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Title Main-chain organometallic polymers comprising redox-active iron(II) centers connected by
ditopic N-heterocyclic carbenes
Authors(s) Mercs, Laszlo; Neels, Antonia; Stoeckli-Evans, Helen; Albrecht, Martin
Publication date 2009
Publication information Dalton Transactions, (35): 7168-7178
Publisher RSC
Item record/more information http://hdl.handle.net/10197/3651
Publisher's version (DOI) 10.1039/B907018D
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Main-chain organometallic polymers comprising redox-active iron(II)
centers connected by ditopic N-heterocyclic carbenes
Laszlo Mercs,
a
Antonia Neels,
b
Helen Stoeckli-Evans
c
and Martin Albrecht*
a,d
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X 5
DOI: 10.1039/b000000x
Main-chain organometallic polymers were synthesized from bimetallic iron(II) complexes
containing a ditopic N-heterocyclic carbene (NHC) ligand [(cp)(CO)LFe(NHC~NHC)Fe
(cp)(CO)L]X
2
(where NHC~NHC represents a bridging dicarbene ligand, L = I
–
or CO). Addition
of a diimine ligand such as pyrazine or 4,4’-bipyridine interconnected these bimetallic complexes 10
and gave the corresponding co-polymers containing iron centers that are alternately linked by a
dicarbene and a diimine ligand. Diimine coordination was depending on the wingtip groups at the
carbene ligands and was accomplished either by photolytic activation of a carbonyl ligand from the
cationic [Fe(cp)(NHC)(CO)
2
]
+
precursor (alkyl wingtips) or by AgBF
4
-mediated halide abstraction
from the neutral complex [FeI(cp)(NHC)(CO)] (mesityl wingtips). Remarkably, the polymeric 15
materials were substantially more stable than the related bimetallic model complexes.
Electrochemical analyses indicated metal-metal interactions in the pyrazine-containing polymers,
whereas in 4,4’-bipyridine-linked systems the metal centers were electronically decoupled.
Introduction
While the impact of N-heterocyclic carbene (NHC) complexes 20
in catalysis has been widely recognized,
1
the application of
organometallic NHC chemistry in other areas of materials
science has been much less developed thus far.
2–4
This is
remarkable, especially when considering the relatively robust
metal-carbon bond in NHC complexes.
5
In addition, 25
theoretical
6
and experimental
7
results have indicated that the
M–C
NHC
bond comprises a significant portion of
π
character
when bound to electron-rich metal centers. Such bonding may
become attractive for designing systems for potential
application in molecular electronics, since facile transfer of 30
electron density along
π
networks may also encompass the
metal center. Specifically when using redox-active metal
centers in the main chain, organometallic polymers have great
potential to surpass the versatility of most common organic or
inorganic conducting systems,
8
as the oxidation state of the 35
metal center in these organometallic polymers represents an
additional function that can be selectively addressed and
reversibly switched.
Initial efforts on using organometallic NHC chemistry for
polymer synthesis concentrated on metals with low redox 40
activity.
9
Predominantly through the pioneering work of
Bielawski and coworkers, a range of ditopic carbene ligands
became available for the interconnection of two metal
centers.
10
Subsequent exploitation of this approach provided
access to redox-active molecular switches
11
and to molecular 45
squares.
12
Stimulated by these achievements, we aimed at
expanding the use of ditopic carbene ligands for the
fabrication of co-polymers containing redox-acitve NHC
iron(II) units
13
in the main chain.
Our retrosynthetic approach towards such co-polymers was 50
based on the interconnection of a bimetallic synthon with a
Fig. 1 Retrosynthetic approach towards organometallic polymers
comprising NHC metal complexes in the main chain.
ditopic, bridging ligand (Figure 1). Since coordination of a 55
second, non-chelating NHC ligand to a Fe–NHC complex
proved to be difficult, we focussed our attention on co-
polymers that feature two different interconnecting ligands.
Hence, the desired co-polymer can be dissected into two
bimetallic synthons A and B (Figure 1), comprising each a 60
different ditopic ligand. Here we report on the synthesis and
electrochemical properties of iron NHC polymers and their
bimetallic synthons A and B. Metal-metal interactions in these
systems are strongly dependent on the type of diimine ligand
employed. 65
Experimental Section
General Comments
All manipulations were performed using standard Schlenk
techniques under an argon atmosphere unless stated otherwise.
Toluene, THF and CH
2
Cl
2
were dried by passage through 5
solvent purification columns, all other reagents were used
without further purification. The synthesis of the
diimidazolium salts 1
14
and 2
15
and complexes 7
13
and 8
16
are
described elsewhere. Unless stated otherwise, all
1
H and
13
C{
1
H} NMR spectra were recorded at 25 °C on Bruker 10
spectrometers operating at 360 or 400 MHz (
1
H NMR) and at
100 MHz (
13
C NMR), respectively. Resonance frequencies
were referenced to residual solvent
1
H or
13
C resonances.
Chemical shifts (
δ
) are given in ppm, coupling constants (J) in
Hz. Assignments are based either on distortionless 15
enhancement of polarization transfer (DEPT) experiments or
on homo- and heteronuclear shift correlation spectroscopy.
Elemental analyses were performed by the Microanalytical
Laboratory of Ilse Beetz (Kronach, Germany) and by the
Microanalytical Laboratory of the ETH Zürich (Switzerland). 20
A commercially available Hg lamp was used for irradiation.
Absorption spectra were measured on an Agilent 8453 UV-
Vis spectrophotometer. IR spectra were recorded on a Mattson
5000 FT-IR and a Bruker Tensor 27 FT-IR instrument for
solutiona and solid state measurements, respectively. 25
Syntheses
Synthesis of 3. To a suspension of the diimidazolium salt 1
(0.32 g, 0.5 mmol) in dry THF (10 mL) was added nBuLi (1.6
M in hexanes, 0.63 mL, 1.0 mmol) at –78 °C. After stirring
for 30 min at RT, the mixture was frozen and overlaid with a 30
solution of [FeI(cp)(CO)
2
] (0.76 g, 2.5 mmol) in dry toluene
(30 mL). The reaction mixture was gradually warmed to RT
and stirred for 16 h. The formed precipitate was separated by
centrifugation, washed once with dry toluene (30 mL) and
then extracted with dry CH
2
Cl
2
(2 × 20 mL). The combined 35
CH
2
Cl
2
fractions were reduced to ca. 20 mL and subsequently
irradiated for 16 h. All volatiles were removed in vacuo, and
the residue was purified by gradient column chromatography
(SiO
2
, CH
2
Cl
2
/acetone) and by subsequent precipitation from
CH
2
Cl
2
/pentane. The product was obtained as a green powder 40
(0.32 g, 68%) as a mixture of diastereoisomers. X-ray quality
crystals were grown by slow diffusion of pentane into a
CH
2
Cl
2
solution of 3 at –20 °C. Analytically pure material
was obtained by recrystallisation from toluene/pentane.
1
H
NMR of major isomer (CDCl
3
, 400 MHz):
δ
8.49 (s, 2H, 45
NCH
2
N), 7.34 (d,
3
J
HH
= 2.1 Hz, 2H, H
NHC
), 7.05, 7.03 (2 × s,
4H, H
Mes
), 6.79 (d,
3
J
HH
= 2.1 Hz, 2H, H
NHC
), 4.56 (s, 10H,
H
cp
), 2.41 (s, 6H, p-CH
3
), 1.90, 1.88 (2 × s, 12H, o-CH
3
).
1
H
NMR of minor isomer (CDCl
3
, 400 MHz):
δ
8.42, 8.20 (2 × d,
2
J
HH
= 12.8 Hz, 2H, NCH
2
N), 7.81, 7.08 (2 × d,
3
J
HH
= 2.1 Hz, 50
2H, H
NHC
), 7.05, 7.03 (2 × s, 4H, H
Mes
), 7.01, 6.92 (2 × d,
3
J
HH
= 2.1 Hz, 2H, H
NHC
), 4.47 (s, 10H, H
cp
), 2.42 (s, 6H, p-CH
3
),
1.97, 1.94, 1.92 (3 × s, 12H, o-CH
3
).
13
C{
1
H} NMR (CDCl
3
,
100 MHz): δ 220.3 (CO), 192.1 (C
carbene
), 139.7, 138.1, 137.0,
134.7 (4 × C
Mes
), 129.9, 129.1 (2 × C
Mes
–H), 124.6, 124.1 (2 × 55
C
NHC
–H), 80.2 (C
cp
), 68.8 (NCH
2
N), 21.4, 18.6, 18.0 (3 ×
CH
3
). IR (neat, cm
–1
): 1939 ν (CO). Anal. Calcd. for
C
37
H
38
Fe
2
I
2
N
4
O
2
(936.22) × ½ C
6
H
5
CH
3
: C 49.52, H 4.31, N
5.70. Found: C 49.49, H 4.28, N 5.67.
Synthesis of 4. To a suspension of the diimidazolium salt 2 60
(1.16 g, 2.0 mmol) in dry THF (20 mL) was added
LiN(SiMe
3
)
2
(1 M in THF, 4.0 mL, 4.0 mmol) at RT. After
stirring for 1 h, this solution was added to a solution of
[FeI(cp)(CO)
2
] (1.15 g, 3.8 mmol) in dry toluene (40 mL) and
stirring was continued for 16 h. The formed precipitate was 65
separated by centrifugation, washed once with dry toluene (30
mL) and then with dry CH
2
Cl
2
(2 × 20 mL). The product was
obtained as a yellow powder (0.31 g, 67%). Single crystals
suitable for X-ray diffraction analysis were grown in the dark
by slow Et
2
O diffusion into a MeNO
2
solution.
1
H NMR 70
(DMSO-d
6
, 400 MHz):
δ
7.99 (s, 2H, H
NHC
), 7.78 (br, 6H, H
Ph
and H
NHC
), 5.45 (s, 10H, H
cp
), 4.23 (br, 4H, NCH
2
), 1.89 (br,
4H, NCH
2
CH
2
), 1.50 (br, 4H, CH
2
CH
3
), 1.01 (t,
3
J
HH
= 7.2 Hz,
6H, CH
2
CH
3
).
13
C{
1
H} NMR (DMSO-d
6
, 100 MHz): δ 211.6
(CO), 165.8 (C
carbene
), 140.8 (C
Ph
), 130.0 (C
Ph
–H), 128.3, 125.1 75
(2 × C
NHC
–H), 87.8 (C
cp
), 51.1 (NCH
2
), 32.3 (NCH
2
CH
2
), 19.3
(CH
2
CH
3
), 13.8 (CH
2
CH
3
). IR (neat, cm
–1
): 2049, 1981 ν(CO).
Anal. Calcd. for C
34
H
36
Fe
2
I
2
N
4
O
4
(930.17): C 43.90, H 3.90,
N 6.02. Found: C 43.79, H 3.98, N 5.98.
Synthesis of 5. Complex 4 (0.28 g, 0.3 mmol) and AgBF
4
80
(0.12 g, 0.6 mmol) were stirred in MeNO
2
and CH
2
Cl
2
(20
mL, 1:1 V/V) in the dark for 3 h. The suspension was filtered
through Celite and evaporated to dryness, affording the crude
product as a yellow powder (0.24 g, 96%). An analytically
pure sample was obtained by recrystallisation of 5 from 85
MeNO
2
/Et
2
O.
1
H NMR (DMSO-d
6
, 360 MHz):
δ
7.99 (s, 2H,
H
NHC
), 7.78 (s, 6H, H
Ph
and H
NHC
), 5.44 (s, 10H, H
cp
), 4.22 (br,
4H, NCH
2
), 1.88 (br, 4H, NCH
2
CH
2
), 1.50 (br, 4H, CH
2
CH
3
),
1.01 (t,
3
J
HH
= 7.3 Hz, 6H, CH
2
CH
3
). IR (neat, cm
–1
): 2051,
1984 ν(CO). Anal. Calcd. for C
34
H
36
B
2
F
8
Fe
2
N
4
O
4
(849.97) × 90
¼ MeNO
2
: C 47.54, H 4.28, N 6.88. Found: C 47.57, H 4.63,
N 7.14.
Synthesis of 6. A solution of 4 (0.186 g, 0.2 mmol) in dry
CH
2
Cl
2
(10 mL) was irradiated for 16 h, upon which the
initially yellow solution became green. Evaporation of the 95
solvent gave the crude product as a green powder, which was
purified by precipitation from CH
2
Cl
2
/pentane (0.14 g, 80%).
Recrystallisation from CH
2
Cl
2
/toluene/Et
2
O at +4 °C gave an
analytically pure sample.
1
H NMR (CDCl
3
, 400 MHz):
δ
7.33
(d,
3
J
HH
= 2.0 Hz, 2H, H
NHC
minor
), 7.31 (d,
3
J
HH
= 2.0 Hz, 2H, 100
H
NHC
major
), 7.30–7.13 (br, 4H, H
Ph
), 7.21 (d,
3
J
HH
= 2.0 Hz,
2H, H
NHC
minor
), 7.09 (d,
3
J
HH
= 2.0 Hz, 2H, H
NHC
major
), 5.10–
4.95, 4.91–4.79 (2 × m, 4H, NCH
2
), 4.44, 4.43 (2 × s, 10H,
H
cp
), 2.13–1.87 (2 × m, 4H, NCH
2
CH
2
), 1.73–1.56 (m, 4H,
CH
2
CH
3
), 1.11 (t,
3
J
HH
= 7.4 Hz, 6H, CH
2
CH
3
).
13
C{
1
H} NMR 105
(CDCl
3
, 100 MHz): δ 222.5, 221.7 (2 × CO), 189.1, 188.8 (2 ×
C
carbene
), 142.3, 142.1 (2 × C
Ph
), 129.2, 128.4 (2 × C
Ph
–H),
125.6, 125.4, 123.0, 122.9 (4 × C
NHC
–H), 80.5, 80.4 (2 × C
cp
),
54.1, 54.0 (2 × NCH
2
), 33.6 33.6 (2 × NCH
2
CH
2
), 20.4
(CH
2
CH
3
), 14.3 (CH
2
CH
3
). IR (CH
2
Cl
2
, cm
–1
): 1941 ν(CO). 110
Anal. Calcd. for C
32
H
36
Fe
2
I
2
N
4
O
2
(874.15) × C
6
H
5
CH
3
: C
46.33, H 4.38, N 6.09. Found: C 46.29, H 4.74, N 6.10.
Synthesis of 9a. A solution of 7 (0.208 g, 0.5 mmol) and
pyrazine (0.020 g, 0.25 mmol) in dry CH
2
Cl
2
(10 mL) was
irradiated for 16 h. During this time the initially yellow
solution turned purple and a precipitate formed. Separation of
the precipitate by filtration and washing with dry CH
2
Cl
2
gave
9a as a purple solid (0.116 g, 54%), which was precipitated
from MeNO
2
/acetone/Et
2
O at 0 °C to give an analytically pure 5
sample.
1
H NMR (MeNO
2
-d
3
, 400 MHz, 258 K):
δ
8.12–7.63
(br, 4H, H
pz
), 7.63–7.40 (m, 4H, H
NHC
), 5.1–4.9 (br, 2H,
CHMe
2
), 4.98 (s, 10H, H
cp
), 4.78–4.60 (br, 2H, CHMe
2
),
1.71–1.57, 1.57–1.43, 1.43–1.27, 0.85–0.66 (4 × m, 24H,
CH(CH
3
)
2
).
13
C{
1
H} NMR (MeNO
2
-d
3
, 100 MHz, 258 K): δ 10
221.8 (CO), 174.3 (C
carbene
), 153.9 (C
pz
–H), 123.2, 122.7 (2 ×
C
NHC
–H), 85.4 (C
cp
), 54.2, 53.5 (2 × CHMe
2
), 24.4, 23.9, 23.5,
22.3 (4 × CH(CH
3
)
2
). The resonances of the second isomer are
strongly overlapping, additional signals were detected at
δ
C
221.7 (CO), 174.4 (C
carbene
), 85.4 (C
cp
), 54.3, 53.4 (2 × 15
CHMe
2
). IR (neat, cm
–1
): 1952 ν(CO). Anal. Calcd. for
C
34
H
46
B
2
F
8
Fe
2
N
6
O
2
(856.07): C 47.70, H 5.42, N 9.82. Found:
C 47.61, H 5.48, N 9.83.
Synthesis of 9b. To a solution of 8 (0.174 g, 0.3 mmol) and
pyrazine (0.013 g, 0.17 mmol) in dry CH
2
Cl
2
(10 mL) was 20
added AgBF
4
(0.058 g, 0.3 mmol) and the resulting
suspension was stirred for 16 h. During this time the initially
yellow solution turned red and a precipitate formed.
Separation of the precipitate by filtration and subsequent
repeated precipitation from CH
2
Cl
2
/MeNO
2
/Et
2
O at 0 °C gave 25
9b as an analytically pure solid (0.110 g, 63%). Crystals
suitable for an X-ray diffraction analysis were obtained from
acetone/pentane at +4 °C.
1
H NMR (MeNO
2
-d
3
, 400 MHz, 258
K):
δ
7.48 (s, 4H, H
NHC
), 7.44 (s, 4H, H
pz
), 7.26, 6.85 (2 × s,
8H, H
Mes
), 4.72 (s, 10H, H
cp
), 2.43, 2.28, 1.53 (3 × s, 36H, 30
CH
3
).
13
C{
1
H} NMR (MeNO
2
-d
3
, 100 MHz, 258 K): δ 219.3
(CO), 177.2 (C
carbene
), 153.3 (C
pz
–H), 141.9, 137.7, 137.2 (3 ×
C
Mes
), 131.0, 129.9 (2 × C
Mes
–H), 129.5 (C
NHC
–H), 85.5 (C
cp
),
21.2, 18.7, 17.9 (3 × CH
3
). IR (neat, cm
–1
): 1946 ν(CO). Anal.
Calcd. for C
58
H
62
B
2
F
8
Fe
2
N
6
O
2
(1160.45) × ½ CH
2
Cl
2
: C 35
58.41, H 5.28, N 6.99. Found: C 58.85, H 5.33, N 7.10.
Synthesis of 10a. This complex was prepared in a manner
similar to that for 9a using 7 (0.38 g, 0.9 mmol) and 4,4’-
bipyridine (0.071 g, 0.46 mmol) in degassed acetone (20 mL).
Precipitation gave an analytically pure red solid (0.174 g, 40
41%).
1
H NMR (MeNO
2
-d
3
, 400 MHz, 258 K):
δ
8.22 (br, 4H,
H
bpy
), 7.60–7.4 (br, 4H, H
NHC
), 7.42 (d,
3
J
HH
= 6.9 Hz, 4H,
H
bpy
), 5.22 (br, 2H, CHMe
2
), 4.99 (s, 10H, H
cp
), 4.93 (br, 2H,
CHMe
2
), 1.63, 1.50, 1.39, 0.49 (4 × s, 24H, CH(CH
3
)
2
).
13
C{
1
H} NMR (MeNO
2
-d
3
, 100 MHz, 258 K): δ 223.6 (CO), 45
177.0 (C
carbene
), 159.8 (C
bpy
–H), 145.1 (C
bpy
), 123.7 (C
bpy
–H),
122.8, 122.3 (2 × C
NHC
–H), 84.3 (C
cp
), 54.0, 53.4 (2 ×
CHMe
2
), 24.4, 23.9, 23.6, 21.8 (4 × CH(CH
3
)
2
). IR (neat, cm
–
1
): 1937 ν(CO). Anal. Calcd. for C
40
H
50
B
2
F
8
Fe
2
N
6
O
2
(932.16)
× ½ CH
2
Cl
2
: C 49.91, H 5.27, N 8.62. Found: C 49.72, H 5.47, 50
N 8.87..
Synthesis of 10b. This complex was prepared in a manner
similar to that for 9b using 8 (0.174 g, 0.3 mmol), 4,4’-
bipyridyl (0.027 g, 0.17 mmol) and AgBF
4
(0.058 g, 0.3
mmol) in dry CH
2
Cl
2
(10 mL). The reaction mixture was 55
filtered through Celite at 0 °C, evaporated to dryness and then
precipitated twice from CH
2
Cl
2
/MeNO
2
/Et
2
O at 0 °C to give
an analytically pure orange solid (0.104 g, 56%).
1
H NMR
(MeNO
2
-d
3
, 400 MHz, 258 K):
δ
8.18 (s, 4H, H
bpy
), 7.48 (s,
4H, H
NHC
), 7.30 (s, 4H, H
Mes
), 7.18 (s, 4H, H
bpy
), 6.95–6.71 60
(m, 4H, H
Mes
), 4.61 (s, 10H, H
cp
), 2.80–1.79, 1.64–1.27 (2 ×
m, 36 H, CH
3
).
13
C{
1
H} NMR (MeNO
2
-d
3
, 100 MHz, 258 K):
δ 220.7 (CO), 180.8 (C
carbene
), 160.5 (C
bpy
–H), 144.6 (C
bpy
),
141.4, 137.6 (2 × C
Mes
), 130.7, 130.3 (2 × C
Mes
–H), 129.0
(C
NHC
–H), 122.8 (C
bpy
–H), 84.3 (C
cp
), 21.2, 18.7, 17.8 (3 × 65
CH
3
). IR (neat, cm
–1
): 1947 ν (CO). Anal. Calcd. for
C
64
H
66
B
2
F
8
Fe
2
N
6
O
2
(1236.55) × 3/4 CH
2
Cl
2
: C 59.81, H 5.23,
N 6.46. Found: C 59.79, H 5.28, N 6.69.
Synthesis of 11. Irradiation of a solution of 7 (0.17 g, 0.4
mmol) in dry MeCN (10 mL) for 16 h and subsequent solvent 70
evaporation gave 11 as an orange solid in quantitative yield
(0.18 g). Analytically pure material was obtained by
recrystallisation of 11 from CH
2
Cl
2
/Et
2
O at +4 °C.
1
H NMR
(acetone-d
6
, 400 MHz):
δ
7.74 (s, 2H, H
NHC
), 5.35–5.05 (br,
2H, CHMe
2
), 4.96 (s, 5H, H
cp
), 2.46 (s, 3H, CH
3
CN), 1.59, 75
1.41 (2 × d,
3
J
HH
= 6.7 Hz, 12H, CH(CH
3
)
2
).
13
C{
1
H} NMR
(acetone-d
6
, 100 MHz): δ 221.4 (CO), 173.7 (C
carbene
), 136.1
(CH
3
CN), 122.4 (C
NHC
–H), 83.2 (C
cp
), 53.1 (CHMe
2
), 23.8,
23.7 (2 × CH(CH
3
)
2
), 4.8 (CH
3
CN). IR (neat, cm
–1
): 1964
ν(CO). Anal. Calcd. for C
17
H
24
BF
4
FeN
3
O (429.04): C 47.59, 80
H 5.64, N 9.79. Found: C 47.72, H 5.46, N 9.68.
Synthesis of 12. In analogy to the preparation of 5, irradiation
of a solution of 7 (0.11 g, 0.3 mmol) and pyridine (0.11 g, 1.3
mmol) in dry CH
2
Cl
2
(10 mL) for 16 h and subsequent
evaporation of the volatiles yielded the crude product as a 85
dark brown solid in quantitative yield (0.13 g).
Recrystallisation from acetone/pentane at +4 °C gave an
analytically pure sample.
1
H NMR (acetone-d
6
, 400 MHz):
δ
8.26 (d,
3
J
HH
= 5.3 Hz, 2H, o-H
py
), 7.81 (m, 1H, p-H
py
), 7.74
(s, 2H, H
NHC
), 7.27 (m, 2H, m-H
py
), 5.20 (m, 2H, CHMe
2
), 90
5.10 (s, 5H, H
cp
), 1.63 (d,
3
J
HH
= 6.5 Hz, 6H, CH(CH
3
)
2
), 0.99
(br, 6H, CH(CH
3
)
2
).
13
C{
1
H} NMR (acetone-d
6
, 100 MHz): δ
223.4 (CO), 177.3 (C
carbene
), 159.1 (o-C
py
–H), 138.6 (p-C
py
–H),
126.8 (m-C
py
–H), 122.7 (C
NHC
–H), 83.8 (C
cp
), 53.3 (CHMe
2
),
24.2, 22.7 (2 × CH(CH
3
)
2
). IR (neat, cm
–1
): 1927 ν(CO). Anal. 95
Calcd. for C
20
H
26
BF
4
FeN
3
O (467.09): C 51.43, H 5.61, N
9.00. Found: C 51.65, H 5.55, N 9.12.
General procedure for the preparation of polymers 13a
and 14a. An acetone solution (10 mL) containing complex 5
(0.15 mmol) and diimine (0.15 mmol) was irradiated for 16 h. 100
The product precipitated from the reaction mixture and was
collected by filtration and washed with CH
2
Cl
2
(2 × 2 mL).
Analytical data are collected in Table 6.
General procedure for the preparation of polymers 13b
and 14b. Complex 3 (0.15 mmol), diimine (0.15 mmol) and 105
AgBF
4
(0.33 mmol) were stirred in dry CH
2
Cl
2
(10 mL) for 16
h. The reaction mixture was filtered through Celite,
evaporated to dryness and then precipitated twice form
CH
2
Cl
2
/MeNO
2
/Et
2
O to give the desired polymers. Analytical
data are collected in Table 6. 110
Electrochemical Measurements
Electrochemical studies were carried out using an EG&G
Princeton Applied Research Potentiostat Model 273A
employing a gastight three-electrode cell under an argon
atmosphere. For compounds 3, 6, 11 and 12 a Pt disk with a 115
3.80 mm
2
surface area or a glassy-carbon disk with a 3.14
mm
2
surface area was used as the working electrode and was
polished before each measurement. The reference was a
Ag/AgCl electrode; the counter electrode was a Pt wire.
Bu
4
NPF
6
(0.1 M) in dry CH
2
Cl
2
was used as a base electrolyte 5
with analyte concentrations of approximately 1 × 10
–3
M.
Measurements were carried out at RT and at a 100 mV s
–1
sweep rates unless stated otherwise. The redox potentials were
measured against ferrocenium/ferrocene (Fc
+
/Fc; E
1/2
= 0.46 V
vs. SCE)
17
or against [Ru(bpy)
3
]
3+
/[Ru(bpy)
3
]
2+
(E
1/2
= 1.39 V 10
vs. SCE),
18
which were used as internal standards. For
compounds 9, 10, 13 and 14 a similar setup was used except
that measurements were carried out in MeNO
2
at –20 °C and
referenced to Fc
+
/Fc as internal standard (E
1/2
= 0.35 V vs.
SCE in MeNO
2
). 15
Crystal structure determination
Suitable single crystals were mounted on a Stoe Mark II-
Imaging Plate Diffractometer System (Stoe & Cie, 2002; for
3, 4, 9b and 12) and on a Nonius KappaCCD area-detector
diffractometer (for 11) equipped with a graphite-20
monochromator. Data collections were performed at –100 ˚C
(for 3, 4, 9b and 12) and at –113 °C (for 11) using Mo-Kα
radiation (λ = 0.71073 Å). All structures were solved by direct
methods using SHELXS-97
19
(for 3, 4, 9b and 12) or SIR92
20
(for 11) and refined by full-matrix least-squares on F
2
with 25
SHELXL-97.
19
Hydrogen atoms were included in calculated
positions and treated as riding atoms using SHELXL-97
default parameters. All non-hydrogen atoms were refined
anisotropically. A semi-empirical absorption correction was
applied for 4, 9b, 11 and 12 using Sortav
21
or MULscanABS 30
as implemented in PLATON.
22
An empirical absorption
correction was applied for 3 using DELrefABS as
implemented in PLATON.
For complex 3, a region of electron density related to a
disordered molecule of CH
2
Cl
2
was squeezed out using the 35
SQUEEZE routine in PLATON03 (82 electrons for 305.7 Å
3
per unit cell). In complex 12, the BF
4
–
anion is disordered.
Further details on data collection and refinement parameters
are collected in Table 1. Crystallographic data (excluding
structure factors) for the structures 3, 4, 9b, 11, and 12 have 40
been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC 726548-
726552. Copies of the data can be obtained free of charge on
application to CCDS, 12 Union Road, Cambridge CB2 1EZ,
UK [Fax: (int.) +44-1223-336-033; E-mail: 45
deposit@ccds.cam.ac.uk].
Table 1 Crystallographic data for complexes 3, 4, 9b, 11, and 12
3
4
9b
11
12
color, shape
green block
yellow rod
red rod
red prism
red block
size/mm
0.50 × 0.40 × 0.35
0.45 × 0.25 × 0.15
0.20 × 0.13 × 0.11
0.18 × 0.15 × 0.13
0.23 × 0.18 × 0.16
empirical formula
C
37
H
38
Fe
2
I
2
N
4
× CH
2
Cl
2
C
34
H
36
Fe
2
I
2
N
4
O
4
C
58
H
62
B
2
F
8
Fe
2
N
6
O
2
× 2 C
3
H
6
O
C
17
H
24
BF
4
FeN
3
O
C
20
H
26
BF
4
FeN
3
O
Fw
1021.14
930.17
1276.61
429.05
467.10
T/K
173(2)
173(2)
173(2)
160(1)
173(2)
crystal system
Triclinic
Triclinic
Monoclinic
Monoclinic
Monoclinic
space group
P–1 (No. 2)
P–1 (No. 2)
P2
1
/c (No. 14)
P2
1
/n (No. 14)
P2
1
/n (No. 14)
unit cell
a/Å
11.6665(8)
7.8161(13)
12.1129(17)
9.3055(2)
11.7156(9)
b/Å
12.7160(7)
9.7331(15)
20.0683(19)
21.7066(4)
14.0710(7)
c/Å
15.2275(8)
12.836(2)
14.218(2)
10.3747(2)
13.7433(10)
α
/deg
66.698(4)
99.738(13)
90.00
90.00
90.00
β
/deg
87.838(5)
104.479(13)
115.165(11)
108.404(1)
105.542(6)
γ
/deg
79.813(5)
89.843(13)
90.00
90.00
90.00
V/Å
3
2040.8(2)
931.1(3)
3128.1(7)
1988.41(7)
2182.7(3)
Z
2
1
2
4
4
D
calc
/g cm
–3
1.662
1.659
1.355
1.433
1.421
µ/mm
–1
(Mo K
α
)
2.391
2.477
0.539
0.806
0.740
measured reflections
26602
8252
14935
45202
30379
unique reflns, R
int
10857, 0.092
3307, 0.085
4089, 0.135
4541, 0.101
5895, 0.060
obsd. reflections [I > 2σ(I)]
9449
2608
2168
3725
3905
transmission range
0.054–0.483
0.366–0.912
0.992–1.010
0.757–0.906
0.984–1.016
no. parameters, restraints
430, 0
209, 0
396, 0
249, 0
330, 0
R,
a
Rw,
b
0.082, 0.231
0.060, 0.155
0.053, 0.074
0.054, 0.129
0.032, 0.060
GOF
1.024
0.982
0.824
1.141
0.831
min/max resid density/e Å
–3
–3.559, 2.160
–1.504, 1.199
–0.235, 0.259
–0.509, 0.575
–0.309, 0.284
a
R
1
= Σ||F
O
|–|F
C
||/Σ|F
O
| for all I > 2σ(I)
b
wR
2
= [Σw(F
O
2
–F
C
2
)
2
/Σ(w(F
O
4
)]
1/2
Results and Discussion
Carbene-connected dinuclear iron(II) complexes (Synthon A) 50
Formation of synthon A required a ditopic carbene ligand
precursor. For this purpose, we investigated the diimidazolium
salts 1 and 2 comprising a flexible methylene and a rigid
phenylene linker, respectively. The bimetallic iron(II)
complexes 3–6 containing these bridging NHC ligands were 55
synthesized via the free carbene route (Scheme 1). In order to
prevent chelation of the flexible dicarbene ligand derived
from 1,
13
metal coordination was performed at low ligand
concentrations. After deprotonation of 1 by BuLi in THF, the
reaction mixture was thus frozen and overlayered with an 60
excess of the metal precursor [FeI(cp)(CO)
2
] dissolved in
toluene. Upon gradual melting and warming of the resulting