EPR Spectroscopy Investigation of Cu 2+ Ions Adsorbed in Tannin-Formaldehyde Resins of Mimosa ( Acacia mearnsii ) Bark Extracts

The chemical environment of Cu 2+ ions adsorbed in tannin-formaldehyde resins of Acacia mearnsii bark extracts, prepared from polybutene, is investigated by EPR spectroscopy at 300 K. The spectrum is simulated considering two isolated axial Cu(II) species (Species 1: A ∥ = 16.50 ( 5 ) mT , A ⊥ = 1.20 ( 10 ) mT , g ∥ = 2.28 ( 1 ) , and g ⊥ = 2.08 ( 1 ) ; Species 2: A ∥ = 15.00 ( 5 ) mT , A ⊥ = 1.50 ( 10 ) mT , g ∥ = 2.32 ( 1 ) , and g ⊥ = 2.05 ( 2 ) ), superposed to a broad line ( g iso = 2.14 ( 1 ) ; linewidth = 30.0(5) mT) assigned to Cu(II) aggregates. Measurements at 77 K did not improve spectral resolution. Heating at 413 K changes the hyperfine parameters of Species 2 ( A ∥ = 14.20 ( 5 ) mT , A ⊥ = 1.20 ( 10 ) mT , g ∥ = 2.33 ( 1 ) , and g ⊥ = 2.07 ( 2 ) ) and slightly modifies the parameters of the broad line ( g iso = 2.11 ( 2 ) ; linewidth = 40.00(50) mT) but does not change Species 1, assigned to Cu(II) species immobilized into resin pores in cathecolate-type coordination sites. Species 2 is assigned to Cu(II) species immobilized at the outer resin surface. Upon extended heating, a reversible formation of semiquinone-type paramagnetic radicals ( g iso = 2.0037 ( 1 ) ) is observed, assigned to the partial collapse of the resin polymeric network.