C(sp²)-coupled nitronyl nitroxide and iminonitroxide diradicals

Spin-labelled compounds are widely used in chemistry, physics, biology and the materials sciences but the synthesis of stable high-spin organic molecules is still a challenge. We succeeded in synthesising heteroatom analogues of the 1,1,2,3,3-pentamethylenepropane (PMP) diradicals with two nitronyl nitroxide (DR¹) and with two iminonitroxide (DR²) fragments linked through the C(sp²) atom of the nitrone group. According to magnetic susceptibility measurements, EPR data and ab initio calculations at the (8,6)CASSCF and (8,6)NEVPT2 levels, DR¹ and DR² have singlet ground states. The singlet-triplet energy splitting (2J) is low (J/k=-7.4 for DR¹ and -6.0K for DR²), which comes from the disjoint nature of these diradicals. The reaction of [Cu(hfac)₂] with DR¹ gives rise to different heterospin complexes in which the diradical acts as a rigid ligand, retaining its initial conformation. For the [{Cu(hfac)₂}₂(DR¹)(H₂O)] complex, sufficiently strong ferromagnetic interactions (J₁/k=42.7 and J ₂/k=14.1K) between two coordinating Cu^II ions and DR ¹ were revealed. In [{Cu(hfac)₂}₂(DR ¹)(H₂O)][Cu(hfac)₂(H₂O)], the very strong and antiferromagnetic (J/k=-416.1K) exchange interaction between one of the coordinating Cu^II ions and DR¹ is caused by the very short equatorial Cuï-O bond length (1.962Å). Playing with spins: New C(sp²)-spaced nitronyl nitroxide and iminonitroxide diradicals are reported. The exciting feature of these diradicals inherent in their nature is the very low singlet-triplet energy splitting. A small difference in the structure of Cu(hfac)₂ complexes with the nitronyl nitroxide diradical gives rise to essentially different exchange interactions between the copper and nitroxide spins (see scheme; hfac=hexafluoroacetylacetonate).