Reaction of singlet oxygen with Ir(I) and Rh(I) thiolato complexes: Oxidative addition vs. S-oxidation

Article abstract of DOI:10.1039/b110396m

Singlet oxygen reacts with Ir(I) and Rh(I) thiolato complexes to form the corresponding Ir(III) and Rh(III) peroxo thiolato complexes which do not undergo intramolecular oxidation of the thiolate moiety.

Full text of DOI:10.1039/b110396m

Reaction of singlet oxygen with Ir(
I
) and Rh( ) thiolato complexes:
I
oxidative addition vs. S-oxidation†
David G. Ho,a Rehana Ismail,a Nestor Franco,a Ruomei Gao,a Edward P. Leverich,a Irina Tsyba,b
Nam Nhat Ho,b Robert Baub and Matthias Selke*a
a Department of Chemistry and Biochemistry, California State University, Los Angeles, Los Angeles, CA
90032, USA. E-mail: mselke@calstatela.edu
b Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA
Received (in Purdue, IN, USA) 13th November 2001, Accepted 12th February 2002
First published as an Advance Article on the web 27th February 2002
Singlet oxygen reacts with Ir(
I
) and Rh(
I
) thiolato complexes
ligands. Support for this hypothesis is derived from the
observation that the peroxo complex 3 itself slowly reacts with
singlet oxygen resulting in the same intractable mixture of
decomposition products. X-Ray molecular structures have been
obtained for both peroxo thiolato complexes, and the ORTEP
diagrams of 3 and 4 are shown in Fig. 1.‡ The peroxo complexes
3 and 4 are remarkably stable, and despite the proximity of the
peroxo ligand to the sulfur of the thiolato group, no intra-
molecular attack on the thiolate ligand is observed. Even upon
to form the corresponding Ir(III) and Rh(III) peroxo thiolato
complexes which do not undergo intramolecular oxidation
of the thiolate moiety.
There have been a number of reports of reactions of triplet1,2
and singlet1f,3,4 dioxygen with a variety of metal thiolate
complexes leading to formation of isolable sulfenato and
sulfinato complexes. In all cases, the primary site of attack by
the dioxygen molecule appears to have been the thiolate moiety
rather than the metal atom; no peroxo thiolato complexes have
been observed as intermediates. For coordinatively unsaturated
thiolato complexes, however, attack at either the metal or the
thiolate ligand is a priori possible. We reasoned that studying
the reactivity of singlet dioxygen with such complexes would be
particularly interesting, since singlet oxygen tends to be more
reactive both with thiolato ligands1f,4 and with late transition
metal centers such as Rh(
I) and Ir(
I
)5 than ground state oxygen.
) and Ir( ) thiolato
We have therefore prepared a number of Rh(
I
I
complexes containing electron-rich and electron-poor thiolato
ligands, and have studied their reactivity with triplet and singlet
dioxygen. Such thiolato complexes are of great interest as their
high catalytic activity in hydroformylation is well documented.6
We now report that oxidative addition of the dioxygen molecule
to the metal center of such complexes is generally preferred
over S-oxidation, and that the resulting peroxo thiolato
complexes are in fact remarkably resistant towards intra-
molecular oxidation.
The mononuclear Ir(
I
)
thiolato complexes trans-Ir-
(CO)(PPh3)2(SR) (1: R = Me,7 2: R = C6F5 ) react with triplet
8
dioxygen to form stable peroxo thiolato complexes Ir(III)-
(CO)(PPh3)2(SR)O2 (3: R = Me, 4: R = C6F5 ) [eqn. (1)]. No
9
oxidation of the thiolate moiety is observed during the
reaction.
(1)
For complex 2, the same peroxo thiolato complex 4 is cleanly
obtained upon reaction with singlet dioxygen; no oxidation at
the sulfur is observed. Reaction of complex 1 with singlet
dioxygen also leads to some formation of the corresponding
peroxo thiolato complex 3, accompanied by extensive decom-
position and formation of triphenylphosphine oxide. Since the
peroxo thiolato complex 3 itself is very stable (see below), the
decomposition must result from attack of a second singlet
oxygen molecule on the thiolate ligand. Since triphenylphos-
phine oxide is one of the reaction products, we hypothesize that
Fig. 1 (Top): ORTEP diagram of the peroxo thiolato complex 3. Selected
bond lengths (Å) and angles (°): O(1)…S(1) 3.69, O(1)–O(2) 1.473(3),
Ir(1)–C(38) 1.917(7), Ir(1)–S(1) 2.375(2), Ir(1)–O(1) 2.027(2), Ir(1)–P(1)
2.3575(8), Ir(1)–P(2) 2.3563(8); O(1)–Ir(1)–S(1) 116.70(8), C(38)–Ir(1)–
S(1) 92.2(2). (Bottom): ORTEP diagram of peroxo thiolato complex 4.
Selected bond lengths (Å) and angles (°): O(6)…S(3) 3.55, O(5)–O(6)
1.474(9), Ir(1)–C(12) 1.891(9), Ir(1)–S(3) 2.436(3), Ir(1)–O(5) 2.2029(6),
Ir(1)–O(6) 2.017(6), Ir(1)–P(1) 2.388(3), Ir(1)–P(2) 2.400(3); O(6)–Ir(1)–
S(3) 105.2(3), C(12)–Ir(1)–S(3) 92.4(8).
1
the intermediate persulfoxide formed by attack of O2 on the
sulfur is trapped intramolecularly by one of the phosphine
† Electronic supplementary information (ESI) available: experimental
crystallographic details. See http://www.rsc.org/suppdata/cc/b1/b110396m/
570
CHEM. COMMUN., 2002, 570–571
This journal is © The Royal Society of Chemistry 2002

refluxing in benzene under nitrogen or irradiation under
nitrogen, complex 4 reductively eliminates dioxygen to re-form
complex 1, rather than attacking the thiolate ligand. Complex 3
is stable in refluxing benzene and under irradiation without
significant loss of dioxygen or oxidation of the thiolate
ligand!
This research was supported by an award from the Research
Corporation. Acknowledgement is made to the Donors of The
Petroleum Research Fund, administered by the American
Chemical Society, for partial support of this research. Support
by the NIH-NIGMS MBRS program (Award number GM
08101) is also gratefully acknowledged.
The related Rh( ) complex trans-Rh(CO)(PPh3)2(SR) (5: R =
I
8
C6F5 ) also reacts with singlet dioxygen to form the correspond-
ing previously unknown peroxo thiolato complex Rh(III-
)(CO)(PPh3)2(SC6F5)O2 (6). This complex is unstable at room
temperature, and decomposes into a mixture of the starting
Notes and references
‡
Crystal data for 3: C38H33IrO3P2S, M = 823.84, monoclinic, space
group P21/c (no. 14), a = 9.7548(8), b = 17.652(1), c = 22.680(2) Å, b =
99.540(1)°, V = 3851.4(5) Å3, Z = 4, Dc = 1.776 g cm23, T = 293(2) K,
m = 4.545 mm21, 8454 total reflections, 5625 observed reflections, 487
parameters, R1 (all data) = 0.0528, wR2 (all data) = 0.0769.
complex
[Rh(CO)(PPh3)(SC6F5)]2. Again, no oxidation of the thiolate
ligand is observed. Mononuclear Rh( complexes
5
and
the
dinuclear
Rh(I)
complex
¯
I)
For 4: C43H30F5IrO3P2S, M = 975.87, triclinic, space group P1 (no. 2),
Rh(CO)(PPh3)2(SR) bearing more electron-rich thiolate ligands
cannot be isolated, as they rapidly dimerize even at very low
temperature.10 Such dimers might react with singlet oxygen by
four different pathways, namely (i) oxidative addition at one of
a = 9.878(8), b = 11.854(6), c = 18.538(9) Å, a = 99.98(4), b =
101.32(5), g = 111.39(5)°, V = 1909.2(20) Å3, Z = 2, Dc = 1.698 g cm23
,
T = 293(2) K, m = 3.699 mm21, 4895 total reflections, 4010 observed
reflections, 472 parameters, R1 (all data) = 0.0699, wR2 (all data) =
0.1131.
the metal centers, leading to a dimer containing one Rh( ) and
I
CCDC reference numbers 179090 and 179091. See http://www.rsc.org/
suppdata/cc/b1/b110396m/ for crystallographic data in CIF or other
electronic format.
one Rh(III) center; (ii) oxidative addition at both metal centers;
(iii) formation of a m-peroxo bridged dimer, or (iv) oxidation of
the bridging thiolate ligands. We therefore studied the reaction
of trans-[Rh(CO)(PPh3)(StBu)]2 (7) with singlet oxygen. At
11
1 (a) P. J. Farmer, T. Solouki, D. K. Mills, T. Soma, D. H. Russell, J. H.
Reibenspies and M. Y. Darensbourg, J. Am. Chem. Soc., 1992, 114,
4601; (b) P. J. Farmer, T. Solouki, D. K. Mills, T. Soma, D. H. Russell
and M. Y. Darensbourg, Inorg. Chem., 1993, 32, 4171; (c) R. M.
Buomono, I. Font, M. J. Maguire, J. H. Reibenspies, T. Tuntulani and
M. Y. Darensbourg, J. Am. Chem. Soc., 1995, 117, 963; (d) R. M.
Buomono, I. Font, M. J. Maguire, J. H. Reibenspies, T. Tuntulani and
M. Y. Darensbourg, J. Am. Chem. Soc., 1995, 117, 5427; (e) C. A.
Grapperhaus, M. Y. Darensbourg, L. W. Sumner and D. H. Russell, J.
Am. Chem. Soc., 1996, 118, 1791; (f) C. A. Grapperhaus, M. J. Maguire,
T. Tuntunlani and M. Y. Darensbourg, Inorg. Chem., 1997, 36, 1860.
2 Y. Zhang, K. D. Ley and K. S. Schanze, Inorg. Chem., 1996, 35,
7102.
240 °C, reaction of 7 with 1O2 leads to formation of the
remarkable mixed dimer Rh(III)(O2)(CO)(PPh3)(m-StBu)2Rh(
)
I
(CO)(PPh3) (8)12 [eqn. (2)]. Formation of this species is
(2)
reversible, and warming leads to re-formation of starting
material, implying that no oxidation of the bridging thiolato
ligand occurs. Singlet oxygen luminescence quenching studies
are consistent with this reactivity, as the rate of singlet oxygen
removal by 7 is approximately twice that of the mononuclear
species 5, indicating that the quenching of singlet oxygen occurs
by a similar mechanism as in the mononuclear complexes
(Table 1).
3 W. B. Connick and H. B. Gray, J. Am. Chem. Soc., 1997, 119,
11 620.
4 C. Galvez, D. G. Ho, A. Azod and M. Selke, J. Am. Chem. Soc., 2001,
123, 3381.
5 M. Selke, C. S. Foote and W. L. Karney, Inorg. Chem., 1995, 34,
5215.
6 J. R. Dilworth, D. Morales and Y. Zheng, J. Chem. Soc., Dalton Trans.,
2000, 3007; C. Claver, Ph. Kalck, M. Ridmy, A. Thorez, L. A. Oro, M.
T. Pinillos, M. C. Apreda, F. H. Cano and C. Foces-Foces, J. Chem.
Soc., Dalton Trans., 1988, 1523.
7 Complex 1 was prepared by treatment of IrCl(CO)(PPh3)2 with a
stoichiometric amount of AgBF4 in CH3CN followed by addition of
ethanolic NaSCH3.
Table 1 Singlet oxygen luminescence quenching constants for Ir(
I) and
Rh( ) thiolato complexes
I
8 This was prepared as previously described: M. H. B Stiddard and R. E.
Townsend, J. Chem. Soc. (A), 1970, 2719.
Compound
KT 3 108 M21 s21 in CDCl3
9 Complex 4 has been previously obtained by slow reaction of 2 with
ground state dioxygen. However, only IR data have been reported. See
ref. 8.
Ir(CO)(PPh3)2(SCH3) (1)
Ir(CO)(PPh3)2(SC6F5) (2)
Rh(CO)(PPh3)2(SC6F5) (5)
[Rh(CO)(PPh3)(StBu)]2 (7)
2.0 0.3
1.3 0.1
1.6 0.2
3.3 0.2
10 Treatment of complexes [Rh(CO)(PPh3)2(NCCH3)]+(BF4)2 with etha-
t
nolic NaSR (R = Me, Bu) should initially lead to formation of the
mononuclear species; however, even at 280 °C, only the corresponding
dimers were observed.
Singlet oxygen luminescence quenching rates by all other
complexes have also been obtained and are summarized in
Table 1.14 All complexes remove singlet oxygen with very large
rates, comparable with those of Vaska’s complex and deriva-
tives,5 consistent with attack of singlet dioxygen at the metal.
11 D. de Montauzon, P. Kalck and R. Poilblanc, J. Organomet. Chem.,
1980, 186, 121.
12 Complex 8 is easily identified by its 31P NMR spectrum which gives two
doublets of doublets [d 34.34 (dd, 1P, JRh–P 144 Hz, JP–P 7 Hz); 27.37
(dd, 1P, JRh–P 102 Hz, JP–P 7 Hz)]. The Rh–P coupling constant of 144
Hz is consistent with a square planar Rh(I) center, whereas the smaller
Rh–P coupling constant is consistent with an octahedral Rh(III) center.
See ref. 13.
The lack of reactivity of the thiolate ligands of the Ir(
I
) and Rh( )
I
complexes is in remarkable contrast with that of several Co and
Ni complexes.1,4 The peroxo ligand on group VIII peroxo
complexes has often been considered to be nucleophilic. The
lack of intramolecular oxidation of both electron-poor and
electron-rich thiolate ligands by the peroxo group indicates that
this group should at times be considered unreactive rather than
nucleophilic.
13 L. Carlton, Magn. Res. Chem., 1997, 35, 153; ; see also ref. 5.
14 Time-resolved singlet oxygen luminescence quenching experiments
were conducted by exciting a solution containing the sensitizer (TPP or
methylene blue) and varying amounts of substrate (quencher) with a
short (a few ns) laser pulse and monitoring the singlet oxygen
luminescence decay at a right angle.
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