Metal-free binding and coupling of carbon monoxide at a boron-boron triple bond

Article abstract of DOI:10.1038/nchem.1778

Many metal-containing compounds, and some metal-free compounds, will bind carbon monoxide. However, only a handful of metal-containing compounds have been shown to induce the coupling of two or more CO molecules, potentially a method for the use of CO as a one-carbon-atom building block for the synthesis of organic molecules. In this work, CO was added to a boron-boron triple bond at room temperature and atmospheric pressure, resulting in a compound into which four equivalents of CO are incorporated: a flat, bicyclic, bis(boralactone). By the controlled addition of one CO to the diboryne compound, an intermediate in the CO coupling reaction was isolated and structurally characterized. Electrochemical measurements confirm the strongly reducing nature of the diboryne compound.

Full text of DOI:10.1038/nchem.1778

ARTICLES
PUBLISHED ONLINE: 13 OCTOBER 2013 | DOI: 10.1038/NCHEM.1778
Metal-free binding and coupling of carbon
monoxide at a boron–boron triple bond
Holger Braunschweig1 , Theresa Dellermann1, Rian D. Dewhurst1, William C. Ewing1, Kai Hammond1,
*
J. Oscar C. Jimenez-Halla1,2, Thomas Kramer1, Ivo Krummenacher1, Jan Mies1, Ashwini K. Phukan1,3
and Alfredo Vargas1,4
Many metal-containing compounds, and some metal-free compounds, will bind carbon monoxide. However, only a handful
of metal-containing compounds have been shown to induce the coupling of two or more CO molecules, potentially a
method for the use of CO as a one-carbon-atom building block for the synthesis of organic molecules. In this work, CO
was added to a boron–boron triple bond at room temperature and atmospheric pressure, resulting in a compound into
which four equivalents of CO are incorporated: a flat, bicyclic, bis(boralactone). By the controlled addition of one CO to the
diboryne compound, an intermediate in the CO coupling reaction was isolated and structurally characterized.
Electrochemical measurements confirm the strongly reducing nature of the diboryne compound.
arbon monoxide has long been available on an industrial scale to B(III), corresponding to a six-electron reduction of CO, and is
in the form of synthesis gas (syngas), a mixture of CO and H2 one of only a few instances of undisputable boron-centred
C
traditionally produced from coal. Efforts to use CO as a one- reduction. This rare reactivity pattern has only been observed pre-
carbon-atom (or C1) building block by coupling multiple CO units viously with electron-rich compounds such as the boryl anions20
arguably began in the early 1800s, with reports of reductive coupling and boron radicals21 of Yamashita and Nozaki, and the base-stabil-
of CO molecules by both Gmelin1 and Liebig2 to form salts of the ized borylene22 of Bertrand.
dianion [CnOn]22. Currently, CO is the major C1 feedstock of the
chemical industry, partly due to the prominence of the Fischer– Results and discussion
Tropsch process for the production of alkanes of varying lengths As a preliminary experiment to test the reactivity of our recently
using CO and dihydrogen. Although the Fischer–Tropsch process reported diboryne compound 123 with simple small molecules, we
generally uses late transition-metal catalysts, coupling of CO is treated a benzene solution of this with an excess of CO (Fig. 2) at
also possible using strongly-reducing early transition-metal com- room temperature. A red-orange solid sample of B2C4O4(IDip)2
plexes, beginning with samarium and later lanthanum complexes (3, IDip ¼ 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene; Fig. 2a,
(Evans and Atwood)3,4 (I, Fig. 1). Other reductive CO coupling reac- Supplementary Fig. S1) was isolated with one signal in its 11B
tions have involved yttrium, niobium and tantalum (Lippard)5, NMR spectrum (d ¼ 23.6) found at a much lower frequency than
uranium (Cloke, II, III, V; Arnold, VI; Fig. 1)6–9, rhenium that of precursor 1 (d ¼ 39), in the region usually restricted to tetra-
(Labinger and Bercaw)10 and tantalum (Kawaguchi, IV; Fig. 1)11.
hedrally coordinated boron atoms. The 1H-decoupled 13C spectrum
Reduction of CO with neutral p-block compounds is potentially of 3 indicated the addition of two new quaternary carbon nuclei to
a route to neutral CO-coupled products. However, to our knowl- the molecule. The connectivity of compound 3 was identified by
edge, the only CO-coupling processes using p-block (albeit metallic) single-crystal X-ray crystallography (Fig. 3). The most salient fea-
elements are those presented by Power with a germylene compound, tures of the molecular structure of 3 are the planarity of the bicyclic
which can head-to-head couple two CO units (VII, Fig. 1)12,13
.
C6B2O4 bis(boralactone) core including the carbene carbon atoms
Siebert showed that CO could be inserted into a cyclic diborane, and the multiple bonding in the B–C–C–B portion of the molecule.
but this led to unusual rearrangements rather than CO coupling14. The boron atoms are in an environment reminiscent of borata-
Transient triplet carbenes15 and certain stable singlet carbenes16 alkenes of the form [R2B ¼ CR2′ ]2, previously reported by the groups
react with CO, in some cases leading to reduction, but no coupling of Power24 and Gabba¨ı25,26. In both cases, planar, tricoordinate
has been observed. Some ‘frustrated Lewis pairs’ (FLPs)17 also react boron atoms bear one very short B–C distance that can be described
with CO, and a compound was recently reported in which two CO as a double bond (B1–C1 distance in 3, 1.4507(18) Å; B–C distance
units and 1 equiv. of H2 derived from syngas were coupled in a in borataalkenes, 1.438(9)–1.475(6) Å). However, the similarity
head-to-tail manner using such an FLP18. The disilyne (Si;Si) of between 3 and borataalkenes does not extend to their 11B NMR
Sekiguchi has been found to couple some unsaturated compounds, parameters, signals of which range from 35.0 to 45.5 ppm in the
but no reactivity with CO has been reported19. Here, we provide a latter. The C1–C1′ distance (1.426(2) Å) is much shorter than a
report of CO coupling by a single main-group compound, a reaction regular C–C single bond, and is even shorter than the central
in which a boron–boron triple bond is broken and four CO mol- C–C single bond in a comparable bis(lactam) derived from pulvinic
ecules assemble a planar, carbene-stabilized, bis(boralactone). The acid (1.489(4) Å)27, suggesting a certain amount of double-bond
reaction is formally an oxidation of two zerovalent boron atoms character between these two atoms. Overall, the three B–C–C–B
1Institut fu¨r Anorganische Chemie, Julius-Maximilians-Universita¨t Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany, 2Department of Chemistry (DCNE),
Universidad de Guanajuato, Noria Alta s/n 36050, Guanajuato, Mexico, 3Department of Chemical Sciences, Tezpur University, Napaam 784028, Assam,
India, 4Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, Sussex, UK. e-mail: h.braunschweig@uni-wuerzburg.de
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1778
ARTICLES
O
O
THF
L
C
C
M
R
R
L
M
L
L
O
O
L'
O
O
O
L
C
C
C
C
C
O
C
C
C
R
R
U
U
U
U
O
C
O
R
R
O
R
L'
L
C
C
L
L
M
L
R
M
II
III
L
THF
O
O
I
R
L
R
2–
H
U
O
C
C
O
U
C
O
R
L
Ta(OOO)
O
O
C
V
THF
Ge
O
R
O
C
C
(OOO)Ta
C
C
C
C
O
Ta(OOO)
THF
N(SiMe3)2
O
(Me3Si)2N
VII
N(SiMe3)2
U
O
C
C
O
U
O
(OOO)Ta
H
(Me3Si)2N
(Me3Si)2N
N(SiMe3)2
VI
IV
M = La, Sm; L = C5Me5; L' = C5HMe4; R = SiiPr3
H3(OOO) = 2,6-bis(3-tert-butyl-5-methyl-2-hydroxybenzyl)-
4-tert-butylphenol
Figure 1 | Selected products of published metal-mediated CO-coupling reactions. Note that most of these coupling procedures function in a head-to-head
(C-to-C) manner. The coupled CO units are highlighted in red.
N
Dip
N
O
a
Dip
Dip
Dip
Dip
N
Dip
N
C
N
N
N
N
O
B
2 equiv. CO,
–78 ºC
C
C
1 atm, CO, r.t.
B
B
B
B
O
C
C
O
B
O
N
N
Dip
Dip
Dip
Dip
Dip
N
N
Dip
1
2
3
Dip = 2,6-diisopropylphenyl
1 atm, CO, r.t.
b
N
Dip
N
Dip
N
Dip
N
N
N
Dip
Dip
Dip
O
B
O
B
O
B
O
O
O
O
O
O
B
O
B
O
B
O
Dip
Dip
Dip
N
N
N
N
Dip
N
Dip
N
Dip
3a
3c
3b
Figure 2 | Synthesis of 2 and 3 and some plausible electronic descriptions of 3. a, Ambient temperature and pressure CO coupling by a main group species.
It should be noted that 2 equivalents of CO were added to the reaction forming 2, but only 1 equivalent is incorporated into the product, presumably due to its low
solubility and precipitation from the solvent, and/or the high stability of 2 relative to any dicarbonyl compounds. b, Some plausible electronic descriptions of 3.
bonds are too short to be considered conventional single bonds, core of the molecule as a 1,4-dibora-1,3-butadiene as in structure
leading us to infer some delocalization over this area of the molecule 3b. The central B2C2 core of the HOMO-1, HOMO and LUMO
and propose the two resonance forms 3b and 3c (Fig. 2b) as possible (Supplementary Fig. S4) resemble the same three orbitals of
contributors to the electronic structure of 3. At ambient tempera- 1,3-butadiene, albeit with a small amount of participation from
ture, no signal corresponding to a diradical as in resonance form p orbitals of the carbene and carboxylate carbons in 3. The
3c could be detected in the electron paramagnetic resonance bis(boralactone) 3 was also found to be fluorescent, in analogy to
(EPR) spectrum of 3, although this does not exclude a diradical the structurally very similar pulvinic acid lactones found in
structure completely.
fluorescent lichens28,29. Ultraviolet-visible (UV-vis) and fluorescent
Determination of the frontier orbital topology of 3 by density spectra, as well as time-dependent DFT calculations of 3, are
functional theory (DFT) confirms the description of the B–C–C–B provided in Supplementary Fig. S6 and Table S2.
2
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ARTICLES
O1
C1
O1
O2
C2
C2
B1
C11'
C1'
C2'
B2
C3
B1'
O2'
C1
O1'
B1
C11
2
3
Figure 3 | Molecular structures of 2 and 3. Thermal ellipsoids in the structures represent the 50% probability level. For clarity, hydrogen atoms and one
molecule of benzene solvent (structure of 3) have been removed. Bond lengths (Å) and angles (deg) relevant to the B–B–C–O core of 2: B1–B2, 1.549(3);
C1–B1, 1.565(3); C1–B2, 1.485(3); O1–C1, 1.249(2); O1–C1–B2, 155.38(17); O1–C1–B1, 143.62(16); B2–C1–B1, 61.00(13). The CO is bound unsymmetrically, with a
C1–B2 distance that is significantly shorter than the C1–B1 distance. Nevertheless, the B–B distance still corresponds to a double-bond description. Bond
lengths (Å) relevant to the B–C–C–B core of 3: B1–C1, 1.4507(18); B1–C2, 1.5742(18); B1–C11, 1.5471(18); C1–C1′, 1.426(2). The three B–C–C–B bonds are very
short—too short to be considered conventional single bonds, and we propose some delocalization as illustrated by the resonance forms in Fig. 2b.
Although the aforementioned reaction of 1 with a vast excess of Fig. S2). The potential of the first oxidation step is close to those
CO proceeds rapidly at room temperature, we sought to slow the of cobaltocene [(h5-C5H5)2Co]32 and samarium diiodide33, the
reaction down by adding discrete amounts of CO at lower tempera- latter a common reagent for organic reductions, indicating that 1
tures. Two equivalents of CO were added using a glass tube of is a potent neutral, metal-free reductant. Strong main-group reduc-
measured volume to a hexane solution of 1 at 278 8C, leading to tants such as the bis(monovalent) magnesium (MgIMgI) reagent of
the precipitation of an orange-brown solid (2, Fig. 2a). This solid Jones34 have attracted much attention lately and, based on the results
was stable at room temperature under inert conditions. Although herein, diboryne 1 is potentially a useful new member of this family.
a single 11B NMR signal (d ¼ 17) was observed for 2, multinuclear
The reaction of CO with the triply bonded diboryne compound 1
NMR techniques did not provide enough information to unambigu- shows that CO coupling is both possible and highly favoured ener-
ously ascertain its structure. However, single-crystal X-ray crystallo- getically on a non-metallic framework. The reaction is facile under
graphy confirmed that 2 was the product of a single CO addition to mild conditions and at atmospheric pressure. Both the CO-coupling
1 containing an unsymmetrically bridging CO (Fig. 3). The B–B dis- reaction and electrochemical measurements confirm the strongly
tance in 2 (1.549(3) Å) corresponds to a double-bond description reducing nature of 1. Additionally, the binding of one molecule of
(B ¼ B bond distances in doubly-carbene-stabilized diborenes: CO to 1 elucidates the first step of the CO-coupling reaction.
1.546(6), 1.561(18) Å)23,30. The C–O distance of 2 (1.249(2) Å) is There is mounting recent evidence that low-valent boron species
much longer than those calculated for CO adducts of boranes are able to act like transition metals by binding traditional ligands
(F3B ꢀ CO, 1.128 Å; (F5C6)3B ꢀ CO, 1.131 Å)31, clearly indicating of inorganic chemistry35. Both intermediate 2 and product 3 pre-
strong backdonation from the B2 unit into the p* orbital of CO. A sented herein provide further support for this hypothesis.
band in the infrared spectrum of 2 at 1,926 cm21 is clearly attribu-
Methods
table to the semi-bridging carbonyl group.
Synthetic methods. All manipulations were conducted under an atmosphere of
Compound 2 could then be converted quantitatively to the
dry argon or under vacuum using standard Schlenk line or glovebox techniques.
bis(boralactone) 3 by treatment with an excess of CO at room
temperature, confirming its intermediacy in the CO-coupling
reaction. Alternatively, when 1 was treated with 2 equiv. of CO in
toluene at room temperature, a mixture of 2 and 3 was observed
by 11B NMR. The coexistence of intermediate 2 and product 3 in
Solvents (pentane, hexane, toluene and tetrahydrofuran) were purified by distillation
from appropriate drying agents (NaK2.8 alloy and sodium/benzophenone) under
dry argon immediately before use. C6D6 was degassed by three freeze–pump–thaw
cycles and stored over molecular sieves.
Spectroscopic methods. NMR spectra were acquired on a Bruker Avance 500
1
this reaction suggests that 2 is probably the most stable, and NMR spectrometer. 1H and 13C{ H} NMR spectra were referenced to external
tetramethylsilane via the residual protons of the solvent (1H) or the solvent itself
perhaps the only isolable, intermediate species in this reaction.
11
.
(
13C). B NMR spectra were referenced to external BF3 OEt2. UV-vis spectra
Compounds 1–3 were evaluated using Kohn–Sham DFT methods
at the OLYP/DZP level, giving excellent reproduction of the
geometries (Supplementary Table S1, Figs S3, S5 and S7). These
calculations showed that the conversion of 1 þ CO to 2 is exother-
mic by 26.23 kcal mol21, and the conversion of 2 þ 3CO to 4 is
exothermic by 66.50 kcal mol21, indicating that the overall reaction
were measured on a JASCO V-660 UV-vis spectrometer. Elemental analyses were
measured on an Elementar Vario MICRO cube instrument. Starting material
1 was synthesized according to literature methods23
.
Crystallographic methods. The crystal data of 2 and 3 were collected on a Bruker
X8Apex II diffractometer with a charge-coupled device (CCD) area detector and
multilayer mirror monochromated MoKa radiation. The structure was solved using
direct methods, refined with the Shelx software package and expanded using
Fourier techniques. All non-hydrogen atoms were refined anisotropically. Hydrogen
atoms were included in structure factor calculations. All hydrogen atoms were assigned
to idealized geometric positions. Crystallographic data have been deposited with the
Cambridge Crystallographic Data Centre, and allocated the reference numbers CCDC
935712 (2) and 913268 (3). These data can be obtained free of charge from
www.ccdc.cam.ac.uk/data_request/cif.
(1 þ 4CO to 4) is exothermic by 92.73 kcal mol21
.
To quantify the apparent strongly reducing properties of 1, cyclic
voltammetric measurements on diboryne compound 1 show an
extremely facile and reversible one-electron oxidation step
(E1/2 ¼ 21.3 V versus ferrocene/ferrocenium, Fc/Fcþ) followed by
a second, irreversible oxidation (Epa ¼ þ0.1 V) (Supplementary
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1778
ARTICLES
Electrochemical methods. Cyclic voltammetry experiments were performed using a
Gamry Instruments Reference 600 potentiostat. A standard three-electrode cell
configuration was employed using a platinum disk working electrode, a platinum
wire counter electrode, and a silver wire, separated by a Vycor tip, serving as the
reference electrode. Formal redox potentials are referenced to the Fc/Fcþ redox
couple. Tetra-n-butylammonium hexafluorophosphate ([n-Bu4N][PF6]) was used
as the supporting electrolyte.
18. Dobrovetsky, R. & Stephan, D. W. Stoichiometric metal-free reduction of CO
in syn-gas. J. Am. Chem. Soc. 135, 4974–4977 (2013).
19. Asay, M. & Sekiguchi, A. Recent developments in the reactivity of stable
disilynes. Bull. Chem. Soc. Jpn 85, 1245–1261 (2012).
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boryllithium: synthesis, structure, and reactivity. J. Am. Chem. Soc. 130,
16069–16079 (2008).
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with amines. Science 333, 610–613 (2011).
23. Braunschweig, H. et al. Ambient-temperature isolation of a compound
with a boron–boron triple bond. Science 336, 1420–1422 (2012).
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Received 12 May 2013; accepted 5 September 2013;
published online 13 October 2013
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Acknowledgement
This work was supported by the Deutsche Forschungsgemeinschaft
(grant no. BR 1149/13-1).
Author contributions
H.B. conceived and supervised the study. J.M., T.D., K.H. and W.C.E. performed the
syntheses and spectroscopic studies. J.O.C.J.H., A.V. and A.K.P. performed the DFT
computational studies. I.K. performed the EPR spectroscopic and cyclic voltammetry.
T.K. performed the X-ray crystallographic measurements. R.D.D. wrote the paper.
All authors discussed the results and commented on the manuscript.
Additional information
15. Sander, W., Bucher, G. & Wierlacher, S. Carbenes in matrices—spectroscopy,
structure, and reactivity. Chem. Rev. 93, 1583–1621 (1993).
Supplementary information and chemical compound information are available in the
online version of the paper. Reprints and permissions information is available online
at www.nature.com/reprints. Correspondence and requests for materials should be
addressed to H.B.
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Competing financial interests
The authors declare no competing financial interests.
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