A β-Diketiminate-Stabilized Sila-Acyl Chloride: Systematic Access to Base-Stabilized Silicon Analogues of Classical Carbonyl Compounds

Alkyl backbone variations in common β-diketiminate ligands and applications to N-heterocyclic silylene chemistry

We report the extension of the common β-diketimine proligand class, RArnacnacH (HC(RCNAr)2H), where R is an alkyl group such as Et or iPr, plus Ph, and Ar is a sterically demanding aryl substituent such as Dip = 2,6-diispropylphenyl, Dep = 2,6-diethylphenyl, Mes = 2,4,6-trimethylphenyl or mesityl, Xyl = 2,6-dimethylphenyl, via one-pot condensation procedures. When a condensation reaction is carried out using the chemical dehydrating agent PPSE (polyphosphoric acid trimethylsilylester), β-diketiminate phosphorus(V) products such as (iPrMesnacnac)PO2 can also be obtained, which can be converted to the respective proligand iPrMesnacnacH via alkaline hydrolysis. The RArnacnacH proligands can be converted to their alkali metal complexes with common methods and we have found that deprotonation of iPrDipnacnacH is significantly more sluggish than that of related β-diketimines with smaller backbone alkyl groups. The basicity of the RArnacnac- anions can play a role in the success of their salt metathesis chemistry and we have prepared and structurally characterised the EtDipnacnac-derived silicon(II) compounds (EtDipnacnac)SiBr and (EtDipnacnac')Si, where EtDipnacnac' is the deprotonated variant MeCHC(NDip)CHC(NDip)Et.

Reactions of (mesityl)n(methyl)2-nsilylene complexes with pyridine-N-oxide (n = 1 and 0): formation of silanone complexes and a disiloxanyloxy complex

Silanones (OSiR2), a heavier congener of ketones (R2CO), are highly reactive species that are readily converted to oligomeric siloxane (O-SiR2)n. Coordination of silanones to the transition-metal fragments to afford silanone-coordinated complexes is a reliable silanone stabilization method. Recently, our group reported the synthesis, structures, and reactivity of dimesityl-substituted silanone complexes Cp*(OC)2M{OSiMes2(L)}(SiMe3) (M = W, Mo, L: Lewis base, Cp*: η5-C5Me5, Mes: 2,4,6-Me3C6H2). Herein, to investigate the effect of substituents on the silicon atom during the formation of a silanone complex, we demonstrated the use of Mes and smaller Me groups. As a result, the formation of Mes(Me)-substituted silanone molybdenum complex Cp*(OC)2Mo{OSiMes(Me)(py)}(SiMe3) (5b, py: pyridine) was suggested, the silanone tungsten complex Cp*(OC)2W{OSiMes(Me)(DMAP)}(SiMe3) (4a, DMAP: 4-(dimethylamino)pyridine) was obtained, and a dimethyl-substituted disiloxanyloxy(dioxo) complex Cp*(O)2W(OSiMe2OSiMe3) (9) was formed. The reaction of 4a with PMe3 proceeded via the elimination of DMAP and migration of the SiMe3 group to the oxygen atom of the silanone ligand to afford Cp*(OC)2W(SiMes(Me)OSiMe3)(PMe3) (11a). The Mo complex Cp*(OC)2Mo(SiMes(Me)OSiMe3)(PMe3) (11b) was produced by the reaction of Cp*(OC)2Mo{SiMes(Me)}(SiMe3) (7b) with pyridine-N-oxide in the presence of PMe3.

Stabilization and One Electron Reduction of a Silicon Analogue of a Carboxylic Acid Anhydride

Reaction of the 1,2-disilylenes {(DipAr Am)Si}2 (DipAr Am=[(NDip)2 CAr]- , Dip=2,6-diisopropylphenyl, Ar=4-C6 H4 But (Ar') 1 a or Ph 1 b) and two abnormal N-heterocyclic silylenes, (DipAr Am)SiOCSi{(NDip)2 CAr} (Ar=Ar' 3 a or Ph 3 b) with N2 O led to formation of unprecedented examples of uncoordinated silicon analogues of carboxylic acid anhydrides, (DipAr Am)(O=)SiOSi(=O)(DipAr Am) (Ar=Ar' 2 a or Ph 2 b). Both compounds have been fully characterized, and the mechanism of formation of one explored using DFT calculations. Reduction of sila-acid anhydride 2 a with a dimagnesium(I) compound, [{(Mes Nacnac)Mg}2 ] (Mes Nacnac=[(MesNCMe)2 CH]- , Mes=mesityl), led to the one-electron reduction of the anhydride and formation of a magnesium complex of a sila-acid anhydride radical anion [(Mes Nacnac)Mg{(OSi(DipAr' Am)}2 O] 5. A combination of EPR spectroscopic studies and DFT calculations reveal the unpaired electron to largely reside on one of the amidinate ligands of the complex.

Recent advances in the chemistry of heavier group 14 analogues of carbonyls

Heavier analogues of carbonyls, in the form of "R2E=O" (E = Si, Ge, Sn, Pb), feature a high polar E=O double bond. In contrast to carbonyl compounds, heavier analogues are extremely unstable and prone to proceed head-to-tail oligomerization. Thus, the isolation of such species under ambient conditions is a challenging synthetic target in main group chemistry. In recent years, much progress has been achieved in the synthesis and isolation of a variety of Lewis base/acid, Lewis base-stabilized and even Lewis acid/base free heavier analogues. These compounds exhibit interesting reactivities, such as small molecule activation and metathesis reactions, indicating the potential of heavier analogues in synthetic chemistry. This review summarizes the recent achievements in the chemistry of Lewis base and/or acid stabilized heavier analogues of carbonyls, including synthetic approaches, structural parameters and reactivity of these isolable compounds.

Halogen-Exchange Reactions of Iminophosphonamido-Chlorosilylenes with Alkali Halides: Convenient Synthesis of Heavier Halosilylenes

Halogen-substituted silylenes are an important building block for synthesizing silicon-based low-valent and multiple-bond species. However, the number of reports on heavier halosilylenes that contain bromine and iodine is still limited. Here, we present a convenient synthesis for bromo- and iodosilylenes supported by an iminophosphonamide ligand. The heavier halosilylenes [Ph₂P(^tBuN)₂]SiX (2: X = Br, 3: X = I) were successfully synthesized via the halogen-exchange reaction of chlorosilylene 1 with alkali halides in THF. As a demonstration of the reactivity of 2 and 3, oxidative addition reactions of 2 and 3 with elemental selenium in C₆D₆ afforded the corresponding bromo- (5) or iodosilylene-selone (6) as colorless crystals. The molecular structures of 2, 3, 5, and 6 were fully characterized by spectroscopic means and single-crystal X-ray diffraction analysis. Furthermore, the effects of the halogen atom on the electronic state of halosilylenes 1-3 and halosilylene-selones 4-6 were investigated using density functional theory (DFT) calculations.

Synthesis, Structure, and Reactivity of Acid-Free Neutral Oxoborane

An efficient synthesis of an acid-free neutral oxoborane of the type carboranyl-B(carbene)=O has been developed via a serendipitous discovery from the reaction of 1,2-[BBr(carbene)]-o-carborane with AgOTf. This represents a new type of oxoborane. The stabilization of this oxoborane may be attributed to 1) kinetic stabilization provided by a bulky 3D carboranyl ligand and 2) thermodynamic stabilization offered by a carbene ligand. Crystallographic analyses support the presence of the shortest terminal B=O double bond ever reported thus far. Its reactivity has also been examined.

Formation of silaimines from a sterically demanding iminophosphonamido chlorosilylene via intramolecular N-P bond cleavage

The sterically demanding iminophosphonamido chlorosilylene [Ph2P(DipN)2]SiCl (Dip = 2,6-diisopropylphenyl) was synthesized and fully characterized using NMR spectroscopy and X-ray crystallography. Substitution reactions of [Ph2P(DipN)2]SiCl with N- and Fe-nucleophiles led to the unexpected formation of the corresponding silaimine derivatives. This process involves the ring-opening rearrangement of three-coordinated silylene intermediates that proceeds via intramolecular N-P bond cleavage.

Distinctly different reactivity of bis(silylenyl)-versus phosphanyl-silylenyl-substituted o-dicarborane towards O2, N2O and CO2

In stark contrast to the reactivity of the bis-silylenyl dicarborane CB-Si2 (1) [CB = ortho-C,C'-C2B10H10, Si = PhC(tBuN)2Si] towards O2, N2O, and CO2, yielding the same dioxygenation product CB-Si2O2 (2) with a four-membered 1,3,2,4-disiladioxetane ring, the activation of the latter small molecules with the phosphanyl-silylenyl-functionalised CB-SiP (3) {P[double bond, length as m-dash]P[N(tBu)CH2]2} affords with O2 the CB-Si([double bond, length as m-dash]O)P([double bond, length as m-dash]O) silanone-phosphine oxide (4), with N2O the CB-Si([double bond, length as m-dash]O)P silanone-phosphine (5), and with CO2 the CB-Si(O2C[double bond, length as m-dash]O)P silicon carbonate-phosphine (6) and CB-C([double bond, length as m-dash]O)OSiOP ester (7), respectively.

Acid-Base Free Main Group Carbonyl Analogues

Main group carbonyl analogues (R2 E=O) derived from p-block elements (E=groups 13 to 15) have long been considered as elusive species. Previously, employment of chemical tricks such as acid- and base-stabilization protocols granted access to these transient species in their masked forms. However, electronic and steric effects inevitably perturb their chemical reactivity and distinguish them from classical carbonyl compounds. A new era was marked by the recent isolation of acid-base free main group carbonyl analogues, ranging from a lighter boracarbonyl to the heavier silacarbonyls, phosphacarbonyls and a germacarbonyl. Most importantly, their unperturbed nature elicits exciting new chemistry, spanning the vista from classical organic carbonyl-type reactions to transition metal-like oxide ion transfer chemistry. In this Review, we survey the strategies used for the isolation of such systems and document their emerging reactivity profiles, with a view to providing fundamental comparisons both with carbon and transition metal oxo species. This highlights the emerging opportunities for exciting "crossover" reactivity offered by these derivatives of the p-block elements.

Main Group Multiple Bonds for Bond Activations and Catalysis

Since the discovery that the so-called "double-bond" rule could be broken, the field of molecular main group multiple bonds has expanded rapidly. With the majority of homodiatomic double and triple bonds realised within the p-block, along with many heterodiatomic combinations, this Minireview examines the reactivity of these compounds with a particular emphasis on small molecule activation. Furthermore, whilst their ability to act as transition metal mimics has been explored, their catalytic behaviour is somewhat limited. This Minireview aims to highlight the potential of these complexes towards catalytic application and their role as synthons in further functionalisations making them a versatile tool for the modern synthetic chemist.

Reactivity of 1,3-dichloro-1,3-bis(dimethylamino)-propenium salts with primary amines

1,3-Dichloro-1,3-bis(dimethylamino)propenium salts react with primary amines to give different products depending on properties of the amine used, including the size of the R groups, whether it is aromatic, and if there is a hydrogen on the α-carbon.

Cleavage of C–O and C–H bonds in ethers by a genuine SiO bond

A dialkylsilanone with a genuine SiO bond undergoes the cleavage of an ethereal C–O bond which is initiated by the coordination of the ethereal O atom to the electrophilic Si atom in the SiO bond.

A Neutral Silicon(II) Half-Sandwich Compound

Metathesis reaction of a dilithio borole dianion, a cyclic π-ligand isoelectronic to ubiquitous cyclopentadienyls, with two equivalents of "silicocenium" cation [Cp*Si]⁺ as a source of low-valent Si(II), cleanly gives a borole half-sandwich π-complex of Si(II) and silicocene. The resulting half-sandwich complex is a neutral isoelectronic analogue to the iconic silicocenium cation and features the rare structural motif of an apical silicon(II) atom with an energetically high lying lone pair of electrons that is shown to be accessible for coordination chemistry toward tungsten carbonyl. Protonation at the Si(II) atom with [H(OEt₂)₂][Al{OC(CF₃)₃}₄] induces formal oxidation, and the compound rearranges to incorporate the Si atom into the carbocyclic base to give an unprecedented cationic 5-sila-6-borabicyclo[2.1.1]hex-2-ene. This rearrangement is accompanied by drastic changes in the ¹¹B and ²⁹Si NMR chemical shifts.

Probing the non-innocent nature of an amino-functionalised β-diketiminate ligand in silylene/iminosilane systems

Electron-rich β-diketiminate ligands, featuring amino groups at the backbone β positions ("N-nacnac" ligands) have been employed in the synthesis of a range of silylene (Si^II) complexes of the type (N-nacnac)SiX (where X = H, Cl, N(SiMe₃)₂, P(SiMe₃)₂ and Si(SiMe₃)₃). A combination of experimental and quantum chemical approaches reveals (i) that in all cases rearrangement to give an aza-butadienyl Si^IV imide featuring a contracted five-membered heterocycle is thermodynamically favourable (and experimentally viable); (ii) that the kinetic lability of systems of the type (N-nacnac)SiX varies markedly as a function of X, such that compounds of this type can be isolated under ambient conditions for X = Cl and P(SiMe₃)₂, but not for X = H, N(SiMe₃)₂ and Si(SiMe₃)₃; and (iii) that the ring contraction process is most facile for systems bearing strongly electron-donating and sterically less encumbered X groups, since these allow most ready access to a transition state accessed via intramolecular nucleophilic attack by the Si^II centre at the β-carbon position of the N-nacnac ligand backbone.

Oxidation reactions of a versatile, two-coordinate, acyclic iminosiloxysilylene

Due to their outstanding reactivity, acyclic silylenes have emerged as attractive organosilicon alternatives for transition metal complexes on the way to metal-free catalysis. However, exploration of their reactivity is still in its infancy, as only a few derivatives of this unique compound class have been isolated so far. Here, we present the results of an extensive reactivity investigation of the previously reported acyclic iminosiloxysilylene 1. Divalent silylene 1 proved to be a versatile building block for a plethora of novel organosilicon compounds. Thus, not only the activation of the rather challenging targets NH₃ and P₄ could be achieved, but also the conversion into a reactive donor-free silaimine, which itself turned out to be a useful reagent for small molecule activation. In addition, 1 served as an excellent precursor for gaining access to donor-stabilized heavier carbonyl compounds. Our results thus provide further insights into the chemistry of low-valent silicon at the interface between carbon and transition metals.

An Isolable Bis(Silanone-Borane) Adduct

The reaction of bis(silylenyl)-substituted ferrocene 1 with two molar equivalents of BPh3 yields the corresponding bis(silylene-borane) Lewis adduct 2. The latter is capable to activate CO2 to furnish the borane-stabilized bis(silanone) 3 through mono-oxygenation of the dative SiII →B silicon centers under release of CO. Removal of BPh3 from 3 with PMe3 affords the corresponding 1,3,2,4-cyclodisiloxane and the Me3 P-BPh3 adduct. All isolated new compounds were characterized and their molecular structures were determined by single-crystal X-ray diffraction analyses.

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.

Electronically Induced Steric Clash: Synthesis of NMe2-Modified β-Diketiminate-Supported Boron Difluoride Compounds

We report on the synthesis and structural features of NMe2-modified β-diketiminate-supported boron difluoride compounds (LArBF2: LAr=[HC(NAr)2(CNMe2)2]–; LPh: Ar=Ph; LTol: Ar=p-tolyl; LXyl: Ar=m-xylyl). The title compounds were prepared in moderate yields (~65%) by insitu deprotonation of the corresponding ligands LArH using KH, followed by the addition of BF3OEt2. According to solid-state and theoretical analyses of the BF2 compounds, the lone pair at each NMe2 group is involved in electron delocalization within the central BC3N2 ring. As a result, the N-aryl substituents sterically clash with the NMe2 groups, causing this central ring to pucker. Several attempts were made to prepare heavy analogues (e.g. LArBX2, X=Cl, Br, I) but only unidentifiable product mixtures were observed. It appears that the observed steric clash between the N-aryl substituents and the NMe2 groups prevented the formation of these heavy analogues.

N–H cleavage vs. Werner complex formation: reactivity of cationic group 14 tetrelenes towards amines

Two-coordinate cations [(N-nacnac)E]⁺ (E = Si, Ge, Sn) react differently towards ammonia/amines: simple adduct formation (Ge,Sn) contrasts with N–H activation (Si).

Reactions of a Silicon-Based Cyclic Silylone with Chalcogens

Tetrasilasilylone N is composed of a silicon(0) atom, two NHC-coordinated silylenes, and a silaimine (Si═N) group. It reacts rapidly and selectively with the chalcogens selenium and tellurium to afford corresponding dichalcogenides 1 and 2 in high yields of 69-79%. In these compounds, one chalcogen atom is connected to the silicon(0) atom with a short bond in the range of Si═E double bonds. The second chalcogen atom adopts the bridging position between the Si(0) atom and the Si═N group. Using only 1 equiv of tellurium affords the selective formation of monotelluride 3 in 84% yield. In this monotelluride, the tellurium atom adopts a bridging position between the silylone functionality and the Si═N group.

Donor-acceptor-stabilised germanium analogues of acid chloride, ester, and acyl pyrrole compounds: synthesis and reactivity

Germaacid chloride, germaester, and N-germaacyl pyrrole compounds were not known previously. Therefore, donor-acceptor-stabilised germaacid chloride (i-Bu)₂ATIGe(O)(Cl) → B(C₆F₅)₃ (1), germaester (i-Bu)₂ATIGe(O)(OSiPh₃) → B(C₆F₅)₃ (2), and N-germaacyl pyrrole (i-Bu)₂ATIGe(O)(NC₄H₄) → B(C₆F₅)₃ (3) compounds, with Cl-Ge[double bond, length as m-dash]O, Ph₃SiO-Ge[double bond, length as m-dash]O, and C₄H₄N-Ge[double bond, length as m-dash]O moieties, respectively, are reported here. Germaacid chloride 1 reacts with PhCCLi, KOt-Bu, and RLi (R = Ph, Me) to afford donor-acceptor-stabilised germaynone (i-Bu)₂ATIGe(O)(CCPh) → B(C₆F₅)₃ (4), germaester (i-Bu)₂ATIGe(O)(Ot-Bu) → B(C₆F₅)₃ (5), and germanone (i-Bu)₂ATIGe(O)(R) → B(C₆F₅)₃ (R = Ph 6, Me 7) compounds, respectively. Interconversion between a germaester and a germaacid chloride is achieved; reaction of germaesters 2 and 5 with TMSCl gave germaacid chloride 1, and 1 reacted with Ph₃SiOLi and KOt-Bu to produce germaesters 2 and 5. Reaction of N-germaacyl pyrrole 3 with thiophenol produced a donor-acceptor-stabilised germaacyl thioester (i-Bu)₂ATIGe(O)(SPh) → B(C₆F₅)₃ (10). Furthermore, the attempted syntheses of germaamides and germacarboxylic acids are also discussed.

The Quest for Stable Silaaldehydes: Synthesis and Reactivity of a Masked Silacarbonyl

The first donor-acceptor complex of a silaaldehyde, with the general formula (NHC)(Ar)Si(H)OGaCl₃ (NHC=N-heterocyclic carbene), was synthesized using the reaction of silyliumylidene-NHC complex [(NHC)₂ (Ar)Si]Cl with water in the presence of GaCl₃ . Conversion of this complex to the corresponding silacarboxylate dimer [(NHC)(Ar)SiO₂ GaCl₂ ]₂ , free silaacetal ArSi(H)(OR)₂ , silaacyl chloride (NHC)(Ar)Si(Cl)OGaCl₃ , and phosphasilene-NHC adduct (NHC)(Ar)Si(H)PTMS unveil its true potential as a synthon in silacarbonyl chemistry.