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.

This article deals with the S-S bond scission of the model substrate 2,2'-dithiodipyridine (DTDP) in the presence of a selective set of metal precursors: Ru II (acac) 2 , [Ru II Cl 2 (PPh 3 ) 3 ], [Ru II HCl(CO)(PPh 3 ) 3 ], [Ru II (H) 2 (CO)(PPh 3 ) 3 ], [Ru II (bpy) 2 Cl 2 ], [Ru II (pap) 2 Cl 2 ], [Os II (bpy) 2 Cl 2 ], and [Os II (pap) 2 Cl 2 ] (acac, acetylacetonate; bpy, 2,2'-bipyridine; pap, 2-phenylazopyridine). This led to the eventual formation of the corresponding mononuclear complexes containing the cleaved pyridine-2-thiolate unit in 1 - 4 /[ 5 ]ClO 4 -[ 8 ]ClO 4 . The formation of the complexes was ascertained by their single-crystal X-ray structures, which also established sterically constrained four-membered chelate (average N1-M-S1 angle of 67.89°) originated from the in situ-generated pyridine-2-thiolate unit. Ruthenium(III)-derived one-electron paramagnetic complexes 1-2 ( S = 1/2, magnetic moment/B.M. = 1.82 ( 1 )/1.81( 2 )) exhibited metal-based anisotropic electron paramagnetic resonance (EPR) (Δ g : 1 / 2 = 0.64/0.93, ⟨ g ⟩: 1 / 2 = 2.173/2.189) and a broad 1 H nuclear magnetic resonance (NMR) signature due to the contact shift effect. The spectroelectrochemical and electronic structural aspects of the complexes were analyzed experimentally in combination with theoretical calculations of density functional theory (DFT and TD-DFT). The unperturbed feature of DTDP even in refluxing ethanol over a period of 10 h can be attributed to the active participation of the metal fragments in facilitating S-S bond cleavage in 1 - 4 /[ 5 ]ClO 4 -[ 8 ]ClO 4 . It also revealed the following three probable pathways toward S-S bond cleavage of DTDP as a function of metal precursors: (i) the metal-to-ligand charge-transfer (MLCT) (Ru II → σ* of DTDP)-driven metal oxidation (Ru II → Ru III ) process in the case of relatively electron-rich metal fragments {Ru II (acac) 2 } or Ru II Cl 2 in 1 or 2 , respectively; (ii) metal hydride-assisted formation of 3 or 4 with the concomitant generation of H 2 ; and (iii) S-S bond reduction with the simultaneous oxidation of the solvent benzyl alcohol to benzaldehyde.