Electronic communication in oligonuclear ferrocene complexes with anionic four-coordinate boron bridges

Article abstract of DOI:10.1039/b821406a

The di- and trinuclear ferrocene species Li[Fc-BPh₂-Fc] (Li[9]) and Li₂[Fc-BPh₂-fc-BPh₂-Fc] (Li ₂[10]) have been investigated with regard to their electrochemical properties and the degree of interval

Full text of DOI:10.1039/b821406a

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www.rsc.org/dalton | Dalton Transactions
Electronic communication in oligonuclear ferrocene complexes with anionic
four-coordinate boron bridges†
Linda Kaufmann,a Jens-Michael Breunig,a Hannes Vitze,a Frauke Scho¨del,a Israel Nowik,b Markus Pichlmaier,c
Michael Bolte,a Hans-Wolfram Lerner,a Rainer F. Winter,c Rolfe H. Herberb and Matthias Wagner*a
Received 1st December 2008, Accepted 2nd February 2009
First published as an Advance Article on the web 27th February 2009
DOI: 10.1039/b821406a
The di- and trinuclear ferrocene species Li[Fc-BPh2-Fc] (Li[9]) and Li2[Fc-BPh2-fc-BPh2-Fc] (Li2[10])
have been investigated with regard to their electrochemical properties and the degree of intervalence
charge-transfer after partial oxidation. Li[9] shows two distinct one-electron redox waves for its
chemically equivalent ferrocenyl substituents in the cyclic voltammogram (E1/2 = -0.38 V, -0.64 V; vs.
FcH/FcH+). The corresponding values of Li2[10] are E1/2 = -0.45 V (two-electron process) and -1.18 V.
All these redox events are reversible at r. t. on the time scale of cyclic voltammetry. X-ray
crystallography on the mixed-valent FeII2FeIII complex Li(12-c-4)2[10] reveals the centroid–centroid
˚
distance between the cyclopentadienyl rings of each of the terminal ferrocenyl substituents (3.329 A) to
˚
be significantly smaller than in the central 1,1¢-ferrocenediyl fragment (3.420 A). This points towards a
charge-localized structure (on the time scale of X-ray crystallography) with the central iron atom being
in the FeIII state. Mo¨ßbauer spectroscopic measurements on Li(12-c-4)2[10] lend further support to this
interpretation. Spectroelectrochemical measurements on Li[9] and Li2[10] in the wavelength range
between 300–2800 nm do not show bands interpretable as intervalence charge-transfer absorptions for
the mixed-valent states. All data accumulated so far lead to the conclusion that electronic interaction
between the individual Fe atoms in Li[9] and Li2[10] occurs via a through-space pathway and/or is
electrostatic in nature.
adducts between poly(ferrocenylene)s with three coordinate boron
Introduction
bridges and Lewis bases (L) influences the degree of electronic
communication along the polymer backbone, thereby offering an
opportunity to design novel sensor compounds and switchable
nanowires.
Poly(ferrocenylene)s A (Fig. 1) represent an important class of
processable metal-containing polymers with applications rang-
ing from molecular electronics to the preparation of magnetic
ceramics.1–4 The materials properties of macromolecules A depend
to a large extent on the nature of the bridging element ERx, which
can be varied over a broad range (e.g. ERx = SiMe2,5 SntBu2,6
PPh,7 S8). For the following reasons, our group is particularly inter-
ested in boron-bridged poly(ferrocenylene)s:9 (i) Three-coordinate
boron atoms (ERx = BR¢) possess an empty p-orbital that is well-
suited for p-conjugation with the cyclopentadienyl rings and thus
able to act as an efficient transmitter of electronic interactions
between the individual 1,1¢-ferrocenediyl moieties.10,11 (ii) Four-
coordinate boron atoms (ERx = BR¢2) can bear a positive, a
negative, or zero charge which provides a powerful set-screw for
tuning the FeII/FeIII redox potentials of corresponding ferrocene
oligomers by electrostatic means.12,13 (iii) The formation of B–L
Given this background, a thorough evaluation of the degree
of ferrocene–ferrocene interaction via three- and four-coordinate
boron linkers and a comprehension of the underlying transmission
pathways is essential for further rational developments in this
area. We have already reported evidence for pronounced electronic
communication along the chains of A-type polymers in which
ERx equals BMes10 or BO(CH2)4Br11 (Mes = mesityl). There is
also a strong indication for charge-transfer interactions between
ferrocene and p*-orbitals of the 4,4¢-bipyridyl units in polymeric
B–N adducts B (Fig. 1) even though the boron atoms are
four-coordinate.14–17 The electronic structure, photophysics, and
relaxation dynamics of charge-transfer excited states in soluble
model systems of type [C](PF6)2 (Fig. 1) have been investi-
gated using cyclic voltammetry, spectroelectrochemistry and laser
spectroscopy.18 These results led to the conclusion that a four-
coordinate boron atom is not necessarily an insurmountable bar-
rier to electronic interactions between its redox-active substituents.
This conclusion is further substantiated by the fact that two
different redox potentials are observed for the FeII/FeIII transitions
in dinuclear complexes like Li[D] (Fig. 1) even though the two
ferrocenyl substituents are chemically equivalent.12,13
aInstitut fu¨r Anorganische Chemie, Goethe-Universita¨t Frankfurt, Max-
von-Laue-Strasse 7, 60438, Frankfurt (Main), Germany. E-mail:
Matthias.Wagner@chemie.uni-frankfurt.de
bRacah Institute of Physics, The Hebrew University of Jerusalem, 91904,
Jerusalem, Israel
cInstitut fu¨r Anorganische Chemie, Universita¨t Regensburg, Univer-
sita¨tsstrasse 31, 93040, Regensburg, Germany
† Electronic supplementary information (ESI) available: Experimental
details. CCDC reference numbers 699649 (3), 699647 (6), 699646 (7),
699650 ((Li(12-c-4)(THF))2[10]), 699648 (Li(12-c-4)2[10]) and 710783
(Li(THF)4[11]). For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/b821406a
Most importantly in this context, a broad band with a maximum
near l = 2200 nm has been reported to appear in the UV/vis/NIR
spectrum of the related ferricenyltris(ferrocenyl)borate zwitterion
2940 | Dalton Trans., 2009, 2940–2950
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and Li2[Fc-BPh2-fc-BPh2-Fc] (Li2[10]; Scheme 2) are outlined in
this paper.
Results and discussion
Synthesis and NMR spectroscopy
Compounds Li[Fc-BMe2-Fc] (Li[D]) and Li2[Fc-BMe2-fc-BMe2-
Fc] are readily accessible via B–C adduct formation between
FcBMe2 and FcLi or 1,1¢-fcLi2 ¥ 2/3 TMEDA.13 We therefore
considered the corresponding diphenylborylferrocenes 3 and 6
(Scheme 1) useful building blocks for the synthesis of BPh2-bridged
oligo- and polyferrocenes. FcBPh2 (3) has already been prepared
in the form of a red oil by Herberhold and Wrackmeyer, who
treated FcLi with MeOBPh2.20 In our hands, the reaction between
FcHgCl (1)21 and BrBPh2 (2)22 resulted in better yields and higher
purity of 3.
The 1,1¢-diborylated derivative 6 is not known in the literature so
far, and its preparation turned out to be cumbersome. A synthesis
approach analogous to the preparation of 3 is not practical due
to the poor solubility of the doubly mercurated ferrocene 1,1¢-
fc(HgCl)2. The following potential alternative routes resulted in
inseparable product mixtures rather than in the formation of
pure 6: (i) 1,1¢-fc(BBr2)2 and PhLi or SnPh4 or PhSiMe3, (ii) 1,1¢-
fc(B(OMe)2)2 and PhLi, (iii) 1,1¢-fcLi2 ¥ 2/3 TMEDA and XBPh2
(X = Br, OiPr), (iv) Li2[1,1¢-fc(BPh3)2] and ClSiMe3. We were
finally able to synthesize 6 from 1,1¢-fcLi2 ¥ 2/3 TMEDA (4) and
2 equiv. of MeOBPh2, however, the target compound was contam-
inated with substantial amounts of Li(TMEDA)[MeOBPh3] (7;
Scheme 1). Both components precipitated from hexane as single
crystals. Since it was not possible to completely separate the two
products by fractional crystallization, we had to rely on manual
crystal selection in order to obtain samples of reasonable purity
for NMR-spectroscopic characterization.
Fig. 1 General representation of ERx-bridged poly(ferrocenylene)s A;
coordination polymer B showing charge-transfer interactions between the
ferrocenylene fragments and the 4,4¢-bipyridyl bridge; [C](PF6)2, a soluble
model system of B; dinuclear BMe2-bridged complex Li[D] possessing two
different FeII/FeIII redox potentials.
([FcIII(FcII)3B]) and was interpreted as intervalence charge-transfer
absorption (Fc = (C5H5)Fe(C5H4)).19 The question thus arises
whether the electrochemical behaviour of Li[D]-type oligofer-
rocenes is merely governed by electrostatic interactions or whether
charge delocalization also plays a significant role. In addition
to Li[D], our group has recently published the synthesis and
structural characterization of its trinuclear congener Li2[Fc-BMe2-
fc-BMe2-Fc] (fc = (C5H4)2Fe).13 Unfortunately, both compounds
tend to decompose upon iron oxidation, so that cyclic voltammo-
grams had to be recorded at -78 ◦C and all efforts regarding the
isolation of mixed-valent species were unsuccessful.
Given the apparent stability of [FcIII(FcII)3B], we postulated
that replacement of the methyl substituents by phenyl groups in
Li[Fc-BMe2-Fc] (Li[D]) and Li2[Fc-BMe2-fc-BMe2-Fc] will lead to
increased stability and thus allow us to assess the level of electronic
communication between the redox-active sites in greater detail.
The results of our studies on Li[Fc-BPh2-Fc] (Li[9]; Scheme 2)
Scheme 1 Synthesis of the borylated ferrocenes 3 and 6. (i) hexane, -78 ◦C
to r. t.
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The dinuclear BPh2-bridged compound Li[9] is readily accessi-
ble from Fc2BBr (8) and 2 equiv. of PhLi (Scheme 2). Details of
the synthesis protocol as well as an X-ray crystal structure deter-
mination of Li(OBu2)[9] have already been published elsewhere.23
Attempts at the synthesis of the FeIIFeIII mixed-valent species 9 by
reaction of Li[9] with AgBF4, AgPF6 or I2, always resulted in the
formation of a red–brown oil that could not be transformed into an
analytically pure solid. However, the UV/vis spectra of chemically
oxidized Li[9] proved to be identical to the spectrum obtained
during electrochemical oxidation of this compound (see below).
Due to its paramagnetic nature, interpretable NMR spectra of 9
could not be acquired.
Scheme 3 Synthesis of Li[11]; oxidation of Li[11] to its mixed-valent state
11. (i) THF/pentane, r. t.; (ii) CH2Cl2, r. t.
also to synthesize and structurally characterize the fully reduced
form Li[11]. Li[11] is accessible from BF3·OEt2 and excess FcLi,
provided that strictly anaerobic conditions are maintained. Single
crystals of Li(THF)4[11] were grown from THF/pentane. The
targeted oxidation of Li(THF)4[11] to 11 was performed essentially
as described in the literature (Scheme 3).19
The NMR data of 320 and Li[9]23 are in accord with published
values. The 11B NMR resonance of 6 is broadened beyond
detection, probably as a result of slow intramolecular motion
(note that already the monosubstituted analog 3 shows a very
broad signal (h1/2 = 600 Hz; d(11B) = 63.3). All 1H and 13C NMR
resonances of 6 are similar to those of 3 and therefore do not
merit further discussion. The 11B NMR spectrum of Li2[10] is
characterized by a signal at -11.7 ppm which lies in a range
typical of four-coordinate boron nuclei.26 All proton resonances
are broadened at r. t. A hump lacking any fine structure appears in
the region of the C5H4 and C5H5 signals; the phenyl resonances are
better resolved and appear at 6.63 ppm, 6.79 ppm, and 7.35 ppm.
Similar to Li2[10], Li[11] gives rise to a signal at -15.3 ppm in the
11B NMR spectrum. All four ferrocenyl substituents are chemically
equivalent (d(1H) = 3.69 (C5H5), 3.87, 4.34 (C5H4)). In the 13C
NMR spectrum, both C5H4 resonances of Li[11] show a multiplet
structure due to partially resolved 11B coupling (the ipso-carbon
signal is not observed due to quadrupolar broadening). Similar
to 9, the paramagnetic nature of Li[10] and 11 precluded their
characterization by NMR spectroscopy.
Scheme 2 Synthesis of the di- and trinuclear BPh2-bridged ferrocene
aggregates Li[9] and Li2[10]; oxidation of Li2[10] to its mixed-valent state
Li[10]. (i) toluene/OBu2, -78 ◦C to r. t.; (ii) THF, -78 ◦C to r. t.; (iii) THF,
r. t.
The trinuclear species Li2[10] is formed from 2 equiv. of FcBPh2
(3) and 1,1¢-fcLi2 ¥ 2/3 TMEDA (4)24,25 under mild conditions and
in good yields. In the presence of trace amounts of oxygen, Li2[10]
is immediately transformed into its mixed-valent state Li[10]
(Scheme 2). Li2[10] and Li[10] crystallized from THF/hexane in
the presence of crown ether (12-c-4) as ether adducts (Li(12-c-
4)(THF))2[10] and Li(12-c-4)2[10], respectively.
Crystal structure determinations
For reasons of comparison, we decided to revisit the ferricenyl-
tris(ferrocenyl)borate inner salt [FcIII(FcII)3B]19 (11; Scheme 3) and
Selected crystallographic data of 3, 6, (Li(12-c-4)(THF))2[10],
Li(12-c-4)2[10], and Li(THF)4[11] are summarized in Tables 1
2942 | Dalton Trans., 2009, 2940–2950
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Table 1 Crystallographic data of 3 and 6
C–C bond length alternation within the cyclopentadienyl rings of
3 and 6, contributes to the electronic saturation of the electron-
deficient boron atoms. In addition to that, there is a second type
of interaction in borylated ferrocenes that involves filled d-type
orbitals at iron and the empty p-orbital at boron and manifests
itself by a bending of the boryl substituent out of the plane of the
cyclopentadienyl ring towards the iron atom.27,28 In the cases of
3 and 6, the corresponding dip angles a* amount to 13.0◦ in 3
and 10.8◦ in the C2-symmetric molecule 6 (a* = 180◦ - a, a =
COG(C5H4)–Cipso–B; COG(C5H4): centroid of a cyclopentadienyl
ring). As usual, the degree of bending is higher in the monobory-
lated than in the diborylated species. The a* value of 3 is the same
as in FcBMe2 (a* = 13.0 ◦),28 but it is smaller than the dip angle
of FcBBr2 (a* = 18.3◦)27 and FcB(C6F5)2 (a* = 16◦).29 No X-ray
crystal structure analyses of 1,1¢-fc(BMe2)2 or 1,1¢-fc(B(C6F5)2)2
are available to date. We therefore compare 6 with 1,1¢-fc(BBrMe)2
(a* = 9.4◦)28 and 1,1¢-fc(BBr2)2 (a* = 9.1◦)30 which reveal a
similar degree of ligand bending. The ferrocenyl substituent in
3 as well as the 1,1¢-ferrocenediyl backbone in 6 adopt an eclipsed
conformation (3: C(1)–COG(1)–COG(11)–C(11) = 7.2◦, 6: C(1)–
COG(1)–COG(4A)–C(4A) = -4.0◦; COG(X): centroid of the
cyclopentadienyl ring containing the carbon atom C(X)). As a
consequence, the torsion angle between the two boryl substituents
of 6 has a value of B(1)–COG(1)–COG(1A)–B(1A) = 140.9◦.
In the solid state, both trinuclear aggregates (Li(12-c-
4)(THF))2[10] (Fig. 4) and Li(12-c-4)2[10] (Fig. 5) possess an
inversion centre located at Fe(1). The Li+ ions are wrapped
by ether ligands and do not establish short contacts with the
anionic molecules. (Li(12-c-4)(THF))2[10] contains two Li+ ions
per oligoferrocene moiety, whereas in Li(12-c-4)2[10] the cation :
anion ratio is 1 : 1. This is in accord with the presence of three
FeII ions in (Li(12-c-4)(THF))2[10] but points towards two FeII
ions and one FeIII centre in Li(12-c-4)2[10]. This conclusion is
compound
3
6
formula
fw
C22H19BFe
350.03
red, needle
173(2)
tetragonal
P43
11.2142(7)
11.2142(7)
13.5550(11)
90
90
90
1704.7(2)
4
1.364
C34H28B2Fe
514.03
red, needle
173(2)
orthorhombic
Fdd2
18.425(4)
30.690(6)
9.2970(19)
90
colour, shape
temperature (K)
crystal system
space group
˚
a (A)
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
˚
90
90
3
V (A )
5257.1(18)
8
1.299
2144
0.595
Z
Dcalcd. (g cm-3)
F(000)
728
0.883
m (mm-1)
crystal size (mm3)
no. of rflns collected
no. of indep rflns (Rint
0.24 ¥ 0.06 ¥ 0.05 0.24 ¥ 0.11 ¥ 0.10
21782
3214 (0.0716)
3214/1/217
1.025
7049
)
2204 (0.0758)
2204/1/168
1.010
0.0372, 0.0710
0.0521, 0.0788
0.196, -0.236
data/restraints/parameters
GOOF on F2
R1, wR2 (I>2s(I))
R1, wR2 (all data)
largest diff peak and hole (e A ) 0.461, -0.476
0.0433, 0.1033
0.0486, 0.1069
-3
˚
and 2; details of the crystal structure analysis of 7 are compiled in
the ESI (Fig. 1S, Table 1S†).
3 (Fig. 2) and 6 (Fig. 3) feature planar three-coordinate boron
atoms. In both compounds, the B–Cp bonds are shorter by about
˚
0.04 A than the B–Ph bonds, thereby indicating that the ferrocene
fragment acts as a stronger p-donor than a phenyl ring. The
resulting borafulvene character, which also results in a distinctive
Table 2 Crystallographic data of (Li(12-c-4)(THF))2[10], Li(12-c-4)2[10] and Li(THF)4[11]
compound
(Li(12-c-4)(THF))2[10]
Li(12-c-4)2[10]
Li(THF)4[11]
formula
fw
C78H94B2Fe3Li2O10
1394.58
C70H78B2Fe3LiO8 ¥ C4H8O
1315.54
C56H68BFe4LiO4
1046.25
colour, shape
temperature (K)
crystal system
space group
orange, block
173(2)
monoclinic
P21/n
14.9703(10)
10.3522(6)
22.9360(18)
90
105.526(5)
90
3424.8(4)
2
1.352
1472
0.687
brown, needle
173(2)
orthorhombic
Pnma
17.4879(19)
27.436(3)
15.5563(17)
90
red, block
173(2)
monoclinic
P21/n
14.9829(9)
20.0854(10)
16.7066(9)
90
100.813(5)
90
4938.4(5)
4
1.407
2192
1.198
˚
a (A)
˚
b (A)
˚
c (A)
a (◦)
b (◦)
g (◦)
90
90
3
˚
V (A )
7463.9(14)
4
1.171
2772
0.626
0.30 ¥ 0.12 ¥ 0.11
33604
6709 (0.1866)
6709/33/378
0.979
Z
Dcalcd. (g cm-3)
F(000)
m (mm-1)
crystal size (mm3)
no. of rflns collected
no. of indep rflns (Rint
0.23 ¥ 0.21 ¥ 0.18
29466
6418 (0.1120)
6418/48/539
0.971
0.37 ¥ 0.33 ¥ 0.32
30721
9233 (0.0628)
9233/0/595
0.961
)
data/restraints/parameters
GOOF on F2
R1, wR2 (I>2s(I))
R1, wR2 (all data)
largest diff peak and hole (e A )
0.0510, 0.0971
0.0973, 0.1107
0.720, -0.433
0.1029, 0.1944
0.2222, 0.2438
0.718, -0.539
0.0394, 0.0901
0.0586, 0.0965
0.490, -0.521
-3
˚
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Fig. 2 Structure of 3 in the crystal. Displacement ellipsoids are drawn at the 30% probability level. H atoms omitted for clarity. Selected bond lengths
◦
◦
˚
(A), bond angles ( ), and the dip angle a* ( ): B(1)–C(1) = 1.542(5), B(1)–C(21) = 1.579(5), B(1)–C(31) = 1.581(5); C(1)–B(1)–C(21) = 122.4(3),
C(1)–B(1)–C(31) = 121.1(3), C(21)–B(1)–C(31) = 116.5(3); a* = 13.0.
Fig. 3 Structure of 6 in the crystal. Displacement ellipsoids are drawn at the 50% probability level. H atoms omitted for clarity. Selected bond lengths
◦
◦
˚
(A), bond angles ( ), and the dip angle a* ( ): B(1)–C(1) = 1.542(5), B(1)–C(11) = 1.585(6), B(1)–C(21) = 1.584(6); C(1)–B(1)–C(11) = 119.2(3),
C(1)–B(1)–C(21) = 123.1(3), C(11)–B(1)–C(21) = 117.6(3); a* = 10.8.
further supported by the observation of almost equal distances
between the cyclopentadienyl rings of the three ferrocene frag-
the same in (Li(12-c-4)(THF))2[10] and in Li(12-c-4)2[10] (cf .
Fig. 4 and 5). This is in agreement with a priori expectations,
because differences in the p-donor strengths of both aromatic
substituents are only relevant for three-coordinate boranes like
3 and 6 but not for compounds containing four-coordinate
boron atoms. The central 1,1¢-ferrocenediyl fragments in (Li(12-c-
4)(THF))2 [10] and Li(12-c-4)2[10] adopt staggered conformations
with boron substituents pointing in opposite directions. The
major difference in the overall conformations of both oligomers
lies in the position of the terminal ferrocenyl substituents
˚
ments in (Li(12-c-4)(THF))2[10] (COG(1)–COG(1A) = 3.330 A,
˚
COG(11)–COG(21) = 3.325 A), with values characteristic of
ferrocenes containing FeII ions.19,31 In contrast, Li(12-c-4)2[10]
exhibits similar centroid—centroid distances only for the terminal
˚
ferrocenyl substituents (COG(21)–COG(31) = 3.329 A), while the
central moiety is significantly expanded (COG(11)–COG(11A) =
˚
3.420 A). We take this as evidence that Fe(1) in Li(12-c-4)2[10]
is an FeIII centre.19,31 The B–Cp and B–Ph bond lengths are
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Fig. 4 Structure of (Li(12-c-4)(THF))2[10] in the crystal. Displacement ellipsoids are drawn at the 50% probability level. H atoms and Li(12-c-4)(THF)+
◦
˚
˚
counterions omitted for clarity. Selected bond lengths (A), atom ◊ ◊ ◊ atom distances (A), and bond angles ( ): B(1)–C(1) = 1.658(4), B(1)–C(11) = 1.646(5),
B(1)–C(31) = 1.647(5), B(1)–C(41) = 1.663(4), COG(1)–COG(1A) = 3.330, COG(11)–COG(21) = 3.325, Fe(1) ◊ ◊ ◊ Fe(2) = 6.112(1); C(1)–B(1)–C(11) =
105.3(3), C(31)–B(1)–C(41) = 106.9(3). COG(X): centroid of the cyclopentadienyl ring containing the carbon atom C(X). Symmetry transformations
used to generate equivalent atoms: A: -x + 1, -y, -z + 1.
The crystal lattice of the tetrakis(ferrocenyl)borate salt
Li(THF)4[11] contains tetrahedrally coordinated cations and
anions; the molecular structure of the [BFc4]- ion is shown in
Fig. 6. We note a nearly threefold axis normal to the plane defined
by Fe(1), Fe(2), and Fe(3). As a result, the fourth ferrocenyl unit,
i.e. that labeled Fe(4) in Fig. 6, is distinct from the others. Thus,
the conformation of [BFc4]- in Li(THF)4[11] is strikingly similar
to that of the neutral mixed-valent species 11.19 All B–C bond
lengths of Li(THF)4[11] fall in the interval between B(1)–C(21) =
˚
˚
1.634(4) A and B(1)–C(11) = 1.653(4) A; the smallest C–B–C
angle is C(11)–B(1)–C(31) = 103.2(2)◦, the largest C(21)–B(1)–
C(31) = 113.0(2)◦. Most importantly, all four centroid-to-centroid
˚
distances within the ferrocenyl substituents lie between 3.312 A
and 3.318 A and are thus characteristic of Fe states. This is in
II
˚
contrast to the molecular structure reported for the mixed-valent
species 11, which contains three ferrocenyl groups with short
II
˚
˚
centroid-to-centroid distances (3.291 A to 3.320 A; Fe states),
and one substituent with a significantly elongated centroid-to-
III
centroid distance (3.428 A; Fe state).19
˚
Fig. 5 Structure of Li(12-c-4)2[10] in the crystal. Displacement ellipsoids
+
are drawn at the 50% probability level. H atoms, the Li(12-crown-4)2
Electrochemical and spectroelectrochemical investigations
counterion and the THF molecule omitted for clarity. Selected bond
◦
˚
˚
lengths (A), atom ◊ ◊ ◊ atom distances (A), and bond angles ( ): B(1)–C(11) =
The electrochemical parameters of the redox events exhibited by
Li[9],23 Li2[10], Li[10] and the related complexes Li[Fc-BMe2-Fc]13
and Li2[Fc-BMe2-fc-BMe2-Fc]13 are summarized in Table 3.
Each of the dinuclear complexes Li[9] and Li[Fc-BMe2-Fc] dis-
plays two oxidation processes of relative intensity 1:1, assignable as
successive one-electron transitions at the two ferrocenyl moieties.
In the case of the phenyl derivative Li[9], both electron transitions
are reversible at r. t. on the cyclic voltammetric timescale, whereas
the methyl derivative Li[Fc-BMe2-Fc] has to be examined at
-78 ◦C in order to obtain cyclic voltammograms showing features
1.646(12), B(1)–C(21) = 1.644(12), B(1)–C(41) = 1.649(13), B(1)–C(51) =
1.649(12), COG(11)–COG(11A)
=
3.420, COG(21)–COG(31)
=
3.329, Fe(1) ◊ ◊ ◊ Fe(2)
=
5.477(1); C(11)–B(1)–C(21)
=
112.4(6),
C(41)–B(1)–C(51) = 111.3(6). COG(X): centroid of the cyclopenta-
dienyl ring containing the carbon atom C(X). Symmetry transformations
used to generate equivalent atoms: A: -x + 1, -y + 1, -z.
with respect to the central ferrocene moiety (dihedral angles:
(Li(12-c-4)(THF))2[10]: Cp(C(1))//Cp(C(11)) = 83.0◦; Li(12-c-
4)2[10]: Cp(C(11))//Cp(C(21)) = 41.1◦).
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Table 3 Formal electrode potentials E1/2 (vs. FcH/FcH+) and peak-to-peak separations DE (at 0.1 V s-1) for the FeII/FeIII redox processes exhibited by
compounds Li[9],23 Li2[10], Li[10], Li[Fc-BMe2-Fc],13 and Li2[Fc-BMe2-fc-BMe2-Fc]13
E1/2 [V]
DE [mV]
DE1/2 [mV]
solvent
DE(FcH) [mV]
Li[9]
-0.38/-0.64
-0.45/-1.18
-0.45/-1.18
-0.43/-0.64
-0.51/-1.21
99/97
260
730
730
210
700
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
103
122
108
100
250
Li2[10]
125/82
119/85
90/100
330/210
Li[10]
Li[Fc-BMe2-Fc]a
Li2[Fc-BMe2-fc-BMe2-Fc]a
a recorded at a scan rate of 0.2 Vs-1 and at a temperature of -78 ◦C.
Fig. 7 Cyclic voltammogram of Li2[10] (CH2Cl2, [NBu4][PF6] as support-
ing electrolyte (0.1 M), scan rate 0.1 V s-1; vs. FcH/FcH+).
found for the internal ferrocene standard (DE(FcH), Table 3;
theoretically expected value for a chemically and electrochemically
reversible one-electron step: 59 mV). The more anodic two-
electron transfer at E1/2 = -0.45 V can be attributed to the terminal
ferrocenyl moieties, while the less anodic one-electron redox event
at E1/2 = -1.18 V takes place at the interior iron centre. These data
agree with the results obtained from electrochemical investigations
of Li2[Fc-BMe2-fc-BMe2-Fc]. However, as in the case of Li[9]
and Li[Fc-BMe2-Fc], the E1/2 values of Li2[10]/Li[10] are slightly
shifted to the anodic regime compared to the redox potentials of
Li2[Fc-BMe2-fc-BMe2-Fc] (Table 3).
Oxidation of the 1,1¢-ferrocenylene unit inLi2[10] takes place at a
much more cathodic redox potential than oxidation of the terminal
ferrocenyl groups (DE1/2 = 730 mV) which is clearly due to the
fact that the former has two negatively charged substituents, while
each of the latter bears only one such group. The Fc moieties of
Li[9] are, however, chemically equivalent. The comparatively large
differences of more than 200 mV between the redox potentials
of the two FeII/FeIII transitions in Li[9] as well as Li[Fc-BMe2-
Fc] indicate that the two Fc subunits are mutually interacting.
The question thus arises whether this interaction is entirely
electrostatic in nature or whether there is a certain degree of charge
delocalization via four-coordinate boron linkers or even through-
space. The latter assumption is supported by the finding of a
very broad band with a maximum near 2200 nm in the electronic
spectrum of the FeII3FeIII species 11.19 It has been suggested that
this absorption is due to intervalence charge-transfer processes
which may proceed by a through-space mechanism.
Fig.
6 Structure of Li(THF)4[11] in the crystal. Displacement
ellipsoids are drawn at the 50% probability level.
and the Li(THF)4 counterion omitted for clarity. Selected bond
H atoms
+
lengths (A), atom ◊ ◊ ◊ atom distances (A), and bond angles (◦):
B(1)–C(1) = 1.648(4), B(1)–C(11) = 1.653(4), B(1)–C(21) = 1.634(4),
B(1)–C(31) = 1.644(4), COG(1)–COG(6) = 3.312, COG(11)–COG(16) =
3.318, COG(21)–COG(26) = 3.314, COG(31)–COG(36) = 3.314, av.
Fe ◊ ◊ ◊ Fe = 5.675(1); C(1)–B(1)–C(11) = 109.6(2), C(1)–B(1)–C(21) =
109.6(2), C(1)–B(1)–C(31) = 111.8(2), C(11)–B(1)–C(21) = 109.4(2),
C(11)–B(1)–C(31) = 103.2(2), C(21)–B(1)–C(31) = 113.0(2). COG(X):
centroid of the cyclopentadienyl ring containing the carbon atom C(X).
˚
˚
of chemical reversibility. Moreover, when also determined at a
temperature of -78 ◦C, both E1/2 values of Li[9] (-0.35 V/
-0.57 V) are anodically shifted by about 0.08 V with respect to the
redox potentials of Li[Fc-BMe2-Fc] (-0.43 V/-0.64 V), which can
easily be explained by the greater electronegativity of the phenyl
rings as compared to methyl substituents.
The cyclic voltammograms of the trinuclear complexes Li2[10]
and Li[10] are congruent to each other and reveal two redox events
with an intensity ratio of 1 : 2 (Fig. 7; note that the different
oxidation states of Li2[10] and Li[10] have been confirmed by linear
sweep voltammetry).
Both these processes are chemically reversible as evidenced by
In view of this background, we decided to carry out spec-
troelectrochemical measurements on the dinuclear compound
Li[9] and to look for intervalence charge-transfer (IVCT) bands.
To this end, we have performed a coulometrically controlled
the following criteria: the current ratios ipc/ipa are constantly equal
1
2
to 1, the current functions ipa/v remain constant, and the peak-
to-peak separations (DE) do not depart appreciably from the value
2946 | Dalton Trans., 2009, 2940–2950
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one-electron oxidation at a working potential of -0.5 V (CH2Cl2,
[NBu4][PF6] (0.1 M)) and simultaneously recorded the changes in
the UV/vis/NIR spectrum of the solution. Prior to oxidation, we
observe the lowest-energy band at lmax = 470 nm which compares
reasonably well to the absorption band at lmax = 442 nm exhibited
by parent ferrocene under the same conditions. After exhaustive
one-electron oxidation, the resulting FeIIFeIII species 9 still showed
a band at lmax = 471 nm and, in addition, an absorption at lmax
=
698 nm (cf . [(C5H5)2Fe]PF6: lmax = 635 nm). After exhaustive two-
electron oxidation at an applied voltage of 0.1 V, only a very broad
band at lmax = 619 nm remained in this region of the spectrum.
Most importantly, at no stage did we observe any feature at longer
wavelengths than 900 nm that might be interpretable as FeII/FeIII
IVCT band. An IVCT absorption is also absent in the electronic
spectrum of the FeII2FeIII mixed-valent complex Li[10]. In order
to test our spectroelectrochemical setup we also re-investigated
the tetraferrocenylborate Li[11] and have fully reproduced the
published spectral data19 including the IVCT band. We have
moreover observed that upon further oxidation of mixed-valent
11 (FeII3FeIII) to [11]+ (FeII2FeIII2) and to [11]2+ (FeIIFeIII3) the
broad featureless NIR band first intensifies and then decreases
in intensity (cf . Fig. 2S of the ESI†). Upon potential reversal and
subsequent reduction these changes are reversible.
57Fe Mo¨ßbauer spectroscopy
Since there are no analytically pure and solid samples of the mixed-
valent FeIIFeIII species 9 available, we had to restrict our Mo¨ßbauer
spectroscopic studies to the trinuclear FeII2FeIII compound Li[10].
Mo¨ßbauer data were acquired on single crystals of Li(12-c-
4)2[10] in the temperature range 97.5 K ≤ T ≤ 304 K. We first
consider the spectrum taken at 97.5 K which is shown in the lower
trace of Fig. 8. The spectrum consists of two distinct iron sites,
Fe and Fe¢, and the relative area under the resonance curve is Fe :
Fe¢ ª 2 : 1. This already suggests Fe to correspond to the FeII
sites and Fe¢ to the unique FeIII site of Li(12-c-4)2[10]. At 90 K,
the isomer shifts (IS) are 0.529 0.002 mm s-1 (Fe) and 0.50
0.04 mm s-1 (Fe¢) and the corresponding quadrupole splittings
(QS) are 2.379 0.002 mm s-1 (Fe) and -0.24 0.02 mm s-1
(Fe¢; Note: It is not always possible to record Mo¨ßbauer spectra
at exactly 90 K. To effect intersample comparison, the hyperfine
parameter data have been linearly extrapolated to 90 K and those
values are reported herein). The QS of the Fe¢ site is negative,
as has previously been observed for ferricinium centres in related
compounds.32
The qualitative picture remains the same irrespective of the
temperature applied during measurements. This leads to the
conclusion that there is no electron delocalization over the three
iron centres in the entire interval 97.5 K ≤ T ≤ 304 K. However,
it should be noted that at higher temperatures the FeIII resonance
sharpens and becomes a well-defined doublet, as shown in the
upper trace of Fig. 8. This observation is consistent with a spin–
lattice relaxation process (vide infra).
Fig. 8 57Fe Mo¨ßbauer spectrum of Li(12-c-4)2[10] at 97.5 K (lower trace).
The velocity scale is with respect to the centroid of a r. t. a-Fe spectrum.
The corresponding spectrum at 304 K is shown in the upper trace and
confirms the absence of electron delocalization between the iron sites, as
discussed in the text.
averaged over both types of iron sites (as determined from the areas
under the resonance curves) is linear over the whole temperature
range (slope = -(9.89 0.44) · 10-3 K-1; correlation coefficient =
0.99 for 7 data points). From these data it is possible to calculate
the parameter FM173 = k2 <xave 2> (k: wave vector of the Mo¨ßbauer
g-ray; xave 2: average root-mean-square amplitude of vibration of
the iron atom), which can be compared to the parameter FX173
as it has been extracted from the Uij value determined by X-
ray crystallography on Li(12-c-4)2[10] at 173 K. This comparison
reveals the two parameters, FM173 = 1.70 and FX173 = 1.73, to
be in excellent agreement with each other. The root-mean-square
amplitudes of vibration of the Fe atoms in Li(12-c-4)2[10] derived
˚
˚
˚
from the Mo¨ßbauer data are 0.193 A, 0.215 A, and 0.236 A at
200 K, 250 K, and 300 K, respectively.
As mentioned above, the paramagnetic FeIII centre Fe¢ relaxes by
spin–lattice relaxation and obeys a fifth order power law, indicative
of a Raman process, as has previously been reported for other S =
5/2 spin systems.33 The relaxation is slow (on the Mo¨ßbauer time
scale) below 150 K, but becomes rapid at higher temperatures (cf .
Fig. 9 for a plot of the temperature dependence of the relaxation
rate).
The temperature dependence of the IS for the FeII site Fe can be
fitted by a linear regression with a correlation coefficient of 0.992
for 11 data points (slope = -(4.12 0.15) · 10-4 mm s-1 K-1). From
this temperature dependence, the effective vibrating mass of the
metal centre is calculated to be Meff = 101 3 Daltons. Likewise,
the ln of the temperature dependence of the recoil-free fraction
Conclusion
We have shown that the molecular framework of BPh2-bridged
oligoferrocenes Li[Fc-BPh2-Fc] (Li[9]) and Li2[Fc-BPh2-fc-BPh2-
Fc] (Li2[10]) remains intact when the iron atoms are oxidized (Fc:
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conjugated, whereas in 9 and 10 there is no p-conjugation between
the redox-active moieties.
Experimental
General considerations
All reactions were carried out under a nitrogen atmosphere using
Schlenk tube techniques. Solvents were freshly distilled under
argon from Na/benzophenone (diethyl ether, THF, d8-THF),
Na/Pb alloy (pentane, hexane) or CaH2 (CH2Cl2, CDCl3) prior
to use. NMR spectrometers: Bruker AM 250, AV 300 and AMX
400. Chemical shifts are referenced to residual solvent peaks (1H,
13C{ H}) or external BF3·Et2O (11B{ H}). Abbreviations: s =
singlet, d = doublet, tr = triplet, mult = multiplet, br = broad,
o = ortho, m = meta, p = para, n.o. = not observed, n.r. =
not resolved. All NMR spectra were run at r. t. Electrochemical
measurements: Potentiostat EG&G Princeton Applied Research
263 A. Compounds FcHgCl (1),21 BrBPh2 (2),22, FcLi,35 and
1,1¢-fcLi2 ¥ 2/3 TMEDA (4)24,25 were synthesized according to
literature procedures. The synthesis of MeOBPh2 (5) is described
in the ESI.†
1
1
Fig. 9 Temperature dependence of the spin–lattice relaxation process
for the FeIII site in Li(12-c-4)2[10]. The solid line represents a fifth order
temperature dependence indicative of a Raman-type relaxation process.
(C5H5)Fe(C5H4)). A corresponding FeII2FeIII mixed-valent species
Li[Fc-BPh2-fc-BPh2-Fc] (Li[10]) has been structurally character-
ized by X-ray crystallography. Both the crystallographical data
and 57Fe Mo¨ßbauer spectroscopy on Li[10] point towards a largely
localized electronic structure with the central iron atom adopting
an oxidation state of +III. Moreover, spectroelectrochemical
measurements on Li[9] and Li2[10] in the UV/vis/NIR region
do not reveal absorption bands assignable to intervalence charge-
transfer processes after partial oxidation of the compounds.
This is in striking contrast to published data (which have been
confirmed by our own measurements) on the FeII3FeIII mixed-
valent complex Fc4B (11) for which electron delocalization has
been observed. To account for this different behaviour, we offer
two explanations: (i) In the dinuclear molecule [FcIIIFcIIBPh2]
(9), FeII→FeIII charge transfer is three times less likely than
in the tetranuclear compound [FcIII(FcII)3B] (11). As a result,
the absorbance of a hypothetical IVCT band of 9 should be
considerably smaller than the corresponding value of the IVCT
band of 11, which is already rather low (e = 300). (ii) If electron
transfer in 11 proceeds via a through-space mechanism (as has
been suggested by Cowan et al.19) the average Fe ◊ ◊ ◊ Fe distance
becomes a decisive factor. Since the ferrocenyl substituents in 11
are much more densely packed than in 9, a through-space charge-
transfer operative in 11 may well no longer be possible in 9 (cf . the
solid state structure of Li(OBu2)[9]23 shows that conformations of
this molecule are possible in which the Fe ◊ ◊ ◊ Fe distance is as long
Synthesis of 3. A solution of BrBPh2 2 (0.53 g, 2.16 mmol)
in hexane (12 mL) was added dropwise with stirring at -78 ◦C
to a suspension of FcHgCl 1 (0.91 g, 2.16 mmol) in hexane
(35 mL). The reaction mixture was slowly warmed to r. t. and
stirred overnight, whereupon a grey solid precipitated. After
filtration, the filtrate was slowly evaporated in vacuo to a volume of
5 mL whereupon single crystals of 3 formed. Yield: 0.60 g (80%).
1
1
11B{ H} NMR (128.4 MHz, CDCl3): d 63.3 (h1/2 = 600 Hz). H
NMR (400.1 MHz, CDCl3): d 4.18 (s, 5H, C5H5), 4.54, 4.82 (2 ¥
n.r., 2 ¥ 2H, C5H4), 7.40–7.46 (mult, 6H, m-Ph, p-Ph), 7.75 (d, 4H,
3JHH = 7.6 Hz, o-Ph). 13C{ H} NMR (100.6 MHz, CDCl3): d 69.4
1
(C5H5), 75.8, 78.4 (C5H4), 127.2 (m-Ph), 129.4 (p-Ph), 135.4 (o-
Ph), n.o. (BC). Elemental analysis: Calcd. for C22H19BFe (350.03):
C, 75.49; H, 5.47. Found: C, 75.23; H, 5.50%.
Synthesis of 6 and 7. A solution of MeOBPh2 5 (0.36 g,
1.86 mmol) in hexane (10 mL) was added dropwise with stirring
at -78 ◦C to a suspension of 1,1¢-fcLi2 ¥ 2/3 TMEDA 4 (0.26 g,
0.93 mmol) in hexane (15 mL). The reaction mixture was slowly
warmed to r. t. and stirred overnight. The resulting orange
suspension was filtered and the filtrate was evaporated to a volume
of 5 mL. Red needles of 6 and colourless plates of 7 crystallized
◦
after the solution had been stored for several months at -35 C.
˚
as 6.679(1) A; in contrast, the average Fe ◊ ◊ ◊ Fe distance in the less
The crystals were separated by manual selection in a glovebox.
1
1
˚
flexible molecule Li(THF)4[11] is only 5.675(1) A).
NMR data of 6: 11B{ H} NMR (128.4 MHz, CDCl3): d n.o. H
We therefore conclude that electronic interaction between the
individual iron sites in Li[9], 9, Li2[10], and Li[10] does probably
not occur by charge delocalization via the BPh2-bridge, but is
either a through-space process or of an electrostatic nature. In this
context, it is interesting to compare the results of Curtis et al.
on the degree of electronic communication within mixed-valent
poly(ferrocenylenearylene)s.34 These authors find very similar
behaviour to our oligomers (e.g. significant ferrocene–ferrocene
interaction as measured by cyclic voltammetry, but little electron
transfer as measured by NIR and Mo¨ßbauer spectroscopy), even
though the polymer chains in poly(ferrocenylenearylene)s are
NMR (400.1 MHz, CDCl3): d 4.54, 4.75 (2 ¥ n.r., 2 ¥ 4H, C5H4),
7.39 (mult, 8H, m-Ph), 7.47 (mult, 4H, p-Ph), 7.68 (d, 8H, 3JHH
=
7.2 Hz, o-Ph). 13C{ H} NMR (100.6 MHz, CDCl3): d 76.6, 79.4
1
(C5H4), 127.3 (m-Ph), 129.9 (p-Ph), 135.5 (o-Ph), n.o. (BC).
NMR data of 7: 11B{ H} NMR (128.4 MHz, d8-THF): d 2.5
1
(h1/2 = 150 Hz). 1H NMR (400.1 MHz, d8-THF): d 2.15 (s, 12H,
NMe), 2.31 (s, 4H, NCH2), 3.11 (s, 3H, OMe), 6.86 (tr, 3H, 3JHH
=
7.6 Hz, p-Ph), 7.00 (mult, 6H, m-Ph), 7.41 (d, 6H, 3JHH = 7.2 Hz,
o-Ph). 13C{ H} NMR (100.6 MHz, d8-THF): d 46.4 (NMe), 59.1
1
(NCH2), 52.2 (OMe), 124.0 (p-Ph), 126.9 (m-Ph), 135.3 (o-Ph),
n.o. (BC).
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Synthesis of Li2[10] and Li[10]. A solution of FcBPh2
3
0.51(1) and 0.49(1)). The Flack-x-parameters for structures 3 and
6 are 0.00(2) and -0.02(3), respectively.
CCDC reference numbers: 699649 (3), 699647 (6), 699646
(7), 699650 ((Li(12-c-4)(THF))2[10]), 699648 (Li(12-c-4)2[10]) and
710783 (Li(THF)4[11]).†
(0.15 g, 0.43 mmol) in THF (8 mL) was added dropwise with
stirring at -78 ◦C to a solution of 1,1¢-fcLi2 ¥ 2/3 TMEDA 4
(0.06 g, 0.22 mmol) in THF (12 mL). The reaction mixture was
slowly warmed to r. t. and stirred overnight. The volume of the
solution was first reduced to 4 mL in vacuo and then 12-crown-
4 (0.27 mL) and hexane (10 mL) were added. The resulting red
precipitate was extracted with hexane (3 ¥ 10 mL). Single crystals
of (Li(12-c-4)(THF))2[10] were grown by gas-phase diffusion of
hexane into a THF solution of the crude product under strict
exclusion of air. Single crystals of Li(12-c-4)2[10] were grown
under similar conditions but without strict exclusion of air. Yield
of (Li(12-c-4)(THF))2[10]: 0.10 g (33%). Yield of Li(12-c-4)2[10]:
Electrochemical measurements. All electrochemical measure-
ments were performed by using an EG&G Princeton Applied
Research 263A potentiostat with glassy carbon or platinum disc
working electrode. Carefully dried (CaH2) and degassed CH2Cl2
was used as the solvent and [NBu4][PF6] as the supporting
electrolyte (0.1 M). All potential values are referenced against
the FcH/FcH+ couple. Spectroelectrochemical measurements
were performed in a home-built optically transparent thin-layer
electrolysis (OTTLE) cell following the design of Hartl et al.40
with a Bruins Instruments Omega 20 UV/vis/NIR spectrometer.
1
0.030 g (10%). NMR data of (Li(12-c-4)(THF))2[10]: 11B{ H}
1
NMR (128.4 MHz, d8-THF): d -11.7 (h1/2 = 70 Hz). H NMR
(400.1 MHz, d8-THF): d 3.59 (s, 32H, 12-c-4), 3.68 (very br,
C5H4/C5H5), 6.63 (br, 4H, p-Ph), 6.79, 7.35 (2 ¥ br, 2 ¥ 8H, o,m-
Ph). Elemental analysis: Calcd. for C78H94B2Fe3Li2O10 (1394.58):
C, 67.18; H, 6.79. Found: C, 67.40; H, 6.89%. Elemental analysis:
Calcd. for C70H78B2Fe3LiO8 (1243.43) ¥ C4H8O (72.11): C, 67.56;
H, 6.59. Found: C, 67.71; H, 6.62%.
Mo¨ßbauer spectra. The details of 57Fe temperature-dependent
Mo¨ßbauer spectroscopy have been described earlier.41–43 Due to the
air- and moisture sensitivity of the compounds, sample transfer to
perspex sample holders, lubricated with high-temperature silicone
grease and sealed with O-rings, was effected in an inert-atmosphere
glove box (VAC model DLX-001-S-P) having an oxygen partial
pressure of less than 0.5 ppm and less than 1 ppm H2O. The
filled and sealed sample holders were removed from the glove
box, immediately cooled to liquid nitrogen temperature, and then
placed into the Mo¨ßbauer spectrometer pre-cooled to ª 90 K. Data
accumulation (in the first instance) was effected in a warming mode
as discussed above. All isomer shifts are reported with respect to
the centroid of a room temperature a-Fe absorber spectrum which
was also used for spectrometer calibration.
Synthesis of Li[11]. Ferrocene (5.00 g, 26.88 mmol) was treated
with THF (25 mL) and the mixture cooled to 0 ◦C. tert-BuLi
in pentane (1.6 M; 14 mL, 22.4 mmol) was added dropwise
with stirring to the amber coloured slurry, whereupon the colour
changed to red. The reaction mixture was stirred at 0 ◦C for
another 15 min and allowed to warm to r. t. BF3·OEt2 (0.37 g,
2.61 mmol) in THF (5 mL) was added slowly over a period of
1 h. The mixture was stirred for 15 h, the solvents were removed
under vacuum and the residue extracted with Et2O (40 mL). The
remaining solid was kept under vacuum overnight to remove
residual ferrocene. Single crystals of Li(THF)4[11] were grown
by gas-phase diffusion of pentane into a THF solution of the
Acknowledgements
1
crude product. Yield of Li(THF)4[11]: 0.70 g (26%). 11B{ H}
M. W. is grateful to the “Deutsche Forschungsgemeinschaft”
(DFG) and the “Fonds der Chemischen Industrie” (FCI) for finan-
cial support. L. K. wishes to thank the “Hessisches Ministerium
fu¨r Wissenschaft und Kunst” for a Ph. D. grant. The authors
are also indebted to A. Aharoni for effecting the careful sample
transfers in an inert atmosphere glovebox to avoid oxidative
degradation prior to spectral examination of the samples referred
to herein.
1
NMR (96.3 MHz, d8-THF): d -15.3 (h1/2 = 10 Hz). H NMR
(300.0 MHz, d8-THF): d 3.69 (s, 20H, C5H5), 3.87, 4.34 (2 ¥
1
n.r., 2 ¥ 8H, C5H4). 13C{ H} NMR (75.5 MHz, d8-THF): d 66.0
(mult, C5H4), 68.1 (C5H5), 74.7 (mult, C5H4), n.o. (BC). Elemental
analysis: Calcd. for C56H68BFe4LiO4 (1046.25): C, 64.29; H, 6.55.
Found: C, 63.88; H, 6.54%.
Crystal structure determinations of 3, 6, 7, (Li(12-c-4)(THF))2[10],
Li(12-c-4)2[10] and Li(THF)4[11]
References
Single crystals of 3, 6, 7, (Li(12-c-4)(THF))2[10], Li(12-c-4)2[10]
and Li(THF)4[11] were analyzed with a STOE IPDS II two-
circle diffractometer with graphite-monochromated MoKa radia-
tion. Empirical absorption corrections were performed using the
MULABS36 option in PLATON.37 The structures were solved by
direct methods using the program SHELXS38 and refined against
F2 with full-matrix least-squares techniques using the program
SHELXL-97.39 All non-hydrogen atoms (except disordered atoms
in Li(12-c-4)2) were refined with anisotropic displacement param-
eters. Hydrogen atoms were refined using a riding model. The
crown ether molecules of (Li(12-c-4)(THF))2[10] are disordered
over two positions (occupancy factors 0.663(5) and 0.337(5)).
Li(12-c-4)2[10] contains one equivalent of non-coordinating THF
in the crystal lattice. One of the two crown ether molecules of
Li(12-c-4)2[10] is disordered over two positions (occupancy factors
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