L-Edge X-ray Absorption Spectroscopic Investigation of {FeNO}6: Delocalization vs Antiferromagnetic Coupling.

NO is a classic non-innocent ligand, and iron nitrosyls can have different electronic structure descriptions depending on their spin state and coordination environment. These highly covalent ligands are found in metalloproteins and are also used as models for Fe-O2 systems. This study utilizes iron L-edge X-ray absorption spectroscopy (XAS), interpreted using a valence bond configuration interaction multiplet model, to directly experimentally probe the electronic structure of the S = 0 {FeNO}6 compound [Fe(PaPy3)NO]2+ (PaPy3 = N,N-bis(2-pyridylmethyl)amine-N-ethyl-2-pyridine-2-carboxamide) and the S = 0 [Fe(PaPy3)CO]+ reference compound. This method allows separation of the σ-donation and π-acceptor interactions of the ligand through ligand-to-metal and metal-to-ligand charge-transfer mixing pathways. The analysis shows that the {FeNO}6 electronic structure is best described as FeIII-NO(neutral), with no localized electron in an NO π* orbital or electron hole in an Fe dπ orbital. This delocalization comes from the large energy gap between the Fe-NO π-bonding and antibonding molecular orbitals relative to the exchange interactions between electrons in these orbitals. This study demonstrates the utility of L-edge XAS in experimentally defining highly delocalized electronic structures.

Photoactive metal carbonyl complexes as potential agents for targeted CO delivery.

The surprising discovery of carbon monoxide (CO) as a signaling molecule in mammalian physiology has recently raised interest in this toxic gas among researchers in biochemical and pharmaceutical community. CO is endogenously produced mainly from catabolism of heme by the enzyme heme oxygenase (HO) and participates in a myriad of anti-inflammatory, anti-proliferative, and vasoregulatory pathways. In animal models, low doses of CO have exhibited beneficial effects in suppression of organ graft rejection and safeguarding the heart during reperfusion after cardiopulmonary bypass surgery. The salutary effects of CO have naturally drawn attention of the pharmaceutical industry for its use as a cytoprotective agent. Safety-related concerns of the use of this noxious gas have prompted research in the area of syntheses of CO-releasing molecules (CORMs) and to date, several metal carbonyls (metal complexes of CO) have been employed as CORMs in promoting prolonged (and safe) delivery of low doses of CO to cellular targets. Because many carbonyl complexes release CO upon illumination, investigators have recently began to explore the possibility of "controlled CO delivery" through the use of light. During the past few years, a number of photoactive CORMs or "photoCORMs" have been synthesized that release CO upon illumination with UV or visible light. The utility of these photoCORMs in CO delivery has also been confirmed. Novel design principles for isolation of photoCORMs have started to appear in recent reports. Scrutiny of the literature reveals the emergence of a new exciting area of drug development in such efforts. The potential of photoCORMs as CO-donating pharmaceuticals along with a brief overview of the physiological roles of CO is presented in this review.

Light-triggered carbon monoxide delivery with Al-MCM-41-based nanoparticles bearing a designed manganese carbonyl complex.

A photoactive manganese carbonyl complex namely, fac-[Mn(pqa)(CO)3]ClO4 (abbreviated as {Mn-CO}, pqa = (2-pyridylmethyl)(2-quinolylmethyl)amine) has been incorporated in the pores of 60 nm mesoporous Al-MCM-41 nanoparticles. Strong electrostatic interactions with the negatively charged walls of the aluminosilicate host entrap the cationic carbonyl complex in the resulting material {Mn-CO}@Al-MCM-41 which has been characterized by various physical techniques and chemical analysis. The sample morphology and microstructure of the material have been established by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Results of powder X-ray diffraction (PXRD) data and the SEM-EDX elemental maps of Si, O, Al and Mn confirm that the carbonyl complex is within the pores of the nanoparticles. The 2% manganese content of {Mn-CO}@Al-MCM-41, determined by acid digestion followed by inductively coupled plasma-optical emission spectroscopy (ICP-OES), indicates efficient loading of the carbonyl complex in the nanoparticles. In biological buffer solutions, less than 2% leaching of {Mn-CO} from the nanoparticles was noted within a period of 24 h. When exposed to a broadband visible light source (λ > 350 nm), {Mn-CO}@Al-MCM-41 rapidly releases CO, as confirmed by the reduced myoglobin assay. The utility of light-induced CO delivery by {Mn-CO}@Al-MCM-41 has been established by its capacity to relax rat aorta muscle rings in tissue bath experiments.