Structural, magnetic, and computational correlations of some imidazolo-fused 1,2,4-benzotriazinyl radicals

1,3,7,8-Tetraphenyl-4,8-dihydro-1H-imidazolo[4,5g][1,2,4]benzotriazin-4-yl (5), 8-(4-bromophenyl)-1,3,7-triphenyl-4,8-dihydro-1H-imidazolo[4,5g][1,2,4] benzotriazin-4-yl (6), and 8-(4-methoxyphenyl)-1,3,7-triphenyl-4,8-dihydro-1H- imidazolo[4,5g][1,2,4]benzotriazin-4-yl (7) were characterized by using X-ray diffraction crystallography, variable-temperature magnetic susceptibility studies, and DFT calculations. Radicals 5-7 pack in 1 D π stacks made of radical pairs with alternate short and long interplanar distances. The magnetic susceptibility (χ vs. T) of radicals 5 and 6 exhibit broad maxima at (50±2) and (50±4) K, respectively, and are interpreted in terms of an alternating antiferromagnetic Heisenberg linear chain model with average exchange-interaction values of J=-31.3 and -35.4 cm^-1 (g _solid=2.0030 and 2.0028) and an alternation parameter a=0.15 and 0.38 for 5 and 6, respectively. However, radical 7 forms 1 D columns of radical pairs with alternating distances; one of the interplanar distances is significantly longer than the other, which decreases the magnetic dimensionality and leads to discrete dimers with a ferromagnetic exchange interaction between the radicals (2J=23.6 cm^-1, 2zJ′=-2.8 cm^-1, g _solid=2.0028). Magnetic exchange-coupling interactions in 1,2,4-benzotriazinyl radicals are sensitive to the degree of slippage and inter-radical separation, and such subtle changes in structure alter the fine balance between ferro- and antiferromagnetic interactions. Bigger is stronger: π-Extension of 1,2,4-benzotriazin-4-yl radicals leads to an enhanced overlap between SOMO orbitals and, therefore, to a strong effective exchange coupling within alternating 1 D chains of imidazolo-benzotriazinyls. However, subtle changes in interplanar distances and slippage angles switch the observed magnetic properties (see figure).