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 Ru-2 Complexes
Mandal, S; Samanta, S; Mondal, TK; Goswami, S
The chemical reactions of a selection of ortho-mono- and disubstituted anilines with two ruthenium polyene mediator complexes, (CpRuCl)-Cl-II(PPh3)(2) (Cp- = cyclopentadienyl anion) and (Bnz)(2)(Ru2Cl4)-Cl-II (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 (HL1a), 2-methylaniline (HL1c), and 2-methylthioaniline (HL1b), with (CpRuCl)-Cl-II(PPh3)(2) yielded mononuclear complexes [CpRu(II)L2(a-c)Cl] (1, 3, and 5), containing in situ generated ligands N-(aryl)-ortho-quinonediimine, L2a-c, along with anilido-bridged Ru-2(III) complexes (2, [CpClRuIII{mu-eta(2)-(L-1a)(-)}](2); 4, [CpClRuIII{mu-eta(2)- (L-1c)(-)}](2); and [6]Cl-2, [CpRuIII{mu-eta(2):eta(1)-(L-1b)(-)}](2)), respectively. The new ligands, L2a-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 (CpRuCl)-Cl-II(PPh3)(2) and ortho-disubstituted aniline 2,6-dichloroaniline (HL3a) in more forceful conditions. The ruthenium(III) binuclear complex [CpRuIII{mu-eta(2):eta(1)-(L-3a)(-)}(mu-eta(2):eta(1)-L-2d)(mu-eta(2)-acetate)(RuCl)-Cl-III]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 eta(2)-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)(2)(Ru2Cl4)-Cl-II (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-2(II) compound, [9]Cl-2. 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.