Syntheses and Electronic Properties of Rhodium(III) Complexes Bearing a Redox-Active Ligand

A series of rhodium­(III) complexes of the redox-active
ligand,
H­(L = bis­(4-methyl-2-(1H-pyrazol-1-yl)­phenyl)­amido),
was prepared, and the electronic properties were studied. Thus, heating
an ethanol solution of commercial RhCl3·3H2O with H­(L) results in the precipitation of insoluble
[H­(L)]­RhCl3, 1. The reaction
of a methanol suspension of [H­(L)]­RhCl3 with
NEt4OH causes ligand deprotonation and affords nearly quantitative
yields of the soluble, deep-green, title compound (NEt4)­[(L)­RhCl3]·H2O, 2·H2O. Complex 2·H2O
reacts readily with excess pyridine, triethylphosphine, or pyrazine
(pyz) to eliminate NEt4Cl and give charge-neutral complexes trans-(L)­RhCl2(py), trans-3, trans-(L)­RhCl2(PEt3), trans-4,
or trans-(L)­RhCl2(pyz), trans-5, where the incoming Lewis base is trans- to the amido nitrogen of the meridionally coordinating
ligand. Heating solutions of complexes trans-3 or trans-4 above about 100
°C causes isomerization to the appropriate cis-3 or cis-4. Isomerization
of trans-5 occurs at a much lower temperature
due to pyrazine dissociation. Cis-3 and cis-5 could be reconverted to their respective trans- isomers in solution at 35 °C by visible light
irradiation. Complexes [(L)­Rh­(py)2Cl]­(PF6), 6, [(L)­Rh­(PPh3)­(py)­Cl]­(PF6), 7, [(L)­Rh­(PEt3)2Cl]­(PF6), 8, and [(L)­RhCl­(bipy)]­(OTf
= triflate), 9, were prepared from 2·H2O by using thallium­(I) salts as halide abstraction agents
and excess Lewis base. It was not possible to prepare dicationic complexes
with three unidentate pyridyl or triethylphosphine ligands; however,
the reaction between 2, thallium­(I) triflate, and the
tridentate 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine
(ttpy) afforded a high yield of [(L)­Rh­(ttpy)]­(OTf)2, 10. The solid state structures of nine new
complexes were obtained. The electrochemistry of the various derivatives
in CH2Cl2 showed a ligand-based oxidation wave
whose potential depended mainly on the charge of the complex, and
to a lesser extent on the nature and the geometry of the other supporting
ligands. Thus, the oxidation wave for 2 with an anionic
complex was found at +0.27 V versus Ag/AgCl in CH2Cl2, while those waves for the charge-neutral complexes 3–5 were found between +0.38 to +0.59
V, where the cis- isomers were about 100 mV more
stable toward oxidation than the trans- isomers.
The oxidation waves for 6–9 with
monocationic complexes occurred in the range +0.74 to 0.81 V while
that for 10 with a dicationic complex occurred at +0.91
V. Chemical oxidation of trans-3, cis-3, and 8 afforded crystals
of the singly oxidized complexes, [trans-(L)­RhCl2(py)]­(SbCl6), cis-[(L)­RhCl2(py)]­(SbCl4)·2CH2Cl2, and [(L)­Rh­(PEt3)2Cl]­(SbCl6)2, respectively. Comparisons of structural
and spectroscopic features combined with the results of density functional
theory (DFT) calculations between nonoxidized and oxidized forms of
the complexes are indicative of the ligand-centered radicals in the
oxidized derivatives.