Capture and complexation of carbon monoxide using NHC and its boron analogs: a computational study

Capturing carbon monoxide (CO) via ligand complexation offers a strategy to mitigate and control CO emissions. In this study, the interaction of CO with N-heterocyclic carbenes (NHCs) and their isoelectronic boron-substituted analogs (NHBs) is examined using density functional theory. Computed geometries, natural bond orbital (NBO) analysis, topology analysis, electron localization functions (ELF), and charge density difference (CDD) maps are employed to characterize the nature of CO binding. The boron-based systems consistently formed more stable CO adducts than traditional NHC ligands. Building upon this, a mixed ligand approach combining NHC and NHB on either side of CO resulted in exceptional stabilization for 1NO₂-CO-2NH₂ and 1NO₂-CO-2CH₃ complexes, with the reaction mechanism suggesting that NHC facilitates initial CO capture, followed by complex stabilization through NHB coordination. Charge analyses showed a clear redistribution of electron density in CO upon coordination, with reduced C-O bond order and distinct C-C and B-O bonds, while Natural orbitals for chemical valence (NOCV), and Charge decomposition analysis (CDA) confirmed predominant covalent interaction with an electron transfer from NHB to NHC fragment through CO and π-backdonation from NHC to CO. Such charge transfer and back bonding exist in pure NHC complexes; however, the extent of orbital interaction is significantly enhanced in the pure NHB and NHC-NHB mixed systems. These findings highlight the superior ability of NHC and NHB ligand combinations, particularly those with electron-withdrawing and electron-donating substituents, to trap CO, offering valuable insights into the design of main-group frameworks for toxic gas capture applications.