Tuning the Nature and Formation of Bis(dihydrogen)–Osmium Species

The influence of
chelate ligands in the formation and nature of
bis­(dihydrogen) units of OsH4 complexes has been studied.
The classical trihydride OsH3{κ2-C,N-(C6H4-py)}­(PiPr3)2 (1) reacts with HBF4·OEt2 to give the Kubas-type dihydrogen–elongated
dihydrogen derivative [Os­{κ2-C,N-(C6H4-py)}­(η2-H2)2(PiPr3)2]­BF4 (2), as a result of the protonation of one of
the hydride ligands. Triflate (OTf) displaces the Kubas-type dihydrogen
and elongates the elongated dihydrogen ligand, which is converted
into a compressed dihydride. Thus, the addition of 1 equiv of HOTf
to 1 leads to Os­(H···H)­{κ2-C,N-(C6H4-py)}­(OTf)­(PiPr3)2 (3). Similar to [OTf]−, acetone reacts with 2 to afford the related compressed dihydride [Os­(H···H)­{κ2-C,N-(C6H4-py)}­(κ1-OCMe2)­(PiPr3)2]­BF4 (4), whereas acetonitrile
leads to a 1:8 mixture of the monohydride [OsH­(CH3CN)3(PiPr3)2]­BF4 (5) and the trihydride [OsH3(CH3CN)2(PiPr3)2]­BF4 (6). Reactions of 2 with toluene and p-xylene yield the half-sandwich derivatives [Os­{κ2-C,N-(C6H4-py)}­(η6-toluene)­(PiPr3)]­BF4 (7) and [Os­{κ2-C,N-(C6H4-py)}­(η6-p-xylene)­(PiPr3)]­BF4 (8), respectively. The acyl oxygen atom of the C,C-chelate ligand of the trihydride OsH3{κ2-C,C-[C­(O)­CH2ImMe]}­(PiPr3)2 (10; Im = imidazolylidene) provides reliable and effective
protection of the hydride ligands against the protonation. Thus, the
addition of HBF4·OEt2 or HOTf to 10 leads to the trihydride-hydroxycarbene cation [OsH3{κ2-C,C-[C­(OH)­CH2ImMe]}­(PiPr3)2]+ (11). The hydroxycarbene–NHC ligand
of the latter is unstable, undergoing an intramolecular 1,3-hydrogen
shift, which produces the rupture of the chelate ligand and the formation
of the cis-hydride dihydrogen (12a)
and trans-hydride–dihydrogen (12b) derivatives [OsH­(η2-H2)­(CO)­(Me2Im)­(PiPr3)2]+ (12). The protonation of 11 with a second equivalent
of HOTf affords the transient bis­(Kubas-type dihydrogen) [Os­(η2-H2)2{κ2-C,C-[C­(OH)­CH2ImMe]}­(PiPr3)2]­(OTf)2, which undergoes the displacement
of a coordinated hydrogen molecule by a [OTf]− anion
to give the Kubas-type dihydrogen [Os­(OTf)­(η2-H2)­{κ2-C,C-[C­(OH)­CH2ImMe]}­(PiPr3)2]­OTf (13). The addition of MeOTf to 10 leads
to [OsH3{κ2-C,C-[C­(OMe)­CH2ImMe]}­(PiPr3)2]­OTf (14) which, in contrast to 11, is stable. Similar to the latter, the protonation of 14 with HOTf yields the Kubas-type dihydrogen complex [Os­(OTf)­(η2-H2)­{κ2-C,C-[C­(OMe)­CH2ImMe]}­(PiPr3)2]­OTf (15).