Insight into the Hydrogen Migration Processes Involved in the Formation of Metal–Borane Complexes: Importance of the Third Arm of the Scorpionate Ligand

The
reactions of [Ir­(κ3N,N,H-Tai)­(COD)] and [Ir­(κ3N,N,H-PhBai)­(COD)] (where Tai =
HB­(azaindolyl)3 and PhBai = Ph­(H)­B­(azaindolyl)2) with carbon monoxide
result in the formation of Z-type iridium–borane complexes
supported by 7-azaindole units. Analysis of the reaction mixtures
involving the former complex revealed the formation of a single species
in solution, [Ir­(η1-C8H13)­{κ3N,N,B-B­(azaindolyl)3}­(CO)2], as confirmed by NMR spectroscopy. In the
case of the PhBai complex, a
mixture of species was observed. A postulated mechanism for the formation
of the new complexes has been provided, supported by computational
studies. Computational studies have also focused on the reaction step
involving the migration of hydrogen from boron (in the borohydride
group) to the iridium center. These investigations have demonstrated
a small energy barrier for the hydrogen migration step (ΔG298 = 10.3 kcal mol–1). Additionally,
deuterium labeling of the borohydride units in Tai and PhBai confirmed the final position
of the former borohydride hydrogen atom in the resulting complexes.
The importance of the “third azaindolyl” unit within
these transformations and the difference in reactivity between the
two ligands are discussed. The selective coordination properties of
this family of metallaboratrane complexes have also been investigated
and are discussed herein.