Journal of the Chemical Society, Dalton Transactions p. 1215 - 1221 (1996)
Update date:2022-08-02
Topics:
Nivorozhkin, Alexander L.
Toftlund, Hans
Jorgensen, Per Lauge
Nivorozhkin, Leonid E.
Two series of copper(II) and nickel(II) complexes [ML1]-[ML6] containing deprotonated tetradentate N,N′-bis(5-aminopyrazol-4-ylmethylene)polymethylene diamine ligands L1-L6 with varying n (2-4) of the bridge -HC=N(CH2)nN=CH- have been prepared and investigated by 1H NMR, UV/VIS and ESR spectroscopy and cyclic voltammetry measurements. Complexes [CuL4]-[CuL6] display a gradual alteration of the configuration at the metal centre from planar to pseudo-tetrahedral when going from n = 2 to 4 as indicated by the variations in their ESR spectral parameters. This was also reflected in the electronic spectra which exhibited a red shift of the ligand-field bands in this series. The nickel(II) complexes with n = 2 or 3 possess a planar structure both in solution and in the solid state, whereas in the case of [ML6] (n = 4), diamagnetic in the solid state, a rapid (on the NMR time-scale) equilibrium between planar low-spin (S = 0) and tetrahedral high-spin (S = 1) forms was found in chloroform solution with 45% of the high-spin species at 331 K. At low temperatures the planar ? tetrahedral interconversion is not accompanied by R ? S enantiomerization at the metal centre due to the higher barriers to inversion of the seven-membered metallocycte which was followed at elevated temperatures by coalescence of the diastereotopic NCH2 and CCH2 protons. The crystal structure of [NiL6] [H2L6 = N,N′-bis(5-amino-1,5-diphenylpyrazol-4-ylmethylene)butane-1,4-diamine] revealed a planar-to-tetrahedral distorted metal configuration with an angle between the N(1)NiN(2) and N(5)NiN(8) planes of 21° and twist-chair conformation of the seven-membered metallocycle. Cyclic voltammetry showed a reversible NiII → NiIII oxidation at potentials 0.47-0.51 V and irreversible CuII → CuI transformations. .
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