NiII, CuII and ZnII complexes with a sterically hindered scorpionate ligand (TpmsPh) and catalytic application in the diasteroselective nitroaldol (Henry) reaction

Abstract

The Ni^II and Zn^II complexes [MCl(Tpms^Ph)] (Tpms^Ph = SO₃C(pz^Ph)₃, pz = pyrazolyl; M = Ni 2 or Zn 3) and the Cu^II complex [CuCl(Tpms^Ph)(H₂O)] (4) have been prepared by treatment of the lithium salt of the sterically demanding and coordination flexible tris(3-phenyl-1-pyrazolyl)methanesulfonate (Tpms^Ph)⁻ (1) with the respective metal chlorides. The (Tpms^Ph)⁻ ligand shows the N₃ or N₂O coordination modes in 2 and 3 or in 4, respectively. Upon reaction of 2 and 3 with Ag(CF₃SO₃) in acetonitrile the complexes [M(Tpms^Ph)(MeCN)](CF₃SO₃) (M = Ni 5 or Zn 6, respectively) were formed. The compounds were obtained in good yields and characterized by analytic and spectral (IR, ¹H and ¹³C{¹H} NMR, ESI-MS) data, density functional theory (DFT) methods and {for 4 and [ⁿBu₄N](Tpms^Ph) (7), the latter obtained upon Li⁺ replacement by [ⁿBu₄N]⁺ in Li(Tpms^Ph)} by single crystal X-ray diffraction analysis. The Zn^II and Cu^II complexes (3 and 4, respectively) act as efficient catalyst precursors for the diastereoselective nitroaldol reaction of benzaldehydes and nitroethane to the corresponding β-nitroalkanols (up to 99% yield, at room temperature) with diastereoselectivity towards the formation of the anti isomer, whereas the Ni^II complex 2 only shows a modest catalytic activity.