Metal‐to‐Ligand Charge Transfer Induced Valence Tautomeric Forms of Non‐Innocent 2,2′‐Azobis(benzothiazole) in Ruthenium Frameworks

Abstract

The impact of metal-to-ligand charge transfer towards the redox noninnocence of 2,2'-azobis(benzothiazole) (abbt) has been highlighted on coordination to {Ru II (acac) 2 } (acac = 2,4-pentanedionato). It led to the authentication of a series of mononuclear and dinuclear complexes incorporating variable oxidation states of abbt (abbt 0/ C À/2À ). Mononuclear 1 was identified as [Ru III (abbtC À )], a MLCT excited state of [Ru II (abbt)]. Dinuclear 2 was however recognized as two discrete redox isomers: (i) radical bridged mixed-valent meso-[Ru 2.5 (m-abbtC À )Ru 2.5 ] (2a) and (ii) dianionic ligand bridged isovalent meso-[Ru III (m-abbt 2À )Ru III ] (2b), demonstrating unprecedented structural confirmation of valence tautomerism in azo-based ligand systems. A crystal structure of [2]ClO 4 validated the formation of [Ru III (m-abbtC À ) Ru III ]ClO 4 . Analysis of electronic structural forms of 1 and 2 in accessible redox states via spectroelectrochemistry and DFT revealed their electron reservoir feature.

Inner-sphere electron transfer at the ruthenium-azo interface

“…Earlier reports pertaining to the distortion of the azo bonds of abpy and abbt in {Ru(acac) 2 }-derived complexes as a consequence of IET led to the ground-state stabilisation of the MLCT exited state being considered (Scheme 2). 11,12 Thus, the isolated two-step IET-induced acceptor-bridge-derived symmetrical dinuclear complex 2 (Ru III -abim 2− -Ru III ) could also be interpreted as being a MLCT excited electronic configuration of Ru II -abim 0 -Ru II .…”

“…Therefore, the observed elongation of the NN bond in the mononuclear complex 1 beyond the well-established limit of 1.30 Å (ref. 11 and 12) could be addressed in terms of a synergistic impact from ligand modification and partial ruthenium( ii )-to-abim back-bonding. On the contrary, the azo distance of abim in 2 (1.430(3) Å) was appreciably longer than that calculated for the ground state (1.391 Å), thus implying an inner-sphere two-electron-transfer (Ru to abim) induced hydrazido bridged configuration.…”

“…The DFT-calculated NN bond lengths in the range of 1.35 Å–1.39 Å for the abpy/abbt/abim (L) bridged symmetrical dinuclear {Ru(acac) 2 } complexes (Table 1) led to a consideration of the radical-anion-bridged delocalised {(acac) 2 Ru 2.5 (μ-L˙ − )Ru 2.5 (acac) 2 } or localised {(acac) 2 Ru III (μ-L˙ − )Ru II (acac) 2 } form 5 a as the electronic ground state, obtained via a one-step IET-induced process from {(acac) 2 Ru II (μ-L 0 )Ru II (acac) 2 }. Hence, the experimental azo distance of >1.40 Å seen for dinuclear [(acac) 2 Ru III (μ-L 2− )Ru III (acac) 2 ] was conceived as arising from the MLCT-driven excited-state electronic form (Table 1), 11,12 which was isolated in the ground state via a two-step IET process relating to [(acac) 2 Ru II (μ-L 0 )Ru II (acac) 2 ]. The extended conjugation due to electronically rich benzimidazole and benzothiazole rings in abim and abbt, respectively, might have facilitated the two-step IET process in the corresponding dinuclear complexes, while the electronically poor pyridine substitute abpy was restricted to a one-step IET triggered process.…”

“…11 In this context, the introduction of abbt (azobis(benzothiazole)) led to structural evidence for both one- and two-step IET induced tautomeric forms in the shape of {(acac) 2 Ru 2.5 (μ-abbt˙ − )Ru 2.5 (acac) 2 } and {(acac) 2 Ru III (μ-abbt 2− )Ru III (acac) 2 }, specifically in the absence of any external redox equivalent . 12 This initiated the present approach of employing electron-donating N -methyl-benzimidazole-derived azo functionality (abim, Fig. 1) to evaluate its effects on tuning the electronic form of the azo functionality based on the same {Ru(acac) 2 } metal platform.…”

Metal‐to‐Ligand Charge Transfer Induced Valence Tautomeric Forms of Non‐Innocent 2,2′‐Azobis(benzothiazole) in Ruthenium Frameworks

“…Hydrogen atoms are omitted for clarity;ellipsoids are set at 25 % probability. Only azo bond lengths are shown here [16].…”