Solvent-Mediated Functionalization of Benzofuroxan on Electron-Rich Ruthenium Complex Platform

Sequential Oxygenation of Bis(β-diketiminate) on a Selective Diruthenium Platform

This article demonstrated the redox-noninnocent phenylene-linked bis(β-diketiminate) (L2-)-bridged first example of isomeric diruthenium(III)-acac species (acac = acetylacetonate) and its ability to activate dioxygen. The coordination of deprotonated L2- to the {Ru(acac)2} in bis(bidentate) mode led to isomeric {(acac)2RuIII}2(μ-L2-) (S = 1, 1-trans/1-cis, green). 1 displayed Ru(III)-based anisotropic EPR in CH3CN but without the resolution of the forbidden (ΔMs = 2) g1/2 signal at 77 K. 1-cis, however, slowly transformed to the energetically favored 1-trans form. 1 underwent two-step oxygenation at the Cβ sites of L2- to form the β-diketiminate/α-ketodiimine (L'-)-bridged mixed valent (acac)2RuIII(μ-L'-)RuII(acac)2 (2, S = 1/2, pink) followed by bis(α-ketodiimine) (L″)-bridged isovalent (acac)2RuII(μ-L″)RuII(acac)2 (3, S = 0, red). The role of O2 toward 1 → 2/3 was corroborated by 18O2 labeling experiment. Redox steps of 1-3 varied as a function of isomeric identity, bridge, and metal oxidation state. The calculated MOs and Mulliken spin densities attributed to the noninnocence of L2-, L'-, and L″ in the respective complexes. Spectrophotometric monitoring of 1 → 2 revealed pseudo-first-order rate constants (105k s-1) of 1.8 (303 K), 3.5 (313 K), 7.7 (323 K), and 17.0 (333 K) and ΔH⧧/ΔS⧧/ΔG⧧ of 14.3 kcal mol-1/-33.1 cal mol-1 K-1/24.2 kcal mol-1 (298 K), respectively. Moreover, characterization of the short-lived blue intermediate obtained during the conversion of 1 → 2/3 upon exposure to O2 supported its valence tautomeric form (VT1, RuIII-L2--RuIII ↔ RuIII-L•--RuII, S = 1), which in effect facilitated oxygen activation at the ligand backbone.

Inner-Sphere Electron Transfer Induced Reversible Electron Reservoir Feature of Azoheteroarene Bridged Diruthenium Frameworks

This article demonstrates the stabilization of ground- and redox-induced metal-to-ligand charge transfer excited states on coordination of azo-coupled bmpd(L4) [bmpd = (E)-1,2-bis(1-methyl-1H-pyrazol-3-yl)diazene; L4 = -N═N-] to the electron-rich {Ru(acac)₂} (acac = acetylacetonate) unit in mononuclear Ru^II(acac)₂(L4) (1) and diastereomeric dinuclear (acac)₂Ru^2.5(μ-L4^•-)Ru^2.5(acac)₂ [rac, ΔΔ/ΛΛ (2a)/meso, ΔΛ (2b)] complexes, respectively. It also develops further one-step intramolecular electron transfer induced L4^•- bridged isovalent higher analogue [(acac)₂Ru^III(μ-L4^•-)Ru^III(acac)₂]ClO₄ in diastereomeric forms, rac-[2a]ClO₄/meso-[2b]ClO₄. On the contrary, under identical reaction conditions electronically and sterically permuted bimpd [L5, (E)-1,2-bis(4-iodo-1-methyl-1H-pyrazol-3-yl)diazene)] delivered mononuclear Ru^II(acac)₂(L5) (3) as an exclusive product. Further, the generation of unprecedented heterotrinuclear complex [(acac)₂Ru^II(μ-L4)Ag^I(μ-L4)Ru^II(acac)₂]ClO₄ ([4]ClO₄) involving unreduced L4 via the reaction of 1 and AgClO₄ revealed the absence of any inner-sphere electron transfer (IET) as in precursor 1, which in turn reaffirmed an IET (at the interface of electron-rich Ru(acac)₂ and acceptor L4) mediated stabilization of 2. Structural authentication of the complexes with special reference to the tunable azo distance (N═N, N-N^•-, N-N^2-) of L and their spectro-electrochemical events in accessible redox states including the reversible electron reservoir feature of 2 → 2⁺/2⁺ → 2 were evaluated in conjunction with density functional theory/time-dependent density functional theory calculations. The varying extent of IET as a function of heteroaromatics appended to the azo group of L (L1 = abpy = 2,2'-azobipyridine, L2 = abbt = 2,2'-azobis(benzothiazole), L3 = abim = azobis(1-methylbenzimidazole), L4 and L5, Schemes 1 & 2) in the Ru(acac)₂-derived respective molecular setup has been addressed.

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.

Inner-sphere electron transfer at the ruthenium-azo interface

Metal complexes exhibiting multiple reversible redox states have drawn continuing research interest due to their electron reservoir features. In this context, the present article described ruthenium-acac complexes (acac=acetylacetonate) incorporating redox-active...

Redox induced oxidative C-C coupling of non-innocent bis(heterocyclo)methanides

Redox driven C-C bond formation has gained recent attention over the traditional sequence of oxidative addition, insertion and reductive elimination reactions. In this regard, the transient radical mediated diverse reactivity profile of bis(heterocyclo)methanes (H-BHM: HL1-HL4) has been demonstrated as a function of varying metal ions and ligand backbones. It highlighted the following events: (a) redox induced homocoupling of deprotonated HL1 and HL4 on coordination to M(OAc)₂ precursors (M = Cu^II, Zn^II, Pd^II, Ag^I), including the effective role of molecular oxygen in the transformation process; (b) steric inhibition of C-C coupling of HL1 or HL4 on inserting the substituent at the bridged methylene centre (Ph in HL2 or CH₃ in HL3); (c) competitive C-C coupling versus oxygenation of free HL1 with varying concentrations of Pd^II(OAc)₂ as the ease of oxygenation over dimerisation of the deprotonated HL1 was corroborated by the DFT calculated lower activation barrier and greater thermodynamic stability of the former; and (d) redox non-innocence of BHMs on a coordinatively inert ruthenium platform, which in turn favored the involvement of a radical pathway for the aforestated coupling or oxygenation process. A combined structural, spectroscopic and DFT calculated transition state analysis demonstrated the mechanistic outline for the metal assisted oxidative coupling of BHMs.

Ruthenium-Benzothiadiazole Building Block Derived Dynamic Heterometallic Ru-Ag Coordination Polymer and Its Enhanced Water-Splitting Feature

This article deals with the development of the unprecedented redox-mediated heterometallic coordination polymer {[Ru^III(acac)₂(μ-bis-η¹-N,η¹-N-BTD)₂Ag^I(ClO₄)]ClO₄}_n (3) via the oxidation of the monomeric building block cis-[Ru^II(acac)₂(η¹-N-BTD)₂] (1) by AgClO₄ (BTD = exodentate 2,1,3-benzothiadiazole, acac = acetylacetonate). Monomeric cis-[Ru^II(acac)₂(η¹-N-BTD)₂] (1) and [Ru^II(acac)₂(η¹-N-BTD)(CH₃CN)] (2) were simultaneously obtained from the electron-deficient BTD heterocycle and the electron-rich metal precursor Ru^II(acac)₂(CH₃CN)₂ in refluxing CH₃CN. Molecular identities of 1-3 were authenticated by their single-crystal X-ray structures as well as by solution spectral features. These results also reflected the elusive trigonal-planar geometry of the Ag ion in Ru-Ag-derived polymeric 3. Ru(III) (S = 1/2)-derived 3 displayed metal-based anisotropic EPR with ⟨g⟩/Δg = 2.12/0.56 and paramagnetically shifted ¹H NMR. Spectroelectrochemistry in combination with DFT/TD-DFT calculations of 1ⁿ and 2ⁿ (n = 1+, 0, 1-) determined a metal-based (Ru^II/Ru^III) oxidation and BTD-based reduction (BTD/BTD^•-). The drastic decrease in the emission intensity and quantum yield but insignificant change in the lifetime of 3 with respect to 1 could be addressed in terms of static quenching and/or a paramagnetism-induced phenomenon. A homogeneously dispersed dumbbell-shaped morphology and the particle diameter of 3 were established by microscopic (TEM-EDX/SEM) and DLS analysis, respectively. Moreover, the dynamic nature of polymeric 3 was highlighted by its degradation to the η¹-N-BTD coordinated monomeric fragment 1, which could also be followed spectrophotometrically in polar protic EtOH. Interestingly, both monomeric 1 and polymeric 3 exhibited efficient electrocatalytic activity toward water oxidation processes (OER, HER) on immobilization on an FTO support, which also divulged the better intrinsic water oxidation activity of 3 in comparison to 1.

Ruthenium-Hydride Assisted Remarkable Diversity Towards Non-Spectator Feature of Benzodifuroxan

The present article demonstrates that ruthenium-hydride [Ru^II (H)(Cl)(CO)(PPh₃ )₃ ] mediated diverse functionalization modes of benzodifuroxan (BDF) encompassing two furoxan rings. Hydride transfer from the metal precursor facilitated multiple cascade reactions involving unsymmetrical cleavage of the furoxan rings of BDF, leading to the one-pot formation of a series of ruthenium (II) coordinated functionalized ligands exhibiting bidentate κ² -N,O, κ² -N,N' and bis-bidentate μ-bis(κ² -N,O) modes. Further, a moderately stable intermediate species was also encountered in the reaction sequence in which the transformed deoxygenated ligand coordinated to the metal ion via the rarely manifested furazan ring (κ² -N,N'' mode). The products were authenticated by their single-crystal X-ray structures and other spectroscopic/analytical techniques. Redox non-innocence of the functionalized ligands in the complexes was illustrated by spectroelectrochemistry (cyclic voltammmetry, UV-Vis. and EPR) in conjunction with DFT/TD-DFT calculations. Mechanistic outline for the facile ring opening processes of BDF including interconversions of complexes (e. g. reductive ring opening) were also addressed.

Homo- and Hetero-Oligonuclear Complexes of Platinum Group Metals (PGM) Coordinated by Imine Schiff Base Ligands

Chemistry of Schiff base (SB) ligands began in 1864 due to the discovery made by Hugo Schiff (Schiff, H., Justus Liebigs Ann. der Chemie 1864, 131 (1), 118-119). However, there is still a vivid interest in coordination compounds based on imine ligands. The aim of this paper is to review the most recent concepts on construction of homo- and hetero-oligonuclear Schiff base coordination compounds narrowed down to the less frequently considered complexes of platinum group metals (PGM). The combination of SB and PGM in oligonuclear entities has several advantages over mononuclear or polynuclear species. Such complexes usually exhibit better electroluminescent, magnetic and/or catalytic properties than mononuclear ones due to intermetallic interactions and frequently have better solubility than polymers. Various construction strategies of oligodentate imine ligands for coordination of PGM are surveyed including simple imine ligands, non-innocent 1,2-diimines, chelating imine systems with additional N/O/S atoms, classic N2O2-compartmental Schiff bases and their modifications resulting in acyclic fused ligands, macrocycles such as calixsalens, metallohelical structures, nano-sized molecular wheels and hybrid materials incorporating mesoionic species. Co-crystallization and formation of metallophilic interactions to extend the mononuclear entities up to oligonuclear coordination species are also discussed.