Three-Dimensional Magnetic Exchange Networks in Trigonal Bisdithiazolyl Radicals

The N-methyl-4-phenyl-pyridine-bridged bisdithiazolyl radical PhBPMe is polymorphic, crystallizing from cold acetonitrile in a trigonal α-phase, space group P3₁21, and from hot dichloroethane in an orthorhombic β-phase, space group Pca2₁. The crystal structures of both phases consist of slipped π-stacks of undimerized radicals aligned laterally into herringbone arrays. In the β-phase, there are two independent radicals in the asymmetric unit, and the resulting π-stacks form corrugated layers interspersed by methyl and phenyl groups which block the approach of neighboring radicals. In the α-phase, the methyl/phenyl groups and the radical π-stacks separately form spirals about 3₁ axes, the latter giving rise to a 3D network of close radical/radical contacts. Variable temperature magnetic susceptibility measurements on the β-phase indicate strong antiferromagnetic coupling. Weaker but predominantly antiferromagnetic interactions (θ = -20.7 K) are observed in the α-phase. A high temperature series expansion analysis of the magnetic data for the α-phase affords antiferromagnetic exchange energies for the one- and two-step radical/radical interactions about the 3₁ spirals ( J₁ = -1.2 K, J₂ = -10.9 K, respectively), with weak ferromagnetic interactions along the π-stacks ( J_π = +1.8 K). Despite the presence of a 3D network based on the dominant J₂ interactions, which affords two independent bipartite sublattices, no evidence of bulk antiferromagnetic order has been observed above T = 2 K. The magnetic results are discussed in light of exchange energies calculated using density functional theory broken symmetry methods.