Naphthyldiamine diradical dications. Triplet dications of 2,7- bis(amino)naphthalene and 2,7-bis(phenylenediamino)naphthalene [2]

Full text of DOI:10.1021/ja984169c

7152
J. Am. Chem. Soc. 1999, 121, 7152-7153
Naphthyldiamine Diradical Dications. Triplet
Dications of 2,7-Bis(amino)naphthalene and
2,7-Bis(phenylenediamino)naphthalene
Trent D. Selby and Silas C. Blackstock*
Department of Chemistry, The UniVersity of Alabama
Tuscaloosa, Alabama 35487-0336
ReceiVed December 4, 1998
ReVised Manuscript ReceiVed March 6, 1999
As part of an effort to construct and characterize redox-
switchable high-spin organic molecules,1 we have begun a study
of diaminonaphthalene diradical dications isoelectronic with the
non-Kekule´ dimethylenenaphthalene structures 1-4. The 1,8-
naphthyl diyl2,3 (1) and its alkyl derivatives4,5 have been previously
observed by matrix ESR and fluorescence spectroscopy6 and
assigned as ground-state triplet diradicals. Naphthyl diyls 2-4
have been prepared by matrix photolysis of the corresponding
dichlorides and were also assigned as triplet diyls on the basis of
their fluorescence spectra.7 Here, we report on 2,7-diaminonaph-
thalene diradical dications 52+ and 62+ which are found to be
solution-stable triplets.
Figure 1. Cyclic voltammogram in CH2Cl2 (0.1 M Bu4NBF4) at 20 mV
s-1 scan rate at 298 K of (a) 1.0 mM 5 (b) 1.0 mM 6.
coupling of the resulting diamine 7 with p-iodoanisole or di-p-
anisyl-p-iodophenylamine (8) as shown below. Iodoamine 8 was
made by Ullmann coupling of di-p-anisylamine with excess 1,4-
diiodobenzene.
Oxidation of 5 by cyclic voltammetry (CV) shows two
chemically reversible waves (Figure 1a) yielding E°′ values of
(n,+) 0.74 and (+,2+) 0.93 V vs SCE. CV oxidation of 6 shows
(2) Pagni, R. M.; Burnett, M. N.; Dodd, J. R. J. Am. Chem. Soc. 1977, 99,
1972.
Substrates 5 and 6 were prepared by condensation8 of p-
anisidine with 2,7-dihydroxynaphthalene followed by Ullmann
(3) Platz, M. S. J. Am. Chem. Soc. 1979, 101, 3398.
(4) Watson, C. R., Jr.; Pagni, R. M.; Dodd, J. R.; Bloor, J. E. J. Am. Chem.
Soc. 1976, 98, 2551.
(1) (a) Stickley, K. R.; Blackstock, S. C. J. Am. Chem. Soc. 1994, 116,
11576. (b) Stickley, K. R.; Selby, T. D.; Blackstock, S. C. J. Org. Chem.
1997, 62, 448.
(5) Muller, J.-F.; Muller, D.; Dewey, H. J.; Michl, J. J. Am. Chem. Soc.
1978, 100, 1629.
10.1021/ja984169c CCC: $18.00 © 1999 American Chemical Society
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Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 30, 1999 7153
signal width of ∼73 G along with a half-field ∆ms ) 2 absorption
centered at 1601 G that verifies the triplet nature of this dication
(Figure 2b).
The half-field ESR signal intensities for 52+ and for 62+ obey
the Curie law and increase linearly from 120 to 90 K which
suggests that their triplet populations are not changing detectably
in this limited temperature range. This observation is consistent
with (a) triplet ground states and singlet/triplet energy gaps (∆ES,T
)
greater than RT (i.e., >250 cal mol-1) or (b) with ∆ES,T values
,RT and either singlet or triplet ground states for these dications.
The solution stability of 52+ and 62+ allows the determination
of their solution magnetic susceptibility by the Evans11 NMR shift
method. The effective magnetic moments (µeff) measured for 5+
and 52+ (prepared by TH+ oxidation in CF3CO2D) were 1.75 (
0.04 and 2.64 ( 0.04, respectively. For 6+PF6 and 62+(PF6
)
2
-
-
dissolved in CDCl3,12 µeff values were measured to be 1.73 (
0.04 and 2.40 ( 0.03, respectively. Theoretical µeff values for
doublet and triplet molecules are 1.73 and 2.83, respectively. The
µ
eff values of 52+ and 62+ solutions suggests that they are singlet/
triplet mixtures at 301 K. Assuming a weighted admixture of
singlet (µeff ) 0) and triplet (µeff ) 2.83) states for these dication
solutions yields populations of 87% triplet, 13% singlet for 52+
and 72% triplet, 28% singlet for 62+. After correction for the 3:1
statistical triplet:singlet state preference, the data implies a triplet
ground state for 52+ with ∆ES,T 0.48 kcal mol-1 in CF3CO2D at
301 K and nearly degenerate singlet and triplet states with a
possible singlet ground state for 62+ in CDCl3 at 301 K.
A high-spin ground state for 52+ is consistent with AM1
calculations that predict a strong triplet preference for 2,7-
diaminonaphthalene dication (92+) (UHF, 13.5 and π-CI, 14.9
kcal mol-1). Although 2,7-aza substitution of the perinaphthalene
nucleus alters the π MO structure from that calculated for the
non-Kekule´ hydrocarbon, the nondisjoint13 nature of the (nearly)
degenerate singly occupied frontier MOs is preserved and thus
so is also the energetic preference for spin alignment of the diyl
electrons. The two highest energy occupied MOs of 9 (C2ν) are
shown below. Spin dilution at R-nitrogens in 52+ and 62+ will
diminish ∆ES,T relative to that for 92+; however, enough of an
spin alignment preference apparently remains in the former to
yield a triplet ground state structure.
Figure 2. ESR spectra of (a) 0.5 mM 6+PF6- in fluid CH2Cl2 at 298 K
and (b) 0.5 mM 62+(PF6-)2 in solid PrCN at 90 K.
four chemically reversible waves (Figure 1b), with E°′ values of
(n,+) 0.50, (+,2+) 0.59, (2+,3+) ∼0.97, and (3+,4+) ∼1.02 V
vs SCE. The mono- and dication hexafluorophosphate salts of 6
are isolable and have been prepared by NOPF6 oxidation of 6 in
CH2Cl2 solutions.9
Oxidation of 5 and 6 by 1 or 2 molar equiv of thianthrenium
(TH+) perchlorate10 yields ESR-active solutions. The ESR
spectrum of 5+ in CH2Cl2 at 25 °C is a broad 5-line spectrum
with overlaid fine structure (presumably due to multiple small
hydrogen splittings). The 5-line splitting of ∼4.6 G is assigned
as the a(2N) coupling due to two equivalent nitrogens. The
dication 52+ in frozen PrCN shows a broad signal in the ∆ms )
1 region with a signal width of ∼95 G and a strong half-field
∆ms ) 2 absorption centered at 1600 G. For 6+ in CH2Cl2 at 25
°C, a broad, apparent 9-line spectrum (Figure 2a) is observed.
The apparent spiltting is consistent with nearly equal spin density
on all four nitrogens of 6+ with a splitting constant of ∼3.0 G.
For 62+ in frozen PrCN, a ∆ms ) 1 signal is observed with a
(6) Gisin, M.; Rommel, E.; Wirz, J.; Burnett, M. N.; Pagni, R. M. J. Am.
Chem. Soc. 1979, 101, 2216.
(7) Biewer, M. C.; Biehn, C. R.; Platz, M. S.; Despre´s, A.; Migirdicyan,
E. J. Am. Chem. Soc. 1991, 113, 616.
In summary, we conclude that 2,7-bis(amino)naphthalene
substrates are a viable structural unit for preparing redox-activated
stable diradical dication triplets.14 Further work to investigate other
patterns of polyradical polycation spin-coupling about a naph-
thalene nucleus is in progress.
(8) Buu-Ho¨ı, N. P. J. Chem. Soc. 1952, 4346.
(9) Anal. Calcd (%) for 62+(PF6-)2: C, 60.67; H, 4.46; N, 4.42. Found:
C, 60.90; H, 4.38; N, 4.56.
(10) Caution! Thianthrenium perchlorate (TH+ClO4-) is a shock-sensitive
explosive solid and should be handled with due care. For preparation and
properties see Murata, Y.; Shine, H. J. J. Org. Chem. 1969, 34, 3368.
(11) (a) Evans, D. F. J. Chem. Soc. 1959, 2003. (b) Live, D. H.; Chan, S.
I. Anal. Chem. 1970, 42, 791.
Acknowledgment. We are grateful for financial support from the
NSF-MRSEC (DMR-9809423).
(12) These solutions are normally stabilized with up to 5% trifluoroacetic
acid and 5% trifluoroacetic anhydride. However, runs in the absence of these
additives were checked and gave unaltered µeff values for the cations and
dications.
(13) Borden, W. T. Diradicals; Wiley: New York, 1982; Chapter 1.
(14) For a potential application of 2,7-naphthyl-connected polyarylamine
polymers as dopable high-spin cluster materials see: Bushby, R. J.; Gooding,
D. J. Chem. Soc., Perkin Trans. 2 1998, 1069.
Supporting Information Available: Synthesis and spectroscopic
analysis of 5 and 6 and experimental details for ESR and NMR shift
measurements (PDF). This material is available free of charge via the
Internet at http://pubs.acs.org.
JA984169C