Microwave spectroscopic study of the BBr molecule

Oxy-Borylenes as Photoreductants: Synthesis and Application in Dehalogenation and Detosylation Reactions

While the range of accessible borylenes has significantly broadened over the last decade, applications remain limited. Herein, we present tricoordinate oxy-borylenes as potent photoreductants that can be readily activated by visible light. Facile oxidation of CAAC stabilized oxy-borylenes (CAAC)(IPr2 Me2 )BOR (R=TMS, CH2 CH2 C6 H5 , CH2 CH2 (4-F)C6 H4 ) to their corresponding radical cations is achieved with mildly oxidizing ferrocenium ion. Cyclovoltammetric studies reveal ground-state redox potentials of up to -1.90 V vs. Fc+/0 for such oxy-borylenes placing them among the strongest organic super electron donors. Their ability as photoreductants is further supported by theoretical studies and showcased by the application as stoichiometric reagents for the photochemical hydrodehalogenation of aryl chlorides, aryl bromides and unactivated alkyl bromides as well as the detosylation of anilines.

On Neutral Unsaturated Ouroboric Borylenes

The search is on for stable isolated borylenes. Potential roles in modern synthetic chemistry for boron analogues of carbenes continue to motivate interest in locating them. Using density functional and ab initio methods, we posit and examine the thermochemistry, and chemical bonding, including aromaticity, of several classes of 5- and 6-membered borylenic rings. In these systems, cyclization relies on dative bonding (ouroboric coordination) and π donation to a monovalent boron center from an adjacent O center. Certain neutral five-membered rings (heterocyclic cyclopentadienyl analogues) in particular are found to exhibit exceptionally strong preferences for the singlet multiplicity, each with singlet-triplet (S-T) gaps in excess of 40 kcal·mol^-1. The singlet five-membered rings with the largest S-T gaps and some of the six-membered rings show evidence of weak aromaticity. Relationships of the form N = A·r^-b, in line with Gordy's and other functions linking bond order, N, and covalent bond length, are identified for dative B←O contacts, r, reinforced in rings by π-delocalization.

Ab initio investigation on the low-lying electronic states of thallium bromide

Thirteen Λ-S states of TlBr molecule are calculated by the method of multireference configuration interaction (MRCI) plus Davidson correction (+Q), and the spectroscopic constants of these states are fitted. The dipole moment of thirteen Λ-S states are also included and analyzed in this calculation. Two bound states split into five Ω states with the SOC effect. The interaction between the molecular configurations of TlBr is analyzed with spin orbit coupling. Considering the forbidden transition, transition dipole moment with SOC effect is considered. On account of the accurate potential energy curves and the transition dipole moment, the Franck-Condon factors and radiative lifetime of the a3Π0+ ↔ Χ1Σ0++ transition are calculated. The feasibility of laser cooling of TlBr molecule is analyzed by comparing with the thallium compounds and the bromides of the group-IIIA.

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)₂BF] (1) stabilized by cyclic (alkyl)(amino) carbene (cAAC). The fluoroborylene 1 is obtained by the reductive defluorination of Me-cAAC:BF₃ with 2.0 equivalents of KC₈ in the presence of 1.0 equivalent of Me-cAAC. Due to its highly electron rich nature, 1 underwent one-electron oxidation with 1.0 equivalent of lithium tetrakis(pentafluorophenyl)borate [LiB(C₆F₅)₄] to form the radical cation [(Me-cAAC)₂BF]˙⁺[B(C₆F₅)₄]^- (2). DFT studies suggested that the lone pair of electrons is localized on the boron atom in 1, which explains its unprecedented reactivity. Both compounds 1 and 2 were characterized by X-ray crystallography. The radical cation 2 was studied by EPR spectroscopy.

Synthesis and reduction chemistry of mixed-Lewis-base-stabilised chloroborylenes

Mixed-base-stabilised chloroborylenes are easily accessed by twofold reduction of a cyclic (alkyl)(amino)carbene-supported trichloroborane in the presence of a second Lewis base, thus enabling fine-tuning of the electronic properties of the electron-rich borylene centre.

The one-electron reduction of (CAAC^Me)BCl₃ (CAAC^Me = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene) yields the dichloroboryl radical [(CAAC^Me)BCl₂]˙. Furthermore, the twofold reduction of (CAAC^Me)BCl₃ in the presence of a range of Lewis bases (L = CAAC^Me, N-heterocyclic carbene, phosphine) yields a series of doubly base-supported (CAAC^Me)LBCl chloroborylenes, all of which were structurally characterised. NMR and UV-vis spectroscopic and electrochemical data for (CAAC^Me)LBCl show that the boron centre becomes more electron-rich and the HOMO–LUMO gap widens as L becomes less π-accepting. A [(CAAC^Me)BCl₂]^– boryl anion coordination polymer was isolated as a potential intermediate in these reductions. In most cases the reduction of the chloroborylenes resulted in the formation of the corresponding hydroborylenes or derivatives thereof, as well as ligand C–H activation products.

Borylenes: An Emerging Class of Compounds

Free borylenes (R-B:) have only been spectroscopically characterized in the gas phase or in matrices at very low temperatures. However, in recent years, a few mono- and bis(Lewis base)-stabilized borylenes have been isolated. In both of these compounds the boron atom is in the formal oxidation state +I which contrasts with classical organoboron derivatives wherein the element is in the +III oxidation state. Mono(Lewis base)-stabilized borylenes are isoelectronic with singlet carbenes, and their reactivity mimics to some extent that of transition metals. They can activate small molecules, such as H₂ , and coordinate an additional ligand; in other words, they are boron metallomimics. Bis(Lewis base)borylene adducts are isoelectronic with amines and phosphines. In contrast to boranes, which act as electron acceptors and thus Lewis acids, they are electron-rich and act as ligands for transition metals.

Electronically Excited States of Borylenes

Borylenes, RB, are elusive reactive intermediates. Still not much is known about their excited states from spectroscopic experiments, and existing knowledge is limited to diatomic borylenes only. The electronic structure and geometry of borylenes with diverse substituents on boron (where R = H, F, Cl, CH3, CF3, tBu, NH2, Ph, and SiMe3) were studied by means of computational chemistry. For this purpose, geometries of borylenes in their lowest singlet and triplet states were optimized at the B3LYP/def2-TZVP level of theory. Additionally, the influence of substitution on the energies of frontier molecular orbitals, HOMO-LUMO energy gaps, singlet-triplet energy splittings, and excitation energies was investigated. Two lowest vertical singlet-singlet excitations were computed using EOM-CCSD and TD-DFT (using hybrid B3LYP, and long-range separated CAM-B3LYP and ωB97X functionals) in combination with the aug-cc-pVTZ basis set. The electronic transitions involve excitations from nonbonding sp boron orbital (HOMO) mainly to empty p(B) orbitals and partially to the orbitals of the substituent, and are of n → π* type. The results can facilitate prospective identification of borylenes, e.g., in UV-vis matrix isolation or time-resolved spectroscopy experiments.

Microwave spectroscopy of the PBr radical in the X (3)Sigma(-) state

The microwave spectrum of the PBr radical in the X (3)Sigma(-) ground electronic state has been observed by a source modulated spectrometer. The PBr radical was generated in a free space cell by an acdc glow discharge in a mixture of PBr(3) with He andor H(2). A spectrum with three spin components for each of the two isotopomers, P(79)Br and P(81)Br, was observed. The spectrum showed hyperfine splitting caused by interactions due to both bromine and phosphorus nuclei. The molecular constants including the magnetic hyperfine and nuclear quadrupole hyperfine interaction constants were determined by analyzing the observed spectrum. The spin density of the unpaired electrons was estimated from the observed hyperfine coupling constants to be 85.4% and 16.3% on the phosphorus and bromine atoms, respectively.

Extensive ab initio study of the valence and low-lying Rydberg states of BBr including spin-orbit coupling

The electronic states of the BBr molecule, including 12 valence states and 12 low-lying Rydberg states, have been studied at the theoretical level of MR-CISD+Q with all-electron aug-cc-pVQZ basis sets and Douglas-Kroll [Ann. Phys. (N.Y.) 82, 89 (1974)] scalar relativistic correction. The spin-orbit coupling effect in the valence states was calculated by the state interaction approach with the full Breit-Pauli Hamiltonian. This is the first multireference ab initio study of the excited electronic states of BBr. Potential energy curves of all states were plotted with the help of the avoided crossing rule between electronic states of the same symmetry. The structural properties of these states were analyzed. Computational results reproduced most experimental data well. The transition properties of the a (3)Pi(0(+) ), a (3)Pi(1), and A (1)Pi(1) states to the ground state X (1)Sigma(0(+) ) (+) transitions were obtained, including the transition dipole moments, the Franck-Condon factors, and the radiative lifetimes. The evaluated radiative lifetime of the a (3)Pi(0(+) ), and a (3)Pi(1) states are near 1 ms, much longer than that of the A (1)Pi(1) state.

Infrared Diode Laser Spectroscopy of Fundamental and Hot Bands of BBr (X1Sigma+)

Infrared absorption spectra of the four common isotopic forms of the transient molecule BBr (11B79Br, 11B81Br, 10B79Br, and 10B81Br) have been observed in natural abundance. The spectra were detected in the region between 650 and 720 cm-1 using diode laser spectroscopy of a BBr3/He ac discharge. Over 150 lines consisting of both fundamental and hot bands up to v" = 5 have been fitted to Watson's isotopically invariant coefficients (Ukl). Spectroscopic parameters for each isotopomer could be derived from these coefficients; for 11B79Br, omegae = 685.1892(15) cm-1 and omegaexe = 3.73632(95) cm-1. The equilibrium dissociation energy obtained from Dunham's expanded Morse potential is 32 711(43) cm-1. Copyright 1998 Academic Press.

Microwave Spectroscopy of BrBO

The microwave spectra of the four isotopomers of the BrBO molecule (79Br10BO, 81Br10BO, 79Br11BO, and 81Br11BO) were observed in a dc glow discharge plasma of a mixture of boron tribromide vapor and oxygen gas. Rotational transitions of BrBO were measured for the ground state as well as for the vibrationally excited states, nu2 (bend) and nu3 (Br-B str.), for all of four isotopomers, and associated 2nu2, nu2 + nu3, and 2nu3 for the 11B species. The l = 0 substate of the 2nu2 state interacts with the nu3 state through the Fermi resonance. The rotational constants determined for the ground states of the four isotopomers yield the substitution structure, rs(Br&sbond;B) = 1.835791(70) Å and rs(B&dbond;O) = 1.20472(25) Å. The pi character and ionic character of the Br-B bond, which are estimated from the bromine quadrupole coupling constant eQq, are discussed through the comparison with those of related molecules such as BrCN and BrBS. Copyright 1998 Academic Press.