Linear free-energy relationships in semiquinone species and their Mn(II) and Cu(II) complexes

• Published in: Journal of physical organic chemistry 2012-02, Vol.25 (2), p.101-109
• Main Authors: Sloop, Joseph C., Shultz, David A., Marcus, Mithra Beikmohamadi, Shepler, Benjamin
• Format: Article
• Language: English
• Subjects: Hammett correlation; hydro-tris(3-cumenyl-5-methylpyrazolyl)borate; hyperfine coupling; semiquinone
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.1878

Linear free‐energy relationships for a series of functionalized semiquinone ligands and their MnII‐ and CuIIhydro‐tris(3‐cumenyl‐5‐methylpyrazolyl) borate complexes were examined. Quinone–semiquinone cycle half‐wave reduction potentials and semiquinone hydrogen hyperfine coupling constants (aH) were determined and their correlation with Hammett σ parameters reported. A new σ parameter, σaH, has been proposed. MnII and CuII metal complex metal–ligand charge transfer and n → π* UV transitions were found to be modulated by substituents. Satisfactory Hammett correlations between UV transitions and various σ values have been determined and compared in a number of instances. Copyright © 2011 John Wiley & Sons, Ltd. Substituent effects in semiquinone species and their MnII and CuII metal complexes have been examined. Reduction potentials, hyperfine coupling constants, metal–ligand charge transfer and n → π* transitions are modulated by substituents and gave satisfactory Hammett correlations. A new σ parameter based on electron paramagnetic resonance hyperfine coupling, log (aHxR/aHoR) = σaH, where aHxR = Σ{aH(substituted semiquinone)−aH(2a)} is proposed.