Cyclic (Amino)(Phosphonium Bora-Ylide)Silanone: A Remarkable Room-Temperature-Persistent Silanone

Base-Stabilized Neutral Oxoborane and Thioxoborane Supported by a Bis(oxazolinyl)(phenyl)methanide Ligand

Base-stabilized neutral oxoborane and thioxoborane supported by a bis(oxazolinyl)(phenyl)methanide ligand have been synthesized and structurally characterized. While previous synthetic attempts of oxoborane ligated by β-diketiminate (NacNac) did not allow for its isolation in acid-free form, oxoborane supported by a bis(oxazolinyl)(phenyl)methanide ligand is isolable, due to the absence of imine ɑ-protons and steric protection of enamine carbon. Crystallographic analysis revealed the presence of a B-O double bond close to the shortest end of the reported lengths. Its reactivity has also been examined, and it was majorly governed by the nucleophilicity and basicity of the oxygen atom. The chemical inertness and synthetic convenience of the bis(oxazolinyl)(phenyl)methanide scaffold presented in this work suggest its utility as an innocent alternative to the NacNac scaffold.

Dipyrromethane‐Based PGeP Pincer Germyl Rhodium Complexes

A family of germyl rhodium complexes derived from the PGeP germylene 2,2’‐bis(di‐isopropylphosphanylmethyl)‐5,5’‐dimethyldipyrromethane‐1,1’‐diylgermanium(II), Ge(pyrmPⁱPr₂)₂CMe₂ (1), has been prepared. Germylene 1 reacted readily with [RhCl(PPh₃)₃] and [RhCl(cod)(PPh₃)] (cod=1,5‐cyclooctadiene) to give, in both cases, the PGeP‐pincer chloridogermyl rhodium(I) derivative [Rh{κ³P,Ge,P‐GeCl(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] (2). Similarly, the reaction of 1 with [RhCl(cod)(MeCN)] afforded [Rh{κ³P,Ge,P‐GeCl(pyrmPⁱPr₂)₂CMe₂}(MeCN)] (3). The methoxidogermyl and methylgermyl rhodium(I) complexes [Rh{κ³P,Ge,P‐GeR(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] (R=OMe, 4; Me, 5) were prepared by treating complex 2 with LiOMe and LiMe, respectively. Complex 5 readily reacted with CO to give the carbonyl rhodium(I) derivative [Rh{κ³P,Ge,P‐GeR(pyrmPⁱPr₂)₂CMe₂}(CO)] (6), with HCl, HSnPh₃ and Ph₂S₂ rendering the pentacoordinate methylgermyl rhodium(III) complexes [RhHX{κ³P,Ge,P‐GeMe(pyrmPⁱPr₂)₂CMe₂}] (X=Cl, 7; SnPh₃, 8) and [Rh(SPh)₂{κ³P,Ge,P‐GeMe(pyrmPⁱPr₂)₂CMe₂}] (9), respectively, and with H₂ to give the hexacoordinate derivative [RhH₂{κ³P,Ge,P‐GeMe(pyrmPⁱPr₂)₂CMe₂}(PPh₃)] (10). Complexes 3 and 5 are catalyst precursors for the hydroboration of styrene, 4‐vinyltoluene and 4‐vinylfluorobenzene with catecholborane under mild conditions.

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.

An NHC-Stabilized H2 GeBH2 Precursor for the Preparation of Cationic Group 13/14/15 Hydride Chains

The synthesis, characterization and reactivity studies of the NHC‐stabilized complex IDipp ⋅ GeH₂BH₂OTf (1) (IDipp=1,3‐bis(2,6‐diisopropylphenyl)imidazolin‐2‐ylidene) are reported. Nucleophilic substitution of the triflate (OTf) group in 1 by phosphine or arsine donors provides access to the cationic group 13/14/15 chains [IDipp ⋅ GeH₂BH₂ERR¹R²]⁺ (2 E=P; R, R¹=H; R²=^tBu; 3 E=P; R=H; R¹, R²=Ph; 4 a E=P; R, R¹, R²=Ph; 4 b E=As; R, R¹, R²=Ph). These novel cationic chains were characterized by X‐ray crystallography, NMR spectroscopy and mass spectrometry. Moreover, the formation of the parent complexes [IDipp ⋅ GeH₂BH₂PH₃][OTf] (5) and [IDipp ⋅ GeH₃][OTf] (6) were achieved by reaction of 1 with PH₃. Accompanying DFT computations give insight into the stability of the formed chains with respect to their decomposition.

Computational Exploration of Mechanistic Avenues in Metal-Free CO2 Reduction to CO by Disilyne Bisphosphine Adduct and Phosphonium Silaylide

Recent years have seen a growing interest in metal-free CO₂ activation by silylenes, silylones, and silanones. However, compared to mononuclear silicon species, CO₂ reduction mediated by dinuclear silicon compounds, especially disilynes, has been less explored. We have carried out extensive computational investigations to explore the mechanistic avenues in CO₂ reduction to CO by donor-stabilized disilyne bisphosphine adduct (R1^M ) and phosphonium silaylide (R2) using density functional theory calculations. Theoretical calculations suggest that R1^M exhibits donor-stabilized bis(silylene) bonding features with unusual Si-Si multiple bonding. Various modes of CO₂ coordination to R1^M have been investigated and the coordination of CO₂ by the carbon center to R1^M is found to be kinetically more facile than that by oxygen involving only one or both the silicon centers. Both the theoretically predicted reaction mechanisms of R1^M and R2-mediated CO₂ reduction reveal the crucial role of silicon-centered lone pairs in CO₂ activations and generation of key intermediates possessing enormous strain in the Si-C-O ring, which plays the pivotal role in CO extrusion.

Single‐Site and Cooperative Bond Activation Reactions with Ylide‐Functionalized Tetrylenes: A Computational Study

Due to their transition metal‐like behavior divalent group 14 compounds bear huge potential for their application in bond activation reactions and catalysis. Here we report on detailed computational studies on the use of ylide‐substituted tetrylenes in the activation of dihydrogen and phenol. A series of acyclic and cyclic ylidyltetrylenes featuring various α‐substituents with different σ‐ and π‐donating capabilities have been investigated which demonstrate that particularly π‐accepting boryl groups lead to beneficial properties and low barriers for single‐site activation reactions, above all in the case of silylenes. In contrast, for the thermodynamically more stable germylenes and stannylenes an alternative mechanism involving the active participation of the ylide ligand in the E−H bond (E=H or PhO) activation process by addition across the element carbon linkage was found to be energetically favored. Furthermore, the boryl substituted tetrylenes allowed for a further activation pathway involving the active participation of the boron element bond. These cooperative mechanisms are especially attractive for the heavier cyclic ylidyltetrylenes in which the loss of the protonated ylide group is prevented due to the cyclic framework. Overall, the present studies suggest that cyclic ylide‐substituted germylenes and stannylenes bear huge potential for cooperative bond activations at mild conditions which should be experimentally addressed in the future.

An Isolable Three-Coordinate Germanone and Its Reactivity

A rare three‐coordinate germanone [IPrN]₂Ge=O (IPrN=bis(2,6‐diisopropylphenyl)imidazolin‐2‐imino) was successfully isolated. The germanone has a rather high thermal stability in arene solvent, and no detectable change was observed at 80 °C for at least one week. However, high thermal stability of [IPrN]₂Ge=O does not prevent its reactivity toward small molecules. Structural analysis and initial reactivity studies revealed the highly polarized nature of the terminal Ge=O bond. Besides, the addition of phenylacetylene, as well as O‐atom transfer with 2,6‐dimethylphenyl isocyanide make it a mimic of nucleophilic transition‐metal oxides. Mechanism for O‐atom transfer reaction was investigated via DFT calculations, which revealed that the reaction proceeds via a [2+2] cycloaddition intermediate.

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.

Strained and Reactive Donor/Acceptor-Supported Metallasilanone

A novel stable donor/acceptor-supported Mn^I -metallasilanone 3 was synthesized. The intramolecular silanone-Mn^I interaction induces a highly strained three-membered cyclic structure, leading to an exceptionally high reactivity of 3 as a donor/acceptor complex of silanone. Indeed, metallasilanone 3 readily reacts with various small molecules such as H₂ or ethylene gas in mild conditions.

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.

Superseding β-Diketiminato Ligands: An Amido Imidazoline-2-Imine Ligand Stabilizes the Exhaustive Series of B=X Boranes (X=O, S, Se, Te)

Boron reluctantly forms B=X (X=O, S, Se, Te) moieties, which has stimulated the quest for such species in the past few years. Based on the N,N'-chelating β-diketiminato ligand (HNacNac), a new amido imidazoline-2-imine ligand system (HAmIm) is presented, giving rise to the isolation of an exhaustive series of Lewis acid free, monomeric chalcogen B=X boranes with documented π-bond character between boron and the chalcogen. The chalcogenoboranes are isoelectronic and isolobal to the respective ketones. The chemical behavior of the oxoborane (B=O) strongly resembles the classical carbonyl reactivity in C=O bonds. The improved stability provided by HAmIm arises from the formation of more-stable five-membered boron chelates versus the six-membered NacNac analogues and from the imidazoline-2-imine moiety providing enhanced σ- and π-donation. The HAmIm ligand class may supersede the widely employed NacNac system in certain applications.

Bonding Relationship between Silicon and Germanium with Group 13 and Heavier Elements of Groups 14-16

The topic of heavier main group compounds possessing multiple bonds is the subject of momentous interest in modern organometallic chemistry. Importantly, there is an excitement involving the discovery of unprecedented compounds with unique bonding modes. The research in this area is still expanding, particularly the reactivity aspects of these compounds. This article aims to describe the overall developments reported on the stable derivatives of silicon and germanium involved in multiple bond formation with other group 13, and heavier groups 14, 15, and 16 elements. The synthetic strategies, structural features, and their reactivity towards different nucleophiles, unsaturated organic substrates, and in small molecule activation are discussed. Further, their physical and chemical properties are described based on their spectroscopic and theoretical studies.

Small Molecule Activation by Two-Coordinate Acyclic Silylenes

In recent decades, the chemistry of stable silylenes (R2Si:) has evolved significantly. The first major development in this chemistry was the isolation of a silicocene which is stabilized by the Cp* (Cp* = η5-C5Me5) ligand in 1986 and subsequently the isolation of a first N-heterocyclic silylene (NHSi:) in 1994. Since the groundbreaking discoveries, a large number of isolable cyclic silylenes and higher coordinated silylenes, i.e. Si(II) compounds with coordination number greater than two, have been prepared and the properties investigated. However, the first isolable two-coordinate acyclic silylene was finally reported in 2012. The achievements in the synthesis of acyclic silylenes have allowed for the utilization of silylenes in small molecule activation including inert H2 activation, a process previously exclusive to transition metals. This minireview highlights the developments in silylene chemistry, specifically two-coordinate acyclic silylenes, including experimental and computational studies which investigate the extremely high reactivity of the acyclic silylenes.

Phosphorus-ylides: powerful substituents for the stabilization of reactive main group compounds

Phosphorus ylides are 1,2-dipolar compounds with a negative charge on the carbon atom. This charge is stabilized by the neighbouring onium moiety, but can also be shifted towards other substituents thus making ylides strong π donor ligands and hence ideal substituents to stabilize reactive compounds such as cations and low-valent main group species. Furthermore, the donor strength and the steric properties can easily be tuned to meet different requirements for stabilizing reactive compounds and for tailoring the properties and reactivities of the main group element. Although the use of ylide substituents in main group chemistry is still in its infancy, the first examples of isolated compounds impressively demonstrate the potential of these ligands. This review summarizes the most important discoveries also in comparison to other substituents, thus outlining avenues for future research directions.

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.

Reduction of Carbon Oxides by an Acyclic Silylene: Reductive Coupling of CO

Reactions of a boryl-substituted acyclic silylene with carbon dioxide and monoxide are reported. The former proceeds through oxygen atom abstraction, generating CO (with rearrangement of the putative silanone product through silyl-group transfer). The latter is characterized by reductive coupling of CO to give an ethynediolate fragment, which undergoes formal insertion into the Si-B bond. The net conversion of carbon dioxide with two equivalents of silylene offers a route for the three-electron reduction of CO₂ to [C₂ O₂ ]^2- .

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.

Lewis Acid-Mediated One-Electron Reduction of Nitrous Oxide

The one-electron reduction of nitrous oxide (N₂ O) was achieved using strong Lewis acids E(C₆ F₅ )₃ (E=B or Al) in combination with metallocenes. In the case of B(C₆ F₅ )₃ , electron transfer to N₂ O required a powerful reducing agent such as Cp*₂ Co (Cp*=pentamethylcyclopentadienyl). In the presence of Al(C₆ F₅ )₃ , on the other hand, the reactions could be performed with weaker reducing agents such as Cp*₂ Fe or Cp₂ Fe (Cp=cyclopentadienyl). The Lewis acid-mediated electron transfer from the metallocene to N₂ O resulted in cleavage of the N-O bond, generating N₂ and the oxyl radical anion [OE(C₆ F₅ )₃ ]^⋅- . The latter is highly reactive and engages in C-H activation reactions. It was possible to trap the radical by addition of the Gomberg dimer, which acts as a source of the trityl radical.

Precise Activation of Ammonia and Carbon Dioxide by an Iminodisilene

The activation of NH₃ and CO₂ is still an ambitious target for multiply bonded sub-valent silicon compounds. Now, the precise splitting of the N-H bond of ammonia by (Z)-imino(silyl)disilene 1 to give trans-1,2-adduct 2 a at low temperatures (-78 °C) is presented. According to DFT calculations, the stereospecific hydroamination follows a similar mechanism as the recently reported anti-addition of H₂ to the Si=Si bond of 1. The aminosilane 2 b could also be obtained as the formal silylene addition product under thermodynamic reaction control. By applying low temperatures, the activation of CO₂ with 1 selectively afforded the cis-oxadisilacyclobutanone 7-c as [2+2] cycloadduct. By performing the reaction directly at ambient temperatures, a mixture of three different-sized silacycles (4-6) was observed. Their formation was investigated theoretically and their structures were revealed with separate experiments using 1 and the oxygenation agents N₂ O and O₂ .