2H-Azirines as ligands: Synthesis and structural characterization of (2H-3-arene-azirine)ReI(CO)3Br complexes

Article abstract of DOI:10.1016/j.inoche.2008.03.035

The synthesis and structural characterization of four 2H-azirine rhenium(I) complexes are described. Stabilization of the highly reactive and strained 3-arene-2H-azirine ligand through N-coordination to the transition metal is observed. The reaction of [Re(CO)₃(THF)₂Br] with 3-arene-2H-azirine ligands (az) in THF at -20 °C in a 1:2 molar ratio afforded the neutral bis-azirine complexes [Re(CO)₃(az)₂Br] (az = NCH₂CC₆H₄R; R = -H, -Me, -Cl, -Br). After purification, all complexes have been fully characterized. The molecular structures of the novel rhenium(I) complexes exhibit slightly distorted octahedral coordination geometries around the metal centers with cis-bonded az ligands.

Full text of DOI:10.1016/j.inoche.2008.03.035

Inorganic Chemistry Communications 11 (2008) 829–831
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
2H-Azirines as ligands: Synthesis and structural characterization
of (2H-3-arene-azirine)ReI(CO)3Br complexes
*
J. Nicolas Roedel, Xaver Wurzenberger, Ingo-Peter Lorenz
Department of Chemistry and Biochemistry, Ludwig-Maximilians-University, Butenandtstraße 5ꢀ13 (Building D), 81377 Munich, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and structural characterization of four 2H-azirine rhenium(I) complexes are described. Sta-
bilization of the highly reactive and strained 3-arene-2H-azirine ligand through N-coordination to the
transition metal is observed. The reaction of [Re(CO)3(THF)2Br] with 3-arene-2H-azirine ligands (az) in
THF at ꢀ20 °C in a 1:2 molar ratio afforded the neutral bis-azirine complexes [Re(CO)3(az)2Br] (az =
NCH2CC6H4R; R = –H, –Me, –Cl, –Br). After purification, all complexes have been fully characterized.
The molecular structures of the novel rhenium(I) complexes exhibit slightly distorted octahedral coordi-
nation geometries around the metal centers with cis-bonded az ligands.
Received 27 February 2008
Accepted 27 March 2008
Available online 4 April 2008
Dedicated to Prof. Dr. Dr. h.c. mult. Heinrich
Nöth on the occasion of his 80th birthday.
Ó 2008 Elsevier B.V. All rights reserved.
Keywords:
Rhenium
CO complexes
Azirine
Pi stacking
X-ray crystallography
2H-Azirines have attracted considerable interest from the syn-
thetic, mechanistic and theoretical point of view, especially be-
cause of their versatile reactivity [1–3]. The reason for the
increased reactivity of 2H-azirines is mainly due to high ring strain
energy. Following von Baeyer [4] the forced deformation of bond
angles in this unsaturated three membered ring system is the main
reason for the ring strain. 2H-Azirines as reactive and versatile
starting materials in organic synthesis can react as nucleophiles,
electrophiles and dienophiles or dipolarophiles in cyclo addition
reactions [5–7]. Interestingly, all three bonds of the azirine ring
can be cleaved depending on the experimental conditions used
[1,2]. Moreover, this highly strained imines are suitable precursors
in the synthesis of an impressive number of heterocycles [5–10]. In
this context transition metal catalyzed ring expansion or rear-
rangement reactions of azirines have been reported recently, for
example the rearrangement of 2H- and 3H-azirines to indoles cat-
alyzed by [Rh(CO)2Cl]2 and [Rh2(OCOCF3)4] [6,11–18].
barely investigated class of highly strained ligands. In this commu-
nication we report the synthesis and structural characterization of
four 2H-azirine rhenium(I) complexes and the stabilization of the
highly reactive 2H-azirine ligands in the coordination sphere of
the ReI(CO)3Br fragment.
2H-Azirines 3a–d (az) and [Re(CO)5Br] (1) were prepared
according to literature methods and stored in dry argon atmo-
sphere [26,27]. Additionally the az ligands 3a–d were cooled to
ꢀ32 °C due to the thermic instability at r.t. (caution: 2H-3-phe-
nyl-azirine and 2H-3-(4-methyl-phenyl)-azirine are highly irri-
tant). The novel azirine transition metal complexes 4a–d have
been prepared as follows and illustrated in Scheme 1. Complex 2
was synthesized by solving one equivalent of 1 in dry THF and
refluxing the colourless solution for 20 h. The conversion of com-
plexes 1–2 is monitored by liquid phase IR spectroscopy in THF.
Complex 2 can be assigned to the point group Cs and shows the
absorption of three CO valence vibrations deduced to the symme-
try race 2A0 + A00 at 2030, 1915 and 1894 cmꢀ1. The vigorously stir-
red solution of 2 was cooled at ꢀ20 °C and 3a–d was added rapidly.
The mixture was stirred at ꢀ20 °C for 45 min and then warmed up
to room temperature and stirred for 30 min. After removal of half
of the solvent in vacuo and subsequent addition of 30–40 mL of
dry n-heptane the complexes 4a–d precipitate as off-white (4a,b)
to yellow (4c,d) solids. After filtration the solids were washed
two times with 8 mL of dry n-heptane and dried in vacuo. All iso-
lated complexes 4a–d are soluble in polar solvents like chloroform
and dichlormethane and stable in the solid state. All complexes
Nevertheless little is known about the coordination chemistry
of azirines. To the best of our knowledge only two X-ray structure
analyses of ZnBr2 and PdCl2 azirine complexes have been reported
[1,19–21]. Inspired by our recent studies on the coordination
chemistry of saturated three membered aziridines [22–25], we
chose to study the coordination chemistry of 2H-azirines as a
* Corresponding author. Tel.: +49 89 2180 77486; fax: +49 89 2180 77867.
E-mail address: ipl@cup.uni-muenchen.de (I.-P. Lorenz).
1387-7003/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2008.03.035

830
J. Nicolas Roedel et al. / Inorganic Chemistry Communications 11 (2008) 829–831
Scheme 1. Reaction of [Re(CO)5Br] (1) with THF and 2H-3-arene-azirines (az) (3a–d) yielding [fac-Re(CO)3(az)2Br] (4a–d).
4a–d as well as the corresponding ligands 3a–d are sensitive to
protic solvents.
ranges. Re–CCO and Re–Br bond lengths are comparable to those
in similar ReI(CO)3L2Br complexes [29]. The average value for the
Re(1)–Br(1) bonds is 2.599(1) Å and those for the Re(1)–CCO bonds
lie within the range of 1.92(1) Å and 2.01(1) Å. In all cases the Re–
CCO bond lengths trans to the Re–Naz positions are significantly
shorter (average 1.92(1) Å) than those trans to Re–Br positions
(average 2.01(1) Å). Obviously the azirine ligands act as r-donor–
p-donor ligands and favour the double bond of the p-acceptor CO
in trans position. The Re–Nsp2 bond lengths are shortened with
2.150 Å for Re(1)ꢀN(1) and 2.145 Å for Re(1)ꢀN(2) (4a) compared
to analogous aziridine complexes with Re–Nsp3 2.199 Å and
2.202 Å [29]. Further bond lengths and angles of 4a–d are listed
in Table 1. The C@N, CꢀN, CꢀC bond lengths in the az ligands
are comparable to bond lengths observed in uncoordinated 2H-azi-
rines using single crystal X-ray diffraction methods [30]. The devi-
ation between the mean planes of the azirine rings (defined by
N(1), C(1), C(2) and N(2), C(9), C(10)) towards the equatorial coor-
dination plane of Re(1) (defined by N(1), N(2), C(17) and C(18)) lies
between 9.61° (4a) and 45.99° (4d). The azirine ring plane and the
phenyl ring plane of the az ligands in the complexes 4a–d are
slightly bent out of a coplanar arrangement and exhibit angles be-
tween both ring planes of 4.27° and 10.10° (4a), 9.73° and 15.90°
(4b), 0.94° and 8.31° (4c), 11.27° and 17.28° (4d) due to steric
requirements of the az ligands. In addition in compound 4d an
intermolecular plane to plane p-stacking between the phenyl moi-
eties of two az ligands is observed. The distance between the cen-
The connectivity within complexes 4a–d was deduced by single
crystal X-ray crystallography, NMR and IR spectroscopy and MS
spectrometry. Single crystals of 4a–d suitable for structural deter-
mination by X-ray diffraction were obtained by isothermic diffu-
sion of n-pentane into dichlormethane solutions of 4a–d. The 1H
NMR spectra of 4a–d show the expected azirine signals shifted to
lower field due to deshielding caused by transition metal coordina-
tion. A good indication for the N-coordination of the az ligands 3a–
d to Re(I) is the chemical shift of the CH2 ring signals. The 13C sig-
nals of 4a–d are slightly shifted to lower field in comparison to the
signals observed for 3a–d. For example, the CH2 ring carbon signals
are observed at 22.1 ppm (4a) and 21.9 ppm (4b), respectively,
compared to 19.5 ppm (3a) and 18.8 ppm (3b). Likewise in the
1H NMR spectra the tendency of signal shift to lower field, com-
pared to the corresponding signals of the free azirine, is observed
[28].
Mass spectrometric investigations in the FAB+ mode of the com-
plexes 4a–d exhibit similar fragmentation patterns. For example
compound 4a shows a fragmentation pattern with m/z peaks at
584, 556 and 528, 505, 500, 477 and 421 corresponding to [M]+,
[MꢀBr]+, [MꢀnCO]+ (n = 1, 2, 3) and [MꢀnCO–Br]+ (n = 1, 3),
respectively. The fragmentation pattern for the complexes 4a–d
is characterized by the successive loss of CO and Br or both kinds
of ligands.
The IR spectra of the complexes 4a–d show sharp absorption
bands for the azirine C@N stretching vibrations in the range of
1760 cmꢀ1 (4b)–1777 cmꢀ1 (4d). The tendency of a shift to higher
wavenumbers as a result of transition metal coordination with a
shift of 22 cmꢀ1 (4a) compared to m(C@N) = 1740 cmꢀ1 of az (3a)
is detected [20]. Like complex 2 the complexes 4a–d can be as-
signed to the point group Cs and show three CO valence vibrations
of the symmetry race 2A0 + A00 at for example 2025 cmꢀ1, 1928 and
1898 cmꢀ1 (4b) in CH2Cl2. As expected, the m(CꢀH) absorptions of
troids of both aromatic moieties is 3.740 Å and a parallel
arrangement between the planes of the two phenyl rings is formed.
Concluding we have reported the synthesis of four 2H-3-
(arene)-azirine complexes of ReI. Of note, these complexes are
the first fully characterized transition metal carbonyl 2H-azirine
Table 1
Selected bond lengths (Å) and angles (°) of the complexes 4a–d
4a–d were observed in the range of 3000 cmꢀ1
.
4a
4b
4c
4d
The X-ray structure analysis of 4a–d revealed the formation of
distorted octahedral coordination geometries. The Re(I) centers
are coordinated besides the three CO ligands in facial positions
by the nitrogen atoms of two az and one bromido ligand in com-
plexes 4a–d presenting a distorted octahedral geometry (Fig. 1).
The Reꢀligand bond lengths in 4a–d lie within the expected
Bond lengths (Å)
Re(1)–N(1)
Re(1)–N(2)
Re(1)–Br(1)
Re–C(17)
Re–C(18)
Re–C(19)
N(1)ꢀC(1)
N(1)ꢀC(2)
C(1)ꢀC(2)
2.150(10)
2.145(10)
2.615(2)
1.928(14)
1.906(14)
1.957(14)
1.535(15)
1.260(16)
1.456(17)
2.137(5)
2.151(5)
2.606(1)
1.921(7)
1.908(7)
1.983(8)
1.521(8)
1.266(8)
1.458(9)
2.135(6)
2.148(5)
2.585(1)
1.915(7)
1.933(8)
2.047(10)
1.525(10)
1.262(9)
1.445(10)
2.142(9)
2.143(9)
2.592(2)
1.932(13)
1.939(12)
2.070(13)
1.518(12)
1.290(13)
1.467(15)
Bond angles (°)
N(1)ꢀRe(1)ꢀN(2)
N(1)ꢀRe(1)ꢀBr(1)
C(17)ꢀRe(1)ꢀN(1)
C(19)ꢀRe(1)ꢀBr(1)
C(2)ꢀC(1)ꢀN(1)
C(2)ꢀN(1)ꢀC(1)
N(1)ꢀC(2)ꢀC(1)
84.2(4)
84.1(3)
95.6(5)
176.7(4)
49.7(7)
61.9(8)
68.4(9)
9.61
85.0(2)
84.03(13)
94.5(3)
175.0(2)
50.3(4)
62.3(4)
67.4(5)
36.84
83.5(2)
81.82(16)
93.7(3)
175.9(2)
50.2(4)
61.6(5)
68.2(5)
22.55
82.1(3)
86.7(2)
94.7(4)
175.0(3)
51.2(6)
62.4(7)
66.4(8)
34.02
a
D1
b
D2
32.34
40.27
32.12
45.99
a Angle between the equatorial coordination plane of Re(1) (defined by N(1), N(2),
C(17), C(18)) and the azirine ring plane defined by N(1), C(1), C(2) or that b defined
by N(2), C(9), C(10).
Fig. 1. Molecular structure of 4b with thermal ellipsoids drawn at the 30% proba-
bility level. Hydrogen atoms are omitted for clarity.

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Acknowledgment
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Appendix A. Supplementary data
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