RhCl3-assisted C-H and C-S bond scissions: Isomeric self-association of organorhodium(III) thiolato complex. Synthesis, structure, and electrochemistry

Article abstract of DOI:10.1021/ic7016006

The ligating properties of alkyl 2-(phenylazo)phenyl thioether 1 (HL ^R; R = Me, CH₂Ph) 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 RhCl₃·3H₂O in the presence of excess PPh ₃ via in situ C(sp²)-H and C(sp³)-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)Cl₂(PPh₃)] 6 with 1 equiv of PPh₃ relative to both 1 and RhCl₃· 3H₂O in the course of the synthesis of the S-dealkylated product. PPh₃ plays a crucial role in the C(sp³)-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(PPh₃)]₂ 7 and syn-[Rh(L)Cl(PPh₃)]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 monomelic 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.

Full text of DOI:10.1021/ic7016006

Inorg. Chem. 2008, 47, 429−438
RhCl3-Assisted C−H and C−S Bond Scissions: Isomeric
Self-Association of Organorhodium(III) Thiolato Complex. Synthesis,
Structure, and Electrochemistry
Kausikisankar Pramanik,*,† Ujjwal Das,† Basab Adhikari,† Deepak Chopra,‡ and Helen Stoeckli-Evans§
Department of Chemistry, Inorganic Chemistry Section, JadaVpur UniVersity,
Kolkata 700032, India, Solid State and Structural Chemistry Unit, Indian Institute of Science,
Bangalore 560012, India, and Institut de Chemie, UniVersite´ de Neuchaˆtel,
AV. de BelleVaux 51, 2000, Neuchaˆtel, Switzerland
Received August 11, 2007
The ligating properties of alkyl 2-(phenylazo)phenyl thioether 1 (HLR; R
examined. A novel hexacoordinated orthometalated rhodium(III) thiolato complex trans-[Rh(L)Cl(PPh3)2] 5 has been
synthesized from 1 and RhCl3 S bond
3H2O in the presence of excess PPh3 via in situ C(sp2) H and C(sp3)
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(LR)Cl2(PPh3)] 6 with 1 equiv of PPh3 relative to both 1 and RhCl3
3H2O in the course of the synthesis of the S-dealkylated product. PPh3 plays a crucial role in the C(sp3)
S cleavage
process. A plausible mechanistic pathway is presented for C S bond cleavage, and reductive cleavage by single-
) Me, CH2Ph) toward Rh(III) have been
‚
−
−
‚
−
−
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(PPh3)]2 7 and syn-[Rh(L)Cl(PPh3)]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.
Introduction
respectively. In pursuing our search for alkyl 2-(phenylazo)-
phenyl thioether 1, a typical ligand of this kind, we found
that no metal-assisted S-dealkylation occurred with noble
metal halides and only chelation modes of types 2 and 3
(Scheme 1) are cited in the literature.5 Great attention
has been paid to metal-assisted C-S bond activation from
the standpoint of synthetic, catalytic, bioinorganic, and
theoretical chemistry.1-4 Interestingly, such S-dealkylation
strategy (chelation mode 4) has indeed materialized success-
fully with RhCl3‚3H2O in the presence of excess PPh3,
affording organorhodium(III) thiolato complexes. Recently,
rhodium thiolato and organorhodium compounds have been
found to exhibit various bioactivities, including anticancer
activity.8
Platinum group metals display efficacy in various organic
transformations via activation of C-H and C-S bonds; the
latter process is less common.1-7 In this context, suitable
multidentate ligands incorporating both phenyl and thioether
functions are good candidates for providing organothiolates
of noble metals via C-H and C-S bond cleavages,
* To whom correspondence should be addressed. E-mail: kpramanik@
hotmail.com.
† Jadavpur University.
‡ Indian Institute of Science.
§ Universite´ de Neuchaˆtel.
(1) Transition Metal Sulfur Chemistry; Stiefel, E. I., Matsumoto, K., Eds.;
ACS Symposium Series 653; American Chemical Society: Washing-
ton, DC, 1996.
10.1021/ic7016006 CCC: $40.75
Published on Web 12/28/2007
© 2008 American Chemical Society
Inorganic Chemistry, Vol. 47, No. 2, 2008 429

Pramanik et al.
Scheme 1. Alkyl 2-(Phenylazo)phenyl Thioether and Its Different
Chelation Modes with Noble Metals
it has been recognized that the high nucleophilicity of
coordinated thiolato function may furnish S-bridged poly-
nuclear structures with other metal centers.9 It is now of
interest to stabilize and isolate the intermediate mononuclear
rhodium thiolato compounds as well as to explore their
chemistry during di- and polymerization processes. To the
best of our knowledge, only a few instances are reported
where mononuclear rhodium thiolato precursors are isolated,
which ultimately afford S-bridged self-assembled products
in due course of the reaction.10,11 It is worth noting that the
aforesaid precursors contain η5-Cp ligation, and these
examples of monomers can be rationalized on the basis of
the coordinated Cp ligand.12 It is now of significance to
isolate and study monomeric intermediates without Cp
ligand. Here we demonstrate the convenient synthesis and
characterization of electronically and coordinatively saturated
cyclometalated rhodium(III) thiolato complex 5 and its
spontaneous self-association in solution, which has led to
the isolation of an isomeric pair of dimers anti-7 and syn-8.
Significantly, both stacking and CH/π interactions of aryl
rings with chelate rings were observed.13 In this perspective,
an account of intramolecular stacking interaction between
two chelate rings in dinuclear complexes (not joined by direct
metal-metal bond) reveals another structural evidence of
the noncovalent interaction. In this work, the availability of
the isomeric dimers 7 and 8 provides an opportunity to
scrutinize the presence of noncovalent intramolecular met-
allochelate-metallochelate interaction.
Moreover, interest in the polymetallic complexes arises
from the possible “cooperative effects”.1 The self-assembly
process appears as rational means to construct metallo
supramolecular architectures. In rhodium thiolato chemistry,
(2) (a) Santra, B. K.; Lahiri, G. K. J. Chem. Soc., Dalton Trans. 1998,
1613-1618. (b) Giner-Planas, J.; Marumo, T.; Ichikawa, Y.; Hirano,
M.; Komiya, S. J. Chem. Soc., Dalton Trans. 2000, 2613-2625. (c)
Komiya, S.; Hirano, M. J. Chem. Soc., Dalton Trans. 2003, 1439-
1453. (d) Furuya, M.; Tsutsuminai, S.; Nagasawa, H.; Komine, N.;
Hirano, M.; Komiya, S. Chem. Commun. 2003, 2046-2047. (e)
Chehata, A.; Oviedo, A.; Are´valo, A.; Berne`s, S.; Garc´ıa, J. J.
Organometallics 2003, 22, 1585-1587. (f) Rauchfuss, T. B. Inorg.
Chem. 2004, 43, 14-26. (g) Shimizu, D.; Takeda, N.; Tokitoh, N.
Chem. Commun. 2006, 177-179. (h) Goj, L. A.; Lail, M.; Pittard, K.
A.; Riley, K. C.; Gunnoe, T. B.; Petersen, J. L. Chem. Commun. 2006,
982-984. (i) Siedle, G.; Kersting, B. J. Chem. Soc., Dalton Trans.
2006, 2114-2126. (j) Helmstedt, U.; Lo¨nnecke, P.; Hey-Hawkins, E.
Inorg. Chem. 2006, 45, 10300-10308. (k) Hassan, M. R.; Kabir, S.
E.; Nicholson, B. K.; Nordlander, E.; Uddin, M. N. Organometallics
2007, 26, 4627-4233.
(3) (a) Osakada, K.; Matsumoto, K.; Yamamoto, T.; Yamamoto, A.
Organometallics 1985, 4, 857-862. (b) Bianchini, C.; Meli, A. Acc.
Chem. Res. 1998, 31, 109-116. (c) Angelici, R. J. Organometallics
2001, 20, 1259-1275. (d) Giner-Planas, J.; Prim, D.; Rose-Munch,
F.; Rose, E.; Thouvenot, R.; Vaissermann, J. Organometallics 2002,
21, 4385-4389. (e) Bianchini, C.; Meli, A.; Vizza, F. J. Organomet.
Chem. 2004, 689, 4277-4290. (f) Jones, W. D. Inorg. Chem. 2005,
44, 4475-4484.
(4) (a) Mullen, G. E. D.; Fa¨ssler, T. F.; Went, M. J.; Howland, K.; Stein,
B.; Blower, P. J. J. Chem. Soc., Dalton Trans. 1999, 3759-3766. (b)
Maresca, O.; Maseras, F.; Lledo´s, A. New J. Chem. 2004, 28, 625-
630. (c) Magistrato, A.; Maurer, P.; Fa¨ssler, T.; Rothlisberger, U. J.
Phys. Chem. A 2004, 108, 2008-2013.
(5) (a) Mahapatra, A. K.; Dutta, S.; Goswami, S.; Mukherjee, M.;
Mukherjee, A. K.; Chakravorty, A. Inorg. Chem. 1986, 25, 1715-
1721. (b) Mahapatra, A. K.; Bandyopadhyay, D.; Bandyopadhyay, P.;
Chakravorty, A. Inorg. Chem. 1986, 25, 2214-2221. (c) Chatto-
padhyay, S.; Sinha, C.; Basu, P.; Chakravorty, A. Organometallics
1991, 10, 1135-1139.
(6) (a) Gerisch, M.; Krumper, J. R.; Bergman, R. G.; Tilley, T. D.
Organometallics 2003, 22, 47-48. (b) Rybtchinski, B.; Cohen, R.;
Ben-David, Y. J.; Martin, M. L.; Milstein, D. J. Am. Chem. Soc. 2003,
125, 11041-11050. (c) Krumper, J. R.; Gerisch, M.; Magistrato, A.;
Rothlisberger, U.; Bergman, R. G.; Tiley, T. D. J. Am. Chem. Soc.
2004, 126, 12492-12502. (d) Panda, M.; Das, C.; Lee, G.-H.; Peng,
S.-M.; Goswami, S. J. Chem. Soc., Dalton Trans. 2004, 2655-2661.
(e) Frech, C. M.; Ben-David, Y.; Weiner, L.; Milstein, D. J. Am. Chem.
Soc. 2006, 128, 7128-7129. (f) Ben-Ari, E.; Cohen, R.; Gandelman,
M.; Shimon, L. J. W.; Martin, J. M. L.; Milstein, D. Organometallics
2006, 25, 3190-3210.
(7) (a) Hoare, R. J.; Mills, O. S. J. Chem. Soc., Dalton Trans. 1972, 2141-
2145. (b) Bruce, M. I.; Humphrey, P. A.; Skelton, B. W.; White, A.
H. J. Organomet. Chem. 1989, 361, 369-390. (c) Aulwurm, U. R.;
Knoch, F.; Kisch, H. Z. Kristallogr. New Cryst. Struct. 1997, 212,
179-180. (d) Huang, L.-Y.; Aulwurm, U. R.; Heinemann, F. W.;
Knoch, F.; Kisch, H. Chem.-Eur. J. 1998, 4, 1641-1646. (e) Dutta,
S.; Peng, S.-M.; Bhattacharya, S. J. Chem. Soc., Dalton Trans. 2000,
4623-4627. (f) Pratihar, J. L.; Maiti, N.; Chattopadhyay, S. Inorg.
Chem. 2005, 44, 6111-6114. (g) Pratihar, J. L.; Patra, D.; Chatto-
padhyay, S. J. Organomet. Chem. 2005, 690, 4816-4821.
In addition, the high nucleophilicity of the nonbridged S
atom of 5 vis-a`-vis high electron density on the thiolato group
has been reflected in the electrochemical oxidation of
rhodium sulfur compounds.14 Redox chemistry of the afore-
(9) (a) Yamanari, K.; Fukuda, I.; Yamamoto, S.; Kushi, Y.; Fuyuhiro,
A.; Kubota, N.; Kukuo, T.; Arakawa, R. J. Chem. Soc., Dalton Trans.
2000, 2131-2136. (b) Hirotsu, M.; Kobayashi, A.; Yoshimura, T.;
Konno, T. J. Chem. Soc., Dalton Trans. 2002, 878-884. (c) Casado,
M. A.; Pe´rez-Torrente, J. J.; Ciriano, M. A.; Lahoz, F. J.; Oro, L. A.
Inorg. Chem. 2004, 43, 1558-1567. (d) Chikamoto, Y.; Kawamoto,
T.; Igashira-Kamiyama, A.; Konno, T. Inorg. Chem. 2005, 44, 1601-
1610. (e) Hirata, K.; Suzuki, T.; Noya, A.; Takei I.; Hidai, M. Chem.
Commun. 2005, 3718-3720.
(10) Eighteen-electron monomeric precursors coordinated with labile water
molecule: (a) reference 9a. (b) Yamanari, K.; Ito, R.; Yamamoto, S.;
Konno, T.; Fuyuhiro, A.; Fujioka, K.; Arakawa, R. Inorg. Chem. 2002,
41, 6824-6830.
(11) Sixteen-electron monomeric precursors: (a) Jones, W. D.; Chin, R.
M. J. Am. Chem. Soc. 1992, 114, 9851-9858.
(12) Rauchfuss, T. B.; Roundhill, D. M. J. Am. Chem. Soc. 1975, 97, 3386-
3392.
(13) (a) Castin˜eiras, A.; Sicilia-Zafra, A. G.; Gonza´lez-Pe´rez, J. M.;
Choquesillo-Lazarte, D.; Niclo´s-Gutie´rrez, J. Inorg. Chem. 2002, 41,
6956-6958. (b) Tomic´, Z. D.; Novakovic´, S. B.; Zaric´, S. D. Eur. J.
Inorg. Chem. 2004, 2215-2218. (c) Jiang, Y.; Xi, C.; Liu, Y.; Niclo´s-
Gutie´rrez, J.; Choquesillo-Lazarte, D. Eur. J. Inorg. Chem. 2005,
1585-1588. (d) Yang, X.-J.; Drepper, F.; Wu, B.; Sun, W.-H.;
Haehnel, W.; Janiak, C. J. Chem. Soc., Dalton Trans. 2005, 256-
267. (e) Milcˇicˇ, M. K.; Medakovic´, V. B.; Sredojevic´, D. N.; Juranic´,
N. O.; Zaric´, S. D. Inorg. Chem. 2006, 45, 4755-4763.
(14) (a) Yamada, Y.; Uchida, M.; Miyashita, Y.; Fujisawa, K.; Konno, T.;
Okamoto, K Bull. Chem. Soc. Jpn. 2000, 73, 913-922. (b) Chohan,
B. S.; Shoner, S. C.; Kovacs, J. A.; Maroney, M. J. Inorg. Chem.
2004, 43, 7726-7734. (c) Gonza´lez-Duarte, P.; Lledo´s, A.; Mas-
Balleste´, R. Eur. J. Inorg. Chem. 2004, 3585-3599. (d) Mas-Balleste´,
R.; Capdevila, M.; Gonza´lez-Duarte, P.; Hamidi, M.; Lledo´s, A.;
Me´gret, C.; Montauzon, D. J. Chem. Soc., Dalton Trans. 2004, 706-
712.
(8) (a) Sorasaenee, K.; Gala´n-Mascaro´s, J. R.; Dunbar, K. R. Inorg. Chem.
2002, 41, 433-436. (b) Sorasaenee, K.; Gala´n-Mascaro´s, J. R.;
Dunbar, K. R. Inorg. Chem. 2003, 42, 661-663. (c) Dorcier, A.; Ang,
W. H.; Bolan˜o, S.; Gonsalvi, L.; Juillerat-Jeannerat, L.; Laurenczy,
G.; Peruzzini, M.; Phillips, A. D.; Zanobini, F.; Dyson, P. J.
Organometallics 2006, 25, 4090-4096.
430 Inorganic Chemistry, Vol. 47, No. 2, 2008

RhCl3-Assisted C-H and C-S Bond Scissions
II Princeton Applied Research potentiostat/galvanostat under argon.
The cell contained a Pt working electrode and a Pt wire auxiliary
electrode. Tetraethylammnium perclorate (NEt4ClO4) was used as
a supporting electrolyte, and the potentials were referenced to the
Ag/AgCl electrode without junction correction. Coulometric oxida-
tion was performed at a constant potential of 1.0 V vs Ag/AgCl at
300 K in 1:1 CH2Cl2/CH3CN in the presence of NEt4ClO4 under
argon. EPR spectrum was recorded on a Varian E-109C X-band
spectrometer. Magnetic susceptibilities were measured on a PAR-
155 vibrating-sample magnetometer.
Computational Study. Ground-state electronic structure calcula-
tions of the organosulfur complexes using the X-ray coordinate were
carried out by density functional theory (DFT)21 method employing
the Gaussian 03 (G03) program.22 The functional used throughout
the study was the B3LYP, consisting of a nonlocal hybrid exchange
functional as defined by Becke’s three-parameter equation23 and
nonlocal Lee-Yang-Parr correlation functional.24 The basis set
used for all non-metal atoms except hydrogen was 6-31G(d) and
6-31G for hydrogen. LANL2DZ valance and effective core potential
functions were used for rhodium.
said 5-8 compounds is scrutinized, revealing the formation
of rhodium-bound thiyl radical in the case of 5.15
Experimental Section
Materials. The chemicals used were purchased from the fol-
lowing sources: 2-aminothiophenol, triphenylphosphine, tripheny-
larsine, and phenyl sulfide from Aldrich Chemical Co., and RhCl3‚
3H2O from Arora Matthey Limited. All solvents were used as
received.
Physical Measurements. The elemental analyses (C, H, N) were
performed with a PerkinElmer model 2400 series II elemental
analyzer. IR spectra were recorded on a PerkinElmer L-0100
spectrometer. The electronic spectra in dichloromethane solution
were obtained using a PerkinElmer Lambda 25 spectrophotometer
1
with a solute concentration of about 10-3 M. H NMR spectral
measurements were carried out on a Bruker av FT 300 MHz
spectrometer with TMS as an internal reference. 31P NMR spectral
measurements were carried out on a Bruker AMX 500 spectrometer
operating at 202.42 MHz. The electrospray ionization mass spectra
(EI-MS positive) were measured in acetonitrile on a Micromass
Qtof YA263 mass spectrometer. GC analysis of the solution was
performed on a PerkinElmer Clarus 500 gas chromatograph
equipped with a 30-m (0.25 mm i.d., 0.25-µm film thickness) ELITE
5MS column. Carrier gas He, T1 ) 40 °C, initial isotherm time )
2 min, heating rate ) 8, Tfinal ) 300 °C, final isotherm time ) 5,
flow ) 1 mL/min. The mass spectrometry data were collected after
electron ionization at 70 eV. GC/MS analysis was performed on a
PerkinElmer Clarus 500 GC/MS apparatus. X-ray intensity data
for compounds 5 and 7 were measured at 293 (2) K on a Bruker
AXS SMART APEX CCD diffractometer (Mo KR, λ ) 0.71073
Å). Metal atoms were located by direct methods, and the rest of
the non-hydrogen atoms emerged from successive Fourier synthesis.
The structures were refined by full-matrix least-squares procedures
on F2. All non-hydrogen atoms were refined anisotropically. All
the hydrogen atoms were included in calculated positions for
compound 5, while those of 7 were located from difference maps
and refined isotropically. X-ray intensity data for compounds 6b
and 8 were measured at 153(2) K on a Stoe Mark II-Image plate
diffraction system16 equipped with a two-circle goniometer and
using Mo KR (λ ) 0.71073 Å) graphite monochromatic radiation.
Image plate distance 135 mm. Metal atoms were located and solved
by direct methods using the program SHELXS-97.17 The refinement
and all further calculations were carried out using SHELXL-97.18
The hydrogen atoms were included in calculated positions and
treated as riding atoms using SHELXL default parameters. Calcula-
tions were performed using the SHELXTL v5.03 program pack-
age.19 Thermal ellipsoids were drawn at the 35% probability level.
Molecular structure plots were drawn using ORTEP.20 Electro-
chemical measurements were carried out at 27 °C with VersaStat
Syntheses of Ligands. (1). HLR (R ) Me, CH2Ph) was
synthesized following the methods previously described.25
Syntheses of Compounds. [Rh(L)Cl(PPh3)2] (5). A mixture of
1 (R ) CH2Ph) (118 mg, 0.388 mmol), RhCl3‚3H2O (100 mg, 0.380
mmol), and triphenylphosphine (1.0 g, 3.816 mmol) in ethanolic
(50 mL) solution was heated to reflux for 16 h until a dark green
crystalline solid was precipitated. The green mass was filtered,
washed with ethanol, and dried under reduced pressure. Upon
chromatographic purification of the solid on neutral silica gel with
1:3 hexane/benzene mixture as eluent, green 5 was obtained. Yield
(41%). An analogous reaction was made with 1 (R ) Me). Yield
(39%). Anal. Calcd for C48H38ClN2P2RhS: C, 65.87; H, 4.38; N,
3.20. Found: C, 65.69; H, 4.32; N, 3.26%. UV-vis spectra (CH2-
Cl2): λmax, nm (ꢀ dm-3 mol-1 cm-1): 711 (3600). IR (KBr disk,
cm-1): 1393 (νNN), 743, 693 and 518 (νRhP). 1H NMR (300 MHz,
CDCl3): δ 7.612 (d) 7.542-7.486 (m), 7.436 (d), 7.198-7.056
(m), 6.872 (t), 6.697 (d), 6.556-6.460 (m), 6.377 (d), 6.087 (t);
1
31P NMR (CDCl3): δ 16.87 (d, JRh-P ) 97.6 Hz). HRMS (EI)
calcd for C48H38ClN2P2RhS (M+) 874.0938, found (m/z) 874.0875.
[Rh(LR)Cl2(PPh3)] (6). To an ethanolic (50 mL) solution of 1
(R ) CH2Ph) (116 mg, 0.382 mmol) and RhCl3‚3H2O (100 mg,
(20) Johnson, C. K. ORTEP; Report ORNL-5138; Oak Ridge National
Laboratory: Oak Ridge, TN, 1976.
(21) Parr, R. G.; Yang, W. Density functional theory of atoms and
molecules; Oxford University Press: New York, 1989.
(22) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G.
A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.;
Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R.
E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J.
W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.;
Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.;
Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari,
K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.;
Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A.
Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004.
(23) Becke, A. D. J. Chem. Phys. 1993, 98, 5648-5652.
(15) (a) Kimura, S.; Bill, E.; Bothe, E.; Weyhermuller, T.; Wieghardt, K.
J. Am. Chem. Soc. 2001, 123, 6025-6039. (b) Journaux, Y.; Glaser,
T.; Steinfeld, G.; Lozana, V.; Kersting B. J. Chem. Soc., Dalton Trans.
2006, 1738-1748. (c) McDonough, J. E.; Weir, J. J.; Sukcharoenphon,
K.; Hoff, C. D.; Kryatova, O. P.; Rybak-Akimova, E. V.; Scott, B.
L.; Kubas, G. J.; Mendiratta, A.; Cummins, C. C. J. Am. Chem. Soc.
2006, 128, 10295-10303. (d) Grapperhaus, C. A.; Venna, K. B.;
Mashuta, M. S. Inorg. Chem. 2007, 46, 8044-8050.
(16) X-Area V1.17 and X-RED32 V1.04 Software; Stoe & Cie: Darmstadt,
Germany, 2002.
(17) Sheldrick, G. M. SHELXS-97, Program for Crystal Structure Solution;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(18) Sheldrick, G. M. SHELXL-97, Program for Refining X-ray Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(19) Sheldrick, G. M. SHELXTL, version 5.03; Siemens Analytical Instru-
ments: Madison, WI, 1994.
(24) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1998, 37, 785-789.
(25) Shriner, R. L.; Struck, H. C.; Jorison, W. J. J. Am. Chem. Soc. 1930,
52, 2060-2069. (b) Burawoy, A.; Vellins, C. E. J. Chem. Soc. 1954,
90-95. (c) Livingstone, S. E. J. Chem. Soc. 1956, 437-440.
Inorganic Chemistry, Vol. 47, No. 2, 2008 431

Pramanik et al.
Scheme 2. Syntheses and Structures of Orthometalated Rhodium(III)
Thiolato 5 and Thioether 6 Compounds
0.380 mmol), triphenylphosphine (102 mg, 0.389 mmol) was added
and heated to reflux for 4 h until a dark orange solution was
obtained. The resulting solution was evaporated to dryness. Upon
chromatographic purification of the solid on neutral silica gel with
1:8 acetonitrile/benzene mixture as eluent, orange 6b was obtained.
Yield (62%). An analogous reaction was made with 1 (R ) Me).
Yield (58%). For compound 6b: Anal. Calcd for C37H30Cl2N2-
PRhS: C, 60.09; H, 4.08; N, 3.79. Found: C, 60.58; H, 4.29; N,
3.65%. UV-vis spectra (CH2Cl2): λmax, nm (ꢀ dm-3 mol-1 cm-1):
470 (2500). IR (KBr disk, cm-1): 1411 (νNN), 696 and 528 (νRhP).
1H NMR (300 MHz, CDCl3): δ 8.107 (d), 8.033 (d), 7.689 (d),
7.481-7.035 (m), 5.556 (d), 4.624 (d); 31P NMR (CDCl3): δ 22.21
(d, 1JRh-P ) 118.9 Hz). For compound 6a: Anal. Calcd for C31H26-
Cl2N2PRhS: C, 56.12; H, 3.95; N, 4.22. Found: C, 55.79; H, 4.14;
N, 4.14%. UV-vis spectra (CH2Cl2): λmax, nm (ꢀ dm-3 mol-1
cm-1): 471 (2100). IR (KBr disk, cm-1): 1416 (νNN), 696, 535
(νRhP). 1H NMR (300 MHz, CDCl3): δ 8.121 (d), 8.018 (d), 7.662
(d), 7.528-7.082 (m), 2.175 (s); 31P NMR (CDCl3): δ 21.86 (d,
1JRh-P ) 117.1 Hz).
Conversion of 6 to 5. 6a (45 mg, 0.068 mmol) and triph-
enylphosphine (144 mg, 0.551 mmol) were dissolved in ethanol
(30 mL), and the mixture was refluxed for 10 h. During this time,
green precipitate of compound 5 was formed. Chromatographic
purification was made by the above procedure. Yield (57%). An
analogous reaction with 6b produced 5 in 55% yield.
afforded an emerald solid 5 in 40% yield (Scheme 2). Both
C(sp2)-H and C(sp3)-S bonds were cleaved in situ during
the course of the synthesis. The general mechanism of
formation of orthometalated azobenzene derivatives was
described to proceed via initial coordination of azo-nitrogen
followed by electrophilic substitution at the pendant phenyl
ring.7g,26
anti-[Rh(L)Cl(PPh3)]2 (7) and syn-[Rh(L)Cl(PPh3)]2 (8). In a
typical reaction, toluene solution (150 mL) of compound 5 (100
mg, 0.114 mmol) was refluxed for 100 h. The original green
solution slowly turned dark red. Organic solvent was evaporated
under reduced pressure, and the resulting dark mass was subjected
to column chromatography. Compounds 8 and 7 were eluted as a
brown band with benzene and red band with 1:10 acetonitrile/
benzene mixture, respectively. Yield for 7 (58%) and for 8 (8%).
An analogous reaction was performed at room temperature in
toluene for 4 months. Then the compounds were isolated as above.
Yield for 7 (55%) and for 8 (8%). For compound 7: Anal. Calcd
for C60H46Cl2N4P2Rh2S2: C, 58.78; H, 3.78; N, 4.58. Found: C,
58.65; H, 3.84; N, 4.49%. UV-vis spectra (CH2Cl2): λmax, nm (ꢀ
dm-3 mol-1 cm-1): 529 (5400). IR (KBr disk, cm-1): 1385 (νNN),
It is worth noting that the C-S bond scission was not
observed when reactions were performed with 1 equiv of
PPh3 relative to both 1 and RhCl3‚3H2O. In that case, orange
organothioether rhodium(III) compounds 6 (chelation mode
3) are obtained. Although the S-dealkylation is not observed
with RhCl3 in the absence of PPh3, Wilkinson’s catalyst, viz.
[RhCl(PPh3)3], alone is competent to furnish 5 in the
analogous conditions in low yield. Interestingly, the thioether
functions of 6 are converted to thiolato group in presence of
excess PPh3 to afford organothiolato rhodium 5 (Scheme 2).
Although the oxidation states of rhodium remain the same
in both the starting material and organothiolato product, we
speculate it passes through an intermediate rhodium(II)
species that could possibly be involved in the C-S bond
cleavage process. However, attempts to observe any Rh(II)
intermediates along the reaction pathway were unsuccessful.
Interestingly, use of less π-acidic SPh2 and AsPh3 instead
of PPh3 appears ineffective for such bond cleavage, indicating
that the C-S bond activation is sensitive to the π-acidity of
the added coligand. The reason behind this observation
possibly lies in the stabilization of the proposed Rh(II)
intermediate. Tertiary phosphine ligands have a strong
stabilizing effect on transition-metal complexes in low
oxidation states owing to their π-acceptance nature.27,28 The
stability of such complexes has been exploited in various
1
694 and 526 (νRhP). H NMR (300 MHz, CDCl3): δ 8.351 (d),
7.734 (d), 7.468-7.407 (m), 7.160-7.001 (m), 6.695-6.502 (m),
1
5.684 (d); 31P NMR (CDCl3): δ 23.52 (d, JRh-P ) 115.2 Hz).
HRMS (EI) calcd for C60H46Cl2N4P2Rh2S2 (M+) 1224.0156, found
(m/z) 1224.0077. For compound 8: Anal. Calcd for C60H46Cl2N4P2-
Rh2S2: C, 58.78; H, 3.78; N, 4.58. Found: C, 58.61; H, 3.72; N,
4.48%. UV-vis spectra (CH2Cl2): λmax nm (ꢀ dm-3 mol-1 cm-1):
508 (3800). IR (KBr disk, cm-1): 1388 (νNN), 694 and 522 (νRhP).
1H NMR (300 MHz, CDCl3): δ 8.178 (d), 7.660 (d), 7.273 (t),
7.153-7.039 (m), 6.991-6.940 (m), 6.827 (t); 31P NMR (CDCl3):
1
δ 23.11 (d, JRh-P ) 116.6 Hz). HRMS (EI) calcd for C60H46-
Cl2N4P2Rh2S2 (M+) 1224.0156, found (m/z) 1224.0089.
Conversion of Dimers (7 and 8) to Monomer (5). Compound
7 (20 mg, 0.016 mmol) and triphenylphosphine (25 mg, 0. 095
mmol) were dissolved in toluene (5 mL), and the mixture was
refluxed for 48 h. The solution turned dark green, and compound
5 was isolated by the above procedure. Yield (71%). An analogous
reaction was performed with 8. Yield (68%).
(26) (a) Aulwurm, U. R.; Melchinger, J. U.; Kisch, H. Organometallics
1995, 14, 3385-3395. (b) Lersch, M.; Tilset, M. Chem. ReV. 2005,
105, 2471-2526. (c) Dupont, J.; Consorti, C. S.; Spencer, J. Chem.
ReV. 2005, 105, 2527-2571. (d) Patra, D.; Pratihar, J.; Shee, B.;
Pattanayak, P.; Chattopadhyay, S. Polyhedron 2006, 25, 2637-2642.
(27) Al-Jeboori, M. J.; Dilworth, J. R.; Zheng, Y. J. Chem. Soc., Dalton
Trans. 1998, 3215-3218.
Results and Discussion
Synthesis and Spectroscopic Characterization. Treat-
ment of an alcoholic solution of RhCl3‚3H2O with 1 in the
presence of 10-fold excess of PPh3 under refluxing condition
432 Inorganic Chemistry, Vol. 47, No. 2, 2008

RhCl3-Assisted C-H and C-S Bond Scissions
Scheme 3. Suggested Mechanism of C-S Cleavage
Scheme 4. Syntheses and Structures of Organorhodium(III) Thiolato
Dimers
and R-R. The chromatogram is consistent with reductive
cleavage by single-electron transfer (SET) mechanism since
the products (comparable yield of toluene, benzyl chloride,
and ethyl benzyl ether and a little bibenzyl) appear to be
derived from radical species.33,34
carbon-heteroatom bond activation processes via oxidative
addition.2b-d,29 This strategy may be utilized as a useful tool
for a variety of C-heteroatom bond-breaking processes.
Moreover, ample precedent exists for internal oxidative
addition of one such C-S bond of coordinated thiophene to
Rh(III) where reductive cleavage of C-heteroatom bond
occurs.3b-f In our case, selective C-S bond viz. C(sp3)-S
is cleaved consistent with the bond strength order (C(sp3)-S
< C(sp2)-S). The reductive cleavage of C(sp3)-S bond of
coordinated thioether moiety has been explored as an
attractive route to functional ligands and new materials.
A plausible mechanism of the C-S cleavage reaction
process is proposed as shown in Scheme 3.30 We anticipate
that, after initial coordination of HLR with Rh(III), the
resultant organothioether rhodium(III) compounds 6 get
reduced in the presence of excess phosphine to the interme-
diate organothioether rhodium(II) complex. Although no low
valence reaction intermediates have been observed or
isolated, a rhodium(II) intermediate is a likely active species.
In view of the proposed mechanism presented in Scheme 3,
it is reasonable to conclude that the conversion of 6 to 5
proceeds through the intermediacy of the Rh(II) fragment
[Rh(LR)Cl(PPh3)] (electron-rich d7 center), which is active
toward the one-electron reduction of the coordinated S-C(sp3)
bond of thioether group from LR (via electron donation into
the C-S σ* antibonding orbital)4c,31 and causes reductive
cleavage of pendant S-C(alkyl) bond with concomitant
extrusion of alkyl radical (R•). To obtain support for this
proposed mechanism, GC/MS analysis (Figure S1 in the
The mononuclear thiolato complex was found to be
indefinitely stable in the solid state. Surprisingly, electroni-
cally and coordinatively saturated complex 5 appeared to
be unstable in solvents such as toluene, dichloromethane,
and so forth at ambient conditions and undergoes dimeriza-
tion. Nonetheless, spectral study ensures the substantial
6
stability of 5 (t2 , substitutionally inert)28 in solution. It slowly
self-dimerized via dissociation of one coordinated PPh3
ligand in solution to thiolato-bridged magenta 7 and brown
8 dimers in approximately 7:1 ratio (Scheme 4). Interestingly,
upon boiling the concentrated toluene solution of respective
dimer with 6 equiv of PPh3, mononuclear 5 was regenerated.
Phosphine-mediated mononuclear-dinuclear interconversion
occurs presumably via a 16-electron five-coordinated inter-
mediate [RhIII(L)Cl(PPh3)].
All three organorhodium(III) thiolates were isolated as both
air- and moisture-stable solids and characterized both in
solution by UV-vis, NMR (1H and 31P), and mass spec-
troscopies and in the solid state by IR, susceptibility
measurement, and single-crystal X-ray diffraction studies.
The thioether complexes 6a and 6b were analogously
characterized, and X-ray structure of one representative
compound was determined. Additionally, ligand redox
behavior of the organosulfur compounds was scrutinized by
(33) While open questions remain about the preferred pathway for C-S
bond cleavage of coordinated thioether moiety of 6, the observed
products from the GC/MS study do provide some insight into the
mechanism of cleavage for these organothioether compounds. Forma-
tion of R+ by heterolytic cleavage is not a reasonable mechanism for
the C-S bond rupture of organothioether rhodium(III) systems since
the generation of R-R is rather unexpected via alkyl cation. In
addition, formation of the corresponding alkane via hydride abstraction
appears less likely. Alternative mechanisms include (1) two electrons
reductive cleavage and the formation of R- proceeds through the
intermediacy of the Rh(I) fragment [Rh(LR)(PPh3)] and (2) an initial
formal reduction at the metal center to the unstable intermediate [RhII-
(LR)Cl(PPh3)] (phosphines are well known reducing agent) followed
by SET to the S-C(alkyl) σ* antibonding orbital with the release of
R• as cleavage product. Since the observed products derived from
alkyl group contain benzyl chloride and ethyl benzyl ether as well
(Figure S1 in the Supporting Information), we suggest that the latter
reaction pathway is most likely to be operative. The plausible radical
reactions can be written as: PhCH2• + EtOH f PhCH3 + EtO•; EtO•
+ PhCH2• f EtOCH2Ph; PhCH2• + [Cl-RhIII] f PhCH2Cl + [RhII]34;
Supporting Information) of the reaction mixture was per-
formed for HLCH Ph 32
.
The reactive alkyl radical is trans-
2
formed to various products, namely R-H, R-Cl, R-OEt,
(28) Cotton, F. A.; Wilkinson, G.; Murrilo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; Wiley-Interscience: New York, 1999.
(29) (a) Ohta, T.; Michibata, T.; Yamada, K.; Omori, R.; Furukawa, I.
Chem. Commun. 2003, 1192-1193. (b) Li, M.; Ellern, A.; Espenson,
J. H. Angew. Chem., Int. Ed. 2004, 43, 5837-5839. (c) Li, M.; Ellern,
A.; Espenson, J. H. J. Am. Chem. Soc. 2005, 127, 10436-10447.
(30) We are thankful for the helpful mechanistic discussion with one of
the reviewers.
(31) (a) Jones, W. D.; Dong, L. J. Am. Chem. Soc. 1991, 113, 559-564.
(b) Dong, L.; Duckett, S. B.; Ohman, K. F.; Jones, W. D. J. Am. Chem.
Soc. 1992, 114, 151-160.
(32) We could not perform GC/MS analysis in the case of HLMe since the
products derived from methyl radical are highly volatile and were
unable to arrest during the synthesis of organothiolato compound 5.
•
2PhCH2 f PhCH2CH2Ph.
(34) Bacha, J. D.; Kochi, J. K. J. Org. Chem. 1968, 33, 83-93.
Inorganic Chemistry, Vol. 47, No. 2, 2008 433

Pramanik et al.
cyclic voltammetric study. The radical cation 5•+ complex
generated by electrochemical means was characterized by
EPR spectroscopy.
Infrared spectra of the complexes exhibit many sharp and
strong vibrations within 1750-400 cm-1. A sharp vibration
observed near 1400 cm-1 for all the complexes is assigned
to the νNN. The expected lowering of νNN values of complexes
as compared with those of 1 is consistent with the Rh(III)
f π*(azo) back-bonding.5a Strong bands near 520 and 690
cm-1 are indicative of Rh-PPh3 ligation in the complexes,
while an additional intense feature at 743 cm-1 is observed
in the case of 5, which may be attributed to the presence of
a trans-Rh(PPh3)2 moiety.
Thiolato and thioether complexes are soluble in both
nonpolar and polar organic solvents such as benzene, toluene,
dichloromethane, acetone, and so forth. Electronic spectra
of all the complexes have been recorded in dichloromethane
solution (Figure S2 in the Supporting Information). The
mononuclear rhodium thiolato compound shows absorption
near 700 nm in the visible region. The lowest energy
absorptions of the organothiolato complexes in the visible
region are probably due to intraligand charge-transfer transi-
tions taking place in the three-coordinated 2-(phenylazo)-
thiolate ligand and are corroborated with the nature of FMOs
(vide supra).7e Observed hypsochromic shift of the lowest
energy excitation in UV-vis spectra of 7 and 8 (529 and
508 nm, respectively) relative to that of 5 is ascribed to the
weaker charge-transfer transition upon µ-SAr dimerization.
The lowest energy electronic excitation of thioether com-
plexes is shifted to higher energy (∼470 nm) as compared
with that of thiolato compounds, indicating that facile CT
transition occurs in S-dealkylated complexes.
Figure 1. X-band EPR spectrum of 5•+ in CH2Cl2 at 300 K. Instrument
settings: microwave frequency, 9.1 GHz; microwave power, 28 dB;
modulation frequency, 100 kHz. Voltammogram of 5 in 1:1 CH2Cl2/CH3-
CN at 50 mV s-1 is shown in the inset.
All the synthesized complexes gave satisfactory elemental
analyses and were diamagnetic, which corresponds to the
trivalent rhodium (low-spin d6, S ) 0) in these complexes.
1
Absence of alkyl resonance in the H NMR spectrum of 5
validates the occurrence of S-dealkylation. Alkyl proton
signals of thioether compounds 6a and 6b appear as a singlet
at δ 2.2 (SCH3) and a pair of doublets centered at δ 5.1
(SCH2Ph, 1JHH ) 12.3 Hz) resonances, respectively. The 1H
NMR spectra of 7 and 8 show a 1:1 relation of ligand/
triphenylphosphine signals, corresponding to the thiolato-
bridged dinuclear rhodium(III) hexacoordinated species with
loss of one PPh3 molecule per metal center of the parent
monomer. The 31P NMR spectra of the thiolato complexes
containing two phosphine groups exhibit only single doublet
resonances, indicating equivalent phosphine environment
(i.e., trans and anti dispositions of PPh3 groups in monomeric
and dimeric complexes, respectively). The formation of
dimers results in a significant deshielding (dimers δ ≈ 23
Figure 2. HOMO and LUMO of organothiolato 5 (top), 7, 8, and
organothioether 6b (bottom) compounds (isosurface cutoff value ) 0.02).
Color code for atom: gray, C; blue, N; orange, P; yellow, S; green, Cl;
aquamarine, Rh.
S3 and S4 in the Supporting Information). These attributions
are consistent with the crystal structures showed in Figures
3 (compound 5), 4(compound 6b), and 5(compounds 7 and
8).
Ligand Redox and EPR. Electron-transfer property of
the organosulfur rhodium(III) complexes was studied by
using cyclic voltammetry in 1:1 dichloromethane/acetonitrile
solution (0.1 M NEt4ClO4) by using a platinum working
electrode (Table 1). The reported potential is referenced to
1
vs monomer δ ≈ 17) in the 31P signals. The higher JRh-P
values of dimers by ca. 18 Hz in comparison to those of 5
are indicative of the shorter Rh-P lengths in the former
compounds (vide supra). The 31P signals of the thioether
complexes occur near δ ≈ 22.
Both the NMR and MASS spectroscopic studies unequivo-
cally established the substantial stability of cyclometalated
rhodium thiolato compounds in the solution phase (Figures
434 Inorganic Chemistry, Vol. 47, No. 2, 2008

RhCl3-Assisted C-H and C-S Bond Scissions
conditions. Coulometric study at 300 K confirms the oxida-
tion to be a one-electron process and demonstrates a fully
reversibile 5/5•+ couple. It is very likely that this process
corresponds to the oxidation of the thiophenolate sulfur atom,
yielding a rhodium-bound thiyl radical (eq 1).15
[RhIII(L-II)Cl(PPh3)2] - e- T [RhIII(L-I)Cl(PPh3)2]+ : E1/2
(1)
The redox noninnocence of the thiophenolate donor in the
mononuclear species is supported by the EPR and theoretical
studies. It should be noted that the dinuclear species 7 and
8 display a single, irreversible redox wave near Epa ) +1.2
V vs Ag/AgCl. The oxidative response can be assigned
primarily to a ligand-centered redox reaction. In contrast,
all the complexes show one-electron irreversible reduction
waves between -1.0 and -1.5 V vs Ag/AgCl. These
responses are believed to be centered on the coordinated
ligand with substantial contribution of the azo moiety (Figure
2).5a
Figure 3. Molecular structure of 5. Solvate and H atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Rh1-C1 2.025(6),
Rh1-N1 1.990(5), Rh1-Cl1 2.368(2), Rh1-P1 2.363(2), Rh1-S1 2.452-
(2), N1-N2 1.280(7), C12-S1 1.761(7); N1-Rh1-S1 83.85(15), P1-
Rh1-P2 174.85(9).
The fluid solution X-band EPR spectrum of 5•+ radical
cation complex generated electrochemically in the presence
of NEt4ClO4 exhibits a single line at g ) 2.003 with the
peak-to-peak line width of 28 G (Figure 1). The isotropic
and intense feature of the spectrum signifies little or no
contribution of metal to the spin-bearing orbital.15,35 We
believe that the thiyl radical in the one-electron oxidized 5•+
form is stabilized by coordination to a kinetically inert RhIII
6
(t2 ) ion.15b
To gain insight into the nature of the redox orbitals, we
performed DFT calculation (vide infra) on compounds 5-8
(Figures 2 and S5 in the Supporting Information). Calculation
indicates that HOMOs of organothiolato compounds are
exclusively distributed over the ligand with an appreciable
contribution from the 3p AO of thiolato sulfur. The oxidative
response is believed to be largely centered on the S atom of
coordinated 2-(phenylazo)thiolate ligand (eq 1). It is worth
noting that the absence of oxidative response for thioether
complexes is nicely corroborated with the insignificant
contribution of S atom in the HOMO of 6b. In contrast, the
components of LUMOs are essentially AOs of azo ligands,
especially N atoms with no significant participation of S
orbitals.
Figure 4. Molecular structure of 6b. H atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Rh1-C1 2.003(1), Rh1-N1
2.028(1), Rh1-Cl1 2.341(1), Rh1-Cl2 2.338(1), Rh1-P1 2.336(1), Rh1-
S1 2.472(1), N1-N2 1.265(2), C12-S1 1.785(1), C13-S1 1.839(1); Cl1-
Rh1-Cl2 173.22(1), N1-Rh1-S1 82.51(3), N1-Rh1-P1 174.60(3).
Ligand-centered electrochemical oxidation reveals the
noninnocent nature of the thiolato coordination in mono-
nuclear species. Thus, the high electron density on the S atom
of 5 facilitates the formation of S-bridged dinuclear com-
pounds.9,14
the Ag/AgCl electrode. The value for the ferrocenium-
ferrocene couple, under our experimental conditions, was
0.52 V.
Unlike organothioether complexes, the mononuclear [RhIII-
(L)Cl(PPh3)2] shows an oxidative response at E1/2 ) +0.82
V vs Ag/AgCl and the voltammogram is reversible in nature,
characterized by a rather small peak-to-peak separation (∆Ep
) Epa - Epc) of 80 mV. This indicates the oxidative potential
is sensitive to the nature of the metal-sulfur (thioether vs
thiolato) bonding. Voltammogram of the mononuclear thi-
olato complex is shown in the inset of Figure 1. The one-
electron nature of this oxidation has been verified by
comparing its current height (ipa) with that of the standard
ferrocene-ferrocenium couple under identical experimental
Description of the Crystal Structures. The X-ray crystal
structures of 5.2CH2Cl2, 6b, 7.2CH2Cl2, and 8.CH2Cl2 were
determined to establish the geometries about the metal ions
as well as the bonding modes of the LR-, L2-, and the
coligands. Single crystals of the respective complex suitable
for X-ray analysis were grown by slow diffusion of hexane
into dichloromethane solution at room temperature. Experi-
(35) (a) Pramanik, K.; Shivakumar, M.; Ghosh, P.; Chakravorty, A. Inorg.
Chem. 2000, 39, 195-199. (b) Pramanik, K. Ph.D. Thesis, Jadavpur
University, Kolkata, India, 1997.
Inorganic Chemistry, Vol. 47, No. 2, 2008 435

Pramanik et al.
Figure 5. Molecular structure of 7 (left) and 8 (right). Solvate and H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg) of 7:
Rh1-C1 2.014(3), Rh1-N1 1.971(3), Rh1-Cl1 2.373(1), Rh1-P1 2.342(1), Rh1-S1 2.452(1), Rh1-S1A 2.478(1), N1-N2 1.274(4), C12-S1 1.776(4),
Rh1‚‚‚Rh1A 3.675(1), S1‚‚‚S1A 3.287(2); N1-Rh1-Cl1 178.04(8), N1-Rh1-S1 83.75(9); Compound 8: Rh1-C1 2.004(2), Rh1-N1 1.983(2), Rh1-Cl1
2.378(1), Rh1-P1 2.322(1), Rh1-S1 2.473(1), Rh1-S2 2.409(1), N1-N2 1.274(3), Rh2-C31 2.002(2), Rh2-N3 1.983(2), Rh2-Cl2 2.368(1), Rh2-P2
2.316(1), Rh2-S2 2.471(1), Rh2-S1 2.418(1), N3-N4 1.273(3), C12-S1 1.775(3), C42-S2 1.763(2), Rh1‚‚‚Rh2 3.593(1), S1‚‚‚S2 3.309(1); N1-Rh1-
Cl1 173.59(6), N3-Rh2-Cl2 174.55(6), N1-Rh1-S1 83.11(6), N3-Rh2-S2 83.22(5).
Table 1. Electrochemical Data for 5-8a
thiolato 5 and thioether 6b complexes, notably: (i) Rh1-
S1(thioether) distance is longer by 0.02 Å than the analogous
Rh1-S1(thiolato) distance, indicating enhanced bonding
ability of the latter; (ii) Rh1-N1(trans to π-acid ligand PPh3)
length of 6b is significantly longer (∼0.04 Å) than Rh1-
N1(trans to chloride ligand) length of 5, which is consistent
with the superior overlap of the PPh3 d/σ* orbital with metal
d orbital compared to the poorer overlap between chloride
d and metal d orbitals; (iii) N1-N2 azo distances in thioether,
compound
E1/2/V (∆Ep/mV)
5 [Rh(L)Cl(PPh3)2]
-1.32,b +0.82(80)
6a [Rh(LMe)Cl2(PPh3)]
-1.34b
6b [Rh(LCH Ph)Cl2(PPh3)]
-1.28b
2
7 anti-[Rh(L)Cl(PPh3)]2
8 syn-[Rh(L)Cl(PPh3)]2
-1.21,b -1.02,b +1.24c
-1.19,b -1.01,b +1.12c
a Solute concentration ≈ 10-3 mol dm-3; scan rate, 50 mV s-1; E1/2
)
0.5(Epa - Epc) for reversible one-electron process, where Epa and Epc are
the anodic and cathodic peak potentials, respectively; ∆Ep ) Epa - Epc.
b Epc value. Epa value.
c
thiolato, and free ligand HLCH Ph (Figure S6 in the Supporting
2
Information) are 1.265(2), 1.280(7), and 1.251(3) Å, respec-
tively, signifying that RhIII(4d) f N(azo π*) back-bonding
severely impedes in presence of π-acceptor PPh3 ligands in
the trans position; (iv) Rh1-P1(trans to azo) length of 6b is
significantly shorter by ca. 0.03 Å than Rh1-P1(trans to PPh3
ligand) length of 5 (π-acidity order: PPh3 > azo); (v) Rh1-
Cl(trans to chloride) average length of 6b is significantly
shorter by ca. 0.03 Å than Rh1-Cl1(trans to azo) length of
5 (trans influence order: azo > chloride); and (vi) in the
structure of 5 the N, S chelate bite angle of the bianionic
ligand [L]2-, 83.85(15)°, is marginally larger than that of
mental crystallographic data are summarized in Table 2.
Further data are available in the Supporting Information.
5.2CH2Cl2. The unit cell of 5 consists of [Rh(L)Cl(PPh3)2]
neutral molecules, with dichloromethane as solvate. In this
complex, the central rhodium(III) is surrounded with a
distorted octahedral coordination environment by tridentate
bianionic ligand (C, N, S), two trans-PPh3, and one chloride
ligands. The ligand, [L]2-, is orthometalated and coordinates
to rhodium(III) using the ortho-carbon C(1), azo-nitrogen
N(1), and thiolato sulfur S(1) so as to form two five-
membered chelates. Primary valence satisfying ligands (L2-
and Cl-) and metal center lie on a crystallographic plane
indicating the Cs symmetry of 5. We wish to note here that
such a coordination mode of the reference ligand via both
C-H and C-S bond dissociations has not been observed
previously. The bianionic ligand of type 4 is new, and
rhodium(III) cyclometalates with azo function are sparse.7
6b. Figure 4 shows the molecular view and atom number-
the monoanionic ligand [LCH Ph]- of 6b, 82.51(3)°.
2
7.2CH2Cl2 and 8.CH2Cl2. The molecules 7 and 8 are
isomeric dimers and crystallize in monoclinic C2/c and P21/n
space group, respectively, with Z values of 4 indicating the
former contains a crystallographically imposed inversion
center. The asymmetric unit contains dichloromethane as
solvent of crystallization for both compounds. Crystal-
lographic analysis reveals both are edge-shared bioctahedra
consisting of a Rh2S2 core, in which two bridging ligands
vis-a`-vis Cl atoms adopt anti and syn positions, respectively.
The valence electron count for Rh atoms (18 electrons per
metal) and long Rh‚‚‚Rh separations (Rh-Rh single bond
distance, 2.4-2.9 Å)36 rule out any metal‚‚‚metal covalent
interaction in the dimers. Significantly, the Rh‚‚‚Rh distance
of 8 is shorter by ca. 0.08 Å than that of 7. The anti isomer
has a crystallographically imposed inversion center at the
midpoint of the Rh1-Rh1A vector signifying the exact
planarity of the Rh2S2 four-membered ring, where the
ing scheme of a representative 6b (HLR ) HLCH Ph) complex.
2
Crystallographic analysis of the thioether complex reveals a
distorted octahedral structure with meridional C, N, S
coordination of type 3 and the ligand, [LCH Ph]-, acts as a
2
tridentate monoanionic donor. The coordination sphere of
the metal atom is completed by one triphenylphosphine and
two trans-chloride ligands. Expectedly, S1-C12 length is
shorter than S1-C13 length (1.785(1) and 1.839(1) Å,
respectively, C(sp2) vs C(sp3)). Both S1-C12 and S1-C13
bond lengths of the complex are longer by ca. 0.025 and
0.022 Å compared to the respective bonds of the free ligand
(Table S1 and Figure S6 in the Supporting Information).
Several significant differences are observed between the
(36) Cotton, F. A.; Walton, R. A. Multiple bonds between metal atoms,
2nd ed.; Oxford University Press: New York, 1993.
436 Inorganic Chemistry, Vol. 47, No. 2, 2008

RhCl3-Assisted C-H and C-S Bond Scissions
Table 2. Summarized Crystallographic Data for 5.2CH2Cl2, 6b, 7.2CH2Cl2, and 8.CH2Cl2
params
5.2CH2Cl2
6b
7.2CH2Cl2
8.CH2Cl2
empirical formula
fw
C50H42Cl5N2P2RhS
1045.02
C37H30Cl2N2PRhS
739.47
C62H50Cl6N4P2Rh2S2
1395.64
C61H48Cl4N4P2Rh2S2
1310.71
T/K
293(2)
153(2)
293(2)
153(2)
cryst syst
space group
a/Å
b/Å
c/Å
orthorhombic
Cmc21
14.995(10)
18.277(12)
17.451(11)
90.0
90.0
90.0
4783(5)
4
triclinic
P1h
monoclinic
C2/c
24.242(11)
0.311(5)
25.882(11)
90.0
114.169(7)
90.0
5902(4)
monoclinic
P21/n
11.3564(5)
24.2964(8)
20.4698(9)
90.0
100.944(4)
90.0
5545.3(4)
4
9.6036(6)
11.1046(7)
16.5264(10)
92.453(5)
105.235(5)
109.209(5)
1589.59(17)
2
R/deg
â/deg
γ/deg
V/Å3
Z
4
Dc/Mg m-3
µ/mm-1
1.451
0.784
1.545
0.852
1.571
1.000
1.570
0.966
cryst size/mm3
θ/deg
0.40 × 0.30 × 0.20
1.76-24.99
16967
4170/0.0245
1.119
0.30 × 0.25 × 0.20
2.19-29.47
20562
8616/0.0215
1.028
0.35 × 0.21 × 0.16
1.8-25.4
21294
5399/0.0301
1.122
0.32 × 0.31 × 0.08
1.31-25.50
41222
10241/0.0319
1.029
measured reflns
unique reflns/Rint
GOF on F2
R1,a wR2b
[I > 2σ(I)]
R indices (all data)
R1, wR2
largest diff. peak,
hole/e Å3
0.0376, 0.0957
0.0217, 0.0565
0.0360, 0.0821
0.0260, 0.0621
0.0380, 0.0960
0.0234, 0.0573
0.0423, 0.0852
0.0358, 0.0647
0.509 and -0.531
0.378 and -0.873
0.661 and -0.425
1.069 and -0.818
2
2
2
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
corresponding ring for syn isomer slightly deviates from
planarity (md 0.022 Å). Although Rh2S2 has been a long-
standing motif in rhodium chemistry with physical data
generally consistent with bisquare planar and bitetrahedral
configurations, the structurally authenticated bioctahedra
containing Rh2S2 core are scarce.8a,37
Metrical parameters associated with Rh-C, Rh-N, Rh-
Cl, and N-N linkages in the three thiolato complexes are
comparable. The Rh(III) f π*(azo) description (vide infra)
is consistent with the lengthening of N-N distance by 0.02-
0.03 Å upon coordination. Slight shortening of Rh-P
distances of 7 by ca. 0.02 Å as compared to that of 5 is
corroborated with the weaker trans influence of the bridging
thiolato than PPh3 ligand. Additional shortening of Rh-P
lengths by 0.02-0.03 Å of 8 is caused by the π-π stacking
interaction between phenyl(thiolato) and phenyl(PPh3) rings
(Table S2 in the Supporting Information).38 The syn isomer
differs appreciably in the orientation of the phenyl rings of
PPh3 groups (Figures S7 and S8 in the Supporting Informa-
tion).
Each µ-SAr group unsymmetrically bridges the two Rh
centers in both dimers. The Rh1-S1 distance of the anti
isomer remains practically unaltered with that of 5, while
the Rh1-S1A distance is elongated by 0.026 Å. In contrast,
anomalous shortening of the Rh1-S2 distance of the syn
isomer by ca. 0.06 Å than both Rh1-S1 and Rh1-S1A
distances of 8 and 7, respectively, may plausibly be explained
by invoking the “metal chelate ring-ring” interaction.
Structural study of 8 provides the significance of intramo-
lecular noncovalent “metallochelate-metallochelate” and
“phenyl-phenyl” stacking interactions (Table S2 in the
Supporting Information) toward the stabilization of sterically
encumbered syn isomer (Figures S7 and S8 in the Supporting
Information). It is worth noting that the stacking tendencies
diverge significantly in the two isomeric forms. In conse-
quence, the sterically hindered isomer is also formed during
the self-association process since L2- exhibits face-selective
dimerization as well.
Conclusions
The main findings of the present work can be summarized
as follows: (a) rhodium-assisted in situ formation of new
C-, N-, and S-coordinating ligand (L2-) via C-H and C-S
bond scissions; PPh3 plays a crucial role in the latter bond-
breaking process. (b) The preferred pathway for S-C(sp3)
bond cleavage of the coordinated thioether moiety is likely
to be the reductive cleavage by SET mechanism. (c) The
superior nucleophilicity of thiolato group can be exploited
to assemble electronically as well as coordinatively saturated
compounds; 5 appears as a viable precursor to tailored
S-bridged dimers 7 and 8. (d) Phosphine mediated monomer-
dimer interconversion of hexacoordinated rhodium thiolates.
(e) Unlike thioether and µ-S bridged dinuclear species,
mononuclear thiolato complex exhibits a fully reversible
oxidative response that is supposed to be primarily centered
on the thiophenolate sulfur atom. EPR spectroscopy of the
oxidized form formed by electrochemical means along with
DFT study reveal the formation of rhodium-bound thiyl
radical cation 5•+. (f) Noncovalent “phenyl-phenyl” and
“metallochelate-metallochelate” interactions are important
toward the stabilization of sterically unfavorable syn isomer
8.
(37) (a) Mueting, A. M.; Boyle, P.; Pignolet, L. H. Inorg. Chem. 1984, 23,
44-48. (b) Bianchini, C.; Mealli, C.; Meli, A.; Sabat, M. Inorg. Chem.
1986, 25, 4617-4618. (c) Dilworth, J. R.; Morales, D.; Zheng, Y. J.
Chem. Soc., Dalton Trans. 2000, 3007-3015. (d) Seino, H.; Yoshikaya,
T.; Hidai, M.; Mizobe, Y. J. Chem. Soc., Dalton Trans. 2004, 3593-
3600.
(38) Janiak, C. J. Chem. Soc., Dalton Trans. 2000, 3885-3896.
Inorganic Chemistry, Vol. 47, No. 2, 2008 437

Pramanik et al.
We anticipate that this information will guide the prepara-
tion of the analogous systems for other noble metals. Work
is now in progress to prepare a new class of organothiolates
and discover their reactivity.
Jadavpur University. We thank Mr. Vivek Dhyani, Perki-
nElmer India, for the GC/MS experiment. We thank Dr. K.
K. Rajak, Jadavpur University, for help in computations. B.A.
thanks the UGC, New Delhi, for his fellowship.
Supporting Information Available: Crystallographic data
(CIF) for 5, 6b, 7, and 8, structural data for 1 (R ) CH2Ph),
characterization data, and structural analysis of 8. This material is
available free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. We thank the Department of Science
and Technology, New Delhi, for financial assistance (Grant
SR/FTP/CS-20/2005), and data collection of two crystals on
CCD facility setup (IISc, Bangalore, India) under the IRPHA-
DST program. This research has also been funded by
IC7016006
438 Inorganic Chemistry, Vol. 47, No. 2, 2008