Photooxidation of a platinum-anthracene pincer complex: formation and structures of pt"-anthrone and -ketal complexes

Article abstract of DOI:10.1021/ic900479w

Irradiating the cyclometalated pincer complex Pt^II(DPA)Cl (1, DPA = 1, 8-bis(dlphenylphosphlno)anthracene) In the presence of O₂ led to three sequential oxidations of the anthracenyl ring. The first photoproduct, a Pt^II-9, 10-endoperoxide complex, was converted photochemically to a Pt^II-9-hydroxyanthrone complex A which was further oxygenated to a Pt^II-hemlketal (B). The oxidation of A, which could be accelerated by light irradiation, probably Involved a Pt ^II-anthraqulnone Intermediate. B underwent acid-catalyzed ketalizatlon to form a binuclear Pt₂^II-diketal (B1). The photolysis was followed by UV-vls absorption and NMR spectroscopy, and the structures of A and B1 were characterized by single crystal X-ray diffraction.

Full text of DOI:10.1021/ic900479w

9684 Inorg. Chem. 2009, 48, 9684–9692
DOI: 10.1021/ic900479w
Photooxidation of a Platinum-Anthracene Pincer Complex: Formation
and Structures of PtII-Anthrone and -Ketal Complexes
Jian Hu, Huan Xu, Minh-Hai Nguyen, and John H. K. Yip*
Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 11754
Received March 13, 2009
Irradiating the cyclometalated pincer complex PtII(DPA)Cl (1, DPA = 1,8-bis(diphenylphosphino)anthracene) in the
presence of O2 led to three sequential oxidations of the anthracenyl ring. The first photoproduct, a PtII-9,
10-endoperoxide complex, was converted photochemically to a PtII-9-hydroxyanthrone complex A which was further
oxygenated to a PtII-hemiketal (B). The oxidation of A, which could be accelerated by light irradiation, probably
involved a PtII-anthraquinone intermediate. B underwent acid-catalyzed ketalization to form a binuclear PtII -diketal
2
(B1). The photolysis was followed by UV-vis absorption and NMR spectroscopy, and the structures of A and B1 were
characterized by single crystal X-ray diffraction.
Introduction
related to the present study is photooxidation of anthracene
in which O2 undergoes cycloaddition with the polycyclic
1
Photooxidation has important applications in catalysis,
oxidation of biomolecules and organic synthesis.1 Many
photooxidations involve oxygen in its lowest energy singlet
excited state (1Δg) which is usually produced by energy
transfer from an electronic excited state of a dye to the triplet
ground state of oxygen (3Σg-).2 Reactions of 1O2 and organic
molecules such as polycyclic aromatics, olefins, and hetero-
cyclics have been well examined.3 A paradigmatic example
compound to give 9,10-endoperoxide (EP) as the primary
photoproduct.2a,4
While photooxygenation of organic molecules has been
extensively studied, the scope of photooxygenation of transi-
tion metal complexes is rather limited even though many
emissive metal complexes can photosensitize O2 efficiently.5
Most of the studies focused on thiolate complexes,6 especially
d8 square planar MII-cis-dithiolates (M = NiII, PdII, PtII).
Except the photooxidation of (dibutyl-2,20-bipyridine)PtII-
(1,2-diphenyl-1,2-ethanedithiolate)6a in which the dithiolate
was dehydrogenated, reactions between metal-thiolates and
1O2 led to oxidation of the coordinated sulfur atoms to
sulfinate (OdSdO).
*To whom correspondence should be addressed. E-mail: chmyiphk@
nus.edu.sg. Fax: 65-67791691.
(1) (a) Maldotti, A.; Molinari, A.; Amadelli, R. Chem. Rev. 2002, 102,
3811. (b) Meunier, B. Chem. Rev. 1992, 92, 1411. (c) Foote, C. S. Oxygen Oxy-
Radicals Chem. Biol. 1981, 425. (d) George, M. V.; Bhat, V. Chem. Rev. 1979,
79, 447.
(2) (a) Foote, C. S.; Wexler, S. J. Am. Chem. Soc. 1964, 86, 3879. (b) Foote,
C. S. Acc. Chem. Res. 1968, 1, 104–110. (c) Greer, A. Acc. Chem. Res. 2006, 39,
797.
Interactions between metal and organic chromophores can
influence the photophysics and the photochemistry of the
latter.7 Our recent study showed that attaching PtII ion to
(3) (a) Adam, W.; Griesbeck A. G. In CRC Handbook of Organic
Chemistry and Photobiology; Horspool, W.M., Song, P.-S., Eds.; CRC Press
Inc.: London 1995; pp 311-324. (b) Foote, C. S. Pure Appl. Chem. 1971, 45,
635. (c) Sivaguru, J.; Solomon, M. R.; Poon, T.; Jockusch, S.; Bosio, S. G.; Adam,
W.; Turro, N. J. Acc. Chem. Res. 2008, 41, 387. (d) Clennan, E. L. Tetrahedron
1991, 47, 1343.
(4) (a) Corey, E. J.; Taylor, W. C. J. Am. Chem. Soc. 1964, 86, 3881. (b)
Aubry, J.-M.; Pierlot, C.; Rigaudy, J.; Schmidt, R. Acc. Chem. Res. 2003, 36, 668.
(c) Sparfel, D.; Gobert, F.; Rigaudy, J. Tetrahedron 1980, 36, 2225.
(5) (a) Gafney, H. D.; Adamson, A. W. J. Am. Chem. Soc. 1972, 94, 8238.
(6) (a) Zhang, Y.; Ley, K. D.; Schanze, K. S. Inorg. Chem. 1996, 35, 7102.
(b) Connick, W. B.; Gray, H. B. J. Am. Chem. Soc. 1997, 119, 11620. (c) Galvez,
C.; Ho, D. G.; Azod, A.; Selke, M. J. Am. Chem. Soc. 2001, 123, 3381. (d)
Anbalagan, V.; Srivastava, T. S. J. Photochem. Photobiol., A: Chem. 1995, 89,
113. (e) Anbalagan, V.; Srivastava, T. S. J. Photochem. Photobiol., A: Chem.
1994, 77, 141. (f ) Anbalagan, V.; Srivastava, T. S. Polyhedron 1994, 13, 291. (g)
Farmer, P. J.; Solouki, T.; Mills, D. K.; Soma, T.; Russell, D. H.; Reibenspies,
J. H.; Darensbourg, M. Y. J. Am. Chem. Soc. 1992, 114, 4601. (h) Darensbourg,
M. Y.; Tuntulani, T.; Reibenspies, J. H. Inorg. Chem. 1995, 34, 6287.
(7) (a) Sayer, P.; Gouterman, M.; Connell, C. R. Acc. Chem. Res. 1982, 15,
73. (b) Wrighton, M. S.; Morse, D. L; Pdungsap, L. J. Am. Chem. Soc. 1975, 97,
2073. (c) Lee, P. H.-M.; Ko, C. C.; Zhu, N.; Yam, V. W.-W. J. Am. Chem. Soc.
2007, 129, 6058. (d) Hu, J.; Lin, R.; Yip, J. H. K.; Wong, K.-Y.; Ma, D.-L.; Vittal,
J, J. Organometallics 2007, 26, 6533. (e) Heng, W. Y.; Hu, J.; Yip, J. H. K.
Organometallics 2007, 26, 6760. (f ) Hu, J.; Yip, J. H. K.; Ma, D.-L.; Wong,
K.-Y.; Chung, W.-H. Organometallics 2009, 28, 51. (g) Wang, B.-Y.;
Karikachery, A. R.; Li, Y.; Singh, A.; Lee, H. B.; Sun, W.; Sharp, P. R. J. Am.
Chem. Soc. 2009, 131, 3150. (h) Partyka, D. V.; Esswein, A. J.; Zeller, M.;
Hunter, A. D.; Gray, T. G. Organometallics 2007, 26, 3279.
(b) Szacilowski, K.; Macyk, W.; Drzewiecka-Matuszek, A.; Brindell, M.; Stochel,
e
G. Chem. Rev. 2005, 105, 2647. (c) Demas, J. M.; Harris, E. W.; McBride, R. P.
J. Am. Chem. Soc. 1977, 99, 3547. (d) Djurovich, P. I.; Murphy, D.; Thompson,
M. E.; Hernandez, B.; Gao, R.; Hunt, P. L.; Selke, M. Dalton Trans. 2007, 34,
3763. (e) Abdel-Shafi, A. A.; Worrall, D. R.; Ershov, A. Y. Dalton Trans. 2004,
30. (f ) Muro, M. L.; Diring, S.; Wang, X.; Ziessel, R.; Castellano, F. N. Inorg.
Chem. 2008, 47, 6796. (g) Liu, Y.; Hammitt, R.; Lutterman, D. A.; Joyce, L. E.;
Thummel, R. P.; Turro, C. Inorg. Chem. 2009, 48, 375. (h) Mulazzani, Q. G.; Sun,
H.; Hoffman, M. Z.; Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1994, 98,
1145.
r
pubs.acs.org/IC
Published on Web 09/23/2009
2009 American Chemical Society

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9685
anthracene changes the electronic spectroscopy of the
organic chromophore significantly.7d,e This led us to con-
template the effect of metals on excited state chemistry of
anthracene, especially the well-known photooxygenation. In
search for a system suitable for studying photooxidation of
metal-anthracene, our attention was drawn to the pincer
complex PtII(DPA)X (X = Cl (1), Br (2), I (3), DPA = 1,8-
bis(diphenylphosphino)anthracene) (Scheme 1).8 DPA and
its cyclometalated NiII and PdII complexes were first synthe-
sized by Haenel.9 Our interest in the PtII pincer complex was
aroused by the intriguing reactivity10 and rich photochem-
istry11 of PtII complexes. Most importantly, being attached to
the anthracenyl ring at the C(9) position, the PtII ion can
influence the course of the photooxidation. Indeed, the
results of the present study showed that unlike the photo-
oxidation of anthracene, which gives rise to EP and a myriad
of secondary photoproducts and thermal products such as
10-hydroxyanthrone, anthraquinone, and diepoxides,4c
photooxygenation of 1 produces three oxidized complexes
sequentially.
Scheme 1
Experimental Section
General Method. All syntheses were carried out in N2 atmo-
sphere with standard Schlenck techniques. K2PtCl4 was pur-
chased from Oxkem. CHCl3 (GR) and CDCl3 were used as
the incident light (λ = 300-500 nm) is 1.42 ꢀ 10-8 Einstein/s.
Photolysis of NMR samples was carried out using xenon lamp
with 300-500 nm filters. The UV-vis absorption changes were
monitored by a Hewlett-Packard HP8452A diode array spectro-
photometer. The quantum yields of the reactions were deter-
mined by ferrioxalate actinometry.13 For preparative
photolysis, solutions of metal complexes were irradiated by a
tungsten lamp of 100 W.
received. 1,8-Difluoroanthracene9b and NaPPh2 were pre-
12
pared according to reported methods. The following chemicals
were used as received: PPh2Cl (Acros), 1,8-dichloroanthraqui-
none and CeF2 (Aldrich). Anthracene (Aldrich) was used as
standard for emission quantum yield and was purified by
recrystallization from ethanol.
Physical Measurements. The UV/vis absorption and emission
spectra of the complexes were recorded on a Hewlett-Packard
HP8452A diode array spectrophotometer and a Perkin-Elmer
LS-50D fluorescence spectrophotometer, respectively. Anthra-
cene was used as a standard in measuring the emission quantum
yields. 1H and 31P{1H} NMR spectra were recorded at on either
a Bruker ACF 300 or an AMX500 spectrometer. All chemical
shifts are quoted relative to SiMe4 (1H) or H3PO4 (31P). Ele-
mental analyses of the complexes were carried out in the
microanalysis laboratory in the Department of Chemistry at
National University of Singapore.
Synthesis. Synthesis of 1, 8-Bis(diphenylphosphino)anth-
racene (DPA). A solution of 1,8-difluoroanthracene (1.20 g,
5.6 mmol) in 20 mL of 1,4-dioxane was slowly added into a
solution NaPPh2, which was prepared in situ by reacting sodium
(1.25 g, 54 mmol) and excess PPh2Cl (7.3 mL, 41 mmol) in
refluxing 1,4-dioxane. The mixture was refluxed for 4 h and
then cooled down to room temperature, followed by addition of
∼10 mL of CH3OH to quench excess NaPPh2. All solvents were
removed under vacuum; 100 mL of H2O was then added to the
resulting mixture, and the product was extracted with CH2Cl2
and purified by column chromatography (toluene/cyclohexane
10:1, v/v). The yellow solid obtained was washed with 2,2,
4-trimethylpentane to remove any HPPh2. Yield: 1.97 g, 65%.
Anal. Calcd (%) for C38H28P2: C, 83.52; H, 5.13; Found (%): C,
Steady-State Photolysis. A LPS 300 SM xenon arc lamp
controlled by LPS 300SM switched mode (75 to 300 W) was
used as the light source in the photolysis. Optical filters were
used to select the wavelength of the irradiation. For a typical
photolysis, the power of the lamp was 80 W and the intensity of
1
84.12; H, 5.34. H NMR (CDCl3, 300 M Hz, δ/ppm): 9.77 (t,
4JPH = 5 Hz, 1H, H9), 8.44 (s, 1H, H10), 7.96 (d, 3JHH = 9 Hz,
2H, H4,5), 7.49-7.44 (m, 2H, H2,7), 7.33-7.23 (m, 20H, Ph),
7.03-6.99 (m, 2H, H3,6). 31P{1H}NMR (CDCl3, 121.5 M Hz,
δ/ppm): -14.06 (s). 13C{1H} NMR (CDCl3, 75.48 M Hz, δ/
ppm): 124.78 (t, 3JC-P = 27.9 Hz, C9), 125.97 (s, Ph C3), 128.31
(8) During the preparation of this manuscript, Gelman et al reported the
synthesis of Pt(DPA)Cl and its reaction with acetylene, see: Azerraf, C.;
Shpruhman, A.; Gelman, D. Chem. Commun. 2009, 466.
(9) (a) Haenel, M. W.; Jakubik, D.; Betz, P. Chem. Ber. 1991, 124, 333. (b)
::
Haenel, M. W.; Oevers, S.; Bruckmann, J.; Kuhnigk, J.; Kruger, C. Synlett 1998,
1
(s, C10), 129.06, (d, JC-P =7.5 Hz, C2,7), 129.29 (s, Ph C4),
301.
129.91 (s, C4,5), 132.24 (d, 3JC-P = 3.8 Hz, C11,13), 132.71 (s,
=
(10) For reviews on pincer complexes, see: (a) van der Boom, M. E.;
Milstein, D. Chem. Rev. 2003, 103, 1759. (b) Benito-Garagorri, D.; Kirchner, K.
Acc. Chem. Res. 2008, 41, 201. (c) Gossage, R. A.; van de Kuil, L. A.; van Koten,
G. Acc. Chem. Res. 1998, 31, 423. (d) Albrecht, M.; van Koten, G. Angew.
Chem., Int. Ed. 2001, 49, 3750.
C3,6), 133. 91 (d, 2JC-P = 22.6 Hz, C8,14), 134.79 (d, 2JC-P
20.4 Hz, Ph C2), 136.82 (d, 1JC-P = 15.3 Hz, C1, C8), 137.05 (d,
1JC-P = 10.6 Hz, Ph C1).
Synthesis of PtII(DPA)Cl (1). To a solution of K2PtCl4 (0.277
g, 0.667 mmol) in 80 mL of CH3CN/H2O (1:1, v/v) was added
DPA (0.365 g, 0.668 mmol). The mixture was refluxed under N2
for 18 h. The resulting yellow suspension was extracted with
CH2Cl2. Addition of excess Et2O to the concentrated extraction
precipitated the product as yellow solids. Single crystals were
obtained from vapor diffusion of Et2O into its CH2Cl2 solution.
(11) (a) Jude, H.; Krause Bauer, J. A.; Connick, W. B. J. Am. Chem. Soc.
2003, 125, 3446. (b) Harkins, S. B.; Peters, J. C. Inorg. Chem. 2006, 45, 4316. (c)
Du, P.; Knowles, K.; Eisenberg, R. J. Am. Chem. Soc. 2008, 130, 12576. (d)
Wong, K. M.-C.; Yam, V. W.-W. Coord. Chem. Rev. 2007, 251, 2477. Lai, S.-W.;
Che, C.-M. Top. Curr. Chem. 2004, 241, 27. (e) Shavaleev, N.; Adams, H.; Best,
J.; Edge, R.; Navaratnam, S.; Weinstein, J. A. Inorg. Chem. 2006, 45, 9410. (f )
Wan, K.-T.; Che, C.-M. Chem. Commun. 1990, 140. (g) Hill, R. H.; Puddephatt,
R. J. J. Am. Chem. Soc. 1985, 107, 1218.
::
(12) Kuhl, O.; Sieler, J.; Baum, G.; Hey-Hawkins, E. Z. Anorg. Allg.
Chem. 2000, 626, 605.
(13) Parker, C. A. Proc. R. Soc. London, Ser. A 1953, 104, 220.

9686 Inorganic Chemistry, Vol. 48, No. 20, 2009
Hu et al.
Yield: 0.45 g, 87%. Anal. Calcd (%) for C38H27ClP2Pt CH2Cl2:
7 Hz, 2H, H4,5), 7.90-7.83 (m, 4H, Ph), 7.51-7.42 (m, 18 H, Ph,
1 3
C, 54.40; H, 3.39; Found (%): C, 54.35; H, 3.05. H NMR-
(CDCl3, 500 M Hz, δ/ppm) 8.35 (t, 5JPt-H = 2.4 Hz, 1H, H10),
8.11 (d, JH-H = 8.1 Hz, 2H, H4,5), 7.95(m, 8H, phenyl H),
7.79(m, 2H, H2,7), 7.54 (dd, JH-H = JH-H = 8.1 Hz, 2H, H3,6),
7.48-7.43(m, 12H, Phenyl H). 31P{1H} NMR(121.5 M Hz,
H
2,7), 7.00-6.97 (m, 2H, H3,6), 3.06 (s, 1H, OH). 31P{1H} NMR
1
(CDCl3, 121.5 MHz, δ/ppm): 18.57 (s, JP-Pt = 2870 Hz).
13C{1H} NMR (CDCl3, 75.48 M Hz, δ/ppm): 97.5 (s, ketal C),
124.7-156.0 (aromatic C), 186.8 (s, ketone C).
X-ray Crystallography. The diffraction experiments were
carried out on a Bruker AXS SMART CCD 3-circle diffract-
ometer with a sealed tube at 223 K using graphite-monochro-
1
CDCl3, δ/ppm): 40.22 (s, JPt-P = 2995 Hz). 13C{1H} NMR
(CDCl3, 75.48 M Hz, δ/ppm): 121.33 (s, Ph, C3), 126.77 (s,
C11,13), 129.41 (d, 2JC-P=10.6 Hz, Ph C2), 129.40 (s, Ph C4),
˚
mated Mo KR radiation (λ = 0.71073 A). The software used
1
were SMART13a for collecting frames of data, indexing reflec-
tion and determination of lattice parameters; SAINT14a for
integration of intensity of reflections and scaling; SADABS14b
for empirical absorption correction; and SHELXTL14c for space
group determination, structure solution, and least-squares re-
finements on |F|.2 Anisotropic thermal parameters were refined
for the rest of the non-hydrogen atoms. The hydrogen atoms
were placed in their ideal positions. The crystallographic data
and experimental details for 1, A•CH3CN and B1•4CH2Cl2 are
given in the Table 1.
131.52 (s, C3,6), 131.79 (d, JC-P = 40 Hz, C1,8), 134.61 (s,
C4,5), 134.64 (d, 2JC-P = 13.4 Hz, C2,7), 134.56 (s, C10), 135.05
(d, 1JC-P = 150 Hz, Ph C1), 139.36, (s, C9), 143.36 (s, C12,14).
FAB-MS: m/z (r.a.) 776 ([M]þ, 93%), 740.1([M - Cl]þ, 100%).
Synthesis of PtII(DPA)Br (2) and PtII(DPA)I (3). A mixture of
1 (0.10 g, 0.129 mmol) and excess KBr (0.5 g, 4.1 mmol) or KI (2
g, 12.1 mmol) was stirred in 80 mL CH2Cl2/MeOH (ratio is 1:1)
at room temperature for 48 h. Yellowish solids were obtained
after the solvent was evaporated. The solids were then extracted
with CH2Cl2. The yellow solution was concentrated to 5 mL.
Addition of excess diethyl ether to the solution precipitated the
product as yellow solids. Crystals of both compounds were
obtained from slow diffusion of hexane into a CH2Cl2 solution
of the complexes. Yields for 2 and 3 are 43% and 57%,
respectively. 2: for C39H29PtBrCl2P2, Calcd (%) C, 51.73; H,
3.23; found (%); C, 51.24; H, 3.44. 1H NMR (CDCl3, 300 M Hz,
δ/ppm): 8.38 (t, 5JPt,H = 2.4 Hz, 1H, anthracene H10), 8.10 (d,
JH,H = 8.4 Hz, 2H, H4,5), 7.97-7.88 (m, 8H, phenyl H), 7.75 (m,
2H, H2,7), 7.51 (m, 2H, H3,6), 7.48-7.24 (m, 12H, phenyl H).
Results and Discussion
Synthesis. 1,8-bis(diphenylphosphino)anthracene (DPA)
was first synthesized by Haenel et al.9 Our work showed
that treating 1,8-difluoroanthracene with excess NaPPh2
afforded DPA in reasonably good yield (65%). The complex
Pt(DPA)Cl (1) was synthesized by refluxing a CH3CN/H2O
mixture of DPA and K2PtCl4. The bromide (2) and iodide
(3) analogues were prepared from 1 by ligand substitution.
All three complexes show a singlet with Pt satellites in their
31P{1H} NMR spectra. The chemical shifts (δ ∼ 40) and
JPt-P constants (2926-2994 Hz) of the signals are similar.
Structures of Pt(DPA)X (X = Cl, Br, I). Figure 1 shows
31P{1H} NMR (CDCl3, 121.5 MHz, δ/ppm): 40.31 (s, JPt-P
=
2968 Hz). 13C{1H} NMR (CDCl3, 75.48 M Hz, δ/ppm): 121.41
(s, Ph, C3), 126.90 (s, C11,13), 129.28 (d, 2JC-P = 10.2 Hz, Ph
C2), 129.31 (s, Ph C4), 131.56 (s, C3,6), 131.89 (d, 1JC-P = 25.7
Hz, C1,8), 132.00 (s, C4,5), 134.63 (d, 2JC-P = 13.5 Hz, C2,7),
134.90 (d, 1JC-P = 75.5 Hz, Ph C1), 134.62 (s, C10), 139.93, (s,
C9), 145.97 (s, C12,14). ESI MS: m/z 823 ([M]þ, 93%). 3: for
C39H29PtICl2P2. Calcd (%) C, 49.18; H, 3.06, found (%);
C, 49.52; H, 3.32. 1H NMR (CDCl3, 300 MHz, δ/ppm): 8.44 (t,
5JPt,H = 2.4 Hz, 1H, anthracene H10), 8.11 (d, JH-H = 7.6 Hz,
2H, H4,5), 7.96-7.86 (m, 8H, phenyl H), 7.73(m, 2H, H2,7), 7.51
(dd, JH-H = JH-H= 7.6 Hz, 2H, H3,6), 7.48-7.25 (m, 12H,
phenyl H). 31P{1H} NMR (CDCl3, 121.5 M Hz, δ/ppm) 41.02 (s,
the X-ray crystal structure of 1 CH3CN (see Table 2 for
3
selected structural parameters and Supporting Informa-
tion for the structures of 2 and 3). The structures of 1, 2,
and 3 are very similar, all showing a 4-coordinated Pt ion
bonded to the C(9) atom of the anthracenyl ring, the two
P atoms at the 1,8-positions, and a halide ion that is trans
to the metalated carbon atom.
JPt-P=2926 Hz). 13C{1H} NMR (CDCl3, 75.48 M Hz, δ/ppm):
The local geometry of the central Pt ion is distorted
square planar in 1-3 and the anthracenyl ring is planar; in
fact except the phenyl rings, all atoms of the molecule lie
nearly on the same plane. The P-Pt-P angles
(166.86(5)-167.34(3)°) in the complexes deviates from
linearity, and the C-Pt-P angles are ∼83.6°. The Pt-C
2
121.47 (s, Ph, C3), 127.04 (s, C11,13), 129.12 (d, JC-P
=
10.4 Hz, Ph C2), 129.15 (s, Ph C4), 131.57 (s, C3,6), 131.75 (d,
2
1JC-P = 52.8 Hz, C1,8), 134.84 (d, JC-P = 13.2 Hz, C2,7),
134.86 (s, C4,5), 134.11 (d, 1JC-P=34.7 Hz, Ph C1), 134.86 (s,
C10), 141.04, (t, 3JC-P=26.4, 1JC-Pt=110 Hz, C9), 145.56 (s,
C12,14). ESI MS: m/z: 867 ([M]þ, 50%).
˚
˚
(1.991(3)-2.015(5) A) and Pt-P (2.2664(9)-2.2784(4) A)
bond distances are typical for PtII-PCP pincers.15 The
C-Pt-X angles are close to 180°. Despite the different
trans-influence of the halides, the Pt-C distances in the
complexes are rather close (1991(3), 1.997(4), and
Photoproduct A. Compound 1 (0.05 g, 0.067 mmol) was
dissolved in 20 mL of CHCl3 and irradiated with a tungsten lamp
(100 W) for 24 h. The resulting solution was filtered, and the
solvent was removed by rotaevaporation. Solids obtained were
dissolved in CH3CN and filtered. Violet block crystals were
obtained by slow diffusion of diethyl ether into a CH3CN
solution. Yield = 56%. C46H32PtP2 Calcd (%): C, 65.55; H,
3.83. found (%); C, 65.38; H, 3.86. 1H NMR(CDCl3, 500 M Hz,
δ/ppm): 8.29 (dd, 2H, JH-H=7.4 Hz, H4,5), 7.88 (m, 4H, Ph),
7.82(m, 4H, Ph), 7.71(m. 2H, H2,7), 7.60 (dd, J=7.4 Hz, 2H, H3,6),
7.48-7.43 (m, 12H, Ph), 31P{1H} NMR (CDCl3, 121.5 MHz,
δ/ppm): 39.34 (s), (1JPt-P= 3171 Hz). ESI MS: m/z 851([M]þ,
100%). 13C{1H} NMR (CDCl3, 75.48 M Hz, δ/ppm): 78.9
(s, platinatedC(9)), 129.4-161.4(aromatic C), 184.8 (s, ketone C).
˚
2,015(5) A for 1, 2, and 3, respectively), indicating that
it is fixed by the rigid pincer coordination. On the other
hand, because of the strong trans-influence of the carba-
˚
nion, the Pt-X distances (2.3914(9), 2.4799(6), 2.6645(5) A
for Cl, Br and I for X = Cl, Br, I) are consistently longer
(14) (a) SMART & SAINT Software Reference Manuals, version 4.0;
Siemens Energy & Automation, Inc., Analytical Instrumentation: Madison, WI,
1996. (b) Sheldrick, G. M. SADABS: A Software for Empirical Absorption
Photoproduct B1 4CH2Cl2. It was synthesized by photolysis
3
::
::
Correction; University of Gottingen: Gottingen, Germany, 1996. (c) SHELXTL
Reference Manual, version 5.03; Siemens Energy & Automation, Inc., Analytical
Instrumentation: Madison, WI, 1996.
(Tungsten lamp, 100 W) of an aerated CHCl3 solution of A for 2
days. The solvent was purified by fractional distillation to
remove EtOH. The compound was isolated and purified by
recrystallization from CH2Cl2/Et2O. Yield = 62%. C76H52-
Pt2P4O5Cl2 Calcd (%): C, 56.00; H, 3.19. found (%); C, 56.81;
(15) (a) Schwartsburd, L.; Poverenov, E.; Shimon, L. J. W.; Milstein, D.
Organometallics 2007, 26, 2931. (b) Adams, J. L.; Arulsamy, N.; Roddick, D. M.
Organometallics 2009, 28, 1148. (c) Poverenov, E.; Leitus, G.; Shimon, L. J. W.;
Milstein, D. Organometallics 2005, 24, 5937.
H, 3.42. 1H NMR (300 Mz, in CDCl3, δ/ppm): 8.07 (d, JH-H
=

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9687
B1•4CH2Cl2
Table 1
complex
1•CH3CN
C40H30ClP2Pt
817.13
monoclinic
P21/n
a = 12.7731(6) A
A•CH3CN
empirical formula
formula weight
crystal system
space group
C40H30ClNO2P2Pt
849.13
C80H60Cl10O5P4Pt2
1969.84
orthorhombic
Pbca
a = 18.7678(6) A
monoclinic
C2/c
a = 29.565(2) A
b = 12.9527(7) A
c = 23.9535(15) A
˚
˚
˚
˚
˚
unit cell dimensions
˚
b = 17.7800(8) A
˚
c = 14.4478(7) A
b = 16.9665(6) A
˚
c = 21.6485(8) A
˚
R = 90°
β = 96.539(1)°
γ = 90°
3259.8(3)
R = 90°
R = 90°
β = 125.014(1)°
γ = 90°
7512.8(8)
β = 90°
γ = 90°
6893.4(4)
3
volume (A )
˚
Z
4
1.665
4.515
1608
0.20 ꢀ 0.20 ꢀ 0.14
1.82 to 27.50°
-16 e h e 16
-21 e k e 23
-15 e l e 18
22701
8
1.636
4.278
3344
0.40 ꢀ 0.30 ꢀ 0.12
1.87 to 27.50°
-24 e h e 24
-22 e k e 22
-28 e l e 28
86389
4
1.742
4.214
3864
0.10 ꢀ 0.06 ꢀ 0.04
1.68 to 27.50°
-34 e h e 38
-12 e k e 16
-31 e l e 29
26185
density (calculated, g cm-3
)
Absorption coefficient (mm-1
F(000)
)
crystal size (mm3)
θ range for data collection
index ranges
Reflections collected
independent reflections
max. and min transmission
data/restraints/parameters
Goodness-of-fit (GOF)b
7470 [R(int) = 0.0439]
0.5705 and 0.4654
7470/0/526
1.007
R1 = 0.0315, wR2 = 0.0637
1.102 and -0.770
7920 [R(int) = 0.0544]
0.6278 and 0.2795
7920/0/426
1.236
R1 = 0.0404, wR2 = 0.0934
3.621 and -1.428
8608 [R(int) = 0.0460]
0.8495 and 0.6780
8608/8/456
1.067
R1 = 0.0561, wR2 = 0.1555
2.054 and -2.010
final R indicesa[I > 2σ(I)]
-3
˚
largest diff. peak and hole (e A
)
P
P
P
P
P
a R1 = ||Fo| - |Fc||/ |Fo|; wR2 = [ w(Fo2 - Fc2)2/ w(Fo4)]1/2
.
b GOF = [ [w(Fo2 - Fc2)2]/(n - p)]1/2. For all crystal determinations, scan type
˚
and wavelength of radiation used are ω and 0.71073 A, respectively.
Figure 1. ORTEP plot of 1 CH3CN (thermal ellipsoids are drawn with
3
50% probability). The hydrogen atoms and solvent are omitted for
clarity.
Table 2. Selected Bond Lengths and Angles Pt(DPA)Cl•CH3CN
Figure 2. UV-vis absorption spectra of DPA (green), 1 (black), 2
(blue), and 3 (red) in CH2Cl2 at 298 K.
Pt(1)-C(9)
Pt(1)-P(2)
Pt(1)-Cl(1)
1.991(3)
2.2664(9)
2.3914(9)
C(9)-Pt(1)-P(2)
P(1)-Pt(1)-P(2)
C(9)-Pt(1)-Cl(1)
P(2)-Pt(1)-Cl(1)
83.79(10)
167.34(3)
177.54(10)
94.53(3)
tion bands, suggesting that the spectra are dominated by
πfπ* transitions of the anthracenyl rings.17 Notably,
there is a progressive red-shift of the absorptions from
anthracene to DPA to the Pt complexes: the vibronic
bands of DPA and 1 are shifted from that of anthracene
by 1580 cm-1 and 4430 cm-1, respectively. It indicates
increasing perturbations of the electronic structures of the
anthracenyl ring as it is substituted first with PPh2 groups
and then the PtII ion. No distinct metal-to-ligand-charge-
transfer (MLCT) or ligand-to-ligand-charge-transfer
(LLCT) band is found in the spectra as they are probably
masked by the intense ligand-centered transitions.
2-
than the corresponding bonds in PtIIX4
(2.308(2),
16
˚
2.418(9), and 2.606(2) A).
Electronic Spectroscopy. Figure 2 shows the UV-vis
absorption spectra of the PtII-complexes and DPA. The
absorption spectra of the complexes are very similar, all
showing a moderately intense vibronic band in 390-480
nm (λmax = 454 nm, εmax=7.6-9.2 ꢀ 103 M-1 cm-1), and
a very intense band at ∼280 nm (ε
=5.2-5.9 ꢀ 104
max
M-1 cm-1). Both anthracene (λ max =255 nm, 358 nm)
and DPA (λ max=266 nm, 380 nm) show similar absorp-
(16) (a) Britten, J.; Lock, C. J. L. Acta Crystallogr. B 1979, 35, 3065. (b)
Melanson, R.; Rochon, F. D.; Hubert, J. Acta Crystallogr. B 1979, 35, 736. (c)
Olsson, L. F.; Oskarsson, A. Acta Chem. Scand. 1989, 43, 811.
(17) (a) Kessinger, M.; Michl, J. Excited States and Photochemistry of
Organic Molecules; VCH: New York, 1995. (b) Turro, N. J. Modern Molecular
Photochemistry; Benjamin: Menlo Park, 1978.

9688 Inorganic Chemistry, Vol. 48, No. 20, 2009
Hu et al.
Figure 4. UV-visible absorption spectra of an air-saturated CDCl3
solution of 1 at different irradiation time. Conditions: light intensity =
80 W, wavelength = 300-500 nm. The arrows indicate directions of
absorbance changes.
Figure 3. Emission spectra of 1 in a degassed CH2Cl2 solution (black)
and in an oxygen-saturated CH2Cl2 solution (red) at room temperature
(excitation wavelength = 390 nm).
However, in the C2v symmetry of the complexes, the
5dxz and the 6pz orbitals of the PtII ion and the npz (n = 3,
4, or 5) orbital of the halide have b1 symmetry as the
highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) of the
anthracenyl ring.18 It is expected that mixing of the
orbitals would introduce MLCT and LLCT character
to the πfπ* transitions.
Emission. The Pt complexes are luminescent in solution
and solid state at room temperature. Irradiating degassed
CH2Cl2 solutions of the complexes at 390 nm give emis-
sions at λmax = 474 nm (Figure 3) with vibronic shoulder
at 520 nm. The small Stokes shift (∼900 cm-1) between
the emission and the πfπ* absorption suggests the
emission arises from the lowest energy ππ* excited state
of the anthracenyl ring. The complex 1 displays strong
emission in degassed CH2Cl2 with quantum yield of 0.22.
The emissions of the complexes can be quenched
by oxygen. Bubbling O2 into a CH2Cl2 solution of 1 for
15 min reduces the emission intensity to ∼70% of that of
the degassed solution. The fluorescence restores nearly to
its original intensity after removal of oxygen by freeze-
pump-thaw cycles.
Photolysis of 1. Upon irradiation (λ = 300-500 nm),
an oxygen-saturated CHCl3 solution of 1 turned from
yellow to purple. Figure 4 shows the UV-vis spectral
changes during photolysis. “Wet chloroform” containing
adventitious water was used as solvent for the photolysis.
It was shown later that water took part in the formation
of photoproducts.
Figure 5. 31P{1H} NMR spectral changes during the photolysis of an
oxygen-saturated CDCl3 solution of 1 (irradiation wavelength =
300-500 nm, power = 80 W). Inset shows magnified P1 and PA.
31P{1H} NMR spectroscopy provided useful informa-
tion about the photolysis (Figure 5). The starting complex
1 showed a singlet (labeled P1) at δ 40.22 (1JPt-P = 2995
Hz). A new singlet PA at δ 39.34 (1JPt-P = 3171 Hz)
appeared after irradiation for 5 min. As the photolysis
continued, the intensity of the PA grew at the expense of
that of the P1. The P1 completely disappeared after 4 h of
irradiation and a new singlet PB at δ 18.57 (1JPt-P = 2870
Hz) started to appear. The intensity of the PB grew very
slowly at the expense of that of the PA, and after 30 h of
irradiation, the PB became the most intense signal. The
species that give rise to the PA and the PB are labeled A
and B, respectively. The results of the 31P NMR experi-
ment showed that the formation of A and B was sequen-
tial, that is, 1fAfB. Clearly, both A and B are
Pt-phosphine complexes as the PA and the PB display
Pt satellites. Furthermore, since both signals are singlet, it
implies the P atoms in each of the two complexes are
equivalent, as confirmed later by the crystal structures of
A and B.
The vibronic πfπ* band in 390-480 nm disappeared
in 1 h. Concurrently, two weak absorption bands at 538
and 706 nm developed. The bleaching of the anthracenyl
πfπ* transitions indicates the π-conjugation of the
anthracenyl ring is partially disrupted. No isosbestic
point was observed in the overlaid spectra, indicating
that more than one species was produced in the photo-
chemical reaction.
The sequential formation of A and B was also observed
in the H NMR spectra of the photolysis (Supporting
Information) which showed a new set of signals after
1
(18) The short and long axes of the anthracenyl ring are defined as x- and
y-axes.

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9689
ring. The C(10)-H bond in 1 is converted to a ketone
carbonyl group C(10)dO(1). The two lateral rings of the
anthrone are bent toward each other (dihedral angle =
15.3°). Similar conformation is found in other anthrones
such as 10-bromoanthrone (dihedral angle = 16°).19
While the C(9) remains coordinated to the Pt ion, it
becomes four-coordinate with an additional hydroxyl
group. Because of structural constraints imposed by the
planar geometry of the PtII ion and the bent conformation
of the anthrone, the bonding geometry of the metalated
C atom deviates from the ideal geometry for sp3 hybridi-
zation. For example, the O(2)-C(9)-C(11) and
C(13)-C(9)-Pt(1) angles are 117.1(5) and 114.4(3), re-
spectively. The hydroxyl group is hydrogen bonded to the
N atom of a CH3CN molecule (calcd N H distance =
˚
3 3 3
˚
2.022 A). The C(10)-O(1) distance of 1.223(7) A is typical
for CdO double bond and the C(9)-O(2) distance of
1.487(6) A signifies a single bond. The central ring of the
˚
anthrone displays a boat conformation with the hydroxyl
group occupying the pseudo-axial position and the car-
bonyl bond being tilted slightly away from the two lateral
rings. The carbonyl C(10) atom and the C(9) atom in the
central ring are located out of the planes defined by the
˚
other 4 carbon atoms by 0.158 and 0.380 A (c.f. 0.12 and
II
0.19 A for 10-bromoanthrone). The Pt ion is chelated by
˚
the two P atoms and the C(9) carbanion, forming a
distorted square planar geometry. The Pt(1)-C(9) bond
II
3
˚
distance (2.051(5) A) is in the range of normal Pt -C(sp )
˚
bond lengths (2.03-2.07 A).
The C(9)-Pt(1)-Cl(1) axis is slightly tilted from the
central C(9)-C(10) axis of the anthrone. Interactions
between the Pt ion and the hydroxyl O atom seem possible
as the atoms are separated by 2.892 A, slightly shorter
than the sum of the van der Waals radii of the two atoms
˚
(3.24 A).
Figure 6. (a) ORTEP diagram of A CH3CN. Thermal ellipsoids are
3
drawn with 50% probability. All the hydrogen atoms are omitted except
the OH group. (b) Side-view showing the butterfly shape of the anthrone
ligand and the stacking of the molecules. H atoms and MeCN were
omitted.
˚
Structure of B1. As previously mentioned, prolonged
1
˚
Table 3. Selected Bond Lengths (A) and Angles (deg) for A•CH3CN
photolysis gave the photoproduct B (δ 18.57, JPt-P
=
a diketal derivative of B,
2870 Hz). However,
B1 4CH2Cl2, was isolated by recrystallization of the
product in CH2Cl2/Et2O (Figure 7, Table 4). The struc-
Pt(1)-C(9)
Pt(1)-P(1)
Pt(1)-P(2)
Pt(1)-Cl(1)
C(9)-O(2)
C(10)-O(1)
2.051(5)
C(9)-Pt(1)-P(2)
C(9)-Pt(1)-Cl(1)
P(1)-Pt(1)-P(2)
P(1)-Pt(1)-Cl(1)
O(2)-C(9)-C(13)
O(2)-C(9)-C(11)
O(2)-C(9)-Pt(1)
C(11)-C(9)-C(13)
C(11)-C(9)-Pt(1)
C(13)-C(9)-Pt(1)
C(12)-C(10)-C(14)
83.94(14)
174.32(14)
166.29(5)
96.00(5)
104.3(4)
102.6(4)
108.6(3)
111.5(4)
114.2(3)
114.4(3)
116.5(5)
2.2670(13)
2.2650(13)
2.3967(13)
1.487(6)
3
ture of the complex shows a binuclear PtII complex in
2
which two anthrone units are joined by an unusual
platinated diketal PtO-C-O(3)-C-OPt linkage. The
two halves of the molecule are related by a C2 axis passing
through the central O(3) atom which is bonded to the C(9)
and C(9A) atoms at the flagpole positions of the two
anthrone rings.
1.223(7)
5 min of irradiation. Although the signals of 1 and A
overlapped extensively in the aromatic region, the forma-
tion of A was shown by the appearance of a doublet of
doublets at δ 8.29 (H4,5 of A) and disappearance of the
signals for H10 (δ 8.35) and H4,5 (δ 8.11) of 1. The signals
of 1 vanished after 4 h, and prolonged photolysis saw
weakening of the signals of A (i.e., dd at δ 8.29) and
growth of signals belonging to B.
Structure of Photoproduct A. The photoproduct A was
isolated and X-ray crystal analysis showed that it is a PtII-
9-hydroxyanthrone pincer complex (Figure 6 and
Table 3). To our knowledge, A is the first metal-anthrone
complex ever synthesized and structurally characterized.
The structure confirmed that the photochemical reac-
tion is an oxygenation in which two O atoms are intro-
duced to the C(9) and C(10) positions of the anthracenyl
Comparing the structures of A and B1 shows that an
oxygen atom O(2) is inserted in each Pt-C bond. The
˚
Pt-O(2) bond (1.996(4) A) is slightly shorter than the one
II
20
in a reported Pt -ketal complex (2.027(3) A). The PtII
ion displays a severely distorted square planar geometry
where the P-Pt-P angle facing the O(2) is 155.79(6)°
only.
˚
The bonding geometry of the flagpole carbon atom
C(9) is distorted from ideal tetrahedral. The Pt-Cl bonds
˚
in B1 (2.3022(19) A) are shorter than the Pt-Cl bonds
in 1 (2.3914(9) A) and A (2.3967(13) A), suggesting the
˚
˚
(19) Destro, R.; D’Alfonso, T. B.; Simonetta, M. Acta Crystallogr. B
1973, 29, 2214.
(20) Bergamini, P.; Marchesi, E.; Bertolasi, V.; Fogagnolo, M.; Scarpantonio,
L.; Manfredini, S.; Vertuani, S.; Canella, A. Eur. J. Inorg. Chem. 2008, 529.

9690 Inorganic Chemistry, Vol. 48, No. 20, 2009
Hu et al.
Scheme 2
˚
central rings by 0.659 and 0.335 A, respectively. The
highly distorted coordination geometry of the Pt ion and
anthrone ring suggests that B1 is more strained than A.
Electronic Spectroscopy of A and B1. In contrast to the
yellow color of 1, A and B1 are deep red in color. The
absorption spectra of the complexes show a weak, low
energy absorption band in ∼450-700 nm (Figure 8). The
energies and intensities of the absorption bands are
similar: the absorption maxima εmax (M-1 cm-1) for A
and B1 are 540 nm (302) and 555 nm (226), respectively.
The bands show weak vibronic features. In contrast,
10-hydroxyanthrone shows a weak shoulder at 402 nm
and does not absorb at >430 nm.
Figure 7. ORTEP plot of B1 4CH2Cl2 (30% thermal ellipsoids); H
3
atoms and solvent molecules are omitted.
˚
Table 4. Selected Bond Lengths (A) and Angles (deg) for B1 4CH2Cl2
3
Pt(1)-P(1)
Pt(1)-O(2)
Pt(1)-Cl(1)
C(9)-O(2)
C(9)-O(3)
C(10)-O(1)
2.2991(17)
1.996(4)
2.3022(19)
1.363(7)
1.459(7)
1.210(8)
P(1)-Pt(1)-Cl(1)
P(1)-Pt(1)-P(2)
P(1)-Pt(1)-O(2)
Cl(1)-Pt(1)-P(2)
Cl(1)-Pt(1)-O(2)
Pt(1)-O(2)-C(9)
C(9A)-O(3)-C(9)
O(2)-C(9)-O(3)
O(2)-C(9)-C(11)
O(3)-C(9)-C(11)
O(3)-C(9)-C(13)
C(11)-C(9)-C(13)
99.23(7)
155.79(6)
84.44(14)
97.14(7)
170.88(15)
114.4(4)
123.3(6)
110.2(5)
117.1(5)
106.0(4)
97.7(4)
It has been established that the lowest energy spin-
1
allowed transition of anthrones is nfπ*(CdO, phenyl
rings).21 The low intensities of the 540 and 555 nm
absorption bands of the complexes are in accord with
1
the orbital forbidden nature of nfπ* transition. Both
complexes display very weak absorption in 670-750 nm.
The absorption of A is most intense and distinct, showing
a peak at 706 nm (εmax = 64 M-1 cm-1). It is tentatively
3
assigned to the spin-forbidden nfπ* transition as the
108.5(5)
lowest energy excited states of aromatic ketones such as
benzophenone and anthrone have been shown to be
3nπ*.21a Degassed solution of A exhibits a weak emission
at 732 nm (Φem = 3 ꢀ 10-3). The very small Stokes
3
shift between the emission and the nfπ* absorption
(∼500 cm-1) suggests the luminescence is derived from
the 3nπ* excited state.
Mechanistic Postulate. 10-Hydroxylanthrone (Scheme 1),
one of the products of photooyxgenation of anthracene,
arises from photoinduced homolytic cleavage of the peroxi-
dic bond in the primary photoproduct 9,10-endoperoxide
(EP) followed by rearrangement. That the hydroxyanth-
rone complex A is formed in the photooxygenation of 1
argues strongly that a PtII-endoperoxide complex (Pt-EP,
Scheme 2) is formed in the reaction between oxygen and 1.
It is possible that the Pt-EP is produced from reaction
of 1 and 1O2, as in the case for EP. The involvement of 1O2
is evident from the fact that bleaching of the πfπ* band
is faster in CDCl3 than in CHCl3 (Figure 9). The observa-
Figure 8. UV-vis absorption spectra of A (black), B1 (green), and 10-
hydroxyanthrone (blue) in CH2Cl2 at room temperature, and emission
spectrum of a degassed CH2Cl2 solution of A (orange, excitation wave-
length = 600 nm).
1
tion is in accord with the fact that the lifetime of O2 in
CDCl3 (300 μs) is longer than in CHCl3 (30 μs).22 The 1O2
could arise from photosensitization of the ground state
O2 by the excited state of 1. The Pt-EP was not observed
in the NMR spectrum of the photolyzed solution,
trans-influence of the carbanion is stronger than the
alkoxide ion in the ketal group. The central ring of the
anthrone shows a boat conformation whereby the O(3) is
in the axial position. The anthrone ring in B1 is severely
bent, showing a dihedral angle of 43.6° between the two
lateral aromatic rings. The C(9) and C(10) atoms of the
central ring deviated from the other four C atoms in the
(21) (a) Lipson, M.; McGarry, P. F.; Koptyug, I. V.; Staab, H. A.; Turro,
N. J.; Doetschman, D. C. J. Phys. Chem. 1994, 98, 7504. (b) Hoffmann, R.;
Swenson, J. R. J. Phys. Chem. 1970, 74, 415.
(22) Kearns, D. R.; Merkel, P. B.; Nilsson, R. J. Am. Chem. Soc. 1972, 94,
7244.

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9691
Scheme 3
into two parts, one was exposed to visible light while the
other was kept in darkness. The 31P{1H} NMR spectrum
of the solution irradiated for 2 days indicated 85%
conversion whereas only 6% conversion was observed
for the solution kept in darkness for 2 weeks. The result
demonstrates clearly that the oxidation of A to B is very
slow but can be accelerated by photoexcitation. Examples
of direct O atom insertion into transition-metal-carbon
bond have been reported for some cyclometalated
palladium complexes26 but strong oxidant such as me-
tal-peroxide or t-butyl hydroperoxide is needed to
insert O atom into the Pd-C(sp2) bond. Oxygenation of
the metal-C(sp3) in (t-Bu3CO)2M(Me)2 (M=Ti, Zr, Hf)27
involves direct insertion of O2 to generate metal per-
oxides or alkoxides. However, for late transition
metals, the reactions are limited to the first row and
there is no example of direct insertion of O2 into Pt-C
bond to form Pt-alkoxide. Recently, Goldberg et al.
showed direct insert of O2 into Pt-C bond to form a
Pt-alkylperoxide28 but not a Pt-alkoxide as in the pre-
sent case.
Figure 9. Absorbance of452 nmbandat different timeofphotolysis of1
in aerated CHCl3 (red) and CDCl3 (black).
indicating that conversion from A to Pt-EP is fast and
efficient.
We proposed that the oxidation of A to B involves
oxidation of the anthrone by oxygen to the PtII-anthra-
quinone intermediate which is the same as the one pro-
posed for the ketalization of B (Scheme 3). Nucleophilic
attack of H2O at the intermediate gives rise to the hemi-
ketal B (Scheme 3). The proposed mechanism can
explain the fact that the oxidation can be accelerated by
irradiation as the emissive excited state of A can sensitize
In view of facile acid-catalyzed ketalization of hemi-
ketal, we believe that the diketal B1 originates from
dimerization of a hemiketal B. On the basis of the well-
established mechanism for ketalization,23 a mechanism
for the formation of B and B1 is postulated (Scheme 3).
First of all, in the presence of HCl, which is a common
side product in photochemical reactions in chloroform,24
a water molecule is eliminated from B to generate a PtII-
anthraquinone intermediate which consists of a PtII-(η2-
OdC)þ π-bonding.
1
conversion of ground state O2 to O2 which is more
oxidizing than the former.
It is noted that Milstein et al. reported an isoelectronic
IrI pincer complex that exhibits IrI-(η2-OdC) quinonoid
coordination mode.25 Nucleophilic attack of the hydro-
xyl group of another molecule of B at the electrophilic
PtII-(η2-OdC)þ unit of the intermediate would lead to B1.
Light can accelerate the oxidation of A to B conversion.
In one experiment, an aerated solution of A was divided
Conclusion
Photooxidation of the cyclometalated PtII-anthracene pin-
cer complex leads to a PtII-9-hydroxyanthrone complex that
can be further oxidized to a PtII-hemiketal complex. It is
believed a PtII-9,10-endoperoxide complex is formed from
the reaction between 1 and singlet oxygen. The PtII-hemiketal
complex undergoes ketalization to form a PtII2-diketal via a
(23) (a) Fife, T. H. Acc. Chem. Res. 1972, 5, 264. (b) Anslyn, E. V.;
Dougherty, D. A. Modern Physical Organic Chemistry; University Science
Books: Sausalito, CA, 2004, p 578.
(24) (a) Hoggard, P. E.; Bridgman, A. J.; Kunkely, H.; Vogler, A. Inorg.
Chim. Acta 2004, 357, 639. (b) Hautecloque, S. J. Photochem. 1980, 14, 157.
(25) Vigalok, A.; Rybtchinski, B.; Gozin, Y.; Koblenz, T. S.; Ben-David,
Y.; Rozenberg, H.; Milstein, D. J. Am. Chem. Soc. 2003, 125, 15692.
(26) (a) Alsters, P. L.; Boersma, J.; van Koten, G. Organometallics 1993,
12, 629. (b) Alsters, P. L.; Teunissen, H. T.; Boersma, J.; Spek, A. L.; van Koten,
G. Organometallics 1993, 12, 4691.
(27) (a) Lubben, T. V.; Wolczanski, R. T. J. Am. Chem. Soc. 1987, 109,
424. (b) Brindley, P. B.; Hodgson, J. C. J. Organomet. Chem. 1974, 65, 57.
(28) Grice, K. A.; Goldberg, K. I. Organometallics 2009, 28, 953.

9692 Inorganic Chemistry, Vol. 48, No. 20, 2009
Hu et al.
PtII-anthraquinone intermediate which is proposed as the
intermediate in the oxidation of the PtII-9-anthrone to the
PtII-hemiketal.
and National University of Singapore are thanked for pro-
viding financial support.
Supporting Information Available: CIF of 1, A, and B1, X-ray
crystal structures of 2 and 3, and 1H NMR spectra of photolysis
of 1. This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We thank Ms Tan Geok Kheng and
Prof Koh Lip Lin for determining the X-ray crystal struc-
tures. Ministry of Education, Singapore (R-143-000-331-112)