Electrophilic behavior of coordinated nitric oxide

Nitric oxide has recently been found to be important messenger molecule mediating critical physiological functions such as neurotransmission and muscle relaxation. The nitroprusside ion, which is clinically used as a vasodilator, functions by releasing NO in vivo. The reactivities of these molecules are surveyed with a view toward the development of metallonitrosyls as pharmaceutical agents.

Chemistry relevant to the biological effects of nitric oxide and metallonitrosyls

The influence of the solvent on the structure and IR spectrum of the [Fe(CN)(5)NO](2)(-) ion is investigated by using gradient corrected density functional theory. IR spectra are also measured on different solvents and the results obtained are compared with the predicted ones. We have treated the solvent effects with a continuum model, based on the Onsager's reaction field approach; in order to mimic strong specific interactions, calculations were also performed on the complex protonated at the cyanide trans to the nitrosyl group. The reaction field calculations predict only qualitatively the most important observed trends, e.g., the shifts in the nitrosyl stretching wavenumber, but fail in accounting quantitatively for the differences between the spectra in water and acetonitrile. The possible role of specific interactions is consistently accounted for by interpreting the experimental shifts of the NO stretching wavenumber nu(NO), as well as the visible absorption energies, when changing the Lewis acidity of the solvent, as measured by the Gutmann's acceptor number. Ligand population analysis was performed to relate the solvent effects with the sigma donor and pi acceptor behavior of cyanide and nitrosyl ligands. The significance of nu(NO) shifts as a result of changes in the medium is discussed in view of the physiological relevance of transition-metal nitrosyl chemistry.

Theoretical and Experimental Study of Medium Effects on the Structure and Spectroscopy of the [Fe(CN)(5)NO](2)(-) Ion.

The oxidation and reduction behavior of complexes [(mpz)M(CN)5]2- between the group 8 pentacyanometalates(II) and the N-methylpyrazinium ion mpz+ was studied using spectroelectrochemical techniques (UV/vis/NIR, IR, EPR) in nonaqueous and, in part, aqueous media. As a further source of information on the electronic structure, we studied the variable solvatochromism of MLCT absorption bands in the precursor ions [(mpz)M(CN)5]2-. The mpz-centered one-electron reduction showed a tendency for concomitant loss of cyanide (EC process) to yield ions [(mpz)M(CN)4]2-, the lability increasing in the order M = Os << Ru < Fe. In contrast to the EPR spectra obtained from reduction with S2O42- in aqueous media, which showed no evidence for close mpz•/metal association, the intra muros electrolysis experiments in DMSO or acetonitrile yielded the following results:  The EPR spectrum that was previously assigned to [(mpz)Fe(CN)5]3- is now ascribed to [(mpz)Fe(CN)4]2-. Whereas the dissociatively inert osmium radical complex...

The mpz(+)/mpz(*) Pair as Organic Analogue of the NO(+)/NO(*) Ligand Redox System (mpz = N-Methylpyrazinium). A Combined Electrochemical and Spectroscopic Study (UV/Vis/NIR, IR, EPR) of Complexes [(mpz)M(CN)(5)](2)(-)(/3)(-) (M = Fe, Ru, Os).

Publisher Summary This chapter discusses redox chemistry of small nitrogen-containing L ligands with bioinorganic relevance, particularly nitric oxide, NO, and its redox-interconverted forms NO + (nitrosyl) and NO - (nitroside, or its protonated form, HNO, nitroxyl). The coordination chemistry of transition metal cyanometallates has been of great and long-standing matter because of the fundamental issues related to their electronic structure and reactivity. The chemistry of redox-active ligands L in the pentacyano(L) ferrate(II) and -(III) complexes is of fundamental concern for disclosing the mechanistic features which are operative in stoichiometric and catalytic processes associated with oxidation, reduction, and disproportionation of L. This is particularly important for small molecules displaying a versatile coordination behavior, such as the N-binding species associated with the nitrogen redox cycle, which display complex, multi-electronic paths for oxidation or reduction in most cases, generally under kinetic rather than thermodynamic control. Another crucial factor influencing the redox reactivity of cyano complexes is the possibility of bridging one (or more) cyano ligands to an acceptor moiety.

Redox reactivity of coordinated ligands in pentacyano(l)ferrate complexes

: The electrochemical reduction of the pentacyanonitrosylferrate(II) anion has been studied in nonaqueous aprotic media by polarography, cyclic voltammetry and coulometry. In contrast to results from aqueous solution, the initial one electron reduction proceeds with a rapid loss of cyanide to give the blue five-coordinate tetracyanonitrosylferrate(I) anion. The tetracyanonitrosylferrate(I( anion undergoes a further quasireversible reduction and an oxidation which is reversible at fast sweep rates.

Peter. Bonnar

Papers

Electrochemistry of the nitroprusside ion in nonaqueous solvents

Electrochemistry of the nitroprusside ion in nonaqueous solvents