N-heterocyclic olefins as dative carbon donor ligands for diaminoplumbylenes: Syntheses and crystal structures of adducts with 1,3,4,5-tetramethyl-2-methyleneimidazoline

Abstract

The aim of this study was to shed more light on the potential of N-heterocyclic olefins as carbon donor ligands for the coordination/stabilization of divalent Group 14 element compounds. Stable adducts of the N-heterocyclic olefin 1,3,4,5-tetramethyl-2-methyleneimidazoline (MeIMeCH2) were isolated with the cyclic diaminoplumbylenes ph(NSiMe3)2Pb (ph = 1,2-phenylene) and fc(NSiRMe2)2Pb (fc = 1,1′-ferrocenylene; R = Me, tBu) as well as with the acyclic congener [(Me3Si)2N]2Pb. In contrast, the cyclic (alkyl)(amino)carbene 1-(2,6-diisopropylphenyl)-3,3-diethyl-5,5-dimethylpyrrolidin-2-ylidene (CAACEt) gave rise to a stable adduct only with ph(NSiMe3)2Pb. The ferrocene-based N-heterocyclic olefin fc(NCH2tBu)2CCH2 was synthesised. It afforded a stable adduct with triphenylborane, but not with the diaminoplumbylenes of this study. All stable adducts were structurally characterized by single-crystal X-ray diffraction. An essentially perpendicular orientation of the Pb-C bond vector with respect to the PbN2 plane was found for the diaminoplumbylene adducts. 207Pb NMR spectroscopic data suggest, but do not prove, that the strength of the donor-acceptor interaction of the MeIMeCH2 adducts is lower than that of adducts with the related N-heterocyclic carbene 1,3,4,5-tetramethylimidazolin-2-ylidene, but higher than that of adducts with the popular N-donor 4-(dimethylamino)pyridine.