Luigi Alvarado - Supramolecular assemblies prepared from an iron(II) tripodal complex, tetrafluoroborate, and alkali cations. The effect of cations size on coordination number, anion disorder and hydrogen bonding

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      Publication Details (including relevant citation   information):

      Alvarado, L.; Brewer, C.; Brewer, G.; Butcher, R.J.; Straka, A.;   Viragh, C. CrystEngComm 11 (2009) 2297-2307

      Abstract:

      The iron(II) complex cation, 12+, of the 1 : 3 Schiff base   condensate of tris(2-aminoethyl)amine

      (tren) with imidazole-2-carboxaldehyde (H3L1) was reacted with   MBF4 (M ¼ Na, K, Rb, Cs

      and NH4). The products were double salts of the formula   {[FeH3L1](BF4)2}$MBF4, 1[M(BF4)3].

      The complexes have been characterized by EA, IR, X-ray   crystallography, and M€ossbauer

      spectroscopy. The resulting complexes crystallize in P-3 and the   cations are located at the origin (0, 0, 0) and c/2 (0, 0, 0.5)   on the c axis. Six tetrafluoroborate anions surround the cation   sites and each binds to the sodium in a monodentate fashion and   to the other cations in a bidentate fashion resulting in   distorted octahedral and icosahedral complexes, respectively.   Disorder is observed in the tetrafluoroborate anion of both   1[Na(BF4)3] and 1[Cs(BF4)3], resulting in two sets of fluorine   atom positions (F set A and F set B), but is absent in the   ammonium, potassium, and rubidium double salts. The disorder is   attributed to coordination number preference and poor size match,   respectively. The tetrafluoroborate anions are involved in   extensive hydrogen bonding with the imidazole NH and imine CH of   the iron complex. The hydrogen bonding arrangements observed with   potassium, rubidium, cesium (F set A) and ammonium are almost   identical with one another, but are altered in 1[Na(BF4)3] and   1[Cs(BF4)3] (F set B) complexes. These effects are explained on   the basis of cation size. The formation of the double salt   results in a shift in the spin equilibrium of the parent iron(II)   complex from 62% high spin: 38% low spin at 295 K to pure low   spin in the double salts.

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