Facile synthetic access to rhenium(II) complexes: Activation of carbonbromine bonds by single-electron transfer

Article abstract of DOI:10.1002/chem.200902600

The five-coordinated Re¹ hydride complexes [Re(Br)(H)(NO) (PR³)₂] (R = Cy la, iPr lb) were reacted with benzylbromide, thereby affording the 17-electron mononuclear ReII hydride complexes [Re(Br)2(H)(NO)(PR₃)₂] (R = Cy 3a, iPr 3b), which were characterized by EPR, cyclic voltammetry, and magnetic susceptibility measurements. In the case of dibromomethane or bromoform, the reaction of 1 afforded Re^II hydrides 3 in addition to Re¹ carbene hydrides [Re(= CHR¹)(Br)(H)(NO)(PR₃)₂,] (R ¹ = H 4, Br 5; R = Cy a, iPr b) in which the hydride ligand is positioned cis to the carbene ligand. For comparison, the dihydrogen Re ¹ dibromide complexes [Re(Br)₂(NO)(PR₃MIf- H₂)] (R = Cy 2 a, iPr 2 b) were reacted with allyl- or benzylbromide, thereby affording the monophosphine Re^II complex salts [R ₃PCH₂R'][Re(Br)₄(NO)(PR₃)] (R' = -CH=CH₂ 6, Ph 7). The reduction of Re^II complexes has also been examined. Complex 3 a or 3 b can be reduced by zinc to afford la or lb in high yield. Under catalytic conditions, this reaction enables homocoupling of benzylbromide (turnover frequency (TOF): 3a 150, 3b 134 h^-1) or allylbromide (TOF: 3a 150, 3b 562 h^-1). The reaction of 6 a and 6 b with zinc in acetonitrile affords in good yields the monophosphine Re ¹ complexes [Re(Br)₂(NO)(MeCN)₂(PR ₃)] (R = Cy 8a, iPr 8b), which showed high catalytic activity toward highly selective dehydrogenative silylation of styrenes (maximum TOF of 61 h-1). Single-electron transfer (SET) mechanisms were proposed for all these transformations. The molecular structures of 3 a, 6 a, 6 b, 7 a, 7 b, and 8 a were established by single-crystal X-ray diffraction studies.

Full text of DOI:10.1002/chem.200902600

DOI: 10.1002/chem.200902600
Facile Synthetic Access to Rhenium(II) Complexes: Activation of Carbon–
Bromine Bonds by Single-Electron Transfer
Yanfeng Jiang, Olivier Blacque, Thomas Fox, Christian M. Frech, and Heinz Berke*[a]
Abstract: The five-coordinated ReI hy-
dride complexes [Re(Br)(H)(NO)-
drogen ReI dibromide complexes
[Re(Br)2(NO)(PR3)2A (R=Cy
(h2-H2)]
2a, iPr 2b) were reacted with allyl- or
benzylbromide, thereby affording the
monophosphine ReII complex salts
benzylbromide (turnover frequency
(TOF): 3a 150, 3b 134 hÀ1) or allylbro-
mide (TOF: 3a 575, 3b 562 hÀ1). The
reaction of 6a and 6b with zinc in ace-
tonitrile affords in good yields the
G
CHTUNGTRENNUNG
A
ed with benzylbromide, thereby afford-
ing the 17-electron mononuclear ReII
hydride complexes [Re(Br)2(H)(NO)-
[R3PCH2R’][Re(Br)4(NO)
N
monophosphine
[Re(Br)2(NO)(MeCN) CHTUNGTRENNUNG
ReI
2A(PR3)] (R=Cy
complexes
À
A
CH=CH2 6, Ph 7). The reduction of
ReII complexes has also been exam-
ined. Complex 3a or 3b can be re-
duced by zinc to afford 1a or 1b in
high yield. Under catalytic conditions,
this reaction enables homocoupling of
U
were characterized by EPR, cyclic vol-
tammetry, and magnetic susceptibility
measurements. In the case of dibromo-
methane or bromoform, the reaction of
1 afforded ReII hydrides 3 in addition
8a, iPr 8b), which showed high catalyt-
ic activity toward highly selective dehy-
drogenative silylation of styrenes (max-
imum TOF of 61 hÀ1). Single-electron
transfer (SET) mechanisms were pro-
posed for all these transformations.
The molecular structures of 3a, 6a, 6b,
7a, 7b, and 8a were established by
single-crystal X-ray diffraction studies.
to ReI carbene hydrides [Re
CHR1)(Br)(H)(NO)(PR3)2] (R1 =H 4,
ACHTUNGERTN(NUNG =
ACHTUNGTRENNUNG
Keywords: carbenes
·
electron
Br 5; R=Cy a, iPr b) in which the hy-
dride ligand is positioned cis to the car-
bene ligand. For comparison, the dihy-
transfer · hydrides · phosphanes ·
rhenium
Introduction
tion-state complexes such as the ReIII compound [Re-
(OPPh3)Cl3(L)];[4] and 3) selective oxidation of lower oxida-
tion-state complexes such as [NBu4]trans-[Re(CN)2A(dppe)2]
(dppe=1,2-bis(diphenylphosphino)ethane),[5] fac-[Re
(PPh3)-
(PF3)
(dien)(N2)]+ (dien=diethylenetriamine),[6] and those
AHCTUNGTRENNUNG
Rhenium is an extraordinarily redox-active element span-
ning a wide range of oxidation states from ÀIII to VII.[1]
Among these redox states, the 17-electron ReII chemistry is
particularly interesting due to its potential application in in-
organic medicinal chemistry, and other studies have demon-
strated its potential application in molecular magnetism.[2]
However, ReII chemistry is surprisingly scarce with just a
few mononuclear ReII compounds reported; these were
CHTUNGTRENNUNG
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
with fac-[Re(CO)3]+ units.[7] In contrast to the manganese
congeners, organometallic ReII compounds possess inherent
stability, which greatly depends on the ligand environment.
Pseudo-octahedral, d7 complexes, such as ReII species, could
in principle take over an important role in redox catalysis
and homogeneous catalysis.[8] To be able to exploit ReII
chemistry for catalysis, we also needed to know about its
prevalent radical character: s or p type. Therefore we
sought to explore organometallic ReII chemistry and per-
haps get an idea about the stability criteria for organometal-
lic ReII complexes. In addition, such knowledge was re-
quired to develop appropriate synthetic access.
mainly accessed by the following approaches: 1) homolytic
scission of Re Re bonds of dinuclear Re units such as
II
À
[3]
[Re
(NCCH3)10]4+
;
2) selective reduction of higher oxida-
[a] Y. Jiang, Dr. O. Blacque, Dr. T. Fox, Dr. C. M. Frech,
Prof. Dr. H. Berke
Anorganisch-Chemisches Institut, Universitꢀt Zꢁrich
Winterthurerstrasse 190, 8037 Zꢁrich (Switzerland)
Fax : (+41)1-63-568-02
E-mail: hberke@aci.uzh.ch
Homepage: http://www.aci.uzh.ch/
In recent years, our group has focused on the exploration
of low-valent rhenium hydride chemistry.[9] Lately, it was
found that five-coordinate, 16-electron ReI hydride com-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200902600.
plexes [Re(Br)(H)(NO)ACHTNUTGRNEUNG
(PR3)2] (R=Cy 1a, iPr 1b),[10] de-
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Chem. Eur. J. 2010, 16, 2240 – 2249

FULL PAPER
rived from easily accessible ReI dihydrogen dibromides
[Re(Br)2(NO)(PR3)2A
(h2-H2)] (R=Cy 2a, iPr 2b), exhibited
A
CHTUNGTRENNUNG
a versatile reactivity toward H2, olefins, nBuLi, N-heterocy-
clic carbenes (NHCs), carbenes, and terminal alkynes.[11] In
an extension of this work and to account for the aspects of
organometallic radical chemistry just discussed, we report
here the reactions of 1 and 2 toward the alkyl bromides in-
cluding allyl-, benzylbromide, dibromomethane and bromo-
form, which are often seen as radical initiators.
Results and Discussion
Reaction of [Re(Br)(H)(NO)ACHTNUGTRNEUNG(PR3)2] (1) with benzylbro-
mide, CH2Br2, and CHBr3: Alkyl halides are known as
single-electron acceptors in reductive electron-transfer proc-
esses.[12,8g,h] The C X bonds next to p-conjugate groups such
À
as benzyl are particularly weak.[13] Treatment of a violet so-
lution of 1a or 1b in benzene with benzylbromide
(1.1 equiv) at room temperature immediately afforded a
brown solution, from which the mononuclear bisphosphine
Figure 1. Molecular structure of [Re(H)(Br)2(NO)ACHTUNGRTNEUNG(PCy3)2] (3a) at 30%
probability ellipsoids (selected hydrogen atoms and the trans-NO/Br po-
sitional disorder have been omitted for clarity). Selected bond lengths
ReII hydride complexes [Re(Br)2(H)(NO)
ACTHNUGTRNEUGN(PR3)2] (R=Cy
À
À
À
À
[ꢃ]: P1 Re1 2.4709(7), P2 Re1 2.4732(8), Br1A Re1 2.5213(6), N1A
À
O1A 1.229(6), Br2 Re1 2.5809(4). Selected bond angles [8]: P1-Re1-P2
3a, iPr 3b) were isolated in 88 (3a) or 80% (3b) yield
(Scheme 1). The presence of 1,2-diphenylethane in the reac-
147.94(2), O1A-N1A-Re1 179.0(4), N1A-Re1-Br1A 178.87(16), P1-Re1-
H 74.6(11), P2-Re1-H 73.4(11), P1-Re1-Br2 106.486(18), P2-Re1-Br2
105.45(2), P1-Re1-Br1A 90.00(2), Br2-Re1-H 172.0(11).
pseudo-octahedral towards a “pentagonal bipyramidal” ge-
ometry. The “pentagonal” equatorial plane would be de-
fined by P1, P2, Br2, and H, and a sterically demanding s-
type radical. The P1-Re1-H (74.6(11)8) and P2-Re1-H
(73.4(11)8) angles are indeed close to the angle of 728 ex-
pected for this pseudogeometry. This may point to a soft
energy surface for the movement of Breq toward P1 or P2
reaching the extreme position of Breq with a P-Re(1)-Br(2)
angle of 728 and a small activation barrier for the conversion
of a p-type (Breq in symmetric pseudo-octahedral position)
to a s-type radical.
Scheme 1.
1
tion mixture was confirmed by both H NMR spectroscopy
and GC–MS analyses. In the IR spectra of 3a and 3b,
strong n(NO) absorptions were observed at 1703 (3a) or
1694 cmÀ1 (3b), thus indicating the presence of a single
mononitrosyl species; despite the radical character of the
species and expected thermal lability, satisfactory elemental
analyses were also obtained. Single crystals of 3a were ob-
tained by means of slow diffusion of pentane into a solution
of 3a in benzene, and the molecular structure of 3a was es-
tablished by an X-ray diffraction study, as depicted in
Figure 1.[14] The molecule adopted a pseudo-octahedral ge-
ometry around the rhenium center with the hydride being
trans to one bromide ligand (Br2-Re1-H angle of
172.0(11)8). The NO group is linear with the trans-bromide
ligand with a N1A-Re1-Br1A angle of 178.87(16)8. The two
trans-phosphine ligands are bent strongly toward the rheni-
um hydride ligand with a P2-Re1-P1 angle of 147.94(2)8,
presumably mainly due to the presence of the p-donating
bromide ligand trans to the hydride ligand.[15] The asymme-
try in the different angles of Br2-Re1-P2 (105.45(2)8) and
EPR measurements were carried out on 3b in toluene at
293 and 98 K. The spectra are shown in Figure 2. At room
temperature, a well-resolved sextet was observed due to the
hyperfine coupling of the unpaired electron with the mag-
netic nuclei of rhenium
(
185Re, 37.4%, spin (I)=5= ,
2
mN(nuclear magnetic moment)=3.187; 187Re, 62.6%, I=5= ,
2
mN =3.220). The hyperfine coupling constants ai are unequal
and increase from low-field to high-field spanning in the
range of 22.0–29.1 mT (average: 25.5 mT) due to second-
order effects (hyperfine splitting non-negligible versus exter-
nal field).[16] Such a pattern resembles closely those of re-
ported ReVI compounds.[17] The observed center field g
value of 2.0018 is surprisingly but only coincidentally close
to the free-electron value of 2.00232. The magnitude of ai in-
dicates that the spin density is mainly concentrated on the
metal center, which speaks more to a s-type radical. The
EPR measurement in toluene glass at 98 K showed a com-
plex spectrum with overlapping signals, which was caused by
P1-Re1-Br1A (90.00(2)8) indicates a distortion from
a
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2241

H. Berke et al.
THF, most probably due to further reactions of the hydride
ligand in the cation proton loss[18] or of irreversible BrÀ
ligand loss from the anion.
The reactivity of other alkyl halides towards 1a or 1b was
also examined. Bromoethane and tert-butylbromide proved
to be inert toward 1a or 1b even at higher temperatures
(808C). The reaction with allylbromide was found to be
complicated and could not be unraveled, since the allyl
group might also coordinate to the rhenium vacant site,
thereby leading to a 18-electron ReI complex. Interestingly,
dibromomethane does react with 1a or 1b, but in a different
manner than benzylbromide did. Treatment of a violet solu-
tion of 1a,b in benzene with CH2Br2 (0.5 equiv) immediate-
ly afforded a deep brown solution, from which a mixture
containing the ReII hydride 3a,b and a ReI carbene hydride
complex [Re
ACHTUTGNRENUNG(=CH2)(Br)(H)(NO)ACHTUNTGRENN(UGN PCy3)2] (R=Cy 4a, iPr
4b) was obtained (Scheme 2). In the 1H NMR spectra, a
doublet at d=14.79 (4a, JACTHNUTRGNE(UNG H,H)=4 Hz) or 14.92 ppm (4b,
Figure 2. a) X-band EPR spectra of a solution of 3b in toluene at 293 K
(top). b) Low-temperature glass spectrum of 3b at 98 K (bottom).
g anisotropy rather than by hyperfine coupling with the hy-
dride (1H, I=1= ), phosphine (31P, I=1= ), bromide (79,81Br,
2
2
I=3= ), or nitrosyl ligand (14N, I=1). The extracted values
2
are g1 =1.98, g2 =2.05, g3 =1.98, and a1 =43, a2 =18, a3 =
15 mT for the 185,187Re coupling. These results further stress
that spin density is mainly localized on the rhenium center
and not to a significant extent on the ligands.
Scheme 2.
Magnetic susceptibility data of 3a,b were measured in the
temperature range of 5–300 K and are shown in Figure 3. At
low temperature, the effective magnetic moment (meff)
values of 1.61 mB for 3a and of 1.64 mB for 3b (mB Bohr mag-
neton) were determined in accordance with a d5 system
bearing an unpaired electron. The electrochemical behavior
of 3b has also been investigated by cyclic voltammetry.
However, both oxidation and reduction are irreversible in
JACHTUNGTRENNUNG
ACHTUNGTRENNUNG
G
ACHTUNGTRENNUNG
AHCTUNGTREGUN(NN =CH,ReH)=10 Hz). This is interpreted in terms of
a transoid 3J coupling with the rhenium hydride (see
Scheme 2), which was observed as a triplet of doublets at
d=2.42 (4a, 2J
G
ACHTUNGTRENNUNG
2
3
2.40 ppm (4b, J
(H,P)=34 Hz, J
U
indicates that the carbene shows hindered rotation around
À
the Re=CH2 bond and the two Ccarbene H bonds are in a
same plane with the H-Re-NO plane so as to maximize the
p back-donation from rhenium to the empty p orbital of the
carbene carbon atom. This hypothesis was further substanti-
1
ated by a H TOCSY experiment for 4a, in which the dipo-
lar H···H coupling (NOE) detectable through irradiation of
the carbene proton signal at d=14.59 ppm indicates their
spatial proximity. This demands the cis position of the hy-
dride and the carbene ligand. The preference for this geom-
etry requires rearrangement of the initial coordination
sphere, which stresses the need for geometrical flexibility of
such rhenium nitrosyl systems. One main driving force for
Figure 3. Temperature-dependent magnetic susceptibility data of 3a re-
corded in the range of 5–300 K.
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Rhenium(II) Complexes
FULL PAPER
such a geometrical change could be the “push–pull” type
trans arrangement of the p-donor bromide and the p-accept-
or carbene ligand (=CH2). In the 13C{1H} NMR spectra, the
multiplet resonance at d=278.1 (4a) or 279.0 ppm (4b) was
assigned to the Ccarbene atom, which was further supported by
13C,1H correlation spectra that demonstrated a correlation
between this signal and the proton signal of the rhenium hy-
dride. In the 31P{1H} NMR spectra, a singlet at d=46.0 (4a)
or 52.6 ppm (4b) was observed in accord with the trans dis-
position of the phosphine ligands. In the IR spectra, besides
the n(NO) absorption for 3a,b, a strong stretching vibration
of the NO group at 1670 (4a) or 1661 cmÀ1 (4b) was ob-
served. It should be noted that products based on the ap-
tected by 1H NMR spectroscopy (d=5.58 ppm, s; CH2).
Similarly, formation of 4a was not observed, even when
other single-electron transfer (SET) scavengers such as p-di-
nitrobenzene were used.[19] These results indicate that 4a is
C
derived from the bromomethylene radical CH2Br through
the bromomethyl rhenium(II) intermediate.
A plausible pathway for the reaction of 1 with PhCH2Br
and CH2Br2 was thus proposed. Single-electron transfer
À
(SET) from 1 to the C Br bond of benzylbromide occurs to
afford 3 along with the formation of the benzyl radical
C
(PhCH2 ), which undergoes dimerization to afford 1,2-diphe-
nylethane. In the case of CH2Br2, the reaction takes place
À
probably through the coordination of the C Br bond to the
C
pearance of a free bromomethylene radical ( CH2Br), like,
vacant site of 1. The formed intermediate reacts with anoth-
for instance, its recombination product dibromoethane, were
not observed by in situ NMR spectroscopy and GC–MS. No
obvious formation of other organic products was observed.
We therefore assume primary intermediacy of the bro-
moalkyl ReII species (A) shown in Scheme 2, which is
thought to be subjected to another bromide abstraction by 1
so that an overall stoichiometry of 2:1 arises between the
products 3 and 4. Although 3a and 3b could be isolated
from the mixture in 27 and 34% yield, respectively, attempts
to separate 4a and 4b failed.
er equivalent of 1, thus yielding 3 and the bromomethylene
À
C
radical ( CH2Br) coordinated species. As one C Br bond is
still present, it further accepts one electron from 1, thereby
resulting in the formation of the carbene hydride 4 and an-
other equivalent of 3. The geometry change might also
occur in this process. It should also be pointed out that acti-
vation of gem-dihalide by late-transition-metal compounds
such as those with rhodium centers have been widely ex-
plored.[20] However, they all take place by means of oxida-
tive addition of carbon halogen bonds to metals, thus result-
ing in oxidation states increased by two. Furthermore, one-
pot syntheses of carbene compounds from the reactions of
ruthenium complexes with gem-dihalide have also been re-
ported through oxidative addition followed by an a-halide
elimination process.[21] In comparison, CH2Br2 or CHBr3 act
in the reaction with 1a,b as a double single-electron accept-
or.
Similarly, the reaction of 1a,b with bromoform (0.5 equiv)
afforded within 5 min a mixture that contained 3a,b and a
ReI carbene hydride complex [Re
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
spectra, a doublet at d=14.20 (5a) or 14.30 ppm (5b) was
assigned to the carbene proton. The magnitude of the cou-
pling constant (11 Hz) indicated, like in complexes 4, a
À
À
transoid alignment of the carbene C H and the Re H
ligand. The resonance for the hydride was observed as a
(h2-H2)] (2) with allyl-
Reactions of [Re(Br)2(NO)ACHTUGNTRENNU(G PR3)2CAHTUNGTRENNUNG
and benzylbromide: Treatment of a solution of 2a in toluene
with an excess amount of allylbromide (10 equiv) afforded
at 708C within 24 h the monophosphine-coordinated ReII
complex salt [Cy3PCH2CH=CH2][Re(Br)4(NO)ACTHNUGTRENNG(U PCy3)] (6a),
which was isolated as a red solid in 53% yield (Scheme 3).
triplet of doublets at d=2.20 (5a, 2J
G
G
CH,ReH)=11 Hz) or 2.04 ppm (5b, 2J
ACHTUNGTRENNUNG
CH,ReH)=11 Hz). In the 13C{1H} NMR spectra, a singlet at
d=250.8 (5a) or 252.3 ppm (5b) was assigned to the Ccarbene
atom. The 31P{1H} NMR spectra showed a singlet at d=
38.8 ppm for 5a and at 46.9 ppm for 5b.
In the presence of radical
By comparison, the reaction of 2b bearing PiPr3 ligands was
scavengers such as tetramethyl-
piperidine oxide (TEMPO), the
reaction of 1a with PhCH2Br
afforded 3a and the primary
radical
trapped
product
TEMPO–CH2Ph, as proven by
1H NMR spectroscopy (d=
4.89 ppm, s; CH2). Formation of
1,2-diphenylethane was not ob-
served. In the reaction of 1a
with CH2Br2, the presence of
TEMPO in benzene blocked
the formation of the carbene
compound 4a completely. The
C
CH2Br radical was trapped by
TEMPO to give the adduct
TEMPO–CH2Br, which was de- Scheme 3.
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2243

H. Berke et al.
much faster, and
time of 2 h was enough to ach-
ieve [iPr3PCH2CH=CH2]
[Re(Br)4(NO)(PiPr3)] (6b) in
a reaction
ACHTUNGTRENNUNG
90% isolated yield. In both
cases, one phosphine ligand dis-
sociated from the rhenium
center and was found bonded
to the cationic allyl group. The
allyl-dimerization product hexa-
1,5-diene was detected by GC–
MS in both reaction mixtures.
Compounds 6a and 6b were
characterized by various spec-
troscopic methods, gave correct
elemental analyses, and were
studied by single-crystal X-ray
diffraction. IR spectroscopy
Figure 4. Molecular structure of [Cy3PCH2CH=CH2][Re(Br)4(NO)
ment ellipsoids. Two of the four independent molecules are shown. All hydrogen atoms have been omitted for
ACHTUNGRTENN(UNG PCy3)] (6a) with 20% probability displace-
À
À
À
À
clarity. Selected bond lengths [ꢃ]: Re1 N1 1.793(11), Re1 Br2 2.5147(11), Re1 Br4 2.5215(11), Re1 P1
À
À
À
2.546(3), Re1 Br3 2.5618(11), Re1 Br1 2.5718(11), N1 O1 1.046(12). Selected bond angles [8]: N1-Re1-Br1
confirmed the presence of one 169.8(3), P1-Re1-Br3 176.32(6), N1-Re1-Br2 89.3(3), N1-Re1-Br4 97.8(3), O1-N1-Re1 173.6(11).
nitrosyl ligand that displayed
strong n(NO) absorptions at
1728 for 6a and at 1731 cmÀ1
for 6b. The X-ray diffraction study of 6a (Figure 4) revealed
a
angles of 169.8(3) and 170.8(3)8 were found to deviate some-
what from the ideal 1808. The nitrosyl ligands were only
slightly bent with O-N-Re angles of 173.6(11) and 171.6(11)8
a redox process to the formation of the neutral transient
2
pseudo-octahedral rhenium center.[22] The ON-Re-Br
[ReIIBr3(NO)
(PR3)2] complexes and alkyl radicals (sp
C
À
C
CHR ), which undergo recombination (Scheme 3). Presuma-
bly due to the presence of three bromide ligands, which cu-
mulatively exert a strong cis-labilization effect, one phos-
phine ligand dissociates from the rhenium center of
À
with relatively short N O distances, which spoke to linear
nitrosyl ligands.[23]
[ReBr3(NO)ACHTUGNTERNNU(G PR3)2] and gets replaced by a bromide ion. The
In the same manner as allylbromide, 2a and 2b reacted
bromide originates from the allyl- or benzylbromides by
phosphine substitution to generate the phosphonium cations,
with benzylbromide to give the monophosphine ReII salts
[R3PCH2Ph][Re(Br)4(NO)(PR3)] (R=Cy 7a, iPr 7b) in
G
ACTHNGUTERNNUG
which together with the anionic [ReIIBr4(NO)(PR3)]À com-
good yields. 1,2-Diphenylethane was detected by GC–MS in
the reaction mixtures. Satisfactory elemental analyses were
obtained for both compounds. The molecular structures of
7a and 7b were also established by X-ray diffraction studies
given in the Supporting Information.
It should be noted at this point that other alkyl bromides
such as bromoethane, tert-butylbromide, and dibromome-
thane were also tested in their reactions with 2a and 2b, but
were found inert under various more forceful conditions,
ponent form salts that precipitate from the toluene solution.
Reduction of ReII complexes and their applications in catal-
yses: For the reduction of the ReII complex, the brown solu-
tion of 3a or 3b was treated in THF with an excess of zinc
at room temperature. A purple solution was formed within
30 min with the recovery of ReI hydride in 82 (1a) or 78%
(1b) yield obtained in situ by means of NMR spectroscopy.
This result enables the development of rhenium-catalyzed
homocoupling reactions of benzylbromide. As sketched in
Scheme 4, the reaction of benzylbromide with zinc
(0.5 equiv) in the presence of 1% of 3a or 3b as a catalyst
in THF afforded 1,2-diphenylethane in a turnover frequency
À
presumably due to stronger C Br bonds in these cases and
the accompanying higher barriers to homolysis or heteroly-
1
sis relative to those of allyl- and benzylbromide. H NMR
spectroscopy analyses carried out on samples in the pres-
ence of TEMPO shows that the reaction of 2a with allylbro-
mide proceeded through the intermediacy of the allyl radi-
cal, thereby affording in benzene at 708C the radical trap-
ping product TEMPO–CH2CH=CH2. No evidence for allyl
dimerization was given. Thus, a SET pathway is proposed in
which the allyl- or benzylbromide acts not only as a single-
electron acceptor but also as a phosphine trap. Therefore,
for proper reactivity, 2 equiv of these reagents are needed.
The reaction presumably proceeds through the initial loss of
the labile H2 ligand to afford five-coordinated, 16-electron
intermediates, which further interact with the alkyl bromides
À
by SET from rhenium to the C Br bond, thereby leading in
Scheme 4.
2244
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Chem. Eur. J. 2010, 16, 2240 – 2249

Rhenium(II) Complexes
FULL PAPER
(TOF) value of 150 (3a) or 134 hÀ1 (3b). Interestingly, de-
spite the complexity of their stoichiometric reactions with
allylbromide, 3a and 3b proved to be highly active catalysts
for the homocoupling of allylbromide. One percent of 3a or
3b with 0.25 equiv of zinc are enough to afford hexa-1,5-
diene in THF at room temperature with good TOF values of
575 (3a) and 562 hÀ1 (3b), respectively. The catalytic reac-
tion was found to be solvent dependent. No conversion was
achieved when the reaction was carried out in nonpolar sol-
vents such as benzene or toluene. This might be due to pas-
sivation of zinc caused by the formed zinc bromide that is
insoluble in nonpolar solvents.
adopts a pseudo-octahedral geometry around the rhenium
center with the phosphine in trans position to one of the
bromides (P1-Re1-Br2 with an angle of 178.461(18)8). The
trans-acetonitrile ligands are slightly bent with a N3-Re1-N2
angle of 168.66(9)8. The NO group is practically linearly
aligned with the trans bromide with an average N-Re-Br
angle of 177.58.
Recently we reported that the bisphosphine compounds
2a and 2b are efficient catalysts for highly selective dehy-
drogenative silylation of alkenes.[10] However, the reaction
can only be conducted at temperatures over 1008C, which
presumably was connected to the quite high energy barrier
for the required dissociation of one phosphine ligand from
the precatalyst to initiate the catalytic cycle. The application
of the monophosphine compounds 8a and 8b as catalyst
precursors would allow one to circumvent this difficulty in,
for instance, dehydrogenative silylations of styrenes with
Et3SiH. Indeed, using 8a, we found that the reaction tem-
perature for this type of catalysis could be lowered
For comparison, the reduction of the orange-red solution
of 6a or 6b with an excess of Zn powder (10 equiv) in ace-
tonitrile took 1 h at room temperature to produce yellow
mixtures, from which the monophosphine ReI complexes
[Re(Br)2(NO)ACHTUNGTRENNUNG(MeCN)2CAHUTNGTRENN(UGN PR3)] (R=Cy 8a, iPr 8b) were iso-
lated in 61 (8a) and 79% (8b) yield, respectively. Both com-
pounds were fully characterized by elemental analyses and
various spectroscopic methods. In the IR spectra, strong
n(NO) absorptions appeared at 1682 for 8a and at
Table 1. Dehydrogenative silylation of styrenes catalyzed by ReI com-
plexes.[a]
1
1685 cmÀ1 for 8b. In the H NMR spectra, singlets were de-
tected at d=1.26 for 8a and at 1.25 ppm for 8b, which were
assigned to the protons of two identical acetonitrile ligands.
This implies that the two acetonitrile ligands are arranged in
a trans orientation. Integrations of the 1H NMR spectro-
scopic signals of 8b indicated a 2:1 ratio of the acetonitrile
to phosphine ligands. In the 31P NMR spectra, the resonan-
ces of the phosphorus ligands are shifted in comparison with
the related bisphosphine monoacetonitrile ReI complexes to
lower field to d=3.2 ppm for 8a and 13.9 ppm for 8b.[9f] The
molecular structure of 8a was also established by an X-ray
diffraction study presented in Figure 5.[24] The molecule
Entry R1
[Re]
T
t
Yield dehydroACTHNGUTERNNUG
(E/Z)/hydro[c]
[8C] [h]
[%][b]
1
p-CH3C6H4
2a
2b
8a
8b
8a
8a
8a
8a
70 24
70 24
70
70
70
70 19
100
100
70 20
70 20
70
70
100 18
700 20
70
70
30
25
–
–
2
3
4
5
p-CH3C6H4
p-CH3C6H4
p-CH3C6H4
p-CH3C6H4
p-CH3C6H4
p-CH3C6H4
p-CH3C6H4
p-CH3OC6H4 8a
p-CH3OC6H4 8b
p-FC6H4
p-FC6H4
1.0
6.0
5.0
61 99
ACHTUNGTRENNUNG
81 98ACHTGNUTRENNUNG
91 99ACHTGNUTRENNUNG
6
>99 99
ACHTUNGTRENNUNG
7[d]
8[d]
9
0.5
77 99ACHTGNUTRENNUNG
2.0 >99 99ACTHUNGTRENNNUG
92 99
90 99
92 99
89 99
64 79
51 75
ACHTUNGTRENNUNG
10
11
12
13[d]
14[d]
15
16
ACHTUNGTRENNUNG
8a
8b
8a
8b
8a
8b
5.0
4.0
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
G
ACHTUNGTRENNUNG
T
ACHTUNGTRENNUNG
H
H
4.0 >99 68:32
7.0 >99 79:21
[a] 0.25 mmol of silane and 0.50 mmol of olefin. [b] From 1H NMR spec-
troscopic integration. Hydrosilylation byproducts add up to 100%.
[c] Determined by GC–MS. [d] In [D8]toluene.
(Table 1). An exemplary experiment with 8a (1.6 mg, 1.0%)
dissolved in [D6]benzene (0.5 mL) containing Et3SiH
(0.25 mmol) and p-methylstyrene (0.50 mmol) was carried
out at 708C. It reached a TOF value of 61 hÀ1 after 1 h and
91% conversion within 6 h. This amounts to a reaction rate
over 12 times faster than that catalyzed by 2a under other-
1
wise similar conditions. H NMR spectroscopy and GC–MS
Figure 5. Molecular structure of [Re(Br)2(NO)ACHTUNTRGENN(UG MeCN)2ACHTUTGNREN(NUGN PCy3)] (8a) with
30% probability displacement ellipsoids. The trans-NO/Br disorder and
all hydrogen atoms have been omitted for clarity. Selected bond lengths
indicated a selectivity of 99% for dehydrogenative silylation
with the product 1-(p-methylstyryl)-2-(triethylsilyl)ethylene,
for which E and Z isomers were determined in a 96:4 ratio.
The formation of branched dehydrogenative silylation prod-
ucts was not detected. The high selectivity remained even
À
À
À
À
[ꢃ]: P1 Re1 2.4311(7), N1A Re1 1.831(7), N2 Re1 2.060(2), N3 Re1
À
2.054(2), N1A O1A 1.226(8). Selected bond angles [8]: P1-Re1-Br2
178.461(18), N1A-Re1-Br1A 178.7(3), N3-Re1-N2 168.66(9), O1A-N1A-
Re1 173.5(10), Br1A-Re1-P1 92.28(5).
Chem. Eur. J. 2010, 16, 2240 – 2249
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2245

H. Berke et al.
N2 by employing standard procedures and were degassed by freeze–thaw
cycles prior to use. The deuterated solvents were dried with sodium/ben-
zophenone ([D8]THF, [D8]toluene, [D6]benzene) and vacuum-transferred
for storage in Schlenk flasks fitted with Teflon valves. 1H, 13C{1H}, and
31P{1H} NMR spectroscopic data were recorded using a Varian Gemini-
300, Varian Mercury 200, or Bruker DRX 500 spectrometers by using
5 mm diameter NMR spectroscopy tubes equipped with Teflon valves,
which allow degassing and further introduction of gases into the probe.
Chemical shifts are expressed in parts per million (ppm). 1H and
13C{1H} NMR spectra were referenced to the residual proton or 13C reso-
nances of the deuterated solvent. All chemical shifts for the
31P{1H} NMR spectroscopic data are reported downfield in ppm relative
to external 85% H3PO4 at d=0.0 ppm. Signal patterns are reported as
follows: s, singlet; d, doublet; t, triplet; m, multiplet. IR spectra were ob-
tained by ATR methods using a Bio-Rad FTS-45 FTIR spectrometer. Mi-
croanalyses were carried out at the Anorganisch-Chemisches Institut of
the University of Zurich. Benzylbromide, allylbromide, TEMPO, and p-
dinitrobenzene were purchased from Aldrich and used without further
purification. Dibromomethane and bromoform were used after distilla-
tion.
when the reaction was conducted at 1008C, and conversion
over 99% was then achieved within 2 h (Table 1, entries 7,
8). Thus we can conclude that addition of 2 equiv of olefin
is enough to achieve high selectivity. Similarly, rate enhance-
ment was observed for the dehydrogenative silylation of p-
methylstyrene at 708C by using 8b as a catalyst (entry 4). In
general, 8b was found to be less active than 8a. Other sub-
stituted styrenes, such as p-methoxy- and p-fluorostyrene,
have also been tested as substrates, which not only gave
high yields, but also higher selectivities (over 99.6%) for
trans-dehydrogenative silylation products (entries 9–12).
With aliphatic alkenes such as triethoxylvinylsilane, both the
conversion rate and the selectivity for dehydrogenative sily-
lation became poor (entries 13, 14). In the case of ethylene
gas, a high yield but low dehydrogenative silylation/hydrosi-
lylation selectivity (less than 4:1) was observed (entries 15,
16). Based on all these results, it is evident that monophos-
phine ReI complexes exhibit higher activity than bisphos-
phine ones. The rate enhancement is anticipated to originate
from the more facile dissociation of an acetonitrile ligand in
comparison with a phosphine ligand, which facilitates the in-
itial coordination of alkenes and the oxidative addition of
[Re(Br)2(H)(NO)ACHTUNGTENRU(NG PCy3)2] (3a): In a 20 mL vial in a glove box,
[Re(Br)(H)(NO)(PCy3)2] (86 mg, 0.10 mmol) was dissolved in benzene
AHCTUNGTRENNUNG
(2 mL). Then benzylbromide (13 mL, 0.11 mmol) was added and the
violet solution immediately turned dark brown. After 5 min, the solution
was evaporated in vacuo. The residue was washed with pentane (2ꢄ
2 mL) and dried in vacuo to give a brown solid. Yield: 83 mg, 89%. The
formation of 1,2-diphenylethane was proven by 1H NMR spectroscopy,
with characteristic resonances at d=6.97 (m; Ph), 2.73 ppm (s; PhCH2-),
and by GC–MS measurements: m/z: 182.2 [M+]. IR (ATR): n˜ =2924 (n-
À
the Si H bond to the rhenium center.
(Re-H)), 1703 cmÀ1 (n(NO)); EPR (tolu-
AHCTUNGERTG(NUNN C-H)), 2848 (nACHTUNGTREG(NNUN C-H)), 2013 (nACHTNGUTRENNGUN
ene, 298 K): ai =24.5–31.5 mT (sextet), central field: 3480 G, g=2.0018;
elemental analysis calcd (%) for C36H67Br2NOP2Re (936.26): C 46.10, H
7.20, N 1.49; found: C 46.26, H 7.12, N 1.47.
Conclusion
A facile synthetic route to mononuclear ReII hydride com-
plexes [Re(H)(Br)2(NO)ACHTNUGTRENUNG(PR3)2] (3) was developed by ex-
[Re(Br)2(H)(NO)ACHTUNGTENRU(NG PiPr3)2] (3b): In a 20 mL vial in a glove box,
[Re(Br)(H)(NO)(PiPr3)2] (31 mg, 0.05 mmol) was dissolved in benzene
AHCTUNGTRENNUNG
ploiting the propensity of ReI complexes of type 1 for SET
pathways with benzyl bromide. In comparison, dibromome-
thane or bromoform act as a double SET acceptor toward 1
to lead to the formation of ReI carbene hydrides compounds
4 or 5 along with 3. In contrast, the SET reaction mode 2 of
(1 mL). Then benzylbromide (7 mL, 0.06 mmol) was added and the violet
solution immediately turned dark brown. After 5 min, the solution was
evaporated in vacuo. The residue was washed with pentane (1ꢄ2 mL)
and dried in vacuo to give a dark-brown solid. Yield: 28 mg, 80%. IR
(ATR): n˜ =2963 (nACHTUNGTERN(NNUG C-H)), 2927 (nCAHUTGTNRENNUG(C-H)), 2872 (nAHCTUNGERTG(NNNU C-H)), 2032 (nACHTUNGTRENNUNG(Re-
H)), 1694 cmÀ1 (n(NO)); elemental analysis calcd (%) for
C18H43Br2NOP2Re (696.07): C 31.00, H 6.21, N 2.01; found: C 31.12, H
6.29, N 2.00; EPR (toluene, 298 K): ai =22.0–29.1 mT (sextet), central
field=3480 G, g=2.0018.
allyl- or benzylbromide with the [ReIBr(H2)(NO)
hydrogen complex led to the paramagnetic monophosphine
salts 6 and 7 bearing the [ReIIBr4(NO)(PR3)]À anions. Com-
pound 6 was found to be an appropriate precursor to pre-
pare the monophosphine [ReIBr2(NO)
(MeCN)2A(PR3)] (8),
which exhibit improved catalytic activity in highly selective
dehydrogenative silylations of styrenes. In combination with
zinc, complexes 3 were also applied as highly active catalysts
for homocoupling of allyl- and benzylbromide. This work
therefore not only presents the first example of ReII hydride
complexes in the realm of ReII chemistry, but also offers a
practical synthetic route to monophosphine ReI compounds.
Finally, evidence was provided that ReII hydrides of type
3 may be converted to s-type radicals with small activation
barriers. Therefore their applications in new radical-driven
catalyses are now under investigation in our group.
ACHTUGNERTN(NUNG PR3)2] di-
AHCTUNGTRENNUNG
Reaction of 1a with CH2Br2: In
a 20 mL vial in a glove box,
[Re(Br)(H)(NO)(PCy3)2] (35 mg, 0.04 mmol) was dissolved in benzene
AHCTUNGTRENNUNG
G
CHTUNGTRENNUNG
(1 mL). Then CH2Br2 (2 mL) was added and the violet solution immedi-
ately turned dark brown. After 5 min, the solution was evaporated in
vacuo. The residue was washed with pentane (2ꢄ2 mL) and dried in
vacuo to give a green-brown solid that contained a mixture of [Re
ACHTUNGTRENNUNG(=
CH2)(Br)(H)(NO)
AHCTUNGTRENNUNG
benzene/pentane mixing solution (1:20, 2ꢄ2 mL), pure 3a could be ob-
tained as a brown solid in 27% yield (10 mg). However, attempt to iso-
late pure 4a failed. In situ spectroscopic data for 4a: 1H NMR
(500.25 MHz, [D8]toluene): d=14.79 (d, J
14.59 (dd, J(H,H)=4 Hz, J(=CH,ReH)=10 Hz, 1H; Re=CH2), 2.42 (dt,
(=CH,ReH)=10 Hz, (H,P)=34 Hz, 1H; Re-H), 1.13–2.78 ppm (m,
66H; P
(C6H11)3); 13C{1H} NMR (125.8 MHz, [D8]toluene): d=278.1 (brs;
ACHTUNGTREN(NUGN H,H)=4 Hz, 1H; Re=CH2),
A
ACHTUNGTRENNUNG
J
N
JACHTUNGTRENNUNG
AHCTUNGTRENNUNG
Re=CH2), 33.7 (t, J=12 Hz; P-CH), 30.2, 29.7, 29.5, 27.9 (t, J=5 Hz),
27.2, 26.8, 19.0 ppm; 31P{1H} NMR (202.5 MHz, [D8]toluene): d=
46.0 ppm (s; 2P); IR (ATR): n˜ =2924 (n
1670 cmÀ1 (n(NO)).
ACHUTGTNRENN(GU C-H)), 2850 (nACHTUNGTRENNUNG(C-H)),
Reaction of 1b with CH2Br2: In
a 20 mL vial in a glove box,
Experimental Section
[Re(Br)(H)(NO)(PiPr3)2] (31 mg, 0.05 mmol) was dissolved in benzene
AHCTUNGTRENNUNG
(1 mL). Then CH2Br2 (2 mL) was added and the violet solution immedi-
ately turned dark brown. After 5 min, the solution was evaporated in
vacuo. The residue was washed with pentane (2ꢄ2 mL) and dried in
General: All manipulations were performed under an atmosphere of dry
nitrogen using standard Schlenk techniques or in a glove box (M. Braun
150B-G-II) filled with dry nitrogen. Solvents were freshly distilled under
vacuo to give a green-brown solid that contained a mixture of [ReACHTUNGTRENNUNG(=
2246
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Chem. Eur. J. 2010, 16, 2240 – 2249

Rhenium(II) Complexes
FULL PAPER
CH2)(Br)(H)(NO)
G
analysis calcd (%) for C43H73Br4NOP2Re (1184.15): C 43.48, H 6.19, N
1.18; found: C 43.62, H 6.34, N 1.21.
with pentane (5ꢄ2 mL), pure 3b could be obtained as a brown solid in
34% yield (12 mg). However, pure 4b could not be obtained. In situ
spectroscopic data for 4b: 1H NMR (500.25 MHz, [D8]toluene): d=14.92
[iPr3PCH2Ph][Re(Br)4(NO)
(PiPr3)] (7b): In
a
50 mL Schlenk tube
equipped with Young tap, [Re(Br)2(NO)
a
N
CHTUNGTRENNUNG
(d,
CH,ReH)=10 Hz, 1H; Re=CH2), 2.47 (m, 6H; P-CH
(=CH,ReH)=10 Hz, J(H,P)=34 Hz, 1H; Re-H), 1.25 ppm (m, 36H; P-
CH
(CH3)2); 13C{1H} NMR (125.8 MHz, [D8]toluene): d=279.0 (brs; Re=
CH2), 24.0 (t, J(P,C)=11 Hz; P-CH
(CH3)2), 19.1 (s), 18.7 ppm (s); 31P{1H}
NMR (202.5 MHz, [D8]toluene): d=52.65 ppm (s, 2P); IR (ATR): n˜ =
2961 (n(C-H)), 2927 (n(C-H)), 2030 (n
(Re-H)), 1661 cmÀ1 (n(NO)).
Reaction of 1a with CHBr3: In 20 mL vial in glove box,
[Re(Br)(H)(NO)(PCy3)2] (35 mg, 0.04 mmol) was dissolved in benzene
J
N
0.03 mmol) and benzylbromide (24 mL, 0.20 mmol) were mixed in toluene
(5 mL). The mixture was stirred at 708C for 24 h. After cooling, a blood-
red precipitate was formed. The precipitate was separated and washed
with toluene (1ꢄ2 mL) and pentane (5ꢄ2 mL) and dried in vacuo to
give an orange-red solid. Yield: 15 mg, 53%. IR (ATR): n˜ =1728 cmÀ1
(n(NO)); elemental analysis calcd (%) for C25H49Br4NOP2Re (947.40): C
31.69, H 5.21, N 1.48; found: C 31.79, H 5.31, N 1.50.
N
ACHTUNGTRENNUNG
R
G
[Re(Br)2(NO)
ACHUTGTNERN(NUG MeCN)2ACHTUNGTREN(NNGU PCy3)] (8a): In a 30 mL Schlenk tube equipped
ACHTUNGTRENNUNG
with Young tap, 6a (50 mg, 0.05 mmol) and Zn powder (26 mg,
a
(1 mL). Then CHBr3 (1.5 mL) was added, and the violet solution immedi-
ately turned dark brown. After 5 min, the solution was evaporated in
vacuo. The residue was washed with pentane (2ꢄ2 mL) and dried in
vacuo to give a green-brown solid that contained a mixture of ReII hy-
0.4 mmol) were mixed in acetonitrile (5 mL). The mixture was stirred at
room temperature for 1 h to afford a yellow mixture. The excess of Zn
was removed by filtration through Celite. The yellow filtrate was dried in
vacuo and was further extracted with benzene (3ꢄ1 mL). The benzene
solution was dried in vacuo again and further washed with cold benzene
(1ꢄ0.5 mL). The residue was dried in vacuo to afford an orange solid.
Yield: 23 mg, 61%. 1H NMR (200.0 MHz, [D6]benzene): d=1.07–2.73
dride 3a and [Re
tic data for 5a: 1H NMR (500.25 MHz, [D6]benzene): d=14.20 (d, J
CH,ReH)=11 Hz, 1H; Re=CHBr), 2.20 (dt, J(=CH,ReH)=11 Hz, J-
(H,P)=35 Hz, 1H; Re-H), 1.14–2.86 ppm (m, 66H; P
(C6H11)3); 13C{1H}
ACHTUNGTRENNUNG(=CHBr)(Br)(H)(NO)ACTHUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
(m, 33H;
PACTHNGUTERNNUG
(C6H11)2), 1.26 ppm (s, 6H; CH3CN); 13C{1H} NMR
A
ACHTUNGTRENNUNG
NMR (125.8 MHz, [D6]benzene): d=250.8 (brs; Re=CHBr), 34.7 (t, J=
12 Hz; P-CH), 30.1, 29.5, 29.4, 27.8 (t, J=5 Hz), 27.5, 26.8, 21.4 ppm;
31P{1H} NMR (202.5 MHz, [D6]benzene): d=38.8 ppm (s; 2P); IR (ATR,
cmÀ1): n˜ =1728 cmÀ1 (n(NO)).
(75.5 MHz, [D6]benzene): d=134.6 (s; CH3CN), 36.4 (s), 36.0 (s), 28.9
(s), 27.7 (s), 26.8 (s), 26.7 (s), 2.1 ppm (br; CH3CN); 31P{1H} NMR
(80.9 MHz, [D6]benzene): d=3.2 ppm (s); IR (ATR): n˜ =2928 (n
ACHTUNGTRENNUNG(C-H)),
2913 (n(C-H)), 2849 (n
N
ACHTUNGTRENNUNG
mental analysis calcd (%) for C22H39Br2N3OPRe (738.55): C 35.78, H
5.32, N 5.69; found: C 36.21, H 4.99, N 5.28.
Reaction of 1b with CHBr3: In
a 20 mL vial in a glove box,
[Re(Br)(H)(NO)(PiPr3)2] (25 mg, 0.04 mmol) was dissolved in benzene
ACHTUNGTRENNUNG
(1 mL). Then CHBr3 (1.5 mL) was added and the violet solution immedi-
ately turned dark brown. After 5 min, the solution was evaporated in
vacuo. The residue was washed with pentane (2ꢄ2 mL) and dried in
vacuo to give a green-brown solid that contained a mixture of ReII hy-
[Re(Br)2(NO)
ACHUTGTNERN(NUG MeCN)2ACHTUNGTREN(NGUN PiPr3)] (8b): In a 50 mL Schlenk tube equipped
with Young tap, 6b (96 mg, 0.11 mmol) and Zn powder (65 mg,
a
1.0 mmol) were mixed in acetonitrile (5 mL). The mixture was stirred at
room temperature for 1 h to afford a yellow mixture. The excess of Zn
was removed by filtration through Celite. The yellow filtrate was dried in
vacuo and was further extracted with benzene (6ꢄ2 mL). The benzene
solution was concentrated to 1 mL and pentane (6 mL) was added to
afford orange precipitates, which were isolated and dried in vacuo to give
dride 3b and [Re
tic data for 5b: 1H NMR (500.25 MHz, [D6]benzene): d=14.30 (d, J
CH,ReH)=11 Hz, 1H; Re=CHBr), 2.50 (m, 6H; P-CH(CH3)2), 2.04 (dt,
(=CH,ReH)=11 Hz, J(H,P)=35 Hz, 1H; Re-H), 1.23 (m, 18H; P-CH-
(CH3)2), 1.09 ppm (m, 18H; P-CH
(CH3)2); 13C{1H} NMR (125.8 MHz,
[D6]benzene): d=252.3 (brs; Re=CHBr), 25.4 (t, J(P,C)=12 Hz; P-CH-
(CH3)2), 19.0 (s), 18.9 ppm (s); 31P{1H} NMR (202.5 MHz, [D6]benzene):
d=47.9 ppm (s; 2P); IR (ATR): n˜ =1734 cmÀ1 (n(NO)).
[Cy3PCH2CH=CH2][Re(Br)4(NO)(PCy3)] (6a): In a 50 mL Schlenk tube
equipped with Young tap, [Re(Br)2(NO)(PCy3)2A
(h2-H2)] (160 mg,
ACHTUNGTRENNUNG(=CHBr)(Br)(H)(NO)ACTHUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
J
N
ACHTUNGTRENNUNG
1
A
ACHTUNGTRENNUNG
an orange solid. Yield: 52 mg, 79%. H NMR (200.0 MHz, [D6]benzene):
AHCTUNGTRENNUNG
d=2.78 (m, 3H; P-CH
P-CH
(CH3)2); 13C{1H} NMR (75.5 MHz, [D6]benzene): d=134.5 (s;
CH3CN), 26. 6 (s), 26.3 (s), 18.8 (t, J(P,C)=5 Hz; P-CH(CH3)2), 2.6 ppm
(br; CH3CN); 31P{1H} NMR (80.9 MHz, [D6]benzene): d=13.9 ppm (s);
ACHTNUGRTNE(NUGN CH3)2), 1.25 (s, 6H; CH3CN), 1.17 ppm (m, 18H;
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
A
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
IR (ATR): n˜ =2961 (nACHTUNGRTEN(NNUG C-H)), 2906 (nACHNUTRTGEG(NNUN C-H)), 2874 (nCAHTNUGTRENUN(GN C-H)), 2275
a
N
CHTUNGTRENNUNG
(n(CN)), 1685 cmÀ1 (n(NO)); elemental analysis calcd (%) for
C13H27Br2N3OPRe (618.36): C 25.25, H 4.40, N 6.80; found: C 25.42, H
4.48, N 6.47.
0.16 mmol) and allylbromide (16 mL, 1.60 mmol) were mixed in toluene
(5 mL). The mixture was stirred at 708C for 24 h. During the reaction, a
blood-red leaflike precipitate was formed gradually. The precipitate was
separated and washed with toluene (2ꢄ5 mL) and dried in vacuo to give
Proof of SET mechanism for the formation of 6b: In a 3 mL NMR spec-
troscopic tube equipped with a Young tap, [Re(Br)2(NO)ACHTNUTRGENN(UG PiPr3)2ACHTUNGTRENNUNG
(h2-H2)]
a blood-red solid. Yield: 93 mg, 53%. IR (ATR): n˜ =2924 (n
ACHTUNGTRENUN(NG C-H)), 2846
(n
ACHTUNGTRENNUNG
(7 mg, 0.01 mmol), TEMPO (3.1 mg, 0.02 mmol), and allylbromide (1 mL,
C39H71Br4NOP2Re (1137.76): C 41.17, H 6.29, N 1.23; found: C 41.30, H
6.23, N 1.26.
0.01 mmol) were mixed in [D6]benzene (0.5 mL). The mixture was kept
at 708C for 5 h to give a deep purple solution. H NMR spectroscopy in-
1
dicated the formation of allyl-radical trapped product TEMPO–CH2CH=
CH2 in 41% yield. 1H NMR (200.0 MHz, [D6]benzene): d=5.85 (m, 1H;
[iPr3PCH2CH=CH2][Re(Br)4(NO)
tube equipped with a Young tap, [Re(Br)2(NO)
G
a
50 mL Schlenk
2ACHTUNGTRENNUNG
(h2-H2)] (65 mg,
G
ACTHNUTRGNEUNG
-CH=CH2), 5.28 (d, 3J(H,H)=18H, 1H; -CH=CHH(trans)), 5.04 (d, 3J-
0.10 mmol) and allylbromide (70 mL, 0.7 mmol) were mixed in toluene
(5 mL). The mixture was stirred at 708C. After 30 min, the color of the
solution changed from light yellow to blood-red. After another 1.5 h, a
blood-red leaflike precipitate was formed, which was separated and
washed with toluene (2ꢄ5 mL) and dried in vacuo to give a blood-red
ACHUTNGERNNUG ACHTUNGTNER(NUGN H,H)=4H, 2H; CH2-CH=
(H,H)=10H, 1H; -CH=CHH(cis)), 4.36 (d, 3J
CH2), 0.93–1.48 (m, 6H; -CH2-), 1.17 (s, 6H; CH3), 1.22 ppm (s, 6H;
CH3).
Blank reaction of TEMPO and allylbromide: In a 3 mL NMR spectro-
scopic tube, TEMPO (4.5 mg, 0.03 mmol) and allylbromide (1 mL,
0.01 mmol) were mixed in [D6]benzene (0.5 mL) and were kept at 708C
for 5 h. 1H NMR spectroscopy indicated that no reaction occurred.
solid. Yield: 75 mg, 90%. IR (ATR): n˜ =2970 (nACTHNUGRTENNU(G C-H)), 2886 (nHCATUNGTRNEN(UGN C-H)),
1731 cmÀ1 (n(NO)); elemental analysis calcd (%) for C21H47Br4NOP2Re
(897.38): C 28.11, H 5.28, N 1.56; found: C 28.39, H 5.44, N 1.62.
Homocoupling of benzyl- or allylbromide with Zn catalyzed by ReII com-
[Cy3PCH2Ph][Re(Br)4(NO)
T
a
50 mL Schlenk tube
equipped with Young tap, [Re(Br)2(NO)
a
G
CHTUNGTRENNUNG
plexes: In
a 20 mL vial in a glove box, benzyl- or allylbromide
(0.5 mmol), zinc (13 mg, 0.25 mmol), and ReII catalyst 3a or 3b
(0.005 mmol) were mixed in [D8]THF (0.5 mL). The mixture was violent-
ly stirred at room temperature for 10 min. The yield of the homocoupling
product was determined by 1H NMR spectroscopy and GC–MS. In the
case of allylbromide, the color of the solution was observed to turn from
brown to gray-yellow and the zinc was almost consumed within 10 min.
0.10 mmol) and benzylbromide (50 mL, 0.40 mmol) were mixed in toluene
(5 mL). The mixture was stirred at 708C for 15 h. After cooling to room
temperature, a blood-red precipitate was formed. The precipitate was
separated and washed with toluene (2ꢄ2 mL) and pentane (5ꢄ2 mL)
and dried in vacuo to give an orange-red solid. Yield: 60 mg, 51%. IR
(C-H)), 1730 cmÀ1 (n(NO)); elemental
(ATR): n˜ =2930 (nACHTUNGTRENNUNG(C-H)), 2849 (nCAHTUNGTRENNUGN
Chem. Eur. J. 2010, 16, 2240 – 2249
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2247

H. Berke et al.
Dehydrogenative silylation of alkenes with Et3SiH catalyzed by mono-
phosphine ReI complexes: In a Schlenk tube equipped with a Young tap,
Et3SiH (0.25 mmol), alkene (0.50 mmol), and an appropriate amount of
rhenium catalyst (0.0025) were mixed in [D6]benzene (0.5 mL). The mix-
ture was kept stirring in the closed system at 70 or 1008C. After an ap-
propriate reaction time, the yield and product distribution were deter-
R. Poli, J. Am. Chem. Soc. 2001, 123, 9513; e) C. Aguilar-Lugo,
R. L. Lagadec, D. R. Alexander, G. C. Valverde, S. L. Morales, L.
Alexandrova, J. Polym. Sci. Part A 2009, 47, 3814; f) Y. Kotani, M.
Kamigaito, M. Sawamoto, Macromolecules 1999, 32, 2420; g) K. Ma-
tyjaszewski, J. Xia, Chem. Rev. 2001, 101, 2921; h) M. Kamigaito, T.
Ando, M. Sawamoto, Chem. Rev. 2001, 101, 3689; i) H. Takahashi, T.
Ando, M. Kamigaito, M. Sawamoto, Macromolecules 1999, 32, 6461;
j) I. del Rio, G. van Koten, M. Lutz, A. L. Spek, Organometallics
2000, 19, 361.
1
mined by H NMR spectroscopy and GC–MS.
Crystal structure analyses of 3a, 6a, and 8a: Crystallographic data were
collected at 183(2) K using an Oxford Xcalibur diffractometer (4-circle
kappa platform, Ruby CCD detector and a single-wavelength Enhance
X-ray source with MoKa radiation, l=0.7107 ꢃ).[25] The selected suitable
single crystals were mounted using polybutene oil on the top of a glass
fiber fixed on a goniometer head and were immediately transferred to
the diffractometer. Pre-experiment, data collection, face-indexing analyti-
cal absorption correction,[26] and data reduction were performed using
the Oxford program suite CrysAlisPro.[27] The structures were initially
solved from heavy-atom location by Patterson interpretation (SHELXS-
97) and were refined by full-matrix least-squares methods on F2
(SHELXL-97).[28] All programs used during the crystal-structure determi-
nation processes are included in the WINGX software.[29] The program
PLATON[30] was used to check the result of the X-ray analyses. More de-
tailed information about the structure refinements of 3a, 6a, and 8a and
the crystal structures of 6b, 7a, and 7b are given in the Supporting Infor-
mation. The X-ray crystal-structure data for 3a, 6a, and 8a can be found
in references [14,22,24], respectively. CCDC-748180 (3a), -748181 (6a),
-748182 (6b), -748183 (8a), -748184 (7a), and -748185 (7b) contain the
supplementary crystallographic data for this paper. These data can be ob-
tained free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
[9] a) A. Choualeb, O. Blacque, H. W. Schmalle, T. Fox, T. Hiltebrand,
H. Berke, Eur. J. Inorg. Chem. 2007, 5246; b) A. Llamazares, H. W.
Schmalle, H. Berke, Organometallics 2001, 20, 5277; c) D. Gusev, A.
Llamazares, G. Artus, H. Jacobsen, H. Berke, Organometallics 1999,
18, 75; d) H. Jacobsen, V. Jonas, D. Werner, A. Messmer, J. C.
Panitz, H. Berke, W. Thiel, Helv. Chim. Acta 1999, 82, 297; e) H.
Berke, P. Burger, Comments Inorg. Chem. 1994, 16, 279; f) A.
Choualeb, E. Maccaroni, O. Blacque, H. W. Schmalle, H. Berke, Or-
ganometallics 2008, 27, 3474; g) Y. Jiang, H. Berke, Chem. Commun.
2007, 3571.
[10] Y. F. Jiang, O. Blacque, T. Fox, C. M. Frech, H. Berke, Chem. Eur. J.
2009, 15, 2121.
[11] a) Y. F. Jiang, O. Blacque, T. Fox, C. M. Frech, H. Berke, Organome-
tallics 2009, 28, 5493; b) Y. F. Jiang, O. Blacque, T. Fox, C. M. Frech,
H. Berke, Organometallics 2009, 28, 4670.
[12] A. Houmam, Chem. Rev. 2008, 108, 2180.
[13] J. M. Khurana, S. Chauhan, G. C. Maikap, Org. Biomol. Chem. 2003,
1, 1737.
[14] X-ray
crystal-structure
data
for
3a:
brown
crystals;
C36H67Br2NOP2Re; Mr =937.86; crystal size: 0.36ꢄ0.15ꢄ0.13 mm3;
orthorhombic, space group P212121; a=12.5510(1), b=17.0330(2),
c=18.3518(2) ꢃ; V=3923.27(7) ꢃ3; Z=4; 1calcd =1.588 MgmÀ3; F-
AHCTUNGTRENNUNG
(000)=1892; m=5.243 mmÀ1; 58426 reflections (2qmax =618), 11968
unique; (Rint =0.0528), 406 parameters; R1 (I>2s(I))=0.0252; wR2
(all data)=0.0514; GOF=0.989; largest difference peak and hole:
Acknowledgements
1.192 and À1.129 eꢃÀ3
.
We thank Prof. Wolfgang Kaim from University of Stuttgart, Germany,
for his help with the interpretation of the ReII EPR spectra. Support
from the Swiss National Science Foundation, the European COST Pro-
gramme, and the Funds of the University of Zꢁrich are gratefully ac-
knowledged.
[15] D. G. Gusev, R. Kuhlman, J. R. Rambo, H. Berke, O. Eisenstein,
K. G. Caulton, J. Am. Chem. Soc. 1995, 117, 281.
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[22] X-ray crystal-structure data for 6a: red crystals; C39H71Br4NOP2Re;
Mr =1139.73; crystal size: 0.22ꢄ0.12ꢄ0.10 mm3; triclinic, space
2248
www.chemeurj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 2240 – 2249

Rhenium(II) Complexes
FULL PAPER
¯
group P1; a=10.94810(10); b=14.1262(3), c=28.4646(7) ꢃ; a=
0.0661; GOF=0.917; largest difference peak and hole: 1.468 and
93.6436(19), b=91.4048(15), g=90.2123(14)8; V=4391.91(15) ꢃ3;
À0.968 eꢃÀ3
.
Z=4; 1calcd =1.721 MgmÀ3; F
(000)=2260; m=6.509 mmÀ1; 51426 re-
[25] Oxford Diffraction, Xcalibur CCD system, Oxford Diffraction Ltd,
Abingdon, England, 2007.
[26] R. C. Clark, J. S. Reid, Acta Crystallogr. Sect. A 1995, 51, 887–897.
[27] CrysAlisPro, Versions 1.171.32/34d, Oxford Diffraction Ltd, Abing-
don, England.
flections (2qmax =548); 18732 unique (Rint =0.0386); 865 parameters;
R1 (I> 2s(I))=0.0652; wR2 (all data)=0.1754; GOF=1.109; larg-
est difference peak and hole: 3.061 and À1.889 eꢃÀ3
.
[23] B. Machura, Coord. Chem. Rev. 2005, 249, 2277.
[24] X-ray
crystal-structure
data
for
8a:
brown
crystals,
[28] G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, 64, 112–122.
[29] L. J. Farrugia, J. Appl. Crystallogr. 1999, 32, 837.
[30] A. L. Spek, J. Appl. Crystallogr. 2003, 36, 7–13.
C40H57Br2N3OPRe; Mr =972.98; crystal size: 0.45ꢄ0.19ꢄ0.10 mm3;
¯
triclinic, space group P1; a=12.3974(2), b=13.5874(2), c=
14.2977(3) ꢃ; a=68.242(2), b=71.012(2), g=83.211(1)8; V=
2115.12(8) ꢃ3; Z=2; 1calcd =1.528 MgmÀ3
;
FACHTUNGTRENNUNG
=
Received: September 22, 2009
Published online: January 11, 2010
Chem. Eur. J. 2010, 16, 2240 – 2249
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2249