Synthesis, structural characterization and NMR studies of group 10 metal complexes with macrocyclic amine N-heterocyclic carbene ligands

Abstract

A series of Ni(II), Pd(II) and Pt(II) complexes [ML][PF₆]₂ [L = L¹, M = Ni (1), Pd (2), Pt (3); L = L², M = Ni (4), Pd (5), Pt (6)] and [Pt(L²)(acac)] (7) have been prepared by the reactions of two tetradentate macrocyclic amine-NHC ligand precursors, [H₂L¹][PF₆]₂ and [H₂L²][PF₆]₂, with Ni(OAc)₂·4H₂O, Pd(OAc)₂ and Pt(acac)₂ in the presence of NaOAc. Complex 7 is isolated along with 6 from the same reaction between [H₂L²][PF₆]₂ and Pt(acac)₂. There are two atropisomers in 1–3 and two achiral conformers in 4–6. The crystal structures of 1–3 and one conformer of 4–6 (4a–6a) have been determined by single-crystal X-ray diffraction studies. The metal ion is found to reside in the cavity of the macrocyclic ring and adopts a square-planar configuration. Detailed NMR studies including variable-temperature NMR spectroscopy reveal a dynamic interconverting process between two atropisomers of 1–3 in the solutions via a ring twisting mechanism. Two conformers in the equilibrated solution of 4–6, probably arising from the orientation of two amine N–H bonds with respect to the coordination plane, exchange slowly. Time-dependent ¹H NMR spectra show that one conformer (4a–6a) in solution converts into the other (4b–6b) via the inversion of the nitrogen atom.