Ashwini K. Phukan

Isolation of a Neutral Boron‐Containing Radical Stabilized by a Cyclic (Alkyl)(Amino)Carbene

Utilizing a cyclic (alkyl)(amino)carbene (CAAC) as a ligand, neutral CAAC-stabilized radicals containing a boryl functionality could be prepared by reduction of the corresponding haloborane adducts. The radical species with a duryl substituent was fully characterized by single-crystal X-ray structural analysis, EPR spectroscopy, and DFT calculations. Compared to known neutral boryl radicals, the isolated radical species showed larger spin density on the boron atom. Furthermore, the compound that was isolated is extraordinarily stable to high temperatures under inert conditions, both in solution and in the solid state. Electrochemical investigations of the radical suggest the possibility to generate a stable formal boryl anion species.

Metal-free binding and coupling of carbon monoxide at a boron–boron triple bond

Many metal-containing compounds, and some metal-free compounds, will bind carbon monoxide. However, only a handful of metal-containing compounds have been shown to induce the coupling of two or more CO molecules, potentially a method for the use of CO as a one-carbon-atom building block for the synthesis of organic molecules. In this work, CO was added to a boron-boron triple bond at room temperature and atmospheric pressure, resulting in a compound into which four equivalents of CO are incorporated: a flat, bicyclic, bis(boralactone). By the controlled addition of one CO to the diboryne compound, an intermediate in the CO coupling reaction was isolated and structurally characterized. Electrochemical measurements confirm the strongly reducing nature of the diboryne compound.

Direct Hydroboration of BB Bonds: A Mild Strategy for the Proliferation of BB Bonds

Synthetic access to electron-precise boron chains is hampered by the preferential formation of nonclassical structures. The few existing strategies for this involve either strongly reducing reagents or transition-metal catalysts, both with distinct disadvantages. The synthesis of new furyl- and thienyl-substituted diborenes is presented, along with their direct hydroboration with catecholborane (CatBH) to form a new electron-precise B-B bond and a B3 chain. The reaction is diastereoselective and proceeds under mild conditions without the use of strong reducing agents or transition-metal catalysts commonly used in B-B coupling reactions.

Is delocalization a prerequisite for stability of ring systems? A case study of some inorganic rings

A combined DFT, AIM and ELF study has been carried out on borazine and its heavier analogs containing both the pnictogens and chalcogens as the ring constituent. Compared to the pnictogen substituted rings, chalcogen substituted rings are found to be less aromatic. Except for a few systems, the computed aromatic stabilization energies (ASE) do not correlate with the NICS values. For these ring systems, NICS and bond length equalization are found to be better indicators of aromaticity than ASE. It was found that bulky electronegative substituents at the metal atom dramatically increases the stability and aromaticity of these molecules. AIM and ELF analysis predicts that boron and gallium based heterocycles are moderately aromatic while the aluminium analogs are significantly less aromatic.

New Routes to a Series of σ-Borane/Borate Complexes of Molybdenum and Ruthenium

A series of agostic σ-borane/borate complexes have been synthesized and structurally characterized from simple borane adducts. A room-temperature reaction of [Cp*Mo(CO)3 Me], 1 with Li[BH3 (EPh)] (Cp*=pentamethylcyclopentadienyl, E=S, Se, Te) yielded hydroborate complexes [Cp*Mo(CO)2 (μ-H)BH2 EPh] in good yields. With 2-mercapto-benzothiazole, an N,S-carbene-anchored σ-borate complex [Cp*Mo(CO)2 BH3 (1-benzothiazol-2-ylidene)] (5) was isolated. Further, a transmetalation of the B-agostic ruthenium complex [Cp*Ru(μ-H)BHL2 ] (6, L=C7 H4 NS2 ) with [Mn2 (CO)10 ] affords a new B-agostic complex, [Mn(CO)3 (μ-H)BHL2 ] (7) with the same structural motif in which the central metal is replaced by an isolobal and isoelectronic [Mn(CO)3 ] unit. Natural-bond-orbital analyses of 5-7 indicate significant delocalization of the electron density from the filled σBH orbital to the vacant metal orbital.

Gauging metal Lewis basicity of zerovalent iron complexes via metal-only Lewis pairs

A range of pentacoordinate iron complexes of the form [Fe(CO)5−nLn] (L = neutral donor, n = 0–2) were treated with gallium chloride or bromide, leading to hexacoordinate metal-only Lewis pairs (MOLPs) of the form [Ln(OC)5−nFe → GaX3] (X = Cl, Br). These complexes were used to gauge the Lewis basicity of the pentacoordinate iron precursors through comparison of their experimentally- and computationally-derived Fe–Ga distances and the degree of pyramidalization at the Ga center. The data indicate that consecutive replacement of CO groups with donor ligands increases the Lewis basicity of [Fe(CO)5−nLn] complexes, with the largest effect seen upon going from n = 0 to n = 1. While the Lewis basicity differences are clear between tri-, tetra- and pentacarbonyl complexes, the difference between the ligands within each group is less clear. A series of transfer experiments were also performed, in which the GaCl3 was transferred consecutively to Fe complexes with increasing numbers of donor ligands.

Exclusive π Encapsulation of Light Alkali Metal Cations by a Neutral Molecule.

Cation-π interactions are one of the most important classes of noncovalent bonding, and are seen throughout biology, chemistry, and materials science. However, in almost every documented case, these interactions play only a supporting role to much stronger covalent or dative bonds, thus making examples of exclusive cation-π bonding exceedingly rare. In this study, a neutral diboryne molecule is found to encapsulate the light alkali metal cations Li(+) and Na(+) in the absence of a net charge, covalent bonds, or lone-pair donor groups. The resulting encapsulation complexes are, to our knowledge, the first structurally authenticated species in which a neutral molecule binds the light alkali metals exclusively through cation-π interactions.

Strongly Phosphorescent Transition Metal π-Complexes of Boron–Boron Triple Bonds

Herein are reported the first π-complexes of compounds with boron-boron triple bonds with transition metals, in this case CuI. Three different compounds were isolated that differ in the number of copper atoms bound to the BB unit. Metalation of the B-B triple bonds causes lengthening of the B-B and B-CNHC bonds, as well as large upfield shifts of the 11B NMR signals, suggesting greater orbital interactions between the boron and transition metal atoms than those observed with recently published diboryne/alkali metal cation complexes. In contrast to previously reported fluorescent copper(I) π-complexes of boron-boron double bonds, the Cun-π-diboryne compounds (n = 2, 3) show intense phosphorescence in the red to near-IR region from their triplet excited states, according to their microsecond lifetimes, with quantum yields of up to 58%. While the Cu diborene bond is dominated by electrostatic interactions, giving rise to S1 and T1 states of pure IL(π-π*) nature, DFT studies show that the CuI π-complexes of diborynes reported herein exhibit enhanced metal d orbital contributions to HOMO and HOMO-1, which results in S1 and T1 having significant MLCT character, enabling strong spin-orbit coupling for highly efficient intersystem-crossing S1 → Tn and phosphorescence T1 → S0.

A B–C Double Bond Unit Coordinated to Platinum: An Alkylideneboryl Ligand that Is Isoelectronic to Neutral Aminoborylene Ligands

The reaction of [Pt(PCy3)2] with Br2B-CH(SiMe3)2 resulted in generation of the first alkylideneboryl complex, trans-[Br(Cy3P)2Pt{B=CH(SiMe3)}], with concomitant elimination of Me3 SiBr. The trans bromide ligand of the alkylideneboryl complex was readily substituted by a methyl group upon treatment with methyllithium, leading to another alkylideneboryl complex, trans-[Me(Cy3P)2Pt{B=CH(SiMe3)}]. Various spectrochemical techniques, single-crystal X-ray crystallography, and quantum chemical calculations confirmed the formulation of a double bond between the boron and the carbon atom. The theoretical studies also provided evidence for the stronger trans influence of the alkylideneboryl ligand over iminoboryl and oxoboryl ligands.

Synthesis and characterization of a mercury-containing trimetalloboride

The reaction of phenylmercuric chloride with an anionic dimanganese borylene (Li(+)[Cp2(CO)4Mn2B](-)) led to the formation of a trimetalloboride featuring the first reported bond between mercury and a non-cluster boron atom. Examination by (199)Hg NMR indicated (11)B-(199)Hg scalar coupling. Theoretical calculations indicated the nature of bonding to be σ-donation from a B-Mn π-orbital to Hg, in conjunction with weak Hgd→π* back-donation.