2-(4'-nitrenophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3- oxide-1-oxyl: Photogeneration of a quartet state organic molecule with both localized and delocalized spins [6]

Full text of DOI:10.1021/ja980120v

5122
J. Am. Chem. Soc. 1998, 120, 5122-5123
2-(4′-Nitrenophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-
1H-imidazole-3-oxide-1-oxyl: Photogeneration of a
Quartet State Organic Molecule with Both Localized
and Delocalized Spins
Paul M. Lahti,* Burak Esat, and Richard Walton
Department of Chemistry, UniVersity of Massachusetts
Amherst, Massachusetts 01003
ReceiVed January 12, 1998
In hopes of gaining insight for the design of new molecular
magnetic materials,1 many open-shell molecules have been
constructed by linking hypovalent spin carrier (SC) units with
connectivities determined by parity methods.2 Most workers have
used the same types of SC units, rather than linking different
types of SCs (heterospin approach). The latter approach allows
the study of heteroatom substitution effects on spin multiplicity
as well as expanding the number of available high-spin molecules.
We have studied numerous dinitrenes in order to investigate
effects of connectivity, conformation, and heteroatom substitution
on their exchange behavior.3,4 In the present study, we report
the extension of this methodology to produce the novel heterospin
system 2-(4′-nitrenophenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-
imidazole-3-oxide-1-oxyl(1).
Figure 1. 9.5 GHz ESR spectra for 2 at 77 K before (a) and after (b)
photolysis, compared to the S ) 3/2 simulated spectrum with |D/hc| )
0.277 cm-1 and |E/hc| ) 0.0002 cm-1 (c).
followed by oxidation with aqueous NaIO4/chloroform.5 Radical
2 is an indigo-black solid that is stable for weeks under nitrogen
at <0 °C. In degassed benzene solution, 2 displays a typical
R-nitronylnitroxide (NN) electron spin resonance (ESR) spectrum
with aN ) 7.4 G. When 2 is dissolved in 2-methyltetrahydrofuran
(MTHF), degassed, frozen to 77 K, and photolyzed for 30 s at
>300 nm (Pyrex filter, 1000 W xenon arc), the originally blue
matrix changes color to purple.
Figure 1 shows the prephotolysis and postphotolysis 9.5 GHz
ESR spectra of the frozen matrix sample.5 The initial spectrum
shows only the radical peaks at g ∼ 2 expected from the NN SC
unit, while the postphotolysis spectrum also shows a series of
new peaks (1230 (wk), 1830, 5225, and 6056 G) that are stable
at 77 K, but which are immediately quenched by thawing of the
matrix above 90 K. More extended photolysis of dilute samples
of 2 lead to >80% disappearance of the g ∼ 2 radical peaks and
concurrent appearance of the new spectral peaks. The new
spectrum can be fit by the eigenfield method6 to a quartet
randomly oriented spectrum with the following parameters: S )
3/2, giso ) 2.003, |D/hc| ) 0.277 cm-1, |E/hc| e 0.003 cm-1
.
Except for the g ∼ 2 region from remaining unphotolyzed 2, the
simulated positions fit to within 25 G for all observable peaks
with good agreement for line shape and intensity (Figure 1). There
is no additional peak in the 6000-7000 G region attributable to
an isolated triplet phenyl mononitrene unit, ruling out production
of an observable conformational isomer such as 3.7
Precursor 2 was synthesized by treatment of 4-azidobenzalde-
hyde with 2,3-bis(hydroxylamino)-2,3-dimethylbutane sulfate,
Additional information about structure 1 is given by changes
in the UV-vis spectrum upon photolysis5 of a frozen MTHF
sample at 77 K (Figure 2). The initial spectrum is consistent
with expectations for the aryl azide chromophore plus the NN
chromophore, with major absorbances at 280-300, 537, 584, 638,
(1) For summaries and many leading papers with references: (a) Lahti, P.
M. In Molecule-Based Magnetic Materials. Theory, Techniques, and Applica-
tions; Turnbull, M. M., Sugimoto, T., Thompson, L. K., Eds.; ACS Symposium
Series 644, Washington, DC, 1996; pp 218-235. (b) Magnetic Molecular
Materials; Gattechi, D., Kahn, O., Miller, J. S., Palacio, F., Eds.; NATO ASI
Series, Kluwer: Dordrecht, 1991; Vol. 198E. (c) Iwamura, H., Miller, J. S.,
Eds.; Mol. Cryst. Liq. Cryst., Sect. A 1993, 232-233, 1ff. (d) Miller, J. S.,
Epstein, J. S., Eds.; Mol. Cryst. Liq. Cryst., Sect. A 1995, 271-274, 1ff. (e)
Itoh, K., Miller, J. S., Takui, T.; Eds.; Mol. Cryst. Liq. Cryst., Sect. A 1997,
305-306, 1ff.
(5) Experimental details. 4-Azidobenzaldehyde was prepared by the method
of Walton, R.; Lahti, P. M. Synth. Comm. 1998, 28, 0000 (in press). For
compound 2: blue-black solid mp 120-123 °C; UV-vis (chloroform, nm
(ꢀ)) - - 299 (15, 400), 372 (5700), 625 (400); IR (KBr, cm-1) - - 2945 (CH3
str), 2115 (-N3 str), 1255, 1090, 1010, 795 (p-phenylene), no OH band
observed. HR-MS (EI) Calcd for C13H16N5O2 274.1304; Found 274.1304.
High-resolution mass spectrometry was performed at the Nebraska Center
for Mass Spectrometry.
(2) Cf. for example, (a) Dougherty, D. A. Mol. Cryst. Liq. Cryst., Sect. A
1989, 176, 25. (b) Lahti, P. M.; Ichimura, A. S. Mol. Cryst. Liq. Cryst., Sect.
A 1989, 176, 125.
(3) (a) Lahti, P. M.; Minato, M.; Ling, C. Mol. Cryst. Liq. Cryst., Sect. A
1995, 271, 147 summarizes much of the recent aryl dinitrene work by ourselves
and others. However, zfs descriptions in this reference will require revision:
see: Fukuzawa, T. A.; Sato, K.; Ichimura, A. S.; Kinoshita, T.; Takui, T.;
Itoh, K.; Lahti, P. M. Mol. Cryst. Liq. Cryst., Sect. A 1996, 278, 253. (b)
Lahti, P. M.; Kalgutkar, R.; Walton, R. A.; Murcko, M. A. Mol. Cryst. Liq.
Cryst., Sect. A, 1997, 305, 553. (c) Kalgutkar, R. S.; Lahti, P. M. J. Am. Chem.
Soc. 1997, 119, 4771.
(6) (a) Belford, G. G.; Belford, R. L.; Burkhalter, J. F. J. Magn. Reson.
1973, 11, 251. (b) Teki, Y.; Takui, T.; Yagi, H.; Itoh, K.; Iwamura, H. J.
Chem. Phys. 1985, 83, 539. (c) Teki, Y.; Takui, T.; Itoh, K. J. Chem. Phys.
1988, 88, 6134. (d) Teki, Y.; Fujita, I.; Takui, T.; Kinoshita, T.; Itoh, K. J.
Am. Chem. Soc. 1994, 116, 11499. (e) Teki Y., Ph.D. Thesis, Osaka City
University, Osaka, Japan, 1985. (f) Theory and practical details of the line
shape program used by us are described in Sato, K. Ph.D. Thesis, Osaka City
University, Osaka, Japan, 1994. We thank Prof. Sato for allowing us the use
of this program.
(4) Ichimura, A. S.; Sato, K.; Kinoshita, T.; Takui, T.; Itoh, K.; Lahti, P.
M. Mol. Cryst. Liq. Cryst., Sect. A 1995, 272, 57.
(7) Cf. also: Ling, C.; Lahti, P. M. Chem. Lett., 1993, 769.
S0002-7863(98)00120-6 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/12/1998

Communications to the Editor
J. Am. Chem. Soc., Vol. 120, No. 20, 1998 5123
parameter sufficiently small that it was not resolved in frozen
solution spectra, but 5 has a zfs of |D/hc| ) 0.107 cm-1. The
X-ray structure of 4 suggests it to have only minor quinonoidal
character in terms of bond alternation, so this system is shown
as a nitroxide/NN structure.
Photolysis of the azide group in 2 produces a π-electron on a
nitrene site which we expected to be delocalized onto the NN
group (resonance structure 1a). The DFT calculations show 0.7
of an unpaired R-electron on each N-O unit of the NN in 1
(Mulliken population numbers). The exocyclic nitrene nitrogen
has a spin density of 1.5. In bisected structure 3 each N-O unit
has a spin density of 0.6, while the exocyclic nitrogen has a spin
density of 1.6. Structure 3 is a reasonable benchmark for an
enforced nitrene/NN structure. Although the trends of the
numbers are in accord with greater quinonoidal character in 1
than in 3, the magnitude of the change is surprisingly small. The
DFT computed geometry of 4A1 1 has a moderate bond alternation
of about 0.05 Å in the phenylene ring, very similar to that in 3.
For the important exocyclic nitrene site, rC-N ) 1.31 Å for 1
versus rC-N ) 1.33 Å in 3. Similarly, the interannular bond
connecting the two rings is rCC ) 1.44 Å for 1 versus 1.47 Å
for 3. Again, the trends in bond lengths are consistent with those
expected, but the magnitudes of change are quite small.
The electronic natures of systems with different mulitiplicities
are properly compared by use of the quantity (2S-1)|D/hc|, where
S is the spin quantum number of the state considered.12 Arylni-
trenes with conjugated substitutents in the 4-position have8e,13 |D/
hc| ∼ 0.75-0.88 cm-1. For 1 (2S-1)|D/hc| ) 0.55, while for a
typical para-conjugated arylnitrene (2S-1)|D/hc| ) 0.75-0.88,
where S ) 1.5 and 1.0, respectively. By this criterion, the zfs
for quartet 1 is appreciably smaller than that for a conjugated
triplet arylnitrene. The balance of evidence from computation
and from evaluation of ESR zfs data thus seems consistent with
the planar nitrene/NN resonance structure 1, but with significant
contribution from quinonoidal structure.
The closest structural analogue to 1 is quartet carbene-nitroxide
6.14 This species has zfs parameters of |D/hc| ) 0.113 cm-1 and
|E/hc| ) 0.006 cm-1. The role of the extra phenyl group in 6
gives rise to additional electron delocalization in accord with the
observed decrease of zfs relative to 1, in addition to breaking the
axial symmetry of 6 (E * 0).
Mataga has argued that networks possessing localized and
delocalized electronic band structure should have improved
prospects to exhibit molecular ferromagnetism.15 Although 1 is
an isolated molecule and not an extended material, it is a readily
studied model for this genre of organic open-shell networks.
Additional studies of analogous heterospin open-shell organic
molecules are ongoing in efforts to generalize the application of
photochemical cleavage toward producing new models for fer-
romagnetic materials.
Figure 2. UV-vis spectra for 2 at 67 K before (a) and after (b) photolysis
at >300 nm for 60 s. Absorbance scale is appropriate for the main curves;
the expanded portions of curves (a) and (b) are displayed at approximately
14× and 4× expansion, respectively.
and 703 nm: the latter four appear to be due to vibronic spacing
of about 1500 ( 60 cm-1
. The postphotolysis spectrum is
completely changed, showing spectral features at 320 and 445
(wk) nm as well as at 486, 528, and 578 nm. The latter
absorbances are responsible for the purple color of the photolyzed
sample and correspond to a vibronic spacing of about 1640 ( 20
cm-1. The new peaks are stable at 77 K but disappear irreversibly
at >90 K. The spectra may be compared to those obtained from
phenyl azide photolysis,8a-d which have strong absorbance at
300-380 and only weak peaks at 500 nm. The conjugation effect
of the NN unit connected para to the nitrene in 1 may be gauged
by the qualitatively similar matrix UV-vis spectra obtained from
photolysis of 4-amino-4′-azido-(E)-stilbene.8e
Density functional calculations (6-31G**/B3LYP) were carried
out9 on model systems for 1 where all methyl groups were
replaced by hydrogen atoms. The cylindrical computed structure
4
for A1 planar 1 is consistent with the near-zero zfs E-value of
2
the observed ESR spectrum. The excited, low-spin A1 state is
5.3 kcal/mol higher in energy, while bisected 4A1 3 is 18.7 kcal/
mol higher. Details are given in the Supporting Information.
Quartet 1 is an unusual organic system in having a localized
σ-electron interacting with a pair of delocalized triplet π-electrons.
The computed singly occupied molecular orbital occupancies for
4A1 1 are b2(R)b1(R)a2(R) ) π n1π . “Y-conjugated” biradicals
410 and 511 are close structural analogues to 1, although they are
triplet rather than quartet systems. Triplet 4 has a zfs |D/hc|
(8) (a) Phenyl azide photochemistry. Azides and Nitrenes, ReactiVity and
Utility; Scriven, E. F. V., Ed.; Academic Press: 1984. (b) Reiser, A.; Bowes,
G.; Horne, P. J. Trans. Faraday Soc. 1966, 62, 3162 (c) Levya, E.; Platz, M.
S.; Persy, G.; Wirz, J. J. Am. Chem. Soc., 1986, 108, 3783. (d) While multiple
UV-vis bands are produced in the 300-370 nm region during photolysis of
phenyl azide, not all are definitively assignable to to triplet phenylnitrene,
see: Cullin, D. W.; Yu, L.; Williamson, J. M.; Platz, M. S.; Miller, T. A. J.
Phys. Chem. 1990, 94, 3387 and Kim, S-j.; Hamilton, T. P.; Schaeffer, H.
F., III J. Am. Chem. Soc. 1992, 114, 5349. (e) Harder, T.; Bendig, J.; Scholz,
G.; Sto¨sser, R. J. Am. Chem. Soc. 1996, 118, 2497.
(9) All calculations were carried out using the program Gaussian 94,
Revision D.4: Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.;
Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.;
Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.;
Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.; Nanayakkara,
A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.; Wong, M. W.;
Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley,
J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez,
C.; Pople, J. A. Gaussian, Inc.: Pittsburgh, PA, 1995.
Acknowledgment. This work was supported in part by the National
Science Foundation (CHE-9521594). Support from the donors of the
Petroleum Research Fund administered by the American Chemical Society
is gratefully acknowledged.
Supporting Information Available: 6-31G**/B3LYP computed
coordinates and energies for the 4A1 and 2A1 states of 1, the 4A1 geometry
for 3, plus an edited output file for a 6-31G*/B3LYP frequency calculation
on 1 (21 pages print/PDF). See any current masthead page for ordering
information and Web access instructions.
JA980120V
(12) Itoh, K. Magnetic Coupling in High Spin Carbenes. In Magnetic
Molecular Materials, NATO ASI Series; Gatteschi, D., Kahn, O., Miller, J.
S., Palacio, F. Eds.; Kluwer: Dordrecht, The Netherlands, 1991; p 67.
(13) (a) Minato, M.; Lahti, P. M. J. Am. Chem. Soc. 1993, 115, 4532 (b)
Minato, M.; Lahti, P. M. J. Am. Chem. Soc. 1997, 119, 2187.
(14) Matsuda, K.; Iwamura, H. Mol. Cryst. Liq. Cryst., Sect. A 1997, 306,
89.
(10) Inoue, K.; Iwamura, H. Angew. Chem., Int. Ed. Engl. 1995, 34, 927.
(11) Ishiguro, K.; Ozaki, M.; Kamerku, Y.; Sekine, N.; Sawaki, Y. Mol.
Cryst. Liq. Cryst., Sect. A 1997, 306, 75.
(15) Mataga, N. Theor. Chim. Acta 1968, 10, 273.