Lone Pairs in Confrontation in [M(S.CS.XR(n))3] Systems. Crystal and Molecular Structure of Tris(N-methyl-N-phenyldithiocarbamato) antimony(III)

Abstract

Tris(dithiochelate) complexes of metal ions exhibit a diversity of interesting behaviours.[1] The particular ligand form R(n)XCS2 (L) may be varied by changing X sequentially (e.g.) from C (dithiocarboxylate) to N (dithiocarbamate) to O (xanthate) among the first row elements, with a consequent change in electron density at the sulfur atoms. This in turn influences features such as the crystal-field strength experienced at the metal in those complexes where it forms symmetrically bonded chelates. A change in the metal atom may cause a change in bonding mode. For example, trivalent transition metal and group 13 species ML3 exhibit metal core environments of 32 symmetry, whereas with group 15 metals, the core symmetry is degraded to 3 with the bonding in the chelate becoming unsymmetrical, possibly with concomitant effects attributable to a stereochemically active lone pair. In general terms, among the now numerous array of such complexes structurally characterized, a further degradation in symmetry about the metal atom is often found among complexes of the larger metals Sb and Bi where scope for increased coordination number is enhanced with increase in metal size. This leads to the observation of loosely[2] or strongly[3] bonded dimeric forms, due to the effect of M· · ·S intermolecular interactions between the two component molecules. Among the structurally characterized ML3 where M is a trivalent group 15 metal, a number of examples may be identified where the potential threefold symmetry is realized by location of the metal atom on a crystallographic symmetry axis. The majority of such examples, interestingly, are found in space group R3,[4–7] with the implication that molecules so disposed will lie in confrontation with their inversion images. This may be a consequence of the dominance of ‘packing forces’ at the supramolecular level. Alternatively, it may be a manifestation of some tendency towards dimerization by metal–metal bond formation.This raises the possibility of the presence of some interaction competing with the confrontation of any postulated lone pairs, which should lie coincident with the molecular/crystallographic axis, or, alternatively, a diminution in lone-pair/lone-pair repulsion. A vehicle which drew our attention to this curiosity was the observation of apparent rhombohedral symmetry in nicely crystalline, golden specimens of the title compound, [Sb(S2CNMePh)3] (m.p. 216–218◦C. Found: C, 42.9; H, 3.9; N, 6.3%. C24H24N3S6Sb requires: C, 43.1; H, 3.6; N, 6.3%.) prompting us to determine its crystal structure. Our hopes that the molecule would be disposed on a crystallographic 3-axis (indeed, a 3-axis in space group R3) were realized, one third of the molecule comprising the asymmetric unit of the structure, the first example of this type for X=N. The molecule is depicted, together with its inversion image in Figure 1, normal to and down the crystallographic 3-axis, with the phenyl substituents of the ‘tails’of successive pairs interleaving. Sb–S(1,2) are 2.528(2), 3.000(3) A, with S(1)–Sb–S(1) 83.34(7), S(2)–Sb–S(2) 116.75(8)◦, values in which it is of interest to compare with their counterparts in the recently determined structure of [Sb(S2CNMeCy)3],[8] the molecules of which are stacked successively up the symmetry axis in space group P63 (2.530(1), 2.975(1)) A, 87.1(2), 117.2(2)◦). These differences are quite minor, suggesting that the mode of stacking—successive, compared with confrontational—plays little part as a determinant of these values. (Interestingly, the substituent dispositions— methyl versus ring—are interchanged in the two compounds, so that parameter is not constant throughout.) We also note the numerous array of related structures that have been determined which, although not in space group R3 (or of similar centrosymmetric, threefold axis combinations), do crystallize in centrosymmetric space groups with some form of binuclear interaction across the centres. Generally, these are of a lower symmetry with significant M· · ·S intermolecular component, and we do not consider such examples further here. Table 1 marshals relevant parameters for all examples of group 15 [M(S2CXR(n))3] complexes which have been structurally defined as R3 (Z 2 (rhombohedral cell)), these being confined to the cases M=As or Sb. (There is an X=P example,[9] but P is disordered).Although for neither of these is there a complete X= (C,) N, or O array, the examples