Knoevenagel condensation of the diphosphine ligand 4,5- bis(diphenylphosphino)-4-cyclopentene-1,3-dione with 9-anthracenecarboxaldehyde: Synthesis of the second-generation ligand 2-(anthracen-9-ylidene)-4,5- bis(diphenylphosphino)-4-cyclopentene-1,3-dione

Article abstract of DOI:10.1007/s10870-006-9125-1

The Knoevenangel condensation between 9-anthracenecarboxaldehyde and the diphosphine ligand 4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-dione (bpcd) takes place rapidly in CH₂Cl₂/MeOH solution in the presence of molecular sieves (4

Full text of DOI:10.1007/s10870-006-9125-1

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C
Journal of Chemical Crystallography, Vol. 36, No. 11, November 2006 ( 2006)
DOI: 10.1007/s10870-006-9125-1
Knoevenagel condensation of the diphosphine ligand
4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-dione with
9-anthracenecarboxaldehyde: Synthesis of the
second-generation ligand 2-(anthracen-9-ylidene)-4,5-
bis(diphenylphosphino)-4-cyclopentene-1,3-dione
William H. Watson,(1) Bhaskar Poola,(2) and Michael G. Richmond(2)∗
Received January 29, 2006; accepted June 23, 2006
Published Online August 18, 2006
TheKnoevenangel condensation between9-anthracenecarboxaldehydeand thediphosphine
ligand 4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-dione (bpcd) takes place rapidly in
˚
CH2Cl2/MeOH solution in the presence of molecular sieves (4 A) to produce the func-
tionalized ligand 2-(anthracen-9-ylidene)-4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-
dione. The title compound has been isolated and characterized in solution by IR, NMR,
and UV-vis spectroscopies, and the solid-state structure has been established by X-ray
diffraction analysis. 2-(anthracen-9-ylidene)-4,5-bis(diphenylphosphino)-4-cyclopentene-
˚
˚
1,3-dione crystallizes in the triclinic space group P − 1, a = 10.227(2) A, b = 13.865(2) A,
◦
◦
◦
3
˚
˚
c = 15.905(2) A, α = 112.157(2) , β = 101.424(2) , γ = 100.065(3) , V = 1968.5(5) A ,
Z = 2, and dcalc = 1.101 Mg/m3; R = 0.0873, Rw = 0.2604 for 7452 reflections with I>2σ(I).
The cyclic voltammetric behavior for 2-(anthracen-9-ylidene)-4,5-bis(diphenylphosphino)-
4-cyclopentene-1,3-dione has been studied, and the observed redox data and results from
extended Hu¨ckel MO calculations are discussed relative to the parent ligand bpcd.
KEY WORDS: diphosphine ligands; Knoevenagel condensation reaction; redox chemistry; MO calculations.
Introduction
the last decade.1 Coordination of this ligand to
polynuclear cluster systems has furnished un-
The coordination chemistry of the redox
equivocal evidence for the dynamic isomerization
of the bpcd ligand between bridging and chelating
binding modes at an Os3 cluster polyhedron,2 in
addition to serving as a model probe ligand for the
study on the deactivation paths that diphosphine
ligands participate in during productive cataly-
sis.2,3
The parent ligand bpcd also exhibits
interesting redox chemistry. Here a relatively
low-lying π∗ LUMO that is localized on the two
-active diphosphine ligand 4,5-bis(diphenylpho-
sphino)-4-cyclopentene-1,3-dione (bpcd) has
been extensively investigated by our groups over
(1)
Department of Chemistry, Texas Christian University, Fort Worth,
TX 76129.
(2)
Department of Chemistry, University of North Texas, Denton,
TX 76203.
∗
To whom correspondence should be addressed; e-mail: cobalt@
unt.edu.
715
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1074-1542/06/1100-0715/0 2006 Springer Science+Business Media, Inc.

716
Watson, Poola, and Richmond
carbonyl moieties and the carbon-carbon alkene
bond has been verified by EPR spectroscopy and
MO calculations.4 This LUMO bears a close
resemblance to the ψ4 LUMO exhibited by the
related 6π-electron systems maleic anhydride
and hexatriene.5 Modification of the electronic
and redox properties of the parent ligand would
give rise to a second-generation of diphosphine
ligands that could be used for the synthesis of new
organometallic compounds. Such second-
generation systems, especially ones that contain
highly conjugated π arrays, would allow us the
opportunity to construct novel photosynthetic
mimics for light-to-chemical applications6 and
electroluminscent dyes.7 Our goal of bpcd
modification has been achieved through the
Knoevenagel condensation reaction of bpcd and
various aldehydes. To our knowledge, this
protocol has not been employed for the func-
tionalization of the bpcd ligand.8 Our choice of
9-anthracenecarboxaldehyde for our initial study
was driven, in part, by its aromaticity and
cocks.10 The tetra-n-butylammonium perchlo-
rate (TBAP) electrolyte was purchased from
Johnson Matthey Electronics and recrystallized
from ethyl acetate/hexane (1:1), followed by
drying for at least 30 h under high vacuum.
The reported melting point was recorded on
a Meltemp apparatus in a sealed tube and is
uncorrected.
The infrared spectrum was recorded on a
Nicolet 20 SXB FT-IR spectrometer in a 0.1 mm
NaCl cell, using PC control and OMNIC soft-
ware, with the 1H and 31P NMR spectra recorded
at 200 MHz on a Varian Gemini-200 spectrometer
and 121 MHz on a Varian 300-VXR spectrometer,
respectively. The reported 31P chemical shifts are
referenced to external H3PO4 (85%), taken to have
δ = 0.
Reaction of 9-anthracenecarboxaldehyde
with bpcd
To 0.20 g (0.43 mmol) of bpcd in 30 mL
of CH2Cl2 was added under argon flush 89 mg
(0.43 mmol) of 9-anthracenecarboxaldehyde, ca.
well-known
excited-state
photochemistry.
Herein we report our data on the synthesis,
characterization, and redox properties of the
new diphosphine ligand 2-(anthracen-9-ylidene)-
4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-
dione.
˚
5 g of molecular sieves (4 A), and 5 mL of MeOH.
The reaction solution was stirred overnight at
room temperature and then examined by TLC
analysis, which revealed the near complete con-
sumption of the starting materials and the pres-
ence of a new orange-red spot (Rf = 0.6 in 1:1
CH2Cl2/hexane). At this point, the solution was
filtered to remove the molecular sieves, followed
by solvent removal. The desired product was iso-
lated by column chromatography over silica gel
using a 7:3 mixture of CH2Cl2/hexane as the elu-
ent. Yield: 0.21 g (75%). mp: 171–174◦C. IR
(CH2Cl2): ν(CO) 1720 (w, sym, dione), 1686 (s,
antisym, dione) cm−1. 1H NMR (CDCl3): δ 6.95–
8.45 (m, aryl and alkenyl). 31P NMR (CDCl3):
δ − 21.63 (s). 1H NMR (C6D6): δ 6.82–8.44 (m,
aryl and alkenyl). 31P NMR (C6D6): δ − 20.56
(AB quartet, JP−P = 61 Hz). UV-vis (CH2Cl2):
λmax 374 (ε = 7600), 458 (ε = 7910); (acetone):
λmax 370 (ε = 7230), 438 (ε = 7530).
Experimental section
General
The parent diphosphine ligand was synthe-
sized from the starting compound 4,5-dichloro-4-
cyclopentene-1,3-dione and Ph2PSiMe3,9 while
the 9-anthracenecarboxaldehyde was purchased
from Aldrich Chemical Co. and used as received.
All reaction and spectroscopy solvents were de-
gassed with argon prior to use. The CH2Cl2 em-
ployed in the cyclic voltammetry studies was
distilled from an appropriate drying agent un-
der argon using Schlenk techniques and stored
in Schlenk storage vessels with Teflon stop-

Knoevenagel condensation of the diphosphine ligand bpcd
717
X-ray diffraction data for 2-(anthracen-9-
ylidene)-4,5-bis(diphenylphosphino)-4-
cyclopentene-1,3-dione
Cyclic voltammetry data
The cyclic voltammetric data were recorded
on a PAR Model 273 potentiostat/galvanostat,
equipped with positive feedback circuitry to com-
pensate for iR drop. The airtight CV cell used was
based on a three-electrode design, and a platinum
disk was employed as the working and auxiliary
electrodes. The reference electrode utilized a sil-
ver wire as a quasi-reference electrode, and the re-
ported potential data are referenced to the formal
potential of the Cp2Fe/Cp2Fe+ (internally added)
redox couple, taken to have E1/2 = 0.307 V.14
Single crystals of 2-(anthracen-9-ylidene)-
4,5-bis(diphenylphosphino)-4-cyclopentene-
1,3-dione suitable for X-ray crystallography
were both grown from a CH2Cl2 solution that
had been layered with hexane. The X-ray
data were collected on a Bruker SMARTTM
1000 CCD-based diffractometer at 213 K.
The frames were integrated with the available
SAINT software package using a narrow-frame
algorithm,11 and the structure was solved and
refined with the aid of the SHELXTL program
package.12 The molecular structure was checked
by using PLATON,13 and all non-hydrogen atoms
were refined anisotropically. All carbon-bound
hydrogen atoms in the title compound were
assigned calculated positions and allowed to
ride on the attached heavy atom. The structure
packs with large holes partially filled with the
solvent molecules hexane and CH2Cl2 used
in the recrystallization. Some of these solvent
molecules diffused from the structure before
the X-ray data were collected leaving partial
occupancy and disorder. The distribution about
the centers of symmetry further complicates the
specific assignments so that no recognizable
disordered model could be generated. It appears
that the two solvent molecules are superimposed
and disordered. Identical results were obtained
using the Squeeze routine in Platon. Ten of
the largest peaks were arbitrarily assigned
as carbon atoms during refinement. These
disordered solvent molecules were consequently
ignored during the data reduction since the
structure of the diphosphine ligand and its
chemistry were of primary importance to this
body of work. 2-(anthracen-9-ylidene)-4,5-
bis(diphenylphosphino)-4-cyclopentene-1,3-
dione afforded convergence values of R = 0.0873
and Rw = 0.2604 for 7452 independent reflections
with I > 2σ(I).
¨
Extended Huckel MO calculations
The extended Hu¨ckel calculations on 2-
(anthracen-9-ylidene)-4,5-bis(diphenylphosphi-
no)-4-cyclopentene-1,3-dione were conducted
with the original program developed by Hoff-
mann,15 as modified by Mealli and Proserpio,16
using weighted H’ijs. The input Z-matrix for the
model compound was constructed from the X-ray
fractional coordinates, with the phenyl groups
replaced by hydrogen atoms in order to simplify
the calculations. The P–H bond lengths were
17
˚
assigned a distance of 1.42 A.
Discussion
Synthesis, spectroscopic data, and molecular
structure
The condensation of bpcd with 9-anthra-
cenecarboxaldehyde was found to be sensitive
to the reaction conditions employed. Good
conversion to the title compound was achieved
through the use of the mixed-solvent system
composed of CH2Cl2/MeOH in the presence
of added molecular sieves. The condensation
reaction is driven to completion by the presence of
the molecular sieves, whose role is to irreversibly

718
Watson, Poola, and Richmond
the 0–0 and 0–1 π → π∗ transitions localized on
the anthracene ring. These spectral assignments
are supported by the magnitudes of the molar
absorptivities, relative insensitivity to solvent
polarity,19 and the close similarity to the reported
absorption data for anthracene.20 The UV-vis
spectral data for the bpcd ligand and the starting
9-anthracenecarboxaldehyde are also included in
Fig. 1 for comparative purposes.
remove the liberated water. The presence of
MeOH was also found to be critical as reactions
run in the absence of MeOH failed to give any
product (<1% conversion by TLC analysis). It
is believed that the protic solvent helps generate
the bpcd-derived enol, which in turn functions
as the nucleophile and that adds to the aldehyde
moiety in an aldol-type reaction. Dehydration
of the intermediate aldol species affords the
conjugated product 2-(anthracen-9-ylidene)-
4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-
dione. The condensation reaction is depicted in
Eq. (1). This new diphosphine ligand was isolated
by column chromatography over silica gel and
characterized in solution.
The solid-state structure of 2-(anthracen-
9-ylidene)-4,5-bis(diphenylphosphino)-4-
cyclopentene-1,3-dione was determined by
X-ray crystallography. Single crystals of the
title compound were found to exist as discrete
molecules in the unit cell with no unusually short
inter- or intramolecular contacts. Tables 1 and 2
report the X-ray data collection and processing
parameters and selected bond distances and
angles, respectively.
The formal union of the pendant anthracen-
9-ylidene moiety with the active methylene car-
bon atom of the bpcd ligand was verified by
X-ray diffraction analysis. The thermal ellip-
soid plot of this second-generation diphosphine is
shown in Fig. 2. The two alkene units defined by
CHO
CH2Cl2/MeOH
-H2O
O
O
(1)
O
O
PPh2
Ph2
P
PPh2
Ph2
P
1
The H NMR spectrum for 2-(anthracen-
9-ylidene)-4,5-bis(diphenylphosphino)-4-cyclo-
pentene-1,3-dione in CDCl3 revealed only
alkenyl and aromatic hydrogens from δ 6.95–
8.45. The absence of resonances attributed to
the aldehyde and the methylene hydrogens
from the bpcd moiety provided initial evidence
for the condensation process. Consistent with
the proposed structure are the 31P NMR data
for the title compound, where a high-field
singlet at δ − 21.63 (in CDCl3) and an AB
quartet at δ − 20.56 (in C6D6) were recorded
at room temperature. The two vibrationally
coupled carbonyl stretches associated with the
dione moiety at 1720 and 1686 cm−1, while
shifted slightly to lower energy relative to
the parent bpcd ligand, are in accord with the
nature of the new diphosphine ligand.18 The
UV-vis spectrum of 2-(anthracen-9-ylidene)-
4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-
dione recorded in CH2Cl2 from 300–750 nm
is shown in Fig. 1. The two, medium intensity
bands found at 374 and 458 nm are assigned to
2.5
2.0
1.5
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. UV-vis spectra of the diphosphine ligand bpcd
(red), 9-anthracenecarboxaldehyde (blue), and 2-(anthracen-
9-ylidene)-4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-
dione (green) recorded in CH2Cl2 at room temperature. All
compounds were recorded at a concentration of ca. 10−4 M.

Knoevenagel condensation of the diphosphine ligand bpcd
719
◦
Table 1. X-ray Crystallographic Data and Processing Parameters
for 2-(anthracen-9-ylidene)-4,5-bis(diphenylphosphino)-4-cyclopen
tene-1,3-dione
˚
Table 2. Selected Bond Distances (A) and Angles ( ) in 2-(anthra
cen-9-ylidene)-4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-
dionea
CCDC entry no.
Space group
288259
Bond distances
Triclinic, P − 1
10.227(2)
13.865(2)
15.905(2)
112.157(2)
101.424(2)
100.065(3)
1968.5(5)
–
–
P(2) C(32)
1.821(5) P(2) C(18)
1.825(4)
1.823(5)
1.845(5)
1.332(7)
1.483(6)
1.364(6)
˚
a (A)
–
–
1.836(5) P(1) C(17)
–
1.833(5) P(1) C(20)
–
1.464(6) C(15) C(44)
–
1.464(7) C(15) C(19)
P(2) C(38)
˚
b (A)
–
P(1) C(26)
˚
c (A)
–
C(1) C(44)
α (◦)
β (◦)
γ (◦)
–
C(15) C(16)
–
–
C(16) C(17)
1.513(6) C(17) C(18)
–
C(18) C(19)
1.496(7)
3
˚
V (A )
Bond angles
Mol formula
fw
C44H30O2P2
1128.02
2
1.101
0.71073
0.143
0.0540
0.9438/0.7598
13284
7452
–
–
–
–
–
–
–
–
–
–
–
–
–
C(1) C(44) C(15) 131.5(5) C(44) C(15) C(16) 131.6(5)
–
–
–
C(44) C(15) C(19) 122.3(5) C(16) C(15) C(19) 105.7(4)
C(18) C(17) P(1) 130.3(3) C(16) C(17) P(1) 119.8(3)
C(17) C(18) P(2) 130.0(4) C(19) C(18) P(2) 120.4(3)
formula units per cell (Z)
Dcalcd (Mg/m3)
˚
λ (Mo Kα) (A)
aNumbers in parentheses are estimated standard deviations in the
least significant digits.
µ (mm−1
)
Rmerge
Abs corr factor
Total reflections
Independent reflections
Completeness to theta (26.00◦) 96.1%
Absorption correction
Data/res/parameters
Waals’ contacts between the C(16)O(1) carbonyl
group and the C(3)–H(3) linkage. The remain-
ing distances and angles are unexceptional and
require no comment.
empirical
7452/0/474
0.0873
0.2604
1.014
R
Rw
GOF on F2
Largest diff. peak and hole
1.027, − .392
−3
˚
¨
Cyclic voltammetry and extended Huckel
(e A
)
MO calculations
The redox properties of the title diphosphine
were next examined by cyclic voltammetry in
CH2Cl2 solvent containing 0.2 M TBAP at a
platinum electrode given the rich electrochemical
behavior displayed by the parent ligand bpcd and
related dione-based disulfide platforms.4a,b,23 In
all of the documented examples to date, electron
accession has been shown to take place at the
ψ4 LUMO that is localized on the dione ring.
Scanning the sample at room temperature at a
rate of 0.1 V/s over the potential range of 1.0 V
to − 1.3 V showed the presence of a single,
quasi-reversible reduction at − 1.01 V that is
readily attributed to the 0/1− redox couple.(3) The
˚
the atoms C(17)–C(18) [1.364(6) A] and C(15)–
˚
C(44) [1.332(7) A] and the arthacenyl moiety all
exhibit carbon-carbon double distances in agree-
ment with such linkages.17 The six different P–C
˚
bond distances reveal a mean distance of 1.831 A
consistent with those P–C bond distances re-
ported in numerous alkenyl- and aryl-substituted
phosphine ligands.21 The carbon-carbon single
bonds in the carbocyclic dione moiety range from
1.464(7) A [C(15)–C(16)] to 1.513(6) A [C(16)–
C(17)] and exhibit an average length of 1.489 A, in
keeping with those distances found in other bpcd-
substituted structurally characterized by us.1,22
The five-membered dione ring defined by carbons
C(15)–C(19) and the three-fused aromatic rings
belonging to the anthracene carbons C(1)–C(14)
display a dihedral angle of 59.7(3)◦. This tipping
or canting of the anthracene ring relative to the
dione ring helps to relieve unfavorable van der
˚
˚
˚
(3) The quasi-reversible nature was verified by the less than unity cur-
rent ratio (Ipa/Ipc = 0.65) under the specified conditions. Increas-
ing the scan rate to 1.0 V(s led to an electrochemically reversible
redox couple based on Ipa/Ipc = 0.99. For discussions and applica-
tions related to the determination of electrochemical reversibility,
see: ref. 14.

720
Watson, Poola, and Richmond
Fig. 2.
Thermal ellipsoid plot of 2-(anthracen-9-ylidene)-4,5-
bis(diphenylphosphino)-4-cyclopentene-1,3-dione showing the thermal ellipsoids at
the 50% probability level
CV behavior found for 2-(anthracen-9-ylidene)-
4,5-bis(diphenylphosphino)-4-cyclopentene-1,3-
dione parallels those data reported for bpcd and
supports a common acceptor LUMO for this
genre of diphosphine ligand.4
consistent with that of ψ4 found for in the
6π-electron systems maleic anhydride and
hexatriene.5 In fact, the composition and energy
level of the LUMO for our model compound are
identical in all respects to data calculated for the
parent ligand bpcd-H4, whose LUMO occurs at
− 10.19 eV. The HOMO in the 2-(anthracen-
9-ylidene)-4,5-bis(diphenylphosphino-H4)-4-
cyclopentene-1,3-dione, which was found at
− 11.52 eV, was also investigated and found
to be essentially localized on the anthracenyl
moiety and composed of π orbitals possessing
idealized b2g symmetry. The orbital composition
of the HOMO and its energy level are in excellent
agreement with published theoretical data at
both the extended Hu¨ckel and B3LYP/6-31G(d)
computational levels.15b,24 The important HOMO
and LUMO levels for 2-(anthracen-9-ylidene)-
4,5-bis(diphenylphosphino-H4)-4-cyclopentene
Support for our contention concerning the
nature of the LUMO and the site of electron
accession was established by carrying out ex-
tended Hu¨ckel MO calculation on 2-(anthracen-
9-ylidene)-4,5-bis (diphenylphosphino)-4-cyclo-
pentene-1,3-dione. Here the model compound
2-(anthracen-9-ylidene)-4,5-bis(diphenylphosphi
no-H4)-4-cyclopentene-1,3-dione was employed
in our calculations, where the four ancillary
phenyl groups were replaced with hydrogen
atoms. The LUMO in the model compound,
which occurs at an energy of − 10.29 eV, is
best described as a π∗ system that is localized
over dione moiety and having a nodal pattern

Knoevenagel condensation of the diphosphine ligand bpcd
721
-1,3-dione, which are depicted below, are in full
accord with the redox and UV-vis properties
already discussed.
References
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Conclusions
The condensation reaction between the
diphosphine ligand bpcd with 9-anthracene-
carboxaldehyde occurs in high yield to furnish
the title ligand 2-(anthracen-9-ylidene)-4,5-
bis(diphenylphosphino)-4-cyclopentene-1,3-
dione. This new diphosphine ligand has
been isolated and fully characterized in
solution and its solid-state structure deter-
mined by X-ray analysis. The redox and
MO properties of 2-(anthracen-9-ylidene)-4,5-
bis(diphenylphosphino)-4-cyclopentene-1,3
-dione have been investigated and contrasted with
the parent ligand bpcd. Future studies will explore
the coordination chemistry of 2-(anthracen-9-
ylidene)-4,5-bis(diphenylphosphino)-4-cyclopen
tene-1,3-dione with mono- and polynuclear metal
compounds, the results of which will be made
public in due course.
8. For an earlier report where the Knoevenagel-type conden-
sation reaction has been employed in the functionalization
of the platform compound 4,5-dichloro-4-cyclopentene-1,3-
dione, see: Alfred, R.; Hellmut, Z. Chem. Ber. 1961, 94,
1800.
9. (a) Fenske, D.; Becher, H. Chem. Ber. 1974, 107, 117. (b) Fenske,
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Financial support from the Robert A. Welch
Foundation (Grants P-0074-WHW and B-1093-
MGR) is appreciated.

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