Synthesis and Characterization of Mixed-Phosphine Osmium Polyhydrides:  Hydrogen Delocalization in [OsH₅P₃]⁺ Systems

The hexahydrido complex OsH 6 (P i Pr 3 ) 2 ( 1 ) reacts with PHPh 2 to give molecular hydrogen and the tetrahydride OsH 4 (PHPh 2 )(P i Pr 3 ) 2 ( 2 ). However, the formation of OsH 2 (PHPh 2 ) 2 (P i Pr 3 ) 2 ( 3 ) as a consequence of the substitution of a second hydrogen molecule from 2 by PHPh 2 does not occur. The treatment of 2 with 1.0 equiv of PHPh 2 in toluene at 80 °C leads after 3 days to OsH 2 (PHPh 2 ) 3 (P i Pr 3 ) ( 4 ). The preparation of 3 requires the previous acidolysis of 2 with HBF 4, which gives [OsH 5 (PHPh 2 )(P i Pr 3 ) 2 ]BF 4 ( 5 ). In contrast to 2, the addition of PHPh 2 to 5 affords [OsH 3 (PHPh 2 ) 2 (P i Pr 3 ) 2 ]BF 4 ( 6 ) and molecular hydrogen. Deprotonation of 6 with Et 3 N yields 3 . The skeleton of the cation of 5 has been determined by X-ray diffraction. The configuration is consistent with a Y-shaped OsP 3 disposition with the osmium atom in the common vertex. Complex 5 also reacts with methanol and water to give [OsH 5 {P(OMe)Ph 2 }(P i Pr 3 ) 2 ]BF 4 ( 7 ) and [OsH 5 {P(OH)Ph 2 }(P i Pr 3 ) 2 ]BF 4 ( 8 ), respectively. The addition of Et 3 N to 7 affords OsH 4 {P(OMe)Ph 2 }(P i Pr 3 ) 2 ( 9 ). A theoretical study on the OsH 5 (PH 3 ) 3 + model complex reveals that although a static description is fully consistent with a classical pentahydride assignment, the formation of a dihydrogen is a very low energy costing process, on both thermodynamic and kinetic grounds. Thus, these polyhydride systems might be better described as possessing delocalized hydrogen atoms. A further QM/MM IMOMM study on the actual OsH 5 (PR 3 ) + systems indicates that the inclusion of bulky phosphine substituents plays a role against the stability of dihydrogen forms, because of the higher steric congestion of lower coordination number complexes arising from repulsions between bulky phosphines. Although IMOMM calculations improve significantly the agreement with experimental structures, they do not change the validity of the aforementioned statement concerning delocalization.