An Unexpected Family of Heterometallic Rings {Cr₆M₂} with 3,5-bis(trifluoromethyl)benzoate

Three heterometallic rings, [Pr2NH2][(Pr2NH2)CrIII6MII2F8(BTFB)16] (M is Ni, Co or Zn, BTFBH = 3,5-bis(trifluoromethyl)benzoic acid) are reported, templated about a di-n-propylammonium cation. The ring contains six Cr(III) and two M(II) at the corners of an octagon, with the divalent metal ions disordered around the ring. Each edge of the octagon is bridged by a fluoride and two bridging BTFB– ligands. One of the two ammonium cations in the crystal structure is located at the centre of the octagonal ring, and the other outside the ring. Two divalent metals in the ring leads to the presence of isomers which can be seen in the magnetic and EPR studies for MII = Ni or Zn, and 19F NMR spectra for MII = Co. The data are interpreted as due to an approximately 50:50 mixture of isomers where the divalent ions are at the 1,5- and 1,2- positions in the ring. A different reaction was pursued which gave [(Pr2NH2)][CrIII7MIIF8(BTFB)16] rings; these could not be fully characterised by crystallography but their formation is confirmed by elemental analysis, mass spectrometry, NMR spectroscopy (for M = Co), magnetic studies and EPR spectroscopy (for M = Ni, Zn).