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

Abstract

Ferrocene-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 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 has been addressed experimentally and computationally. It critically depends on the steric demand of the N-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.