Synthesis and structure of tetracoordinated nickel(II) complexes of deprotonated chelated aminoazo ligands: X-ray crystal structure of bis[1-isopropyl-3-methyl-4-(4-methylphenylazo)-5-(4-methoxyphenylamidato)pyrazole] nickel(II)

Summary Tetracoordinate nickel(II) complexes NiL₂ derived from the deprotonated forms of aminoazoligands HL were prepared and investigated by spectroscopy and magnetic susceptibility measurements. The magnetic moments, which lie in the 3.1–3.6 B.M. range correspond to the occurrence both in solution and in the solid state of the high spin (S = 1) form and a tetrahedral configuration at the metal centre. The u.v. spectra exhibit three ligand field bands at 1020–1280 nm characteristic of high spin nickel(II) complexes. The large isotropic chemical shifts found in the ¹H n.m.r. spectra are consistent with partial delocalization of unpaired electron spin density to the ligand HOMO. The X-ray single crystal structure of NiL ₂ ⁴ L⁴= 1-isopropyl-3-methyl-4-(4-methylphenylazo)-5-(4-methoxyphenylamino)-pyrazole] reveals that the metal is coordinated by four nitrogen atoms in tetrahedral configuration with an angle of 90° between the N(1)NiN(2) and N(6)NiN(7) planes belonging to the different almost planar metallocycles. The rates of RS interconversion of the tetrahedral configuration for NiL ₂ ⁴ and NiL ₂ ⁵ L⁵ = 1-isopropyl-3-methyl-4-phenylazo-d ₅-5-(4-methoxyphenylamino)pyrazole] are slow on the n.m.r. timescale. In contrast to NiL ₂ ¹ -Ni ₂ ⁶ , NiL ₂ ⁷ ], which contains coordinated NH-groups instead of NAr-groups, is planar.