Heteroferrocene: The synthesis of bis[(3a,4,5,6,6a-η)-1,3,4,5,6- pentamethylcyclopenta[d]imidazo-2-thionoyl]iron(II)

Article abstract of DOI:10.1246/cl.2005.1010

The synthesis of bis[(3a,4,5,6,6a-η)-1,3,4,5,6-pentamethylcyclopenta[d] imidazo-2-thionoyl]iron(II) is reported. Structural determination of the unusual hetero-substituted metallocene by X-ray crystallographic analysis is described. Preliminary electrochemical studies reveal that oxidation and reduction potentials for the subject ferrocene lie midway between those of ferrocene and decamethylferrocene. Copyright

Full text of DOI:10.1246/cl.2005.1010

1010
Chemistry Letters Vol.34, No.7 (2005)
Heteroferrocene: The Synthesis of Bis[(3a,4,5,6,6a-ꢀ)-1,3,4,5,6-
pentamethylcyclopenta[d]imidazo-2-thionoyl]iron(II)
Anthony J. Arduengo, III,ꢀ Thomas P. Bannenberg, Daniela Tapu, and William J. Marshally
Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487
yDuPont Central Research and Development, Experimental Station, Wilmington, Delaware 19880, U. S. A.
(Received April 18, 2005; CL-050524)
The synthesis of bis[(3a,4,5,6,6a-ꢀ)-1,3,4,5,6-pentamethyl-
methyl-3-penten-2-one affords alcohol 2 in 80% yield. Naza-
rov-type cyclization6 of 2 produces the cyclopentadienylimida-
zolethione 3. Deprotonation of 3 and addition of anhydrous
iron(II) chloride yields the target ferrocene 4 (Scheme 1).
Ferrocene 4 is an orange solid melting over 380 ꢁC with
solubility in range of polar organic solvents (e.g. methylene
chloride, chloroform, and tetrahydrofuran).7 The solubility in
benzene and toluene is, however, only slight. Under a nitrogen
atmosphere 4 is stable for months, but slowly decomposes on
prolonged exposure to air. The 1H NMR spectrum of 4 in CDCl3
shows 3 singlets at ꢃ 3.65, 1.97, and 1.62. The imidazole 13C2
center resonates at ꢃ 176.72 ppm (about 14 ppm downfield of
the same position in 1,3-dimethylimidazole-2-thione).5
Crystals suitable for X-ray diffraction studies were grown by
diffusion of hexane into a saturated chloroform solution of 4.
Ferrocene 4 crystallizes in the monoclinic space group C2=c.8
The iron center resides on a 2-fold axis so that half of the mole-
cule comprises the asymmetric unit. The KANVAS9 drawing in
Figure 1 depicts the solid-state structure of 4. Each fused ring is
distinctly planar with no ring atom deviating from the best plane
by more than 4.74 pm and an average deviation of 2.43 pm.
The two bicyclic rings are parallel and adopt an antiperiplanar
orientation. Selected bond distances and angles are presented
cyclopenta[d]imidazo-2-thionoyl]iron(II) is reported. Structural
determination of the unusual hetero-substituted metallocene
by X-ray crystallographic analysis is described. Preliminary
electrochemical studies reveal that oxidation and reduction
potentials for the subject ferrocene lie midway between those
of ferrocene and decamethylferrocene.
Relatively little is reported regarding the chemistry of
ferrocenes containing fused heterocyclic rings.1 Ferrocenes con-
taining atoms with good donor abilities have attracted strong in-
terest, since these complexing moieties are able to act as ligands
toward transition metals. Ferrocenes with electron releasing sub-
stituents are reported to be relatively unstable.1,2 Aminoferro-
cene is stable only as its N-acetyl derivates1 and 1,10-diaminofer-
rocene is characterized solely on the basis of its reaction prod-
ucts.2 There are only a few examples of heteroferrocenes, but
they too lack great thermal stability.3 Imidazole-2-thiones and
imidazol-2-ylidenes show coordination chemistry in which they
act as ꢁ-donors for many transition metals and main-group ele-
ments.4 The ꢁ-ligation of imidazole moieties offer an interesting
compliment to the cyclopentadienyl ꢂ-bonding observed in met-
allocenes. Our interest in imidazole-based carbenes led us to
consider the fusion of a metallocene with an imidazole. We here-
in report a synthetic strategy that produces these fused-metallo-
cenes in three steps. A ferrocenyl-fused imidazole-2-thione is
characterized and its crystal structure is reported.
in Table 1. The average r
(205.8 pm) is comparable to
ðFe{CÞ
the distances found in Cp2Fe [204(2) pm]10 and (Me5Cp)2Fe
[205.0(2) pm].11
A comparative cyclic voltammetry study (Figure 2) was
done for ferrocene, decamethylferrocene, and 4.12 The data are
recorded in Table 2. The oxidation and reduction potentials for
the subject ferrocene lie midway between those of ferrocene
The title compound is obtained from the readily available
1,3-dimethylimidazole-2-thione.5 Deprotonation of thione 1
with 1 equiv. of n-butyl lithium followed by treatment with 3-
Scheme 1. Synthesis of 4. Reagents and conditions: (i) 1. n-
BuLi, THF; 2. 3-Methyl-3-penten-2-one; (ii) TsOH, dichloro-
methane; (iii) 1. n-BuLi, THF; 2. FeCl2.
Figure 1. KANVAS9 depiction of bis(imidazolyl)ferrocene 4.
Copyright Ó 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.7 (2005)
1011
Table 1. Selected bond lengths (pm) and Angles (deg) in 4
R. V. Golovnuja, and L. S. Shilovtseva, Dokl. Akad. Nauk
SSSR, 102, 535 (1955); Chem. Abstr., 50, 4925 (1956).
a) G. R. Knox, Proc. Chem. Soc., 1959, 56. b) G. R. Knox
and P. L. Pauson, J. Chem. Soc., 1961, 4615. c) A. N.
Nesmeyanov, V. N. Drozd, and V. A. Sazonova, Dokl. Akad.
Nauk SSSR, 150, 321 (1963); Chem. Abstr., 59, 5196 (1963).
a) F. Popp and E. B. Moynahan, in ‘‘Advances in Heterocy-
clic Chemistry,’’ ed. by A. R. Katritzky and A. J. Boulton,
Academic Press, New York (1971), Bd. 13, S1, p 1. b) H.
Volz and H. Kowarsch, J. Organomet. Chem., 136, C27
(1977). c) H. Volz and R. Draese, Tetrahedron Lett., 37,
3209 (1975). d) J. C. Ruble and G. C. Fu, J. Org. Chem.,
61, 7230 (1996). e) G. H. Hwang, E. S. Ryu, D. K. Park,
S. C. Shim, C. S. Cho, T. J. Kim, J. H. Jeong, and M. Cheong,
Organometallics, 20, 5784 (2001). f) N. Nagahora, S.
Ogawa, Y. Kawai, and R. Sato, Tetrahedron Lett., 43,
5825 (2002).
Distance
4
Angle
4
2
3
Fe–C
C2–S1
C2–N1ð3Þ
205.8
C2–N1ð3Þ–C6að3aÞ
108.96(15), 108.92(15)
167.7(19) N1ð3Þ–C6að3aÞ–C3að6aÞ 106.89(15), 106.96(15)
136.9(2)
C6ð4Þ–C6að3aÞ–C3að6aÞ 109.79(16), 110.14(15)
N1ð3Þ–C6að3aÞ 140.2(2)
C3a–C6a
140.3(3)
C3að6aÞ–C4ð6Þ 142.7(2)
C6að3aÞ–C6ð4Þ–C5
C4–C5–C6
N1–C2–N3
105.04(16), 104.83(16)
110.18(16)
108.25(16)
C4ð6Þ–C5
145.0(3),
144.9(3)
4
a) D. J. Williams, P. H. Pool, G. Ramirez, and B. L. Heyl,
Inorg. Chim. Acta, 147, 221 (1988). b) S. K. Hadjikakou,
P. Aslanidis, P. Karagiannidis, A. Aubry, and S. Skoulika,
Inorg. Chim. Acta, 193, 129 (1992). c) D. J. Williams,
V. L. H. Bevilacqua, P. A. Morson, K. J. Dennison, W. T.
Pennington, G. L. Schimek, D. VanDerveer, J. S. Kruger,
and N. T. Kawai, Inorg. Chim. Acta, 285, 217 (1999). d)
N. Kuhn, R. Fawzi, T. Kratz, M. Steimann, and G. Henkel,
Phosphorus, Sulfur Silicon Relat. Elem., 108, 107 (1996).
B. L. Benac, E. M. Burgess, and A. J. Arduengo, III, Org.
Synth., 64, 92 (1986).
5
6
7
Figure 2. Comparative cyclic voltammetry studies on 4 and
reference compounds.
I. N. Nazarov and I. F. Zaretskoya, J. Gen. Chem. USSR, 29,
1532 (1959).
Analytical Data for 4. mp > 380 ꢁC 1H NMR (360 MHz,
CDCl3): ꢃ 3.65, (s, 12H), 1.97 (s, 12H) 1.62 (s, 6H).
13C NMR (90 MHz, CDCl3): ꢃ 176.75, 96.96, 78.54, 60.79,
32.85, 9.92, 9.72. Anal. Calcd for C22H30FeN4S2: C,
56.165; H, 6.427; N, 11.908%. Found: C, 56.35; H, 6.49;
N, 11.64%.
Table 2. Cyclic voltammmetry data on ferrocene systems
Compound
Epa/V
Epc/V
E01/V
Cp2Fe
0.674
0.336
0.123
0.472
0.232
0.573
0.284
0.035
Ferrocene 4
Cpꢀ2Fe
ꢂ0:053
8
Selected crystallographic data of 4: a ¼ 1781:36ð4Þ, b ¼
870:90ð2Þ, c ¼ 1509:68ð3Þ pm, ꢄ ¼ 113:7600ð10Þꢁ, V ¼
214358ð8Þ pm3, Dcalcd ¼ 1:458 mg/m3, monoclinic, C2=c,
Z ¼ 4, ꢅðMoÞ ¼ 0:92 mmꢂ1, Mo Kꢆ radiation, 1678 reflec-
tions (I > 4ꢁðIÞ) R1 ¼ 0:0301 and wR2 ¼ 0:0831. GOF ¼
1:09 Further details of the crystal structure have been depos-
ited with the Cambridge Crystallographic Data Centre
(CCDC deposition number 269184).
and decamethylferrocene. These data suggest that the thiourea
moiety is approximately electro-neutral in its effect of the cyclo-
pentadienyl group so that the redox potentials roughly track the
number of methyl substituents on the Cp-ring.
This new imidazole-2-thione fused metallocene not only
provides entry into imidazole thione complexes bearing a metal-
locene moiety, but also offers an opportunity for elaboration of
the structure into a class of metallocene-fused imidazole-2-
ylidenes. In future reports we will elaborate on this novel class
of carbenes and their metal complexes.
9
This drawing was made with the KANVAS computer
graphics program. This program is based on the program
SCHAKAL of E. Keller (Kristallographisches Institute der
Universitat Freiburg, Germany), which was modified by
¨
A. J. Arduengo, III (The University of Alabama), to produce
the back and shadowed planes. The planes bear a 50-pm grid,
and the lighting source is at infinity so that shadow size is
meaningful.
We gratefully acknowledge Atotech U.S.A. for a Fellowship
to DT in support of her Ph.D. Dissertation Research, The
Alexander von Humboldt Foundation for a Feodor-Lynen
Fellowship to TPB, The Saxon Endowment of The University
of Alabama (AJA), and E. I. du Pont de Nemours and Co.
This work was supported by grants from the National Science
Foundation (CHE-0413521 and CHE-0115760).
10 a) J. D. Dunitz, L. E. Orgel, and A. Rich, Acta Crystallogr.,
9, 373 (1956). b) P. Seiler and J. D. Dunitz, Acta Crystal-
logr., 35, 1068 (1979).
11 D. P. Freyberg, J. L. Robbins, K. N. Raymond, and J. C.
Smart, J. Am. Chem. Soc., 101, 892 (1979).
References and Notes
1
12 Cyclic voltammograms were measured in CH2Cl2 at
20 ꢁC containing 0.1 mol dmꢂ3 n-Bu4NBF4 as a supporting
a) F. S. Arimoto and A. C. Haven, J. Am. Chem. Soc., 77,
6295 (1955). b) A. N. Nesmeyanov, E. G. Perevalova,
electrolyte using a Pt electrode; scan rate was 200 mV sꢂ1
.
Published on the web (Advance View) June 11, 2005; DOI 10.1246/cl.2005.1010