Substituent Effects on the Properties of Borafluorenes

Abstract

A series of substituted 9-borafluorenes were studied both experimentally and computationally in order to assess substituent effects on the optical and electronic properties and the stability of 9-borafluorenes. The previously unknown 9-substituted-9-borafluorenes Mes F BF (MesF = 2,4,6-tris­(trifluoromethyl)­phenyl), TipBF­(OMe) 2 (Tip = 2,4,6-tris­(triisopropyl)­phenyl, (OMe)2= methoxy at the borafluorene 3 and 6 positions), and i Pr 2 NBF (iPr2N = diisopropylamino) were synthesized and structurally characterized. The previously reported TipBF, ClBF (9-chloro-9-borafluorene) and t BuOBF (9-(tert-butoxy)-9-borafluorene) were also included in this study. All of the aryl borafluorenes (TipBF, TipBF­(OMe) 2 , Mes F BF), and t BuOBF are moderately air-stable. Both i Pr 2 NBF and ClBF degrade rapidly in air. Cyclic voltammogram measurements and density functional theory (DFT) calculations reveal that (a) borafluorenes have higher electron affinities relative to comparable boranes and (b) substituents have a strong influence on the lowest unoccupied molecular orbital (LUMO) levels of borafluorenes but less influence over the highest occupied molecular orbital (HOMO) levels. The DFT calculations show that, in general, borafluorenes exhibit low electron reorganization energies, a predictor of good electron mobility. However, the MesF group, which is finding popularity as a stabilizing group in borane chemistry, significantly increases the electron reorganization energy of Mes F BF compared to the other borafluorenes. The Lewis acidities of the borafluorenes were probed using Et3PO as a Lewis base (the Gutmann–Beckett method) and found to be dictated primarily by steric considerations. Calculated fluoride affinities (Lewis acidities) correlate with the LUMO energies of the borafluorenes. UV–visible and fluorescence spectroscopic measurements showed that compared to the Tip substituent, the MesF, Cl, and methoxy groups only cause subtle changes to the optical properties of the borafluorenes. The absorption spectra of both i Pr 2 NBF and t BuOBF are blue-shifted due to substituent π-backbonding with the p-orbital on boron. The results of this study provide insights into substituent effects on conjugated boron systems and will help in the design of future boron containing materials.

Doping Polycyclic Aromatics with Boron for Superior Performance in Materials Science and Catalysis

“…The borafluorene scaffold is also compatible with a wide variety of substituents at the boron atom . Contrary to parent DBA 52 , however, parent 9 H ‐9‐borafluorene 54 exhibits extensive dynamic covalent behavior.…”

Electronic Push–Pull Modulation by Peripheral Substituents in Pentaaryl Boroles

“…[87] To form dianionic precursors for coordination chemistry,u sually amino-3-borolenes are deprotonated with no need to isolate the free aminoborole. [83] Therefore, despite their establishmenti nc oordination chemistry,o nly few derivatives of isolated free aminoboroles (apart from af ew amino derivativeso f9 -borafluorenes) [60,88] are known (Scheme 6, B-F). [34,50,51,89] They are scarcer than the more reactive free boroles without BÀX p-stabilization.…”

“…[42,43] However,t he diversity in substitution patternsa tt he butadiene backbone is lessd eveloped. Apart from benzannelated boroles (9-borafluorenes and boraindenes), [55][56][57][58][59][60][61] which are known to reveal different electronic properties, only af ew derivatives of stable free boroles with ab ackboneo ther than a tetraphenylbutadiene are documentedi nt he literature (Scheme 1). Yamaguchi reported on electron-rich thienyl groups in the butadieneb ackbone, whichl ed to markedly altered HOMO-LUMO gaps.…”

Electronically Driven Regioselective Iridium‐Catalyzed C−H Borylation of Donor‐π‐Acceptor Chromophores Containing Triarylboron Acceptors

“…This can be avoided by using bulky substituents or by fixing the boron center in a rigid scaffold . Only recently the use of ortho ‐trifluoromethylated aryls in triarylboranes has been established as a strategy to improve acceptor strength as well as stability . The improved acceptor strength can be attributed to the electron‐withdrawing nature of the trifluoromethyl groups, while the increase in stability is due to steric shielding of the boron center and its empty p z ‐orbital, as well as a direct interaction of the fluorine lone pairs with the empty p z ‐orbital on boron.…”