Computational design of cooperatively acting molecular catalyst systems: carbene based tungsten- or molybdenum-catalysts with rhodium- or iridium-complexes for the ionic hydrogenation of N2 to NH3.

This density functional theory (DFT) study explores the efficacy of cooperative catalytic systems in enabling the ionic hydrogenation of N2 with H2, leading to NH3 formation. A set of N-heterocyclic carbene-based pincer tungsten/molybdenum metal complexes of the form [(PCP)M1(H)2] (M1 = W/Mo) were chosen to bind N2 at the respective metal centres. Simultaneously, cationic rhodium/iridium complexes of type [Cp*M2{2-(2-pyridyl)phenyl}(CH3CN)]+ (Cp* = C5(CH3)5 and M2 = Rh/Ir), are employed as cooperative coordination partners for heterolytic H2 splitting. The stepwise transfer of protons and hydrides to the bound N2 and intermediate NxHy units results in the formation of NH3. Interestingly, the calculated results reveal an encouraging low range of energy spans ranging from ∼30 to 42 kcal mol-1 depending on different combinations of ligands and metal complexes. The optimal combination of pincer ligand and metal center allowed for an energy span of unprecedented 29.7 kcal mol-1 demonstrating significant potential for molecular catalysts for the N2/H2 reaction system. While exploring obvious potential off-cycle reactions leading to catalyst deactivation, the computed results indicate that no increase in energy span would need to be expected.

Small molecule activation by sila/germa boryne species.

The inability of p-block elements to participate in π-backbonding restricts them from activating small molecules like CO, H2 , and so forth. However, the development of the main group metallomimetics became a new pathway, where the main-group elements like boron can bind and activate small molecules like CO and H2 . The concept of the frustrated Lewis pair, Boron-Boron multiple bonds, and borylene are previously illustrated. Some of these reported classes of boron species can mimic the jobs of the metal complexes. Hence, we have theoretically studied the binding of CO/N2 molecules at B-center of elusive species like sila/germa boryne stabilized by donor base ligands (cAAC)BE(Me)(L), where E  Si, L  cAACMe , NHCMe , PMe3 , E  Ge, L  cAACMe and (NHCMe )BE(Me)(cAACMe )). The substitutional analogues of (cAACR )BSiR1 (cAAC) and E  P, L  cAACMe ) have been studied by density functional theory (DFT), natural bond orbital, QTAIM calculations and energy decomposition analysis (EDA) coupled with natural orbital for chemical valence (NOCV) analyses. The computed bond dissociation energy and inner stability analyses by the EDA-NOCV method showed that the CO molecule can bind at the B-center of the above-mentioned species due to stronger σ-donor ability while binding of N2 has been theoretically predicted to be weak. The energy barrier for the CO binding is estimated to be 13-14 kcal/mol by transition state calculation. The change of partial triple bond character to single bond nature of the BSi bond and the bending of CBSi bond angle of sila-boryne species are the reason for the activation energy. Our study reveals the ability of such species to bind and activate the CO molecule to mimic the transition metal-containing complexes. We have additionally shown that binding of Fe(CO)4 and Ni(CO)3 is feasible at Si-center after binding of CO at the B-center.

Enhancing the catalytic OER performance of MoS2via Fe and Co doping.

Enhancing the sluggish kinetics of the electrochemical oxygen evolution reaction (OER) is crucial for many clean-energy production technologies. Although much progress has been made in recent years, developing active, stable, and cost-effective OER electrocatalysts is still challenging. The layered MoS2, based on Earth-abundant elements, is widely explored as a promising hydrogen evolution electrocatalyst but exhibits poor OER activity. Here, we report a facile strategy to improve the sluggish OER of MoS2 through co-doping MoS2 nanosheets with Fe and Co atoms. The synergistic effect obtained by adjusting the Co/Fe ratio in the Fe-Co doped MoS2 induces electronic and structural modifications and a richer active surface area morphology resulting in a relatively low OER overpotential of 380 mV (at 10 mA cm-2). The electronic modulation upon doping was further supported by DFT calculations that show favorable interaction with the OER intermediate species, thus reducing the energy barrier for the OER. This work paves the way for future strategies for tailoring the electronic properties of transition-metal dichalcogenides (TMDCs) to activate the structure for the sluggish OER with the assistance of non-noble-metal materials.

Modulating the radical reactivity of phenyl radicals with the help of distonic charges: it is all about electrostatic catalysis.

This manuscript reports the modulation of H-abstraction reactivity of phenyl radicals by (positive and negative) distonic ions. Specifically, we focus on the origins of this modulating effect: can the charged functional groups truly be described as "extreme forms of electron-withdrawing/donating substituents" - implying a through-bond mechanism - as argued in the literature, or is the modulation mainly caused by through-space effects? Our analysis indicates that the effect of the remote charges can be mimicked almost perfectly with the help of a purely electrostatic treatment, i.e. replacing the charged functional groups by a simple uniform electric field is sufficient to recover the quantitative reactivity trends. Hence, through-space effects dominate, whereas through-bond effects play a minor role at best. We elucidate our results through a careful Valence Bond (VB) analysis and demonstrate that such a qualitative analysis not only reveals through-space dominance, but also demonstrates a remarkable reversal in the reactivity trends of a given polarity upon a rational modification of the reaction partner. As such, our findings demonstrate that VB theory can lead to productive predictions about the behaviour of distonic radical ions.

Interplay of Hückel and Möbius Aromaticity in Metal-Metal Quintuple Bonded Complexes of Cr, Mo, and W with Amidinate Ligand: Ab†initio DFT and Multireference Analysis*.

The aromaticity of metal-metal quintuple bonded complexes of the type M2 L2 (M=Cr, Mo, and W; L=amidinate) are studied employing gauge including magnetically induced ring current (GIMIC) analysis and electron density of delocalized bonds (EDDB). It is found that the complexes possess two types of aromaticity: i) Hückel aromaticity through delocalization of ligand π electrons with metal-metal δ-bond-forming 6 conjugated electrons (4π and 2δ) ring; ii) Craig-Möbius aromaticity through delocalization of π electrons of both the ligands with metal d-orbitals in Craig type orientation forming 10π electrons ring with a double twist. Extended transition state natural orbital chemical valence (ETS-NOCV) and canonical molecular orbital natural chemical shielding (CMO-NCS) analysis confirm the Craig-Möbius type arrangement of the orbitals. Furthermore, the unprecedented Hückel and Möbius type aromaticity is confirmed from the plot of the current pathways using 3D line integral convolution (3D-LIC) plots. The metal-metal bond order also increases down the group as justified from the complete active space self-consistent field (CASSCF) analysis. Due to an increase in the π and δ electron conjugation, both the Hückel and Möbius aromaticity increase down the group.

On the Differences in the Mechanisms of Reduction of AuCl2- and Ag(H2O)2+ with BH4.

The mechanism of reduction of AuCl4-/AuCl3OH- by BH4- was analyzed by density functional theory (DFT). The results point out that Auatoms0 are not intermediates in the process. The derived mechanism differs considerably from that reported for the analogous process involving the reduction of Ag(H2O)2+ by BH4-. Thus, though both processes follow the Creighton procedure, the detailed mechanism differs significantly. For Au, the agglomeration starts with AuH2-, whereas for Ag, it starts with (H2O)AgH. Stopped-flow measurements support the complicated mechanism.

On the mechanism of reduction of M(H2O)mn+ by borohydride: the case of Ag(H2O)2.

The redox potentials of M(H2O)mn+/M0(atom) couples are often far too negative to enable the formation of M0(atom) by most reducing agents. Therefore, one has to reconsider the mechanism of formation of M0-NPs by the bottom-up procedure. A deep and detailed theoretical analysis of the reduction of Ag(H2O)2+ by BH4- points out that silver cations act mainly as catalysts of the reactions BH4- + 4H2O → B(OH)4- + 4H2. However, the transition states of the catalyzed process differ from those of the un-catalyzed process. The formation of (H2O)Ag-H, which is the starting stage for the formation of intermediates with Ag-Ag bonds, is only a side reaction in the process. Experimental evidence of the complexity of the process is presented, by stopped-flow; at least four processes are observed prior to the formation of Ag0-NPs. The spectra of these intermediates differ from those of Ag0atom and Ag2+aq. Though DFT calculations were performed only for silver cations, it is believed that analogous mechanisms are involved in the reductions of other cations.

Computational Exploration of Mechanistic Avenues in C-H Activation Assisted Pd-Catalyzed Carbonylative Coupling.

The detailed mechanism of the intermolecular Pd-catalyzed carbonylative coupling reaction between aryl bromides and polyfluoroarenes relying on C(sp2)-H activation was investigated using state-of-the-art computational methods (SMD-B3LYP-D3(BJ)/BS2//B3LYP-D3/BS1). The mechanism unveils the necessary and important roles of a slight excess of carbon monoxide: acting as a ligand in the active catalyst state, participating as a reactant in the carbonylation process, and accelerating the final reductive elimination event. Importantly, the desired carbonylative coupling route follows the rate-limiting C-H activation process via the concerted metalation-deprotonation pathway, which is slightly more feasible than the decarboxylative route leading to byproduct formation by 1.2 kcal/mol. The analyses of the free energies indicate that the choice of base has a significant effect on the reaction mechanism and its energetics. The Cs2CO3 base guides the reaction toward the coupling route, whereas carbonate bases such as K2CO3 and Na2CO3 switch toward an undesired decarboxylative path. However, K3PO4 significantly reduces the C-H activation barrier over the decarboxylation reaction barrier and can act as a potential alternative base. The positional influence of a methoxy substituent in bromoanisole and different substituent effects in polyfluoroarenes were also considered. Our results show that different substituents impose significant impact on the desired carbonylative product formation energetics. Considering the influence of several ligands leads to the conclusion that other phosphine and N-heterocyclic carbene, such as P nBuAd2 and IMes, can be used as an efficient alternative than the experimentally reported P tBu3 ligand exhibiting a clear preference for C-H activation (ΔΔ⧧ GLS) by 7.1 and 10.9 kcal/mol, respectively. We have also utilized the energetic span model to interpret the experimental results. Moreover, to elucidate the origin of activation barriers, energy decomposition analysis calculations were accomplished for the critical transition states populating the energy profiles.

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.

Mechanistic Exploration of the Transmetalation and Reductive Elimination Events Involving PdIV -Abnormal NHC Complexes in Suzuki-Miyaura Coupling Reactions: A DFT Study.

A comprehensive DFT (M06-L-D3(SMD)/BS2//M06-L/BS1 level) investigation has been carried out to explore in detail the mechanism of the transmetalation and reductive elimination reactions of abnormal N-heterocyclic carbene (aNHC) palladium(IV) complexes within the framework of Suzuki-Miyaura cross-coupling reactions. Emphasis was placed on the role of base and the effect of countercations on the critical transmetalation and reductive elimination events involving palladium(IV) complexes. Of the two competing roles of the base, the route involving boronate formation followed by halide exchange prevails over that of direct halide exchange for the intermediates [PdIV (aNHC)(OMe)2 Cl]- Na+ (pathway A), [PdIV (aNHC)(OMe)(Cl)2 ]- Na+ (pathway B), and [PdIV (aNHC)Cl3 ]- Na+ (pathway C) emanating from the oxidative addition reaction. The results of the calculations are in accordance with our previous theoretical findings of favorable energetics for palladium intermediates incorporating two coordinated methoxy groups. The negative role played by the countercation in the transmetalation step is mainly due to the overstabilization of the pre-transmetalation intermediate, which is in line with experimental kinetic results. The anionic complexes exhibit greater affinity for the transmetalation and reductive elimination reactions than the neutral variants.

Reagent for Introducing Base-Stabilized Phosphorus Atoms into Organic and Inorganic Compounds.

The cyclic alkyl(amino) carbene (cAAC) stabilized monoanionic phosphorus atom in the form of lithium phosphinidene [cAACPLi(THF)2]2 (1) has been isolated as a molecular species and characterized by single crystal X-ray structure analysis. Furthermore, the structure and bonding of compound 1 has been investigated by theoretical methods. The utilization of the lithium phosphinidene as a phosphorus transfer reagent for a wide range of organic and inorganic substrates has been investigated. Herein, we report on the preparation of fascinating compounds containing P-C, P-Si, P-Ge, and P-P bonds using a single step with a base-stabilized phosphorus atom.

Synthesis and characterization of Lewis base stabilized mono- and di-organo aluminum radicals.

Two cyclic (alkyl)(amino)carbene (cAAC) stabilized mononuclear neutral radicals of aluminum have been synthesized. They contain an ethyl [(cAAC)2AlClEt (1)] and as well a diethyl group [(cAAC)2AlEt2 (2)], and have been prepared from the reduction of EtAlCl2 and Et2AlCl, respectively, with KC8. Compounds 1 and 2 are monoradicals, which were confirmed by EPR measurements to have the spin located on the carbene carbon of one of the cAAC ligands.

DFT Study on C-F Bond Activation by Group 14 Dialkylamino Metalylenes: A Competition between Oxidative Additions versus Substitution Reactions.

The C-F bond activation of pentafluoropyridine (PFP) by group 14 dialkylamino metalylenes has been studied employing DFT calculations. Emphasis is placed on the group 14 central atom (M = SiII, GeII, and SnII) and substituents (-NMe2, -NiPr2, -Cl, -NH2, and -PH2) dependent switching of oxidative addition to the metathesis/substitution reaction route, using state-of-the-art theoretical methods (M062X/def2-QZVP(SMD)//M062X/def2-TZVP) to provide a systematic classification of the individual mode of reactions. Moreover, an energy decomposition analysis (EDA) is implemented to get a brief insight into the physical factors that control the activation barriers originating via the different mode of reactions, viz., oxidative addition and metathesis routes. The key finding is that the distortion of PFP is the principal guiding factor in the oxidative addition reaction, while distortions imposed on both the PFP and metalylenes are inevitable toward the origin of the metathesis reaction barrier. The preferable oxidative addition reaction over metathesis of substituted silylenes can be explained on the basis of electron concentration and the HOMO-LUMO gap between the reacting substrates. However, the dramatic switch between oxidative addition and metathesis reaction in substituted germylenes depends on both the electronic and steric nature of the substituents. Similar observations are also noted for the reactivity of substituted stannylenes.

Diverse Bonding Activations in the Reactivity of a Pentaphenylborole toward Sodium Phosphaethynolate: Heterocycle Synthesis and Mechanistic Studies.

The reaction of the pentaphenylborole [(PhC)4BPh] (1) with sodium phosphaethynolate·1,4-dioxane (NaOCP(1,4-dioxane)1.7) afforded the novel sodium salt of phosphaboraheterocycle 2. It comprises anionic fused tetracyclic P/B-heterocycles that arise from multiple bond activation between the borole backbone and [OCP]-anion. Density functional theory calculations indicate that the [OCP]- anion prefers the form of phosphaethynolate -O-C≡P over phosphaketenide O═C═P- to interact with two molecules of 1, along with various B-C, C-P, and C-C bond activations to form 2. The calculations were verified by experimental studies: (i) the reaction of 1 with NaOCP(1,4-dioxane)1.7 and a Lewis base such as the N-heterocyclic carbene IAr [:C{N(Ar)CH}2] (Ar = 2,6-iPr2C6H3) and amidinato amidosilylene [{PhC(NtBu)2}(Me2N)Si:] afforded the Lewis base-pentaphenylborole adducts [(PhC)4B(Ph)(LB)] (LB = IAr (3), :Si(NMe2){(NtBu)2CPh} (4)), respectively; (ii) the reaction of 1 with the carbodiimide ArN═C═NAr afforded the seven-membered B/N heterocycle [B(Ph) (CPh)4C(═NAr)N(Ar)] (5). Compounds 2-5 were fully characterized by NMR spectroscopy and X-ray crystallography.

A phenoxo-bridged dicopper(ii) complex as a model for phosphatase activity: mechanistic insights from a combined experimental and computational study.

A µ-phenoxo-bis(µ2-1,3-acetato)-bridged dicopper(ii) complex [Cu(L1)(µ-O2CMe)2][NO3] (1) has been synthesized from the perspective of modeling phosphodiesterase activity. Structural characterization was done initially with 1·3Et2O (vapour diffusion of Et2O into MeOH solution of 1; poor crystal quality) and finally with its perchlorate salt [Cu(L1)(µ-O2CMe)2][ClO4]·1.375MeCN·0.25H2O, crystallized from vapour diffusion of n-pentane into a MeCN-MeOH mixture (comparatively better crystal quality). An asymmetric unit of such a crystal contains two independent molecules of compositions [Cu(L1)(µ-O2CMe)2][ClO4] and [Cu(L1)(µ-O2CMe)2(MeCN)][ClO4] (coordinated MeCN with 0.75 occupancy), and two molecules of MeCN and H2O (each H2O molecule with 0.25 occupancy) as the solvent of crystallization. These two cations, each having five-coordinate (µ-phenoxo)bis(µ-acetato)-bridged CuII ions, differ by only the coordination environment of only one CuII ion, which has a weakly coordinated acetonitrile molecule in its sixth position. Temperature-dependent magnetic studies on 1 reveal that the copper(ii) centres are antiferromagnetically coupled with the exchange-coupling constant J = -124(1) cm-1. Theoretically calculated J = -126.51 cm-1, employing a broken-symmetry DFT approach, is in excellent agreement with the experimental value. The dicopper(ii) complex has been found to be catalytically efficient in the hydrolysis of 2-hydroxypropyl-p-nitrophenylphosphate (HPNP). Detailed kinetic experiments and solution studies (potentiometry, species distribution and ESI-MS) were performed to elucidate the reaction mechanism. DFT calculations were performed to discriminate between different possible mechanistic pathways. The free-energy barrier for HPNP hydrolysis catalyzed by 1 is comparable to that obtained from the experimentally-determined value. The involvement of non-covalent (hydrogen-bonding) interaction has also been probed by DFT calculations. The activity of 1 is found to be the highest, compared to the structurally-characterized Mn, Co, Ni and Zn complexes of L1(-) reported earlier, under identical experimental conditions, in which each metal centre is six-coordinate.

Exploring the Oxidative-Addition Pathways of Phenyl Chloride in the Presence of PdII Abnormal N-Heterocyclic Carbene Complexes: A DFT Study.

DFT calculations were performed to elucidate the oxidative addition mechanism of the dimeric palladium(II) abnormal N-heterocyclic carbene complex 2 in the presence of phenyl chloride and NaOMe base under the framework of a Suzuki-Miyaura cross-coupling reaction. Pre-catalyst 2 undergoes facile, NaOMe-assisted dissociation, which led to monomeric palladium(II) species 5, 6, and 7, each of them independently capable of initiating oxidative addition reactions with PhCl. Thereafter, three different mechanistic routes, path a, path b, and path c, which originate from the catalytic species 5, 7, and 6, were calculated at M06-L-D3(SMD)/LANL2TZ(f)(Pd)/6-311++G**//M06-L/LANL2DZ(Pd)/6-31+G* level of theory. All studied routes suggested the rather uncommon PdII /PdIV oxidative addition mechanism to be favourable under the ambient reaction conditions. Although the Pd0 /PdII routes are generally facile, the final reductive elimination step from the catalytic complexes were energetically formidable. The PdII /PdIV activation barriers were calculated to be 11.3, 9.0, 26.7 kcal mol-1 (ΔΔ≠ GLS-D3 ) more favourable than the PdII /Pd0 reductive elimination routes for path a, path b, and path c, respectively. Out of all the studied pathways, path a was the most feasible as it comprised of a PdII /PdIV activation barrier of 24.5 kcal mol-1 (ΔGLS-D3 ). To further elucidate the origin of transition-state barriers, EDA calculations were performed for some key saddle points populating the energy profiles.

Activation of Elemental Sulfur at a Two-Coordinate Platinum(0) Center.

Platinum dichalcogenides have been known to exhibit two-dimensional layered structures. Herein, we describe the syntheses, isolation, and characterization of air-stable crystalline cyclic alkyl(amino) carbene (cAAC)-supported monomeric platinum disulfide three-membered ring complex [(cAAC)2 Pt(S2 )] (2). The highly reactive platinum(0) [(cAAC)2 Pt] complex (1) with two-coordinate platinum activates elemental sulfur to give 2. The brown crystals of bis-carbene platinum(II)monosulfate [(cAAC)2 Pt(SO4 )x (S2 )1-x ] (4) have been isolated when the reaction was performed in air. The dioxygen analogue of 2 was formed upon exposing the THF solution of 1 to aerial oxygen (O2 ). The binding of oxygen at the Pt(0) center was found to be reversible. Additionally, DFT study has been performed to elucidate the electronic structure and bonding scenario of 2, 3, and 4. Quantum chemical calculations showed donor-acceptor-type interaction for the Pt-S bonds in 2 and Pt-O bonds in 3 and 4.

Addition Reactions of Me3 SiCN with Aldehydes Catalyzed by Aluminum Complexes Containing in their Coordination Sphere O, S, and N Ligands.

The reaction of one equivalent of LAlH2 (1; L=HC(CMeNAr)2 , Ar=2,6-iPr2 C6 H3 , ß-diketiminate ligand) with two equivalents of 2-mercapto-4,6-dimethylpyrimidine hydrate resulted in LAl[(µ-S)(m-C4 N2 H)(CH2 )2 ]2 (2) in good yield. Similarly, when N-2-pyridylsalicylideneamine, N-(2,6-diisopropylphenyl)salicylaldimine, and ethyl 3-amino-4,5,6,7-tetrahydrobenzo[b]thiophene-2-carboxylate were used as starting materials, the corresponding products LAl[(µ-O)(o-C6 H4 )CN(C5 NH4 )]2 (3), LAlH[(µ-O)(o-C4 H4 )CN(2,6-iPr2 C6 H3 )] (4), and LAl[(µ-NH)(o-C8 SH8 )(COOC2 H5 )]2 (5) were isolated. Compounds 2-5 were characterized by (1) H and (13) C NMR spectroscopy as well as by single-crystal X-ray structural analysis. Surprisingly, compounds 2-5 exhibit good catalytic activity in addition reactions of aldehydes with trimethylsilyl cyanide (TMSCN).

Monomeric siliconthiodichloride trapped by a Lewis base.

Thiophosgene (CSCl2), a chemical reagent used in numerous organic syntheses, exists in the monomeric form while its heavier silicon analogue [siliconthiodichloride (SiSCl2)] has been isolated so far as a dimer at room temperature and as a tetramer at 180 °C. Herein, we report on the first synthesis, isolation, and characterization of cyclic alkyl(amino) carbene (cAAC) stabilized siliconthiodichloride (cAAC)SiSCl2 (3) in the neutral monomeric form. 3 is synthesized via reaction of (cAAC˙)2Si2Cl4 (1) or (cAAC)2Si2Cl2 (2) with S8 in the temperature range of -78 to 20 °C. An NHC [NHC = N-heterocyclic carbene] analogue of 3 is not isolated when (NHC)SiCl2 is reacted with S8. The bright yellow colored compound 3 is soluble in polar organic solvents. It is stable at room temperature for a month under an inert atmosphere. 3 decomposes above 160 °C. The monomeric molecular structure of 3 has been unambiguously confirmed by X-ray single crystal diffraction. 3 is also characterized by NMR, UV-vis, and IR spectroscopy. The bonding and electron density distributions of 3 have been further studied by theoretical calculations.

Carbene supported dimer of heavier ketenimine analogue with p and si atoms.

A cyclic alkyl(amino) carbene (cAAC) stabilized dimer [(cAAC)Si(P-Tip)]2 (2) (Tip = 2,4,6-triisopropylphenyl) is reported. 2 can be considered as a dimer of the heavier ketenimine (R2C═C═N-R) analogue. The dark-red rod-shaped crystals of 2 were synthesized by reduction of the precursor, cAAC-dichlorosilylene-stabilized phosphinidene (cAAC)SiCl2→P-Tip with sodium napthalenide. The crystals of 2 are storable at room temperature for several months and stable up to 215 °C under an inert atmosphere. X-ray single-crystal diffraction revealed that 2 contains a cyclic nonplanar four-membered SiPSiP ring. Magnetic susceptibility measurements confirmed the singlet spin ground state of 2. Cyclic voltammetry of 2 showed a quasi-reversible one-electron reduction indicating the formation of the corresponding radical anion 2(•-), which was further characterized by EPR measurements in solution. The electronic structure and bonding of 2 and 2(•-) were studied by theoretical calculations. The experimentally obtained data are in good agreement with the calculated values.