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.

Light‐Induced Rearrangement of Thioether‐Substituted Phosphanide Ligands: Scope and Limitations of a Remarkable Isomerization

“…More recently, Pramanik and co-workers have reported the thermally induced C À S cleavage of alkyl-2-(phenylazo)thioether ligands complexed to Rh III . [12] These rhodiummediated cleavage reactions generate a variety of organic side-products, consistent with a radical-based mechanism; this was attributed to PPh 3 -induced reduction of the Rh III center to Rh II , followed by elimination of an alkyl radical and re-oxidation of the metal center to Rh III (Scheme 2).…”

“…The majority of previously reported C À S cleavage reactions observed for phosphane-thioether ligands are thermally induced, although even these are relatively small in number. [8][9][10]12] However, the photolytic dealkylation of transition-metal-coordinated tertiary phosphane-thioether lig-A C H T U N G T R E N N U N G ands has been reported by Darensbourg and co-workers (see above). [11] Such transition-metal-mediated reactions differ markedly from the rearrangements described here: the light-induced demethylation of (Ph 2 PC 6 H 4 -2-SMe) 2 Ni (III) yields the bisA C H T U N G T R E N N U N G (thiolate) complex (Ph 2 PC 6 H 4 -2-S) 2 Ni; that is, elimination of the S-methyl groups is accompanied by oxidation from Ni 0 to Ni II .…”

“…Compound 5 P c crystallizes as a separated ion pair. The lithium cation is coordinated by the eight O atoms of two molecules of [12]crown-4 in the typical manner. The two aromatic rings of the isolated anion are not coplanar; the dihedral angle between the mean planes of these rings is 52.48.…”

“…www.chemeurj.org (Li À S 2.380-2.660(7) ) and lie in the typical range for Li À SA C H T U N G T R E N N U N G (thiolate) bonds. [29] For example, the Li À S distances in the two independent molecules of (2,4,6-tBu 3 C 6 H 2 S)Li-A C H T U N G T R E N N U N G (THF) 3 are 2.449(11) and 2.459 (12) . [30] The Li À P distance (2.653(7) ) falls in the usual range of distances for bonds between lithium and a tertiary phosphane center; [20] for example, the LiÀP distance in [Li{OCA C H T U N G T R E N N U N G (tBu) 2 CH 2 PPh 2 }] 2 is 2.651 (6) .…”

“…a); R 2 = Ph, R 1 = Me, (L) n = ([12]crown-4) 2 (5 P c)]. Treatment of 1 P H with either PhCH 2 Na or PhCH 2 K yields the heavier alkali metal complexes[{(Me 3 Si) 2 CH}P(C 6 H 4 -2-SMe)]MA C H T U N G T R E N N U N G (THF) n [M = Na (1 P d), K (1 P e)].With the exception of 2 P a and 2 P b, photolysis of these complexes with white light proceeds rapidly to give the thiolate species[R 2 P(R 1 )A C H T U N G T R E N N U N G (C 6 H 4 -2-S)]M(L) n [M = Li, L = THF (1 S a, 3 S a-5 S a); M = Li, L = tmeda (1 S b, 3 S b-5 S b); M = Na, L = THF (1 S d); M = K, L = THF (1 S e)]as the sole products.…”