Reactivity of seven-membered azazirconacycloallenes and four-membered zirconacycles toward diphenylacetonitrile

Article abstract of DOI:10.1021/om300949a

The reaction of the seven-membered azazirconacycloallene 2 with Ph ₂CHCN was carried out to yield a zirconocene complex with three fused rings and a keteniminate ligand. Further reactivity toward propargyl bromide or propionyl chloride shows that the keteniminate ligand can be replaced by halogen atoms (Br or Cl). In addition, the reaction of the strained four-membered zirconacycle 1 with Ph₂CHCN gives zirconocenes possessing vinyl-imine and keteniminate species.

Full text of DOI:10.1021/om300949a

Article
pubs.acs.org/Organometallics
Reactivity of Seven-Membered Azazirconacycloallenes and Four-
Membered Zirconacycles toward Diphenylacetonitrile
Jing Zhao,† Shaoguang Zhang,‡ Wen-Xiong Zhang,*,‡ and Zhenfeng Xi*,‡,§
†Beijing National Laboratory of Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences (CAS), Beijing
100190, People's Republic of China
‡Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education, College of Chemistry, Peking
University, Beijing 100871, People's Republic of China
§State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, People's Republic of China
S
* Supporting Information
ABSTRACT: The reaction of the seven-membered azazirco-
nacycloallene 2 with Ph2CHCN was carried out to yield a
zirconocene complex with three fused rings and a keteniminate
ligand. Further reactivity toward propargyl bromide or
propionyl chloride shows that the keteniminate ligand can
be replaced by halogen atoms (Br or Cl). In addition, the
reaction of the strained four-membered zirconacycle 1 with Ph2CHCN gives zirconocenes possessing vinyl-imine and
keteniminate species.
Scheme 1. Reactions of Complex 2 with Different Nitriles
he synthesis and reactivity of zirconacycles have been
Tincreasingly important issues because of their broad
applications in stoichiometric and catalytic transformations.1−5
We have been interested in the isolation, structural character-
ization, and reaction of five-membered or strained four-
membered zirconacycles since 2003.6 Recently, we reported
the reaction of zirconacyclopentene or zirconacyclopentadiene
with 2 equiv of diphenylacetonitrile (Ph2CHCN) bearing an
acidic α-proton to furnish zirconocenes possessing vinyl-imine
and keteniminate species.7 Two molecules of Ph2CHCN were
found to go through a CN insertion reaction, deprotonation,
and intramolecular protonation. In contrast to the versatile
chemistry of zirconacyclopentene or zirconacyclopentadiene,2,3
the reactivity of zirconacycles containing allenyl moieties has
been rarely explored.8 In 2010, we reported the synthesis of
well-defined seven-membered azazirconacycloallenes 2 by the
sequential reaction of the zirconacyclobutene−silacyclobutene
t
fused complex 1 with 2 equiv of bulky BuCN and 1 equiv of
aromatic nitrile.9a The preliminary insertion/rearrangement
reaction of nitriles into the allenic C−Zr bond of 2 generated
complexes 3 (Scheme 1, path a).9b
When azazirconacycloallene 2 was treated with Ph2CHCN,
the analogous 3 was not observed. Interestingly, the unexpected
complex 4 was obtained. Single crystals of 4 suitable for X-ray
analysis were grown in benzene/THF (1/1) mixed solvents at
room temperature. X-ray analysis of 4 reveals that it consists of
three fused rings: one five-membered azasilacyclopentene, one
five-membered azacyclopentene, and one four-membered ring
which contains a neutral N-donor ligand (Figure 1). The
zirconium center in 4 adopts a nine-coordinated environment
bonded with two Cp, one nitrogen from the keteniminate
ligand, and one chelating vinyl-imine fragment. The amino
Inspired by the special reactivity of five-membered zircona-
cycles with Ph2CHCN, we envisioned the reactivity of
azazirconacycloallene 2 and strained four-membered zircona-
cycle 1 toward Ph2CHCN. Herein we report the reaction of 2
with Ph2CHCN to yield the unexpected complex 4 (Scheme 1,
path b). Further reactivity of 4 with propargyl bromide or
propionyl chloride is shown to provide the halogenated
zirconocenes. In addition, the reaction of the strained four-
membered zirconacycle 1 with Ph2CHCN is also explored to
give zirconocene complexes possessing vinyl-imine and
keteniminate species.
Received: October 9, 2012
Published: November 29, 2012
© 2012 American Chemical Society
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Scheme 2. Two Possible Mechanisms for the Formation of 4
Figure 1. ORTEP drawing of 4 with 30% thermal ellipsoids. Hydrogen
atoms are omitted for clarity, except for H3a on the N3 atom. Selected
bond lengths (Å) and angle (deg): Zr1−N1 = 2.222(5), N1−C41 =
1.175(6), C41−C42 = 1.395(7), Zr1−C11 = 2.339(5), Zr1−N2 =
2.465(4), N2−C15 = 1.297(6), N2−C12 = 1.458(6), C11−C12 =
1.384(7), C12−C13 = 1.532(7), C13−C16 = 1.341(6), N3−H3a =
0.91(5), N3−C16 = 1.429(7), Si1−N3 = 1.738(5); N1−C41−C42
177.2(7).
hydrogen neighboring the silicon atom in the azasilacyclopen-
1
tene ring displayed a singlet at δ 2.48 ppm in the H NMR
spectrum. In 13C NMR spectroscopy, two singlets at δ 180.49
and 57.29 ppm were unambiguously assigned to the C(sp) and
C(sp2) atoms in the keteniminate unit, respectively.
Two possible mechanisms for the formation of 4 are
proposed (Scheme 2). The coordination of the CN group of
Ph2CHCN to the zirconium center would give rise to the
intermediate A. A goes through intramolecular proton transfer
to yield the intermediate B. Then the intramolecular hydro-
amination of N−H across the allenyl moiety in B should
generate the zwitterionic complex C.10 Further intramolecular
proton transfer gives the final product 4 (path a). Another
possible mechanism for the formation of 4 cannot be ruled out
(pathway b). The coordination of the CN group of Ph2CHCN
to the zirconium center would give rise to the intermediate A′.
A′ goes through intermolecular proton transfer to yield the
zwitterionic intermediate B′, which should generate the final
product 4.
Scheme 3. Formation of Zirconocene Complexes 5 and 6 by
Reaction of 4 with Propargyl Bromide or Propionyl Chloride
The reactivity of 4 toward propargyl bromide and propionyl
chloride was investigated. Interestingly, the keteniminate ligand
was replaced by halogen atoms to provide zirconocene
complexes 5 and 6 in high yield, respectively (Scheme 3).
Compounds 7 and 8 were isolated and characterized by
combining the H NMR, 13C NMR, and HRMS data. The
1
single-crystal structures of 5 and 6 confirmed their zirconium−
halogen bonds (Figure 2). The zirconium center in 5 and 6 also
adopts a nine-coordinated environment bonded with two Cp,
one halogen ligand (Br or Cl), and one chelating vinyl-imine
fragment. The lengths of the Zr−Br bond in 5 and Zr−Cl bond
in 6 are 2.7741(8) and 2.573(1) Å, respectively. The bond
lengths Zr1−N1 (2.414(3) Å) in 5 and Zr1−N1 (2.438(4) Å)
in 6 are slightly shorter than that found in 4 (Zr1−N2 =
2.465(4) Å). The Zr1−C15 distance (2.319(3) Å) in 5 and
Zr1−C1 distance (2.314(5) Å) in 6 are also comparable with
Zr1−C11 distance (2.339(5) Å) in 4.
zirconocene species 9, which contains vinyl-imine and
keteniminate species (Scheme 4). An X-ray analysis of 9a
unambiguously revealed the structure of the keteniminate unit
and one chelating vinyl-imine fragment (Figure 3). The bond
lengths N2−C38 (1.288(3) Å), Zr1−N2 (2.2629(18) Å), and
Zr1−N1 (2.267(2) Å) are comparable with those found in
[Cp2Zr{η2(C,N)-C(TMS)C(CH3)C(Ph2CH)NH}{NCC-
(C6H5)2}] (1.290(5), 2.265(4), and 2.290(4) Å).7a The
terminal keteniminate Ph2CCN unit has a nearly linear
N1−C11−C12 angle (178.6(3)°). This Ph2CCN unit was
also confirmed by comparing the bond lengths N1−C11
(1.176(3) Å) and C11−C12 (1.370(3) Å) with those in
[Cp2Zr{η2(C,N)-C(TMS)C(CH3)C(Ph2CH)NH}{NCC-
(C6H5)2}] (1.159(5) and 1.386(6) Å).7a
In addition, the zirconacyclobutene−silacyclobutene fused
complex 1, first reported by Takahashi and co-workers,11 was
allowed to react with 2.0 equiv of diphenylacetonitrile to give
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Figure 2. ORTEP drawings of 5 and 6 with 30% thermal ellipsoids. Hydrogen atoms are omitted for clarity except for H2 in 5 and H3 in 6. Selected
bond lengths (Å): 5, Zr1−Br1 = 2.7741(8), Zr1−C15 = 2.319(3), Zr1−N1 = 2.414(3), N1−C11 = 1.310(4), N1−C14 = 1.448(4), C13−C14 =
1.480(5), C13−C22 = 1.361(5), N2−H2 = 0.8601, N2−C22 = 1.409(5), Si1−N2 = 1.742(3); 6, Zr1−Cl1 = 2.573(1), Zr1−N1 = 2.438(4), Zr1−C1
= 2.314(5), N1−C3 = 1.306(6), N1−C2 = 1.444(6), C2−C5 = 1.473(6), C5−C6 = 1.341(7), N2−H3 = 0.69(5), N2−C6 = 1.420(7), N2−Si1 =
1.729(6).
Scheme 4. Reaction of Complex 1 with Ph2CHCN to Yield
Zirconocene Species 9
Scheme 5. Proposed Mechanism for the Formation of 9
complex with three fused rings and a keteniminate ligand.
Further reactivity of 2 toward propargyl bromide or propionyl
chloride shows that the keteniminate ligand can be replaced by
halogen atoms (Br or Cl). Moreover, the four-membered
zirconacycles 1 have been found to react with 2 equiv of
Ph2CHCN to yield the unexpected keteniminate species 9.
Studies of the further reactivity of these new complexes are in
progress.
EXPERIMENTAL SECTION
■
Synthesis of Complex 4. In a 20 mL Schlenk tube,
diphenylacetonitrile (39 mg, 0.2 mmol) and 2 (134 mg, 0.2 mmol)
were added into the benzene solvent (5 mL). After the reaction
mixture was stirred at room temperature for 5 min, it was dried under
vacuum and the residue was washed with hexane. After filtering, the
solid was dried under vacuum to give 4 as a red solid (156 mg, 91%
Figure 3. ORTEP drawing of 9a with 30% thermal ellipsoids.
Hydrogen atoms are omitted for clarity, except for H11 on the
azazirconacycle. Selected bond lengths (Å) and angle (deg): Zr1−C46
= 2.403(2), Zr1−N2 = 2.2629(18), N2−C38 = 1.288(3), Zr1−N1 =
2.267(2), N1−C11 = 1.176(3), C11−C12 = 1.370(3); N1−C11−C12
= 178.6(3).
1
based on 0.2 mmol scale). H NMR (400 MHz, C6D6): δ −0.43 (s,
3H, SiMe2), 0.12 (s, 3H, SiMe2), 0.84 (s, 9H, CMe3), 2.48 (br s, 1H,
NH), 5.51 (s, 5H, C5H5), 6.08 (s, 5H, C5H5), 6.91−7.06 (m, 8H,
C6H5), 7.18−7.26 (m, 5H, C6H5), 7.30 (t, J = 7.8 Hz, 4H, C6H5), 7.50
(d, J = 7.2 Hz, 4H, C6H5), 7.60 (d, J = 7.2 Hz, 2H, C6H5), 7.77 (d, J =
7.2 Hz, 2H, C6H5); 13C NMR (100 MHz, C6D6): δ −3.77 (s, 1 CH3),
−2.26 (s, 1 CH3), 28.42 (s, 3 CH3), 35.02 (s, 1 quat C), 57.29 (s, 1
quat C), 61.77 (s, 1 quat C), 109.90 (s, 5 CH), 111.55 (s, 5 CH),
118.73 (s, 1 quat C), 120.28 (s, 2 CH), 124.19 (s, 4 CH), 125.18 (s, 1
CH), 126.64 (s, 1 CH), 127.08 (s, 2 CH), 127.29 (s, 2 CH), 128.40 (s,
2 CH), 128.53 (s, 2 CH), 128.61 (s, 2 CH), 128.80 (s, 4 CH), 130.05
(s, 2 CH), 131.51 (s, 1 CH), 134.48 (s, 1 quat C), 142.30 (s, 2 quat
Formation of 9 takes place through a nitrile-induced skeletal
rearrangement to generate the azazirconacyclopentadiene 10.
The successive coordination of the second nitrile to 10 and
intramolecular proton transfer gives the final product 9
(Scheme 5).
In summary, we report the reaction of the seven-membered
azazirconacycloallene 2 with Ph2CHCN to yield a zirconocene
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C), 144.36 (s, 1 quat C), 145.90 (s, 1 quat C), 149.35 (s, 1 quat C),
149.86 (s, 1 quat C), 159.55 (s, 1 quat C), 177.58 (s, 1 quat C), 180.49
(s, 1 quat C). Anal. Calcd for C54H51N3SiZr: C, 75.30; H, 5.97; N,
4.88. Found: C, 75.36; H, 5.96; N, 4.89. Recrystallization of 4 from
benzene/THF (1/1) mixed solvents at room temperature gave single
crystals suitable for X-ray analysis.
The preparation of 9b was similar to that of 9a. Green solid, isolated
yield 88%. 1H NMR (300 MHz, C6D6): δ 0.24 (s, 6H, SiMe2), 0.75 (t,
J = 7.4 Hz, 3H, CH2CH2CH3), 0.84 (t, J = 7.2 Hz, 3H, CH2CH2CH3),
1.35−1.42 (m, 2H, CH2CH2CH3), 1.45−1.53 (m, 2H, CH2CH2CH3),
2.28 (t, J = 7.7 Hz, 2H, CH2CH2CH3), 2.40 (t, J = 7.4 Hz, 2H,
CH2CH2CH3), 4.99 (s, 1H, CH), 6.16 (s, 10H, 2 C5H5), 6.65 (d, J =
7.8 Hz, 2H, C6H4), 6.78−6.98 (m, 16H, C6H5 and C6H4), 7.31 (t, J =
7.8 Hz, 4H, C6H5), 7.48 (d, J = 8.1 Hz, 2H, C6H4), 7.59 (d, J = 7.5 Hz,
4H, C6H5), 8.59 (br s, 1H, NH). 13C NMR (75 MHz, C4D8O): δ 4.33
(s, 2 CH3), 14.02 (s, 1 CH3), 14.16 (s, 1 CH3), 25.32 (s, 1 CH2),
25.57 (s, 1 CH2), 38.47 (s, 1 CH2), 38.73 (s, 1 CH2), 57.56 (s, 1 quat
C), 59.32 (s, 1 CH), 98.22 (s, 1 quat C), 107.33 (s, 1 quat C), 111.38
(s, 10 CH), 120.05 (s, 2 CH), 124.05 (s, 4 CH), 128.45 (s, 2 CH),
128.76 (s, 4 CH), 129.41 (s, 2 CH), 129.62 (s, 4 CH), 129.68 (s, 4
CH), 129.74 (s, 2 CH), 131.88 (s, 2 CH), 132.34 (s, 2 CH), 137.93 (s,
1 quat C), 138.42 (s, 2 quat C), 139.84 (s, 1 quat C), 142.28 (s, 1 quat
C), 142.57 (s, 2 quat C), 144.37 (s, 1 quat C), 155.00 (s, 1 quat C),
155.18 (s, 1 quat C), 192.87 (s, 1 quat C), 234.23 (s, 1 quat C). Anal.
Calcd for C62H60N2SiZr: C, 78.18; H, 6.35; N, 2.94. Found: C, 78.31;
H, 6.37; N, 2.93.
Formation of Complex 5 and Compound 7. In a 20 mL
Schlenk tube, propargyl bromide (17 μL, 0.2 mmol) and 4 (172 mg,
0.2 mmol) were added into the benzene solvent (5 mL). After the
reaction mixture was stirred at room temperature for 5 min, it was
dried under vacuum and the residue was washed with hexane.
Filtration gave an orange solid and a clear filtrate. The filtrate was
reduced under vacuum and was subjected to SiO2 column
chromatography with petroleum ether/ethyl acetate (100/5) as the
eluent to give compound 7 as a colorless oil (see the Supporting
Information). The orange solid was dried under vacuum to give 5 (111
1
mg, 74% based on a 0.2 mmol scale). H NMR (400 MHz, C6D6): δ
−0.46 (s, 3H, SiMe2), 0.13 (s, 3H, SiMe2), 0.83 (s, 9H, CMe3), 2.45
(br s, 1H, NH), 5.69 (s, 5H, C5H5), 6.20 (s, 5H, C5H5), 6.81 (d, J =
7.2 Hz, 2H, C6H5), 6.92−6.94 (m, 1H, C6H5), 7.01 (t, J = 7.7 Hz, 3H,
C6H5), 7.17−7.22 (m, 5H, C6H5), 7.62 (d, J = 7.6 Hz, 2H, C6H5), 7.91
(d, J = 7.2 Hz, 2H, C6H5); 13C NMR data were not collected because
of the poor solubility of 5 in C6D6, THF-d8, toluene-d8, etc. Anal.
Calcd for C40H41BrN2SiZr: C, 64.14; H, 5.52; N, 3.74. Found: C,
64.31; H, 5.63; N, 3.81. Recrystallization of 5 from benzene/THF (1/
1) mixed solvents at room temperature gave single crystals suitable for
X-ray analysis.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving crystallographic data
for 4−6 and 9a, NMR data, further experimental details, and
spectra for all new compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
Formation of Complex 6 and Compound 8. In a 20 mL
Schlenk tube, propionyl chloride (17 μL, 0.2 mmol) and 4 (171 mg,
0.2 mmol) were added into the benzene solvent (5 mL). After the
reaction mixture was stirred at room temperature for 5 min, it was
dried under vacuum and the residue was washed with hexane.
Filtration gave a yellow solid and a clear filtrate. The filtrate was
reduced under vacuum and was subjected to SiO2 column
chromatography with petroleum ether/ethyl acetate (100/10) as the
eluent to give compound 8 as a colorless oil (see the Supporting
Information). The yellow solid was dried under vacuum to give 6 (111
AUTHOR INFORMATION
Corresponding Author
*E-mail: wx_zhang@pku.edu.cn (W.-X.Z.); zfxi@pku.edu.cn
(Z.X.).
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
1
■
mg, 79% based on 0.2 mmol scale). H NMR (400 MHz, C6D6): δ
This work was supported by the Natural Science Foundation of
China, and the “973” program from the National Basic
Research Program of China (2012CB821600).
−0.43 (s, 3H, SiMe2), 0.16 (s, 3H, SiMe2), 0.84 (s, 9H, CMe3), 2.46
(br s, 1H, NH), 5.66 (s, 5H, C5H5), 6.21 (s, 5H, C5H5), 6.81 (d, J =
7.2 Hz, 2H, C6H5), 6.95−7.03 (m, 3H, C6H5), 7.12−7.22 (m, 6H,
C6H5), 7.62 (d, J = 7.2 Hz, 2H, C6H5), 7.96 (d, J = 7.2 Hz, 2H, C6H5);
13C NMR data were not collected because of the poor solubility of 6 in
C6D6, THF-d8, toluene-d8, etc. Anal. Calcd for C40H41ClN2SiZr: C,
68.19; H, 5.87; N, 3.98. Found: C, 68.39; H, 5.94; N, 3.99.
Recrystallization of 6 from THF/toluene (1/1) mixed solvents at −20
°C gave single crystals suitable for X-ray analysis.
Formation of Complexes 9a,b. In a 20 mL Schlenk tube,
diphenylacetonitrile (77 mg, 0.4 mmol) and 1a (96 mg, 0.2 mmol)
were added into the benzene solvent (5 mL). After the reaction
mixture was stirred at 50 °C for 1 h, it was dried under vacuum and the
residue was washed with hexane. After filtering, the solid was dried
under vacuum to give 9a as a green solid (130 mg, 75% based on 0.2
mmol scale). 1H NMR (300 MHz, C6D6): δ 0.17 (s, 6H, SiMe2), 4.90
(s, 1H, CH), 6.12 (s, 10H, 2 C5H5), 6.65 (d, J = 6.9 Hz, 2H, C6H5),
6.76 (d, J = 6.9 Hz, 4H, C6H5), 6.90−7.01 (m, 14H, C6H5), 7.30 (t, J =
7.5 Hz, 4H, C6H5), 7.44−7.47 (m, 2H, C6H5), 7.58 (d, J = 7.5 Hz, 4H,
C6H5), 8.59 (br s, 1H, NH); 13C NMR (75 MHz, C4D8O): δ 4.28 (s, 2
CH3), 57.60 (s, 1 quat C), 59.30 (s, 1 CH), 98.87 (s, 1 quat C), 107.10
(s, 1 quat C), 111.37 (s, 10 CH), 120.06 (s, 2 CH), 124.01 (s, 4 CH),
127.82 (s, 2 CH), 128.21 (s, 2 CH), 128.37 (s, 4 CH), 128.40 (s, 1
CH), 128.75 (s, 4 CH), 129.28 (s, 2 CH), 129.44 (s, 1 CH), 129.63 (s,
4 CH), 129.70 (s, 1 quat C), 131.92 (s, 2 CH), 132.36 (s, 2 CH),
138.28 (s, 2 quat C), 142.41 (s, 1 quat C), 142.47 (s, 2 quat C), 155.02
(s, 1 quat C), 155.08 (s, 1 quat C), 192.56 (s, 1 quat C), 233.79 (s, 1
quat C). Anal. Calcd for C56H48N2SiZr: C, 77.46; H, 5.57; N, 3.23.
Found: C, 77.62; H, 5.65; N, 3.31. Recrystallization of 9a from
benzene at room temperature gave single crystals suitable for X-ray
analysis.
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dx.doi.org/10.1021/om300949a | Organometallics 2012, 31, 8370−8374