Coordination and conversion of urea at dinuclear mu-acetato nickel(II) complexes with symmetric and asymmetric cores

A series of symmetric and asymmetric pyrazolate-based dinuclear Ni(II) comp lexes 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 com partments of the primary pyrazolate ligand matrixes. X-ray crystallographic structures of the acetone adduct [(LNi2)-Ni-1(mu-OAc)(acetone)(2)](ClO4)(2 ) (1) as well as of the urea adducts [(LNi2)-Ni-1(mu-OAc)(benzylurea)(2)](C lO4)(2) (2c), [(LNi2)-Ni-2(mu-OAc)(urea)](ClO4)(2) (3a), and [(LNi2)-Ni-3(m u-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 th e urea NH and the O atoms of the bridging acetate. In a related complex [(L Ni2)-Ni-3(mu-OAc)(OAc)(2)Na]BPh4 (5) a sodium ion is associated with the di nickel framework via binding to one O atom of each of the three acetates. T he 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 cor relate with the tilting of the acetate moiety with respect to the plane of the pyrazolate. The superexchange in 3a and 1 is even weaker. The ability o f the new complexes to mediate the ethanolysis of urea is examined and is f ound to be dependent on the number and stereochemical arrangement of the ac cessible coordination sites at the dinuclear core: the asymmetric species 3 a is not capable of inducing any solvolysis of the substrate, and the activ ity of the symmetric systems 1 and 2b is less than stoichiometric, whereas 4 displays higher activity, albeit this is still very low and possibly proc eeds via a one metal ion mechanism.