Synthesis and Properties of [Ru2(acac)4(bptz)]n+(n= 0, 1) and Crystal Structure of [Ru2(acac)4(bptz)]

Abstract

The neutral complex [Ru₂(acac)₄(bptz)] (I) has been prepared by the reaction of Ru(acac)₂(CH₃CN)₂ with bptz (bptz = 3,6-bis(2-pyridyl)-1,2,4,5-tetrazine) in acetone. The diruthenium(II,II) complex (I) is green and exhibits an intense metal−ligand charge-transfer band at 700 nm. Complex I is diamagnetic and has been characterized by NMR, optical spectroscopy, IR, and single-crystal X-ray diffraction. Crystal structure data for I are as follows: triclinic, P1̄, a = 11.709(2) Å, b = 13.487(3) Å, c = 15.151(3) Å, α = 65.701(14)°, β = 70.610(14)°, γ = 75.50(2)°, V = 2038.8(6) ų, Z = 2, R = 0.0610, for 4397 reflections with F_o > 4σF_o. Complex I shows reversible Ru₂(II,II)−Ru₂(II,III) and Ru₂(II,III)−Ru₂(III,III) couples at 0.17 and 0.97 V, respectively; the 800 mV separation indicates considerable stabilization of the mixed-valence species (K_com > 10¹³). The diruthenium(II,III) complex, [Ru₂(acac)₄(bptz)](PF₆) (II) is prepared quantitatively by one-electron oxidation of I with cerium(IV) ammonium nitrate in methanol followed by precipitation with NH₄PF₆. Complex II is blue and shows an intense MLCT band at 575 nm and a weak band at 1220 nm in CHCl₃, which is assigned as the intervalence CT band. The mixed valence complex is paramagnetic, and an isotropic EPR signal at g = 2.17 is observed at 77 and 4 K. The solvent independence and narrowness of the 1200 nm band show that complex II is a Robin and Day class III mixed-valence complex.