Two Structurally Characterized Conformational Isomers with Different C−P Bonds

A Digermanium(III) 1,2-Dication Stabilized by Amidinate and cAAC-Phosphinidenide Ligands

A digermanium(III) 1,2-dication comprises two cationic centers located at two interconnected Ge atoms. The strong Coulombic repulsion between two positively charged germanium cations hinders their bond formation. Balancing these two oppositions was achieved by using amidinate and cyclic (alkyl)amino carbene (cAAC)-phosphinidenide ligands, where an amidinato cAAC-phosphinidenidogermylene complex, [LGeP(cAACMe)] (2, where L = PhC(NtBu)2, cAACMe = :C{C(Me)2CH2C(Me)2NAr}, and Ar = 2,6-iPr2C6H3), underwent one-electron oxidation with a bis(phosphinidene) radical cation, [(cAACMe)P]2•+, to form a digermanium(III) 1,2-dication, [LGeP(cAACMe)]22+, in compound 4.

Coordination Chemistry of Bis-Cyclic Alkyl(Amino) Carbene (cAAC)-Supported Di-Phosphorus (P2 ): An Efficient Route to Donor Base-Stabilized Elusive Di-Phosphorus-Monoxide(P2 O)-Gold Complex

The stability and reactivity studies of heavier di-atomic group-15 congeners of alkynes, e. g., the di-phosphorus (P≡P) compounds have been the topic of huge interest because of their contrasting transient properties and lower stability compared to those of the stable molecular di-nitrogen (N≡N). Herein, we depict the reactivity studies of the bis-cAAC-stabilized di-phosphorus (P2 ) having an inversely polarized phosphaalkene nature featuring the C=P double bonds with Au(I)Cl. Both the mono-, and the di-aurated phosphaalkenes with the formulae [(Me2 -cAAC=P)2 (AuCl)] (2), and [(Me2 -cAAC=P)2 (AuCl)2 ] (3), respectively have been isolated in the solid state. Moreover, for the first time, we have been able to isolate the cAAC-stabilized tetra-aurated elusive di-phosphorus-monoxide (P2 O) with the formula [(Cy-cAAC=P)-O-(P=cAAC-Cy)(AuCl)4 ] (5) in presence of oxygen. Complexes 2-3, 5 have been structurally characterized by single crystal X-ray diffraction, and further studied by NMR spectroscopy. Our findings reveal significant elongation of the CcAAC -P bonds in 2-3, 5, and the presence of aurophilic interaction in 5. Quantum chemical calculations, including density functional theory (DFT), and energy decomposition analysis coupled with natural orbitals for chemical valence (EDA-NOCV) have been performed to study the electron densities distribution and nature of bonding in 2-3, 5.

Bicyclic (alkyl)(amino)carbene (BICAAC)-supported phosphinidenes

Herein, the synthesis and characterization of bicyclic (alkyl)(amino)carbene (BICAAC)-stabilized phosphinidenes (1-4) are reported. Compounds 1-3 were obtained by reacting trihalophosphine [PX3, X = Cl (1), Br (2), I (3)] with BICAAC in THF. A BICAAC-stabilized bis-phosphinidene (4) was obtained from the reduction of compound 2. All four compounds were characterized by X-ray crystallography and heteronuclear NMR spectroscopy. Theoretical calculations indicated the predominant C(carbene)P double bond characteristic in compounds 1-4.

Isolation of (Aryl)-(Imino) Phosphide and (Aryl)-(Phosphaalkene) Amide Complexes of Alkali Metals from Carbene-Phosphinidenes under Reductive-Thermal Rearrangements

Two-electron reduction of cyclic alkyl(amino) carbene (cAAC)-supported chloro-phosphinidene cAAC=P-Cl (1) followed by unprecedented thermal rearrangements afforded the alkali metal complexes of (aryl)-(cyclic alkyl(imino)) phosphides 3 a-3 c, 4 a-4 b through migration of the 2,6-diisopropylphenyl (dipp) group from N to the P centre, and the (aryl)-(cyclic alkyl(phosphaalkene)) amide 5 through cleavage of the CMe2 -N bond followed by energetically favoured 5-exo-tet ring-closure in the presence of the alkali metals Cs (3 a-3 c), K (4 a, 4 b), and Li (5). Compound 3 a was found to be photoluminescent (PL), emitting bright orange light under a laboratory UV lamp of wavelength 365 nm with PL quantum yield (ϕPL ) of 2.6 % (λem =600 nm), and an average lifetime (τ) of 4.8 μs. Reaction of 3 a with CuCl and AgOTf afforded (aryl)-(cyclic alkyl(imino)) phosphide-stabilized tetra-nuclear CuI (6), and octa-nuclear AgI (7) clusters, respectively. Moreover, complexes 3 a-3 c provided a direct route for the stabilization of cyclic alkyl(aminoboryl) phosphaalkenes 8 a-8 c when treated with 1-bromo-N,N,N',N'-tetraisopropylboranediamine.

Stabilisation and reactivity studies of donor-base ligand-supported gallium-phosphides with stronger binding energy: a theoretical approach

Gallium phosphide is a three-dimensional polymeric material of the hetero-diatomic GaP unit, which has a wurtzite type structure, and captivating application as a light emitting diode (LED). As a result, there is a constant search for suitable precursors to synthesise GaP-based materials. However, the corresponding monomeric species is exotic in nature due to the expected Ga[triple bond, length as m-dash]P multiple bond. Herein, we report on the theoretical studies of stability, chemical bonding, and reactivity of the monomeric gallium phosphides with two donor base ligands having tuneable binding energies. We have performed detailed investigations using density functional theory at three different levels (BP86/def2-TZVPP, B3LYP/def2-TZVPP, M06-2X/def2-TZVPP), QTAIM and EDA-NOCV (BP86-D3(BJ)/TZ2P, M06-2X/TZ2P) to analyse various ligand-stabilised GaP monomers, which revealed the synthetic viability of such species in the presence of stable singlet carbenes, e.g., cAAC, and NHC as ligands [cAAC = cyclic alkyl(amino) carbene, NHC = N-heterocyclic carbene] due to the larger bond dissociation energy compared to a phosphine ligand (PMe₃). The calculated bond dissociation energies between a pair of ligands and the monomeric GaP unit are found to be in the range of 87 to 137 kcal mol^-1, predicting their possible syntheses in the laboratory. Further, the reactivity of such species with metal carbonyls [Fe(CO)₄, and Ni(CO)₃] have been theoretically investigated.

Redox Active cAAC-Fluorene/Indene Systems Displaying Solvatochromism, Green Luminescence and pH Sensing: Functionalization of Fluorenyl/Indenyl Rings with Radical Carbene

Two new series of air stable compounds of cAAC^X = fluorene/indene (X = Me₂ , Et₂ , Cy) [cAAC = cyclic (alkyl) amino carbene] have been isolated and well characterized by X-ray single crystal diffraction, photoluminescence, cyclic voltammogram (CV) and electron paramagnetic resonance (EPR) studies. Fluorescence studies reveals green light emission of cAAC bonded fluorene, whereas free fluorene generally displays a violet emission. Interestingly, the sterically crowded cAAC-fluorene analogue display solvatochromism and CF₃ CO₂ H sensing in solution. CV of the these compounds show a quasi-reversible electron transfer process, indicating the functionalization of fluorene/indene with radical anionic form of carbene, confirmed by CV/EPR measurements. DFT/TDDFT calculations and energy decomposition analysis coupled with natural orbital for chemical valence (EDA-NOCV) have been carried out to study different aspects of bonding and electronic transitions. Such a class of redox active and thermally stable organic molecules may be suitable for molecule based spin memory devices in future.

Carbene-Anchored Boryl- and Stibanyl-Phosphaalkenes as Precursors for Bis-Phosphaalkenyl Dichlorogermane and Mixed-Valence AgI/AgII Phosphinidenide

Cyclic alkyl(amino) carbene (cAAC)-anchored boryl- and stibanyl-phosphaalkenes with general formula cAAC = P-ER₂ [E = B, R = (NⁱPr₂)₂ (3a-c); E = Sb, R = 2,4,6-triisopropylphenyl (5a-b)] have been synthesized and utilized as precursors for the bis-phosphaalkenyl dichlorogermane [(cAAC = P)₂GeCl₂] (6) and the first molecular example of a neutral polymeric mixed-valence Ag^I/Ag^II phosphinidenide complex [(cAACP)₂Ag₄^IAg^IICl₄]_n (7). All compounds have been characterized by single-crystal X-ray diffraction and further investigated by nuclear magnetic resonance (NMR), mass spectrometric analysis, and UV-vis/fluorescence measurements. The paramagnetic complex 7 has been characterized by ESR spectroscopy. Cyclic voltammetry studies of compounds 3/5 have suggested possible one-electron quasi-reversible reductions, indicating their redox noninnocent behavior in solution. Quantum chemical studies revealed the electron-sharing nature of the P-B and P-Sb σ bonds in compounds 3 and 5, and the polar C_cAAC = P bonds in compounds 3, 5, and 6 prevailing their phosphaalkene structures over phosphinidenes.

Recent Advances in the Domain of Cyclic (Alkyl)(Amino) Carbenes

Isolation of cyclic (alkyl) amino carbenes (cAACs) in 2005 has been a major achievement in the field of stable carbenes due to their better electronic properties. cAACs and bicyclic(alkyl)(amino)carbene (BicAAC) in essence are the most electrophilic as well as nucleophilic carbenes are known till date. Due to their excellent electronic properties in terms of nucleophilic and electrophilic character, cAACs have been utilized in different areas of chemistry, including stabilization of low valent main group and transition metal species, activation of small molecules, and catalysis. The applications of cAACs in catalysis have opened up new avenues of research in the field of cAAC chemistry. This review summarizes the major results of cAAC chemistry published until August 2021.

EDA-NOCV Analysis of Donor-Base-Stabilized Elusive Monomeric Aluminum Phosphides [(L)P-Al(L'); L, L' = cAACMe, NHCMe, PMe3]

Herein, we report on the stability and bonding analysis of donor-base-stabilized monomeric AlP species (1-6) of the general formula (L)P-Al(L'); [L = cAACMe, L' = cAACMe, NHCMe, PMe3, (N i Pr2)2 (1-4); L = L' = NHCMe, PMe3 (5 and 6); cAAC = cyclic alkyl(amino) carbene; NHC = N-heterocyclic carbene]. Energy decomposition analysis coupled with natural orbitals for chemical valence (EDA-NOCV) analysis indicates the synthetic viability of this class of species, stabilized in their singlet ground state, in the laboratory. The CL-P bond is found to be a partial double bond (WBI ∼ 1.45), while the CL/PL-Al bond is a single bond (WBI ∼ 0.42-0.69). These bonds are mostly covalent or dative σ/π bonds depending upon the ligands attached. The central P-Al bond is an electron-sharing covalent polar single bond (WBI ∼ 0.80; P-Al) for 1-4 and a dative σ bond for 5 and 6 (WBI ∼ 0.89-0.93; P-Al). The calculated intrinsic interaction energies of the central P-Al bonds are found to be in the range from -116 to -216 kcal/mol (1-3 and 5 and 6). This value is the highest for compound 3, possibly due to the push and pull effects from the ligands PMe3 and cAAC, respectively.

A Brønsted Acidic Gallium Hydride: Facile Interconversion of NNNN-Macrocycle Supported [GaI]+ and [GaIIIH]2

Protonolysis of [Cp*M] (M=Ga, In, Tl) with [(Me₄TACD)H][BAr₄^Me] (Me₄TACD=N,N′,N′′,N′′′‐tetramethyl‐1,4,7,10‐tetraazacyclododecane; [BAr₄^Me]⁻=[B{C₆H₃‐3,5‐(CH₃)₂}₄]⁻) provided monovalent salts [(Me₄TACD)M][BAr₄^Me], whereas [Cp*Al]₄ yielded trivalent [(Me₄TACD)AlH][BAr₄^Me]₂. Protonation of [(Me₄TACD)Ga][BAr₄^Me] with [Et₃NH][BAr₄^Me] gave an unusually acidic (pK_a(CH₃CN)=24.5) gallium(III) hydride dication [(Me₄TACD)GaH][BAr₄^Me]₂. Deprotonation with IMe₄ (1,3,4,5‐tetramethyl‐imidazol‐ylidene) returned [(Me₄TACD)Ga][BAr₄^Me]. These reversible processes occur with formal two‐electron oxidation and reduction of gallium. DFT calculations suggest that gallium(I) protonation is facilitated by strong coordination of the tetradentate ligand, which raises the HOMO energy. High nuclear charge of [(Me₄TACD)GaH]²⁺ facilitates hydride‐to‐metal charge transfer during deprotonation. Attempts to prepare a gallium(III) dihydride cation resulted in spontaneous dehydrogenation to [(Me₄TACD)Ga]⁺.

Recent Developments in the Chemistry of Pn(I) (Pn=N, P, As, Sb, Bi) Cations

The Group 15 Pn(I) cations (Pn=N, P, As, Sb and Bi), which are isoelectronic with the donor-stabilized carbones, have emerged recently. Despite the presence of two lone pair of electrons, the Pn(I) cations are weakly nucleophilic due to their inherent positive charge. Strongly electron-donating supporting ligands including zwitterionic forms have been used to enhance their Lewis basicity. Furthermore, the chelating effect of cyclic ligand systems proved effective in increasing their nucleophilicity. The strategies involved in successfully isolating the fleeting Sb(I) and Bi(I) cations as the recent most achievements in this field have been discussed. The syntheses, structure, bonding situations and reactivity of the Pn(I) cations are discussed. An outlook on the periodic trends and future applications of these electronically unique electron-rich cationic moieties have been provided.

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.

An NHC-Stabilised Phosphinidene for Catalytic Formylation: A DFT-Guided Approach

In recent years, the applications of low-valent main group compounds have gained momentum in the field of catalysis. Owing to the accessibility of two lone pairs of electrons, NHC-stabilised phosphinidenes have been found to be excellent Lewis bases; however, they cannot yet be used as catalysts. Herein, an NHC-stabilised phosphinidene, 1,3-dimethyl-2-(phenylphosphanylidene)-2,3-dihydro-1H imidazole (1), for the activation of CO₂ is reported.A closer inspection of the CO₂ activation process by DFT calculations along with intrinsic bond orbital analysis shows that phosphinidene is associated with phenylsilane through a noncovalent π-π interaction between two phenyl rings which activates the Si-H bond facilitating hydride transfer to the CO₂ molecule. Detailed DFT studies along with spectroscopic experiments were combined to understand the mechanism of CO₂ activation and its catalytic reductive functionalisation leading to the formylation of a range of chemically inert primary amides under mild reaction conditions.

Bonding and stability of donor ligand-supported heavier analogues of cyanogen halides (L')PSi(X)(L)

Fluoro- and chloro-phosphasilynes [X-Si[triple bond, length as m-dash]P (X = F, Cl)] belong to a class of illusive chemical species which are expected to have Si[triple bond, length as m-dash]P multiple bonds. Theoretical investigations of the bonding and stability of the corresponding Lewis base-stabilized species (L')PSi(X)(L) [L' = cAACMe (cyclic alkyl(amino) carbene); L = cAACMe, NHCMe (N-heterocyclic carbene), PMe3, aAAC (acyclic alkyl(amino) carbene); X = Cl, F] have been studied using the energy decomposition analysis-natural orbitals for chemical valence (EDA-NOCV) method. The variation of the ligands (L) on the Si-atom leads to different bonding scenarios depending on their σ-donation and π-back acceptance properties. The ligands with higher lying HOMOs prefer profoundly different bonding scenarios than the ligands with lower lying HOMOs. The type of halogen (Cl or F) on the Si-atom was also found to have a significant influence on the overall bonding scenario. The reasonably higher value and endergonic nature of the dissociation energies along with the appreciable HOMO-LUMO energy gap may corroborate to the synthetic viability of the homo and heteroleptic ligand-stabilized elusive PSi(Cl/F) species in the laboratory.

Synthesis and Reactivity of Heteroleptic Ga-P-C Allyl Cation Analogues

Oxidative addition of cyclic alkyl(amino)carbene-coordinated phosphinidenes (Me cAAC)PX to LGa affords gallium-coordinated phosphinidenes LGa(X)-P(Me cAAC) (L=HC[C(Me)N(2,6-i-Pr2 C6 H3 )]2 ; X=Cl 1, Br 2), which react with NaBArF 4 and LiAl(ORF )4 to [LGaP(Me cAAC)][An] (An=B(C6 H3 (CF3 )2 )4 3, B(C6 F5 )4 4, Al(OC(CF3 )3 )4 5). The cations in 3-5 show substantial Ga-P double bond character and represent heteronuclear analogues of allyl cations according to quantum chemical calculations. The reaction of 4 with 4-dimethylaminopyridine (dmap) to adduct 6 confirms the strong electrophilic nature of the gallium center, whereas 5 reacts with ethyl isocyanate with C-C bond formation to the γ-C atom of the β-diketiminate ligand and formation of compound 7.

Solid-State Isolation of Cyclic Alkyl(Amino) Carbene (cAAC)-Supported Structurally Diverse Alkali Metal-Phosphinidenides

Cyclic alkyl(amino) carbene (cAAC)-supported, structurally diverse alkali metal-phosphinidenides 2-5 of general formula ((cAAC)P-M)_n (THF)_x [2: M=K, n=2, x=4; 3: M=K, n=6, x=2; 4: M=K, n=4, x=4; 5: M=Na, n=3, x=1] have been synthesized by the reduction of cAAC-stabilized chloro-phosphinidene cAAC=P-Cl (1) utilizing metallic K or KC₈ and Na-naphthalenide as reducing agents. Complexes 2-5 have been structurally characterized in solid state by NMR studies and single crystal X-ray diffraction. The proposed mechanism for the electron transfer process has been well-supported by cyclic voltammetry (CV) studies and Density Functional Theory (DFT) calculations. The solid state oligomerization process has been observed to be largely dependent on the ionic radii of alkali metal ions, steric bulk of cAAC ligands and solvation/de-solvation/recombination of the dimeric unit [(cAAC)P-M(THF)_x ]₂ .

The Reactivity of Phosphanylphosphinidene Complexes of Transition Metals Toward Terminal Dihaloalkanes

The reactivities of phosphanylphosphinidene complexes [(DippN)₂W(Cl)(η²-P–PtBu₂)]⁻ (1), [(pTol₃P)₂Pt(η²-P=PtBu₂)] (2), and [(dppe)Pt(η²-P=PtBu₂)] (3) toward dihaloalkanes and methyl iodide were investigated. The reactions of the anionic tungsten complex (1) with stochiometric Br(CH₂)_nBr (n = 3, 4, 6) led to the formation of neutral complexes with a tBu₂PP(CH₂)₃Br ligand or neutral dinuclear complexes with unusual tetradentate tBu₂PP(CH₂)_nPPtBu₂ ligands (n = 4, 6). The methylation of platinum complexes 2 and 3 with MeI yielded neutral or cationic complexes bearing side-on coordinated tBu₂P–P-Me moieties. The reaction of 2 with I(CH₂)₂I gave a platinum complex with a tBu₂P—P—I ligand. When the same dihaloalkane was reacted with 3, the P—P bond in the phosphanylphosphinidene ligand was cleaved to yield tBu₂PI, phosphorus polymers, [(dppe)PtI₂] and C₂H₄. Furthermore, the reaction of 3 with Br(CH₂)₂Br yielded dinuclear complex bearing a tetraphosphorus tBu₂PPPPtBu₂ ligand in the coordination sphere of the platinum. The molecular structures of the isolated products were established in the solid state and in solution by single-crystal X-ray diffraction and NMR spectroscopy. DFT studies indicated that the polyphosphorus ligands in the obtained complexes possess structures similar to free phosphenium cations tBu₂P⁺=P−R (R = Me, I) or (tBu₂P⁺=P)₂.

The phosphanylphosphinidene complexes of tungsten and platinum as building blocks for syntheses of polydentate phosphorus ligands.

N-Heterocyclic Carbene Adducts of Main Group Elements and Their Use as Ligands in Transition Metal Chemistry

N-Heterocyclic carbenes (NHC) are nowadays ubiquitous and indispensable in many research fields, and it is not possible to imagine modern transition metal and main group element chemistry without the plethora of available NHCs with tailor-made electronic and steric properties. While their suitability to act as strong ligands toward transition metals has led to numerous applications of NHC complexes in homogeneous catalysis, their strong σ-donating and adaptable π-accepting abilities have also contributed to an impressive vitalization of main group chemistry with the isolation and characterization of NHC adducts of almost any element. Formally, NHC coordination to Lewis acids affords a transfer of nucleophilicity from the carbene carbon atom to the attached exocyclic moiety, and low-valent and low-coordinate adducts of the p-block elements with available lone pairs and/or polarized carbon-element π-bonds are able to act themselves as Lewis basic donor ligands toward transition metals. Accordingly, the availability of a large number of novel NHC adducts has not only produced new varieties of already existing ligand classes but has also allowed establishment of numerous complexes with unusual and often unprecedented element-metal bonds. This review aims at summarizing this development comprehensively and covers the usage of N-heterocyclic carbene adducts of the p-block elements as ligands in transition metal chemistry.

Synthesis of Cyclic Alkyl(amino) Carbene Stabilized Silylenes with Small N-Donating Substituents

Lewis base cAACs stabilized monomeric silylenes with halogen or methyl substituents at the silicon center have not been reported due to the strong σ-donor and π-acceptor character of cAAC. To prepare these monomeric silylenes, we used the silicon(IV) precursors 5 and 6 with a nitrogen donor group L (L=o-C₆ H₄ NMe₂ ). The cAAC-stabilized (cAAC=C(CH₂ )(CMe₂ )₂ N-Ar, Ar=2,6-iPr₂ C₆ H₃ ) silylenes LSiCl(cAAC) (7) and LSiMe(cAAC) (8) were synthesized by reduction of LSiCl₃ and LSiMeCl₂ with two equivalents of KC₈ in the presence of one equivalent of cAAC, respectively. Compounds 7 and 8 were characterized by single-crystal X-ray crystallography, NMR spectroscopy, and elemental analysis. Compounds 7 and 8 are stable in the solid state as well as in solution at room temperature for at least four months under inert conditions.

Donor Stabilized Diatomic Gr.14 E2 (E = C-Pb) Molecule D-E2-D (D = NHC, aNHC, NNHC, NHSi, NHGe, cAAC, cAASi, cAAGe): A Theoretical Insight

Quantum chemical calculations have been carried out to explore the detailed electronic structure and bonding scenario in various bis-donor stabilized E₂ compounds (E = C-Pb). Our computational findings reveal that the thermodynamic stabilities of the E₂ core gradually decrease as we move down the group. A linear D-E-E'-D framework is observed for C₂ systems, while the heavier group 14 analogues possess trans-bent geometries. Consideration of few compounds as viable targets for synthesis is suggested by their corresponding calculated formation energies. In addition, the thermodynamic stabilities of C₂ systems notably increase with the saturation of the donor ring framework and are even more pronounced for boron-substituted saturated NHD ligand. QTAIM calculations affirmed that the covalent nature of E-E' bonds shifts toward the donor-acceptor region as one traverses from top to bottom along group 14. The E-D and E'-D bonds in the C₂ systems have covalent nature, whereas those in Si₂-Pb₂ systems are characterized by donor-acceptor bonds. In addition, we have computed proton affinities and vertical ionization potentials (VIPs) of these compounds. An excellent correlation was obtained between calculated VIPs and orbital energies of HOMOs. Furthermore, in the present study, we also explored the effect of bis-donors in the stabilization of heterodiatomic SiC compounds. Our calculations indicate that a typical bonding description of the SiC(D)₂ compounds should be represented by a combination of a classical double bond between C-D with significant donor-acceptor interaction in Si-D, i.e., D → Si═C═D. The SiC(D)₂ systems are found to be less stable than the corresponding dicarbon compounds C₂(D)₂, but they show significant stabilization compared to the corresponding disilicon systems Si₂(D)₂.

Comparison of Two Phosphinidenes Binding to Silicon(IV)dichloride as well as to Silylene

The cyclic alkyl(amino) carbene (cAAC) anchored silylene with two phosphinidenes was isolated as (cAAC)Si{P(cAAC)}₂ (3) at room temperature, which was synthesized from the reduction of (Cl₂)Si{P(cAAC)}₂ (2) using 2 equiv of KC₈. Compound 2 resulted from the reaction of 2 equiv of (cAAC)PK (1) with 1 equiv of SiCl₄. Compounds 2 and 3 are the first examples where two terminal phosphinidenes are binding each to a silicon center characterized by single crystal X-ray structural analysis. Furthermore, the structure and bonding of compounds 2 and 3 have been investigated by theoretical methods for comparison.

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.