Complexes with magnetic bistability are attracting significant attention for their potential applications in sensors, processing units and information storage devices. In that context, the classic rigid and air-stable bis(trispyrazolylborate) ligand system is particularly attractive, as it affords both spin crossover complexes (with Fe2+) and single molecule magnets (with Co2+ and some lanthanoids). We report here on a family of nitro and amino-functionalized complexes based on this framework. Direct exchange reactions between nitro-pyrazole and the tetrabutylammonium trispyrazolylborate (Tp-) ligand or unfunctionalized [Tp2Fe] and [Tp2Co] complexes gave us access to a variety of nitro-Tp ligands and nitro-complexes. Amino-functionalized derivatives could then be readily obtained by reduction of the nitro groups. We will show how the variation of the nature, position and number of the functionalities impacts the electronic and steric properties of the ligands and in turn the magnetic properties of the complexes in the solid state and in solution. The electronic and steric effects of the substitution can be disentangled by a systematic investigation of the properties by XRD, electrochemistry, IR, UV-vis spectrometry, SQUID and paramagnetic NMR. For each iron complex, we extrapolated the thermodynamic parameters—enthalpy and entropy of spin crossover, cooperativity— from variable temperature SQUID and UV-vis data, allowing us to decouple the position and abruptness of the spin transition. For the SMM cobalt complexes, we used SQUID AC measurements to investigate the magnetic relaxation: field-dependent Raman process were found to be dominating.

Synthesis and magnetic properties of some functionalized trispyrazolylborate complexes of iron and cobalt

ACS Fall 2021

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