Conversion of metallabenzynes into carbene complexes

Article abstract of DOI:10.1002/anie.201101844

Osmabenzynes are shown to rearrange to carbene complexes. This previously unknown reaction of metallabenzynes, in analogy to the well-known formation of cyclopentadienyl complexes from metallabenzenes, has thus now been achieved. A subtle change in the su

Full text of DOI:10.1002/anie.201101844

DOI: 10.1002/anie.201101844
Metallacycles
Conversion of Metallabenzynes into Carbene Complexes**
Jiangxi Chen, Chuan Shi, Herman H. Y. Sung, Ian D. Williams, Zhenyang Lin,* and
Guochen Jia*
The chemistry of transition metal-containing metallaben-
zenes has attracted considerable attention.[1] Previous studies
have led to the isolation and characterization of a number of
stable metallabenzenes, especially those of osmium,[2–4] iri-
dium,[5–8] platinum,[9] ruthenium,[10,11] and rhenium.[12] Many
interesting chemical properties of metallabenzenes have also
been discovered. For example, it has been demonstrated that
metallabenzenes can undergo electrophilic substitution reac-
tions,[2a,5d] cycloaddition reactions,[8d,9c] nucleophilic addition
reactions,[13] and nucleophilic aromatic substitution of hydro-
gen.[14] Another common reactivity of metallabenzenes is that
they can undergo migratory insertion reactions to give
cyclopentadienyl complexes. The transformation has been
demonstrated with well-characterized metallabenzenes[2e,6b,-
ClC6H4)2}(PPh3)2] produced a osmanaphthalyne complex.[19]
The reaction was proposed to proceed through a 16e four-
coordinate square planar osmium carbyne complex, which
À
undergoes an oxidative addition reaction involving a C Cl
bond. Inspired by the observation, we envisioned that
reactions of zinc with carbyne complexes of the type
ꢀ À
=
À
=
[OsCl3{ C CH CR CR’ CClR’’}(PPh3)2] might lead to the
formation of new osmabenzynes.
To test this hypothesis, we first prepared the meridional
osmium carbyne complex 1 and then treated it with zinc in
THF at room temperature. An in situ 31P NMR study showed
that osmabenzyne 2 with substituents on the C3 and C5
positions was produced as the major phosphorus-containing
product (Scheme 1), which can be isolated in 62% yield after
column chromatography.
d,e,10b]
as well as a spectroscopically characterized ruthena-
benzene,[15] and it has been proposed as a key step in the
formation of cyclopentadienyl complexes.[1]
Compounds closely related to metallabenzenes are metal-
labenzynes.[16] Compared with the chemistry of metallaben-
zenes, that of metallabenzynes is much less developed, which
is partly due to the lack of convenient methods to synthesize
such compounds.[17] Structurally, metallabenzynes are similar
to metallabenzenes in that both have a delocalized structure.
Thus it might be expected that metallabenzynes should have
properties similar to those of metallabenzenes. Indeed,
previous studies have demonstrated that metallabenzynes,
like metallabenzenes, can also undergo electrophilic substi-
tution reactions[18] and nucleophilic addition reactions.[17c] As
formation of cyclopentadienyl complexes from metallaben-
zenes by migratory insertion reactions is well-known, one
might expect that metallabenzynes could also undergo
migratory insertion reactions to give carbene complexes.
However, such reactions have not been previously observed.
Herein, we present a reliable method to prepare osmaben-
zynes along with the first examples of conversion of metal-
labenzynes into carbene complexes.
Scheme 1. Preparation of complexes 2, 4, and 5.
We recently observed that reaction of zinc with the
ꢀ À
=
osmium vinyl carbyne complex [OsCl3{ C CH C(2-
The osmabenzyne 2 has been characterized by NMR,
elemental analysis, and X-ray diffraction.[20] As shown in
Figure 1, it contains an essentially planar six-membered
metallacycle. The sum of angles in the six-membered ring of
2 is 720.08 and the maximum deviation from the least-squares
[*] Dr. J. Chen, C. Shi, Dr. H. H. Y. Sung, Prof. Dr. I. D. Williams,
Prof. Dr. Z. Lin, Prof. Dr. G. Jia
Department of Chemistry
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon (Hong Kong)
E-mail: chzlin@ust.hk
À
À
plane is 0.020 ꢀ for the C3 atom. The Os C5 and Os C1
bond lengths of 2 are 2.097(5) and 1.751(7) ꢀ, respectively.
À
The ring C C distances for 2 (1.361–1.419 ꢀ) are typical of a
chjiag@ust.hk
regular aromatic system. The overall geometry of 2 resembles
[**] This work was supported by the Hong Kong Research Grant Council
(HKUST601007 and HKU1/CRF/08).
that of other reported osmabenzynes.[16c]
Consistent with the solid-state structure, the 1H NMR
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101844.
ꢀ À
spectrum of 2 has signals for the two protons of Os C CH
Angew. Chem. Int. Ed. 2011, 50, 7295 –7299
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7295

Communications
Figure 1. ORTEP drawing of 2. Ellipsoids are set at 35% probability;
Figure 2. ORTEP drawing of 5. Ellipsoids are set at 35% probability;
the H atoms on PPh3 are omitted for clarity. Selected bond lengths [ꢀ]
and angles [8]: Os1–C1 1.897(5), C1–C2 1.483(7), C1–C5 1.486(7), C2–
C3 1.337(8), C3–C4 1.474(8), C4–C5 1.344(8); Cl1-Os1-Cl2 140.85(5),
P2-Os1-P1 167.78(5), C1-Os1-Cl1 109.07(15), C1-Os1-Cl2 110.08(15),
C1-Os1-P1 94.90(15), C1-Os1-P2 97.28(15), C2-C1-Os1 126.5(4), C2-
C1-C5 103.5(5), C5-C1-Os1 130.0(4), C3-C2-C1 110.2(5), C2-C3-C4
108.0(5), C5-C4-C3 109.3(5), C4-C5-C1 109.0(5).
the H atoms on PPh3 and the n-pentyl group are omitted for clarity.
Selected bond lengths [ꢀ] and angles [8]: Os1–C1 1.751(7), Os1–C5
2.097(5), C1–C2 1.375(8), C2–C3 1.374(8), C3–C4 1.419(8), C4–C5
1.361(8); C1-Os1-C5 79.8(2), C2-C1-Os1 153.3(4), C3-C2-C1 112.3(6),
C2-C3-C4 121.4(5), C5-C4-C3 128.4(5), C4-C5-Os1 124.8(4).
=
and OsC CH at d = 3.93 and 6.72 ppm, respectively. The
13
1
ꢀ
C{ H} NMR spectrum of 2 (in CD2Cl2) has an Os C signal at
a signal for Os C at d = 224.8 ppm. The 13C signals for the
=
d = 297.0 ppm, and the signal corresponding to the other
metal-bound carbon atom (OsC) at d = 224.0 ppm. The 13C
signals for the remaining carbon atoms of the metallacycle
can also be assigned unambiguously, with the aid of DEPT135
and 1H–13C HSQC techniques, at d = 183.8, 123.5 and
103.7 ppm, corresponding to C3, C4, and C2, respectively.
Inspired by the success of synthesis of 2, we then tried to
prepare osmabenzyne with only one substituent at the C3
position by using osmium carbyne complex 3 as the starting
material (Scheme 1). The in situ 31P{1H} NMR study shows
that osmabenzyne 4 was produced in 2 h as the major
phosphorus-containing product. However, the complex is
unstable in solution and completely isomerized to osmium
carbene complex 5 in 24 h. Complex 5, a rare osmium
analogue of Grubbs catalysts,[21] can be isolated as a brownish-
yellow solid in 72% yield after column chromatography.
The carbene complex 5, which is structurally related to the
remaining carbon atoms of the cyclopentadienylene ring were
observed at d = 142.7, 142.4, 137.8, and 117.6 ppm corre-
sponding to C3, C5, C2, and C4, respectively.
While there are many reported examples of rearrange-
ment of metallabenzenes to h5-cyclopentadiene complexes by
migratory insertion, formation of 5 from 4 is the first example
of conversion of a metallabenzyne into a metal carbene by
migratory insertion.
Compound 2 can be stored as a solid at room temperature
for at least two weeks without decomposition. In a toluene
solution, it also survived when heated at 1008C for 50 min.
The question arises as to why 2 is more stable than 4. A close
examination of the structure of 5 shows that the carbene
ligand is oriented parallel to the P1-Os-P2 axis. The protons
attached to the C2 and C5 carbons are pointing to the PPh3
ligands. If 2 underwent migratory insertion to form a similar
carbene complex, it would be expected that the pentyl group
will have a steric interaction with one of the two PPh3 ligands.
Such a steric effect may help to stabilize the metallabenzyne
complex 2 relative to the corresponding carbene complex.
In an effort to confirm this proposition, we studied zinc
reduction reactions of analogous carbyne complexes 6 and 8.
As expected, reaction of 6 with zinc gave osmabenzyne
complex 7, which can be isolated as a green solid and stored as
a solid at room temperature for at least two weeks without
decomposition. Reaction of 8 with zinc gave osmabenzyne 9,
which slowly isomerized to carbene complex 10 (Scheme 2).
It is of interest to note that osmabenzynes 4 and 9, which
have a tert-butyl (4) or a 1-adamantyl (9) group at the C3
position, are unstable at room temperature and rearrange to
= =
known five-coordinate vinylidene complexes OsCl2( C
CHR)(PR’3)2,[22] was also characterized by NMR, elemental
analysis, and X-ray diffraction. As shown in Figure 2, the
complex can be described as a distorted trigonal bipyramidal
complex with two PPh3 ligands at the axial positions, and two
chloride atoms and the carbene ligand at the equatorial
positions. The Cl-Os-Cl angle is 140.85(5)8, which is close to
= =
that observed in the related vinylidene complex [OsCl2( C
CHCMe3)(PPh3)2] (149.31(2)8),[22a] and significantly smaller
[21b]
=
than that in [RuCl2( CH-p-C6H4Cl)(PCy3)2] (167.6(1)8).
Consistent with the solid-state structure, the 1H NMR
spectrum of 5 (in CD2Cl2) had three proton signals at d = 6.23,
6.54, and 6.70 ppm for the protons attached to C2, C5, and C4,
respectively. The 13C{1H} NMR spectrum of 5 (in CD2Cl2) had
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ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 7295 –7299

Scheme 3. Short H···H contacts in the optimized structures of 4, 5, 11,
and 12.
positions of the metallacycle and the tert-butyl or 1-adamantyl
group. Conversion into the carbene structural form releases
the steric congestion.
Scheme 2. Preparation of complexes 7, 9, and 10.
As mentioned previously, sterically repulsive interaction
in the resultant carbene complex involving pentyl and one of
the two PPh3 ligands may prevent the metallabenzyne
complex 2 from conversion into the corresponding carbene
complex. Indeed, the calculation shows that the carbene
ligand in the resultant carbene complex adopts the electroni-
cally unfavorable conformation in which the carbene ligand is
oriented perpendicular to the P-Os-P axis to avoid unfavor-
able steric repulsions,[26] and for this reason the rearrange-
ment of 2 to the corresponding carbene complex is unfavor-
able. The same argument can be used to explain the failure of
7 to rearrange to the corresponding carbene complex.
In summary, we have demonstrated that zinc reduction of
the carbene complexes 5 and 10, respectively. In contrast, the
ꢀ À
=
À
=
reported osmabenzyne [OsCl2( C CH CMe CH CH)-
(PPh3)2] (11)[18] with a methyl group at the C3 position is
relatively stable and does not rearrange to the corresponding
carbene complex, even when heated in toluene at 1008C for
4 h.
To understand how the subtle change in the substituents
on the metallacycle influences the stability of the metalla-
benzynes studied in this work, we have calculated themody-
namics and kinetics of the conversion of 2, 4, and 11 into their
corresponding carbene complexes.[23,24] The theoretical cal-
culations reproduced the stability trend observed experimen-
tally. Compound 4 was calculated to be less stable than the
corresponding carbene complex 5, while 2 and 11 were
ꢀ À
=
osmium vinylcarbyne complexes of the type [OsCl3{ C CH
À
=
CR CR’ CClR’’}(PPh3)2] can be a reliable method to pre-
pare osmabenzynes. We also observed for the first time that
osmabenzynes can rearrange to carbene complexes. A subtle
change in the substituents in the metallacyle can have a
drastic effect on the transformation.
calculated to be more stable than their corresponding carbene
[23]
complexes. Significant barriers (ca. 35 kcalmolÀ1
)
were
calculated for these conversions. The significant barrier for
the conversion of 4 is in agreement with the experimental
observation that 4 can be detected spectroscopically.
A plausible explanation for the difference in the stability
of the metallabenzynes is as follows. The optimized structure
of osmabenzyne 4 has very short H···H contacts of 2.199 and
2.230 ꢀ involving the tert-butyl group and H atoms of the
metallacycle at the C2- and C4-positions (Scheme 3). The
contacts are shorter than those in the eclipsed conformer of
ethane (2.36 ꢀ). The carbene complex 5, the conversion
product from 4, shows the shortest H···H distance of 2.468 ꢀ,
which is close to that in the staggered conformer of ethane
(2.54 ꢀ). In contrast, the shortest H···H distances in the
structures of 11 (2.417 ꢀ) and 12 (a hypothetical carbene
complex formed from the conversion of 11; 2.657 ꢀ) are all
close to or longer than that in the staggered conformer of
ethane. It is well known that eclipsed conformer of ethane is
less stable than the staggered conformer by 12 kJmolÀ1, which
is primarily due to steric effects.[25] Therefore, the lower
stability of 4 and 9 compared to 11 could be related to the
steric repulsion between the hydrogens at the C2- and C4-
ExperꢀimÀental Section
=
À
=
À
[OsCl2{ C CH CtBu CH C(nC5H11) }(PPh3)2] (2): A mixture of 1
(501 mg, 0.48 mmol) and zinc dust (35 mg, 0.53 mmol) in THF
(30 mL) was stirred at room temperature for 12.5 h to give a brown
solution. The solvent then was removed completely under vacuum.
The residue was dissolved in benzene (ca. 5 mL) and was loaded onto
a silica gel column. The column was flashed with benzene and
dichloromethane subsequently, and then eluted with dichlorome-
thane/diethyl ether (8:1) to give a purple solution. The solvent was
removed under vacuum to give a purple solid, which was dried under
vacuum. Yield: 290 mg, 62.2%. 31P{1H} NMR (161.98 MHz, CD2Cl2):
d = À3.8 ppm (s). 1H NMR (400.13 MHz, CD2Cl2): d = 7.68 (dt,
J(HH) = 7.1 Hz, J(PH) = 5.2 Hz, 12H, PPh), 7.40 (t, J(HH) =
7.0 Hz, 6H, PPh), 7.34 (t, J(HH) = 7.2 Hz, 12H, PPh), 6.72 (s,1H,
=
ꢀ
Os-C CH), 3.93 (s, 1H, Os CCH), 2.83 (t, J(HH) = 7.6 Hz, 2H,
CH2CH2CH2CH2CH3), 1.10 (qn, J(HH) = 7.2 Hz, 2H,
CH2CH2CH2CH2CH3), 0.99 (s, 9H, C(CH3)3), 0.91 (m, 2H,
CH2CH2CH2CH2CH3), 0.83 ppm (m, 5H, CH2CH2CH2CH2CH3).
13C{1H} NMR (100.62 MHz, CD2Cl2): d = 297.0 (t, J(PC) = 11.8 Hz,
Angew. Chem. Int. Ed. 2011, 50, 7295 –7299
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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Communications
ꢀ
=
Os C), 224.0 (t, J(PC) = 5.8 Hz, Os-C CH), 183.8 (s, CC(CH3)3),
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=
ꢀ
123.5
(s,
Os-C CH),
103.7
37.8
29.2
(s,
Os CCH),
C(CH3)3),
C(CH3)3),
52.2
31.4
29.0
(s,
(s,
(s,
CH2CH2CH2CH2CH3),
CH2CH2CH2CH2CH3),
(s,
(s,
CH2CH2CH2CH2CH3), 22.0 (s, CH2CH2CH2CH2CH3), 13.4 (s,
CH2CH2CH2CH2CH3), 134.3 (t, J(PC) = 4.9 Hz, PPh), 130.7 (t,
J(PC) = 26.3 Hz, PPh), 129.5 (s, PPh), 126.7 ppm (t, J(PC) = 4.9 Hz,
PPh). Anal. calcd (%) for C50H52Cl2OsP2: C 61.53, H 5.37; found:
C 61.70, H 5.37.
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13742.
=
À
=
=
À
[OsCl2{ C( CH C(tBu)CH CH )}(PPh3)2] (5): A mixture of 3
(450 mg, 0.46 mmol) and zinc dust (46 mg, 0.70 mmol) in THF
(30 mL) was stirred at room temperature for 24 h to give a brown
solution. The solvent was removed completely under vacuum. The
residue was dissolved in benzene (ca. 5 mL) and was loaded onto a
silica gel column. The column was flashed with benzene/hexane (15:1)
and then eluted with benzene to give a brownish-yellow solution. The
solvent was removed under vacuum to give a brownish-yellow solid,
which was dried under vacuum. Yield : 301 mg, 72.2%. 31P{1H} NMR
(161.98 MHz, CD2Cl2): d = À4.2 ppm (s). 1H NMR (400.13 MHz,
=
CD2Cl2): d = 0.86 (s, 9H, C(CH3)3), 6.23 (s, 1H, Os C-CH), 6.54 (dd,
J(HH) = 5.6 Hz, 2.0 Hz, 1H, Os C-CH CH), 6.70 (d, J(HH) =
=
=
=
=
5.2 Hz, 1H, Os C-CH CH), 7.38–7.40 (m, 12H, PPh), 7.45–7.50
(m, 6H, PPh), 7.62 (br, 12H, PPh). 13C{1H} NMR (100.62 MHz,
=
CD2Cl2): d = 224.8 (t, J(PC) = 5.6 Hz, Os C), 142.7 (s, CtBu), 142.4 (t,
J(PC) = 9.1 Hz, Os C-CH CH), 137.8 (t, J(PC) = 9.4 Hz, Os C-
CH), 134.1 (br, PPh), 129.5 (s, PPh), 127.6 (t, J(PC) = 4.9 Hz, PPh),
117.6 (s, Os C-CH CH), 31.4 (s, C(CH3)3), 22.0 ppm (s, C(CH3)3).
Anal. calcd (%) for C45H42Cl2OsP2: C 59.66, H 4.67; found: C 59.87,
H 4.43.
Details of the preparation and characterization of 1, 3, 4, and 6–10
can be found in the Supporting Information.
=
=
=
=
=
Received: March 15, 2011
Revised: April 28, 2011
Published online: June 17, 2011
Keywords: carbenes · metallabenzenes · metallabenzynes ·
.
metallacycles · osmium
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0.0727. A portion of the pentyl group of 2 was disordered over
two positions; C11, C12, and C13 were refined as 0.6 occupan-
cies, and C11 A, C12 A, and C13 A were refined as 0.4
occupancies. Crystal data for 5: C45H42Cl2OsP2 (from C6H6),
Mr = 905.83, monoclinic, P21, a = 9.9551(11), b = 15.5004(18),
c = 12.5658(14) ꢀ, b = 95.781(2)8, V= 1929.1(4) ꢀ3, Z = 2,
1calcd = 1.559 gcmÀ3, m = 3.558 mmÀ1, F(000) = 904, qmax = 28.098,
11273 reflections, 6212 independent (Rint = 0.0347), R1 = 0.0298,
wR2 = 0.0627. CCDC 816621 (2) and CCDC 816622 (5) contain
the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
[20] Single-crystal X-ray diffraction intensity data of 2 was collected
with an Oxford Diffraction Gemini S Ultra X-ray diffractometer
with CuKa radiation (l = 1.54178 ꢀ) at T= 133 K. The intensity
data of 5 was collected with a Bruker Smart APEX CCD X-ray
diffractometer with MoKa (l = 0.71073 ꢀ) at T= 100 K. Lattice
determination, data collection, and data reduction of 2 were
carried out using CrysAlisPro 171.33.46, and absorption correc-
tion was performed using the built-in SADABS in the CrysA-
lisPro program suite. Lattice determination and data collection
of 5 were carried out using SMART v.5.625 software. Data
reduction and absorption correction by semi-empirical methods
were performed using SAINT v.6.26 and SADABS v.2.03,
respectively. Structure solution and refinement for both 2 and
5 were performed using the OLEX2 software package. All of the
structures were solved by direct methods, expanded by differ-
ence Fourier syntheses, and refined by full-matrix least-squares
on F2. All non-hydrogen atoms were refined anisotropically with
a riding model for the hydrogen atoms, except for those
disordered groups that have been re-examined and refined as
disorder model with appropriate partial occupancies applied.
Crystal data for 2: C50H52Cl2OsP2 (from CH2Cl2/hexane), Mr =
975.96, monoclinic, P21/c, a = 14.3894(5), b = 10.5071(5), c =
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[23] See the Supporting Information.
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to h5-cyclopentadiene complexes, see for example: M. A. Iron,
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28.4259(12) ꢀ, b = 95.863(4)8, V= 4275.3(3) ꢀ3, Z = 4, 1calcd
=
1.516 gcmÀ3, m = 7.744 mmÀ1, F(000) = 1968, qmax = 67.58, 17173
reflections, 7569 independent (Rint = 0.0638), R1 = 0.0367, wR2 =
Angew. Chem. Int. Ed. 2011, 50, 7295 –7299
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 7299