The Suppression of Columnar π-Stacking in 3-Adamantyl-1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl

3-Adamantyl-1-phenyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl (4) crystallizes as chains of radicals where the spin bearing benzotriazinyl moieties are isolated from each other. Magnetic susceptibility studies in the 5–300 K temperature region indicate that radical 4 demonstrates typical paramagnetic behavior stemming from non-interacting S = ½ spins.


Magnetic Susceptibility Studies
Variable-temperature magnetic-susceptibility measurements on radical 4 were obtained using a SQUID magnetometer in the temperature region 5-300 K in an applied field of 0.4 T. Data were collected in both warming and cooling modes with no significant differences in sample susceptibility. The temperature dependence of χT is shown in Figure 5.

Magnetic Susceptibility Studies
Variable-temperature magnetic-susceptibility measurements on radical 4 were obtained using a SQUID magnetometer in the temperature region 5-300 K in an applied field of 0.4 T. Data were collected in both warming and cooling modes with no significant differences in sample susceptibility. The temperature dependence of χT is shown in Figure 5. columns without π-π interactions between the benzotriazinyl moieties [5,20]. This radical demonstrated typical paramagnetic behavior as it followed the Curie-Weiss law with θ = −0.3 K [5].

Magnetic Susceptibility Studies
Variable-temperature magnetic-susceptibility measurements on radical 4 were obtained using a SQUID magnetometer in the temperature region 5-300 K in an applied field of 0.4 T. Data were collected in both warming and cooling modes with no significant differences in sample susceptibility. The temperature dependence of χT is shown in Figure 5.  The inverse of molar susceptibility (1/χ) followed the Curie-Weiss law ( Figure 5, inset) with C = 0.377 emu K¨mol´1 and θ =´0.61 K. On cooling from 300 K down to ca. 50 K, χT remained stable with a value of 0.375 emu K¨mol´1 expected for an S = ½ paramagnet. Below 50 K, χT gradually decreased as antiferromagnetic interactions along the a-axis chain (weak hydrogen d N6 . . . C34 contacts) became dominant.

Experimental Sections
Synthetic procedure: The synthesis of radical 4 was published previously [10]. Instrumental analyses: Cyclic voltammetry (CV) measurements were performed on a Princeton Applied Research Potentiostat/Galvanostat 263A apparatus (Oak Ridge, TN, USA). The concentration of the benzotriazinyl radical 4 used was 1 mM in CH 2 Cl 2 . A 0.1 M CH 2 Cl 2 solution of tetra-butylammonium tetrafluoroborate (n-Bu 4 BF 4 ) was used as electrolyte. The electrolyte was dried for four days in the vacuum oven at 100˝C prior to the use. The reference electrode was Ag/AgCl and the scan rate was 50 mV/s. Ferrocene was used as an internal reference; the E 1/2 (ox) of ferrocene in this system was 0.352 V [26]. EPR spectra were recorded on a Bruker EMXplus X-band EPR spectrometer (Bruker, Billerica, MA, USA) at room temperature in dilute solution of CH 2 Cl 2 . For the EPR spectrum, the microwave power was in the region 5-70 mW with modulation frequencies of 50 or 100 kHz and modulation amplitudes of 0.5-1.0 G pp . Simulations of the solution spectrum was made using Winsim software [27]. X-ray data of 4 (CCDC 1474192) [28] were collected on an Oxford-Diffraction Supernova diffractometer, equipped with a CCD area detector utilizing Mo-Kα radiation (λ = 0.71073 Å). A suitable crystal was attached to glass fibers using paratone-N oil and transferred to a goniostat where they were cooled for data collection. Unit cell dimensions were determined and refined by using 4562 (3.85 ď θ ď 27.68˝) reflections. Empirical absorption corrections (multi-scan based on symmetry-related measurements) were applied using CrysAlis RED software [29]. The structures were solved by direct method and refined on F 2 using full-matrix least squares using SHELXL97 [30,31]. Software packages used: CrysAlis CCD [29] for data collection, CrysAlis RED [29] for cell refinement and data reduction, WINGX for geometric calculations [32] and Mercury 3.1 (CCDC, Cambridge, UK) [33]. The non-H atoms were treated anisotropically. The hydrogen atoms were placed in calculated, ideal positions and refined as riding on their respective carbon atoms. Magnetic properties were studied by using a Quantum Design SQUID MPMS 2 field-shielded magnetometer (Quantum Design Inc., San Diego, CA, USA). The DC (direct current) magnetic moment was measured for 43 mg sample of radical 4, placed in gelatin capsules held by polyethylene straw. The magnetic susceptibilities were measured in the temperature range of 5-300 K in an applied field of 0.4 T. Data were collected in both warming and cooling modes with no significant differences in sample susceptibility.

Conclusions
Introduction of the bulky substituent adamantyl to the C-3 position of the 1,2,4-benzotriazinyl framework successfully inhibited the formation of π-stacked radical columns, but instead of promoting higher dimensional frameworks, only isolated radicals were obtained. This led to non-interacting S = ½ spins and a typical paramagnetic behavior. Our results indicate that there is a fine balance to be reached between steric perturbation and π-stacking interactions if materials with interesting magnetic and transport properties are to be developed. and the following organizations in Cyprus for generous donations of chemicals and glassware: the State General Laboratory, the Agricultural Research Institute, the Ministry of Agriculture, MedoChemie Ltd. and Biotronics Ltd. Furthermore, we thank the A. G. Leventis Foundation for helping to establish the NMR facility in the University of Cyprus.
Author Contributions: Christos P. Constantinides and Panayiotis A. Koutentis have contributed to the conception, drafting and critical revision of the submitted manuscript. Andrey A. Berezin and Georgia A. Zissimou have contributed to the synthesis of the radical and collection of data. Maria Manoli has acquired the X-ray diffraction data and solved the structure. Gregory M. Leitus has acquired the magnetometry data.

Conflicts of Interest:
The authors declare no conflict of interest.