Synthesis, Structure, and Properties of the Stable and Highly Acidic Dihydrogen Complex <i>trans</i>-[Os(η²-H₂)(CH₃CN)(dppe)₂](BF₄)₂. Perspectives on the Influence of the <i>trans</i> Ligand on the Chemistry of the Dihydrogen Ligand

Abstract

The complex trans-[Os(H)(CH3CN)(dppe)2]BF4 (2) is prepared from the known complex trans-[Os(η2-H2)(H)(dppe)2]BF4 (1), dppe = Ph2PCH2CH2PPh2, by substitution of the η2-H2 ligand with CH3CN. 2 was identified as a potential precursor for a highly acidic and stable η2-H2 complex on the basis of its electrochemical potential E1/2(OsIII/OsII). The stable complex trans-[Os(η2-H2)(CH3CN)(dppe)2](BF4)2 (3) is formed when 2 is protonated with excess HBF4·Et2O in anhydrous CH2Cl2. The pKa of 3 is estimated to be −2 because it is partially deprotonated by Et2O. This makes it the most acidic, stable dihydrogen complex to be fully characterized. Its properties are compared to the known complex trans-[Os(η2-H2)(CH3CN)(en)2](CF3SO3)2 (5), en = H2NCH2CH2NH2, to illustrate the influence of the chelating ligand (π-acidic dppe versus σ-basic en), and to the complexes trans-[Os(η2-H2)(X)(dppe)2]PF6, X = H (1), Cl (4), and Br (6), to illustrate the influence of the trans ligand on the characteristic properties of the η2-H2 ligand. The ligand field strength of X is an important factor. The structures of 2 and 3a,b (3 crystallizes in 2 forms) were determined by X-ray diffraction. In 3a the hydrogen atoms of the η2-H2 ligand were isotropically refined, resulting in an H−H distance of 0.9(1) Å. In 3b there is residual electron density associated with the η2-H2 ligand, but the hydrogen atoms were not located. There is a close dihydrogen−fluorine contact of approximately 2.4 Å in 3a.