An Extended Rudolph Diagram: B3H5 and B3H6+ Relate 3D-, 2D-, 1D-, and 0D-Boron Allotropes.

The structural chemistry of boron goes beyond the sp, sp2, and sp3 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 B3H5 and the nonplanar B3H6+ constitute the missing links. The nonplanar B3H6+ (C3v) is the starting point for construction of polyhedral boranes; e.g., fusion of two of them leads to octahedral B6H62-. On the other hand, planar B3H6+ and B3H5 relate to borophenes with hexagonal holes. These borophene sheets can be further stacked with diverse interlayer BB bonds, ranging from bilayers to infinite layers. The tendency to achieve electron sufficiency as in the parent C3H3+ dictates the preference for hexagonal holes in the constituent layers and the interlayer bonds between them in multilayer borophenes. The design principles and theoretical validations for the formation of multilayer borophenes are also presented, indicating the variety and complexities involved.

Rearrangement of dicarboranyl methyl cation to icosahedral C3 B9 H12 + : 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-CH2 7,9-nido-C2 B9 H10 + ) using ab initio molecular dynamics calculations with metadynamics. Our simulations confirmed the concerted mechanism of the rearrangement. We believe this work will resume the interest in its synthesis and carborane cations in general.

Lewis Acid Stabilized Diatomic Molecules of Group 14: A Computational Study on [(CO)4Fe]2E2 (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)4] as Lewis acid. Computational studies on [(CO)4Fe]2E2, E = C, Si, Ge, Sn, and Pb, predict five plausible isomers on its potential energy surface: linear (E2_L), bent (E2_B), three-membered (E2_T), dibridged (E2_D), and four-membered (E2_F). For the carbon analogue, the lowest energy configuration is linear and has a typical cumulenic structure, while silicon and germanium analogues favor three-membered cyclic isomers. Four-membered cyclic isomers are the most stable for tin and lead analogues.

Pn Ring Size Dependence on NHC-Induced Ring Contraction Reactions of [CoCp‴(η4-P4)], [Ta(CO)2Cp″(η4-P4)], and [FeCp*(η5-P5)]: A DFT Study.

The facile ring contraction of [CoCp‴(η4-P4)] and [Ta(CO)2Cp″(η4-P4)] to [CoCp‴(η3-P3)][(MeNHC)2P] and [Ta(CO)2Cp″(η3-P3)] [(MeNHC)2P] induced by MeNHC and the absence of the ring contraction of [FeCp*(η5-P5)] 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 P3 and P4 rings and the lower strain energy of the P5 ring play a decisive role in the ring contraction capability of these [TM-cyclo-Pn] complexes. Theoretical approaches involving NBO and IBO analysis have been employed to provide a qualitative picture of the overall reactions. The role of substituents and the nature of transition metals in determining the energetics of these reactions has also been studied and an isolobal perspective on these systems affords a simplified picture.

Hexagonal Planar [B6 H6 ] within a [B6 H12 ] Borate Complex: Structure and Bonding of [(Cp*Ti)2 (µ-ɳ6 : ɳ6 -B6 H6 )(µ-H)6 ].

Isolation of planar [B6 H6 ] is a long-awaited goal in boron chemistry. Several attempts in the past to stabilize [B6 H6 ] were unsuccessful due to the domination of polyhedral geometries. Herein, we report the synthesis of a triple-decker sandwich complex of titanium [(Cp*Ti)2 (µ-η6 : η6 -B6 H6 )(µ-H)6 ] (1), which features the first-ever experimentally achieved nearly planar six-membered [B6 H6 ] ring, albeit within a [B6 H12 ] borate. The small deviation from planarity is a direct consequence of the predicted structural pattern of the middle ring in 24 Valence Electron Count (VEC) triple-decker complexes. The large ring size of [B6 H6 ] in 1 brings the metal-metal distance into the bonding range. However, significant electron delocalization from the M-M bonding orbital to the bridging hydrogen and B-B skeleton in the middle decreases its bond strength.

Metal-Stabilized [B8 H8 ]2- Derivatives with Dodecahedral Structure in the Solid and Solution States: [(Cp2 MBH3 )2 B8 H6 ] (Cp=η5 -C5 H5 ; M=Zr (1-Zr) and Hf (1-Hf)).

Despite the synthesis and structural characterization of closo-hydroborate dianions, [Bn Hn ]2- (n=6-12) more than 50 years ago, some ambiguity remains about the structure of [B8 H8 ]2- . Although the solid-state structure of [B8 H8 ]2- 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 [B8 H8 ]2- by using Cp2 MBH3 and structurally characterized two new octaborane analogues, [(Cp2 MBH3 )2 B8 H6 ] (Cp=η5 -C5 H5 ; M=Zr (1-Zr) and Hf (1-Hf)), so that the dynamics of the B8 skeleton were arrested. The solid-state structures of both 1-Zr and 1-Hf comprise a dodecahedron core protected by {Cp2 MBH3 } moieties on both sides of the cluster. Spectroscopic characterization (11 B NMR) validates the intactness of the B8 dodecahedron core in solution as well. Theoretical calculations establish that the two exo-{Cp2 MBH3 } fragments provide structural and electronic structural stability to the B8 core and its intact dodecahedral dianionic nature. Furthermore, we propose isodesmic equations for the formation of higher analogues of the Bn core (n>8) guarded by different group 4 transition metals. Our analysis suggests that, as we move to higher polyhedra (n>10), the formation becomes unfavourable irrespective of metal.

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.

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 [(Me2 NC6 H4 )Bi]2+ possessing three electrophilic sites is experimentally evidenced by the isolation of [{Me2 NC6 H4 }Bi{OP(NMe2 )3 }3 ][B(3,5-C6 H3 Cl2 )4 ]2 . These observations led us to generate dicationic organobismuth catalyst, [(Me2 NC6 H4 )Bi(L)3 ]2+ (L=aldehyde/ketone), evidenced by NMR spectroscopy in solution and by single-crystal X-ray diffraction in the solid state. It efficiently catalyzes hydrosilylation of aldehydes and ketones resulting in silyl ethers as the only products in high yields. Our investigations support a carbonyl activation mechanism at the bismuth center followed by Si-H addition.

A Neutral Three-Membered 2π Aromatic Disilaborirane and the Unique Conversion into a Four-Membered BSi2 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. Trimethylsilylnitrene, generated in situ, inserts in the Si-Si bond of disilaborirane to obtain a four-membered heterocycle 1-aza-2,3-disila-4-boretidine derivative. Both the heterocycles are fully characterized by X-ray crystallography.

Overlap of Radial Dangling Orbitals Controls the Relative Stabilities of Polyhedral B nH n- x 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. Stabilization of these molecular orbitals depends on their overlap. The lateral (sideways) overlap of dangling orbitals decreases with the decreasing cluster size from 12 to 5 boron atoms as the orbitals become more and more splayed out. Thus, as the number of dangling orbitals increases, the destabilization of their combinations increases at a higher rate for smaller polyhedral boranes, leading to flat structures with the removal of a fewer number of hydrogens. Though exohedral orbitals form better overlap in larger polyhedral clusters, the increase of electrons with the removal of H atoms results in occupancy of antibonding skeletal orbitals (beyond Wade's rules) and leads to flat structures. The reverse happens when hydrogens are added to a flat cluster. Substitution of BH by Si does not change structural patterns.

A theoretical analysis of the structure and properties of B26H30 isomers. Consequences to the laser and semiconductor doping capabilities of large borane clusters.

Decaborane(14), nido-B10H14, is the major commercially available molecular building block in boron cluster chemistry. The condensation of two such {nido-B10} blocks gives the known isomers of B18H22- 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 B26H30 regioisomers constructed from the fusion of three {nido-B10} blocks with the three subclusters conjoined by two-boron atom shared edges. In addition, density functional theory, time-dependent (TD)-DFT and multiconfigurational CASPT2 methods have been used to model and investigate the physical and photophysical properties of the three most stable of these isomers. Our findings predict these isomers to be potentially useful materials for the semiconductor industry, as high boron-content doping agents, and in the fabrication of new optical materials.

Stabilization of Classical [B2 H5 ]- : Structure and Bonding of [(Cp*Ta)2 (B2 H5 )(µ-H)L2 ] (Cp*=η5 -C5 Me5 ; L=SCH2 S).

The room-temperature reaction of [Cp*TaCl4 ] with LiBH4 ⋅THF followed by addition of S2 CPPh3 results in pentahydridodiborate species [(Cp*Ta)2 (µ,η2 :η2 -B2 H5 )(µ-H)(κ2 ,µ-S2 CH2 )2 ] (1), a classical [B2 H5 ]- ion stabilized by the binuclear tantalum template. Theoretical studies and bonding analysis established that the unusual stability of [B2 H5 ]- in 1 is mainly due to the stabilization of sp2 -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)4 } fragment (M=Mo or W) resulted in simple adducts, [{(Cp*Ta)(CH2 S2 )}2 (B2 H5 )(H){M(CO)3 }] (6: M=Mo and 7: M=W), that retained the diborane(5) unit.

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)3 {B(Dur)B(N(SiMe3 )2 )}] (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)3 and (B-Dur)(B-N(SiMe3 )2 ) units; the short B-B distance in 1 a is due to this π delocalization. The π-donor ligand N(SiMe3 )2 on the boron provides a decisive stability to the complex 1 a. The LUMO of 1 a has B-B σ-bonding character. Hence B-B coupling is facilitated by filling the LUMO. Strong σ-donating ligands, such as PMe3 or PCy3 , induce B-B coupling. Expulsion of one CO from 1 a followed by dimerization leads to [Fe(CO)2 {B(Dur)B(N(SiMe3 )2 )}]2 (3 a) with a short Fe-Fe distance of 2.355 Å. A detailed mechanism for the reaction of 3 a with CO to give the B-B catenation product 2 f is presented. The bonding of all intermediates is compared to their isolobal main-group analogues.

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 [2.2.2]propellane, especially their highest occupied molecular orbital (HOMO), shows the existence of significant electronic congestion at their bridge bond. The HOMO of [1.1.1]propellane is a spread-out orbital of its inverted tetrahedral bridgehead atoms. The HOMO of the open form of [2.2.2]propellane is an anti-bonding combination of its bridgehead atoms due to the stabilizing through-bond interaction. This unique spatial disposition of the HOMO enables a high electron density at the bridgehead atoms. Herein, we utilize the electron scavenging power of halogen bond donors to extract a fraction of destabilizing electrons from the bridge bond with the aim to alleviate its electronic congestion, which results in shortening and strengthening of the bridge bond with a reduction in the bond order. This result answers the seminal question raised by K. B. Wiberg in 1983, "how can one have a relatively 'strong bond' without much bonding character?"

Nanoisozymes: Crystal-Facet-Dependent Enzyme-Mimetic Activity of V2 O5 Nanomaterials.

Nanomaterials with enzyme-like activity (nanozymes) attract significant interest owing to their applications in biomedical research. Particularly, redox nanozymes that exhibit glutathione peroxidase (GPx)-like activity play important roles in cellular signaling by controlling the hydrogen peroxide (H2 O2 ) level. Herein we report, for the first time, that the redox properties and GPx-like activity of V2 O5 nanozyme depends not only on the size and morphology, but also on the crystal facets exposed on the surface within the same crystal system of the nanomaterials. These results suggest that the surface of the nanomaterials can be engineered to fine-tune their redox properties to act as "nanoisozymes" for specific biological applications.

Synthesis, Structure, Bonding, and Reactivity of Metal Complexes Comprising Diborane(4) and Diborene(2): [{Cp*Mo(CO)2 }2 {µ-η2 :η2 -B2 H4 }] and [{Cp*M(CO)2 }2 B2 H2 M(CO)4 ], M=Mo,W.

The reaction of [(Cp*Mo)2 (µ-Cl)2 B2 H6 ] (1) with CO at room temperature led to the formation of the highly fluxional species [{Cp*Mo(CO)2 }2 {µ-η2 :η2 -B2 H4 }] (2). Compound 2, to the best of our knowledge, is the first example of a bimetallic diborane(4) conforming to a singly bridged Cs structure. Theoretical studies show that 2 mimics the Cotton dimolybdenum-alkyne complex [{CpMo(CO)2 }2 C2 H2 ]. In an attempt to replace two hydrogen atoms of diborane(4) in 2 with a 2e [W(CO)4 ] fragment, [{Cp*Mo(CO)2 }2 B2 H2 W(CO)4 ] (3) was isolated upon treatment with [W(CO)5 ⋅thf]. Compound 3 shows the intriguing presence of [B2 H2 ] with a short B-B length of 1.624(4) Å. We isolated the tungsten analogues of 3, [{Cp*W(CO)2 }2 B2 H2 W(CO)4 ] (4) and [{Cp*W(CO)2 }2 B2 H2 Mo(CO)4 ] (5), which provided direct proof of the existence of the tungsten analogue of 2.

A DFT Study on the Stabilization of the B≡B Triple Bond in a Metallaborocycle: Contrasting Electronic Structures of Boron and Carbon Analogues.

The electronic structure of (η5 -Cp)2 Zr(NH2 -BB-NH2 ) (3 b) suggests that it could be a candidate for having a boron-boron triple bond in the cyclic system; however, computational studies shows that 3 b is a very high energy isomer on its potential energy surface. Replacement of amines with tricoordinate nucleophilic boron groups (η5 -Cp)2 Zr[B(PH3 )2 -BB-B(PH3 )2 ] (3 c) reduces the relative energy dramatically. The B≡B triple bond arises through the donation of two electrons from the metal fragment, ZrCp2 , to the in-plane π-bonding orbital of the central B-B unit. Strong σ-donating and chelating bis-phosphine ligands (Me2 P(CH2 )n PMe2 ), which stabilize donor-acceptor bonding interaction in gem-diborene L2 B-BBr2 (10), would be a good choice along the synthetic path towards 3 d, (η5 -Cp)2 Zr[B4 (Me2 P(CH2 )3 PMe2 )2 ]. A comparison of the energetics of the reaction leading to a cyclic boryne system (3 d), with the linear boryne isomer [(B2 NHCPh )2 ] shows that the angle strain from cyclization is compensated by stabilization from the metal.

Contrasting Behavior of the Z Bonds in X-Z···Y Weak Interactions: Z = Main Group Elements Versus the Transition Metals.

In contrast to the increasing family of weak intermolecular interactions in main-group compounds (X-Z···Y, Z = main-group elements), an analysis of the Cambridge Structural Database indicates that electron-saturated (18-electron) transition-metal complexes show reluctance toward weak M bond formation (X-M···Y, M = transition metal). In particular, weak M bonds involving electron-saturated (18-electron) complexes of transition metals with partially filled d-orbitals are not found. We propose that the nature of valence electron density distribution in transition-metal complexes is the primary reason for this reluctance. A survey of the interaction of selected electron-saturated transition-metal complexes with electron-rich molecules (Y) demonstrates the following: shielding the possible σ-hole on the metal center by the core electron density in 3d series, and enhanced electronegativity and relativistic effects in 4d and 5d series, hinders the formation of the M bond. A balance in all the destabilizing effects has been found in the 4d series due to its moderate polarizability and primogenic repulsion from inner core d-electrons. A changeover in the donor-acceptor nature of the metal center toward different types of incoming molecules is also unveiled here. The present study confirms the possibility of M bond as a new supramolecular force in designing the crystal structures of electron-saturated transition-metal complexes by invoking extreme ligand conditions.

Consequence of Ligand Bite Angle on Bismuth Lewis Acidity.

Ligand bite angle, a common parameter to fine-tune reactivity in transition-metal chemistry, is used for the first time in main-group chemistry to control and tune the Lewis acidity in organobismuth cations bearing 2-[(dimethylamino)methyl]phenyl (Me2NCH2C6H4) and 2-(dimethylamino)phenyl (Me2NC6H4) ligands. The latter chelating ligand induces a shorter C-Bi-N bite angle, leading to a weaker Bi-N bond with a corresponding lower Bi-N σ*-acceptor orbital and hence exhibiting remarkably higher Lewis acidity. The Gutmann-Beckett method is successfully employed to quantify the Lewis acidity in organobismuth cations.

The Role of Holes in Borophenes: An Ab†Initio Study of Their Structure and Stability with and without Metal Templates.

An electron-counting strategy starting from magnesium boride was used to show the inevitability of hexagonal holes in 2D borophene. The number (hole density, HD) and distribution of the hexagonal holes determine the binding energy per boron atom in monolayer borophenes. The relationship between binding energy and HD changes dramatically when the borophene is placed on a Ag(111) surface. The distribution of holes in borophenes on Ag(111) surfaces depends on the temperature. DFT calculations show that aside from the previously reported S1 and S2 borophene phases, other polymorphs may also be competitive. Plots of the electron density distribution of the boron sheets suggest that the observed STM image of an S2 phase corresponds to a sheet with a HD of 2/15 instead of a sheet with a HD of 1/5. The hole density and the hole distribution echo the distribution of vacancies and extra occupancies in complex ß-rhombohedral boron.