From bis(silylene) and bis(germylene) pincer-type nickel(II) complexes to isolable intermediates of the nickel-catalyzed sonogashira cross-coupling reaction

Article abstract of DOI:10.1021/ja408137t

The first [ECE]Ni(II) pincer complexes with E = Si^II and E = Ge^II metallylene donor arms were synthesized via C-X (X = H, Br) oxidative addition, starting from the corresponding [EC(X)E] ligands. These novel complexes were fully characterized (NMR, MS, and XRD) and used as catalyst for Ni-catalyzed Sonogashira reactions. These catalysts allowed detailed information on the elementary steps of this catalytic reaction (transmetalation → oxidative addition → reductive elimination), resulting in the isolation and characterization of an unexpected intermediate in the transmetalation step. This complex, {[ECE]Ni acetylide → CuBr} contains both nickel and copper, with the copper bound to the alkyne π-system. Consistent with these unusual structural features, DFT calculations of the {[ECE]Ni acetylide → CuBr} intermediates revealed an unusual E-Cu-Ni three-center-two-electron bonding scheme. The results reveal a general reaction mechanism for the Ni-based Sonogashira coupling and broaden the application of metallylenes as strong σ-donor ligands for catalytic transformations.

Full text of DOI:10.1021/ja408137t

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Article
From Bis(silylene) and Bis(germylene) Pincer-Type
Nickel(II) Complexes to Isolable Intermediates of the
Nickel-Catalyzed Sonogashira Cross-Coupling Reaction
Daniel Gallego, Andreas Brueck, Elisabeth Irran, Florian
Meier, Martin Kaupp, Matthias Driess, and John F. Hartwig
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja408137t • Publication Date (Web): 20 Sep 2013
Downloaded from http://pubs.acs.org on September 21, 2013
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From Bis(silylene) and Bis(germylene) Pincer-Type Nickel(II)
Complexes to Isolable Intermediates of the Nickel-Catalyzed
Sonogashira Cross-Coupling Reaction
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Daniel Gallego, Andreas Brück, Elisabeth Irran, Florian Meier, Martin Kaupp, Matthias
†
‡
Driess,* and John F. Hartwig*
†
Department of Chemistry: Metalorganics and Inorganic Materials, Technische Universität Berlin, Straße des 17.
Juni 135, Sekr. C2, 10623 Berlin, Germany
University of California, Department of Chemistry, 718 Latimer Hall, Berkeley, CA 94720, United States
‡
KEYWORDS silicon — germanium — catalysis — DFT calculations— C-C coupling reactions
II
II
ABSTRACT: The first [ECE]Ni(II) pincer complexes with E = Si and E = Ge metallylene donor arms were syntheꢀ
sized via CꢀX (X = H, Br) oxidative addition, starting from the corresponding [EC(X)E] ligands. These novel complexes
were fully characterized (NMR, MS, and XRD) and used as catalyst for Niꢀcatalyzed Sonogashira reactions; these cataꢀ
lysts allowed detailed information on the elementary steps of this catalytic reaction (transmetallation→oxidative addiꢀ
tion→reductive elimination), resulting in the isolation and characterization of an unexpected intermediate in the
transmetallation step. This complex, {[ECE]Niꢀacetylide→CuBr} contains both nickel and copper, with the copper
bound to the alkyne πꢀsystem. Consistent with these unusual structural features, DFT calculations of the {[ECE]Niꢀ
acetylide→CuBr} intermediates revealed an unusual EꢀCuꢀNi threeꢀcenterꢀtwoꢀelectron bonding scheme. The results
reveal a general reaction mechanism for the Niꢀbased Sonogashira coupling and broaden the application of metallylenes
as strong σꢀdonor ligands for catalytic transformations.
INTRODUCTION
Transition metal (TM) centers create a vast range of applications in organometallic chemistry, but the ligands conꢀ
trol the reactivity of these sites. During the last decades, many ligands have been developed that have created a wide
range of new reactivities. For example, the pincerꢀtype motif [EDE], a tridentate, meridional coordinating ligand
framework offers a myriad of opportunities to fineꢀtune the steric and electronic properties of TM complexes.1
Generally, the arms (E) of a pincer ligand consist of neutral, twoꢀelectron Lewis donor moieties (e.g. E = PR
or SR), which are connected over a linker group (often CH or O) to the neutral or monoꢀanionic anchoring site (D; e.g.
a pyridyl or phenyl group). Highly electronꢀrich [PCP] pincer complexes of Ir have been applied to activate the strong
2
, NR ,
2
2
2
3
4
bonds of ammonia and alkanes, and [PCP]Pd systems catalyze CꢀC coupling reactions.
σꢀStabilized metallylene (silylene or germylene) donor ligands also have given rise to unique properties and reactiviꢀ
ties.5,6 It is noteworthy that bisꢀsilylene and ꢀgermylene pincer moieties are more σ–donating than P ꢀbased ligands.
This difference in σ donation was established through structural and spectroscopic investigation of the series of iridium
III
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olefin [ECE]IrHCl(coe) (E = Si, Ge, P) complexes. In addition, the first experiments with the bisꢀsilylene pincer arene
II
SiCHSi and the group 10 metal precursor Pd(PPh
3
)
4
led to the unexpected formation of a mixed silylene(Si )ꢀ
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silyl(Si )ꢀpincer Pd(II) complex (Scheme 1). This unexpected result raised the question of whether isolable group 10
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II
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metal [ECE]M(II)X complexes (E = Si , Ge ; X = halogen) with E:ꢀM(II):E coordination are at all accessible.
Scheme 1. Earlier and present complexation studies of ECXE pincer ligands toward group 10 metals
.
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On the other hand, cross coupling reactions are one of the most important breakthroughs in chemical synthesis.9,10
Although, the catalytic pathways remain unclear and in rare cases reaction intermediates have been isolated.9e,10b,f Lateꢀ
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ly the Sonogashira reaction, normally catalyzed by Pd species with CuI as cocatalyst (Figure 1), has been catalyzed usꢀ
ing nickelꢀbased catalysts.10g,h Nickel pincer complexes were recently reported to catalyze Sonogashira reactions10 beꢀ
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3 11
tween a terminal alkyne and a sp (or sp ) hybridized carbon electrophile (typically in the presence of a copper coꢀ
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IV
catalyst). The authors of the later work proposed that the catalytic cycle might involve a Ni ꢀNi couple, but no experꢀ
imental evidence for the formation of NiIV was gained. Therefore, it is important to gain access to intermediates that
could reveal the connections between the structures of the intermediates and the rates and selectivity of the intermediꢀ
ates.
Pd(0) or Pd(II) cat. / ligand
Cu(I)ꢀsalt (cat.) / base / solvent
1
R2
R1
R2
R ꢀX
+
H
1
2
R = aryl, alkenyl, R = H, alkyl, aryl,
heteroaryl
X = Cl, Br, I, OTf
alkenyl, SiR3
Figure 1. General conditions in Sonogashira cross coupling.
Herein, we report the synthesis of the novel bromo bisꢀgermylene GeCBrGe pincer arene and the straightforward
coordination chemistry of the ECHE (E = Si, Ge) and GeCBrGe ligands towards nickel, a nonꢀprecious metal center.
These studies led to the formation of [ECE]NiBr complexes, and these complexes catalyzed the Sonogashira coupling
reaction (Scheme 1). Most important, reactions catalyzed by these strong σꢀdonor based pincer ligands enabled the
isolation of elusive intermediates of the Sonogashira coupling. The results demonstrate for the first time, that chemical
transformations at a nonꢀprecious metal center with pincerꢀlike metallylene donor arms are viable processes.
RESULTS AND DISCUSSION
Synthesis of the GeCBrGe pincer ligand
group 10 TM using Pd(PPh
.
From our previous metallation studies on the SiCHSi ligand with a
IV
3
)
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as the metal source, we observed an unexpected formation of silylene(SiII)ꢀsilyl(Si )ꢀ
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pincer Pd(II) complex (Scheme 1). This occurred through a hydride migration from the Pd(II)ꢀH transient species to
one of the silylene arms of the ligand. Therefore, we explored alternative routes to [ECE]MX complexes avoiding the
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intermediacy of MꢀH bonds. Parallel, we sought to expand the coordination chemistry of these ligands to a non precious
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metal center (e.g. nickel) and explore their potential to increase the electron density on the metal for catalytic applicaꢀ
tions.
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A common way to synthesize complexes bearing a C(sp )ꢀM ꢀX motif is the oxidative addition of a suitable M preꢀ
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cursor into a C(sp )ꢀX bond. Consequently we envisioned preparing the nickel complexes of ECE pincer ligands from
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the reaction of ECBrE (E = Si, Ge) compounds with Ni(cod) (cod = 1,5ꢀcyclooctadiene) as a Ni source. However, the
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synthesis of the ECBrE starting material was challenging, and just successful results were obtained for the GeCBrGe
ligand. Slow addition of two molar equiv of LHMDS in toluene to a mixture of 2ꢀbromoꢀ4,6ꢀdiꢀtertꢀbutylresorcinol and
two molar equiv of the N,N’ꢀdiꢀtertꢀbutylchloro(phenylamidinate)germanium(II) at room temperature produced a new
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species. The H NMR spectrum of this compound contained two singlets for the tertꢀbutyl groups with the relative ratio
of 1:2. Purification by extraction with nꢀhexane and recrystallization produced the desired ligand GeCBrGe in 68%
yield as a colorless solid (Figure 2). GeCBrGe was fully characterized by H and 13C NMR, mass spectrometry and sinꢀ
gleꢀcrystal Xꢀray diffraction analysis (See experimental section and supporting information, SI). Its structural features
1
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are similar to those of the GeCHGe ligand. Reversing the addition sequence of the reactants led to the formation of an
insoluble purple product of unknown composition. Unfortunately, attempts to obtain the silicon analogue SiCBrSi
from the bromoresorcinol repeatedly led to undefined products.
Figure 2. Synthesis and molecular structure of the novel GeCBrGe pincer ligand. Selected distances [Å] and angles [°] C1ꢀ
Br1 1.905(3), O1ꢀGe1 1.862(2), O2ꢀGe2 1.868(2), C2ꢀO1 1.347(3), C6ꢀO2 1.355(3), Ge1ꢀN1 2.031(3), Ge2ꢀN4 2.021(2), C2ꢀO1ꢀ
Ge1 142.6(2), C6ꢀO2ꢀGe2 136.3(2), N1ꢀGe1ꢀN2 64.8(1), N3ꢀGe2ꢀN4 64.8(1). Thermal ellipsoids are drawn at a 50% probability
level; hydrogen atoms are omitted for clarity. See Supporting Information for details.
Synthesis of [ECE]NiBr complexes. The reaction of GeCBrGe with Ni(cod) in toluene solution at ꢀ30°C furꢀ
2
1
nished a new species, as determined by H NMR spectroscopy. The C2v symmetry determined by the two singlets for the
+.
tertꢀbutyl groups with the relative integral ratio of 1:2 and its mass spectrum (M : exp. 966.20721 ; cald. 966.20510)
showed unambiguously the formation of the desired [GeCGe]NiBr pincer complex. This complex was isolated by exꢀ
traction and recrystallization from nꢀhexane as a red powder in 88% yield (Scheme 2).
Because the silicon analogue could not be prepared by this route, we explored the possibility of synthesizing the pincerꢀ
type [ECE]NiBr complexes via reaction of the ECHE ligands (E = Si, Ge) with NiBr
2
(dme) (dme = 1,2ꢀdimethoxyethane).
in a refluxing
This reaction conducted with the SiCHSi ligand precursor in the presence of 10 molar equivalents of NEt
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THF/toluene solvent mixture led to change in color from dark blue to yellow within 4 hours and with GeCHGe under the
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same conditions to a change in color from dark blue to dark red. The H NMR spectroscopic investigation of the isolated Ni
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complexes showed the same features for the [GeCGe]NiBr complex obtained by oxidative addition of the C(sp )ꢀBr bond
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described above. The absence of the phenyl CꢀH H NMR resonance corresponding to the proton between the silylene donor
arms and the sole resonance at δ = 20.2 ppm in the 29Si{ H} NMR spectrum of [SiCSi]NiBr are consistent with the formation
of the desired pincer complex having a squareꢀplanar coordinated Ni(II) (Scheme 2). Single crystals suitable for additional
characterization and structural elucidation by Xꢀray diffraction analysis were obtained for both complexes at ambient temperꢀ
ature in concentrated nꢀhexane solutions or by layering toluene solutions of [SiCSi]NiBr and [GeCGe]NiBr with nꢀhexane,
respectively (Figure 3).12 Their molecular structures bearing squareꢀplanar Ni(II) sites were confirmed by Xꢀray diffraction
analyses.
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Scheme 2. Synthesis of the [ECE]NiBr pincer-like complexes (E = Si, Ge).
For direct comparison with the well known phosphane pincer ligands, we prepared the analogous nickel complex containꢀ
III
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ing the sterically and isoelectronically related P ꢀ pincer analogue PCHP. Following the nickel metallation procedure for the
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ligands ECHE using NiBr (dme) (Scheme 2), the [PCP]NiBr complex was obtained in 95% yield (Figure 3c). Its H NMR
spectrum is consistent with a C2v symmetric structure. This spectrum contained one doublet and one septet for the isopropyl
group, and a single singlet resonance for the tertꢀbutyl groups in the relative ratio of 12:2:9, as well as a singlet in the 31P NMR
spectrum at 134.6 ppm. This structural assignment was confirmed by Xꢀray diffraction analysis. As observed for the
[
ECE]NiBr complexes (E = Si, Ge), the Ni(II) sites have a square planar geometry (Figure 3c). Comparing the bond distances
in the crystal structures depicted in Figure 3, the EꢀNi distances vary in accordance with the covalent radii of the donor atom.
However, there is a slight difference in the NiꢀBr distances depending on the donor atom E at the pincer arms [ECE]. The Niꢀ
Br and CipsoꢀNi distances increased accordingly to the σꢀdonor strength of the ligand. This is in accordance with the order esꢀ
tablished previously for the same ligand series on the iridium(III) olefinic complexes [ECE]IrHCl(coe) (E = P < Ge ≤ Si). The
III
CꢀNi bond is longer in about 5ꢀ8 pm for the complexes with Si and Ge as donors comparing with the P ꢀisoelectronic complex,
respectively. Moreover, this effect is slightly lower for the NiꢀBr bond distance where an increment of 1 pm is observable for
the [SiCSi]NiBr complex.
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Figure 3. Molecular structures with selected distances [Å] and angles [°] of a) [SiCSi]NiBr: C1ꢀNi1 1.927(2), Br1ꢀNi1
.3410(5), Si1ꢀNi1 2.1737(7), Si2ꢀNi1 2.1716(7), C1ꢀNi1ꢀBr1 178.51(7), Si1ꢀNi1ꢀSi2 161.75(3), C6ꢀO1ꢀSi1 110.8(1), C2ꢀO2ꢀSi2
10.5(1), Σ<Ni1 360.02(7); b) [GeCGe]NiBr: C1ꢀNi1 1.961(3), Br1ꢀNi1 2.3351(5), Ge1ꢀNi1 2.2113(6), Ge2ꢀNi1 2.2190(6), C1ꢀ
Ni1ꢀBr1 178.5(1), Ge1ꢀNi1ꢀGe2 165.42(2), C2ꢀO1ꢀGe1 109.9(2), C6ꢀO2ꢀGe2 110.7(2), Σ<Ni1 360.0(1); c) [PCP]NiBr: C1ꢀNi1
.881(4), Br1ꢀNi1 2.3297(7), P1ꢀNi1 2.136(2), P2ꢀNi1 2.151(2), C1ꢀNi1ꢀBr1 178.0(2), P1ꢀNi1ꢀP2 165.36(5), C2ꢀO1ꢀP1 111.6(3), C6ꢀ
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Nickel-Catalyzed Sonogashira Cross Coupling. We initially assessed the reactivity of the [ECE]NiBr pincer
complexes as catalyst for the Sonogashira reaction of (E)ꢀ1ꢀiodoꢀ1ꢀoctene with phenylacetylene as test substrates
(
Scheme 3). The reaction conducted with 5 mol % of E = Si, Ge and an excess of the halide substrate occurred in moderꢀ
III
ate yields. These yields are comparable to those of the isoelectronic P ꢀbased reference system [PCP]NiBr. The cataꢀ
lytic reaction mixtures with the [ECE]NiBr (E = Si, Ge, P) complexes turned dark to black, indicating the formation of
0
Ni . This observation raised the question of whether the metallylene systems are stable under the reaction conditions
and if the typical elementary steps of oxidative addition, transmetallation, and reductive elimination account for the
catalytic activity. To the best of our knowledge, a defined chemical transformation at a metal center decorated with
metallylene scaffolds has not been reported in the literature yet. This is of general interest since metallylenes are prone
to undergo chemical reactions with great variety of functional groups.5,6 Therefore we explored stoichiometric reactions
to identify possible intermediates of the Sonogashira reaction, including i) transmetallation on the Ni(II) center with
the copper acetylide, ii) oxidative addition of the alkenyl iodide and iii) reductive elimination to produce the coupling
product and regenerate the Ni(II) active species.
Scheme 3. Evaluation of the [ECE]NiBr (E = Si, Ge, P) complexes as preꢀcatalysts in the Sonogashira cross coupling reacꢀ
tion.
Stepwise Stoichiometric Reaction with Copper Acetylides. To investigate the first elementary step of the
mechanism (i.e. transmetallation), copper phenylacetylide, as well as the 4ꢀmethoxy and 3,5ꢀbisꢀtrifluoromethyl substiꢀ
tuted derivatives were synthesized following the reported procedures.13 The Cuꢀacetylides are not soluble in either benꢀ
zene or toluene, but two molar equiv reacted as a slurry with [SiCSi]NiBr in C
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D over 3ꢀ4 hours. NMR spectroscopic
characterization of the reaction mixture after filtration through Celite showed that a new product with C2v symmetry
1
was formed. This symmetry was shown by one tertꢀbutyl resonance in the H NMR spectrum and one singlet resonance
in the 29Si{ H} NMR spectrum (Scheme 4).
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Scheme 4. i) Investigation of possible elementary steps through stoichiometric transformations. ii) Sequential H and 29Si
1
NMR spectra for the course of the coupling reaction.
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The products of this transmetallation were only moderately stable in solution for a couple of hours and gave a black
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precipitate upon evaporation of the solvent. The original signals for [SiCSi]NiBr reappeared partially in the H NMR
spectra and the homocoupled phenylacetylenes (C≡CꢀPhꢀR)
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not all of the CuBr formed in the transmetallation could be removed by simple filtration, and ii) that the intermediate
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Niꢀacetylides – if formed – reacted in a bimolecular reaction to form the bisꢀacetylenes, Ni and undefined organic
products from the ligand.
To probe for a bimolecular decomposition pathway, the solutions generated by reaction of [SiCSi]NiBr with [Cuꢀ
C≡CꢀPh]
n
and [CuꢀC≡Cꢀ3,5ꢀ(CF
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)
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Ph]
n
were combined. The reaction products were analyzed by GCꢀMS after one day at
Ph was detected. Further characterization of the crude Niꢀ
room temperature. The mixed diyne PhꢀC≡CꢀC≡Cꢀ3,5ꢀ(CF
3 2
)
acetylides in solution by Atmospheric Pressure Chemical Ionization – Mass Spectrometry (APCIꢀMS) showed three
molecular ions in each case: [SiCSi]NiBr as the most intense signal, one signal for the expected Niꢀacetylides [SiCꢀ
Si]NiꢀC≡CꢀPhꢀR and one signal for an adduct {[SiCSi]NiꢀC≡CꢀPhꢀR→CuBr} (Figure 4a). The signals for the latter two
species were approximately equal in intensity.
Single crystals were obtained from the reaction of [SiCSi]NiBr with (CuꢀCCꢀPh) after microfiltration of the reacꢀ
n
tion mixture and storage in a mixture of nꢀpentane and toluene as solvent at ꢀ78 °C over approximately one week. Interꢀ
estingly, the structure determined by Xꢀray diffraction corresponded to the {[SiCSi]Ni-C≡C-Ph→CuBr} adduct (Figꢀ
ure 5a). This structure consists of a copper center in close proximity to the C≡C bond and the silylene unit (vide infra for
a detailed discussion). Moreover, after reꢀdissolving the crystals in C
and the symmetric NMR data as described above were obtained.
D
6 6
, the same mass spectrum with the three species
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59
60
Figure 4. APCIꢀMS and calculated spectra for the transmetallation intermediates a) {[SiCSi]Ni-CC-Ph→CuBr} and b)
{[SiCSi]Ni-CC-terPh→CuBr}
Transmetallation reactions between the synthesized Cuꢀphenylacetylides and [GeCGe]NiBr occurred in a fashion
similar to the reactions with [SiCSi]NiBr, but full conversion to the Niꢀphenylacetylides intermediates were not obꢀ
served. However, the products were less stable and decomposed in the presence of larger quantities of the Cuꢀ
phenylacetylides. The dependence of the stability of the transmetallation products on the amount of Cuꢀphenylacetylide
implies that at least one of the decomposition pathways of the Niꢀphenylacetylide complexes is bimolecular. Thus, we
sought to sterically block the Ni center by conducting analogous reactions with a metaꢀterphenylacetylide compound
(Scheme 5).
Scheme 5. Synthesis of kinetic stabilized [ECE]NiꢀC≡CꢀterPh→CuBr intermediates.
Copperꢀterphenylacetylide reacted cleanly with each of the metallylene pincer [ECE]NiBr (E = Si, Ge) complexes.
1
As anticipated, the products were stable in solutions for several days without noticeable decomposition. The H NMR
spectrum of the bisꢀgermylene derivative showed the same C2v symmetry as described for the intermediates above, but
the signals for the tertꢀbutyl groups on the amidinate arms in the bisꢀsilylene complex were as broad singlets. This line
shape could be due to i) steric interactions between the silylene subunits and the terphenyl group, or ii) the reversible
formation of an adduct with CuBr, thereby breaking the C2v symmetry. The APCIꢀMS of the crude reaction contained
three sets of signals for the [ECE]NiBr precursors, the [ECE]NiꢀC≡CꢀterPh transmetallation product, and the CuBr
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adduct of the transmetallation product (the signal due to the adduct was more intense than it was for the nonꢀsterically
hindered acetylides; Figure 4b). This kinetic stabilization of the transmetallation product by steric hindrance allowed
us to crystallize the copper acetylene adduct bound by the bisꢀgermylene ligand at room temperature without any noꢀ
ticeable decomposition. The structure determined by Xꢀray diffraction data again revealed the formation of the adduct
1
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{
[GeCGe]Ni-C≡C-terPh→CuBr} (Figure 5b).
Transmetallation experiments with the phosphane analogue [PCP]NiBr with [CuꢀC≡CꢀPh]
n
and [CuꢀC≡CꢀterPh]
n
showed similar reactivity to the [ECE]NiBr complexes. However, the equilibrium is strongly shifted to the starting Niꢀ
0
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Br complex (see SI for NMR and MS data) and no desired product could be isolated. Reaction with 5 molar equiv of
[
CuꢀC≡CꢀPh]
at 60°C for 12 h furnished the transmetallation product in 5% conversion determined by 31P NMR specꢀ
n
troscopy. Analysis by APCIꢀMS showed two molecular ion peaks for the [PCP]NiBr and [PCP]NiCCPh complexes, the
latter with lower intensity. No CuBr adduct could be observed in this case. The difference in reactivity can be explained
by the higher electron density at the Ni center in the [ECE]NiBr complexes which enables a stronger πꢀbackbonding
interaction with the C≡C bond, leading to a higher stability of the transmetallation product.14
Figure 5. Molecular structures with selected bond lengths [Å] and angles [°] of a) {[SiCSi]Ni-C≡C-Ph→CuBr}: Ni1ꢀSi1
2.296(1), Ni1ꢀSi2 2.137(1), Ni1ꢀC9 1.940(4), Ni1ꢀC1 1.860(4), Ni1ꢀCu1 2.4628(9), Si1ꢀCu1 2.508(1),C1ꢀCu1 1.976(4), C2ꢀCu1
2
.420(4), C1ꢀC2 1.213(6), C2ꢀC3 1.451(6), Cu1ꢀBr1 2.2855(7), Si1ꢀNi1ꢀSi2 160.93(5), C1ꢀNi1ꢀC9 162.9(2), Ni1ꢀSi1ꢀCu1 61.49(3),
C10ꢀO1ꢀSi1 115.4(2), C14ꢀO2ꢀSi2 110.4(2), Σ<Ni1 360.1(1); b) {[GeCGe]Ni-C≡C-terPh→CuBr}: Ni1ꢀGe1 2.3254(6), Ni1ꢀ
Ge2 2.1786(6), Ni1ꢀC9 1.985(3),Ni1ꢀC1 1.890(4), Ni1ꢀCu1 2.5208(7), Ge1ꢀCu1 2.5450(6),C1ꢀCu1 1.951(3), C2ꢀCu1 2.403(3), C1ꢀ
C2 1.216(5), C2ꢀC3 1.437(5), Cu1ꢀBr1 2.2855(7), Ge1ꢀNi1ꢀGe2 161.09(3), C1ꢀNi1ꢀC9 164.1(2), Ni1ꢀGe1ꢀCu1 62.15(2), C10ꢀO1ꢀGe1
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1
13.4(2), C14ꢀO2ꢀGe2 109.0(2), Σ<Ni1 360.4(1). Thermal ellipsoids are drawn at a 50% probability level; hydrogen and solꢀ
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6
vent atoms are omitted for clarity. See Supporting Information for details.
Isolated Intermediates, Their Structural Features and DFT Calculations. The coordination of the copper
to the acetylide unit renders the structures in Figure 5 unsymmetrical. The E-Ni (E = Si, Ge) distances between the
CuBrꢀcoordinated side (d(NiꢀSi1) = 2.296 Å, d(NiꢀGe1) = 2.325 Å) and the nonꢀcoordinated side (d(NiꢀSi2) = 2.137Å,
d(NiꢀGe2) = 2.179 Å) are different in both {[ECE]Ni-C≡C-Ph/terPh→CuBr} complexes. In addition, the copper
atom is much closer to the C1 atom (d(CuꢀC1) = 1.976 Å, 1.944 Å for E = Si, Ge, respectively) of the acetylide ligand than
to the remote C2 atom (d(CuꢀC2) = 2.420 Å, 2.441 Å for E = Si, Ge, respectively). The unsymmetrical binding of the
0
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0
15
copper is different from a “classicalꢀsideꢀon” coordination in which Δd(CuꢀC1 vs. CuꢀC2) is less than 0.150 Å. Addiꢀ
tionally, the <(C1≡C2ꢀC3) and <(NiꢀC1≡C2) angles are indicative of a C≡C→Cu bond (sideꢀon: ca. 156ꢀ165°; endꢀon: ca.
1
1
70°ꢀ180° 15f,g) and show, if at all, only minor perturbation of the C≡C bond (Si: 170.5° and 174.9°; Ge: 171.5° and
76.4°). The arms of the pincer ligands open towards the Cu atom in accordance with a reduction of the <(O1ꢀE1ꢀN)
angles (average change: Si: 106.75°→99.45°; Ge: 106.41°→97.12°) and shortening of the CuꢀE distances (d(CuꢀSi) =
.5080 Å; d(CuꢀGe) = 2.5704 Å), indicating a E→Cu bond
2
.
To understand the bonding in the four member ring (C1ꢀNiꢀSi1ꢀCu), we conducted detailed density functional theory
DFT) calculations for both {[ECE]Ni-C≡C-Ph/terPh→CuBr} intermediates (E = Si, Ge) with B3LYPꢀD3(BJ)/def2ꢀ
(
TZVP16ꢀ18 functionals. For comparison, the corresponding [ECE]Ni-C≡C-Ph/terPh complexes lacking bound CuBr
were also computed (see experimental section for computational details). Computations were conducted on i) fully opꢀ
timized structures (fullopt) and ii) the Xꢀray crystal structures after reoptimization of the hydrogenꢀatom positions
(
crystal/Hꢀopt). We focused mostly on atomic charges from natural population analysis (NPA) and on a real space deꢀ
19
20
scription by the electron localization function (ELF) or the related electron localizability indicator (ELIꢀD). The bond
distances of the fullopt structure agree well with those of the crystal/Hꢀopt structure (Table 1). The main effect of CuBr
coordination in all cases is lengthening of the E1-Ni bond and concomitant shortening of the E2-Ni bond opposite to
the CuBr fragment. Interestingly, the bending of the phenylacetylide ligand out of a straight CipsoꢀNiꢀCꢀC arrangement in
both complexes with CuBr is accompanied by a slight but significant lengthening of the alkyne triple bond and of the
C2ꢀC3 single bond.
Table 1. Selected bond lengths in complexes {[ECE]Ni-CC-R→CuBr} and {[ECE]Ni-CC-R} (E=Si, R=Ph / E=Ge,
R=terPh)a
bond length [Å]
{[ECE]NiꢀCCꢀR→CuBr}
{[ECE]NiꢀCCꢀR}
crystal/H-optb
fullopt-D3b
fullopt-D3b
Bond
E=Si
E=Ge
E=Si E=Ge
E=Si
E=Ge
E1-Ni
E2-Ni
2.296
2.137
2.317
2.181
2.319 2.329
2.181
2.182
2.199
2.196
2.131
2.178
Ni-C1
C1-C2
1.861
1.212
1.452
1.976
1.881
1.219
1.444
1.948
1.866 1.885
1.233 1.232
1.420 1.423
2.020 1.998
1.848
1.222
1.413
ꢀꢀꢀ
1.858
1.224
1.414
ꢀꢀꢀ
C2-C3 (ipso,Rc)
Cu-C1
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Cu-C2
Cu-E1
2.420
2.422
2.528 2.551
ꢀꢀꢀ
ꢀꢀꢀ
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2.508
2.462
2.286
2.558
2.526
2.285
2.496 2.548
2.485 2.531
2.318 2.306
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
Cu-Ni
Cu-Br
a
b
For atom number assignment see Figure 5.
B3LYP/def2ꢀTZVP results (cf. Computational details). C ipso of the phenylacetylide ligand.
c
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The ELF plot displayed in Figure 6 shows that the Cu atom is involved in a threeꢀcenter bond with E1 and Ni (ELIꢀD
gives a qualitatively similar bonding description, Table S6 in SI). While no interaction of Cu with the acetylide ligand is
apparent in the ELF, an appreciable Cu-C1 Mayer bond order (Si: 0.963; Ge: 0.855) suggests that there are bonding
interactions between Cu and the acetylide ligand (See Table S4 for further values). The reduced C≡C bond order in the
full optimized structures containing the bound CuBr (Si: 0.728, Ge: 1.321), as compared to the C≡C bond order in the
system lacking CuBr (Si: 1.737; Ge: 2.076) is in agreement with a bonding interaction between C1 and the Cu center.
NPA charges (Tables S2 and S3) show that CuBr receives about 0.2 electrons from the complex, and E1 becomes more
negative by about 0.18 electrons for E = Si and by about 0.13 electrons for E = Ge. Closer examination shows that this
charge results mainly from Ni and the E2 atom on the opposite side, with smaller contributions from other parts of the
ligand framework. This charge distribution is consistent with a charge transfer towards the newly formed E1-Cu-Ni
threeꢀcenter bond (Figures 5 and 6). Overall, a pattern of delocalized interactions emerges that allows the CuBr fragꢀ
ment to bond to the combination of Ni, one metallylene arm from the pincer moiety, and the acetylide coligand.
{
[SiCSi]NiꢀCCꢀPh→CuBr}
{[GeCGe]NiꢀCCꢀterPh→CuBr}
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Figure 6. ELF plot in the main Ni coordination plane for {[ECE]Ni-CC-Ph/terPh→CuBr} (E= Si, Ge) (B3LYPꢀD3/def2ꢀ
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6
TZVP level).
After the complete analysis by the spectroscopic methods and DFT calculations of the {[ECE]Ni-CC-
Ph/terPh→CuBr} complexes, we conclude that the first step of the catalytic reaction occurs via a transmetallation process
forming the CuBr adduct in solution with a medium halfꢀlife time. Consequently, to study the next elementary step the
transmetallation products were generated in situ and used after microfiltration.
0
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Stepwise Stoichiometric Reaction with (E)-1-Iodo-1-octene. One a possible scenario for the observed Soꢀ
nogashira coupling is the sequence of transmetallation, oxidative addition, and reductive elimination. In this sequence,
2
oxidative addition of the substrate containing a C(sp )ꢀhalide bond would occur to the nickelꢀacetylide. Indeed, addition
of three molar equiv of (E)ꢀ1ꢀiodoꢀ1ꢀoctene to solutions of the in situ generated {[SiCSi]Niꢀphenylacetylides→CuBr} in
C
6
D formed the CꢀC coupled products in yields from 80ꢀ95% (determined by GC/MS) after a few hours at 50 °C in
6
combination with [SiCSi]NiI (Scheme 4). The spectroscopic features for the latter species were confirmed by the indeꢀ
pendent synthesis of the nickel iodide complex. This complex was prepared by the same procedure applied for the broꢀ
1
mide analogue but using NiI
2
(dme) as the metal precursor instead, and fully characterized by H and 29Si NMR specꢀ
troscopies, APCIꢀMS and Xꢀray analysis (see experimental section and SI for further details). The formation of the couꢀ
pled product and the nickel iodide is consistent with a combination of oxidative addition and reductive elimination. The
Xꢀray structure of [SiCSi]NiI resembles the structural features of [SiCSi]NiBr concluding that no alteration on the
ligand backbone has occurred after closing the catalytic cycle. At this stage, there is no evidence of the reaction intermeꢀ
1
diate for this elementary step. However, the perseverance of the E stereochemistry on the final product evidenced by H
NMR spectroscopy suggested the oxidative addition and subsequent reductive elimination as the most likely pathway.
Reaction Mechanism. The investigation of stoichiometric transformations reveals a general mechanism for the
CꢀC coupling between phenylacetylene and (E)ꢀ1ꢀiodoꢀ1ꢀoctene catalyzed by [ECE]NiBr complexes (Scheme 6). The
[ECE]NiX (E = Si or Ge, X = Br or I) complex reacts with the Cuꢀphenylacetylide generated in situ between the phenylaꢀ
cetylene and the CuI in the presence of Cs
2
CO
3
. The product of this transmetallation process binds CuBr to form the
{
[ECE]NiꢀC≡CꢀPh→CuBr} species. This species reacts with the alkenyl halide to form the organic product and regenerꢀ
ate the nickel halide. One possible sequence to form these products is oxidative addition and reductive elimination to
form [ECE]NiI and (E)ꢀdecꢀ3ꢀenꢀ1ꢀynylꢀbenzene.
Scheme 6. Proposed catalytic cycle
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CONCLUSIONS
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6
In summary, the first silylene and germylene nickelꢀpincer complexes [ECE]NiBr (E = Si, Ge) were synthesised by
either a sequence of CꢀH activation and HBr elimination between ECHE and NiBr (dme) or CꢀBr oxidative addition of
GeCBrGe by Ni(cod) . These novel structurally characterized complexes catalyze the Sonogashira coupling between
2
2
phenylacetylene and (E)ꢀ1ꢀiodoꢀ1ꢀoctene to form (E)ꢀdecꢀ3ꢀenꢀ1ꢀynylꢀbenzene. Most important, investigation of the
proposed mechanism of this unusual nickelꢀcatalyzed Sonogashira reaction by stoichiometric transformations allowed
the isolation and structural characterization of a copperꢀbound nickel acetylide as the sole transmetallation product.
2
0
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This Ni(II) species reacts with alkenyl iodides (C(sp )ꢀI) to form a Ni(II) iodide and the coupled product. These results
suggest that the electronꢀrich nickelꢀpincer complexes with neutral Ge and Si donor atoms can react by a mechanism in
which transmetallation precedes reaction with the haloarene. Moreover, the stoichiometric reaction raises the question
of the oxidation state of the nickel intermediate that results from CꢀI bond cleavage and forms the CꢀC bond of the orꢀ
ganic product. Further studies to elucidate the identity of such an intermediate are ongoing.
EXPERIMENTAL SECTION
General Considerations. All experiments and manipulations were conducted under dry oxygenꢀfree nitrogen using standꢀ
ard Schlenk techniques or in a MBraun drybox with an atmosphere of purified nitrogen. Solvents were dried by standard
methods and freshly distilled prior use. H, 13C, 31P and 29Si NMR spectra were recorded on Bruker AV 400 ( H: 400.13 MHz,
1
1
13C: 100.61 MHz; 29
Si: 79.49 MHz) or AFM 200 ( H: 200.13 MHz, 13C: 50.32 MHz, 19F: 188.33 MHz, 31P: 81.01 MHz) specꢀ
1
1
: 7.15 ppm; 13C:
6 6
D
trometers. The NMR signals are reported relative to the residual solvent peaks ( H: CDCl
3
: 7.26 ppm; C
0.0 ppm; 29Si: TMS: 0.0 ppm). SingleꢀCrystal Xꢀ
ray Structure Determinations: Crystals were mounted on a glass capillary in perfluorinated oil and measured in a cold N flow.
3
1
3 6 6 3 4
CDCl : 77.0 ppm; C D : 128.0 ppm), or an external standard ( P: 85% H PO
2
The data was collected either on an Agilent Technologies Xcalibur S Sapphire at 150 K (MoꢀKα radiation, λ = 0.71073 Å) or an
Agilent Technologies SuperNova (single source) at 150K (CuꢀKα radiation, λ = 1.5418 Å). The structures were solved by direct
methods and refined on F2 with the SHELXꢀ97 software package. The positions of the H atoms were calculated and considꢀ
ered isotropically according to a riding model. Mass spectra were recorded on a Finnigan MAT95S. IR spectra were recorded
on a Perkin Elmer Spectrum 100 FTꢀIR. GCꢀMS measurements were conducted on a Shimadzu GCꢀ2010 gas chromatograph
(
30 m Rxiꢀ5ms column) linked with a Shimadzu GCMAꢀQP 2010 Plus mass spectrometer. NiBr
2
, NiI
2
, were purchased from
Aldrich. NiBr NiI
2
(dme),21 (dme),21 2ꢀbromoꢀ4,6ꢀdiꢀtertꢀbutylresorcinol,22
2
N,N’ꢀdiꢀtertꢀ
2
3
1
1
b
u
t
y
l
c
h
l
o
r
o
(
p
h
e
n
y
l
a
m
i
d
i
n
a
t
e
)
g
e
r
m
a
n
i
u
m
(
I
I
)
w
e
r
e
p
r
e
p
a
r
e
d
a
c
c
o
r
d
i
n
g
t
o
t
h
e
r
e
p
o
r
t
e
d
p
r
o
c
e
d
u
r
e
s
,
a
s
w
e
l
l
a
s
t
h
e
S
i
C
H
S
i
and GeCHGe10 and PCHP10 ligands.
Synthesis of GeCBrGe. A solution of 2ꢀbromoꢀ4,6ꢀdiꢀtertꢀbutylresorcinol (0.69 g, 2.3 mmol) in 20 mL of toluene was slowꢀ
ly added to a solution N,N’ꢀdiꢀtertꢀbutylchloro(phenylamidinate)germanium(II) (1.54 g, 4.5 mmol) in 20 mL of toluene at
room temperature forming a strong yellow reaction mixture. After stirring for 30 min, a solution of LHMDS (0.77 g, 4.6 mmol)
in 10 mL of toluene was added dropwise in a period of 30 min with concomitant formation of turbidity and color change to
terracotta. All volatiles were removed in vacuo after stirring overnight at room temperature and the product was extracted
with hot hexane (1x60 mL, 2x20 mL) via cannula filtration. The product was concentrated up to 10 mL and crystallized overꢀ
1
night at ꢀ3°C as white crystals. Further filtration and dry in vacuo produced 1.40 g of the desired product (68% yield). H
NMR (400.13 MHz, C
.29ꢀ7.32 (m, 2 H, arom. H), 7.61 (s, 1 H, ArꢀH). 13C{ H} NMR (100.61 MHz, C
NC(CH ), 35.8 (ArC(CH ), 53.1 (NC(CH ), 111.6 (CaromBr), 123.4 (CaromH), 129.6 (CaromtBu), 127.4, 129.2, 129.9, 136.0
(CaromH), 156.9 (CaromO), 170.3 (NCN). APCI-MS (m/z): calcd for [C44 65BrGe2N
●+] 908.26975; found 908.26978 (correct
isotope pattern). Elemental analysis for C44 65BrGe : calc: C 58.25, H 7.22, N 6.18; found: C 58.45, H 7.35, N 6.18.
6
D
6
, 298K): δ(ppm) = 1.07 (s, 36 H, NC(CH
3
)
3
), 1.83 (s, 18 H, ArC(CH
3
)
3
), 6.90ꢀ7.04 (m, 8 H, arom. H),
1
7
6
D
6
, 298K): δ(ppm) = 31.5 (ArC(CH
3 3
)
), 32.2
(
3
)
3
3
)
3
3 3
)
H
4 2
O
H
2 4 2
N O
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Synthesis of [ECE]NiX (E = Si, Ge, P; X = Br, I) complexes. General procedure with NiBr
2 3
(dme): NEt (10.0
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equiv.) were added to a suspension of NiBr (dme) (1.1 equiv.) in THF forming a dark blue solution. After stirring for 20 min, a
2
solution of ECHE (1.0 equiv.) in toluene was added via cannula and the reaction mixture was heated to reflux for 4 h. The
mixture was allowed to cool to room temperature, filtered through a short plug of Celite and all volatiles were removed in
vacuo. The residue was extracted with hexane (1x50 mL, 2x20 mL) at 50°C. The combined organic solutions were concentratꢀ
ed slowly in vacuo until small crystals formed at the glass wall. Further cooling in the freezer at ꢀ30°C resulted in the crystalliꢀ
zation of the pure products in form of needles, which were collected by filtration and dried in vacuo for 2 h.
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[
SiCSi]NiBr (1.4 mmol scale: 70% yield; yellow needles): H NMR (400.13 MHz, C
NC(CH ), 1.75 (s, 18 H, ArC(CH
NMR (100.61 MHz, C , 298K): δ(ppm) = 30.8 (s, ArC(CH
23.8 (s, CHarom), 126.5 (s, CaromtBu), 128.4 (s, CHarom), 130.3 (s, 2C, CHarom), 131.3 (s, CNi), 131.4 (s, Carom), 162.7 (s, CaromO),
6
D
6
, 298K): δ(ppm) = 1.34 (s, 36 H,
)
3
3
)
3
), 6.79ꢀ6.91 (m, 8 H, CHarom), 7.48 (s, 1 H, CHarom), 7.69ꢀ7.71 (m, 2 H, CHarom). 13C{ H}
1
3
6
D
6
3 3 3 3 3 3 3 3
) ), 31.5 (s, NC(CH ) ), 35.3 (s, ArC(CH ) ), 54.1 (s, NC(CH ) ),
1
1
73.1 (s, NCN). 29Si{ H} NMR (79.49 MHz, C
74.31774; found 874.31805 (correct isotope pattern). Elemental analysis for C44
8.27, N 5.82; found: C 62.72, H 8.43, N 6.22.
1
, 298K): δ (ppm) = 20.22. APCI-MS (m/z): calcd for [C44
H
65BrN
4 2 2
NiO Si
●+]
6
D
6
8
H
65BrN NiO Si .C H14: calcd: C 62.36, H
4 2 2 6
1
[
SiCSi]NiI. NiI
2
(dme) as the precursor (0.3 mmol scale: 66% yield; orange needles): H NMR (200 MHz, C
6 6
D , 298K):
δ(ppm) = 1.33 (s, 36 H, NC(CH
3
)
3
), 1.77 (s, 18 H, ArC(CH
3
3
) ), 6.80ꢀ6.97 (m, 8 H, CHarom), 7.54 (s, 1 H, CHarom), 7.85ꢀ7.88 (m, 2
H, CHarom). 13C{ H} NMR (50.32 MHz, C
1
, 298K): δ(ppm) = 30.8 (s, ArC(CH
)
3
), 31.5 (s, NC(CH ), 35.4 (s, ArC(CH ),
)
6
D
6
3
3
3
3 3
)
5
4.1 (s, NC(CH
3
)
3
), 123.9 (s, CHarom), 126.6 (s, CaromtBu), 130.2 (s, CHarom), 130.3 (s, CHarom), 131.4 (s, Carom), 135.7 (s, CNi),
1
62.5 (s, CaromO), 173.4 (s, NCN). 29Si{ H} NMR (79.49 MHz, C
1
D
6 6
, 298K): δ (ppm) = 30.70. APCI-MS (m/z): calcd for
2
NiO Si :
[
C
44
H65IN
4
NiO
2
Si
2
●+] 922.30387; found 922.30396 (correct isotope pattern). Elemental analysis for C44
H
65IN
4
2
calc.: C 57.21, H 7.09, N 6.06; found: C 56.44, H 6.47, N 7.17.
1
[
GeCGe]NiBr (0.5 mmol scale, 57% yield; red needles): H NMR (400.13 MHz, C
NC(CH ), 1.83 (s, 18 H, ArC(CH
NMR (100.61 MHz, C , 298K): δ (ppm) = 31.0 (s, ArC(CH
25.1 (s, CHarom), 125.4 (s, CaromCtBu), 126.8 (s, Carom), 128.7 (s, CHarom), 128.7 (s, CHarom), 130.0 (s, CHarom), 132.3 (s, CNi),
62.6 (s, CaromO), 175.0 (s, NCN). APCI-MS (m/z): calcd for [C44 65BrGe NiO
●+] 966.20510; found 966.20721 (correct
65BrGe : calc. C 54.71, H 6.78, N 5.80; found: C 54.22, H 7.06, N 5.51.
6
D
6
, 298K): δ (ppm) = 1.21 (s, 36 H,
)
3
3
)
3
), 6.78ꢀ6.91 (m, 8 H, CHarom), 7.06ꢀ7.10 (m, 2 H, CHarom), 7.56 (s, 1 H, CHarom). 13C{ H}
1
3
D
6 6
3 3 3 3 3 3 3 3
) ), 31.5 (s, NC(CH ) ), 35.8 (s, ArC(CH ) ), 54.4 (s, NC(CH ) ),
1
1
H
2
N
4
2
isotope pattern). Elemental analysis for C44
H
2 4 2
N O
1
), 1.32 (d, 3JHH
3 3
)
[
PCP]NiBr (95% yield, yellow needles). H NMR (200.13 MHz, CDCl
3
, 298K): δ (ppm) = 1.31 (s, 18H, C(CH
), 3.25 – 3.32 (m, 4H, CH2), 3.33 – 3.39 (m, 4H, CH ), 3.73 – 3.86 (m, 4H, CH(CH
, 298K): δ (ppm) = 21.8 (t, 2JCP = 2.5 Hz, PC(CH ), 22.6 (t, 2JCP = 2.8 Hz,
), 123.8 (s, CHarom.), 126.2 (t, 3JCP = 5.7 Hz, Carom.),
, 298K): δ (ppm) = 134.6 (s). HR-
=
6.6 Hz, 24 H, CH(CH
3
)
2
2
3 2
) ), 6.93 (br. s,
1
1
H, CHarom). 13C{ H} NMR (50.32 MHz, CDCl
3
3 3
)
PC(CH
3
)
3
), 29.9 (s, ArC(CH
3
)
3
), 43.5 (s, CH
2
), 47.1 (t, 2JCP = 7.1 Hz, PC(CH
)
3 3
1
1
34.2 (t, 1JCP = 24.7 Hz, CNi), 156.5 (t, 2JCP = 13.2 Hz, CO). 31P{ H} NMR (81.01 MHz, CDCl
3
EI-MS (m/z): calcd for [C30
H55BrN
4
NiO
2
P
2
●+] 704.23166; found 704.17770 (correct isotope pattern).
Procedure with GeCBrGe and Ni(cod)
2
: Ni(cod) (0.15 g, 0.53 mmol) was dissolved in 20 mL of toluene at ꢀ30°C. A
2
solution of GeCBrGe (0.53 g, 0.58 mmol) in 20 mL of toluene was added dropwise through a syringe. The stirred reaction
mixture was slowly warmed up to room temperature over the course of 8 h, resulting in a dark red solution, which was filtered.
All volatiles from the filtrate were removed in vacuo, and the solid was washed with cold hexane (2x10 mL). The residue was
dried in vacuo for 2 h, obtaining [GeCGe]NiBr as a dark red powder (0.45 g, 88 % yield).
Catalytic Sonogashira Cross Coupling. In a nitrogen filled drybox, [ECE]NiBr (5 molꢀ%; 11.4 ꢀmol), CuI (1.1 mg, 5 molꢀ
%
; 11.4 ꢀmol), Cs
2
CO
3
(74.0 mg, 228.0 ꢀmol), phenylacetylene (11.6 mg, 114 ꢀmol) and (E)ꢀ1ꢀiodooctꢀ1ꢀene (1ꢀ5 eq) were
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weighted in a 4.0 mL screw cap vial containing a stir bar, and dissolved in 1.5 mL 1,4ꢀdioxane. The sealed vial was removed
from the drybox and heated to 100°C for 24h in a temperature controlled heating block. After cooling, the mixture was filtered
through short plug of silica and all volatile materials were evaporated in vacuo. The yields of the products were determined by
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3 2 2
redissolving the crude product in CDCl and adding a defined amount of CH Br as internal reference; and/or by addition of a
defined amount of dodecane as internal standard to the reaction mixture before heating and taking small aliquots for analysis
by GCꢀMS.
Stoichiometric Reactions with Copper Acetylides. In a nitrogen filled drybox, [ECE]NiBr (0.02ꢀ0.07 mmol), [CuꢀCCꢀ
0
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PhꢀR] (0.03ꢀ0.09 mmol) were weighted in a Schlenk flask or in a capped vial containing a stir bar and dissolved in the approꢀ
6 6
priate solvent (toluene or C D ). The sluggish reaction mixture was stirred for 4h and filtered through a short plug of Celite.
Suitable crystals of {[SiCSi]NiꢀCCꢀPh →CuBr} for Xꢀray analysis were grown at ꢀ78°C after several micro filtrations in a solꢀ
vent mixture nꢀpentane (1:1). For the complex {[GeCGe]NiꢀCCꢀterPh→CuBr} the crystals were grown layering nꢀpentane on
the toluene solution at room temperature.
Quantum Chemical Calculations. Computational details. All structures were optimized at the B3LYP/def2ꢀTZVP level
of theory using the Turbomole 6.31 program package.24 That is, the global hybrid functional B3LYP25 with 20% of the exact
HartreeꢀFock exchange admixture, in conjunction with standard Turbomole allꢀelectron def2ꢀTZVP basis sets26 for all atoms.
For the full optimization Grimme’s dispersion correction 3 with the BeckeꢀJohnson potential was used.27 The crystal structures
of {[ECE]Ni-CC-Ph/terPh→CuBr} complexes were used as initial structures for full optimization (denoted as “fulloptꢀ
D3”). For comparison, partial optimization of only the hydrogenꢀatom positions for the Xꢀrayꢀbased structure of {[ECE]Ni-
CC-Ph/terPh→CuBr} was also done at the B3LYP/def2ꢀTZVP level (denoted as “crystal/optꢀH”). For full optimization of
the {[ECE]Ni-CC-Ph/terPh} complexes the optimized structures of {[ECE]Ni-CC-Ph/terPh→CuBr} were taken and the
CuBr removed.
Atomic charges were evaluated at the B3LYP/def2ꢀTZVP level of theory by means of natural population analyses (NPA), using
the builtꢀin NBO subroutines of the Gaussian 09 program.28 Mayer bond orders29 were evaluated using the program
BORDER.30 The wavefunctions were also analyzed in the DGrid program30 by means of the electron localization function
3
1
32
(
ELF) and the electron localizability indicator based on the parallelꢀspin electron pair density (ELIꢀD). For this purpose, the
KohnꢀSham orbitals of the (Gaussian 09) single point calculations were transferred to the DGrid and the examined property
was calculated on a grid with 100 points per Bohr. The results of ELF analyses was visualized using the Paraview program.33
ASSOCIATED CONTENT
Experimental details about synthesis of organic precursors. NMR, APCIꢀMS, XRD experimental data and calculated Mayer
bond orders, NPA charges and ELFꢀD, ELIꢀD plots. This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*
matthias.driess@tuꢀberlin.de, jhartwig@berkeley.edu.
Author Contributions
#These authors contributed equally. All authors have given approval to the final version of the manuscript.
ACKNOWLEDGMENT
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Financial support by the Einstein Foundation Berlin and the Cluster of Excellence UniCat (financed by the Deutsche Forꢀ
schungsgemeinschaft and administered by the TU Berlin) is gratefully acknowledged. A. B. thanks the Deutsche Forꢀ
schungsgemeinschaft for an initial postdoctoral fellowship to work with J.F.H. (BR 4069/1ꢀ1). We also thank the reviewers of
the manuscript for helpful comments and suggestions.
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ABBREVIATIONS
APCIꢀMS, Atmospheric Pressure Chemical IonizationꢀMass Spectrometry; NPA, Natural Population Analyses; ELF, Electron
Localization Function; ELI, Electron Localizability Indicator.
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REFERENCES
(1)
Reviews on pincer complexes: a) Albrecht, M.; van Koten, G. Angew. Chem. 2001, 113, 3866; Angew. Chem. Int. Ed. 2001, 40,
3
750. b) van der Boom, M. E.; Milstein, D., Chem. Rev. 2003, 103, 1759. c) The Chemistry of Pincer Compounds; Eds. Moralesꢀ
Morales, D.; Jensen, C. M.; Elsevier, Amsterdam, 2007. d) Albrecht, M.; Linder, M. Dalton Trans. 2011, 40, 8733. e) Zargariꢀ
an, D.; Castonguay, A.; Spasyuk, D. M. Top. Organomet. Chem. 2013, 40, 131.
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(
(
3) Review on alkane dehydrogenation: Choi, J.; MacArthur, A. H. R.; Brookhart, M.; Goldman, A. S. Chem. Rev. 2011, 111, 1761.
4)
5)
a) Eberhard, M. R.; Wang, Z.; Jensen, C. M. Chem. Commun. 2002, 818. b) SolanoꢀPrado, M. A.; EstudianteꢀNegrete, F.;
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(
Reviews on group 14 metallylenes: a) Mizuhata, Y.; Sasamori, T.; Tokitoh, N. Chem.Rev. 2009, 109, 3479. b) Mandal, S. K.;
Roesky, H.W. Chem. Commun. 2010, 46, 6016. c) Yao, S., Xiong, Y., Driess, M. Organometallics 2011, 30, 1748. d) Asay, M.;
Jones, C.; Driess, M. Chem. Rev. 2011, 111, 354.
(6) Review on metallylene complexes: Blom, B.; Stoelzel, M.; Driess, M. Chem. Eur. J. 2013, 19, 40.
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2012, 124, 11645.
(
(
8) Wang, W.; Inoue, S.; Irran, E.; Driess, M. Angew. Chem. Int. Ed. 2012, 51, 3691; Angew. Chem. 2012, 124, 3751.
9) Reviews about CꢀC coupling reactions: a) Hassan, J.; Sévignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M. Chem. Rev. 2002, 102,
1359. b) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem. Int. Ed. 2005, 44, 4442; c) Suzuki, A. Angew. Chem. Int. Ed.
2011, 50, 6722. d) Negishi, Eꢀi. Angew. Chem. Int. Ed. 2011, 50, 6738. e) Seechurn, C. C. C. J.; Kitching, M. O.; Thomas J.
Colacot, T. J.; Snieckus, V. Angew. Chem. Int. Ed. 2012, 51, 5062.
(
10) Reviews on the Sonogashira reaction: a) Sonogashira, K. In Handbook of Organopalladium Chemistry for Organic Synthesis;
Eds. Negishi, E.; de Meijere, A.; Wileyꢀinterscience: New York, 2002; p 493. b) Sonogashira, K. J. Organomet. Chem. 2002,
653, 46. c) Tykwinski, R. R. Angew. Chem. 2003, 115, 1604; Angew. Chem. Int. Ed. 2003, 42, 1566. d) Negishi, E.; Anastasia,
L. Chem. Rev. 2003, 103, 1979. e) Chinchilla, R.; Nájera, C. Chem. Rev. 2007, 107, 874. f) Chinchilla, R.; Nájera, C. Chem. Soc.
Rev. 2011, 40, 5084. Examples using Niꢀbased catalyst: g) Wang, L.; Li, P.; Zhang, Y. Chem. Commun. 2004, 514. h) Beletskaꢀ
ya, I. P.; Latyshev, G. V.; Tsvetkov, A. V.; Lukashev, N. V. Tetrahedron Lett. 2003, 44, 5011. Relevant example of sulphurꢀ
carbon coupling reaction catalyzed by phosphinite Ni pincer complex: i) GómezꢀBenítez, V.; BaldovinoꢀPantaleón, O.; Herreraꢀ
Álvarez, C.; Toscano, R. A.; MoralesꢀMorales, D. Tetrahedron Lett. 2006, 47, 5059.
(
11) Vechorkin, O.; Barmaz, D.; Proust, V.; Hu, X. J. Am. Chem. Soc. 2009, 131, 12078.
(12) CCDC 942871 (GeCBrGe), 942872 (terphenylacetylene), and 942864 ([GeCGe]NiBr), 942865 ([SiCSi]NiC≡CterPh-CuBr),
42866 ([PCP]NiBr), 942867 ([SiCSi]NiBr), 942868 ([SiCSi]NiC≡CPh-CuBr), and 942869 ([SiCSi]NiI) contain the
9
supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystalloꢀ
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. See Supporting Information for further information.
13) General procedure for the synthesis of copper phenylcetylides in analogy to: Takeda Pharmaceutical Company Limited patent,
EP1559422 A1, 2005 and Osakada, K.; Sakata, R.; Yamamoto, T. Organometallics 1997, 16, 5354.
(
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(
(
14) Manna, J.; John, K. D.; Hopkins, M. D. Adv. Organomet. Chem. 1995, 38, 79.
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15) The C≡C triple bond distances are not very indicative for a sideꢀon or endꢀon coordination mode: Si =1.213 Å; Ge = 1.222 Å; sideꢀ
on: 1.207–1.239 Å; endꢀon: 1.193[15f]. a) Janssen, M. D.; Köhler, K.; Herres, M.; Dedieu, A.; Smeets, W. J. J.; Spek, A. L.; Grove,
D. M.; Lang, H.; van Koten, G. J. Am. Chem. Soc. 1996, 118, 4817. b) Lang, H.; Köhler, K.; Rheinwald, G. Organometallics
1
999, 18, 598. c) Köhler, K.; Eichhorn, J.; Meyer, F.; Vidovic, D. Organometallics 2003, 22, 4426. d) Thompson, J. S.; Bradley,
A. Z.; Park, K.ꢀH.; Dobbs, K. D.; Marshall, W. Organometallics 2006, 25, 2712. e) Dias, H. V. R.; Flores, J. A.; Wu, J.; Kroll, P.
J. Am. Chem. Soc. 2009, 131, 11249. f) Vechorkin, O.; Godinat, A.; Scopelliti, R.; Hu, X. Angew. Chem. Int. Ed. 2011, 50, 11777;
Angew. Chem. 2011, 123, 11981. g) Zhao, N.; Zhang, J.; Ying, Y.; Chen, G.; Zhu, H.; Roesky, H. W. Organometallics, 2013, 32,
762;
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0
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2
3
4
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0
(
(
16) a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. b) Lee, C. T.; Yang, W. T.; Parr, R. G. Phys. Rev. B 1988, 37, 785. c) Stephens, P.
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