Cationic rhenium complexes ligated with N-heterocyclic carbenes – an overview

Abstract

This review provides an overview of the currently known cationic rhenium NHC complexes. Synthesis, structures and properties are described. The title compounds are potential candidates for both catalytic and medical applications. Besides the variety of ancillary ligands, which are in some cases easily substituted, functionalization can be carried out in the side chain or at the backbone of the carbene ligand as well as - in the case of biscarbene ligands - at the bridging moiety. Cationic Re NHC complexes are promising precursors for radiopharmaceuticals and diagnostics - not only because of the possibility to radiolabel the metal (steps in this direction have been made and described already) - but rather the opportunity to link the complexes to biomolecules via the different possibilities provided by the ligands. The development of OLEDs based on luminescent Re(i) carbene complexes renders another potential application.

Recent progress on NHC-stabilized early transition metal (group 3–7) complexes: Synthesis and applications

“…Yet, since NHCs are exceptionally strong s-donating ligands, they also impart enhanced stability to most oxophilic and electron-poor metal centers, including group 3-7 metal centers. 6,7,8,9,10,11 Beside the electronic features of the NHC unit, appropriate ligand design, such as the use of NHC-bearing chelating ligand, may be crucial for further stability and robustness of the resulting M-NHC complexes, in particular in the case of oxophilic metal centers. 12 Since the last comprehensive review on the subject in 2014, 6 numerous developments on NHC-stabilized early transition metal (group 3-7) complexes for use in various applications ranging from catalysis to optical and medical applications have been achieved.…”

From betaines to anionic N-heterocyclic carbenes. Borane, gold, rhodium, and nickel complexes starting from an imidazoliumphenolate and its carbene tautomer

“…Consequently books [14–16] and reviews cover the range from NHC structures in the light of their early history [17], syntheses [6,18], coordination chemistry [19–20], and catalysis [21–22], to biological activities of NHC complexes [23–24]. In the last decade, attention has also been directed to anionic N-heterocyclic carbenes [25].…”

Hybrids of cationic [4]helicene and N-heterocyclic carbene as ligands for complexes exhibiting (chir)optical properties in the far red spectral window

“…The NHC steric and electronic properties are discussed. Low energy absorption and emission of the cationic [4]helicene moiety are maintained. Regardless of the organometallic species, intense (chir)optical properties are observed in the far-red spectral region, which can be beneficial for bio-related applications.…”

“…In recent decades, N-heterocyclic carbenes (NHCs) have become ubiquitous ligands able, in the context of optical devices, to form robust organometallic complexes with luminescent properties. [1][2][3][4][5][6] In most cases, the luminescence arises from the metal center and its coordination sphere, and it occurs predominantly at high energy (UV or blue-green spectral range). In the meantime, new NHC structures affording unique stereo-electronic properties have been prepared, 7,8 and unsymmetrical unsaturated N-heterocyclic carbenes (U 2 -NHCs) in particular, [9][10][11][12][13] allowing for instance the introduction of cycloalkyl or cationic cyclopropenium chains onto the diaminocarbene unit (e.g., A and B, Fig.…”

“…19,20,23,24 Herein, a different approach is detailed based on ligand-controlled luminescence. Synthetically, using a one-pot multicomponent procedure, 13 cationic [4]helicene aniline 1-NH 2 (BF 4 ) 38,40 and cyclododecylamine are combined to generate imidazolium 2ÁH(PF 6 ) 2 (Fig. 1).…”