Fully Borylated Methane and Ethane by Ruthenium-Mediated Cleavage and Coupling of CO

Article abstract of DOI:10.1002/anie.201601121

Many transition-metal complexes and some metal-free compounds are able to bind carbon monoxide, a molecule which has the strongest chemical bond in nature. However, very few of them have been shown to induce the cleavage of its C-O bond and even fewer are those that are able to transform CO into organic reagents with potential in organic synthesis. This work shows that bis(pinacolato)diboron, B₂pin₂, reacts with ruthenium carbonyl to give metallic complexes containing borylmethylidyne (CBpin) and diborylethyne (pinBC≡CBpin) ligands and also metal-free perborylated C₁ and C₂ products, such as C(Bpin)₄ and C₂(Bpin)₆, respectively, which have great potential as building blocks for Suzuki-Miyaura cross-coupling and other reactions. The use of ¹³CO-enriched ruthenium carbonyl has demonstrated that the boron-bound carbon atoms of all of these reaction products arise from CO ligands.

Full text of DOI:10.1002/anie.201601121

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International Edition: DOI: 10.1002/anie.201601121
German Edition: DOI: 10.1002/ange.201601121
Borylation
Fully Borylated Methane and Ethane by Ruthenium-Mediated
Cleavage and Coupling of CO
Andrei S. Batsanov, Javier A. Cabeza,* Marco G. Crestani, Manuel R. Fructos, Pablo García-
lvarez, Marie Gille, Zhenyang Lin, and Todd B. Marder*
Abstract: Many transition-metal complexes and some metal-
free compounds are able to bind carbon monoxide, a molecule
which has the strongest chemical bond in nature. However,
very few of them have been shown to induce the cleavage of its
industrial processes, the latter is the only one in which the
À
C O bond of carbon monoxide is cleaved.
From the point of view of fundamental research, the
cleavage of CO under mild reaction conditions has long been
an attractive challenge because it could provide new ways of
transforming CO into highly functionalized C1, C2, and Cn
molecules. To date, apart from some ruthenium and osmium
carbonyl clusters, which are able to transform two CO ligands
into CO2 and a carbide ligand,[2,3] only a handful of transition-
metal complexes are known to cleave CO ligands with[4–6] or
without[7–16] the help of other reagents. These reactions have,
in some cases, afforded metal-free CO-derived organic
products,[4–6,13,16] but all these products are hydrocarbons.
In contrast, it has recently been shown that some reagents
of low-valent p-block elements are able to activate free or
metal-coordinated CO. Regarding boron-containing reagents,
Stephan and co-workers and Erker and co-workers have
reported that some frustrated Lewis pairs promote the
reduction of CO with either hydrogen[17] or HB(C6F5)2,[18,19]
Siebert and co-workers have reported the insertion of CO
into a cyclic diborane,[20] and Braunschweig and co-workers
have reported the coupling of CO at a boron–boron triple
bond[21] and at a metal–boron double bond.[22] By using other
p-block elements, we note that CO can be reduced by certain
singlet carbenes[23] and transient triplet carbenes,[24] a germy-
lene compound can couple two CO units head-to-head,[25,26]
and CO and N2 can react on a hafnium complex to give an
oxamide precursor.[27]
À
C O bond and even fewer are those that are able to transform
CO into organic reagents with potential in organic synthesis.
This work shows that bis(pinacolato)diboron, B2pin2, reacts
with ruthenium carbonyl to give metallic complexes containing
ꢀ
borylmethylidyne (CBpin) and diborylethyne (pinBC CBpin)
ligands and also metal-free perborylated C1 and C2 products,
such as C(Bpin)4 and C2(Bpin)6, respectively, which have great
potential as building blocks for Suzuki–Miyaura cross-cou-
pling and other reactions. The use of 13CO-enriched ruthenium
carbonyl has demonstrated that the boron-bound carbon
atoms of all of these reaction products arise from CO ligands.
T
he strongest chemical bond in nature is that of carbon
monoxide, which has a dissociation energy of 1076 kJmolÀ1 at
298 K. The robustness of this triple bond has a key influence
on the chemistry of this simple molecule.
Although carbon monoxide is inexpensive and readily
available, the synthetic chemical industry only uses it as
a feedstock in very few processes, such as the water-gas shift
reaction (synthesis of CO2 and H2 from CO and water), the
hydroformylation of olefins, the preparation of phosgene,
methanol, and acetic acid, and the Fischer–Tropsch synthesis
of hydrocarbons (hydrogenation of CO).[1] Among these
We now report that, by studying the reactivity of the
diborane(4) compound B2pin2 (pin = pinacolato) with ruthe-
[*] Prof. Dr. J. A. Cabeza, Dr. P. García-lvarez, Dr. M. Gille
Departamento de Química Orgµnica e Inorgµnica-IUQOEM
Centro de Innovación en Química Avanzada (ORFEO-CINQA)
Universidad de Oviedo-CSIC, 33071 Oviedo (Spain)
E-mail: jac@uniovi.es
À
nium carbonyl, we have discovered that the C O bond of
coordinated CO can be cleaved by borylation with B2pin2 to
give metallic complexes containing borylmethylidyne
ꢀ
(CBpin) and diborylethyne (pinBC CBpin) ligands and also
perborylated metal-free C1 and C2 products, that is, C(Bpin)4
and C2(Bpin)6, respectively, which have a great potential in
organic synthesis as highly functionalizable C1 and C2 building
Dr. A. S. Batsanov, Dr. M. G. Crestani, Dr. M. R. Fructos,
Prof. Dr. T. B. Marder
Department of Chemistry, Durham University
South Road, Durham DH1 3LE (UK)
À
blocks for Suzuki–Miyaura C C bond coupling and other
reactions.[28–31]
Prof. Z. Lin
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong (P.R. China)
Reactions of [Ru3(CO)12] with excess B2pin2 in octane at
1308C led to dark-red solutions which contained mixtures of
products (experimental details and characterization data are
given in the Supporting Information). Preparative column
and/or thin-layer chromatography allowed the isolation of
three novel tetraruthenium clusters, namely, [Ru4(m-k1O-
Prof. Dr. T. B. Marder
Current address:
Institut für Anorganische Chemie
Julius-Maximilians-Universität Würzburg
Am Hubland, 97074 Würzburg (Germany)
E-mail: todd.marder@uni-wuerzburg.de
OBpin)(m4-k1C-CBpin)(CO)12]
(1),
[Ru4(m-H)(m4-k1C-
CBpin)(CO)12] (2), and [Ru4{m4-k2C,C-C2(Bpin)2}(CO)12] (3;
Figure 1), in yields (5–35%) which depended upon the
concentration of the reagents and the (trace) water content
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under http://dx.doi.org/10.
1002/anie.201601121.
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The formation of carbide ligands (and
CO2) from coordinated CO is well known
in transition-metal carbonyl cluster
chemistry and proceeds by thermal activa-
tion,[2] but we do not think that 1–3 arise
from reactions of B2pin2 with carbide
cluster intermediates because this proposal
does not explain the formation of the
OBpin fragment of 1 (which is the major
reaction product), and because we have
experimentally checked that the treatment
of [Ru5(m5-C)(CO)12] with an excess of
B2pin2 in octane at 1308C does not lead to
1–3. In fact, we believe that the mechanism
À
of the carbonyl C O activation process
which leads to 1–3 may be similar to that
proposed for the synthesis of the tetrairon
cluster [Fe4(m-H)(m4-k1C-CAuPh3)(CO)12],
and involves intermediates having k2C,O-
coordinated CO ligands which are prone to
Figure 1. Carbon-borylated ruthenium clusters (1–3) and metal-free carbon-borylated prod-
ucts (4–7) isolated from the reaction of [Ru3(CO)12] with B2pin2.
À
suffer C O bond activation in the presence
of Lewis acids[38] (B2pin2 in our case).
of the solvent. Two clusters of higher nuclearity, [Ru5-
(m5-C)(CO)12][32] and [Ru6(m6-C)(CO)17],[33] which are known
to arise from the thermolysis of [Ru3(CO)12], were also
formed in these reactions in small amounts (< 6%).
With the aim of shedding light on the origin of the hydride
ligand of 2, we treated 1 with a small amount of D2O in octane
at 1308C, but no reaction occurred. However, an analogous
reaction carried out in the presence of B2pin2 allowed the
The structures of compounds 1–3, shown in Figure 1, were
determined by X-ray diffraction[42] (see the Supporting
Information). They all are tetranuclear, have a common
butterfly-type metallic skeleton, and are stabilized by twelve
terminal CO ligands, but they differ in their bridging ligands.
While 1 contains a quadruply-bridging borylated methylidyne
ligand (CBpin) and an edge-bridging oxoboryl ligand
(OBpin), 2 can be described as resulting from the formal
substitution of a hydride for the edge-bridging OBpin ligand
of 1. Remarkably, 3 possesses a diborylated alkyne ligand,
isolation
of
[Ru4(m-D)(m4-k1C-CBpin)(CO)12],
while
(pinB)2O was detected in the resulting solution by GC/MS
analysis. Therefore, 2 arises from 1 and its formation requires
the presence of both water and B2pin2. While moisture is the
source of the hydride ligand of 2, the role of the additional
B2pin2 should be the abstraction of the OBpin ligand from 1.
The generation of the diborylated acetylene ligand of 3,
which formally arises from the coupling of two CBpin units on
an undetected intermediate, represents a remarkable exam-
ple of reductive coupling and deoxygenation of CO ligands.
Although some boron-[21] and metal-mediated (see citations
in Ref. [20]) reductive couplings of two or more CO ligands
have already been reported, oxygen-free coupled products
are formed only when strong reducing conditions are
used,[5,6,8,12,13] but this is not the case in our system. For
example, the reductive coupling of the two pairs of carbonyl
ligands of the cluster [Fe4(h5-C5H5)4(m3-CO)4], to form the
bis(acetylene) derivative [Fe4(h5-C5H5)4(m4-C2H2)2], is only
achieved when a large excess of LiAlH4 is used.[12]
The results described above confirm the ability of B2pin2
to convert coordinated CO into three different types of
ligands, namely, borylalkylidyne, boryloxo, and diborylal-
kyne, but these already significant findings were surpassed
when the crude reaction solutions from the treatment of
[Ru3(CO)12] with B2pin2 were analyzed by GC/MS and NMR
spectroscopy (see the Supporting Information). These anal-
yses indicated that the cluster complexes described above
were accompanied by O(Bpin)2 and by small amounts of
various metal-free C-borylated products, namely, C(Bpin)4
(4), C2(Bpin)6 (5), HC(Bpin)3 (6), and H2C(Bpin)2 (7;
Figure 1; individual spectroscopic yields of < 5% are based
on initial B2pin2). To establish unequivocally their nature,
they were isolated by preparative HPLC and characterized by
ꢀ
pinBC CBpin, thus forming part of a distorted C2Ru4
octahedron. Metallic clusters containing borylated alkylidyne
ligands are rare, with the iron compounds [Fe4(m-H)(m4-k1C-
BXY)(CO)12] (X = Y= H, Cl, Br; X = H, Y= Cl, Br,
OH)[34,35] being the closest to 1 and 2, but they are formed
by borylation of carbon–halogen bonds. Although dibory-
lated alkynes are known,[36] they have never been prepared
from CO and their coordination to transition metals has been
hitherto observed only in the dicobalt complex [Co2{m-k2C,C-
C2(Bcat)2}(CO)6] (cat = catecholato).[37] The 13C NMR signals
of the ruthenium-bound carbon atoms of the CBpin frag-
ments of 1–3 (d = 303.8, 275.9, and 180.2 ppm, respectively)
were only observed in samples prepared using 60% 13CO-
enriched ruthenium carbonyl as a starting material. This fact
unequivocally demonstrates that the ruthenium-bound
carbon atoms of the CBpin fragments of 1–3 arise from CO
groups. A reaction carried out in the presence of 75% 18O-
enriched water helped determine that the ruthenium-bound
oxygen atom of the OBpin ligand in 1 does not arise from
water, because the mass spectra of samples of this compound
prepared with and without 18O-enriched water are identical.
Therefore, the CBpin and OBpin fragments of 1–3 arise from
CO ligands.
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standard analytical methods. We also determined the molec-
Erasmus Programme). We thank NetChem Inc. and Ally-
ular structures of 4 and 5 by single-crystal X-ray diffraction
(see the Supporting Information).[42] Compounds 4 and 6 have
been previously prepared in several steps by borylation of
CCl4 or CHCl3,[30,31] and compound 7 by treating B2pin2 with
diazomethane in the presence of [Pt(PPh3)4].[39] Compound 5
is a novel product, although a platinum-catalyzed double
diboration of the alkyne C2(Bcat)2 with B2cat2 has been
reported to afford the Bcat analogue of 5, C2(Bcat)6.[40]
Isotopic-labelling experiments (see the Supporting Infor-
mation) were used to determine the origin of the boron-
bound carbon atoms of 4–7 and the carbon-bound hydrogen
atoms of 6 and 7. The use of 13CO-enriched [Ru3(CO)12]
helped establish that the former arise from CO ligands of
[Ru3(CO)12] (13C-enriched 4–7 were obtained), while the
addition of D2O to the initial reaction mixture led to the
deuterium-labelled compounds DC(Bpin)3 and D2C(Bpin)2,
thus confirming that water is the source of the carbon-bound
hydrogen atoms of 6 and 7.
The use of small amounts of solvent in the reactions
(concentrated solutions) increased the yields of 1, 3, 4, and 5,
while diluted solutions favored the formation of 2, 6, and 7.
This observation not only supports that hydrolytic processes
participate in the synthesis of 2, 6, and 7 (the more dilute the
reaction solution the greater the water/reactants ratio), but
also demonstrates that, when working on a small scale,
residual water from “dry solvent” and/or from the glassware
surface allows the formation of hydrolyzed products. In fact,
the amount of water necessary to make 26 mg (0.030 mmol)
of 2 is only 0.27 mL.
Chem Co. Ltd. for gifts of diboron reagents and Dr. D. S. Yufit
for assistance with one of the X-ray structures.
Keywords: boron · borylation · carbon monoxide ·
cluster compounds · ruthenium
How to cite: Angew. Chem. Int. Ed. 2016, 55, 4707–4710
Angew. Chem. 2016, 128, 4785–4788
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Received: January 31, 2016
Published online: March 8, 2016
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