Bioinspired catalytic conjugate additions of thiophenols to α,β-enones by a disubstituted benzoate-bridged nickel mimic for the active site of urease

A disubstituted benzoate polydentate ligand, 2,6-bis[bis(pyridinyl-2- methyl)aminoethoxyl]benzoate (HL), was prepared to synthesize nickel mimics for the active site of urease. Reaction of the deprotonated L^- with Ni(ClO⁴)² · 6H²O afforded a dinickel complex, [LNi²(CH³CN)(THF)](ClO⁴)³ (1), characterized by UV/vis spectroscopy and X-ray crystallography. Addition of urea to an acetonitrile solution of 1 afforded a dinickel urea adduct, [LNi²(urea)²](ClO⁴)³ · 2CH³CN (2), which was structurally and spectroscopically characterized. ¹H NMR and ESI-MS spectra of 2 both evidenced that urea molecules remained coordinated to the nickel centers of 2 in solution. With the inspiration of urea coordination to the nickel centers of 1, the conjugate additions of thiophenols to α,β-enones catalyzed by complex 1 were examined and found to proceed in good yields. In contrast, the same catalytic reaction by Ni(ClO₄)₂ · 6H₂O and HL was far less effective. Also, addition of NaOAc or NaOAcPh₂ to an acetonitrile solution of 1 gave tetranuclear nickel complexes, [LNi ₂-(μ-OAc)]₂(ClO₄)₄ · 3H₂O (3) and [LNi₂(μ-OAcPh₂)] ₂(ClO₄)₄ · 5CH₃CN · 2THF (4), and their molecular structures determined by X-ray diffraction can be described as dimers of dimers. Each dinickel core of 3 and 4 closely mimics the active site of urease. The magnetic data of 1, 3, and 4 exhibited a very weak antiferromagnetic coupling (J =-0.72 cm^-1 for 1,-0.65 cm^-1 for 3 and 4) between two metal centers in the dinickel core.