Ferromagnetically Coupled S=1 Chains in Crystals of Verdazyl‐Nitronyl Nitroxide Diradicals

Abstract

Thermally stable organic diradicals with at riplet ground state along with large singlet-triplet energy gap have significant potential for advanced technological applications.A series of phenylene-bridged diradicals with oxoverdazyla nd nitronyl nitroxide units were synthesized via ap alladiumcatalyzedc ross-coupling reaction of iodoverdazyls with an itronyl nitroxide-2-ide gold(I) complex with high yields.T he diradicals exhibit high stability and do not decompose in an inert atmosphere up to 180 8 8C. Fort he diradicals,b oth substantial AF (DE ST %À64 cm À1)a nd FM (DE ST ! 25 and 100 cm À1)intramolecular exchange interactions were observed. The sign of the exchange interaction is determined both by the bridging moiety (para-or meta-phenylene) and by the type of oxoverdazylb lock(C-linked or N-linked). Upon crystallization, diradicals with the triplet ground state form unique onedimensional exchange-coupled chains with strong intra-and weak inter-diradical ferromagnetic coupling.

Synthesis and Thin Films of Thermally Robust Quartet (S = 3/2) Ground State Triradical

“…41,42 Tretyakov and coworkers also utilized complex 4 for the Pd(0)-catalyzed cross-coupling with iodo-oxoverdazyls, which led to oxoverdazylnitronyl nitroxide diradicals in high yields. 33 These results provide valuable insight and motivation to further develop the crosscoupling methodology for a variety of stable organic radicals. We focus on the cross-coupling reactions between Blatter radicals and nitronyl nitroxide radicals (Scheme 1).…”

“…The degree of spin density delocalization correlates with the strength of exchange coupling. The oxoverdazyl-nitronyl nitroxide diradicals have the smaller singlet-triplet energy gaps, EST < 0.3 kcal mol -1 , 33 compared to EST ≈ 0.5 kcal mol -1 for 1 and EST ≈ 1.7 kcal mol -1 for 2. 31,32 Thus, the spin density in the Blatter radicals is more delocalized than that in oxoverdazyls, leading to greater densities at the carbons of the C-I bonds in the starting Blatter radicals such as 5 and 6 (Scheme 1).…”

“…15,[28][29][30] Recently, we reported the robust triplet ground state diradicals 1 and 2 (Figure 1). 31,32 To our knowledge, these and the analogous oxoverdazyl-based diradicals 33 are the only neutral highspin diradicals that are well characterized by thermogravimetric analysis (TGA), to establish firmly their robust thermal stability. 34 Diradicals 1 and 2 have an onset of decomposition at 175 and 160 C, while the onset at 192 C was reported for the oxoverdazylbased diradical.…”

“…34 Diradicals 1 and 2 have an onset of decomposition at 175 and 160 C, while the onset at 192 C was reported for the oxoverdazylbased diradical. [31][32][33] Diradical 2, which has a 95+% thermal population of the triplet state at room temperature, can be evaporated under ultra-high vacuum (UHV) to form thin films on silicon. 32 The next challenge is a high-spin triradical, with a significant population of quartet ground state at room temperature, that is suitable for thin film fabrications.…”

“…The Blatter moiety is considerably more electron-rich, compared to typical diamagnetic -systems or oxoverdazyl radicals (vide infra, Electrochemistry section). Thus, the common Pd(0) catalyst, such as Pd(PPh3)4 that was used both by Okada and Tretyakov, 33,41,42 may not be sufficiently powerful. In addition, spin delocalization in the Blatter radicals could be an impediment.…”