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 [Pr₂III(NO₃)₄(L)₂] (isolated as MeCN and MeNO₂ solvates; 1 and 2), [Pr₄III(OH)₂(NO₃)₄(dpkox)₆(EtOH)₂] (3) and [Pr₈IIIPrIVO₄(OH)₄(NO₃)₄(dpkox)₁₂(H₂O)₄] (4). A novel PrIII-assisted/promoted ligand transformation has occurred in the dinuclear complexes, where L⁻ is the anion of di(pyridin-2-yl)methanone O-(1-hydroxy-1,1-di(pyridin-2-yl)) methyl oxime (HL). Mechanistic schemes have been proposed. In 1 and 2, the two PrIII centers are bridged by two deprotonated oxygen atoms of two "head-to-head" 2.2011110 (Harris notation) L⁻ ligands. The tetranuclear molecule 3 is held together by two μ₃-hydroxido groups, two 2.2110, two 2.1110 and two 3.2110 dpkox⁻ ligands. The {Pr₄III(μ₃-OH)₂}¹⁰⁺ unit of the {Pr₄III(μ₃-OH)₂(μ₂-oximato)₄}⁶⁺ core comprises four coplanar PrIII ions in a "butterfly"-type disposition. In the highly symmetrical cluster 4, the nine Pr ions are held together by four μ₃-O²⁻ and four μ₃-OH⁻ groups, as well as four 2.1110, four 3.2111 and four 2.2110 dpkox⁻ ligands. The inorganic {Pr₈IIIPrIV(μ₃-O)₄(μ₃-OH)₄}¹⁶⁺ unit of the core can be described as four fused "butterflies", the PrIV center being the common site at one of their "body" positions. The X-band (∼9.5 GHz) EPR spectrum of 4 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 PrIV center.