From arylcarbenes to borazyl and fluorinated borazylcarbene analogues: Structure, multiplicity and stability at DFT

Abstract

In analogy to many arylcarbenes, in this research 12 borazyl and fluorinated borazylcarbenes are investigated at DFT-B3LYP/6-311 + G∗ level of theory. Hence, the N, B and N/B attached monoborazylcarbenes as well as their corresponding diborazylcarbenes, in each singlet (s) and triplet (t) electronic states, are considered to delineate geometry, relative stability, electronic ground state and aromaticity in comparison to those of phenylcarbene and diphenylcarbene. Our data show that B-attached borazylcarbenes appear more stable than N-attached ones. Moreover, the multiplicities of the B-attached borazylcarbenes are triplet in contrast to those of N-attached ones with the singlet ground states. Fluorine substitutions drastically affect the stability of borazylcarbenes. This is in a way that N-attached borazylcarbenes become more stable especially when fluorine bonded to boron instead of nitrogen at the ring. However, fluorine substitution does not change the multiplicities of borazylcarbenes. In conclusion, fluorination helps the synthesis of N-attached monoborazylcarbenes 6s, 10s and 12s along with the B-attached monoborazylcarbene 7t among other isomeric carbene species. Calculated NICS values indicate that all borazyl rings studied appear to be moderately aromatic and in most cases triplet state borazylcarbenes are more aromatic than their corresponding singlet states.