Isotopic Probing of Molecular Oxygen Activation at Copper(I) Sites

Lanci, M. P.; Smirnov, V. V.; Cramer, C. J.; Gauchenova, E. V.; Sundermeyer, J.; Roth, J. P.
J. Am. Chem. Soc. 2007, 129, 14697.

Copper-oxygen (CuO₂) adducts are frequently proposed as intermediates in enzymes yet their electronic and vibrational structures have not always been understood. [Cu(η¹-O₂)TMG₃tren]⁺ (TMG₃tren = 1,1,1-tris{2-[N²-(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, most likely 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 defined as K(¹⁶O¹⁶O)/K(¹⁶O¹⁸O) = 1.0148 +/- 0.0012) is much 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 O₂ 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.

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