RhCl3-Assisted C−H and C−S Bond Scissions: Isomeric Self-Association of Organorhodium(III) Thiolato Complex. Synthesis, Structure, and Electrochemistry

Abstract

The ligating properties of alkyl 2-(phenylazo)phenyl thioether 1 (HL(R); R = Me, CH(2)Ph) toward Rh(III) have been examined. A novel hexacoordinated orthometalated rhodium(III) thiolato complex trans-[Rh(L)Cl(PPh3)2] 5 has been synthesized from 1 and RhCl(3).3H(2)O in the presence of excess PPh(3) via in situ C(sp(2))-H and C(sp(3))-S bond scissions, which is the first example for a coordination compound of [L](2-). We were also able to isolate the intermediate organothioether rhodium(III) compound trans-[Rh(L(R))Cl(2)(PPh(3))] 6 with 1 equiv of PPh(3) relative to both 1 and RhCl(3).3H2O in the course of the synthesis of the S-dealkylated product. PPh(3) plays a crucial role in the C(sp(3))-S cleavage process. A plausible mechanistic pathway is presented for C-S bond cleavage, and reductive cleavage by single-electron transfer mechanism is likely to be operative. The electronically and coordinatively saturated thiolato complex 5, indefinitely stable in the solid state, undergoes spontaneous self-dimerization in solution via dissociation of one coordinated PPh3 molecule to afford edge-shared bioctahedral anti-[Rh(L)Cl(PPh(3))]2 7 and syn-[Rh(L)Cl(PPh(3))]2 8 isomers. All the synthesized organosulfur rhodium(III) compounds were isolated as both air- and moisture-stable solids and spectroscopically characterized in both solution and solid states. In addition, all the representative members have been authenticated by single-crystal X-ray structure analyses. Availability of the isomeric dimers provides an opportunity to recognize the presence of noncovalent intramolecular "metallochelate-metallochelate" interaction in the sterically encumbered syn isomer. Unlike other organosulfur rhodium complexes, the monomeric thiolato complex 5 exhibits a fully reversible oxidative wave at 0.82 V vs Ag/AgCl, which is supposed to be primarily centered on the thiolato sulfur atom, and such perception is consistent with the DFT study. Formation of rhodium-bound thiyl radical cation 5(*+) by electrochemical oxidation was scrutinized by EPR spectroscopy.