Direct Evidence for Extremely Facile 1,2- and 1,3-Group Migrations in an FeSi2 System

Article abstract of DOI:10.1002/anie.200352519

Facile 1,2- and 1,3-group migrations: The first donor-free silyl(silylene)-iron complex 1 has been prepared and structurally characterized (see scheme). This complex reacts with tBuNC under mild heating to give a disilanyl complex 2 through 1,3-alkyl and aryl migration and 1,2-silyl migration processes. Complex 2 is an example of how organosilicon species bound to transition-metal centers can change their form in a dynamic fashion (Mes = 2,4,6-trimethylphenyl).

Full text of DOI:10.1002/anie.200352519

Angewandte
Chemie
Silyl(silylene)–Iron Complexes
Direct Evidence for Extremely Facile 1,2- and 1,3-
Group Migrations in an FeSi2 System**
Hiromi Tobita,* Akihisa Matsuda, Hisako Hashimoto,
Keiji Ueno, and Hiroshi Ogino
The formation and high reactivity of transition-metal–ele-
ment multiple bonds plays an important role in transition-
metal-catalyzed reactions, in particular, by facilitating the
cleavage and formation of usually robust bonds. Olefin
metathesis is a typical and very useful example of this type
of reaction, in which carbene complexes, which have a metal–
carbon double bond, are not only key intermediates but may
also act as high-performance catalysts.[1] In contrast to
metal–carbon multiple bonds, metal–element multiple
bonds, where the element is from the third or subsequent
row of the periodic table, have been much less widely
investigated. Among them, silylene complexes, which possess
a metal–silicon double bond, have been the most extensively
studied,[2–9] but the mechanisms of their reactions remain
rather unclear.
Both ourselves and Pannell's group have insisted, through
the generation of silyl(silylene) complexes with transition
metals from groups 6 to 9 and the preparation of their donor-
stabilized forms, that 1,2- and 1,3-group migrations of these
systems (Scheme 1) occur very easily under mild conditions,
and cause the metal-catalyzed oligomerization/deoligomeri-
zation, isomerization, and redistribution of organosilicon
Scheme 1. Illustrating the 1,2- and 1,3-group migrations in silyl(sily-
lene) complexes with metals of groups 6 to 9.
[*] Prof. H. Tobita, A. Matsuda, Dr. H. Hashimoto
Department of Chemistry, Graduate School of Science
Tohoku University, Sendai 980-8578 (Japan)
Fax: (+81)22-217-6543
E-mail: tobita@mail.tains.tohoku.ac.jp
Prof. K. Ueno
Department of Chemistry, Faculty of Engineering
Gunma University, Kiryu 376-8515 (Japan)
Prof. H. Ogino
Miyagi Study Center, The University of the Air, Sendai 980-8577
(Japan)
[**] This work was supported by the Ministry of Education, Culture,
Sports, Science and Technology, Japan [Grants-in-Aid for Scientific
Research Nos. 13440193, 14204065, and 14078202]
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DOI: 10.1002/anie.200352519
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221

Communications
compounds.[2,3,10–12] In fact, this mechanism was notably
successful in explaining the redistribution reactions of various
organosilicon, -germanium, and -phosphorus systems.[13–15] We
have previously given direct experimental evidence for 1,2-
silyl-migration from Si to the metal M (A!B or D!B’) via
isolating complexes of the type B’ or C, which are formed in
reactions of complexes of type A or D.[2,3,16] Furthermore, we
have observed fluxional behavior in the 1,3-migration of
methyl groups on an externally donor-stabilized silyl(silyl-
!
=
ene)iron complex [Cp(CO)Fe( SiMe2 HMPA)SiMe3]
(HMPA = hexamethyl phosphoramide) by variable-temper-
ature NMR spectroscopy.[17] In this process, we assumed that
the coordinated HMPA dissociates at elevated temperatures
to generate a donor-free silyl(silylene) complex. We now give
direct evidence for 1,3-alkyl migration (B!B’ and vice versa)
and 1,2-silyl migration from M to Si (B!A or B’!D) by
employing newly synthesized, donor-free silyl(silylene)iron
complexes.
Figure 1. ORTEP drawing of 3a showing thermal ellipsoids at the 50%
probability level. Selected bond lengths [Š] and angles [8]: Fe(1)-Si(1)
2.154(1), Fe(1)-Si(2) 2.343(1), Fe(1)-C(11) 1.724(4); Si(1)-Fe(1)-Si(2)
93.15, Fe(1)-Si(1)-C(12) 127.8(1), Fe(1)-Si(1)-C(21) 127.2(1), C(12)-
Si(1)-C(21) 104.3(2).
Photolysis of [Cp’Fe(CO)2Me] (1a: Cp’ = h5-C5Me5
(Cp*); 1b: Cp’ = h5-C5H5 (Cp)) in the presence of HSiMe2-
SiMes2Me (2; Mes = mesityl (2,4,6-trimethylphenyl)) pro-
duced the first donor-free silyl(silylene)iron complexes
structures of 3a and 3b lead to hindered rotation around
=
=
ꢀ
[Cp’Fe(CO)( SiMes2)SiMe3] (3a: Cp’ = Cp*, 60%; 3b:
both the Fe Si and the Si C(mesityl) bonds.
Cp’ = Cp, 38% yield, calculated by NMR spectroscopy
[Eq. (1)]). Complex 3a could be isolated as orange crystals
in 40% yield, whereas isolation of 3b was unsuccessful
Sharma and Pannell previously reported that the photol-
ysis of linear oligosilanyl–[Fe(CO)2Cp] complexes containing
more than three silicon atoms produces highly branched,
tris(silyl)silyl iron complexes in high yields, for example,
ꢀ
[{Me3Si(Me2Si)3}Fe(CO)2Cp] is converted to [(Me3Si)3Si
Fe(CO)2Cp] on irradiation.[12] In this reaction, the 1,2-silyl
migration from the Fe center to the silylene silicon atom on
the silyl(silylene) iron intermediates (corresponding to B!A
or B’!D; Scheme 1) could play an important role.
To confirm this hypothesis, thermolysis of 3a in the
presence of several two-electron-donor ligands was carried
out. As a result, when 3a was heated to 808C for 6 h in the
presence of tBuNC, a disilanyl complex [Cp*Fe(CO)(CN-
because of its extreme instability. We have previously
synthesized the tungsten analogue of 3a by a similar
method, but the chemistry has not been thoroughly inves-
tigated.[16]
The molecular structure of 3a is shown in Figure 1.[18] The
two mesityl groups are on the silylene ligand, while all of the
three methyl groups are on the silyl ligand. The iron–silylene
ꢀ
bond (Fe Si(1) 2.154(1) Š) is about 9% shorter than the
ꢀ
iron–silyl bond (Fe Si(2) 2.343(2) Š) and is the shortest
tBu)SiMesMeSiMesMe2] (4) was isolated as a main product in
25% yield [Eq. (2)]. The 29Si NMR signals of 4 appear in the
reported bond of this type.[19] The silylene silicon atom is
tricoordinate and its geometry is almost planar (sum of the
three bond angles around Si(1) = 359.3 (2)8). No intermolec-
ular bonding interaction was found. The 29Si NMR spectra of
3a and 3b show signals for the dimesitylsilylene ligand at
extremely low field (365.8 ppm for 3a and 372.0 ppm for 3b),
which is characteristic of the donor-free dialkyl- or diaryl-
silylene complexes.[5,16] Also present are the resonances for
the trimethylsilyl ligand (28.4 ppm for 3a and 31.0 ppm for
3b). These data unambiguously demonstrate the donor-free
ꢀ
normal range of disilanyl iron complexes (9.5 ppm for Fe Si
and ꢀ11.2 ppm for terminal Si atoms). The molecular
structure of 4 is shown in Figure 2.[18] A tBuNC molecule is
terminally coordinated to the iron center, and each of the a-
and b-Si atoms of the disilanyl ligand is coordinated to a
ꢀ
ꢀ
mesityl group. The Fe Si(1) and Si(1) Si(2) bond lengths are
2.4107(7) and 2.4004(9) Š, respectively, which are normal
values for single bonds.
A mechanism that can rationalize the reactions in
Equations (1) and (2) is illustrated in Scheme 2. From 1a,
successive CO dissociation, oxidative addition of 1, methane
reductive elimination, 1,2-silyl migration, and 1,3-methyl
migration occur to afford 3a. Three isomeric donor-free
silyl(silylene) complexes (3a, 3a’, and 3a’’) are in rapid
1
silyl(silylene)iron structures. In each of the H NMR spectra
of 3a and 3b, all the four o-Me groups, four m-H atoms, and
two p-Me groups in two mesityl groups are inequivalent at
room temperature. Apparently, the extremely congested
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Angewandte
Chemie
of 3a and its conversion to 4 involves 1,2-silyl migration and
1,3-alkyl and/or aryl migration processes. These are consid-
ered to be concerted processes with low energy barriers.[20]
ꢀ
Importantly, through the latter process, usually robust Si C
bonds readily cleave under extremely mild conditions: The
ꢀ
typical bond dissociation energy of the Si C single bond is
301kJmol ꢀ1, which is comparable to that of the C C single
ꢀ
bond (346 kJmolꢀ1).[21]
In this paper, we have provided the most straightforward
evidence for extremely facile 1,2- and 1,3-group migrations in
silyl(silylene) complex systems. These observations clearly
show how organosilicon species bound to a transition-metal
center can change their structures in an amazingly dynamic
ꢀ
ꢀ
fashion through extremely facile Si C and Si Si bond fission
and formation processes. A more detailed elucidation of the
dynamic behavior is underway.
Experimental Section
3a: A pentane solution (3 mL) of [Cp*Fe(CO)2Me] (1a; 1.02 g,
3.89 mmol) and HSiMe2SiMes2Me (2; 1.01 g, 2.96 mmol) in a pyrex
sample tube with a teflon vacuum valve was irradiated for 80 min with
a 450 W medium-pressure Hg lamp immersed in a water bath (48C).
The reaction mixture was degassed every 20 min by a conventional
freeze-pump-thaw cycle on a vacuum line. The reaction mixture was
filtered through a glass filter and volatiles were removed from the
filtrate under reduced pressure. The residue was recrystallized from
pentane at ꢀ308C to afford orange crystals of [Cp*Fe(CO)-
Figure 2. ORTEP drawing of 4 showing thermal ellipsoids at the 50%
probability level. Selected bond lengths [Š] and angles [8]: Fe(1)-Si(1)
2.4107(7), Fe(1)-C(11) 1.732(2), Fe(1)-C(33) 1.808(2), Si(1)-Si(2)
2.4004(9), N(1)-C(33) 1.174(3); Fe(1)-Si(1)-Si(2) 118.74(3), Si(1)-
Fe(1)-C(11) 84.23(9), Si(1)-Fe(1)-C(33) 94.92(7), C(11)-Fe(1)-C(33)
92.5(1), Fe(1)-C(33)-N(1) 173.4(2), C(33)-N(1)-C(34) 162.0(3).
( SiMes2)SiMe3] (3a) in 40% yield (0.660 g, 1.18 mmol). 1H NMR
=
(300 MHz, [D6]benzene): d = 0.59 (s, 9H, SiMe3), 1.56 (s, 15H,
C5Me5), 2.05 (s, 3H, o-Me), 2.10 (s, 3H, o-Me), 2.12 (s, 3H, o-Me),
2.15 (s, 3H, o-Me), 2.74 (s, 3H, p-Me), 3.05 (s, 3H, p-Me), 6.51(s, 1H,
m-H), 6.56 (s, 1H, m-H), 6.79 (s, 1H, m-H), 6.86 ppm (s, 1H, m-H);
13C{1H} NMR (75.5 MHz, [D6]benzene): d = 9.5 (SiMe3), 10.1
(C5Me5), 21.1 (p-Me), 23.7 (o-Me), 24.0 (o-Me), 24.7 (o-Me), 24.9
(o-Me), 93.7 (C5Me5), 128.7 (C6H2Me3), 129.2 (C6H2Me3), 138.7
(C6H2Me3), 138.9 (C6H2Me3), 139.2 (C6H2Me3), 139.3 (C6H2Me3),
142.5 (C6H2Me3), 142.8 (C6H2Me3), 145.3 (C6H2Me3), 145.6
(C6H2Me3), 220.2 ppm (CO); 29Si{1H} NMR (59.6 MHz, [D6]ben-
zene): d = 28.4 (SiMe3), 365.8 ppm (SiMes2); IR ([D6]benzene sol-
ution): n˜ = 1905 cmꢀ1 (s, nCO); MS (EI, 70 eV) 558 (M+, 8), 543
(M+ꢀCH3, 30), 515 (M+ꢀCH3-CO, 12), 73 (SiMe3, 100); elemental
analysis calcd (%) for C32H46FeOSi2: C 68.79, H 8.30; found: C 69.07,
H 8.41.
4: A toluene solution (5 mL) of 3a (0.103 g, 0.184 mmol) and
tBuNC (0.0730 g, 0.878 mmol) in a pyrex tube with a teflon vacuum
valve was heated to 808C for 6 h. After removal of volatiles, the
yellow residue was recrystallized from toluene/hexane to afford
yellow crystals of [Cp*Fe(CO)(CNtBu)SiMesMeSiMesMe2] (4) in
25% yield (0.030 g, 0.047 mmol). 1H NMR (300 MHz, [D6]benzene):
d = 0.69 (s, 3H, SiMesMe2), 0.97 (s, 3H, SiMesMe2), 1.09 (s, 3H,
SiMesMe), 1.46 (s, 15H, C5Me5), 2.15 (s, 3H, p-Me), 2.23 (s, 3H, p-
Me), 2.45 (s, 6H, o-Me), 2.52 (s, 3H, o-Me), 2.64 (s, 3H, o-Me), 6.78 (s,
2H, m-H), 6.84 (s, 1H, m-H), 6.88 ppm (s, 1H, m-H); 13C{1H} NMR
(75.5 MHz, [D6]benzene): d = 6.4 (SiMe), 6.7 (SiMe), 9.9 (C5Me5),
11.7 (SiMe), 21.1 (C6H2Me3), 25.8 (C6H2Me3), 26.8 (C6H2Me3), 28.0
(C6H2Me3), 31.2 (CMe3), 56.3 (CMe3), 92.7 (C5Me5), 128.9 (C6H2Me3),
129.1 (C6H2Me3), 129.3 (C6H2Me3), 130.1 (C6H2Me3), 136.3
(C6H2Me3), 136.9 (C6H2Me3), 137.2 (C6H2Me3), 140.3 (C6H2Me3),
144.5 (C6H2Me3), 145.3 (C6H2Me3), 176.6 (FeCN), 222.3 ppm (CO);
29Si{1H} NMR (59.6 MHz, [D6]benzene): d = ꢀ11.2 (SiMesMe2),
9.5 ppm (SiMesMe); IR ([D6]benzene solution): n˜ = 1907 cmꢀ1 (s)
(nCO); MS (EI, 70 eV) 641( M+, 0.3), 626 (M+ꢀMe, 0.6), 556
(M+ꢀCOꢀtBu, 4), 515 (M+ꢀCOꢀCNtBuꢀMe, 7), 464
=
Scheme 2. A mechanism for the formation of [Cp*Fe(CO)( SiMes2)-
SiMe3] (3a) and [Cp*Fe(CO)(CNtBu)SiMesMeSiMesMe2] (4).
equilibrium at room temperature, where 3a is the major and
only observable isomer. When this equilibrium mixture is
heated in the presence of tBuNC, 1,2-migration of the silyl
ligand onto the silylene ligand followed by coordination of
tBuNC to the unsaturated iron center occurs to produce 4. It
should be noted that both 3a and 4 take the structures that
obviously minimize the steric repulsion between the bulky
groups, namely, the two mesityl groups and a pentamethylcy-
clopentadienyl group. In other words, 3a and 4 are the
thermodynamically controlled products. Both the formation
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Communications
(M+ꢀCOꢀ2MeꢀMes, 100); elemental analysis calcd (%) for
C37H55FeONSi2: C 69.24, H 8.64, N 2.18; found: C 69.31, H 8.66, N
2.28.
Received: July 30, 2003 [Z52519]
Keywords: group migration · iron · ligands · rearrangement ·
.
transition metals
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[18] 3a: monoclinic; P21/c; a = 10.4593(2), b = 18.3566(6), c =
16.0338(4) Š,
b = 96.5724(7)8;
V= 3058.2(1) Š3;
Z = 4;
C32H46FeOSi2; T= 150 K, 26965 reflections, 6692 independent
(Rint = 0.043); R1 = 0.046 (I > 3s(I)), Rw = 0.106; m = 5.93 cmꢀ1
;
Full-matrix least-squares on F2. 4: monoclinic; P21; a =
8.8296(2), b = 20.6795(5), c = 9.9852(2) Š, b = 99.970(1)8; V=
1795.68(7) Š3; Z = 2; C37H55FeNOSi2; T= 150 K, 17660 reflec-
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Rw = 0.063; m = 5.14 cmꢀ1
;
Full-matrix least-squares on F2.
CCDC-215618 (3a) and CCDC-215619 (4) contains the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrie-
ving.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB21EZ, UK; fax: (+ 44)1223-
336-033; or deposit@ccdc.cam.ac.uk).
[19] Based on a search of the Cambridge Structural Database, CSD
version 5.24 (November 2002).
[20] K. Morokuma, private communication; Z. Liu, PhD Thesis,
Emory
University,
2000,
chap. 3.
MO
calculation
(B3LYPLANL2DZ) for the 1,3-migration of a Me group from
=
a silyl ligand to a silylene ligand in [CpFe(CO)( SiMe2)SiMe3]
showed that this is a concerted process with no intermediate and
the activation energy of this process is 12.0 kcalmolꢀ1
.
[21] J. Emsley, The Elements, 3rd ed., Oxford University Press,
Oxford, 1998.
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