Hydroxo radicals, C-H activation, and Pt-C bond formation from 77 K photolysis of a platinum(IV) hydroxo complex

Article abstract of DOI:10.1021/ic502325r

Photolysis (380 nm) of trans,cis-Pt(PEt₃)₂(Cl)₂(OH)(4-tft) (4-tft = 4-trifluoromethylphenyl) at 77 K in 2-methyltetrahydrofuran gives triplet emission, platinum(III), and a hydroxo radical. Benzyl radical emission is observed in toluene from the reaction of a portion of the OH radicals with toluene. Warming the photolyzed solutions gives platinacycle trans-Pt(CH₂CH₂PEt₂)(PEt₃)(Cl)₂(4-tft) by hydrogen-atom abstraction from a PEt₃ ligand and trans-Pt(PEt₃)₂(Cl)(4-tft) from net HOCl photoelimination. The platinacycle undergoes thermal reductive elimination at 298 K or photolytic reductive elimination, even at 77 K.

Full text of DOI:10.1021/ic502325r

Communication
pubs.acs.org/IC
Hydroxo Radicals, C−H Activation, and Pt−C Bond Formation from
77 K Photolysis of a Platinum(IV) Hydroxo Complex
Lasantha A. Wickramasinghe and Paul R. Sharp*
Department of Chemistry, University of Missouri, 125 Chemistry Building, Columbia, Missouri 65211-7600, United States
*
S Supporting Information
New 2 is unstable at room temperature (∼4 h to decay) and
was not isolated but is readily characterized by NMR
spectroscopy as the phosphaplatinacycle pictured in Scheme 1.
ABSTRACT: Photolysis (380 nm) of trans,cis-Pt-
PEt ) (Cl) (OH)(4-tft) (4-tft = 4-trifluoromethylphenyl)
at 77 K in 2-methyltetrahydrofuran gives triplet emission,
platinum(III), and a hydroxo radical. Benzyl radical
emission is observed in toluene from the reaction of a
portion of the OH radicals with toluene. Warming the
photolyzed solutions gives platinacycle trans-Pt-
(
3
2
2
31
Most diagnostic for the formation of 2 is the P NMR shift for
the phosphorus atom in the four-membered ring. Four-
membered-ring 31P NMR signals are strongly upfield-shifted
26
from those not in a ring, and this is observed in 2, where the
ring−P signal is at δ −65, while that for the unaltered PEt ligand
b
3
(
CH CH PEt )(PEt )(Cl) (4-tft) by hydrogen-atom ab-
2 2 2 3 2
(P ) is at δ −7.7, near that of 1. The phosphorus centers are
a
2 195
straction from a PEt ligand and trans-Pt(PEt ) (Cl)(4-tft)
3
3 2
coupled to each other (doublet, JPP = 513 Hz) and to Pt with
1
1
1
1
from net HOCl photoelimination. The platinacycle
undergoes thermal reductive elimination at 298 K or
photolytic reductive elimination, even at 77 K.
JPtP = 1853 Hz (PEt ) and 1410 Hz (P ). H− H COSY and
3
b
13
H− C HMQC NMR spectroscopy and comparisons to data for
27−30
platinum(II) phosphaplatinacycle complexes
tification and assignment of all H NMR shifts for 2. The ring-
allowed iden-
1
system methylene protons appear as diastereotopic pairs. The
ransition-metal hydroxo complexes are attracting consid-
platinum-bonded methylene group protons, H1a and H , are
1b
1
−4
T
erable interest as key species in hydrocarbon oxidation,
found at δ 2.71 and 1.73, and are bonded to the same carbon
5
−11
12,13
13
C−H activation,
water oxidation,
solar energy con-
atom, C1, found at δ −0.2 in the C NMR spectrum. The P-
14−18
19−24
version and storage,
and other catalytic schemes.
We
bonded methylene group protons are located at δ 2.98 and 2.37
13
recently reported the synthesis and room-temperature photo-
chemistry of platinum(IV) hydroxo complexes trans-Pt-
and are bonded to C2, found at δ 29.7 in the C NMR spectrum.
The diastereotopic nature of the methylene protons establishes a
cis configuration for the chloro ligands and the stereochemistry
for 2 given in Scheme 1.
(
PEt ) (X)(Y)(OH)(R) (X and Y = Cl, Br). These complexes
3 2
undergo net photoreductive elimination of HOX with oxidation
25
After several hours at room temperature in toluene-d or
8
of alkenes or solvent. We now report the unusual 77 K
photochemistry of one of these platinum(IV) hydroxo
complexes, which leads to a sequence of hydroxo radical
formation, C−H activation, Pt−C bond formation, and C−Cl
reductive elimination.
MeTHF, or more rapidly in chloroform, platinacycle 2
completely converts to Pt(PEt )(Et PCH CH Cl)(Cl)(4-tft)
3
2
2
2
3
1
(
4) by reductive elimination (Scheme 2). The P NMR
Scheme 2
Irradiation (380 nm) of trans,cis-Pt(PEt ) (Cl) (OH)(4-tft)
3
2
2
25
(
1; 4-tft = 4-trifluoromethylphenyl) for several hours in glassy
2-methyltetrahydrofuran (MeTHF) or toluene at 77 K followed
by warming to room temperature gives a mixture of a new
complex 2 and trans-Pt(PEt ) (Cl)(4-tft) (3) (Scheme 1).
3
2
1
(
Yields in Scheme 1 were determined by H NMR integration
against an internal standard.) Complex 3 is the sole platinum-
containing product at room temperature, and it is produced in
minutes instead of hours.
spectrum of 4 shows two closely spaced signals at δ 13.7 and
13.0 with 1 Pt satellites (JPtP = 2770 and 2722 Hz, respectively).
The shifts and coupling constants are very similar to those of 3,
25
95
25
Scheme 1
consistent with the formation of an analogous platinum(II)
complex. The H NMR spectrum shows a distinctive set of
1
multiplets at δ 3.87 and 2.25 assigned to the chloroethyl group. A
crystal grown from the mixture of 3 and 4 was subjected to X-ray
crystal structure analysis, which showed that 3 and 4 cocrystal-
lized in an 11:14 ratio (Figure 1). Complex 2 also photo-
Received: September 24, 2014
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/ic502325r | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Scheme 3
Figure 1. Drawing of the solid-state structure of cocrystallized 3 (44%)
and 4 (56%).
decomposed (380 nm) to 4 at room temperature or at 77 K. (A
gold(III) complex was recently reported to undergo photolytic
31
reductive elimination of aryl chloride. )
In addition to its photochemical reactivity, 1 is photo-
luminescent in MeTHF glass at 77 K. Irradiation at 380 nm gives
visible salmon-pink emission (λmax = 620 nm). The emission
lifetime (38 μs) and intensity decrease as the photolysis
progresses indicate that emission is from a triplet excited state
of a species that is photodecaying; in other words, the emission is
from a triplet of 1. Remarkably, our previously reported density
functional theory calculations of the lowest-energy triplet and
singlet of the PMe analogue of 1 gave a ground-state singlet to
part of Scheme 3) or the PEt
3
ligand (lower part of Scheme 3),
3
25
lowest-energy triplet gap corresponding to 630 nm. The
calculations also indicate high spin density on a loosely bonded
OH ligand in the triplet excited state. Switching the solvent from
with the ratio depending on the solvent. Greater solvent reaction
occurs with the weak C−H bonds of the toluene methyl group
and less with the stronger 2-MeTHF bonds (Scheme 1).
Warming the frozen matrix results in carbon/platinum(III)
radical pair (B) coupling to form platinacycle 2. Platinacycle 2
formation must occur as the frozen matrix is warmed because 2 is
photosensitive to reductive elimination, even at 77 K. The
possibility that coupling of the solvent and platinum(III) radicals
occurs upon warming to give a platinum/solvent complex (E)
that then reductively eliminates to give chlorinated solvent and 3
can be eliminated because E complexes are not observed by low-
temperature NMR of a photolyzed sample warmed from 77 K to
MeTHF to toluene-d changes the emission to blue, which, in
8
contrast to the salmon-pink emission, brightens as photolysis
progresses. The blue emission spectrum (Figure 2) is character-
32−34
istic of the benzyl radical.
The picture (Scheme 3) that thus emerges is that photolysis of
in a 77 K matrix generates a triplet excited state (1 ) that decays
T
1
to the ground state by emission or progresses to a geminate
platinum(III)/OH radical pair (A). The highly reactive OH
radical then abstracts hydrogen atoms from the solvent (upper
−
3
60 °C; only 3 and 4 are detected. Another possible pathway to
is the abstraction of a solvent hydrogen atom by the phosphine
ethyl radical in B to give a solvent radical that then abstracts a
chlorine radical from platinum(III) complex C (lower part of
Scheme 3). This can be eliminated because deuterium
incorporation into the phosphine ligand of 3 is predicted (3-d)
in deuterated solvents but is not observed (toluene-d8).
A question that arises is, why is the 77 K frozen matrix
photochemistry of 1 different from the room temperature
25
solution photochemistry (net HOCl elimination )? A likely
explanation is that in the frozen matrix the PEt ligand may be
3
locked into a position favorable for abstraction and the chlorine
ligand may be inhibited from attaining a position favorable for
abstraction.
The relatively efficient formation of 2 and the subsequent
reductive elimination of the C−Cl bond are notable. Although
we did not observe the intermolecular equivalent reactions with
solvent, the formation of 2 does suggest that other photo-
reductions of transition-metal complexes that are accompanied
Figure 2. Benzyl radical emission spectrum from a toluene-photolyzed
sample of 1 at 77 K. Inset: visible blue emission during photolysis (λ =
2
5,35−39
3
80 nm).
by hydrocarbon oxidation
may occur by a similar process,
B
dx.doi.org/10.1021/ic502325r | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
that is, metal−ligand radical pair generation, ligand radical−
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formation), and reductive elimination. It should also be noted
that the photochemistry described here shows parallels to the
thermal chemistry of iron halogenases, where hydrogen-atom
transfer to an oxo ligand generates the carbon radical that then
couples with a halogen ligand.
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dx.doi.org/10.1021/ic502325r | Inorg. Chem. XXXX, XXX, XXX−XXX