Difluorination at Boron Leads to the First Electrophilic Ligated Boryl Radical (NHC‐BF₂^.)

Abstract

1,3-Dimethylimidazol-2-ylidene difluoroborane (NHC-BF 2 H) was prepared in ao ne-pot, two-step reaction from the parent ligated borane (NHC-BH 3 ). The derived difluoroboryl radical (NHC-BF 2 C)was generated by laser flash photolysis experiments and characterized by UV spectroscopy and rate-constant measurements.I ti st ransient and reacts quickly with O 2 .U nusually,i ta lso reacts more rapidly with ethyl vinyl ether than with methyl acrylate.Bythis measure,itis the first electrophilic ligated boryl radical. Both NHC-BH 3 and NHC-BF 2 Hs erve as co-initiators in bulk photopolymerizations,c onverting both electron-poor and electron-richm onomers at roughly similar rates.H owever,t he difluorinated coinitiator provides polymers with dramatically increased chain lengths from both monomers.

Matrix Infrared Spectra and Theoretical Calculations of Fluoroboryl Complexes F2B–MF (M = C, Si, Ge, Sn and Pb)

“…The calculations predict that there is no dative bonding in either CH 2 BF or CHBF, despite the empty p-orbital on B and the filled p-orbitals on the halogen atom . In addition, the reactions of laser-ablated boron atoms with C 2 H 4 , C 2 H 6 , CH 3 NH 2 , and CH 3 OH in solid noble gas matrices have also produced the BC, BN, or BO bond, respectively. Some boron cation complexes, B 3 (CO) n + and [ArB 3 O 4 ] + , [ArB 3 O 5 ] + , [ArB 4 O 6 ] + , and [ArB 5 O 7 ] + were prepared in gas phase, in which B 3 (CO) 4 + has a chemical bonding pattern similar to allene, and [ArB 3 O 5 ] + , [ArB 4 O 6 ] + , and [ArB 5 O 7 ] + cation complexes have planar structures each involving anaromatic boroxol ring.…”

“…The fluorine substituted boryl, fluoroboryl, has been synthesized through boron trifluoride activated by oxidative addition to the transition-metal complex [(Cy 3 P) 2 Pt] to form [(Cy 3 P) 2 Pt­(BF 2 )­(FBF 3 )] . Very recently a fluoroboryl radical (NHC-BF 2 ) was generated from 1,3-dimethylimidazol-2-ylidene difluoroborane (NHC–BF 2 H) by laser flash photolysis experiments and characterized by UV-spectroscopy and rate constant measurements …”

Thermolysis of Organofluoroborate Ionic Liquids to NHC-Organofluoroborates

“…Six [C 4 C 1 Im]­[BR x F 4‑ x ] ILs were also identified as key precursors to novel NHC-BR x F 3‑ x compounds because NHC-BR x F 3‑ x was either produced at low temperatures or in high quantity. Although some ILs did not produce NHC-BR x F 3‑ x , they may serve as ideal precursors to NHC-BF 3 compounds because of their potential to liberate NHC-BF 3 at low temperatureslikely because B–C bonds are significantly weaker than B–F bonds . Dicationic [C 12 (C 1 Im) 2 ]­[BF 4 ] 2 decomposed by chain fragmentation, but hydroxyl-functionalized [(C 2 OH)­C 1 Im]­[BF 4 ] produced significant quantities of a cyclic NHC-BR x F 3‑ x where the organic group was from the imidazolium alkyl chain.…”

“…NHC-trifluoroborates and NHC-organofluoroborates (i.e., NHC-BR x F 3‑ x ) are emerging as valuable new molecules with unique reactivity, which are impacting areas such as synthetic organic chemistry, , polymer chemistry, photoelectrochemistry, and PET imaging. , In many cases, NHC-BR x F 3‑ x compounds are replacing expensive metal catalysts, and their applications are rapidly growing as their chemistry is unraveled. NHC-BR x F 3‑ x compounds can be prepared from free NHCs and BR x F 3‑ x ; however, this method requires strong bases which limits the flexibility of the organic groups .…”

Formation and reactivity of NHC-boryl radicals: insight into substituent effect from theoretical calculations

“…25,26 It is widely accepted that almost all reported NHC-boryl radicals are nucleophilic, and it was not until 2018 that the first electrophilic NHC-boryl radical, NHC-BF 2 with two highly electronegative fluorine atoms on boron, was prepared. 27 Here the introduction of two fluorine substituents on boron dramatically changes the properties of NHC-boryl radicals, showing that the structural modification could effectively regulate the chemical properties of the NHC-boryl radicals, such as electro-and nucleophilicity, acidity and alkalinity, etc. Inspired by this work, computational screening and evaluation of structural modifications at the Lewis-donor (C-and N-sites) and Lewis-acceptor (B-site) moieties of the simplest NHC-BH 3 were performed (see Scheme 1).…”