Reversible Cleavage/Formation of the Chromium–Chromium Quintuple Bond in the Highly Regioselective Alkyne Cyclotrimerization

Article abstract of DOI:10.1002/anie.201709583

Herein we report the employment of the quintuply bonded dichromium amidinates [Cr{κ²-HC(N-2,6-ⁱPr₂C₆H₃)(N-2,6-R₂C₆H₃)}]₂ (R=iPr (1), Me (7)) as catalysts to mediate the [2+2+2] cyclotrimerization of terminal alkynes giving 1,3,5-trisubstituted benzenes. During the catalysis, the ultrashort Cr?Cr quintuple bond underwent reversible cleavage/formation, corroborated by the characterization of two inverted arene sandwich dichromium complexes (μ-η⁶:η⁶-1,3,5-(Me₃Si)₃C₆H₃)[Cr{κ²-HC(N-2,6-ⁱPr₂C₆H₃)(N-2,6-R₂C₆H₃)}]₂ (R=ⁱPr (5), Me (8)). In the presence of σ donors, such as THF and 2,4,6-Me₃C₆H₂CN, the bridging arene 1,3,5-(Me₃Si)₃C₆H₃ in 5 and 8 was extruded and 1 and 7 were regenerated. Theoretical calculations were employed to disclose the reaction pathways of these highly regioselective [2+2+2] cylcotrimerization reactions of terminal alkynes.

Full text of DOI:10.1002/anie.201709583

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Accepted Article
Title: Reversible Cleavage/Formation of the Cr-Cr Quintuple Bond in
the Highly Regioselective Alkyne Cyclotrimerization
Authors: Yi-Chou Tsai, Jen-Shiang Kenny Yu, Yu-Siang Huang, Gou-
Tao Huang, and Yao-Lun Liu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201709583
Angew. Chem. 10.1002/ange.201709583
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10.1002/anie.201709583
Angewandte Chemie International Edition
COMMUNICATION
Reversible Cleavage/Formation of the Cr-Cr Quintuple Bond in
the Highly Regioselective Alkyne Cyclotrimerization
Yu-Siang Huang,[a+] Gou-Tao Huang,[b+] Yao-Lun Liu,[a] Jen-Shiang K. Yu*,[b] and Yi-Chou Tsai*[a]
Dedication ((optional))
Abstract: Here we report the employment of the quintuply bonded
dichromium amidinates [Cr{κ2-HC(N-2,6-iPr2C6H3)(N-2,6-R2C6H3)}]2
other hand, all the N donor-supported Cr-Cr quintuple bonds
were even shorter[10] but more reactive;[11] they were readily
cleaved upon reacting with small inorganic molecules and
organic functionalities,[10b, 11d, 11e, 11i] in contrast with the quintuply
bonded molybdenum amidinate dimers.[12]
Applications of bimetallic complexes featuring direct metal-
metal bonding in catalysis have been documented;[13] most of
them contain a metal-metal single bond. For example, the
paddlewheel dirhodium(II) carboxylates,[14] which retain their
dinuclear structures during catalysis, have been widely used in a
(R
= iPr (1), Me (7)) as catalysts to mediate the [2+2+2]
cyclotrimerization of terminal alkynes giving 1,3,5-trisubstituted
benzenes. During the catalysis, the ultrashort Cr-Cr quintuple bond
underwent reversible cleavage/formation, corroborated by the
characterization of two inverted arene sandwich dichromium
complexes (μ-η6:η6-1,3,5-(Me3Si)3C6H3)[Cr{κ2-HC(N-2,6-iPr2C6H3)(N-
2,6-R2C6H3)}]2 (R = iPr (5), Me (8)). In the presence of σ donors such
as THF and 2,4,6-Me3C6H2CN, the bridging arene 1,3,5-
(Me3Si)3C6H3 in 5 and 8 was extruded and 1 and 7 were regenerated. variety of catalytic organic transformations. In contrast,
Theoretical calculations were employed to disclose the reaction
pathways of these highly regioselective [2+2+2] cylcotrimerization
reactions of terminal alkynes.
numerous multiply bonded dinuclear complexes have been
reported in the past five decades,[1] but only limited quadruply
bonded dimolybdenum paddlewheel complexes have been
utilized in catalysis.[15] During catalysis, their dinuclear structures
were proposed to be maintained. It was recently demonstrated
that the quintuply bonded Cr2 and Mo2 complexes underwent
[2+2] cylcoaddition with alkynes.[11j, 11h, 12e] More interestingly, we
reported that the quintuply bonded dimolybdenum amidinates
underwent [2+2+2] cycloaddition with 2 equiv of terminal
alkynes,[12e] and subsequently reacted with an excess amount of
terminal alkyne to selectively produce 1,3,5-trisubstituted
benzenes.[12e] Theoretical calculations suggested the formal Mo-
Mo bond order varied between 3.5 and 5 over the course of
catalysis.[16] Herein, we wish to report terminal alkyne
cyclotrimerization catalyzed by the quintuply bonded dichromium
complex [Cr(μ-HC(N-2,6-iPr2C6H3)2)]2 (1), in which an
unprecedented reversible cleavage/formation of the ultrashort
Cr-Cr quintuple bond was involved in the catalysis.
Of the group VI elements, chromium is the most unique in
quadruple bonding. For example, among the tetragonal
dinuclear species M2X4 (M = Cr, Mo, W; X is a monoanionic
bidentate ligand), a wide range of Cr-Cr bond lengths between
1.83 and 2.00 Å have been documented,[1] while the variation of
M-M distances in the other two heavier congeners is relatively
small.[1] The Cr-Cr quadruple bond is typically considered to be
strong, but some examples showed short but weak Cr-Cr
quadruple bonds.[2-6] For instance, the [Cr2Me8]4ˉ in
[Li(THF)]4[Cr2Me8], where the Cr-Cr distance is 1.980(5) Å,
readily fell apart to two mononulcear species [CrMe4]2ˉ upon
treatment with Me2NCH2CH2NMe2.[5] Another example is the
dichromium
amidinate
cis-[Cr(μ-κ2-tBuNC(Me)NEt)(κ2-
tBuNC(Me)NEt)]2, which has a Cr-Cr bond length of 1.9601(12)
Å in the solid state, but readily decomposed in benzene and
cyclohexane solutions to give a mononuclear compound Cr(κ2-
tBuNC(Me)NEt)2.[3]
A similar contradiction was observed in Cr-Cr quintuple
bonding. For example, the energy of the very short Cr-Cr
quintuple bond (1.8351(4) Å) in the dimeric complex [Cr(μ-2,6-
[7]
{2,6-iPr2C6H3}2C6H3)]2 was 76 kcal/mol based on theoretical
calculations.[8] However, the employment of the more sterically
congested terphenyl ligand 2,6-(2,4,6-iPr3C6H2)2C6H3 effectively
prevented two Cr atoms from coupling to each other.[9] On the
[a] Y.-S. Huang[+], Y.-L. Liu, Prof. Dr. Y.-C. Tsai
Department of Chemistry and Frontier Research Center on
Fundamental and Applied Sciences and Matters,
National Tsing Hua University
101, Sec. 2, Guang-Fu Road, Hsinchu 300 (Taiwan)
E-mail: yictsai@mx.nthu.edu.tw
[b] Dr. G.-T. Huang[+], Prof. Dr. J.-S. K. Yu
Institute of Bioinformatics and Systems Biology and Department of
Biological Science and Technology, National Chiao Tung University
75 Bo-Ai Street, Hsinchu 300 (Taiwan)
Scheme 1. Reactions of
stoichiometry, and the preparation of the inverted arene sandwich dichromium
complexes.
1 and 7 with terminal alkynes of different
E-mail: jsyu@mail.nctu.edu.tw
[+] These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of the
document.
As reported previously, 1 reacted with one equiv of alkyne
R1CCR2 (R1 = R2 = Et, Ph; R1 = nPr, R2 = H) to produce the [2+2]
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10.1002/anie.201709583
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3
5
8
Figure 1. The solid-state molecular structures of 3, 5, and 8.
cycloadducts
(μ-η2-R1CCR2)[Cr(μ-HC(N-2,6-iPr2C6H3)2)]2.[12e]
arranged in the same side along the Cr···Cr axis with the torsion
angle N(2)–Cr(1)–Cr(2)–N(3) of 60.0(2)°. All six C−C bond
lengths of the central arene rings in 6 and 8 are very close and
provide an average C−C separation of 1.45 Å for both
complexes. These bond lengths are comparable with those
Three additional [2+2] cycloadducts (μ-η2-RCCH)[Cr(μ-HC(N-
2,6-iPr2C6H3)2)]2 (R = Ph (2), SiMe3 (3), Bu (6)) were prepared
t
herein. Compounds 2, 3 and 6 displayed similar 1H NMR
features and the solid-state crystal structure of 3 is presented in
Figure 1. Interestingly, the addition of three equiv of RCCH (R =
nPr, Ph) to 1 led to the formation of a trace amount of 1,3,5-
R3C6H3 (R = nPr, Ph) as assayed by 1H NMR spectroscopy. The
reported values found for the μ-η6:η6-arene ligands in inverted
18]
sandwich compounds,[17,
but slightly longer than those
reported for free benzene (1.397(1) Å)[19] and toluene (1.39(1)
Å).[20] Moreover, the bridging arene rings in 5 and 8 are not
planar as indicated by inequivalent distances from two Cr
centers to a carbon atom attached by a SiMe3 group. For
example, the Cr(1)-C(54) bond length in 5 is 2.227(3) Å, and the
Cr(2)-C(54) distance is 2.341(3) Å. Observed Cr-C bond lengths
vary from 2.194(3) to 2.341(3) Å in 5 and from 2.208(3) to
2.347(3) Å in 8. The distances between the bridging arene
n
isolated yields of 1,3,5-R3C6H3 increased to 67% (R = Pr) and
86% (R = Ph), if 20 equiv of RCCH were added to 1 in Et2O at
room temperature (Scheme 1). On the other hand, reactions of 1
and 3 equiv of Me3SiCCH in Et2O unexpectedly produced a
nondiamagnetic species and
a
trace amount of 1,3,5-
1
(Me3Si)3C6H3 revealed by H NMR spectroscopy. It turned out
that an inverted sandwich dichromium complex with a 1,3,5-
(Me3Si)3C6H3 molecule bridging two Cr(κ2-HC(N-2,6-iPr2C6H3)2
fragments in an η6,η6 fashion was obtained. In addition, a similar
compound was isolated from the reaction of the newly prepared
quintuply bonded dichromium complex [Cr{κ2-HC[(N-2,6-
iPr2C6H3)(N-2,6-Me2C6H3)]}]2 (7), where the Cr-Cr distance is
1.7444(15) Å, and three equiv of Me3SiCCH in Et2O. Both
inverted sandwich complexes (μ-η6:η6-1,3,5-(Me3Si)3C6H3)[Cr{κ2-
centroid and both chromium centers in
5 and 8 are
approximately equivalent, with average Cr-centroid separation of
1.746 Å (5) and 1.734 Å (8). By way of contrast, the benzene
centroid is not equidistant from two Cr atoms in the more
sterically congested complexes (μ-η6:η6-Arene)[Cr(nacnac)]2, the
Cr-centroid distances of 1.755 and 1.853 Å being longer.[17]
The electronic structures and unusual high spin state of 5
HC[(N-2,6-iPr2C6H3)(N-2,6-R2C6H3)]}]2 (R = Pr (5), Me (8)) were
and 8 were elucidated by performing DFT/BP86–GD3BJ
i
isolated in a respective 58 and 38% yield as dark crystalline
solids. Both displayed paramagnetic nature and exhibited a
respective solid-state magnetic moment of 6.45 and 6.83 μB as
measured by SQUID magnetometry, in line with an S = 3 ground
state (see Figure S1 in the Supporting Information).
We have demonstrated that 1 was prepared from KC8
reduction of the mononuclear amidinate species CrCl2(THF)(κ2-
HC(N-2,6-iPr2C6H3)2.[10d] Since the inverted arene sandwich
calculations to the geometry optimization of 5 in the septet state.
The optimized structure showed good agreements with the
experimental geometry (see Table S1 in the SI). The energies of
various spin states of 5 were also computed on the model
compound (μ-η6:η6-C6H6)[Cr{κ2-HC(NMe)2}]2 (5m) in D2 symmetry
by using the CASSCF method (see Figure S3 in the SI). Shown
in Figure S4, ten electrons in ten orbitals φ5, φ6, φ7, φ8, φ11, φ12,
φ13, φ14, φ15, and φ16 were involved in the active space of the
CASSCF calculation. Both natural orbitals Φ5 and φ6 revealed
complexes were typically prepared from reduction of
18]
metal−halide precursors in aromatic solvents,[17,
5 was
back donations from two groups of out of phase dx2 2···dx2 2
-y -y
alternatively obtained in a good yield (67%) from KC8 reduction
of CrCl2(THF)(κ2-HC(N-2,6-iPr2C6H3)2 (9)[10d] with five equiv of
1,3,5-(Me3Si)3C6H3 in Et2O. The molecular structures of 5 and 8
were deciphered by single crystal X-ray crystallography and are
depicted in Figure 1. Two CN2Cr four-membered rings are
arranged in an orientation with a N(1)–Cr(1)–Cr(2)–N(4) torsion
angle of 54.9(2) (5) and 67.4(2)° (8). The most salient structural
feature of 8 is that two less bulky 2,6-Me2C6H3 substituents are
and dxy···dxy orbitals to the doubly degenerate LUMOs of the
benzene ligand in a δ manner. The φ11, φ12, φ13, φ14, φ15, and φ16
orbitals were essentially nonbonding between benzene and two
sextet Cr[κ2-HC(NMe)2] fragments, and they are singly occupied
by the remaining 6 electrons. These results revealed that three
unpaired spins on each Cr centre in 5 and 8 were coupled in a
ferromagnetic fashion (vide infra). The calculated occupation
numbers for both natural bonding orbitals φ5 and φ6 (1.68),
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10.1002/anie.201709583
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those mainly Cr d character orbitals φ11, φ12, φ13, φ14, φ15, and
φ16 (1.00), and both antibonding orbitals φ7 and φ8 (0.32) led the
bond order between the Cr atom and benzene to be 1.34, much
smaller than its formal bond order of 2, resulted from two doubly
occupied bonding orbitals between them.
(95%) of 1,3,5-(Me3Si)3C6H3 was achieved when the reaction
was carried out in THF.
To understand how the Cr-Cr quintuple bond of 1 was
cleaved and restored during catalysis, DFT calculations were
invoked to elucidate the possible mechanism. Since the [2+2]
reaction involving 1 and one molecule of alkyne has been
investigated,[16] the calculations essentially started from the
singlet 3 (13) (see Figure S6 in the SI). Axial coordination of one
molecule of Me3SiCCH to the less sterically encumbering Cr1 of
13 (Cr-Cr = 1.75 Å) (Figure S6a) elicit a dramatic change of the
ligation mode of two amidinates from a bridging to chelating
fashion (Figure S6b). Meanwhile, two alkyne molecules
underwent a head-to-tail coupling at Cr2 to give the dinuclear
chromacyclopentadiene complex 1bs4 (Figure S6b), which was
the rate-determining step during catalysis. In 1bs4, the chromole
Scheme 2. Reversible cleavage/formation of the Cr-Cr quintuple bond in the
cyclotrimerization of Me3SiCCH.
C4Cr ring was bonded to Cr1 in
a
fashion of η5-
chromacyclopentadiene. The Cr-Cr distance was drastically
elongated to 2.57 Å, and antiferromagnetic coupling between the
unpaired spins on the two metal centers led to an open shell
singlet ground state. It is interesting to note that magnetic
exchange behaviors between two Cr centers of all intermediates
during catalysis were dependent on the Cr-Cr distances. As the
Cr-Cr separation increased, ferromagnetic coupling became
more significant (see Figure S3), as was the case with both 74b
and
75
(see
Figure
S6b).
Although
dinuclear
metallacyclopentadienes have been well documented,[22] only
dinuclear Co,[23] Ta,[24] and Ni[25] complexes were involved in
catalytic cyclotrimerization of alkynes. It should be pointed out
that subsequent reactions of these dinculear Co, Ta, and Ni
metallacyclopentadienes with one molecule of alkyne uniformly
afforded the arene-capped dinuclear complexes, where their
metal-metal bonds and coordination modes of ligands were
retained during catalysis. On the contrary, 1bs4 reacted with the
third equivalent of alkyne to give the inverted arene sandwich
dichromium species 5 (75 in Figure S6b) instead of the singlet
arene-capped dichromium compound containing two chelating
amidinates (1bs4f in Figure S7) with the Cr-Cr bond length of 2.07
Å. Compound 75 was energetically lower than 1bs4f by 13.8
kcal/mol (Figure S7). In THF or the presence of MesCN,
Scheme 3. No cross reactions between 5 and 8 in THF.
Further analysis of CASSCF wave function on the model
complex (μ-η6:η6-C6H6)[Cr{κ2-HC(NMe)2}]2 indicated that the
charge of each Cr center is +1.52 and that of the bridging
benzene is ‒1.03 (Figure S5). It is thus presumed that strong
electrophilicity of three silyl groups played an important role to
stabilize the bridging benzene of anionic character in 5 and 8.[21]
Interestingly, the bridging 1,3,5-(Me3Si)3C6H3 of 5 in Et2O was
liberated in the presence of half an equiv of MesCN (Mes =
2,4,6-Me3C6H2) or in THF (Scheme 2). Upon the elimination of
1,3,5-(Me3Si)3C6H3 from 5, 1 was unexpectedly regenerated. It is
worth noting that only 1 and 7 were detected by 1H NMR
spectroscopy from the mixture of 5 and 8 in THF (Scheme 3),
which implicates the mononuclear fragments “Cr(amidinate)”
were not produced in THF. Complex 5 is indeed an intermediate
in the reaction mediated by 1, and 8 in the analogous process
promoted by 7. As assayed by 1H NMR spectroscopy, only a
trace amount of 1,3,5-(Me3Si)3C6H3 was produced from the
reaction between 5 and 20 equiv of terminal alkynes Me3SiCCH
in Et2O, while the addition of half an equiv of MesCN
dramatically increased the yield of 1,3,5-(Me3Si)3C6H3 to 60%
along with a 14% yield of 1,2,4-(Me3Si)3C6H3. The best yield
7
coordination of these donors to Cr centers of 5 led to extrusion
of the bridging arene molecule and regeneration of 1 (Figures
S8 and S9). As a result, a catalytic cycle of cyclotrimerization of
alkynes mediated by 1 was thus proposed in Scheme 4.
In summary, we have demonstrated for the first time that
the ultrashort Cr-Cr quintuple bonds stabilized by amidinates
could be readily cleaved and restored upon reacting with
terminal alkynes, from which two unusual inverted arene
sandwich dichromium amidinates 5 and 8 with a high spin state
were characterized. Both 5 and 8 are the first two examples of
inverted arene sandwich dinuclear complexes with two strained
four-membered
[Cr(κ2-amidinate)]
fragments.
The
characterization of 5 and 8 provides an alternative pathway to
prepare the quintuply bonded dichromium complexes.
Furthermore, the structural elucidation of 5 and 8 accounts for
the excellent regioselectivity in the catalytic cyclotrimerization of
terminal alkynes mediated by 1; 1,3,5-R3C6H3 are better arene
ligands than 1,2,4-R3C6H3 in stabilizing two [Cr(κ2-amidinate)]
moieties by minimizing steric repulsions among these three
fragments. Exploring the implications of the reversible cleavage
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For support of this work, Y.-C.T. is grateful to the Ministry of
Science and Technology (MOST), Taiwan (Grant 105-2113-M-
007-009-MY3) and the Frontier Research Center on
Fundamental and Applied Sciences of Matters of National Tsing
Hua University. J.-S.K.Y. is indebted to the MOST (Grants 103-
2113-M-009-014-MY3 and 106-2113-M-009-018-MY3) for
financial support. G.-T.H. acknowledges postdoctoral fellowships
from MOST 106-2811-M-009-040. We also thank Ting-Shen
Kuo of Department of Chemistry, National Taiwan Normal
University for assistance with X-ray crystallography.
[13] For the most relevant review article, see: I. G. Powers, C. Uyeda, ACS
Catal. 2017, 7, 936-958.
Keywords: Cr-Cr quintuple bond • catalysis • alkyne • [2+2+2]
cyclotrimerization • DFT calculations
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Entry for the Table of Contents
COMMUNICATION
The quintuply bonded dichromium
bisamidinate catalyzes the highly
regioselective cyclotrimerization of
terminal alkynes involving reversible
cleavage of the Cr-Cr quintuple bond.
From these catalytic reactions, two
inverted arene sandwich dichromium
amidinates were isolated, where the
bridging arene is 1,3,5-(Me3Si)3C6H3,
which account for the catalytic
mechanism of terminal alkyne
Yu-Siang Huang, Kuo-Tao Hwang, Yao-
Lun Liu, Jen-Shiang K. Yu,* Yi-Chou
Tsai *
Page No. – Page No.
Reversible Cleavage/Formation of the
Cr-Cr Quintuple Bond in the Highly
Regioselective Alkyne
Cyclotrimerization
cyclotrimerization.
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