Oxidative purification of halogenated ferrocenes

Article abstract of DOI:10.1039/c2dt32779a

We report the large scale syntheses and 'oxidative purification' of fcI₂, fcBr₂ and FcBr (fc = ferrocene-1,1′-diyl, Fc = ferrocenyl). These valuable starting materials are typically laborious to separate via conventional techniques, but can be readily isolated by taking advantage of their increased E_1/2 relative to FcH/FcX contaminants. Our work extends this methodology towards a generic tool for the separation of redox active mixtures.

Full text of DOI:10.1039/c2dt32779a

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Oxidative purification of halogenated ferrocenes
Michael S. Inkpen, Shuoren Du, Mark Driver, Tim Albrecht,* Nicholas J. Long*
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
We report the large scale syntheses and ‘oxidative purification’ of fcI , fcBr and FcBr (fc = ferroceneꢀ
2
2
1
,1’ꢀdiyl, Fc = ferrocenyl). These valuable starting materials are typically laborious to separate via
conventional techniques, but can be readily isolated by taking advantage of their increased E1/2 relative to
FcH/FcX contaminants. Our work extends this methodology towards a generic tool for the separation of
redox active mixtures.
1
0
Background
Haloferrocenes have a broad reactivity profile, permitting facile
preparation of numerous ferroceneꢀincorporated compounds. For
1
2
example, direct reactions from Fc–I include Suzuki, Ullmann,
3
4
5
Negishi, Stille or Sonogashira couplings, Pdꢀcatalysed
6
7
8
1
2
2
3
3
4
4
5
0
5
0
5
0
5
carbonylations, aminocarbonylations or phosphinations, and
copper mediated processes forming ferrocenyl carboxylic acids
or aryl ethers. Indirectly, synthetic possibilities are extended by
9
10
11
ready conversion of halogen functionalities to lithium,
12
13
14
12
cyanide, magnesium halide, azide or acetyl species.
Ferrocenyl iodides may also be converted to their bromides and
12
chlorides via exchange with CuX (X = Br, Cl) in pyridine.
Despite their versatility, obtaining large quantities of
monohaloferrocenes (FcX) or 1,1’ꢀdihaloferrocenes (fcX ) in
2
Scheme 1 Synthesis and oxidative purification of (a) FcI (Goeltz and
21
–
–
–
high purity has never been straightforward. Typical syntheses of
Kubiak), and fcBr, and (b) fcI
2
and fcBr
2
(this work) (A = Cl , [FeCl
3
]
1
5
–
5
0
or [FeCl
4
] ).
the latter (X = Br, I) – via 1,1’ꢀchloromercuriferrocene, 1,1'ꢀ
1
6
17
dilithioferrocene,
1,l’ꢀbis(triꢀnꢀbutylstannyl)ferrocene,
1,1'ꢀ
lithioferrocene/iodine), they isolated the halogenated product by
1
8
19
ferrocenylenediboronic acid, or other reagents – involve toxic
starting materials/intermediates or provide the desired products in
difficult to separate mixtures.
repeatedly washing the solution with aq. FeCl (a mild oxidant).
3
This converted the FcH into [FcH]Cl, which was readily
extracted into the water layer and removed. By eliminating the
need for careful column chromatography/recrystallization,
syntheses could be run at large scale – i.e. their published
procedure generates 7.03 g FcI from 14.98 g FcH (28% yield).
Here we extend this approach, preparing and purifying large
1
7a
Indeed, it has been our
experience that reactions of elemental iodine with 1,1’ꢀ
55
dilithioferroceneꢀTMEDA
tetramethylethylenediamine)
(TMEDA
generate
=
N,N,N’,N’ꢀ
crude product
1
6a
a
consisting of ferrocene (FcH), FcI and fcI (in addition to 1,1’’’ꢀ
2
diiodobiferrocene and higher order ferrocenes), which is tedious
to purify via conventional techniques. In telling examples from
the literature, Kovar et al. recommend column chromatography of
quantities of fcI , fcBr2 and FcBr via oxidation and aqueous
2
60
extraction of contaminants (Scheme 1). Goeltz and Kubiak’s
original purification of FcI is revised (in light of new
observations), and the compound is produced in improved yield
crude fcI using a 2″ x 18″ column, or fractional sublimation of
2
1
6a
crude fcBr2 followed by successive recrystallization. Others
have suggested recrystallization from ethanol/methanol at ꢀ
(
47% based on FcH) following a readily scalable procedure
23
adapted from Sünkel and Bernhartzeder for FcBr.
1
4,20
30°C.
Purification problems are likely a result of the
monoferrocene series having comparable polarities/solubilities,
65
Results and discussion
exacerbated in the case of fcI , a liquid at room temperature (in
2
23ꢀ24
16a
contrast to FcI, FcBr and fcBr , which are solids).
Mixtures containing FcH, FcX
and fcX2 (X = Br, I) were
2
Similar issues with the isolation of FcI were recently discussed
prepared as shown in Scheme 1. Based on the previously reported
2
1
21
by Goeltz and Kubiak, who developed an improved purification
oxidative procedure for purifying FcI, it was considered that an
22
25
procedure based on oxidation potentials (Scheme 1a, X = I).
Taking a FcH/FcI mixture in pentane (from
oxidant stronger than FeCl (E° Fe(III) + e ⇄ Fe(II) = 0.771 V)
3
70 would be needed to oxidize the FcX components of crude fcX2.
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oxidation potentials of FcH/FcX and 0.13ꢀ0.15 V between those
of FcX/fcX . The difference in these E values indicates that
Table 1 Experimental details for the optimized oxidative purifications of
different haloferrocene mixtures.
2
1/2
separation of the oxidised species is possible with sufficient
45 selectivity. If the E1/2 values are too close then oxidation of one
will also lead to oxidation of the other.
3
compound FcH molar ratios of [FeCl ] # washes aq. pure yield
c
a
(g)
FcH/FcX or
(M)
FeCl
3
b
(g)
(%)
FcH/FcX/fcX
2
FcI
FcBr
6.0
1:17
1:11
0.9:1.5:1
0:1:5.5
0.2
0.2
0.5
sat.
1
2
10
2
4.7
12.6
9.5
47
88
19
67
Conclusions
10.0
21.1
18.6
fcI
fcBr
2
We have developed a methodology towards the large scale
synthesis of fcI , fcBr , and FcBr. We stress that concentration
2
23.0
2
2
a
b
1
Indicative of reaction scale. Estimated from H NMR spectroscopy of
the crude product. 1 wash = 200 mL.
50
(and quantity) of oxidizing solutions are crucial to success, and
accordingly detail a revised procedure for FcI from previous
c
21
literature. We suggest that this new purification technique is
broadly applicable to redox active mixtures in general (when
species are stable in both solution phases, with appropriate ꢁE1/2),
and should prove a useful addition to the toolkit of methods
already available to the practicing synthetic chemist.
55
Experimental
General considerations
All reactions were performed under an atmosphere of nitrogen.
Solvents used in reactions were sparged with nitrogen and dried
with alumina beads or Q5 copper catalyst on molecular sieves,
where appropriate. Silica and neutral alumina of Brockmann
5
0
2
Fig. 1 (a) The decreasing percentage of FcBr in crude fcBr with 4
6
6
7
7
8
8
9
9
0
5
0
5
0
5
0
5
washings of 200 mL 0.5 M (squares) followed by 2 washings of 200 mL
.0 M aq. FeCl (triangles). (b) The decreasing percentage of FcI in crude
fcI with successive washings of 20 mL (squares) or 200 mL (triangles)
.5 M aq. FeCl
2
3
2
1
0
3
. Compositions estimated by H NMR, where % FcX + %
fcX taken as 100%.
activity II (3% H
O) were used for chromatographic separations.
2
1
2
Mass spectrometry analyses were conducted by the Mass
1
13
Spectrometry Service, Imperial College London. H and
NMR were recorded on a Bruker 400 MHz spectrometer and
referenced to the residual solvent peaks of CDCl at 7.26 and
C
Initial work with nꢀhexane solutions of crude fcI2 thus
successfully employed AgOTf (E° Ag(I) + e ⇄ Ag = 0.7996
25
3
V), forming ferrocenium triflates and precipitating silver metal.
However, it was soon realized that (the much cheaper) FeCl3
7
7.16 ppm, respectively. Microanalyses were carried out by
Stephen Boyer of the Science Centre, London Metropolitan
1
2
2
3
3
4
5
0
5
0
5
0
would also oxidize FcX (and given the chance, fcX ). This
2
University. FcI was prepared according to literature
critical result indicated that FcH/FcX samples were at risk of
complete oxidation if the original procedure (using saturated aq.
16a,21
procedures
or via the modified route detailed below, whilst
all other reagents were commercially available and used as
received.
FeCl ) was applied too rigorously.
3
Further investigations showed that selective oxidation could be
achieved simply by varying the concentration of aq. FeCl3.
Procedures are summarized in Table 1, with each washing
performed by vigorous shaking or magnetic stirring of the
biphasic mixture for ca. 2 min. Whilst washes with 0.2 M FeCl3
Cyclic voltammograms were recorded under an atmosphere of
n
argon in MeCN/0.1 M Bu NPF on a CHI760C potentiostat (CH
4
6
Instruments, Austin, Texas) with a glassy carbon disc as working
electrode (diameter = 2.5 mm), and Ptꢀwire as reference and
counter electrodes respectively. Analyte solutions were between
(
200 mL) proved sufficient for removing FcH from mixtures
0
.1ꢀ1 mM. Potentials are reported relative to an internal
(without significantly affecting FcX), higher concentrations such
as 0.5 M or ≥2.0 M could safely be used in purifications of fcX2,
more efficiently oxidizing FcI or FcBr, respectively. This is
exemplified in Figure 1a, where changing from 0.5 M to 2.0 M
+
[FcH] /[FcH] reference.
When washing with solutions of aq. FeCl , extent of oxidation
3
1
was most accurately monitored by H NMR spectroscopy.
However, removal of contaminants was also indicated by specific
experimental observations. Aqueous solutions containing
primarily [FcH]A were characterized by a blueꢀgreen colour,
whereas dark green aqueous solutions containing solid material
FeCl greatly reduced the number of washes required to purify a
3
sample of fcBr . The volume of oxidant solution was also found
2
to play a role, as shown in Figure 1b, where washings with 200
mL 0.5 M FeCl were preferable to washings with 20 mL 0.5 M
3
were typical of [FcI]A. In purifications of fcI , a lightening of
2
FeCl in purifications of crude fcI .
3
2
colour and clearing of debris proved indicative of the end point.
In all purifications, it was found necessary to elute materials
through a short column (e.g. 3″ x 4″, silica/nꢀhexane) to remove
oxidized components (as well as higher order ferrocenes and
other contaminants, where appropriate). The purity and identify
As expected, the observed order of oxidation, FcH > FcX >
fcX2 (and the easier oxidation of iodinated vs. brominated
materials), follows the increasing redox potential of these species
as measured by cyclic voltammetry (E1/2(V) = 0.16 (FcI), 0.17
+
†
(
FcBr), 0.29 (fcI ) and 0.32 (fcBr ) vs. [FcH] /[FcH]). Each
1
13
2
2
of all products was established by H/ C NMR spectroscopy,
accurate mass and elemental analyses.
1
/2
complex exhibits reversible behaviour (ipa/ipc ≈ 1, i ∝ Vs , ꢁE ~
p
0.060 V, as expected for a oneꢀelectron process), with measured
nꢀHexane solutions of pure FcH, FcBr and FcI were each
oxidised with aq. FeCl3 and the resulting aqueous phases
differences of E1/2 (in CH CN) of 0.16ꢀ0.17 V between the
3
2
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vacuo to yield pure fcBr as an orange solid (23.00 g, 67 %). H
+
1
analysed by mass spectrometry. Peaks attributable to [FcH] ,
2
+
+
[
FcBr] and [FcI] were observed in positive ion mode, with those
NMR (400 MHz, CDCl ): δ (ppm) 4.17 (pseudoꢀt, 4H, CpꢀH, J =
3
–
–
attributable to FeCl4 and FeCl3 seen in negative ion mode.
60 1.82 and 2.18 Hz), 4.43 (pseudoꢀt, 4H, CpꢀH, J = 1.82 and 2.18
13
1
1
6a
Hz). C{ H} NMR (100 MHz, CDCl ): δ (ppm) 70.07 (Cp–Br,
3
1
,1’-Diiodoferrocene (fcI2)
CH), 72.81 (Cp–Br, CH), 78.40 (Cp–Br, CBr). HRꢀMS EI+: m/z
+
5
A mixture of ferrocene (21.14 g, 113.6 mmol), nꢀhexane (100
mL) and TMEDA (37.7 mL, 251.4 mmol) was stirred in an ovenꢀ
dried 1 L threeꢀnecked flask and cooled to 0°C (iceꢀbath). 2.5 M
341.8354, ([M] calc.: 341.8342). (Found: C, 35.02; H, 2.38.
Calc. for C H FeBr : C, 34.93; H, 2.35%).
1
0
8
2
23
65
Bromoferrocene (FcBr)
n
BuLi in hexanes (100 mL, 250 mmol) was added via cannula
over 5 min, whereby the suspension was slowly raised to ambient
temperature and stirred overnight. The resulting bright orange
suspension (1,1’ꢀdilithioferroceneꢀTMEDA) was cooled to ꢀ78°C
A mixture of ferrocene (10.00 g, 53.75 mmol), potassium tertꢀ
butoxide (0.75 g, 6.68 mmol) and THF (500 mL) was stirred in
an ovenꢀdried 1 L threeꢀnecked flask. After cooling to ꢀ78 °C
1
1
2
2
3
0
5
0
5
0
t
(
acetone/dry ice) and a solution of I (62.51 g, 246.3 mmol) in
(acetone/dry ice), 1.9 M BuLi in pentane (57 mL, 108.3 mmol)
2
diethyl ether (350 mL) added over ca. 15 min. After warming to 70 was added dropwise. The resulting orange suspension was
0
°C, the reaction was quenched with water (100 mL) and stirred
for a further 15 min. The mixture was extracted with water (3 x
00 mL), dried over MgSO , and filtered through Celite to
vigorously stirred for 2 h whereby 1,2ꢀdibromotetrachloroethane
(26.25 g, 80.61 mmol) was added against nitrogen. The reaction
mixture was stirred for a further 30 minutes at ꢀ78°C before
slowly warming to ambient temperature, at which point the
2
4
provide a dark red oil (ca. 33 g) after solvent removal.
The crude product was extracted into nꢀhexane (300 mL) and 75 orange/brown solution was carefully quenched with water,
washed successively with 0.5 M aqueous FeCl (ca. 10 x 200
extracted two times with CH Cl and the solvent removed.
2 2
The crude product was extracted into nꢀhexane (300 mL), and
3
mL). When FcH/FcI contaminants had been removed
1
(
composition monitored by H NMR spectroscopy between
washed with 0.2 M aq. FeCl (ca. 2 x 200 mL). When ferrocene
3
1
washings), the organic phase was extracted with water until the
had been removed (composition monitored by
H NMR
washings were colourless, dried over MgSO , filtered through 80 spectroscopy between washings), the organic phase was extracted
4
Celite and evaporated to reveal a brown oil. Further purification
via column chromatography (silica/nꢀhexane, 3″ x 4″) and drying
with water until the washings were colourless, dried over MgSO4
and filtered (50 g silica/nꢀhexane). Evaporation of solvent
in vacuo at 50°C yielded pure fcI as a dark orange oil (9.54 g,
provided pure FcBr as an orange crystalline solid (12.56 g, 88 %).
2
1
1
1
9%). H NMR (400 MHz, CDCl ): δ (ppm) 4.18 (pseudoꢀt, 4H,
H NMR (400 MHz, CDCl ): δ (ppm) 4.10 (pseudoꢀt, 2H, CpꢀH,
3
3
CpꢀH, J = 1.68 and 1.78 Hz), 4.37 (pseudoꢀt, 4H, CpꢀH, J = 1.88 85 J = 1.87 Hz), 4.23 (s, 5H, CpꢀH) 4.41 (pseudoꢀt, 2H, CpꢀH, J =
13
1
13
1
and 1.81 Hz). C{ H} NMR (100 MHz, CDCl ): δ (ppm) 40.42
1.84 and 1.87 Hz). C{ H} NMR (100 MHz, CDCl ): δ (ppm)
3
3
(Cp–I, CI), 72.41 (Cp–I, CH), 77.72 (Cp–I, CH). HRꢀMS EI+:
67.21 (Cp–Br, CH), 70.24 (Cp–Br, CH), 70.73 (Cp, CH), 77.76
+
+
m/z 437.8065, ([M] calc.: 437.8065). (Found: C, 27.58; H, 1.72.
(Cp–Br, CBr). HRꢀMS EI+: m/z 263.9258, ([M] calc.:
Calc. for C H FeI : C, 27.43; H, 1.84%).
263.9237). (Found: C, 45.27; H, 3.40. Calc. for C H FeBr: C,
10
8
2
10
9
1
6a
90 45.34; H, 3.42%).
1
,1’-Dibromoferrocene (fcBr2)
Iodoferrocene (FcI)
A mixture of ferrocene (18.60 g, 100 mmol), nꢀhexane (100 mL)
and TMEDA (35 mL, 233 mmol) was stirred in an ovenꢀdried 1 L
threeꢀnecked flask equipped with a pressureꢀequalizing dropping
funnel. After cooling to 0°C (iceꢀbath), 2.5 M BuLi in hexanes
(
3
4
4
5
5
5
0
5
0
5
With an identical procedure to the synthesis of FcBr, the crude
solution was produced using ferrocene (6.0 g, 32.3 mmol),
n
potassium tertꢀbutoxide (0.45 g, 4.01 mmol), THF (300 mL), 1.9
t
85 mL, 212.5 mmol) was added dropwise and the suspension 95 M BuLi in pentane (34 mL, 64.6 mmol) and I2 (12 g, 47.3
was slowly raised to ambient temperature and stirred overnight.
The resulting bright orange precipitate was filtered via cannula,
reꢀsuspended in diethyl ether (300 mL), and cooled to ꢀ78°C
mmol). This was additionally washed with saturated sodium
thiosulfate prior to solvent removal.
The crude product was extracted into nꢀhexane (300 mL),
(acetone/dry ice), whereby the funnel was charged with a solution
filtered, and washed with 0.2 M aq. FeCl (1 x 200 mL). When
3
1
of 1,1,2,2ꢀtetrabromoethane (25 mL, 215 mmol) in diethyl ether 100 ferrocene had been removed (composition monitored by H NMR
(
100 mL). This was added to the reaction mixture over a period
of 4 h (with vigorous stirring), so as to maintain a temperature of
ꢀ70°C. After slowly raising the reaction to ambient temperature
and stirring overnight, a twoꢀphase system was produced. The top
spectroscopy between washings), the organic phase was extracted
with water until the washings were colourless, dried over MgSO4,
and filtered (30 g silica/nꢀhexane). Evaporation of solvent
<
1
provided pure FcI as a dark orange solid (4.7 g, 47 %). H NMR
darkꢀred layer was decanted (from the bottom black one), 105 (400 MHz, CDCl ): δ (ppm) 4.15 (pseudoꢀt, 2H, CpꢀH, J = 1.50
quenched with water (50 mL) and separated, providing a dark
orange solid after solvent removal.
The crude product was extracted into nꢀhexane (300 mL),
filtered through Celite and washed successively with saturated
3
and 1.74 Hz), 4.19 (s, 5H, CpꢀH), 4.41 (pseudoꢀt, 2H, CpꢀH, J =
13
1
1.75 and 1.51 Hz). C{ H} NMR (100 MHz, CDCl ): δ (ppm)
3
39.85 (Cp–I, CI), 68.97 (Cp–I, CH), 71.19 (Cp, CH), 74.61 (Cp–
+
I, CH). HRꢀMS EI+: m/z 311.9104, ([M] calcd: 311.9098).
aq. FeCl (ca. 2 × 200 mL). When FcH/FcBr contaminants had 110 (Found: C, 38.12; H, 2.97. Calc. for C H FeI: C, 38.50; H,
3
10
9
1
been removed (composition monitored by H NMR spectroscopy
between washings), the organic phase was extracted with water
2.91%).
until the washings were colourless, dried over MgSO , and
filtered (50 g silica/nꢀhexane). The red solution was dried in
Notes and references
4
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Department of Chemistry, Imperial College London, London SW7 2AZ,
U.K.; Tel: +44 (0)20 7594 5781; Eꢀmail: n.long@imperial.ac.uk;
t.albrecht@imperial.ac.uk.
25. CRC Handbook of Chemistry and Physics (92nd ed.), ed. W. M.
Haynes, CRC Press, 2011ꢀ2012.
†
Electronic Supplementary Information (ESI) available: Electrochemical
1 13
5
0
5
0
5
0
5
0
5
0
5
0
5
0
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H/ C NMR spectra for all compounds. See
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Large scale syntheses and ‘oxidative purification’ of fcI , fcBr and FcBr is reported, providing a methodology towards
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generic tools for the separation of redox active mixtures.