Chemistry of the Si-Si and Fe-Fe bonds in the cyclic structure (Me2SiSiMe2) [η5-C5H4Fe(CO)2]2. Selective cleavage of the Fe-Fe bond by I2 and unexpected properties of the iodide

Article abstract of DOI:10.1021/ic034156m

The Si-Si bond in the title cyclic structure (1) exhibited unexpected stability toward I₂. Thus, the reaction of 1 with 1 equiv of I₂ in chloroform resulted in selective cleavage of the Fe-Fe bond to afford diiodide (Me₂SiSiMe₂) [η⁵-C₅H₄Fe(CO)₂I]₂ (2) with retention of the Si-Si bond. When excess (2-4 equiv) I₂ was used to react with 1 in either benzene or chloroform, iodonium-bridged diiron complex {(Me₂SiSiMe₂) [η⁵-C₅H₄Fe(CO)₂] ₂I⁺}(I₅^-) (4) was obtained, in which the Si-Si bond was still retained. It is noteworthy that 4 contains a counteranion I₅^- rather than the expected I₃^-, which is the first example for an iodonium-bridged diiron complex to combine a polyiodide anion larger than I₃^-. UV irradiation of 2 did not affect the stability of the silicon-silicon bond and, in the presence of PR₃, resulted in CO substitution to give (Me₂SiSiMe₂) [η⁵-C₅H₄Fe(CO) (PR₃)I]₂ (5 R = Ph; 6, R = OPh). The molecular structure of 2 was determined by the x-ray diffraction method. It is noteworthy that the structure of 2 does not take the expected anti conformation but adopts a gauche one. The length of the Si-Si bond of 2 [2.353(3) A] is about the same as that of 1 [2.346(4) A], which can be direct evidence to demonstrate that the Si-Si bond in the cyclic structure of 1 is not subject to significant ring strain. The molecular structure of 4 was also determined by the x-ray diffraction method. It is noted that the structure of 4 contains an abnormally large Fe-I⁺-Fe bond angle of 121.25(7)°. Of particular interest is the observation that the I₅^- anions of 4 are self-assembled into novel layered, two-dimensional networks with the (Me₂SiSiMe₂)[η⁵-C₅ H₄Fe(CO)₂]₂I⁺ cations as the template.

Full text of DOI:10.1021/ic034156m

Inorg. Chem. 2003, 42, 4076−4081
Chemistry of the Si−Si and Fe−Fe Bonds in the Cyclic Structure
(Me2SiSiMe2)[η5-C H Fe(CO)2]2. Selective Cleavage of the Fe−Fe Bond by
5 4
I2 and Unexpected Properties of the Iodide
Huailin Sun,* Zhensheng Zhang, Yanbin Pan, Jian Yang, and Xiuzhong Zhou
Department of Chemistry, Nankai UniVersity, Tianjin 300071, P. R. China
Received February 13, 2003
The Si−Si bond in the title cyclic structure (1) exhibited unexpected stability toward I2. Thus, the reaction of 1 with
1 equiv of I2 in chloroform resulted in selective cleavage of the Fe−Fe bond to afford diiodide (Me2SiSiMe2)[η5-
C H Fe(CO)2I]2 (2) with retention of the Si−Si bond. When excess (2−4 equiv) I2 was used to react with 1 in either
5
4
benzene or chloroform, iodonium-bridged diiron complex {(Me2SiSiMe2)[η5-C H Fe(CO)2]2I+}(I5-) (4) was obtained,
5
4
in which the Si−Si bond was still retained. It is noteworthy that 4 contains a counteranion I5- rather than the
expected I3-, which is the first example for an iodonium-bridged diiron complex to combine a polyiodide anion
larger than I3-. UV irradiation of 2 did not affect the stability of the silicon−silicon bond and, in the presence of
PR , resulted in CO substitution to give (Me2SiSiMe2)[η5-C H Fe(CO)(PR )I]2 (5, R ) Ph; 6, R ) OPh). The
3
5
4
3
molecular structure of 2 was determined by the X-ray diffraction method. It is noteworthy that the structure of 2
does not take the expected anti conformation but adopts a gauche one. The length of the Si−Si bond of 2 [2.353-
(3) Å] is about the same as that of 1 [2.346(4) Å], which can be direct evidence to demonstrate that the Si−Si bond
in the cyclic structure of 1 is not subject to significant ring strain. The molecular structure of 4 was also determined
by the X-ray diffraction method. It is noted that the structure of 4 contains an abnormally large Fe−I+−Fe bond
angle of 121.25(7)°. Of particular interest is the observation that the I5- anions of 4 are self-assembled into novel
layered, two-dimensional networks with the (Me2SiSiMe2)[η5-C H Fe(CO)2]2I+ cations as the template.
5
4
Scheme 1
Introduction
Silicon-silicon bond-bridged dicyclopentadienyltetracar-
bonyldiiron complexes have received considerable attention
recently due to coexistence of the silicon-silicon and iron-
iron bonds in the cyclic structure which leads to the
interesting thermal metathesis reaction between the two
bonds (Scheme 1). This reaction was first reported by us
for the title complex (1) in 1993.1 Since then, much effort
in this area has been devoted to explore the scope of this
reaction in related systems. For example, the possibilities of
the reaction in acyclic systems2,3 and in cyclic structures
similar to that of 1 but containing a Ge-Ge bond in place
of the Si-Si bond4-6 or a Ru-Ru bond in place of the Fe-
Fe bond7,8 have been examined. Mechanistic details have
also been under thorough investigation, which has resulted
in recognition of the intramolecular nature9,10 and the
interesting regioselective features10 of this reaction. All the
evidence obtained up to now has demonstrated that it is the
(4) Zhou, X.; Xie, W.; Xu, S. Chin. Chem. Lett. 1996, 7, 385-386.
(5) Xie, W.; Wang, B.; Dai, X.; Xu, S.; Zhou, X. J. Chem. Soc., Dalton
Trans. 1999, 7, 1141-1146.
(6) Xie, W.; Wang, B.; Xu, S.; Zhou, X. Polyhedron 1999, 18, 1647-
* To whom correspondence should be addressed. E-mail: sunhl@
nankai.edu.cn. Tel.: +86-22-23498132. Fax: +86-22-23502458.
(1) Sun, H.; Xu, S.; Zhou, X.; Wang, H.; Yao, X. J. Organomet. Chem.
1993, 444, C41-C43.
(2) Sun, H.; Zhou, X.; Yao, X.; Wang, H. Polyhedron 1996, 15, 4489-
4495.
1651.
(7) Zhang, Y.; Xu, S.; Zhou, X. Organometallics 1997, 16, 6017-6020.
(8) Zhang, Y.; Wang, B.; Xu, S.; Zhou, X.; Sun, J. J. Organomet. Chem.
1999, 584, 356-360.
(9) Zhou, X.; Zhang, Y.; Xie, W.; Xu, S.; Sun, J. Organometallics 1997,
16, 3474-3481.
(3) Sun, H.; Teng, X.; Huang, X.; Hu, Z.; Pan, Y. J. Organomet. Chem.
2000, 595, 268-275.
(10) Sun, H.; Huang, X.; Hu, Z.; Zhang, Z.; Leng, X.; Weng, L. Inorg.
Chim. Acta 2003, 348, 8-14.
4076 Inorganic Chemistry, Vol. 42, No. 13, 2003
10.1021/ic034156m CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/24/2003

Cyclic Structure (Me2SiSiMe2)[η5-C5H4Fe(CO)2]2
gave 0.18 g (77% yield) of 2 as dark crystals, mp 143-145 °C. 1H
NMR: CDCl3, δ 0.46 (s, 12H, SiMe), 4.90 (dd, J ) 1.5, 1.5 Hz,
4H, Cp), 5.16 (dd, J ) 1.5, 1.5 Hz, 4H, Cp); acetone-d6, δ 0.52 (s,
12H, SiMe), 5.30 (dd, J ) 2.0, 2.0 Hz, 4H, Cp), 5.38 (dd, J ) 2.0,
2.0 Hz, 4H, Cp). 13C NMR (CDCl3): δ -3.75 (SiMe), 85.50, 85.68,
cyclic structure that is responsible for this reaction’s taking
place. But the specific aspect of the cyclic structure that
allows the reaction to take place is still in question.
To examine whether the silicon-silicon and iron-iron
bonds in the cyclic structure have unusual properties, we
have undertaken a study on the reaction of 1 with I2. This
study is so devised because both silicon-silicon and iron-
iron bonds in other systems have been shown to be
susceptible to cleavage by I2. For example, the reaction of
Me3SiSiMe3 with I2 is an effective route to generate Me3-
SiI.11 On the other hand, the reaction of [CpFe(CO)2]2 with
I2 has been well established to produce the iodide CpFe-
(CO)2I12 and in the presence of excess I2 in nonpolar solvent
(such as benzene) to afford the iodonium-bridged diiron
92.55 (Cp), 213.08 (CO). IR (KBr): νCO 2024 (s), 1983 (s) cm-1
.
Anal. Calcd for C18H20O4Fe2I2Si2: C, 29.94; H, 2.79; I, 35.15.
Found: C, 30.44; H, 2.98; I, 35.29.
Synthesis of {(Me2SiSiMe2)[η5-C5H4Fe(CO)2]2I+}(I5-) (4). To
a Schlenk flask were added 0.12 g of 1 (0.25 mmol) and 5 mL of
benzene, to which a solution of 0.13 g of I2 (0.50 mmol) in 5 mL
of benzene was added. The mixture was left at room temperature
for 30 min. During this time black crystals were formed, which
was collected by filtration to give 0.19 g (62%, based on 1) of 4,
mp 93-95 °C. 1H NMR (acetone-d6): δ 0.65 (s, 12H, SiMe), 5.45
(dd, J ) 2.0, 2.0 Hz, 4H, Cp), 5.86 (dd, J ) 2.0, 2.0 Hz, 4H, Cp).
IR (KBr): νCO 2011 (s), 1996 (s) cm-1. Anal. Calcd for C18H20O4-
Fe2I6Si2: C, 17.58; H, 1.64; I, 61.92. Found: C, 18.11; H, 1.72; I,
61.97.
-
complex {[CpFe(CO)2]2I+}(I3 ) containing I3- as the coun-
teranion.13 Thus, the coexistence of the silicon-silicon and
iron-iron bonds in the molecule of 1 raises a question about
which of the two bonds will be preferentially cleaved upon
interaction with I2. Herein, we report the results of our study,
which demonstrates that only the iron-iron bond can be
cleaved by I2 whereas the silicon-silicon bond remains
unchanged, showing unexpected stability of the silicon-
silicon bond. The selective cleavage of the iron-iron bond
provides the ring-opened product. The molecular structure
of this product has been determined and compared with the
structure of 1 to get information about the suspected ring
strain in the cyclic structure, especially at the silicon-silicon
bond. Photochemical properties of the ring-opened product
have also been examined. Also reported in this paper is the
isolation of the iodonium-bridged diiron complex which
unexpectedly contains I5- as the counteranion. The molecular
structure study of this product has revealed that the I5- anions
are self-assembled into novel layered two-dimensional
networks composed solely of iodine atoms.
When 3 or 4 equiv of I2 was used in above reaction, quantitative
yield of 4 could be obtained. Similar treatment of 1 with 2, 3, and
4 equiv of I2 in chloroform resulted in isolation of 4 in 7%, 56%,
and 83% yield, respectively.
Synthesis of (Me2SiSiMe2)[η5-C5H4Fe(CO)(PR3)I]2 (5; R )
Ph). A solution containing 0.14 g of 2 (0.20 mmol), 0.16 g of PPh3
(0.6 mmol) and 20 mL of benzene in a Pyrex tube was irradiated
using a high-pressure mercury lamp (500 W) for 3 h at room
temperature. The resulting solution was transferred to another flask,
where the solvent was removed under vacuum. The residue was
purified through a column (neutral Al2O3, 1:1 petroleum ether/ether)
to give 0.16 g (66% yield) of 5 as a green powder, mp 130 °C
1
(dec). H NMR (CDCl3): δ 0.51, 0.52, 0.53, 0.56 (s, 3H:3H:3H:
3H, SiMe), 3.50, 3.51, 3.78, 4.58, 4.61, 5.17, 5.27 (br s, 1H: 1H:
2H: 1H: 1H: 1H: 1H, Cp), 7.38, 7. 55 (m, 18H:12H, Ph). 13C
NMR (CDCl3): δ -3.90, -3.66, -3.61 (SiMe), 78.12, 78.41,
81.18, 84.48, 84.77, 88.08, 102.12 (Cp), 128.07-136.28 (Ph),
221.38 (d, J ) 33.05 Hz, CO). 31P NMR (CDCl3): δ 68.93. IR
(KBr): νCO 1967 (s), 1978 (s) cm-1. Anal. Calcd for C52H50O2-
Fe2I2Si2P2: C, 52.46; H, 4.23. Found: C, 52.37; H, 4.36.
Experimental Section
Synthesis of (Me2SiSiMe2)[η5-C5H4Fe(CO)(PR3)I]2 (6; R )
OPh). A solution containing 0.14 g of 2 (0.20 mmol), 0.19 g of
P(OPh)3 (0.6 mmol) and 20 mL of benzene in a Pyrex tube was
irradiated with a high-pressure mercury lamp (500 W) for 5 h at
room temperature. The solvent was removed, and the residue was
purified through a short column (neutral Al2O3, 1:1 ether/CH2Cl2).
The product obtained was washed with hexane to give 0.17 g (67%
General Methods. All reactions were carried out under an argon
atmosphere by using standard Schlenk techniques. 1H NMR spectra
were recorded on a Varian Unity-plus 400 or a Bruker AC-P200
spectrometer. IR spectra were recorded using a Bruker Smart 1000
instrument. Elemental analyses were performed using a Vario EL
instrument. Chloroform was dried by refluxing over P2O5 and
distilled under an inert atmosphere. Benzene was dried by refluxing
with sodium in the presence of benzophenone and distilled before
use. Complex 1 was synthesized as reported previously.1 Other
chemicals were purchased and used without further purification.
Synthesis of (Me2SiSiMe2)[η5-C5H4Fe(CO)2I]2 (2). To a Schlenk
flask were added 0.24 g of 1 (0.50 mmol), 0.13 g of I2 (0.50 mmol),
and 10 mL of chloroform. The mixture was stirred at room
temperature overnight. The resulting solution was washed with a
saturated solution of sodium sulfite and then with water and dried
over anhydrous sodium sulfate. After removal of sodium sulfate
by filtration, the solvent was evaporated under reduced pressure to
give a solid residue, which was purified through a column (neutral
Al2O3, chloroform) to give a brown band. Collection of this band
1
yield) of 6 as a yellowish powder, mp 105-107 °C. H NMR
(CDCl3): δ 0.15, 0.20, 0.30, 0.32 (s, 3H:3H:3H:3H, SiMe), 2.53,
2.57, 4.04, 4.08, 4.32, 4.34, 4.65, 4.77 (m, 1H:1H:1H:1H:1H:1H:
1H:1H, Cp), 7.21, 7.35 (m, 6H:24H, Ph). 13C NMR (CDCl3): δ
-4.38, -4.23, -4.13 (SiMe), 75.94, 82.45, 82.89, 84.54, 86.34,
97.61 (Cp), 121.57-151.45 (Ph), 217.81 (d, J ) 47.69 Hz, CO).
31P NMR (CDCl3): δ 131.30. IR (KBr): νCO 1932 (s), 1944 (s)
cm-1. Anal. Calcd for C52H50O8Fe2I2Si2P2: C, 48.54; H, 3.92.
Found: C, 49.11; H, 4.06.
X-ray Crystallography. Single crystals of 2 and 4 suitable for
X-ray analyses were obtained from hexane and hexane/CH2Cl2
solution, respectively. Data collections were performed on a Brucker
SMART 1000 diffractometer using a 2θ/ω scan technique with Mo
KR (λ ) 0.710 73 Å) radiation at room temperature. The structures
were solved using SHELX-90 program and refined by full matrix
least-squares methods on F2. All non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were added theoretically and
(11) Razuraev, G. A.; Brevnova, T. N.; Semenov, V. V. Russ. Chem. ReV.
(Engl. Transl.) 1986, 55, 606-621.
(12) Hallam, B. F.; Pauson, P. L. J. Chem. Soc. 1956, 3030-3037.
(13) Haines, R. J.; du Preez, A. L. J. Am. Chem. Soc. 1969, 91, 769-770.
Inorganic Chemistry, Vol. 42, No. 13, 2003 4077

Sun et al.
Scheme 2
Table 1. Crystal Data and Structure Refinement for
(Me2SiSiMe2)[η5-C5H4Fe(CO)2I]2 (2) and
{(Me2SiSiMe2)[η5-C5H4Fe(CO)2]2I+}(I5-) (4)
2
4
formula
fw
cryst size (mm)
cryst system
space group
a (Å)
C18H20Fe2I2O4Si2
722.02
0.22 × 0.20 × 0.10
monoclinic
C2/c
11.5768(11)
17.4149(17)
13.6659(15)
113.956(6)
2517.8(4)
4
C18H20Fe2I6O4Si2
1229.62
0.24 × 0.20 × 0.10
monoclinic
P21
8.434(4)
18.966(8)
10.336(5)
96.686(9)
1642.1(13)
2
b (Å)
c (Å)
â (deg)
V (Å3)
Z
F
calc (g/cm-3
)
1.905
3.719
2.487
6.617
abs coeff (mm-1
)
θ range (deg)
no. of reflcns collcd
no. of indpt reflcns
2.25-26.35
1.98-26.38
5167
9591
2524
5632
no. of obsd reflcns
1641
1.014
0.0509
0.1355
3616
0.992
0.0504
0.1088
goodness of fit on F2
R1 indices [I > 2σ(I)]
wR2 indices (all data)
largest peak/hole (e Å-3
)
0.634/-1.083
0.872/-0.810
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
(Me2SiSiMe2)[η5-C5H4Fe(CO)2I]2 (2) and
{(Me2SiSiMe2)[η5-C5H4Fe(CO)2]2I+}(I5-) (4)
be the increased steric hindrance at the silicon-silicon bond
compared to the case of Me3SiSiMe3. Electronic effects of
the cyclopentadienylmetal moieties on the reactivity of the
silicon-silicon bond may also exist. Similar steric and
electronic effects on the reactivity of silicon-silicon bonds
toward halogens have been reported in the literature.14
When the reaction was performed in benzene in the
presence of 2 equiv of I2, iodonium-bridged diiron complex
Compound 2
I(1)-Fe(1)
Fe(1)-C(1)
Si(1)-C(8)
Si(1)-C(15)
Fe(1)-C(4)
Fe(1)-C(6)
2.5969(10)
1.804(8)
1.857(7)
1.883(6)
2.085(6)
2.103(7)
Si(1)-Si(1A)
Fe(1)-C(2)
Si(1)-C(9)
Fe(1)-C(3)
Fe(1)-C(5)
Fe(1)-C(7)
2.353(3)
1.810(10)
1.872(8)
2.100(6)
2.087(7)
2.103(6)
C(1)-Fe(1)-I(1)
C(1)-Fe(1)-C(2)
C(8)-Si(1)-Si(1)
88.1(2)
93.6(4)
112.2(3)
C(2)-Fe(1)-I(1)
C(3)-Si(1)-Si(1A)
C(9)-Si(1)-Si(1)
89.0(3)
104.52(19)
111.1(3)
-
{(Me2SiSiMe2)[η5-C5H4Fe(CO)2]2I+}(I5 ) (4) was obtained
(62% yield, based on 1) (Scheme 2). It was found that the
Compound 4
-
counteranion in this complex appeared to be I5 , rather than
Fe(1)-I(1)
Si(1)-C(5)
Si(1)-C(11)
Si(2)-C(13)
Fe(1)-C(1)
Fe(2)-C(4)
Si(1)-Si(2)
I(3)-I(4)
2.622(2)
1.888(13)
1.828(14)
1.866(13)
1.78(2)
1.766(16)
2.336(5)
2.8596(19)
Fe(2)-I(1)
Si(2)-C(14)
Si(1)-C(10)
Si(2)-C(12)
Fe(1)-C(2)
Fe(2)-C(3)
I(2)-I(3)
2.614(2)
1.858(13)
1.869(15)
1.880(13)
1.799(16)
1.80(2)
-
-
the expected I3 as in the case of {[CpFe(CO)2]2I+}(I3 ).13
The same product was obtained in quantitative yield when
3 or 4 equiv of I2 was used. During these processes, the
silicon-silicon bond still remained unchanged.
3.003(2)
2.785(3)
To confirm the stability of the silicon-silicon bond
observed above, the reaction was monitored by the 1H NMR
spectroscopic method in chloroform-d. It was found that,
upon being mixed with 1 equiv of I2, 1 was quickly
transformed into the iodonium-bridged intermediate (Me2-
SiSiMe2)[η5-C5H4Fe(CO)2]2I+ (3) (Scheme 1), accompanied
by formation of a small amount of 2. It was noted that the
1H NMR spectrum of 3 exhibited a singlet for SiMe protons
at 0.62 ppm and two pseudotriplets for Cp groups at 5.16
and 5.50 ppm, which was completely different in chemical
shifts from those of 2 as well as of 1. As the time prolonged,
3 was gradually transformed into 2 in about 5 min. No
product from cleavage of the silicon-silicon bond was
observed. When 2 equiv of I2 was used, 3 was formed
immediately but transformed into 2 slowly with a half-life
time of about 40 min. This is due presumably to the
I(5)-I(6)
I(4)-I(3)-I(2)
177.40(6)
91.7(7)
96.2(4)
90.7(5)
114.8(4)
Fe(1)-I(1)-Fe(2)
C(1)-Fe(1)-I(1)
C(4)-Fe(2)-C(3)
C(3)-Fe(2)-I(1)
C(14)-Si(2)-Si(1)
121.25(7)
88.2(5)
93.5(7)
96.0(5)
114.2(4)
C(1)-Fe(1)-C(2)
C(2)-Fe(1)-I(1)
C(4)-Fe(2)-I(1)
C(5)-Si(1)-Si(2)
included in the refinement processes in an isotropic manner. Crystal
data and refinement details for the two structures are given in Table
1. Selected bond lengths and angles for 2 and 4 can be found in
Table 2. Additional information may be found in the Supporting
Information.
Results and Discussion
Iodination Reactions. The reaction of 1 with 1 equiv of
I2 in chloroform resulted in cleavage of the iron-iron bond
to give diiodide (Me2SiSiMe2)[η5-C5H4Fe(CO)2I]2 (2) as the
only isolated product (Scheme 2). No product from cleavage
of the silicon-silicon bond was obtained. This result is
consistent with the expected property of the iron-iron bond,
but the stability of the silicon-silicon bond is somewhat
unexpected. A probable reason for this phenomenon may
-
formation of I3 which is less reactive than I- as a
nucleophile to react with 3.13 No cleavage of the silicon-
(14) West, R. In ComprehensiVe Organometallic Chemistry; Stone, F. G.
A., Abel, E. W., Eds.; Pergamon: Oxford, U.K., 1983; Vol. 2, pp
365-397.
4078 Inorganic Chemistry, Vol. 42, No. 13, 2003

Cyclic Structure (Me2SiSiMe2)[η5-C5H4Fe(CO)2]2
Scheme 3
silicon bond was observed although excess I2 was present
in the reaction process. When 3 equiv of I2 was used in the
1H NMR experiment, 3 disappeared quickly in about 2 min.
In this case, however, instead of being transformed into 2 it
precipitated out through combination with I5- to form a large
amount of crystals of 4 in the NMR tube at the end of the
experiment. This formation of 4 from chloroform is unex-
pected and very interesting. It means that for preparation of
the iodonium-bridged intermediate product it is not necessary
to use a nonpolar solvent. According to this finding, larger
scale preparation of 4 in chloroform was carried out. It was
found that the same product 4 containing the counteranion
system, CO substitution for the PR3 ligand took place to give
the product (Me2SiSiMe2)[η5-C5H4Fe(CO)(PR3)I]2 (5, R )
Ph; 6, R ) OPh) (Scheme 3). The formation of 5 and 6
indicates that CO loss can occur during the irradiation
processes but the coordinatively unsaturated iron center thus
formed is not able to cleave the silicon-silicon bond. Similar
photochemical stability of silicon-silicon bonds has been
observed in the system (η5- Me3SiSiMe2C5H4)Fe(CO)2-
Me,15,16 although silicon-silicon bonds linked to the metal
atoms [e.g. CpFe(CO)2SiMe2SiMe2Fe(CO)2Cp17,18] are easily
cleaved by the coordinatively unsaturated iron centers formed
via CO loss under the same conditions.19,20 It seems that the
photochemical stability of silicon-silicon bonds linked
indirectly to the metal atoms through Cp ligands is a common
feature.
5 and 6 were characterized by elemental analyses as well
as by spectroscopic methods. 1H NMR spectrum of 5 showed
that it was a mixture of rac and meso isomers in a ratio of
about 1:1. Each of the isomers should give rise to two singlets
for SiMe2 protons due to influence of the chiral center, and
four such singlets were indeed observed for the mixture of
the two isomers. The Cp groups which should exhibit four
peaks for each isomer, however, gave only seven peaks for
-
I5 could be obtained in 7%, 56%, and 83% yield when 2,
3, and 4 equiv of I2 were used, respectively.
All the results obtained above evidently demonstrate that
the silicon-silicon bond is stable toward I2. According to
this stability, it looks like that there is no significant
molecular strain at the silicon-silicon bond. This question
will be further clarified through study of the molecular
structure of 2 (see below). It is noteworthy that 4 containing
-
the counteranion I5 is the first example for an iodonium-
bridged diiron complex to combine with a polyiodide anion
-
-
larger than I3 . The fact that only I5 but neither a smaller
nor larger polyiodide anion was produced when different
amounts of I2 were used is quite interesting. The formation
-
of the stable well-organized network of I5 anions (see
below), especially the template effect of the (Me2SiSiMe2)-
[η5-C5H4Fe(CO)2]2I+ cations on the formation of the net-
works, should be responsible for this phenomenon.
2 and 4 were characterized by conventional elemental (C
and H) analyses. For the products of 4, iodine contents were
1
the two isomers due to accidental degeneracy. H NMR
spectrum of 6 showed that it was also a mixture of rac and
meso isomers similar to the case of 5. In this case, four
singlets for SiMe protons and eight peaks for the Cp groups
were all observed. The 13C NMR spectra of 5 and 6 showed
the presence of rac and meso isomers, although the numbers
of the peaks were generally less than expected due to
degeneracy. Only one peak could be observed for the rac
and meso isomers in the 31P NMR spectra of either 5 or 6
also due to degeneracy. The IR spectrum of 5 gave a narrow
and strong νCO band which was split into two on the top
due to the presence of the rac and meso isomers. The IR
spectrum of 6 showed a situation similar to that of 5, but
the corresponding peaks appeared at higher frequencies due
to lower electron-donating ability of P(OPh)3 compared to
that of PPh3. Separation of the rac and meso isomers proved
to be difficult due to their similarity in physical properties.
Molecular Structures. The molecular structure of 2 was
determined by the X-ray diffraction method. A molecule of
2 (Figure 1) has C2 symmetry. The conformation around the
silicon-silicon bond appeared to be gauche (Figure 2) rather
1
also determined. The H NMR spectrum of 2 recorded in
chloroform-d showed, as expected, a singlet for SiMe groups
1
and two sets of pseudotriplets for the Cp protons. The H
NMR spectrum of 4 in chloroform-d could not be obtained
because it became sparingly soluble in chloroform after being
precipitated from the solution. However, acetone-d6 was
found to have good solubility for 4. The spectrum thus
obtained in acetone-d6 also showed a singlet for SiMe groups
and two sets of pseudotriplets for the Cp protons. But the
chemical shifts were obviously different from those of 2
recorded in chloroform-d as well as in acetone-d6. The 13C
NMR spectrum of 2 exhibited one signal for SiMe, two
signals for Cp, and one signal for CO carbon atoms at the
expected chemical shifts. The 13C NMR spectrum of 4 was
not obtained because it was transformed gradually into 2
upon being dissolved in acetone-d6. The IR spectra of 2 and
4 gave generally two absorption bands due to symmetrical
and asymmetrical vibrations of two germinal CO groups,
which is in accord with the structures.
Photochemical Reactions. The silicon-silicon bond in
the cyclic structure of 1 has been known to be stable under
photochemical conditions.1 After the cyclic structure has been
opened, reexamination of the stability of the silicon-silicon
bond of 2 under photochemical conditions should also be of
interest. It was found that upon being irradiated with a high-
pressure mercury lamp the silicon-silicon bond of 2 was
not affected. When the PR3 ligand was added to the reaction
(15) Pannell, K. H.; Cervantes, J.; Hernandez, C.; Cassias, J.; Vincenti, S.
Organometallics 1986, 5, 1056-1057.
(16) Pannell, K. H.; Castilo-Ramirez, J.; Cervantes-Lee, F. Organometallics
1992, 11, 3139-3143.
(17) Pannell, K. H.; Sharma, H. K. Organometallics 1991, 10, 954-959.
(18) Ueno, K.; Hamashima, N.; Kurita, H.; Ogino, H. Organometallics
1991, 10, 959-962.
(19) Schubert, U. Angew. Chem., Int. Ed. Engl. 1994, 33, 419-421.
(20) Sharma, H. K.; Pannell, K. H. Chem. ReV. 1995, 95, 1351-1374.
Inorganic Chemistry, Vol. 42, No. 13, 2003 4079

Sun et al.
Figure 1. ORTEP plot of (Me2SiSiMe2)[η5-C5H4Fe(CO)2I]2 (2). Thermal
ellipsoids are given at the 30% probability level.
Figure 3. ORTEP plot of the cation part of {(Me2SiSiMe2)[η5-C5H4Fe-
(CO)2]2I+}(I5-) (4). The counteranion is omitted. Thermal ellipsoids are
given at the 30% probability level.
errors, indicating that there is no obvious molecular strain
at the Si-Si bond in the cyclic structure of 4. It is noteworthy
that the bond angle of Fe-I+-Fe is 121.25(7)°, which is
abnormally large compared to the reported angle of 110.8(1)°
for the case of 7. This difference between the two complexes
indicates that the bond angle at the iodonium center is quite
flexible. It has been reported21 that the bond angle is generally
sensitive to steric effects between groups attached to the
iodonium center and when the attached groups are not
particular bulky, this angle tends to take a value of 90° to
optimize the employment of its p-orbitals. This has been
observed in the case of diphenyliodonium ion (8), where the
C-I+-C angle is 92°.22 It is noteworthy that if solely
determined according to these angles, the orbital hybridiza-
tion at the iodonium center should be sp2, sp3, and pure p in
the cases of 4, 7, and 8, respectively. Thus, it will be
interesting to see in the future if an even larger bond angle
of 180° featuring an sp hybridization at the iodine atom could
be reached in case of special steric demands.
Figure 2. Newmann projection along the silicon-silicon bond axis of
(Me2SiSiMe2)[η5-C5H4Fe(CO)2I]2 (2).
than the expected anti one. The reasons for this unexpected
conformation were not determined in this work. It may be
stabilized either by an intermolecular interaction of crystal
packing or by an intramolecularly attractive interaction
between groups at two silicon atoms. The two Cp groups
that are nearly parallel to each other with a dihedral angle
of 10.18° may be responsible for such an attractive inter-
action, if it exists. The length of the Si-Si bond was
determined to be 2.353(3) Å. It is interesting to compare
this bond length after the cyclic structure was opened with
that before opening of the cyclic structure [2.346(4) Å in
11]. In doing so, it is found that the lengths of the two
Si-Si bonds are about the same within the limits of
experimental errors. This is direct evidence to show that the
silicon-silicon bond in the cyclic structure of 1 is not subject
to significant ring strain. Although it has previously been
noted that the boatlike conformation of cyclic structure of 1
may cause a certain extent of molecular strain,1 it seems that
this kind of molecular strain caused by the ring conformation
is not strong enough to lead to detectable weakening of the
silicon-silicon bond. This is in accord with the observed
stability of the silicon-silicon bond of 1 toward I2, taking
into account the contribution of the steric and electronic
effects (see above).
-
The structure of the I5 anion is shown in Figure 4. As
-
-
usually seen, the I5 anion consists of a linear I3 anion
[I(2)-I(3)-I(4), 177.4°] and an I2 molecule.23 The bond
-
lengths within the I3 anion [I(2)-I(3), 2.8596(19) Å, and
I(3)-I(4), 3.003(2) Å] and within the I2 molecule [I(5)-
I(6), 2.785(3) Å] are also normal.
-
Between the I3 anion and the I2 molecule, significant
interaction [I(2)-I(6), 3.460 Å] and weak secondary inter-
action [I(3)-I(6), 4.157 Å] exist to combine the two species
-
into a relatively discrete unit of I5 having approximately
an L-shape [I(3)-I(2)-I(6), 79.7°].24 It is interesting that
these I5- units are linked together through different interac-
tions to result in formation of a two-dimensional layered
structure. The interaction scheme has been shown in Figure
The molecular structure of 4 was also determined by the
X-ray diffraction method. The structure of the cationic part
of 4 (Figure 3) has C1 symmetry. The lengths of the Fe-I
bonds are 2.622(2) and 2.614(2) Å, which are slightly longer
than the terminal Fe-I bond of 2 and the bridging Fe-I
-
4, in which each I5 unit is linked to the next one through
significant interaction [I(2)-I(5′)/I(2′′′)-I(5′′), 3.578 Å] to
form branched infinite chains. Cross-linking of these chains
-
bonds [2.581(2) and 2.595(2) Å] of {[CpFe(CO)2]2I+}(BF4 )
(7).21 The length of the Si-Si bond is 2.336(5) Å, also quite
close to those of 2 and 1 within the limits of experimental
(22) Wright, W. B.; Meyers, E. A. Cryst. Struct. Commun. 1972, 1, 95-
97.
(23) Tebbe, K.-F.; Buchem, R. Angew. Chem., Int. Ed. Engl. 1997, 36,
1345-1346.
(24) Kloo, L.; Svensson, P. H.; Taylor, M. J. J. Chem. Soc., Dalton Trans.
2000, 7, 1061-1065.
(21) Cotton, F. A.; Frenz, B. A.; White, A. J. J. Organomet. Chem. 1973,
60, 147-152.
4080 Inorganic Chemistry, Vol. 42, No. 13, 2003

Cyclic Structure (Me2SiSiMe2)[η5-C5H4Fe(CO)2]2
-
Figure 4. Structure and cross-linking scheme of the I5 anion, showing
interactions ranging up to the van der Waals distance of 4.30 Å. Thermal
ellipsoids are given at the 50% probability level.
Figure 6. View of the I5- networks along the a-axis, showing the channels
surrounded by 11 iodine atoms. The good fit of the shape of the channels
with the cation is shown in the central unit cell.
Conclusions
The silicon-silicon bond in the cyclic structure of title
complex 1 has been found to be stable toward I2. This has
resulted in selective cleavage of the iron-iron bond by I2 to
give the ring-opened diiodide 2. The molecular structure of
2 determined by the X-ray diffraction method provides direct
evidence to conclude that the silicon-silicon bond in the
cyclic structure of 1 is not subject to significant ring strain.
This conclusion will be important for further understanding
the mechanism of the thermal rearrangement reaction that
occurred for the cyclic structure. Photochemical stability of
the silicon-silicon bond of 2 has also been observed. It
confirms that a silicon-silicon bond linked to a Cp ligand
cannot be cleaved by the transition metal center even if CO
loss has occurred. The study on the reaction of 1 with excess
I2 has also resulted in the isolation of the iodonium-bridged
Figure 5. View of the layers of the I5- networks along the c-axis, showing
location of the cations between the layers in the central unit cell.
through weak secondary interactions [I(4)-I(6′′′)/I(4′′)-I(6′),
4.194 Å] forms the layered structure. The distance between
two adjacent layers (Figure 5) is 8.434 Å. The (Me2SiSiMe2)-
[η5-C5H4Fe(CO)2]2I+ cations are located between the layers
to form a huge sandwich-like structure. The top view of the
layers is illustrated in Figure 6. It shows the structure and
stacking of the networks which form channels surrounded
by 11 iodine atoms with a diameter of about 10 Å [I(4)-
I(4′′), 9.652 Å, and I(5′)-I(5′′), 10.336 Å]. The cations reside
in the channels. It is very impressive that the shape of the
channels has a good fit with the dimensions of the cation’s
structure, which unambiguously demonstrates the template
effects of the cations on the formation the iodine networks.
Weak interactions that exist between the bridging iodonium
and the network iodine atoms [I(1)-I(2), 4.210 Å, and I(1)-
I(5), 4.424 Å] may contribute to the realization of the
template effects. Also noticeable in Figure 4 are the bond
angles [e.g.: I(3)-I(2)-I(5′), 123.8°; I(2)-I(5′)-I(6′), 169.4°;
I(5′)-I(6′)-I(4′′), 80.5°; I(3)-I(4)-I(6′′′), 149.9°; I(4)-
I(6′′)-I(2′′), 107.7°; I(5′′)-I(2′′′)-I(6′′′), 153.1°] which vary
considerably. This indicates that the two-dimensional net-
works constructed solely from iodine atoms could be rather
flexible. This may provide opportunity for further tuning the
network structure through modifying the cationic template.
-
diiron complex 4 containing unexpectedly I5 rather than
-
I3 as the counteranion. The molecular structure of 4
determined by the X-ray diffraction method shows self-
-
assembly of the I5 anions to form novel layered, two-
dimensional networks composed solely of iodine atoms. The
cationic part of the complex demonstrates obvious template
effects on formation of the network structure. This template
effect can well explain why the I5- anions have always been
produced when different equivalents of I2 are used to react
with 1.
Acknowledgment. We thank the National Natural Sci-
ence foundation of China for financial support (Project Nos.
29604003 and 29872020).
Supporting Information Available: X-ray crystallographic files
in CIF format for the structural determinations of 2 and 4. This
material is available free of charge via the Internet at http://
pub.acs.org.
IC034156M
Inorganic Chemistry, Vol. 42, No. 13, 2003 4081