[(18-C-6)K][(NC)CuI-SiMe2Ph], a Potassium Silylcyanocuprate as a Catalyst Model for Silylation Reactions with Silylboranes: Syntheses, Structures, and Catalytic Properties

Article abstract of DOI:10.1021/acs.inorgchem.7b00749

Cu^I-catalyzed silylation reactions involving silylboranes (in particular, pinB-SiMe₂Ph (1)) as silyl sources have recently gained considerable attention. One of the most efficient and versatile and yet simplest catalyst systems consi

Full text of DOI:10.1021/acs.inorgchem.7b00749

Article
pubs.acs.org/IC
[(18-C-6)K][(NC)CuI−SiMe2Ph], a Potassium Silylcyanocuprate as a
Catalyst Model for Silylation Reactions with Silylboranes: Syntheses,
Structures, and Catalytic Properties
Jacqueline Plotzitzka and Christian Kleeberg*
Institut fur Anorganische und Analytische Chemie, Technische Universitat Carolo-Wilhelmina zu Braunschweig, Hagenring 30, 38106
̈
̈
Braunschweig, Germany
S
* Supporting Information
ABSTRACT: CuI-catalyzed silylation reactions involving silyl-
boranes (in particular, pinB−SiMe2Ph (1)) as silyl sources have
recently gained considerable attention. One of the most efficient
and versatile and yet simplest catalyst systems consists of
CuCN/NaOMe; however, nothing is known about the catalyti-
cally relevant species. Using an NHC-based model catalyst,
copper silyl complexes of the type [(NHC)Cu−SiMe2Ph] have
been established to be crucial species in these catalytic processes.
The well-defined and spectroscopically and structurally charac-
terized complex [(18-C-6)K][NC−Cu−OtBu] (2), as a model
for the catalytic system, CuCN/NaOMe, shows comparable
catalytic activity toward established, exemplary substrates
(aldehydes, imines, α,β-unsaturated carbonyls) and in extension
allows the efficient silylation of ketones. In addition, a number of peculiarities of the catalytic reaction are readily rationalized on
the basis of the mechanistic insight already established using [(NHC)Cu−SiMe2Ph] as a model catalyst. Analogously to the
NHC model system, the reaction of 2 with the silylborane 1 furnishes the silylcyanocuprate [(18-C-6)K][NC−Cu−SiMe2Ph]
(3) as a potential crucial intermediate in these silylation reactions also suggesting mechanistic similarities between (NHC)Cu-
and CuCN/NaOMe-based catalyst systems. Moreover, 3 and [(NHC)Cu−SiMe2Ph] complexes also share structurally
distinctive features. In the solid state 3 either exists as a linear, two-coordinated copper complex or, depending on the conditions
of crystallization, forms binuclear μ-silyl bridged dimers exhibiting very short Cu···Cu distances. Both structural motifs are also
known for [(NHC)Cu−SiR3] complexes. These findings give an initial insight into the versatile structural chemistry of certain
silylcyanocuprates; in particular, the finding of dinuclear silylcuprates gives rise to the question whether these dimeric species are
of mechanistic relevance for the catalytic processes. However, all peculiarities of the investigated catalytic reaction can readily be
rationalized on the basis of the mechanistic details established using (NHC)Cu model complexes.
order to elucidate mechanistic details of the respective catalytic
reactions.2c,5 In these catalytic processes, a crucial step is the
B−Si bond activation of the silylboranes by a Cu-alkoxido
complex (e.g., [(IDipp)Cu−OtBu]) via σ-bond metathesis to
give the copper silyl complex (e.g., [(IDipp)Cu−SiMe2Ph]).5c
For similar catalytic reactions, a catalyst system consisting of
CuCN and NaOMe has been developed by Oestreich and co-
workers, exhibiting a wide substrate scope and a high reactivity
(Scheme 1).2 However, for this catalytic system, the relevant
Cu species has not been investigated, but silylcyanocuprates
[NC−Cu−SiR3]− appearin analogy to (NHC)Cu−SiR3
complexes (vide supra)feasible as catalytic active species.
Besides, silylcyanocuprates have not, to the best of our
knowledge, been structurally investigated, whereas only a few
silylcuprates [R3Si−Cu−SiR3]− and [X−Cu−SiR3]−, respec-
tively, have been structurally characterized.7 In general, the
INTRODUCTION
■
CuI silyl complexes are central for a number of transformations
as catalytically active intermediates. In particular, silylation of
substrates such as imines, aldehydes, and α,β-unsaturated
carbonyl/carboxyl compounds with silylboranes has recently
been a particularly active field of research (Scheme 1).1−3 For
these types of reactions, CuI -silyl complexes of different types
have been proposed as the catalytically crucial intermediates.
The structural chemistry of NHC-CuI silyl complexes such as
[(IDipp)Cu−SiR3], [(IMes)Cu−SiR3] (SiR3 = SiMe2Ph,
SiPh3), [(ItBu)Cu−SiR3] (SiR3 = SiMe2Ph, SiPh3, SiEt(TMS)2,
Si(TMS)3), [(IMe)Cu−Si(TMS)3], [(IMe)Cu−SiEt(TMS)2]2,
and [(Me2IMe)Cu−SiR3]2 (SiR3 = SiMe2Ph, SiPh3) has been
explored.4−6 These complexes exhibit monomeric linear or
dimeric structures depending on the steric demand of the NHC
ligand, the latter comprising μ-silyl ligands bridging two Cu
ions with extraordinarily short Cu···Cu distances of <2.3 Å.
Moreover, we have used the complex [(IDipp)Cu−SiMe2Ph]
as a model for various silylation processes with silylboranes in
Received: March 27, 2017
© XXXX American Chemical Society
A
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individual signals (1H−1H NOESY (1 s mixing time), 1H−1H
COSY, 1H−13C HSQC, and 1H−13C HMBC). GC/MS measurements
were performed using a Shimadzu GCMS-QP2010SE instrument
operating in positive EI mode (70 eV). An Agilent 6890 gas
chromatograph coupled to a JMS-T100GC (GCAccuTOF, JEOL)
time-of-flight mass spectrometer operating in EI mode (70 eV) was
used for HRMS measurements. Melting points were determined in
Scheme 1. Selected Examples of the CuCN/NaOMe-
Catalyzed Silylation with pinB−SiMe2Ph (1)1,2
flame-sealed capillaries under nitrogen using a Buchi 535 apparatus
̈
and are uncorrected. Elemental analyses were performed at the Institut
fur Anorganische and Analytische Chemie of the Technische
̈
Universitat Carolo-Wilhelmina zu Braunschweig. IR spectra were
̈
measured on a Bruker Vertex 70 spectrometer. ATR measurements
were conducted using a PIKE MIRacle ATR device. These IR spectra
were recorded in air under ambient conditions; it was ensured that no
appreciable decomposition occurred on the time scale of the
measurement. IR spectra of solutions were recorded using a cuvette
of approximately 0.5 mm optical path length equipped with NaCl
windows.
X-ray Structure Determinations. The crystals were transferred
into inert perfluoroether oil inside a nitrogen-filled glovebox and,
outside of the glovebox, rapidly mounted on top of a human hair and
placed in the cold nitrogen gas stream on the diffractometer.12a The
data were either collected on an Oxford Diffraction Xcalibur E
instrument using monochromated MoKα radiation or on an Oxford
Diffraction Nova A instrument, using mirror-focused CuKα radiation.
The reflections were indexed and integrated, and an empirical
absorption correction was applied as implemented in the CrysAlisPro
software.12b The structures were solved employing the programs
SHELXS, SHELXT, or SIR-92 and refined anisotropically for all non-
hydrogen atoms by full-matrix least-squares on all F2 using SHELXL
software.12c−e Generally, hydrogen atoms were refined employing a
riding model; methyl groups were treated as rigid bodies and were
allowed to rotate about the E−CH3 bond. During refinement and
analysis of the crystallographic data the programs WinGX, PLATON,
Mercury, and Diamond were used.12f−i Unless otherwise noted,
ellipsoids are shown at the 50% probability level, and hydrogen atoms
are omitted for clarity.
majority of organo(cyano)cuprates studied contain lithium ions
that are of relevance to their structure and reactivity; hence,
their properties cannot be directly transferred to Li-free
silyl(cyano)cuprates.8
Considering this, and the potential relevance of silylcyano-
cuprates for stoichiometric reactions, we endeavored to develop
a molecular (model) system mimicking the catalytic system
CuCN/NaOMe and to study its reactivity toward silylboranes
with emphasis on the isolation and structural characterization of
the complexes formed. Beyond these fundamental questions is
the potential impact on the further development of Cu-
catalyzed silylation reactions with silylboranes.
Moreover, our studies with [(IDipp)Cu−SiMe2Ph] as a
model catalyst showed that strong similarities exist between
copper catalyst silylation reactions with silylboranes and CuI-
catalyzed borylation reactions with diboron reagents, with
regard to their general reactivity, substrate scope and in
particular their reaction pathway details (e.g., Brook rearrange-
ment, alcohol as additive; vide infra).2c,5b,c,9
[(18-C-6)K][NC−Cu−OtBu] (2). In a nitrogen-filled glovebox,
CuCN (250 mg, 2.81 mmol, 1.0 equiv) and [(18-C-6)K]OtBu (1057
mg, 2.81 mmol, 1.0 equiv) were combined in dry THF (3 mL). After
30−60 min of stirring at rt, a clear yellow solution was obtained. After
thorough drying in vacuo, the material was dissolved in hot PhMe (2
mL) and filtered through a plug of Celite. Upon cooling to −40 °C for
48 h, 2 was obtained as colorless to off-white crystals that were washed
with n-pentane (3 × 2 mL) and dried in vacuo (1018 mg, 1.99 mmol,
71%). X-ray quality crystals were obtained as described above by slow
1
cooling of the PhMe solution. H NMR (600 MHz, THF-d8, rt): δ
3.65 (s, 24 H, CH2O), 1.13 (s, 9 H, OC(CH3)3). 13C{1H} NMR (150
MHz, THF-d8, rt): 141.7 (CN), 70.9 (CH2O), 68.9 (OC(CH3)3), 37.2
(OC(CH3)3). IR: 2119 cm−1 (ν CN, in THF), 2120 cm−1 (ν CN,
solid ATR). m.p.: decomp. > 180 °C. Anal. Calcd for C17H33CuKNO7
C, 43.81; H, 7.14; N, 3.01. Found: C, 43.65; H, 7.15; N, 3.43.
[(18-C-6)K(thf)][NC−Cu−OtBu] (2(thf)). In a nitrogen-filled
glovebox CuCN (100 mg, 1.12 mmol, 1.0 equiv) and [(18-C-
6)K]OtBu (422 mg, 1.12 mmol, 1.0 equiv) were combined in dry
THF (2 mL). After 30−60 min of stirring at rt, a clear yellow solution
was obtained, and the mixture was left to crystallize at −40 °C. After 2
days, the supernatant solution was decanted from colorless crystals of
2(thf) (suitable for single crystal X-ray diffraction). Drying in vacuo at
rt furnished 2 with varying amounts of cocrystallized THF depending
on the time and conditions of drying; after prolonged drying, virtually
THF free 2 was obtained (354 mg, 0.76 mmol, 68%). Anal. Calcd for
C21H41CuKNO8 (2(thf)) C, 46.86; H, 7.68; N, 2.60. Anal. Calcd for
C17H33CuKNO7 (2) C, 43.81; H, 7.14; N, 3.01. Found: C, 44.13; H,
7.05; N, 3.04.
EXPERIMENTAL SECTION
■
General Considerations. The compounds pinB−SiMe2Ph (1),
pinB−SiPh3, [(18-C-6)K]OtBu, and (4-methylphenyl)-N-phenylme-
thanimine were prepared according to literature procedures.10 All
other compounds were commercially available and were used as
received; their purity and identity were checked by appropriate
methods. All solvents were dried using MBraun solvent purification
systems, deoxygenated using the freeze−pump−thaw method, and
stored under purified nitrogen. All manipulations were performed
using standard Schlenk techniques under an atmosphere of purified
nitrogen or in a nitrogen-filled glovebox (MBraun). NMR spectra were
recorded on Bruker Avance 400, Avance 300, or Avance 600
spectrometers. For air sensitive samples, NMR tubes equipped with
screw caps (WILMAD) were used, and the solvents were dried over
potassium/benzophenone and degassed. Chemical shifts (δ) are given
in ppm, using the (residual) resonance of the solvents for calibration
1
1
(C6D6: H NMR: 7.16 ppm, 13C NMR: 128.06 ppm; THF-d8: H
NMR: 1.72, 3.58 ppm, 13C NMR: 25.31, 67.21 ppm, obtained from
Eurisotop with 99.5% deuteration).11 29Si chemical shifts are reported
relative to external Me4Si. 13C and 29Si NMR spectra were recorded
employing (29Si: inverse-gated) composite pulse 1H decoupling. If
necessary, 2D NMR techniques were employed to assign the
{[(18-C-6)K][NC−Cu−SiMe2Ph]}2 (3)2. In a nitrogen-filled glove-
box, 2 (40 mg, 86 μmol, 1.0 equiv) was dissolved in toluene (2 mL),
and 1 (25 mg, 95 mmol, 1.1 equiv) was added. After 0.5 h of stirring at
rt, the initially yellow, then purple solution was, if necessary, decanted
from small amounts of a dark precipitate and layered with n-pentane
B
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(2 mL). After crystallization at −40 °C for 48 h, the supernatant
solution was decanted, and the pale yellow crystalline residue (suitable
for single crystal X-ray diffraction) was washed with n-pentane (2 × 2
mL). After drying in vacuo, the product was obtained as pale yellow
crystals (29 mg, 27 μmol, 64%). 1H NMR (400 MHz, C6D6, rt): δ 8.13
(dq*, JHH = 8.2, 1.5 Hz, 2 H, o-CHPh), 7.46 (tt*, JHH = 7.3, 1.4 Hz, 2
H, m-CHPh), 7.33 (tt*, JHH = 7.3, 1.5 Hz, 1 H, p-CHPh), 3.23 (s, 24 H,
OCH2), 0.90 (s, 6 H, Si(CH3)2). 13C{1H} NMR (100 MHz, C6D6, rt):
155.3 (CPh), 149.4 (CN), 135.3 (CHPh), 127.3 (CHPh), 125.8 (CHPh),
70.1 (OCH2), 5.0 (Si(CH3)2). 29Si{1H} NMR (79.5 MHz, C6D6, rt):
1
−16.8. H NMR (400 MHz, THF-d8, rt): δ 7.45 (dd*, JHH = 8.1, 1.5
Hz, 2 H, o-CHPh), 7.02 (t*, JHH = 7.3 Hz, 2 H, m-CHPh), 6.92 (tt*, JHH
= 7.3, 1.5 Hz, 1 H, p-CHPh), 3.58 (s, 24 H, OCH2), 0.05 (s, 6 H,
Si(CH3)2) (*approximate signal shape; the signal exhibits additional
fine structure due to higher-order coupling). 13C{1H} NMR (100
MHz, THF-d8, rt): 156.5 (CPh), 147.3 (CN), 135.3 (CHPh), 127.0
(CHPh), 125.3 (CHPh), 71.2 (OCH2), 4.9 (Si(CH3)2). IR: 2113 cm−1
(ν CN). m.p.: decomp.
> 60 °C. Anal. Calcd for
(C21H35CuKNO6Si)2 C, 47.75; H, 6.68; N, 2.65. Found: C, 47.36;
H, 6.61; N, 2.74.
Figure 1. Left: Detail of the packing pattern of 2. Right: View of a
stack of 2 along the b axis including disorder (parts with minor
occupancy in gray). Selected bond lengths (Å) and angles (deg):
Cu1−C1 1.847(5), Cu1−O1 1.822(3), C1−N1 1.155(8), O1−K1
2.667(3), K1−C21 3.309(5), K1−[O6]K1 0.556(1), C1−Cu1−O1
177.4(2), N1−C1−Cu1 178.8(4); Cu2−C18 1.842(5), Cu2−O8
1.816(3), C18−N2 1.154(8), O8−K2 2.630(3), K2(x,y‑1,z)−C4
3.424(8), K2−[O6]K2 0.471(1), C18−Cu2−O8 176.5(2), N2−C18−
Cu2 178.5(5).13
[(18-C-6)K(thf)][NC−Cu−SiMe2Ph] 3(thf). In a nitrogen-filled
glovebox, 2 (40 mg, 86 μmol, 1.0 equiv) was dissolved in THF (2 mL)
and 1 (25 mg, 95 mmol, 1.1 equiv) was added. The yellowish solution
was layered with n-pentane (2 mL) after 0.5 h stirring at rt. After
crystallization at −40 °C for 48 h, the supernatant solution was
decanted, and the crystalline residue (suitable for single crystal X-ray
diffraction) was washed with n-pentane (2 × 2 mL). Drying in vacuo at
rt furnished 3 with varying amounts of cocrystallized THF depending
on the time and conditions of drying. Thorough drying leads to
virtually complete loss of THF; hence, 3 is obtained (23 mg, 43 μmol,
50%). Anal. Calcd for C25H43CuKNO7Si (3(thf)) C, 50.02; H, 7.22;
N, 2.33. Anal. Calcd for C21H35CuKNO6Si (3) C, 47.75; H, 6.68; N,
2.65. Found: C, 47.83; H, 6.64; N, 2.33.
Crystallization of 2 from THF furnishes the solvate 2(thf),
which crystallizes in a monoclinic space group P21/c and is
highly disordered due to its virtual inversion symmetry. The
asymmetric unit contains one molecule of 2(thf) in which
[NC−Cu−OtBu]− and the THF moiety are situated above and
below the plane of the [(18-C-6)K]+ moiety. Moreover, in each
moiety of 2(thf) the [NC−Cu−OtBu]− and the THF ligand,
above and below the [(18-C-6)K]+ plane are disordered with
each other, accompanied by disorder of the [(18-C-6)K]+
moiety; additionally the THF ligand is disordered over two
sites in itself (Figure 2). In other words, the two disordered
units of 2(thf) are almost related by an inversion center.
However, it has to be emphasized that the occupancies of the
RESULTS AND DISCUSSION
■
[(18-C-6)K][NC−Cu−OtBu] (2) and 2(thf). The reaction
of CuCN with an equimolar amount of [(18-C-6)K]OtBu
results in the formation of the alkoxy-cyanocuprate [(18-C-
6)K][NC−Cu−OtBu] (2) which may be considered to be a
model for the catalyst system CuCN/NaOMe (vide supra).1,2
Using [(18-C-6)K]+ improves the crystallization process and
adds a spectroscopic handle to the counterion. Single crystals
suitable for X-ray diffraction of 2 were obtained from PhMe and
THF solutions, respectively. For the latter case, the solvate
2(thf) was obtained.13
From PhMe solution, 2 crystallizes in the space group Pna21
as an inversion twin.13,14 The asymmetric unit contains two
independent molecules exhibiting similar geometrical parame-
ters (Figure 1). The copper ions are virtually linearly
coordinated, exhibiting Cu−O distances in the range observed
for linear [(NHC)Cu−OAlkyl] complexes (1.79−1.82 Å) as
well as Cu-CN and CN distances similar to those in
polymeric CuCN (Cu−C/N 1.84−1.90 Å, CN 1.12−1.18
Å).5a,15,16 Within the crystal, 2 forms stacks parallel to the b axis
consisting of alternating units of the two independent
molecules of 2. The individual units within each stack are
associated by K−H3C interactions. However, the K−H3C
distances are longer than in potassium trimethylstannyl
complexes (<3.3 Å).17 The eight-coordinate potassium atoms
are situated slightly out of the crown ether plane ([O6]X,
defined by the mean plane of the six oxygen atoms bound to
atom X) toward the oxygen ligand. Within the stacks of 2, the
[NC−Cu−OtBu]− moieties show positional disorder over two
positions, associated with a corresponding disorder of the
potassium atoms (occupancy of the minor components around
6%).13
Figure 2. Left: Molecular structure of 2(thf) with the disordered THF
molecule. Atoms of the THF molecule shown with arbitrary radii.
Right: Details of the packing pattern of 2(thf); only disorder due to
the pseudo-inversion symmetry is shown (gray). Selected bond lengths
(Å) and angles (deg): Cu1−C1 1.846(3), Cu1−O1 1.823(3), C1−N1
1.145(5), O1−K1 2.658(3), O8−K1 2.909(9), K1−[O6]K1 0.221(2),
C1−Cu1−O1 178.9(1), N1−C1−Cu1 178.3(4).13
C
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two disordered 2(thf) units refined to 0.62 and 0.38, deviating
significantly from 1/2 as inevitable for (crystallographic)
inversion symmetry.
On a molecular level, 2(thf) exhibits a structure similar to the
one which was found for 2 (vide infra), comprising an
approximately linear [NC−Cu−OtBu]− ion and an eight-
coordinate potassium ion (Figure 2). The potassium ion is
coordinated to the oxygen atoms of the cuprate and the THF
ligand as well as the crown ether ligand. While the geometrical
data, in particular of the THF moiety, must be treated with
considerable care, the rather long K1−O8 distance suggests
weak bonding of the THF moiety. In agreement with that,
release of THF is observed upon drying 2(thf) in vacuo. Similar
to 2, the packing of the solvate 2(thf) may be described as
stacks of 2(thf) parallel to the [1 0 1] vector. However, within
̅
these stacks, the individual units of 2(thf) show no particular
interactions.
Figure 3. Asymmetric unit of 3(thf) and an overlay (best fit of the
K[O6] units) of the two independent molecules (inset). Selected bond
length (Å) and angles (deg): Cu1−C1 1.916(5), Cu1−Si1 2.273(1),
C1−N1 1.147(6), O7−K1 2.801(4), N1−K1 2.830(4), K1−[O6]K1
0.0692(9), C1−Cu1−Si1 173.0(1), N1−C1−Cu1 176.3(4), K1−N1−
C1 112.7(3); Cu1A−C1A 1.911(5), Cu1A−Si1A 2.269(1), C1A−
N1A 1.144(6), O7A−K1A 2.799(4), N1A−K1A 2.825(4), K1A−
[O6]K1A 0.1820(9), C1A−Cu1A−Si1A 176.3(1), N1A−C1A−Cu1A
176.9(4), K1A−N1A−C1A 109.5(3).13
[(18-C-6)K][NC−Cu−SiMe2Ph] (3). As reported for a
number of [(NHC)Cu−OtBu] complexes, 2 reacts readily
with 1 with activation of the B−Si bond leading to the
silylcyanocuprate [(18-C-6)K][NC−Cu−SiMe2Ph] (3). Crys-
tallization from the reaction mixture in PhMe leads to the
isolation of dimeric {[(18-C-6)K][NC−Cu−SiMe2Ph]}2
((3)2), while crystallization from THF solution leads to the
linear coordinated solvate [(18-C-6)K(thf)][NC−Cu−Si-
Me2Ph] (3(thf)) (Scheme 2). However, the cocrystallized
distance, however, is in the range typically observed for [Li···
NC−Cu−R] complexes and 2(thf).18 The Si−Cu distance
and the C−Cu−Si angle are in good agreement with those
observed for various linear [(NHC)Cu−SiMe2Ph] complexes
(2.26−2.29 Å, 171−178°).5a
Scheme 2. Formation of Silylcyanocuprates by Reaction of 2
with 1
The potassium ion is coordinated by the crown ether moiety
and deviates only slightly from its mean plane [O6]K. The
coordination sphere of the potassium ion is completed by a
THF ligand, exhibiting significantly smaller O−K distances than
in 2(thf), and an N-coordinated K···Cu bridging cyanide ligand.
The CN−K moiety is significantly bent suggesting a
significant electron donation via a π-orbital of the CN ligand
(Figure 3). This mode of coordination is not unprecedented
within [(crown−ether)K···NC−M] fragments (M = metal),
though more linear geometries appear to be more common.19
Complex (3)2 shows a dimeric μ-silyl bridged structure. As
we and others reported recently, this is often observed for
[(NHC)Cu−SiR3]2 complexes with sterically undemanding
NHC ligands.5a,6 The Si2Cu2 core found for (3)2 resembles
those complexes closely. The dimer (3)2 crystallizes in the
space group P21/c on a center of inversion, resulting in half a
molecule in the asymmetric unit. The CuI ion is trigonal-planar,
coordinated by a CN ligand and two μ-silyl ligands. The
coordination to the two silyl moieties is unsymmetric; small but
significant differences in Cu−Si distance and C−Cu−Si angle
are found (Figure 4). This unsymmetrical coordination
resembles the findings for the structurally analogous [(NHC)-
Cu−SiR3]2 complexes (Cu−Si: 2.39−2.49 Å and 2.50−2.61 Å;
C−Cu−Si: 106.9−124.8° and 121.1−126.5°), albeit significant
variations depending on the silyl group were observed for the
NHC complexes.5a,6 A most remarkable feature of the structure
of (3)2 is the very short Cu···Cu distance of 2.2543(8) Å,
significantly below twice the van der Waals radius of 2.8 Å, or
the interatomic distance in elemental copper of 2.56 Å.20 While
the occurrence of close Cu···Cu distances in general, as well as
bridging by silyl moieties, is well established, the distances
found in (3)2 and the related [(NHC)Cu−SiR3]2 complexes
THF is readily removed upon evacuation. Hence, intriguingly,
the structure of complex 3 depends on the solvate; the THF
solvate 3(thf) comprises a linear coordinated CuI ion, while the
dimeric (3)2 comprises two three-coordinate CuI ions bridged
by two silyl moieties.
The solvate 3(thf) may be compared to structurally related
2(thf) as well as to linear complexes of the type [(NHC)Cu−
SiMe2Ph]. The crystal structure of 3(thf) in the space group
P21 comprises two independent moieties 3(thf) in the
asymmetric unit (Z = 4, Z′ = 2). The two independent
moieties exhibit similar structures with approximately linear
two-coordinate CuI ions. The most significant difference is the
slightly different orientation of the cuprate and the THF moiety
with respect to the 18-C-6 moiety (Figure 3). Compared to
2(thf) and 2 and also complexes of the type [Li···NC−Cu−
R] (R: Aryl, C(SiR’3)3) (1.84−1.89 Å), a significant elongation
of the NC−Cu bond is observed, in agreement with the
increased trans-influence of the silyl ligand.18 The CN
D
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out with a slight excess of 2, we observed the crystallization of
the linear alkoxysilylcuprate [(18-C-6)K][tBuO−Cu−Si-
Me2Ph].13 Moreover, during our work, we also obtained and
characterized, by single crystal X-ray structure determination,
the complexes [(18-C-6)K][Cu(CN)2], [(18-C-6)K]2[Cu-
(CN)3] and [(18-C-6)K][Cu(SiMe2Ph)2] though the crystal-
lization of individual species cannot be assigned to specific
experimentally readily controllable reaction parameters.13
Therefore it must be stressed that this behavior is intrinsic
and reflects directly the dynamic nature of the cuprates in
solution. Hence, the crystallization conditions have a crucial
impact on the final product obtained. This is also reflected in
the isolation of 3(thf) and (3)2 from THF and PhMe,
respectively.
In addition to the silylcuprates 3 and (3)2 containing
SiMe2Ph groups, the investigation was extended to the
synthesis of sterically more congested silylcyanocuprates
containing the SiPh3 moiety by the reaction of 2 with the
silylborane pinB−SiPh3. Although, in principal, a very similar
reactivity is foundfacile B−Si bond activation and formation
of the corresponding boric acid ester pinB−OtBu along with
silylcupratesthe reproducible crystallization of defined
species was found to be exceedingly challenging. We obtained
silylcyano-, disilyl-, and trisilylcuprates and characterized those
structurally; however, the exact nature of the cuprate obtained
from a particular reaction was, in our hands, not readily
controllable. A discussion of selected aspects of the structures
of the SiPh3 complexes obtained can be found in the
Supporting Information.13
Catalysis Experiments. Having the isolated and well-
characterized cyanocuprate 2 available, we investigated the use
of 2 as a precatalyst in silylation reactions of exemplary
substrates (Table 1). Performing the reaction with 4-methyl
benzaldehyde, under the conditions described by Oestreich and
co-workers for CuCN/NaOMe as precatalyst, delivered indeed
the α-silylbenzyl alcohol product in 67% yield after chromato-
graphic workup (Table 1 entry 1). This compares well with the
Figure 4. Molecular structure of (3)2. Disordered parts as capped-
sticks-model, symmetry equivalent atoms denoted by a prime. Selected
bond lengths (Å) and angles (deg): Cu1−C1 1.9196(3), Cu1−Si1
2.4805(9), Cu1−Si1′ 2.525(1), C1−N1 1.146(4), N1−K1 2.735(3),
K1−[O6]K1 0.5912(6), C1−Cu1−Si1 120.04(9), C1−Cu1−Si1′
113.48(9), N1−C1−Cu1 175.1(3), K1−N1−C1 151.5(2).13
are far shorter than typically found (2.36−2.45 Å). This may be
explained by three-center-two-electron bonds within the Cu2Si2
motif, possibly accompanied by close-shell Cu−Cu interaction-
s.5a,6,7,21,22 The NC−Cu distance is not found to be strongly
affected by the change in coordination number at the CuI ion.
With respect to the coordination of the potassium ion, a
more linear arrangement of the CN−K unit, as in 3(thf), is
found, with a significantly shorter K−N distance. The latter
may be associated with the reduced coordination number at the
potassium ion in (3)2, which may also account for the
pronounced displacement of K1 from the [O6]K1 plane.
It should be emphasized that the synthesis of 3(thf) and (3)2
is highly reproducible, albeit the purity of the starting materials
and the adherence to the experimental procedures are essential.
For example, if the reaction in PhMe leading to (3)2 is carried
Table 1. Catalytic Silylation of Carbonyl Compounds Using 2 as Precatalyst
substrate
conditions
a
b
entry
R
R′
E
t/h
solv.
T/C
yield (%)
1
H
4-Me-C6H4
4-Me-C6H4
4-Me-C6H4
4-CF3-C6H4
4-CF3-C6H4
4-Me-C6H4
4-Me-C6H4
Et
O
2
THF
0°
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
4 equiv of MeOH
67
87
16
88
57
84
67
71
69
0
2
H
O
O
O
O
NPh
O
O
O
O
O
O
O
O
O
3
THF
3
H
24
2
PhMe
THF
4
H
5
H
3
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
6
H
0.5
3
1.1 equiv of iPrOH
7
Me
Et
8
5
9
iBu
iPr
Me
Me
Me
Me
iPr
Me
5
10
11
12
13
14
15
iPr
5
1-Me-C6H4
Bn
4
0
4
0
c
1-Me-C6H4
Bn
4
1.1 equiv of iPrOH
1.1 equiv of iPrOH
1.1 equiv of iPrOH
74
4
84
0
iPr
5
a
b
c
Unoptimized reaction time; full conversion may be reached earlier. Isolated yield after chromatographic workup. O-Silylation product.
E
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Article
yield obtained by Oestreich and co-workers for related
benzaldehydes.2c
boric acid ester IV is formed as the primary catalysis product.
The latter may be converted, in a subsequent hydrolysis step, to
the isolated reaction product V. This “default” reaction pathway
has been established and studied in some detail using
(IDipp)Cu−SiMe2Ph as a model catalyst.2c It is apparently
operative for aldehydes and sterically undemanding ketones
(Table 1, entries 2−5 and 7−9).
Pathway 2 (Scheme 3, Orange). Alternatively, III may
undergo a Brook rearrangement to the isomeric Cu−C
complex IIIa. This complex does not react rapidly with 1 or
the substrate and, hence, does not lead to a productive reaction
pathway as shown for (IDipp)Cu−SiMe2Ph as model catalyst.2c
However, in the presence of an alcohol, IIIa may be converted
to the silyl ether VII and the alkoxido complex VI which again
regenerates the silyl complex I upon reaction with 1 and
concludes a catalytic cycle. Nonetheless, the reaction product is
now the silyl ether VII, formally the product of inverted
insertion selectivity. This reaction pathway is apparently
operative for sterically demanding ketones for which the direct
reaction of III with 1 is slow compared to the Brook
rearrangement (Table 1, entry 13).
Pathway 3 (Scheme 3, Green). Insertion products III that
do not readily react with 1 and do not exhibit competing Brook
rearrangement may react with an alcohol to give the C-silylated
product V and the alkoxido complex VI. This complex may
then react with 1 to regenerate I and close the catalytic cycle.
This reaction pathway appears to be operative for imines, where
the driving force of B−O bond formation (e.g., III → IV + I) is
missing, or for ketones where this reaction hampered for other,
possibly steric, reasons (Table 1, entries 6 and 14). This
reaction pathway may also be operative in the presence of an
alcohol, in parallel or exclusively, for substrates that can also
react following pathway 1 (Table 1, entries 1).
We found that performing the reaction at ambient
temperature in the absence of MeOH as an additive resulted
in slightly better yields, while using PhMe as solvent reduced
the yield significantly (Table 1, entries 1−3). Similar results
were obtained with the more reactive trifluoromethyl derivative
(Table 1, entries 4−5). Also, for an imine as an exemplary
substrate comparable reactivity as established by Oestreich and
co-workers was observed. However, in this case, the presence of
an equivalent of an alcohol as additive proved to be essential
(Table 1, entry 6).
To our delight we found that not only aldehydes and imines,
but also ketones, a substrate class previously not reported for
these reactions, are efficiently silylated to give α-Si-alcohols
using 2 as precatalyst (Table 1, entries 7−9). Here, the steric
demand of the substituents appears to be crucial; effective
conversion is only observed for sterically undemanding ketones,
while sterically more demanding ketones were unreactive under
the given conditions (Table 1, entries 10−12). Interestingly,
addition of 1 equiv of iPrOH as an additive facilitated the
conversion of phenylacetone and 1-methylacetophenone
(Table 1, entries 13−14). Surprisingly, while for phenylacetone
the expected α-silylalcohol is obtained in good yields, for 1-
methylbenzaldehyde, the isomeric O-silylation product is
obtained.
While the necessity of alcohol addition for the silylation of
the imine, phenylacetone, and 1-methylacetophenone may
appear unexpected, it can be fully rationalized on the basis of
the reaction pathways established using (NHC)Cu complexes
as model catalysts and also indicates strong similarities to the
related CuI catalyzed borylation reactions of similar substrates
(Scheme 3).2c,9d
It should be emphasized that the important reaction steps
and intermediates included in pathways 1−3, though not
established for cuprate-based catalytic systems, have been
established and experimentally studied for aldehydes using the
model catalyst [(IDipp)Cu−SiMe2Ph].2c Moreover, virtually
identical reaction pathways, in particular, the Brook rearrange-
ment and the role of the alcohol additive for certain substrates,
have also been established and studied experimentally and
computationally, for the related CuI-catalyzed borylation
reactions with tetraalkoxydiborane reagents.9b−d Hence, it
may be concluded that, despite the difference in the auxiliary
ligand in NHC-containing and CuCN-based catalytic systems,
the fundamental reaction steps are identical, or at least very
similar. In other words, (NHC)Cu complexes are good catalyst
models for CuCN-based catalytic systems, despite the fact that
we have still no direct experimental evidence of the actual
catalytically active species in the latter.
Scheme 3. Possible Reaction Pathways for the Silylation of
Aldehydes, Ketones and Imines
The reaction in the presence of the sterically demanding di-
isopropyl ketone (Table 1, entry 10 and 15) was investigated
further. The reaction of 1 with this ketone in the presence of
catalytic amounts of 2 showed no conversion (Table 1, entry
10). However, in the presence of iPrOH, consumption of 1 is
observed while no silylated ketone is formed (Table 1, entry
15). Following this reaction by NMR spectroscopy (Figure 5)
revealed that 1 is quantitatively converted to pinB−OiPr and
the silane PhMe2SiH after 2 h, hence, on a time scale typical
also for the silylation reactions (Table 1).13 This reaction is
easily rationalized by solvolytic cleavage of the Cu−Si bond in
an initially formed copper silyl species I (e.g., 3) to give the
silane and a copper isopropoxido complex that regenerates the
Central to all observed silylation reactions (Table 1) are
copper silyl species, either an NHC complex (NHC)Cu−
SiMe2Ph or, for the reactions discussed here, a silylcyanocup-
rate such as 3 or an other feasible copper silyl species [Cu]−
SiMe2Ph (I).23 Compounds containing C = E (E = O, N)
double bonds II insert into the Cu−Si bond of I with complete
selectivity for the Cu−E complex III (Scheme 3, red). The
latter complex, however, can participate in various further
reaction pathways.
Pathway 1 (Scheme 3, Blue). If E = O III reacts with the
silylboranes 1 via σ-bond metathesis, I is regenerated, and the
F
DOI: 10.1021/acs.inorgchem.7b00749
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
model system, α,β-unsaturated esters are only converted in the
presence of an alcohol. This can again be rationalized by the
formation of the 3,4-insertion product, in contrast to the 1,4-
insertion product observed for α,β-unsaturated aldehydes and
ketones, and its lack of reactivity toward 1.5b Intriguingly, and
in contrast to the (IDipp)Cu catalytic system, the bis-β-
substituted aldehyde is converted to the 1,4-addition product,
and the 1,2-addition product is only obtained in minor
amounts, possibly an effect of reduced steric demand of the
catalytically active Cu−Si species.5b
Again the close similarities of the CuI-catalyzed silylation and
borylation reactions must be emphasized. Reactions pathways
such as the occurrence of 1,4- and 3,4-addition depending on
the substrate as well as the role of the alcohol additive are also
reported for the related borylation reactions.9e−g
CONCLUSION
■
The alkoxycyanocuprate [(18-C-6)K][NC−Cu−OtBu] (2)
was synthesized as a well-defined catalyst model resembling
the established catalytic system CuCN/NaOMe used in various
silylation reactions with the silylborane pinB−SiMe2Ph (1).1,2
The reaction of 2 with 1 leads, as described in some detail for
the related NHC complexes [(NHC)Cu−OtBu], to the
formation of copper silyl complexes, and thus significantly
extends the scope of this versatile and easy-to-use access to
copper silyl complexes.5a,c However, in contrast to [(NHC)-
Cu−SiMe2Ph] complexes, the nature of the isolated com-
plexeslinear, mononuclear, or μ-silyl bridged dimersis not
readily determined by the steric demand of the auxiliary ligand.
Depending on the crystallization conditions, from THF the
linear mononuclear complex [(18-C-6)K(thf)][NC−Cu−Si-
Me2Ph] (3(thf)) was obtained, while from PhMe the solvent
free dimeric μ-silyl bridged complex (3)2 with a very short Cu···
Cu distances of 2.2543(8) Å was obtained.5a Noteworthy, both
complexes exhibit significant anion cation interactions via
cyanide coordination to the [(18-C-6)K]-cation. Similar
complexes were also obtained employing pinB−SiPh3 as
silylboranes; however, the reactions proved exceedingly difficult
to control and were of low reproducibility.13
Figure 5. Details of in situ 1H NMR spectra of a mixture of 1 (1 equiv,
orange), iPrOH (1 equiv, purple), and 2 (0.05 equiv) in the absence
(top) and presence (bottom) of di-isopropylketone (1 equiv, blue);
300 MHz, C6D6.
copper silyl species upon reaction with 1. Hence, in the absence
of a suitable substrate, the catalytic H−O activation of the
alcohol additive is observed as an unproductive background
process. It is important to note that this unproductive reaction
isapparentlytypically slower than the substrate silylation;
otherwise the described silylation reactions with alcohol as
additive would not generally give favorable yields (Table 1 and
refs 1 and 2). It has further been shown that a mixture of pinB−
OiPr and PhMe2SiH in the presence of catalytic 2 appears to be
unreactive in the silylation of aldehydes.13
As alternative substrates, α,β-unsaturated carbonyl com-
pounds were employed. Again, substrates were chosen that
were previously extensively studied with [(IDipp)Cu−
SiMe2Ph] as (model) catalyst (Table 2).5b For all substrates,
efficient conversion at ambient temperature to the expected
1,4-silylation products was observed, and the silylated
compounds were isolated in good (unoptimized) yields
(Table 2). As described previously for the (IDipp)Cu catalytic
At this point it may be cautiously suggested that silylation
reactions employing the CuCN/NaOMe-based catalyst system
may well proceed via silylcyanocuprates. However, no
conclusion may at present be drawn on the nature of the
exact species present in solution either for the isolated cuprates
or during catalytic silylations using CuCN/NaOMe.
Table 2. Catalytic Silylation of α,β-Unsaturated Carbonyl
Compounds Using 2 as a Precatalyst
The catalytic properties of the silylcyanocuprates were
investigated using 2 as a precatalyst in silylation reactions of
well-established substrates with 1 as silyl source. With
aldehydes, imines, and α,β-unsaturated carbonyl and carboxyl
compounds, a similar reactivity as described for (NHC)Cu-
based catalysts as well as the CuCN/NaOMe catalyst system
was observed, suggesting the suitability of 2 as a molecular
model catalyst for the CuCN/NaOMe catalyst system.
Moreover, using 2 as precatalyst, the unprecedented copper-
catalyzed silylation of ketones with 1 furnishes tertiary α-silyl
alcohols in good yields. However, for two particular substrates,
the reaction takes a different turn; phenylacetone and 1-
methylacetophenone are only reactive in the presence of an
alcohol as an additive. While the former then furnishes the
expected α-silylalcohol, the latter gives the corresponding silyl
ether. Strikingly, this behavior is easily rationalized assuming
reaction steps studied in detail for a (NHC)Cu model
catalyst.13 This confirms the value of this model catalyst and,
substrate
conditions
T/C
b
yield
a
entry
R
R′
t /h
solv.
(%)
1
2
3
4
5
6
cyclohexanone
5
PhMe
PhMe
PhMe
PhMe
PhMe
PhMe
rt
rt
rt
rt
rt
rt
70
77
73
75
Me
H
Me
Et
16
3
H
H
1
c
Me
H
H
6
62
OMe
6
1.1 equiv of
iPrOH
89
a
Unoptimized reaction time; full conversion may be reached earlier.
Isolated yield after chromatographic workup. The 1,2-silylation
product was isolated in 10% yield.
b
c
G
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Wu, H.; Haeffner, F.; Hoveyda, A. H. NHC−Cu-Catalyzed Silyl
Conjugate Additions to Acyclic and Cyclic Dienones and Dienoates.
Efficient Site-, Diastereo- and Enantioselective Synthesis of Carbonyl-
Containing Allylsilanes. Organometallics 2012, 31, 7823−7826.
(c) Lee, K-s.; Hoveyda, A. H. Enantioselective Conjugate Silyl
Additions to Cyclic and Acyclic Unsaturated Carbonyls Catalyzed by
Cu Complexes of Chiral N-Heterocyclic Carbenes. J. Am. Chem. Soc.
2010, 132, 2898−2900. (d) Delvos, L. B.; Vyas, D. J.; Oestreich, M.
Asymmetric Synthesis of α-Chiral Allylic Silanes by Enantioconvergent
γ-Selective Copper(I)-Catalyzed Allylic Silylation. Angew. Chem., Int.
Ed. 2013, 52, 4650−4653. (e) Pace, V.; Rae, J. P.; Procter, D. J.
Cu(I)−NHC Catalyzed Asymmetric Silyl Transfer to Unsaturated
Lactams and Amides. Org. Lett. 2014, 16, 476−479.
therefore, advocates the close mechanistic relation between
(NHC)Cu, 2, and CuCN/NaOMe-based (pre)catalyst systems.
The findings are of direct relevance to the growing field of
CuI-catalyzed silylation reactions illustrating on one hand the
similarities of at first sight quite different catalytic
systems(NHC)Cu- and CuCN/NaOMe-basedand, on
the other hand illustrating again the diverse structural nature
of copper silyl complexes. In particular, it must be emphasized
that, while at present, numerous solid state structures of copper
silyl complexes are known, their solution state behavior remains
to be explored, a subject which is currently under investigation
in our laboratory.
(4) IDipp = 1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene, IMes
= 1,3-bis(2,4,6-trimethyl-phenyl)imidazol-2-ylidene, IMe = 1,3-bis(-
methyl)imidazol-2-ylidene, ItBu = 1,3-bis(tert-butyl)imidazol-2-yli-
dene, Me2IMe = 1,3,4,5-tetra(methyl)imidazol-2-ylidene.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00749.
■
S
(5) (a) Plotzitzka, J.; Kleeberg, C. [(NHC)CuI−ER3] Complexes
(ER3 = SiMe2Ph, SiPh3, SnMe3): From Linear, Mononuclear
Complexes to Polynuclear Complexes with Ultrashort CuI···CuI
Distances. Inorg. Chem. 2016, 55, 4813−4823. (b) Plotzitzka, J.;
Kleeberg, C. CuI-Catalyzed Conjugate Addition of Silyl Boronic
Esters: Retracing Catalytic Cycles Using Isolated Copper and Boron
Enolate Intermediates. Organometallics 2014, 33, 6915−6926.
(c) Kleeberg, C.; Cheung, M. S.; Lin, Z.; Marder, T. B. Copper-
Mediated Reduction of CO2 with pinB-SiMe2Ph via CO2 Insertion
into a Copper−Silicon Bond. J. Am. Chem. Soc. 2011, 133, 19060−
19063.
Additional experimental, analytical and crystallographic
data (PDF)
Accession Codes
CCDC 1539040 and 1539070−1539084 contain the supple-
mentary crystallographic 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.
(6) Sgro, M. J.; Piers, W. E.; Romero, P. E. Synthesis, structural
characterization and thermal properties of copper and silver silyl
complexes. Dalton Trans. 2015, 44, 3817−3828.
(7) (a) Heine, A.; Herbst-Irmer, R.; Stalke, D. [Cu2R2BrLi(thf)3], R
= Si(SiMe3)3 − a complex containing five-coordinate silicon in a three-
centre two-electron bond (thf = tetrahydrofuran). J. Chem. Soc., Chem.
Commun. 1993, 1729−1731. (b) Klinkhammer, K. W. Synthesis and
Crystal Structure of the two Lithium Hypersilylcuprates LiCu2[Si-
(SiMe3)3]3 and [Li7(OtBu)6] [Cu2{Si(SiMe3)3}3]. Z. Anorg. Allg.
Chem. 2000, 626, 1217−1223. (c) Heine, A.; Stalke, D. Preparation
and Structure of Two Highly Reactive Intermediates: [Li(thf)4]-
[Cu5Cl4R2] and [Li(thf)4][AlCl3R], R = Si(SiMe3)3. Angew. Chem., Int.
Ed. Engl. 1993, 32, 121−122. (d) Lerner, H.-W.; Scholz, S.; Bolte, M.
The Sodium Cuprate (tBu3Si)2CuNa: Formation and X-ray Crystal
Structure Analysis. Organometallics 2001, 20, 575−577. (e) Klink-
hammer, K. W.; Klett, J.; Xiong, Y.; Yao, S. Homo- and Heteroleptic
Hypersilylcuprates − Valuable Reagents for the Synthesis of Molecular
Compounds with a Cu−Si Bond. Eur. J. Inorg. Chem. 2003, 2003,
3417−3424.
AUTHOR INFORMATION
Corresponding Author
*Fax: +49 (0) 531 391 5387. E-mail: ch.kleeberg@tu-
braunschweig.de.
ORCID
Christian Kleeberg: 0000-0002-6717-4086
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
■
C.K. thanks the Fonds der Chemischen Industrie for the
generous support by a Liebig Fellowship. J.P. and C.K.
gratefully acknowledge support by a Research Grant (KL
2243/4-1) of the Deutsche Forschungsgemeinschaft (DFG).
We thank Andreas Reding and Tim Harig for help with the
laboratory work.
(8) For an overview see: (a) Woodward, S. Decoding the ‘black box’
reactivity that is organocuprate conjugate addition chemistry. Chem.
Soc. Rev. 2000, 29, 393−401. (b) Davies, R. P. The structures of
lithium and magnesium organocuprates and related species. Coord.
Chem. Rev. 2011, 255, 1226−1251.
REFERENCES
■
(9) Selected references regarding CuI-catalyzed borylation reactions:
(1) For a recent overview see: Oestreich, M.; Hartmann, E.; Mewald,
M. Activation of the Si−B Interelement Bond: Mechanism, Catalysis,
and Synthesis. Chem. Rev. 2013, 113, 402−441.
(2) For examples see: (a) Vyas, D. J.; Oestreich, M. Copper-
Catalyzed Si−BB and Activation in Branched-Selective Allylic
Substitution of Linear Allylic Chlorides. Angew. Chem., Int. Ed. 2010,
(a) Laitar, D. S.; Muller, P.; Sadighi, J. P. Efficient Homogeneous
̈
Catalysis in the Reduction of CO2 to CO. J. Am. Chem. Soc. 2005, 127,
17196−17197. (b) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. Catalytic
Diboration of Aldehydes via Insertion into the Copper−Boron Bond.
J. Am. Chem. Soc. 2006, 128, 11036−11037. (c) Zhao, H.; Dang, L.;
Marder, T. B.; Lin, Z. DFT Studies on the Mechanism of the
Diboration of Aldehydes Catalyzed by Copper(I) Boryl Complexes. J.
Am. Chem. Soc. 2008, 130, 5586−5594. (d) McIntosh, M. K.; Moore,
C. M.; Clark, T. B. Copper-Catalyzed Diboration of Ketones: Facile
Synthesis of Tertiary α-hydroxyboronate Esters. Org. Lett. 2010, 12,
1996−1999. (e) Lee, J.-E.; Yun, J. Catalytic Asymmetric Boration of
Acyclic α,β-Unsaturated Esters and Nitriles. Angew. Chem., Int. Ed.
2008, 47, 145−147. (f) Lee, J.-E.; Kwon, J.; Yun, J. Copper-catalyzed
addition of diboron reagents to α,β-acetylenic esters: efficient synthesis
of β-boryl-α,β-ethylenic esters. Chem. Commun. 2008, 733−734.
(g) Dang, L.; Lin, Z.; Marder, T. B. DFT Studies on the Borylation of
49, 8513−8515. (b) Vyas, D. J.; Frohlich, R.; Oestreich, M. Activation
̈
of the Si−B Linkage: Copper-Catalyzed Addition of Nucleophilic
Silicon to Imines. Org. Lett. 2011, 13, 2094−2097. (c) Kleeberg, C.;
Feldmann, E.; Hartmann, E.; Vyas, D. J.; Oestreich, M. Copper-
Catalyzed 1,2-Addition of Nucleophilic Silicon to Aldehydes:
Mechanistic Insight and Catalytic Systems. Chem. - Eur. J. 2011, 17,
13538−13543.
(3) For selected examples see: (a) Hensel, A.; Nagura, K.; Delvos, L.
B.; Oestreich, M. Enantioselective Addition of Silicon Nucleophiles to
Aldimines Using a Preformed NHC−Copper(I) Complex as the
Catalyst. Angew. Chem., Int. Ed. 2014, 53, 4964−4967. (b) Lee, K-s;
H
DOI: 10.1021/acs.inorgchem.7b00749
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
α,β-Unsaturated Carbonyl Compounds Catalyzed by Phosphine
Copper(I) Boryl Complexes and Observations on the Interconver-
sions Between O- and C-Bound Enolates of Cu, B, and Si.
Organometallics 2008, 27, 4443−4454.
Monomeric Two-Coordinate Copper(I) Systems. Inorg. Chem. 2005,
44, 8647−8649.
(17) Kleeberg, C.; Grunenberg, J.; Xie, X. K−H3C and K−Sn
Interactions in Potassium Trimethylstannyl Complexes: A Structural,
Mechanochemical, and NMR Study. Inorg. Chem. 2014, 53, 4400−
4410 and references cited therein..
(10) (a) Andersen, M. W.; Hildebrandt, B.; Koster, G.; Hoffmann, R.
̈
W. Stereoselective synthesis of alcohols, XXX: E- and Z-
pentenylboronates, reagents for simple diastereoselection on addition
to aldehydes. Chem. Ber. 1989, 122, 1777−1782. (b) Suginome, M.;
Matsuda, T.; Ito, Y. Convenient Preparation of Silylboranes.
Organometallics 2000, 19, 4647−4649. (c) Kleeberg, C. Synthesis
and Characterisation of [K(18-Crown-6)(OtBu)]: An Useful Reagent
in Nonpolar Solvents. Z. Anorg. Allg. Chem. 2011, 637, 1790−1794.
(18) (a) Hwang, C.-H.; Power, P. P. Isolation and Structural
Characterization of the Lithium Cyanoorganocuprate Salt [Li-
(THF)2{Cu(CN)C6H3-2,6-Trip2}]2 (Trip = − C6H2-2,4,6-i-Pr3). J.
Am. Chem. Soc. 1998, 120, 6409−6410. (b) Eaborn, C.; El-Hamruni, S.
M.; Hill, M. S.; Hitchcock, P. B.; Smith, J. D. Syntheses and structures
of lithium cyanocuprates containing the C(SiMe3)3, C-
(SiMe3)2(SiMe2NMe2) and C(SiMe3) (SiMe2OMe)2 groups. J.
Chem. Soc., Dalton Trans. 2002, 3975−3979. (c) Bosold, F.; Marsch,
M.; Harms, K.; Boche, G. Crystal structure of bis[bis(diethyl ether)
lithium (trimethylsilyl)methylcyanocuprate], (C4H10O)2Li(C3H9)-
SiCH2CuCN]2. Z. Kristallogr. - New Cryst. Struct 2001, 216, 143−
144. (d) Hwang, C.-H.; Power, P. P. Synthesis and Characterization of
(THF)3Li(NC)Cu(C6H3-2,6-Mes2) and Br(THF)2Mg (C6H3-2,6-
Trip2) (Mes = C6H2-2,4,6-Me3; Trip = C6H2-2,4,6-i-Pr3): The
Structures of a Monomeric Lower-Order Lithium Organocyanocup-
rate and a Bulky Terphenyl Grignard Reagent in the Solid State. Bull.
Korean Chem. Soc. 2003, 24, 605−609. (e) Eaborn, C.; Hill, M. S.;
Hitchcock, P. B.; Smith, J. D. Structural Diversity in Organolithium,
-sodium, and -potassium Cyanocuprates. Organometallics 2000, 19,
5780−5783.
(d) Neuvonen, K.; Fulop, F.; Neuvonen, H.; Koch, A.; Kleinpeter, E.;
̈
̈
Pihlaja, K. Comparison of the Electronic Structures of Imine and
Hydrazone Side-Chain Functionalities with the Aid of 13C and 15N
NMR Chemical Shifts and PM3 Calculations. The Influence of CN-
Substitution on the Sensitivity to Aromatic Substitution. J. Org. Chem.
2003, 68, 2151−2160.
(11) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.;
Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. NMR
Chemical Shifts of Trace Impurities: Common Laboratory Solvents,
Organics, and Gases in Deuterated Solvents Relevant to the
Organometallic Chemist. Organometallics 2010, 29, 2176−2179.
(12) (a) Stalke, D. Cryo crystal structure determination and
application to intermediates. Chem. Soc. Rev. 1998, 27, 171−178.
(b) CrysAlisPro; Agilent Technologies, Version 1.171.34.44−
1.171.36.24; 2010−2012. (c) Sheldrick, G. M. A short history of
SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−
122. (d) Sheldrick, G. M. Crystal structure refinement with SHELXL.
Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (e) Altomare, A.;
Cascarano, G.; Giacovazzo, C.; Guagliardi, A. Completion and
refinement of crystal structures with SIR92. J. Appl. Crystallogr.
1993, 26, 343−350. (f) Farrugia, L. J. WinGX suite for small-molecule
single-crystal crystallography. J. Appl. Crystallogr. 1999, 32, 837−838.
(g) Spek, A. L. Single-crystal structure validation with the program
PLATON. J. Appl. Crystallogr. 2003, 36, 7−13. (h) Macrae, C. F.;
Bruno, I. J.; Chisholm, J. A.; Edgington, P. R.; McCabe, P.; Pidcock, E.;
Rodriguez-Monge, L.; Taylor, R.; van de Streek, J.; Wood, P. A.
Mercury CSD 2.0 - new features for the visualization and investigation
of crystal structures. J. Appl. Crystallogr. 2008, 41, 466−470. (i)
Diamond 4.2.2. - Crystal and Molecular Structure Visualization; Crystal
Impact - H. Putz and K. Brandenburg GbR: Bonn, Germany, 2016.
(13) See Supporting Information for further details.
(19) (a) Pickardt, J.; Wischlinski, P. Crystal Structure of the
“Supramolecular” Complex [K(benzo-18-crown-6)][Zn(CN)3]·H2O
with a Polymeric Tricyano Zincate Anion with Chain Structure. Z.
Naturforsch., B: J. Chem. Sci. 1999, 54, 747−750. (b) Ben
́
aud, O.;
Berthet, J.-C.; Thuery, P.; Ephritikhine, M. Iodide, Azide, and Cyanide
́
Complexes of (N,C), (N,N), and (N,O) Metallacycles of Tetra- and
Pentavalent Uranium. Inorg. Chem. 2011, 50, 12204−12214.
(c) Whaley, C. M.; Rauchfuss, T. B.; Wilson, S. R. Coordination
Chemistry of [HFe(CN)2(CO)3]− and Its Derivatives: Toward a
Model for the Iron Subsite of the [NiFe]-Hydrogenases. Inorg. Chem.
2009, 48, 4462−4469. (d) Schlueter, J. A.; Geiser, U. Potassium
dicyanoargentate(I) 1,4,7,10,13,16-hexaoxacyclooctadecane. Acta Crys-
tallogr., Sect. C: Cryst. Struct. Commun. 2003, 59, m325−m327.
(e) Mok, K. F.; McKee, V.; Robinson, W. T. Structure of a
dipotassium tetracyanomercurate(II) salt with dibenzo-18-crown-6.
Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1987, 43, 2093−2096.
(f) Whaley, C. M.; Rauchfuss, T. B.; Wilson, S. R. Bis[(1,4,7,10,13,16-
hexaoxacyclooctadecane)potassium] carbonyltricyanorhodate(I) ace-
tonitrile solvate. Acta Crystallogr., Sect. E: Struct. Rep. Online 2005, 61,
m1918−m1919. (g) Li, J.; Noll, B. C.; Schulz, C. E.; Scheidt, W. R.
New Insights on the Electronic and Molecular Structure of Cyanide-
Ligated Iron(III) Porphyrinates. Inorg. Chem. 2007, 46, 2286−2298.
(h) Gloaguen, F.; Lawrence, J. D.; Schmidt, M.; Wilson, S. R.;
(14) In addition, on occasion, a second polymorph of 2 was obtained
from PhMe solutions. Though the quality of this structure is
unsatisfactory, the crystallographic data for this polymorph and a
cautious discussion are given in the Supporting Information.
(15) (a) Hibble, S. J.; Cheyne, S. M.; Hannon, A. C.; Eversfield, S. G.
CuCN: A Polymorphic Material. Structure of One Form Determined
from Total Neutron Diffraction. Inorg. Chem. 2002, 41, 4990−4992.
(b) Reckeweg, O.; Lind, C.; Simon, A.; DiSalvo, F. J. Synthesis,
Thermal and X-Ray Investigations of the High-Temperature Phase of
Copper(I) Cyanide. Z. Naturforsch. B 2003, 58, 155−158. (c) Hibble,
S. J.; Eversfield, S. G.; Cowley, A. R.; Chippindale, A. M. Copper(I)
Cyanide: A Simple Compound with a Complicated Structure and
Surprising Room-Temperature Reactivity. Angew. Chem., Int. Ed. 2004,
43, 628−630.
(16) (a) Mankad, N. P.; Laitar, D. S.; Sadighi, J. P. Synthesis,
Structure, and Alkyne Reactivity of a Dimeric (Carbene)copper(I)
Hydride. Organometallics 2004, 23, 3369−3371. (b) Goj, L. A.; Blue,
E. D.; Delp, S. A.; Gunnoe, T. B.; Cundari, T. R.; Pierpont, A. W.;
Petersen, J. L.; Boyle, P. D. Chemistry Surrounding Monomeric
Copper(I) Methyl, Phenyl, Anilido, Ethoxide, and Phenoxide
Complexes Supported by N-Heterocyclic Carbene Ligands: Reactivity
Consistent with Both Early and Late Transition Metal Systems. Inorg.
Chem. 2006, 45, 9032−9045. (c) Goj, L. A.; Blue, E. D.; Munro-
Leighton, C.; Gunnoe, T. B.; Petersen, J. L. Cleavage of X−H Bonds
(X = N, O, or C) by Copper(I) Alkyl Complexes To Form
Rauchfuss, T. B. Synthetic and Structural Studies on
x−
[Fe2(SR)2(CN)x(CO)6−x
]
as Active Site Models for Fe-Only
Hydrogenases. J. Am. Chem. Soc. 2001, 123, 12518−12527.
(i) Vostrikova, K. E.; Peresypkina, E. V. Facile Preparation of
Paramagnetic RuIII and OsIII Hexacyanides. Eur. J. Inorg. Chem. 2011,
2011, 811−815. (j) Hoshino, M.; Sonoki, H.; Miyazaki, Y.; Iimura, Y.;
Yamamoto, K. Structure and Photochemistry of Dicyanocobalt(III)
Tetraphenylporphyrin. Photochromic Reaction Caused by Photo-
dissociation of Axial Ligand. Inorg. Chem. 2000, 39, 4850−4857.
(k) He, G.-F. Synthesis and crystal structure of polymer complex
{[K(18-crown-6)] [K(H2O)2]}[Cd(CN)4]. Guangpu Shiyanshi 2003,
20, 716−718.
(20) Wiberg, N. Holleman-Wiberg, Lehrbuch der Anorganischen
Chemie; Walter de Gruyter: Berlin, 2007; pp 2002−2006.
(21) (a) Pyykko, P. Strong Closed-Shell Interactions in Inorganic
̈
Chemistry. Chem. Rev. 1997, 97, 597−636. (b) Davenport, T. C.;
Tilley, T. D. Dinucleating Naphthyridine-Based Ligand for Assembly
of Bridged Dicopper(I) Centers: Three-Center Two-Electron Bonding
Involving an Acetonitrile Donor. Angew. Chem., Int. Ed. 2011, 50,
12205−12208. (c) Che, C.-M.; Mao, Z.; Miskowski, V. M.; Tse, M.-C.;
I
DOI: 10.1021/acs.inorgchem.7b00749
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Chan, C.-K.; Cheung, K.-K.; Phillips, D. L.; Leung, K.-H.
Cuprophilicity: Spectroscopic and Structural Evidence for Cu−Cu
Bonding Interactions in Luminescent Dinuclear Copper(I) Complexes
with Bridging Diphosphane Ligands. Angew. Chem., Int. Ed. 2000, 39,
4084−4088. (d) Kolmel, C.; Ahlerichs, R. An ab initio investigation of
̈
copper complexes with supershort copper-copper distances. J. Phys.
Chem. 1990, 94, 5536−5542. (e) Stollenz, M.; Meyer, F.
Mesitylcopper − A Powerful Tool in Synthetic Chemistry. Organo-
metallics 2012, 31, 7708−7727.
(22) (a) Klett, J.; Klinkhammer, K. W.; Niemeyer, M. Ligand
Exchange between Arylcopper Compounds and Bis(hypersilyl)tin or
Bis(hypersilyl)lead: Synthesis and Characterization of Hypersilylcop-
per and a Stannanediyl Complex with a Cu−Sn Bond. Chem. - Eur. J.
1999, 5, 2531−2536. (b) Cowley, A. H.; Elkins, T. M.; Jones, R. A.;
Nunn, C. M. Phosphane-Stabilized CuI and AgI Silanes. Angew. Chem.,
Int. Ed. Engl. 1988, 27, 1349−1350.
(23) An initial experiment to illustrate the reactivity of 3 as a
potentially catalytically relevant species has been performed: The
1
reaction of (3)2 with excess mesityl oxide was followed by in situ H
NMR spectroscopy, and the resulting reaction mixture was analyzed by
GC/MS.13 While the NMR spectrum indicates immediate reaction of
(3)2 with mesityl oxide, the formed species are not readily identified;
however, the GC/MS analysis indicates the formation of the β-
silylated ketone as found for the respective catalytic reaction (Table 2,
entry 2).
J
DOI: 10.1021/acs.inorgchem.7b00749
Inorg. Chem. XXXX, XXX, XXX−XXX