Inorganic reaction mechanisms. A personal journey

A Personal Account on Inorganic Reaction Mechanisms

The presented Review is focused on the latest research in the field of inorganic chemistry performed by the van Eldik group and his collaborators. The first part of the manuscript concentrates on the interaction of nitric oxide and its derivatives with biologically important compounds. We summarized mechanistic information on the interaction between model porphyrin systems (microperoxidase) and NO as well as the recent studies on the formation of nitrosylcobalamin (CblNO). The following sections cover the characterization of the Ru(II)/Ru(III) mixed-valence ion-pair complexes, including Ru(II)/Ru(III)(edta) complexes. The last part concerns the latest mechanistic information on the DFT techniques applications. Each section presents the most important results with the mechanistic interpretations.

The inorganic chemistry of the cobalt corrinoids - an update

The inorganic chemistry of the cobalt corrinoids, derivatives of vitamin B₁₂, is reviewed, with particular emphasis on equilibrium constants for, and kinetics of, their axial ligand substitution reactions. The role the corrin ligand plays in controlling and modifying the properties of the metal ion is emphasised. Other aspects of the chemistry of these compounds, including their structure, corrinoid complexes with metals other than cobalt, the redox chemistry of the cobalt corrinoids and their chemical redox reactions, and their photochemistry are discussed. Their role as catalysts in non-biological reactions and aspects of their organometallic chemistry are briefly mentioned. Particular mention is made of the role that computational methods - and especially DFT calculations - have played in developing our understanding of the inorganic chemistry of these compounds. A brief overview of the biological chemistry of the B₁₂-dependent enzymes is also given for the reader's convenience.

Ligand exchange at tetra-coordinated beryllium centres

Mono and dinuclear phosphine complexes of beryllium halides [(PMe₃)₂BeX₂], [(PMe₃)BeX₂]₂ and [(PCy₃)BeX₂]₂ (X = Cl, Br, I) were synthesised and characterised via NMR and IR spectroscopy as well as single crystal X-ray diffraction experiments. Dissociation and ligand exchange processes at these complexes were investigated through variable temperature NMR experiments in combination with line shape analysis and complemented by quantum chemical calculations. The PMe₃ dissociation energy is smallest in [(PMe₃)₂BeCl₂], while PMe₃ exchange is similar in energy in all mononuclear [(PMe₃)₂BeX₂] complexes and follows an interchange mechanism. While [(PMe₃)BeX₂]₂ dissociates homolytically, [(PCy₃)BeX₂]₂ cleaves one phosphine ligand. These distinctive dissociation processes account for the different chemical behaviour of these complexes.

Reactivity of non-organometallic ruthenium(II) polypyridyl complexes and their application as catalysts for hydride transfer reactions

Recently, we investigated the substitution behavior of a series of ruthenium(II) complexes of the general formula [RuII(terpy)(N∧N)Cl]Cl, where terpy = 2,2′:6′,2″-terpyridine, N∧N = bidentate ligand, in aqueous solutions. We have shown that the most and least reactive complexes of the series are [RuII(terpy)(en)Cl]Cl (en = ethylenediamine) and [RuII(terpy)(phen)Cl]Cl (phen = 1, 10-phenantroline), respectively, as a result of different electronic effects provided by the bidentate spectator chelates. Polypyridyl amine Ru(II) complex, viz. [Ru(terpy)(en)Cl]Cl and [Ru(terpy)(ampy)Cl]Cl (where ampy = 2-(aminomethyl)pyridine), in which the terpy chelate labilizes the metal center, are able to catalyze the conversion of NAD+ to 1,4-NADH using sodium formate as a source of hydride. We showed that this complex can control the [NAD+]/[NADH] ratio and potentially induce reductive stress in living cells, which is accepted as an effective method to kill cancer cells. Polypyridyl Ru(II) complexes, characterized in terms of the behavior in aqueous solutions, can be used as model systems to monitor heterogeneous multiphase ligand substitution reactions at the solid-liquid interface. Colloidal coordination compounds in the submicron range were synthesized from Ru(II)-aqua derivatives of starting chlorido complexes via the anti-solvent procedure and stabilized by a surfactant shell layer.

Electronic effects on the mechanism of the NAD+ coenzyme reduction catalysed by a non-organometallic ruthenium(ii) polypyridyl amine complex in the presence of formate

In the present study, electronic effects on the mechanism of the NAD⁺ coenzyme reduction in the presence of formate, catalysed by a non-organometallic ruthenium(ii) polypyridyl amine complex, were investigated. The [Ru^II(terpy)(ampy)Cl]Cl (terpy = 2,2′:6′,2′′-terpyridine, ampy = 2-(aminomethyl)pyridine) complex was employed as the catalyst. The reactions were studied in a water/ethanol mixture as a function of formate, catalyst, and NAD⁺ concentrations at 37 °C. The overall process was found to be 11 to 18 times slower than for the corresponding ethylenediamine (en) complex as the result of π-back bonding effects of the ampy ligand. The mechanistic studies revealed a complete set of reactions that accounted for the overall catalytic cycle based on a formate-induced hydride transfer reaction to form the reduced coenzyme, NADH. The geometries of the ruthenium(ii)-ampy complexes involved in the catalytic cycle and free energy changes for the main steps were predicted by DFT calculations. Similar calculations were also performed for the analogues ruthenium(ii)-en and ruthenium(ii)-bipy complexes (bipy = 2,2′-bipyridine). The DFT calculated energies show that both the solvent-formato exchange and the formato-hydrido conversion reactions have negative (favourable) energies to proceed spontaneously. The reactions involving the en complex have the more negative (favourable) reaction energies, followed by the ampy complex, in agreement with faster reactions for en complexes and slower reactions for bipy complexes than for ampy complexes.

The graphical abstract represents the overall catalytic cycle in which the non-organometallic Ru(ii) formato complex releases CO₂ and transfers hydride to NAD⁺ to form NADH coenzyme.

Mechanistic Studies on the Reaction between Aquacobalamin and the HNO Donor Piloty's Acid over a Wide pH Range in Aqueous Solution

Detailed kinetic and mechanistic studies have been carried out on the reaction between aquacobalamin/hydroxocobalamin (CblOH₂⁺/CblOH) and nitroxyl (HNO) generated by Piloty's acid (PA, N-hydroxybenzenesulfonamide) over a wide pH range (3.5-13). The resulting data showed that in a basic solution HNO can react with hydroxocobalamin to form nitrosylcobalamin despite the inert nature of CblOH. It was shown that at low PA concentrations the rate-determining step is the decomposition of PhSO₂NHO^- to release HNO, whereas the reaction between CblOH and HNO becomes the rate-determining step at high PA concentrations. Data from kinetic studies on the reaction of CblOH with an excess of HNO enabled us to experimentally determine the pK_a(HNO) value from initial rate data as a function of pH, giving pK_a(HNO) = 11.47 ± 0.04. An especially interesting observation was made in the neutral pH range, where PA is stable and does not produce HNO. Under such conditions, rapid formation of CblNO was observed in the studied system. The obtained data suggest that CblOH₂⁺ reacts directly with PA to form a Piloty's acid-bound cobalamin intermediate, which deprotonates rapidly at neutral pH followed by rate-determining S-N bond cleavage to give CblNO and release PhSO₂^-.

Complexation and bonding studies on [Ru(NO)(H2O)5]3+ with nitrate ions by using density functional theory calculation

Complexation reactions of ruthenium–nitrosyl complexes in HNO₃ solution were investigated by density functional theory (DFT) calculations in order to predict the stability of Ru species in high-level radioactive liquid waste (HLLW) solution. The equilibrium structure of [Ru(NO)(NO₃)₃(H₂O)₂] obtained by DFT calculations reproduced the experimental Ru–ligand bond lengths and IR frequencies reported previously. Comparison of the Gibbs energies among the geometrical isomers for [Ru(NO)(NO₃)_x(H₂O)_5−x]^(3−x)+/− revealed that the complexation reactions of the ruthenium–nitrosyl complexes with NO₃⁻ proceed via the NO₃⁻ coordination to the equatorial plane toward the Ru–NO axis. We also estimated Gibbs energy differences on the stepwise complexation reactions to succeed in reproducing the fraction of Ru–NO species in 6 M HNO₃ solution, such as in HLLW, by considering the association energy between the Ru–NO species and the substituting ligands. Electron density analyses of the complexes indicated that the strength of the Ru–ligand coordination bonds depends on the stability of the Ru species and the Ru complex without NO₃⁻ at the axial position is more stable than that with NO₃⁻, which might be attributed to the difference in the trans influence between H₂O and NO₃⁻. Finally, we demonstrated the complexation kinetics in the reactions x = 1 → x = 2. The present study is expected to enable us to model the precise complexation reactions of platinum-group metals in HNO₃ solution.

Density functional study on the complexation of [Ru(NO)(H₂O)₅]³⁺ with NO₃⁻ ions reproduced the stabilities of the geometrical isomers and the stepwise substitution reactivities by combining the association energy with the leaving/entering ligands.