How to capture C2O2: structures and bonding of neutral and charged complexes [(NHC)–C2O2–(NHC)]q (NHC = N-heterocyclic carbene; q = 0, 1+, 2+)

Abstract

We present the results of DFT calculations and a thorough bonding analysis of the neutral and charged complexes of the elusive C₂O₂ species stabilized by two NHC ligands. It is shown that the thermodynamic stability of the neutral complex [(NHC)–C₂O₂–(NHC)] is due to the low-lying triplet state of [NHC–CO] (T), which is only 3.2 kcal mol⁻¹ higher in energy than the singlet state [NHC–CO] (S), while the triplet state of CO is 131.9 kcal mol⁻¹ above the singlet. The much lower S/T gap of [NHC–CO] than in CO comes from the charge donation of NHC into the degenerate π* LUMO of CO and the concomitant mixing of the LUMO of NHC with the degenerate π* LUMO of CO, which strongly lowers the energy difference between HOMO and LUMO in the complex. The energy gain resulting from the formation of the CC double bond compensates the singlet–triplet gap and the thermodynamic instability of the fragments [NHC–CO] (S). The dissociation of neutral [(NHC)–C₂O₂–(NHC)] to 2NHC and 2CO molecules is calculated to be endothermic by D_o = 78.2 kcal mol⁻¹. The bonding analysis indicates that the neutral and the charged molecules [(NHC)–C₂O₂–(NHC)]^q have a central unit with C–C single bonds, where a combination of electron sharing and s dative interactions leads to very strong carbon–carbon bonds complemented by minor π-donation, which make all systems stable with respect to dissociation reactions. The central C₂O₂ fragment carries a large negative partial charge in the neutral and singly charged compounds [(NHC)–C₂O₂–(NHC)]^0,1+, while it is neutral in the dication [(NHC)–C₂O₂–(NHC)]²⁺.