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

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-(OCH₃) ₁₄, octacosaether (28-ether) 4-(OCH₃)₂₈, and dotetracontaether (42-ether) 5-(OCH₃)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 ¹H 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-(OCH₃)₂₈, 5-(OCH₃) ₄₂, and polymer 6-(OCH₃)_n, respectively. The corresponding polyarylmethyl polyradicals 3 and 4, and polymer 6 possess average values of S ≈ 6-7, S ≈ 10, and S ≈ 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 ≈ 6-7), annealing at room temperature for 0.5 h leads to a polyradical with S ≈ 5.