Unsymmetric (μ-oxido)/(μ-pyrazolato) and Symmetric (μ-pyrazolato)2 Bridged Diosmium Frameworks: Electronic Structure and Magnetic Properties

Fine Tuning between Radical versus Nonradical States of Azoheteroarenes on Selective Osmium Platforms

The article highlights the cooperative impact of azoheteroarenes [abbt: 2,2'-azobis(benzothiazole), L1-L3; bmpd: (E)-1,2-bis(1-methyl-1H-pyrazole-3-yl) diazene, L4] and coligands [bpy: 2,2'-bipyridine; pap: 2-phenylazopyridine] in tuning radical (N-N•-) versus nonradical (N═N0) states of L on selective OsII-platforms in structurally/spectroscopically characterized monomeric [1]ClO4-[6]ClO4 and [1](ClO4)2-[2](ClO4)2/[7](ClO4)2-[8](ClO4)2, respectively. The preferred syn-configuration of L in the complexes prevented obtaining ligand bridged dimeric species. It revealed that {Os(bpy)2} facilitated the stabilization of both nonradical ([1](ClO4)2-[2](ClO4)2) and radical ([1]ClO4-[2]ClO4) states of L1/L2, while it delivered exclusively the radical form for L3 in [3]ClO4. In contrast, {Os(pap)2} generated radical states of L1-L3 in [4]ClO4-[6]ClO4, respectively, without any alteration of the redox state of OsII and azo (N═N0) function of the pap coligand. The neutral state of L4 was, however, ascertained in [7](ClO4)2 or [8](ClO4)2 irrespective of the nature of the metal fragment {Os(bpy)2} or {Os(pap)2}, respectively. Switching between radical and nonradical forms of L in the complexes as a function L and coligand could be addressed based on their relative FMO (frontier molecular orbital) energies. Multiple close redox steps of the complexes extended a competitive electron transfer scenario between the redox active components including metal/L/bpy/pap, leading to delicate electronic forms in each case.

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.

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}).