Extending π-conjugation of triarylborons with a 2,2-bpy core: Impact of donor-acceptor geometry on luminescence, anion sensing, and metal ion binding

Article abstract of DOI:10.1021/ic1004159

Four new 2,2′-bipyridine-based molecules functionalized by either BMes₂-phenyl (Mes = mesityl) or NPh₂-phenyl - 5,5′-(p-BMes₂-phenyl)₂-2,2′-bpy (5,5′-BP2bpy, 1), 4,4′-(p-BMes₂-phenyl) ₂-2,2

Full text of DOI:10.1021/ic1004159

4394 Inorg. Chem. 2010, 49, 4394–4404
DOI: 10.1021/ic1004159
Extending π-Conjugation of Triarylborons with a 2,2-Bpy Core: Impact of
Donor-Acceptor Geometry on Luminescence, Anion Sensing, and Metal Ion
Binding
Yi Sun and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
Received March 2, 2010
Four new 2,20-bipyridine-based molecules functionalized by either BMes2-phenyl (Mes=mesityl) or NPh2-phenyl;
5,50-( p-BMes2-phenyl)2-2,20-bpy (5,50-BP2bpy, 1), 4,40-( p-BMes2-phenyl)2-2,20-bpy (4,40-BP2bpy, 2), 4-( p-BMes2-
phenyl)-40-( p-NPh2-phenyl)-2,20-bipy (4,40-BPNPbpy, 4), and 4,40-( p-NPh2-phenyl)2-2,20-bpy (4,40-NP2bpy, 5);
have been synthesized. Their complexes with PtPh2 have been synthesized and fully characterized. The electronic
and photophysical properties of the new molecules have been examined by electrochemical, absorption, and
luminescence spectroscopic analysis and DFT calculations, which show significant differences from those of the
related but smaller 2,20-bpy derivatives functionalized directly by either BMes2 or NPh2 groups that we reported
previously. Molecules 1, 2, and 4 and their Pt(II) complexes respond to fluoride ions in both absorption and emission
modes. The donor-acceptor molecule 4 and its Pt(II) complex have a distinct fluorescence/phosphorescence turn-on
response toward fluoride or cyanide ions. Molecules 1, 2, 4, and 5 also respond to Zn(II) ions in both absorption and
emission modes. The diboryl molecules 1 and 2 have a distinct and contrasting fluorescence response toward Zn(II)
ion;turn-off for 1 and turn-on for 2;demonstrating the significant impact of molecular geometry on metal ion binding
and fluorescence.
Introduction
chelation of such functionalized bipy ligands to a metal ion
can not only further increase the electron and anion affinity
of the molecule but also introduce distinct colors or bright
phosphorescence to the molecule, making the metal com-
plexes attractive sensory materials and potential bifunctional
phosphorescent emitters in organic light-emitting diodes.
Nonetheless, the BMes2 attachment at the 5 and/or 50 posi-
tion of 2,2-bipy does limit the use of the resulting molecules as
ligands for binding to metal ions that have a high coordina-
tion number due to strong interligand steric interactions.
One obvious solution for this problem is to move the BMes2
unit to the 4,40 position, away from the metal center. Un-
fortunately our attempts to synthesize molecules such as
π-Conjugated triarylboron compounds have attracted
much recent research attention because of their broad appli-
cations in optoelectronic devices1,2 and their ability to func-
tion as highly selective sensors3,4 for certain anions via the
disruption of the pπ-π conjugation around the boron center.
A number of research groups have shown recently that the
attachment of a metal center to a triarylboron group can have
a significant impact on the properties of the molecule.5,6 We
have shown previously that the direct attachment of a boron
unit such as BMes2 (Mes=mesityl) to a 2,2-bipy core at the 5
and 50 positions (e.g., 5,50-(BMes2)2-2,20-bpy, 5,50-B2bpy)
can greatly enhance both electron and fluoride affinity of
the boron center.6a,b We have further demonstrated that the
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*To whom correspondence should be addressed. E-mail: wangs@
chem.queensu.ca.
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pubs.acs.org/IC
Published on Web 04/23/2010
2010 American Chemical Society

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4395
Chart 1
4,40-(BMes2)2-2,2-bpy have not been successful. Our recent
efforts have therefore focused on extending the π-conjuga-
tion of the BMes2 with the bipy core by inserting a phenyl
group so that the bulky BMes2 group is farther away from
the metal center, compared to those in 5,50-B2bpy. We
have succeeded in synthesizing 5,50-( p-BMes2-phenyl)2-
2,20-bpy (5,50-BP2bpy, 1), 4,40-( p-BMes2-phenyl)2-2,20-
bpy (4,40-BP2bpy, 2), the donor-acceptor molecule
4-( p-BMes2-phenyl)-40-( p-NPh2-phenyl)-2,20-bipy (4,40-
BPNPbpy, 4), and the donor-only compound 4,40-( p-
NPh2-phenyl)2-2,20-bpy (4,40-NP2bpy, 5) (Chart 1) and
the corresponding Pt(II) complexes. We have compared
the photophysical and electronic properties of the new
ligands and their Pt(II) complexes with those based on 5,50-
B2bpy and 5,50-BNbpy reported by us earlier,6a,b which
revealed a significant difference between these two classes of
molecules. In addition to Pt(II) ions, we have investigated and
compared the interactions of this new group of ligands with
Zn(II) ions. Distinct differences have also been observed
between 4,40-BP2bpy and 5,50-BP2bpy and their complexes.
The full account is presented herein.
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Experimental Section
All reactions were performed under dry N2 with stan-
dard Schlenk techniques unless otherwise noted. All starting
materials were purchased from Aldrich Chemical Co.
and used without further purification. THF, toluene, and
CH2Cl2 were purified using the solvent purification system
(Innovation Technologies Co.). Deuterated solvents CDCl3
and CD2Cl2 (Cambridge Isotopes) were used as received
without further drying. NMR spectra were recorded on a
Bruker Avance 400 spectrometer (400.13 MHz for 1H, 100.62
MHz for 13C). UV-vis spectra were recorded on an Ocean
Optics ISS-UV-vis spectrophotometer. Cyclic voltammetry
was performed using a BAS CV-50W analyzer with a scan
rate of 500 mV/s to 4 V/s and a typical concentration of 5 mg
of the compound in 3 mL of DMF, at room temperature
using 0.10 M NBu4PF6 as the supporting electrolyte. The
electrolytic cell used was a conventional three-compartment
cell, in which a Pt working electrode, a Pt auxiliary elec-
trode, and an Ag/AgCl reference electrode were employed.
The ferrocenium/ferrocene couple was used as the internal
standard (E0=0.55 V). Elemental analyses were performed
at Canadian Microanalytical Service Lto., Delta, British
Columbia. 5,50-Br2-2,20-bipy,7 4,40-Br2-2,20-bipy,7 p-Br-phenyl-
BMes2,1e 2-(4-diphenylaminophenyl)-4,4,5,5,-tetramethyl-
1,3,2-dioxaborolane,8 and [PtPh2(SMe2)]n (n = 2 or 3)9
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4396 Inorganic Chemistry, Vol. 49, No. 10, 2010
Sun and Wang
were prepared by modified methods described in the
literature.
before a THF (2 mL) solution of Pd(PPh3)4 (5 mol %, 20 mg)
and 3 (162 mg, 0.34 mmol) was added. After the mixture was
stirred at 55 °C for 48 h, a saturated aqueous NH4Cl solution
was added and the product was extracted with CH2Cl2. All
organic portions were combined and dried with MgSO4. After
removing the solvent, the residue was purified by column
chromatography in silica gel, eluting with 20% (v/v) ethyl
acetate/CH2Cl2 to give 130 mg (53%) of 4 as a yellow solid.
1H NMR (400 MHz, CD2Cl2): 8.78 (d, J=1.2 Hz, 1H, Ha), 8.74
(d, J=1.2 Hz, 1H, Hf), 8.73 (d, J=5.2 Hz, 1H, Hh), 8.67 (d, J=
5.2 Hz, 1H, Hc), 7.78 (d, J=8.4 Hz, 2H, He), 7.69 (d, J=8.4 Hz,
2H, Hd), 7.62 (d, J=8.4 Hz, 2H, Hi), 7.61 (dd, J=2.0 Hz, J=5.2
Hz, 1H, Hb), 7.54 (dd, J=2.0 Hz, J=5.2 Hz, 1H, Hg), 7.31 (t, J=
7.6 Hz, 4H, ph), 7.15 (m, 6H, Hj þ ph), 7.09 (t, J=7.6 Hz, 2H,
ph), 6.86 (s, 4H, H of Mes), 2.32 (s, 6H, CH3 of Mes), 2.03 (s,
12H, CH3 of Mes). 13C NMR (100 MHz, CD2Cl2): 157.1, 156.7,
150.0, 149.3, 149.1, 148.7, 147.6, 146.9, 141.9, 141.8, 141.1,
139.2, 137.2, 131.5, 129.7, 128.5, 128.1, 127.0, 125.3, 123.9,
123.2, 121.9, 121.2, 119.3, 119.2, 118.4, 118.3 (aryl C), 23.5,
21.3 (C of Mes). HRMS: calcd for C52H47BN3, [Mþ1]þ
724.3872; found 724.3862.
Synthesis of 5,50-BP2bpy (1). To a solution of p-Br-phenyl-
BMes2 (505 mg, 1.25 mmol) in 2 mL of THF was added
dropwise n-BuLi (0.86 mL, 1.30 mmol) at -78 °C, and the
reaction mixture was stirred at this temperature for 1 h. Anhy-
drous ZnCl2 (255 mg, 1.85 mmol) was then added, and the
reaction mixture was allowed to warm to room temperature and
stirred for an additional 2 h. A THF (2 mL) solution of Pd-
(PPh3)4 (5 mol %, 145 mg) and 5,50-Br2-2,20-bipy (157 mg, 0.50
mmol) was then added. After the mixture was stirred at 55 °C for
48 h, a saturated aqueous NH4Cl solution was added and the
product was extracted with CH2Cl2. The organic portions were
combined and dried with MgSO4. After removing the solvent,
the residue was separated by column chromatography in silica
gel, eluting with 10% (v/v) ethyl acetate/hexane, to give 262 mg
(65%) of 2 as white solids. 1H NMR (400 MHz, CD2Cl2): 8.99
(d, J=1.5 Hz, 2H, Ha), 8.56 (d, J=6.3 Hz, 2H, Hb), 8.12 (dd, J=
1.5 Hz, J=6.3 Hz, 2H, Hc), 7.70 (d, J=6.3 Hz, 4H, Hd), 7.61 (d,
J=6.3 Hz, 4H, He), 6.86 (s, 8H, H of Mes), 2.32 (s, 12H, CH3 of
Mes), 2.04 (s, 24H, CH3 of Mes). 13C NMR (100 MHz, CD2Cl2):
154.6, 148.2, 142.9, 141.1, 139.3, 137.5, 135.6, 128.6, 126.8,
121.2, 112.9, 111.0 (aryl C), 23.6, 21.3 (CH3 of Mes). HRMS:
calcd for C58H59B2N2, [M þ 1]þ 805.4864; found 805.4874.
4,40-BP2bpy (2). This compound was prepared as white solids
from 4,40-Br2-2,20-bipy (157 mg, 0.50 mmol) using the same
procedure as described for 1: yield 242 mg (60%). 1H NMR (400
MHz, CD2Cl2): 8.80 (d, J=1.5 Hz, 2H, Ha), 8.73 (d, J=5.1 Hz,
2H, Hc), 7.78 (d, J=8.1 Hz, 4H, Hd), 7.62 (d, J=8.1 Hz, 4H, He),
7.71 (d, J=5.1 Hz, 2H, Hb), 6.86 (s, 8H, H of Mes), 2.32 (s, 12H,
CH3 of Mes), 2.02 (s, 24H, CH3 of Mes). 13C NMR (100 MHz,
CD2Cl2): 153.9, 149.7, 148.9, 142.9, 141.8, 141.2, 139.7, 139.2,
137.3, 133.5, 132.4, 129.0, 128.9, 128.7, 127.4, 125.5, 120.4 (aryl
C), 23.6, 21.3 (CH3 of Mes). HRMS: calcd for C58H59B2N2,
[M þ 1]þ 805.4864; found 805.4866.
Pt(5,50-BP2bpy)Ph2 (1a). Ligand 1 (80 mg, 0.10 mmol) and
[PtPh2( μ-SMe2)]n (n=2 or 3) (41 mg, 0.10 mmol) were mixed in
20 mL of CH2Cl2 and stirred for 12 h under nitrogen at room
temperature. After removing the solvent, the red residue was
washed by Et2O and dried under vacuum to afford 1a in 82%
yield. 1H NMR (400 MHz, CD2Cl2): 8.88 (d, J=1.6 Hz, 2H, Ha),
8.38 (dd, J=1.2 Hz, J=6.8 Hz, 2H, Hb), 8.17 (d, J=6.8 Hz, 2H,
Hc), 7.56 (d, J=6.4 Hz, 4H, He), 7.51 (d, satellite, J=5.2 Hz,
J
Pt-H=64 Hz, 4H, Pt-Ph), 7.56 (d, J=6.4 Hz, 4H, Hd), 7.02 (t,
J=6.0 Hz, 4H, Pt-Ph), 6.87 (m, 2H, Pt-Ph), 6.85 (s, 8H, H of
Mes), 2.32 (s, 12H, CH3 of Mes), 1.99 (s, 24H, CH3 of Mes). 13
C
NMR (100 MHz, CD2Cl2): 154.9, 148.6, 141.9, 141.2, 139.5,
139.1, 138.6, 138.5, 137.5, 135.4, 132.4, 132.3, 128.9, 128.8,
128.6, 127.4, 126.5, 122.8, 122.4 (aryl C), 23.6, 21.3 (CH3 of
Mes). Anal. Calcd for C70H68B2N2Pt: C, 72.86; H, 5.94; N, 2.43.
Found: C, 72.96; H, 5.80; N, 2.25.
4-Br-40-( p-NPh2-phenyl)-2,20-bpy (3) and 4,40-NP2bpy (5).
The mixture of 4,40-Br2-2,20-bipy (471 mg, 0.50 mmol), Pd-
(PPh3)4 (45 mg g, 0.04 mmol), and toluene (20 mL) was stirred
for 10 min. 2-(4-Diphenylaminophenyl)-4,4,5,5,-tetramethyl-
1,3,2-dioxaborolane (220 mg, 0.6 mmol) in 10 mL of EtOH
and NaOH (800 mg) in 10 mL of H2O were subsequently added.
The mixture was stirred and heated at reflux for 40 h. After
cooling to room temperature, the water layer was separated and
extracted with CH2Cl2 (3 ꢁ 30 mL). The combined organic
layers were dried over MgSO4, and the solvents were evaporated
under reduced pressure. Purification of the crude product by
column chromatography (ethyl acetate/hexane, 1:5) afforded 3
as a yellow solid in 32% yield and 4 as a yellow solid in 23%
yield. For 3, 1H NMR (400 MHz, CDCl3): 8.67 (m, 2H, HaþHc),
8.63 (s, 1H, Hf), 8.49 (d, J=4.8 Hz, 1H, Hh), 7.64 (d, J=8.1 Hz,
2H, Hd), 7.52 (dd, J=1.2 Hz, J=4.8 Hz, 1H, Hb), 7.49 (dd, J=
1.2 Hz, J=4.8 Hz, 1H, Hg), 7.30 (t, J=7.6 Hz, 4H, ph), 7.15 (m,
6H, He þ ph), 7.08 (t, J=7.6 Hz, 2H, ph). 13C NMR (100 MHz,
CD2Cl2): 157.8, 155.5, 150.1, 150.0, 149.3, 149.1, 147.6, 134.3,
131.3, 129.8, 128.2, 127.2, 125.3, 125.0, 123.9, 123.2, 121.7,
118.9. HRMS: calcd for C28H20BrN3, [M]þ 477.0845; found
477.0841. For 5, 1H NMR (400 MHz, CDCl3): 8.69 (d, 2H, Hc),
8.68 (s, 2H, Ha), 7.66 (d, J=6.8 Hz, 4H, Hd), 7.51 (dd, J=1.2 Hz,
J=4.8 Hz, 2H, Hb), 7.30 (t, J=7.6 Hz, 8H, ph), 7.15 (m, 12H,
He þ ph), 7.08 (t, J=7.6 Hz, 4H, ph). 13C NMR (100 MHz,
CD2Cl2): 157.1, 149.3, 149.1, 147.7, 147.0, 131.8, 129.8, 128.3,
125.4, 125.2, 123.8, 123.5, 123.4, 121.3, 120.6, 119.0. HRMS:
calcd for C46H35N4, [M þ 1]þ 643.2861; found 643.2883.
4,40-BPNPbpy (4). To a solution of p-Br-phenyl-BMes2
(207 mg, 0.51 mmol) in 2 mL of THF was added dropwise
n-BuLi (0.35 mL, 0.57 mmol) at -78 °C, and the reaction
mixture was stirred for 1 h. After the addition of anhydrous
ZnCl2 (83 mg, 0.56 mmol), the reaction mixture was allowed to
warm to room temperature and stirred for an additional 2 h
Pt(4,40-BP2bpy)Ph2 (2a). Ligand 2 (80 mg, 0.10 mmol) and
[PtPh2( μ-SMe2)]n (n=2 or 3) (41 mg, 0.10 mmol) were mixed in
20 mL of CH2Cl2 and stirred for 12 h under nitrogen at room
temperature. After removing the solvent, the red residue was
washed by Et2O and dried under vacuum to afford 2a in 75%
yield. 1H NMR (400 MHz, CD2Cl2): 8.56 (d, J=5.6 Hz, 2H, Ha),
8.41 (s, 2H, Hc), 7.74 (d, J=8.4 Hz, He), 7.66 (dd, J=1.2 Hz, J=
5.6 Hz, 2H, Hb), 7.63 (d, J=8.4 Hz, 2H, Hd), 7.47 (d, satellite, J=
6.8 Hz, JPt-H=64 Hz, 4H, Pt-Ph), 7.01 (t, J=7.6 Hz, 4H, Pt-Ph),
6.87 (m, 2H, Pt-Ph), 6.86 (s, 8H, H of Mes), 2.32 (s, 12H, CH3 of
Mes), 2.00 (s, 24H, CH3 of Mes). 13C NMR (100 MHz, CD2Cl2):
156.9, 150.4, 150.0, 146.5, 141.8, 141.2, 140.1, 139.6, 138.6,
137.3, 128.7, 127.4, 127.0, 125.4, 122.1, 120.8 (aryl C), 23.6,
21.3 (CH3 of Mes). Anal. Calcd for C70H68B2N2Pt: C, 72.86; H,
5.94; N, 2.43. Found: C, 72.51; H, 5.83; N, 2.39.
Pt(4,40-BPNPbpy)Ph2 (4a). Ligand 4 (72 mg, 0.10 mmol) and
[PtPh2( μ-SMe2)]n (n=2 or 3) (41 mg, 0.10 mmol) were mixed in
20 mL of CH2Cl2 and stirred for 12 h under nitrogen at room
temperature. After the removal of the solvent, the red residue
was washed by Et2O (10 mL) and dried under vacuum to afford
4a in 71% yield. 1H NMR (400 MHz, CD2Cl2): 8.54 (d, J=5.6
Hz, 2H, Ha), 8.45 (d, J=5.6 Hz, 2H, Hf), 8.38 (s, 2H, Hc), 8.33 (s,
2H, Hh), 7.74 (d, J=8.4 Hz, He), 7.64 (m, 5H, Hd þ Hi þ Hg),
7.57 (dd, J=1.2 Hz, J=6.0 Hz, 1H, Hb), 7.47 (m, 4H, Pt-Ph),
7.33 (t, J=7.2 Hz, 4H, N-Ph), 7.15 (m, 6H, Hj þ N-Ph), 7.00 (m,
6H, N-Ph þ Pt-Ph), 6.88 (m, 2H, Pt-Ph), 6.86 (s, 4H, H of Mes),
2.31 (s, 6H, CH3 of Mes), 2.01 (s, 12H, CH3 of Mes). 13C NMR
(100 MHz, CD2Cl2): 156.1, 154.1, 151.8, 150.4, 150.2, 149.9,
147.3, 145.2, 141.2, 140.1, 139.5, 138.7, 137.3, 132.5, 129.9,
128.7, 128.1, 127.3, 125.8, 125.1, 124.6, 122.4, 120.8 (aryl C),
23.6, 21.3 (CH3 of Mes). Anal. Calcd for C64H56BN3Pt: C,
71.64; H, 5.26; N, 3.92. Found: C, 71.91; H, 5.42; N, 4.30.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4397
Scheme 1
Pt(4,40-NP2bpy)Ph2 (5a). Ligand 5 (64 mg, 0.10 mmol) and
reaction of p-Br-phenyl-BMes2 with 5,50-Br2-2,20-bpy or
4,40-Br2-2,20-bpy using a Negishi coupling procedure10
with Pd(PPh3)4 as the catalysts (Scheme 1). The donor-
only molecule 4,40-NP2bpy (5) ligand was obtained as a
byproduct in 23% yield from the Suzuki-Miyaura cou-
pling reaction11 for the synthesis of 3, a precursor for the
synthesis of the donor-acceptor molecule 4,40-BPNPbpy
(4). Negishi coupling of 3 with p-Br-phenyl-BMes2 pro-
duced compound 4 in 53% yield (Scheme 1). Compounds
1 and 2 are white solids, while compounds 4 and 5 are
both yellow solids. The PtPh2 complexes Pt(5,50-BP2-
bpy)Ph2 (1a), Pt(4,40-BP2bpy)Ph2 (2a), Pt(4,40-BPNP-
bpy)Ph2 (4a), and Pt(4,40-NP2bpy)Ph2 (5a) were obtained
by the reaction of the chelate ligands with [PtPh2(SMe2)]n
in high yields. The structures of all Pt(II) complexes
are shown in Chart 2. They all have an orange color and
are air-stable in both solution and the solid state. The
ligands and their Pt(II) complexes have been fully char-
acterized by NMR spectroscopy and HR-MS or element
analyses. The crystal structure of 2 was determined by
X-ray diffraction and is shown in Figure 1. One half of
molecule 2 is related to the other half by an inversion
[PtPh2( μ-SMe2)]n (n=2 or 3) (41 mg, 0.10 mmol) were mixed in
20 mL of CH2Cl2 and stirred for 12 h under nitrogen at room
temperature. After the removal of the solvent, the red residue was
washed by Et2O (10 mL) and dried under vacuum to afford 5a in
65% yield. 1H NMR (400 MHz, CD2Cl2): 8.43 (d, J=5.6 Hz, 2H,
Ha), 8.30 (s, 2H, Hc), 7.63 (d, J=8.8 Hz, 4H, Hd), 7.54 (d, J=5.6 Hz,
2H, Hb), 7.47 (d, J=5.2 Hz, 4H, Pt-Ph), 7.33 (t, J=7.6 Hz, 8H,
N-Ph), 7.15 (m, 12H, N-Ph þ He), 7.01 (m, 8H, N-Ph þ Pt-Ph), 6.87
(m, 2H, Pt-Ph). 13C NMR (100 MHz, CD2Cl2): 156.2, 149.8, 149.5,
148.8, 146.6, 146.3, 138.1, 129.3, 128.7, 127.5, 126.7, 125.4, 125.2,
123.9, 121.8, 121.3, 118.9 (aryl C). Anal. Calcd for C58H44N4Pt: C,
70.22; H, 4.47; N, 5.65. Found: C, 70.55; H, 4.64; N, 5.32.
[Zn(4,40-BP2bpy)2][ClO4]2 (2b). Ligand 2 (80 mg, 0.10 mmol)
and Zn(ClO4)2 (18.6 mg, 0.05 mmol) were mixed in 20 mL of
CH2Cl2 and stirred for 2 h at room temperature. After removal of
solvent, the residue was washed by diethyl ether (10 mL) and dried
1
under vacuum to afford 2b in 82% yield. H NMR (400 MHz,
CD2Cl2): 8.70 (m, 8H, HaþHc), 7.98 (d, J=6.0 Hz, 4H, Hb), 7.82
(d, J=8.0 Hz, 8H, He), 7.68 (d, J=8.0 Hz, 8H, Hd), 6.86 (s, 16H, H
C
of Mes), 2.32 (s, 24H, CH3 of Mes), 2.02 (s, 48H, CH3 of Mes). 13
NMR (100 MHz, CD2Cl2): 155.9, 151.4, 150.5, 146.1, 141.5,
141.2, 140.6, 139.3, 138.7, 136.3, 128.2, 127.2, 126.8, 124.4,
122.5, 120.4 (aryl C), 23.6, 21.3 (CH3 of Mes). HRMS: calcd for
C
116H116B4ClN4O4Zn, [M - ClO4]þ 1771.8348; found 1771.8402.
Results and Discussions
Syntheses and Structures. The new bpy derivatives 5,50-
BP2bpy (1) and 4,40-BP2bpy (2) were synthesized by the
€
(10) Kiehne, U.; Bunzen, J.; Staats, H.; Lutzen, A. Synthesis 2007, 1061.
(11) (a) Kotha, S.; Lahiri, K.; Kashinath, D. Tetrahedron 2002, 58, 9633.
(b) Suzuki, A. Acc. Chem. Res. 1982, 15, 178. (c) Miyaura, N.; Suzuki, A. Chem.
Rev. 1995, 95, 2457.

4398 Inorganic Chemistry, Vol. 49, No. 10, 2010
Sun and Wang
Chart 2
Figure 1. Diagram showing the structure of ligand 2 with 35% thermal
ellipsoids.
Figure 2. UV-vis absorption spectra of free ligands (1, 2, 4, and 5) and
their Pt(II) complexes (1a, 2a, 4a, and 5a) recorded in CH2Cl2.
center. The central bpy unit is coplanar. The phenyl ring
has a dihedral angle of 53.8° with the bpy ring (see
Supporting Information (SI) for details). None of the
Pt(II) complexes produced single crystals suitable for
X-ray diffraction analysis.
Reduction Potentials. The reduction potential of the
free ligands and the corresponding Pt(II) complexes were
recorded by cyclic voltammetry and are shown in Figure
3. Ligands 1, 2, and 4 all have two reversible reduction
peaks with a similar first reduction peak (-2.04 V, vs
FeCp20/þ), which is attributed to the reduction of the
boron center and the conjugated biphenylbpy linker. In
contrast, ligand 5 has only one reversible reduction at
-2.47 V (vs FeCp20/þ), much more negative than the first
reduction potential of other ligands, supporting that the
boron center enhances the electron-accepting ability in
compounds 1, 2, and 4. The first reduction peaks for the
four Pt(II) complexes were observed at -1.61 V (1a),
-1.73 V (2a), -1.78 V (4a), and -1.89 V (5a), respec-
tively, which are all more positive than that of the
corresponding free ligand, in agreement with the trend
observed for the previously reported PtPh2 complexes6a,b
of 5,50-B2bpy, 5,50-BNbpy, and 5,50-N2bpy. However,
compared to that of Pt(5,50-B2bpy)Ph2 (-1.34 V), the
reduction potential of 1a and 2a is much more negative,
attributed to the diminished impact of the BMes2 group
on the electron-accepting ability of the molecule in 1a and
2a due to the longer separation distance from the chelate
site. Using the CV and UV-vis data, the HOMO and
Absorption Spectra. In CH2Cl2, the free ligands show a
strong absorption between 290 and 360 nm. Among the 4,40-
functionalized bpy ligands, the λmax of the low-energy
absorption band follows the order 4,40-BP2bpy (2) (321
nm)< 4,40-BPNPbpy (4) (337 nm)< 4,40-NP2bpy (5)
(358 nm). As shown in Figure 2 and Table 1, the absorption
energy of 1 is about 30 nm lower than that of 2, attributable
to the more effective π-conjugation in 5,50-BP2bpy. The
corresponding Pt(II) complexes all have a broad and weak
absorption band, characteristic of metal-to-ligand charge-
transfer (MLCT) transitions. The MLCT energy for the 4,40-
substituted bpy complexes follows the order 5a < 4a
(418 nm) < 2a (460 nm), in agreement with the electron-
accepting nature of the boron center that lowers the MLCT
energy in 2a and 4a. The MLCT energy (468 nm) of 1a is
much higher than that (525 nm) of Pt(5,50-B2bpy)Ph2, where
two BMes2 are directly attached to the bpy group,6a,b
indicating that the impact of the BMes2 as an electron-
accepting group decreases significantly as its distance from
the Pt(II) center increases.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4399
Table 1. Absorption and Luminescence Data
UV-vis, nm
(ε, M-1 cm-1
λ
em, nm/τ,a μs
CH2Cl2, rt
λ
em, nm/τ, ms,
compound
)
CH2Cl2, 77 K
Φ,b CH2Cl2
1
2
4
5
1a
2a
4a
5a
350 (71 589)
390/7.7(2)
415/5.8(1)
459/6.6(2)
455/7.0(1)
N/A
N/A
577/8.5(1)
577/7.6(1)
549/0.5(1)
0.35
321 (52 745)
337 (32 253)
358 (35 646)
369 (47 451), 468 (2929)
330 (55 328), 460 (6842)
345 (30 168), 418 (17 564)
360 (35 573), 409 (27 579)
466, 500/1.5(3), 1.1(2)
503, 531/0.8(1), 0.9(1)
507/3.0(1)
547/0.010(1)
540/0.009(1)
0.076
0.14
0.52
N/A
N/A
537/0.022(1)
545/0.027(1)
0.0017
0.0066
a Decay lifetime at rt for the Pt(II) complexes was measured under N2. b The quantum efficiency for the free ligands was obtained under air using
9,10-diphenylanthracene as the standard. For the Pt(II) complexes, the quantum efficiency was measured under N2 using Ir(ppy)3 as the standard
(Φ = 0.40).12 The quantum efficiencies of complexes 4a and 5a versus Ir(ppy)3 under air are 0.0014 and 0.0056, respectively (Φ = 0.0068 for Ir(ppy)3
under air, assuming Φ = 0.40 for Ir(ppy)3 under N2).
Figure 4. HOMO and LUMO energy levels of the free ligands 1, 2, 4,
and 5 and the Pt(II) complexes 1a, 2a, 4a, and 5a.
a π* orbital dominated by a bpy unit. Hence, the lowest
Figure 3. CV diagrams for all ligands (1, 2, 4, and 5) and complexes (1a,
2a, 4a, and 5a) recorded in DMF. The potential is relative to that of
electronic transition in these molecules can be considered
as a π to π* transition (1) and charge-transfer transition
(mesityl to B in 2, N to bpy in 5), respectively. The HOMO
level of 4 involves the NPh2 group, while the LUMO is
dominated by the phenyl-BMes2 group, supporting that
the lowest electronic transition of this molecule is charge
transfer from the amino group to the B center, mediated
by the bpy unit. For the Pt(II) complexes, the HOMO
level consists of π orbitals of the two phenyl ligands and
the d orbital of Pt(II) center, while the LUMO level
involves π* orbitals of the chelate ligand with some d
orbital contributions of the Pt(II) center. The lowest
electronic transition of all Pt complexes can be therefore
considered as charge transfer from PtPh2 to the π* orbital
of the ligands.
Luminescence. All four free ligands are blue fluorescent
in solution, the solid state with 5 being the brightest (φ=
0.52). Consistent with the lowest electronic transition
being charge transfer, the emission band of 2, 4, and 5
shows dependence on the solvent polarity (e.g., ∼50 nm
red shift from toluene to CH3CN for 2, 100 nm for 4 and
5, see SI). In contrast, the emission of 1 has little depen-
dence on solvent polarity. The Pt(4,40-BPNPbpy)Ph2 (4a)
and Pt(4,40-NP2bpy)Ph2 (5a) complexes display ambient-
temperature phosphorescence with λmax = 577 nm, τ =
8.5(1) μs and λmax=577 nm, τ=7.6(1) μs, respectively, in
CH2Cl2 (Figure 6), which is most likely from a ligand-
centered triplet state, due to the similarity of the emission
peak profile with the phosphorescence emission peak of
þ/0
FeCp2
.
LUMO energies of the ligands and their complexes were
estimated and are shown in Figure 4 (see also Table 3 in
the SI). The data show consistently the stabilization of the
LUMO level of free ligands by metal chelation.
DFT Calculations. Molecular orbital calculations were
performed for all compounds using the Gaussian suite of
programs (Gaussian 03)13 and the basis set B3LYP-6-
311G(d). The calculated HOMO and LUMO energy level
and the energy gaps for the ligands and the complexes are
shown in Figure 4. The calculated HOMO-LUMO trend
is consistent with that observed experimentally. The
HOMO and LUMO orbitals of the free ligands and all
metal complexes are shown in Figure 5a and b, respec-
tively.
The HOMO level for 1 is a π orbital dominated by the
bpy unit, while the HOMO level of 2 has contributions
almost exclusively from the mesityls. The LUMO levels of
1 and 2 are π* orbitals dominated by the two boron atoms
and the bpy units. For 5, the HOMO level is a π orbital
involving the two NPh2 groups, while the LUMO level is
(12) (a) King, K. A.; Spellane, P. J.; Watts, R. J. J. Am. Chem. Soc. 1985,
107, 1431. (b) Lamansky, S.; Djurovich, P.; Murphy, D.; Abdel-Razzaq, F.;
Kwong, R.; Tsyba, I.; Bortz, M.; Mui, B.; Bau, R.; Thompson, M. E. Inorg. Chem.
2001, 40, 1704.
(13) Frisch, M. J.; et al. Gaussian 03, Revision C.02; Gaussian, Inc.:
Wallingford, CT, 2004.

4400 Inorganic Chemistry, Vol. 49, No. 10, 2010
Sun and Wang
Figure 5. (a) HOMO and LUMO diagrams of ligands 1, 2, 4, and 5. (b) HOMO and LUMO diagrams of the Pt(II) complexes 1a, 2a, 4a, and 5a.
the corresponding free ligand at 77 K (see SI). The
phosphorescent decay lifetimes of these two Pt(II) com-
plexes at 77K are all much shorter than those of the free
ligands, consistent with the heavy atom effects in the
complexes. Although under nitrogen the emission quan-
tum efficiency of 4a and 5a is much lower than that of
Ir(ppy)3, under air their emission quantum efficiency is
∼21% and 82% of that of Ir(ppy)3, due to their insensi-
tivity toward air. This insensitivity may be attributed to
the short decay lifetime of the Pt(II) complexes. At 77 K,
in frozen CH2Cl2, complexes 1a and 2a also display
phosphorescence (see Table 1 and SI).
Interactions with Zn(II) Ions. In addition to forming
Pt(II) complexes, the new ligands are also capable of
binding to Zn(II) ions, which can be monitored readily
Figure 6. Emission spectra of the free ligands 1, 2, 4, and 5 and
complexes 4a and 5a in THF (1.0 ꢁ 10-5 M) at ambient temperature.
by UV-vis, fluorescence, and NMR spectra. Because of
the intense current interest in Zn(II) ion sensing,14 we
decided to examine the interaction of Zn(ClO4)2 with our
As shown in Figure 7, the binding of the three 4,40-
functionalized bpy ligands to a Zn(II) ion generates a
four new ligands in THF.
distinct spectral change. For both 4 and 5, the λmax of the
low-energy absorption band is red-shifted by 40-60 nm
upon Zn(II) binding, resulting in a change of the solution
(14) (a) Bozym, R. A.; Thompson, R. B.; Stoddard, A. K.; Fierke, C. A.
Chem. Biol. 2006, 1, 103. (b) Kimura, E.; Kikuta, E. J. Biol. Inorg. Chem. 2000,
color from colorless to yellow. A similar absorption spectral
red shift has been observed in 4,40-aminoaryl-functionalized
5, 139. (c) Nolan, E. M.; Jaworski, J.; Okamoto, K.-I.; Hayachi, Y.; Sheng, M.;
Lippard, S. J. J. Am. Chem. Soc. 2005, 127, 16812. (d) Jamieson, E. R.; Lippard,
S. J. Chem. Rev. 1999, 99, 2467. (e) Hang, L.; Murphy, C. S.; Kuang, G. C.;
Hazelwood, K. L.; Constantino, M. H.; Davidson, M. W.; Zhu, L. Chem.
Commun. 2009, 7408. (f) Hang, L.; Clark, R. J.; Zhu, L. Chem.;Eur. J.
2008, 14, 2894.
ꢀ ꢀ
(15) Senechal, K.; Maury, O.; Le Bozec, H. Ledoux, I.; Zyss, J. J. Am.
Chem. Soc. 2002, 124, 4560, and references therein.

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4401
Figure 7. UV-vis (right) and fluorescence (left) spectral change of ligands 4,40-BP2bpy (2), 4,40-BPNPbpy (4), and 4,40-NP2bpy (5) with the addition of
Zn(ClO4)2 in THF (∼1.0 ꢁ 10-5 M).
2,20-bpy Zn(II) complexes and attributed to an intraligand
charge-transfer transition.15 The emission spectra of 4 and 5
are quenched by Zn(II) ions. The response of 2 toward
Zn(II) ions is unique among the 4,40-functionalized ligands:
its absorption spectrum only experiences a ∼5 nm red shift,
while its emission spectrum has a ∼40 nm red shift with a
distinct emission color change from deep blue to sky blue, as
shown in Figure 7. The lack of an amino donor group in 2 is
clearly responsible for its distinct fluorescence “turn-on”
response to Zn(II) ions, although the details are been
understood.
The absorption spectral change of 1 with Zn(II) ions is
similar to that of 2 with Zn(II) ions (see SI). However, the
emission spectrum of 1 undergoes a complete quenching
in the same manner as that of 4 and 5, thus demonstrating
that the fluorescence response of the bpy derivative
ligands toward Zn(II) can be greatly influenced by not
only the nature of the substituent group but also the
geometry of the ligand. Stern-Volmer plots (see SI)
indicate that the Zn(II) ion forms a 1:2 complex with
the bpy derivative ligands ([ZnL2]2þ), which is also sup-
ported by NMR and MS spectral data for the isolated
compound [Zn(4,40-BP2bpy)](ClO4)2. Using the absorp-
tion spectral titration data, the overall binding constants
of ligands 1, 2, 4, and 5 with Zn(II) ions were estimated by
using a general fitting equation.16 The results of the fitting
support that the Zn(II) ion indeed forms a 1:2 complex
with the bpy ligands. The overall binding constant K was
found to be similar for all boryl ligands (1.2 ꢁ 1011 M-2
(16) (a) Connors, K. A. Binding Constants; John Wiley and Sons: New York,
1987. (b) Valeur, B. Molecular Fluorescence: Principles and Applications;
Wiley-VCH: Weinheim, 2002. (c) Peng, X.; Du, J.; Fan, J.; Wang, J.; Wu, Y.;
Zhao, J.; Sun, S.; Xu, T. J. Am. Chem. Soc. 2007, 129, 1500.

4402 Inorganic Chemistry, Vol. 49, No. 10, 2010
Sun and Wang
Figure 8. UV-vis (left) and fluorescence (right) spectral change of 4,40-BPNPbpy (4) (∼1.0 ꢁ 10-5 M) with the addition of NBu4F (λex = 350 nm) in
CH2Cl2 (∼1.0 ꢁ 10-5 M).
Figure 9. Absorption spectral change of 5,50-BP2bpy (1) and its Pt(II) complex Pt(5,50-BP2bpy)2Ph2 (1a) with the addition of NBu4F in CH2Cl2
(∼1.0 ꢁ 10-5 M).
for 1, 2.7 ꢁ 1011 M-2 for 2 and 4). The donor-only ligand 5
was found to have a greater binding strength toward
Zn(II), compared to the boryl ligands, with K = 7.8 ꢁ
1011 M-2. For details, please see section S8 in the SI. The
magnitude of these binding constants is consistent with
those reported previously for Zn(II) complexes with bpy
derivative ligands.14
toward fluoride is reminiscent of that of 4,40-BNbpy we
reported recently6b and can be attributed to the switching
from a charge-transfer transition of the ligand to a πfπ*
transition and the relative high emission quantum effi-
ciency of the fluoride adduct. The difference is that the
emission peak of 4,40-BNbpy experiences a much greater
blue shift (∼100 nm) than that of 4 upon binding with
fluoride ions, thus resulting in a distinct color change
from blue-green to deep blue, which can be attributed to
the much shorter conjugated linker in 4,40-BNbpy, com-
pared to that in 4. Efforts to obtain the stepwise binding
constants (K1 and K2) for the two diboryl molecules 1 and
2 with fluoride ions were unsuccessful due to the lack of a
distinct stepwise change of the titration spectra. None-
theless, we were able to obtain the overall binding con-
stant K for these two molecules using fluorescence
titration data.16 The magnitude of the overall binding
constants (∼2 ꢁ 108 M-2 for 1 and ∼6 ꢁ 108 M-2 for 2,
∼2 ꢁ 105 M-1 for 4 (K=K1)) indicates that the binding
strength of these new boron compounds with fluoride
ions is similar to that of typical triarylboron compounds
B(Mes)2(Ar),4 but much less than that of 5,50-B2bpy,
where the two B(Mes)2 groups are directly attached to
the bpy ring.6a,b
Response to Fluorides and Cyanides. The Zn(II) com-
plexes are unstable toward fluorides, which causes the
dissociation of the bpy derivative ligand from the Zn(II)
center. Our study has therefore focused on the interaction
of the boron-containing free ligands 1, 2, and 4 and the
corresponding Pt(II) complexes with fluoride ions.
For diboryl compound 1, the addition of [NBu4]F
causes an intensity decrease of the intense absorption
band at 350 nm and the appearance of a weak shoulder
band at ∼430 nm. For 2, the absorption band at 322 nm is
quenched and blue-shifted (see SI). Unlike 5,50-B2bpy,
which shows a distinct stepwise formation of the 1:1 and
1:2 fluoride adducts,6a,b 1 and 2 do not show a stepwise
addition of fluoride ions, an indication of a much dimini-
shed electronic communication between the two boron
centers in 1 and 2 due to the much longer linker group.
For both molecules, the emission band is completely
quenched by the addition of the fluoride ions (see SI).
In contrast, for the donor-acceptor molecule 4, the
addition of F- causes a substantial intensity increase of
the emission peak with a ∼10 nm blue shift (from 457 to
447 nm) and a small intensity decrease of the low-energy
absorption band (Figure 8). This “turn-on” response
For complex 1a, the addition of fluoride ions causes the
decrease of the absorption band at 369 nm and the
appearance of a new band at 395 nm, a behavior similar
to that of the free ligand, as shown in Figure 9, and
attributable to the narrowing of the π-π* gap caused by
the switching of two electron-withdrawing BMes2 groups

Article
Inorganic Chemistry, Vol. 49, No. 10, 2010 4403
Figure 10. UV-vis (left) and emission (right) spectral change of Pt(4,40-BPNPbpy)2Ph2 (4a) with the addition of NBu4F in CH2Cl2.
Figure 11. UV-vis spectral change of Pt(4,40-BP2bpy)2Ph2 (2a) with the addition of NBu4F (left) and NBu4CN (right) in CH2Cl2.
to electron-donating BMes2(F) groups. In contrast to
Pt(5,50-B2bpy)Ph2, which has a distinct MLCT color
change from red to orange, then to light yellow with the
sequential addition of one and two equivalents of fluoride
ions,6a 1a does not have such a distinct color change due
to the lack of a strong and distinct MLCT band.
Conclusions
Two diboryl molecules, one donor-acceptor molecule
and one donor-only molecules based on either a 5,50-
biphenyl-2,20-bpy or a 4,40-biphenyl-2,20-bpy core, have
been achieved. Compared to 5,50-B2bpy, which contains
the 2,20-bpy core only, extending the π-conjugation via the
phenyl ring, in general, diminishes the electronic communi-
cation between the two boron centers and their electron-
accepting ability/Lewis acidity, although it does signifi-
cantly enhance the emission quantum efficiency of the 5,50-
diboryl molecule 1. The molecular geometry has been
found to have a significant impact on the properties of
the molecule, as illustrated by the much lower quantum
efficiency of the 4,40-diboryl-substituted molecule 2, com-
pared to 1, and the fact that molecule 1 is a “turn-off”
fluorescence sensor while 2 is a “turn-on” sensor for Zn(II)
ions. Extending the π-conjugation of the bpy-based
ligands also significantly blue-shifts the energy of the
MLCT band of the Pt(II) complex of the 5,50-diboryl
molecule 1, causing the loss of a dramatic visual color
change upon binding with fluoride that is characteristic of
the Pt(II) complexes of 5,50-B2bpy. Extending the π-con-
jugation does not change the fluorescence/phosphore-
scence “turn-on” response of the donor-acceptor mole-
cule 4 and its Pt(II) complex toward fluoride, but does
cause the loss of a distinct emission color change, com-
pared to 5,50-BNbpy and its Pt(II) complex. Finally, cya-
nide ions can bind to the boron center of the PtPh2
For complexes 2a (Figures 10 and 11) and 4a, the
MLCT absorption band is quenched by fluorides with a
blue shift of the spectrum. The emission spectrum of 4a
experiences a dramatic increase in intensity in the same
manner as that of the free ligand (Figures 8 and 10),
further supporting that the origin of the emission is
ligand-centered. In addition, the Pt(II) complexes have
consistently a stronger binding with the fluoride ion,
compared to the free ligand, as shown by the Stern-
Volmer plots (see SI). Using the absorption titration data,
the overall binding constants for the Pt(II) complexes were
estimated16 to be ∼2ꢁ 109 M-2 for 1a, ∼7ꢁ 108 M-2 for 2a,
and ∼6 ꢁ 105 M-1 for 4a.
Interestingly, the boron center in the Pt(II) complexes is
also capable of binding to CN- without the dissociation
of the bpy ligand. The spectral change with the cyanide
addition resembles that of fluoride addition, as shown by
the UV-vis titration diagrams of 2a by F- and CN-, in
Figure 11. The overall binding constant K (∼2 ꢁ 109 M-2
)
for the CN- binding was found to be similar to that of F-
binding with 2a (see SI). Using triarylboron-functiona-
lized metal complexes to detect CN- anions has been
demonstrated recently.3-5

4404 Inorganic Chemistry, Vol. 49, No. 10, 2010
Sun and Wang
complexes based on the boryl-functionalized 2,20-bpy
ligands in the same manner as fluoride ions without
disrupting the Pt-bpy chelate bonds. Using the new
ligands in the synthesis of polyboryl transition metal
complexes such as Ru(II) is in progress, and the results
will be reported in due course.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial
support.
Supporting Information Available: This material is available
free of charge via the Internet at http://pubs.acs.org.