N,O- and S,O-bonded Hg(II) complexes with S-substituted amino acids: synthesis and crystal structures of bis(S-methyl-L-cysteinato)mercury(II) and bis(D,L-ethioninato)-mercury(II) diperchlorate

Abstract

Complexes Hg[CH3SCH2CH(NH2)COO]2 and Hg[CH3CH2SCH2CH2CH(NH3COO]2(ClO4)2 obtained from reactions of yellow HgO with S-methyl-L-cysteine and ethionine respectively have been characterized by X-ray diffraction methods. The former is monoclinic, space group P21, with a = 13.790(4), b = 4.994(1), c = 9.898(2) Å, and β = 106.20(2)°. The R and Rw factors were 0.063 and 0.075 respectively for 980 independent reflections. The S-methyl-L-cysteine complex is N,O bonded with two Hg—N bond lengths of 2.16(3) and 2.13(3) Å and [Formula: see text]. There are eight [Formula: see text] distances within the range of 2.54–3.75 Å. The three shortest of these are distributed in an equatorial plane with respect to the N—Hg—N moiety. The Hg—O bond length of 2.54(2) Å is associated with the chelate ligand.The ethionine complex is triclinic, space group [Formula: see text] with a = 11.376(7), b = 12.009(8), c = 9.495(6) Å, α = 94.74(6), β = 108.01(5), and γ = 71.78(5)°. The R and Rw values for all 3526 reflections were 0.083 and 0.10 respectively but R became 0.058 for only observed reflections (I > 3 σ(I)). The Hg2+ is bonded to two sulfur, Hg—S = 2.436(5) and 2.537(4) Å, and two oxygen, Hg—O = 2.28(1) and 2.259(9) Å, atoms in a distorted tetrahedron. The Hg2+–carboxylate interactions are stronger in the S-bonded than the N-bonded complex probably indicating a preference for Hg–amine coordination. At low pH when the amino group is protonated, Hg–thioether coordination becomes important for S-substituted amino acids.