Activation of Small Molecules by Heavier Analogs of Cyclic(Alkyl)(Amino)Carbenes (CAACs): A DFT Study

Cyclic (alkyl)(amino)carbenes (CAACs), a subclass of N-heterocyclic carbenes (NHCs), are powerful tools in coordination chemistry due to their strong σ-donor and π-acceptor abilities. The heavier analogs of carbenes CAAEs (E = Si-Pb), however, are not as ubiquitous but may have potential in small-molecule activation and catalysis. Here, quantum chemical computations using density functional theory (DFT) were utilized to investigate the analogs of CAACs (CAAE, E = C-Pb), in which the compounds were assessed to determine the electronic properties and reactivity toward small molecules H2, NH3, and CO2. The activation energies for small-molecule bond cleavage were found to increase as the central element E became heavier, which may be attributed to changes in the Lewis basicity. In addition, the differences in NH3 oxidative addition, Werner coordination complex-type adduct formation, and proton-shuttling mechanisms were investigated. Carbon dioxide could not be activated by the heaviest germylene, stannylene, or plumbylene derivatives, but the CAAC and CAASi species interacted with CO2 via different coordination modes. The results were comparable to those of experimentally assessed systems, indicating the feasibility of synthesizing and isolating these heavier CAAC derivatives and their ability to activate small molecules.