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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 −6.0 K 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 J2/k=14.1 K) between two coordinating CuII ions and DR1 were revealed. In [{Cu(hfac)2}2(DR1)(H2O)][Cu(hfac)2(H2O)], the very strong and antiferromagnetic (J/k=−416.1 K) exchange interaction between one of the coordinating CuII ions and DR1 is caused by the very short equatorial Cu[BOND]O bond length (1.962 Å).