Synthesis,Structure, and Reactivities of Iminosulfane‐ and Phosphane‐Stabilized Carbones Exhibiting Four‐Electron Donor Ability

Iminosulfane(phosphane)carbon(0) derivatives (iSPCs; Ar₃P→C←SPh₂(NMe); Ar=Ph ( 1 ), 4‐MeOC₆H₄ ( 2 ), 4‐(Me₂N)C₆H₄ ( 3 )) have been successfully synthesized and the molecular structure of 3 characterized. Carbone 3 is the first thermally and hydrolytically stable carbone stabilized by phosphorus and sulfur ligands. DFT calculations reveal the electronic structures of 1 – 3 , which have two lone pairs of electrons at the carbon center. First and second proton affinity values are theoretically calculated to be in the range of 286.8–301.1 and 189.6–208.3 kcal mol^?1, respectively. Cyclic voltammetry measurements reveal that the HOMO energy levels follow the order of 3 > 2 > 1 and the HOMO of 3 is at a higher energy than those of bis(chalcogenane)carbon(0) (BChCs). The reactivities of these lone pairs of electrons are demonstrated by the C‐diaurated and C‐proton‐aurated complexes. These results are the first experimental evidence of phosphorus‐ and sulfur‐stabilized carbones behaving as four‐electron donors. In addition, the reaction of hydrochloric salts of the carbones with Ag₂O gives the corresponding Ag^I complexes. The resulting silver(I) carbone complexes can be used as carbone transfer agents. This synthetic protocol can also be used for moisture‐sensitive carbone species.     

Design,Synthesis, and Structural Analysis of Divalent NI Compounds and Identification of a New Electron‐Donating Ligand

The dative‐bond representation (L→E) in compounds with main group elements (E) has triggered extensive debate in the recent past. The scope and limits of this nonclassical coordination bond warrant comprehensive exploration. Particularly compounds with (L→N←L′)⁺ arrangement are of special interest because of their therapeutic importance. This work reports the design and synthesis of novel chemical species with the general structural formula (L→N←L′)⁺ carrying the unusual ligand cyclohexa‐2,5‐diene‐4‐(diaminomethynyl)‐1‐ylidene. Four species belonging to the (L→N←L′)⁺ class carrying this unconventional ligand were synthesized. Quantum chemical and X‐ray diffraction analyses showed that the electronic and geometric parameters are consistent with those of already reported divalent N^I compounds. The molecular orbital analysis, geometric parameters, and spectral data clearly support the L→N and N←L′ interactions in these species. The newly identified ligand has the properties of a reactive carbene and high nucleophilicity.     

Syntheses,Structures, and Reactivities of Two Chalcogen‐Stabilized Carbones

Electronic effects on the central carbon atom of carbone, generated by the replacement of the S^IV ligand of carbodisulfane (CDS) with other chalcogen ligands (Ph₂E, E=S or Se), were investigated. The carbones Ph₂E→C←SPh₂(NMe) [E=S( 1 ) or Se( 2 )] were synthesized from the corresponding salts, and their molecular structures and electronic properties were characterized. The carbone 2 is the first carbone containing selenium as the coordinated atom. DFT calculations revealed the electronic structures of 1 and 2 , which have two lone pairs of electrons at the carbon center. The trend in HOMO energy levels, estimated by cyclic voltammetry measurements, for the carbones and CDS follows the order of 2 > 1 >CDS. Analysis of a doubly protonated dication and trication complex revealed that the central carbon atom of 2 behaves as a four‐electron donor.     

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.     

Stabilization of cyclic and acyclic carbon(0) compounds by differential coordination of heterocyclic carbenes: a theoretical assessment

Recently, donor stabilized divalent carbon(0) compounds have undergone intense experimental and theoretical investigation due to their strong electron rich character. In this Article, some new cyclic and acyclic carbon(0) compounds stabilized by differential coordination modes (such as abnormal, remote and a mixture of both) of N-heterocyclic carbenes are studied theoretically. The cyclic carbon(0) compounds proposed in this study are unusual in the sense that they contain a five membered ring consisting of only carbon atoms with a central carbon atom in the formal oxidation state of zero. All these compounds are found to be very strong nucleophiles which might have wide implications in catalysis. Calculation of first proton affinities of these molecules reveal that they are better σ donors than the carbon(0) compound supported by normal N-heterocyclic carbenes. Quantum chemical calculations indicate that these molecules possess very high donor-acceptor L → C bond strengths and are thermodynamically stable. Calculation of the bond dissociation energies for the complexation of one and two molecules of AuCl indicates the possible isolation of their gem dimetalated derivatives.     

Carbodiphosphorane analogues E(PPh3)2 with E=C-Pb: a theoretical study with implications for ligand design

Quantum-chemical calculations at the BP86/TZVPP level have been carried out for the heavy Group 14 homologues of carbodiphosphorane E(PPh(3))(2), where E=Si, Ge, Sn, Pb, which are experimentally unknown so far. The results of the theoretical investigation suggest that the tetrelediphosphoranes E(PPh(3))(2) (1E) are stable compounds that could become isolated in a condensed phase. The molecules possess donor-acceptor bonds Ph(3)P→E←PPh(3) to a bare tetrele atom E, which retains its four valence electrons as two electron lone pairs. The analysis of the bonding situation and the calculation of the chemical reactivity indicate that the molecules 1E belong to the class of divalent E(0) compounds (ylidones). All molecules 1C-1Pb have very large first but also very large second proton affinities, which distinguishes them from the N-heterocyclic carbene homologues, in which the donor atom is a divalent E(II) species that possesses only one electron lone pair. Compounds 1E are powerful double donors that strongly bind Lewis acids such as BH(3) and AuCl in the complexes 1E(BH(3))(n) and 1E(AuCl)(n) (n=1, 2). The bond dissociation energies (BDEs) of the second BH(3) and AuCl molecules are only slightly less than the BDE of the first BH(3) and AuCl. The results of this work are a challenge for experimentalists.     

Syntheses,Structures, and Reactivities of Two Chalcogen‐Stabilized Carbones

Electronic effects on the central carbon atom of carbone, generated by the replacement of the S^IV ligand of carbodisulfane (CDS) with other chalcogen ligands (Ph₂E, E=S or Se), were investigated. The carbones Ph₂E→C←SPh₂(NMe) [E=S( 1 ) or Se( 2 )] were synthesized from the corresponding salts, and their molecular structures and electronic properties were characterized. The carbone 2 is the first carbone containing selenium as the coordinated atom. DFT calculations revealed the electronic structures of 1 and 2 , which have two lone pairs of electrons at the carbon center. The trend in HOMO energy levels, estimated by cyclic voltammetry measurements, for the carbones and CDS follows the order of 2 > 1 >CDS. Analysis of a doubly protonated dication and trication complex revealed that the central carbon atom of 2 behaves as a four‐electron donor.     

Bonding in titanocenyl complexes containing O,O′‐cyclic ligands

Density functional theory calculations show that the formal 16‐electron count of d⁰ [Cp₂Ti^IV(O,O′‐BID)]^0/1 complexes containing a O,O′‐chelated bidentate ligand O,O′‐BID of different ring size, is increased via Ti←O π bonding when both the O donor atoms carry a formal negative charge. The Ti←O π bonding occurs by symmetry lowering of the complex by either symmetrical (C_s) or unsymmetrical (C₂) folding of the O,O′‐BID ligand round the O···O axis. An NBO analysis confirms the Ti←O π charge transfer via back‐bonding. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Existence of dynamic tautomerism and divalent N(I) character in N‐(pyridin‐2‐yl)thiazol‐2‐amine

N‐(Pyridin‐2‐yl)thiazol‐2‐amine is a versatile chemical functional unit present in many therapeutically important species. Quantum chemical analysis shows that there are six competitive isomeric structures possible for this class of compounds within a relative energy difference of ～4 kcal/mol. Some of the isomeric structures possess divalent N(I) character. There appears to be a competition between the thiazole and pyridine groups to accommodate the tautomeric hydrogen, and consequently show electron donating property in the structure with R‐N←L representation. Details of electron distribution, tautomeric preferences, protonation energy, and divalent N(I) character, and so on, of this class of compounds are presented in this article. Subsequently, upon protonation, (L→N←L)^⊕ character is clearly evident in these moieties as molecular orbital analysis clearly shows two lone pairs of electrons on the central nitrogen, in this system. © 2013 Wiley Periodicals, Inc.     

Donor→acceptor coordination interactions in 1,3-bis(NHC)triazenyl Cations: An electronic structure analysis

Donor→acceptor coordination interactions (L → N) between ligands and nitrogen center as in L → N^⊕ ← L were reported in the recent past. This article describes the possibility of L → N coordination interactions in triazenyl cation species L → N₃^⊕ ← L. A few 1,3-bis(NHC)triazenyl cation species were experimentally known, the electronic structure analysis reported in this work reveals the presence of L → N (donor→acceptor) interactions in these species. Molecular orbital analysis, NBO charge analysis, energy decomposition analysis, and so forth, confirm the possibility of L → N coordination bond character. Ten molecules with the general formula L → N₃^⊕ ← L have been designed carrying L → N₃^⊕ ← L interactions. © 2019 Wiley Periodicals, Inc.     

The Search for Enhanced σ-Donor Ligands to Stabilize Boron-Boron Multiple Bonds

Boron-boron multiple bonds, such as those found in diborenes and diborynes, are typically stabilized by σ-donor ligands that furnish electron density to these otherwise electron-deficient species. These compounds are not only of fundamental importance in the study of chemical bonding, but can also activate small molecules in a chemistry reminiscent of that carried out by transition metals. In the pursuit of designing new and improved σ-donor ligands to stabilize diborenes and diborynes suitable to activate small molecules, we performed density functional calculations to evaluate the Lewis basicity of a series of σ-donor ligands. For this evaluation, we analysed the interaction between the boranes and the σ-donor ligands in model systems L→BX₃ (X=F and Me) using energy decomposition analyses. We found that electronic bond energies of the L→BX₃ adducts correlate well with the ionization energies of the ligands and that ligands with high or medium basicity stabilize diborynes better than ligands with low basicity. We also learnt that beryllium-based ligands are promising since they are able to stabilize L→B≡B←L diborynes without significantly reducing the triple bond character of the B≡B bond.     

Divalent NI Compounds: Identifying new Carbocyclic Carbenes to Design Nitreones using Quantum Chemical Methods

Nitreones are compounds with oxidation state 1 at the nitrogen, these compounds carry formal positive charge as well as two lone pairs of electrons at nitrogen center. These compounds are also known as divalent N^I compounds and can be represented with the general formula L → N⁺ ← L, where L is an electron donating ligand. In the recent past, several divalent N^I compounds have been reported with L = N-heterocyclic carbene (NHC), remote N-heterocyclic carbene (rNHC), carbocyclic carbene (CCC) and diaminocarbene. Recently, our group reported that a novel six-membered CCC (cyclohexa-2,5-diene-4-[diaminomethynyl]-1-ylidene) can stabilize N⁺ center in nitreones. As an independent carbene, this species is very unstable. In this work, modulation of this CCC using (a) annulation, (b) heterocyclic ring modification, (c) substitutions adjacent to the carbenic carbon, (d) exocyclic double bond insertion and (e) ring contraction, has been reported. These modulations and quantum chemical analyses helped in the identification of five new six-membered CCCs which carry improved donation and stability properties. Further, these CCCs were employed in the design of new divalent N^I compounds (nitreones) which carry coordination bonds between ligands and N⁺ center. The molecular and electronic structure properties, and the donor→acceptor coordination interactions present in the resultant low oxidation state divalent N^I compounds have been explored.     

Isolation of Elusive Phosphinidene-Chlorotetrylenes: The Heavier Cyanogen Chloride Analogues

The elusive phosphinidene-chlorotetrylenes, [PGeCl] and [PSiCl] have been stabilized by the hetero-bileptic cyclic alkyl(amino) carbene (cAAC), N-heterocyclic carbene (NHC) ligands, and isolated in the solid state at room temperature as the first neutral monomeric species of this class with the general formulae (L)P-ECl(L′) (E=Ge, 3 a – 3 c ; E=Si, 6 ; L=cAAC; L′=NHC). Compounds 3 a – 3 c have been synthesized by the reaction of cAAC-supported potassium phosphinidenides [cAAC=PK(THF)_x]_n ( 1 a – 1 c ) with the adduct NHC:→GeCl₂ ( 2 ). Similarly, compound 6 has been synthesized via reaction of 1 a with NHC:→SiCl₂ adduct ( 4 ). Compounds 3 a – 3 c , and 6 have been structurally characterized by single-crystal X-ray diffraction, NMR spectroscopy and mass spectrometric analysis. DFT calculations revealed that the heteroatom P in 3 bears two lone pairs; the non-bonding pair with 67.8 % of s- and 32 % of p character, whereas the other lone pair is involved in π backdonation to the C_cAAC-N π* of cAAC. The Ge atom in 3 contains a lone pair with 80 % of s character, and slightly involved in the π backdonation to C_NHC. EDA-NOCV analyses showed that two charged doublet fragments {(cAAC)(NHC)}⁺, and {PGeCl}⁻ prefer to form one covalent electron-sharing σ bond, one dative σ bond, one dative π bond, and a charge polarized weak π bond. The covalent electron-sharing σ bond contributes to the major stabilization energy to the total orbital interaction energy of 3 , enabling the first successful isolations of this class of compounds ( 3 , 6 ) in the laboratory.     

The Fate of NHC‐Stabilized Dicarbon

The attempted synthesis of NHC‐stabilized dicarbon (NHC?C?C?NHC) through deprotonation of a doubly protonated precursor ([NHC?CH?CH?NHC]²⁺) is reported. Rather than deprotonation, a clean reduction to NHC?CH?CH?NHC is observed with a variety of bases. The apparent resistance towards deprotonation to the target compound led to a reinvestigation of the electronic structure of NHC→C?C←NHC, which showed that the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) gap is likely too small to allow for isolation of this species. This is in contrast to the recent isolation of the cyclic alkylaminocarbene analogue (cAAC?C?C?cAAC), which has a large HOMO–LUMO gap. A detailed theoretical study illuminates the differences in electronic structures between these molecules, highlighting another case of the potential advantages of using cAAC rather than NHC as a ligand. The bonding analysis suggests that the dicarbon compounds are well represented in terms of donor–acceptor interactions L→C₂←L (L=NHC, cAAC).     

Magnetic circular dichroism and molecular orbital studies on bicyclo[6,2,0] decapentaene

The magnetic circular dichroism (MCD) spectrum of bicyclo[6,2,0] decapentaene has revealed four skeletal π → π^* electronic transitions in the visible and ultraviolet region. The four MCD bands are assigned to the B₂ ← A₁, A₁ ← A₁. B₂ ← A₁ and A₁ ← A₁ electronic transitions in increasing order of energy.     

Molecular electrostatic potentials of divalent carbon(0) compounds

The molecular electrostatic potentials of divalent carbon(0) and divalent carbon(ii) compounds are calculated and the results are compared with theoretically predicted proton affinities and complexation energies with BH(3).     

Reactivity of a N-Coordinated Germylene-Borane Complex: From Ge→B to Ge→Ga Coordination

Reactivity studies of the Ge^II→B complex L(Cl)Ge⋅BH₃ ( 1 ; L=2-Et₂NCH₂-4,6-tBu₂-C₆H₂) were performed to determine the effect on the Ge^II→B donation. N-coordinated compounds L(OtBu)Ge⋅BH₃ ( 2 ) and [LGe⋅BH₃]₂ ( 3 ) were prepared. The possible tuning of the Ge^II→B interaction was proved experimentally, yielding compounds 1-PPh₂-8-(LGe)-C₁₀H₆ ( 4 ) and L(Cl)Ge⋅GaCl₃ ( 5 ) without a Ge^II→B interaction. In 5 , an unprecedented Ge^II→Ga coordination was revealed. The experimental results were complemented by a theoretical study focusing on the bonding in 1 − 5 . The different strength of the Ge^II→E (E=B, Ga) donation was evaluated by using energy decomposition analysis. The basicity of different L(X)Ge groups through proton affinity is also assessed.     

The Quadruple Bonding in C2 Reproduces the Properties of the Molecule

Ever since Lewis depicted the triple bond for acetylene, triple bonding has been considered as the highest limit of multiple bonding for main elements. Here we show that C₂ is bonded by a quadruple bond that can be distinctly characterized by valence‐bond (VB) calculations. We demonstrate that the quadruply‐bonded structure determines the key observables of the molecule, and accounts by itself for about 90 % of the molecule's bond dissociation energy, and for its bond lengths and its force constant. The quadruply‐bonded structure is made of two strong π bonds, one strong σ bond and a weaker fourth σ‐type bond, the bond strength of which is estimated as 17–21 kcal mol^?1. Alternative VB structures with double bonds; either two π bonds or one π bond and one σ bond lie at 129.5 and 106.1 kcal mol^?1, respectively, above the quadruply‐bonded structure, and they collapse to the latter structure given freedom to improve their double bonding by dative σ bonding. The usefulness of the quadruply‐bonded model is underscored by “predicting” the properties of the ³ state. C₂’s very high reactivity is rooted in its fourth weak bond. Thus, carbon and first‐row main elements are open to quadruple bonding!     

Reactivity of Dicoordinated Stannylones (Sn0) versus Stannylenes (SnII): An Investigation Using DFT‐Based Reactivity Indices

The reactivity of dicoordinated Sn⁰ compounds, stannylones, is probed using density functional theory (DFT)‐based reactivity indices and compared with the reactivity of dicoordinated Sn^II compounds, stannylenes. For the former compounds, the influence of different types of electron‐donating ligands, such as cyclic and acyclic carbenes, stannylenes and phosphines, on the reactivity of the central Sn atom is analyzed in detail. Sn⁰ compounds are found to be relatively soft systems with a high nucleophilicity, and the plots of the Fukui function f^? for an electrophilic attack consistently predict the highest reactivity on the Sn atom. Next, complexes of dicoordinated Sn compounds with different Lewis acids of variable hardness are computed. In a first part, the double‐base character of stannylones is demonstrated in interactions with the hardest Lewis acid H⁺. Both the first and second proton affinities (PAs) are high and are well correlated with the atomic charge on the Sn atom, probing its local hardness. These observations are also in line with electrostatic potential plots that demonstrate that the tin atom in Sn⁰ compounds bears a higher negative charge in comparison to Sn^II compounds. Stannylones and stannylenes can be distinguished from each other by the partial charges at Sn and by various reactivity indices. It also becomes clear that there is a smooth transition between the two classes of compounds. We furthermore demonstrate both from DFT‐based reactivity indices and from energy decomposition analysis, combined with natural orbitals for chemical valence (EDA‐NOCV), that the monocomplexed stannylones are still nucleophilic and as reactive towards a second Lewis acid as towards the first one. The dominating interaction is a strong σ‐type interaction from the Sn atom towards the Lewis acid. The interaction energy is higher for complexes with the cation Ag⁺ than with the non‐charged electrophiles BH₃, BF₃, and AlCl₃.