Structural and magnetic properties of semiquinonate based Al(iii) and Ga(iii) complexes

The reaction of anhydrous MCl3 (M = Al(III) or Ga(III)) with one-electron-reduced 3,5-di-tert-butyl-1,2-ortho- benzoquinone (using metallic sodium) led us to isolate two distinct metal complexes of Al(III) and Ga(III), which were structurally and magnetically characterized. Complex 1 crystallized in the monoclinic P2(1)/n space group, whereas 2 crystallized in the triclinic P (1) over bar space group. Interestingly, whereas the Al(III) derivative was obtained as a dimer with the molecular formula [Al-2(mu-HL-)(2)(L-)(4)] (1) (where L- is a semiquinonate radical and HL- is a monoanionic catecholate ligand), the Ga(III) derivative crystallized as [Ga (L-)(3)] (2), which is a polymorph of a previously reported complex. The presence of both catecholate and/or semiquinonate ligands in 1 and 2 was confirmed by single-crystal X-ray diffraction, mass spectrometry, and NMR and infrared spectroscopy techniques. The crystalline phase purity of the complexes was confirmed by powder X-ray diffraction (PXRD). Measurements of direct-current magnetic susceptibility, which were performed on a polycrystalline samples, revealed that in both complexes the semiquinonate radical anions are coupled ferromagnetically via the diamagnetic metal ion. The magnetism data of both complexes were modelled using the Heisenberg-Van Vleck-Dirac (HDVV) Hamiltonian, and the extracted parameters are consistent with the literature reports. The details of the electronic structures of the ground states of 1 and 2 were further investigated via X-band (ca. 9 GHz) electron paramagnetic resonance (EPR). The EPR spectrum of 2 could be reproduced by considering a quartet ground state with zero-field splitting and hyperfine coupling, whereas attempts to simulate all the EPR spectral features observed in a frozen solution of 1 by assuming it was a pure phase failed. A correct simulation required the simultaneous inclusion of contributions from a quartet and a triplet state. This evidently suggests that the dimeric complex of 1 is in equilibrium with a monomeric [Al(L-)(3)] complex in solution.