Hydrogen Deuterium Isotope Effect on Exchange Rates in η² Bond Transition Metal Dihydrogen Complexes Revealed by ²H Solid State NMR Spectroscopy

A selectively η2-D2 labeled isotopomer of the complex W(PCy3)2 (CO)3) (η2-D2) has been synthesized and the 2H NMR spectra and spin lattice relaxation rates of this complex have been measured in the temperature regime of 50 K to 300 K. The spectra have been analyzed employing a model of a combination of homonuclear dipolar D−D interaction and deuterium quadrupolar interaction and a D−D distance of 0.89 ± 0.1 Å. The line width of the spectra exhibits a weak temperature dependence at temperatures above 150 K. This temperature dependence is interpreted as a slight decrease of the quadrupolar coupling with increasing temperature, which is an indication of a change of the M−D2 distance with changing temperatures. The spin lattice relaxation data of the complex exhibit pronounced deviations from a simple Arrhenius behavior at lower temperatures, indicating the presence of a quantum mechanical tunneling process. This process is analyzed in terms of a simple one-dimensional Bell tunnel model. A comparison with INS data from the H2 complex reveals a strong isotope effect of 2 × 103 for the exchange rates of the deuterons.