Aromaticity in small rings containing boron and carbon, ((CH)2(BH)n, n = 1,2): comarisons with isoelectronic carbocations. The decisive roles of orbital mixing and nonbonded 1,3-interactions in the structures of four-membered rings

Isolation of Fluorescent 2π-Aromatic 1,3-Disiladiboretenes

Herein, we reported the isolation of 2π-aromatic disiladiboretenes (L2Si2B2Ph2) [L = ArC(NtBu)2, Ar = Ph (1), Mes (2)], which have been synthesized from the straightforward reduction of silylene-borane adducts (LSiX → BX2Ph) [X = Cl, Br] with potassium graphite (KC8). X-ray diffraction analysis of 1 and 2 revealed that the Si2B2 units are completely planar, and DFT calculations suggested delocalization of 2π-electrons over the Si2B2 rings. Moreover, their photophysical properties and reactivity toward sulfur were also investigated in detail.

A Journey from Benzoborirene to Benzoborole-Supported 1,2-Diimine and Antiaromatic Borolediide

The reaction of benzoborirene with one equivalent of isocyanides leads to benzene-fused boretes bearing one imine function, while the reaction with two equivalents of isocyanide affords 2,3-dihydro-2,3-diiminoboroles with perfect regioselectivity. The isocyanide double insertion products represent a novel type of 1,2-diimine with a benzoborole backbone. The reduction chemistry of the benzoborole-supported 1,2-diimine was investigated. Single- and two-electron reduction allow for the isolation and full characterization of a radical anion and a dianion, respectively. In stark contrast to the ordinary boroles, which should turn aromatic by accepting two electrons, the antiaromatic character of the benzoborole backbone is highlighted upon reduction, thus presenting a rare example of antiaromatic borole dianion. Detailed quantum chemical calculations provide a rationale for the observed regioselectivity and the electronic structure of the reduced borole diimine species.

Dimorpholinoacetylene and Its Use for the Synthesis of Tetraaminocyclobutadiene Species

The new diaminoacetylene (DAA) dimorpholinoacetylene (3) was prepared from 1,1-dimorpholinoethene (1) by bromination to form the dibromoketene aminal 2, which upon lithiation afforded 3 through a Fritsch-Buttenberg-Wiechell rearrangement. Heating 3 at elevated temperatures resulted in a complete conversion into the dimer 1,1,2,4-tetramorpholino-1-buten-3-yne (4), which was used for the synthesis of four-membered cyclic bent allene (CBA) transition-metal complexes of the type [(CBA)MLn ] (5-7; MLn =AuCl, RhCl(COD), RhCl(CO)2 ; CBA=1,3,4,4-tetramorpholino-1,2-cyclobutadiene; COD=1,5-cyclooctadiene). The reaction of 3 with tetraethylammonium bromide gave 1,2,3,4-tetramorpholinocyclobutenylium bromide (8), which reacted with bromine to form 1,2,3,4-tetra(morpholino)cyclobutenediylium bis(tribromide) (9). Compound 9 represents the first fully characterized compound containing a tetraaminocyclobutadiene dication and displays a nearly planar C4 N4 core as shown by X-ray diffraction analysis. Detailed quantum chemical calculations were performed to assess the aromaticity of tetraaminocyclubutadiene dications by employing the Nucleus Independent Chemical Shift (NICS) method and current density analysis.

Accurate Ring Strain Energies of Unsaturated Three-Membered Heterocycles with One Group 13-16 Element

High-quality ring strain energy (RSE) data for 1H-unsaturated (CH)₂X parent rings, where X is a group 13–16 element, are reported in addition to the 2H-isomers of the pnictogenirene rings. RSE data are obtained from appropriate homosdesmotic reactions and calculated at the DLPNO-CCSD(T)/def2-TZVPP//B3LYP-D3/def2-TZVP(ecp) level. 1H-Tallirene and 1H-plumbirene have unique donor–acceptor structures between an acetylene π(CC) orbital and an empty p orbital of a metallylene subunit (a Dewar–Chatt–Duncanson description) and therefore cannot be described as proper rings but as pseudocyclic structures. Also, 1H-indirene and 1H-oxirene lack ring critical points and constitute borderline cases of pseudorings. 1H-Unsaturated rings exhibit enhanced RSE compared to their saturated homologues. The mechanism of ring strain relaxation by increasing the s character in the lone pair (LP) of group 15–16 elements is remarkable and increases on descending the groups. Furthermore, RSE is affected by the aromatic character of group 13 rings and certain aromatic or antiaromatic character in group 14 or 15–16 rings, respectively, which tend to vanish on descending the group as shown by NICS(1) values. 2H-Unsaturated rings were found only for group 15 elements (although only 2H-azirine shows a proper cyclic structure) and displayed lower RSE (higher stability) than the corresponding 1H-isomers.

Accurate ring strain energies (RSE) are calculated for three-membered 1H-unsaturated rings containing a 13−16 group element and group 15 2H-isomers. The presence or absence of lone pairs at the heteroatom and the existence of intrinsic or residual (anti)aromaticity were shown to affect the RSE.

Comparison of RNC Coupling and CO Coupling Mediated by Cr-Cr Quintuple Bond and B-B Multiple Bonds: Main Group Metallomimetics

A theoretical analysis of reductive coupling of isocyanide and CO mediated by a Cr-Cr quintuple bonded complex and B-B multiple bonded complexes shows how the difference in donor-acceptor capability of isocyanide and CO ligands controls the product distributions. In the case of CO, the Cr-Cr quintuple bonded complex is unable to show C-C coupling due to the high π- back bonding possibility of CO and the reaction follows the singlet potential energy surface throughout, whereas, in the case of isocyanide, less π- back bonding possibility allows the reactions to undergo a spin transition and gives a series of products with different spin multiplicities. Similarly, reactions of B-B multiple bonded complexes with CO and isocyanides are also controlled by donor-acceptor capabilities of ligands, and the C-C coupling takes place by changing the oxidation state of the boron centers from +I to +II, in contrast to the classical main group mediated reactions where stable oxidation states are always preserved. This part of the main group chemistry which is dominated by donor-acceptor bonding interaction is more likely to follow transition metal behavior.

Dithiophene-Fused Oxadiborepins and Azadiborepins: A New Class of Highly Fluorescent Heteroaromatics

Access to dithiophene-fused oxadiborepins and the first azadiborepins attained via a modular synthesis route are presented. The new compounds emit intense blue light, some of which demonstrate fluorescence quantum yields close to unity. Cyclic voltammetry (CV) revealed electrochemically reversible one-electron reduction processes. The weak aromatic character of the novel 1,2,7-azadiborepin ring is demonstrated with in-depth theoretical investigations using nucleus-independent chemical shift (NICS) scans and anisotropy of the induced current density (ACID) calculations.

Solution-Phase Synthesis of a Base-Free Benzoborirene and a Three-Dimensional Inorganic Analogue

The base-free benzoborirene 1,2-BR-1,2-C₆H₄ (7) and its three-dimensional inorganic analogue 1,2-BR-1,2-C₂B₁₀H₁₀ (13) have been successfully synthesized by Cp₂ZrBr₂ and LiCl elimination, respectively. The Cl analogue of the key intermediate for the formation of benzoborirene 7 has been isolated and structurally characterized, thus suggesting the reaction pathway via benzyne Zr complex formation, B-Br/C_benzyne-Zr σ-bond metathesis, and a Cp₂ZrBr₂ elimination/ring-closing process. The rationality of the reaction pathway has been confirmed by DFT calculations. In addition, the title compounds shared the same reactivity pattern (i.e., 1,3-silyl migration) toward ^MeIiPr (8), thus allowing for the synthetic approach to the first carborane-substituted iminoborane 14.

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.

Four Boron Atoms, Four Positive Charges, and Four Skeletal Electrons: A Fluorescent σ-Aromatic Tetraborane(4)

Reaction of a ditriflatodiborane compound with the Lewis acids AlCl₃ or GaCl₃ leads to abstraction of the two triflate substituents and dimerization of the resulting dicationic diborane to give a σ-aromatic tetracationic tetraborane with a planar, rhomboid B₄ core. The compound exhibits four skeletal σ-electrons involved in two (3c,2e) bonds and represents the first stable fourfold base-stabilized [B₄ H₄ ]⁴⁺ analogue. The product is isolated from the reaction mixture in the form of bright orange crystals that display fluorescence. Further analysis shows that the new tetraborane(4) is stabilized in the solid state by the lattice energy. It exhibits an extremely high electron affinity and is only stable in solution after one-electron reduction to the radical cation.

Computational Study of the Isomerization Reactions of Borirane

Borirane is isoelectronic to the cyclopropyl cation, but is stable toward electrocyclic ring opening to 2-bora-propa-1,3-diyl A, the boron analogue of the allyl cation. A computational investigation using density functional theory (B3LYP) in combination with highly correlated electronic structure theory methods of the coupled-cluster [CCSD(T)] and multireference configuration interaction (MRCI) type in conjunction with basis sets of up to quadruple-ζ quality reveal that formation of A is endothermic by roughly 15 kcal mol^-1 and that A collapses almost without barrier (0.2 kcal mol^-1) to borirane. The vinylborane isomer B is more stable than borirane and its formation is associated with a barrier of 36-38 kcal mol^-1. Methyl methylideneborane E, only slightly less stable than B, can only be accessed by pathways involving barriers of at least 60 kcal mol^-1.

DFT Studies of Dimerization Reactions of Boroles

Boroles undergo dimerization reactions to give Diels-Alder (DA) dimers, bridged-bicyclic (BB) dimers or spiro dimers (SD) depending on the substituents on the borole. We performed DFT calculations to investigate how different substituents at the carbon atoms of the butadiene backbone as well as at the boron atom influence the dimerization reaction pathways. The DFT results show that, in general, both the DA and BB dimers are easily accessible kinetically, and the DA dimers are thermodynamically more stable than the BB dimers. When the substituent-substituent repulsive steric interactions are alleviated to a certain extent, the BB dimers are more stable than the DA dimer, and become accessible. The SD dimers are generally kinetically difficult to obtain. However, we found that aryl substituents promote the formation of the SD dimers.

Orbital phase design of diradicals

Over the last three decades the rational design of diradicals has been a challenging issue because of their special features and activities in organic reactions and biological processes. The orbital phase theory has been developed for understanding the properties of diradicals and designing new candidates for synthesis. The orbital phase is an important factor in promoting the cyclic orbital interaction. When all of the conditions: (1) the electron-donating orbitals are out of phase; (2) the accepting orbitals are in phase; and (3) the donating and accepting orbitals are in phase, are simultaneously satisfied, the system is stabilized by the effective delocalization and polarization. Otherwise, the system is less stable. According to the orbital phase continuity requirement, we can predict the spin preference of π-conjugated diradicals and relative stabilities of constitutional isomers. Effects of the intramolecular interaction of bonds and unpaired electrons on the spin preference, thermodynamic and kinetic stabilities of the singlet and triplet states of localized 1,3-diradicals were also investigated by orbital phase theory. Taking advantage of the ring strains, several monocyclic and bicyclic systems were designed with appreciable singlet preference and kinetic stabilities. Substitution effects on the ground state spin and relative stabilities of diradicals were rationalized by orbital interactions without loss of generality. Orbital phase predictions were supported by available experimental observations and sophisticated calculation results. In comparison with other topological models, the orbital phase theory has some advantages. Orbital phase theory can provide a general model for both π-conjugated and localized diradicals. The relative stabilities and spin preference of all kinds of diradicals can be uniformly rationalized by the orbital phase property. The orbital phase theory is applied to the conformations of diradicals and the geometry-dependent behaviors. The insights gained from the orbital phase theory are useful in a rational design of stable 1,3-diradicals.

Why do two π-electron four-membered Hückel rings pucker?

Notwithstanding their two (i.e., 4n + 2) π electrons, four-membered ring systems, 1-4, favor puckered geometries (1a-4a) despite the reduction in vicinal π overlap and in the ring atom bond angles. This nonplanar preference is due to σ → π* hyperconjugative interactions across the ring (A) rather than to partial 1,3-bonding (B). Electronegative substituents (e.g., F in C(4)F(4)(2+)) reduce the σ → π* electron delocalization, and planar geometries result. In contrast, electropositive groups (e.g., SiH(3) in C(4)(SiH(3))(4)(2+)) enhance hyperconjugation and increase the ring inversion barriers substantially.

Structure and bonding in cyclic isomers of BAl2Hnm (n=3-6, m=-2 to +1): preference for planar tetracoordination, pyramidal tricoordination, and divalency

The structure and energetics of cyclic BAl2Hnm (n=3-6, m=-2 to +1), calculated at the B3LYP/6-311+G** and QCISD(T)/6-311++G** levels, are compared with their corresponding homocyclic boron and aluminium analogues. Structures in which the boron and aluminium atoms have coordination numbers of up to six are found to be minima. There is a parallel between structure and bonding in isomers of BAl2H(3)2- and BSi2H3. The number of structures that contain hydrogens out of the BAl2 ring plane is found to increase from BAl2H3(2-) to BAl2H6+. Double bridging at one bond is common in BAl2H5 and BAl2H6+. Similarly, species with lone pairs on the divalent boron and aluminium atoms are found to be minima on the potential energy surface of BAl2H(3)2-. BAl2H4- (2 b) is the first example of a structure with planar tetracoordinate boron and aluminium atoms in the same structure. Bridging hydrogen atoms on the B--Al bond prefer not to be in the BAl2 plane so that the pi MO is stabilised by pi-sigma mixing. This stabilisation increases with increasing number of bridging hydrogen atoms. The order of stability of the individual structures is decided by optimising the preference for lower coordination at aluminium, a higher coordination at boron and more bridging hydrogen atoms between B--Al bonds. The relative stabilisation energy (RSE) for the minimum energy structures of BAl2Hnm that contain pi-delocalisation are compared with the corresponding homocyclic aluminium and boron analogues.