Redox-active tin(ii) complexes with sterically demanding o-phenylenediamido ligands and their reactivity with organic azides

Abstract

A series of tin(II) complexes ^R1 supported by phenylene-1,2-diamido ligands containing a bulky N-substituent TIPT (2,4,2′′,4′′-tetraisopropyl-[1,1′:3′,1′′]terphenyl) and different aromatic substituents R (Cl, H, Me, OMe) at the 4,5-positions and by a naphthalene-2,3-diamido ligand with the TIPT substituent ^naph1 are synthesised and characterised. Tin(II) complexes SnL^Me and SnL^Ph(tBu)2 supported by phenylene-1,2-diamido ligands with sterically less hindered N-substituents, Ph or 3,5-di-tert-butylphenyl, are also prepared as reference complexes. Crystal structures of ^R1 and ^naph1 show that the tin(II) centers are coordinated with the two amido nitrogen atoms of the respective deprotonated chelating ligand and two solvent molecules such as tetrahydrofuran and/or acetonitrile. On the other hand, the tin(II) complex SnL^Ph(tBu)2 contains only one coordinating solvent molecule, and its tin(II) center exhibits intermolecular interaction with the aromatic ligand moiety of a neighboring tin(II) complex through a 5p–π interaction. The ¹¹⁹Sn NMR signal of ^R1 in C₆D₆ shifts from the lower- to higher-magnetic field region as R becomes more electron-withdrawing, which can be explained by assuming that the naked two-coordinate tin(II) complex and solvent-involving three- and/or four-coordinate tin(II) complex exist in equilibrium in solution. The tin(II) complexes with the sterically demanding ligands (^R1 and ^naph1) are more stabilised against hydrolysis when compared with SnL^Ph and SnL^Ph(tBu)2. The tin(II) complexes ^R1 and ^naph1 undergo one-electron quasi-reversible ligand-based redox oxidation in a range from −0.45 V to +0.13 V vs. Fc/Fc⁺. The tin(II) complexes having electron-donating groups (R = OMe, Me, H) exhibit intramolecular C–H amination reactivity when treated with trisylazide (2,4,6-triisopropylphenylsulfonyl azide), giving sultam (5,7-diisopropyl-2,3-dihydro-3,3-dimethyl-1,2-benzothiazole-1,1-dioxide) as the product. On the other hand, the tin(II) complexes having electron-withdrawing groups (R = Cl, naph) do not show such reactivity. Such a difference in the reactivity is attributed to availability of a nitrene-radical bound species formed by the reaction. The formation step of the nitrene-radical bound species is analyzed kinetically to reveal that the reaction comprises two steps: binding of the organic azide to the tin(II) centre and successive intramolecular electron transfer from the ligand to the azide moiety to induce dinitrogen (N₂) elimination and nitrene-radical formation.