Reactions of metal-metal quintuple bonds with alkynes: [2+2+2] and [2+2] cycloadditions

Article abstract of DOI:10.1002/anie.201205027

High five. The quintuply bonded molybdenum amidinate 1 reacts with two equivalents of 1-pentyne through a [2+2+2] cycloaddition reaction to give the first examples of the benzannulated product 2. The C₄Mo₂ rings exhibit aromatic character. Treatment of 1 with internal alkynes afforded the 2:1 and 1:1 [2+2] cycloadducts, which are mechanistically associated with the formation of 2. Copyright

Full text of DOI:10.1002/anie.201205027

Angewandte
Chemie
DOI: 10.1002/anie.201205027
Aromaticity
Reactions of Metal–Metal Quintuple Bonds with Alkynes: [2+2+2]
and [2+2] Cycloadditions**
Hong-Zhang Chen, Shih-Cheng Liu, Chun-Hsu Yen, Jen-Shiang K. Yu,* Yun-Jen Shieh, Ting-
Shen Kuo, and Yi-Chou Tsai*
Benzene, C6H6, is one of the most important molecules in
organic chemistry. The term aromaticity[1] was developed
from the study of benzene and has become an important and
fundamental concept in contemporary chemistry since
Kekulꢀ proposed a ringlike structure for benzene in 1865.[2]
Today, aromatic compounds are not only limited to carbo-
cycles, but also extended to heterocycles containing main-
group elements. For example, one or two CH groups of
benzene can be replaced by group 14 and 15 elements, such as
silicon, nitrogen, and phosphorus, to give heterocyclic aro-
matics.[3] Furthermore, an aromatic compound can be a tran-
sition-metal-containing organometallic species. Hoffmann
et al. predicted that a metallabenzene could be possible by
replacing one CH group with a transition-metal fragment.[4]
On the experimental side, since the recognition of the first
metallabenzene, osmabenzene, in 1982,[5] many metallaben-
zenes have been subsequently synthesized and structurally
characterized.[6]
The chemistry of transition-metal multiple bonding was
recently reawakened by the recognition the first isolable
quintuply bonded dimeric chromium complex Cr2Ar’2 (Ar’ =
C6H3-2,6-(C6H3-2,6-iPr2)2 by Power and co-workers.[10] Since
then, several quintuply bonded dinuclear group VI metal[11,12]
complexes stabilized by nitrogen donors have been
reported.[13] Besides interesting bonding, the chromium
derivatives have recently been shown to display novel
reactions with AlMe3,[11f] N2O, RN3,[14] and P4.[15] Reactions
of the chromium dimers with alkynes through a [2+2] cyclo-
addition process to give 1:1 adducts were recently explored as
well.[16] In light of the bonding analogy between the C C
À
p components of alkynes and metal–metal d components in
the quintuply bonded species (Figure 1), we became inter-
There are many ways to synthesize aromatic compounds.
Among the methods, the [2+2+2] cycloaddition reaction of
alkynes is a powerful tool to construct highly substituted
homo- and heterocyclic aromatic molecules. Such a reaction is
thermally allowed,[7] but it does not proceed at ambient
temperature without transition-metal catalysts.[8] It is how-
ever worth noting that Sekiguchi et al. have demonstrated
a catalyst-free [2+2+2] cycloaddition reaction for the syn-
thesis of 1,2-disilabenzene by reacting a kinetically stable
ꢀ
disilyne RSi SiR (R = SiiPr[CH(SiMe3)2]2) and phenylacety-
À
À
Figure 1. Frontier orbitals of the M M quintuple bond and C C triple
bond.
lene at room temperature.[9]
[*] Dr. H.-Z. Chen, S.-C. Liu, C.-H. Yen, Y.-J. Shieh, Prof. Dr. Y.-C. Tsai
Department of Chemistry and Frontier Research Center
on Fundamental and Applied Sciences of Matters
National Tsing Hua University, Hsinchu 30013 (Taiwan)
E-mail: yictsai@mx.nthu.edu.tw
ested in exploring reactions between them. Herein, we
present the characterization of the first two complexes
containing a cyclically delocalized six-membered C4Mo2
À
aromatic ring incorporating a quadruple Mo Mo bond, and
the aromaticity involves d electrons. These aromatics were
obtained by reacting the quintuply bonded dimolybdenum
amidinates [Mo2{m-k2-RC(N-2,6-iPr2C6H3)2}2] [R = H (1), Ph
(2)][12a] with two equivalents of a terminal alkyne through
a [2+2+2] cycloaddition pathway.
Prof. Dr. J.-S. K. Yu
Institute of Bioinformatics and Systems Biology and
Department of Biological Science and Technology
National Chiao Tung University, Hsinchu, 30010 (Taiwan)
E-mail: jsyu@mail.nctu.edu.tw
T.-S. Kuo
As illustrated in Scheme 1, reaction of a THF solution of
1 and an ethereal solution of 2 with two equivalents of 1-
pentyne at room temperature yielded the diamagnetic
[2+2+2] cycloaddition products [Mo2(m-k1:k1-1,3-nPr2C4H2)-
{m-k2-RC(N-2,6-iPr2C6H3)2}2] (3 and 4, respectively), which
were isolated as green crystalline solids in 59.8% and 66.8%
yield, respectively. Both 3 and 4 are air and moisture sensitive.
Department of Chemistry, National Taiwan Normal University
Taipei 11677 (Taiwan)
[**] We are grateful to the National Science Council, Taiwan for financial
support under Grants NSC 99-2113-M-007-012-MY3 (YCT) and 100-
2627-B-009-001 (JSKY), and the “Center for Bioinformatics Research
of Aiming for the Top University Program” of NCTU and MoE,
Taiwan.
It is worth pointing out that monitoring these two reactions by
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201205027.
1
À
H NMR spectroscopy indicated that 3 and 4 (head-to-tail C
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structure, whereby each Mo2 unit is span-
ned by the newly formed 1,3-butadienyl
group and two amindinato ligands, and is
therefore three-coordinate with respect to
the ligands. The structural arrangement at
each Mo center is better described as
a
distorted T-shaped geometry, which
results from the steric interactions between
the 1,3-butadienyl group and the bulky
À
amidinato ligands. The average Mo N
bond length (2.142 ꢁ for 3 and 2.144 ꢁ
for 4) is only slightly longer than that of
1 and 2 (2.117 ꢁ for both).
Scheme 1. Synthesis of complexes 3 and 4. THF=tetrahydrofuran.
The most salient structural feature of
both 3 and 4 is the six-membered C4Mo2
metallacycle. These two rings are almost
C coupling regioisomer) were the sole products as evidenced
by signals for only one amidinato ligand environment and two
sets of nPr groups.
Recrystallization of 3 from a diethyl ether/n-hexane (1:1)
solution at room temperature and 4 from n-hexane at À358C
furnished green crystals suitable for X-ray diffraction stud-
ies.[17] Both molecular structures of 3 and 4 are presented in
Figure 2. In principle, both 3 and 4 display a lantern-type
planar with an average internal torsion angle of 1.65 and 2.658
for 3 and 4, respectively. It is interesting to note that two nPr
substituents in both compounds are on the same face of the
six-membered rings. Such an arrangement accounts for the
less planar C4Mo2 ring of 4, and is attributed to the stronger
steric repulsion between the two nPr groups and two 2,6-
iPr2C6H3 substituents of the amidinates. The bond lengths of
Mo1-C54 [1.988(5) ꢁ] and Mo2-C57 [1.982(5) ꢁ] in 3 and
Mo1-C66 [1.999(5) ꢁ] and Mo2-C69 [1.988(5) ꢁ] in 4 are
approximately the same, and they are intermediate between
the lengths of Mo C single and double bonds.[18] In addition,
À
À
all C C bond lengths in these two metallacycles are essen-
tially the same and range from 1.375(7) to 1.402(7) ꢁ, which is
consistent with the C C bond length (1.39–1.40 ꢁ)
[19]
À
in
benzene. As is usually the case for a dinuclear complex, the
most intriguing metric parameter is the metal–metal separa-
À
tion. The Mo Mo bond length in 3 and 4 is 2.1525(6) and
2.1344(7) ꢁ, respectively. These values are equal to those
5+
(2.119–2.164 ꢁ) of the paddlewheel Mo2 complexes having
the Mo–Mo formal bond order of 3.5.[20–27] For instance, the
noncarbon aromatic six-membered dimolybdenum/chalcogen
Mo2E2Mo2 clusters in [{Mo2(HC(N-4-MeOC6H4)2)3}2(E)2]
(E = O,[25] S,[26] Se[27]) supported by amidinates with a Mo–
À
Mo bond order of 3.5 have Mo Mo bond lengths which fall
within the range of 2.135–2.155 ꢁ.
Besides structural parameters, both 3 and 4 display
interesting chemical shifts in their 1H NMR spectra. Most
importantly, two distinct resonance signals corresponding to
the C4Mo2 ring protons were observed in the low-field
aromatic region at d = 12.6 and 8.3 ppm (3) and d = 12.5 and
8.3 ppm (4) as a result of the ring current. It is interesting to
note that these spectra show an unusually large NMR
displacement for two protons in a ring, Dd = 4.2 ppm. The
more downfield shifted signals are assigned to the meta
protons, whereas the ortho protons in the vicinity of Mo2 are
more shielded and observed upfield in the NMR spectra.
Moreover, the signals of ring carbon atoms resonate in
a typical aromatic region as well. Therefore, consistent with
the archetype of Hꢂckel-type aromaticity, the C4Mo2 rings in
complexes 3 and 4 are aromatic and represent the first
example of stable quadruply bonded 1,2-dimolybdabenzene
derivatives.
Figure 2. Molecular structures of a) 3 and b) 4 with thermal ellipsoids
at 35% probability. Hydrogen atoms have been omitted for clarity.
Selected bond lengths [ꢀ] and angles [8]: 3: Mo1–Mo2 2.1525(6),
Mo1–C54 1.988(5), Mo2–C57 1.982(5), C54–C55 1.391(7), C55–C56
1.396(7), C56–C57 1.387(7); N1-Mo1-N4 155.40(14), N2-Mo2-N3
152.40(14); 4: Mo1–Mo2 2.1339(7), Mo1–C66 1.999(5), Mo2-C69
1.988(5), C66–C67, 1.400(7), C67–C68 1.402(8), C68–C69 1.375(7);
N1-Mo1-N4 147.59(16), N2-Mo2-N3 148.62(16).
2
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In addition to experimental evidence, the electronic
delocalization in the C4Mo2 metallacycles of 3 and 4 can be
further realized by theoretical calculations. In this regard, we
performed DFT calculations on compound 4 at the BP86/
def2-TZVP level of theory to reproduce an optimized
structure, whose geometry and structural parameters are in
good agreement with the X-ray data (see Table S16 in the
Supporting Information). For example, two nPr groups are
also on the same side of the C4Mo2 ring in the computed
intermediate metallacyclopentadiene system.[7] We then won-
dered if 3 and 4 could subsequently react with another
equivalent of an alkyne to produce a trisubstituted benzene.
To our delight, 3 is indeed capable of catalyzing cyclo-
trimerization of 1-pentyne at ambient temperature, whereas 4
is inactive towards this reaction, presumably because of its
more sterically congested ligands. When a diethyl ether
solution of 3 was treated with 20 equivalents of 1-pentyne,
1,3,5-nPr3C6H3 was obtained as the sole product in 88% yield
upon isolation. The same product in a high yield was also
observed when 20 equivalents of 1-pentyne was added to
a THF solution of 1. The high regioselectivity of alkyne
cyclotrimerization is controlled by the bulky amidinato
ligands of 3. A similar effect was also recently demonstrated
by a bulky b-diketiminate-supported monovalent vanadium
complex.[29]
À
structure, and the Mo–Mo distance is 2.154 ꢁ, and the Mo C
bond lengths are 2.019 and 1.982 ꢁ. The molecular orbitals
depicting the bonding schemes of the aromatic C4Mo2 ring are
shown in Figure 3. Of particular interest is the HOMO, which
The reactions of 1 and 2 with an internal alkyne, 3-hexyne,
were also performed. In sharp contrast to the reactions of
1 and 2 with 1-pentyne giving the [2+2+2] cycloaddition
products 3 and 4, respectively, treatment of 1 in THF or 2 in
diethyl ether with 2 equivalents of 3-hexyne afforded the
diamagnetic [2+2] cycloaddition adducts Mo2(H)(m-k2:k2-
EtCCEt)2[k2-HC(N-2,6-iPr2C6H3)2][k3-HC(N-2,6-iPr2C6H3)-
(N-2-CH(CH2)CH3-6-iPrC6H3)] (5) and [Mo2(m-k2-EtCCEt)2-
{m-k2-PhC(N-2,6-iPr2C6H3)2}2] (6; Scheme 2), respectively. In
addition, reactions of the quintuply bonded dimeric chro-
mium amidinate [Cr2{m-k2-HC(N-2,6-iPr2C6H3)2}2] (7)[11d] with
either internal or terminal alkynes exclusively yielded the
[2+2] complexation adducts [Cr2(m-k2-R2CCR3)2{m-k2-HC(N-
2,6-iPr2C6H3)2}2] (8: R2 = H, R3 = nPr; 9: R2 = R3 = Et; 10:
R2 = R3 = Ph). Their formulations are evidenced by 1H NMR
spectroscopy. For example, two distinct ligand environments
in 5 are supported by two resonance signals (d = 7.58 and
7.55 ppm) corresponding to two methine (NCHN) protons of
the formamidinate ligands. Two diastereotopic protons of the
metalated CH(CH3)CH2Mo fragment appear at d = 2.29 (dd)
and 0.77 (dd) ppm and the singlet hydride signal is at d =
1
2.44 ppm. In the room-temperature H NMR spectrum of 6,
four unequivalent isopropyl groups are observed, with CH
resonances at d = 2–4 ppm and diastereotopic Me groups at
d = À0.46–1.59 ppm.
Figure 3. The frontier orbitals of 4. a) HOMO. b) HOMOÀ1.
c) HOMOÀ21.
The molecular structures of 5–6 and 8–10 were deter-
mined by single-crystal X-ray diffraction,[17] and are depicted
in the Supporting Information (Figures S1–5). The X-ray
study of 5 indicates two terminally chelating amidinates and
corroborates the presence of a pair of uncoupled and
unequivocally indicates the participation of the Mo-Mo
d bonding in pp(C)-dd(Mo) for C66-Mo1-Mo2 conjugation
and the pp–pp p interaction between C67 and C68. The
HOMOÀ1 is mainly C66-C67 and C68-C69 p bonding with
some contribution from the Mo atomic orbitals. A combina-
tion of in-phase atomic orbitals of pp(C) and dd(Mo) from the
C4Mo2 ring is observed in HOMOÀ21. As a result, the C4Mo2
ring is a six-electron six-center p-d conjugation system, which
comprise four p electrons from the 1,3-butadienyl fragment
and two electrons from the Mo-Mo d (dxy-dxy) bond. In
addition, the pp(C)-dd(Mo) p conjugation within the C4Mo2
ring can be further corroborated by the smaller effective Mo–
Mo bond order of 2.66 based on NBO analyses.[28]
À
perpendicularly bridged 3-hexyne ligands. The Mo Mo
bond length of 2.4843(11) ꢁ is in the double bond range.[30]
À
The bridging central alkyne C C bond lengths, 1.297(16) and
1.348(18) ꢁ, are comparable to those in ethylene
[1.339(1) ꢁ][31] and the only two examples of bis(alkyne)
ditungsten adducts.[32] The C-C-C angles of the coordinated
alkynes are in the range of 137.5(12)–142.5(15)8, which are
close to that of an sp2-hybridized carbon atom, thus indicating
extensive bonding to the dimolybdenum center. Another
prominent structural feature in 5 is the presence of one
From the mechanistic point of view of the metal-catalyzed
alkyne [2+2+2] cycloaddition reactions, two monoynes react
with the metal catalyst to engender the formation of an
cyclometalated amidinate ligand resulting from an intramo-
3
À
lecular sp C H oxidative addition to two Mo atoms. The
hydrido ligand (located from Fourier difference maps and
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Scheme 2. Synthesis of complexes 5–6 and 8–10.
refined by least-squares) is terminally bound to Mo1 with
of a terminal alkyne through the [2+4] cycloaddition reaction
to yield the [2+2+2] cycloaddition products 3 and 4.
Reactivity studies of 3 and 4 are currently underway and
will be reported in due course.
À
a Mo1 H bond length of 1.852(3) ꢁ.
In contrast, all the other [2+2] cycloaddition alkyne
adducts 6 and 8–10 exhibit a similar molecular structure:
a central four-membered C2Mo2 metallacycle supported by
two bridging amidinates. The most interesting structural
feature in these adducts is the bridging alkyne ligands skewed
with respect to the metal–metal bond axes. The formation of
compounds 6 and 8–10 is essentially the result of the oxidative
addition of 2 and 7 to the alkynes, which are reduced by two
electrons. This reaction is supported by the elongation of the
Received: June 27, 2012
Published online: && &&, &&&&
Keywords: aromaticity · cycloaddition · electronic structure ·
.
multiple bonds · molybdenum
À
coordinated alkyne C C bonds [6: 1.364(5) ꢁ, 8: 1.324(4) ꢁ,
9: 1.341(10) ꢁ, 10: 1.356(4) ꢁ], and the hybridization around
the carbon atoms changes from sp to sp2, because the C-C-R
bond angles decrease from 1808 to between 1258 and 1308.
Furthermore, these complexes have very short metal–metal
quadruple bond lengths of 2.0603(4) (6), 1.8054(6) (8),
1.7981(14) (9), and 1.8233(7) ꢁ (10). In combination with
Kempe and Theopoldꢃs analogues,[16] reactions between
quintuply bonded dichromium complexes and internal or
terminal alkynes solely yield the 1:1 [2+2] cycloaddition
products.
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
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.
Angewandte
Communications
Communications
Aromaticity
H.-Z. Chen, S.-C. Liu, C.-H. Yen,
J.-S. K. Yu,* Y.-J. Shieh, T.-S. Kuo,
Y.-C. Tsai*
&&&& — &&&&
Reactions of Metal–Metal Quintuple
Bonds with Alkynes: [2+2+2] and [2+2]
Cycloadditions
High five! The quintuply bonded molyb-
denum amidinate 1 reacts with two
equivalents of 1-pentyne through
a [2+2+2] cycloaddition reaction to give
the first examples of the benzannulated
product 2. The C4Mo2 rings exhibit aro-
matic character. Treatment of 1 with
internal alkynes afforded the 2:1 and 1:1
[2+2] cycloadducts, which are mechanis-
tically associated with the formation of 2.
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!