Highly luminescent dinuclear platinum(II) complexes incorporating bis-cyclometallating pyrazine-based ligands: A versatile approach to efficient red phosphors

Article abstract of DOI:10.1021/ic401131x

A series of luminescent dinuclear platinum(II) complexes incorporating diphenylpyrazine-based bridging ligands (LⁿH₂) has been prepared. Both 2,5-diphenylpyrazine (L²H₂) and 2,3-diphenylpyrazine (L³H₂) are able to undergo cyclometalation of the two phenyl rings, with each metal ion binding to the two nitrogen atoms of the central heterocycle, giving, after treatment with the anion of dipivaloyl methane (dpm), complexes of formula {Pt(dpm)} ₂Lⁿ. These compounds are isomers of the analogous complex of 4,6-diphenylpyrimidine (L¹H₂). Related complexes of dibenzo(f,h)quinoxaline (L⁴H₂), 2,3-diphenyl-quinoxaline (L⁵H₂), and dibenzo[3,2-a:2′,3′-c]phenazine (L⁶H₂) have also been prepared, allowing the effects of strapping together the phenyl rings (L⁴H₂ and L ⁶H₂) and/or extension of the conjugation from pyrazine to quinoxaline (L⁵H₂ and L⁶H₂) to be investigated. In all cases, the corresponding mononuclear complexes, Pt(dpm)LⁿH, have been isolated too. All 12 complexes are phosphorescent in solution at ambient temperature. Emission spectra of the dinuclear complexes are consistently red shifted compared to their mononuclear analogues, as are the lowest energy absorption bands. Electrochemical data and TD-DFT calculations suggest that this effect arises primarily from stabilization of the LUMO. Introduction of the second metal ion also has the effect of substantially increasing the molar absorptivity and, in most cases, the radiative rate constants. Meanwhile, extension of conjugation in the heterocycle of L⁵H₂ and L⁶H₂ and planarization of the aromatic system favored by interannular bond formation in L ⁴H₂ and L⁶H₂ leads to further red shifts of the absorption and emission spectra to wavelengths that are unusually long for cyclometalated platinum(II) complexes. The results may offer a versatile design strategy for tuning and optimizing the optical properties of d-block metal complexes for contemporary applications.

Full text of DOI:10.1021/ic401131x

Article
pubs.acs.org/IC
Highly Luminescent Dinuclear Platinum(II) Complexes Incorporating
Bis-Cyclometallating Pyrazine-Based Ligands: A Versatile Approach
to Efficient Red Phosphors
†
†
‡
†
,‡
̈
Stacey Culham, Pierre-Henri Lanoe, Victoria L. Whittle, Marcus C. Durrant, J. A. Gareth Williams,*
,†
and Valery N. Kozhevnikov*
†Department of Applied Sciences, Northumbria University, Newcastle upon Tyne, NE1 8ST, United Kingdom
‡Department of Chemistry, Durham University, Durham, DH1 3LE, United Kingdom
*
S Supporting Information
ABSTRACT: A series of luminescent dinuclear platinum(II)
complexes incorporating diphenylpyrazine-based bridging
n
ligands (L H ) has been prepared. Both 2,5-diphenylpyrazine
2
2
3
(
L H ) and 2,3-diphenylpyrazine (L H ) are able to undergo
2
2
cyclometalation of the two phenyl rings, with each metal ion
binding to the two nitrogen atoms of the central heterocycle,
giving, after treatment with the anion of dipivaloyl methane
n
(
dpm), complexes of formula {Pt(dpm)} L . These com-
2
pounds are isomers of the analogous complex of 4,6-
1
diphenylpyrimidine (L H ). Related complexes of dibenzo-
2
4
5
(
f,h)quinoxaline (L H ), 2,3-diphenyl-quinoxaline (L H ), and
2
2
6
dibenzo[3,2-a:2′,3′-c]phenazine (L H ) have also been pre-
2
pared, allowing the effects of strapping together the phenyl
4
6
5
6
rings (L H and L H ) and/or extension of the conjugation from pyrazine to quinoxaline (L H and L H ) to be investigated. In
2
2
2
2
n
all cases, the corresponding mononuclear complexes, Pt(dpm)L H, have been isolated too. All 12 complexes are phosphorescent
in solution at ambient temperature. Emission spectra of the dinuclear complexes are consistently red shifted compared to their
mononuclear analogues, as are the lowest energy absorption bands. Electrochemical data and TD-DFT calculations suggest that
this effect arises primarily from stabilization of the LUMO. Introduction of the second metal ion also has the effect of
substantially increasing the molar absorptivity and, in most cases, the radiative rate constants. Meanwhile, extension of
conjugation in the heterocycle of L H and L H and planarization of the aromatic system favored by interannular bond
formation in L H and L H leads to further red shifts of the absorption and emission spectra to wavelengths that are unusually
5
6
2
2
4
6
2
2
long for cyclometalated platinum(II) complexes. The results may offer a versatile design strategy for tuning and optimizing the
optical properties of d-block metal complexes for contemporary applications.
INTRODUCTION
of light from this region of the solar spectrum is required if
efficiencies are to be significantly improved.
One of the most studied classes of phosphorescent materials
is Ru(II) complexes of polypyridines, since they often emit in
■
7
Phosphorescent red and near-infrared emitters are desired for a
variety of applications. For example, in the field of phosphors
for organic light-emitting diodes (OLEDs) and light-emitting
electrochemical cells (LECs), red emitters are required for full-
color displays and white-light systems destined for lighting
the red region, albeit with modest quantum yields of a few
percent, from low-energy 3MLCT transition states. More
8
recently, cyclometalated complexes of the phenylpyridine type
have become increasingly popular, particularly with third-row
1
−3
applications.
Yet the currently available red-emitting
9
,10
11,12
compounds are not able to match the high performance of
metals such as iridium(III)
and platinum(II).
The
4
−
green emitters. Meanwhile, in the field of luminescent sensors
combination of (i) a strong σ donor in the C -ligating unit
that raises the energy of metal-centered orbitals, thus stabilizing
the MLCT states, (ii) a strong ligand field that destabilizes
potentially deactivating d−d states, and (iii) efficient spin−orbit
coupling (SOC) induced by the heavy metal ion to promote
5
for bioimaging and sensing there is a need for compounds that
not only emit in the red or near-infrared (NIR) regions but also
absorb at long wavelengths in order to benefit from the window
of greater optical transparency of biological tissue and
decreased scattering, which favors delivery and collection of
13
radiative decay of the triplet state renders many such
6
light. There is a similar pressing need for red- and NIR-
absorbing materials in development of new dyes for dye-
Received: May 6, 2013
sensitized solar cells (DSSCs), where more efficient collection
©
XXXX American Chemical Society
A
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n
n
n
Figure 1. Structures of the proligands, L H , mononuclear platinum(II) complexes, PtL H, and dinuclear platinum(II) complexes, Pt L .
2
2
−
3
complexes very intensely luminescent. Moreover, the emission
energy can be quite easily tuned over a wide range by
systematic modifications to the ligand, owing to the HOMO
and LUMO being localized on mutually different parts of the
emitters, albeit with low quantum yields on the order of 10 or
20
less. We have been interested in the design of new
luminescent materials based on polynuclear assemblies with
cyclometalated metal centers. Recently, we reported a versatile
strategy to access efficient red emitters based on polynuclear
Pt(II) and heterometallic Pt(II)/Ir(III) complexes in which
4,6-di(4-tert-butyl-phenyl)pyrimidine was used as the bridging
ligand. The general observation was that introduction of a
second cyclometalated metal center is accompanied by a
significant red shift in absorption and emission, mainly due to
stabilization of the LUMO level. Motivated by these results, the
present study seeks to extend the methodology to other
systems based on related pyrazine rather pyrimidine bridging
ligands. Synthesis of five dinuclear platinum(II) complexes
based on diphenylpyrazine-based bridging ligands is described,
together with their mononuclear analogues. The approach leads
to efficient deep red emission, and the influence of isomerism
and extended conjugation/rigidification in the bridging ligand is
explored.
1
4
molecule. In order to shift to the red region of the spectrum,
the usual approaches are (i) to lower the LUMO energy
through the use of a more electron-deficient heterocycle such as
21
4
b,c,e,15
an isoquinoline,
using a more electron-rich aromatic unit such as a
(ii) to raise the energy of the HOMO
1
6
benzothiophene, and (iii) to extend the conjugation within
17
the ligand such that the HOMO−LUMO gap decreases. The
problem with these approaches, particularly ii and iii, is that as
the ligand-based filled orbitals rise in energy, the amount of
metal character in the excited state concomitantly decreases, so
that one can expect less efficient SOC, smaller radiative decay
constants, and even fluorescence from the ligands due to
18
retarded intersystem crossing. For example, introduction of
additional pendent thiophene groups into thienylpyridine
complexes of iridium and platinum leads to a switch from
efficient phosphorescence to fluorescence accompanied by only
19
RESULTS AND DISCUSSION
very weak, albeit red shifted, phosphorescence.
■
An emerging alternative approach to achieve red-shifted
emission is to introduce two or more metal centers into one
molecule by means of a bridging ligand. For example, Zhong
1. Synthesis. The family of mono- and dinuclear complexes
investigated is shown in Figure 1, together with the
corresponding proligands. PtL H and Pt L are derived from
1
1
2
and Abrun
̃
a recently demonstrated that combination of Pt(II)
4,6-diphenylpyrimidine and are essentially the same as the
complexes we reported previously, except that the phenyl
rings do not incorporate tert-butyl substituents, and dipivaloyl-
21
and Ru(II) metal centers, linked through a dipyridyldiphe-
nylpyrazine bridging ligand, can provide deep red and NIR
B
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a
Table 1. UV−Vis Absorption Data and Ground-State Reduction Potentials for the Complexes
complex
λmax/nm (ε/M−1cm−1
)
E0red/V [ΔE/mV]
1
PtL H
256 (25 200), 301 (31600), 338sh (10 600), 408 (9550)
−2.21 [160]
−1.83 [160]
−2.13 [180]
−1.79 [100]
−2.14 [150]
−1.81 [90]
−2.02 [150]
−1.70 [120]
−1.83 [100]
−1.46 [90]
−1.66 [90]
−1.43 [90]
1
Pt2L
257 (47 000), 326 (63 200), 430 (17 100), 481 (24 300), 532sh (1990)
2
PtL H
254 (26 800), 299 (25 400), 342 (17 300), 398 (6430), 435 (5240)
2
Pt2L
253 (35 300), 296 (26 500), 333 (21 900), 372 (16 600), 453 (15 900), 503 (6830), 618 (346)
252 (23 800), 292 (16 900), 340 (11 900), 398 (6170), 449 (2170)
3
PtL H
3
Pt2L
245 (36 700), 301 (20 300), 319 (20 700), 390 (19 600), 451 (15 200), 572 (310)
251 (66 000), 278 (42 100), 344 (18 900), 407 (11 800), 463 (3190)
4
PtL H
4
Pt2L
258 (59 300), 295 (25 900), 308 (25 700), 350 (25 800), 428 (10 100), 462 (19 400), 509 (11 500)
252 (40 000), 294 (20 000), 373sh (9380), 388 (10 600), 445 (6470), 488sh (3790)
251 (42 000), 313 (21 600), 387 (14 200), 428 (8610), 496 (1360)
5
PtL H
5
Pt2L
6
PtL H
251 (52 300), 282 (38 000), 383sh (7870), 401 (10 300), 463 (7790), 511 (4520)
Pt2L6
266 (29 000), 301sh (13 600), 324sh (11 500), 396 (3070), 422 (3890), 476sh (3350), 519 (7780), 569 (6690)
a
In CH Cl at 298 ± 3 K. Reduction potentials obtained by cyclic voltammetry using Bu NPF (0.1 M) as the supporting electrolyte. Potentials are
2
2
4
6
+
given relative to the Fc|Fc couple measured under the same conditions. Peak-to-peak separation is given in parentheses.
methane (dpm) is used as the ancillary ligand in place of
acetylacetonate (acac). The change from acac to dpm allows
the complexes to retain good solubility without the need for
tert-butyl groups in the cyclometallating ligands, facilitating
comparison with the new compounds. All other complexes are
based on a diphenylpyrazine core, cyclometalated either once
the dinuclear complex was the main product, the corresponding
monoplatinum complex PtL H was still present in the mixture.
n
In general, it proved to be most convenient to prepare the two
n
complexes simultaneously by reaction of the proligand L H
2
with 1.5 equiv of K PtCl , targeting a statistical 1:1 mixture.
2 4
After introduction of the dpm ancillary ligand under conven-
tional conditions, the mono- and dinuclear platinum complexes
are readily separated by column chromatography (typically on
silica gel eluting with dichloromethane) to give both products
2
6
(
mononuclear series, PtL H−PtL H) or twice (dinuclear series,
2 6 4 5
Pt L −Pt L ). L H is dibenzo(f,h)quinoxaline, L H is 2,3-
2
2
2
2
6
diphenylquinoxaline, and L H is dibenzo[3,2-a:2′,3′-c]-
2
1
2
1
phenazine. Note that the compounds based on L H , L H ,
in one procedure. All products were characterized by H NMR
2
2
3
and L H2 are isomers of one another, so that a direct
spectroscopy, mass spectrometry, and elemental analysis.
2. Electrochemistry. The electrochemical behavior of the
complexes was examined by cyclic voltammetry in dichloro-
comparison can be made of the influence of (i) changing from
1
2
3
pyrimidine to pyrazine (L H versus L H /L H ) and (ii) the
2
2
2
methane solution using Bu NPF as the supporting electrolyte
substitution pattern in the pyrazine ring (2,5-substitution in
4
6
2
3
and the ferrocene/ferrocenium couple as the reference. All 12
complexes display one reversible or quasi-reversible reduction
wave in the range from −2.2 to −1.4 V versus ferrocene; the
reduction potentials are given in Table 1. Four trends emerge
upon inspection of the data.
L H versus 2,3-substitution in L H ). Meanwhile, it may be
2
2
3
6
seen that the complexes of ligands L H −L H form a series of
2
2
closely related complexes in which either a C−C bond is
4
introduced between the two pendent phenyl rings (L H versus
2
3
5
L H ) or the pyrazine is replaced by a quinoxaline (L H versus
2
2
3
6
n
L H ), while complexes of L H incorporate both these
(i) For any given ligand L , the reduction potential of the
2
2
2
3
n
features. It may also be noted that ligands L H and L H are
dinuclear complex Pt L is significantly less negative than
2
2
2
n
related to 2,5-bis(2-pyridyl)pyrazine and 2,3-bis(2-pyridyl)-
pyrazine, respectively. These ligands, which can act as bridging
analogues of 2,2′-bipyridine, have been previously explored by
others for preparation of multinuclear ruthenium(II) and
osmium(II) complexes in a strategy somewhat similar to that
for the corresponding mononuclear complex PtL H by
around 200−400 mV.
1
3
(
ii) Among the three isomeric complexes PtL H−PtL H,
there is little difference between the reduction potentials,
all being within 80 mV. Similarly, the three dinuclear
complexes of these ligands have reduction potentials that
are within 40 mV of one other. Apparently, then, the
change from pyrimidine to pyrazine has rather little effect
on the reduction, as does the substitution pattern within
the pyrazine.
2
2
used here.
,3-Diphenylpyrazine (L H ) and 2,3-diphenylquinoxaline
3
2
2
5
(
L H ) are commercially available, while the other proligands
2
were prepared by following established procedures (details and
references are given in the Experimental Section). The
∧
monometallic Pt(II) complexes, Pt(N C-LH)(dpm) (which
(
iii) Strapping together the two phenyl rings in the 2,3-
n
will be henceforth be abbreviated as PtL H), were obtained by
4
3
substituted pyrazine system (PtL H versus PtL H, and
heating at reflux a mixture of the proligand and 1 equiv of
4
3
Pt L versus Pt L ) facilitates reduction by around 100
2
2
K PtCl in acetic acid for up to 3 days. The resulting dichloro-
2
4
mV, with a qualitatively similar effect in the quinoxaline
bridged dinuclear intermediates were then reacted with
dipivaloylmethane in the presence of base to give, after
purification by column chromatography, the monoplatinum
6
5
analogues (L versus L ).
(
iv) The change from pyrazine to quinoxaline (complexes of
5
3
6
4
n
L versus L and of L versus L ) is accompanied by a
more significant anodic shift of the reduction on the
order of 300 mV.
complexes PtL H in low to moderate yield. Formation of a
small amount of the corresponding diplatinum complexes was
registered in each case.
The dinuclear platinum(II) complexes (which will be
Trends i, iii, and iv are consistent with the intuitively
n
referred to as Pt L ) were synthesized either by reaction of
reasonable notion that the LUMO in such complexes (which
would be expected to be essentially ligand based, according to
previous experimental and theoretical work on cyclometalated
2
n
n
PtL H with 1 equiv of K PtCl or by reacting L H with 2 equiv
2
4
2
of K PtCl . Under these conditions, it was found that although
2
4
C
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1
1,12
Pt(II) complexes
) will be stabilized by the extension of
conjugation that accompanies (i) introduction of the second
metal ion, (ii) incorporation of the interannular bond which
facilitates a planar conformation, and (iii) change from pyrazine
to quinoxaline.
As is typically found for cyclometalated platinum(II)
complexes, the first oxidation process observed for each of
the complexes studied was irreversible and, in several cases,
rather ill defined. Indeed, we found an unsatisfactory level of
variation in the peak potential according to the conditions, not
only of the scan rate but even from one scan to the next,
despite extensive cleaning of the electrode surface between
scans. However, the data showed unequivocally that there is
significantly less variation in the oxidation potentials with
molecular structure than in the reduction potentials: the values
+
were generally around 0.8 V versus Fc|Fc , a value quite typical
for cyclometalated platinum complexes with arylpyridine
ligands.
Figure 3. UV−vis absorption spectra of PtL3−6H (black, blue, green,
3. Absorption Spectroscopy. Absorption data for all 12
3
−6
complexes is compiled in Table 1, and spectra are shown in
Figures 2 and 3. In order to facilitate the discussion, we shall
and red solid lines, respectively) and Pt
2
L
(correspondingly colored
dashed lines) in CH Cl solution at 298 K.
2 2
bands can be attributed to charge-transfer transitions from the
aryl−metal unit to the heterocyclic ring, reflecting the
predominant localization of the frontier orbitals on these two
different parts of the molecule.
The dinuclear complexes Pt2L1 display very different
−3
spectra from their corresponding mononuclear analogues
1
−3
1
PtL H (Figure 2, Table 1). For complexes of L H and
2
2
L H , the set of lower energy bands in the visible region is
2
substantially red shifted in the dinuclear compared to the
mononuclear complexes (lowest energy band shifts by around
−1
4
000 cm ) and there is a large increase in the molar
absorptivity of most of the bands. The visible-region bands in
3
Pt L , on the other hand, are scarcely shifted compared to
2
3
PtL H, although there is again a very large increase in the molar
1
absorptivities. The red shift in the dinuclear complexes of L
2
and L is likely to be associated with the planarization and
extension of conjugation that would be expected to accompany
3
introduction of the second metal ion. In the case of Pt L , on
2
the other hand, it seems likely that attainment of a fully planar
structure will be inhibited by steric crowding of the C−H bonds
of the pendent phenyl groups, which would account for the lack
of red shift in that case.
Figure 2. UV−vis absorption spectra of PtL1−3H (blue, red, and black
1
−3
solid lines, respectively) and Pt L (blue, red, and black short-dashed
2
lines, respectively) in CH Cl solution at 298 K. Low-energy region of
2
2
2
3
the spectra of Pt L and Pt L is shown on an arbitrarily expanded
2
2
A further feature of the dinuclear complexes is the
appearance of a very weak transition on the low-energy side
scale (red and black long-dashed lines, respectively).
1
3
of the main bands. In Pt L and Pt L , it appears as a shoulder
2
2
2
first consider the behavior of the two sets of three isomeric
(λ = 532 and 572 nm respectively) but for Pt L as a well-
2
complexes, PtL1 H and Pt L , and then examine how
−3
1−3
defined narrow band (λmax = 618 nm) (shown on an expanded
scale in Figure 2). It is likely that these bands are due to direct
excitation to the triplet state (S → T ), a formally forbidden
2
3
elaboration of the L H ligand influences the properties on
2
4
6
moving to complexes of ligands L H −L H . The mononuclear
2
2
0
1
complexes PtL1 H display absorption spectra that are quite
−3
process that is facilitated by the spin−orbit coupling associated
∧
23
typical of platinum(II) complexes with N C-cyclometallating,
with the presence of the heavy metal ions.
4
−6
aryl−heterocycle ligands, such as Pt(ppy)(acac) and related
Absorption spectra of all other complexes, PtL H and
1
1,12
4−6
3
derivatives.
There are intense bands in the UV region, <350
Pt2L , are shown in Figure 3, together with those of PtL H
3
nm, with molar absorptivities ε in the range 10 000−30 000
and Pt L again as a point of comparison. Considering first the
2
−
1
−1
1
M
cm , corresponding to π−π* transitions associated with
mononuclear complexes, it can be seen that all four display
spectra that are quite similar to one another in form, but the
visible-region bands move progressively further to the red. The
the ligands (Figure 2, Table 1). Somewhat weaker bands, ε <
1
−1
1
0 000 M− cm , in the visible region (390−500 nm) have no
3
4
counterparts in the free proligands. On the basis of the
displacement is small on going from PtL H to PtL H (∼700
12
−1
conclusions of many previous studies and on the results of
time-dependent density functional theory calculations for the
present series of complexes (vide infra), at least some of these
cm ), but nevertheless, the trend would be consistent with the
planarization of the structure that becomes possible upon
formation of the interannular C−C bond and loss of the
D
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sterically unfavorable interaction between the C−H bonds on
the two rings mentioned above. A much larger red shift
3
accompanies the change from the pyrazine unit of PtL H to the
5
−1
quinoxaline of PtL H (around 1800 cm for the lowest energy
−1
band and around 2600 cm for the next lowest). This trend is
consistent with the trend in reduction potentials, which
suggests a significant stabilization of the lowest unoccupied
molecular orbital on going from the pyrazine to the quinoxaline
and hence, if the filled orbitals are not substantially lowered in
energy, an expected red shift in the lowest energy absorption
band. Indeed, the energy of the lowest energy absorption
maxima for the mononuclear complexes correlates well with the
reduction potentials (Figure 4, top, data points represented by
6
6
blue circles). Notably, PtL H and Pt L , which feature both the
2
interannular bond and the quinoxaline unit, display the lowest
energy absorption bands among the two series of complexes
(
mononuclear and binuclear, respectively), indicating a
qualitatively additive effect.
Again, it is clear that introduction of the second metal ion
leads to a red shift in the visible-region bands and to a large
increase in the molar absorptivity of these bands. The
correlation between the energy of the absorption maxima and
the reduction potentials for the set of complexes as a whole,
including the binuclear complexes, remains quite clear, albeit
with rather more scatter than for the series of mononuclear
complexes alone (Figure 4, top, all data points).
4. Photoluminescence Properties. All 12 complexes are
luminescent in degassed solution at room temperature, most of
them intensely so. Emission data for the compounds are
compiled in Table 2, and spectra are shown in Figures 5 and 6.
The trends in emission maxima are mostly quite similar to
those observed in the absorption spectra. In particular, it may
be immediately seen that, for a given ligand, the spectrum of
n
the binuclear complex Pt2L is significantly red shifted
compared to that of the corresponding mononuclear complex
n
−1
PtL H. The shift is in the range 1600−1900 cm for the five
pairs of pyrazine-based complexes (based on the emission
maxima) and a little less, 1270 cm , for the pyrimidine pair
PtL H and Pt L ). Overall, there is again a quite strong
−1
1
1
(
2
correlation between the emission energy and the reduction
potential of the complexes (Figure 4, bottom), suggesting that
the structural variations among the complexes predominantly
influence the LUMO. The gradient of the best-fit line is very
similar to that in the plot for absorption, indicating that the
emissive triplet state has similar orbital parentage to the lowest
energy singlet state formed upon absorption.
Figure 4. (Top) Plot of the energy of the lowest energy absorption
maximum in CH Cl2 at 298 K (derived from λmax) versus the
2
+
reduction potential (values versus Fc|Fc ) obtained by cyclic
voltammetry. Data for the mononuclear complexes are shown as
blue circles and for the binuclear complexes as red squares. Dashed
line represents the best fit of the data for all 12 complexes; gradient =
Among the three isomeric complexes PtL1−3H, the
1
pyrimidine complex PtL H emits at higher energy (λ
=
−1
max
6410 cm /V. (Bottom) Corresponding plot for the emission
5
21 mm) while the two pyrazine complexes emit in a similar
maximum under the same conditions; gradient of best-fit line =
2
−
1
region to one another, with PtL H displaying a more structured
6460 cm /V.
profile. Meanwhile, the trend among the four complexes
PtL3 H is similar to that in absorption, except that PtL H
−6
5
6
emits at marginally lower energy than PtL H, the opposite of
Two such instances of a reversal in the relative order of
the trend in absorption, and its spectrum is markedly broader. If
we consider the difference in energy between the lowest energy
absorption maximum and the emission band (which we will
emission energies versus absorption energies are observed
3
among the binuclear complexes. Thus, Pt L emits at
2
1
significantly lower energy than Pt L , contrary to the absorption
2
A−E
refer to as ΔE ; note that it is not strictly a Stokes’ shift, since
being significantly lower in energy for the latter. Indeed, we
3
the absorption is to the singlet state, whereas the emission
occurs from a triplet state), it can be seen that this value is
noted in section 3 that Pt L seemed somewhat anomalous in
2
that its lowest energy absorption band was not significantly
shifted compared to that of its mononuclear analogue, and we
tentatively attributed this to a lack of planarity in the ground
state. This complex may undergo a more profound change in
geometry toward planarity prior to emission to lead to a lower
5
largest for PtL H among the mononuclear complexes and
6
particularly compared to PtL H, suggesting that there is a more
profound excited-state reorganization in this complex prior to
emission. We return to this point below.
E
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Inorganic Chemistry
Article
Table 2. Emission Data for the Complexes in Degassed CH Cl at 298 K except Where Stated Otherwise
2
2
g
emission at 77 K
kQSQ /10 M
c
7
−1
k /10
d
4
Σknr /104
d
kQO2 /10 M
e
8
−1
ΔE
A−E
−1
f
/
r
a
b
−1
−1
−1
−1
complex
λmax/nm
521
Φlum
τ/ns
s
s
s
s
cm
λmax/nm
τ/μs
1
PtL H
0.25
0.36
0.30
0.41
0.43
0.37
0.15
0.20
0.14
1600 [460]
1100 [670]
8900 [840]
3200 [680]
7700 [850]
4000 [750]
2600 [510]
1800 [650]
3700 [700]
910 [490]
87
h
16
47
58
7.9
18
7.4
16
33
44
23
110
23
49
7.0
2.6
4.9
5.3
4.8
4.9
7.2
4.5
5.3
4.3
5.3
5320
2870
5290
3960
4220
5830
3990
3720
5520
6810
3920
3840
501, 527, 573sh
539, 582, 630
554, 603, 654
617, 676, 742
536, 576, 618
586, 634, 693
528, 566, 615
601, 654, 716
625, 677, 747sh
694, 760
6.4
4.3
11
1
Pt L
558, 594
565, 608
628, 685, 753
554, 578
612
33
2
2
PtL H
71
h
3.4
13
2
Pt L
3.4
11
2
3
PtL H
210
3.5
270
27
15
h
5.6
9.3
5.8
11
3
Pt L
6.1
11
2
4
PtL H
568
4
Pt L
628
7.0
5.7
3.5
1.3
1.5
2
5
PtL H
668
3.8
5
I
I
I
Pt L
749
0.025
0.13
2.7
2
6
PtL H
Pt2L6
639, 686
728, 799
3800 [700]
2000 [580]
180
h
3.4
617, 676, 746
712, 793
I
0.027
1.4
5.6
a
b
c
Measured using [Ru(bpy) ]Cl as the standard. Values in air-equilibrated solution in parentheses. Self-quenching rate constant, estimated from a
3
2
−1
d
plot of τ versus concentration. k and Σk are the radiative and nonradiative decay rate constants, estimated from the quantum yield and lifetime
assuming that the emissive state is formed with unitary efficiency. Bimolecular rate constant for quenching by O , estimated from the luminescence
lifetimes in degassed and air-equilibrated solutions and taking [O ] = 2.2 mM in CH Cl at p = 1 atm air and T = 298 K. Difference between the
r
nr
e
2
f
2
2
2
g
energy of the lowest energy (singlet) absorption band and the emission band using λ values. In diethyl ether/isopentane/ethanol (2:2:1 v/v).
max
h
The change in lifetime with concentration was too small to estimate a self-quenching rate constant, being scarcely larger than the uncertainty on the
I
measurements. Quantum yields of these two complexes will be underestimated owing to a significant part of the emission falling out of the range of
detection of our equipment. Radiative rate constants will be underestimated as a result.
energy triplet excited state from which emission occurs. The
Luminescence lifetimes are on the order of a few
microseconds in degassed solution, and for each pair, the
lifetime of the binuclear complex is shorter than that of its
5
6
second instance is the pair of complexes Pt L and Pt L , where
2
2
the emission of the former has a slightly longer λmax, despite
having an absorption maximum at an unequivocally higher
energy. Note that the order is switched round in the more rigid
medium of the glass at 77 K (compare Figure 6, top and
bottom). Excited-state reorganization at room temperature can
mononuclear analogue. By estimating the radiative, k , and
r
nonradiative, Σknr, decay rate constants from the quantum yield
and lifetime data (Table 2), it can be seen that introduction of
the second metal ion facilitates the radiative decay process by a
factor of around 2. Apparently, the emissive transition becomes
more allowed in the binuclear complex. Normally, for a given
type of excited state in structurally related complexes, a
5
3
be anticipated for Pt L similar to that proposed for Pt L given
2
2
the expected sterically unfavorable interannular interactions in
the ground state. In line with this explanation, it is notable that
A−E
decrease in k with decreasing energy is anticipated, since the
ΔE
is larger for these two complexes than for any others.
r
3
oscillator strength of a transition is dependent upon ν in the
At 77 K, the emission spectra show otherwise similar trends
as at room temperature but the spectra become more
vibrationally resolved. The observed vibrational spacing is
around 1200−1400 cm , as typically expected for coupling to
aromatic ring vibrations. In each case, there is a blue shift in the
emission maximum upon cooling, as invariably observed for
such cyclometalated complexes in which emission emanates
from excited states having appreciable charge-transfer character.
Complexes of 2,5-diphenylpyrazine display the smallest shifts
Einstein coefficient for spontaneous emission. Comparison with
diruthenium complexes of bis-pyridylpyrazine ligands is
revealing. For example, based on lifetime and quantum yield
−1
data reported by Denti et al., the k value for the dinuclear
r
4+
4
complex [{Ru(bpy) } (μ-2,3-dpp)] is approximately 2 × 10
2
2
−1
4
−1
s , compared to a larger value of 6 × 10 s for the
2+
mononuclear analogue [Ru(bpy) (2,3-dpp)] {2,3-dpp = 2,3-
2
22c
bis(2-pyridyl)pyrazine}.
In that case, the anticipated
reduction is observed and the quantum yield for the dinuclear
complex is lower.
−1
between 77 K and room temperature (around 300 cm ), while
4
5
PtL H and complexes of L H have the largest shifts, a little in
2
It may noted that the molar absorptivities in the present set
of Pt(II) complexes are also increased by a comparable factor of
around 2-fold in the bimetallic compared to monometallic
compounds (Table 1 and Figures 2 and 3), possibly suggesting
a similar effect on the oscillator strength of the S → S
−1
excess of 1000 cm . In the latter case, at least, this may be
indicative of a greater degree of excited-state distortion at room
temperature, as noted above.
The room-temperature photoluminescence quantum yields,
0
1
Φ
lum, are high for many of the complexes (Table 2) falling off
transition to the T → S . However, such an increase might be
1
0
to <0.1 only for the most red-emitting complexes, where much
of the emission lies in the near-infrared. Binuclear complexes
anticipated simply on the basis that there are now two metallic
units contributing to the absorption bands, as observed in the
2
3
2
2
such as Pt L and Pt L are particularly striking in that they
2
2
diruthenium(II) systems. Thus, it is possible that the
have quantum yields of around 0.40 at a λmax that matches well
with that of the standard red emitters in RGB displays (e.g.,
observed increase in k in the dimetallic complexes may reflect
r
the higher degree of spin−orbit coupling expected from the
presence of the second heavy metal ion, promoting radiative
decay of the triplet state. Nevertheless, facilitation of the
transition by other mechanisms, for example, a change in
symmetry to a less centrosymmetric environment at the metal,
cannot be ruled out.
3+
Eu phosphors emit at around 617 nm). Indeed, these
complexes are brighter than many of the widely investigated
iridium(III) complexes of phenylisoquinoline and benzothie-
4
a,b,e
nylpyridine ligands,
which emit in a similar region, and are
4c,d
competitive with quinoxaline-based iridium(III) complexes.
F
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Inorganic Chemistry
Article
Figure 5. (Top) Normalized emission spectra of PtL1−3H (blue, red,
and black solid lines, respectively, legend as in Figure 2) and Pt2L1
−3
(
blue, red, and black short-dashed lines, respectively) in CH Cl2
Figure 6. (Top) Normalized emission spectra of Pt3−6H (black, blue,
green, and red solid lines, respectively) and Pt2L (correspondingly
2
solution at 298 K. (Bottom) Corresponding spectra in an EPA glass
at 77 K (EPA = diethyl ether/isopentane/ethanol, 2:2:1 v/v).
3−6
colored dashed lines) in CH Cl2 solution at 298 K. (Bottom)
2
Corresponding spectra in an EPA glass at 77 K.
5
. Time-Dependent Density Functional Theory (TD-
In addition to spectroscopic analysis, DFT calculations
provide a measure of the relative ground-state stabilities of the
mononuclear and dinuclear complexes of a given ligand. For
each ligand, the energy changes have been calculated for the
hypothetical reactions 1 and 2, generating the mono- and
dinuclear complexes, respectively
DFT) Calculations. Density functional theory calculations
have been carried out on all of the compounds shown in Figure
, but with the dpm ligands truncated to acac in order to
facilitate the calculations. It is well established that changing
from dpm to acac in Pt(N C)(O O) complexes has very little
influence on the photophysical properties, and indeed, we
confirmed this experimentally in the case of PtL H and Pt L ,
where the corresponding acac derivatives have also been
studied. The advantage of the dpm ligand is in enhancing
solubility. All calculations were performed with Gaussian09W
using the B3LYP functional and 6-31+G(d) basis set for all
atoms except Pt, for which the LanL2DZ basis set was
employed. After full geometry optimization, visible spectra
were calculated by TD-DFT both in vacuum and in
dichloromethane solution using the polarized continuum
1
∧
∧
12b
n
n
1
1
L H + Pt(acac) → Pt(acac)L H + acacH
(1)
(2)
2
2
2
n
n
Pt(acac)L H + Pt(acac) → {Pt(acac)} L + acacH
2
1
2
2
24
The energy changes (ΔE) associated with these reactions are
shown in Table 3. The first notable feature is that introduction
of the second Pt center is always more favorable thermody-
namically than the first, indicating a synergy in the binding. The
trends in complex stabilities for the different ligands can be
understood in terms of steric effects: the least sterically
25
2
6
27
1
2
model for the solvent.
hindered ligands L H2 and L H2 give the most stable
G
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Inorganic Chemistry
Article
Table 3. Calculated Gas-Phase Energy Changes for
Reactions 1 and 2 Generating the Mono- or Dinuclear
Complex
by 0.12 Å above and below the mean plane of the ring,
respectively, and the dihedral angle between the two
coordinated phenyl rings is 56°. The calculated structure of
3
Pt L shows slightly smaller distortions: the corresponding
cpecies Ln,
n
n
2
reaction 1 → Pt(acac)L H,
reaction 2 →{Pt(acac)} L ,
2
−1
ΔE/kJ mol−
1
carbons in the pyrazine ring are displaced by 0.09 Å out of the
mean plane, and the dihedral angle between the two phenyl
rings is 43°. This is very similar to the distortions observed in
n =
ΔE/kJ mol
1
2
3
4
5
6
−13.4
−10.3
−4.2
−6.5
+25.8
−42.0
−46.7
−32.8
−33.0
−12.4
−23.6
the X-ray crystal structure of the analogous palladium complex
3
[
{Pd(acac)} L ], for which the equivalent carbons lie 0.15 Å
2
out of the pyrazine mean plane and the phenyl rings subtend an
28
angle of 46°.
+14.9
For all complexes, the calculations reveal that the lowest
4
6
energy singlet transition (S → S ) is primarily HOMO →
LUMO in nature in each case (Supporting Information). The
0 1
complexes, and the fused ring systems L H and L H are a
2
2
3
5
little better than their ortho-diphenyl analogues L H and L H ,
where the complexes show twisting of the ligands to avoid close
contacts in the coordinated hydrocarbon rings. Complexes of
2
2
frontier orbitals for the mono- and dinuclear complexes of
2
L H , which are shown in Figure 8, are representative of all six
2
5
6
L H and L H are the least stable among the two series, which
2
2
can be explained by the close contacts between the O atoms of
the acac ligands and the CH groups of the extra ring (closest
5
6
CH···O contacts are 1.96−2.04 Å for L H and L H , compared
2
2
3
4
to 2.35−2.58 Å for L H and L H ). This is illustrated for
{Pt(acac)} L ] in Figure 7. This steric congestion is also
2
2
5
[
2
2
Figure 8. Frontier orbital diagrams for PtL H (left) and Pt L (right).
2
2
pairs of complexes; frontier orbital plots for all the other
complexes are shown in the Supporting Information for
reference. For all 12 complexes, the HOMOs are derived
mainly from combinations of platinum d orbitals with π orbitals
of the surrounding ligands, with only small contributions from
the pyrazine and pyrimidine rings, whereas these heterocyclic
rings are much more important components of the LUMOs.
The frontier orbitals of the dinuclear complexes are all
symmetrically distributed over both halves of the molecules,
while those of the mononuclear complexes are all qualitatively
similar to their dinuclear counterparts but of course limited to
the one available metal center; this affects the composition of
the occupied frontier orbitals more than the virtual orbitals.
Nevertheless, the energies of the HOMOs for all 12 complexes
are very similar, varying by only 0.077 eV, whereas the energies
of the LUMOs vary by 0.784 eV. This is consistent with the
conclusions from the experimental electrochemical measure-
ments described in section 2, which showed little variation in
the oxidation potential between the complexes but a greater
range in the reduction potential. Moreover, on going from
5
Figure 7. Calculated structure of Pt L showing the steric distortion
2
centered on the pyrazine ring. Dotted lines show the close contacts
between the C−H groups and O atoms; distance = 2.00 Å. Atom
colors: C, gray; N, blue; O, red; Pt, magenta.
reflected in the calculated Pt−N bond lengths, which are
5
6
2
.064−2.088 Å for complexes of L H and L H , significantly
2
2
longer compared to the range of 2.009−2.027 Å for complexes
of the other four ligands; similarly, the Pt−O bonds trans to the
5
metalated carbon are 2.175−2.179 Å for complexes of L H and
2
6
L H , compared to 2.142−2.160 Å for the other complexes.
2
The calculated Pt−C bond lengths for all 12 complexes are in
the range of 1.970−2.003 Å, while the Pt−O bonds trans to the
nitrogen are in the range 2.031−2.043 Å. All of these values are
comparable to those obtained in our previous calculations and
for experimentally determined structures of related mono-
n
n
PtL H to Pt L , incorporation of the second metal lowers the
2
energy of the LUMO by around 0.3 eV within each pair of
complexes (the change is in the range 0.237−0.373 eV; see
Supporting Information). This observation also correlates well
with the electrochemical data, which showed a less negative
reduction potential (by around 0.3 eV) for each dinuclear
complex compared to its mononuclear analogue (Table 1; the
range is 0.23−0.38 eV).
There is a good correlation between the energy of the
LUMO and the energy of the lowest energy singlet transition in
the calculated spectra. Calculated absorption spectra are shown
in the Supporting Information and are in generally good
21
nuclear complexes.
As expected, the proligands whose rings are linked by
rotatable C−C bonds are nonplanar: the dihedral angles
between the heterocyclic and phenyl rings vary from 19° for
L H to 42° for L H and L H . In the mononuclear complexes
of L H and L H and all complexes of the other ligands, the
coordinated rings are essentially coplanar; however, some
distortion is observed for the dinuclear complexes of L H and
L H . This is greatest for Pt L , as shown in Figure 7; the
pyrazine ring is twisted, such that carbons 1 and 2 are displaced
1
3
5
2
2
2
3
5
2
2
3
2
5
5
2
2
H
dx.doi.org/10.1021/ic401131x | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
agreement with the experimentally measured spectra. Calcu-
on a Thermo Scientific LTQ Orbitrap XL Mass Spectrometer using
low-resolution electrospray (ESI) or high-resolution nanoelectrospray
(NSI) ionization techniques. Elemental analyses were carried out using
a Carlo Erba 1108 Elemental Analyzer controlled with CE Eager 200
software, run in accordance with the manufacturer’s instructions and
samples weighed using a certified Mettler MX5Microbalance.
1
2
lations including the triplet excited states for Pt L , Pt L , and
2
2
3
Pt L in dichloromethane placed the formally spin-forbidden S
2
0
→
T transitions at 538, 622, and 591 nm, respectively, in good
1
agreement with the experimental phosphoresence data
discussed in section 4.
3
5
Proligands L H and L H were purchased from Aldrich and used
2
2
4
6
without additional purification, while L H and L H were prepared
following literature procedures.
2
2
2
9,30
CONCLUDING DISCUSSION
■
Photophysical Measurements. Absorption spectra were meas-
ured on a Biotek Instruments XS spectrometer using quartz cuvettes of
1 cm path length. Steady-state luminescence spectra were measured
using a Jobin Yvon FluoroMax-2 spectrofluorimeter, fitted with a red-
sensitive Hamamatsu R928 photomultiplier tube; spectra shown are
corrected for the wavelength dependence of the detector, and quoted
emission maxima refer to the values after correction. Samples for
emission measurements were contained within quartz cuvettes of 1 cm
path length modified with appropriate glassware to allow connection
to a high-vacuum line. Degassing was achieved via a minimum of three
freeze−pump−thaw cycles while connected to the vacuum manifold;
We previously showed how 4,6-diphenylpyrimidine can be used
as a bridging ligand to prepare dinuclear cyclometalated
21
platinum(II) complexes. The present study shows that
diphenylpyrazines can likewise generate dinuclear platinum(II)
complexes through metalation of the two pendent phenyl rings
and binding of the metal ions to the two nitrogen atoms of the
central heterocycle. Both the 2,3- and the 2,5-substituted
isomers of diphenylpyrazine can be employed, along with
diphenylquinoxaline and derivatives in which the phenyl rings
are interlinked through their ortho positions. Mono- and
dinuclear complexes can be isolated from a single reaction by
chromatographic separation.
−2
the final vapor pressure at 77 K was <5 × 10 mbar, as monitored
using a Pirani gauge. Luminescence quantum yields were determined
using [Ru(bpy) ]Cl in degassed aqueous solution as the standard
3
2
Introduction of the second metal ion leads, for all six ligands
investigated, to a significant stabilization of the LUMO of
around 0.3 eV, according to both cyclic voltammetry and TD-
DFT calculations. In contrast, the HOMO energy seems to be
influenced rather little. These contrasting effects are intuitively
in line with the typical distribution of frontier orbitals in
cyclometalated complexes of aryl−heterocycles, in that the ring
to which two metal ions are bound is the heterocycle, on which
the LUMO is predominantly localized. The decreased
HOMO−LUMO gap that ensues leads to red-shifted
absorption and emission spectra for the dinuclear complexes
compared to their mononuclear analogues. Meanwhile, the
31
(
Φ
= 0.042 ); estimated uncertainty in Φ is ±20% or better,
lum
lum
except where indicated otherwise. The luminescence lifetimes of the
complexes were measured by time-correlated single-photon counting,
following excitation at 374.0 nm with an EPL-375 pulsed diode laser.
The emitted light was detected at 90° using a Peltier-cooled R928
PMT after passage through a monochromator. The estimated
uncertainty in the quoted lifetimes is ±10% or better. Bimolecular
rate constants for quenching by molecular oxygen, k , were
Q
determined from the lifetimes in degassed and air-equilibrated
solution, taking the concentration of oxygen in CH Cl at 0.21 atm
2
2
−3 32
O to be 2.2 mmol dm .
2
Electrochemistry. Cyclic voltammetry was carried out using a
μAutolab Type III potentiostat with computer control and date
storage via GPES Manager software. Solutions of concentration 1 mM
increased conjugation within the heterocycle on going from a
6
pyrazine through quinoxaline to a phenazine unit (as in L H )
in CH Cl at 298 ± 3 K were used, containing [Bu N][PF ] as the
2
2
4
6
2
likewise has a much more significant effect on the LUMO,
leading to the progressive shift toward the red region observed
in the optical spectra.
supporting electrolyte. A three-electrode assembly was employed,
consisting of a glassy carbon working electrode and platinum wire
counter and reference electrodes. Solutions were purged for at least 5
min with solvent-saturated nitrogen gas with stirring, prior to
measurements being taken under a nitrogen atmosphere without
stirring. Voltammograms were referenced to the ferrocence−
ferrocenium couple measured under the same conditions.
Of particular interest is the effect of the second metal ion on
the radiative rate constant k . For all of the systems in which
r
this parameter can be estimated with confidence, the value
increases upon introduction of the second metal ion, despite
the red shift. Normally, for a given type of excited state in
Density Functional Theory Calculations. All DFT calculations
2
4
were carried out using Gaussian 09W. The B3LYP hybrid functional
was used together with the LanL2DZ basis set for Pt and 6-31+G(d)
for all other atom types. Geometries were fully optimized in vacuo.
UV−vis absorption spectra were obtained by single-point TD-DFT
calculations at the optimized geometries using the same functional and
structurally related complexes, a decrease in k with decreasing
r
energy is anticipated, since the oscillator strength of a transition
3
is dependent upon ν in the Einstein coefficient for
spontaneous emission. It is possible that the observed effect
may be due to the higher degree of spin−orbit coupling
expected from the presence of the second heavy metal ion.
In summary, dinuclear cyclometalated complexes are shown
to offer interesting potential for obtaining phosphorescent
materials that both absorb strongly and emit brightly at unusually
low energies. Further derivatization of the diarylpyrazine unit can
be confidently expected to lead to further new compounds with
opportunities for controlling excited states for contemporary
applications, such as in imaging, solar energy conversion, and
light-emitting displays.
27
basis sets, either in vacuo or with a PCM solvent correction for
dichloromethane. Molecular orbitals and spectra were visualized using
GaussView 5.0,34 with Gaussian line shapes and an arbitrary half-
−1
width at half-height of 1000 cm for the latter.
1
4
,6-Diphenylpyrimidine L H . Phenylboronic acid (1 g, 8.20 mmol)
2
and 4,6-dichloropyrimidine (469 mg, 3.15 mmol) were dissolved in
3
1,4-dioxane (25 cm ), and the reaction mixture was then deaerated by
bubbling N
through the mixture for 10 min. Aqueous K CO solution
2
2 3
3
(
2 M, 9.4 cm , 18.8 mmol) and tetrakis(triphenylphosphine)-
palladium(0) (220 mg, 0.19 mmol) were added, and the mixture
was deaerated for a further 15 min. The reaction mixture was then
stirred at 95 °C under N for 24 h. Solvents were removed under
2
3
vacuum, and DCM (35 cm ) was added. The organic layer was
EXPERIMENTAL SECTION
3
■
separated, washed with water (3 × 35 cm ), dried over MgSO
4
,
1
H and 13C NMR spectra were recorded on JEOL Delta-270 or JEOL
filtered, and evaporated to dryness. The product was then purified by
ECS-400 spectrometers operating at the frequencies indicated.
Chemical shifts (δ) are in ppm, referenced to tetramethylsilane
Si(CH ) , and coupling constants are in Hertz. Mass spectra were
column chromatography using silica gel and ethyl acetate/DCM (1:4
1
ratio) as the eluent; yield 680 mg, 93%. H NMR (CDCl , 270 MHz)
3
δ: 9.31 (1H, d, J = 1.2 Hz), 8.16−8.12 (4H, m), 8.10 (1H, d, J = 1.5
3
4
13
recorded at the EPSRC National Mass Spectrometry Service Centre
Hz), 7.55−7.50 (6H, m). C NMR (CDCl , 68 MHz) δ : 164.8,
3
C
I
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Inorganic Chemistry
Article
1
59.3, 137.2, 131.0, 129.1, 127.3, 112.9. MS (ESI): m/z 233 [M]+.
Anal. Calcd for C H N : C, 82.73; H, 5.21; N, 12.06. Found: C,
Calcd for C H N O Pt : C, 46.24; H, 4.90; N, 2.84. Found: C, 46.36;
38
48
2
4
2
H, 5.27; N, 2.60.
16
12
2
3
8
2.85; H, 5.33; N, 11.92.
PtL H. A solution of potassium tetrachloroplatinate (270 mg, 0.65
2
3
3
2
,5-Diphenylpyrazine L H . Phenylboronic acid (1 g, 8.20 mmol)
mmol) in water (2 cm ) was added to a stirred solution of L H (150
2
2
3
and 2,5-dibromopyrazine (750 mg, 3.15 mmol) were dissolved in 1,4-
dioxane (25 cm ), and the reaction mixture was then deaerated by
bubbling N through the mixture for 10 min. Aqueous K CO solution
mg, 0.65 mmol) in acetic acid (60 cm ). The mixture was heated at
3
reflux under an argon atmosphere for 19 h. The precipitate was filtered
3
off, washed successively with acetic acid (10 cm ) and ethanol (10
2
2
3
3
3
(
2 M, 9.0 cm , 18.8 mmol) and tetrakis(triphenylphosphine)-
cm ), and dried to give the dichlorobridged dimer. A mixture of the
palladium(0) (6% mol, 220 mg, 0.19 mmol) were added, and the
mixture was deaerated for a further 15 min. The reaction mixture was
dimer (150 mg, 0.30 mmol), 2,2,6,6-tetramethyl-3,5-heptanedione
3
(166 mg, 0.90 mmol), aqueous K CO solution (2M, 1 cm , 1.8
2
3
3
then stirred at 95 °C for 24 h under N . On cooling, water was added
mmol), and ethoxyethanol (70 cm ) was heated at reflux under argon
2
and the resulting precipitate was filtered and dried. The residue was
for 15 h. Solvent was evaporated, and the solid was washed in water
then recrystallized from ethanol; yield 238 mg, 33%. 1H NMR
3
(70 cm ). The solid was then filtered, dried, and purified by column
(
7
1
CDCl , 270 MHz) δ : 9.07 (2H, d, J = 0.7 Hz), 8.08−8.03 (4H, m),
chromatography using silica gel and dichloromethane as the eluent; Rf
3
H
1
3
1
.56−7.46 (6H, m). C NMR (CDCl , 100 MHz) δ : 150. 8, 141.4,
= 0.4 yield 140 mg, 35%. H NMR (CDCl , 400 MHz) δ : 9.10 (1H,
3
C
3
H
3
36.4, 129.9, 129.2, 126.9.
d, J = 3.4 Hz, J
= 47 Hz), 8.33 (1H, d, J = 4.5 Hz), 7.69 (1H, dd, J
H−Pt
1
1
PtL H and Pt L . A solution of potassium tetrachloroplatinate (1 g,
= 7.9, 1.4 Hz), 7.57−7.49 (5H, m), 7.13 (1H, ddd, J = 7.8, 7.2, 1.8
2
3
1
2
.42 mmol) in water (4 cm ) was added to a stirred solution of L H
Hz), 6.75−6.23 (2H, m), 5.84 (1H, s), 1.29 (9H, s), 1.28 (9H, s). MS
2
3
+
(375 mg, 1.61 mmol) in acetic acid (80 cm ). The mixture was heated
(ESI): m/z 609 [M + H] . Anal. Calcd for C H N O Pt: C, 53.20;
27
30
2
2
at reflux under a nitrogen atmosphere for 3 days. The precipitate was
H, 4.96; N, 4.60. Found: C, 53.64; H, 5.18; N, 4.52.
3
3
filtered off and washed successively with acetic acid (10 cm ) and
Pt L . A solution of potassium tetrachloroplatinate (540 mg, 1.30
2
3
3
3
ethanol (10 cm ) to give the dichlorobridged intermediate. A mixture
mmol) in water (4 cm ) was added to a stirred solution of L H (150
mg, 0.65 mmol) in acetic acid (60 cm ). The mixture was heated at
2
3
of the intermediate (1.07 g, 1.29 mmol), 2,2,6,6-tetramethyl-3,5-
heptanedione (949 mg, 5.16 mmol), aqueous K CO solution (2 M, 5
2
3
reflux under N for 24 h. The precipitate was filtered off, washed
2
3
3
3
3
cm , 10.32 mmol), and acetone (80 cm ) was heated at reflux under
successively with acetic acid (10 cm ) and ethanol (10 cm ), and dried
to give the dichlorobridged dimer. A mixture of the dimer (200 mg,
0.26 mmol), 2,2,6,6-tetramethyl-3,5-heptanedione (144 mg, 0.78
N for 24 h. Solvent was removed by rotary evaporation, the residue
2
was treated with DCM and filtered, and the filtrate was evaporated to
dryness. The residue was then purified by column chromatography
using silica gel and a mixture of dichloromethane/hexane, 1/2, as the
eluent to give both the mononuclear and the dinuclear products.
3
mmol), aqueous K CO solution (2M, 1 cm , 1.56 mmol), and
2
3
3
ethoxyethanol (70 cm ) was heated under reflux under nitrogen for 15
h. Solvent was evaporated, and the solid was washed with water (70
1
1
3
PtL H, R = 0.1, yield 150 mg, 15%. H NMR (CDCl , 400 MHz) δ :
cm ). The solid was then filtered off and dried. The product was
f
3
H
9
7
7
7
.62 (1H, d, J = 0.9 Hz), 8.17−8.14 (2H, m), 7.90 (1H, d, J = 0.9 Hz),
.72 (1H, dd, J = 7.6, 0.8 Hz), 7.64 (1H, dd, J = 7.6, 1.0 Hz), 7.56−
.54 (3H, m), 7.31 (1H, ddd, J = 7.6, 7.6, 1.0 Hz), 7.15 (1H, ddd, J =
purified by column chromatography using silica gel and dichloro-
1
methane as an eluent; R = 0.9, yield 60 mg, 9%. H NMR (CDCl ,
f
3
400 MHz) δ : 8.69 (2H, s), 8.03 (2H, dd, J = 8.5, 2.0 Hz), 7.73 (2H,
H
.6, 7.6, 0.8 Hz), 5.84 (1H, s), 1.29 (9H, s), 1.28 (9H, s). HRMS
dd, J = 8.5, 2.0 Hz), 7.31−7.25 (2H, m), 7.00 (2H, ddd, J = 7.6, 7.8,
+
(
6
NSI): m/z 609.2003 [M + H] . Calcd for C H N O Pt m/z
09.2007. Anal. Calcd for C H N O Pt: C, 53.20; H, 4.96; N, 4.60.
1.8 Hz), 5.86 (2H, s), 1.31 (18H, s), 1.30 (18H, s). MS (ESI): m/z
27
31
2
2
+
27
30
2
2
985 [M + H] . Anal. Calcd for C H N O Pt : C, 46.24; H, 4.90; N,
38 48
2
4
2
1
Found: C, 53.31; H, 4.92; N, 4.49. Pt L , R = 0.2, yield 29 mg, 2.4%.
2.84. Found: C, 46.31; H, 5.05; N, 2.79.
2
f
1
4
4
H NMR (CDCl , 400 MHz) δ : 9.68 (1H, s), 7.71 (2H, br.d, J = 7.6
PtL H and Pt L . A solution of potassium tetrachloroplatinate (452
3
H
2
3
Hz), 7.70 (1H, s), 7.62 (2H, br.d, J = 7.6 Hz), 7.34 (2H, br t, J = 7.6
Hz), 7.17 (2H, br t, J = 7.6 Hz), 5.84 (2H, s), 1.28 (18H, s) 1.26
mg, 1.09 mmol) in water (2 cm ) was added to a stirred solution of
L H (250 mg, 1.09 mmol) in acetic acid (75 cm ). The mixture was
heated at reflux under N
2
4
3
2
+
(
18H, s). HRMS (NSI): m/z 985.2934 [M + H] . Calcd for
for 24 h. The precipitate was filtered off,
3
3
C H N O Pt H m/z 985.2941. Anal. Calcd for C H N O Pt : C,
washed successively with acetic acid (10 cm ) and ethanol (10 cm ),
38
48
2
4
2
38 48
2
4
2
4
6.24; H, 4.90; N, 2.84. Found: C, 46.30; H, 5.25; N, 2.64.
and dried to give the dichloro-bridged dimer. A mixture of the dimer
2
2
PtL H and Pt L . A solution of potassium tetrachloroplatinate (896
(270 mg, 0.55 mmol), 2,2,6,6-tetramethyl-3,5-heptanedione (304 mg,
2
3
3
mg, 2.16 mmol) in water (4 cm ) was added to a stirred solution of
L H (250 mg, 1.08 mmol) in acetic acid (80 cm ). The mixture was
heated at reflux under N for 3 days. The precipitate was filtered off,
washed consecutively with acetic acid (10 cm ) and ethanol (10 cm ),
and dried to give the dichlorobridged dimer. A mixture of the dimer
1.65 mmol), aqueous K
2
CO
3
solution (2M, 2 cm , 3.3 mmol), and
2
3
3
acetone (70 cm ) was heated under reflux under N for 23 h. The
mixture was filtered, and water (70 cm ) was added to the filtrate. The
2
2
3
2
3
3
precipitated solid was filtered off, washed with water, and dried. The
product was then purified by column chromatography using silica gel
and dichloromethane as the eluent to give both the mononuclear and
(
680 mg, 1.32 mmol), 2,2,6,6-tetramethyl-3,5-heptanedione (972 mg,
3
4
1
5
.28 mmol), aqueous K CO solution (2M, 5 cm , 10.56 mmol), and
the dinuclear products. For PtL H, R = 0.7, yield 100 mg, 15%. H
2
3
f
3
acetone (80 cm ) was heated at reflux under N for 24 h. Acetone was
NMR (CDCl , 400 MHz) δ : 9.13 (1H, dd, J = 7.7, 1.3 Hz), 9.09 (1H,
2
3
H
removed by rotary evaporation, and the residue was then washed with
water, filtered, and dried. The solid was purified by column
d, J = 3.4 Hz), 8.75 (1H, d, J = 3.4 Hz), 8.60 (1H, dd, J = 8.2, 1.3 Hz),
8.19 (1H, dd, J = 8.2, 1.3 Hz), 7.86−7.63 (4H, m), 5.91 (1H, s), 1.18,
+
chromatography using silica gel and dichloromethane as an eluent to
(18H, s). MS (ESI): m/z 607 [M + H] . Anal. Calcd for
2
give both the mononuclear and dinuclear products. For PtL H, R =
C H N O Pt: C, 53.37; H, 4.64; N, 4.61. Found: C, 53.65; H,
f
27 28
2
2
1
4
1
0
.6, yield 230 mg, 35%. H NMR (CDCl , 400 MHz) δ : 9.54 (1H, d,
4.75; N, 4.48. For Pt L , R = 0.9, yield 50 mg, 9%. H NMR (CDCl ,
3
H
2
f
3
3
4
J = 1.2 Hz, J
= 46 Hz), 8.95 (1H, d, J = 1.2 Hz, JH−Pt = 13 Hz),
400 MHz) δ : 8.69 (2H, s), 8.09 (2H, d, J = 7.6 Hz), 7.75 (2H, d, J =
H−Pt
H
8
7
7
.06−8.01 (2H, m), 7.70 (1H, dd, J = 7.8, 1.6 Hz), 7.60 (1H, dd, J =
8.4 Hz), 7.62 (2H, t, J = 8.0, 7.6 Hz), 5.91 (2H, s), 1.36 (18H, s), 1.35
(18H, s). MS (ESI): m/z 982 [M + H] . Anal. Calcd for
+
.8, 1.6 Hz), 7.58−7.48 (3H, m), 7.29 (1H, ddd, J = 7.4, 7.4, 1.2 Hz),
.17 (1H, ddd, J = 7.4, 7.4, 1.2 Hz), 5.86 (1H, s), 1.34 (9H, s), 1.31
C H N O Pt: C, 46.69; H, 4.95; N, 2.81. Found: C, 46.86; H,
38
46
2
4
+
(
9H, s). HRMS (NSI): m/z 609.2003 [M+H] . Calcd for
C H N O PtH m/z 609.2007. Anal. Calcd for C H N O Pt: C,
4.87; N, 2.82.
5
5
PtL H. Under N atmosphere, L H (220 mg, 0.78 mmol) and
27
31
2
2
27 30
2
2
2
2
5
3.20; H, 4.96; N, 4.60. Found: C, 52.52; H, 4.47; N, 4.15%. For
potassium tetrachloroplatinate (420 mg, 1.01 mmol) were dissolved in
acetic acid (50 cm ) and the mixture was heated at reflux overnight.
1
3
Pt L , R = 0.9, yield 150 mg, 14%. H NMR (CDCl , 400 MHz) δ :
2
2
f
3
H
9
.44 (2H, s), 7.72 (2H, dd, J = 6.7, 1.5 Hz), 7.43 (2H, dd, J = 6.7, 1.5
The mixture was cooled to room temperature, and the black solid was
filtered off, washed with ethanol (20 cm ), and air dried. The crude
3
Hz), 7.29 (2H, ddd, J = 7.7, 7.7, 1.5 Hz), 7.17 (2H, ddd, J = 7.7, 7.7,
1
9
.5 Hz), 5.88 (2H, s), 1.37 (18H, s), 1.30 (18H, s). HRMS (NSI): m/z
product was added to a solution of 2,2,6,6-tetramethyl-3,5-
heptanedione (432 mg, 2.34 mmol) in acetone (60 cm ), followed
+
3
85.2940 [M + H] . Calcd for C H N O Pt H m/z 985.2941. Anal.
38
48
2
4
2
J
dx.doi.org/10.1021/ic401131x | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
by aqueous K CO solution (2M, 5 cm ). The mixture was heated at
Notes
2
3
reflux overnight. Acetone was removed under reduced pressure, and
the crude product was dissolved in DCM. The organic layer was
The authors declare no competing financial interest.
washed with brine and dried with MgSO . Solvent was removed under
4
ACKNOWLEDGMENTS
■
vacuum, and the product was then purified by column chromatog-
1
raphy using silica gel and CHCl . R = 0.4, yield 231 mg, 45%. H
We thank EPSRC for funding (grants EP/I014942/1 and EP/
H051902/01) and the EPSRC National Mass Spectrometry
Service Centre, Swansea for recording mass spectra.
3
f
NMR (CDCl , 400 MHz) δ : 9.42 (1H, dd, J = 8.2 Hz, J = 1.4 Hz),
3
H
8
7
5
.09 (1H, dd, J = 7.6 Hz, J = 1.6 Hz), 7.83 (1H, d, J = 7.8 Hz), 7.68−
.76 (4H, m), 7.50−7.56 (3H, m), 7.14 (1H, m), 6.71−6.77 (2H, m),
+
.93 (1H, s), 1.32 (9H, s), 1.27 (9H, s). MS (ESI): m/z 659 [M + H] .
REFERENCES
■
Anal. Calcd for C H N O Pt: C, 56.44; H, 4.89; N, 4.25. Found: C,
31
32
2
2
(
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5
6.84; H, 5.12; N, 4.08.
5
5
Pt L . Under N atmosphere, PtL H (200 mg, 0.30 mmol) and
2
2
(
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potassium tetrachloroplatinate (415 mg, 0.36 mmol) were dissolved in
3
acetic acid (40 cm ), and the mixture was heated at reflux 15 h. The
(
mixture was cooled to room temperature, and the black solid was
3
filtered off, washed with ethanol (20 cm ), and air dried. The crude
product was added to a solution of 2,2,6,6-tetramethyl-3,5-
3
(4) (a) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.;
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heptanedione (74 mg, 0.4 mmol) in acetone (50 cm ), followed by
3
aqueous K CO solution (2M, 5 cm ). The mixture was heated at
2
3
reflux for 16 h. Acetone was removed under reduced pressure, and the
crude product was dissolved in DCM. The organic layer was washed
with brine, dried over MgSO , filtered, and evaporated to dryness. The
4
residue was then purified by column chromatography using silica gel
and CHCl as the eluent to give the product; R = 0.8 yield 102 mg,
3
f
1
3
=
3%. H NMR (CDCl , 400 MHz) δ 9.49 (2H, m), 8.00 (2H, br d, J
3 H
10.0 Hz), 7.85 (2H, dd, J = 12.0 Hz, J = 2.4 Hz), 7.64 (2H, m), 7.26
(
2H, ddd, J = 12.0 Hz, J = 11.6 Hz, J = 1.6 Hz), 7.00 (2H, ddd, J = 11.6
Hz, J = 10.4 Hz, J = 2.4 Hz), 5.96 (2H, s), 1.34 (18H, s), 1.28 (18H,
+
s). MS (ESI): m/z 1035 [M + H] . Anal. Calcd for C H N O Pt : C,
42
50
2
4
2
4
8.64; H, 4.86; N, 2.70. Found: C, 48.86; H, 5.38; N, 2.48.
6
6
6
PtL H and Pt L . A mixture of L H (93 mg, 0.33 mmol), potassium
2
2
3
tetrachloroplatinate (150 mg, 0.4 mmol), and acetic acid (35 cm ) was
heated at reflux under nitrogen for 14 h. The mixture was allowed to
cool to room temperature, and the black solid was filtered off, washed
(6) Wang, L. V.; Wu, H. Biomedical Optics: Principles and Imaging;
John Wiley: Hoboken, NJ, 2007.
3
with ethanol (20 cm ), and air dried. The crude product was added to
a solution of 2,2,6,6-tetramethyl-3,5-heptanedione (431 mg, 2.34
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2011, 40, 12421.
3
mmol) in acetone (60 cm ), followed by aqueous K CO solution
2
3
3
(
2M, 5 cm ). The mixture was heated at reflux for 14 h. Acetone was
removed under reduced pressure, and DCM (30 cm ) and brine (15
cm ) were added. The organic layer was dried over MgSO4 and
3
3
(8) Juris, A.; Balzani, V.; Barigelletti, F.; Campagna, S.; Belser, P.;
filtered. Solvent was removed by rotary evaporation, and the residue
was then purified by column chromatography using silica gel and
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CHCl /hexane, 7/3 v/v, as the eluent to give both the mononuclear
3
6
1
and the dinuclear products. For PtL H, R = 0.7, yield 24 mg, 11%. H
f
NMR (CDCl , 400 MHz) δ : 10.07 (1H, dd, J = 8.7 Hz, J = 1.4 Hz),
3
H
9
.31 (1H, dd, J = 9.2 Hz, J = 1.4 Hz), 8.50 (1H, d, J = 7.8 Hz), 8.36
1H, dd, J = 8.7 Hz, J = 1.4 Hz), 8.12 (1H, d, J = 7.8 Hz), 7.99 (1H, d,
J = 6.9 Hz), 7.69−7.92 (4H, m), 7.62 (1H, t, J = 7.8 Hz), 5.99 (1H, s),
(
+
1
.38 (9H, s), 1.37 (9H, s). MS (ESI): m/z 657 [M + H] . Anal. Calcd
for C H N O Pt: C, 56.61; H, 4.60; N, 4.26. Found: C, 56.59; H,
3
1
30
2
2
6
1
4
4
.63; N, 4.18. For Pt L , R = 0.8, yield 2 mg, 1%. H NMR (CDCl ,
2
f
3
00 MHz) δ : 10.15 (2H, m), 8.06 (2H, d, J = 7.8 Hz), 7.96 (2H, d, J
H
=
7.3 Hz), 7.87 (2H, m), 7.61 (2H, t, J = 7.8 Hz), 5.99 (2H, s), 1.38
+
(
18H, s), 1.37 (18H, s). MS (ESI): m/z 1033 [M + H] .
ASSOCIATED CONTENT
■
(d) Zhao, Q.; Jiang, C.-Y.; Shi, M.; Li, F.-Y.; Yi, T.; Cao, Y.; Huang, C.-
*
S
Supporting Information
H NMR and mass spectra; data from DFT calculations;
H. Organometallics 2006, 25, 3631. (e) Xu, M. L.; Wang, G. Y.; Zhou,
R.; An, Z. W.; Zhou, Q.; Li, W. L. Inorg. Chim. Acta 2007, 360, 3149.
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c) Williams, J. A. G.; Develay, S.; Rochester, D. L.; Murphy, L. Coord.
frontier orbital plots for all complexes generated by TD-DFT
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
2
(
Chem. Rev. 2008, 252, 2596. (d) Murphy, L.; Williams, J. A. G. Top.
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AUTHOR INFORMATION
(12) (a) Balashev, K. P.; Puzyk, M. V.; Kotlyar, V. S.; Kulikova, M. V.
Coord. Chem. Rev. 1997, 159, 109. (b) Brooks, J.; Babayan, Y.;
Lamansky, S.; Djurovich, P. I.; Tsyba, I.; Bau, R.; Thompson, M. E.
Inorg. Chem. 2002, 41, 3055. (c) Yin, B. L.; Niemeyer, F.; Williams, J.
A. G.; Jiang, J.; Boucekkine, A.; Toupet, L.; Le Bozec, H.; Guerchais, V.
■
*
*
Corresponding Authors
E-mail: j.a.g.williams@durham.ac.uk.
E-mail: valery.kozhevnikov@northumbria.ac.uk.
K
dx.doi.org/10.1021/ic401131x | Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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dx.doi.org/10.1021/ic401131x | Inorg. Chem. XXXX, XXX, XXX−XXX