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

Two series of heavy alkaline earth metal pyrazolates, [M(Ph(2)pz)(2)(thf)(4)] 1a-c (Ph(2)pz = 3,5-diphenylpyrazolate, M = Ca, Sr, Ba; THF = tetrahydrofuran) and [M(Ph(2)pz)(2)(dme)(n)] (M = Ca, 2a, Sr, 2b, n = 2; M = Ba, 2c, n = 3; DME = 1,2-dimethoxyethane) have been prepared by redox transmetallation/ligand exchange utilizing Hg(C6F5)(2). Compounds 1a and 2b were also obtained by redox transmetallation with Tl(Ph(2)pz). 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(Me(2)pz)(2)(Me(2)pzH)(4)] 3a-c (M = Ca, Sr, Ba; Me(2)pzH = 3,5-dimethylpyrazole) were prepared by protolysis of [M{N(SiMe3)(2)}(2)(thf)(2)] (M = Ca, Sr, Ba) with Me(2)pzH in THF and by direct metallation with Me(2)pzH in liquid NH3/THF. Compounds 1a-c and 2a-c display eta(2)-bonded pyrazolate ligands, while 3a,b exhibit eta(1)-coordination. Complexes 1a-c have transoid Ph(2)pz ligands and an overall coordination number of eight with a switch from mutually coplanar Ph(2)pz 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 eta(1)-Me(2)pzH donors, which are hydrogen-bonded to the uncoordinated nitrogen atoms of the two trans Me(2)pz 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.