title: Syntheses and electronic properties of rhodium(III) complexes bearing a redox-active ligand.
abstract: 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 RhCl(3)·3H(2)O with H(L) results in the precipitation of insoluble [H(L)]RhCl(3), 1. The reaction of a methanol suspension of [H(L)]RhCl(3) with NEt(4)OH causes ligand deprotonation and affords nearly quantitative yields of the soluble, deep-green, title compound (NEt(4))[(L)RhCl(3)]·H(2)O, 2·H(2)O. Complex 2·H(2)O reacts readily with excess pyridine, triethylphosphine, or pyrazine (pyz) to eliminate NEt(4)Cl and give charge-neutral complexes trans-(L)RhCl(2)(py), trans-3, trans-(L)RhCl(2)(PEt(3)), trans-4, or trans-(L)RhCl(2)(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)(2)Cl](PF(6)), 6, [(L)Rh(PPh(3))(py)Cl](PF(6)), 7, [(L)Rh(PEt(3))(2)Cl](PF(6)), 8, and [(L)RhCl(bipy)](OTf = triflate), 9, were prepared from 2·H(2)O 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 CH(2)Cl(2) 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 CH(2)Cl(2), 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)RhCl(2)(py)](SbCl(6)), cis-[(L)RhCl(2)(py)](SbCl(4))·2CH(2)Cl(2), and [(L)Rh(PEt(3))(2)Cl](SbCl(6))(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.
authors.affiliation: Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201-1881, USA.
authors.forename: Sarath
authors.initials: S
authors.lastname: Wanniarachchi
authors.forename: Brendan J
authors.initials: BJ
authors.lastname: Liddle
authors.forename: Brandon
authors.initials: B
authors.lastname: Kizer
authors.forename: Jeewantha S
authors.initials: JS
authors.lastname: Hewage
authors.forename: Sergey V
authors.initials: SV
authors.lastname: Lindeman
authors.forename: James R
authors.initials: JR
authors.lastname: Gardinier