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

Abstract

The novel rhenium pentahydride complex [ReH 5(PPh 3) 2(PTA)] ( 2) was synthesized by dihydrogen replacement from the reaction of [ReH 7(PPh 3) 2] with PTA in refluxing THF. Variable temperature NMR studies indicate that 2 is a classic polyhydride ( T 1(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 HBF 4 · Et 2O at 183 K afford [ReH 5(PPh 3) 2{PTA(H)}] + ( 3) via protonation of one of the nitrogen atoms on the PTA ligand. When 5 equivalents of HBF 4 · Et 2O are used, additional protonation of one hydride ligand takes place to generate the thermally unstable dication [ReH 4(η 2-H 2)(PPh 3) 2{PTA(H)}] 2+ ( 4), as confirmed by 1H NMR and T 1 analysis. IR monitoring of the reaction between 2 and CF 3COOD at low temperature shows the formation of the hydrogen bonded complex [ReH 5(PPh 3) 2{PTA⋯DOC(O)CF 3}] ( 5) and of the ionic pair [ReH 5(PPh 3) 2{PTA(D)⋯OC(O)CF 3}] ( 6) preceding the proton transfer step leading to 3.