Rhodium and Iridium Complexes of Lutidine-Based NHC/Amino Pincer Ligands: From Monodentate to Tetradentate Coordination Enabled by C–H Activation

A series of rhodium and iridium complexes derived from
lutidine-based
ligands (lutidine, 2,6-dimethylpyridine) with NHC and amino side-donor
functions have been prepared and characterized. Deprotonation of the
functionalized imidazolium salts, [tBuHImCH2PyCH2NR2]Br, by the bridging methoxo
ligands of the dinuclear complexes [M(μ-OMe)(cod)]2 affords [MBr(cod)(κC-tBuImCH2PyCH2NR2)] (M = Rh
and Ir) complexes from which a series of Rh(I) and Ir(I) complexes
including [M(cod)(κ3C,N,N′-tBuImCH2PyCH2NR2)]+, [Rh(CO)(κ3C,N,N′-tBuImCH2PyCH2NR2)]+, and [IrBr(CO)2(κC-tBuImCH2PyCH2NR2)] are readily accessible by halide abstraction and/or carbonylation
reactions. In contrast, direct metalation of imidazolium salts with
the dinuclear compounds [M(μ-Cl)(cod)]2 (M = Rh and
Ir) in the presence of potassium acetate and potassium iodide, a well-established
synthetic route to M(III) species, provides access to unusual di-iodido
M(III) cyclometalated compounds [MI2{κ4C,C′,N,N′-(CH2CMe2ImCH2PyCH2NR2)}] in low yield. Experimental studies
combined with DFT calculations suggest that cyclometalated M(III)
hydrido [MH(CH3CN){κ4C,C′,N,N′-(CH2CMe2ImCH2PyCH2NR2)}]+ compounds and square-planar cyclometalated
M(I) [M{κ4C,C′,N,N′-(CH2CMe2ImCH2PyCH2NR2)}] species resulting
from their deprotonation by acetate could be intermediates involved
in the formation of these compounds. Based on the observed formation
of elemental rhodium, disproportionation of square-planar cyclometalated
M(I) complexes to afford M(0) and the cationic M(II) species [M{κ4C,C′,N,N′-(CH2CMe2ImCH2PyCH2NR2)}]+ is proposed.
Reaction of the latter with iodide (I–) would regenerate
the M(I) intermediate to give an iodo radical (I•) that in
turn could dimerize to form diiodine I2. In this regard,
DFT calculations have shown that the oxidative addition of diiodine
to the cyclometalated M(I) intermediates leading to the di-iodido
M(III) cyclometalated compounds is a highly exergonic process.