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Monomeric organoantimony(I) and organobismuth(I) compounds stabilized by an NCN chelating ligand: syntheses and structures.

The synthesis and structure of compounds containing multiple bonds between heavier Group 15 (Sb, Bi) elements, or analogous low-valent compounds with the central atom in the formal oxidation state + 1, have been among the most exciting targets in the chemistry of main-group elements. However, these compounds are often very unstable under normal conditions. The introduction of sufficiently large substituents is necessary for kinetic stabilization of such reactive species. Utilizing very effective and bulky ligands, such as 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, 2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)methyl]phenyl, or various m-terphenyl ligands, allowed isolation of stable heavier-element dipnictenes RMMR (M = Sb, Bi). Related ligands are able to stabilize unsymmetric compounds, where two different heavier Group 15 elements are connected. Using less stericaly crowded ligands, such as bis(trimethylsilyl)methyl resulted in formation of various cluster compounds, and coordination of a transition metal was necessary for stabilization of the RMMR fragment. Nevertheless, monomeric compounds of the type RM (stibinidenes or bismuthinidenes) have not been isolated in the condensed phase to date, although the existence of such compounds with two lone pairs for bismuth has been predicted based on relativistic effects. We and others have recently demonstrated that using of so-called NCN pincer-type ligand, [2,6bis(dimethylamino)methyl]phenyl (denoted as Ar hereafter), is an alternative to sterically overcrowded ligands for stabilization of low-valent antimony and bismuth compounds and the terminal ArSb=E (E = S, Se, Te) bonds. In view of this fact, it was believed that using of even more rigid and more sterically demanding NCN pincer ligand may lead to a stabilization of monomeric RM species (stibinidenes or bismuthinidenes) by combination of thermodynamical and kinetic influence of the ligand. Herein, we present the syntheses and structures of unprecedented monomeric stibinidene and bismuthinidene by taking advantage of using of 2,6-bis[N-(2’,6’-dimethylphenyl)ketimino]phenyl (denoted as L hereafter; Scheme 1).