Novel phosphorescent tetradentate bis-cyclometalated ?C *N?N-coordinated platinum complexes: Structure, photophysics, and a synthetic adventure

Article abstract of DOI:10.1016/j.poly.2012.06.078

A long but adventurous journal to the synthesis of tetradentate bis-cyclometalated ?C^*N?N-coordinated platinum complexes is described, where ?C and N?N denote a five-membered chelation and C^*N denotes a six-membered chelation. Initial ligand designs resulted in the selective cycloplatination to form a tridentate coordination complex. The structures of the tridentate coordination complexes were confirmed by the X-ray crystallographic studies. The selective formation of the tridentate coordination complexes was analyzed, which led us to an ultimate design of the ligand 15 and 16. Ligands 15 and 16 differ only in the N?N motif; a bipyridine-based N?N for 15 and a pyrazolylpyridine-based N?N for 16, respectively. In both cases, a forced consecutive double cycloplatination would be possible to form a ?C^*N?N- coordinated platinum complex. As expected, the reaction of ligand 15 and 16 with K₂PtCl₄ in acetic acid at reflux produced 17 and 18, respectively. The coordination geometry was confirmed by the X-ray crystallography of the complex 17. The square planar geometry and dual intermolecular π-π and Pt-Pt interactions were revealed in the molecular packing. The photophysical properties of the complexes were studied. It was found that the complex 18 emitted intensely yellow phosphorescence at room temperature in deoxygenated solution, while complex 17 emitted red emission in a frozen glass matrix. Both compounds have the lifetime of the excited states in the scale of micro seconds. The DFT calculations were carried out to elucidate the frontier orbitals of the complexes, which are used in interpreting the electronic spectra of the complexes.

Full text of DOI:10.1016/j.poly.2012.06.078

Polyhedron 52 (2013) 1030–1040
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Novel phosphorescent tetradentate bis-cyclometalated C^C⁄N^N-coordinated
platinum complexes: Structure, photophysics, and a synthetic adventure
a
a
a
a
Shouquan Huo a, , Caleb F. Harris , Dileep A.K. Vezzu , Joshua P. Gagnier , Meredith E. Smith ,
⇑
Robert D. Pike b, Yumin Li a
a Department of Chemistry, East Carolina University, Greenville, NC 27858, USA
b Department of Chemistry, College of William and Mary, Williamsburg, VA 23185, USA
a r t i c l e i n f o
a b s t r a c t
A long but adventurous journal to the synthesis of tetradentate bis-cyclometalated C^C⁄N^N-coordinated
platinum complexes is described, where C^C and N^N denote a five-membered chelation and C⁄N
denotes a six-membered chelation. Initial ligand designs resulted in the selective cycloplatination to form
a tridentate coordination complex. The structures of the tridentate coordination complexes were con-
firmed by the X-ray crystallographic studies. The selective formation of the tridentate coordination com-
plexes was analyzed, which led us to an ultimate design of the ligand 15 and 16. Ligands 15 and 16 differ
only in the N^N motif; a bipyridine-based N^N for 15 and a pyrazolylpyridine-based N^N for 16, respec-
tively. In both cases, a forced consecutive double cycloplatination would be possible to form a C^C⁄N^N-
coordinated platinum complex. As expected, the reaction of ligand 15 and 16 with K2PtCl4 in acetic acid at
reflux produced 17 and 18, respectively. The coordination geometry was confirmed by the X-ray crystal-
Article history:
Available online 10 July 2012
Dedicated to Alfred Werner on the 100th
anniversary of his Nobel Prize in chemistry
in 1913.
Keywords:
Platinum
Cyclometalation
Tetradentate coordination
Phosphorescence
Crystal structure
lography of the complex 17. The square planar geometry and dual intermolecular p–p and Pt–Pt interac-
tions were revealed in the molecular packing. The photophysical properties of the complexes were
studied. It was found that the complex 18 emitted intensely yellow phosphorescence at room tempera-
ture in deoxygenated solution, while complex 17 emitted red emission in a frozen glass matrix. Both
compounds have the lifetime of the excited states in the scale of micro seconds. The DFT calculations
were carried out to elucidate the frontier orbitals of the complexes, which are used in interpreting the
electronic spectra of the complexes.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
N^C^N (1,3-dipyridylbenzene) [9] ligands. Among various types
of phosphorescent cyclometalated platinum complexes, those
Cyclometalated platinum complexes have attracted a great deal
of interest recently. One of the major driving forces can be attrib-
uted to a number of applications demonstrated by those lumines-
cent complexes that can emit intense phosphorescence light at
ambient temperature in the visible region. They have found poten-
tial applications in the areas of OLED (organic light emitting diode)
[1], chemical sensing [2], biological labeling, and imaging [3].
Cyclometalated platinum complexes have also found applications
in other important fields such as photocatalysis [4] and anticancer
drug developments [5]. Two classes of cyclometalated platinum
complexes have been extensively investigated, i.e., bidentate
cyclometalated complexes such as cis-Pt(ppy)2 (Hppy = 2-phenyl-
pyridine) and (ppy)Pt(acac) [6] and tridentate cyclometalated
complexes such as those derived from tridentate C^N^N (6-phe-
nyl-2,20-bipyridine) [7], C^N^C (2,6-diphenylpyridine) [8], and
based on 1,3-dipyridylbenzene and its analogs have displayed very
high phosphorescent efficiency and been used as triplet emitters in
the OLED devices [10]. More recently, we have reported highly
emissive tridentate cyclometalated platinum complexes featuring
in a five-six-fused metallacycle [11]. Those complexes, unlike other
platinum complexes, do not show significant concentration
quenching.
In contrast to the bidentate or tridentate cyclometalated plati-
num complexes, tetradentate cyclometalated platinum complexes
are considerably underdeveloped [12]. We have recently reported
that tetradentate bis-cyclometalated C^N⁄N^C and N^C⁄C^N-
coordinated platinum complexes [12a], where N^C or C^N and
C⁄N or N⁄C denote a five and six-membered chelation with the
platinum, respectively. These complexes have
a fused five-
six-five-membered metallacycle and have demonstrated very high
photoluminescent quantum yields. By changing the coordination
geometry, for example, from C^N⁄N^C to N^C⁄C^N-coordination,
and modifying the molecular structure, a broad spectrum of emis-
sions from blue to red have been achieved. The use of tetradentate
⇑
Corresponding author. Tel.: +1 252 328 9784; fax: +1 252 328 2610.
E-mail address: huos@ecu.edu (S. Huo).
0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2012.06.078

S. Huo et al. / Polyhedron 52 (2013) 1030–1040
1031
C^N⁄N^C-coordinated platinum complex as highly efficient triplet
emitter in an OLED device has also been demonstrated [12a]. In this
paper, we will report the design, synthesis, structure, and photo-
physical properties of tetradentate C^C⁄N^N-coordinated platinum
complexes.
not transferred. Pd(PPh3)4 (0.29 g, 0.25 mmol) was then added
and the reaction was refluxed for 48 h. After cooling to room tem-
perature, 25 mL of ethyl acetate was added and stirred for a while.
The precipitate formed were filtered and the filtrate was evapo-
rated. The crude product was purified by chromatography on silica
gel with a mixture of hexane and ethyl acetate (v/v = 4:1) to give a
white solid, 0.54 g, yield 29%. 1H NMR (500 MHz, CDCl3) d 8.60 (d,
J = 4.5 Hz, 1H), 8.03 (d, J = 8 Hz, 1H), 7.95 (d, J = 8 Hz, 1H), 7.61–
7.52 (m, 5H), 7.41–7.27 (m, 9H), 7.23–7.14 (m, 3H), 6.77 (d,
J = 8 Hz, 1H). 13C NMR (75 MHz, CDCl3), d 158.0, 156.2, 154.0,
148.9, 146.3, 145.8, 142.2, 140.7, 138.4, 136.8, 129.5, 129.3,
128.7, 127.4, 127.0, 126.6, 125.1, 124.6, 123.4, 123.2, 121.0,
113.4, 113.1. MS, m/z calcd for C28H21N3: 400.18 (M+H+). Found:
400.18. Anal. Calc. for C28H21N3: C, 84.18; H, 5.30; N, 10.52. Found:
C, 84.29; H, 5.30; N, 10.24%.
2. Experimental
2.1. Synthesis
All reactions involving moisture- and/or oxygen-sensitive orga-
nometallic complexes were carried out under nitrogen or argon
atmosphere and anhydrous conditions. Tetrahydrofuran (THF) and
2-methyltetrahydrofuran were distilled from sodium and benzo-
phenone under nitrogen before use. All other anhydrous solvents
were purchased from Aldrich Chemical Co. and were used as re-
ceived. All other reagents were purchased from chemical companies
and were used as received. Mass spectra were measured on a Waters
spectrometer. NMR spectra were measured on a Mercury VX 300,
Bruker 400, or a Varian 500 spectrometer. Spectra were taken in
CDCl3 or CD2Cl2 using tetramethylsilane as reference for 1H NMR
chemical shifts and the solvent peak (CDCl3, 77.0 ppm; CD2Cl2,
53.8 ppm) as reference for 13C NMR chemical shifts. Elemental anal-
yses were performed at Atlantic Microlab, Inc. Norcross, Georgia.
2.1.4. Preparation of platinum complex (7)
General procedure B. To a dry, 100 mL argon-flushed flask were
charged ligand 6 (0.27 g, 0.68 mmol), K2PtCl4 (0.28 g, 0.68 mmol),
and glacial acetic acid (25 mL). The mixture was degassed and re-
fluxed under argon for 12 h. After cooling to room temperature, the
glacial acetic acid was removed by evaporation. The crude materi-
als were purified by flash chromatography on silica gel with
dichloromethane and ethyl acetate (v/v = 15:1) to yield an or-
ange-red solid, 0.3 g, 75%. 1H NMR (400 MHz, CD2Cl2) d 9.95 (d,
3
J = 5.6 Hz, 1H), 8.79 (d, J = 8 Hz, JPt–H = 18.3 Hz, 1H), 8.20–8.10
2.1.1. Preparation of N-phenyl-[1,10-biphenyl]-3-amine [13] (4)
General procedure A. A dry, 250 mL nitrogen flushed flask was
charged with aniline (2.09 g, 22.5 mmol), 3-bromo-1,10-biphenyl
(3.46 g, 15 mmol), sodium tert-butoxide (2.20 g, 22.5 mmol,),
Pd(dba)2 (0.35 g, 0.6 mmol), DPPF (0.34 g, 0.6 mmol), and toluene
(50 mL). The reaction mixture was refluxed for 2 h. After cooling
to room temperature, the reaction mixture was diluted with ethyl
acetate (50 mL). The precipitate was filtered off and the filtrate was
collected and evaporated. The crude product was purified by chro-
matography on silica gel with a mixture of hexane and dichloro-
methane (v/v = 1:1) to yield a white solid, 3.5 g, 95%. 1H NMR
(300 MHz, CDCl3), d 7.57 (d, J = 7.5 Hz, 2H), 7.42 (t, J = 7.2 Hz, 2H),
7.36–7.29 (m, 5H), 7.13 (t, J = 9.3 Hz, 3H), 7.06 (d, J = 9 Hz, 1H),
6.94 (t, J = 10 Hz, 1H), 5.78 (s, 1H). 13C NMR (75 MHz, CDCl3), d
143.6, 143.0, 142.5, 141.14, 129.7, 129.4, 128.7, 127.3, 127.1,
121.2, 119.9, 118.0, 116.6, 116.4.
(m, 2H), 7.80–7.6 (m, 4H), 7.44 (d, J = 8.4 Hz, 2H) 7.35–7.30 (m,
4H), 7.25–7.20 (m, 1H), 7.14 (dd, J = 9.2 Hz, 2 Hz, 1H), 6.80 (dd,
J = 9 Hz, 1.1 Hz, 1H), 6.62 (d, J = 1.9 Hz, 1H). 13C NMR (125 MHz,
CD2Cl2) d 148.7, 140.3, 137.7, 136.7, 134.0, 131.4, 130.5, 129.2,
128.5, 126.7, 126.4, 125.6, 121.5, 120.6, 119.1, 116.2, 114.5. MS,
m/z calcd for C28H20N3ClPt: 629.10. Found: 629.09. Anal. Calc. for
C
28H20N3ClPtꢀ0.5CH2Cl2: C, 50.98; H, 3.15; N, 6.26. Found: C,
50.55; H, 3.04; N, 6.28%.
2.1.5. Preparation of N-butyl-[1,10:30,100-terphenyl]-50-amine (8)
This compound was prepared by the cross coupling of 50-bro-
mo-m-terphenyl (1.24 g, 4 mmol) [14] with butylamine (0.44 g,
6 mmol) following general procedure A, white solid, 0.72 g, yield
60%. 1H NMR (400 MHz, CDCl3): d 7.61 (d, J = 7.4 Hz, 4H), 7.43 (t,
J = 7.3 Hz, 4H), 7.34, (t, J = 7.4 Hz, 2H), 7.13 (s, 1H), 6.80 (s, 2H),
3.92 (bs, 1H), 3.22 (t, J = 7.1 Hz, 2H), 1.60–1.70 (m, 2H), 1.40–1.50
(m, 2H), 0.97 (t, J = 7.3 Hz, 3H). 13C NMR (100 MHz, CDCl3): d
149.0, 142.8, 141.8, 128.6, 127.2, 115.8, 110.7, 43.9, 31.6, 20.3,
13.9. MS, m/z calcd for C22H23N: 302.19 (M+H+). Found: 302.20.
Anal. Calc. for C22H23Nꢀ0.05CH2Cl2: C, 86.64; H, 7.62; N, 4.58.
Found: C, 86.66; H, 7.58; N, 4.61%.
2.1.2. Preparation of N-([1,10-biphenyl]-3-yl)-6-bromo-N-
phenylpyridin-2-amine (5)
This compound was prepared by the cross coupling of 4 (2.45 g,
10 mmol) with 2,6-dibromopyridine (3.5 g, 15 mmol) using general
procedure A, yellow solid, 3.6 g, yield 90%. 1H NMR (300 MHz,
CDCl3), d 7.52 (d, J = 6 Hz, 2H), 7.46–7.31 (m, 7H), 7.26–7.14 (m,
6H), 6.93 (d, J = 7.5 Hz, 1H) 6.58 (d, J = 8.4 Hz, 1H). 13C NMR
(75 MHz, CDCl3), d 158.5, 145.5, 145.0, 142.4, 140.6, 139.8, 139.3,
129.6, 129.5, 128.9, 128.7, 127.4, 127.1, 126.6, 125.3, 125.1,
125.0, 123.8, 119.3, 111.3. MS, m/z calcd for C23H17N2Br: 401.06
(M+H+). Found: 401.06.
2.1.6. Preparation of N-([1,10:30,100-terphenyl]-50-yl)-6-bromo-N-
butylpyridin-2-amine (9)
This compound was prepared by the cross coupling of 8 (0.6 g,
2.0 mmol) with 2,6-dibromopyridine (0.71 g, 3.0 mmol) following
general procedure A, yellow solid, 0.87 g, yield 95%. 1H NMR
(400 MHz, CDCl3): d 7.64 (t, J = 1.6 Hz, 1H), 7.55–7.60 (m, 4H),
7.35–7.40 (m, 6H), 7.30–7.35 (m, 2H), 7.00 (dd, J = 7.4 Hz, 1H),
6.64 (d, J = 7.4 Hz, 1H), 6.27 (d, J = 7.8 Hz, 1H), 3.93 (t, J = 7.5 Hz,
2H), 1.55–1.65 (m, 2H), 1.25–1.35 (m, 2H), 0.86 (t, J = 7.4 Hz, 3H).
13C NMR (100 MHz, CDCl3): d 157.5, 144.4, 142.5, 139.2, 138.9,
137.5, 127.8, 126.7, 126.1, 124.2, 123.0, 114.6, 105.9, 49.0, 28.9,
19.0, 12.9. MS, m/z calcd for C27H25N2Br: 459.13 (M+H+). Found:
459.13.
2.1.3. Preparation of N-([1,10-biphenyl]-3-yl)-N-phenyl-[2,20-
bipyridin]-6-amine (6)
A dry, 100 mL nitrogen flushed flask was charged with 2-bro-
mopyridine (0.92 mL, 9.4 mmol) and tetrahydrofuran (10 mL).
The zinc suspension in THF (12.5 mL, 5 g/100 mL) was added to
the reaction mixture via a syringe. The reaction mixture was stir-
red at 50 °C for 3 h to ensure that all the bromopyridine was con-
verted to pyridylzinc reagent. The resulting pyridylzinc bromide
was then transferred to a separate flask that was charged with 5
(1.88 g, 4.7 mmol) and THF (10 mL). Proper care was taken such
that unreacted zinc, which was left at bottom of the flask, was
2.1.7. Preparation of 10ꢀZnBr2
To
a 50 mL argon-flushed flask was charged 9 (1.01 g,
2.2 mmol), Pd(PPh3)4 (0.13 g, 0.11 mmol), and THF (15 mL). The
pyridylzinc bromide (0.6 M, 11 mL, prepared from 2-bromopyri-

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S. Huo et al. / Polyhedron 52 (2013) 1030–1040
dine and Rieke’s zinc [15]) was then added dropwise to the reac-
tion flask at ambient temperature. The reaction was then heated
to 50 °C and stirred for 8 h. After cooling to room temperature,
the reaction mixture was poured into water, and the aqueous
phase was extracted with ethyl acetate (EtOAc) (3 ꢁ 35 mL). The
combined organic extracts were then washed with brine, dried
over MgSO4, filtered, and evaporated. The crude product was puri-
fied by chromatography on silica gel with a mixture of hexanes and
ethyl acetate (v/v = 2:1) to provide a yellow-green solid, 1.29 g.
This compound might be contaminated with other Zn adducts,
since the element analysis did not give the result consistent with
the structure of 10ꢀZnBr2 and 1H NMR spectrum also shown extra
protons. However, the crystals prepared from the diffusion of hex-
anes to a solution of the yellow-green solid in dichloromethane
was determined to be 10ꢀZnBr2 by single crystal X-ray crystal
structure analysis. Mass spectrum of the solid showed the mass
of the compound 10. MS, m/z calcd for C32H29N3: 456.22 (M+H+).
Found: 456.25.
was purified by chromatography on silica gel with a mixture of
hexane and ethyl acetate (v/v = 4:1) to give a white solid, 0.37 g,
84%. 1H NMR (400 MHz, CDCl3): d 7.61 (d, J = 7.7 Hz, 4H), 7.43 (t,
J = 7.2 Hz, 4H), 7.34 (t, J = 7.3 Hz, 2H), 7.20 (s, 1H), 6.89 (s, 2H),
3.82 (s, 2H). 13C NMR (100 MHz, CDCl3): d 147.1, 143.0, 141.4,
128.7, 127.3, 127.2, 117.0, 112.9. MS, m/z calcd for C18H15N:
246.13 (M+H+). Found: 246.12.
2.1.11. Preparation of di([1,10:30,100-terphenyl]-50-yl)amine (14)
This compound was prepared by the palladium catalyzed cross-
coupling of 13 (0.25 g, 1 mmol) with 50-bromo-m-terphenyl
(0.46 g, 1.5 mmol) following general procedure A, white solid,
0.27 g, yield 62%. 1H NMR (400 MHz, CDCl3): d 7.63 (d, J = 7 Hz,
8H), 7.40–7.45 (m, 10H), 7.30–7.35 (m, 8H), 5.99 (s, 1H). 13C
NMR (100 MHz, CDCl3): d 143.4, 143.1, 141.0, 128.7, 127.5, 127.2,
119.4, 115.8. MS, m/z calcd for C36H27N: 474.22 (M+H+). Found:
474.22. Anal. Calc. for C36H27N: C, 91.30; H, 5.75; N, 2.96. Found:
C, 91.12; H, 5.81; N, 3.10%.
2.1.8. Preparation of platinum complex (11)
2.1.12. Preparation of N,N-di([1,10:30,100-terphenyl]-50-yl)-[2,20-
bipyridin]-6-amine (15)
This complex was prepared by reacting 10ꢀZnBr2 (0.55 g) with
K2PtCl4 (0.42 g, 1 mmol) following general procedure B, orange so-
lid, 0.27 g, yield 32% based on K2PtCl4. 1H NMR (400 MHz, CD2Cl2):
d 9.30 (d, J = 5.3 Hz, 1H), 8.08 (t, J = 7.7 Hz, 1H), 7.97 (d, J = 8 Hz,
1H), 7.85 (s, 1H), 7.55–7.70 (m, 8H), 7.49 (t, J = 8.4 Hz, 4H), 7.40
(t, J = 8.1 Hz, 2H), 7.13 (d, J = 7.4 Hz, 1H), 6.55 (d, J = 8.8 Hz, 1H),
This compound was prepared by the palladium catalyzed cross-
coupling of 14 (0.11 mg, 0.24 mmol) with 6-bromo-2,20-bipyridine
(0.085 mg, 0.36 mmol) following general procedure A, off-white so-
lid, 0.07 g, 86%. 1H NMR (400 MHz, CDCl3): d 8.63 (d, J = 4.8 Hz, 1H),
8.13 (d, J = 8 Hz, 1H), 8.00 (d, J = 7.5 Hz, 1H), 7.55–7.70 (m, 16H),
7.40 (t, J = 7.1 Hz, 8H), 7.33 (t, J = 7.3 Hz, 4H), 7.20–7.25 (m, 1H),
6.92 (d, J = 8.8 Hz, 1H). 13C NMR (100 MHz, CDCl3): d 158.0,
156.1, 154.1, 149.0, 146.6, 142.8, 140.7, 138.6, 136.8, 128.8,
127.6, 127.2, 124.2, 123.5, 122.5, 121.1, 113.6, 113.4. MS, m/z calcd
3
6.19 (s, JPt–H = 49 Hz, 1H), 2.00–2.10 (m, 1H), 1.85–1.95 (m, 1H),
1.45–1.55 (m, 2H), 0.85 (t, J = 7.2 Hz, 3H). 13C NMR (100 MHz,
CD2Cl2): d 160.5, 157.1, 151.3, 149.2, 143.8, 141.7, 140.5, 138.3,
136.0, 129.3, 128.2, 127.5, 127.2, 124.9, 122.1, 110.4, 109.8, 52.9,
39.3, 22.7, 14.6. MS, m/z calcd for C32H28N3BrPt: 729.11. Found:
729.11. Anal. Calc. for C32H28BrN3Pt: C, 52.68; H, 3.87; N, 5.76.
Found: C, 52.42; H, 3.66; N, 5.76%.
for
46H33N3: C, 88.01; H, 5.30; N, 6.69. Found: C, 87.86; H, 5.24; N,
C
46H33N3: 628.28 (M+H+). Found: 628.28. Anal. Calc. for
C
6.82%.
2.1.9. Preparation of N-([1,10:30,100-terphenyl]-50-yl)-2-
phenylacetamide (12)
2.1.13. Preparation of N,N-di([1,10:30,100-terphenyl]-50-yl)-6-(1H-
pyrazol-1-yl)pyridin-2-amine (16)
This compound was prepared according to Buchwald’s proce-
dure [16]. A dry, 50 mL argon flushed flask was charged with benz-
amide (0.44 g, 3.6 mmol), 50-bromo-m-terphenyl (0.92 g, 3 mmol),
Cs2CO3 (1.95 g, 6 mmol), Pd(dba)2 (0.052 g, 0.09 mmol), Xantphos
(0.078 g, 0.13 mmol), and dioxane (8 mL). The reaction mixture
was heated to 100 °C for 24 h. After cooling to room temperature,
20 mL of ethyl acetate was added and stirred for 5 min. The reac-
tion was poured into water and extracted with ethyl acetate
(3 ꢁ 35 mL). The organic layers were washed with brine (30 mL)
and dried over magnesium sulfate overnight. After filtration and
removal of the solvents, the crude product was purified by chroma-
tography on silica gel with a mixture of hexane and dichlorometh-
ane (v/v = 1:4) to give a white solid, 1.0 g, 96%. 1H NMR (400 MHz,
CDCl3): d 8.07 (s, 1H), 7.85–7.95 (m, 4H), 7.64 (d, J = 7.3 Hz, 4H),
7.50–7.60 (m, 2H), 7.40–7.50 (m, 6H), 7.36 (t, J = 7.1 Hz, 2H). 13C
NMR (100 MHz, CDCl3): d 165.8, 142.6, 140.6, 138.7, 134.8, 131.9,
128.8, 127.6, 127.2, 127.0, 122.4, 117.9. MS, m/z calcd for
This compound was prepared palladium catalyzed cross-cou-
pling of 14 (0.73 g, 1.68 mmol) with 2-bromo-6-(1H-pyrazol-1-
yl)pyridine [18] (0.36 g, 2.02 mmol) following general procedure
A, off-white solid, 0.45 g, yield 52%. 2-Bromo-6-(1H-pyrazol-1-
yl)pyridine was prepared from 2,6-dibromopyridine and pyrazole
according to Buchwald’s procedure [19]. Analytical data for 16:
1H NMR (400 MHz, CDCl3): 1H NMR (400 MHz, CDCl3): d 8.20 (dd,
J = 2.6 Hz, 0.6 Hz, 1H), 7.65–7.70 (m, 3H), 7.60–7.65 (m, 9H), 7.55
(d, J = 1.6 Hz, 4H), 7.48 (d, J = 7.6 Hz, 1H), 7.42 (t, J = 7.2 Hz, 8H),
7.30–7.35 (m, 4H), 6.74 (d, J = 8.0 Hz, 1H), 6.25–6.30 (m, 1H). 13C
NMR (100 MHz, CDCl3): d 157.2, 149.8, 146.1, 142.9, 141.8, 140.5,
140.2, 128.8, 127.6, 127.1, 127.0, 124.4, 122.9, 109.7, 107.4,
103.3; MS, m/z calcd for
617.28. Anal. Calc. for C44H32N4: C, 85.69; H, 5.23; N, 9.08. Found:
C, 85.54; H, 5.21; N, 8.96%.
C
44H32N4: 617.27 (M+H+). Found:
2.1.14. Preparation of platinum complex (17)
C
C
25H19NO: 372.14 (M+Na+). Found: 372.13. Anal. Calc. for
To a dry, 50 mL argon-flushed flask were charged ligand 15
(0.1 g, 0.16 mmol), K2PtCl4 (0.087 g, 0.21 mmol), and acetonitrile
(10 mL). The mixture was degassed and refluxed under argon for
two weeks. After cooling to room temperature, the acetonitrile
was removed by evaporation. The crude materials were purified
by flash chromatography on silica gel with dichloromethane and
hexane (v/v = 15:1). Dark red solid, 0.05 g, yield 38%. 1H NMR
(400 MHz, CD2Cl2): d 9.72 (d, J = 5.28 Hz, 1H), 8.31 (d, J = 8.1 Hz,
1H), 8.18 (t, J = 7.9 Hz, 1H), 8.12 (s, 1H), 7.70–7.85 (m, 10H),
7.40–7.55 (m, 10H), 7.28 (t, J = 7.2 Hz, 2H), 7.19 (t, J = 7.2 Hz, 1H),
7.10 (t, J = 6.6 Hz, t), 7.02 (t, J = 7.2 Hz, 1H), 6.97 (d, J = 8.1 Hz,
25H19NO: C, 85.93; H, 5.48; N, 4.01. Found: C, 85.68; H, 5.48; N,
3.86%.
2.1.10. Preparation of [1,10:30,100-terphenyl]-50-amine [17] (13)
A
50 mL flask was charged with compound 12 (0.63 g,
1.8 mmol), 12 M HCl (6 mL), and glacial acetic acid (6 mL). The
reaction was stirred at reflux for 18 h. After cooling to room tem-
perature, the reaction was neutralized with a saturated sodium
carbonate solution. The reaction mixture was extracted with
dichloromethane (3 ꢁ 40 mL) and the organic layers were washed
with brine (45 mL) and dried over magnesium sulfate overnight.
After filtration and removal of the solvents, the crude product
3
1H), 6.5 (s, JPt–H = 10 Hz, 1H). 13C NMR (100 MHz, CD2Cl2): d
158.1, 153.7, 149.7, 145.2, 142.8, 151.6, 140.0, 137.6, 137.3,

S. Huo et al. / Polyhedron 52 (2013) 1030–1040
1033
134.7, 133.0, 129.4, 128.9, 128.5, 128.0, 126.9, 126.7, 126.5, 126.1,
2.3. Photophysical experiments
125.9, 124.1, 123.3, 120.4, 119.5, 115.6, 114.4, 113.8. MS, m/z calcd
for C46H31N3Pt: 821.22. Found: 821.21. Anal. Calc. for C46H31N3Pt:
C, 67.31; H, 3.81; N, 5.12. Found: C, 67.04; H, 3.68; N, 5.04%.
Absorption spectra were recorded using a Shimadzu 2445 UV–Vis
spectrophotometer, using 1 cm path-length quartz cuvettes. The
steady state emission spectra were measured using a PTI QM-4CW
system and an excitation source with the spectral bandpass of 2 nm
was used. Quantum yields were measured using comparative meth-
od at room temperature in a dichloromethane solution. The solution
was deoxygenated by purging with argon gas for 20 min. A long
necked 1 cm quartz cuvette was used for measurements. The cuvette
was cooled with a dry ice–acetone bath to prevent solvent loss. The
optical density of both sample and reference solutions was main-
tained below 0.1 AU at and above the excitation wavelength. Emis-
sion spectra in a frozen glass were recorded in 2-MeTHF at 77 K.
Phosphorescence lifetime measurements were performed on the
same fluorimeter equipped with variable high rep rate pulsed xenon
2.1.15. Preparation of platinum complex (18)
This complex was prepared from 16 (0.26 g, 0.5 mmol) and
K2PtCl4 (0.21 g, 0.5 mmol) following similar procedure to that for
complex 17 except that a few of tertabutylammonium chloride
was added as a phase transfer catalyst, orange solid, 0.09 g, yield
22%. 1H NMR (400 MHz, CD2Cl2): d 8.38 (dd, J = 18.3 Hz, 2 Hz,
2H), 8.11 (s, 1H), 7.65–7.75 (m, 8H), 7.40–7.50 (m, 10H), 7.27 (t,
J = 7.3 Hz, 2H), 7.18 (t, J = 7.2 Hz, 1H), 7.08 (d, J = 7.7 Hz, 1H),
3
6.95–7.05 (m, 2H), 6.79 (s, 1H), 6.59 (d, J = 9 Hz, 1H), 6.5 (s, JPt–
H = 11 Hz, 1H). 13C NMR (100 MHz, CD2Cl2): d 158.2, 155.8, 149.3,
148.7, 147.0, 145.2, 144.8, 142.7, 141.3, 140.9, 139.9, 137.3,
136.3, 134.1, 132.7, 129.4, 128.9, 128.5, 127.9, 127.4, 126.7,
126.5, 126.2, 124.0, 120.2, 115.9, 115.0, 114.2, 109.9, 100.4. MS:
m/z calcd for C44H30N4Pt was 809.21. Found: 809.21. Anal. Calc.
for C44H30N4Pt: C, 65.26; H, 3.73; N, 6.92. Found: C, 65.17; H,
3.77; N, 6.96%.
source for excitation and were limited to lifetimes >0.4 ls.
2.4. X-ray crystallography
All crystals were grown by diffusing hexanes to a dichlorometh-
ane solution of the complexes. A suitable crystal was selected and
mounted on a glass fiber. All measurements were made using
2.2. DFT calculations
graphite-monochromated Cu Ka radiation (1.54178 Å) on a Bru-
ker-AXS three-circle diffractometer, equipped with a SMART Apex
II CCD detector. In each case, initial space group determination was
based on a matrix consisting of 120 frames. The data were reduced
using SAINT+ [22], and empirical absorption correction applied using
SADABS [23]. Structures were solved using direct methods. Least-
squares refinement for all structures was carried out on F2. All
non-hydrogen atoms were refined anisotropically. Hydrogen
Density functional theory (DFT) calculations were performed on
complexes 17 and 18. The quantum mechanical computer software
package G03 and the B3LYP exchange–correlation functional [19]
were used to perform all calculations. The DEF2_TZVP basis set
[20] was used for platinum, while the cc-pvdz basis set [21] was
used for all other atoms.
Table 1
Crystal data and structure refinement for the compound 7, 11, 10ꢀZnBr2, and 17.
Compound
7
11
10ꢀZnBr2
17
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
Unit cell dimensions
a (Å)
C
57H41Cl4N6Pt2
C32H28BrN3Pt
729.57
296(2)
C32.50H30Br2ClN3Zn
723.24
100(2)
C46H31N3Pt
820.83
296(2)
1.54178
orthorhombic
Pna2(1)
1341.94
296(2)
1.54178
triclinic
1.54178
triclinic
1.54178
triclinic
ꢀ
ꢀ
ꢀ
P1
P1
P1
9.86690(10)
10.7199(2)
12.0140(2)
88.4230(10)
70.9140(10)
88.5010(10)
1200.24(3)
9.2524(2)
8.69480(10)
12.6994(2)
13.9372(2)
90.6750(10)
101.0420(10)
93.1300(10)
1507.80(4)
18.8103(4)
10.1842(2)
36.9117(7)
90
90
90
7071.1(2)
8
b (Å)
c (Å)
10.4128(2)
15.1210(2)
76.0020(10)
83.8520(10)
74.5260(10)
1360.90(4)
2
1.780
11.553
708
a
(°)
b (°)
c
(°)
V (A3)
Z
1
2
Calculated density (mg mꢂ3
)
1.857
12.163
649
1.591
5.251
724
1.542
7.702
3248
Absorption coefficient (mmꢂ1
F(000)
)
Crystal size (mm)
h range for data collection (°)
Limiting indices
0.39 ꢁ 0.26 ꢁ 0.21
3.89–66.99
ꢂ11 6 h 6 11
ꢂ12 6 k 6 12
ꢂ14 6 l 6 12
20200
0.22 ꢁ 0.10 ꢁ 0.09
3.01–67.00
ꢂ11 6 h 6 9
ꢂ12 6 k 6 12
ꢂ18 6 l 6 18
22699
0.505 ꢁ 0.156 ꢁ 0.055
0.27 ꢁ 0.26 ꢁ 0.03
2.39–64.25
ꢂ21 6 h 6 18
ꢂ11 6 k 6 11
ꢂ42 6 l 6 41
71266
3.23–67.00
ꢂ10 6 h 6 10
ꢂ15 6 k 6 15
ꢂ16 6 l 6 16
25381
Reflections collected
Independent reflections (Rint
Completeness to h = 67.0 (%)
Absorption correction
)
4140 (0.0344)
96.7
numerical
4691 (0.0277)
96.9
numerical
5216 (0.0272)
97.0
numerical
11127 (0.0480)
98.2
numerical
Maximum and minimum transmission
Refinement method
0.1884 and 0.0882
full-matrix least-squares on
F2
0.4365 and 0.1817
full-matrix least-squares on
F2
0.85778 and 0.36159
full-matrix least-squares on
F2
0.8249 and 0.2267
full-matrix least-squares on
F2
Data/restraints/parameters
4140/1/311
1.109
4691/0/335
1.004
5216/0/371
1.049
11127/19/902
1.103
Goodness-of-fit on F2
Final R indices (I > 2
R indices (all data)
r
(I))
R1 = 0.0279, wR2 = 0.0699
R1 = 0.0282, wR2 = 0.0702
0.892 and ꢂ1.395
R1 = 0.0180, wR2 = 0.0447
R1 = 0.0185, wR2 = 0.0450
0.854 and ꢂ0.637
R1 = 0.0280, wR2 = 0.0715
R1 = 0.0294, wR2 = 0.0727
1.280 and ꢂ0.986
R1 = 0.0537, wR2 = 0.1543
R1 = 0.0631, wR2 = 0.1650
2.878 and ꢂ1.361
Largest difference in peak and hole
(e Åꢂ3
)

1034
S. Huo et al. / Polyhedron 52 (2013) 1030–1040
atoms were placed in calculated positions and allowed to be re-
fined isotropically as riding models. Structure solutions, refine-
ment and the calculation of derived results were performed
using the SHELXTL package of computer programs [24]. Crystal data
and structure refinement for the compound 7, 11, 10ꢀZnBr2, and
17 are listed in Table 1.
3. Results and discussion
3.1. Initial attempt to synthesize a C^C⁄N^N-coordinated platinum
complexes
There are four possible geometrical variations on the tetraden-
tate bis-cyclometalated platinum complexes based on the carbon
and nitrogen donors, namely C^N⁄N^C, N^C⁄C^N, C^C⁄N^N, and
C^N⁄C^N. Among one of many possible designs shown in Chart 1,
compounds C^N⁄N^C–Pt (1) and N^C⁄C^N–Pt (2) have been re-
ported previously [12a]. The other two types of complexes have
not been prepared. Our efforts started with attempting to prepare
the C^C⁄N^N-coordinated complex (3).
The preparation of the corresponding ligand 6 is shown in
Scheme 1. The synthesis was readily achieved by two consecutive
Pd-catalyzed C–N bond formations [25] starting from 3-bromo-
1,10-biphenyl followed by the Negishi coupling [26] of the interme-
diate bromide 5 with 2-pyridylzinc bromide. However, when the
ligand 6 was reacted with K2PtCl4 in acetic acid, the major product
of the reaction was not the desired tetradentate coordination com-
plex 6. The X-ray single crystal structure determination confirmed
that the product was a tridentate N^N⁄C-coordinated platinum
complex 7 (Fig. 1). In addition to the 3 and 7, there is another pos-
sible metalation site in ligand 6 (Scheme 1) for the formation of the
N^N⁄C-coordinated complexes, which is on the N-phenyl ring. The
Fig. 1. ORTEP drawing of 7. Ellipsoids are at the 50% probability level. Bond lengths:
Pt(1)–C(1) 1.982 (4), Pt(1)–N(2) 1.993 (3), Pt(1)–N(3) 2.088 (4), Pt(1)–Cl(1) 2.3066
(11) Å. Bond angles: C(1)–Pt(1)–N(2) 93.31 (15)°, C(1)–Pt(1)–N(3) 168.74 (15)°,
N(2)–Pt(1)–N(3) 80.97 (143)°, C(1)–Pt(1)–Cl(1) 93.70 (12)°, N(2)–Pt(1)–Cl(1)
169.76 (10)°, N(3)–Pt(1)–Cl(1) 93.32 (11)°.
preferential formation of 7 may be due to the reduced steric hin-
drance (kinetic factor) and the extended conjugation of coordina-
tion plane (thermodynamic factor).
The crystal data are listed in Table 1 and the molecular structure
of 7 was shown in Fig. 1. The platinum coordinates to the two nitro-
gen atoms of the bipyridine ring and one carbon of the biphenyl ring
to form a tridentate N^N⁄C-coordinated complex with a slightly dis-
torted square-planar geometry. The N-phenyl ring is nearly perpen-
dicular with the coordination plane. The amino nitrogen adopts a
trigonal planar geometry, implying a conjugation with the bipyri-
dine ring and the cyclometalated phenyl ring, or an extended elec-
tron delocalization including the center platinum ion. The two
phenyl rings in the biphenyl moiety are slightly twisted with a small
torsion angle of 11.8°, as a result of the balance between the steric
N
N
N
N
N
N
N
N
N
Pt
Pt
Pt
Pt
N
N
N
effect and the extension of
p conjugation. The extended conjugation
is likely responsible for the preferential formation of the complex 7
over the other similar tridentate N^N⁄C-coordinated platinum com-
plex that could be formed from the ligand 6 with the metalation on
the N-phenyl ring as indicated above.
1
2 3
N^C*C^N-Pt ( ) C^C*N^N-Pt ( )
C^N*N^C-Pt ( )
C^N*C^N-Pt (?)
Chart 1. Coordination patterns for a platinum complex with two carbon and two
nitrogen donors.
Br
ZnCl
N
Br
N
NH
N
N
NH2
Br
N
N
N
Pd(dba)2, DPPF
NaOtBu, Toluene
Pd(dba)2, DPPF
NaOtBu, Toluene
Pd(PPh3)4
Br
4
5
6
K2PtCl4
AcOH
115 o
K2PtCl4
AcOH
N
N
N
N
C
115 oC
N
N
N
Pt
Pt
65%
N
N
Cl
7
3
6
Scheme 1. Synthesis of ligand 6 and its reaction with K2PtCl4.

S. Huo et al. / Polyhedron 52 (2013) 1030–1040
1035
Br
ZnBr
Br
N
N
NH
N
N
BuNH2
Br
N
N
N
Br2Zn
Pd(dba)2,DPPF
Pd(PPh3)4
Pd(dba)2, DPPF
NaOtBu, Toluene
Br
NaOtBu, Toluene
8
9
10-ZnBr2
Scheme 2. Preparation of ligand 10.
3.2. Further attempt to synthesize a C^C⁄N^N-coordinated platinum
complex
After failing to obtain the tetradentate C^C⁄N^N-coordinated
platinum complex 3, we changed our strategy of the ligand design.
A new ligand 10 was designed with the following considerations.
Firstly, a butyl group was used to replace the N-phenyl group in
the ligand 6, because it was expected that a preferential aromatic
C–H bond activation over the sp3 C–H bond activation would occur.
Secondly, introduction of the terphenyl group makes a forced dou-
ble cyclometalation possible. The preparation of the ligand 10 is
shown in Scheme 2, which is similar to that of ligand 6. The palla-
dium catalyzed C–N cross coupling of 50-bromo-m-terphenyl with
butylamine gave 8, which was cross coupled with 2,6-dibromo-
pyridine to give the intermediate 9. The Negishi coupling of 9 with
pyridylzinc bromide produced the 10, which was isolated as an ad-
duct with zinc bromide, as revealed later by the X-ray crystal struc-
ture determination.
Initially we thought that a free ligand 10 had been prepared, so
the complexation with platinum was carried out and an orange so-
lid was isolated as the single product. However, the 1H NMR spec-
trum of the product was not consistent with the structure of
expected tetradentate coordination platinum complex. The X-ray
crystal structure analysis confirmed that, once again, a tridentate
coordination platinum complex 11 was formed (Scheme 3).
The crystal data for the compound 11 are listed in Table 1 and
the molecular structure is shown in Fig. 2. The cycloplatination oc-
curred on the butyl group to form a tridentate C^N^N-coordinated
complex with a fused five-five-membered metallacyle. The coordi-
nation geometry is significantly deviated from ideal square for the
platinum (II) complex. For example, the angle of C(29)–Pt(1)–N(1)
is 163.33° compared with an ideal 180°. N(2)–Pt(1)–N(1) of 79.58
(9) and C(29)–Pt(1)–Br(1) of 98.95 (8)° are nearly 10° off the ideal
90°. Comparatively, the N^N⁄C-coordinated complex 7 has a geom-
etry closer to the ideal square plane. The amino nitrogen is conju-
gated with the bipyridine ring as indicated by its trigonal planar
geometry and the fact that the N-phenyl ring is nearly perpendic-
ular to the trigonal plane.
Fig. 2. ORTEP drawing of 11. Ellipsoids are at the 50% probability level. Bond lengths:
Pt(1)–N(2) 1.942 (2), Pt(1)–C(29) 2.034 (3), Pt(1)–N(1) 2.119 (2), Pt(1)–Br(1) 2.4302
(3) Å. Bond angles: N(2)–Pt(1)–C(29) 83.76 (10)°, N(2)–Pt(1)–N(1) 79.58 (9)°,
C(29)–Pt(1)–N(1) 163.33°, N(2)–Pt(1)–Br(1) 176.76 (6)°, C(29)–Pt(1)–Br(1) 98.95
(8)°, N(1)–Pt(1)–Br(1) 97.72 (6)°.
lated, even by column chromatography on silica gel, was actually
its adduct with ZnBr2. The structure of the 1:1 adduct was con-
firmed by the X-ray crystallography. The molecular structure was
depicted in Fig. 3. It can be seen that the bipyridine ligand coordi-
nates to the zinc bromide to form a bidentate coordination com-
plex. The complex has distorted tetrahedral coordination
geometry. The bite angle in this bidentate complex, N(1)–Zn(1)–
N(2), is 79.79 (8)°, while the angle Br(1)–Zn(1)–Br(2) is 116.470
(15)°. The amino nitrogen is considerably deviated from a trigonal
planar geometry found in the platinum complexes 7 and 11, with
the distance of the nitrogen from the trigonal plane defined by
C(10), C(11) and C(15) being 0.24 Å. This is obviously attributed
to the fact that the cyclometalation, which formed a fused metalla-
cycle in the platinum complexes, is absent in the ZnBr2 adduct.
The cyclometalation occurred at the butyl group rather than the
aromatic ring, which is in opposition to a generally accepted notion
that aromatic C–H bonds are more reactive than sp3 C–H bonds.
This unexpected result also prompted us to further study the selec-
tivity in the carbon–hydrogen bond activation by platinum, and
eventually led to the discovery of a solvent-controlled switch of
From the molecular structure it was also discovered that the ha-
lide attached to the platinum was not a chloride from the K2PtCl4.
It was a bromide! It turned out that the ligand 10 prepared and iso-
Br
K2PtCl4
AcOH
K2PtCl4
Pt
N
AcOH
115 oC
115 oC
N
N
N
N
N
N
N
N
Pt
Br2Zn
11
10-ZnBr2
Scheme 3. Reaction of 10 with K2PtCl4.

1036
S. Huo et al. / Polyhedron 52 (2013) 1030–1040
K2PtCl4
CH3CN
reflux
N
N
one week
N
N
N
N
Pt
17
15
K2PtCl4
CH3CN
Bu4N+Cl-
Reflux
N
N
one week
N
N
N
N
Fig. 3. ORTEP drawing of 10ꢀZnBr2. Ellipsoids are at the 50% probability level. Bond
lengths: Zn(1)–Br(1) 2.3638 (4), Zn(1)–Br(2) 2.3230 (4), Zn(1)–N(1) 2.061 (2),
Zn(1)–N(2) 2.0839 (19) Å. Bond angles: N(1)–Zn(1)–N(2) 79.79 (8)°, N(1)–Zn(1)–
Br(2) 111.19 (6)°, N(1)–Zn(1)–Br(1), 112.95 (6)°, N(2)–Zn(1)–Br(1) 102.46 (5)°,
N(2)–Zn(1)–Br(2) 128.60 (5)°, Br(1)–Zn(1)–Br(2) 116.470 (15)°.
Pt
N
N
16
18
selectivity between sp2 and sp3 C–H bond activation by platinum
[27]. It was found that the cyclometalation reaction of N-alkyl-N-
phenyl-2,20-bipyridin-6-amine with K2PtCl4 occurred predomi-
nantly at the alkyl group when the reaction was carried out in ace-
tic acid, while the cyclometalation on the phenyl ring was
predominant when the reaction was carried out in acetonitrile.
The selectivity switch was rationalized by the kinetic and thermo-
dynamic control of the reactions [27]. However, when the ligand
10ꢀZnBr2 was reacted with K2PtCl4 in acetonitrile, no desired tetra-
dentate C^C⁄N^N-coordinated complex was formed, presumably
due to the higher energy barrier caused by the steric hindrance
for the aromatic C–H bond activation compared to the sp3 C–H
bond activation for this ligand.
Scheme 5. Synthesis of C^C⁄N^N-coordinated platinum complexes.
consecutive Pd-catalyzed amination reactions of 3,5-diphenylani-
line 13 with 50-bromo-m-terphenyl and 6-bromo-2,20-bipyridine
led to the rapid construction of ligand 15. 3,5-Diphenyl aniline
13 was prepared by Pd-catalyzed amidation of 50-bromo-m-ter-
phenyl followed by the acid-promoted hydrolysis of the amide
12. The ligand 16 was prepared similarly by the cross coupling of
14 and 2-bromo-6(1H-pyrazol-1-yl)pyridine.
To our great delight, the reactions of ligand 15 and 16 with
K2PtCl4 in acetonitrile at reflux proceeded to form the desired
C^C⁄N^N-coordinated platinum complexes 17 and 18, respectively
(Scheme 5). The reaction was very slow, taking a week or so to attain
reasonable conversion. Addition of phase transfer catalyst tetrabu-
tylammonium chloride accelerated the reaction to some extent.
3.3. First synthesis of C^C⁄N^N-coordinated platinum complexes
After all unsuccessful, albeit fruitful, attempts of synthesizing a
C^C⁄N^N-coordinated platinum complex, we arrived at our ulti-
mate design of the ligand 15 where a forced double cyclometala-
tion would be, if not the only possibility, a viable alternative. The
synthesis of ligand 15 is shown in Scheme 4. An efficient use of
3.4. Crystal structure of C^C⁄N^N-coordinated platinum complex 17
The structure of 17 was determined by single crystal X-ray dif-
fraction experiment. The crystal data are summarized in Table 1.
Br
Br
N
N
Br
NHCOPh
HCl-AcOH
NH2
NH
N
PhCONH2
N
N
Pd(dba)2,DPPF
Pd(dba)2,DPPF
NaOtBu, Toluene
Pd2(dba)3, XthanPhos
NaOtBu, Toluene
NaOtBu, Toluene
12
13
14
15
Br
N
N
N
Br
CuI
Cs2CO3
NH
N
Br
H
N
N
N
+
N
N
N
N
Pd(dba)2,DPPF
NaOtBu, Toluene
N
Br
N
MeHN
NHMe
14
16
Scheme 4. Preparation of ligands 15 and 16.

S. Huo et al. / Polyhedron 52 (2013) 1030–1040
1037
Fig. 5. DFT calculated HOMO (bottom) and LUMO (top) for 17 and 18, showing
localization of the orbitals.
in the unit cell. The flat shape of the molecule with extended con-
jugation facilitates the intermolecular stacking through
action. As shown in Fig. 4(b), the coordination planes of two
molecules have a distance of 3.4 Å, indicating a significant
interaction in the solid state. The interaction can be further
p–p inter-
p–p
p–p
rationalized by the HOMO–LUMO interaction of the two molecules
as the bipyridine moiety in one molecule is overlapping with the
biphenyl moiety of the other molecule (see DFT calculations of
HOMO and LUMO of 17, Fig. 5 in the following section). The Ptꢀ ꢀ ꢀPt
distance was measured to be 3.72 Å, implying a platinum–plati-
num interaction, which has not been found in C^N⁄N^C and
N^C⁄C^N-coordinated platinum complexes 1 and 2 [12]. The Pt–
N and Pt–C bonds and the bond angles around the core coordina-
tion sphere are similar to those found in the C^N⁄N^C and
N^C⁄C^N-coordinated platinum complexes [12].
3.5. DFT calculations on C^C⁄N^N-coordinated platinum complexes 17
and 18
Density functional theory (DFT) calculations were performed on
complexes 17 and 18. The optimized ground state geometry of 17
compares satisfactorily to that determined by X-ray crystallography.
The HOMO and LUMO orbitals are shown in Fig. 5. The compo-
sition of frontier molecular orbitals has been very useful to inter-
preting the nature of the excited states and substituent effect on
the photophysical properties of platinum complexes. The HOMO
orbitals for both complexes are mainly localized on the C^C rings
(the two phenyl rings forming bond with the platinum), with con-
tribution from the platinum. The LUMOs are localized on the N^N
rings, with very small contribution from the platinum. Both HOMO
Fig. 4. (a) ORTEP drawing of 17. Ellipsoids are at the 50% probability level. Bond
lengths: Pt(1)–C(40) 1.938 (12), Pt(1)–C(46) 2.006 (15), Pt(1)–N(2) 2.049 (11),
Pt(1)–N(1) 2.114 (12) Å. Bond angles: C(40)–Pt(1)–C(46) 82.3 (6)°, C(40)–Pt(1)–
N(2) 92.8 (5)°, C(46)–Pt(1)–N(2) 175.1 (6)°, C(40)–Pt(1)–N(1) 172.6 (5)°, C(46)–
Pt(1)–N(1) 104.5 (6)°, N(2)–Pt(1)–N(1) 80.3 (5)°. (b) Intermolecular
of 17. (c) Ptꢀ ꢀ ꢀPt interaction in the solid state of complex 17.
p–p interaction
The unit cell of the complex 17 contains two independent mole-
cules with essentially identical geometry. The structure of one
molecule is depicted in Fig. 4. The tetradentate C^C⁄N^N-coordina-
tion was confirmed and the molecule has a square planar coordina-
tion geometry. From the mean coordination plane composed of 26
atoms including two phenyl rings and two pyridine rings as well as
the amino nitrogen and the platinum, the largest deviations are
less than 0.15 Å in one molecule and 0.25 Å in the other molecule
and LUMO orbitals have
p symmetry. The nature of the HOMO and
LUMO of the complexes provide an excellent basis for explaining
the intermolecular interaction found in the crystal packing of com-
plex 17, namely a HOMO–LUMO interaction. The calculated ener-
gies of HOMO and LUMO for the compound 17 are ꢂ4.65 and
ꢂ2.39 eV, while those for complex 18 are ꢂ4.66 and ꢂ2.00 eV.

1038
S. Huo et al. / Polyhedron 52 (2013) 1030–1040
Table 2
Photophysical data for complexes 17 and 18.
a
kabs, nm (
e
ꢁ 103 Mꢂ1 cmꢂ1
)
298 Ka
77 Kb
kem (nm)
Uc
s
(l
s)d
kem (nm)
s (ls)
17
18
261 (27.3), 392 (10.3), 418 (8.5), 519 (2.3)
260 (26.5), 371 (12.9), 456 (2.7)
660
555
0.001
0.17
0.85
4.4
607, 663
535, 577
6.3
14.3
a
b
c
Measurements were carried out in a solution of dichloromethane.
Measurements were carried out in a solution of 2-methyltetrahydrofuran.
Quantum yields were measured in deoxygenated dichloromethane. For 17, a solution of Ru(bpy)3Cl2 in water (/ = 0.028) [28] was used as the reference and for 18, a
deoxygenated solution of Pt(dpyb)Cl in dichloromethane (/ = 0.6) [9c] was used as the reference.
Measured in deoxygenated dichloromethane.
d
The HOMOs of 17 and 18 have essentially the same energy level,
which is consistent with the same composition of the HOMO orbi-
tals for both compounds. However, the LUMO energy of 18 is sig-
nificantly higher than that of 17, which is consistent with the
poorer electron acceptability of the pyrazolyl ring compared to
the pyridine ring.
molecule. The substantial red shift of the 1p p⁄ absorptions of
–
complexes 17 and 18 compared to those of their ligands 15 and
16 can be attributed to the extended conjugation through the ami-
no nitrogen after the cyclometalation as the crystal structure anal-
ysis revealed.
3.6.2. Emission
3.6. Photophysical properties of C^C⁄N^N-coordinated platinum
Both 17 and 18 are emissive in solution at room temperature.
The emissions shifted drastically to the lower energy compared
to the fluorescent emissions of the ligands and were sensitive to
the oxygen, indicating that they are phosphorescent emissions.
The emission spectra of 17 and 18 are shown in Fig. 7. Compound
17 only emitted very weak, deep red light with an emission max-
imum of 660 nm, whereas complex 18 exhibited very bright yellow
emission with a maximum of 550 nm, which seems to be ruled by
the energy gap law [29]. The lifetime for the excited triplet states
complexes 17 and 18
The photophysical data of 17 and 18 including absorptions,
emissions at room temperature and 77 K, quantum yields, and
the lifetimes of the excited states are summarized in Table 2.
3.6.1. Absorption
The absorption spectra of complexes 17 and 18 are shown in
Fig. 6. Also included in Fig. 6 are the spectra of the corresponding
ligands 15 and 16 for comparison. The intense absorptions in high-
er energy region <450 nm in the spectrum of 17 can be assigned as
were estimated to be 0.9 and 4.4 ls for 17 and 18, respectively.
The higher energy emission of 18, compared to that of 17, is con-
sistent with the larger HOMO–LUMO gap of 18, indicating that
the emission is from the lowest triplet state involving HOMO–
LUMO transition, namely a ILCT state mixed with MLCT states. In
the frozen glass matrix, both 17 and 18 were intensely emissive
(Fig. 8). The rigidochromic shift to the higher energy was observed
for both compounds. The spectra of 17 and 18 are highly struc-
the 1p p⁄ transitions. The absorptions appearing at lower energy
–
can be assigned as charge transfer bands, as their intensities are
moderate and they do not appear in the spectrum of the ligand.
Based on the HOMO and LUMO characteristics of the complex 17,
the lowest energy absorption may be assigned as intra ligand
charge transfer (1ILCT) (predominant) and metal to ligand charge
transfer (1MLCT) (minor). The complex 18 displayed similar
spectroscopy features, except for the blue shifted spectrum com-
pared to that of 17. The solvent effect on the absorption spectrum
of 18 was studied, and it was found that by changing the solvent
from toluene to acetonitrile, the lowest absorption was blue shifted
from 483 to 445 nm, which is consistent with the assignment of
the charge transfer state that changed the dipole moment of the
tured, displaying vibronic progression of 1392 and 1361 cmꢂ1
,
respectively, which are characteristic of aromatic ring breathing.
The excited state lifetimes at 77 K were estimated to be 6.3 and
14.3 ls for 17 and 18, respectively. These results further suggested
that the emissions are primarily ligand centered transition (ILCT)
with admixture of MLCT. The relatively large solvent effect on
the emission (25 nm of hypsochromic shift when changing the sol-
vent from toluene to acetonitrile) is indicative of a charge transfer
Fig. 6. Absorption spectra of 15–18 in dichloromethane.
Fig. 7. Emission spectra of 17 and 18 in dichloromethane at room temperature.

S. Huo et al. / Polyhedron 52 (2013) 1030–1040
1039
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
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Sciences, Petroleum Research Fund (PRF# 51147-UR3), Burroughs
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