The samples 1-5,c-d were studied at room temperature by Fe Mössbauer spectroscopy, and sample 5e was additionally analyzed by temperature-dependent measurements between 80 K and 300 K. As a local nuclear probe technique, Mössbauer spectroscopy offers a deeper insight on the oxidation and spin states of the iron centers and by this supports the interpretation of the magnetic measurements of composite materials, where the diamagnetic polymer matrix needs to be considered. With the exception of the as-synthesized sample 5e (see below), the room temperature Mössbauer spectra of all investigated compounds show only one distinct doublet with an isomer shift of δ = 0.93 mm s -1 -0.96 mm s -1 and a quadrupole splitting of ΔEQ = 2.15 mm s -1 -2.20 mm s -1 that clearly corresponds to a Fe(II) HS species. The spectra of all samples are presented in Figures S18-S22 and Figure along with the determined isomer shift δ, the quadrupole splitting ΔEQ and the line width Γ (Table and Table ).