Quantum Mechanical Exchange Coupling in Trihydridoosmium Complexes Containing Azole Ligands

The reaction of the hexahydrido complex OsH 6 (P i Pr 3 ) 2 ( 1 ) with 2,2‘-biimidazole (H 2 biim) leads to the trihydrido complex OsH 3 (Hbiim)(P i Pr 3 ) 2 ( 2 ). Complex 2 reacts with the dimers [M(μ-OMe)(COD)] 2 (COD = 1,5-cyclooctadiene) to afford the heterobimetallic derivatives (P i Pr 3 ) 2 H 3 Os(μ-biim)M(COD) (M = Rh ( 3 ), Ir ( 4 )). The structure of 4 was determined by an X-ray investigation. Complex 4 crystallizes in the triclinic space group P 1̄ (No. 2) with a = 8.978(2) Å, b = 13.629(3) Å, c = 15.369(3) Å, α = 79.34(2)°, β = 86.31(2)°, γ = 72.43(1)°, and Z = 2. The coordination geometry around the osmium atom can be described as a distorted pentagonal bipyramid with the two phosphorus atoms of the phosphine ligands occupying trans positions. The osmium coordination sphere is completed by the hydrido ligands and by the chelating Ir(biim)(COD) group, bonded through two nitrogen atoms. The H−H separations are 1.57(5) and 1.59(7) Å. Complex 1 also reacts with pyrazole (Hpz). The reaction gives OsH 3 (pz)(Hpz)(P i Pr 3 ) 2 ( 5 ), which affords [OsH 3 (Hpz) 2 (P i Pr 3 ) 2 ]BF 4 ( 6 ) and OsH 3 Cl(Hpz)(P i Pr 3 ) 2 ( 7 ) by reaction with HBF 4 and HCl, respectively. The 1 H NMR spectra of 2 − 4 in the hydrido region at low temperature display AM 2 X 2 spin systems (X = 31 P), which are simplified to AM 2 spin systems in the 1 H{ 31 P} spectra. The values for J A - M are abnormally large (between 31.7 and 76.0 Hz) and temperature dependent. Furthermore, they are inversely proportional to the electron density on the osmium atom, increasing in the sequence 2 < 3 < 4 . These results are interpreted in terms of the operation of quantum mechanical coupling between the hydrogen nuclei in the OsH 3 unit.