Five-coordinated halogenomercurate(II) complexes: crystal structures of bis(L-tryptophanium) trichloromercurate(II) and bis(D,L-homocysteine thiolactonium) tetrabromomercurate(II)

Abstract

Reactions of L-tryptophan and D,L-homocysteine with mercury(II) halides in acidic ethanol/water solutions result in the formation of bis(L-tryptophanium) trichloromercurate(II) (TPTC) and bis(D,L-homocysteine thiolactonium) tetrabromomercurate(II) (HTTB) respectively. Their crystal structures consist of halide-bridged polymeric chains of quasi HgX3− units. The coordination geometry around Hg(II) in both anions is distorted trigonal bipyramidal. There are no chemical bonds between the amino acid cations and the mercury(II) ions. TPTC is monoclinic, space group P21, with a = 16.688(5), b = 5.626(1), c = 14.022(5) Å, β = 108.17(3)°. The structure was solved and refined from 2433 independent counter measured reflections to an R factor of 0.062. The two independent tryptophan molecules per asymmetric unit form a single cation via a strong hydrogen bond between the carboxylate groups. Thus the compound can be represented by MHgCl3 where M is the cation. One of the three Cl− ions is strongly bonded to one Hg(II) ion and weakly bonded to two other Hg(II) ions. In addition to the three short Hg—Cl distances of 2.403(3), 2.376(4), and 2.614(4) Å, there are two long Hg—Cl bonds of 2.833(4) and 2.888(4) Å to each Hg(II), from the weakly bridged chloride ions. HTTB is orthorhombic, space group Pnma with a = 8.068(3), b = 24.985(7), and c = 8.828(3) Å. The structure was refined to an R value of 0.066 for 819 observed reflections (I ≥ 3σ(I)), however, inclusion of unobserved reflections did not change the parameters. The homocysteine free base used in the reaction was lactonised under the experimental conditions to a thiolactonium ion. There are three relatively short Hg—Br interactions (2.566(3) twice, 2.662(4) Å) within the distorted trigonal bipyramidal stereochemistry of each mercury atom and two longer Hg—Br bonds (2.917(6) and 3.087(5) Å). The polymeric, anion chains arrange in layers on the mirror planes (y = ±1/4) with these layers sandwiched by two layers of cations.