New platinum compounds containing the diphosphine ligand 2-(ferrocenylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (fbpcd): Synthesis, redox behavior, and X-ray diffraction structures of PtCl2(fbpcd) and Pt(mnt)(fbpcd)

Article abstract of DOI:10.1016/j.poly.2008.09.005

The reaction of the redox-active diphosphine ligand 2-(ferrocenylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (fbpcd) with PtCl₂(1,5-cod) furnishes the platinum(II) compound PtCl₂(fbpcd) (2). Treatment of 2 with disodium

Full text of DOI:10.1016/j.poly.2008.09.005

Polyhedron 27 (2008) 3693–3699
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
New platinum compounds containing the diphosphine ligand
2-(ferrocenylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione
(fbpcd): Synthesis, redox behavior, and X-ray diffraction structures of PtCl2(fbpcd)
and Pt(mnt)(fbpcd)
,1
*
*
Bhaskar Poola, Sean W. Hunt, Xiaoping Wang , Michael G. Richmond
Department of Chemistry, University of North Texas, Denton, TX 76203, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of the redox-active diphosphine ligand 2-(ferrocenylidene)-4,5-bis(diphenylphosphino)-4-
cyclopenten-1,3-dione (fbpcd) with PtCl2(1,5-cod) furnishes the platinum(II) compound PtCl2(fbpcd)
(2). Treatment of 2 with disodium maleonitriledithiolate (Na2mnt) yields the chelating thiolate com-
pound Pt(mnt)(fbpcd) (3). Both 2 and 3 have been fully characterized in solution by IR, UV–Vis, and
NMR spectroscopies, and their molecular structures established by X-ray crystallography. The redox
properties of the fbpcd ligand and compounds 2 and 3 have been investigated by cyclic voltammetry,
and the composition of the HOMO and LUMO levels in these systems have been determined by extended
Hückel MO calculations, the results of which are discussed with respect to electrochemical data.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 16 June 2008
Accepted 13 September 2008
Available online 22 October 2008
Keywords:
Diphosphine ligand
Platinum compounds
Redox chemistry
MO calculations
1. Introduction
N)(S–S) complexes that possess a p* LUMO that is localized on
the
a-diimine residue. The absence of an energetically accessible
The synthesis and photophysical study of four-coordinate plat-
inum(II) compounds continue to command the attention of many
research groups. The commonly observed d8 electronic structure
associated with this genre of compounds gives rise to interesting
optoelectronic properties, electrical conductivity, and liquid crystal
behavior [1]. Several of these molecules have also been reported to
serve as suitable intercalating agents that function as probes for
the binding environment of DNA in model biological systems [2].
Prevalent among the Pt(II) compounds that have received long-
LUMO in the diphosphine ligand of Pt(diphosphine)(S–S) com-
pounds severely limits the generation of long-lived charge-transfer
species needed for photosynthetic and OLED devices [5]. Moreover,
the scarcity of Pt(diphosphine)(S–S) complexes that possess a low-
lying diphosphine-based p* LUMO has effectively prevented
researchers from systematically exploring the photophysical
behavior of this family of compounds.
Our groups have had a longstanding interest in the synthesis
and physical study of mono- and polynuclear complexes contain-
ing the redox-active diphosphine ligand 4,5-bis(diphenylphos-
phino)-4-cyclopenten-1,3-dione (bpcd) [6]. This particular ligand
undergoes a facile one-electron reduction at E1/2 = 0.73 V (in MeCN
term exploration are those that possess chelating
a-diimine and
dithiolate ligands. For example, Eisenberg and coworkers have pre-
pared sundry highly luminescent Pt(N–N)(S–S) derivatives (where
N–N =
a-diimine; S–S = chelating dithiolate) that exhibit direc-
versus Ag/AgCl reference) at a p
*-based LUMO that is localized over
tional charge transfer through a Pt(dithiolate) ? diimine manifold
[3].
the two carbonyl groups and the alkene moiety of the carbocyclic
ring [7]. Recently, we have prepared a series of second-generation
redox-active ligands based on bpcd by employing the Knoevenagel
condensation [8]. Here the reaction of 9-anthracenecarboxalde-
hyde, thiophene-2-carboxaldehyde, and ferrocenecarboxaldehyde
affords the new ligands 2-(anthracenyl-9-ylidene)-4,5-bis(diphen-
ylphosphino)-4-cyclopenten-1,3-dione (abpcd), 2-(2-thienylidene)-
4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (tbpcd), and
2-(ferrocenylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-
1,3-dione (fbpcd), respectively, whose structures are depicted in
Scheme 1. Some of our preliminary findings on the coordination
chemistry and redox reactivity of these systems have already been
published [9–11].
Structurally related Pt(diphosphine)(S–S) complexes have also
been synthesized and their solution and solid-state emissive prop-
erties investigated [4]. The absence of a low-lying p* acceptor
LUMO radically changes the emissive behavior of the diphos-
phine-substituted compounds vis-á-vis the aforementioned Pt(N–
* Corresponding authors. Tel.: +1 940 369 8489 (X. Wang); tel.: +1 940 565 3515;
fax: +1 940 565 4318 (M.G. Richmond).
E-mail addresses: xpwang@unt.edu (X. Wang), cobalt@unt.edu (M.G. Richmond).
Present address: Oak Ridge National Laboratory, P.O. Box 2008 MS6460, Oak
1
Ridge, TN 37831-6460, United States.
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.09.005

3694
B. Poola et al. / Polyhedron 27 (2008) 3693–3699
Ph2P
O
PPh2
O
bpcd
O
H
O
H
Fe
O
S
H
O
O
O
Ph2P
Ph2P
Ph2P
Ph2P
Ph2P
Ph2P
O
O
S
O
Fe
abpcd
tbpcd
fbpcd
Scheme 1.
Given the dearth of Pt(diphosphine)(S–S) compounds that pos-
sess an energetically accessible phosphine-based LUMO, we have
prepared and investigated the redox properties of Pt(fbpcd)(mnt)
through the use of cyclic voltammetry and molecular orbital calcu-
lations at the extended Hückel level. The electrochemical and MO
results are discussed with respect to related dithiolate-substituted
platinum compounds.
ligand fbpcd, followed by 20 mL of CH2Cl2 via syringe. The reaction
mixture was stirred at room temperature for ca. 0.5 h, and then sol-
vent was removed under vacuum and the residue washed with
2 ꢁ 10 mL portions of hexane to remove the liberated cod ligand.
The crude product was recrystallized from a 1:1 mixture of CH2Cl2/
hexane to afford 0.22 g (90% yield) of 2. IR (CH2Cl2):
sym dione), 1676 (vs, antisym dione) cmꢂ1. UV–Vis (CH2Cl2): kmax
356 ( = 12149), 584 (
= 2192). 1H NMR (CDCl3): d 4.22 (s, 5H, Cp),
m(CO) 1718 (w,
e
e
4.93 (s, br, 2H, Cp), 5.21 (s, br, 2H, Cp), 7.25–8.30 (m, 21H, aryls
and alkenyl). 31P NMR (CDCl3): d 26.51 (s, JPt–P = 1838 Hz), 27.44 (s,
JPt–P = 1838 Hz). ESI-MS (m/z): 949.27 [2 + Na]+.
2. Experimental
2.1. General
2.3. Synthesis of Pt(mnt)(fbpcd)
The PtCl2(cod) starting material was prepared from
H2PtCl6 ꢀ xH2O and 1,5-cyclooctadiene (cod) according to the
known procedure [12], and the fbpcd ligand synthesized via the
condensation reaction of bpcd and ferrocenecarboxaldehyde [13].
The disodium maleonitriledithiolate (Na2mnt) was prepared
according the published literature procedure [14]. The chemicals
cod and ferrocenecarboxaldehyde were purchased from Aldrich
Chemical Co. All reaction and NMR solvents were distilled from
an appropriate drying agent under argon using Schlenk techniques
and stored under argon in storage vessels equipped with high-vac-
uum Teflon stopcocks [15].
All IR spectra were recorded on a Nicolet 20 SXB FT-IR spectrom-
eter in amalgamated0.1 mmNaCl cells, while the absorptionspectra
were recorded on a HP 8425A diode array spectrometer using 1.0 cm
quartz UV–Vis cells that were equipped with a high-vacuum Teflon
stopcock to facilitate handling on the vacuum line. The 1H spectra
were recorded at 200 MHz on a Varian Gemini-200 spectrometer,
and the 31P NMR spectra were collected in the proton-decoupled
mode on a Varian 300-VXR spectrometer at 121 MHz, with the re-
ported chemical shifts referenced to external H3PO4 (85%), taken
to have d = 0. The ESI mass spectrum for 2 was recorded at the
UNT mass spectrometry facility in the positive ion mode using a ma-
trix composed of MeCN and 1% trifluoroacetic acid (UNT), while the
mass spectral data for 3 were recorded at the University of the Pacific
mass spectrometry facility in the negative ion mode.
To 0.10 g (0.11 mmol)of PtCl2(fbpcd)ina Schlenk tubewasadded
30 mL of acetone, after which 20 mg (0.11 mmol) of Na2mnt was
added in oneportion. The solutionwas stirred for 3 h and thenexam-
ined by TLC analysis, which revealed the desired product as a blue
spot at Rf = 0.40 using CH2Cl2 as the eluent. The solvent was removed
under vacuum and the crude product was purified by flash-column
chromatography over silica gel. The isolated product was recrystal-
lized from CH2Cl2 and hexane to afford compound 3 in 83% yield
(90 mg). IR (CH2Cl2):
dione), 1679 (s, antisym dione) cmꢂ1. UV–Vis (CH2Cl2): kmax 360
= 12288), 582 (
= 2750). 1H NMR (CDCl3): d 4.20 (s, 5H, Cp),
m(CN and CO) 2209 (m, nitrile), 1718 (w, sym
(e
e
4.93 (s, b, 2H, Cp), 5.20 (b, 2H, Cp), 7.45–7.95 (m, 21H, aryls and
alkenyl). 31P NMR (CDCl3): d 30.26 (s, JPt–P = 1420 Hz), 31.08 (s,
JPt–P = 1420 Hz). ESI-MS (m/z): 874.04 [3-CpFe]ꢂ.
2.4. X-ray diffraction data for clusters 2 and 3
Tables 1 and 2 contain the X-ray data and processing parame-
ters and selected bond distances and angles, respectively, for com-
pounds 2 and 3. Single crystals of 2 and 3 suitable for X-ray
diffraction analysis were grown at 5 °C from CH2Cl2 solutions con-
taining each compound that had been layered with hexane. The X-
ray data for 2 ꢀ CH2Cl2 and 3 ꢀ CH2Cl2 were collected on an APEX II
CCD-based diffractometer at 100 K. The frames were integrated
with the available APEX2 [16] software package using a narrow-
frame algorithm, and the structures were solved and refined using
the SHELXTL program package [17]. The molecular structures were
checked using PLATON [18], and all nonhydrogen atoms were refined
2.2. Synthesis of the PtCl2(fbpcd) (2)
To
a small Schlenk flask under argon was added 0.10 g
(0.27 mmol)of PtCl2(cod)and 0.17 g (0.27 mmol) of thediphosphine

B. Poola et al. / Polyhedron 27 (2008) 3693–3699
3695
Table 1
X-ray crystallographic data and processing parameters for compounds 2 ꢀ CH2Cl2 and 3 ꢀ CH2Cl2
CCDC entry number
Crystal system
Space group
a (Å)
b (Å)
c (Å)
683128
triclinic
P1
683129
monoclinic
P21/n
10.1301(5)
29.531(2)
14.5702(8)
ꢀ
10.112(1)
14.481(2)
14.585(2)
100.194(2)
101.582(2)
107.055(2)
1935.7(4)
C40H30Cl2FeO2P2Pt ꢀ CH2Cl2
1011.35
a
(°)
b (°)
106.700(1)
4174.9(4)
c
(°)
V (Å3)
Molecular formula
Fw
C44H30FeN2O2P2PtS2 ꢀ CH2Cl2
1080.63
4
Formula units per cell (Z)
2
Dcalc (Mg/m3)
1.735
1.719
k (Mo K
l
a
)
) (Å)
0.71073
4.379
0.71073
4.041
(mmꢂ1
Absorption correction
Absorption correction factor
Total reflections
numerical
0.7791/0.3968
21544
analytical
0.7936/0.4197
49948
Independent reflections
Data/restraints/parameters
7512
7512/0/460
0.0286
0.0698
1.063
8733
8733/0/514
0.0211
0.0453
1.035
R1a(I P 2
r(I)]
b
wR2
Goodness-of-fit on F2
D
q(max),
D
q
(min) (e/Å3)
2.807, ꢂ0.764
0.892, ꢂ0.450
P
P
a
R1
=
kFoj ꢂ jFck/ jFoj.
P
P
b
R2 = { [w(Fo2 ꢂ F2c)2]/ [w(F2o)2]}1/2
.
Table 2
Selected bond distances (Å) and angles (°) in compounds 2 ꢀ CH2Cl2 and 3 ꢀ CH2Cl2
Compound 2 ꢀ CH2Cl2
Bond distances
Pt(1)–P(1)
Pt(1)–Cl(1)
C(1)–C(2)
C(2)–C(3)
C(4)–C(6)
2.217(1)
2.352(1)
1.331(6)
1.526(5)
1.365(6)
1.427(6)
1.640(2)
Pt(1)–P(2)
Pt(1)–Cl(2)
C(1)–C(5)
C(3)–C(4)
C(4)–C(5)
2.218(1)
2.342(1)
1.518(6)
1.478(6)
1.468(6)
1.436(7)
1.659(2)
C(6)–C(7)
C(7)–C(11)
Cp(centroid)-Fea
Cp0(centroid)-Fea
Bond angles
P(1)–Pt(1)–P(2)
P(2)–Pt(1)–Cl(2)
P(2)–Pt(1)–Cl(1)
C(1)–P(1)–Pt(1)
C(2)–C(1)–P(1)
C(4)–C(6)–C(7)
89.42(4)
176.51(4)
88.64(4)
105.7(1)
119.0(3)
P(1)–Pt(1)–Cl(2)
P(1)–Pt(1)–Cl(1)
Cl(2)–Pt(1)–Cl(1)
C(2)–P(2)–Pt(1)
C(1)–C(2)–P(2)
89.94(4)
173.88(4)
92.34(4)
105.5(1)
119.9(3)
Compound 3 ꢀ CH2Cl2
Bond distances
Pt(1)–P(1)
Pt(1)–S(2)
S(1)–C(1)
N(1)–C(2)
C(1)–C(3)
C(3)–C(4)
C(29)–C(33)
C(31)–C(32)
C(32)–C(33)
Cp(centroid)-Feb
2.2589(6)
2.3027(6)
1.737(3)
1.145(3)
1.365(4)
1.426(4)
1.516(3)
1.482(3)
1.470(3)
1.641(1)
Pt(1)–P(2)
Pt(1)–S(1)
S(2)–C(3)
N(2)–C(4)
2.2572(7)
2.3047(6)
1.736(3)
1.152(3)
1.433(4)
1.339(3)
1.514(3)
1.356(3)
1.437(3)
1.655(1)
C(1)–C(2)
C(29)–C(30)
C(30)–C(31)
C(32)–C(34)
C(34)–C(35)
Cp0(centroid)-Feb
Bond angles
P(2)–Pt(1)–P(1)
P(1)–Pt(1)–S(2)
P(1)–Pt(1)–S(1)
C(1)–S(1)–Pt(1)
C(2)–C(1)–S(1)
N(1)–C(2)–C(1)
C(32)–C(34)–C(35)
88.46(2)
90.51(2)
174.08(2)
101.47(9)
116.4(2)
178.4(3)
131.9(2)
P(2)–Pt(1)–S(2)
P(2)–Pt(1)–S(1)
S(2)–Pt(1)–S(1)
C(3)–S(2)–Pt(1)
C(4)–C(3)–S(2)
N(2)–C(4)–C(3)
178.71(2)
90.69(2)
90.26(2)
101.46(9)
117.2(2)
178.9(3)
a
2 ꢀ CH2Cl2: Cp centroid defined by the atoms C(7)–C(11) and Cp0 centroid defined by the atoms C(12)–C(16).
3 ꢀ CH2Cl2: Cp centroid defined by the atoms C(35)–C(39) and Cp0 centroid defined by the atoms C(40)–C(44).
b
anisotropically. All hydrogen atoms were assigned calculated posi-
tions and allowed to ride on the attached carbon atom. The refine-
ment for 2 ꢀ CH2Cl2 converged at R1 = 0.0286 and wR2 = 0.0698 for
7512 independent reflection with I > 2
convergence values of R1 = 0.0211 and wR2 = 0.0453 for 8733 inde-
pendent reflection with I > 2r(I).
r
(I), with 3 ꢀ CH2Cl2 yielding

3696
B. Poola et al. / Polyhedron 27 (2008) 3693–3699
2.5. Electrochemical studies
Cl
Cl
Pt
All cyclic voltammograms were recorded on a PAR Model 273
potentiostat/galvanostat, equipped with positive feedback cir-
cuitry to compensate for iR drop. The homemade three-electrode
cell that was employed allowed the cyclic voltammograms to be
recorded under oxygen- and moisture-free conditions. A platinum
disk (area = 0.0079 cm2) was utilized as the working and auxiliary
electrode, and the reference electrode utilized a silver wire as a
quasi-reference electrode, with the reported potential data stan-
dardized against the formal potential of the Cp*2Fe/Cp2* Fe+ (inter-
nally added) redox couple, taken to have E1/2 = ꢂ0.20 V [19].
fbpcd
CH2Cl2
Ph2
O
P
Cl
Cl
2.6. Extended Hückel MO calculations
Pt
The extended Hückel calculations reported here were carried
out using the original program developed by Hoffmann [20], as
modified by Mealli and Proserpio [21]. The weighted Hij’s con-
tained in the program were used in the calculations, and the input
P
Ph2
O
Fe
Z-matrices for 2 and 3 were constructed from the available X-ray
data. The Z-matrix for the free ligand fbpcd was constructed from
the appropriate bpcd and ferrocene fragments.
Na2mnt
acetone
3. Results and discussion
3.1. Synthesis and spectroscopic characterization of compounds 2 and
3
Ph2
O
Treatment of PtCl2(cod) at room temperature in CH2Cl2 with an
equimolar amount of the diphosphine ligand 2-(ferrocenylidene)-
4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (fbpcd) fur-
nishes the expected product of cod displacement, namely
PtCl2(fbpcd). The structure of 2 is depicted in Scheme 2. Compound
2 was isolated in greater than 90% yield after recrystallization and
was characterized initially by traditional spectroscopic methods
and mass spectrometry. The IR spectrum reveals the presence of
two vibrationally coupled carbonyl bands associated with the
dione ring at 1718 (w) and 1676 (vs) cmꢂ1, whose frequencies
are in good agreement with the data reported by us for the iso-
meric Os3(CO)10(fbpcd) clusters containing chelating and bridging
fbpcd ligands [10]. The electronic absorption spectrum of 2 re-
corded in CH2Cl2 exhibits a moderately intense near UV–Vis absor-
bance at 356 nm in excellent agreement with the optical data
reported by Eisenberg and coworkers for sundry Pt(mnt)(diphos-
phine) complexes [4b], with the weak visible band at 584 nm as-
cribed to a d–d transition localized on the pendant ferrocene
moiety [22]. The four sets of 1H NMR resonances at d 4.22 (s, 5H,
Cp), 4.93 (s, br, 2H, Cp), 5.21 (s, br, 2H, Cp), 7.25–8.30 (m, 21H, aryls
and alkenyl) and the two inequivalent 31P singlets at d 26.51 and
27.44 are in accord with the formulated structure. The ESI mass
spectrum of PtCl2(fbpcd) displayed a prominent m/z peak at
949.27 consistent with the sodiated species [2 + Na]+. The solid-
state structure of 2, as the CH2Cl2 solvate, confirms the presence
of a four-coordinate, square-planar platinum center possessing
two ancillary chlorine groups and a chelating fbpcd ligand. Fig. 1
shows the thermal ellipsoid plot of 2. The Pt–P and Pt–Cl bonds
display a mean distance of 2.218 Å and 2.347 Å, respectively, and
agree with those bond distances in the related platinum(II) com-
pounds PtCl2[1,2-bis(diphenylphosphino)ethane], PtCl2[(Z)-1,2-
bis(diphenylphosphino)ethene], and derivatives based on diphos-
phine ligands having an ethano backbone [23]. The remaining bond
distances and angles are unremarkable and require no comment.
NC
NC
S
S
P
Pt
P
Ph2
O
Fe
Scheme 2.
The new dithiolate complex was isolated as a navy blue colored so-
lid that is stable under argon in the solid state but solutions con-
taining 3 that have been exposed to oxygen are less stable, with
decomposition observed after several hours. 3 was fully character-
ized by IR, UV–Vis, and NMR spectroscopies, and by ESI mass spec-
trometry and X-ray diffraction analysis. The IR spectrum of 3
recorded in CH2Cl2 revealed a moderately intense
m
(CN) stretch
(CO) bands
at 2209 cmꢂ1 for the two nitrile groups and a pair of
m
at 1718 and 1679 cmꢂ1 for the kinematically coupled dione groups
in the fbpcd residue [24]. The UV–Vis and 1H NMR spectral data for
3 parallel those data found in the dichloride precursor 2. The 31P
NMR spectrum of 3 exhibits two singlets at d 30.26 and 31.08, both
of which display a pair of satellites from geminal coupling with a
195Pt center. Interesting, the JP–Pt coupling of 1420 Hz recorded
1
in 3 is 418 Hz smaller than that found in 2, and strongly supports
the existence of longer Pt–P bond distances in compound 3 com-
pared to 2. This empirical trend was initially discovered by Pidcock
and coworkers in a series of four- and six-coordinated phosphine-
substituted platinum compounds and was corroborated by the X-
ray diffraction structures of compounds 2 and 3 [25]. Finally, 3
was examined by ESI mass spectrometry in the negative ion mode,
where a peak at m/e 874.04 corresponding to [3-CpFe]ꢂ was found.
Attempts to observe the molecular ion derived from 3 were unsuc-
cessful as a result of the facile loss of the ‘‘CpFe” fragment, a phe-
nomenon with ample literature precedent [26]. The X-ray
structure of 3, as the CH2Cl2 solvate, that is shown in Fig. 1 con-
The reaction of
2 with disodium maleonitriledithiolate
(Na2mnt) in acetone at room temperature afforded the correspond-
ing mnt-substituted complex Pt(mnt)(fbpcd) (3) in good yield after
flash-column chromatographic separation and recrystallization.

B. Poola et al. / Polyhedron 27 (2008) 3693–3699
3697
Fig. 1. Thermal ellipsoid plot of 2 ꢀ CH2Cl2 (left) and 3 ꢀ CH2Cl2 (right) showing the thermal ellipsoids at the 50% probability level. The CH2Cl2 solvent molecule in each plot has
been omitted for clarity.
firms the formal replacement of the chlorine ligands by the mnt
group. The two Pt–P bond lengths of 2.2589(6) Å [Pt(1)–P(1)] and
2.2572(7) Å [Pt(1)–P(2)] exhibit an average distance of 2.2581 Å
1
and account for the aforementioned trend in the JP–Pt coupling
constants in 2 and 3. The mnt and fbpcd moieties in 3 exhibit bond
distances and angles consistent with related derivatives containing
these ligands.
3.2. Cyclic voltammetric and MO data on the ligand fbpcd and
compounds 2 and 3
Given the rich redox history of the organometallic compounds
containing ferrocene, mnt, and bpcd ligands, we next investigated
the cyclic voltammetric behavior of the fbpcd ligand and com-
pounds 2 and 3. The CV data were recorded at platinum electrodes
in CH2Cl2 at room temperature using 0.2 M TBAP as the supporting
electrolyte. The CV of the free ligand fbpcd when scanned at a rate
of 250 mV/s over the potential range of ꢂ1.0 V to 1.5 V revealed
1.0
0.5
0.0
—0.5
—1.0
two reversible, diffusion-controlled processes at E1/2 = 0.63 and
ꢂ1.08 V for the 0/1+ and 0/1ꢂ redox couples [27]. The assignments
for these redox couples are straightforward and ascribed to an oxi-
dation and reduction of the ferrocene appendage [i.e., Fe(II) to
Potential (V)
Fig. 2. Anodic scan cyclic voltammogram of 2 (ca. 10ꢂ3 M) in CH2Cl2 containing
0.2 M TBAP at 0.25 V/s at 298 K.
Fe(III)] and dione moiety (i.e., eꢂ occupation of the
w
4-based p*
MO) moiety, respectively, in good agreement with the reported re-
dox potentials of related ferrocene derivatives and the parent li-
gand bpcd [7,10,28]. The assignments for the redox data were
also corroborated by extended Hückel MO calculations, which
yielded the expected ferrocene-based HOMO at ꢂ11.80 eV and a
p* LUMO at ꢂ10.40 eV at the dione platform of the fbpcd ligand.
Examination of 2 by CV in a manner identical to that of the free
fbpcd ligand revealed the presence of two well-defined redox
waves at E1/2 = 0.62 and ꢂ0.63 V, as depicted in Fig. 2. While the
oxidation potential of the ferrocene portion of the molecule re-
mains unchanged relative to the free fbpcd ligand, an anodic shift
of 0.45 V in the 0/1ꢂ redox couple is observed upon coordination of
the fbpcd ligand to the Pt(II) center. The electrochemical assign-
ments in 2 were verified by carrying out extended Hückel MO cal-
culations. The orbital composition and approximate energy of the
HOMO and LUMO levels in 2, which are shown below, are in keep-
ing with the nature of the ancillary fbpcd ligand. The HOMO occurs
at ꢂ11.82 eV and resembles the universally seen a1g metal-based
orbital for ferrocene and its derivatives [22a,29]. The LUMO in 2 oc-
of this p
* orbital makes it a superb electron reservoir vis-á-vis com-
mon diphosphine ligands such as dppe and (Z)-Ph2PCH@CHPPh2
that do not possess an electrochemically accessible
p
* counterpart.
Ph2
Cl
Cl
P
Pt
LUMO (-10.35 eV)
P
Ph2
Fe
Ph2
P
O
Cl
Cl
HOMO (-11.82 eV)
Pt
P
Ph2
O
curs at ꢂ10.35 eV and traces its parentage to w4 of the
p system
associated with the dione moiety [30]. The relatively low energy

3698
B. Poola et al. / Polyhedron 27 (2008) 3693–3699
The CV of 3 exhibited three redox waves when examined
4. Conclusions
over the potential range of ꢂ1.0 V to 1.5 V at a scan rate of
250 mV/s, with the one-electron reduction wave at E1/
2 = ꢂ0.57 V unambiguously assigned to an electron accession
at the dione residue. Unlike the straightforward assignment of
the oxidation in compound 2 that had single, well-defined oxi-
dation center, the diffusion-controlled waves at E1/2 = 0.65 (fully
reversible) and 1.27 (quasi-reversible) V in 3 could not be made
with certainty because dithiolate ligands in other four-coordi-
nate Pt(II) compounds have also been reported to undergo oxi-
dation in the same vicinity as the E1/2 value displayed by 3. The
reported oxidation values for Pt(N–N)(S–S) and Pt(P–P)(S–S)
compounds (where N–N = bpy and phen derivatives; S–
S = mnt, dmit, ecda, tdt; P–P = 2PR3, diphosphine) vary consider-
ably, ranging from ca. 0.30 V to 1.0 V depending on the donor/
acceptor properties of the dithiolate ligand. For example, the
platinum compounds Pt(dppe)(dmit) and Pt(bpy)(tdt) reveal a
0/1+ redox response at ca. 0.5 V that mirrors the oxidation po-
tential recorded for 3 [3f,4a]. Oxidation of the HOMO in this lat-
The synthesis and spectroscopic properties of the new platinum
compounds PtCl2(fbpcd) (2) and Pt(mnt)(fbpcd) (3) have been pre-
sented and discussed. The solid-state structures and the redox
properties of PtCl2(fbpcd) (2) and Pt(mnt)(fbpcd) (3) have been
established through a combination of electrochemical and compu-
tational methods. Future studies will focus on the photophysical
investigation of these and related compounds, and the luminescent
properties will be reported in due course.
Acknowledgments
Financial support from the Robert A. Welch Foundation (B-
1093-MGR) is much appreciated, and Prof. Guido F. Verbeck and
graduate student Ms. Nicole Ledbetter (UNT) and Prof. Andreas
H. Franz (University of the Pacific) are thanked for recording the
mass spectra of compounds 2 and 3, respectively.
ter genre of compounds is controlled exclusively by dp–pp
interactions between the ancillary dithiolate ligand and plati-
Appendix A. Supplementary data
num metal [3c].
CCDC 683128 and 683129 contain the supplementary crystallo-
graphic data for 2 ꢀ CH2Cl2 and 3 ꢀ CH2Cl2. These data can be ob-
tained free of charge via http://www.ccdc.cam.ac.uk/conts/
retrieving.html, or from the Cambridge Crystallographic Data Cen-
tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-
033; or e-mail: deposit@ccdc.cam.ac.uk. Cyclic voltammogram of 3
recorded in CH2Cl2 and 0.2 M TABP. Supplementary data associated
with this article can be found, in the online version, at doi:10.1016/
j.poly.2008.09.005.
The nature and orbital energies of the important HOMO and
LUMO levels in 3 were established by MO analysis, as illustrated
below. The LUMO in 3 is concentrated on the dione moiety and
is unremarkable in comparison to the energy level and orbital com-
position of the LUMO found in free fbpcd ligand and compound 2.
The HOMO is best viewed as arising from an out-of-phase union of
the 2b1 and dzy orbitals of the [mnt]2ꢂ and [Pt(fbpcd)]2+ fragments,
respectively [31]. This particular interaction accounts nicely for ob-
served pp–dp bond that is exhibited by the HOMO in compound 3
and that has been similarly found by Eisenberg in the compounds
References
Pt(mnt)(bpy), Pt(SPh)2(bpy), and other Pt(II) derivatives [3c]. In
fact, the filled-filled or 4eꢂ interaction in play between these
p-
[1] (a) C.N. Pettijohn, E.B. Jochnowitz, B. Chuong, J.K. Nagle, A. Vogler, Coord.
Chem. Rev. 171 (1998) 85;
based fragments allows one, in theory, to control or tune the en-
ergy level associated with the HOMO in this family of compounds
[32]. Our calculations on 3 reveal a HOMO energy of ꢂ11.45 eV
that is in excellent agreement with those HOMO energies reported
for Pt(mnt)(bpy) and Pt(SPh)2(bpy) [3c]. Accordingly, the observed
0/1+ redox process at E1/2 = 0.65 in 3 is assigned to the oxidation of
(b) C.J. Adams, N. Fey, J.A. Weinstein, Inorg. Chem. 45 (2006) 6105;
(c) M. Kato, Bull. Chem. Soc. Jpn. 80 (2007) 287;
(d) G.D. Batema, K.T.L. van de Westelaken, J. Guerra, M. Lutz, A.L. Spek, C.A. van
Walree, C. de Mello Donegá, A. Meijerink, G.P.M. van Klink, G. Van Koten, Eur. J.
Inorg. Chem. (2007) 1422;
(e) P. Du, J. Schneider, P. Jarosz, J. Zhang, W.W. Brennessel, R. Eisenberg, J. Phys.
Chem. B 111 (2007) 6887;
(f) B.W. Smucker, J.M. Hudson, M.A. Omary, K.R. Dunbar, Inorg. Chem. 42
(2003) 4714.
the mnt
p
*-based orbital. Finally, the Hückel calculations demon-
strate that the subjacent highest occupied molecular orbital (SHO-
MO) at ꢂ11.81 eV corresponds to the anticipated ferrocence-based
a1g molecular orbital, whose oxidation potential has been shifted to
more positive potential relative to compound 2 that is dithiolate
free.
[2] (a) F. Puntoriero, S. Campagna, M.L. Di Pietro, A. Giannetto, M. Cusumano,
Photochem. Photobiol. Sci. 6 (2007) 357;
(b) M. Cusumano, M.L. Di Pietro, A. Giannetto, Inorg. Chem. 46 (2006) 230.
[3] (a) J.A. Zuleta, C.A. Chesta, R. Eisenberg, J. Am. Chem. Soc. 111 (1989) 8916;
(b) J.A. Zuleta, M.S. Burberry, R. Eisenberg, Coord. Chem. Rev. 97 (1990) 47;
(c) J.A. Zuleta, J.M. Bevilacqua, D.M. Proserpio, P.D. Harvey, R. Eisenberg, Inorg.
Chem. 31 (1992) 2396;
(d) J.M. Bevilacqua, R. Eisenberg, Inorg. Chem. 33 (1994) 1886. 2913;
(e) S.D. Cummings, R. Eisenberg, Inorg. Chem. 34 (1995) 3396;
(f) S.D. Cummings, R. Eisenberg, J. Am. Chem. Soc. 118 (1996) 1949;
(g) W.B. Connick, D. Geiger, R. Eisenberg, Inorg. Chem. 38 (1999) 3264;
(h) J. Zhang, P. Du, J. Schneider, P. Jarosz, R. Eisenberg, J. Am. Chem. Soc. 129
(2007) 7726.
Ph2
NC
P
S
S
Pt
LUMO (-10.40 eV)
P
[4] (a) For reports on the synthesis, structural characterization, and photophysical
properties of Pt(diphosphine)(S–S) compounds, see: R. Vicente, J. Ribas, X.
Solans, M. Font-Altaba, A. Mari, P. De Loth, P. Cassoux, Inorg. Chim. Acta 132
(1987) 229;
NC
Ph2
Fe
(b) J.M. Bevilacqua, J.A. Zuleta, R. Eisenberg, Inorg. Chem. 33 (1994) 258;
(c) S. Takemoto, S. Kuwata, Y. Nishibayashi, M. Hidai, Inorg. Chem. 37 (1998)
6428;
(d) K.G. Landis, A.D. Hunter, T.R. Wagner, L.S. Curtin, F.L. Filler, S.A. Jansen-
Varnum, Inorg. Chim. Acta 282 (1998) 155;
(e) D.-Y. Noh, E.-M. Seo, H.-J. Lee, H.-Y. Jang, M.-G. Choi, Y.H. Kim, J. Hong,
Polyhedron 20 (2001) 1939;
(f) M. Nihei, M. Kurihara, J. Mizutani, H. Nishihara, J. Am. Chem. Soc. 125
(2003) 2964.
Ph2
P
O
HOMO (-11.45 eV)
[5] R. Saha, M.A. Qaium, D. Debnath, M. Younus, N. Chawdhury, N. Sultana, G.
Kociok-Köhn, L.-L. Ooi, P.R. Raithby, M. Kijima, Dalton Trans. (2005) 2760.
[6] (a) For bpcd ligand reactivity studies, see: H. Shen, S.G. Bott, M.G. Richmond,
Organometallics 14 (1995) 4625;
P
Ph2
O
Fe
(b) S.G. Bott, H. Shen, R.A. Senter, M.G. Richmond, Organometallics 22 (2003)
1953;

B. Poola et al. / Polyhedron 27 (2008) 3693–3699
3699
(c) W.H. Watson, G. Wu, M.G. Richmond, Organometallics 25 (2006) 930;
(d) W.H. Watson, S.G. Bodige, K. Ejsmont, J. Liu, M.G. Richmond, J. Organomet.
Chem. 691 (2006) 3609;
Chem. 37 (1984) 2193;
(d) M.H. Johansson, T. Malmstrom, O.F. Wendt, Inorg. Chim. Acta 316 (2001)
149;
(e) W.H. Watson, B. Poola, M.G. Richmond, J. Organomet. Chem. 691 (2006)
5579.
[7] (a) H.J. Becher, D. Fenske, M. Heymann, Z. Anorg. Allg. Chem. 475 (1981) 27;
(b) R. Meyer, D.M. Schut, K.J. Keana, D.R. Tyler, Inorg. Chim. Acta 240 (1995)
405;
(e) J. Powell, M.J. Horvath, A. Lough, J. Chem. Soc., Dalton Trans. (1996) 1679.
[24] The observation of only one nitrile stretch for the two nitrile groups in
compound 3, as opposed to a set of vibrationally coupled symmetric and
antisymmetric
m(CN) bands, is expected if the oscillator strength of the higher
energy (CN) symmetric stretch is weak. See: N.B. Coltrup, L.H. Daly, S.E.
m
(c) N.W. Duffy, R.R. Nelson, M.G. Richmond, A.L. Rieger, P.H. Rieger, B.H.
Robinson, D.R. Tyler, J.C. Wang, K. Yang, Inorg. Chem. 37 (1998) 4849.
[8] For a report on Knoevenagel condensation reactions using 4,5-dichloro-4-
cyclopenten-1,3-dione, the immediate precursor in the synthesis of the
diphosphine ligand bpcd, see: R. Alfred, Z. Hellmut, Chem. Ber. 94 (1961) 1800.
[9] W.H. Watson, B. Poola, M.G. Richmond, J. Chem. Crystallogr. 36 (2006) 715.
[10] W.H. Watson, D. Wiedenfeld, A. Pingali, B. Poola, M.G. Richmond, Polyhedron
26 (2007) 3577.
Wiberley, Introduction to Infrared and Raman Spectroscopy, Academic Press,
New York, 1990.
[25] G.G. Mather, A. Pidcock, G.J.N. Rapsey, J. Chem. Soc., Dalton Trans. (1973) 2095.
[26] (a) S. Barfuss, K.-H. Emrich, W. Hirschwald, P.A. Dowben, N.M. Boag, J.
Organomet. Chem. 391 (1990) 209;
(b) E. Bullita, S. Tamburini, P.A. Vigato, M. Carbini, S. Catinella, P. Traldi, Rapid
Commun. Mass Spectrom. 10 (1996) 490.
[27] (a) Electrochemical reversibility and electron stoichiometry were determined
via the ususal methods, see: A.J. Bard, L.R. Faulkner, Electrochemical Methods,
Wiley, New York, 1980;
[11] W.H. Watson, B. Poola, M.G. Richmond, Polyhedron 26 (2007) 3585.
[12] D. Drew, J.R. Doyle, Inorg. Synth. 28 (1990) 346.
[13] W.H. Watson, B. Poola, M.G. Richmond, Polyhedron 26 (2007) 3585.
[14] A. Davison, R.H. Holm, Inorg. Synth. 10 (1967) 8.
[15] D.F. Shriver, The Manipulation of Air-Sensitive Compounds, McGraw-Hill, New
York, 1969.
[16] APEX2 Version 2.14, Bruker AXS, Inc. Copyright 2007, Madison, WI.
[17] SHELXTL Version 6.14, Bruker AXS, Inc. Copyright 2003, Madison, WI.
[18] A.L. Spek, PLATON – A Multipurpose Crystallographic Tool, Utrecht University,
Utrecht, The Netherlands, 2006.
[19] M.F. Ryan, D.E. Richardson, D.L. Lichtenberger, N.E. Gruhn, Organometallics 13
(1994) 1190.
(b) P.H. Rieger, Electrochemistry, Chapman and Hall, New York, 1994.
[28] (a) G. Gritzner, J. Kuta, Pure Appl. Chem. 56 (1984) 461;
(b) I. Ratera, C. Sporer, D. Ruiz-Molina, N. Ventosa, J. Baggerman, A.M.
Brouwer, C. Rovira, J. Veciana, J. Am. Chem. Soc. 129 (2007) 6117;
(c) S.M. Batterjee, M.I. Marzouk, M.E. Aazab, M.A. El-Hashash, Appl.
Organomet. Chem. 17 (2003) 291.
[29] (a) J.W. Lauher, R. Hoffmann, J. Am. Chem. Soc. 98 (1976) 1729;
(b) P. Zanello, G. Opromolla, F.F. de Biani, A. Ceccanti, G. Giorgi, Inorg. Chim.
Acta 225 (1997) 47;
(c) M. Lein, J. Frunzke, A. Timoshkin, G. Frenking, Chem. Eur. J. 7 (2001) 4155.
[30] T.A. Albright, J.K. Burdett, M.H. Whangbo, Orbital Interactions in Chemistry,
Wiley-Interscience, New York, 1985.
[20] (a) R. Hoffmann, W.H. Lipscomb, J. Chem. Phys. 36 (1962) 2179;
(b) R. Hoffmann, J. Chem. Phys. 39 (1963) 1397.
[21] C. Mealli, D.M. Proserpio, J. Chem. Ed. 67 (1990) 399.
[22] (a) Y.S. Sohn, D.N. Hendrickson, H.B. Gray, J. Am. Chem. Soc. 93 (1971) 3603;
(b) G.L. Geoffroy, M.S. Wrighton, Organometallic Photochemistry, Academic
Press, New York, 1979.
[23] (a) W. Oberhauser, C. Bachmann, P. Brüggeller, Inorg. Chim. Acta 238 (1995)
35;
[31] (a) For some references dealing with the frontier orbitals of these fragments,
see: T.A. Albright, R. Hoffmann, J.C. Thibeault, D.L. Thorn, J. Am. Chem. Soc. 101
(1979) 3801;
(b) T.A. Albright, R. Hoffmann, Y.-C. Tse, T. O’Ottavio, J. Am. Chem. Soc. 101
(1979) 3812;
(c) R. Hoffmann, Angew. Chem., Int. Ed. Engl. 21 (1982) 711;
(d) T.A. Albright, Acc. Chem. Res. 15 (1982) 149;
(b) W. Oberhauser, C. Bachmann, T. Stampfl, R. Haid, C. Langes, A. Rieder, P.
Brüggeller, Inorg. Chim. Acta 274 (1998) 143;
(c) L.M. Engelhardt, J.M. Patrick, C.L. Raston, P. Twiss, A.H. White, Aust. J.
(e) S. Alvarez, R. Vicente, R. Hoffmann, J. Am. Chem. Soc. 107 (1985) 6253.
[32] K.G. Caulton, New J. Chem. 18 (1994) 25.