Nonconjugated dimesitylboryl-functionalized phenylpyridines and their cyclometalated platinum(II) complexes

Article abstract of DOI:10.1021/om200539r

To investigate possible through-space charge transfer transitions between a Pt(II) ion and a triarylborane unit, the two new nonconjugated molecules 1 and 2 have been synthesized and fully characterized. Compound 1 has a V-shaped geometry, while 2 has a U-shaped geometry. In 1 a BMes₂Ar group and a ppy group (Mes = mesityl, ppy = phenylpyridine) are linked together by a SiPh₂ unit, while in 2 these groups are joined together by a 1,8-naphthyl linker. The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analyses. Their cyclometalated compounds Pt-1 and Pt-2 with a Pt(acac) unit chelated to the ppy site have been synthesized. Computational and experimental examinations on the photophysical properties of the free ligands and the Pt(II) compounds revealed that the molecular shape and geometry of the molecule have a distinct impact on the fluorescence and phosphorescence of these molecules. Pt-1 is a bright phosphorescent emitter with _em 490 nm and Φ = 66% while Pt-2 is very weakly emissive with _em 567 nm and Φ = ～0.05%. Anions such as fluoride were found to have no impact on the phosphorescence of these two Pt(II) compounds, thus establishing that phosphorescence of these molecules does not involve a through-space charge transfer transition between the Pt(ppy)(acac) unit and the BMes₂Ar unit.

Full text of DOI:10.1021/om200539r

ARTICLE
pubs.acs.org/Organometallics
Nonconjugated Dimesitylboryl-Functionalized Phenylpyridines and
Their Cyclometalated Platinum(II) Complexes
Zachary M. Hudson and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario, Canada K7L 3N6
S Supporting Information
b
ABSTRACT: To investigate possible through-space charge transfer
transitions between a Pt(II) ion and a triarylborane unit, the two new
nonconjugated molecules 1 and 2 have been synthesized and fully
characterized. Compound 1 has a V-shaped geometry, while 2 has a
U-shaped geometry. In 1 a BMes2Ar group and a ppy group (Mes = mesityl,
ppy = phenylpyridine) are linked together by a SiPh2 unit, while in 2 these
groups are joined together by a 1,8-naphthyl linker. The crystal structures of
1 and 2 were determined by single-crystal X-ray diffraction analyses. Their
cyclometalated compounds Pt-1 and Pt-2 with a Pt(acac) unit chelated to
the ppy site have been synthesized. Computational and experimental
examinations on the photophysical properties of the free ligands and the Pt(II) compounds revealed that the molecular shape
and geometry of the molecule have a distinct impact on the fluorescence and phosphorescence of these molecules. Pt-1 is a bright
phosphorescent emitter with λem 490 nm and Φ = 66% while Pt-2 is very weakly emissive with λem 567 nm and Φ = ∼0.05%. Anions
such as fluoride were found to have no impact on the phosphorescence of these two Pt(II) compounds, thus establishing that
phosphorescence of these molecules does not involve a through-space charge transfer transition between the Pt(ppy)(acac) unit and
the BMes2Ar unit.
’ INTRODUCTION
example, compounds B and C incorporate distinct donor and
acceptor chromophores connected by rigid and nonrigid linkers,
respectively. In the absence of fluoride, both exhibit through-
space charge transfer as broad green emission. When fluoride is
added, this emission readily switches to the blue fluorescence of
the arylamine chromophore.
More recent research has shown that the triarylboron group is
capable of greatly enhancing metal-to-ligand charge transfer
(MLCT) in transition-metal compounds, leading to higher
luminescent quantum yields and brighter phosphorescent
emission.7 In combination with the improved electron-transport-
ing and film-forming properties afforded by the triarylboron
group, this enhanced phosphorescence has allowed for the
fabrication of OLEDs with exceptional performance.7cꢀf Further-
more, several reports have described the use of organometallic
triarylboranes for phosphorescent sensing,6 in which interference
from background fluorescence or scattering may be removed by
time-gated detection.
Optoelectronic materials containing a triarylboron group have
been the subject of considerable recent research due to the
electron-accepting ability of the boron center. When accompa-
nied by appropriate electron donors such as amines, the triaryl-
boron center promotes intense donorꢀacceptor charge transfer
luminescence in π-conjugated materials.1 The empty pπ orbital
of triarylboranes stabilizes the formation of radical anions and has
led to their successful use in nonlinear optical materials2 and as
emissive and electron-transport materials in organic light-emitting
diodes (OLEDs).3
Triarylboranes have also been extensively investigated as
chemical sensors for anions. While bulky mesityl groups protect
the boron center from most nucleophiles, small anions such as
fluoride and cyanide are capable of binding to the boron center
despite this steric protection.4ꢀ6 As the empty p orbital on boron
typically makes a large contribution to the LUMO of the
molecule, this binding event may be observed by colorimetric,
luminescent, or electrochemical changes. Luminescent sensing is
particularly attractive, as it allows the detection of an analyte with
high sensitivity. Directly conjugated donorꢀacceptor boranes
typically act as “switch-off” luminescent sensors, as binding of an
anion to the boron center blocks the empty p orbital, deactivating
any charge-transfer emission. (compound A, Chart 1)
Despite advances in related organic systems, however, non-
conjugated triarylboranes incorporating a metal center as donor
have not been widely studied, and little is known about the
through-space interaction between metal centers and the boron
group. Only one example of such a compound, Pt(N,C-Si-
BNPA)(SMe2)Ph, is known to date, and this material was found
to exhibit simultaneous singlet and triplet dual emission from
We have recently shown, however, that nonconjugated do-
norꢀacceptor triarylboranes can act as effective “switch-on”
sensors, in which binding of an analyte may be clearly observed
by a change in luminescence from one color to another.5 For
Received: June 22, 2011
Published: August 05, 2011
r
2011 American Chemical Society
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Chart 1
Scheme 1a
a Reagents and conditions: (i) n-BuLi, THF, ꢀ78 °C; (ii) FBMes2, ꢀ78 °C to room temperature; (iii) 2-(SnBu3)pyridine, Pd(PPh3)4, THF, reflux;
(iv) PtCl(DMSO)(acac), NaOAc, THF/MeOH, reflux; (v) ZnCl2, 0 °C, then 1,8-diiodonaphthalene, Pd(PPh3)4, 0 °C to room temperature;
(vi) 4-(20-py)phenylzinc bromide, Pd(PPh3)4, THF, room temperature.
both chromophores.5c To further investigate these systems, we
designed ligands 1 and 2 (Scheme 1) that incorporate a flexible
silane and a rigid naphthyl linker for a BMes2Ar unit and a
Pt(ppy)(acac) unit (ppy =2-pyridylphenyl), which may readily
serve as cyclometalating ligands to a variety of transition-metal
atoms. Herein we report the synthesis and photophysical proper-
ties of these compounds and their Pt(II) complexes.
both cases by Negishi coupling via organozinc intermediates.
The Pt(II) acetylacetonate (acac) complexes of these ligands
were prepared under mild conditions on stirring with PtCl-
(DMSO)(acac) in the presence of NaOAc in refluxing
methanol.7c,d,9 The use of acac as an ancillary ligand has several
advantages, giving stable complexes that may be isolated in high
purity by column chromatography. In addition, the rigidity and
high triplet level of the acac ligand minimize its impact on the
quantum yield of the phosphorescent phenylpyridine Pt(II)
unit.10
’ RESULTS AND DISCUSSION
Synthesis. The syntheses of compounds Pt-1 and Pt-2 are
shown in Scheme 1. Pt-1 incorporates a silane linker with a high
degree of rotational flexibility, while Pt-2 includes a more rigid
naphthyl spacer that both decreases the metalꢀboron separation
distance and increases the steric congestion of the molecule.
Intermediate 1a is prepared by introducing the boryl substituent
to bis(p-bromophenyl)diphenylsilane by lithiumꢀhalogen ex-
change and substitution with FBMes2. Subsequent Stille cou-
pling with the appropriate pyridyl stannane gives 1 in good yield.
It should be noted that more common Suzuki coupling reactions
should be avoided once the dimesitylboron group is introduced,
as the basic conditions required for this reaction can lead to
unwanted side products in the presence of Pd catalyst.8 Ligand 2
is most efficiently prepared by synthesis of first the arylboron and
phenylpyridine arms of the molecule, followed by their stepwise
coupling to 1,8-diiodonaphthalene. This is readily achieved in
Crystal Structures. Single crystals suitable for X-ray diffrac-
tion analysis were obtained for the free ligands 1 and 2 by slow
evaporation from CH2Cl2/hexanes, while efforts to obtain single
crystals of the Pt(II) complexes were unsuccessful. The large
difference in steric congestion between ligands 1 and 2 is readily
apparent on comparison of these structures (Figure 1). In the
crystal structure of 1, the boron and phenylpyridine arms of the
molecule show no intramolecular steric interaction, with the
shortest separation distance being 9.89 Å from the boron atom to
the pyridyl ring. In contrast, in ligand 2 this distance is much
shorter (5.55 Å on average for the two independent molecules in
the lattice), as shown in Figure 1. The crystal structure of 2
reveals considerable strain, with the naphthyl linker being out of
coplanarity by approximately 14° (Figure 2). Though joined by a
continuous path of sp2-hybridized atoms, the boron and phe-
nylpyridine arms of 2 show torsion angles of 40.8 and 44.8° with
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the naphthyl ring, expected to greatly reduce electronic commu-
nication through the linker. The CꢀC bond distances between
the phenyl groups and the naphthyl ring are 1.486(4) and
1.490(4) Å, respectively, further supporting this poor conjuga-
tion. Molecules of 2 pack in such a manner in the crystal lattice
that the BMes2 groups are all on the same side while the
pyridylphenyl groups are parallel to each other (Figure 2).
Photophysical and Electrochemical Properties. Ligands 1
and 2 both display broad absorptions centered around 325 nm,
characteristic of charge transfer from the filled π orbitals of
adjacent aryl groups to the empty p orbital on the boron center
(Table 1). The molar extinction coefficient of this band in 2 is
more than twice as intense as that in 1, owing to the additional
contribution from the electron-rich naphthyl ring (Figure 3).
Chelation to Pt(II) introduces a distinct low-energy absorption
band in both cases, characteristic of MLCT from the Pt(II)
center to the phenylpyridine chelate.7
close proximity of the boron center to a relatively electron rich
naphthyl and ppy group in 2. The Pt(acac) compounds, however,
both have very similar reduction potentials. For Pt-2, two well-
resolved reduction peaks are observed, which may be assigned to
the reduction of the Pt(ppy) leg and the BMes2 leg, respectively.
Using the CV and UVꢀvis data, HOMO and LUMO energies for
all compounds have been estimated and provided in Table 2.
DFT Calculations. To understand the impact exerted by Pt(II)
chelation, we performed DFT calculations at the B3LYP level of
theory14 using LANL2DZ as the basis set for Pt and 6-31G* for
all other atoms (Figure 5). The computational results show that
while both ligands possess a distinct boron-centered LUMO, the
LUMOs of the respective Pt complexes are instead centered on
the ppy chelate and occur at similar energies, consistent with the
CV data. Furthermore, in both cases the energy of the MO based
Table 1. Photophysical Properties of All Compoundsa
The CV diagrams of both ligands and their Pt(II) complexes
are shown in Figure 4. Ligand 2 has a poor solubility in DMF and
THF, resulting in a weak reduction peak. Nonetheless, the data
show that ligand 1 has a somewhat more positive reduction
potential than 2 (by ∼20 mV). This may be attributed to the
abs λmax (nm)
(ε (104 Mꢀ1 cmꢀ1))
compd
λem (nm)
Φb
τP (μs)
1
271 (3.31), 285 (2.63),
401
0.21
304 (1.95), 325 (1.32)
Pt-1
2
262 (4.57), 285 (3.55), 325 (2.67)
286 (2.29), 331 (3.41), 400 (0.45)
287 (2.67), 333 (3.86), 413 (0.44)
494
406
473
0.66
1.0
6.3
5.7
Pt-2
0.005
a Obtained at 1.0 ꢁ 10ꢀ5 M in CH2Cl2 at 298 K. b All quantum yields (10%.
Figure 1. Diagrams showing the crystal structures of ligands 1 (left) and
2 (right) with 35% thermal ellipsoids: (red) Si; (pink) B; (blue) N.
Important bond lengths (Å) and angles (deg) for 1: SiꢀC(Ph), 1.855(4),
1.868(4); SiꢀC(ppy), 1.871(4); SiꢀC(ArꢀB), 1.863(4); BꢀC, 1.570(5),
1.571(6), 1.572(6); CꢀSiꢀC, 108.36(15), 108.55(18), 109.46(16),
114.20(7), 107.76(16), 108.74(18); CꢀBꢀC, 116.2(3), 120.4(3),
123.4(3). Important bond lengths (Å) and angles (deg) for the two
independent molecules of 2: BꢀC, 1.552(4)/1.557(4), 1.581(4)/
1.581(4), 1.584(4)/1.582(4); CꢀBꢀC, 116.5(2)/117.7(2), 119.0(2)/
117.7(2), 124.4(2)/124.7(2).
Figure 3. UVꢀvis absorption spectra of the boryl ligands and their
Pt(acac) compounds in CH2Cl2.
Figure 2. Views of 2: (left) side view showing the bending of the naphthyl ring; (right) diagram showing the orientation and packing of molecules 2 in
the crystal lattice.
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on the empty p orbital on boron has been destabilized consider-
ably. This suggests that Pt(II) chelation has a significant impact
on the electronic properties of the BMes2 center, despite the lack
of direct conjugation between these two portions of the
molecule. This is in contrast to the effect observed in fully
conjugated systems, where it has been shown that metal chela-
tion can substantially improve the electron-accepting ability of
triarylboranes by Coulombic and inductive effects.7,11 The
HOMO for 1 is localized on the mesityl π orbitals, while that
of 2 is localized mostly on the naphthyl ring. Introduction of the
metal center is found to have the largest impact on the HOMO
energies of these molecules, raising this level by 0.5ꢀ0.7 eV. This
results in a clear change in the lowest energy electronic transition
for both nonconjugated boranes, from one based on the triar-
ylboron group itself to a transition that is largely based on the
metal chelate site. It is possible that the increased electron density
of the ppy chelate in the complex increases repulsive interactions
between the two different arms, thus increasing the π* energy
level of the triarylboron unit.
Luminescence. Ligands 1 and 2 both show high-energy
purple fluorescence upon irradiation with UV light, with little
change in emission maximum despite considerable differences in
structure (Figure 6). However, while 1 displays only a moderate
fluorescence quantum yield of 0.21, that of 2 is near unity,
consistent with previously reported naphthyl-functionalized
triarylboranes.12 Despite the similarities in the fluorescence of
the ligands, the phosphorescent properties of Pt-1 and Pt-2 are
dramatically different. Both display long-lived phosphorescent
emission, with decay lifetimes of 6.3 and 5.7 μs, respectively. Pt-1
shows bright blue-green emission in the solid state and solution
(λem 490 nm), similar to that of Pt(ppy)(acac),10 with a
remarkable quantum yield of 0.66 in degassed CH2Cl2. The
vibrational features clearly visible in the emission spectrum of
Pt-1 indicate that this phosphorescence may be attributed to a
mixture of ligand-centered (LC) and MLCT excited states. Pt-2,
however, exhibits only very weak, broad orange phosphorescence
(λem 567 nm), with a quantum yield of approximately 0.005,
attributable to the considerable strain in this molecule. This low-
energy emission likely has contributions from through-space
intramolecular charge transfer from the Pt(ppy)(acac) unit to the
Figure 4. Cyclic voltammetry diagrams of the boryl ligands and their
Pt(acac) compounds in DMF (with ∼10% THF) with NBu4PF6 as the
electrolyte.
Table 2. Electrochemical Properties and Experimental
HOMO and LUMO Energiesa
compd
E1/2red (V)b
HOMO (eV)
LUMO (eV)
1
ꢀ2.41
ꢀ5.84
ꢀ5.21
ꢀ5.71
ꢀ5.17
ꢀ2.39
ꢀ2.36
ꢀ2.37
ꢀ2.34
Pt-1
2
ꢀ2.46
ꢀ2.43
ꢀ2.46, ꢀ2.67
Pt-2
a The LUMO energy level was estimated using the first reduction
potential, while the HOMO energy level was obtained by using the
optical energy gap (absorption edge) and the LUMO value. b Recorded
+/0
in DMF with NBu4PF6 as the electrolyte, relative to FeCp2
.
Figure 5. Molecular orbital surfaces and energies obtained from DFT calculations.
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Figure 6. Excitation spectra (dashed lines) and emission spectra (solid lines) for 1 and 2 (left) and their Pt(II) complexes (right), obtained at 10ꢀ5 M
in CH2Cl2 at 298 K.
BMes2Ph unit, since there are considerable contributions from
the BMesPh unit at the LUMO level of Pt-2 (Figure 5), while no
similar contributions are present for Pt-1. The distinct phos-
phorescence of the two Pt(II) complexes illustrates the impor-
tance of donorꢀacceptor geometry on modulating phosphore-
scent color and efficiency. The photophysical properties of all
compounds are summarized in Table 1.
mostly localized on the Pt(ppy)(acac) portion of the molecule,
consistent with DFT calculations which show the HOMO and
LUMO located on this arm of the complex in both cases. The
phosphorescent response of Pt-1 toward fluoride ions is in sharp
contrast with that of the closely related V-shaped molecule Pt(N,
C-Si-BNPA)(SMe2)Ph we reported recently,5c where a BMes2Ar
group and a Pt(N,C-NPA)(SMe2)Ph unit (NPA = N-(20-
pyridyl)-7-azaindole) are linked together by a SiPh2 group.
Pt(N,C-Si-BNPA)(SMe2)Ph displays simultaneous singlet and
triplet dual emission originating from the boryl and Pt arms of the
molecule, respectively, and has a distinct response to the addition
of fluoride ions.5c Compounds Pt-1 and Pt-2, however, demon-
strate that the dual emissive properties of a nonconjugated
system are highly dependent on the nature and the energetic
state of the chelate group around the metal center.
To further explore the electronic structure of these nonconju-
gated molecules, we performed luminescent titration experi-
ments using NBu4F (TBAF) in CH2Cl2. Both boryl ligands 1
and 2 display a fluorescent quenching response with the addition
of fluoride (see the Supporting Information). This suggests that
the fluorescence of these two molecules does not involve
through-space charge transfer from the ppy group to the boron
center, as such transitions are typically characterized by activation
of a higher energy emission band based on the donor group once
fluoride blocks the boron site.5 This is also in agreement with
DFT results, which suggest that the HOMO and LUMO of both
molecules are based on the boron leg. In contrast to diarylamino
groups, which are excellent electron donors and promote
through-space intramolecular charge transfer in nonconjugated
systems,5a,b the ppy ligand cannot serve as an appropriate
electron donor in molecules 1 and 2. In the case of 1, the
fluorescent emission is completely quenched on saturation with
Fꢀ, while 2 experiences only ∼60% emission quenching. This is
consistent with titrations of both molecules in absorption mode.
Compound 1 displays complete quenching of the 325 nm
absorption band, indicating that the boryl unit is the sole
contributor to this electronic transition. Compound 2, however,
only shows partial quenching of the 325 nm band, confirming
that the naphthyl linker plays a role in the fluorescent emission of
the molecule.
’ CONCLUSIONS
A pair of nonconjugated dimesitylboron-containing Pt(II)
phosphors have been achieved, with remarkably different proper-
ties based on the nature of the linker. The V-shaped Pt-1 shows
bright green phosphorescence at room temperature, while the
U-shaped Pt-2 is almost nonemissive under ambient conditions,
indicating that the molecular geometry and shape have a
significant impact on the phosphorescence of BMes2-function-
alized Pt(II) compounds. With an improved understanding of
their synthesis and photophysical properties, future studies will
examine the impact of the spatially proximate Lewis acidic boron
center on the reactivity of Pt(II) complexes.
’ EXPERIMENTAL SECTION
General Considerations. All reactions were carried out under a
nitrogen atmosphere. Reagents were purchased from Aldrich Chemical
Co. and used without further purification. Thin-layer and flash chroma-
tography were performed on silica gel. 1H and 13C NMR spectra were
recorded on Bruker Avance 400, 500, and 600 MHz spectrometers.
Deuterated solvents were purchased from Cambridge Isotopes and used
without further drying. Excitation and emission spectra were recorded
using a Photon Technologies International QuantaMaster Model 2
spectrometer. UVꢀvisible absorbance spectra were recorded using a
Varian Cary 50 UVꢀvisible absorbance spectrophotometer. Cyclic
voltammetry experiments were performed using a BAS CV-50W
analyzer with a scan rate of 0.2ꢀ1.0 V/s using 1 mg of sample in
0.5 mL of dry DMF. The electrochemical cell was a standard three-
compartment cell composed of a Pt working electrode, a Pt auxiliary
electrode, and an Ag/AgCl reference electrode. CV measurements were
When metal complexes Pt-1 and Pt-2 are titrated with
fluoride, however, both show partial quenching of the broad
absorption at 325 nm, while the MLCT band is in both cases
totally unaffected (see the Supporting Information). This is in
contrast to directly conjugated systems, suggesting that the
presence of a spatially proximate but electronically separated
boryl acceptor does not perturb electronic excited states at lower
energy elsewhere in the molecule. Indeed, the low-energy
phosphorescence displayed by Pt-1 and Pt-2 is totally unaffected
by saturation of the BMes2 binding site with fluoride. These
results are quite unexpected, and to our knowledge this is the first
case in which the phosphorescence of BMes2-containing metal
compounds could not be affected by anion binding. These results
also support that the phosphorescence of both Pt-1 and Pt-2 is
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carried out at room temperature with 0.1 M NBu4PF6 as the supporting
electrolyte, with ferrocene/ferrocenium as internal standard (E° = 0.55
V). Solution quantum yields were calculated using optically dilute
solutions (A ≈ 0.1), using anthracene as the standard for 1 and 2 and
naph, ꢀC6H4ꢀ), 7.09 (d, J = 7.9 Hz, 2H, ꢀC6H4ꢀ), 6.72 (s, 4H, Mes),
2.27 (s, 6H, Mes), 1.75 (s, 12H, Mes) ppm. 13C NMR (125 MHz,
CDCl3): δ 157.0, 149.5, 147.6, 144.1, 141.5, 140.6, 140.2, 139.9, 138.1,
136.6, 136.5, 136.3, 135.6, 131.6, 131.2, 130.0, 129.2, 129.0, 128.8, 128.6,
127.9, 127.9, 125.9, 125.4, 125.2, 121.8, 23.4, 21.1 ppm. HRMS: m/z
calcd for C45H40BN 605.3254, found 605.3248.
13
Ir(ppy)3 for Pt-1 and Pt-2. Elemental analyses were performed by
Canadian Microanalytical Service Ltd., Delta, British Columbia, Canada.
Molecular orbital and molecular geometry calculations were performed
using the Gaussian 03 program suite. DFT calculations were performed
at the B3LYP level of theory14 using LANL2DZ as the basis set for Pt and
6-31G* for all other atoms. The synthesis of 1a,5b 2a,5b 2-(40-bromophenyl)-
pyridine,7d and PtCl(DMSO)(acac)9 have been reported previously.
Synthesis of 1. In a 50 mL Schlenk flask with a stir bar and
condenser were added 1a (150 mg, 0.23 mmol), Pd(PPh3)4 (13.1 mg,
0.011 mmol), and 30 mL of degassed THF. 2-(Tributylstannyl)pyridine
(0.35 mL, 0.91 mmol) was then added via syringe, and the mixture was
heated at reflux for 48 h. The solvent was removed under reduced
pressure and the residue purified on silica (2/1 hexanes/CH2Cl2 as
eluent) to give 76 mg of 1 as a white solid (51% yield). 1H NMR (400
MHz, CDCl3): δ 8.72 (d, J = 4.7 Hz, 1H, Py), 8.00 (d, J = 8.1 Hz,
2H, ꢀC6H4ꢀ), 7.80 (t, J = 7.6 Hz, 1H, Py), 7.76 (d, J = 7.7 Hz, 1H, Py),
7.68 (d, J = 8.1 Hz, 2H, ꢀC6H4ꢀ), 7.62ꢀ7.56 (m, 6H, ꢀPh, ꢀC6H4ꢀ),
7.48 (d, J = 7.8 Hz, 2H, ꢀC6H4ꢀ), 7.44 (t, J = 7.3 Hz, 2H, ꢀPh), 7.37
(t, J = 7.4 Hz, 4H, ꢀPh), 7.28 (dd, J = 6.5 Hz, J = 4.7 Hz, 1H, Py), 6.80
(s, 4H, Mes), 2.29 (s, 6H, Mes), 2.01 (s, 12H, Mes) ppm. 13C NMR (100
MHz, CDCl3): δ 156.9, 149.2, 147.2, 141.7, 140.8, 139.7, 138.7, 138.2,
137.4, 136.9, 135.8, 135.5, 135.0, 134.0, 133.8, 129.7, 128.1, 127.9, 126.4,
122.5, 121.0, 23.4, 21.2 ppm. HRMS: m/z calcd for C47H44BNSi
661.3345, found 661.3347.
Synthesis of Pt-1. In a 50 mL Schlenk flask with a stir bar and
condenser were added 1(70 mg, 0.11 mmol), PtCl(DMSO)(acac) (43 mg,
0.11 mmol), NaOAc (8.7 mg, 0.11 mmol), and 20 mL of degassed
1/1 THF/MeOH. The mixture was heated at reflux for 3 days, and then
the solvent was removed under reduced pressure and the residue
purified on silica (1/1 hexanes/CH2Cl2 as eluent) to give 38 mg of
Pt-1 as a yellow solid (38% yield). 1H NMR (400 MHz, C6D6): δ 9.06
(d, sat, J = 5.4 Hz, 1H, Py), 8.67 Hz (s, sat, 1H, PyꢀPh), 7.95 (d, J = 7.5
Hz, 2H, ꢀC6H4ꢀ), 7.93ꢀ7.89 (m, 4H), 7.69 (d, J = 7.6 Hz, 2H, ꢀ
C6H4ꢀ), 7.63 (d, J = 7.6 Hz, 1H, PyꢀPh), 7.34ꢀ7.24 (m, 6H), 6.91 (d,
J = 7.8 Hz, 1H, PyꢀPh), 6.86 (s, 4H, Mes), 6.83 (t, J = 7.2 Hz, 1H, Py),
6.23 (t, J = 7.2 Hz, 1H, Py), 5.25 (s, 1H, acac), 2.27 (s, 6H, Mes), 2.21 (s,
12H, Mes), 1.79 (s, 3H, acac), 1.63 (s, 3H, acac) ppm. 13C NMR (100
MHz, C6D6): δ 185.7, 184.7, 169.0, 147.9, 147.7, 146.7, 142.7, 141.4,
140.6, 140.3, 140.2, 139.1, 138.0, 137.5, 137.1, 136.0, 135.9, 135.6, 132.1,
130.0, 129.1, 128.3, 122.9, 121.4, 118.8, 102.9, 28.5, 27.3, 24.1, 21.7 ppm.
Anal. Calcd for C52H50BNO2PtSi: C, 65.40; H, 5.28; N, 1.47. Found: C,
64.30; H, 4.80; N, 1.43. The low carbon content may be caused by
solvent molecules (CH2Cl2) trapped in the solid or incomplete
combustion of boron. The purity of the sample was verified by a
1H NMR spectrum (see the Supporting Information, Figure S2).
Synthesis of 2. In a 50 mL Schlenk flask with a stir bar was added
2-(40-bromophenyl)pyridine (181 mg, 0.77 mmol) and 30 mL of dry,
degassed THF. The solution was cooled to ꢀ78 °C, and then n-BuLi
(0.53 mL, 0.85 mmol, 1.6 M in hexanes) was added dropwise via syringe.
After the mixture was stirred for 1 h, anhydrous ZnCl2 (137 mg, 1.01
mmol) was added. The mixture was stirred for 1 h at ꢀ78 °C and then
1 h at 0 °C, and then Pd(PPh3)4 (45 mg, 0.39 mmol) and 2a (300 mg,
0.52 mmol) were added. The mixture was stirred for 1 h at 0 °C and then
slowly warmed to room temperature and stirred for 48 h. After removal
of the solvent under reduced pressure, the residue was purified on silica
(30/1 hexanes/EtOAc as eluent) to give 253 mg of 2 as a white solid
(81% yield). 1H NMR (500 MHz, CDCl3): δ 8.66 (d, J = 4.1 Hz, 1H,
Py), 7.95 (d, J = 7.9 Hz, 2H, ꢀC6H4ꢀ), 7.68 (t, J = 7.6 Hz, 1H, Py), 7.64
(d, J = 8.3 Hz, 2H, ꢀC6H4ꢀ), 7.59 (d, J = 6.1 Hz, 2H, naph), 7.56 (d, J =
7.2 Hz, 1H, Py), 7.45 (t, J = 7.8 Hz, 2H, naph), 7.23ꢀ7.14 (m, 5H, Py,
Synthesis of Pt-2. In a 50 mL Schlenk flask with a stir bar and
condenser was added 2(50 mg, 0.083 mmol), PtCl(DMSO)(acac) (34 mg,
0.083 mmol), NaOAc (6.8 mg, 0.083 mmol), and 20 mL of degassed
1/1 THF/MeOH. The mixture was heated at reflux for 3 days, and then
the solvent was removed under reduced pressure and the residue
purified on silica (2/1 hexanes/CH2Cl2 as eluent) to give 27 mg of
Pt-2 as a yellow solid (36% yield). 1H NMR (500 MHz, CDCl3): δ 9.09
(d, J = 5.4 Hz, 1H, Py), 8.04 (s, 1H, PyꢀPh), 7.72 (d, J = 8.5 Hz, 1H,
naph), 7.71 (d, J = 8.5 Hz, 1H, naph) 7.68 (d, J = 6.6 Hz, 1H, PyꢀPh),
7.54 (d, J = 7.7 Hz, 1H, naph), 7.43 (d, J = 7.7 Hz, 1H, Naph), 7.41 (d, J =
6.6 Hz, 1H, PyꢀPh), 7.36ꢀ7.28 (m, 4H), 6.87 (t, J = 7.4 Hz, 1H, Py),
6.85ꢀ6.79 (m, 3H), 6.73 (s, 4H), 6.26 (t, J = 6.2 Hz, 1H, Py), 5.11 (s,
1H, acac), 2.15 (s, 6H, Mes), 2.01 (s, 12H, Mes), 1.69 (s, 3H, acac), 1.53
(s, 3H, acac) ppm. 13C NMR (125 MHz, C6D6): δ 185.7, 184.7, 169.0,
147.9, 147.7, 146.7, 142.7, 141.4, 140.6, 140.2, 139.1, 138.0, 137.5, 137.1,
136.0, 135.6, 132.1, 130.0, 129.1, 122.9, 121.4, 118.8, 102.9, 28.5, 27.3,
24.1, 21.7 ppm (several quaternary carbons could not be detected due to
poor solubility). Anal. Calcd for C50H46BNO2Pt: C, 66.82; H, 5.16; N,
1.56. Found: C, 66.62; H, 5.31; N, 1.46.
X-ray Crystallographic Analysis. Single crystals of 1 and 2 were
mounted on glass fibers and were collected on a Bruker Apex II single-
crystal X-ray diffractometer with graphite-monochromated Mo KR
radiation, operating at 50 kV and 30 mA and at 180 K. Data were
processed on a PC with the aid of the Bruker SHELXTL software
package (version 5.10)15 and corrected for absorption effects. All non-
hydrogen atoms were refined anisotropically. Complete crystal structure
data can be found in the Supporting Information. The crystal data of 1
and 2 have been deposited at the Cambridge Crystallographic Data
Center (CCDC No. 830933 and 830934).
’ ASSOCIATED CONTENT
S
Supporting Information. Figures, tables, and CIF files
b
giving electrochemical data, fluoride titration data, and complete
crystal structure data. This material is available free of charge via
the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: wangs@chem.queensu.ca.
’ ACKNOWLEDGMENT
We thank the Natural Sciences and Engineering Research
Council for financial support.
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