Zwitterionic Forms of Salicylaldimine Donor Ligands in Unusual Adduct Formation with Organotin(IV) Lewis Acids

The focus of this investigation was on adduct formation, and its consequences, between organotin(IV) Lewis acids and the tetradentate salicylaldimine ligands H₂3-MeOsalen [N,N‘-bis(3-methoxysalicylidine)1,2-ethane diamine] and H₂3-MeOsalbiphen [N,N‘-bis(3-methoxysalicylidine)2,2‘-biphenyl diamine]. SnBuⁿ₂Cl₂ reacted with H₂3-MeOsalen to give a 1/1 adduct, A, containing a dangling salicylaldimine ligand but failed to yield a solid state adduct with H₂3-MeOsalbiphen, even though it exists in dynamic equilibrium with the latter ligand in solution. The stronger Lewis acids SnPh₂Cl₂ and SnBuⁿCl₃ yielded 2/1 solid state adducts D and E, respectively, with H₂3-MeOsalbiphen in each of which the ligand bridges to six-coordinated tin centers and donor bonds are via phenolic and methoxy oxygen atoms. SnBuⁿ₂(NCS)₂ reacted with H₂3-MeOsalen to give an adduct, B, having a solid state ionic structure of formulation {[SnBuⁿ₂(NCS)(H₂3-MeOsalen)]₂(μ-H₂3-MeOsalen)}[SnBuⁿ₂(NCS)₄]. The cation of B displays both bridging and dangling ligands and seven-coordinated tin. The ionic structure gives way to a nonionic symmetric cyclic trimeric structure of formulation [SnBuⁿ₂(NCS)₂(μ-H₂3-MeOsalen)]₃ in solution. The latter reacted with NaCl and SnBuⁿ₂Cl₂ to yield a new adduct, C, which in the solid state has an the ionic formulation [SnBuⁿ₂Cl(μ-H₂3-MeOsalen)]₂[SnBuⁿ₂(NCS)₄], the cation of which has a centrosymmetric structure and seven-coordinated tin. The solid state ionic structure of C is not retained in solution. Crystallographic data are also reported for the free ligands H₂3-MeOsalen and H₂3-MeOsalbiphen (two crystal modifications of the latter were identified). The structural data clearly identify that donor bond formation through the salicylaldimine phenolic oxygen has the concomitant effect of phenolic hydrogen transfer to the imine nitrogen, a process that accounts for the imine hydrolysis which frequently occurs when the salicylaldimine ligands react with Lewis acids. Salicylaldehyde does not assume a zwitterionic form in its 1/1 adduct, F, with SnMe₂Cl₂ (the structure of this adduct was reinvestigated in the present study), and consequently the role of the phenolic oxygen is reduced to that of a very weak donor, participating merely in secondary bonding.