Triphos iridium(III) halide complex photochemistry: Triphos arm dissociation

Article abstract of DOI:10.1021/ic401835d

Photolysis of Ir(triphos)X₃ (triphos = 1,1,1- tris(diphenylphosphinomethyl)ethane; X = Cl, Br) yields an insoluble product believed to be oligomeric [Ir(triphos)X₃]_n with bridging triphos and halide ligands. Refluxing pyridine (py) dissolves the insoluble photoproducts ultimately yielding the dangling triphos complexes mer-Ir(κ²-triphos)(py)X₃. Oxidation of the P center of the dangling arm of Ir(κ²-triphos)(py)Cl₃ yields mer-Ir(κ²-P,P-triphosO)(py)Cl₃ (triphosO = MeC(CH₂P(O)Ph₂)(CH₂PPh₂) ₂), which was characterized by single-crystal X-ray diffraction. mer-Ir(κ²-triphos)(py)Cl₃ is also formed when Ir(triphos)Cl₃ is photolyzed in the presence of py (φ = 26%). Both mer-Ir(κ²-triphos)(py)Cl₃ and mer-Ir(κ²-P,P-triphosO)(py)Cl₃ photoisomerize in pyridine to their thermally unstable fac-isomers. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations suggest triphos ligand arm dissociation occurs along a triplet pathway from an initial Franck-Condon ligand-field excited state that relaxes to a Jahn-Teller axially distorted octahedral triplet with a long Ir-P bond. Subsequent triphos arm dissociation yields a distorted trigonal-bipyramidal triplet that undergoes intersystem crossing to a square pyramidal singlet.

Full text of DOI:10.1021/ic401835d

Article
pubs.acs.org/IC
Triphos Iridium(III) Halide Complex Photochemistry: Triphos Arm
Dissociation
Andreas Ross and Paul R. Sharp*
Department of Chemistry, University of Missouri-Columbia, 125 Chemistry Building, Columbia, Missouri 65211-7600, United States
S
* Supporting Information
ABSTRACT: Photolysis of Ir(triphos)X3 (triphos = 1,1,1-
tris(diphenylphosphinomethyl)ethane; X = Cl, Br) yields an
insoluble product believed to be oligomeric [Ir(triphos)X3]n
with bridging triphos and halide ligands. Refluxing pyridine
(py) dissolves the insoluble photoproducts ultimately yielding
the dangling triphos complexes mer-Ir(κ2-triphos)(py)X3.
Oxidation of the P center of the dangling arm of Ir(κ2-
triphos)(py)Cl3 yields mer-Ir(κ2-P,P-triphosO)(py)Cl3 (tri-
phosO = MeC(CH2P(O)Ph2)(CH2PPh2)2), which was
characterized by single-crystal X-ray diffraction. mer-Ir(κ2-triphos)(py)Cl3 is also formed when Ir(triphos)Cl3 is photolyzed in
the presence of py (ϕ = 26%). Both mer-Ir(κ2-triphos)(py)Cl3 and mer-Ir(κ2-P,P-triphosO)(py)Cl3 photoisomerize in pyridine to
their thermally unstable fac-isomers. Density functional theory (DFT) and time-dependent DFT (TDDFT) calculations suggest
triphos ligand arm dissociation occurs along a triplet pathway from an initial Franck−Condon ligand-field excited state that
relaxes to a Jahn−Teller axially distorted octahedral triplet with a long Ir−P bond. Subsequent triphos arm dissociation yields a
distorted trigonal-bipyramidal triplet that undergoes intersystem crossing to a square pyramidal singlet.
disfavor ligand exchange and isomerization and perhaps show
other photochemical processes such as halogen photoelimina-
tion (eq 4, X = Y = a halogen). Herein we report our
photochemical investigation of iridium(III) trihalide complexes
with the tripodal ligand triphos (1,1,1-tris-
(diphenylphosphinomethyl)ethane), which locks in a fac
configuration of the halide ligands. (For other examples of Ir
tripodal phosphine systems see references 11−14.) While the
triphos complexes show rich dissociative photochemistry,
halogen elimination is not observed.
INTRODUCTION
■
We recently reported efficient net photochemical bromine
elimination from Pt(IV) polyhalide complexes (eq 1),1 an
important process in potential solar energy conversion and
storage.2 Wishing to expand this chemistry to other transition
metal d6-systems we became interested in the photochemistry
of octahedral Ir(III) polyhalide complexes.
light
trans‐PtL2(R)Br ⎯⎯⎯→ trans‐PtL2(R)Br + Br2
3
(Br2 trapped by alkenes)
(1)
Iridium(III) complexes are involved in a number of
photochemical processes. Recently, they have been studied
especially as phosphorescent dyes in the field of organometallic
light emitting diodes (OLEDs) because of their excellent color
tuning.3,4 Ford and co-workers5,6 observed photoaquation (eq
2) and photoinduced ligand exchange of iridium(III)
complexes. Photoisomerization (eq 3)7−9 and photochemical
reductive elimination of molecular oxygen, molecular hydrogen,
and hydrogen chloride have also been reported (eq 4, XY = O2,
H2, HCl),10 the latter suggesting that Ir(III) complexes may
also undergo halogen photoelimination.
RESULTS
■
Complex Synthesis and Characterization. Known
trichloro complex Ir(triphos)Cl3 (1) was prepared by a
literature procedure (eq 5)15 and its molecular structure
determined by single-crystal X-ray analysis (Figure 1). (The
synthesis and structure of analogous fac-Ir(PMe3)3Cl3 was
recently reported.16) Although not mentioned in the original
synthesis of 1, in our hands known17 hydride complex
Ir(triphos)Cl2H (2) is always present as a byproduct. This
hydride complex is probably formed from the reaction of 1 with
the 2-methoxyethanol solvent, and prolonged heating of 1 in 2-
methoxyethanol gives good yields of 2 (eq 6). This probably
also explains the need for NaCl in the synthesis of 1, excess Cl−
would inhibit formation of alkoxo complexes with the 2-
methoxyethanol and subsequent hydride formation. Leaving
out the NaCl in the synthesis of 1 yields a black precipitate of
light
[IrL6]3+ + H2O HoooI [IrL5(H2O)]3+ + L
(2)
light
mer‐IrCI3L3 ⎯⎯⎯→ fac‐IrC3L3
(3)
light
L3IrCI(X)(Y) ⎯⎯⎯→ L3IrCI + XY
(4)
To suppress isomerization (eq 3), we chose to investigate
octahedral Ir(III) complexes with tridentate ligands that would
Received: July 16, 2013
Published: October 18, 2013
© 2013 American Chemical Society
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1 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯K⎯⎯B⎯r⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ir(triphos)Br H
2
40°C,48 h, “bulk” acetone
(8)
4
Characterization of new Ir(triphos)Br3 (3) and Ir(triphos)-
Br2H (4) is through 31P and 1H NMR spectroscopy and single
crystal X-ray diffraction (4, see Supporting Information).
Complex 3 has a single resonance at δ −37 in its 31P{1H}
NMR spectrum (CH2Cl2). The 31P{1H} NMR spectrum
(CD2Cl2) of 4 is very similar to that of 2 and shows two
resonances in a 2:1 ratio, a doublet (JPP = 11.2 Hz) at δ −13.1
1
and a triplet at δ −50.8. The H NMR spectrum (CD2Cl2)
shows a hydride resonance at δ −8.78 as a doublet of triplets
(JHP = 181 Hz, JHP = 8.8 Hz).
UV−vis spectra for the trihalo complexes 1 and 3 are
presented in Figure 2. As expected for the white to pale yellow
complexes, strong absorption is only observed in the uv region
with weak tail-offs into the visible.
Photochemistry. Photolysis (313 nm) of 1 or 3 in the
presence or absence of 1-hexene causes the yellow solution to
bleach and pale yellow precipitates (5 and 6, respectively) to
form (eq 9). Precipitates 5 and 6 are formulated as oligomeric
forms of 1 and 3 (see below) and can be converted back to 1
and 3 simply by refluxing in 2-methoxyethanol for 16 h (eq 9).
(Refluxing for longer periods (48 h) yields hydride complexes
Ir(triphos)(H)Cl2 (2) and Ir(triphos)Br2H (4) (see eq 6 and
8).)
Figure 1. Solid-state structure of Ir(triphos)Cl3 1 (50% thermal
ellipsoids, hydrogen atoms omitted and carbon atoms unlabeled).
unknown composition, probably from reduction of starting
IrCl3(H2O)x prior to formation of 1.
313 nm,CH2CI2
Ir(triphos)X3 XoooooooooooooooooY [Ir(triphos)X3]n
reflux,16 h
1(X=CI)
3(X=Br)
5(X=CI)
6(X=Br)
IrCI3(H2O)x ⎯⎯⎯⎯⎯⎯⎯⎯⎯t⎯r⎯i⎯p⎯h⎯o⎯⎯s⎯,N⎯⎯a⎯C⎯⎯I⎯⎯⎯⎯⎯⎯⎯→ Ir(triphos)CI3
2‐methoxyethanol
(9)
reflux,16 h, 2‐methoxyethanol
(5)
(6)
1
Elemental analysis, solid state IR, and 31P NMR spectroscopy
were used to more completely characterize photoproduct 5.
Elemental analysis of 5 is essentially identical to 1 except with
incorporation of small and variable amounts of solvent
(CH2Cl2) that could not be removed in vacuo but were
detected (1H NMR) by dissolving (see below) the solid in
pyridine-d5. The KBr pellet IR spectrum of 5 and 1 are also
essentially identical (Supporting Information) suggesting that
the triphos ligand is intact in the photoproduct. The far IR
(mineral oil mull) spectra, however, differ (Supporting
Information). That of 1 shows two Ir−Cl stretching bands at
303 and 278 cm−1, characteristic of facial terminal chloro
reflux,24 h
1 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ir(triphos)CI2H
2‐methoxyethanol
2
The bromo analogue of 1, Ir(triphos)Br3 (3), is readily
obtained from 1 in quantitative yield by heating (40 °C) 1 with
KBr in freshly distilled acetone (eq 7). The same reaction in
“bulk” acetone yields the bromohydride Ir(triphos)Br2H (4)
(eq 8). Again, hydride formation most likely involves alkoxo
complexes, in this case from alcohol impurities in the acetone.
1 ⎯⎯⎯⎯⎯⎯⎯⎯K⎯⎯B⎯⎯r⎯⎯⎯⎯⎯→⎯ Ir(triphos)Br
3
40°C,24 h acetone
(7)
3
Figure 2. UV−vis spectra for Ir(triphos)Cl3 1 (black dashed line) and Ir(triphos)Br3 3 (red solid line) in dichloromethane.
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ligands trans to phosphine ligands.18−21 The far IR spectrum of
5 also shows bands at 303 and 278 cm−1 but strong, broad
bands also appear at higher energy (324 and 317 cm−1) and
lower energy (264 cm−1). The higher energy bands indicate the
presence of chloro ligands trans to weaker donors than a
phosphine ligand (e.g., terminal or bridging chloro ligands) and
the lower energy band is consistent with bridging chloro
ligands. The solid-state 31P NMR spectrum of 5 shows multiple
broad peaks from δ 9 to −90 suggesting multiple environments
for the P centers in solid 5. The largest peak occurs at δ −32 in
a region similar to that for 1 and 2 in solution.
Precipitates 5 and 6 are generally insoluble but dissolve in
pyridine to give initial 31P NMR spectra with a plethora of
peaks (Supporting Information) indicating a complex product
or mixtures. Heating a pyridine solution of 5 at 120 °C (sealed
tube) under argon overnight gives a single product identified as
the dangling triphos complex mer-Ir(κ2-triphos)(py)Cl3 (7) (eq
10). Complex 1 is stable under these conditions indicating that
Figure 3. Solid-state structure of mer-Ir(κ2-P,P-triphosO)(py)Cl3
8
(50% thermal ellipsoids, hydrogen atoms omitted, and carbon atoms
unlabeled).
1 is not a product of dissolving 5 in pyridine. The 31P NMR
spectrum of 7 in CDCl3 or CH3NO2 shows the dangling
phosphine group as a broad peak at δ −26.6, near the signal for
free triphos (δ −25.3). The coordinated phosphine group
signals are found at δ −29.1 and −41.0 and are coupled to each
other (2JPP = 24 Hz). Surprisingly, the signal at δ −29.1 shows
weak coupling to the dangling phosphine group (4JPP = 2 Hz).
In pyridine or benzene, the coupling increases to 4 Hz, and the
dangling phosphine group signal sharpens into a doublet, also
although the change in oxidation state of the phosphorus center
could also affect coupling. As with 8, heating the NMR sample
causes collapse of the dangling phosphine oxide group signal
indicating the presence of multiple conformations.
Complex 1 also dissolves in pyridine but without reaction.
However, photolysis results in efficient conversion, first to 7,
but with continued irradiation a new product (9) grows in
(Scheme 1). Continued irradiation, after 1 is completely
1
with 4 Hz coupling. H NMR signals for one of the three
diastereotopic triphos methylene groups are broad in CDCl3
and probably belong to the dangling phosphine group. The
changes in P−P coupling and broadness of the dangling
phosphine group signals is likely due to hindered rotation about
the C−C and P−C bonds of the dangling phosphine group
with two or more rotational conformations. The lowest energy
conformation must be such that four-bond P−P coupling is
possible, but the coupling is lost in other conformations.
(Conformation-dependent, long-range P−P coupling has been
previously observed.22) Consistent with this, heating a
CH3NO2 NMR sample to 80 °C causes collapse of the
dangling phosphine group signal and complete loss of coupling
to the Ir-bonded phosphine group. The favoring of the one
conformation by pyridine and benzene is likely due to π−π
interactions between the pyridine and the triphos phenyl rings.
Complex 7 is oxygen sensitive and tends to undergo slow
oxidation of the dangling phosphine group. (Dangling triphos
ligand oxidation has been reported.23,24) Rapid oxidation of 7
with hydrogen peroxide gives mer-Ir(κ2-P,P-triphosO)(py)Cl3
(8, triphosO = MeC(CH2P(O)Ph2)(CH2PPh2)2) (eq 11),
which readily yielded crystals suitable for X-ray diffraction
analysis (Figure 3). NMR data for 8 are similar to 7 except that
the dangling phosphine group signal is absent and converted to
a dangling phosphine oxide group signal at δ 27.1, and all
signals are sharp. Presumably, conversion of the dangling
phosphine group to a phosphine oxide group results in a
conformation that does not allow four-bond P−P coupling,
Scheme 1
consumed, converts 7 into 9. The 31P NMR spectrum of 9 in
CDCl3 shows two peaks in a 1:2 ratio with the smaller peak (δ
−27.8) in the same region as the dangling phosphine group of
7. The larger peak (δ −38.7) is in a similar region to that of one
of the bonded phosphine groups of 7 and 8. (These peaks are
assigned to a P atom trans to a Cl ligand.) Complex 9 is
assigned as the facial isomer of meridional 7, that is fac-Ir(κ2-
triphos)(py)Cl3 with the py trans to a Cl instead of a P group as
in 7 and 8. Similar to 7, 9 shows evidence of a conformation of
the dangling phosphine group that gives rise to four-bond P−P
coupling. In pyridine and benzene, coupling between the
dangling phosphine group and the coordinated phosphine
groups is evident, and the coordinated phosphine group signal
at δ −38.7 becomes a doublet and the smaller signal for the
dangling phosphine group becomes a triplet (JPP = 4 Hz).
Complex 9 slowly (days) reverts to 7 in solution and was
always isolated with small amounts of 7. Complex 7 is also
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obtained from the 380 nm photolysis of 1 in dichloromethane
with added pyridine. These conditions were used to avoid
absorption by pyridine and to obtain the quantum yield (0.26
0.04) for the photolysis.
When a CH2Cl2 solution of 1 is photolyzed with PPh2Et a
new soluble product is formed (eq 12). Although not isolated,
Table 1. Experimental and Calculated Mean Metrical
Parameters (Å and deg) for Ir(triphos)Cl3
a
distance/angle
experimental (1)
1S6
Ir−P
Ir−Cl
P−Ir−P
Cl−Ir−Cl
P−Ir−Cl
2.303(4)
2.414(8)
89.0(4)
2.391(15)
2.475(10)
89.0(17)
86.6(13)
92.2(32)
175.7(9)
84.1(6)
93.4(16)
176.0(4)
a
Mean values with standard deviations.
ring arrangement resulting in different intramolecular contacts.)
Using the X-ray crystal structure coordinates of 1 as starting
coordinates for triplet optimization yielded 5-coordinate 1T5
(Figure 4). Again starting from the X-ray crystal structure
the 31P NMR spectrum is fully consistent with the formation of
the PPh2Et analogue of 7, mer-Ir(κ2-triphos)(PPh2Et)Cl3 (10).
The dangling phosphine signal is a singlet at δ −28, the PPh2Et
signal is a double doublet at δ −16.8 and shows very strong
trans coupling (442 Hz) to one of the coordinated triphos P
atoms and much weaker cis coupling (16 Hz) to the other
coordinated triphos P atom. The coordinated triphos P atom
signals are also double doublets (trans to PPh2Et: δ −35.6, trans
to Cl: δ −48.8) and with cis coupling to each other of 19 Hz.
Prolonged (24 h) photolysis of a pyridine solution of 1 under
air results in phosphine oxidation and formation of the facial
isomer of 8, fac-Ir(κ2-P,P-triphosO)(py)Cl3 (11, Scheme 2).
Scheme 2
Figure 4. Optimized DFT (M06/LANL2DZ) 5- and 6-coordinate
triplets 1T5 and 1T6 (hydrogen atoms and phenyl rings omitted).
coordinates of 1 but changing the chloro ligands to bromo
ligands (i.e., 3) gave 6-coordinate 3T6. Exchanging the bromo
ligands of 3T6 for chloro ligands followed by optimization gave
6-coordinate triplet structure 1T6 (Figure 4) and exchanging the
chloro ligands of 1T5 for bromo ligands gave 5-coordinate
triplet 3T5. Metrical parameters for the triplet structures are
listed in Table 2. The 5-coordinate triplet structures (1T5 and
3T5) show a 5-coordinate, distorted trigonal bipyramidal Ir
center geometry with one arm of the triphos ligand completely
dissociated. The coordinated portion of the triphos ligand
occupies an axial (P1) and an equatorial (P2) position. The 6-
coordinate triplet structures (1T6 and 3T6) show an axially
distorted octahedral geometry with elongation of one set of
trans Ir−P and Ir−X bonds (Ir−P3 and Ir−X3). The 5-
coordinate chloro triplet (1T5) is isoenergetic with its 6-
coordinate analogue (1T6) while the bromo 5-coordinate triplet
(3T5) is 12 kcal lower than the 6-coordinate triplet (3T6).
However, it is doubtful that the DFT energies are sufficiently
reliable to determine relative ordering.25,26 A potential energy
scan (redundant coordinate) extending the axial Ir−P distance
in 1T6 indicates a low barrier of ∼1 kcal for complete triphos
arm dissociation and a facile conversion to 5-coordinate triplet
1T5. The transition state was located (Supporting Information)
for the bromo system and is 1.0 kcal above the 6-coordinate
triplet (3T6).
Complex 11 is also formed in three other ways: when
photoproduct 5 is dissolved in pyridine and then the solution
is exposed to sunlight for 6 days in air, by photolysis of the
meridional isomer 8, or by air oxidation of 9. The 31P NMR
spectrum of 11 shows two peaks in a 1:2 ratio. The smaller
peak is found at δ 25.7, in the same region as the dangling PO
group of 8. The larger peak is observed at δ −39.3, in the same
region as that for the bonded P groups of 9. Facial 11 slowly
(days) converts to meridional 8 at room temperature in
dichloromethane solution, or rapidly (min) at 100 °C in
pyridine, indicating that, as with 7 and 9, the meridional isomer
is the thermally more stable isomer.
DFT Calculations. To better understand the photoactivity
of the iridium(III) complexes a density functional theory
(DFT) study (M06/LANL2DZ (X = Cl) or B3LYP/
LANL2DZ (X = Br)) was undertaken. The mean metrical
parameters (Table 1) for the DFT optimized geometry 1S6 (3S6
for the bromo analogue) and the X-ray structure of 1 are in
good agreement with the Ir-ligand distances being slightly
longer (4% or less) in the DFT structure. (The greater variation
in the equivalent bond distances for the DFT structure (see
esd’s) may be at least partly due to a slightly different phenyl
Mulliken atomic spin densities for the triplets are given in
Table 3. In the 6-coordinate triplets, spin density is located
primarily on the Ir center, two of the halogen atoms, and the
elongated P atom (P3). Of the two halogen atoms with
significant spin density, the halogen atom (X3) with the
elongated bond dominates. This distribution and the geometry
are consistent with a d6 Jahn−Teller distorted triplet excited
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Table 2. Metrical Parameters (Å and deg) for the 5- and 6-
Coordinate Triplets
distance/
a
angle
1T6 (X = Cl) 1T5 (X = Cl) 3T6 (X = Br) 3T5 (X = Br)
Ir−P1
Ir−P2
Ir−P3
Ir−X1
Ir−X2
Ir−X3
2.5006
2.3901
2.7768
2.4170
2.4941
2.6194
88.09
2.3857
2.4258
4.7007
2.4360
2.4836
2.4388
88.64
2.5054
2.4495
2.8652
2.5471
2.6369
2.8293
88.15
2.4120
2.4460
5.1938
2.6344
2.5766
2.5827
89.29
b
b
P1−Ir−P2
P1−Ir−P3
P2−Ir−P3
X1−Ir−X2
X1−Ir−X3
X2−Ir−X3
P1−Ir−X1
P1−Ir−X2
P1−Ir−X3
P2−Ir−X1
P2−Ir−X2
P2−Ir−X3
P3−Ir−X1
P3−Ir−X2
P3−Ir−X3
78.60
83.17
Figure 5. Optimized DFT (M06/LANL2DZ) 5-coordinate singlet
structures for 1 (hydrogen atoms and phenyl rings omitted, gas phase
for 1S5, in dichloromethane for 1S5′.DCM).
88.16
86.15
86.76
93.00
94.00
85.52
90.58
90.85
97.00
97.74
86.26
113.96
179.00
86.00
84.03
113.28
177.62
87.54
Time-dependent DFT (TDDFT) (CAM-B3LYP/
LANL2DZ) calculations were used to obtain vertical transition
energies and oscillator strengths for 1 and 3 and to simulate the
absorption spectra (Supporting Information). A good match
with the experimental spectra is obtained with a set of weak
transitions at the UV−vis edge and stronger transitions deeper
into the uv. The four lowest vertical singlet and triplet
transitions for 1 are listed in Table 4. Each singlet transition has
a matching triplet transition at slightly lower energy, implying
facile intersystem crossing from the singlet to the triplet
manifold.
178.08
94.91
177.87
93.06
82.18
86.40
84.68
88.56
90.38
91.77
93.06
92.80
171.69
101.84
102.52
84.85
113.37
131.88
172.62
103.34
95.15
113.20
133.32
86.76
158.01
164.33
a
b
Atom numbering from Figure 4. Dangling P.
The orbital make up of the transitions from the TDDFT
calculations is quite complex. Simplification was obtained
through a natural transition orbital (NTO) treatment.27 Each of
the transitions is described by two dominant NTO sets. Those
for the first singlet (similar for the first triplet) are pictured in
Figure 6. The “destination” orbitals (top) are eg-type and show
Ir-ligand σ-antibonding character. The departure orbitals are
t2g-type with contributions from the π-type p-orbitals of the Cl
atoms. The NTO sets for the other three transitions
(Supporting Information) are closely related, differing mostly
in which Ir d- and Cl p-orbitals are involved. Thus, all of the
transitions can be described as LF transitions.
a
Table 3. Triplet Mulliken Atomic Spin Densities
a
atom
1T6
1T5
3T6
3T5
Ir
1.27
0.01
1.39
−0.03
0.10
1.17
0.01
1.23
b
b
P1
P2
P3
X1
X2
X3
−0.02
0.08
−0.01
0.17
−0.01
0.17
c
a
a
c
0.00
0.00
b
b
0.15
0.06
0.20
0.09
0.02
0.27
0.01
0.35
0.25
a
0.34
0.19
0.42
a
b
c
Atom numbering from Figure 4. Axial ligand. Dangling P.
DISCUSSION
■
state of primarily ligand field (LF) character with an unpaired
In keeping with the known photochemistry of many Ir(III),
Rh(III), and other low spin d6 complexes,28,29 the photolysis of
1 and 3 is dominated by ligand dissociation. Either halide ligand
or triphos ligand arm dissociation is possible, but the
photoproducts for 1 and 3 clearly indicate that triphos ligand
arm dissociation dominates. (The alternative of halide
dissociation/substitution followed by thermal halide displace-
ment of the triphos ligand arm is very unlikely given the
stability of Ir(triphos)X3 to py.) This is not unexpected as
halide photodissociation is disfavored in organic solvents
because of poor charge solvation.30
Once formed, the photogenerated 5-coordinate intermediate
would be susceptible to ligand coordination. In the absence of
added ligand, several possibilities exist. Recoordination of the
triphos arm would return the intermediate to 1 or 3. If
recoordination is slow, halide bridges with other Ir centers
could form or the dangling triphos arm could coordinate to
another Ir center forming a triphos bridge. Such bridged
structures (Figure 7) are likely elements of photoproducts 5
and 6 and these compounds are formulated as oligomeric
[Ir(triphos)Cl3]n and [Ir(triphos)Br3]n. (Dimerization of mer-
Ir(Et2S)3Cl3 by photodissociation of Et2S has been reported.31)
electron in a t2g-type orbital on the Ir center and an unpaired
2
electron in an eg-type M-L antibonding orbital (dz ). On
moving to the more stable trigonal-bipyramidal 5-coordinate
triplet, spin density on the Ir center increases slightly and shifts
away from the dangling P atom (P3) and the axial P (P1) and
halogen (X1) atoms.
Optimization of the triplet structure of 1T5 as a singlet
yielded square-pyramidal, 5-coordinate Ir(κ2-triphos)Cl3 (1S5,
Figure 5) at 25.3 kcal above 1S6 putting it 17.8 kcal lower in
energy than triplet 1T5. It is presumably 5-coordinate 1S5 that is
trapped by py or PPh2Et or, in the absence of a trapping ligand,
oligomerizes to produce 5. It should be noted that trapping 1S5
with py or PPh2Et should give the meridional isomer, as is
observed experimentally. Attempts to optimize (gas phase) the
alternate isomer with a Cl atom in the axial position yielded a
structure with the same Ir center geometry as in 1S5 (axial P)
but with the dangling P group in a slightly more stable (2 kcal)
orientation. However, optimization of the axial-Cl isomer in
CH2Cl2 (pcm model) did yield a stationary point structure
(1S5′.DCM, Figure 5), evidently stabilized by the polar medium
but less stable than the geometry of 1S5.
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Table 4. Lowest Four Singlet Vertical Transitions for 1S6 and Their Corresponding Triplet Transitions
singlets
osc. strength
triplets
contributions (≥5%)
no.
1
λ, nm
contributions (≥5%)
no.
2
λ, nm
S-T gap (kcal/mol)
380
0.0015
0.0139
0.0115
0.0006
H-3→LUMO (46%)
H-2→L+1 (9%)
H-3→L+3 (7%)
416
H-3→LUMO (36%)
H-2→L+1 (8%)
−6.5
H-3→L+3 (7%)
H-1→LUMO (9%)
H-2→LUMO (40%)
H-2→L+3 (10%)
H-3→L+1 (6%)
2
3
4
377
374
344
H-2→LUMO (41%)
H-2→L+3 (5%)
1
3
4
418
410
391
−7.4
−6.5
−9.9
H-3→L+1 (11%)
H-5→LUMO (5%)
H-4→L+1 (42%)
HOMO→L+1 (22%)
H-3→LUMO (6%)
H-4→L+1 (28%)
HOMO→L+1 (28%)
H-1→L+1 (6%)
H-21→L+1 (5%)
HOMO→LUMO (26%)
H-4→LUMO (19%)
H-3→L+1 (6%)
HOMO→LUMO (32%)
H-4→LUMO (24%)
H-3→L+1 (11%)
HOMO→L+3 (5%)
H-4→L+1 (5%)
H-21→LUMO (5%)
Figure 6. Natural transition orbital (NTO) sets for the first singlet
excited state of 1S6 (isovalue = 0.04). The “destination” orbital is on
the top and the “departure” orbital is on the bottom, and λ is the NTO
eigenvalue for each set. The X, Y, and Z axes were arbitrarily chosen
but are consistent for all drawings. (C and H atoms omitted.).
Figure 7. Proposed structural elements of 5 and 6.
The solid-state NMR data, the far IR data, the formation of
complex mixtures of products when 5 and 6 are first dissolved
in pyridine, and the trapping of various amounts of solvent in
the solid are all consistent with irregular bridged oligomeric
structures. Interestingly, formation of 5 and 6 appears to be
exclusively from oligomerization of the photogenerated 5-
coordinate intermediate. If 1 or 3 were to react with the 5-
coordinate intermediate through halide bridge formation
dissolving 5 or 6 in pyridine would rupture the halide bridge
and regenerate 1 or 3. This is not observed. The implication is
that the 5-coordinate intermediate is remarkably long-lived and
recoordination of the dangling triphos arm relatively slow,
allowing bimolecular reactions of the 5-coordinate intermediate
to occur.
dangling phosphine group and would require geometric
isomerization to an alternative square-pyramidal structure
(1S5′.DCM) for recoordination of the dangling phosphine
group. That the 5-coordinate intermediate has the structure of
1S5 is indicated by the initial formation of pyridine trapping
product mer-Ir(κ2-triphos)(py)Cl3 (7) and PPh2Et trapping
product mer-Ir(κ2-triphos)(PPh2Et)Cl3 (10) both of which can
be envisioned as forming by pyridine or PPh2Et coordination to
the vacant site of 1S5.
In agreement with previous conclusions on low-spin d6 halide
complex photochemistry,28,29 the DFT and TDDFT calcu-
lations indicate that the phosphine group dissociation occurs
through low-energy triplet excited states. These would be
accessed through rapid internal conversion/intersystem cross-
ing from initial excitation into the singlet manifold or possibly
through direct excitation into the triplet manifold. This is
consistent with the photochemistry wavelength independence
No doubt contributing to slow arm recoordination in the 5-
coordinate intermediate is the square pyramidal geometry
predicted by DFT. The open site of 1S5 is not adjacent to the
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(313 and 380 nm and sunlight) of the complexes. The initially
formed triplet LF Franck−Condon state would relax by Jahn−
Teller distortion yielding 6-coordinate triplet structures 1T6 or
3T6. The 6-coordinate triplets then progress to 5-coordinate 1T5
or 3T5 through complete triphos ligand arm dissociation. (A
similar 6-coordinate triplet was located by DFT for d6
[Ru(bpy)2L2]2+ and found to give ligand (L) dissociation.32)
Subsequent intersystem crossing to singlet 1S5 or 3S5 would
then open the complex to coordination of another ligand or
reformation of 1 or 3. This sequence, depicted in Scheme 3 for
states have significant halogen lone-pair-to-Pt charge transfer
(LMCT) character yielding radical density at the halogens in
conjunction with a weakening of the Pt−X bonds.1 We are still
working out the likely pathways, but the result of radical
halogen character in the excited state appears to be net
elimination of X2. Although less common than heterolytic
ligand dissociation, photolytic radical reactivity has been
observed for some d6 complexes, especially on excitation into
higher energy CT bands.34−37
In contrast to 1 and 3, which photoeliminate a triphos arm,
complexes 7 and 8 probably photoeliminate a chloride ligand
along the Cl−Ir−Cl axis. This is suggested by Adamson’s Rules,
developed for d3 complexes but also applicable to d6 complexes,
which state that ligand dissociation will occur along the weakest
ligand field axis.38−40 The isomerization of 7 and 8 to their
facial isomers 9 and 11 likely occurs by a pathway similar to
that in the photochemistry of 1 and 3. This is illustrated for 8 in
Scheme 4, where 5-coordinate triplet 8T5 undergoes
intersystem crossing to 5-coordinate singlet 8S5. Recoordination
of the chloride then yields 11.
Scheme 3
Finally, given the strong current interest in photoluminescent
Ir(III) complexes, a comment on the different photoexcitation
results for the above complexes should be made. A key
difference is the nature of the lowest triplet excited state. The
photoluminescent complexes generally contain ligands with
extended π-systems.3,4,41 This introduces low-lying intraligand
triplet excited states that are inactive to ligand dissociation and
phosphorescence dominates.
CONCLUSIONS
■
the chloro complexes, is analogous to that recently reported for
PtBr62−, a d6 octahedral complex related to 1 and 3.33
Computational chemistry supported transient absorption
spectroscopy of this Pt(IV) system indicates that initial
excitation into the singlet manifold is followed by rapid
intersystem crossing to the lowest-energy triplet with expulsion
The photochemistry of d6 Ir(triphos)Cl3 (1) and Ir(triphos)Br3
(3) is dominated by triphos ligand arm dissociation.
Dissociation is proposed to occur through a triplet pathway,
going first to a 6-coordinate Jahn−Teller distorted LF triplet
excited state that transits by triphos arm dissociation to a 5-
coordinate distorted trigonal bipyramidal triplet. Subsequent
intersystem crosses then gives a long-lived, square pyramidal, 5-
coordinate singlet that oligomerizes or is trapped by added
ligands. No evidence was found for halogen elimination, as was
observed for d6 trans-Pt(PEt3)2(R)X3 (R = Br, Cl, aromatic
group; X = Cl, Br). The difference in the photochemistry of 1
and 3 and the Pt(IV) complexes is attributed to greater MLCT
character in the lowest energy excited states.
of a Br− to give triplet 5-coordinate PtBr5 . The DFT calculated
−
geometry for the relaxed 5-coordinate triplet is similar to that
calculated here for 1T5 and 3T5 (distorted trigonal bipyramid).
−
Intersystem crossing to square-pyramidal singlet PtBr5 then
opens the complex to ligand coordination (aquation or
recombination with a Br−) as predicted here for 1S5 and 3S5.
As mentioned in the introduction we began this study with
the rather naive expectation of observing halogen photo-
elimination from 1 and 3. This expectation was partly based on
the halogen photoelimination from the related Pt(IV) d6
complexes PtL2X3R (L = a phosphine; X = Cl, Br; R = Cl,
Br, aromatic group).1 At this juncture, we believe the difference
between the Pt complexes and 1 and 3 is the character of the
lowest-energy excited states. All of the above data point to a
high degree of LF character in the lowest-energy excited states
of 1 and 3. In contrast, the Pt complex lowest-energy excited
Photoisomerization of meridional pyridine complexes (7 and
8) to the facial isomers (9 and 11) probably occurs by halide
photodissociation where the 5-coodinate cationic intermediate
is captured by the halide.
EXPERIMENTAL SECTION
■
General Procedures. Ir(triphos)Cl3 (1)15 and concentrated
solutions of hydrogen peroxide in diethyl ether42 were prepared by
Scheme 4
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reported procedures. Reagents were purchased from commercial
sources (Aldrich or Acros) and used as received. Experiments were
performed under a dinitrogen or argon atmosphere in a Vacuum
Atmospheres Corporation drybox or on a Schlenk line unless
otherwise indicated. Solvents were dried, degassed, and stored under
dinitrogen over 4 Å molecular sieves or sodium metal unless otherwise
noted. Solution NMR spectra were recorded on Bruker AMX-250 or
-300 spectrometer at ambient probe temperatures. Solid state 31P
NMR spectra were recorded on the Bruker AMX-300 spectrometer
with MAS (magic angle spinning) at 5 kHz. NMR shifts are given in δ
with positive values downfield of TMS (1H and 13C), external H3PO4
(31P) and external ADP (ammonium dihydrogen phosphate) reference
(solid state 31P, δ 0). 13C and 31P NMR spectra were recorded in
proton-decoupled mode. ALS Environmental performed the micro-
analyses. UV−vis spectra were recorded on a Cary 50 or Hewlett-
Packard 8452 diode array spectrophotometer in 1 cm quartz cells.
Photolyses were performed in 5 mm NMR tubes or 4 or 8 mL vials
(borosilicate glass) using a Philips PL-S 9W/01, 9 W lamp (313 nm)
or a Super Bright LEDs, RL5-UV0315-380 LED (380 nm) operated at
30 mA. The LED photon flux for the quantum yield determination was
measured with a Newport 841-PE power meter equipped with an 818-
UV detector.
C41H39Cl3IrP3·0.3CH2Cl2: C: 53.34 (52.29), H: 4.26 (4.21). The
1
presence of the 0.3CH2Cl2 was confirmed by H NMR spectroscopy
of the analytical sample dissolved in pyridine-d5. A second sample was
similarly analyzed and showed 0.45CH2Cl2. See Supporting
Information for solid-state 31P NMR and IR spectra.
[Ir(triphos)Br3]n (6). This was made in the same way as 5 with 11
mg (10.4 μmol) of Ir(triphos)Br3 3. The yellow solid product was
obtained in quantitative yield and is insoluble in common organic
solvents. The results were identical when the experiment was repeated
in air or with added 1-hexene (100 fold excess).
mer-Ir(κ2-triphos)(pyridine)Cl3 (7). A 50 mL sealable tube was
charged with 64.4 mg (69.92 μmol) of 5, and 25 mL of previously
degassed pyridine was added. The tube was sealed, and the
homogeneous mixture was heated at 120 °C overnight. (Caution!
This is above the boiling point of pyridine and the tube will be pressurized.)
The volatiles were then removed in vacuo yielding the yellow, air-
sensitive, solid product in quantitative yield. Complex 7 was difficult to
purify being always contaminated with small amounts of 8.
31P NMR (CDCl3, 101 MHz): −26.5 (br s, 1P), −29.1 (dd, JPP = 24
Hz, JPP = 2 Hz, 1P), −41.0 (d, JPP = 24 Hz, 1P).
31P NMR (pyridine, 101 MHz): −26.3 (d, JPP = 4 Hz, 1P), −27.9
(dd, JPP = 24 Hz, JPP = 4 Hz, 1P), −41.1 (d, JPP = 24 Hz, 1P).
31P NMR (C6D6, 101 MHz): −26.0 (d, JPP = 4 Hz, 1P), −28.1 (dd,
JPP = 24 Hz, JPP = 4 Hz, 1P), −41.5 (d, JPP = 24 Hz, 1P).
31P NMR (CD3NO2, 101 MHz): −26.2 (br d, JPP = 3 Hz, 1P),
−29.0 (dd, JPP = 24 Hz, JPP = 3 Hz, 1P), −40.9 (d, JPP = 24 Hz, 1P).
1H NMR (CDCl3, 300 MHz): 8.97 (m, 2H), 8.07 (m, 2H), 7.71
(m, 7H), 7.26 (m, 22), 6.82 (m, 2H), 3.25 (m, 2H), 2.74 (m, 2H),
2.17 (br d, J = 2.4, 1H), 1.91 (br d, J = 2.4, 1H) and 1.23 (br s, 3H).
mer-Ir(κ2-P,P-triphosO)(pyridine)Cl3 (8). Complex 7 was dis-
solved in 2−3 mL of dichloromethane, and a few drops of a
concentrated solution of hydrogen peroxide in diethyl ether was added
with stirring. The volatiles were removed in vacuo yielding yellow solid
8 in quantitative yield. Crystals for the X-ray analysis were grown by
slow evaporation of a benzene solution of 8. Anal. Calc. (found) for
C46H44Cl3IrNOP3: C: 54.25 (53.63, 53.48), H: 4.36 (4.66, 4.66), N:
1.38 (1.41, 1.42).
Ir(triphos)Cl2H (2). A 100 mL round-bottom flask was charged
with a stirring bar and 38 mg (41 μmol) of 1 in 33 mL of 2-
methoxyethanol. The mixture was refluxed under argon for 12 h. The
volatiles were then removed in vacuo. The crude product was
dissolved in 2 mL of dichloromethane and 75 mL of diethylether
added to give a precipitate. The mixture was stored in a freezer for 30
min and then filtered through a fritted funnel (M) to isolate the
precipitate, which was washed with 20 mL of diethylether and then
extracted with acetonitrile. The volatiles were removed in vacuo from
the extract giving 14.8 mg (41%) of solid pale yellow Ir(triphos)Cl2H.
17
1
31P NMR and H NMR data match the literature values.
31P NMR (CD2Cl2, 101 MHz): −12.5 (d, JPP = 11 Hz, 2P) −43.1
(t, JPP = 11 Hz, 1P).
1H NMR (CD2Cl2, 300 MHz): −7.91 (dt, JHP = 188 Hz and JHP
=
10 Hz, 1H), 1.54 (s, 3H), 2.41 (m, 2H), 2.60 (m, 4H), 7.34 (m, 30H).
Ir(triphos)Br3 (3). A 20 mL vial was equipped with a stirring bar
and charged with 18.9 mg (20.5 μmol) of Ir(triphos)Cl3 and 2.0 g (17
mmol) of KBr in 2−3 mL of freshly distilled acetone. The reaction
mixture was stirred overnight at 40 °C. The completion of the reaction
was confirmed by 31P NMR spectroscopy. All volatiles were removed
in vacuo, and the residue was extracted with dichloromethane (3 × 5
mL). The volatiles were removed from the extract in vacuo leaving a
solid, which was washed with 2 × 1 mL of acetonitrile, 0.5 mL of
water, and 2 mL of diethylether and dried in vacuo to give 18.7 mg
(17.7 μmol, 86.3%) of solid pale yellow 3.
31P NMR (CDCl3, 101 MHz): 27.1 (s, 1P), −27.8 (d, JPP = 24 Hz,
1P), −42 (d, JPP = 23 Hz, 1P).
1H NMR (CDCl3, 300 MHz): 8.95 (m, 2H), 8.2 (m, 2H), 8.1 (m,
2H), 7.9 (m, 2H), 7.7 (m, 1H), 7.6 (s, 1H), 7.30 (m, 19), 6.97 (m,
2H), 6.9 (t, 2H), 3.7 (m, 1H), 2.95 (m, 2H), 2.7 (m, 1H), 2.5 (m,
1H), 1.7 (m, 1H) and 1.55 (s, 3H).
Photolysis of mer-Ir(κ2-P,P-triphosO)Cl3 (8). Complex 8 (10
mg, 10 μmol) was dissolved in 0.5 mL of pyridine and photolyzed at
313 nm in 3 min intervals in a standard 5 mm NMR tube (N2
atmosphere). The complex underwent photochemical isomerization to
fac-Ir(κ2-P,P-triphosO)(pyridine)Cl3 (11) as shown by 31P NMR
spectroscopy. Similar experiments with 8 dissolved in dichloro-
methane, CDCl3, and benzene gave no conversion after 30 min of
irradiation.
31P NMR (CH2Cl2, 101 MHz): −37 (s).
1H NMR (CDCl3, 300 MHz): 1.25 (s, 3H), 2.68 (d, JPH = 7.06 Hz,
6H), 7.16 (m, 15H), 7.74 (br s, 15H).
Ir(triphos)Br2H (4). A 50 mL round-bottom flask equipped with a
stirring bar was charged with 32 mg (34.7 μmol) of 1, 2 g of KBr, and
25 mL of bulk acetone. The mixture was stirred at 40 °C for 2 days. All
volatiles were removed in vacuo, and the residue was extracted with
dichloromethane. Removing the volatiles from the extract in vacuo
yielded 28.3 mg (83.6%) of pale yellow Ir(triphos)Br2H (4).
31P NMR (CD2Cl2, 101 MHz): −13.1 (d, JPP = 11 Hz, 2P) −50.8
(t, JPP = 11 Hz, 1P).
Photolysis of Ir(triphos)Cl3 (1) in pyridine. Complex 1 (6.0 mg,
6.5 μmol) was added to 0.5 mL of pyridine and photolyzed at 313 nm
in a standard 5 mm NMR tube under an N2 atmosphere. The progress
of the photolysis was periodically monitored by 31P NMR spectros-
copy. The first two minutes of photolysis showed only formation of
mer-Ir(κ2- triphos)(pyridine)Cl3 (7). After 7 min, both 7 and fac-Ir(κ2-
triphos)(pyridine)Cl3 (9) were present along with remaining 1. After
20 min, 1 was completely consumed and the 7:9 (mer:fac) ratio was
1:1.3. Continued photolysis completely converted the mixture to the
fac isomer 9. Air exposure of solutions of 9 gave fac-Ir(κ2-P,P-
triphosO)(pyridine)Cl3 (11).
1H NMR (CD2Cl2, 300 MHz): −8.78 (dt, JHP = 181, 9 Hz, 1H),
1.55 (s, 3H), 2.39 (m, 2H), 2.67 (m, 4H), 7.68−6.89 (m, 30H).
[Ir(triphos)Cl3]n (5). A 4 mL vial was charged with 10 mg (10.9
μmol) of Ir(triphos)Cl3 1 and 1 mL of dichloromethane. The resulting
yellow solution was photolyzed with a 313 nm light source. The yellow
solution slowly bleached, and a yellowish precipitate formed. The
photolysis was judged complete when an aliquot showed no 31P NMR
peaks. The volatiles were then removed in vacuo to yield the yellow
solid product in quantitative yield. The product is insoluble in
common organic solvents but dissolves readily in pyridine (see below).
The results were identical when the experiment was repeated in air or
with added 1-hexene (100 fold excess). Anal. Calc. (found) for
fac-Ir(κ2-triphos)(pyridine)Cl3 (9). A 50 mL Schlenk flask was
charged with 34.0 mg (36.9 μmol) of 1 in 8 mL of pyridine and a stir
bar. The sample was photolyzed under N2 at 313 nm for 4 h with
stirring. The completion of the reaction was determined by 31P NMR
spectroscopy. The volatiles were then removed in vacuo (1 day). The
resulting yellow solid was dissolved in dichloromethane. The solution
was filtered through diatomaceous earth and reduced to ∼0.5 mL.
Addition of 3−4 mL of hexane resulted in an immediate precipitate.
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The mixture was allowed to stand at −20 °C for 3 h and then filtered
through diatomaceous earth. The precipitate/diatomaceous earth mix
was washed with 2−3 mL of hexane and then extracted with 3−4 mL
of dichloromethane. The volatiles were removed in vacuo from the
extract yielding 32.1 mg of yellow solid 9 (32.1 μmol, yield 87%).
During isolation, 9 isomerized to small amounts of mer-Ir(κ2-
triphos)Cl3(py) 7.
using gas phase energies was tested by including a solvent correction
(pcm model) for the gas phase energies of 1, 1T6, and 1T5. This did not
significantly alter their relative energies (3 kcal/mol or less) indicating
that the gas phase relative energies are adequate. Coordinates and
energies are given in the Supporting Information.
ASSOCIATED CONTENT
■
31P NMR (CDCl3, 101 MHz): −27.8 (d, JPP = 2 Hz, 1P), −38.7 (d,
JPP = 2 Hz, 2P).
S
* Supporting Information
1
CIF files, H and 31P NMR spectra, IR spectra, and DFT data
31P NMR (pyridine, 101 MHz): −27.8 (d, JPP = 2 Hz, 1P), −38.7
(d, JPP = 2 Hz, 2P).
(coordinates, energies, NTO figures). This material is available
free of charge via the Internet at http://pubs.acs.org.
31P NMR (benzene, 101 MHz): −27.8 (d, JPP = 2 Hz, 1P), −38.7
(d, JPP = 2 Hz, 2P).
1H NMR (CDCl3, 300 MHz): 0.56 (s, 3H), 2.33 (br s, 2H), 2.39
(m, 2H), 3.73 (m, 2H), 7.20 (m, 30), 7.61 (t, JHH = 7.5 Hz, 1H), 7.93
(m, 2H), 8.48 (br s, 2H).
AUTHOR INFORMATION
Corresponding Author
*E-mail: sharpp@missouri.edu.
■
Photolysis of Ir(triphos)Cl3 with PPh2Et. Observation of mer-
Ir(κ2-triphos)(PPh2Et)Cl3 (10). A standard 5 mm NMR tube was
charged with 7.0 mg (7.6 μmol) of 1 and 1 M PPh2Et in ∼0.5 mL of
CH2Cl2. The sample was photolyzed at 380 nm for 33 min. The 31P
NMR spectrum confirmed full conversion. The product, mer-Ir(κ2-
P,triphos)Cl3(PPh2Et), was not isolated or further characterized.
31P NMR (CH2Cl2, 101 MHz): −16.8 (dd, JPP = 442 Hz, 16 Hz, 1P,
PPh2Et), −28.0 (s, 1P), −35.6 (dd, JPP = 442 Hz, 19 Hz, 1P, P trans to
PPh2Et), −48.8 (overlapping dd, JPP = 19 Hz, 16 Hz, 1P).
fac-Ir(κ2-P,P-triphosO)(pyridine)Cl3 (11). (a) A 20 mL bor-
osilicate vial was charged with 20 mg (22 μmol) of 5 dissolved in 5−6
mL of pyridine. The sample was photolyzed in sunlight through a
window for 6 days. After completion of the reaction (31P NMR
spectroscopy), the volume was reduced to about 1.5 and 15 mL of
hexane was added. The resulting milky solution and precipitate were
stored in freezer overnight. The precipitate was then recovered by
filtration, washed with hexane, and dried in vacuo to yield 15.2 mg
(69%) of yellow solid 11. (b) A 20 mL vial was charged with 44.6 mg
(48.4 μmol) of 1 in 10 mL of pyridine. The sample was photolyzed at
313 nm for 24 h and judged complete by 31P NMR spectroscopy. The
volume of the solution was then reduced to 2 mL, and about 15 mL of
diethyl ether was added to precipitate the product. The product was
recovered by filtration through a pipet filled with diatomaceous earth
and washed several times with diethyl ether. Extraction with
dichloromethane and final removal of the volatiles in vacuo yielded
10.3 mg (21%) of yellow solid 11.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
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 Barns for X-ray data collection and
processing, Dr. Wei Wycoff for assistance with the NMR
measurements, and Dr. Richard Martin for helpful discussions
on the TDDFT calculations. The computations were
performed on the HPC resources at the University of Missouri
Bioinformatics Consortium (UMBC).
REFERENCES
■
(1) Karikachery, A. R.; Lee, H. B.; Masjedi, M.; Ross, A.; Moody, M.
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(4) Lowry, M. S.; Bernhard, S. Chem.Eur. J. 2006, 12, 7970.
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1976, 15, 1980.
31P NMR (dichloromethane, 101 MHz): 25.7 (s, 1P), −39.3 (s,
2P).
1H NMR (CD2Cl2, 250 MHz): 1.26 (s, 3H), 2.54 (d, JPH = 11 Hz,
2H), 2.96 (m, 2H), 3.92 (m, 2H), 7.29 (m, 29H), 7.75 (m, 2H), 7.96
(m, 2H), 8.6 (m, 2H).
(6) Talebinasab-Sarvari, M.; Zanella, A. W.; Ford, P. C. Inorg. Chem.
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(7) Brookes, P. R.; Masters, C.; Shaw, B. L. J. Chem. Soc. A 1971,
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Quantum Yield. A quartz UV/vis cuvette was charged with a
magnetic stir bar and 2.0 to 2.5 mL of a 1 M pyridine solution in
dichloromethane with 4.0 mg (4.3 μmol) of Ir(triphos)Cl3 (greater
than 99% absorbance at 380 nm). The sample was photolyzed with the
380 nm LED at a distance of 1.2 cm from the cuvette (flux = 2.84 ×
1015 photons/sec) with periodic 31P NMR spectroscopic monitoring.
Two measurements gave quantum yields of 0.23 and 0.29 (0.25
0.04).
DFT Calculations. Gaussian 0943 with the M0644 (X = Cl) or
B3LYP45 (X = Br) functional was used for all calculations (gas phase),
except for the TDDFT calculation, which employed the CAM-
B3LYP46 functional. The LANL2DZ47,48 basis set was employed for all
atoms. Initial structures were derived from crystal coordinates and
were modified with Gaussview49 without simplification. All geometries
were optimized without symmetry constraints. Analytical frequency
calculations gave no imaginary frequencies for the complexes except
for Ir(triphos)Cl3 (1), which had a small imaginary frequency of −21
cm−1 associated with a phenyl ring rotation and for the transition state
for the interconversion of 3T6 and 3T5. The redundant coordinate
potential energy scan for 1T6 employed the “ModRedundant” keyword
and increased the Ir−P distance in 0.1 Å increments with a maximum
of 10 refinement cycles between increments. Free energies, enthalpies,
and entropies were calculated at 298.15 K and 1 atm. The validity of
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