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

Chengxiang Ding; Lei Yao; Lili Zhao; Gernot Frenking

doi:10.1039/d2cp02598a

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

Phys Chem Chem Phys. 2022 Jul 13;24(27):16732-16745. doi: 10.1039/d2cp02598a.

Authors

Chengxiang Ding¹, Lei Yao¹, Lili Zhao^{1

2}, Gernot Frenking^{1

2

3

4}

Affiliations

¹ Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816, China.
² Institute of Advanced Synthesis, School of Chemistry and Molecular Engineering, State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, Nanjing 211816, China. ias_llzhao@njtech.edu.cn.
³ Fachbereich Chemie, Philipps-Universität Marburg, Hans-Meerwein-Strasse 4, D-35043 Marburg, Germany.
⁴ Donostia International Physics Center (DIPC), P.K. 1072, 20080 Donostia, Euskadi, Spain.

PMID: 35770713
DOI: 10.1039/d2cp02598a

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 CC 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⁺.