Heavy Alkaline Earth Metal Pyrazolates: Synthetic Pathways, Structural Trends, and Comparison with Divalent Lanthanoids

• Published in: Chemistry : a European journal 2004-07, Vol.10 (13), p.3315-3323
• Main Authors: Hitzbleck, Julia, O'Brien, Anna Y., Forsyth, Craig M., Deacon, Glen B., Ruhlandt-Senge, Karin
• Format: Article
• Language: English
• Subjects: alkaline earth metals; coordination modes; lanthanides; N ligands; structure elucidation; synthetic methods
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.200400076

Two series of heavy alkaline earth metal pyrazolates, [M(Ph2pz)2(thf)4] 1 a–c (Ph2pz=3,5‐diphenylpyrazolate, M=Ca, Sr, Ba; THF=tetrahydrofuran) and [M(Ph2pz)2(dme)n] (M=Ca, 2 a, Sr, 2 b, n=2; M=Ba, 2 c, n=3; DME=1,2‐dimethoxyethane) have been prepared by redox transmetallation/ligand exchange utilizing Hg(C6F5)2. Compounds 1 a and 2 b 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 1 a–c and 2 a–c upon extraction with THF or DME. By contrast, [M(Me2pz)2(Me2pzH)4] 3 a–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 1 a–c and 2 a–c display η2‐bonded pyrazolate ligands, while 3 a,b exhibit η1‐coordination. Complexes 1 a–c have transoid Ph2pz ligands and an overall coordination number of eight with a switch from mutually coplanar Ph2pz ligands in 1 a,b to perpendicular in 1 c. In eight coordinate 2 a,b the pyrazolate ligands are cisoid, whilst 2 c has an additional DME ligand and a metal coordination number of ten. By contrast, 3 a,b have octahedral geometry with four η1‐Me2pzH donors, which are hydrogen‐bonded to the uncoordinated nitrogen atoms of the two trans Me2pz 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. The synthesis of families of heavy alkaline earth metal pyrazolates using metal‐based preparative methods developed for rare earth metals allows the direct comparison of the reactivity between the two groups of metals. While the alkaline earth metals appear to be more reactive than the europium, samarium, and ytterbium counterparts, as also evidenced by their increased electropositive character, the structural features between the two groups of metals are clearly related (two examples of such compounds are illustrated here).