ABSTRACT
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 [L(1)Ni(2)(&mgr;-OAc)(acetone)(2)](ClO(4))(2) (1) as well as of the urea adducts [L(1)Ni(2)(&mgr;-OAc)(benzylurea)(2)](ClO(4))(2) (2c), [L(2)Ni(2)(&mgr;-OAc)(urea)](ClO(4))(2) (3a), and [L(3)Ni(2)(&mgr;-OAc)(N,N'-dimethylurea)(2)(MeOH)(2)](ClO(4))(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 [L(3)Ni(2)(&mgr;-OAc)(OAc)(2)Na]BPh(4) (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.
ABSTRACT
A series of pyrazolate-based dinuclear Ni(II) complexes relevant to the active site of urease are reported. Deprotonation of HL(1) [HL(1) = 3,5-bis(R(2)NCH(2))-pyzH; R(2)N = Me(2)N(CH(2))(3)NMe] by means of 1 equiv of BuLi and subsequent reaction with 2 equiv of [Ni(H(2)O)(6)](ClO(4))(2) in the presence of NEt(i)Pr(2) affords the dinuclear complex [L(1)Ni(2)(OH)(MeCN)(2)](ClO(4))(2) (1). This is shown crystallographically to contain two five-coordinate nickel ions bridged by both the pyrazolate and a hydroxide, with an acetonitrile solvent molecule bound to each metal center. When HL(2) is employed {HL(2) = 3,5-bis(R(2)NCH(2))-pyzH; R(2)N = [Me(2)N(CH(2))(3)](2)N}, the additional ligand side arms act as proton acceptors forming an intramolecular N.H.N bridge to yield the complex [HL(2)Ni(2)(OH)(MeCN)(2)](ClO(4))(3) (2), whose basic bimetallic framework is essentially identical to 1. The two Ni(II) centers in 2 exhibit strong antiferromagnetic coupling (J = -46.7 cm(-)(1)). The labile acetonitrile donors in 2 are easily replaced by either neutral ligands such as dmf or anions such as thiocyanate, giving rise to the formation of complexes [HL(2)Ni(2)(OH)(dmf)(2)](ClO(4))(3) (3) and [HL(2)Ni(2)(OH)(NCS)(2)](ClO(4)) (4), respectively, where the overall dinuclear framework of 2 remains unchanged upon the substitution reaction.
