Generation of trans-dioxoruthenium(VI) porphyrins: A photochemical approach

Article abstract of DOI:10.1016/j.ica.2011.06.002

trans-Dioxoruthenium(VI) porphyrin complexes have been developed as one of the best-characterized model systems for heme-containing enzymes. Traditionally, this type of compounds can be prepared by oxidation of ruthenium(II) precursors with peroxyacids an

Full text of DOI:10.1016/j.ica.2011.06.002

Inorganica Chimica Acta 376 (2011) 152–157
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Generation of trans-dioxoruthenium(VI) porphyrins: A photochemical approach
⇑
⇑
Rui Zhang , Yan Huang, Chris Abebrese, Helen Thompson, Eric Vanover, Cathleen Webb
Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101-1079, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
trans-Dioxoruthenium(VI) porphyrin complexes have been developed as one of the best-characterized
model systems for heme-containing enzymes. Traditionally, this type of compounds can be prepared
by oxidation of ruthenium(II) precursors with peroxyacids and other terminal oxidants under different
conditions, depending on the porphyrin ligands. In this work, a new photochemical generation of
trans-dioxoruthenium(VI) porphyrins has been developed by extension of the known photo-induced
ligand cleavage reactions. Refluxing ruthenium(II) carbonyl porphyrins [RuII(Por)(CO)] in carbon tetra-
chloride afforded dichlororuthenium(IV) complexes [RuIV(Por)Cl2]. Facile exchange of the counterions
in [RuIV(Por)Cl2] with Ag(ClO3) or Ag(BrO3) gave the corresponding dichlorate [RuIV(Por)(ClO3)2] or dibro-
mate [RuIV(Por)(BrO3)2] salts. Visible-light photolysis of the photo-labile porphyrin–ruthenium(IV) dichl-
orates or dibromates resulted in homolytic cleavage of the two O–Cl or O–Br bonds in the axial ligands to
produce trans-dioxoruthenium(IV) species [RuVI(Por)O2] bearing different porphyrin ligands.
Ó 2011 Elsevier B.V. All rights reserved.
Received 18 March 2011
Received in revised form 20 May 2011
Accepted 2 June 2011
Available online 12 June 2011
Keywords:
trans-Dioxoruthenium(VI) porphyrin
Photolysis
Homolytic cleavage
Photo-oxidation
1. Introduction
concentrations of active oxidants will not build up to concentra-
tions that permit detection and direct kinetic studies. Moreover,
Catalytic oxidations are core transformations in organic synthe-
sis. Millions of tons of oxygenated compounds are annually pro-
duced and applied worldwide, ranging from pharmaceutical to
large-scale commodities [1–4]. In Nature, an ubiquitous type of
monooxygenase is the cytochrome P-450 enzyme, which features
an iron porphyrin core, and can catalyze a wide variety of oxidation
reactions including unreactive hydrocarbons with exceptionally
high reactivity and selectivity [5–7]. Many transition metal cata-
lysts, with a core structure closely resembling that of the iron por-
phyrin core of the P450s, have been designed as models to probe
the sophisticated mechanism of molecular oxygen activation to-
gether with a more goal oriented approach to invent enzyme-like
oxidation catalysts [1,8].
a high valent metal-oxo species detected in a reaction might not
be the true oxidant but a precursor to the true oxidant that is
formed in small, undetectable amounts [12]. The resulting lack of
kinetic and mechanistic information complicates attempts to de-
duce the identities of the active oxidants. Indeed, most commonly,
the nature of the active oxidants in homogeneous catalysis has
been inferred indirectly from product studies [13–16]. The success-
ful generation and characterization of the reactive metal-oxo spe-
cies will allow a direct assessment of their reactivities and provide
important insight into chemical modeling of the enzyme-like oxi-
dants, and ultimately lead to catalyst development for the selective
oxidation of organic substrates in large-scale industrial processes.
Ruthenium porphyrin complexes are among the most exten-
sively studied biomimetic catalysts for hydrocarbon oxidation be-
cause of their rich coordination and redox chemistry [1,8,9,17]. In
particular, trans-dioxoruthenium(VI) porphyrin complexes, abbre-
viated as RuVI(Por)(O)2, have received considerable attention as
model systems for heme-containing enzymes [18–20]. This inter-
esting family of high-valent ruthenium complexes exhibits signifi-
cant reactivity toward organic substrates such as phosphines,
sulfides, alcohols and hydrocarbons [21–23]. Notably, trans-
dioxoruthenium(VI) porphyrins have been shown to catalyze the
clean aerobic epoxidation of olefins in the absence of a reducing
agent under mild conditions [24,25]. Although trans-dioxorutheni-
um(VI) derivatives are the best characterized oxidizing intermedi-
ates, their involvement as the active oxidant in the catalytic cycle
of ruthenium porphyrins with aromatic N-oxides as the oxygen
In both synthetic and natural catalysts, high-valent transition
metal-oxo species have been implicated as the active oxidizing
species [9–11]. In
a typical catalytic reaction, however, the
Abbreviations: Por, porphyrin; TMP, 5,10,15,20-tetramesitylporphyrin dianion;
D4-Por⁄, 5,10,15,20-tetrakis[(1S,4R,5R,8S)-1,2,3,4,5,6,7,8-octahydro-1,4:5,8dimet-
hanoanthracen-9-yl]porphyrin dianion; TPP, 5,10,15,20-tetraphenylporphyrin
dianion; 4-MeO-TPP, 5,10,15,20-tetra(4-methoxyphenyl)porphyrin dianion; 4-
CF3-TPP, 5,10,15,20-tetra(4-trifuoromethylphenyl)porphyrin dianion; TPFPP,
5,10,15,20-tetrapentafluorophenylporphyrin dianion; m-CPBA, meta-chloroperoxy-
benzoic acid.
⇑
Corresponding authors. Address: Department of Chemistry, Western Kentucky
University, 1906 College Heights Blvd. #11079, Bowling Green, KY 42101-1079,
USA. Tel.: +1 270 745 8899; fax: +1 270 745 3803.
E-mail addresses: rui.zhang@wku.edu (R. Zhang), cathleen.webb@wku.edu
(C. Webb).
0020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.06.002

R. Zhang et al. / Inorganica Chimica Acta 376 (2011) 152–157
153
source has been ruled out in view of the low reactivity and enanti-
oselectivity [26,27].
development of the requisite photochemical methods and espe-
cially the photo-labile precursors. For examples, photochemical
cleavages of porphyrin-manganese(III) nitrates [33] or chlorates
[12], corrole-manganese(IV) chlorates [36] and corrole-iron(IV)
chlorates [37], give neutral porphyrin-manganese(IV)-oxo, cor-
role-manganese(V)-oxo and corrole-iron(V)-oxo derivatives by
homolytic cleavage of the O–Cl or O–N bond in the axial ligand.
On the other hand, photolysis of porphyrin-manganese(III) perchlo-
rate complexes gave much more reactive porphyrin-manga-
nese(V)-oxo transients by heterolytic cleavage of the O–Cl bond in
perchlorate [12,38].
The first isolation and characterization of the sterically hindered
trans-dioxoruthenium(VI) porphyrin complex RuVI(TMP)(O)2
(TMP = 5,10,15,20-tetramesityporphyrinato dianion) by oxidation
of RuII(TMP)(CO) with meta-chloroperoxybenzoic acid (m-CPBA)
was reported by Groves and Quinn [28]. The steric hindrance im-
posed by the ortho substituents at the phenyl groups prevents
the facile dimerization to give the inert diruthenium(IV)
l-
oxo-bridged complex. Che and co-workers had reported the
syntheses of non-sterically encumbered trans-dioxoruthenium(VI)
porphyrins RuVI(TPP)(O)2 (TPP = 5,10,15,20-tetraphenylporphyrin-
ato dianion) and RuVI(OEP)(O)2 (OEP = octaethylporphyrinato dian-
ion) in the presence of coordinating solvents (such as methanol)
[29]. The choice of solvent is very important to the success of the
synthesis, presumably due to the coordination of alcohol on the
ruthenium(IV) intermediate which can inhibit the formation of
Recently, we have extended the photo-induced ligand cleavage
reactions to produce the well-known trans-dioxoruthenium(VI)
complexes by irradiation of porphyrin–ruthenium(IV) dichlorate
complexes with visible light (Scheme 2) [39]. This photochemical
method can efficiently generate the trans-dioxoruthenium(VI)
complexes in sterically bulky and non-bulky porphyrins without
the limitation of porphyrin ligands as observed in chemical meth-
ods [39]. Herein, we report our full findings on the photochemical
generation of trans-dioxoruthenium(VI) complexes by irradiation
of porphyrin–ruthenium(IV) dichlorate or dibromate complexes
that result in homolytic cleavage of the O–X bond in both axial li-
gands (Scheme 2). We show this photochemical method can be
used to generate the trans-dioxoruthenium(VI) in various porphy-
rin systems under similar condition, in particular the very elec-
l-oxo dimer. In addition to peroxyacids, other sacrificial oxidants
such as iodosylbenzene (PhIO), periodate, and tert-butyl hydroper-
oxide (TBHP) were able to produce the trans-dioxoruthenium(VI)
species [9,28]. Increasingly electronegative iodosylbenzene
substituents like pentafluoroiodosylbenzene are seen to increase
dramatically the rate of dioxo formation. Later, chiral trans-diox-
oruthenium(VI) complexes were prepared in similar manners and
characterized with satisfactory spectroscopic properties [30–32].
Some trans-dioxoruthenium(IV) porphyrin complexes are even
determined by X-ray crystal structures [20,25].
tron-demanding
pentafluorophenylporphyrinato dianion).
RuVI(TPFPP)O2
(TPFPP = tetrakis-
Our particular interest in the context of metal-oxo chemistry is
to explore the photochemical approach that targets the direct
detection and kinetic study of high-valent transition metal-oxo
derivatives. With photochemical production of reactive metal-oxo
transients, one has access to time scales that are much shorter than
the fastest mixing experiments and kinetics of oxidation reactions
of the transients of interest are not convoluted with the kinetics
of reactions that form the transients. Recently, the photo-induced
ligand cleavages reactions have been developed to generate a vari-
ety of metal-oxo species [33–35]. The concept of photo-induced li-
gand cleavage reactions is very straightforward as illustrated in
Scheme 1. The precursors are metal complexes with the metal in
the n oxidation state and an oxygen-containing ligand such as per-
chlorate, chlorate, or nitrate. Photolysis can result in homolytic
cleavage of the O–X bond in the ligand to give an (n + 1) oxidation
state metal-oxo species (one electron photo-oxidation) or hetero-
lytic cleavage of the O–X bond in the ligand to give an (n + 2) oxida-
tion state metal-oxo species (two electron photo-oxidation).
Although straightforward in concept, the creation of photochemical
methods for formation of metal-oxo species requires considerable
The porphyrin systems that we have studied are shown in
Scheme 2, using abbreviations that follow those conventionally
established. Of these macrocyclic ligands, ligands a and b are gen-
erally considered as a sterically-encumbered porphyrin due to the
presence of relatively large substituents on the ortho positions of
the meso-phenyl groups, whereas ligands c–f are all typical steri-
cally-unencumbered porphyrins. The different aromatic groups
on the porphyrins also result in varying electron demands with
the phenyl system least electron withdrawing, the 4-methoxy-
phenyl system, and the pentafluorophenyl system most electron
withdrawing.
2. Experimental
2.1. General
All commercial reagents were of the best available purity and
were used as supplied unless otherwise specified. HPLC grade ace-
tonitrile (99.93%) was distilled over P2O5 prior to use. Free porphy-
rin ligand, H2TPFPP (f), was purchased from Aldrich and used as
received. Others including H2TMP (a) [40], H2(D4-Por⁄) (b) [41],
H2TPP (c) [42], H2(4-MeO-TPP) (d) [42], and H2(4-CF3-TPP) (e)
[42] were prepared according to the known methods. The corre-
sponding ruthenium(II) carbonyl complexes RuII(Por)(CO)
(Por = a–f) used for generation of RuIV(Por)Cl2 (1) were prepared
by literature methods [20] and fully characterized, matching those
reported [20,24,29]. UV–Vis spectra were recorded on an Agilent
8453 diode array spectrophotometer. IR spectra were obtained
on a Bio-Rad FT-IR spectrometer. 1H NMR was performed on a JEOL
ECA-500 MHz spectrometer at 298 K with tetramethylsilane (TMS)
as internal standard. Chemical shrifts (ppm) are reported relative
to TMS.
2.2. Preparation of dichlororuthenium(IV) porphyrin [RuIV(Por)Cl2]
[43]
In a typical run, RuIV(Por)Cl2 (1) was prepared by refluxing RuII
Scheme 1. Generation of high-valent metal-oxo species by photo-induced ligand
cleavage reactions.
(Por)(CO) complexes (50 mg) in CCl4 (30 mL) monitored by UV–Vis

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R. Zhang et al. / Inorganica Chimica Acta 376 (2011) 152–157
Scheme 2. Photochemical generation of trans-dioxoruthenium(VI) porphyrins.
spectroscopy. The refluxing time, ranging from 2 h to overnight, de-
pended on the porphyrin system employed. The RuIV(Por)Cl2 with
electron-deficient sterically unhindered porphyrin (4-CF3-TPP and
TPFPP) were readily purified by refluxing the crude mixture in a
2.7. [RuIV(4-CF3-TPP)Cl2] (1e)
Yield = 43%. 1H NMR (500 MHz, CDCl3) d, ppm: À57.6 (s, 8H, b-
pyrrole). IR (KBr, cmÀ1): 1006 (oxidation state marker band). UV–
Vis (CH3CN) kmax/nm: 406 (Soret), 530.
Soxhlet apparatus with diethyl ether to remove the
byproducts [44].
l-oxo dimer
2.8. [RuIV(TPFPP)Cl2] (1f)
2.3. [RuIV(TMP)Cl2] (1a)
Yield = 48%. 1H NMR (500 MHz, CDCl3) d, ppm: À53.3 (s, 8H, b-
pyrrole). IR (KBr, cmÀ1): 1009 (oxidation state marker band). UV–
Vis (CH3CN) kmax/nm: 405 (Soret), 525.
Yield = 96%. 1H NMR (500 MHz, CDCl3) d, ppm: À55.8 (s, 8H,
b-pyrrole). IR (KBr, cmÀ1): 1008 (oxidation state marker band).
UV–Vis (CH3CN) kmax/nm: 407 (Soret), 529.
2.9. Generation procedure for photosynthesis of trans-
dioxoruthenium(VI) porphyrins [RuVI(Por)(O)2]
2.4. [RuIV(D4-Por⁄)Cl2] (1b)
Treatment of compounds 1 (ca. 10 mg) with 2–5 folds excess of
AgClO3 in CH3CN gave the dichlorate complexes 2 that were gener-
ated in situ in most runs, and directly used in subsequent photo-
chemical reactions immediately after preparation. When the
solution of 2 at desired concentration (ꢀ1 Â 10À5 M) was irradi-
ated with a 100 W of visible light from a tungsten lamp at ambient
temperature, the formation of 3 was complete in ca. 50 min mon-
itored by UV–Vis spectroscopy. The dioxo complexes (3) were iso-
lated as dark red–purple crystalline solids, and can be readily
purified by passing through a short dry column (basic alumina)
to give satisfactory spectroscopic characterization spectra.
Yield = 98%. 1H NMR (500 MHz, CDCl3) d, ppm: À52.3 (s, 8H,
b-pyrrole). IR (KBr, cmÀ1): 1010 (oxidation state marker band).
UV–Vis (CH3CN) kmax/nm: 409 (Soret), 532.
2.5. [RuIV(TPP)Cl2] (1c)
Yield = 95%. 1H NMR (500 MHz, CDCl3) d, ppm: À58.2 (s, 8H,
b-pyrrole). IR (KBr, cmÀ1): 1005 (oxidation state marker band).
UV–Vis (CH3CN) kmax/nm: 408 (Soret), 528.
2.10. [RuVI(TMP)O2] (3a)
2.6. [RuIV(4-MeO-TPP)Cl2] (1d)
Yield = 92%. 1H NMR (500 MHz, CDCl3) d, ppm: 8.79 (s, 8H), 7.23
Yield = 86%. 1H NMR (500 MHz, CDCl3) d, ppm: À53.8 (s, 8H,
b-pyrrole). IR (KBr, cmÀ1): 1006 (oxidation state marker band).
UV–Vis (CH3CN) kmax/nm: 406 (Soret), 525.
(s, 8H), 2.62 (s, 12H), 1.89 (s, 24H). IR (KBr, cmÀ1): 1018 (oxidation
state marker band) and 820 (mRuO ). UV–Vis (CH2Cl2) kmax/nm: 422
(Soret), 521.
2

R. Zhang et al. / Inorganica Chimica Acta 376 (2011) 152–157
155
Scheme 3. Synthesis of dichlororuthenium(IV) porphyrins complexes.
2.11. [RuVI(D4-Por⁄)O2] (3b)
3. Results and discussion
Yield = 88%. 1H NMR (500 MHz, C6D6): d 9.24 (s, 8H), 7.40 (s,
4H), 3.46 (s, 8H), 2.78 (s, 8H), 2.03 (m, 8H), 1.66 (m, 8H), 1.41–
1.11 (m, 24H) and 0.96 (m, 8H). IR (KBr, cmÀ1): 1019 (oxidation
3.1. Preparation of dichlororuthenium(IV) porphyrin [RuIV(Por)Cl2]
Our study started with the preparation of dichlororutheni-
um(IV) porphyrins, RuIV(Por)Cl2 (1), that can be readily prepared
by heating the carbonyl precursor RuII(Por)(CO) in CCl4 in air as
described by Gross and Barzilay (Scheme 3) [43]. As expected,
RuII(Por)CO with sterically hindered porphyrin (Por = TMP, D4-
Por⁄) could be readily converted to the desired dichlororutheni-
um(IV) products (1a and 1b) in over 95% yields. The oxidative
transformation is characterized by a distinct color change from
orange red to dark brown, accompanied by the blue shift of
the Soret bands (CHCl3). Interestingly, RuII(Por)(CO) with steri-
cally unhindered ligands (Por = TPP and 4-MeO-TPP) also gave
the desired products 1c and 1d predominately. However, the
same procedure for the sterically unhindered porphyrins with
electron-withdrawing substituents (Por = 4-CF3-TPP and TPFPP)
state marker band) and 822 (mRuO ). UV–Vis (CH2Cl2): kmax/nm
2
(log e) 424 (5.38), 521 (4.35).
2.12. [RuVI(TPP)O2] (3c)
Yield = 86%. 1H NMR (500 MHz, CDCl3): d 9.1 (s, 8H), 8.6 (d, 8H).
8.4 (d, 4H), 7.8 (m, 8H). IR (KBr, cmÀ1): 1016 (oxidation state mar-
ker band) and 819 (mRuO ). UV–Vis (CH2Cl2) kmax/nm: 420 (Soret
2
band), 520 (Q band).
2.13. [RuVI(4-MeOTPP)(O)2] (3d)
led to the formation of substantial amounts of
l-oxo dimer
Yield = 94%.1H NMR (500 MHz, CDCl3): d 9.1 (s, 8H), 8.2 (d, 8H),
byproducts, [RuIV(Por)Cl]2O, along with RuIV(Por)Cl2 (1c and
1d). The former byproducts were identified by its diamagnetic
1H NMR signals similar to those of [RuIV(TPP)Cl]2O [45]. Using
an alternative method by reacting the corresponding RuVI(-
Por)(O)2 with Me3SiCl or HCl gave a similar result [46]. The rea-
7.3 (d, 8H), 4.2 (s, 12H). IR (KBr, cmÀ1): 1017 (oxidation state mar-
ker band) and 819 (mRuO ). UV–Vis (CHCl3) kmax/nm: 425 (Soret
2
band), 525 (Q band), 550 (sh).
son for the marked dependence of formation of
the porphyrin substituents is not yet clear. It seems that elec-
l-oxo dimer on
2.14. [RuVI (4-CF3TPP)O2] (3e)
tron-withdrawing substituents such as CF3 and F on porphyrin
Yield = 89%. 1H NMR (500 MHz, CDCl3): d 9.0 (s, 8H), 8.5 (d, 8H),
ligand favor
l-oxo dimer formation. Presumably, the electron-
8.1 (d, 8H). IR (KBr, cmÀ1): 1018 (oxidation state marker band) and
withdrawing substituents would stabilize the ruthenium(IV)
complex in a dimer form by reducing the electron density of me-
819 (mRuO ). UV–Vis (CHCl3) kmax/nm: 418 (Soret band), 520 (Q
band).
2
tal atoms. We also found the amount of
l-oxo dimer was highly
dependent on the reaction concentration, i.e. refluxing the car-
bonyl ruthenium(II) precursors in higher concentration in CCl4
2.15. [RuVI(TPFPP)O2] (3f)
gave a less amount of
able effect of substituent and concentration on the formation of
-oxo dimer is not apparent, and this subject deserves further
l-oxo dimers. The origin of the consider-
Yield = 82%. 1H NMR (300 MHz, CDCl3): d, ppm: 9.18 (s, 8H). IR
l
(KBr, cmÀ1): 1022 (oxidation state marker band) and 827 (mRuO ).
study. Following a known procedure [44], complexes 2e and 2f
with electron-deficient sterically unhindered porphyrin were
2
UV–Vis (CH2Cl2) kmax/nm: 412 (Soret), 506.
Fig. 1. (A) UV–Vis spectra of RuIV(TMP)Cl2 (1a, dashed) and RuIV(TMP)(ClO3)2 (2a, solid) in CH3CN. (B) 1H NMR spectrum of RuIV(TMP)Cl2 (1a, top) and RuIV(TMP)(ClO3)2 (2a,
bottom) in CDCl3.

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R. Zhang et al. / Inorganica Chimica Acta 376 (2011) 152–157
purified by refluxing the crude mixtures in a Soxhlet apparatus
with diethyl ether to remove the -oxo dimer byproducts.
spectra of 1a and 2a (see, for example, Fig. 1B), the paramagnetic
pyrollic signal of 2a is located at ca. À20 ppm (Fig. 1B bottom),
slightly more downfield than that of paramagnetic 1a (Fig. 1B,
top), consistent with weaker binding ability of chlorate than that
of chloride.
l
3.2. Preparation of porphyrin–ruthenium(IV) dichlorates
[RuIV(Por)(ClO3)2]
Each of the dichlororuthenium(IV) complexes (1) were charac-
terized by its distinct paramagnetically shifted pyrrolic protons
(d ranging from À52.3 to À58.2 ppm) [43]. Facile exchange of the
counterions in 1 with Ag(ClO3) gave the corresponding dichlorate
salts 2 that were characterized by UV–Vis and 1H NMR spectra.
Complexes 2 are photo-labile and can be handled for a short time
in solutions exposed to light. Interestingly, ruthenium(IV) dichl-
orate complexes 2 show featuring UV–Vis and paramagnetic 1H
NMR spectra, similar to those of dichloro compounds (1). The spec-
tra of 1a and 2a are shown in Fig. 1A as an example. In the 1H NMR
3.3. Photolysis of porphyrin–ruthenium(IV) dichlorates
[RuIV(Por)(ClO3)2]
Irradiation of chlorate complexes (2) in anaerobic CH3CN with
visible light from a tungsten lamp (100 W) resulted in homolytic
cleavage of the O–Cl bond in the two chlorate counterions to pro-
duce neutral dioxoruthenium(VI) species (3). Fig. 2 shows three
representative time-resolved formation spectra of RuVI(Por)(O)2 in
three different porphyrin systems, i.e. sterically encumbered 3b,
non-sterically hindered 3c and electron-efficient 3f. These time-re-
solved UV–Vis spectra show decay of photo-labile precursors (2)
and growth of the products (3) in absorption spectra with clean
isosbestic points. In each case, the formed species 3 display a stron-
ger red-shifted Soret and a blue-shifted weaker Q bands that are
characteristic for the corresponding trans-dioxoruthenium(VI) por-
phyrins [28,29]. This general approach is applicable to the evidently
most reactive 3f bearing the most electron demanding porphyrin
(TPFPP) [26]. The formation of 3f by photochemical oxidation was
also observed with a slowest rate among all systems studied
(Fig. 2C); this is in line with the expected highest oxidation poten-
tial of the RuIV species (2f) with the most electron-withdrawing
porphyrin ring of TPFPP [20]. The identities of complexes 3 as
RuVI(Por)(O)2 generated in photochemical reactions were further
confirmed by UV–Vis, 1H NMR and IR spectra (see Section 2). Con-
trol experiments showed that no dioxo species was formed in the
absence of light. As expected, using more intensive light
(kmax = 420 nm, ca. 300 W) resulted in faster formation of 3 in the
same solvent. The use of other non-coordinating solvent such as
CH2Cl2 or THF as the solvent also gave similar results. However,
no reaction was observed when methanol or ethanol was used. Pre-
sumably, the weakly binding chlorate counterions were displaced
by these coordinating solvents.
It is noteworthy that the photochemical reactions of chlorates 2
appears to present a reaction manifold similar to that of porphyrin-
manganese(III) chlorates [12], corrole-manganese(IV) chlorates
[36] and corrole-iron(IV) chlorates [37], which result in homolytic
cleavage of the O–Cl bond in the chlorate. Photolysis of MnIII(Por)
(NO3) complexes was reported to give MnIV(Por)(O) species by
homolytic cleavage of an O–N bond [33], similar to those of
chlorate complexes albeit with considerably less efficiency [12].
However, we found that photolyses of dinitrate ruthenium(IV)
complexes did not afford the dioxo complexes 3 even with pro-
longed irradiation and higher-energy light (kmax = 350 nm). The
reaction generated an unknown product with slightly blue shifted
Soret band (data not shown). We did not observe any photochem-
ical reactions when we irradiated the ruthenium(IV) diperchl-
orates, i.e. RuVI(Por)(ClO4)2 under similar conditions.
3.4. Photolysis of porphyrin–ruthenium(IV) dibromates
[RuIV(Por)(BrO3)2]
Photolysis of RuIV(Por)(BrO3)2 complexes producing dioxoru-
thenium(VI) species also were possible. We produced the photo-
labile ruthenium(IV) dibromates in situ from the chlorides (1) by
counterion exchange with AgBrO3. Irradiation of resulting dibro-
mate species with visible light gave trans-dioxoruthenium(VI) por-
phyrin with well-anchored isosbestic points (Fig. 3). The
photochemical reaction that gave homolytic cleavage of the O–Br
bonds is directly the same as that of the ruthenium(IV) chlorates.
Fig. 2. Examples of generation of trans-dioxoruthenium(VI) species (3) by photol-
ysis of the corresponding dichlorate precursors (2). In these time-resolved
difference spectra, species 2 are decaying away with time, and species 3 with
red-shifted Soret absorbance are growing in with time. (A) Spectrum of
RuVI(TPP)(O)2 (3c) following photolysis of chlorate salt (2c) in CH3CN over a total
time of 45 min. (B) Spectrum of RuVI(D4-Por⁄)(O)2 (3b) following photolysis of
chlorate salt (2b) in CH3CN over
a total time of 60 min. (C) Spectrum of
RuVI(TPFPP)(O)2 (3f) following photolysis of chlorate salt (2f) in CH3CN over a total
time of 160 min.

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157
Fig. 3. UV–Vis spectral change of 1a (8 Â 10À6 M) with 5-fold excess of AgBrO3 in anaerobic CH3CN solution upon irradiation with a 100 W tungsten lamp at 22 °C over
80 min.
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In conclusion, we have reported a photochemical approach to
prepare trans-dioxoruthenium(VI) porphyrin complexes by photol-
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with visible light. With this method, we have produced a variety of
trans-dioxoruthenium(VI) porphyrins without the limitation of
porphyrin ligands. This work also demonstrates, for the first time,
that a two electron photo-oxidation can be achieved by two simul-
taneous homolytic bond cleavages, i.e. two one-electron photo-
oxidation. We are currently investigating the photo-synthetic
methodology to produce other high-valent metal-oxo complexes.
Given that porphyrin–ruthenium(V)-oxo transients are more
attractive candidates for oxidations [26,44,47], the extension of
this method for generation of the elusive ruthenium(V)-oxo spe-
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This work was supported by the Petroleum Research Fund (PRF
48764-GB4) and an internal Grant from WKU Office of Research
(RCAP).
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