Azole Assisted C–H Bond Activation Promoted by an Osmium-Polyhydride: Discerning between N and NH

The capacity of the
hexahydride complex OsH6(PiPr3)2 (1) to discern between the
nitrogen atom and the NH unit in the azole assisted aryl C–H
bond activation has been investigated. Complex 1 reacts
with 2-phenylimidazole to give OsH3{κ2-C,N-(C6H4-imidazole)}­(PiPr3)2 (2), which has been characterized by X-ray diffraction analysis. The
structure proves the higher affinity of the metal center for the N
atom in the presence of the NH unit, which remains unchanged, and
reveals that in the solid state the molecules of this complex form
infinite chains by means of intermolecular asymmetric 3-center bifurcated
dihydrogen bonds. In solution, 1H-DOSY NMR experiments
suggest that the association degree decreases as the temperature increases.
The fused six-membered ring of benzimidazole weakens the NH bond,
enhancing its reactivity. As a consequence, complex 1 cannot discern between the N atom and the NH unit of 2-phenylbenzimidazole.
Thus, the treatment of 1 with this substrate leads to
a mixture of OsH3{κ2-C,N-(C6H4-benzimidazole)}­(PiPr3)2 (3) and the dinuclear
species (PiPr3)2H3Os­(C6H4-benzimidazolate)­OsH­(η2-H2)­(PiPr3)2 (4). The latter is the result of a N-assisted ortho-C–H bond activation of the phenyl group promoted by 0.5 equiv
of 1 and the N–H bond activation promoted by
the remaining 0.5 equiv of hexahydride 1 along with the
agostic coordination of the remaining ortho-C–H
bond to the metal center of the unsaturated fragment OsH­(η 2-H2)­(PiPr3)2.
The comparison of the redox properties of 3 and 4 suggests that the interaction between the metal centers
in the dinuclear compound is negligible. The replacement of the NH
group of the azoles by a sulfur atom does not modify the behavior
of the substrates. Thus, the reactions of 1 with 2-phenylthiazole
and 2-phenylbenzothiazole afford OsH3{κ 2-C,N-(C6H4-thiazole)}­(PiPr3)2 (5) and OsH3{κ2-C,N-(C 6H4-benzothiazole)}­(PiPr3)2 (6). In turn, complexes 2, 3, 5, and 6 are
phosphorescent.