Contrasting binding modes in lanthanoid pyrazolate and pseudo-pyrazolates prepared by oxidation of lanthanoid(II) complexes and lanthanoid metals with thallium(I) reagents

Article abstract of DOI:10.1021/om990932f

Oxidation of Yb(C₅Me₅)₂ with [Tl(Ph₂pz)] and [Tl(azin)] (Ph₂pz = 3,5-diphenylpyrazolate, azin = 7-azaindolate) yields [Yb(C₅Me₅)₂(Ph₂pz)] (1) and [Yb(C₅Me₅)₂(azin)] (2). X-ray crystal structures of 1 and 2 reveal monomeric eight-coordinate complexes with η²-Ph₂pz or azin ligands. Treatment of neodymium metal with [Tl(Ph₂pz)] yields [Nd(Ph₂pz)₃(dme)₂] (3), which is a nine-coordinate monomer with three η²-pyrazolate ligands and a chelating and a unidentate dme ligand. Oxidation of Sm(C₅Me₅)₂ with [TI(1,4,2-P₂SbC₂Bu^t₂)]/[TI(1,2,4-P₃C₂ Bu^t₂)] (approximately 4:1) gave a mixture of [Sm(C₅Me₅)₂(1,2,4-P₃C₂Bu^t₂)] (4) and [Sm(C₅Me₅)₂(1,4,2-P₂SbC₂Bu^t₂)]. The former was shown to be a nine-coordinate monomer with novel pseudo-pyrazolate η²-(P₂)-[1,2,4-P₃C₂Bu^t₂] coordination of the triphosphacyclopentadienide ligand. After metathesis routes to Ln(1,4,2-P₂SbC₂Bu^t₂)_n complexes failed, redox transmetalation between ytterbium metal and [Tl(1,4,2-P₂EC₂Bu^t₂)] (E = Sb, P) containing a substantial impurity of [Li(tmeda)₂][1,4,2-P₂EC₂Bu^t₂] yielded the first lanthanoid diphosphastibacyclopentadienide complex in a mixture with [1,2,4-P₃C₂Bu^t₂] species. An X-ray investigation of single crystals containing an isomerically diverse mixture enabled the structure of [Li(thf)₄]-[Yb(1,4,2-P₂SbC₂Bu^t₂)₃] (5) to be determined, and it was shown to contain a π-η² and two η⁵-[1,4,2-P₂SbC₂Bu^t₂] ligands. In the series 1, 4, and 5, the η²-ring varies from edge-on to side-on, reflecting a progression from σ to π coordination. [Li(tmeda)₂][1,4,2-P₂SbC₂Bu^t₂] (6) has been successfully crystallized from a [Li(tmeda)₂][P₂EC₂Bu^t₂] mixture and structurally characterized.