No abstract

The portfolio of acyclic diaminocarbenes (ADACs) has been substantially expanded, owing to the synthesis of eleven new formamidinium salts, mostly of the type [(iPr2N)CH(NRR')][PF6], for use as immediate carbene precursors. The corresponding ADACs (iPr2N)C(NRR') were sufficiently stable for isolation in the case of NRR' = 2-methylpiperidino (13), 3-methylpiperidino (14), 4-methylpiperidino (15), morpholino (17) and NiPrPh (20), but had to be trapped in situ in the case of NRR' = 2,2,6,6-tetramethylpiperidino (12) and NiPrMe (19). The tetraaryl-substituted ADACs (Ph2N)2C (22) and (Ph2N)C[N(C6F5)2] (24) also could only be generated and trapped in situ. Trapping with elemental selenium was particularly efficient, affording the corresponding selenourea derivative in all cases, whereas trapping with [{Rh(μ-Cl)(cod)}2] did not work for 12 and 24. The (77)Se NMR chemical shifts, δ((77)Se), of the selenourea compounds derived from the new ADACs lie in the range 450-760 ppm, which indicates a much higher electrophilicity and π-accepting capability of ADACs in comparison with NHCs, which typically exhibit δ((77)Se)<200 ppm. The extreme low-field shift of 758 ppm observed for 12Se can be rationalised by the results of DFT calculations, which revealed that ADAC 12 has a minimum energy conformation with the 2,2,6,6-tetramethylpiperidino unit perpendicular to the N2C plane, which suppresses the π donation of this amino group and causes an unusually low LUMO energy and high electrophilicity.

We describe herein N-heterocyclic carbenes (NHCs) with a 1,1Ј-ferrocenylene backbone that contain substituents at the nitrogen atoms relevant for applications in the area of metallopharmaceuticals, namely benzyl units functionalised with dimethylamino and methoxy groups at the para position. These carbenes are too unstable for isolation. They were generated from the corresponding formamidinium tetrafluoroborates by deprotonation and efficiently trapped in situ. Trapping with elemental selenium afforded the corre-

This study was motivated by our recent observation that the analytical reagent Nitron (2) is an “instant carbene”, whose reaction with coinage metal salts MX afforded complexes of its carbenic tautomer 1,4-diphenyl-3-phenylamino-1,2,4-triazol-5-ylidene (2′). Our aim was to establish an alkyl homologue of 2 in order to achieve a carbenic tautomer of higher donicity. For this purpose 1-tert-butyl-4-methyl-1,2,4-triazol-4-ium-3-tert-butylaminide (6) was synthesized. Its reactions with MX afforded complexes of the carbenic tautomer 1-tert-butyl-3-tert-butylamino-4-methyl-1,2,4-triazol-5-ylidene (6′). With a stoichiometric ratio of 1:1 complexes of the type [MX(6′)] were obtained. A ratio of 2:1 furnished complexes of the type [MX(6′)2] or [M(6′)2]X. 6′ is a better σ-donor and less electrophilic than 2′ according to NMR spectroscopic data of 6H[BF4] and 6′ = Se, respectively, and IR spectroscopic data of [RhCl(6′)(CO)2] confirm that its net electron donor capacity is superior to that of 2′. A comparison of the complexes of 2′ and 6′ reveals two pronounced structural differences. [CuX(6′)2] (X = Cl, Br) exhibit more acute C–Cu–C bond angles than [CuX(2′)2]. In contrast to [CuCl(2′)], [CuCl(6′)] aggregates through Cu···Cu contacts of ca. 2.87 Å, compatible with cuprophilic interactions. These differences may be explained by the complementary steric requirements of the t-Bu and the Me substituent of 6′.

The cover picture shows the core of the molecular structure of metal complexes containing a ferrocene‐based N‐heterocyclic carbene ligand. Such compounds are prospective metallopharmaceuticals. Substituents that can be used for tuning the lipophilicity, which is an important factor for cellular uptake through biological membranes, are indicated symbolically. The cover background (courtesy of Prof. Angela Wöhrmann‐Repenning) shows the statue of Hercules, which is a famous landmark of the city of Kassel, the home of our university. It is in Wilhelmshöhe Park, Europe?s largest hill park and a UNESCO World Heritage Site. This mythological figure is associated with particularly challenging tasks, and finding new and potent pharmaceuticals is one of today?s biggest challenges. The cross‐section of a lipid bilayer schematically shown in the cover image alludes to another main attraction of Wilhelmshöhe Park, Devil?s Bridge. Details are discussed in the article by U. Siemeling et al. on . For more on the story behind the cover research, see the .

Invited for the cover of this issue is the group of Professor Ulrich Siemeling from the University of Kassel, Germany. The cover image shows the core of a metal complex containing a ferrocene-based N-heterocyclic carbene ligand with substituents that can be used to tune the lipophilicity, an important property in crossing biological membranes. In the background, the statue of Hercules symbolises the challenges of drug discovery.