Preparation and Degradation of Rhodium and Iridium Diolefin Catalysts for the Acceptorless and Base-Free Dehydrogenation of Secondary Alcohols

Rhodium
and iridium diolefin catalysts for the acceptorless and
base-free dehydrogenation of secondary alcohols have been prepared,
and their degradation has been investigated, during the study of the
reactivity of the dimers [M­(μ-Cl)­(η4-C8H12)]2 (M = Rh (1), Ir
(2)) and [M­(μ-OH)­(η4-C8H12)]2 (M = Rh (3), Ir (4)) with 1,3-bis­(6′-methyl-2′-pyridylimino)­isoindoline
(HBMePHI). Complex 1 reacts with HBMePHI, in dichloromethane,
to afford equilibrium mixtures of 1, the mononuclear
derivative RhCl­(η4-C8H12)­{κ1-Npy-(HBMePHI)} (5), and the binuclear species [RhCl­(η4-C8H12)]2{μ-Npy,Npy-(HBMePHI)} (6). Under
the same conditions, complex 2 affords the iridium counterparts
IrCl­(η4-C8H12)­{κ1-Npy-(HBMePHI)} (7) and [IrCl­(η4-C8H12)]2{μ-Npy,Npy-(HBMePHI)} (8). In contrast to chloride,
one of the hydroxide groups of 3 and 4 promotes
the deprotonation of HBMePHI to give [M­(η4-C8H12)]2(μ-OH)­{μ-Npy,Niso-(BMePHI)} (M = Rh
(9), Ir (10)), which are efficient precatalysts
for the acceptorless and base-free dehydrogenation of secondary alcohols.
In the presence of KOtBu, the [BMePHI]− ligand undergoes three different degradations: alcoholysis
of an exocyclic isoindoline-N double bond, alcoholysis of a pyridyl-N
bond, and opening of the five-membered ring of the isoindoline core.