When crystallography can use help from tin-119 Mössbauer spectroscopy

Crystallography, the most powerful method for obtaining structural data, can benefit from help from other techniques. In this work, 119Sn Mössbauer spectroscopy was used to assist crystallography, for finding the tin(II) positions in the unit-cell and determine a tin(II) coordination in agreement with both the diffraction data and the tin electronic structure. Even high quality single crystal data do not guarantee that the right solution will be obtained. A first attempt at the structure of α–SnF2 yielded the tin positions with very reasonable R and Rw residuals, 0.23-0.25. However, the fluorine positions could not be found (Bergerhoff, 1962). After many other attempts, the full crystal structure was finally solved 14 years later (R.C. McDonald et al. 1976). The difference in the tin position with the initial solution (1962) was that, in the latter, half of the tin atoms were on special sites, however, the tin sublattice was identical. Because the tin sites in the initial solution gave very reasonable residuals, 14 years of hopeless efforts were wasted. The presentation will show that this could have been avoided using 119Sn Mössbauer spectroscopy. This was possible since the spectrum had already been recorded (A.J.F. Boyle et al., 1962). Mössbauer spectroscopy can also help determine the tin coordination, when combined with powder diffraction data, in case of disordered structures. The presence of tin(II), disordered with a metal ion in cubic coordination, when diffraction shows there is no lattice distortion and no superstructure, suggests that tin has also a cubic coordination. This would require the tin lone pair to be non-stereoactive; however Mössbauer spectroscopy shows it is stereoactive.

Tetrakis(μ-2-phenylacetato-κ2O:O′) bis{(DMSO κO′)]copper(II)}(Cu-Cu)]

Carboxylate groups may interact as bridging ligands with divalent transition metals present in biological environments, thereby altering the bioavailability of drugs. Moreover, it is well known that many complexes of divalent transition metals are capable of catalyzing the hydrolysis of RNA (Stem et al., 1990; Kimura, 1994). The coordination chemistry of Cu2+ complexes bridged by phenylacetate has been reported. We have found only two reports of a dinuclear Co2+ complexes, namely tetrakis (phenylacetato)bis[(quinoline-N)-cobalt(II)](Cui et al.,1999),μ-aqua-κ2O:O-di-μ-phenylacetato-κ4O:O′-bis[(1,10-phenanthroline-κ2N,N′) (phenyl acetato-κO)cobalt(II)](Kong et al., 2005) and dinuclear Cu2+ complex, namely tetrakis (phenylacetato)bis-[(N,N-dimethylformamide)copper(II)], in which all phenylacetate groups are in bidendate bridging modes. In this presentation, the crystal structure of a new dimeric complex obtained by reaction of phenylacetic acid with copper(II) acetate is described. Each Cu(II) atom is six-coordinated by five O atoms from carboxylate groups of the phenylacetate and DMSO ligands and is completed by a Cu-Cu bond in a strongly distorted octahedral coordination, in which an inversion center is located at the mid-point of the Cu-Cu bond with a Cu...Cu distance of 2.6321(4) Å. This is longer than the 2.251(2)Å distance found in the polymeric complex [Cu2(C8H7O2)4]n. However, it is similar to the 2.6414(8) Å and 2.6261(8)Å distances found in the complex [Cu2(C8H7O2)4(C3H7NO)2] (Kong et al., 2005). The Cu-O phenylacetate bond length lies in the range 1.9644(14) to 1.9734 (14) Å and the Cu-ODMSO bond length is 2.1319(13) Å.

Complexes of Mn(II) with imidazole and 2-thiophene acetic acid ligands

The reactions of 2-thiophene acetic acid and imidazole with manganese (II) chloride resulted in mononuclear [Mn(C6H5O2S)2(C3H4N2)6] (1), or binuclear [Mn2(C6H5O2S)4(C3H4N2)4] (2) and [Mn2(C6H5O2S)4(C3H4N2)4(H2O)] (3) complexes. In the complex (1), the Mn ion is octahedral coordinated by six nitrogen atoms from six imidazole rings and the Mn-O bond lengths are in the range 2.261(4) to 2.275(6) Å. The crystal packing is stabilized by weak C-H...O and N-H...O hydrogen bonds. In the structure of (2), the asymmetric unit is formed from an Mn ion bonded to two N atoms from two imidazole ligands and to three O atoms from three different thiophene acetic acid ligands. Two of these ligands are deprotonated and bridge by the same oxygen atoms between the second manganese ion giving rise to an binuclear complex. In this complex, each Mn cation is located in a slightly distorted square-planar environment and the Mn-N bond lengths are in the range 1,960(3) to 1.976(2) Å. The Mn-O bonds lengths in the square base of the two pyramids are in the range 1.956(5) to 2.000(1) Å. The Mn-O axial bond distance is quite longer than the Mn-O equatorial bond distances and is 2.458(2)Å [1,2]. The crystal packing is stabilized by weak C-H...O and N-H...O hydrogen bonds forming connected layers parallel to (001) planes. When an oxygen of one molecule of water bridges the two metal centers in the complex (2), the environment of the two Mn becomes octahedral and thus we obtain the complex (3). In this latter complex, the Mn-N bonds are in the range 2.192(4) to 2.245(4). All Mn-O bonds are between 2.148(6) and 2.232(7)Å. All structures are disordered in all thiophenyl rings occupy alternatively two positions related to one another by an 180o rotation about the C-C sigma bond [3]. The sulfur and one carbon atom of the ring occupy the same position. The complexes were structurally characterized by single crystal X-ray diffraction analyses, infrared spectroscopy (IR), elemental analyses and thermogravimetric analyses (TGA). Cristal data: (1): MnC30H34N12O4S2 in C2/c, a=11.3492(6), b=13.9186(7), c=22.2101(13), beta= 94.963(4) (2): Mn2C36H36N8O8S4 in P21/n, a=15.996(2), b=10.274(13), c=26.141(3), beta=97.715(9) (3): Mn2C24H38N8O9S4 in P21/n, a=26.206(2), b=8.5421(8), c=19.0694(17), beta= 92.229(4)