Heavy alkaline earth metal pyrazolates: Synthetic pathways, structural trends, and comparison with divalent lanthanoids

Two series of heavy alkaline earth metal pyrazolates, [M(Ph ₂pz)₂(thf)₄] 1a-c (Ph₂pz: = 3,5-diphenylpyrazolate, M = Ca, Sr, Ba; THF = tetrahydrofuran) and [M(Ph ₂pz)₂(dme)_n] (M = Ca, 2a, Sr, 2b, n = 2; M = Ba, 2c, n = 3; DME = 1,2-di-methoxyethane) have been prepared by redox transmetallation/ligand exchange utilizing Hg(C₆F₅) ₂. Compounds 1a and 2b were also obtained by redox transmetallation with Tl(Ph₂pz). Alternatively, direct reaction of the alkaline earth metals with 3,5-diphenylpyrazole at elevated temperatures under solventless conditions yielded compounds 1a-c and 2a-c upon extraction with THF or DME. By contrast, [M(Me₂pz)₂(Me₂pzH)₄] 3a-c (M = Ca, Sr, Ba; Me₂pzH = 3,5-dimethylpyrazole) were prepared by protolysis of [M{N(SiMe₃)₂}₂(thf)₂] (M = Ca, Sr, Ba) with Me₂pzH in THF and by direct metallation with Me₂pzH in liquid NH₃/THF. Compounds 1a-c and 2a-c display η²-bonded pyrazolate ligands, while 3a,b exhibit η¹-coordination. Complexes 1a-c have transoid Ph₂pz ligands and an overall coordination number of eight with a switch from mutually coplanar Ph₂pz ligands in 1a,b to perpendicular in 1c. In eight coordinate 2a,b the pyrazolate ligands are cisoid, whilst 2c has an additional DME ligand and a metal coordination number of ten. By contrast, 3a,b have octahedral geometry with four η¹-Me₂pzH donors, which are hydrogen-bonded to the uncoordinated nitrogen atoms of the two trans Me ₂pz ligands. The application of synthetic routes initially developed for the preparation of lanthanoid pyrazolates provides detailed insight into the similarities and differences between the two groups of metals and structures of their complexes.