Stable BH3adducts to rhodium amide bonds

Article abstract of DOI:10.1016/j.jorganchem.2016.05.019

Rh(I) diolefin amides, [Rh(trop₂N)(L)] (trop₂N?=?bis(5H-dibenzo[a,d]cyclohepten-5-yl)amide), form the corresponding hydrido amine species, [RhH(trop₂NH)(L)], by reaction with Me₂HN-BH₃(DMAB). Both amide and amine complexes are active dehydrocoupling catalysts, forming the monomer [Me₂N?=?BH₂], the linear [Me₂NHBH₂NMe₂BH₃] and the cyclic dimer [Me₂BNH₂]₂. Good catalytic activity was observed especially for complexes which contain a metal hydride unit, Rh–H, in a co-planar cis-arrangement with respect to the N-H unit and for those bearing an N-heterocyclic carbene ligand (IMe) in trans-position to the active basic site of the ligand. Four-membered Rh–N–B–H metallacycles [Rh{(μ-H)BH₂}(Ntrop₂)(L)] (L?=?PPh₃, IMe) were isolated by direct reaction of the amide complex with BH₃(THF). These stable species are not active in the dehydrogenation of DMAB. Their isolation and lack of reactivity gives some indication for a possible catalyst deactivation. This observation is consistent with a mechanism in which the unprotected amine or amide ligand is essential for N–H and B–H bond cleavage.

Full text of DOI:10.1016/j.jorganchem.2016.05.019

Journal of Organometallic Chemistry xxx (2016) 1e8
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Stable BH3 adducts to rhodium amide bonds
,
Fabian Müller a, Monica Trincado a *, Bruno Pribanic a, Matthias Vogt b,
a, **
€
Hansjorg Grützmacher
a Laboratory of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1, 8093 Zürich, Switzerland
b
€
€
Institut für Anorganische Chemie und Kristallographie, Universitat Bremen, Leobener Straße e Gebaude NW2, 28359 Bremen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Rh(I) diolefin amides, [Rh(trop2N)(L)] (trop2N ¼ bis(5H-dibenzo[a,d]cyclohepten-5-yl)amide), form the
corresponding hydrido amine species, [RhH(trop2NH)(L)], by reaction with Me2HN-BH3 (DMAB). Both
amide and amine complexes are active dehydrocoupling catalysts, forming the monomer [Me2N ¼ BH2],
the linear [Me2NHBH2NMe2BH3] and the cyclic dimer [Me2BNH2]2. Good catalytic activity was observed
especially for complexes which contain a metal hydride unit, RheH, in a co-planar cis-arrangement with
respect to the N-H unit and for those bearing an N-heterocyclic carbene ligand (IMe) in trans-position to
Received 16 February 2016
Received in revised form
23 May 2016
Accepted 24 May 2016
Available online xxx
the active basic site of the ligand. Four-membered RheNeBeH metallacycles [Rh{(m-H)BH2}(Ntrop2)(L)]
Keywords:
(L ¼ PPh3, IMe) were isolated by direct reaction of the amide complex with BH3(THF). These stable
species are not active in the dehydrogenation of DMAB. Their isolation and lack of reactivity gives some
indication for a possible catalyst deactivation. This observation is consistent with a mechanism in which
the unprotected amine or amide ligand is essential for NeH and BeH bond cleavage.
© 2016 Elsevier B.V. All rights reserved.
BeH activation
NeH activation
Cooperative ligand
Rhodium
Olefin ligand
1. Introduction
BeH bond can be induced with the help of a basic ligand in an
adjacent or remote position to the metal center. The heterolytic
Binding and activation of molecular hydrogen (H2) by metal
complexes leading to homolytic or heterolytic bond cleavage has
been intensively investigated due to its relevance for catalytic hy-
cleavage of the BeH bond (or s-bond metathesis) depends on the
nature of the metal center and the cooperative ligand (Scheme 1).
Addition of a BeH bond across a metal oxygen bond is rather
common [9]. For example, a bifunctional pincer ligand with
pendent hydroxyl groups coordinated to Ru(II) allowed the addi-
tion of HeBPin (pinacol borane) across a Ru(II)eO bond. This step is
involved in the catalytic hydroboration of nitriles [10]. Stradiotto
[11] and Oestreich [12] showed that [Ru]eX (X ¼ O or S) units
interact with BeH bonds of borane esters. The addition of the BeH
bond of boronic esters across the metal center and a basic site in a
dearomatized pincer (PNP or PNN) ligand was reported by Milstein
et al. [13] Love et al. reported on rhodium phosphoramidate com-
plexes where a B-H bond coordinates to a RheN or IreN bond. From
these intermediates, a borane group can be transferred chemo-
selectively to organic substrates [14]. An early example of BeH
bond activation in a boron ester was reported by Fryzuk et al. who
described the addition across a TaeN bond leading to a complex
with a functionalized N2 ligand and a terminal hydride ligand as
intermediate, which underwent spontaneous NeN bond cleavage
[15]. Other examples of BeH bond addition across MeN moieties
concern the reversible interaction of boranes with non-innocent
palladium or ruthenium pincer complexes, where the boron atom
adds to the basic amine moiety [16].
drogenations [1]. The s-coordination and activation of BeH bonds
of boranes or borane-Lewis base adducts resemble closely the one
of dihydrogen, and complexes with coordinated BeH bonds serve
as models for the activation and functionalization of other XeH
bonds (X ¼ Si, C) in silanes and hydrocarbons, especially methane
[2]. Furthermore, the activation of the BeH bond is relevant for
catalytic processes including the borylation of hydrocarbons [3],
the hydroboration of unsaturated organic molecules [4] and
dehydrogenation of boranes and borane adducts [5].
Typically, coordination of the borane compound to a metal
center precedes the activation process and many
s-borane com-
plexes have been isolated [6,7]. In the activation step, the BeH bond
is oxidatively added to the metal center under the formation of
hydrido boryl intermediates [8]. Alternatively, the activation of the
* Corresponding author.
** Corresponding author.
E-mail addresses: trincado@inorg.chem.ethz.ch (M. Trincado), hgruetzmacher@
ethz.ch (H. Grützmacher).
http://dx.doi.org/10.1016/j.jorganchem.2016.05.019
0022-328X/© 2016 Elsevier B.V. All rights reserved.
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2
F. Müller et al. / Journal of Organometallic Chemistry xxx (2016) 1e8
BHRNR3
H
amide, IMe ¼ 1,3,4,5-tetramethylimidazole-2-ylidene) are acces-
LnM
sible by deprotonation of the amine precursors 1a and 1b with base
[tBuOK for 2a or Na[N(SiMe3)2 for 2b] [25]. An alternative and high-
yielding synthesis of 2b is a ligand exchange reaction whereby the
PPh3 ligand in the amide complex 2a is displaced by addition of a N-
heterocyclic carbene (Scheme 2) [26]. These tetra-coordinated 16
electron rhodium(I) amido complexes 2 adopt a butterfly-type
geometry (that is a trigonal bipyramidal structure with one
missing ligand in the equatorial plane). The rhodium centers are
BHRNR3
H
BR2
H
Shimoi
BHR2
H
BR2
H
LnM
LnM
LnM
R
H
H
LnM
B
oxidative addition
-borane
NR3
Weller
BR2NR3
BR2
coordinated to two olefinic units as
positions and to an amido and phosphane or N-heterocyclic car-
bene as -donor groups placed mutually trans in the axial positions.
p-acceptors in the equatorial
H
LnM
LnM
X
H
X
LnM
H
X
LnM
X
H
BR2
H
BR2
BR2
s
NR3
The corresponding amino hydride complexes 3a or 3b are formed
by a rapid reaction of 2a and 2b with H2. In a similar manner,
addition of 1.0 equivalents of Me2HNeBH3 (DMAB) to a green so-
lution of [Rh(trop2N)(PPh3)] (2a) results in an immediate colour
change to yellow. The formation of a mixture of hydride complexes
[Rh(eq-H)(trop2NH)(PPh3)] (3aeq) and [Rh(ax-H)(trop2NH)(PPh3)]
(3aax) in a 10:1 ratio and the cyclic dimer [Me2NBH2]2 is observed.
The isomerization of the equatorial hydride complex to the com-
plex with a hydride in axial position, which is 40 kcal molꢀ1 more
stable, has been observed previously [22]. This isomerization re-
action is catalyzed by base and/or accelerated thermally. In the
reaction of the carbene analog 2b, only the isomer with the hydride
ligand in equatorial position (3beq) is formed. Complex 3beq is
stable and no conversion to its isomer 3bax was observed even after
prolonged reaction times. Reactions of amide complexes 2a,b with
the adduct BH3(THF) proceed to completion after few minutes at
Scheme 1. Different coordination modes of BeH bonds ranging from
s
-type com-
plexes to oxidative addition. Top: Interaction of single metal centers with BeH bonds;
bottom: Interaction of complexes showing metal ligand cooperativity, MeX, with BeH
units.
The interaction of Lewis baseeborane complexes with metal
carbonyl fragments has been studied by Shimoi et al. (Scheme 1)
[17]. In s-borane complexes (see Scheme 1 right), back-donation of
electron density from the metal into the unoccupied p orbital of
boron is highly efficient. In contrast, in metal-complexes of Lewis-
base borane-adducts, electron donation of the BeH
metal, with minor electron back-donation into the
s
-bond to the
s
*BeH orbital is
involved in Me(BeH) bonding, which results in highly labile li-
gands. Significant efforts were made in order to investigate and
understand the coordination and activation of amine boranes by
metal complexes, which involve successive BeH and NeH bond
activation [18]. Mechanistic studies focused on the catalytic dehy-
dropolymerization of R3-xHxN-BH3 [19]. Subtle differences in the
structure of the catalyst control the selectivity of these reactions
and lead to drastic structural differences of the final products and
the degree of dehydrogenation. Fagnou [20], Schneider [21] and
Williams [22] reported on different ruthenium-catalyzed amine
borane dehydrogenation reactions that proceed through an outer-
sphere hydrogen transfer mechanism involving a cooperative
ligand. A concerted Hþ/Hꢀ transfer from an amine borane to a Ru-
amide moiety under the release of R2N ¼ BH2 and rate-determining
H2 elimination from the Ru hydrido amine complex was proposed.
Furthermore, the catalyst deactivation by formation of a four-
membered Ru-N-B-H metallacycle was observed spectroscopically
[21b].
We have previously described a rhodium(I) amide diolefin
complex that splits molecular hydrogen heterolytically in a one-
step process across the RheN bond resulting in the formation of
the hydride species [Rh(eq-H)(trop2NH)(PPh3)] (3aeq) [23]. Both,
the rhodium amide and the corresponding amine hydride species
are active catalysts for the hydrogenation of ketones and imines.
The complexes are also highly efficient catalysts for the dehydro-
genative coupling (DHC) of alcohols to acids, esters and amides
under mild conditions using a hydrogen acceptor [24]. The
reversible formation of the hydride amine complex 3aeq is the key
transformation in these catalytic processes. Herein, we report on
the interaction of amine borane adducts, Me2HNeBH3 (DMAB) and
BH3(THF), with rhodium(I) amide complexes and their catalytic
activity in the dehydrogenative coupling of DMAB. Possible reasons
for catalyst deactivation are suggested based on the isolation of two
stable complexes in which a BeH bond coordinates across the
RheN moiety. These complexes with a planar NeRheHeB cycle are
remarkably stable and show no catalytic activity.
room temperature to give the yellow complexes [Rh{(
BH2}(Ntrop2)(PPh3)] (4a) and [Rh{( -H)BH2}(Ntrop2)(IMe)] (4b)
which were isolated as single crystals in good yields.
m-H)
m
The solid-state structures of complexes 4a and 4b were deter-
mined by single-crystal X-ray diffraction analyses (Fig. 1, Table 1, 3
and 4). Both complexes show a five-coordinated d8-Rh center co-
ordinated to two olefinic units, the N atom of the trop ligand, the
ligand L ¼ PPh3 or IMe, respectively and a hydride stemming from
the BH3 unit. The B1eN1 interatomic distances [1.551(3) Å for 4a
and 1.562(8) Å for 4b] are comparable to the lengths of dative bonds
in other R3NeBH3 metal complexes [18a] and metal amidotrihy-
droborates (1.56e1.52 Å) [27a] but significantly shorter than in free
amine borane adducts (e.g. 1.617(4) in Me3NeBH3) [27b]. The co-
ordination of BH3 leads only to slight variations of the RheN1, co-
ordinated C ¼ Ctrop, and RheL (L ¼ P1 or C31) bond lengths with
respect to the amides 2a,b (Table 1).
Differences in the binding of the BH3 molecules in 4a and 4b are
indicated by the NMR spectra in solution of both complexes. A
solution of complex 4a in d8-THF shows 1H NMR resonances in
accordance with the solid state structure. A broad resonance at
d
¼ ꢀ6.96 ppm (at 298 K) is assigned to the bridging hydrogen
between the boron and rhodium center, while the terminal BH2
group exhibits a chemical shift at higher frequencies (ꢀ0.55 ppm).
In contrast, the 1H NMR spectrum of complex 4b exhibits only one
broad resonance at ꢀ1.79 ppm at 298 K for the BH3 group. At lower
temperatures (<193 K), this signal splits into one high-field reso-
nance at ꢀ5.76 ppm and one at 0.22 ppm in approx. 1:2 intensity
ratio corresponding to the hydride ligand and terminal BH2
respectively (Fig. 2). The bridging hydride and the terminal hy-
drides of the BH2 group in 4b undergo an exchange process, which
formally can be described as a rotation of the BH3 unit over the
RheN bond. This phenomenon suggests a weaker coordination of
the borane in the N-heterocyclic carbene complex 4b.
Temperature-dependent 1H 1D selective gradient EXSY spectra
were recorded for solutions of compound 4b which allowed to
estimate experimentally the barrier to rotation around the B-N
2. Results and discussion
The complexes [Rh(trop2NH)(PPh3)] (2a) and [Rh(trop2N-
H)(IMe)] (2b) (trop2N ¼ bis(5-H-dibenzo[a,d]cyclohepten-5-yl)
bond as E# ¼ (5.5 0.5) kcal/mol, which is about twice as high as
rot
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F. Müller et al. / Journal of Organometallic Chemistry xxx (2016) 1e8
3
Scheme 2. Synthesis of rhodium amides 2a and 2b and their reaction with amino boranes or BH3 to 2a,b and 3a,b, respectively.
Fig. 1. Molecular structures derived from single crystal X-ray diffraction analysis. ORTEP plot of 4a at 50% ellipsoid probability. Non-relevant H atoms and solvent molecules are
omitted for clarity. Selected bond lengths (Å) and angles (ꢁ): A) Rh1-N1 2.085(1), Rh1-P 2.2730(6), N1-B1 1.551(3), Rh1-H1B 1.69(3), Rh1-ct 2.081(1), P1-Rh1-N1 165.08(5), B1-N1-
Rh1 79.43(12). Complex 4b: B) Rh1-N1 2.089(5), Rh1-C31 2.042(6), N1-B1 1.562(8), Rh1-H1 1.74(5), Rh1-ct 2.042(1), Rh1-ct 2.047(1), C31-Rh1-N1 170.65(19), B1-N1-Rh1 80.9(3).
in the adducts H3NeBH3 (2.04 kcal/mol) [28] or Me3NeBH3
(3.39 kcal/mol) [26].
d
(
11B) ¼ ꢀ13.2 ppm which is in the range of four-coordinated boron
compounds. In both 4a and 4b, the hydride ligand in the pseudo-
equatorial position provokes a shift of the olefinic 13C NMR reso-
nances to lower frequencies by about 15 ppm (average) as a
The higher degree of activation of the BeH unit in 4a is also
supported by the 11B NMR spectrum, which shows a significant
shift to higher frequency at
for three-coordinate boron compounds compared to 4b at
d
(
11B) ¼ ꢀ0.4 ppm - the expected range
consequence of
s-electron donation of the hydride to the metal
center which in turn induces higher electron back-donation into
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4
F. Müller et al. / Journal of Organometallic Chemistry xxx (2016) 1e8
Table 1
Comparison of bond distances and NMR chemical shifts of complexes 2, 3 and 4.
Complex
RheN
Rhect
C]Ctrop
RheX
BeN
d
(
11B)
d
(
13C)C]Ctrop (ppm)
d
(1H)H/M (ppm)
2aa
2.00
2.07
2.06
2.04
2.06
e
1.41
1.42
1.41
1.44
e
2.32
e
e
76.2, 84.5
e
2b
2.05
2.18
e
2.02
2.23
e
e
e
e
e
e
75.7, 81.3
57.8, 60.6
57.1, 61.2
52.3, 54.1
65.2, 66.5
62.4, 62.5
e
3a_eq
3a_ax
3b_eq
4a
ꢀ8.15
e
ꢀ21.37
e
e
e
e
e
ꢀ6.30
2.08
2.089
2.08
2.04, 2.05
1.42
1.43
2.27
2.04
1.55
1.56
ꢀ0.4
ꢀ6.96 (298 K)
ꢀ1.79 (298 K)
ꢀ5.76 (193 K)
4b
ꢀ13.2
a
The bond lengths of the [Rh(trop2N)(PR3)] complex with R ¼ 4-Me-C6H4 are given [22].
Fig. 2. a) High field region of the 1H NMR spectra (500 MHz, d8-THF) of complex 4b as a function of temperature. b) Section of the IR spectra section of complexes 4a, 4b, Me3NeBH3
and K[Me2NeBH3] showing the stretching vibrations of the BeH groups, (BeH).
n
the
p
*-orbitals of the coordinated C ¼ Ctrop units. Note, however,
indicating a significantly stronger rhodium hydride interaction in
these compounds.
that the corresponding resonances in the amino hydride complexes
3aeq and 3beq are shifted by another 10 ppm to lower frequencies
The IR spectra of the borane complexes 4a and 4b together with
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F. Müller et al. / Journal of Organometallic Chemistry xxx (2016) 1e8
5
Table 3
the spectra of the potassium dimethylamidotrihydroborate K
[Me2NBH3] [
borane Me3NeBH3 [
n
(BeH) ¼ 2192, 2120, and 2052 cmꢀ1] and of the amino
Crystallographic data for complex 4a.
n
(BeH) ¼ 2350, 2310, 2282 and 2263 cmꢀ1] are
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
C50H44BCl4NPRh
945.35
100(2)
Orthorhombic
Pnma
25.9576(15)
13.4699(8)
11.9483(7)
90.00
90.00
90.00
4177.7(4)
4
1.503
0.741
1936.0
shown in Fig. 2b. The Rh-H stretching vibration in 3aeq is observed
at 1727 cmꢀ1 [29,30]. In the IR spectra of 4a,b there is no absorption
band observed which can be associated with such a Rh-H stretching
vibration. For complex 4a, four absorptions at 2447, 2374, 2278 and
2213 cmꢀ1 are recorded which are tentatively assigned to vibra-
b/Å
tions of the m
2-bridging BeH and BH2 group. In complex 4b, these
c/Å
ꢁ
ꢁ
a
b
g
/
/
stretching vibrations are observed at slightly higher wave numbers
at 2457, 2428 and 2365 cmꢀ1. If one assumes that an increasing
strength of the N/B donor-acceptor interaction is reflected in an
increasing B-H bond length and a concomitant decrease of the wave
numbers for the B-H stretching vibration, these results indicate a
comparatively weaker N/B interaction in 4a,b [31]. That is the
ꢁ
/
Volume/Å3
Z
rcalc g/cm3
m
/mmꢀ1
F(000)
Crystal size/mm3
Radiation
0.50 ꢂ 0.36 ꢂ 0.32
N/B
bond
strength
decreases
in
the
order
[Me2N/BH3]ꢀ > Me3N/BH3 > Rhtrop2N/BH3 (4a,b), although
this statement requires a detailed analysis which is beyond the
scope of this work. In summary, the IR spectra in combination with
the NMR spectra indicate that the borane adducts 4a,b are best
described as (metallo)amino borane adducts with a relatively
weaker N/B bond and a weak but significant RheH interaction.
Next, we studied the catalytic dehydrocoupling of DMAB by
complexes 2a-b, 3a-b and 4a-b as catalysts (Table 2 and Fig. 3). In
an open system purged by a stream of argon, the addition of the
rhodium amides 2a or 2b (0.2 mol%) to a toluene solution of DMAB
at 50 ꢁC results in Analysis of the reaction mixtures revealed the
formation of the linear and cyclic dimers (C and D) as major
products. Moderate turnover frequencies (TOFs) of up to 185 hꢀ1
were achieved. An increase in the catalyst loading of 2b to 10 mol%
resulted in a nearly complete conversion of DMAB (A) after 90 min
(entry 3).
MoK
3.14 to 56.64
a
(
l
¼ 0.71073)
2
Q
range for data collection/ꢁ
Index ranges
ꢀ34 ꢃ h ꢃ 34, ꢀ17 ꢃ k ꢃ 17, ꢀ15 ꢃ l ꢃ 15
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
41959
5392 [Rint ¼ 0.0429, Rsigma ¼ 0.0257]
5392/0/370
1.059
R1 ¼ 0.0285, wR2 ¼ 0.0717
R1 ¼ 0.0333, wR2 ¼ 0.0735
1.06/ꢀ0.99
Final R indexes [I ꢄ 2
s
(I)]
Final R indexes [all data]
Largest diff. peak/hole/e Åꢀ3
Table 4
Crystallographic data for complex 4b.
Empirical formula
Formula weight
Temperature/K
Crystal system
Space group
a/Å
C39H41BCl4N3Rh
807.27
103.4
triclinic
P-1
11.0484(11)
11.6239(10)
14.3834(15)
76.298(4)
87.498(5)
82.833(4)
1780.4(3)
2
The highest conversion of the substrate was obtained with the
equatorial hydride complexes 3aeq and 3beq. However, the best
selectivity with respect to the four-membered B2N2 heterocycle D
was achieved with the carbene complex 2b and the corresponding
equatorial hydride complex 3beq (entries 3 and 5). A small ratio of
b/Å
c/Å
ꢁ
ꢁ
ꢁ
/
a
b
g
/
/
the dehydrogenated monomer Me2N ¼ BH2 (B)
d
¼ 33.1 (d,
Volume/Å3
Z
rcalcg/cm3
1.506
0.813
Table 2
/mmꢀ1
Catalytic dehydrocoupling of Me2HNeBH3 (DMAB) by rhodium diolefin complexes.a
m
F(000)
828.0
Crystal size/mm3
Radiation
0.22 ꢂ 0.04 ꢂ 0.04
MoK
a
(l
¼ 0.71073)
2
Q
range for data collection/ꢁ
4.69 to 49.59
Index ranges
ꢀ13 ꢃ h ꢃ 13, ꢀ13 ꢃ k ꢃ 13, ꢀ16 ꢃ l ꢃ 16
Reflections collected
Independent reflections
Data/restraints/parameters
Goodness-of-fit on F2
33958
6105 [Rint ¼ 0.1663, Rsigma ¼ 0.1090]
6105/0/407
1.011
Final R indexes [I ꢄ 2
s
(I)]
R1 ¼ 0.0556, wR2 ¼ 0.1111
R1 ¼ 0.1041, wR2 ¼ 0.1292
0.69/ꢀ1.08
Final R indexes [all data]
Largest diff. peak/hole/e Åꢀ3
Entry Catalyst tR(h) Conversion (%) B (%) C (%) D (%) E (%) F (%)
1b
2b
3
2a
2b
2b
3aeq
3beq
3aax
4a
3
3
33
37
95
97
>99
48
3
<1
<1
e
18
22
8
<1
12
2
10
14
76
52
76
39
<1
e
5
<1
11
15
12
4
e
JBH123 Hz) is typically observed when the reactions are carried out
at room temperature. At 50 ꢁC besides C and D, small amounts of
other amine boranes like Me2NeBHeNMe2 (E) and B(NMe2)3 (F)
are also detected (Fig. 3). The formation of these products requires
BeN bond cleavage and formation of free BH3 in the reaction media
[32]. Because the reaction of the amido complexes 2a and 2b with
BH3 is fast and the borane complexes 4a and 4b show almost no
activity, it is plausible that the dissociation of amine boranes in
amine and BH3 is responsible for the catalyst deactivation. This
dissociation may occur either within the free amine borane or, even
e
1.5
1.5
1.5
1.5
1.5
1.5
e
4c
5
29
e
<1
e
6c
7
8
e
2
<1
e
e
e
2
e
4b
0
e
e
a
Conditions: 10 mol% catalyst, Me2HNeBH3 (0.06 M) in d8-THF, open system to
an argon stream.
b
Conditions: 0.2 mol% catalyst, Me2HNeBH3 (0.5 M) in toluene, open system to
an argon stream.
c
Additional unidentified minor product also detected11B NMR
d
¼ 27.1 ppm (d,
more likely, from [Rh{(
m-H)BH2NMe2H}(Ntrop2)(L)] species where
JBH ¼ 140 Hz).
the amine boranes coordinate with a BeH bond to the Rh-N bond of
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6
F. Müller et al. / Journal of Organometallic Chemistry xxx (2016) 1e8
Fig. 3. 11B{1H} NMR spectra of catalytic dehydrocoupling of Me2HN-BH3 (DMAB) by rhodium diolefin complexes at 50 ꢁC after 90 min. A: Me2HN-BH3; C: Me2HNeBH2eNMe2eBH3;
D: [Me2NBH2]; E: (Me2N)2BH, F: (Me2N)3B. Additional unidentified minor product (*) detected by 11B NMR
¼ 27.1 ppm (d, JHB ¼ 140 Hz).
d
2a or 2b. These assumptions are bolstered by the observation that
the reaction between 2a and Me3NeBH3, which has no NeH bond,
likewise gives the borane adduct 4a.
in 2a,b behaves as a cooperating ligand in this dehydrogenation
step. However, while a hydrogen acceptor is necessary in the
dehydrogenation reactions of alcohols in order to convert the
amine hydride complex [RhH(trop2NH)(L)] 3a,b back to the amides
2a,b, this is not a requirement in the catalytic dehydrogenation of
DMAB. The amine hydride complexes 3a,b readily react with DMAB
to produce hydrogen. We propose the activated complex TS2 as the
transition state for this reaction as shown in Scheme 3. Two re-
actions were identified which can reduce the catalytic reactivity: i)
If the free amino hydride complex 3aeq with L ¼ PPh3 is formed, this
may rearrange to its isomer 3aax with an axial hydride ligand. In
this case, a much lower conversion of only 39% to the cyclic product
(D) is observed (entry 6, Table 2). The mechanism by which the
Based on the results from this work in combination with our
previously reported calculations [22,23], we propose the mecha-
nism shown in Scheme 3. Me2HNeBH3 (A) is dehydrogenated by
the rhodium amide 2 to the corresponding Me2 ¼ BH2 (B) and the
amino hydride complex 3 is formed. Me2N ¼ BH2 may either
dimerize to cyclic D or react with A to give the linear dimer
Me2HeBH2eMeNeBH3 (C) which is likewise dehydrogenated by 2
to the cyclic product D [33]. We suggest that these dehydrogenation
steps proceed via cyclic activated complexes at the transition states
TS1 as shown on the bottom of Scheme 3. Thus, the amide function
Scheme 3. A plausible simplified sketch of key-transformations in the catalyzed dehydrogenation of amino borane A by rhodium amide complexes 2.
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F. Müller et al. / Journal of Organometallic Chemistry xxx (2016) 1e8
7
4
hydride complex 3aax evolves hydrogen from aminoborane A re-
1H NMR (300 MHz, d8-THF)
d
¼ 4.82 (d, JPH ¼ 13.5 Hz, 2H,
mains speculative but may involve a slow re-isomerization to 3aeq
as the true catalyst. ii) The predominant deactivation reaction,
however, is blocking the active RheN site by BH3 in the complexes
4a and 4b. These latter species are remarkably stable and attempts
to displace the BH3 group by amides [NaN(TMS)2] or amines (N-
methylmorpholine, 2,6-dimethylpiperidine or NMe3) added in
excess gave no reaction. Additionally, transfer of the boron frag-
ment to an unsaturated organic substrate (acetophenone) was also
unsuccessful.
CHbenzyl), 5.17 (s, 4H, CHolefin), 6.72e6.84 (m, 12H, CHarom),
7.00e7.02 (m, 4H, CHarom), 7.49e7.51 (m, 9H, CHarom), 7.61e7.68 (m,
6H, CHarom) ppm.
4.3. Synthesis of [Rh(eq-H)(trop2NH)(PPh3)] (3aeq
)
To a solution of 2a (24.0 mg, 0.03 mmol, 1.0 equiv) in d8-THF
(2 ml) was added Me2HN-BH3 (1.8 mg, 0.03 mmol, 1.0 equiv). After
stirring at room temperature for 10 min, the resulting yellow so-
lution was analysed and used for the catalytic experiments.
1
3. Conclusions
1H NMR (300 MHz, d8-THF)
d
¼ ꢀ8.29 (t, JRhH ¼ 23.1 Hz, 1H,
RhH), 3.54 (s, 2H, CHolefin), 3.86e3.90 (m, 2H, CHolefin), 4.45 (d,
4JPH ¼ 8.1 Hz, CHbenzyl), 5.04 (d, 3JPH ¼ 4.6 Hz,1H, NH), 6.16e7.81 (m,
31H, CHarom) ppm.
The rhodium amide complexes [Rh(trop2N)(L)] and their cor-
responding amine hydride complexes [RhH(trop2NH)(L)] catalyze
the dehydrogenation of amine boranes. Likely this process proceeds
via an outer-sphere hydrogen transfer mechanism which was also
proposed for alcohol dehydrogenations. The best catalytic perfor-
mance (given by the highest yield of 76% of the dehydrogenated
cyclic azaboroethene D and concomitant formation of hydrogen) is
achieved with the carbene complex [Rh(trop2N)(IMe)] 2b and the
corresponding amino hydride [RhH(trop2NH)(IMe)] 3b. In the case
of the hydride complex, the reaction proceeds with a slightly better
overall conversion of the starting material but a higher amount of
the linear dimer is formed. The performance is, however, magni-
tudes of order lower than the one observed with a paramagnetic
Ni(I) complex carrying the same trop2NH ligand [34]. Remarkably
4.4. Synthesis of [Rh(ax-H)(trop2NH)(PPh3)] (3aax
)
A solution of 3aeq (0.004 mmol) in d8-THF (0.6 mL) was heated
to 50 ꢁC for 16 h. Complete isomerization was observed as indicated
by the NMR spectrum.
1H NMR (300 MHz, d8-THF)
d
¼ ꢀ21.52 (t, 1JRhH ¼ 17.7 Hz, 1H,
3
RhH), 0.89 (s, 1H, NH), 4.12 (s, 2H, CHbenzyl), 4.43 (t, JHH ¼ 8.5 Hz,
2H, CHolefin), 5.15e5.19 (m, 2H, CHolefin), 6.20e7.47 (m, 31H, CHarom
)
ppm.
4.5. Synthesis of [Rh(eq-H)(trop2NH)(IMe)] (3beq
)
stable borane complexes [Rh{(m-H)BH2}(Ntrop2)(L)] were isolated.
In these species, a BeH bond binds to the RheN bond configuring a
planar Rh-N-H-B metallacycle. This observation strongly bolsters
calculations which indicate that HeH bond in dihydrogen is
directly heterolytically cleaved via a comparable activated complex
To a solution of 2b (18.7 mg, 0.03 mmol, 1.0 equiv) in d8-THF
(2 ml) was added Me2HN-BH3 (1.8 mg, 0.03 mmol, 1.0 equiv). After
stirring at room temperature for 10 min, the resulting yellow so-
lution was analysed and used for the catalytic experiments.
with a RheNeHeH ring and no
s
-type H2 bonded is formed as
1H NMR (300 MHz, d8-THF)
d
¼ ꢀ6.00 (br, 1H, RhH), 2.13 (s, 3H,
intermediate which is intra- or intermolecularly deprotonated [22].
CH3), 2.16 (s, 3H, CH3), 3.32 (d, 3JHH ¼ 9.2 Hz, 2H, CHolefin), 3.84 (d,
3JHH ¼ 9.2 Hz, 2H, CHolefin), 3.93 (s, 3H, CH3), 4.12 (s, 3H, CH3), 4.32
(s, 2H, CHbenzyl), 6.58e7.16 (m, 16H, CHarom) ppm.
4. Experimental section
4.1. General methods
4.6. Synthesis of [Rh{(m-H)BH2}(Ntrop2)(PPh3)] (4a)
All reactions were carried out under an inert atmosphere using
standard vacuum Schlenk line techniques or in a MBraun glovebox.
THF and n-hexane were dried over sodium, CH2Cl2 was purified
using an Innovative Technologies PureSolv system. Starting mate-
rials were obtained from Sigma Aldrich or Johnson Mattey or syn-
thesized according to the literature, [Rh(COD)Cl]2 [35],
To a solution of [Rh(trop2N)(PPh3)] (25 mg, 0.0328 mmol,
1.0 equiv) in THF (1 mL) was added BH3(THF) (360 L of 1 M in THF,
0.0361 mmol, 1.1 equiv). The green solution immediately turned
yellow forming [Rh{( -H)BH2}(Ntrop2)(PPh3)] as only product. The
m
m
complex was recrystallized from DCM/n-hexane. Yield: 87%, 22 mg.
1H NMR (300 MHz, d8-THF):
d
¼ ꢀ6.96 (br, 1H, BH2H), ꢀ0.55 (br,
3
[Rh(trop2NH)Cl]2
[29],
[RhCl(trop2NH)PPh3]
[24],
2H, BH2H), 4.03 (d, JHH
¼
10.9 Hz, 2H, CHolefin), 4.59 (d,
3
[Rh(trop2N)(PPh3)] [22], [Rh(trop2N)(IMe)] [25] and IMe [36]. NMR
spectra were recorded on Bruker instruments at 300 or 500 MHz.
For 1H and 13C NMR spectra, SiMe4, for 11B NMR spectra BF3(OEt2),
for 31P NMR spectra H3PO4 and for 103Rh NMR spectra Rh(acac)3 was
used as an external standard. Data collections for single-crystal X-
ray diffractions were performed using a Bruker APEX I or Bruker
4JPH ¼ 8.8 Hz, 2H, CHbenzylic), 4.78 (d, JHH ¼ 10.9 Hz, 2H, CHolefin),
6.54e7.79 (m, 31H, CHarom) ppm. 13C{1H}-NMR (75 MHz, CD2Cl2):
d
¼ 65.3 (d, 1JRhC ¼ 10.4 Hz, 2C, Colefin), 66.5 (d, 1JRhC ¼ 8.2 Hz, 2C,
olefin), 77.0 (d, 2JRhC ¼ 2.4 Hz, 2C, Cbenzyl), 125.1e139.9 (Carom) ppm.
C
31P{1H}-NMR (121 MHz, d8-THF):
d
¼ 56.5 (d, 1JRhP ¼ 136.1 Hz) ppm.
11B{1H}-NMR (96 MHz, d8-THF)
(18.8 MHz, CD2Cl2):
¼ ꢀ8078 ppm.
d
¼ ꢀ0.4 ppm (br). 1H103Rh
VENTURE diffractometer (MoKa,
solved with direct methods using SHELX-97 [37] or SHELXT [38]
l
¼ 0.71073 Å). The structures were
d
software and refined via full-matrix least-squares methods [39].
4.7. Synthesis of [Rh{(m-H)BH2}(Ntrop2)(IMe)] (4b)
4.2. Synthesis of [Rh(trop2N)(PPh3)] (2a)
To a cooled solution of [Rh(trop2N)PPh3] (54 mg, 0.071 mmol,
1 equiv) in THF (2 ml) at ꢀ30 ꢁC was added a solution of IMe
(8.8 mg, 0.071 mmol, 1 equiv) in THF (2 ml). After stirring the
To a solution of 1a (150 mg, 0.19 mmol, 1.0 equiv) in THF (2 ml)
was added KOtBu (21.1 mg, 0.19 mmol,1 equiv). After stirring for 1 h
at room temperature, the solvent was removed. The dark green
residue was dissolved in THF (2 ml) and filtered over Celite. The
solution was concentrated under vacuum and layered with toluene
(1 ml) and n-hexane (5 mL). The product was obtained as dark
green microcrystals (110 mg, 0.144 mmol, 77%).
resulting green solution for 10 min at rt, BH3(THF) (0.2 ml of 0.5
M in
THF, 0.1 mmol, 1.4 equiv) was added. The green solution turns
immediately yellow. After removing the solvent in vacuo, the res-
idue was recrystallized from DCM/n-hexane. The product was ob-
tained as pale yellow plates (30 mg, 0.047 mmol, 66%).
1H NMR (500 MHz, CD2Cl2, 298 K):
d
¼ ꢀ1.79 (s, br, 3H, BH3), 2.14
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8
F. Müller et al. / Journal of Organometallic Chemistry xxx (2016) 1e8
f) J.F. Hartwig, S.R.J. De Gala, Am. Chem. Soc. 116 (1994) 3661.
[8] a) For isolated examples of hydrido boryl complexes, see: G.J. Irvine,
M.J.G. Lesley, T.B. Marder, N.C. Norman, C.R. Rice, E.G. Robins, W.R. Roper,
G.R. Whittell, L.J. Wright Chem. Rev. 98 (1998) 2685;
(s, 3H, CH3), 2.17 (s, 3H, CH3), 3.73 (s, 3H, NCH3), 4.00 (dd,
3JHH ¼ 9.3 Hz, 2JRhH ¼ 1.5 Hz, 2H, CHolefin), 4.05 (s, 3H, NCH3), 4.44 (s,
2H, CHbenzylic), 5.17 (dd, 3JHH ¼ 9.3 Hz, 2JRhH ¼ 2.4 Hz, 2H, CHolefin),
6.70e6.76 (m, 6H, CHarom), 6.92e7.10 (m, 10H, CHarom) ppm. 13C
b) H. Braunschweig, C. Kollann, D. Rais, Angew. Chem. Int. Ed. 45 (2006) 5254.
[9] a) M.A. Salomon, T. Braun, A. Penner, Angew. Chem. Int. Ed. 47 (2008) 8867;
{1H}-NMR (75 MHz, CD2Cl2):
d
¼ 10.0 (s, 1C, CH3), 10.4 (s, 1C, CH3),
~
b) A.C. Fernandes, J.A. Fernandes, F.A.A. Paz, C.C. Romao, Dalton Trans. (2008)
1
36.1 (s, 1C, CH3), 38.4 (s, 1C, CH3), 62.42 (d, JRhC ¼ 1.3 Hz, 2C,
CHolefin), 62.55 (d, 1JRhC ¼ 2.5 Hz, 2C, CHolefin), 76.6 (s, 2C, CHbenzylic),
124.4 (s, 2C, Caroma), 125.1 (s, 2C, Carom), 125.5 (s, 1C, Cquart), 125.7 (s,
1C, Cquart),126.9 (s, 2C, Carom),127.0 (s, 2C, Carom), 127.4 (s, 2C, Carom),
127.8 (s, 2C, Carom), 128.1 (s, 2C, Carom), 130.0 (s, 2C, Carom), 137.3 (d,
1JRhC ¼ 1.4 Hz, 1C, Cquart), 138.3 (s, 1C, Cquart), 139.4 (s, 1C, Cquart),
139.5 (s,1C, Cquart),173.1 (d, 1JRhC ¼ 49.8 Hz,1C, Cquart) ppm. 11B{1H}-
6686.
[10] J.B. Geri, N.K. Szymczak, J. Am. Chem. Soc. 137 (2015) 12808.
[11] M.A. Rankin, K.D. Hesp, G. Schatte, R. McDonald, M. Stradiotto, Dalton Trans.
(2009) 4756.
[12] T. Stahl, K. Mu€ther, Y. Ohki, K. Tatsumi, M. Oestreich, J. Am. Chem. Soc. 135
(2013) 10978.
[13] A. Anaby, B. Butschke, Y. Ben-David, L.J.W. Shimon, G. Leitus, M. Feller,
D. Milstein, Organometallics 33 (2014) 3716.
[14] M.W. Drover, L.L. Schafer, J.A. Love, Angew. Chem. Int. Ed. 55 (2016) 1e7.
[15] M.D. Fryzuk, B.A. MacKay, S.A. Johnson, B.O. Patrick, Angew. Chem. Int. Ed. 41
(2002) 3709.
NMR (160.50 MHz, CD2Cl2):
d
¼ ꢀ13.22 ppm (br). 1H103Rh
(18.8 MHz, CD2Cl2):
d
¼ ꢀ7915 ppm.
[16] a) Y. Zhu, C.-H. Chen, C.M. Fafard, B.M. Foxman, O.V. Ozerov, Inorg. Chem. 50
(2011) 7980;
4.8. General procedure for catalytic dehydrogenation of DMAB
b) Yijing Xu, A. Christoph, Rettenmeier, Gudrun, T. Plundrich,
Hubert Wadepohl, Markus Enders, Lutz H. Gade, Organometallics 34 (2015)
5113e5118.
A solution of the catalyst (0.004 mmol, 1 equiv) in THF (0.6 mL)
was added to Me2HN-BH3 (0.04 mmol, 2.7 mg, 10 equiv). The re-
action solution was warmed to 50 ꢁC in an open system connected
to an argon stream. After 90 min the conversion was determined
via 11B NMR spectroscopy.
[17] M. Shimoi, S. Nagai, M. Ichikawa, Y. Kawano, K. Katoh, M. Uruichi, H. Ogino,
J. Am. Chem. Soc. 121 (1999) 11704.
[18] a) A.B. Chaplin, A.S. Weller, Angew. Chem. 122 (2010) 591e594. Angew.
Chem. Int. Ed. 2010, 49, 581 e 584;
b) G. Alcaraz, L. Vendier, E. Clot, S. Sabo-Etienne, Angew. Chem. 122 (2010)
930e932. Angew. Chem. Int. Ed. 2010, 49, 918 e 920;
c) H.C. Johnson, T.N. Hooper, A.S. Weller, Top. Organomet. Chem. 49 (2015)
153.
Acknowledgments
[19] a) A. Staubitz, M.E. Sloan, A.P.M. Robertson, A. Friedrich, S. Schneider,
P.J. Gates, J. Schmedt auf der Gunne, Manners, I. J. Am. Chem. Soc. 132 (2010)
13332;
b) A. Staubitz, A.P.M. Robertson, I. Manners, Chem. Rev. 110 (2010) 4079.
[20] N. Blaquiere, S. Diallo-Garcia, S.I. Gorelsky, D.A. Black, K. Fagnou, J. Am. Chem.
Soc. 130 (2008) 14034e14035.
[21] a) M. Kaess, A. Friedrich, M. Drees, S. Schneider, Angew. Chem. 121 (2009)
922e924. Angew. Chem. Int. Ed. 2009, 48, 905 e 907;
b) A. Friedrich, M. Drees, S. Schneider, Chem. Eur. J. 15 (2009) 10339.
[22] Xingyue Zhang, Zhiyao Lu, Lena K. Foellmer, Travis J. Williams, Organome-
tallics 34 (15) (2015) 3732e3738.
This work was supported by the Swiss National Science Foun-
dation (SNF) (grant no: 200021-162437) and the ETH Zürich. We
thank especially Dr. Rene Verel and Alex Lauber for help with the
spectroscopic analyses.
Supplementary data
[23] P. Maire, T. Büttner, F. Breher, P. Le Floch, H. Grützmacher, Angew. Chem. 117
(2005) 6477. Angew. Chem. Int. Ed. 2005, 44, 6318.
[24] a) T. Zweifel, J. Naubron, H. Grützmacher, Angew. Chem. 121 (2009) 567.
Angew. Chem. Int. Ed. 2009, 48, 559;
The crystallographic data is deposited free of charge at the Cam-
bridge Crystallographic Data Center (CCDC-1451490 (4a), 1450882
(4b) via https://summary.ccdc.cam.ac.uk/structure-summary-form).
b) T. Zweifel, J. Naubron, T. Büttner, T. Ott, H. Grützmacher, Angew. Chem. 120
(2008) 3289. Angew. Chem. Int. Ed. 2008, 47, 3245;
References
€
c) M. Trincado, Hansjorg Grützmacher, F. Vizza, C. Bianchini, Chem. Eur. J. 16
(2010) 2751;
d) M. Trincado, K. Kühlein, H. Grützmacher, Chem. Eur. J. 17 (2011) 11905.
[25] S. Annen, T. Zweifel, F. Ricatto, H. Grützmacher, ChemCatChem 2 (2010)
1286e1295.
[26] T.L. Gianetti, S.P. Annen, G. Santiso-Quinones, M. Reiher, M. Driess,
H. Grützmacher, Angew. Chem. 128 (2016) 1886e1890. Angew. Chem. Int. Ed.
Engl. 2016, 55, 1854 e 1858.
[27] a) For a review on the synthesis and structure of s-block amidoboranes, see:
Tom E. Stennett, Sjoerd Harder Chem. Soc. Rev. 45 (2016) 1112;
b) Aldridge, A.J. Downs, C.Y. Tang, S. Parsons, M.C. Clarke, R.D.L. Johnstone,
H.E. Robertson, D.W.H. Rankin, D.A. Wann, J. Am. Chem. Soc. 131 (2009) 2231.
[28] L.R. Thorne, R.D. Suenram, F.J.J. Lovas, Chem. Phys. 78 (1983) 167.
[29] The observed frequency for the hydride-metal vibrations are in the range of
values obtained for other terminal Rh-H complexes. See for example: C. Kohrt,
S. Hansen, H. Drexler, U. Rosenthal, A. Schulz, D. Heller Inorg. Chem. 51 (2012)
7377e7383 (and references cited herein).
[30] J.D. Odom, J.A. Barnes, B.A. Hudgens, J.R. Durig, J. Phys. Chem. 78 (1974) 1505.
[31] a) For quantitative estimation of the strength of electrostatic bonding and
donor-acceptor bonding of a series of amine ligands and Lewis acids (BH3),
see: P.R. Rablen J. Am. Chem. Soc. 119 (1997) 8350e8360;
b) F. Bessac, G. Frenking, Inorg. Chem. 45 (2006) 6956e6964.
[32] a) (The spectroscopic observation of byproducts E and F as source for free
borane, which leads to catalyst deactivation has been previously proposed:)
X. Chen, J.-C. Zhao, S.G. Shore, J. Am. Chem. Soc. 132 (2010) 10658;
b) A. Glu€er, M. Fo€rster, V.R. Celinski, J. Schmedt auf der Gu€nne,
M.C. Holthausen, S. Schneider, ACS Cat. 5 (2015) 7214.
[33] A.P.M. Robertson, E.M. Leitao, T. Jurca, M.F. Haddow, H. Helten, G.C. Lloyd-
Jones, I. Manners, J. Am. Chem. Soc. 135 (2013) 12670.
[34] M. Vogt, B. de Bruin, H. Berke, M. Trincado, H. Grützmacher, Chem. Sci. 2
(2011) 723e727.
[1] a) G.J. Kubas, Metal Dihydrogen and
New York, 2000;
s-Bond Complexes, Kluwer Academic,
b) M. Peruzzini, R. Poli (Eds.), Recent Advances in Hydride Chemistry, Elsevier,
Amsterdam, 2001;
c) H. Berke, ChemPhysChem 11 (2010) 1837.
[2] C.M. Nagaraja, P. Parameswaran, E.D. Jemmis, B.R. Jagirdar, J. Am. Chem. Soc.
129 (2007) 5587.
[3] a) I.A.I. Mkhalid, J.H. Barnard, T.B. Marder, J.M. Murphy, J.F. Hartwig, Chem.
Rev. 110 (2010) 890;
ꢀ
b) A. Ros, R. Fernandez, J.M. Lassaletta, Chem. Soc. Rev. 43 (2014) 3229;
c) I.A.I. Mkhalid, J.H. Barnard, T.B. Marder, J.M. Murphy, J.F. Hartwig, Chem.
Rev. 110 (2010) 890;
d) J.Y. Cho, M.K. Tse, D. Holmes, R.E. Maleczka, M.R. Smith, Science 295 (2002)
305;
e) M.A. Larsen, C.V. Wilson, J.F.J. Hartwig, Am. Chem. Soc. 137 (2015) 8633;
f) M.A. Larsen, S. Hwan Cho, J. Hartwig, J. Am. Chem. Soc. 138 (2016) 762.
[4] a) K. Burgess, M.J. Ohlmeyer, Chem. Rev. 91 (1991) 1179;
b) L. Dang, Z. Lin, T.B. Marder, Chem. Commun. (2009) 3987.
[5] a) C.W. Hamilton, R.T. Baker, A. Staubitz, I. Manners, Chem. Soc. Rev. 38 (2009)
279;
b) A. Staubitz, A.P.M. Robertson, M.E. Sloan, I. Manners, Chem. Rev. 110 (2010)
4023;
c) R. Waterman, Chem. Soc. Rev. 42 (2013) 5629;
d) A.S. John, K.I. Goldberg, D.M. Heinekey, Top. Organomet. Chem. 40 (2013)
271;
e) E.M. Leitao, T. Jurca, I. Manners, Nat. Chem. 5 (2013) 817.
[6] R.N. Perutz, S. Sabo-Etienne, Angew. Chem. Int. Ed. 46 (2007) 2578.
[7] a) C.N. Muhoro, X. He, J.F.J. Hartwig, Am. Chem. Soc. 121 (1999) 5033e5046;
b) S. Schlecht, J.F. Hartwig, J. Am. Chem. Soc. 122 (2000) 9435;
c) V. Montiel-Palma, M. Lumbierres, B. Donnadieu, S. Sabo-Etienne,
B.J. Chaudret, Am. Chem. Soc. 124 (2002) 5624;
[35] T.G. Schenck, J.M. Downes, C.R.C. Milne, P.B. Mackenzie, H. Boucher, J. Whelan,
B. Bosnich, Inorg. Chem. 24 (1985) 2334e2337.
[36] N. Kuhn, T. Kratz, Synth. (Stuttg) (1993) 561e562.
[37] G.M. Sheldrick, T.R. Schneider, Methods Enzymol. 277 (1997) 319e343.
[38] G.M. Sheldrick, Acta Crystallogr. Sect. A Found. Adv. 71 (2015) 3e8.
[39] G.M. Sheldrick, Acta Crystallogr. Sect. C Struct. Chem. 71 (2015) 3e8.
d) S. Lachaize, K. Essalah, V. Montiel-Palma, L. Vendier, B. Chaudret, J.-
C. Barthelat, S. Sabo-Etienne, Organometallics 24 (2005) 2935;
e) M.G. Crestani, M. Munoz-Hernandez, A. Arevalo, A. Acosta-Ramırez,
J.J. Garcıa, J. Am. Chem. Soc. 127 (2005) 18066;
Please cite this article in press as: F. Müller, et al., Journal of Organometallic Chemistry (2016), http://dx.doi.org/10.1016/
j.jorganchem.2016.05.019