Unveiling the Anomaly of Reduction of Carborane-bis-silylene-Stabilised Silylone/Germylone Leading to Unusual Oxidation of Si0 /Ge0 to SiI /GeI with EDA-NOCV Analyses

Researchers have successfully isolated Si0 /Ge0 species, termed silylone and germylone, with two lone pairs of electrons on them. These elusive compounds have been stabilised in singlet ground states by using different donor base ligands. Driess et al. in particular have made strides in this area, isolating carborane-bis-silylene-stabilised silylone/germylone and their N+ /Pb analogues. Carborane (C2 B10 H10 ) plays a pivotal role as a redox-active ligand, converting from closo-carborane to nido-carborane with the addition of two electrons. Notably, anomalous oxidation of Si0 /Ge0 centres in carborane-bis-silylene-stabilised species to SiI /GeI has been reported, resulting in the formation of dimeric SiI -SiI /GeI -GeI di-cationic units. The energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) study focuses on the carborane-bis-silylene ligand in the free state, and its three other species, including silylone/germylone species. Interestingly, it reveals that the carborane unit in an anionic doublet state tends to form one electron-sharing bond and one dative bond with the counter fragment in its cationic doublet state. This helps us to rationalise why the carborane unit undergoes intramolecular electronic rearrangements leading to the formation of a di-anionic carborane unit with a significantly elongated C-C bond (2.38-2.68 Å) and undergoes unusual oxidation of Si0 /Ge0 to SiI /GeI .

σ versus π-radical: Tuning the electronic nature of neutral carbon (I) compounds with three non-bonding electrons

The bonding and reactivity of the hypo-coordinated compounds with one, two, and four non-bonding electrons namely, carbon-centered free radical, carbenes, and carbones were well earlier established. Here, we report stability, bonding and reactivity of compounds RCL, where R is one-electron donor group (R = CH₃ (a), CHO (b), and NO₂ (c)) and L is two-electron donor ligand (L = cAAC (1), CO (2), NHC (3) and PMe₃ (4)), having three non-bonding electrons. The ground states of molecules exist in a doublet with a lone pair of electrons and an unpaired electron at the central carbon atom (C1). The spin hops over from π- to σ-type orbitals is observed as the π-acceptor strength of the donor ligand increases. The replacement of the methyl group by CHO and NO₂ indicate that the cAAC and CHO substituted compounds gives a σ-radical except in compound 2c. These molecules show very high proton affinity and exothermic reaction energy for the hydrogen atom addition indicating dual reactivity namely, radical and lone pair reactivity.

Carbodicarbenes and Striking Redox Transitions of their Conjugate Acids: Influence of NHC versus CAAC as Donor Substituents

Herein, a new type of carbodicarbene (CDC) comprising two different classes of carbenes is reported; NHC and CAAC as donor substituents and compare the molecular structure and coordination to Au(I)Cl to those of NHC-only and CAAC-only analogues. The conjugate acids of these three CDCs exhibit notable redox properties. Their reactions with [NO][SbF₆ ] were investigated. The reduction of the conjugate acid of CAAC-only based CDC with KC₈ results in the formation of hydrogen abstracted/eliminated products, which proceed through a neutral radical intermediate, detected by EPR spectroscopy. In contrast, the reduction of conjugate acids of NHC-only and NHC/CAAC based CDCs led to intermolecular reductive (reversible) carbon-carbon sigma bond formation. The resulting relatively elongated carbon-carbon sigma bonds were found to be readily oxidized. They were, thus, demonstrated to be potent reducing agents, underlining their potential utility as organic electron donors and n-dopants in organic semiconductor molecules.

Low-Valent Germanylidene Anions: Efficient Single-Site Nucleophiles for Activation of Small Molecules

Rare mononuclear and helical chain low-valent germanylidene anions supported by cyclic (alkyl)(amino)carbene and hypermetallyl ligands were synthesised by stepwise reduction from corresponding germylene precursors via stable and isolable germanium radicals. The electronic structures of the anions can be described with ylidene and ylidone resonance forms with the Ge-C π-electrons capable of binding even weak electrophiles. The germanylidene anions reacted with CO2 to give μ-CO2 -κC:κO complexes, a rare coordination mode for low-valent germanium and inaccessible for the related neutral germylones. These results implicate low-valent germanylidene anions as efficient single-site nucleophiles for activation of small molecules.

Stabilization of group 14 elements E = C, Si, Ge by hetero-bileptic ligands cAAC, MCOn with push-pull mechanism

The stability and bonding of a series of hetero-diatomic molecules with general formula (cAAC)EM(CO)_n , where cAAC = cyclic alkyl(amino) carbene; E = group 14 elements (C, Si, and Ge); M = transition metal (Ni, Fe, and Cr) have been studied by quantum chemical calculations using density functional theory (DFT) and energy decomposition analysis-natural orbital chemical valence (EDA-NOCV). The equilibrium geometries were calculated at the BP86/def2-TZVPP level of theory. The tri-coordinated group 14 complex (1a, 4a, and 7a) in which one of the CO groups is migrated to the central group 14 element from adjacent metal is theoretically found to be more stable when the central atom (E) is carbon. On the other hand, the two-coordinate group 14 element containing metal-complexes (2, 5, 8, 3, 6, and 9) are found to be more stable with their corresponding heavier analogues. The electronic structures of all the molecules have been analyzed by molecular orbital, topological analysis of electron density and natural bond orbital (NBO) analysis at the M06/def2-TZVPP//BP86/def2-TZVPP level of theory. The nature of the cAACE and EM bonds has been studied by EDA-NOCV calculations at BP86-D3(BJ)/TZ2P level of theory. The EDA analysis suggests that the bonding of cAACC(CO) can be best represented by electron sharing σ and π interactions, whereas, C(CO)M(CO)_n-1 by dative σ and π interactions. On the other hand, EDA-NOCV calculations suggests both dative σ and π interactions for cAACE and EM(CO)_n bonds of the corresponding Si and Ge analogues having stronger σ- and relatively weaker π-bonds. The topological analysis of electron density supports the closed-shell interaction for the Si and Ge complexes and open-shell interaction for the carbon complexes. The calculated proton affinity and hydride affinity values corroborated well with the present bonding description. This class of complexes might act as efficient future catalysts for different organic transformations due to the presence of electron rich group 14 element and metal carbonyl.

Revisiting the Bonding Scenario of Two Donor Ligand Stabilized C2 Species

Quantum chemical calculations using density functional methods were performed for complexes of type L₂C₂ with L = NHC^Me (1), SNHC^Me (2) (S = saturated), cAAC^Me (3), and diamidocarbene (DAC^Me) (4). The equilibrium structures of 1-4 possess almost linear C₄ cores. A high thermochemical stability of the complexes with respect to dissociation, L₂C₂ → C₂ + 2L, is indicated by the large bond dissociation energy following the order 3 > 4 > 2 > 1. The results show that the use of SNHC^Me and DAC^Me as ligands is preferable over NHC^Me. The bonding analysis using charge and energy decomposition methods reveals that (cAAC^Me)₂C₂ and (DAC^Me)₂C₂ possess genuine cumulene C₄ moieties, which results from the electron-sharing bonding between quintet L₂ and quintet C₂ fragments. In contrast, the bonding in (NHC^Me)₂C₂ and (SNHC^Me)₂C₂ comes from a combination of dative and electron-sharing interactions between doublet L₂⁺ and doublet C₂^- fragments.

Stabilization of Linear C3 by Two Donor Ligands: A Theoretical Study of L-C3 -L (L=PPh3 , NHCMe , cAACMe )*

Quantum chemical studies using density functional theory and ab initio methods have been carried out for the molecules L-C3 -L with L=PPh3 (1), NHCMe (2, NHC=N-heterocyclic carbene), and cAACMe (3, cAAC=cyclic (alkyl)(amino) carbene). The calculations predict that 1 and 2 have equilibrium geometries where the ligands are bonded with rather acute bonding angles at the linear C3 moiety. The phosphine adduct 1 has a synclinal (gauche) conformation whereas 2 exhibits a trans conformation of the ligands. In contrast, the compound 3 possesses a nearly linear arrangement of the carbene ligands at the C3 fragment. The bond dissociation energies of the ligands have the order 1<2<3. The bonding analysis using charge and energy decomposition methods suggests that 3 is best described as a cumulene with electron-sharing double bonds between neutral fragments (cAACMe )2 and C3 in the respective electronic quintet state yielding (cAACMe )=C3 =(cAACMe ). In contrast, 1 and 2 possess electron-sharing and dative bonds between positively charged ligands [(PPh3 )2 ]+ or [(NHCMe )2 ]+ and negatively charged [C3 ]- fragments in the respective doublet state.

In-Depth Investigation of a Donor-Acceptor Interaction on the Heavy-Group-14@Group-13-Diyls in Transition-Metal Tetrylone Complexes: Structure, Bonding, and Property

Stabilization for tetrylone complexes, which carry ylidone(0) ligands [(CO)₅W-X (YCp*)₂] (X = Ge, Sn, Pb; Y = B-Tl), has become an active theoretical research because of their promising application. Structure, bonding, and quantum properties of the transition-metal donor-acceptor complexes were theoretically investigated at the level of theory BP86 with several types of basis sets including SVP, TZVPP, and TZ2P+. The optimized structures reveal that all ligands X (YCp*)₂ are strongly bonded in tilted modes to the metal fragment W(CO)₅, and Cp* rings are mainly η₅-bonded to atom X. DFT-based bonding analysis results in an implication that the stability of W-X bond strength primarily stems from the donation (CO)₅W ← X(YCp*)₂ formed by both σ- and π-bondings and the electrostatic interaction ΔE _elstat. The W-X bond possesses a considerable polarizability toward atom X, and analysis on its hybridization is either sp²-characteristic or mainly p-characteristic. EDA-NOCV-based results further imply that the ligands XY perform as significant σ-donors but minor π-donors. The visual simulations of NOCV pairs and the deformation densities assemble a comprehensive summary on different components of the chemical bond via σ- and π-types in the complexes. This work contributes to the literature as an in-depth overview on predicted molecular structures and quantum parameters of the complexes [(CO)₅W-X(YCp*)₂] (X = Ge, Sn, Pb; Y = B-Tl), conducive to either further theoretical reference or extending experimental research.

How to Bend a Cumulene

Allenes (carbodicarbenes) and [3]cumulenes are linear carbon chains that can be bent when the terminal group has a strong carbene nature. This bending can be quite pronounced in allenes but not in [3]cumulenes. In this study, how N-heterocyclic or cyclic (alkyl)(amino) carbene (NHC and CAAC, respectively) terminal groups can modify the linear structure of [n]cumulenes has been analyzed. A low π acidity of the terminal carbene affects the linearity of [2n]cumulenes. Indeed, it has been found that the NHC [4]cumulene is extremely bent, contrary to classical [4]cumulenes. The predicted NHC [4]cumulene or tricarbodicarbene has two lone pairs and the π electrons are delocalized over the whole molecule. More significantly, DFT calculations have shown that this bent [4]cumulene is very stable, considerably more so than the corresponding [3]cumulene, which has been elusive to synthesize. Remarkably, calculations have shown that all the NHC [2n]cumulenes are more than 25 kcal mol^-1 more stable than the [2n-1]cumulenes.

NHCs in Main Group Chemistry

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 useful for the broad research community.

Synthesis, Electronic Structure, and Reactivities of Two-Sulfur-Stabilized Carbones Exhibiting Four-Electron Donor Ability

Bis(sulfane)carbon(0) (BSC; Ph₂ S→C←SPh₂ (1)) is successfully synthesized by deprotonation of the corresponding protonated salt 1⋅HTfO. The diprotonated salt 1⋅(HTfO)₂ as the starting material can be also easily accessed by the deimination of iminosulfane(sulfane)carbon(0) (iSSC)⋅HBF₄ . Density functional theory calculations revealed the peculiar electronic structure of 1, which has two lone pairs of electrons at the central carbon atom. The largest proton affinities (PA(1): 297.5 kcal mol^-1 ; PA(2): 183.7 kcal mol^-1 ) and the highest energy levels of the HOMOs (HOMO: -4.89 eV; HOMO-1: -5.02 eV) for 1 among the two-sulfur-stabilized carbones clearly indicate the strong donor ability of carbon center stabilized by two S^II ligands. The donating ability of these lone pairs of electrons is demonstrated by the C-diaurated and C-proton-aurated complexes, which provide the first experimental evidence for two-sulfurstabilized carbones behaving as four-electron donors. Furthermore, the syntheses and application of Ag^I carbone complexes as carbone transfer agents are also reported.