Photoreduction of Pt(IV) halo-hydroxo complexes: Possible hypohalous acid elimination

Article abstract of DOI:10.1021/ic402358s

Concentrated hydrogen peroxide addition to trans-Pt(PEt₃) ₂Cl(R) [1 (R = 9-phenanthryl), 2 (R = 4-trifluoromethylphenyl)] yields hydroxo-hydroperoxo complexes trans-Pt(PEt₃) ₂(Cl)(OOH)(OH)(R) [5 (R = 9-phenanthryl), 4 (R = 4- trifluoromethylphenyl)], where the hydroperoxo ligand is trans to R. Complex 5 is unstable and reacts with solvent CH₂Cl₂ to give trans,cis-Pt(PEt₃)₂(Cl)₂(OH)(9-phenanthryl) (3). Treatment of 4 with HCl yields analogous trans,cis-Pt(PEt₃) ₂(Cl)₂(OH)(4-trifluoromethylphenyl) (6) and HBr gives trans-Pt(PEt₃)₂(Br)(Cl)(OH)(4-trifluoromethylphenyl) (7), where the Br and 4-trifluoromethylphenyl ligands are trans. Photolysis of 3 or 6 at 313 or 380 nm causes reduction to trans-Pt(PEt₃)₂Cl(R) (1 or 2, respectively). Expected coproduct HOCl is not detected, but authentic solutions of HOCl are shown to decompose under the reaction conditions. Chlorobenzene and other unidentified products that oxidize PPh₃ to OPPh₃ are detected in photolyzed benzene solutions. Photolysis of 3 or 6 in the presence of 2,3-dimethyl-2-butene (TME) yields the chlorohydrin (2-chloro-2,3-dimethyl-3-butanol), 3-chloro-2,3-dimethyl-1-butene, and acetone, all expected products from HOCl trapping, but additional oxidation products are also observed. Photolysis of mixed chloro-bromo complex 7 with TME yields the bromohydrin (2-bromo-2,3-dimethyl-3-butanol) and 2, consistent with cis-elimination of HOBr. Computational results (TDDFT and DFT) and photochemistry of related complexes suggest a dissociative triplet excited state reaction pathway and that HOCl elimination may occur by an incipient hydroxo radical abstraction of an adjacent halogen atom, but a pathway involving hydroxo radical reaction with solvent or TME to generate a carbon-based radical followed by halogen abstraction from Pt cannot be eliminated.

Full text of DOI:10.1021/ic402358s

Article
pubs.acs.org/IC
Photoreduction of Pt(IV) Halo-Hydroxo Complexes: Possible
Hypohalous Acid Elimination
Lasantha A. Wickramasinghe and Paul R. Sharp*
Department of Chemistry, University of MissouriColumbia, 125 Chemistry Building, Columbia, Missouri 65211-7600,
United States
S
* Supporting Information
ABSTRACT: Concentrated hydrogen peroxide addition to trans-Pt(PEt3)2Cl(R)
[1 (R = 9-phenanthryl), 2 (R = 4-trifluoromethylphenyl)] yields hydroxo-
hydroperoxo complexes trans-Pt(PEt3)2(Cl)(OOH)(OH)(R) [5 (R = 9-phenanthryl),
4 (R = 4-trifluoromethylphenyl)], where the hydroperoxo ligand is trans to R.
Complex 5 is unstable and reacts with solvent CH2Cl2 to give trans,cis-Pt(PEt3)2-
(Cl)2(OH)(9-phenanthryl) (3). Treatment of 4 with HCl yields analogous
trans,cis-Pt(PEt3)2(Cl)2(OH)(4-trifluoromethylphenyl) (6) and HBr gives trans-
Pt(PEt3)2(Br)(Cl)(OH)(4-trifluoromethylphenyl) (7), where the Br and 4-trifluoro-
methylphenyl ligands are trans. Photolysis of 3 or 6 at 313 or 380 nm causes
reduction to trans-Pt(PEt3)2Cl(R) (1 or 2, respectively). Expected coproduct HOCl is not detected, but authentic solutions of HOCl are
shown to decompose under the reaction conditions. Chlorobenzene and other unidentified products that oxidize PPh3 to OPPh3 are
detected in photolyzed benzene solutions. Photolysis of 3 or 6 in the presence of 2,3-dimethyl-2-butene (TME) yields the chlorohydrin
(2-chloro-2,3-dimethyl-3-butanol), 3-chloro-2,3-dimethyl-1-butene, and acetone, all expected products from HOCl trapping, but
additional oxidation products are also observed. Photolysis of mixed chloro-bromo complex 7 with TME yields the bromohydrin
(2-bromo-2,3-dimethyl-3-butanol) and 2, consistent with cis-elimination of HOBr. Computational results (TDDFT and DFT)
and photochemistry of related complexes suggest a dissociative triplet excited state reaction pathway and that HOCl elimination
may occur by an incipient hydroxo radical abstraction of an adjacent halogen atom, but a pathway involving hydroxo radical
reaction with solvent or TME to generate a carbon-based radical followed by halogen abstraction from Pt cannot be eliminated.
(detected by 1H NMR).7 Thermal elimination was also
reported suggesting a small free energy change for the reaction.
Curiously, the H2O2 elimination is thought to occur from an
INTRODUCTION
Photoreductive elimination of an oxidant (XY) from a high
oxidation state transition metal complex (eq 1) has been identified
■
oxo-bridged binuclear complex. Binuclear Tp*Cu(OH)2CuTp*
as an important step in potential solar energy conversion and
(Tp* = hydrotris(3,5-dimethyl-1-pyrazolyl)borate)8 and main
group polyhydroxo complexes9,10 have also been reported to
photoeliminate hydrogen peroxide.
storage.1,2 To be useful in such a scheme, these reactions should be
endergonic and, as such, are plagued by the exergonic back reaction,
reoxidation of the reduced transition metal complex, LnM.
We recently reported efficient molecular bromine photoelimina-
tion from the mononuclear Pt(IV) complexes trans-Pt(PEt3)2(R)-
(Br)3 (R = Br, aromatic carbyl ligand).11 These reactions suffer from
back reaction of Br2 with coproduct, trans-Pt(PEt3)2(R)(Br), and
hence bromine traps must be added. With the idea of H2O2 instead
of Br2 photoelimination, we set out to prepare the analogous
dihydroxo complexes trans-Pt(PEt3)2(R)(X)(OH)2 (X = Cl, Br).
Instead, we isolated monohydroxo complexes. While not the desired
complexes, they are also highly photochemically active to net
elimination, surprisingly of hypohalous acid (HOX, X = Cl, Br).
Their synthesis and photochemistry is reported herein.
hν
LnM(X)(Y) → LnM + XY
(1)
This problem may be able to be overcome by separation of the
products into different phases.3,4 An alternative strategy is to follow
the reductive elimination with conversion of the oxidant into a
kinetically less reactive species. This could be the situation with
hydrogen peroxide photoreductive elimination from a dihydroxo
complex (eq 1, X = Y = OH) where the hydrogen peroxide
decomposes into water and O2.5 The O2 is still a thermodynamically
good oxidant, but it is kinetically less reactive than hydrogen peroxide
and less likely to react with the reduced metal complex (LnM).
There have been a few reports of hydrogen peroxide
photoreductive elimination from dihydroxo complexes. Mil-
stein5 reported apparent photoreductive elimination of hydro-
gen peroxide from a Ru dihydroxo complex, but only O2 was
detected, and computational studies6 dispute the intermediacy
of hydrogen peroxide. More recently, the Pt(IV) dihydroxo
complex cis,trans,cis-[PtCl2(OH)2(cis-1,4-DACH)] (DACH =
diaminocyclohexane) was reported to photoeliminate H2O2
RESULTS
■
Complex Synthesis and Characterization. Pt(II)
complexes trans-Pt(PEt3)2Cl(R) [1 (R = 9-phenanthryl), 2
(R = 4-trifluoromethylphenyl)] are resistant to oxidation by
Received: September 17, 2013
Published: January 21, 2014
© 2014 American Chemical Society
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hydrogen peroxide and solutions (CH2Cl2, THF, acetone) of
the complexes remain unchanged when stirred with excess 30%
aqueous hydrogen peroxide. However, slow oxidation does
occur with concentrated diethyl ether solutions of hydrogen
peroxide (Scheme 1). The reactions are accompanied by
Scheme 1
Figure 1. Solid-state structure of the hydrogen-bonded dimer of 3
(30% probability ellipsoids, hydrogen atoms omitted except for the
OH-group hydrogen atoms, which are represented as arbitrary
spheres).
extensive decomposition of the hydrogen peroxide and can
require multiple additions to complete. The final product
depends on the R group. With R = 9-phenanthryl, the product
is trans,cis-Pt(PEt3)2(Cl)2(OH)(9-phenanthryl) (3). The
source of the additional chloride is unknown, but because the
isolated yield (68%) exceeds 50%, at least a portion of the
chloride must originate from the CH2Cl2 solvent.
The reaction is more rapid with 2 than with 1. However, the
product is now the hydroxo-hydroperoxo complex trans-
Pt(PEt3)2(Cl)(OH)(OOH)(4-trifluoromethylphenyl) (4).
The NMR properties of 4 are similar to those of an inter-
mediate (5) observed in the preparation of 3 and assigned as
the 9-phenanthryl analogue of 4. Complex 4 is readily converted
to trans,cis-Pt(PEt3)2(Cl)2(OH)(4-trifluoromethylphenyl) (6), an
analogue of 3, simply by the addition of aqueous HCl (eq 2,
X = Cl). The same reaction with HBr(aq) yields the mixed
bromo-chloro-hydroxo complex trans-Pt(PEt3)2(Br)(Cl)(OH)-
(4-trifluoromethylphenyl) (7).
Figure 2. Solid-state structure of the centrosymmetric hydrogen
bonded dimer of 4 (50% probability ellipsoids, hydrogen atoms
omitted except for the OH and OOH group hydrogen atoms, which
are represented as arbitrary spheres).
Complexes 3, 4, 6, and 7 gave crystals suitable for X-ray
analysis. The solid-state structures of 3, 4, and 6 are shown in
Figures 1, 2, and 3. Complex 7 is isomorphous and isostructural
with 6 and also cocrystallized with ∼20% 6 that was present as
an impurity in the reaction mixture. Although the presence of a
Br atom trans to the 4-trifluoromethylphenyl group is
confirmed, the cocrystallization with 6 and disorder make the
metrical parameters unreliable, and the structure of 7 will not
be discussed. Selected metrical parameters for the other
complexes are listed in Table 1 and have been averaged
where chemically equivalent parameters are present.
Figure 3. Solid-state structure of 6 (one of two independent
molecules, 50% probability ellipsoids, hydrogen atoms omitted, except
for the OH ligand hydrogen atom, which is represented as an arbitrary
sphere). Rotational disorder in the CF3 group is not shown.
hydroxo-group hydrogen atoms participates in the hydrogen
bonding. In contrast, a centrosymmetric dimer is observed for
4, and both the hydroxo and the hydroperoxo groups are
involved in hydrogen bonding. However, only the hydroperoxo
group participates in the intermolecular hydrogen bonding and
Complexes 3 and 4 crystallized as hydrogen-bonded dimers.
In the case of 3, the dimer is asymmetric and only one of the
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a
parameters unreliable. Comparison of the O−O bond distance
and the Pt−O−OH angle in the Tp* complex to those in 4
shows that they are essentially identical; however, the Pt−OOH
distances differ markedly being longer by ∼0.1 Å in 4. This is
likely due to a difference in the donor trans to the OOH ligand.
In 4, it is a strong trans-influence aryl ligand, while in the Tp*
complex it is a weaker trans-influence pyrazolyl group of the
Tp* ligand.
The hydroxo complexes 3 and 6 are new members of a very small
group of structurally characterized Pt(IV) hydroxo complexes
containing a phosphine ligand. While there are many structurally
characterized Pt(IV) hydroxo complexes with a variety of ligands,14
only fac-Pt(dppbz)Me3(OH) (dppbz = o-bis(diphenylphosphino)-
benzene)15 and [Pt(PCN)(OH)2(H2O)]BF4 (PCN = C6H3[CH2P-
(t-Bu)2](CH2)2N(CH3)2)16 contain a phosphine ligand. Unfortu-
nately, the hydroxo and aqua ligands were not differentiated in the
PCN complex. In the dppbz complex, the OH group is trans to a
strong trans-influence methyl group, and the Pt−OH distance,
2.116(7) Å, is consequently longer than the 2.018(2) and 2.028(5) Å
distances observed in 3 and 6.
The stereochemistry of the Pt centers in 3, 4, 6, and 7 is
established by the X-ray structures and places the phosphine
ligands trans to each other and the hydroxo group cis to the R
group. While less definitive, the NMR data for the complexes is
consistent with the solid-state results. A single phosphine 31P
NMR resonance with 195Pt satellites is observed. The JPPt values
are typical of similar Pt(IV) complexes (1700−1800 Hz) with
trans-PEt3 ligands and, as expected, are reduced from the parent
Pt(II) complexes 1 and 2.11 Separate signals are observed for
the phenyl ring protons in the 1H NMR spectra of 4, 6, and 7.
This signifies a nonrotating phenyl ring on the NMR time scale
and an asymmetric environment for the aryl ligand. For 6, this
would only be consistent with the OH ligand and a Cl ligand
being cis to the phenyl ring, as observed in the solid-state
structure.
Table 1. Selected Mean Metrical Parameters for Chloro-
Hydroxo Complexes 3 and 6 and Hydroperoxo-Hydroxo
Complex 4
3
6
4
b
Pt−Cl1
Pt−Cl2
Pt−P
Pt−OH
Pt−OOH
Pt−C
2.356(6)
2.444(2)
2.374(4)
2.018(2)
2.349(1)
2.442(9)
2.371(6)
2.028(5)
2.3541(11)
2.36(1)
b
2.032(3)
b
2.104(3)
b
2.088(6)
2.050(1)
2.040(4)
b
O−O
1.472(4)
Cl1−Pt−Cl2
Cl1−Pt−P
88.98(3)
87.5(8)
92.0(6)
90(1)
b
88.23(4) (P1)
c
b
95.6(4)
93.05(4) (P2)
mean = 91(3)
Cl2−Pt−P
87(1)
89(3)
c
89(1)
b
Cl1−Pt−OH
Cl1−Pt−OOH
Cl2−Pt−OH
Cl1−Pt−C1
Cl2−Pt−C1
P−Pt−P
175(1)
178.2(6)
176.47(9)
b
84.98(9)
86(1)
86(1)
b
98(1)
93.3(1)
174(1)
177.8(1)
90(2)
94.96(13)
172(1)
174.7(4)
91(1)
b
177.51(4)
P−Pt−OH
89(3)
c
85.4(5)
b
P−Pt−OOH
P−Pt−C1
92.11(9) (P1)
b
85.87(9) (P2)
90.9(6)
90.8(6)
88.4(4)
91(2)
c
92.8(5)
b
HO−Pt−C1
HO−Pt−OOH
C1−Pt−OOH
Pt−O1−O2
86.5(4)
88.54(15)
b
91.54(12)
b
178.14(14)
b
110.4(2)
a
Mean of chemically equivalent parameters with standard deviation in
parentheses. Single value with estimated error in parentheses.
atom of PEt3 ligand near phenanthryl peri-hydrogen atom.
b
c
P
The complexes also show evidence of hydrogen bonding and
exchange in their solution NMR spectra. 31P NMR chemical
shifts for 3 are sensitive to the presence of H2O such that there
are slight differences in the shift (δ ∼1) observed for the
isolated complexes and those for the reaction mixtures that
contain water from H2O2 decomposition. In addition, an OH
signal (δ −0.13) is not observed in the 1H NMR spectrum of 3
unless protic impurities are removed by treatment of the NMR
forms a six-membered ring through head-to-tail dimerization of
the OOH group. Meanwhile, the hydroxo group is intra-
molecularly hydrogen bonded to the β-O of the hydroperoxo
group. A similar hydrogen bonding arrangement is found in the
dimer structure of Tp*Pt(OOH)(CH3)2 (Tp* = hydrotris(3,5-
dimethylpyrazolyl)borate).12
1
sample with base. For 4, 6, and 7, H NMR spectra in CDCl3
show peaks for the OH ligand at δ 0.20 (4), −0.48 (6), and
−0.6 (7). The OOH ligand of 4 is found at δ 6.42. The OH and
OOH signals are concentration dependent for 4 and 6. (This is
presumably also true of 7, but this was not investigated.) With
increasing concentration the signals broaden and the OH
signals, which have 195Pt satellites (40 Hz), begin to lose their
coupling to the Pt center. In C6D6, the OH signal for 6 lacks
satellites, even in dilute solutions, suggesting more rapid
exchange than that in CDCl3, probably resulting from a greater
tendency to form hydrogen-bonded dimers.
The UV−vis absorption spectra for 3 and 6 are given in
Figure 4. As expected from their pale yellow color, the spectra
show only a tail-off from the UV into the visible. The spectrum
of 3 is much more intense in the UV than that for 6 and is likely
dominated by absorptions associated with the phenanthryl
moiety. In particular, the vibronically coupled π−π* transitions
are usually located in the 300 nm region, and the bands in this
region in the spectrum of 3 can be thus assigned. In contrast,
the higher energy aryl group π-system in 6 will shift such bands
Metrical parameters for the complexes are much as expected
with essentially identical Pt−P and Pt−Cl distances for all three
complexes. The Pt−OH and Pt−C distances in the phenanthryl
complex (3) differ slightly from those in the CF3−phenyl
complexes (4 and 6). The Pt−C distance is longer in 3,
possibly due to steric interactions of the peri-hydrogen atom of
the phenanthryl group with the cis-PEt3 and -Cl ligands,
although electronic factors may also be involved. The steric
effect of the peri-hydrogen atom is evident in the angles around
the Pt center, which deviate more from the ideal 90° and 180°
angles in 3 than in 6. (For examples of peri-hydrogen steric
interaction in Pt(IV) organometallic complexes, see ref 11.)
Larger deviations are also seen in 4 and may be associated with
the hydrogen bonding.
Complex 4 is preceded by two crystallographically
characterized Pt(IV) η1-hydroperoxo complexes: Pt(tmeda)-
13
(OOH)(OCH3)(CH3)2 and Tp*Pt(OOH)(CH3)2 (Tp* =
hydrotris(3,5-dimethylpyrazolyl)borate).12 Crystallographic
problems with the tmeda structure make the metrical
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Figure 5. UV−visible absorption spectrum of 6 in dichloromethane
with TDDFT (CAM-B3LYP, pcm) vertical transitions for 6′ (red
vertical lines with the height corresponds to the oscillator strength).
Figure 4. UV−visible absorption spectra of 3 and 6 in dichloro-
methane.
to higher energy exposing more Pt centered transitions. because
we are more interested in the Pt centered transitions, we will
focus on the spectrum of 6.
To help in the assignment of the low-UV bands in 6, a
TDDFT (CAM-B3LYP, pcm) study was undertaken with
model complex trans,cis-Pt(PMe3)2(Cl)2(OH)(4-trifluoro-
methylphenyl), 6′, where the PEt3 ligands of 6 are replaced
by PMe3 ligands. The calculated vertical singlet transition
energies and oscillator strengths (Table 2) over the region of the
Table 2. Vertical Singlet Transitions for 6′ in
a
Dichloromethane
transition
1
wavelength
365
osc. strength
0.0082
contributions (>3%)
H − 3 → LUMO (56%)
HOMO → LUMO (20%)
H − 1 → LUMO (17%)
H − 4 → LUMO (40%)
H − 7 → LUMO (15%)
HOMO → LUMO (12%)
H − 5 → LUMO (8%)
HOMO → LUMO (49%)
H − 3 → LUMO (19%)
H − 4 → LUMO (12%)
HOMO → L + 1 (7%)
H − 6 → LUMO (5%)
H − 1 → LUMO (65%)
H − 3 → LUMO (13%)
H − 1 → L + 1 (4%)
2
3
311
304
0.0054
0.0313
Figure 6. Natural transition orbital (NTO) set for the first singlet
excited state of 6′ (isovalue = 0.04). The “arrival” orbital is on the top
and the “departure” orbital is on the bottom (H atoms omitted).
four sets, the arrival orbital for the first transition is an eg*-type
(pseudo-octahedral symmetry) orbital with strong antibonding
interactions between the Pt and the OH and Cl ligands along
the HO−Pt−Cl axis. The departure orbital is largely composed
of an OH lone-pair and a matching t2g-type Pt orbital, with
minor P, C, and Cl components. The departure orbitals for the
other three transitions are various combinations of Cl lone-pair,
t2g-type Pt, P, and π-aryl orbitals. The first five triplet excited-
state transitions were also calculated (Table S22, Supporting
Information), and the compositions and energies of the two
lowest match closely to those of the corresponding singlets,
suggesting facile singlet−triplet intersystem crossing at this
level. Also possible is direct excitation into the triplet excited
states at lower energies.
4
292
0.009
a
CAM-B3LYP, pcm.
experimental spectrum are displayed in Figure 5 along with the
experimental spectrum. A good fit of the calculated transitions
for 6′ to the experimental spectrum of 6 is observed, especially
in the lower-energy region. The obtained orbital contributions
to the transitions are fairly complex (Table 2 and Table S21,
Supporting Information) and a natural transition orbital
(NTO) analysis17 was applied for the five lowest-energy
transitions. This yielded one dominant (eigenvalue ≥0.96)
contributing orbital set for each transition. The NTO set for
the first (lowest-energy) transition is graphically presented in
Figure 6, and the remaining four are provided in the Supporting
Information (Figures S36−S39). In common with the other
Photochemistry. Complexes 3 and 6 are photosensitive to
light in the blue and UV regions. In C6D6 (λ = 313 nm), clean
conversion of 3 to Pt(II) complex 1 is observed (eq 3) as
indicated by 1H and 31P NMR spectroscopy and by the
observation of isosbestic points when the reaction is monitored
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by UV−vis absorption spectroscopy (Figure S34, Supporting
Information). Two Pt products are obtained for 6 in C6D6 and
for both 3 and 6 in CDCl3 and CD2Cl2. The major products
(60−80% yield) are again 1 and 2, but significant amounts
(10−35% yield) of the Pt(IV) complexes trans,mer-Pt-
(PEt3)2(Cl)3R [R = 9-phenanthryl (8), R = 4-trifluoromethyl-
phenyl (9)] are also obtained (eq 4).
successful, although chlorobenzene was detected in 50% yield
(relative to 2). Photolysis of HOCl solutions in C6H6 also
produced chlorobenzene.
The absence of HOCl in photolyzed solutions of 3 and 6 is
consistent with the low stability of HOCl in the presence of Pt
complexes and under photolysis.21 Fresh benzene solutions of
HOCl start out a characteristic yellow color but become
colorless within 15 min and lose their ability to oxidize TME.
Dichloromethane and chloroform solutions appear to be more
stable (hours) but quickly decolorize under irradiation (313 or
380 nm), and the resulting solutions do not oxidize PPh3.
Addition of trans-Pt(PEt3)2Cl(4-trifluoromethylphenyl) (2) to
a fresh benzene solution of HOCl results in the immediate
formation of trichloro Pt(IV) complex 9, and subsequent TME
addition does not give oxidation products indicating that HOCl
has been consumed. Thus, HOCl photoelimination from 3 and
6 may occur, but the HOCl would not survive the reaction
conditions.
To further investigate the role of 2 in the possible
decomposition of photoeliminated HOCl, the photolysis of 6
in the presence of an equimolar amount of 2 was examined.
Photolysis proceeds as without added 2 (similar time scale), but
the yield (based on 6) of trichloro Pt(IV) complex 9 increases
to 48%. From the amount of chlorine in 6, the maximum
possible yield of 9 is 50%. If this represents trapping of
photoeliminated HOCl by 2, the capture efficiency is 96%.
TME was added to photolysis solutions of 3 and 6 to serve as
an HOCl trap. Photolysis proceeds as without added trap, but
now the amount of 8 and 9 formed in CDCl3 and CD2Cl2 is
reduced to just traces. TME oxidation is observed and products
from the CD2Cl2 and C6D6 reactions are listed in Tables 3 (3)
and 4 (6). In common with HOCl, 11 and acetone are
observed for 3, but only small amounts of chlorohydrin (10)
are detected, and several new products (12−14) are formed.
By stoichiometry, the formation of 1 and 2 implies that 3 and
6 photoeliminate HOCl. To test for the possible presence of
HOCl, PPh3 was added to C6D6 solutions of 3 immediately
after photoconversion (313 nm) to 1. A 22% yield (based on 3)
of OPPh3 (eq 5) was indicated by 31P NMR spectroscopy. No
OPPh3 is observed for the CDCl3 and CD2Cl2 photoreaction
mixtures. To help identify the oxidant in the photolyzed C6D6
solutions, 2,3-dimethyl-2-butene (TME) was added to a
photolyzed solution of 3 in place of PPh3. The expected
HOCl oxidation product, chlorohydrin 10, is not observed, nor
are other possible oxidation products (see below).
Because reaction conditions typically used to generate
chlorohydrins from an alkene employ a large excess of HOCl
in aqueous acid,18 very different from our potential reactions
conditions of low HOCl concentration and nonprotic solvents,
we studied HOCl reactions with TME under conditions similar
to ours. Gaseous HOCl was generated by treating solid
Ca(OCl)2 with acid (H2SO4 or HCl)19 and was vacuum
transferred into solution. Alternatively, HOCl solutions were
prepared by the reaction of HgO and Cl2 in water-saturated
solvents.20 HOCl formation was confirmed by UV−vis
absorption spectroscopy after transfer to water (Figure S2,
Supporting Information). Approximately 12 mol % ClO2 was
also detected in the HOCl from Ca(OCl)2.
1
Another product, formed in low yield, has a single H NMR
signal and could be triacetone triperoxide,22 but this was not
confirmed. In contrast to 3, photolysis of 6 with TME in CDCl3
and CD2Cl2 produces chlorohydrin 10 as a major product in
addition to 11, 12, 14, and acetone. Chlorohydrin 10 is the only
detected TME product for 6 in C6D6, but the yield is only 5%.
Hydroperoxide 13 is reported to be the major product of the
TME reaction with singlet oxygen.23 Its formation in the
photolysis of 3 (Table 3) suggests triplet sensitization of O2
with formation of singlet oxygen.24 To investigate this
possibility, the photolysis of 3 and TME was conducted in
O2-saturated C6D6 solutions. (All other photolyses were
conducted in vacuum-degassed solutions backfilled with Ar.)
The yield of 13 increased greatly such that the total yield of
oxygen-containing products exceeds that possible from 3 alone,
clearly indicating the involvement of the added O2 in product
formation. Photolysis of an O2-saturated TME C6D6 solution
does not produce 13. Thus, if 13 is produced from singlet
oxygen in the degassed solutions of 3, the source of the O2
must be 3, possibly from decomposition of photoeliminated
HOCl into O2 and HCl. Photolysis of 6 with TME in O2-free
and O2-saturated solutions (Table 4) does not produce 13,
indicating a difference in the photochemistry of 3 and 6.
Complex 7 is also photochemically active. Photolysis of a
mixture of 6 (17%) and 7 (83%) gives predominately 2 (69%)
with lesser amounts of trans-Pt(PEt3)2(Br)(4-trifluoromethyl-
phenyl) 15 (31%), both in the presence and in the absence of
TME (eq 6). Seventeen percent of the yield of 2 is from 6 with
the remaining 52% coming from 7. Thus, the photolysis of 7
1
TME reactions with HOCl solutions were analyzed by H
NMR spectroscopy and showed that chlorohydrin 10 is formed
along with 3-chloro-2,3-dimethyl-1-butene (11) and acetone in
about a 1:2:0.2 ratio (Chart 1). Thus, the oxidant in the
a
Chart 1. TME−HOCl Reaction Products
a
HOCl from H2SO4 and Ca(OCl)2.
photolyzed solutions of 3 and 6 (eq 5) is not HOCl but
probably a solvent-derived species. Attempts to detect 1H NMR
signals for this species in photolyzed C6H6 solutions were not
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a
a
Table 3. TME (0.1 M) Product Yields from Photolysis of 3
(10 mM)
Table 4. TME Product Yields from Photolysis of 6 (18 mM,
313 nm)
a
Yields are relative to the molar amount of 2 with an estimated error
b
of 1%. TME impurities at this high concentration are significant and
a
c
d
Yields are relative to the molar amount of 2 with an estimated error
may interfere. 13 mM 6 (3% epoxide also formed). O atom
yield = 10 yield + 12 yield + acetone yield; Cl atom yield = 10 yield +
11 yield + 14 yield.
b
c
of 1%. Short (<6 min) high-intensity irradiations. O atom yield =
10 yield + 12 yield + 2(13 yield) + acetone yield; Cl atom yield = 10 yield +
11 yield + 14 yield.
Chart 2
Scheme 2. DFT Gas-Phase Reductive Elimination Free
Energies (25 °C, 1 atm)
produces 2 and 15 in a 1:0.6 ratio suggesting that the reaction is
primarily a net cis-photoelimination of the Br and OH groups.
Consistent with HOBr photoelimination, TME products are
primarily (21%) bromohydrin 16 (Chart 2) with no detectable
chlorohydrin 10.
Computational Modeling. Previously, we found that the
M06 functional gave gas-phase thermodynamic estimates for Br2
oxidative addition to trans-PtL2Br(R) complexes that matched well
with toluene solution experimental measurements.11 Extending
these calculations to the current system using PMe3 model com-
plexes gave the free energies for reductive elimination of HOCl
and Cl2 in Scheme 2 (L = PMe3). An estimate of the reductive
elimination/oxidative addition barrier would be useful. However,
to computationally determine the barrier, the mechanism must be
known, and with the many possibilities, such a computational
study is beyond the scope of this manuscript. We did try to induce
reductive elimination by a stepwise reduction of the OH−Cl
distance in 6′, but no conversion was observed and the potential
energy steadily increased by over 100 kcal/mol. We also looked for
HOCl adducts of the Pt(II) complexes but found only very weak
interactions (1−3 kcal/mol).
As expected, the reductive-elimination free energies for HOCl
and Cl2 from trans,cis-Pt(PMe3)2(Cl)2(OH)(9-phenanthryl) (3′)
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and trans,cis-Pt(PMe3)2(Cl)2(OH)(4-trifluoromethylphenyl)
(6′) are positive, indicating thermodynamically unfavorable
reactions but favorable reverse oxidative-addition reactions.
Chlorine elimination is disfavored over HOCl elimination by
∼20 kcal. The Cl−Cl bond enthalpy is ∼2 kcal/mol greater
than the HO−Cl bond enthalpy, slightly favoring Cl2
elimination.25 A Pt−OH bond that is significantly weaker
than the Pt−Cl bond must be invoked to explain
the favored HOCl elimination. Indeed, the calculated (M06)
gas-phase bond dissociation enthalpies for 3′ and 6′ are 63.6
and 55.6 kcal/mol for the Pt−OH bond and 78.9 and 69.8 for
the Pt−Cl bond. Thus, the Pt−Cl bond (trans to OH) in 3′ and
6′ is stronger than the Pt−OH bond by 14−15 kcal/mol.
The relaxed triplet structures for the Pt(IV) complexes were
also explored. Two triplet structures (3′T1 and 3′T2) from 3′
and one (6′T2) from 6′ were located at 40−45 kcal above the
singlets in the gas phase. Inclusion of dichloromethane solvent
(pcm, gas phase structures) in the calculation did not sig-
nificantly alter the energy difference (by 2 kcal/mol for 6′T2).
Drawings of 3′T1 and 3′T2 are given in Figure 7. That for 6′T2 is
Figure 8. Doublet structures 21′ and 23′ (carbon-bonded H atoms
omitted for clarity).
3′ and 6′. The resulting structures for R = 4-trifluoromethylphenyl
are displayed in Figure 8. The optimized structures retain the
parent configuration and are square pyramidal with a chloro or an
hydroxo ligand in the axial position, although facile isomerization is
expected for a square-pyramidal complex. Atomic spin density
values (Table S20, Supporting Information) reveal spin density on
the axial OH (0.3 e−) and Cl (0.5 e−) ligands suggesting
substantial radical character for these ligands. (This is consistent
with the first excited state NTO set in Figure 6.) Energetically, the
loss of an OH group from either triplet, 3′T1 or 3′T2, is nearly
energetically neutral, while that from 6′T2 is slightly (6 kcal)
endergonic. Loss of a Cl atom from the triplets is unfavorable
being ∼18 kcal endergonic for 3′T1 or 3′T2 and 22 kcal endergonic
for 6′T2.
Heterolytic bond cleavage with chloride and hydroxide ion
dissociation was examined by optimization of the cations
[Pt(PMe3)2(Cl)(OH)(4-trifluoromethylphenyl)]+ (24′) and
[Pt(PMe3)2(Cl)2(4-trifluoromethylphenyl)]+ (25′), derived
from Cl1− and OH− removal from 6′. The gas-phase free
energies to dissociate an OH− from 6′ to give 24′ and a Cl− to
give 25′ are, of course, very high (>100 kcal) due to charge
separation without solvation. Applying a dichloromethane
solvent correction (pcm model) lowers the free energies to
90.2 (24′) and 24.2 (25′) kcal. Relative to triplet 6′T2, the
energies are 46.5 and −19.5 kcal. Clearly, hydroxide ion
dissociation to 24′ is not favorable and unlikely to occur in this
system. On the other hand, chloride ion dissociation appears
favorable, at least in dichloromethane. However, with benzene
as the solvent, an unfavorable energy of 60.4 kcal relative to 6′
(15.9 kcal relative to 6′T2) is obtained, indicating that even
chloride ion dissociation is probably unfavorable in benzene.
Figure 7. Triplet 3′T1 and 3′T2 and singlet 3′ structures (distances in
Å, carbon-bonded H atoms omitted for clarity).
given in the Supporting Information (Figure S35). The
structures 3′T2 and 6′T2 are similar and only 3′T1 and 3′T2 will
be discussed. Triplets 3′T1 and 3′T2 have distorted octahedral
geometries resulting from single occupancy of an eg-type orbital.
The two structures differ primarily in the extent of OH and Cl
dissociation along the HO−Pt−Cl1 axis in the distorted
octahedron. Both the Pt−OH and Pt−Cl1 distance increase by
ca. 0.3 Å on going from 3′ to 3′T1. These distances also increase
in 3′T2 but the increase is dominated by the Pt−Cl1 distance,
which increases by nearly 0.4 Å. Accompanying the Pt−Cl1
distance increase is a bending of the PMe3 ligands in the
direction of Cl1 such that the P1−Pt−P2 angle decreases from
near linearity in 3′ to 159.6° in 6′T2. Changes in the Pt−C and
Pt−Cl2 distances are less than 0.1 Å. Both triplets appear to
have weak hydrogen bonding between the OH group and Cl2.
Dissociative triplet excited states are a common feature of
d6-octahedral metal complexes and can result in ligand dissociation
reactions, either with heterolytic or homolytic bond cleav-
age.26−32 The loss of an OH and a Cl radical from the triplets was
examined by optimizing the Pt(III) doublets Pt(PMe3)2(Cl)2(R)
(20′, R = 9-phenanthryl; 21′, R = 4-trifluoromethylphenyl) and
Pt(PMe3)2(Cl)(OH)(R) (22′, R = 9-phenanthryl; 23′, R =
4-trifluoromethylphenyl) derived from OH or Cl1 removal from
DISCUSSION
■
Syntheses. Pt(IV)-hydroxo complexes have been previously
prepared by oxidative addition of H2O2 to Pt(II) com-
plexes.33−39 However, this has been mostly restricted to Pt(II)
complexes with amine or diimine ligands, research driven
largely by the antitumor activity of Pt amine complexes. These
reactions generally occur at, or slightly above, room temper-
ature with 30% aqueous H2O2 to give dihydroxo complexes.
The resistance of 1 and 2 to oxidation by 30% H2O2 was
therefore unexpected but could be overcome with higher H2O2
concentrations. The Pt(II) oxidative addition mechanism for
H2O2 has been proposed to be a three-centered concerted
process40 similar to C−H oxidative addition or to follow an ionic
process similar to that for halogen oxidative addition: (a) oxidant
electrophilic attack at an axial position with heterolytic cleavage of
the oxidant and then (b) capture of the five-coordinate Pt(IV)
cation by the oxidant anion or another available anion or ligand
(Scheme 3).13,38,41 The capturing anion or ligand may also participate
in the oxidation step by interaction with the Pt center opposite to the
attacking oxidant, thereby bypassing the five-coordinate Pt(IV)
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would have been consumed in the formation of 9 from 2. With
added TME, only traces of 9 form and the expected TME
HOCl reaction products, chlorohydrin 10 and chlorinated 11,
are observed. However, the yield is rather low, especially for 3,
and a number of other products (12−14) are also produced,
but these could arise from HOCl decomposition products.
Alternatives to HOCl elimination to consider are ionic and
radical processes. Bromide photodissociation is reported for
[PtBr6]2− in water and MeOH,31,32,43−45 and we have shown
that net bromide photodissociation also occurs in CH2Cl2.11
Chloride or hydroxide photodissociation from 3 or 6 would
produce a Pt(IV) complex,46 perhaps in an excited state and
capable of alkene or even benzene oxidation. Alkene
chlorination by Pd(IV) chloro complexes has been reported,
and Pt(IV) complexes are involved in hydrocarbon oxidation in
Shilov-type chemistry.47−50 Arguing against an ionic pathway is
the absence of any significant change in reactivity and products
on going from CH2Cl2 to benzene (at least for 3) where ion
formation should be disfavored. The DFT modeling of chloride
and hydroxide photodissociation suggests that chloride
dissociation is possible in CH2Cl2, though probably not in
benzene. Hydroxide dissociation is, however, unlikely.
Scheme 3
Scheme 4
Another ionic pathway to consider is excited-state, single-
electron transfer (SET). This has been proposed for photo-
51
platination of aromatic compounds with [PtCl6]2− and for
[PtBr6]2− photoreduction in the presence of high bromide ion
concentrations52 or in methanol solution.53 However, it was
recently concluded that SET follows Br− photodissociation for
[PtBr6]2−.32 The photoreduction of 3, 6, and 7 proceeds
equally well in the presence of electron-rich TME or simply in
toluene, benzene, CH2Cl2, or CDCl3 solvent. Oxidation by SET
in these various solvents seems unlikely.
cation. In addition, isomerization can occur such that the final
product does not contain a trans disposition of the added ligands.
Applying the ionic mechanism to the current system gives
the proposed pathway to 3 and 4 given in Scheme 4. Here,
capture of the five-coordinate cation (or assistance in the oxidation)
is by high-concentration H2O2, giving first trans-hydroperoxo-
hydroxo 26, which then isomerizes to 4 or 5. Complex 5 is
unstable, possibly due to the strong labilizing effect of the trans-9-
phenanthryl group, and reaction of the hydroperoxo ligand with
CH2Cl2 yields chloride ion for formation of 3. A relatively strong
basic character for the hydroperoxo group is indicated by its
selective protonation over the hydroxo group in the synthesis of 6
and 7 from 4 (eq 3).
Photochemistry. Understanding the photoreduction of 3
and 6 is challenging. Stoichiometry indicates HOCl elimination,
but without direct detection of HOCl, other possibilities exist
that could give the final stoichiometry without HOCl ever
being formed. (A previous report of HOCl photoelimination
from prolonged irradiation of Pt(IV) complexes assumes HOCl
elimination by stoichiometry and bleaching of a dye.42)
Unfortunately, HOCl is not stable in the presence of Pt(II)
complexes 1 and 2, making direct detection difficult. Clearly, a
strong oxidant is photochemically produced that oxidizes
solvent benzene to chlorobenzene and other unidentified
products that retain some oxidizing power and oxidize added
PPh3. This behavior parallels that of a benzene solution of
HOCl, which decomposes rapidly under photolysis (313 nm)
and, just like in the photolysis of 3 and 6, produces
chlorobenzene and a solution that oxidizes PPh3.
A radical reaction pathway would be consistent with the near
solvent independence of the photochemistry of 3, 6, and 7, and
radical photoelimination from Pt(IV) is known. [PtCl6]2− is
thought to photoeliminate a chlorine atom,32,54−57 and Pt(IV)
methyl complexes photoeliminate a methyl radical.58 In
addition, the DFT calculations on triplets 3′T1, 3′T2, and 6′T2
show large spin density on the OH and Cl axial ligands and
elongation of the Pt−OH and Pt−Cl bonds, suggesting insipient
formation of a geminate Pt(III)/OH radical pair from the triplet
excited state. An OH or Cl radical would certainly be reactive
enough to drive the photochemistry under a variety of conditions.
According to the DFT calculations, OH radical formation is more
favorable and the resulting Pt(III) complexes (modeled with 20′
and 21′) have large spin densities on the axial Cl atoms, suggesting
possible Cl atom abstraction from Pt by the OH radical (probably
within the solvent cage) with net HOCl elimination. Dissociation of
a Cl atom from the Pt(III) doublet complexes 20′ and 21′ is
endergonic (32.6 and 33.3 kcal/mol, respectively) in the gas phase.
Solvent inclusion (pcm, dichloromethane) gave only minor changes
(<3 kcal/mol) in the dissociation energies. (Teets and Nocera
suggested something similar for molecular halogen photoelimina-
tion from Au(III) halide complexes.4) To account for the preferred
cis-elimination indicated by the photochemistry of bromo-hydroxo
complex 7, we propose that geometric isomerization of the five-
coordinate Pt(II) complexes (20′ and 21′) interconverts the axial
and equatorial chloro ligands bringing the former cis-chloro ligand
into the axial position for abstraction by the OH radical. This
pathway is summarized in Scheme 5 and assumes either direct
excitation into the triplet manifold or rapid intersystem crossing
following singlet excitation. TDDFT does indicate matching triplet
Further support for HOCl formation is provided by the
trapping experiments with 2 and TME. Complex 2 reacts with
HOCl to produce trichloro complex 9. Any HOCl produced in
the photolysis of 6 could react with photogenerated 2 to
produce 9. In fact, 9 is observed in a 35% yield in the absence
of added TME. Adding 2 at the beginning of the reaction
increases the yield of 9 such that 96% of the theoretical HOCl
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conversion and intersystem crossing) followed by geminate
Pt(III)/hydroxo radical pair formation. The hydroxo radical can
then either abstract a halogen atom from the Pt(III) center
forming HOX that decomposes and reacts with solvent or trap or
react directly with the alkene trap or solvent to produce a carbon-
based radical that then abstracts a halogen from the Pt(III) center.
Both processes could be operating simultaneously.
Scheme 5. Possible Photoelimination Pathway with DFT
Gas-Phase Free Energies in kcal/mol for Model Complexes
a
EXPERIMENTAL SECTION
■
General Considerations. Pt(PEt3)459 was prepared by a reported
procedure. Reagents and solvents were purchased from commercial
sources (Aldrich or Acros). Synthetic procedures were performed
without exclusion of air with unpurified solvents unless otherwise
noted. Platinum complex photolysis samples were prepared under a
dinitrogen atmosphere in a Vacuum Atmospheres Corporation drybox
or on a Schlenk line. Photolysis solvents were dried, degassed, and
stored under dinitrogen over 4 Å molecular sieves. NMR spectra were
recorded on Bruker AMX-250, -300, or -500 spectrometers at ambient
probe temperatures except as noted. NMR shifts are given in δ with
positive values downfield of tetramethylsilane, Si(CH3)4 (1H and 13C),
external H3PO4 (31P), external CFCl3 (19F), or external K2PtCl4 (aq)
(
195Pt, δ −1630). 13C, 19F, 195Pt, and 31P NMR spectra were recorded in
a
Black for R = 9-phenanthryl, blue for R = 4-CF3C6H4.
proton-decoupled mode. Microanalyses were completed by ALS Environ-
mental or Atlantic Microlab. UV−visible absorption spectra were recorded
on a Cary 50 or Hewlett-Packard 8452 diode array spectrophotometer in
quartz cells. Photolyses were performed in quartz (UV) or borosilicate glass
vessels using a Philips PL-S 9W/01 9 W lamp (313 nm emission) or LEDs
(superbrightleds.com) of the indicated wavelength.
Scheme 6. Substrate-Assisted cis-Elimination Pathway from
Hydroxo Radical/Pt(III) Geminate Pair
trans-Pt(PEt3)2Cl(4-trifluoromethylphenyl) (2).
A solution of 1-chloro(4-trifluoromethy)benzene (23 mg, 0.13 mmol)
in THF (∼2 mL) was added to a clear orange solution of Pt(PEt3)4
(70 mg, 0.10 mmol) in THF (∼2 mL). The resulting clear orange
solution was stirred for ∼20 h at 140 °C in a sealed tube to yield a pale
yellow solution. The mixture was cooled to ambient temperature, and
the volatiles were removed in vacuo. The solid residue was dissolved in
∼2 mL of CH2Cl2 and transferred to a 4 mL vial. The volume was
reduced in vacuo to ∼0.5 mL followed by the addition of ∼1 mL of
methanol. The vial was capped and stored at −20 °C overnight to
afford colorless crystals. The mother liquor was pipetted out, and the
crystals were dried in vacuo to yield 40 mg (63%) of 2. 31P NMR
(101 MHz, CDCl3): 14.5 (s with satellite, JPtP = 2733 Hz). 19F NMR
transitions for the first two singlet transitions and small energy gaps,
consistent with facile intersystem crossing.
An alternative to HOX elimination that could also follow
from OH radical formation is depicted in Scheme 6. In this scheme,
the OH radical either abstracts a hydrogen atom from solvent or
TME or adds to solvent or TME. In either case, a carbon-based
radical is then generated, which can abstract a halogen atom from
the Pt(III) center. This scheme accounts for solvent oxidation,
TME chlorination, and formation of the chlorohydrin.
1
(235 MHz, CDCl3,): −62.0 (s). H NMR (250 MHz, CDCl3): 7.47 (d
with satellites, JHH = 7.95 Hz, JPtH = 65 Hz, 2H), 7.12 (m, 2H), 1.78−1.53
(m, 12H, CH2), 1.14−1.01 (app quintet, Japp = 8.27 Hz, 18H, CH3).
trans-Pt(PEt3)2(Cl)(9-phenanthryl) (1). This complex was prepared
from 9-chlorophenanthrene by a procedure similar to that for 2. 31P
NMR (101 MHz, CD2Cl2): 14.10 (s with satellite, JPtP = 2735 Hz). 31P
NMR (101 MHz, C6D6): 13.97 (s with satellite, JPtP = 2736 Hz). 1H NMR
(250 MHz, CD2Cl2): 8.97−8.93(m, 1H), 8.62−8.57 (m, 2H), 7.82 (s with
satellites, JPtH =75 Hz, 1H), 7.69−7.65 (m, 1H), 7.57−7.46 (m, 4H), 1.70−
1.40 (m, 12H, CH2), 1.08−0.96 (app quintet, J = 8.2 Hz, 18H, CH3). 1H
NMR (250 MHz, C6D6): 9.28 (d, JHH = 7.50 Hz, 1H), 8.59 (t, JHH = 7.50
Hz, 2H), 8.09 (s with satellites, JPtH = 75.0 Hz, 1H), 7.75 (d, JHH = 7.50
Hz, 1H), 7.58 (t, JHH = 7.50 Hz, 1H), 7.49−7.37 (m, 3H), 1.62−1.28 (m,
12H, CH2), 0.89−0.77 (app quintet, J = 8.2 Hz, 18H, CH3).
CONCLUSIONS
■
The low kinetic reactivity of the Pt(II) phosphine complexes
with H2O2 bodes well for H2O2 photoelimination from Pt(IV)
dihydroxo complexes because this is the back reaction and
H2O2 disproportionation may not be required to prevent it.
When oxidation of the Pt(II) center by H2O2 does occur, the
required high H2O2 concentrations result in trapping of the initial
oxidation product with H2O2 and hydroperoxo-hydroxo complexes
result instead of the expected dihydroxo complexes. Selective
replacement of the hydroperoxo ligand with a halide ligand can
occur spontaneously in CH2Cl2 or by treatment with HX.
The photochemistry of the hydroxo-halo complexes is net
HOX elimination. The photochemical pathway most consistent
with the behavior of the system (trapping products and solvent
insensitivity) and the DFT modeling is excitation into the
lowest energy triplet excited state (either directly or via internal
Preparation of H2O2 in Ether.60 Approximately 7 mL of 30% H2O2
was placed in a 20 mL vial, and then 7 mL of diethyl ether was added.
The vial was capped tightly, and the contents were stirred vigorously
for a minimum of 2 h. (Caution! While we never observed H2O2
decomposition and O2 evolution, this is a possibility and a mechanism for
pressure release should be provided.) The ether layer (upper layer) was
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then pipetted into another 20 mL vial. The solution was then
concentrated in vacuo to 0.5−1 mL. The concentrated solution was
not stored but used immediately in the following syntheses.
trans,cis-Pt(PEt3)2Cl2(OH)(9-phenanthryl) (3).
solid was redissolved in about 0.5 mL of dichloromethane, and 1 mL of
hexane was added. Evaporation of the solution at −20 °C yielded 13 mg
(89%) of pale yellow crystals that were suitable for X-ray analysis. 31P
NMR (101 MHz, CDCl3): 1.09 (s with satellite, JPtP = 1770 Hz). 19F
1
NMR (235 MHz, CDCl3): −62.27 (s). H NMR (250 MHz, CDCl3):
8.05 (d with satellites, JHH = 7.50 Hz, JPtH = 45 Hz, 1H), 7.88 (d with
satellites, JHH = 7.50 Hz, JPtH = 45 Hz, 1H), 7.29 (m, 2H), 6.42 (s, 1H,
OOH), 2.00−1.89 (m, 12H, CH2), 1.11−1.00 (app quintet, Japp = 7.9 Hz,
18H, CH3), 0.20 (s with satellites, JPtH = 40 Hz, 1H, OH). The OH group
signal is concentration dependent in CDCl3 (and CD2Cl2). As
concentration increases (above ∼0.01 mol L−1), the signal broadens
and the 195Pt satellites move in and eventually merge with the main peak.
The OOH signal also broadens. D2O addition causes complete loss of the
OH and OOH signals.
trans-Pt(PEt3)2Cl(9-phenanthryl) (1) (10.1 mg, 0.0157 mmol) in
1.5 mL of dichloromethane was mixed in a 20 mL vial with 1 mL of a
freshly prepared H2O2 solution in diethyl ether. (The preparation of
the H2O2 solution is described above.) The vial was capped with a
rubber septum, and a needle was inserted to release the pressure
generated during the reaction. The mixture was stirred and monitored
by 31P NMR spectroscopy. The colorless solution became yellowish
orange, and conversion of 1 (δ 14.1) to product 3 along with small
amounts of OPEt3 (δ 50−60) was observed. (31P NMR shifts varied
with the water content of the reaction mixture.) An intermediate (5)
was detected at δ 6−7 and was gone by the end of the reaction. If the
reaction was not complete after 10−12 h, additional H2O2 solution
was added. Once the reaction was complete, the dichloromethane
layer was separated from the aqueous layer (from H2O2 decom-
position) and washed with 3 × 5 mL of deionized water and then dried
over MgSO4. After filtering, the volatiles were removed in vacuo. The
resulting solid was redissolved in about 0.5 mL of dichloromethane,
and 1 mL of hexane was added. A pale orange precipitate (not
identified) formed and was removed by filtration. The yellow filtrate
was checked by 31P NMR spectroscopy and showed only product 3.
The filtrate was concentrated and then stored at −20 °C to obtain 7.4
mg (68%) of yellow 3. Yellow crystals for the X-ray analysis were
grown similarly but in an open vial in the freezer. Anal. Calcd (found)
for PtP2Cl2OC26H40·0.3MeOH·0.5H2O: C, 44.26 (44.03); H, 5.86
trans,cis-Pt(PEt3)2(Cl)2(OH)(4-trifluoromethylphenyl) (6).
Compound 4 was dissolved in 1 mL of dichloromethane and
approximately 2 mL of deionized water, 0.05 mL of dilute HCl (50 μL
of conc HCl in 1−2 mL water) was added, and the mixture was stirred
for 1 min. 31P NMR spectroscopy showed complete conversion of 4 to
6 (δ 1.09). The dichloromethane layer was washed with 10 mL of
deionized water and dried with MgSO4. After filtration, the volatiles
were removed in vacuo. The resulting solid was washed with 1.0 mL of
cold hexane and dried in vacuo to give 12.0 mg (88%) of 6. Pale yellow
crystals for the X-ray analysis were grown in ether/heptane (1:5) by
slow evaporation in the freezer. 31P NMR (101 MHz, CDCl3 or
CD2Cl2): 1.09 (s with satellites, JPtP = 1708 Hz). 31P {1H} NMR
(101 MHz, C6D6): 0.16 (s with satellites, JPtP = 1708 Hz). 19F NMR
(235 MHz, CDCl3 or CD2Cl2): −62.30. 195Pt NMR (64 MHz,
1
1
CDCl3): −1944 (t, JPtP =1720 Hz). H NMR (250 MHz, CDCl3):
(5.90). (The water and MeOH content was determined by H NMR
spectroscopy.) MS (APCI) m/z: [M − H]+ 695, [(M − H) − HCl]+
659, [(M − H) − HOCl]+ 643, [(M − H) − OCHl2]+ 608. The
fragment isotope patterns matched those predicted. 31P NMR
(101 MHz, CD2Cl2): 3.22 (s with satellites, JPtP = 1740 Hz). 31P
NMR (101 MHz, C6D6): 2.29 (s with satellites, JPtP = 1757 Hz). 195Pt
NMR (64 MHz, CD2Cl2): −1754 (t, JPtP = 1751 Hz). 1H NMR
8.16 (d with satellites, JHH = 7.50 Hz, JPtH = 42 Hz, 1H), 7.93 (d with
satellites, JHH = 7.50 Hz, JPtH = 42 Hz, 1H), 7.33 (m, 2H), 2.00−1.85
(m, 12H, CH2), 1.13−1.03 (app quintet, Japp = 8.0 Hz, 18H, CH3),
−0.48 (s with satellites, JPtH = 40 Hz, 1H, OH). The OH group signal
is concentration dependent in CDCl3 (and CD2Cl2). As concentration
increases (above ∼0.01 mol L−1), the signal broadens and the 195Pt
satellites move in and eventually merge with the main peak. D2O
(250 MHz, CD2Cl2, CDCl3): 9.14 (d, JHH = 7.50 Hz, 1H), 8.54 (t, JHH
7.50 Hz, 2H), 8.18 (s with satellites, JPtH = 50.02 Hz, 1H), 7.78 (d, JHH
=
=
1
addition causes complete loss of the OH signal. H NMR (250 MHz,
C6D6): 8.41 (d with satellites, JHH = 8.25 Hz, JPtH = 42 Hz, 1H), 8.25
7.50 Hz, 1H), 7.64−7.52 (m, 4H), 2.06−1.99 (m, 12H, CH2), 1.02−0.89
(app quintet, JHH = 8.0 Hz, 18H, CH3), −0.13 (s, 1H, OH). The OH
group signal was observed only after treating the sample with polymer
bound diethylamine and could be eliminated by D2O addition.
trans-Pt(PEt3)2Cl(OH)(OOH)(4-trifluoromethylphenyl) (4).
(d with satellites, JHH = 8.25 Hz, JPtH = 38.5 Hz, 1H), 7.36 (d, JHH
=
8.25 Hz,1H), 7.31 (d, JHH = 8.25 Hz, 1H), 1.78−1.63 (m, 12H, CH2),
0.81−0.68 (app quintet Japp = 8.0 Hz, 18H, CH3), −0.4 (br s, 1H,
OH).
trans-Pt(PEt3)2(Br)(Cl)(OH)(4-trifluoromethylphenyl) (7).
trans-Pt(PEt3)2Cl(4-trifluoromethylphenyl) (2) (13.5 mg, 0.022 mmol)
in 1.5 mL of dichloromethane was mixed in a 20 mL vial with a freshly
prepared H2O2 solution in diethyl ether (1 mL). The vial was capped with
a rubber septum, and a needle was inserted to release the pressure
generated during the reaction. The mixture was stirred for 3 h at room
temperature during which time the solution turned pale yellow and
conversion of 2 (δ 14.5) to trans-Pt(PEt3)2Cl(OH)(OOH)(4-trifluor-
omethylphenyl) (4) (δ 5−4) was observed by 31P NMR spectroscopy
(31P NMR shifts varied slightly with the water content of the reaction
mixture). Once the reaction was complete, the dichloromethane layer was
separated, washed with 3 × 5 mL of deionized water and dried with
MgSO4. After filtering, the volatiles were removed in vacuo. The resulting
Compound 4 was dissolved in 1 mL of dichloromethane,
approximately 2 mL of deionized water and 0.05 mL of 50% HBr
was added, and the mixture was stirred for 1 min. 31P NMR
spectroscopy showed complete conversion of 4 to 7 (δ −2.09). The
dichloromethane layer was washed with 10 mL of deionized water and
dried with MgSO4. After filtering, the volatiles were removed in vacuo.
The resulting solid was washed with 1.0 mL of cold hexane and dried
in vacuo to give 7. Pale yellow crystals for the X-ray analysis were
grown in CH2Cl2/heptane (1:2) by slow evaporation in the
refrigerator. 31P {1H} NMR (101 MHz, CD2Cl2): −2.09 (s with
satellite, JPtP = 1703 Hz). 1H NMR (250 MHz, CD2Cl2): 8.15 (d with
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satellites, JHH = 7.50 Hz, JPtH = 44 Hz, 1H), 7.95 (d with satellites,
JHH = 9.50 Hz, JPtH = 39 Hz, 1H), 7.38−7.34 (m, 2H), 2.00−1.86 (m,
12H, CH2), 1.11−0.98 (app quintet, Japp = 7.75 Hz, 18H, CH3), −0.6
(brs,1H, OH).
portions until the 31P NMR signal for PPh3 (δ −4.9) disappeared. The
only product detected by 31P NMR spectroscopy was OPPh3 (δ 28.0).
Reaction of HOCl with 1 or 2. A solution of 1 (0.02 M) in C6D6
was mixed with a 0.05 mL aliquot of HOCl made in C6D6 (prepared
from Ca(OCl)2). The 31P NMR signal for 8 was observed. The only
product detected by 31P NMR spectroscopy was 8 and remaining 1. A
similar experiment was carried out with 2 and yielded 9.
Reaction of photolyzed 3 with PPh3. A sample of 3 (2.0 mg,
0.0029 mmol) was photolyzed at 313 nm in ∼0.5 mL of C6D6 until all
3 had been consumed. Excess PPh3 (5 mg, 0.019 mmol) was added. A
22% yield of OPPh3 was observed by 31P NMR spectroscopy.
Photolysis of trans,cis-Pt(PEt3)2Cl2(OH)(9-phenanthryl) (3).
Photolysis experiments were performed at 313 and 380 nm in C6D6
and CD2Cl2. A 0.011 M solution of 3 was added to a 5 mm J. Young
NMR tube. Photolysis was then carried out with periodic 31P NMR
and 1H NMR monitoring. Compound 3 was observed to convert to 1
(colorless) although the solution remained yellow orange and did not
become colorless as expected for 1.
Photolysis of trans,cis-Pt(PEt3)2Cl2(OH)(4-trifluoromethyl-
phenyl) (6). The photolysis experiments were performed at 313 nm
in CDCl3, CD2Cl2, and C6D6. Solutions of 6 (0.018 M) were
photolyzed in the presence of variable amounts of TME. Irradiation
times were 14−17 min. Spectroscopic analysis (NMR) was carried out
within 5 min of the photolysis, and the results are given in Table 4.
Photolysis of 6 in the Presence of 2. A 5 mm NMR tube was
charged with 0.5 mL of a CH2Cl2 solution that was 0.018 M in 6 and
0.016 M in 2. A capillary tube containing a solution of PPh3 was added
to the tube to serve as an integration standard. The sample was then
photolyzed at 313 nm. 31P NMR analysis was carried out at 30 s
intervals until all 6 was consumed (13 min total). At the end of the
photolysis, a 48% conversion of 2 into trichloro complex 9 was
indicated. A maximum conversion of 50% is expected from the
available chlorine in 6.
Photolysis of trans-Pt(PEt3)2(Br)(Cl)(OH)(4-trifluoromethyl-
phenyl) (7). The photolysis experiments were performed similarly
to those for 6 in CD2Cl2 both with and without TME. The
bromohydrin 16 was identified by comparison of the NMR properties
(1H NMR in CDCl3: δ 1.84, 1.35) with an authentic sample prepared
by HBr addition to the epoxide. NMR data for 17 have been
reported.62
UV−Visible Absorption Spectra. trans,cis-Pt(PEt3)2(OH)Cl2(9-
phenanthryl) (3) (3.2 mg, 0.0046 mmol) was dissolved in HPLC
grade CH2Cl2 and diluted to the mark in a 5.00 mL volumetric flask.
This solution (0.92 mM) was used as a stock solution for the analysis.
A series of concentrations was prepared by diluting 40, 50, 60, 70, and
80 μL into 5.0 mL volumetric flasks. The solvent blank was obtained,
and spectra were recorded from 200 to 800 nm for each sample.
trans-Pt(PEt3)2(OH)Cl2(4-trifluoromethylphenyl) (6) (20.1 mg,
0.0303 mmol) was dissolved in HPLC grade CH2Cl2 and diluted to
the mark in a 10.00 mL volumetric flask. This stock solution
(3.03 mM) was used to prepare a series of concentrations by diluting
20, 30, 40, and 50 μL into 5.0 mL volumetric flasks. All the
experimental conditions were kept identical for 3 and 6 except for the
concentrations. The absorbance data collected for the above samples
were converted to molar extinction coefficient, averaged, and plotted
(Figure 4).
Preparation of HOCl from Ca(OCl)2. Solid Ca(OCl)2 (100 mg,
0.70 mmol) was added to a 100 mL Schlenk flask. The stopcock plug
was removed from the flask and replaced with a rubber septum. The
flask was then connected to a vacuum line. Solvent (H2O, C6D6, or
CDCl3; 0.50 mL) was added into a 5 mm NMR tube. The tube was
connected to the vacuum line and immersed in LN2. The frozen NMR
tube and the Schlenk flask were connected and kept under vacuum for
10 min. After reaching a vacuum of 50 mTorr, the connection to the
vacuum line was closed and concentrated H2SO4 or HCl (0.10 mL)
was injected through the rubber septum onto the solid Ca(ClO)2.
(The Schlenk flask was isolated from the NMR tube prior to the
addition of H2SO4 and reconnected 5 min postinjection.) The gaseous
products were condensed into the NMR tube. The tube was thawed,
and the solution was transferred into a 1 cm path length quartz cuvette
for recording of the UV−vis absorption spectrum (Figure S2,
Supporting Information). Solid NaOH (∼20 mg, 0.50 mmol) was
added to the cuvette with stirring. The spectrum was recorded and
showed λmax for the ClO− at 292 nm, as reported in previous studies.61
Synthesis of HOCl from HgO and Cl2.20 Freshly prepared HgO
(100 mg, 0.46 mmol) was added to 2 mL of double distilled water
(DDW) in an 8 mL vial. Chlorine gas (1 mL, 0.045 mmol), dissolved
in 1 mL of DDW, was immediately added, and the mixture was stirred
for 30 min. Filtration through diatomaceous earth yielded a clear
solution. The solution was transferred to a 1 cm path length cuvette,
and the UV−vis absorption spectrum was recorded (Figure S2,
Supporting Information).
NMR Data for trans-Pt(PEt3)2(Br)(4-trifluoromethylphenyl)
(15). 31P {1H} NMR (101 MHz, CD2Cl2): 12.4 (s with satellite, JPtP
2709 Hz). 1H NMR (250 MHz, CD2Cl2): 7.50 (d with satellites, JHH
=
=
10 Hz, JPtH = 67.50 Hz, 2H), 7.17−7.11 (m, 2H), 1.74−1.59 (m, 12H,
CH2), 1.12−0.99 (app quintet, Japp = 8.10 Hz, 18H, CH3).
Quantum Yield Determinations. Irradiations were performed in
the compartment of a Cary 50 UV−vis spectrometer equipped with a
magnet stirrer and temperature control. The sample was contained in a
1 cm quartz cuvette sealed at the top with a quartz microscope slide.
The light source was positioned over the sample compartment,
allowing irradiations through the top of the cuvette.63 The photon flux
was measured (Fe oxalate actinometry64,65) before and after each
sample irradiation, and the average of the before and after
measurements was used as the photon flux during the sample
irradiation. Sample solution concentrations were sufficiently high to
ensure complete photon absorption (absorbance ≥2 at the irradiation
wavelength) over the ∼3 cm depth of the solution. Reaction progress
was monitored by the UV−vis absorbance of product trans-
Pt(PEt3)2Cl3(4-trifluoromethylphenyl) (8) over the region of 310−
400 nm where trans-Pt(PEt3)2Cl(4-trifluoromethylphenyl) (2) does
not absorb and trans-Pt(PEt3)2Cl2(OH)(4-trifluoromethylphenyl) (6)
absorbance is relatively weak. The yield of 8 from the UV−vis
(0.3 mM) data is 34% and matches the yield determined by 31P NMR
spectroscopy (1 mM). The absorbance increase at three wavelengths
(365, 375, and 390 nm) during the first 10% of the reaction was used
to calculate the quantum yield. The average of three runs gave a final
quantum yield for 6 of 51% 2% in dichloromethane.
Reaction of HOCl with TME. (A) HOCl was generated from
Ca(OCl)2 as described above, but the gaseous products were
condensed into a mixture of CDCl3 (0.50 mL) and TME (2,3-
dimethyl-2-butene, ∼30 μL) instead of water. The mixture was
thawed, and 1H NMR analysis indicated the formation of TME
products (Chart 1). (B) A solution of HOCl from HgO and Cl2 was
generated as described above but with water-saturated CDCl3 as the
solvent instead of DDW. After the NMR tube was thawed, TME was
1
added, and the mixture was analyzed by H NMR spectroscopy.
Photolysis of HOCl. Three identical samples of HOCl in C6D6
were prepared from Ca(OCl)2 as described above. TME (∼30 μL)
1
was added to one, and the products were determined by H NMR
spectroscopy (Chart 1). Another sample was photolyzed at 313 nm for
8 min, and then TME was added. No TME products were observed
except for 11 (TME and Cl2 yields 11). The third sample was
photolyzed at 380 nm for 8 min, and then TME was added. No TME
oxidation was detected. In other experiments, C6H6 samples of HOCl
were photolyzed at 313 and 380 nm without trap. Chlorobenzene was
1
detected by H NMR in both samples.
Reaction of HOCl with PPh3 and 1. PPh3 (3.5 mg, 0.015 mmol)
was dissolved in 0.8 mL of C6D6. A dilute C6D6 solution of HOCl,
prepared by diluting a 0.05 mL aliquot of a C6D6 HOCl solution
(prepared from Ca(OCl)2 as described above) to 0.4 mL, was added in
Computational Details. Gaussian 09 (revision A.1 or C.1)66 with
the M0667 or CAM-B3LYP68 (TDDFT) functional was used for all
calculations. The LANL2DZ69−72 basis set was employed for Pt, Cl,
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and P with an added d function (α = 0.05) for Pt, d (α = 0.648) and p
(α = 0.0467) functions for Cl, and d (α = 0.434) and p (α = 0.0376)
functions for P.73 The 6-31G(d) basis set was used for all other atoms.
Initial structures were derived from crystal coordinates and were
modified with Gaussview.74,75 All geometries were optimized with no
symmetry constraints in the gas phase. Free energies, enthalpies, and
entropies were calculated at 298.15 K and 1 atm. Analytical frequency
calculations gave no imaginary frequencies for the complexes except
for 3′, which had a small imaginary frequency of −11 cm−1 associated
with a methyl group rotation. Coordinates and energies are given in
Tables S1−S19, Supporting Information. TDDFT and other indicated
calculations included solvent (dichloromethane) with the polarized
continuum model (pcm)76 and employed the gas-phase optimized
structures.
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■
S
* Supporting Information
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CIF files, additional experimental details, NMR spectra, and
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AUTHOR INFORMATION
Corresponding Author
*Paul R. Sharp. E-mail: sharpp@missouri.edu.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
■
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Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
(32) Glebov, E. M.; Kolomeets, A. V.; Pozdnyakov, I. P.; Plyusnin, V.
F.; Grivin, V. P.; Tkachenko, N. V.; Lemmetyinen, H. RSC Adv. 2012,
2, 5768−5778.
■
Support was provided by the U.S. Department of Energy,
Office of Basic Energy Sciences (Grant DE-FG02-88ER13880).
We thank Dr. Charles Barns for X-ray data collection and
processing, Tharushi Perera for the synthesis of 1, Dr. Wei
Wycoff for assistance with the NMR measurements, and
Dr. Nathan Lee for mass spectral analyses. The computations
were performed on the HPC resources at the University of
Missouri Bioinformatics Consortium (UMBC).
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