Photochemical reactions of [(η5-C5R5)Re(CO)3] (R = Me or H) with aryl halides in benzene: Stoichiometric formation of biphenyl

Article abstract of DOI:10.1002/ejic.201402793

Photolysis of [Cp ??Re(CO)₃] (Cp? = η⁵-C₅Me₅) with bromobenzene or chlorobenzene in benzene results in biphenyl and the corresponding rhenium hydride halide complex trans-[Cp?Re(CO)₂H(X)], which further converts into the dihalide complexes cis/trans-[Cp?Re(CO)₂X₂] (X = Br, Cl). Additional experiments, including isotopic labeling experiments, indicate that one phenyl ring in biphenyl comes from the aryl halide and the other one comes from solvent benzene. Photolysis of [CpRe(CO)₃] (Cp = η⁵-C₅H₅) with aryl halide in benzene similarly generates biphenyl, the corresponding rhenium hydride halide complex. Furthermore, the reaction also affords small amounts of rhenium dihydride complex [CpRe(CO)₂H₂]. Plausible mechanisms for the formation of the different types of products are proposed that involve intermolecular C-X and C-H activation. Molecular structures of trans-[Cp?Re(CO)₂(Br)(Cl)] (trans-6) and trans-[CpRe(CO)₂(Br)(Cl)] (trans-12) determined by X-ray diffraction are also presented.

Full text of DOI:10.1002/ejic.201402793

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DOI:10.1002/ejic.201402793
Photochemical Reactions of [(η5-C5R5)Re(CO)3] (R = Me
or H) with Aryl Halides in Benzene: Stoichiometric
Formation of Biphenyl
Bolin Zhu,*[a] Xinwei Huang,[a] and Xiaoting Hao[a]
Keywords: C–H activation / Rhenium / Photolysis / Halides / Isotopic labeling
Photolysis of [Cp*Re(CO)3] (Cp* = η5-C5Me5) with bromo-
benzene or chlorobenzene in benzene results in biphenyl
and the corresponding rhenium hydride halide complex
trans-[Cp*Re(CO)2H(X)], which further converts into the di-
ide in benzene similarly generates biphenyl, the correspond-
ing rhenium hydride halide complex. Furthermore, the reac-
tion also affords small amounts of rhenium dihydride com-
plex [CpRe(CO)2H2]. Plausible mechanisms for the formation
halide complexes cis/trans-[Cp*Re(CO)2X2] (X = Br, Cl). Ad- of the different types of products are proposed that involve
ditional experiments, including isotopic labeling experi- intermolecular C–X and C–H activation. Molecular structures
ments, indicate that one phenyl ring in biphenyl comes from
of trans-[Cp*Re(CO)2(Br)(Cl)] (trans-6) and trans-[CpRe-
the aryl halide and the other one comes from solvent benz- (CO)2(Br)(Cl)] (trans-12) determined by X-ray diffraction are
ene. Photolysis of [CpRe(CO)3] (Cp = η5-C5H5) with aryl hal-
also presented.
Sutton et al. reported that photolysis of [Cp*Re(CO)3] with
allyl chloride affords the η3-allyl chloride complex
[Cp*Re(η3-C3H5)(CO)Cl], whereas the similar reaction with
allyl bromide results in the dibromide complex [Cp*Re(CO)2-
Br2].[12e] In both cases, the dicarbonylrhenium dihalide
complexes were generated, but the formation mechanism
has never been discussed. To better understand this type of
reaction, we have studied the photochemical reactions of
[(η5-C5R5)Re(CO)3] (R = H or Me) with aryl halides in
benzene in this paper and proposed the plausible mecha-
nism for the generation of the different types of rhenium
complexes and an unexpected organic product, biphenyl.
Introduction
The chemistry of the “classic” transition-metal com-
plexes [(η5-C5R5)Re(CO)3] [R = H (Cp) or Me (Cp*)] have
been extensively studied ever since they were synthesized
half a century ago.[1] In addition to the interesting photo-
chemical reactivity of [(η5-C5R5)Re(CO)3] itself in different
solvents,[2] their application has also received much atten-
tion as useful precursors to produce complexes of the type
[(η5-C5R5)Re(CO)2L] when they are irradiated in the pres-
ence of PR3,[3] alkene,[4] and especially THF. [(η5-C5R5)-
Re(CO)2(thf)] has been used as a common starting material
in various kinds of coordination reactions.[5] More import-
antly, these complexes could be used in the catalytic
borylation of alkanes;[6] activation of the C–H bond in
alkanes[7] or cycloalkanes;[8] activation of the Si–H bond in
silanes;[9] and activation of the C–H,[10] C–F,[11] or C–X (X
= Cl, Br)[12] bond in aryl halides and allyl halides. Among
these prior investigations, Klahn et al. reported that longer
irradiation time of [(η5-C5R5)Re(CO)3] in neat aryl chloride
or bromide or their saturated solution in hexane would gen-
erate the dihalide complex [(η5-C5R5)Re(CO)2X2] in ad-
dition to the normal product [(η5-C5R5)Re(CO)2(Ar)X]
formed by the oxidative addition of the C–X bond.[12a,12d]
Results and Discussion
Photolysis of [Cp*Re(CO)3] with Bromobenzene in Benzene
Photolysis of [Cp*Re(CO)3] (λmax = 365 nm) with
bromobenzene (5 equiv.) in benzene under N2 at room tem-
perature for 2 h produces a yellow solution. After removal
of the solvent under vacuum, the residue was carefully sepa-
rated in air by column chromatography on silica gel, and
biphenyl (97% yield based on Re), and two dibromide com-
plexes, trans-[Cp*Re(CO)2Br2] (trans-1, major) and cis-
[Cp*Re(CO)2Br2] (cis-1, minor), were obtained successively
(Scheme 1; Table 1, Entry 1). To determine whether any air-
sensitive species formed in the photolysis, an NMR spec-
troscopy tube reaction of [Cp*Re(CO)3] with bromobenz-
ene in C6H6/C6D6 (1:1) was conducted. After irradiation for
3 h, a singlet at δ = 1.60 ppm for methyl protons and a
singlet at δ = –10.02 ppm for the Re–H proton were ob-
[a] Tianjin Key Laboratory of Structure and Performance for
Functional Molecules, Key Laboratory of Inorganic–Organic
Hybrid Functional Material Chemistry,
Ministry of Education, College of Chemistry,
Tianjin Normal University,
Tianjin 300387, People’s Republic of China
E-mail: hxxyzbl@gmail.com
http://59.67.75.245/college/hxxy/skin/one/index.asp
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402793.
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Scheme 1.
served from the 1H NMR spectrum of the reaction mixture, the absence of product trans-2 when the separation was car-
in addition to the peaks for the trace amounts of dibromide ried out by routine column chromatography in air. The final
complexes trans- and cis-1 and the unreacted [Cp*Re- products, the rhenium dibromide complexes trans- and cis-
1
(CO)3] when comparing their H NMR spectra with those 1, are supposed to come from the further reaction of the
of authentic samples (see Figure S1 in the Supporting Infor- rhenium hydride bromide complex trans-2 when exposed to
1
mation). On the basis of the reported H NMR spectro- air.
scopic data, this compound was assigned as the rhenium
hydride bromide complex trans-[Cp*Re(CO)2H(Br)] (trans-
Photolysis of [Cp*Re(CO)3] with Chlorobenzene in Benzene
1
2) [literature[13] values: H NMR (C6D6): δ = –9.98 (s, 1 H,
Re–H), 1.64 (s, 15 H, Cp*) ppm]. Sutton et al. previously
reported the synthesis of trans-2 by the reaction of
[Cp*Re(CO)2(N2)] with HBr under scrupulously anhydrous
conditions and demonstrated that the product readily reacts
even in the presence of traces of water to give the corre-
sponding dibromide products 1.[13] Cowley and Richmond
et al. also described that this compound clearly begins to
decompose after 6 h at room temperature even in a sealed
tube.[14] These results might provide a good explanation for
Similarly, photolysis of [Cp*Re(CO)3] with chloro-
benzene (5 equiv.) in benzene for 4 h afforded biphenyl
(78% yield based on Re) and two dichloride complexes,
trans-[Cp*Re(CO)2Cl2] (trans-3, major) and cis-[Cp*Re-
(CO)2Cl2] (cis-3, minor) (Scheme 1; Table 1, Entry 2).
Again, an NMR spectroscopy tube reaction of [Cp*Re-
(CO)3] with chlorobenzene in C6H6/C6D6 (1:1) was carried
out. After irradiation for 3 h, two distinct resonances for
methyl protons and an Re–H proton at δ = 1.60 and
1
–9.15 ppm, respectively, were observed from the H NMR
Table 1. Photolysis of [(η5-C5R5)Re(CO)3] with aryl halides in
spectrum of the reaction mixture in addition to the peaks
for the unreacted [Cp*Re(CO)3] and trace amounts of by-
products. This new compound was similarly assigned as the
rhenium hydride chloride complex [Cp*Re(CO)2H(Cl)] (4),
like its bromide analogue 2 (Scheme 1). To confirm the
identification of 4, we tried to synthesize it by a different
method. However, to the best of our knowledge, complex 4
has never been synthesized and fully characterized before.
We prepared the dichloride complex trans-3 and the di-
hydride complex trans-[Cp*Re(CO)2H2] (trans-5) by pub-
lished procedures,[15a,23] mixed them in a 1:1 ratio in C6D6,
and irradiated the mixture with UV light for 2 h to check
whether the chloride exchanges with the hydride. From the
1H NMR spectrum of the reaction mixture, the same two
resonances were observed at δ = 1.60 and –9.15 ppm in a
perfect integration ratio of 15:1, in addition to the peaks for
the dichloride complexes trans- and cis-3, and the dihydride
complexes trans- and cis-5 (Scheme 2). Much longer irradi-
ation times did not make trans-3 and trans-5 completely
convert to complex 4, but they did produce equilibrium
mixtures, as evidenced by integration of the Cp* resonances
benzene.[a]
Entry
R
RЈ
X
t [h]
Product[b]
Yield[c] [%]
1
2
3
4
5
6
7
8
Me
Me
H
H
Me
Me
H
H
H
H
p-CF3
p-Br
Br
Cl
Br
Cl
Br
Br
Br
2
4
2
biphenyl
biphenyl
biphenyl
97
78
91
71
95
96
85
92
82
85
88
96
4
biphenyl
2.5
2.5
2.5
2.5
3
p-CF3-biphenyl
p-Br-biphenyl
p-MeO-biphenyl
m-MeO-biphenyl
biphenyl
Me p-MeO
Me m-MeO Br
Me
Me
Me
Me
9[d]
10[e]
11[f]
12[g]
H
H
H
H
Br
Br
Br
Br
2.5 2,4,6-Me3-biphenyl
1.5
5
2,5-Me2-biphenyl
biphenyl
[a] Reaction conditions: irradiation of [(η5-C5R5)Re(CO)3]
(0.10 mmol) and Ar–X (0.50 mmol) in benzene solvent (10 mL)
with a 500 W high-pressure Hg lamp (λmax = 365 nm) in an ice/
water bath. [b] Biphenyl products were isolated by routine silica gel
column chromatography and identified by comparing their respec-
tive 1H NMR spectra with authentic samples. [c] Isolated yield
{based on unrecovered [(η5-C5R5)Re(CO)3]}. [d] Addition of
(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) (0.20 mmol).
[e] Using mesitylene as solvent instead of benzene. [f] Using p-xyl-
ene as solvent instead of benzene. [g] [(η5-C5R5)Re(CO)3]
(1.0 mmol) and Ar–X (5.0 mmol) in benzene (50 mL).
1
in the H NMR spectrum. Interestingly, complex 4 exists
only as one isomer. Since complex 4 could not be separated
efficiently, the reaction mixture was treated with CHBr3 to
transform the hydride complexes into the corresponding
bromide complexes.[11a] By using column chromatography,
the expected complex [Cp*Re(CO)2(Br)(Cl)] (trans-6) was
separated as well as the dichloride complexes trans-3 and
the dibromide complexes trans-1; only traces of cis-3 and
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Scheme 2.
cis-1 were obtained. In general, the dicarbonylrhenium di- upon mild heating, which indeed yielded the expected di-
halide complex undergoes the isomerization under UV light bromide product trans-1 quantitively; however, no halogen
or upon mild heating to give a mixture of cis and trans exchange was observed when mixing dichloride compound
isomers, and the trans isomer is always the main form.[15] trans-3 with CHBr3 upon heating.
To verify the isomerization of the dihydride complex trans-
The molecular structure of trans-6 was determined by X-
5, irradiation of trans-5 in C6D6 was carried out, which af- ray single-crystal diffraction (Figure 1). Two CO ligands are
forded less cis-5 after 20 min. This result indicated that located in trans configuration. The Re–Br bond length is
cis/trans isomerization indeed occurred for trans-5 under 2.558(9) Å, and the [Re–Cp*(centroid)] distance is 1.958 Å,
the photochemical conditions.[16] In addition, to confirm which are consistent with those values [2.579(2) and
whether the rhenium hydride complex converts into the cor- 1.945(7) Å] in the dibromide analogue trans-[Cp*Re(CO)2-
responding rhenium bromide in the presence of CHBr3, a Br2].[15a]
separate reaction of trans-5 with CHBr3 was carried out
Photolysis of [CpRe(CO)3] with Bromobenzene or
Chlorobenzene in Benzene
To develop a wider generality of the above reactions,
photoreaction of [CpRe(CO)3] with bromobenzene or chlo-
robenzene in benzene was also explored. Similarly, UV irra-
diation of [CpRe(CO)3] with aryl halide (5 equiv.) in benz-
ene under the above conditions for 2–4 h produced a yellow
solution. After removal of the solvent, the residue was sepa-
rated in air by column chromatography on silica gel; bi-
phenyl (91% for bromobenzene, 71% for chlorobenzene,
yields based on Re) and two corresponding dihalide com-
plexes, trans-7 or -10 (major) and cis-7 or -10 (minor), were
obtained. In addition, the reaction also afforded small
amounts of dihydride complex trans-9, which is different
from the photolysis of [Cp*Re(CO)3] with aryl halides
(Scheme 3; Table 1, Entries 3 and 4). When the reaction of
[CpRe(CO)3] with bromobenzene was performed in an
NMR spectroscopy tube using C6H6/C6D6 (1:1) as solvents
1
and irradiated for 3 h, the H NMR spectrum of the reac-
Figure 1. Thermal ellipsoid drawing of [Cp*Re(CO)2(Br)(Cl)]
tion mixture exhibited a singlet at δ = 4.31 ppm for the Cp
protons and a singlet at δ = –10.08 ppm for the hydride
resonance in addition to the peaks for the aforementioned
complexes and unreacted [CpRe(CO)3] when comparing
their 1H NMR spectra with those of authentic samples (see
Figure S2 in the Supporting Information). This new species
(trans-6) showing the labeling scheme and 50% probability ellip-
soids. Hydrogen atoms are omitted for clarity. Selected bond
lengths [Å] and angles [°]: Re(1)–C(11) 1.960(5), Re(1)–C(12)
1.969(6), Re(1)–Br(1) 2.558(9), Re(1)–Cl(1) 2.504(15), Re(1)–
Cp(centroid) 1.958; Re(1)–C(11)–O(1) 175.5(5), Re(1)–C(12)–O(2)
176.1(5), Cl(1)–Re(1)–C(11) 75.7(5), C(11)–Re(1)–Br(1) 77.3(2),
Br(1)–Re(1)–C(12) 79.0(2), C(12)–Re(1)–Cl(1) 80.2(5).
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was similarly assigned as the rhenium hydride bromide the dichloride complexes trans-10. Only a trace of cis-7 and
complex [CpRe(CO)2H(Br)] (8).[17] Again, an NMR spec- cis-10 was obtained, again because the cis isomers are al-
troscopy tube reaction of [CpRe(CO)3] with chlorobenzene ways the minor form in the equilibrium of isomerization
1
in C6H6/C6D6 (1:1) was carried out. From the H NMR under UV light (Scheme 4).[15]
spectrum of the reaction mixture, two distinct resonances
The molecular structure of trans-12 was determined by
for the Cp protons and the Re–H proton at δ = 4.32 and X-ray single-crystal diffraction (Figure 2); it is similar to
–9.18 ppm, respectively, were observed, in addition to the that of trans-6. The Re atom exhibits a four-legged piano-
peaks for the aforementioned products and unreacted stool geometry, and two CO ligands are located in trans
[CpRe(CO)3]. Also this new compound was assigned as the configuration.
rhenium hydride chloride complex [CpRe(CO)2H(X)] (11),
like its bromide analogue 8 (Scheme 3).
Scheme 3.
The further conformation of 11 was similarly made by
the photolysis of the mixture of dichloride complex trans-
1
10 and dihydride complex trans-9 in C6D6. From the H
NMR spectrum of the reaction mixture, the same two reso-
nances were observed at δ = 4.32 and –9.18 ppm in an inte-
gration ratio of 5:1 in addition to the peaks for the dichlo-
Figure 2. Thermal ellipsoid drawing of [CpRe(CO)2(Br)(Cl)] (trans-
ride complexes trans- and cis-10, and the dihydride complex
trans- and cis-9 (Scheme 4). In a similar manner the above
mixture was treated with CHBr3 to transform the hydride
complexes into the corresponding bromide complexes. The
expected complex trans-[CpRe(CO)2(Br)(Cl)] (trans-12) was
12) showing the labeling scheme and 30% probability ellipsoids.
Hydrogen atoms are omitted for clarity. Selected bond lengths [Å]
and angles [°]: Re(1)–C(6) 1.941(15), Re(1)–C(7) 1.968(16), Re(1)–
Br(2) 2.51(2), Re(1)–Cl(1) 2.41(3), Re(1)–Cp(centroid) 1.943;
Re(1)–C(6)–O(1) 178.2(14), Re(1)–C(7)–O(2) 177.4(13), Cl(1)–
Re(1)–C(7) 77.4(12), C(7)–Re(1)–Br(2) 80.8(6), Br(2)–Re(1)–C(6)
separated, as well as the dibromide complexes trans-7 and 79.3(7), C(6)–Re(1)–Cl(1) 73.5(9).
Scheme 4.
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Scope Extension and Mechanism Discussion
[Cp*Re(CO)3].[4] Then the two Ph groups in C are coupled
to give Ph–Ph and the intermediate D. In the final step,
recoordination of CO to D results in the Re product E,
which is unstable when exposed to air and converts into the
final product F. In addition, the above explanation for the
formation of intermediate C would involve an ReV oxidative
level and require the loss of a second CO ligand, which
seems less common, and the intermediate C has never been
isolated or observed during the reaction, so other possible
pathways in which intermediate B converts into E should
not be ruled out, such as a radical aromatic substitution
that involves the aromatic radical with the solvent benz-
ene.[19]
To further support this proposed mechanism, we tried to
isolate the intermediate B by a different method; however,
we failed to obtain the product [Cp*Re(CO)2Ph(Cl)] from
the photolysis of [Cp*Re(CO)3] in neat chlorobenzene or its
solution in hexane. Therefore, we prepared the analogous
compound trans-[CpRe(CO)2(2,3,4,5-Cl4C6H)Cl] according
to a literature method[12d] and studied its photochemical
reaction in benzene. As expected, the photolysis afforded
the rhenium dichloride product trans-10, the hydride chlor-
ide complex 3, and trace amounts of dihydride complex
trans-9, as well as 2,3,4,5-tetrachlorobiphenyl. It is worth
mentioning that the reaction also gave small amounts of
cross-coupling product pentachlorobenzene (Scheme 6).
The most interesting part in the above photoreactions
of [(η5-C5R5)Re(CO)3] with aryl halides in benzene is the
generation of biphenyl. To gain insight into the reaction
mechanism, addition experiments were carried out. At first,
to investigate the source of biphenyl, two experiments were
conducted. One was photolysis of [Cp*Re(CO)3] with a
series of substituted bromobenzene in C6H6, which gener-
ated the corresponding monosubstituted biphenyl, absol-
utely no biphenyl, or disubstituted biphenyl (Table 1, En-
tries 5–8). The second was photolysis of [Cp*Re(CO)3] with
4-bromoanisole in C6D6, which only provided the 4-[D5]-
methoxybiphenyl (p-CH3O–C6H4–C6D5). These results re-
veal that one phenyl ring in the resulting biphenyl comes
from the aryl halide and the other one comes from benzene
as solvent.
Similar photochemical reactions of [Cp*Re(CO)3] with
substituted chlorobenzene in hexane were reported by
Klahn and co-workers,[12] which yielded trans-[Cp*Re(CO)2-
(Ph)Cl] by activation of the C–Cl bond with the photo-
generated rhenium fragment {Cp*Re(CO)2}. Furthermore,
in terms of average bond dissociation energies, the C–Cl
bond energy on chlorobenzene (95 kcalmol–1) and C–Br
bond energy on bromobenzene (84 kcalmol–1) are much
lower than that of the C–H bond on benzene
(110 kcalmol–1).[18] Based on the experimental results and
theoretical values mentioned above, a plausible mechanism
for the formation of the biphenyl is proposed in Scheme 5.
First, CO is lost from [(η5-C5R5)Re(CO)3] under UV irradi-
ation to give the 16e– unsaturated species A; this is followed
by oxidative addition of a C–X bond of the aryl halide (Ph–
X) to form B (ReIII). In the next step, another CO is lost
followed by oxidative addition of a C–H bond of benzene
to B, which results in intermediate C (ReV). The reason that
the oxidative addition of the aryl halide takes place first is
mainly due to the weaker dissociation energy of the C–X
bond relative to the C–H bond. In the structure of C, the Re
atom exhibits a five-legged piano-stool geometry; a similar
intermediate has never been proposed in the photochemical
reaction of B2Pin2 (Pin = 4,4,4Ј,4Ј,5,5,5Ј,5Ј-octamethyl-
2,2Ј-bi-1,3,2-dioxaborolane) with alkanes catalyzed by
Scheme 6.
To further understand the reaction mechanism, a similar
photolysis of [Cp*Re(CO)3] with bromobenzene in benzene
was conducted in the presence of the free-radical scavenger
TEMPO, which also afforded the corresponding dibromo
product 9 and biphenyl in slightly lower yield (82%)
(Table 1, Entry 9). This result suggested that the reaction
might involve the radical mechanism, but the resulting radi-
cals have not diffused adequately in the transition state and
therefore recombined quickly before they reacted with
TEMPO, so the radical scavenger did not affect the reaction
significantly. Arndtsen and Bengali et al. have just proposed
a similar mechanism in oxidative addition reactions of
dichloromethane to [(η5-C5R5)Re(CO)3] (R = H or Me).[20]
In addition, a competition experiment was performed to
study the kinetic isotopic effect. The photolysis of
[Cp*Re(CO)3] with 4-bromobenzotrifluoride in 50% of
C6H6 and 50% of C6D6 was carried out, which gave a small
kH/kD value of about 1.02 (by 1H NMR spectroscopic
analysis of the mixtures of p-CF3–C6H4–C6H5 and p-CF3–
C6H4–C6D5). Therefore, the C–H bond cleavage is not the
rate-determining step.
Scheme 5.
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[CpRe(CO)2H2],[17a,22] trans/cis-[Cp*Re(CO)2X2] (X = Cl, Br),[15a]
and trans/cis-[Cp*Re(CO)2H2][23] were prepared according to litera-
ture methods.
The only difference between the photolysis of
[Cp*Re(CO)3] and [CpRe(CO)3] with aryl halides was that
the rhenium dihydride product was obtained in the latter
reaction. On the basis of the mechanism we proposed in
Scheme 5, we consider that the formation of the rhenium
dihydride compound involves two C–H bond activations on
the solvent benzene, without C–X bond activation on aryl
halide. This hypothesis is supported by two blank experi-
ments: the photolysis of [CpRe(CO)3] and [Cp*Re(CO)3] in
neat benzene (50% of C6H6 and 50% of C6D6), respectively.
The former reaction indeed affords small amounts of rhe-
nium dihydride compound trans-9 and biphenyl, whereas
the latter mainly affords the products [Cp*Re(CO)2(η2-
C6H6)] and [Cp*Re(η6-C6H6)], which is basically consistent
with the results Pasman et al. had reported previously.[2]
In the end, we replaced the solvent benzene with mesityl-
ene and p-xylene, and carried out a similar photolysis of
[Cp*Re(CO)3] with bromobenzene to see which kind of C–
H bond is activated: the C(sp2)–H bond on the phenyl ring
or the C(sp3)–H bond on the methyl group in the solvents.
We observed the formation of 2,4,6-trimethylbiphenyl
(85%) and 2,5-dimethylbiphenyl (88%), respectively. It is
apparent that the C(sp2)–H bond of the solvent is selectively
activated when compared to the C(sp3)–H bond (Table 1,
Entries 10 and 11).
Photolysis of [Cp*Re(CO)3] with Bromobenzene in Benzene:
[Cp*Re(CO)3] (41 mg, 0.10 mmol) and bromobenzene (79 mg,
0.50 mmol) were dissolved in benzene (10 mL). The resulting solu-
tion was bubbled with N2 for 10 min and then irradiated for 2 h.
The solvent was pumped off from the resulting yellow solution,
and the residue was chromatographed on silica gel with petroleum
ether as eluant; a colorless band was eluted and collected. Then an
orange band was eluted with petroleum ether/CH2Cl2 (3:1). Finally,
another orange band was eluted with pure CH2Cl2. After vacuum
removal of the solvents from the above three eluents, the first frac-
tion gave 11 mg (97%) of biphenyl as a white solid, the second
fraction gave 26 mg (48%) of trans-1 as a red solid, and the final
fraction afforded a trace amount of cis-1 as a red solid. All prod-
ucts were confirmed by comparison of their 1H NMR spectra with
those of authentic samples.
Photolysis of [Cp*Re(CO)3] with Chlorobenzene in Benzene:
[Cp*Re(CO)3] (41 mg, 0.10 mmol) and chlorobenzene (56 mg,
0.50 mmol) were dissolved in benzene (10 mL). The resulting solu-
tion was bubbled with N2 for 10 min and then irradiated for 4 h.
By using the same separation method described above, the first
fraction gave 12 mg (78%) of biphenyl as a white solid, the second
fraction gave 10 mg (23%) of trans-3 as a red solid, and the final
fraction afforded a trace amount of cis-3 as a red solid. All prod-
ucts were confirmed by comparison of their 1H NMR spectra with
those of authentic samples.
Photolysis of trans-[Cp*Re(CO)2Cl2] (trans-3) and trans-
[Cp*Re(CO)2H2] (trans-5) in C6D6: Compounds trans-3 (13 mg,
0.03 mmol) and trans-5 (11 mg, 0.03 mmol) were dissolved in C6D6
(1.0 mL). The resulting solution was bubbled with N2 for 10 min
and then irradiated for 3 h. On the basis of 1H NMR spectroscopic
analysis of the mixtures, the ratio among the three rhenium hy-
drides trans-5/cis-5/4 was ca. 2.1:1.0:5.7. CHBr3 (76 mg,
0.30 mmol) was then added, and the mixture was kept at 45 °C for
6 h. The solvent was pumped off from the resulting red solution.
By using the same separation method described above, the first
fraction gave 6 mg of trans-1 as a red solid, the second fraction
gave 13 mg of trans-6 as a red solid, the third fraction gave 5 mg
of trans-3 as a red solid, and the final two fractions afforded trace
amounts of cis-1 and cis-3 as red solids. Compound trans-6: 1H
NMR (400 MHz, CDCl3): δ = 1.94 (s, 15 H, Me) ppm. 13C NMR
(100 MHz, CDCl3): δ = 187.30 (CO), 105.70 (Cp–C), 9.92 (Me)
Conclusion
Once we had shown that the photochemical reactions of
[Cp*Re(CO)3] with aryl halides in benzene could generate
biphenyl almost stoichiometrically, and the studies con-
firmed that one phenyl ring in the resulting biphenyl comes
from the aryl halide and the other phenyl ring comes from
the solvent benzene, we tried to scale up the photolysis of
[Cp*Re(CO)3] with bromobenzene (Table 1, Entry 12). It
still gave biphenyl in almost quantitive yield (96%). Appar-
ently, the above reaction still performs well on a larger scale.
In addition, plausible mechanisms for the formation of the
different types of products are proposed that involve C–X
and C–H activation. Activation of other inert bonds and
the development of a catalytic system with [Cp*Re(CO)3]
are in progress.
ppm. IR (νCO, KBr): ν = 2043 (s), 1967 (s) cm–1. C H BrClO Re
˜
12 15
2
(492.81): calcd. C 29.25, H 3.07; found C 99.27, H 3.14. All other
1
products were confirmed by comparison of their H NMR spectra
with those of authentic samples.
Experimental Section
Photolysis of [CpRe(CO)3] with Bromobenzene in Benzene:
[CpRe(CO)3] (34 mg, 0.10 mmol) and bromobenzene (79 mg,
0.50 mmol) were dissolved in benzene (10 mL). The resulting solu-
tion was bubbled with N2 for 10 min and then irradiated for 2 h.
By using the same separation method described above, the first
fraction gave 14 mg (91%) of biphenyl as a white solid, the second
fraction gave a trace amount of trans-9 as a white solid, the third
fraction gave 21 mg (45%) of trans-7 as a red solid, and the final
fraction afforded a trace amount of cis-7 as a red solid. All prod-
ucts were confirmed by comparison of their 1H NMR spectra with
those of authentic samples.
General Procedures: Schlenk and vacuum-line techniques were em-
ployed for all manipulations. All solvents were distilled from appro-
priate drying agents under nitrogen prior to use. 1H and 13C NMR
spectra were recorded with a Bruker AV400 instrument at room
temperature with TMS as internal standard. IR spectra were re-
corded as KBr disks with a Nicolet 560 ESP FTIR spectrometer.
Elemental analyses were performed with a Perkin–Elmer 240C ana-
lyzer. Photochemical reactions were carried out at room tempera-
ture with a water-jacketed 500 W high-pressure Hg lamp (λmax
=
365 nm) as the UV source. The irradiation was conducted in a
quartz tube, which was placed adjacent to the lamp, or in a 5 mm
Pyrex NMR spectroscopic tube. Complexes [(η5-C5R5)Re(CO)3] (R
= H, Me),[21] trans/cis-[CpRe(CO)2X2] (X = Cl, Br),[15a,22] trans/cis-
Photolysis of [CpRe(CO)3] with Chlorobenzene in Benzene:
[CpRe(CO)3] (34 mg, 0.10 mmol) and chlorobenzene (56 mg,
0.50 mmol) were dissolved in benzene (10 mL). The resulting solu-
Eur. J. Inorg. Chem. 0000, 0–0
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Table 2. Crystal data and summary of X-ray data collection for trans-6 and trans-12.
trans-6
trans-12
Empirical formula
Mr
Crystal system
Space group
a [Å]
b [Å]
c [Å]
C12H15BrClO2Re
492.80
monoclinic
P21/c
8.4426(10)
12.3459(16)
13.5408(17)
90
102.272(2)
90
C7H5BrClO2Re
422.67
triclinic
¯
P1
6.858(5)
8.551(6)
8.776(6)
90.038(14)
90.046(14)
112.001(12)
477.2(6)
2
α [°]
β [°]
γ [°]
V [Å3]
1379.1(3)
4
2.373
Z
Dcalcd. [gcm–3]
2.941
17.152
μ [mm–1]
11.889
F(000)
920
380
Crystal size [mm]
Max. 2θ [°]
No. of reflns. collected
No. of indep. reflns (Rint
No. of params.
GoF on F2
R1, wR2 [IϾ2σ(I)]
R1, wR2 (all data)
0.18ϫ0.17ϫ0.06
50.02
7688
2423 (0.0462)
166
1.066
0.0307, 0.0764
0.0332, 0.0784
1.689/–3.252
0.16ϫ0.15ϫ0.12
50.00
2718
1692 (0.0533)
107
1.110
0.0632, 0.1774
0.0709, 0.1866
3.212/–3.066
)
Largest diff. peak/hole [eÅ–3]
tion was bubbled with N2 for 10 min and then irradiated for 4 h.
By using the same separation method described above, the first
fraction gave 11 mg (71%) of biphenyl as a white solid, the second
fraction gave 5 mg (16%) of trans-9 as a white solid, the third frac-
tion gave 10 mg (27%) of trans-10 as a red solid, and the final
fraction afforded a trace amount of cis-10 as a red solid. All prod-
ucts were confirmed by comparison of their 1H NMR spectra with
those of authentic samples prepared according to the published
procedures.
zation from n-hexane/CH2Cl2 (1:1). Data collection was performed
with a Bruker SMART 1000 using graphite-monochromated Mo-
Kα radiation (ω–2θ scans, λ = 0.71073 Å). Semiempirical absorp-
tion corrections were applied for all complexes. The structures were
solved by direct methods and refined by full-matrix least-squares
procedures. For all calculations the SHELXTL-97 program system
was used.[24] The crystal data and summary of X-ray data collection
are presented in Table 2. CCDC-1018975 (6) and -1018976 (12)
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
Photolysis of trans-[CpRe(CO)2Cl2] (trans-10) and trans-[CpRe-
(CO)2H2] (trans-9) in C6D6: trans-10 (11 mg, 0.03 mmol) and trans-
9 (10 mg, 0.03 mmol) were dissolved in C6D6 (1.0 mL). The re-
sulting solution was bubbled with N2 for 10 min and then irradi-
ated for 2 h. On the basis of 1H NMR spectroscopic analysis of the
mixtures, the ratio among the three rhenium hydrides trans-9/cis-9/
11 was ca. 7.4:1.0:2.9. CHBr3 (76 mg, 0.30 mmol) was then added,
and the mixture was kept at 45 °C for 0.5 h. The solvent was
pumped off from the resulting red solution. By using the same sep-
aration method described above, the first fraction gave 10 mg of
trans-7 as a red solid, the second fraction gave 4 mg of trans-12 as
a red solid, the third fraction gave 7 mg of unreacted trans-10 as a
red solid, and the final two fractions afforded trace amounts of cis-
Supporting Information (see footnote on the first page of this arti-
cle): 1H NMR spectra for the reaction of [Cp*Re(CO)3] or
[CpRe(CO)3] with bromobenzene in C6H6/C6D6 (1:1).
Acknowledgments
This work was financially supported by the National Natural Sci-
ence Foundation of China (grant no. 21002069), the Natural Sci-
ence Foundation of Tianjin (grant no. 14JCYBJC20300), and the
Program for Innovative Research Team in University of Tianjin
(TD12-5038).
7
and cis-10 as red solids. Compound trans-12: 1H NMR
(400 MHz, CDCl3): δ = 5.74 (s, 5 H, Cp–H) ppm. 13C NMR
(100 MHz, CDCl3): δ = 95.23 (Cp–C), 183.60 (CO) ppm. IR (νCO
KBr): ν = 2067 (s), 2025 (s) cm–1. C H BrClO Re (422.67): calcd.
,
˜
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Received: August 21, 2014
Published Online: ᭿
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Rhenium Complexes
B. Zhu,* X. Huang, X. Hao ............. 1–9
Photochemical Reactions of [(η5-C5R5)-
Re(CO)3] (R = Me or H) with Aryl Halides
in Benzene: Stoichiometric Formation of
Biphenyl
Photolysis of [Cp*Re(CO)3] with bromo-
or chlorobenzene in benzene results in the
formation of biphenyl and the correspond-
ing rhenium hydride halide complex trans-
in the resulting biphenyl comes from the
aryl halide and the other one from the sol-
vent benzene. Plausible mechanisms are
proposed that involve intermolecular C–X
and C–H activation.
Keywords: C–H activation / Rhenium /
Photolysis / Halides / Isotopic labeling
[Cp*Re(CO)2H(X)].
Isotopic
labeling
experiments indicate that one phenyl ring
Eur. J. Inorg. Chem. 0000, 0–0
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