Quest for Double Möbius Aromaticity.

Neutral four-membered rings with 4n electrons are generally Hückel antiaromatic. On the other hand, 4n electron system can also be Möbius aromatic, although identification has escaped so far. A recent study of double Möbius aromaticity has been put forwarded in the D2h symmetric singlet ground state of four member Pa2B2 ring. Although interesting, but the synthesis possesses a significant challenge as Pa is rare, highly radioactive and toxic. Herein, a synthetically viable four membered Rh2B2 cluster is proposed which contains double Möbius aromaticity. Interestingly, the three membered RhB2 - cluster also possess Möbius aromaticity and is the smallest ring to show such phenomenon.

Enhancing Precatalyst Performance and Robustness through Aromaticity: Insights from Iridaheteroaromatics.

Despite the inherent stability-enhancing benefits of dπ-pπ conjugation-induced aromaticity, metallaaromatic catalysts remain underutilized in this context, despite their reactivity with organic functionalities in stoichiometric reactions. We present a strategy for synthesizing a diverse range of iridaheteroaromatics, (L^L)IrIII(Cp*)I, including iridapyridylidene-indole, iridapyridene-indole, and iridaimidazole, via in situ deprotonation/metalation reactions utilizing [Cp*IrCl2]2 and the respective ligands. These catalysts exhibit enhanced σ-donor and π-acceptor properties, intrinsic σ-π continuum attributes, and versatile binding sites, contributing to stability through enhanced dπ-pπ conjugation-induced aromaticity. Spectroscopic data, X-ray crystallographic data, and density functional theory calculations confirm their aromaticity. These iridaheteroaromatics exhibit formidable catalytic ability across a spectrum of transformations under industrially viable conditions, notably excelling in highly selective cross alkylation and ß-alkylation of alcohols and an eco-friendly avenue for quinolone synthesis, achieving remarkably high turnover frequencies (TOFs). Additionally, this method extends to the self-condensation of bioalcohols like ethanol, n-butanol, and n-hexanol in water, replicating conditions frequently encountered in primary fermentation solutions. These iridaheteroaromatics exhibit strong catalytic activity with fast reaction rates, high TOFs, broad substrate compatibility, and remarkable selectivity, displaying their potential as robust catalysts in large-scale applications and emphasizing their practical significance beyond their structural and theoretical importance.

Spodium bonding with noble gas atoms.

The nature of the bonding between a neutral group 12 member (Zn3, Cd3 and Hg3) ring and a noble gas atom was explored using quantum chemical simulations. Natural bond orbital, quantum theory of atoms in molecules, symmetry-adapted perturbation theory, and molecular electrostatic potential surface analysis were also used to investigate the type of interaction between the noble gas atom and the metal rings (Zn3, Cd3 and Hg3). The Zn3, Cd3 and Hg3 rings are bonded to the noble gas through non-covalent interactions, which was revealed by the non-covalent interaction index. Additionally, energy decomposition analysis reveals that dispersion energy is the key factor in stabilizing these systems.

Beryllium carbonyl Be(CO)n (n = 1-4) complex: a p-orbital analogy of Dewar-Chatt-Duncanson model.

Transition metal-carbonyl bonds are rationalized by M ← CO σ donation and M → CO π back donation where the d orbital of the transition metal is involved. This bonding model provided by Dewar, Chatt and Duncanson (DCD) has rationalized many transition metal-ligand bonds. The involvement of p orbital in such a DCD model can be intriguing. Alkaline earth metals with ns2np0 configuration may appear suitable as ns0np2 excitation has been recognized in many complexes. Herein, a theoretical study is presented for the Be(CO)n (n = 1-4) complex to verify this assumption. Detailed electronic structure analyses confirmed the involvement of the p orbital of beryllium in M → CO π back donation, thereby supporting the hypothesis. EDA-NOCV results reveal that the π-back donation from the central Be atom to CO ligands significantly predominates over the σ donation from the ligands for both Be(CO)3 and Be(CO)4. Our calculations reveal that Be(CO)4 is the highest carbonyl that may be experimentally detected.

BeM(CO)3- (M = Co, Rh, Ir) and BeM(CO)3 (M = Ni, Pd, Pt): Triply bonded terminal beryllium in zero oxidation state.

We perform detailed potential energy surface explorations of BeM(CO)3- (M = Co, Rh, Ir) and BeM(CO)3 (M = Ni, Pd, Pt) using both single-reference and multireference-based methods. The present results at the CASPT2(12,12)/def2-QZVPD//M06-D3/def2-TZVPPD level reveal that the global minimum of BeM(CO)3- (M = Co, Rh, Ir) and BePt(CO)3 is a C3v symmetric structure with an 1A1 electronic state, where Be is located in a terminal position bonded to M along the center axis. For other cases, the C3v symmetric structure is a low-lying local minimum. Although the present complexes are isoelectronic with the recently reported BFe(CO)3- complex having a B-Fe quadruple bond, radial orbital-energy slope (ROS) analysis reveals that the highest occupied molecular orbital (HOMO) in the title complexes is slightly antibonding in nature, which bars a quadruple bonding assignment. Similar weak antibonding nature of HOMO in the previously reported BeM(CO)4 (M = Ru, Os) complexes is also noted in ROS analysis. The bonding analysis through energy decomposition analysis in combination with the natural orbital for chemical valence shows that the bonding between Be and M(CO)3q (q = -1 for M = Co, Rh, Ir and q = 0 for M = Ni, Pd, Pt) can be best described as Be in the ground state (1S) interacting with M(CO)30/- via dative bonds. The Be(spσ) → M(CO)3q σ-donation and the complementary Be(spσ) ← M(CO)3q σ-back donation make the overall σ bond, which is accompanied by two weak Be(pπ) ← M(CO)3q π-bonds. These complexes represent triply bonded terminal beryllium in an unusual zero oxidation state.

Pushing The Extreme of Multicentre Bonding: Planar Pentacoordinate Hydride.

Planar hypercoordination has sparkled interest among the researchers from last few decades. Most of the elements in the Periodic Table have shown this remarkable structural feature. However, the smallest element, hydrogen, is missing in the list. No evidence is there in the literature. Herein, we introduce the first planar pentacoordinate hydrogen atom (ppH) in the global minimum geometry of Li5 H6 - cluster. Bonding analysis indicates that the central hydrogen atom is stabilized by multicentre bonding with five surrounding Li atoms. Natural charge analysis reveals that the central hydrogen is acting like a hydride which is strongly attracted by the positively charged surrounding lithium centres. The ppH structure is stabilized by strong electrostatic attraction as well as extensive multicentre bonding. Aromaticity has no role to play here. The cluster is dynamically stable and is expected to be detected in gas phase.

Hetero-Polar Sextuple Bond: A Relativistic Quantum Chemical Study.

Number of bonds formed by sharing an electron pair between two atoms is not restricted to one, it can go beyond four and six is the maximum. While homopolar sextuple bond in Mo2 and W2 has been reported, such a high bond order in heteropolar diatomics has remained elusive. In the pursuit of the sextuple bond in polar diatomics, the present study depicts the existence of such multiple bonds in Rhodium-Scandium hetero-diatom based on relativistic quantum chemical calculations. The bonding comprises of three normal electron sharing covalent bonds and three dative covalent bonds.

Rare Spin Avoided σ-σ Diradical Planar Tetracoordinate Boron Cluster: A Proto-Star for Planar Pentacoordination.

We report a global planar star-like cluster B3 Li3 featuring three planar tetracoordinate boron centres with a rare spin avoided σ-σ diradical character. The cluster was found to be stable towards dissociation into different fragments. The spin density was found to be localized solely on the three boron atoms in the molecular plane. This spin avoided σ-σ diradical character leads to the extension of the coordination number to yield a neutral B3 Li3 H3 and a cationic B3 Li3 H3 + cluster with three planar pentacoordinate boron centres in their global minimum structures. The planar geometry of the aninonic B3 Li3 H3 - cluster is slightly higher in energy. The planar global clusters were found to maintain planarity in their ligand protected benzene bound complexes, B3 Li3 (Bz)3 , B3 Li3 H3 (Bz)3 and B3 Li3 H3 (Bz)3 + with high ligand dissociation energies offering candidature for experimental detection.

Inorganic Bergman Cyclization: An Appeal From Theory.

The scope of Bergman cyclization is expanded computationally by exploring the cyclization in inorganic B, N substituted derivative. This substitution has introduced polarity into the transition state, which resulted in dramatic lowering of the activation barrier. Natural charge distribution throughout the reaction profile has ascertained this hypothesis. Single B and N atom substitution at 1 and 6 terminal positions lowers the activation barrier by almost half of the original value which becomes even lower in the complete B, N analogue. The parent Bergman and single B, N substituted products were characterized by significant biradical character while the complete B, N substituted analogue was characterized by significant zwitterionic character. Reduction in electron delocalization is also observed in the complete B, N substituted analogue.

Palladium-Catalyzed Annulative Coupling of Spirovinylcyclopropyl Oxindoles with p-Quinone Methides.

Pd-catalyzed annulative coupling of spirovinylcyclopropyl oxindoles with p-quinone methides has been accomplished via cascade carbon-carbon bond formation to afford bis-spirooxindole scaffolds. The mild reaction conditions, diastereoselectivity, functional group diversity, post-synthetic transformations, and mechanistic studies using DFT calculations are the important practical features.

Cobalt-Catalyzed Stereospecific C-N/C-O Bond Formation of Oxiranes with Diaziridines.

Co-catalyzed stereospecific C-N and C-O bond formation of oxiranes with diaziridines has been accomplished to furnish tetrahydro-[1,3,4]-oxadiazines at room temperature. Optically active oxiranes can be coupled with high optical purities (>96% ee). Stereoselectivity, functional group tolerance, mechanistic studies using DFT, and natural product modification are the important practical features.

Alkali Metal Decorated Planar Tetracoordinate Hydrogen.

Despite the long list of planar tetracoordinate atoms, hydrogen is elusive. This is especially due to the inherent ability of hydrogen to form multicenter bonds with other centers. Herein, we introduce the first planar tetracoordinate hydrogen atom (ptH) in the global minimum geometry of a C2v symmetric Li4H4- cluster. Bonding analysis indicates that the central hydrogen atom is stabilized by multicenter bonding with four surrounding Li atoms. Natural charge analysis reveals that the central hydrogen is acting like a hydride, which is strongly attracted by the positively charged surrounding lithium centers. The ptH structure is stabilized by strong electrostatic attraction as well as extensive multicenter bonding. Aromaticity has no role to play here. The cluster is dynamically stable and is expected to be detected in the gas phase. Introduction of a heavier alkali metal such as sodium makes the planar C2v cluster a local minimum with slightly higher energy than the linear global minimum geometry.

Sixth Ligand Induced HNO/NO- Release by a Five-Coordinated Cobalt(II) Nitrosyl Complex Having a {CoNO}8 Configuration.

Nitroxyl (HNO) and nitroxide (NO-) anion, the one-electron-reduced form of nitric oxide (NO), have been shown to have distinct advantages over NO from pharmacological and therapeutic points of view. However, the role of nitroxyl in chemical biology has not yet been studied as extensively as that of NO. Consequently, only a few examples of HNO donors such as Angeli's salt, Piloty's acid, or acyl- and acyloxynitroso derivatives are known. However, the intrinsic limitations of all of these hinder their widespread utility. Metal nitrosyl complexes, although few examples, could serve as an efficient HNO donor. Here, a cobalt nitrosyl complex of the {CoNO}8 (1) configuration has been reported. This complex in the presence of a sixth ligand [BF4-, DTC- (diethyldithiocarbamate anion), or imidazole] releases/donates HNO/NO-. This has been confirmed using well-known HNO/NO- acceptors like [Fe(TPP)Cl] and [Fe(DTC)3]. The HNO release has been authenticated further by the detection and estimation of N2O using gas chromatography-mass spectroscopy as well as its reaction with PPh3.

Three-membered beryllium ring, Be3: not just a hydrogen bond acceptor.

Quantum chemical calculations predict that the three-membered beryllium ring, Be3 is not just a hydrogen bond acceptor. It is a bond breaker. Calculations reveal that the global minimum geometry of Be3HX (X = F, Cl, OH, CN) features a linear geometry with complete cleavage of the H-X bond along with linearization of the Be3 unit. The linear geometry is radical stabilized Be trimer, R-Be-Be-Be-R. Thus, the present study clearly reveals that the Be3 ring has more to offer than just showing hydrogen bond accepting ability.

Planar pentacoordinate carbon in a 16-electron neutral CCu2Be3H4 cluster.

Planar hypercoordination in carbon is a fascinating area, which departs from the usual view of coordination geometry in carbon. Herein, we propose a planar pentacoordinate carbon species in the global minimum of the CCu2Be3H4 cluster. The cluster is a 16-electron species that is thermodynamically and kinetically very stable. Bonding analyses reveal 2π/6σ double aromaticity in the cluster. A low-energy isomerization pathway also reveals that the cluster has a sufficient lifetime for its experimental detection.

An in silico study of the selective adsorption and separation of CO2 from a flue gas mixture (CH4, CO2, N2) by ZnLi5+ clusters.

Due to the increasing concentration of CO2 in the atmosphere and its negative effect on the environment, selective adsorption of CO2 from flue gas has become significantly important. In this study, we have considered a Zn-doped lithium cluster, ZnLi5+ cluster, featuring a planar pentacoordinate Zn centre, as a potential candidate for selective CO2 capture and separation from a flue gas mixture (CH4, CO2, N2). The binding energy calculation and non-covalent interaction study showed that CO2 molecules bind relatively strongly as compared to N2 and CH4 molecules. The metal cluster can bind five CO2, five CH4, and four N2 molecules with average binding energies of -9.2, -4.4, and -6.1 kcal mol-1, respectively. Decomposition of the binding energy through symmetry-adapted perturbation theory analysis reveals that the electrostatic component plays a major role. The cationic cluster may be a promising candidate for selective CO2 capture and can be used as a pollution-controlling agent. The calculated adsorption energy of H2S is quite closer to that of CO2, suggesting competitive adsorption between CO2 and H2S. The adsorption energies of H2O and NH3 are higher compared to CO2, indicating that these gases may be a potential threat to CO2 capture.

Identification of Global Minimum of HNBeCO Complex.

Multiple bonding has always excited chemists. Recently, triple bonding between beryllium and N atoms in the HNBeCO complex has been reported based on experimental infrared spectroscopy and theoretical calculations. However, the present work reports a different structure based on a detailed potential energy surface scan. The global minimum geometry features only a weak partial Be-N double bond. The global minimum geometry lies very deep in the potential energy surface with respect to the reported one. Isomerization kinetics reveals that the reported structure has to overcome a very small barrier (5.4 kcal/mol) to isomerize to the global one. Although the previously reported structure is a real minimum, the present study identifies a much lower energy structure. A re-examination of the experimental spectra might show that the global minimum has also been observed.

Hydrogen storage capacity of Be2 (NLi)2 cluster with ultra-short beryllium-beryllium distance.

Quantum chemical calculations have been carried out to investigate the hydrogen storage capacity of Be2 (NLi)2 cluster. Calculations reveal that the cluster can take up to eight H2 molecules reaching a maximum gravimetric density of 21.04 wt%. Six H2 molecules bind at the Li atoms and two H2 bind at the Be atoms with moderate binding energy which is required for reversible storage of H2 . Symmetry-adapted perturbation analysis reveals the significant contribution of electrostatic and induction and very minor contribution of dispersion toward the total interaction energy. Molecular dynamics simulations reveal that the H2 molecules are strongly bound at 77 K and get slowly released at elevated temperatures.

Tetrametallic Copper Complex to Nanoscale Copper: Selective and Switchable Dehydrogenation-Hydrogenation under Light.

A discrete, photoactive, ultrafine copper nanocluster of fewer than hundreds of atoms with stimuli-responsive switchable redox-active states is highly desired to control two different antagonistic reactions. Herein, a mixed-valent tetrametallic copper complex (C-1) of N-O-N Schiff base ligand is disclosed, in which the five different Cu-Cu interactions were used to generate photoactive nanoscale copper [LCu0 n , S-1] through the reduction of coordinated imine to the amine of C-1. The presence of a ligand provides stability and helps to homogenize the material (S-1) in the organic solvent. The cluster showed stimuli (O2 /light)-responsive switching between its reduced (S-1) and oxidized [LCu0 n-m CuOm , S-2] states that allows it to serve as a highly and poorly active (bistate, relative rate >5-12 fold) catalyst for the dehydrogenation of alcohols to aldehydes and hydrogenation of nitroaromatics to amino aromatics under the light.

Planar Pentacoordinate Zinc Group Elements Stabilized by Multicentric Bonds.

Planar pentacoordinate zinc group elements, (M = Zn, Cd, Hg) were computationally found to be at a global minimum in Li5M+ clusters. The stability of these clusters is due to the presence of multicentric bonds. The central element (Zn, Cd, Hg) in each cluster features a negative oxidation state owing to the in-plane electron donation by the Li5+ framework. A similar global minimum planar pentacoordinate structure is found in Na5Zn+ and Na5Cd+ clusters.