Small Ring Molecules Comprising 3-6 Boron Atoms: An Account on Synthesis, Structure, and Orbital Engineering.

ConspectusUnlike carbon, boron does not usually form ring compounds due to its electron-deficiency-driven affinity toward polyhedral geometries. The polyhedral boranes having -, -, -, or -shapes can be structurally and electronically correlated using various electron counting rules developed by Wade, Mingos, and one of us. However, in the last few decades, boron chemistry progressed significantly toward ring systems.

The Quest for Stable Borozene Core in Main-Group Capped Inverse Sandwich Complexes, [(HE)BH] (E=B, Al, Ga, In, and Tl).

The ubiquitous chemistry of benzene led us to explore ways to stabilise analogous borozene, by capping them with appropriate groups. The mismatch in overlap of ring-cap fragment molecular orbitals in [(HB)BH] is overcome by replacing the two BH caps with higher congeners of boron. We calculated the relative energies of all the polyhedral structural candidates for [(HE)BH] (E=Al-Tl) and found hexagonal bipyramid (HBP) to be more stable with Al-H caps.

An Extended Rudolph Diagram: BH and BH Relate 3D-, 2D-, 1D-, and 0D-Boron Allotropes.

The structural chemistry of boron goes beyond the sp, sp, and sp hybridization paradigms of carbon chemistry. We relate the apparently unconnected polyhedral boranes and 3D allotropes on the one hand and 2D clusters, borophenes, and multilayer borophenes on the other hand, through an extended Rudolph diagram. All-boron equivalents of cyclopropenium cation viz the flat BH and the nonplanar BH constitute the missing links.

Lewis Acid Stabilized Diatomic Molecules of Group 14: A Computational Study on [(CO)Fe]E (E = C, Si, Ge, Sn, Pb).

A Lewis base and acid combination has been effectively employed to stabilize and isolate the low-valent group 14 compounds. We report DFT studies on stabilizing low-valent group 14 diatomics as adducts of Lewis acids employing transition metal carbonyl fragment iron tetracarbonyl [Fe(CO)] as Lewis acid. Computational studies on [(CO)Fe]E, E = C, Si, Ge, Sn, and Pb, predict five plausible isomers on its potential energy surface: linear (), bent (), three-membered (), dibridged (), and four-membered ().

P Ring Size Dependence on NHC-Induced Ring Contraction Reactions of [CoCp‴(η-P)], [Ta(CO)Cp″(η-P)], and [FeCp*(η-P)]: A DFT Study.

The facile ring contraction of [CoCp‴(η-P)] and [Ta(CO)Cp″(η-P)] to [CoCp‴(η-P)][(NHC)P] and [Ta(CO)Cp″(η-P)] [(NHC)P] induced by NHC and the absence of the ring contraction of [FeCp*(η-P)] under the same conditions are studied by density functional theory (DFT) computations. The latter is estimated to be thermodynamically the least favorable reaction and also has a very high energy barrier. The similar strain energies of P and P rings and the lower strain energy of the P ring play a decisive role in the ring contraction capability of these [TM-cyclo-P] complexes.

Crystallographic evidence for a continuum and reversal of roles in primary-secondary interactions in antimony Lewis acids: applications in carbonyl activation.

Primary and secondary interactions form the basis of substrate activation in Lewis-acid mediated catalysis, with most substrate activations occurring at the secondary binding site. We explore two series of antimony cations, [(NMeCHCH)(mesityl)Sb] (A) and [(NMeCH)(mesityl)Sb] (B), by coordinating ligands with varying nucleophilicity at the position to the N-donor. The decreased nucleophilicity of the incoming ligands leads to reversal from a primary bond to a secondary interaction in A, whereas a constrained N-coordination in B diminishes the border between primary and secondary bonding.

From a Möbius-aromatic interlocked MnBH wheel to the metal-doped boranaphthalenes M@BH and MB 2D-sheets (M = Mn and Fe): a molecules to materials continuum using DFT studies.

The inherent tendency of BR fragments to undergo coupling is utilized to predict MBH and M@BH complexes (where M = Mn and Fe). Electronic structure analysis of MnBH (7) shows that the metal d-orbitals stabilize the interlocked boron wheel structure, forming an unprecedented geometrical pattern with Möbius aromaticity. The two additional electrons in Fe@BH (8) stabilize a twisted [10]boraannulene structure.

An electron counting formula to explain and to predict hydrogenated and metallated borophenes.

A generalized electron counting rule for borophene with varying hole density explains the preference for hydrogenation in β borophene of hole density ⅙. Interplay between bridging and terminal BH bonds controls the strength of interaction of the 2D layer with the metal surface.

Hexagonal Planar [B H ] within a [B H ] Borate Complex: Structure and Bonding of [(Cp*Ti) (μ-ɳ : ɳ -B H )(μ-H) ].

Isolation of planar [B H ] is a long-awaited goal in boron chemistry. Several attempts in the past to stabilize [B H ] were unsuccessful due to the domination of polyhedral geometries. Herein, we report the synthesis of a triple-decker sandwich complex of titanium [(Cp*Ti) (μ-η  : η -B H )(μ-H) ] (1), which features the first-ever experimentally achieved nearly planar six-membered [B H ] ring, albeit within a [B H ] borate.

Rearrangement of dicarboranyl methyl cation to icosahedral C B H : An ab initio dynamics view.

Closo-carborane anions are prominent, whereas the cations of the same are less abundant in the literature. As these ions have similar size and are weakly coordinating, the ionic liquids of these two ions could have important applications in many areas of chemistry. In view of limited number of polyhedral carborane cations available, we revisited the rearrangement of dicarboranyl methyl cation (7-CH 7,9-nido-C B H ) using ab initio molecular dynamics calculations with metadynamics.

Metal-Stabilized [B H ] Derivatives with Dodecahedral Structure in the Solid and Solution States: [(Cp MBH ) B H ] (Cp=η -C H ; M=Zr (1-Zr) and Hf (1-Hf)).

Despite the synthesis and structural characterization of closo-hydroborate dianions, [B H ] (n=6-12) more than 50 years ago, some ambiguity remains about the structure of [B H ] . Although the solid-state structure of [B H ] was established by single-crystal X-ray studies in 1969, fast rearrangements in solution at accessible temperatures prevented its detailed characterization. We therefore stabilized a derivative of [B H ] by using Cp MBH and structurally characterized two new octaborane analogues, [(Cp MBH ) B H ] (Cp=η -C H ; M=Zr (1-Zr) and Hf (1-Hf)), so that the dynamics of the B skeleton were arrested.

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.

Reversing Lewis acidity from bismuth to antimony.

Investigations on the boundaries between the neutral and cationic models of (Mesityl)EX (E = Sb, Bi and X = Cl, OTf) have facilitated reversing the Lewis acidity from bismuth to antimony. We use this concept to demonstrate a higher efficiency of (Mesityl)SbOTf over (Mesityl)BiOTf in the catalytic reduction of phosphine oxides to phosphines. The experiments supported with computations described herein will find use in designing new Lewis acids relevant to catalysis.

A Neutral Three-Membered 2π Aromatic Disilaborirane and the Unique Conversion into a Four-Membered BSi N-Ring.

We report the design, synthesis, structure, bonding, and reaction of a neutral 2π aromatic three-membered disilaborirane. The disilaborirane is synthesized by a facile one-pot reductive dehalogenation of amidinato-silylene chloride and dibromoarylborane with potassium graphite. Despite the tetravalent arrangement of atoms around silicon, the three-membered silicon-boron-silicon ring is aromatic, as evidenced by NMR spectroscopy, nucleus independent chemical shift calculations, first-principles electronic structure studies using density functional theory (DFT) and natural bond orbital (NBO) based bonding analysis.

Electrophilic Organobismuth Dication Catalyzes Carbonyl Hydrosilylation.

Bismuth compounds are gaining importance as potential alternatives to transition-metal complexes and electron deficient lighter p-block compounds in homogeneous catalysis. Computational analysis on the two-coordinate [(Me NC H )Bi] possessing three electrophilic sites is experimentally evidenced by the isolation of [{Me NC H }Bi{OP(NMe ) } ][B(3,5-C H Cl ) ] . These observations led us to generate dicationic organobismuth catalyst, [(Me NC H )Bi(L) ] (L=aldehyde/ketone), evidenced by NMR spectroscopy in solution and by single-crystal X-ray diffraction in the solid state.

Structures and bonding in [L]M(μ-CCR)M[L] and [L]M(μ-RCR)M[L]: requirements for C-C coupling.

A theoretical analysis of [L]M(μ-CCR)M[L] and [L]M(μ-RCR)M[L], where M represents the selected elements from the main group, transition metals, lanthanides and actinides, shows how the central (μ-CCR) and (μ-RCR) units reorganize as M traverses across the periodic table. In this context transition metal and actinide complexes are similar in nature, while lanthanide and main group complexes show similarity. The ground state electronic configuration and thus the metal oxidation state control these striking differences.

Organoaluminum cations for carbonyl activation.

In search of stable, yet reactive aluminum Lewis acids, we have isolated an organoaluminum cation, [(Me2NC6H4)2Al(C4H8O)2]+, coordinated with two labile tetrahydrofuran ligands. Its catalytic performance in aldehyde dimerization reveals turn-over frequencies reaching up to 6000 h-1, exceeding that of the reported main group catalysts. The cation is further demonstrated to catalyze hydroelementation of ketones.

Stabilization of Classical [B H ] : Structure and Bonding of [(Cp*Ta) (B H )(μ-H)L ] (Cp*=η -C Me ; L=SCH S).

The room-temperature reaction of [Cp*TaCl ] with LiBH ⋅THF followed by addition of S CPPh results in pentahydridodiborate species [(Cp*Ta) (μ,η :η -B H )(μ-H)(κ ,μ-S CH ) ] (1), a classical [B H ] ion stabilized by the binuclear tantalum template. Theoretical studies and bonding analysis established that the unusual stability of [B H ] in 1 is mainly due to the stabilization of sp -B center by electron donation from tantalum. Reactions to replace the hydrogens attached to the diborane moiety in 1 with a 2 e {M(CO) } fragment (M=Mo or W) resulted in simple adducts, [{(Cp*Ta)(CH S )} (B H )(H){M(CO) }] (6: M=Mo and 7: M=W), that retained the diborane(5) unit.

A theoretical analysis of the structure and properties of BH isomers. Consequences to the laser and semiconductor doping capabilities of large borane clusters.

Decaborane(14), nido-BH, is the major commercially available molecular building block in boron cluster chemistry. The condensation of two such {nido-B} blocks gives the known isomers of BH- a molecule used in the fabrication of p-type semiconductors and capable of blue laser emission. Here, we computationally determine the structures and thermodynamic stabilities of 20 possible BH regioisomers constructed from the fusion of three {nido-B} blocks with the three subclusters conjoined by two-boron atom shared edges.

Isolation of base stabilized fluoroborylene and its radical cation.

Herein, we report the synthesis and characterization of the metal free low valent fluoroborylene [(Me-cAAC)BF] (1) stabilized by cyclic (alkyl)(amino) carbene (cAAC). The fluoroborylene 1 is obtained by the reductive defluorination of Me-cAAC:BF with 2.0 equivalents of KC in the presence of 1.

Overlap of Radial Dangling Orbitals Controls the Relative Stabilities of Polyhedral B H Isomers ( n = 5-12, x = 0 to n - 1).

The removal of H atoms from polyhedral boranes results in the formation of dangling radial orbitals with one electron each. If there is a requirement of electrons for skeletal bonding to meet the Wade's rule, these are provided from the exohedral orbitals. Additional electrons occupy a linear combination of the dangling orbitals.

B-B Coupling and B-B Catenation: Computational Study of the Structure and Reactions of Metal-Bis(borylene) Complexes.

A detailed molecular orbital analysis of the metal-bis(borylene) complex [Fe(CO) {B(Dur)B(N(SiMe ) )}] (Dur=2,3,5,6-tetramethylphenyl) (1 a) serves as a focal point of recent developments in this area of chemistry, such as B-B coupling and B-B catenation reactions. There is strong a π delocalization between the Fe(CO) and (B-Dur)(B-N(SiMe ) ) units; the short B-B distance in 1 a is due to this π delocalization. The π-donor ligand N(SiMe ) on the boron provides a decisive stability to the complex 1 a.

Halogen bond shortens and strengthens the bridge bond of [1.1.1]propellane and the open form of [2.2.2]propellane.

Detailed electronic structural analysis of [1.1.1]propellane and the open form of [2.

Synthesis, Structure, Bonding, and Reactivity of Metal Complexes Comprising Diborane(4) and Diborene(2): [{Cp*Mo(CO) } {μ-η :η -B H }] and [{Cp*M(CO) } B H M(CO) ], M=Mo,W.

The reaction of [(Cp*Mo) (μ-Cl) B H ] (1) with CO at room temperature led to the formation of the highly fluxional species [{Cp*Mo(CO) } {μ-η :η -B H }] (2). Compound 2, to the best of our knowledge, is the first example of a bimetallic diborane(4) conforming to a singly bridged C structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum-alkyne complex [{CpMo(CO) } C H ].