Noble Metal Complexes of a Bis-Caffeine Containing NHC Ligand

Abstract

N-Heterocyclic carbenes (NHCs) have seen more and more use over the years. The go-to systems that are usually considered are derivatives of benzimidazole or imidazole. Caffeine possesses an imidazole unit and was already utilized as a carbene-type ligand; however, its use within a tridentate bis-NHC system has—to the best of our knowledge—not been reported so far. The synthesis of the ligand is straightforward and metal complexes are readily available via silver-salt metathesis. A platinum(II) and a palladium(II) complex were isolated and a crystal structure of the former was examined. For the Pt(II) complex, luminescence is observed in solid state as well as in solution.

Luminescent Platinum(II) Complexes with a Tridentate Caffeine‐Based NHC‐Pincer Ligand: Synthesis, Electrochemistry and Photophysics

“…Protoligand 1 and Pt(II) complex 2 were synthesized following the procedure published beforehand. [67] In all cases, the reaction mixtures were purged with N 2 for 20 min before starting the reactions. 1 Steady-state absorption measurements were performed in 1 cm quartz cells and recorded with a V-760 (JASCO) spectrophotometer.…”

“…All complexes were characterized by 1 H and 13 C nuclear magnetic resonance (NMR) spectroscopy as well as highresolution electrospray-ionization mass spectrometry (HR-ESI- [67] Figure reproduced from the original data (CCDC entry no. 2157149) set using the Mercury 2022.3.0 software.…”

“…[65][66] In previous work, we reported the synthesis and structural characterization of complexes 2 and 3, which contained a C N C coordinating tridentate, bis-NHC ligand (Figure 1a). [67] The ligand design allowed, due to the CH 2 bridges, for an almost ideal square-planar coordination geometry (Figure 1b). Pd(II) complex 3 displayed a remarkably high catalytic activity in various cross-coupling reactions -even at low catalyst loading in the ppm range.…”

“…Most of these studies were devoted to biomedical applications. More recently, such complexes have also been employed as catalysts in various organic reactions: Hydrogenation of ketones, [60] cross‐coupling reactions, [61–63] and Diels‐Alder reactions [64] …”

“… [67] The ligand design allowed, due to the CH 2 bridges, for an almost ideal square‐planar coordination geometry (Figure 1b). Pd(II) complex 3 displayed a remarkably high catalytic activity in various cross‐coupling reactions – even at low catalyst loading in the ppm range [61] . In continuation of this work, we now turn back to the Pt(II) complex 2 and present a study of its arylacetylide complexes 4 – 8 (Scheme 1).…”