Selective One-Pot Syntheses of Mixed Silicon-Germanium Heteroadamantane Clusters

Synthesis and functionalization of the six-vertex anionic amido-substituted silicon cluster [Si6{N(SiMe3)Ph}5]. Dalton Trans 2023;52:14949-14955. [PMID: 37800884 DOI: 10.1039/d2dt03952d] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

The reaction of the six-vertex amido-substituted silicon cluster Si6{N(SiMe3)Ph}6 1 with two equiv. of KC8 results in the abstraction of K{N(SiMe3)Ph} and leads to the contact ion pair 2 including an anionic silicon cluster with two unsubstituted pyramidal vertices. Facile functionalization of 2 was achieved with MeI, SiCl4 and SiBr4 and results in neutral two-fold functionalized silicon clusters.

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.

An Improved Synthesis of PN-adamantanoid Cages P4 (NR)6 and a Mechanistic Study of their Fourfold Oxidation

Phosphorus-nitrogen (PN) adamantanoid cages are valuable precursors for materials chemistry, but their syntheses are based on harsh methods that sometimes require access to restricted reagents. We report a new and scalable synthesis of PN adamantanoid compounds by chlorosilane elimination between bis-silylated amines and phosphorus trichloride. We further study the mechanism of the recently-reported four-fold oxidation of such cages with Me3 SiN3 to yield tetravalent tetrahedral connectors for materials chemistry. Reaction monitoring and kinetic modelling revealed the key rate-limiting step, but attempts to accelerate this using Lewis acid additives were unsuccessful. Nevertheless, a new four-fold oxidized PN-adamantanoid cage has been prepared and structurally characterized.

Selective synthesis of germasila-adamantanes through germanium-silicon shift processes

The regioselective synthesis of germasila-adamantanes with the germanium atoms in the bridgehead positions is described starting from cyclic precursors by a cationic sila-Wagner-Meerwein (SWM) rearrangement reaction. The SWM rearrangement allows also a deliberate shift of germanium atoms from the periphery and within the cage structures into the bridgehead positions. This opens the possibility for a synthesis of germasila-adamantanes of defined germanium content and controlled regiochemistry. In the same way that sila-adamantane can be regarded as a molecular building block of elemental silicon, the germasila-adamantane molecules represent cutouts of silicon/germanium alloys.

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.

Group IV heteroadamantanes: synthesis of Si6Sn4 and site-selective derivatization of Si6Ge4

The mixed heteroadamantanes Si₆Ge₄ and Si₆Sn₄ are readily accessible from Me₂ECl₂/Si₂Cl₆/cat. Cl^- (4 × EMe₂, 2 × SiCl₂, 4 × Si-SiCl₃ vertices; E = Ge, Sn). Different from Si₆Ge₄, two skeletal isomers are formed in the case of Si₆Sn₄. Site-selective SiCl₃-methylation of Si₆Ge₄ was achieved, leaving the SiCl₂ groups untouched.

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.

Mixed-Substituted Single-Source Precursors for Si1-xGex Thin Film Deposition

A series of new mixed-substituted heteronuclear precursors with preformed Si-Ge bonds has been synthesized via a two-step synthesis protocol. The molecular sources combine convenient handling with sufficient thermal lability to provide access to group IV alloys with low carbon content. Differences in the molecule-material conversion by chemical vapor deposition (CVD) techniques are described and traced back to the molecular design. This study illustrates the possibility of tailoring the physical and chemical properties of single-source precursors for their application in the CVD of Si_1-xGe_x coatings. Moreover, partial crystallization of the Si_1-xGe_x has been achieved by Ga metal-supported CVD growth, which demonstrated the potential of the presented precursor class for the synthesis of crystalline group IV alloys.

Unsaturated Amido-Substituted Six-Vertex Mixed Silicon Germanium Clusters

The synthesis of mixed silicon and germanium clusters SixGe6-x{N(SiMe3)Dipp}4 1-3 (x = 3.7, 3.1, 2.1) with amido-substituents and two unsubstituted germanium atoms was achieved in co-reductions using the tribromosilane {N(SiMe3)Dipp}SiBr3...

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.