Synthesis and characterization of a rhenabenzyne complex

Article abstract of DOI:10.1002/anie.201104587

Complex rearrangements: Rhenabenzynes, which are metallaaromatic compounds related to metallabenzenes (see scheme), can be obtained from thermally unstable rhenium-carbyne complexes through rearrangement reactions. The rhenabenzyne complex was characterized by elemental analysis, ¹H NMR spectroscopy, and X-ray diffraction analysis. The results show that it is possible to prepare metallabenzynes with different transition metals. Copyright

Full text of DOI:10.1002/anie.201104587

DOI: 10.1002/anie.201104587
Metallacycles
Synthesis and Characterization of a Rhenabenzyne Complex**
Jiangxi Chen, Herman H. Y. Sung, Ian D. Williams, Zhenyang Lin,* and Guochen Jia*
There has been much interest in the chemistry of transition-
metal-containing metallaaromatic compounds. In particular
metallabenzenes, which are organometallic compounds
À
derived by the formal replacement of a C H group in
benzene with an isolobal transition metal fragment, have
attracted considerable attention.[1] Previous studies have led
to the isolation and characterization of an impressive number
of stable metallabenzenes, especially those of osmium,[2–4]
iridium,[5–8] platinum,[9] ruthenium,[10,11] and rhenium.[12]
Metallabenzynes,[13] which are organometallic compounds
À
derived from formal replacement of a carbon atom or a C H
group in benzyne with an isolobal transition metal fragment,
are closely related to metallabenzenes. At first sight, one
might expect that metallabenzynes may be too unstable to be
ꢀ
isolated, because organic compounds with a C C bond in the
six-membered ring, for example, benzyne and cyclohexyne,
are thermally highly unstable because of ring strain. Never-
theless, in recent years, several stable osmabenzynes have
been isolated[14] and their interesting chemical properties
have been demonstrated. For example, osmabenzynes, such as
metallabenzenes and aromatic compounds, can undergo
electrophilic substitution reactions.[14b]
Compared with the chemistry of metallabenzenes, that of
metallabenzynes is much less developed. For instance, while
stable metallabenzenes with several metals are known, to
date, isolated metallabenzynes are limited to those containing
an osmium center. The synthesis and characterization of
metallabenzynes with metals other than osmium remain
important goals in the development of the chemistry of these
compounds. Herein, we report the preparation and structural
characterization of the first stable rhenabenzyne complex.
The rhenabenzyne complex was synthesized by the
synthetic route outlined in Scheme 1. Reaction of [ReH5-
(PMe2Ph)3] (1) with alkynol 2 in the presence of HCl
produced the dichlorocarbyne complex 3. Treatment of 3
with tert-butylmagnesium chloride produced the hydrido-
chlorocarbyne complex 4, likely through the intermediate
Scheme 1. Preparation of rhenabicyclo[3.1.0]hexatriene 6 and rhena-
benzyne 7.
spectroscopy. Complex 4 was isolated as a blue solid in 63%
yield, and could be stored under nitrogen at room temper-
ature for a week without appreciable decomposition. The
Me3Si group in 4 could be easily removed by treatment of 4
with nBu4NF to give carbyne complex 5. The new carbyne
complexes 3–5 can be readily identified by NMR spectrosco-
py. For example, the 1H NMR spectrum of 5 shows the Re–H
signal at 1.85 ppm as a doublet of triplets with P–H coupling
ꢀ
=
ꢀ
[Re( C-CH C(tBu)C CSiMe3)Cl(tBu)(PMe2Ph)3], which
underwent b-H elimination. Experimentally, the side product
CH2 CMe2 has indeed been detected by in situ 1H NMR
=
ꢀ
constants of 50.0 and 28.0 Hz, the Re CCH signal at
[*] Dr. J. Chen, Dr. H. H. Y. Sung, Prof. Dr. I. D. Williams,
Prof. Dr. Z. Lin, Prof. Dr. G. Jia
ꢀ
4.42 ppm, and the C CH signal at 3.22 ppm; the
13
1
Department of Chemistry
ꢀ
C{ H} NMR spectrum of 5 shows the Re C signal at
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon (Hong Kong)
E-mail: chjiag@ust.hk
ꢀ
254.1 ppm and the C CH signals at 88.4 and 83.8 ppm.
In the solid state, complex 5 can be stored under nitrogen
for at least three days without any decomposition, however, it
is thermally unstable in solution. When a solution of 5 in
benzene was stored at room temperature for three days,
complex 5 completely rearranged to form complex 6. The
rearrangement reaction was complete within 4 h when a
solution of 5 in benzene/hexane was heated at 508C. Complex
chzlin@ust.hk
[**] This work was supported by the Hong Kong Research Grant Council
(HKUST601007, 602611 and HKU1/CRF/08). We thank Chuan Shi
for his help in the NBO analysis of complex 6.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201104587.
Angew. Chem. Int. Ed. 2011, 50, 10675 –10678
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10675

Communications
6 could be isolated as a red solid in 74% yield. In the solid
resonance form that contributes to the overall structure to
give the shorter C1–C2 bond and the almost linear C1-C2-C3
moiety. We know that the four substituents on an allene
moiety in the ground state are not in the same plane. As the
atoms bonded to C1 and C3 in complex 6 are nearly coplanar,
6b cannot be a stable resonance form, and is therefore not
expected to make a significant contribution to the overall
structure. The molecular structure is better described by a
resonance hybrid of 6 and 6a shown in Scheme 1. Natural
bond orbital (NBO) analyses suggest that Re–C1 can be
considered to have double-bond character, while Re–C5 and
Re–C2 have single-bond character. The calculated Wiberg
bond indices (bond orders, which are a measure of bond
strength) of Re–C1, Re–C2, and Re–C5 are 1.22, 0.70, and
0.91, respectively (see the Supporting Information for the
calculated bond indices of the remaining bonds in the
metallacycle).
state, complex 6 remained unchanged when it was stored
under nitrogen for several days. When a solution of 6 in
benzene was stored for 20 days at room temperature, complex
6 completely rearranged to complex 7.
The structure of complex 6 has been confirmed by an X-
ray diffraction study,[15] and contains an essentially planar
bicyclic metallacycle (Figure 1). The maximum deviation
from the least-squares plane through Re1 and C1–C5 is
0.028 ꢀ for C1. The C1–C2, C2–C3, C3–C4, and C4–C5 bond
lengths are 1.310(11), 1.337(11), 1.426(12), and 1.350(14) ꢀ,
respectively. The Re–C1 bond length of 1.933(7) ꢀ is within
=
the range of those reported for typical Re CHR(carbene)
bonds (1.850–2.143 ꢀ),[16,17] and shorter than those reported
for typical Re–C(vinyl) bonds (1.996–2.305 ꢀ).[16,18] The Re–
C5 bond length of 2.158(8) ꢀ is within the range of those
reported for typical Re–C(vinyl) bonds,[16,18] and longer than
those reported for typical Re CHR(carbene) bonds.[16,17] Our
Consistent with the solid-state structure, the 1H NMR
spectrum of 6 showed signals at 8.79, 9.47, and 14.92 ppm for
the three protons on C4, C5, and C1 of the metallacycle,
respectively. The 13C{1H} NMR spectrum of 6 showed signals
at 208.0 (C1), 169.9 (C5), 151.0 (C2), 147.3 (C4), and
132.9 ppm (C3).
Rhenabenzyne 7 has been characterized by NMR spec-
troscopy, elemental analysis, as well as X-ray diffraction. The
1H NMR spectrum of 7 showed the signals of the three
protons on C2, C4, and C5 of the metallacycle at 4.92, 8.06,
and 12.28 ppm, respectively. The 13C{1H} NMR spectrum of 7
=
preliminary DFT calculations show that the optimized
structure agrees well with the structural features of 6
described above (see the Supporting Information for details).
Scheme 1 shows two resonance forms that contribute to
the overall structure of the metallacycle of complex 6:
metallabicyclo[3.1.0]hexatriene complex 6 and alkyne–car-
bene complex 6a.[19] The structural feature suggests that
either one of the two resonance forms alone cannot properly
describe the molecular structure. The resonance form 6 can
account for the bond lengths of the Re–C1, Re–C2, Re–C5,
C2–C3, C3–C4, and C4–C5 bonds, but cannot adequately
account for the fact that C1–C2 is even shorter than C2–C3
and that the C1-C2-C3 moiety is almost linear. The shorter
C1–C2 bond and the almost linear C1-C2-C3 moiety can be
related to the contribution from the resonance form 6a.
Alternatively, one might consider 6b (see Scheme 1), in which
C1, C2, and C3 are part of an allenyl ligand, as a possible
ꢀ
showed the Re C signal at 289.4 ppm. The signal correspond-
ing to the other metal-bound carbon atom (ReC) was
1
observed at 210.4 ppm. With the aid of DEPT135 and H–
13C HSQC techniques, the 13C signals for the remaining
carbon atoms of the metallacycles were located at 173.6,
124.2, and 110.7 ppm, corresponding to C3, C4, and C2,
respectively.
The structure of 7 has also been confirmed by X-ray
diffraction analysis. As shown in Figure 2, 7 contains an
essentially planar six-membered metallacycle. The maximum
deviation from the least-squares plane through Re1 and C1-
C5 is 0.020 ꢀ for C5. The Re-C1-C2 angle (150.8(3)8) is
significantly smaller than that expected for either a carbyne or
a vinylidene complex, because of the constraint of the six-
membered ring. The C–C distances in the ring in 7 (1.367–
1.457 ꢀ) are typical of a regular aromatic system. The Re–C1
bond length (1.774(4) ꢀ) is in the high end of the observed
[16,20]
ꢀ
range for typical Re C bond lengths (1.672–1.802 ꢀ),
=
and shorter than the lengths found for typical Re C bonds in
rhenium vinylidene complexes (1.925–2.046 ꢀ).[16,21] The Re–
C5 bond length (2.107(4) ꢀ) is within the range of typical Re–
C(vinyl) bond lengths (1.996–2.305 ꢀ),[16,18] and in the high
=
end of the reported range for typical Re CH(carbene) bonds
(1.850–2.143 ꢀ).[16,17] The structural data indicate that the
metallacycle of 7 has a delocalized structure with contribu-
tions from resonance structures 7 and 7a, with 7 being more
important.
In summary, we have successfully isolated and structurally
characterized the first rhenabenzyne complex. We are cur-
rently exploring the possibility of isolating metallabenzynes
of other middle or early transition metals.
Figure 1. ORTEP drawing of 6 with thermal ellipsoids at 35% proba-
bility level. The hydrogen atoms on PMe2Ph ligands are omitted for
clarity. Selected bond lengths [ꢀ] and angles [8]: Re1–C1 1.933(7), Re1–
C2 2.104(8), Re1–C5 2.158(8), C1–C2 1.310(11), C2–C3 1.337(11), C3–
C4 1.426(12), C4–C5 1.350(13); C1-Re1-C2 37.6(3), C1-Re1-C5
107.4(3), C2-Re1-C5 69.9(3), C2-C1-Re1 78.3(5), C1-C2-C3 170.0(7), C1-
C2-Re1 64.1(4), C3-C2-Re1 125.7(5), C2-C3-C4 109.5(7), C5-C4-C3
115.0(8), C4-C5-Re1 119.9(6).
10676 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 10675 –10678

Keywords: carbynes · metallabenzynes · metallacycles ·
.
rhenium · transition metals
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Figure 2. ORTEP drawing of 7 with thermal ellipsoids at 35% proba-
bility level. The hydrogen atoms on PMe2Ph ligands are omitted for
clarity. Selected bond lengths [ꢀ] and angles [8]: Re1–C1 1.774(4), Re1–
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1.369(6); C1-Re1-C5 79.08(17), C2-C1-Re1 150.8(3), C2-C3-C4 120.6(4),
C3-C2-C1 115.4(4), C4-C5-Re1 128.7(3), C5-Re1-Cl1 87.51(11), C5-Re1-
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ExpꢀeriÀmental Section
À
=
=
[Re( C CH C(tBu)CH CH )Cl(PMe2Ph)3] (7): A solution of 6
(433 mg, 0.57 mmol) in benzene (5 mL) was stored at room temper-
ature for 20 days; NMR experiments suggest that 6 was completely
converted to 7 after this time. The mixture was stored for additional
20 days to give a crystalline solid together with some oily material.
The crude solid was washed with hexane (3 ꢁ 3 mL) under sonication,
then collected by filtration and dried under vacuum. The crystals
(179 mg, 41.3%) were separated by hand with the help of a
microscope. An analytically pure sample was obtained by column
chromatography on silica gel with benzene as eluent. The blue band
was collected, and the solvent was removed completely to give a
cadet-blue solid. 31P{1H} NMR (161.98 MHz, C6D6): d = À23.1 (t,
J(PP) = 19.9 Hz), À24.5 ppm (d, J(PP) = 20.4 Hz); 1H NMR
(400.13 MHz, C6D6): d = 1.40 (d, J(PH) = 6.8 Hz, 6H, PMe2Ph), 1.40
(t, J(PH) = 3.4 Hz, 6H, PMe2Ph), 1.47 (s, 9H, C(CH3)3), 1.78 (t,
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ꢀ
J(PH) = 3.8 Hz, 6H, PMe2Ph), 4.92 (dt, J(PH) = 4.4, 2.0 Hz, 1H, Re
À
C CH), 7.00–7.08 (m, 9H, Ph), 7.38–7.46 (m, 4H, Ph), 7.56–7.66 (m,
À
2H, Ph), 8.06 (ddt, J(HH) = 10.7 Hz, J(PH) = 6.1, 1.9 Hz, 1H, Re
=
CH CH), 12.28 ppm (ddt, J(HH) = 10.8 Hz, J(PH) = 7.1, 1.7 Hz, 1H,
Re CH); 13C{1H} NMR (100.62 MHz, C6D6): d = 289.4 (dt, J(PC) =
À
ꢀ
À
12.0, 11.9 Hz, Re C), 210.4 (dt, J(PC) = 46.8, 12.1 Hz, Re CH), 173.6
(t, J(PC) = 3.5 Hz, CtBu), 141.7 (dt, J(PC) = 33.6, 2.2 Hz, Ph), 140.8
(dt, J(PC) = 18.9, 1.6 Hz, Ph), 129.6 (d, J(PC) = 10.0 Hz, Ph), 129.6 (t,
J(PC) = 4.2 Hz, Ph), 127.5 (s, Ph), 127.3 (s, Ph), 127.2 (d, J(PC) =
7.2 Hz, Ph), 126.9 (t, J(PC) = 4.0 Hz, Ph), 124.2 (t, J(PC) = 2.9 Hz,
À
=
ꢀ À
Re CH CH), 110.7 (s, Re C CH), 36.3 (s, tBu), 30.0 (s, tBu), 18.3 (d,
J(PC) = 22.1 Hz, PMe2Ph), 14.6 (dt, J(PC) = 1.8 Hz, 16.7 Hz,
PMe2Ph), 14.5 ppm (t, J(PC) = 16.4 Hz, PMe2Ph); elemental analysis
(%) calcd for C33H45ClP3Re: C 52.41, H 6.00; found: C 52.16, H 6.16.
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Supporting Information.
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Received: July 3, 2011
Published online: September 14, 2011
Angew. Chem. Int. Ed. 2011, 50, 10675 –10678
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 10677

Communications
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