mer, fac, and Bidentate Coordination of an Alkyl-POP Ligand in the Chemistry of Nonclassical Osmium Hydrides

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Nonclassical and classical osmium polyhydrides containing the diphosphine 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(P i Pr 2 ) 2 ), coordinated in κ 3 - mer, κ 3 - fac, and κ 2 - P, P fashions, have been isolated during the cyclic formation of H 2 by means of the sequential addition of H + and H – or H – and H + to the classical trihydride OsH 3 Cl{xant(P i Pr 2 ) 2 } ( 1 ). This complex adds H + to form the compressed dihydride dihydrogen complex [OsCl(H···H)(η 2 -H 2 ){xant(P i Pr 2 ) 2 }] + ( 2 ). Under argon, cation 2 loses H 2 and the resulting unsaturated fragment dimerizes to give [(Os(H···H){xant(P i Pr 2 ) 2 }) 2 (μ-Cl) 2 ] 2+ ( 3 ). During the transformation the phosphine changes its coordination mode from mer to fac . The benzofuran counterpart of 1, OsH 3 Cl{dbf(P i Pr 2 ) 2 } ( 4; dbf(P i Pr 2 ) 2 = 4,6-bis(diisopropylphosphino)dibenzofuran), also adds H + to afford the benzofuran counterpart of 2, [OsCl(H···H)(η 2 -H 2 ){xant(P i Pr 2 ) 2 }] + ( 5 ), which in contrast to the latter is stable and does not dimerize. Acetonitrile breaks the chloride bridge of 3 to form the dihydrogen [OsCl(η 2 -H 2 )(CH 3 CN){xant(P i Pr 2 ) 2 }] + ( 6 ), regenerating the mer coordination of the diphosphine. The hydride ion also breaks the chloride bridge of 3 . The addition of KH to 3 leads to 1, closing a cycle for the formation of H 2 . Complex 1 reacts with a second hydride ion to give OsH 4 {xant(P i Pr 2 ) 2 } ( 7 ) as consequence of the displacement of the chloride. Similarly to the latter, the oxygen atom of the mer -coordinated diphosphine of 7 has a tendency to be displaced by the hydride ion. Thus, the addition of KH to 7 yields [OsH 5 {xant(P i Pr 2 ) 2 }] − ( 8 ), containing a κ 2 - P, P -diphosphine. Complex 8 is easily protonated to afford OsH 6 {xant(P i Pr 2 ) 2 } ( 9 ), which releases H 2 to regenerate 7, closing a second cycle for the formation of molecular hydrogen.