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, [LNi2(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.