Postsynthetic Systematic Electronic Tuning of Organoplatinum Photosensitizers via Secondary Coordination Sphere Interactions

Article abstract of DOI:10.1021/acs.organomet.6b00332

In this work we show that postsynthetic addition of borane Lewis acids to Lewis base decorated organoplatinum photosensitizers induces significant changes in the optical and electrochemical properties. In particular, the charge transfer (CT) energies of these chromophores are significantly modified by these outer-sphere interactions. The direction of the CT shift depends on the site of Lewis acid binding, which occurs either at the diimine ligand in bipyrazine-linked molecules or at an ancillary acetylide ligand in pyridyl-substituted bis(acetylide) molecules. The magnitude of the shift depends on the Lewis acidity of the borane and the number of equivalents added and is comparable to the perturbation brought on by covalent substituent modification of supporting ligands in related complexes. This approach offers a new means of tuning the properties of organometallic phosphors that complements the traditional approach of covalent modification and other postsynthetic modification strategies.

Full text of DOI:10.1021/acs.organomet.6b00332

Article
pubs.acs.org/Organometallics
Postsynthetic Systematic Electronic Tuning of Organoplatinum
Photosensitizers via Secondary Coordination Sphere Interactions
Hanah Na, Ayan Maity, and Thomas S. Teets*
Department of Chemistry, University of Houston, 3585 Cullen Boulevard, Room 112, Houston, Texas 77204-5003, United States
S
* Supporting Information
ABSTRACT: In this work we show that postsynthetic addition
of borane Lewis acids to Lewis base decorated organoplatinum
photosensitizers induces significant changes in the optical and
electrochemical properties. In particular, the charge transfer
(CT) energies of these chromophores are significantly modified
by these outer-sphere interactions. The direction of the CT shift
depends on the site of Lewis acid binding, which occurs either at
the diimine ligand in bipyrazine-linked molecules or at an
ancillary acetylide ligand in pyridyl-substituted bis(acetylide)
molecules. The magnitude of the shift depends on the Lewis
acidity of the borane and the number of equivalents added and is
comparable to the perturbation brought on by covalent
substituent modification of supporting ligands in related complexes. This approach offers a new means of tuning the properties
of organometallic phosphors that complements the traditional approach of covalent modification and other postsynthetic
modification strategies.
enhanced reactivity. The “weak-link approach” has been
popularized in supramolecular chemistry, whereby coordina-
tion-driven structural changes engender synthetic catalysts with
allosteric properties.14 In the context of olefin polymerization
catalysis, remote Lewis acid triggers have been used to alter the
electron density at the metal center and enhance reactivity.15−19
Along the same lines, rhenium hydrogenation catalysts have
been enhanced by coordination to Lewis acids through a
nitrosyl oxygen,20−22 and the reactivity of hydrogenase model
complexes can be altered when boranes are coordinated to
terminal cyanide ligands.23,24 In other recent accounts,
secondary coordination sphere interactions in macrocyclic
cation receptors have led to altered reactivity patterns brought
on either by interaction of the bound Lewis acid with substrate
molecules or by decoordination of the hemilabile crown
receptor from the metal center.25−29 In a similar vein, the
interaction of a bipyrazine-ligated platinum diaryl complex with
INTRODUCTION
■
Molecular inorganic and organometallic complexes with
structurally and electronically tunable supporting ligands play
prominent roles in a number of important chemical subfields,
including catalysis,1−3 small-molecule activation,4,5 and photo-
sensitization.6,7 A unifying theme in all of these examples is the
central role played by the supporting ligands in controlling the
electronic structure, reactivity, and photophysical properties of
the metal complexes. Traditionally, targeted modification of a
complex is brought on by one or more covalent modifications
to the supporting ligand environment, requiring ground-up
synthesis of the modified ligand and complex. Systematic ligand
modification has been used to understand reaction mechanisms
and develop improved homogeneous catalysts,8,9 as well as to
elucidate the electronic structure and tailor the photophysical
properties of photosensitizer molecules.7,10,11 This controlled
ligand modification is particularly fruitful when ligands
containing aromatic groups are involved, where the well-
known Hammett substituent constants12,13 can be leveraged to
quantitatively rationalize and predict the effects of the structural
perturbations. In many cases such alterations to the supporting
ligands are synthetically trivial, but in others this approach is
derailed by synthetic limitations or results in only marginal
gains after much synthetic effort. An alternative strategy would
be to incorporate functional groups into supporting ligands that
allow for the facile postsynthetic modification of the metal
complex, permitting subtle, systematic changes to the ligand
environment that can be rapidly implemented.
B(C6F5)3 (abbreviated herein as BArF ) gives rise to a
3
substantial increase in the rate of biaryl reductive elimination.30
Careful control of the redox, absorption, and emission
properties of photosensitizer molecules is a requisite for their
continued development and implementation in devices, and
there are multiple examples where covalent substituent
modification has led to pronounced changes in fundamental
properties and in the performance of the photosensitizer in a
device7 or photochemical process.10 There are also many
noteworthy examples, mostly in the context of metal ion,
There are examples where postsynthetic modification of
Received: April 25, 2016
transition-metal complexes and ligands leads to altered or
© XXXX American Chemical Society
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solvent, and pH sensing, of phosphorescent metal complexes
with properties that can be modified in response to external
additives.31,32 Platinum acetylide complexes33−40 and com-
plexes of other transition metals41,42 can be solvatochromic40,42
and have also been designed with various functional groups that
can interact with cationic Lewis acids35−37,39,41,42 or pro-
tons.33,36 In many of these examples, the colorometric and
luminescence response is brought on by controlling aggrega-
tion, which turns on or off absorption/emission from an
aggregated state, or by inhibiting photoinduced electron
transfer (PET), which quenches the emission in the unbound
state.31 In these mechanisms the identity of the Lewis or
Brønsted acid additive does not affect the wavelength of
absorption/emission, and the response is on/off in nature.
Furthermore, sensors are often designed to be reversible, such
that the adducts with Lewis acids are often not able to be
isolated. We propose here that secondary coordination sphere
Lewis acid−base interactions involving pyridines and sub-
stituted boranes could be a complementary approach for
modulating the physical properties of photosensitizer molecules
and offer some potential advantages of this strategy. Unlike
alkali-metal and alkaline-earth-metal Lewis acids, boranes are
available with numerous organic substituents, giving them a
potentially wider range of available Lewis acidities and
engendering much more precise tunability over the electronic
structure than is possible with simple metal cations. In addition,
in contrast to the cation receptors, which often utilize crown
ethers that are size-matched to only certain cations, pyridine
receptors should be able to coordinate to virtually any borane
Lewis acid (and many others), allowing the design of versatile
platforms for studying the effects of Lewis acid additives on the
electrochemical and photophysical properties. Finally, pyr-
idine−borane interactions are thermodynamically stable,43
permitting isolation of the adducts and their further
manipulation and processing. For these reasons, we believe
that phosphors with strategically placed pyridines would permit
careful postsynthetic alteration of both redox potentials and
excited-state energies upon interaction with boranes, which
could allow for rapid optimization of the properties for a given
application.
perturbations of the electrochemical and optical properties,
(iii) the magnitudes of the observed effects are influenced by
the stoichiometry of the borane additive, and (iv) the pyridine−
borane interactions form instantly and cleanly, allowing for the
properties to be tuned rapidly with no additional workup or
purification needed.
RESULTS AND DISCUSSION
■
Synthesis of Complexes and Their Borane Adducts.
The complexes Pt(bpy)(CCpy)2 (1)45 and Pt(bpz)(4-
C6H4CF3)2 (2)30 were prepared following previously published
procedures. The crystal structure of complex 2, which was not
previously reported, has been determined and is presented in
Figure S1 in the Supporting Information, showing the expected
square-planar coordination environment of PtII. Whereas
complex 1 has been shown to bind lanthanide ions in the
construction of platforms for sensitized emission,45 its
coordination chemistry with boranes has not been explored.
As summarized in Scheme 1, complex 1 reacts cleanly and
Scheme 1. Synthesis of Borane Adducts of Complex 1
In this work, we demonstrate the ability of borane−pyridine
Lewis acid−base interactions in the secondary coordination
sphere to measurably alter the electrochemical and photo-
physical properties of organometallic platinum diimine photo-
sensitizers. In complexes of this type, the luminescent excited
states are triplet charge transfer (3CT) states, involving Pt d
orbitals and diimine π* orbitals, and it has been shown that
covalent modifications to both the diimine ligand and the
acetylide or aryl ancillary ligand can induce profound changes
in the excited-state properties.10,44 We describe one complex,
Pt(bpy)(CCpy)2 (1; bpy = 2,2′-bipyridine, CCpy = 4-
pyridylacetylide), where the Lewis acidic boranes interact with
the acetylide ancillary ligands, and a second, Pt(bpz)(4-
C6H4CF3)2 (2; bpz = 2,2′-bipyrazine), where boranes
coordinate to the diimine ligand. For both of these complexes,
we study the effects of triarylborane additives on the
electrochemical and optical properties of the platinum
complexes, which are perturbed with magnitudes rivaling
those obtainable by covalent substituent modification. Our
investigations reveal several attractive features of this strategy:
(i) the site of borane binding determines the direction of the
shift of the excited-state energy, (ii) the identity of the borane
and its Lewis acidity determine the magnitude of the
successively with 2 equiv of the boranes BPh3 and BArF to
3
furnish adducts where either 1 or 2 equiv of the borane is
1
bound to the pyridyl nitrogen. H NMR studies demonstrate
quantitative and irreversible binding of both BPh3 and BArF .
3
Figures S2−S4 in the Supporting Information show stacked
NMR spectra for the successive addition of 1, 2, and 2.5 equiv
of BPh3 and BArF to complex 1. With 1 equiv, the C2
3
symmetry is lost, and an asymmetric spectrum results that is
consistent with one bound pyridine and one free pyridine. The
1
asymmetry is evident in H NMR peaks assigned to both the
bipyridyl ligand and to the pyridylacetylide. For example, the
most downfield resonance in complex 1, assigned to the bpy H6
position (ortho to nitrogen) occurs at 9.64 ppm. Addition of 1
equiv of BPh3 splits this resonance, resulting in peaks at 9.62
ppm, minimally perturbed from complex 1, and 9.48 ppm,
B
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substantially upfield. The upfield shift is even more pronounced
when BArF is added, and with 1 equiv added the H6 protons
3
resonate at 9.61 and 9.41 ppm, consistent with stronger binding
of the more Lewis acidic borane. With 2 equiv of borane only
one upfield-shifted H6 resonance is observed. For both BPh3
1
and BArF , no further changes in the H spectra are observed
3
after addition of more than 2 equiv of borane. The borane
adducts of complex 1 can all be isolated. The NMR spectra of
the isolated products are identical with the spectra obtained by
treating 1 with BAr3 in situ (see Figure S5 in the Supporting
Information for a representative example), and the isolated
powders gave satisfactory elemental analyses for all four 1-
(BAr3)n (n = 1, 2) complexes.
As observed by Tilley and co-workers, bipyrazine complex 2
binds up to 2 equiv of BArF at the free bipyrazine nitrogens,
3
summarized in Scheme 2.30 Addition of >2 equiv results in no
Scheme 2. Synthesis of Borane Adducts of Complex 2
Figure 1. X-ray crystal structure of the complex 1-(BArF )2, depicted
3
with ellipsoids at the 50% probability level. Hydrogen atoms and
dichloromethane solvent molecules are omitted. Selected bond lengths
(Å): Pt(1)−N(1) 2.057(2), Pt(1)−N(2) 2.065(2), Pt(1)−C(20)
1.944(3), Pt(1)−C(30) 1.947(3), N(3)−B(1) 1.624(4), N(4)−B(2)
1.613(4).
the pyridyl rings of the acetylide. In complex 1 both pyridine
rings are nearly orthogonal to the PtN2C2 coordination plane,
with dihedral angles of 83.4 and 79.1°, whereas in 1-(BArF )2
3
one of the pyridine rings shows a shallow dihedral angle of
30.06° and the other a nearly orthogonal angle of 87.36°. This
is likely a crystal-packing phenomenon, as one of the two
dichloromethane solvate molecules is sandwiched between the
two BArF moieties, and in solution the apparent C2 symmetry
3
in the 1H and 19F NMR spectra are consistent with free
rotation of the borane-bound pyridine rings.
Effect of Borane Binding on Electronic Absorption
and Emission. Diimine platinum acetylide complexes akin to
1 have been long known as photosensitizers, with absorption
that tails into the visible region and efficient triplet
emission.10,48,49 The low-energy absorption and emission
features in these complexes arise from charge transfer (CT)
excited states. Previous experimental and theoretical work on
closely related bis-acetylide complexes has demonstrated that
the CT transitions involve occupied orbitals of mixed Pt dπ/
acetylide π parentage (HOMO and HOMO-1) and the bpy π*
LUMO50,51 and as such can be characterized more precisely as
mixed metal−ligand to ligand charge transfer states
(MMLL′CT). Binding of boranes to the pyridylacetylide
ligands perturbs the frontier orbital manifold by influencing
the mixing between the acetylide orbitals and the Pt dπ orbitals,
resulting in changes in the absorption spectra as complex 1 is
titrated with boranes. Figure 2 displays the evolution of the
absorption spectra as complex 1 is titrated with BPh3 and
further changes in the NMR spectra of the adduct, but some
additional aromatic peaks grow in which suggest decomposition
in the presence of excess BArF . However, addition of BPh3 to
3
1
bipyrazine complex 2 only resulted in minimal shifts of the H
NMR peaks ([2] ≈ 6 mM), and no asymmetry was observed
after addition of 1 equiv, suggesting weak binding of BPh3 that
is rapidly reversible. This observation of weak binding of BPh3
by bipyrazine complex 2 was further verified by UV−vis
titrations (see below), and for this reason adducts of bipyrazine
complex 2 with BPh3 were not studied further.
One of the borane adducts, 1-(BArF )2, was crystallized by
3
layering hexane into a concentrated dichloromethane solution
and characterized by single-crystal X-ray diffraction. The
structure is shown in Figure 1. The N(pyridine)−B bond
distances are 1.624(4) and 1.613(4) Å, very similar to the
46
1.628(2) Å N−B distance in C6H5N→BArF
and the
BArF . For the latter, the borane was added in increments of
3
3
1.620(3) Å distance in an ethynylpyridine−BArF adduct,47
0.25 equiv, showing the gradual change that occurs as complex
3
suggesting strong interaction between the pyridine and borane
that is not significantly weakened by the presence of the PtII
center. In a comparison of bond metrics to those previously
determined for complex 1,45 nearly identical Pt−C distances
are observed1.939(8) and 1.956(8) Å for 1 and 1.944(3) and
1 converts first to 1-(BArF ) and then to 1-(BArF )2. In both
3
3
cases, addition of borane induces a hypsochromic shift in the
1
low-energy CT absorption band, and no further changes are
noted after addition of >2 equiv. The low-energy band appears
at 389 nm (ε = 6600 M−1 cm−1) in complex 1 (CH2Cl2
1.947(3) Å for 1-(BArF )2. Similarly, the Pt−N distances in 1
solvent), shifting to 376 nm (ε = 9100 M−1c m−1) in 1-(BArF )
3
3
are negligibly perturbed when 2 equiv of BArF is bound. One
and 370 nm (ε = 13000 M−1 cm−1) in 1-(BArF )2. The results
3
3
difference between the structures of 1 and 1-(BArF )2 is the
are qualitatively similar when BPh3 is added, although with this
weaker Lewis acid additive the magnitudes of the shifts are not
3
dihedral angle between the platinum’s coordination plane and
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Figure 3. Emission spectra for complex 1 on titration with BArF3 (top)
and BPh3 (bottom). Spectra were recorded in CH2Cl2 at room
temperature with λex 388 nm. For the upper graph, the borane was
Figure 2. Electronic absorption spectra for complex 1 when titrated
with BArF (top) and BPh3 (bottom). Spectra were recorded in
3
added successively in 0.25 equiv aliquots, and the spectra for 1-(BArF )
3
CH2Cl2 at room temperature. For the upper graph, the borane was
and 1-(BArF )2 are shown as thicker lines.
added successively in 0.25 equiv aliquots, and the spectra for 1-(BArF )
3
3
and 1-(BArF )2 are shown as thicker lines.
3
emission maximum occurs at 465 nm (Φ = 0.0036) with
vibronic structure observed. The vibronic spacing of ∼1300
cm−1 between the first two maxima is consistent with aromatic
stretching vibrations52 and suggests that when 2 equiv of
borane is bound a LC state mixes with the low-energy CT
state, demonstrating that these secondary coordination sphere
interactions can not only perturb the energy of the excited state
but also influence the excited-state character. Similar shifts are
observed upon addition of BPh3, but again the magnitude is not
as large. The complex 1-(BPh3) has a featureless emission
spectrum with λmax 523 nm (Φ = 0.025), and the spectrum for
1-(BPh3)2 shows poorly resolved vibronic structure, with
overlapping maxima at 485 and 508 nm (Φ = 0.0074).
as pronounced. The complex 1-(BPh3) has an absorption
maximum of 382 nm (ε = 7600 M−1 cm−1), and for 1-(BPh3)2
the value is 379 nm (ε = 11000 M−1 cm−1). The magnitude of
the effect depends on the strength of the Lewis acid, with BArF
3
3
3
perturbing the absorption band by ca. 1300 cm−1 when 2 equiv
is bound, as opposed to a perturbation of only ca. 680 cm−1
when 2 equiv of BPh3 is coordinated. These shifts are
consistent with a stabilization of the Pt dπ orbital brought on
by coordination of the borane to the pyridyl acetylide. The
higher-energy region of the absorption spectrum, λ <350 nm, is
dominated by diimine and acetylide-centered π → π*
transitions, which are also perturbed by the coordination of
boranes. In this region new bands grow in at lower energy when
borane is added, and it is most likely the acetylide-centered
transitions that are affected most strongly.
One potential concern with this approach is the instability of
the borane adducts relative to covalent substituent bonds,
which would limit the utility of the strategy described here.
Acknowledging that pyridine−borane interactions (ΔH°f for
Ph3B·NC5H5 from pyridine and BPh3 is −17.9 kcal/mol)43 are
considerably weaker than covalent carbon−heteroatom bonds,
all of our experiments indicate good stability of the adducts of
complex 1 under the experimental conditions. In addition to
the titration experiments described in Figures 2 and 3, we
carried out a number of experiments on isolated borane adducts
of 1. As shown in Figures S6 and S7 in the Supporting
Information, when isolated solids of 1-(BArF ) and 1-(BArF )2
Addition of boranes to complex 1 also induces changes in the
emission spectrum. Figure 3 shows the effect of borane binding
on the emission spectrum, when complex 1 is titrated with
BArF3 and BPh3. At room temperature in dichloromethane with
[1] = 1.2 × 10−5 M, complex 1 exhibits a broad emission
spectrum with λmax 551 nm and an emission quantum yield (Φ)
of 0.044. We note some asymmetry in the spectrum of 1 at this
concentration, possibly caused by unresolved vibronic structure,
which is clearly observed in low-temperature emission spectra
of related diimine platinum bis-acetylide chromophores.50,51
Addition of BArF3 induces a hypsochromic shift in the emission
spectrum, mirroring the absorption spectral changes for the
1CT band described above, and a concomitant decrease in
3
3
are dissolved in CH2Cl2, the absorption and emission spectra
are nearly identical with those of the samples prepared by
titration. In addition, as depicted in Figures S8 and S9 in the
Supporting Information, the absorption spectra of the isolated
compounds obey Beer’s law, and the emission wavelengths are
largely concentration independent, although there is some red
quantum yield is noted as well. In 1-(BArF ) the emission
3
maximum has shifted to 518 nm (Φ = 0.019), and the spectrum
remains broad and is featureless. In the complex 1-(BArF )2 the
shifting in complex 1-(BArF ) at higher concentration which is
3
3
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likely caused by aggregation through Pt···Pt and/or π···π
interactions.48,53 And finally, when complex 1-(BArF )2 is
3
treated with additional amounts of BArF , the absorption
3
spectrum is unchanged and no emission quenching is observed
(Figure S10 in the Supporting Information). These experi-
ments reveal several key points: (i) the binding of boranes to
complex 1 is strong and irreversible, (ii) as low as 10−6 M there
is no significant borane dissociation in these samples, (iii) the
borane adducts are stable as solids and retain their properties
upon dissolution, and (iv) excess borane in solution is optically
inert and does not act as a bimolecular quencher.
It is instructive to compare these borane-induced spectral
changes to those that result from covalent modification of the
4-position of the phenylacetylide. Eisenberg et al. have studied
the effects of systematic substituent modification on the
photophysical properties of platinum bis-acetylide chromo-
phores of the type Pt(dtbpy)(CCC6H4X)2, where dtbpy is
4,4′-di-tert-butyl-2,2′-bipyridine and X is a variable substitu-
ent.48 They found that the CT absorption is fairly insensitive
1
to covalent modification of the 4-position of the phenyl-
acetylide. Substitution of phenylacetylide with (4-nitrophenyl)-
acetylide induces a ca. 1000 cm−1 hypsochromic shift in the
low-energy absorption band, smaller than the 1300 cm−1 shift
in λmax that is achieved by adding 2 equiv of BArF to complex
1. However, this band is believed to be a π → π*3transition of
1
the nitrophenyl group and the position of the CT band was
not clear. Furthermore, the emission energy in this example is
unchanged by nitro substitution, in contrast to the over 3300
cm−1 perturbation of the emission maximum brought on by
Figure 4. Electronic absorption spectra for the titration of complex 2
with BArF in benzene, shown in two stages for clarity. The top plot
3
shows the spectra for the addition of up to 2 equiv in 0.25 equiv
addition of 2 equiv of BArF . In related work on terpyridyl
increments, which forms 2-(BArF ), and the bottom plot shows the
3
3
platinum complexes with a single phenylacetylide ligand,54 the
CT character of the low-energy excited state is preserved upon
introduction of electron-withdrawing groups. Incorporation of a
nitro group (σp = 0.78) at the 4-position induced a 2000 cm−1
blue shift in the 3CT emission band, whereas chloro
substitution (σp = 0.23) caused only a 250 cm−1 change. In
addition of 2−7 equiv in increments of 1 equiv, which forms 2-
(BArF )2 as the final product.
3
quantitatively form 2-(BArF )2 at UV−vis concentrations.
3
This corresponds to an estimated Keq ≈ 103 for the conversion
of 2-(BArF ) to 2-(BArF )2 (see Figure S13 in the Supporting
3
3
the complex 1-(BArF ), which has only one borane
Information), indicating that the second equivalent of borane
3
“substituent”, the blue shift is 1200 cm−1; thus, from these
binds with lower affinity. Significant spectral changes
accompany the formation of 2-(BArF )2, with a new low-
comparisons we conclude that the electron-withdrawing power
3
of BArF coordinated to pyridine is more potent than a chloro
energy band growing in at 699 nm (ε = 4600 M−1 cm−1). To
verify that the new absorption bands shown in Figure 4 can be
attributed to borane adducts of 2, the solution prepared by
3
substituent and less potent than a nitro group.
Bipyrazine complex 2 weakly interacts with BPh3, and
addition of up to 5 equiv induced no changes in the UV−vis
spectrum (Figure S11 in the Supporting Information). Binding
of BArF3 to bipyrazine-ligated complex 2 does induce significant
spectral changes, but in this case the low-energy 1CT transition
bathochromically shifts, consistent with stabilization of the bpz-
centered π* LUMO. Figure 4 shows the evolution of the
absorption spectrum as 2 is treated with successive amounts of
titrating 2 with 7 equiv of BArF was treated with excess 4-
3
(dimethylamino)pyridine (DMAP), which sequestered the
borane and quantitatively regenerated the spectral features of
complex 2 (see Figure S14 in the Supporting Information).
Complex 2 is nonemissive at room temperature in fluid
solution. We do observe luminescence at 77 K, as shown in
Figure S15 in the Supporting Information, with structured
emission maxima at 551 and 592 nm, similar to the case for
other organoplatinum diimine complexes.55,56 However,
BArF . Complex 2, previously prepared30 but not characterized
3
1
by electronic spectroscopy, is red and has a low-energy CT
absorption band at 496 nm (ε = 4300 M−1 cm−1), with a
shoulder at 473 nm (ε = 3700 M−1 cm−1). Coordination of
BArF3 is not as strong as is the case with complex 1, and excess
borane is needed to reach full conversion at concentrations
appropriate for UV−visible spectroscopy. When complex 2 (1.9
addition of BArF to complex 2 completely quenches the
3
luminescence.
Effect of Borane Binding on Redox Properties. To
further examine the perturbation of the electronic structure by
remote borane binding, cyclic voltammetry studies of 1 and its
borane adducts were carried out, as summarized in Figure 5.
Complex 1 shows a single quasi-reversible reduction wave, with
E1/2 = −1.78 V vs Fc+/Fc in CH2Cl2 with 0.2 M
tetrabutylammonium hexafluorophosphate as the supporting
electrolyte. On a sweep in the anodic direction, no clear
oxidation wave was observed as far out as +2.4 V vs Fc+/Fc (see
Figure S16 in the Supporting Information). The reduction is
× 10−4 M) is titrated with BArF , ca. 2 equiv is required to reach
3
full conversion to 2-(BArF ), corresponding to an estimated Keq
3
≈ 104 M−1 (see Figure S12 in the Supporting Information).
The complex 2-(BArF ) has two CT absorption bands in the
1
3
low-energy region, one very similar to free 2 with λmax 504 nm
(ε = 3900 M−1 cm−1), and a second at λmax 616 nm (ε = 4400
M−1cm−1). A total of 7 equiv of BArF is required to
3
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Complex 2 shows a similar pseudoreversible one-electron
reduction at −1.43 V vs Fc+/Fc as well as a second chemically
irreversible wave (Figure S18 in the Supporting Information),
but detailed studies of the effects of added BArF on the redox
3
properties were obviated by the apparent instability of the
borane adducts of 2 in the electrolyte solution, as addition of
BArF to 2 gave rise to numerous overlapping CV waves,
3
suggesting multiple decomposition products.
CONCLUSIONS
■
This work demonstrates the concept that borane−pyridine
Lewis acid−base interactions in the secondary coordination
sphere can bring about significant and systematic changes in the
electrochemical and photophysical properties of photosensitizer
molecules, providing an approach more rapid and systematic
than that offered by traditional covalent substituent mod-
ification. We have studied two different platinum(II)
chromophores with Lewis acid binding sites located on
different sites of the molecule. The results show that the
optical properties of the complexes can be tuned in different
directions depending on the site of the Lewis acid−base
interaction. Coordination to the ancillary acetylide ligands
raises the energy of the CT state by stabilizing the filled Pt(dπ)
orbital, and coordination to the diimine ligand lowers the CT
energy by stabilizing the π* LUMO of the diimine. Complexes
1 and 2 both can bind 1 or 2 equiv of borane, and in each case
the magnitude of the shift in excited state energy depends on
how many equivalents of borane are bound. Furthermore,
complex 1 can bind either BArF3 or the weaker Lewis acid BPh3,
and we showed in studies of both the electrochemical and
photophysical properties that the magnitude of the perturba-
tion depends on the Lewis acidity of the borane additive,
empowering synthetic chemists with an additional degree of
fine control in this approach.
Figure 5. Cyclic voltammograms of complex 1 and its borane adducts,
recorded in CH2Cl2 with 0.2 M TBAPF6 supporting electrolyte, using
a glassy-carbon working electrode, platinum-wire counter electrode,
and silver-wire pseudoreference electrode. The scan rate was 0.1 V/s.
Potentials were referenced to an internal standard of ferrocene, and
currents are normalized to the cathodic peak current ip,c to assist
interpretation.
centered on the bipyridine ligand, and coordination of BArF
3
induces a small but significant change in the reduction
One limitation of our approach is that we can only introduce
electron-withdrawing groups in the form of external Lewis
acids, whereas covalent modification can also include electron-
donating groups. As an example, p-methoxy substitution in
Pt(dtbpy)(CCC6H4X)2 complexes induces a 1900 cm−1
bathochromic shift in the emission energy, similar in magnitude
but opposite in direction to the change induced by 2 equiv of
potential. The complex 1-(BArF ) has a reduction potential
3
that is 50 mV more positive, with E1/2 = −1.73 V, and in 1-
(BArF )2 the value is anodically shifted by another 60 mV to
3
−1.67 V. A second wave also grows in at more negative
potentials, but we believe this is a pyridine/BArF -centerd
3
reduction, as it does not occur in the BPh3 adducts and the
current value is augmented when the second equivalent of
BArF . Conceding this limitation, a unique advantage offered by
3
BArF is added. Also consistent with this assignment, the
our strategy is the ability to more finely tune the properties by
addition of only 1 instead of 2 equiv of borane; synthesis of
asymmetrically substituted platinum bis-acetylide complexes
would be difficult by conventional means, which involves
copper-mediated acetylation of platinum dichloride precursors.
These comparisons show that postsynthetic modification by
Lewis acid−base interactions can be a synthetic strategy which
complements covalent modification, perturbing the properties
by a similar amount.
Addition of boranes to complex 1 can tune the emission
maximum from the yellow-green region into the blue region of
the spectrum, revealing the potential utility of this strategy for
tuning the color of luminescent compounds, but one final
limitation which presents itself is that coordination of borane
reduces the emission intensity, by approximately 1 order of
magnitude in 1 and completely in complex 2. To address this
limitation and assess its generality, we are beginning to install
analogous design elements into complexes with larger quantum
yields and are also currently investigating alternative ligand
platforms, such as 2,2′-bipyrimidine, which can accommodate
Lewis acids in addition to boranes and may be more amenable
3
pyridine adduct of BArF shows two irreversible reduction
3
waves in a similar region, with Ep,c,= −1.75 and −2.20 V (Figure
S17 in the Supporting Information). In a similar fashion, the
reduction potential for 1-(BPh3) is 30 mV more positive than
that of 1, with E1/2 = −1.75 V, and for 1-(BPh3)2 the potential
is shifted by another 50 mV to −1.70 V. As above in the
absorption and emission studies, the perturbation of the redox
potential is more pronounced with 2 equiv of borane bound. In
addition, the shifts are seemingly larger with BArF in
3
comparison to BPh3, though the differences between BPh3
and BArF are quite small in this case, presumably because the
3
boranes are bound remote to the bipyridine ligand, which is the
site of reduction. Eisenberg has shown that the reduction
potentials of platinum bis-acetylide complexes are relatively
insensitive to substitution on the acetylide ligands,48 and
though the potential shifts we see are subtle, they are more
substantial than those induced by covalent substituent
modification; they demonstrate that electrochemical properties
can be systematically tuned by the coordination of Lewis acids
in the secondary coordination sphere.
F
DOI: 10.1021/acs.organomet.6b00332
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
to study in polar, coordinating solvents.57 The initial results
presented here speak to the potential utility and versatility of
this approach, which we will continue to investigate in other
contexts.
(24000), 285 (26000). Anal. Calcd for C42H16BF15N4Pt·0.4CH2Cl2: C,
46.24; H, 1.54; N, 5.09. Found: C, 46.12; H, 1.46; N, 4.88.
1
1-(BArF ) . H NMR (500 MHz, CD2Cl2, 20 °C, TMS): δ 9.41 (d,
3 2
J(H,H) = 4.5 Hz, 2H, bpy H6), 8.32 (d, J(H,H) = 6.3 Hz, 4H, py H2),
8.26 (td, J(H,H) = 8.0 Hz, 1.5 Hz, 2H, bpy H4), 8.18 (d, J(H,H) = 8.0
Hz, 2H, bpy H3), 7.70 (m, 2H, bpy H5), 7.55 (d, J(H,H) = 6.9 Hz, 4H,
py H3). 19F NMR (470 MHz, CD2Cl2, 20 °C, CFCl3): δ −131.79 (br,
s, 12F), −157.80 (t, J(F,F) = 20 Hz, 6F), −164.10 (m, 12F). UV/vis
(CH2Cl2): λmax (nm) (ε (M−1 cm−1)) 370 (13000), 321 (40000), 311
(37000), 265 (19000). Anal. Calcd for C60H16BF30N4Pt: C, 45.63; H,
1.02; N, 3.55. Found: C, 45.46; H, 1.04; N, 3.49.
EXPERIMENTAL SECTION
■
Materials. Reactions were performed in a nitrogen-filled glovebox
or using standard Schlenk techniques unless otherwise stated.
Anhydrous solvents were obtained from Grubbs-type solvent
purification systems and degassed with N2. Unless otherwise stated,
starting materials and reagents were obtained from commercial sources
and used without further purification. The 2,2′-bipyrazine ligand was
prepared according to the reported procedures.58 Pt(bpy)(CCpy)2
(1; bpy = 2,2′-bipyridine, CCpy = 4-pyridylacetylide)45 and
Pt(bpz)(4-C6H4CF3)2 (2; bpz = 2,2′-bipyrazine)30 were prepared as
described previously. Spectral data for complexes 1 and 2 and the
BArF (ArF C6F5) adducts of 2 were in good agreement with
1
1-(BPh3). H NMR (600 MHz, CD2Cl2, 20 °C, TMS): δ 9.62 (s,
1H, bpy H6), 9.48 (s, 1H, bpy H6), 8.43 (d, J(H,H) = 4.8 Hz, 2H, py
H2), 8.30 (s, 2H, py H2), 8.20 (s, 2H, bpy H4), 8.13 (s, 2H, bpy H3),
7.66 (d, J(H,H) = 6.2 Hz, 2H, bpy H5), 7.45 (m, 2H, py H3), 7.28−
7.38 (m, 2H, py H3), 7.17−7.21 (m, 12H, o,m-BPh3), 7.09−7.12 (m,
3H, p-BPh3). UV/vis (CH2Cl2): λmax (nm) (ε (M−1 cm−1)) 382
(7600), 309 (28000), 289 (32000). Anal. Calcd for C42H31BN4Pt: C,
63.24; H, 3.92; N, 7.02. Found: C, 63.05; H, 3.90; N, 6.75.
3
=
previously reported data and are provided below for easy reference.
1
1-(BPh3)2. H NMR (600 MHz, CD2Cl2, 20 °C, TMS): δ 9.48 (d,
1
Physical Methods. H and 19F NMR spectra were recorded at
J(H,H) = 4.8 Hz, 2H, bpy H6), 8.31 (d, J(H,H) = 6.2 Hz, 4H, py H2),
8.21 (td, J(H,H) = 7.8 Hz, 1.8 Hz, 2H, bpy H4), 8.13 (d, J(H,H) = 8.3
Hz, 2H, bpy H3), 7.66 (t, J(H,H) = 6.5 Hz, 2H, bpy H5), 7.41−7.44
(m, 4H, py H3), 7.17−7.21 (m, 24H, o,m-BPh3), 7.09−7.12 (m, 6H, p-
BPh3). UV/vis (CH2Cl2): λmax (nm) (ε (M−1 cm−1)) 379 (11000),
320 (40000), 311 (44000). Anal. Calcd for C60H46BN4Pt·0.2CH2Cl2:
C, 68.42; H, 4.43; N, 5.30. Found: C, 68.66; H, 4.45; N, 5.19.
room temperature using a JEOL ECA-500 or ECA-600 NMR
spectrometer. UV−vis absorption spectra were recorded in CH2Cl2
or benzene solutions in screw-capped 1 cm quartz cuvettes using an
Agilent Carey 8454 UV−vis spectrophotometer. Steady-state emission
spectra were recorded using a Horiba FluoroMax-4 spectrofluor-
ometer. To exclude air, samples for emission spectra were prepared in
a nitrogen-filled glovebox using dry, deoxygenated solvents. Samples
for room-temperature emission were housed in 1 cm quartz cuvettes
with septum-sealed screw caps, and samples for low-temperature
emission were contained in a custom quartz EPR tube with high-
vacuum valve and immersed in liquid nitrogen using a finger Dewar.
Emission quantum yields were determined relative to a standard of
quinine sulfate in 0.05 M sulfuric acid, which has a reported
fluorescence quantum yield (ΦF) of 0.52.59 Cyclic voltammetry
(CV) experiments were performed with a CH Instruments 602E
potentiostat using a three-electrode system under an N2 atmosphere. A
3 mm diameter glassy-carbon electrode, Pt wire, and silver wire were
used as working electrode, counter electrode, and pseudoreference
electrode, respectively. Measurements were carried out in dichloro-
methane solution with 0.2 M TBAPF6 as a supporting electrolyte at
scan rate of 0.1 V/s. Ferrocene was used as an internal standard, and
potentials were referenced to the ferrocene/ferrocenium couple.
Elemental analyses were performed by Midwest Microlab, LLC.
1
NMR Data for Pt(bpz)(4-C6H4CF3)2 (2). H NMR (600 MHz,
C6D6, 20 °C, TMS): δ 7.95 (s, 2H, bpz H3), 7.77 (d, J(H,H) = 1.4 Hz,
2H, bpz H5), 7.65 (d, J(H,H) = 7.6 Hz, 2H, bpz H6), 7.48 (d, J(H,H)
= 8.3 Hz, 4H, Ar−H2), 7.42 (d, J(H,H) = 2.8 Hz, 4H, Ar−H3). 19F
NMR (565 MHz, C6D6, 20 °C, CFCl3): δ −61.21 (s, 6F).
Preparation of BArF Adducts of 2. These were prepared as
3
described by Tilley,30 by combining complex 2 (3−4 mg) and 1 or 2
equiv of BArF in C6D6. NMR data are in good agreement with those
3
in the previous report and are summarized below. UV−vis data for
these adducts are also summarized below.
1
2-(BArF ). H NMR (600 MHz, C6D6, 20 °C, TMS): δ 8.47 (br),
3
8.14 (br), 7.80 (br), 7.61 (br), 7.44−7.48 (br), 7.37 (br), 7.22 (br). 19F
NMR (565 MHz, C6D6, 20 °C, TMS): δ −61.41 (br, 3F), −61.55 (br,
3F), −130.87 (br, s, 6F), −152.1 (br, s, 3F), −160.63 (br, s, 6F). UV/
vis (C6H6): λmax (nm) (ε (M−1 cm−1)) 295 (18000), 322 (17000), 504
(3900), 616 (4400).
2-(BArF ) . 1H NMR (600 MHz, C6D6, 20 °C, TMS): δ 8.49 (s, 2H,
3 2
bpz H3), 8.40 (s, 2H, bpz H5), 7.43 (d, J(H,H) = 7.6 Hz, 4H), 7.25
(br, 6H). 19F NMR (565 MHz, C6D6, 20 °C, TMS): δ −61.66 (s, 6F),
−131.04 (br, s, 12F), −151.56 (br, s, 6F), −160.27 (br, s, 12F). λmax
(nm) (ε (M−1 cm−1)) 388 (sh) (11000), 472 (7100), 567 (3600), 699
(4600).
1
1H NMR Data for Pt(bpy)(CCpy)2 (1). H NMR (600 MHz,
CD2Cl2, 20 °C, TMS): δ 9.64 (d, J(H,H) = 5.5 Hz, 2H, bpy H6), 8.43
(d, J(H,H) = 5.5 Hz, 4H, py H2), 8.19 (td, J(H,H) = 7.8 Hz, 1.4 Hz,
2H, bpy H4), 8.12 (d, J(H,H) = 8.3 Hz, 2H, bpy H3), 7.65 (t, J(H,H) =
7.2 Hz 2H, bpy H5), 7.29 (m, 4H, py H3).
General Procedure for Preparation of Borane Adducts 1-
(BAr3)n (Ar = Ph, C6F5; n = 1, 2). For NMR-scale reactions, a
solution of complex 1 (3−5 mg) in 0.5 mL of CD2Cl2 was treated with
1 or 2 equiv of the appropriate borane, also dissolved in 0.5 mL of
CD2Cl2. The solution was transferred to an NMR tube, and NMR
analysis showed immediate, near-quantitative formation of the adducts.
The borane adducts could be prepared on a larger scale and isolated;
in a typical preparation 20−30 mg of 1 was combined with 1 or 2
equiv of the borane in minimum CH2Cl2 to ensure complete solubility.
After stirring at room temperature for ∼4 h, the solvent was removed
under vacuum, and the product was washed with pentane to yield
analytically pure material in quantitative yield. NMR and analytical
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00332.
Crystallographic data table, crystal structure of complex
2, NMR spectra for borane addition to complex 1,
additional UV−vis absorption and emission spectra, and
cyclic voltammograms of complexes 1 and 2 and
pyridine·BArF (PDF)
data for each of the adducts are given below.
3
1
1-(BArF ). H NMR (600 MHz, CD2Cl2, 20 °C, TMS): δ 9.61 (d,
Crystallographic data for complexes 1-(BArF )2 and 2
3
J(H,H) = 4.8 Hz, 1H, bpy H6), 9.41 (d, J(H,H) = 4.8 Hz, 1H, bpy H6),
8.41 (s, 2H, py H2), 8.30 (d, J(H,H) = 5.5 Hz, 2H, py H2), 8.19−8.25
(m, 2H, bpy H4), 8.12−8.17 (m, 2H, bpy H3), 7.66−7.70 (m, 2H, bpy
H5), 7.51 (d, J(H,H) = 6.9 Hz, 2H, py H3), 7.32 (d, J(H,H) = 4.1 Hz,
2H, py H3). 19F NMR (565 MHz, CD2Cl2, 20 °C, CFCl3): δ −131.84
(br, s, 6F), −157.85 (t, J(F,F) = 20 Hz, 3F), −164.14 (m, 6F). UV/vis
(CH2Cl2): λmax (nm) (ε (M−1 cm−1)) 376 (9100), 322 (25000), 311
3
(CIF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for T.S.T.: tteets@uh.edu.
G
DOI: 10.1021/acs.organomet.6b00332
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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(32) Eryazici, I.; Moorefield, C. N.; Newkome, G. R. Chem. Rev.
Notes
The authors declare no competing financial interest.
2008, 108, 1834−1895.
ACKNOWLEDGMENTS
(33) Farley, S. J.; Rochester, D. L.; Thompson, A. L.; Howard, J. A.
K.; Williams, J. A. G. Inorg. Chem. 2005, 44, 9690−9703.
(34) Wong, K. M.-C.; Tang, W.-S.; Lu, X.-X.; Zhu, N.; Yam, V. W.-
W. Inorg. Chem. 2005, 44, 1492−1498.
■
We acknowledge the Welch Foundation (Grant No. E-1887)
and the University of Houston for funding this research.
(35) Lo, H.-S.; Yip, S.-K.; Wong, K. M.-C.; Zhu, N.; Yam, V. W.-W.
Organometallics 2006, 25, 3537−3540.
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DOI: 10.1021/acs.organomet.6b00332
Organometallics XXXX, XXX, XXX−XXX