Functionalization of Nitronyl-Nitroxide Ligands via Schiff Condensations: Rational Synthesis of Binuclear Complexes with Bis-Nitronyl-Nitroxide Ligands

Starting from 2,6-diformyl-<i>p</i>-cresol, only one formyl group has been transformed into the mono nitronyl-nitroxide radical, <b>HL</b>^<b>2</b> (2-(2-hydroxy-3-formyl-5-methylphenyl)-4,4,5,5-tetramethyl-4,5-dihydro-1<i>H</i>-imidazolyl-1-oxyl 3-oxide). The presence of the formyl group in <b>HL</b>^<b>2</b> allows further functionalization using Schiff condensation reactions. <b>HL</b>^<b>2</b> has been reacted with 1,3-diaminopropane, benzylamine, and 2-picolylamine, generating a family of three new ligands, <b>H</b>_<b>2</b><b>L</b>^<b>4</b>, <b>HL</b>^<b>5</b>, and <b>HL</b>^<b>6</b>, with <b>H</b>_<b>2</b><b>L</b>^<b>4</b> being a biradical. Using <b>HL</b>^<b>2</b> and <b>H</b>_<b>2</b><b>L</b>^<b>4</b> as ligands, five binuclear complexes, [Zn₂(L²)₂(hfac)₂] (<b>1</b>) and [M₂(L⁴H₂)­(hfac)₄], where M = Mn (<b>2</b>), Co (<b>3</b>), Ni (<b>4</b>), and Zn (<b>5</b>), hfac^– = hexafluoroacetylacetonate ligand, have been obtained. The crystal structures of complexes <b>1</b>–<b>5</b> have been solved. Compound <b>1</b> presents two polymorphs. The metal ions from compounds <b>2</b>–<b>5</b> adopt an octahedral geometry, being coordinated by phenoxido and aminoxyl oxygen atoms from the ligand and four oxygen atoms from the hfac^– ligand. The cryomagnetic investigation of the three similar compounds, <b>2</b>–<b>4</b>, reveals relatively strong antiferromagnetic M^II-Rad interactions (<i>J</i>_MnRad = −82 cm^–1, <i>J</i>_CoRad = −27.6 cm^–1, and <i>J</i>_NiRad = −185 cm^–1, with <i>H</i> = −2<i>JS</i>_M<i>S</i>_Rad).