Copper complexes of "superpodal" amine ligands and reactivity studies towards dioxygen

The results of studies focussed on copper complexes of a variety of ligands with an NN₄ donor set are reported. The permethylated tetrapodal ligand 2 forms a complex with copper(I) which, upon reaction with dioxygen at -90 °C, yields a product having a bis(μ-oxido)dicopper(III) core ("O-type" product, 10), as inferred from UV/Vis and resonance-Raman spectroscopic data. The UV/Vis spectrum of 10 has two bands at 300 and 404 nm, with extinction coefficients of 9400 and 10400 L mol^-1 cm ^-1, respectively. Resonance-Raman spectra display two ¹⁶O/¹⁸O-sensitive bands which, based on the isotopic shifts and the absolute frequencies, are attributed to the Cu-O stretching modes of the O-type product. Complex 10 shows tyrosinase-like activity, as its reaction with sodium p-tert-butylphenolate at -90 °C in THF yields p-tert-butylcatechol, in an ortho-hydroxylation reaction (yield: 30 %). Two new rigid tetrapodal pentadentate ligands (the "superpods" 3 and 4) can be synthesized by condensation of the primary polyamine 1 with paraformaldehyde. Their copper(II) complexes (5 and 6) have been spectroscopically characterized. As ascertained by X-ray crystallography, 5 has the Cu^II ion in a tetragonal-pyramidal environment, with almost uniform Cu-N bond lengths (basal bonds: 2.052 Å and 2.070 Å; apical bond: 2.077 Å). No significant Jahn-Teller distortion is observed here. In 6, the ligand acts as a multinucleating donor, which leads to the formation of a ladder-like cluster of [Cu(μ₃-OH)] units containing a total of two ligands, six copper(II) ions, four hydroxido ligands and eight trifluoroacetate ions. Two of the trifluoroacetate ions are non-coordinating. Variable-temperature magnetic susceptibility data are reported for this hexanuclear copper(II) cluster. Copper(I) complexes of 1 and 3 have been characterized and allowed to react with molecular oxygen, which caused the decomposition of the complexes. The IR spectra of the oxygenation products have bands at 1652 and 1632 cm^-1, respectively, which are absent in the spectra of 1 and 3, suggesting that amine functions have been oxidized to imines.