Dihydrido and Trihydrido Diolefin Complexes Stabilized by the Os(PiPr3)2 Unit: New Examples of Quantum Mechanical Exchange Coupling in Trihydrido Osmium Compounds

The hexahydrido complex OsH 6 (PiPr 3 ) 2 ( 1 ) reacts with tetrafluorobenzobarrelene (TFB), 2,5-norbornadiene (NBD), and 1,3-cyclohexadiene to afford OsH 2 (η 4 -TFB)(PiPr 3 ) 2 ( 2 ), OsH 2 (η 4 -NBD)(PiPr 3 ) 2 ( 3 ), and OsH 2 (η 4 -cyclohexadiene)(PiPr 3 ) 2 ( 4 ), respectively. The protonation of 2 and 3 with HBF 4 yields [OsH 3 (η 4 -TFB)(PiPr 3 ) 2 ]BF 4 ( 5 ) and [OsH 3 (η 4 -NBD)(PiPr 3 ) 2 ]BF 4 ( 6 ). The 1 H NMR spectra of 5 and 6 in the hydrido region at low temperature display AM 2 X 2 spin systems ( X = 31 P), which are simplified to AM 2 spin systems in the 1 H{ 31 P} spectra. The values for J AM are temperature dependent, increasing from 13.1 to 35.9 Hz ( 5 ) and from 11.0 to 17.7 Hz ( 6 ) as temperature is increased from 190 to 230 K and from 180 to 240 K, respectively. The reaction of 4 with HBF 4 leads to the cyclohexenyl complex [OsH 2 (η 3 -C 6 H 9 )(PiPr 3 ) 2 ]BF 4 ( 7 ), which shows an agostic interaction between the osmium center and one of the two endo -CH bonds adjacent to the π-allyl unit. In solution complex 7 is fluxional. The fluxional process involves the exchange between the relative positions of the hydrido ligands and the endo -CH hydrogen atoms of the cyclohexenyl ligand and, at the same time, the exchange between the CH allyl and the exo -CH hydrogen atoms inside the cyclohexenyl ligand. The structure of 7 in the solid state has been determined by X-ray diffraction. The distribution of ligands around the osmium atom can be described as a piano stool geometry with the agostic hydrogen atom and the midpoints of the carbon−carbon bonds involved in the π-allyl unit forming the three-membered face, while both the hydrido and phosphine ligands lie in the four-membered face.