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

The present article deals with the structurally characterized unsymmetric oxido/pyrazolato-bridged [(bpy)2OsIII(μ-oxido)­(μ-pz)­OsIII­(bpy)2]­(ClO4)3 ([1]­(ClO4)3) and symmetric dipyrazolato-bridged [(bpy)2OsII(μ-pz)2­OsII(bpy)2]­(ClO4)2 ([2]­(ClO4)2) (pz = pyrazolato, bpy = 2,2′-bipyridine) complexes with the Os···Os separations of 3.484 and 4.172 Å, respectively. The anti-ferromagnetically coupled OsIII centers [E(S = 1)-E(BS­(1,1) S = 0) = 322.504 cm–1] in 13+ and diamagnetic (S = 0) 22+ exhibit well-resolved 1H NMR resonances. [1]­(ClO4)3 shows temperature- and magnetic field-dependent paramagnetism at low magnetic field and diamagnetism at high magnetic field. 13+ and 22+ display successive metal-based oxidation processes involving the intermediate mixed-valent states and isovalent congeners: OsIVOsIV (15+)→OsIIIOsIV (14+)⇌OsIIIOsIII (13+)⇌OsIIIOsII (12+) and OsIIIOsIII (24+)→OsIIOsIII (23+)⇌OsIIOsII (22+) as well as bpy-centered reductions. The effect of π donor O2– and σ/π-donating pz– in 13+ and 22+, respectively, leads to varying oxidation state of the metal ions in the isolated complexes: OsIIIOsIII versus OsIIOsII. UV–visible–near-IR–electron paramagnetic resonance spectro-electrochemistry and density functional theory (DFT)/time-dependent DFT calculations collectively reveal overlapping of the metal- and ligand (pz, O, bpy)-based frontier orbitals in the delocalized mixed-valent states in 14+ and 12+ with comproportionation constant (Kc) value > 1 × 1014 as well as in isovalent 13+, resulting in mixed metal/ligand to metal/ligand near-IR transitions in all the three states. The mixed-valent OsIIOsIII state in 23+ exhibits high Kc value of 1 × 1022 corresponding to a strong electrochemical coupling situation. However, closeness of the bandwidth (Δν1/2, 4861 cm–1) of broad and weak intervalence charge transfer transition of 23+ at 1360 nm (ε/M–1 cm–1: 490) with the calculated Δν1/2 of 4121 cm–1 based on the Hush formula as well as spin-density distributions of Os1: 0.811/0.799, Os2: 0.045/0042, and pz: 0.162/0.173 in meso and rac diastereomeric forms, respectively, attribute its localized class II state.