NITROXYL LIGAND (HNO) ON A PENTACIANOFERRATE(II) PLATFORM

ANDREA C. MONTENEGRO; SARA E. BARI; LEONARDO D. SLEP; DANIEL H. MURGIDA; JOSÉ A. OLABE

Jerusalem, Israel

Conferencia; 38th International Conference on Coordination Chemistry; 2008

The coordination chemistry of nitroxyl (HNO/NO-) in iron complexes is relevant to the characterization of intermediates in nitrogen metabolism. Only the myoglobin complex (MbIIHNO) has been characterized to date. We focus in the preparation of another nitroxyl-complex, by reducing nitroprusside (Na2 [Fe(CN)5NO], SNP) with dithionite, under excess cyanide (pH 10, anaerobiosis). An intense band at l = 445 nm (e = 3600 M–1cm–1) forms upon the two-electron successive reduction of SNP, with the intermediacy of the well characterized [Fe(CN)5NO•]3–. Oxidants (eg.: hexacyanoferrate(III)) revert the process, suggesting the reversible interconversion equilibria:   [Fe(CN)5NO]2– ↔ [Fe(CN)5NO•]3– ↔ [Fe(CN)5HNO]3–   Previous DFT/ZINDO calculations allowed for trustable structural parameters for bound HNO, and predicted a MLCT transition at 460 nm. Resonance Raman spectra (lexc, 458 nm) displayed typical bands for the cyano-ligands (2050-2100 cm–1), and for N=O (1300-1400 cm–1), shifted upon labeling with 15N. Additional excess of cyanide provided kinetic evidence of the slow, irreversible dissociation of HNO, with k-HNO= 10-4 s-1 (250C, pH 7), forming [Fe(CN)6]4- and N2O, as evidenced by ATR spectra (2038 and 2230cm-1, respectively). The nitroxyl complex reacts slowly with O2, giving mainly SNP. Further studies on this and other reactivity modes are currently ongoing.