Isolation of four-/five-coordinate manganese( ii )-bis(dithiolene-radical) complexes: exchange coupling

Dithiolene ligands have long fascinated chemists for their non-innocent redox properties. The transition metal complexes with delocalized unpaired electrons on the ligand find applications in catalysis, magnetism, and molecular electronics. Despite extensive studies on Fe, Co, Ni, Pd, Pt, Mo, and W congeners featuring monoanionic dithiolene radicals, first-row analogues, particularly manganese with its versatile MnII redox chemistry, remain elusive due to synthetic challenges in accessing the radical centre in the dithiolene ligand, which favours over-reduction to dianions during metalation. Herein, we present two elusive manganese(ii)-bis(dithiolene) diradical complexes, synthesised via a protocol that selectively generates and stabilises the monoanionic radical state, five-coordinate [Mn(ii)(LS˙−)2(THF)] (1), alongside a four-coordinate tetrahedral [Mn(ii)(LSe˙−)2] (2). They are unprecedented to date. Single-crystal X-ray diffraction reveals that the coordination geometries of Mn(ii) ions are distorted square-pyramidal and tetrahedral in 1 and 2, respectively. These complexes were characterized by UV-vis, IR, Raman, XPS, and EPR spectroscopy. ... mehrThe local spin state of Mn(ii) ions, the spin ground states of the complexes and the nature of Mn(ii)–radical interactions were studied by temperature-dependent magnetic susceptibility measurements. Combined magnetic and EPR measurements suggest that the central Mn(ii) ion of this series of Mn(ii)-bis(dithiolene) diradical complexes (1 and 2) most likely possesses SMn2+ = 5/2 (high spin). The central Mn(ii) ions of 1 and 2 are antiferromagnetically coupled with two unpaired electrons on the dithiolene ligands, leading to spin ground states ST = 3/2. Broken-symmetry DFT and ab initio CASSCF calculations reproduce this spin ground state and quantify the three exchange interactions within the radical–Mn(ii)–radical core. For 2, the CASSCF Mn–radical coupling (−44.8 cm−1) agrees with the value fitted to the magnetic data (−44.01 cm−1), and the direct radical–radical interaction is found to be negligible in tetrahedral 2 but substantial in square-pyramidal 1.