Syntheses, Structures, and Reactivity of NHC Copper(I) Boryl Complexes: A Systematic Study

Article abstract of DOI:10.1021/acs.organomet.8b00672

Five novel NHC copper(I) boryl complexes were synthesized by B-B activation via σ-bond metathesis of symmetrical tetraalkoxy and unsymmetrical dialkoxy diamino diborane(4) derivatives. Despite their low stability, the NHC copper boryl complexes were thoroughly characterized spectroscopically and structurally. Variation of the NHC ligand (ItBu or Me₂IiPr) as well as of the boryl ligand (Bpin, Bdmab, or BiPrEn) allowed, for the first time systematically, a study in such complexes of the dependence on steric encumbrance. For sterically more demanding ligand combinations, mononuclear linear complexes were obtained, while with less demanding ligand combinations, dimeric dinuclear complexes with two bridging μ-boryl ligands were obtained, exhibiting extremely short Cu···Cu distances (<2.26 ?). The decomposition of all these complexes was found to proceed via a common pathway, leading ultimately to elemental copper, the free NHC ligand, and the respective symmetrical diborane(4) derivative. The rate of decomposition depended strongly on the steric encumbrance of the individual complex. Two apparently low-valent copper clusters were observed and suggested to be relevant species with respect to the reductive decomposition of the copper(I) boryl complexes.

Full text of DOI:10.1021/acs.organomet.8b00672

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pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Syntheses, Structures, and Reactivity of NHC Copper(I) Boryl
Complexes: A Systematic Study
Christian Kleeberg* and Corinna Borner
̈
Institut fur Anorganische und Analytische Chemie, Technische Universitat Carolo-Wilhelmina zu Braunschweig, 38106
̈
Braunschweig, Germany
*
S Supporting Information
ABSTRACT: Five novel NHC copper(I) boryl complexes
were synthesized by B−B activation via σ-bond metathesis of
symmetrical tetraalkoxy and unsymmetrical dialkoxy diamino
diborane(4) derivatives. Despite their low stability, the NHC
copper boryl complexes were thoroughly characterized
spectroscopically and structurally. Variation of the NHC
ligand (ItBu or Me IiPr) as well as of the boryl ligand (Bpin,
2
Bdmab, or BiPrEn) allowed, for the first time systematically, a
study in such complexes of the dependence on steric
encumbrance. For sterically more demanding ligand combi-
nations, mononuclear linear complexes were obtained, while
with less demanding ligand combinations, dimeric dinuclear
complexes with two bridging μ-boryl ligands were obtained,
exhibiting extremely short Cu···Cu distances (<2.26 Å). The decomposition of all these complexes was found to proceed via a
common pathway, leading ultimately to elemental copper, the free NHC ligand, and the respective symmetrical diborane(4)
derivative. The rate of decomposition depended strongly on the steric encumbrance of the individual complex. Two apparently
low-valent copper clusters were observed and suggested to be relevant species with respect to the reductive decomposition of
the copper(I) boryl complexes.
INTRODUCTION
The steric protection of the reactive Cu−B bond appears to be
vital for the stabilization of these complexes. Consequently, X-
ray single-crystal structures could be obtained exclusively for
the sterically highly demanding complexes 1a, 1b, 1e, and 1f
■
Copper boryl complexes comprising three coordinate dialkoxy
boryl ligands of the type B(OR) [in particular Bpin, where pin
2
=
(OCMe ) ] are boron nucleophiles, and their properties
2 2
with dipp (dipp = 2,6-iPr C H )-substituted carbenes; the
contrast the ubiquitous Lewis acidic properties of three
coordinate boron atoms.
their unique reactivity have been proposed as central
intermediates in copper-catalyzed borylation reactions with
diborane(4) derivatives since the first independent reports of
those reactions by Miyaura et al. and Hosomi et al. in 2000.
Since then, a plethora of such borylation reactions have been
reported on substrates such as CO , carbonyls, α,β-unsaturated
2
6
3
1
−4
Copper boryl complexes with
mesityl-substituted complex 1c was isolated and characterized
spectroscopically, and the cyclohexyl-substituted derivative 1d
1a−c
could be prepared only in situ.
In agreement with that, 1a and 1b were reported to be stable
2
,4
for several hours in solution at room temperature, whereas 1c
1
a,c
is reported to be less stable.
Moreover, this σ-bond
2
metathesis route to copper boryl complexes was extended by
carbonyls, dienes, allenes, alkynes, and aryl and alkyl
Sadighi to the cationic μ-boryl complex 3 in 2016 (Scheme
1
a−c,2,5,6
halides.
Despite the widely accepted relevance of
1d
1
). Furthermore, it should be noted that a different class of
copper(I) boryl complexes, reports of isolated and well-
characterized copper boryl complexes are still scarce. Sadighi
and co-workers reported on the first isolation of a Cu(I) boryl
complex as well as initial reactivity studies in 2005. This
complex, (IDipp)Cu−Bpin (1a), and subsequently a number
of closely related complexes 1b−f were prepared, in analogy to
the proposed reaction pathway in catalysis, by a σ-bond
metathesis reaction of a copper alkoxido precursor and B pin
Cu(I) boryl complexes with a weakly demanding ancillary
ligand (e.g., halide or cyanide) but sterically demanding
diaminoboryl ligands have been reported by Yamashita and co-
workers. Contrary to the information presented above, these
complexes were obtained via the reaction of the respective
boryl lithium species with a copper(I) precursor. Those
complexes exhibit either terminal or μ-boryl ligands, depending
2
2
9
on the nature of the auxiliary ligands.
(
(
2a) or unsymmetrical diborane(4) derivatives pinB−B-
1a,b,6,7
(NR) C H ) (Scheme 1).
As a common feature, the
2
6
4
isolated complexes 1a−c, 1e, and 1f contain sterically
demanding NHC ligands with aromatic substituents (R).
Received: September 13, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00672
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Scheme 1. Example of Copper(I) Boryl Complexes
Prepared from Diborane(4) Derivatives and Copper(I)
Alkoxido Complexes
C H (NtBu) ] with copper(I) tert-butoxido complexes of the
2
4
2
two NHC ligands, ItBu and Me IiPr [ItBu = 1,3-bis(tert-
2
1
,7,8
butyl)imidazol-2-ylidene, and Me IiPr = 1,3-bis(isopropyl)-
2
imidazol-4,5-dimethyl-2-ylidene], we were able to isolate a
number of copper(I) boryl complexes (Scheme 2).
Scheme 2. NHC Cu Boryl Complexes Prepared from
Diboranes 2a−c and Copper tert-Butoxido Complexes
At this point, it should be stated that the copper boryl
complexes isolated so far are kinetically stabilized by the steric
encumbrance of either the NHC ligands or the boryl ligand
itself; nonetheless, despite the steric protection, it has been
stated that these copper boryl complexes are highly reactive
1a,c,5
and decompose readily to elemental copper.
We have
recently reported on the isolation and characterization of
unprecedented phosphine copper(I) boryl complexes follow-
ing the same σ-bond metathesis route outlined above. Here
phosphine ligands that were only intermediately sterically
demanding [(iPr) P and (Ph P) C H ] were used, and highly
With the sterically comparably more demanding NHC
ligand of the pair, ItBu, the respective boryl complexes, (4a)2,
4b, and 4c, could be isolated upon reaction of [(ItBu)Cu−
OtBu] with the diborane(4) derivatives 2a−c (Scheme 2).
3
2
2
6
4
Using the sterically less demanding NHC ligand Me IiPr, the
2
reactive but isolable dinuclear (or even more aggregated)
analogous precursor alkoxido complex [(Me IiPr)Cu−OtBu]
2
complexes with bridging μ-boryl ligands were obtained
was not readily obtained; however, upon treatment of
8
(
Scheme 1). Furthermore, it was shown that those complexes
equimolar mixtures of the free carbene Me IiPr and CuOtBu
2
are efficient stoichiometric boron nucleophiles under con-
with the diborane(4) derivatives 2a and 2b, two copper boryl
complexes (5a) and (5b) were isolated (Scheme 2). All new
8
ditions resembling catalytic conditions.
2
2
Following this, we were interested in the properties and in
particular the coordination chemistry of NHC copper boryl
complexes with NHC ligands that were less demanding than
those employed so far (vide infra). Moreover, having
established synthetic routes to unsymmetrical dialkoxy
diamino diborane(4) derivatives, we are for the first time
able to study the impact of a systematic variation of the (steric)
properties of the boryl ligand on the properties of NHC copper
boryl complexes (4a) , 4b, 4c, (5a) , and (5b) were fully
2 2 2
characterized, including single-crystal X-ray structure determi-
nations. No reactions were observed with the sterically more
encumbered diborane 2d, while all attempts to obtain a
BiPrEn-derived boryl complex from 2c with Me IiPr as
2
11,12
auxiliary ligand failed.
It has to be emphasized that all
those copper(I) boryl complexes are highly reactive com-
pounds of quite limited stability at room temperature (vide
infra), in particular in solution. While all of these complexes
are reproducibly obtained in moderate to good yields, the low
stability of these complexes in solution challenges the
experimentalist.
7
,10
boryl complexes.
RESULTS AND DISCUSSION
Synthesis of Boryl Complexes. Reacting the four
■
diborane(4) derivatives, B pin (2a), pinB−Bdmab (2b),
Structural Characterization. Among the structures of the
2
2
pinB−BiPrEn (2c), and pinB−BtBuEn (2d) [dmab =
C H (NMe) , iPrEn = C H (NiPr) , and tBuEn =
novel boryl complexes (4a) , 4b, 4c, (5a) , and (5b) , two
2
2
2
distinct structural motifs are apparent: linear mononuclear
6
4
2
2
4
2
B
DOI: 10.1021/acs.organomet.8b00672
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Organometallics
Article
1
1
11
complexes with terminal boryl ligands and dinuclear bis-μ-
boryl complexes. The first structural motif is analogous to the
one previously described for NHC Cu boryl complexes,
whereas the latter somewhat resembles the cationic mono-μ-
boryl complex reported by Sadighi and phosphine copper boryl
4, and Z′ = / ; and (5b) (PhMe) , P1
̅
, Z = 1, and Z′ = / .
2
2
2
2
While (4a) crystallizes free of solvent and (5b) was obtained
2
2
as a toluene solvate, (5a) was obtained free of co-crystallized
2
11
solvent as well as the THF solvate (5a) (THF) (not shown).
2
In all of these structures, the bis-μ-boryl complexes are situated
on centers of inversion in the middle of the Cu···Cu vector
1d,8
complexes reported by us (Scheme 1).
The mononuclear linear complexes 4b and 4c crystallize in
monoclinic space group I2 and orthorhombic space group
(
Figure 2).
11
C222 , respectively. The asymmetric unit of 4b (Figure 1,
1
Figure 2. Molecular structures of (4a) (top left), (5a) (top right),
2
2
(
5b)2 (bottom left), and [(5a)((Me P) Cu(Bpin))] (6) (bottom
3 2
right). Co-crystallized solvent molecules and disorder have been
11
Figure 1. Asymmetric unit of 4b (top) and molecular structure of 4c
omitted for the sake of clarity.
11
(
bottom).
The structural parameters of the structures of (4a) , (5a) ,
2
2
(
5a) (THF), and (5b) (PhMe) are quite similar (Table 1)
2 2 2
top) contains two independent molecules (Z = 8, and Z′ = 2)
with respect to the CNHC−Cu and B−Cu distances as well as
the relative orientation of the ligands, as indicated by the
angles CNHC−Cu−B, Cu−B−Cu, and B−Cu−B and the
interplanar angles τNHC and τ . However, for (4a) , the
without further molecular symmetry and a highly disordered
11
solvent (presumably n-pentane). In contrast, the asymmetric
unit of 4c comprises only half a molecule of 4c (Z = 4, and Z′
EBE
2
1
=
/ ) situated on a 2-fold axis through atoms B1, Cu1, and
2
boryl ligands deviate slightly more from a perpendicular
orientation with respect to the Cu B core than the average,
C5. A comparison of the structural parameters of 4b and 4c
2
2
with those of other linear NHC Cu boryl complexes such as
while the same is true for the orientation of the NHC ligand in
1
a, 1b, 1e, and 1f as well as [(IMes)Cu−B((Ndipp) C H )]
(5b) (PhMe) as evidenced by angles τ and τNHC. The most
2
2
4
2
2
EBE
and [(IMes)Cu−B((Ndipp) C H )] may be fruitful (C
−
remarkable feature of this series of complexes is certainly the
2
2
2
NHC
Cu, 1.92−1.94 Å; Cu−B, 1.98−2.00 Å; C −Cu−B, 165.5−
Cu B core with an extremely short Cu···Cu distance of <2.26
NHC
2 2
1a,c,7,9c
1
79.4°; τ , 35.5−87.5°).
The B−Cu distances in 4b
Å, significantly shorter than the double van der Waals radius of
NHC
and 4c are on the long side of the reported range (and also
2.8 Å and the interatomic distance in elemental copper of 2.56
13
similar to the distances found in linear boryl cyano cuprates of
Å. The geometrical parameters of complexes (4a) , (5a) ,
2 2
1
.98−2.00 Å), whereas the C −Cu distances are slightly
and (5a) resemble those of the closely related phosphine-
NHC
2
9b,d
outside the reported range.
The coordination angle at the
containing bis-μ-boryl complexes [(iPr P) Cu (Bdmab) ]
3
2
2
2
copper atoms in 4b and 4c is, as to be expected for the
sterically comparably demanding diaminoboryl ligands, com-
[Cu−Cu, 2.2359(4) Å; Cu−B, 2.179(2)/2.263(2) Å; Cu−P,
2.1989(4) Å; P−Cu−B, 123.52(4)/116.89(4)°; Cu−B−Cu,
parably linear (for Bpin complexes C −Cu−B angles of
60.41(4)°; B−Cu−B, 119.59(4)°; and τ , 86.35(7)°] and
NHC
EBE
1a,c
<
170° are observed). The angle included for the BEB and
[(dppbz)Cu−Bpin] [dppbz = 1,2-bis(diphenylphosphino)-
2
NCN planes is also within the reported range. However, an
effect of the spatial construction of the NHC ligand is
apparent, while in [(IDipp)Cu−Bdmab] (1e), a nearly
benzene] [Cu−Cu, 2.2706(6) Å; Cu−B, 2.203(3)/2.311(3)
Å; Cu−P, 2.2706(6)/2.2826(6) Å; Cu−B−Cu, 60.41(7)°; B−
8
Cu−B, 119.59(7)°; and τ , 86.35(7)°]. In fact, the very
EBE
perpendicular orientation of the ligand planes (τ , 87.53°)
similar structures of the NHC complexes (4a) , (5a) , and
NHC
2
2
is observed in 4b and 4c and only a moderate twist is found.
The bis-μ-boryl complexes (4a) , (5a) , and (5b) crystallize
(5b) and their phosphine congeners not only illustrate their
2
structural relationship but also allow a transfer of the
interpretation of the bonding situation in the NHC complexes
2
2
2
in triclinic or monoclinic space groups each with half a
1d,8
molecule in the asymmetric unit: (4a) , P2 /n, Z = 2, and Z′ =
and in particular of the Cu B core.
Hence, the bonding
2
1
2 2
1
1
/
2
; (5a) , P2 /c, Z = 2, and Z′ = / ; (5a) (THF), C2/c, Z =
within the Cu B core may be described as a multicenter bond
2
1
2
2
2 2
C
DOI: 10.1021/acs.organomet.8b00672
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11
2
Table 1. Selected Structural Parameters of (4a) , 4b, 4c, (5a) , (5a) (THF), and (5b) (PhMe)
2
2
2
2
C
−Cu
Cu···Cu
(Å)
CNHC−Cu−B
Cu−B−Cu
τNHC
(deg)
τEBE
b
NHC
a
(
Å)
Cu−B (Å)
(deg)
(deg)
B−Cu−B (deg)
(deg)
(
4a)2
1.9349(18)
2.236(2) [B1]
2.2338(5)
115.50(7) [B1]
125.07(8) [B1′]
175.49(16)
175.61(16)
180.0
125.16(5) [B1]
115.33(5) [B1′]
124.95(14) [B1]
115.81(12) [B1′]
123.66(11) [B1]
117.81(11) [B1′]
126.22(3)
60.64(5)
119.36(5)
86.66(6)
80.6(1)
2
.188(2) [B1′]
4
4
4
b (A)
b (B)
c
1.953(4)
1.948(4)
1.9444(15)
1.9120(11)
1.993(4)
1.997(4)
2.0019(16)
2.1760(14) [B1]
32.9(2)
30.8(2)
15.35(4)
86.82(7)
(
(
(
5a)2
2.2243(3)
2.2352(7)
2.2592(7)
2.2734(3)
60.51(4)
60.76(11)
61.48(8)
61.47(3)
119.49(4)
119.24(11)
118.52(8)
86.67(8)
85.4(2)
89.7(2)
85.56(7)
2
.2373(13) [B1’]
2.176(4) [B1]
.242(4) [B1′]
2.178(3) [B1]
.240(3) [B1′]
2.3402(13) [Cu2]
.0875(12) [Cu1]
5a) (THF)
1.930(3)
1.939(3)
1.9269(16)
85.5(2)
76.5(2)
85.32(5)
2
2
5b) (PhMe)2
2
2
c
6
129.49(7) [Cu1]
107.56(6) [Cu2]
2
a
τNHC is the angle between the NCN plane of the NHC and the EBE plane of the boryl ligand (4b and 4c) or mean Cu B plane [(4a) , (5a) , and
2
2
2
2
b
c
(5b) ]. τ is the angle between the EBE plane of the boryl ligand and the mean Cu B plane. P1−Cu1, 2.2080(3)°; P1−Cu1−P1′, 115.58(2)°.
2
EBE
2 2
formally composed of occupied σ-type orbitals at the boryl
ligands and vacant orbitals at the copper atoms, including
close-shell interaction of the Cu atoms.
position of these complexes was studied in detail. Following
1
11
1
the decomposition of 4b and 4c by in situ H and B{ H}
NMR spectroscopy reveals immediately that the stability of 4c
is much higher than that of 4b. While 4b decomposes virtually
instantaneously at room temperature (Figure 3), 4c undergoes
The findings described herein are not limited to the bis-
NHC complexes and also apply to the mixed phosphine/NHC
complex [(5a)((Me P) Cu(Bpin))] (6) (vide infra). Generally
3
2
similar interatomic distances as in the symmetrical bis-NHC
complexes are found; however, the angles are more influenced
by the now unsymmetrical ligand set. The molecule of 6 no
longer adopts inversion symmetry but is now situated on a 2-
fold axis through atoms C7, Cu2, and Cu1 (space group C2/
11
c). The structural parameters are also comparable to those
found in the symmetrical dinuclear alkylphosphine complexes
8
[
(iPr P)Cu−Bdmab] and [(dppbz)Cu−Bpin] .
3
2
2
Spectroscopic Characterization. All described boryl
complexes, (4a) , 4b, 4c, (5a) , (5b) , and 6, have been
2
2
2
characterized by solution state nuclear magnetic resonance
NMR) spectroscopy. However, the low stability (vide infra)
of the complexes at ambient temperature allowed NMR
(
1
13
11
spectroscopic characterization, including H, C, B, and two-
dimensional (2D) NMR spectra (and 31P, if applicable), only
11
at low temperatures, typically below −40 °C. B NMR shifts
were obtained for the complexes (4a) (49 ppm; Δw = 2500
2
1/2
Hz), 4a (45 ppm; Δw = 1000 Hz), and 4c (49 ppm; Δw
1/2
1/2
11
=
1500 Hz); for the other complex, no B NMR shifts could
be obtained because of the severe line broadening of the
1
1
quadrupolar B nucleus, in particular at low temperatures.
1
1
1
11
1
Those B NMR chemical shifts fit nicely into the range of the
Figure 3. H and B{ H} NMR spectra of 4b at −40 °C and after its
decomposition at room temperature (THF-d (*), 400/128 MHz and
00/96 MHz).
1
1
few reported B NMR shifts for copper boryl complexes with
8
3
dialkoxyboryl and diaminoboryl ligands [(IDipp)Cu−Bpin]
(
41.7 ppm), [(IDipp)Cu−Bdmab] (44.1 ppm; Δw = 1020
1/2
Hz), [(IDipp)Cu−B((NBn) C H )] (47.0 ppm; Δw = 2140
2
6
4
1/2
slow decomposition over several days (Figure 4). For 4b, a
clean decomposition to the symmetrical diboron compound
Hz), [(IMes)Cu−B((Ndipp) C H )] (44.7 ppm), and
2
2
4
1a,7,9c
1
11
1
[
(IMes)Cu−B((Ndipp) C H )] (38.9 ppm).
In light of
2
2
2
B dmab and the free NHC is indicated by H and B{ H}
2 2
14
the low stability of the complexes (4a) , (5a) , (5b) , and 6
2
2
2
NMR spectroscopy (Figure 3). Furthermore, the appearance
of a copper mirror indicates the formation of elemental
1
1
(
vide infra), we emphasize that H− H NOESY NMR data as
13
15
well as the C chemical shift of the carbene carbon atoms
copper. This decomposition pathway, the reductive B−B
3
1
(
and P shift for 6) suggest the presence of the intact copper
coupling resulting in a symmetrical diborane(4) derivative and
the free ligand, has been reported for related phosphine
copper(I) boryl complexes, whereas the formation of elemental
copper as a decomposition product of copper boryl complexes
has also been mentioned for other NHC copper boryl
boryl complexes bearing NHC ligands.
Stability and Decomposition Pathways. As mentioned
above, none of the mononuclear linear complexes (4b and 4c)
or the dinuclear complexes (4a) , (5a) , (5b) , and 6 are stable
2
2
2
1a,5,8
at ambient temperature in solution. Therefore, the decom-
complexes.
D
DOI: 10.1021/acs.organomet.8b00672
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Figure 5. 1H and B{ H} NMR spectra of (4a)
11
1
and its
1
11
1
2
Figure 4. H and B{ H} NMR spectra of 4c and its decomposition
decomposition products at different temperatures (THF-d8 (*),
00/128 MHz). The times given refer to times at a certain
temperature. The inset shows the X-ray powder diffraction pattern
of the decomposition product of (4a)2.
at room temperature (PhMe-d , 300/96 MHz). The inset shows a
plot of the relative amounts of 4c, B (iPrEn) , and ItBu during the
decomposition of 4c.
8
4
2
2
The analogous decomposition products have also been
identified for 4c, but the rate of decomposition is greatly
reduced; full decomposition is observed only after >3 days at
black precipitate formed during the decomposition of (4a)2
(Figure 5, inset). The crystallite size was roughly estimated
using the Scherrer equation to be 35 ± 10 Å and thus clearly in
the nanoscale range. It should be noted that this is still
significantly larger than the diameter of the copper core in
copper clusters 8 (4.1 Å) and 9 (8 Å) discussed below.
For the dinuclear complexes (5a) and (5b) , a fundamen-
15
room temperature (Figure 4). Moreover, the decomposition
is less clean, and additional unidentified (secondary)
1
1
1
compounds are also apparent (in particular in the B{ H}
NMR spectrum). In agreement with this is the substoichio-
2
2
metric amount of B (iPrEn)2 formed during the decom-
tally similar behavior like that observed for (4a) is found. The
2
2
position as observed by monitoring the relative amounts of 4c,
complexes are stable enough to record a full set of NMR
spectra at low temperatures, while at room temperature,
1
B (iPrEn) , and the free NHC ItBu by in situ H NMR
2
2
1
1
spectroscopy (Figure 4, inset). A kinetic analysis of these data
did not lead to a conclusive picture. The unclear formation of
byproducts as well as the potential involvement of more
aggregated species (vide infra) in this reductive decomposition
process renders complicated kinetics likely. The much higher
stability of 4c compared to that of 4b, and presumably also
those of 1a−d (no detailed data exist for these complexes), is
readily explained by the increased level of kinetic stabilization
immediate decomposition is observed (Figures S4 and S5).
1
1
For (5a) after decomposition two B NMR signals at 27.7
2
ppm (Δw = 1150 Hz) and 4.0 ppm (Δw = 80 Hz) are
1
/2
1/2
observed that are not indicative of the formation of the
11
expected diborane(4) derivative 2a (Figure S4). However,
considering the established ability of 2a to form a Lewis acid/
base adduct with free NHCs, these NMR data suggest the
formation of the pinB−B(Me IiPr)pin adduct from the
2
1
3,16
by the sterically more demanding boryl ligand B(iPrEn).
primary decomposition products.
In agreement with that
With the sterically less demanding pinacolyl boryl ligand, the
is the fact that (5b) gives after decomposition free 2b, in
2
bis-μ-boryl-bridged complex (4a) exhibits a stability signifi-
accordance with the less Lewis acidic character of tertraamino
2
14
11
cantly lower than those of 4c and 4b (Figure 5). Measuring
one sample at different, increasing temperatures reveals that
only slow decomposition occurs at −46 °C (∼1% after 1 h),
while decomposition becomes appreciable at −10 °C (∼8%
within 40 min) and fast at 10 °C (∼40% within 10 min). The
decomposition products are again the same as those described
earlier, the symmetrical diborane(4) derivative and the free
NHC identified by their distinct NMR spectra (Figure 5).
Moreover, the elemental copper as a decomposition product
could here also be identified by X-ray powder diffraction of the
diborane(4) compounds. Nonetheless, for (5b) , the free
2
NHC is not readily identified as a decomposition product; the
broad signals assigned to the Me IiPr moiety suggest its
2
involvement in additional dynamic processes (vide infra)
(Figure S5). A mixture of (5a) and (5b) results in the
2
2
formation of the symmetrical diboranes(4) as for (5a) and
2
(5b) independently as well as the unsymmetrical diborane(4)
2
11
2b clearly indicating the molecular exchange in solution.
One of the NHC ligands in (5a)2 is replaced upon
recrystallization of (5a) from PMe /THF at −40 °C, and
2
3
E
DOI: 10.1021/acs.organomet.8b00672
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
the unsymmetrical dinuclear complex [(5a)((Me P) Cu-
of (5b) in THF and n-pentane at −40 °C and 6 in PhMe at
3
2
2
(
Bpin))] (6) is obtained in an 18% yield. The unsymmetrical
ambient temperature, respectively.
nature of this complex allows the elucidation of its solution
As a common feature, both compounds comprise metal
clusters of five and 23 copper atoms, respectively (Figures 7
1
1
state structure. H− H NOESY NMR contacts between the
Bpin methyl groups (④) and both phosphine (②) and NHC
hydrogen atoms (① and ③) as well as contacts between the
isopropyl group of the NHC (①) and phosphine hydrogen
atoms (②) indicate the presence of all three moieties within
one molecule (Figure 6). Hence, it may be concluded that 6
1
1
F i g u r e 7 . M o l e c u l a r s t r u c t u r e a n d d e t a i l s o f
Figure 6. H− H NOESY NMR spectrum of 6 (PhMe-d , 400 MHz,
8
[
Cu (Me IiPr) (Bdmab) ] (8). Co-crystallized solvent molecules
5 2 3 3
−
47 °C). Arrows symbolize observed NOESY contacts, and the
have been omitted for the sake of clarity, and ellipsoids are drawn
distances given are the minimal H···H distances found in the solid
11
at the 30% probability level.
state structure.
and 8). Complex 8 contains three neutral NHC as well as three
anionic boryl ligands; hence, the three negative charges of the
ligands are compensated by five low-valent copper atoms with
an average formal oxidation state of +0.6. In 9, no anionic
ligands were found, and hence, the formal oxidation state of
adopts a similar molecular structure in solution as in the solid
state. The interatomic H···H distances found in the solid state
agree qualitatively with the observed NOESY contacts.
Furthermore, it may be suggested that the dimeric complexes
(
4a) , (5a) , and (5b) are also dimers in solution.
2 2 2
Changing the reaction conditions under which (5b) is
2
obtained and employing ∼3 equiv of the NHC per equivalent
of copper tert-butoxide ligand lead to the isolation of the ionic
+
−
species {[(Me IiPr) Cu] [pinB(OtBu)−Bdmab] } (7). Com-
2
3
plex 7 was fully characterized and is surprisingly stable in
solution at room temperature (complete decomposition after
1
1
∼
1 h). While no primary products were isolated, B dmab as
2 2
well as pinB−OtBu was identified by in situ NMR spectros-
copy, suggesting the intermediate formation of a copper boryl
complex [e.g., (5b) ] and its subsequent decomposition as
2
1
1
described above. The spectroscopic and structural data agree
well with the data reported earlier for the anion [pinB(OtBu)−
−
7
Bdmab] with a different counterion.
From the decomposition of (5b) and 6, the decomposition
2
product [Cu (Me IiPr) (Bdmab) ] (8) [as 8(THF) ] and
5
2
3
3
2
1
1
[
(Me IiPr) Cu (PMe ) ] (9) (as the PhMe solvate) were
2 10 23 3 2
isolated as single crystals and characterized by single-crystal X-
ray diffraction analysis. However, both compounds could not
be obtained in yields allowing any further characterizationm,
nor did the analysis of the mother liquor provide any valuable
information (e.g., mass spectrometry). As both compounds
may provide essential insight into the decomposition pathways
of copper boryl complexes, may have implications for the
catalytic applications of copper boryl complexes, and have
fascinating structures, the complexes are briefly discussed here.
Single crystals of [Cu (Me IiPr) (Bdmab) ] (8) and
Figure 8. [(Me IiPr) Cu (PMe ) ] (9): copper core (top) and
2
10
23
3 2
5
2
3
3
molecular structure (bottom). Disorder has been omitted for the sake
of clarity, and ellipsoids are drawn at the 30% probability level.
11
[
(Me IiPr) Cu (PMe ) ] (9) were obtained from solutions
2
10
23
3 2
F
DOI: 10.1021/acs.organomet.8b00672
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
the copper atoms would be zero. However, it should be noted
that on the basis of the single-crystal X-ray diffractions the
presence of additional hydride ligands in 8 and 9 cannot be
been reported as a decomposition product of an indium bis-
boryl complex.
With regard to compounds 8 and 9, it must be emphasized
that single crystals of both compounds were obtained from
17d
8
excluded. The observation of low-valent polynuclear copper
complexes as possible decomposition products of copper(I)
boryl complexes is not unprecedented; this was also true of the
phosphine complex [(iPr P) Cu (Bdmab) ], where the
more than one independently prepared sample of (5b) and 6,
2
respectively. Nonetheless, their synthesis cannot be considered
reproducible, as only approximately two of 10 runs furnish the
products and in only marginal yields. Moreover, the crystal
structure determinations of 8 and 9 were not without
difficulties, and in particular, the structure of 8 is of only low
3
2
2
2
octanuclear low-valent complex [(iPr P) Cu (B-
3
2
8
8
(
iPrEn)) (OtBu) ] is observed. Complex 8 comprises a
3 3
distorted trigonal bipyramid of copper atoms with the apical
and one of the equatorial copper atoms (Cu3−Cu5)
coordinated by one NHC ligand (Figure 7). Two μ-boryl
ligands coordinate to Cu2 and Cu3 and to Cu2 and Cu4, while
the third boryl ligand coordinates the remaining equatorial
copper atom (Cu1). The CNHC−Cu distances are slightly
longer than those found for the mono- and dinuclear
complexes addressed above. The μ-boryl and terminal B−Cu
distances are also in the range observed for these complexes
and significantly unsymmetrical in the case of μ-boryl ligands.
The Cu−Cu distances in 8 cover a wide range from 2.30 to
11
quality.
CONCLUSION
Five NHC copper(I) boryl complexes, (4a) , 4b, 4c, (5a) ,
■
2
2
and (5b) , were obtained by σ-bond metathesis reaction of
2
suitable NHC copper(I) tert-butoxido precursors with sym-
metrical tetraalkoxy or unsymmetrical dialkoxy diamino
diborane(4) derivatives. In contrast to established NHC
copper(I) boryl complexes obtained via this route, the NHC
ligands in the presented complexes are sterically comparably
weakly demanding. Varying the steric demand of the NHC
2
.72 Å, whereby the shortest distances are observed for μ-
boryl-bridged dicopper moieties (Cu2−Cu3 and Cu2−Cu4),
though they are slightly longer than those found in the
dinuclear complexes mentioned above, whereas the longest
distances are found for nonbridged Cuapical/Cuequatorial atom
pair moieties (>2.6 Å), larger that that found in elemental
ligand (ItBu or Me IiPr) and the boryl ligand (Bpin, Bdmab, or
2
BiPrEn) allows a first systematic study of the stability,
reactivity, and structures of NHC copper(I) boryl complexes.
Despite the low stability of the obtained copper boryl complex,
these complexes were thoroughly characterized NMR spec-
troscopically and structurally in the solid state (single-crystal
X-ray diffraction). Depending on the steric demand of both the
NHC and the boryl ligand, the complexes are either
mononuclear linear complexes (4b and 4c) or with less
demanding ligands dimeric dinuclear complexes with two
bridging μ-boryl ligands [(4a) , (5a) , and (5b) ] according to
1
3
copper (2.56 Å). It should be noted that the copper(I)
hydride [(PPh )CuH] also comprises a distorted trigonal-
3
5
bipyramidal copper core; however, in the complexes,
tentatively longer and less distorted Cu−Cu distances are
1
7a
observed.
Complex 9 comprises a centered icosahedral core of 13
copper atoms (Figure 8). Of these atoms, the apical copper
atoms are coordinated by additional phosphine ligands (purple
in Figure 8), whereas the equatorial copper atoms are not
coordinated by additional ligands (orange in Figure 8). The 10
remaining copper atoms are localized above the 10 trigonal
faces formed by the 10 equatorial copper atoms and are
coordinated by one NHC ligand each (gray in Figure 8). The
entire cluster is localized on a center of inversion (space group
P2 /c). The C −Cu distances are in the range of 1.95−1.97
2
2
2
single-crystal X-ray diffraction studies. However, the NOESY
NMR spectroscopic analysis of the unsymmetrical complex
[(5a)((Me P) Cu(Bpin))] (6), obtained from (5a) upon
3
2
2
recrystallization from PMe /THF, indicates a bis-μ-boryl
3
structure in solution, as found in the solid state. Hence, it
may be concluded that also the other symmetrical bis-μ-boryl
dicopper complexes comprise similar structures in the solution
as in the solid state. The decomposition of all obtained NHC
copper boryl complexes occurs readily at ambient temperature
in solution, and its rate depends strongly on the steric
properties of the complex. Nonetheless, as a common
decomposition pathway, the formation of elemental copper,
the free NHC ligand, and the symmetrical diborane(4)
derivative derived by reductive B−B coupling from the boryl
ligand is observed. Moreover, the isolation and structural
characterization of two apparently low-valent copper clusters,
[Cu (Me IiPr) (Bdmab) ] (8) and [(Me IiPr) Cu (PMe ) ]
1
NHC
Å and therefore comparable to those in 8. The P−Cu distance
2.204(1) Å], however, is significantly shorter than that
reported for [(iPr P) Cu (B(iPrEn)) (OtBu) ] [2.300(4) Å]
[
3
2
8
3
3
and quite comparable to distances found in
8
[
(iPr P) Cu (Bdmab) ] [2.1989(4) Å] and 6 (Table 1).
3 2 2 2
The Cu−Cu distances within the icosahedron (Cu6−Cu11)
are in the range of the distance found for elemental copper of
2
.56 Å, while the distances to the central copper atom (Cu12)
5
2
3
3
2
10
23
3 2
are significantly shorter, in the range of 2.42−2.44 Å. The Cu−
Cu distances involving the NHC-coordinated copper atoms
(9), suggest that those clusters are related to the reductive
decomposition of NHC copper(I) boryl complexes, potentially
as intermediates. The structures and decomposition of the
presented NHC copper(I) boryl complexes resemble those
reported recently for phosphine copper(I) boryl complexes
and, hence, may be considered quite general for copper(I)
13
(
Cu1−Cu5) are in a wide range of 2.4−2.7 Å. Thereby, the
distances on the outer perimeter are around 2.7 Å, while one
shorter distance (2.4 Å) and two longer distances (2.56−2.58
Å) are found between the NHC-coordinated copper atoms
8
(
Cu1−Cu5, gray in Figure 8) and the copper atoms in the
boryl complexes. We may conclude that, contrary to our
icosahedron (Cu6−11, orange in Figure 8). It should be noted
that a related structural motif based on a centered icosahedron
has been reported for the low-valent copper hydrido clusters
present understanding, NHC copper(I) boryl complexes, in
particular those with sterically not terribly demanding ligands,
are not mononuclear linear complexes. This appears to be
especially relevant as the latter types of complexes are widely
assumed in mechanistic considerations to be the pivotal
reactive intermediates in various copper-catalyzed borylation
reactions, while dinuclear copper(I) or even low-valent copper
[
Cu H (PPh ) ]Cl and [Cu Cl H (Ph phen) ]Cl; how-
25 22 3 12 29 4 22 2 12
ever, in the complexes, characteristically longer Cu−Cu
17b,c
distances are observed.
It is also worth noting that the
formally low-valent indium cluster In [B(NDippCH) ] has
19
2 6
G
DOI: 10.1021/acs.organomet.8b00672
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
complexes have, to the best of our knowledge, not yet been
considered.
indexed and integrated, and a multiscan absorption correction was
20b
1
−6
applied as implemented in CrysAlisPro.
The structures were
determined by employing SHELXT or SIR-92 and refined anisotropi-
2
cally for all non-hydrogen atoms by full-matrix least squares on all F
using SHELXL.
EXPERIMENTAL SECTION
20c−e
■
Generally, hydrogen atoms were refined
employing a riding model; methyl groups were treated as rigid bodies
General Considerations. [(ItBu)CuOtBu], Me IiPr, CuOtBu,
pinB−Bdmab (2b), pinB−BiPrEn (2c), and pinB−BtBuEn (2d) were
prepared according to literature procedures.
2
and allowed to rotate about the E−CH bond. During refinement and
3
7
,10,18
analysis of the crystallographic data, WinGX, PLATON, Mercury, and
All other com-
20f−i
Diamond were used.
Unless noted otherwise, the shown ellipsoids
pounds 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). Reactions in the glovebox were performed
in scintillation vials (ø 18 mm × 58 mm). To handle temperature
sensitive compounds inside the glovebox at low temperatures, a brass
block with holes fitting to the scintillation vials precooled to −40 °C
was used. NMR spectra were recorded on a Bruker Avance II 300, an
Avance III HD 300, or an Avance III 400 spectrometer. For air
sensitive samples, NMR tubes equipped with screw caps (WILMAD)
were used and the solvents were dried over potassium/benzophenone
and degassed. Low-temperature NMR spectra of temperature
sensitive compounds were obtained as follows. Inside a nitrogen-
filled glovebox, a precooled NMR tube was placed in a brass block
with a suitable hole (ø 5.1 mm × 6 cm) cooled for >10 h to −40 °C.
Inside the NMR tube, the respective compound was placed and the
precooled solvent was added using a precooled pipet. The NMR tube
was brought, still inside the brass bock, outside of the glovebox and
immediately immersed in an appropriate cooling bath. The NMR
tube was removed from the cooling bath, wiped, and inserted into the
precooled magnet of the NMR instrument. Despite the handling time
at room temperature being kept to a minimum, a brownish coloration
of the sample was frequently observed during handling. Chemical
shifts (δ) are given in parts per million, using the (residual) resonance
signal of the solvents (99.5% deuteration, Eurisotop) for calibration
represent the 50% probability level and the hydrogen atoms are
omitted for the sake of clarity. Powder X-ray diffraction measurements
were conducted using an Oxford Diffraction Nova A instrument with
mirror-focused Cu Kα radiation with a transmission geometry,
20b
employing CrysAlisPro.
The sample was, inside a nitrogen-filled
glovebox, mixed with inert perfluoroether oil and, outside of the
glovebox, attached to a glass fiber and placed in the cold nitrogen gas
stream on the diffractometer. The resulting powder pattern was
compared with a powder pattern computed on the basis of a single-
20h
crystal X-ray structure determination (Mercury).
[(ItBu)Cu−Bpin] (4a) . In a nitrogen-filled glovebox, B pin (2a)
2 2
2
2
(40.2 mg, 158.1 μmol, 1.0 equiv) and [(ItBu)CuOtBu] (50.0 mg,
158.1 μmol, 1.0 equiv) were dissolved separately in dry THF (0.3
mL) and cooled to −40 °C for 4 h. The solutions were combined in
the cold and layered with cold n-pentane (∼2 mL) at −40 °C. After
48 h at −40 °C, X-ray quality crystals had separated. The supernatant
solution was decanted from off-white crystals that were subsequently
washed with cold n-pentane (2 × 2 mL, −40 °C) and briefly dried in
1
vacuo at ambient temperature (39.1 mg, 52.8 μmol, 67%): H NMR
(400 MHz, THF-d
NC(CH ], 0.89 [br s, 24 H, OC(CH
MHz, THF-d , −46 °C) δ 186.5 (CCarbene), 116.4 (CHN), 79.4
[OC(CH ], 57.5 [NC(CH ], 31.0 [OC(CH ], 26.5 [NC-
]; B{ H} NMR (128 MHz, THF-d , −10 °C) δ 49 (s,
Δw1/2 = 2500 Hz) (no signal was detectable at lower temperatures);
mp >80 °C dec. Anal. Calcd for [C17 32BCuN : C, 55.06; H,
8.70; N, 7.55. Found: C, 54.73; H, 8.74; N, 7.62.
, −46 °C) δ 7.15 (s, 4 H, CHN), 1.74 [s, 36 H,
8
13 1
)
3
) ]; C{ H} NMR (100
3 2
3
8
)
)
3
)
3 2
3
2
3
1
1
1
(CH
)
3
3
8
H
O ]
2 2 2
1
13
1
(
C D , H NMR 7.16 ppm, C NMR 128.06 ppm; THF-d , H
[(ItBu)Cu−Bdmab] (4b). In a nitrogen-filled glovebox, pinB−
Bdmab (2b) (36.0 mg, 132.3 μmol, 1.05 equiv) and [(ItBu)CuOtBu]
(40.0 mg, 126.5 μmol, 1.0 equiv) were dissolved separately in dry
PhMe (0.85 mL) and cooled to −40 °C for 12 h. The solutions were
combined in the cold and after 60 min at −40 °C layered with cold n-
pentane (∼4 mL). After 48 h at −40 °C, X-ray quality crystals had
separated. The supernatant solution was decanted from dark bronze
crystals that were subsequently washed with cold n-pentane (2 × 2
6
6
8
1
3
1
NMR 1.72, 3.58 ppm, C NMR 25.31, 67.21 ppm; PhMe-d , H
8
NMR 2.08 ppm, 13C NMR 20.43 ppm).
19
11
B and P NMR chemical
31
shifts are reported relative to external BF ·Et O and 85% aqueous
3
2
1
3
11
31
H PO , respectively. C, B, and P NMR spectra were recorded by
employing composite pulse H decoupling. If necessary, 2D NMR
3
4
1
1
1
techniques were employed to assign the individual signals [ H− H
1
1
1
13
NOESY (1 s mixing time), H− H COSY, H− C HSQC, and
1
13
H− C HMBC]. Melting points were determined in flame-sealed
mL, −40 °C) and briefly dried in vacuo at ambient temperature (37.0
1
̈
chi 535 apparatus and are not
mg, 95.3 μmol, 75%): H NMR (400 MHz, THF-d , −40 °C) δ 7.29
8
capillaries under nitrogen using a Bu
(
s, 2 H, CHN), 6.72−6.65 (m, 4 H, HC ), 3.48 (s, 6 H, NCH ), 1.87
corrected. Elemental analysis (EA) was used to establish the purity of
new compounds; however, for compounds 4b, 8, and 9, no
satisfactory elemental analysis can be provided. Copper clusters 8
and 9 were obtained in only minute yields and could be characterized
only by single-crystal X-ray diffraction. The highly unstable 4b,
despite several independent attempts, gave reproducibly overly low
carbon values, possibly because of the presence of decomposition
Ar
3
1
3
1
[s, 18 H, NC(CH ) ]; C{ H} NMR (100 MHz, THF-d , −40 °C) δ
3 3 8
182.1 (CCarbene), 141.4 (CAr), 116.7 (HCAr), 116.6 (CHN), 106.2
(HCAr), 58.4 [NC(CH
)
], 33.9 (NCH
B{ H} NMR (128 MHz, THF-d , −40 °C) δ 44.9 (s, Δw1/2
1000 Hz); mp >70 °C dec. Anal. Calcd for C19 30BCuN
), 32.6 [NC(CH
)
];
=
3
3
3
3
3
1
1
1
8
H
: C, 58.69;
4
H, 7.78; N, 14.41. Found: C, 56.40; H, 7.85; N, 14.60. All attempts to
perform more accurate elemental analysis failed, presumably because
of the presence of impurities that were inevitable for this highly
unstable compound (vide supra).
products. Elemental analyses were performed at the Institut fu
Anorganische and Analytische Chemie of the Technische Universitat
̈
r
̈
Carolo-Wilhelmina zu Braunschweig using an Elementar vario
MICRO cube instrument.
[(ItBu)Cu−B(iPrEn)] (4c). In a nitrogen-filled glovebox, pinB−
B(iPrEn) (2c) (44.2 mg, 158.0 μmol, 1.0 equiv) was dissolved in n-
pentane (0.15 mL) and [(ItBu)CuOtBu] (50.0 mg, 158 μmol, 1.0
equiv) was dissolved separately in a mixture of PhMe (0.65 mL) and
n-pentane (0.15 mL). The solutions were combined to give a red-
pinkish solution that turned purple within 20 min. The solution was
cooled to −40 °C after 30 min at ambient temperature (occasional
shaking). After 24−72 h at −40 °C, cuboid colorless crystals that were
suitable for X-ray diffraction had separated. The supernatant solution
was decanted, and the crystals were washed with cold n-pentane (2 ×
It should be emphasized that the synthesis of all boryl complexes is
highly reproducible, though the purity of the starting materials and
the adherence to the experimental procedures, in particular with
respect to the temperature and the amount of solvent(s), proved to be
essential.
X-ray Diffraction Studies. The crystals were, inside a nitrogen-
filled glovebox, transferred into inert perfluoroether oil and, outside of
the glovebox, rapidly mounted on top of a human hair and placed in
20a
the cold nitrogen gas stream on the diffractometer. The data were
either collected on an Oxford Diffraction Xcalibur E instrument using
graphite monochromated Mo Kα radiation (conventional sealed X-
ray tube) or an Oxford Diffraction Nova A instrument using mirror-
focused Cu Kα radiation (micro focus source). The reflections were
1 mL, −40 °C) and briefly dried in vacuo at ambient temperature
1
(25.0 mg, 63.0 μmol, 40%): H NMR (400 MHz, PhMe-d , −47 °C)
8
δ 6.26 (s, 2 H, CHN), 4.52 [sept, J = 6.7 Hz, 2 H, NCH(CH ) ], 3.54
3
2
(s, 4 H, NCH ), 1.54 [d, J = 6.7 Hz, 12 H, NCH(CH ) ], 1.52 [s, 18
2
3 2
H
DOI: 10.1021/acs.organomet.8b00672
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
1
3
1
H, NC(CH ) ]; C{ H} NMR (100 MHz, PhMe-d , −47 °C) δ
n-pentane (2 × 1 mL, −40 °C) and dried in vacuo at −40 °C for 1 h
to give (5b) (PhMe) as a brownish solid (74.3 mg, 77.4 μmol, 62%):
3
3
8
1
83.1 (CCarbene), 114.7 (CHN), 57.4 [NC(CH ) ], 49.7 [NCH-
CH ) ], 43.8 (NCH ), 31.8 [NC(CH ) ], 23.6 [NCH(CH ) ];
B{ H} NMR (128 MHz, PhMe-d , −47 °C) δ 49 (s, Δw = 1500
Hz); H NMR (300 MHz, PhMe-d , room temperature) δ 6.36 (s, 2
3 3
2
2
1
(
H NMR (400 MHz, THF-d , −46 °C) δ 6.6 (br s, 8 H, CH ), 4.73
3
2
2
3
3
3
2
8
Ar
1
1
1
8
1/2
[br s, 4 H, NCH(CH ) ], 3.47 (s, 12 H, NCH ), 2.04 [br s, 12 H,
3 2 3
NC(CH )], 0.88 [br s, 24 H, NCH(CH ) ]; C{ H} NMR (100
3 3 2
1
13
1
8
H, CHN), 4.33 [sept, J = 6.7 Hz, 2 H, NCH(CH ) ], 3.42 (s, 4 H,
MHz, THF-d , −46 °C) δ 142.0 (C ), 122.7 [C(CH )N], 116.0
Ar Ar 3 2 3
[NCH(CH ) ], 9.5 [C(CH )N] [carbene carbon atom not detected;
3 2 3
3
2
8
Ar
3
NCH ), 1.56 [s, 18 H, NC(CH ) ], 1.43 [d, J = 6.7 Hz, 12 H,
(HC ), 104.6 (HC ), 52.8 [NCH(CH ) ], 33.9 (NCH ), 22.5
2
3
3
13
1
NCH(CH ) ]; C{ H} NMR (75 MHz, PhMe-d , room temper-
3
2
8
ature) δ 114.8 (CHN), 57.6 [NC(CH ) ], 49.8 [NCH(CH ) ], 44.4
in addition, resonances indicative for PhMe (1 equiv) were observed];
3
3
3 2
1
1
1
(
NCH ), 32.1 [NC(CH ) ], 23.8 [NCH(CH ) ] (carbene carbon
B{ H} NMR (128 MHz, THF-d , −46 °C) δ no signal detected; mp
2
3
3
3
2
8
1
1
1
atom not detected); B{ H} NMR (96 MHz, PhMe-d , room
>60 °C dec. Anal. Calcd for C H B Cu N [(5b) (PhMe) ]: C,
8
52 76 2 2 8 2 2
temperature) δ 44.6 (s, Δw = 400 Hz); mp >90 °C dec. Anal.
64.93; H, 7.96; N, 11.65. Found: C, 64.94; H, 8.12; N, 11.46. This
composition agrees with the composition established by X-ray
diffraction and by NMR spectroscopy.
1
/2
Calcd for C H BCuN : C, 57.50; H, 9.65; N, 14.12. Found: C,
19
38
4
5
7.67; H, 9.76; N, 14.09.
(Me IiPr)Cu−Bpin] [(5a) ]. In a nitrogen-filled glovebox, B pin
[
[(Me IiPr) Cu][pinB(OtBu)−Bdmab] (7). In a nitrogen-filled
2
2
2
2
2
2
3
(2a) (70.0 mg, 275.4 μmol, 1.0 equiv) was dissolved in PhMe (0.5
glovebox, pinB−Bdmab (2b) (22.0 mg, 80.8 μmol, 1.0 equiv) was
mL) and cooled to −40 °C (16 h). Me IiPr (49.6 mg, 275.3 μmol, 1.0
dissolved in THF (0.5 mL) and cooled to −40 °C (16 h). Me IiPr
2
2
equiv) and CuOtBu (37.4 mg, 275.0 μmol, 1.0 equiv) were combined
in PhMe (1.0 mL) and, after complete dissolution (3−6 h), cooled to
(45.0 mg, 249.8 μmol, 3.1 equiv) and CuOtBu (11.0 mg, 80.9 μmol,
1.0 equiv) were dissolved in THF (1.0 mL) and, after complete
dissolution, cooled to −40 °C for 16 h. The solutions were combined
in the cold to give a yellow solution, which was layered with n-pentane
after 4−16 h at −40 °C. After 16−48 h, colorless prisms (suitable for
X-ray diffraction) had separated. The supernatant solution was
decanted, and the residue washed with cold n-pentane (3 × 1 mL,
−40 °C for 5−16 h. The solutions were combined in the cold to give
a reddish solution that was kept at −40 °C for 30 h before it was
layered with cold n-pentane (∼3 mL). After 24−60 h at −40 °C, well-
developed light yellow prisms (suitable for X-ray diffraction) had
separated (in some instances, the crystals were embedded in a
brownish oil that was readily removed by washing). The supernatant
solution was decanted, and the residue washed with cold n-pentane (3
−40 °C) and dried in vacuo at room temperature (60.9 mg, 64.2
1
μmol, 79%): H NMR (400 MHz, THF-d , −43 °C) δ 6.56 (br s, 4
8
×
1 mL, −40 °C) and briefly dried in vacuo at ambient temperature to
give the product as a brown-yellowish solid (26.3 mg, 35.5 μmol,
6%). Alternatively, the residue after decanting may be dissolved in
H, CH ), 4.88 [br sept, J = 6.8 Hz, 6 H, NCH(CH ) ], 3.46 (s, 6 H,
Ar
3 2
NCH ), 2.18 [br s, 18 H, NC(CH )], 1.35 [br d, J = 6.8 Hz, 36 H,
3
3
2
NCH(CH ) ], 1.06 [s, 6 H, OC(CH ) ], 0.99 [s, 9 H, OC(CH ) ],
3 2 3 2 3 3
1
3
1
cold THF (0.5 mL) and recrystallized by being layered with cold n-
0.83 [s, 6 H, OC(CH ) ]; C{ H} NMR (100 MHz, THF-d , −43
3
2
8
pentane (3 mL) at −40 °C to give the THF solvate (5a) (THF)
°C) δ 185.4 (CCarbene), 142.0 (C ), 124.8 [C(CH )N], 115.9 (HC ),
105.4 (HC ), 76.8 [OC(CH ) ], 66.7 [OC(CH ) ], 53.4 [NCH-
Ar 3 2 3 3
2
Ar 3 Ar
1
(
4
15.9 mg, 21.5 μmol, 16%): H NMR (400 MHz, THF-d , −60 °C) δ
.23 [br sept, J = 7 Hz, 4 H, NCH(CH ) ], 2.07 [s, 12 H, C(CH )N],
8
3
2
3
(CH ) ], 33.6 [OC(CH ) ], 31.2 (NCH ), 27.7 [OC(CH ) ], 27.5
3 2 3 3 3 3 2
1
1
1
1
.66 [d, J = 6.6 Hz, 24 H, NCH(CH ) ], 0.88 [s, 24 H, OC(CH ) ];
[OC(CH ) ], 22.5 [NCH(CH ) ], 9.9 [C(CH )N]; B{ H} NMR
3 2 3 2 3
3
2
3
2
13
1
C{ H} NMR (100 MHz, THF-d , −60 °C) δ 183.2 (CCarbene), 122.3
(128 MHz, THF-d , −43 °C) δ 6.4 [s, Δw = 300 Hz, pinB(OtBu)−
8
8
1/2
1
[
C(CH )N], 79.3 [OC(CH ) ], 49.9 [NCH(CH ) ], 26.3 [OC-
CH ) ], 23.7 [NCH(CH ) ], 9.2 [NC(CH )]; B{ H} NMR (128
Bdmab] (only one signal detected); H NMR (300 MHz, THF-d
room temperature) δ 6.60−5.52 (m, 4 H, CHAr), 4.88 [br s, 6 H,
NCH(CH ), 2.15 [br s, 18 H,
)], 1.38 [d, J = 7.0 Hz, 36 H, NCH(CH ], 1.10 [s, 6 H,
], 1.01 [s, 9 H, OC(CH ], 0.85 [s, 6 H, OC(CH ];
, room temperature) δ 30.4 [s,
= 7 Hz,
,
8
3
3
2
3 2
1
1
1
(
3
2
3
2
3
MHz, THF-d , −60 °C) δ no signal detected; mp >90 °C dec. Anal.
3
)
2
, Δw1/2 = 45 Hz], 3.48 (s, 6 H, NCH
3
8
Calcd for [C H BCuN O ] : C, 55.06; H, 8.70; N, 7.55. Found: C,
NC(CH
OC(CH
3
)
3 2
17
32
2
2 2
5
4.96; H, 8.81; N, 7.22.
((Me IiPr)Cu)((Me P) Cu)(Bpin) ] (6). (5a) was prepared as
3
)
2
)
3
3
)
3 2
1
1
1
[
B{ H} NMR (96 MHz, THF-d
8
2
3
2
2
2
described above (starting from 550 μmol of Me IiPr). The mother
Δw
= 200 Hz, pinB(OtBu)−Bdmab], 5.7 [s, Δw
2
1/2
1/2
liquor was decanted, and the residue was washed with cold n-pentane.
pinB(OtBu)−Bdmab]; mp >80 °C dec. Anal. Calcd for
After addition of precooled PMe (0.5 mL, −40 °C), THF (room
C H B CuN O : C, 64.51; H, 9.66; N, 11.80. Found: C, 64.28;
51 91
3
2
8
3
temperature) was added in a dropwise manner to the slurry at room
H, 9.83; N, 11.45.
temperature until it was clear (∼0.3 mL); the solution was cooled to
[Cu (Me IiPr) (Bdmab) ] (8). In a nitrogen-filled glovebox,
5
2
3
3
−
40 °C. After 48−72 h, colorless crystals suitable for X-ray diffraction
(5b) (PhMe) (30 mg, 31 μmol) was dissolved in dry THF (0.9
2 2
had separated. The mother liquor was decanted, and the crystals were
mL) and layered with n-pentane (3.0 mL) at room temperature. After
1−2 days at −40 °C, a small amount of optically uniform single
washed with cold n-pentane (2 × 1 mL) and dried briefly in vacuo
1
(
36.2 mg, 50.7 μmol, 18%): H NMR (400 MHz, PhMe-d , −47 °C)
crystals of 8(THF) may have separated. It must be emphasized that
8
2
δ 3.95 [br sept, J = 6.7 Hz, 2 H, NCH(CH ) ], 1.89 [d, J = 6.7 Hz, 12
8(THF) is obtained in only approximately two of 10 runs.
3
2
2
H, NCH(CH ) ], 1.51 (br s, 18 H, PMe ), 1.40 [s, 6 H, C(CH )N],
[(Me IiPr) Cu (PMe ) ] (9). In a nitrogen-filled glovebox, 6 (20
3
2
3
3
2
10
23
3 2
1
3
1
1
.22 [s, 24 H, OC(CH ) ]; C{ H} NMR (100 MHz, PhMe-d , −47
mg, 28 μmol) was dissolved in dry PhMe (0.7 mL). After ∼24 h at
ambient temperature, a small amount of optically uniform single
crystals of 9 was deposited on the glass surface. It must be emphasized
that 9 is obtained in only approximately two of 10 runs.
3
2
8
°
C) δ 178.4 (CCarbene), 120.7 [C(CH )N], 79.1 [OC(CH ) ], 49.4
NCH(CH ) ], 26.6 [OC(CH ) ], 24.3 [NCH(CH ) ], 19.3 (t, J =
3 3 2
[
3 2 3 2 3 2
1
1
1
8
−
−
.1 Hz, PMe ), 8.8 [NC(CH )]; B{ H} NMR (128 MHz, PhMe-d ,
3 3 8
3
1
1
47 °C) δ no signal detected; P{ H} NMR (162 MHz, PhMe-d ,
8
47 °C) δ −32.0 (s, Δw = 80 Hz); mp >60 °C dec. Anal. Calcd for
1
/2
ASSOCIATED CONTENT
■
C H B Cu N O P : C, 48.82; H, 8.76; N, 3.93. Found: C, 48.63; H,
29
62
2
2
2
4 2
*
S
Supporting Information
8
.71; N, 4.03.
[
(Me IiPr)Cu−Bdmab] [(5b) ]. In a nitrogen-filled glovebox,
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00672.
2
2
2
pinB−Bdmab (2b) (68.0 mg, 249.8 μmol, 1.0 equiv) was dissolved in
PhMe (1.0 mL) and cooled to −40 °C (2−4 h). Me IiPr (45.0 mg,
2
2
49.8 μmol, 1.0 equiv) and CuOtBu (34.0 mg, 250.0 μmol, 1.0 equiv)
Additional analytical (in situ NMR spectra) and
crystallographic data (PDF)
were combined in a mixture of PhMe (1.0 mL) and THF (0.5 mL)
and, after complete dissolution, cooled to −40 °C for 2−4 h. The
solutions were combined in the cold to give a reddish turbid solution
that was kept at −40 °C. After 16−30 h at −40 °C, well-developed
orange prisms (suitable for X-ray diffraction) had separated. The
supernatant solution was decanted, and the residue washed with cold
Accession Codes
CCDC 1859613−1859622 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
I
DOI: 10.1021/acs.organomet.8b00672
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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.
Diboration of Aldehydes Catalyzed by Copper(I) Boryl Complexes. J.
Am. Chem. Soc. 2008, 130, 5586−5594. (d) Moon, J. H.; Jung, H.-Y.;
Lee, Y. J.; Lee, S. W.; Yun, J.; Lee, J. Y. Origin of Regioselectivity in
the Copper-Catalyzed Borylation Reactions of Internal Aryl Alkynes
with Bis(pinacolato)diboron. Organometallics 2015, 34, 2151−2159.
(7) Borner, C.; Kleeberg, C. Selective Synthesis of Unsymmetrical
AUTHOR INFORMATION
Corresponding Author
II
I
■
Diboryl Pt and Diaminoboryl Cu Complexes by B−B Activation of
Unsymmetrical Diboranes(4) {pinB−B[(NR) C H ]}. Eur. J. Inorg.
2
6
4
*
Institut fu
sche Universitat
̈
r Anorganische and Analytische Chemie, Techni-
Carolo-Wilhelmina zu Braunschweig, Hagenr-
ing 30, 38106 Braunschweig, Germany. Fax: +49 (0) 531 391
387. E-mail: ch.kleeberg@tu-braunschweig.de.
Chem. 2014, 2014, 2486−2489.
̈
(8) Borner, C.; Anders, L.; Brandhorst, K.; Kleeberg, C. Elusive
Phosphine Copper(I) Boryl Complexes: Synthesis, Structures, and
Reactivity. Organometallics 2017, 36, 4687−4690.
5
(
9) (a) Kajiwara, T.; Terabayashi, T.; Yamashita, M.; Nozaki, K.
ORCID
Syntheses, Structures, and Reactivities of Borylcopper and -zinc
Compounds: 1,4-Silaboration of an α,β-Unsaturated Ketone to Form
a γ-Siloxyallylborane. Angew. Chem., Int. Ed. 2008, 47, 6606−6610.
(b) Okuno, Y.; Yamashita, M.; Nozaki, K. One-Pot Carboboration of
Alkynes Using Lithium Borylcyanocuprate and the Subsequent
Suzuki−Miyaura Cross-Coupling of the Resulting Tetrasubstituted
Alkenylborane. Eur. J. Org. Chem. 2011, 2011, 3951−3958.
Christian Kleeberg: 0000-0002-6717-4086
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors gratefully acknowledge support from a Research
Grant (KL 2243/5-1) of the Deutsche Forschungsgemein-
schaft (DFG) and from the Fonds der Chemischen Industrie.
The authors thank AllyChem Co. Ltd. for a generous gift of
diboron(4) reagents.
(c) Segawa, Y.; Yamashita, M.; Nozaki, K. Boryl Anion Attacks
Transition-Metal Chlorides To Form Boryl Complexes: Syntheses,
Spectroscopic, and Structural Studies on Group 11 Borylmetal
Complexes. Angew. Chem., Int. Ed. 2007, 46, 6710−6713.
(d) Okuno, Y.; Yamashita, M.; Nozaki, K. Borylcyanocuprate in a
One-Pot Carboboration by a Sequential Reaction with an Electron-
Deficient Alkyne and an Organic Carbon Electrophile. Angew. Chem.,
Int. Ed. 2011, 50, 920−923.
REFERENCES
1) (a) Laitar, D. S.; Mu
Catalysis in the Reduction of CO to CO. J. Am. Chem. Soc. 2005,
■
(
̈
ller, P.; Sadighi, J. P. Efficient Homogeneous
(10) Oschmann, W.; Borner, C.; Kleeberg, C. Unsymmetrical
2
diborane(4) derivatives by copper mediated B−B coupling. Dalton
1
27, 17196−17197. (b) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P.
Trans. 2018, 47, 5318−5327.
Catalytic Diboration of Aldehydes via Insertion into the Copper−
Boron Bond. J. Am. Chem. Soc. 2006, 128, 11036−11037. (c) Semba,
K.; Shinomiya, M.; Fujihara, T.; Terao, J.; Tsuji, Y. Highly Selective
Copper-Catalyzed Hydroboration of Allenes and 1,3-Dienes. Chem. -
Eur. J. 2013, 19, 7125−7132. (d) Wyss, C. M.; Bitting, J.; Bacsa, J.;
Gray, T. G.; Sadighi, J. P. Bonding and Reactivity of a Dicopper(I) μ-
Boryl Cation. Organometallics 2016, 35, 71−74.
(
(
11) See the Supporting Information for details.
12) While no product could be isolated from reactions involving
Me IiPr, CuOtBu, and 2c, the formation of a black precipitate,
2
presumably elemental copper, is in agreement with the formation of a
copper(I) boryl complex as a transient species.
(13) Wiberg, N. Holleman-Wiberg, Lehrbuch der Anorganischen
Chemie; Walter de Gruyter: Berlin, 2007; pp 2002−2006.
(
2) For recent reviews, see: (a) Neeve, E. C.; Geier, S. J.; Mkhalid, I.
(
14) It is noted that also in the spectrum at −40 °C a small amount
A. I.; Westcott, S. A.; Marder, T. B. Diboron(4) Compounds: From
Structural Curiosity to Synthetic Workhorse. Chem. Rev. 2016, 116,
of decomposition products of 4b is observed (∼15%). However, this
is attributed to decomposition during the preparation of the NMR
sample; no appreciable decomposition was observed within 6 h at
9
091−9161. (b) Semba, K.; Fujihara, T.; Terao, J.; Tsuji, Y. Copper-
catalyzed borylative transformations of non-polar carbon−carbon
unsaturated compounds employing borylcopper as an active catalyst
species. Tetrahedron 2015, 71, 2183−2197. (c) Fujihara, T.; Semba,
K.; Terao, J.; Tsuji, Y. Regioselective transformation of alkynes
catalyzed by a copper hydride or boryl copper species. Catal. Sci.
Technol. 2014, 4, 1699−1709. (d) Dang, L.; Lin, Z.; Marder, T. B.
Boryl ligands and their roles in metal-catalysed borylation reactions.
Chem. Commun. 2009, 3987−3995.
−
40 °C.
(
15) The elemental copper formed during the decomposition of the
boryl complexes is formed either as a black precipitate or as a copper
mirror on the vessel used; concomitant formation of both has also
been observed. In our hands, the formation of elemental copper in
one form or the other was neither reproducible nor predictable.
(16) (a) Kleeberg, C.; Crawford, A. G.; Batsanov, A.; Hodgkinson,
P.; Apperley, D. C.; Cheung, M. S.; Lin, Z.; Marder, T. B.
Spectroscopic and Structural Characterization of the CyNHC Adduct
of B pin in Solution and in the Solid State. J. Org. Chem. 2012, 77,
(
3) Cid, J.; Carbo, J. J.; Fernandez, E. Disclosing the Structure/
́ ́
Activity Correlation in Trivalent Boron-Containing Compounds: A
Tendency Map. Chem. - Eur. J. 2012, 18, 12794−12802.
2
2
7
85−789. (b) Eck, M.; Wurtemberger-Pietsch, S.; Eichhorn, A.;
̈
(
4) (a) Takahashi, K.; Ishiyama, T.; Miyaura, N. Addition and
Coupling Reactions of Bis(pinacolato)diboron Mediated by CuCl in
the Presence of Potassium Acetate. Chem. Lett. 2000, 29, 982−983.
Berthel, J. H. J.; Bertermann, R.; Paul, U.; Schneider, H.; Friedrich, A.;
Kleeberg, C.; Radius, U.; Marder, T. B. B−B bond activation and
NHC ring-expansion reactions of diboron(4) compounds, and
(
b) Ito, H.; Yamanaka, H.; Tateiwa, J.-i.; Hosomi, A. Boration of an
α,β-enone using a diboron promoted by a copper(I)−phosphine
mixture catalyst. Tetrahedron Lett. 2000, 41, 6821−6825.
accurate molecular structures of B
(NMe ) , B eg , B neop and
2 2 4 2 2 2 2
B
pin . Dalton Trans. 2017, 46, 3661−3680.
2
2
(
5) Kleeberg, C.; Dang, L.; Lin, Z.; Marder, T. B. A facile route to
(17) (a) Albert, C. F.; Healy, P. C.; Kildea, J. D.; Raston, C. L.;
Skelton, B. W.; White, A. H. Lewis-base adducts of Group 11 metal(I)
compounds. 49. Structural characterization of hexameric and
pentameric (triphenylphosphine)copper(I) hydrides. Inorg. Chem.
1989, 28, 1300−1306. (b) Nguyen, T.-A. D.; Jones, Z. R.; Goldsmith,
B. R.; Buratto, W. R.; Wu, G.; Scott, Z. L.; Hayton, T. W. A Cu25
Nanocluster with Partial Cu(0) Character. J. Am. Chem. Soc. 2015,
137, 13319−13324. (c) Nguyen, T.-A. D.; Jones, Z. R.; Leto, D. F.;
Wu, G.; Scott, S. L.; Hayton, T. W. Ligand-Exchange-Induced Growth
of an Atomically Precise Cu29 Nanocluster from a Smaller Cluster.
Chem. Mater. 2016, 28, 8385−8390. (d) Protchenko, A. V.; Dange,
aryl boronates: room-temperature, copper-catalyzed borylation of aryl
halides with alkoxy diboron reagents. Angew. Chem., Int. Ed. 2009, 48,
5
(
350−5354.
6) (a) Zhao, H.; Lin, Z.; Marder, T. B. Density Functional Theory
Studies on the Mechanism of the Reduction of CO to CO Catalyzed
2
by Copper(I) Boryl Complexes. J. Am. Chem. Soc. 2006, 128, 15637−
1
5643. (b) Dang, L.; Zhao, H.; Lin, Z.; Marder, T. B. DFT Studies of
Alkene Insertions into Cu−B Bonds in Copper(I) Boryl Complexes.
Organometallics 2007, 26, 2824−2832. (c) Zhao, H.; Dang, L.;
Marder, T. B.; Lin, Z. DFT Studies on the Mechanism of the
J
DOI: 10.1021/acs.organomet.8b00672
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
D.; Blake, M. P.; Schwarz, A. D.; Jones, C.; Mountford, P.; Aldridge,
S. Oxidative Bond Formation and Reductive Bond Cleavage at Main
Group Metal Centers: Reactivity of Five-Valence-Electron MX2
Radicals. J. Am. Chem. Soc. 2014, 136, 10902−10905.
I
(
18) (a) Plotzitzka, J.; Kleeberg, C. [(NHC)Cu −ER ] Complexes
3
(
ER = SiMe Ph, SiPh , SnMe ): From Linear, Mononuclear
3 2 3 3
I
I
Complexes to Polynuclear Complexes with Ultrashort Cu ···Cu
Distances. Inorg. Chem. 2016, 55, 4813−4823. (b) Badiei, Y. M.;
Warren, T. H. Bis[copper 2,4-bis-(2,4,6-trimethylphenylimido)-
Me,Me3
2
2
pentyl] toluene, (L
1. (c) Kuhn, N.; Kratz, T. Synthesis of Imidazol-2-ylidenes by
Reduction of Imidazole-2(3H)-thiones. Synthesis 1993, 1993, 561−
62.
19) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.;
Cu (μ-η :η -C H ). Inorg. Synth. 2010, 35,
2 7 8
5
5
(
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.
(
20) (a) Stalke, D. Cryo crystal structure determination and
application to intermediates. Chem. Soc. Rev. 1998, 27, 171−178.
b) CrysAlisPro, version 1.171.34.44−1.171.36.24; Agilent Technol-
(
ogies, 2010−2012. (c) Sheldrick, G. M. SHELXT - Integrated space-
group and crystal-structure determination. Acta Crystallogr., Sect. A:
Found. Adv. 2015, 71, 3−8. (d) Altomare, A.; Cascarano, G.;
Giacovazzo, C.; Guagliardi, A.; Burla, M. C.; Polidori, G.; Camalli, M.
SIR92: A program for automatic solution of crystal structures by
direct methods. J. Appl. Crystallogr. 1994, 27, 435. (e) Sheldrick, G.
M. Crystal structure refinement with SHELXL. Acta Crystallogr., Sect.
C: Struct. Chem. 2015, 71, 3−8. (f) Farrugia, L. J. WinGX suite for
small-molecule single-crystal crystallography. J. Appl. Crystallogr.
1
999, 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.
2
008, 41, 466−470. (i) Putz, H.; Brandenburg, K. Diamond 4.2.2:
Crystal and Molecular Structure Visualization; Crystal Impact: Bonn,
Germany, 2016.
K
DOI: 10.1021/acs.organomet.8b00672
Organometallics XXXX, XXX, XXX−XXX