Synthesis of tetranuclear rhodium and iridium complexes directed by 6-mercaptopyridin-2-ol: Electrochemical behavior, chemical oxidation, and coordination chemistry

Article abstract of DOI:10.1021/ic9005289

The new ligand 6-mercapto-2(1 H)-pyridone (H₂PySO) has been prepared in good yield by reaction of 6-chloro-pyridin-2-ol with NaSH. Reaction of the salt K₂PySO, generated in situ, with the appropriate complex [M(μ-C1)(diolefin)]₂affords the tetranuclear complexes [M ₄(μ-PySO)2(diolefin)₄] [M = Rh, diolefin = 1,5-cyclooctadiene (cod) (1), tetrafluorobenzobarralene (tfbb) (2); M = lr, diolefin = cod (3)]. The molecular structure of complex 1 has been determined by X-ray diffraction methods. The tetranuclear structure is supported by two S, N, Otridentate ligands exhibiting a 1κO, 2κN, 3:4κ² S coordination mode. Carbonylation of the rhodium diolefin complexes at atmospheric pressure gives [Rh₄(μ-PyS0)2(C0)₈] (4). The carbonylation of 1 is partially reversible, and the mixed-ligand complex [Rh₄(μ-PySO)₂(cod)₂(CO)₄] (5) has been obtained as a single isomer. The reaction of 4 with triphenylphosphine gives the compound [Rh₄(μ-PySO)2(CO)₄(PPh ₃)₄] (6) which also exists as a single isomer of C ₂ symmetry. The diolefin complexes are redox active and exhibit two one-electron oxidations at a platinum disk electrode in dichloromethane separated by approximately 0.5 V at potentials accessible by chemical oxidants. The tetranuclear complexes were selectively oxidized to the 63-electron mixed-valence cationic complexes [M₄(μ-PySO)₂(diolefin) ₄]⁺(1a⁺, 2⁺, and 3⁺) by using AgCF₃SO₃ as oxidant and isolated as the triflate salts. Alternatively, the oxidation with [Cp₂Fe]PF₆ gives [Rh₄(μ-PySO)2(cod)₄][PF₆] (1b⁺). The parameters obtained from the simulation of the electron paramagnetic resonance spectra of the oxidized species strongly suggest that the unpaired electron is delocalized over only two metal atoms in the complexes.

Full text of DOI:10.1021/ic9005289

7984 Inorg. Chem. 2009, 48, 7984–7993
DOI: 10.1021/ic9005289
Synthesis of Tetranuclear Rhodium and Iridium Complexes Directed by
6-Mercaptopyridin-2-ol: Electrochemical Behavior, Chemical Oxidation, and
Coordination Chemistry
‡
†
†
†
,†
,†
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ꢀ
´
´
Pablo J. Alonso, Oscar Benedı, Marıa J. Fabra, Fernando J. Lahoz, Luis A. Oro,* and Jesus J. Perez-Torrente*
†
´
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ꢀ
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Departamento de Quımica Inorganica, Instituto Universitario de Catalisis Homogenea, Instituto de Ciencia de
‡
ꢀ
Materiales de Aragon, Universidad de Zaragoza-C.S.I.C., E-50009 Zaragoza, Spain, and Departamento de
´
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Fısica de la Materia Condensada, Instituto de Ciencia de Materiales de Aragon, Universidad de
Zaragoza-C.S.I.C., E-50009 Zaragoza, Spain
Received March 18, 2009
The new ligand 6-mercapto-2(1H )-pyridone (H2PySO) has been prepared in good yield by reaction of 6-chloro-pyridin-
2-ol with NaSH. Reaction of the salt K2PySO, generated in situ, with the appropriate complex [M(μ-Cl)(diolefin)]2
affords the tetranuclear complexes [M4(μ-PySO)2(diolefin)4] [M = Rh, diolefin = 1,5-cyclooctadiene (cod) (1),
tetrafluorobenzobarralene (tfbb) (2); M = Ir, diolefin = cod (3)]. The molecular structure of complex 1 has been
determined by X-ray diffraction methods. The tetranuclear structure is supported by two S,N,O-tridentate ligands
exhibiting a 1κO, 2κN, 3:4κ2S coordination mode. Carbonylation of the rhodium diolefin complexes at atmospheric
pressure gives [Rh4(μ-PySO)2(CO)8] (4). The carbonylation of 1 is partially reversible, and the mixed-ligand complex
[Rh4(μ-PySO)2(cod)2(CO)4] (5) has been obtained as a single isomer. The reaction of 4 with triphenylphosphine
gives the compound [Rh4(μ-PySO)2(CO)4(PPh3)4] (6) which also exists as a single isomer of C2 symmetry. The
diolefin complexes are redox active and exhibit two one-electron oxidations at a platinum disk electrode in
dichloromethane separated by approximately 0.5 V at potentials accessible by chemical oxidants. The tetranuclear
complexes were selectively oxidized to the 63-electron mixed-valence cationic complexes [M4(μ-PySO)2(diolefin)4]þ
(1aþ, 2þ, and 3þ) by using AgCF3SO3 as oxidant and isolated as the triflate salts. Alternatively, the oxidation with
[Cp2Fe]PF6 gives [Rh4(μ-PySO)2(cod)4][PF6] (1bþ). The parameters obtained from the simulation of the electron
paramagnetic resonance spectra of the oxidized species strongly suggest that the unpaired electron is delocalized
over only two metal atoms in the complexes.
Introduction
control the structure and more importantly, to impart the
necessary flexibility to adapt both to variations of metal-
metal separations and to the coordination geometry about
Polynuclear transition metal complexes have attracted
considerable attention because of their potential chemical
reactivity based on a cooperative action of the metal centers
and the promising applications as electronic and optoelec-
tronic materials and catalyst.1 The design of suitable poly-
dentate bridging ligands plays an important role in the
construction of polynuclear transition metal complexes to
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r
pubs.acs.org/IC
Published on Web 07/16/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7985
the metal atoms.2-5 In this context, oligo-(R-pyridyl)amido
ligands represent an outstanding example of ligand design for
the synthesis of extended metal atom chains (EMAC).3
Interestingly, some rigid tridentate ligands based on a 2,6-
difunctionalized pyridine scaffold, as, for example, 2,6-bis-
(diphenylphosphino)pyridine and 6-diphenylphosphino-2-
pyridonate, with a linear disposition of the P, N, P and P,
N, O donor atoms set imposed by the rigid pyridine and
pyridone frameworks, respectively, have also met success in
the stabilization of linear metal arrays.4,5
Scheme 1. Synthesis of Tetranuclear Complexes Directed by 2,6-
Dimercaptopyridine (a) and 6-Mercapto-pyridin-2-ol (b)
We have been interested for some time in the application of
small bite polydentate ligands for the construction of poly-
nuclear complexes.6 In particular, we have been intensively
exploring the potential of polydentate ligands containing
sulfur atoms in the donor set like pyridine-2-thiolate, ben-
zothiazole-2-thiolate, or benzimidazole-2-thiolate.7 This strat-
egy led us to study the coordination chemistry of the doubly
thiolate functionalized pyridine ligand 2,6-pyridinedithiolate
(PyS22-) and the synthesis of [M4(μ-PyS2)4(diolefin)4] (M=
Rh, Ir) tetranuclear complexes having an unusual structure
that results from the coordination of each tridentate 2,6-
pyridinedithiolate ligand to the four d8 metal centers arranged
in a bent-zigzag disposition.8 This coordination mode pro-
duces a tetranuclear framework that contains available co-
ordination donor sites on the peripheral sulfur atoms
oriented in a divergent fashion (Scheme 1a).
Metal containing building blocks are of particular interest
in supramolecular chemistry, and the utilization of molecular
complexes as ligands is a well-established methodology for
the formation of heterobimetallic extended solids.9 However,
transition-metal clusters and polynuclear complexes are
also an attractive class of structural and functional building
blocks in supramolecular chemistry.10 The ability of these
tetranuclear complexes to act as S,S coordination entities
have been demonstrated by the assembly of coordination
polymers using the complexes [Rh4(μ-pyS2)4(diolefin)4]
(diolefin=cod, tfbb) as building blocks. Thus, cationic one-
dimensional coordination polymers such as [AgRh4(μ-PyS2)2-
(diolefin)4]n[BF4]n and [Rh5(μ-PyS2)2(diolefin)5]n[BF4]n re-
sulted from the zigzag chain arrangement of alternating
[Rh4] tetranuclear units and Agþ (d10) and [Rh(diolefin)]þ
(d8) metal fragments.11 In addition, the determination of the
structure of the polymer [ClCuRh4(μ-PyS2)4(cod)4]n has
revealed that the self-assembly process of coordination poly-
mers containing trigonal-planar metal fragments as linkers is
chiroselective since the one-dimensional chains are formed
exclusively by homochiral [Rh4] building blocks.12
(5) (a) Mashima, K.; Shimoyama, Y.; Kusumi, Y.; Fukumoto, A.;
Yamagata, T.; Ohashi, M. Eur. J. Inorg. Chem. 2007, 235. (b) Ruffer, T.;
Ohashi, M.; Shima, A.; Mizomoto, H.; Kaneda, Y.; Mashima, K. J. Am. Chem.
Soc. 2004, 126, 12244. (c) Mashima, K.; Fukumoto, A.; Nakano, H.; Kaneda, Y.;
Tani, K.; Nakamura, A. J. Am. Chem. Soc. 1998, 120, 12151. (d) Mashima, K.;
Tanaka, M.; Tani, K.; Nakamura, A.; Takeda, S.; Mori, W.; Yamaguchi, K. J. Am.
Chem. Soc. 1997, 119, 4307. (e) Mashima, K.; Nakano, H.; Nakamura, A. J. Am.
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L. A.; Lanfranchi, M.; Tiripicchio, A.; Tiripicchio-Camellini, M. Inorg.
Chem. 1998, 37, 3954. (b) Tejel, C.; Villarroya, B. E.; Ciriano, M. A.; Oro, L. A.;
Lanfranchi, M.; Tiripicchio, A.; Tiripicchio-Camellini, M. Inorg. Chem. 1996,
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35, 4360. (c) Ciriano, M. A.; Perez-Torrente, J. J.; Lahoz, F. J.; Oro, L. A. Inorg.
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Chem. 1992, 31, 969. (d) Ciriano, M. A.; Perez-Torrente, J. J.; Oro, L. A.;
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225. (e) Ciriano, M. A.; Perez-Torrente, J. J.; Viguri, F.; Lahoz, F. J.; Oro, L. A.;
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The tetranuclear complexes [M4(μ-PyS2)4(cod)4] behave as
encapsulating agents for the thallium(I) ion through the
formation of cationic pentametallic species [TlM4(μ-PyS2)4-
(cod)4]þ (M=Rh, Ir) involving a structural reorganization of
the molecular framework that makes possible the existence of
d8-s2-d8 bonding interactions.13 Besides, these compounds
are redox active and the 63-electron mixed-valence paramag-
netic complexes [M4(μ-PyS2)4(cod)4]þ are easily obtained by
chemical oxidation of the corresponding tetranuclear com-
plexes using mild one-electron oxidants.14
On the basis of this interesting coordination chemistry
exhibited by these tetranuclear complexes, we have envisaged
the design of tetranuclear O,O coordination entities as struc-
tural motifs for supramolecular chemistry. These new tetra-
nuclear complexes should be accessible from the doubly depro-
tonated 6-mercaptopyridin-2-ol (H2PySO) ligand (Scheme 1b).
ꢀ
1493. (f) Ciriano, M. A.; Viguri, F.; Perez-Torrente, J. J.; Lahoz, F. J.; Oro, L. A.;
Tiripicchio, A.; Tiripicchio-Camellini, M. J. Chem. Soc., Dalton Trans. 1989,
25.
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(8) Perez-Torrente, J. J.; Casado, M. A.; Ciriano, M. A.; Lahoz, F. J.;
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Coord. Chem. Rev. 2006, 250, 2501. (c) Przychodzen, P.; Korzeniak, T.;
Podgajny, R.; Sieklucka, B. Coord. Chem. Rev. 2006, 250, 2234. (d) Sieklucka,
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7986 Inorganic Chemistry, Vol. 48, No. 16, 2009
Alonso et al.
Herein we report on the synthesis of 6-mercaptopyridin-2-ol
and their application for the construction of new redox-active
[M4(μ-PySO)2L4] tetranuclear complexes structurally related
to [M4(μ-PyS2)4L4] (M=Rh, Ir). In addition, a comparison
of the chemical behavior of both types of complexes is
described.
Scheme 2. Synthesis of Rhodium Diolefin Complexes [Rh4( μ-PySO)2-
(diolefin)4], Carbonylation, and Replacement Reactions
Results and Discussion
Synthesis and Properties of 6-Mercapto-pyridin-2-ol.
Several mercaptopyridinol derivatives have been de-
scribed but only the coordination chemistry of 2-mercap-
topyridin-3-ol has been investigated.15 In contrast, the
compound 6-mercaptopyridin-2-ol was mentioned but
no experimental details were reported.16 The ligand
6-mercaptopyridin-2-ol has been prepared by reaction
of the commercially available 6-chloropyridin-2-ol with a
moderate excess of NaSH in refluxing N,N-dimethylfor-
mamide (DMF). The compound was isolated as a hygro-
scopic pale-yellow solid in good yield and has been fully
characterized by HRMS, IR, and NMR.
Although 6-mercaptopyridin-2-ol can be in equilibri-
um with the amide and thioamide tautomers,17 namely,
6-mercapto-2(1H)-pyridone and 6-hydroxy-2(1H)-pyri-
1
dinethione respectively, the H NMR in DMSO-d6 at
CDCl3. These data indicate the chemical equivalence of
both PySO2- ligands and the presence of two types of
diolefin ligands, facts that are compatible with the ex-
istence of a C2 axis in the molecule.
room temperature (RT) showed exclusively the presence
of the 6-mercapto-2(1H)-pyridone tautomer. In addition,
the IR spectrum showed several strong absorptions in the
region 1650-1530 cm-1 that suggests the presence of this
tautomer also in the solid state. Interestingly, the com-
pounds 2-mercaptopyridin-3-ol and 3-mercaptopyridin-
2-ol also exist mainly as the tautomers 3-hydroxy-2(1H)-
pyridinethione18 and 3-mercapto-2(1H)-pyridone,19 re-
spectively.
Synthesis and Characterization of Tetranuclear Diolefin
Complexes. The reaction of 6-mercaptopyridin-2-ol
(H2PySO) with two molar-equiv of a solution of KOH
in methanol gave a pale yellow solution of the salt
K2PySO. Further reaction with a dichloromethane solu-
tion of complex [Rh(μ-Cl)(cod)]2 gave an orange-red
suspension of the tetranuclear complex [Rh4(μ-PySO)2-
(cod)4] (1) and KCl as byproduct (Scheme 2). Complex 1
was isolated as a red microcrystalline solid in high yield
after the separation of the KCl by extraction of the crude
with dichloromethane.
The related diolefin complexes [Rh4(μ-PySO)2(tfbb)4]
(2) and [Ir4(μ-PySO)2(cod)4] (3) were obtained as purple
and green microcrystalline solids, respectively, in good
yield following a similar synthetic procedure. The tetra-
nuclear formulation of the complexes is supported by the
microanalysis data and the FAB mass spectra, which
showed the molecular ions at the expected values of m/z
(1566 and 1452, respectively). In addition, the NMR data
strongly suggest that 1-3 are isostructural. In particular,
the two PySO2- ligands were found to be equivalent and
two types of diolefin ligands were observed both in the 1H
and 13C{1H} NMR spectra, in agreement with structures
with C2 symmetry.
Assuming that the tetranuclear complexes [M4(μ-PySO)2-
(diolefin)4] and [M4(μ-PyS2)2(diolefin)4] (M=Rh, Ir) are
isostructural, there are for the former complex two dif-
ferent coordination modes possible for the S,N,O-triden-
tate PySO2- ligands that would result in tetranuclear
structures of C2 symmetry: the 1κO, 2κN, 3:4κ2S coordi-
nation mode that produces tetranuclear complexes hav-
ing oxygen atoms on the periphery, and the less probable
1κS, 2κN, 3:4κ2O that involve an oxygen atom coordi-
nated in a μ2 fashion. To establish the coordination mode
of the bridging ligands in the tetranuclear complexes,
the molecular structure of complex 1 has been studied by
X-ray diffraction methods.
The tetranuclear formulation of 1 is supported by the
FABþ mass spectrum in which the molecular ion was
observed at m/z 1094 (100%). The aromatic region of the
1H NMR spectrum showed three characteristic reso-
nances (d, dd and d) for the aromatic protons of both
PySO2- ligands. In addition, the olefinic protons and
carbons (=CH) of the four cod ligands were observed as
eight resonances in the 1H and 13C{1H} NMR spectra in
(15) (a) Reynolds, J. G.; Sendlinger, S. C.; Murray, A. M.; Huffman, J. C.;
Christou, G. Inorg. Chem. 1995, 34, 5745. (b) Bodini, M. E.; Estefo, C.; Del
Valle, M. A. Polyhedron 1992, 11, 2203. (c) Wen, T. B.; Shi, J. C.; Huang, X.;
Chen, Z. N.; Liu, Q. T.; Kang, B. S. Polyhedron 1988, 17, 331. (d) Howlin, B.;
Hider, R. C.; Silver, J. J. Chem. Soc., Dalton Trans. 1982, 1433.
(16) Sasaki, H.; Watanabe, H. Fuji Photo Film Co., Ltd., Japan Patent JP
9211811, 1997.
(17) (a) Moran, D.; Sukcharoenphon, K.; Puchta, R.; Schaefer, H. F., III;
Schleyer, P. V.; Hoff, C. D. J. Org. Chem. 2002, 67, 9061. (b) Wong, M. W.;
Wiberg, K. B.; Frish, M. J. J. Am. Chem. Soc. 1992, 114, 1645.
(18) Sato, N.; Nagano, E. J. Heterocyclic Chem. 1993, 30, 691.
(19) Smith, K.; Anderson, D.; Matthews, I. J. Org. Chem. 1996, 61, 662.
Molecular Structure of [Rh4(μ-PySO)2(cod)4] (1). The
molecular framework of the compound consists of four
rhodium atoms, arranged in a zigzag chain disposition,
supported by two S,N,O-tridentate doubly deprotonated
6-mercaptopyridin-2-ol ligands acting as six electron
donors (Figure 1). The molecule exhibits a crystallogra-
phic imposed C2 symmetry, with two “Rh2(μ-PySO)-
(cod)2” moieties forming the tetranuclear complex. Each
mercaptopyridinol ligand is bridging two rhodium atoms

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7987
these sulfur atoms. On the other hand, the exocyclic C-O
distances of 1.299(5) A suggest weak electron delocaliza-
tion of the oxygen lone pairs into the aromatic ring.
Synthesis and Characterization of Rhodium Tetranuc-
lear Carbonyl Complexes. The reaction of complex [Rh4-
(μ-PySO)2(cod)4] (1) with carbon monoxide under atmo-
spheric pressure in dichloromethane gave an air-sensitive
deep violet solution of complex [Rh4(μ-PySO)2(CO)8] (4)
(Scheme 2). However, the isolation of 4 from this solution
was not possible because of the interference of the re-
placed cod in the reaction media. An alternative synthetic
approach is the reaction of the salt K2PySO with the
dinuclear complex [Rh(μ-Cl)(CO)2]2 under a carbon
monoxide atmosphere. The compound was isolated as
fairly stable violet microcrystals after the separation of
the KCl byproduct.
The FABþ spectrum of compound 4 showed the
molecular ion at m/z 885 with additional peaks resulting
from the sequential loss of all the carbonyl ligands, which
confirms that the nuclearity is maintained upon carbo-
nylation. The available spectroscopic data indicate that 4
also has C2 symmetry indicating that its structure is
identical to those of the parent diolefin complex replacing
each diolefin ligand by two carbonyl groups. Thus, in the
13C{1H} NMR spectrum the expected four resonances for
the terminal carbonyl ligands (JRh-C=72-76 Hz) were
observed between δ 185-180 ppm. The aromatic protons
of the equivalent PySO2- ligands showed two resonances
at δ 6.82 and 5.98 (dd) ppm (2:1 ratio) in CDCl3. The
complexity of the resonance at δ 6.82 ppm is a conse-
quence of second order effects, and standard coupling
constants were obtained from the simulated spectrum (see
Experimental Section).
We have found that the carbonylation of complex
[Rh4(μ-PySO)2(cod)4] (1) is partially reversible. When a
freshly prepared dichloromethane solution of complex 4,
obtained from the carbonylation of complex 1, was
stirred under argon a deep blue solution of complex
[Rh4(μ-PySO)2(cod)2(CO)4] (5) was obtained. The for-
mation of 5 is a consequence of the replacement of four
carbonyl ligands by two molecules of cod in 4. Obviously,
complex 4 can also be obtained by carbonylation of 5
(Scheme 2). The tetranuclear formulation of compound 5
relies on the FABþ mass spectrum, were the molecular
ion was observed at m/z 990. Although 5 can exist as
four different isomers resulting from the location of the
cod and carbonyl ligands in the molecular framework,
the compound is obtained as a single isomer having C2
symmetry. Thus, equivalent PySO2- and cod ligands were
observed both in 1H and 13C{1H} NMR spectra, whereas
the four carbonyl ligands exhibited two doublets at δ
187.6 (JRh-C=64 Hz) and 184.7 ppm (JRh-C=71 Hz) in
the 13C{1H} NMR spectrum in CDCl3.
Figure 1. Molecular structure of compound [Rh4(μ-PySO)2(cod)4] (1).
Selected bond distances (A) and angles (deg): Rh(1)-S 2.3890(10), Rh-
(1)-O 2.110(3), Rh(1)-C(6) 2.129(4), Rh(1)-C(7) 2.138(4), Rh(1)-
C(10) 2.085(4), Rh(1)-C(11) 2.093(4), Rh(2)-S 2.3716(10), Rh(2)-N
2.093(3), Rh(2)-C(14) 2.139(4), Rh(2)-C(15) 2.134(4), Rh(2)-C(18)
2.137(4), Rh(2)-C(19) 2.142(4); S-Rh(1)-O 96.86(8), S-Rh(2)-N
86.97(9), S-Rh(1)-M(1) 176.65(11), S-Rh(2)-M(3) 170.53(13), S-
Rh(1)-M(2) 90.07(12), S-Rh(2)-M(4) 94.4(6), O-Rh(1)-M(1)
85.26(13), N-Rh(2)-M(3) 91.28(14), O-Rh(1)-M(2) 170.59(13), N-
Rh(2)-M(4) 178.6(8), Rh(1)-S-Rh(2) 83.52(3), Rh(1)-O-C(1)
128.2(2) (M labels represent the midpoints of the olefinic bonds).
through the pyridinic nitrogen and oxygen donor atoms
(1κO, 2κN) and connecting the other two metals through
a sulfur atom coordinated in a μ2 fashion (3:4κ2S).
All the rhodium atoms exhibit slightly distorted square-
planar environments resulting from their coordination to
two donor atoms of different bridging ligands and a cod
molecule chelated through the two olefinic bonds. In
agreement with the spectroscopic information obtained
in solution, the solid state structure displays two different
metal co-ordinations: the external rhodium centers,
which are bonded to sulfur and to an oxygen atom, and
the internal ones, which are linked to nitrogen and sulfur
donor atoms. The differences of these two metal co-
ordination spheres are slightly evidenced in the dissimilar
Rh-S bond distances, 2.3890(10) A and 2.3716(11) A,
but markedly in the bonding of the olefin trans to the
oxygen of the bridging ligand (Rh-midpoint distances
1.969(4) vs 2.021(3) A).
The whole tetranuclear molecule closely resembles the
related 2,6-pyridinedithiolate analogue, [Rh4(μ-PyS2)2-
(cod)4].8 As in this previous structure, the external inter-
metallic Rh(1) Rh(2) separation is the shortest one,
3.1708(5) A, being very similar to that reported in the
dithiolate complex, 3.1435(5) A; however, with no appar-
3 3 3
ent reason, the internal Rh(2) Rh(20) distance is mark-
3 3 3
edly shorter in 1, 3.5028(5) A, than in the dithiolate
analogue, 3.9210(6) A.8 Within the metal core the torsion
angle Rh(1)-Rh(2)-Rh(20)-Rh(10) is 108.11(1)°, while
in the dithiolate complex this parameter becomes
125.84(1)°.
The exocyclic C(5)-S distance, 1.777(4) A, comparable
to those observed for the μ2-S atoms in [Rh4(μ-PyS2)2-
(cod)4], 1.789(4) A,8 and in the hexanuclear structur-
ally related [(PPh3)2Au2)Rh4(μ-PyS2)2(tfbb)4] complex,
1.757(10) and 1.790(10) A,11 is slightly shorter than
typical C-S single bonds (1.83(3) A in alkanethiolates,20
for instance) and indicative of the thiolate character of
The two structures compatible with the spectroscopic
data are shown in Figure 2. Unfortunately, neither the
magnitude of the JRh-C coupling constant nor the che-
mical shifts of the olefinic resonances (δ 4.98, 4.85, 4.28,
and 3.27 ppm) are fully reliable parameters to make a
precise structural assignment. However, the pattern of
resonances compares well with that observed for the cod
ligands coordinated to the outer rhodium atoms in the
complex [Ir4(μ-PyS2)2(cod)4], if the shielding effect in-
duced by the O atom trans to one of the CdC bonds is
(20) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D.;
Taylor, R. J. Chem. Soc., Dalton Trans. 1991, S1.

7988 Inorganic Chemistry, Vol. 48, No. 16, 2009
Alonso et al.
Table 1. Redox Potentials (E° vs SCE, in V) and Peak-to-Peak Separation (ΔEp
in mV) for Complexes [M4( μ-PySO)2(diolefin)4] and [M4( μ-PyS2)2(diolefin)4]8 in
0.1 M TBAH/CH2Cl2 at 100 mV s-1
[M4]f[M4]þ
[M4]þf[M4]2þ
compound
E° (V) ΔEp (mV) E° (V) ΔEp (mV)
[Rh4( μ-PySO)2(cod)4] (1)
[Rh4( μ-PySO)2(tfbb)4] (2)
[Ir4( μ-PySO)2(cod)4] (3)
0.25
0.49
0.12
78
0.77
0.84
0.64
0.90
0.58
0.79
0.53
84
Figure 2. Possible C2 isomers of compound [Rh4(μ-PySO)2(cod)2-
(CO)4] (5).
100
105
95
63
80
110
102
105
70
80
80
taken into consideration.21 The proposed structure
(Figure 2a) is probably also the preferred on steric
grounds as the cod ligands are located further apart into
the tetranuclear framework.
[Rh4( μ-PySO)2(cod)2(CO)4] (5) 0.41
[Rh4( μ-pyS2)2(cod)4]
[Rh4( μ-pyS2)2(tfb)4]
[Ir4( μ-pyS2)2(cod)4]
0.16
0.37
0.08
79
The reaction of [Rh4(μ-PySO)2(CO)8] (4), obtained by
carbonylation of complex 1 in dichloromethane, with
four molar-equiv of triphenylphosphine gave the com-
pound [Rh4(μ-PySO)2(CO)4(PPh3)4] (6) which was iso-
lated as an air-sensitive violet solid in good yield
(Scheme 2). Compound 6 can be alternatively prepared
by reaction of trans-[RhCl(CO)(PPh3)2] with the salt
K2PySO (2:1 molar ratio). The spectroscopic data for 6
indicate than the compound is tetranuclear and exists as a
single isomer with C2 symmetry. The molecular ion was
observed in the FABþ mass spectrum at m/z 1822, and
the broad band at 1982 cm-1 in the IR spectrum evide-
nced that each rhodium center is coordinated both to a
carbonyl and a triphenylphosphine ligand. The 31P{1H}
NMR spectrum of 6 consisted of two doublets at δ 39.76
and 38.69 ppm (JRh-P=163 and 175 Hz, respectively) as
expected for a C2 symmetry. The proposed structure for
complex 6 is shown in Scheme 2 and is based on the
electronic and steric effects influencing the substitution
processes. In particular, the replacement of the carbonyl
ligands trans to the sulfur atoms in the two inner rhodium
centers is driven by the trans effect of the sulfur atom22
and is in agreement with the substitution pattern found in
related dinuclear complexes having short-bite N,S biden-
tate ligands.23 However, the values of the observed JRh-P
coupling constants strongly suggest that the substitution
on the external rhodium atoms takes place trans to
the oxygen atoms. Typical values of JRh-P for P trans to
S in mono- or dinuclear Rh(I) complexes are around
160 Hz7f,8,23a whereas the JRh-P for P trans to O are
175 Hz.24 This proposal is also supported by steric con-
siderations since the substitution trans to the oxygen
atoms place further apart the PPh3 ligands on neighbor-
ing rhodium atoms.
chemically reversible one-electron charge transfers that
can be assigned to the electrogeneration of the mono
[M4(μ-PySO)2(diolefin)4]þ and dicationic [M4(μ-PySO)2-
(diolefin)4]2þ (M=Rh, Ir) species, respectively. The formal
electrode potentials (E°) for the processes [M4] f [M4]þ
and [M4]þ f [M4]2þ are shown in Table 1. In addition, the
complexes 1 and 3 displayed one additional irreversible
anodic process at 1.18 and 1.17 V, respectively. The
analysis of both waves in complex [Rh4(μ-PySO)2(cod)4]
(1) with scan rates varying from 0.05 to 0.20 V s-1 was
indicative of their reversible character.25 However, the
ΔEp of both waves in 2, 3, and 5 increased by 20-30 mV
when the scan rate was increased from 0.05 to 0.20 V s-1
,
which is a diagnostic of an electronically quasi-reversible
one-electron redox processes.26
The formal electrode potentials are strongly dependent
on the nature of both the metallic center and the auxiliary
ligands. Thus, the formal electrode potentials in complex
[Rh4(μ-PySO)2(tfbb)4] (2) were anodically shifted 240
and 70 mV from those observed for [Rh4(μ-PySO)2-
(cod)4] (1) which is in agreement with a reduction of the
electronic density on the rhodium atoms because of the
greater electron-withdrawing character of the tfbb li-
gands. This effect was also observed in complex [Rh4(μ-
PySO)2(cod)2(CO)4] (5) which exhibites oxidation waves
anodically shifted 160 and 130 mV with reference to those
of compound 1, a fact that is fully consistent with the
presence of four π-acid carbonyl ligands. Both oxidation
waves in compound [Ir4(μ-PySO)2(cod)4] (3) are catho-
dically shifted 130 mV compared with those of compound
1 as expected for the easiest oxidation of the iridium
centers.27 Interestingly, similar trends have been observed
along the series of complexes [M4(μ-PyS2)2(diolefin)4]
(M=Rh, Ir; diolef=cod, tfbb) although as can be obser-
ved in Table 1, in general, the complexes [M4(μ-PySO)2-
(diolefin)4] were found harder to oxidize. The larger
differences in the formal electrode potentials, 90 and
190 mV, were observed between complex 1 and [Rh4(μ-
PyS2)2(cod)4]. This observation can be rationalized by
the superior donor ability of sulfur versus oxygen that
facilitates the elimination of electrons from the metal
centers.
Electrochemical Properties of the Tetranuclear Diolefin
Complexes. The electrochemical behavior of complexes
1-3 and 5 is similar to that exhibited by the tetranuclear
complexes supported by 2,6-pyridinedithiolato ligands.
The cyclic voltammograms (CV) of the complexes re-
corded in dichloromethane at 100 mV s-1 showed two
(21) Rodman, G. S.; Mann, K. R. Inorg. Chem. 1988, 27, 338.
(22) Botha, L. J.; Basson, S. S.; Leipoldt, J. G. Inorg. Chim. Acta 1987,
126, 25.
ꢀ
(23) (a) Ciriano, M. A.; Perez-Torrente, J. J.; Lahoz, F. J.; Oro, L. A.
(25) (a) Brown, E. R.; Sandifer, J. R. In Physical Methods of Chemistry:
Electrochemical Methods; Rossiter, B.W., Hamilton, J. F., Eds.; Wiley: New
York, 1986; Vol. 2. (b) Piraino, P.; Bruno, G.; Tresoldi, G.; Lo Schiavo, S.;
Zanello, P. Inorg. Chem. 1987, 26, 91.
J. Organomet. Chem. 1993, 455, 225. (b) Claver, C.; Kalck, P.; Ridmy, M.;
Thorez, A.; Oro, L. A.; Pinillos, M. T.; Apreda, M. C.; Cano, F. H.; Foces-Foces,
C. J. Chem. Soc., Dalton Trans. 1988, 1523.
(24) (a) Conradie, J.; Lamprecht, G. J.; Otto, S.; Swarts, J. C. Inorg. Chim.
Acta 2002, 328, 191. (b) Purcell, W.; Basson, S. S.; Leipoldt, J. G.; Roodt, A;
Preston, H. Inorg. Chim. Acta 1995, 234, 153. (c) Trzeciak, A. M.; Ziolkowski,
J. J. J. Organomet. Chem. 1992, 429, 239.
(26) Geiger, W. E., Jr. Prog. Inorg. Chem. 1985, 23, 275.
(27) (a) Connelly, N. G.; Garcia, G. J. Chem. Soc., Dalton Trans. 1987,
2737. (b) Connelly, N. G.; Garcia, G.; Gilbert, M.; Stirling, J. S. J. Chem. Soc.,
Dalton Trans. 1987, 1403.

Article
Interestingly the two reversible one-electron processes
Inorganic Chemistry, Vol. 48, No. 16, 2009 7989
in the [M4(μ-PySO)2(diolefin)4] complexes occur at for-
mal electrode potentials that are accessible by chemical
reagents. In addition, the large difference between the
formal potentials of both oxidation processes (350-
520 mV) ensures that the [M4]þ species should be stable
with respect to disproportionation into the [M4] and
[M4]2þ complexes. In fact, the calculated Kdisp values
for the disproportionation equilibria are in the range
10-6-10-9 28
.
Synthesis of Paramagnetic Tetranuclear Complexes by
Chemical Oxidation. The oxidation of the tetranuclear
complexes with mild one-electron oxidants resulted in the
formation of cationic 63-electron mixed-valence para-
magnetic complexes [M4(μ-PySO)2(diolefin)4]þ. The re-
action of complexes [M4(μ-PySO)2(diolefin)4] (1-3) with
one molar equiv of AgCF3SO3 in dichloromethane gave
the cationic complexes [Rh4(μ-PySO)2(cod)4][CF3SO3]
(1aþ), [Rh4(μ-PySO)2(tfbb)4][CF3SO3] (2þ), and [Ir4(μ-
PySO)2(cod)4][CF3SO3] (3þ) which were isolated as deep
colored microcrystalline solids in good yield. Alterna-
tively, compound [Rh4(μ-PySO)2(cod)4][PF6] (1bþ) can
be obtained by reaction of 1 with [Cp2Fe]PF6 in dichlor-
omethane (1:1 molar ratio).
Figure 3. Q-band (34 GHz) EPR spectra of polycrystalline powdered
sample of 1aþ and 2þ. Dotted lines correspond to calculated spectra with
the parameters given in Table 2 (see text for details). These last ones have
been shifted down a 25% of their whole amplitude.
The paramagnetic complexes 1þ, 2þ, and 3þ have been
characterized by elemental analyses, FABþ, and electron
paramagnetic resonance (EPR) spectroscopy. The com-
plexes behave as 1:1 electrolytes in acetone and display the
peaks corresponding to molecular ions [M4(μ-PySO)2-
(diolefin)4]þ in the FABþ mass spectra. Furthermore,
the formulation of complexes as paramagnetic mono-
oxidized species was established electrochemically by
linear voltammetry at RDE.
EPR spectra of polycrystalline samples of 1aþ, 2þ, and
3þ have been measured in X- and Q-band spectra at RT.
The X-band spectra consist of a slightly asymmetric line
centered at a g-value between 2.1 and 2.3. On the other
hand two clearly resolved features appear in the Q-band
spectra (see Figures 3 and 4) suggesting that the local
symmetry is close to axial. In fact the spectra can be
associated to an S=1/2 paramagnetic entity and a slightly
orthorhombic g-tensor has to be considered. So, for
describing them, the following spin-Hamiltonian has
been used:
Figure 4. Q-band (34 GHz) EPR spectrum of polycrystalline powdered
sample of 3þ. Dotted line corresponds to the calculated spectrum with the
parameters given in Table 2 (see text for details). This last one has been
shifted down a 25% of its whole amplitude.
Table 2. Principal Values of the g-Tensora, Isotropic Contribution to the g-tensor
(g0), and Halfwidth of the Lorentzian Lineshape used in the Simulation Spectra of
the Complexes [M4(μ-PySO)2(diolefin)4]þ
compound
gx, Wx (mT)
gy, Wy (mT)
gz, Wz (mT)
g0
H ¼ μBBfgxlxSxþgylySyþgzlzSzg
1aþ
2þ
2.145, 10.6
2.100, 5.0
2.201, 13.0
2.158, 6.5
2.108, 4.5
2.221, 15.0
2.205, 6.3
2.145, 3.0
2.371, 13.0
2.169
2.118
2.264
3þ
where μB is the Bohr magneton, B is the strength of the
applied magnetic field and (lx, ly, lz) are the director
cosines that the magnetic field makes with the principal
axis of the g-tensor whose principal values are gx, gy, gz.
To estimate these values several spectra have been
calculated taking a Lorentzian line shape with an aniso-
tropic halfwidth. In this way the values given in Table 2
have been obtained, and the calculated spectra are repre-
sented by dotted lines in Figures 3 and 4. The values of the
isotropic contribution of the g-tensor, g0=(gx þ gy þ gz)/3,
are also given in Table 2.
a The estimated uncertainty in the g-values is ( 0.008 in all the cases.
reported for [Rh4(μ-PyS2)2(cod)4]þ (g0=2.16 ( 0.01) and
[Rh4(μ-PyS2)2(tfbb)4]þ (g0=2.11 ( 0.01).14 In particular,
when a frozen solution of [Rh4(μ-PyS2)2(cod)4]þ was
measured a hyperfine structure corresponding to an
interaction with two equivalent 103Rh nuclei was re-
solved. That was interpreted considering that the un-
paired electron was mainly located in two equivalent
rhodium centers. The similar value of g0 strongly suggests
that the same occurs in 1aþ. A trickier fact is the different
g0 value in 1aþ and 2þ, suggesting a higher unpaired
electron delocalization when tfbb ligand replaces cod.
Although the lack of resolved hyperfine structure in 2þ as
It is interesting to note that the g0 value obtained for
1aþ and 2þ (Table 2) was similar to that previously
ꢀ
(28) Gagne, R. R.; Spiro, C. L.; Smith, T. J.; Hamann, C. A.; Thies, W. R.;
Shiemke, A. K. J. Am. Chem. Soc. 1981, 103, 4073.

7990 Inorganic Chemistry, Vol. 48, No. 16, 2009
Alonso et al.
well as in [Rh4(μ-PyS2)2(tfbb)4]þ, even in a frozen solu-
tion, prevent the determination of the number of metal
centers sharing the unpaired electron; the decrease of the
g0 value as compared with that found in 1aþ and [Rh4(μ-
PyS2)2(cod)4]þ could be a consequence of the different
electron acceptor character of the diolefin ligand, higher
in tfbb than in cod. This also can account for the decrease
of the g-tensor anisotropy.
On the other hand, the observed decrease of the g-
tensor anisotropy in complexes [Rh4(μ-PySO)2(diolefin)4]þ
when compared with [Rh4(μ-PyS2)2(diolefin)4]þ could be
associated with some structural modification, probably
related with the metal-metal separation, driven by the
different bridging ligands (μ-PySO and μ-PyS2) because
the g0 value does not change. In the case of the related
iridium tetranuclear compounds, the differences ob-
served between [Ir4(μ-PyS2)2(cod)4]þ and [Ir4(μ-PySO)2-
(cod)4]þ can be explained similarly.
The reaction of [Rh4(μ-PySO)2(cod)4] (1) with the
solvated species [Au(PPh3)(Me2CO)X]þ in acetone/dichlo-
romethane gave a deep-red solution. The 1H NMR
spectrum of the isolated red-brick solid in CDCl3 at 223 K
showed six well-defined resonances of equal intensity for
the PySO2- bridging ligands which is compatible with the
formation of the pentanuclear cation [(PPh3)AuRh4(μ-
PySO)2(cod)4]þ (7). However, we observed that com-
pound 1 crystallized out from these solutions which is
an indication of the weak coordination of the fragment
[Au(PPh3)]þ to one of the O donor sites in 1. In contrast,
the protonation of complex 1 with CF3SO3H in CH2Cl2
resulted in the breakdown of the tetranuclear structure
and the formation of unidentified species.
Concluding Remarks
We have described the high yield synthesis of rhodium
and iridum tetranuclear complexes [M4(μ-PySO)2(diolefin)4]
(M=Rh, Ir; diolefin=cod, tfbb) from the doubly deproto-
nated form of the new ligand 6-mercapto-2(1H)-pyridone.
The C2 symmetry tetranuclear structure is supported by two
S,N,O-tridentate ligands acting as six electron donor ligands.
The coordination mode of the ligand determines the forma-
tion of the isomer having two peripheral oxygen donor atoms
oriented in a divergent fashion. However, both sites have a
marginal nucleophilic character and only a weak coordina-
tion with selected metalion fragmentshas been observed. The
tetranuclear complexes are redox-active species that undergo
two stepwise one-electron oxidation processes, and the mono-
oxidized mixed-valence paramagnetic tetranuclear species
have been prepared by chemical oxidation. Interestingly,
the integrity of the rhodium tetranuclear framework is also
sustained upon carbonylation, and the carbonyl complex
undergoes stereoselective replacement reactions by 1,5-cy-
clooctadiene and triphenylphosphine ligands.
The chemical reversibility associated to the redox pro-
cesses leading to the oxidized species [M4(μ-PySO)2-
(diolefin)4]þ and [M4(μ-PySO)2(diolefin)4]2þ suggests that
the tetranuclear framework is maintained upon oxida-
tion. Thus, the main structural changes probably affect
the metal-metal separations as it has been found in
related di- and trinuclear complexes.29 An extended
::
Huckel molecular orbital calculation carried out on
complex [Rh4(μ-PyS2)2(cod)4] showed the antibonding
character of the highest occupied molecular orbital
(HOMO) in the tetranuclear species, and thus a short-
ening of the metal-metal distances upon oxidation
should be also expected in the complexes [M4(μ-PySO)2-
(diolefin)4]. Unfortunately, we have failed to obtain good
quality crystals of any of the oxidized species, even using
different anions, to confirm this result. In addition,
attempts to prepared the dicationic complexes [M4(μ-
PySO)2(diolefin)4]2þ using silver salts as oxidants were
unsuccessful.
Experimental Section
Coordination Chemistry of the complexes [M4(μ-
pySO)2L4]. The coordination mode of the 6-mercapto-
pyridin-2-ol ligands in the complexes [M4(μ-PySO)2L4]
(M=Rh, Ir) produces a tetranuclear framework with two
oxygen atoms oriented in a divergent fashion available for
coordination (Scheme 1b). Although these complexes are
potential O,O donor entities they have shown weaker
coordination ability than the S,S donor [M4(μ-PyS2)2L4]
tetranuclear counterparts, previously reported.11,12 Thus,
the compound [Rh4(μ-PySO)2(cod)4] (1) failed to react
with the species [Rh(cod)(NCCH3)2]BF4 that has two
labile acetonitrile ligands. However, evidence of a weak
interaction between and 1 and Tlþ was obtained from 1H
NMR data in solution (CDCl3) because a pronounced
low field shift (0.22 ppm) of the resonance attributable to
the meta protons adjacent to the oxygen atoms of the
bridgind ligands has been observed.30
General Methods. All manipulations were performed under a
dry nitrogen atmosphere using Schlenk-tube techniques. Sol-
vents were dried by standard methods and distilled under argon
immediately prior to use. Standard literature procedures were
used to prepare the starting materials [Rh(μ-Cl)(diolefin)]2
(diolefin=cod,31 tfbb),32 [Ir(μ-Cl)(cod)]2,33 [Rh(μ-Cl)(CO)2]2,34
trans-[RhCl(CO)(PPh3)2],35 [AuCl(PPh3)],36 and [Cp2Fe][PF6].37
AgCF3SO3 and 6-chloro-2-pyridinol were purchased from Flu-
ka Chem. and Aldrich, respectively. NaSH H2O (Aldrich) was
3
used immediately on delivery.
Physical Measurements. IR spectra were recorded on a Per-
kin-Elmer FT-IR Spectrum One. Elemental C, H, N, and S
analysis were performed in a 240-C Perkin-Elmer microanaly-
zer. Conductivities were measured in about 5 ꢀ 10-4 M acetone
solutions using a Philips PW 9501/01 conductimeter. Mass
spectra were recorded in a VG Autospec double-focusing mass
spectrometer operating in the FAB or EI modes. The ions were
produced by the standard Csþ gun at about 30 KV; 3-nitroben-
zyl alcohol (NBA) was used as matrix. Electrospray mass
spectra (ESI-MS) were recorded in methanol on a Bruker
1
MicroTof-Q using sodium formiate as reference. H, 13C{1H},
(29) (a) Connelly, N. G.; Hayward, O. D.; Klangsinsirikul, P.; Orpen,
A. G.; Rieger, P. H. Chem. Commun. 2000, 963. (b) Villarroya, B. E.; Oro,
L. A.; Lahoz, F. J.; Edwards, A. J.; Ciriano, M. A.; Alonso, P. J.; Tiripicchio, A.;
Tiripicchio-Camellini, M. Inorg. Chim. Acta 1996, 250, 241. (c) Boyd, D. C.;
Neil, G.; Connelly, N. G.; Garcia Herbosa, G.; Hill, M. G.; Mann, K. R.; Mealli,
C.; Orpen, A. G.; Richardson, K. E.; Rieger, P. H. Inorg. Chem. 1994, 33, 960.
(30) (a) Isab, A. A.; Wazeer, M. I. M. Spectrochim. Acta A 2006, 65, 191.
(b) Alizadeh, N.; Bordbar, M.; Shamsipur, M. Bull. Chem. Soc. Jpn. 2005, 78,
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Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7991
and 31P{1H} NMR spectra were recorded on a Varian Gemini
300 spectrometer operating at 300.08, 75.46, and 121.47 MHz
respectively. Chemical shifts are reported in ppm and referenced
to SiMe4 using the residual resonances of the deuterated solvents
(1H and 13C) and 85% H3PO4 (31P) as external reference
respectively. Assignments in complex NMR spectra were done
by simulation with the program gNMR v 3.6 (Cherwell Scien-
tific Publishing Limited) for Macintosh. EPR spectra were
measured in a Bruker ESP380E spectrometer working either
in X-band (≈ 9.5 GHz) or Q-band (≈ 34 GHz). Powdered
polycrystalline samples were introduced in standard EPR
quartz tubes, and the spectra were run at RT. The magnetic
field was measured with a Bruker ER035 M NMR gaussmeter,
and a 5350B HP frequency counter was used for determining the
microwave frequency. Cyclic voltammetric experiments were
performed with an EG&G PARC Model 273 potentiostat/
galvanostat using a three-electrode glass cell consisting of a
platinum-disk working electrode, a platinum-wire auxiliary
electrode, and a standard calomel reference electrode (SCE).
Linear voltamperometry was performed using a rotating plati-
num electrode (RDE) as the working electrode. Tetra-n-buty-
lammoniumhexafluorophosphate (TBAH) was employed as
supporting electrolyte. Electrochemical experiments were car-
ried out under nitrogen in about 5 ꢀ 10-4 M dichloromethane
solutions of the complexes and 0.1 M in TBAH. The
[Fe(C5H5)2]þ/[Fe(C5H5)2] couple is observed at þ0.47 V under
these experimental conditions.
(Mþ - cod, 10%), 878 (Mþ - 2cod, 7%). Mol. Weight (CHCl3).
1
Found: 1065 (Calcd. 1094). H NMR (CDCl3, 293 K) δ: 7.76
(d, 2H, JH-H=7.1 Hz), 6.77 (dd, 2H, JH-H=8.7 Hz, JH-H=7.1
Hz), 5.74 (d, 2H, JH-H=8.7 Hz) (pySO); 4.95 (m, 2H, =CH),
4.89 (m, 2H, =CH), 4.79 (m, 2H, =CH), 4.75 (m, 2H, =CH),
4.14 (m, 2H, =CH), 3.95 (m, 2H, =CH), 3.91 (m, 2H, =CH),
3.38 (m, 2H, >CH2), 3.31 (m, 2H, =CH), 2.80-2.35 (m, 14H,
>CH2), 2.22 (m, 2H, >CH2), 2.05 (m, 2H, >CH2) 1.94 (m, 6H,
>CH2), 1.76 (m, 6H, >CH2) (cod). 13C{1H} NMR (CDCl3,
293 K) δ: 170.7 (C-O), 161.6 (C-S), 137.9, 117.2, 111.9 (CH)
(pySO); 91.3 (d, JRh-C =12 Hz), 88.0 (d, JRh-C 12 Hz), 82.9
(d, JRh-C 13 Hz), 81.3 (d, JRh-C = 11 Hz), 79.0 (d, JRh-C
=
12 Hz), 76.7 (d, JRh-C=12 Hz), 68.9 (d, JRh-C=14 Hz), 67.2
(d, JRh-C=15 Hz) (=CH, cod); 34.2, 32.5, 31.3, 30.9, 30.4, 30.2,
30.6, 28.8 (>CH2, cod).
[Rh4(μ-PySO)2(tfbb)4] (2). To a solution of K2PySO (0.274
mmol) in methanol (5 mL), obtained by reaction of H2PySO
with a solution of KOH in methanol, was added a solution
of [Rh(μ-Cl)(tfbb)]2 (0.200 g, 0.274 mmol) in dichloromethane
(10 mL). The mixture was reacted for 12 h to give a purple
suspension. Work-up as described above gave the compound as
a purple microcrystalline solid. Yield: 0.183 g (85%). Anal.
Calcd for C58H30F16N2O2Rh4S2: C, 44.47; H, 1.93; N, 1.79; S,
4.09. Found: C, 44.51; H, 1.83; N, 1.73; S, 4.18. MS (FABþ,
CH2Cl2, m/z): 1566 (Mþ, 100%), 1340 (Mþ-tfbb, 6%), 1237
(Mþ-Rh(tfbb), 4%), 1114 (Mþ-2tfbb, 11%), 888 (Mþ-3tfbb,
1
6%), 784 (Mþ - 2Rh(tfbb) - pySO, 40%). H NMR (CDCl3,
Synthesis of 6-Mercaptopyridin-2-ol (H2PySO). NaSH H2O
293 K) δ: 7.38 (d, 2H, JH-H =6.9 Hz), 6.82 (dd, 2H, JH-H
=
3
(8.50 g, 0.115 mol) was dissolved in refluxing DMF (90 mL)
under argon. After cooling, a solution of 6-chloropyiridin-2-ol
(5.0 g, 0.038 mol) in dimethylformamide (10 mL) was added,
and the solution heated for 16 h at 418 K. The mixture was
cooled, and the precipitated salt was filtered off to obtain an
orange-brown solution. The solvent was mostly removed by
distillation under vacuum, and the oily brown residue dissolved
in 40 mL of a methanol-THF mixture (1:3). The solution was
filtered through Celite, and the resulting orange solution
brought to dryness under vacuum. The residue was stirred with
diethylether to give a cream solid that was filtered, washed with
diethylether, and dried under vacuum. The solid was dissolved
in the minimum amount of a dichlorometane-methanol mixture
(4:1) and then eluted through a silica gel column using dichlo-
rometane/methanol (2:1). The eluted orange solution was
brought to dryness under vacuum, and the residue stirred in
tetrahydrofuran (THF). Slow addition of diethyl ether gave
the compound as a hygroscopic pale-yellow solid that was
filtered, washed with diethyl ether, and dried under vacuum.
Yield 3.650 g (74%). 1H NMR (DMSO-d6, 293 K) δ: 10.56 (br,
8.5 Hz, JH-H=6.9) (pySO); 6.40 (m, 2H, CH), 5.84 (m, 4H, CH)
(tfbb), 5.79 (d, 2H, JH-H=8.5) (pySO), 5.50 (m, 2H, CH), 5.34
(m, 2H, =CH), 4.66 (m, 2H, =CH), 4.50 (m, 2H, =CH), 4.34
(m, 2H, =CH), 3.98 (m, 2H, =CH), 3.84 (m, 4H, =CH), 2.97
(m, 2H, =CH) (tfbb).
[Ir4(μ-PySO)2(cod)4] (3). K2PySO (0.223 mmol) and [Ir(μ-
Cl)(cod)]2 (0.150 g, 0.223 mmol) were reacted for 12 h in a
dihloromethane/methanol mixture following the procedure de-
scribed above. The resulting blue-green suspension was evapo-
rated to dryness and the residue extracted with dichloromethane
(3 ꢀ 15 mL), and the combined solutions were filtered through
Celite. Work-up as above afforded the compound as green
microcrystalline solid. Yield: 0.138 g (85%). Anal. Calcd for
C42H54Ir4N2O2S2: C, 34.75; H, 3.75; N, 1.93; S, 4.42. Found: C,
34.80; H, 3.65; N, 2.01; S, 4.11. MS (FABþ, CH2Cl2, m/z): 1452
(Mþ, 69%), 1151 (Mþ - Ir(cod), 62%). 1H NMR (CDCl3,
293 K) δ: 7.48 (dd, 2H, JH-H = 7.2, JH-H = 1.1 Hz), 6.85
(dd, 2H, JH-H=8.7 Hz, JH-H=7.2 Hz), 5.89 (dd, 2H, JH-H
=
8.70, JH-H=1.1 Hz) (pySO); 4.68 (m, 2H, =CH), 4.43 (m, 6H,
=CH), 4.03 (m, 2H, =CH), 2.80 (m, 4H, =CH), 2.39 (m, 14H,
>CH2), 2.35 (m, 2H, =CH), 2.10 (m, 2H, >CH2), 1.83 (m, 6H,
>CH2), 1.69 (m, 2H, >CH2), 1.44 (m, 8H, >CH2) (cod).
13C{1H} NMR (CDCl3, 293 K) δ: 170.2 (C-O), 160.1 (C-S),
138.4, 118.1, 114.4 (CH) (pySO); 75.1, 71.3, 67.5, 66.1, 65.1,
64.5, 54.4, 49.1 (=CH, cod), 36.4, 32.2, 31.5, 31.2(2C), 30.9,
29.6, 29.1 (>CH2, cod).
NH), 6.96 (dd, JH-H=8.4 Hz, JH-H=7.2 Hz), 5.96 (d, JH-H
=
7.2 Hz), 5.51 (d, JH-H=8.4 Hz), 3.51 (br, SH). 13C{1H} NMR
(DMSO-d6, 293 K) δ: 169.8 (C=O), 164.6 (C-S), 140.1, 109.0,
103.5. IR (cm-1): 3300-2400 (br, SH, NH), 1648, 1601, 1537.
MS (EI, CH3OH, m/z): 127 (H2pySO, 100%), 94 (HpyO, 20%).
HRMS calcd for C5H6NOS 128.01646, found 128.01652.
Synthesis of the Complexes. [Rh4(μ-PySO)2(cod)4] (1).
H2PySO (0.052 g, 0.406 mmol) was reacted with a solution of
KOH in methanol (2.91 mL, 0.278 M, 0.811 mmol) in methanol
(5 mL) to give a pale-yellow solution of K2PySO. This solution
was further reacted with a solution of [Rh(μ-Cl)(cod)]2 (0.200 g,
0.406 mmol) in dichloromethane (10 mL) to give an orange
suspension after stirring for 12 h. The solvent was removed
under vacuum, and the residue dissolved in dichloromethane
(25 mL) and then filtered through Celite. Concentration of this
solution to about 1 mL and slow addition of methanol afforded
the compound as a red-orange microcrystalline solid which was
collected by filtration, washed with methanol, and dried under
vacuum. Yield: 0.200 g (90%). Anal. Calcd for C42H54N2O2Rh4S2:
C, 46.08; H, 4.97; N, 2.56; S, 5.86. Found: C, 46.43; H, 4.63; N,
2.67; S, 5.93. MS (FABþ, CH2Cl2, m/z): 1094 (Mþ, 100%), 986
[Rh4(μ-PySO)2(CO)8] (4). A solution of K2PySO (0.386
mmol) in methanol (5 mL) was prepared following the proce-
dure described above. The solvent was removed under vacuum,
and the residue suspended in dichlomethane (10 mL). Solid
[Rh(μ-Cl)(CO)2]2 (0.150 g, 0.386 mmol) was added under a
carbon monoxide atmosphere to give a deep violet solution
which was stirred for 1 h. The solution was filtered through
Celite and then concentrated by continuous bubbling of carbon
monoxide to about 1 mL. Slow addition of n-hexane (10 mL)
and cooling to 258 K under carbon monoxide afforded the
compound as violet microcrystals which were collected by
filtration, washed with n-hexane, and then dried under vacuum.
Yield: 0.139 g (81%). Anal. Calcd for C18H6N2O10Rh4S2: C,
24.40; H, 0.68; N, 3.16; S, 7.24. Found: C, 24.23; H, 0.78; N,
3.06; S, 7.46. MS (FABþ, CH2Cl2, m/z): 885 (Mþ, 22%), 829

7992 Inorganic Chemistry, Vol. 48, No. 16, 2009
Alonso et al.
(Mþ - 2CO, 94%), 801 (Mþ - 3CO, 100%), 773 (Mþ - 4CO,
90%), 745 (Mþ-5CO, 63%), 717 (Mþ-6CO, 85%), 689 (Mþ-
7CO, 74%), 661 (Mþ - 8CO, 53%). 1H NMR (CDCl3, 293 K) δ:
6.82 (m, 4H), 5.98 (dd, 2H, JH-H = 8.45, JH-H = 0.97 Hz)
(1) (0.300 g, 0.274 mmol) in dichloromethane (15 mL), and the
mixture was stirred for 1 h with exclusion of light. The dark
suspension was filtered through Celite under argon to remove
the metallic silver, and the resulting bright garnet solution
concentrated under vacuum to about 1 mL. Slow addition of
diethyl ether (10 mL) gave 1aþ as a garnet microcrystalline solid.
Yield: 0.268 g (79%). Anal. Calcd for C43H54F3N2O5Rh4S3: C,
41.53; H, 4.38; N, 2.25; S, 7.73. Found: C, 41.25; H, 4.09; N,
2.17; S, 7.65. MS (FABþ, CH2Cl2, m/z): 1094 (Mþ, 72%), 986
(Mþ-cod, 8%), 883 (Mþ-Rh(cod), 37%), 758 (Mþ-Rh(cod)
-pySO, 15%). ΛM (Ω-1 cm2 mol-1): 109 (acetone, 4.85 ꢀ 10-4 M).
[Rh4(μ-PySO)2(cod)4][PF6] (1bþ). Solid [Cp2Fe]PF6 (0.045 g,
0.137 mmol) was added to a solution of [Rh4(μ-PySO)2(cod)4]
(1) (0.150 g, 0.137 mmol) in dichloromethane (15 mL). The
mixture was stirred for 30 min, and the resulting bright garnet
solution concentrated under vacuum to about 3 mL. Slow
addition of diethyl ether (10 mL) gave 1bþ as a garnet micro-
crystalline solid that was collected by filtration, washed repeat-
edly with diethyl ether, and then vacuum-dried. Yield: 0.132 g
(78%). Anal. Calcd for C42H54F6N2O2PRh4S2: C, 40.70; H,
4.39; N, 2.26; S, 5.17. Found: C, 40.58; H, 4.28; N, 2.24; S, 5.15.
MS (FABþ, CH2Cl2, m/z): 1094 (Mþ, 72%), 883 (Mþ - Rh -
cod, 37%), 758 (Mþ-Rh(cod)-pySO, 15%). ΛM (Ω-1 cm2 mol-1):
118 (acetone, 5.6 ꢀ 10-4 M).
(pySO). Calculated spectrum, δ: 6.791 (JH-H=6.99 Hz, JH-H
=
=
0.97 Hz), 6.842 (JH-H=6.99 Hz, JH-H=8.45 Hz), 5.980 (JH-H
8.45 Hz, JH-H =1.24 Hz). 13C{1H} NMR (CDCl3, 293 K) δ:
184.9 (d, JRh-C=62 Hz), 183.7 (d, JRh-C =63 Hz), 182.5 (d,
JRh-C =72 Hz), 181.0 (d, JRh-C=76 Hz) (CO); 170.2 (C-O),
152.7 (C-S), 139.6, 117.4, 115.1 (CH) (pySO). IR (CH2Cl2, cm-1):
2099 (m), 2079 (s), 2068(s), 2029 (s), and 2013 (m).
[Rh4(μ-PySO)2(cod)2(CO)4] (5). Carbon monoxide was
bubbled through a solution of [Rh4(μ-PySO)2(cod)4] (1) (0.100 g,
0.091 mmol) in dichloromethane (15 mL) to give a deep violet
solution of complex [Rh4(μ-PySO)2(CO)8] in 15 min. The solution
was slowly concentrated under vacuum (5 mL) to give a deep red
solution that was stirred under argon for 72 h. The dark blue
solution obtained was concentrated under vacuum to about 1 mL
to give a purple microcrystalline solid by slow addition of methanol
(5 mL). The solid was collected by filtration, washed with methanol,
and then dried under vacuum. Yield: 0.079 g (87%). Anal. Calcd for
C30H30N2O6Rh4S2: C, 36.38; H, 3.05; N, 2.83; S, 6.47. Found: C,
36.02; H, 2.90; N, 2.84; S, 6.36. MS (FABþ, CH2Cl2, m/z):990(Mþ,
32%), 962 (Mþ -CO, 8%), 934 (Mþ - 2CO, 100%), 906 (Mþ -
3CO, 17%), 878 (Mþ - 4CO, 11%). 1H NMR (C6D6, 293 K) δ:
6.86 (dd, 2H, JH-H=7.1; JH-H=0.9 Hz), 6.39 (dd, 2H, JH-H=8.45
Hz, JH-H=7.1 Hz), 6.00 (dd, 2H, JH-H=8.5 Hz, JH-H=0.9 Hz)
(pySO); 4.98 (m, 2H, =CH), 4.85 (m, 2H, =CH), 4.28 (m, 2H,
=CH), 3.27 (m, 2H, =CH), 2.60 (m, 4H, >CH2), 2.14 (m, 4H,
>CH2), 2.02 (m, 2H, >CH2), 1.77 (m, 2H, >CH2), 1.46 (m, 4H,
[Rh4(μ-PySO)2(tfbb)4][CF3SO3] (2þ). [Rh4(μ-PySO)2(tfbb)4]
(0.100 g, 0.064 mmol) and AgCF3SO3 (0.017 g, 0.064 mmol)
were reacted in dichloromethane (15 mL) for 1 h with exclusion
of light to give a purple solution. Work-up as described above
gave the compound as a purple microcrystalline solid. Yield:
0.095 g (87%). Anal. Calcd for C59H30F19N2O5Rh4S3: C, 41.30;
H, 1.76; N, 1.63; S, 5.61. Found: C, 41.15; H, 1.53; N, 1.59; S,
>CH2) (cod). 13C{1H} NMR (CDCl3, 293 K) δ: 187.6 (d, JRh-C
=
64 Hz), 184.7 (d, JRh-C=71 Hz) (CO); 171.0 (C-O), 157.4 (C-S),
5.56. MS (FABþ, CH2Cl2, m/z): 1566 (Mþ, 100%), 1340 (Mþ
-
138.9, 115.2, 113.9 (CH) (pySO); 90.4 (d, JRh-C=12 Hz), 88.6 (d,
JRh-C=12 Hz), 73.2 (d, JRh-C=12 Hz), 66.5 (d, JRh-C=12 Hz)
tfbb, 20%). ΛM (Ω-1 cm2 mol-1): 102 (acetone, 4.92 ꢀ 10-4 M).
[Ir4(μ-PySO)2(cod)4][CF3SO3] (3þ). [Ir4(μ-PySO)2(cod)4]
(0.100 g, 0.069 mmol) and AgCF3SO3 (0.018 g, 0.069 mmol)
were reacted in dichloromethane (15 mL) for 1 h with exclusion
of light to give a green suspension. Work-up as describe above
gave the compound as a green microcrystalline solid. Yield:
0.074 g (67%). Anal. Calcd for C43H54F3Ir4N2O5S3(%): C,
32.26; H, 3.40; N, 1.75; S, 6.01. Found: C, 32.01; H, 3.32; N,
1.73; S, 6.08. MS (FABþ, CH2Cl2, m/z): 1452 (Mþ, 100%), 1152
(Mþ-Ir(cod), 29%), 743 (Mþ-2Ir-3cod, 21%). ΛM (Ω-1 cm2
mol-1): 105 (acetone, 2.99 ꢀ 10-4 M).
(=CH, cod); 35.0, 32.7, 27.9, 27.0 (CH2, cod). IR (CH2Cl2, cm-1):
2089 (sh), 2075 (s), 2057(m) y 2010 (s).
[Rh4(μ-PySO)2(CO)4(PPh3)4] (6). Method A. Carbon mon-
oxide was bubbled through a solution of [Rh4(μ-PySO)2(cod)4]
(0.150 g, 0.137 mmol) in dichloromethane (10 mL) for 15 min to
give a deep violet solution of the complex [Rh4(μ-PySO)2(CO)8].
Further addition of PPh3 (0.144 g, 0.548 mmol) gave a brown-
reddish solution after evolution of carbon monoxide. The solution
was stirred for 10 min and then concentrated under vacuum to
about 1 mL. Slow addition of methanol (10 mL) gave the complex
as a violet solid which was isolated by filtration, washed with
methanol, and dried under vacuum. Yield: 0.169 g (68%).
Method B. A solution of K2PySO (0.144 mmol) in methanol
(5 mL) was added to a solution of trans-[RhCl(CO)(PPh3)2]
(0.200 g, 0.290 mmol) in dichloromethane (15 mL) to give a red-
brown solution which was stirred for 1 h. The solvent was
removed under vacuum, and the residue dissolved in dichlor-
omethane (15 mL) and then filtered through Celite. The solution
was concentrated under vacuum to about 2 mL, and a violet
solid began to crystallize. The crystallization was completed by
addition of ethanol (10 mL), and the solid was collected by
filtration, washed with methanol, and dried under vacuum.
Yield: 0.087 g (66%). Anal. Calcd for C86H66N2O6P4Rh4S2:
C, 56.67; H, 3.65; N, 1.54; S, 3.52. Found C, 56.76; H, 3.57; N,
1.50; S, 3.48. MS (FABþ, CH2Cl2, m/z): 1822 (Mþ, 100%), 1794
(Mþ-CO, 5%), 1504 (Mþ-PPh3-2CO, 5%), 1476 (Mþ-PPh3
-3CO, 5%), 1448 (Mþ-PPh3-4CO, 5%), 1297 (Mþ-2PPh3,
56%), 1214 (Mþ - 2PPh3 - 3CO, 16%), 1185 (Mþ - 2PPh3 -
4CO, 24%). 1H NMR (CDCl3, 293 K) δ: 7.70-7.01 (m, 60 H)
(PPh3); 6.49 (d, 2H, JH-H=6.8 Hz), 6.15 (dd, 2H, JH-H=8.2 Hz,
JH-H=6.8), 4.86 (d, 2H, JH-H=8.2) (pySO). 31P{1H} NMR
Crystal Structure determination of [Rh4(μ-PySO)2(cod)4] (1).
Single crystals for the X-ray diffraction study of compound 1
were obtained by slow diffusion of ethanol into a dichloro-
methane solution of the complex at 258 K. Crystal data for 1:
C42H54N2O2Rh4S2 2(CH2Cl2), fw 1264.48, space group C2/c,
3
a=9.3868(11), b=18.516(2), c=25.869(3) A, β=98.994(2)°, V=
4441.0(9) A3, Z = 4, Dcalcd 1.891 g cm-3, μ=1.836 mm-1. X-ray
data were collected for an irregular block (0.24ꢀ0.20ꢀ0.11 mm)
at low temperature (100(2) K) on a Bruker SMART APEX
CCD difractometer. Data were collected using graphite-mono-
chromated Mo KR radiation (λ=0.71073 A; 7268 measd. reflns.
(2.2 e θ e 28.47°), 5191 unique (Rint=0.0351)). An absorption
correction for 1 was applied by using the SADABS routine38
(min, max. transm. factors 0.6598, 0.8194). The structure was
solved by direct methods, completed by subsequent difference
Fourier techniques and refined by full-matrix least-squares on
F2 (SHELXL-97)39 with initial isotropic thermal parameters.
Anisotropic thermal parameters were used in the last cycles of
refinement for all non-hydrogen atoms (no. data/restraints/
parameters 5191/0/291). Two solvent molecules of dichloro-
methane were found. All hydrogens were found in the difference
(CDCl3, 293 K) δ: 39.76 (d, JRh-P=163 Hz), 38.69 (d, JRh-P
=
(38) (a) SADABS: Area detector absorption; Bruker-AXS: Madison, WI,
1996.(b) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33.
175 Hz). IR (CH2Cl2, cm-1): 1982 (s), 1974 (sh).
[Rh4(μ-PySO)2(cod)4][CF3SO3] (1aþ). Solid AgCF3SO3 (0.071
g, 0.274 mmol) was added to a solution of [Rh4(μ-PySO)2(cod)4]
(39) Sheldrick, G. M. SHELXL-97 Program for Crystal Structure Refine-
::
::
ment; University of Gottingen: Gottingen, Germany, 1997.

Article
Inorganic Chemistry, Vol. 48, No. 16, 2009 7993
Fourier maps and refined with positional parameters riding
on carbon atoms, but with free displacement parameters. Final
agreement factors were R1(F) (F2 g 2σ(F2)) 0.0412, wR2(F2)
0.0955 and GOF 1.044.
BQU and Grant CSD2006-0015 Consolider Ingenio 2010 is
gratefully acknowledged.
Supporting Information Available: An X-ray crystallographic
file in CIF format for the structure determination of complex
[Rh4(μ-PySO)2(cod)4] (1). This material is available free of
charge via the Internet at http://pubs.acs.org.
Acknowledgment. The financial support from Ministerio de
ꢀ
Educacion y Ciencia (MEC/FEDER) Project CTQ2006-03973/