N-Heterocyclic carbenes (NHCs) have become one of the most widely studied class of ligands in molecular chemistry and have found applications in fields as varied as catalysis, the stabilization of reactive molecular fragments, and biochemistry. More recently, NHCs have found applications in materials chemistry and have allowed for the functionalization of surfaces, polymers, nanoparticles, and discrete, well-defined clusters. In this review, we provide an in-depth look at recent advances in the use of NHCs for the development of functional materials.

The formation of organic films on gold employing N-heterocyclic carbenes (NHCs) has been previously shown to be a useful strategy for generating stable organic films. However, NHCs or NHC precursors typically require inert atmosphere and harsh conditions for their generation and use. Herein we describe the use of benzimidazolium hydrogen carbonates as bench stable solid precursors for the preparation of NHC films in solution or by vapour-phase deposition from the solid state. The ability to prepare these films by vapour-phase deposition permitted the analysis of the films by a variety of surface science techniques, resulting in the first measurement of NHC desorption energy (158±10 kJ mol−1) and confirmation that the NHC sits upright on the surface. The use of these films in surface plasmon resonance-type biosensing is described, where they provide specific advantages versus traditional thiol-based films.

The development of novel chemical transformations calls for environmentally friendly synthetic methods. Visible light photoredox catalysis has gained attention as a versatile tool to address these challenges. Due to the well‐defined structure of homogeneous transition metal‐based catalysts and organic dyes, they have been widely used to drive a wide variety of organic transformations. On the other hand, due to low cost, simple preparation procedure, and chemical stability of carbon nitride semiconductors, they have been known almost exclusively in environmental applications and water splitting. Until recently the applicability of carbon nitride semiconductors in organic synthesis has remained hidden from the broad community of organic chemists. In this review, we summarize reactions mediated by heterogeneous carbon nitride photocatalysts and common homogeneous photocatalysts in the same organic reactions, such as carbon‐carbon or carbon‐heteroatom bond formation reactions. We compare performance of both groups of catalysts in the discussed organic reactions.

A set of aryl-substituted pyridylideneamide (PYA) ligands with variable donor properties owing to a pronounced zwitterionic and a neutral diene-type resonance structure were used as electronically flexible ligands at a pentamethylcyclopentadienyl (Cp*) iridium center. The straightforward synthesis of this type of ligand allows for an easy incorporation of donor substituents such as methoxy groups in different positions of the phenyl ring of the C,N-bidentate chelating PYA. These modifications considerably enhance the catalytic activity of the coordinated iridium center toward the catalytic aerobic transfer hydrogenation of carbonyls and imines as well as the hydrosilylation of phenylacetylene. Moreover, these PYA iridium complexes catalyze the base-free transfer hydrogenation of aldehydes, and to a lesser extent also of ketones. Under standard transfer hydrogenation conditions including base, aldehydes are rapidly oxidized to carboxylic acids rather than reduced to the corresponding alcohol, as is observed under base-free conditions.

The synthetic versatility of pyridylidene amide (PYA) ligands has been exploited to prepare and evaluate a diverging series of iridium complexes containing C,N-bidentate chelating aryl-PYA ligands for water oxidation catalysis. The phenyl-PYA lead structure 1 was modified (i) electronically through introduction of one, two, or three electron-donating methoxy substituents on the aryl ring, (ii) by incorporating long aliphatic chains to the pyridyl fragment of the PYA unit, and (iii) by altering the PYA positions from para-PYA to its ortho- and meta-isomers. Electrochemistry indicated no substantial electronic effect of the aliphatic chains, and only minor changes of the electron density at iridium when modifying the aryl ligand site, yet substantial alteration if the PYA ligand is the ortho- (E =+0.72 V), para- (E =+0.64 V), or meta-isomer (E =+0.56 V vs. saturated calomel electrode; SCE). In water oxidation catalysis, the long alkyl chains did not induce any rate enhancement compared with the phenyl-PYA lead compound, whereas MeO groups incorporated in the aryl group enhanced the catalytic activity from a turnover frequency (TOF )=1600 h in the original Ph-PYA system gradually as more MeO groups were introduced up to a TOF =3300 h for a tris(MeO)-substituted aryl-PYA system. The variation of the PYA substitution had only a minor impact on catalytic activity and revealed only a weak trend in the sequence ortho>meta>para. The high activity of the tris(MeO) system and the ortho-PYA isomer were attributed to efficient hydrogen bonding, which assists O-H bond activation and proton transfer. Remarkably, merging of the two optimized motifs, that is, an aryl unit with three MeO substituents and the PYA as the ortho isomer, into a single new aryl-PYA ligand system failed to improve the catalytic activity. Computational analysis suggests too much congestion at the active site, which hinders catalytic turnover. These results illustrate the complexity of ligand design and the subtle effects at play in water oxidation catalysis.