Ruthenium-Hydride Mediated Unsymmetrical Cleavage of Benzofuroxan to 2-Nitroanilido with Varying Coordination Mode

Abstract

The reaction of R-benzofuroxan (R = H, Me, Cl) with the metal precursor [Ru(Cl)(H)(CO)(PPh)] (A) or [Ru(Cl)(H)(CHCN)(CO)(PPh)] (B) in CHCN at 298 K resulted in the intermediate complex [Ru(Cl)(L)(CHCN)(CO)(PPh)] (L = monodentate 2-nitroanilido) (1, pink), which however underwent slow transformation to the final product [Ru(Cl)(L)(CO)(PPh)] (L = bidentate 2-nitroanilido) (2, green). On the contrary, the same reaction in refluxing CHCN directly yielded 2 without any tractable intermediate 1. Structural characterization of the intermediates 1a-1c and the corresponding final products 2a-2c (R = H, Me, Cl) authenticated their identities, revealing ruthenium-hydride mediated unsymmetrical cleavage of benzofuroxan to hydrogen bonded monodentate 2-nitroanilido (L) in the former and bidentate 2-nitroanilido (L) in the latter. The spectrophotometric monitoring of the transformations of B → 1 as well as 1 → 2 with time and temperature established the first order rate process with associatively activated pathway for both cases. Both 1 and 2 exhibited one reversible oxidation and an irreversible reduction within ±1.5 V versus saturated calomel reference electrode in CHCN with slight variation in potential based on substituents in the benzofuroxan framework (R = H, Me, Cl). Spectroscopic (electron paramagnetic resonance and UV-vis) and density functional theory calculations collectively suggested varying contribution of metal based orbitals along with the ligand in the singly occupied molecular orbital of 1 or 2, ascertaining the noninnocent potential of the in situ generated (L) or (L).

Solvent‐Mediated Functionalization of Benzofuroxan on Electron‐Rich Ruthenium Complex Platform

“…In continuation of recent revelationso ft he reactivity profile of benzofuroxan (Scheme 1), [3] herein, we have reinstated the nonspectatorb ehavior of the benzofuroxan moiety as af unction of the electronic features of the ruthenium metal precursor.A lthough ruthenium chelation mediated symmetrical and unsymmetrical cleavage of benzofuroxani nto 1,2-dinitosobenzene and 2-nitroanilido,r espectively (Scheme 1) were reported previously, [3] the generationo fs olvent-inserted (1E,2E)-N 1 ,N 2bis(2-nitrosophenyl)ethane-1,2-diimine derivatives in 2 on the electron-rich {Ru(acac) 2 }p latform, through ac ascade sequence involving 2-nitrosoanilido and reactive intermediate 1,2-dinitrosobenzene,i sn ew.F inally,t he nonspectator behavior of benzofuroxan, along with potential redoxn oninnocent features of its transformed moieties, would extend the scope of future exploration towards metal-driven substrate activation.…”

“…[8] The NÀOd istances of 1.251(3) (2a)and 1.257(8) (2b)implied the neutral state, as in 1. [5] Appreciable Ru II (dp)!p*(N1 = O1) back-bondingi nteractions imitated nicely in the shorter Ru1ÀN1(N=O) distance (1.900(2) (2a)a nd 1.910(7) (2b)), [3,9] relative to that of the Ru1À N2(imine) distance (2.017(2) , 2a/2.013(7) , 2b), as well as in al onger Ru1ÀO3 bond (trans to N1;2 .083(2) (2a)a nd 2.063(6) (2b)) with respectt ot hose of Ru1ÀO4 (2.064(2) (2a) and 2.055(6) (2b)), Ru1ÀO5 (2.014(2)( 2a)a nd 2.004(6) (2b)), Ru1ÀO2 (2.041(2) (2a)a nd 2.034(6) (2b); cis to Ru1À N1). Furthermore, ac omparison of bond parameters of the chelate rings involving L 1 and L 2 in 1a and 2a,r espectively,r evealed the shortening of C6ÀN2 (1.329(14) versus1 .430(4) ) and C1ÀN1 (1.385(14) versus 1.436(4) )b onds and lengthening of the C1ÀC6 (1.427 (14) versus 1.399(4) )b ondo f 1a,r elative to 2a,o wing to partial delocalization of the negative charge of N2 of the former in the direction of N=O.…”

“…The reaction of in situ generated short-lived A (i.e., Ru-coordinated 1,2-dinitrosobenzene [3] ), as confirmed by MS analysis( m/z 437.01;F igure S1 in the Supporting Information) of the initial reaction,w ith ethanol possibly led to the formationofr uthenium(III)-chelated 2-nitrosoanilido (1)a nd CH 3 CHO as ab yproduct of ethanol oxidation [13] (Scheme 4a). The change in metal redox in the reaction sequence, ruthenium(II) in {Ru(acac) 2 }p recursor to ruthenium(III) in 1,c ould be favoredb ye lectron-rich acac and L 1 .…”

“…[1] 1,2-Dinitrosobenzene, which is the tautomeric form of biologicallyi mportant benzofuroxan, [2] has attracted attention in coordination chemistry recently, owing to its diverseb inding modes with varying metal precursors (Scheme 1) and their redox noninnocent features. [3] Previous studies have revealed that electrophilicN 1a nd ambiphilic N3 centerso fb enzofuroxan facilitated symmetrical versusu nsymmetrical cleavage to afford differentr eactivity profiles with selectiver uthenium precursors (Scheme 1). [3] However,t he coupling reactiona tt he re-active N-centers of benzofuroxan or its tautomeric 1,2-dinitrosobenzenei ss till undetermined.…”

“…[3] Previous studies have revealed that electrophilicN 1a nd ambiphilic N3 centerso fb enzofuroxan facilitated symmetrical versusu nsymmetrical cleavage to afford differentr eactivity profiles with selectiver uthenium precursors (Scheme 1). [3] However,t he coupling reactiona tt he re-active N-centers of benzofuroxan or its tautomeric 1,2-dinitrosobenzenei ss till undetermined. In this regard, herein, we demonstrate intermolecular coupling of the N-centero fb enzofuroxanw ith the solvent( ethanol) upon integration with the {Ru(acac) 2 }m etal fragment, which incorporates the electronrich acetylacetonate( acac À )l igand.T his leads to the formation of monomeric [Ru III (acac) 2 (L 1 )] (1;L 1 = 2-nitrosoanilido)a nd dimeric [{Ru II (acac) 2 } 2 (m-L 2 )] (2;L 2 = (1E,2E)-N 1 ,N 2 -bis(2-nitrosophenyl)ethane-1,2-diimine) in varying metal redox states through a cascade sequence (Scheme 2).…”