Synthesis of the donor-acceptor ligand 2-(4-dimethylaminobenzylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (dbpcd) and X-ray diffraction structure of the platinum(II) compound PtCl2(dbpcd)·1.5CH2Cl2

Article abstract of DOI:10.1016/j.ica.2009.11.011

Knoevenagel condensation of 4-(dimethylamino)benzaldehyde with 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) gives the donor-acceptor ligand 2-(4-dimethylaminobenzylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (dbpcd). The reaction of dbpcd with PtCl₂(cod) affords the platinum(II) complex PtCl₂(dbpcd) in high yield. The free dbpcd ligand and PtCl₂(dbpcd) have been isolated and fully characterized in solution by IR and NMR spectroscopies, and the solid-state structure of PtCl₂(dbpcd) determined by X-ray diffraction analysis. PtCl₂(dbpcd), as the 1.5CH₂Cl₂ solvate, crystallizes in the triclinic space group P over(1, ?), a = 11.7412(7) ?, b = 12.0486(7) ?, c = 14.4781(9) ?, α = 82.866(1), β = 75.049(1), γ = 83.905(1), V = 1957.6(2) ?³, Z = 2, and D_calc = 1.678 mg/m³, R = 0.0291 and wR₂ = 0.0723 for 8315 reflections with I > 2σ(I). The molecular structure of PtCl₂(dbpcd)·1.5CH₂Cl₂ consists of a square-planar platinum architecture containing two chlorines and the ancillary dbpcd diphosphine ligand. The redox properties of the dbpcd ligand and PtCl₂(dbpcd) have been explored by cyclic voltammetry, and these data are discussed with respect to extended Hückel MO calculations.

Full text of DOI:10.1016/j.ica.2009.11.011

Inorganica Chimica Acta 363 (2010) 418–423
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis of the donor–acceptor ligand 2-(4-dimethylaminobenzylidene)-
4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (dbpcd) and X-ray
diffraction structure of the platinum(II) compound PtCl2(dbpcd)Á1.5CH2Cl2
c,
Silvia Atim a, Xiaoping Wang b, , Michael G. Richmond
*
*
a Department of Chemistry and Physics, Coastal Carolina University, Conway, SC 29528, United States
b Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, United States
c 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:
Received 3 May 2009
Received in revised form 26 October 2009
Accepted 10 November 2009
Available online 14 November 2009
Knoevenagel condensation of 4-(dimethylamino)benzaldehyde with 4,5-bis(diphenylphosphino)-4-
cyclopenten-1,3-dione (bpcd) gives the donor–acceptor ligand 2-(4-dimethylaminobenzylidene)-4,5-
bis(diphenylphosphino)-4-cyclopenten-1,3-dione (dbpcd). The reaction of dbpcd with PtCl2(cod) affords
the platinum(II) complex PtCl2(dbpcd) in high yield. The free dbpcd ligand and PtCl2(dbpcd) have been
isolated and fully characterized in solution by IR and NMR spectroscopies, and the solid-state structure
of PtCl2(dbpcd) determined by X-ray diffraction analysis. PtCl2(dbpcd), as the 1.5CH2Cl2 solvate, crystal-
Keywords:
ꢀ
lizes in the triclinic space group P1, a = 11.7412(7) Å, b = 12.0486(7) Å, c = 14.4781(9) Å,
a = 82.866(1),
Platinum compounds
Diphosphine ligand
Redox chemistry
MO calculations
Crystallography
b = 75.049(1),
for 8315 reflections with I > 2
c
= 83.905(1), V = 1957.6(2) Å3, Z = 2, and Dcalc = 1.678 mg/m3, R = 0.0291 and wR2 = 0.0723
r(I). The molecular structure of PtCl2(dbpcd)Á1.5CH2Cl2 consists of a
square-planar platinum architecture containing two chlorines and the ancillary dbpcd diphosphine
ligand. The redox properties of the dbpcd ligand and PtCl2(dbpcd) have been explored by cyclic voltam-
metry, and these data are discussed with respect to extended Hückel MO calculations.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
ligand have been fully described by several different groups, we
now have turned our attention toward the functionalization of
The synthesis and photophysical study of new inorganic and
organometallic compounds that can efficiently promote energy-
and electron-transfer reactions remain under are active investiga-
tion due to their importance as integral components in artificial
photosynthetic mimics, molecular wires, and directional charge-
transfer processes [1–4]. The preparation of new luminescent com-
plexes continues to command the attention of researchers in the
inorganic and materials communities, with square-planar plati-
num(II) compounds accounting for the vast majority of published
reports [5–7]. The ability to control or manipulate the physiochem-
ical properties of platinum(II) compounds through the tuning of
the HOMO and/or LUMO levels is critical if the excited-state prop-
erties of a molecule are to be successful exploited for applications
involving pH sensors and redox switches.
the dione ring of the parent diphosphine. Our goal of ligand mod-
ification is readily accomplished through the Knoevenagel conden-
sation reaction, with the diphosphine ligand serving as the active
hydrogen compound given the presence of the 1,3-dione moiety.
We have coupled the bpcd ligand with sundry aldehydes to pro-
duce the new, second-generation ligands depicted in Scheme 1
[12–14]. Depending on the extent of conjugation between the
dione platform and aldehyde appendage, these dyad arrays range
from yellow-orange to dark red in color and exhibit rich redox
behavior. Here the p*-based LUMO remains localized on the dione
platform in each case and it is the HOMO that is easily tuned via
the chosen aldehyde. For example, the ferrocene condensation product,
2-(ferrocenylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-
1,3-dione (fbpcd), reveals an iron-based HOMO and a 0/1+ redox
couple displaying an E1/2 value of 0.63 V. The HOMO–LUMO energy
gap may be further tuned and/or altered upon coordination of the
redox-active diphosphine ligand to a transition-metal compound,
especially if a filled metal orbital supplants the original ligand-
based HOMO upon metal coordination.
Herein we report the synthesis of the new diphosphine ligand
2-(4-dimethylaminobenzylidene)-4,5-bis(diphenylphosphino)-4-
cyclopenten-1,3-dione (dbpcd) from bpcd and 4-(dimethyl-
amino)benzaldehyde and its use as an ancillary ligand in the
The redox-active ligand 4,5-bis(diphenylphosphino)-4-cyclop-
enten-1,3-dione (bpcd) is of interest to our groups because of the
low-lying
p* orbital that renders this diphosphine an excellent
electron reservoir in chemical and electrochemical reactions
involving electron transfer [8–11]. Since the redox properties of
the parent ligand and organometallic compounds containing this
* Corresponding authors. Tel.: +1 940 565 3548 (M.G. Richmond).
E-mail addresses: wangx@ornl.gov (X. Wang), cobalt@unt.edu (M.G. Richmond).
0020-1693/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2009.11.011

S. Atim et al. / Inorganica Chimica Acta 363 (2010) 418–423
419
O
O
O
Ph2P
Ph2P
Ph2P
Ph2P
S
S
S
S
Ph2P
Ph2P
O
S
O
O
Fe
ttbpcd
tbpcd
fbpcd
Scheme 1.
3
platinum(II) compound PtCl2(dbpcd). The solid-state structure of
the latter compound has been crystallographically determined,
and the redox properties of these new systems have been investi-
gated by cyclic voltammetry. The oxidation and reduction waves
recorded by CV are discussed relative to MO calculations performed
at the extended Hückel level.
PPh2 aryl hydrogens), 8.11 (d, 2H, JH–H = 9 Hz). 31P NMR (CDCl3):
3
d À23.03 (AB quartet, JP–P = 67 Hz).
2.3. Synthesis of PtCl2(dbpcd)
To 0.36 g (0.60 mmol) of dbpcd in 20 mL of CH2Cl2 was added
0.22 g (0.60 mmol) of PtCl2(cod). The slurry was stirred at room
temperature and monitored by TLC using neutral alumina plates
until the starting materials were completely consumed (ca. 1 h).
The volatiles were next removed under vacuum and the desired
product purified by chromatography over neutral alumina using
CH2Cl2/ethyl acetate (4:1). PtCl2(dbpcd) was obtained as a red-or-
ange solid in 87% yield (0.45 g). Single crystals of PtCl2(dbpcd) suit-
able for X-ray diffraction analysis were grown at room temperature
from a CH2Cl2 solution of PtCl2(dbpcd) that had been layered with
2. Experimental
2.1. Materials and equipment
The PtCl2(cod) was prepared from chloroplatinic acid and
1,5-cyclooctadiene (cod) [15], while the 4,5-bis(diphenylphos-
phino)-4-cyclopenten-1,3-dione (bpcd) was synthesized from
4,5-dichloro-4-cyclopenten-1,3-dione and Ph2PSiMe3 [16,17]. The
4-(dimethylamino)benzaldehyde and potassium tert-butoxide
were purchased from Aldrich Chemical Co. and used as received.
The latter chemical was stored in the dry box when not in use.
All reaction solvents were distilled from an appropriate drying
agent under inert atmosphere or obtained from an Innovative
Technology solvent purification system. All distilled solvents were
handled via inert-atmosphere techniques, and when not in use
these solvents were stored in Schlenk vessels equipped with Teflon
stopcocks [18]. The tetra-n-butylammonium perchlorate (TBAP)
electrolyte was purchased from Johnson Matthey Electronics and
recrystallized from a 1:1 mixture of hexane/ethyl acetate and dried
under vacuum for at least 44 h prior to use.
The IR spectra were recorded on a Nicolet 6700 FT-IR spectrom-
eter in amalgamated NaCl cells capable of handling air-sensitive
samples. The 1H and 31P NMR spectral data were recorded at 500
and 201 MHz, respectively, on a Varian VXR-500 spectrometer.
The 31P NMR spectra were collected in the proton-decoupled mode
and the reported chemical shifts referenced to external H3PO4
(85%), taken to have d = 0. The ESI-APCI mass spectrum of
PtCl2(dbpcd) was recorded at the UNT mass spectrometry facility
in the positive ionization mode, using MeOH as the sample matrix.
hexane. IR (CH2Cl2):
dione) cmÀ1 1H NMR (CDCl3): d 3.07 (s, 6H, NMe2), 6.58 (d, 2H,
3JH–H = 9 Hz), 7.20–8.16 (m, 23H, alkenyl and aryl). 31P NMR
m(CO) 1726 (w, sym dione), 1668 (s, antisym
.
1
1
(CDCl3): d 25.94 (s, JPt–P = 3660 Hz), 27.51 (s, JPt–P = 3660 Hz).
ESI-MS: m/z 826.53 for [MÀCl]+ and 1686.73 for [2MÀCl]+.
2.4. Electrochemical studies
The quoted cyclic voltammetric data were recorded on a PAR
Model 273 potentiostat/galvanostat, equipped with positive feed-
back circuitry to compensate for IR drop. The cyclic voltammo-
grams were recorded under oxygen- and moisture-free
conditions in a homemade three-electrode cell. A platinum disk
was utilized as the working and auxiliary electrode, and the refer-
ence electrode utilized a silver wire as a quasi-reference electrode,
with the reported potential data standardized against the formal
potential of the Cp 2Fe/Cp 2Fe+ (internally added) redox couple, ta-
*
*
ken to have E1/2 = À0.20 V [19].
2.5. Extended Hückel MO calculations
The extended Hückel calculations on the dbpcd ligand and
PtCl2(dbpcd) were carried out using the original program devel-
oped by Hoffmann [20,21], as modified by Mealli and Proserpio
[22]. The weighted Hij’s contained in the program were used in
the calculations, and the input Z-matrices for the two compounds
dbpcd-H4 and PtCl2(dbpcd-H4) were constructed by using bond
distances and angles from the available X-ray structure. The phenyl
groups associated with the phosphorus atoms were replaced with
hydrogens and P–H bond distances of 1.41 Å were employed in our
calculations [23].
2.2. Synthesis of 2-(4-dimethylaminobenzylidene)-4,5-
bis(diphenylphosphino)-4-cyclopenten-1,3-dione (dbpcd)
To a large Schlenk flask under argon was charged 0.20 g
(0.43 mmol) of bpcd, 10 mL of CH2Cl2 and 10 mL of MeOH each
by syringe, after which 57 mg (0.52 mmol) of KOBut was intro-
duced in one portion. 64 mg (0.43 mmol) of 4-(dimethyl-
amino)benzaldehyde was added after 0.5 h and the solution was
then gently heated at 40–45 °C for 4 h with stirring. TLC analysis
at this point using neutral alumina plates and CH2Cl2/hexane
(9:1) as the eluent confirmed the consumption of the starting
materials and the presence of the desired product at Rf = 0.67.
The solvents were removed under vacuum and the crude residue
purified by column chromatography over neutral alumina using
the aforementioned eluent to afford the condensation product
2.6. X-ray crystallographic data
The X-ray data for PtCl2(dbpcd)Á1.5CH2Cl2 were collected on an
APEX II CCD-based diffractometer at 100(2) K. The frames were
integrated with the available APEX2 [24] software package using
a narrow-frame algorithm, and the structure was solved and re-
fined using the SHELXTL program package [25]. The phenyl groups
on P(2) were found disordered in two positions and refined accord-
dbpcd in 95% yield (0.24 g). IR (CH2Cl2):
m
(CO) 1713 (w, sym dione),
1662 (vs, antisym dione) cmÀ1
.
1H NMR (CDCl3): d 2.99 (s, 6H,
3
NMe2), 6.53 (d, 2H, JH–H = 9 Hz), 7.15–7.35 (m, 21H, alkenyl and

420
S. Atim et al. / Inorganica Chimica Acta 363 (2010) 418–423
Table 2
ingly with distance restraints. The molecular structure was
checked using PLATON [26], and all non-hydrogen atoms were
refined anisotropically. All hydrogen atoms were assigned calcu-
lated positions and allowed to ride on the attached carbon atom.
The refinement for PtCl2(dbpcd)Á1.5CH2Cl2 converged at
R = 0.0291 and wR2 = 0.0723 for 8315 independent reflection with
I > 2r(I). The X-ray data and processing parameters are reported
in Table 1, with selected bond distances and angles quoted in
Table 2.
Selected bond distances (Å) and angles (°) in PtCl2(dbpcd)Á1.5CH2Cl2.
Bond distances
Pt(1)–P(2)
Pt(1)–Cl(1)
N(1)–C(10)
N(1)–C(13)
C(1)–C(5)
C(3)–C(4)
C(4)–C(5)
P(1)Á Á ÁP(2)
Pt(1)–P(1)
Pt(1)–Cl(2)
N(1)–C(14)
C(1)–C(2)
C(2)–C(3)
C(4)–C(6)
C(6)–C(7)
2.217(1)
2.341(1)
1.346(5)
1.458(6)
1.530(5)
1.474(6)
1.463(6)
3.126(1)
2.2206(9)
2.3565(9)
1.445(6)
1.328(6)
1.510(5)
1.371(6)
1.426(6)
3. Results and discussion
3.1. Synthesis and spectroscopic properties of 2-(4-
dimethylaminobenzylidene)-4,5-bis(diphenylphosphino)-4-
cyclopenten-1,3-dione (dbpcd)
Bond angles
The Knoevenagel condensation between bpcd and 4-(dimethyl-
amino)benzaldehyde proceeds smoothly in MeOH solvent with a
measured excess of KOBut to furnish the new diphosphine ligand
reaction 2-(4-dimethylaminobenzylidene)-4,5-bis(diphenylphos-
phino)-4-cyclopenten-1,3-dione (dbpcd), whose structure is de-
picted below. The reaction requires KOBut because control
experiments conducted in the absence of KOBut gave only trace
amounts of products. The dbpcd ligand is readily isolated by col-
umn chromatography over neutral alumina as a red solid in essen-
tially quantitative yield. The choice of chromatographic support is
important in this case because the ligand was found to bind irre-
versibly to silica gel. The vibrationally coupled dione carbonyl
P(2)–Pt(1)–P(1)
P(1)–Pt(1)–Cl(1)
P(1)–Pt(1)–Cl(2)
C(1)–P(1)–Pt(1)
C(10)–N(1)–C(14)
C(6)–C(4)–C(5)
C(4)–C(6)–C(7)
P(2)–Pt(1)–Cl(1)
P(2)–Pt(1)–Cl(2)
Cl(1)–Pt(1)–Cl(2)
C(2)–P(2)–Pt(1)
C(10)–N(1)–C(13)
C(6)–C(4)–C(3)
89.59(4)
178.15(4)
90.02(3)
105.4(1)
121.2(4)
133.5(4)
134.6(4)
88.88(4)
176.83(4)
91.45(4)
105.7(1)
120.7(4)
119.4(4)
groups exhibit m
(CO) bands at 1713 (w,) and 1662 (vs) cmÀ1 and
lapping multiplet from d 7.15–7.35. The 31P NMR spectrum
value of 0.51
for the inequivalent PPh2 moieties in agreement with the formu-
these frequencies are in accord with other derivatives prepared
by us [12–14,27]. The 1H NMR spectrum recorded in CDCl3 con-
firmed the absence of the signature resonances of the bpcd meth-
ylene group and the formyl moiety of the 4-Me2NC6H4CHO. The
observed singlet at d 2.99 is readily assigned to NMe2 group, and
the two doublets at d 6.53 and 8.11, each of which integrates for
2H, represent the pairwise equivalent hydrogens on the pendant
benzylidene moiety. The aryl hydrogens associated with the phos-
phine moieties and the lone alkenyl hydrogen appear as an over-
reveals a classic AB quartet at d À23.03 with a J/
Dm
lated structure [28].
O
Ph2P
dbpcd
Ph2P
O
Table 1
NMe2
X-ray crystallographic data and processing parameters for PtCl2(dbpcd)Á1.5CH2Cl2.
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Triclinic
ꢀ
P1
11.7412(7)
12.0486(7)
14.4781(9)
82.866(1)
75.049(1)
83.905(1)
1957.6(2)
C39.5H34Cl5NO2P2Pt
988.96
3.2. Synthesis, spectroscopic data, and X-ray diffraction structure of
PtCl2(dbpcd)
a
(°)
The dbpcd ligand was next studied for its reactivity with
PtCl2(cod) since the desired compound PtCl2(dbpcd) can serve as
platform for further architectural modification through halide
replacement. Treatment of dbpcd with one equivalent of PtCl2(cod)
at room temperature gives the corresponding diphosphine-substi-
tuted compound PtCl2(dbpcd) without complications. PtCl2(dbpcd)
was conveniently isolated by chromatographic separation over
neutral alumina, after which the desired product was characterized
by a combination of IR and NMR spectroscopies, mass spectrome-
try, and X-ray diffraction analysis. The IR spectrum for
b (°)
c
(°)
V (Å3)
Molecular formula
Formula weight
Formula units per cell (Z)
Dcalc (mg/m3)
2
1.6788
0.71073
4.043
k (Mo K
l
a
)
) (Å)
(mmÀ1
Absorption correction
F(0 0 0)
Semi-empirical from equivalents
974
0.18 Â 0.12 Â 0.11
0.6739/0.5364
23486
8315
8315/15/508
0.0291
0.0723
1.119
1.924, À2.175
Crystal size (mm3)
Absorption correction factor
Total reflections
PtCl2(dbpcd) displays dione m
(CO) bands at 1726 and 1668 cmÀ1
that are shifted slightly to higher energy vis-á-vis the free ligand.
The 1H NMR resonances observed at d 3.07 and 6.58 are assigned
to the NMe2 group and a pair of aryl hydrogens on the benzylidene
ring, respectively. The remaining alkenyl and aryl hydrogens ap-
pear as a multiplet from d 7.20–8.16. The two 31P singlets recorded
at d 25.94 and 27.51 and their accompanying 195Pt satellites of
3660 Hz are consistent with the proposed structure and in keeping
with other platinum(II) compounds containing a chelating diphos-
Independent reflections
Data/restraints/parameters
a
R1 (I P 2
r
(I)]
b
wR2
Goodness-of-fit on F2
D
qmax
,
D
qmin (e Å3)
a
R1
R2 = {
=
R
R
||Fo| À |Fc||/
R|Fo|.
[w(F2o À F2c)2/
R .
[w(F2o)2]}1/2
b

S. Atim et al. / Inorganica Chimica Acta 363 (2010) 418–423
421
phine and two chlorine ligands [29]. The ESI mass spectrum of
PtCl2(dbpcd) recorded in MeOH gave a weak m/z peak at 826.53
[MÀCl]+ for the loss of a chloride ion from PtCl2(dbpcd), along with
a strong m/z peak at 1686.73 [2MÀCl]+ arising from the complex-
ation of [PtCl(dbpcd)]+ with PtCl2(dbpcd). The ability of the ESI-
MS technique to control aggregation states and guide chemical
synthesis as a function of the applied cone angle voltage is a
well-established phenomenon [30].
Fig. 1 shows the thermal ellipsoid plot of the molecular structure
of PtCl2(dbpcd), as the 1.5CH2Cl2 solvate, and confirms the coordi-
nation of the dbpcd ligand to a square-planar platinum(II) center.
The Pt–P and Pt–Cl bonds display a mean distance of 2.219 Å and
2.349 Å, respectively, and agree with those distances reported by
us for PtCl2(bpcd), PtCl2(fbpcd), and PtCl2(abpcd) [14,27]. The
P(1)–Pt(1)–P(2) bond angle of 89.59(4) is typical for this genre of
compound and is in excellent agreement with the P–Pt–P angle
exhibited by the aforementioned bpcd and bpcd-substituted Pt(II)
compounds and the related diphosphine-substituted compounds
PtCl2(dppe) and PtCl2[(Z)–Ph2PCH@CHPPh2] [31–33]. PtCl2(dbpcd)
exhibits an extended planar array of atoms involving the platinum
metal, the platinum-bound chlorine and phosphorus atoms, and the
À11.69 eV and À10.41 eV, respectively. The HOMO is best de-
scribed as a ligand-based orbital involving the benzylidene and
p
dione rings, as depicted below. In terms of the fragment contribu-
tions to the HOMO, molecular orbital overlap population (MOOP)
analysis indicated approximately 80% and 20% contributions from
the Me2N-substituted benzylidene and dione rings, respectively.
The orbital composition of the Me2N-substituted benzylidene ring
closely resembles the b1 HOMO reported for dimethylaniline, a fea-
ture consistent with perturbation molecular orbital (PMO) theory
[35], with the
consistent with the W3 MO exhibited by related 6e-
as hexatriene and maleic anhydride [36]. The * LUMO found for
p
system on the dione ring showing a nodal pattern
p
systems such
p
the new ligand is localized exclusively on the dione moiety with
small orbital contributions (<5%) from the PH2 moieties, as estab-
lished by MOOP analysis. The energy level and W4 parentage of
the LUMO are features that have already been described in the lit-
erature and require no additional comments [10–12].
H2P
LUMO (-10.41 eV)
p
system defined by the dione and benzylidene fragments. If the
four phenyl rings and all of the hydrogen atoms are excluded, the
remaining atoms are planar with a p = 0.02 Å. The bond distances
H2P
r
N
and angles associated with the dione and benzylidene moieties are
unremarkable and require no comment.
H2P
3.3. Cyclic voltammetric properties and MO calculations on dbpcd and
PtCl2(dbpcd)
HOMO (-11.69 eV)
The redox properties of the dbpcd ligand and PtCl2(dbpcd) were
next examined by cyclic voltammetry (CV) in CH2Cl2 containing
0.25 M TBAP as the supporting electrolyte. The CV of dbpcd (not
shown) recorded over the potential range of 1.0 V to À1.3 V and
at a scan rate of 200 mV/s revealed two diffusion-controlled waves
at Eap = 0.85 and À1.14 V, assigned to a multielectron oxidation pro-
cess and a reversible 0/1À redox couple, respectively [34]. The oxi-
dation wave showed no sign of reversibility up to a scan rate of
1.0 V/s and was not examined further. The site of these redox waves
was verified by extended Hückel MO calculations on the model
compound dbpcd-H4, which revealed HOMO and LUMO levels at
H2P
The CV of PtCl2(dbpcd) recorded at 200 mV/s in CH2Cl2 containing
0.25 M TBAP as the supporting electrolyte over the potential range
of 1.5 V to À1.0 V showed two distinct waves at E1/2 = 1.07 and
À0.65 V that are ascribed to the 0/1+ and 0/1À redox couples,
respectively. While the oxidation couple in PtCl2(dbpcd) exhibits
greater reversibility compared to oxidation wave recorded for
dbpcd, it is best viewed as quasi-reversible given the observed cur-
rent ratio (Icp=Iap) of 0.80. Increasing the scan rate up to 1.0 V/s led to
a slight enhancement in the reversibility of the oxidation based on
an Icp=Iap value of 0.90. The one-electron reduction was judged to be
reversible, on the basis of unity current ratios and current calibra-
tion against the one-electron standard ferrocene. Extended Hückel
calculations were performed to clarify the nature of the HOMO
and LUMO in PtCl2(dbpcd). The formal coordination of the PtCl2
fragment to dbpcd does not alter the nature of the HOMO in
PtCl2(dbpcd-H4), as its orbital composition and À11.69 eV energy
level are identical to the dbpcd ligand discussed previously. The
LUMO level in PtCl2(dbpcd) also mimics the LUMO computed for
dbpcd in all respects (i.e., a
The computational data suggest that a HOMO–LUMO transition
would involve a * excitation, and this prediction is supported
experimentally by UV–vis spectroscopy where PtCl2(dbpcd) shows
a solvent-independent visible band at 484 nm ( = 1480) that is
p* dione-based orbital at À10.41 eV).
p
? p
e
consistent with such an intraligand (IL) electronic transition
[37,38]. In comparison, the compound PtCl2(bpcd) exhibits a near-
UV absorbance at ca. 320 nm that has been ascribed to
Pt ?
sence of a long-wavelength IL transition in PtCl2(bpcd) viz-à-viz
PtCl2(dbpcd) underscores the importance of the extended system
in the latter compound and its ability to help tune the HOMO–
LUMO transition in this family of compounds.
p*(bpcd) MLCT, on the basis of MO calculations [27]. The ab-
p
Fig. 1. Thermal ellipsoid plot of PtCl2(dbpcd)Á1.5CH2Cl2 at the 50% probability level
with the hydrogen atoms shown as small spheres of arbitrary radii. The CH2Cl2
solvent has omitted for clarity.

422
S. Atim et al. / Inorganica Chimica Acta 363 (2010) 418–423
The HOMO-1 level for PtCl2(dbpcd) at À12.07 eV is of interest
4. Conclusions
because it is primarily a metal-based orbital (dz2 with some
dx2–y2 hybridization) that contains minor antibonding chlorine
contributions, as shown below. Conceptually speaking, a low-en-
ergy excitation from the HOMO-1 level would be expected to pop-
ulate the LUMO and give rise to a MLCT band in the near-UV
spectrum of PtCl2(dbpcd). The dynamics concerning the HOMO–
LUMO transition in the present case are easily manipulated, espe-
cially if the Me2N moiety in the ligand-based HOMO is chemically
altered by protonation or quaternization. The loss of the lone-elec-
tron pair on the nitrogen atom will lead to a decrease in the energy
of the HOMO, as predicted by PMO theory, and promote the
HOMO-1 level to that of the new HOMO. In fact, this prediction
has been demonstrated by performing MO calculations on the
methylated species [PtCl2(dbpcd-Me)]+, whose nitrogen contribu-
tion to the HOMO has been eliminated by methylation. The HOMO
in the cationic compound corresponds to the metal-based MO
shown below. This is important since the dbpcd-substituted com-
plex may be used as a pH sensor and multipurpose molecular
switch through the use of the different IL and MLCT spectral
responses.
The new diphosphine ligand 2-(4-dimethylaminobenzylidene)-
4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (dbpcd) has
been synthesized and used as a ligand in the preparation of the
platinum(II) compound PtCl2(dbpcd), whose solid-state structure
has been determined by X-ray crystallography. The redox properties
of dbpcd and PtCl2(dbpcd) have been explored by electrochemical
measurements, and these properties are correlated with the HOMO
and LUMO levels established by Hückel MO calculations. The
presence of the electron-donating 4-dimethylaminobenzylidene
moiety in the dbpcd ligand gives rise to an energetically accessible
HOMO in both the free dbpcd ligand and the title Pt(II) compound
relative to the parent diphosphine ligand (bpcd) and the PtCl2(bpcd).
Futurephotophysical and luminescence studies are planned, and the
use of PtCl2(dbpcd) as a precursor for the synthesis of more highly
conjugated, visible-light absorbing systems will be pursued. The
details from these studies will be disseminated in due course.
5. Supplementary material
CCDC 728874 contains the supplementary crystallographic data
for PtCl2(dbpcd)Á1.5CH2Cl2. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
H2
O
P
Acknowledgments
P
O
H2
N
Financial support from the Robert A. Welch Foundation (Grant
B-1093-MGR) is greatly appreciated. X. Wang acknowledges the
support by the US Department of Energy, Office of Science, under
Contract No. DE-AC05-00OR22725 managed by UT Battelle, LLC.
Ms. Nicole Ledbetter is thanked for recording the ESI mass spec-
trum of PtCl2(dbpcd).
HOMO-1 for PtCl2(dbpcd)
Finally, the behavior of PtCl2(dbpcd) may be contrasted with the
redox and MO data exhibited by the parent compound PtCl2(bpcd)
compound and the Knoevenagel-modified derivatives containing
the ferrocene-substituted ligand fbpcd (vide supra) and the ligand
2-(9-anthracenylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-
1,3-dione (abpcd) [27]. Table 3 summarizes the redox and MO data
for the related Pt(II) compounds. While the LUMO remains un-
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