Synthesis and Reactivity of Heptamethylcyclohexadienyl Rhodium(III) Complexes

We report the synthesis and reactivity of the half-sandwich
rhodium­(III) complexes with the fully methylated cyclohexadienyl ligand
C6Me7, which is analogous to the classical Cp*.
The starting complex [(C6Me7)­RhCl2]2 (4) was obtained in high yield (92%) by
the reaction of [(cyclooctene)2RhCl]2 with the
readily available 6-methylenehexamethylcyclohexadiene-1,4 (C6Me6CH2) followed by addition of HCl.
Reactions of complex 4 with common two-electron ligands
L gave the expected adducts (C6Me7)­RhCl2L (L = pyridine, P­(OEt)3, PPh3) in high
yields (80–90%). At the same time, the interaction of 4 with the stronger ligand tBuNC led to the replacement
of the C6Me7 ligand. The cationic complex [(C6Me7)­Rh­(dppe)­Cl]­PF6 and the dicationic
complex [(C6Me7)­Rh­(C6Me6)]­(BF4)2 were obtained by abstraction of chlorides
from 4 with TlPF6 or AgBF4 in the
presence of the corresponding ligands. The reaction of 4 with 2-phenylpyridine in the presence of CsOAc proceeded via CH
activation and gave the cyclometalated product (C6Me7)­Rh­(C6H4-Py)Cl in 85% yield. Accordingly,
the catalytic reaction of 2-phenylpyridine with 3-hexyne in the presence
of 4 (5 mol %) gave the 9,10-diethyl-8a-azaphenanthrene
cation in 61% yield. However, the catalytic efficientcy of 4 was lower than that of the classical catalyst [Cp*RhCl2]2, possibly because the displacement of the cyclohexadienyl
ligand interrupted the catalytic cycle. The DFT calculations suggested
that the electron-donating ability of cyclic π ligands in the
rhodium complexes (CnRn)­RhCl2CO decreases in the order CnRn = C5Me5 > C6Me7 > C5H4OMe ≈ C5Me4CF3 > C5Me3(COOMe)2 ≈ C5H4Me > C5H5 > C5H4F ≈ C5H4COOMe > C5H4CF3.