Steric influence and controlled dynamic process in a chiral square planar nickel(ii) complex supported by a dipyrromethane-based NNNN tetradentate ligand

Abstract

The equimolar reaction between 1,9-bis(N,N-dimethylaminomethyl)-5,5′-diphenyldipyrromethane, H₂L1 or 1,9-bis(3,5-dimethylpyrazolylmethyl)-5,5′-diethyldipyrromethane, H₂L2 and [NiCl₂(DME)] in the presence of KH yielded the neutral mononuclear nickel(II) complexes [Ni(L1)-κ⁴N,N,N,N], 1 and [Ni(L2)-κ⁴N,N,N,N], 2, respectively. Their X-ray structures revealed the formation of triple chelation by the ligand in both structures with quite different nickel(II)-surrounded square planar geometries owing to the steric difference between –NMe₂ and 3,5-dimethylpyrazolyl groups. While the –NMe₂ group substituted ligand L1 afforded nickel complex 1 with a plane of symmetry, 3,5-dimethylpyrazole substituted L2 rendered helical chirality in the structure of complex 2. Both the enantiomers (Δ and Λ) of complex 2 were observed in the crystal lattice. Their solution state behaviors were consistent with the solid-state structures as monitored by ¹H NMR spectroscopy. Both the enantiomers of complex 2 were present in a dynamic equilibrium in solution and studied by variable temperature ¹H NMR spectroscopy. Similar to the chiral complex, the corresponding ligand H₂L2 also exhibited chirality as shown by its crystal structure and the two ligand enantiomers participated in a very fast dynamic interconversion, which became slower upon metalation.