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RuII(α-diimine) or RuIII(α-diimine·–)? Structural, Spectroscopic, and Theoretical Evidence for the Stabilization of a Prominent Metal-to-Ligand Charge-Transfer Excited-State Configuration in the Ground State


  • Dedicated to Werner Uhl on the occasion of his 60th birthday


The new compounds [Ru(R-DAB)(acac)2] (R-DAB = 1,4-diorganyl-1,4-diazabuta-1,3-diene; R = tert-butyl, 4-methoxyphenyl, 2,6-dimethylphenyl; acac = 2,4-pentanedionate) exhibit intrachelate ring bond lengths 1.297 < d(CN) < 1.344 Å and 1.382 < d(CC) < 1.425 Å, which suggest a RuIII(R-DAB·) oxidation state formulation. This notion is confirmed by the negligible solvatochromism of the intense (ϵ ≈ 104 M–1 cm–1) charge-transfer absorption band in the visible region and by DFT calculations. Oxidation of the compounds occurs mainly at the R-DAB· radical ligand to produce UV/Vis/NIR and electron paramagnetic resonance (EPR) spectroelectrochemically detectable RuIII species, whereas the reduction proceeds less reversibly and yields predominantly (R-DAB)-ligand-based spin for the 4-methoxyphenyl derivative, measured at low temperature. The results are discussed with respect to metal-to-ligand charge-transfer (MLCT) excited states of conventional (α-diimine)ruthenium(II) complexes and in view of other (α-diimine)metal complexes with ambiguous oxidation-state assignments.