The quest for triplet ground state silylenes

Experimental and theoretical study of the Sn-O bond formation between atomic tin and molecular oxygen

The merging of the electronic structure calculations and crossed beam experiments expose the reaction dynamics in the tin (Sn, 3Pj) - molecular oxygen (O2, X3Σ-g) system yielding tin monoxide (SnO, X1Σ+) along with ground state atomic oxygen O(3P). The reaction can be initiated on the triplet and singlet surfaces via addition of tin to the oxygen atom leading to linear, bent, and/or triangular reaction intermediates. On both the triplet and singlet surfaces, formation of the tin dioxide structure is required prior to unimolecular decomposition to SnO(X1Σ+) and O(3P). Intersystem crossing (ISC) plays a critical role in the reaction mechanism and extensively interosculates singlet and triplet surfaces. The studied reaction follows a mechanism parallel to that for the gas phase reaction of germanium and silicon with molecular oxygen, however, the presence of the tin atom enhances and expands ISC via the "heavy atom effect".

Recent advances in the chemistry of isolable carbene analogues with group 13-15 elements

Carbenes (R2C:), compounds with a divalent carbon atom containing only six valence shell electrons, have evolved into a broader class with the replacement of the carbene carbon or the RC moiety with main group elements, leading to the creation of main group carbene analogues. These analogues, mirroring the electronic structure of carbenes (a lone pair of electrons and an empty orbital), demonstrate unique reactivity. Over the last three decades, this area has seen substantial advancements, paralleling the innovations in carbene chemistry. Recent studies have revealed a spectrum of unique carbene analogues, such as monocoordinate aluminylenes, nitrenes, and bismuthinidenes, notable for their extraordinary properties and diverse reactivity, offering promising applications in small molecule activation. This review delves into the isolable main group carbene analogues that are in the forefront from 2010 and beyond, spanning elements from group 13 (B, Al, Ga, In, and Tl), group 14 (Si, Ge, Sn, and Pb) and group 15 (N, P, As, Sb, and Bi). Specifically, this review focuses on the potential amphiphilic species that possess both lone pairs of electrons and vacant orbitals. We detail their comprehensive synthesis and stabilization strategies, outlining the reactivity arising from their distinct structural characteristics.

Gas-Phase Preparation of Subvalent Germanium Monoxide (GeO, X1+) via Non-Adiabatic Reaction Dynamics in the Exit Channel

The subvalent germanium monoxide (GeO, X¹Σ⁺) molecule has been prepared via the elementary reaction of atomic germanium (Ge, ³P_j) and molecular oxygen (O₂, X³Σ_g^-) with each reactant in its electronic ground state by means of single-collision conditions. The merging of electronic structure calculations with crossed beam experiments suggests that the formation of germanium monoxide (GeO, X¹Σ⁺) commences on the singlet surface through unimolecular decomposition of a linear singlet collision complex (GeOO, i1, C_∞v, ¹Σ⁺) via intersystem crossing (ISC) yielding nearly exclusively germanium monoxide (GeO, X¹Σ⁺) along with atomic oxygen in its electronic ground state [p1, O(³P)]. These results provide a sophisticated reaction mechanism of the germanium-oxygen system and demonstrate the efficient "heavy atom effect" of germanium in ISC yielding (nearly) exclusive singlet germanium monoxide and triplet atomic oxygen compared to similar systems (carbon dioxide and dinitrogen monoxide), in which non-adiabatic reaction dynamics represent only minor channels.

N-Heterocyclic silylenes as ambiphilic activators and ligands

Recent developments of the use of N-heterocyclic silylenes (NHSis), higher homologues of Arduengo-carbenes, as ambiphilic activators and ligands are highlighted and a comparison of NHSi ligands with NHC and phosphine ligands is provided.

Activation of small molecules by cyclic alkyl amino silylenes (CAASis) and germylenes (CAAGes): a theoretical study

Quantum chemical calculations have been carried out on a series of skeletally modified cyclic alkyl amino silylenes (CAASis) and germylenes (CAAGes) to understand their ligand properties and reactivity towards the activation of a variety of small molecules. The installation of boron or silicon atoms into the ring framework of these silylenes/germylenes led to a dramatic increase in their σ-basicity while the incorporation of ylidic moieties resulted in a sharp reduction of their π-acidity although it did help in increasing the electron donation ability. The calculated values of energy barriers for the activation of H-H, N-H, C-H and Si-H bonds by many of the cyclic silylenes considered here are found to be comparable to those for experimentally evaluated systems, indicating the potential of these computationally designed molecules in small molecule activation and calling for synthetic efforts towards their isolation. Furthermore, activations employing CAAGes are found to be more demanding than those with CAASis which may be attributed to the significantly lower Lewis basicity of the former than the latter.

A 1,5-Oligosilanylene Dianion as Building Block for Oligosiloxane Containing Cages, Ferrocenophanes, and Cyclic Germylenes and Stannylenes

Starting out from dipotassium 1,5-oligosiloxanylene diide 2, a 3,7,10-trioxa-octasilabicyclo[3.3.3]undecane was prepared, which represents the third known example of this cage structure type. Reaction of 1,3-dichlorotetramethyldisiloxane with 1,1'-bis[bis(trimethylsilyl)potassiosilyl]ferrocene gave a ferrocenophane with a disiloxane containg bridge. The compound can be further derivatized by conversion into a 1,5-oligosilanyl diide. Reacting 1,5-oligosiloxanylene diide 2 with SnCl2 or GeCl2·dioxane in the presence of PMe3 gave cyclic disilylated tetrylene PMe3 adducts. Release of the base-free stannylene led to a dimerization process which gave a bicyclic distannene as the final product. Abstraction of the PMe3 from the cyclic disilylated germylene PMe3 adduct with B(C6F5)3 caused oxidative addition of the germylene into a para-C-F bond of Me3P·B(C6F5)3.

Where silylene–silicon centres matter in the activation of small molecules

Small molecules such as H₂, N₂, CO, NH₃, O₂ are ubiquitous stable species and their activation and role in the formation of value-added products are of fundamental importance in nature and industry.

Novel triplet germylenes in focus: normal vs. abnormal triplet exocyclic tetrazol-5-vinylidene germylenes at DFT

Substituent effects on stability (assumed as the singlet and triplet energy gaps, ΔΕ_S-T) of novel 1,4-disubstituted-tetrazole-5-vinylidene germylenes (normal, 1_R) and their corresponding 1,3-disubstituted-tetrazole-5-vinylidene germylenes (abnormal, 2_R) are computed and compared, at B3LYP/6-311++G** and M06/6-311++G**, where R = H, CN, CF₃, F, SH, C₆H₆, OMe, and OH. Interestingly, every triplet vinylidene germylene shows more stability than its corresponding singlet. Also, every triplet abnormal isomer (2_R) emerges to be more stable than its corresponding normal (1_R). All abnormal 2_R isomers show broader band gaps (ΔE_HOMO-LUMO) and higher nucleophilicity (N), but less electrophilicity (ω) than their corresponding normal 1_R isomers. The NICS (nuclear-independent chemical shift) results indicate that every 1_R (except singlet 2_Ph) emerges more aromatic than its corresponding 2_R. Our Hammet studies indicate that 1_R is more sensitive to the electronic effects of substituents, R, than 2_R. Electron-donating species increase N in both 1_R and 2_R, while electron-withdrawing groups increase stability.

Electron binding capabilities of some silylenes having small singlet-triplet splittings or triplet ground states

Several silyl and alkaline metal substituted silylenes have been investigated using the CAS-ACPF method in conjunction with the aug-cc-pVTZ basis sets. Silylsilylene and disilylsilylene are found to have singlet ground states with DeltaEST(-) values of 0.676 and 0.319 eV, respectively. The adiabatic ground state electron affinities are found to be 1.572 and 2.361 eV for HSiSiH(3) and Si(SiH(3))(2). respectively. Both silylenes possesses a stable 2A1 excited negative ion state, with respective adiabatic EA values of 0.037 and 1.000 eV. In contrast, all silylenes with at least one akaline metal substituent exhibit triplet neutral ground states. The metalated silylenes HSiLi, HSiNa, LiSiSiH(3), NaSiLi, SiLi(2), and SiNa(2) have adiabatic ground state EAs somewhat below 1 eV, but each of these negatively charged system possesses up to three bound excited negative ion states, some of which are dipole-bound states.

Mechanism of Silver-Mediated Di-tert-butylsilylene Transfer from a Silacyclopropane to an Alkene

Kinetic studies of the reactions of cyclohexene silacyclopropane 1 and monosubstituted alkenes in the presence of 5 mol % of (Ph3P)2AgOTf suggested a possible mechanism for silver-mediated di-tert-butylsilylene transfer. The kinetic order in cyclohexene silacyclopropane 1 was determined to be one. Inverse kinetic saturation behavior (rate inhibition) was observed in monosubstituted alkene and cyclohexene concentrations. Saturation kinetic behavior in catalyst concentration was observed. A reactive intermediate, a silylsilver complex, was observed using low temperature 29Si NMR spectroscopy. Competition experiments between substituted styrenes and a deficient amount of 1 correlated well with the Hammett equation and provided a rho value of -0.62 +/- 0.02 using sigmap constants. These data support a mechanism involving reversible silver-promoted di-tert-butylsilylene extrusion from 1 followed by irreversible concerted electrophilic attack of the silylsilver intermediate on the alkene.