Dedicated to Professor Karl Wieghardt on the occasion of his 60th birthdayNitric oxide (NO) is the first gaseous molecule known to act as a biological messenger and it participates in several important functions including control of blood pressure, neurotransmission, and inhibition of tumor growth. [1] The tumoricidal property of NO has raised interest in the use of organic [1] and metal ± nitrosyl complexes [2] that release NO upon illumination as agents in photodynamic therapy (PDT). [3] Such compounds can deliver NO to biological targets on demand and are preferred over conventional photodynamic agents such as photofrin in treating malignant tumors with hypoxic locales. In recent years, iron ± nitrosyl complexes such as Na 2 [Fe(CN) 5 NO] [4] and Roussin×s salts [5] [8] M.

Two new iron nitrosyls derived from two designed pentadentate ligands N,N-bis(2-pyridylmethyl)-amine-N'-(2-pyridylmethyl)acetamide and N,N-bis(2-pyridylmethyl)-amine-N'-[1-(2-pyridinyl)ethyl]acetamide (PcPy(3)H and MePcPy(3)H, respectively, where H is the dissociable amide proton) have been structurally characterized. These complexes are similar to a previously reported (Fe-NO)6 complex, [(PaPy(3))Fe(NO)](ClO(4))(2) (1) that releases NO under mild conditions. The present nitrosyls, namely [(PcPy(3))Fe(NO)](ClO(4))(2) (2) and [(MePcPy(3))Fe(NO)](ClO(4))(2) (3), belong to the same (Fe-NO)6 family and exhibit (a) clean (1)H NMR spectra in CD(3)CN indicating S = 0 ground state, (b) almost linear Fe-N-O angles (177.3(5) degrees and 177.6(4) degrees for 2 and 3, respectively), and (c) N-O stretching frequencies (nu(NO)) in the range 1900-1925 cm(-)(1). The binding of NO at the non-heme iron centers of 1-3 is completely reversible and all three nitrosyls rapidly release NO when exposed to light (50 W tungsten bulb). In addition to acting as photoactive NO-donors, these complexes also nitrosylate thiols such as N-acetylpenicillamine, 3-mercaptopropionic acid, and N-acetyl-cysteine-methyl-ester in yields that range from 30 to 90% in the absence of light. The addition of alkyl or aryl thiolate (RS(-)) to the (Fe-NO)6 complexes in the absence of dioxygen results in the reduction of the iron metal center to afford the corresponding (Fe-NO)7 species.

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The non‐heme iron(III) complex [(PaPy3)Fe(NO2)](ClO4) (1) transfers oxygen in the temperature range 45–65 °C to PPh3 to afford OPPh3 in MeCN. The product, [(PaPy3)Fe(NO)](ClO4), a {Fe‐NO}7 species, reverts rapidly back to 1 in the presence of dioxygen, thus making the system catalytic (see scheme). Formation of an oxo‐bridged species terminates the oxo‐transfer process (PaPy3H=N‐[N,N‐bis(2‐pyridylmethyl)aminoethyl]‐2‐pyridinecarboxamide).