A polar copper-Boron One-Electron σ-bond

Article abstract of DOI:10.1021/ja4006578

Virtually all chemical bonds consist of one or several pairs of electrons shared by two atoms. Examples of σ-bonds made of a single electron delocalized over two neighboring atoms were until recently found only in gas-phase cations such as H₂⁺ and Li₂ ⁺ and in highly unstable species generated in solid matrices. Only in the past decade was bona fide one-electron bonding observed for molecules in fluid solution. Here we report the isolation and structural characterization of a thermally stable compound featuring a Cu-B one-electron bond, as well as its oxidized (nonbonded) and reduced (two-electrons-bonded) congeners. This triad provides an excellent opportunity to study the degree of σ-bonding in a metalloboratrane as a function of electron count.

Full text of DOI:10.1021/ja4006578

Communication
pubs.acs.org/JACS
A Polar Copper−Boron One-Electron σ‑Bond
Marc-Etienne Moret,† Limei Zhang, and Jonas C. Peters*
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
spectroscopy supported their respective one-electron σ-
bonding descriptions.
ABSTRACT: Virtually all chemical bonds consist of one
or several pairs of electrons shared by two atoms.
Examples of σ-bonds made of a single electron delocalized
over two neighboring atoms were until recently found only
in gas-phase cations such as H2 and Li2 and in highly
unstable species generated in solid matrices. Only in the
past decade was bona fide one-electron bonding observed
for molecules in fluid solution. Here we report the
isolation and structural characterization of a thermally
stable compound featuring a Cu−B one-electron bond, as
well as its oxidized (nonbonded) and reduced (two-
electrons-bonded) congeners. This triad provides an
excellent opportunity to study the degree of σ-bonding
in a metalloboratrane as a function of electron count.
In this Communication we describe the structural character-
ization of a formal Cu(0) adduct of a tris(phosphine)borane
ligand,9 (TPB)Cu (TPB = tris[2-(diisopropylphosphino)-
phenyl]borane). Because Cu(0) would be exceptional among
monomeric copper coordination complexes, we sought to
develop a detailed picture of the electronic structure of this
complex, with an alternative limiting description being that of
Cu(II) wherein the borane ligand accepts a pair of electrons
from the Cu center.10 In valence terminology, zerovalent
copper would imply no bonding between the Cu and B centers
in (TPB)Cu, whereas divalent copper would imply a Z-type
borane ligand accepting a pair of electrons via σ-back-donation
from the copper center (Cu→B).11 As discussed below, neither
of these formulations proves most apt. We instead prefer a
description of (TPB)Cu that invokes a polarized one-electron
σ-bond between the Cu and B centers (i.e., Cu·B).
Spectroscopic and theoretical data for (TPB)Cu, in addition
to related data for its structurally characterized one-electron
oxidized and reduced partners, {(TPB)Cu}+ and {(TPB)Cu}−
(Figure 1, bottom), in sum support this view. While there are a
few known families of metallaboratranes related by formal
redox processes,12 the existence of the (TPB)Cu scaffold in
three isolable oxidation states with the same ligand set allows
for a uniquely detailed characterization of the electronic
structure of metal−boron bonds.
+
+
inus Pauling proposed in 1931 the possibility of one-
L
electron σ-bonding.1 While such a bond is expected to be
much weaker than a prototypical two-electron σ-bond, the gas-
2
+
phase characterization of the diatomic cations H2+ and Li2 , in
addition to the matrix identification of anionic {(MeO)3B·B-
(OMe)3}− and larger radical cations of the type {Me3E·EMe3}+
(E = C, Si, Ge),3−5 helped place this supposition on firm
experimental ground after the middle of the 20th century.
Nonetheless, it was only more recently that bona fide one-
electron σ-bonding was observed for molecules in fluid
solution. The two most definitive systems of this type are
depicted in Figure 1 (top) and exhibit nonpolar one-electron σ-
bonding between a pair of P or B atoms, respectively.6−8
Generation of these one-electron σ-bonded systems was aided
by the use of covalent tethers, helping to hold the P·P and B·B
subunits together. While these systems were not structurally
characterized, solution electron paramagnetic resonance (EPR)
The preparation of (TPB)Cu was readily accomplished by
stirring a solution of TPB and CuBr in tetrahydrofuran over an
excess of sodium/mercury amalgam. Such a procedure yielded
an inky purple suspension from which (TPB)Cu could be
isolated as black crystals in 50% yield. The cyclic voltammo-
gram of (TPB)Cu exhibits two quasi-reversible waves at −1.60
and −2.76 V vs Fc/Fc+ (Fc = ferrocene) that correspond to the
{(TPB)Cu}+/0 and {(TPB)Cu}0/− redox couples, respectively.
We hence sought synthetic access to these redox partners.
Stirring a mixture of TPB, CuBr, and Na{BArF } in diethyl
4
ether for 2.5 h ({BArF }− = tetrakis[3,5-bis(trifluoromethyl)-
4
phenyl]borate) followed by removal of the insoluble NaBr by
filtration and slow concentration of the filtrate afforded
{(TPB)Cu}{BArF } as yellow blocks (70% yield). Its anionic
4
cousin {(TPB)Cu}− could be generated as an inky blue
solution of the salts {Na}{(TPB)Cu} or {K}{(TPB)Cu} by
stirring a solution of (TPB)Cu in the presence of either metallic
sodium or potassium. Crystallization of {(TPB)Cu}− was most
readily accomplished as the potassium salt, wherein the
potassium countercation is encapsulated by 2 equiv of benzo-
15-crown-5, {K(benzo-15-C-5)2}{(TPB)Cu}. The solid-state
Figure 1. (Top) Symmetrical one-electron-bonded species previously
characterized by EPR spectroscopy in fluid solution. (Bottom) Copper
complexes described herein.
Published: February 18, 2013
© 2013 American Chemical Society
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Communication
crystal structures of {(TPB)Cu}{BArF }, (TPB)Cu, and
{K(benzo-15-C-5)2}{(TPB)Cu} were determined and are
shown in Figure 2A. Key NMR, UV−vis, and X-ray diffraction
over the B and Cu atoms. This interpretation was corroborated
by a natural atomic orbital13 population analysis of the spin
density obtained from DFT calculations that reveal a spin
population of 0.57 on B (0.06 2s, 0.51 2p) and 0.13 on Cu
(0.01 4s, 0.10 4p, 0.02 3d), the remaining spin being
delocalized over the phosphine P atoms (2% each) and the
phenylene linkers. The fact that the spin population of the 4p
orbital of Cu largely exceeds that of the 3d orbital supports the
description of the Cu·B bond in (TPB)Cu as a one-electron
bond with only marginal participation of internal 3d orbitals of
Cu.
4
The comparative XRD, NMR, and UV−vis data obtained for
the redox series {(TPB)Cu}+, (TPB)Cu, and {(TPB)Cu}−
shed additional light on the nature of the Cu−B interaction.
The XRD structure of {(TPB)Cu}{BArF } exhibits a long Cu−
4
B distance of 2.495 Å and a nearly planar boron center with a
sum of the C−B−C angles (∑(C−B−C)) of 355°, indicating a
very weakif anyretrodative B←Cu interaction. This
suggestion is additionally corroborated by a 11B chemical shift
of 67 ppm, virtually equal to that measured for triphenylborane
(67.4 ppm)14 and that calculated for the metal-free TPB ligand
(65.4 ppm).9 The yellow color of {(TPB)Cu}+, constrasting
with the generally colorless appearance of tris(phosphine)-
Figure 2. (A) Ellipsoid representation (50% probability) of the XRD
crystal structures of {(TPB)Cu}+, (TPB)Cu, and {(TPB)Cu}−. For
clarity only the core atoms are plotted. (B) Experimental (black) and
simulated (red) X-band EPR spectrum of (TPB)Cu at room
temperature in toluene solution. (C) Spin density of (TPB)Cu
calculated by DFT at the B3LYP/6-311+G(d,p)//6-31+G(d) level.
copper(I) cations,15,16 is caused by an additional band (λmax
=
403 nm) assigned to a copper-to-boron charge-transfer
transition.17 Bourissou has previously reported17 the related
neutral complex (TPB)CuCl, in which a weak B←Cu
interaction was inferred on the basis of the 11B chemical shift
at 53.8 ppm and pyramidalization (∑(C−B−C) = 347.0°),
despite a long Cu−B distance of 2.508 Å. Evidently, formal
abstraction of the Cl− from (TPB)CuCl decreases the Lewis
basicity of the Cu(I) center to the point where electron
donation to the unsaturated boron atom becomes close to
negligible. Accordingly, an NBO analysis of the DFT-calculated
electron density associates an energy of only 3.6 kcal/mol to
Table 1. NMR, UV−Vis, and Metrical Parameters Relevant
to the Description of the Cu−B Interaction
{(TPB)Cu}+
(TPB)Cu
{(TPB)Cu}−
δ(11B) [ppm]
δ(31P) [ppm]
67
paramagnetic
paramagnetic
6.7
19.3
20.2
λmax [nm]
430 {sh}, 403 665 {2300}, 485
780 {sh}, 615
{4900}, 530
{5100}, 350
{sh}
{ε [cm−1 M−1]}
{850}, 320
{8000}
{4200}, 345
2
the Cu(3dz )→B interaction, to be compared with the ca. 8
{6400}, 310 {sh}
kcal/mol calculated for (TPB)CuCl.17
As electrons are added to yellow {(TPB)Cu}+ to generate
dark purple (TPB)Cu, and then its dark indigo anion
{(TPB)Cu}−, intense bands appear in the visible region of
the spectrum (Table 1). These new absorptions are tentatively
attributed to charge-transfer transitions from the high-lying
σ(Cu−B) orbital to π* orbitals of the phenylene linkers of the
TPB ligand. The main geometrical changes (Table 1) that are
manifest include a marked shortening of the Cu−B distance
from 2.495 Å in {(TPB)Cu}+ to 2.289 Å in (TPB)Cu, and then
to 2.198 Å in {(TPB)Cu}−. Also evident in the series is a
gradual pyramidalization of the boron center as the electron
count is increased. This is reflected in the sum of the C−B−C
angles (∑(C−B−C)), which decreases from 355.0° to 347.1°
to 338.9° in the series. Whereas the hybridization at boron
changes markedly, the geometry of the P3Cu subunit in each
structure shows negligible variation. Accordingly, the 11B NMR
resonance shifts from 67 ppm in {(TPB)Cu}+ to 6.7 ppm in
{(TPB)Cu}−, whereas the 31P chemical shift varies a negligible
amount, from 19.3 to 20.2 ppm, respectively. These X-ray and
NMR data collectively suggest that the Cu−B interaction
increases in the order {(TPB)Cu}+ < (TPB)Cu < {(TPB)-
Cu}−. The Cu−B bonding in {(TPB)Cu}+ is insignificant but
becomes more prevalent in (TPB)Cu with the onset of one-
electron σ-bonding, and it becomes that of a prototypical two-
electron bond in {(TPB)Cu}−.
Cu−B [Å]
⟨Cu−P⟩ [Å]
∑(P−Cu−P) [°] 356.7
∑(C−B−C) [°] 355.0
2.495
2.295
2.289
2.198
2.244
357.5
338.9
2.270
359.0
347.1
(XRD) data are summarized in Table 1. In addition, the
geometries of {(TPB)Cu}+, (TPB)Cu, and {(TPB)Cu}− were
optimized by density functional theory (DFT), and a natural
bonding orbitals (NBO) analysis13 was applied. These data are
discussed below.
(TPB)Cu is a stable radical, and its room-temperature EPR
spectrum is shown in Figure 2B. The spectrum shown displays
a 13-line pattern whose faithful simulation requires the
inclusion of hyperfine coupling to both 63Cu (I = 3/2,
Aiso(63Cu) = 191 MHz) and 11B (I = 3/2, Aiso(11B) = 64 MHz),
in addition to the minor isotopes 65Cu (I = 3/2) and 10B (I =
3) at natural abundance. The doubling of the outmost lines
due to the different gyromagnetic ratios of 63Cu and 65Cu
confirms the assignment of the larger coupling constant to Cu.
An EPR spectrum of (TPB)Cu recorded at 77 K in frozen
toluene displays an axial signal with virtually isotropic g values
(g∥ = 2.006, g⊥ = 2.010) but strongly anisotropic hyperfine
tensors (A∥(63Cu) = 335 MHz, A⊥(63Cu) = 93 MHz, A∥(11B) =
110 MHz, A⊥(11B) = 40 MHz), consistent with the unpaired
electron being located in an orbital of σ symmetry delocalized
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Cu K-edge X-ray near-edge absorbtion spectroscopy
(XANES) data were obtained on single-crystal samples of
delocalization of that orbital into Cu 4s and 4p orbitals,
respectively, comparable in magnitude to those associated with
the P→Cu interactions (72.7 kcal/mol to Cu(4s), 82.5 kcal/
mol to Cu(4p)). While boron-centered radical anions had been
long known as solution species,22 and one example had been
isolated as a crystalline salt,23 the few known analogous
boron(I) dianions are highly unstable and involve extensive π-
delocalization.24,25
{(TPB)Cu}{BArF }, (TPB)Cu, and {K(benzo-15-C-5)2}-
4
{(TPB)Cu} to further explore how the relative state of
oxidation of the copper centers changes across the series, if at
all (Figure 3, left). Owing to the high oxygen sensitivity of these
ASSOCIATED CONTENT
* Supporting Information
Detailed experimental and spectroscopic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
jpeters@caltech.edu
■
Present Address
Figure 3. Cu K-edge XAS spectra collected on single-crystal samples
†Organic Chemistry & Catalysis, Debye Institute for Nanoma-
terials Science, Faculty of Science, Utrecht University,
Universiteitsweg 99, 3584 CG Utrecht, The Netherlands
of (TPB)Cu, {(TPB)Cu}{BArF }, and {K(benzo-15-C-5)2}{(TPB)-
4
Cu} (left), and a schematic orbital-interaction diagram illustrating
assigned transitions (right).
Notes
The authors declare no competing financial interest.
copper complexes, particularly anion 5, the single-crystal XAS
data were collected under a nitrogen atmosphere at 100 K.
Potential photon damage to the complexes during XANES
collection was ruled out by comparing the multiple sweeps of
XAS scans, as well as comparing the single-crystal XRD data
collected at the end of the XANES scans to the XRD data
independently collected and already described above. The
XANES spectrum of {(TPB)Cu}− is qualitatively similar to
those of other tetracoordinate Cu(I) complexes18 and exhibits a
first peak at ca. 8984 eV that is assigned to the dipole-allowed
Cu(1s)→Cu(4p) transition (labeled b, Figure 3). Formal one-
and two-electron oxidation of {(TPB)Cu}− to generate (TPB)
Cu and {(TPB)Cu}+, respectively, results in the appearance of
an additional band of comparable intensity but at appreciably
lower energy, ca. 8981 eV (labeled a, Figure 3). The band at ca.
8984 eV is preserved. The appearance of the new band at 8981
eV is unexpected for a copper center that is being formally
oxidized, but such a band is consistent with oxidation of
electrons in a B→Cu dative bond that is heavily polarized
toward boron. This additional band is therefore tentatively
assigned to a transition from Cu(1s) to an orbital of B(2p)
parentage, whose large intensity is due to strong mixing with
the Cu(4p) orbital (Figure 3 right). In {(TPB)Cu}− this
transition is not observed, presumably because the bonding
orbital is filled by two electrons.19
To summarize, we have provided the first structural snapshot
of one-electron σ-bonding in a complex, whose characterization
is complemented by the structures of its one-electron oxidized
and reduced relatives. The structural, spectroscopic, and
theoretical data in hand collectively suggest that sequential
reduction of the cation {(TPB)Cu}+ to (TPB)Cu and then to
{(TPB)Cu}− results in the gradual formation of a polar Cu−B
σ-bond to which the ionic resonance structures R3B•− Cu+ for
(TPB)Cu and R3B:2− Cu+ for {(TPB)Cu}− strongly contribute,
in agreement with the location of the spin density in (TPB)Cu
residing mostly on B (vide supra). In additional support of this
view, NBO analysis carried out on the anion {(TPB)Cu}−
identifies an occupied lone pair with a large p character (13.3%
s, 86.8% p) on the boron atom. Second-order perturbation
analysis associates energies of 69.2 and 56.5 kcal/mol with the
ACKNOWLEDGMENTS
■
This work was supported by the Gordon and Betty Moore
foundation. M.-E.M. acknowledges a Fellowship for Advanced
Researchers from the Swiss National Science Foundation. L.Z.
acknowledges a postdoctoral fellowship from Natural Sciences
and Engineering Research Council of Canada. We acknowledge
the Gordon and Betty Moore Foundation, the Beckman
Institute, and the Sanofi-Aventis BRP at Caltech for their
generous support of the Molecular Observatory at Caltech.
SSRL is operated for the DOE and supported by its Office of
Biological and Environmental Research, and by the NIH,
NIGMS (including P41GM103393), and the NCRR
(P41RR001209).
REFERENCES
■
(1) Pauling, L. J. Am. Chem. Soc. 1931, 53, 3225−3237.
(2) Clark, T. J. Am. Chem. Soc. 1988, 110, 1672−1678.
(3) Hudson, R. L.; Williams, F. J. Am. Chem. Soc. 1977, 99, 7714−
7716.
(4) Shida, T.; Kubodera, H.; Egawa, Y. Chem. Phys. Lett. 1981, 79,
179−182.
(5) Wang, J.; Williams, F. E. S. R. J. Chem. Soc., Chem. Commun.
1981, 666−668.
́
(6) Cataldo, L.; Choua, S.; Berclaz, T.; Geoffroy, M.; Mezailles, N.;
Ricard, L.; Mathey, F.; Le Floch, P. J. Am. Chem. Soc. 2001, 123,
6654−6661.
(7) Hoefelmeyer, J. D.; Gabbaï, F. P. J. Am. Chem. Soc. 2000, 122,
9054−9055.
(8) Canac, Y.; Bourissou, D.; Baceiredo, A.; Schoeller, W. W.;
Bertrand, G. Science 1998, 279, 2080−2082.
(9) Bontemps, S.; Bouhadir, G.; Dyer, P. W.; Miqueu, K.; Bourissou,
D. Inorg. Chem. 2007, 46, 5149−5151.
(10) Amgoune, A.; Bourissou, D. Chem. Commun. 2011, 47, 859−
871.
(11) (a) Hill, A. F.; Owen, G. R.; White, A. J. P.; Williams, D. J.
Angew. Chem., Int. Ed. 1999, 38, 2759−2761. (b) Parkin, G.
Organometallics 2006, 25, 4744−4747.
(12) (a) Crossley, I. R.; Hill, A. F.; Willis, A. C. Organometallics 2008,
27, 312−315. (b) Moret, M.-E.; Peters, J. C. Angew. Chem., Int. Ed.
2011, 50, 2063−2067. (c) Moret, M.-E.; Peters, J. C. J. Am. Chem. Soc.
3794
dx.doi.org/10.1021/ja4006578 | J. Am. Chem. Soc. 2013, 135, 3792−3795

Journal of the American Chemical Society
Communication
2011, 133, 18118−18121. (d) Suess, D. L. M.; Tsay, C.; Peters, J. C. J.
Am. Chem. Soc. 2012, 134, 14158−14164.
(13) Weinhold, F.; Landis, C. R. Valency and Bonding: A Natural
Bond Orbital Donor-Acceptor Perspective; Cambridge University Press:
Cambridge, UK, 2005.
(14) Brown, H. C.; Racherla, U. S. J. Org. Chem. 1986, 51, 427−432.
(15) Baiada, A.; Jardine, F. H.; Willett, R. D. Inorg. Chem 1990, 29,
4805−4808.
(16) Gaughan, A. P., Jr.; Dori, Z.; Ibers, J. A. Inorg. Chem. 1974, 7,
1657−1667.
(17) Sircoglou, M.; Bontemps, S.; Bouhadir, G.; Saffon, N.; Miqueu,
K.; Cu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.;
Ozerov, O. V.; Bourissou, D. J. Am. Chem. Soc. 2008, 130, 16729−
16738.
(18) George, G. N.; Pickering, I. J.; Yu, E. Y.; Prince, R. C.; Bursakov,
S. A.; Gavel, O. Y.; Moura, I.; Moura, G. J. G. J. Am. Chem. Soc. 2000,
122, 8321−8322.
(19) As a cautionary note, the polarization vector e was aligned nearly
⃗
perpendicular to the Cu-B bond for {K(benzo-15-C-5)2}{(TPB)Cu}.
Therefore, an alternative explanation for the absence of transition a in
{(TPB)Cu}− could be due to the polarization effect.20,21 For
complexes (TPB)Cu and {(TPB)Cu}+, however, e was aligned
⃗
predominantly along the Cu−B bond, and hence enhancement of
transition a would be anticipated.
(20) Templeton, D. H.; Templeton, L. K. Acta Crystallogr., Sect. A
1980, 36, 237−241.
(21) Smith, T. A.; Pennerhahn, J. E.; Berding, M. A.; Doniach, S.;
Hodgson, K. O. J. Am. Chem. Soc. 1985, 107, 5945−5955.
(22) Leffler, J. E.; Watts, G. B.; Tanigaki, T.; Dolan, E.; Miller, D. S. J.
Am. Chem. Soc. 1970, 92, 6825−6830.
(23) Olmstead, M. M.; Power, P. P. J. Am. Chem. Soc. 1986, 108,
4235−4236.
(24) Okada, K.; Kawata, T.; Oda, M. J. Chem. Soc., Chem. Commun.
1995, 233−234.
(25) Kinjo, R.; Donnadieu, B.; Celik, M. A.; Frenking, G.; Bertrand,
G. Science 2011, 333, 610−613.
3795
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