Benzvalene-like Carboranes: Genuine Rule Breakers and Good Kinetic Stability.

Substituents are widespread in chemistry, but it has remained quite difficult to reliably determine the thermodynamic and kinetic stabilities of substituted compounds, though they are key to helping establish a structural rule and synthetic viability, respectively. As an important class of valence isomers in the benzene family, benzvalene-like structures have been extensively studied in systems associated with electron-neutral (i.e., C, Si, Ge, Pb, and Sn) and electron-rich (e.g., P) skeletons. However, stable benzvalene-like examples associated with electron-deficient skeletons have been very limited, possibly due to the very complicated bonding patterns of electron-deficient elements. Here, we performed an extensive structural search at the density functional theory (DFT) and CBS-QB3 level for the well-known six-vertex dicarboranes (C2B4R6), one of the central families of boranes and carboranes chemistry. We unexpectedly found that all of the previously reported benzvalene-like structures III (C2B4R6) as the long-chased "rule breaker" examples of the Wade-Mingos rule (W-M rule) are not the lowest-lying structures. Promisingly, for the first time, we succeeded in identifying several substituted III as the genuine lowest-lying structures and thus true "rule breakers." Thus, "benzvalenes" present hitherto the fourth member of the lowest-lying structural patterns for the family of six-vertex dicarboranes. Moreover, the presently revealed good kinetic stability of III' (C2B4R2R'4) over a wide range of substituents promoted us to recommend a novel kind of synthesizable carboranes beyond the Wade-Mingos rule, i.e., "benzvalene-like carboranes" with all of the classical skeletal atoms.

Double Direct C-H Bond Arylation of Thiophenes with Aryl Chlorides Catalyzed by the N-Heterocyclic Carbene-PdCl2-1-methylimidazole Complex.

With the combination of the N-heterocyclic carbene-PdCl2-1-methylimidazole complex and Cu2O, we succeeded in the first example of double direct C-H bond arylation reactions between thiophenes and aryl chlorides, giving the desired 2,5-diarylated thiophenes in moderate to high yields under suitable conditions, consistent with the density functional theory calculations.

Building a New Platform for Significantly Improving Performance of Hartree-Fock and CCSD(T) Correlation Energy Based on Two-Point Complete Basis Set Extrapolation Schemes.

The leading cause of high expense in gold standard coupled cluster theory is that calculations of electronic energies converge exceedingly slowly with an increased basis set size. Extrapolation principally allows for achieving higher-quality outcomes at reduced costs. Numerous extrapolation formulas have been developed, with attempts to predict energies up to the complete basis set limit. Unfortunately, since the intricate shape of the function hinges on the molecular properties with the highest angular momentum of the basis set, the accuracy of the extrapolated energies highly depends on the fitted empirical parameters, which rely on the quality of the data sets for fitting. In this work, to overcome the extrapolation deficiency caused by the very limited data sets and smaller basis sets in the early stages, we constructed a new benchmark platform that includes a broader data set of 183 species (containing open-shell, closed-shell, ionic, and neutral species) and a larger basis set up to aug-cc-pV6Z. The newly optimized parameters can significantly improve the energy-predictive abilities of ten published formulas. Notably, all ten formulas perform quite similarly under the new platform with the reoptimized parameters. Finally, we built an online calculator for researchers to use for these extrapolation schemes. Our work would reignite the interest and applications of the underestimated formulas.

Achieving Accuracy and Economy for Calculating Vertical Detachment Energies of Molecular Anions: A Model Chemistry Composite Methods.

The vertical detachment energy (VDE) is a vital factor for predicting the stability of anions that have important applications in the atom, molecule and cluster science. Due to the synthetic or characterization difficulty of anions, accurate and efficient predictions of VDE independent of laboratory data have always been an appealing task to remedy the experimental deficiencies. Unfortunately, the generally adopted CCSD(T) and electron propagator theory (EPT) methods have respectively been proven to be reliable but very cost-expensive, and cost-effective but sometimes problematic when Koopman's theorem is invalid. Here, we for the first time introduced and benchmarked a series of model chemistry composite methods (e. g., CBS-QB3, G4 and W1BD) on calculating VDE for 57 molecular anions. Notably, CBS-QB3 exceeds the accuracy of CCSD(T) while approaching the economy of EPT. Therefore, we highly recommend the composite method CBS-QB3 to compute VDEs for molecular anions in the attractive "killing two birds with one stone" manner.

Free and Complexed Fluoropentazoles: Anomalous Ring Charge and Appreciable Kinetic Stability.

Fluoronitrogens are strongly related to high-energy density materials. Fluoropentazole (cyclo-FN5) with hitherto the highest N/F ratio at ambient pressure belongs to the family of well-known and 119-year-old pentazoles. Due to the presence of the overwhelmingly electronegative F element, cyclo-FN5 is the only pentazole to date that contains a cationic N5 ring. However, also due to F's large electronegativity, the reported ring-destruction barrier of cyclo-FN5 is nearly the lowest (6.7 kcal/mol), which has made its synthesis and characterization quite difficult. Here, cyclo-FN5 was re-predicted to bear an almost doubled ring-destruction barrier (∼12-14 kcal/mol) at the CBS-QB3, G4, W1BD, CCSD(T)/CBS//CCSD/cc-pVTZ, and CCSD(T)/CBS//CCSD(T)/aug-cc-pVTZ levels. Promisingly, upon further complexation with Lewis acid(s), the ring-destruction barrier of cyclo-FN5 might reach 23 kcal/mol, which is comparable to that of the well-known and already synthesized arylpentazoles (∼20 kcal/mol), and the positive charge on the N5 ring can be increased to 0.66 |e|.

All-nitrogen spiropentadiene-N5.

Of the pentanitrogen cation (N5 +) family, the only experimentally known isomer is the V-shaped structure 01. Here, we showed that a super-high-energy (∼100 kcal/mol above 01) all-nitrogen spiropentadiene 02 with considerable σ-delocalization deserves pursuit as the first spirocyclic all-nitrogen molecule, at least spectroscopical.

How stable can the pentanitrogen cation be in kinetics?

For the 22 year-old pentanitrogen cation N5+ (01), we surprisingly found that the previously reported transition states (TSs) do not correspond to N2-extrusion. We located the real N2-extrusion TS, which can well reconcile the hitherto remaining inconsistency between the gas-phase and salt-like forms of 01 both in structure and energetics.

Planar Tetracoordinate Carbons in Allene-Type Structures.

The exhaustive exploration of the potential energy surfaces of CE2M2 (E = Si-Pb; M = Li and Na) revealed seven global minima containing a planar tetracoordinate carbon (ptC). The design, based on a π-localization strategy, resulted in a ptC with two double bonds forming a linear or a bent allene-type E═C═E motif. The magnetic response of the bent E═C═E fragments support a σ-aromaticity. The bonding analysis indicated that the ptCs form C-E covalent bonds and C-M electrostatic interactions.

Arylpentazoles with surprisingly high kinetic stability.

The more-than-one-century-old arylpentazoles can only be used in situ in generating the pentazole anion due to their unfavourable kinetic stability. We successfully increased the N2-leaving barrier to reach hitherto the highest value of 40.83 kcal mol-1 at the CBS-QB3 level via a newly proposed co-stabilization method, making the broader applications of arylpentazoles feasible.

A more effective catalysis of the CO2 fixation with aziridines: computational screening of metal-substituted HKUST-1.

A vital issue for the fixation and conversion of CO2 into useful chemical products is to find effective catalysts. In this work, in order to develop more effective and diverse catalysts, we implemented the first computational screening study (at M06-2X//B3LYP level) on the cycloaddition of CO2 with aziridines under eighteen metal-substituted HKUST-1 MOFs and tetrabutylammonium bromide (TBAB) as a co-catalyst. For all considered catalytic systems, the ring-opening of aziridine is calculated to be the rate-determining step. Up to 11 M-HKUST-1 systems, i.e., Rh (31.87 kcal mol-1), Y (31.02), Sc (30.50), V (30.02), Tc (29.90), Cd (29.80), Ti (29.32), Mn (29.05), Zn (28.29), Fe (27.85) and Zr (25.09), possess lower ring-opening barrier heights than the original Cu-HKUST-1 (32.90), indicative of their superior catalytic ability to the original Cu-HKUST-1 in theory. With the lowest ring-opening barrier, Zr-HKUST-1 is strongly advocated for future synthetic and catalytic studies.

A motif for heteronuclear C[triple bond, length as m-dash]E (E = Si, Ge, Sn, Pb) bonding: Lewis acid-base pair strategy.

The design and characterization of the heteronuclear group 14 C[triple bond, length as m-dash]E (E = Si, Ge, Sn, Pb) triple bonds have attracted intensive interest in the past few decades. In the current work, utilizing the advantages of N-heterocyclic carbenes (NHCs) and Lewis acid-base pair strategy, we theoretically designed a new class of compounds III-1, i.e., (NHCAR)C[triple bond, length as m-dash]E(Al(C6F5)3). Quantum chemical calculations showed that these singlet compounds possess very favourable isomerization, fragmentation and dimerization stabilities at the B3LYP/def2-TZVPP//B3LYP/def2-SVP level. The calculated bond lengths of CE in III-1 are 1.63 Å for Si, 1.70 Å for Ge, 1.91 Å for Sn and 2.01 Å for Pb, respectively, which are close to or even shorter than the known C[triple bond, length as m-dash]E bond lengths. In addition, the significant Mayer bond order values, two orthogonal π orbitals and one σ orbital between the C and E atoms also indicate the characteristics of triple bonds. Based on several bonding analyses, strong delocalization is found to exist between the C[triple bond, length as m-dash]E core and NHCAR forming a weak C[double bond, length as m-dash]C double bond. Hence, such obtained C[triple bond, length as m-dash]E species also can be described by their resonace structures as cunmulene analogs. In all, III-1 proposed here not only presents a universal C[triple bond, length as m-dash]E motif for all the heavier group 14 elements, but also provides a new strategy for the design and synthesis of heteronuclear group 14 triple bonds in the future.

Planar pentacoordinate silicon and germanium atoms.

The global minimum of XMg4Y- (X = Si, Ge; Y = In, Tl) and SiMg3In2 contains a planar pentacoordinate atom of group 14 other than carbon. Its design is based on the "localization" approach, replacing one or two peripheral atoms in XMg52- by more electronegative ones. This change diminishes the repulsion and leads to stronger covalent X-Y bonds, stabilizing the planar pentacoordinate atom species.

Avoided spin coupling: an unexpected σ-σ diradical in global planar pentacoordinate carbon.

We present a global planar pentacoordinate carbon (ppC) featuring a hitherto unreported σ-σ diradical characteristic. Using the multi-reference approach combined with the CCSD(T)/aug-cc-pVTZ method, the ppC C3Li3- was found to be an intriguing triplet ground state, in which the unpaired density is mostly located at three Li ligands. Chemical bonding analysis reveals that the 2pzπ electrons of C3Li3- are fully located at the C3 ring formed by C-C multiple bonds, in contrast to the perfect 2pzπ-delocalization found in the well-known ppCs.

A sixteen-valence-electron carbon-group 13 family with global penta-atomic planar tetracoordinate carbon: an ionic strategy.

The design of planar tetracoordinate carbon (ptC) has always been a challenge due to its unique bonding mode that necessitates the perfect balance between the carbon center and surrounding ligands both electronically and mechanically. A unique type of 18-valence-electron (18ve) template, i.e., CAl42-, has been found to be very effective in designing various novel 18ve-species upon skeletal substitution. In this work, we showed that though ptC is not the global structure for the parent 16ve-CAl4, suitable skeletal substitution can allow for a series of global minimum ptC species. Theoretical calculations at the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP for 35 carbon-group 13 systems with 16-ve, i.e., CXaYbZcKd (X, Y, Z, K = Al/Ga/In/Tl; 0 ≤ a, b, c, d ≤ 4, a + b + c + d = 4), showed that 9 systems (CAl3Tl, CGa3Tl, CGa2Tl2, CAl2GaTl, CAl2InTl, CGa2InTl, CAlGa2Tl, CGa2InTl and CAlGaInTl) possess global minimum ptC and 2 systems (CAl3In and CAl2Tl2) have quasi-GM ptC. Except for CAl3Tl and CAl3In, all the ptCs were predicted for the first time. All these stable ptC structures have the same skeleton and can be described as the same ionic sub-structure, i.e., [A-]B+. This study not only enriches 16ve-ptC, but also directly demonstrates that utilizing an ionic strategy, non-ptC CAl4 also can be used as a template to extend the ptC family.

Mono-silicon isoelectronic replacement in CAl4 : van't hoff/le bel carbon or not?

In cluster studies, the isoelectronic replacement strategy has been successfully used to introduce new elements into a known structure while maintaining the desired topology. The well-known penta-atomic 18 valence electron (ve) species C Al 4 2 - and its Al- /Si or Al/Si+ isoelectronically replaced clusters CAl3 Si- , CAl2 Si2 , C AlSi 3 - , and C Si 4 2 + , all possess the same anti-van't Hoff/Le Bel skeletons, that is, nontraditional planar tetracoordinate carbon (ptC) structure. In this article, however, we found that such isoelectronic replacement between Si and Al does not work for the 16ve-CAl4 with the traditional van't Hoff/Le Bel tetrahedral carbon (thC) and its isoelectronic derivatives CAl3 X (X = Ga/In/Tl). At the level of CCSD(T)/def2-QZVP//B3LYP/def2-QZVP, none of the global minima of the 16ve mono-Si-containing clusters CAl2 SiX+ (X = Al/Ga/In/Tl) maintains thC as the parent CAl4 does. Instead, X = Al/Ga globally favors an unusual ptC structure that has one long C─X distance yet with significant bond index value, and X = In/Tl prefers the planar tricoordinate carbon. The frustrated formation of thC in these clusters is ascribed to the CSi bonding that prefers a planar fashion. Inclusion of chloride ion would further stabilize the ptC of CAl2 SiAl+ and CAl2 SiGa+ . The unexpectedly disclosed CAl2 SiAl+ and CAl2 SiGa+ represent the first type of 16ve-cationic ptCs with multiple bonds. © 2019 Wiley Periodicals, Inc.

Globally stabilized bent carbon-carbon triple bond by hydrogen-free inorganic-metallic scaffolding Al4F6.

For over 100 years, known bent C[triple bond, length as m-dash]C compounds have been limited to those with organic (I) and all-carbon (II) scaffoldings. Here, we computationally report a novel type (III) of bent C[triple bond, length as m-dash]C compound, i.e., C2Al4F6-01, which is the energetically global minimum isomer and bears an inorganic-metallic scaffolding and unexpected click reactivity.

New global minima of 6-vertex dicarboranes: classical but unexpected.

Dicarboranes generally adopt global minimum predicted by the well-known Wade-Mingos rules, although one classical non-closo structure in the benzvalene form has long been pursued and later synthesized. Here we predicted two new non-closo global minima for 6-vertex dicarboranes (C2B4R6), i.e., trigonal bipyramid (R = SH) and butterfly (R = Cl, NH2, OH, F). The long expected classical benzvalene-like structure, however, is not the global minimum for any of the nine substituents.

Global Isomeric Survey of Elusive Cyclopropanetrione: Unknown but Viable Isomers.

Despite the great interest in energy storage application, stable neutral CnOn (n > 1) structures either in thermodynamics or kinetics have yet been largely limited due to the rather high tendency to release the very stable CO molecule. The neutral cyclopropanetrione (C3O3) cluster has long remained elusive since no isomer with sufficient kinetic stability has been found either experimentally or theoretically. In this work, we constructed the first global potential energy surface of singlet C3O3 at the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level, from which the kinetic stability of a wide range of C3O3 isomers can be determined by investigating their isomerization and fragmentation pathways. Amongst, a three-membered ring structure 01 is the global C3O3 isomer with a barrier of 10.6 kcal/mol at the sophisticated W1BD level. In particular, two carbene-type isomers 02 and 04 possess appreciable destruction barriers of 20.3 and 24.7 kcal/mol at W1BD, respectively. Thus, 02 and 04 can be useful building blocks for constructing larger high-energy density carbon-oxygen clusters. Moreover, with the carbene center, both might effectively functionalize various nano-materials while retaining the electrochemical active carbonyl and epoxyl moieties that are very desirable in alkali metal-ion batteries.

A kinetically persistent isomer found for pentazole: a global potential energy surface survey.

Among the N5R family, pentazoles (A1) with a 103-year-old research history remain the only class of kinetically persistent isomers. Aided by the first global isomeric survey and substitution study of N5H at the composite CBS-QB3 level, we predicted a new N5R isomer (C1) with kinetic stability close to pentazoles.

Predicting viable isomers of [X,C,N] and [H,X,C,N] (X = Sn, Pb).

Metal cyanide/isocyanide and hydrometal cyanide/isocyanide compounds are key metal-carriers in interstellar space. Lighter group 14 elements (X = C/Si/Ge) cyanides/isocyanides and hydrocyanides/hydroisocyanides have been studied theoretically and experimentally. However, no reports are available on the analogues of tin (Sn) and lead (Pb). In this work, we carried out the first theoretical study on the structures and stabilities of [X,C,N] and [H,X,C,N] (X = Sn/Pb) at the CCSD(T)/def2-QZVPP//B3LYP/def2-QZVPP level. Comparisons were made with the lower analogues (X = C/Si/Ge) concerning the structural, energetic and bonding properties. Significantly different from that of c-C2N, a dative-bonded valence structure of c-XCN for heavier X was revealed for the first time, which can account for the rather worse kinetic stability of cyclic [X,C,N] for heavier X = Si/Ge. A unique kind of agostic bonding was found within three isomers of [H,Pb,C,N], whereas it is absent for X = C/Si/Ge/Sn. The computed structural and spectroscopic data could aid future laboratory and astrophysical detection of the [X,C,N] and [H,X,C,N] (X = Sn/Pb) isomers.