Overlap of Radial Dangling Orbitals Controls the Relative Stabilities of Polyhedral B n H 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. © Copyright © 2019 American Chemical Society

Aza-bowls: synthesis and molecular structure of triaza-[3]-peristylane

A one-pot synthesis of the first aza-bowl, [3]-aza-[3]- eristylane system, from bullvalene is described. The molecular structure of this novel entity has been probed by X-ray crystallography and theoretical calculations

Vertex-Fused Metallaborane Clusters: Synthesis, Characterization and Electronic Structure of [(eta(5)-C(5)Me(5)Mo)(3)MoB(9)H(18)]

The reaction of the [(eta(5)-C(5)Me(5))MoCl(4)] complex with [LiBH4 - TH F] in toluene at - 70 degrees C, followed by pyrolysis at 110 degrees C, afforded dark brown [(eta(5)-C(5)Me(5)Mo)(3)MoB(9)H(18)], 2, in parallel with the known [(eta(5)-C(5)Me(5)Mo)(2)B(5)H(9)], 1. Compound 2 has been characterized in solution by (1)H, (11)B, and (13)C NMR spectroscopy and elemental analysis, and the structural types were unequivocally established by crystallographic studies. The title compound represents a novel class of vertex-fused clusters in which a Mo atom has been fused in a perpendicular fashion between two molybdaborane clusters. Electronic structure calculations employing density functional theory yield geometries in agreement with the structure determinations, and on grounds of density functional theory calculations, we have analyzed the bonding patterns in the structure

Generation of cationic two-coordinate group-13 ligand systems by spontaneous halide ejection: remarkably nucleophile-resistant (dimethylamino)borylene complexes.

Spontaneous ejection of chloride from a three-coordinate boron Lewis acid can be effected by employing very electron rich metal substituents and leads to the formation of a sterically unprotected terminal (dimethylamino)borylene complex that has a short metal-boron bond and remarkable resistance to attack by nucleophilic and protic reagents

Reactivity of Cationic Terminal Borylene Complexes: Novel Mechanisms for Insertion and Metathesis Chemistry Involving Strongly Lewis Acidic Ligand Systems

The reactions of terminal borylene complexes of the type [CpFe(CO) 2(BNR2)]+ (R = i1Pr, Cy) with heteroallenes have been investigated by quantum-chemical methods, in an attempt to explain the experimentally observed product distributions. Reaction with dicyclohexylcarbodiimide (CyNCNCy) gives a bis-insertion product, in which 1 equiv of carbodiimide is assimilated into each of the Fe=B and B=N double bonds to form a spirocyclic boronium system. In contrast, isocyanates (R'NCO, R' = Ph, 2,6XyI, Cy; XyI = CoH3Me2) react to give isonitrile complexes of the type [CpFe(CO)2(CNR')]+, via a net oxygen abstraction (or formal metathesis) process. Both carbodiimide and isocyanate substrates are shown to prefer initial attack at the Fe=B bond rather than the B=N bond of the borylene complex. Further mechanistic studies reveal that the carbodiimide reaction ultimately leads to the bis-insertion compounds [CpFe(CO)2C(NCy)2B(NCy)2CNR2] +, rather than to the isonitrile system [CpFe(CO)2(CNCy)] +, on the basis of both thermodynamic (product stability) and kinetic considerations (barrier heights). The mechanism of the initial carbodiimide insertion process is unusual in that it involves coordination of the substrate at the (borylene) ligand followed by migration of the metal fragment, rather than a more conventional process: i.e., coordination of the unsaturated substrate at the metal followed by ligand migration. In the case of isocyanate substrates, metathesis products are competitive with those from the insertion pathway. Direct, single-step metathesis reactivity to give products containing a coordinated isonitrile ligand (i.e. [CpFe(CO)2(CNRO]+) is facile if initial coordination of the isocyanate at boron occurs via the oxygen donor (which is kinetically favored); insertion chemistry is feasible when the isocyanate attacks initially via the nitrogen atom. However, even in the latter case, further reaction of the monoinsertion product so formed with excess isocyanate offers a number of facile (low energetic barrier) routes which also generate [CpFe(CO)2(CNRO]+, rather than the bis-insertion product [CpFe(CO)2C(NRO(O)B(NRO(O)CNR2]+ (i.e., the direct analogue of the observed products in the carbodiimide reaction). © 2009 American Chemical Society

Half-Sandwich Group 8 Borylene Complexes: Synthetic and Structural Studies and Oxygen Atom Abstraction Chemistry

Cationic terminal borylene complexes, synthesized by halide abstraction, offer a versatile platform on which to gauge the effects on the electronic structure of the metal-ligand bond brought about by variation in the borylene substituent and the metal/ligand framework. While the borylene substituent exerts a strong influence on boron-centered electrophilicity and hence on metal-ligand n character and bond length (e.g., from 1.792(8) Å for [Cp Fe(CO)2(BMes)]-to 2.049(4) Å for [CpFe(CO) 2{ B(NCy2)(4pic)}]+), much smaller changes are effected by changes in the metal/ligand set. Introduction of stronger n donor ruthenium- and/or phosphine-containing fragments is readily brought about by extension of the halide abstraction approach; phosphines are readily introduced by carbonyl ligand substitution at the boryl precursor stage. Thus, the novel systems [CpRu(CO)2(B(NCy2)J]+[BAr4]-, [CpM(CO)(PMe3)(B(NCy2)J]+[BArJ 4]" (M = Fe, Ru), and [(CpFe(CO){B(NCy2)J)2(-dmpe)]2+[BAr 4]-2 have been synthesized and structurally characterized. The effects of substitution at the metal on the M-B and B-N bonds are relatively minor {e.g., Fe-B and B-N bond lengths of 1.821(4), 1.347(5) Å and 1.859(6), 1.324(7) Å for [CpFe(CO)(PMe3)(B(NCy 2)J]+[BM]" and [CpFe(CO)2(B(NCy2)J] +[BAr4]-, respectively}, presumably reflecting, at least in part, the mutually cis disposition of the borylene and phosphine/carbonyl ligands. The utility of cationic complexes containing formally subvalent boron-based ligands in oxygen atom abstraction chemistry has been demonstrated by the conversion of a range of isocyanates, R'NCO, to the corresponding (metalcoordinated) isonitriles, [CpFe(CO)2(CNR')]+. Moreover, with a view to modeling potential intermediates, the mechanism of related chemistry with carbodiimide substrates, R'NCNR', has been investigated by structural and in situ ESI-MS approaches. While reactivity toward carbodiimides leads to the formation of a novel spirocyclic boronium complex, [CpFe(CO)2C(NCy) 2B(NCy)2CNCy2]+[BAr4]- (for R' = Cy), by a double-insertion process, DFT studies imply that the analogous product of isocyanate insertion is unlikely to be an intermediate on the pathway to isonitrile formation. The presence of a number of facile competing reaction pathways (including metathesis) and the thermodynamic stability of the spirocyclic product mean that the product distribution is better explained in terms of competing pathways, rather than differing extents of reaction along similar traiectories. © 2009 American Chemical Society

Chlorinated Hypoelectronic Dimetallaborane Clusters: Synthesis, Characterization, and Electronic Structures of (eta(5)-C5Me5W)(2)B5HnClm (n=7, m=2 and n=8, m=1)

Pyrolysis of (eta(5)-C5Me5WH3)B4H8, 1, in the presence of excess BHCl2 center dot SMe2 in toluene at 100 degrees C led to the isolation of (eta(5)-C5Me5W)(2)B5H9, 2, and B-Cl inserted (eta(5)-C5Me5W)(2)B5H8Cl, 3, and (eta(5)-C5Me5W)(2)B5H7Cl2, (four isomers). All the Chlorinated tungstaboranes were isolated as red and air and moisture sensitive solids. These new compounds have been characterized in solution by H-1, B-11, C-13 NMR, and the structural types were unequivocally established by crystallographic analysis of compounds 3, 4, and 7. Density functional theory (DFT) calculations were carded out on the model molecules of 3-7 to elucidate the actual electronic structures of these chlorinated species. On grounds of DFT calculations we demonstrated the role of transition metals, bridging hydrogens, and the effect of electrophilic substitution of hydrogens at B-H vertices of metallaborane structures

Chlorinated Hypoelectronic Dimetallaborane Clusters: Synthesis, Characterization, and Electronic Structures of (eta(5)-C5Me5W)(2)B5HnClm (n=7, m=2 and n=8, m=1)

Pyrolysis of (eta(5)-C5Me5WH3)B4H8, 1, in the presence of excess BHCl2 center dot SMe2 in toluene at 100 degrees C led to the isolation of (eta(5)-C5Me5W)(2)B5H9, 2, and B-Cl inserted (eta(5)-C5Me5W)(2)B5H8Cl, 3, and (eta(5)-C5Me5W)(2)B5H7Cl2, (four isomers). All the Chlorinated tungstaboranes were isolated as red and air and moisture sensitive solids. These new compounds have been characterized in solution by H-1, B-11, C-13 NMR, and the structural types were unequivocally established by crystallographic analysis of compounds 3, 4, and 7. Density functional theory (DFT) calculations were carded out on the model molecules of 3-7 to elucidate the actual electronic structures of these chlorinated species. On grounds of DFT calculations we demonstrated the role of transition metals, bridging hydrogens, and the effect of electrophilic substitution of hydrogens at B-H vertices of metallaborane structures