REACTIVITY OF NITRIC OXIDE AND S-NITROSOTHIOLS WITH LOW-COORDINATE ZINC AND NICKEL CENTERS
Varonka, Matthew Stephen
Nitric oxide (NO) and biologically relevant sources of NO such as S-nitrosothiols (RSNOs) have become a topic of great interest due to their regulation of biological functions including vasorelaxation, neurotransmission, and bronchodilation. NO and RSNOs are also thought to interact with zinc-containing metalloproteins, such as metallothionein and zinc fingers, both of which contain Zn-S bonds due to coordination to cysteine residues within the proteins.In an effort to gain a better molecular level understanding of NO and RSNO reactivity with Zn-SR bonds, three families of zinc thiolate model complexes simulating His2Cys2Zn, His3Cys1Zn, and His1Cys3Zn sites are synthesized using beta-diketiminate, tris(pyrazolyl)borate, and N-heterocyclic carbene ligands. Nitric oxide itself is shown to be inert toward these complexes, however in the presence of an oxidant such as dioxygen, the Zn-S bonds are cleaved and an S-nitrosothiol is produced. S-nitrosothiols can interact with Zn-SR linkages to undergo transnitrosation. Thermodynamic preferences and kinetic parameters of this reaction are explored.The remediation of NO is also important in ecology as it is a component of photochemical smog. NO and NOx are produced as a byproduct of high temperature combustion in automobiles, and catalytic converters cleave the N-O bond to product dinitrogen and dioxygen. These catalysts require high temperatures and expensive precious metals such as Pt, Pd, and Rh to cleave the N-O bond, however.Activation of the N-O bond by Ni could prove useful for new more cost effective catalysts. Low-coordinate nickel nitrosyl complexes are explored using beta-diketiminate and N-heterocyclic carbene ligands. Calculations and preliminary experimental data are presented for the photochemical linkage isomerization of the Ni-N-O bond from linear to a metastable side-on binding mode.
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