Side-Arm Functionalized Silylene Copper(I) Complexes in Catalysis

Article abstract of DOI:10.1021/acs.inorgchem.9b00629

A convenient new method was added to the toolbox of the ligand design of N-heterocyclic silylenes and their transition-metal complexes. Herein we report on six novel compounds of two novel classes of copper(I) complexes based on the benzamidinate silylene

Full text of DOI:10.1021/acs.inorgchem.9b00629

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Side-Arm Functionalized Silylene Copper(I) Complexes in Catalysis
Alexander N. Paesch, Anne-Kathrin Kreyenschmidt, Regine Herbst-Irmer, and Dietmar Stalke*
Institut fur Anorganische Chemie, Georg-August-Universitat Gottingen, Tammannstraße 4, 37077 Gottingen, Germany
̈
̈ ̈ ̈
S
* Supporting Information
ABSTRACT: A convenient new method was added to the
toolbox of the ligand design of N-heterocyclic silylenes and
their transition-metal complexes. Herein we report on six
novel compounds of two novel classes of copper(I) complexes
based on the benzamidinate silylene (Cl)Si(PhC(NtBu)2). By
taming the high reactivity of the free electron pair of the Si(II)
atom via a preset metalation with a desired metal precursor
(in this case copper(I) halides) we can easily introduce novel
pyridyl-based groups in the subsequent functionalization of
the chloro group and undergo coordination of the metal atom
at the same time. The resulting pseudocubane-like tetramer
[XCu(I) ← (Cl)Si(PhC(NtBu)2)]4 2a−2c and the trinuclear dimer [(XCu(I))3(PyNMes)Si(PhC(NtBu)2)] 3a−3c (with X =
Cl (2a/3a), Br (2b/3b), I (2c/3c)) were fully characterized via X-ray diffraction analysis, NMR spectroscopy, mass
spectrometry, and elemental analysis. Moreover, we took a look into the catalytic potential of the Cu(I) complexes 2b and 3b
by testing them under the conditions of the renowned copper(I)-catalyzed alkyne−azide cycloaddition and observed an
increased activity of the functionalized species.
the chemistry of N-heterocyclic silylenes and are the basis of
many following results.
INTRODUCTION
■
Over the past decades, N-heterocyclic silylenes (NHSi) have
been intensively studied and became more and more a suitable
alternative for traditional ligands in organometallic chemistry,
not only but primarily in their role as strong σ-donors in metal
complexes.1−3 For several years, only group 5−10 transition-
metal complexes stood in the focus of research, whereas group
11 or 12 metal complexes were still comparatively rare.1 Early
examples of Frenking and Boehme about theoretical analysis of
coinage metal NHSi complexes4 were followed by single
synthetic approaches by Lappert et al., who, for example,
published a tetrahedral copper NHSi complex employing the
Arduengo type of motif (I, Scheme 1).5 Ten years later the
group of Driess started new synthetic approaches on group 11
complexes to further explore the fundamentals of copper NHSi
complexes (II−III).6 These results were expanded through the
very recent work of Khan et al., who published certain reports
on group 11 transition-metal complexes and inter alia one of
the first gold and silver complexes (IV−VI).7−13 Almost all
these metal complexes are based on the same core unit, the
benzamidinate silylene,14 which had its breakthrough in 2010,
when the group of Roesky was able to publish an improved
synthesis of this ligand, which allowed facile access to the high-
yield synthesis of the chlorobenzamidinate silylene (NHSiCl,
1) ligand.15
Of course there are also other ways of substituting the chloro
group, which appeared in the literature, for example, through
rearrangement. Roesky et al. observed the coordination of
ZnEt2/Zn(arene)2 and the subsequent rearrangement of the
alkyl/aryl group with the chloro group to eventually coordinate
ZnCl2.17 Nevertheless, such methods did not gain broad
acceptance in the field of versatile functionalization. The focus
was set on the substitution of the chloro group due to the
possibility of flexible ligand design.
Herein, we concentrated on the enhanced functionalization
of the chlorobenzamidinate ligand by introducing additional
donor sites. The idea of functionalizing silylenes for a bi- or
tridentate coordination of metal atoms is not new to silylene
chemistry but barely applied. In fact, some of the first
transition-metal complexes were designed as pincer-like ligands
(e.g., III, Scheme 1; VII & X, Scheme 3).18−23 One of the very
few examples that illustrates the possibility to form new NHSi
bearing an additional coordination site on a nitrogenous base
was published by Tacke et al. (Scheme 2B).24−26 Referring
back to the previous work on the heavier Ge(II) analogues,27,28
they were using 2 equiv of benzamidinate as building blocks
right from the start, whereby one-half can function as the
pendent donating side arm. However, alongside the benefits of
this synthesis, this method is limited to a symmetrical design of
the ligand, and therefore objects our intentions. As we were
searching for a suitable route to substitute the chloro group of
Shortly after, certain groups independently published similar
methods on the facile functionalization via simple substitution
of the chloro group with seemingly any other monoanionic
group bearing nitrogen, phosphor, or even oxygen atoms
(Scheme 2A).16 These achievements boosted the research of
Received: March 4, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00629
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Scheme 1. Overview of Early to Recent N-Heterocyclic
Silylene Group 11 Transition-Metal Complexes
Scheme 3. Examples of Silylene Transition-Metal
Complexes in Catalysis
a
a
a
VII: Driess et al., Sonogashira cross coupling;20 VIII: Roesky et al.,
Heck coupling;37 IX: Driess et al., ketone hydrosilylation;44 X: Driess
et al., [2 + 2 + 2] cyclotrimerization;22 XI: Tilley et al., hydrosilylation
of alkenes.36.
a
By Lappert et al. (I),5 Driess et al. (II, III),6 and Khan et al. (IV−
group with more complex features. In this work we will report
our results of this method featuring copper(I) halide
precursors.
VI).7,8,10
Scheme 2. Comparison of Different Methods of
Functionalization and Metalation
Simultaneous to our investigations, the group of Li and Sun
reported their work on a similar structure motif, featuring a
functional group of the type MeNPy instead but employing the
conventional route via previous isolation of the free silylene
and following metalation.34 Within their work they investigated
the catalytical properties of corresponding iron hydride
complexes in hydroboration reactions, thus further illustrating
the potential of such bidentate NHSis.
a
Besides new synthetic approaches, it was always our
intention to investigate the reactivity of new NHSi
transition-metal complexes. In direct comparison with their
lighter homologue, the ubiquitous N-heterocyclic carbenes
(NHCs), their applications in catalysis are still comparatively
rare, yet emerging in various fields (e.g., Co,22 Rh,35,36 Ir,18,35
Fe,32 Pd,19,37 and Ni21,23). The group of Tilley not only made a
significant contribution to the elucidation of early transition-
metal silylene complexes in the early 1990s38,39 but has also
contributed to a large number of investigations of catalytic
applications over the past decades.36,40,41 Likewise, Driess and
his co-workers illustrated the potential of NHSi transition-
metal complexes in certain reviews and were able to show that,
in some cases, the catalytic activity and even selectivity can be
increased and consequently prove that silylenes are not only
isoelectronic replacements for classical phosphines or
NHCs.42,43 For these reasons we analyzed the catalytical
potential of our newly characterized NHSi copper(I)
complexes by checking their reactivity in the field of the
renowned “Click” chemistry with a scope of commonly used
azides and acetylenes at ambient temperature.
a
(A) Preset substitution of the free chloro benzamidinate silylene,
followed by the coordination of the metal precursor; (B) direct
functionalization and generation of the free silylene with subsequent
metalation; (C) preset metalation with simple metal precursors and
subsequent substitution of the chloro group and accompanied metal
coordination.
NHSiCl with pyridyl-based groups, we were inspired to
investigate alternative methods for the functionalization.
Because of the high reactivity of the free electron pair of the
silicon(II) atom we changed the conventional synthetic route
described above to an early metalation of the neat chloro
silylene, and therefore tame the lone pair through the
coordination of transition-metal atoms and postpone the
functionalization (Scheme 2C). Of course there are already
numerous reports on NHSiCl transition-metal complexes (e.g.,
Co,29 Cr,30 Mo,30 W,30 Mn,31 Fe,32 Cu,6 Ni33) but never
under the aspect to perform further reactions with it like
functionalization. With these metal complexes in hand, we
were able to concentrate on the substitution of the chloro
RESULTS AND DISCUSSION
■
For the synthesis of [(XCu(I))3 ← (PyNMes)Si(PhC-
(NtBu)2)] 3a−c we started with the metalation of the
chlorinated silylene PhC(NtBu)2SiCl 1. Treatment of a
tetrahydrofuran (thf) suspension of the metal halide precursors
(CuCl, CuBr(SMe2), CuI) with 1 equiv of NHSiCl 1 at
B
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ambient temperature leads to the formation of the
pseudocubane Cu4(μ3-X)4 [XCu(I) ← (Cl)Si(PhC(NtBu)2)]4
2a−c in the form of slightly yellowish crystals after storage at
−30 °C for 1 d. Compounds 2a−2c (Scheme 4) have been
Scheme 4. Synthesis of 2a−2c and 3a−3c via Reaction of the
Chlorosilylene Benzamidinate 1 and the Respective
a
Copper(I) Precursors
Figure 1. Molecular structure of 2a. Anisotropic displacement
parameters are depicted at the 50% probability level. Hydrogen
atoms, tert-butyl, phenyl groups, and solvent molecules are omitted
for clarity. Structural data and selected bond lengths [Å] and angles
[deg] are given in the Experimental Section and in Table 1,
respectively (The molecular structures and full lists of bond lengths
and angles of 2b, 2c, and 2a are listed in the Supporting Information).
a
Reaction of the chlorosilylene benzamidinate 1 and the respective
copper(I) precursors, and the subsequent functionalization to the
complexes 3a−3c via the in situ deprotonation of the amine
MesNHPy using LiHMDS.
1
characterized by elemental analysis, H, 13C, and 29Si NMR
(NtBu)2)][OTf] (II, Scheme 2) with a similar 29Si shift of 32.9
ppm.6
spectroscopy, mass spectrometry, and single-crystal X-ray
diffraction (XRD) analysis. The latter confirms the compounds
in their pseudocubane nature. Each copper(I) atom of this
alternating tetramer is coordinated by an unsubstituted silylene
ligand 1. The formation of such pseudocubanes is not
uncommon for [MX]4Ln complexes with M = Cu, Ag; X =
Cl, Br, I; and L = neutral donor ligands.45,46 Nevertheless, until
now there are no reports on such tetramers bearing silylenes
and especially the well-known chlorosilylene benzamidinate
ligand. However, looking at the higher homologue germanium,
there are a few recent reports about the synthesis and
characterization of such tetramers by Polo et al. using
germylene benzamidinate ligands or six-membered germylenes
by Mak et al.47,48 The molecular structures of 2a−2c (shown
in Figure 1; see also Table 1) crystallize in the monoclinic
space groups C2/c. The bond lengths and angles of M−X and
M−X−M, respectively, correlate to the respective halides and
therefore lay in a typical range of such pseudocubanes. The
Si−M bond lengths (2.1889(9)−2.2306(13) Å) are compara-
ble to cubane systems with common phosphine ligands (e.g.,
[CuX]4Ln; L = PPh3, X = Br; Cu−P = 2.209 Å).46,49
To further investigate the structure of 2a−2c we tried
various mass spectrometry techniques (LIFDI, ESI, MALDI,
and EI) but were unable to obtain any informative mass
spectrum of these compounds. We assume that, as a result of
the comparatively high molecular weight, the neutral charge of
the complex, and the air and moisture sensitivity of the
tetramers 2a−2c they decomposed during the processes.
However, it was possible to identify a side product containing
the characteristic pseudocubane core unit. A reaction of 2b
with a slight excess of NaSbF6 led to the oxidized product
[(NHSiCl)4(Cu(I))2(Cu(II))2Br3(SbF6)2]+. Even though this
structure contains oxidized copper(II) atoms the matching
isotope pattern of m/z = 2144.78 compared to the simulated
spectrum confirms the presence of the core moiety known by
XRD experiments.
In the following functionalization steps, the corresponding
complexes 2a−2c were dissolved in thf, and 4 equiv of the
amine MesNHPy were added. At a temperature of −30 °C the
mixture was treated with LiHMDS to gain the target
compounds as bright yellow crystals. Likewise, compounds
1
3a−3c were characterized by elemental analysis, H, 13C, and
Contrary to the XRD experiments, diffusion-ordered spec-
troscopy (DOSY) analysis indicates the presence of a dimeric
structure in solution. With the help of the external calibration
curve method to determine the molecular weight of
compounds and intermediates via DOSY experiments, which
was established in our group by Neufeld, we were able to
detect a molecular weight in the area of a dimeric structure
29Si NMR spectroscopy, mass spectrometry, and single-crystal
XRD analysis. The most prominent feature of the resulting
dimeric structure, the triple copper(I) core unit, was initially
illustrated by XRD experiments (see Figure 2). The silicon(II)
atoms of both ligands are coordinating not one but two
copper(I) atoms, whereby one copper atom (the “tip” of the
Cu3 triangle) is mutually coordinated by both silylenes at a
time in a Si−Cu−Si fashion, and the additional copper atoms
(the “base” of the Cu3 triangle) are supported by the
coordination of the corresponding pyridyl group of the
newly introduced functional group. The tip of the triangle
bears a terminal halide, whereas the two base copper(I) atoms
are μ-bridged by two halide atoms in a Cu2(μ2-X)2 manner.
accompanied by thf molecules.50,51 The H and 13C NMR
1
spectra show one set of slightly shifted signals characteristic for
the benzamidinate-like ligand. But more importantly, the 29Si
NMR show one broadened singlet at 27.73, 26.09, and 22.62
ppm, respectively. In 2014 Driess et al. published the first
heteroleptic NHSi-Cu(I) complexes containing the first
chlorosilylene copper complex [Cu(I)(tmeda) ← (Cl)Si(PhC-
C
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a
Table 1. Selected Bond Lengths [Å] and Angles [deg] for Complexes 2a−2c
2a
2b
2c
Si−Cu
Cu−X
Cu−X−Cu
X−Cu−X
2.1889(8)−2.1930(9)
2.3097(8)−2.7844(10)
69.96(2)−90.07(3)
84.81(3)−111.93(3)
2.2068(7)−2.2085(7)
2.4632(8)−2.7860(10)
68.02(2)−85.16(2)
88.59(2)−114.22(2)
2.2285(13)−2.2306(13)
2.5993(9)−2.8559(8)
65.48(3)−78.57(4)
93.21(4)−116.56(3)
a
The Cu−X bond lengths show wide ranges caused by the distortion of the pseudocubane-like structure.
which also results in diverging angles (e.g., 3a, 61.98(2)° (tip),
59.19(3)°/58.82(3)° (base)). Furthermore, it is to emphasize
that the Si−Cu1 bond length is significantly longer in
comparison to the Si−Cu2/3 bond length (e.g., 3a,
2.5427(13)/2.5569(14) Å vs 2.2564(14)/2.2647(15) Å).
The aforementioned and closely related copper(I) complexes
by Khan et al.8,9 and Driess et al.6 show a narrow range of Si−
Cu (2.199−2.231 Å) and N4−Si (1.710−1.737 Å) bond
lengths, for which the shorter Si−Cu bond fits quite well, and
the substituted amine is slightly elongated in the case of 3a−3c
(1.813(4)−1.828(4) Å). The range of the Si−Cu1 bond
lengths (2.5427(13)−2.6063(8) Å; Table 2) is more
Table 2. Selected Bond Lengths [Å] and Angles [deg] for
Complexes 3a−3c
3a
3b
3c
Si−Cu1
2.5427(13)/
2.5569(14)
2.5806(14)/
2.5812(14)
2.5985(8)/
2.6063(8)
Si−Cu2/3
Cu1−Cu2/3
2.2564(14)/
2.2647(15)
2.4305(10)/
2.4399(10)
2.2678(15)/
2.2689(15)
2.4289(11)/
2.4323(11)
2.2819(11)/
2.2838(11)
2.4227(9)/
2.4333(9)
Cu2−Cu3
2.5078(7)
2.5350(9)
2.5458(5)
N4−Cu2/3
1.995(4)/
1.998(4)
2.005(4)/
2.014(4)
2.009(2)/
2.014(2)
N3−Si
1.813(4)/
1.828(4)
1.814(4)/
1.823(4)
1.815(2)/
1.821(2)
Cu−X
2.2240(11)
2.307(5)
2.550(5)
Cu−(μ2-X)
2.3699(13)−
2.5043(10)−
2.6629(6)−
2.4333(13)
2.5308(9)
2.6698(8)
Cu2−Cu1−Cu3
Cu1−Cu2/3−Cu3/2 58.82(3)/
61.98(2)
62.86(3)
58.50(3)/
58.63(3)
59.71(4)/
59.78(4)
85.39(11)/
85.59(12)
63.24(2)
58.18(3)/
58.59(3)
59.08(3)/
59.26(3)
84.83(7)/
84.92(7)
Figure 2. Molecular structure of 3a. Central view at the Cu triangle
(top); view along the triangle Cu2/3 → Cu1 (bottom). Anisotropic
displacement parameters are depicted at the 50% probability level.
Hydrogen atoms and solvent molecules are omitted for clarity.
Structural data and selected bond lengths [Å] and angles [deg] are
given in the Experimental Section and in Table 2, respectively (The
molecular structures and full lists of bond lengths and angles of 3b, 3c,
and 3a are listed in the Supporting Information.).
59.19(3)
60.41(4)/
60.53(4)
85.82(12)/
85.94(12)
62.64(3)/
63.31(3)
Cu1−Si−Cu2/3
Si−Cu−N4
Cu−(μ2-X)−Cu
60.25(2)/
60.69(2)
56.986(16)/
57.039(12)
To the best of our knowledge, there is no previous report on
such a [(XCu)(Cu2(μ2-X)2)] moiety. The formation of copper
triangles itself, however, is not new in coordination chemistry,
yet rarely observed, especially in low-valent compounds like
carbenes, silylenes, or germylenes. Recently Lu et al. and Chen
et al. published certain structures with pyridyl-functionalized
NHCs that bind to a copper triangle in a similar fashion.52−54
In both cases the copper triangle is fully coordinated by the
carbene carbon and pyridyl nitrogen atoms, whereas the
carbon atoms are also coordinating two copper atoms at a
time. Because of their C3-like composition, it is no surprise that
comparable to bridging silyl groups, like in dimeric NHC
copper(I) complexes reported by Romero et al. and Kleeberg
et al.55,56 They illustrate a variety of dimeric structures,
whereby bulky silyl groups (e.g., Si(TMS)2(Et)) (TMS =
tetramethylsilane) are bridging Cu−Cu bonds in a wide range
from 2.385 to 2.618 Å. The wide range is caused due to the
asymmetric bonding nature of silyl groups, which is often
observed in M2(μ2-SiR3) moieties and is pursued in 3a−3c.
Noteworthy, in comparison to other five-coordinated silicon
atoms coordinating two Cu(I) atoms in the same manner, here
the Si−Cu bonds are significantly elongated. We characterized
certain multinuclear copper halide complexes earlier on,
showing μ-bridging Si(TMS)3 groups with bond lengths
from 2.406(2) to 2.283(2) Å and from 2.348(3) to 2.334(3)
Å, respectively.57,58
the triangles are nearly isosceles (e.g., 60
0.25°, 2.490
0.007 Å).52 The C2-symmetric structure of compounds 3a−3c
shows a slightly distorted triangle, with a shortened bond
length to the tip (e.g., 3a, 2.4305(10)−2.4399(10) Å) and an
elongated bond length of the base (e.g., 3a, 2.5078(7) Å),
D
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The dimeric structure holding the trinuclear copper(I) core
of 3a−3c could also be verified by LIFDI mass spectrometry
by identifying the ion peaks corresponding to the cationic
moieties of [M−X]+ (X = Cl, Br, I) with m/z = 1117.2, 1291.2
and 1385.3, respectively. The 1H NMR spectra of 3a−3c show
the expected additional aromatic signals of the pyridyl and
mesityl group. Although the solid-state structure shows a
butterfly-like structure the NMR spectra show one set of
signals, which indicates that the structure must be flattened
under the NMR time scale in solution and therefore
symmetric. The usually sharp multiplets of the pyridyl
hydrogens are partially broadened and upfield-shifted caused
by the functionalization and metal coordination (e.g., 8.09 →
8.72 ppm or 6.26 → 6.80 ppm). The 13C NMR spectra of 3a−
3c exhibit corresponding signals, which are consistent with
screening are provided in the Supporting Information).
Thereby it must be considered that the catalytic application
within the context of this work was not intended to offer a
comprehensive comparison to other literature-known applica-
tions of copper(I) based catalysts, nor was it intended to
provide a wide-ranging scope of subjected substrates or
reaction conditions. The subjected complexes 2b and 3b
should rather demonstrate the potential of the silylene ligands
in a currently uncharted area and, above all, the influence of
the functionalization on the reactivity among the two
complexes. The progress of the reactions was monitored by
1H NMR spectroscopy at ambient temperature. To a solution
of benzyl azide (1.00 equiv), the appropriate alkyne (1.00
equiv), and adamantane as an internal standard in deuterated
dichloromethane (dcm) complex 2b/3b was added (Scheme
5). For the alkyne scope we chose commonly used small
1
their H NMR resonances. Unfortunately, under no circum-
stances were we able to detect any 29Si NMR signals. It is
assumed that the low natural abundance of 29Si in combination
with the already broadened signals known from 2a−2c got
further broadened due to the functionalization and enhanced
metal coordination, therefore making it nearly impossible to
detect any 29Si signals. Broadened signals are often
accompanied by fluctuating effects of compounds in solution.
It is possible that the NPy−Cu coordination opens up along
with a transformation of the bridging halide to a terminal one.
Thus, conceiving this scenario as a dynamic complexation and
decomplexation in solution the signal could be heavily
broadened. Nevertheless, the remaining analyses confirm the
presence of the silicon atoms.
Scheme 5. General Synthesis of the Triazoles P1-5 via the
CuAAC Conducted Here
a
Catalysis. After the full characterization of our newly
designed NHSi: → copper(I) complexes via the alternative
functionalization we wanted to further investigate its behavior
by checking possible catalytic activity. Former publications of
low-valent silicon chemistry have illustrated the great potential
of silylenes as new and improved ligands and also found
various applications in catalysis.42,59−61 However, in the field of
silylene copper(I) chemistry there has been only one report on
catalytic applications yet. In 2013 Driess and Hartwig
characterized a novel silylene nickel(II) complex and
investigated its reactivity and copper(I) intermediates toward
the Sonogashira coupling.20
Our overall idea was to test and compare possible catalytic
behavior toward renowned reactions. Therefore, we chose the
copper(I)-catalyzed alkyne−azide cycloaddition (CuAAC) also
known as the Huisgen 1,3-dipolar cycloaddition.62,63 It is one
of the most established and investigated transformations of the
so-called Click chemistry and has noteworthy applications in
nearly every field of chemistry.64−67 The analogous NHC-
Cu(I) complexes have been extensively investigated in Click
chemistry and illustrate the great potential in this field of
catalysis.68,69
Putting Click chemistry in simple terms, any Cu(I) source
can be used as a catalyst; however, well-designed Cu(I)
complexes can increase the selectivity, decrease catalyst
loadings, or establish milder reaction conditions.68−70 When
Fokin and Finn revealed crucial details of the CuAAC
mechanism in 200571 and therefore the mandatory role of
copper(I), following studies increasingly concentrated on
dinuclear copper species as the active species and illustrated
their potential.62,65,70,72,73
a
Catalyst = 2b/3b; R = Ph−CCH, Py-CCH, 3-butyn-1-ol, tBu-CCH,
TMS-CCH.
alkynes like phenylacetylene, 2-ethynylpyridine, and 3-butyn-1-
ol and sterically more demanding ones like tert-butylacetylene
and ethynyltrimethylsilane. The results of our time-resolved
measurements are depicted in Chart 1 and summarized in
Table 3, respectively. Within the framework of these catalytic
investigations, blank tests were performed for cross-checking to
ensure that the determined conversion rates did not originate
from possible traces of neat Cu(I)Br (see Supporting
Information for further details).
Both complexes show catalytic activity toward the formation
of triazoles in CuAAC reactions. A more detailed look at the
smaller triazoles P1−P3 reveals that the trinuclear complex 3b
shows significantly improved conversion times than the
tetrameric complex 2b. The quantitative conversion of P2
and P3 was already achieved after 1.5 and 0.5 h, respectively,
and was therefore 3 to 8 times faster as with 2b. The formation
of P1 shows a half-conversion time of ∼8.5 h using 2b.
Changing to complex 3b leads to a full conversion within 8 h.
At this point, it must be noted that conversion times in this
area can still not fully compete when compared to the
literature, but they illustrate the potential of silylenes and more
important functionalized silylenes as active catalysts. Moreover,
the decrease in conversion times between complex 2b and 3b
shows how the ligand design can increase the reactivity.
Dimeric NHC-Cu(I) systems most likely tend to undergo
decomplexation in solution during CuAAC reactions to form
more reactive species.70 It is questionable if the tetrameric
pseudocubane 2b undergoes similar transformations. However,
the increased activity of the trinuclear complex 3b indicates an
improved ligand design. The above-mentioned possibility to
Following these examples, we wanted to test and compare
the catalytic activity of our new multinuclear complexes 2b and
3b in certain CuAAC reactions (the details of catalysts
E
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Chart 1. Time−Conversion Diagram of the CuAAC Reactions of Benzyl Azide with the Corresponding Alkynes (right) in the
Presence of 1 mol % 2b (hollow icons) or 3b (full icons) under Inert Conditions, in dcm-d2 at Ambient Temperature
which is accompanied with the introduction of the pyridyl-
based group and therefore the advanced coordination of the
new metal atom, illustrates the potential of functionalized
bidentate N-heterocyclic silylene copper(I) complexes.
Table 3. Yields and Conversion Times of the Triazoles P1−
a
1
P5, Monitored by H NMR Spectroscopic Analysis
2b
3b
P1 (R = Ph)
P2 (R = Py)
P3 (R = C2H6OH)
P4 (R = tBu)
P5 (R = TMS)
∼46% (8 h)
>99% (6 h)
>99% (4 h)
∼33% (10 h)
∼20% (10 h)
>99% (8 h)
>99% (1.5 h)
>99% (0.5 h)
∼29% (10 h)
∼10% (10 h)
EXPERIMENTAL SECTION
■
General Procedures. All reactions were performed under an
atmosphere of N2 and Ar by Schlenk techniques.74 All solvents were
distilled from Na or K before used for synthesis. Starting materials
were purchased commercially and used without further purification.
MesNHPy and PhC(NtBu)2SiCl were synthesized according to
literature procedures.14,15,75,76 1H, 13C, and 29Si NMR spectroscopic
data were recorded on a Bruker Avance 401 MHz and a Bruker
Avance 300 MHz spectrometer and referenced to the deuterated
solvent (benzene-d6, dcm-d2, thf-d8). Deuterated solvents were dried
over activated molecular sieves (3 Å) and stored in an argon drybox.
Elemental analyses (C, H, N) were performed on a Vario EL3 at the
a
1
Yields were determined by the integration of the corresponding H
NMR spectra normalized on the internal standard (adamantane).
cleave the copper nitrogen coordination of the pyridyl groups
to form reaction intermediates could be a reason for this
observation.
The rather sterically demanding substrates (P4, P5) showed
significantly longer conversion times in both cases. We suggest
that the steric bulk of the tert-butyl and TMS group hinders the
substrates from forming the reactive intermediates and
therefore the products. Especially since complex 3b shows
even slightly slower conversion times it is logical to assume
that the more complex design of the trinuclear compound
hinders the substrate from reacting or that the reaction
intermediates are more shielded toward substrates.
Mikroanalytisches Labor, Institut fur Anorganische Chemie, Uni-
̈
̈
versity of Gottingen. LIFDI-MS spectra were measured on a Jeol
AccuTOF spectrometer.
Shock-cooled crystals were selected from a Schlenk flask under
argon atmosphere using the X-TEMP2 device.77−79 The data were
collected with a Mo−IμS microfocus source.80 All data were
integrated with SAINT,81 and a multiscan absorption correction
(SADABS)82 was applied. For structures 2b, 3a, and 3b, a 3λ
correction was applied.82 The structures were solved by direct
methods (SHELXT)83 and refined on F2 using the full-matrix least-
squares methods of SHELXL84 within the SHELXLE GUI.85 The
Crystallographic Information Files (CIF) can be obtained free of
charge as indicated in the Supporting Information.
General Procedure for the Preparation of 2. The copper
precursor (1.00 equiv) was dissolved in thf (2 mL) to form a white
suspension and was subsequently cooled to 0 °C. The chlorosilylene
PhC(NtBu)2SiCl 1 (1.00 equiv) was separately dissolved in thf (5
mL) and slowly added under vigorous stirring. The reaction mixture
was allowed to warm to room temperature and stirred overnight. After
filtration the yellow filtrate was concentrated and stored at −35 °C for
1 d to gain the target compounds in the form of crystals suitable for X-
ray analysis.
CONCLUSIONS
■
In this work, we present the successful synthesis of six novel N-
heterocyclic silylene group 11 transition-metal complexes 2a−
2c and 3a−3c, respectively. These complexes were obtained by
applying a new synthetic approach of a preset metalation of the
neat chlorobenzamidinate silylene 1 resulting in the
pseudocubane-like compounds 2a−2c followed by the
subsequent functionalization via the substitution of the chloro
group with the lithiated N-mesityl-N-(2-pyridyl)amine to
isolate the complexes 3a−3c. Each of the resulting complexes
[XCu(I) ← (Cl)Si(PhC-(NtBu)2)]4 2a−2c and [(XCu(I))3
← (PyNMes)Si(PhC(NtBu)2)] 3a−3c (with X = Cl (2a/3a),
Br (2b/3b), I (2c/3c)) was characterized in solid state via
XRD analysis and in solution via different NMR techniques as
well as via mass spectrometry and elemental analysis. The
increase of the catalytic activity from compound 2b to 3b,
Preparation of 2a. Following the general procedure described
above CuCl (168 mg, 1.70 mmol) and PhC(NtBu)2SiCl 1 (500 mg,
1.70 mmol) were used and resulted in pale yellow crystals (448.8 mg,
0.285 mmol, 67%).
1H NMR (300.13 MHz, dcm-d2, ppm): 7.72−7.69 (m, 1H, CPh),
7.60−7.49 (m, 4H, CPh), 1.26 (s, 18H, NtBu). 13C{1H} NMR (75.48
F
DOI: 10.1021/acs.inorgchem.9b00629
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
MHz, dcm-d2, ppm): 172.11 (CPh), 131.21 (CPh), 130.59 (CPh),
128.68 (CPh), 128.38 (CPh), 128.07 (CPh), 54.33 (NC(CH3)3),
30.53 (NtBu). 29Si{1H} NMR (79.49 MHz, dcm-d2, ppm): 27.73.
Elemental analysis (%) calcd for C60H92N8Si4Cl8Cu4: C, 45.74; H,
5.89; N, 7.11; found C, 45.44; H, 5.65; N, 7.31.
Preparation of 2b. Following the general procedure described
above CuBr(SMe2) (348 mg, 1.70 mmol) and PhC(NtBu)2SiCl 1
(500 mg, 1.70 mmol) were used and resulted in yellowish crystals
(550 mg, 0.302 mmol, 71%).
(NC(CH3)3), 31.06 (NtBu), 20.54 (MesMe), 19.70 (MesMe). MS
(LIFDI[+], thf): m/z = 1435.0 (100) [M-Br+2thf]+, 1291.2 (5) [M-
Br]+. Elemental analysis (%) calcd for C58H76N8Si2Br3Cu3(C7H8)2: C,
55.57; H, 5.96; N, 7.20; found C, 58.26; H, 5.71; N, 7.19.
Preparation of 3c. Following the general procedure described
above, 2c (334 mg, 0.172 mmol), MesNHPy (146 mg, 0.688 mmol),
and LiHMDS (115 mg, 0.688 mmol) were used and resulted in
yellow crystals (159 mg, 0.105 mmol, 31%).
1H NMR (300.13 MHz, dcm-d2, ppm): 8.72 (br s, 1H, NPy),
7.59−7.40 (m, 5H, CPh), 7.48−7.42 (m, 1H, NPy), 7.08 (s, 2H,
NMes), 6.79 (ddd, J = 6.8, 5.4, 1.1 Hz, 1H, NPy), 6.00 (d, J = 8.5 Hz,
1H, NPy), 2.41−2.37 (m, 9H, MesCH3), 1.04 (s, 18H, NtBu).
13C{1H} NMR (75.48 MHz, dcm-d2, ppm): 172.68 (CPh), 161.13
(NPy), 148.57 (NPy), 139.93 (NMes), 138.96 (NPy), 137.93 (NMes),
137.34 (NMes), 131.68 (CPh), 130.47 (CPh), 129.82 (NMes), 128.96
(CPh), 128.15 (CPh), 114.46 (NPy), 110.31 (NPy), 54.68-
(NC(CH3)3), 31.28 (NtBu), 20.60 (MesMe), 20.12 (MesMe). MS
(LIFDI[+], thf): m/z = 1385.3 (10) [M-I]+, 1003.6 (<5) [M-Cu2I3]+.
Elemental analysis (%) calcd for C58H76N8Si2I3Cu3(C7H8): C, 48.64;
H, 5.28; N, 6. 98; found C, 53.53; H, 5.21; N, 7.11.
General Procedure for Catalysis. A NMR Young tube was
charged with benzyl azide (1.00 equiv), the appropriate alkyne (1.00
equiv), adamantane as an internal standard, and deuterated
dichloromethane. After an initial measurement of the neat reactants,
the complex 2b/3b was added. The progress of the reaction was
monitored via 1H NMR spectroscopic measurements in 10 min steps
for at least 10 h.
1H NMR (400.13 MHz, benzene-d6, ppm): 7.35 (d, J = 8.1 Hz, 1H,
CPh), 7.13−6.98 (m, 4H, CPh), 1.21 (s, 18H, NtBu). 13C{1H} NMR
(100.62 MHz, benzene-d6, ppm): 171.52 (CPh), 131.08 (CPh),
130.09 (CPh), 128.35 (CPh), 128.32 (CPh), 127.66 (CPh), 54.29
(NC(Me)3), 30.75 (NtBu). 29Si{1H} NMR (79.49 MHz, benzene-d6,
ppm): 26.09. MS (ESI[+], thf): m/z = 2144.78 (<5) [M-
Br−+2SbF6]+. Elemental analysis (%) calcd for C60H92N8Si4-
Br4Cl4Cu4(C7H8): C, 43.60; H, 5.46; N, 6.07; found C, 43.00; H,
5.40; N, 5.96.
Preparation of 2c. Following the general procedure described
above CuI (323 mg, 1.70 mmol) and PhC(NtBu)2SiCl 1 (500 mg,
1.70 mmol) were used and resulted in yellow crystals (454 mg, 0.234
mmol, 55%).
1H NMR (300.13 MHz, dcm-d2, ppm): 7.66−7.48 (m, 5H, CPh),
1.30 (s, 18H, NtBu). 13C{1H} NMR (75.48 MHz, dcm-d2, ppm):
172.14 (CPh), 130.87 (CPh), 130.63 (CPh), 128.63 (CPh), 128.50
(CPh), 128.16 (CPh), 54.58 (NC(CH3)3), 31.11 (NtBu). 29Si{1H}
NMR (79.49 MHz, dcm-d2, ppm): 22.62. Elemental analysis (%)
calcd for C60H92N8Si4I4Cl4Cu4(C7H8): C, 39.57; H, 4.96; N, 5.51;
found C, 38.92; H, 4.61; N, 5.42.
ASSOCIATED CONTENT
* Supporting Information
■
General Procedure for the Preparation of 3. The compound 2
(1.00 equiv) and the amine MesNHPy (4.00 equiv) were dissolved in
a mixture of toluene (3 mL) and thf (3 mL) in a ratio of 1:1 and
cooled to −30 °C. A separately prepared solution of LiHMDS (4.00
equiv) in thf (2 mL) was added dropwise and subsequently allowed to
warm to ambient temperature under constant stirring for ∼16 h. The
resulting suspension was concentrated, until a yellow solid
precipitated, which was filtered off and dissolved in dcm to fully
remove the LiCl salt in a further filtration step. The concentrated
solution was stored at −35 °C for 1 d. The target compounds were
isolated in form of yellow crystals suitable for X-ray analysis.
Preparation of 3a. Following the general procedure described
above, 2a (213 mg, 0.135 mmol), MesNHPy (115 mg, 0.540 mmol),
and LiHMDS (90.4 mg, 0.540 mmol) were used and resulted in
yellow crystals (183 mg, 0.148 mmol, 55%).
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00629.
Discussion of experimental details, X-ray crystallographic
and NMR spectroscopic analyses, additional references
(PDF)
Accession Codes
CCDC 1898639−1898644 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1H NMR (300.13 MHz, dcm-d2, ppm): 8.39 (br s, 1H, NPy),
7.61−7.42 (m, 5H, CPh), 7.45 (ddd, J = 8.8 Hz, 6.2, 1.9 Hz, 1H,
NPy), 7.06 (s, 2H, NMes), 6.80 (t, J = 6.2 Hz, 1H, NPy), 6.01 (dt, J =
8.5, 1.0 Hz, 1H, NPy), 2.39−2.35 (m, 9H, MesCH3), 1.11 (br s, 18H,
NtBu). 13C{1H} NMR (75.48 MHz, dcm-d2, ppm): 172.09 (CPh),
160.05 (NPy), 146.05 (NPy), 138.71 (NMes), 137.92 (NMes), 137.24
(NMes), 136.46 (NMes), 130.57 (NPy), 129.62 (CPh), 128.96
(NMes), 128.38 (CPh), 128.15 (CPh), 128.00 (CPh), 114.42 (NPy),
108.16 (NPy), 54.73 (NC(CH3)3), 31.32 (NtBu), 20.66 (MesMe),
19.38 (MesMe). MS (LIFDI[+], thf): m/z = 1117.2 (5) [M-Cl]+,
919.4 (100) [M-Cu2Cl3]+. Elemental analysis (%) calcd for
C58H76N8Si2Cl3Cu3(C7H8)2: C, 60.78; H, 6.52; N, 7.88; found C,
61.38; H, 6.33; N, 7.70.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dstalke@chemie.uni-goettingen.de.
■
ORCID
Regine Herbst-Irmer: 0000-0003-1700-4369
Dietmar Stalke: 0000-0003-4392-5751
Notes
The authors declare no competing financial interest.
Preparation of 3b. Following the general procedure described
above, 2b (234.0 mg, 0.134 mmol), MesNHPy (114 mg, 0.535
mmol), and LiHMDS (89.5 mg, 0.535 mmol) were used and resulted
in yellow crystals (77.8 mg, 0.0567 mmol, 42%).
ACKNOWLEDGMENTS
■
We thank the Danish National Research Foundation
(DNRF93) funded Center for Materials Crystallography
(CMC) for partial support.
1H NMR (300.13 MHz, dcm-d2, ppm): 8.27−8.17 (m, 1H, NPy),
7.66−7.49 (m, 3H, CPh), 7.45 (ddd, J = 8.8, 7.1, 1.9 Hz, 1H, NPy),
7.42−7.35 (m, 2H, CPh), 7.06 (s, 2H, NMes), 6.68 (ddd, J = 6.6, 5.3,
1.1 Hz, 1H, NPy), 6.17 (d, J = 8.5 Hz, 1H, NPy), 2.35 (s, 3H,
MesCH3), 2.31 (s, 6H, MesCH3), 1.05 (s, 18H, NtBu). 13C{1H} NMR
(75.48 MHz, dcm-d2, ppm): 170.81 (CPh), 162.30 (NPy), 148.10
(NPy), 139.35 (NMes), 138.74 (NPy), 136.20 (NMes), 136.10
(NMes), 131.63 (CPh), 130.74 (CPh), 129.81 (NMes), 128.38
(CPh), 127.83 (CPh), 114.16 (NPy), 111.45 (NPy), 54.16
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