Ferrocen-basierte N-heterocyclische Tetrylene mit tertiären Alkylsubstituenten an den Stickstoff-Atomen

In 2009, our group published two stable N-heterocyclic carbenes (NHCs) with a 1,1'-ferrocenediyl backbone (fc), which formally exhibit a six-membered FeIIC2N2CII-heterocycle. The stabilization of the fcNHCs is favored by bulky N-substituents. So far neopentyl and 2-adamantyl have been used for this purpose - a bulky primary and secondary alkyl substituent. In contrast to other cyclic diaminocarbenes, the fcNHCs are capable of activating small molecules such as NH3 or CO. The reason for this remarkable ability is the ambiphilic character of these fcNHCs, wich is similar to that of cyclic (alkyl)(amino)carbenes. We expect that tertiary alkyl substituents such as, for example, tert-butyl, will be even more beneficial for the stability of such fcNHCs and also of their heavier analogues, the N-heterocyclic tetrylenes with 1,1´-ferrocenediyl backbone (fcNHTs); fcNHTs with primary and secondary alkyl substituents tend to undergo a β-hydride elimination, which is suppressed in the case of tertiary alkyl substituents. During the course of this research four new tertiary alkyl-substituted ferrocene-based diamines (1) (fc(NHR)2; R = tBu (a), CMe2Ph (b), CMePh2 (c), CMe2tBu (d)) were synthetized and fully characterized. By using triethyl orthoformate the formamidinium ring closure was successfully performed for diamines 1a and 1b. The deprotonation of the thus obtained carbene precursors with NaHMDS cleanly afforded fcNHCs 2a and 2b. In solution both carbenes suffer a slow alkene elimination (H2C=CMe2 and H2C=CMePh, respectively). Starting from 1 we have been able to obtain germylenes fcNHGe 3a  d, stannylenes fcNHSn 4a  d as well as plumbylene fcNHPb 5d. 3a and 3d reacted with (SePh)2 and S8 to derivates of the type 3(SePh)2 and (3S)2, respectively. An analogous reaction with Se8 could only be observed with 3d. On top of that, an unprecedented addition of 2 equivs. of Ph2C=C=O was observed for 3a. The 119Sn NMR shifts of 4a and 4b are in the usual range for diaminostannylenes known from the literature. In contrast, abnormally upfield-shifted 119Sn NMR signals were detected for 4c and 4d, which is apparently related to the particularly bulky N-substituents. In the case of the molecular structure of 4d in the crystal, these substituents cause a folded FeIIC2N2SnII ring with a short Fe∙∙∙Sn distance, so that the SnII atom is no longer strictly dicoordinated. The analogous plumbylene 5d shows a signal pattern for the cyclopentadienyl protons in the 1H NMR spectrum which also clearly indicates a folding of the FeIIC2N2PbII ring, and the unusual upfield-shift of the 207Pb NMR signal suggests tricoordinate PbII (Fe∙∙∙Pb interaction). In the presence of ϭ-donors L (L = 4-(dimethyl-amino)pyridine, 1,3,4,5-tetramethylimidazolin-2-ylidene) corresponding plumbylene adducts [5d(L)] could be synthesized and structurally characterized.