Aluminacyclopropene: Syntheses, characterization, and reactivity toward terminal alkynes

Article abstract of DOI:10.1021/ja057731p

Reactions of LAl with ethyne, mono- and disubstituted alkynes, and diyne to aluminacyclopropene LAl[η²-C₂(R¹)(R ²)] ((L = HC[(CMe)(NAr)]₂, Ar = 2,6-iPr₂C ₆H₃); R¹ = R² = H, (1); R ¹ = H, R² = Ph, (2); R¹ = R² = Me, (3); R¹ = SiMe₃, R² = C≡CSiMe ₃, (4)) are reported. Compounds 1 and 2 were obtained in equimolar quantities of the starting materials at low temperature. The amount of C ₂H₂ was controlled by removing an excess of C ₂H₂ in the range from -78 to -50 °C. Compound 4 can be alternatively prepared by the substitution reaction of LAl[η²- C₂(SiMe₃)₂] with Me ₃SiC≡CC≡CSiMe₃ or by the reductive coupling reaction of LAlI₂ with potassium in the presence of Me ₃SiC≡CC≡CSiMe₃. The reaction of LAl with excess C₂H₂ and PhC≡CH (<1:2) afforded the respective alkenylalkynylaluminum compounds LAl(CH=CH₂)(C≡CH) (5) and LAl(CH=CHPh)(C≡CPh) (6). The reaction of LAl(η²- C₂Ph₂) with C₂H₂ and PhC≡CH yielded LAl(CPh=CHPh)(C≡CH) (7) and LAl(CPh=CHPh)(C≡CPh) (8), respectively. Rationally, the formation of 5 (or 6) may proceed through the corresponding precursor 1 (or 2). The theoretical studies based on DFT calculations show that an interaction between the Al(I) center and the C≡C unit needs almost no activation energy. Within the AlC₂ ring the computational Al-C bond order of ca. 1 suggests an Al-C σ bond and therefore less π electron delocalization over the AlC₂ ring. The computed Al-η²-C₂ bond dissociation energies (155-82.6 kJ/mol) indicate a remarkable reactivity of aluminacyclopropene species. Finally, the ¹H NMR spectroscopy monitored reaction of LAl(η²-C₂Ph₂) and PhC≡CH in toluene-d₈ may reveal an acetylenic hydrogen migration process.

Full text of DOI:10.1021/ja057731p

Published on Web 03/24/2006
Aluminacyclopropene: Syntheses, Characterization, and
Reactivity toward Terminal Alkynes
Hongping Zhu,† Rainer B. Oswald,‡ Hongjun Fan,§ Herbert W. Roesky,*,†
Qingjun Ma,† Zhi Yang,† Hans-Georg Schmidt,† Mathias Noltemeyer,†
Kerstin Starke,† and Narayan S. Hosmane|
Contribution from the Institut fu¨r Anorganische Chemie der UniVersita¨t Go¨ttingen,
Tammannstrasse 4, D-37077 Go¨ttingen, Germany, Institut fu¨r Physikalische Chemie der
UniVersita¨t Go¨ttingen, Tammannstrasse 6, D-37077 Go¨ttingen, Germany, Labor fu¨r
Physikalische und Theoretische Chemie, UniVersita¨t Siegen, 57068 Siegen, Germany,
and Department of Chemistry and Biochemistry, Northern Illinois UniVersity,
Dekalb, Illinois 60115-2862
Received November 25, 2005; E-mail: hroesky@gwdg.de
Abstract: Reactions of LAl with ethyne, mono- and disubstituted alkynes, and diyne to aluminacyclopropene
LAl[η2-C2(R1)(R2)] ((L ) HC[(CMe)(NAr)]2, Ar ) 2,6-iPr2C6H3); R1 ) R2 ) H, (1); R1 ) H, R2 ) Ph, (2); R1
) R2 ) Me, (3); R1 ) SiMe3, R2 ) CtCSiMe3, (4)) are reported. Compounds 1 and 2 were obtained in
equimolar quantities of the starting materials at low temperature. The amount of C2H2 was controlled by
removing an excess of C2H2 in the range from -78 to -50 °C. Compound 4 can be alternatively prepared
by the substitution reaction of LAl[η2-C2(SiMe3)2] with Me3SiCtCCtCSiMe3 or by the reductive coupling
reaction of LAlI2 with potassium in the presence of Me3SiCtCCtCSiMe3. The reaction of LAl with excess
C2H2 and PhCtCH (<1:2) afforded the respective alkenylalkynylaluminum compounds LAl(CHdCH2)(Ct
CH) (5) and LAl(CHdCHPh)(CtCPh) (6). The reaction of LAl(η2-C2Ph2) with C2H2 and PhCtCH yielded
LAl(CPhdCHPh)(CtCH) (7) and LAl(CPhdCHPh)(CtCPh) (8), respectively. Rationally, the formation of
5 (or 6) may proceed through the corresponding precursor 1 (or 2). The theoretical studies based on DFT
calculations show that an interaction between the Al(I) center and the CtC unit needs almost no activation
energy. Within the AlC2 ring the computational AlsC bond order of ca. 1 suggests an AlsC σ bond and
therefore less π electron delocalization over the AlC2 ring. The computed Alsη2-C2 bond dissociation
energies (155-82.6 kJ/mol) indicate a remarkable reactivity of aluminacyclopropene species. Finally, the
1H NMR spectroscopy monitored reaction of LAl(η2-C2Ph2) and PhCtCH in toluene-d8 may reveal an
acetylenic hydrogen migration process.
clohexadienes or related dimers.5,6 In view of these reports on
the proposed AlC2 ring intermediates, the bulky chelating
Introduction
Aluminacyclopropene, a new entry to the organoaluminum
family, is exhibiting a remarkable reactivity and versatile
reaction patterns.1-3 By the reductive coupling reaction, several
aluminacyclopropenes LAl[(η2-C2(R1)(R2)] ((L ) HC[(CMe)-
(NAr)]2, Ar ) 2,6-iPr2C6H3); R1, R2: SiMe3, Ph) were suc-
cessfully isolated and characterized.1 This indicates an experi-
mental access to the olefinic bond-containing AlC2 ring
compounds. For a long time such species were thought to be
unstable due to their highly strained structure4 and only assumed
as reactive intermediates in the formation of 1,4-dialuminacy-
â-diketiminato ligand L utilized here undoubtedly protects the
formed AlC2 ring system from its ring-opening dimerization
induced by the Lewis acidic Al center.5,6
Already in 1990 Schaefer III and co-workers demonstrated
through theoretical studies that an AlC2 ring model HAl(η2-
C2H2) was an energetically lower lying system than the separated
AlH and C2H2 (DHAlsC H ) ca. 126 kJ/mol) and therefore could
2
2
be preparatively accessible.7 However, the question is arising
whether the bulkiness of the ligand (L) is responsible for the
reactivity of an aluminum(I) monomer LAl8 directly toward
† Institut fu¨r Anorganische Chemie der Universita¨t Go¨ttingen.
‡ Institut fu¨r Physikalische Chemie der Universita¨t Go¨ttingen.
§ Universita¨t Siegen.
(4) Eisch, J. J. In ComprehensiVe Organometallic Chemistry II; Wilkinson,
G., Stone, F. G. A., Abel, E. W., Eds.; Elsevier: Oxford, UK; 1982, pp
555-682.
| Northern Illinois University.
(5) (a) Hoberg, H.; Gotor, V.; Milchereit, A.; Kru¨ger, C.; Sekutowski, J. C.
Angew. Chem. 1977, 89, 563-564; Angew. Chem., Int. Ed. Engl. 1977,
16, 539-540. (b) Hoberg, H.; Aznar, F. J. Organomet. Chem. 1979, 164,
C13-C15.
(1) Cui, C.; Ko¨pke, S.; Herbst-Irmer, R.; Roesky, H. W.; Noltemeyer, M.;
Schmidt, H.-G.; Wrackmeyer, B. J. Am. Chem. Soc. 2001, 129, 9091-
9098.
(2) Zhu, H.; Chai, J.; Ma, Q.; Jancik, V.; Roesky, H. W.; Fan, H.; Herbst-
Irmer, R. J. Am. Chem. Soc. 2004, 126, 9472-9473.
(3) Zhu, H.; Chai, J.; Fan, H.; Roesky, H. W.; He, C.; Jancik, V.; Schmidt,
H.-G.; Noltemeyer, M.; Merrill, W. A.; Power, P. P. Angew. Chem. 2005,
117, 5090-5093; Angew. Chem., Int. Ed. 2005, 44, 5220-5223.
(6) (a) Schno¨ckel, H.; Leimku¨hler, M.; Lotz, R.; Mattes, R. Angew. Chem.
1986, 98, 929-930; Angew. Chem., Int. Ed. Engl. 1986, 25, 921-922. (b)
U¨ ffing, C.; Ecker, A.; Ko¨ppe, R.; Merzweiler, K.; Schno¨ckel, H. Chem.s
Eur. J. 1998, 4, 2142-2147.
(7) Xie, Y.; Schaefer, H. F., III. J. Am. Chem. Soc. 1990, 112, 5393-5400.
9
5100
J. AM. CHEM. SOC. 2006, 128, 5100-5108
10.1021/ja057731p CCC: $33.50 © 2006 American Chemical Society

Aluminacyclopropene: Reactivity toward Terminal Alkynes
A R T I C L E S
alkynes.1 In fact, a more recent result reported from our group3
has shown that LAl can react with acetylene at low temperature
(even ca. -100 °C) to form an aluminacyclopropene LAl(η2-
C2H2) with the simplest Al(η2-C2H2) moiety. Subsequently, the
LAl(η2-C2H2) reacts further with another molecule of C2H2
leading to an alkenylalkynylaluminum compound LAl(CHd
CH2)(CtCH) when an excess of C2H2 is used. The structural
and multinuclear NMR (1H, 13C) data of the LAl(η2-C2H2)
provide unambiguous evidence for such an olefinic bond in
metallacyclopropene and are of importance in disclosing the
ring property.9 These reactions can be considered prebiotic as
Al(I) species and acetylene are found in the interstellar space.10,11
All these facts led us to investigate the reaction of LAl with
other alkynes (mono- and disubstituted ones and diyne) sys-
tematically, as well as further studies of their reactivity.
Furthermore, theoretical studies (DFT calculations) were per-
formed in order to understand the AlC2 ring bond character and
the interaction mode between the Al(I) center and the CtC
carbon atoms. The estimation of the Alsη2-C2 bond dissociation
energy in a series of aluminacyclopropenes by calculation may
essentially reveal their reactivities as well as the reaction
23.5, 24.4, 24.8, 29.0 (CH(CH3)2, â-CH3), 97.1 (γ-C), 124.4, 125.3,
127.0, 128.4, 139.0, 143.2, 143.8 (Ph-C, Ar-C), 165.4, 170.2 (broad,
Alsη2-C2), 172.8 (CN). 27Al NMR (78.02 MHz, C6D6, 298 K, ppm):
no resonances were observed. EI-MS: m/z (%) 445 (100, [M+ - PhCt
CH + 1]), 545 (5, [M+ - 1]). Anal. Calcd (%) for C37H47AlN2 (Mr )
546.78): C, 81.28; H, 8.67; N, 5.12. Found: C, 80.83; H, 8.48; N,
5.23.
Synthesis of LAl(η2-C2Me2) (3). To a toluene solution (25 mL) of
LAl (0.22 g, 0.5 mmol) at -78 °C an excess of distilled MeCtCMe
was added, and an immediate color change of the solution was observed
from red to orange. The mixture was allowed to warm to room
temperature and stirred for 12 h. All volatiles were removed in vacuum,
and the residue was immediately washed with n-hexane (1 mL). An
orange crystalline solid of 3 was afforded in almost quantitative yield
(>95%). Mp: 182 °C. 1H NMR (500.13 MHz, toluene-d8, 298 K,
ppm): δ 1.17 (d, 3JHH ) 6.8 Hz, 4 × 3 H, CH(CH3)2), 1.47 (d, 3JHH
)
6.8 Hz, 4 × 3 H, CH(CH3)2), 1.57 (s, 2 × 3 H, â-CH3), 1.89 (s, 2 ×
3 H, dCMe), 3.37 (sept, 4 × 1 H, CH(CH3)2), 4.92 (s, 1 H, γ-CH),
7.02-7.14 (m, 6 H, Ar-H). 13C {1H} NMR (125.77 MHz, toluene-d8,
298 K, ppm): δ 16.1, 23.0, 23.4, 24.2, 24.7, 28.9, 32.0 (CH(CH3)2,
â-CH3, dCMe), 96.5 (γ-C), 124.2, 137.4, 137.5, 139.6, 143.8 (Ar-C),
169.4 (broad, Alsη2-C2), 172.3 (CN). EI-MS: m/z (%) 468 (100, [M+
- 2 Me]), 483 (7, [M+ - Me]), 498 (3, [M+]). Anal. Calcd (%) for
C33H47AlN2 (Mr ) 498.74): C, 79.47; H, 9.50; N, 5.62. Found: C,
79.01; H, 9.38; N, 5.73.
1
patterns. The H NMR spectra monitored reaction of LAl(η2-
C2Ph2) and PhCtCH in toluene-d8 is hereby reported.
Synthesis of LAl[η2-C2(SiMe3)(CtCSiMe3)] (4). Method A: A
toluene solution (10 mL) of Me3SiCtCCtCSiMe3 (0.38 g, 2 mmol)
was added to a toluene solution (30 mL) of LAl[η2-C2(SiMe3)2] (1.23
g, 2 mmol) at room temperature. By stirring, the solution slowly
changed from dark brown into an orange color within 10 min. After
additional stirring for 12 h, all volatiles were removed in vacuum. The
residue was washed with n-hexane (2 × 2 mL) to afford an orange
crystalline solid of 4. Yield: 1.19 g, 93%. Mp: 198 °C. 1H NMR
(300.13 MHz, C6D6, 298 K, ppm): δ 0.03 (s, 3 × 3 H, SiMe3), 0.34
(s, 3 × 3 H, SiMe3), 0.98 (d, 2 × 3 H, 3JHH ) 6.8 Hz, CH(CH3)2), 1.10
Experimental Section
General Procedures. All manipulations were carried out under a
purified nitrogen atmosphere using Schlenk techniques or inside a
Mbraun MB glovebox filled with dry nitrogen where the calibrated
values of O2 and H2O were strictly controlled below 1 ppm. All solvents
were dried according to standard methods prior to use. Commercially
available chemicals were purchased from Aldrich or Fluka and used
as received except for phenyl acetylene and acetylene. Phenyl acetylene
was distilled under N2 prior to use, and acetylene was dried over a
P4O10 filled column. The other compounds mentioned in this paper
were prepared according to published procedures: LAl(η2-C2Ph2), LAl-
[η2-C2(SiMe3)2],1 LAlI2, LAl,8 (L ) HC[(CMe)(NAr)]2, Ar ) 2,6-
iPr2C6H3). Elemental analyses were performed by the Analytisches
Labor des Instituts fu¨r Anorganische Chemie der Universita¨t Go¨ttingen.
1H (200.13, 300.13, and 500.13 MHz), 13C (125.77 MHz), 27Al (78.02
MHz), and 29Si (99.36 MHz) NMR spectra were recorded on a Bruker
AM 200, 300, or 500 spectrometer, and IR spectra, on a Bio-Rad
Digilab FTS-7 spectrometer. EI mass spectra were measured on a
Finnigan MAT 8230 or a Varian MAT CH5 instrument. Melting points
were measured in sealed glass tubes and were not corrected.
3
(d, 4 × 3 H, JHH ) 6.8 Hz, CH(CH3)2), 1.45 (s, 2 × 3 H, â-CH3),
3
3
1.47 (d, 2 × 3 H, JHH ) 6.8 Hz, CH(CH3)2), 1.53 (d, 2 × 3 H, JHH
) 6.8 Hz, CH(CH3)2), 3.18 (sept, 2 × 1 H, 3JHH ) 6.8 Hz, CH(CH3)2),
3
3.31 (sept, 2 × 1 H, JHH ) 6.8 Hz, CH(CH3)2), 4.87 (s, 1 H, γ-CH),
7.00-7.05 (m, 6 H, Ar-H). 13C {1H} NMR (125.77 MHz, C6D6, 298
K, ppm): δ 0.29, 0.65 (SiMe3), 23.4, 24.0, 24.7, 25.1, 25.2, 28.8, 29.0,
30.2 (CH(CH3)2, â-CH3), 97.2 (γ-C), 107.6, 108.8 (CtC), 123.9, 125.0,
127.9, 128.0, 138.7, 142.6, 145.0 (Ar-C), 173.5 (CN), 190.0, 193.2
(broad, Alsη2-C2). 29Si {1H} NMR (99.36 MHz, C6D6, 298 K, ppm):
δ -20.9, -16.4. 27Al NMR (77.13 MHz, C6D6, 298 K, ppm): no
resonances were observed. IR (KBr plate, Nujol mull, cm-1): ν˜ 2065
(CtC). EI-MS: m/z (%) 429 (100, [M+ - Me3SiCtCCtCSiMe3 -
Me]), 638 (15, [M+]). Anal. Calcd (%) for C39H59AlN2Si2 (Mr )
639.069): C, 73.30; H, 9.31; N, 4.38. Found: C, 72.49; H, 9.24; N,
4.26. X-ray quality crystals were obtained by recrystallization from a
mixture of toluene and n-hexane (1:3).
Synthesis of LAl[η2-C2(H)(Ph)] (2). To a toluene solution (25 mL)
of LAl (0.41 g, 0.92 mmol) at -78 °C a toluene solution (5 mL) of
distilled PhCtCH (0.101 mL, 0.92 mmol) was added, and an immediate
color change of the solution was observed from red to orange. The
mixture was allowed to warm to room temperature and stirred for 12
h. The volatiles were removed in vacuum, and the residue was washed
with n-hexane (2 mL). An orange crystalline solid of 2 was obtained.
Yield: 0.44 g (88%). Mp: 148 °C. 1H NMR (500.13 MHz, C6D6, 298
K, ppm): δ 1.12 (d, 3JHH ) 6.8 Hz, 4 × 3 H, CH(CH3)2), 1.23 (d, 3JHH
Method B: To a mixture of LAl (0.22 g, 0.5 mmol) and Me3SiCt
CCtCSiMe3 (0.095 g, 0.5 mmol) was added toluene (20 mL). The
solution was stirred for ca. 1 h without changing its red color.
Afterwards the solution was allowed to heat to reflux, and a color
change of the solution to orange was observed. Finally the solution
was refluxed for 4 h and then cooled to room temperature. All solvents
were removed in vacuum. The residue was washed with a little n-hexane
to afford an orange crystalline solid (almost in a quantitative yield,
>95%). Melting point measurement and spectral analysis confirmed
compound 4.
3
) 6.8 Hz, 2 × 3 H, CH(CH3)2), 1.40 (d, JHH ) 6.8 Hz, 2 × 3 H,
CH(CH3)2), 1.53 (s, 2 × 3 H, â-CH3), 3.39 (m, 4 × 1 H, CH(CH3)2),
4.92 (s, 1 H, γ-CH), 6.94-7.26 (m, 11 H, Ph-H, Ar-H), 8.66 (s, 1 H,
Al-η2-C2H). 13C {1H} NMR (125.77 MHz, C6D6, 298 K, ppm): δ
(8) Cui, C.; Roesky, H. W.; Schmidt, H.-G.; Noltemeyer, M.; Hao, H.;
Cimpoesu, F. Angew. Chem. 2000, 112, 4444-4446; Angew. Chem., Int.
Ed. 2000, 39, 4274-4276.
Method C: A suspension of LAlI2 (1.40 g, 2 mmol), K (0.17 g, 4.2
mmol), and Me3SiCtCCtCSiMe3 (0.38 g, 2 mmol) in toluene (50
mL) was vigorously stirred for 3 d. After filtration the orange solution
was concentrated (8 mL), and to it n-hexane (5 mL) was added. The
solution was kept at 4 °C for 1 week to afford orange crystals (in a
(9) Roskamp, E.; Roskamp C. In ComprehensiVe Heterocyclic Chemistry II;
Katritzky, A. R., Rees, C. W., Scriven, E. F. V., Eds.; Pergamon: Oxford,
1996; Vol. 2, pp 305-332.
(10) Gail, H.-P.; Sedlmayr, E. Faraday Discuss. 1998, 109, 303-319.
(11) Smith, I. W. M. Chem. Soc. ReV. 2002, 31, 137-146.
9
J. AM. CHEM. SOC. VOL. 128, NO. 15, 2006 5101

A R T I C L E S
Zhu et al.
relatively low yield, 20%). Melting point measurement and spectral
analysis proved compound 4.
Cd, AlsCt), 171.1 (CN). IR (KBr plate, Nujol mull, cm-1): ν˜ 2124
(CtC). EI-MS: m/z (%) 545 (100, [M+ - C(Ph)dCH(Ph)]), 724 (4,
[M+ - 1]). Anal. Calcd (%) for C54H64AlN2 (8‚0.5 n-hexane, Mr )
768.102): C, 84.44; H, 8.40; N, 3.65. Found: C, 84.81; H, 8.42; N,
3.61.
Synthesis of LAl(CHdCHPh)(CtCPh) (6). To a toluene solution
(30 mL) of LAl (0.57 g, 1.28 mmol) at -78 °C a toluene solution (5
mL) with a little excess of distilled PhCtCH (0.40 mL, 3.64 mmol)
was added. The mixture was allowed to warm to room temperature
and stirred for 3 d; a color change of the solution from red to orange,
to yellow, and finally to almost colorless was observed. All volatiles
were removed in vacuum. The residue was washed with n-hexane (2
× 2 mL) to afford an off-white solid of 6. Yield: 0.62 g (75%). Mp:
X-ray Structure Determination and Refinement. The crystal-
lographic data for compounds 2‚4, 6, and 8‚0.5 n-hexane were collected
on a Stoe IPDS II-array detector system using graphite-monochromated
Mo KR radiation (λ ) 0.710 73 Å) and for compound 7 on a Bruker
three-circle detector system using Cu KR radiation (λ ) 1.541 78 Å).
All structures were solved by direct methods (SHELXS-96)12 and
refined against F2 using SHELXL-97.13 The non-hydrogen atoms were
located by difference Fourier synthesis and refined anisotropically. In
6 the CtCPh and CHdCHPh groups both are disordered and located
into two positions with the same occupation ratio of 0.671(13)/0.329-
(13). The hydrogen atoms were included in geometrically idealized
positions with the Uiso tied to that of the parent atoms and were refined
with the riding model except for the acetylenic hydrogen in 7, which
was located by difference Fourier synthesis and refined isotropically.
A summary of cell parameters, data collection, and structure solution
and refinement is given in Table 1.
Computational Details. The calculations for the bond situation of
a series of aluminacyclopropenes as well as the corresponding Al-
η2-C2 bond dissociation energies made use of the established DFT
variant B3LYP method.14,15 The computations were carried out with
the Gaussian G03 program suite.16 All the molecule geometries were
fully optimized to the equilibrium structures according to their respective
real or related ones. To get a suitable description of the binding situation
of the AlN2C3 ring, a modified 6-31-G basis set extended with additional
diffuse functions was employed.17 To get a clear picture of the binding
situation of the AlC2 ring, a method established by Mayer18 was used,
and then a complete NBO analysis19 was performed. The Al-η2-C2
bond dissociation energy was calculated by analyzing the energy
difference between LAl[η2-C2(R1)(R2)] and the separated LAl and R1Ct
CR2 species.
1
3
346 °C. H NMR (500.13 MHz, C6D6, 298 K, ppm): δ 1.10 (d, JHH
3
) 6.8 Hz, 2 × 3 H, CH(CH3)2), 1.24 (d, JHH ) 6.8 Hz, 4 × 3 H,
CH(CH3)2), 1.56 (d, 3JHH ) 6.8 Hz, 2 × 3 H, CH(CH3)2), 1.60 (s, 2 ×
3
3 H, â-CH3), 3.35 (sept, JHH ) 6.8 Hz, 2 × 1 H, CH(CH3)2), 3.94
(sept, 3JHH ) 6.8 Hz, 2 × 1 H, CH(CH3)2), 4.98 (s, 1 H, γ-CH), 6.67-
6.72, 6.94-7.26 (m, 16 H, Ph-H, Ar-H), 7.41, 7.42 (d, 2 × 1 H, CHd
CH). 13C {1H} NMR (125.77 MHz, C6D6, 298 K, ppm): δ 23.4, 23.7,
24.7, 24.9, 25.0, 26.4, 28.8, 28.9 (CH(CH3)2, â-CH3), 98.2 (γ-C), 107.8
(tCPh), 124.4, 124.8, 126.1, 126.3, 127.1, 127.2, 127.5, 132.0, 132.2,
140.3, 140.6, 144.2, 145.3, 148.7 (dCHPh, Ph-C, Ar-C), 131.0, 134.0
(AlsCd, AlsCt), 170.8 (CN). EI-MS: m/z (%) 545 (100, [M+
-
HCdCHPh]), 648 (10, [M+]). IR (KBr plate, Nujol mull, cm-1): ν˜
2128 (CtC). Anal. Calcd (%) for C45H53AlN2 (Mr ) 648.92): C, 83.29;
H, 8.23; N, 4.31. Found: C, 82.93; H, 8.36; N, 4.25. Single crystals of
X-ray quality of 6 were obtained by recrystallization from a mixture
of n-hexane and toluene.
Synthesis of LAl(CPhdCHPh)(CtCH) (7). A toluene solution (30
mL) of LAl(η2-C2Ph2) (0.62 g, 1 mmol) was exposed to dried HCt
CH under reduced pressure and stirred for 12 h. After workup, all
volatiles were removed in vacuum, and the residue was extracted with
a 1:5 mixture of toluene and n-hexane (15 mL). The extract was kept
at 4 °C for a week to afford colorless X-ray quality crystals of 7.
1
Yield: 0.27 g, 42%. Mp: 200 °C. H NMR (300.13 MHz, C6D6, 298
K, ppm): δ 1.08 (d, 2 × 3 H, 3JHH ) 6.8 Hz, CH(CH3)2), 1.26 (d, 4 ×
3
3
3 H, JHH ) 6.8 Hz, CH(CH3)2), 1.30 (d, 2 × 3 H, JHH ) 6.8 Hz,
CH(CH3)2), 1.59 (s, 2 × 3 H, â-CH3), 1.82 (s, 1 H, CtCH), 3.38 (sept,
The calculations of the reaction system energy changes of LAl and
C2H2 based on Al‚‚‚CC2H2 distances were carried out on DFT level (RI-
BP86) with the SV(P) basis sets (double-ú quality with one polarized
function). The program used is TURBOMOLE 5.5.20
3
3
2 × 1 H, JHH ) 6.8 Hz, CH(CH3)2), 3.86 (sept, 2 × 1 H, JHH ) 6.8
Hz, CH(CH3)2), 5.07 (s, 1 H, γ-CH), 6.66 (broad, 1 H, CdCH), 6.40-
6.52, 6.80-7.00 (m, 10 H, C(Ph)dCH(Ph)), 7.04-7.12 (m, 6 H, Ar-
H). 13C {1H} NMR (125.77 MHz, C6D6, 298 K, ppm): δ 23.5, 24.5,
24.6, 24.8, 26.5, 28.3, 29.1 (CH(CH3)2, â-CH3), 93.9 (broad, tCH),
99.6 (γ-C), 124.4, 124.3, 125.4, 126.4, 127.1, 127.4, 129.6, 139.0, 141.3,
141.5, 143. 4, 145.9, 146.2 (Ph, Ar-C, dC), 153.0, 156.0 (broad, Als
Cd, AlsCt), 171.2 (CN). IR (KBr plate, Nujol mull, cm-1): ν˜ 1996
(CtC), 3270 ((t)CsH). EI-MS: m/z (%) 469 (100, [M+ - C(Ph)d
CH(Ph)]), 648 (2, [M+]). Anal. Calcd (%) for C45H53AlN2 (Mr )
648.92): C, 83.29; H, 8.23; N, 4.31. Found: C, 83.16; H, 8.18; N,
4.34.
Results and Discussion
Synthesis and Characterization of Aluminacyclopropene
LAl[η2-C2(R1)(R2)]. As shown in Scheme 1, LAl (L ) HC-
[(CMe)(NAr)]2, Ar ) 2,6-iPr2C6H3) reacts readily with the
respective ethyne, mono- and disubstituted alkynes, and diyne,
leading to aluminacyclopropene LAl[η2-C2(R1)(R2)] (R1 ) R2
) H, 1; R1 ) H, R2 ) Ph, 2; R1 ) R2 ) Me, 3; R1 ) SiMe3,
R2 ) CtCSiMe3, 4) in excellent yield. Compounds 1 and 2
were obtained when the precursor reagents were used in exactly
equimolar quantities at low temperature. Without this equimolar
ratio, the products obtained were different (vide infra). On the
other hand, when LAl was reacted further with an excess
Synthesis of LAl(CPhdCHPh)(CtCPh) (8). To a toluene solution
(30 mL) of LAl(η2-C2Ph2) (1.24 g, 2 mmol) a toluene solution (10
mL) of an excess of PhCtCH (0.33 mL, 3 mmol) was added. After
stirring for 12 h, the solution was dried in vacuum, and the residue
was extracted with n-hexane (20 mL). The extract was kept at 4 °C for
a week to afford colorless X-ray quality crystals of 8‚0.5 n-hexane.
1
Yield: 1.12 g, 73%. Mp: 187 °C. H NMR (300.13 MHz, C6D6, 298
(12) Sheldrick, G. M. SHELXS-90, Program for Structure Solution. Acta
Crystallogr., Sect. A 1990, 46, 467-475.
3
K, ppm): δ 0.86∼0.90 (m, 7 H, n-hexane), 1.11 (d, 2 × 3 H, JHH
)
(13) Sheldrick, G. M. SHELXL-97, Program for Crystal Structure Refinement;
University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(14) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785-790.
(15) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157,
200-206.
6.8 Hz, CH(CH3)2), 1.24 (d, 4 × 3 H, 3JHH ) 6.8 Hz, CH(CH3)2), 1.35
3
(d, 2 × 3 H, JHH ) 6.8 Hz, CH(CH3)2), 1.61 (s, 2 × 3 H, â-CH3),
3
3.44 (sept, 2 × 1 H, JHH ) 6.8 Hz, CH(CH3)2), 4.01 (sept, 2 × 1 H,
3JHH ) 6.8 Hz, CH(CH3)2), 5.08 (s, 1 H, γ-CH), 6.72 (broad, 1 H,
CdCH), 6.40∼6.52, 6.80-7.00 (m, 10 H, C(Ph)dCH(Ph)), 7.08∼7.24,
7.42∼7.54 (m, 6 H, Ar-H). 13C {1H} NMR (125.77 MHz, C6D6, 298
K, ppm): δ 14.3 (n-hexane), 23.0, 24.5, 24.8, 24.9, 26.1, 28.7, 29.2,
31.9 (CH(CH3)2, â-CH3), 99.5 (γ-C), 106.6 (broad, tCPh), 124.1,
124.2, 125.4, 126.4, 127.1, 127.4, 128.0, 129.6, 131.4, 132.0, 139.1,
141.3, 143.2, 145.9, 146.7 (Ph, Ar-C, dC), 144.5, 153.8 (broad, Als
(16) Frisch, M. J. et al. Gaussian 03, revision C.02. Gaussian, Inc.: Wallingford,
CT, 2004.
(17) (a) Petersson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081-
6090. (b) Petersson, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham, M.
A.; Shirley, W. A.; Mantzaris, J. J. Chem. Phys. 1988, 89, 2193-2218.
(18) Mayer, I. Chem. Phys. Lett. 1983, 97, 270-274.
(19) Reed, A. E.; Weinhold, F. J. Chem. Phys. 1985, 83, 1736-1740.
(20) Ahlrichs, R. et al. TURBOMOLE 5.5; University of Karlsruhe, Germany,
2002.
9
5102 J. AM. CHEM. SOC. VOL. 128, NO. 15, 2006

Aluminacyclopropene: Reactivity toward Terminal Alkynes
A R T I C L E S
Table 1. Crystallographic Data for Compounds 4, 6, 7, and 8‚0.5 n-Hexane
2‚
4
6
7
8
‚
0.5 n-hexane
formula
fw
C78H118Al2N4Si4
1278.08
C45H53AlN2
648.87
C45H53AlN2
648.87
C54H64AlN2
768.05
temp (K)
crystal syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å3)
133(2)
orthorombic
Pca2(1)
40.070(8)
18.332(4)
10.556(2)
90.00
90.00
90.00
7754(3)
4
100(2)
triclinic
P1h
11.118(2)
12.440(3)
15.279(3)
73.57(3)
75.60(3)
72.74(3)
1904.2(7)
2
1.132
0.086
700
100(2)
monoclinic
P2(1)/n
15.768(3)
12.973(3)
18.678(4)
90.00
98.39(3)
90.00
3779.8(14)
4
1.140
133(2)
monoclinic
P2(1)/n
14.641(3)
19.674(4)
16.162(3)
90.00
99.78(3)
90.00
4587.7(16)
4
1.112
Z
Fc (Mg/m3)
1.095
0.142
2784
µ (mm-1
F(000)
)
0.703
1400
0.081
1660
θ range (deg)
1.22-24.81
-47 e h e 45
-21 e k e 21
-12 e l e 12
43 701
1.76-26.39
-13 e h e 13
-15 e k e 15
-19 e l e 19
36 703
3.43-60.33
-17 e h e 17
-14 e k e 14
-21 e l e 21
17 868
1.65-24.89
-17 e h e 17
-23 e k e 23
-19 e l e 19
49 895
index ranges
no. of reflns collected
no. of indep reflns (Rint
)
13 073 (0.1187)
13073/1/825
0.721
0.0494, 0.0628
0.1310, 0.0782
0.187/-0.242
7784 (0.0381)
7784/0/443
1.221
0.0789, 0.1670
0.0871, 0.1714
0.612/-0.467
5560 (0.0288)
5560/1/447
1.016
0.0328, 0.0766
0.0415, 0.0814
0.236/-0.242
7919 (0.0648)
7919/0/524
0.986
0.0430, 0.1020
0.0671, 0.1100
0.170/-0.204
no. of data/restraints/params
GOF/F2
R1a, wR2b(I > 2σ(I ))
R1a, wR2b(all data)
largest diff peak/hole (e‚Å-3
)
2
2
2
a R ) Σ||Fo| - |Fc||/Σ|Fo|. b wR2 ) [Σw(Fo - Fc )2/Σw(Fo )]1/2
.
Scheme 1
Scheme 2
quantity of MeCtCMe and Me3SiCtCCtCSiMe3, there were
no more surprises; the expected compounds 3 and 4 were
isolated. This indicates an inertness of 3 and 4 toward further
reaction with the respective disubstituted alkyne precursor.
Compounds 1-3 can be formed at low temperature (even at
ca. -100 °C), while compound 4 is generated under refluxing
conditions. Alternatively, 4 can be obtained by the substitution
reaction using LAl[η2-C2(SiMe3)2] and Me3SiCtCCtCSiMe3
or by the reductive coupling reaction of LAlI2 with potassium
in the presence of Me3SiCtCCtCSiMe3 both at room tem-
perature (Scheme 2). Reductive couplings are not successful in
the presence of C2H2 and PhCtCH for the preparation of
aluminacyclopropene derivatives due to the metalation of the
acetylenic hydrogen.
solvent mixture of n-hexane and toluene. Attempts to grow
single crystals of 2 were not successful.
1
In the H NMR spectra the proton(s) at the AlC2 carbon
atom(s) resonate(s) at 8.82 ppm for 1 and at 8.66 ppm for 2. In
the 13C NMR spectra the AlC2 carbon atoms resonate in the
range of 165.4 to 193.2 ppm (177.2, 1; 165.4, 170.2, 2; 169.4,
3; 190.0, 193.2 ppm, 4). These chemical shifts both appear
significantly downfield when compared to those of the respective
protons and carbons in normal organic alkenyl systems.21 The
13C resonances are comparable to that found in such a SnC2
ring compound (163.9 ppm).22 These suggest a speciality of
the olefinic bond in metallacyclopropene. The observation of
two kinds of carbon resonances in 2 and 4, respectively,
indicates an asymmetric AlC2 ring induced by the different
groups (H and Ph, 2; SiMe3 and CtCSiMe3, 4) at the AlC2
carbon atoms. Furthermore, the environmentally different SiMe3
groups in 4 are evident from its 29Si NMR spectrum, in which
two silicon resonances were observed (-20.9 and -16.4 ppm).
Moreover, the CtC functionality in 4 is confirmed from its
Compounds 1-4 are easily separated as orange crystalline
solids by removing all volatiles and then by washing the residue
with n-hexane. They are air-sensitive and highly soluble in
aromatic solvents, but slightly soluble in aliphatic hydrocarbons.
Compounds 1-4 have been fully characterized by mass
spectrometry and IR and multinuclear NMR (1H, 13C, or/and
29Si) spectroscopy as well as by X-ray crystallography in the
cases of 1 and 4. X-ray quality crystals were grown from a
(21) Vollhardt, K. P. C.; Schore, N. E. Organic Chemistry: Structure and
Function, 3rd ed.; W.H. Freeman: New York, 1999; pp 438-483.
(22) Sita, L. R.; Bickerstaff, R. D. J. Am. Chem. Soc. 1988, 110, 5208-5209.
9
J. AM. CHEM. SOC. VOL. 128, NO. 15, 2006 5103

A R T I C L E S
Zhu et al.
ref
Table 2. Geometric Parameters within the AlC2 Ring of Aluminacyclopropenes
compound
Al
sC (Å)
C
s
C (Å)
CsAls
C (deg)
AlsCsC (deg)
HAl(η2-C2H2)a
1.844-1.852
1.362-1.384
1.358(2)
1.382(4)
1.356(5)
1.369(11) 1.395(11)
7
3
1
1
LAl(η2-C2H2)b
1.878(2), 1.885(2)
1.899(3), 1.908(3)
1.889(4), 1.894(3)
1.862(8), 1.909(9)
1.904(8), 1.918(9)
42.30(7)
42.57(11)
42.02(14)
42.0(3)
68.57(10), 69.13(10)
68.39(15), 69.04(15)
68.80(19), 69.20(20)
66.5(5), 70.1(5)
LAl(η2-C2Ph2)
LAl[η2-C2(SiMe3)2]
LAl[η2-C2(SiMe3)(CtCSiMe3)]c
this work
43.4(3)
68.5(5), 69.6(5)
a This species is based on a theoretical model, and the corresponding data are obtained by calculation. b L ) HC[(CMe)(NAr)]2, Ar ) 2,6-iPr2C6H3. c By
X-ray structural analysis the two independent molecules are revealed, corresponding to two sets of structural data.
Table 3. Geometric Parameters of the AlsCdC and AlsCtC Functionalities in Compounds 5-8
compound
Al
s
CC C (Å)
Al
s
CC C (Å)
t
CdC (Å)
C
tC (Å)
Al
sCd
C (deg)
Al
s
CtC (deg)
ref
d
LAl(CHdCH2)(CtCH)a,b
LAl(CHdCHPh)(CtCPh)b
LAl(CPhdCHPh)(CtCH)
LAl(CPhdCHPh)(CtCPh)
1.944(11)
1.952(3)
1.961(2)
1.972(2)
1.962(11)
1.941(3)
1.952(2)
1.941(2)
1.323(18)
1.334(11)
1.349(2)
1.343(3)
1.170(14)
1.189(11)
1.196(2)
1.217(2)
124.6(14)
121.8(9)
119.53(11)
119.69(13)
175.4(19)
169.3(6)
173.11(14)
169.07(17)
3
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a L ) HC[(CMe)(NAr)]2, Ar ) 2,6-iPr2C6H3. b Both CHdCH2 (or CHdCHPh) and CtCH (or CtCPh) groups are disordered in two positions, and
therefore the averaged CsC bond lengths and AlsCsC angles are given.
Scheme 3
are comparable. In a group 14 metal-containing MC2 ring, the
C-C bond lengths are observed in the range from 1.331(9) to
1.39(1) Å.22,24 Finally, it is important to note that the AlC2 ring
is nearly perpendicular to its fused AlN2C3 ring formed by the
chelating L groups at the Al center (the dihedral angle 88.6°,
1; 88.3°, 88.8°, 4).
Figure 1. Molecular structure of 4. Thermal ellipsoids are drawn at the
Synthesis and Characterization of Alkenylalkynylalumi-
num Compound LAl(CR1dCHR2)(CtCR3). The reaction of
LAl with excess C2H2 and PhCtCH (<1:2) affords LAl(CHd
CH2)(CtCH) (5) and LAl(CHdCHPh)(CtCPh) (6), respec-
tively. Compounds 1 and 2 have been isolated from the
corresponding reactions in an exact molar ratio of 1:1 at low
temperature. Therefore we rationalized that 5 and 6 are formed
proceeding through 1 and 2 as the respective intermediates.
Nonetheless, we have still investigated the reaction of LAl[η2-
C2Ph2] with C2H2 and PhCtCH, and as expected compounds
LAl(CPhdCHPh)(CtCH) (7) and LAl(CPhdCHPh)(CtCPh)
(8) were isolated, respectively (Scheme 3).
Compounds 5-8 were obtained as colorless crystalline solids.
The molecular structures of 5-8 are shown in Figures 2-4.
The chelating character of the L ligand by the two N atoms at
the Al center is known,1-3,26 and the geometric data for the
functionalities Al(CR1dCHR2)(CtCR3) are listed in Table 3.
50% level, and the hydrogen atoms are omitted for clarity.
characteristic carbon resonances (107.6 and 108.8 ppm) and IR
absorption band (2065 cm-1).
The structural analysis, with two independent molecules of
4 that crystallize in space group Pca2(1), unambiguously
exhibits this structural feature. The structure of one molecule
is shown in Figure 1. The geometric parameters of the AlC2
ring in 4 and the other aluminacyclopropenes including the
computationally derived one are shown in Table 2. These data
exhibit that the Al-C bond lengths within the AlC2 ring range
from 1.844 to 1.918(9) Å and the C-C bond distances from
1.356(5) to 1.395(11) Å. These Al-C bond lengths are shorter
than the experimental ones (1.95 to 2.05 Å, the value of 2.07
Å is based on the covalent radii).23 However, the C-C bond
distances appear longer than those observed for terminal
aluminum vinyl system (1.325(17) to 1.349(2) Å, Table 3), and
are ranging between C-C double and single bonds. A com-
parison of the bond lengths in four-,3 five-,1 and six-membered5,6
aluminum-atom-contained ring compounds (Al-C, 1.932(3) to
1.992(2) Å; C-C, 1.342(4) to 1.366(2) Å) shows that the Al-C
bond lengths are still shorter, while the C-C bond distances
(24) (a) Egorov, M. P.; Kolesnikov, S. P.; Struchkov, Yu. T.; Antipin, Y. M.;
Sereda, S. V.; Nefedov, O. M. J. Organomet. Chem. 1985, 290, C27-
C30. (b) Ando, W.; Ohgaki, H.; Kabe, Y. Angew. Chem. 1994, 96, 723-
725; Angew. Chem., Int. Ed. Engl. 1994, 33, 659-661.
(25) Lide, D. R. In Handbook of Chemistry and Physics 2003-2004, 84,
(9-)16-17.
(26) Qian, B.; Ward, D. L.; Smith, M. R., III. Organometallics 1998, 17, 3070-
3076.
(23) Brothers, P. J.; Power, P. P. AdV. Organomet. Chem. 1996, 39, 1-69.
9
5104 J. AM. CHEM. SOC. VOL. 128, NO. 15, 2006

Aluminacyclopropene: Reactivity toward Terminal Alkynes
A R T I C L E S
Figure 2. Molecular structure of 6. Thermal ellipsoids are drawn at the
50% level. The olefinic hydrogen atoms are presented, and the other ones
are omitted for clarity. The CHdCHPh and CtCPh groups both are in the
occupancies of 0.671(13).
Figure 4. Molecular structure of 8. Thermal ellipsoids are drawn at the
50% level. The olefinic hydrogen atom is presented, and the other ones are
omitted for clarity.
The selected spectral data (IR, 1H and 13C NMR) of the Al-
(CR1dCHR2)(CtCR3) functionalities in compounds 5-8 are
shown in Table 4. The olefinic proton resonances range from
5.79 to 7.42 ppm with a variety of signals due to the adjacent
HsH coupling. A similar pattern was observed for R-methyl-
styrene (9%) (5.20 ppm) and trans-â-methylstyrene (7%) (6.15
ppm), the hydrolysis products of the reaction of AlMe3 with
monophenyl acetylene.27 The acetylenic protons resonate at
around 1.80 ppm as a singlet (1.73, 5; 1.82 ppm, 7) and the
corresponding carbons (tCR3) in the characteristic range 93.9
to 107.8 ppm as one broad resonance. In the IR spectra, the
CtC and (t)CsH vibrations are clearly observed in the
respective modes both as weak absorption bands (νCtC: 1992
5, 2128 6, 1996 7, 2124 cm-1 8; ν(t)CsH: 3277 5, 3270 cm-1
7).
Bonding Character of AlsN and the AlC2 Ring in
Aluminacyclopropenes. We have previously discussed, using
1
variable-temperature H NMR investigation and ESR studies
Figure 3. Molecular structure of 7. Thermal ellipsoids are drawn at the
50% level. The acetylenic and olefinic hydrogen atoms are presented, and
the other ones are omitted for clarity.
as well as 27Al NMR spectral analysis, that the AlC2 ring
compound like LAl[η2-C2R2] (R ) Ph, SiMe3) can be better
described as a metallacyclopropene (Al(III)) more than as an
alkyne π-complex.1 Furthermore, the AlC2 ring structural data
in a series of aluminacyclopropenes clearly indicate a shortening
of the AlsC bond length and an elongation of the CsC bond
distance when compared with the covalent AlsC single and
CdC double bonds. These structural findings can be compared
with those in the corresponding borirenes, in which π-electron
delocalization over the BC2 ring is invoked whether by
experimental data28 or by calculations.29 Therefore it is interest-
ing for us to investigate the bonding properties of this AlC2
ring system by DFT calculations. Comparable features are also
observed in group 14 metal MC2 ring compounds.22,24 The
Precluding the phenyl groups in these functionalities, the CsC
bond lengths are ranging from 1.171(4) to 1.217(2) Å and from
1.325(17) to 1.349(2) Å, respectively. They essentially fall in
the range of the respective sp hybridized organic acetylenic
(1.174-1.183 Å) and sp2 hybridized olefinic linkage (1.299-
1.331 Å),25 indicating the respective CtC triple and CdC
double bond system. Correspondingly, the AlsCtC (169.07-
(17)-173.9(2)°) and AlsCdC bond angles (119.53(11)-124.6-
(12)°) are close to the ideal 180° and 120°, respectively. The
AlsC bond lengths (AlsCCtC, 1.941(2)-1.962(11); AlsCCd
C, 1.944(11)-1.972(2) Å) are comparable to those found in the
literature (1.95-2.05 Å)23 and are also close to those in
â-diketiminato aluminum alkyls LAlMe2 (1.955(4), 1.961(3) Å)
and L′AlMe2 (L′ ) HC[C(Me)N-p-toluene]2, 1.958(3), 1.970-
(3) Å).26 This may imply here much lesser interaction of π
electrons in CsC multiple bonds extending to its adjacent Lewis
acidic Al atom.
(27) Mole, T.; Surtees, J. R. Aust. J. Chem. 1964, 17, 1229-1235.
(28) Eisch, J. J.; Shafii, B.; Odom, J. D.; Rheingold, A. L. J. Am. Chem. Soc.
1990, 112, 1847-1853.
(29) (a) Pittman, C. U., Jr.; Kress, A.; Patterson, T. B.; Walton, P.; Kispert, L.
D. J. Org. Chem. 1974, 39, 373-378. (b) Krogh-Jespersen, K.; Cremer,
D.; Dill, J. D.; Pople, J. A.; Schleyer, P. v. R. J. Am. Chem. Soc. 1981,
103, 2589-2594. (c) Jug, K. J. Org. Chem. 1984, 49, 4475-4478.
9
J. AM. CHEM. SOC. VOL. 128, NO. 15, 2006 5105

A R T I C L E S
Zhu et al.
Table 4. Selected Spectral Data (IR, 1H and 13C NMR) of the AlsCR1dCHR2 and AlsCtCR3 Functionalities in Compounds 5-8a
IR
˜, cm-
13C NMR
, ppm)
1
(
ν
)
1H NMR (
δ, ppm)
(δ
compound
Ct
C
(
t
)Cs
H
t
CH
dCH
t
CR3
ref
LAl(CHdCH2)(CtCH)
LAl(CHdCHPh)(CtCPh)
LAl(CPhdCHPh)(CtCH)
LAl(CPhdCHPh)(CtCPh)
1992
2128
1996
2124
3277
3270
1.73 (s)
1.82 (s)
5.79 (dd), 6.04 (dd), 6.12 (dd)
7.41 (d), 7.42 (d)
6.66 (s)
94.6
107.8
93.9
3
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6.72 (s)
106.6
a L ) HC[(CMe)(NAr)]2, Ar ) 2,6-iPr2C6H3.
Table 5. Selected Computational Bond Orders and Dissociation
Scheme 4
2
Energies (DAl-η -C , ∆E, kJ/mol) of Aluminacyclopropenes
2
bond order
compound
Als
N
Al
sC
C
s
Ca
∆
Eb
LAl(η2-C2H2)c
0.610, 0.610 1.103, 1.103 1.667 142.2, 155d
0.612, 0.612 1.076, 1.083 1.573 134.3
0.614, 0.615 1.037, 1.058 1.518 107.9
LAl[η2-C2(H)(Ph)]
LAl(η2-C2Ph2)
LAl[η2-C2(SiMe3)2] 0.612, 0.613 1.030, 1.032 1.483
82.6
92.1
LAl[η2-C2(SiMe3)-
(CtCSiMe3)]
0.623, 0.624 1.005, 1.063 1.436
a CsC in the AlC2 ring. b ∆E ) E(LAl + alkyne) - E(aluminacyclo-
propene); the calculations made use of the established DFT variant B3LYP
method. c L ) HC[(CMe)(NAr)]2, Ar ) 2,6-iPr2C6H3. d This value is
calculated on DFT level (RI-BP86) with the SV(P) basis sets.
of aluminacyclopropene must await further investigation on such
a compound with a planar three-coordinate Al atom.32
calculations have been carried out using the established DFT
variant B3LYP method,15 and the computational bond order data
(the sum of σ and π contributions) for all bonds in a series of
aluminacyclopropene molecules are obtained. The bond order
values of the AlsN as well as those of the AlsC and CsC
bonds within the AlC2 ring are listed in Table 5.
The two AlsN bond orders of these aluminacyclopropenes
are calculated in the range of 0.610 to 0.623 and of 0.610 to
0.624, respectively. Due to the bonding character of monovalent
â-diketiminato L with Al by the two adjacent N atoms, the
general σ-bond order of two AlsN bonds should be around 1.
The total calculated AlsN bond order is ca. 1.23. This value is
a little high and yet normal when compared with calculated data
for those of ethane (1.25) and ethylene (2.16).30 These two sets
of data are prone to be equal, suggesting the equivalence and
therefore give not a clear distinction of the two AlsN bonds.
In fact, the bidentate â-diketiminato skeleton is a conjugated
system, and in the majority of â-diketiminato metal complexes
the two AlsN bond lengths are almost equal based on the
structural data.31
In the AlC2 ring, the AlsC bond orders range from 1.01 to
1.10, and those of the CsC bond from 1.67 to 1.44 with the
variation of substituents at the C atoms. The former is close to
1 and could be related to indicate the AlsC σ-bond. This
suggests less π electron delocalization over the AlC2 ring.
However it is in contrast to the lower CsC bond orders, since
the results differ when compared with those of the calculated
congeneric BC2 ring model, HB(CH)2 (BsC vs CsC bond
order: 1.623 vs 1.737, by INDO; 1.614 vs 1.899, by SINDO1
calculations).29 Thus, the corresponding structural and multi-
nuclear NMR (1H, 13C) data, which are most important for the
ring description, may hide some complexity of the MC2 ring in
comparison with the organic ones. Insight into the ring properties
Interaction of the Al(I) Center with the CtC Carbon
Atoms. LAl reacts with 1 equiv of alkyne leading to alumina-
cyclopropene. A formation of AlsC σ bonds proposed above
and the conversion of the CtC triple bond to the CdC double
bond, in which an elongation of the CdC bond length is evident,
indicate that this reaction is better described as an oxidative
cycloaddition. Some related calculations demonstrate that there
is a complexation energy responsible for such a reaction.2,7 This
implies that an interaction that occurs between the Al(I) center
and the CtC carbon atoms may correlate with the energy
changes of the reaction system. Thus an approach of an alkyne
molecule to the Al(I) center to form the final AlC2 ring and the
energy changes for this system based on DFT calculations have
been investigated. The unlimited Al‚‚‚C separation between LAl
and C2H2 is set as zero potential energy surface, and the settled
one is proposed to indicate one state that will correspond to a
relative potential energy surface. In the meantime, two possible
approaching modes are considered: a CtC center is set close
to the Al atom (channel a), and one terminal CtC atom to the
Al atom (channel b) (Scheme 4).
By calculations, a series of potential energy surface points
versus Al‚‚‚C distances (4.00-1.80 Å) were obtained. With a
line drawn according to these points, the potential energy curve
is given in Figure 5 (the black curve corresponds to channel a,
and the red one, to b). In Figure 5 we find that the reaction
system energy varies with the separation between LAl and C2H2.
Of which there is an energy barrier (the most height is at ca.
145 kJ/mol, which was found by calculating the cross section
of the potential energy surfaces of the two adjacent states) in a,
while almost no barrier is formed in b. Channel b is energetically
(32) In a series of aluminacyclopropenes presented in this work, the Al center
is involved in two fused ring systems, in which C-C or C-N multiple
bonds are contained, in a tetrahedral coordinative geometry. This may give
rise to the complexity of the AlC2 ring properties in calculations. Volpin,
M. E.; Koreshkov, Y. D.; Dulova, V. G.; Kursanov, D. N. Tetrahedron
1962, 18, 107-122.
(30) Jug, K. J. Org. Chem. 1983, 48, 1344-1348.
(31) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. ReV. 2002, 102,
3031-3065.
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5106 J. AM. CHEM. SOC. VOL. 128, NO. 15, 2006

Aluminacyclopropene: Reactivity toward Terminal Alkynes
A R T I C L E S
Figure 5. The potential energy curve of the reaction system of LAl and
C2H2 versus Al‚‚‚CC H distances.
2
2
favored, which means that almost no activation energy is needed
for the reaction of LAl with C2H2 to form LAl(η2-C2H2). This
is in good agreement with our experimental observation that
LAl can react with C2H2 at low temperature (ca. -100 °C).
Channel a may be depicted as a π-complexation process. Ab
initio MO calculations have suggested that a lone pair of
electrons is accommodated at the Al atom of LAl,8 and therefore
an electronic interaction between Al(I) center and CtC bond
could be involved. This would conversely favor the occurrence
of channel b.
1
Figure 6. The H NMR spectroscopy studies of a reaction of LAl(η2-C2-
Ph2) with a small excess of PhCtCH in toluene-d8. This reaction was kept
at 40 °C (behaved like a sample under stirring). (I) and (II) record the
resonance changes of the respective γ-CH proton of L and olefinic proton
of CPhdCHPh (ppm) with reaction time (h).
Scheme 5
The relative potential energy corresponding to the final LAl-
(η2-C2H2) state is about 155 kJ/mol, which is suggestive of the
reaction complexation energy of LAl and C2H2 (that is the Al-
η2-C2 bond dissociation energy, DAl-η -C ). This value can be
2
2
compared with that of HAl(η2-C2H2) (av 126 kJ/mol).7 Moreover
2
we computed such DAl-η -C values of other aluminacyclopro-
2
penes for comparison. The results are shown in Table 4. On
Some experimental data support this assumption.6,35 Further Als
H or AlsC addition to the CtC triple bond can result in
aluminum vinyl derivatives,4,34 while the corresponding meta-
lation can occur with acetylenic hydrogen leading to aluminum
acetylides.4,36 The reaction of aluminacyclopropene with ter-
minal alkynes generating alkenylalkynylaluminum compounds
may proceed through such a preliminary π-complexation,
followed by an acetylenic hydrogen migration to one of the
olefinic carbons under AlC2 ring opening. The proposed
mechanism is shown in Scheme 5.
2
one hand, the DAl-η -C values range from 155 to 82.62 kJ/mol
2
and are much smaller than that of Al-Cmethyl in AlMe3 (281.4
kJ/mol).4 This implies a weak Al-η2-C2 bonding and essentially
indicates a remarkable reactivity of the AlC2 ring compound.
Indeed, the insertion of small molecules or substitution of the
AlC2 ring has been experimentally observed.1-3 On the other
2
hand, the DAl-η -C varies with the substituents at the AlC2
2
carbon atoms. This shows an influence of such substituents (for
example, steric bulk and electronic factors) on the Al-η2-C2
bond dissociation energy and hence an effect on the complex-
ation of LAl with alkynes where such a bulkiness of L actually
exists. While aluminacyclopropenes 1-3 are formed at low
temperature (-100 °C), the LAl[(η2-C2(R1)(R2)] (R1, R2: SiMe3,
Ph) could be made at room temperature.33 Thus, 4 is obtained
under refluxing conditions.
1
To gain further insight into such a reaction process, the H
NMR studies on the reaction of LAl(η2-C2Ph2) and PhCtCH
in toluene-d8 was accomplished. Figure 6 shows the resonance
changes of the γ-CH proton in L, which is a good indicator for
the reactant and product, and the olefinic one in the AlCPhd
CHPh group with reaction time. In (I), the gradual consumption
of LAl(η2-C2Ph2) and the corresponding generation of 8 are
clearly observed. In the meantime, the integral intensity ratios
of resonances of PhCtCH and referenced toluene-d8 methyl
protons varied from 0.37:1, 0.34:1, 0.33:1, ‚ ‚ ‚, and finally to
0.27:1, indicating the reduction of PhCtCH. Instead, in (II),
An Acetylenic Hydrogen Migration to One of the AlC2
Carbon Atoms. Among the reactions of aluminum(III) alkyls
or hydrides with alkynes, a preliminary π-complexation is
generally suggested due to an interaction of the π electrons of
the CtC bond with a Lewis acidic aluminum(III) center.4,34
(33) The respective NMR tube reaction of LAl with a little excess of Me3SiCt
CSiMe3 and PhCtCPh in toluene-d8 at room temperature has been
accomplished. The solution color changes and 1H NMR spectral analysis
indicates the occurrence of both reactions and the formation of the
corresponding aluminacyclopropenes LAl[(η2-C2(SiMe3)2] and LAl(η2-C2-
Ph2).
(35) (a) Stucky, G. D.; McPherson, A. M.; Rhine, W. E.; Eisch, J. J.; Considine,
L. J. Am. Chem. Soc. 1974, 96, 1941-1942. (b) Dolzine, T. W.; Oliver, J.
P. J. Am. Chem. Soc. 1974, 96, 1737-1740. (c) Hausen, H.-D.; To¨dtmann,
J.; Weidlein, J. Z. Naturforsch. 1994, 49B, 430-433.
(36) (a) Mole, T.; Surtees, J. R. Chem. Ind. (London) 1963, 1727-1728. (b)
Zheng, W.; Mo¨sch-Zanetti, N. C.; Roesky, H. W.; Hewitt, M.; Cimpoesu,
F.; Schneider, T. R.; Stasch, A.; Prust, J. Angew. Chem. 2000, 112, 3229-
3231; Angew. Chem., Int. Ed. 2000, 39, 3099-3101.
(34) Ziegler, K. In Organometallic Chemistry; Zeiss, H. H., Ed.; Reinhold
Publishing Corp.: New York, 1960; pp 195-269.
9
J. AM. CHEM. SOC. VOL. 128, NO. 15, 2006 5107

A R T I C L E S
Zhu et al.
the concomitant increasing of the olefinic proton resonance
corresponds to the formation of the terminal AlCPhdCHPh
group of 8. This undoubtedly draws a picture of the PhCtCH
reactivity of such AlC2 species, which have been viewed from
its highly strained structure.1,3,4 This is well demonstrated by
its reaction chemistry reported in recent years1-3,5,6 and in this
work. The existing Lewis acidity of the Al(III) center (although
four coordinate) of the AlC2 ring indicates a large possibility
that the Al center can allow an attack from other Lewis basic
reagents. Therefore, the reaction of aluminacyclopropene with
terminal alkyne forming alkenylalkynylaluminum compounds
may proceed through an initial π-complexation, following with
an acetylenic hydrogen migration under opening of the AlC2
ring.
1
hydrogen migration process. However, in the H NMR moni-
tored reaction, no resonances were observed, which could be
assigned to be those of the proposed π-complex of PhCtCH.
This could explain that the π-complexation of alkyne to LAl is
only a transition state, which is extremely unstable, in the
formation of the product.
Conclusions
In summary, the Al(I) center stabilized by the bulky
â-diketiminato ligand L can readily react with the CtC of
various alkynes, resulting in the formation of aluminacyclopro-
penes. DFT calculations suggest that the CtC unit approaching
mode to the Al(I) center needs almost no activation energy.
The temperature variation in the formation of the AlC2 ring by
the reaction of LAl with various alkynes reflects an influence
of the substituents at the CtC linkage in the presence of the
bulky L. Similar reaction behavior has been widely observed
in organic cycloaddition of transient carbene to the CsC
multiple bonds.37 Based on the energy calculations, much lower
Alsη2-C2 bond dissociation energies reveal a remarkable
Acknowledgment. We are grateful for financial support of
the Go¨ttinger Akademie der Wissenschaften.
Supporting Information Available: CIF files for compounds
2‚4, 6, 7, and 8‚0.5n-hexane, selected results of DFT calcula-
tions, and a detailed author name list of refs 16 and 20. This
material is available free of charge via the Internet at
http://pubs.acs.org.
JA057731P
(37) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000,
100, 39-91.
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5108 J. AM. CHEM. SOC. VOL. 128, NO. 15, 2006