A Luminescent Platinum(II) 2,6-Bis(N-pyrazolyl)pyridine Complex

Article abstract of DOI:10.1021/ic035066k

Four platinum(II) cationic complexes were prepared with the mer-coordinating tridentate ligands 2,6-bis(N-pyrazolyl)-pyridine (bpp) and 2,6-bis(3,5-dimethyl-N-pyrazolyl)pyridine (bdmpp): [Pt(bpp)Cl]Cl·H ₂O; [Pt(bdmpp)Cl]Cl·H₂O; [Pt(bpp)(Ph)](PF ₆); [Pt(bdmpp)(Ph)](PF₆). The complexes were characterized by ¹H NMR spectroscopy, elemental analysis, and mass spectrometry, and the structures of the bpp derivatives were determined by X-ray crystallography. [Pt(bpp)Cl]Cl·2H₂O: monoclinic, P2 ₁/n, a = 11.3218(5) A, b = 6.7716(3) A, c = 20.6501(6) A, β = 105.883(2)°, V = 1522.73(11) A³, Z = 4. The square planar cations stack in a head-to-tail fashion to form a linear chain structure with alternating Pt...Pt distances of 3.39 and 3.41 A. [Pt(bpp)(Ph)](PF₆)·CH₃CN: triclinic, P1, a = 8.3620(3) A, b = 10.7185(4) A, c = 13.4273(5) A, α = 96.057(1)°, β = 104.175(1)°, γ = 110.046(1)°, V = 1072.16(7) A³, Z = 2. Cyclic voltammograms indicate all four complexes undergo irreversible reductions between -1.0 and -1.3 V vs Ag/AgCl (0.1 M TBAPF₆/CH₃CN), attributable to ligand- and/or metal-centered processes. By comparison to related 2,2′:6′,2″ -terpyridine complexes, the electrochemical and UV-visible absorption data are consistent with bpp being both a weaker σ-donor and π-acceptor than terpyridine. Solid samples of [Pt(bpp)(Ph)]-(PF6) at 77 K exhibit a remarkably intense, narrow emission centered at 655 nm, whereas the other three complexes exhibit only very weak emission.

Full text of DOI:10.1021/ic035066k

Inorg. Chem. 2004, 43, 2548−2555
A Luminescent Platinum(II) 2,6-Bis(N-pyrazolyl)pyridine Complex
Stuart A. Willison, Hershel Jude, Ryan M. Antonelli, Jeffrey M. Rennekamp, Nathan A. Eckert,
Jeanette A. Krause Bauer, and William B. Connick*
Department of Chemistry, UniVersity of Cincinnati, P.O. Box 210172,
Cincinnati, Ohio 45221-0172
Received September 9, 2003
Four platinum(II) cationic complexes were prepared with the mer-coordinating tridentate ligands 2,6-bis(N-pyrazolyl)-
pyridine (bpp) and 2,6-bis(3,5-dimethyl-N-pyrazolyl)pyridine (bdmpp): [Pt(bpp)Cl]Cl‚H O; [Pt(bdmpp)Cl]Cl‚H O;
2
2
[Pt(bpp)(Ph)](PF ); [Pt(bdmpp)(Ph)](PF ). The complexes were characterized by 1H NMR spectroscopy, elemental
6
6
analysis, and mass spectrometry, and the structures of the bpp derivatives were determined by X-ray crystallography.
[Pt(bpp)Cl]Cl‚2H O: monoclinic, P21/n, a ) 11.3218(5) Å, b ) 6.7716(3) Å, c ) 20.6501(6) Å, â ) 105.883(2)°,
2
3
V ) 1522.73(11) Å , Z ) 4. The square planar cations stack in a head-to-tail fashion to form a linear chain
structure with alternating Pt‚‚‚Pt distances of 3.39 and 3.41 Å. [Pt(bpp)(Ph)](PF )‚CH CN: triclinic, P1h, a )
6
3
8.3620(3) Å, b ) 10.7185(4) Å, c ) 13.4273(5) Å, R ) 96.057(1)°, â ) 104.175(1)°, γ ) 110.046(1)°, V )
3
1072.16(7) Å , Z ) 2. Cyclic voltammograms indicate all four complexes undergo irreversible reductions between
−1.0 and −1.3 V vs Ag/AgCl (0.1 M TBAPF /CH CN), attributable to ligand- and/or metal-centered processes. By
6
3
comparison to related 2,2′:6′,2′′-terpyridine complexes, the electrochemical and UV−visible absorption data are
consistent with bpp being both a weaker σ-donor and π-acceptor than terpyridine. Solid samples of [Pt(bpp)(Ph)]-
(PF ) at 77 K exhibit a remarkably intense, narrow emission centered at 655 nm, whereas the other three complexes
6
exhibit only very weak emission.
Introduction
emission lifetimes of some platinum(II) terpyridyl com-
plexes,6d,8 though only indirect evidence for these states is
presently available. For these reasons, we were surprised to
observe intense emission from solid samples of the hexafluo-
Since Jameson and Goldsby’s report on the regulation of
redox potentials of ruthenium(II) complexes with tridentate
bis(N-pyrazolyl)pyridyl ligands,1 including bpp and bdmpp,
there has been growing interest in using these ligands as
easily synthesized substitutes for terpyridyl chelating groups,
such as tpy.2-5 Whereas luminescent complexes with tpy
ligands are well-known,6,7 there have been no reports of
emissive transition metal complexes with bis(N-pyrazolyl)-
pyridyl ligands. In the case of ruthenium(II) complexes, this
result is not surprising since bpp is regarded as both a poorer
σ-donor and π-acceptor than tpy.1 Consequently, complexes
(2) See for example: (a) Bessel, C. A.; See, R. F.; Jameson, D. L.;
Churchill, M. R.; Takeuchi, K. J. J. Chem. Soc., Dalton Trans. 1993,
10, 1563-1576. (b) Fung, W.-H.; Cheng, W.-C.; Yu, W.-Y.; Che,
C.-M.; Mak, T. C. W. Chem. Commun. 1995, 2007-2008. (c) Fung,
W.-H.; Yu, W.-Y.; Che, C.-M. J. Org. Chem. 1998, 63, 7715-7726.
(d) Begley, M. J.; Hubberstey, P.; Stroud, J. J. Chem. Soc., Dalton
Trans. 1996, 4295-4301. (e) Ru¨lke, R. E.; Kaasjager, V. E.; Kliphuis,
D.; Elsevier: C. J.; van Leeuwen, P. W. N. M.; Vrieze, K.; Goubitz,
K. Organometallics 1996, 15, 668-677. (f) Holland, J. M.; Kilner,
C. A.; Thornton-Pett, M.; Halcrow, M. A. Polyhedron 2001, 20, 2829-
2840. (g) Chryssou, K.; Stergiopoulos, T.; Falaras, P. Polyhedron 2002,
21, 2773-2781. (h) Chryssou, K.; Catalano, V. J.; Kurtaran, R.;
Falaras, P. Inorg. Chim. Acta 2002, 328, 204-209. (i) Calderazzo,
F.; Englert, U.; Hu, C.; Marchetti, F.; Pampaloni, G.; Passarelli, V.;
Romano, A.; Santi, R. Inorg. Chim. Acta 2003, 344, 197-206. (j)
Beach, N. J.; Spivak, G. J. Inorg. Chim. Acta 2003, 343, 244-252.
(k) Ayers, T.; Turk, R.; Lane, C.; Goins, J.; Jameson, D.; Slattery, S.
J. Inorg. Chim. Acta 2004, 357, 202-206.
2+
such as Ru(bpp)2 are expected to have low-lying, triplet
ligand field states (3LF) that facilitate rapid nonradiative
decay, decreasing the probability of radiative emission. The
situation will not necessarily improve for platinum(II)
complexes. In fact, low-lying ligand field states have been
suggested to account for low quantum yields and short
(3) Bessel, C. A.; See, R. F.; Jameson, D. L.; Churchill, M. R.; Takeuchi,
K. J. J. Chem. Soc., Dalton Trans. 1991, 2801-2805.
* Author to whom correspondence should be addressed. E-mail:
bill.connick@uc.edu.
(1) Jameson, D. J.; Blaho, J. K.; Kruger, K. T.; Goldsby, K. Inorg. Chem.
1989, 28, 4312-4314.
(4) Catalano, V. J.; Kurtaran, R.; Heck, R. A.; Ohman, A.; Hill, M. G.
Inorg. Chim. Acta 1999, 286, 181-188.
(5) Slattery, S. J.; Bare, W. D.; Jameson, D. L.; Goldsby, K. A. J. Chem.
Soc., Dalton Trans. 1999, 1347-1352.
2548 Inorganic Chemistry, Vol. 43, No. 8, 2004
10.1021/ic035066k CCC: $27.50 © 2004 American Chemical Society
Published on Web 03/20/2004

A Luminescent Pt(II) 2,6-Bis(N-pyrazolyl)pyridine Complex
a SPEX Fluorolog-3 fluorometer as previously described11 and
corrected for instrumental response. Emission samples for lifetime
measurements were excited using 4-6 ns pulses from a Continuum
Panther optical parametric oscillator (500 nm, <0.3 mJ), pumped
with the third harmonic (355 nm) of a Surelite II Nd:YAG laser.
Emission transients were detected with a modified PMT connected
to a Tektronix TD5580D oscilloscope, and data were modeled by
a nonlinear least-squares fitting procedure using in-house software
on a Microsoft Excel platform. Time-resolved emission spectra were
collected using a similar setup, except the PMT/oscilloscope was
replaced with an Andor DH520-25U ICCD (25 mm, 1024 × 256
pixels).
rophosphate salt of Pt(bpp)(Ph)+ at 77 K. Here we report
the first examples of platinum(II) complexes with bis(N-
pyrazolyl)pyridyl ligands and their luminescence properties.
[Pt(bpp)Cl]Cl‚H2O. K2PtCl4 (0.50 g, 1.20 mmol) and bpp (0.30
g, 1.42 mmol) were refluxed in water (100 mL) for 4 days. The
yellow-tan mixture was filtered, and the filtrate was rotoevaporated
to dryness. The yellow solid was washed with ether and
hexanes. Yield: 0.484 g (1.01 mmol, 84%). Anal. Calcd for
PtC11H9N5Cl2‚H2O: C, 26.68; H, 2.24; N, 14.14. Found: C, 26.57;
Experimental Section
K2PtCl4 was purchased from Pressure Chemical, and COD (1,5-
cyclooctadiene) was obtained from Aldrich. All other reagents were
obtained from Acros. The ligands, 2,6-bis(N-pyrazolyl)pyridine
(bpp)9 and 2,6-bis(3,5-dimethyl-N-pyrazolyl)pyridine (bdmpp),9 as
well as Pt(COD)(Ph)Cl,10 were prepared according to published
1
H, 2.23; N, 14.08. H NMR (CD3OD, δ): 7.04 (2 H, m), 8.05 (2
H, d, J ) 3 Hz), 8.13 (2 H, d, J ) 9 Hz), 8.65 (1 H, t, J ) 9 Hz),
9.11 (2 H, d, J ) 4 Hz). MS-ESI (acetonitrile) (m/z): 441.014 (Pt-
(bpp)Cl+ calcd 441.019).
1
procedures. H NMR spectra were recorded at room temperature
using a Bruker AC 250 MHz instrument. Deuterated solvents were
purchased from Cambridge Isotope Laboratories. Mass spectra were
obtained by electrospray ionization using either an Ionspec HiRes
ESI-FTICRMS instrument or a Micromass Q-TOF-II instrument.
Observed isotope patterns agreed well with predicted patterns on
the basis of natural isotopic abundancies. UV-visible absorption
spectra were obtained using a HP8453 diode array spectrometer.
Elemental analyses were performed by Atlantic Microlab (Norcross,
GA). Electrochemical measurements were recorded using a standard
three-electrode cell and CV50w potentiostat from Bioanalytical
Systems. Scans were recorded in distilled acetonitrile solution
[Pt(bdmpp)Cl]Cl‚H2O. This was prepared by the same proce-
dure as for [Pt(bpp)Cl]Cl, substituting the following materials:
K2PtCl4 (0.250 g, 0.602 mmol); bdmpp (0.193 g, 0.722 mmol); 50
mL of water. Yield: 0.223 g (0.418 mmol, 69%). Anal. Calcd for
PtC15H17N5Cl2‚H2O: C, 32.68; H, 3.47; N, 12.70. Found: C, 32.43;
H, 3.27; N, 12.65. 1H NMR (CD3OD, δ): 2.49 (6 H, s), 3.30 (6 H,
s), 6.59 (2 H, s), 7.91 (2 H, d, J ) 9 Hz), 8.51 (1 H, dd, J ) 9, 9
Hz). MS-ESI (acetonitrile) (m/z): 498.077 (Pt(bdmpp)Cl+ calcd
498.081).
[Pt(bpp)(Ph)](PF6). A mixture of 0.246 g (0.592 mmol) of
Pt(COD)(Ph)Cl and 0.150 g (0.593 mmol) of AgPF6 in 10 mL of
acetone was stirred at room temperature for 30 min in the dark.
The mixture was filtered, and 0.125 g (0.592 mmol) of bpp was
added to the filtrate. The pale yellow solution was stirred for 1
day at room temperature and reduced to dryness by rotary
evaporation to give a yellow-orange solid. The crude product was
recrystallized from acetonitrile and diethyl ether. Yield: 0.247 g
(0.393 mmol, 66%). Anal. Calcd for PtC17H14N5PF6: C, 32.49; H,
containing 0.1
M tetrabutylammonium hexafluorophosphate
((TBA)PF6), which was recrystallized at least twice from methanol
and dried under vacuum prior to use. All scans were recorded using
a Pt wire auxiliary electrode, a Ag/AgCl (3.0 M NaCl) reference
electrode, and a 0.79 mm2 Au working electrode. Reported
potentials are referenced vs Ag/AgCl (3.0 M NaCl). Potentials for
irreversible reduction couples are reported as the peak potential of
the cathodic wave (Epc). Reduction of these complexes resulted in
a return oxidation wave at positive potentials characteristic of
adsorption. Therefore, the working electrode was polished between
scans with 0.05 µm alumina, rinsed with distilled water, and dried
using a Kimwipe. Steady-state emission data were collected using
1
2.25; N, 11.15. Found: C, 32.58; H, 2.25; N, 11.27. H NMR
(CD3CN, δ): 6.89 (2 H, dd, J ) 3, 3 Hz, with Pt satellites, JH-Pt
) 16 Hz), 7.18 (3 H, m), 7.49 (2 H, d, J ) 7 Hz, with Pt satellites,
JH-Pt ) 33 Hz), 7.93 (4 H, m, J ) 9 Hz), 8.53 (1 H, dd, J ) 9, 9
Hz), 8.76 (2 H, d, J ) 3 Hz). MS-ESI (acetonitrile) (m/z): 483
(Pt(bpp)(Ph)+ calcd 483).
(6) See for example: (a) Creutz, C.; Chou, M.; Netzel, T. L.; Okumura,
M.; Sutin, N. J. Am. Chem. Soc. 1980, 102, 1309-1319. (b) Juris,
A.; Campagna, S.; Bidd, I.; Lehn, J. M.; Ziessel, R. Inorg. Chem.
1988, 27, 4007-4011. (c) Ayala, N. P.; Flynn, C. M., Jr.; Sacksteder,
L.; Demas, J. N.; DeGraff, B. A. J. Am. Chem. Soc. 1990, 112, 3837-
3844. (d) Aldridge, T. K.; Stacy, E. M.; McMillin, D. R. Inorg. Chem.
1994, 33, 722-727. (e) Crites Tears, D. K.; McMillin, D. R. Coord.
Chem. ReV. 2001, 211, 195-205.
[Pt(bdmpp)(Ph)](PF6). This was prepared by the same procedure
as for [Pt(bpp)(Ph)](PF6), substituting the following materials: 0.200
g (0.481 mmol) of Pt(COD)(Ph)Cl; 0.122 g (0.483 mmol) of AgPF6;
10 mL of acetone; 0.129 g (0.483 mmol) of bdmpp. Yield: 0.131
g (0.191 g, 40%). Anal. Calcd for PtC21H22N5PF6: C, 36.85; H,
3.24; N, 10.23. Found: C, 36.85; H, 3.11; N, 9.91. 1H NMR
(CD3CN, δ): 1.71 (6 H, s), 2.78 (6 H, s), 6.31 (2H, s, with Pt
satellites, JH-Pt ) 19 Hz), 6.96 (1 H, t, J ) 7 Hz), 7.11 (2 H, dd,
J ) 7, 7 Hz), 7.55 (2 H, d, J ) 8 Hz, with Pt satellites JH-Pt ) 33
Hz), 7.78 (2 H, d, J ) 8 Hz), 8.35 (1 H, dd, J ) 9, 9 Hz). MS-ESI
(acetonitrile) (m/z): 539.155 (Pt(bdmpp)(Ph)+ calcd 539.153).
X-ray Crystallography. Crystals of [Pt(bpp)Cl]Cl‚2H2O were
grown by slow evaporation of a methanol solution, and intensity
data were collected using a Bruker SMART 1K CCD diffractometer.
(7) Bailey, J. A. H., M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer,
W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591-4599.
(8) (a) Yip, H.-K.; Cheng, L.-K.; Cheung, K. K.; Che, C. M. J. Chem.
Soc., Dalton Trans. 1993, 2933-2938. (b) Crites Tears, D. K.;
Cunningham, C. T.; McMillin, D. R. Inorg. Chim. Acta 1998, 273,
346-353. (c) Lai, S.-W.; Chan, M. C. W.; Cheung, K.-K.; Che, C.-
M. Inorg. Chem. 1999, 38, 4262-4267. (d) Michalec, J. F.; Bejune,
S. A.; McMillin, D. R. Inorg. Chem. 2000, 39, 2708-2709. (e)
Michalec, J. F.; Bejune, S. A.; Cuttell, D. G.; Summerton, G. C.;
Gertenbach, J. A.; Field, J. S.; Haines, R. J.; McMillin, D. R. Inorg.
Chem. 2001, 40, 2193-2200. (f) Yang, Q.-Z.; Wu, L.-Z.; Wu, Z.-X.;
Zhang, L.-P.; Tung, C.-H. Inorg. Chem. 2002, 41, 5653-5655.
(9) Goldsby, K. A. J., D. L. J. Org. Chem. 1990, 55, 4992-4994.
(10) Eaborn, C. O., K. J.; Pidcock, A. J. Chem. Soc., Dalton Trans. 1978,
357-368.
(11) Jude, H.; Krause Bauer, J. A.; Connick, W. B. Inorg. Chem. 2002,
41, 2275-2281.
Inorganic Chemistry, Vol. 43, No. 8, 2004 2549

Willison et al.
prepared by allowing Pt(COD)(Ph)Cl10 to react with silver
hexafluorophosphate. After removal of silver chloride by
filtration, 1 equiv of bpp or bdmpp was added to give Pt-
(bpp)(Ph)+ or Pt(bdmpp)(Ph)+, respectively. All four com-
pounds gave satisfactory elemental analyses and mass
spectra. Both chloro products were found to retain 1 equiv
of H2O with tenacity reminiscent of [Pt(tpy)Cl]Cl‚2H2O.15
The 1H NMR spectra exhibit the expected patterns of
resonances. For each complex, characteristic doublet and
triplet pyridyl proton resonances occur in the ranges of 7.2-
8.2 and 8.4-8.7 ppm, respectively. In the case of the bpp
complexes, resonances for the â-pyrazolyl protons occur
between 6.3 and 7.0 ppm and show only weak splitting. For
the phenyl derivatives, resonances for the phenyl protons
occur from 6.9 to 7.6 ppm. Distinct Pt satellites are associated
with the R-proton resonances (JPt-H ) 33 Hz). Surprisingly,
Pt satellites also are resolved for the â-pyrazolyl proton
resonance in the spectrum of [Pt(bdmpp)(Ph)](PF6), resulting
from four bond coupling (JPt-H ) 19 Hz).
Table 1. Crystallographic Data for [Pt(bpp)Cl]Cl‚2H2O and
[Pt(bpp)(Ph)](PF6)‚CH3CN
[Pt(bpp)Cl]Cl‚2H2O [Pt(bpp)(Ph)](PF6)‚CH3CN
formula
fw
space group
a, Å
b, Å
c, Å
[C11H9N5ClPt]Cl‚2H2O [C17H14N5Pt]PF6‚CH3CN
513.25
P21/n
669.45
P1h
11.3218(5)
6.7716(3)
20.6501(6)
90
105.883(2)
90
8.3620(3)
10.7185(4)
13.4273(5)
96.057(1)
104.175(1)
110.046(1)
1072.16(7)
2.074
R, deg
â, deg
γ, deg
V, Å3
1522.73(11)
2.239
F
calcd, g cm-3
µ, mm-1
Z
9.576
6.690
4
2
T, K
150(2)
15 406
3735
0.0591
1.034
150(2)
14 810
5330
0.0245
1.040
0.0218/0.0498
0.0277/0.0521
reflcns collcd
indpnt reflcns
Rint
GOF on F2
R1/wR2 [I > 2σ(I)]a 0.0310/0.0727
R1/wR2 (all data)a 0.0529/0.0825
a R1 ) ∑|Fo| - |Fc|/∑|Fo|; wR2 ) [∑w(Fo - Fc )2/∑w(Fo )2]1/2
.
2
2
2
Crystals of [Pt(bpp)(Ph)](PF6)‚CH3CN were grown by diffusion of
ether into an acetonitrile solution, and intensity data were collected
using a SMART6000 CCD diffractometer. For both crystals,
graphite-monochromated Mo KR radiation (λ ) 0.710 73 Å) was
used. Data frames were processed using the SAINT program,12 and
reflection data were corrected for decay, Lorentz, and polarization
effects. Absorption and beam corrections were applied on the basis
of the multiscan technique using SADABS.13 For the chloro
complex, the structure was solved by a combination of direct
methods using SHELXTL v5.0314 and the difference Fourier
technique. For the phenyl complex, the structure was solved by a
combination of the Patterson method using SHELXTL v6.1214 and
the difference Fourier technique. In both cases, the models were
refined by full-matrix least squares on F2. Non-hydrogen atoms
were refined with anisotropic parameters. Ligand-based H atoms
were located either directly or calculated on the basis of geometric
criteria. The isotropic factors for H atoms were defined as 1.2
times Ueq of the adjacent atom. The water H atoms for
[Pt(bpp)Cl]Cl‚2H2O were held fixed where located with minor
adjustment to the positions of H30 and H31. The Uiso was set at
Crystal Structures. The molecular structures of the
cations Pt(bpp)Cl+ and Pt(bpp)(Ph)+ were confirmed by
X-ray crystallographic studies of crystals of [Pt(bpp)Cl]Cl‚
2H2O and [Pt(bpp)(Ph)](PF6)‚CH3CN at 150 K. ORTEP
diagrams are shown in Figures 1 and 2, and data are
summarized in Tables 1 and 2.
-
0.05. For [Pt(bpp)(Ph)](PF6)‚CH3CN, the PF6 counterion shows
In both crystals, the bpp ligands adopt a tridentate
coordination geometry, bonding to the Pt center through two
pyrazolyl N atoms and the pyridine N atom to give a nearly
planar Pt(bpp) unit. The fourth coordination site is occupied
by the anionic ligand. Bond distances and angles of the
cations are normal.16 The central Pt-N1 distance is shorter
than the peripheral Pt-N3 and Pt-N5 distances, as expected
for the constrained bite angle of the bpp ligand.4 The Pt-
N1 distance is shorter for the chloro complex (1.950(4) Å)
than for the phenyl derivative (1.999(3) Å), in keeping with
the relative trans influence of the anionic ligands. The
resulting N3-Pt-N5 angles (chloro, 161.3(2)°; phenyl,
158.7(1)°) lie outside the range observed for Ru(II) com-
plexes with similar ligands (154.9-158.0°),2a-c,3,4 but in the
case of the chloro complex, the value is comparable to those
typical disorder. Crystallographic data are summarized in Table 1.
Results and Discussion
Synthesis and Characterization. The chloride salts of
the two chloro complexes, Pt(bpp)Cl+ and Pt(bdmpp)Cl+,
were prepared by refluxing the free bis(N-pyrazolyl)pyridine
ligands with K2PtCl4. The hexafluorophosphate salts of
phenyl derivatives, Pt(bpp)(Ph)+ and Pt(bdmpp)(Ph)+, were
(12) Bruker SMART (v5.051 for [Pt(bppy)Cl]Cl‚2H2O.; v5.628 for
[Pt(bppy)(Ph)](PF6)‚CH3CN) and SAINT (v5.A06 for [Pt(bppy)Cl]-
Cl‚2H2O.; v6.28A for [Pt(bppy)(Ph)](PF6)‚CH3CN) programs were
used for data collection and data processing, respectively. Bruker
Analytical X-ray Instruments, Inc., Madison, WI.
(13) SADABS was used for the application of semiempirical absorption
and beam corrections for the CCD data. G. M. Sheldrick, University
of Goettingen, Goettingen, Germany, 1996.
(14) SHELXTL v5.03 ([Pt(bppy)Cl]Cl‚2H2O) and SHELXTL v6.12
([Pt(bppy)(Ph)](PF6)‚CH3CN) were used for the structure solutions
and generation of figures and tables. G. M. Sheldrick, University of
Goettingen, Goettingen, Germany, and Siemens Analytical X-ray
Instruments, Inc., Madison, WI.
-
-
observed for Pt(tpy)Cl+ salts (CF3SO3 , 161.8(2)°;8a ClO4 ,
(15) Howe-Grant, M.; Lippard, S. J. Inorg. Synth. 1980, 20, 101-105.
(16) Bessel, C. A.; See, R. F.; Jameson, D. L.; Churchill, M. R.; Takeuchi,
K. J. J. Chem. Soc., Dalton Trans. 1992, 3223-3228.
2550 Inorganic Chemistry, Vol. 43, No. 8, 2004

A Luminescent Pt(II) 2,6-Bis(N-pyrazolyl)pyridine Complex
Figure 1. (a) ORTEP diagram of the cation in crystals of [Pt(bpp)Cl]Cl‚
2H2O with 50% probability ellipsoids and (b) diagram showing columnar
packing parallel to b Pt‚‚‚Pt spacings of 3.39 and 3.41 Å. H-atoms are
omitted for clarity.
Figure 2. (a) ORTEP diagram of the cation in crystals of [Pt(bpp)(Ph)]-
(PF6)‚CH3CN with 50% probability ellipsoids and (b) diagram showing
columnar packing parallel to a axis with Pt‚‚‚Pt spacings of 4.48 and 4.71
Å. H-atoms are omitted for clarity.
163.5(7), 161.2(6)°).7 Similarly, the N1-Pt-N3 and N1-
Pt-N5 angles (79.3-80.7°) for Pt(bpp)Cl+ and Pt(bpp)(Ph)+
are less acute than those generally observed for Ru(II)
complexes (77.3-79.6°)2a-c,3,4 but smaller than observed for
Pt(tpy)Cl+ salts (79.8-82.6°),7,8a,17 in agreement with the
relative ligand bite angles.16
In crystals of [Pt(bpp)Cl]Cl‚2H2O, the approximately
square planar Pt(bpp)Cl+ cations stack in a head-to-tail
fashion to give columns parallel to the b axis (Figure 1b).
Consecutive complexes along the stack are related by an
inversion center, resulting in spacings of approximately 3.33
Å between the coordination planes, defined by the Pt atom
and the four coordinating atoms. The stacking arrangement
results in a nearly linear chain of Pt atoms (Pt‚‚‚Pt‚‚‚Pt,
170.2°) with alternating short Pt‚‚‚Pt distances (3.39, 3.41
Å) that are characteristic of platinum(II) linear-chain struc-
tures with intermolecular metal‚‚‚metal interactions.18 The
salt crystallizes as a dihydrate, resulting in a hydrogen-
bonding network linking the anion and water subunits. The
hydrogen-bonding scheme was inferred from short inter-
molecular contacts between the chloride anion (Cl2) and
water O atoms, Cl2‚‚‚O1 (3.204(5) Å) and Cl2‚‚‚O2
(3.172(5), 3.203(5) Å), as well as between the two water O
atoms, O1‚‚‚O2 (2.982(7) Å). There also are short contacts
between the bpp H atoms and the Cl atoms, C4‚‚‚Cl1
(3.456(6) Å), C5‚‚‚Cl2 (3.648(6) Å), and C10‚‚‚Cl1
(3.548(7) Å), as well as between the bpp H atoms and the
water groups, C8‚‚‚O1 (3.423(8) Å), C9‚‚‚O1 (3.432(8) Å),
and C12‚‚‚O2 (3.483(8) Å).
In crystals of [Pt(bpp)(Ph)](PF6)‚CH3CN, the Pt(bpp)(Ph)+
cations also stack in a head-to-tail fashion, forming columns
parallel to the a axis (Figure 2b). The coordination plane is
slightly canted with respect to the bc plane, resulting in a
19.3(1)° dihedral angle. Consecutive complexes along the
stack are related by an inversion center, resulting in alternat-
ing interplanar spacings of ∼4.38 and ∼4.58 Å. The resulting
Pt‚‚‚Pt separations of 4.48 and 4.71 Å are considerably longer
than observed for crystals containing Pt(bpp)Cl+. Neverthe-
less, it is noteworthy that the approximately planar phenyl
group forms a 36.9(2)° dihedral angle with the coordination
plane. This value lies outside the 55-90° range typically
observed for platinum(II) phenyl complexes with bidentate
or tridentate ligands.19,20 Kaim et. al. have suggested that a
dihedral angle of ∼60° between the pyridyl and phenyl rings
(17) Wong, Y.-S.; Lippard, S. J. Chem. Commun. 1977, 824-825.
(18) (a) Miller, J. S. Extended Linear Chain Compounds; Miller, J. S., Ed.,
Ed.; Plenum Press: New York, 1982; Vols. 1-3. (b) Connick, W. B.;
Marsh, R. E.; Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1997, 36,
913-922.
Inorganic Chemistry, Vol. 43, No. 8, 2004 2551

Willison et al.
Table 2. Selected Distances (Å) and Angles (deg) for
[Pt(bpp)Cl]Cl‚2H2O and [Pt(bpp)(Ph)](PF6)‚CH3CN
the observed processes are irreversible and the Epc values
do not necessarily represent thermodynamic potentials. In
2
2
[Pt(bpp)Cl]Cl‚2H2O
[Pt(bpp)(Ph)](PF6)‚CH3CN
addition, both the unoccupied ligand π* and dx -y (Pt) levels
are arguably accessible at these potentials. To illustrate this
point, we can obtain crude estimates of the relative orbital
energies from accumulated electrochemical data. The cyclic
voltammogram of Ru(bpp)22+ exhibits an irreversible ligand-
centered reduction wave at -1.66 V vs SSCE (0.1 M
(TBA)PF6/CH3CN).1 Ru(tpy)22+ is reversibly reduced at more
positive potentials (-1.25 V vs SSCE;1 -1.26 V vs Ag/
AgCl (1.0 M KCl), 0.1 M (TBA)PF6/DMF),22 reflecting the
relative energies of the π*(bpp) and π*(tpy) orbitals when
bonded to Ru(II).1 In the case of Pt(tpy)Cl+ in DMF, ligand-
centered reduction occurs at -0.74 V vs Ag/AgCl (1.0 M
KCl).22 Hill and co-workers22 have attributed the 0.52 V
anodic shift of the Pt(tpy)Cl+ reduction potential with respect
to that of Ru(tpy)22+ to stabilization of the π*(tpy) levels as
a result of mixing with the 6pz(Pt) orbital.22 Similar stabiliza-
tion of the π*(bpp) level of Pt(bpp)Cl+ with respect to Ru-
(bpp)22+ would shift the thermodynamic reduction potential
of the platinum complex to -1.14 V vs SSCE, in the vicinity
of the irreversible reductions observed for this series of
platinum(II) bis(N-pyrazolyl)pyridyl complexes. On the other
hand, Pt(tpy)Cl+ undergoes a second chemically reversible
reduction in DMF at -1.30 V, generally attributed to addition
Pt-N1
Pt-N3
Pt-N5
Pt-La
N1-C6
N1-C2
N2-C6
N4-C2
N2-N3
N4-N5
1.950(4)
2.000(5)
1.991(5)
2.298(1)
1.331(6)
1.337(7)
1.421(7)
1.408(6)
1.400(6)
1.391(6)
1.999(3)
2.012(3)
2.006(2)
2.007(3)
1.334(4)
1.334(4)
1.407(4)
1.402(4)
1.393(4)
1.390(4)
N1-Pt-N5
N1-Pt-N3
N5-Pt-N3
N1-Pt-La
N5-Pt-La
N3-Pt-La
C6-N1-Pt
C2-N1-Pt
N3-N2-C6
N5-N4-C2
N2-N3-Pt
N4-N5-Pt
80.6(2)
80.7(2)
79.39(10)
79.34(10)
158.73(11)
176.38(10)
101.26(12)
99.97(11)
118.7(2)
118.6(2)
118.7(3)
119.1(2)
111.82(18)
111.68(19)
161.3(2)
178.91(13)
98.35(14)
100.34(14)
119.3(4)
118.7(3)
118.1(4)
117.9(4)
110.8(3)
111.4(3)
a L ) chlorine (Cl1) for [Pt(bpp)Cl]Cl‚2H2O and carbon (C13) for
[Pt(bpp)(Ph)](PF6)‚CH3CN.
in Pt(diimine)(Ph)2 complexes provides optimal overlap
between the phenyl rings and the π-system of the diimine
ligand.20 The angle for Pt(bpp)(Ph)+ is remarkably similar
to that found for two Pt(Ph)2 units bridged by 1,2,4,5-tetrakis-
(1-N7-azaindolyl)benzene (36.0°), in which intramolecular
steric constraints presumably are responsible for the relatively
small angle.21 Similarly, it is conceivable the relatively acute
angle found for Pt(bpp)(Ph)+ is a consequence of the
compressed columnar packing arrangement of the cations.
Cyclic Voltammetry. To investigate the electronic prop-
erties of these complexes, their cyclic voltammograms were
recorded in 0.1 M (TBA)PF6 acetonitrile solution. None of
the complexes was oxidized at <1.0 V vs Ag/AgCl.
However, all four compounds underwent a chemically
irreversible process between -1.0 and -1.3 V (0.25 V/s).
Even at 50 V/s, the reductions were completely irreversible.
For the chloro complexes, a second irreversible process
occurred near -1.45 V. The -1.0 to -1.3 V processes
involve reduction of the bis(N-pyrazolyl)pyridine ligand
and/or reduction of the metal center. Distinguishing between
these two possibilities is presently difficult, in part because
of an electron to the dx -y (Pt) orbital.22,23 From Hill and co-
2
2
workers’ 0.38 eV estimate for the energy gap between π*-
24
2
2
(tpy) and dx -y (Pt) levels and assuming similar crystal field
strengths, we obtain a crude estimate of -1.12 V vs Ag/
AgCl (1.0 M KCl) for the one-electron reduction of the
2
2
d
x -y (Pt) orbital. The reduction could be expected to occur
at even more positive potentials for Pt(bpp)Cl+ since bpp is
weaker σ-donor than tpy.
Though the observed reductions are irreversible, it is
noteworthy that within this series of platinum(II) bis(N-
pyrazolyl)pyridyl complexes, the peak potentials follow
expected trends on the basis of inductive effects, which are
2
2
expected to shift the energies of the dx -y (Pt) and π*(bpp)
levels in the same direction. Reduction of the bdmpp
complexes is slightly less favorable than their bpp counter-
parts. Similarly, reduction of the phenyl derivatives is ∼0.2
V less favorable than the chloro complexes, as expected for
the relative donor properties of the ancillary ligands. Overall,
the reduction potentials of these complexes are cathodically
shifted from those of related Pt(II) terpyridyl com-
plexes,8b-d,22,25 including Pt(tpy)(Ph)+ (-0.91, -1.42 V vs
Ag/AgCl, 1.0 M (TBA)PF6/CH3CN),26 in keeping with the
(19) (a) Bruno, G.; Lanza, S.; Nicolo, F. Acta Crystallogr. 1990, C46, 765-
767. (b) Deacon, G. B.; Hilderbrand, E. A.; Tiekink, E. R. T. Z.
Kristallogr. 1993, 205, 340-342. (c) Ito, M.; Ebihara, M.; Kawamura,
T. Inorg. Chim. Acta 1994, 218, 199-202. (d) Sato, M.; Mogi, E.;
Kumakura, S. Organometallics 1995, 14, 3157-3159. (e) Ng, Y.-Y.;
Che, C.-M.; Peng, S.-M. New J. Chem. 1996, 20, 781-789. (f) Morton,
M. S.; Lachicotte, R. J.; Vicic, D. A.; Jones, W. D. Organometallics
1999, 18, 227-234. (g) Muller, C.; Iverson, C. N.; Lachicotte, R. J.;
Jones, W. D. J. Am. Chem. Soc. 2001, 123, 9718-9719. (h) Johansson,
M. H.; Malmstro¨m, T.; Wendt, O. F. Inorg. Chim. Acta 2001, 316,
149-152. (i) Zucca, A.; Doppiu, A.; Cinellu, M. A.; Stoccoro, S.;
Minghetti, G.; Manassero, M. Organometallics 2002, 21, 783-785.
(j) Harkins, S. B.; Peters, J. C. Organometallics 2002, 21, 1753-
1755. (k) Romeo, R.; Scolaro, L. M.; Plutino, M. R.; Romeo, A.;
Nicolo, F.; Del Zotto, A. Eur. J. Inorg. Chem. 2002, 629-638.
(20) Klein, A.; McInnes, E. J. L.; Kaim, W. J. Chem. Soc., Dalton Trans.
2002, 2371-2378.
(22) Hill, M. G.; Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem.
1996, 35, 4585-4590.
(23) Liu, X.; McInnes, E. J. L.; Kilner, C. A.; Thornton-Pett, M.; Halcrow,
M. A. Polyhedron 2001, 20, 2889-2900.
+
2
2
(24) The π*(tpy)-dx -y (Pt) energy gap for Pt(tpy)Cl was estimated in
ref 20 from the difference of the first two reduction potentials (0.56
V), crudely corrected for electrostatic effects using the difference
2+
between the first two reduction potentials of Ru(bpy)3 (0.18 V).
(25) (a) Yam, V. W.-W.; Tang, R. P.-L.; Wong, K. M.-C.; Cheung, K.-K.
Organometallics 2001, 20, 4476-4482. (b) Jude, H.; Krause Bauer,
J. A.; Connick, W. B. J. Am. Chem. Soc. 2003, 125, 3446-3447. (c)
Jude, H.; Carroll, G. T.; Connick, W. B. Chemtracts: Inorg. Chem.
2003, 16, 13-22.
(21) Song, D.; Sliwowski, K.; Pang, J.; Wang, S. Organometallics 2002,
21, 4978-4983.
2552 Inorganic Chemistry, Vol. 43, No. 8, 2004

A Luminescent Pt(II) 2,6-Bis(N-pyrazolyl)pyridine Complex
Table 3. UV-Visible Absorptiona and Electrochemical Datab for [Pt(bpp)Cl]Cl, [Pt(bdmpp)Cl]Cl, [Pt(bpp)(Ph)](PF6), and [Pt(bdmpp)(Ph)](PF6)
a
compd
abs λmax, nm (ꢀ, cm-1 M-1
)
Epc, Vb
bpp
bdmpp
239 (27 700), 245 (34 000), 264 (12 300), 270 sh (11 300), 301 (18 700)
247 (22 700), 265 sh (11 000), 295 (14 400)
[Pt(bpp)Cl]Cl
222 (21 500), 267 (33 800), 278 (25 200), 318 (11 300), 340 sh (4100)
229 (36 000), 266 (37 800), 288 (22 300), 300 sh (20 000), 320 sh (11 300), 360 (5500)
228 (17 900), 270 (28 400), 316 sh (9100), 330 (12 900)
-1.07, -1.44
-1.24
-1.12, -1.48
-1.34
[Pt(bpp)(Ph)](PF6)
[Pt(bdmpp)Cl]Cl
[Pt(bdmpp)(Ph)](PF6)
224 (27 900), 239 (23 500), 268 (35 100), 295 sh (11 400), 318 sh (12 500), 330 (14 800)
a In methanol solution. b Cyclic voltammograms were recorded in 0.1 M (TBA)PF6/acetonitrile at 0.25 V/s and referenced vs Ag/AgCl (3.0 M NaCl).
to the donor properties of the monodentate ligand and to
solvent (e.g., Pt(bdmpp)(Ph)+: MeOH, 330 nm; CH2Cl2; 333
nm) is suggestive of a bdmpp-centered π-π* transition. The
corresponding transition in the bpp complex may occur at
shorter wavelengths, possibly associated with the feature near
320 nm, coincident in the spectra of Pt(bpp)Cl+ and
Pt(bpp)(Ph)+.
At longer wavelengths (λ > 330 nm), the bpp complexes
exhibit an additional absorption band, whereas the bdmpp
complexes exhibit a tailing absorption profile. For both
Pt(bpp)(Ph)+ and Pt(bdmpp)(Ph)+, the phenyl complexes
absorb at longer wavelengths than the chloro adducts.
Interpretation is complicated by the prospect that, in addition
to bis(N-pyrazolyl)pyridine-centered transitions, metal-to-
ligand (bis(N-pyrazolyl)pyridine) charge transfer (MLCT),
Figure 3. UV-visible absorption spectra of salts of (a) Pt(bpp)(Cl)+ (---)
and Pt(bpp)(Ph)+ (s) and (b) Pt(bdmpp)(Cl)+ (---) and Pt(bdmpp)(Ph)+
(s) in methanol solution.
chloride or phenyl ligand-to-metal charge-transfer (LMCT),
and chloride or phenyl ligand-to-ligand (bis(N-pyrazolyl)-
pyridine) charge transfer (LLCT) transitions can conceivably
occur in this region.30,31 Additionally, Klein, Za´lisˇ, van
Slageren, Kaim, and co-workers32 have noted that orbital
mixing in related platinum(II) complexes can result in excited
states with significant admixtures of these states.
The absorption shift to longer wavelengths associated with
substitution of the chloro ligand with more electron-releasing
phenyl groups is consistent with stabilization of MLCT,
LMCT, and LLCT states. In the case of Pt(bpp)(Ph)+, a
distinct, solvent-sensitive band maximizes at 360 nm
(CH3CN, 359 nm, 7000 M-1 cm-1; CH2Cl2, 377 nm, 5800
M-1 cm-1). Though a definitive assignment is not possible,
the bandshape is suggestive of vibronic structure, resulting
from bis(N-pyrazolyl)pyridyl involvement. The bathochromic
shift with decreasing solvent polarity and the band intensity
resemble the lowest spin-allowed metal-to-ligand(tpy) charge-
transfer transition of Pt(tpy)Cl+ (CH3CN, 377 nm, 2200 M-1
cm-1, 390 sh; CH2Cl2, 388 sh, 405 nm).8b,d Thus, in keeping
established view that the bpp ligand is a weaker π-acceptor
than tpy.
Absorption Spectroscopy. The yellow complexes dissolve
to give pale yellow methanol solutions, and their UV-visible
absorption spectra are reported in Table 3 and Figure 3. The
UV region for each complex is dominated by strong
absorptions from 230 to 330 nm. The spectra of the free
ligands confirm ligand-centered bands occur in this region,
though other charge-transfer transitions also may contribute
significant intensity. As observed for the four platinum(II)
complexes, the spectrum of Ru(bpp)(1,3-dimethyl-â-dike-
tonate)Cl+ exhibits an intense absorption near 270 nm
(∼27 000 M-1 cm-1),5,27 suggesting this transition is centered
on the bis(N-pyrazolyl)pyridine ligand.28 For comparison,
coordination of 2,2′-bipyridine to an acidic metal center is
known to result in intense and structured π-π* absorption
in the vicinity of 290-310 nm that is very different from
the spectrum of the free ligand.29 Related transitions in the
spectra of all four platinum(II) complexes are expected to
be essentially unperturbed by the electronic properties of the
monodentate ligand. For the two bdmpp complexes, a band
occurs near 330 nm (12 000-15 000 M-1 cm-1) with a
shoulder at ∼317 nm. The relative insensitivity of this band
(30) The possibility of Pt d f p transitions can be excluded on energetic
grounds since these transitions in Pt(II) complexes generally occur at
wavelengths shorter than 250 nm (Mason, W. R.; Gray, H. B. J. Am.
Chem. Soc. 1968, 90, 5721-5729. Patterson, H. H.; Tewksbury, J.
C.; Martin, M.; Krogh-Jesperson, M.-B.; LoMenzo, J. A.; Hooper, H.
O.; Viswanath, A. K. Inorg. Chem. 1981, 20, 2297-2301. Roberts,
D. A.; Mason, W. R.; Geoffroy, G. L. Inorg. Chem. 1981, 20, 789-
796. Isci, H.; Mason, W. R. Inorg. Chem. 1984, 23, 1565-1569.
Mason, W. R. Inorg. Chem. 1986, 25, 2925-2929).
(31) Triplet components of these charge-transfer transitions and spin-
allowed Pt-centered d-d transitions also are candidates but likely to
have relatively weak intensities (<1000 M-1 cm-1).
(32) (a) Klein, A.; van Slageren, J.; Za´lisˇ, S. Eur. J. Inorg. Chem. 2003,
1917-1928. (b) Klein, A.; van Slageren, J.; Za´lisˇ, S. Inorg. Chem.
2002, 41, 5216-5225. (c) Kaim, W.; Dogan, A.; Wanner, M.; Klein,
A.; Tiritiris, I.; Schleid, T.; Stufkens, D. J.; Snoeck, T. L.; McInnes,
E. J. L.; Fiedler, J.; Za´lisˇ, S. Inorg. Chem. 2002, 41, 4139-4148. (d)
van Slageren, J.; Klein, A.; Za´lisˇ, S. Coord. Chem. ReV. 2002, 230,
193-211.
(26) Jude, H.; Krause Bauer, J. A., Connick, W. B. Manuscript in
preparation.
(27) It is assumed the concentration of the solution reported in Figure 3 of
ref 5 was 5.5 × 10-5 M.
(28) A similar band assignment has been made for a Eu(III) and Tb(III)
complexes with a related ligand, 2,6-bis(3-carboxy-1-pyrazolyl)-
pyridine (Remuin˜a´n, M. J.; Roma´n, H.; Alonso, M. T.; Rodr´ıguez-
Ubis, J. C. J. Chem. Soc., Perkins Trans. 2 1993, 1099-1102).
(29) (a) Bray, R. G.; Ferguson, J.; Hawkins, C. J. Aust. J. Chem. 1969, 22,
2091-2103. (b) Ohno, T.; Kato, S. Bull. Chem. Soc. Jpn. 1974, 47,
2953-2957.
Inorganic Chemistry, Vol. 43, No. 8, 2004 2553

Willison et al.
ments show that the 77 K emission signal decay is not single-
exponential. The data are adequately described with a
biexponential function corresponding to emission lifetimes
of approximately 120 and 1550 ns, consistent with a
predominantly spin-forbidden process. Despite the complex
decay kinetics, time-resolved emission spectra indicate the
shape of the emission profile remains essentially constant
during the decay, though the maximum undergoes a gradual
shift from 650 to 660 nm during the first several hundred
nanoseconds (Figure 4). The emission properties of a 10-4
M butyronitrile 77 K glassy solution of Pt(bpp)(Ph)+ are
strikingly different. The emission is exceedingly weak but
discernibly shifted to much shorter wavelengths (λmax ) 401,
424, 450 nm; Figure 4) than that of the solid. The apparent
vibronic structure with spacings of ∼1350 cm-1 is suggestive
of emission originating from a lowest ligand-centered π-π*
state.
Figure 4. 77 K emission (s, λex ) 410 nm) and excitation (- -, λem
)
640 nm) spectra of solid [Pt(bpp)(Ph)](PF6) and emission spectrum of a
butyronitrile glassy solution (---, λex ) 330 nm). The inset shows time-
resolved emission spectra recorded during 90 ns integration windows at
500 ns intervals from 0-4 µs, following a 500 nm excitation pulse.
with the notion of a relatively low-lying unoccupied bpp π*
orbital, this band is tentatively assigned as having significant
charge-transfer-to-ligand(bpp) character. The corresponding
transition in the spectrum of Pt(bpp)Cl+ must occur at
wavelengths <345 nm, shifted at least 0.28 eV to the blue
of that observed for Pt(tpy)Cl+.8b,d For Pt(bpp)(Ph)+ in
alcohol solution, the transition is shifted by ∼0.5 eV to the
blue of that observed for Pt(tpy)(Ph)+ (4:1 methanol-ethanol,
λmax ) 424 nm, 2000 M-1 cm-1),33 in accord with the view
that the poorer σ-donor and π-acceptor properties of bpp will
tend to destabilize MLCT states. Similar features are less
pronounced and apparently weaker in the spectra of
Pt(bdmpp)(Ph)+. However, in CH2Cl2 solution, a distinct
shoulder is evident near 380 nm (2000 M-1 cm-1), suggesting
a transition energy similar to that of the bpp complex.
Evidently destabilization of the π* level by the methyl groups
of the bdmpp ligand is approximately offset by increased
electron donation to the metal center.
Emission Spectroscopy. At room temperature, emissions
from samples of each of the four complexes under UV
irradiation are exceedingly weak. Even upon cooling to 77
K, salts of Pt(bpp)Cl+, Pt(bdmpp)(Ph)+, and Pt(bdmpp)Cl+
only exhibit very weak emissions that could not be
reliably recorded. In contrast, the emission from solid
[Pt(bpp)(Ph)](PF6) is a remarkably intense pink color at 77
K, and this observation prompted further investigation.
The accumulated data are largely consistent with the
presence of intermolecular interactions in low-temperature
solid samples of [Pt(bpp)(Ph)](PF6) that perturb the emission
properties of the complexes. The 77 K emission properties
of solid [Pt(bpp)(Ph)](PF6) bear a striking qualitative resem-
blance to those of dimers34,35 and linear chain materials8a,36,37
composed of weakly interacting platinum(II) complexes. In
2
these materials, the occupied 5dz (Pt) orbitals of adjacent
complexes interact to form an antibonding (dσ*) HOMO,
as well as a lower-energy, bonding (dσ) molecular orbital.
Similar interactions between unoccupied 6pz levels of
adjacent complexes yields pσ and pσ* molecular orbitals,
and intense emission from these solids often originates from
3
a lowest (dσ* f pσ) state. In the case of platinum(II)
terpyridyl complexes, the LUMO has mostly π*(tpy) char-
acter, and the resulting intense emission arises from a lowest
3(dσ* f π*) state.36 For both classes of these materials, the
∼80 K emission profiles tend to be nearly symmetrical with
fwhm of 1000-1600 cm-1.8a,34-37 At first glance, either of
these excited states seems a tempting candidate for assign-
ment of the emission from [Pt(bpp)(Ph)](PF6).38 The excita-
tion band at 500 nm could be reasonably assigned as the
corresponding spin-allowed absorption, which is consistent
with typical Stokes shifts and apparent singlet-triplet
The emission from 77
K
solid samples of
(34) Yip, H.-K.; Che, C. M.; Zhou, Z.-Y.; Mak, T. C. W. J. Chem. Soc.,
Chem. Commun. 1992, 1369-1371.
[Pt(bpp)(Ph)](PF6) is centered at 655 nm (Figure 4). The
maximum is independent of excitation wavelength, and the
band is nearly symmetrical and relatively narrow as indicated
by the full-width at half-maximum (fwhm) of ∼1660 cm-1.
The excitation spectrum is in disagreement with the solution
absorption spectrum, showing significant emission intensity
for excitation wavelengths as long as 520 nm. Notably, the
lowest energy band in the excitation spectrum occurs near
500 nm with an estimated fwhm of ∼1200 cm-1. Interest-
ingly, the solid still appeared yellow at 77 K. When the
sample was removed from the liquid-nitrogen bath and
allowed to warm slightly above 77 K, the emission intensity
weakened considerably. Luminescence lifetime measure-
(35) Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1993, 32,
369-370.
(36) (a) Field, J. S.; Haines, R. J.; McMillin, D. R.; Summerton, G. C. J.
Chem. Soc., Dalton Trans. 2002, 1369-1376. (b) Field, J. S.;
Gertenbach, J.-A.; Haines, R. J.; Ledwaba, L. P.; Mashapa, N. T.;
McMillin, D. R.; Munro, O. Q.; Summerton, G. C. J. Chem. Soc.,
Dalton Trans. 2003, 1176-1180. (c) Bu¨chner, R.; Cunningham, C.
T.; Field, J. S.; Haines, R. J.; McMillin, D. R.; Summerton, G. C. J.
Chem. Soc., Dalton Trans. 1999, 711-718. (d) Bu¨chner, R.; Field, J.
S.; Haines, R. J.; Cunningham, C. T.; McMillin, D. R. Inorg. Chem.
1997, 36, 3952-3956.
(37) Gliemann, G.; Yersin, H. Struct. Bonding 1985, 62, 87-153.
(38) The dσ* f π* assignment appears more likely since the energies of
the emissions from tetracyanoplatinate salts with predominant lowest
dσ* f pσ excited state character only approach that observed here
for very short metal‚‚‚metal spacings ∼3.1 Å.37 The absence of a
vibronic component characteristic of ligand excited-state distortion is
consistent with the ∼80 K emission spectra of Pt(II) tpy dimers and
linear chain compounds, in which this feature is sometimes, but not
always, evident.35,36
(33) Arena, G.; Calogero, G.; Campagna, S.; Scolaro, L. M.; Ricevuto,
V.; Romeo, R. Inorg. Chem. 1998, 37, 2763-2769.
2554 Inorganic Chemistry, Vol. 43, No. 8, 2004

A Luminescent Pt(II) 2,6-Bis(N-pyrazolyl)pyridine Complex
splittings in chain materials.39 However, a troubling incon-
sistency is the absence of unusually short intermolecular
interactions in crystals of [Pt(bpp)(Ph)](PF6) at 150 K. In
fact, the shortest Pt‚‚‚Pt interactions are considerably longer
(4.48, 4.71 Å) than those of crystalline [Pt(bpp)Cl]Cl‚2H2O
(3.39, 3.41 Å), which is an exceedingly weak emitter. One
possibility is that crystals of [Pt(bpp)(Ph)](PF6) undergo a
phase change below 150 K, resulting in an arrangement of
cations having significantly shorter Pt‚‚‚Pt distances. Indeed,
phase changes in linear chain materials are well docu-
mented;40 however, in the present case there is no cor-
roborating evidence to support this suggestion and further
investigation is necessary.
emission onset (e16 900 cm-1).43 The relative donor proper-
ties of the bpp and tpy ligands suggest that the lowest LF
states of platinum(II) terpyridyl complexes are likely to lie
at even higher energies than those of related bpp complexes.
Acknowledgment.
Diffraction
data
for
[Pt(bpp)Cl]Cl‚2H2O were collected through the Ohio Crys-
tallographic Consortium, funded by the Ohio Board of
Regents 1995 Investment Fund (CAP-075) and located at
the University of Toledo, Instrumentation Center in A&S,
Toledo, OH 43606. We thank the National Science Founda-
tion (Grants CHE-0215950, CHE-0134975) and the Univer-
sity of Cincinnati for support of this research. W.B.C. thanks
the Arnold and Mabel Beckman Foundation for a Young
Investigator Award. H.J. and W.B.C. are grateful to the
University of Cincinnati University Research Council for
summer research fellowships. H.J. thanks the University of
Cincinnati Department of Chemistry for the Stecker Fellow-
ship, the Link Foundation for an Energy Fellowship, and
the University of Cincinnati for a Distinguished Dissertation
Fellowship. We thank Drs. M. J. Baldwin, E. Brooks, and
K. Jayasimhulu, as well as Mr. Sudhir Shori, for helpful
discussions and expert technical assistance.
The intense emission from 77 K samples of solid
[Pt(bpp)(Ph)](PF6) contrasts sharply with the relatively broad
3
and weak LF emissions observed for many platinum(II)
complexes.11,41,42 Low-lying LF excited states has been
proposed to account for low quantum yields and short
emission lifetimes of some platinum(II) terpyridyl com-
plexes,6d,8 and a similar explanation for weak emission from
several of these platinum(II) bis(N-pyrazolyl)pyridyl com-
plexes also may apply. In the case of [Pt(bpp)(Ph)](PF6),
assuming the emission originates from the lowest excited
state, we can confidently conclude that the lowest LF states
for this complex must be near or at higher energies than the
Supporting Information Available: Tables of crystallographic
data, structure refinement details, atomic coordinates, interatomic
distances and angles, anisotropic displacement parameters, and
calculated hydrogen parameters in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
(39) Miskowski, V. M.; Houlding, V. H. Inorg. Chem. 1991, 30, 4446-
4452.
(40) Yersin, H.; Gliemann, G.; Ra¨de, H. S. Chem. Phys. Lett. 1978, 54,
111-116.
IC035066K
(41) Miskowski, V. M.; Houlding, V. H.; Che, C.-M.; Wang, Y. Inorg.
Chem. 1993, 32, 2518-2524.
(42) (a) Yersin, H.; Otto, H.; Zink, J. H.; Gliemann, G. J. Am. Chem. Soc.
1980, 102, 951-955. (b) Miskowski, V. M.; Houlding, V. H. Inorg.
Chem. 1989, 28, 1529-1533.
(43) The weak emission observed from glassy solutions suggests that the
lowest LF states could lie at significantly higher energies (g25 000
cm-1).
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