Synthesis, properties and structures of eight-coordinate zirconium(iv) and hafnium(iv) halide complexes with phosphorus and arsenic ligands

Abstract

Eight-coordinate [MX₄(L–L)₂] (M = Zr or Hf; X = Cl or Br; L–L = o-C₆H₄(PMe₂)₂ or o-C₆H₄(AsMe₂)₂) were made by displacement of Me₂S from [MX₄(Me₂S)₂] by three equivalents of L–L in CH₂Cl₂ solution, or from MX₄ and L–L in anhydrous thf solution. The [MI₄(L–L)₂] were made directly from reaction of MI₄ with the ligand in CH₂Cl₂ solution. The very moisture-sensitive complexes were characterised by IR, UV/Vis, and ¹H and ³¹P NMR spectroscopy and microanalysis. Crystal structures of [ZrCl₄{o-C₆H₄(AsMe₂)₂}₂], [ZrBr₄{o-C₆H₄(PMe₂)₂}₂], [ZrI₄{o-C₆H₄(AsMe₂)₂}₂] and [HfI₄{o-C₆H₄(AsMe₂)₂}₂] all show distorted dodecahedral structures. Surprisingly, unlike the corresponding Ti(IV) systems, only the eight-coordinate complex was found in each system. In contrast, the ligand o-C₆H₄(PPh₂)₂ forms only six-coordinate complexes [MX₄{o-C₆H₄(PPh₂)₂}] which were fully characterised spectroscopically and analytically. Surprisingly the tripodal triarsine, MeC(CH₂AsMe₂)₃, also produces eight-coordinate [MX₄{MeC(CH₂AsMe₂)₃}₂] in which the triarsines bind as bidentates in a distorted dodecahedral structure. There is no evidence for seven-coordination as found in some thioether systems.