New rhenium(I) compounds containing the donor-acceptor diphosphine ligands 2-(ferrocenylidene)-4,5-bis(diphenylphosphino)4-cyclopenten-1,3-dione (fbpcd) and 2-(3-ferrocenylprop-2-ynylidene)-4,5-bis(diphenylphosphino)4-cyclopenten- 1,3-dione (fpbpcd): Elec

Article abstract of DOI:10.1016/j.poly.2009.05.049

Displacement of the labile THF molecules in BrRe(CO)₃(THF)₂ (1) by the diphosphine ligands 2-(ferrocenylidene)-4,5-bis(diphenylphosphino)4-cyclopenten-1,3-dione (fbpcd) and 2-(3-ferrocenylprop-2-ynylidene)-4,5-bis(diphenylphosphino)4

Full text of DOI:10.1016/j.poly.2009.05.049

Polyhedron 28 (2009) 2619–2624
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
New rhenium(I) compounds containing the donor–acceptor diphosphine ligands
2-(ferrocenylidene)-4,5-bis(diphenylphosphino)4-cyclopenten-1,3-dione (fbpcd)
and 2-(3-ferrocenylprop-2-ynylidene)-4,5-bis(diphenylphosphino)4-cyclopenten-
1,3-dione (fpbpcd): Electrochemical behavior, MO properties, and X-ray
diffraction structure of fac-BrRe(CO)3(fpbpcd)
a,
Bhaskar Poola a, Xiaoping Wang b, , Michael G. Richmond
*
*
a Neutron Scattering Science Division, Oak Ridge, National Laboratory, Oak Ridge, TN 37831, United States
b 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:
Displacement of the labile THF molecules in BrRe(CO)3(THF)2 (1) by the diphosphine ligands 2-(ferroceny-
lidene)-4,5-bis(diphenylphosphino)4-cyclopenten-1,3-dione (fbpcd) and 2-(3-ferrocenylprop-2-ynylid-
ene)-4,5-bis(diphenylphosphino)4-cyclopenten-1,3-dione (fpbpcd) yields the mononuclear compounds
fac-BrRe(CO)3(fbpcd) (2) and fac-BrRe(CO)3(fpbpcd) (3), respectively. The new ligand fpbpcd ligand has
been synthesized from 3-ferrocenylpropynal and the parent diphosphine ligand 4,5-bis(diphenylphos-
phino)-4-cyclopenten-1,3-dione (bpcd) through a Knoevenagel condensation. 2 and 3 have been isolated
and fully characterized by IR and NMR spectroscopies (1H and 31P), ESI mass spectrometry, and X-ray dif-
fraction analysis in the case of 3. The electrochemical properties of compounds 2 and 3 have been examined
by cyclic voltammetry, and the nature of the HOMO and LUMO levels in these systems has been confirmed
by MO calculations at the extended Hückel level. The redox and MO data are discussed relative to the redox
and orbital properties of related functionalized diphosphines based on the bpcd platform.
Received 22 April 2009
Accepted 20 May 2009
Available online 28 May 2009
Keywords:
Rhenium(I) compounds
Diphosphine ligands
Redox chemistry
Crystal structure
MO calculations
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
factor responsible for the disappointing photophysical data (i.e.,
efficient non-radiative decay) experimentally found in the over-
New inorganic and organometallic systems that can promote
energy- and electron-transfer reactions are highly sought after as
device components in optoelectronic applications, photosynthetic
mimics, redox switches, and model systems for directional
charge-transfer processes [1]. The synthesis and photophysical
study of new luminescent organometallic complexes continue to
command the attention of a diverse group of researchers. Many
dyad and triad arrays possessing an energetically accessible LUMO
typically give rise to long-lived charge-transfer species, a condition
tantamount for the successful fabrication of photosynthetic and
OLED devices [2]. While many derivatives exhibit a low-lying
LUMO and promote long-lived charge separated species, the search
for new systems with tunable LUMO levels remains a universal
priority.
whelming majority of diphosphine-substituted metal compounds
is the absence of a low-lying * acceptor LUMO on the diphosphine
ligand. The absence of an energetically accessible diphosphine
p
LUMO radically changes the emissive behavior relative to those
compounds containing an
that possess a
* LUMO that is localized on the ancillary heterocy-
clic residue [4].
a-diimine or related heterocyclic ligand
p
Our groups maintain a longstanding interest in the synthesis and
physical study of organometallic complexes containing the
unsaturated diphosphine ligands 2,3-bis(diphenylphosphino)
maleic anhydride (bma) and 4,5-bis(diphenylphosphino)-4-cyclop-
enten-1,3-dione (bpcd) [5,6]. These particular ligands are
unique vis-á-vis the common diphosphine ligands dppm, (Z)-1,2-
Ph2PCH@CHPPh2, and 1,2-bis(diphenylphosphino)benzene because
both bma and bpcd exhibit a one-electron reduction at relatively
While diphosphine ligands are ubiquitous in transition-metal
chemistry, only a few documented paradigms exist that display
significant luminescent behavior [3]. Undoubtedly, the principal
low potential (<ꢀ1.1 V), with electron accession occurring at a
p*-
based LUMO that is localized about the central platform ring [7].
The superb electron reservoir properties displayed by bma and
bpcd have been shown to modulate ligand substitution in
selected mononuclear compounds having electron counts in excess
of 18e- [8].
* Corresponding authors. Tel.: +1 940 565 3548; fax: +1 940 565 4318 (M.G.
Richmond).
E-mail addresses: wangx@ornl.gov (X. Wang), cobalt@unt.edu (M.G. Richmond).
0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2009.05.049

2620
B. Poola et al. / Polyhedron 28 (2009) 2619–2624
In our quest to prepare new, second-generation ligands based on
vent purification system. All distilled solvents were stored in
Schlenk storage vessels equipped with high-vacuum Teflon stop-
cocks [18]. The tetra-n-butylammonium perchlorate (TBAP) elec-
trolyte was purchased from Johnson Matthey Electronics and
recrystallized from hexane/ethyl acetate and dried under vacuum
for at least 36 h prior to use.
The IR spectra were recorded on a Nicolet 6700 FT–IR spec-
trometer in amalgamated NaCl cells capable of handling air-sen-
sitive samples. The quoted 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 pro-
ton-decoupled mode and the reported chemical shifts referenced
to external H3PO4 (85%), taken to have d=0. UV–Vis spectra for 2
and 3 were recorded on a Hewlett–Packard 8452A diode array
spectrometer in Schlenk-modified 1.0 cm quartz UV–Vis cells.
The ESI-APCI mass spectra were recorded at the University of
the Pacific (2) and UNT (3) mass spectrometry facilities, respec-
tively, in the positive ionization mode. The matrices employed
for the collection of the mass spectra were MeOH (2) and
MeCN/KI (3).
bpcd, we have instituted Knoevenagel condensation reactions be-
tween bpcd and selected aldehydes [9]. Here the union of the bpcd
platform with a redox-active appendage and/or light-harvesting
moiety facilitates, in theory, the tuning of the HOMO–LUMO gap
and emission properties in a given metal complex. We have success-
fully employed this concept in the synthesis of several new ligands
[10], with Eq. (1) showing the reaction for the preparation of 2-(fer-
rocenylidene)-4,5-bis(diphenylphosphino)-4-cyclopenten-1,3-
dione (fbpcd) [11]. The dynamic isomerization of this particular
ligand has been investigated in the triosmium clusters 1,2-Os3(-
CO)10(fbpcd) and 1,2-Os3(CO)10(fbpcd) [11], and its redox properties
have been recently reported in the case of the platinum(II) complex
Pt(mnt)(fbpcd) [10b].
O
O
molecular sieves
CH2Cl2/-H2O
Ph2P
Ph2P
Ph2P
Ph2P
OHC
Fe
O
O
Fe
bpcd
fbpcd
2.2. Synthesis of fac-BrRe(CO)3(fbpcd) (2) from BrRe(CO)3(THF)2 (1)
and fbpcd
ð1Þ
To a small Schlenk tube under argon flush was charged 0.10 g
(0.20 mmol) of 1 and 0.14 g (0.21 mmol) of fbpcd, followed the
addition of 25 mL of CH2Cl2 via syringe. The solution was stirred
at room temperature for 1 h and then examined by IR analysis,
which revealed the consumption of 1 and the presence of the de-
sired product. The volatiles were removed under vacuum and the
crude residue recrystallized from toluene at ꢀ20 °C. 2 was isolated
by filtration under argon as an air-sensitive, purple-blue solid in
Given the importance of d6 complexes in multicomponent re-
dox relays [12], coupled with our interest in the coordination
chemistry of the fbpcd ligand with different metal systems, we
have prepared and examined the redox properties of the new rhe-
nium(I) compound fac-BrRe(CO)3(fbpcd) (2). The synthesis of the
conjugated enyne derivative of fbpcd, whose structure is depicted
below, and its use as a ligand in the synthesis of fac-BrRe
(CO)3(fpbpcd) (3) are also described herein.
95% yield (0.19 g). IR (CH2Cl2):
(s), 1720 (w, sym dione), 1676 (s, antisym dione) cmꢀ1. UV–Vis
(CH2Cl2): kmax 358 ( =6100), 578 (
=1500). 1H NMR (CDCl3): d
m(CO) 2040 (vs), 1966 (s), 1912
O
e
e
Ph2P
4.19 (s, 5H, Cp), 4.81 (s, 1H, Cp), 4.85 (s, 1H, Cp), 4.98 (s, 1H, Cp),
5.35 (s, 1H, Cp), 7.36–7.91 (m, 21H, alkenyl and aryl). 31P NMR
3
3
(CDCl3): d 23.03 (d, JPꢀP=11 Hz), 23.55 (s, JPꢀP=11 Hz). ESI–MS:
m/z 1033.59 for [2+Na]+ and 1012.90 for [2+H]+.
Ph2P
O
fpbpcd
Fe
2.3. Synthesis of fpbpcd from bpcd and 3-ferrocenylpropynal
0.10 g (0.22 mmol) of bpcd and 51 mg (0.22 mmol) of 3-ferroce-
nylpropynal in 30 mL of a 5:1 mixture of CH2Cl2/MeOH were stir-
red in the presence of 3 Å molecular sieves for 48 hr at room
temperature. The solution slowly acquired a green–blue color
indicative of the condensation product fpbpcd, and TLC analysis
using CH2Cl2 as an eluent confirmed the presence of trace amounts
of the starting materials and a prominent green–blue streak
belonging to the desired ligand (Rf = 0.5). The solution was filtered
under argon and the volatiles removed under vacuum, after which
the residue was washed with hexane to remove the unreacted
bpcd and 3-ferrocenylpropynal. Yield of green–blue fpbpcd:
0.10 g (68%). Given the air sensitivity of the product and the poor
quality NMR spectra obtained due to paramagnetic impurities,
the fpbpcd ligand was characterized by IR spectroscopy and used
2. Experimental
2.1. Materials and equipment
The BrRe(CO)5 employed in the synthesis of BrRe(CO)3(THF)2 (1)
was prepared from Re2(CO)10 [13]. 1 was prepared according to the
published procedure and typically used within one week of
preparation [14]. The bpcd ligand used in the synthesis of the title
diphosphines fbpcd and fpbpcd was synthesized from 4,5-di-
chloro-4-cyclopenten-1,3-dione and Ph2PSiMe3 [15], with the
former ferrocene-containing ligand prepared from ferrocenecarb-
oxaldehyde and bpcd [11]. The acetylferrocene used in the prepara-
tion of 3-ferrocenylpropynal [16] was synthesized according the
procedure of Parshall et al. [17]. Re2(CO)10 was purchased from
Pressure Chemical Co., while the chemicals ferrocene, ferrocene-
carboxaldehyde, hexachlorocyclopentadiene, and diethyl chloro-
phosphate were purchased from Aldrich Chemical Co. and used as
received. The reaction and NMR solvents employed in these studies
were distilled from an appropriate drying agent under argon using
Schlenk techniques or obtained from an Innovative Technology sol-
directly in the next step. IR (CH2Cl2):
m(CO) 1715 (w, sym dione),
1674 (s, antisym dione) cmꢀ1
.
2.4. Synthesis of fac-BrRe(CO)3(fpbpcd) (3) from BrRe(CO)3(THF)2 (1)
and fpbpcd
The synthesis of 3 followed the procedure outlined for 2. Here
0.10 g (0.20 mmol) of 1, 0.14 g (0.20 mmol) of fpbpcd, and 25 mL
of CH2Cl2 were stirred at room temperature for 1 h. IR analysis at
this point confirmed the presence of the desired product, after

B. Poola et al. / Polyhedron 28 (2009) 2619–2624
2621
Table 2
which the solution was dried under vacuum to afford crude 3.
Selected bond distances (Å) and angles (deg) in 3 ꢂ 1/2H2O.
Recrystallization of 3 from toluene at ꢀ20 °C, followed by filtration
under argon, gave 3 as an air-sensitive, green–blue solid in 91%
Bond distances
Re(1)–C(1)
Re(1)–C(3)
Re(1)–P(1)
C(4)–C(8)
C(7)–C(8)
C(5)–C(6)
C(33)–C(34)
C(35)–C(36)
Fe(3)-centroida
1.952(2)
1.918(2)
2.4373(6)
1.349(3)
1.513(3)
1.478(3)
1.405(3)
1.419(3)
1.654(3)
Re(1)–C(2)
Re(1)–Br(1)
Re(1)–P(2)
C(4)–C(5)
1.970(2)
(0.19 g). IR (CH2Cl2):
sym dione), 1687 (s, antisym dione) cmꢀ1. UV–Vis (CH2Cl2): kmax
394 ( = 12000), 608 (
= 3800). 1H NMR (CDCl3): d 4.27 (s, 5H,
m(CO) 2041 (vs), 1967 (s), 1914 (s), 1729 (w,
2.6333(2)
2.4601(6)
1.507(3)
1.475(3)
1.349(3)
1.204(3)
3.1810(8)
1.660(3)
e
e
Cp), 4.54 (bs, 20H, Cp), 4.67 (bs, 2H, Cp), 6.80 (s, 1H, alkenyl),
C(6)–C(7)
3
7.15–7.81 (m, 2H, aryl). 31P NMR (CDCl3): d 22.76 (d, JPꢀP=12 Hz),
C(6)–C(33)
C(34)–C(35)
P(1) ꢂ ꢂ ꢂ P(2)
Fe(3)-centroidb
3
23.90 (s, JPꢀP=12 Hz). ESI–MS: m/z 1073.13 for [3+K]+.
2.5. Electrochemical studies
Bond angles
C(2)–Re(1)–P(1)
P(1)–Re(1)–P(2)
P(2)–Re(1)–Br(1)
O(2)–C(2)–Re(1)
C(33)–C(6)–C(7)
C(6)–C(33)–C(34)
C(34)–C(35)–C(36)
173.17(7)
81.01(2)
80.29(2)
179.6(3)
128.0(2)
126.1(2)
177.6(3)
C(1)–Re(1)–P(2)
P(1)–Re(1)–Br(1)
O(1)–C(1)–Re(1)
O(3)–C(3)–Re(1)
C(33)–C(6)–C(5)
C(35)–C(34)–C(33)
166.09(7)
88.89(1)
176.8(2)
177.8(2)
124.3(2)
173.6(3)
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 Cpꢁ2Fe=Cp2ꢁ Feþ (internally added) redox couple, ta-
ken to have E1/2=ꢀ0.20 V [19].
a
Mean Fe(3)–C distance for the Cp carbons C(36)–C(40).
Mean Fe(3)–C distance for the Cp carbons C(41)–C(45).
b
integrated with the available APEX2 [22] software package using
a narrow-frame algorithm, and the structure was solved and re-
fined using the SHELXTL program package [23]. The molecular
structure was checked using PLATON [24], and all nonhydrogen
atoms were refined anisotropically. Except for the water solvent
whose hydrogens were not located, all other hydrogen atoms
were assigned calculated positions and allowed to ride on the at-
tached carbon atom. The refinement for 3 ꢂ 1/2H2O converged at
R = 0.0165 and wR2 = 0.0432 for 8316 independent reflection with
2.6. Extended Hückel MO calculations
The extended Hückel calculations on compounds 2 and 3 were
carried out using the original program developed by Hoffmann
[20], as modified by Mealli and Proserpio [21]. The weighted Hij’s
contained in the program were used in the calculations, and the in-
put Z-matrices were constructed by using the available X-ray data
for 3 and related compounds.
I > 2r(I). The X-ray data and processing parameters are reported
in Table 1, with selected bond distances and angles quoted in
2.7. X-ray crystallographic data
Table 2.
Single crystals of 3 ꢂ 1/2H2O suitable for X-ray diffraction anal-
ysis were grown from an acetone solution containing 3 that had
been layered with Et2O. The X-ray data were collected on an
APEX II CCD-based diffractometer at 100(2) K. The frames were
3. Results and discussion
3.1. Synthesis, spectroscopic data, and redox properties
of fac-BrRe(CO)3(fbpcd) (2)
Displacement of the THF ligands in BrRe(CO)3(THF)2 (1) by
fbpcd is rapid and gives 2 in essentially quantitative yield as as-
sessed by IR spectroscopy. While 2 may be isolated by column
chromatography over silica gel, this leads to considerable mate-
rial loss and oxidation of the ferrocene appendage, which in turn
introduces unwanted paramagnetic impurities in the NMR sam-
ple. Alternatively, 2 could be isolated by recrystallization in tolu-
ene at low temperature in high yield, making this the preferred
method of isolation. We would add that 2 may also be prepared
by heating BrRe(CO)5 in the presence of fbpcd but the yield of 2
is lower and the reaction is accompanied by decomposition, com-
plicating the purification of the desired product. 2 is mildly air
sensitive, especially in solution, but may be handled in the atmo-
sphere for a short period of time. The structure of 2 is shown
below.
Table 1
X-ray crystallographic data and processing parameters for 3 ꢂ 1/2H2O.
CCDC entry number
Cryst system
Space group
a (Å)
b (Å)
c (Å)
727855
triclinic
P1
10.0766(3)
10.5463(4)
19.0710(7)
92.612(1)
101.511(1)
98.580(1)
ꢀ
a
(°)
b (°)
c
(°)
V (Å3)
1957.8(1)
Mol formula
C44H30BrFeO5P2Re ꢂ 1/2H2O
Formula weight
Formula units per cell (Z)
Dcalcd (Mg/m3)
1043.60
2
1.770
0.71073
4.610
k (Mo K
a
)
) (Å)
l
(mmꢀ1
O
Br
Re
CO
Ph2
P
Absorption correction
Absorption correction factor
Total reflections
semi-empirical from equivalents
0.8371/0.4042
23813
8316
8316/0/505
0.0165
0.0432
1.103
1.142, ꢀ0.503
OC
OC
Independent reflections
Data/res/parameters
P
Ph2
P
O
a
R1 [I P 2 (I)]
Fe
b
wR2 (all data)
GOF on F2
compound 2
D
q
(max),
D
q
(min) (e/Å3)
P
P
a
R1
¼
kFo j ꢀ j Fck= j Fo j.
P
P
2
2
1=2
R2 ¼ f ½wðF2o ꢀ Fc2Þ ꢃ= ½wðF2oÞ ꢃg
:
b

2622
B. Poola et al. / Polyhedron 28 (2009) 2619–2624
2 was thoroughly characterized by IR, NMR, and UV–Vis spec-
troscopies, and mass spectrometry. The IR spectrum revealed ter-
minal carbonyl stretching bands at 2041 (vs), 1967 (s), and
1914 (s) cmꢀ1 characteristic of the signature fac-Re(CO)3 moiety
[25], in addition to two lower energy
m(CO) bands observed at
1729 (w) and 1687 (s) cmꢀ1 ascribed to the vibrationally coupled
symmetric and antisymmetric carbonyl stretching bands associ-
ated with the dione platform [26]. The 1H NMR spectrum exhib-
ited a 5H singlet at d 4.19 for the unsubstituted Cp ring, with the
alkenyl-substituted Cp ring displaying four 1H singlets at d 4.81,
4.85, 4.98, and 5.35. The lone alkenyl hydrogen and the aromatic
hydrogens appear as a set of overlapping resonances from d 7.36–
7.91. Two inequivalent 31P resonances were observed in the 31P
NMR spectrum of 2 at d 23.03 and 23.55 consistent with the for-
mulated structure. The ESI mass spectrum of 2 revealed a strong
molecular ion for the sodium-containing species [2+Na]+ based on
a m/z peak at 1033.59, along with a weak m/z peak at 1012.90 for
[2+H]+. Finally, the UV–Vis spectrum of 2 (Fig. 1) recorded in
CH2Cl2 showed two featureless bands at 358 and 578 nm, whose
assignments are ascribed to a dp(Re) ? p
*(dione) MLCT and a
charge-transfer-to-solvent (CTTS) transition involving the ferro-
cene and CH2Cl2, respectively [5a,27].
Fig. 2. Perspective view of 3 ꢂ 1/2H2O showing the atom-numbering scheme.
Displacement ellipsoids are drawn at the 50% probability level. The solvent
molecule has been omitted for clarity.
The redox properties of 2 were examined by cyclic voltamme-
try (CV) in CH2Cl2 containing 0.25 M TBAP as the supporting
electrolyte. The CV of 2 (not shown) recorded over the potential
range of 1.0 V to ꢀ1.0 V and at a scan rate of 250 mV/s revealed
two diffusion-controlled waves at E1/2 = 0.71 and ꢀ0.65 V belong-
ing to 0/1+ and 0/1ꢀ redox couples [28]. The site of the two re-
dox waves was verified by extended Hückel MO calculations,
which revealed HOMO and LUMO levels at ꢀ11.80 eV and
ꢀ10.42 eV, respectively. The HOMO in 2 is best viewed as an
iron-based dz2 orbital that is similar in nature to the a1g HOMO
found in ferrocene [29]. The computationally observed LUMO
Ph2
P
Br
Re
CO
OC
OC
LUMO (-10.42 eV)
P
Ph2
Fe
traces its parentage to the w4 MO associated with the
p system
of dione ring, whose calculated energy and nodal pattern are in
excellent agreement with those data reported for other bpcd-
substituted compounds prepared by our groups [6a,10,30]. The
relatively low energy of this p* orbital renders it a superb elec-
tron reservoir and accounts nicely for the observed one-electron
reduction found by CV.
Ph2
P
Br
Re
CO
O
OC
OC
HOMO (-11.80 eV)
P
O
Ph2
3.2. Synthesis, spectroscopic data, and redox properties
of fac-BrRe(CO)3(fpbpcd) (3)
2
1
0
Knoevenagel condensation of 3-ferrocenylpropynal with an
equimolar amount of bpcd in a solvent mixture composed of
CH2Cl2/MeOH (5:1) in the presence of 3 Å molecular sieves, whose
function is to absorb the liberated water, furnishes the new ligand
2-(3-ferrocenylprop-2-ynylidene)-4,5-bis(diphenylphosphino)4-
cyclopenten- 1,3-dione (fpbpcd). Isolated yields of fpbpcd were on
the order of ca. 70% for samples that were purified by recrystalliza-
tion. The fpbpcd ligand exhibits a green–blue color and undergoes
facile oxidation in solution upon exposure to oxygen, making char-
acterization of the ligand by NMR spectroscopy problematic. The IR
spectrum of the ligand revealed two m(CO) bands at 1715 (w) and
1674 (s) for the kinematically coupled dione carbonyl groups con-
sistent with the proposed structure.
400
600
800
Treatment of either BrRe(CO)5 or 1 with a slight excess of
fpbpcd affords the corresponding diphosphine-substituted com-
pound 3, whose structure is depicted below. As discussed earlier,
reactions using 1 as a starting material were cleaner and afforded
3 in higher yields. 3 was isolated by recrystallization as a
Wavelength (nm)
Fig. 1. UV–Vis spectra for 2 (red line; ca. 4e-4 M) and 3 (black line; ca. 2e-4 M) in
CH2Cl2 solvent. The ordinate values displayed are for 2, with 3 plotted alongside
using arbitrary absorbance units.

B. Poola et al. / Polyhedron 28 (2009) 2619–2624
2623
[10c], ReCl2[(Z)-Ph2PCH@CHPPh2]2 [31], and ClRe(CO)3[3,4-Me2-
30,40- bis(diphenylphosphino)tetrathiafulvalene] [32]. The Re–CO
bond distances range from 1.970(2) Å [Re(1)–C(2)] to 1.918(2) Å
[Re(1)–C(3)] with the latter axial CO group displaying the shortest
Re–CO bond distance. The Re(1)–Br(1) vector reveals a bond length
of 2.6333(2) Å, whose value is in excellent agreement with the
Re–Br distance found in the diphosphine-substituted compounds
2
0
fac-BrRe(CO)3[
g
2-(Ph2PCH2)3CMe]
[33],
fac-BrRe(CO)3[Ph2PO
(CH2)2OPPh2] [34], and fac-BrRe(CO)3[1,2-(HOCH2P)2C6H4] [35].
The five-membered metallocyclic ring defined by the atoms
Re(1)–P(2)–C(8)–C(4)–P(1) is distinctly non-planar and exhibits a
fold angle of 29.49(4)° along the P(1) ꢂ ꢂ ꢂ P(2) hinge. The enyne car-
bons are nearly planar on basis of the observed torsion angle of ca.
ꢀ165° for the C(6)–C(33)–C(34)–C(35) atoms. The remaining bond
distances and angles are unremarkable and require no comment.
The cyclic voltammetric properties of 3 were examined in
CH2Cl2 under conditions identical to those employed in the investi-
gation of 2. The CV of 3, which is shown in Fig. 3, recorded over the
potential range of 1.0 V to ꢀ1.0 V and at a scan rate of 250 mV/s re-
vealed well-defined diffusion-controlled waves at E1/2 = 0.68 and
ꢀ0.65 V ascribed to 0/1+ and 0/1ꢀ redox couples. These half-wave
potentials are identical to those redox data recorded for 2, suggest-
ing identical HOMO and LUMO levels in the two different rhe-
nium(I) products. This latter assumption was subsequently
corroborated by extended Hückel MO calculations, which revealed
HOMO and LUMO levels at ꢀ11.86 eV and ꢀ10.39 eV, respectively.
The orbital composition of these important frontier molecular orbi-
tals in 3 resembles the HOMO and LUMO levels computed for 2 and
will not be reproduced here.
-2
-4
1.0
0.0
-0.5
-1.0
0.5
E (volts)
Fig. 3. Cathodic scan cyclic voltammogram of 3 (ca. 10ꢀ3 M and 0.25 V/s) in CH2Cl2
containing 0.25 M TBAP at room temperature.
green–blue solid and was characterized by a combination of IR,
NMR, and UV–Vis spectroscopies, mass spectrometry, and X-ray
diffraction analysis. The IR spectrum for 3 reveals terminal
bands at 2041 (vs), 1967 (s), and 1914 (s) cmꢀ1, along with sym-
metric and antisymmetric dione (CO) bands at 1729 and
m(CO)
m
1687 cmꢀ1. These carbonyl frequencies relative to those recorded
for 2 indicate that the electron-donating properties of the two
diphosphine ligands are similar in magnitude. The UV–Vis spec-
trum (Fig. 1) shows a slight bathochromic shift of the two principal
absorption bands compared to 2, with maxima appearing at
394 nm and 608 nm. The spectroscopic assignments for these
UV–Vis bands are assumed to be the same as those in 2 (vide su-
pra). The 1H NMR spectrum shows the cyclopentadienyl hydrogens
at d 4.27, 4.54, and 4.67 in a 5:2:2 integral ratio, and the lone vinyl
hydrogen on the enyne chain appears as a singlet at d 6.80. The aro-
matic hydrogens were observed as a multiplet from d 7.15–7.81.
The inequivalent phosphines in 3 appear as doublets in the 31P
4. Conclusions
The new rhenium(I) compounds fac-BrRe(CO)3(fbpcd) (2) and
fac-BrRe(CO)3(fpbpcd) (3) have been synthesized and their redox
properties investigated by cyclic voltammetry. Both compounds
exhibit well-defined 0/1+ and 0/1ꢀ redox couples attributed to fer-
rocene oxidation and diphosphine reduction, respectively, with the
HOMO and LUMO compositions in 2 and 3 corroborated by ex-
tended Hückel MO calculations. The p
*-based LUMO in compounds
2 and 3 is localized on the dione platform and remains unchanged
to that found in the parent bpcd ligand. The selection of aldehydes
more electron rich than ferrocenecarboxaldehyde will facilitate the
tuning of the HOMO level in our bpcd-derived condensation prod-
ucts, and this aspect is under active investigation. Future time-re-
solved photophysical studies are planned and these data will be
disseminated in due course.
3
NMR spectrum at d 22.76 and 23.90 and exhibit a JPꢀP coupling
of 12 Hz. The ESI mass spectrum recorded in MeCN/KI gave a
strong molecular ion at m/z 1073.13 for [3+K]+ consistent with
the formulated structure.
O
Br
Re
CO
Ph2
P
OC
OC
P
5. Supplementary data
Ph2
O
CCDC 727855 contains the supplementary crystallographic data
for 3 ꢂ 1/2H2O. 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, Cam-
bridge CB2 IEZ, UK; Fax: (+44) 1223-336-033; or email:
deposit@ccdc.cam.ac.uk.
Fe
compound 3
The unequivocal identity of 3 and the presence of the enyne
linking moiety that serves to tether the bpcd and ferrocene groups
were established by X-ray diffraction analysis. The solid-state
structure of 3 ꢂ 1/2H2O is depicted in Fig. 2. The rhenium center
is six-coordinate with the fpbpcd ligand situated trans to two of
the three facial CO groups. The mean Re–P bond distance of
2.4487 Å and the bond angle of 81.01(2)° for the P(1)–Re(1)–P(2)
linkage agree with those values reported for the octahedral rhe-
nium(I) compounds fac-BrRe(CO)3(bma) [5a], fac-BrRe(CO)3(tbpcd)
Acknowledgement
Continued financial support from the Robert A. Welch Founda-
tion (Grant B-1093) is greatly appreciated. Prof. Andreas H. Franz
(UP) and Ms. Nicole Ledbetter (UNT) are thanked for their expertise
and help in recording the mass spectra for compounds 2 and 3.

2624
B. Poola et al. / Polyhedron 28 (2009) 2619–2624
(c) W.H. Watson, D. Wiedenfeld, A. Pingali, B. Poola, M.G. Richmond,
References
Polyhedron 26 (2007) 3577;
(d) W.H. Watson, B. Poola, M.G. Richmond, J. Chem. Crystallogr. 36 (2006)
715.
[1] (a) D.S. Tyson, C.R. Luman, X. Zhou, F.N. Castellano, Inorg. Chem. 40 (2001)
4063;
[11] W.H. Watson, B. Poola, M.G. Richmond, Polyhedron 26 (2007) 3585.
[12] (a) P. Thanasekaran, R.-T. Liao, B. Manimaran, Y.-H. Liu, P.-T. Chou, S.
Rajagopal, K.-L. Lu, J. Phys. Chem. A 110 (2006) 10683;
(b) R. Ziessel, A. Juris, M. Venturi, Inorg. Chem. 37 (1998) 5061;
(c) W.K. Chan, P.K. Ng, X. Gong, S. Hou, Appl. Phys. Lett. 75 (1999) 3920.
[13] S.P. Schmidt, W.C. Trogler, F. Basolo, Inorg. Synth. 28 (1990) 160.
[14] D. Vitali, F. Calderazzo, Gazz. Chim. Acta 102 (1972) 587.
[15] (a) D.T. Mowry, J. Am. Chem. Soc. 72 (1950) 2535;
(b) D. Fenske, H.J. Becher, Chem. Ber. 107 (1974) 117;
(c) D. Fenske, H.J. Becher, Chem. Ber. 108 (1975) 2155.
[16] G. Doisneau, G. Balavoine, T. Fillebeen-Khan, J. Organomet. Chem. 425 (1992)
113.
[17] P.J. Graham, R.V. Lindsey, G.W. Parshall, M.L. Peterson, G.M. Whitman, J. Am.
Chem. Soc. 79 (1957) 3416.
[18] D.F. Shriver, The Manipulation of Air-Sensitive Compounds, McGraw-Hill, New
York, 1969.
[19] M.F. Ryan, D.E. Richardson, D.L. Lichtenberger, N.E. Gruhn, Organometallics 13
(1994) 1190.
[20] (a) R. Hoffmann, W.H. Lipscomb, J. Chem. Phys. 36 (1962) 2179;
(b) R. Hoffmann, J. Chem. Phys. 39 (1963) 1397.
[21] C. Mealli, D.M. Proserpio, J. Chem. Ed. 67 (1990) 399.
[22] APEX2 Version 2.14, Bruker Advanced Analytical X-ray Systems, Inc. Copyright
2007, Madison, WI.
[23] G.M. Sheldrick, Acta Cryst. A64 (2008) 112.
[24] A.L. Spek, J. Appl. Cryst. 36 (2003) 7.
[25] C.M. Lukehart, Fundamental Transition Metal Organometallic Chemistry,
Brooks/Cole Publishing Co., Monterey, CA, 1985.
[26] N.B. Coltrup, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman
Spectroscopy, Academic Press, New York, 1990.
[27] (a) G.L. Geoffroy, M.S. Wrighton, Organometallic Photochemistry, Academic
Press, New York, 1979;
(b) C.J. McAdam, J.L. Morgan, B.H. Robinson, J. Simpson, P.H. Rieger, A.L. Rieger,
Organometallics 22 (2003) 5126;
(c) J. Vicente, P. González-Herrero, M. Pérez-Cadenas, P.G. Jones, D. Bautista,
Inorg. Chem. 44 (2005) 7200;
(d) K. Heinze, S. Reinhardt, Organometallics 26 (2007) 5406;
(e) S. Rau, D. Walther, J.G. Vos, Dalton Trans. (2007) 915;
(f) J. Zhang, P. Du, J. Schneider, P. Jarosz, R. Eisenberg, J. Am. Chem. Soc. 129
(2007) 7726.
[2] (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.
[3] (a) V.W.W. Yam, W.K. Lee, T.F. Lai, J. Chem. Soc., Chem. Commun. (1993) 1571;
(b) Q.-H. Wei, G.-Q. Yin, L.-Y. Zhang, L.-X. Shi, Z.-W. Mao, Z.-N. Chen, Inorg.
Chem. 43 (2004) 3484;
(c) I.O.A.J. Karttunen, S.P. Tunik, M. Haukka, S.I. Stanislav, A.S. Alexei, P.Y.
Serdobintsev, M.A. Khodorkovskiy, T.A. Tapani, Inorg. Chem. 47 (2008) 9478.
[4] J.A. Zuleta, M.S. Burberry, R. Eisenberg, Coord. Chem. Rev. 97 (1990) 47.
[5] (a) For bma ligand studies, see: K. Yang, S.G. Bott, M.G. Richmond,
Organometallics 13 (1994) 3788;
(b) K. Yang, S.G. Bott, M.G. Richmond, Organometallics 14 (1995) 2387;
(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;
(d) S.G. Bott, K. Yang, M.G. Richmond, J. Organomet. Chem. 689 (2004) 791;
(e) S.G. Bott, K. Yang, M.G. Richmond, J. Organomet. Chem. 690 (2005) 3067;
(f) S.G. Bott, K. Yang, M.G. Richmond, J. Organomet. Chem. 691 (2006) 3771.
[6] (a) For bpcd ligand studies, see: 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;
(b) Y. Yamaguchi, W. Ding, C.T. Sanderson, M.L. Borden, M.J. Morgan, C. Kutal,
Coord. Chem. Rev. 251 (2007) 515.
[28] Electrochemical reversibility and electron stoichiometry were determined via
the usual methods, see: P.H. Rieger, Electrochemistry, Chapman and Hall, New
York, 1994.
[29] (a) Y.S. Sohn, D.N. Hendrickson, H.B. Gray, J. Am. Chem. Soc. 93 (1971) 3603;
(b) J.W. Lauher, R. Hoffmann, J. Am. Chem. Soc. 98 (1976) 1729.
[30] W.H. Watson, S. Kandala, M.G. Richmond, J. Chem. Crystallogr. 36 (2006)
813.
[31] M. Bakir, P.E. Fanwick, R.A. Walton, Polyhedron 6 (1987) 907.
[32] N. Avarvari, D. Martin, M. Fourmiqué, J. Organomet. Chem. 643–644 (2002)
292.
(e) W.H. Watson, B. Poola, M.G. Richmond, J. Organomet. Chem. 691 (2006)
5579.
[7] (a) H.J. Becher, D. Fenske, M. Heymann, Z. Anorg. Allg. Chem. 475 (1981) 27;
(b) R. Meyer, D.M. Schut, K.J. Keana, D.R. Tyler, Inorg. Chim. Acta 240 (1995) 405.
[8] (a) F. Mao, D.R. Tyler, D. Keszler, J. Am. Chem. Soc. 111 (1989) 130;
(b) F. Mao, D.R. Tyler, M.R.M. Bruce, A.E. Bruce, A.L. Rieger, P.H. Rieger, J. Am.
Chem. Soc. 114 (1992) 6418.
[9] 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.
[10] (a) S.W. Hunt, X. Wang, M.G. Richmond, J. Mol. Struct. 919 (2009) 34;
(b) B. Poola, S.W. Hunt, M.G. Richmond, Polyhedron 27 (2008) 3693;
[33] D.H. Gibson, H. He, M.S. Mashuta, Organometallics 20 (2001) 1456.
[34] S. Bolano, J. Bravo, R. Carballo, S. Garcia-Fontan, E.M. Vazquez-Lopez,
Polyhedron 18 (1999) 1431.
[35] R. Schibli, K.V. Katti, W.A. Volkert, C.L. Barnes, Inorg. Chem. 40 (2001) 2358.