Quantum Hydrogen Tunneling in an Iridium Dihydride Complex
Abstract
Quantum mechanical tunneling (QMT) is the mechanism by which a particle can pass through a high potential energy barrier. Although rooted in quantum physics, QMT influences key chemical reactions in a number of ways. However, the significance and scope of QMT are not adequately considered by our current knowledge of chemical reactivity and catalysis where its contribution remains challenging to assess. Here, we show that a new iridium dihydride complex IrH2 bearing a N,B-bidentate pyridine carboranyl ligand [(C 5 H 4 N)CB 11 H 10 ] undergoes H•••H exchange coupling via QMT, as supported by variable temperature NMR studies showing large temperature-dependent exchange coupling constants J H-H (99-162 Hz), non-linear Arrhenius behavior of the exchanging hydrogens and the absence of detectable J H-D coupling in the deuterium enriched complex IrHD. These observations agree with the predicted existence of quantum exchange coupling in metal dihydrides reported by Zilm and Heinekey [J. Am. Chem. Soc. 1990, 112, 3, 920-929]. The observed high relaxation rates T 1,min (0.250-0.262 seconds) support the assignment for IrH2 as being a metal dihydride rather than a nonclassical dihydrogen complex, thus ruling out any major involvement from a classical scalar coupling to the observed large J H-H coupling constants. The reactivity of complex IrH2 against various bases, nucleophiles and electrophiles was investigated, and X-ray photoelectron spectroscopy (XPS) as well as computational studies were conducted, all of which support an Ir in the formal +III oxidation state for IrH2.
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