Mixed valency in ligand-bridged diruthenium frameworks: divergences and perspectives

Azo-Cyanamide Bridged Dinuclear Iron Complexes Exhibiting no Electronic Coupling but Moderate Magnetic Coupling between the two Iron Centers

To investigate the effect of long-distance organic ligand on electronic coupling between metallic atoms, the mononuclear and dinuclear complexes [Cp(dppe)Fe(apc)] (1), [{Cp(dppe)Fe}2(μ-adpc)] (2), [{CpMe5(dppe)Fe}2(μ-adpc) (3) and their oxidized complexes [Cp(dppe)Fe(apc)][PF6] (1[PF6]), [{Cp(dppe)Fe}2(μ-adpc)][PF6] (2[PF6]2), [{CpMe5(dppe)Fe}2(μ-adpc)][PF6]2 (3[PF6]2) (Cp=1,3-cyclopentadiene, CpMe5=1,2,3,4,5-pentamethylcyclopentadiene, dppe=1,2-bis(diphenylphosphino)ethane), apc-=4-azo(phenylcyanamido)benzene and adpc2-=4,4'-azodi(phenylcyanamido)) were synthesized and characterized by cyclic voltammetry, UV-vis, single-crystal X-ray diffraction and Mössbauer spectra. Electrochemical measurements showed no electronic coupling between the two terminal Fe units, However, the investigation results of the magnetic properties of the two-electron oxidized complexes indicate the presence of moderate antiferromagnetic coupling across 18 Å distance.

Mixed-Valence BN-Doped Corannulene Trimer Radical Cations

Mixed-valence (MV) dimers have been extensively investigated, however, the structure and properties of purely organic MV trimers based on open-shell polycyclic aromatic hydrocarbons remain elusive. Herein, unprecedented MV BN-doped corannulene radical cations [BN-Cor1]3 ⋅⋅2+  ⋅ 2[BArylF 4 ]- and [BN-Cor2]3 ⋅⋅2+  ⋅ 2[BArylF 4 ]- were synthesized via chemical oxidation, and their structures were unambiguously confirmed by single-crystal X-ray diffraction. These uncommon radical cations consist of three corannulene cores and two [BArylF 4 ]- anions, and three corannulene motifs [BN-Cor1]3 ⋅⋅2+ and [BN-Cor2]3 ⋅⋅2+ in the unit cell exhibit a trimer structure with a slipped π-stacking configuration. Detailed structural analyses further revealed that the corannulene cores exhibit an infinite layered self-assembly configuration, allowing their potential applications as building blocks for molecular conductors. The detection of a forbidden transition (Δms =±2) by electron paramagnetic resonance (EPR) spectroscopy further confirmed the existence of two unpaired electrons in the π-trimers and the MV characteristic of these two species. Variable-temperature EPR and conductivity measurements suggested that the BN-doped π-trimers exhibited antiferromagnetic coupling and conductivity properties.

Redox-Transmetalation Reactions: Easy Access to Homo- and Heterodimetallic d8,d10 Complexes

The ability of the imine PyCH═N-CH2Py (Py = 2-pyridyl, bpi) to behave as a heteroditopic ligand, which is suitable for creating two separate compartments to host metals in different oxidation states, has been developed by studying the reactions of the mixed-valence complexes [(cod)M-Ι(μ-bpi)MΙ(cod)] (M = Rh, Ir) with [M'(Cl)2(PPh3)2] (M' = Pd, Ni). The results depend on the molar ratio of the reagents used (1:1 or 1:2) to give the heterometallic complexes {d10-M',d8-M}-[(PPh3)(Cl)M'0(μ-bpi)MΙ(cod)] (Pd,Rh, 4; Pd,Ir, 5; Ni,Rh, 8; Ni,Ir, 9) and the two-electron mixed-valent compounds [(PPh3)(Cl)M'0(μ-bpi)M'ΙΙ(Cl)] (M' = Ni, 10; Pd, 11), respectively. A redox process occurs in the replacement of the low-valent [(cod)M-I] fragment, whereas the exchange of the [(cod)MI] fragment is redox-neutral. The metal with a d8 configuration in the products exhibits a square-planar geometry coordinated to two (Rh/Ir) or three (Ni/Pd) nitrogen atoms of the bridging bpi ligand. Conversely, the metal with a d10 configuration adopts trigonal-planar geometries, π-bonded to the imine C═N bond. The isolated complexes 4/5 and 10/11, along with the hypothetical heterometallic Pd,Ni compound (12), were studied by DFT methods. Additionally, the T-shaped moiety 'M'ΙΙ(PPh3)(Cl)(η1-CH-N(bpi))', stabilized by a secondary γ-agostic interaction, and the 'M'II(Cl)(κ3N-bpi)' fragment was found to be accessible redoxomers of complexes 10 and 11 by DFT calculations.

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.

Diruthenium and triruthenium compounds of the potential redox active non-chelated η1-N,η1-N-benzothiadiazole bridge

In the present study, a series of non-chelated BTD (2,1,3-benzothiadiazole)-bridged diruthenium(II) ([{(CH₃CN)(acac)₂Ru^II}₂(μ-BTD)] 1, [{CH₃CN(acac)₂Ru^II}(μ-BTD){Ru^II(acac)₂(η¹-N-BTD)}] 2, [{(η¹-N-BTD)(acac)₂Ru^II}₂(μ-BTD)] 3), and triruthenium ([{(acac)₂Ru^II}₃(μ-BTD)₂(η¹-N-BTD)₂] 4) complexes with varying ratios of η¹-N and μ-bis-η¹-N,η¹-N modes of BTD were studied. Complexes 1-4 (S = 0) were obtained via the one-pot reaction of electron-rich Ru(acac)₂(CH₃CN)₂ and electron-deficient BTD in refluxing acetone. The relatively low Ru(II)/Ru(III) potential of 1-4 (0.08-0.44 V versus SCE) further facilitated the isolation of the corresponding mixed valent Ru^IIRu^III (S = 1/2) and Ru^IIRu^IIRu^III (S = 1/2)/Ru^IIRu^IIIRu^III (S = 1) forms [1]ClO₄-[3]ClO₄ and [4]ClO₄/[4](ClO₄)₂, respectively. The single-crystal X-ray structures of the representative mixed valent [1]ClO₄ and [3]ClO₄ established (i) Ru⋯Ru distances of 6.227 Å and 6.256 Å (molecule A)/6.184 Å (molecule B), respectively, (ii) a significant variation of the N-S distance of BTD in [3]ClO₄ as a function of its binding mode μ versus η¹ and (iii) similar Ru-N (μ-BTD) distances in each case corresponding to a valence delocalised situation. The mixed valent diruthenium (1+-3+) and triruthenium (4+/42+) complexes exhibited metal-based anisotropic electron paramagnetic resonance (EPR) and moderately intense low-energy intervalence charge-transfer (IVCT) transitions in the near-infrared region of 1730-1890 nm. Analysis of the IVCT band using the Hush treatment revealed a valence delocalised class III mixed valent state with the electronic coupling V_ab of ≈2640-2890 cm^-1, as also corroborated by the K_c values of 10⁵-10⁸, solvent independency of the IVCT band and uniform spin distribution between the metal ions in the singly occupied state(s). Furthermore, the involvement of the BTD (η¹ and μ)-based orbitals in the reduction processes was evident by its free radical EPR feature.

Structural and electronic forms of doubly oxido/Pz and triply oxido/(Pz)2 bridged mixed valent and isovalent diruthenium complexes (Pz = pyrazolate)

The article reported on the diastereomeric dinuclear mixed-valent complexes [(acac)₂Ru(III)(μ-O)(μ-Pz^R)Ru(IV)(acac)₂] (R = H, Me, meso: ΔΛ, 1a-1c; rac: ΔΔ/ΛΛ, 2a-2c) and rac-[(acac)₂Ru(III)(μ-O)(μ-Iz)Ru(IV)(acac)₂], (2d) (HPz = pyrazole, HIz = indazole, acac = acetylacetonate). Moreover, the diruthenium(II,II) complexes [(HPz)₃Ru(II)(μ-O)(μ-Pz)₂ Ru(II)(HPz)₃] (3a) and [(HIz)₃Ru(II)(μ-O)(μ-Iz)₂Ru(II)(HIz)₃] (3d) were presented. The analogous form of 3a, i.e., [(HPz)₂(Pz)Ru(III)(μ-O)(μ-Pz)₂Ru(III)(Pz)(HPz)₂], was previously reported. Single crystal X-ray structures of 1a-1c/2a-2d and representative 3a showed their molecular forms, including the diastereomeric nature of the former. The Ru-O-Ru angle decreased appreciably on switching from doubly bridged 1 and 2 (128-135°) to triply bridged 3a (114°). Both series of complexes displayed rhombic symmetry in their EPR spectra, with g₁ and g₂ being very similar for 1a-1c with an almost axial look. The mixed-valence complex with a Ru(III)Ru(IV) (S = 1/2) state of 1 and 2 would lead to iso-valence complexes of Ru(III)Ru(III) and Ru(IV)Ru(IV) with an EPR inactive state by one electron redox reaction. On the other hand, metal based {Ru(II)Ru(II)/Ru(II)Ru(III), 3a/3a⁺} and terminal ligand (HPz/HPz^-, 3a/3a^-) based redox processes displayed anisotropic and free radical EPR, respectively. An IVCT (intervalence charge transfer) band was found for the delocalised mixed valent 1 and 2 {Ru(III)Ru(IV)} or 3a⁺ {Ru(II)Ru(III)} in the NIR region. The intense metal-to-ligand charge transfer (MLCT) transitions of 1-3 in the visible region varied systematically as a function of the metal oxidation state.

Synthesis and electronic coupling studies of cyclometalated diruthenium complexes bridged by 3,3',5,5'-tetrakis(benzimidazol-2-yl)-biphenyl

Three cyclometalated diruthenium complexes bridged by 3,3',5,5'-tetrakis(benzimidazol-2-yl)biphenyl (H-tbibp) and capped with different terminal ligands have been synthesized and examined. In addition, two monoruthenium complexes with H-tbibp have been prepared for the purpose of comparison studies. The degree of Ru-Ru electronic coupling of these diruthenium complexes has been investigated by electrochemical and intervalence charge-transfer (IVCT) analyses. These results suggest that when the same or similar terminal ligands are used, the strength of H-tbibp in mediating the Ru-Ru coupling is enhanced with respect to that of the previously reported bridging ligand 3,3',5,5'-tetrakis(N-methylbenzimidazol-2-yl)biphenyl, but it is slightly inferior to that of the classical bridging ligand 3,3',5,5'-tetrakis(pyrid-2-yl)biphenyl. This trend is also supported by CNS analyses based on the hole-superexchange mechanism. In addition, DFT calculations have been performed to probe the spin density distributions of the singly-oxidized diruthenium complexes with H-tbibp and TDDFT calculations are used to reproduce the IVCT transitions.

Unprecedented metal-metal bonded {Ru4(μ3-O)2} butterfly core in oxido-carboxylato bridged mixed valence cluster-structural elucidation and electronic forms in accessible redox states

Unprecedented metal-metal bonded oxido-carboxylato bridged mixed valence tetraruthenium cluster [(acac)₆Ru(μ₃-O)₂(μ-CH₃COO)₃] 1 (S = 1/2) (acac = acetylacetonate) with {Ru₄(μ₃-O)₂} "butterfly" core has been achieved via the reaction of Ru(acac)₂(CH₃CN)₂ with excess CH₃COONa (Ru : CH₃COONa = 1 : 15) in refluxing EtOH-H₂O (5 : 1). Structural analysis of 1 ascertained a unique Ru-Ru bonded (Ru2-Ru3: 2.5187(6) Å (DFT: 2.560 Å)) {Ru₂(Ru-Ru)(μ₃-O)₂} butterfly core, unlike the reported other Fe₄ or Mn₄ derived "butterfly" core. The connectivity of Ru₄(μ₃-O)₂ core in 1 with three Ru₂(μ-CH₃COO)₃ and two each Ru-(acac)₂/Ru-acac units resulted in interconnected four RuO₆ octahedral entities. The doubly bridged μ₃-O^2- ions of the nearly planar central metal-metal bonded Ru₂O₂ core (Ru2-Ru3, "body" or "hinge") linked to the remaining two "wing-tip" Ru atoms (Ru1 and Ru4). Complex 1 with a S = 1/2 spin state displayed paramagnetically shifted ¹H NMR over a wide chemical shift range in CDCl₃ (δ, 13 to -30 ppm) and a metal based anisotropic EPR (g₁ = 2.17, g₂ = 2.01, g₃ = 1.86; Δg = g₁-g₃ = 0.31 and 〈g〉 = [1/3(g₁² + g₂² + g₃²]^1/2 = 2.01) at 100 K in CH₃CN-toluene. The metal based one-electron reversible oxidation at 0.49 V and reduction at 0.485 V versus SCE of 1 led to the EPR inactive (even at 4 K) spin-coupled Ru^IIIRu^IIIRu^IVRu^IV (S = 0) and Ru^IIIRu^IIIRu^IIIRu^III (S = 0) electronic configurations for 1⁺ and 1^-, respectively. Mixed valence 1 and 1⁺ exhibited low-energy near-infrared (NIR) absorption bands at 1350 nm and 1156 nm, respectively, in CH₃CN. A combined experimental (UV-vis-NIR and EPR spectroelectrochemistry) and theoretical (DFT) analysis indicated a delocalised mixed valence form of 1 ({Ru₂(Ru-Ru)(μ₃-O)₂}).

Characterization of a Mixed-Valence Ru(II)/Ru(III) Ion-Pair Complex. Unexpected High-Frequency Electron Paramagnetic Resonance Evidence for Ru(III)-Ru(III) Dimer Coupling

In this contribution, we report the synthesis and full characterization of the first mixed-valence Ru(II)/Ru(III) ion-pair complex, [Ru^II(bipy)₂(pic)]⁺[cis-Ru^IIICl₂(pic)₂]^-, in the solid state and in aqueous solution, where bipy = 2,2'-bipyridine and pic^- = picolinate. In addition, unexpected high-frequency electron paramagnetic resonance evidence for interactions between two neighboring Ru(III) ions, resulting in a triplet state, S = 1, was found.

Ruthenium coordination preferences in imidazole-containing systems revealed by electrospray ionization mass spectrometry and molecular modeling: Possible cues for the surprising stability of the Ru (III)/tris (hydroxymethyl)-aminomethane/imidazole complexes

Ruthenium is a platinoid that exhibits a range of unique chemical properties in solution, which are exploited in a variety of applications, including luminescent probes, anticancer therapies, and artificial photosynthesis. This paper focuses on a recently demonstrated ability of this metal in its +3 oxidation state to form highly stable complexes with tris (hydroxymethyl)aminomethane (H₂ NC(CH₂ OH)₃ , Tris-base or T) and imidazole (Im) ligands, where a single Ru^III cation is coordinated by two molecules of each T and Im. High-resolution electrospray ionization mass spectrometry (ESI MS) is used to characterize Ru^III complexes formed by placing a Ru^II complex [(NH₃ )₅ Ru^II Cl]Cl in a Tris buffer under aerobic conditions. The most abundant ionic species in ESI MS represent mononuclear complexes containing an oxidized form of the metal, ie, [X_n Ru^III T₂ - 2H]⁺ , where X could be an additional T (n = 1) or NH₃ (n = 0-2). Di- and tri-metal complexes also give rise to a series of abundant ions, with the highest mass ion representing a metal complex with an empirical formula Ru₃ C₂₄ O₂₁ N₆ H₆₆ (interpreted as cyclo(T₂ RuO)₃ , a cyclic oxo-bridged structure, where the coordination sphere of each metal is completed by two T ligands). The empirical formulae of the binuclear species are consistent with the structures representing acyclic fragments of cyclo(T₂ RuO)₃ with addition of various combinations of ammonia and dioxygen as ligands. Addition of histidine in large molar excess to this solution results in complete disassembly of poly-nuclear complexes and gives rise to a variety of ionic species in the ESI mass spectrum with a general formula [Ru^III His_k T_m (NH₃ )_n - 2H]⁺ , where k = 0 to 2, m = 0 to 3, and n = 0 to 4. Ammonia adducts are present for all observed combinations of k and m, except k = m = 2, suggesting that [His₂ Ru^III T₂ - 2H]⁺ represents a complex with a fully completed coordination sphere. The observed cornucopia of Ru^III complexes formed in the presence of histidine is in stark contrast to the previously reported selective reactivity of imidazole, which interacts with the metal by preserving the RuT₂ core and giving rise to a single abundant ruthenium complex (represented by [Im₂ Ru^III T₂ - 2H]⁺ in ESI mass spectra). Surprisingly, the behavior of a hexa-histidine peptide (HHHHHH) is similar to that of a single imidazole, rather than a single histidine amino acid: The RuT₂ core is preserved, with the following ionic species observed in ESI mass spectra: [HHHHHH·(Ru^III T₂ )_m - (3m-1)H]⁺ (m = 1-3). The remarkable selectivity of the imidazole interaction with the Ru^III T₂ core is rationalized using energetic considerations at the quantum mechanical level of theory.