Ferrocene linked to PtL2 fragments (L2 = Cl 2, fumaronitrile, 3,6-Di-tert-butylcatecholato): A spectroscopic and theoretical investigation of redox behavior and charge distributions

Article abstract of DOI:10.1021/om700585v

The optical and redox properties of a ferrocene-appended diimine ligand (1) and its PtL₂ complexes [L₂ = Cl₂ (2), fumaronitrile (3), 3,6-di-tert-butylcatecholato (4)] are reported. The chromophores are characterized by NMR, UV/vis, and IR spectroscopy as well as cyclic voltammetry. Preparative oxidations to radical cations using Magic Blue [N(p-C₆H₄Br)₃][SbCl₆] or silver triflate in conjunction with UV/vis, NMR, and EPR spectroscopy were used to probe the chloride-dependent redox chemistry of ligand 1 and platinum complexes 2-4. Density functional calculations were employed to corroborate geometric and electronic descriptions of 1-4 and their oxidized counterparts.

Full text of DOI:10.1021/om700585v

5406
Organometallics 2007, 26, 5406-5414
Ferrocene Linked to PtL2 Fragments (L2 ) Cl2, Fumaronitrile,
3,6-Di-tert-butylcatecholato): A Spectroscopic and Theoretical
Investigation of Redox Behavior and Charge Distributions†
Katja Heinze* and Sven Reinhardt
Department of Inorganic Chemistry, UniVersity of Heidelberg, Im Neuenheimer Feld 270,
D-69120 Heidelberg, Germany
ReceiVed June 14, 2007
The optical and redox properties of a ferrocene-appended diimine ligand (1) and its PtL2 complexes
[L2 ) Cl2 (2), fumaronitrile (3), 3,6-di-tert-butylcatecholato (4)] are reported. The chromophores are
characterized by NMR, UV/vis, and IR spectroscopy as well as cyclic voltammetry. Preparative oxidations
to radical cations using “Magic Blue” [N(p-C6H4Br)3][SbCl6] or silver triflate in conjunction with UV/
vis, NMR, and EPR spectroscopy were used to probe the chloride-dependent redox chemistry of ligand
1 and platinum complexes 2-4. Density functional calculations were employed to corroborate geometric
and electronic descriptions of 1-4 and their oxidized counterparts.
the catalytic properties of (diimine)MLn complexes (M ) Ni,
Introduction
Pd, etc.).16-18
Square planar platinum(II) complexes have found widespread
applications as cytostatic drugs,1-4 as potential photocatalysts,5-8
as artificial photosynthetic devices,9 as NLO materials,10,11 as
electrophosphorescent dopants in OLEDs,12-14 and as sensitizers
in dye-sensitized solar cells.15 In many cases chelating diimine
ligands serve as ancilliary ligands while the remaining coligands
tune the properties of the resulting complex; e.g., Pt(dithiolate)-
(diimine) complexes are luminescent while Pt(diolate)(diimine)
and Pt(catecholate)(diimine) complexes usually are nonlumi-
nescent at ambient temperature. On the other hand, the steric
and electronic specifications of diimine ligands often determine
Ferrocenes have been appended to diimine ligands and to
coligands to tune the catalytic performance of homogeneous
catalysts and to install redox-switchable tags onto catalysts to
simplify catalyst recovery.19-24 To fully exploit the redox
properties of ferrocenes appended to transitions metal complexes
for tuning catalytic and optical properties, it is essential to gain
insight into the electronic situation of ferrocene appended
transition metal complexes and to understand their redox
behavior. Ferrocenes as electron donors have also been con-
nected to the quinone/semiquinone/hydroquinone redox system
with proton-dependent redox chemistry.25-27 The study of such
dyads has provided new insight into unusual charge distributions
(valence tautomerism, bistability), proton-coupled electron-
transfer reactions, and the role of hydrogen bonding in excited
charge-separated states. Ligand- versus metal-centered redox
chemistry of transition metal complexes with catecholato and/
* To whom correspondence should be addressed. Fax: int + 49 6221
548587. Tel.: int
urz.uni-heidelberg.de.
+ 49 6221 545707. E-mail: katja.heinze@
† This article is dedicated to Professor Gottfried Huttner on the occasion
of his 70th birthday.
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10.1021/om700585v CCC: $37.00 © 2007 American Chemical Society
Publication on Web 09/28/2007

Ferrocene Linked to PtL2 Fragments
Scheme 1. Reactions of 1a
Organometallics, Vol. 26, No. 22, 2007 5407
a Key: (i) PtCl2(dmso)2; (ii) Pt(nb)3, fumaronitrile; (iii) Pt(nb)3, 3,6-
di-tert-butyl-1,2-benzoquinone.
or o-semiquinonato ligands28 has been of considerable debate
during the last decades and has been recently systematically
addressed by Wieghardt and co-workers for the Ni, Pd, and Pt
triad.29-31
Herein we present a study on the electronic interaction
between ferrocene and PtL2 connected by a diimine chelate with
platinum in the formal oxidation states of zero and +II and the
coligands varying among chloride, fumaronitrile, and a poten-
tially noninnocent catecholato/o-semiquinonato/o-quinonato coli-
gand. The electronic structure of oxidized species is probed by
electrochemical measurements and preparative oxidation with
“Magic Blue” [N(p-C6H4Br)3][SbCl6] or silver triflate and
subsequent UV/vis, NMR, and EPR spectroscopic analyses.
Interpretations concerning geometric and electronic situations
will be aided by density functional calculations.
Figure 1. Partial 1H NMR spectra of 2-4 in CD2Cl2 at 400 MHz
(asterisk denotes residual CDHCl2).
(3) is conveniently prepared from 1, Pt(nb)3 (nb ) nor-
bornene),33 and fumaronitrile via ligand exchange in 86%
isolated yield (Scheme 1). The 3,6-di-tert-butylcatecholato
complex (4) is accessible from 1, Pt(nb)3, and 3,6-di-tert-butyl-
[1,2]benzoquinone34 by oxidative addition of the quinone to
platinum(0) in 45% yield (Scheme 1). All new compounds have
been characterized by one- and two-dimensional NMR spec-
troscopy (Tables 1 and 2), IR and UV/vis spectroscopy, high-
resolution mass spectrometry, cyclic voltammetry, and elemental
analyses.
Results and Discussion
Synthesis and Characterization of PtL2(Fc-N∩N′) [L2 )
Cl2 (2), Fumaronitrile (3), 3,6-Di-tert-butylcatecholato (4)].
The ferrocenyl-appended diimine ligand Fc-N∩N′ (1) has been
prepared according to the literature.18 Dichloroplatinum(II)
complex (2) has been obtained in 69% yield from 1 and PtCl2-
(dmso)2,32 while the platinum(0) complex with fumaronitrile
(28) Pierpont, C. G.; Lange, C. W. Prog. Inorg. Chem. 1994, 41, 331-
442.
(29) Ghosh, P.; Begum, A.; Herebian, D.; Bothe, E.; Hildenbrand, K.;
Weyhermu¨ller, T.; Wieghardt, K. Angew. Chem., Int. Ed. 2003, 42, 563-
567.
In the proton NMR spectra all complexes 2-4 display 195Pt
3
satellites for resonances of H5 and H10 with JPtH5 ) 88, 54,
and 69 Hz and 3JPtH10 ) 37, 27, and 31 Hz, respectively (Figure
(30) Herebian, D.; Bothe, E.; Neese, F.; Weyhermu¨ller, T.; Wieghardt,
K. J. Am. Chem. Soc. 2003, 125, 9116-9128.
(31) Bachler, V.; Olbrich, G.; Neese, F.; Wieghardt, K. Inorg. Chem.
2002, 41, 4179-4193.
(33) Crascall, L. E.; Spencer, J. L. Inorg. Synth. 1990, 28, 126-132.
(34) Paquette, L. A.; Hefferon, G. J.; Samodral, R.; Hanzawa, Y. J. Org.
Chem. 1983, 48, 1262-1266.
(32) Romeo, R.; Scolaro, L. M. Inorg. Synth. 1998, 32, 153-158.

5408 Organometallics, Vol. 26, No. 22, 2007
Heinze and Reinhardt
Table 1. 1H NMR Data for 2-4 in CD2Cl2
proton
H1
H3/H3
H4/H4
H5
2
3
4
4.37 (s, 5H)
5.12 (s, 2H)
4.53 (s, 2H)
4.37 (s, 5H)
4.32 (s, 5H)
4.98 (pt, 2H)
4.63 (pt, 2H)
5.14 (pt, 1H)/4.96 (pt, 1H)
4.68 (m, 1H)/4.65 (m, 1H)
9.34 (s, 3JPtH ) 54 Hz, 1H)
7.81 (d, 3JHH ) 7.9 Hz, 1H)
8.09 (pt, 1H)
9.35 (s, 1H, 3JPtH ) 88 Hz)
7.88 (d, 3JHH ) 7.1 Hz, 1H)
8.15 (pt, 1H)
9.16 (s, 3JPtH ) 69 Hz, 1H)
7.76 (d, 3JHH ) 7.0 Hz, 1H)
7.96 (pt, 1H)
H7
H8
H9
H10
H11/H11
7.71 (pt, 1H)
7.62 (pt, 1H)
7.62 (pt, 1H)
9.74 (d, 3JHH ) 5.6 Hz, 3JPtH ) 37 Hz, 1H)
9.21 (d, 3JHH ) 5.1 Hz, 3J(PtH) ) 27 Hz, 1H)
2.81 (d, 3JHH ) 7.8 Hz, 2JPtH ) 89 Hz, 1H)/
2.86 (d, 3JHH ) 7.8 Hz, 2JPtH ) 90 Hz, 1H)
8.71 (d, 3JHH ) 5.2 Hz, 3JPtH ) 31 Hz, 1H)
H13/H13
H15/H15
5.85 (s, 5JPtH ) 19 Hz, 2H)
1.23 (s, 18H)
Table 2. 13C NMR Data for 2-4 in CD2Cl2
for olefin rotation is estimated as ∆Gqrot ) 77 kJ mol-1 on the
basis of the equations kTc ) 2.22(∆ν2 + 6J2)1/2 and ∆Gq
)
-RTc ln[kTcNAh/(RTc)] and the experimental values Tc ≈ro3t 63
K, ∆ν ) 24.0 Hz, and J ) 7.72 Hz. The estimated rotation
barrier for the fumaronitrile ligand in 3 is in a range typical for
PtL2(olefin) complexes.35 Interestingly, for Pt(η2-fn)(tBu-N∩N′)
no fluxional behavior has been observed in CDCl3.36
carbon
2
3
4
C1
C2
C3/C3
C4/C4
C5
C6
C7
C8
C9
71.8 (s)
106.6 (s)
68.7 (s)
71.5 (s)
103.2 (s)
67.3 (s)/66.6 (s)
71.4 (s)/71.0 (s)
156.2 (s)
156.4 (s)
126.0 (s)
139.4 (s)
128.5 (s)
71.3 (s)
103.3 (s)
67.2 (s)
69.3 (s)
70.7 (s)
165.0 (s)
157.7 (s)
126.8 (s)
140.0 (s)
127.9 (s)
150.7 (s)
155.4 (s)
157.2 (s)
126.2 (s)
138.4 (s)
127.4 (s)
148.6 (s)
178.5 (s)
109.2 (s)
90.3 (s)
However, the molecular structure of Pt(η2-fn)(tBu-N∩N′)
has been determined by X-ray crystallography which allows us
to compare and validate the metric data of the DFT-optimized
minimum structure of 3 (Figure 3; B3LYP, LanL2DZ) with
those of Pt(η2-fn)(tBu-N∩N′).36 The DFT calculations will be
used to interpret fluxional behavior and electronic structure of
the platinum complexes (vide infra). In fact, the overall geometry
is well reproduced by the calculation. The calculated CdC bond
length of 3 (1.500 Å) compares well with the experimental one
for Pt(η2-fn)(tBu-N∩N′) [1.43(2) Å] as do the Pt-Npy and Pt-
Nim distances [3-DFT, 2.122, 2.169 Å; Pt(η2-fn)(tBu-N∩N′),
2.105(15), 2.134(12) Å], while the Pt-C11 and Pt-C11′
distances are somewhat overestimated by the calculation
employing this level of theory [3-DFT, 2.079, 2.099 Å; Pt(η2-
fn)(tBu-N∩N′), 2.007(15), 2.011(14) Å]. For our purpose,
however, the accuracy is sufficient. Also shown in Figure 3 are
selected H‚‚‚H distances relevant for the interpretation of the
NOE cross-peaks and the conformation in solution (vide supra).
The calculated (in the gas phase) CN stretching vibrations
(νCN,asym ) 2238 cm-1, νCN,sym ) 2232 cm-1; not scaled) are
also in line with the experimental IR data (in the solid state,
νCN,asym ) 2203 cm-1, νCN,sym ) 2195 cm-1; in CH2Cl2, νCN,asym
) 2202 cm-1, νCN,sym ) 2193 cm-1). The transition state for
olefin rotation has been calculated by DFT methods to lie at
105 kJ mol-1 above the ground state in good agreement with
the experimental value. Likewise, the barrier for rotation of the
ferrocenyl substituent (Scheme 2) has been probed by DFT
calculations. As expected, an asymmetric energy profile for the
torsion around C5-Nim-C2-C3 is calculated with two transi-
tion states at -100 and 70° corresponding to barriers of 31 and
C10
154.2 (s)
-0.8 (s)/0.7 (s)
C11/C11
C12/C12
C13/C13
C14/C14
C15/C15
34.7 (s)
29.5 (s)
1). For 3 and 4 195Pt satellites are also observed for the
2
doublets of olefin protons H11 and H11′ with JPtH ≈ 90 Hz
(3) and the singlet resonance of the catecholate protons H13/
5
H13′ with JPtH ) 19 Hz (4) (Figure 1). All Pt-H couplings
have been checked by measuring proton NMR spectra at 200
and 400 MHz.
Significant differences in proton chemical shifts are observed
for pyridine proton H10 and ferrocene protons H3 and H4 in
2-4 (Figure 1). For the olefin complex 3 the protons H3/H3′
and H4/H4′ as well as the corresponding carbon atoms C3/C3′
and C4/C4′ are diastereotopic due to the asymmetric coordina-
tion of the (E)-olefin relative to the Pt-N-N′ plane which
renders the platinum atom chiral (Tables 1 and 2). Thus, 3 exists
as an enantiomeric pair 3 and 3′ (by rotation of the ferrocenyl
substituent, Scheme 2). 2D correlation and NOE spectroscopy
allowed us to assign all signals in the NMR spectrum of 3 to
individual protons of 3 as NOE cross-peaks are observed
between H3 and H5 as well as between H3′ and H11′. Variable-
temperature 1H NMR spectra in [D6]DMSO remain unchanged
up to 90 °C except for the two doublets for H11 and H11′ which
coalesce at around 90 °C (Figure 2). The spectral changes are
reversible indicating no substantial degradation of the complex.
Ligand dissociation up to 90 °C is negligible as all signals
including platinum satellites of H5 and H10 and diasterotopic
splitting of H3 and H3′ are preserved. The coalescence is
attributed to beginning olefin rotation. The activation barrier
50 kJ mol-1
.
Geometries of dichloroplatinum(II) complexes with asym-
metric diimine ligands have been calculated recently by DFT
methods and evaluated against experimental molecular struc-
Scheme 2. Enantiomeric Pair 3 and 3′ a
a Key: (i) Fc rotation; (ii) molecule rotation.

Ferrocene Linked to PtL2 Fragments
Organometallics, Vol. 26, No. 22, 2007 5409
Figure 5. UV/vis absorption spectra of 1-4 in CH2Cl2 (and
Gaussian deconvolution of the absorption bands of 4 in the visible
region; dotted line).
1
Figure 2. Variable-temperature H NMR spectra of 3 in [D6]-
DMSO at 200 MHz.
Figure 3. DFT-calculated minimum geometry of 3 with relevant
H‚‚‚H distances indicated.
Figure 6. Ferrocene and diimine frontier Kohn-Sham orbitals of
1 (isosurfaces at 0.07 au).
C12/C12′, and C13/C13′ are magnetically indistinguishable in
the NMR spectra (vide supra). The calculated C-O distances
are in the typical range for coordinated catecholato ligands and
justify the description of 4 as a catecholato complex.38 The
calculated Pt-N distances are the shortest in the series of
complexes 2-4 (Pt-Npy 2.007 Å; Pt-Nim 2.048 Å), which may
point to some charge delocalization in the ground state from
the catecholato to the diimine ligand mediated by Pt(dπ) orbitals.
Optical Spectroscopy of 1-4. All compounds 2-4 are deep
green in the solid state and in solution, while the ferrocene-
appended ligand 1 itself is orange-red with a ferrocene f
diimine charge-transfer band [“eπ(HOMO-Fc)” f π*(LUMO-
diimine)] at 493 nm (ꢀ ) 1890 M-1 cm-1). Upon coordination
of PtL2 to the diimine, this charge-transfer band shifts to 608,
621, and 686 nm for 4, 3, and 2, respectively (in CH2Cl2; Figure
5). These bands are negatively solvatochromic as in the more
polar solvent THF they shift to lower energy by 500-1000 cm-1
which is indicative for different dipole moments of the ground
and exited states and typical for charge-transfer transitions. If
the assignment of the band as a ferrocene f diimine charge
transfer is correct it is expected that the transition energy is
correlated to the energy difference of the diimine LUMOs and
the ferrocene HOMOs (Figure 6; B3LYP; LanL2DZ). Indeed,
good correlations are observed between the energy differences
of the highest occupied ferrocene-based molecular orbitals [“eπ-
(HOMO-Fc)”] and the two lowest empty diimine-based mo-
lecular orbitals as calculated by DFT methods (Figure 7). Thus,
the observed absorption band can be ascribed to transitions from
the ferrocene “eπ” orbitals to the diimine π* orbitals and the
Figure 4. DFT-calculated minimum geometry of 4.
tures.37 So a comparison of the DFT-optimized geometry of 2
and experimental values is omitted. All values are in the
expected range (Pt-Npy 2.043 Å; Pt-Nim 2.079 Å; Pt-Cl1
2.401 Å; Pt-Cl1′ 2.402 Å).
The DFT-optimized minimum geometry of 4 is depicted in
Figure 4. There appears to be a negligible difference in trans
influences of Nim and Npy in 4 as all Pt-O, C-O, and
symmetry-related C-C distances in the catecholato ligand are
very similar (Pt-O1 1.995 Å; Pt-O1′ 2.023 Å; C11-O1 1.389
Å; C11′-O1′ 1.391 Å; C11-C12 1.420 Å; C11′-C12′ 1.422
Å; C12-C13 1.408 Å; C12′-C13′ 1.408 Å). Accordingly, the
protons H13 and H13′ as well as the carbon atoms C11/ C11′,
(35) van Asselt, R.; Elsevier, C. J.; Smeets, W. J. J.; Spek, A. L. Inorg.
Chem. 1994, 33, 1521-1531.
(36) Canovese, L.; Visentin, F.; Chessa, G.; Santo, C.; Uguagliati, P.;
Maini, L.; Polito, M. J. Chem. Soc., Dalton Trans. 2002, 3696-3704.
(37) Reinhardt, S.; Heinze, K. Z. Anorg. Allg. Chem. 2006, 632, 1465-
1470.
(38) Pal, S.; Das, D.; Sinha, C.; Kennard, C. H. L. Inorg. Chim. Acta
2001, 313, 21-29.

5410 Organometallics, Vol. 26, No. 22, 2007
Heinze and Reinhardt
Figure 7. Correlation of the low-energy absorptions of 1-4 with
energy differences of molecular orbitals [FcHOMO-1 f π*LUMO (b),
FcHOMO f π*LUMO+1 (O), FcHOMO f π*LUMO+1 (9), FcHOMO-1
π*LUMO+1 (0)]. (The solid lines are a guide to the eye.)
Figure 9. Titration of 1 with “Magic Blue” in CH2Cl2.
f
Figure 10. Titration of 2 with “Magic Blue” in CH2Cl2.
Figure 8. UV/vis absorption spectrum of 4 and a reference solar
AM 1.5 spectrum.40
in intensity. The new band is tentatively assigned a ferrocenium
absorption mixed with some charge-transfer character which
accounts for the rather broad band. Tight isosbestic points are
observed during oxidation at 438 and 561 nm suggesting a clean
oxidation of 1 to 1•+ by [N(p-C6H4Br)3]•+. Proton NMR spectra
of 1•+ obtained by oxidation of 1 with 1 equiv of [N(p-C6H4-
Br)3]•+ in THF show only paramagnetically broadened signals
which could not be assigned with confidence.
absorption energy can be tuned by coordination of PtL2
fragments to the diimine over a range of 5700 cm-1 (Figure 7).
For 4 an additional absorption band is observed at 496 nm
with ꢀ ) 4435 M-1 cm-1 (obtained by spectral deconvolution).
This absorption band is ascribed to a ligand-to-ligand charge
transfer from the catecholate to the diimine ligand. For Pt(cat)-
(phen) (phen ) phenanthroline, cat ) catecholato) the corre-
sponding band is found at 540 nm.39 Thus, in 4 the dual charge
transfer to the diimine results in covering the whole visible
region of the solar spectral irradiance distribution with extinction
coefficients of more than 1500 M-1 cm-1 (Figure 8). In this
sense the two appended moieties at the diimine (Fc and
catecholate) are able to harvest light energy accompanied with
a charge shift to the diimine in the excited states.
Redox Chemistry of 1-4. The cyclic voltammogram of 1
in CH2Cl2 reveals a reversible oxidation of the ferrocene moiety
at 505 mV vs SCE, which is excellent agreement with the
reported value.18 Chemical oxidation of 1 was performed using
“Magic Blue” [N(p-C6H4Br)3][SbCl6] with a formal redox
potential of 0.7 V higher than that of ferrocene.41 In the range
0-1 equiv of [N(p-C6H4Br)3]•+ added to 1 the characteristic
bands of the radical cation at 727 nm41 are absent implying
that the blue radical cation is reduced to the aromatic amine
while 1 is oxidized to 1•+ (Figure 9). The absorption band at
493 nm decreases while a new broad band at 688 nm increases
In a CV experiment 2 displays a reversible Fc/Fc+ oxidation
at 615 mV vs SCE, which is similar to that observed for the
palladium analogue PdCl2(Fc-N∩N′) (630 mV vs SCE).18
Preparative oxidation was again accomplished using [N(p-C6H4-
Br)3][SbCl6] in CH2Cl2. During oxidation the Fc f diimine CT
band at 686 nm disappears as does the ferrocene band at 445
nm, while new bands at 462 and 650 nm appear (Figure 10).
Again, the latter weak band is consistent with a ferrocenium
moiety. Clean isosbestic points are found at 459 and 538 nm.
Adding more than 1 equiv of [N(p-C6H4Br)3][SbCl6] results in
appearance of the typical band of the triarylamine radical cation
at 727 nm41 showing that 2•+ is not oxidized further by [N(p-
1
C6H4Br)3]•+. Similar to 1•+, H NMR spectra of 2•+ obtained
by oxidation of 2 with 1 equiv of [N(p-C6H4Br)3]•+ in THF
display only paramagnetically broadened signals which could
not be assigned with confidence.
The cyclic voltammogram of the platinum(0) complex 3
displays a quasireversible oxidation at 655 mV vs SCE.
However, for scan rates below 100 mV s-1 the reduction peak
current is no more proportional to the square root of the scan
rate (Figure 11). This finding suggests that a slow EC mecha-
nism is operative giving an oxidized product which is reduced
below 600 mV (Figure 11). The quasireversiblity also accounts
(39) Kamath, S. S.; Uma, V.; Srivastava, T. S. Inorg. Chim. Acta 1989,
166, 91-98.
(40) http://rredc.nrel.gov/solar/spectra/am1.5/.
(41) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877-910.

Ferrocene Linked to PtL2 Fragments
Organometallics, Vol. 26, No. 22, 2007 5411
AgOTf induces a bathochromic and hyperchromic shift of this
band to 640 nm. This finding also suggests a more complicated
reaction than just a simple oxidation of Fc to Fc+.
To further clarify the initial oxidation events a proton NMR
spectrum of 3 and 0.5 equiv of [N(p-C6H4Br)3][SbCl6] was
measured in CD2Cl2. All the products are diamagnetic with
chemical shifts in the typical diamagnetic range (see above).
The reduction product N(p-C6H4Br)3 shows two doublets at δ
3
) 6.93 and 7.37 ppm with JHH ) 8.7 Hz. Two products each
containing a Pt(Fc-N∩N′) unit are observed in the ratio 0.75:
0.25 (internally referenced to the integral of the triarylamine
doublets as 0.5 equiv; 6H) in addition to a singlet of free
fumaronitrile at δ ) 6.31 ppm with an intensity corresponding
to 0.75 equiv (1.5 H). The signals of the major product have
striking similarity to those of PtCl2(Fc-N∩N′) (2) in terms of
chemical shifts and Pt coupling constants, while the signals of
the minor product bear resemblance to those of the platinum-
(0) complex 3. From a stoichiometric point of view [N(p-C6H4-
Br)3][SbCl6] acts as a three-electron oxidant toward 3. In this
Figure 11. Cyclic voltammograms of 3 in CH2Cl2 at different scan
rates.
-
redox reaction the SbCl6 counterion is noninnocent and acts
as a source of nucleophilic chloride ions furnishing SbCl5 which
is itself a two-electron oxidizing agent forming SbCl3.41 The
following mechanism is proposed: After one-electron transfer
from 0.5 equiv of 3 to 0.5 equiv of [N(p-C6H4Br)3]•+, the
generated cation 3•+ disproportionates to 0.25 equiv of 3 and
0.25 equiv of 32+ with the latter loosing fumaronitrile (0.25
equiv). The slow disproportionation is also consistent with the
electrochemical experiments. Chloride is then abstracted from
-
the SbCl6 anion to form 0.25 equiv of PtCl2(Fc-N∩N′) (2)
and 0.5 equiv of antimony(V) chloride. The SbCl5 oxidizes
another 0.5 equiv of Pt(η2-fn)(Fc-N∩N′) (3) to 0.5 equiv of
PtCl2(Fc-N∩N′) (2) and fumaronitrile (0.5 equiv) giving SbCl3
(0.5 equiv). The remaining 0.25 equiv of 3 is attacked by SbCl3
to give a Pt0 complex tentatively formulated as Pt(SbCl3)(Fc-
N∩N′).42 The reaction of 3 with SbCl3 was analyzed indepen-
dently by IR and NMR spectroscopy. In the IR spectrum the
characteristic asymmetric and symmetric CN absorption bands
of 3 vanish and a new CN stretching absorption band appears
at 2227 cm-1 (at the same position as a solution of fumaronitrile
1
and SbCl3 in CH2Cl2), and in the H NMR spectrum of 3 and
SbCl3 in CD2Cl2 a signal for uncoordinated fumaronitrile is
observed at δ ) 6.31. Thus, SbCl3 is indeed able to displace
the olefin from 3.
Figure 12. Titration of 3 with “Magic Blue” in CH2Cl2 (top, 0-0.7
equiv; bottom, 0.7-1.7 equiv).
The UV/vis spectroscopic results confirm this stoichiometry.
Up to ca. 0.7 equiv of [N(p-C6H4Br)3][SbCl6] 3 is oxidized to
2, and further addition of [N(p-C6H4Br)3][SbCl6] results in
oxidation of 2 to 2•+. The primary reaction of 3 with [N(p-
C6H4Br)3][SbCl6] is summarized as follows:
for the high peak-to-peak separation (the peak-to-peak separation
of ferrocene itself under these conditions amounts to 90 mV).
In spite of the instability of electrochemically generated 3•+,
we were interested in the possible oxidation products. Thus, 3
was oxidized using [N(p-C6H4Br)3][SbCl6] in CH2Cl2. The Fc
f diimine CT band decreases in intensity while concomitantly
a new CT band at 669 nm rises in addition to a sharp band at
441 nm (Figure 12). Isosbestic points are observed at 459 and
665 nm up to ca. 0.7 equiv of oxidant. The UV/vis spectrum
after consumption of ca. 0.7 equiv of [N(p-C6H4Br)3]•+ is quite
similar to that of the dichloride complex 2. In fact further
oxidation with [N(p-C6H4Br)3]•+ results in spectral changes also
similar to those observed for the oxidation of 2 to 2•+, namely,
reduction of the 669 and 459 nm bands, while new bands around
460 and 642 nm rise giving isosbestic points at 418 and 537
nm, respectively.
3 + 2/3[N(p-C6H4Br)3][SbCl6] f
2 + 2/3N(p-C6H4Br)3 + 2/3SbCl3 + fn (1)
The catecholato complex 4 displays two oxidation waves in
the cyclic voltammogram at 505 and 770 mV vs SCE, the first
one being quasi-reversible (Figure 13). For the comparable
complexes Pt(4-tert-Bu-cat)(bpy) (4-tert-Bu-cat ) 4-tert-butyl-
catecholato, bpy ) 2,2′-bipyridine)39 and Pt(3,5-dtcat)(dpphen)
(3,5-dtcat ) 3,5-di-tert-butylcatecholato, dpphen ) 4,7-diphe-
nyl-1,10-phenanthroline),43 the catecholate oxidations are ob-
served at 430 and 470 mV vs SCE, respectively; thus, the first
oxidation of 4 is believed to be the catecholate to o-semiquinone
Oxidation with innocent (chloride-free) silver triflate as
oxidant was performed to support the chloride-dependent
oxidation with [N(p-C6H4Br)3][SbCl6]. With up to 1 equiv of
AgOTf, the Fc f diimine CT band decreases with isosbestic
points at 376, 573, and 684 nm while the second 1 equiv of
(42) Garrou, P. E.; Hartwell, G. E. Inorg. Chem. 1976, 15, 730-732.
(43) Weinstein, J. A.; Tierney, M. T.; Davies, E. S.; Base, K.; Robeiro,
A. A.; Grinstaff, M. W. Inorg. Chem. 2006, 45, 4544-4555.

5412 Organometallics, Vol. 26, No. 22, 2007
Heinze and Reinhardt
Figure 13. Cyclic voltammogram of 4 in CH2Cl2.
Figure 15. Titration of 4 with silver triflate in CH2Cl2.
) 6.46 and 1.32 ppm (0.25 equiv) ascribed to a complex formed
from 3,6-tBu2cat and SbCl6- which is tentatively formulated as
Sb2(3,6-tBu2cat)Cl8.44,45 Thus, after one-electron transfer from
the amine radical cation to give 4•+ the coordinated o-
semiquinone 3,6-tBu2sq is displaced by chloride ions to form
2. The o-semiquinone disproportionates to the o-benzoquinone
and the catecholate with the latter coordinating to antimony.
The observation that 1 equiv of [N(p-C6H4Br)3][SbCl6]/4 is
sufficient for the primary redox process is consistent with the
results from the UV/vis data. Thus, [N(p-C6H4Br)3][SbCl6] acts
as a one-electron oxidant and chloride donor toward 4. The
overall stoichiometric reaction is summarized as follows:
4 + [N(p-C6H4Br)3][SbCl6] f 2 + N(p-C6H4Br)3 +
1/2Sb2(3,6-tBu2cat)Cl8 + (1/2)3,6-tBu2bq (2)
From the cyclic voltammogram of 4, it is expected that the
first (quasireversible) oxidation is localized on the catecholato
ligand to give the o-semiquinone while the second oxidation
corresponds to the ferrocene/ferrocenium oxidation. Using the
innocent (chloride-free) one-electron oxidant silver triflate, 4
is cleanly oxidized to 4•+ in CH2Cl2. During oxidation the
catecholate to diimine CT band at 496 nm loses intensity while
the ferrocene to diimine CT band shifts from 608 to 617 nm
(Figure 15). Isosbestic points are observed at 297, 355, 475,
and 583 nm. Interestingly, for the related electrochemically
generated complex [Pt(3,5-dtsq)(dpphen)]•+ (3,5-dtsq ) 3,5-
di-tert-butylsemiquionato, dpphen ) 4,7-diphenyl-1,10-phenan-
throline) with a symmetric diimine ligand and an asymmetrically
substituted o-semiquinone ligand, a new band around 478 nm
(ꢀ ≈ 10 000 M-1 cm-1) is observed which has been assigned
dπ(Pt)-π*(o-semiquinone) MLCT character.43 For 4•+ such a
discrete band is not observed, but it might be hidden under the
intense absorption band at 346 nm.
Figure 14. Titration of 4 with “Magic Blue” in CH2Cl2 (top, 0-1
equiv; bottom, 1-2 equiv).
oxidation and the second one the Fc/Fc+ oxidation. To gain
insight into the oxidation processes, chemical oxidation was
performed with “Magic Blue” in CH2Cl2 and studied by UV/
vis spectroscopy. Both CT bands at 496 and 608 nm lose
intensity while new bands at 443 and 675 nm arise (Figure 14).
Up to 1 equiv, isosbestic points are observed at 467 and 682
nm. The final spectrum bears resemblance to that of 2. Thus
the new bands are assigned ferrocene transitions and Fc f
diimine CT transitions. Further addition of [N(p-C6H4Br)3]-
[SbCl6] results in spectral changes similar to those observed
during oxidation of 2 to 2•+, namely diminishing the intensity
of 443 and 675 nm bands and rise of weak bands at 475 and
660 nm.
The proton NMR spectrum of a solution of 4 and a
substoichiometric amount of [N(p-C6H4Br)3][SbCl6] (0.5 equiv)
shows signals only in the diamagnetic region. In fact, resonances
of the dichloro complex 2 (0.5 equiv) are observed with
characteristic platinum coupling patterns in addition to signals
corresponding to remaining 4 (0.5 equiv). Furthermore, signals
for 3,6-di-tert-butyl-1,2-benzoquinone (3,6-tBu2bq, 0.25 equiv)
are found at δ ) 6.78 and 1.24 ppm together with signals at δ
The nature of the radical cation 4•+ was probed by EPR
spectroscopy. The X-band EPR spectrum (Figure 16) of 4•+ at
room temperature is a superposition of two spectra correspond-
ing to species with a platinum nucleus with I ) 0 (66.3%) and
I ) 1/2 (33.7%, 195Pt). The spectra show typical π radical signals
with g around 2.0 which could be simulated using giso ) 2.0030,
Aiso(195Pt) ) 24.2 G, Aiso(1H) ) 4.25 G, Aiso(1H) ) 4.55 G, and
Aiso(2 × 14N) ) 1.75 G (line width 1.55 G). The unpaired
electron in 4•+ couples to two different hydrogen nuclei (H13,
H13′) of the o-semiquinonato ligand comparable to the coupling
found in the 1,4-benzosemiquinone-appended ferrocene deriva-
(44) Malhotra, K. C.; Mahajan, V. P., Miss; Chaudhry, S. C. Curr. Sci.
1981, 50, 89-91.
(45) Tian, Z.; Tuck, D. G. J. Chem. Soc., Dalton Trans. 1993, 1381-
1385.

Ferrocene Linked to PtL2 Fragments
Organometallics, Vol. 26, No. 22, 2007 5413
Table 3. DFT-Calculated Natural Charges of Selected
Fragments
complex
FeCp2
diimine
L2
Pt
Fe
1
0.21
1.02
0.34
1.15
0.33
1.15
0.31
0.43
1.23
0.70
-0.21
-0.02
-0.05
0.08
-0.13
0.00
-0.21
0.06
0.20
0.24
0.68
0.25
0.71
0.24
0.69
0.25
0.26
0.73
0.37
1•+
2
-1.01
-0.95
-0.71
-0.65
-1.08
-0.44
-0.36
0.14
0.72
0.73
0.51
0.50
0.97
0.95
0.94
0.97
2•+
3
3•+
4
4•+
4
4
2+(triplet)
2+(singlet)
0.19
EPR hyperfine coupling parameters were calculated by a
single-point DFT calculation using the EPR-II basis set of
Barone for H, C, N, and O,47 the 6-31+g(d,p) for Fe, and the
WTBS basis set48-50 for Pt on the B3LYP/LanL2DZ-optimized
geometry. Hyperfine splittings (hfc) to H13 and H13′ were
estimated as 3.6 and 3.3 G, respectively, in reasonable good
agreement with the experimental values while coupling to 14N
and 195Pt is somewhat underestimated (0.36, 0.42, and 15 G,
respectively). The latter is probably due to the employed
nonoptimized basis set for platinum.51 However, delocalization
of spin density toward platinum and the diimine moiety is
supported by the calculations.
Figure 16. X-band EPR spectrum of 4•+ at 300 K in CH2Cl2
(bottom) and simulation (top). Conditions: frequency, 9.855 78
GHz; power, 19.92 mW; modulation amplitude, 10 G.
In frozen glass matrix, 4•+ is EPR-silent precluding observa-
tion of individual components of g and A tensors, while on
warming, the isotropic spectrum is recovered. This finding is
probably due to aggregation/spin pairing in the solid matrix
which has been observed previously in the crystal structure of
PdL2(bpy) (L2 ) N-phenyl-o-iminobenzosemiquinonato).29
Figure 17. Singly occupied molecular Kohn-Sham orbital of 4•+
(isosurface at 0.03 au).
Conclusion
tive [Fc-C6H4NHCOC6H3O2]•- (Aiso(1H) ) 4.60, 2.05, 1.75
G).26 For Pd(3,6-dtsq)(bpy) (3,6-dtsq ) 3,6-di-tert-butylsemi-
quinonato), coupling of the unpaired electron to two equivalent
protons of the o-semiquinone has been reported with Aiso(2 ×
1H) ) 3.5 G29 while for Ni(3,5-dtsq)(PyBz2) (PyBz2 ) N,N-
bis(benzyl)-N-[(2-pyridyl)methyl]amine) coupling to one proton
is observed with Aiso(1H) ) 3.8 G.46
In this study of ferrocene-appended Pt(L2)(diimine) complexes
with L2 ) Cl2 (2), fumaronitrile (3), and 3,6-di-tert-butylcat-
echolato (4), the presence of a ferrocene moiety induces
solvatochromic ferrocene to diimine charge-transfer transitions
in the range 490-690 nm. With the additional catecholato donor
as coligand, a second charge-transfer absorption to the diimine
is present at 496 nm. The redox potential of the ferrocene unit
strongly depends on the presence and type of PtL2 moieties
attached to the diimine chelate. Oxidation of the dichloroplati-
num(II) complex 2 results in the formation of the expected
ferrocenium cation 2•+. On the other hand, oxidation of the
platinum(0) complex 3 in the presence of chloride labilizes the
olefin coligand eventually furnishing the platinum(II) chloride
complex 2 which is subsequently oxidized to 2•+. The first
oxidation of the catecholato complex 4 is not ferrocene-centered
at all but largely localized on the catecholato coligand giving a
platinum-coordinated o-semiquinonato ligand. In the presence
of chloride the o-semiquinonato ligand is replaced by chloride
under formation of 2. Under chloride-free conditions the
The hyperfine couplings to platinum and nitrogen show that
the unpaired electron also interacts with the platinum as well
as with the diimine nitrogen nuclei. The coupling constant to
platinum is of somewhat lower magnitude than that found for
[Pt(3,5-dtsq)(dpphen)]•+ [Axx(195Pt) ) 52 G, Ayy(195Pt) ) 33 G,
Azz(195Pt) ) 21 G; Aiso(195Pt) ) 35 G)].43 These coupling
constants have been interpreted as indicative of a SOMO mainly
localized on the o-semiquinone with partial metal character
which thus also applies for 4•+. In addition the unpaired electron
in 4•+ couples with the two nitrogen nuclei of the diimine ligand
providing evidence for some delocalization of the unpaired
electron onto the diimine ligand via the platinum atom.
Indeed, DFT calculations support the interpretations on the
basis of experimental data. The HOMO of 4 (and the SOMO
of 4•+) is mainly localized on the catecholato/o-semiquinonato
ligand with contributions of platinum, the diimine ligand, and
iron (Figure 17). Natural charge analyses of neutral and oxidized
species reveal that the first oxidation of 4 takes place on the
catecholato ligandsin contrast to complex 2 and ligand 1 which
display a ferrocene-based oxidation. Table 3 summarizes the
natural charges of selected fragments of 1/1•+, 2/2•+, 3/3•+, and
4/4•+/42+. In all cases the platinum retains its charge; i.e., it is
hardly affected by the initial oxidation process.
(47) Barone, V. In Recent AdVances in Computational Methods; Chong,
D. P., Ed.; World Scientific: Singapore, 1995; Part 1.
(48) Huzinaga, S.; Miguel, B. Chem. Phys. Lett. 1990, 175, 289-291.
(49) Huzinaga, S.; Klobukowski, B. Chem. Phys. Lett. 1993, 212, 260-
264.
(50) The WTBS basis set was obtained from the Extensible Computa-
tional Chemistry Environment Basis Set Database, Version 02/02/06, as
developed and distributed by the Molecular Science Computing Facility,
Environmental and Molecular Sciences Laboratory, which is part of the
Pacific Northwest Laboratory, P.O. Box 999, Richland, WA 99352, and
funded by the U.S. Department of Energy. The Pacific Northwest Laboratory
is a multiprogram laboratory operated by Battelle Memorial Institute of
the U.S. Department of Energy under Contract DE-AC06-76RLO.
(51) Munzarova´, M.; Kaupp, M. J. Phys. Chem. A 1999, 103, 9966-
9983.
(46) Ohtsu, H.; Tanaka, K. Chem. Eur. J. 2005, 11, 3420-3426.

5414 Organometallics, Vol. 26, No. 22, 2007
Heinze and Reinhardt
WTBS basis set48-50 and for Fe the 6-31+g(d,p) basis set were
employed, since no EPR-optimized basis sets are available for
transitions metals.50
coordinated o-semiquinonato ligand in 4•+ has been observed
by EPR spectroscopy. The unpaired electron of 4•+ is largely
localized on the o-semiquinonato ligand but also interacts with
the platinum nucleus and the diimine ligand. In summary, the
outcome of asat first sightssimple ferrocene-based oxidation
in a complex system might well largely depend on the type of
system under study as well as the oxidation conditions employed
(noninnocent vs innocent oxidizing agent).
(Ferrocenylpyridin-2-ylmethyleneamine)dichloroplatinum-
(II) [PtCl2(Fc-N∩N′), 2]. A mixture of Fc-N∩N′ (1; 100 mg,
0.34 mmol) and bis(dimethyl sulfoxide)dichloroplatinum(0) (127
mg, 0.30 mmol) in methanol (40 mL) was stirred at 25 °C for 18
h to give a blue-green suspension. After removal of the solvent in
vacuo, the residue was washed with pentane and diethyl ether,
suspended in tetrahydrofuran, and precipitated with diethyl ether.
Drying the residue in vacuo gave the product as a deep green solid
in 69% yield (115 mg, 0.21 mmol).
Experimental Section
General Procedures. All reactions were carried out under an
atmosphere of dry argon. All solvents were dried according to
standard procedures and saturated with argon prior to use. Chemi-
cals were obtained from commercial suppliers and used without
further purification. Ferrocenylpyridin-2-ylmethyleneamine18 (Fc-
N∩N′, 1), bis(dimethyl sulfoxide)dichloroplatinum(II),32 tris(bicyclo-
[2.2.1]heptene)platinum(0),33 and 3,6-di-tert-butyl[1,2]benzoquinone34
were synthesized as reported. IR spectra were recorded on a BioRad
Excalibur FTS 3000 spectrometer using cesium iodide disks. UV/
vis spectra were recorded on a Perkin-Elmer Lambda 19 in 1.0 cm
cells (Hellma, suprasil). Wavelengths (λmax) are reported in nm,
and extinction coefficients (ꢀ), in M-1 cm-1. High-resolution mass
spectra (FAB+) were recorded on a JEOL JMS-700 instrument
with 4-nitrobenzyl alcohol as matrix. Elemental analyses were
performed by the Mikroanalytisches Laboratorium der Chemischen
Institute der Universita¨t Heidelberg. NMR spectra were obtained
IR (CsI; cm-1): 1614 (m, νimine). UV/vis (CH2Cl2; λmax (ꢀ)): 263
(6355), 295 sh (4915), 341 (4005), 363 (3890), 442 (1510), 686
(835). CV (CH2Cl2; vs SCE): E1/2 ) 615 mV (rev). Mp: 257-
35
259 °C (262 °C dec). MS (HR-FAB+; m/z (%)): calcd for C16H14
-
-
35
Cl2FeN2196Pt 555.9533, found 555.9542 (77); calcd for C16H14
ClFeN2195Pt519.9839,found519.9841(44);calcdforC16H14FeN2194Pt
484.0133, found 484.0085 (100). Anal. Calcd for C16H14Cl2FeN2-
Pt: C, 34.56; H, 2.54; N, 5.04. Found: C, 35.06; H, 2.81; N, 5.06.
(Ferrocenylpyridin-2-ylmethyleneamine)(η2-fumaronitrile)-
platinum(0) [Pt(Fc-N∩N′)(fn), 3]. A colorless solution of fuma-
ronitrile (39 mg, 0.50 mmol) and tris(bicyclo[2.2.1]heptene)-
platinum(0) (200 mg, 0.42 mmol) in tetrahydrofuran (20 mL) was
treated with Fc-N∩N′ (1; 119 mg, 0.41 mmol). The deep red
solution was stirred at 25 °C for 16 h. Evaporation of the solvent,
washing with pentane and diethyl ether, and drying in vacuo gave
the product as a deep green solid in 86% yield (199 mg, 0.35 mmol).
1
on a Bruker Avance DPX 200 or Avance II 400 at 25 °C. H and
13C chemical shifts are reported in ppm and calibrated to TMS on
the basis of the solvent as an internal standard (δ ) 5.31 ppm [1H]
and δ ) 53.80 ppm [13C] for dichloromethane). Assignments of
13C NMR spectra were made with the aid of 2D correlation spectra.
Cyclic voltammetry was performed on a Metrohm “Universal Mess-
and Titriergefaess”, Metrohm GC electrode RDE 628, platinum
electrode, SCE electrode, and Princeton Applied Research poten-
tiostat model 273; the substrate concentration was 1 mM in 0.1 M
nBu4NPF6/CH2Cl2. All potentials are given relative to that of SCE.
Melting points were determined with a Gallenkamp capillary
melting point apparatus MFB 595 010 and are not corrected. EPR
spectra were recorded on a Bruker ELEXSYS E500 spectrometer
(X-band). Xsophe, version 1.0.2â, was used for simulation of the
spectra.
IR (CsI; cm-1): 2203 (vs, νCN,asym), 2195 (s, νCNsym), 1609 (m,
imine). UV/vis (CH2Cl2; λmax (ꢀ)): 298 (11 510), 346 (10 805), 366
ν
sh (9360), 437 (2625), 621 (2930). CV (CH2Cl2; vs SCE): E1/2
)
655 mV (qrev). Mp: 274-276 °C. MS (HR-FAB+; m/z (%)):
calcd for C20H17FeN4195Pt 564.0450, found 564.0488 (100). Anal.
Calcd for C20H16FeN4Pt: C, 42.64; H, 2.86; N, 9.95. Found: C,
42.37; H, 3.00; N, 9.78.
(Ferrocenylpyridin-2-ylmethyleneamine)(3,6-di-tert-butyl-o-
catecholato)platinum(II) [Pt(tBu2Cat)(Fc-N∩N′), 4]. A solution
of Fc-N∩N′ (1; 139 mg, 0.48 mmol), bicyclo[2.2.1]heptene (66
mg, 0.70 mmol), and tris(bicyclo[2.2.1]heptene)platinum(0) (230
mg, 0.48 mmol) in tetrahydrofuran (20 mL) was stirred at 25 °C
for 4 h. After addition of 3,6-di-tert-butyl-1,2-benzoquinone (106
mg, 0.48 mmol), the mixture was stirred for 16 h at 25 °C.
Evaporation of all volatiles, washing with pentane and diethyl ether,
and drying the residue gave the product as a deep green solid in
45% yield (151 mg, 0.21 mmol).
IR (CsI; cm-1): 2955 (s, νCH-aliph.), 1624 (s, νimine). UV/vis (CH2-
Cl2; λmax (ꢀ)): 319 (15 755), 365 sh (13 550), 491 (4410), 608
(3505). CV (CH2Cl2; vs SCE): E1/2 ) 505 mV (qrev), E1/2 ) 770
mV (rev). Mp: 230-232 °C (dec). MS (HR-FAB+; m/z (%)):
calcd for C30H35FeN2O2195Pt 706.1696, found 706.1797 (100); calcd
for C25H29FeN2O2195Pt 640.1227, found 640.1130 (70). Anal. Calcd
for C30H34FeN2O2Pt: C, 51.07; H, 4.86; N, 3.97. Found: C, 50.93;
H, 4.96; N, 3.85.
Computational Method. Density functional calculations were
carried out with the Gaussian03/DFT52 series of programs. The
B3LYP formulation of density functional theory was used employ-
ing the LanL2DZ basis set.52 All structures were characterized as
minima (Nimag ) 0) or first-order saddle points (Nimag ) 1) by
frequency analysis. Natural atomic orbital and natural bond orbital
analyses were calculated using Gaussian NBO, version 3.1.
Hyperfine coupling constants were obtained using the EPR-II basis
set of Barone for H, C, N, and O, which is specifically optimized
for the evaluation of hyperfine coupling constants.47 For Pt the
(52) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
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Acknowledgment. We thank the Deutsche Forschungsge-
meinschaft (Heisenberg Fellowship to K.H.) and the Graduate
College 850 “Modeling of Molecular Properties” (fellowship
to S.R.) for financial support.
Supporting Information Available: DFT-calculated geometries
of 1/1•+, 2/2•+, 3/3•+, and 4/4•+/42+ and the calculated transition
states for olefin and ferrocene rotation of 3. This material is
available free of charge via the Internet at http://www.pubs.acs.org.
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