Direct observation of a photoinduced nonstabilized nitrile imine structure in the solid state

Article abstract of DOI:10.1021/ja9094523

(Chemical Equation Presented) We report the direct observation of a bent geometry for a nonstabilized nitrile imine in a metal-coordination crystal. The photoinduced tetrazole ring rupture to release N₂ appears to depend on the size of voids around the N³-N⁴ bond in the crystal lattice. We further observed the selective formation of the 1,3-addition product when a reactive nitrile imine was photogenerated in water. Overall, the bent nitrile imine geometry agrees with the 1,3-dipolar structure, a transient reactive species that mediates the photoinduced 1,3-dipolar cycloaddition in the aqueous medium.

Full text of DOI:10.1021/ja9094523

Published on Web 11/24/2009
Direct Observation of a Photoinduced Nonstabilized Nitrile Imine Structure in
the Solid State
Shao-Liang Zheng, Yizhong Wang, Zhipeng Yu, Qing Lin,* and Philip Coppens*
Department of Chemistry, State UniVersity of New York at Buffalo, Buffalo, New York 14260
Received September 22, 2009; E-mail: qinglin@buffalo.edu; coppens@buffalo.edu
The photoinduced ring opening of 2,5-diphenyltetrazole with the
generation of N2 and a nitrile imine was first reported by Huisgen
and co-workers in 1967.1 As a highly reactive dipole, nitrile imine
reacts readily with a variety of dipolarophiles2 to form five-
membered-ring heterocycles.3 Recently, we employed photogener-
ated nitrile imines for functionalization of an alkene-containing
Figure 1. Structures of the tetrazole compounds used in this study.
protein in living cells.4 Whereas crystal structures of stabilized
nitrile imines have been reported,5 nonstabilized N-arylnitrile imines
have only been spectroscopically observed as transient intermediates
in low-temperature matrices.6
Four alternative structures have been postulated for nonstabilized
nitrile imines: propargylic, allenic, 1,3-dipolar, and carbenic (Scheme
1). To date, theoretical calculations of nitrile imine structures have
Scheme 1
generated conflicting results in the literature. For example, in 1993, a
high-level computational study using configuration interaction (QCISD)
and a large basis set concluded that the stable nitrile imine structure
has a nonplanar, allenic geometry and that the propargylic structure
does not correspond to a local minimum on the potential energy
surface.7 More recent density functional theory calculations in com-
bination with natural resonance theory indicated that all four resonance
structures are necessary for a full description and that the carbenic
form dominates for F-CNN-F and H2N-CNN-NH2.8 In contrast,
a spin-coupled valence bond calculation using the geometry from a
CASSCF calculation suggested that the stable electronic structure of
H-CNN-H is predominantly propargylic.9 To provide direct evi-
dence, herein we report the use of photocrystallography10 to observe
for the first time the structure of a nonstabilized nitrile imine
photochemically generated in situ in the solid state.
Figure 2. Crystal structures of Zn-tetrazole complexes Zn-1 to Zn-6: (a)
Zn(1)2(H2O)2; (b) Zn(2)2; (c) Zn(3)(H2O); (d) Zn(4)(NH3)2; (e)
Zn(5)2(H2O)4(CH3OH)2; (f) Zn(6)2(H2O)4(NH4)2. Zn is shown in silver. The
distances between N3-N4 and the nearest surrounding atoms are marked
on the structures.
to Zn-6 were obtained by allowing ∼17 mM tetrazole solutions
[dissolved in 2:1 MeOH/H2O and mixed with Zn(NO3)2 and NH4OH]
to stand in air at room temperature for 2 weeks (Figure 2; also see
Table S1 for crystal data and structural refinement). Structural analyses
indicated the Zn to be tetracoordinated with two carboxylates and two
water molecules in all structures except that of Zn-tetrazole 4, in which
two NH3 molecules serve as the ligands (Figure 2d). A close
examination of the tetrazole packing revealed that the empty spaces
(voids) varied significantly, with the distances between N3-N4 and
the nearest surrounding atoms being 2.66, 2.90, 3.21, 2.43, 2.70, and
2.35 Å, respectively (Figure 2).
To test the photoreactivity, all six crystals were exposed to the 325
nm He-Cd laser beam. Whereas the crystals of Zn-2-5 showed slow
decay indicated by color darkening (Figure 3a), only the Zn-3 crystal
afforded a discrete photodifference map after 2 min of photoirradiation
at 90 K (Figure 3b). This is consistent with the fact that crystal Zn-3
has the largest void around N3-N4 (3.21 Å in Figure 2c). Subsequent
In our initial study, a crystal of 2-(4′-methoxyphenyl)-5-(2′′-
isopropoxy-4′′-methoxyphenyl)tetrazole2b was photoirradiated with a
325 nm He-Cd laser (45 mW/cm2) at 280 K for 12 h. While the
crystal showed a darkening of its color, no products could be detected
in the X-ray photodifference map, defined as the difference in electron
densities after and before laser exposure. A closer examination revealed
that the tetrazole molecules are tightly packed in the crystal lattice
with the distance between the dissociating N3-N4 atoms and an
adjacent methyl group on the neighboring tetrazole equal to 2.65 Å,
resulting in a very small void for N2 to escape (Figure S1 in the
Supporting Information). To overcome this problem, we envisioned
that the void next to N3-N4 could be enlarged by the use of rigid
supramolecular frameworks formed by complexation of carboxylates
with Zn.11 To test this, we prepared a small panel of N-aryl tetrazoles
1-6 carrying carboxyl groups and potential H-bond donors such as
NH2 and OH (Figure 1). Crystals of the Zn-tetrazole complexes Zn-1
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18036 J. AM. CHEM. SOC. 2009, 131, 18036–18037
10.1021/ja9094523  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 2
followed its decay in the NMR tube in 1:1 CD3CN/D2O. We found the
major product to be benzaldehyde with no traces of hydrazide (Figure
S5). Hence, we propose that the 1,3-dipolar structure represents the major
electronic structure of the photogenerated nitrile imine. The bent geometry
in the 1,3-dipolar structure can explain the high reactivity of the
photogenerated nitrile imines in cycloaddition reactions in aqueous media4
because of the lower activation barriers that result from dipole structural
preorganization.16
In summary, we have reported the direct observation of a
photogenerated, bent nitrile imine structure in a Zn coordination
crystal. The efficiency of tetrazole ring rupture in the solid state
appears to depend on the size of the void around the N3-N4 bond.
A water-quenching study suggested that the bent geometry repre-
sents the 1,3-dipolar form, a major electronic structure involved in
the photoinduced 1,3-dipolar cycloaddition in aqueous media.
Figure 3. Photocrystallography of complex Zn-3. (a) Color change of the
crystal upon laser exposure. (b) Photodifference map based on Fo(after) -
Fo(before): blue, 2.0; light-blue, 1.0; orange, -1.0; red, -2.0 e/Å3. Only
half of the map is shown because of the twofold symmetry. (c) ORTEP
representation of the geometry-refined nitrile imine structure. (d) Packing
of the nitrile imines and molecular N2 in the crystal lattice. The Nt N bonds
are perpendicular to the plane of view.
Acknowledgment. We acknowledge the National Science
Foundation (CHE0236317 and CHE0843922 to P.C.) and the NIH
(GM 85092 to Q.L.) for financial support.
least-squares refinement gave a 13% yield of the corresponding nitrile
imine product. The dissociated N2 was visible in the photodifference
map with a bond length of 0.89(9) Å (Figure 3c), which is within
experimental error of its value in molecular N2 (1.09 Å). The occupancy
of N2 in the crystal lattice was 8%, less than the value of 13% for the
nitrile imine, suggesting that some of the N2 escaped from the crystal
lattice. Since apart from the CNN center the structure of tetrazole 3 is
symmetric, the photodifference map showed twofold symmetry (Figure
3b). Using a free geometry refinement model,12 we fit the electron
density to two symmetry-related nitrile imine geometries (only one is
shown in Figure 3c).13 Evidently, in the solid state, the nitrile imine
adopted a bent geometry with an increased twisting of the flanking
phenyl rings (dihedral angle ) 62.1° for the nitrile imine vs 38.8° for
3; compare Figure 3c to Figure 2c), to allow the trapping of the escaping
N2 in the intrastrand space (Figure 3d). The photoreactivity of 3 is not
due to the electronic effect of the carboxylic groups, as photoirradiation
of the Zn-free crystal of 3, which has smaller voids around N3-N4 in the
crystal (Figure S2), did not yield a recognizable photodifference map.
The bent geometry of the nitrile imine can be ascribed to either the
1,3-dipolar or carbenic structure (Scheme 1). These two could be
distinguished by a water-quenching experiment: it was expected that
the dipolar structure would undergo 1,3-addition to generate a
hydrazonic acid intermediate, which would tautomerize to afford the
stable hydrazide, while the carbenic structure would undergo 3,3-
addition14 to generate a metastable R-hydroxyazobenzene, which would
decompose slowly to produce benzaldehyde and phenyldiazene15 (Scheme
2). When tetrazole 3 was irradiated with a handheld UV lamp (UVP, 302
nm, 0.16 A) in 1:1 acetonitrile/water, the 1,3-addition product was found
to be the major product in the product mixture on the basis of 1H NMR
analysis (Figure S3). Moreover, water quenching of the nitrile imine
derived from reactive 2-phenyl-5-p-methoxyphenyl tetrazole4b yielded
exclusively the 1,3-addition product with no traces of benzaldehyde (Figure
S4), thereby excluding the existence of the carbenic structure. To ensure
that there is no crossover between the 1,3- and 3,3-addition pathways, we
prepared R-hydroxyazobenzene separately from R-azohydroperoxide and
Supporting Information Available: Experimental procedures,
characterization data, structural information, and CIF files. This material
is available free of charge via the Internet at http://pubs.acs.org.
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