New [LNi<sup>II</sup><sub>2</sub>]<sup>+</sup> Complexes Incorporating 2-Formyl or 2,6-Diformyl-4-methyl Phenol as Inhibitors of the Hydrolysis of the Ligand L<sup>3-</sup>:  Ni···Ni Ferromagnetic Coupling and <i>S</i> = 2 Ground States

Reaction of the dinucleating ligand H<sub>3</sub>L (2-(2‘-hydroxyphenyl)-1,3-bis[4-(2-hydroxyphenyl)-3-azabut-3-enyl]-1,3-imidazolidine) with Ni(NO<sub>3</sub>)<sub>2</sub>·6H<sub>2</sub>O produces the dimer of monomers [Ni(HL1)]<sub>2</sub>(NO<sub>3</sub>)<sub>2</sub>·4H<sub>2</sub>O (<b>1</b>·4H<sub>2</sub>O) following the hydrolysis of H<sub>3</sub>L. 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 [Ni<sub>2</sub>L(fp)(H<sub>2</sub>O)]·3H<sub>2</sub>O (<b>2</b>·3H<sub>2</sub>O) and [Ni<sub>2</sub>L(dfp)]·4.5H2O (<b>3</b>·4.5H<sub>2</sub>O), respectively. Complexes <b>2</b> and <b>3</b> 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 <b>2</b>, coordination of terminal water reproduces the binding of this substrate of the enzyme to the active site. In both dinuclear complexes, the Ni<sup>II</sup> ions are coupled ferromagnetically to yield <i>S</i> = 2 ground states, whereas complex <b>1</b> 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.