Zofia Stasicka

Light and metal complexes in medicine

Abstract This article surveys some medical aspects of inorganic photochemistry, in particular those involving the use of coordination compounds. Examples of beneficial (therapeutic, diagnostic) and deleterious effects of the interaction between light and metallopharmaceuticals have been selected for presentation. The use of light as a tool in studying and modelling the different biochemical processes is also discussed.

Mechanism of photochemical reduction of chromium(VI) by alcohols and its environmental aspects

Abstract The idea of photochemical abatement of the Cr(VI) pollution has been verified by investigating the photoreduction mediated by aliphatic alcohols under conditions mimicking the environmental ones. Effects of the alcohol nature, pH and presence of oxygen are analysed. The time-resolved spectra are used to follow the generation of the transient chromium Cr(V), Cr(IV) and Cr(II) species and the R 1 R 2 CHOH + radicals. A direct interaction between chromate(VI) and an electron donor is a precondition of the photoreduction via the photoinduced electron transfer (PET). Two pathways of the PET are identified: one-electron transfer for intermolecular and two-electron transfer for the intramolecular systems. In the case of the alcohol mediators the option is pH-controlled.

S-NITROSOTHIOLS: MATERIALS, REACTIVITY AND MECHANISMS

The article provides a comprehensive view of S-nitrosothiols, chemical behaviour, the pathways leading to their synthesis, their spectral properties, analytical methods of detection and determination, chemical and photochemical reactivity, kinetic aspects and suggested mechanisms. The structure parameters of S-nitrosothiols and the parent thiols are analysed with respect to their effect on the strengthening or weakening the S–NO bond, and in consequence on the S-nitrosothiol stability. This depends also on the ease of S–S bond formation in the product disulphide. These structural features seem to be crucial both to spontaneous as well as to Cu-catalysed decomposition. Principal emphasis is given here to the S-nitrosothiols’ ability to act as ligands and to the effect of coordination on the ligand properties. The chemical and photochemical behaviours of the complexes are described in more detail and their roles in chemical and biochemical systems are discussed. The aim of the article is to demonstrate that the contribution of S-nitrosothiols to chemical and biochemical processes is more diverse than supposed hitherto. Nevertheless, their role is predictable and, based on the correlation between structure and reactivity, many important mechanisms of biochemical processes can be interpreted and various applications designed.

Ligand and medium controlled photochemistry of iron and ruthenium mixed-ligand complexes: prospecting for versatile systems

Abstract Selected Fe and Ru systems, whose photochemical behaviour is sensitive to numerous parameters, are presented. These systems, containing multiple species in equilibrium, are versatile enough to be adapted to special tasks and may also be used to model the phenomena and mechanisms occurring in nature. The role of various parameters is analysed and principal emphasis is given to the ligand sphere influence on the nature of the excited state and thereby on the photochemical mode. This is crucial in the case of Fe(II) complexes of the type [Fe(CN) 5 L] n − , whereas in the carbolyl–cyclopentadienyl complexes, represented by [cpRu(CO) 2 ] 2 , the nature of the excited state is of less importance than for pentacyanoferrates(II). The photochemistry of the carbonyl–cyclopentadienyl complexes is more susceptible to the impact of the medium and the role of the secondary processes is more significant.

Reactions of the [Fe(CN)5NO]2− complex with biologically relevant thiols

Reactions of the [Fe(CN)5NO]2− complex with biologically relevant thiols (HnRS = cysteine, N-acetylcysteine, ethyl cysteinate and glutathione) are initiated by the nucleophilic attack of a thiolate (RSn−) on the N atom of the NO+ ligand in the complex to form [Fe(CN)5N(O)SR](n+2)−. The N–S bond in the latter complex is, however, weak and can undergo both heterolytic and homolytic splitting. The former process makes the synthesis reaction reversible, whereas the latter is responsible for the spontaneous redox decomposition: [Fe(CN)5N(O)SR](n+2)− → [FeI(CN)5NO]3− + RS˙(n−1)−. The rate of the monomolecular reaction is controlled by an inductive effect in the thiol with an additional stabilisation coming from formation of a six-membered ring in the case of the N-acylated compounds. In the presence of thiolate excess, the RS˙(n−1)− radicals are transformed into the more stable RSSR˙(2n−1)− radicals, which are scavenged by both [Fe(CN)5N(O)SR](n+2)− and [Fe(CN)5NO]2−. The former reaction initiates, whereas the latter terminates, chain reactions of the catalysed redox decomposition. The catalytic decomposition (in the thiol excess) is much faster than the spontaneous decay (in the nitroprusside excess) but leads to the same final products. The Fe(I) reduction product is identified by UV/Vis, IR, electrochemical and EPR methods. The effect of molecular oxygen is investigated and explained. The mechanism is interpreted in terms of intermediate [Fe(CN)5N(O)(SR)2](2n+2)− complex formation via nucleophilic attack and its decay mainly via homolytic splitting of the N–S bond. To verify the mechanism, a simple reaction model is constructed, based on the assumption that the RSNO(n−1)− ligands are mostly responsible for the [Fe(CN)5N(O)(SR)](n+2)− reactivity and their electronic properties are discussed within the DFT framework.

PHOTOSUBSTITUTION AND PHOTOREDOX BEHAVIOUR OF CYANOMETALLATES : REACTION MODELS

Abstract This paper surveys thermal and photochemical reactivities of the cyanometallates. The thermal reactivity is described in connection with the unique properties of the cyanide ligand, whereas photochemical behaviour is discussed with reference to characteristics of the relevant excited states. For some hexacyano and pentacyanonitrosyl complexes the reactivity is interpreted using a simple INDO-type method. The modelling procedure for the thermal reactivity consists in interpreting the characteristics of the frontier orbitals, bond orders and charge distributions in the ground state, whereas photochemical reactivity is modelled by calculating the electronic structure of the selected excited states and analyzing the differences between the features of the ground and excited states.

Molecular switches based on cyanoferrate complexes

Abstract A new system able to play the role of a molecular switch has been characterised. The system consists of the [Fe(CN) 5 NO] 2− –[Fe(CN) 5 N(O)SR] 3− complexes in equilibrium. The constituent complexes differ considerably in electronic absorption (especially in the visible region) and undergo photochemical reactions proceeding in distinct spectral ranges and yielding different products. The equilibrium can be shifted by several physical and chemical stimuli changing thereby absorption and/or modifying the photochemical behaviour of the system. To the stimuli belong: pH, thiol concentration, ion strength, nature and concentration of cations, temperature and pressure. All these features make the system suitable for switching purposes and for different kinds of signal processing in digital, analogue and integrating circuits. As the [Fe(CN) 5 NO] 2− –[Fe(CN) 5 N(O)SR] 3− system photochemically produces NO-donors and/or nitric oxide, its phototherapeutic application is considered.

Structure and UV-Vis spectroscopy of the iron-sulfur dinuclear nitrosyl complexes [Fe2S2(NO)4]2− and [Fe2(SR)2(NO)4]

Calculations of the electronic structure, geometry and electronic spectra of Roussin’s red salt dianion (RRS) and Roussin’s red diester (RRE) were carried out with the RB3LYP and UB3LYP methods. The electronic structure emerging from these calculations may be described as composed of two {Fe(NO)2}9 units, in which ferric ion (S = 5/2) is antiferromagnetically coupled to two NO− ligands (each with S = 1), giving S = 1/2; the units are antiferromagnetically coupled to each other yielding a total S = 0. The S2− bridges (in RRS) or SR− bridges (RRE) mediate the antiferromagnetic coupling. The character of the frontier orbitals controls the dinuclear species’ reactivity, which is initiated by electrophilic attack on S-localized HOMO orbitals (RRS) or nucleophilic attack on the Fe–S antibonding LUMO orbital (RRE). The contrasting susceptibility to electrophilic/nucleophilic attack is also assisted by the sulfur charge, which is negative in RRS and positive in RRE. The calculated spectra of RRS and RRE show substantial resemblance to the experimental spectra. The calculated transitions are mainly of charge transfer character: At long wavelengths they are described as π*NO → d (LMCT), at short wavelengths (below 250 nm) the most intense transitions are d → π*NO (MLCT). In the middle part of the spectra both types of transitions are present. Some contribution of sulfur to the transitions throughout the whole spectrum is observed. The π*NO → d transitions are assumed to be responsible for the photochemical reactivity of both compounds, which is initiated by photodissociation of the NO group.

Photochemistry of the [Fe(CN)5N(O)SR]3- complex : A mechanistic study

Photochemical behaviour of the title complex (RS − = mercaptosuccinate) was defined as photodissociation and photooxidation–

Cyanonitrosylmetallates as potential NO-donors

The [M(CN)xNOy]n- complexes (where M = Cr(I), Mn(I), Mn(II), Fe(I), Fe(II), Fe(III)) were studied as potential NO-donors using both pharmacological and theoretical semi-empirical methods. Only iron complexes appeared to be pharmacologically active. The quantum chemical calculations indicated that these complexes have the highest predisposition to undergo a nucleophilic attack followed by the NO+ release. The results allowed us to interpret the metabolism of the [M(CN)xNOy]n- complexes in terms of the NO(+)-donation.