Redox‐Induced Oxidative C−C Bond Cleavage of 2,2′‐Pyridil in Diruthenium Complexes

Abstract

In the recent years, there has been an emerging research interest in the domain of C−C bond‐cleavage reactions. The present contribution deals with the redox‐mediated dioxygen activation and C−C bond cleavage in a diruthenium complex [(acac)2RuII(μ‐L1)RuII(acac)2], 1 (acac=acetylacetonate) incorporating 2,2′‐pyridil (L1) as the bridging ligand. The above process leads to a C−C‐cleaved monomeric product [(acac)2RuIII(pic−)], 2 (pic−=piconilate). Intriguingly, similar diastereomeric complexes [(acac)2RuII(μ‐L2)RuII(acac)2], meso (ΔΛ): 3 a and rac (ΔΔ/ΛΛ): 3 b, involving an analogous diimine bridge (L2=N1,N2‐diphenyl‐1,2‐di(pyridin‐2‐yl)ethane‐1,2‐diimine), were stable towards such oxidative transformations. Electrochemical and spectroelectrochemical studies, in combination, establish the potential non‐innocent feature of the 2,2′‐Pyridil (L1) and its derivative (L2) both in oxidation and reduction processes. Additionally, theoretical calculations have been employed to verify the redox states and their behavior. Furthermore, transition state (TS) calculations at the M06L/6‐31G*/LANL2DZ level of theory together with detailed kinetic studies outline a putative mechanism for the selective transformation of 1→2 involving the formation of an intermediate bearing peroxide linkage to complex 1.

Redox Induced Tunable Functionalization of Picolylamines on Selective Ru‐Platform

“…Interestingly, the structure of 1 also showed that β‐ketoiminate moiety allied to the picolinoyl fragment of coordinated L1′ dangling outward from the chelate ring formed by L1′, which might have extended the preferred orientation for the facile oxygenation at its C α instead of its cyclization as reported earlier (Scheme 1). [4a] Structural parameters involving {Ru(acac) 2 } fragment in both 1 and 3 are in agreement with the reported analogous systems [4a,5] …”

“…Metal based anisotropic EPR (Δ g =0.38–0.57, Figure 2 and Table 2) along with metal dominated calculated Mulliken spin density (Ru>0.75, Figure 3 and Table 3) implied {Ru III −L′ − } and {Ru III −L′} electronic form for 1 , 3 and 2 + , respectively [5,8,9] . Moreover, the higher g anisotropy along with greater extent of spin localization onto the metal center in complex 3 supported the decreasing covalency with respect to 1 and 2 + which in effect disfavored the delocalization of spin into the ligand backbone.…”

“…Further, 1 in refluxing toluene under atmospheric condition transformed to [Ru III (acac) 2 (L3)] ( 3 , orange) involving coordinated picolinate (L3) moiety along with β‐ketoimine by‐product ( m / z =99.42 in GC‐MS) as a consequence of C−N bond cleavage of L1′ in 1 (Experimental Section). It is to note that complex 3 was also recently reported to form via the redox mediated dioxygen activation and C−C bond cleavage of 2,2′‐Pyridil (BL) in a diruthenium complex [(acac) 2 Ru II (μ‐BL)Ru II (acac) 2 ] [5] …”

“…Ru(III) ( t 2g 5 ) derived 1 and 3 displayed proton resonances in the wide chemical shift ( δ ) region of −25−+15 ppm as a consequence of paramagnetic contact shift (Figures S2–S4, Supporting Information and Experimental Section) [5,7] . Though we failed to obtain suitable single crystal of Ru(II) ( t 2g 6 ) derived 2 for the structural elucidation, the identity of 2 could be ascertained by accessing its well resolved 1 H/ 13 C−NMR spectral features (Figure S3, Supporting Information).…”

Bond Activations Assisted by Redox Active Ligand Scaffolds

“…In the following year, Lahiri and co‐workers reported dioxygen activaton by a 2,2'‐pyridyl bridged diruthenium framework (13) (Figure c) . The redox accessibility at both the metal and the ligand centers allows for the formulation of the redox tautomeric form 14 which in turn activates the dioxygen leading to the formation of a mononuclear ruthenium picolinate complex by an oxidative cleavage of C–C bond.…”