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

Abstract

The hexahydrido complex OsH6(PiPr3)2 (1) reacts with tetrafluorobenzobarrelene (TFB), 2,5-norbornadiene (NBD), and 1,3-cyclohexadiene to afford OsH2(η4-TFB)(PiPr3)2 (2), OsH2(η4-NBD)(PiPr3)2 (3), and OsH2(η4-cyclohexadiene)(PiPr3)2 (4), respectively. The protonation of 2 and 3 with HBF4 yields [OsH3(η4-TFB)(PiPr3)2]BF4 (5) and [OsH3(η4-NBD)(PiPr3)2]BF4 (6). The 1H NMR spectra of 5 and 6 in the hydrido region at low temperature display AM2X2 spin systems (X = 31P), which are simplified to AM2 spin systems in the 1H{31P} spectra. The values for JAM 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 HBF4 leads to the cyclohexenyl complex [OsH2(η3-C6H9)(PiPr3)2]BF4 (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.