Recent developments in the chemistry of stable silylenes

Elusive 2H-1,2-oxasiletes through reactions of an isolable dialkylsilylene with diazocarbonyl compounds

The reactions of isolable dialkysilylene 1 with compounds 5 and 6 gave elusive silacycles, 2H-1,2-oxasiletes 2 and 3, respectively, with releasing dinitrogen. In contrast, six-membered cyclic diazo compound 8 was obtained in a good yield by the reaction of 1 with less reactive 7.

Reactivity of an NHC-stabilized silylene towards ketones. Formation of silicon bis-enolates vs. bis-silylation of the CO bond

The reaction of NHC-stabilized silylaminosilylene 1 with enolizable ketones rapidly led to the regio- and stereoselective formation of silicon bis-enolates in excellent yields under mild conditions.

Self-assembly of N-heterocyclic derivatives of divalent germanium, tin, and lead

A new class of exceptionally stable asymmetric N-heterocyclic germylenes, stannylenes, and plumbylenes has been successfully isolated and characterized by single-crystal X-ray diffraction analysis and multinuclear NMR spectroscopy. Their stability results from tetrameric supramolecular aggregation through strong intermolecular Npy →E(II) (E=Ge, Sn, Pb) interactions involving the nitrogen atom of a neighboring pyridine moiety. The electronic structures and stabilities of the prepared divalent derivatives of Ge, Sn, and Pb in monomeric and aggregated forms are discussed based on theoretical investigations.

A theoretical investigation of photochemical reactions of an isolable silylene with benzene

The mechanisms of photochemical insertion reactions are investigated theoretically using the model system, an isolable dialkylsilylene and benzene, using the CAS(10,10)/6-31G(d) and MP2-CAS-(10,10)/6-311++G(df,pd)//CAS(10,10)/6-31G(d) methods. The structures of the conical intersections, which play a key role in such photoinsertion reactions, are determined to provide a qualitative explanation of the reaction pathways. The model investigation demonstrates that the preferred reaction route for the isolable dialkylsilylene with benzene is as follows: reactants → Franck-Condon region → conical intersection → seven-membered-ring photoproduct. The theoretical findings suggest that the singlet excited dialkylsilylene should attack benzene in the perpendicular conformation, and that no silyl radicals should exist during these photoinsertion reactions. The results obtained allow a number of predictions to be made.

Mechanistic investigation of the reactivity of disilene with nitrous oxide: A DFT study

We have reported the mechanistic investigation of the reaction of N2O addition to disilene, trans-[(TMS)2N(η(1)-Me5C5)SiSi(η(1)-Me5C5)N(TMS)2] (1t), employing density functional theory (BP86/TZVP//BP86/SVP) calculations. The potential energy surfaces of the title reaction are broadly classified under three pathways. Pathway I deals with the direct N2O additions to 1t affording the trans-dioxadisiletane ring compound Pt whereas in the same pathway we report a different bifurcation route from intermediate 2t. This route portrays the isomerization of trans-monooxadisiletane species 2t prior to the second N2O addition, finally leading to the cis-isomeric product Pc. Different possibilities for isomerization of disilene 1t to 1c were studied in pathway II. The cis-disilene (1c) formed can subsequently react with two N2O molecules affording the cis-product Pc. Pathway III details the formation of silanone type intermediate 6, which subsequently combine with another silanone to afford loosely bound intermediates 7 and 8 respectively. The two separated silanone fragments in the isomeric intermediates 7 and 8 can then dimerizes to furnish the desired products. Among all the calculated potential energy surfaces, pathway III remains the most preferred route for disilene oxygenation under normal experimental condition. The present investigation about disilene reactivity will provide a deeper understanding on silylene chemistry and will exhibit promising applicability in main group chemistry as a whole.

Chemical tricks to stabilize silanones and their heavier homologues with E=O bonds (E=Si-Pb): from elusive species to isolable building blocks

In contrast to the well-established chemistry of ketones (R2C=O), the reactivity of the elusive heavier congeners R2E=O (E=Si, Ge, Sn, Pb) is far less explored because of the high polarity of the E=O bonds and hence their tendency to oligomerize with no activation barrier. Very recently, great advances have been achieved in the synthesis of isolable compounds with E=O bonds, including the investigation of donor-stabilized isolable silanones and the first stable "genuine" germanone. These compounds show drastically different reactivities compared to ketones and represent versatile building blocks in silicon-oxygen and germanium-oxygen chemistry. This and other exciting achievements are described in this Minireview.

Mechanisms for the reactions of group 10 transition metal complexes with metal-group 14 element bonds, Bbt(Br)E═M(PCy3)2 (E = C, Si, Ge, Sn, Pb; M = Pd and Pt)

The electronic structures of the Bbt(Br)E═M(PCy(3))(2) (E = C, Si, Ge, Sn, Pb and M = Pt, Pd) complexes and their potential energy surfaces for the formation and water addition reactions were studied using density functional theory (B3LYP/LANL2DZ). The theoretical evidence suggests that the bonding character of the E═M double bond between the six valence-electron Bbt(Br)E: species and the 14 valence-electron (PCy(3))(2)M complexes has a predominantly high s-character. That is, on the basis of the NBO, this theoretical study indicates that the σ-donation from the E element to the M atom prevails. Also, theoretical computations suggest that the relative reactivity decreases in the order: Bbt(Br)C═M(PCy(3))(2) > Bbt(Br)Si═M(PCy(3))(2) > Bbt(Br)Ge═M(PCy(3))(2) > Bbt(Br)Sn═M(PCy(3))(2) > Bbt(Br)Pb═M(PCy(3))(2), irrespective of whether M = Pt or M = Pd is chosen. Namely, the greater the atomic weight of the group 14 atom (E), the larger is the atomic radius of E and the more stable is its Bbt(Br)E═M(PCy(3))(2) doubly bonded species toward chemical reactions. The computational results show good agreement with the available experimental observations. The theoretical results obtained in this work allow a number of predictions to be made.

Theoretical investigation of the mechanisms for the reaction of fused tricyclic dimetallenes containing highly strained E═E (E = C, Si, Ge, Sn, and Pb) double bonds

The potential energy surfaces for the reactions of fused tricyclic dimetallenes that feature a highly strained E═E double bond, Rea-E═E, where E = C, Si, Ge, Sn, and Pb, were studied using density functional theory (B3LYP/LANL2DZ). Three types of chemical reactions (i.e., a self-isomerization reaction, a [2 + 2] cycloaddition with a ketone and a methanol 1,2-addition reaction) were used to determine the reactivity of the Rea-E═E molecules. The theoretical findings reveal that the smaller the singlet-triplet splitting of the Rea-E═E, the lower are its activation barriers and, in turn, the more rapid are its chemical reactions with other chemical molecules. Theoretical observations suggest that the relative reactivity increases in the following order: C═C ≪ Si═Si < Ge═Ge < Sn═Sn < Pb═Pb. Namely, the smaller the atomic weight of the group 14 atom (E), the smaller is the atomic radius of E and the more stable is its fused tricyclic Rea-E═E to chemical reaction. It is thus predicted that the fused tricyclic Rea-C═C and Rea-Si═Si molecules should be stable and readily synthesized and isolated at room temperature. The computational results show good agreement with the available experimental observations. The theoretical results obtained from this work allow a number of predictions to be made.