Quantum Mechanical Exchange Coupling in Trihydridoosmium Complexes Containing Azole Ligands

The reaction of the hexahydrido complex
OsH6(PiPr3)2
(1) with 2,2‘-biimidazole (H2biim) leads to
the trihydrido
complex
OsH3(Hbiim)(PiPr3)2
(2). Complex 2 reacts with the dimers
[M(μ-OMe)(COD)]2 (COD = 1,5-cyclooctadiene) to afford the heterobimetallic derivatives
(PiPr3)2H3Os(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
P1̄ (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
OsH3(pz)(Hpz)(PiPr3)2
(5), which affords
[OsH3(Hpz)2(PiPr3)2]BF4
(6) and OsH3Cl(Hpz)(PiPr3)2
(7) by reaction with HBF4 and HCl, respectively.
The 1H NMR spectra of 2−4 in
the hydrido
region at low temperature display AM2X2 spin
systems (X = 31P), which are simplified to
AM2 spin systems in
the 1H{31P} 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.