N-Heterocyclic Olefins as Electron Donors in Combination with Triarylborane Acceptors: Synthesis, Optical and Electronic Properties of D–π–A Compounds

Abstract

N-heterocyclic olefins (NHOs), relatives of N-heterocyclic carbenes (NHCs), exhibit high nucleophilicity and soft Lewis basic character. To investigate their π-electron donating ability, NHOs were attached to triarylborane π-acceptors (A) giving donor (D)–π–A compounds 1–3. In addition, an enamine π-donor analogue (4) was synthesized for comparison. UV–visible absorption studies show a larger red shift for the NHO-containing boranes than for the enamine analogue, a relative of cyclic (alkyl)(amino) carbenes (CAACs). Solvent-dependent emission studies indicate that 1–4 have moderate intramolecular charge-transfer (ICT) behavior. Electrochemical investigations reveal that the NHO-containing boranes have extremely low reversible oxidation potentials (e.g., for 3, (Formula presented.) =−0.40 V vs. ferrocene/ferrocenium, Fc/Fc+, in THF). Time-dependent (TD) DFT calculations show that the HOMOs of 1–3 are much more destabilized than that of the enamine-containing 4, which confirms the stronger donating ability of NHOs.