Stannylene or metallastanna(IV)ocane: a matter of formalism.

The s basicity of electron-rich transition metals (TMs) plays a crucial role in Brønsted acid–base reactions of TM complexes, such as [H2Fe(CO)4] and [HCo(CO)4] (strong acids, poor s-basicity of the corresponding conjugate bases) and was shown to increase upon coordination of good donor ligands L, such as phosphines; that is, lowered acidity of [H2Fe(CO)3(PPh3)] or [HCo(CO)3(PPh3)]. [2] Thus, P and/or S donors bearing electron-rich TM centers have been shown to support s donation towards other main-group-element (E) Lewis acidic centers, for example in the so-called metallaboratranes I and II and Be, Al, and Ga compounds of type III (Scheme 1). Very recently, we have described compounds IV–VII comprising {L5TM(d )} moieties that exhibit s donation towards electronically saturated Lewis acidic centers E, that is, Si and Sn. Gabbaï et al. have reported similar intermetallic interactions in the heterobimetallic complexes VIII–X (Scheme 1), which comprise d TM donor sites with an almost square-planar coordination sphere. Whereas compounds IV–X were obtained by a straightforward route starting from sources that comprise TM and E in the desired oxidation states, herein we present a (formal) redox approach, which involves a reaction sequence starting from a stannylene (SnCl2) and yielding hypercoordinate tin compounds that can be regarded as palladastanna(IV)ocanes. In a convenient one-pot synthesis, [PdCl2(PPh3)2] was treated with the potassium salt of 1-methyl-2-mercaptoimidazole (methimazole, Hmt) and [SnCl2(dioxane)] (Scheme 2) to afford compound 1. Substitution of the tin-bound chlorine atoms with a dianionic tridentate ligand afforded compound 2, which comprises a hexacoordinate tin atom (Scheme 2). Reference compounds 3 and 4 (comprising Sn and Sn, respectively, and the same tridentate ONN ligand as 2) were prepared as references for spectroscopic properties. The molecular structures of 1–4 were confirmed crystallographically (see Figure 1 and the Supporting Information). Scheme 1. Selected examples of TM–base complexes with electrophilic main-group-element sites (“Z-type ligands”). Cy=cyclohexyl.