Synthesis and structural characterization of the first trialkylguanidinate and hexahydropyramidopyramidinate complexes of tin

The first guanidinate complexes of tin have been prepared using N,N′,N′′-trialkylguanidinates ([(RN)₂C(NRH)]⁻; R=cyclohexyl; isopropyl) and 1,3,4,6,7,8-hexahydro-2H-pyramido[1,2-a]pyramidinate (hpp⁻) as ligands. The direct reaction between triisopropylguanidine and SnCl₄ provided the complex {[ⁱPrN]₂C[NHⁱPr]}SnCl₃ (1) along with concomitant formation of the guanidinium salt {C[NHⁱPr]₃}⁺[SnCl₅(THF)]⁻ (2). The Sn(II) guanidinate complexes {(C₆H₁₁N)₂C[NH(C₆H₁₁)]}₂Sn (3) and {CN[N(CH₂)₃]₂}₂Sn (4) were prepared through metathesis reactions between 0.5 equiv. SnCl₂ and (C₆H₁₁N)₂C[NH(C₆H₁₁)]Li or hppLi, respectively. Complex 4 is the first reported mononuclear complex of this ligand. In contrast, the reaction of hppLi with 1 equiv. SnCl₂ afforded a bridging dinuclear species, {CN[N(CH₂)₃]₂SnCl}₂ (5). A second mononuclear complex of the hpp⁻ ligand, {CN[N(CH₂)₃]₂}₂SnCl₂ (6), was the product obtained from the reaction of 2 equiv. of hppLi with SnCl₄. The full structural details of compounds 1 and 3–6 are reported. In the case of compounds 1 and 3 these results revealed a distinctly unsymmetrical bonding mode for the bidentate guanidinate ligand and suggest variable degrees of π delocalization with the ligand. The geometries of the Sn centers in 3, 4 are derived from distorted trigonal bipyramidal coordination with a stereochemically active lone pair occupying one coordination site. In contrast, complex 5 displayed a geometry derived from a tetrahedral ligand array with one vertex occupied by a lone pair of electrons. Complex 6 is six coordinate and possesses 2 equiv. chelating bidentate hpp⁻ ligands and two cis-chloro groups.