New Homoleptic Rare Earth 3,5‐Diphenylpyrazolates and 3,5‐Di‐tert‐butylpyrazolates and a Noteworthy Structural Discontinuity

Direct thermally induced reactions between rare earth metals (Ln = Y,Ce, Dy, Ho, and Er) activated by Hg metal and 3,5‐diphenylpyrazole (Ph₂pzH) or 3,5‐di‐tert‐butylpyrazole (tBu₂pzH) yielded either homoleptic complexes Ln_n(R₂pz)_3n] or a heteroleptic complex Ln(Ph₂pz)₃(Ph₂pzH)₂] From Ph₂pzH, Ce₃(Ph₂pz)₉], Dy₂(Ph₂pz)₆], Ho₂(Ph₂pz)₆], and Y(Ph₂pz)₃(Ph₂pzH)₂] were isolated. The first has a bowed trinuclear Ce₃ backbone with two η² pyrazolate ligands on the terminal metal atoms and one on the middle, and bridging by both μ‐η²:η² and μ‐η²:η⁵ ligands between the terminal and the central Ce atoms. Although both the Dy and Ho complexes are dinuclear, the former has the rare μ‐η²:η¹ bridging whilst the latter has μ‐η²:η² bridging. Thus the dysprosium complex is seven‐coordinate and the holmium is eight‐coordinate, in contrast to any correlation with Ln³⁺ ionic radii, and the series has a remarkable structural discontinuity. The heteroleptic Y complex is eight coordinate with three chelating Ph₂pz and two transoid unidentate Ph₂pzH ligands. From tBu₂pzH, dimeric Ln₂(tBu₂pz)₄] (Ln = Ce, Er) were isolated and are isomorphous with eight coordinate Ln atoms ligated by two chelating terminal tBu₂pz and two μ‐η²:η² tBu₂pz donor groups. They are also isomorphous with previously reported La, Nd, Yb, and Lu complexes.