Base-Free Molybdenum and Tungsten Bicyclic Silylene Complexes Stabilized by a Homoaromatic Contribution

Heavy Tetrel Clusters Based on the Bicyclobutane Framework: Bicyclo[1.1.0]butanes, [1.1.1]Propellanes, and Tricyclo[2.1.0.02,5 ]pentanes

In this review, the current state of affairs in the novel field of the group 14 element clusters based on the bicyclo[1.1.0]butane framework, namely, bicyclo[1.1.0]butane, [1.1.1]propellane, and tricyclo[2.1.0.02,5 ]pentane derivatives, are overviewed. Structural similarities and differences between these three classes of highly strained polycyclic compounds are considered from the viewpoint of the critically important nature of their bridgehead bonds. Synthetic strategies towards bicyclo[1.1.0]butanes, [1.1.1]propellanes, and tricyclo[2.1.0.02,5 ]pentanes, as well as their peculiar structural and bonding features, and specific reactivity, are also presented and discussed in this review.

Neutral Homoaromatic Diboradisilacyclobutene: Synthesis, Structure, and Reactivity

A formally neutral homoaromatic diboradisilacyclobutene was prepared and structurally characterized via the reduction of borylaminobromosilane with potassium graphite, which represents the first neutral boron and silicon mixed analogue of the homocyclopropenylium ion. Single-crystal X-ray analyses revealed a folded four-membered B₂Si₂ ring (26.2°) and a remarkably short transannular B-Si distance of 2.306(3) Å. DFT calculations showed that two π-electrons delocalize over the BSi₂ moiety. Both structural feature and DFT calculations show the significant 2π-homoaromatic nature of the diboradisilacyclobutene. Furthermore, diboradisilacyclobutene reacted readily with CuCl or HCl to afford the diboradisilacyclobutanes.

Metallacyclosilanes of Calcium, Yttrium, and Iron

Utilizing a choice of α,ω-oligosilanylene diides, it is possible to synthesize a number of heterocyclosilanes with heteroelements of calcium, yttrium, and iron by metathesis reactions with respective metal halides CaI₂, YCl₃, and FeBr₂. ²⁹Si NMR spectroscopic analysis of the calcacyclosilanes suggests that these compounds retain a strong oligosilanylene dianion character, which is more pronounced than in the analogous magnesacyclosilanes. As the electronegativity of calcium lies between potassium and magnesium, silyl calcium reagents should be considered as building blocks with an attractive reactivity profile. Reaction of a 1,4-oligosilanylene diide with YCl₃ gave the five-membered yttracyclosilane as an ate-complex with two chlorides still attached to the yttrium atom. Reaction of the obtained compound with two equivalents of NaCp led to another five-membered yttracyclosilane ate-complex with an yttracene fragment. When using a dianionic oligosilanylene ligand containing a siloxane unit, the siloxane oxygen acted as an additional coordination site for Ca and Y. When the same ligand was used to prepare a cyclic 1-ferra-4-oxatetrasilacyclohexane, an analogous transannular interaction between the iron and oxygen atoms is missing.

Reactions of some α,ω-oligosilanylene diides with CaI₂, YCl₃, and FeBr₂ allow convenient access to metallocyclosilanes with Ca, Y, and Fe as heteroatoms. The calcium and yttrium compounds resemble previously prepared magnesacyclosilanes, retaining a strong silanide character. The only related compounds to the described ferracyclosilanes are acyclic examples.

Synthesis and Reactivity of a Neutral Homocyclic Silylene

Isolation of the neutral homocyclic silylene 2 is possible via amine ligand abstraction with potassium graphite (KC8 ) and subsequent reaction with SiMe3 Cl from a bicyclic silicon(I) amide J. This reaction proceeds via an anionic homoaromatic silicon ring compound 1 as an intermediate. The twofold-coordinated silicon atom in the homocyclic silylene 2 is stabilized by an allyl-type π-electron delocalization. 2 reacts in an oxidative addition with two equivalents of MeOH and in cycloadditions with ethene, phenylacetylene, diphenylacetylene and with 2,3-dimethyl-1,3-butadiene to afford novel functionalized ring compounds.

Titanium Germylidenes

Two novel 18-electron titanium germylene complexes, Cp₂ Ti(L)=Ge[Si₃ (SiMe^t Bu₂ )₄ ] 3 b (L=Me₃ P) and 3 c (L=XylNC), were synthesized, isolated, and structurally characterized. The length of the titanium-germanium bonds of 2.5387(3) Å and 2.5276(3) Å (in 3 b and 3 c, respectively) well match those expected for the double bond, which was further supported by the DFT study. Based on their structural characteristics, as well as their atomic charges calculations which revealed Ti(δ+)=Ge(δ-) bond polarization, both 3 b and 3 c are classified as the Schrock-type titanium germylidenes, as the germanium analogues of the widely known titanium alkylidenes. By contrast, germylene complexes of the group 6 metals 8 a (M=Mo) and 8 b (M=W) are better described as Fischer-type germylene complexes.

An Air-Stable Heterobimetallic Si2 M2 Tetrahedral Cluster

Air- and moisture-stable heterobimetallic tetrahedral clusters [Cp(CO)2 MSiR]2 (M=Mo or W; R=SitBu3 ) were isolated from the reaction of N-heterocyclic carbene (NHC) stabilized silyl(silylidene) metal complexes Cp(CO)2 M=Si(SitBu3 )NHC with a mild Lewis acid (BPh3 ). Alternatively, treatment of the NHC-stabilized silylidene complex Cp(CO)2 W=Si(SitBu3 )NHC with stronger Lewis acids such as AlCl3 or B(C6 F5 )3 resulted in the reversible coordination of the Lewis acid to one of the carbonyl ligands. Computational investigations revealed that the dimerization of the intermediate metal silylidyne (M≡Si) complex to a tetrahedral cluster instead of a planar four-membered ring is due to steric bulk.

Interconversion between a planar 1,3-dichlorobicyclo[1.1.0]tetrasilane and a (chloro)(chlorosilyl)cyclotrisilene

A 1,3-dichlorotetrasilabicyclo[1.1.0]butane with a π-type bridgehead Si–Si bond undergoes a skeletal isomerisation to a 1-chloro-2-(chlorosilyl)cyclotrisilene.

Facile Access to an NHC-Coordinated Silicon Ring Compound with a Si=N Group and a Two-Coordinate Silicon Atom

Reaction of the bicyclo[1.1.0]tetrasilatetraamide Si₄ {N(SiMe₃ )Dipp}₄ 1 (Dipp=2,6-diisopropylphenyl) with 5 equiv of the N-heterocyclic carbene NHC^Me4 (1,3,4,5-tetramethylimidazol-2-ylidene) affords a bifunctional carbene-coordinated four-membered-ring compound with a Si=N group and a two-coordinate silicon atom Si₄ {N(SiMe₃ )Dipp}₂ (NHC^Me4 )₂ (NDipp) 2. When 2 reacts with 0.25 equiv sulfur (S₈ ), two sulfur atoms add to the divalent silicon atom in plane and perpendicular to the plane of the Si₄ ring, which confirms the silylone character of the two-coordinate silicon atom in 2.

Reductive Cleavage of Carbon Monoxide by a Disilenide

The complete reductive cleavage of the triple bond in carbon monoxide was achieved using a lithium disilenide at room temperature. The C-C-coupled product can be regarded as a silanone dimer with pending alkyne and silirene moieties and incorporates two equivalents of CO per disilenide unit. A formation mechanism via ketenyl intermediates is proposed on the basis of DFT calculations and elucidated experimentally by employing Group 6 metal carbonyls as both stabilizing entity and source of CO in the reaction with disilenide. The isolation of cyclic silylene complexes with weakly donating ketenyl donor groups further supports the mechanistic scenario.

Isolation of a hydrogen-bridged bis(silylene) tungsten complex: a snapshot of a transition state for 1,3-hydrogen migration

A hydrogen-bridged bis(silylene) complex, which can be viewed as a snapshot of a transition state for 1,3-hydrogen migration, was isolated, and its unprecedented WSi2H four-membered-ring structure with a short diagonal Si-Si distance was revealed by X-ray crystallography. NMR studies including determination of the W-Si, Si-Si, and Si-H coupling constants and theoretical calculations suggest that a novel multicenter bond is formed in the WSi2H system, in which the bridging hydrogen takes on a hydridic nature.

Unprecedented silicon(ii)→calcium complexes with N-heterocyclic silylenes

The first N-heterocyclic silylene (NHSi) complexes of any s-block element to date are reported for calcium: [(η⁵-C₅Me₅)₂Ca←:Si(O-C₆H₄-2-^tBu){(N^tBu)₂CPh}] and [(η⁵-C₅Me₅)₂Ca←:Si(N^tBuCH)₂]. The synthesis, structure, bonding analysis by DFT methods and the reactivity towards oxygen containing substrates is discussed.

Five-coordinate silicon(II) compounds with Si-M bonds (M = Cr, Mo, W, Fe): Bis[N,N'-diisopropylbenzamidinato(-)]silicon(II) as a ligand in transition-metal complexes

Reaction of the donor-stabilized silylene 1 with [Cr(CO)6], [Mo(CO)6], [W(CO)6], or [Fe(CO)5] leads to the formation of the transition-metal silylene complexes 2-5, which contain five-coordinate silicon(II) moieties with Si-M bonds (M = Cr, Mo, W, Fe). These compounds were characterized by NMR spectroscopic studies in the solid state and in solution and by crystal structure analyses. These experimental investigations were complemented by computational studies to gain insight into the bonding situation of 2-5. The nature of the Si-M bonds is best described as a single bond.