Crystal structure and spectroscopic studies of the complexes [HgX(NO3)(PPh3)](X = Cl, Br or I)

Abstract

The structures of the complexes [HgX(NO3)(PPh3)](X = Cl, Br or I) have been determined by single-crystal X-ray diffraction. All three crystallize in the monoclinic space group P21/c with one formula unit comprising the asymmetric unit of the structure. Their structures are quite similar, but that of the chloride differs in a number of respects from those of the bromide and iodide, which are isostructural. In each complex the mercury atom environment is dominated by close associations with the halide and phosphorus atoms in a quasi-linear array, but significant deviations from linearity arise, presumably because of contacts normal to the P–Hg–X array by the nitrate oxygens [Hg–X 2.328(2), 2.4386(8) and 2.601(1); Hg–P 2.372(1), 2.388(1) and 2.402(2)A; P–Hg–X 158.44(6), 166.05(4) and 166.17(5)° for X = Cl, Br and I respectively]. The nitrate moieties link successive mercury atoms in a one-dimensional polymeric array. In all three structures one of the Hg ⋯ O contacts, designated O(1), is shorter [2.527(5), 2.594(6) and 2.586(7)A respectively] than those to the other two oxygen atoms which lie at 2.7–2.9 A. In the bromide and iodide complexes both of these are to O(2), which bridges adjacent mercury atoms; in the chloride the nitrate bridges to an adjacent mercury atom by way of a bidentate O(2,3) interaction. These structural features are reflected in the infrared spectra in the ν(HgX) and nitrate ν(NO) regions. Raman measurements yield assignments for ν(HgP) in the range 147–164 cm–1. The solution 31P NMR spectra in MeCN show that [HgX(NO3)(PPh3)] is the only species in solution for X = Cl, but for X = Br or I a partial symmetrization to [Hg(NO3)2(PPh3)2] and HgX2 occurs. The 31P cross polarization magic angle spinning (CP MAS) NMR spectra of the solids exhibit a single line due to species which contain non-magnetic isotopes of mercury, in agreement with the existence of only one molecule in the asymmetric unit, and satellite lines due to 1J(199Hg31P) coupling. Broader satellites which can be shown to be due to the presence of scalar spin–spin coupling between 31P and the quadrupolar 201Hg nucleus (I=, natural abundance = 13.22%) are also observed. This agrees well with the predicted form of the spectrum of a spin ½ nucleus coupled to a spin nucleus for the case of a small dipolar coupling interaction between the nuclei and a large nuclear quadrupole interaction at the I= nucleus. The 31P NMR parameters measured for [HgX(NO3)(PPh3)] in the solid state are compared with those observed for the corresponding complexes in MeCN solution. The chemical shifts are not strongly dependent on X, but the variation with X is in opposite directions for the solution and solid samples. The 1J(199Hg31P) values show opposite trends for the chloride (increasing from the solution to the solid state) relative to the bromide and iodide complexes (where a decrease in observed). This may be related to the difference in the crystal structures of the complexes.