Cooperative carbon-atom abstraction from alkenes in the core of a pentanuclear nickel cluster

Article abstract of DOI:10.1038/nchem.2840

Although the cleavage of C-C bonds in unactivated hydrocarbons by soluble transition-metal complexes remains a challenge, such reactions hold the potential to provide access to previously inconceivable skeletal transformations. For instance, one can imagine the dismantling and reassembly of C-C and C-H bonds commonly observed in surface catalysis, but with the increased control innate to homogeneous catalysis. Here we report a pentanuclear nickel cluster that is unreactive to functional groups, such as alcohols, amines and even water, but selectively cleaves the C=C bonds of simple alkenes, such as ethylene, styrene and isobutylene, at temperatures as low as 30 °C and in near-quantitative yields. The isolation of intermediates in reactions with styrene and isobutylene demonstrates that the five nickel centres cooperatively activate three C-H bonds of the alkene substrate before cleaving the C-C bond in the core of the cluster to give a pentanuclear nickel carbide. The net organic product transformation is the abstraction of a carbon atom from an alkene.

Full text of DOI:10.1038/nchem.2840

ARTICLES
PUBLISHED ONLINE: 7 AUGUST 2017 | DOI: 10.1038/NCHEM.2840
Cooperative carbon-atom abstraction from alkenes
in the core of a pentanuclear nickel cluster
Manar M. Shoshani and Samuel A. Johnson
*
Although the cleavage of C–C bonds in unactivated hydrocarbons by soluble transition-metal complexes remains a
challenge, such reactions hold the potential to provide access to previously inconceivable skeletal transformations. For
instance, one can imagine the dismantling and reassembly of C–C and C–H bonds commonly observed in surface catalysis,
but with the increased control innate to homogeneous catalysis. Here we report a pentanuclear nickel cluster that is
unreactive to functional groups, such as alcohols, amines and even water, but selectively cleaves the C=C bonds of simple
alkenes, such as ethylene, styrene and isobutylene, at temperatures as low as −30 °C and in near-quantitative yields. The
isolation of intermediates in reactions with styrene and isobutylene demonstrates that the five nickel centres cooperatively
activate three C–H bonds of the alkene substrate before cleaving the C–C bond in the core of the cluster to give a
pentanuclear nickel carbide. The net organic product transformation is the abstraction of a carbon atom from an alkene.
olecular clusters have the potential to display reactivity ethylene, one of which is hydrogenated to give ethane. The
comparable to that of metal surfaces in the catalysis of second equivalent of ethylene is converted into methane via the
M
difficult bond transformations that feature multiple C−H net loss of a carbon atom. The reaction of 1 with ethylene was
bond activations and C−C bond skeletal rearrangements1–6. The conducted in an NMR probe cooled to –80 °C, and revealed that
low activation barriers for these transformations on surfaces can cluster 2, ethane and methane are produced on warming above −30 °C.
be ascribed to cooperative reactivity between metal centres;
An ORTEP (Oak Ridge thermal-ellipsoid plot program) depic-
however, the presence of multiple adjacent metal sites is insufficient tion of the solid-state structure of 2, as determined by X-ray
for cooperative catalysis, and the full potential of cooperative reactiv- crystallography, is shown in Fig. 2. The 68-electron [Ni5]8+
ity is rarely actualized. Even on heterogeneous surface catalysts, reac- carbide cluster 2 features a central carbide, C(1), which lies 0.372
tion sites are often structure sensitive7–9. For example, catalysis on (2) Å above the approximate square base of Ni(2), Ni(3), Ni(4)
Ni(111) surfaces frequently occurs at steps rather than on terraces; and Ni(5). Ni(1) sits above this plane, bound only to C(1), Ni(3),
these sites are so reactive that they facilitate the room-temperature Ni(1) and Ni(2). The Ni−C(1) distances to the Ni(2), Ni(3),
cleavage of ethylene to give nickel carbide from multiple C−H and Ni(4) and Ni(5) square base vary from 1.818(2) to 1.854(2) Å.
C=C bond cleavages10. Model complexes for surface C–C bond clea- The Ni(1) site is disordered, and was modelled with capping
vage and the more intriguing mechanistic microscopic reverse, C–C Ni(1A), Ni(1B) and Ni(1C) that bridge Ni(2) and N(3), Ni(2)
bond formation from C1 feedstocks, are entirely lacking11. Although and Ni(5), or Ni(4) and Ni(5), respectively.
mononuclear complexes that cleave C−C bonds are known, many of
Solution NMR spectroscopy of the crude reaction mixture
them rely on activated substrates12–15, or on highly reduced metal demonstrates a clean conversion into 2, which is fluxional. The
centres incompatible with the majority of functional 31P{1H} NMR spectrum has a single resonance at δ = 52.1. The 1H
groups1,3,6,16,17. Despite focused efforts to design reactive polynuclear and 13C{1H} NMR spectra confirm the conversion to a single
complexes with both monodentate18–24 and polydentate ligands25–32
,
cluster, with a set of resonances that correspond to five equivalent
few molecular clusters are known that selectivity react with hydro- fluxional iPr3P moieties and an upfield signal at δ = −7.15 that inte-
carbons via multiple C−H and C–C bond activation2, and invariably grate to four hydride ligands. The hydride signal is a sextet from
the voracious reactivity and incompatibility of these complexes with coupling to five 31P nuclei with an average JPH of 2.5 Hz. The H
1
functional groups limits their catalytic potential.
NMR spectrum of the crude reaction mixture also has two singlets
The pentanuclear cluster [(iPr3P)Ni]5H6 (1) has been shown to at δ = 0.84 and δ = 0.16 from ethane and methane, respectively. As
perform catalytic H/D exchange with unactivated arenes33 and for 1, cooling solutions of 2 to as low as –90 °C did not slow the flux-
ultradeep hydrodesulfurization34. Both reactions are slow at room ional process enough to cause decoalescence of any of the 1H or 31
temperature. The cleavage of strong unactivated C=C bonds in NMR resonances.
P
alkenes is a difficult transformation, because of the substantial
The reaction of 1 with 13C-labelled ethylene provided the
bond-dissociation energy35 and the anticipated involvement of an 13C-labelled cluster (2-13C). The 13C{1H} NMR spectrum of 2-13C
intermediate with an unreactive C–C bond. These C=C cleavages has a sextet for the carbide from coupling to five phosphines, at a
are typically accomplished via oxidative processes36. The addition downfield shift of δ = 446.4 with JCP = 20.5 Hz. The 31P{1H}
2
of two equivalents of ethylene to a solution of the pentanuclear NMR spectrum of 2-13C features a doublet instead of a singlet,
hydride 1 in n-pentane at room temperature gave an immediate with JCP = 20.5 Hz. The 13C NMR spectrum of the crude reaction
2
and clean conversion into [(iPr3P)Ni]5(µ2-H)4(µ5-C) (2), as mixture allowed confirmation of the hydrocarbon products 13CH4
shown in Fig. 1a. Cluster 2 has two fewer hydrides than 1, in agree- and 13C2H6 as a pentet at δ = −4.90 and a second-order multiplet
ment with the stoichiometric requirement of two equivalents of at δ = 6.79 in deuterated tetrahydrofuran (THF-d8), respectively.
Department of Chemistry and Biochemistry, University of Windsor, Sunset Avenue 401, Windsor, Ontario N9B 3P4, Canada.
e-mail: sjohnson@uwindsor.ca
*
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1

NATURE CHEMISTRY DOI: 10.1038/NCHEM.2840
ARTICLES
PiPr3
c
b
+ H2
C
+ 2PiPr3
25 °C
Ni
+ 5 PhCH=CH2
–40 °C
C
H
Ni
Ni
– 3 PhCH2CH3
– 2PiPr3
–PhEt
Ni
H
iPr3P
iPr3P
Ni
–
H
3
CH3
iPr3P
PiPr3
Ni
a
Ni
+ 2 H2C=CH2
>–30 °C
H
H
H
H
C
iPr3P
H
PiPr3
iPr3P
Ni
PiPr3
H
PiPr3
Ni
Ni
Ni
Ni
Ni
H
– C2H6
– CH4
H
Ni
H
iPr3P
Ni
[(iPr3P)Ni]5H6
iPr3P
PiPr3
H
1
2
PiPr3
d
e
+ H2
25 °C
CH2
H3C
+ 2 Me2C=CH2
0 °C
Ni
C
H
– Me3CH
– H2 (removed
by vacuum)
–
H3C
CH3
C
H
iPr3P
Ni
Ni
Ni
Ni
H
PiPr3
H
H
PiPr3
iPr3P
4
Figure 1 | Cooperative carbon-atom abstraction from alkenes by [(iPr3P)Ni]5H6 (1). a, Synthesis of 2 from the reaction of ethylene with 1. b, Synthesis of
3 from the reaction of styrene with 1. c, Conversion of 3 into 2 by reaction with H2 and iPr3P. d, Synthesis of 4 from the reaction of isobutene with 1.
e, Conversion of 4 into 2 by reaction with H2. The cleavage of the C=C bonds in these substrates under such mild conditions is mediated by the cooperative
activation of the C–H and C–C bonds by all five nickel centres, as evidenced by the structures of intermediates 3 and 4.
3-hexyne and neopentyl chloride. With two exceptions, the addition
P(1)
of ethylene to these mixtures at –40 °C produced an equivalent of
ethane, methane and the carbide cluster 2, in greater than 70%
NMR yield (Supplementary Table 1). One exception was
3-hexyne, which also generated methane and 2, but an equivalent
of 3-hexyne was hydrogenated to give 3-hexene in lieu of ethane.
Ni(1)
The second exception was the reaction in the presence of neopentyl
chloride, which still provided ethane and methane from the selective
activation of ethylene, but neopentyl chloride was observed to react
H(4)
P(2)
Ni(2)
Ni(5)
P(5)
with cluster 2; further work is underway to examine the reactivity of
2 and the nature of this product. Only a few functional groups were
H(1)
C(1)
H(3)
not tolerated. For example, both benzyl chloride and propionitrile
were observed to react rapidly with 1 at –40 °C.
Ni(3)
Ni(4)
The isolation of intermediates in the reactions with alkenes other
P(3)
P(4)
than ethylene gave insight into how the five nickel centres cooperatively
abstract C from alkenes. When 1 was reacted with styrene, the immedi-
ate 1H and 31P{1H} NMR spectra displayed an intermediate that con-
verted into 2 over the course of 16 hours. The addition of excess
styrene to 1 in a solution of n-pentane at −40 °C followed by recrystal-
lization provided the intermediate to the C=C bond cleavage,
(iPr3P)3Ni5(µ2-H)3(CCPh)(H2C=CHPh) (3), as shown in Fig. 1b.
The stoichiometry of the reaction involves five equivalents of styrene.
Two equivalents of styrene are incorporated in the product. One is
triply C−H activated to give a bent PhCC fragment, and the second
styrene acts as bridging ligand π bound over three nickel centres.
H(2)
Figure 2 | ORTEP depiction of the solid-state molecular structure of 2,
with 50% probability ellipsoids. Carbons and hydrogens associated with the
phosphine ligands and disorder of Ni(1) are omitted for clarity. The carbon
atom abstracted from the alkene is in a distorted five-coordinate geometry,
with the low-coordinate Ni(1) centre shifted towards the Ni(2)–Ni(3) edge
of the square base of the 68-electron [Ni5]8+ carbide cluster.
To demonstrate the remarkable selectivity in the C=C bond The complex has only three hydride ligands, and along with the
activation, cluster 1 was mixed with a variety of molecules that three hydrogens lost from the activated styrene, the total of six hydro-
contained functional groups, which included n-butanol, diisopro- gens lost are all incorporated into ethyl benzene from the hydrogen-
plyamine, 3-pentanone, isopropyl acetate, N,N-dimethylformamide, ation of three equivalents of styrene, as quantified by integration of
2
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2840
ARTICLES
Cluster 3 demonstrates how the Ni5 core of 1 can open to
cooperatively triply C–H activate alkenes, and ultimately facilitate
C−C bond cleavage to give 2 and the formation of three new C–H
bonds to give toluene; however, the additional π-bound styrene is
a factor in the isolation of 3 that may not be relevant to the
activation of ethylene or other alkenes. To demonstrate the scope
of substituted alkenes that can undergo carbon-atom abstraction
using 1 and to reveal mechanistic similarities, the isolation of an
intermediate using a different substrate was explored. The reaction
of 1 with isobutylene yielded 2 along with an equivalent of both
isobutane and propane; however, a short-lived intermediate (4)
was also observed when the reaction was monitored by 31P{1H}
C(7)
C(6)
C(8)
C(5)
C(3)
C(2)
H(1)
C(4)
P(1)
C(14)
Ni(1)
Ni(5)
C(13)
P(2)
Ni(2)
C(15)
C(16)
1
and H NMR spectroscopy. Under typical reaction conditions, the
C(1)
isolation of 4 was impossible because of its rapid conversion into
2; however, the reaction of a large excess of isobutylene with 1 at
0 °C followed by the application of a vacuum allowed the characteri-
zation of [(iPr3P)Ni]5(µ2-H)5(CC(Me)=CH2) (4), as shown in
Fig. 1d. The removal of H2 gas from the reaction using a vacuum
is necessary for the crystallization of 4, although the thermal
instability of 4 prevented its isolation in the absence of 2. When
hydrogen gas is added to a solution of 4, conversion into 2 along
with propane occurs via C–C bond cleavage and the formation of
three new C−H bonds, as shown in Fig. 1e.
C(12)
H(2)
C(11)
Ni(3)
Ni(4)
H(3)
C(10)
C(9)
P(3)
Figure 3 | ORTEP depiction of the solid-state molecular structure of 3,
with 50% probability ellipsoids. Carbons and hydrogens associated with the
phosphine ligands and disorder of C(9) and C(10) are omitted for clarity.
This 72-electron [Ni5]6+ cluster reveals the triple C–H bond activation of one
styrene vinyl moiety, shown in blue, which is coordinated to all five nickel
centres prior to C–C cleavage. Two iPr3P groups are replaced by a
coordinated styrene moiety, shown in red.
The solid-state structure of 4 is shown in Fig. 4. The structure
features multiple similarities to clusters 2 and 3, but with Ni(1)
attached only to Ni(2). A hydride bridges this connection to the
approximately square base of Ni(2), Ni(3), Ni(4) and Ni(5). As
for cluster 3, triple C−H bond activation of isobutylene is observed,
which includes the vinylic C=CH2 bonds and an sp3 C−H bond of
the 1H NMR spectrum of the crude reaction mixture. The unactivated one of the Me groups. Two-site disorder was modelled for the
bridging styrene displaces two iPr3P ligands from the cluster.
bound isobutylene fragment. The C(2)−C(3) bond distance is
The solid-state structure of 3 was determined by X-ray crystallo- 1.380(14) Å and the C(1)−C(2) bond distance is 1.475(9) Å. Short
graphy, and is displayed in Fig. 3. The solid-state structure of cluster bonding contacts are observed between C(1) and all the nickel
3 features many similarities to that of cluster 2. The capping Ni(1) centres, with Ni(1) η3 bound to the C(1)−C(2)−C(3) fragment.
bonds to C(2) and retains short distances to only two of the nickel
The 31P{1H} NMR spectrum of the 70-electron [Ni5]8+ inter-
centres in the base, which opens up the cluster to provide a binding mediate 4 features a singlet at δ = 47.3, and selective 1H decoupling
pocket for the substrate. The Ni(2), Ni(3), Ni(4) and Ni(5) centres of the isopropyl substituent hydrogens resulted in a sextet with a JPH
distort away from a square planar to a butterfly geometry, and all of 6.5 Hz. The 1H NMR spectrum displays a similar 6.5 Hz sextet at
bind to the terminal C(1) of the PhCC fragment. Ni(5) features δ = –22.05 for the hydrides. This is consistent with the solid-state
nearly equal bond lengths to both C(1) and C(2). The C(1)−C(2) structure undergoing a rapid fluxional exchange of the five
1
bond length of 1.397(5) Å along with the bent C(1)−C(2)−C(3) hydride and phosphine environments. The H NMR spectrum of
bond angle are suggestive that some double-bond character of
styrene is maintained after the triple C–H bond activation. The π-
bound styrene exhibits disorder where the two different faces of
the vinyl group are bound. This disorder is maintained in solution,
P(1)
C(4)
C(3)
and results in a major and a minor isomer being observed by 1H, 13
C
{1H} and 31P{1H} NMR spectroscopy. Unlike 1 and 2, the C1
symmetric 72-electron [Ni5]6+ cluster 3 is not fluxional, and all
the anticipated resonances were accounted for and are in agreement
with the presence of two isomers in the solid-state structure of 3. In
an attached proton test 13C{1H} NMR spectrum, the α and β carbons
of the PhCC moiety, located at δ = 150.4 (d) and δ = 242.2 (dd)
for the major isomer, were phased consistent with the absence
of attached hydrogens. A 1H-13C heteronuclear single-quantum
correlation (HSQC) NMR experiment also showed no proton
correlation to the α and β carbons.
Ni(1)
H(1)
C(2)
C(1)
P(3)
H(3)
Ni(3)
Ni(2)
H(2)
H(5)
P(5)
Ni(4)
Ni(5)
H(4)
To confirm that cluster 3 is an intermediate of the C−C bond
cleavage rather than a result of a non-productive side reaction, an
P(4)
P(2)
isolated sample of 3 in a solution of deuterated benzene (C6D6)
i
was exposed to Pr3P and an atmosphere of dihydrogen, as shown Figure 4 | ORTEP depiction of the solid-state structure of cluster 4, with
in Fig. 1c. After four hours, 1H and 31P{1H} NMR spectra revealed 50% probability ellipsoids. Carbons and hydrogens on the phosphine
the conversion of cluster 3 into cluster 2 along with ethyl benzene ligands along with the two-site disorder on the top nickel centre and the
and toluene. The ethyl benzene is probably formed from hydrogen- bound isobutylene fragment are omitted for clarity. This 70-electron [Ni5]8+
ation of the π-bound styrene fragment, whereas the toluene is intermediate reveals triple C–H bond activation of one sp3 and both vinylic
generated from C–C bond cleavage of the PhCC fragment and the C–H bonds of isobutylene. The resultant moiety, shown in blue, is
incorporation of three hydrogen atoms.
cooperatively bound by all five nickel centres prior to C–C bond cleavage.
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.2840
ARTICLES
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Acknowledgements
Acknowledgement is made to the Natural Sciences and Engineering Research
Council (NSERC) of Canada for funding. M.M.S. acknowledges the NSERC
scholarship support.
Author contributions
M.M.S. conducted the experiments and analysed the data. M.M.S. and S.A.J. conceived and
designed the project and prepared the manuscript
Additional information
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. Publisher’s note: Springer Nature remains neutral with regard to
jurisdictional claims in published maps and institutional affiliations. Correspondence and
requests for materials should be addressed to S.A.J.
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cleavage of a cyclopentadienyl ligand. J. Am. Chem. Soc. 125, 9568–9569 (2003).
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Competing financial interests
The authors declare no competing financial interests.
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