Chemistry of bis(o-benzosemiquinonato)bis(triphenylphosphine)ruthenium(II) complexes. Crystal and molecular structure of [Ru(O2C6Cl4)2(PPh3)2]

Abstract

The green complexes [Ru(L^s)₂(PPh₃)₂]·2H₂O (L^s=o-benzosemiquinonate) are obtained in excellent yields by the reaction of catechols H₂L¹–H₂L⁴ with [RuH(Hsa)(PPh₃)₃]{H₂L¹= C₆H₄(OH)₂-1,2; H₂L²= 4-Bu^tC₆H₃(OH)₂-1,2; H₂L³= 3,5-Bu^t₂C₆H₂(OH)₂-1,2; H₂L⁴= C₆Cl₄(OH)₂-1,2; H₂sa = salicylic acid}. The crystals of the tetrachloro complex are triclinic, space group P with a= 9.430(5), b= 11.689(4), c= 12.551 (6)Å, α= 68.58(3), β= 72.07(4), γ= 79.92(4)° and Z= 1; the final R factor is 0.0406 for 4741 observed reflections. The complex is centrosymmetric with a trans-RuO₄P₂ co-ordination sphere; the average C–O distance is 1.310(5)Å. Significant d_π–p_π* back-bonding occurs within the Ru^II(L^s)₂ fragment and metal-to-ligand charge-transfer (m.l.c.t.) transitions are observed in the near-IR region (900–1300 nm). In dichloromethane solution four successive one-electron cyclic voltammetric couples are observed. The product of the first one-electron reduction has been identified as [Ru^III(L^c)₂]^–(L^c= catecholate) from ESR spectra. The probable nature of the other members of the redox series is discussed.