Elusive Phosphine Copper(I) Boryl Complexes: Synthesis, Structures, and Reactivity

Article abstract of DOI:10.1021/acs.organomet.7b00775

We report the first isolation of phosphine copper boryl complexes - species pivotal to numerous copper-catalyzed borylation reactions. The reaction of diboron(4) derivatives with copper tert-butoxide complexes of phosphine ligands allows the isolation of the dimeric μ-boryl-bridged Cu(I) complexes [(iPr₃P)Cu-Bdmab]₂ (4) and [(C₆H₄(Ph₂P)₂)Cu-Bpin]₂ (6) with Cu···Cu distances of 2.24-2.27 ? (dmab = (NMe)₂C₆H₄, pin = (OCMe₂)₂)). A slightly more sterically demanding boryl ligand furnishes the unprecedented multinuclear copper boryl complex [(iPr₃P)₂Cu₈(B(iPrEn))₃(OtBu)₃] (5), a potential intermediate of the decomposition of an initial Cu(I) boryl complex (iPrEn = (NiPr)₂C₂H₄). All complexes were characterized by single-crystal X-ray diffraction, NMR spectroscopy, and elemental analysis. DFT computations support the nature of these unique complexes and give insight into their electronic structures.

Full text of DOI:10.1021/acs.organomet.7b00775

Communication
Cite This: Organometallics XXXX, XXX, XXX−XXX
Elusive Phosphine Copper(I) Boryl Complexes: Synthesis, Structures,
and Reactivity
Corinna Borner, Lisa Anders, Kai Brandhorst, and Christian Kleeberg*
Institut fur Anorganische und Analytische Chemie, Technische Universitat Carolo-Wilhelmina zu Braunschweig, Hagenring 30, 38106
̈
̈
Braunschweig, Germany
S
* Supporting Information
ABSTRACT: We report the first isolation of phosphine copper boryl
complexesspecies pivotal to numerous copper-catalyzed borylation
reactions. The reaction of diboron(4) derivatives with copper tert-
butoxide complexes of phosphine ligands allows the isolation of the
dimeric μ-boryl-bridged Cu(I) complexes [(iPr3P)Cu−Bdmab]2 (4) and
[(C6H4(Ph2P)2)Cu−Bpin]2 (6) with Cu···Cu distances of 2.24−2.27 Å
(dmab = (NMe)2C6H4, pin = (OCMe2)2)). A slightly more sterically
demanding boryl ligand furnishes the unprecedented multinuclear copper
boryl complex [(iPr3P)2Cu8(B(iPrEn))3(OtBu)3] (5), a potential
intermediate of the decomposition of an initial Cu(I) boryl complex
(iPrEn = (NiPr)2C2H4). All complexes were characterized by single-
crystal X-ray diffraction, NMR spectroscopy, and elemental analysis. DFT
computations support the nature of these unique complexes and give
insight into their electronic structures.
opper-catalyzed borylation reactions of e.g. CO2, carbon-
yls, α,β-unsaturated carbonyls, olefins, and alkynes as well
boryl complexes, such sterically encumbered complexes are
C
typically not involved in catalytic processes. Attempts to isolate
and characterize sterically less encumbered and in particular
phosphine-containing copper boryl complexes haveto the best
of our knowledgeso far been unsuccessful. Presumably this is a
consequence of the high reactivity and low stability of these
complexes.2,7,8a
as aryl and alkyl halides employing diboron(4) reagents (in
particular B2pin2 (1), pin = (OCMe2)2) are a currently
flourishing field of research.1−3,4a−c Since the first independent
reports on those reactions by Miyaura, Hosomi, and co-workers
copper(I) boryl complexes have been proposed as the central
reactive intermediates.5 However, only in 2005 did Sadighi and
co-workers report the first isolation of a Cu(I) boryl complex as
well as initial reactivity studies. This complex of the type
(NHC)Cu−Bpin (I), was prepared, in analogy to the proposed
catalytic reaction pathway, by σ-bond metathesis reaction of a
copper alkoxide precursor and the diborane(4) derivative B2pin2
(1).3,4a,b This σ-bond metathesis reaction was later extended to a
variety of related NHC-Cu complexes, including a remarkable
cationic μ-catecholatoboryl complex [((IDipp)Cu)2Bcat]+ (II)
by Sadighi in 2016. As a common feature all of these complexes
employ sterically demanding ancillary NHC ligands to provide
kinetic stabilization.4c,d Cu(I) boryl complexes with ancillary
ligands having little steric demand (e.g., halide, cyanide) have
been obtained by Yamashita and co-workers using sterically
demanding diaminoboryl ligands, by reaction of the respective
boryl lithium species with an appropriate copper precursor, e.g.,
[(C2H4((2,6-iPr2C6H3)N)2B)2Cu4Br2] (III).6 It has to be
emphasized that the known copper boryl complexes are
stabilized either by sterically demanding NHC ligands or by
the steric demand of the boryl ligand itself; nonetheless, despite
the steric protection it has been stated that these copper boryl
complexes are highly reactive and decompose readily to
elemental copper.2,4a,c While these complexes are useful to
establish the general reactivity and structural properties of copper
In order to investigate further the chemistry of copper(I) boryl
complexes, we combined phosphine ligands, a ligand class
frequently employed in catalytic reactions but not yet known in
isolated boryl complexes, with diaminoboryl ligands with
reduced steric demand, in comparison with those derived from
boryl lithium derivatives. The reaction of the dimeric alkoxide-
bridged complex [(iPr3P)Cu−OtBu]2 (2) with the unsym-
metrical diborane(4) derivative pinB-Bdmab (3a; dmab =
(NMe)2C6H4), recently introduced by us as a versatile precursor
for otherwise inaccessible diaminoboryl ligands, results in the
formation of the unprecedented bis-μ-boryl-bridged complex
[(iPr3P)Cu−Bdmab]2 (4) in good yields (Scheme 1).8,9 The
sterically more demanding diboron compound pinB−B(iPrEn)
(3b; iPrEn = (NiPr)2C2H4), however, furnishes under similar
reaction conditions the unprecedented octanuclear boryl
complex [(iPr3P)2Cu8(B(iPrEn))3(OtBu)3] (5) (Scheme
1).9,10 The sterically more encumbered pinB−BtBuEn (3c;
tBuEn = (NtBu)2C2H4) does not react with 2, whereas B2pin2
(1) undergoes facile B−B bond activation.9 While for the latter
no boryl complex could be isolated, the observed (decom-
Received: October 18, 2017
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.7b00775
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Scheme 1. Synthesis of the Boryl Complexes 2, 5, and 69
of 6 and one molecule of cocrystallized PhMe; however, as for 4,
each molecule of 6 is situated on a center of inversion (Figure 1,
right).9 Moreover, the THF solvate 6(thf)4 was obtained from
THF solution, exhibiting geometrical properties very similar to
those of 6(PhMe)2.9 Complexes 4 and 6 both show as a pivotal
structural feature, despite the difference in coordination number
of the copper ion, a planar, slightly unsymmetrical Cu2B2 moiety
exhibiting a μ-boryl coordination and an extremely short Cu···Cu
distance of 2.24−2.27 Å, significantly shorter than twice the van
der Waals radius (2.8 Å) or the interatomic distance in elemental
copper (2.56 Å).11 While μ-boryl-bridged copper complexes
have been reported, they have not been considered as readily
formed, possible intermediates in borylation reactions. In fact,
the reported μ-bridged boryl complexes II and III are structurally
quite distinct from 4 and 6.4d,6a,7 The Cu···Cu distances in II and
III are at 2.4082(2) and 2.312(1) Å, respectively, significantly
longer than those in 4 and 6, whereas the slightly unequal Cu−B
distances are significantly shorter in II (2.051(6), 2.041(6) Å)
and III (2.073(5), 2.093(4) Å), although the Cu−B−Cu angles
are wider in the latter (II 72.1(2)°, III 67.4(1)°).4d,6a These
difference in B−Cu distances are rationalized by assuming a
mutual weakening of the B−Cu bond by the two competing
boryl ligands in 4 and 6, similar to a trans influence,12 whereas the
shorter Cu···Cu distance may be considered a direct result of the
more effective bonding within the B2Cu2 core (vide infra). It is
worth noting that a very similar structural motif, Cu2Si2,
including short Cu···Cu distances, is also found in copper silyl
complexes, once more emphasizing the diagonal relationship
between boron and silicon.13
position) products agree with an intermediate formation of an
(unstable) copper boryl species (vide infra).
Replacing the monodentate phosphine in 2 by the bidentate
1,2-bis(diphenylphosphino)benzene (dppbz) resulted in the
isolation of the complex [(C6H4(Ph2P)2)Cu−Bpin]2 (6) upon
reaction with 1 (Scheme 1).9
It has to be emphasized that the complexes 4−6 are highly
sensitive species and are not stable at ambient temperature in
solution (vide infra). Their isolation was successful as conditions
were found under which crystallization is competitive with
decomposition. However, as solids the compounds may be
stored for several months at −40 °C under an inert nitrogen
atmosphere.
Complex 5 crystallizes from a toluene/n-pentane mixture at
−40 °C as small hexagonal bronze prisms. Each entity of 5 is
situated on a 3-fold axis through the atoms P1/Cu3 and on a
mirror plane perpendicular to this axis (P62c, Z = 1, Z′ = 1/3,
̅
Figure 2).9
Single crystals of 4 were obtained from an n-pentane/PhMe
solution at −40 °C as light orange hexagonal plates. 4 crystallizes
with half a molecule in the asymmetric unit (Pbca, Z = 4, Z′ = 1/
2), each molecule being situated on a center of inversion (Figure
1, left).9 Single crystals of 6(PhMe)2 were obtained from a PhMe
solution at −40 °C as orange rhombohedral plates. The
asymmetric unit (P1, Z = 1, Z′ = 1/2) comprises half a molecule
̅
Figure 2. Two selected views of the molecular structure of 5. Hydrogen
atoms and disordered parts are omitted for clarity.9 Selected distances
(Å) and angles (deg): P1−Cu3 2.300(4), Cu1−B1−Cu2 64.3(4), Cu1−
O1−Cu2″ 86.9(4).
The molecular structure of 5 comprises an octanuclear copper
cluster consisting of a slightly irregular hexagon of copper atoms
with both hexagonal faces capped by two additional copper
atoms. This cluster is decorated with three diaminoboryl and
three alkoxide ligands in the equatorial plane; the apical copper
atoms are coordinated by additional phosphine ligands. The B−
Cu distances are slightly shorter than those observed in the
dinuclear complexes 4 and 6 but longer than those in I (R = dipp)
and III.4d,6a The O−Cu distances, however, are significantly
shorter than those in its precursor 2 (1.983(3) and 1.994(3) Å),
Figure 1. Molecular structures of (left) 4 and (right) 6. Hydrogen atoms
and disordered parts are omitted for clarity.9 Selected distances (Å) and
angles (deg): 4, Cu1−Cu1′ 2.2359(4), B1−Cu1 2.179(2), B1−Cu1′
2.263(2), P1−Cu1 2.1989(4), B1−Cu1−P1 123.52(4), B1−Cu1′−P1′
116.89(4), Cu1−B1−Cu1′ 60.41(4), B1−Cu1−B1′ 119.59(4); 6,
Cu1−Cu1′ 2.2730(5), B1−Cu1 2.203(3), B1−Cu1′ 2.311(3), P1−
Cu1 2.2706(6), P2−Cu1 2.2826(6), P1−Cu1−P2 84.42(2), B1−Cu1−
B1′ 119.59(7), Cu1−B1−Cu1′ 60.41(7).
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whereas the P−Cu distance is longer than in 4 and 6 as well as in
2.9 The Cu···Cu distances within 5 vary greatly in the range of
2.28−2.72 Å depending on the ligands at the copper atoms
(Figure 2). The μ-boryl-bridged Cu1···Cu2 unit exhibits the
shortest Cu···Cu distance, in the range observed for 4 and 6,
whereas the Cu1···Cu2″ distance, bridged by a μ-alkoxide ligand,
is significantly shorter than in the μ-alkoxide precursor complex 2
(2.8565(6) Å).9 The unbridged Cu1/2···Cu3 distances,
however, are only slightly shorter than double the van der
Waals radius of copper (2.8 Å).11 Complex 5 comprises three
anionic diaminoboryl and three anionic alkoxide ligands as well
as two neutral phosphine ligands and eight copper atoms; hence,
formally an averaged oxidation state of each copper atom of +3/4
has to be assigned. It should be emphasized that, as polyhydrido
copper clusters are well established,14 no indication of the
presence of hydride ligands was obtained by an X-ray diffraction
study or by elemental analysis.9 Hence, it may be concluded, in
agreement with computational data (vide infra), that 5 comprises
an unprecedented multinuclear copper boryl complex with an
average formal Cu oxidation number of lower than 1.
The bonding situation in the boryl complexes 4−6 was
analyzed using DFT calculations. For all compounds, starting
from the solid-state structures, the geometries were fully
optimized and were confirmed to be minima on the energy
hypersurface.9 Generally, for 4 as well as for 6, the computed
molecular structure resembles the solid-state structures well.4d
However, the small differences found are more pronounced for 6
than for 4, possibly due to solid-state effects not considered in the
gas-phase computations. To evaluate the bonding in 4−6 the
Laplacians were calculated and analyzed using Bader’s atoms in
molecules method (AIM).15 For complexes 4 and 6 bond critical
points on all B···Cu as well as Cu···Cu bond paths were found
(Figure S35, S40), while quite low electron densities (4, 0.063−
0.070; 6, 0.045−0.080) and positive Laplacians (4, 0.063−0.117;
6, 0.059−0.111) indicate closed-shell interactions (Table S2 in
the Supporting Information).9,15 These findings, indicating a
Cu−Cu bond, are in agreement with the data reported for the
related complex II.4d A qualitative examination of the computed
orbitals of 4 and 6 shows that some of the energetically high lying
orbitals (Figures S34 and S39 in the Supporting Information)
exhibit substantial boron and copper contributions (4, HOMO,
HOMO-5, -11, -16; 6, HOMO, HOMO-2, -32).9 This is in
agreement with a simple rationalization assuming multicentered
bonds within the B2Cu2 core, essentially composed of an
occupied σ-type orbital at the boryl ligands and vacant orbitals at
the copper atoms, a bonding situation similar to that described
for II.4d The computed structure of 5, however, is of lower
symmetry (C3) than its solid-state structure (approximately
D3h).9 The computed Cu···Cu distances, in particular those
bridged by alkoxide ligands, are longer, whereas the Cu···B
distances are shorter than those observed in the solid state.9 For a
deeper understanding of the bonding situation in the octanuclear
cluster 5, a more detailed analysisbeyond the scope of this
communicationis required. It is noted that while no bond
critical points within the equatorial Cu6 unit in 5 were found, the
HOMO of this molecule includes substantial boron and copper
contributions as found for 4 and 6 (Figure S37 in the Supporting
Information).9
containing both a iPr3P and a Bdmab moiety, indicating the
presence of 4. The 31P and 13C NMR data are in full agreement
with this interpretation; unfortunately no 11B NMR signal could
be detected. This may be due to the fast relaxation of the
quadrupolar 11B nucleus in an environment of low symmetry and
the proximity of quadrupolar Cu nuclei.4d,9 Moreover, from the
NMR data two major decomposition products of 4 were
identified: the symmetrical diborane(4) derivative B2dmab2 and
free iPr3P (Figures S1−S5 in the Supporting Information).9,16 In
addition, in working with 4 we observed tiny amounts of a very
fine black precipitate and/or a blackish metallic luster at the
vessels used, which are assigned, in agreement with earlier
reports, to elemental copper formation.4a,c Unfortunately, all
attempts to directly characterize this material, e.g. by X-ray
powder diffraction, failed due to experimental reasons (the small
amounts available, fine nature, etc.). However, the isolation of
the boryl complex 5 suggests that copper clusters, including low-
valent copper atoms, are also possible decomposition products or
intermediates in the reductive decomposition of boryl
complexes.
The solution structure of 4 has not been straightforwardly
elucidated; however, the phosphine-bound isopropyl group
exhibits apparent triplet signals in the 13C NMR spectrum
(Figure S3 in the Supporting Information). Possible explanations
include coupling to two phosphorus atoms or the diastereoto-
picity of the isopropyl groups, both tentatively supportive of a
dimeric (or higher aggregated) rather than a mononuclear linear
solution structure.9
In solution 6 exhibits behavior similar to that for complex 4; it
decomposes at ambient temperature in solution instantaneously.
At −45 °C in THF-d8 and PhMe-d8 solution NMR data can be
obtained along with indications for decomposition even at low
temperatures. However, again no 11B NMR signal was detected
(vide supra) and a complete set of 13C NMR data was not
obtained due to the low solubility of this compound.9 The
symmetrical diborane(4) B2pin2 (1) was unambiguously
identified by NMR spectroscopy as a decomposition product
of 6, suggesting a reductive decomposition pathway analogous to
that described for 4. However, additionally Ph−Bpin along with
PPh3 (and further unidentified phosphorus containing decom-
position products) were observed as decomposition products of
6, indicating the presence of additional decomposition pathways
besides reductive B−B bond formation (Figures S7−S12 in the
Supporting Information).9
Due to its extremely low solubility 5 could not be
comprehensively characterized by NMR spectroscopy. However,
31P NMR spectroscopic data are in agreement with the presence
of a copper phosphine complex in solution at −45 °C and its
decomposition upon heating to ambient temperature (Figure S6
in the Supporting Information).9
An initial study on the reactivity of complexes 4−6 as sources
of a nucleophilic boron moiety was conducted. The copper boryl
complexes were reacted with excess 4-iodotoluene as an
electrophile established as a substrate in borylation catalysis
and used in reactivity studies of [(IDipp)Cu−Bpin] (type I).2
Indeed, for all three boryl complexes 4−6, the expected boronic
acid derivatives 4-Me(C5H4)-Bdmab, 4-Me(C5H4)-B(iPrEn)
and 4-Me(C5H4)-Bpin, respectively, were identified by NMR
spectroscopy and/or GCMS analysis as a major reaction product
(Figures S19−S28 in the Supporting Information).9 It has to be
noted that the borylation of the aryl iodide is competitive with
decomposition of the boryl complexes (vide supra) and hence
difficult to quantify.9 Nonetheless, a mixture of CuOtBu and
Compound 4 is, in solution, unstable at room temperature, but
NMR spectra could be recorded at −47 °C. Nonetheless even at
this temperature 4 shows considerable decomposition within a
few hours (Figures S1 and S2 in the Supporting Information).9,16
However, NOESY NMR contacts prove the presence of a species
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dppbz was shown to be an effective catalyst system for the
copper-catalyzed borylation of 4-iodotoluene, under conditions
virtually identical with those reported elsewhere (53% isolated
yield).2,9 In particular it should be noted that this system
performs better than that in the absence of a phosphine ligand.9
Further details on the reactivity of the presented copper boryl
complexes in stoichiometric as well as catalytic reactions will be
the subject of forthcoming studies.
REFERENCES
■
(1) For recent reviews, see: (a) Neeve, E. C.; Geier, S. J.; Mkhalid, I. A.
I.; Westcott, S. A.; Marder, T. B. Chem. Rev. 2016, 116, 9091−9161.
(b) Semba, K.; Fujihara, T.; Terao, J.; Tsuji, Y. Tetrahedron 2015, 71,
2183−2197. (c) Fujihara, T.; Semba, K.; Terao, J.; Tsuji, Y. Catal. Sci.
Technol. 2014, 4, 1699−1709. (d) Dang, L.; Lin, Z.; Marder, T. B. Chem.
Commun. 2009, 3987−3995.
(2) Kleeberg, C.; Dang, L.; Lin, Z.; Marder, T. B. Angew. Chem., Int. Ed.
2009, 48, 5350−5354.
The findings presented may be summarized in three major
points, all immediately relevant to the vibrant field of copper-
catalyzed borylation reactions. (a) Phosphine copper boryl
complexes are isolable compounds that may be used for
mechanistic studies. Nonetheless, those complexes are intrinsi-
cally highly unstable toward decomposition: e.g., via reductive
B−B coupling. (b) μ-Boryl coordination appears to be the
preferred coordination mode of phosphine copper boryl
complexes. While this does not exclude mononuclear copper
boryl complexes as catalytic relevant intermediates, dimeric and
possibly also higher aggregated species also have to be
considered. (c) So far unprecedented, multinuclear copper
boryl complexes have been observed and are possible
intermediates in the reductive decomposition of Cu(I) boryl
complexes. Moreover, those complexes are formed under
conditions similar to those for Cu(I) boryl complexes and are
sources of boron nucleophiles and, as such, have to be considered
as potential reactive intermediates in borylation reactions.
(3) (a) Zhao, H.; Lin, Z.; Marder, T. B. J. Am. Chem. Soc. 2006, 128,
15637−15643. (b) Dang, L.; Zhao, H.; Lin, Z.; Marder, T. B.
Organometallics 2007, 26, 2824−2832. (c) Zhao, H.; Dang, L.;
Marder, T. B.; Lin, Z. 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.
Organometallics 2015, 34, 2151−2159.
(4) (a) Laitar, D. S.; Muller, P.; Sadighi, J. P. J. Am. Chem. Soc. 2005,
̈
127, 17196−17197. (b) Laitar, D. S.; Tsui, E. Y.; Sadighi, J. P. J. Am.
Chem. Soc. 2006, 128, 11036−11037. (c) Semba, K.; Shinomiya, M.;
Fujihara, T.; Terao, J.; Tsuji, Y. Chem. - Eur. J. 2013, 19, 7125−7132.
(d) Wyss, C. M.; Bitting, J.; Bacsa, J.; Gray, T. G.; Sadighi, J. P.
Organometallics 2016, 35, 71−74.
(5) (a) Takahashi, K.; Ishiyama, T.; Miyaura, N. Chem. Lett. 2000, 29,
982−983. (b) Ito, H.; Yamanaka, H.; Tateiwa, J.-i.; Hosomi, A.
Tetrahedron Lett. 2000, 41, 6821−6825.
(6) (a) Kajiwara, T.; Terabayashi, T.; Yamashita, M.; Nozaki, K. Angew.
Chem., Int. Ed. 2008, 47, 6606−6610. (b) Okuno, Y.; Yamashita, M.;
Nozaki, K. Eur. J. Org. Chem. 2011, 2011, 3951−3958. (c) Segawa, Y.;
Yamashita, M.; Nozaki, K. Angew. Chem., Int. Ed. 2007, 46, 6710−6713.
(7) We reported earlier the crystallographic characterization of the
complex [Cu5(PPh3)2(OtBu)(Bdmab)4]; however, all attempts to
obtain this complex reproducibly failed. This Cu(I) boryl complex
exhibits the same structural motif as 4 and 6: a μ-coordinating boryl
ligand and short Cu···Cu distances.8a
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00775.
(8) (a) Borner, C.; Kleeberg, C. Eur. J. Inorg. Chem. 2014, 2014, 2486−
2489. (b) Borner, C.; Brandhorst, K.; Kleeberg, C. Dalton Trans. 2015,
44, 8600−8604.
(9) See the Supporting Information for details.
Experimental, analytical, computational and crystallo-
(10) Unsymmetrical diborane(4) derivatives of the type pinB-
B(NRR′)2 are readily prepared by reaction of the copper boryl complex
(IDipp)Cu−Bpin with boron electrophiles Br−B(NRR′)2.9 A more
comprehensive discussion of this reaction will be the subject of a
forthcoming report.
graphic data (PDF)
Cartesian coordinates for the calculated structures (MOL)
Accession Codes
(11) Wiberg, N. Holleman-Wiberg, Lehrbuch der Anorganischen Chemie;
Walter de Gruyter: Berlin/New York, 2007; pp 2002−2006 (13).
(12) Zhu, J.; Lin, Z.; Marder, T. B. Inorg. Chem. 2005, 44, 9384−9390.
(13) (a) Plotzitzka, J.; Kleeberg, C. Inorg. Chem. 2017, 56, 6671−6680.
(b) Plotzitzka, J.; Kleeberg, C. Inorg. Chem. 2016, 55, 4813−4823.
(c) Sgro, M. J.; Piers, W. E.; Romero, P. E. Dalton Trans. 2015, 44,
3817−3828.
CCDC 1572689−1572693 contain the supplementary crystallo-
graphic 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.
(14) For recent overviews on copper hydride complexes, see:
(a) Dhayal, R. S.; van Zyl, W. E.; Liu, C. W. Acc. Chem. Res. 2016, 49,
86−95. (b) Jordan, A. J.; Lalic, G.; Sadighi, J. P. Chem. Rev. 2016, 116,
8318−8372.
AUTHOR INFORMATION
■
Corresponding Author
*C.K.: fax, +49 (0) 531 391 5387; e-mail, ch.kleeberg@tu-
braunschweig.de.
(15) (a) Bader, R. F. W. Chem. Rev. 1991, 91, 893−928. (b) Bader, R.
Atoms in Molecules: A Quantum Theory; Oxford University Press: New
York, 1994.
ORCID
(16) Obtaining NMR spectra of 4 and 6 is not straightforward and is
hampered by the instability at room temperature and the low solubility
of the complexes, as well as the time constraints for measurements at low
temperatures. In our hands it was impossible to obtain NMR data of 4
and 6 without substantial decomposition. All possible care was taken to
handle and transfer the NMR samples of boryl complexes below −40
°C; however, ultimately they had to be transferred from a cooling bath
into the precooled NMR instrument and were exposed briefly to
ambient temperature. During this short period of time (∼0.5−1 min)
the samples darkened notably.9
Christian Kleeberg: 0000-0002-6717-4086
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.K. and C.B. thank the Fonds der Chemischen Industrie for the
generous support by a PhD and a Liebig Fellowship. C.K.
gratefully acknowledges support by a Research Grant (KL 2243/
5-1) of the DFG. The authors thank BASF SE for a gift of
B2(NMe2)4.
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DOI: 10.1021/acs.organomet.7b00775
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