Organic spin clusters: annelated macrocyclic polyarylmethyl polyradicals and a polymer with very high spin S=6-18

Abstract

Synthesis and magnetic studies of annelated macrocyclic polyradicals and a related high-spin polymer with macrocyclic repeat units are described. Polyarylmethyl polyether precursors to the polyradicals and the related polymer are prepared by using Negishi cross-coupling of difunctionalized calix[4]arene-based macrocycles. The three lowest homologues, with high degree of monodispersity, are tetradecaether (14-ether) 3-(OCH(3))(14), octacosaether (28-ether) 4-(OCH(3))(28), and dotetracontaether (42-ether) 5-(OCH(3))(42), in which 2, 4, and 6 calix[4]arene-based macrocycles are annelated to the center macrocycle, respectively. The evidence for their annelated structures (ladder connectivities) is based upon FAB-MS and the (1)H NMR based end-group analysis. The absolute masses (4-12 kDa) were determined by FAB-MS and GPC/MALS. Small angle neutron scattering (SANS) provides the radii of gyration of 1.7, 2.0, and 3.2 nm for 4-(OCH(3))(28), 5-(OCH(3))(42), and polymer 6-(OCH(3))(n), respectively. The corresponding polyarylmethyl polyradicals 3 and 4, and polymer 6 possess average values of S approximately 6-7, S approximately 10, and S approximately 18, respectively, as determined by SQUID magnetometry and numerical fits to linear combinations of Brillouin functions. The quantitative values of magnetization at saturation and of magnetic susceptibilities indicate that about 40-60 % of unpaired electrons are present at low temperatures (T=1.8-5 K). For polyradical 3, the variable temperature magnetic data are fit to the Heisenberg Hamiltonian based model. The variable magnetic field data at low temperatures are also fit to a percolation-based model for organic spin cluster, with random distribution of chemical defects, and ferromagnetic versus antiferromagnetic couplings, providing quantitative agreement between the experiment and the theory. For polyradical 3 (with S approximately 6-7), annealing at room temperature for 0.5 h leads to a polyradical with S approximately 5.