Photoreduction of Pt(IV) chloro complexes: Substrate chlorination by a triplet excited state

Article abstract of DOI:10.1021/ic5009413

The Pt(IV) complexes trans-Pt(PEt₃)₂(Cl) ₃(R) 2 (R = Cl, Ph, 9-phenanthryl, 2-trifluoromethylphenyl, 4-trifluoromethylphenyl, 3-perylenyl) were prepared by chlorination of the Pt(II) complexes trans-Pt(PEt₃)₂(R)(Cl) 1 with Cl ₂(g) or PhICl₂. Mixed bromo-chloro complexes trans,trans-Pt(PEt₃)₂(Cl)₂(Br)(R) (R = 9-phenanthryl, 4-trifluoromethylphenyl), trans,cis-Pt(PEt₃) ₂(Cl)₂(Br)(4-trifluoromethylphenyl), trans,trans- Pt(PEt₃)₂(Br)₂(Cl)(R) (R = 9-phenanthryl), and trans,cis-Pt(PEt₃)₂(Br)₂(Cl)(4- trifluoromethylphenyl) were obtained by halide exchange or by oxidative addition of Br₂ to 1 or Cl₂ to trans-Pt(PEt₃) ₂(R)(Br). Except for 2 (R = Ph, 4-trifluoromethylphenyl), all of the Pt(IV) complexes are photosensitive to UV light and undergo net halogen reductive elimination to give Pt(II) products, trans-Pt(PEt₃) ₂(R)(X) (X = Cl, Br). Chlorine trapping experiments with alkenes indicate a reductive-elimination mechanism that does not involve molecular chlorine and is sensitive to steric effects at the Pt center. DFT calculations suggest a radical pathway involving ³LMCT excited states. Emission from a triplet is observed in glassy 2-methyltetrahydrofuran at 77 K where photoreductive elimination is markedly slowed.

Full text of DOI:10.1021/ic5009413

Article
pubs.acs.org/IC
Photoreduction of Pt(IV) Chloro Complexes: Substrate Chlorination
by a Triplet Excited State
Tharushi A. Perera, Mehdi Masjedi, and Paul R. Sharp*
Department of Chemistry, University of Missouri, 125 Chemistry Building, Columbia, Missouri 65211-7600, United States
S
* Supporting Information
ABSTRACT: The Pt(IV) complexes trans-Pt(PEt3)2(Cl)3(R) 2 (R = Cl, Ph, 9-phenanthryl,
2-trifluoromethylphenyl, 4-trifluoromethylphenyl, 3-perylenyl) were prepared by chlorination
of the Pt(II) complexes trans-Pt(PEt3)2(R)(Cl) 1 with Cl2(g) or PhICl2. Mixed bromo−chloro
complexes trans,trans-Pt(PEt3)2(Cl)2(Br)(R) (R = 9-phenanthryl, 4-trifluoromethylphenyl),
trans,cis-Pt(PEt3)2(Cl)2(Br)(4-trifluoromethylphenyl), trans,trans-Pt(PEt3)2(Br)2(Cl)(R)
(R = 9-phenanthryl), and trans,cis-Pt(PEt3)2(Br)2(Cl)(4-trifluoromethylphenyl) were obtained by halide exchange or by oxidative
addition of Br2 to 1 or Cl2 to trans-Pt(PEt3)2(R)(Br). Except for 2 (R = Ph, 4-trifluoromethylphenyl), all of the Pt(IV) complexes
are photosensitive to UV light and undergo net halogen reductive elimination to give Pt(II) products, trans-Pt(PEt3)2(R)(X)
(X = Cl, Br). Chlorine trapping experiments with alkenes indicate a reductive-elimination mechanism that does not involve
molecular chlorine and is sensitive to steric effects at the Pt center. DFT calculations suggest a radical pathway involving 3LMCT
excited states. Emission from a triplet is observed in glassy 2-methyltetrahydrofuran at 77 K where photoreductive elimination is
markedly slowed.
INTRODUCTION
RESULTS
■
■
There is growing interest in the photochemistry of transition
metal complexes in relation to solar energy conversion and
storage.1−9 Several schemes to achieve conversion and storage
center on splitting stable small molecules HX (e.g., H2O,
hydrogen halides) into component oxidant (X2) and reductant
(H2).3,6,8,10−14 A key step in these schemes is the photo-
induced, expectedly endergonic reductive elimination of the
oxidant (X2) from the metal center(s) (eq 1).
Complex Synthesis and Characterization. Platinum(II)
complexes, trans-Pt(PEt3)2(R)Cl 1 (Scheme 1), needed for the
Scheme 1
A number of apparent photoreductive eliminations of this
type have been reported.10,11,15−24 In the case of X = a halogen
(Cl, Br), the reactions are generally studied using a halogen trap
(alkenes or halogen-reactive solvents) to prevent the rapid back
reaction, the recombination of X2 with the reduced metal
center(s). This presents a problem in that the trap renders the
net reaction exergonic and opens the possibility that the trap
reacts not with photoeliminated X2 but with an excited state or
reactive intermediate generated in the photolysis of the metal
complex. Strong support has been found for molecular bromine
(X = Br) photoelimination, although the reactions are not
strongly endergonic and may have a thermal component.19,21,23
Evidence for more strongly endergonic molecular Cl2 photo-
elimination is much weaker with Cl2 only detected by mass
spectral analysis and probably in very low quantities.20,21
Herein, we describe our studies on net Cl2 photoelimination in
the presence of halogen traps and present evidence that the
photoreactions occur not by Cl2 photoelimination but probably
by reaction of the trap with an excited state of the metal
complex that acts as a chlorine atom donor.
synthesis of the targeted Pt(IV) complexes trans-Pt(PEt3)2(R)-
(Cl)3 2, were prepared by three methods. The well-established
oxidative addition of organochlorides (RCl) to Pt(PEt3)4
gave the R = Ph, 2-tft, 4-tft, and phen derivatives. As 3-
chloroperylene is not readily available, trans-Pt(PEt3)2(peryl)Cl
1 (R = peryl) was instead prepared by treating trans-
Pt(PEt3)2(peryl)Br25 with silver triflate followed by KCl. Lastly,
trans-Pt(PEt3)2(Cl)2 1 (R = Cl) was obtained by the thermal
conversion of solid cis-Pt(PEt3)2(Cl)2.26
Chlorination of 1 with Cl2(g) or PhICl2 yields the Pt(IV)
complexes trans-Pt(PEt3)2(R)(Cl)3 2 (R = Cl, Ph, 2-tft, 4-tft,
phen, peryl) (Scheme 1). For R = peryl, chlorination with Cl2 is
unselective and gives a mixture of closely related products that
Received: April 22, 2014
© XXXX American Chemical Society
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could not be separated. These products appear to be a mixture
of 2 and analogues of 2 where the perylene ring has been
chlorinated, presumably in different locations and/or extent. In
contrast, PhICl2 selectively gives only chlorination at the Pt(II)
center. However, the presence of PhI in the reaction mixture
complicates isolation of 2 (R = peryl). Solvent removal triggers
partial reduction of 2 to 1 (R = peryl), and white solid phenol is
detected. Evidently, as concentrations increase adventitious
water combines with the PhI to produce HI and phenol and the
HI then reduces 2 to 1 (R = peryl). Complex 2 (R = peryl) is
successfully isolated by precipitation from the reaction mixture
with hexanes.
silver triflate followed by KCl yields trans,trans-Pt(PEt3)2(R)-
(Br)2(Cl) 5 (R = phen) (eq 3). The selective replacement of
the halide ligand trans to the aromatic group is supported by
the NMR data for 3 and 5. Crystal structures indicate that the
phenanthryl ring in this class of Pt(IV) complexes is in close
contact with the cis-halo and phosphine ligands (see above and
31P NMR spectra of 2 show singlets with satellites in the δ
2−7 region, shifts that are about 10 units negative of those
for the corresponding Pt(II) precursors 1 (δ 11−15). A similar
negative shift is observed in the analogous bromo system.19
The magnitudes of the Pt−P coupling constants are in the
region expected for Pt(IV) complexes with a trans disposition
of the PEt3 ligands and are also similar to those observed in the
analogous bromo system.19 1H NMR spectra show the PEt3
signals in the aliphatic region and signals for the aromatic
ligands (R ≠ Cl). Complexes 2 (R = phen, peryl) yielded
crystals suitable for single crystal X-ray analysis. A drawing of
the phen complex is provided in Figure 1, and one for the peryl
1
refs18 and 19), and therefore, the phen ring H NMR shifts
might be expected to be influenced by the identity of the cis-
1
halo ligands. This is indeed the case. The H NMR signal for
H10 (CDCl3), the vicinal-to-Pt hydrogen atom (Scheme 1 and
Figure 1), appears to be most sensitive and is found at δ 8.28
for chloro complex 2 (R = phen) while that for bromo complex
4 (R = phen) appears at δ 8.59. Replacement of the trans-
chloro ligand in 2 (R = phen) with a bromo ligand would not
be expected to significantly change the shift, and this is
observed; the H10 signal for 3 (R = phen) is at δ 8.26, almost
exactly that of parent 2. Likewise, replacement of the trans-
bromo ligand in 4 (R = phen) with a chloro ligand yields an
H10 shift of δ 8.54 for 5 (R = phen), essentially that of parent
4. Replacement of the halo ligand trans to the carbyl ligand is
expected from the strong trans influence of the carbyl ligand.
In the case of 3 (R = 4-tft), we have previously observed
that 4-tft-ring rotation is restricted on the NMR time scale for
this class of compounds.18 Thus, if the two halide ligands cis to
1
the 4-tft ligand are different, then two H NMR signals are
observed for the ortho-ring protons. Only one signal is observed
for 3 (R = 4-tft), and the shift is similar to that of precursor 2.
Further evidence for the correct identification of 3 (R = 4-tft) is
obtained from the attempted synthesis of 3 (R = 4-tft) and 5
(R = 4-tft) by halogenation of 1 (R = 4-tft) and trans-
Pt(PEt3)2(4-tft)Br 6 (R = 4-tft) (eq 4 and eq 5). In these
Figure 1. Solid-state structure of one of two independent molecules of
2 (R = phen, 50% probability ellipsoids, hydrogen atoms omitted).
The second molecule shows a rotational disorder in the phenanthryl
ring system (Supporting Information).
complex may be found in the Supporting Information. Both
structures are typical of this class of complex19 and include
distortions caused by steric interactions of the peri-to-Pt poly-
cyclic ring hydrogen atom (H8 on C8 in Figure 1) with the cis
ligands (Cl3 and the P1-triethylphosphine ligand in Figure 1).
Metrical parameters and other details of the structures may be
found in the Supporting Information.
Mixed bromo−chloro versions of 2 are accessible by halide
exchange. Thus, trans,trans-Pt(PEt3)2(R)(Cl)2(Br) 3 (R =
phen, 4-tft) are obtained by treating trans-Pt(PEt3)2(R)(Cl)3
with silver triflate followed by addition of tetrabutylammonium
bromide (eq 2), and treating trans-Pt(PEt3)2(R)(Br)3 4 with
reactions, cis-oxidative addition is observed, and the products,
trans,cis-Pt(PEt3)2(4-tft)(Br)2(Cl) 7 and trans,cis-Pt(PEt3)2(4-
tft)(Cl)2(Br) 8 show two different ortho-ring proton signals
(Ha, Hb), consistent only with slow tft-ring rotation and two
different halogen ligands cis to the 4-tft ligand. 31P NMR signals
for 3 (R = 4-tft), 7, and 8 are all singlets with satellites at dif-
ferent chemical shifts indicating that each is isomerically unique
and not a mixture with accidentally coincident shifts.
The UV−vis absorption spectrum for 2 (R = peryl) is given
in Figure 2, and those for the other derivatives are provided in
the Supporting Information (Figures S88−S92). The spectrum
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Scheme 2
With R = phen or peryl, irradiation of 2 at 313 nm in the
presence or absence of an alkene also results in photoreduc-
tion to the corresponding 1. However, also formed is 9, an
analogue of 1 where the R group has been chlorinated (eq 6).
Figure 2. UV−vis absorption spectrum of trans-Pt(PEt3)2(peryl)(Cl)3
2 (R = peryl) in CH2Cl2.
for deep orange 2 (R = peryl) is dominated in the visible region
by the peryl ligand π-to-π* transition which shows character-
istic vibronic coupling.25 The other complexes 2 are pale yellow
and absorb only weakly in the blue region. Stronger absorptions
are found in the UV region, and for R = phen, the phenanthryl
ligand π-to-π* transition appears at about 300 nm with vibronic
coupling (1340 cm−1).27,28 TDDFT calculations for the
complexes are described below.
Photoproducts. Like their bromo analogues,19 the chloro
complexes 2 and the chloro−bromo complexes 3, 5, 7, and 8
undergo photoreduction. The P-containing photoproducts depend
on the complex, solvent, and the presence or absence of an
added alkene (Table 1). For 2 (R = Cl), irradiation at 313 nm in
the presence of excess (0.2 M) alkene yields exclusively 1 (R =
Cl) and its cis isomer along with chlorinated alkene products
(see below) (Scheme 2). If the alkene is omitted or present at
low concentrations, 1 (R = Cl) is again formed but in lower
yields and is accompanied by Cl2PEt3, [Pt(PEt3)(μ-Cl)Cl]2, and
unidentified P-containing products. Similar behavior is observed
for R = 2-tft, but lower yields of 1 and greater amounts of
Cl2PEt3 and unidentified P-containing products are obtained.
The ring-chlorination sites in 9 are shown in eq 6 and were
determined by comparison of the NMR data with those for the
previously reported bromo complexes.19 A single crystal X-ray
structure determination of 9 (RCl = Cl-phen) confirms the regio-
chemistry of the ring chlorination (Figure 3). Similar photoring
bromination was reported for the bromo analogues of 2.19
While 1 and 9 are produced together in the photolysis of 2
(R = phen, peryl) their ratio (1:9) is influenced by the presence
of an alkene and the alkene reactivity. In the absence of an
alkene, the ratio is 1:1.3 in C6D6 for R = phen. Adding trans-2-
hexene suppresses ring chlorination and approximately inverts
Table 1. P-Containing Products from the Photolysis of 2 (∼0.01 M) at 313 or 380 nm
a
P-containing products
R (wavelength)
Cl (313 nm)
solvent
added trap
1
9
Et3PCl2
other
b
b
f
C6D6
none
70%
88%
11%
8%
7% [PtLCl2]2, 12% unknown
4% [PtLCl2]2
CDCl3
C6D6
0.06 M TME
b
b
0.2 M cis- or trans-2-hexene
0.2 M TME
100%
100%
42%
63%
32%
36%
62%
50%
89%
CDCl3
C6D6
f
2-tft (313 nm)
phen (313 nm)
0.5 M 1-hexene
0.4 M TME
20%
39%
38% unknown
f
C6D6
37% unknown
f
CD2Cl2
C6D6
0.3 M trans-3-hexene/0.3 M 1-hexene
none
29% unknown
f
48%
38%
45%
11%
61%
42%
12%
6%
16% unknown
C6D6
0.2 M trans-2-hexene
0.5 M 1-hexene
0.4 M TME
f
CDCl3
CDCl3
CDCl3
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
5% unknown
c
phen (380 nm)
0.5 M 1-hexene
0.08 M 1-hexene
0.08 M TME
58%
88%
94%
70%
0.08 M TME/0.008 M pyridine
0.008 M pyridine
none
f
16%
50%
31%
14% unknown
d
d
d
peryl (380 nm)
50%
d
,
e
,e
0.2 M TME
61%
a
b
c
d
e
Percent of total 31P NMR integration. cis and trans mixture. 1H NMR yield against an internal TMS standard. Ratio by 1H NMR. Isolated yield
f
as mixture of 1 and 9. See experimental details in the Supporting Information for additional details.
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With TME and R = Cl, high combined yields (90−100%) of
the two chlorination products 1-chloro-2,3-dimethyl-2-butene 10
(29−30%) and 3-chloro-2,3-dimethyl-1-butene 11 (60−70%)
are obtained. Both of these chlorination products must be
accompanied by HCl formation, and the TME−HCl addition
product, 2-chloro-2,3-dimethylbutane 12, is therefore also present.
TME chlorination products are the same for R = phen, but the
combined yield decreases to 45%. As with hexene, part of the
decrease can be attributed to chlorine loss (11%) to ring
chlorination.
The reaction of Cl2 with excess TME and cis- and trans-2-
hexene was examined for comparison with the photoreactions.
For TME the exclusive products are 3-chloro-2,3-dimethyl-1-
butene and 2-chloro-2,3-dimethylbutane, as previously reported.29
For cis- and trans-2-hexene, the anti-addition products are the
only detected products.
To further investigate the nature of the photoreduction of 2,
a competition experiment30 was conducted in which 2 (R = Cl,
2-tft) were photolyzed at 313 nm with an excess 1:1 mixture
of 1-hexene and trans-3-hexene in CD2Cl2 (Scheme 3).
Figure 3. Solid-state structure of 9 (RCl = Cl-phen) (30% probability
ellipsoids, hydrogen atoms omitted). A second orientation of one ethyl
group on each phosphine ligand is not shown.
the ratio to 1.6:1. In CDCl3 with 1-hexene the ratio is about
1:1, but with more electron-rich alkene TME there is only a
small amount of ring chlorination, and the ratio is 8.1:1.
The photolysis of 2 with alkenes produces chlorinated
alkenes (Figure 4). For R = Cl with excess (0.2 M) cis-2-hexene
Scheme 3
The resulting product ratio of 3,4-trans-dichlorohexane and
1,2-dichlorohexane was 3:1 for R = Cl and 1:3 for R = 2-tft.
A 12:1 ratio was obtained with Cl2 as the chlorinating agent.
(A solution of Cl2 was slowly added to a rapidly stirred solution
of the alkene mixture in order to approximate the conditions
that would be obtained if molecular Cl2 is photoeliminated
from the complexes.)
Figure 4. TME and cis- and trans-2-hexene products from the
photolysis of 2. Only one enantiomer shown for chiral 2-hexene
products.
Photokinetics. The photolyses of 2 (R = Cl, phen, 2-tft)
and TME at 380 nm in CDCl3 were monitored by 31P NMR
spectroscopy. This wavelength was chosen in an attempt to
avoid absorbance by photoproduct 1 and ring-chlorination
product 9 (RCl = Cl-phen). In addition, absorbance at this
wavelength by 2 is low such that the reaction is not limited by
the photon flux but only by the concentration of 2. Under these
conditions the rate of the photoreaction should be first order in
2. As expected, plots of the concentration of 2 against time for
R = Cl and 2-tft are first order, and log plots give straight lines
(Supporting Information Figures S93 and S94). In contrast,
the plot for R = phen does not show first-order behavior
(Figure 5). Instead, the photoreaction shows a low initial rate
(slope = −0.092) that increases as the reaction progresses
reaching a final rate that is more than 4 times higher (slope =
−0.39) than the initial rate.
in C6D6, about a 1:1 mixture of the anti-chlorine-addition
product (a racemic mixture of (2R,3R)- and (2S,3S)-2,3-
dichlorohexane) and the syn-addition product (a racemic
mixture of (2R,3S)- and (2S, 3R)-2,3-dichlorohexane) were
produced in a combined yield of 51% (based on 1). A small
amount (∼1%) of the remaining cis-2-hexene was isomerized to
trans-2-hexene during the photolysis and cannot account for the
mixture of syn- and anti-addition products. With trans-2-hexene
a similar mixture is obtained (53% yield) with perhaps a slight
favoring of the anti-addition product. The fate of the missing
chlorine in these reactions is unknown but presumably is
present in solvent (or solvent impurity) chlorination products.
For 2 R = phen and trans-2-hexene in C6D6, about a 1:1 mixture
of the anti- and syn-addition products is again observed. In this
case, the alkene chlorination yield is only 26% (based on 1).
However, the maximum possible yield is 62%, since 38% of the
Cl2 from 2 has gone to phenanthryl ring chlorination in the
formation of 9.
Such a rate increase is suggestive of a product-catalyzed
reaction (autocatalysis).31,32 The photoreaction was therefore
repeated with 1 molar equiv of photoproduct 1 (R = phen)
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Solid samples of 2 (R = Cl, phen) were photolyzed at 313 nm in
a vacuum with LN2 trapping of any photoeliminated Cl2 into a
1-hexene or TME solution. Although photoreduction of 2 was
observed, no alkene−chlorination products were detected
leading us to conclude that no significant amounts of Cl2 are
evolved in the photolysis of solid 2. No effort was made to
detect Cl2 by mass spectrometry.
Elimination Stereoselectivity. To explore the halogen
photoelimination stereoselectivity (cis or trans), trans,trans-
Pt(PEt3)2(phen)(Cl)2(Br) 3 (R = phen) and trans,trans-
Pt(PEt3)2(phen)(Br)2(Cl) 5 (R = phen) were separately
photolyzed with excess TME at 313 nm in CDCl3. Under
these conditions (Table 1) the photoreduction of parent 2
(R = phen) is clean with the only P-containing product being 1
(R = phen). This proved also true for 3 and 5 (R = phen), and
the only detected P-containing products are Pt(PEt3)2(Cl)-
(phen) 1 (R = phen) and Pt(PEt3)2(Br)(phen) 6 (R = phen).
For 5 (R = phen), bromo complex 6 (R = phen) is the
exclusive Pt-containing product suggesting a highly selective
cis-elimination of BrCl (eq 7). Complex 3 (R = phen) gives a
Figure 5. Photolysis of 2 (R = phen) (9 mM) with TME (90 mM) in
dichloromethane at 380 nm: black circles for 2, blue squares for 1
(R = phen), green triangles for ring-chlorinated 9 (RCl = Cl-phen).
The solid lines are polynomial fits to the data and are intended only to
aid the eye. Red dashed lines are linear fits (with equations) at the
beginning and end of the reaction.
added at the beginning of the reaction. A linear plot of the
concentration of 2 (R = phen) against time is then observed
(Figure 6) with a reaction rate that is near that of the final rate
mixture of 1 (R = phen) and 6 (R = phen), with 1 (R = phen)
formed in 86% yield and 6 (R = phen) in 14% yield (eq 8).
Figure 6. Photolysis of 2 (R = phen) (9 mM) with TME (90 mM)
and 1 (R = phen) (9 mM) in dichloromethane at 380 nm: black circles
for 2 (R = phen), blue squares for 1 (R = phen), green triangles for
ring-chlorinated 9 (RCl = Cl-phen). The lines are linear fits to the data
with fixed intercepts of 0 or 100%.
This would suggest lower cis-elimination selectivity; however, 6
(R = phen) is likely formed from the reaction of 1 (R = phen)
with HBr from TME oxidation. As with 2, some of the TME
products (Table 2) require the formation of HBr and HCl,
and in separate experiments we find that 1 (R = phen) reacts
rapidly with HBr,33 even in the presence of TME, to produce 6
(R = phen). In contrast, 6 (R = phen) does not react at all with
HCl. Thus, 1 (R = phen) may be the exclusive initial product
of cis-photoelimination, but some (14%) reacts with HBr to
produce 6 (R = phen). TME products from the photolysis of 3
(R = phen) are listed in Table 2 and are formed in similar yields
in the photolysis of 5 (R = phen).
We also hoped to examine the photoelimination stereo-
chemistry in the mixed bromo−chloro complexes 3 (R = 4-tft)
and 7, but the photoeliminations are complicated by photo-
isomerization and halide exchange. Consistent with a preference
for cis elimination, photolysis of 3 (R = 4-tft) with 1-hexene
gives 70% 1 (R = 4-tft) and 30% 6 (R = 4-tft). However,
monitoring the reaction reveals that trans-dichloro 3 (R = 4-tft)
is partially photoisomerized to cis-dichloro 8. Photolysis of pure
8 gives essentially the same final 1 (R = 4-tft)-to-6 (R = 4-tft)
ratio as 3 (R = 4-tft), but again, reaction monitoring indicates
some isomerization to 3 (R = 4-tft) and also formation of
trichloro 2 (R = 4-tft). As a result, it is not possible to establish
the photoelimination stereochemistry for the R = 4-tft system.
Quantum Yields. Quantum yields for the photoreduc-
tions of 2 are listed in Table 3. The highest yield (58%) is for
of the reaction without added 1 (R = phen). These results
indicate that photoproduct 1 is a catalyst for the photoreaction.
Also of note is that the addition of 1 at the beginning of the
reaction does not alter the final Pt-containing product
distribution. In both reactions the yield of the ring chlorination
product 9 (RCl = Cl-phen) is about 10%. Thus, whatever
the role of 1 in accelerating the reaction, it does not affect the
ring chlorination amount. Similar results are obtained using
1-hexene as the chlorine trap (Supporting Information Figures
S95 and S96) but with a ∼40% yield of 9 (RCl = Cl-phen).
A close inspection of the UV−vis absorption data for 1 (R =
phen) reveals that the extinction coefficient near 380 nm is not
zero but approximately half that of 2 (R = phen). We therefore
conclude that 1 (R = phen) is an efficient sensitizer for excita-
tion of 2 (R = phen) into the photochemically active excited
state (most likely the lowest-energy triplet, see below).
Solid-State Photolyses. Previously it was found that solid-
state samples of Au and Pt bromo complexes photoeliminate
chemically detectable amounts of Br2 into the gas phase.19,21 In
contrast, photolysis of solid-state samples of chloro complexes
did not release chemically detectable amounts of Cl2, although an
undetermined quantity was detected by mass spectrometry.20,21
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a
nearly the same in the presence of TME and 1-hexene. Lower
yields are observed for dichloromethane and are essentially
independent of TME concentration over the range examined.
The yields drop considerably for R = phen and peryl, and the
peryl complex shows an excitation wavelength dependence.
Despite strong absorption in the blue region (Figure 2) no photo-
reduction is observed at 400 nm. Decreasing the photolysis
wavelength to 380 nm gives slow conversion with a quantum
yield of 2.6%. The photoreduction is accompanied by strong
visible peryl ligand emission.25 (None of the other complexes 2
display visible room-temperature photoemission in solution.)
Decreasing the photolysis wavelength further to 313 nm
decreases the emission and increases the reaction quantum
yield to 9.4%. Emission spectroscopy (Figure 7) confirms the
Table 2. Photolysis Product Yields for trans,trans-
Pt(PEt3)2(phen)(Cl)2(Br) 3 with 2,3-Dimethyl-2-butene
(TME)
Figure 7. Photoemission spectrum of trans-Pt(PEt3)2(peryl)(Cl)3 2
(R = peryl) in CH2Cl2. Solid red line for 380 nm excitation and dashed
black line for 313 nm excitation.
emission decrease and indicates a 5-fold reduction on going
from 380 to 313 nm, nearly matching the 4-fold reaction
quantum yield increase.
Low-Temperature Photoemission. As mentioned above,
only the peryl complex shows photoemission in solution at
room temperature. However, the other complexes 2 (R = Cl,
2-tft, 4-tft, phen) do show photoemission in a 2-methylte-
trahydrofuran glass at 77 K (Table 4). The emission colors
a
1H NMR yields relative to 1 (R = phen) + 6 (R = phen).
Table 4. Emission Data for 2 in 2-Methyltetrahydrofuran
Glass at 77 K (380 nm excitation)
Table 3. Quantum Yields for Photoreduction of 2 at 313 nm
a
R
solvent
trap
TME
[trap] (M) [Pt] (mM)
QY (%)
a
R
λmax (nm)
fwhm (cm−1
)
lifetime (μs)
b
2-tft
Cl
DCM
0.2
0.58
0.13
0.14
0.13
0.13
0.14
0.25
0.16
0.12
0.12
58
2
1
3
1
1
2
c
c
c
c
c
Cl
590
585
605
580
2150
2660
3060
84
benzene
benzene
DCM
DCM
DCM
DCM
DCM
DCM
DCM
1-hexene
TME
0.05
0.017
0.046
0.15
0.62
0.15
0.13
0.13
0.13
24
25
19
21
20
2-tft
4-tft
phen
66
Cl
64
Cl
TME
b
∼3000
109
Cl
TME
a
b
Errors are estimated to be 1 μs or less. Probable phen ligand emis-
Cl
TME
sion observed at ∼480 nm.
phen
peryl
peryl
peryl
TME
1.0 0.2
9.4 0.2
2.6 0.9
1-hexene
1-hexene
1-hexene
d
range from yellow to orange with the brightest emission seen
for R = Cl. The emission spectra are given in Figure 8 and show
broad (fwhm =2150 to 3060 cm−1), unstructured emission
bands at 580−590 nm. (The differing emission colors appear
to be due more to the degree of tail off into the red than to
differing maxima.) The same emission bands are observed
with 320, 380, or 390 nm excitation and show exponential
decay with mean lifetimes of 66−109 μs. An emission band is
also observed at ∼480 nm for R = phen and is assigned to
phenanthrenyl ligand phosphorescence.34 Along with emission
turn-on in the frozen matrix there is a strong decrease in the
photoreduction of 2 with conversion rates that are reduced by
e
0
a
b
Average of three runs at three monitoring wavelengths. DCM =
c
dichloromethane. Single monitoring wavelength (see experimental
details in the Supporting Information). At 380 nm. At 400 nm.
d
e
2 (R = 2-tft) and is the highest yet reported for net chlorine
photoelimination from a transition metal chloro complex.
(The highest previous yield of 38% is claimed by the dinuclear
complex Pt2(tfepma)2Cl6, tfepma = [P(CF3CH2O)2]2NMe.20)
Yields are somewhat lower for R = Cl and are solvent
dependent. The best yields are obtained in benzene and are
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interaction of the peri-to-Pt, phenanthryl-hydrogen atom (H8),
and the 2-CF3 group in 2′ (see above and ref 19) which is at
least partly relieved in the square planar products 1′. Also of
note is the more favorable entropy term (TΔS) for R = phen
and 2-tft, which are 2−3 kcal/mol more favorable than for
R = Cl and 4-tft. The more favorable entropy term for R = phen
and 2-tft is attributed to an increase in rotational motion of
the R group on going from very crowded 6-coordinate Pt(IV) to
4-coordinate Pt(II).
We focus first on 2 (R = Cl) as this is the simplest photo-
active system and does not have the complication of an
aromatic group. Calculated vertical electronic transitions
(TDDFT, CAM-B3LYP) for 2′ (R = Cl) in dichloromethane
(pcm) using the gas-phase optimized (M06) structure (see
below) are shown in Table 6. The calculated transition energies
and oscillator strengths are compared to the experimental
UV−vis absorption spectrum of 2 (R = Cl) in Figure 9.
Figure 8. Photoemission spectrum of trans-Pt(PEt3)2R(Cl)3
2
(11−17 mM) in 2-methyltetrahydrofuran glass at 77 K with 390 or
380 nm excitation. Intensity traces are scaled to minimize overlap and
do not represent relative intensities.
at least a factor of 3 from the room-temperature rates. Of note
is that the amount of ring chlorination for R = phen is reduced
by at least 50% from that in 2-methyltetrahydrofuran at room
temperature suggesting that constraints imposed by the rigid
solvent matrix inhibit ring chlorination.
DFT Modeling. Model complexes where the PEt3 ligands
are replaced with PMe3 ligands were used in all calculations and
are given the same number as the experimental complexes (where
applicable) but with a prime added. Thus, model complexes for 2
are designated as 2′ and have the formula trans-Pt(PMe3)2(Cl)3R.
Calculated (M06) gas-phase molecular chlorine reductive-
elimination enthalpies and free energies for 2′ (R = Cl, phen,
2-tft, 4-tft) are shown in Table 5. As expected, the eliminations
Table 5. Calculated (M06, Gas Phase, 298 K, 1 atm) ΔH,
ΔG, and TΔS Values (kcal/mol) for Chlorine Reductive
Elimination from trans,trans-Pt(PMe3)2Cl2(R) 2′ (R = Cl,
phen, 2-tft, 4-tft)
Figure 9. UV−vis absorption spectrum of trans-Pt(PEt3)2(Cl)4
2
(R = Cl) in dichloromethane (black line) and TDDFT (CAM-B3LYP,
pcm) vertical transitions for trans-Pt(PMe3)2(Cl)4 2′ (R = Cl) (red
vertical lines, height corresponds to the oscillator strength) in
dichloromethane. Yellow circles mark calculated triplet transitions,
and green squares mark calculated dark (oscillator strength <0.01)
singlet transitions at >350 nm.
a
a
R
ΔH
TΔS
ΔG
Cl
41.1
41.1
33.2
50.0
10.9
14.0
13.2
10.6
30.2
27.1
20.1
39.4
phen
2-tft
4-tft
a
ΔG and TΔS values are expected to decrease by about 3 kcal/mol
The match is quite good with excellent correspondence of
maxima and minima and relative intensities. The five lowest-
energy singlet transitions (Table 6 and Figure 9) are dark
(oscillator strength ∼0) but should be accessible through
internal conversion from bright, higher-energy, singlet states.
Matching triplet transitions for the dark singlet transitions are
found at somewhat lower energy with a slightly different
ordering where the first and second transitions are switched for
the triplets (Table 6 and Figure 9). The destination orbital for
all five transitions is the LUMO (Figure 10), which is Pt−Cl
when corrected for the smaller entropy increase in solution as
compared to the gas phase.19,35,36
are endergonic (22−39 kcal/mol), and the free energy values
are 2−4 times greater than for related bromo complexes.19
Despite the expectedly greater electron density on the Pt centers
in the carbyl complexes, only for R = 4-tft is the reductive
elimination enthalpy greater than that for the expectedly less-
electron-rich chloro complex (R = Cl). The low values for the
R = phen, 2-tft complexes can be attributed to the strong steric
Table 6. Five Lowest TDDFT (CAM-B3LYP, pcm) Vertical Transitions for trans-Pt(PMe3)2(Cl)4 2′ (R = Cl) in
a
Dichloromethane
singlets
triplets
cm−1
nm
contribution (>5%)
cm−1
nm
contribution (>5%)
S−T gap cm−1 (eV)
20 393
21 770
24 009
24 156
25 593
490
459
416
413
390
HOMO → LUMO (99%)
H-4 → LUMO (93%)
H-2 → LUMO (88%)
H-3 → LUMO (90%)
H-1 → LUMO (95%)
19 523
18 578
20 054
20 185
22 605
512
538
498
495
442
HOMO → LUMO (97%)
H-4 → LUMO (91%)
H-2 → LUMO (83%)
H-3 → LUMO (85%)
H-1 → LUMO (89%)
870 (0.11)
3192 (0.40)
3955 (0.49)
3971 (0.49)
2988 (0.37)
a
All singlets are “dark” (oscillator strength <0.001).
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Figure 10. Selected orbitals (isovalue = 0.04 e/Å3, CAM-B3LYP) for
trans-Pt(PMe3)2(Cl)4 2′ (R = Cl) in dichloromethane (pcm). Hydrogen
atoms omitted. Atom colors: blue = Pt, orange = P, green = Cl, gray = C.
Numbers in parentheses are orbital energies in Hartrees.
antibonding and, in pseudo-octahedral symmetry, can be
Figure 11. Singlet 2′, triplet 2′T1 (R = Cl) and 2′T2 (R = Cl), and
doublet 16′ (R = Cl) structures from trans-Pt(PMe3)2(Cl)4 2′ (R =
Cl). Hydrogen atoms omitted. Distances in angstroms, gas-phase
energies above singlet 2′ (R = Cl) in parentheses, and that for 16′
(R = Cl) includes a Cl radical.
2
2
identified as a dx −y , eg*-like orbital. (The other member of
2
the eg*-like set, the dz orbital, is the LUMO + 1 and is oriented
along the P−Pt−P axis at higher energy due to the stronger
phosphine ligand field.) The departure orbitals are predom-
inantly (88% or greater) the HOMO through HOMO − 4.
The HOMO is mostly phosphine ligand based with some con-
tribution from Cl lone-pair p-orbitals, while the HOMO − 1 is
purely Cl lone-pair. HOMO − 2, − 3, and − 4 are also Cl lone-
pair based but with some Pt d-orbital contribution (dxy-, dxz-,
dyz-like orbitals). Thus, the five lowest-energy transitions
(triplets and singlets) involve electron density transfer from
ligand-based (mostly Cl) orbitals to a Pt−Cl antibonding
orbital. The small energy gap between the singlet and matching
triplets (especially the HOMO−LUMO transitions) suggests
facile intersystem crossing and the likely involvement of a
triplet excited state in the photochemistry of these systems.
Singlet-to-triplet intersystem crossing occurs in less than 150 fs
in energy than triplet 2′T1 (R = Cl). There is again Pt−Cl bond
elongation from singlet 2′, but all Pt−Cl distances are the same
and at 2.50 Å are close to the average of those (2.52 Å) in 2′T1
(R = Cl). At first glance, the symmetry of both 2′T2 (R = Cl)
and singlet 2′ is D4h. Closer inspection reveals a tilt of the
P1−Pt−P2 axis (both P1−Pt−P2 angles are 180°) in both
structures such that this axis is not perpendicular to the plane
containing Pt, Cl1, Cl2, Cl3, and Cl4. The tilt is more severe in
triplet 2′T2 (R = Cl) but can be attributed in both complexes to
steric interactions of the PMe3-methyl groups with the Cl atoms.
In the direction of the tilt, a methyl group on each phosphine
ligand nestles between two cis-Cl atoms (C1 and Cl2 for P1, Cl3
and Cl4 for P2), relieving steric interactions between the other
methyl groups and Cl atoms which are directly under them.
The greater tilt in 2′T2 (R = Cl) is apparently allowed by the
longer Pt−Cl distances, which open the gaps between the Cl
atoms for deeper nestling of the methyl groups. In both
complexes the average C···Cl distance for the methyl groups in
contact with the Cl atoms is identical.
Mulliken atomic spin densities (see ref 38 for a review of
spin-density distribution in transition metal complexes) for
the two triplets are given in Table 7. Of note is the larger spin
density on the Cl atoms in 2′T1 (R = Cl). This is especially
so for Cl1, the Cl atom with the very long distance to Pt,
and suggests strong Cl radical character and the possibility that
this triplet is a good Cl atom donor. The distribution is also
consistent with a triplet that might be derived from Cl-to-Pt
charge-transfer originating from HOMO − 1 (Figure 10).
37
2−
in PtBr6
.
Gas-phase relaxed triplets derived from 2′ were calculated
(M06). For R = Cl, two triplet minima were located, and the
structures are shown in Figure 11. Triplet 2′T1 (R = Cl) is
lowest and lies 39.9 kcal/mol (40.8 kcal/mol in dichloro-
methane) above singlet 2′. An increase in one Pt−Cl distance
(Pt−Cl1) from 2.40 Å in 2′ (R = Cl) to 2.69 Å in 2′T1 (R = Cl)
is evident. Along with the increased Pt−Cl distance is a bending
of the cis-Cl atoms, Cl2 and Cl4, toward the elongated position
giving a Cl2−Pt−Cl4 angle of 154°. The other Pt−Cl distances
have also lengthened but not to the degree observed for
Pt−Cl1. In contrast, the Pt−P bonds have slightly shortened
from 2.42 Å in 2′ (R = Cl) to 2.40 Å in 2′T1 (R = Cl), and the
phosphine ligands have bent slightly away from Cl1 toward Cl3,
giving a P1−Pt−P2 angle of 175°.
Triplet 2′T2 (R = Cl) is 42.2 kcal/mol (42.8 kcal/mol in
dichloromethane) above 2′ placing it about 2 kcal/mol higher
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Table 7. Triplet and Doublet Mulliken Atomic Spin
a
Densities (Gas Phase) for R = Cl
atom
Pt
2′T1 (R = Cl)
2′T2 (R = Cl)
16′ (R = Cl)
0.622
0.484
0.345
0.209
0.345
1.383
0.865
0.289
0.287
0.289
0.287
1.152
0.550
Cl1
Cl2
0.063
0.320
0.063
0.446
Cl3
Cl4
Cl total
a
Spin densities greater than 0.01.
The much greater spin density on Pt in 2′T2 (R = Cl) suggests
that this triplet may instead originate from HOMO − 2,
HOMO − 3, or HOMO − 4 or a combination of these. The
low spin density on the P atoms (<0.01) in both of these
triplets is not consistent with contributions from a HOMO-to-
LUMO transition. Since HOMO-to-LUMO transition is the
lowest-energy vertical transition, either we have not found the
lowest-energy relax triplet structure or there is a reordering in
the excited states in the relaxation process. Alternatively, either
the vertical-transition energy or the triplet-state energy ordering
is incorrect.
Complete dissociation of a Cl atom from the triplets was
examined by optimizing doublet trans-Pt(PMe3)2(Cl)3 16′ (R =
Cl) (Figure 11). The doublet and a Cl atom are 53.7 kcal/mol
(51.6 in dichloromethane) above singlet 2′ (R = Cl) or
11.5 kcal/mol (10.8 in dichloromethane) above 2′T2 (R = Cl)
and 13.8 kcal/mol above 2′T1 (R = Cl). Of note is that
the approximately square-pyramidal geometry of doublet 16′
(R = Cl) is very close to that of triplet 2′T1 (R = Cl) after
removal of Cl1 (Figure 11). Thus, the Pt−Cl1 bond in 2′T1
(R = Cl) and 2′T2 (R = Cl) is very weak, and the triplets should
be a very good Cl atom donor with minimal geometric
rearrangement for triplet 2′T2 (R = Cl).
Figure 12. UV−vis absorption spectrum of trans-Pt(PEt3)2-
(Cl)3(phen) 2 (R = phen) in dichloromethane (black line) and
TDDFT (CAM-B3LYP, pcm) vertical transitions for trans-Pt(PMe3)2-
(Cl)3(phen) 2′ (R = phen) (red vertical lines, height corresponds to
the oscillator strength). Yellow circles mark calculated triplet
transitions (>300 nm), and green squares mark calculated dark
(oscillator strength <0.01) singlet transitions (>370 nm).
lowest-energy singlet transitions find an approximate match in
the five lowest-energy triplet transitions. With the exception of
the second triplet at 492 nm, all of the transitions have the
LUMO as the destination orbital. As for 2′ (R = Cl), the
LUMO (Figure 13) is predominantly an eg*-like Pt−Cl anti-
2
bonding orbital, but in this case it can be described as a Pt dz
based orbital directed along the Cl−Pt−Cl axis. (The higher-
lying LUMO + 1 is a Pt dx2−y2, eg*-like orbital in the phen
and phosphine ligand plane.) The HOMO and HOMO − 1 are
almost pure phen π orbitals, and the HOMO − 2 through
HOMO − 8 are similar to the orbitals for 2′ (R = Cl) (large
contributions from Cl lone-pair orbitals) with some contribu-
tion from phen ligand π orbitals. Thus, the excitation picture is
similar to that for 2′ (R = Cl) with electron density transfer
from primarily ligand-based orbitals to an antibonding Pt−Cl
orbital, but in contrast to 2′ (R = Cl) the antibonding orbital
is directed along the Cl−Pt−Cl axis and there is signifi-
cant involvement of the phen π system. Similar conclusion
can be made for 2′ (R = 4-tft, 2-tft) (Supporting Information
Tables S28 and S29, Figures S101 and S103).
Relaxed triplet calculations for 2′ (R = 4-tft) located three
different structures (Figure 14). Structure 2′T1 (R = 4-tft), at
43.2 kcal/mol above the singlet, is similar to 2′T1 (R = Cl)
in having a unique elongated Pt−Cl1 bond (2.96 Å) along a
Cl−Pt−Cl axis, slightly longer other Pt−Cl bonds, and slightly
shorter Pt−P distances relative to singlet 2′ (R = 4-tft). In
addition, the Pt−C distance is slightly shorter, and the tft ring
is rotated into the Pt, C1, Cl1, Cl2, Cl3 plane. The other
two triplets, 2′T2 (R = 4-tft) and 2′T3 (R = 4-tft), resemble 2′T2
(R = Cl) in having equal or nearly equal Pt−Cl distances along
the Cl−Pt−Cl axis. All other distances and the orientation of
the tft ring in 2′T2 (R = 4-tft) are essentially the same as in 2′T1
(R = 4-tft), as is the energy (42.7 kcal/mol above the singlet).
In contrast, 2′T3 (R = 4-tft) is significantly higher in energy
(47.1 kcal/mol above the singlet) and has the tft ring rotated
out of the Pt/Cl plane. The Pt−C distance is also reduced, and
as shown below, this ring orientation and the shorter Pt−C
distance is likely associated with high tft-ring spin density.
As with R = Cl, complete loss of a Cl atom from all of the
triplets is relatively facile with doublet, square-pyramidal trans-
Pt(PMe3)2R(Cl)2 16′ (R = 4-tft) and a Cl atom 16 kcal/mol
above 2′T1 (R = 4-tft) and 2′T2 (R = 4-tft) and 12 kcal/mol
Removal of a Cl atom from 16′ (R = Cl), though not as facile
as for the triplets, requires only 29.5 kcal/mol (31.5 kcal/mol
in dichloromethane). With a reported Cl2 bond enthalpy of
58.0 kcal/mol39 and a calculated (M06) ΔG of 53.0 kcal/mol, it
seems a Cl radical is capable of abstracting a Cl atom from 16′
(R = Cl) with formation of Cl2. Abstraction of the Cl atom by
a carbon radical would be even more favorable. Spin densities
in 16′ (R = Cl) show the largest Cl spin density (0.32) on axial
Cl3 (Table 7) suggesting that this would be the Cl atom to be
removed. However, isomerization in square-pyramidal com-
plexes is usually a low-energy process, and transition state 17′
(R = Cl) for axial−equatorial Cl exchange in 16′ (R = Cl) is
indeed only 0.8 kcal/mol above 16′ (R = Cl). Applying the
Eyring equation gives a picosecond room-temperature lifetime
for axial−equatorial Cl atom exchange in 16′ (R = Cl) indicat-
ing that abstraction of any of the Cl atoms is possible after
rearrangement to an axial position.
In contrast to 2′ (R = Cl), the TDDFT results for 2′ (R =
4-tft, 2-tft, phen) do not fully match the experimental spectra
(Figure 12, R = phen; Supporting Information Figure S99,
R = 4-tft; Supporting Information Figure S100, R = 2-tft). The
match is good for the weak transitions in the low-energy region
(>300 nm), but the energies of the high-intensity transitions
deeper in the UV appear to be overestimated. In addition,
orbital compositions (Table 8 and Supporting Information
Tables S26−S29) for the first five transitions are more complex
than for R = Cl, and most are without dominant (>80%)
contributions. For R = phen (Table 8), only four of the five
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Table 8. Five Lowest-Energy TDDFT (CAM-B3LYP, pcm) Vertical Transitions for trans-Pt(PMe3)2R(Cl)3 2′ (R = phen) in
Dichloromethane
singlets
osc
triplets
cm−1
nm
contribution (>5%)
cm−1
nm
contribution (>5%)
T−S gap cm−1
20 664
484
0.0034
HOMO → LUMO (86%)
19 178
521
HOMO → LUMO (74%)
H-7 → LUMO (7%)
HOMO → L+2 (65%)
H-1 → L+3 (23%)
1486
20 304
22 638
492
442
26 393
379
0.0044
H-7 → LUMO (53%)
H-4 → LUMO (9%)
H-2 → LUMO (7%)
H-7 → LUMO (49%)
H-4 → LUMO (9%)
H-8 → LUMO (8%)
H-2 → LUMO (7%)
HOMO → LUMO (6%)
H-8 → LUMO (76%)
H-2 → LUMO (11%)
H-2 → LUMO (59%)
H-7 → LUMO (11%)
H-3 → LUMO (7%)
3755
27 161
27 646
368
362
0.0147
0.0329
H-8 → LUMO (64%)
H-2 → LUMO (27%)
H-2 → LUMO (51%)
H-8 → LUMO (22%)
H-7 → LUMO (9%)
H-3 → LUMO (8%)
H-1 → LUMO (76%)
H-3 → LUMO (14%)
23 290
24 240
429
412
3871
3406
29 443
340
0.0046
Figure 13. LUMO (isovalue = 0.04 e/Å3, CAM-B3LYP, pcm) for
trans-Pt(PMe3)2(Cl)3(phen) 2′ (R = phen) in dichloromethane. Atom
colors: blue = Pt, orange = P, green = Cl, gray = C.
above 2′T3 (R = 4-tft). Although two doublet structures
are possible with equatorial and axial 4-tft ligand, only the
equatorial structure is a minimum. Exchange between the
axial and equatorial Cl atoms in doublet 16′ (R = 4-tft) is facile
with transition state 17′ (R = 4-tft) only 2.9 kcal/mol above 16′
(R = 4-tft). The structure of 17′ (R = 4-tft) is nearly that
expected of the “missing” square-pyramidal isomer of 16′ (R =
4-tft) with an axial 4-tft ligand (Figure 14, Cl2−Pt−Cl3 angle =
136.7°).
Spin densities for the R = 4-tft triplets are listed in Table 9.
Triplet 2′T1 (R = 4-tft), like 2′T1 (R = Cl), shows large spin
density on the Cl atom with the elongated Pt−Cl distance
(Cl1). Triplet 2′T2 (R = 4-tft), like 2′T2 (R = Cl), has a more
even spin distribution over the Cl atoms with slightly more
density on the axial Cl atoms Cl1 and Cl3. Both triplets thus
appear to be Cl-to-Pt charge-transfer in character. The spin-
density distribution in triplet 2′T3 (R = 4-tft) is quite different.
The total Cl spin density is reduced from that in the other
triplets with what remains mostly on the axial Cl atoms Cl1 and
Cl3. The “missing” spin density is found mostly in the tft ring
(0.465 with greatest density at the ortho and para positions)
suggesting significant tft-to-Pt charge-transfer character. The
spin-density distribution in optimized square-pyramidal doublet
16′ (R = 4-tft) (Table 9), derived by Cl1 atom removal from
Figure 14. Singlet 2′ (R = 4-tft), triplet 2′T1 (R = 4-tft), 2′T2 (R =
4-tft), 2′T3 (R = 4-tft), doublet 16′ (R = 4-tft), and doublet
isomerization transition state 17′ (R = 4-tft) structures from trans-
Pt(PMe3)2(Cl)4 2′ (R = 4-tft). Hydrogen atoms omitted. Distances in
angstroms, gas-phase energies above singlet 2′ (R = 4-tft) in
parentheses (energies for 16′ and 17′ include a Cl radical).
triplet 2′T1 (R = 4-tft), closely resembles that in 16′ (R = Cl) in
that the Cl spin density is predominantly located on the axial Cl
atom (Cl3).
Relaxed triplet calculations for 2′ (R = phen, 2-tft) were also
completed and revealed structures that resemble 2′T1 (R = Cl,
4-tft) in having a unique elongated Pt−Cl bond along the
Cl−Pt−Cl axis. However, due to the asymmetry of the R group,
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Table 9. Triplet and Doublet Mulliken Atomic Spin
a
Densities (Gas Phase, > 0.01) for R = 4-tft
atom
Pt
2′T1
2′T2
2′T3
16′
0.564
0.499
0.016
0.016
0.642
0.488
0.289
1.419
0.495
0.020
0.020
0.489
0.444
0.486
1.419
0.512
0.096
0.092
0.352
0.109
0.328
0.789
P1
P2
Cl1
NA
Figure 16. MOs (isovalue = 0.04 e/Å3, CAM-B3LYP, pcm) for triplet
2′T3 (R = phen) showing ring-Pt and ring-Cl overlap. Same molecular
orientation as in Figure 15 (SOMOs = 98a, 99a).
Cl2
0.050
0.306
0.356
Cl3
Cl total
a
Spin densities on the 2′T3 tft ring: C1, 0.153; C2, 0.102; C3, −0.066;
the upper 16 occupied α and β MOs for 2′T3 (R = phen)
showed only the two orbitals (Figure 16) that might favor the
fold. Of particular interest is 91a, which shows a small bonding
overlap between chlorine atom Cl1 and phen carbon atom C10.
C4, 0.213; C5, −0.084; C6, 0.147, C total = 0.465 (atoms numbered
sequentially around ring with C1 bonded to Pt).
there are two isomers differing by the side of the R group
where the elongated bond is located. Triplets 2′T1A (R = phen)
and 2′T1B (R = phen) are shown in Figure 15 along with the
C10 is the ring-chlorination sight in photoproduct 9 (RCl
=
Cl-phen). Mulliken spin densities for 2′T3 (R = phen) are
also interesting in this regard and show, with the exception
of the Pt center, the largest spin densities on Cl1 and C10
(Table 10). The implication is that this triplet is a model for the
a
Table 10. Triplet and Doublet Mulliken Atomic Spin
Densities (Gas Phase, >0.01) for R = phen
T1A
T1B
T3
A
B
atom
Pt
2′
2′
2′
16′
16′
0.607
0.505
0.014
0.012
0.607
0.521
0.300
1.428
0.505
0.024
0.024
0.353
0.024
0.266
0.643
0.899
0.608
0.590
P1
P2
Cl1
0.301
0.426
0.628
1.355
0.304
0.035
NA
NA
Cl2
0.043
0.304
0.347
Cl3
Cl total
C total
0.339
b
a
Doublets 16′A and 16′B are derived by removal of the weakly bonded
b
Cl atoms from 2′T1A and 2′T1B, respectively. Spin densities >0.100 for
phen C atoms: C1, 0.121; C3, 0.123; C8, 0.108; C9, 0.247; C10, 0.300.
Doublet.
Figure 15. Singlet 2′, triplet 2′T1A, triplet 2′T1A, and triplet 2′T3 (R =
phen) structures. Hydrogen atoms omitted. Distances in angstroms,
gas-phase energies above singlet 2′ (R = phen) in parentheses.
structure of singlet 2′ (R = phen). Triplet 2′T1A has the lowest
energy of the two and has the elongated Pt−Cl bond (Pt−Cl1)
on the same side as the peri-hydrogen atom of the phen group.
Triplet 2′T1B has the elongated Pt−Cl bond (Pt−Cl3) on the
opposite side from the peri-hydrogen atom of the phen group.
This leaves the more strongly bonded Cl1 in close proximity
to the peri-hydrogen atom, and this probably accounts for the
excited-state species involved in the ring-chlorination photo-
chemistry of 2.
DISCUSSION
■
As shown in Scheme 2 and eq 6 the overall photochemical
process for complexes 2 is photoreduction by chlorine elimina-
tion. The eliminated chlorine is found in ring chlorination
(Table 1), phosphine ligand chlorination (Table 1), alkene
chlorination (Scheme 3 and Figure 4), and, most likely, solvent
chlorination products. The alkene products clearly indicate
that molecular chlorine (Cl2) is not the chlorinating agent. In
particular, the observation of an approximately 50−50 mixture
of syn and anti 2-hexene addition products is not consistent
with Cl2 addition. Chlorination of 2-hexene with Cl2 gives only
the anti addition product under similar conditions. The chlorina-
tion competition experiment with a mixture of 1-hexene and
trans-3-hexene further confirms that Cl2 is not the chlorinating
agent in the photochemical reactions and indicates an R group
dependent steric factor. With Cl2 as the chlorinating agent
the more electron-rich, but sterically more demanding, trans-3-
hexene is the favored (by 12:1) chlorination target. Photo-
chlorination with 2 (R = Cl), the least sterically encumbered of
the complexes, still favors trans-3-hexene, but the bias is much
reduced (3:1). With crowded 2 (R = 2-tft) the selection is
higher energy of 2′T1B
.
Yet a third triplet structure, 2′T3 (R = phen), was located for
the phen complex. This triplet resembles triplet 2′T3 (R = 4-tft)
in having elongated Pt−Cl distances along the Cl1−Pt−Cl3
axis and other bond distances reduced from the singlet. Unique
to 2′T3 (R = phen) is a distortion of the Pt−phen bonding.
In 2′ (R = phen); the Pt atom and the phen ring carbon
atoms all lie in a plane, as would be expected. In contrast, 2′T3
(R = phen) has the Pt atom 0.18 Å and C9 0.83 Å out of the
plane formed by all the other phen carbon atoms (largest
deviation from plane is 0.06 Å for the other C atoms). Another
way of describing the distortion is as a 17° fold of the phen
ligand along the C10−C14 vector as measured by the angle
between the planes formed by the phen carbon atoms except
C9 and the plane formed by Pt, C9, C10, and C14. (See
Supporting Information Figure S104 for a drawing of the planes
and distances to the planes in 2′T3 (R = phen).) Examination of
K
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inverted (1:3), and the less-electron rich, but sterically less
demanding, 1-hexene is the favored chlorination target. This
implies that the chlorinating agent is an activated Pt complex
which transfers a chlorine atom to the alkene in a step that is
susceptible to steric hindrance. The observed mixture of syn- and
anti-addition products is consistent with isomerization of an
alkene Cl radical-addition intermediate.
Table 11. Energy (kcal/mol) Comparison for the Emission,
the Lowest-Energy TDDFT Singlet−Triplet Vertical Transi-
tions and Lowest-Energy DFT Relaxed Triplets for 2 or 2′
a
R
emission
lowest vertical transition
lowest relaxed triplet
4-tft
Cl
47.3
48.6
49.0
49.4
64.4
53.2
63.1
55.0
42.7
39.9
31.7
28.2
2-tft
phen
The evident absence of Cl2 elimination from 2 is in contrast
to the analogous bromine systems trans-Pt(PEt3)2(R)(Br)3
where strong evidence that molecular bromine (Br2) is the
alkene brominating agent was found.19 However, even in this
system, highly reactive alkenes give evidence of radical-like
reactivity ascribed to direct reaction of the alkene with a bromo
complex excited state. Direct reaction with an excited state is
apparently the only alkene reaction pathway for 2. Interestingly,
a similar dichotomy in the aqueous photochemistry of the
a
With respect to the ground-state singlet.
changes involved in the triplet relaxation giving a strained triplet.
Finally, frozen solvents are oriented to the ground state and
not the excited state, and this tends to blue-shift emissions.42
However, the rather small emission energy spread (∼2 kcal/mol)
over the R groups is notable and in contrast to the much larger
spread (15 kcal/mol) in the relaxed-triplet energies.
2−
simple halogen complexes PtX6 is observed: The X = Br
complex photoeliminates a bromide anion37 while the X = Cl
The 2′ triplet excited states revealed in the DFT calculations
appear to model the reactive excited states of 2. High spin
density is located on the Cl atoms of the triplets, and Cl radical
loss is a facile process indicating that the triplets should be
good Cl atom donors. The finding of multiple triplets with
close energies indicates that if the lifetimes are sufficient then a
triplet equilibrium mixture would be attained. (Multiple triplets
are a feature of Ru(II) polypyridyl systems.43−46) The R = phen
system is a particularly interesting example as triplet 2′T3
(R = phen) appears to be a model for a precursor to phen
ligand chlorination. Suppression of ring chlorination when
reactive substrates are present can then be explained either by
reaction of the substrate with 2′T3 at a competitive rate to the
ring chlorination or by reaction of the substrate with triplets
2′T1A and/or 2′T1B thereby shifting the equilibrium away from
2′T3 and slowing the rate of ring chlorination. The latter case is
depicted in the proposed Jablonski-type diagram for 2 (R =
phen) in Scheme 4 and more generally in the reaction pathway
complex photoeliminates a chlorine radical.40
Although we have not probed it in detail, it appears that the
photochemistry for 2 (except R = peryl) is essentially the same
at 313 and 380 nm (Table 1) suggesting that the complexes
follow Kasha’s rule41 that excitation to upper energy levels is
followed by rapid internal conversion to the lowest-energy
state. Singlet TDDFT calculations indicate that this state is dark
for 2 and is found at 521 nm for R = phen in dichloromethane.
This value may be underestimated as suggested by the behavior
of 2 (R = peryl). Transition energies involving the Pt and Cl
ligand orbitals should be similar for the peryl and phen
complexes while transitions involving the π systems should be
at substantially lower energies for the peryl complex. Excitation
at 313 nm into the phen ligand π−π* transition of 2 (R =
phen) does not give detectable phen ligand fluorescence but
does give Pt center reduction indicating internal conversion to
the lower energy Pt/Cl-centered excited states. In contrast,
excitation into the peryl ligand π−π* transition of 2 (R = peryl)
at 400 nm gives only peryl-ligand fluorescence. No Pt center
photoreduction is observed indicating little to no internal
conversion and suggesting that the π−π* excited state is at
similar or lower energy to the Pt/Cl-centered excited state(s).
The photoreduction chemistry of the peryl complex can still be
accessed by irradiation into higher energy bands (380 nm), but
the quantum yields are low and peryl moiety fluorescence is still
observed but at lower quantum yield than with direct excitation
into the π−π* band. Population of both the π−π* and the
photoreduction-active excited states from these higher energy
states is therefore indicated. Consistent with this interpretation,
the analogous bromo complex trans-Pt(PEt3)2(peryl)(Br)3,19
which should have lower energy Pt/Br centered excited states
(due to the lower energy bromine orbitals), shows strong
photoreduction activity with no visible peryl moiety fluo-
rescence even when the peryl π−π* band is irradiated at as low
an energy as 470 nm.
a
Scheme 4. Proposed Jablonski-type Diagram for 2 (R = phen)
a
Triplets 2T3, 2T1A, and/or 2T1B are PEt3 analogues of model triplets
The 77 K emission wavelengths give a measure of the triplet
excited-state energies in 2. The microsecond lifetimes indicate
triplet emission, and assuming that the associated excited states
are the lowest-energy triplets, they can be compared to the
calculated lowest vertical triplet transitions and the lowest
relaxed triplet energies (Table 11). In every case, the emission
energy lies between the vertical-transition energy and the
relaxed triplet energy. This is to be expected since emission is
likely from a vibrationally relaxed triplet to a vibrationally excited
singlet. In addition, the rigid matrix should inhibit geometric
2′T3, 2′T1A, and/or 2′T1B
.
for all 2 in Scheme 5. The role of symmetric triplets 2′T2 (R =
Cl, 4-tft) in the photochemistry is unclear. The overall spin-
density distribution in these triplets is similar to that in 2′T1
with the difference that the spin density is the same for each of
the Cl atoms and not concentrated on a single Cl atom as in
2′T1. This may make 2′T2 weaker Cl-atom donors, although,
energetically, loss of a Cl atom is no more difficult than for 2′T1.
L
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Scheme 5. Proposed Photoreaction Pathway for 2
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CONCLUSIONS
■
Solution photoreduction of 2 does not involve molecular
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ASSOCIATED CONTENT
* Supporting Information
Experimental details, CIF files, NMR spectra, and DFT data
(coordinates, energies, transitions, NTO figures, structures).
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
*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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ACKNOWLEDGMENTS
■
Support was provided by the U.S. Department of Energy,
Office of Basic Energy Sciences (DE-FG02-88ER13880). We
thank Dr. Charles Barns for X-ray data collection and
processing, Dr. Wei Wycoff for assistance with the NMR
measurements, and Alice R. Karikachery for assistance with
experimental procedures. The computations were performed
on the HPC resources at the University of Missouri
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̈
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