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

• Published in: RSC advances 2016-01, Vol.6 (16), p.13224-13233
• Main Authors: Rezaee, Nasibeh, Ahmadi, Aliakbar, Kassaee, Mohammad Zaman
• Format: Article
• Language: English
• Subjects: Bonding; Carbenes; Carbon; Energy gap; Isomers; Stability; Steric effects; Transition metals
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/c5ra21247b

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 (Δ E 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- 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 Δ E 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). Nucleophilicity of both normal ( 1 R ) and abnormal ( 2 R ) N-heterocyclic carbene (NHC); every 2 R showing a higher nucleophilicity than its corresponding 1 R isomer (R = H, methyl, ethyl, i -propyl, and t -butyl).