Metal-assisted unusual hydroxylation at the carbon atom of the triazine ring in dinuclear Ruthenium(II) and Osmium(II) complexes bridged by 2,4,6-Tris(2-pyridyl)-1,3,5-triazine: Synthesis, structural characterization, stereochemistry, and electrochemical studies

Abstract

The reaction of cis-[M(bpy)₂Cl₂] (M = Ru(II), and Os(II)) with 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz) in refluxing ethanol−water resulted in the formation of dinuclear complexes of the composition [{M(bpy)₂}₂(tptz-OH)](PF₆)₃·nH₂O (n = 1 for Ru and n = 0 for Os). In this reaction an unusual metal-induced hydroxylation at the carbon atom of the triazine ring of bridged tptz occurred. However, hydroxylation did not occur in the corresponding mononuclear complexes under similar reaction condition. A comparative study revealed that sufficient electrophilicity on the carbon atom and free movement of the attached pyridyl ring promoted the hydroxylation reaction. The hydroxylated dinuclear complexes exist in two stereoisomeric forms, a rac form (ΔΔ/ΛΛ) and a meso form (ΔΛ/ΛΔ). Both diastereoisomers have been isolated in pure form and characterized. The molecular structures of the rac form of Ru(II) complex (3-II) and meso form of the Os(II) complex (4-I) have been established by single-crystal X-ray studies. Crystal data:  complex 3-II, monoclinic, C2/c, a = 24.584(7) Å, b = 14.309(4) Å, c = 41.044(13) Å, β = 92.84(2)°, V = 14420.0(7) ų, Z = 8, R = 0.179, wR2 = 0.479; complex 4-I, triclinic, P1̄, a = 13.444(7) Å, b = 14.576(5) Å, c = 19.641(7) Å, α = 98.21(3)°, β = 101.67(4)°, γ = 105.80(4)°, V = 3546.0(3) ų, Z = 2, R = 0.093, wR2 = 0.279. The poor data quality of 3-II did not allow anisotropic refinement of non-hydrogen atoms except Ru and P. A PLUTO drawing of this compound is given only to support the molecular structure. ¹H NMR data have been used to characterize the diastereoisomers. The dinuclear complexes exhibit unusual electrochemical behavior; cathodic shifts of the metal-centered oxidations and ligand-based first reduction compared to mononuclear complexes have been observed. There is a splitting in the metal-centered oxidation potentials, indicating strong electronic communication between the metal centers. Comproportionation constants (K_com) of the mixed-valence species have been calculated; the values are in the range 6.03 × 10⁴−4.7 × 10⁶. It appears that a metal−metal interaction occurred by an electron-transfer mode across the low-lying π* orbital of the bridged tptz.