Preparation and structures of coordination complexes of the very hard Lewis acids ZrF _4 and HfF _4

摘要

[MF _4(dmso) _2] (M = Zr or Hf) and [MF _4(dmf) _2], prepared by dissolving MF _4· nH _2O in the appropriate solvent, have been used as synthons for a range of complexes of these otherwise intractable tetrafluorides. These reagents react with OPR _3 (R = Me or Ph) or OAsPh _3 (L) in anhydrous CH _2Cl _2 to form six-coordinate [MF _4L _2] which exist as a mixture of cis (predominant form) and trans isomers in CH _2Cl _2 solution but which crystallise as trans (OPPh 3, OAsPh 3) or cis (OPMe _3) forms. Cis-[ZrF _4(OAsPh _3) _2] crystals were obtained from MeCN. Cis-[MF _4(pyNO) _2] and eight-coordinate (distorted dodecahedral) [MF _4(L-L) _2] (L-L = 2,2′-bipy, or 1,10-phen), and [MF _4(Me _4-cyclam)] were also obtained. Attempts to prepare complexes with the N-heterocyclic carbene, 1,3-(2,6-di-isopropylphenyl)imidazol-2-ylidene (IDiPP) or alkyl diphosphines were unsuccessful. Crystal structures are reported for trans-[ZrF _4(OPPh _3) _2], cis- and trans-[ZrF _4(OAsPh _3) _2], cis-[HfF _4(OPMe _3) _2], [ZrF _4(2,2′-bipy) _2], cis-[HfF _4(dmf) _2], and geometric isomers (both pentagonal bipyramidal) of [(dmso) _2F _3M(μ-F) _2MF _3(dmso) _2]. The failed attempts to make IDiPP adducts led to crystals of [IDiPPH] _3[M _3F _(15)] containing discrete anions based upon a triangle of M atoms with single F bridges. The results are compared with previous work on TiF _4 adducts and with complexes of MCl _4, and demonstrate that the MF _4 are very hard Lewis acids, with a marked preference for O- over N-donors.