Selective benzylic C-H activation of solvent toluene and m-xylene by an iron-tin cluster complex: Fe2(μ-SnBu2t)2(CO)8

Article abstract of DOI:10.1016/j.jorganchem.2009.09.024

The bimetallic cluster complex Fe₂(μ-SnBu₂^t)₂(CO)₈, 1, selectively activates the benzylic C-H bond in solvent toluene at reflux conditions to afford the complex Fe₂[μ-SnBu^t(CH₂Ph)]₂(CO)₈, 3, where two of the Bu^t groups in 1 have been replaced with benzyl groups. Similarly 1 also activates the benzylic C-H bond in solvent m-xylene to yield complexes Fe₂[μ-SnBu^t(m-CH₂PhMe)]₂(CO)₈, 4, Fe₂[μ-SnBu^t(m-CH₂PhMe)][μ-Sn(m-CH₂PhMe)₂](CO)₈, 5, and Fe₂[μ-Sn(m-CH₂PhMe)₂]₂(CO)₈, 6, where two, three and four of the Bu^t groups in 1 have been replaced by m-tolyl groups, respectively. A mechanism based on a radical pathway is proposed for the C-H activation by 1.

Full text of DOI:10.1016/j.jorganchem.2009.09.024

Journal of Organometallic Chemistry 695 (2010) 1–5
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Communication
Selective benzylic C–H activation of solvent toluene and m-xylene
by an iron–tin cluster complex: Fe2(
l
-SnBut )2(CO)8
2
Lei Zhu a, Veeranna Yempally a, Derek Isrow a, Perry J. Pellechia b, Burjor Captain a,
*
a Department of Chemistry, University of Miami, Coral Gables, FL 33124, USA
b Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 August 2009
Received in revised form 17 September 2009
Accepted 18 September 2009
Available online 27 September 2009
The bimetallic cluster complex Fe2(
l
-SnBut )2(CO)8, 1, selectively activates the benzylic C–H bond in sol-
2
vent toluene at reflux conditions to afford the complex Fe2[
But groups in 1 have been replaced with benzyl groups. Similarly 1 also activates the benzylic C–H bond
in solvent m-xylene to yield complexes Fe2[
-SnBut(m-CH2PhMe)]2(CO)8, 4, Fe2[ -SnBut
l
-SnBut(CH2Ph)]2(CO)8, 3, where two of the
l
l
(m-CH2PhMe)][l-Sn(m-CH2PhMe)2](CO)8, 5, and Fe2[l-Sn(m-CH2PhMe)2]2(CO)8, 6, where two, three
and four of the But groups in 1 have been replaced by m-tolyl groups, respectively. A mechanism based
on a radical pathway is proposed for the C–H activation by 1.
Keywords:
Tin
Iron
Ó 2009 Elsevier B.V. All rights reserved.
Bimetallic cluster
C–H activation
Radical mechanism
1. Introduction
There are also instances that demonstrate an aryl C–H to aliphatic
C–H (benzylic C–H) [3] rearrangement and vice versa [4] in iridium
The activation of the carbon–hydrogen bond in organic
molecules for the purpose of functionalization by transition metal
complexes has always been one of the fore-fronts of chemical
research [1]. Selective C–H activation is especially important when
the substrate contains both sp2 hybridized aryl C–H bonds as well
as sp3 hybridized aliphatic C–H bonds. It has been shown that tran-
sition metal complexes are capable of activating both the aryl C–H
as well as the aliphatic C–H bonds, for example see Eq. (1) [2].
complexes, for example see Eq. (2).
P
Ir
CO
P
P
C2H4
(2)3
Ir
P
C6H6
90 °C
CO
R
N
R
N
Recent studies have shown that even some cyclopentadienyl com-
plexes of Ytterium and Gadolinium can also activate C–H bonds,
Eq. (3) [5]. The most common main group elements known to met-
alate arene rings are those of complexes containing Tl [6] and Pb [7].
Tin has also been shown to selectively activate the aryl C–H bonds in
alkylaromatic compounds. For example, the complex ‘‘(CF3COO)4Sn”
[8] activates the aryl C–H bond in benzene as well as in p-xylene.
CH3
OH2
MeCN
+
+
Pt
Pt
- CH4
TFE-d3
25 °C
N
R
N
R
NCMe
R = 3,5-(CF3)2C6H3: Nf-Nf
R = 2,6-(CH3)2C6H3:N'-N'
93% (Nf-Nf)
10% (N'-N')
R
N
Cp*
Cp*
Cp*
Cp*
+
(1)1b, 2k
Pt
(3)5
Y
Cp*
Y
Cp*
C6D12
25 °C
H
N
R
NCMe
7% (Nf-Nf)
90% (N'-N')
Recently, a bimetallic Ir–Sn complex was shown to be a catalyst for
the alkylation of arenes, which proceeds via aryl C–H activation of
the arene substrate at an Ir center [9]. We have now found that
* Corresponding author. Tel.: +1 305 284 3059.
E-mail address: captain@miami.edu (B. Captain).
the bimetallic Fe–Sn cluster Fe2(
l
-SnBut )2(CO)8, 1, is capable of
2
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.09.024

2
L. Zhu et al. / Journal of Organometallic Chemistry 695 (2010) 1–5
selectively activating only the benzylic C–H bond in toluene and m-
xylene, and propose a radical mechanism for the reaction. This is
the first example of selective benzylic C–H activation of alkylaro-
matic compounds at Sn centers, and we wish to report our preli-
minary findings in this communication.
To further investigate the ability of complex 1 to activate the al-
kyl groups in alkylaromatic compounds, we investigated the reac-
tion of 1 with m-xylene. When a solution of 1 in m-xylene solvent
was heated to reflux, the compound, Fe2[l
-SnBut(m-CH2PhMe)]2
(CO)8, 4, was obtained in 20% yield, see Fig. 3. Like in compound
3, compound 4 has replaced one of the But groups on each of the
two Sn atoms with a m-tolyl (tolyl = CH2PhMe) group. The m-tolyl
groups are trans with respect to each other. In addition to 4
where two But groups were replaced, this reaction afforded com-
2. Results and discussion
The bimetallic cluster complex 1 was obtained from the reac-
tion of Fe2(CO)9 and But3SnH at 97 °C in 41% yield. Compound 1
was prepared several years ago from the reaction of Na2Fe(CO)4
and But2SnCl2 in 50% yield and its structure was formulated accu-
rately based on infrared spectroscopy and molecular weight mea-
surements [10]. We have obtained the structure of compound 1
by single crystal X-ray diffraction and its molecular structure is
shown in Fig. 1. The structure is essentially the same as the di-
pounds Fe2[
and Fe2[ -Sn(m-CH2PhMe)2]2(CO)8, 6, in 19% and 8% yields, res-
l l-Sn(m-CH2PhMe)2](CO)8, 5,
-SnBut(m-CH2PhMe)][
l
pectively. For compound 5 three of the But groups have been
replaced by m-tolyl groups, see Fig. 3, and in 6 all of the But groups
in 1 have been replaced with m-tolyl groups, see Fig. 4. The reason
multiple addition products with m-xylene solvent were obtained is
probably due to the higher boiling point of m-xylene (139 °C) to
that of toluene (110 °C).
methyl tin analog Fe2(l-SnMe2)2(CO)8, 2 [11]. The two iron atoms
0
The reaction of 1 with solvents toluene and m-xylene furnished
products which were a result of activation of the benzylic C–H bond.
We have not seen any evidence for products that may have been
formed as a result of aryl C–H activation. When the reaction of 1 in
refluxing toluene was carried out in the presence of a radical scaven-
ger TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl) for 1 h, com-
pound 3 was not formed and all of the starting 1 was consumed.
Furthermore, light is not required to generate the radical species
in solution as we found no significant difference in the rate of forma-
tion for 3 when the reaction of 1 in refluxing toluene was carried out
in the dark verses when this reaction was performed under hood
light. The effect of toluene-d8 on the reaction was investigated and
only trace amount of 3 was found. Only when this reaction was
sealed in a Parr reactor and heated to 150 °C were we able to obtain
the desired product where two of the But groups in 1 had been re-
placed by two deuterated benzyl groups. 1H NMR shows it is a mix-
ture of two isomers like compound 3, and X-ray diffraction analysis
of 3-d14 for crystals obtained from a methylene chloride/hexane sol-
vent mixture at ꢀ20 °C, revealed the structure of the cis isomer, see
SI for an ORTEP of 3-d14. This observation is consistent with the
mechanism in which C–H bond activation plays a prominent role.
To detect the formation of isobutane-d1 (D-But), compound 1
in toluene-d8 was sealed in an NMR tube under partial vacuum
and heated at 150 °C for 2 h. The 2H{1H} NMR spectrum of the
reaction mixture showed a singlet at 1.63 ppm confirming appro-
priately formation of isobutane-d1 (D-But). However, it should
be noted that the 1H NMR spectrum showed large amounts of
non-deuterated isobutane, which made it very difficult to detect
isobutane-d1. (D-But), as well as isobutylene. The observance of
non-deuterated isobutane in the reaction mixture can be ex-
plained due to the possibility that two But radicals can transform
to isobutylene and isobutane by losing and gaining one hydrogen
atom, respectively, see Eq. (4). Pryor and Tang also observed dis-
proportionation of But radicals (generated from photolysis of AIB
(azoisobutane) in neat toluene and substituted toluene) to isobu-
tene and isobutane [12]. Since the yield of the reaction is only
21%, we can account for the mass balance only by the fact that
there is a lot of decomposition, thus it is not surprising that large
quantities of isobutane and isobutylene were observed.
which are apart by 4.225(1) ÅA (4.153 (1) Å in molecule 2, there are
two molecules in the asymmetric unit) are non-bonding, and is
0
similar to the iron–iron non-bonding distance in 2, 4.139(15) ÅA.
The electron count around each iron atom is 18.
Interestingly, when the reaction of Fe2(CO)9 and But3SnH was
carried out in refluxing toluene solvent (110 °C), compound 1
was not formed, instead the new compound Fe2[l-
SnBut(CH2Ph)]2(CO)8, 3 was obtained in 4% yield. Compound 3
was characterized by a combination of IR, NMR and single crystal
X-ray diffraction analyses. An ORTEP showing the molecular struc-
ture of compound 3 is shown in Fig. 2. Compound 3 is very similar
to the structure of 1 except that one But group on each of the Sn
atoms in 1 has been replaced with a benzyl group from the solvent
toluene. Indeed when compound 1 is dissolved in toluene solvent
and heated to reflux, compound 3 is obtained as the sole metal
complex product in 21% yield. As seen in Fig. 2 the benzyl groups
are located trans to each other, however 1H NMR after TLC work-
up indicates the presence of another resonance which is attributed
to the cis isomer which we are not able to separate by chromatog-
raphy. The relative proportions of the trans and cis isomer by 1H
NMR are approximately in a 50:50 ratio. However, the trans isomer
can be obtained in pure form by fractional crystallizations at
ꢀ20 °C from a methylene chloride/hexane solvent mixture. We
have been unable to obtain a crystal structure of this cis isomer
at this time.
H3C
H3C
H3C
H3C
H3C
H3C
H3C
H3C
2
(4)
C
C
H
C
CH2 +
Fig. 1. An ORTEP of the molecular structure of Fe2(
probability thermal ellipsoids. Selected bond distances (in Å) are as follows:
(molecule 1) Sn(1)–Fe(1) = 2.7216(9), Sn(1)–Fe(2) = 2.7074(9), Sn(2)–
Fe(1) = 2.7156(9), Sn(2)–Fe(2) = 2.7288(9), (molecule 2) Sn(3)–Fe(3) = 2.7248(10),
Sn(3)–Fe(4) = 2.7236(10), Sn(4)–Fe(3) = 2.7207(11), Sn(4)–Fe(4) = 2.7106(9).
l
-SnBut )2(CO)8, 1 showing 30%
2
As possible control experiments, there was no reaction of Fe2(CO)9 in
refluxing toluene, and the same was observed when But SnCl2 was
2
refluxed in toluene, or when But3SnH was heated in toluene at
95 °C. Furthermore, when But3SnH in toluene was heated at reflux,

L. Zhu et al. / Journal of Organometallic Chemistry 695 (2010) 1–5
3
Fig. 2. An ORTEP of the molecular structure of Fe2[
1) Sn(1)–Fe(1) = 2.6749(10), Sn(1)–Fe(1) = 2.6958(11), (molecule 2) Sn(2)–Fe(2) = 2.6837(10), Sn(2)–Fe(2) = 2.6951(10).
l
-SnBut(CH2Ph)]2(CO)8, 3 showing 30% probability thermal ellipsoids. Selected bond distances (Å) are as follows: (molecule
*
*
O
O
O
O
Me
Me
C
C
C
C
C
O
O C Fe
O
Sn
O
O C Fe C
But
But
CH2
But
CH2
Sn
Sn
Sn
H2C
H2C
CH2
O
Fe
O
Fe
O
C
C
C
C
Me
Me
4
5
Me
C
C
C
C
O
O
O
O
Fig. 3. Line structures for compounds 4 and 5. See supplementary material for ORTEPs of the molecular structures of compounds 4 and 5.
Fig. 4. An ORTEP of the molecular structure of Fe2[
(molecule 1) Sn(1)–Fe(1) = 2.6340(3), Sn(1)–Fe(1) = 2.6901(3), Sn(1) –Fe(1) = 2.6901(3), Sn(1) –Fe(1) = 2.6340(3).
l
-Sn(m-CH2PhMe)2]2(CO)8, 6 showing 30% probability thermal ellipsoids. Selected bond distances (Å) are as follows:
*
*
*
*
-SnBut )(CO)4] units could transform to
no reaction was observed after 1.5 h, however, after prolonged time
mer comprised of two [Fe(
l
2
(over 3 h) the 1H NMR spectrum of the reaction mixture was full of a
the monomer by cleavage of a Fe–Sn bond. Marks et al. have shown
that homolysis of the Fe–Sn bond in 1 is facile and reversible in
coordinating solvents such as THF, pyridine, acetone and others,
while homolysis of the Fe–Sn bond was not observed in non-coor-
dinating solvents such as benzene or cyclohexane [10]. We thus
suspect that the dimer does remain intact in solution as toluene
and m-xylene are non-coordinating solvents. The next step is the
abstraction of a hydrogen atom from the methyl group by the
number of products, which we have not been able to identify. The
role of the Fe-cluster framework we believe is to provide stability
of the Sn radical that is produced in the first step by cleavage of
the Sn–C bond in the mechanism of this reaction.
Based on these observations a radical mechanism is proposed,
see Scheme 1, with the first step being homolysis of the Sn–C bond
in 1 to expel a But radical. Alternatively, compound 1 which is a di-

4
L. Zhu et al. / Journal of Organometallic Chemistry 695 (2010) 1–5
Fe
Fe
Fe
But
But
But
But
But
But
But
Sn
Sn
+
Sn
Sn
But
Fe
1
But
But
+
CH2
H
CH3
+
X
X
X = H or Me
Fe
Fe
Fe
But
But
But
But
CH2
But
Sn
Sn
+
CH2
Sn
Sn
But
X
Fe
X
Scheme 1.
But radical to form isobutane, and the benzyl or m-tolyl radical. The
presence of isobutane was detected by 1H NMR. The benzyl or
m-tolyl radical then combines with the tin radical in the Fe2Sn2
cluster complex to yield the benzylic C–H activated complex. This
process is then repeated to replace another But group with a benzyl
or m-tolyl group.
97 Hz). Elemental Anal. Calc.: C, 41.43; H, 3.71. Found: C, 41.12; H,
3.97%.
3.2. Synthesis of Fe2[
Fe2[
-SnBut(m-CH2PhMe)][
(m-CH2PhMe)2]2(CO)8, 6
l
-SnBut(m-CH2PhMe)]2(CO)8, 4,
-Sn(m-CH2PhMe)2](CO)8, 5, and Fe2[l-Sn
l
l
It is interesting that the bimetallic FeSn cluster Fe2(
l
-SnBut )2
2
(CO)8 reacts with solvents toluene and m-xylene to afford com-
pounds containing benzyl or m-tolyl groups. It is proposed that
the But radical expelled from 1 attacks the benzylic C–H bond in
toluene and m-xylene. There is evidence in the literature that But
radicals can abstract hydrogens from toluene and substituted tolu-
enes [12,13]. Thus, it should be possible to selectively activate the
benzylic C–H bond in other alkyl aromatic compounds as well. Pre-
liminary work now underway has also shown similar reactivity
with ortho and para-xylene solvent. Bimetallic catalytic systems
have attracted much attention because their catalytic properties
are often superior to that of their components [14]. It has been pro-
posed that the different metals may exhibit ‘‘synergy” or bifunc-
tional cooperativity such that one metal performs one role in a
catalytic reaction and the other performs a second function.
A 30 mg amount of Fe2(l
-SnBut )2(CO)8 (0.037 mmol) was
2
dissolved in 7 mL of m-xylene in a 100 mL 3-neck flask. The reac-
tion was heated to reflux for 100 min. The solvent was removed
in vacuo and the product was separated by TLC by using hexane
solvent to yield in order of elution 6.9 mg (yield 20%) of pale yellow
Fe2[
low Fe2[
l
-SnBut(m-CH2PhMe)]2(CO)8, 4, 6.7 mg (yield 19%) of pale yel-
l
-SnBut(m-CH2PhMe)][
l-Sn(m-CH2PhMe)2](CO)8, 5, and
3.1 mg (yield of 8.3%) of colorless Fe2[l-Sn(m-CH2PhMe)2]2(CO)8,
6. Spectral data for 4: IR mCO (cmꢀ1 in hexane): 2043 (vs), 1996
(m), 1992 (m, sh), 1979 (s). 1955 (w). 1H NMR (C6D6, in ppm,
2
300 MHz): d = 7.13–6.85 (m, 8H, Ph), 3.51 (s, 4H, CH2, JSn–H
=
2
34 Hz), 3.49 (s, 4H, CH2, JSn–H = 34 Hz), 2.20 (s, 6H, CH3), 2.15
3
(s, 6H, CH3), 1.33 (s, 18H, CH3, JSn–H = 95 Hz), 1.32 (s, 18H, CH3,
3JSn–H = 95 Hz). Note: This is a mixture of cis- and trans-isomers.
Elemental Anal. Calc.: C, 42.81; H, 4.04. Found: C, 42.99; H, 3.80%.
Spectral data for 5: IR mCO (cmꢀ1 in hexane): 2065 (w), 2045 (vs),
2006 (m), 1997 (m), 1979 (s), 1955 (w). 1H NMR (C6D6, in ppm,
3. Experimental
2
300 MHz): d = 7.13–6.84 (m, 12H, Ph), 3.44 (s, 2H, CH2, JSn–H
=
=
3
3
3.1. Synthesis of Fe2[l
-SnBut(CH2Ph)]2(CO)8, 3
35 Hz), 3.42 (s, 2H, CH2, JSn–H = 39 Hz), 3.33 (s, 2H, CH2, JSn–H
38 Hz), 2.18 (s, 9H, CH3), 1.30 (s, 9H, CH3, 3JSn–H = 96 Hz). Elemental
Anal. Calc.: C, 45.71; H, 3.83. Found: C, 45.67; H, 3.54%. Spectral
data for 6: IR mCO (cmꢀ1 in hexane): 2068 (w), 2049 (vs), 2009
A 30 mg amount of Fe2(CO)9 (0.082 mmol), 50 mg amount of
But3SnH (0.17 mmol) and 10 mL of toluene were charged into a
50 mL 3-neck flask. The reaction was heated to reflux for 3 h, at
which time IR showed complete consumption of the starting mate-
rial. The solvent was removed in vacuo and the product was sepa-
rated by TLC by using hexane solvent to yield 3.1 mg (yield 4%) of
colorless product, 3. 1H NMR indicates the presence of another res-
onance which is attributed to the cis isomer, which we are not able
(m), 2003 (m), 1979 (s). 1955 (w). 1H NMR (C6D6, in ppm,
2
300 MHz): d = 7.13–6.85 (m, 16H, Ph), 3.28(s, 8H, CH2, JSn–H
=
38.6 Hz), 2.17(s, 12H, CH3). Elemental Anal. Calc.: C, 48.34; H,
3.65. Found: C, 48.62; H, 3.12%.
Acknowledgement
to separate by chromatography. IR m
CO (cmꢀ1 in hexane) for the mix-
ture: 2044 (vs), 1998 (m), 1979 (s). By means of fractional crystalli-
This research was supported by Start-up Funds provided by the
University of Miami.
zations at ꢀ20 °C from a methylene chloride/hexane solvent
mixture, 5 mg of pure trans-Fe2[l
-SnBut(CH2Ph)]2(CO)8 is separated
from 40 mg of the mixture. Spectral data for trans-3: IR
m
CO (cmꢀ1 in
Appendix A. Supplementary data
hexane): 2044 (vs), 1997 (m), 1992 (w), 1980 (s). 1H NMR (toluene-
d8, in ppm, 400 MHz): d = 7.27–6.98 (m, 10H, Ph), 3.44(s, 4H, CH2,
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.09.024.
3
3
2JSn–H = 34 Hz), 1.31(s, 18H, CH3, J117Sn–H = 94 Hz, J119Sn–H
=

L. Zhu et al. / Journal of Organometallic Chemistry 695 (2010) 1–5
5
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