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

Abstract

This article demonstrates the stabilization of ground-and redoxinduced metal-to-ligand charge transfer excited states on coordination of azocoupled 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) 2 Ru 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 L4On 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) 2 Ru II (μ-L4)Ag I (μ-L4)Ru II (acac) 2 ]ClO 4 ([4]ClO 4 ) involving unreduced L4 via the reaction of 1 and AgClO 4 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.

Azo Appended Modified Lawsone Derived Ru‐Systems with Multi‐Redox Entities. Competitive Electronic Form and the Question of Azo Reduction

“…The collective consideration of MOs and ≈80 % spin localization onto the metal ion supported the similar electronic form of [Ru III (acac) 2 (L 1 − /L 2 − )] 0 for 1 / 5 ( S =1/2) (Scheme 7) and accordingly it exhibited Ru(III) based anisotropic EPR with < g >/Δ g =2.089/0.36 or 2.097/0.25, respectively, (Figure 5, Table 3). [29] Primarily ligand (L 1 − /L 2 − ) based (phenolate to phenoxide) oxidation of 1 / 5 extended dominating electronic form of [Ru III (acac) 2 (L 1 ⋅/L 2 ⋅)] + ( S =1) for 1 + / 5 + . Though 1 + was unstable at the voltammetric as well as electrolysis time scale, the electrogenerated corresponding 5 + displayed primarily metal based EPR but with the low g anisotropy (< g >/Δ g =2.011/0.15) as compared to that of 5 due to the participation of L 2 along with the metal ion in the singly occupied molecular orbitals (SOMOs) [30] .…”

Ruthenium-hydride-mediated stabilisation of the azo anion radical of azobis(benzothiazole) and its reversible electron-reservoir feature

“…12 The ν (CO), ν (NN), and ν (ClO 4 − ) vibrations appeared in the ranges of 2020–1950 cm −1 , 1460–1390 cm −1 , and 1130–1096 cm −1 , respectively (see Experimental section). 5–10,16…”

“…5 The overlapping of the energetically compatible frontier molecular orbitals (FMOs) of a metal ion and azo function indeed stimulates intramolecular metal–ligand electron transfer (IET). 6…”

“…Under the pretext of redox active azoheteroarenes, metallated abpy (abpy = 2,2′-azobis(pyridine)) has accomplished a distinct position due to its facile sequential electron-uptake feature. 5 a – e ,6,7 The recent exploration of analogous azo-based ligand, i.e. abbt (2,2′-azobis (benzothiazole)) with an electron-rich {Ru(acac) 2 } metal unit, however, has reported varying extents of intramolecular electron transfer (IET) in the monomeric and dimeric forms (Scheme 1).…”

The isomer-sensitive electrochemical HER of ruthenium(ii)–hydrido complexes involving redox-active azoheteroaromatics

“…Partial charge transfer from (dπ)Ru(II) to π*(azo) (backbonding) 20 was reflected in the increase of the azo (NvN) bond length in [1a] 1.304(12)/1.279(6) and 1.296(5)/1.298(6) Å, respectively, (Table 1), 21,22 as compared to the corresponding free ligands L1/L2 (∼1.25 Å). 17,23 The collective impact of the back-bonding…”