Comparative Study on ortho-C–H vs ortho-C–X (X = C, Cl, S) Bond Activation in ortho-C_aromatic–N Bond Fusion in Substituted Anilines Using Ruthenium(II) Mediators: Isolation and Characterization of Unusual Ru₂ Complexes

The chemical reactions of a selection of ortho-mono- and disubstituted anilines with two ruthenium polyene mediator complexes, CpRu^IICl­(PPh₃)₂ (Cp^– = cyclopentadienyl anion) and (Bnz)₂Ru^II₂Cl₄ (Bnz = benzene), have been undertaken with a primary aim to make a comparison between ortho-C–H and ortho-C–X (X = Cl, C, S) bond activation processes in ortho-C–N bond fusion reactions. The reaction of ortho-monosubstituted anilines, viz., 2-chloroaniline (HL^1a), 2-methylaniline (HL^1c), and 2-methylthioaniline (HL^1b), with CpRu^IICl­(PPh₃)₂ yielded mononuclear complexes [CpRu^IIL^2a–cCl] (1, 3, and 5), containing in situ generated ligands N-(aryl)-ortho-quinonediimine, L^2a–c, along with anilido-bridged Ru^III₂ complexes (2, [CpClRu^III{μ-η²-(L^1a)⁻}]₂; 4, [CpClRu^III{μ-η²-(L^1c)^–}]₂; and [6]­Cl₂, [CpRu^III{μ-η²:η¹-(L^1b)^–}]₂), respectively. The new ligands, L^2a–c are formed via ortho-C–H bond activation reactions, whereas ortho-C–X bonds remained unaffected. However, the ortho-C–Cl bond activation reaction is also noted in the reaction between CpRu^IICl­(PPh₃)₂ and ortho-disubstituted aniline 2,6-dichloroaniline (HL^3a) in more forceful conditions. The ruthenium­(III) binuclear complex [CpRu^III{μ-η²:η¹-(L^3a)^–}­(μ-η²:η¹-L^2d)­(μ-η²-acetate)­Ru^IIICl]­Cl, [7]­Cl, of an in situ generated N-(2,6-dichlorophenyl)-6-chloro-ortho-quinonediimine ligand, L^2d, has been isolated from the above reaction. The ligand L^2d coordinates in a η²-binding mode through an imine (NH) nitrogen atom. The coordination mode of 2,6-dichloroanilide, (L^3a)^–, in [7]Cl is unusual in that an aromatic-C–Cl group is coordinated to a Ru­(III) center, and it represents the first authentic crystallographic evidence of such a coordination mode in a transition metal complex. Similar reactions on a redox-inert mediator complex, (Bnz)₂Ru^II₂Cl₄ (Bnz = benzene), with the aforesaid aromatic amines failed to result in ortho-C–N bond fusion reactions and afforded the mononuclear anilino complexes and an anilido-bridged Ru^II₂ compound, [9]­Cl₂. The complexes have been characterized by using a host of physical methods as well as single-crystal X-ray structure determination. Their redox and spectroscopic properties have been thoroughly characterized by cyclic voltammetry and UV–vis and electron paramagnetic resonance spectroscopy. Density-functional theory calculations were employed to confirm their structural features and to support the spectral and redox properties.