New [LNi^II₂]⁺ Complexes Incorporating 2-Formyl or 2,6-Diformyl-4-methyl Phenol as Inhibitors of the Hydrolysis of the Ligand L^3-:  Ni···Ni Ferromagnetic Coupling and S = 2 Ground States

Reaction of the dinucleating ligand H₃L (2-(2‘-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) with Ni(NO₃)₂·6H₂O produces the dimer of monomers [Ni(HL1)]₂(NO₃)₂·4H₂O (1·4H₂O) following the hydrolysis of H₃L. If the reaction occurs in the presence of 2-formylphenol (Hfp) or 2,6-diformyl-4-methylphenol (Hdfp), this hydrolysis is prevented by incorporation of these co-ligands into the structure and stabilization of the new complexes [Ni₂L(fp)(H₂O)]·3H₂O (2·3H₂O) and [Ni₂L(dfp)]·4.5H2O (3·4.5H₂O), respectively. Complexes 2 and 3 may be considered to be structural models of the active site of urease, where coordination of the carbonyl ligand mimics binding of urea. In complex 2, coordination of terminal water reproduces the binding of this substrate of the enzyme to the active site. In both dinuclear complexes, the Ni^II ions are coupled ferromagnetically to yield S = 2 ground states, whereas complex 1 exhibits weak intradimer antiferromagnetic exchange through hydrogen bonds. The magnetic data can be modeled by using the Van Vleck equation, incorporating intermolecular interactions, or by diagonalization of a spin Hamiltonian that includes single-ion anisotropy.