Unusual Reactivity of Group 14 Hydrides toward Organic Halides: Synthetic Studies and Application to Functional Materials

Hydrosilane/Organoiodine Coupling-Enabled Studies of Organosilane Physical Properties

ConspectusThis Account summarizes recent developments in metal-mediated coupling reactions between hydrosilanes and aryl iodides in the presence of Pd(P(t-Bu)3)2 and base. Alkylated organosilanes are obtained when Pt(P(t-Bu)3)2 and a base are employed in reactions between hydrosilanes and aliphatic iodides. These transformations show unusual reactivity compared to the typical behavior of hydrosilanes toward organoiodides in the presence of PdCl2 or PtCl2, and they proceed in high yields under mild conditions. In addition, the reaction demonstrates (1) high functional group tolerances, (2) stepwise introduction of substituents onto silicon atoms from secondary silanes, and (3) transformation without cleavage of weak Si-Si σ bonds in the molecules.This transformation can serve as a powerful tool for the synthesis of functional organosilicon compounds. The advantage is the flexibility in the molecular structure due to the large size compared to carbon. These compounds have relatively sparse packing in the crystalline state, unlike π-conjugated molecules, which induce dense π-π interactions. Consequently, they have significantly different physical properties in solution and in the solid state. Among them, aromatic disilanes and oligosilanes are important substance groups, because the conjugated chain within these molecules expands due to σ-π interaction between Si-Si σ bond and π orbital of aromatic ring. σ-π Conjugation is most efficient when the dihedral angle between the aromatic ring and the Si-Si bond is 90°, resulting in the overlap of σ orbital and π orbital. The conformational structure, packing, and physical properties of these compounds can change in tandem in response to external stimuli through a crystal phase transition. The interlocking changes in structure and physical properties are reversible, easily returning to their original state with different external stimulus. This account covers several important aspects, including solid-state emission with high fluorescence intensity, aggregation-induced emission (AIE) in water-THF system, mechanochromic fluorescence, organic light emitting diode (OLED), second harmonic generation (SHG) and thermosalient phenomena.This reaction can synthesize optically active tertiary and quaternary silanes by the enantioselective arylation of secondary silanes with aryl iodides using a palladium catalyst modified with a TADDOL-derived amide phosphoric acid ester as a chiral ligand. These optically active compounds can be used as useful circularly polarized luminescence (CPL) materials due to their strong luminescence intensity (Φ) and luminescence dissymmetry factor (glum) attributed to the chiral silicon atom. The efficient synthesis of sila-pharmaceuticals using this method as a key step is also described.The technique enables the design and synthesis of various silicon-containing bioactive substances and medical chemicals. Through the synthesis of organosilane compounds using this method, it is anticipated that the development of functional organic silanes will accelerate their practical applications in a wide range of fields.

Nickel-Catalyzed Reductive Coupling of Chlorosilanes

Organosilanes play essential roles in many important research areas. The use of readily available chlorosilanes to catalytically access these compounds is synthetically appealing but remains a long-standing challenge. Nickel-catalyzed reductive cross-coupling reaction has recently emerged as a promising protocol to arrive at this goal. This strategy allows the chlorosilanes to be coupled with various carbon electrophiles under mild conditions. These reactions afford organosilanes with improved molecular diversity, structural complexity, and functional group compatibility. This Concept article summarizes the recent advance on nickel-catalyzed reductive C-Si couplings of chlorosilanes.

A Series of D-A-D Structured Disilane-Bridged Triads: Structure and Stimuli-Responsive Luminescence Studies

A series of σ-π extended octamethyltetrasilanes, which have phenothiazine, 9,9-dimethyl-9,10-dihydroacridine, or phenoxazine (1, 2, and 3) groups as donor moieties and thienopyrazine or benzothiadiazole (a and b) groups as acceptor fragments, has been prepared, and their optical properties have been studied as an extension of our work. All six compounds exhibited fluorescence in the solid state with maximum wavelengths centered in the range of 400 and 650 nm upon excitation by a UV lamp. Compound 2b showed apparent dual emission behavior in solution, which depends on solvent polarity, and a reversible photoluminescent change under mechanical and thermal stimuli in the solid state. Quantum chemical calculations suggest the contribution of a quasi-axial conformer of the 9,9-dimethyl-9,10-dihydroacridine moiety in 2b to the dual emission in solution and the mechanofluoroluminescence in the solid state, similarly to 1a. These studies provide new insight into the preparation of disilane-bridged triads capable of responding to multiple stimuli.

Nickel-Catalyzed Reductive C(sp2 )-Si Coupling of Chlorohydrosilanes via Si-Cl Cleavage

We report here a new method for the synthesis of organohydrosilanes from phenols and ketones. This method is established through reductive C-Si coupling of chlorohydrosilanes via unconventional Si-Cl cleavage. The reaction offers access to aryl- and alkenylhydrosilanes with a scope that is complementary to those of the established methods. Electron-rich, electron-poor, and ortho-/meta-/para-substituted (hetero)aryl electrophiles, as well as cyclic and acyclic alkenyl electrophiles, were coupled successfully. Functionalities, including Grignard-sensitive groups (e.g., primary amine, amide, phenol, ketone, ester, and free indole), acid-sensitive groups (e.g., ketal and THP protection), alkyl-Cl, pyridine, furan, thiophene, Ar-Bpin, and Ar-SiMe₃ , were tolerated. Gram-scale reaction, incorporation of -Si(H)R₂ into complex biologically active molecules, and derivatization of formed organohydrosilanes are demonstrated.

Synthesis, Structure, and Photophysical Properties of Yellow-Green and Blue Photoluminescent Dinuclear and Octanuclear Copper(I) Iodide Complexes with a Disilanylene-Bridged Bispyridine Ligand

The synthesis, structural, and photophysical investigations of CuI complexes with a disilanylene-bridged bispyridine ligand 1 are herein presented. Dinuclear (2) and ladder-like (3) octanuclear copper(I) complexes were straightforwardly prepared by exactly controlling the ratio of CuI/ligand 1. Single-crystal X-ray analysis confirmed that dinuclear complex 2 had no apparent π…π stacking whereas octanuclear complex 3 had π…π stacking in the crystal packing. In the solid state, the complexes display yellow-green (λ_em = 519 nm, Φ = 0.60, τ = 11 µs, 2) and blue (λ_em = 478 nm, Φ = 0.04, τ = 2.6 µs, 3) phosphorescence, respectively. The density functional theory calculations validate the differences in their optical properties. The difference in the luminescence efficiency between 2 and 3 is attributed to the presence of π…π stacking and the different luminescence processes.

Luminescent Behavior Elucidation of a Disilane-Bridged D-A-D Triad Composed of Phenothiazine and Thienopyrazine

A σ-π extended aryldisilane, comprising a thienopyrazine group as an acceptor fragment and phenothiazine groups as the donor moiety, has been prepared through the introduction of two Si-Si bridges (compound 1). X-ray diffraction analysis determined the crystal structure of 1, and experimental and theoretical approaches investigated its optical properties. Solvatochromic studies revealed the dual emission of 1 in all solvents tested. Compound 1 also exhibited fluorescence in the solid state upon excitation with a hand-held UV lamp, as well as mechanochromic luminescent properties. The packing mode in the crystal structure, variation of phenothiazine conformation, morphological changes between crystalline and amorphous phases are the major factors showing reversible fluorescence under external stimuli. A theoretical conformer study found that 1 exists in distinct conformational groups differing in Gibbs free energy by less than 3 kcal mol^-1 . The conformer in the crystalline state of 1 can promote the complete separation of the HOMO and LUMO between the phenothiazine donor and the thienopyrazine acceptor, linked by the disilane linker. HOMO-LUMO energy transition in the crystalline state is forbidden due to the lack of frontier orbital overlap. Crystalline state emission showed LUMO → HOMO-1 transition (locally excited (LE) state). In the amorphous state, the partial presence of quasi-axial conformers allows intramolecular charge-transfer type emission via energy transfer from dominant quasi-equatorial conformers. The strategy proposed in this work provides important guidance for developing stimuli-responsive materials with controlled excited states.

Cross-Electrophile C(sp2 )-Si Coupling of Vinyl Chlorosilanes

The cross-electrophile coupling has become a powerful tool for C-C bond formation, but its potential for forging the C-Si bond remains unexplored. Here we report a cross-electrophile Csp² -Si coupling reaction of vinyl/aryl electrophiles with vinyl chlorosilanes. This new protocol offers an approach for facile and precise synthesis of organosilanes with high molecular diversity and complexity from readily available materials. The reaction proceeds under mild and non-basic conditions, demonstrating a high step economy, broad substrate scope, wide functionality tolerance, and easy scalability. The synthetic utility of the method is shown by its efficient accessing of silicon bioisosteres, the design of new BCB-monomers, and studies on the Hiyama cross-coupling of vinylsilane products.

Effects of Substituents on the Blue Luminescence of Disilane-Linked Donor‒Acceptor‒Donor Triads

A series of disilane-linked donor‒acceptor‒donor triads (D‒Si‒Si‒A‒Si‒Si‒D) was synthesized to investigate the effects of substituents on the photophysical properties. The triads were prepared by metal-catalyzed diiodosilylation of aryl iodides using a Pd(P(t-Bu)₃)₂/(i-Pr)₂EtN/toluene system that we previously developed. Optical measurements, X-ray diffraction analysis, and density functional theory calculations revealed relationships between the photophysical properties and molecular structures of these triads in solution and in the solid state. The compounds emitted blue to green fluorescence in CH₂Cl₂ solution and in the solid state. Notably, compound 2 showed fluorescence with an absolute quantum yield of 0.17 in the solid state but showed no fluorescence in CH₂Cl₂. Our findings confirmed that the substituent adjacent to the disilane moiety affects the conformations and emission efficiencies of compounds in solution and in the solid state.

Aggregation-Induced Emission Enhancement from Disilane-Bridged Donor-Acceptor-Donor Luminogens Based on the Triarylamine Functionality

Six novel donor-acceptor-donor organic dyes containing a Si-Si moiety based on triarylamine functionalities as donor units were prepared by Pd-catalyzed arylation of hydrosilanes. Their photophysical, electrochemical, and structural properties were studied in detail. Most of the compounds showed attractive photoluminescence (PL) and electrochemical properties both in solution and in the solid state because of intramolecular charge transfer (ICT), suggesting these compounds could be useful for electroluminescence (EL) applications. The aggregation-induced emission enhancement (AIEE) characteristics of 1 and 3 were examined in mixed water/THF solutions. The fluorescence intensity in THF/water was stronger in the solution with the highest ratio of water because of the suppression of molecular vibration and rotation in the aggregated state. Single-crystal X-ray diffraction of 4 showed that the reduction of intermolecular π-π interaction led to intense emission in the solid state and restricted intramolecular rotation of the donor and acceptor moieties, thereby indicating that the intense emission in the solid state is due to AIEE. An electroluminescence device employing 1 as an emitter exhibited an external quantum efficiency of up to 0.65% with green light emission. The emission comes solely from 1 because the EL spectrum is identical to that of the PL of 1. The observed luminescence was sufficiently bright for application in practical devices. Theoretical calculations and electrochemical measurements were carried out to aid in understanding the optical and electrochemical properties of these molecules.

Effects of Substitution on Solid-State Fluorescence in 9-Aryl-9-methyl-9H-9-silafluorenes

Aromatic groups were incorporated into 9H-9-silafluorene units at the 9-position (mono-9H-silafluorenes) and 9,9'-positions (di-9H-9-silafluorenes). The aryl substituents showed weak conjugation to the 9H-9-silafluorene for 9-aryl substituted ones 1-7 and a 9,9'-phenylene substituted one (compound 8) and they exhibited similar absorption and emission spectra. The 9H-9-silafluorene 10 containing a 5,5'-(2,2'-bithiophenyl) group showed a significantly red-shifted absorption and fluorescence maxima in the solid-state. Single-crystal X-ray diffraction studies found J-type aggregated structures formed by intermolecular CH-π interactions (ca. 2.6-2.7 Å). Density functional theory (DFT), time-dependent DFT (TD-DFT), and configuration interaction single (CIS) calculations were conducted to explain the observed optical properties.