N-Heterocyclic germylenes in focus: Steric effects on nucleophilicity of novel tetrazole-5-germylenes at DFT

Discovery of N-heterocyclic germylenes (NHGes) began years before Arduengo̕ s report on N-hetrocyclic carbenes (NHCs) in 19911. In this work 20 novel NHGes are characterized at B3LYP/6-311++G**level of theory. Nucleophilicity (N) is anticipated to be a crucial factor for coordination of NHGes to transition metal complexes2. Hence,comparison is made between N of a series of 1,4-disubstituted tetrazol-5-germylidens ( normal , 1R), and their corresponding ten 1,3-disubstituted isomers ( mesoionic, or abnormal , 2R), where R = H, methyl, ethyl, i-propyl, and t-butyl. In contrast to their corresponding carbenes where a singlet is more nucleophilic than its corresponding triplet, all our ten triplet NHGes appear more nucleophilic than their related singlet isomers. Similarly, singlet NHGes appear rather aromatic (NICS ≥ -20.7) while their corresponding triplets are nonaromatic (NICS ≥ 0.23). In addition, N increases as the size of the substituent increases. The global electrophilicity (ω) trend takes on an exactly opposite direction. Stabilities of 1R and 2R are presumed to be related to their singlet–triplet energy gaps (ΔES-T, kcal mol-1). Every normal 1R appears more stable than its corresponding abnormal 2R isomer. This is except for 1ethyl which is less stable than 2ethyl. The most stable germylenes among the normal and abnormal series are 1H and 2H; while the least stable ones are 1ethyl and 2methyl, respectively.