Synthesis and crystal structure of the double
cluster [Cp
3
Fe
4
(CO)
4
(C
5
H
4
)]
2
(p-C
6
H
4
)
q
Wen-Yann Yeh
a,*, Yen-Chung Liu
a, Shie-Ming Peng
b, Gene-Hsiang Lee
baDepartment of Chemistry, National Sun Yat-Sen University, 70 Lan-Hai Road, Kaohsiung 804, Taiwan bDepartment of Chemistry, National Taiwan University, Taipei 106, Taiwan
Received 12 September 2003; accepted 25 December 2003
Abstract
[Cp4Fe4(CO)4] (1) reacts with p-BrC6H4Li and MeOH in sequence to afford the functionalized cluster [Cp3Fe4(CO)4(C5H4
-p-C6H4Br)] (2), while the reaction of 2 with n-BuLi and MeOH produces [Cp2Fe4(CO)4(C5H4Bu)(C5H4-p-C6H4Br)] (3). The double
cluster [Cp3Fe4(CO)4(C5H4)]2(p-C6H4) (4) has been prepared by treatment of [Cp4Fe4(CO)4] with p-C6H4Li2 and MeOH in
se-quence. The electrochemistry of 2 and 4, as well as the crystal structure of 4 have been investigated. 2004 Elsevier B.V. All rights reserved.
Keywords: Iron; Tetrairon cluster
1. Introduction
[Cp4Fe4(CO)4] (1), originally reported by King [1], is one of the first substance containing a tetrahedral cluster of metal atoms. A unique feature of this stable cluster is that it is electroactive, reversibly undergoing both re-duction and oxidation [2,3], which property is essential to perform important functions such as solar energy conversion and multielectron catalysis [4,5]. Recently, assembling higher nuclearity clusters with well-defined dimensions provides a new field of chemistry with pro-spective application in areas including molecular rec-ognition and nanotechnology [6–10]. It is therefore of interest to construct oligomers of 1 and study their electroactivity [11]. We have previously prepared the
double clusters [Cp3Fe4(CO)4(C5H4)]2 and [Cp3Fe4
(CO)4(C5H4)]2 [(C5H4)2Fe] by treating the anion
[Cp3Fe4(CO)4(C5H4)] with 1 and dibromoferrocene,
respectively [12]. Now we report an arene-bridged
dou-ble cluster [Cp3Fe4(CO)4(C5H4)]2(p-C6H4) (4) but with a different synthetic approach.
2. Results and discussion
Compound 1 reacted with p-BrC6H4Li and MeOH in
sequence to afford the functionalized cluster [Cp3Fe4
-(CO)4(C5H4-p-C6H4Br)] (2) in 30% yield (Eq. (1)), where
the nucleophile p-BrC6H4 added to a cyclopentadienyl
ring of 1. It was thought that subsequent treatment of 2 with n-BuLi might abstract the bromine atom to gen-erate [Cp3Fe4(CO)4(C5H4-p-C6H4)], which can then
react with 1 to produce 4. In fact, the n-Bu anion
attacked a separate Cp group to produce [Cp2Fe4(CO)4
-(C5H4Bu)(C5H4-p-C6H4Br)] (3) in 40% yield (Eq. (2)).
A reverse way by treating 2 with the anion [Cp3Fe4
-(CO)4(C5H4)] did not afford 4, too. We then prepared
the dianion p-C6H4Li2 by treating p-C6H4Br2 with two
equivalent of n-BuLi [13], which reacted with 1 and MeOH in sequence to give 4 in 24% yield after purifi-cation by column chromatography and crystallization (Eq. (3)). In these reactions, however, the starting clus-ters were recovered in 48–57% yield even though the reactions monitored by IR showed no presence of them
q
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.jorganchem.2003.12.040.
*
Corresponding author. Tel.: +886752520003927; fax: +886752 53908.
E-mail address:wenyann@mail.nsysu.edu.tw(W.-Y. Yeh).
0022-328X/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2003.12.040
after introduction of the nucleophiles. Since these car-bonyl clusters are easily reduced, as the electrochemical studies have shown, reduction of them probably com-petes with nucleophilic addition here and limits the yields of products [11]
ð1Þ
ð2Þ
ð3Þ The new clusters 2–4 form air-stable, dark green crystalline solids which have been characterized by ele-mental analyses, mass, IR and NMR. Their IR spectra in the carbonyl region present one broad absorption
around 1635 cm1 for the triply bridging carbonyl
li-gands, suggesting that their tetrahedral iron cores
re-main intact. Their1H NMR spectra are closely related,
where the unsubstituted Cp groups display a singlet
resonance at ca. 4.5 ppm and each substituted C5H4
group shows two sets of multiplet resonances in the
range 4.9–4.3 ppm. The C6H4Br group in 2 and 3
pre-sents two doublet resonances at 7.6 and 7.5 ppm, while
the bridging C6H4 group of 4 shows a singlet resonance
at 7.78 ppm. The13C{1H} NMR spectrum of 2 displays
the triply bridging carbonyl signals at 290.8 and 290.5 ppm in an approximate ratio of 1:3, four signals for the
C6H4Br group in the range 131.5–122.3 ppm, three
signals for the C5H4group at 104.4, 100.6 and 95.2 ppm,
and the Cp group resonance at 99.1 ppm, consistent
with a molecule of idealized Cs symmetry in solution.
The molecular structure of 4 is illustrated in Fig. 1. Selected bond distances and bond angles are collected in Table 1. There is a crystallographic center of symmetry imposed on the molecule. The coordination about each
Fe4cluster shows great resemblance to that of 1 [14,15].
The two Fe4 clusters are located in opposite sides of the
C5H4–C6H4–C5H4link. The two C5H4groups are about
coplanar, while the bridging C6H4 group is tilted from
the plane by 11.5. The average Fe–Fe lengths and Fe–
C(Cp) lengths are 2.52 and 2.12 A, respectively. The
individual Fe–CO distances range from 1.964(3) to
1.999(3) A and the C–O distances from 1.197(4) to
1.202(4) A, while Fe–C–O angles are in the range
131.7(2)–134.6(2). The C–C bond lengths within the cyclopentadienyl and bezene rings are averaged 1.42 and
1.39 A, respectively, and the C(9)–C(10) length is
1.470(4) A.
Cyclic voltammogram studies of 2 and 4 were taken
in dry, oxygen-free CH2Cl2 at 27 C. The E1=2 values
(versus Cp2Fe/Cp2Feþ couple) relating each oxidation
state are depicted in Scheme 1. Analogous to 1, compound 2 also exists in four electrochemically
reversible oxidation states, [Cp3Fe4(CO)4(C5H4
-p-C6H4Br)]2þ=þ=0=, while the redox potentials are shifted anodically by 56–130 mV due to the electron-with-drawing substituent. On the other hand, compound 4 displays three redox waves in correspondence to a
43þ$ 42þ$ 40$ 42 transformation. The two-electron
reduction (and oxidation) wave is likely the overlap of two closely spaced one-electron redox couples for each
Fe4 cluster with slight electronic interactions between
them, presumably because the C5H4–C6H4–C5H4link is
not planar and therefore is not in full conjugation [16,17].
In summary, the double cluster 4 has been prepared
by the reaction of p-C6H4Li2 with two molecules of 1.
Since compound 1 is susceptible to two nucleophilic additions to form 3, it is promising that further
treat-ment of 4 and 1 with p-C6H4Li2 or other dianionic
nucleophiles could lead to higher cluster oligomers. The investigation is in progress in our laboratory.
3. Experimental 3.1. General methods
All manipulations were carried out under an atmo-sphere of purified dinitrogen with standard Schlenk
techniques. [Cp4Fe4(CO)4] (1) was prepared as described
in the literature [12]. 1,4-dibromobenzene (from Aldrich) and n-butyl lithium (2.5 M in n-hexane, from Merck) were used as received. Solvents were dried over appropriate reagents under dinitrogen and distilled immediately before use. Infrared spectra were recorded with a 0.1
mm-path CaF2 solution cell on a Hitachi I-2001 IR
spec-trometer.1H and13C NMR spectra were obtained on a
Varian Unity INOVA-500 spectrometer at 500 and 125.7 MHz, respectively. Fast-atom-bombardment (FAB) mass spectra were recorded on a JEOL JMS-SX102A mass spectrometer. Elemental analyses were performed at the National Chen-Kung University, Tainan, Taiwan. 3.2. Preparation of 2
Under a nitrogen atmosphere, n-butyl lithium
(0.68 mmol) was slowly added into a solution of
Table 1
Selected bond distances (A) and bond angles () for 4 Bond distances Fe(1)–C(2) 1.979(3) Fe(1)–C(4) 1.983(3) Fe(1)–C(1) 1.991(3) Fe(1)–C(7) 2.102(3) Fe(1)–C(6) 2.108(3) Fe(1)–C(8) 2.114(3) Fe(1)–C(5) 2.117(3) Fe(1)–C(9) 2.137(3) Fe(1)–Fe(4) 2.5071(6) Fe(1)–Fe(3) 2.5130(6) Fe(1)–Fe(2) 2.5168(6) Fe(2)–C(1) 1.964(3) Fe(2)–C(2) 1.986(3) Fe(2)–C(3) 1.991(3) Fe(2)–C(13) 2.109(3) Fe(2)–C(14) 2.119(3) Fe(2)–C(16) 2.123(3) Fe(2)–C(15) 2.123(3) Fe(2)–C(17) 2.123(3) Fe(2)–Fe(3) 2.5120(6) Fe(2)–Fe(4) 2.5406(6) Fe(3)–C(4) 1.968(3) Fe(3)–C(2) 1.980(3) Fe(3)–C(3) 1.993(3) Fe(3)–C(22) 2.100(3) Fe(3)–C(21) 2.105(4) Fe(3)–C(19) 2.111(3) Fe(3)–C(20) 2.112(3) Fe(3)–C(18) 2.116(4) Fe(3)–Fe(4) 2.5026(6) Fe(4)–C(3) 1.972(3) Fe(4)–C(4) 1.987(3) Fe(4)–C(1) 1.999(3) Fe(4)–C(27) 2.107(3) Fe(4)–C(23) 2.108(3) Fe(4)–C(24) 2.113(3) Fe(4)–C(26) 2.116(3) Fe(4)–C(25) 2.120(3) O(1)–C(1) 1.198(4) O(2)–C(2) 1.197(4) O(3)–C(3) 1.201(4) O(4)–C(4) 1.202(4) Bond angles Fe(4)–Fe(1)–Fe(2) 60.755(17) Fe(3)–Fe(1)–Fe(2) 59.924(17) Fe(4)–Fe(1)–Fe(3) 59.802(18) Fe(3)–Fe(2)–Fe(4) 59.377(17) Fe(1)–Fe(2)–Fe(4) 59.436(17) Fe(1)–Fe(2)–Fe(3) 59.965(17) Fe(4)–Fe(3)–Fe(2) 60.879(17) Fe(4)–Fe(3)–Fe(1) 59.982(17) Fe(2)–Fe(3)–Fe(1) 60.111(17) Fe(3)–Fe(4)–Fe(2) 59.744(17) Fe(1)–Fe(4)–Fe(2) 59.810(17) Fe(1)–Fe(4)–Fe(3) 60.216(18) O(1)–C(1)–Fe(2) 131.8(2) O(1)–C(1)–Fe(1) 133.4(3) Fe(2)–C(1)–Fe(1) 79.03(12) O(1)–C(1)–Fe(4) 133.2(2) Fe(2)–C(1)–Fe(4) 79.74(12) Fe(1)–C(1)–Fe(4) 77.86(11) O(2)–C(2)–Fe(1) 133.8(3) O(2)–C(2)–Fe(3) 131.8(2) Fe(1)–C(2)–Fe(3) 78.82(11) O(2)–C(2)–Fe(2) 133.0(2) Fe(1)–C(2)–Fe(2) 78.80(12) Fe(3)–C(2)–Fe(2) 78.60(12) O(3)–C(3)–Fe(4) 132.8(3) O(3)–C(3)–Fe(2) 132.4(3) Fe(4)–C(3)–Fe(2) 79.76(12) O(3)–C(3)–Fe(3) 133.6(3) Fe(4)–C(3)–Fe(3) 78.28(12) Fe(2)–C(3)–Fe(3) 78.19(12) O(4)–C(4)–Fe(3) 134.6(2) O(4)–C(4)–Fe(1) 131.7(3) O(4)–C(4)–Fe(4) 132.6(2) Fe(3)–C(4)–Fe(4) 78.53(12) Fe(1)–C(4)–Fe(4) 78.32(11) Scheme 1.
1,4-dibromobenzene (160 mg, 0.68 mmol) in 5 ml of
toluene at 0C. The resulting p-BrC6H4Li reagent was
then added into a solution of 1 (200 mg, 0.336 mmol) in
20 ml of THF. The mixture was stirred at 50C for 5 h,
followed by addition of MeOH (2 ml). The solvent was removed under vacuum and the residue subjected to column chromatography (silica gel), with n-hexane/di-chloromethane/ethyl acetate (3:1:1) as eluant. Com-pound 2 (75 mg, 30%) was obtained from the second
green band. Anal. Calc. for C30H23BrFe4O4: C, 47.99;
H, 3.09. Found: C, 47.62; H, 3.04%. IR (CH2Cl2,mCO): 1636 cm1. 1H NMR (CDCl 3, 25 C): 7.62 (d, 2H, JH–H ¼ 10 Hz), 7.55 (d, 2H, JH–H¼ 10 Hz, C6H4), 4.96 (br, 2H), 4.86 (br,2H, C5H4), 4.59 (s, 15H, Cp) ppm. 13C{1H} NMR(CDCl 3, 25 C): 290.8, 290.5 (l3-CO), 131.5, 131.3, 127.9, 122.3 (C6H4), 104.4, 100.6, 95.2 (C5H4), 99.1 (Cp) ppm. MS (FAB) m=z 750 [Mþ,79Br]. 3.3. Preparation of 3
Under a nitrogen atmosphere, n-butyl lithium (0.35 mmol) was slowly added into a solution of 2 (126 mg,
0.16 mmol) in 5 ml of toluene at 0C. The mixture was
stirred at room temperature for 1 h, followed by addi-tion of MeOH (1 ml). The reacaddi-tion was worked up in a fashion identical with that above. Compound 3 (56 mg, 40%) was obtained from the first green band. Anal.
Calc. for C34H31BrFe4O4: C, 50.61; H, 3.87. Found: C,
51.03; H, 3.95%. IR (CH2Cl2, mCO): 1634 cm1. 1H NMR (CDCl3, 25 C): 7.61 (d, 2H), 7.56 (d, 2H, JH–H ¼ 10 Hz, C6H4), 4.93 (m, 2H,), 4.82 (m, 2H, C5H4), 4.52 (s, 10H, Cp), 4.46 (m, 2H), 4.30 (m, 2H, C5H4), 2.43 (t, 2H, JH–H ¼ 12 Hz), 1.26 (m, 4H), 0.93 (t, 3H, JH–H¼ 12 Hz, Bu) ppm. MS (FAB) m=z 806 [Mþ,79Br]. 3.4. Preparation of 4
Under a nitrogen atmosphere, n-butyl lithium (1.74 mmol) was slowly added into a solution of 1,4-dib-romobenzene (200 mg, 0.85 mmol) in 5 ml of toluene at
0C. The mixture was heated at 50 C for 4 h to result in
a pale yellow precipitate of p-C6H4Li2. The supernatant
was removed by a syringe, and the solid washed with
freshly distilled toluene (3· 5 ml). A solution of 1 (150
mg, 0.251 mmol) in 40 ml of THF was added. The
re-sulting mixture was vigorously stirred at 50C for 5 h,
followed by addition of MeOH (2 ml). The reaction was worked up in a fashion identical with that above. Compound 4 (38 mg, 24%) was obtained from the
fourth green band. Anal. Calc. C54H42Fe8O8: C, 51.24;
H, 3.34. Found: C, 50.93; H, 3.58%. IR (CH2Cl2, mCO):
1632 cm1. 1H NMR (CDCl
3, 25 C): 7.78 (s, 4H,
C6H4), 5.01 (br, 4H), 4.86 (br, 4H, C5H4), 4.55 (s, 30H,
Cp) ppm. MS (FAB) m=z 1266 [Mþ].
3.5. Cyclic voltammetric measurements for 2 and 4 Electrochemical measurements were taken with a CV 50 W system. Cyclic voltammetry was performed with a Pt button working electrode, a Pt-wire auxiliary elec-trode, and an Ag/AgCl reference electrode. The experi-ments were carried out with 1 mM of 2 and 4,
respectively, in dry CH2Cl2solvent containing 0.1 M
(n-C4H9)4NPF6as the supporting electrolyte. Potential was
scanned at 100 mV s1 at 27 C.
3.6. Structure determination for 4
A crystal of 4 with approximate dimensions of
0.5· 0.08 · 0.08 mm3was mounted in a thin-walled glass
capillary and aligned on the Bruker Smart ApexCCD
diffractometer with graphite-monochromated Mo–Ka
radiation (k¼ 0:71073 A). The data were collected at
150 K. All data were corrected for the effects of ab-sorption. The structures were solved by the direct
method and refined by full-matrix least-square on F2.
The program used was the S H E L X T LS H E L X T L package [18]. All non-hydrogen atoms were refined with anisotropic dis-placement parameters. Hydrogen atoms were included but not refined. A summary of relevant crystallographic date is provided in Table 2.
4. Supplementary material
Crystallographic data for the structural analysis of 4 has been deposited with the Cambridge Crystallographic Data Centre, CCDC No. 217582. Copy of this infor-mation may be obtained free of charge from The Di-rector, CCDC, 12 Union Road, Cambridge CB2 1EZ,
Table 2
Crystal data and refinement details for 4 Formula C54H42Fe8O8
T (K) 150(1)
Crystal system Rhombohedral Crystal solvent 4(C6H6) + 0.67(CHCl3)
Space group R3 Unit cell dimensions
a(A) 33.696(1) b(A) 33.696(1) c(A) 15.0885(5) c() 120 V (A3) 14836.6(8) Z 9 Dcalc(g cm3) 1.670 Fð0 0 0Þ 7602 Radiation k (A) 0.71073 l(mm1) 1.849 hrange () 1.21–27.50 R1 0.0470 wR2 0.1133 Goodness-of-fit on F2 1.118
UK (fax: +44-1223-336033 or e-mail: deposit@ccdc.
cam.ac.uk or www:http://www.ccdc.cam.ac.uk).
Acknowledgements
We are grateful for support of this work by the Na-tional Science Council of Taiwan.
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