Hydroxyl Radical Control through Hydrogen Bonding: Photolysis of Platinum(IV)hydroxido Complexes with Intramolecular H-Bonding

Article abstract of DOI:10.1021/acs.organomet.5b00292

By introducing hydrogen-bonding groups into the coordination sphere of Pt(IV) hydroxido complexes photogenerated hydroxyl radicals are tethered and directed to abstract a hydrogen atom from the ethyl group of a triethylphosphine ligand, even at 25 °C, to yield phosphaplatinacycle complexes. (Chemical Equation Presented).

Full text of DOI:10.1021/acs.organomet.5b00292

Communication
pubs.acs.org/Organometallics
Hydroxyl Radical Control through Hydrogen Bonding: Photolysis of
Platinum(IV)hydroxido Complexes with Intramolecular H‑Bonding
Lasantha A. Wickramasinghe and Paul R. Sharp*
125 Chemistry, University of Missouri, Columbia, Missouri 65211, United States
S
* Supporting Information
ABSTRACT: By introducing hydrogen-bonding groups into the coordination sphere
of Pt(IV) hydroxido complexes photogenerated hydroxyl radicals are tethered and
directed to abstract a hydrogen atom from the ethyl group of a triethylphosphine
ligand, even at 25 °C, to yield phosphaplatinacycle complexes.
he hydroxyl (OH) radical is a notoriously promiscuous
outcome of the photochemistry.16 This inspired us to
synthesize new Pt(IV) hydroxido complexes with carboxylate
ligands (5, 6, and 7) in place of the hydroperoxo ligand by
simple protonation of the OOH ligand in 4 with acids (Scheme
2). (A sulfate-bridged dimer, [trans-Pt(PEt3)2Cl(4-tft)-
T
species reacting at near-diffusion-controlled rates with
many compounds1 and is important in atmospheric and
combustion chemistry2,3 and in biological systems.4−6 It is
believed to be involved in metal−oxygen chemistry including
Fenton-type chemistry,7,8 Shilov hydrocarbon oxidation,9 and
cytochrome P450 hydrocarbon hydroxylation.10
Scheme 2
Recently, we discovered that photolysis of Pt(IV) hydroxido
complexes generates OH radicals that can be constrained into
intramolecular C−H abstraction from a PEt3 ligand by
photolyzing in a 77 K glassy matrix (Scheme 1, 4-tft = 4-
Scheme 1
(OH)]2SO4, and a boric acid product that incorporates the
OOH ligand, trans-Pt(PEt3)2Cl(4-tft)(OB(OH)OO), were also
prepared by this method and characterized by X-ray
crystallography.17)
As shown by the solid-state structures for 5 (Figure 1) and
6,17 there is strong intramolecular hydrogen bonding between
the carboxylato and the hydroxido ligands.18 Complex 6 shows
a slightly longer hydrogen bond to the carbonyl group than in 5
(O1--O3 = 2.840(6) Å for 6, 2.815(3) Å for 5), consistent with
the expected weaker hydrogen bond acceptor properties of the
trifluoroacetato ligand. The hydroxido ligand 1H NMR
resonances in 5, 6, and 7 also indicate hydrogen bonding and
are downfield shifted compared to the hydroxido signal in non-
hydrogen-bonded 1.
trifluoromethylphenyl).11 (Photolysis of organic compounds
can also produce OH radicals.12,13) The constraints of the
glassy matrix are thought to inhibit OH radical escape and/or
its abstraction of an adjacent chlorido ligand, processes that
occur at room temperature to give exclusively formal HOCl
reductive-elimination product 3.14,15 Seeking to gain further
control of the OH radical, we modified the complex by
introducing ligands that form intramolecular hydrogen bonds
to the hydroxido ligand. (Hydrogen bonding of the OH radical
is believed to be important in enzyme systems.4,10) We now
report reactivity for these new complexes that is similar to that
of 1 in the 77 K matrix but in solution at room temperature.
The potential importance of intramolecular hydrogen
bonding in the photochemistry of Pt(IV) hydroxido complexes
was recently highlighted for trans-Pt(PEt3)2(Cl)(OH)(OOH)-
(4-tft) (4) (tft = trifluoromethylphenyl), where DFT
calculations suggested that the hydrogen-bonding pattern of
the hydroxido and hydroperoxido ligands could affect the
Remarkably, photolysis (380 nm) of 5−7 at room temper-
ature in CD2Cl2 gives C−H activation products phosphapla-
tinacycles 9−11 in yields as high as 50% (Scheme 3, Table 1).
At −78 °C the yields of the phosphaplatinacycles increase to as
high as 90%. The solvent choice for the photolysis is critical,
Received: April 7, 2015
Published: July 2, 2015
© 2015 American Chemical Society
3451
DOI: 10.1021/acs.organomet.5b00292
Organometallics 2015, 34, 3451−3454

Organometallics
Communication
platinum coupling and two-bonded diastereotopic protons.
1
What has changed with the new complexes is the H NMR
signals for the 4-tft ligand ortho-protons. Only one signal is
observed for 2, whereas 9 has two signals due to slow 4-tft
rotation on the NMR time scale. As this change is most likely
due to a change in the ligand cis to the 4-tft ligand, we assign
the structures of 9−11 as shown in Scheme 3.
Monitoring the photolysis of 5 shows the formation of a
second phosphaplatinacycle, 9′. Both 9 and 9′ initially grow in
together in a 1:1 ratio, but as the photolysis continues, 9′
converts to 9 (Scheme 4). We assign the structure of 9′ to the
Scheme 4
Figure 1. Drawing of the solid-state structure of trans-Pt(PEt3)2(4-
tft)(Cl)(OH)(OAc), 5 (50% probability ellipsoids, C-bonded hydro-
gen atoms omitted). Rotational disorder in the CF3 group is not
shown. Distances (Å): O3−O1 = 2.815(3), O1−H1O1 = 0.75(3),
O3--H1O1 = 2.11(3).
isomer of 9 where the chlorido ligand, instead of the OAc
ligand, is cis to the 4-tft ligand (Scheme 4). Consistent with this
assignment, the 1H NMR spectrum of 9′ shows only one ortho-
tft proton signal, the same as 2, which also has a chlorido ligand
cis to the 4-tft ligand. Careful examination of the 31P NMR
spectra17 of 11 also reveals low concentrations of a second
phosphaplatinacycle, which we assign as the isomer (11′) with
the chlorido ligand cis to the 4-tft ligand, the same as 9′. No
other isomer is detected for 10.
Scheme 3
The above results suggest that intramolecular hydrogen
bonding to the hydroxido group is an important factor in the
formation of the phosphaplatinacycles. However, the cis ligand,
which is subject to abstraction, has also been changed, and this
may have suppressed the competitive formal reductive
elimination to give 3. We investigated this possibility by DFT
(gas phase) and by the synthesis and photolysis of two other
complexes. The DFT-calculated free energy of AcOOH
reductive elimination for 5 was compared to the previously
calculated value for HOCl reductive elimination from 1. The
values are essentially identical at 32.8 (5) and 32.3 kcal/mol
(1), indicating that the net thermodynamics of the elimination
are unchanged. The Cl radical and OAc radical removal
energies from the Pt(III) doublets resulting from OH
dissociation from 1 and 5 were also evaluated. Acetate radical
removal from trans-Pt(PEt3)2(Cl)(OAc)(4-tft) takes only 17.3
kcal/mol, while Cl radical loss from trans-Pt(PEt3)2Cl2(4-tft)
requires 33.3 kcal/mol.19 Clearly, replacement of the chlorido
ligand in 1 with the acetatato ligand in 5 does not appear likely
to inhibit reductive elimination and explain the enhanced yield
of phosphaplatinacycles for carboxylate complexes 5−7.
In an attempt to experimentally suppress elimination, 8 was
prepared with a cyanido ligand (Scheme 5). Despite the
strongly bonded cyanido ligand cis to the OH ligand, photolysis
a
Table 1. Photolysis Products (%) from trans-
Pt(PEt3)2(Cl)(X)(OH)(4-tft) in CD2Cl2
−78 °C
platinacycle
25 °C
platinacycle
b
b
X
Pt(II)
Pt(II)
OAc (5)
90
90
81
30
15
50
10
10
19
70
50
15
50
0
50
85
50
O2CCF3 (6)
BBzO (7)
e
CN (8)
95
c
c
Cl (1)
45
0
100
100
d
d
OAc (5) (in C7H8)
50
0
a
b
Yields determined by 31P NMR spectral integration. Pt(II) = trans-
Pt(PEt3)2(4-tft)X or trans-Pt(PEt3)2(4-tft)Cl. OPEt3 (24%) and other
c
d
unidentified products also observed. Same at 77 K (LN2).
e
Remaining unidentified product observed at δ 17 (s) in the 31P
NMR spectrum.
and in toluene the yield of 9 from 5 is 0% at 25 °C and 50% at
−78 °C. Since our original studies of 1 were in 2-MeTHF and
toluene, we repeated the photolysis of 1 in CD2Cl2. As before,
at 25 °C phosphaplatinacycle 2 is not detected and 3 is the only
product. At −78 °C, some 2 is formed (15%), but the major
products are 3 (45%) and OPEt3 (24%).
Unfortunately, the phosphaplatinacycles are unstable to
isolation17 and were characterized in solution. The 31P and
1H NMR spectra17 (for 9) of 9−11 are similar to those for
previously reported 2.11 Most diagnostic is the 31P NMR high-
field shift of the P atom in the four-membered ring. Strong
coupling between this P atom and that of the PEt3 ligand
establishes a trans relationship. Signals for the platinum-bonded
phosphaplatinacycle carbon atoms (C1) are also evident in the
13C-DEPT NMR and HMQC NMR spectra17 and show
Scheme 5
3452
DOI: 10.1021/acs.organomet.5b00292
Organometallics 2015, 34, 3451−3454

Organometallics
Communication
of 8 at 25 °C yields only reductive elimination product trans-
Pt(PEt3)2(CN)(4-tft) (Scheme 5) and no phosphaplatinacycle.
Thus, with a strongly bonded cis ligand, and in the absence of
hydrogen bonding, an alternate trans elimination occurs.
However, at −78 °C a 30% yield of phosphaplatinacycle 12
is obtained. While not as high a phosphaplatinacycle yield as
the hydrogen-bonded carboxylic acid systems, it is double the
yield from 1, suggesting that strong bonding of the cis ligand
does slow reductive elimination and allow greater C−H
activation.
are the net reductive elimination pathways A and C. In pathway
A the OH radical abstracts a ligand from the Pt(III) center,
while in pathway C a solvent hydrogen atom is first abstracted
by the OH radical and then the resulting solvent radical
abstracts a ligand from the Pt(III) center. Pathway C is favored
with toluene and other solvents that contain relatively weakly
bonded hydrogen atoms and when the OH radical escapes the
coordination sphere of the Pt center. Pathway A generally
results in abstraction of a cis ligand (cis elimination in closely
related systems has been observed14,16,19), but in the case of 8,
where the CN ligand is strongly bonded, the OH radical can
migrate and abstract the trans Cl ligand. Pathway B then is
favored by OH radical tethering to the complex, preventing
escape, migration, and ligand abstraction. Consistent with
entropically unfavorable tethering, pathway B is more sensitive
to temperature than pathways A and C, and the amount of
pathway B product (phosphaplatinacycle) decreases from −78
°C to 25 °C. Indeed, the strength of the hydrogen-bond
tethering is reflected in the yields. At −78 °C all three
carboxylate complexes give comparable yields of phosphapla-
tinacycle that drop as the temperature is raised and the other
pathways began to operate. However, the strongest hydrogen
bonding is expected to be present in 5 and 7, and these give
50% at 25 °C. Weaker hydrogen bonding in 6 limits the yield to
15% at 25 °C, where OH radical escape, migration, and/or
ligand abstraction dominate.
The above results can be interpreted with the three
photolysis pathways presented in Scheme 6. All three pathways
Scheme 6
The lowest energy triplet states (T5 for 5 and T6 for 6) were
modeled by DFT to examine the hydrogen bonding and the
spin density distribution. The optimized structures are given in
Figure 2 and generally resemble the triplet previous found for
start from the lowest energy triplet excited state, which has
been previously modeled11,14 with DFT for 1 and now for 5
and 6. The model complexes (see below and Figure 2) show
elongation of the Pt−OH bond and near-unity spin density on
the OH group, indicating a weakly interacting Pt(III)/OH
geminate radical pair. In pathway B a phosphine hydrogen atom
abstraction by the OH radical gives water and, after ring
closing, the phoshaplatinacycle. Competitive with this pathway
1.11,14 Both 5 and 6 show intramolecular hydrogen bonding
between the carboxylate and hydroxido groups with distances
that are shorter than in the singlet17 model complexes.
(Stronger hydrogen bonding is attributed to a more polar
T
T
20
T
O−H bond in the radical. ) The OAc complex 5 appears to
have a particularly strong interaction with a remarkably short
H--O distance of 1.65 Å (1.92 Å in the singlet17), consistent
with strong tethering of the OH radical. The expected weaker
hydrogen bond acceptor ability of the CF3OAc ligand is evident
in the longer O--H distance of 1.73 Å (1.97 Å in the singlet17),
indicating a weaker tethering and more facile migration or
escape of the OH radical. Spin density values (Figure 2) show
the expected near-unity spin density on the OH moiety,
indicating high radical character. Finally, escape of the OH
T
radical from 5 requires 8.7 kcal/mol, most of which would
involve breaking the hydrogen bonding.
Finally, we sought to incorporate alternative hydroxido
hydrogen bonding to enhance pathway B to the phosphapla-
tinacycle and replaced the 4-tft ligand with a 2-methoxyphenyl
group to give 1417 (Scheme 7). Photolysis of 14 at −78 °C
gives a 40% yield of phosphaplatinacycle 15, improved from the
30% yield for analogous 4-tft complex 8. This is not to the 90%
level observed for 6, but the hydrogen bonding of the
Scheme 7
Figure 2. Drawings of the DFT triplets trans-Pt(PEt3)2(4-tft)(Cl)-
(OH)(OAc) (T5, top) and trans-Pt(PEt3)2(4-tft)(Cl)(OH)(O2CCF3)
(T6, bottom) (hydrogen atoms omitted except for OH and OAc).
Distances (Å) in black and Mulliken electron spin densities on Cl, Pt,
and the OH in blue and brackets. Pt = blue, P = orange, Cl = green, F
= light blue, O = red, C = gray.
3453
DOI: 10.1021/acs.organomet.5b00292
Organometallics 2015, 34, 3451−3454

Organometallics
Communication
(15) Net HOCl photoreductive elimination has been observed in the
gas phase for AuCl2(OH)2 : Marcum, J. C.; Kaufman, S. H.; Weber, J.
methoxyphenyl ligand to the hydroxido ligand should be
relatively weak compared to that in the carboxylato complexes
and the effect of changing the aryl group on the other pathways
is unknown and could be significant.
In conclusion, we have shown that by incorporating
hydrogen bonding into hydroxido complexes that are photo-
active for OH radical generation we can tether the OH radical
and direct its reactivity toward one pathway out of several, thus
harnessing the radical to do selective chemistry. (Hydrogen-
bonded hydroxyl radicals are thought to be involved in the
mechanism of monooxygenases.4)
−
M. J. Phys. Chem. A 2011, 115, 3006−3015,.
(16) Wickramasinghe, L. A.; Sharp, P. R. J. Am. Chem. Soc. 2014, 136,
13979−13982,.
(17) See Supporting Information.
(18) A similar intramolecular hydrogen-bonding pattern is observed
in a platinum(IV) hydroxido phthalato complex: Pellarin, K. R.;
McCready, M. S.; Sutherland, T. I.; Puddephatt, R. J. Organometallics
2012, 31, 8291−8300,.
(19) Perera, T. A.; Masjedi, M.; Sharp, P. R. Inorg. Chem. 2014, 53,
7608−7621,.
(20) Johnson, E. R.; Dilabio, G. A. Interdiscip. Sci.: Comput. Life Sci.
2009, 1, 133−140,.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details, decomposition data for 9 and 9′, NMR
spectra, structural drawings, DFT energies and coordinates, X-
ray structural details (cif). The Supporting Information is
available free of charge on the ACS Publications website at
DOI: 10.1021/acs.organomet.5b00292.
AUTHOR INFORMATION
Corresponding Author
*E-mail: sharpp@missouri.edu.
■
Author Contributions
All authors have given approval to the final version of the
manuscript and contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support was provided by the U.S. Department of Energy,
Office of Basic Energy Sciences (DE-FG02-88ER13880). We
thank Dr. Charles Barnes for X-ray data collection and
processing and Dr. Wei Wycoff for assistance with the NMR
measurements.
REFERENCES
■
(1) Mitroka, S.; Zimmeck, S.; Troya, D.; Tanko, J. M. J. Am. Chem.
Soc. 2010, 132, 2907−2913,.
(2) Allodi, M. A.; Kirschner, K. N.; Shields, G. C. J. Phys. Chem. A
2008, 112, 7064−7071,.
(3) Carr, S. A.; Still, T. J.; Blitz, M. A.; Eskola, A. J.; Pilling, M. J.;
Seakins, P. W.; Shannon, R. J.; Wang, B.; Robertson, S. H. J. Phys.
Chem. A 2013, 117, 11142−11154,.
(4) Badieyan, S.; Bach, R. D.; Sobrado, P. J. Org. Chem. 2015, 80,
2139−2147,.
(5) Burrows, C. J.; Muller, J. G. Chem. Rev. 1998, 98, 1109−1152,.
(6) Pogozelski, W. K.; Tullius, T. D. Chem. Rev. 1998, 98, 1089−
1108,.
(7) Fenton, H. J. H. J. Chem. Soc., Trans. 1894, 65, 899−910,.
(8) Bataineh, H.; Pestovsky, O.; Bakac, A. Chem. Sci. 2012, 3, 1594−
1599,.
(9) Rudakov, E. S.; Shul’pin, G. B. J. Organomet. Chem. 2015,
DOI: 10.1016/j.jorganchem.2015.01.032.
(10) Bach, R. D.; Dmitrenko, O. J. Am. Chem. Soc. 2006, 128, 1474−
1488,.
(11) Wickramasinghe, L. A.; Sharp, P. R. Inorg. Chem. 2014, 53,
11812−11814,.
(12) Boivin, J.; Crep
6869−6872,.
́
on, E.; Zard, S. Z. Tetrahedron Lett. 1990, 31,
(13) Zhou, C.-H.; Cheng, S.-B.; Yin, H.-M.; He, G.-Z. J. Phys. Chem.
A 2011, 115, 5062−5068,.
(14) For efforts to detect photogenerated HOCl see:
Wickramasinghe, L. A.; Sharp, P. R. Inorg. Chem. 2014, 53, 1430−
1442,.
3454
DOI: 10.1021/acs.organomet.5b00292
Organometallics 2015, 34, 3451−3454