A Thiazyl‐Based Organic Ferromagnet

Abstract

In 1928 Heisenberg proposed [1] that bulk ferromagnetic order would only ever be achieved in systems containing heavy atoms, that is, metals, their oxides, and related derivatives. Indeed it was not until 1991 that the first organic ferromagnet was reported; the b-polymorph of the para-nitrophenyl nitronyl nitroxide radical (p-NPNN, 1; Scheme 1) was shown to order below 0.6 K. [2] Since then a number of other purely organic radicals have been found to undergo bulk ferromagnetic order, although the majority order below 1 K. Exceptions include the radical cation salts [C 60 ][TDAE] [3] (TDAE = tetrakis(dimethylamino)ethylene) and [BBDTA]-GaCl 4 [4](3; BBDTA = benzobis(1,3,2-dithiazolyl), which order as ferromagnets at 16 and 6.7 K, respectively. Of the neutral radicals, only the nitroxide-based diradical DOTM-DAA (2; DOTMDAA = N,N'-dioxy-1,3,5,7-tetramethyl-2,6diazaadamantane) orders ferromagnetically above 1 K (T c = 1.48 K), [5] although the dithiadiazolyl radical p-NCC 6 F 4 CNSSNC (4) orders as a weak ferromagnet at 36 K under ambient pressure, [6] the highest reported temperature for magnetic ordering in an organic radical. We have sought to prepare new dithiadiazolyl radicals, closely related to 4, in which small structural changes may lead to new magnetically ordered systems. Here we report the structure and magnetic properties of 5, which is only the second neutral organic radical to order as a ferromagnet above 1 K.

Synthesis and Magnetic Properties of the Novel Dithiadiazolyl Radical, p-NCC6F4C6F4CNSSN

“…Previous EPR studies [12,7] on 1 -3 indicate that the unpaired spin density distribution in these systems is almost invariant of the substituent on the perfluoroaryl group. In addition there is excellent agreement between the spin densities calculated from EPR and ab initio calculations.…”

“…Many of these radicals have been isolated in the solid state. Whilst the majority form diamagnetic closed-shell dimers via a π*-π* dimerisation process, [ 4 ] a small number of perfluorophenyl derivatives 1-3 have recently been found which retain their paramagnetism in the solid state [5,6,7]. Radical 1 is polymorphic; the α-phase exhibits a maximum in χ around 8 K indicative of antiferromagnetic ordering, [5a] whilst the β-phase undergoes a phase transition to a canted antiferromagnetic phase at 36 K [5b].…”

“…Radical 1 is polymorphic; the α-phase exhibits a maximum in χ around 8 K indicative of antiferromagnetic ordering, [5a] whilst the β-phase undergoes a phase transition to a canted antiferromagnetic phase at 36 K [5b]. Radical 2 also exhibits short range antiferromagnetic interactions, [6] but 3 has recently been reported to order as a ferromagnet at 1.3 K [7]. In both polymorphs of 1 [5], as well as other cyano-functionalised dithiadiazolyl radicals [ 8 ], an electrostatically favourable CN···S interaction is persistently observed.…”

Spin density studies onp-O2NC6F4CNSSN: A heavypLuzón1, Campo2, Palácio3 et al. 2010Phys. Rev. BSelf Cite19090View full textAdd to dashboardCite

“…The preliminary x-ray and magnetic studies of 2 have been reported in a previous communication. 11 Large single crystals for the work described here were prepared by annealing a small sample ͑approximately 60 mg͒ in a sealed tube under N 2 for 3-4 hours at 100°C. While there was evidence for some decomposition ͑formation of an intractable brown oil on the surface of the glass͒, a small number of large single crystals were also formed with upper dimensions in the region 1 ϫ 1 ϫ 1 mm 3 .…”

“…In this paper, we implement polarized neutron diffraction, electron paramagnetic resonance ͑EPR͒, and electron-nuclear double resonance ͑ENDOR͒ spectroscopies to experimentally probe the spin density distribution in p-O 2 NC 6 F 4 CNSSN, which some of us have previously reported to exhibit ferromagnetism below 1.3 K. 11 In addition these experimental studies are supported by ab initio densityfunctional theory ͑DFT͒ calculations ͑Preliminary reports of selected aspects of this study have been presented elsewhere 9,12 ͒. This combination of experimental and theoretical methods provides a self-consistent unified view of the spin density distribution within the dithiadiazolyl ring.…”