Biomimetic dipicolylamine nickel complexes: Structure and reactivity

This document comprises the results of over seven years of research in the area of the urease enzyme and its structural modeling. The research ranges from direct kinetic studies of the urease enzyme, to a series of synthetic model complexes. These featured nickel complexes derive from the 2,2'-dipicolylamine (RDPA) tridentate ligand moiety, with a variety of coordination geometries, nuclearity, and reactivity demonstrated.;In Chapter 2, several nickel systems are described, beginning with kinetic studies of the urease enzyme itself, demonstrating that the cyanate anion is not an inhibitor of the enzyme. Spectral studies of multiple nickel complexes are then described, all based on the RDPA ligand moiety, demonstrating urea binding to the 1:1 nickel:ligand complex formed from nickel(II) and tert-butyl-dipicolylamine (tBuDPA) and no observed binding in the 1:2 nickel:ligand complex formed from benzyl-dipicolylamine (BzDPA). The differences in reactivity between these two species are further explored with regards to radical aliphatic hydroxylation. Chapter 2 closes with two stopped-flow studies of nickel species related to the DPA species discussed elsewhere: urea binding to a nickel complex containing a dinucleating ligand with DPA moieties (in collaboration with Prof. Franc Meyer), and oxygen dependence on the reactivity of a trinuclear nickel complex towards an ARD-analog (in collaboration with Prof. Lisa Berreau).;Chapter 3 describes the synthesis and characterization of an unusual 4Ni-4OH cubanetype tetrameric species supported by a dipicolylamine ligand DPA lacking a substitutent on its central amine (HDPA). The cubane is characterized crystallographically and magnetically; it demonstrates antiferromagnetic exchange across its bridging hydroxide anions. A brief review of the magnetochemistry of 4Ni-4OR cubane-type clusters is included. In Chapter 4, the effects of a sterically bulky tert-butyl group on the coordination chemistry described in Chapter 3 are presented. Unlike HDPA, tBuDPA prevents formation of a 4Ni-4OH cubane, instead limiting the polynuclearity of the resulting complex to a dimer; the resulting 2Ni-2OH species and its 2Ni-2OMe analog are described. The tetrameric carbonate-bridged species resulting from carbon dioxide fixation by the 2Ni-2OH species is also described. The carbonate tetramer and the hydroxide dimer are both characterized magnetically.;Chapter 5 describes an unusual nitrile condensation by the 2Ni-2OH species characterized in Chapter 4. Urea, as well as dimethylurea and acetamide participate in this reaction, leading to an imidoylamidine nickel complex, which is crystallographically characterized. Finally chapter 6 describes four nickel complexes each containing an RDPA ligand species. 1:2 nickel:ligand complexes containing BzDPA and isopropyl-dipicolylamine (iPrDPA) are characterized, as well as two novel tBuDPA nickel complexes with different counterions. Nickel(II) chloride gives rise to an unusual 2Ni-3Cl dimer complex, which nickel(II) nitrate gives rise to a 1:1 nickel:ligand complex which crystallizes with both fac and mer conformations in the same unit cell. In this way are the structures and reactivities of these urease model complexes examined.