Synthesis, electrochemistry, MO properties, and X-ray diffraction structures of the new redox-active diphosphine ligand 2-(2-thienylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (tbpcd) and the rhenium compound fac-BrRe(CO)3(tbpcd)

Article abstract of DOI:10.1016/j.poly.2007.03.039

Knoevenagel condensation of 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) with thiophene-2-carboxaldehyde furnishes the second-generation unsaturated diphosphine ligand 2-(2-thienylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (1, tbpcd) in high yield. The substitution chemistry of the rhenium compounds BrRe(CO)₅ and BrRe(CO)₃(THF)₂ with tbpcd has been investigated and found to produce fac-BrRe(CO)₃(tbpcd) (2). Compounds 1 and 2 have been isolated and fully characterized in solution by IR and NMR (¹H and ³¹P) spectroscopies, in addition to mass spectrometry, and X-ray crystallography. The redox properties of 1 and 2 have been examined by cyclic voltammetry, and these data are discussed relative to the results obtained from extended Hückel MO calculations and emission spectroscopic studies, as well as related ligand derivatives previously prepared by us. Our data indicate that the lowest excited state in tbpcd and fac-BrRe(CO)₃(tbpcd) arises from a π → π^* intraligand (IL) transition confined exclusively to the tbpcd ligand.

Full text of DOI:10.1016/j.poly.2007.03.039

Polyhedron 26 (2007) 3577–3584
www.elsevier.com/locate/poly
Synthesis, electrochemistry, MO properties, and X-ray
diffraction structures of the new redox-active diphosphine ligand
2-(2-thienylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione
(tbpcd) and the rhenium compound fac-BrRe(CO)3(tbpcd)
a,
b,
c
William H. Watson *, David Wiedenfeld *, Aparna Pingali ,
c
c,
*
Bhaskar Poola , Michael G. Richmond
a
Department of Chemistry, Texas Christian University, Fort Worth, TX 76129, United States
b
Department of Chemistry, Marshall University, Huntington, WV 25755, United States
Department of Chemistry, University of North Texas, Denton, TX 76203, United States
c
Received 9 November 2006; accepted 25 March 2007
Available online 14 April 2007
Abstract
Knoevenagel condensation of 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (bpcd) with thiophene-2-carboxaldehyde furnishes
the second-generation unsaturated diphosphine ligand 2-(2-thienylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione (1, tbpcd)
in high yield. The substitution chemistry of the rhenium compounds BrRe(CO)5 and BrRe(CO)3(THF)2 with tbpcd has been investigated
and found to produce fac-BrRe(CO)3(tbpcd) (2). Compounds 1 and 2 have been isolated and fully characterized in solution by IR and
NMR (1H and 31P) spectroscopies, in addition to mass spectrometry, and X-ray crystallography. The redox properties of 1 and 2 have
been examined by cyclic voltammetry, and these data are discussed relative to the results obtained from extended Huckel MO calcula-
¨
tions and emission spectroscopic studies, as well as related ligand derivatives previously prepared by us. Our data indicate that the lowest
excited state in tbpcd and fac-BrRe(CO)3(tbpcd) arises from a p ! p* intraligand (IL) transition confined exclusively to the tbpcd ligand.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Diphosphine ligand; Electrochemistry; MO calculations; Photophysics; Rhenium complex
1. Introduction
described in these studies is the presence of a relatively
low-lying p* acceptor orbital associated with the a-diimine
ligand that participates in metal-to-ligand charge transfer
(dp ! p*) upon optical excitation. Knowledge of the life-
time and the exact nature of the lowest emitting state(s)
is crucial if the controlled manipulation of energy transfer
and/or electron transfer manifolds are to be achieved.
Our groups have had an interest in the synthesis and
photophysical study of new systems based on XRe
(CO)3(a-diimine) but where the ancillary a-diimine ligand
has been replaced by a bidentate ligand possessing different
electron-reservoir properties. To this end, we have pre-
pared new rhenium(I) compounds containing the redox-
active ligands 2,3-bis(diphenylphosphino)maleic anhydride
Over the course of the last several decades, numerous
rhenium(I) compounds based on XRe(CO)3(a-diimine)
have been synthesized and examined as luminescent sensing
devices [1], scaffolds for the construction of redox-active
molecular switches and wires [2], and excited-state oxidants
for the study of proton-coupled electron transfer processes
[3]. One commonality between the different compounds
*
Corresponding authors. Tel.: +1 817 257 7195 (W.H. Watson), +1 304
696 2307 (D. Wiedenfeld), +1 940 565 3548 (M.G. Richmond).
E-mail addresses: w.watson@tcu.edu (W.H. Watson), wiedenfeld@
marshall.edu (D. Wiedenfeld), cobalt@unt.edu (M.G. Richmond).
0277-5387/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2007.03.039

3578
W.H. Watson et al. / Polyhedron 26 (2007) 3577–3584
(bma) [4], 4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-
dione (bpcd) [5], and 2,3-bis(methylthio)pyrrolo[1,2-a]ben-
zimidazol-1-one (bmpb) [6], the structures of which are
shown below.
the condensation reaction between thiophene-2-carboxal-
dehyde and bpcd. Herein we present our data on the
synthesis of the new diphosphine ligand 2-(2-thienylid-
ene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-dione
(tbpcd). The reactivity of this ligand with BrRe(CO)5 and
BrRe(CO)3(THF)2 has been examined and found to give
fac-BrRe(CO)3(tbpcd) in high yield. Both new compounds
have isolated and their solid-state structures determined.
The redox properties and photophysics of the tbpcd
ligand and its rhenium(I) complex have been investigated
and the results are discussed relative to MO calculations
Ph2
P
Ph2
P
O
O
O
Br
Br
Cl
N
N
Re
Re
O
Re
P
P
S
Ph2
Ph2
Me
SMe
O
O
at the extended Huckel level.
¨
fac-BrRe(CO)3(bma)
fac-BrRe(CO)3(bmpb)
fac-ClRe(CO)3(bpcd)
2. Experimental
2.1. General
In terms of the diphosphine ligand bpcd, we recently
confirmed that facile and significant structural modifica-
tions of the central dione platform could easily be achieved
through Knoevenagel condensation reactions. Two such
reactions involving the union of 9-anthracenecarboxaldeh-
dye and ferrocenecarboxaldehyde are depicted in Scheme 1
[7,8]. The resulting second-generation ligands derived from
bpcd can be custom- or fine-tuned to produce compounds
that have a fixed or specific redox potential(s) by simply
controlling the extent of p-electron delocalization within
the diphosphine ligand platform. With the attachment of
suitable donor–acceptor groups to the original dione scaf-
fold, new ligands with the ability to function as directed
energy transfer agents can be expected.
The starting diphosphine ligand bpcd was prepared
from 4,5-dichloro-4-cyclopenten-1,3-dione and Ph2PSiMe3
[9], while BrRe(CO)3(THF)2 was synthesized from
BrRe(CO)5 [10]. The thiophene-2-carboxaldehyde was pur-
chased from Aldrich Chemical Co. and used as received.
All reaction and NMR solvents were distilled from an
appropriate drying agent using Schlenk techniques and
stored under argon in storage vessels equipped with high-
vacuum Teflon stopcocks [11]. The tetra-n-butylammo-
nium perchlorate (TBAP) electrolyte was purchased from
Johnson Matthey Electronics and was recrystallized from
ethyl acetate/hexane, followed by drying under high
vacuum for at least 36 h.
Wishing to extend our functionalization studies of the
bcpd ligand to other aldehydes that could, in principle,
function as multidentate ligand systems, we have studied
The IR spectral data were recorded on a Nicolet 20 SXB
FT-IR spectrometer in sealed 0.1 mm NaCl cells, while the
Ph2P
O
PPh2
bpcd
O
O
H
H
Fe
molecular sieves
CH2Cl2/-H2O
O
O
O
Ph2P
Ph2P
Ph2P
Ph2P
O
O
Fe
Scheme 1.

W.H. Watson et al. / Polyhedron 26 (2007) 3577–3584
3579
1H spectral data were recorded at 200 MHz on a Varian
2.4. X-ray diffraction structures
Gemini-200 spectrometer. The 31P NMR spectra were col-
lected in the proton-decoupled mode on a Varian 300-VXR
spectrometer at 121 MHz, with the reported chemical shifts
referenced to external H3PO4 (85%), taken to have d = 0.
The absorption spectra were recorded on a HP 8453 Chem-
Station (Marshall) or a HP 8425A diode array spectrome-
ter (UNT). The emission data were recorded on a Jobin
Fluorolog FL 3-22 spectrometer equipped with DataMax
software. The ESI-APCI mass spectra were recorded at
the University of the Pacific in the positive ionization mode
for 1 and the negative ionization mode for 2.
Single crystals of the tbpcd ligand were grown from a
CH2Cl2 solution containing tbpcd that had been layered
with hexane, while X-ray quality crystals of fac-BrRe-
(CO)3(tbpcd) were grown from an acetone solution con-
taining fac-BrRe(CO)3(tbpcd) that was layered with
Et2O. The reported X-ray data for both compounds were
collected on a Bruker SMARTä 1000 CCD-based diffrac-
tometer at 213 K. The frames were integrated with the
available SAINT software package using a narrow-frame
algorithm [12], and the structures were solved and refined
using the SHELXTL program package [13]. The molecular
structures were checked by using PLATON [14], and solved
by direct methods with all nonhydrogen atoms refined
anisotropically. All carbon-bound hydrogen atoms were
assigned calculated positions and allowed to ride on the
attached heavy atom, unless otherwise noted. Refinement
for tbpcd converged at R = 0.0641 and Rw = 0.1187 for
6379 independent reflections with I > 2r(I), with 2 Æ acetone
yielding convergence values of at R = 0.0358 and
Rw = 0.0873 for 8558 independent reflections with
I > 2r(I). Tables 1 and 2 summarize the pertinent X-ray
data for compounds 1 and 2 Æ acetone.
2.2. Synthesis of tbpcd (1)
To a small Schlenk vessel was added 0.20 g (0.43 mmol)
of bpcd, followed by 25 mL of CH2Cl2, 0.39 mL (excess)
thiophene-2-carboxaldehyde, and ca. 4.0 g of molecular
˚
sieves (4 A). Stirring over night under argon led to the com-
plete consumption of the bpcd ligand, as assessed by TLC
analysis. The presence of the desired product was con-
firmed by TLC examination of the reaction solution. Here
the tbpcd ligand was observed as an orange spot having a
Rf = 0.30 using CH2Cl2/petroleum ether (3:2) as the eluent.
The molecular sieves were filtered away and the solution
concentrated to ca. 0.5 mL, after which the crude product
was subjected to chromatographic separation over silica
gel using the aforementioned solvent system to give tbpcd
as an orange solid in 90% yield (0.22 g). IR (CH2Cl2):
m(CO) 1719 (m, symm dione), 1673 (vs, antisymm dione)
2.5. Electrochemistry
The cyclic voltammetric data were recorded on a PAR
Model 273 potentiostat/galvanostat, equipped with posi-
tive feedback circuitry to compensate for iR drop. The
cmꢀ1 1H NMR (CDCl3): d 7.05 (dd, 1H, thiophene,
.
3JH–H = 7, 5 Hz), 7.22–7.46 (m, 20H, phenyls), 7.56
3
Table 1
(s, 1H, vinyl), 7.68 (d, 1H, thiophene, JH–H = 7 Hz), 7.90
3
(d, 1H, thiophene, JH–H = 5 Hz). 31P NMR (CDCl3):
X-ray crystallographic data and processing parameters for the ligand
tbpcd (1) and fac-BrRe(CO)3(tbpcd) Æ acetone (2 Æ acetone)
d ꢀ24.23 (AB quartet, JP-P = 25 Hz). ESI-APCI MS (m/
z): 581.086 [1+Na]+, 558.995 [1]+.
Compound
1
2 Æ acetone
623342
monoclinic
P21/c
15.986(3)
15.761(3)
14.522(3)
95.267(3)
3643(1)
CCDC entry no.
Crystal system
Space group
623341
monoclinic
P21/c
13.049(2)
10.878(2)
19.582(2)
93.461(3)
2774.5(7)
C34H24O2P2S
2.3. Synthesis of fac-BrRe(CO)3(tbpcd) (2) from tbpcd and
BrRe(CO)3(THF)2
˚
a (A)
˚
b (A)
˚
c (A)
To a Schlenk tube under argon was charged 0.10 g (0.20
mmol) of BrRe(CO)3(THF)2 and 0.11 g (0.20 mmol) of 1,
after which 20 mL of CH2Cl2 was added via cannula.
The contents were stirred at room temperature for 3 h
and examined by TLC, which revealed the complete con-
sumption of the starting materials and only the presence
of the desired product as a yellow spot at Rf = 0.20 using
CH2Cl2 as the mobile phase. The solvent was concentrated
and compound 2 subsequently isolated by column chroma-
tography over silica gel using CH2Cl2. Yield of 2: 0.14 g
(80%). IR (CH2Cl2): m(CO) 2039 (s), 1966 (s), 1912 (s),
b (°)
3
˚
V (A )
Molecular formula
C37H24BrO5P2ReS Æ
acetone
Formula weight
558.53
966.75
Formula units per cell (Z)
Dcalc (Mg/m3)
4
4
1.337
0.71073
0.263
1.763
0.71073
4.625
˚
k (Mo Ka) (A)
l (mmꢀ1
Rmerge
)
0.0550
0.0783
Absorption correction
Absorption correction factor
Total reflections
empirical
0.8149/0.6572
21540
empirical
1.0000/0.4525
38899
1728 (vw, symm dione), 1684 (s, antisymm dione) cmꢀ1
.
Independent reflections
Data/restraints/parameters
R
6379
6379/0/353
0.0641
8558
8558/0/462
0.0358
3
1H NMR (CDCl3): d 7.21 (dd, 1H, thiophene, JH–H = 7,
5 Hz), 7.32–7.90 [m, 22H, vinyl (1H), thiophene (1H),
3
Rw
Goodness-of-fit on F2
Dqmaximum, Dqminimum (e/A )
0.1187
1.005
0.490 and ꢀ0.261
0.0873
0.993
1.978 and ꢀ1.290
and phenyls (20H)], 8.06 (d, 1H, thiophene, JH–H
=
5 Hz). 31P NMR (CDCl3): d 18.09 (s, 1P), 19.18 (s, 1P).
ESI-APCI MS (m/z): 907.821 [2]ꢀ, 938.922 [2+MeO]ꢀ.
3
˚

3580
W.H. Watson et al. / Polyhedron 26 (2007) 3577–3584
Table 2
1 and 2 Æ acetone, employing hydrogens in the place of
the four ancillary phenyl groups and setting the P–H bond
˚
Selected bond distances (A) and angles (°) in ligand tbpcd (1) and fac-
BrRe(CO)3(tbpcd) Æ acetone (2 Æ acetone)
˚
distances to 1.41 A [18].
tbpcd (1)
Bond distances
S(1)–C(34)
C(1)–C(30)
C(1)–C(2)
1.693(3)
1.352(3)
1.473(3)
1.355(3)
1.436(3)
1.398(4)
3.330(1)
S(1)–C(31)
C(1)–C(5)
C(2)–C(3)
C(4)–C(5)
C(31)–C(32)
C(33)–C(34)
S(1)ꢁ ꢁ ꢁO(2)
1.727(3)
1.466(4)
1.514(4)
1.526(3)
1.411(4)
1.351(4)
2.861(2)
3. Results and discussion
3.1. Synthesis, spectroscopic data, and X-ray diffraction
structure of tbpcd (1)
C(3)–C(4)
C(30)–C(31)
C(32)–C(33)
P(1)ꢁ ꢁ ꢁP(2)
Knoevenagel condensation of the diphosphine ligand
bpcd with thiophene-2-carboxaldehyde (excess) proceeds
readily in CH2Cl2 at room temperature in the presence of
added molecular sieves to afford the new diphosphine
Bond angles
C(34)–S(1)–C(31)
C(30)–C(1)–C(2)
C(4)–C(3)–P(2)
C(3)–C(4)–P(1)
C(1)–C(30)–C(31)
91.5(1)
121.9(2)
121.7(2)
122.9(2)
132.8(2)
C(30)–C(1)–C(5)
C(5)–C(1)–C(2)
C(2)–C(3)–P(2)
C(5)–C(4)–P(1)
C(30)–C(31)–S(1)
131.1(2)
107.0(2)
128.4(2)
128.2(2)
127.6(2)
ligand
2-(2-thienylidene)-4,5-bis(diphenylphosphino)-4-
cyclopenten-1,3-dione (tbpcd) in near quantitative yield.
Simply stirring the two reagents without added molecular
sieves showed no evidence for the formation of tbpcd (1),
as assessed by TLC and IR analyses, which clearly under-
scores the need to drive the reaction to completion through
the removal of the water that accompanies the Knoevena-
gel reaction. Compound 1 was subsequently isolated as a
relatively air-stable orange solid after chromatographic
separation over silica using CH2Cl2/petroleum ether (3:2)
as the eluent. Eq. (1) illustrates the reaction leading to
the tbpcd ligand.
fac-BrRe(CO)4(tbpcd) Æ acetone (2 Æ acetone)
Bond distances
Re(1)–P(2)
Re(1)–Br(1)
S(1)–C(34)
C(1)–C(5)
C(3)–C(4)
C(4)–C(5)
C(34)–C(35)
C(36)–C(37)
S(1)ꢁ ꢁ ꢁO(2)
2.449(1)
2.6530(6)
1.714(5)
1.526(5)
1.473(6)
1.469(6)
1.467(6)
1.361(8)
2.912(3)
Re(1)–P(1)
S(1)–C(37)
C(1)–C(2)
2.468(1)
1.676(5)
1.342(5)
1.513(6)
1.365(6)
1.430(6)
1.355(7)
3.213(2)
C(2)–C(3)
C(4)–C(33)
C(33)–C(34)
C(35)–C(36)
P(1)ꢁ ꢁ ꢁP(2)
O
Bond angles
O
molecular sieves
CH2Cl2/-H2O
Ph2P
Ph2P
C(32)–Re(1)–C(31)
C(31)–Re(1)–C(30)
C(31)–Re(1)–P(2)
C(32)–Re(1)–P(1)
C(30)–Re(1)–P(1)
C(32)–Re(1)–Br(1)
C(30)–Re(1)–Br(1)
P(1)–Re(1)–Br(1)
C(2)–C(1)–P(1)
91.0(2)
90.1(2)
92.5(1)
95.4(1)
95.4(1)
178.3(1)
93.5(1)
83.95(3)
120.2(3)
121.2(3)
107.9(3)
C(32)–Re(1)–C(30)
C(32)–Re(1)–P(2)
C(30)–Re(1)–P(2)
C(31)–Re(1)–P(1)
P(2)–Re(1)–P(1)
C(31)–Re(1)–Br(1)
P(2)–Re(1)–Br(1)
C(37)–S(1)–C(34)
C(5)–C(1)–P(1)
88.1(2)
95.6(1)
175.4(1)
171.7(1)
81.59(3)
89.5(1)
82.69(3)
91.7(3)
130.5(3)
126.6(3)
O
H
S
Ph2P
Ph2P
S
O
O
tbpcd (1)
ð1Þ
The tbpcd ligand was fully characterized in solution by IR
and NMR spectroscopies, ESI mass spectrometry, and X-
ray diffraction analysis. The H NMR spectrum of 1 con-
C(1)–C(2)–P(2)
C(5)–C(4)–C(3)
C(3)–C(2)–P(2)
1
firmed the loss of the methylene hydrogens from the bpcd
ligand and the formyl hydrogen from the thiophene-2-car-
boxaldehyde. The 31P NMR spectrum displays a classic
four-line AB quartet centered at d ꢀ24.23 in keeping with
the inequivalent phosphine moieties in 1. The vibrationally
coupled dione carbonyl groups of the tbpcd ligand give rise
to symmetric and antisymmetric m(CO) bands at 1719 (m)
and 1673 (vs) cmꢀ1 [19], that are 23 and 32 cmꢀ1 lower in
energy, respectively, relative to the m(CO) bands of bpcd li-
gand [9,20], which in turn indicates that the tbpcd ligand is
slightly more electron rich than the bpcd ligand. The UV-
vis spectrum of 1 in CH2Cl2 is shown in Fig. 1, where a sin-
gle absorbance at 374 nm (e = 18000) is observed. The
absorbance is insensitive to the nature of the solvent as
the UV–vis band exhibits a small shift to 371 nm
(e = 20000) when the spectrum was recorded in MeCN sol-
vent. The ESI mass spectrum of 1 that was recorded in the
positive ion mode revealed m/z peaks at 581.086 and
558.995 for the singly charged sodium adduct of tbpcd
[1 + Na]+ and the parent ion of [1]+, respectively, consis-
tent with the proposed ligand product.
homemade three-electrode cell that was used allowed the
cyclic voltammograms to be recorded under oxygen- and
moisture-free conditions.
A
platinum disk (area =
0.0079 cm2) was employed as the working and auxiliary
electrode. The reference electrode utilized a silver wire as
a quasi-reference electrode, with the reported potential
data standardized against the formal potential of the
Cp2Fe/Cp2Fe+ (internally added) redox couple, taken to
have E1/2 = 0.307 V [15].
2.6. Extended Hu¨ckel MO calculations
The extended Huckel calculations on tbpcd and fac-
¨
BrRe(CO)3(tbpcd) were carried out using the original pro-
gram developed by Hoffmann [16], as modified by Mealli
and Proserpio [17]. The weighted Hij’s contained in the pro-
gram were used in the calculations. The input Z-matrices
for the model compounds tbpcd-H4 and fac-BrRe-
(CO)3(tbpcd-H4) were constructed from the X-ray data of

W.H. Watson et al. / Polyhedron 26 (2007) 3577–3584
3581
phine ligand (Z)-Ph2PCH@CHPPh2 [21], while the
˚
O(2)ꢁ ꢁ ꢁS(2) internuclear distance of 2.861(2) A is shorter
1.6
1.2
0.8
0.4
0.0
than the sum of the van der Waals radii for oxygen and sul-
fur atoms by ca. 0.36 A [22]. These structural trends indi-
˚
cate that the molecule is not subject to significant adverse
perturbations. The two five-membered rings are essentially
coplanar based on an interplanar angle of 9.1(1)°. The
remaining bond distances and angles are unremarkable
and require no comment.
3.2. Syntheses, spectroscopic data, and X-ray diffraction
structure of fac-BrRe(CO)3(tbpcd) (2)
300
400
500
600
The reactivity of the tbpcd ligand was next examined
with the rhenium(I) compounds BrRe(CO)5 and BrRe-
(CO)3(THF)2 given our aforementioned interest in ligand-
substituted rhenium compounds [4,6]. Our initial substitu-
tion studies employed BrRe(CO)5 as a starting material.
Heating an equimolar mixture of BrRe(CO)5 and tbpcd
at ca. 60–70 °C in either toluene or 1,2-dichloroethane fur-
nished fac-BrRe(CO)3(tbpcd) in good yields. TLC and IR
analyses confirmed that 2 was formed as the major prod-
uct. The same product was also obtained using the labile
compound BrRe(CO)3(THF)2 in place of BrRe(CO)5. In
the case of BrRe(CO)3(THF)2, the reaction with added
tbpcd ligand proceeded rapidly at room temperature to
produce 2 in quantitative yield. While both rhenium start-
ing materials give compound 2, the use of the BrRe
(CO)3(THF)2 has a slight advantage in that the reaction
is cleaner and the solvent can be removed directly to give
pure 2 for immediate use, if desired. The thermolysis reac-
tions were accompanied by small amounts of unreacted
Wavelength (nm)
Fig. 1. UV–Vis spectra of the tbpcd ligand (green) and fac-BrRe-
(CO)3(tbpcd) (red) in CH2Cl2 at room temperature. Both compounds
were recorded at a concentration of ca. 10ꢀ4 M. (For interpretation of the
references to colour in this figure legend, the reader is referred to the web
version of this article.)
The solid-state structure of tbpcd was determined by
single-crystal X-ray diffraction analysis, with the thermal
ellipsoid plot of 1 shown in Fig. 2. The union of the bpcd
ligand to the formyl carbon atom of the thiophene-2-car-
boxaldehyde is confirmed by the presence of the exocyclic
alkene moiety defined by the C(1)–C(30) vector, whose
˚
˚
1.352(3) A bond length is in excellent agreement with the
1.355(3) A bond distance found for the C(3)–C(4) p bond
in the dione platform. The internuclear P(1)ꢁ ꢁ ꢁP(2) bond
˚
distance of 3.330(1) A is slightly longer than the
˚
3.278(3) A Pꢁ ꢁ ꢁP distance reported for unsaturated diphos-
Fig. 2. Thermal ellipsoid plots of the ligand tbpcd (left) and fac-BrRe(CO)3(tbpcd) Æ acetone (right) showing the thermal ellipsoids at the 50% probability
level.

3582
W.H. Watson et al. / Polyhedron 26 (2007) 3577–3584
starting materials and unknown material(s) that remained
at the origin of the TLC, necessitating a chromatographic
separation. Compound 2 is air stable in the solid state,
but solutions containing 2 that have been exposed to the
atmosphere exhibit signs of decomposition on standing
for several hours.
3.3. Cyclic voltammetry, MO calculations, and emission
spectroscopy data
The cyclic voltammetric properties of the tbpcd ligand
and compound 2 were next probed at a platinum disk
electrode in CH2Cl2 solvent containing 0.2 M TBAP as
the supporting electrolyte. Initially scanning a tbpcd
sample at room temperature at a rate of 0.5 V/s over
the potential range of 1.2 to ꢀ1.8 V revealed the pres-
ence of two reduction processes attributed to the 0/1ꢀ
and 1ꢀ/2ꢀ redox couples. The latter reduction exhibits
a forward wave at Epc = ꢀ1.45 with no accompanying
2 was characterized in solution by traditional spectro-
scopic methods, mass spectrometry (negative-ion mode),
and X-ray crystallography. The IR spectrum exhibits
three terminal rhenium carbonyl stretching bands at
2039 (s), 1966 (s), and 1912 (s) cmꢀ1, whose frequencies
and symmetry closely match the IR data reported by us
for the related compounds fac-BrRe(CO)3(bma) and fac-
ClRe(CO)3(bpcd) [4,5]. Coordination of the tbpcd ligand
to the BrRe(CO)3 fragment is accompanied by a slight
decrease in the electron density associated with tbpcd
ligand based on the ca. 10 cmꢀ1 shift of the dione
m(CO) bands to higher energy. The symmetric and anti-
symmetric dione carbonyl stretching bands appear at
1728 and 1684 cmꢀ1, respectively. The 1H NMR data
are fully consistent with the coordination of the tbpcd
ligand to the fac-BrRe(CO)3 fragment, as is the 31P
NMR spectrum, where two down-field singlets at d
18.09 and 19.18 support the existence of a chelating tbpcd
ligand that binds the rhenium atom via the two phospho-
rus moieties [23]. The UV–Vis spectrum of 2 shown in
Fig. 1 exhibits a strong absorbance at 383 nm (e =
28000) and a weaker band at 308 (e = 9500). The
UV–vis spectrum recorded in MeCN afforded the same
two-band pattern [378 nm (e = 35000), 308 (e = 13000)]
for 2 as found in CH2Cl2. The ESI mass spectrum of 2
run in MeOH solvent revealed m/z peaks at 907.821
and 938.922 for the parent compound [2]ꢀ and its meth-
oxide adduct [2 + MeO]ꢀ, respectively, in agreement with
the formulated structure for 2.
a
reverse wave (Ep ), leading to an irreversible reduction
under these conditions. Narrowing the potential window
to 0 to ꢀ1.3 V and scanning just over the first reduction
wave confirmed that 0/1ꢀ redox couple, which displays
an E1/2 = ꢀ1.09 V, is fully reversible when analyzed by
standard electrochemical criteria [15]. The electron stoi-
chiometry for the first reduction process (1eꢀ) was also
verified by comparison of the current function of the
known one-electron standard ferrocene with that from
tbpcd ligand. The recorded E1/2 value for the tbpcd
ligand closely matches that cyclic voltammetically deter-
mined E1/2 value for the diphosphine ligand bpcd by
Tyler and co-worker and the anthracene-functionalized
bpcd ligand (E1/2 = ꢀ1.01 V) prepared by us and
depicted in Scheme 1 [7,20,25].
The CV of compound 2 was recorded under identical
conditions to those employed for 1. Here the two reversible
one-electron waves found at E1/2 = ꢀ0.68 and ꢀ1.39 V are
readily assignable to the 0/1ꢀ and 1ꢀ/2ꢀ redox couples,
respectively, and parallel those data reported earlier by us
for the related diphosphine complex fac-BrRe(CO)3(bma)
and the compound fac-BrRe(CO)3(bpcd) [4,26]. It is inter-
esting to note that the coordination of the tbpcd ligand to
the rhenium center in 2 leads to enhanced stability of the
1ꢀ/2ꢀ redox couple.
The adopted coordination mode of the tbpcd ligand in
2 was established by X-ray diffraction analysis. Single
crystals of 2, as the acetone solvate, were found to exist
as discrete molecules in the unit cell. The thermal ellip-
soid plot of 2 is shown in the right-hand side of
Fig. 1, where the chelation of the tbpcd ligand to the
rhenium center by the two phosphine moieties is con-
firmed. The rhenium atom is six-coordinate and possesses
a distorted octahedral geometry. The Re(1)–P(1) and
That the CV data from 1 and 2 are similar and in con-
cert with the UV–vis data for both compounds suggest that
the free ligand and its rhenium complex possess common
HOMO and LUMO levels. This premise was established
by carrying out extended Huckel MO calculations on
¨
compounds 1 and 2. Here the model compound 2-(2-thie-
nylidene) -4,5-bis(diphenylphosphino-H4)-4-cyclopenten -1,
3-dione (tbpcd-H4) was employed in our calculations,
where the four ancillary phenyl groups were replaced with
hydrogen atoms. The HOMO for tbpcd-H4 occurs at
ꢀ12.00 eV and is confined to the p system of the dione
and thiophene rings, as shown below. The dominant con-
tributions from the C@C p bond of the dione moiety and
the carbon framework of the thiophene ring, the latter
which displays a strong resemblance to w2 of cyclopentadi-
ene and other four-p-electron systems, are evident. The
LUMO occurs at ꢀ10.45 eV and is strongly p* in character
and exclusively localized on the four carbon and two oxy-
gen atoms associated with the dione platform. The nodal
pattern of the LUMO is consistent with theoretical expec-
˚
Re(1)–P(2) distances of 2.468(1) and 2.449(1) A, respec-
tively, and the observed bond angle of 81.59(3)° for the
P(2)–Re(1)–P(1) linkage exhibit acceptable values in com-
parison to those bond distances and angles in related
rhenium compounds structurally characterized by us
˚
[24]. The P(1)ꢁ ꢁ ꢁP(2) [3.213(2) A] and S(1)ꢁ ꢁ ꢁO(2)
˚
[2.912(3) A] bond distances and the interplanar angle of
5.8(2)° are in agreement with those values of the free
tbpcd ligand and confirm that the ancillary ligand does
not experience any major structural perturbations upon
coordination to the rhenium center. The remaining bond
distances and angles are unexceptional and require no
comment.

W.H. Watson et al. / Polyhedron 26 (2007) 3577–3584
3583
tations for a six-p-electron system and the MO results from
maleic anhydride [7,27].
Finally, the photophysics for compounds 1 and 2 were
explored through the use of emission spectroscopy as this
would allow us to corroborate experimentally the p ! p*
HOMO/LUMO transition indicated by the extended Huc-
¨
H2P
H2P
H2P
H2P
kel calculations. The emissive properties of compounds 1
and 2 were examined at room temperature in both CH2Cl2
and MeCN with excitation into the long wavelength band
at ca. 380 nm. However, any emission displayed from
either compound was not resolvable from the baseline,
indicating very weak emission at best and fully consistent
with the proposed intraligand p ! p* transition in the free
ligand and its rhenium(I) compound [29].
S
S
HOMO
LUMO
The HOMO and LUMO levels in fac-BrRe(CO)3-
(tbpcd-H4) were next determined so that a direct MO
comparison with the free tbpcd-H4 ligand could made.
The calculated energies and orbital composition of the
important HOMO and LUMO levels in fac-BrRe(-
CO)3(tbpcd-H4) were unchanged relative to the free
ligand tbpcd-H4. The presence of a low-lying p* LUMO
that is localized on the dione moiety in fac-BrRe(-
CO)3(tbpcd-H4) is expected and has been theoretical
and experimentally confirmed in all compounds contain-
ing an ancillary bpcd ligand investigated to date [20,28].
The surprising feature here was the composition of the
HOMO level in 2 since our earlier report on fac-BrRe-
(CO)3(bma) revealed that the HOMO was associated with
an out-of-phase interaction involving the rhenium dyz and
bromine py orbitals with no contribution from the bma p
4. Conclusions
The synthesis of the new unsaturated diphosphine ligand
2-(2-thienylidene)-4,5-bis(diphenylphosphino)-4-cyclopen-
ten-1,3-dione (tbpcd) from 4,5-bis(diphenylphosphino)-4-
cyclopenten-1,3-dione (bpcd) and thiophene- 2-carboxalde-
hyde is described. The ligand substitution chemistry of the
tbpcd ligand with the rhenium compounds BrRe(CO)5
and BrRe(CO)3(THF)2 has been examined and found to
furnish fac-BrRe(CO)3(tbpcd) as the sole observable reac-
tion product. Both new compounds have been fully charac-
terized and their physical properties studied by
combination of methods including cyclic voltammetry,
emission spectroscopy, and MO calculations.
a
system. Accordingly, we ran comparative Huckel calcula-
¨
5. Supplementary material
tions on fac-BrRe(CO)3(bpcd-H4) and confirmed that the
LUMO (ꢀ10.44 eV) was identical to that calculated for
fac-BrRe(CO)3(bma-H4) and facBrRe(CO)3(tbpcd-H4).
The HOMO in the unfunctionalized diphosphine com-
pound fac-BrRe(CO)3(bpcd-H4) occurs at ꢀ12.35 eV
and exhibits the same antibonding rhenium dyz and bro-
mine py interaction as found in fac-BrRe(CO)3(bma-H4)
[26]. The dichotomy in the HOMO composition between
fac-BrRe(CO)3(tbpcd-H4) and the corresponding bma
and bpcd derivatives is readily traced to p interactions
between the dione and thiophene rings in the tbpcd
ligand. The extended p conjugation in the functionalized
or second-generation tbpcd ligand leads to a high-lying p
orbital that has no counterpart in the simple bpcd ligand.
Examination of the subjacent highest-occupied molecular
orbital (sHOMO) at ꢀ12.35 eV in fac-BrRe(CO)3(tbpcd-
H4), which is depicted below, reveals the presence of
the expected metal-based rhenium dyz and bromine py
antibonding orbital in excellent agreement with the MO
data from fac-BrRe(CO)3(bma-H4) [4] and fac-BrRe(CO)3-
(bpcd-H4) [26].
CCDC 623341 and 623342 contain the supplementary
crystallographic data for tbpcd and fac-BrRe(CO)3(tbpcd).
These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or
e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
Financial support from the Robert A. Welch Founda-
tion (Grants P-0074-WHW and B-1093-MGR) is appreci-
ated. Prof. Andreas H. Franz (University of the Pacific)
is thanked for recording the ESI-ACPI mass spectral data
for the tbpcd ligand and fac-BrRe(CO)3(tbpcd).
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