Ferrocene‐Based N‐Heterocyclic Plumbylenes [Fe{(η5‐C5H4)NSiMe2R}2Pb:]: Influence of the Steric Demand of theN‐Substituents on Their Dimerization via C−H Activation with PbII

Abstract

AbstractFerrocene‐based N‐heterocyclic plumbylenes fc[(NSiMe2R)2Pb:] (1; fc=1,1’‐ferrocenylene) are easily accessible by transamination from [(Me3Si)2N]2Pb and the corresponding 1,1’‐diaminoferrocene derivatives fc(NHSiMe2R)2. They may form unconventional dimers2by 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 (21⇆2) has been addressed experimentally and computationally. It critically depends on the steric demand of theN‐substituents SiMe2R, 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.