Ferrocene-Based N-Heterocyclic Plumbylenes [Fe{(η 5 -C 5 H 4 )NSiMe 2 R} 2 Pb:]: Influence of the Steric Demand of the N-Substituents on Their Dimerization via C-H Activation with Pb II .

Ferrocene-based N-heterocyclic plumbylenes fc[(NSiMe 2 R) 2 Pb:] (1; fc=1,1'-ferrocenylene) are easily accessible by transamination from [(Me 3 Si) 2 N] 2 Pb and the corresponding 1,1'-diaminoferrocene derivatives fc(NHSiMe 2 R) 2 . They may form unconventional dimers 2 by a process, which causes the cleavage of a cyclopentadienyl C-H bond and the formation of a Pb-C and an N-H bond. The monomer-dimer equilibrium (2 1⇆2) has been addressed experimentally and computationally. It critically depends on the steric demand of the N-substituents SiMe 2 R, which has been varied systematically by using homologues with aliphatic (R=methyl, ethyl, isopropyl, tert-butyl) and aromatic units (R=phenyl, mesityl, ferrocenyl). Even in the sterically least congested case (R=methyl), dimerization is only slightly exergonic. It eventually becomes prohibitively endergonic with increasingly larger substituents and is thus not observed for R=tert-butyl, mesityl, and ferrocenyl. R=phenyl represents a borderline case, where the dimer is still detectable in the equilibrium mixture, albeit as a very minor component, in accord with the slightly endergonic Gibbs free energy change calculated for its formation. Addition of 4-dimethylaminopyridine (DMAP) to the monomer-dimer equilibrium mixtures cleanly affords the corresponding adducts [1(DMAP)], irrespective of the equilibrium composition.