Cobalt-promoted B-H and C-H activation in the three-component reactions of 16-electron cobalt carboranedithiolate, alkyne and bronsted acids

Article abstract of DOI:10.1016/j.jorganchem.2015.06.013

The three-component reactions of Cp^#Co(S₂C₂B₁₀H₁₀) (Cp^# = Cp, 1a; MeCp, 1b; Me₄Cp, 1c; Me₅Cp, 1d), methyl propiolate (2) and Bronsted acid organic ligands (L1-L8) are

Full text of DOI:10.1016/j.jorganchem.2015.06.013

Journal of Organometallic Chemistry xxx (2015) 1e8
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Cobalt-promoted BeH and CeH activation in the three-component
reactions of 16-electron cobalt carboranedithiolate, alkyne and
bronsted acids
Rui Zhang, Lin Zhu, Zhenzhong Lu, Hong Yan*
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu, China
a r t i c l e i n f o
a b s t r a c t
The three-component reactions of Cp#Co(S2C2B10H10) (Cp# ¼ Cp, 1a; MeCp, 1b; Me4Cp, 1c; Me5Cp, 1d),
methyl propiolate (2) and Bronsted acid organic ligands (L1eL8) are reported. Only can 1a and 1b lead to
selective B-functionalization at carborane with cyclopentadienyl or methyl-cyclopentadienyl as a func-
tional group at ambient temperature in good yields. Four types of products containing a coupled BeC
bond are obtained dependent on the type of the used Bronsted acids (L1eL8). If stronger coordinating
ligands L1eL3 are chosen compounds 3a(L1eL3) and 3b(L1eL3) are isolated where L1eL3 loses one
proton to provide 3 electrons to metal and alkyne is reduced to olefin. If L4 and L5 are used, products 4a
or 4b are generated where the Bronsted acid is not observed but the alkyne is reduced to sp3 and forms a
five-membered ring with Co center. Allenes (L6 and L7) lead to 5a(L6) and 5a(L7) where an allyl unit is
coordinated to metal. In case of CpH (L8), compounds 6a and 6b are produced which contains an in-situ
generated unusual carborane-functionalized dithiolate ligand from 4 þ 2 cycloaddition of alkyne and the
added cyclopentadiene. All products were fully characterized by spectroscopic techniques and elemental
analysis. All products were fully characterized and some typical solid-state structures were further
determined by X-ray crystallographic analysis. The co-promoted mechanisms by metal and Bronsted
acids to lead both BeH and CeH activation are proposed on the basis of deuterium labeling as well as
NMR, MS, GC monitoring experiments.
Article history:
Received 28 March 2015
Received in revised form
6 June 2015
Accepted 8 June 2015
Available online xxx
Dedicated to Prof. Russell Grimes on the
occasion of his 80th birthday.
Keywords:
BeH activation
Cobalt
Carborane
Alkyne
Bronsted acid
© 2015 Published by Elsevier B.V.
1. Introduction
dithiolate compounds are more reactive than diselenolate species;
electron-deficient alkynes are more reactive than electron-rich al-
Derivatives of polyhedral boranes have been widely applied in
the areas of materials, medicine, catalysis, etc, [1e15] and arouse
great interests among the researches on their functionalization.
However, selective and straightforward boron substitutions have
been proven difficult [16,17], although significant advances in
metal-promoted or metal-catalyzed hydroboration of polyhedral
boranes have been achieved [18e37]. Cp#M(E2C2B10H10) (M ¼ Co,
Rh, Ir; E ¼ S, Se) reacted with active alkynes to achieve the selective
hydroboration of o-carborane have been well known [26e32]. The
metal center, chalcogen element and reagents are all important to
decide the reactivity of these 16e half-sandwich compounds. It has
been found that cobalt species are much more reactive than their
analogous rhodium and iridium species [33,34,37,38]; the
kynes [39,40].
In the case of Cp#Co(S2C2B10H10) (Cp# ¼ Cp, MeCp, Me4Cp,
Me5Cp) [41], different size of the ancillary ligand could lead to
different type of products (Scheme 1). Compounds with smaller
size ligands Cp and MeCp prefer the B-substituted products at
carborane, whereas the larger ligands Me4Cp, Me5Cp give rise to
the products of alkyne two-fold insertion into a MeS bond. In
three-component reactions if a third chelating dithio-ligand is
added, the reactivity deceases with the size increase of ancillary
ligand. The type of generated products is dependent on the
competition between alkyne and dithio-ligand, and determined by
steric and electronic effects of these reagents.
Through control of reaction ratio of the three-component re-
actions of Cp#Co(S2C2B10H10) (Cp# ¼ Cp, MeCp, Me4Cp, Me5Cp),
methyl propiolate and dithio ligands the three-component prod-
ucts G, H and two-component compounds F were obtained
(Scheme 1) [41]. Also, we have isolated novel compounds with BeC
* Corresponding author.
E-mail address: hyan1965@nju.edu.cn (H. Yan).
http://dx.doi.org/10.1016/j.jorganchem.2015.06.013
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R. Zhang et al. / Journal of Organometallic Chemistry xxx (2015) 1e8
2,2-bipyridine and 1,10-phenanthroline did not lead to similar
products, reflecting the requirement of electron accounting of the
metal center. Owing to the different strength of these potential 3e
donor ligands (L1eL8) in coordination to Co metal center, four
types of products were defined (Scheme 2). All the products were
fully characterized by spectroscopic data and the solid-state
structures of some typical compounds were determined by using
X-ray crystallography. No BeC coupling products could be isolated
in the cases of 1c and 1d starting compounds because of the steric
problem of the ancillary ligands.
2.1.1. 3a(L1eL3) and 3b(L1eL3)
As shown in Scheme 2 and Fig. 1 3a(L1) is the first example of
Scheme 1. Two-component reaction between 1ae1d and 2 (up); two component-
reaction between 1ae1d and L1/L2 (left); and three-component reaction of 1ae1d, 2
and L1/L2 (right) [41].
coupling which are generated via both CeH and BeH activation
[42]. Herein, the range of reactions and details are reported.
2. Results and discussions
2.1. Synthesis and structure
The Bronsted acid organic ligands (L1eL8) were introduced to
two component reactions of 16e complexes Cp#Co(S2C2B10H10
)
(Cp# ¼ Cp, 1a; MeCp, 1b; Me4Cp, 1c; Me5Cp, 1d) and methyl pro-
piolate (2), as a result, a series of novel products containing BeC
coupling between carborane cage and Cp# (1a and 1b) unit were
generated at ambient temperature (Scheme 2).
Fig. 1. Molecular structure of 3a(L1); ellipsoids show 30% probability levels, and some
of the hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and
angles (ꢀ): Co1ering centroid 1.682(1), C1eC2 1.843(6), B3eC7 1.569(6), C1eS1
1.740(4), C2eS2 1.778(4), C8eC9 1.333(5), C8eS2 1.730(4), C12eS3 1.696(4), C12eS4
1.708(4), C12eN1 1.314(5), Co1eS4 2.246(1), Co1eS3 2.258(1), Co1eS1 2.272(1),
B3eC2 1.724(6), C3eB3eC2 124.8(4), B3eC2eS2 114.2(3), C9eC8eS1 124.1(3),
C8eC9eC10 123.8(4), S3eC12eS4 111.1(3), C3eCo1eS2 89.4(1), S4eCo1eS3 77.1(1).
Small organic ligands play vital roles in the formation of adducts
with a coupled BeC bond. The 4e donor chelating ligands such as
Scheme 2. Reactions of 1a/1b, 2 and L1eL8 at ambient temperature lead to four types of products.
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R. Zhang et al. / Journal of Organometallic Chemistry xxx (2015) 1e8
3
this type of products. The carborane cage is linked to the Cp ring
through a covalent BeC bond with a length of 1.569(6) Å. The
substituted Cp ring remains coordinated to Co metal center in an
h5emode. Pyrrolidine-1-carbodithioic acid (L1) has lost one
hydrogen atom and chelated to Co as a 3e donor. The alkyne has
been reduced to an olefinic unit in a Z configuration and is con-
nected to one S atom rather than a B atom as in compounds B and C
(Scheme 1) [41] (Scheme 2). Clearly, the breakage of the rigid S-
chelating binding in 1a is essential for constructing the intra-
molecular coupling of the carborane-dithiolate and cyclo-
pentadienyl moieties.
Spectroscopic analysis of 3a(L1) confirms that the relevant
features of its solid-state structure are retained in solution. In
contrast to one singlet at 5.26 ppm for Cp group in 1a [43] in the 1H
NMR spectrum, four well-separated signals at 6.01, 5.92, 5.24 and
4.97 ppm were assigned to the substituted Cp group in 3a(L1). The
13C resonances of the substituted Cp ring appear at 91.0, 84.7, 84.3
and 83.8 ppm for those four CeH bonds, which was only one signal
in 1a [43] at 81.8 ppm. Moreover the characteristic broad 13C
resonance of the BeC bond is remarkably low-field shifted to
127.0 ppm, corresponding to the electron-deficient nature of B
atom. The 11B signal of the BeC bond (2.1 ppm) is also low-field
shifted by 7e11 ppm in comparison to those in 1a [43] further
supporting the formation of a BeC bond. The reactions of 1a, 2, L2
(or L3) led to 3a(L2) and 3a(L3) which have the similar spectro-
scopic data as for 3a(L1) (see Supporting information II). The single-
crystal X-ray diffraction of 3a(L2) presents in Fig. 2, revealing the
similar Cpecarborane cage coupling moiety.
Compounds 3b(L1eL3) have the similar structures as
3a(L1eL3). The only difference is that Cp ring is replaced by MeCp.
But interestingly, the CeH bond of the ortho-position of the methyl
unit is activated as shown in 3b(L3) (Fig. 3) with the BeC bond
length of 1.586(7) Å. The NMR data support the solid-state struc-
ture. In the 1H NMR spectrum only three peaks show at 6.10, 5.55,
4.31 ppm corresponding to the three CeH units of MeCp ring. The
13C NMR data display a broad resonance at 126.2 ppm for CeB bond,
a sharp resonance at 108.6 ppm for another quaternary carbon and
three sharp resonances at 96.6, 83.5 and 77.6 ppm for CeH units, as
confirmed by 2D 13C/1H HETCOR (HMQC) experiments.
Fig. 3. Molecular structure of 3b(L3); ellipsoids show 30% probability levels, and the
hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles
(ꢀ): Co1ering centroid 1.696(1), B3eC3 1.586(7), C1eS1 1.764(6), C1eC2 1.788(6),
C2eS2 1.769(4), C4eC8 1.476(7), C9eN1 1.313(6), C9eS3 1.710(5), C9eS4 1.722(5),
C10eN1 1.462(6), C11eN1 1.453(6), C12eC13 1.267(7), C12eS2 1.720(5), C13eC14
1.453(7), C14eO1 1.260(12), C14eO2 1.294(6), Co1eS3 2.256(1), Co1eS4 2.274(1),
Co1eS1 2.297(1), C3eB3eC1 112.2(4), S3eC9eS4 109.8(3), C13eC12eS2 125.0(4),
C12eC13eC14 125.1(5), C11eN1eC10 116.9(4), C1eS1eCo1 104.4(1), C12eS2eC2
104.4(2).
2.1.2. 4a and 4b
The reactions of 1a/1b, methyl propiolate (2) and L4/L5 gave rise
to the new compounds 4a/4b (Scheme 2). The weakly coordinating
L4 and L5 also lead to products with the BeC bond formation (4a
and 4b), but the ligand does not appear in the structure. Here, L4
and L5 might be considered as Bronsted acid catalysts. The solid-
state structure of 4a is given in Fig. 4. Instead of the strong ligand
chelating to Co in 3a(L1eL3)/3b(L1eL3), the newly generated unit
donates 3e to metal and form the five-membered ring
Co1eC3eC4eC5eO1. The alkyne is reduced to alkane with 1.526 Å
bond length of C3eC4. The NMR data are consistent with the solid
structure of 4a. The substituted Cp ring shows four signals at 6.40,
6.10, 5.49, 5.15 ppm in 1H NMR and at 104.9, 100.8, 96.1, 78.6 ppm in
13C NMR. The resonance of BeC bond presents remarkably low-
field shifted as a characteristic broad signal at 127.2 ppm in 13C
NMR. The 11B signal of the BeC bond appears at 4.5 ppm in 11B
NMR. The reduced alkyne coordinating to metal forms a five-
membered ring which makes the structure more rigid. The
Fig. 4. Molecular structure of 4a; ellipsoids show 30% probability levels, and some of
the hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and an-
gles (ꢀ): Co1ering centroid 1.686(1), B3eC7 1.549(3), C1eC2 1.680(3), C1eS1 1.768(2),
C2eS2 1.786(2), C3eC4 1.526(3), C3eS1 1.799(2), C3eCo1 1.962(2), C4eC5 1.496(4),
C5eO1 1.233(3), C5eO2 1.299(3), C6eO2 1.445(3), Co1eO1 1.983(8), Co1eS2 2.286(1),
C7eB3eC2 119.3(1), C7eB3eC1 112.9(2), C1eC2eS2 117.3(1), C4eC3eS1 112.4(1),
C4eC3eCo1 106.4(1), S1eC3eCo1 118.1(1), C5eC4eC3 106.3(2), O1eC5eO2 122.4(2),
O1eC5eC4 120.4(2), C3eCo1eO1 84.1(1), O1eCo1eS2 94.1(1), C5eO1eCo1 112.5(1),
C1eS1eC3 109.7(1), C2eS2eCo1 97.7(1).
Fig. 2. Molecular structure of 3a(L2); ellipsoids show 30% probability levels, and some
of the hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and
angles (ꢀ): Co1ering centroid 1.685(1), C1eC2 1.793(8), B3eC7 1.564(1), C1eS2
1.780(8), C2eS1 1.761(8), C3eC4 1.321(1), C3eS1 1.752(7), C4eC5 1.486(1), C12eC13
1.523(1), C12eS4 1.659(8), C12eS3 1.677(8), Co1eS3 2.244(2), Co1eS4 2.254(2),
Co1eS2 2.270(2), C7eB3eC2 112.8(6), S1eC1eC2 116.9(5), S2eC2eC1 118.4(5),
C4eC3eS1 121.6(7), C3eC4eC5 117.0(8), S4eC12eS3 110.9(5), S3eCo1eS4 75.29(8),
C3eS1eC1 102.7(4), C2eS2eCo1 104.2(3), C12eS3eCo1 86.9(3), C12eS4eCo1 87.0(3).
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R. Zhang et al. / Journal of Organometallic Chemistry xxx (2015) 1e8
proton at C(3) is coupled by the two adjacent non-equivalent
hydrogen atoms, displaying dd coupling pattern at 6.85 ppm with
J1 ¼ 6.2 Hz and J2 ¼ 9.6 Hz. The chemical shifts of the two hydrogen
atoms at C(4) show at 3.17 ppm (dd, J1 ¼ 9.6 Hz, J2 ¼ 18.5 Hz) and
2.70 ppm (dd, J1 ¼ 6.2, J2 ¼ 18.5 Hz), respectively, which were
further confirmed by 2D COSY and HMQC (SI-Figure 1). Compound
4b is fully characterized by spectroscopic data (see Supporting
information II).
2.1.3. 5a(L6) and 5a(L7)
The reactions of 1a, 2 and L6/L7 led to 5a(L6)/5a(L7). The solid-
state structure of 5a(L6) (Fig. 5) shows similar structural type to
3a(L1eL3) except that the coordinating ligand is the 3e donor of
allyl unit instead of dithio ligand in 3a(L1eL3). The 1H signal of
C9eH in 5a(L6) shows dd coupling pattern at 6.10 ppm with
J1 ¼ 8 Hz and J2 ¼ 14 Hz. The chemical shifts of the two hydrogen
atoms at C8 atom appear at 5.20 ppm (J ¼ 8 Hz) and 1.74 ppm
(J ¼ 14 Hz), respectively, further confirming the existence of the
allyl group generated from ligand L6. The characteristic broad
resonance at 126.8 ppm is attributed to the CeB bond in the 13C
NMR and the 11B signal at 2.2 ppm corresponds to the BeC bond in
11B NMR, indicating that the Cp has been connected to carborane at
boron site. Compound 5a(L7) is also fully characterized by spec-
troscopic data, similar to 5a(L6) (see Supporting information).
Fig. 6. Molecular structure of 6a; ellipsoids show 30% probability levels, and the
hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles
(ꢀ): Co1ering centroid 1.705(1), C5eB13 1.576(5), B3eC14 1.559(5), S1eC1 1.814(3),
S1eC10 1.858(3), S1eCo1 2.284(1), S2eC2 1.782(3), S2eCo1 2.261(1), S3eC3 1.771(3),
S3eCo1 2.275(1), S4eC19 1.752(4), S4eC4 1.766(4), C1eC2 1.665(4), C3eC4 1.806(5),
C10eC11 1.542(4), C10eC14 1.576(4), C11eC12 1.497(5), C11eC16 1.567(4), C14eC15
1.527(4), C14eC18 1.541(4), C15eC16 1.532(4), C16eC17 1.515(4), C17eC18 1.316(5),
C19eC20 1.288(5), C20eC21 1.460(6), C14eB3eC1 109.6(3), C14eB3eC2 122.6(3),
C5eB13eC3 111.9(3), C5eB13eC4 123.6(3), C1eS1eC10 92.0(1), C1eS1eCo1 103.9(1),
C10eS1eCo1 120.3(1), C2eS2eCo1 104.2(1), C3eS3eCo1 104.7(1), C19eS4eC4
103.6(2), S2eCo1eS3 85.4(4), S2eCo1eS1 89.7(3), S3eCo1eS1 103.5(4).
2.1.4. 6a and 6b
The hydrogens generated from [4 þ 2] cycloaddition of the new o-
carboranedithio moiety show corresponding resonance peaks, as
shown in SI-Figure 3. The two new BeC bonds appear at 127.0 ppm
for B(13)eC(5) and 52.3 ppm for B(3)eC(14) as characteristic broad
signals in 13C NMR. The B(3) and B(13) atoms show low-field shift
around 0.8 ppm in the 11B NMR, indicating the formation of these
BeC bonds. The substituent at C(11) is not big enough to freeze
configuration of the bicyclo-[2,2,1] unit. As the result, the solution
of 6a contains both endo and exo isomers, but exo species is the
preferential configuration. The isolated exo-6a shows exo:
endo ¼ 3:1 in solution, whereas 1:1 ratio for the isolated endo-6a in
solution. Each signal was identified by 2D COSY and HMQC. Com-
pound 6b was confirmed by spectral data (see Supporting
information) which is similar to 6a in structure.
The reactions of 1a/1b, 2 and L8 at ambient temperature led to
6a/6b (Scheme 2). Cyclopentadiene (L8) is a very weak Bronsted
acid, to our surprise, it can also produce the BeC coupled com-
plexes. Two carborane cages shows in the solid structure of 6a in
each molecule (see Fig. 6). An unusual in situ-generated 3e donor o-
carborane dithio ligand appears which bears a moiety formed by a
[4 þ 2] DielseAlder cycloaddition, similar to the dithio ligands in
3a(L1eL3)/3b(L1eL3). Cp ring remains the h5emode binding to
metal and the lengths of the newly-generated characteristic BeC
bonds are 1.576 Å for B(13)eC(5) and 1.559 Å for B(3)eC(14),
respectively.
The NMR data are in agreement with the solid-state structure.
2.1.5. Ligand exchange reactions
The chelating ability of in situ generated bulky dimercapto
ligand in 6a/6b is much weaker because of the electron deficiency
of the adjacent carborane cage, allowing to be quickly and quanti-
tatively replaced by the stronger chelating ligands L1eL3 to yield
3a(L1eL3) and 3b(L1eL3) (Scheme 2 and Supporting information).
2.2. Proposed mechanisms
Both BeH and CeH activation as well as hydrogen transfer play
key roles in the information of BeC coupled products. Therefore, a
series of deuterium-labeling experiments and MS, NMR, GC
monitoring reactions were performed to confirm the origin and
destination of hydrogen transfer. Based on the series reactions of
1a, the possible formation mechanisms of all the four types BeC
coupled complexes were proposed.
Fig. 5. Molecular structure of 5a(L6); ellipsoids show 30% probability levels, and some
of the hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and
angles (ꢀ): Co1ering centroid 1.684(1), B3eC3 1.596(4), C1eS1 1.753(3), C1eC2
1.802(4), C2eS2 1.750(3), C8eC9 1.434(4), C8eCo1 2.105(3), C9eC10 1.411(4), C9eCo1
2.017(3), C10eC18 1.469(4), C10eC11 1.554(4), C10eCo1 2.174(3), C11eC12 1.507(4),
C21eC22 1.325(4), C21eS2 1.734(3), C22eC23 1.455(4), Co1eS1 2.308(1), C3eB3eC1
108.7(2), C3eB3eC2 119.1(2), S2eC2eC1 117.5(2), C9eC8eCo1 66.4(2), C10eC9eC8
118.8(3), C9eC10eC11 122.7(3), C18eC10eC11 113.7(3), C12eC11eC10 110.3(2),
C22eC21eS2 118.3(2), C21eC22eC23 119.4(3), C9eCo1eS1 112.9(1).
2.2.1. Formation of 3a(L1)
2.2.1.1. Deuterium-labeled reaction. The reaction of 1a, 2 and L1
gave rise to product 3a(L1). From the X-ray structure of 3a(L1)
(Fig. 1) we can see that both BeH bond of carborane and CeH bond
of Cp unit in 1a have been activated to lose one hydrogen atom for
each. Bronsted acid (L1) also loses one hydrogen atom as a
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R. Zhang et al. / Journal of Organometallic Chemistry xxx (2015) 1e8
5
hydrogen source. The alkyne 2 is reduced by receiving one
hydrogen atom as a hydrogen acceptor. Two hydrogen atoms might
combine and become H2 molecule. Therefore, a series of reactions
were designed to prove possible pathways.
Firstly, we used the deuterated d1-L1 [(CH2)4NCS2D] to react
with 1a and 2. As a result, no proton was deuterated in the isolated
product 3a(L1), indicating that d1-L1 is not the hydrogen source to
reduce alkyne. We then used fully deuterated 16e starting com-
pound d5-1a [(d5-Cp)CoS2B10H10, D ꢁ 75%] to react with 2 and L1.
The product d4-3a(L1) was isolated where no proton of the olefinic
unit has been deuterated (see Fig. 7). This demonstrates that the Cp
ring of 1a is not the hydrogen source to reduce the alkyne either
even although the Cp has lost one hydrogen atom in the reaction. If
all the three reagents were deuterated, that is, d5-1a (D ꢁ 75%), d1-
2 [DC^CCO2Me] and d1-L1 were used d5-3a(L1) was isolated as
determined by 1H NMR (Fig. 8). This reveals that the normal proton
at C(9) which has reduced the terminal alkyne come from the
activated B(3)eH bond of the carborane cage.
Fig. 8. 1H NMR spectra (3.8e7.8 ppm) of compounds 3a(L1) (bottom) and d5-3a(L1)
(top) in CDCl3 at 20 ꢀC.
2.2.1.2. Monitoring by EI-MS and GC. EI-MS and GC methods were
used to monitor the reaction process of 1a, 2 and L1. Initially a
strong peak containing carborane and alkyne at 232.1 (m/z) was
observed which corresponds to a stable organic fragment from
intermediates I or/and II or/and III (Scheme 3). Later, the molecular
ion peak at 559.1 (m/z) of 3a(L1) and its fragments appeared. Note
that the product 3a(L1) started to form around 45 min and the
reaction almost completed in 3 h by TLC test. By GC detection H2
was unambiguously observed, as shown in SI-Chart 1 if the reaction
was done in an autoclave.
analog [29]. However, shimming problems, due to the generation of
paramagnetic Co2þ species in the by-reactions, led to a gradual
worsening of the NMR spectrum. Thus we could not identify more
additional intermediates by NMR. This effect was found to be
reproducible.
On the basis of above experimental results, a plausible mecha-
nism for the generation of 3a(L1) is summarized in Scheme 3. The
BeH activation at B(3) of the carborane and hydrogen transfer to
the alkyne should occur as the earlier steps (IeIII) after alkyne
insertion into one CoeS bond, as previously reported [26e37,41,42]
and revealed by these capture experiments. It should note that the
iridium analog of intermediate III is rather stable [26e32], sug-
gesting that the proposed cobalt intermediate is reasonable. The
chelating coordination of L1 would induce a change in the binding
2.2.1.3. Monitoring by 1H NMR. The reaction of 1a, 2 and L1 in a J.
Young valve NMR tube was detected by 1H NMR at ambient tem-
perature. The assignments for the characteristic protons of the in-
termediates I, II and III (Scheme 3) were shown in SI-Figure 2. The
intermediate I from the alkyne addition was detected in the very
earlier stage by its characteristic 1H resonance at 6.65 ppm assigned
to the HC]CCO2Me unit as compared to a Me5CpCo analog [44].
Later, broad hydride signals appeared in the region from ꢂ4
to ꢂ8 ppm, may be better expressed as CoeHeB. Very shortly
(around 20 min) these hydride signals completely disappeared. At
the same time III was observed owing to its characteristic doublets
at 5.93 and 3.94 ppm with J ¼ 10 Hz, parallel to those of a Me5CpIr
mode of Cp from h5 to h1 (IV-A) [45e50], followed by
aeH elimi-
nation (V-A) [51e54], H2 release, CoeB and CoeC cleavage, BeC
formation, and Cp switching back to h5 coordination [45e50],
finally leading to 3a(L1).
2.2.2. Formation of 4a
In contrast to 3a(L1), 4a contains no chelating ligand L4. How-
ever, in the absence of L4, 4a can not be generated. Series of re-
actions by deuterium-labeling reagents were designed to
demonstrate hydrogen transfer, as shown in Fig. 9. For example, the
1H NMR spectrum of d5-4a generated from d5-1a (No.2 in Fig. 9)
shows that one deuterium of d5-Cp unit has been transferred to
C(4) atom (see Fig. 4). Reaction of 1a, 2 and d1-L4 led to non-
deuterated 4a (No. 3 in Fig. 9), which supports that the other
hydrogen atom at C(4) coming from carborane, the same as in
3a(L1), other than from L4. Reaction of 1a, d1-2 and L4 gave rise to
d1-4a containing the deuterium at C(3). Thus the formation of 4a is
proposed in Scheme 3. L4 chelates to metal (IV-B), same as L1 in the
formation of 3a(L1), to induce Cp ring slippage. Then the reduced
alkyne replaces the weakly coordinating ligand L4, followed by the
activated CeH from Cp transferring to C(4) to generate 4a.
2.2.3. Formation of 5a(L6)/5a(L7)
The same early stage for BeH activation as in the formation of
3a(L1), addition of (L6)/(L7) as 4e donor led to (IV-C) (Scheme 3),
then the activated CeH from Cp migrates to form an allyl ligand in
5a(L6)/5a(L7).
Fig. 7. 1H NMR spectra (4.8e8.5 ppm) of compounds 3a(L1) (bottom) and d4-3a(L1)
(top) in CDCl3 at 20 ꢀC.
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6
R. Zhang et al. / Journal of Organometallic Chemistry xxx (2015) 1e8
Scheme 3. Proposed mechanisms for the formation of BeC containing carborane derivatives.
2.2.4. Formation of 6a
fragment from I or II or III (2 h). But differently, in this reaction the
molecular ion peak at 414.1 (m/z) for intermediate III could be
observed for a long time (6 h) since this is a slow reaction in which
III can survive longer. This is in a sharp contrast to the formation of
3a(L1), where the fast reaction makes observation of species III
impossible. Compound 6 was observed from 2.5 h and 6a was
observed starting from 4.5 h, as confirmed by TLC. Note that only
the fragment of 6a can be observed by MS because of the weaker
binding of the new-generated bulky dimercapto ligand. Production
of H2 as a by-product was confirmed by GC (see SI-Chart 2). Addi-
tionally, intermediate I was clearly observed by 1H NMR in the
earlier time, but the generated paramagnetic species badly
disturbed shimming which prevented observation of more species.
Concerning the formation of 6a (Scheme 3), the coordination of
2.2.4.1. Deuterium-labeled reaction. If d5-1a (D-incorporation
ꢁ95%) was used to react with 2 and L8 product d9-6a was isolated
where the Cp ring and the DielseAlder cycloaddition moiety were
deuterated (see 1H NNR in SI-Figure 3). This demonstrates that the
existing d5-Cp ligand and the coordinating CpH (L8) are
exchangeable. This experiment also supports oxidative addition of
free CpH to metal to produce Cp as well as the exchange of
h
5-and
h
1-Cp binding modes and hydrogen 1,5-Sigmatropic shift over Cp
ring driven by the further DielseAlder cycloaddition. Using d1-2 led
to d2-6a. 1H NMR (Fig. 10) shows that the hydrogen atoms at C(19)
and C(10) are deuterated, reflecting that they originates from the
terminal alkyne. But the hydrogen atoms at C(20) and C(11) are not
deuterated.
L8 to Co induces the existing
form IV-D. Subsequent oxidative addition of the bound CpH to the
metal might occur [51e61], and the resulting
1-Cp might undergo
h h
5-Cp to change binding mode to 1 to
2.2.4.2. Monitoring by EI-MS, GC and 1H NMR. In the earlier stage
the peak at 232.1 (m/z) was observed which corresponds to a
h
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R. Zhang et al. / Journal of Organometallic Chemistry xxx (2015) 1e8
7
MeCp ring cannot undergo 1,5-Sigmatropic rearrangement over the
ring, thus excluding the direct [4 þ 2] addition of free MeCpH to the
olefinic unit.
3. Conclusions
One-pot three-component reactions of 16-electron complex
Cp#Co(S2C2B10H10) (Cp# ¼ Cp, 1a; MeCp, 1b; Me4Cp, 1c; Me5Cp, 1d),
methyl propiolate (HC^CCO2Me) and Bronsted acids (L1eL8) have
been studied. A facile BeC bond formation between carbor-
anedithiolate and Cp/MeCp has been observed through cobalt-
induced and ligand-assistant both BeH and CeH activation at
ambient temperature. This leads to selective B-functionalization of
carborane by a metal-bound
h
5-Cp group in good yields. The
methodology requires the suitable size of ancillary ligand, electron-
deficient alkyne and 3e-donating Bronsted acid ligands, thus suit-
able for a broad range of substrates. Four types of products were
obtained dependent on Bronsted acids. This work opens a way to
development of new types of functional groups at a boron site of
carborane.
4. General methods and synthesis
All manipulations were carried out under an argon atmosphere
by using standard Schlenk techniques unless otherwise noted. Pe-
troleum ether, diethyl ether, and tetrahydrofuran were dried over
sodium and distilled under nitrogen. Hexanes and CH2Cl2 were
dried over Na or CaH2, respectively, and distilled under nitrogen.
Methyl propiolate (Alfa Aesar) and n-BuLi (2.0 M in hexanes,
Fig. 9. 1H NMR spectra (2.5e7.5 ppm) of the products from deuterium-labeling re-
actions in CDCl3 at 20 ꢀC. (1) 4a (1a þ 2 þ L4); (2) d5 ¡ 4a (d5 ¡ 1a þ 2 þ L4); (3) 4a
(1a þ 2 þ d1 ¡ L4); (4) d1 ¡ 4a (1a þ d1 ¡ 2 þ L4).
Aldrich)
were
used
as
supplied.
[CpCo(CO)I2]
and
Cp#Co(S2C2B10H10) (1a e 1d) were prepared by literature pro-
cedures [1]. The isotope-labeled analogs were prepared using the
commercially available isotope-labeled starting materials.
All NMR spectra were obtained at ambient temperature on
Bruker DRX-500 spectrometer. Chemical shifts are reported relative
to CHCl3/CDCl3
Et2OeBF3
(
d
1H ¼ 7.26 ppm,
d
13C ¼ 77.0 ppm) and external
(d
11B ¼ 0 ppm), respectively. The IR spectra were
recorded on a Bruker Vector 22 spectrophotometer with KBr pellets
in the 4000e400 cm-1 region. The mass spectra were recorded on a
Micromass GC-TOF for EI-MS (70 ev). Elemental analysis was per-
formed in an elementar vario EL III elemental analyzer. X-ray
crystallographic data were collected on a Bruker SMART Apex II
CCD diffractometer using graphite-monochromated Mo-K
a
(
l
¼ 0.71073 Å) radiation. The intensities were corrected for
Lorentz-polarization effects and empirical absorption with the
SADABS program [2]. The structures were solved by direct methods
using the SHELXL-97 program [3]. Details regarding data collection
of six complexes (3a(L1), 3a(L2), 3b(L3), 4a, 5a, and 5a(L6)) are
provided in the CIF files (CCDC 809345, 774630, 828292, 812791,
774631, and 832876).
Fig. 10. 1H NMR spectra (4.5e7.9 ppm) of 6a (bottom) and d2-6a (top) in CDCl3 at
20 ꢀC.
Preparation of 3a(L1-L3) and 3b(L1-L3). To a mixture of 1a/1b
(0.4 mmol) and 2 (2 mmol) in CH2Cl2 (20 mL) L1(L2,L3) (0.6 mmol)
was added under argon. The resulting solution was stirred at
ambient temperature for 7 h. After removal of solvent, the residue
was column chromatographed on silica and eluted with hexanes/
CH2Cl2 (1: 2) to give 3a(L1-L3) and 3b(L1-L3), respectively (54%e
60%).
Preparation of 4a and 4b. To a mixture of 1a/1b (0.4 mmol) and
L4/L5 (8 mmol) in CH2Cl2 (30 mL) 2 (0.8 mmol) was added under
argon. The resulting solution was stirred at ambient temperature
for 15 h. After removal of solvent, the residue was column chro-
matographed on silica and eluted with hexanes/CH2Cl2 (2: 1) to
give 4a/4b (45%e53%).
hydrogen 1,5-sigmatropic rearrangement [62,63] over the ring
driven by the intramolecular [4 þ 2] cycloaddition [64] with the
olefinic unit (V-D). As a result, the hydride species (VI-D) is pro-
duced after cleavage of the CoeB and CoeC bonds. Its reductive
elimination leads to paramagnetic species 6 and its further reaction
with III gives rise to 6a.
Using (MeCp)CoS2C2B10H10 (1b) instead of 1a led to 6b. This
example exhibited an alternative pathway. MeCp remains intact
and free CpH is oxidized to metal for further DielseAlder addition
because of forbidden methyl shift over the ring. Following the
above procedure instead of L8 by fresh MeCpH, no DielseAlder
reaction product could be isolated. This demonstrates that the
Preparation of 5a(L6)/5a(L7). To a mixture of 1a (0.4 mmol) and
L6/L7 (8 mmol) in CH2Cl2 (20 mL) 2 (0.8 mmol) was added under
Please cite this article in press as: R. Zhang, et al., Journal of Organometallic Chemistry (2015), http://dx.doi.org/10.1016/
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8
R. Zhang et al. / Journal of Organometallic Chemistry xxx (2015) 1e8
1004e1005.
argon. The resulting solution was stirred at ambient temperature
for 18 h. After removal of solvent, the residue was column chro-
matographed on silica and eluted with hexanes/CH2Cl2 (1: 1) to
give 5a(L6)/5a(L7) (25%e51%).
Preparation of 6a/6b and 6. To a mixture of 1a/1b (0.4 mmol) and
L8 (0.4 mmol) in CH2Cl2 (20 mL) 2 (0.8 mmol) was added under
argon. The resulting solution was stirred at ambient temperature
for 18 h. After removal of solvent, the residue was column chro-
matographed on silica and eluted with hexanes/CH2Cl2 (1: 1) to
give 6a/6b (10%e23%) and 6 (10%~30%).
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The authors declare no competing financial interest.
Acknowledgment
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This work was supported by the National Natural Science
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Appendix A. Supplementary data
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