Coordination and Conversion of Urea at Dinuclear μ-Acetato Nickel(II) Complexes with Symmetric and Asymmetric Cores

A series of symmetric and asymmetric pyrazolate-based dinuclear Ni(II) complexes relevant to the active site of
urease is reported, which have acetate ions as secondary bridges and which feature variations in the type (N or
S) and number of donor sites provided within the individual coordination compartments of the primary pyrazolate
ligand matrixes. X-ray crystallographic structures of the acetone adduct [L1Ni2(μ-OAc)(acetone)2](ClO4)2 (1) as
well as of the urea adducts [L1Ni2(μ-OAc)(benzylurea)2](ClO4)2 (2c), [L2Ni2(μ-OAc)(urea)](ClO4)2 (3a), and
[L3Ni2(μ-OAc)(N,N‘-dimethylurea)2(MeOH)2](ClO4)2 (4) have been determined. They reveal that the urea substrates
are tied up with the bimetallic cores by both O-coordination to the metal centers and hydrogen bonding between
the urea NH and the O atoms of the bridging acetate. In a related complex [L3Ni2(μ-OAc)(OAc)2Na]BPh4 (5) a
sodium ion is associated with the dinickel framework via binding to one O atom of each of the three acetates.
The nickel(II) ions in 1 and 2a are weakly antiferromagnetically coupled (J = −2.6 and −1.9 cm-1), where the
magnitude of the coupling appears to correlate with the tilting of the acetate moiety with respect to the plane of
the pyrazolate. The superexchange in 3a and 4 is even weaker. The ability of the new complexes to mediate the
ethanolysis of urea is examined and is found to be dependent on the number and stereochemical arrangement of
the accessible coordination sites at the dinuclear core:  the asymmetric species 3a is not capable of inducing any
solvolysis of the substrate, and the activity of the symmetric systems 1 and 2b is less than stoichiometric, whereas
4 displays higher activity, albeit this is still very low and possibly proceeds via a one metal ion mechanism.