Radical versus Nonradical States of Azobis(benzothiazole) as a Function of Ancillary Ligands on Selective Ruthenium Platforms

Abstract

The paper deals with the electronic impact of ancillary ligands on the varying redox features of azobis(benzothiazole) (abbt) in the newly introduced mononuclear ruthenium complexes [Ru(pap) 2 (abbt)] n (1 n ) and [Ru(bpy) 2 (abbt)] n (2 n ), where pap = 2phenylazopyridine and bpy = 2,2′-bipyridine. In this regard, the complexes [Ru II (pap) 2 (abbt •− )]ClO 4 ([1]ClO 4 ), [Ru II (pap) 2 (abbt 0 )](ClO 4 ) 2 ([1](ClO 4 ) 2 ), [Ru II (bpy) 2 (abbt 0 )](ClO 4 ) 2 ([2](ClO 4 ) 2 ), and [Ru II (bpy) 2 (abbt •− )]ClO 4 ([2]ClO 4 ) were structurally and spectroscopically characterized. Unambiguous assignments of the aforestated radical and nonradical forms of abbt in 1 + /2 + and 1 2+ /2 2+ , respectively, were made primarily based on their redox-sensitive azo (NN) bond distances as well as by their characteristic electron paramagnetic resonance (EPR)/NMR signatures. Although the radical form of abbt •− was isolated as an exclusive product in the case of strongly π-acidic pap-derived 1 + , the corresponding moderately π-acidic bpy ancillary ligand primarily delivered an oxidized form of abbt 0 in 2 2+ , along with the radical form in 2 + as a minor (<10%) component. The oxidized abbt 0 -derived [1](ClO 4 ) 2 was, however, obtained via the chemical oxidation of [1]ClO 4 . Both 1 + and 2 2+ displayed multiple closed by reversible redox processes (one oxidation O1 and four successive reductions R1−R4) within the potential window of ±2.0 V versus saturated calomel electrode. The involvement of metal-, ligand-, or metal/ligand-based frontier molecular orbitals along the redox chain was assigned based on the combined experimental (structure, EPR, and spectroelectrochemisry) and theoretical [density functional theory (DFT): molecular orbitals, Mulliken spin densities/time-dependent DFT] investigations. It revealed primarily ligand (abbt/pap or bpy)-based redox activities, keeping the metal ion as a simple spectator. Moreover, frontier molecular orbital analysis corroborated the initial isolation of the radical and nonradical species for the pap-derived 1 + and bpy-derived 2 2+ as well as facile reduction of pap and abbt in 1 + and 2 + , respectively.

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

“…Crystal structures of the newly synthesized free ligand (L5) and complexes in Scheme (Figure , Table , Tables S1–S8, Supporting Information) disclosed the following points: (i) The azo (NN) distance of planar L5 of 1.252(11) Å (molecule A)/1.251(11) Å (molecule B) resembled that of the reported neutral state of L1 (1.246(2) Å), L2 (1.249(7) Å), L3 (1.271(3) Å), and L4 (1.262(4) Å). ,, (ii) The cis -configured neutral L4 or L5 ligand linked with the Ru­(acac) 2 metal unit in mononuclear (acac) 2 Ru­(L4 or L5) (NN: 1.307 Å ( 1 ) or 1.327 Å ( 3 )) through its pyrazolic (N5) and azo nitrogen (N3) donors, forming a five-membered chelate in each case. (iii) Twisting between the coordinated and non-coordinated N -methyl pyrazole rings significantly enhanced on moving from 1 (41.07°) to 3 (53.76°) (Figure S11) due to the steric hindrance imparted by the iodo substituent at the C α position to the azo group of L5.…”

“…(iv) The trans form of L4 symmetrically bridged between the two {Ru­(acac) 2 } fragments through its pyrazolic (N2, N5) and azo (N3, N4) nitrogen donor sets in diastereomeric 2a / 2b and [ 2a ]­ClO 4 /[ 2b ]­ClO 4 . (v) Unlike mononuclear 1 or 3 , radical state of the NN (1.359–1.374 Å) , function of L4 stabilized in the dimeric complexes, leading to the electronic forms of (acac) 2 Ru III (μ-L4 •– )­Ru II (acac) 2 and [(acac) 2 Ru III (μ-L4 •– )­Ru III (acac) 2 ]­ClO 4 for diastereomeric 2 and [ 2 ]­ClO 4 , respectively. (vi) Almost identical Ru–N­(azo) distances (2.007/2.009 Å) in 2a implied a delocalized mixed-valent (acac) 2 Ru II+0.5 (μ-L4 •– )­Ru II+0.5 (acac) 2 configuration in the solid state .…”

“…Earlier explorations relating to 2,2′-azobipyridine (abpy), 2,2′-azobis­(benzothiazole) (abbt), and azobis­(1-methylbenzimidazole) (abim)-derived analogous ruthenium-acac (acac = acetylacetonate) frameworks ascertained diverse electronic formulations including metal-to-ligand charge-transfer (MLCT) driven radical state/radical bridged mixed-valent form, valence tautomerism, and hydrazido-bridged isovalent state, respectively, as a function of the varying extent of IET (Scheme ) due to the closeness in energy of their frontier molecular orbitals (FMOs). , This indeed has initiated the present program of introducing unexplored azobispyrazole (bmpd, L4) comprising of an azo function linked to the peripheral electron-donating N -methyl pyrazole heterocycle and to explore its impact in terms of IET on the same Ru­(acac) 2 platform. In this context, the reaction of {Ru­(acac) 2 } and L4 remarkably led to the one-pot generation of five molecular units: (i) neutral L4 [-NN-] derived mononuclear Ru II -complex 1 , (ii) MLCT-induced one-step IET product, that is, radical [-NN-] •– bridged Ru II Ru III mixed-valent 2a ( rac = ΔΔ/ΛΛ) /2b ( meso = ΔΛ) in diastereomeric forms, and (iii) radical [-NN-] •– bridged isovalent higher (Ru III /Ru III ) congeners ([ 2a ]­ClO 4 ( rac ) / [ 2b ]­ClO 4 ( meso )) (Scheme ).…”

On the Question of S–S Bond Cleavage of 2,2′-Dithiodipyridine on Selective Ru and Os Platforms. MLCT or Hydride or Solvent Mediated Event

“…Although S–S bond cleavage of the relatively electron-rich 10 π-electron-derived 2,2′- dithiobisbenzothiazole (DTBT) on the {Ru II (acac) 2 } platform facilitated the formation of a doubly μ-RS – (R = benzothiazole) bridged diruthenium­(III) species (acac) 2 Ru III (μ-RS – ) 2 Ru III (acac) 2 ( H ), the 6 π-electron-derived analogous DTDP altogether failed to generate the corresponding dimeric complex.…”

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

“…The one-electron paramagnetic ( S = 1/2) isolated mixed valent Ru II Ru III complexes [ 1 ]ClO 4 –[ 4 ]ClO 4 exhibited μ eff = 1.78–1.83 B.M., as determined by Evans method 17 in CDCl 3 at 298 K (see the ESI† and the Experimental section) and metal-based anisotropic electron paramagnetic resonance (EPR) spectra ( vide infra ).…”