Exploratory Synthetic Studies of the Praseodymium/Di-2-Pyridyl Ketoxime System Leads to Unusual Reactivity and Interesting New Molecules

Reactions of lanthanoid(III) ions with di-2-pyridyl ketoxime, dpkoxH, have been studied. Full synthetic investigation of the Pr(NO₃)₃·6H₂O/dpkoxH reaction system has provided access to complexes [Pr₂^III(NO₃)₄(L)₂] (isolated as MeCN and MeNO₂ solvates; <b>1</b> and <b>2</b>), [Pr₄^III(OH)₂(NO₃)₄(dpkox)₆(EtOH)₂] (<b>3</b>) and [Pr₈^IIIPr^IVO₄(OH)₄(NO₃)₄(dpkox)₁₂(H₂O)₄] (<b>4</b>). A novel Pr^III-assisted/promoted ligand transformation has occurred in the dinuclear complexes, where L^- is the anion of di(pyridin-2-yl)methanone <i>O</i>-(1-hydroxy-1,1-di(pyridin-2-yl)) methyl oxime (HL). Mechanistic schemes have been proposed. In <b>1</b> and <b>2</b>, the two Pr^III centers are bridged by two deprotonated oxygen atoms of two "head-to-head" 2.2011110 (Harris notation) L^- ligands. The tetranuclear molecule <b>3</b> is held together by two μ₃-hydroxido groups, two 2.2110, two 2.1110 and two 3.2110 dpkox^- ligands. The {Pr₄^III(μ₃-OH)₂}¹⁰⁺ unit of the {Pr₄^III(μ₃-OH)₂(μ₂-oximato)₄}⁶⁺ core comprises four coplanar Pr^III ions in a "butterfly"-type disposition. In the highly symmetrical cluster <b>4</b>, the nine Pr ions are held together by four μ₃-O^2- and four μ₃-OH^- groups, as well as four 2.1110, four 3.2111 and four 2.2110 dpkox^- ligands. The inorganic {Pr₈^IIIPr^IV(μ₃-O)₄(μ₃-OH)₄}¹⁶⁺ unit of the core can be described as four fused "butterflies", the Pr^IV center being the common site at one of their "body" positions. The X-band (∼9.5 GHz) EPR spectrum of <b>4</b> at 4.2 K shows a sharp peak at 630 G, a derivative feature at 1340 G, and a multitude of weaker signals; the data are compatible with the presence of the half-integer spin expected for the 4f¹ Pr^IV center.