High quantum yield molecular bromine photoelimination from mononuclear platinum(IV) complexes

Article abstract of DOI:10.1021/ic4004998

Pt(IV) complexes trans-Pt(PEt₃)₂(R)(Br)₃ (R = Br, aryl and polycyclic aromatic fragments) photoeliminate molecular bromine with quantum yields as high as 82%. Photoelimination occurs both in the solid state and in solution. Calorimetry measurements and DFT calculations (PMe₃ analogs) indicate endothermic and endergonic photoeliminations with free energies from 2 to 22 kcal/mol of Br₂. Solution trapping experiments with high concentrations of 2,3-dimethyl-2-butene suggest a radical-like excited state precursor to bromine elimination.

Full text of DOI:10.1021/ic4004998

Article
pubs.acs.org/IC
High Quantum Yield Molecular Bromine Photoelimination from
Mononuclear Platinum(IV) Complexes
Alice Raphael Karikachery,† Han Baek Lee,† Mehdi Masjedi,† Andreas Ross,† Morgan A. Moody,†
Xiaochen Cai,‡ Megan Chui,‡ Carl D. Hoff,*,‡ and Paul R. Sharp*,†
†125 Chemistry, University of Missouri, Columbia, Missouri 65211, United States
‡Department of Chemistry, University of Miami, Coral Gables, Florida 33146, United States
S
* Supporting Information
ABSTRACT: Pt(IV) complexes trans-Pt(PEt3)2(R)(Br)3 (R = Br, aryl and polycyclic
aromatic fragments) photoeliminate molecular bromine with quantum yields as high as
82%. Photoelimination occurs both in the solid state and in solution. Calorimetry
measurements and DFT calculations (PMe3 analogs) indicate endothermic and
endergonic photoeliminations with free energies from 2 to 22 kcal/mol of Br2. Solution
trapping experiments with high concentrations of 2,3-dimethyl-2-butene suggest a radical-
like excited state precursor to bromine elimination.
DFT calculations show that these are endergonic reactions that
store between 2 and 22 kcal/mol. Bromine trapping experi-
ments indicate that, in addition to reacting with the released
bromine, more reactive traps also react directly with the excited
state that precedes bromine elimination and the products
indicate that this excited state has radical character.
INTRODUCTION
■
Efficient photochemical solar energy conversion to stored
energy in chemical bonds is an important goal for global energy
needs.1 A promising process for the realization of this goal is
the splitting of stable molecules into a reduced component
(reductant) and an oxidized component (oxidant) that can
then be recombined to release the stored energy.2 Most often
this concept is applied to water splitting into molecular
hydrogen (reductant) and molecular oxygen (oxidant)3−6 but
may also be applied to the simpler and, in some ways, more
advantageous7 process of splitting hydrohalic acids into
molecular hydrogen and molecular halogens (eq 1, X = a
halogen).5,8−11 A closely related alternative water splitting
process, yielding molecular hydrogen and hydrogen peroxide
(eq 1, X = OH), has also been proposed.12
RESULTS
■
Complex Synthesis and Characterization. Details may
be found in the Experimental Section located in the Supporting
Information. Platinum(IV) complexes trans-Pt(PEt3)2(R)(Br)3
(2, Scheme 1) are easily prepared by Br2 addition to the
corresponding Pt(II) complexes, trans-Pt(PEt)2(R)(Br) (1). In
the case of 1(NPh), 1(MeOPh) and 1(Per), simultaneous ring
bromination results in the isolation of 2(BrNPh),
2(BrMeOPh), and 2(BrPer). Ring bromination subsequent to
oxidative addition can also occur and 2(BrNap) is formed when
additional Br2 is added to 2(1-Nap).
A potential key step in a transition metal catalyzed hydrohalic
acid splitting process is the photoelimination of a halogen
molecule from a dihalide complex (eq 2, X = a halide).
31P NMR spectra of 2 are diagnostic of the Pt center
oxidation shown in Scheme 1 and the Pt(IV) complexes 2 all
show singlets with satellites in the δ −8 to −10 region, shifts
that are about 20 units negative of those for the corresponding
Pt(II) precursors 1 (δ 12 to 13). Ring bromination generally
leads to a small, but visible, shift decrease (<1 unit) from the
corresponding unbrominated analog. 195Pt−31P coupling
constants for 2 are in the rather tight range of 1600 to 1636
Hz and are reduced from those for 1 (2680 to 2770 Hz).
Although there have been recent advances in this area,10,11,13−16
halogen photoelimination from transition metal halide
complexes (LnMX2) remains rare and poorly understood.
Herein, we report our studies on a family of mononuclear
platinum(IV) bromo complexes that photoeliminate molecular
bromine with quantum yields as high as 82%. Calorimetry and
Received: February 27, 2013
Published: March 13, 2013
© 2013 American Chemical Society
4113
dx.doi.org/10.1021/ic4004998 | Inorg. Chem. 2013, 52, 4113−4119

Inorganic Chemistry
Article
thracene (Scheme 2 and Table 1). The amount of ring
brominated 1(BrNap) decreases from triphenylmethane to
Scheme 1
Table 1. Photolysis Product Yields for 2(1-Nap) and 2(1-
Phen) in Toluene and with Added Bromine Traps
1(1-
a
trap
no added trap
Nap)
1(BrNap) 1(Phen) 1(BrPhen)
0%
100%
56%
38%
30%
85%
91%
70%
15%
9%
triphenylmethane (0.2 M)
44%
62%
dihydroanthracene (0.2 M)
a
Pt(PEt)2(10-bromo-9-phenanthryl)(Br).
dihydroanthracene following the C−H bond strength (or the
ease of bromination) of the organic molecule. Complex
2(Phen) shows similar behavior but just toluene solutions
give, in addition to ring brominated Pt(PEt)2(10-bromo-9-
phenanthryl)(Br) 1(BrPhen), some (30%) unbrominated
1(Phen) (Table 1). The solvent toluene is presumably trapping
some of the bromine in this case.
With 2(Br-Nap), the ring is already brominated and
photolysis (313 nm) of this complex in the presence of cis-2-
hexene yields 1(BrNap) as the only Pt containing product. The
cis-2-hexene is brominated in 96% yield (NMR) to the anti-
addition product, a racemic mixture of (2R, 3S)- and (2S, 3R)-
2,3-dibromohexane (eq 3). None of the syn-addition product
UV−vis spectra of 2 (Supporting Information), with the
exception of the BrPer and PMI derivatives, show broad and
weak (εmax < 5000 M−1cm−1) blue absorbance in the visible
region tapering off from the UV and leading to light orange to
light red compounds. The absorbance deeper into the UV
region is stronger and, for the naphthalene and phenanthrene
derivatives, vibronically coupled aromatic group π−π* bands
are observed at about 300 nm. Dark brown 2(BrPer) and dark
purple 2(PMI) show their vibronically coupled aromatic group
π−π* bands in the visible region. The UV−vis spectra of the
ring brominated derivatives are virtually indistinguishable from
the spectra for the corresponding unbrominated analogues.
Photolysis. Many of the complexes 2 are light sensitive,
even to ambient light. In the case of 2(1-Nap), room light
exposure of an orange toluene solution in an NMR tube results
in the slow formation (days) of the naphthyl ring brominated
Pt(II) complex 1(BrNap) along with HBr (detected by reaction
with 1,8-bis(dimethyamino)naphthalene) (Scheme 2). A 500
W halogen lamp or a medium pressure Hg lamp gives complete
conversion in minutes. Mixtures of 1(BrNap) and 1(1-Nap) are
also observed when toluene solutions of 2(1-Nap) are
irradiated in the presence of triphenylmethane or dihydroan-
(racemic mixture of (2R, 3R)- and (2S, 3S)-2,3-dibromohex-
ane) is detected.17 Under similar conditions, but in the absence
of cis-2-hexene, 2(BrNap) is only weakly photochemically
active producing a small amount (8%) of 1(BrNap) in the time
it takes for complete conversion in the presence of cis-2-hexene
(10 min). Continued irradiation gives further conversion and
small amounts of additional phosphorus-containing products.
The fate of the eliminated bromine is unknown but solvent may
1
be involved. H NMR spectroscopy after prolonged (1 h)
irradiation also shows small amounts of 1,4-dibromonaph-
thalene.
Phenyl and 2-naphthyl derivatives 2(Ph) and 2(2-Nap) are
nearly inactive even in the presence of a bromine trap. In the
presence of 1-hexene only small amounts of 1(Ph) or 1(2-Nap)
are produced on prolonged irradiation (hours). 2(2-Nap) gives
somewhat higher conversion and another Pt(II)-containing
product that is probably ring brominated. Photoactivity is
greatly increased by substitution at the 2-position of the phenyl
ring. Thus, 2(CF3Ph) is very light sensitive and on irradiation at
313 nm in the presence of 1-hexene converts rapidly to
1(CF3Ph). Similarly, 2(BrMeOPh) readily converts to
1(BrMeOPh). Ring substituted 2(BrNPh) is also photochemi-
cally active but much less so than 2(CF3Ph) and 2(BrMeOPh).
Photoinduced ring bromination is not observed for any of the
phenyl derivatives.
Scheme 2
To put the photoactivity of 2 on a more quantitative basis,
quantum yields for photoreduction of selected 2 in the
presence of 1-hexene were determined (Table 2). The yields
4114
dx.doi.org/10.1021/ic4004998 | Inorg. Chem. 2013, 52, 4113−4119

Inorganic Chemistry
Article
Nocera has shown that halogen photoelimination from metal
halide complexes can occur from the solid state without a
trap.13 We find this is also possible for 2. Vacuum photolysis of
a solid film of 2(BrNap) deposited on the inside walls of a
quartz tube causes the orange-red solid to lighten and release
Br2, which was condensed onto a frozen solution of TME in
Table 2. Quantum Yields for Conversion of 2 to 1 in the
Presence of 1-Hexene
a
R
wavelength (nm)
quantum yield (%)
Br
313
313
313
313
313
313
313
313
440
470
59
<1
82
5
Ph
1
CF3Ph
Br2NPh
1-Nap
BrNap
2-Nap
Phen
CDCl3. The mixture was thawed and H NMR spectroscopy
showed formation of the TME bromination product 3. 31P
NMR analysis of the dissolved film remaining in the quartz tube
showed partial conversion to 1(BrNap). The degree of
photoconversion of 2(BrNap) to 1(BrNap) is dependent on
the film treatment. Much higher conversions are observed if the
film was not exposed to a prolonged (h) dynamic vacuum (50
mtorr) suggesting that partial solvent retention promotes the
photolysis. However, the factors involved have not been
thoroughly investigated.
13
19
<1
b
13, 15
BrPer
PMI
48
4
a
b
CH2Cl2 solvent, [1-hexene] = 0.13 M. [1-hexene] = 1.0 M
bear out the qualitative results and show a remarkable 82%
quantum yield for 2(CF3Ph). In addition, the strong π−π*
transitions of 2(BrPer) and 2(PMI) allow the photolysis to be
conducted in the visible (440 and 470 nm) rather than in the uv
region as used for the other complexes.
To further probe the bromine photoelimination process, the
electron rich trap 2,3-dimethyl-2-butene (TME) was used in
the photolysis of 2(Br) (Table 3). The Pt(II) product,
Thermal Reactions. Complex 2(CF3Ph) slowly reacts with
1-hexene at 25 °C in the dark to give 1(CF3Ph). This thermal
reaction is first order in 2(CF3Ph) and zero order in 1-hexene
over a 1-hexene concentration of 0.096 to 0.38 M with an
average rate constant (kobs) of 1.4(2) × 10−3 s−1 or 2.0(2) d−1
(Supporting Information). All other 2 are thermally stable for
days in the presence of 1-hexene at 25 °C, but show signs of
reaction at temperatures ranging from 50 to 120 °C. In the case
of 2(1-Nap), ring bromination is observed with a ratio of
1(BrNap) to 1(1-Nap) of 10:7, approximately reversed from
the ratio for the photochemical reaction under similar
conditions. In the solid state, 2(1-Nap) decomposes in the
dark over a one-year period at room temperature to yield
exclusively 1(BrNap).
a
Table 3. Photolysis Product Yields for trans-Pt(PEt3)2Br4
b
(2(Br)) with 2,3-Dimethyl-2-butene (TME) Trap
Calorimetry and DFT. The bromine reaction enthalpies for
the three complexes, 1(Ph), 1(CF3Ph) and 1(BrNap) were
determined by calorimetry in toluene (Table 4). Initial gas
Table 4. Thermodynamic Values (kcal/mol) for the
Bromination of 1 and 1′
ΔH
a
b
b
b
R
exp
B3LYP
M06
ΔG M06
Ph
−38.3 1.5
−20.5 1.5
−20.7
−10.6
−2.7
−32.5
−23.6
−16.3
−16.8
−27.4
−22.4
−21.8
−10.5
−4.7
−2.2
−15.3
c
Nap
CF3Ph
BrMeOPh
Br
−17.5
2
−20.9
1
−16.2
−9.9
Per
−10.2
a
b
c
Toluene solvent. Gas phase for trans-Pt(PMe3)2(R)(Br) 1′. R =
BrNap.
a
b
c
d
On the basis of 2(Br). [2(Br)] = 4 mM. Ref 18. Refs 19 and 20.
phase DFT calculation on 1′ (PMe3 used in place of PEt3)
employing the B3LYP functional21 gave a poor match to the
experimental data. Solvent inclusion did not improve the
results. The M06 functional22 is reported to give better energy
values for some transition metal systems and calculated values
with this functional are in much better agreement with our
experimental values. Free energies for the reactions were also
calculated and are less negative than the enthalpies due to the
unfavorable entropy change of going from two molecules to
one. (ΔG is increased by about 3 kcal from the values in Table
4 when a simple correction factor23,24 for the expected smaller
entropy decrease in toluene, as compared to the gas phase, is
applied.)
Pt(PEt3)2Br2 1(Br), is observed as cis and trans isomers with
the trans isomer favored in CDCl3. Several TME products are
detected. The dibromoalkane, 2,3-dimethyl-2,3-dibromobutane
(3), which is the exclusive product of Br2 addition to TME, is
the dominant product (73%), but its yield decreases with
increasing TME concentration. Allylic bromination product, 1-
bromo-2,3-dimethyl-2-butene (4), is the second most abundant
and its yield increases with increasing TME concentration. The
HBr addition product, 2-bromo-2,3-dimethylbutane, is also
detected and presumably originates from the reaction of TME
with HBr generated in the formation of the allylic bromination
product 4. In contrast to 2(Br), photolysis of 2(PMI) in the
presence of TME gives exclusively the Br2 addition product 3
over TME concentrations from 22 to 560 mM.
Structures. The solid-state structures of 1(CF3Ph, NPh,
MeOPh, BrMeOPh, 1-Nap, BrPhen, BrPer, PMI) and 2(Br,
4115
dx.doi.org/10.1021/ic4004998 | Inorg. Chem. 2013, 52, 4113−4119

Inorganic Chemistry
Article
BrNPh, BrMeOPh, Phen, BrNap, BrPer) were determined by
single crystal X-ray diffraction studies. Drawings of 2(BrNap)
and 1(CF3Ph) are given in Figures 1 and 2 while those for the
Table 5. Metrical Parameters Indicative of Steric Strain in 2
(R = BrNap, Phen, BrPer, BrMeOPh) and 2′(1-Nap)
2′(1-
c
a
b
atoms
2(BrNap)
2(Phen)
2(BrPer)
2(BrMeOPh)
Nap)
Pt1−C1
Pt1−C1−
C4
2.107(5)
172.8(4)
2.109(7)
170(1)
2.102(5)
173.2(3)
2.104(2)
177.3(2)
2.124
173.4
Pt1−C1−
115.9(4)
127.4(4)
115.7(2)
127.7(3)
90.3(6)
93.4(4)
89.7(3)
96(2)
116.0(3)
127.4(3)
121.9(2)
121.4(2)
93.72(6)
93.34(6)
90.91(6)
89.55(6)
115.9
126.2
90.9
95.9
90.0
94.3
C2
Pt1−C1−
C9
C1−Pt1−
P1
88.24(14)
94.93(14)
89.38(14)
95.10(14)
92.51(13)
95.47(14)
90.04(13)
93.89(13)
C1−Pt1−
P2
C1−Pt1−
Br2
C1−Pt1−
Br3
a
Numbering from 2(BrNap). Corresponding atoms in other structures
b
may have different numbers. Average and standard deviation of the
two independent molecules. DFT (M06) structure of trans-
c
Figure 1. XSeed/POV-Ray drawing of 2(BrNap). Hydrogen atoms
omitted. Atoms are drawn as 50% probability ellipsoids.
Pt(PMe3)2(1-napthyl)(Br)3 2′(1-Nap).
is compressed to 173°. Similarly, the bond angles around C1
show a larger than 120° angle on the side of the peri-hydrogen
atom (Pt1−C1−C9 = 127.4°) and a smaller than 120° angle on
the opposite side (Pt1−C1−C2 = 115.9°). The strain is also
reflected in the angles around Pt, especially the ones involving
the ligands cis to carbon atom C1. The angles on the side of the
peri-hydrogen atom, C1−Pt−Br3 and C1−Pt−P2, are 95.1°
and 94.9° respectively larger than the angles of 89.4° (C1−Pt−
Br2) and 88.2° (C1−Pt−P1) on the opposite side, which are
close to the expected 90°. In more colloquial terms, the
bromine atom Br3 and the P2-PEt3 ligand are bent away from
the peri-hydrogen atom. These same distortions are observed in
the DFT structures of the PMe3 complexes 2′. The DFT
metrical parameters for trans-Pt(PMe3)2(1-napthyl)(Br)3 2′(1-
Nap) are included in Table 5 and compare well with those from
the solid-state structure of 2(BrNap), differing by 3% or less.
A similar bending away of the cis-ligands is observed in
2(BrMeOPh), but with the MeO substituents on both sides of
the phenyl ring the steric strain occurs with all four ligands cis
to the aryl group C1 atom. In this case, the effect is focused on
the two PEt3 ligands (C1−Pt1−P1 = 93.7° and C1−Pt1−P2 =
93.3°) with the Br ligand angles within 1° of the expected 90°.
The structures of 2(Br) and 1(CF3Ph, NPh, MeOPh,
BrMeOPh, 1-Nap, BrPhen, BrPer, PMI) are more routine
and generally do not require comment. One exception is that of
1(CF3Ph) shown in Figure 2. Like 2 (R = BrNap, Phen,
BrMeOPh), this complex shows signs of steric strain. The
phenyl ring CF3 group lies between the two trans-PEt3 ligands
near the Pt center. The CF3 group is rotated such that the Pt
atom is nestled between two of the F atoms. Whereas this is
favorable for steric interactions with the Pt center, it is less
favorable for PEt3 ligand interactions. As a result, the PEt3
ligands bend away from the CF3 group closing the expected P−
Pt−P 180° angle to 171.95(4)o. Steric stress is also apparent in
the angles around C1 which, like in the structures of 2(BrNap,
Phen, BrPer), are distorted and asymmetric: Pt1−C1−C4 =
174.0(3), Pt1−C1−C2 = 128.3(3), Pt1−C1−C6 = 116.2(3).
Undoubtedly, strain is also present in 2(CF3Ph) giving
distortions similar to that in 2(BrNap, Phen, BrPer). The
DFT structures of 2′(CF3Ph) and 2′(BrNap, Phen, BrPer)
Figure 2. XSeed/POV-Ray drawing of 1(CF3Ph). Hydrogen atoms
omitted. Atoms are drawn as 50% probability ellipsoids.
other structures can be found in the Supporting Information.
Remarkably, the structures of the organometallic complexes 2
(i.e., R ≠ Br) are the first reported for the PtL2X3R (L = a
phosphine, X = a halogen) class and that of 2(Br) is only the
second reported member of the PtL2X4 (L = a phosphine, X = a
halogen) class.25 A notable feature of the organometallic
derivatives is the steric crowding associated with the R group.
In each case, the R groups contain either a hydrogen atom peri
to the Pt atom (R = BrNap, Phen, BrPer) or a substituent at the
2-position or the 2- and 6-positions on a phenyl ring (R =
BrNPh, BrMeOPh), which sterically interfere with the Br and
PEt3 ligands cis to the R group. To minimize the steric
repulsion, the aromatic rings tilt at about 45° from the P−Pt−P
axis such that the peri-hydrogen atom or the substituent(s) is
situated between a cis Br atom and the adjacent PEt3 ligand.
Nonetheless, steric distortion of the structures is still apparent
in the metrical parameters listed in Table 5. (Aryl ring disorder
makes the data unreliable for 2(BrNPh) and this structure is
not included in the table.) The 2(BrNap), 2(Phen) and
2(BrPer) structures are similar and only that of 2(BrNap) will
be discussed. The naphthalene ring system of 2(BrNap) is
visibly distorted (Figure 1). The Pt−C1--C4 angle, which
would be expected to be close to linear in a strain-free complex,
4116
dx.doi.org/10.1021/ic4004998 | Inorg. Chem. 2013, 52, 4113−4119

Inorganic Chemistry
Article
show strain and match the distortions observed in the crystal
structures.
of released Br2. The second product, 1-bromo-2,3-dimethyl-2-
butene (4), is the expected product from a radical bromination
and the yield of this product increases with TME concentration
at the expense of Br2 addition product 3. Scheme 3 is consistent
DISCUSSION
■
Photochemistry. Pt(IV) halide complexes [PtX6]2− have
long been known to be photochemically active.26−46 For X =
Br, the net aqueous photochemical reaction is aquation to give
[Pt(H2O)Br5]1− and a bromine anion. The details of the photo
process have been controversial with proposals of initial Br
Scheme 3
28
anion,29,30 Br atom27 and Br2 elimination. Recent transient
spectroscopic experiments combined with computational
studies strongly support Br anion elimination.45 However,
under some conditions oxidation processes, involving an
excited state Pt complex and oxidizable species, may be
involved.40,44
Complexes 2 are related to [PtBr6]2‑ by substitution of two
bromine anions with two PEt3 ligands and, except for R = Br,
substitution of a third bromine anion with an R group. Yet, Br2
elimination is indicated by our bromine trapping experiments
with cis-2-hexene and TME described above. The high yield of
the cis-2-hexene bromine anti-addition product with no allylic
bromination is expected from the accepted bromonium ion
mechanism of Br2 addition to the double bond of alkenes47,48
and this has been used previously as an indicator for Br2
elimination.14 Why is there this difference in the photo-
chemistry of 2 and [PtBr6]2−? There are three notable
differences in the two systems, one is the ligand-field strength,
another is the charge, and finally there is the solvent. The
insolubility of 2 in water eliminates the possibility of comparing
their photochemistry in water. However, [NBu4]2[PtBr6] is
soluble in dichloromethane (DCM) and we have examined its
photochemistry in this solvent. [PtBr6]2− is photoinactive in
DCM, with or without the Br2 trap TME. [PtBr6]2‑ does,
however, undergo photosubstitution of a bromide by added
DMSO in DCM (experimental section). Thus, it appears that
solvent is not a factor in the different photochemistry of 2 and
[PtBr6]2‑. This leaves charge and the ligand-field strength as
possible factors. It can be argued that Br anion loss from neutral
2 is disfavored by charge separation. On the other hand, anion
loss from [PtBr6]2− should not be difficult as a positive charge is
not formed from Br anion removal. The charge difference may
then be an important factor in the different photochemical
processes. The ligand-field strength is also expected to be
important. The [PtBr6]2− excited state from which Br anion
loss occurs has been assigned to the lowest energy triplet, a
with these results. TME (but not cis-2-hexene) is proposed to
react directly with the excited state of 2 and, consistent with a
CT excited state with radical character, this gives the allylic
radical bromination product 4. This reaction must occur within
the lifetime of the excited state. If not, decay of the excited state
results in Br2 elimination followed by Br2 reaction with TME
(or cis-2-hexene) to give 3. At lower concentrations of TME,
the probability of a TME molecule being in the vicinity of an
excited state is less and greater amounts of Br2 elimination
occur giving more 3. TME oxidation by electron transfer to the
excited state is also possible. However, on the basis of the
known oxidation chemistry of TME,51,52 allylic 4 would not be
expected from the TME radical cation.
Interestingly, 2(PMI) gives only the Br2 addition product 3
at all concentrations of TME. This suggests that, in comparison
to 2(Br), either the excited state is less reactive to radical
abstractions or Br2 elimination is faster and the radical pathway
is not competitive. A faster elimination rate might obtain from
the expected steric crowding in the 2(PMI) excited state
(Structures above). However, the low lying PMI orbitals could
delocalize radical character into the PMI unit, thereby reducing
radical reactivity.
Also observed in the photochemistry is the ring bromination
of 2(1-Nap) and 2(Phen). When does this occur? We observe
ring bromination in the synthesis of 2(BrNap), 2(BrMeOPh)
and 2(BrPer), so it is possible that ring bromination occurs by
reaction of eliminated Br2 with remaining 2. However, this
would result in initially high ratios of ring brominated 1 to
unbrominated 1 and the ratio would decrease as the
concentration of 2 falls. This is not observed and, instead,
the ratio remains relatively constant throughout the photolysis.
Ring bromination could be an alternate excited state decay
pathway (Scheme 3). This is consistent with the data in Table
1, which show that ring bromination is considerably reduced
with strong radical traps that may react, at least partly, directly
with the excited state.
The quantum yield variations of 2 (Table 2) are difficult to
interpret. At first glance, there appears to be a reaction free
energy dependence, as shown by comparison of 2(Ph), 2(1-
Nap), and 2(CF3Ph), where the quantum yield varies inversely
with the free energy of the elimination. However, attributing
quantum yield variations to a single factor is simplistic. This is
illustrated by the high quantum yield of 2(Br), which has a
relatively high Br2 elimination free energy. To help in the
metal-centered, ligand-field T1g state.45 Close by are Br-to-Pt
3
charge-transfer states.49 The greater ligand-field strength in 2,
from replacement of the Br anions with PEt3 and the R group
(except 2(Br)), destabilizes the T1g state (ignoring the change
in symmetry). A likely result is that the lowest energy triplet in
2 is a Br-to-Pt charge-transfer (LMCT) state with access to Br2
elimination. DFT calculations on the lowest-energy triplet of 2,
to be reported later, indicate a CT excited state with Br radical
character. Radical character would also be consistent with
Co(III) halide system photochemistry, which show radical
character in their analogous LMCT excited state.50
Whereas the trapping product (anti-addition product) with
cis-2-hexene indicates Br2 photoelimination from 2, the
products from the more reactive alkene, TME, are more
diverse and, for 2(Br), their yields depend on the TME
concentration (Table 3). The major product is 2,3-dimethyl-
2,3-dibromobutane (3), the same product obtained from Br2
addition to TME, suggesting that this product is from trapping
4117
dx.doi.org/10.1021/ic4004998 | Inorg. Chem. 2013, 52, 4113−4119

Inorganic Chemistry
Article
discussion, a simplified Jablonski diagram is given in Scheme 4.
Excitation into the strong π−π* band to the singlet excited
rotation is a deactivation pathway in 2(Ph), then the enhanced
quantum yield of the substituted aryl complexes and the
polycyclic complexes with a hydrogen atom peri to the Pt may
be associated with restricted rotation about the Pt−C bond.
Thermochemistry. Steric strain in 2 is reflected in the
thermochemistry of Br2 addition to 1. The steric strain revealed
in the X-ray crystal structures of 2(BrNap), 2(Phen), and
2(BrMeOPh) and in the DFT structures destabilizes six-
coordinate 2 relative to four-coordinate 1. In fact, we have
previously shown that in trans-Pt(PEt3)2(9-anthracenyl)(Br)
(1, R = 9-anthracenyl) the two anthracenyl peri-hydrogen
atoms completely block the Pt center axial sites and eliminate
access to the six-coordinate Pt(IV) complex Pt(PEt3)2(9-
anthracenyl)(Br)3 (2(9-anthracenyl)).55 Steric destabilization is
also evident in the thermal stability of 2. Complex 2(CF3Ph),
which both calorimetry and DFT calculations indicate is the
least stable to thermal Br2 elimination, decomposes, even in the
solid state, to 1(CF3Ph). The solution kinetic data (above)
indicate a first-order reaction in 2(CF3Ph) and zero order in 1-
hexene trap suggesting the mechanism in Scheme 5, where kre
Scheme 4
state S2 is shown but this is not necessary (and is impossible for
2(Br), which has no π−π* transition) as the photoeliminations
are still observed when irradiation is into the weak long-
wavelength band(s) of 2. This suggests that there is a lower
energy singlet (S1) through which the system transits. (Most
likely, there are other singlet states between S2 and S1, but these
are not shown.) S1 is probably the LMCT excited state
involving Br ligand lone pairs and a vacant σ* orbital on the Pt
center as discussed above. Rapid intersystem crossing to the
associated triplet state T1 would follow. In complexes 2, instead
of Br anion dissociation character found for [PtBr6]2−, T1 has
more radical character and can be trapped with high
concentrations of radical reactive species (kt1) or can progress
either to ring bromination (krb) or Br2 elimination (ke).
[Pt]Br2* in Scheme 3 can then be identified with T1 in Scheme
4.
Coming back to the quantum yield for the reaction, from
Scheme 4 it is the population and fate of T1 that is central to
the quantum yield. Population of T1 will depend partly on the
various decay pathways for the precursor excited states. One
that is easily investigated is emission (kf1, kf2, kp). Weak
emission (Supporting Information) is observed from solutions
of 2(PMI) with excitation into the π-to-π* band. However,
relatively concentrated solutions must be used and the emission
spectrum can be attributed to low level impurities of highly
emissive 1(PMI) and BrPMI or PMI. Thus, radiative decay
from S2 (kf2), at least for 2(PMI), does not appear to be a
significant decay pathway. Emission from S1 and/or T1 could
still be present but below our observation limit (700 nm).
Initially, we thought that steric crowding was important in
preventing bromine recombination (kb). This idea was based
primarily on the very low quantum yield of 2(Ph) and 2(2-
Nap) as compared to 2(1-Nap) and 2(Phen) and on the
observation that introducing a substituent at the phenyl ring 2-
position “turned-on” the photochemical reaction in 2 (CF3Ph)
and 2(BrNPh). Whereas this may be a factor, the high quantum
yield of sterically unencumbered 2(Br) clearly indicates other
factors, probably related to population and decay of the excited
states, are important. In addition, we have studied the
bromination of 1(CF3Ph), one of the most sterically
encumbered complexes, by low temperature NMR spectrosco-
py and find that, even at −30 °C, bromination of 1(CF3Ph) is
too fast to observe. Aryl ring rotation in some organic
molecules is known to be important in excited state
deactivation and can be sterically blocked.53,54 If phenyl ring
Scheme 5
∼ kobs = 1.4(1) × 10−3 s−1. In conjunction with the calculated
free energy of ∼5 kcal this indicates a koa of at least 10 s−1 M−1,
consistent with our inability to observe the bromination
reaction by NMR spectroscopy.
Electronic factors should also be important in the
thermochemistry but are difficult to separate from steric factors
in these congested complexes. The relatively unhindered 2(Br)
and 2(Ph) that perhaps can be compared. The electron
donating phenyl group of 2(Ph) should stabilize the Pt(IV)
state more than the Br ligand in 2(Br) and this is supported by
the more negative free energy of bromination of 1(Ph) to
2(Ph).
CONCLUSIONS
■
Platinum(IV) complexes trans-Pt(PEt3)2(R)(Br)3 2 photo-
eliminate molecular bromine with quantum yields as high as
82% and reaction free energies from 2 to 22 kcal/mol of Br2.
The photoelimination is independent of wavelength over the
UV−vis region where the complexes absorb and the excitation
wavelength can be shifted into the visible by introducing
polycyclic aromatic chromophores as the R group. The excited
state precursor to molecular bromine elimination can undergo
radical reactions with good hydrogen atom donors and is
probably Br-to-Pt charge transfer (LMCT) in character. Details
of the Br2 elimination mechanism are under investigation and
will be reported in later publications. The change from the
bromine anion elimination photochemistry of related [PtBr6]2−
is probably due to the increased ligand field and the high energy
of
charge separation in neutral 2. Similar molecular
elimination chemistry is possible with chloro and hydroxo
complexes and is currently under investigation.
4118
dx.doi.org/10.1021/ic4004998 | Inorg. Chem. 2013, 52, 4113−4119

Inorganic Chemistry
Article
(20) Clennan, E. L.; Chen, X. J. Am. Chem. Soc. 1989, 111, 5787.
(21) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(22) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157.
(23) Martin, R. L.; Hay, P. J.; Pratt, L. R. J. Phys. Chem. A 1998, 102,
3565.
ASSOCIATED CONTENT
* Supporting Information
CIF files and experimental details, H NMR spectra, crystal
structure drawings, UV−vis spectra for 2, emission spectra for
2(PMI), dark reaction kinetics for 2(CF3Ph), calorimetry
information and DFT data. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
1
(24) Sieffert, N.; Buhl, M. J. Am. Chem. Soc. 2010, 132, 8056.
̈
(25) Allen, F. H. Acta Crystallogr. 2002, B58, 380 (CSD V5.32, Feb
11 and May 11, 2011 updates).
(26) Rich, R. L.; Taube, H. J. Am. Chem. Soc. 1954, 76, 2608.
(27) Adamson, A. W.; Sporer, A. H. J. Am. Chem. Soc. 1958, 80, 3865.
(28) Penkett, S. A.; Adamson, A. W. J. Am. Chem. Soc. 1965, 87,
2514.
(29) Balzani, V.; Manfrin, M. F.; Moggi, L. Inorg. Chem. 1967, 6, 354.
(30) Balzani, V.; Carassiti, V. J. Phys. Chem. 1968, 72, 383.
(31) Wright, R. C.; Laurence, G. S. J. Chem. Soc., Chem. Commun.
1972, 132.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sharpp@missouri.edu (P.R.S.), c.hoff@miami.edu
(C.H.).
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
(32) Rehorek, D.; Dubose, C. M.; Janzen, E. G. Inorg. Chim. Acta
1984, 83, L7.
(33) Cameron, R. E.; Bocarsly, A. B. J. Am. Chem. Soc. 1985, 107,
Notes
6116.
The authors declare no competing financial interest.
(34) Fadnis, A. G.; Kemp, T. J. J. Chem. Soc., Dalton Trans. 1989,
1237.
(35) Grivin, V. P.; Khmelinski, I. V.; Plyusnin, V. F. J. Photochem.
Photobiol., A 1990, 51, 379.
(36) Grivin, V. P.; Khmelinski, I. V.; Plyusnin, V. F.; Blinov, I. I.;
Balashev, K. P. J. Photochem. Photobiol., A 1990, 51, 167.
(37) Grivin, V. P.; Khmelinski, I. V.; Plyusnin, V. F. J. Photochem.
Photobiol., A 1991, 59, 153.
(38) Monreal, O.; Esmaeili, T.; Hoggard, P. E. Inorg. Chim. Acta
1997, 265, 279.
(39) Goursot, A.; Kirk, A. D.; Waltz, W. L.; Porter, G. B.; Sharma, D.
K. Inorg. Chem. 2002, 26, 14.
(40) Glebov, E. M.; Plyusnin, V. F.; Venediktov, A. B.; Korenev, S. V.
Russ. Chem. Bull. 2003, 52, 1305.
ACKNOWLEDGMENTS
■
Support was provided by the U.S. Department of Energy,
Office of Basic Energy Sciences, DE-FG02-88ER13880 (PRS),
and the National Science Foundation, CHE 0615743 (CDH).
We thank Dr. Charles Barns for X-ray data collection and
processing, Dr. Wei Wycoff for assistance with the NMR
measurements, Lasantha Wickramasinghe for the low temper-
ature bromination experiment and Professor Robert Kuntz for
helpful discussions. The computations were performed on the
HPC resources at the University of Missouri Bioinformatics
Consortium (UMBC).
(41) Znakovskaya, I. V.; Sosedova, Y. A.; Glebov, E. M.; Grivin, V. P.;
Plyusnin, V. F. Photochem. Photobiol. Sci. 2005, 4, 897.
(42) Glebov, E. M.; Plyusnin, V. F.; Grivin, V. P.; Venediktov, A. B.;
Korenev, S. V. Russ. Chem. Bull. 2007, 56, 2357.
(43) Pozdnyakov, I. P.; Glebov, E. M.; Plyusnin, V. F.; Tkachenko, N.
V.; Lemmetyinen, H. Chem. Phys. Lett. 2007, 442, 78.
(44) Glebov, E. M.; Grivin, V. P.; Plyusnin, V. F.; Venediktov, A. B.;
Korenev, S. V. J. Photochem. Photobiol., A 2010, 214, 181.
(45) Zheldakov, I. L.; N. Ryazantsev, M.; Tarnovsky, A. N. J. Phys.
Chem. Lett. 2011, 2, 1540.
(46) Ford, P. C.; Hintze, R. E.; Petersen, J. D. In Concepts of Inorganic
Photochemistry; Adamson, A. W., Fleischauer, P. D., Eds.; Wiley-
Interscience: New York, 1975, p 203.
(47) McMurry, J. Organic Chemistry; 3rd ed.; Brooks/Cole: Pacific
Grove, CA, 1992.
(48) McMillen, D. W.; Grutzner, J. B. J. Org. Chem. 1994, 59, 4516.
(49) Zheldakov, I. L. Ph.D., Bowling Green State University, 2010.
(50) Langford, C. H.; Malkhasian, A. Y. S. J. Am. Chem. Soc. 1987,
109, 2682.
(51) Brettle, R.; Sutton, J. R. J. Chem. Soc., Perkin Trans. 1 1975,
1947.
(52) Masui, M.; Hosomi, K.; Tsuchida, K.; Ozaki, S. Chem. Pharm.
Bull. 1985, 33, 4798.
(53) Gautam, P.; Harriman, A. J. Chem. Soc., Faraday Trans. 1994, 90,
697.
REFERENCES
■
(1) Lewis, N. S.; Nocera, D. G. Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
15729.
(2) Nocera, D. G. Inorg. Chem. 2009, 48, 10001.
(3) Eisenberg, R.; Nocera, D. G. Inorg. Chem. 2005, 44, 6799.
(4) Kanan, M. W.; Surendranath, Y.; Nocera, D. G. Chem. Soc. Rev.
2009, 38, 109.
(5) Kunkely, H.; Vogler, A. Angew. Chem., Int. Ed. 2009, 48, 1685.
(6) Likhtenshtein, G. Solar Energy Conversion: Chemical Aspects; John
Wiley & Sons: Hoboken, NJ, USA, 2012.
(7) Livshits, V.; Ulus, A.; Peled, E. Electrochem. Commun. 2006, 8,
1358.
(8) Eidem, P. K.; Maverick, A. W.; Gray, H. B. Inorg. Chim. Acta
1981, 50, 59.
(9) Gray, H. B.; Maverick, A. W. Science 1981, 214, 1201.
(10) Heyduk, A. F.; Nocera, D. G. Science 2001, 293, 1639.
(11) Esswein, A. J.; Veige, A. S.; Nocera, D. G. J. Am. Chem. Soc.
2005, 127, 16641.
(12) Kohl, S. W.; Weiner, L.; Schwartsburd, L.; Konstantinovski, L.;
Shimon, L. J. W.; Ben-David, Y.; Iron, M. A.; Milstein, D. Science 2009,
324, 74.
(13) Cook, T. R.; Surendranath, Y.; Nocera, D. G. J. Am. Chem. Soc.
2009, 131, 28.
(14) Teets, T. S.; Nocera, D. G. J. Am. Chem. Soc. 2009, 131, 7411.
(15) Teets, T. S.; Lutterman, D. A.; Nocera, D. G. Inorg. Chem. 2010,
49, 3035.
(54) Kee, H. L.; Kirmaier, C.; Yu, L.; Thamyongkit, P.; Youngblood,
W. J.; Calder, M. E.; Ramos, L.; Noll, B. C.; Bocian, D. F.; Scheidt, W.
R.; Birge, R. R.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2005, 109,
20433.
(55) 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.
(16) Cook, T. R.; McCarthy, B. D.; Lutterman, D. A.; Nocera, D. G.
Inorg. Chem. 2012, 51, 5152.
(17) Lexa, D.; Saveant, J. M.; Schaefer, H. J.; Su Khac, B.; Vering, B.;
Wang, D. L. J. Am. Chem. Soc. 1990, 112, 6162.
(18) SDBSWeb: http://riodb01.ibase.aist.go.jp/sdbs/ (National
Institute of Advanced Industrial Science and Technology, Nov 11,
2012).
(19) Murray, R. W.; Agarwal, S. K. J. Org. Chem. 1985, 50, 4698.
4119
dx.doi.org/10.1021/ic4004998 | Inorg. Chem. 2013, 52, 4113−4119