The Potential of N-Heterocyclic Carbene Complexes as Components for Electronically Active Materials

Abstract

The application of N-heterocyclic carbene complexes as active sites in materials other than catalysis has been remarkably scarce. Inspired by the – often misleading – 'carbene' label, which implies a substantial degree of M=C ? bonding, we have been interested in evaluating the potential of N-heterocyclic carbene complexes as building blocks for constructing electronically active materials. Electron mobility via the metal–carbon bond has been investigated in monometallic imidazol-2-ylidene complexes and subsequently expanded to polymetallic systems. In particular, ditopic benzobisimidazolium-derived ligands have been explored for the fabrication of bimetallic molecular switches and main-chain conjugated organometallic polymers. Electrochemical analyses have allowed the degree of electronic coupling between the metal sites to be quantified and the key parameters that govern the intermetallic communication to be identified.

Pyrene‐Based Bisazolium Salts: From Luminescence Properties to Janus‐Type Bis‐N‐Heterocyclic Carbenes

“…Taking the DPV signal of this monocarbene‐iridium complex as a reference, the deconvolution of the DPV curve for 9 ‐Et shows two oxidation bands separated by 30 mV. This separation indicates an essentially decoupled system, as compared to other π‐linked bis‐NHCs,5a, 7, 11, 20 and is in agreement with our previous suggestion, that the electronic communication across polyaromatic‐linked bis‐NHCs is mostly due to a σ‐interaction,6 unless a suitable orientation of the carbene and metal orbitals is achieved by the use of the appropriate chelating ligands 8…”

“…Inspired by these considerations, a series of ditopic N‐heterocyclic carbenes connected by π‐delocalized aromatic linkers have been obtained, and probably against predictions, they have mostly provided weak metal–metal interactions 6. A logical explanation for the weak coupling is the lack of an effective d M –p L overlap,5a, 7 which results in poor intramolecular connectivity. However, a strong orbital overlap can be achieved by the use of chelating ligands that constrain the coordination of the carbene, allowing a suitable orientation of the carbene and metal orbitals, and thus providing good metal–metal connectivity, as has been elegantly demonstrated 8…”

Carbenes Made Easy: Formation of Unsymmetrically Substituted N‐Heterocyclic Carbene Complexes of Palladium(II), Platinum(II) and Gold(I) from Coordinated Isonitriles and their Catalytic Activity

“…In addition to this very popular use in catalysis, NHC complexes are equally useful in many other fields like medical applications, [11] nanoparticles and supramolecular chemistry, [12] self-assembly, [13] photochemistry, [14] liquid crystals, [15] polymerisation [16] and electronically active materials [17] as recently sum-A C H T U N G T R E N N U N G marised [18a] and highlighted.…”

Efficient Electronic Communication of Two Ruthenium Centers through a Rigid Ditopic N‐Heterocyclic Carbene Linker

“…135.7 (C Ar ÀN), 128.8 (C Ar ÀN), 124.3 (C py ÀH), 114.9 (C py ÀH), 111.2 (C cym À C), 108.4 (C cym ÀC), 97.8 (C Ar ÀH), 95.9 (C Ar ÀH), 94.3 (C cym ÀH), 94.0 (C cym ÀH), 89.3 (C cym ÀH), 85.4 (C cym ÀH), 37.4 (NÀCH 3 ), 32.1 (CHMe 2 ), 22.6 (CÀCH 3 ), 22.5 (CÀCH 3 ), 19.4 ppm (cymÀCH 3 ); elemental analysis calcd (%) for C 40 : C 39.18, H 3.69, N 6.69; found C 38.98, H 3.90, N 6.97.…”