Evelyn L. Rosen

Diaminocarbene[3]ferrocenophanes and their transition-metal complexes.

N-heterocyclic carbenes (NHCs) form robust complexes with nearly every transition metal. In many instances, these complexes function as catalysts with superior activities and selectivities. The broad utility and application of NHCs can be traced to synthetic advancements which enable access to derivatives with novel N substituents and architectures. We envisioned that it would be possible to create a new class of carbene-based ligands with relatively wide N-C-N angles and redox-switchable functions by linking diaminocarbenes to 1,1’-disubstituted metallocenes. In view of the affinity of carbenes for transition metals, the use of such ligands may lead to new redox-active complexes with tunable electronic properties. Herein, we describe a novel carbene architecture that contains a 1,1’-disubstituted ferrocene moiety and demonstrate its ability to engage in unique electronic interactions with coordinated transition metals. The formamidinium salts 1a and 1b were synthesized in up to 95 % yield by the formylative cyclization of the respective 1,1’-dianilinoor 1,1’-dialkylaminoferrocene precursor upon treatment with trimethylorthoformate and HBF4. [8] Diagnostic formamidinium signals were found at d = 9.3 and 8.8 ppm ([D6]DMSO), respectively, in the H NMR spectra of these compounds. The solid-state structure of 1a reveals eclipsed cyclopentadienyl (Cp) ligands, which are tilted toward each other (f = 15.78) as a result of the tethering effect of the diaminocarbene fragment (Figure 1, left). This strain is manifested in a relatively large N-C-N angle of 129.6(3)8, a value that is comparable to those

Bimetallic N-Heterocyclic Carbene−Iridium Complexes: Investigating Metal−Metal and Metal−Ligand Communication via Electrochemistry and Phosphorescence Spectroscopy

Bimetallic [Ir(COD)Cl] and [Ir(ppy)(2)] (COD = 1,5-cyclooctadiene; ppy = 2-phenylpyridyl) complexes bridged by 1,7-dimethyl-3,5-diphenylbenzobis(imidazolylidene) (1), in addition to their monometallic analogues supported by 1-methyl-3-phenylbenzimidazolylidene (2), were synthesized and studied. Electrochemical analyses indicated that 1 facilitated moderate electronic coupling between [Ir(COD)Cl] units (DeltaE = approximately 60 mV), but not [Ir(ppy)(2)]. The metal-based oxidation potentials for the bimetallic complexes were within 20 mV of those for their monometallic analogues. Furthermore, spectroscopic analyses of the [Ir(ppy)(2)] bimetallic and monometallic complexes revealed nearly identical phosphorescence profiles, indicating that carbene coordination does not affect the energy of the emissive states. Collectively, these results suggest that N-heterocyclic carbenes (NHCs) such as 1 could link together two emissive fragments without altering their fundamental phosphorescence profiles. Ultimately, employing multitopic NHCs as non-interfering molecular connectors could facilitate the rational design of new phosphorescent materials as well as second-generation phosphor dopants.