Synthesis, characterization, protonation studies and X-ray crystal structure of ReH5(PPh3)2(PTA) (PTA = 1,3,5-triaza-7-phosphaadamantane)

The novel rhenium pentahydride complex [ReH5(PPh3)2(PTA)] (2) was synthesized by dihydrogen replacement from the reaction of
[ReH7(PPh3)2] with PTA in refluxing THF. Variable temperature NMR studies indicate that 2 is a classic polyhydride (T1(min) = 133 ms).
This result agrees with the structure of 2, determined by X-ray crystallography at low temperature. The compound shows high conformational
rigidity which allows for the investigation of the various hydride-exchanging processes by NMR methods. Reactions of 2 with
equimolecular amounts of either HFIP or HBF4 Æ Et2O at 183 K afford [ReH5(PPh3)2{PTA(H)}]+ (3) via protonation of one of the nitrogen
atoms on the PTA ligand. When 5 equivalents of HBF4 Æ Et2O are used, additional protonation of one hydride ligand takes place to
generate the thermally unstable dication [ReH4(g2-H2)(PPh3)2{PTA(H)}]2+ (4), as confirmed by 1H NMR and T1 analysis. IR monitoring
of the reaction between 2 and CF3COOD at low temperature shows the formation of the hydrogen bonded complex
[ReH5(PPh3)2{PTA DOC(O)CF3}] (5) and of the ionic pair [ReH5(PPh3)2{PTA(D) OC(O)CF3}] (6) preceding the proton transfer
step leading to 3.