Tunable TACN/pyrazolate hybrid ligands as dinucleating scaffolds for metallobiosite modeling—dinickel(II) complexes relevant to the urease active site

• Published in: Inorganica Chimica Acta 2002-09, Vol.337, p.371-386
• Main Authors: Buchler, Silke, Meyer, Franc, Kaifer, Elisabeth, Pritzkow, Hans
• Format: Article
• Language: English
• Subjects: Bioinorganic chemistry; Dinuclear complexes; Nickel; Urea; Urease
• ISSN: 0020-1693; 1873-3255
• DOI: 10.1016/S0020-1693(02)01101-5

Two new dinucleating pyrazole ligands HL1 and HL2 bearing appended 1,4-diisopropyl-1,4,7-triazacyclononane (iPr2TACN) side arms in the 3- and 5-positions have been synthesized by multi-step synthetic procedures. HL1 and HL2 differ by the length of the spacer between the pyrazole and the iPr2TACN, which is a novel means of tuning the metal–metal-separation in pyrazolate-based bimetallic compounds. Complexes [L1Ni2(H3O2)](ClO4)2 (5) and [L2Ni2(OH)](BPh4)2 (6b) have been characterized structurally, where the bridging H3O2 moiety in 5 is reactive and allows cooperative substrate transformations within the bimetallic pocket, while the tightly bound OH bridge in 6b is inert. Using the dinickel scaffold 5, reactivity relevant to the urease active site has been probed. Under anhydrous conditions, 5 reacts with parent and N-substituted urea to yield complexes 8a–c featuring N,O-bridging ureate anions. At high temperatures, 8a–c lose ammonia to give the cyanate-bridged species 10. This is able to bind a second cyanate at one of the nickel ions forming 11. Complexes with bridging acetate (7), O,O-bridging carbamate (9) and N,O-bridging methyl carbamate (12) have been studied for comparison, and mutual interconversions of the dinickel complexes have been investigated. Complexes 5, 6b, 7, 8a–c, 9, 11 and 12 have been characterized by X-ray crystallography. Novel dinucleating TACN/pyrazolate hybrid ligands allow for a control of the metal–metal separation in dinickel(II) complexes, giving a bridging OH or H3O2 moiety depending on the spacer length between the TACN and the pyrazolate. While the tightly bound OH is inert, the H3O2 group allows for subsequent binding and transformation of small substrates relevant to the urease metalloenzyme.