The Synthesis, Structures and Polymorphism of the Dimeric Trivalent Rare-Earth 3,5-Dimethylpyrazolate Complexes [Ln(Me2pz)3(thf)]2

A variety of rare-earth 3,5-dimethylpyrazolate (Me2pz) complexes have been synthesised by (i) the direct reaction of Hg-activated metal with Me2pzH as a pro-ligand at elevated temperatures, (ii) by redox transmetalation/protolysis with the lanthanoid element, Hg(C6F5)2, and Me2pzH, and (iii) by protolysis of tris[bis(trimethylsilyl)amido]cerium(III) with Me2pzH. Each product, upon crystallisation from tetrahydrofuran (thf), formed a dimeric complex, [Ln(Me2pz)3(thf)]2 (Ln = La, Ce, Pr, Nd, Ho, Yb, or Lu). Despite the common formulation, two completely different structures were observed in two distinct crystallographic "domains of existence", together presumptively spanning the gamut of Ln and Y. For the larger rare-earth ions (La–Pr), there are two terminal η2-Me2pz ligands and one thf donor on each Ln atom, with the metal atoms being linked by a pair of bridging pyrazolate ligands of an uncommon type (η2:η5), resulting in formal ten-coordination. A Me2pzH complex [Ce(Me2pz)3(Me2pzH)], although not isomorphous, has a similar structure. For the smaller rare-earth elements (Nd–Lu), the bridging is entirely different, with two μ-κ1(N):κ1(N) pyrazolate and two unusual bridging thf ligands. Each Ln atom also has two chelating Me2pz ligands, resulting in formal eight-coordination. Crystallisation of [Nd(Me2pz)3(thf)]2 from pyridine yields monomeric, nine-coordinate [Nd(Me2pz)3(pyridine)3] with only chelating Me2pz ligands