Dioxygen Activation by Redox‐Active Bis(aldimine) Ligand Bridged Diruthenium Complex Possessing Singlet Ground State

Abstract

The unexplored 'actor' behavior of redox-active bis(aldimine) congener, p-phenylene-bis(picoline)aldimine (L1), towards dioxygen activation and subsequent functionalization of its backbone was demonstrated on coordination with {Ru(acac) 2 } (acac = acetylacetonate). Reaction under aerobic condition led to the one-pot generation of dinuclear complexes with unperturbed L1, imino-carboxamido (L2 À ), and bis(carboxamido) (L3 2À )-bridged isovalent {Ru II (μ-L1)Ru II }, 1/ {Ru III (μ-L3 2À )Ru III }, 3 and mixed-valent {Ru II (μ-L2 À )Ru III }, 2.Authentication of the complexes along with the redox noninnocence behavior of their bridge have been validated through structure, spectroelectrochemistry and DFT calculations. Kinetic and isotope labelling experiments together with DFT analyzed transition states justified the consideration of redox shuttling at metal/L1 interface for 3 O 2 activation despite of the closed shell configuration of 1 (S = 0) to give carboxamido derived 2/3. Synergistic effort of redox-active metal and ligand is used as a protagonist in numerous biological [1] and catalytic [2] processes. [3] Complex with ligand-centered redox event facilitates shuttling of electron at the metal-ligand interface (M n L p $ M n + 1 L pÀ 1 ) through the effective overlapping of their suitably oriented frontier molecular orbitals and that results in valence tautomerism [4,5] or dynamic resonance. [4,6] This in effect promotes diverse reactivity profile of such ligand systems to manifest their "actor" behavior. [7] In line with this, the present article highlights a unique example of ligand-centered oxygenation for the stoichiometric conversion of bis(aldimine) to bis(carboxamido) on diruthenium platform based on dynamic resonance induced dioxygen activation (Scheme 1). Although metal ion mediated functionalization of imine to carboxamido in presence of exogenous reagents such as H 2 O, H 2 O 2 , acid, Ce IV , [8] are well explored, only a few reports by Manuel and Rohde, [9a] de Bruin and co-workers [9b] (M = Ni, Ir, Rh, Scheme 1) have demonstrated oxygen atom transfer from molecular oxygen to the metal bound imine backbone, which led to an occasional scenario of carboxamido formation.In this context, the present work delineates stepwise oxygenation of the two aldimine functions of p-phenylene-[a

Metal–ligand synergy driven functionalisation of alkylene linked bis(aldimine) on a diruthenium(ii) platform. Cyclisation versus oxygenation

“…(ix) The bond distances related to Ru III -N(aldimine) in [2](ClO 4 ) 2 /[4] (ClO 4 ) 2 /[5](ClO 4 ) 2 , Ru III -N(carboxamido) in 2a/3a, Ru III -N( pyridine) and Ru III -O(acac) were in good agreement with those reported in analogous complexes. 10,14 The crystal structures of the earlier reported [{(PPh 3 ) 2 (CO) (H)Ru II } 2 (µ-L1)](ClO 4 ) 2 ([6](ClO 4 ) 2 ) 10 and the present set of complexes ([8](ClO 4 ) 2 ) established their trans and cis/trans configurations, respectively (Fig. 2, Table 2).…”

“…The bond parameters involving Ru II -N( pyridine/aldimine), Ru II -H, Ru II -PPh 3 , and Ru II -CO in the complexes agreed well with the analogous systems. 10,14 In principle, odd ((CH 2 ) n , n = 3) and even ((CH 2 ) n , n = 2, 4) numbers of the alkylene linker in the ligand frameworks should provide cis and trans isomers, respectively. 15 However,…”

“…In this context, it is to be noted that our previous study involving a phenylene (-C 6 H 4 -) linked bis(aldimine) ligand bridged Ru II (acac) 2 system demonstrated stepwise incorporation of dioxygen at the aldimine C-centres to generate a bis(carboxamido) chelate via the formation of a mono (carboxamido) intermediate, which was also validated by the 18 O 2 labeling experiment. 14 The metal coordination assisted ) inhibited the interaction with molecular oxygen due to the decrease in the nucleophilic character of the overall complex and thus disfavoured the ligand-centred radical generation via intramolecular electronic rearrangement. Though C-C coupling took place for the ethylene (syn conformation) linked bis(aldimine) derived L1 in 1, 10 its higher homologues such as propylene (L2) or butylene (L3) linked complexes failed to participate in the aforestated cyclisation process due to the involved ring strain as well as unfavourable orientation of the coupling centres.…”

Donor–acceptor bridge 2,5-bis(2-oxido-phenyl)thiazolo-[5,4-d]thiazole derived diruthenium and diosmium systems. Structural and competitive electronic events as a function of metal ion, bridge and ancillary ligand

“…The present study of this first set of metal complexes of L 2− emphasises the need for further exploration with other metal ions under diverse coordination situations with the broader perspective of looking into the chemical noninnocence of such molecular forms. 31…”

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

“…Easy accessibility of multiple oxidation states as well as facile shuttling of electron at the metal–ligand interface via IET (inner-sphere or intramolecular electron transfer) in redox-active ligand-derived transition-metal complexes extend a unique opportunity in achieving molecular electron reservoirs in the specific direction of developing advanced-grade storage materials for molecular functionalization and catalysis . The IET process may also lead to the situation of resonance or even valence tautomerism, which in turn promotes ligand-centered unusual transformations . In this regard, azoheteroarenes (Ar–NN–Ar) isovalent with nitrosoarene (ArNO 0 → ArNO •– → ArNO 2– ) and singlet dioxygen (O 2 0 → O 2 •– → O 2 2– ), in principle, are capable of taking up two electrons to its hydrazido [Ar-NN-Ar] 2– form via the intermediate radical [Ar-NN-Ar] •– state particularly on metalation …”