Electrochemistry of metal-metal bonded diruthenium complexes

Non-Medical Applications of Inorganic Medicines. A Switch Based on Mechanistic Knowledge

Metals have been used in medicine for centuries. However, it was not until much later that the effects of inorganic drugs could be rationalized from a mechanistic point of view. Today, thanks to the technologies available, this approach has been functionally developed and implemented. It has been found that there is probably no single biological target for the pharmacological effects of most inorganic drugs. Herein, we present an overview of some integrated and multi-technique approaches to elucidate the molecular interactions underlying the biological effects of metallodrugs. On this premise, selected examples are used to illustrate how the information obtained on metal-based drugs and their respective mechanisms can become relevant for applications in fields other than medicine. For example, some well-known metallodrugs, which have been shown to bind specific amino acid residues of proteins, can be used to solve problems related to protein structure elucidation in crystallographic studies. Diruthenium tetraacetate can be used to catalyze the conversion of hydroxylamines to nitrones with a high selectivity when bound to lysozyme. Finally, a case study is presented in which an unprecedented palladium/arsenic-mediated catalytic cycle for nitrile hydration was discovered thanks to previous studies on the solution chemistry of the anticancer compound arsenoplatin-1 (AP-1).

Pyrazine-Coordinated Dinuclear and Mononuclear Ruthenium Complexes Formed via the Conversion of the Triply Chlorido-Bridged Diruthenium(II) Complex

The pyrazine-coordinated dinuclear and mononuclear ruthenium complexes were synthesized through the framework conversion reactions of the triply chlorido-bridged diruthenium(II) complex [{RuII,II(bbpma)}2(μ-Cl)3]+ (bbpma; benzylbis(2-pyridylmethyl)amine, [1]+) in the presence of pyrazine, which could function as the simple molecular multinucleation ligand of metal compounds. A reduction reaction of fac-[RuIIICl3(bbpma)] with zinc in the presence of hydrochloric acid afforded [1]+ in solution, and the following addition of pyrazine (1 equiv) in the solution led to the formation of a singly pyrazine (pz)-bridged diruthenium complex, [{RuII,II(μ-Cl2ZnCl2)(bbpma)}2(μ-pz)] ([2(II,II)(ZnCl2)2]). The stoichiometric two-electron oxidation of [2(II,II)(ZnCl2)2] was successfully proceeded, and a Ru(III)-Ru(III) species, [{RuIII,IIICl2(bbpma)}2(μ-pz)](PF6)2 ([III,III](PF6)2), was isolated. The reaction of [1]+ with excess amounts of pyrazine without hydrochloric acid afforded mononuclear Ru(III) and Ru(II) complexes containing one or two pyrazine, fac-[RuClm(pz)3-m(bbpma)]+ (m = 2; [3]+; m = 1; [4]+). The details of the electrochemical and spectroscopic properties of [1]+-[4]+ in organic and aqueous solutions were discussed.

Edge-sharing bi-octahedral diruthenium(IV,IV) compounds containing Ru-Ru bonds chelated and bridged by two carbonate and two oxo groups

From paddle-wheel starting material Na3Ru2(CO3)4·6H2O, a family of edge-sharing bi-octahedral (ESBO) diruthenium(IV,IV) compounds formulated as Ru2O2(CO3)2(H2O)2L2·nH2O [L = piperazine (1) or 2-methylpiperazine (2), n = 4, and L = 2,2-dimethylpiperazine (3), n = 12] and Ru2O2(CO3)2(OH)4{M(H2O)4}2·nH2O [M = Mg (4), n = 4, and Ni (5), n = 2] were prepared and structurally characterized. The Ru28+ dimer is chelated and bridged by two CO32- and two μ-O in a trans manner, and the Ru-Ru distances fall in the range 2.3808(6)-2.4001(4) Å. Compound 2 shows the shortest Ru-Ru distance for all known ESBO Ru2 compounds reported thus far. Increasing -CH3 groups of terminal piperazine ligands coordinated to the Ru(μ-O)2(μ-O3C)2Ru core, and according to Raman spectra experiments combined with theoretical calculations, the intense bands of compounds 1-3 appearing at ∼360 cm-1 can be assigned to the stretching of Ru-Ru bonds.

Post-synthetic molecular modifications based on Schiff base condensation reactions for designing functional paddlewheel diruthenium(II,II) complexes

A new synthetic route for constructing functional paddlewheel diruthenium(II,II) complexes ([RuII,II2]) was developed by utilizing Schiff base condensation reactions of formyl-substituted benzoate-bridged [RuII,II2] complexes with various aromatic monoamines under mild conditions. Cyclic voltammetry and DFT calculations revealed that the attached Schiff base groups significantly affected the electronic states of the resulting [RuII,II2] complexes.

Steric hindrance and charge influence on the cytotoxic activity and protein binding properties of diruthenium complexes

Paddlewheel diruthenium complexes are being used as metal-based drugs. It has been proposed that their charge and steric properties determine their selectivity towards proteins. Here, we explore these parameters using the first water-soluble diruthenium complex bearing two formamidinate ligands, [Ru2Cl(DPhF)2(O2CCH3)2], and two derivatives, [Ru2Cl(DPhF)(O2CCH3)3] and K2[Ru2(DPhF)(CO3)3] (DPhF- = N,N'-diphenylformamidinate), with one formamidinate. Their protein binding properties have been assessed employing hen egg white lysozyme (HEWL). The results confirm the relationship between the type of interaction (coordinate/non-coordinate bonds) and the charge of diruthenium complexes. The crystallization medium is also a key factor. In all cases, diruthenium species maintain the M-M bond and produce stable adducts. The antiproliferative properties of these diruthenium complexes have been evaluated on an eukaryotic cell-based model. Our data show a correlation between the number of the formamidinate ligands and the anticancer activity of the diruthenium derivatives against human epithelial carcinoma cells. Increased cytotoxicity may be related to increased steric hindrance and Ru25+ core electronic density. However, the effect of increasing the lipophilicity of diruthenium species by introducing a second N,N'-diphenylformamidinate must be also considered. This work illustrates a systematic approach to shed light on the relevant properties of diruthenium compounds to design metal-based metallodrugs and diruthenium metalloenzymes.

Porous supramolecular gels produced by reversible self-gelation of ruthenium-based metal-organic polyhedra

Supramolecular gels based on metal-organic polyhedra (MOPs) represent a versatile platform to access processable soft materials with controlled porosity. Herein, we report a self-gelation approach that allows the reversible assembly of a novel Ru-based MOP in the form of colloidal gels. The presence of cationic mixed-valence [Ru2(COO)4]+ paddlewheel units allows for modification of the MOP charge via acid/base treatment, and therefore, its solubility. This feature enables control over supramolecular interactions, making it possible to reversibly force MOP aggregation to form nanoparticles, which further assemble to form a colloidal gel network. The gelation process was thoroughly investigated by time-resolved ζ-potential, pH, and dynamic light scattering measurements. This strategy leads to the evolution of hierarchically porous aerogel from individual MOP molecules without using any additional component. Furthermore, we demonstrate that the simplicity of this method can be exploited for the obtention of MOP-based gels through a one-pot synthetic approach starting from MOP precursors.

A family of edge-sharing bi-octahedral diruthenium(III,III) compounds containing Ru-Ru single bonds

From paddlewheel starting reactants Ru₂(R'CO₂)₄⁺, a family of edge-sharing bi-octahedral (ESBO) diruthenium(III,III) compounds has been prepared, formulated as Ru₂(μ-O₂CR')₂(μ-OR)₂(η-L)₂ (1-10) [R' = CH₃, R = CH₃, L = acac (1), tfac (2); R' = CH₃, R = CH₂CH₃, L = hfac (3); R' = CH₂CH₃, R = CH₃, L = acac (4), tfac (5); R' = CH₂CH₃, R = CH₂CH₃, L = hfac (6); R' = CH₂Cl, R = CH₃, L = tfac (7); R' = CH₂Cl, R = CH₂CH₃, L = hfac (8); R' = C₆H₅, R = CH₃, L = tfac (9); and R' = H, R = CH₃, L = acac (10); here, acac, tfac and hfac represent acetylacetone, trifluoroacetylacetone and hexafluoroacetylacetone, respectively]. Compounds 1-10 have a similar ESBO coordination geometry of the Ru(μ-O₂CR')₂(μ-OR)₂Ru core with a Ru-Ru center chelated and bridged by two μ-O₂CR' and two μ-OR in a trans manner, and each Ru center is also coordinated with a η²-L bidentate ligand. The Ru-Ru distances fall in the range of 2.4560(9)-2.4771(4) Å. The investigation of the electronic spectra and vibrational frequencies as well as theoretical studies with density functional theory (DFT) reveal that compounds 1-10 are ESBO bimetallic species of d⁵-d⁵ valence electron counts showing a σ²π²δ²δ*²π*² electronic configuration. Varying -CH₃ to -CF₃ groups on the η²-L bidentate ligands coordinating to the Ru(μ-O₂CR')₂(μ-OR)₂Ru core, and according to Raman spectrum measurements combined with theoretical calculations, the intense bands of compounds 1-10 appearing at ∼345 cm^-1 in the small-wavenumber region can be assigned to the stretching of the Ru-Ru single bond.

Bisaryl and Bisalkynyl Diruthenium (III,III) Compounds Based on an Electron-Deficient Building Block

Reported herein is a new series of diruthenium(III,III) bisalkynyl and bisaryl diruthenium(III,III) compounds supported with 2-amino-3-(trifluoromethyl)pyridinate (amtfmp). Using Ru₂(amtfmp)₄Cl₂ from a modified preparation, cis 2:2 Ru₂(amtfmp)₄(C≡CPh)₂ (1), cis 2:2 Ru₂(amtfmp)₄(Ph)₂ (2), and 3:1 Ru₂(amtfmp)₄(Ph)₂ (3) were synthesized via a lithium-halogen exchange reaction using LiC₂Ph and LiPh, respectively. Compounds 1-3 are all Ru₂(III,III) species with a ground-state configuration of π⁴δ²(π*)⁴ (S = 0) and were characterized via mass spectrometry, electron absorption and ¹H/¹⁹F NMR spectroscopies, and voltammetry. The molecular structures of 1-3 were established using single-crystal X-ray diffraction analysis, and preliminary density functional theory analysis was performed to elaborate the electronic structures of 1 and 2. Comparisons of the electrochemical properties of 1-3 against the Ru₂(amtfmp)₄Cl₂ starting material reveal cathodic shifts of the Ru₂^7+/6+ oxidation and the Ru₂^6+/5+ and Ru₂^5+/4+ reduction potentials. In comparison to related Ru₂(III,III) bisalkynyl and bisaryl compounds, the electrode potentials for 1-3 are anodically shifted up to ca. 0.95 V, highlighting the strong electron-withdrawing nature of the amtfmp ligand.

New insights into progressive ligand replacement from [Ru2Cl(O2CCH3)4]: synthetic strategies and variation in redox potentials and paramagnetic shifts

The complete series of [Ru₂Cl(Dp-FPhF)_x(O₂CCH₃)_4-x] (x = 1-4; Dp-FPhF^- = N,N'-bis(4-fluorophenyl)formamidinate) compounds, has been prepared and characterized by a multi-technique approach, including single crystal X-ray diffraction. A careful study of the different methodologies has allowed us to prepare four compounds with good yields and without an inert atmosphere or further purification. Specifically, [Ru₂Cl(Dp-FPhF)(O₂CCH₃)₃] (1) was obtained using an ultrasound-assisted (USS) method, while [Ru₂Cl(Dp-FPhF)₄] (4) was prepared by microwave assisted solvothermal synthesis (MWS). The intermediate substitution products cis-[Ru₂Cl(Dp-FPhF)₂(O₂CCH₃)₂] (2) and [Ru₂Cl(Dp-FPhF)₃(O₂CCH₃)] (3) have been prepared by conventional heating, controlling the molar ratio of the starting materials. ESI-MS and infrared spectroscopy were used to follow all the reactions and permitted a qualitative evaluation of the axial reactivity in this series. Magnetic and absorption measurements confirmed a high spin σ²π⁴δ²(π*δ*)³ electronic configuration in all cases. However, the effect of the gradual modification of the electronic density in the diruthenium core markedly affects other properties. The cyclic voltammograms of the compounds show a strong decrease in the one electron oxidation potential and an increase in the reduction potential in the series from 1 to 4. Furthermore, despite their paramagnetic nature, ¹H- and ¹⁹F-NMR spectra were recorded, and a correlation between the paramagnetic shift of the signals and the substitution degree of the diruthenium species was observed. These results provide a comprehensive guide to synthesise and understand the effects of equatorial ligand substitution on the properties of Ru₂⁵⁺ compounds.

Phenylene as an efficient mediator for intermetallic electronic coupling

The new compound [(NC)Ru₂(ap)₄]₂(μ-1,4-C₆H₄) (ap = 2-anilinopyridinate) was prepared to address the open question of whether a 1,4-phenylene bridge can mediate intermetallic electronic coupling. As a manifestation of strong coupling, hole delocalization between the Ru₂ centers on the IR time scale (10^-14 s) was established using spectroelectrochemistry. An orbital mechanism for coupling was elaborated with DFT analysis.

Electronic Structure of Ru26+ Complexes with Electron-Rich Anilinopyridinate Ligands

Diruthenium paddlewheel complexes supported by electron-rich anilinopyridinate (Xap) ligands were synthesized in the course of the first in-depth structural and spectroscopic interrogation of monocationic [Ru₂(Xap)₄Cl]⁺ species in the Ru₂⁶⁺ oxidation state. Despite paramagnetism of the compounds, ¹H NMR spectroscopy proved highly informative for determining the isomerism of the Ru₂⁵⁺ and Ru₂⁶⁺ compounds. While most compounds are found to have the polar (4,0) geometry, with all four Xap ligands in the same orientation, some synthetic procedures resulted in a mixture of (4,0) and (3,1) isomers, most notably in the case of the parent compound Ru₂(ap)₄Cl. The isomerism of this compound has been overlooked in previous reports. Electrochemical studies demonstrate that oxidation potentials can be tuned by the installation of electron donating groups to the ligands, increasing accessibility of the Ru₂⁶⁺ oxidation state. The resulting Ru₂⁶⁺ monocations were found to have the expected (π*)² ground state, and an in-depth study of the electronic transitions by Vis/NIR absorption and MCD spectroscopies with the aid of TD-DFT allowed for the assignment of the electronic spectra. The empty δ* orbital is the major acceptor orbital for the most prominent electronic transitions. Both Ru₂⁵⁺ and Ru₂⁶⁺ compounds were studied by Ru K-edge X-ray absorption spectroscopy; however, the rising edge energy is insensitive to redox changes in the compounds due to the broad line shape observed for 4d transition metal K-edges. DFT calculations indicate the presence of ligand orbitals at the frontier level, suggesting that further oxidation beyond Ru₂⁶⁺ will be ligand-centered rather than metal-centered.

Diruthenium aryl compounds - tuning of electrochemical responses and solubility

Reported herein are the two new series of diruthenium aryl compounds: Ru₂(DiMeOap)₄(Ar) (1a-6a) (DiMeOap = 2-(3,5-dimethoxyanilino)pyridinate) and Ru₂(m-ⁱPrOap)₄(Ar) (1b-5b) (m-ⁱPrOap = 2-(3-iso-propoxyanilino)pyridinate), prepared through the lithium-halogen exchange reaction with a variety of aryl halides (Ar = C₆H₄-4-NMe₂ (1), C₆H₄-4-^tBu (2), C₆H₄-4-OMe (3), C₆H₃-3,5-(OMe)₂ (4), C₆H₄-4-CF₃ (5), C₆H₅ (6)). The molecular structures of these compounds were established with X-ray diffraction studies. Additionally, these compounds were characterized using electronic absorption and voltammetric techniques. Compounds 1a-6a and 1b-5b are all in the Ru₂⁵⁺ oxidation state, with a ground state configuration of σ²π⁴δ²(π*δ*)³ (S = 3/2). Use of the modified ap ligands (ap') resulted in moderate increases of product yield when compared to the unsubstituted Ru₂(ap)₄(Ar) (ap = 2-anilinopyridinate) series. Comparisons of the electrochemical properties of 1a-6a and 1b-5b against the Ru₂(ap')Cl starting material reveals the addition of the aryl ligand cathodically shifted the Ru₂^6+/5+ oxidation and Ru₂^5+/4+ reduction potentials. These oxidation and reductions potentials are also strongly dependent on the p-substituent of the axial aryl ligands.

Ultrasound-assisted synthesis of water-soluble monosubstituted diruthenium compounds

The elusive monosubstituted diruthenium complexes [Ru₂Cl(DAniF)(O₂CMe)₃] (1), [Ru₂Cl(DPhF)(O₂CMe)₃] (2), [Ru₂Cl(D-p-CNPhF)(O₂CMe)₃] (3), [Ru₂Cl(D-o-TolF)(O₂CMe)₃] (4), [Ru₂Cl(D-m-TolF)(O₂CMe)₃] (5), [Ru₂Cl(D-p-TolF)(O₂CMe)₃] (6) and [Ru₂Cl(p-TolA)(O₂CMe)₃] (7) have been synthesized using for the first time ultrasound-assisted synthesis to carry out a substitution reaction in metal-metal bonded dinuclear compounds (DAniF^- = N,N'-bis(4-anisyl)formamidinate; DPhF^- = N,N'-diphenylformamidinate; D-p-CNPhF^- = N,N'-bis(4-cyanophenyl)formamidinate; D-o/m/p-TolF^- = N,N'-bis(2/3/4-tolyl)formamidinate; p-TolA^- = N-4-tolylamidate). This is a simpler and greener method than the tedious procedures described in the literature, and it has permitted to obtain water-soluble complexes with good yields in a short period of time. A synthetic study has been implemented to find the best experimental conditions to prepare compounds 1-7. Two different types of ligands, formamidinate and amidate, have been used to check the generality of the method for the preparation of monosubstituted complexes. Five new compounds (2-6) have been obtained using a formamidinate ligand, the synthesis of the previously described compound 1 has been improved, and an unprecedented monoamidate complex has been achieved (7). The crystal structures of compounds 3 and 7 have been solved by single crystal X-ray diffraction. These compounds show the typical paddlewheel structure with three acetate ligands and one formamidinate (3) or amidate (7) bridging ligand at the equatorial positions. The axial positions are occupied by the chloride ligand giving rise to one-dimensional polymer structures that were previously unknown for monosubstituted compounds.

Comparing Isoelectronic, Quadruple-Bonded Metalloporphyrin and Metallocorrole Dimers: Scalar-Relativistic DFT Calculations Predict a >1 eV Range for Ionization Potential and Electron Affinity

A scalar-relativistic DFT study of isoelectronic, quadruple-bonded Group 6 metalloporphyrins (M = Mo, W) and Group 7 metallocorroles (M = Tc, Re) has uncovered dramatic differences in ionization potential (IP) and electron affinity (EA) among the compounds. Thus, both the IPs and EAs of the corrole derivatives are 1 eV or more higher than those of the porphyrin derivatives. These differences largely reflect the much lower orbital energies of the δ- and δ*-orbitals of the corrole dimers relative to those of the porphyrin dimers, which in turn reflect the higher (+III as opposed to +II) oxidation states of the metals in the former compounds. Significant differences have also been determined between Mo and W porphyrin dimers and between Tc and Re corrole dimers. These differences are thought to largely reflect greater relativistic destabilization of the 5d orbitals of W and Re relative to the 4d orbitals of Mo and Tc. The calculated differences in IP and EA should translate to major differences in electrochemical redox potentials-a prediction that in our opinion is well worth confirming.

Control of dominant conduction orbitals by peripheral substituents in paddle-wheel diruthenium alkynyl molecular junctions

Control of charge carriers that transport through the molecular junctions is essential for thermoelectric materials. In general, the charge carrier depends on the dominant conduction orbitals and is dominantly determined by the terminal anchor groups. The present study discloses the synthesis, physical properties in solution, and single-molecule conductance of paddle-wheel diruthenium complexes 1R having diarylformamidinato supporting ligands (DArF: p-R-C6H4-NCHN-C6H4-R-p) and two axial thioanisylethynyl conducting anchor groups, revealing unique substituent effects with respect to the conduction orbitals. The complexes 1R with a few different aryl substituents (R = OMe, H, Cl, and CF3) were fully characterized by spectroscopic and crystallographic analyses. The single-molecule conductance determined by the scanning tunneling microscope break junction (STM-BJ) technique was in the 10-5 to 10-4 G 0 region, and the order of conductance was 1OMe > 1CF3 ≫ 1H ∼ 1Cl, which was not consistent with the Hammett substituent constants σ of R. Cyclic voltammetry revealed the narrow HOMO-LUMO gaps of 1R originating from the diruthenium motif, as further supported by the DFT study. The DFT-NEGF analysis of this unique result revealed that the dominant conductance routes changed from HOMO conductance (for 1OMe) to LUMO conductance (for 1CF3). The drastic change in the conductance properties originates from the intrinsic narrow HOMO-LUMO gaps.