Quantum Hydrogen Tunneling in an Iridium Dihydride Complex

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. Here, we show that a new iridium dihydride complex IrH2 bearing a N,B-bidentate pyridine carboranyl ligand [(C₅H₄N)CB₁₁H₁₀] 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), nonlinear 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 s) 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 as well as computational studies were conducted, all of which support an Ir in the formal + III oxidation state for IrH2.