Observation and Analysis of Large Dynamic Frequency Shifts in the 1H NMR Signals of H–D in Deuterium-Substituted Dihydrogen Complexes

A dynamic frequency shift (DFS) in the 1H NMR resonance of the HD unit of the deuterium-labeled dihydrogen complex [Ru­(D)­(η2-HD)­(P3P3iPr)]­[BPh4] [P3P3iPr = P­(CH2CH2CH2PiPr2)3] has been observed and analyzed. To the best of our knowledge, this is the first demonstration of the DFS for a H–D pair. The observed DFS of the center line relative to the outside lines in the H–D triplet is large, up to ∼11 Hz, because of the short H–D distance encountered in dihydrogen complexes. Analysis of the DFS as a function of the temperature, combined with density-functional-theory-calculated or least-squares-fitted electric-field-gradient (EFG) parameters, suggests an H–D bond length of 0.92–0.94 Å. A DFS was also observed in trans-[Fe­(η2-HD)­(H)­(dppe)2]+, suggesting the DFS will be commonplace in dihydrogen complexes if appropriate conditions are employed for its observation. Possible applications of the DFS as a probe of the bond lengths, EFGs, and molecular motion, particularly in inorganic systems, are discussed.