Crystal Structure of Bis(N,N-dimethylthioformamide)mercury(II) Iodide, Bromide, and Chloride and Solvation of the Neutral Mercury(II) Halide Complexes in N,N-Dimethylthioformamide, a Resonance-Induced Hydrogen-Bonding Solvent and Ligand

Abstract

The isostructural solvated mercury(II) halides HgI2(SCHN(CH3)2)2, HgBr2(SCHN(CH3)2)2, and HgCl2(SCHN(CH3)2)2 are by single-crystal X-ray diffraction methods found to crystallize in the monoclinic space group P21/n (No. 14) with Z = 4 and the unit cell parameters a = 9.757(1), b = 15.546(3), c = 10.416(2) Å and β = 104.47(1)° for the iodide, a = 9.406(2), b = 15.208(3), c = 10.226(2) Å and β = 104.66(1)° for the bromide, and a = 9.193(2), b = 15.094(2), c = 10.121(2) Å and β = 105.10(1)° for the chloride. Discrete mercury(II) complexes with pseudotetrahedral coordination are held together in layers by weak intermolecular interactions. An intramolecular hydrogen bond between one halide and the −CHS group of an N,N-dimethylthioformamide ligand makes the two mercury−halide bond lengths significantly different with 2.762(1) and 2.724(1) Å for the iodide, 2.619(3) and 2.565(3) Å for the bromide, and 2.509(5) and 2.452(5) Å for the chloride compound. Large-angle X-ray scattering of a saturated HgI2 solution in N,N-dimethylthioformamide shows HgI2(SCHN(CH3)2)2 complexes to be formed with the Hg−I bond length 2.740(8) Å. The vibrational mercury−halide stretching frequencies are used to discuss correlations between bond distances and strengths. Raman and infrared vibrational spectra of N,N-dimethylthioformamide ligands in zinc, cadmium, and mercury complexes show the shifts of the C−S and C−N vibrational frequencies to be useful indicators of the metal ion-sulfur bond strength. Calorimetric measurements give strongly exothermic heats of solvation of HgI2 and HgBr2 in liquid N,N-dimethylthioformamide, −146 and −156 kJ·mol-1, respectively. From the corresponding heat of solvation of HgI2 and HgBr2 by N,N-dimethylthioformamide in dilute benzene solution the enthalpy contribution from the resonance-induced C−H···S hydrogen bonding in liquid N,N-dimethylthioformamide is estimated to be about 12 kJ·mol-1. The overall stability constants for the addition of N,N-dimethylthioformamide ligands to HgX2 in benzene solution are determined to be β1 = (2.9 ± 0.8) × 104 and (8.2 ± 1.0) × 104 mol-1·dm3 and β2 = (1.4 ± 0.4) × 107 and (3.6 ± 1.2) × 107 mol-2·dm6 for X = I and Br, respectively.