(Salen)tin complexes: Syntheses, characterization, crystal structures, and catalytic activity in the formation of propylene carbonate from CO₂ and propylene oxide

A series of (salen)tin(II) and (salen)tin(IV) complexes was synthesized. The (salen)tin(IV) complexes, (salen)SnX₂ (X = Br and I), were prepared in good yields via the direct oxidation reaction of (salen)tin(II) complexes with Br₂ or I₂. (Salen)SnX₂ successfully underwent the anion-exchange reaction with AgOTf (OTf = trifluoromethanesulfonate) to form (salen)Sn(OTf)₂ and (salen)Sn(X)(OTf) (X = Br). The (salen)Sn(OTf)₂ complex was easily converted to any of the dihalide (salen)SnX₂ compounds using halide salts. All complexes were fully characterized by ¹H NMR spectroscopy, mass spectrometry, and elemental analysis, while some were characterized by ¹³C, ¹⁹F, and ¹¹⁹Sn NMR spectroscopy. Several crystal structures of (salen)tin(II) and (salen)tin(IV) were also determined. Finally, both (salen)tin(II) and (salen)tin(IV) complexes were shown to efficiently catalyze the formation of propylene carbonate from propylene oxide and CO₂. Of the series, (3,3′,5,5′-Br₄-salen) SnBr₂, 3i, was found to be the most effective catalyst (TOF = 524 h^-1).