Selective Double CH Activation at a Methylene Carbon in Methylenediphenol Derivatives to Generate Carbene-Bridged Dinuclear Iridium Complexes

Double
CH activation at a bridging methylene group in methylenediphenols
to generate carbene-bridged dinuclear iridium complexes is presented.
Reaction of 2,2′-methylenediphenol derivatives CH2(C6H2RR′OH)2 (R, R′
= H, Me, Et, tBu) with [Cp*Ir­(OAc)2]·H2O (2 equiv) at 100 °C resulted in
the formation of the corresponding carbene-bridged dinuclear iridium
complexes [(Cp*Ir)2{(μ-C)­(C6H2RR′O)2-κ2-C,O:κ2-C,O′}] in moderate to good yields. The dinuclear complexes were fully
characterized by spectral and X-ray diffraction studies. Relatively
short Ir–Ir distances of ca. 2.66 Å as well as their characteristic
dark violet color suggest the coordinatively unsaturated character
of the complexes. Notably, the reactions using a triphenol and p-tBucalix­[8]­arene that have
two and eight methylene bridges in a molecule, respectively, led to
the selective formation of the corresponding carbene-bridged dinuclear
iridium complexes, indicating that the second C–H activation
step is considerably faster than the first one. The solid-state structure
of the dinuclear iridium complex of calix[8]­arene was also established
by X-ray diffraction analysis, disclosing that the dinuclear iridium
core resides in the cavity of the calix[8]­arene. The calix[8]­arene
scaffold adopts a mixed structure of pleated loop and partial cone
conformations. Considering that only single C–H activation
proceeded in a similar reaction of p-tBucalix­[4]­arene, conformational flexibility around
the bridging methylene moieties are deduced to be essential for the
double C–H activation. Although the dinuclear iridium complexes
are inert to L type ligands such as CO, PMe3, they readily
react with I2 to give the corresponding mononuclear carbene
complexes.