Molecular Tailoring: Reaction Path Control with Bulky Substituents

Steric groups are often regarded in reactions as chemically irrelevant, inert parts of the molecules, which have no influence on the structure of the forming reactive center of the product but rather on the reaction rate; therefore, they are usually not taken into account in theoretical work. However, in some cases, e.g. in the general reaction scheme of reductive dehalogenation of halosilanes, bulky substituents can cause major structural changes in the product simply by their presence. Our calculations using real substituents suggest that the use of proper substituents can prefer and stabilize only one structure on the potential energy surface (PES), eliminating all other relevant minima, not just increasing activation barriers as chemical intuition dictates. Since the preparation of these compounds are generally unpredictably slow process, the theoretical design may bring fundamental breakthroughs in the field of the synthesis of hitherto unknown reactive compounds. With the help of this concept, one can easily design proper substituents for the synthesis of a specific structure, since the mapping of the reaction routes can be spared and only a few calculations are needed. To illustrate the concept in practice, we suggest substituents, asymmetric R-Ind and terpenyl groups, for the synthesis of hexasilabenzene, which is one of the most desired silicon compounds

Theoretical Assessment of Low-Valent Germanium Compounds as Transition Metal Ligands: Can They Be Better than Phosphines or NHCs?

We analyze the key transition metal ligand properties, σ-donor and π-acceptor ability, ligand-to-metal charge transfer, and steric parameters, using theoretical methods in order to examine 144 synthesized low-valent germanium compounds as potential ligands in homogeneous transition metal catalysis. We compare these features to those of currently widely used ligands (carbenes, phosphines). We find that several low-valent germanium compounds discovered lately, especially Ge(0) and double-donor-stabilized Ge­(II) compounds, can easily outperform phosphine ligands in σ-donor strength or reach the strength of NHCs. We set up a databank in which one can find the most suitable germanium-based ligand to promote different homogeneous catalytic steps. We present the factors behind the favorable features of low-valent germanium compounds, which may help to design new low-valent germanium compounds with enhanced properties in the future

Fluorine Modification of the Surface of Diamondoids: A Time-Dependent Density Functional Study

We systematically study the fluorination of nanometer-sized diamond cages, diamondoids, by time-dependent density functional theory. We find that fluorination affects both the highest occupied and lowest unoccupied molecular orbitals. Partial fluorination may decrease the energy of the excited state, and the lowest unoccupied molecular orbital becomes less exposed to the environment around the fluorinated surface. These new features of fluorinated diamondoids could be very useful in several potential applications of fluorescent nanodiamonds such as nitrogen-vacancy center based sensing at nanoscale

Unique Insertion Mechanisms of Bis-dehydro-β-diketiminato Silylene

The unique insertion reactions of the first, stable six-membered-ring silylene ({HC[CMeN(R)]₂}Si, R = 2,6-diisopropylphenyl) with eight reactants were investigated by the B3LYP/cc-pVTZ method. The initial step (<b>IS</b>) of all the reactions is the formation of an intermediate 1,4-adduct, <b>IM</b>, which will be then the starting point toward the different final states (<b>FS</b>). In this study three different mechanisms were found and studied to the 1,4-adduct and six reaction paths from the 1,4-adduct to the final products. On the basis of the results, the different reaction paths, the experimental insertion products, and the special reactivity of the six-membered-ring silylene have been explained

An Amplified Ylidic “Half-Parent” Iminosilane LSiNH

The reaction of LSiBr­(NH₂) (<b>4</b>) (L = CH­[(CCH₂)­CMe­(NAr)₂]; Ar = 2,6-<i>i</i>Pr₂C₆H₃) with lithium bis­(trimethylsilyl)­amide in the presence of pyridine or 4-dimethylaminopyridine (DMAP) resulted in the activation of the α C–H bond of pyridine or DMAP, affording the products LSi­(dmap)­NH₂ (<b>6</b>) and LSi­(pyridine)­NH₂ (<b>7a</b>), respectively. Remarkably, this metal-free aromatic C–H activation occurs at room temperature. The emerging aminosilanes were isolated and fully characterized. Isotope labeling experiments and detailed DFT calculations, elucidating the reaction mechanism, were performed and provide compelling evidence of the formation of the “half-parent” iminosilane <b>1</b>, LSiNH, which facilitates this transformation due to its amplified ylidic character by the chelate ligand L. Furthermore, the elusive iminosilane <b>1</b> could be trapped by benzophenone and trimethylsilylazide affording the corresponding products, <b>8</b> and <b>9,</b> respectively, thereby confirming its formation as a key intermediate

An NHC-Stabilized Silicon Analogue of Acylium Ion: Synthesis, Structure, Reactivity, and Theoretical Studies

The silicon analogues of an acylium ion, namely, sila-acylium ions <b>2a</b> and <b>2b</b> [RSi­(O)­(NHC)₂]Cl stabilized by two <i>N</i>-heterocyclic carbenes (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene), and having chloride as a countercation were successfully synthesized by the reduction of CO₂ using the donor stabilized silyliumylidene cations <b>1a</b> and <b>1b</b> [RSi­(NHC)₂]­Cl (<b>1a</b>, <b>2a</b>; R = <i>m</i>-Ter = 2,6-Mes₂C₆H₃, Mes = 2,4,6-Me₃C₆H₂ and <b>1b</b>, <b>2b</b>; R <b>=</b> Tipp = 2,4,6-<i>i</i>Pr₃C₆H₂). Structurally, compound <b>2a</b> features a four coordinate silicon center together with a double bond between silicon and oxygen atoms. The reaction of sila-acylium ions <b>2a</b> and <b>2b</b> with water afforded different products which depend on the bulkiness of aryl substituents. Although the exposure of <b>2a</b> to H₂O afforded a stable silicon analogue of carboxylate anion as a dimer form, [<i>m</i>-TerSi­(O)­O]₂^2–·2­[NHC–H]⁺ (<b>3</b>), the same reaction with the less bulkier triisopropylphenyl substituted sila-acylium ion <b>2b</b> afforded cyclotetrasiloxanediol dianion [{TippSi­(O)}₄{(O)­OH}₂]^2–·2­[NHC–H]⁺ (<b>4</b>). Metric and DFT (Density Functional Theory) evidence support that <b>2a</b> and <b>2b</b> possess strong SiO double bond character, while <b>3</b> and <b>4</b> contain more ionic terminal Si–O bonds. Mechanistic details of the formation of different (SiO)_<i>n</i> (<i>n</i> = 2, 3, 4) core rings were explored using DFT to explain the experimentally characterized products and a proposed stable intermediate was identified with mass spectrometry

An NHC-Stabilized Silicon Analogue of Acylium Ion: Synthesis, Structure, Reactivity, and Theoretical Studies

The silicon analogues of an acylium ion, namely, sila-acylium ions <b>2a</b> and <b>2b</b> [RSi­(O)­(NHC)₂]Cl stabilized by two <i>N</i>-heterocyclic carbenes (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene), and having chloride as a countercation were successfully synthesized by the reduction of CO₂ using the donor stabilized silyliumylidene cations <b>1a</b> and <b>1b</b> [RSi­(NHC)₂]­Cl (<b>1a</b>, <b>2a</b>; R = <i>m</i>-Ter = 2,6-Mes₂C₆H₃, Mes = 2,4,6-Me₃C₆H₂ and <b>1b</b>, <b>2b</b>; R <b>=</b> Tipp = 2,4,6-<i>i</i>Pr₃C₆H₂). Structurally, compound <b>2a</b> features a four coordinate silicon center together with a double bond between silicon and oxygen atoms. The reaction of sila-acylium ions <b>2a</b> and <b>2b</b> with water afforded different products which depend on the bulkiness of aryl substituents. Although the exposure of <b>2a</b> to H₂O afforded a stable silicon analogue of carboxylate anion as a dimer form, [<i>m</i>-TerSi­(O)­O]₂^2–·2­[NHC–H]⁺ (<b>3</b>), the same reaction with the less bulkier triisopropylphenyl substituted sila-acylium ion <b>2b</b> afforded cyclotetrasiloxanediol dianion [{TippSi­(O)}₄{(O)­OH}₂]^2–·2­[NHC–H]⁺ (<b>4</b>). Metric and DFT (Density Functional Theory) evidence support that <b>2a</b> and <b>2b</b> possess strong SiO double bond character, while <b>3</b> and <b>4</b> contain more ionic terminal Si–O bonds. Mechanistic details of the formation of different (SiO)_<i>n</i> (<i>n</i> = 2, 3, 4) core rings were explored using DFT to explain the experimentally characterized products and a proposed stable intermediate was identified with mass spectrometry

Synthesis and Unexpected Reactivity of Germyliumylidene Hydride [:GeH]⁺ Stabilized by a Bis(<i>N</i>‑heterocyclic carbene)borate Ligand

Employing the potassium salt of the monoanionic bis­(<i>NHC</i>)­borate <b>1</b> (<i>NHC</i> = <i>N</i>-<i>H</i>eterocyclic <i>C</i>arbene) enables the synthesis and isolation of the bis­(<i>NHC</i>)­borate-stabilized chlorogermyliumylidene precursor <b>2</b> in 61% yield. A Cl/H exchange reaction of <b>2</b> using potassium tri<i>sec</i>.-butylborhydride as a hydride source leads to the isolation of the first germyliumylidene hydride [HGe:⁺] complex <b>3</b> in 91% yield. The Ge­(II)–H bond in the latter compound has an unexpected reactivity as shown by the reaction with the potential hydride scavenger [Ph₃C]⁺[B­(C₆F₅)₄]⁻, furnishing the corresponding HGe: → CPh₃ cation in the ion pair <b>4</b> as initial product. Compound <b>4</b> liberates HCPh₃ in the presence of <b>3</b> to give the unusual dinuclear HGe: → Ge: cation in <b>5</b>. The latter represents the first three-coordinate dicationic Ge­(II) species stabilized by an anionic bis­(<i>NHC</i>) chelate ligand and a Ge­(II) donor. All novel compounds were fully characterized, including X-ray diffraction analyses

An NHC-Stabilized Silicon Analogue of Acylium Ion: Synthesis, Structure, Reactivity, and Theoretical Studies

The silicon analogues of an acylium ion, namely, sila-acylium ions <b>2a</b> and <b>2b</b> [RSi­(O)­(NHC)₂]Cl stabilized by two <i>N</i>-heterocyclic carbenes (NHC = 1,3,4,5-tetramethylimidazol-2-ylidene), and having chloride as a countercation were successfully synthesized by the reduction of CO₂ using the donor stabilized silyliumylidene cations <b>1a</b> and <b>1b</b> [RSi­(NHC)₂]­Cl (<b>1a</b>, <b>2a</b>; R = <i>m</i>-Ter = 2,6-Mes₂C₆H₃, Mes = 2,4,6-Me₃C₆H₂ and <b>1b</b>, <b>2b</b>; R <b>=</b> Tipp = 2,4,6-<i>i</i>Pr₃C₆H₂). Structurally, compound <b>2a</b> features a four coordinate silicon center together with a double bond between silicon and oxygen atoms. The reaction of sila-acylium ions <b>2a</b> and <b>2b</b> with water afforded different products which depend on the bulkiness of aryl substituents. Although the exposure of <b>2a</b> to H₂O afforded a stable silicon analogue of carboxylate anion as a dimer form, [<i>m</i>-TerSi­(O)­O]₂^2–·2­[NHC–H]⁺ (<b>3</b>), the same reaction with the less bulkier triisopropylphenyl substituted sila-acylium ion <b>2b</b> afforded cyclotetrasiloxanediol dianion [{TippSi­(O)}₄{(O)­OH}₂]^2–·2­[NHC–H]⁺ (<b>4</b>). Metric and DFT (Density Functional Theory) evidence support that <b>2a</b> and <b>2b</b> possess strong SiO double bond character, while <b>3</b> and <b>4</b> contain more ionic terminal Si–O bonds. Mechanistic details of the formation of different (SiO)_<i>n</i> (<i>n</i> = 2, 3, 4) core rings were explored using DFT to explain the experimentally characterized products and a proposed stable intermediate was identified with mass spectrometry

From a Zwitterionic Phosphasilene to Base Stabilized Silyliumylidene-Phosphide and Bis(silylene) Complexes

The reactivity of ylide-like phosphasilene <b>1</b> [LSi­(TMS)P­(TMS), L = PhC­(N<i>t</i>Bu)₂] with group 10 d¹⁰ transition metals is reported. For the first time, a reaction of a phosphasilene with a transition metal that actually involves the silicon–phosphorus double bond was found. In the reaction of <b>1</b> with ethylene bis­(triphenylphosphine) platinum(0), a complete silicon–phosphorus bond breakage occurs, yielding the unprecedented dinuclear platinum complex <b>3</b> [LSi­{Pt­(PPh₃)}₂P­(TMS)₂]. Spectroscopic, structural, and theoretical analysis of complex <b>3</b> revealed the cationic silylene (silyliumylidene) character of the silicon unit in complex <b>3</b>. Similarly, formation of the analogous dinuclear palladium complex <b>4</b> [LSi­{Pd­(PPh₃)}₂P­(TMS)₂] from tetrakis­(triphenylphosphine) palladium(0) was observed. On the other hand, in the case of bis­(cyclooctadiene) nickel(0) as starting material, a distinctively different product, the bis­(silylene) nickel complex <b>5</b> [{(LSi)₂P­(TMS)}­Ni­(COD)], was obtained. Complex <b>5</b> was fully characterized including X-ray diffraction analysis. Density functional theory calculations of the reaction mechanisms showed that the migration of the TMS group in the case of platinum and palladium was induced by the oxidative addition of the transition metal into the silicon–silicon bond. The respective platinum intermediate <b>2</b> [LSi­{Pt­(TMS)­(PPh₃)}­P­(TMS)] was also experimentally observed. This is contrasted by the reaction of nickel, in which the equilibrium of phosphasilene <b>1</b> and the phosphinosilylene <b>6</b> [LSiP­(TMS)₂] was utilized for a better coordination of the silicon­(II) moiety in comparison with phosphorus to the transition metal center