Silyl Anions or Silylenoids?—A DFT Study of Silyllithium Compounds with π-Donating Substituents

Diverse reactions of a fluorostannylenoid towards ethynes

Fluoro(dialkyl)stannylenoid 2 exhibits unique reactivity towards ethynes with acetylenic hydrogen and those with trimethylsilyl groups, though the corresponding free dialkylstannylene 1 is inactive against those ethynes. Stannylenoid 2 reacts smoothly with gaseous ethyne and phenylethyne at room temperature, giving the corresponding diethynylstannanes, di(phenylethynyl)stannane 3 and diethynylstannane 6, respectively, in good yields with the concomitant evolution of H₂. Trimethylsilyl-substituted ethynes such as 1-trimethylsilyl-(2-phenyl)ethyne and 1,2-bis(trimethylsilyl)ethyne react similarly to give 3 and bis(trimethylsilylethynyl)stannane 8, respectively. Rather unexpectedly, the reaction of 2 with (trimethylsilyl)ethyne affords 1,2-bis(ethenylstannyl)ethyne 7 in a good yield. The reactions of 2 with methyl and ethyl propynoates give the same products 4 and 5 as those obtained during the reaction of dialkylstannylene 1 without CsF. Pathways involving the nucleophilic attack of cesium acetylide to an ethyne-complexed stannylene were proposed, while the detailed mechanisms remain unknown. The structure of 7 was studied by single crystal X-ray diffraction analysis.

Reversible Stannylenoid Formation from the Corresponding Stannylene and Cesium Fluoride

A fluorostannylenoid (Cs⁺ [R₂ SnF]^- (9), R₂ =(TMS)₂ CCH₂ CH₂ C(TMS)₂ ) was prepared by reacting a stable dialkylstannylene (R₂ Sn (8), R₂ =(TMS)₂ CCH₂ CH₂ C(TMS)₂ ) with cesium fluoride at room temperature in THF. While 9 is stable in THF and DME, removal of the solvent leads to the regeneration of stannylene 8. No reaction occurred when 8 was treated with CsF in a hydrocarbon solvent. Addition of dibenzo-21-crown-7 ether to the THF solution of stannylenoid 9 followed by usual workup affords the corresponding crystalline stannylenoid crown ether complex, the X-ray structural analysis of which revealed a fluorine-bridged contact ion-pair structure. The reaction of 9 with excess phenylacetylene gives the corresponding di(phenylethynyl)stannane.

A mechanistic investigation on the formation and rearrangement of silaspiropentane: A theoretical study

The formation of silaspiropentane from addition of singlet silacyclopropylidene 1 and silacyclopropylidenoid 8 to ethylene has been investigated separately at the B3LYP, X3LYP, WB97XD, and M05-2X theories using the 6-31+G(d,p) basis set. The silacycloproylidenoid addition follows a stepwise route. In contrast, a concerted mechanism occurs for silacyclopropylidene addition. Moreover, the intramolecular rearrangements of silaspiropentane 9 to methylenesilacyclobutane 11 and 2-silaallene + ethylene 12 have been studied extensively. The required energy barrier for the isomerization of 9 to 10 was determined to be 44.0 kcal mol(-1) at the B3LYP/6-31+G(d,p) level. After formation of 10, the rearrangement to methylenesilacyclobutane 12 is highly exergonic by -15.9 kcal mol(-1), which makes this reaction promising. However, the conversion of 9 to 11 is calculated to be quite endergonic, by 26.5 kcal mol(-1).

Theoretical investigation of the addition reaction of the aluminum chlorosilylenoid H(2)SiAlCl(3) with ethylene

The addition reaction of the aluminum chlorosilylenoid H2SiAlCl3 with ethylene was investigated using the M06-2X and QCISD methods for the first time. The calculated results demonstrate that the addition reaction proceeds via two pathways: path I involves just one transition state, while path II involves two transition states. Path I is more feasible dynamically, as it has a lower barrier height than path II. The effect of the solvent CH2Cl2 was taken into consideration using the PCM model. The results indicated that the addition reaction is less likely to occur in CH2Cl2 solvent than in vacuum. This work has therefore highlighted a new pathway for the synthesis of silicon heterocyclic compounds. Graphical Abstract Relative energies (in kJ·mol(-1)) of the stationary points along the potential energy surfaces of the addition reaction of H2SiAlCl3 with C2H4 (values in parentheses were calculated in CH2Cl2 solvent).

A new exploration of the addition reaction of the silylenoid H2SiLiF with ethylene

The addition reactions of the simplest silylenoid H2SiLiF with ethylene were studied theoretically. The geometries of the stationary points along the potential energy surfaces were optimized using DFT B3LYP method with the 6-311+G(d,p) basis set, and the single point energies were calculated at QCISD/6-311++G(d,p) level of theory. The theoretical calculations demonstrated that the addition reaction of H2SiLiF and C2H4 can occur through two different pathways. One is path A via a three-membered ring transition state, the other is path B, while through a four-membered ring transition state. The calculated energy barriers of path A and path B are 58.90 and 248.08 kJ∙mol(-1), respectively. Therefore, pathway A is more favorable than pathway B. The solvent effect on the addition reactions were investigated using the PCM model, and the calculated results indicated that in the THF solvent, the addition reaction is much easier than that in vacuum. The present work provided a new pathway to synthesize silicon heterocyclic compounds.

New insights into the insertion reactions of germylenoid H2GeLiF with RH (R = F, OH, NH2)

In the present work, a new pathway of the insertion reactions of H2GeLiF with RH (R = F, OH, NH2) was investigated using DFT B3LYP and QCISD methods. The geometries of the stationary points on the potential energy surfaces in the reactions were optimized at the B3LYP/6-311 + G (d, p) level and then the single-point energies were calculated at the QCISD/6-311++G (d, p) level. The calculated results indicated that the initial step of all the reactions is the isomerization of the p-complex structure to a three-membered-ring structure. After isomerization, the insertion reactions of three-membered-ring H2GeLiF with RH (R = F, OH, NH2) take place. The QCISD/6-311++G (d, p)//B3LYP/6-311 + G (d, p) calculated potential energy barriers of the three reactions were 89.77, 137.40, and 167.45 kJ mol(-1), respectively. Under the same situation, the insertion reactions should occur easily in the following order H-F > H-OH > H-NH2. Compared with the direct insertion reactions of H2GeLiF with RH (R = F, OH, NH2), the two-step insertions have lower activation barriers and should be more favorable.

Structure ofH2GeFMgFand its insertion reactions withRH(R=F,OH,NH2)

The structures of the germylenoid H2GeFMgF and its insertion reactions with RH ( R = F , OH , NH2) were studied using the DFT B3LYP and QCISD approaches for the first time. The geometries of all of the stationary points were optimized at the B3LYP/6-311+G (d, p) level of theory. And then the QCISD/6-311++G (d, p) single-point energies were calculated. The solvent effects on the geometries and insertion reactions were also computed using the PCM model. The calculated results suggested that H2GeFMgF had three equilibrium configurations, in which the p-complex structure had the lowest energy and was the most stable structure. The isomerization reactions among the three complexes had been studied. For the insertion reactions of H2GeFMgF with RH ( R = F , OH , NH2), along the potential energy surface, there were one transition state and one intermediate which connected the reactants and the products. For the three insertion reactions the mechanisms are identical. However, under the same conditions the insertion reactions should occur easily in the order of H - F > H - OH > H - NH2. The solvent effect calculations suggested the larger the solvent polarity is, the easier the reaction will be.

A new reaction mode of germanium-silicon bond formation: insertion reactions of H₂GeLiF with SiH₃X (X = F, Cl, Br)

A combined density functional and ab initio quantum chemical study of the insertion reactions of the germylenoid H2GeLiF with SiH3X (X = F, Cl, Br) was carried out. The geometries of all the stationary points of the reactions were optimized using the DFT B3LYP method and then the QCISD method was used to calculate the single-point energies. The theoretical calculations indicated that along the potential energy surface, there were one precursor complex (Q), one transition state (TS), and one intermediate (IM) which connected the reactants and the products. The calculated barrier heights relative to the respective precursors are 102.26 (X = F), 95.28 (X = Cl), and 84.42 (X = Br) kJ mol(-1) for the three different insertion reactions, respectively, indicating the insertion reactions should occur easily according to the following order: SiH3-Br > SiH3-Cl > SiH3-F under the same situation. The solvent effects on the insertion reactions were also calculated and it was found that the larger the dielectric constant, the easier the insertion reactions. The elucidations of the mechanism of these insertion reactions provided a new reaction model of germanium-silicon bond formation.

Li/X phosphinidenoid pentacarbonylmetal complexes: a combined experimental and theoretical study on structures and spectroscopic properties

The synthesis of P-F phosphane metal complexes [(CO)5M{RP(H)F}] 2a-c (R = CH(SiMe3)2; a: M = W; b: M = Mo; c: M = Cr) is described using AgBF4 for a Cl/F exchange in P-Cl precursor complexes [(CO)5M{RP(H)Cl}] 3a-c; thermal reaction of 2H-azaphosphirene metal complexes [(CO)5M{RP(C(Ph)═N}] 1a-c with [Et3NH]X led to complexes 3a-c, 4, and 5 (M = W; a-c: X = Cl; 4: X = Br; 5: X = I). Complexes 2a-c, 3a-c, 4, and 5 were deprotonated using lithium diisopropylamide in the presence of 12-crown-4 thus yielding Li/X phosphinidenoid metal complexes [Li(12-crown-4)(Et2O)n][(CO)5M(RPX)] 6a-c, 7a-c, 8, and 9 (6a-c: M = W, Mo, Cr; X = F; 7a-c: M = W, Mo, Cr; X = Cl; 8: M = W; X = Br; 9: M = W; X = I). This first comprehensive study on the synthesis of the title compounds reveals metal and halogen dependencies of NMR parameters as well as thermal stabilities of 6a, 7a, 8, and 9 in solution (F > Cl > Br > I). DOSY NMR experiments on the Li/F phosphinidenoid metal complexes (6a-c; M = W, Mo, Cr) rule out that the cation and anion fragments are part of a persistent molecular complex or tight ion pair (in solution). The X-ray structure of 6a reveals a salt-like structure of [Li(12-crown-4)Et2O][(CO)5W{P(CH(SiMe3)2)F}] with long P-F and P-W bond distances compared to 2a. Density functional theory (DFT) calculations provide additional insight into structures and energetics of cation-free halophosphanido chromium and tungsten complexes and four contact ion pairs of Li/X phosphinidenoid model complexes [Li(12-crown-4)][(CO)5M{P(R)X}] (A-D) that represent principal coordination modes. The significant increase of the compliance constant of the P-F bond in the anionic complex [(CO)5W{P(Me)F}] (10a) revealed that a formal lone pair at phosphorus weakens the P-F bond. This effect is further enhanced by coordination of lithium and/or the Li(12-crown-4) countercation (to 10a) as in type A-D complexes. DFT calculated phosphorus NMR chemical shifts allow for a consistent interpretation of NMR properties and provide a preliminary explanation for the "abnormal" NMR shift of P-Cl derivatives 7a-c. Furthermore, calculated compliance constants reveal the degree of P-F bond weakening in Li/F phosphinidenoid complexes, and it was found that a more negative phosphorus-fluorine coupling constant is associated with a larger relaxed force constant.

The competition between Si-Si and Si-C cleavage in functionalised oligosilanes: their reactivity with elemental lithium

The reaction of aryl-substituted disilanes with elemental lithium is a common method for the preparation of lithiosilanes and the subsequent synthesis of functionalised oligosilanes, especially of enantiomerically pure compounds. A series of alkyl- and arylsubstituted di- and trisilanes has been investigated with respect to their reactivity against elemental lithium. Thereby, depending on the substituents, silicon-silicon bond cleavage of the central Si-Si unit and/or silicon-carbon bond cleavage to arenes are observed. Quantum chemical studies provide a deeper insight into the ongoing processes. The reactive centre can be estimated by both, bond elongation after electron transfer and frontier orbital interactions (pi-bonding and sigma-antibonding part). Aromatic substituents at the silicon atoms proved to be necessary for the processing of any cleavage reaction in the studied systems by stabilising the radical anion after electron transfer at the corresponding di- or trisilane. Yet, selective cleavage reactions do not depend on the number of arenes.

Synthesis, structure and reactions of triarylgermyl anion with α,δ-ambiphilic character

Benzosilagermacyclobutene 2 bearing two Ar groups on the germanium atom [Ar = o-(fluorodimethylsilyl)phenyl] undergoes Ge–Si bond cleavage with KF in the presence of a cryptand to form Ar₃Ge⁻[K⁺(cryptand)] 3, α,δ-ambiphilic species, which is converted back into 2 in the presence of LiBPh₄·3dme or BF₃·Et₂O.

Synthesis, Molecular Structure, and Reactivity of the Isolable Silylenoid with a Tricoordinate Silicon

The first tricoordinate fluorosilylenoid, (t-Bu2MeSi)2SiFLi.3THF (1), was synthesized, and its X-ray molecular structure was determined. 1 was synthesized in 40% yield by a bromine-lithium exchange reaction in THF of the corresponding fluorobromosilane with t-Bu2MeSiLi. 1 is best described as an R2SiF- anion attracted to a (Li.3THF)+ cation with a small contribution of resonance structure that consists of a silylene fragment and FLi.3THF. 1 reacts as a nucleophile with MeCl, PhH2SiCl, H2O, and MeOH, as an electrophile with MeLi, and as a silylene with Li (or t-BuLi) and Na, yielding alpha-lithium and alpha-sodium silyl radicals, respectively. Either photolysis or thermolysis of 1 yields the corresponding disilene R2Si=SiR2 (R = t-Bu2MeSi), probably via dimerization of R2Si:.