Ligand effects on the redox behavior of bimetallic tungsten(0)/ ferrocene(II) complexes

Article abstract of DOI:10.1016/j.poly.2014.01.023

This work presents a synthesis of bimetallic tungsten(0)/ferrocene(II) complex dppf[W(CO)₃PMe₃] and detailed comparisons of the redox behavior of dppf[W(CO)₃PMe₃], dppf[W(CO) ₄], and dppf[W(CO)₃CH₃CN]. Reaction of PMe ₃ with dppf[W(CO)₃CH₃CN] in dichloromethane under an inert atmosphere yields dppf[W(CO)₃PMe₃]. Slow cooling of the column purified product yields yellow blocks suitable for X-ray diffraction analysis. Structural, spectroscopic, electrochemical, and computational studies show that all three complexes have multiple redox events corresponding to tungsten and ferrocene oxidation. The complexes' distal tungsten ligands tune the first redox event over a broad range, from -37 to -200 meV versus ferrocene. Computational studies suggest that the ligand tunes the tungsten moiety's redox potential, changing the first redox event from ferrocene(II) oxidation to tungsten(0) oxidation. This bimetallic complex is thus an interesting candidate for redox-based sensing architectures.

Full text of DOI:10.1016/j.poly.2014.01.023

Polyhedron 72 (2014) 50–55
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Ligand effects on the redox behavior of bimetallic
tungsten(0)/ferrocene(II) complexes
⇑
Abby Moore, Kyle Shufelt, Benjamin G. Janesko, Kayla N. Green
Department of Chemistry, Texas Christian University, 2800 S. University Dr., Fort Worth, TX 76129, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
This work presents a synthesis of bimetallic tungsten(0)/ferrocene(II) complex dppf[W(CO)3PMe3] and
detailed comparisons of the redox behavior of dppf[W(CO)3PMe3], dppf[W(CO)4], and dppf[W(CO)3CH3CN].
Reaction of PMe3 with dppf[W(CO)3CH3CN] in dichloromethane under an inert atmosphere yields
dppf[W(CO)3PMe3]. Slow cooling of the column purified product yields yellow blocks suitable for
X-ray diffraction analysis. Structural, spectroscopic, electrochemical, and computational studies show
that all three complexes have multiple redox events corresponding to tungsten and ferrocene oxidation.
The complexes’ distal tungsten ligands tune the first redox event over a broad range, from ꢀ37 to
ꢀ200 meV versus ferrocene. Computational studies suggest that the ligand tunes the tungsten moiety’s
redox potential, changing the first redox event from ferrocene(II) oxidation to tungsten(0) oxidation. This
bimetallic complex is thus an interesting candidate for redox-based sensing architectures.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 26 November 2013
Accepted 23 January 2014
Available online 31 January 2014
Keywords:
Ferrocene
Electrochemistry
Electronic effects
Infrared
Tungsten
1. Introduction
2. Material and methods
Ferrocene (Fc) derivatives have received substantial attention as
electrochemical biosensors. Their redox potential can significantly
shift when a molecule of interest approaches the ferrocene core [1–
16]. Table 1 illustrates that the FeIII/II potential of ferrocene deriva-
tives is sensitive to substituents on cyclopentadienyl (Cp). Electron
donating (withdrawing) Cp substituents increase (decrease) the
electron density on the iron center and yield negative (positive)
shifts in redox potential. Several previous studies have treated fer-
rocene substituent effects [4,17–19] and bimetallic diphenylphos-
phinoferrocene (dppf) complexes [20–24]. Relatively few studies
treat the detailed redox chemistry of bimetallic dppf complexes.
We report the first structural characterization of dppf[W(CO)
PMe3] (complex 3, Chart 1) and compare its electrochemical
behavior to known complexes 1 and 2. The complexes’ CO bond
lengths and vibrational frequencies serve as secondary probes of
ligand electron donation. The tungsten ligands’ electron donor abil-
ity appears to control the redox properties, with different ligands
tuning the first redox event over ꢁ140 meV.
All reagents were purchased from commercial sources and used
as received unless noted. Complex 2 was produced according to the
methods reported by Hsu et al. [20,21].
2.1. Physical methods
A Varian Mercury 300 spectrophotometer was utilized to obtain
the NMR spectra in deuterated solvent as specified in the sections
below. UV–Vis spectra were collected on an Agilent 8453 UV–Vis
spectrometer using quartz cells, and infrared spectra were
obtained using a Midac Corporation FT-Infrared spectrometer.
2.2. Synthesis of 1,10-bis(diphenylphosphine)ferrocene
trimethylphosphine tungsten tricarbonyl] (3)
A solution of 2 (0.517 mmol, 0.446 g) and trimethylphosphine
(0.8 mmol,0.8 mL) was placed in a Schlenk flask under a nitrogen
blanket. Degassed CH2Cl2 (15 mL) solvent was added. The resulting
yellow solution was stirred for 20 h. The solvent was removed
under reduced pressure yielding a mixture of crude products. Prod-
ucts were separated via chromatography give the desired product
in 89% yield (0.414 g). Chromatography used a silica column in
open air and a 1:1 ratio of CH2Cl2: n-hexane as solvent. Crystals
suitable for X-ray diffraction were obtained by re-dissolving the
⇑
Corresponding author. Tel.: +1 817 257 6220; fax: +1 817 257 5851.
E-mail address: kayla.green@tcu.edu (K.N. Green).
http://dx.doi.org/10.1016/j.poly.2014.01.023
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

A. Moore et al. / Polyhedron 72 (2014) 50–55
51
Table 1
[Bu4N][BF4] in CH3CN. Measurements were performed under a
blanket of nitrogen in 0.1 M [Bu4N][BF4] electrolyte in CH3CN.
Measurements used a scan rate of 100 mV/s. Crystalline solid 3
required treatment with a mortar and pestle prior to dissolution.
Ferrocene was used as an internal standard and reported relative
to NHE (Fc/Fc+ = +692 mV versus NHE) [25].
Measured FeIII/II redox potential of representative Fc derivatives.
Complex
E1/2 (mV)
1,10-Dibromoferrocene
Ferrocene (Fc)
337
0
ꢀ499
Decamethylferrocene [Fe(Cp⁄)2]
2.5. Computational methods
Ph
Ph
O
C
All calculations use the GAUSSIAN 09 suite of programs [26]. Calcu-
lations use nonrelativistic generalized Kohn–Sham density func-
tional theory [27]. The noninteracting Kohn–Sham reference state
wavefunction is expanded using the LANL08 basis set and relativ-
istic effective core potential on W and Fe, and the recommended 6-
311+G(d) basis on other atoms [28]. Basis sets are taken from the
EMSL Basis Set Exchange [29,30]. Calculations on open-shell
systems are performed spin unrestricted [31]. Stability analysis is
performed on the Kohn–Sham wavefunctions [32], and the most
stable SCF solution is used unless noted otherwise. DFT calcula-
tions use the local spin-density approximation LSDA [30], B3LYP
P
CO
CO
W
Fe
P
L
Ph
Ph
Chart 1.
product in a minimum amount of CH2Cl2 and cooling in a refriger-
ator for four days. IR spectrum (KBr pellet) m
CO/cmꢀ1:1927, 1846,
1831. 1H NMR (300 MHz, CD3Cl, d/ppm): 7.62–7.32 (20H, m),
4.36 (2H, m), 4.13 (2H, m), 1.02 (9H, m). 1H NMR (300 MHz,
DMSO-d6, d/ppm): 7.47 (20H, m), 4.472 (2H, m), 4.17 (2H, m),
0.90 (9H, m). 31P NMR spectrum (CD3Cl) d/ppm: ꢀ43.4 (t, JP–P = 25 Hz,
JW–P = 232 Hz) ppm, 18.3 (d, JP–P = 23 Hz, JW–P = 214 Hz) ppm.
[33,34], M06 [35], or
xB97X-D [36] approximate exchange–corre-
lation functionals. All calculations use the self-consistent reaction
field model for continuum acetonitrile solvent [37,38]. Calculations
use M06/LANL08 geometries, vibrational frequencies, and free
energy corrections. (Some test calculations use M06/LANL2DZ
geometries.) Starting geometries of all complexes are taken from
crystal structures and reoptimized. Geometries are optimized from
the most stable electronic state. Calculated harmonic vibrational
frequencies are empirically rescaled by a factor 0.9679 before com-
parison to experiment, following Ref. [39]. The Gibbs free energy in
solution is taken as the total energy evaluated in the continuum
solvent, plus ideal gas, rigid rotor, and quasiharmonic oscillator
zero-point and thermal corrections evaluated at 298 K from the
geometry and vibrational frequencies evaluated in continuum sol-
vent. Put another way, the Gibbs free energy of each species is sim-
ply taken as the ‘‘Sum of electronic thermal and Free Energies’’
printed by GAUSSIAN 09 from a geometry optimization + vibrational
frequency calculation in continuum solvent. Refs. [40–42] discuss
the validity of this approach. Phenyl groups on the dppf ligand
are replaced with methyl for computational convenience. Redox
potentials are computed as the difference in Gibbs free energies
calculated with different electron number. For example, ferro-
cene’s FeIII/II redox potential is computed as the Gibbs free energy
of ferrocene with charge = 1, spin multiplicity = 2; minus the Gibbs
free energy of ferrocene with charge = 0, spin multiplicity = 1. Re-
dox potentials are evaluated versus the calculated redox potential
of ferrocene [43], and are compared to the experimental E1/2 in
Table 1 and the experimental Epa in Table 5. Pictures of calculated
geometries use color coding C(gray), N(blue), H(white), O(red).
Bond orders are drawn as a guide to the eye. Calculated spin den-
sities are plotted with isovalue 0.0004 au.
2.3. Structure solution and refinement
A yellow block crystal of 3 (CCDC 967367) with approximate
dimensions of 0.080 mm ꢂ 0.130 mm ꢂ 0.410 mm, was used for
the X-ray crystallographic analysis. The X-ray intensity data
were measured on a Bruker SMART 1000 CCD system equipped
with
a graphite monochromator and a Mo fine-focus tube
(k = 0.71073 Å). A total of 2400 frames were collected. The total
exposure time was 13.39 h. The frames were integrated with the
Bruker SAINT Software package using a SAINT algorithm. The inte-
gration of the data using a monoclinic unit cell yielded a total
of 174528 reflections to a maximum h angle of 36.41° (0.60 Å
resolution), of which 18354 were independent (average redun-
dancy 9.509, completeness = 99.7%, Rint = 3.95%, Rsig = 3.27%) and
14612 (79.61%) were greater than 2r
(F2). The final cell constants
of a = 11.3247(5) Å, b = 22.5717(9) Å, c = 15.0822(6) Å, b =
101.971(2)°, volume = 3771.4(3) Å3 are based upon the refine-
ment of the XYZ-centroids of reflections above 20
corrected for absorption effects using the multi-scan method
SADABS). The calculated minimum and maximum transmission
coefficients (based on crystal size) are 0.3141 and 0.7501. The
occupancy of the solvent was refined as a free variable (con-
verged at 0.25808).
The structure was solved and refined using the Bruker SHELXTL
Software Package, using the space group P121/n1, with Z = 4 for
the formula unit, C40.25H37.50Cl0.50FeO3P3W. The final aniso-
tropic full-matrix least-squares refinement on F2 with 463 vari-
ables converged at R1 = 3.32%, for the observed data and
wR2 = 7.04% for all data. The goodness-of-fit was 1.065. The largest
peak in the final difference electron density synthesis was
4.822 e Åꢀ3 and the largest hole was ꢀ1.129 e Åꢀ3 with an RMS
deviation of 0.169 e Åꢀ3. On the basis of the final model, the calcu-
lated density was 1.619 g cmꢀ3 and F(000), 1826 eꢀ.
r(I). Data were
(
3. Results and discussion
3.1. Structural studies
Previous studies have shown that 1,10-bis(diphenylphos-
phino)ferrocene coordinates as a bidentate ligand to form octahe-
dral tungsten complexes [11,20,21,24,44–46]. There is limited
literature focusing on the redox behavior of these bimetallic sys-
tems. The current literature attributes the observed redox behavior
of these systems to Fe(II), not to W(0) [24]. Here we investigate a
series of dppf[W(CO)3L] complexes 1–3 (Chart 1) [21,24]. We
report the first full structural and spectroscopic characterization
of 3; and a systematic comparison of the shifts in CO bond length,
2.4. Electrochemistry
Cyclic voltammograms were acquired at room temperature
using a BASi-C3 potentiostat equipped with a 3.0 mm glassy car-
bon working electrode, a platinum wire auxiliary electrode, and
Ag/AgNO3 reference electrode filled with 0.01 M AgNO3 in 0.1 M

52
A. Moore et al. / Polyhedron 72 (2014) 50–55
Ph
Ph
Ph
Ph
Ph
Ph
O
C
O
C
CO
CO
P
P
CO
CO
P
W(CH3CN)3(CO)3
CH3CN
PMe3
W
W
Fe
Fe
Fe
P
P
P
PMe3
CH3CN
Ph
Ph
Ph
Ph
Ph
Ph
dppf
3
2
Scheme 1. Synthesis of complex 3 [21].
m
(CO), and redox potential induced by tungsten ligand L. Complex
Table 2
Crystal data and structure refinement details for 3.
1 was obtained commercially. Complex 2 was synthesized as
shown in Scheme 1 [20,21]. Complex 3 was prepared using a mod-
ified method based on the work of Hsu and coworkers [20,21].
While 3 was prepared previously, this is the first report of its full
spectroscopic and structural characterization and comparison to
1 and 2 [21].
Formula
Molecular weight
Unit cell
Space group
a (Å)
[C40H37FeO3P3W]4ꢃ[CH2Cl2]
919.54
monoclinic
P121/n1
11.3247(5)
22.5717(9)
15.0822(6)
90
b (Å)
c (Å)
Yellow crystals suitable for X-ray analyses were obtained from
column purified 3 dissolved in dichloromethane and cooled over-
night. Fig. 1 shows the single crystal X-ray diffraction structure
of complex 3. This species crystallized as a monoclinic crystal sys-
tem with a space group of P21/n derived from the four molecules
each of 3 and dichloromethane solvent per unit cell. Table 2 out-
lines data collection and refinement details. Structural parameters
of interest appear in Table 3. The Supporting Information contains
full structural data. The Cp rings of the ferrocene core are arranged
in a staggered fashion and provide bidentate complexation of the
tungsten center. The tungsten center is roughly octahedral. The
equatorial plane of tungsten atom W(1) is defined by atoms P(3),
C(2), C(3) and P(2). (Fig. 1 shows atom labels.) The angles between
these atoms are near 90°. The smallest angle P(1)–W(1)–C(2) is
82°, consistent with the steric effects of the bulky P(3)Ph2 moiety.
The W-PPh2 bond lengths are largely consistent with those re-
ported for 1 and 2. The remaining axial ligands are largely collinear
with a P(2)–W(1)–C(2) angle 170°. The three CO groups are essen-
tially linear with W–C–O angles 174–177°. The W–CO bond
lengths increase as 2 < 3 < 1.
a
(°)
b (°)
101.971(2)
90
3771.4(3)
c
(°)
Volume (Å3)
Z
q
l
4
calc(mg mmꢀ3
(mmꢀ1
)
1.619
3.632
)
F(000)
1826.0
Crystal size (mm)
2h range for data collection (°)
Index ranges
0.415 ꢂ 0.125 ꢂ 0.084
5.036–72.824
ꢀ18 6 h 6 18, ꢀ37 6 k 6 37,
ꢀ25 6 l 6 25
174528
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
18354[Rint = 0.0393]
18354/523/464
1.065
Final R indexes [I > 2
r
(I)]
R1 = 0.0332, wR2 = 0.0637
R1 = 0.0531, wR2 = 0.0704
4.84/ꢀ1.17
Final R indexes [all data]
Largest difference in peak/hole (e Åꢀ3
)
An important result in Table 3 is that the tungsten ligand con-
trols the C–O bond lengths. The average C–O bond lengths increase
with increasing ligand donor character: 1.149 Å in 1, 1.154 Å in 2,
1.1652 Å in 3. This indicates that the more electron donating
While the crystal structures are not sufficiently high resolution
to unambiguously quantify these effects, the trend is consistent
with shifts in
mCO and redox potential discussed below. The average
ligands CH3CN and PMe3 increase tungsten-CO
p backbonding.
CO bond lengths increase as 1 < 2 < 3, indicating that we show
Fig. 1. Thermal ellipsoid (50% probability) plots of 3 from two viewpoints with labeling scheme.

A. Moore et al. / Polyhedron 72 (2014) 50–55
53
Table 3
Table 5
Comparison of bond lengths (Å) and angles (°) of interest for 1–3 [11,20].
Redox peaks from Fig. 2, Epc and Epa represent cathodic and anodic peak potentials,
other details are in Section 2.
1
2
3
Epa (mV)
Epc (mV)
P2
P2
W1
W1
C1
C2
C3
C4
C
P1
P3
–
–
98.925(6)
98.429(6)
1.1660(8)
1.1611(9)
1.1684(8)
–
95.25(7)
1.156(9)
1.145(2)
1.153(9)
1.140(9)
1.149
97.40(5)
1.141(8)
1.144(8)
1.174(9)
–
1
2
3
ꢀ37
ꢀ40
388
111
83
ꢀ167
ꢀ176
ꢀ276
O1
O2
O3
O4
O
517
353
996
630
1414
1123
398
501
ꢀ200
Average
1.153
1.1652
W1
W1
C1
C2
C3
C4
P1
P3
P2
2.533(2)
2.563(3)
2.020(7)
1.982(9)
1.964(7)
2.029(7)
–
2.539(2)
2.539(2)
1.906(7)
1.948(7)
1.945(8)
–
2.55531(17)
2.53321(19)
1.9703(6)
1.9699(7)
1.9654(7)
–
3.3. Electrochemical studies
W1
W1
W1
W1
W1
Fig. 2 shows the measured cyclic voltammograms of complexes
1–3. Table 5 quantifies the redox peaks from Fig. 2. The complexes
show one quasi-reversible event located between 0 and ꢀ300 mV,
and up to four other oxidations at more positive potentials. These
presumably combine tungsten and ferrocene oxidation, ultimately
yielding Fe(III)/W(IV). The most negative event had previously
been attributed to oxidation of Fe(II), rather than oxidation of
W(0) [24]. However, Table 5 shows that the position of this redox
event sensitively depends on the tungsten ligand, with the first few
peaks’ Epa and E1/2 (Table SI3) decreasing with increasing ligand
electron donor character as 1 > 2 > 3. Moreover, complexes 2 and
3 show a second redox event within <120 mV of ferrocene. This
suggests that the tungsten(0) species may play an important role
in the complexes’ electrochemistry.
–
2.55086(18)
below that ligand electron donation also correlates with positive
shifts of the redox potential and mCO stretch frequencies.
3.2. Spectroscopic studies
The increased tungsten-CO
should decrease the C-O vibrational stretch frequencies mCO
[47,48]. Table 4 reports CO measured for complexes 1–3. Complex
p backbonding discussed above
m
3.4. Computational studies
4 is included for comparison. Complex 1 shows three stretches as-
signed as A11, A12 + B1, and B2 based on the C2v symmetry sur-
2
Fig. 3 compares experimental and calculated values for the first
two redox potentials of complexes 1–3. For example, ‘‘3, second’’
refers to the second Epa (83 mV versus Fc) tabulated for complex
3 in Table 5. Calculations on the molecules of Table 1 are included
for comparison. The calculations reproduce the experimental
trends. The R2 and root-mean-square error between experimental
and calculated redox potentials versus Fc are 0.76 and 0.25 eV.
These discrepancies arise from the continuum solvent model em-
ployed, the finite basis set, the approximate exchange–correlation
functional, and the experimental error bars. The RMS error is con-
sistent with a recent computational study of monometallic com-
plexes’ one-electron redox potentials versus Fc, while the R2 is
somewhat lower [41]. The lower correlation coefficient is in part
because the calculated redox potentials of W/Fe complexes 1–3
lie somewhat above the trend for ferrocenes, possibly due to differ-
ent scaling factors for tungsten. Supporting Information Tables
rounding the tungsten center. We presume the A1 and B1 bands
overlap, as seen in related Mo complexes [49,50]. Complexes 2–4
have a lower symmetry about the tungsten center resulting in
three mCO bands. Consistent with the previous discussion, increas-
ing tungsten ligand electron donor character (1 < 2 < 3) lowers
CO. The mCO of 4 are above 2 and 3, indicating that the CH3CN
and PMe3 ligands are more electron donating than the bulky
m
PPh2Me.
Table 4
mCO (cmꢀ1) and peak intensities measured for complexes 1–4.
Complex
Solvent
mCO
dppfW(CO)4 (1) (ref [24])
dppfW(CO)3CH3CN (2)
dppfW(CO)3P(Me)3 (3)
CDCl3
2016m, 1915s, 1891vs
1930vs, 1836s, 1812s
1927,1846,1831
1,2-C2H4Cl2
1,2-C2H4Cl2
1,2-C2H4Cl2
dppfW(CO)3PPh2Me (4) (ref 21)
1960w, 1852s, 1842(sh)
Fig. 2. Cyclic voltammograms of 1–3 in CH3CN with 0.1 M [Bu4N][BF4] at a scan rate
of 100 mV/s and referenced to Fc (Fc = 0.00 mV).
Fig. 3. Calculated vs. measured redox potentials.

54
A. Moore et al. / Polyhedron 72 (2014) 50–55
Fig. 4. Calculated M06/LANL08 (left) and
x
B97XD/LANL08//M06/LANL2DZ (right) electron spin densities. One-electron-oxidized 1 (top), 2 (middle), 3 (bottom).
SI4–SI5 show that M06/LANL08 calculations also reproduce bond
lengths, bond angles, and mCO of complexes 1–3 to within approx-
imately 0.05 Å, 4°, and 20 cmꢀ1. These relatively small errors sug-
gest that this level of theory is appropriate for initial
computational studies. Calculations moving ligand L to the equato-
rial position (Fig. SI1) find that, for neutral Fe(II)/W(0), this isomer
is destabilized by 12.1 kcal/mol for 2 and by 8.0 kcal/mol for 3.
Fig. 4 illustrates a possible explanation for the tungsten ligand
dependence of the first redox event. The left column of Fig. 4 shows
the calculated M06/LANL08 electron spin density for one-electron-
oxidized molecules 1–3. The unpaired electron is localized to Fe in
1 and to W in 2 and 3. The latter species are thus predicted to be
Fe(II)/W(I) complexes, rather than Fe(III)/W(0). This suggests that
the tungsten ligand dependence is not a long-range effect on ferro-
cene oxidation, but a tuning of the tungsten(0) moiety. The spin
localization is quite sensitive to the computational method
employed. The right column column illustrates this for DFT
However, these results provide additional evidence that the first
redox event may not always be ferrocene oxidation.
4. Conclusions
The structures, infrared spectra, and electrochemistry of
dppf[W(CO)3R] (R = CO, CH3CN, PMe3) complexes were investi-
gated to explore these complexes’ complicated redox spectra. Re-
sults were supported by electronic structure calculations. The
results indicate that these bimetallic Fe(II)/W(0) complexes have
complex and highly tunable redox chemistry, with tungsten oxida-
tion potentially playing a significant role. This makes them inter-
esting candidates for future studies, particularly as redox-active
sensors.
Acknowledgments
K.G. was supported by the R.A. Welch Foundation grant P-1760
and TCU Research and Creative Activities Fund. K.G. and B.G.J. were
additionally supported by TCU start-up funds, and K.S. was sup-
ported by a TCU SERC grant. The authors thank Tracy Hanna for
useful discussions.
calculations with the long-range-corrected
(Supporting Information Table SI6 shows that
M06L/LANL08 calculations also recover the experimental trends of
Fig. 3, though the absolute values differ.) B97XD predicts that 2
x
B97XD functional.
x
B97XD/LANL08//
x
is Fe(II)/W(I) while 1 and 3 are Fe(III)/W(0). Supporting Information
Fig. SI2 shows that the LSDA predicts that all spin densities are
delocalized over both metal centers, consistent with its well-
known tendency to overdelocalize electrons. The oxidized,
open-shell bimetallic complexes treated here are likely multideter-
minant, and DFT calculations may not even be qualitatively correct.
Appendix A. Supplementary data
CCDC 967367 contains the supplementary crystallographic data
for compound 3. These data can be obtained free of charge via

A. Moore et al. / Polyhedron 72 (2014) 50–55
[25] N.G. Connelly, W.E. Geiger, Chem. Rev. 96 (1996) 877.
55
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336 033; or email: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/
10.1016/j.poly.2014.01.023.
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