Synthesis, reactivity investigation, and X-ray diffraction structures of new platinum(II) compounds containing redox-active diphosphine ligands

Article abstract of DOI:10.1016/j.molstruc.2008.08.018

Substitution of the 1,5-cyclooctadiene (cod) ligand in PtCl₂(cod) (1) by the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) yields PtCl₂(bpcd) (2). Knoevenagel condensation of 2 with 9-anthracenecarboxaldehyde leads to the functionalization of the bpcd ligand and formation of the corresponding 2-(9-anthracenylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (abpcd) substituted compound PtCl₂(abpcd) (3), which is also obtained from the direct reaction of 1 with the abpcd ligand in near quantitative yield. The reaction of 3 with disodium maleonitriledithiolate (Na₂mnt) affords the chelating dithiolate compound Pt(mnt)(abpcd) (4). Compounds 2-4 have been fully characterized in solution by IR and NMR spectroscopies (¹H and ³¹P), and their molecular structures established by X-ray crystallography. The electrochemical properties of 2-4 have examined by cyclic voltammetry, and the nature of the HOMO and LUMO levels in these systems has been established by MO calculations at the extended Hückel level, the results of which are discussed with respect to electrochemical data and related diphosphine derivatives.

Full text of DOI:10.1016/j.molstruc.2008.08.018

Journal of Molecular Structure 919 (2009) 34–40
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, reactivity investigation, and X-ray diffraction structures of new
platinum(II) compounds containing redox-active diphosphine ligands
*
*
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:
Received 9 July 2008
Received in revised form 11 August 2008
Accepted 12 August 2008
Available online 29 August 2008
Substitution of the 1,5-cyclooctadiene (cod) ligand in PtCl2(cod) (1) by the diphosphine ligand 4,5-
bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) yields PtCl2(bpcd) (2). Knoevenagel condensa-
tion of 2 with 9-anthracenecarboxaldehyde leads to the functionalization of the bpcd ligand and forma-
tion of the corresponding 2-(9-anthracenylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione
(abpcd) substituted compound PtCl2(abpcd) (3), which is also obtained from the direct reaction of 1 with
the abpcd ligand in near quantitative yield. The reaction of 3 with disodium maleonitriledithiolate
(Na2mnt) affords the chelating dithiolate compound Pt(mnt)(abpcd) (4). Compounds 2–4 have been fully
characterized in solution by IR and NMR spectroscopies (1H and 31P), and their molecular structures
established by X-ray crystallography. The electrochemical properties of 2–4 have examined by cyclic vol-
tammetry, and the nature of the HOMO and LUMO levels in these systems has been established by MO
calculations at the extended Hückel level, the results of which are discussed with respect to electrochem-
ical data and related diphosphine derivatives.
Keywords:
Platinum(II) compounds
Diphosphine ligand
Redox chemistry
Crystal structure
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
compounds severely limits the generation of long-lived charge-
transfer species crucial for successful photosynthetic and OLED
Luminescent square-planar platinum(II) complexes continue to
command attention as pivotal building blocks for the synthesis of
photosynthetic molecular devices and as model systems for the
study of directional charge-transfer in dyad, triad, and higher or-
der arrays [1]. Rich photophysical behavior and highly efficient
luminescence have been reported in numerous Pt(diimine)(dithi-
olate) compounds prepared by Eisenberg and coworkers [2].
Structurally related but less thoroughly explored are platinum(II)
compounds based on a Pt(diphosphine)(dithiolate) architecture. A
few Pt(diphosphine)(dithiolate) derivatives have been shown to
display interesting solution and solid-state emissive properties,
strongly warranting additional studies on this genre of com-
pounds as a function of the ancillary diphosphine ligand [3].
One of the principal factors responsible for the disappointing
photophysical data (i.e., efficient non-radiative decay) generally
devices [4].
A major thrust of our research program involves the synthesis
and physical study of mono- and polynuclear complexes con-
taining the redox-active diphosphine ligand 4,5-bis(diphenyl-
phosphino)-4-cyclopenten-1,3-dione (bpcd) [5]. This ligand is
unique compared to common diphosphine ligands such as dppm
and dppe because it undergoes a facile one-electron reduction at
*
E1/2 = 0.73 V (in MeCN vs Ag/AgCl reference) at a
p -based LUMO
that is localized over the two carbonyl groups and the alkene
moiety of the dione ring [6]. The bpcd ligand and its relatives
have been shown to function as superb electron reservoirs in
chemical and electrochemical reduction reactions. The favorable
redox property extant in bpcd indicates that it and functional-
ized derivatives could, in theory, furnish new Pt(diphos-
phine)(dithiolate) compounds with a well-defined diphosphine-
based LUMO capable of promoting long-lived charge-transfer
state(s).
observed in Pt(diphosphine)(dithiolate) systems is the absence
*
p acceptor LUMO on the diphosphine ligand,
of a low-lying
which in turn radically changes the emissive behavior vis-á-vis
Recently, we have prepared a series of second-generation re-
dox-active ligands based on bpcd through the Knoevenagel con-
densation with selected aldehydes [7]. The union of the bpcd
platform with an additional redox-active appendage and/or
light-harvesting antenna can be advantageously employed in
the tuning of the electronic absorption and emission properties
in a given metal complex. The aldehydes used in the functionali-
zation of bpcd by us include 9-anthracenecarboxaldehyde, thio-
the aforementioned Pt(diimine)(dithiolate) complexes that pos-
*
p LUMO that is localized on the a-diimine residue. The ab-
sess a
sence of an energetically accessible LUMO in square-planar Pt(II)
* Corresponding authors. Present address: Oak Ridge National Laboratory
(X. Wang), Tel.: +1 940 565 3515 (M.G. Richmond).
E-mail addresses: wangx@ornl.gov (X. Wang), cobalt@unt.edu (M.G. Richmond).
0022-2860/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.molstruc.2008.08.018

S.W. Hunt et al. / Journal of Molecular Structure 919 (2009) 34–40
35
phene-2-carboxaldehyde, and ferrocenecarboxaldehyde, which
2.2. Synthesis of PtCl2(bpcd) from PtCl2(cod) and bpcd
To small Schlenk tube under argon was added 0.10 g
(0.27 mmol) of 1 and 0.13 g (0.28 mmol) of bpcd, followed by
25 mL of CH2Cl2 via syringe. The solution was stirred at room tem-
perature for 2 h, after which the volatiles were removed under vac-
uum to afford the desired product as a yellow solid. The crude
material was washed with hexane and then recrystallized from a
afford the new ligands 2-(9-anthracenylidene)-4,5-bis(diphenyl-
phosphino)-4-cyclopenten-1,3-dione (abpcd), 2-(2-thienylid-
ene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (tbpcd),
and 2-(ferrocenylidene)-4,5-bis(diphenylphosphino)-4-cyclopen-
ten-1,3-dione (fbpcd). The structures of these bpcd derivatives
are shown below. Our preliminary findings on the coordination
chemistry and redox reactivity of these systems have been pub-
lished [8–10].
a
O
O
O
Ph2P
Ph2P
Ph2P
Ph2P
Ph2P
Ph2P
S
O
O
O
Fe
abpcd
tbpcd
fbpcd
It is known that phosphorescence is strongly influenced by
spin–orbit coupling in the presence of a heavy metal such as
platinum, and this phenomenon, when combined with the
well-established chromophoric properties of an alternant aro-
matic hydrocarbon such as anthracene, has real potential to fur-
nish new photonic materials for use in broadband gain media,
communications, and data storage [11,12]. The synthesis of
mixture of CH2Cl2 and hexane to furnish 0.18 g of 2 (yield: 92%). IR
(CH2Cl2): m .
(CO) 1754 (m, sym dione), 1724 (s, antisym dione) cmꢁ1
1H NMR (CDCl3): d 3.49 (s, 2H, methylene), 7.01–7.98 (m, 20H,
1
aryl). 31P NMR (CDCl3): d 24.38 (s, JPtAP = 1836 Hz). UV–vis
(CH2Cl2): kmax 295 (sh,
e
= 6200). ESI-MS: m/z 769.27 for [2+K]+.
2.3. Synthesis of PtCl2 (abpcd)
new platinum compounds possessing highly conjugated
p arrays
is required if a thorough exploration of triplet-light-energy har-
vesting systems having commercially desirable near-IR behavior
is to be undertaken. Herein we present our results on the syn-
thesis, structural characterization, and redox properties of plati-
num(II) compounds containing the anthracene-modified ligand
2-(9-anthracenylidene)-4,5-bis(diphenylphosphino)-4-cyclopen-
ten-1,3-dione (abpcd).
2.3.1. Condensation reaction of 9-anthracenecarboxaldehyde and 2
A MeOH solution (ca. 50 mL) containing 0.10 g (0.14 mmol) of 2
and 28 mg (0.14 mmol) of 9-anthracenecarboxaldehyde under ar-
gon was treated with 16 mg (0.14 mmol) of potassium tert-butox-
ide. The reaction was allowed to stir for 2 h and was then checked
by TLC, which revealed the presence of a new spot (Rf = 0.14 in
CH2Cl2). Compound 3 was subsequently isolated by column chro-
matography over silica gel using CH2Cl2 as the eluent and then
recrystallized from a mixture of CH2Cl2 and hexane to afford
2. Experimental section
PtCl2(abpcd) in 75% yield (94 mg). IR (CH2Cl2):
sym dione), 1695 (s, antisym dione) cmꢁ1 1H NMR (CDCl3): d
7.35–8.72 (m, 21H, alkenyl and aryl). 31P NMR (CDCl3): d 27.45
m(CO) 1733 (w,
2.1. Materials and equipment
.
1
1
The PtCl2(cod) employed in our studies was prepared from
H2PtCl6ꢀxH2O and 1,5-cyclooctadiene (cod) according to the known
procedure [13], while the diphosphine ligands bpcd and abpcd
were synthesized from 4,5-dichloro-4-cyclopenten-1,3-dione and
Ph2PSiMe3 and from the condensation of bpcd with 9-anthracene-
carboxaldehyde, respectively [8,14]. The disodium maleonitriledi-
thiolate (Na2mnt) was prepared according the published
literature procedure [15]. The 1,5-cyclooctadiene and 9-anthracen-
ecarboxaldehyde were purchased from Aldrich Chemical Co. and
used as received. 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 [16]. The tetra-n-butylammonium perchlo-
rate (TBAP) electrolyte was purchased from Johnson Matthey
Electronics and recrystallized from hexane/ethyl acetate and dried
under vacuum for at least 30 h prior to use.
(s, JPtAP = 1835 Hz), 27.57 (s, JPtAP = 1835 Hz). UV–vis (CH2Cl2):
kmax 360 (b, = 9300), 390 ( = 4000), 516 (b, = 10,000). ESI-MS:
m/z 957.47 for [3+K]+.
e
e
e
2.3.2. Reaction of 1 with abpcd
This reaction was conducted in manner similar to that de-
scribed for the synthesis of 2 from 1 and bpcd. Here a mixture of
65 mg (0.10 mmol) of abpcd and 38 mg (0.10 mmol) of 1 in a small
Schlenk vessel was treated with 25 mL of CH2Cl2 and the solution
stirred at room temperature for 0.5 h, after which time TLC analysis
confirmed the complete consumption of the starting materials and
the presence of the desired product. Compound 3 was isolated as
described above in 93% yield (91 mg).
2.4. Synthesis of Pt(mnt)(abpcd)
The IR spectra were recorded on a Nicolet 20 SXB FT-IR spec-
trometer in amalgamated 0.1 mm NaCl cells. The 1H and 31P spec-
tra were recorded on a Varian 300-VXR spectrometer at 300 and
121 MHz, respectively. The 31P NMR spectra were collected in the
proton-decoupled mode and the reported chemical shifts refer-
enced to external H3PO4 (85%), taken to have d = 0. The ESI mass
spectra for compounds 2–4 were recorded at the UNT mass spec-
trometry facility in the positive ion mode using a matrix composed
of MeCN and KI.
To 0.05 g (0.054 mmol) of 3 in ca. 100 mL of MeOH was added
11 mg (0.059 mmol) of Na2mnt in one portion. The red slurry
was stirred overnight at 40 °C, affording a pinkish-red precipitate,
whose TLC revealed a single spot in CH2Cl2 (Rf = 0.31) correspond-
ing to the desired product. The solvent was removed under vac-
uum and the residue chromatographed over silica gel using
CH2Cl2 as the eluent, followed by recrystallization from CH2Cl2/
hexane to furnish Pt(mnt)(abpcd) pink solid in 57% yield (31 mg).

36
S.W. Hunt et al. / Journal of Molecular Structure 919 (2009) 34–40
IR (CH2Cl2):
m
(CN and CO) 2206 (m, nitrile), 1733 (w, sym dione),
crystals of 4 were grown from a solution containing toluene and
1695 (s, antisym dione) cmꢁ1
.
1H NMR (CDCl3): d 7.34–8.71 (m,
hexane at room temperature. The X-ray data for 2, 3ꢀ2.17CH2Cl2,
and 4ꢀtoluene were collected on an APEX II CCD-based diffractom-
eter at 100 K. The frames were integrated with the available APEX2
[20] software package using a narrow-frame algorithm, and the
structures were solved and refined using the SHELXTL program
package [21]. The molecular structures were checked using PLA-
TON [22], and all non-hydrogen atoms were refined anisotropi-
cally, with the exception of the two-site disorder found in the
anthracene group in 4ꢀtoluene that required isotropic refinement
of its carbon atoms with distance restraints. All hydrogen atoms
were assigned calculated positions and allowed to ride on the at-
tached carbon atom during the data reduction process. The refine-
ment for 2 converged at R1 = 0.0170 and wR2 = 0.0442 for 6254
21 H, alkenyl and aryl). 31P NMR (CDCl3):
d
34.65 (s,
1JPtAP = 1407 Hz). UV–vis (CH2Cl2): kmax 350 (sh,
e = 5600), 364
(sh,
e = 5100), 395 (sh, e = 4000), 524 (b, e = 11,000). ESI-MS: m/z
1027.20 for [4+K]+.
2.5. Electrochemical studies
All cyclic voltammetric data 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 was utilized as the working and auxiliary electrode, and the
reference electrode utilized a silver wire as a quasi-reference elec-
trode, with the reported potential data standardized against the
formal potential of the Cp2Fe/Cp2Fe+ (internally added) redox cou-
ple, taken to have E1/2 = 0.307 V [17].
independent reflection with I > 2
at R1 = 0.0497 and wR2 = 0.1384 for 7591 independent reflection
with I > 2
(I). Compound 4ꢀtoluene afforded convergence values
of R1 = 0.0655 and wR2 = 0.2234 for 4591 independent reflection
with I > 2 (I).
r(I), and 3ꢀ2.17CH2Cl2 converged
r
r
2.6. Extended Hückel MO calculations
3. Results and discussion
The extended Hückel calculations on compounds 2–4 were car-
ried out using the original program developed by Hoffmann [18] as
modified by Mealli and Proserpio [19]. The weighted Hij’s con-
tained in the program were used in the calculations, and the input
Z-matrices for 2–4 were constructed from the X-ray data of the
three compounds.
3.1. Synthesis, spectroscopic data, and diffraction structures of
compounds 2–4
Treatment of 1 with a slight excess of bpcd at room temperature
proceeds rapidly to give 2 (Scheme 1) in good yield after column
chromatography over silica gel and recrystallization. Compound
2 was characterized by IR and NMR spectroscopies, in addition to
ESI mass spectrometry and X-ray diffraction analysis. The two
2.7. X-ray crystallographic data
m
(CO) bands recorded at 1733 and 1695 cmꢁ1 in the IR spectrum
Tables 1 and 2 contain the X-ray data and processing parame-
ters and selected bond distances and angles, respectively, for the
three new products. Single crystals of 2 and 3 suitable for X-ray dif-
fraction analysis were grown at 0 °C from CH2Cl2 solutions contain-
ing each compound that had been layered with hexane, while the
represent the vibrationally coupled symmetric and antisymmetric
carbonyl stretching bands associated with the dione platform [23].
These dione frequencies are consistent with those IR data reported
by us in related mono- and polynuclear compounds with a chelat-
ing bpcd ligand [5,24]. The 1H NMR spectrum reveals resonances at
Table 1
X-ray crystallographic data and processing parameters for the platinum compounds 2–4
Compound
2
3
4
CCDC entry no.
T (K)
684901
100(2)
684902
100(2)
684903
100(2)
Cryst syst
Space group
Monoclinic
C2/c
Trigonal
R-3
Monoclinic
P2(1)/c
a (Å)
b (Å)
c (Å)
24.049(1)
14.8771(6)
17.6873(7)
123.630(1)
5269.1(4)
44.746(2)
44.746(2)
11.115(1)
19.633(3)
13.123(2)
22.102(2)
123.324(9)
4758.1(1)
C55H38N2O2P2PtS2
1080.02
b (°)
V (Å3)
19273(2)
C46.17H34.34Cl6.33O2P2Pt
1102.59
Molecular formula
fw
C29H22Cl2O2P2Pt
730.40
Formula units per cell (Z)
8
18
1.710
4
Dcalcd (Mg/m3)
1.841
1.508
k (MoK
a) (Å)
0.71073
5.677
0.0206
0.71073
3.785
0.1051
0.71073
3.148
0.0773
Absorption coefficient (mmꢁ1
Rmerge
)
Abs corr, max/min
Semi-empirical
0.5037/0.2285
33949
Semi-empirical
0.6579/0.5206
43872
Semi-empirical
0.8978/0.5921
28608
Total reflections
Independent reflections
6254
7591
4591
Data/res/parameters
6254/0/325
0.0170
7591/3/537
0.0497
4591/46/476
0.0655
a
R1 [I P 2
r
(I)]
Rw
0.0431
1.062
1.852, ꢁ0.510
0.1384
1.032
1.173, ꢁ1.096
0.2224
1.407
5.045, ꢁ1.564
wR2b(all data)
(max) and
D
q
D
q
(min) (e/Å3)
P
P
a
R1
R2
¼
¼
kFo j ꢁ j Fck= j Fo j.
ꢅ
ꢂ
ꢃꢄ
ꢂ
ꢃꢆ
1=2
ꢀ
ꢁ
ꢀ
ꢁ
2
2
P
P
w
F2o ꢁ F2c
w
Fo2
.
b

S.W. Hunt et al. / Journal of Molecular Structure 919 (2009) 34–40
37
Table 2
duces 3 in good yields. Carrying out the condensation reaction in
the presence of molecular sieves, whose function is to remove
the water that is produced and shift the equilibria to the function-
alized ligand, also works as reported in our original synthesis of the
abpcd ligand, but the yields of 3 have been found to be consistently
lower (ca. 25% conversion) as compared to the KOtBu promoted
condensation reactions. Alternatively, treatment of 1 with the pre-
formed abpcd ligand furnishes compound 3 in near quantitative
yield after chromatography and recrystallization.
Selected bond distances (Å) and angles (°) for platinum compounds 2–4
PtCl2(bpcd) (2)
Bond distances
Pt(1)AP(2)
Pt(1)ACl(2)
C(13)AC(17)
C(14)AC(15)
C(16)AC(17)
2.2177(6)
2.3460(5)
1.343(3)
1.514(3)
1.507(3)
Pt(1)AP(1)
Pt(1)ACl(1)
C(13)AC(14)
C(15)AC(16)
P(1)ꢀ ꢀ ꢀP(2)
2.2203(5)
2.3544(6)
1.514(3)
1.519(3)
3.1174(8)
Bond angles
The spectroscopic data recorded for 3 parallel those data found
P(2)APt(1)AP(1)
P(1)APt(1)ACl(2)
P(1)APt(1)ACl(1)
C(13)AP(1)APt(1)
C(17)AC(13)AP(1)
89.25(2)
178.99(2)
88.72(2)
106.02(7)
119.0(2)
P(2)APt(1)ACl(2)
P(2)APt(1)ACl(1)
Cl(2)APt(1)ACl(1)
C(17)AP(2)APt(1)
C(13)AC(17)AP(2)
91.03(2)
177.47(2)
91.02(2)
106.08(7)
119.4(2)
for 2. Here the symmetric and antisymmetric dione
1733 and 1695 cmꢁ1 in 3 are 21 and 29 cmꢁ1, respectively, lower in
frequency relative to the corresponding (CO) bands of the bpcd li-
m(CO) bands at
m
gand in 2 due to conjugation of the dione and anthracene moieties.
The inequivalent phosphines in 3 appear as singlets in the 31P NMR
PtCl2(abpcd) (3)
1
spectrum at d 27.45 and 27.57 and exhibit a JPAPt coupling of
Bond distances
Pt(1)AP(1)
Pt(1)ACl(1)
C(1)AC(2)
C(2)AC(3)
C(4)AC(6)
C(6)AC(7)
1835 Hz consistent with the formulated structure. The ESI mass
spectrum recorded in MeCN/KI gave a moderate intensity molecu-
lar ion at m/z 957.47 for [3+K]+. The molecular structure of
3ꢀ2.17CH2Cl2 is depicted in Fig. 1. The compound crystallizes as a
CH2Cl2 solvate in the R-3 space group with a molecule of 3 and
two interstitial CH2Cl2 molecules occupying general positions
within the unit cell. The third molecule of CH2Cl2 resides on a spe-
cial position with 1/6 site occupancy in the asymmetric unit of the
trigonal cell. The anthracenyl and dione rings are distinctly non-
planar, displaying a dihedral angle of 51.1(3)° for the planes de-
fined by the dione carbons atoms C(1)AC(5) and the fused aryl car-
bons C(7)AC(19) of the anthracene ring system in order to
minimize unfavorable van der Waals’ contacts between the O(2)
and H(9a) and H(6a) and H(19a) atoms.
2.216(2)
2.347(2)
1.34(1)
1.51(1)
1.33(1)
1.48(1)
Pt(1)AP(2)
Pt(1)ACl(2)
C(1)AC(5)
C(3)AC(4)
C(4)AC(5)
P(1)ꢀ ꢀ ꢀP(2)
2.217(2)
2.348(2)
1.51(1)
1.47(1)
1.48(1)
3.125(3)
Bond angles
P(1)APt(1)AP(2)
P(2)APt(1)ACl(1)
P(2)APt(1)ACl(2)
C(1)AP(1)APt(1)
C(2)AC(1)AP(1)
C(4)AC(6)AC(7)
89.65(8)
177.11(7)
90.22(8)
106.0(3)
118.9(6)
131.2(9)
P(1)APt(1)ACl(1)
P(1)APt(1)ACl(2)
Cl(1)APt(1)ACl(2)
C(2)AP(2)APt(1)
C(1)AC(2)AP(2)
87.47(8)
178.65(8)
92.67(8)
104.7(3)
119.8(6)
Pt(mnt)(abpcd) (4)
Bond distances
Pt(1)AP(1)
Pt(1)AS(2)
S(1)AC(7)
C(1)AC(2)
C(2)AC(3)
C(4)AC(6)
C(6)AC(35)
Replacement of the two chlorine groups in 3 by Na2mnt pro-
ceeds readily in MeOH and furnishes 4 as a pink-colored precipi-
2.270(5)
2.292(5)
1.70(2)
1.38(2)
1.48(2)
1.33(3)
1.48(3)
Pt(1)AP(2)
Pt(1)AS(1)
S(2)AC(8)
C(1)AC(5)
C(3)AC(4)
C(4)AC(5)
P(1)ꢀ ꢀ ꢀP(2)
2.283(5)
2.326(5)
1.72(2)
1.49(3)
1.55(3)
1.51(3)
3.147(6)
tate,
which
was
subsequently
isolated
by
column
chromatography over silica gel using CH2Cl2 as the eluent (Scheme
1). 4 was obtained in 57% yield after recrystallization from CH2Cl2/
hexane. Compound 4 appears to a be relatively air stable in the so-
lid-state but solutions containing 4 are less stable, with noticeable
decomposition observed in those solutions left exposed to the
atmosphere for several hours.
Bond angles
P(1)APt(1)AP(2)
P(2)APt(1)AS(2)
P(2)APt(1)AS(1)
C(7)AS(1)APt(1)
C(1)AP(1)APt(1)
C(2)AC(1)AP(1)
N(2)AC(9)AC(8)
87.5(2)
89.5(2)
176.8(1)
102.2(6)
106.7(6)
121(2)
P(1)APt(1)AS(2)
P(1)APt(1)AS(1)
S(2)APt(1)AS(1)
C(8)AS(2)APt(1)
C(2)AP(2)APt(1)
C(1)AC(2)AP(2)
N(1)AC(10)AC(7)
177.0(2)
93.1(2)
89.8(2)
102.6(6)
107.4(6)
117(1)
Compound 4 was thoroughly characterized by IR, 1H and 31P
NMR, ESI mass spectral, and X-ray diffraction analyses. A moderate
intensity
m
(CN) stretch for the nitriles of the mnt ligand was found
(CO) stretches recorded at 1733 and
at 2206 cmꢁ1 while the dione
m
175(2)
177(2)
1695 cmꢁ1 are unchanged relative to compound 3. A strong molec-
ular ion recorded at m/z 1027.20 in the ESI mass spectrum of 4 is
consistent with the formation of a potassium-containing ion
[4+K]+ and in keeping with the mass spectral behavior exhibited
by 2 and 3 (vide supra). The alkenyl and aryl hydrogens appear as
a set of overlapping multiplets from d 7.34–8.71 in the 1H NMR
spectrum, while the 31P NMR spectrum shows a singlet at d
34.65 with a geminal platinum–phosphorus coupling constant of
d 3.49 (2H) and 7.01–7.98 (20H) that are readily assigned to the
methylene group of the dione ring and the aryl hydrogens, respec-
tively, with the same sample exhibiting a singlet at d 24.38 in the
31P NMR spectrum. The accompanying 195Pt satellites in the 31P
NMR spectrum display a 1JPAPt coupling constant of 1836 Hz in ac-
cord with other diphosphine-chelated platinum(II) compounds in
the literature [25]. The ESI mass spectrum of 2 revealed a strong
m/z peak at 769.27 for the potassium-containing species [2+K]+.
The molecular structure of 2 was unequivocally established by
X-ray diffraction analysis, with the thermal ellipsoid plot of 2 de-
picted in Fig. 1 confirming the presence of a square-planar plati-
1
1407 Hz. The observed JPAPt value in 4 is ca. 428 Hz smaller than
the coupling constant found in compounds 2 and 3 and indicates
the existence of longer PtAP bonds in 4 vis-á-vis the halide-con-
taining compounds 2 and 3. This empirical observation was first
noted by Pidcock et al. in four- and six-coordinate phosphine-
substituted platinum complexes and was corroborated by X-ray
diffraction analyses for compounds 2–4 [27]. Fig. 2 shows the ther-
mal ellipsoid plot of 4ꢀtoluene and confirms the replacement of the
two chlorine groups by the mnt ligand. The Pt(1)AP(1) and
Pt(1)AP(2) bond lengths of 2.270(5) and 2.283(5) Å, respectively,
exhibit a mean length of 2.277 Å that is ca. 0.059 Å longer than
the mean distance computed for the PtAP bond lengths in 2 and
num center and
a chelating bpcd ligand. The Pt(1)AP(2)
[2.2177(6) Å] and Pt(1)AP(1) [2.2203(5) Å] bond distances and
89.25(2)° bond angle for the atoms P(2)APt(1)AP(1) are in agree-
ment with those bond distances and angles reported for the
diphosphined-substituted compounds PtCl2(dppe), PtCl2[(Z)-
Ph2CH = CHPPh2], and Pt(CN)2[1,2-(Ph2P)2C6H4] [26].
The anthracene-substituted platinum compound 3 is readily
prepared by several methods. Knoevenagel condensation between
9-anthracenecarboxaldehyde and 2 in the presence of KOtBu pro-
1
3, substantiating the aforementioned trend in the JPAPt coupling
constants for 2–4. The anthracenyl moiety is canted or tipped out
of the plane defined by the dione platform, leading to a dihedral

38
S.W. Hunt et al. / Journal of Molecular Structure 919 (2009) 34–40
CHO
Ph2
P
Ph2
O
bpcd
CH2Cl2
O
Cl
Cl
Cl
Cl
P
Cl
Cl
Pt
Pt
Pt
-H2O
P
P
Ph2
Ph2
O
O
1
2
3
Na2mnt
MeOH
Ph2
P
O
NC
NC
S
S
Pt
P
Ph2
O
4
Scheme 1.
Fig. 1. Perspective views of PtCl2(bpcd) (left) and PtCl2(abpcd)ꢀ2.17CH2Cl2 (right) showing the atom-numbering scheme. Displacement ellipsoids are drawn at the 50%
probability level and the H atoms are shown as small spheres of arbitrary radii. The solvent molecules in 3 have been omitted for clarity.
angle of 71.0(5)°, which is close to that dihedral angle found in 3
[28]. The bond distances and angles about the mnt ligand are con-
sistent with those data reported for Pt(mnt)(dppe),
Pt(mnt)(MeCN)2, Pt(mnt)(PPhMe2)2, and other related Pt(II) deriv-
atives [2c,3b,3d,29], with no additional comments required for this
structure.
3.2. Cyclic voltammetry and MO studies on compounds 2–4
The redox properties of our platinum(II) compounds were ex-
plored by cyclic voltammetry in CH2Cl2 containing 0.25 M TBAP
as the supporting electrolyte. The cyclic voltammogram (CV) of
2 recorded over the potential range of 1.0 V to ꢁ1.0 V and at a
scan rate of 250 mV/s revealed a single, diffusion-controlled wave
at E1/2 = ꢁ0.50 V readily attributed to a one-electron process [30].
The site of the 0/1ꢁ reduction was subsequently confirmed by ex-
Fig. 2. Perspective view of Pt(mnt)(abpcd)ꢀtoluene showing the atom-numbering
tended Hückel MO calculations, which revealed
ꢁ10.36 eV, whose parentage may be traced to a 4-based
on the dione ring. The calculated LUMO energy and its nodal pat-
a
LUMO at
scheme. Displacement ellipsoids are drawn at the 50% probability level and the H
atoms are shown as small spheres of arbitrary radii. The solvent molecule has been
omitted for clarity.
w
p
MO

S.W. Hunt et al. / Journal of Molecular Structure 919 (2009) 34–40
39
tern are in excellent agreement with those data reported for other
4. Conclusions
bpcd-substituted compounds [6,24b,31]. The relatively low en-
*
ergy of this
p orbital renders it a superb electron reservoir com-
The new platinum compounds 2–4 have been synthesized and
physically characterized by traditional solution spectroscopic
methods and X-ray diffraction analysis. Future work will be de-
voted to the photophysical investigation of 4, the results of which
will be made available in due course.
pared to the diphosphine ligands as dppe and (Z)-
Ph2PCH = CHPPh2 that do not possess an electrochemically acces-
*
sible
p
counterpart. The HOMO occurs at ꢁ11.96 eV and is best
described as a metal-based dxy orbital with minor antibonding
halide interactions [32].
Acknowledgments
Ph2
P
Ph2
P
O
Cl
Cl
Financial support from the Robert A. Welch Foundation (Grant
B-1093) is appreciated, and we extend thanks to Prof. Guido F. Ver-
beck for the use of his mass spectrometer and Ms. Nicole Ledbetter
for recording the ESI mass spectra for the compounds 2–4.
Pt
P
P
Ph2
Ph2
O
HOMO (-11.96 eV)
LUMO (-10.36 eV)
Appendix A. Supplementary information
X-ray crystallographic data, in CIF format, for the structure
The CV recorded for 3 mimicked that of 2 with a single reduc-
tion wave observed at ꢁ0.43 V for electron accession at the dione
platform of the abpcd ligand. The reduction assignment was ver-
ified by computational analysis, revealing a LUMO at ꢁ10.40 eV
identical to that found in 2. Interestingly, the HOMO exhibits an
energy of ꢁ11.67 eV and is essentially localized on the pendant
anathracenyl moiety and is similar to the HOMO computed by
us for the free abpcd ligand that exhibits an energy of
determinations of compounds
2
(684901), 3ꢀ2.17CH2Cl2
(684902), and 4ꢀtoluene (684903) have been deposited with the
Cambridge Crystallographic Data Center, CCDC. Copies of this
information may be obtained free of charge from the Director,
CCDC, 12 Union Road, Cambridge, CB2 1EZ (fax: +44 1223
336033); email: deposit@ccdc.cam.uk or at http://www.ccdc.
cam.ac.uk. Table 1. X-ray crystallographic data and processing
parameters for the platinum compounds 2–4.
ꢁ11.52 eV [8]. The HOMO is best described as a
p-based MO pos-
sessing idealized b2g symmetry, as shown below, and has been
shown to function as the HOMO for anthracene and its deriva-
tives in MO calculations performed at different levels of complex-
ity [33].
References
[1] (a) M. Hissler, J.E. McGarrah, W.B. Connick, D.K. Geiger, S.D. Cummins, R.
Eisenberg, Coord. Chem. Rev. 208 (2000) 115;
(b) D.R. Millin, J.J. Moore, Coord. Chem. Rev. 229 (2002) 113;
(c) B. Yin, F. Niemeyer, J.A.G. Williams, J. Jiang, A. Boucekkine, L. Toupet, H. Le
Bozec, V. Guerchais, Inorg. Chem. 45 (2006) 8584;
Ph2
P
O
Cl
Cl
(d) M. Kato, Bull. Chem. Soc. Jpn. 80 (2007) 287.
HOMO (-11.67 eV)
Pt
[2] (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;
P
Ph2
O
(d) J.M. Bevilacqua, R. Eisenberg, Inorg. Chem. 33 (1994) 1886;
(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.
The CV for 4 revealed a single reduction wave at E1/2 = ꢁ0.30 V
over the potential range of 1.0 V to ꢁ1.0 V. The composition and
orbital energies of the HOMO and LUMO levels in 4 were also estab-
lished by extended Hückel MO calculations. The LUMO (ꢁ10.57 eV)
is concentrated on the dione moiety and is unremarkable when
compared with the energy level and orbital composition of the
LUMO found in 2 and 3. The HOMO depicted below consists of a
[3] For reports on the synthesis, structural characterization, and photophysical
properties of Pt(diphosphine)(dithiolate) compounds, see: (a) R. Vicente, J.
Ribas, X. Solans, M. Font-Altaba, A. Mari, P. De Loth, P. Cassoux, Inorg. Chim.
Acta 132 (1987) 229;
(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;
pp–dp interaction between the mnt ligand and the platinum(II)
(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.
center, whose parentage derives from the out-of-phase union of
the 2b1 and dzy orbitals of the [mnt]2ꢁ and [Pt(abpcd)]2+ fragments,
respectively [34]. This particular dithiolate-Pt(II) interaction has
also been found by Eisenberg in the compounds Pt(mnt)(bpy),
Pt(mnt)(1,2-ethylenediamine), and related platinum derivatives
[2c]. The Hückel calculations on 4 show that the HOMO occurs at
ꢁ11.40 eV in excellent agreement with the computed energies of
ꢁ11.48 eV and ꢁ11.44 eV for the HOMO levels in Pt(mnt)(bpy)
and Pt(mnt)(1,2-ethylenediamine), respectively [2c,35].
[4] (a) S.-Y. Chang, J. Kavitha, J.-Y. Hung, Y. Chi, Y.-M. Cheng, E.Y. Li, P.-T. Chou, G.-
H. Lee, A.J. Carty, Inorg. Chem. 46 (2007) 7064;
(b) C. Baik, W.-S. Han, Y. Kang, S.O. Kang, J. Ko, J. Organomet. Chem. 691 (2006)
5900;
(c) 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.
[5] For bpcd ligand reactivity studies, see: (a) H. Shen, S.G. Bott, M.G. Richmond,
Organometallics 14 (1995) 4625;
(b) S.G. Bott, H. Shen, R.A. Senter, M.G. Richmond, Organometallics 22 (2003)
1953;
(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;
Ph2
P
O
HOMO (-11.40 eV)
(e) W.H. Watson, B. Poola, M.G. Richmond, J. Organomet. Chem. 691 (2006)
5579.
[6] (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;
P
Ph2
O

40
S.W. Hunt et al. / Journal of Molecular Structure 919 (2009) 34–40
(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.
[25] (a) A. Pidcock, R.E. Richards, L.M. Venanzi, J. Chem. Soc. A (1966) 1707;
(b) P.E. Garrou, Chem. Rev. 81 (1981) 229. and references therein..
[26] (a) B. Bovio, F. Bonati, G. Banditelli, Gazz. Chim. Ital. 115 (1985) 613;
(b) L.M. Engelhardt, J.M. Patrick, C.L. Raston, P. Twiss, A.H. White, Aust. J. Chem.
37 (1984) 2193;
[7] 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.
[8] W.H. Watson, B. Poola, M.G. Richmond, J. Chem. Crystallogr. 36 (2006) 715.
[9] W.H. Watson, D. Wiedenfeld, A. Pingali, B. Poola, M.G. Richmond, Polyhedron
26 (2007) 3577.
(c) W. Oberhauser, C. Bachmann, T. Stampfl, R. Haid, C. Langes, A. Rieder, P.
Brüggeller, Inorg. Chim. Acta 274 (1998);
(d) W. Oberhauser, C. Bachmann, P. Brüggeller, Inorg. Chim. Acta 238 (1995)
35;
(e) V. Pawlowski, H. Kunkely, C. Lennartz, K. Bohn, A. Vogler, Eur. J. Inorg.
Chem. (2004) 4242.
[10] W.H. Watson, B. Poola, M.G. Richmond, Polyhedron 26 (2007) 3585.
[11] (a) M. Thompson, MRS Bull. 32 (2007) 694;
(b) O. Elbjeirami, C.N. Burress, F.P. Gabbai, M.A. Omary, J. Phys. Chem. C 111
(2007) 9522.
[12] (a) W.Y. Heng, J. Hu, J.H.K. Yip, Organometallics 26 (2007) 6760;
(b) Y. Arai, T. Suzuki, Y. Ohishi, S. Morimoto, S. Khonthon, Appl. Phys. Lett. 90
(2007) 261110;
[27] G.G. Mather, A. Pidcock, G.J.N. Rapsey, J. Chem. Soc. Dalton Trans. (1973) 2095.
[28] The quoted dihedral angle is for the major component. Here the anthracenyl
ring in 4 is disordered in two orientations, with the major component having a
site occupancy of 0.64(2) and involving the atoms C(35)AC(48). The minor
component exhibits a site occupancy of 0.36(2) with atoms C(35B)AC(48B)
and displays a 59.0(5)° dihedral angle.
(c) V.W.-W. Yam, K.L. Cheung, S.K. Yip, N. Zhu, Photochem. Photobiol. Sci. 4
(2005) 149.
[13] D. Drew, J.R. Doyle, Inorg. Synth. 28 (1990) 346.
[14] (a) D.T. Mowry, J. Am. Chem. Soc. 72 (1950) 2535;
[29] (a) J.C. Fitzmaurice, A.M.Z. Slawin, D.J. Williams, J.D. Woollins, Polyhedron 9
(1990) 1561;
(b) D. Fenske, H.J. Becher, Chem. Ber. 107 (1974) 117;
(b) D. Fenske, H.J. Becher, Chem. Ber. 108 (1975) 2155.
[15] A. Davison, R.H. Holm, Inorg. Synth. 10 (1967) 8.
[16] D.F. Shriver, The Manipulation of Air-sensitive Compounds, McGraw-Hill, New
York, 1969.
(b) E.A.M. Geary, L.J. Yellowlees, L.A. Jack, I.D.H. Oswald, S. Parsons, N. Hirata,
J.R. Durrant, N. Robertson, Inorg. Chem. 44 (2005) 242.
[30] Electrochemical reversibility and electron stoichiometry were determined via
the ususal methods, see: P.H. Rieger, Electrochemistry, Chapman and Hall,
New York, 1994.
[17] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 1980.
[18] R. Hoffmann, W.H. Lipscomb, J. Chem. Phys. 36 (1962) 2179.
[19] C. Mealli, D.M. Proserpio, J. Chem. Ed. 67 (1990) 399.
[20] APEX2 Version 2.14, Bruker AXS, Inc. Copyright 2007, Madison, WI.
[21] SHELXTL Version 6.14, Bruker AXS, Inc. Copyright 2003, Madison, WI.
[22] A.L. Spek, PLATON – A Multipurpose Crystallographic Tool, Utrecht University,
Utrecht, The Netherlands, 2006.
[31] W.H. Watson, S. Kandala, M.G. Richmond, J. Chem. Crystallogr. 36 (2006) 813.
[32] T.A. Albright, J.K. Burdett, M.H. Whangbo, Orbital Interactions in Chemistry,
Wiley Interscience, New York, 1985.
[33] (a) R. Hoffmann, J. Chem. Phys. 39 (1963) 1397;
(b) K.E. Wise, R.A. Wheeler, J. Phys. Chem. 108 (1999) 8279.
[34] For some references dealing with the frontier orbitals of these fragments, see:
(a) T.A. Albright, R. Hoffmann, J.C. Thibeault, D.L. Thorn, J. Am. Chem. Soc. 101
(1979) 3801;
[23] N.B. Coltrup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman
Spectroscopy, Academic Press, New York, 1990.
(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;
[24] (a) H. Shen, S.G. Bott, M.G. Richmond, Inorg. Chim. Acta 241 (1996) 71;
(b) S.G. Bott, J.C. Wang, H. Shen, M.G. Richmond, J. Chem. Crystallogr. 29 (1999) 391;
(c) C.-G. Xia, S.G. Bott, M.G. Richmond, J. Chem. Crystallogr. 33 (2003) 681;
(d) S.G. Bott, H. Shen, M.G. Richmond, Struct. Chem. 12 (2001) 237;
(e) W.H. Watson, S. Kandala, M.G. Richmond, J. Chem. Cyrstallogr. 35 (2005) 157;
(f) W.H. Watson, S. Kandala, M.G. Richmond, J. Chem. Cyrstallogr. 36 (2006) 71.
(e) S. Alvarez, R. Vicente, R. Hoffmann, J. Am. Chem. Soc. 107 (1985) 6253.
[35] The subjacent highest occupied molecular orbital (SHOMO) in 4 occurs at
ꢁ11.72 eV and is orbitally identical to the HOMO found in 3.