h (depending on the pattern of ancillary substituents) with N-to-C2 migration of the boryl function being shown to be the predominant rearrangement pathway. Kinetic studies show this to be a first-order process that occurs with an entropy of activation close to zero. DFT calculations imply that an intramolecular 1,2-shift is mechanistically feasible, with calculated activation energies of the order of 90-100 kJ mol^-1 , reflecting the retention of significant aromatic character in the imidazole ring in the transition state. Trapping of the carbene allows for evaluation of steric and electronic properties through systems of the type LAuCl, LRh(CO)₂ Cl, and LSe. A highly unsymmetrical (but nonetheless bulky) steric profile and moderately enhanced σ-donor capabilities (compared with IMes) are revealed.

Sn20(SitBu3)10Cl2 – the largest metalloid group 14 cluster shows a raspberry-like arrangement of smaller units

The reaction of a metastable Sn(i)Cl solution with NaSi^tBu₃ leads to Sn₂₀(Si^tBu₃)₁₀C_l2, where a raspberry-like cluster of cluster arrangement is realized, giving further insight into the formation process of a metalloid tin cluster from molecular precursors.

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 Re-examination of the Synthesis of Monolayer-Protected Co x(SCH2CH2Ph) m Nanoclusters: Unexpected Formation of a Thiolate-Protected Co(II) T3 Supertetrahedron

Herein, we report a re-examination of the synthesis and characterization of monolayer-protected Co _x(SCH₂CH₂Ph) _m nanoclusters. These clusters were reportedly formed by the reaction of CoCl₂ with NaBH₄ in the presence of HSCH₂CH₂Ph and were suggested to contain between 25 and 30 Co atoms. In our hands, however, we found no experimental evidence to support the existence of these large clusters in the reaction mixture. Instead, this reaction results in the relatively clean formation of the cobalt(II) coordination complex [Co₁₀(SCH₂CH₂Ph)₁₆Cl₄] (1). Complex 1 has been fully characterized using a wide variety of techniques, including single-crystal X-ray crystallography, NMR spectroscopy, mass spectrometry, and magnetometry. This complex represents the first example of a thiolate-protected Co(II) T3 supertetrahedral cluster.

Experimental and quantum chemical studies of anionic analogues of N-heterocyclic carbenes

A combination of quantum chemical and synthetic/crystallographic methods have been employed to probe electronic structure in two series of anionic ligands related to the well-known NHC donor class.