Hydrogen exchange of rhenium(VII) heptahydridobis(triphenylphosphine) with water, aniline, methanol, and itself

The hydride ligands of ReH7(PPh3)2 undergo fast hydrogen exchange with water at room temperature while exchanging with the hydrogen atoms of aromatic solvent molecules at a slower rate. The amount of time required for isotopomer peaks to appear in the 1H NMR hydride resonance of ReH7(PPh3)2 was used to distinguish fast hydrogen exchange from slow hydrogen exchange. The room temperature 1H NMR T1 values of adventitious water resonances were used to distinguish two rhenium heptahydride compounds that participate in fast hydrogen exchange with water, ReH7(PPh3)2 and ReH7(AsPh3)2, from one compound that does not participate, ReH7(PCy3)2. Fast hydrogen exchange between ReH7(PPh3)2 and water is greatly slowed or stopped when the compound is dissolved in a solution that contains DMSO. The compound ReH6D(PPh3)2, an isotopomer of ReH7(PPh3)2 that was prepared in situ, exhibited an isotopomer shift in its 31P–{1H} NMR spectrum as well as D–P coupling when measured in the solvent anisole. In a solvent system containing deuteroaniline, nearly all of the hydride ligands were exchanged with deuterium, from aniline, in less than 2 h. Hydrogen exchange between the hydride ligands of ReH7(PPh3)2 and deuteroaniline was found to be reversible upon the addition of normal aniline. The distribution of hydrogen isotopes in the rhenium coordination spheres reflected the overall composition of the exchangeable hydrogen isotopes present in the bulk sample. In a deuteromethanol-containing solvent system, the exchange of hydride ligands between separate ReH7(PPh3)2 isotopomers was evident. The presumed hydrogen exchange intermediate, [ReH6(H2)(PPh3)2]+, may be responsible for the evident exchange of hydride ligands and may also be important to the thermal loss of dihydrogen from ReH7(PPh3)2.

Hydride ligands of ReH7(PPh3)2 exchange rapidly with water, aniline, or methanol. The hydrogen exchange in aniline occurs in a stepwise fashion and is reversible. The distribution of hydrogen isotopes into the hydride ligands reflects the ratio of exchangeable hydrogen isotopes in the bulk solution. In methanol, hydride ligands are exchanged between ReH7(PPh3)2 isotopomers faster than they exchange with methanol.Figure optionsDownload as PowerPoint slide