Isotopic probing of molecular oxygen activation at copper(I) sites

Copper-dioxygen (CuO₂) adducts are frequently proposed as intermediates in enzymes, yet their electronic and vibrational structures have not always been understood. [Cu(η¹-O₂)TMG ₃tren]⁺ (TMG₃-tren = 1,1,1-tris{2-[N ²-(1,1,3,3-tetramethylguanidino)]ethyl}amine) features end-on (η¹) O₂ coordination in the solid state. Described here is an investigation of the compound's solution properties by nuclear magnetic resonance spectroscopy, density functional calculations, and oxygen isotope effects. The study yields two major findings. First, [Cu(η¹-O₂)TMG₃tren]⁺ is paramagnetic due to a triplet electronic structure; this is in contrast to other copper compounds where O₂ is bound in a side-on manner. Second, the oxygen equilibrium isotope effect upon O₂ binding to copper(I) ( ¹⁸O EIE = K(¹⁶O¹⁶O)/K(¹⁶O ¹⁸O) = 1.0148 ± 0.0012) is significantly larger than those determined for iron and cobalt η¹-O₂ adducts. This result is suggested to reflect greater ionic (Cu^II-O₂ ^-I) character within the valence bond description. A revised interpretation of the physical origins of the ¹⁸O EIEs upon O ₂ binding to redox metals is also advanced along with experimental data that should be used as benchmarks for interpreting ¹⁸O kinetic isotope effects upon enzyme reactions.