Synthesis of substituted 1,1′-diaminoferrocenes from cyclo-2-pentene imines

Article abstract of DOI:10.1016/j.jorganchem.2007.12.020

Condensation of anilines and primary aliphatic amines with 3,4-diphenylcyclo-2-pentenone leads to the corresponding diphenylcyclopentene imines in good yields of 72-90%. Deprotonation of these aminocyclopentadiene tautomers and reaction with FeCl₂ leads to the synthesis of the respective 1,1′-diamino-3,3′,4,4′-tetraphenylferrocenes. Yields increase from 33% to 65% with a decrease in the steric bulk of the amine substituent. The observation that a successful conversion requires two equivalents of base is conceived on the basis of the discussed reaction mechanism. The molecular structure of 1,1′-dianilino-3,3′,4,4′-tetraphenylferrocene (3a), which was determined by single crystal X-ray analysis reveals a trans coordination of the two amine moieties with respect to the central Cp-Fe-Cp axis of the ferrocenyl backbone.

Full text of DOI:10.1016/j.jorganchem.2007.12.020

Available online at www.sciencedirect.com
Journal of Organometallic Chemistry 693 (2008) 2223–2230
www.elsevier.com/locate/jorganchem
Synthesis of substituted 1,10-diaminoferrocenes from
cyclo-2-pentene imines
Katharina R. Nikolaides, Stephan D. Hoffmann, Jo¨rg Eppinger *
Technische Universita¨t Mu¨nchen, Department Chemie, Lichtenbergstr. 4, D-85747 Garching, Germany
Received 21 November 2007; accepted 12 December 2007
Available online 23 December 2007
Abstract
Condensation of anilines and primary aliphatic amines with 3,4-diphenylcyclo-2-pentenone leads to the corresponding diphenylcycl-
opentene imines in good yields of 72–90%. Deprotonation of these aminocyclopentadiene tautomers and reaction with FeCl2 leads to the
synthesis of the respective 1,10-diamino-3,30,4,40-tetraphenylferrocenes. Yields increase from 33% to 65% with a decrease in the steric bulk
of the amine substituent. The observation that a successful conversion requires two equivalents of base is conceived on the basis of the
discussed reaction mechanism. The molecular structure of 1,10-dianilino-3,30,4,40-tetraphenylferrocene (3a), which was determined by sin-
gle crystal X-ray analysis reveals a trans coordination of the two amine moieties with respect to the central Cp–Fe–Cp axis of the ferr-
ocenyl backbone.
Ó 2007 Elsevier B.V. All rights reserved.
Keywords: Ferrocene; Aminoferrocene; Diamines; Silylamide route
1. Introduction
proven to be good ligands for the stabilisation of early
transition metal centers applied in the polymerisation of
olefins [6,7]. The main obstacle for a broader application
of this class of compounds is the lack of an efficient syn-
thetic access. Nearly all synthetic routes to di-nitrogen
substituted ferrocenes proceed via a diiodo- or dibromofer-
rocene [3], which is subsequently either coupled with
amides by an copper-mediated Ullman type reaction [8]
or converted to 1,10-diaminoferrocene [9]. This highly air
sensitive compound may be derivatised by a variety of
methods, e.g. by Hartwig–Buchwald coupling [10], reac-
tions with electrophiles [11] or condensation with carbonyl
functionalities [12]. However, the functionalisation of the
ferrocene moiety is always achieved on the level of the
dihalogenoferrocene precursors. Correspondingly, all pro-
cedures above necessitate multiple successive synthetic
steps and share the lack of flexibility with respect to a func-
tionalisation of the ferrocenyl backbone. An interesting
alternative provides the method introduced by Plenio
et al. [13], which utilizes a retro-synthetically different
approach (Fig. 1): the assembly of 1,1-diaminoferrocenes
in one step from aminocyclopentadienes and an iron(II)
During the last decades, the unique characteristics of
ferrocene derivatives have attracted considerable attention
in material sciences [1] and catalysis [2]. In general, the ferr-
ocenyl backbone is set apart from purely organic moieties
by three properties [3]: (i) chemical stability and electro-
chemical reversibility of the ferrocene/ferrocinium redox
couple, (ii) electronic stabilisation of adjacent electron-defi-
cient centers due to participation of the iron atom in the
dispersal of positive charge and (iii) unique steric bulk
due to the cylindrical shape enabling planar chirality and
a rare co-planar arrangement of substituents. While espe-
cially the last property has founded the success of ferro-
cene-derived chelating phosphine ligands in homogeneous
catalysis [2], the homologue amines have been rarely
applied [4] despite several reports emphasis interesting
applications [5,6]. Especially 1,1-diaminoferrocenes were
*
Corresponding author. Tel.: +49 (0) 89 289 13097; fax: +49 (0) 89 289
13473.
E-mail address: joerg.eppinger@ch.tum.de (J. Eppinger).
0022-328X/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.12.020

2224
K.R. Nikolaides et al. / Journal of Organometallic Chemistry 693 (2008) 2223–2230
R
R
NR1R2
a)
b)
X
X
NR1R2
NR1R2
A) toluene, reflux
molecular sieve
O
N
N
Fe
Fe
Fe + 2
[pTsOH]
+ H2NR
+
Fig. 1. Retro-synthetic conception of possible synthetic routes to 1,10
diaminoferrocenes: (a) via dihalogenoferrocenes and (b) via
aminocyclopentadienes.
B) TiCl4(thf)2
Et2O, reflux
Ph
Ph
Ph
Ph Ph
Ph
trans-2
1
cis-2
Compnd.
R
Yield (A) % Yield (B) % cis/trans
precursor. However, this method so far provided an access
only to tertiary diamines with an ferrocenyl backbone,
since secondary aminocyclopentadienes are not syntheti-
cally accessible, yet. We now herein describe the expansion
of the one-step assembly of a fully functionalised ferrocene
to the synthesis of 1,10-diaminoferrocenes with secondary
amino groups, which offer an N–H for further
derivatisations.
2a
2b
2c
2d
2e
C6H5
85
75
90
-
1.4/1
1.8/1
3.5/1
4.2/1
>20/1
4-MeC6H5
2,4,6-Me3C6H2 77
58
47
-
2,6-iPr2C6H3
C6H5CHMe
72
84
Scheme 1. Synthesis of the cyclo-2-pentene imines 2a–e.
2. Results and discussion
Since most examples of successfully applied secondary
diaminoferrocenes as ligands in homogeneous catalysis
bear arene substituents at the nitrogen, we were primarily
interested in a coupling of anilines to the ferrocenyl back-
bones. To the best of our knowledge, secondary aminocy-
clopentadienes have not been described, so far.
Therefore, we initially intended to use the benzylamino
group as a synthon, which provides a N–H functionality
after hydrogenolysis. We thus attempted to react the read-
ily available 3,4-diphenylcyclo-2-pentenone (1) [14] with
benzylaniline under conditions that are established for
the enamine reaction of sec-amines with cyclo-2-pentenon-
es [13]. However, not even azeotropic distillation in high
boiling solvents such as toluene or xylene yielded signifi-
cant amounts of products after 48 h. The combination of
decreased nucleophilicity of the arylamine and the steric
bulk of the secondary amino group seems to result in a dra-
matically reduced reaction rate. Thus we reconsidered our
approach and decided to modify the strategy towards the
condensation of cyclopentenone 1 with primary amines to
cyclo-2-pentene imines, which represent potential
precursors since they are tautomers of secondary
aminocyclopentadienes.
pletion by azeotropic removal of the reaction water. At
this, we received superior results when the conventional
water separation funnel was replaced by a container filled
˚
with molecular sieve (4 A) positioned between the reflux
condenser and the reaction vessel. This procedure allows
to synthesise the cyclopentene imines 2a–e in yields of
72–85%. Care has to be taken to strictly exclude air from
the condensation reaction, since oxygen seems to promote
the formation of highly coloured, unwanted side product.
In principle, all primary amines can be used in this synthe-
sis. However, since sterically demanding amines require
long reaction times harsh heating we sought for a method
to reduce the reaction temperature, which might enable the
conversion of less thermo-stable amines. Replacing the acid
catalyst by stochiometric amounts of TiCl4(thf)2 as Lewis
acid and oxygen trap allows to perform the synthesis of
the cyclopentene imines 2a–e in refluxing diethyl ether.
However, the large amount of titanium(IV) by-products
complicates the work-up procedure. The necessary aqueous
removal of salt by-products decreases the isolated yields
due to reversal of the imine formation in the presence of
water.
1H and 13C NMR spectra of the cyclo-2-pentene imines
obtained reveal a mixture of the cis- and trans-isomers (cis/
trans-ratio = 1.4 (2a), 1.8 (2b), 3.5 (2c) and 4.2 (2d))
whereas for product 2e only the cis-configuration can be
detected. MM2 calculations [16] reveal that the arene
substituent at the nitrogen has an energetically more disad-
vantageous steric interactions with the methylene group
(trans-isomer) then with the methine moiety (cis-isomer).
The calculated trend for the cis/trans-ratios of 1.6/1 (2a
and 2b), 3.9/1 (2c) and 14.7/1 (2d) at reaction temperature
(120 °C) is in good agreement with the spectroscopic
results. The NMR spectra of the cyclo-2-pentene imines
2c and 2d feature non-equivalent signals for the ortho-
methyl (2c) and isopropyl (2d) groups, as a result of a hin-
dered rotation around the Caryl–N bond of the substituted
aniline moiety.
2.1. Synthesis of cyclo-2-pentene imines
Up to our work, only two publications existed, which
describe cyclo-2-pentene imines [15]. However, in these
reports these imines are formed and stabilised by coordina-
tion to a W(CO)5-fragment. We considered the condensa-
tion of a cyclo-2-pentenone with primary amines to be a
more practicable access to this class of compounds and
thus reacted 3,4-diphenylcyclo-2-pentenone (1) with the
different amines listed in Scheme 1 in the presence of cata-
lytic amounts of acid. Generally, the reaction only proceeds
at a satisfactory rate in refluxing toluene, with the ortho-
substituted mesityl- and 2,6-diisopropylamine necessitating
reaction times of up to 24 h. The reaction is driven to com-

K.R. Nikolaides et al. / Journal of Organometallic Chemistry 693 (2008) 2223–2230
2225
2.2. Synthesis of 1,10-diaminoferrocenes
R
R
R
Li
N
N
N
LDA,
-78 ˚C to r.t., thf
Li
Li+
Ph
We anticipated, that after deprotonation of cyclopen-
tene imines with strong bases in THF, the resulting anion
would be in equilibrium with the corresponding tautomeric
aminocyclopentadienyl anion, which should react with
FeCl2 to yield the desired 1,10-diaminoferrocenes. How-
ever, using n-BuLi, t-BuLi or LDA as deprotonating agent,
neither after conventional deprotonation at ꢀ78 °C and
reaction at room temperature, nor after prolonged reaction
times and refluxing any formation of product could be
detected. Thus we investigated several reaction parameters
using the imine 2a as test system (Scheme 2). Remarkably,
product formation strongly depends on the amount of
added base. As illustrated in Fig. 2, significant product for-
mation is only seen for more then two equivalents of base.
This observation leads us to the following interpretations
(Scheme 3): (i) the desired tautomeric rearrangement of
the initial deprotonation product does not take place; (ii)
addition of a second equivalent of base results in a double
deprotonated intermediate, which now provides an lithium
cyclopentadienies contact ion pair which can readily
undergo trans-metallation with the added iron(II) salt.
Unfortunately, any attempt to isolate the assumed dilithio
intermediate failed and addition of one or two equivalents
of electrophiles (acyl chloride, trimethylsilyl chloride and
2HCl) at low temperatures yielded inseparable mixtures
of products.
or
Li
Ph
Ph
Ph
Ph
Ph
1) 1/2 FeCl2
2) H+
R
H
N
Li+
3a
Ph
Ph
Scheme 3. Mechanism proposed for the formation of diaminoferrocenes
from cyclo-2-pentene imines via a double deprotonated intermediate.
The deprotonation of the cyclopentene imines 2a–e with
LDA and successive addition of FeCl2 leads to the forma-
tion of the respective aminoferrocenes 3a–e (Chart 1).
However, the reaction times required as well as side prod-
uct formation increase significantly with the steric bulk of
the nitrogen substituent. Correspondingly, the isolated
yields for the phenyl and 4-tolyl compounds are around
65% while the mesityl, the 2,6-diisopropyl and the 1-phen-
ylethyl derivates can only be obtained in moderate yields.
Since the first products are collected as highly air sensitive
dilithio-salts, strict exclusion of oxygen is mandatory for a
successful isolation. Only the neutral 1,10-diamnioferroc-
enes 3a–e received after careful protonation are fairly sta-
ble if exposed to air. Trials with other bases then LDA
did not lead to any improvements. While the use of KOt-
Bu and K[N(SiMe3)2] did not yield any product, presum-
ably since the weaker bases do not achieve the necessary
double deprotonation, strongly alkyl lithium bases such
as n-BuLi or t-BuLi lead to the enhanced dimerisation of
the cyclopentene imines.
R
Ph
N
°
1) LDA, -78 C to r.t., thf
NHR
NHR
Ph
°
2) FeCl2 -78 C to r.t.
Fe
3) MeOH
Ph
Ph
Ph
Ph
2
3
To change from the required strongly basic to milder
conditions, we investigated the possibility of adapting the
silylamide route which we had already applied successfully
for the synthesis of rare earth metallocenes [17]. We rea-
soned, that coordination of the cyclopentene imine to an
undercoordinated iron precursor should facilitate the rear-
rangement of the p-system to an cyclopentadiene interme-
diate, which then could be deprotonated by the
silylamine. We thus chose Fe[N(SiMe3)2]2(thf) [18] as iron
source, which has already been applied as precursor for
the synthesis of chelating N-heterocyclic dicarbene com-
plexes of iron (II) [19]. Despite that sterically and electron-
Scheme 2. Synthesis of the diaminoferrocenes 3a–e.
100
80
60
40
2a
3a
20
3a R= C6H5
Ph
3b R = 4-MeC6H5
0
NHR
NHR
Ph
0.0
1.0
2.0
3.0
4.0
3c R = 2,4,6-Me3C6H2
3d R = 2,6-iPr2C6H3
3e R = C6H5CHMe
Fe
eq. LDA
Ph
Fig. 2. Dependence of conversion of the cyclopentene imine 2a to the 1,10-
diaminoferrocene 3a on the amount of base added. Reaction conditions:
see Scheme 2.
Ph
Chart 1. Summery of the synthesised diaminoferrocenes.

2226
K.R. Nikolaides et al. / Journal of Organometallic Chemistry 693 (2008) 2223–2230
ically unsaturated precursor, conversion of the cyclopen-
tene imine 2a was very slow even in refluxing toluene.
Yet, after a reaction time of 14 days, the diaminoferrocene
3a was isolated in 62% yield (Scheme 4). Attempts to con-
vert the sterically demanding imines 2c and 2d by the same
method did not yield any significant amount of product.
Cyclic voltammetric studies in acetonitrile solution
reveal the negative reversible one-electron oxidation, which
is typical for aminoferrocenes [7,20]. The introduction of
two phenyl substituents at the ferrocenyl backbone make
the oxidation more difficult (E00 versus ferrocene = ꢀ0.33 V
(3a) compared to ꢀ0.41 V for bis-1,10-diphenylaminoferro-
cene [7]). This explains the observation that the diaminofer-
rocenes 3a–d are fairly stable if exposed to air. Even at slow
scan rates of 10 mV/s the oxidation of 3a remains fully
reversible, which indicates, that the oxidized complex does
not decompose within the minutes time scale.
more favorable trans conformation. A pseudo-C2-symme-
try of the diaminoferrocene core (axis through Fe-atom
defined by section of the plane perpendicular to both
N–CCp-axis and the plane (idealized) parallel to both Cp
rings) is broken by the orientation of the tilted phenyl sub-
stituents. They are twisted into different directions for both
cyclopentadienyl ring, which results in a double-propeller
structure without helical chirality. The steric interaction
of the phenyl rings does not cause any significant barriers
for the rotation of the cyclopentadienyl rings, since neither
rotamers nor significant signal broadening is evident by 1H
NMR spectroscopy in solution in a temperature range
from ꢀ50 to +90 °C (d8-toluene). A slight tilting of the
cyclopentadienyl rings by 3.1(2)° is common for substituted
ferrocenes. All distances and angles are in a range, which is
typical for aminoferrocenes. Correspondingly, the N–CCp
bond length are relatively short (N1–CCp1 = 1.403(11),
N2–CCp2 = 1.410(10) pm). The CAr–N–CCp-angles are
opened (CAr1–N1–CCp1 = 129.2(8)°, CAr2–N2–CCp2 =
127.3(9)°), reducing the steric pressure between the aniline
and the bulky ferrocene moiety.
2.3. Structure analysis of 3a
The solid structure of the aniline derivate 3a was deter-
mined by X-ray structure analysis. Crystals suitable for
X-ray diffraction studies were grown by diffusion of hexane
into a saturated chloroform solution of 3a at room temper-
ature. Diaminoferrocene 3a crystallises in the monoclinic
space group P21/c. The solid state structure (Fig. 3) reveals
that both amino substituents are oriented in the sterically
3. Conclusion
We have shown, that the condensation of a variety of
primary amines with cyclo-2-pentenones under acid cataly-
sis affords the corresponding cyclo-2-pentene imines of
flexible substitution patterns in good yields. Conversion
of these cyclo-2-pentene imines into the corresponding
1,10-diaminoferrocenes requires the addition of two equiv-
alents of base. This result can be understood by assuming,
that a double deprotonation is essential for the formation
of the reactive anionic cyclopentadienyl species. The silyla-
mide route could be adapted to the synthesis of 1,10-diani-
linoferrocenes using Fe[N(SiMe3)2]2(thf) as iron(II)
precursor. We believe, that the facile access to diaminofer-
rocenes with secondary amino groups, which offer an N–H
for further derivatisation reactions will open the door for
substantial advances in the application of these compounds
as ligands for homogeneous catalysts.
Ph
Ph
N
NHPh
Ph
+ Fe[N(SiMe3)2]2(thf)
Fe
toluene, reflux
14d
Ph
NHPh
Ph
Ph
Ph
2a
4
3a (62)
Scheme 4. Synthesis of 3a from an iron(II) silylamide precursor (silyla-
mide route).
CAr
2
CCp2
4. Experimental
N2
4.1. General procedures
Fe
Commercially available solvents and reagents were puri-
fied according to literature procedures. All reactions were
carried out in dry solvents under an argon atmosphere.
NMR spectra were recorded at 300 K with a Bruker
DPX 400 (1H NMR 400.13 MHz, 13C NMR
100.61 MHz). 1H NMR were referenced to the residual
1H-impurities in the solvent, and 13C NMR, to the solvent
signals: CDCl3 (7.24 ppm, 77.0 ppm), C6D6 (7.15 ppm,
128.0 ppm), d8-THF (3.73 ppm, 1.71 ppm; 68.6 ppm
26.7 ppm). Signals that overlap with solvent signals are
indicated by a cross (#). Mass spectra: Finnigan MAT
90. IR spectra: Perkin–Elmer 1650 FTIR; solid materials
CCp
1
CAr
1
N1
Fig. 3. Crystal structure of the diaminoferrocene 3a (except for N–H are
all hydrogens omitted for clarity). Representative distances (pm)
and angles (°): daverageFe–CCp = 2.054, Fe–Cg1 = 1.669(2), N1–CCp1 =
1.403(11), N2–CCp2 = 1.410(10), Cg1–Fe–Cg2 = 176.9(2), CAr1–N1–
C
Cp1 = 129.2(8), CAr2–N2–CCp2 = 127.3(9); Cg: ring center and CAr
:
phenyl carbon at the N-atom.

K.R. Nikolaides et al. / Journal of Organometallic Chemistry 693 (2008) 2223–2230
2227
as KBr tablets. Element analyses: Mikroanalytisches Labor-
atorium der Technischen Universita¨t Munchen.
PhCHCH2). 13C NMR (CDCl3): d = 177.6, 175.9, 163.5,
143.5, 133.9, 130.9, 128.9, 128.6, 127.5, 127.4, 127.0,
126.7, 123.6, 121.0, 119.9, 48.9, 39.9. ESI-MS, m/z (%):
¨
3,4-Diphenylcyclo-2-pentenone [14] and Fe[N(SiMe3)2]2(thf)
[18] were synthesised according to the literature procedures.
Cyclic voltammetry: the standard electrochemical instru-
mentation consisted of a Princeton Applied Research poten-
tiostat/galvanostat (model 273A). Cyclic voltammograms
were recorded in dry CH3CN under an argon atmosphere
at ambient temperature. A three-electrode configuration
was employed. The working electrode was a Pt disk (diame-
ter 1 mm) sealed in soft glass, and the counter electrode, a Pt
310.2. (100) [(M+H)+]. IR (KBr) [cmꢀ1]: m ¼ 3061 w,
~
3026 w, 1618 s, 1585 vs, 1483 s, 1445 m, 1266 m, 1203 m,
776 m, 756 s, 699 vs. Anal. calc. for C23H19N (309.4): C,
89.28; H, 6.19; N, 4.53. Found: C, 87.88; H, 5.88; N, 4.24%.
4.2.3. N-(4-methyl)phenyl-3,4-diphenylcyclo-2-pentene
imine (2b)
Route A reagents: 3,4-Diphenylcyclo-2-pentenone 1
(1.50 g, 6.40 mmol), 4-methylaniline (2.14 g, 20 mmol),
p-toluenesulfonic acid (75.0 mg, 0.32 mmol), reaction time:
1 h, yield: 1.55 g (75%), pale-green powder. 1H NMR
(CDCl3): cis/trans = 1.8/1; cis-isomer: = 7.52–6.90 (m,
15H, ArH + CNCH), 4.61 (‘‘t”, 1H, PhCHCH2), 3.29
(dd, J = 7.6, 18.4 Hz, 1H, trans-PhCHCH2), 2.59 (dd,
J = 2.1, 18.0 Hz, 1H, cis-PhCHCH2), 2.30 (s, 3H, CH3).
trans-isomer: = 7.52-6.90 (m, 15H, ArH + CNCH), 4.61
(‘‘t”, 1H, PhCHCH2), 3.60 (dd, J = 7.6, 18.4 Hz, 1H,
trans-PhCHCH2), 2.87 (dd, J = 2.1, 18.0 Hz, 1H, cis-
PhCHCH2), 2.37 (s, 3H, CH3). 13C NMR (CDCl3):
d = 142.9, 133.8, 133,5, 130,2, 129.8, 128.7, 128.5, 127.9,
127.8, 127.0, 126.2, 121.8, 121.4, 120.6, 49.3, 48.0, 40.6,
20.9, 15.3. ESI-MS, m/z (%): 324.5. (100) [(M+H)+]. IR
disk (area 3 cm2). Solutions were ca. 1 ꢁ 10ꢀ3 mol dmꢀ3
.
(NBu4)ClO4 (0.2 M) was used as a supporting electrolyte.
4.2. Synthesis
4.2.1. General procedure for the synthesis of cyclo-2-pentene
imines 2a–2e
Route A (acid catalysed): Cyclo-2-pentenone (1 eq.),
amine (3 eq.), p-toluenesulfonic acid (0.05 eq.) were dis-
solved in toluene (2 mL/mmol) under argon atmosphere
and the resulting mixture was refluxed through a mol sieve
˚
(4 A) container at 120–140 °C for the time mentioned.
Reaction always is accompanied by a colour change to
green. After the reaction mixture was cooled to room tem-
perature all volatiles were removed in vacuum (10ꢀ3 mbar)
and the remaining solid was re-crystallised from diethyl
ether.
(KBr) [cmꢀ1]: m ¼ 3022 w, 2925 w, 1621 s, 1590 vs, 1484
~
s, 1445 m, 1265 m, 1205 m, 776 m, 757 s, 699 s. Anal. calc.
for C24H21N (323.4): C, 89.12; H, 6.54; N, 4.33. Found: C,
88.78; H, 6.04; N, 4.17%.
Route B (TiCl4-mediated): Cyclo-2-pentenone (1 eq.)
and amine (6 eq.) were dissolved in diethyl ether (3.5 mL/
mmol) under argon atmosphere before TiCl4(thf)2
(1.09 eq.) was added as a yellow solid. The reaction mixture
was then refluxed for 48 h. The mixture was cooled to room
temperature, evaporated to dryness, re-dissolved in ethyl
acetate and transferred into a separating funnel. The
organic layer was than twice extracted with 5% glacial acid,
twice with saturated aqueous NaOAc and washed with
brine. The organic layer was separated and dried over
MgSO4 and the volatiles were evaporated. The crude prod-
uct was washed with hexane, re-crystallised from ether and
dried under vacuum.
4.2.4. N-mesityl-3,4-diphenylcyclo-2-pentene imine (2c)
Route A reagents: 3,4-Diphenylcyclo-2-pentenone 1
(2.00 g, 8.54 mmol), mesitylamine (3.40 g, 25 mmol), p-tol-
uenesulfonic acid (0.11 g, 0.43 mmol), reaction time: 12 h,
yield: 2.03 g (77%), green powder. Route B reagents:
3,4-Diphenylcyclo-2-pentenone 1 (1.17 g, 5.0 mmol), mesi-
tylamin
(4.05 g,
30.0 mmol),
TiCl4(thf)2
(1.82 g,
5.45 mmol), yield: 1.02 g (58%), light green powder. 1H
NMR (CDCl3): cis/trans = 3.5/1; cis-isomer (* indicates
overlap with cis-isomer): d = 7.55–6.75 (m*, 13H,
ArH + CNCH), 4.50 (d, J = 7.3 Hz, 1H, PhCHCH2),
2.81 (dd, J = 7.4, 18.8 Hz, 1H, trans-PhCHCH2), 2.55
(m* 1H, cis-PhCHCH2), 2.25 (s, 3 H, Ar(CH3)), 2.10 (s, 3
H, Ar(CH3)), 2.01 (s, 3 H, Ar(CH3)). trans-Isomer (* indi-
cates overlap with cis-isomer): d = 7.55–6.75 (m, 13H,
ArH), 6.46 (s, 1H, CNCH), 4.60 (d, J = 7.3 Hz, 1H,
PhCHCH2), 3.59 (dd, J = 7.4, 18.8 Hz, 1H, trans-
PhCHCH2), 2.87 (dd, J = 1.8, 18.8 Hz, 1H, cis-
PhCHCH2), 2.32 (s, 3 H, Ar(CH3)), 2.17 (s, 3 H, Ar(CH3)),
2.05 (s, 3H, Ar(CH3)). 13C NMR (CDCl3): d = 178.3,
176.4, 174.3, 163.5, 159.0, 146.8, 145.2, 143.6, 141.7,
134.3, 133.8, 132.1, 130.3, 125.5, 125.3, 125.0, 124.8,
113.2, 93.8, 48.6, 20.7, 18.3, 17.6. ESI-MS, m/z (%): 352.5
4.2.2. N-phenyl-3,4-diphenylcyclo-2-pentene imine (2a)
Route A reagents: 3,4-Diphenylcyclo-2-pentenone 1
(1.50 g, 6.40 mmol), aniline (1.86 g, 20 mmol), p-toluene-
sulfonic acid (75.0 mg, 0.32 mmol), reaction time: 1 h,
yield: 1.68 g (85%), green powder. Route B reagents:
3,4-Diphenylcyclo-2-pentenone 1 (4.50 g, 19.2 mmol), ani-
line (10.7 g, 115.2 mmol), TiCl4(thf)2 (7.0 g, 20.9 mmol),
1
yield: 5.35 g (90%), pale-green powder. H NMR (CDCl3):
cis/trans = 1.4/1; cis-isomer: d = 7.55–6.83 (m, 16H,
ArH + CNCH), 4.54 (d, J = 7.3 Hz, 1H, PhCHCH2),
3.13 (dd, J = 7.4, 18.8 Hz, 1H, trans-PhCHCH2), 2.46
(dd, J = 2.0, 18.8 Hz, 1H, cis-PhCHCH2). trans-Isomer:
d = 7.55–6.83 (m, 16H, ArH + CNCH), 4.59 (d,
J = 7.3 Hz, 1H, PhCHCH2), 3.48 (dd, J = 7.4, 18.8 Hz,
1H, trans-PhCHCH2), 2.80 (dd, J = 2.0, 18.8 Hz, 1H, cis-
(100) [(M+H)+]. IR (KBr) [cmꢀ1]: m ¼ 2915 w, 2852 w,
~
1643 vs, 1595 s, 1491 s, 1476 s, 1445 s, 1263 m, 1207 s,
1144 m, 1030 m, 853 s, 759 s, 699 s. Anal. calc. for

2228
K.R. Nikolaides et al. / Journal of Organometallic Chemistry 693 (2008) 2223–2230
C26H25N (351.5): C, 88.85; H, 7.17; N, 3.99. Found: C,
88.45; H, 7.30; N, 3.93%.
(2.5 eq.) was added and stirred for 1 h. Successive addition
of FeCl2 (0.5 eq.) led to a colour change to red brown. The
reaction mixture was slowly warmed to room temperature
and after the time mentioned all volatiles were removed in
vacuo. The residue was dissolved in methanol and stirred
for 5 min during which a orange solid precipitated. Differ-
ent work-up procedures followed depending on the desired
products.
4.2.5. N-(2,6-diisopropyl)phenyl-3,4-diphenylcyclo-2-
pentene imine (2d)
Route A reagents: 3,4-Diphenylcyclo-2-pentenone 1
(2.00 g,
8.54 mmol),
2,6-diisopropylaniline
(4.45 g,
25 mmol), p-toluenesulfonic acid (0.11 g, 0.43 mmol),
reaction time: 24 h, yield: 2.41 g (72%), dark green powder.
Route B reagents: 3,4-Diphenylcyclo-2-pentenone 1 (1.17 g,
5.0 mmol), 2,6-diisopropylaniline (5.32 g, 30.0 mmol),
TiCl4(thf)2 (1.82 g, 5.45 mmol), yield: 1.57 g (47%), green
powder. 1H NMR (CDCl3): cis/trans = 4.2/1; cis-isomer
(* indicates overlap with cis-isomer): d = 7.52–7.02 (m*,
11H, Ar-H + CNCH), 4.56 (d, J = 7.3 Hz, 1H,
PhCHCH2), 3.05–2.80 (m*, 3H, CH(CH3)textsub-
script2 + trans-PhCHCH2), 2.20 (dd, J = 2.0, 18.8 Hz,
1H, cis-PhCHCH2), 1.34–1.10 (m*, 12H, CH(CH3)2).
trans-isomer (* indicates overlap with cis-isomer):
d = 7.52–7.02 (m, 10 H, ArH), 6.50 (s, 1H, CNCH), 4.68
(d, J = 7.3 Hz, 1H, PhCHCH2), 3.58 (dd, J = 7.4,
18.8 Hz, 1H, trans-PhCHCH2), 3.08 (sept, J = 6.9 Hz,
1H, CH (CH3)2), 3.05–2.80 (m*, 2H, CH(CH3)2 + cis-
PhCHCH2), 1.34–1.10 (m*, 6H, CH(CH3)2), 0.88 (d,
J = 6.9 Hz, 6H, CH(CH3)2). 13C NMR (CDCl3): = 147.1,
143.7, 143.4, 140.2, 137.9, 137.7, 136.6, 133.9, 132.4,
131.1, 130.8, 129.9, 129.7, 129.5, 129.0, 128.9, 128.7,
128.5, 127.6, 127.4, 127.1, 126.8, 124.1, 123.8, 123.6,
123.3, 123.1, 123.0, 122.8, 122.5, 118.5, 48.6, 47.9, 43.6,
40.9, 28.1, 28.0, 27.8, 23.9, 23.8, 23.6, 23.3, 23.2, 23.1,
22.8, 22.4. ESI-MS, m/z (%): 394.6 (100) [(M+H)+]. IR
4.2.8. Bis(1-anilino-3,4-triphenylcyclopentadienyl) iron(II)
(3a)
Reagents: 2a (2.8 g, 9.05 mmol), LDA (11.3 mL,
22.6 mmol), FeCl2 (573 mg, 4.53 mmol), reaction time:
16 h. The crude product was filtered off, washed with cold
methanol, dried in vacuo and re-crystallised from benzene.
Yield: 1.98 g (65%), orange powder. 1H NMR (C6D6):
d = 7.39–7.36 (m, 8H, ArCpH), 7.05 (m, 4 H, ArNH),
7.01–6.99 (m, 12H, ArCpH), 6.73 (t, J = 7.3 Hz, 2H,
ArNH), 6.58 (6, J = 7.2 Hz, 4H, ArNH), 4.62 (s, 2H,
NH), 4.39 (s, 4H, Cp-H). 13C NMR (d8-THF): d = 146.5,
138.2, 130.6, 129.5, 128.4, 126.6, 119.1, 116.2, 103.0, 83.8,
67.1#. ESI-MS, m/z (%): 672.6 (100) [M+]. Anal. calc. for
C46H36FeN2 (672.6): C, 82.14; H, 5.39; N, 4.16. Found:
C, 81.82; H, 5.41; N, 4.14%.
Synthesis via the silylamide route: A mixture of the imine
2a (2.5 g, 8.1 mmol) and freshly distilled Fe[N(Si-
Me3)2]2(thf) (4, 1.5 g, 4.0 mmol) was refluxed for 14 day
while the colour changed from green to brown. Afterwards,
all volatiles were removed in vacuo and the product was
purified by inert gas column chromatography (CH2Cl2/
EtOAc/Et3N) 1/1/0.02. Yield: 1.74 g (62%), orange powder.
(KBr) [cmꢀ1]: m ¼ 2960 s, 2925 s, 2863 m, 2863 m, 1712
~
w, 1635 s, 1588 m, 1492 m, 1446 m, 758 s, 700 s. Anal. calc.
for C29H31N (393.6): C, 86.10; H, 7.64; N, 3.36. Found: C,
86.44; H, 7.49; N, 3.14%.
4.2.9. Bis(1-(4-methyl)anilino-3,4-triphenylcyclopen-
tadienyl) iron(II) (3b)
Reagents: 2b (2.92 g, 9.05 mmol), LDA (11.3 mL,
22.6 mmol), FeCl2 (573 mg, 4.53 mmol), reaction time:
24 h. The crude product was filtered off, washed with cold
methanol, dried in vacuo and re-crystallised from benzene.
Yield: 2.03 g (64%), orange powder. 1H NMR (C6D6):
d = 7.41–7.37 (m, 8H, ArCpH), 7.02–6.99 (m, 12H, ArCpH),
6.86 (t, J = 7.5 Hz, 4H, ArNH), 6.47 (d, J = 7.5 Hz, 4H,
ArNH), 4.60 (s, 2H, NH), 4.36 (s, 4H, Cp-H), 2.21 (s,
6H, CH3). 13C NMR (d8-THF): d = 144.7, 138.1, 130.6,
130.1, 128.2, 126.5, 125.3, 116.0, 103.9, 83.6, 67.1#, 23.1.
ESI-MS, m/z (%): 700.8 (100) [M+]. Anal. calc. for
C46H36FeN2 (700.7): C, 82.28; H, 5.75; N, 4.00. Found:
C, 81.73; H, 5.31; N, 4.11%.
4.2.6. N-((R)-1-phenylethyl)-3,4-diphenylcyclo-2-pentene
imine (2e)
Route A reagents: 3,4-Diphenylcyclo-2-pentenone 1
(1.00 g, 4.3 mmol), (R)-(+)-1-phenylethylamine (7.82 g,
64.5 mmol), p-toluenesulfonic acid (55.0 mg, 0.22 mmol),
reaction time: 32 h, yield: 1.2 g (84%), yellow oil. 1H
NMR (CDCl3): d = 7.75–6.90 (m, 15H, ArH), 4.85 (d,
1H, PhCHCH2), 4.41 (d, 1H, trans-PhCHCH2), 3,32 (d,
1H, cis-PhCHCH2), 2.61 (m, 1H, NCH), 1.50 (m, 3H,
NCHCH3) cis/trans = >20/1. 13C NMR (CDCl3):
d = 133.1, 127.7, 127.3, 126.8, 126.7, 126.6, 126.5, 125.9,
125.7, 124.2, 110.2, 63.9, 62.3, 49.2, 48.8, 39.2, 25.0. ESI-
MS, m/z (%): 338.4 (100) [(M+H)+]. IR (KBr) [cmꢀ1]:
4.2.10. Bis(1-N-(2,4,6-trimethylphenyl)-3,4-diphenyl-
cyclopentadienyl) iron(II) (3c)
~
m ¼ 1642 m,160 w, 1493 m, 1263 w, 1029 w, 757 s, 724
m, 698 s. Anal. calc. for C25H23N (337.5): C, 88.98; H,
6.87; N, 4.15. Found: C, 88.59; H, 6.77; N, 4.17%.
Reagents: 2c (1.0 g, 2.8 mmol), LDA (3.5 mL,
7.0 mmol), FeCl2 (180 mg, 1.4 mmol), reaction time: 32 h.
The crude product was purified by inert gas column chro-
matography (CH2Cl2/EtOAc/Et3N) 1/1/0.02. Yield:
401 mg (38%), dark-orange powder. 1H NMR (C6D6):
d = 7.43–7.40 (m, 8H, ArCpH), 7.04–7.00 (m, 12H, ArCpH),
6.84 (s, 2 H, ArNH), 4.58 (s, 2H, NH), 4.31 (s, 4H, Cp-H),
4.2.7. General procedure for the synthesis of 1,10-diamino-
3,4-diphenylcyclopentadienyl iron(II) compounds 3a–3e
A solution of the imine ligand (1 eq.) in THF (10 mL/
mmol imine ligand) was cooled to ꢀ78 °C and LDA

K.R. Nikolaides et al. / Journal of Organometallic Chemistry 693 (2008) 2223–2230
2229
2.21 (s, 6H, Ar(CH3)), 2.14 (s, 12H, Ar(C H3)). 13C NMR
Acknowledgements
(d8-THF): d = 139.8, 138.3, 130.4, 128.6, 128.1, 126.3,
125.9, 122.5, 103.0, 83.8, 66.5#, 20.5, 17.7. ESI-MS, m/z
(%): 756.6 (100) [M+]. Anal. calc. for C52H48FeN2
(756.8): C, 82.53; H, 6.39; N, 3.70. Found: C, 81.98; H,
6.23; N, 3.88%.
We thank the Stifterverband fur die deutsche Wissens-
¨
chaft (Projekt-Nr. 11047: ForschungsDozentur Molekulare
Katalyse), the Elitenetzwerk Bayern (Fellowship K.N.) and
the IDK NanoCat for generous support. We are grateful to
Prof. H.-J. Kruger and his co-workers for measurement of
¨
4.2.11. Bis(1-N-(2,6-diisopropylphenyl)-3,4-diphenyl-
cyclopentadienyl) iron(II) (3d)
the cyclic voltammogram.
Reagents: 2d (1.01 g, 2.56 mmol), LDA (3.2 mL,
6.4 mmol), FeCl2 (162 mg, 1.28 mmol), reaction time:
32 h. The crude product was purified by inert gas column
chromatography with n-hexane/EtOAc/Et3N (10/5/1).
Yield: 350 mg (33%), brown-orange powder. 1H NMR
(C6D6): d = 7.46–7.42 (m, 8H, ArCp H), 7.08–7.00 (m,
16H, ArH), 6.89 (t, J = 7.3 Hz, 2H, ArNH), 4.43 (s, 2H,
NH), 4.30 (s, 4H, Cp-H), 2.94 (sept, J = 6.8 Hz, 2H,
CH(CH3)2), 1.28 (d, sept, J = 6.8 Hz, 12H, CH(CH3)2).
13C NMR (d8-THF): d = 139.5, 138.4, 132.7, 130.4,
128.6, 126.7, 119.8, 118.0, 113.1, 84.2, 66.1#, 28.2#, 22.5.
ESI-MS, m/z (%), 840.7 (100) [M+]. Anal. calc. for
C58H60FeN2 (841.0): C, 82.84; H, 7.19; N, 3.33. Found:
C, 82.55; H, 6.94; N, 3.18%.
Appendix A. Supplementary material
CCDC 668199 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Cen-
tre via www.ccdc.cam.ac.uk/data_request/cif. Supplemen-
tary data associated with this article can be found, in the
online version, at doi:10.1016/j.jorganchem.2007.12.020.
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