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

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.