Triamino-s-triazine triradical trications. An experimental study of triazine as a magnetic coupling unit

Article abstract of DOI:10.1021/ol9912736

(equation presented) Triamino-s-triazine derivatives 3a, 4, and 5 have been prepared, and their cationic states have been analyzed electrochemically. At 298 K, 3a⁺ has a limited lifetime in CH₂CI₂ solution. However, 4⁺ and 5⁺ are long-lived under such conditions, and quartet states of 4³⁺ and 5³⁺ are observed by ESR spectroscopy. Variable-temperature ESR analysis and NMR shift susceptibility measurements indicate that 5³⁺ is a doublet ground state with a populated quartet state.

Full text of DOI:10.1021/ol9912736

ORGANIC
LETTERS
2000
Vol. 2, No. 2
171-174
Triamino-s-triazine Triradical Trications.
An Experimental Study of Triazine as a
Magnetic Coupling Unit
Trent D. Selby, Kurt R. Stickley, and Silas C. Blackstock*
Department of Chemistry, The UniVersity of Alabama, Tuscaloosa, Alabama 35487
blackstock@ua.edu
Received November 23, 1999
ABSTRACT
Triamino-s-triazine derivatives 3a, 4, and 5 have been prepared, and their cationic states have been analyzed electrochemically. At 298 K, 3a+
has a limited lifetime in CH Cl2 solution. However, 4+ and 5+ are long-lived under such conditions, and quartet states of 43+ and 53+ are
2
observed by ESR spectroscopy. Variable-temperature ESR analysis and NMR shift susceptibility measurements indicate that 53+ is a doublet
ground state with a populated quartet state.
Ferromagnetic coupling between electron spins in organic
polyradicals is promoted by certain spin-connection geom-
etries.1 For example, m-phenylene connection of amino
radical cations favors spin alignment, and 1,3,5-tris(di-p-
anisylamino)benzene trication (13+) has been shown to be a
ground-state quartet triradical.2
have suggested that amino-substituted s-triazine polycations
will prefer high-spin ground states. In 1989, Miller et al.
proposed that diamino-s-triazines (2a) might have triplet
In this report, we consider amino-type radical cations
connected via an s-triazine nucleus. Theoretical predictions
(1) For reviews, see: (a) Magnetic Properties of Organic Materials;
Lahti, P. M., Ed.; Marcel Dekker: New York, 1999. (b) Rajca, A. Chem.
ReV. 1994, 94, 871. (c) Miller, J. S.; Epstein, A. J. Angew. Chem., Int Ed.
Engl. 1994, 33, 385. (d) Iwamura, H.; Koga, N. Acc. Chem. Res. 1993, 26,
346. (e) Dougherty, D. A. Acc. Chem. Res. 1991, 24, 88.
(2) (a) Stickley, K. R.; Blackstock, S. C. J. Am. Chem. Soc. 1994, 116,
1441. (b) Sato, K.; Yano, M.; Furuichi, M.; Shiomi, D.; Takui, T.; Abe,
K.; Itoh, K.; Higuchi, A.; Katsuma, K.; Shirota, Y. J. Am. Chem. Soc. 1997,
119, 6607.
(3) Dixon, D. A.; Miller, J. S. Mol. Cryst. Liq. Cryst. 1989, 176, 211.
(4) Ito, A.; Miyajima, H.; Yoshizawa, K.; Tanaka, K.; Yamabe, T. J.
Org. Chem. 1997, 62, 38.
(5) Zhang, J.; Baumgarten, M. Chem. Phys. 1997, 214, 291.
(6) For a report of triazinyl nitrenes, see: Nakai, T.; Sato, K.; Shiomi,
D.; Itoh, K.; Kazaki, M.; Okada, K. Synth. Met. 1999, 103, 2265.
dication ground states.3 More recently, Yamabe et al.4
reported ab initio calculations that predicted a 12.5 kcal mol-1
triplet ground-state preference for 2a2+. These workers
concluded s-triazine to be a strong ferromagnetic coupling
unit for radicals derived from aza-substituted systems.
Moreover, Baumgarten and Zhang5 have reported AM1-CI
calculations of triamino-s-triazine trications 2b3+ that suggest
a 23 kcal mol-1 quartet preference for this trication. Here,
10.1021/ol9912736 CCC: $19.00 © 2000 American Chemical Society
Published on Web 12/29/1999

we report preliminary experimental findings regarding amino-
substituted triazine polyradical cations.6
the high oxidation potentials made dication and trication
generation difficult.
Figure 1 shows a cyclic voltammogram of 3a. One-
electron oxidation was quasi-reversible under ambient condi-
tions. In cold CH2Cl2, the first two oxidations became
chemically reversible, yielding E° ′(1) ) 1.12 and E° ′(2) )
1.42 V vs SCE. These values are 0.45 and 0.55 V higher
than those for 1.2a In an attempt to improve the chemical
stability of the aminotriazine cations and lower the Eox values,
we next prepared the corresponding p-phenylenediamine (p-
PD) triazine analogues 4 and 5 (Scheme 1). As previously
noted,8 substitution of arylamino groups by p-PD units
generally lowers the oxidation potentials of the system and
increases the stability of aryl-linked polyradical cations.
Initially, a set of tris(arylamino) triazines (3) (Scheme 1)
were prepared for study.7 Unfortunately, the polyradical
Scheme 1
Electrochemical oxidation of 5 by cyclic voltammetry
(CV) gave two chemically reversible waves at room tem-
perature (Figure 2). Controlled potential coulometry at 0.90
cations in this series were too short-lived for easy handling
(lifetimes e seconds as determined electrochemically), and
Figure 2. Voltammogram of 5 (1.0 mM) at 298 K in CH2Cl2 (0.1
M Bu4NBF4) with a scan rate of 20 mV s-1
.
V indicated that three electrons per molecule were removed
at this potential (Figure 3). The associated formal oxidation
potentials were assigned as E° ′(1) ≈ 0.71, E° ′(2) ≈ E° ′(3)
≈ 0.78 V vs SCE. Oxidation of 4 showed a similar CV trace
with E° ′(1) ≈ 0.90, E° ′(2) ≈ E° ′(3) ≈ 1.03 V vs SCE.
Figure 1. Voltammogram of 3a (2.0 mM) at 298 K in CH2Cl2
(0.1 M Bu4NBF4) with scan rates of (solid) 100, (dash-dot) 50,
and (dot) 20 mV s-1
172
Org. Lett., Vol. 2, No. 2, 2000

prepared, 53+ gave a quartet ESR signal with a D value of
0.0015 cm-1. In frozen media, both 43+ and 53+ are quite
stable, but in fluid solution 43+ decays in minutes at room
temperature as deduced by ESR and NMR (vida infra)
analyses.
To obtain the spin state ordering for 53+, its magnetic
susceptibility was measured by the NMR shift method.10 (The
solution instability of 43+ precluded its NMR analysis.)
Measurements on 5+ and 53+ in CDCl3 at 298 K yielded µeff
values of 1.72 ( 0.06 and 2.43 ( 0.06 µB, respectively.
Theoretical µeff values for pure doublet and quartet species
are 1.73 and 3.87 µB, respectively. Therefore, we conclude
53+ to be a mixture of quartet and doublet states at 298 K
with 24% quartet and 76% doublet, the latter being the
ground state under these conditions.
This result is also consistent with the nonlinear temperature
dependence found for the 53+ ∆ms ) 2 signal intensity
measured by ESR in frozen butyronitrile over the range of
133-87 K (Figure 5). Here, a lower ESR signal intensity
Figure 3. Controlled potential coulometry at 0.9 V vs SCE for 5
(0.0113 mmol) at 298 K in CH2Cl2 (0.1 M Bu4NBF4).
Oxidation of 4 or 5 by 1 molar equiv of thianthrenium
perchlorate9 (TH+ClO4 ) gave green solutions of 4+ or 5+
-
that showed a single broad-line ESR spectrum. Presumably,
no hyperfine coupling is observed in these spectra because
of the numerous small splittings which effectively line
broaden the signal.
-
Treatment of 4 with 3 molar equiv of TH+ClO4 in
butyronitrile gave a 43+ solution that, when frozen, shows a
quartet ESR spectrum at 100 K with a D value of 0.0023
cm-1 (Figure 4). This D value is close to that (0.0026 cm-1)
Figure 5. ESR ∆ms ) 2 signal intensity for 53+ in frozen PrCN
as a function of 1/T.
than expected from linear Curie Law behavior is observed
as the temperature is lowered in this range, indicating an
increase in the doublet/quartet ratio.
In summary, we conclude that p-PD radical cations
attached 1,3,5- to an s-triazine core have a slight energetic
preference for spin pairing, yielding ground state doublet
triradical trications with observable low-lying quartet excited
states. Compared to the 1,3,5-phenyl-substituted tris(p-PD)
structure, the triazine-core compound 5 is harder to oxidize
(∆E° ′(1) ) 0.3 V), yields trications of lower chemical
stability (the former are isoable, whereas the latter are not),
and gives triradical trications with lower µeff values (2.43 vs
3.04). If deconjugation of the p-PD radical cations from the
strongly electron-withdrawing s-triazine nucleus occurs in
these polycations, then the apparent discrepancy between
theoretical predictions of spin alignment in aminotriazine
Figure 4. ESR spectrum of 53+ in frozen PrCN at 100 K.
observed for the m-phenylene analogue 1,3,5-tris(triphenyl-
p-phenylenediamino)benzene triradical trication.8 Similarly
(7) Stickley, K. R. Ph.D. Dissertation, Vanderbilt University, Nashville,
TN, 1996.
(8) Stickley, K. R.; Selby, T. D.; Blackstock, S. C. J. Org. Chem. 1997,
62, 448.
(9) Caution! Thianthrenium perchlorate is a shock-sensitive explosive
solid that should be handled with due care. See: Murata, Y.; Shine, H. J.
J. Org. Chem. 1969, 34, 3368.
(10) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Live, D. H.; Chan,
S. I. Anal. Chem. 1970, 42, 791.
Org. Lett., Vol. 2, No. 2, 2000
173

polyradicals and the experimental observation of a low-spin
Supporting Information Available: Description of syn-
thetic procedures, compound spectral data, and NMR sus-
ceptibility data. This material is available free of charge via
the Internet at http://pubs.acs.org.
ground state for 53+ would be rationalized.
Acknowledgment. We thank the NSF (CHE920144) and
NSF-MRSEC (DMR-9809423) for financial support of this
work and the University of Alabama for providing a graduate
fellowship to T.D.S..
OL9912736
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Org. Lett., Vol. 2, No. 2, 2000