Probing the aromaticity in 2,3-pyrido-annulated N-heterocyclic carbene and its heavier analogues

The aromaticity in 2,3-pyrido-annulated 1,3,2λ²-diazatetroles C₅H₃N(NR)₂E^II (E^II = C, Si, Ge, Sn, Pb) was studied using a set of experimental and calculated criteria: UV-VIS, Raman, ISE, NICS, GIMIC and EDDB. The data obtained indicate either a slight decrease in aromaticity (NICS, GIMIC, ISE methods) or equal aromaticity (UV-VIS, ISE methods) compared to benzo-annulated analogues C₆H₄(NR)₂E. The π-aromaticity increases down the group from Si to Pb.     

Reaction Mechanism of the Hydrogermylation/Hydrostannylation of Unactivated Alkenes with Two‐Coordinate EII Hydrides (E=Ge,Sn): A Theoretical Study

Quantum chemical calculations using density functional theory with the TPSS+D3(BJ) and M06‐2X+D3(ABC) functionals have been carried out to understand the mechanisms of catalyst‐free hydrogermylation/hydrostannylation reactions between the two‐coordinate hydrido‐tetrylenes :E(H)(L⁺) (E=Ge or Sn, L⁺=N(Ar⁺)(SiiPr₃); Ar⁺=C₆H₂{C(H)Ph₂}₂iPr‐2,6,4) and a range of unactivated terminal (C₂H₃R, R=H, Ph, or tBu) and cyclic [(CH)₂(CH₂)₂(CH₂)_n, n=1, 2, or 4] alkenes. The calculations suggest that the addition reactions of the germylenes and stannylenes to the cyclic and acyclic alkenes occur as one‐step processes through formal [2+2] addition of the E?H fragment across the C?C π bond. The reactions have moderate barriers and are weakly exergonic. The steric bulk of the tetrylene amido groups has little influence on the activation barriers and on the reaction energies of the anti‐Markovnikov pathway, but the Markovnikov addition is clearly disfavored by the size of the substituents. The addition of the tetrylenes to the cyclic alkenes is less exergonic than the addition to the terminal alkenes, which agrees with the experimentally observed reversibility of the former reactions. The hydrogermylation reactions have lower activation energies and are more exergonic than the stannylene addition. An energy decomposition analysis of the transition state for the hydrogermylation of cyclohexene shows that the reaction takes place with simultaneous formation of the Ge?C and (Ge)H?C′ bonds. The dominant orbitals of the germylene are the σ‐type lone pair MO of Ge, which serves as a donor orbital, and the vacant p(π) MO of Ge, which acts as acceptor orbital for the π* and π MOs of the olefin. Inspection of the transition states of some selected reactions suggests that the differences between the activation energies come from a delicate balance between the deformation energies of the interacting species and their interaction energies.     

The Triboracyclopropenyl Dianion: The Lightest Possible Main‐Group‐Element Hückel π Aromatic

Hückel π aromaticity is typically a domain of carbon‐rich compounds. Only very few analogues with non‐carbon frameworks are currently known, all involving the heavier elements. The isolation of the triboracyclopropenyl dianion is presented, a boron‐based analogue of the cyclopropenyl cation, which belongs to the prototypical class of Hückel π aromatics. Reduction of Cl₂BNCy₂ by sodium metal produced [B₃(NCy₂)₃]^2?, which was isolated as its dimeric Na⁺ salt (Na₄[B₃(NCy₂)₃]₂?2 DME; 1 ) in 45 % yield and characterized by single‐crystal X‐ray diffraction. Cyclic voltammetry measurements established an extremely high oxidation potential for 1 (E_pc=?2.42 V), which was further confirmed by reactivity studies. The Hückel‐type π aromatic character of the [B₃(NCy₂)₃]^2? dianion was verified by various theoretical methods, which clearly indicated π aromaticity for the B₃ core of a similar magnitude to that in [C₃H₃]⁺ and benzene.     

Experimental and theoretical evidence of aromatic behavior in heterobenzene-like molecules with metal-metal multiple bonds

Binding two quadruply bonded dimolybdenum units [Mo₂(DAniF)₃]⁺ (DAniF=N,N′‐di‐p‐anisylformamidinate) with two chalcogen atoms generated two molecules with a central core composed of a cyclic six‐membered [Mo₂]₂(μ‐EH)₂ species (E=S in 1 and O in 3 , and [Mo₂] is a quadruple‐bonded [Mo₂(formamidinate)₃] unit). Aerobic oxidation of 1 and 3 followed by concomitant deprotonation gave rise to the corresponding [Mo₂]₂(μ‐E)₂ compounds 2 and 4 . The latter show a striking coplanarity and near‐bond equalization of the Mo/E cluster. The oxidized species 2 and 4 are diamagnetic in the measured temperature range of 5 to 300 K, which is somewhat unexpected for molecules that have dimetal units with a σ²π⁴δ¹ electronic configuration. This suggests there are strong interactions between the dimolybdenum units through the E atoms. The large electronic delocalization of the δ electrons over the entire Mo/E core is supported by the exceptionally large potential separation for the two successive one‐electron reductions of the linked Mo₂⁵⁺ units from the oxidized species (ΔE_1/2=1.7 V for the sulfur analogue). This large electronic delocalization has an important effect on the NMR spectroscopic signals for the two sets of methine (N‐(CH)‐N) protons from the DAniF ligands. Those essentially parallel to the core, H_∥, and those essentially perpendicular to the core, H_⊥, exhibit downfield and upfield chemical shifts, respectively, that are separated by δ=1.32 ppm. The structural, electronic, magnetic, and chemical behaviors for 2 and 4 are consistent with aromaticity, with the [Mo₂E₂Mo₂] cores that resemble the prototypical benzene molecule. Theoretical studies, including DFT calculations, natural bond orbital (NBO) analyses, and gauge‐independent atomic orbital (GIAO) NMR spectroscopic calculations, are also consistent with the aromaticity of the [Mo₂]₂(μ‐E)₂ units being promoted by d_δ(Mo₂)–p_π(E) π conjugation. The cyclic π conjugation of the central moiety in 2 and 4 involves a total of six electrons with 2e from δ(Mo₂) and 4e from p_π(E) orbitals, thereby conforming to Hückel’s rule when electrons in the MOs with δ character are considered part of the delocalized system.     

Aromaticity Effects on the Profiles of the Lowest Triplet‐State Potential‐Energy Surfaces for Rotation about the CC Bonds of Olefins with Five‐Membered Ring Substituents: An Example of the Impact of Baird’s Rule

A density functional theory study on olefins with five‐membered monocyclic 4n and 4n+2 π‐electron substituents (C₄H₃X; X=CH⁺, SiH⁺, BH, AlH, CH₂, SiH₂, O, S, NH, and CH^?) was performed to assess the connection between the degree of substituent (anti)aromaticity and the profile of the lowest triplet‐state (T₁) potential‐energy surface (PES) for twisting about olefinic C?C bonds. It exploited both Hückel’s rule on aromaticity in the closed‐shell singlet ground state (S₀) and Baird’s rule on aromaticity in the lowest ππ* excited triplet state. The compounds CH₂?CH(C₄H₃X) were categorized as set A and set B olefins depending on which carbon atom (C2 or C3) of the C₄H₃X ring is bonded to the olefin. The degree of substituent (anti)aromaticity goes from strongly S₀‐antiaromatic/T₁‐aromatic (C₅H₄⁺) to strongly S₀‐aromatic/T₁‐ antiaromatic (C₅H₄^?). Our hypothesis is that the shapes of the T₁ PESs, as given by the energy differences between planar and perpendicularly twisted olefin structures in T₁ [ΔE(T₁)], smoothly follow the changes in substituent (anti)aromaticity. Indeed, correlations between ΔE(T₁) and the (anti)aromaticity changes of the C₄H₃X groups, as measured by the zz‐tensor component of the nucleus‐independent chemical shift ΔNICS(T₁;1)_zz, are found both for sets A and B separately (linear fits; r²=0.949 and 0.851, respectively) and for the two sets combined (linear fit; r²=0.851). For sets A and B combined, strong correlations are also found between ΔE(T₁) and the degree of S₀ (anti)aromaticity as determined by NICS(S₀,1)_zz (sigmoidal fit; r²=0.963), as well as between the T₁ energies of the planar olefins and NICS(S₀,1)_zz (linear fit; r²=0.939). Thus, careful tuning of substituent (anti)aromaticity allows for design of small olefins with T₁ PESs suitable for adiabatic Z/E photoisomerization.     

Structure and Bonding in Stannadiphospholes and their Dianions SnC2P2R2m (R=H,tBu m=0, −2): A Comparative Study with C5H5+ and C5H5− Analogues

The potential energy surfaces of both neutral and dianionic SnC₂P₂R₂ (R=H, tBu) ring systems have been explored at the B3PW91/LANL2DZ (Sn) and 6‐311+G* (other atoms) level. In the neutral isomers the global minimum is a nido structure in which a 1,2‐diphosphocyclobutadiene ring (1,2‐DPCB) is capped by the Sn. Interestingly, the structure established by X‐ray diffraction analysis, for R=tBu, is a 1,3‐DPCB ring capped by Sn and it is 2.4 kcal mol^?1 higher in energy than the 1,2‐DPCB ring isomer. This is possibly related to the kinetic stability of the 1,3‐DPCB ring, which might originate from the synthetic precursor ZrCp₂tBu₂C₂P₂. In the case of the dianionic isomers we observe only a 6π‐electron aromatic structure as the global minimum, similarly to the cases of our previously reported results with other types of heterodiphospholes. 1 , 4 , 19 The existence of large numbers of cluster‐type isomers in neutral and 6π‐planar structures in the dianions SnC₂P₂R₂^2? (R=H, tBu) is due to 3D aromaticity in neutral clusters and to 2D π aromaticity of the dianionic rings. Relative energies of positional isomers mainly depend on: 1) the valency and coordination number of the Sn centre, 2) individual bond strengths, and 3) the steric effect of tBu groups. A comparison of neutral stannadiphospholes with other structurally related C₅H₅⁺ analogues indicates that Sn might be a better isolobal analogue to P⁺ than to BH or CH⁺. The variation in global minima in these C₅H₅⁺ analogues is due to characteristic features such as 1) the different valencies of C, B, P and Sn, 2) the electron deficiency of B, 3) weaker pπ–pπ bonding by P and Sn atoms, and 4) the tendency of electropositive elements to donate electrons to nido clusters. Unlike the C₅H₅⁺ systems, all C₅H₅^? analogues have 6π‐planar aromatic structures as global minima. The differences in the relative ordering of the positional isomers and ligating properties are significant and depend on 1) the nature of the π orbitals involved, and 2) effective overlap of orbitals.     

Liquid density,viscosity and spectroscopic studies of binary mixture of tetraethylene glycol + 1,2-ethanediamine for CO2 capture

Thermophysical properties for binary mixture of tetraethylene glycol (T₄EG) (1) + 1,2-ethanediamine (EDA) (2), a potential scrubbing solution for the absorption of CO₂, are very important as well as lacking in the literatures. This work reports densities and viscosities over the entire concentration range for the binary mixture at T = (293.15-318.15) K under atmospheric pressure. According to the experimental density and viscosity values, the mixtures’ excess molar volume (V_m^E), absolute viscosity deviation (?η), excess free energies of activation (?G*^E), apparent molar volumes, partial molar volumes and isobaric thermal expansion coefficient were calculated, respectively. Meanwhile, the V_m^E, ?η and ?G*^E values were fitted by a Redlich–Kister equation to obtain coefficients. To further study, the Fourier transform infrared, UV-Vis and fluorescence spectra of T₄EG + EDA mixtures with various concentrations were measured, and the intermolecular interaction of T₄EG with EDA was also discussed as the formation of –OCH₂CH₂O–H···N(H₂)CH₂CH₂(H₂)N···.     

Inverse sandwich complexes based on low‐valent group 13 elements and cyclobutadiene: A theoretical investigation on E‐C4H4‐E (E = Al,Ga, In,Tl)

The chemistry of the low‐valent Group 13 elements (E = B, Al, Ga, In, Tl) has formed the recent hot topic. Recently, a series of low‐valent Group 13‐based compounds have been synthesized, i.e., [E‐Cp*‐E]⁺ (E = Al, Ga, In, Tl) cations, which have been termed as the interesting “inverse sandwich” complexes. To enrich the family of inverse sandwiches, we report our theoretical design of a new type of inverse sandwiches E‐C₄H₄‐E (E = Al, Ga, In, Tl) for stabilizing the low‐valent Group 13 elements. The calculated dissociation energies indicate that unlike [E‐Cp‐E]⁺ that dissociates via loss of the charged atom E⁺, E‐C₄H₄‐E dissociates via loss of the neutral atom E with the bond strengths of Al > Ga > In > Tl. Moreover, E‐C₄H₄‐E are more stable in dissociation than [E‐Cp‐E]⁺ cations. By comparing with other various isomers, we found that the inverted E‐C₄H₄‐E should be kinetically quite stable with the least conversion barriers of 33.5, 33.5, 35.2, and 36.9 kcal/mol for E = Al, Ga, In, and Tl, respectively. Furthermore, replacement of cyclobutadiene‐H atoms by the highly electron‐positive groups such as SiH₃ and Si(CH₃)₃ could significantly stabilize the inverted form in thermodynamics. Possible synthetic routes are proposed for E‐C₄H₄‐E. With no need of counterions, the newly designed neutral complexes E‐C₄H₄‐E welcome future synthesis. © 2012 Wiley Periodicals, Inc.     

Facts and artifacts about aromatic stability estimation

The stability of a set of 105 five-membered π-electron systems (involving aromatic, non-aromatic and anti-aromatic species) was evaluated using six isodesmic reactions of which two belong to the subclass of homodesmotic reactions, which are based on cyclic and acyclic reference systems. We demonstrate that the ‘Resonance Energies’ derived from isodesmotic schemes have obvious flaws and do not correct or cancel other contributions to the energy, such as the changes of hybridization, homoconjugation of heterosubstituted cyclopentadienes, conjugative interactions of CC or CX (X=N or P) with a π or pseudo π orbital at Y (Y=O, S, NH, PH), strain, etc. as effectively as possible. Likewise, ‘aromatic stabilization energies (ASE)’ derived from homodesmotic schemes based on the acyclic reference compounds do not give satisfactory results. We strongly recommend that only cyclic reference compounds should be used for ASE and other aromaticity evaluations. The analysis is based on ab initio optimized geometries at B3LYP/6-311+G^∗∗.     

Heavy Carbene Analogues: Donor‐Free Bismuthenium and Stibenium Ions

Kinetically stabilized congeners of carbenes, R₂C, possessing six valence electrons (four bonding electrons and two non‐bonding electrons) have been restricted to Group 14 elements, R₂E (E=Si, Ge, Sn, Pb; R=alkyl or aryl) whereas isoelectronic Group 15 cations, divalent species of type [R₂E]⁺ (E=P, As, Sb, Bi; R=alkyl or aryl), were unknown. Herein, we report the first two examples, namely the bismuthenium ion [(2,6‐Mes₂C₆H₃)₂Bi][BAr^F₄] ( 1 ; Mes=2,4,6‐Me₃C₆H₂, Ar^F=3,5‐(CF₃)₂C₆H₃) and the stibenium ion [(2,6‐Mes₂C₆H₃)₂Sb][B(C₆F₅)₄] ( 2 ), which were obtained by using a combination of bulky meta‐terphenyl substituents and weakly coordinating anions.     

Stabilities,Electronic Structures,and Bonding Properties of Iron Complexes (E1E2)Fe(CO)2(CNArTripp2)2 (E1E2=BF,CO, N2, CN−, or NO+)**

The coordination of 10-electron diatomic ligands (BF, CO N₂) to iron complexes Fe(CO)₂(CNAr^Tripp2)₂ [Ar^Tripp2=2,6-(2,4,6-(iso-propyl)₃C₆H₂)₂C₆H₃] have been realized in experiments very recently (Science, 2019 , 363, 1203–1205). Herein, the stability, electronic structures, and bonding properties of (E₁E₂)Fe-(CO)₂(CNAr^Tripp2)₂ (E₁E₂=BF, CO, N₂, CN⁻, NO⁺) were studied using density functional (DFT) calculations. The ground state of all those molecules is singlet and the calculated geometries are in excellent agreement with the experimental values. The natural bond orbital analysis revealed that Fe is negatively charged while E₁ possesses positive charges. By employing the energy decomposition analysis, the bonding nature of the E₂E₁–Fe(CO)₂(CNAr^Tripp2)₂ bond was disclosed to be the classic dative bond E₂E₁→Fe(CO)₂(CNAr^Tripp2)₂ rather than the electron-sharing double bond. More interestingly, the bonding strength between BF and Fe(CO)₂(CNAr^Tripp2)₂ is much stronger than that between CO (or N₂) and Fe(CO)₂(CNAr^Tripp2)₂, which is ascribed to the better σ-donation and π back-donations. However, the orbital interactions in CN⁻→Fe(CO)₂(CNAr^Tripp2)₂ and NO⁺→Fe(CO)₂(CNAr^Tripp2)₂ mainly come from σ-donation and π back-donation, respectively. The different contributions from σ donation and π donation for different ligands can be well explained by using the energy levels of E₁E₂ and Fe(CO)₂(CNAr^Tripp2)₂ fragments.     

Quantum‐chemical analyses of aromaticity,UV spectra,and NMR chemical shifts in plumbacyclopentadienylidenes stabilized by Lewis bases

We carried out a series of zeroth‐order regular approximation (ZORA)‐density functional theory (DFT) and ZORA‐time‐dependent (TD)‐DFT calculations for molecular geometries, NMR chemical shifts, nucleus‐independent chemical shifts (NICS), and electronic transition energies of plumbacyclopentadienylidenes stabilized by several Lewis bases, (Ph)₂(^tBuMe₂Si)₂C₄PbL₁L₂ (L₁, L₂ = tetrahydrofuran, Pyridine, N‐heterocyclic carbene), and their model molecules. We mainly discussed the Lewis‐base effect on the aromaticity of these complexes. The NICS was used to examine the aromaticity. The NICS values showed that the aromaticity of these complexes increases when the donation from the Lewis bases to Pb becomes large. This trend seems to be reasonable when the 4n‐Huckel rule is applied to the fractional π‐electron number. The calculated ¹³C‐ and ²⁰⁷Pb‐NMR chemical shifts and the calculated UV transition energies reasonably reproduced the experimental trends. We found a specific relationship between the ¹³C‐NMR chemical shifts and the transition energies. As we expected, the relativistic effect was essential to reproduce a trend not only in the ²⁰⁷Pb‐NMR chemical shifts and J[Pb‐C] but also in the ¹³C‐NMR chemical shifts of carbons adjacent to the lead atom. © 2014 Wiley Periodicals, Inc.     

Theoretical Studies of Inorganic Compounds. 341) Energy Decomposition Analysis of E–E Bonding in Planar and Perpendicular X2E−EX2 (E = B,Al, Ga,In, Tl; X = H,F, Cl,Br, I)

The nature of E–E bonding in group 13 compounds X₂E–EX₂ (E = B, Al, Ga, In, Tl; X = H, F, Cl, Br, I) has been investigated by means of an energy decomposition analysis (EDA) at the BP86/TZ2P level of theory. The calculated equilibrium geometries of all molecules B₂X₄?Tl₂X₄ have a perpendicular (D_2d) geometry. The largest energy barriers for rotation about the E‐E bond are predicted for the hydrogen species B₂H₄?Tl₂H₄. The EDA shows that the rotational barriers of B₂X₄?Tl₂X₄ may not be used for an estimate of the hyperconjugative strength in the D_2d structures except for the tetrahydrides. The values for the planar (D_2h) transition states reveals that π conjugation of the halogen lone‐pair electrons stabilizes the transition states. The bonding analysis shows that hyperconjugation in B₂I₄ is stronger than in B₂H₄ although the latter compound has a higher rotational barrier than the former. In B₂F₄, hyperconjugative stabilization of the perpendicular structure and conjugative stabilization of the planar structure nearly cancel each other yielding a nearly vanishing rotational barrier. The heavier analogues Al₂X₄?Tl₂X₄ have low rotational barriers and rather weak hyperconjugative interactions. The larger rotational barriers of the hydrogen systems Al₂H₄?Tl₂H₄ compared with the tetrahalogen compounds is explained with the cooperation of the relatively large hyperconjugation in the perpendicular form and the relatively weak conjugation in the planar transition structures. The EDA also indicates that the electrostatic (ΔE_elstat) and molecular orbital (ΔE_orb) components of the E–E bonding are similar in magnitude.Thecalculated B‐B bond dissociation energies of B₂X₄ (D_e = 93.0–108.4 kcal/mol) show that the bonds are rather strong. The heavier analogues Al₂X₄?Tl₂X₄ have weaker bonds (D_e = 16.6–61.7 kcal/mol). In general, the X₂E‐EX₂ bond dissociation energies follow the trend for atoms E: B ? Al > Ga > In > Tl and for atoms X: H > F > Cl > I.     

Comparative Study of Phosphine and N‐Heterocyclic Carbene Stabilized Group 13 Adducts [L(EH3)] and [L2(E2Hn)]

Quantum chemical calculations using density functional theory at the BP86/TZ2P level have been carried out to determine the geometries and stabilities of Group 13 adducts [(PMe₃)(EH₃)] and [(PMe₃)₂(E₂H_n)] (E=B–In; n=4, 2, 0). The optimized geometries exhibit, in most cases, similar features to those of related adducts [(NHC^Me)(EH₃)] and [(NHC^Me)₂(E₂H_n)] with a few exceptions that can be explained by the different donor strengths of the ligands. The calculations show that the carbene ligand L=NHC^Me (:C(NMeCH)₂) is a significantly stronger donor than L=PMe₃. The equilibrium geometries of [L(EH₃)] possess, in all cases, a pyramidal structure, whereas the complexes [L₂(E₂H₄)] always have an antiperiplanar arrangement of the ligands L. The phosphine ligands in [(PMe₃)₂(B₂H₂)], which has C_s symmetry, are in the same plane as the B₂H₂ moiety, whereas the heavier homologues [(PMe₃)₂(E₂H₂)] (E=Al, Ga, In) have C_i symmetry in which the ligands bind side‐on to the E₂H₂ acceptor. This is in contrast to the [(NHC^Me)₂(E₂H₂)] adducts for which the NHC^Me donor always binds in the same plane as E₂H₂ except for the indium complex [(NHC^Me)₂(In₂H₂)], which exhibits side‐on bonding. The boron complexes [L₂(B₂)] (L=PMe₃ and NHC^Me) possess a linear arrangement of the LBBL moiety, which has a B?B triple bond. The heavier homologues [L₂(E₂)] have antiperiplanar arrangements of the LEEL moieties, except for [(PMe₃)₂(In₂)], which has a twisted structure in which the PInInP torsion angle is 123.0°. The structural features of the complexes [L(EH₃)] and [L₂(E₂H_n)] can be explained in terms of donor–acceptor interactions between the donors L and the acceptors EH₃ and E₂H_n, which have been analyzed quantitatively by using the energy decomposition analysis (EDA) method. The calculations predict that the hydrogenation reaction of the dimeric magnesium(I) compound L′MgMgL′ with the complexes [L(EH₃)] is energetically more favorable for L=PMe₃ than for NHC^Me.     

Three dimensional aromaticity in pyramidanes C4R4E and Ge4R4Ge

Three dimensional aromaticity in pyramidanes C₄(SiMe₃)₄E (E = Ge, Sn, Pb, P⁺, BCl, Mg) and Ge₄(SiMe₃)₄Ge was investigated using gauge-included magnetically induced currents and electron density of delocalized bonds as two criteria. For the C₄(SiMe₃)₄E compounds, different series of the aromaticity degree have been obtained by the two methods, respectively: P > Ge ≥ Sn ∼ Pb > B and Pb ≥ Sn ≥ Ge > B > P. Two isomers of Ge₄(SiMe₃)₄Ge possess nearly equal aromaticity.     

On the Critical Effect of the Metal (Mo vs. W) on the [3+2] Cycloaddition Reaction of M3S4 Clusters with Alkynes: Insights from Experiment and Theory

Whereas the cluster [Mo₃S₄(acac)₃(py)₃]⁺ ([ 1 ]⁺, acac=acetylacetonate, py=pyridine) reacts with a variety of alkynes, the cluster [W₃S₄(acac)₃(py)₃]⁺ ([ 2 ]⁺) remains unaffected under the same conditions. The reactions of cluster [ 1 ]⁺ show polyphasic kinetics, and in all cases clusters bearing a bridging dithiolene moiety are formed in the first step through the concerted [3+2] cycloaddition between the C?C atoms of the alkyne and a Mo(μ‐S)₂ moiety of the cluster. A computational study has been conducted to analyze the effect of the metal on these concerted [3+2] cycloaddition reactions. The calculations suggest that the reactions of cluster [ 2 ]⁺ with alkynes feature ΔG^≠ values only slightly larger than its molybdenum analogue, however, the differences in the reaction free energies between both metal clusters and the same alkyne reach up to approximately 10 kcal mol^?1, therefore indicating that the differences in the reactivity are essentially thermodynamic. The activation strain model (ASM) has been used to get more insights into the critical effect of the metal center in these cycloadditions, and the results reveal that the change in reactivity is entirely explained on the basis of the differences in the interaction energies E_int between the cluster and the alkyne. Further decomposition of the E_int values through the localized molecular orbital‐energy decomposition analysis (LMO‐EDA) indicates that substitution of the Mo atoms in cluster [ 1 ]⁺ by W induces changes in the electronic structure of the cluster that result in weaker intra‐ and inter‐fragment orbital interactions.     

Pentapnictogen heterocyclic monoanions: Structure,stability, and aromaticity

Inorganic planar ring-shape molecules with 4n + 2 π electrons are always the focus of experimental synthesis and theoretical research due to their potential aromaticity and stability. In this work, the whole series of five-membered heterocycle monoanions X _nY_5-n⁻ (X, Y = group 15 elements; n = 1-4) were thoroughly investigated by means of density functional theory calculations. They all have large formation energies and HOMO-LUMO gap energies, suggesting the potential thermodynamic and kinetic stability. Their aromaticities are comparable to that of typical aromatic hydrocarbons. Their thermal stabilities were firmly established by the ab initio molecular dynamics simulations. As most of them are predicted for the first time, their various spectra were simulated for experimental characterization. Furthermore, we demonstrate that these five-membered cyclic anions can be employed as η⁵-ligand to construct novel all-inorganic metallocenes, which may serve as the building blocks of low-dimensional nanomaterials.     

Realization of Rhodium Metal‐Oxide Electrode in Indifferent Electrolytes

The pH‐dependence of the stationary open‐circuit potential E_i=0^st of rhodium electrode with a surface layer of anodically formed insoluble compounds has been studied in sulfate and phosphate solutions by means of cyclic voltammetry and chronopotentiometry. The range of potentials of the investigations performed has been confined to the region of rhodium electrochemical oxidation/reduction, i.e., 0.2<E<1.2 V (RHE) in order to prevent any possible interference of other reactions such as H₂ and O₂ evolution. It has been shown that rhodium electrode with a layer of surface compounds formed anodically at E<<1.23 V (RHE) behaves like a reversible metal‐oxide electrode within the range of pH values from ca. 1.0 to ca. 8.0. It has been presumed that the stationary potential of such electrode is determined by the equilibrium of the following electrochemical reaction: Rh+3H₂O??Rh(OH)₃+3H⁺+3e^?. The pH‐dependence of the reversible potential of Eequation/tex2gif-inf-6.gif electrode has been found to be: Eequation/tex2gif-inf-8.gif=E_i=0^st=0.69?0.059 pH, V. In acid solutions (pH<2.0) rhodium hydroxide dissolves into the electrolyte, therefore, to reach equilibrium, the solution must be saturated with Rh(OH)₃. This has been achieved by adding Rh³⁺ ions in the form of Rh₂(SO₄)₃. The solubility product of Rh(OH)₃, estimated from the experimental Eequation/tex2gif-inf-16.gif?pH dependence obtained, is ca. 1.0×10^?48, which is close to the value given in literature.     

Protonation enthalpies in fluorosulfonic acid using ab initio self-consistent reaction field theory

Electrostatic solvation free energies were computed for several small neutral bases and their conjugate acids using a continuum solvation model called the self-consistent isodensity polarizable continuum model (SCIPCM). The solvation energies were computed at the restricted Hartree–Fock (RHF) and second-order Møller–Plesset (MP2) levels of theory, as well as with the Becke3–Lee–Yang–Parr (B3LYP) density functional theory, using the standard 6–31G** Gaussian basis set. The RHF solvation energies are similar to those computed at the correlated MP2 and B3LYP theoretical levels. A model for computing protonation enthalpies for neutral bases in fluorosulfonic acid solvent leads to the equation ΔH(B)=−PA(B)+ΔE_t(BH⁺)−ΔE_t(B)+β, where PA(B) is the gas phase proton affinity for base B, ΔE_t(BH⁺) is the SCIPCM solvation energy for the conjugate acid, and ΔE_t(B) is the solvation energy for the base. A fit to experimental values of ΔH(B) for 10 neutral bases (H₂O, MeOH, Me₂O, H₂S, MeSH, Me₂S, NH₃, MeNH₂, Me₂NH, and PH₃) gives β=238.4±2.9 kcal/mol when ΔΔE_t is computed using the 0.0004 e⋅bohr⁻³ isodensity surface for defining the solute cavity at the RHF/6–31G** level. The model predicts that for carbon monoxide ΔH(CO)=10 kcal/mol. Thus, protonation of CO is endothermic, and the conjugate acid HCO⁺ (formyl cation) behaves as a strong acid in fluorosulfonic acid. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 250–257, 1998