Nucleophilicity of normal and abnormal N-heterocyclic carbenes at DFT: steric effects on tetrazole-5-ylidenes

Abstract

Nucleophilicity (N) is a crucial factor for the coordination of N-heterocyclic carbenes (NHCs) to transition metal complexes. In this work a comparison is made between N of a series of 1,4-disubstituted, “normal”, tetrazol-5-ylidens (1_R), and their corresponding 1,3-disubstituted, “mesoionic, or abnormal”, isomers (2_R), at the M06/6-311++G** level of theory; where R = H, methyl, ethyl, i-propyl, and t-butyl. Every abnormal 2_R appears more nucleophilic than its related normal 1_R isomer. Similarly, every 2_R (except 2_methyl) emerges as more aromatic than its corresponding normal 1_R isomer. In addition, N increases as the size of the substituent increases. The global electrophilicity (ω) trend takes on an exactly opposite direction. Stabilities of 1_R and 2_R carbenic species are presumed to be related to their singlet–triplet energy gaps (ΔE_S–T, kcal mol⁻¹). Every normal 1_R appears more stable than its corresponding abnormal 2_R isomer. The most stable is 1_ethyl in the normal and 2_ethyl in the abnormal series. The least stable are 1_methyl and 2_methyl structures, respectively. Lower stability of the latter two species is attributed to their relatively lower NICS, higher proton affinity (PA), higher aptitude for dimerization, wider carbenic bond angles (N–Ĉ_carbene–N), and longer C_carbene–N bond lengths. Interestingly, the charge on carbenic carbon is the highest for 1_t-butyl and lowest for 2_t-butyl. Every normal 1_R shows a wider band gap than its corresponding abnormal 2_R, for showing a larger ΔE_HOMO–LUMO. Finally, due to the steric effects, 1_t-butyl does not dimerize, while all other normal 1_R carbenes, as well as abnormal 2_methyl give doubly bonded dimers (head-to-head).