A Three-dimensional Diradical Metallacage With Open-shell Singlet State

In recent years,the marriage of organic radicals and supramolecular chemistry has promoted the vigorous development of supramolecular radical chemistry.The magnetic,photochemical and other excellent properties of supramolecular radical compounds have attracted the research interest of theoretical and synthetic chemists.Unfortunately,the research on supramolecular radical systems are still rare and immature,in particular,the supramolecular coordination radical systems are relatively rare.Given that the absolutely instability of organic radicals and the lack of suitable coordination pathways,the construction of supramolecular coordination complexes（SCCs）,especially radical metallacages,still remains a great challenge.Herein,the neutral or diradical metallacages M1 and M2 with polychlorinated triphenylmethyl tricarboxylate（PTMTC）were successfully prepared through a coordination-driven self-assembly approach.The first part of this paper is the assembly and characterization of metallacages M1 and M2.Firstly,the building blocks of M1 and M2 were synthesized,such as neutral ligand（αH-PTMTC）Na₃,radical ligand（PTMTC^·）Na₃,Pt（Ⅱ）receptor cis-Pt（PEt₃）₂（OTf）₂,etc.Then,the neutral metallacage M1 and diradical meallacage M2 were assembled by neutral ligand or radical ligand with 4,4′-bipyridine and Pt（Ⅱ）receptor in a 2:3:6 molar ratio.Subsequently,M1 and M2 were characterized by nuclear magnetic resonance spectroscopy（NMR）,electrospray ionization time-of-flight mass spectrometry（ESI-TOF-MS）,elemental analysis,and electron paramagnetic resonance spectroscopy（EPR）,which proved the successful assembly of M1 and M2.It is worth noting that ¹H NOESY indicated that there is a NOE effect between Pt（Ⅱ）receptor and 4,4′-bipyridine,suggesting they are very close in space and directly given the local molecular structure information of M1 and M2In the second part of this paper,the properties of diradical metallacage M2 were studied.We studied the diffusion coefficient and particle size distribution,photochemical properties,stability,magnetic properties and cytotoxicity of M2.The reasonable diffusion coefficient and particle size distribution of M2 were obtained by two-dimensional diffusion order spectroscopy（2D DOSY）and dynamic light scattering（DLS）.The Ultraviolet and visible（UV-Vis）spectra showed that the absorption intensity of M2 was stronger than that of its radical precursor,and the characteristic absorptions of radical were observed.¹H NMR,DLS and UV-Vis measurements of time and temperature show that M2 is absolutly stable.The large spectral width,the forbidden transition signal and the spin parameters of the solid-state electron paramagnetic resonance（EPR）spectra of M2 indicated an intramolecular spin-spin interaction,and the negative coupling constant prove that the ground state of this diradical is an open-shell singlet state.superconducting quantum interference device（SQUID）measurements confirmed that M2 is an open-shell singlet diradical with a small single‐triplet energy gap.This study developed a new radical system,which is helpful for studying the intramolecular spin-spin interaction in three-dimensional organic radical metalacages.