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1. Cobalt–N-Heterocyclic Carbene Complexes in Catalysis

   https://doi.org/10.1021/acscatal.1c05869  

   Bera, Sourav Sekhar; Szostak, Michal (March 2022, ACS Catalysis)
2. The importance of N-heterocyclic carbene basicity in organocatalysis

   https://doi.org/10.1039/c8ob01667d  

   Wang, Ning; Xu, Jiahui; Lee, Jeehiun K. (September 2018, Organic & Biomolecular Chemistry)

   N-Heterocyclic carbenes (NHCs) are versatile species that figure prominently as catalysts. Despite their widespread use in organocatalysis, studies of the relationship between the basicity of NHCs and their catalytic ability are limited. Herein we review work on both the examination of NHC basicity as well as its impact on organocatalysis. The review is divided into three main parts: an overview of NHC basicity studies, both in solution and in the gas phase; the role of basicity in Umpolung -type catalysis; and the relationship between NHC basicity and its growing role as a Brønsted base catalyst. This review is not an exhaustive catalog of all NHC catalysis, but rather focuses on work that specifically examines and discusses the effect of NHC basicity on catalyst function. 

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3. N-Heterocyclic Carbene Complexes in C–H Activation Reactions

   https://doi.org/10.1021/acs.chemrev.9b00634  

   Zhao, Qun; Meng, Guangrong; Nolan, Steven P.; Szostak, Michal (February 2020, Chemical Reviews)
4. Slow magnetic relaxation in cobalt N-heterocyclic carbene complexes

   https://doi.org/10.1039/D0DT02286A  

   Saber, Mohamed R.; Przyojski, Jacob A.; Tonzetich, Zachary J.; Dunbar, Kim R. (August 2020, Dalton Transactions)

   The combined experimental and theoretical investigation of the magnetic properties of the cobalt( ii ) NHC complexes (NHC = N-heterocyclic carbene); [Co(CH 2 SiMe 3 ) 2 (IPr)] ( 1 ), [CoCl 2 (IMes) 2 ] ( 2 ) and [Co(CH 3 ) 2 (IMes) 2 ] ( 3 ) revealed a large easy plane anisotropy for 1 ( D = +73.7 cm −1 ) and a moderate easy axis anisotropy for 2 ( D = −7.7 cm −1 ) due to significant out-of-state spin–orbit coupling. Dynamic magnetic measurements revealed slow relaxation of the magnetization for 1 ( U eff = 22.5 K, τ 0 = 3 × 10 −7 s, 1000 Oe) and for 2 ( U eff = 20.2 K, τ 0 = 1.73 × 10 −8 s, 1500 Oe). The molecular origin of the slow relaxation phenomena was further supported by the retention of AC signal in 10% solutions in 2-MeTHF which reveals a second zero field AC signal in 1 at higher frequencies. Compound 3 was found to be an S = 1/2 system. 

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5. Synthesis and characterization of low-dimensional N-heterocyclic carbene lattices

   https://doi.org/10.1126/science.adm9814  

   Qie, Boyu; Wang, Ziyi; Jiang, Jingwei; Zhang, Zisheng; Jacobse, Peter H; Lu, Jiaming; Li, Xinheng; Liu, Fujia; Alexandrova, Anastassia N; Louie, Steven G; et al (May 2024, Science)

   The covalent interaction of N-heterocyclic carbenes (NHCs) with transition metal atoms gives rise to distinctive frontier molecular orbitals (FMOs). These emergent electronic states have spurred the widespread adoption of NHC ligands in chemical catalysis and functional materials. Although formation of carbene-metal complexes in self-assembled monolayers on surfaces has been explored, design and electronic structure characterization of extended low-dimensional NHC-metal lattices remains elusive. Here we demonstrate a modular approach to engineering one-dimensional (1D) metal-organic chains and two-dimensional (2D) Kagome lattices using the FMOs of NHC–Au–NHC junctions to create low-dimensional molecular networks exhibiting intrinsic metallicity. Scanning tunneling spectroscopy and first-principles density functional theory reveal the contribution of C–Au–C π-bonding states to dispersive bands that imbue 1D- and 2D-NHC lattices with exceptionally small work functions. 

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