Quantum mechanical exchange coupling in trihydridoosmium complexes containing azole ligands

The reaction of the hexahydrido complex OsH6(PiPr(3))(2) (1) with 2,2'-biimiazole (H(2)biim) leads to the trihydrido complex OsH3(Hbiim)((PPr3)-Pr-i)(2) (2). Complex 2 reacts with the dimers [M(mu-OMe)(COD)](2) (COD = 1,5-cyclooctadiene) to afford the heterobimetallic derivatives ((PPr3)-Pr-i)(2)H3Os(mu-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(1)over bar> (No. 2) with a = 8.978(2) Angstrom, b = 13.629(3) Angstrom, c = 15.369(3) Angstrom, alpha = 79.34(2)degrees, beta = 86.31(2)degrees, gamma = 72.43(1)degrees, 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 hydride 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) Angstrom. Complex 1 also reacts with pyrazole (Hpz). The reaction gives OsH3(pz)(Hpz)((PPr3)-Pr-i)(2) (5), which affords [OsH3(Hpz)(2)((PPr3)-Pr-i)(2)]BF4 (6) and OsH3Cl(Hpz)((PPr3)-Pr-i)(2) (7) by reaction with HBF4 and HCl, respectively. The H-1 NMR spectra of 2-4 in the hydride region at low temperature display AM(2)X(2) spin systems (X = P-31), which are simplified to AM(2) spin systems in the H-1{P-31} spectra. The values for JA-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 OsH3 unit.