Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC–main group element adducts, and might be...

NHCs in Main Group Chemistry.

Oxidative addition and reductive elimination are key steps in a wide variety of catalytic reactions mediated by transition-metal complexes. Historically, this reactivity has been considered to be the exclusive domain of d-block elements. However, this paradigm has changed in recent years with the demonstration of transition-metal-like reactivity by main-group compounds. This Review highlights the substantial progress achieved in the past decade for the activation of robust single bonds by main-group compounds and the more recently realized activation of multiple bonds by these elements. We also discuss the significant discovery of reversible activation of single bonds and distinct examples of reductive elimination at main-group element centers. The review consists of three major parts, starting with oxidative addition of single bonds, proceeding to cleavage of multiple bonds, and culminated by the discussion of reversible bond activation and reductive elimination. Within each subsection, the discussion is...

Oxidative Addition and Reductive Elimination at Main-Group Element Centers

Lewis acids play a major role in all areas of chemistry. For a long time, toxic, corrosive and oxidizing SbF5 was considered as the strongest Lewis acid known. Lately, species significantly exceeding the Lewis acidity of SbF5 have been realized and were termed Lewis superacids (LSA). Prospective new candidates emerge steadily, which not only outperform SbF5 by their strength, but also in terms of their accessibility and ease of handling. In principle, Lewis superacids allow us to combine the outstanding activity of Bronsted superacids with the excellent selectivity of a common Lewis acid. However, the broad application of Lewis superacids in synthesis is all but popular. The present review deals with strong Lewis acids. First, it critically discusses Lewis acidity scaling methods and suggests an extended definition for Lewis superacidity. It then summarizes the properties and applications of the strongest currently known Lewis acids, indexed by the fluoride ion affinity (FIA). The supporting information contains a comprehensive list of experimentally and theoretically derived FIA data as a guide for the choice of Lewis acidic reagents/catalyst. This contribution shall encourage the search for new Lewis superacids and promote their application in non-specialized laboratories.

Lewis Superacids: Classifications, Candidates, and Applications.

The history of silyl cations has all the makings of a drama but with a happy ending. Being considered reactive intermediates impossible to isolate in the condensed phase for decades, their actual c...

Silylium Ions: From Elusive Reactive Intermediates to Potent Catalysts.

Quantum chemical calculations of the main-group compounds E[C(NHCMe )2 ]2 (E=Be, B+ , C2+ , N3+ , Mg, Al+ , Si2+ , P3+ ) have been carried out using density functional theory at the BP86/def2-TZVPP and BP86-D3(BJ)/def2-TZVPP levels of theory. The geometry optimization at BP86/def2-TZVPP gives equilibrium structures with two-coordinated species E and bending angles C-E-C between 152.5° (E=Be) and 110.5° (E=Al). Inclusion of dispersion forces at BP86-D3(BJ)/def2-TZVPP yields a three-coordinated beryllium compound Be[C(NHCMe )2 ]2 as the only energy minimum form. Three-coordinated isomers are found besides the two-coordinated energy minima for the boron and carbon cations B[C(NHCMe )2 ]2+ and C[C(NHCMe )2 ]22+ . The three-coordinated form of the boron compound is energetically lower lying than the two-coordinated form, while the opposite trend is calculated for the carbon species. The theoretically predicted bond dissociation energies suggest that all compounds are viable species for experimental studies. The X-ray structure of the benzoannelated homologue of P[C(NHCMe )2 ]23+ that was recently reported by Dordevic et al. agrees quite well with the calculated geometry of the molecule. A detailed bonding analysis using charge and energy decomposition methods shows that the two-coordinated neutral compounds Be[C(NHCMe )2 ]2 and Mg[C(NHCMe )2 ]2 possess strongly positively charged atoms Be and Mg. The carbodicarbene groups C(NHCMe )2 serve as acceptor ligands in the compounds and may be sketched with dative bonds (NHCMe )2 C←E→C(NHCMe )2 (E=Be, Mg). Dative bonds in which the carbones C(NHCMe )2 are donor ligands are suggested for the cations (NHCMe )2 C→E←C(NHCMe )2 (E=B+ , Al+ ). The dications and trications possess electron-sharing bonds in which the bonding situation is best described with the formula [(NHCMe )2 C]+ -E-[C(NHCMe )2 ]+ (E=C, Si, N+ , P+ ).

Carbones as Ligands in Novel Main‐Group Compounds E[C(NHC)2]2 (E=Be, B+, C2+, N3+, Mg, Al+, Si2+, P3+): A Theoretical Study

Bis(carbodicarbene)phosphenium trication: the case against hypervalency

In this work, strategic enhancement of electrophilicity of phosphenium cations for the purpose of small-molecule activation was described. Our synthetic methodology for generation of novel two-coordinate phosphorus(III)-based compounds [{C6H4(MeN)2C}2C·PR]2+ ([2a]2+, R = NiPr2; [2b]2+, R = Ph) was based on the exceptional electron-donating properties of the carbodicarbene ligand (CDC). The effects of P-centered substituent exchange and increase in the overall positive charge on small substrate activation were comparatively determined by incorporating the bis(amino)phosphenium ion [(iPr2N)2P]+ ([1]+) in this study. Implemented structural and electronic modifications of phosphenium salts were computationally verified and subsequently confirmed by isolation and characterization of the corresponding E-H (E = B, Si, C) bond activation products. While both phosphenium mono- and dications oxidatively inserted/cleaved the B-H bond of Lewis base stabilized boranes, the increased electrophilicity of doubly charged species also afforded the activation of significantly less hydridic Si-H and C-H bonds. The preference of [2a]2+ and [2b]2+ to abstract the hydride rather than to insert into the corresponding bond of silanes, as well as the formation of the carbodicarbene-stabilized parent phosphenium ion [{C6H4(MeN)2C}2C·PH2]+ ([2·PH2]+) were experimentally validated.

E-H (E = B, Si, C) Bond Activation by Tuning Structural and Electronic Properties of Phosphenium Cations.

C–F bond cleavage by transient phosphorus(III)-based dications [RP(C(PPh3)2)]2+ (4a2+, R = Ph; 4b2+, R = 4-F-Ph) is reported. These dications were generated by reaction of the corresponding monocationic precursors with excess Na[BAr4Cl]. Evidence for the existence of transient dicationic species was obtained by trapping the dication 4a2+ with PMe3. According to theoretical analysis, the low-lying lowest unoccupied molecular orbitals of these species were responsible for the observed activation of C–F bonds.

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C–F Bond Activation by Transient Phosphenium Dications

https://dr.ntu.edu.sg/bitstream/10356/106217/1/Extending%20the%20chemistry%20of%20carbones_P-N%20bond%20cleavage%20via%20an%20SN2-like%20mechanism.pdf

Nemanja Đorđević

Papers

Bis(carbodicarbene)phosphenium trication: the case against hypervalency

E-H (E = B, Si, C) Bond Activation by Tuning Structural and Electronic Properties of Phosphenium Cations.

C–F Bond Activation by Transient Phosphenium Dications

Extending the chemistry of carbones: P–N bond cleavage via an SN2′-like mechanism