Structure and reactivity of [Mo-3-mu S-3-(mu S-2)(3)](4+) complexes. Quantum chemical calculations, X-ray structural characterization, and Raman spectroscopic measurements

摘要

A series of compounds containing the [Mo-3-mu(3)S-(mu S-2)(3)-(dtc)(3)](+) complex (dtc = diethyldithiocarbamate) with the anions I- (1), I- and Br- (2), S2- (3), ClO4- (4), NO3- (5), and SO42- (6) was prepared and characterized by elemental analysis, NMR, LR, and Raman spectroscopy, and FAB mass spectrometry. The previously reported crystal structure of 1 was reinvestigated. The X-ray analysis revealed the incorporation of CH2Cl2 in the crystal having the composition [Mo3S7(dtc)(3)]I . 0.5CH(2)Cl(2) (1a), which was in contradiction to the previous protocol. The corresponding ClO4- compound (4a) is isotypic. Crystal data: C15.5H31Cl2Mo3N3O4S13, orthorhombic space group Aba2, a = 25.816(5) Angstrom, b = 17.761(4) Angstrom, c = 16.250(3) Angstrom, Z = 8. For la, 4a, 6, and the previously analyzed 2 and 3 the crystal structures revealed characteristic interactions between the anions X and the three axial (out-of-plane) sulfur atoms S-ax of the disulfido bridges. The Raman data showed a significant decrease of the S-eq-S-ax stretch resonance frequency in the order 4, 5, 6 > 1 > 3. This decrease is paralleled with a slight increase of the S-eq-S-ax bond length and with a significant shortening of the X ... S-ax distances when compared to the sum of the corresponding van der Waals radii. A comprehensive quantum chemical study, using both density functional theory and semiempirical calculations, revealed that for hard counterions such as NO3- and ClO4- the S-ax... X interactions can be understood in terms of an almost entirely electrostatic interaction, whereas for soft nucleophiles such as I- and S2- significant covalency is observed. In addition, the general reaction of [Mo3S7](4+) complexes with a nucleophile was modeled. With regard to the side-on bonding of the mu-S-2 groups to Mo, the calculations indicated a significantly higher bond energy for the axial (out-of-plane) sulfur atoms, explaining the much higher lability of the sulfur atoms in the equatorial (in-plane) position. Analogous differences for the ligating atoms of the peripheral ligands, having a cis and trans position with respect to mu(3)-S, are less pronounced. [References: 51]