Cyclometalated 6-phenyl-2,2'-bipyridyl (CNN) platinum(ll) acetylide complexes: Structure, electrochemistry, photophysics, and oxidativeand reductive-quenching studies

Article abstract of DOI:10.1021/ic801947v

Three cyclometalated 6-phenyl-4-(p-R-phenyl)-2,2'-bipyrldyl (CNN-Ph-R) Pt(II) acetylide complexes, Pt(CNN-PhR)(CCPh), where R = Me (1), COOMe (2), and P(O)(OEt)₂(3), have been synthesized and studied. Compounds 1 and 3 have been structurally ch

Full text of DOI:10.1021/ic801947v

Inorg. Chem. 2009, 48, 4306-4316
Cyclometalated 6-Phenyl-2,2′-bipyridyl (CNN) Platinum(II) Acetylide
Complexes: Structure, Electrochemistry, Photophysics, and Oxidative-
and Reductive-Quenching Studies
Jacob Schneider, Pingwu Du, Paul Jarosz, Theodore Lazarides, Xiaoyong Wang,
William W. Brennessel, and Richard Eisenberg*
Department of Chemistry, UniVersity of Rochester, Rochester, New York 14627
Received October 13, 2008
Three cyclometalated 6-phenyl-4-(p-R-phenyl)-2,2′-bipyridyl (CNN-Ph-R) Pt(II) acetylide complexes, Pt(CNN-Ph-
R)(CCPh), where R ) Me (1), COOMe (2), and P(O)(OEt)2 (3), have been synthesized and studied. Compounds
1 and 3 have been structurally characterized by single crystal X-ray crystallography and are found to exhibit distorted
square planar geometries about the Pt(II) ions. The electrochemical properties of the compounds, as determined
by cyclic voltammetry, have also been examined. Complexes 1-3 are brightly emissive in fluid CH2Cl2 solution
max
and in the solid state with λem of ca. 600 nm and lifetimes on the order of ca. 500 ns in fluid solution. The
emissions are assigned to a 3MLCT transition. The complexes undergo oxidative quenching by MV2+ with quenching
rates near the diffusion-controlled limit (kq ∼ 1.4 × 1010 M-1 s-1) in CH2Cl2 solution. Reductive-quenching experiments
of complexes 1-3 by the amine donors N,N,N′,N′-tetramethylphenylenediamine (TMPD), phenothiazine (PTZ),
and N,N,N′,N′-tetramethylbenzidine (TMB) follow Stern-Volmer behavior, with very fast quenching rates on the
order of 109-1010 M-1 s-1 in CH2Cl2 solution. When the complexes are employed as the sensitizer in multiple
component systems containing MV2+, TEOA, and colloidal Pt in aqueous media, approximately one turnover of H2
(TN vs mol of chromophore) is produced per hour upon irradiation with λ > 410 nm but only after at least a 2 h
induction period.
Introduction
relay such as methyl viologen (MV2+), a colloidal platinum
catalyst, and a sacrificial electron donor such as ethylene-
diaminetetraacetic acid (EDTA) were used for the photoge-
neration of H2 from water,6-8 newer but related systems are
still emerging. For example, Bernhard et al. recently reported
the photoinduced production of H2 from a multiple compo-
nent system that contains a heteroleptic iridium(III) sensitizer
[Ir(dF(CF3)ppy)2(dtbbpy)]+ (where dF(CF3)ppy ) 2-(2,4-
difluorophenyl)-5-trifluoromethylpyridine and dtbbpy ) 4,4′-
di-tert-butyl-2,2′-bipyridine), a cobalt(II) complex (Co-
(bpy)32+) as the electron-transfer reagent, and triethanolamine
(TEOA) as the sacrificial reductant.9,10 Another account
describes the photogeneration (λ ) 330-700 nm) of H2 from
The conversion of solar energy into chemical energy by
artificial photosynthesis remains one of the great challenges
in the development of sustainable energy resources for the
future.1-3 The photogeneration of hydrogen from water,
which represents the reductive side of water splitting,
produces a carbon-free fuel that is storable, clean burning,
and most importantly, renewable.4 Of particular interest is
the production of H2 by molecular photocatalysis.5 After
nearly 30 years since the first reports of multiple component
systems containing a metal-complex chromophore (specif-
ically Ru(bpy)32+, where bpy ) 2,2′-bipyridine), an electron
* To whom correspondence should be addressed. E-mail: eisenberg@
chem.rochester.edu.
(1) Lewis, N. S; Nocera, D. G Proc. Natl. Acad. Sci. U.S.A. 2006, 103,
15729–15735.
(2) Alstrum-Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Inorg. Chem.
2005, 44, 6802–6827.
(3) Eisenberg, R.; Nocera, D. G. Inorg. Chem. 2005, 44, 6799–6801.
(4) Bard, A. J.; Fox, M. A. Acc. Chem. Res. 1995, 28, 141–145.
(5) Esswein, A. J.; Nocera, D. G. Chem. ReV. 2007, 107, 4022–4047.
(6) Keller, P.; Moradpour, A.; Amouyal, E.; Kagan, H. B. NouV. J. Chim.
1980, 4, 377–384.
(7) Kiwi, J.; Gra¨tzel, M. Nature 1979, 281, 657–658.
(8) Lehn, J.-M.; Sauvage, J.-P. NouV. J. Chim 1977, 1, 449–451.
(9) Lowry, M. S.; Goldsmith, J. I.; Slinker, J. D.; Rohl, R.; Pascal, R. A.,
Jr.; Malliaras, G. G.; Bernhard, S. Chem. Mater. 2005, 17, 5712–
5719.
(10) Goldsmith, J. I.; Hudson, W. R.; Lowry, M. S.; Anderson, T. H.;
Bernhard, S. J. Am. Chem. Soc. 2005, 127, 7502–7510.
4306 Inorganic Chemistry, Vol. 48, No. 10, 2009
10.1021/ic801947v CCC: $40.75  2009 American Chemical Society
Published on Web 04/24/2009

CNN Platinum(II) Acetylide Complexes
water using zinc-protoporphyrin IX in the presence of
colloidal Pt, MV2+, and TEOA.11 Fukuzumi and co-workers
have developed a photocatalytic hydrogen generating system
that combines an organic sensitizer, 9-mesityl-10-methyl-
acridinium (Acr+-Mes), and water soluble Pt nanoclusters
functionalized with MV2+ in the presence of dihydronico-
tinamide coenzyme (NADH) as a sacrificial reagent.12,13 Du
et al. showed that Pt(terpyridyl)(acetylide)+ complexes can
be employed as the chromophores in multiple component
hydrogen-generating systems that contain MV2+ or diquat
cations DQ2+, where DQ2+ is a dialkylated-2,2′-bipyridine,
as the electron relay, TEOA, and Pt colloid as the H2-
evolving catalyst.14,15
This Article describes the synthesis and characterization
of one previously reported and two new cyclometalated Pt(II)
CNN acetylide chromophores, 1-3, for the photogeneration
of H2 from water. The CNN ligands L2 and L3 with ester
groups have been synthesized so that upon hydrolysis the
resulting acid will be an effective anchoring group for
binding to TiO2 semiconductor particles,30 providing an
integrated system.31 In this report, the structural, electro-
chemical, and photophysical properties of these complexes,
as well as oxidative-quenching studies with methyl viologen
(MV2+) and reductive-quenching studies with three aryl
amine donors, are presented and discussed, as are observa-
tions regarding the photogeneration of H2.
The goal of the current study was to build on the
fundamental concepts taken from the seminal work done with
Ru(bpy)32+ and Pt(terpy)(acetylide)+ complexes and examine
them in the context of cyclometalated 6-phenyl-2,2′-bipyridyl
(CNN) Pt(II) acetylide chromophores. The CNN ligand,
similar to 2,2′-6′,2′′-terpyridine, and its derivatives have been
studied extensively as tridentate chelators in d6- and d8-
cyclometalated complexes of iridium(III),16 ruthe-
nium(II),17,18 palladium(II),19-21 and platinum(II).22-27 The
Pt(II) CNN complexes are solution luminescent with a
Experimental Section
Characterization and Instrumentation. All NMR spectra were
recorded on either Bruker Avance 400 or 500 MHz spectrometers.
1H NMR chemical shifts (in ppm) are referenced using the chemical
shift of DMSO-d6. 31P NMR chemical shifts (in ppm) are relative
to an external 85% solution of H3PO4 in the appropriate solvent.
Mass spectra were measured using a Shimadzu 2010 series liquid
chromatograph mass spectrometer (LCMS) operated with an
atmospheric pressure chemical ionization (APCI) quadrupole source
and the detection of positive and negative ions. The samples were
dissolved in CH2Cl2 and pumped through the spray chamber using
100% CH3OH. Elemental analyses were performed by Quantitative
Technologies Inc. (QTI). Absorption spectra were recorded using
a Hitachi U2000 scanning spectrophotometer (200-1100 nm).
Steady-state and frozen emission spectra were obtained using a Spex
Fluoromax-P fluorometer corrected for instrument response with
monochromators positioned with a 2 nm band-pass. The metal
complex concentration of solution samples was 1.0 × 10-5 M in
CH2Cl2 and each sample was degassed by purging with nitrogen.
Solid state emission samples were prepared as a 4% (w/w) mixture
of the complex in a matrix of finely ground KBr.
28,29
3
desirable MLCT excited-state energy.
The energy of
the MLCT absorption band is expected to be lowered relative
to Pt(terpy)(acetylide)+ complexes due to the combination
of strong acetylide and phenyl carbanion donors that raises
the highest occupied molecular orbital (HOMO) localized
on the Pt(II) center and a more accepting π* lowest
unoccupied molecular orbital (LUMO) localized on the
bipyridyl part of the CNN ligand.
(11) Komatsu, T.; Wang, R.-M.; Zunszain, P. A.; Curry, S.; Tsuchida, E.
J. Am. Chem. Soc. 2006, 128, 16297–16301.
(12) Kotani, H.; Ohkubo, K.; Takai, Y.; Fukuzumi, S. J. Phys. Chem. B
2006, 110, 24047–24053.
(13) Kotani, H.; Ono, T.; Ohkubo, K.; Fukuzumi, S. Phys. Chem. Chem.
Phys. 2007, 9, 1487–1492.
(14) Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem. Soc.
2006, 128, 7726–7727.
(15) Du, P.; Schneider, J.; Jarosz, P.; Zhang, J.; Brennessel, W. W.;
Eisenberg, R. J. Phys. Chem. B 2007, 111, 6887–6894.
(16) Whittle, V. L.; Williams, J. A. G. Inorg. Chem. 2008, 47, 6596–6607.
(17) Constable, E. C.; Cargill Thompson, A. M. W.; Greulich, S. J. Chem.
Soc., Chem. Commun. 1993, 1444–1446.
(18) Collin, J. P.; Beley, M.; Sauvage, J. P.; Barigelletti, F. Inorg. Chim.
Acta 1991, 186, 91–93.
The room temperature emission quantum yields (φem) are
measured relative to Ru(bpy)3Cl2 in degassed H2O as the reference
solution (φem ) 0.042).32 They were calculated using eq 1
(19) Kim, Y.-J.; Chang, X.; Han, J.-T.; Lim, M. S.; Lee, S. W. Dalton
Trans. 2004, 3699–3708.
(20) Lai, S.-W.; Cheung, T.-C.; Chan, M. C. W.; Cheung, K.-K.; Peng,
S.-M.; Che, C.-M. Inorg. Chem. 2000, 39, 255–262.
(21) Mathis, M.; Harsha, W.; Hanks, T. W.; Bailey, R. D.; Schimek, G. L.;
Pennington, W. T. Chem. Mater. 1998, 10, 3568–3575.
(22) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Che, C.-M.; Zhu, N.;
Lee, S.-T. J. Am. Chem. Soc. 2004, 126, 4958–4971.
(23) Chan, H.-L.; Ma, D.-L.; Yang, M.; Che, C.-M. ChemBioChem 2003,
4, 62–68.
(24) Cheung, T.-C.; Cheung, K.-K.; Peng, S.-M.; Che, C.-M. J. Chem. Soc.,
Dalton Trans. 1996, 1645–1651.
(25) Hofmann, A.; Dahlenburg, L.; van Eldik, R. Inorg. Chem. 2003, 42,
6528–6538.
(26) Lai, S.-W.; Chan, M. C.-W.; Cheung, T.-C.; Peng, S.-M.; Che, C.-M.
Inorg. Chem. 1999, 38, 4046–4055.
(27) Lai, S. W.; Chan, M. C.-W.; Cheung, K.-K.; Che, C.-M. Organome-
tallics 1999, 18, 3327–3336.
(28) Lu, W.; Mi, B.-X.; Chan, M. C. W.; Hui, Z.; Zhu, N.; Lee, S.-T.;
Che, C.-M. Chem. Commun. 2002, 206–207.
(29) Wu, Z.-X; Wu, L.-Z.; Yang, Q.-Z.; Zhang, L.-P.; Tung, C.-H. Chin.
J. Chem. 2003, 21, 196–199.
φs ) φr(Is/Ir)(Ar/As)(ns2/nr2)
(1)
where the subscripts s and r, respectively, denote sample and
reference; I is the integrated emission intensity; A is the absorbance
in a dilute solution (∼10-5 M, A < 0.5) at the wavelength of
excitation; and n is the refractive index that corrects for differences
between the reference and the sample solutions.33
Excited-state lifetime measurements of deoxygenated CH2Cl2
solution samples (∼10-4 M) of complexes 1-3 were obtained by
(30) Bae, E.; Choi, W.; Park, J.; Shin, H. S.; Kim, S. B.; Lee, J. S. J.
Phys. Chem. B 2004, 108, 14093–14101.
(31) Zhang, J.; Du, P.; Schneider, J.; Jarosz, P.; Eisenberg, R. J. Am. Chem.
Soc. 2007, 129, 7726–7727.
(32) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853–4858.
(33) Morris, J. V.; Mahaney, M. A.; Huber, J. R. J. Phys. Chem. 1976, 80,
969–974.
Inorganic Chemistry, Vol. 48, No. 10, 2009 4307

Schneider et al.
frequency doubling and pulse picking a femtosecond Ti-sapphire
laser at 450 nm with a 3.8 MHz repetition rate. The laser beam
was focused on the samples in a quartz cuvette by a microscope
The final set of refinement cycles, run to convergence, provided
the reported quality assessment parameters.
objective (50×, NA 0.55) with a power density of ∼1 kW/cm-1
.
Details of Photolysis Experiments
The photoluminescence (PL) from the samples was collected by
the same objective and sent through a 0.5 m spectrometer to a
charge coupled device (CCD) camera or a time-correlated single
photon counting system (∼200 ps resolution) for the time-integrated
or time-resolved measurements at room temperature. For the time-
resolved measurements, the collected PL was spectrally selected
by the spectrometer within a narrow bandwidth (∼3 nm) so that
only the PL decay at the peaks of maximum emission was
monitored. The PL decay was measured at 615 nm for complex 1
and 630 nm for complexes 2 and 3.
Oxidative- and reductive-quenching experiments were done in
degassed a CH2Cl2 or CH2Cl2:CH3CN (1:4, v/v) solution at room
temperature. The process was done by adding small aliquots of
standard quencher solution (MV2+ ·2PF6- in CH3CN, and TMPD,
PTZ, or TMB in CH2Cl2) to a solution of 1, 2, or 3 at a known
concentration (∼10-5 M). Excess CH2Cl2 (∼1 mL) was added, and
the solution was purged carefully with nitrogen until the original
volume was obtained.
Cyclic voltammetry experiments were conducted on an EG&G
PAR 263A potentiostat/galvanostat using a three-electrode single
compartment cell. A glassy carbon working electrode, a Pt wire
auxiliary electrode, and a Ag wire reference electrode were used.
For all measurements, samples were dissolved in DMF and degassed
with argon. Tetrabutylammonium hexafluorophosphate (Fluka) was
used as the supporting electrolyte (0.1 M), and ferrocene (ROC/
RIC) was employed as an internal redox reference. All redox
potentials are reported relative to the ferrocenium/ferrocene
(Fc+/0) couple (0.45 V vs SCE34 and SCE vs NHE ) 0.24 V). All
scans were done at 100 mV/s unless otherwise noted.
Crystallographic Structure Determinations. Crystals were
attached to glass fibers and mounted under a stream of N2,
maintained at 100.0(1) K, on a Bruker SMART platform diffrac-
tometer equipped with an APEX II CCD detector, positioned at
-33° in 2θ and 5.0 cm from the samples. The X-ray source,
powered at 50 kV and 30 mA, provided Mo KR radiation (λ )
0.71073 Å, graphite monochromator). Preliminary random sam-
plings of reflections were used to determine unit cells and orientation
matrices.35 Complete data collections were obtained by ω scans of
183° (0.5° steps) at four different φ settings. Final cell constants
were calculated from the xyz centroids of approximately 4000
strong reflections from the actual data collection after integration.36
The data were additionally scaled and corrected for absorption.37
Space groups were determined on the basis of systematic absences,
intensity statistics, and space group frequencies. Structures were
solved by direct or Patterson methods.38,39 Non-hydrogen atoms
were located by difference Fourier syntheses, and their positions
were refined with anisotropic displacement parameters, unless noted
otherwise, by full-matrix least-squares cycles on F2.38 In a similar
fashion, all hydrogen atoms were placed geometrically and refined
as riding atoms with relative isotropic displacement parameters.
Photogeneration of MV+•. Each sample was prepared in
a 50 mL round-bottom flask with a total volume of 25 mL
in CH2Cl2. Concentrations of the various components were
as follows: 2.7-33.5 mM TEOA, 2.2 × 10-5 M chro-
-
mophore, and 3.1 × 10-4 M MV2+ · 2PF6 unless noted
otherwise. About 10 mL of the solution was transferred to a
vacuum sealable flask with a quartz cuvette attachment
equipped with a stir bar, and the solution was degassed by
at least three freeze-pump-thaw cycles. The solution in the
cuvette was then irradiated under a 200 W mercury xenon
lamp (Oriel) with a 410 nm cutoff filter to remove the high
energy light from photolysis, and the absorption spectrum
was measured at different time intervals using a Hitachi
U2000 scanning spectrophotometer (200-1100 nm).
Photogeneration of H2. Each sample was prepared in a
50 mL round-bottom flask with a total volume of 25 mL in
CH3CN/H2O (2:3, v/v). Concentrations of the various
components were as follows: 13.4-26.8 mM TEOA, 2.2 ×
10-5 M chromophore, 6.0 × 10-5 M Pt colloid, and 3.1 ×
10-4 M MV2+ · 2Cl- unless noted otherwise. The pH of the
solution was adjusted to pH ∼7 using 10% aqueous HCl
(v/v). The flask was sealed with a septum and degassed by
bubbling N2 for 20 min under atmospheric pressure at room
temperature. After degassing, 5 mL of N2 was removed with
a gastight syringe, and 5 mL of CH4 (760 Torr) was
subsequently added to serve as an internal standard for
quantitative GC analysis. The sample was then irradiated
under a 200 W mercury xenon lamp (Oriel) with a 410 nm
cutoff filter to remove the high energy light from photolysis.
The reaction vessel was set up to stir approximately 5 cm in
front of the light source in a well-ventilated fume hood. Gas
samples (∼10 µL) were taken from the headspace of the
reaction vessel and injected into the GC using a gastight
syringe. The hydrogen generated from the systems was
measured by a GC-17A Shimadzu gas chromatograph using
nitrogen as the carrier gas with either a molecular sieve 5 Å
column (30 m × 0.53 mm) (Supelco) or a molecular sieve
column (30 m × 0.530 mm) (J&W Scientific) and a thermal
conductivity detector. The amount of H2 produced was
quantified by the ratio of H2/CH4 (CH4 ) internal standard),
followed by conversion to mL, mol, or turnovers (TNs) of
H2. Turnovers were calculated relative to the amount of
chromophore in the system, with errors on the order of ∼15%
(see Figure S14 in the Supporting Information for example
calculations).
Materials. K2PtCl4 (VWR), CuI (Aldrich), phenylacety-
lene (Aldrich), triethylamine (Aldrich), triethanolamine
(TEOA) (Aldrich), N,N,N′,N′-tetramethylphenylenediamine
(TMPD)(AlfaAesar),phenothiazine(PTZ)(Aldrich),N,N,N′,N′-
tetramethylbenzidine (TMB) (Aldrich), DMF (Mallinckrodt),
and DMSO-d6 (Cambridge Isotope Laboratories) were
purchased commercially and used without further purifica-
tion. The ligands CNN-Ph-COOMe and CNN-Ph-P(O)(O-
(34) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877–910.
(35) APEX2 V2.1-0; Bruker AXS: Madison, WI, 2006.
(36) SAINT V7.34A; Bruker AXS: Madison, WI, 2006.
(37) Sheldrick, G. M. SADABS V2004/1; University of Go¨ttingen: Go¨ttin-
gen, Germany, 2004.
(38) SHELXTL V6.14; Bruker AXS: Madison, WI, 2000.
(39) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna,
R. SIR97: A New Program for SolVing and Refining Crystal Structures;
Istituto di Cristallografia, CNR: Bari, Italy, 1999.
4308 Inorganic Chemistry, Vol. 48, No. 10, 2009

CNN Platinum(II) Acetylide Complexes
40
Et)2 and complexes [Pt(CNN-Ph-COOMe)Cl], [Pt(CNN-
elemental analyses, as well as by single crystal X-ray
crystallography for complexes 1 and 3. The elemental
analyses for complexes 2 and 3 are not satisfactory; the low
percent carbon is due in part to difficulties getting complete
combustion of the complexes under experimental conditions.
Crystal Structure Determination. Crystals of 1 suitable
for single crystal X-ray diffraction were grown by slow
evaporation of a dichloromethane solution of the complex,
while crystals for complex 3 were obtained by slow diffusion
of diethyl ether into a dichloromethane solution of the
compound. ORTEP diagrams of complexes 1 and 3 are
shown in Figures 1 and 2, respectively. The unit cell, data
collection, and refinement parameters are summarized in
Table 1 with selected bond lengths and angles for both
structures given in Table 2. Both complexes exhibit distorted
square planar coordination geometries around the Pt(II)
nucleus. For complexes 1 and 3, the Pt-N distances to the
central pyridine and the Pt-C(acetylide) distances, 1.980(5)
and 1.990(3) Å and 1.980(6) and 1.976(3) Å, respectively,
are slightly shorter than the peripheral Pt-N and Pt-C(aryl)
distances of 2.090(6) and 2.122(3) Å and 2.028(6) and
1.992(3) Å, respectively. These values are consistent with
earlier reports of analogous structures having the general
formula Pt(pbpy)(Ct CR) (pbpy ) 6-phenyl-2,2′-bipyridine,
R ) C6H5, 4-MeC6H4, C6F5, SiMe3, Ct CC6H5, Ct CSiMe3,
Ct C-Ct CSiMe3, 9,9-Me2-fluorene, and 9-Me-carba-
zole).22,28,44 In those reported structures, the Pt-C(acetylide)
distances range between 1.956 and 1.98 Å, the Pt-N bond
to the central pyridine is shorter than the Pt-N bond to the
peripheral pyridine ring, and the Pt-C(aryl) bonds are in
the expected range of ∼2.0 Å.
Ph-P(O)(OEt)2)Cl],40 and [Pt(CNN-Ph-Me)(C≡CPh)] (1)22
were prepared as reported previously. All other solvents and
reagents were purchased commercially and used as received
unless otherwise noted.
Synthesis and Characterization of New Pt(II) Complexes.
[Pt(CNN-Ph-COOMe)(Ct CPh)] (2). To a flask charged with
[Pt(CNN-Ph-COOMe)Cl] (101 mg, 0.17 mmol), CuI (11 mg,
5 mg per 50 mg of Pt complex), and phenylacetylene (∼0.06
mL, 0.6 mmol) was added anhydrous DMF (∼15 mL) and
NEt3 (∼5 mL). The mixture was briefly sonicated and then
degassed with nitrogen before stirring at room temperature
for 2.5 days in the absence of light. The bright orange
precipitate was collected by filtration, rinsing with Et2O until
dry. Yield: 108 mg (96%). APCI (m/z): 662.3, [M + H]. 1H
NMR (DMSO-d6, 400 MHz): δ 9.10 (d, 1H, J ) 4.7 Hz),
8.79 (d, 1H, J ) 8.0 Hz), 8.67 (s, 1H), 8.43 (m, 2H), 8.28
(d, 2H, J ) 8.4 Hz), 8.17 (d, 2H, J ) 8.5 Hz), 7.91 (m, 2H),
7.79 (d, 1H, J ) 8.2 Hz), 7.39 (d, 2H, J ) 7.4 Hz), 7.29 (t,
2H, J ) 7.4 Hz), 7.18 (m, 2H), 7.12 (m, 1H), 3.93 (s, 3H,
CNN-Ph-CO2CH3). FT-IR (neat) ν/cm-1: 2097 (νCt C), 1713
(νCdO), 1279 (νC-O). Anal. Calcd for C32H22N2O2Pt: C, 58.09;
H, 3.35; N, 4.24. Found: C, 54.21; H, 2.53; N, 4.02.
[Pt(CNN-Ph-P(O)(OEt)2)(Ct CPh)] (3). This compound
was prepared by the same method as for 2 but on a smaller
scale using [Pt(CNN-Ph-P(O)(OEt)2)Cl] (50 mg, 0.073
mmol), CuI (5 mg), phenylacetylene (∼0.03 mL, 0.27 mmol),
DMF (∼6 mL), and NEt3 (∼2 mL), stirring under nitrogen
for four days at room temperature. Addition of Et2O to the
reaction solution caused precipitation of an orange product.
At times, the precipitate formed was green due to Pt-Pt
stacking interactions.41,42 The precipitate was collected by
filtration, rinsing with Et2O until dry. Yield: 50 mg (94%).
In the crystal structure of complex 1, there exists infinite
stacks of complexes that have weak alternating Pt · · ·π and
π · · · π interactions with interplanar distances in the range of
3.3-3.4 Å. The Pt · · · π interaction involves Pt from one
molecule and the π system of a bipyridine ligand unit from
a neighboring molecule (Figure 1). In a similar fashion, the
π · · · π stacking involves the phenyl ring of the ligand core
from one molecule and the central pyridine ring of the CNN
ligand in a nearest-neighbor molecule. In the crystal structure
of complex 3, there is one cocrystallized ether solvent
molecule per Pt complex. Pyridyl rings of symmetry
equivalent molecules are involved in π · · · π stacking with
distances of 3.4-3.5 Å, but no intermolecular Pt · · ·Pt
contacts are observed (Figure 2); the nearest Pt · · ·Pt distance
is 4.95 Å.
Electrochemistry. The cyclic voltammograms of com-
plexes 1-3 (see Supporting Information) were measured
in 0.1 M n-BuNPF6 in DMF, and the electrochemical data
were summarized in Table 3. All of the redox potentials
are reported relative to NHE with the ferrocenium/
ferrocene (Fc+/0) couple used as an internal redox standard
and a value of 0.45 V vs SCE taken for the Fc+/0 couple34
and a value of 0.24 V taken for SCE vs NHE. Each
complex exhibits two reductions, assigned to the CNN
ligand. The first CNN-based reduction observed at -1.08,
1
APCI (m/z): 740.2, [M + H]. H NMR (DMSO-d6, 500
MHz): δ 9.11 (d, 1H), 8.77 (d, 1H, J ) 8.0 Hz), 8.64 (s,
1H), 8.42 (m, 2H), 8.26 (d, 2H, J ) 4.4 Hz), 7.91 (m, 4H),
7.80 (d, 1H, J ) 7.6 Hz), 7.39 (d, 2H, J ) 7.3 Hz), 7.29 (t,
2H, J ) 7.5 Hz), 7.18 (m, 2H), 7.12 (t, 1H, J ) 6.8 Hz),
4.09 (m, 4H, CNN-Ph-P(O)(OCH2CH3)2), 1.28 (t, 6H, 7.0
Hz, CNN-Ph-P(O)(OCH2CH3)2). 31P{1H} NMR (DMSO-d6,
202 MHz): δ 17.08 (s). FT-IR (neat) ν/cm-1: 2097 (νCt C),
1248 (νP) O). Anal. Calcd for C34H29N2O3PPt: C, 55.21; H,
3.95; N, 3.79. Found: C, 53.01; H, 4.03; N, 4.46.
Results and Discussion
Synthesis and Characterization. Complexes 1-3 were
prepared by the reaction of their [Pt(CNN)Cl] precursors with
phenylacetylene in the presence of triethylamine and catalytic
CuI following a previously reported procedure.43 The
complexes have been characterized by H and 31P NMR
1
spectroscopies, mass spectrometry (APCI), FT-IR, and
(40) Schneider, J.; Du, P.; Wang, X.; Brennessel, W. W.; Eisenberg, R.
Inorg. Chem. 2009, 48, 1498–1506.
(41) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer,
W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591–4599.
(42) Herber, R. H.; Croft, M.; Coyer, M. J.; Bilash, B.; Sahiner, A. Inorg.
Chem. 1994, 33, 2422–2426.
(43) Chakraborty, S.; Wadas, T. J.; Hester, H.; Flaschenreim, C.; Schmehl,
R.; Eisenberg, R. Inorg. Chem. 2005, 44, 6284–6293.
(44) Seneclauze, J. B.; Retailleau, P.; Ziessel, R. New J. Chem. 2007, 31,
1412–1416.
Inorganic Chemistry, Vol. 48, No. 10, 2009 4309

Schneider et al.
Figure 1. (Top) ORTEP drawing of complex 1, hydrogen atoms omitted for clarity. (Bottom) Two views of the intermolecular stacking of complex 1,
showing the infinite alternating Pt · · ·π (green and pink molecules) and π· · ·π stacking interactions (pink molecules).
-1.01, and -1.00 V (vs NHE), for 1, 2, and 3,
respectively, is reversible, while the second ligand-based
reduction observed at -1.67, -1.48, and -1.50 V,
respectively, is quasi-reversible. Complex 1 with R ) Me
is the most difficult to reduce while 2 and 3 are slightly
easier to reduce and have similar reduction potentials,
consistent with the electron donating/withdrawing ability
of the pendant R group (Me in 1, COOMe and P(O)(OEt)2
in 2 and 3, respectively). In an earlier report by Che et
al., the reduction potentials for several Pt(pbpy)(acetylide)
complexes were measured in CH2Cl2 solution and only
one reduction was observed, possibly due to a different
solvent window than for the current study, attributed to a
one-electron reduction of the CNN ligand.22 In that report,
the authors observe similar electronic effects with a
complex having an electron-withdrawing COOEt pendant
group possessing a reduction potential of -1.60 V (vs
Fc+/0), while a complex containing two tert-butyl groups
underwent a more difficult reduction at -1.93 V.
oxidation at +1.48, +1.52, and +1.23 V for complexes 1-3,
respectively, follows no obvious trend and is therefore
uncertain in assignment. In an earlier report by Che and co-
workers, the authors only observed one irreversible oxidation
for the Pt(pbpy)(acetylide) complexes in CH2Cl2, which was
not assigned.22
Absorption and Emission Spectra. The photophysical
data for complexes 1-3 are summarized in Tables 4 and 5.
Complexes 2 and 3 show steady-state electronic spectra
similar to that of complex 1 in CH2Cl2 solution (Figure 3),
which has been discussed elsewhere.22,29 The UV region of
the absorption spectra are dominated by intraligand (IL)
transitions with maxima at ca. 285, 340, and 370 nm, having
molar absorpivities (ε) on the order of 10 000 dm3 mol-1
cm-1. In the visible region of the absorption spectra of
complexes 1-3, there is a broad charge-transfer maximum
at ca. 440 nm with a slight shoulder at ca. 460 nm that has
been previously assigned to a metal-to-ligand charge-transfer
(1MLCT) transition.22,28
The oxidation waves of complexes 1-3 correspond to two
irreversible oxidations. The first oxidation observed at +0.94
V (vs NHE) is the same for complexes 1-3 and has
tentatively been assigned to a PtII/III process.43 The second
All of the Pt(CNN)(CCPh) complexes are brightly lumi-
nescent in dichloromethane solution at room temperature and
at 77 K, in frozen MeOH:EtOH (1:4, v/v) glass, and in the
solid state. The steady-state emission maxima in fluid CH2Cl2
4310 Inorganic Chemistry, Vol. 48, No. 10, 2009

CNN Platinum(II) Acetylide Complexes
Figure 2. (Top) ORTEP drawing of complex 3, hydrogen atoms omitted for clarity. (Bottom) Two views of the intermolecular stacking of complex 3,
showing the pyridyl rings of symmetry equivalent molecules involved in π· · ·π stacking interactions.
Table 1. Unit Cell, Data Collection, and Refinement Parameters for
Complex 1 and Complex 3·Et2O
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes 1
and 3·Et2O
complex 1
complex 3·Et2O
Complex 1
Pt(1)-N(1)
2.090(6)
1.980(5)
1.197(8)
79.3(2)
160.3(2)
172.8(7)
Pt(1)-C(1)
Pt(1)-C(9)
1.980(6)
2.028(6)
formula
fw
T (K)
C31H22N2Pt
C34H29N2O3PPt·Et2O
813.77
100.0(1)
0.71073
monoclinic
C2/c
32.340(4)
6.9509(9)
31.480(4)
90
110.027(2)
90
6648.4(14)
8
1.626
4.313
orange needle
Pt(1)-N(2)
617.60
100.0(1)
0.71073
monoclinic
P21/c
9.579(2)
23.142(5)
11.254(3)
90
114.928(3)
90
2262.3(9)
4
C(1)-C(2)
N(2)-Pt(1)-N(1)
C(9)-Pt(1)-C(9)
C(1)-C(2)-C(3)
N(2)-Pt(1)-C(9) 81.0(2)
C(1)-Pt(1)-N(2) 177.5(2)
λ (Å)
crystal system
space group
a (Å)
C(16)-C(17)-C(25)-C(26) 37.1(8)
Complex 3·Et2O
2.122(3) Pt(1)-C(1)
Pt(1)-C(9)
b (Å)
c (Å)
Pt(1)-N(1)
1.976(3)
1.992(3)
R (deg)
ꢀ (deg)
γ (deg)
V (Å3)
Z
Pt(1)-N(2)
1.990(3)
1.201(5)
C(1)-C(2)
N(2)-Pt(1)-N(1)
C(1)-Pt(1)-N(2)
C(1)-C(2)-C(3)
78.38(11) N(2)-Pt(1)-C(9) 81.87(12)
178.22(12) C(9)-Pt(1)-N(1) 160.19(12)
177.4(4)
Fcald (mg/m3)
1.813
6.225
orange needle
C(16)-C(17)-C(25)-C(26) 38.1(4)
µ (mm-1
)
Table 3. Electrochemical Data for Complexes 1-3 and Quenching
crystal color
crystal size (mm3)
θ range (deg)
no. of data
Reagents, As Determined by Cyclic Voltammetrya
0.22 × 0.08 × 0.04 0.38 × 0.14 × 0.04
E1/2red/V
E1/2ox/V
2.18-32.57
8163
1.38-32.03
11527
419
1.207
0.0377, 0.0725
0.0539, 0.0814
complex/reagent
1
2
3
-1.08, -1.67b
-1.01, -1.48b
-1.00, -1.50b
-0.20, -0.58
+0.94,c +1.48c
+0.94,c +1.52c
+0.94,c +1.23c
no. of parameters
296
1.178
GOFa
R1, wR2 (F2, I > 2σ(I))b 0.0534, 0.1024
-
MV2+ ·2PF6
R1, wR2 (F2, all data)b
0.0691, 0.1071
TMPD
PTZ
TMB
TEOA
+0.39
a
GOF ) S ) [∑w(Fo - Fc2)2/(m - n)]1/2, where m ) number of
2
+0.83
reflections and n ) number of parameters. b R1 ) ∑|Fo| - |Fc|/∑|Fo|. wR2
+0.84
) [∑w(Fo - Fc2)2/∑w(Fo )2]1/2, where w ) 1/[σ2(Fo)2 + (aP) + bP] and
2
2
+1.12,c +1.43c
P ) 1/3 max(0, Fo ) + (2/3)Fc2.
2
a All redox potentials are reported versus NHE relative to the (Fc+/0
)
couple (0.45 V vs SCE34 and SCE vs NHE ) 0.24 V). b Quasi-reversible
solution for complexes 1-3 are 595, 607, and 606 nm, with
excited-state lifetimes determined to be 485, 550, and 605
ns, respectively (λex ) 450 nm), and quantum yields (φem)
between 0.047 and 0.073 measured relative to Ru(bpy)3Cl2
under experimental conditions. c Irreversible under experimental conditions.
in H2O (φem ) 0.042).32 The emissive state in 1-3 has been
assigned to a MLCT transition.
20,29,45
3
Complexes 2 and 3
have slightly red-shifted emission maxima compared with
complex 1, due to the electron-withdrawing ester groups on
(45) Lai, S.-W.; Che, C.-M. Top. Curr. Chem. 2004, 241, 27–63 (Transition
Metal and Rare Earth Compounds III).
Inorganic Chemistry, Vol. 48, No. 10, 2009 4311

Schneider et al.
Table 4. Spectroscopic and Photophysical Data for Phenylacetylide Complexes
c
solution λem/nma b
τ/nsb
φem
,
complex
λ
abs/nm (ε/dm3 mol-1 cm-1
)
1
2
3
283 (42 520), 337 (19 980), 370 (14 570), 441 (9950), 460 (9670)
289 (44 940), 340 (16 730), 373 (12 510), 448 (8450), 461 (8380)
287 (43 040), 340 (13 780), 373 (9820), 447 (6380), 464 (6300)
b
595 (max), 645 (sh)
607 (max), 645 (sh)
606 (max), 645 (sh)
485
550
605
0.073
0.049
0.047
a 1.0 × 10-5 M in CH2Cl2. λex ) 440 nm. c Emission quantum yields measured relative to Ru(bpy)3Cl2 in H2O (φem ) 0.042).32
Table 5. Solid State and Frozen Emission Data, λex ) 440 nm
complex
frozen solution λem/nma
frozen glass λem/nmb
solid state λem/nmc
1
2
3
560 (max), 595, 625, 690
575 (max), 605, 650
570 (max), 605, 650
545, 580 (max), 625
560, 605 (max), 660
555 (max), 585, 640
590 (max), 625 (sh)
610 (max), 650 (sh)
600 (max), 640 (sh)
a CH2Cl2, 77 K. b MeOH:EtOH (1:4, v/v), 77 K. c 4% (w/w) mixture with KBr, rt.
Figure 3. Steady-state absorption and emission spectra of complexes 1-3 in fluid CH2Cl2 solution (1.0 × 10-5 M), and normalized frozen glass (MeOH/
EtOH, 1:4 v/v) emission spectrum of complex 1 (blue dashed).
the CNN ligand. Similar to the electrochemical results, this
observation is consistent with the idea that the LUMO, which
is localized on the CNN ligand, is slightly lower in energy
for complexes 2 and 3 compared with the LUMO energy
for complex 1.
The emission maxima of complexes 1-3 in the solid state
at room temperature exhibit similar behavior to the steady-
state emission spectra in fluid solution, with complexes 2
and 3 having slightly red-shifted emission maxima compared
with complex 1. Their solid state emission maxima of 590,
610, and 600 nm, are accompanied by shoulders at 625, 650,
and 640 nm, for 1, 2, and 3, respectively.
Excited-State Potentials. The excited-state reduction and
oxidation potentials may be estimated using eqs 2 and 3,
respectively, where E* is the excited-state reduction (E1/2*red)
or oxidation (E1/2*ox) potential, E is the ground-state reduction
(E1/2red) or oxidation (E1/2ox) potential as determined electro-
chemically, and wr is an electrostatic work term describing
charge generation and separation within the electron-transfer
complex.51
In frozen CH2Cl2 solution, the emission spectra for
complexes 1-3 are broad, exhibiting little structure in the
range of 550-750 nm, with maxima at ca. 570 nm. However,
the frozen MeOH:EtOH (1:4, v/v) glass emission spectra of
complexes 1-3 have structured emission profiles that span
the range of ca. 525-725 nm (Figure 3 and Figure S2 in
the Supporting Information). From the frozen glass emission,
an estimation of the excited-state energy of the MLCT
emissive state, E00, can be made.46 The E00 energies for
complexes 1-3 were estimated directly from the highest
energy emission maxima in the frozen glass samples and
were calculated to be 2.28, 2.22, and 2.23 eV, respectively.
This method is an alternative to the more thorough derivation
of E00 using a single-mode, Franck-Condon analysis of
emission spectral profiles.47-50
*red
red
E1/2
) E1/2 + E00 + wr
(2)
(3)
*ox
ox
E1/2 ) E1/2 - E00 + wr
For simple estimations of the excited-state potentials of the
complexes in this study, the work terms for these neutral
(46) Turro, N. J. Modern Molecular Photochemistry; Addison-Wesley
Publishing Co.: Reading, MA, 1978; 628 pp.
(47) Caspar, J. V.; Meyer, T. J. J. Am. Chem. Soc. 1983, 105, 5583–5590.
(48) Claude, J. P.; Meyer, T. J. J. Phys. Chem. 1995, 99, 51–54.
(49) Dattelbaum, D. M.; Omberg, K. M.; Schoonover, J. R.; Martin, R. L.;
Meyer, T. J. Inorg. Chem. 2002, 41, 6071–6079.
(50) Kober, E. M.; Caspar, J. V.; Lumpkin, R. S.; Meyer, T. J. J. Phys.
Chem. 1986, 90, 3722–3734.
(51) Jones, J. W. E.; Fox, M. A. J. Phys. Chem. 1994, 98, 5095–5099.
4312 Inorganic Chemistry, Vol. 48, No. 10, 2009

CNN Platinum(II) Acetylide Complexes
Table 6. Estimated Excited-State Energies and Excited-State Redox
Table 8. Electrochemical Driving Force for the Quenching Reaction
Estimated Using the Excited-State Oxidation and Reduction Potentials of
Complexes 1-3
Potentials
complex
E00/eV
E1/2*
red/V
E*ox/V
∆E° for quenching reagent/V
1
2
3
2.28
2.22
2.23
+1.20
+1.21
+1.23
-1.34
-1.28
-1.29
complex
MV2+
TEOAa
TMPD
PTZ
TMB
1
2
3
1.14
1.08
1.09
0.08
0.09
0.11
0.81
0.82
0.84
0.37
0.38
0.40
0.36
0.37
0.39
Table 7. Oxidative and Reductive Quenching of Complexes 1-3 by
MV2+ and by TMPD, PTZ, and TMB, Respectively
a Emission quenching of complexes not observed.
kq(MV2+)/1010 kq(TMPD)/1010kq(PTZ)/109 kq(TMB)/109
complex
M-1 s-1
M-1 s-1
M-1 s-1
M-1 s-1
of MV2+ is -0.20 V (vs NHE, Table 3), the driving force,
∆E°, for complexes 1-3 to undergo oxidative quenching
by MV2+ is ∼1.1 V (vs NHE, Table 8), while ∆E° for
Pt(ttpy)(CCPh)+ to undergo oxidative quenching by MV2+
is 0.94 V. The difference in ∆E° for the two systems is only
0.16 V, but it can explain the approximate 10-fold difference
in quenching rate constants kq based on the simple Marcus
theory. The Marcus expression for the free energy of
activation, ∆Gq, of an outer sphere electron transfer is given
in eq 6. The ratio of the rates of two reactions from the
Eyring theory, assuming both have the same prefactor, is
given by eq 7, and the difference in the free energies of
activation, ∆∆Gq, assuming the same reorganization energy
λ for the two reactions, is found in eq 8.
1
2
3
1.4
1.9
1.1
1.7
1.5
1.4
8.1
7.2
7.9
10.6
8.2
6.9
complexes can be assumed to be zero. For the excited-state
oxidation potential E*ox, only a rough estimate can be
obtained since the one electron oxidation process is irrevers-
ible from an electrochemical standpoint. While only a rough
estimate, the E*ox value thus calculated is useful for
comparing the series of complexes 1-3 in the current study.
*red
red
E1/2
) E1/2 + E00
(4)
(5)
E*ox ) Eox - E00
From eqs 4 and 5, complexes 1-3 have estimated E1/2*red
values of +1.20, +1.21, and +1.23 V (vs NHE, Table 6),
respectively, and E*ox values estimated at -1.34, -1.28,
-1.29 V, respectively. When these excited-state potentials
are compared to those reported for analogous Pt(terpyridyl)-
(acetylide)+ compounds (E1/2*red ca. +1.8 V vs NHE and
E*ox ca. -1.1 V),15,52 complexes 1-3 are substantially
weaker excited-state oxidants and slightly stronger excited-
state reductants.
Oxidative-Quenching Studies. Complexes 1-3 were all
found to undergo oxidative quenching by MV2+ in CH2Cl2
solution, following Stern-Volmer behavior.53 The rates of
quenching are essentially diffusion controlled with kq values
of 1.4, 1.9, and 1.1 × 1010 M-1 s-1 for complexes 1-3,
respectively (Table 7). These results are comparable to an
earlier report of the analogous Pt(ttpy)(CCPh)+ complex that
exhibits oxidative quenching by MV2+ and a number of
diquat cations DQn2+, where DQn2+ is a dialkylated-2,2′-
bipyridine for n ) 1 (ethylene bridge), 2 (propylene), and 3
(butylene), with kq values of 3.3 × 109 M-1 s-1 for oxidative
quenching by MV2+ and 10.0, 7.1, and 6.1 × 109 M-1 s-1
for oxidative quenching by DQ12+, DQ22+, and DQ32+
respectively, in CH3CN/H2O solution.15
(λ + ∆Go)2
∆Gq )
(6)
(7)
4λ
k1
q-∆G1q)/RT
) e-(∆G
2
k2
q
q
∆G2 - ∆G1 ) ∆∆Gq )
o
o
o 2
o 2
{2λ(∆G2 - ∆G1 ) + ((∆G2 ) - (∆G1 ) )}
4λ
(8)
While values of λ for electron-transfer reactions involving
complexes such as 1-3 and Pt(ttpy)(CCPh)+ have not been
determined, we use as an estimate 0.5 eV that was suggested
for electron-transfer-quenching reactions of the Pt(II) com-
plex Pt(Ph2phen)(ecda) (where Ph2phen ) 4,7-diphenylphenan-
throline and ecda ) 1-(ethoxycarbonyl)-1-cyanoethylene-2,2-
dithiolate).54 On the basis of this value of λ and a difference
in driving force for the two quenching reactions of 0.16 V,
values for the ratio of rates ranged up to a value of 13
depending on how far the driving force difference was from
the actual value of λ. We do not know the relative positions
of these electron-transfer reactions in the nomal Marcus
region but think that the difference in the quenching rate
can be rationalized primarily on this basis. Additionally, it
is also possible that the change in kq is due in part to the
difference in charge between the cationic Pt(ttpy)(CCPh)+
and the neutral complexes 1-3, with slower kq values for
the dynamic quenching of cationic Pt(ttpy)(CCPh)+ by the
dicationic MV2+ compared with faster quenching of the
neutral complexes by MV2+.
The neutral complexes 1-3 have faster oxidative-quench-
ing rates than does the cationic Pt(ttpy)(CCPh)+ complex,
∼1.0 × 1010 vs ∼1.0 × 109 M-1 s-1, respectively. Complexes
1-3 are slightly stronger excited-state reductants than
Pt(ttpy)(CCPh)+, with excited-state oxidation potentials of
∼-1.3 V (vs NHE, Table 6), compared with an excited-
state oxidation potential of -1.14 V for Pt(ttpy)(CCPh)+.15
With the assumption that the ground-state reduction potential
Reductive-Quenching Studies. Contrary to earlier reports
(52) Narayana-Prabhu, R.; Schmehl, R. H. Inorg. Chem. 2006, 45, 4319–
of analogous Pt(terpyridyl)(acetylide)+ complexes that un-
4321.
(53) Vincze, L.; Sa´ndor, F.; Pe´m, J.; Bosnya´k, G. J. Photochem. Photobiol.,
A 1999, 120, 11–14.
(54) Bevilacqua, J. M.; Eisenberg, R. Inorg. Chem. 1994, 33, 1886–1890.
Inorganic Chemistry, Vol. 48, No. 10, 2009 4313

Schneider et al.
Figure 4. Stern-Volmer plots of oxidative and reductive quenching of complex 2 by MV2+, TMPD, PTZ, and TMB in a CH2Cl2 solution.
dergo reductive quenching by triethanolamine (TEOA) with
quenching rates on the order of 1.7 × 109 M-1 s-1 in CH3CN/
H2O solution,14,15 none of the Pt CNN complexes in this
study show emission quenching by TEOA in CH2Cl2
solution. Complexes 1-3 are weaker excited-state oxidants
than their terpyridyl analogues, with estimated E1/2*red values
of ∼+1.2 V (vs NHE, Table 6) for complexes 1-3,
compared with estimated E1/2*red values of ∼+1.8 V for the
Pt(terpyridyl)(acetylide)+ complexes.15,52 Whereas the ∆E°
for complexes 1-3 to undergo reductive quenching by
TEOA (Eox ) +1.12 V vs NHE, Table 3) is estimated at
only ∼0.1 V (Table 8), ∆E° for the Pt(terpyridyl)(acetylide)+
complexes is estimated to be significantly higher at ∼0.7 V.
For comparison, Pt(ttpy)(CCPh)+ undergoes reductive quench-
ing with TEOA with a kq value of 1.4 × 109 M-1 s-1 in
CH3CN/H2O solution.14,15
s-1. These data suggest that a ∆E° of ∼0.1 V, which is that
of quenching by TEOA, does not have sufficient driving
force to generate reductive-emission quenching with 1-3.
Photoreduction of MV2+ and H2 Production. Though
there was no reductive quenching of complexes 1-3 by
TEOA, it was found that in a deoxygenated system contain-
ing one of the Pt(CNN) complexes (1, 2, or 3), MV2+, and
TEOA in CH2Cl2 solution, the blue MV+• radical anion
develops within 5 min of irradiation with λ > 410 nm light
(Figure 5). This is comparable to similar systems containing
Pt(terpyridyl)(acetylide)+ chromophores, MV2+, and TEOA
in CH3CN/H2O solution, which are capable of generating
MV+• within 5 min as well. In the current report, it was found
that MV+• generation in the system was dependent on TEOA
concentration, with an optimum [TEOA] of 27 mM, as
determined by monitoring the increasing [MV+•] at 604 nm
with time-resolved UV/vis spectroscopy (Figure S6 in the
Supporting Information). On the basis of the fact that
reductive quenching of complexes 1-3 by TEOA does not
occur, the concentration of MV+• in the system will approach
a limiting value with respect to the Pt complex chromophore
if the latter, once oxidized photochemically by MV2+, is not
reduced by TEOA.
It has been reported by Du et al. that visible-light
photolysis (λ > 410 nm) of Pt(terpyridyl)(acetylide)+ chro-
mophores in CH3CN/H2O in the presence of MV2+, TEOA,
and colloidal Pt leads to the photogeneration of H2.14,15
Analogous experiments with complexes 1-3 used in place
of Pt(terpyridyl)(acetylide)+ chromophores also lead to the
photogeneration of H2 but at much slower rates and in much
lower quantities (Figure 6). While the reported system with
Pt(terpyridyl)(acetylide)+ chromophores are capable of gen-
erating H2 in less than 30 min, at least two hours of
irradiation are required of the analogous systems with
complexes 1, 2, or 3 as sensitizers under the same experi-
mental conditions. Furthermore, the systems with complexes
In contrast to the absence of quenching with TEOA,
complexes 1-3 do undergo emission quenching with the aryl
amines N,N,N′,N′-tetramethylphenylenediamine (TMPD),
phenothiazine (PTZ), and N,N,N′,N′-tetramethylbenzidine
(TMB), which oxidize more easily than TEOA and are
known to reductively quench the emission of derivatives of
Ru(bpy)3 (bpy ) 2, 2′-bipyridine)55 and Pt(terpyridyl)-
2+
(acetylide)+ complexes.52 For complexes 1-3, reductive
quenching by TMPD, PTZ, and TMB in CH2Cl2 solution
follows Stern-Volmer behavior with kq values at or near
the diffusion-controlled limit (Figure 4, Table 7). That
complexes 1-3 are quenched by TMPD, PTZ, and TMB,
but not TEOA, can be conjectured in terms of ∆E°, values
of which are summarized in Table 8. For quenching by
TMPD, ∆E° is ∼0.8 V with kq values of ∼1.4 × 1010 M-1
s-1 for complexes 1-3. The emission quenching of com-
plexes 1-3 by PTZ and TMB, have a ∆E° that is less at
∼0.4 V, with slightly slower kq values of ∼8 × 109 M-1
(55) Calvert, J. M.; Caspar, J. V.; Binstead, R. A.; Westmoreland, T. D;
Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 6620–6627.
4314 Inorganic Chemistry, Vol. 48, No. 10, 2009

CNN Platinum(II) Acetylide Complexes
Figure 5. UV-vis monitoring of the generation of MV+• in a system that contains 2.2 × 10-5 M complex 1, 3.1 × 10-4 M MV2+, and 26.8 mM TEOA
in a CH2Cl2 solution, irradiating with λ > 410 nm light.
the absence of (1) reductive quenching by TEOA and (2)
subsequent electron transfer from reduced chromophore C-
to MV2+. It is also possible that unproductive back-electron
transfer between C+ and MV+• occurs more readily than with
the Pt terpyridyl acetylide systems and that reduction of C+,
produced upon oxidative quenching, proceeds more slowly
by TEOA than the corresponding reaction in the Pt terpyridyl
acetylide systems, since in those latter systems, the oxidized
chromophore is dicationic and more strongly oxidizing.
Figure 6. Hydrogen production using 2.2 × 10-5 M complex 3, (1.5-3.1)
× 10-4 M MV2+, and 26.8 mM TEOA in a CH3CN/H2O solution, irradiating
with λ > 410 nm light. Notice the induction period.
1-3 as chromophores yield only ∼12 turnovers (TNs per
mol of chromophore) of H2 after 12 h of photolysis, an
average of ∼1 TN/h of irradiation, which is much poorer
than results obtained with Pt(terpyridyl)(acetylide)+ chro-
mophores of more than 800 TNs of H2 after 20 h of
photolysis with λ > 410 nm.15 Moreover, systems containing
The consequence of these differences is a lower flux of
a CNN chromophore, 1, 2, or 3, in CH3CN/H2O solution
electrons from MV+• to the colloidal Pt catalyst which
required substantially longer irradiation times (∼35 min) to
functions essentially as a microelectrode for H2 genera-
generate the intense blue coloration of MV+• obtained in
tion.4,56,57 The induction period thus represents the time
analogous Pt(terpyridyl)(acetylide)+-based systems after only
needed to make the electrode potential on the Pt catalyst
5 min of irradiation. While the low TNs of H2 in systems
sufficiently negative for proton reduction to occur, and the
that contain 1 or 2 as sensitizers may be a possible
slow rate of H2 formation indicates that the electron flow
consequence of their low solubilities in CH3CN/H2O resulting
into the Pt catalyst via eq 14 is substantially lower than that
in heterogeneous systems, complex 3 has good solubility in
obtained in the Pt terpyridyl acetylide systems reported
CH3CN/H2O, and therefore its meager H2 photogeneration
earlier. While TEOA is capable of transferring a second and
cannot be rationalized in the same way.
highly reducing electron during its decomposition (eq 15),14
Instead, a different explanation can be drawn on the basis
it needs to lose one electron before this can happen. It thus
of consideration of a reaction scheme for hydrogen-generat-
appears that for productive hydrogen-generating systems on
ing systems containing Pt CNN chromophores (C) in eqs
9-15. Key differences of this scheme from that put forward
previously for the Pt terpyridyl acetylide chromophore are
(56) Miller, D.; McLendon, G. Inorg. Chem. 1981, 20, 950–953.
(57) Kiwi, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1979, 101, 7214–7217.
Inorganic Chemistry, Vol. 48, No. 10, 2009 4315

Schneider et al.
the basis of the Pt CNN chromophores, a more strongly
reducing sacrificial donor, one more easily oxidized than
TEOA, is needed.
(∼1 TN/h). The sluggish performance in the photogeneration
of H2 relates to a low flux of electrons into the Pt colloidal
catalyst, which in turn results from the absence of reductive
quenching and slow regeneration of the chromophore after
oxidative quenching.
Conclusions
The structural, electrochemical, and photophysical proper-
ties of two new cyclometalated Pt(II) CNN acetylide
complexes have been examined in detail and compared with
a closely related but previously reported complex. Two of
the complexes were characterized by single crystal X-ray
crystallography, exhibiting a distorted square planar coor-
dination geometry around the Pt(II) nucleus. The complexes
are brightly emissive in fluid CH2Cl2 solution and in the solid
state at ∼600 nm, and their emissions are assigned to a
3MLCT charge-transfer excited-state. In CH2Cl2 solution the
compounds undergo fast oxidative quenching by MV2+ and
fast reductive quenching by three aryl amine donors, with
quenching rates near the diffusion-controlled limit for both.
However, the complexes do not undergo reductive quenching
by TEOA. When complexes 1-3 are employed as photo-
sensitizers in a multiple component system containing MV2+,
colloidal Pt, and TEOA, a 2 h induction period for H2
formation is seen and only modest amounts of H2 are evolved
Acknowledgment. This work was supported by the
Department of Energy, Division of Basic Sciences (DE-
FG02-90ER14125). The authors also wish to acknowledge
the following support from the University of Rochester:
Graduate Assistance in Areas of National Need (GAANN)
Fellowships (J.S. and P.J.), Elon Huntington Hooker Fel-
lowship (P.D.), and Weissberger Memorial Fellowship (J.S.).
The authors also thank Prof. Todd Krauss for help in making
possible the excited-state lifetime measurements.
Supporting Information Available: Emission spectra, Stern-
Volmer plots, generation of MV+• as dependent on TEOA
concentration, emission spectra of quenching experiments, cyclic
voltammograms, and example calculations of TNs H2 vs mol of
chromophore. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC801947V
4316 Inorganic Chemistry, Vol. 48, No. 10, 2009