A rearrangement of azobenzene upon interaction with an aluminum(I) monomer LAl {L = HC[(CMe)(NAr)]2, Ar = 2,6-iPr2C6H 3}

Article abstract of DOI:10.1002/ejic.200400922

Reaction of LAl (1) or [LAl{η²-C₂(SiMe ₃)₂}] (2) {L = HC[(CMe)-(NAr)]₂, Ar = 2,6-iPr₂C₆H₃} with azobenzene affords a five-membered ring compound [LAl{N(H)-o-C₆H₄N(Ph)}] (3). In the formation of 3 a three-membered intermediate [LAl(η ²-N₂Ph₂)] (A) is suggested by a [1 + 2] cycloaddition reaction; A is not stable and further rearranges to 3. DFT calculations on similar compounds with modified L′ {L′ = HC[(CMe) (NPh)]₂} show that the complexation energy of the reaction of L′Al with azobenzene to form [L′Al(η²-N ₂Ph₂)] is about -39 kcal mol^-1, and the best estimate of the energy difference between [L′Al(η²-N ₂Ph₂)] and [L′Al{N(H)-o-C₆H ₄N(Ph)}] is -76 kcalmol^-1. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005.

Full text of DOI:10.1002/ejic.200400922

FULL PAPER
A Rearrangement of Azobenzene upon Interaction with an Aluminum( )
I
Monomer LAl {L = HC[(CMe)(NAr)]2, Ar = 2,6-iPr2C6H3}
Hongping Zhu,[a] Jianfang Chai,[a] Hongjun Fan,[b] Herbert W. Roesky,*[a]
Umesh N. Nehete,[a] Hans-Georg Schmidt,[a] and Mathias Noltemeyer[a]
Keywords: Aluminum / Azobenzene / N ligands / Rearrangement
Reaction of LAl (1) or [LAl{η2-C2(SiMe3)2}] (2) {L = HC[(CMe)- (NPh)]2} show that the complexation energy of the reaction
(NAr)]2, Ar = 2,6-iPr2C6H3} with azobenzene affords a five-
membered ring compound [LAl{N(H)-o-C6H4N(Ph)}] (3). In
the formation of 3 a three-membered intermediate [LAl(η2-
N2Ph2)] (A) is suggested by a [1 + 2] cycloaddition reaction;
A is not stable and further rearranges to 3. DFT calculations
on similar compounds with modified LЈ {LЈ = HC[(CMe)
of LЈAl with azobenzene to form [LЈAl(η2-N2Ph2)] is about
–39 kcalmol–1, and the best estimate of the energy difference
between [LЈAl(η2-N2Ph2)] and [LЈAl{N(H)-o-C6H4N(Ph)}] is
–76 kcalmol–1
.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
isomerization of azobenzene. Obviously, a rearrangement of
azobenzene is occurring upon interaction with LAl.
Introduction
The reactions of Group 13 metal(i) tetramers (RM)4 (R
= organic group, M = Al, Ga, In) with unsaturated mole-
cules
[H2C=C(Me)–C(Me)=CH2,
PhC(O)–C(O)Ph,
Results and Discussion
RN=C(H)–C(H)=NR, R = Me, iPr][1–3] allow the trapping
of the corresponding monomer RM, and are also an inter-
esting oxidative addition of compounds with unsaturated
bonds to low-valent metal centers.[4] This leads to hetero-
cyclic compounds containing novel heavier main-group ele-
ments that have potential applications in pharmaceutical,
agrochemical, and materials science.[5] We have recently pre-
pared the aluminum(i) monomer LAl {1, L = HC[(CMe)-
(NAr)]2, Ar = 2,6-iPr2C6H3}, which has a singlet carbene
character.[6] The reductive coupling reaction of LAlI2 with
potassium in the presence of alkynes follows a [1 + 2] cyclo-
addition pathway to yield the aluminacyclopropene
[LAl{η2-C2(R)(RЈ)}] (R = RЈ = SiMe3, Ph; R = Ph, RЈ =
SiMe3).[7] A direct coupling reaction between LAl and al-
kyne (Me3SiCϵCCϵCSiMe3) is subsequently realized to
form [LAl{η2-C2(SiMe3)(CϵCSiMe3)}].[8] In this context,
we are interested in the interaction of the AlI center with
compounds containing an N=N double bond. The reaction
of 1 or [LAl{η2-C2(SiMe3)2}] (2) with azobenzene unexpec-
tedly resulted in the formation of the five-membered ring
complex [LAl{N(H)-o-C6H4N(Ph)}] (3). Compound 3 con-
tains an N(H)-o-C6H4N(Ph) moiety which is formed by an
Upon stirring of a toluene solution of 1 and azobenzene
at elevated temperature (80 °C) for 5 h, the red color
changed to orange. Partial removal of the solvent in vacuo
and addition of n-hexane led to the crystallization of 3, at
4 °C, as orange crystals in good yield. An alternative route
to 3 was investigated by treating 2 with azobenzene in the
temperature range from –50 °C to room temperature. The
result indicated that 2 could be used as a good precursor
for 1.
Compound 3 is thermally stable, as indicated by its high
melting point (260–261 °C) and its most intense molecular
ion peak {m/z (%) = 626 (100) [M+]} found in the EI mass
spectrum. Complex 3 has been fully characterized by spec-
troscopic, analytical, and X-ray single-crystal measure-
ments.
The molecular structure of 3 is shown in Figure 1. The
central Al atom is involved as part of two fused five-
(AlN2C2) and six-membered (AlN2C3) rings. The corre-
sponding AlN4 core appears in a distorted tetrahedral ge-
ometry. The Al–N bond lengths within the AlN2C2 ring are
1.807(2) Å [Al–N(H)] and 1.847(1) Å [Al–N(Ph)], and are
similar to those of the AlN4 ring complex [1.815(2),
1.851(2) Å].[9] The Al–Nβ-diketiminato bond lengths [1.893(1)
and 1.862(1) Å] fall in the range [1.874(1)–1.959(3) Å] ob-
served for other four-coordinate (β-diketiminato)aluminum
compounds,[7,10] although one bond is a little shorter than
these values. The AlN2C2 ring is nearly planar (Δ =
0.0719 Å) and this planar character can be extended to the
[a] Institut für Anorganische Chemie der Universität Göttingen,
Tammannstrasse 4, 37077 Göttingen, Germany
Fax: +49-551-39-3373
E-mail: hroesky@gwdg.de
[b] Labor für Physikalische und Theoretische Chemie, Universität
Siegen,
57068 Siegen, Germany
Eur. J. Inorg. Chem. 2005, 2147–2150
DOI: 10.1002/ejic.200400922
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H. W. Roesky et al.
FULL PAPER
adjacent disubstituted phenyl group (Δ = 0.0816 Å). It is
interesting to note that, within the AlN2C2 ring, the C(31)–
C(36) bond [1.429(2) Å] is longer than the remaining ones
of the phenyl ring [1.382(2)–1.393(2) Å]. This obviously
contributes to the AlN2C2 ring formation. The phenyl
groups involved in different structural environments [disub-
stituted C6H4, N(Ph), Ar] exhibit diverse resonances for
1
their aromatic protons in the H NMR spectrum of 3. An
Scheme 1. Proposed formation of 3 from the reaction of 1 and
PhNNPh.
unambiguous assignment of the resonances was not pos-
sible. The NH proton resonates at δ = 3.06 (s) ppm and in
the IR spectrum the absorption at 3220 cm–1 is assignable
to νNH
.
A further insight into this proposed mechanism can be
gained from theoretical calculations.[20] The computed re-
sults show that the complexation energy of the initial reac-
tion of LЈAl with azobenzene to form [LЈAl(η2-N2Ph2)] {LЈ
is HC[(CMe)(NPh)]2 for simplicity of calculation} is
about –39 kcalmol–1. This indicates a reasonable possibility
that A is an intermediate in the reaction of 1 with azoben-
zene. The value of –39 kcalmol–1 is even lower than that
calculated for the complexation energy of LAl with alkyne
(ca. –21 kcalmol–1) using the same method.[10] When
[LЈAl(η2-N2Ph2)] is further converted into [LЈAl{N(H)-o-
C6H4N(Ph)}], a best estimate of the energy difference be-
tween these two isomers is –76 kcalmol–1. This suggests an
energetically favored stable rearrangement, and is also in
agreement with the rearrangement of a three- to a five-
membered ring. Furthermore, the D298(Al–η2-N2) value is
comparable to that of the Al–N bond strength in donor–
acceptor H3Al·NH3 species predicted by ab initio studies
with a zero-point vibrational energy correction [D298(Al–N)
= 26 kcalmol–1].[21] This implies a reasonably strong Al–η2-
N2 bonding. Therefore, the cleavage of the corresponding
N–N bond in the rearrangement of A to 3 is highly favored,
although no such bond dissociation energy data are avail-
able for comparison.[22]
Figure 1. Molecular structure of compound 3. The hydrogen atoms
of the C–H bonds have been omitted for clarity. Selected bond
lengths [Å] and angles [°]: Al(1)–N(1) 1.862(1), Al(1)–N(2)
1.893(1), Al(1)–N(3) 1.807(2), Al(1)–N(4) 1.847(1), N(3)–C(31)
1.386(2), C(31)–C(36) 1.429(2), C(36)–N(4) 1.420(2); N(1)–Al(1)–
N(2) 97.68(6), N(3)–Al(1)–N(4) 90.39(6), Al(1)–N(3)–C(31)
109.86(10), N(3)–C(31)–C(36) 115.57(14), C(31)–C(36)–N(4)
113.40(10), C(36)–N(4)–Al(4) 108.44(10).
Conclusions
In summary, we have investigated the reaction of alumi-
num(i) monomer LAl (1) with azobenzene. The formation
of a five-membered AlN2C2 ring containing product 3 is
different to the [1 + 2] cycloaddition product formed in the
reaction of LAl with alkyne, and indicates an interesting
rearrangement of azobenzene via a possible three-mem-
bered AlN2 intermediate (A) upon interaction with LAl.
The reaction of 1 with azobenzene may initially proceed
through an [LAl(η2-N2Ph2)] (A) intermediate, which is
formed by a [1 + 2] cycloaddition reaction. A is not stable
due to the highly strained, metal-containing, three-mem-
bered AlN2 ring, and therefore rearranges by cleaving the
N–N bond, with migration of a hydrogen atom from the
ortho position of one adjacent phenyl ring to yield 3
(Scheme 1). Compounds with similar structures to A are
known for transition and lanthanide metals, for which dif-
ferent electronic interaction modes (π-bonds and one-elec-
tron transfer) have been discussed.[11–13] A three-membered
AlN2 heterocycle bearing an exocyclic N=C double bond at
one of the two N atoms has also been reported.[14] Corre-
spondingly, an easy cleavage of the N–N bond and the re-
arrangement of the adjacent ortho-phenyl hydrogen atom of
azobenzene have also been observed in the reaction of an
FeH-containing active site with azobenzene,[15,16] and in a
cyclometalation[17,18] and a substitution[19] reaction.
Experimental Section
General: All manipulations were carried out under purified nitro-
gen using Schlenk techniques. The solvents were dried by standard
methods. Chemicals were purchased from Aldrich or Fluka and
were used as received. LAl (1)[6] and [LAl{η2-C2(SiMe3)2}] (2)[7]
were prepared as described in the literature. Elemental analyses
were performed by the Analytisches Laboratorium des Instituts für
Anorganische Chemie der Universität Göttingen. 1H (300.13 MHz)
and 13C (125.76 MHz) NMR spectra were recorded with a Bruker
AM 300 spectrometer and IR spectra with a Bio-Rad Digilab FTS-
7 spectrometer. EI mass spectra were recorded with a Finnigan
2148
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2005, 2147–2150

Rearrangement of Azobenzene upon Interaction with an Aluminum(i) Monomer LAl
FULL PAPER
MAT 8230 or a Varian MAT CH5 instrument. Melting points were
measured in sealed glass tubes and are not corrected.
washed with n-hexane (10 mL) to afford an orange crystalline solid,
which was characterized as 3 by m.p. and EI mass measurements.
X-ray Structure Determination and Refinement: The crystallo-
graphic data for compound 3 were collected with a Stoe IPDS II
array detector system with graphite-monochromated Mo-Kα radia-
tion (λ = 0.71073 Å). The structure was solved by direct methods
(SHELXS-96)[23] and refined against F2 using SHELXL-97.[24] All
non-hydrogen atoms were located by difference Fourier synthesis
and refined anisotropically; hydrogen atoms were included using
the riding model with Uiso tied to the Uiso of the parent atoms. A
summary of cell parameters, data collection, and structure solution
and refinement is given in Table 1. CCDC-253931 (3) contains 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.
Synthesis of [LAl{N(H)-o-C6H4N(Ph)}] (3). Method A: A toluene
solution (30 mL) of LAl (1, 0.22 g, 0.5 mmol) and PhNNPh
(0.09 g, 0.5 mmol) was stirred and heated slowly to 80 °C for 5 h.
The color of the solution changed from red to orange. The volume
of the solution was reduced in vacuo (ca. 10 mL) and n-hexane was
added (10 mL). Upon keeping this solution at 4 °C for one week,
orange X-ray quality crystals of 3 were obtained (0.15 g) and col-
lected. The mother liquor was concentrated again (ca. 4 mL) and
n-hexane added (8 mL). Another crop of orange crystals (0.10 g)
was obtained by keeping the solution at –26 °C for 24 h. Total
yield: 0.25 g (81%). M.p. 260–261 °C. 1H NMR (300.13 MHz,
3
C6D6, 298 K): δ = 0.90 [d, JH,H = 6.8 Hz, 2×3 H, CH(CH3)2],
3
3
0.92 [d, JH,H = 6.8 Hz, 2×3 H, CH(CH3)2], 1.07 [d, JH,H
=
3
6.8 Hz, 2×3 H, CH(CH3)2], 1.23 [d, JH,H = 6.8 Hz, 2×3 H,
CH(CH3)2], 1.47 (s, 2×3 H, β-CH3), 3.05 [sept, JH,H = 6.8 Hz,
2×1 H, CH(CH3)2], 3.06 (s, 1 H, NH), 3.14 [sept, JH,H = 6.8 Hz,
3
Acknowledgments
3
2×1 H, CH(CH3)2], 5.12 (s, 1 H, γ-CH), 6.31–6.34 (m, 1 H), 6.58–
6.66 (m, 2 H), 6.82–6.92 (m, 3 H), 6.94–7.02 (m, 4 H), 7.22–7.30
(m, 4 H), 7.48–7.52 (m, 15 H, Ar-H and Ph-H) ppm. 13C{1H}
NMR (125.77 MHz, C6D6, 298 K): δ = 23.4, 24.2, 24.5, 24.9, 25.1,
28.3, 29.2 [CH(CH3)2, β-CH3], 99.2 (γ-C), 112.3, 114.0, 115.1,
118.8, 124.0, 125.1, 127.9, 128.2, 129.0, 139.5, 143.1, 143.7, 145.3,
This work was supported by the Deutsche Forschungsgemein-
schaft, the Fonds der Chemischen Industrie, and the Göttinger Ak-
ademie der Wissenschaften.
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148.8 (Ar-C, Ph-C), 171.5 (CN) ppm. IR (Nujol mull): ν = 3220
˜
(NH) cm–1. EI-MS: m/z (%) = 626 (100) [M+]. C41H51AlN4
(626.87): calcd. C 78.56, H 8.20, N 8.14; found C 78.23, H 8.18, N
8.24. Method B: A toluene solution (5 mL) of PhNNPh (0.18 g,
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lowed to warm to room temperature whilst being stirred. The solu-
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Table 1. Crystallographic data for compound 3.
3
[8] LAl reacts with Me3SiCϵCCϵCSiMe3 to afford [LAl{η2-
C2(SiMe3)(CϵCSiMe3)}], which contains a three-membered
ring: H. Zhu, J. Chai, H. W. Roesky, H.-G. Schmidt, M. Nolte-
meyer, Organometallics, to be submitted.
Empirical formula
Formula mass
T [K]
Crystal system
Space group
a [Å]
b [Å]
c [Å]
α [°]
β [°]
C41H51AlN4
626.84
133(2)
triclinic
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Chem. 2000, 112, 4705–4707; Angew. Chem. Int. Ed. 2000, 39,
4531–4533.
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Herbst-Irmer, J. Am. Chem. Soc. 2004, 126, 10 194–10 195.
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[14] W. Uhl, F. Hannemann, Eur. J. Inorg. Chem. 1999, 201–207.
[15] G. Guillemot, E. Solari, R. Scopelliti, C. Floriani, Organome-
tallics 2001, 20, 2446–2448.
[16] J. M. Smith, R. J. Lachicotte, P. L. Holland, J. Am. Chem. Soc.
2003, 125, 15 752–15 753.
[17] A. J. Craty, Organomet. Chem. Rev., Sect A 1972, 191–243, and
references cited therein.
[18] a) A. Omenat, M. Ghedini, J. Chem. Soc., Chem. Commun.
1994, 1309–1310; b) S. A. Hudson, P. M. Maitlis, Chem. Rev.
1993, 93, 861–885; c) M. Ghedini, D.Pucci, A. Crispini, G. Bar-
berio, Organometallics 1999, 18, 2116–2124.
¯
P1
11.704(3)
12.612(2)
12.980(4)
86.460(2)
77.11(2)
γ [°]
86.40(2)
1861.7(7)
2
1.118
0.087
V [Å3]
Z
ρcalcd. [Mgm–3]
μ [mm–1]
F(000)
676
θ range [°]
Index ranges
1.61–24.88
–13 Յ h Յ 13
–14 Յ k Յ 14
–15 Յ l Յ 15
27600
6396 (0.0536)
6396/0/424
1.015
No. of reflections collected
No. of independent reflections (Rint
No. of data/restraints/parameters
GoF/F2
)
R1,[a] wR2[b] [I Ͼ 2σ(I)]
0.0391, 0.0915
0.0539, 0.0979
0.210/–0.235
[b]
R1,[a] wR2 (all data)
Largest diff peak/hole [eÅ–3]
[19] A. Risaliti, S. Bozzini, A. Stener, Tetrahedron 1969, 25, 143–
148.
[a] R = Σ||Fo| – |Fc||/Σ|Fo|. [b] wR2 = [Σw(Fo – Fc ) /Σw(Fo2)]1/2
.
2
2 2
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H. W. Roesky et al.
FULL PAPER
[20] The DFT calculations were performed by analyzing the energy
difference of products and reactants in the reaction of LЈAl
and PhNNPh to form [LЈAl(η2-N2Ph2)] and then [LЈAl{N(H)-
o-C6H4N(Ph)}], where LЈ is HC[(CMe)(NPh)]2 for simplicity
of calculation. The corresponding geometries were optimized
according to the real structures or related ones at the BP86/
TZVP level with RI approximation {LЈAl to LAl,[6] PhNNPh
in cis position, [LЈAl(η2-N2Ph2)] to [LAl(η2-C2Ph2)][7] but with
an N–N single bond, and [LЈAl{N(H)-o-C6H4N(Ph)}] to
[LAl{N(H)-o-C6H4N(Ph)}]} using the TURBOMOLE 5.5 pro-
gram: R. Ahlrichs, M. Bär, H.-P. Baron, R. Bauernschmitt, S.
Böcker, P. Deglmann, M. Ehrig, K. Eichkorn, S. Elliott, F. Fur-
che, F. Haase, M. Häser, H. Horn, C. Hättig, C. Huber, U.
Huniar, M. Katannek, A. Köhn, C. Kölmel, M. Kollwitz, K.
May, C. Ochsenfeld, H. Öhm, A. Schäfer, U. Schneider, M. Sie,
O. Treutler, B. Unterreiner, M. V. Arnim, F. Weigand, P. Weis,
H. Weiss, TURBOMOLE 5.5, University of Karlsruhe, Ger-
many, 2002.
[21] C. M. B. Marsh, T. P. Hamilton, Y. Xie, H. F. Schaefer, J.
Chem. Phys. 1992, 96, 5310–5317.
[22] The N–N bond strength for H2N–NHPh (D298
=
57 2 kcalmol–1) has been reported previously: D. F.
McMillen, D. M. Golden, Annu. Rev. Phys. Chem. 1982, 33,
493–496.
[23] SHELXS-90, Program for Structure Solution: G. M. Sheldrick,
Acta Crystallogr. Sect. A 1990, 46, 467–473.
[24] G. M. Sheldrick, SHELXL-97, Program for Crystal Structure
Refinement; University of Göttingen, Göttingen, Germany,
1997.
Received: November 02, 2004
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Eur. J. Inorg. Chem. 2005, 2147–2150