[Cl@Si20H20]-: Parent Siladodecahedrane with Endohedral Chloride Ion

An efficient multi-gram access in a two-step synthesis to soluble, nine-atomic, silylated silicon clusters

Silicon is by far the most important semiconducting material. However, solution-based synthetic approaches for unsaturated silicon-rich molecules require less efficient multi-step syntheses. We report on a straightforward access to soluble, polyhedral Si9 clusters from the binary phase K12Si17, which contains both [Si4]4- and [Si9]4- clusters. [Si4]4- ions, characterised by a high charge per atom ratio, behave as strong reducing agents, preventing [Si9]4- from directed reactions. By the here reported separation of [Si4]4- by means of fractional crystallisation, Si9 clusters of the precursor phase K12Si17 are isolated as monoprotonated [Si9H]3- ions on a multi-gram scale and further crystallised as their 2.2.2-Cryptate salt. 20 grams of the product can be obtained through this two-step procedure - a new starting point for silicon Zintl chemistry, such as the isolation and structural characterisation of a trisilylated [MeHyp3Si9]- cluster.

Confined Lewis Pairs: Investigation of the X- →Si20 Interaction in Halogen-Encapsulating Silafulleranes

A joint theoretical and experimental study on 32 endohedral silafullerane derivatives [X@Si20 Y20 ]- (X=F-I; Y=F-I, H, Me, Et) andT h ${T_h }$ -[Cl@Si20 H12 Y8 ]- (Y=F-I) is presented. First, we evaluated the structure-determining template effect of Cl- in a systematic series of concave silapolyquinane model systems. Second, we investigated the X- →Si20 interaction energy (E int ${E_{{\rm{int}}} }$ ) as a function of X- and Y and found the largestE int ${E_{{\rm{int}}} }$ values for electron-withdrawing exohedral substituents Y. Given that X- ions can be considered as Lewis bases and empty Si20 Y20 clusters as Lewis acids, we classify our inseparable host-guest complexes [X@Si20 Y20 ]- as "confined Lewis pairs". Third, 35 Cl NMR spectroscopy proved to be highly diagnostic for an experimental assessment of the Cl- →Si20 interaction as the paramagnetic shielding and, in turn, δ ${\delta }$ (35 Cl) of the endohedral Cl- ion correlate inversely withE int ${E_{{\rm{int}}} }$ . Finally, we disclose the synthesis of [PPN][Cl@Si20 Y20 ] (Y=Me, Et, Br) and provide a thorough characterization of these new silafulleranes.

Elucidating the Electronic Nature of Rh-based Paddlewheel Catalysts from 103 Rh NMR Chemical Shifts: Insights from Quantum Mechanical Calculations

The tremendous importance of dirhodium paddlewheel complexes for asymmetric catalysis is largely the result of an empirical optimization of the chiral ligand sphere about the bimetallic core. It was only recently that a H(C)Rh triple resonance 103 Rh NMR experiment provided the long-awaited opportunity to examine - with previously inconceivable accuracy - how variation of the ligands impacts on the electronic structure of such catalysts. The recorded effects are dramatic: formal replacement of only one out of eight O-atoms surrounding the metal centers in a dirhodium tetracarboxylate by an N-atom results in a shielding of the corresponding Rh-site of no less than 1000 ppm. The current paper provides the theoretical framework that allows this and related experimental observations made with a set of 19 representative rhodium complexes to be interpreted. In line with symmetry considerations, it is shown that the shielding tensor responds only to the donor ability of the equatorial ligands along the perpendicular principal axis. Axial ligands, in contrast, have no direct effect on shielding but may come into play via the electronicc i s ${cis}$ -effect that they exert onto the neighboring equatorial sites. On top of these fundamental interactions, charge redistribution within the core as well as the electronict r a n s ${trans}$ -effect of ligands of different donor strengths is reflected in the recorded 103 Rh NMR shifts.

Installing Quaternary Germanium Centers in Sila-Diamondoid Cores via Skeletal Isomerization

This manuscript describes skeletal isomerization strategies to install one to four quaternary germanium atoms in the sila-adamantane core, in a cluster analogy to precision germanium doping in silicon-germanium alloys. The first strategy embodies an inorganic variant of single-atom skeletal editing, where we use a sila-Wagner-Meerwein bond shift cascade to exchange a peripheral Ge atom with a core Si atom. We can install up to four Ge atoms at the quaternary diamondoid centers based on controlling the SixGey stoichiometry of our precursor. We find that bridgehead Ge centers can be selectively functionalized over bridgehead Si centers in SiGe adamantanes; we use this chemistry in conjunction with scanning tunneling microscopy break-junction (STM-BJ) measurements to show that Si8Ge2 adamantane wires give a 60% increase in single-molecule conductance compared with Si10 adamantanes. These studies describe the first quantum transport measurements in sila-diamondoid structures, and demonstrate how main-chain Ge doping can be used to increase electronic transmission in sila-diamondoid-based molecular wires.

Cl@Si20X20 cages: evaluation of encapsulation nature, structural rigidity, and 29Si-NMR patterns using relativistic DFT calculations

The experimental characterization of Cl@Si₂₀ endohedral clusters, featuring different ligands such as [Cl@Si₂₀H₂₀]^- (1) [Cl@Si₂₀H₁₂Cl₈]^- (2), and [Cl@Si₂₀Cl₂₀]^- (3), provides insight into the variable encapsulation environment for chloride anions. The favorable formation of such species enables the evaluation of the encapsulation nature and the role of the inner anion in the rigidity of the overall cluster. Our results show a sizable interaction which increases as -66.7, -100.8, and -130.3 kcal mol^-1 from 1 to 3, respectively, featuring electrostatic character. The orbital interaction involves 3p-Cl → Si₂₀X₂₀ and 3s-Cl → Si₂₀X₂₀ charge transfer channels and a slight contribution from London dispersion-type interactions. These results show that the inner bonding environment can be modified by the choice of exobonded ligands. Moreover, ²⁹Si-NMR parameters are depicted in terms of the chemical shift anisotropy (CSA), leading to a strong variation of the three principal tensor components (δ₁₁, δ₂₂, δ₃₃), unraveling the origin of the experimental ²⁹Si-NMR chemical shift (δ_iso) differences along the given series. Thus, the Si₂₀ cage is a useful template to further evaluate different environments for encapsulating atomic species.

Synthesis, bridgehead functionalization, and photoisomerization of 9,10-diboratatriptycene dianions

9,10-Diboratatriptycene salts M2[RB(μ-C6H4)3BR] (R = H, Me; M+ = Li+, K+, [n-Bu4N]+) have been synthesized via [4 + 2] cycloaddition between doubly reduced 9,10-dihydro-9,10-diboraanthracenes M2[DBA] and benzyne, generated in situ from C6H5F and C6H5Li or LiN(i-Pr)2. [HB(μ-C6H4)3BH]2- reacts with CH2Cl2 to form quantitatively the bridgehead-derivatized [ClB(μ-C6H4)3BCl]2-, while twofold H- abstraction with B(C6F5)3 in the presence of SMe2 leads cleanly to the diadduct (Me2S)B(μ-C6H4)3B(SMe2). Photoisomerization of K2[HB(μ-C6H4)3BH] (THF, medium-pressure Hg lamp) provides facile access to diborabenzo[a]fluoranthenes, a little explored form of boron-doped polycyclic aromatic hydrocarbons. According to DFT calculations, the underlying reaction mechanism consists of three main steps: (i) photoinduced di-π-borate rearrangement, (ii) "walk reaction" of a BH unit, and (iii) boryl anion-like C-H activation.

Regioselective Derivatization of Silylated [20]Silafulleranes

Silafulleranes with endohedral Cl^- ions are a unique, scarcely explored class of structurally well-defined silicon clusters and host-guest complexes. Herein, we report regioselective derivatization reactions on the siladodecahedrane [nBu₄N][Cl@Si₂₀(SiCl₃)₁₂Cl₈] ([nBu₄N][1]), which has its cluster surface decorated with 12 SiCl₃ and 8 Cl substituents in perfect T_h symmetry. The room-temperature reaction of [nBu₄N][1] with excess iBu₂AlH in ortho-difluorobenzene (oDFB) furnishes perhydrogenated [nBu₄N][Cl@Si₂₀(SiH₃)₁₂H₈] ([nBu₄N][2]) in 50% yield; the non-pyrophoric [2]^- is the largest structurally authenticated (by X-ray diffraction) hydridosilane known to date. A simple switch from pure oDFB to an oDFB/Et₂O solvent mixture suppresses core hydrogenation and results in the formation of [nBu₄N][Cl@Si₂₀(SiH₃)₁₂Cl₈] ([nBu₄N][3]). In addition to the exhaustive Cl/H exchange at all 44 Si-Cl bonds of [1]^- and the regioselective 36-fold silyl group hydrogenation, we achieved the simultaneous introduction of Me substituents at all 8 SiCl vertices along with the conversion of all 12 SiCl₃ to SiH₃ groups by treating [nBu₄N][1] with Me₂AlH/Me₃Al in oDFB ([nBu₄N][Cl@Si₂₀(SiH₃)₁₂Me₈], [nBu₄N][4]; 73%). Quantum-chemical free-energy calculations find an S_N2-Si-type hydrogenation of the exohedral SiCl₃ moieties in [1]^- (trigonal-bipyramidal intermediate) slightly preferred over metathesis-like S_Ni-Si substitutions (four-membered transition state). Cage hydrogenation likely occurs via S_Ni-Si processes. The experimentally demonstrated influence of an Et₂O co-solvent, which drastically increases the respective reaction barriers, is attributed to the increased stability of the resulting iBu₂AlH-OEt₂ adduct and its higher steric bulk compared to free iBu₂AlH.

Theoretical Study of Si/C Equally Mixed Dodecahedrane Analogues

To gain insight into the effect of Si/C arrangement on the molecular framework of strained polyhedral compounds, dodecahedrane analogues containing equal numbers of carbon and silicon (Si/C equally mixed dodecahedrane analogues) were investigated using the ab initio molecular orbital method. There are 1648 isomers for which the Si/C arrangement on the molecular framework is different. Based on the geometry optimization of all the isomers as well as the carbon and silicon analogues, the characteristics of the structure, relative energy, and strain energy of the Si/C equally mixed compounds are presented. Then, important factors controlling the relative energy, such as strain energy, are proposed through regression analysis. Also discussed is the correlation between the relative energy and the indices of Si/C dispersion, such as the number of skeletal C-Si single bonds and condensed five-membered rings constituting the polyhedral structure.

Dope it with germanium: selective access to functionalized Si5Ge heterocycles

The Cl^- diadduct [nBu₄N]₂[A·2Cl] of the mixed cyclohexatetrelane (SiCl₂)₅(GeMe₂), A, is accessible from Me₂GeCl₂, 6 eq. Si₂Cl₆, and 2 eq. [nBu₄N]Cl in one step (96%). Free, tenfold functionalized A can be released from the primary product by decomplexation with AlCl₃ (78%). Insight into the assembly mechanism of [nBu₄N]₂[A·2Cl] and the reactivity of A is provided.

Tetrel Bonding in Anion Recognition: A First Principles Investigation

Twenty-five molecule-anion complex systems [I₄Tt···X^-] (Tt = C, Si, Ge, Sn and Pb; X = F, Cl, Br, I and At) were examined using density functional theory (ωB97X-D) and ab initio (MP2 and CCSD) methods to demonstrate the ability of the tetrel atoms in molecular entities, I₄Tt, to recognize the halide anions when in close proximity. The tetrel bond strength for the [I₄C···X^-] series and [I₄Tt···X^-] (Tt = Si, Sn; X = I, At), was weak-to-moderate, whereas that in the remaining 16 complexes was dative tetrel bond type with very large interaction energies and short Tt···X close contact distances. The basis set superposition error corrected interaction energies calculated with the highest-level theory applied, [CCSD(T)/def2-TZVPPD], ranged from -3.0 to -112.2 kcal mol^-1. The significant variation in interaction energies was realized as a result of different levels of tetrel bonding environment between the interacting partners at the equilibrium geometries of the complex systems. Although the ωB97X-D computed intermolecular geometries and interaction energies of a majority of the [I₄Tt···X^-] complexes were close to those predicted by the highest level of theory, the MP2 results were shown to be misleading for some of these systems. To provide insight into the nature of the intermolecular chemical bonding environment in the 25 molecule-anion complexes investigated, we discussed the charge-density-based topological and isosurface features that emanated from the application of the quantum theory of atoms in molecules and independent gradient model approaches, respectively.

Rational Design of Endohedral Superhalogens without Using Metal Cations and Electron Counting Rules

Superhalogens, predicted 40 years ago, have attracted considerable attention due to their potential as building blocks of novel materials with various applications. While a large number of superhalogen clusters have been theoretically predicted and experimentally synthesized, they either require the use of a metal cation or electron counting rules. In particular, very rare endohedral cage clusters in defiance of the above requirements have been found to be superhalogens. In this work, motivated by recent experimental advances in endohedral cage clusters, we present a rational design principle for creating a new class of such superhalogens. Focusing on the chemical formula of A@Si₂₀X₂₀ (A = F, Cl, Br, I, BH₄, BF₄; X = H, F, Cl, Br, I, BO, CN, SCN, CH₃), we use first-principles calculations to study 54 different clusters and show that these clusters possess electron affinities as high as 8.5 eV. Some of these clusters with X = BO and CN can even be stable as dianions, with large second electron affinity ∼2 eV. Similarly, Cl@C₆₀ is found to be a superhalogen. This class of superhalogens is different from the conventional ones with chemical formula MX_k+1, where X is a halogen and M is a cation with a formal +k oxidation state. Interestingly, the electron affinities of A@Si₂₀X₂₀ are almost independent of the central A moiety, but are guided by the functional group X. The potential of these endohedral superhalogens as electrolytes in Li-ion batteries is discussed.

Synthetic Doping of Diamondoids through Skeletal Editing

We present a strategy for the skeletal editing of diamondoid structures to selectively displace methylene for heteroatom moieties in the carbon framework. This constitutes a synthetic approach to doping diamond-like structures with electron donor dopants (O, N, and S). The key steps involve two subsequent retro-Barbier fragmentations followed by cage reconstruction in the presence of a dopant. Remarkably, the incorporation of n-dopants reduces the strain of the diamondoid cage as shown through homodesmotic equations.

Subvalent mixed SixGey oligomers: (Cl3Si)4Ge and Cl2(Me2EtN)SiGe(SiCl3)2

(Cl₃Si)₄Ge (1; 91%) is accessible from GeCl₄, the Si₂Cl₆/[nBu₄N]Cl silylation system, and excess SiCl₄. A key intermediate step involves Cl^- sequestration with AlCl₃ in the course of the reaction between the first-formed germanide [(Cl₃Si)₃Ge]^- and SiCl₄. The related adduct Cl₂(Me₂EtN)SiGe(SiCl₃)₂ (2; quantitative conversion) was prepared either by amine-induced cleavage of 1 or by a bottom-up synthesis starting from GeCl₄ and Si₂Cl₆.

The Structure of Bis(catecholato)silanes: Phase Adaptation by Dynamic Covalent Chemistry of the Si-O Bond

Catechols occupy a unique role in the structural, bio-, and geochemistry of silicon. Although a wealth of knowledge exists on their hypercoordinate complexes, the structure of tetracoordinate bis(catecholato)silane, Si(cat^H)₂ 1, has been enigmatic since its first report in 1951. Indeed, the claim of a planar-tetracoordinated silicon in 1 triggered a prominent debate, which is unsettled to this day. Herewith, we present a comprehensive structural study on 1 and derivatives in the gas phase by electron diffraction, in a neon matrix by IR spectroscopy, in solution by diffusion NMR spectroscopy, and in the solid-state by X-ray diffraction and MAS NMR spectroscopy, complemented by high-level quantum-chemical computations. The compound exhibits unprecedented phase adaptation. In the gas phase, the monomeric bis(catecholato)silane is tetrahedral, but in the condensed phase, it is metastable toward oligomerization up to a degree controllable by the type of catechol, temperature, and concentration. For the first time, spectroscopic evidence is obtained for a rapid Si-O σ-bond metathesis reaction. Hence, this study sorts out a long-lasting debate and confirms dynamic covalent features for our Earth's crust's most abundant chemical bond.

Interactions between favipiravir and a BNC cage towards drug delivery applications

Electronic structure analysis of bimolecular formation of favipiravir (Fav) and a representative model of boron-nitrogen-carbon (BNC) cage was performed in this work for providing more insightful information regarding the drug delivery purposes by the importance of Fav drug for medication of COVID-19. To achieve the purpose of this work, density functional theory (DFT) calculations were carried out to obtain the stabilized structures and corresponding molecular and atomic scale descriptors. Six models of BNC-Fav complexes were obtained reading the participation of different atomic positions of Fav to interactions with the BNC cage surface. The results yielded BNC-Fav2 at the highest strength and BNC-Fav4 at the lowest strength of bimolecular formations. Molecular orbital–related features and atomic scale quadrupole coping constants all revealed that BNC-Fav2 complex could be proposed for employing in drug delivery process by managing the loaded Fav contribution to future interactions.