Tetrakis(N-heterocyclic Carbene)-Diboron(0): Double Single-Electron-Transfer Reactivity

Jun Fan; An-Ping Koh; Jingsong Zhou; Zheng-Feng Zhang; Chi-Shiun Wu; Richard D Webster; Ming-Der Su; Cheuk-Wai So

doi:10.1021/jacs.3c01801

Tetrakis(N-heterocyclic Carbene)-Diboron(0): Double Single-Electron-Transfer Reactivity

J Am Chem Soc. 2023 May 31;145(21):11669-11677. doi: 10.1021/jacs.3c01801. Epub 2023 May 18.

Authors

Jun Fan¹, An-Ping Koh¹, Jingsong Zhou¹, Zheng-Feng Zhang², Chi-Shiun Wu², Richard D Webster¹, Ming-Der Su^{2

3}, Cheuk-Wai So¹

Affiliations

¹ School of Chemistry, Chemical Engineering and Biotechnology, Nanyang Technological University, 637371 Singapore.
² Department of Applied Chemistry, National Chiayi University, Chiayi 60004, Taiwan.
³ Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 80708, Taiwan.

PMID: 37201187
DOI: 10.1021/jacs.3c01801

Abstract

The use of 1,3,4,5-tetramethylimidazol-2-ylidene (IMe) to coordinate with diatomic B₂ species afforded a tetrakis(N-heterocyclic carbene)-diboron(0) [(IMe)₂B-B(IMe)₂] (2). The singly bonded B₂ moiety therein possesses a valence electronic configuration 1σ_g²1π_u²1π_g*² with four vacant molecular orbitals (1σ_u*, 2σ_g, 1π_u', 1π_g'*) coordinated with IMe. Its unprecedented electronic structure is analogous to the energetically unfavorable planar hydrazine with a D_2h symmetry. The two highly reactive π_g* antibonding electrons enable double single-electron-transfer (SET) reactivity in small-molecule activation. Compound 2 underwent a double SET reduction with CO₂ to form two carbon dioxide radical anions CO₂^•-, which then reduced pyridine to yield a carboxylated pyridine reductive coupling dianion [O₂CNC₅(H)₅-C₅(H)₅NCO₂]^2- and converted compound 2 to the tetrakis(N-heterocyclic carbene)-diborene dication [(IMe)₂B═B(IMe)₂]²⁺ (3²⁺). This is a remarkable transition-metal-free SET reduction of CO₂ without ultraviolet/visible (UV/vis) light conditions.