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.