ReviewStereochemical activity of lone pairs of electrons and supramolecular aggregation patterns based on secondary interactions involving tellurium in its 1,1-dithiolate structures
Introduction
Metal 1,1-dithiolate compounds continue to attract significant attention owing to their many and varied applications ranging from agriculture, to medicine, and organic synthesis; see Fig. 1 for chemical structures of selected examples of 1,1-dithiolate anions relevant to the present review. In agriculture, a prominent example is the continued use of the pesticide Mancozeb®, an ethylenebis(dithiocarbamate) containing manganese and zinc [1], [2], [3]. In medicine, the aldehyde dehydrogenase inhibitor tetraethylthiuram disulfide (Disulfiram, Antabuse®), an oxidation product of the diethyldithiocarbamate anion, is still used as a treatment for chronic alcoholism and, when co-administered with metals, as an anti-cancer agent [4], [5], [6]. In addition, there is an ever increasing number of metal compounds being evaluated for biological activity with gold [7], [8], [9] and bismuth [10] dithiocarbamates, in particular, attracting attention as potential anti-tumour agents. The 1,1-dithiolate anions themselves may be used in organic synthesis [11], [12], [13]. Many of their metal compounds serve as highly efficient precursors for metal sulphide nanoparticle generation [14], [15], [16], [17], [18]. The 1,1-dithiolate ligands are relatively easy to prepare and once made, many of their metal complexes are stable [19]. As such, and given the practical importance of metal 1,1-dithiolates, crystallographic studies are a favoured mode of structural characterisation [20], [21], [22], [23]. Consequently, the relative ease of crystallisation of metal dithiolate complexes has resulted in their being exploited in the rapidly emerging field of crystal engineering, i.e. in the realm of both transition metal [24], [25], [26], [27] and main group element chemistry [28], [29], [30], [31], [32], [33]. In a related theme and as an example of “data mining”, the availability of a relatively large number of structures has allowed structural correlations to be made whereby different types of intermolecular interactions are investigated in closely related structures. Specifically, the prevalence of secondary bonding interactions operating in the crystal structures of main group element compounds has been investigated [34], [35], [36], [37]. Secondary interactions have been long recognized as important in stabilising crystal structures, see the seminal discussions of Alcock [38], [39], and in their simplest form may be regarded as solid-state Lewis acid (metal centre)⋯Lewis base (sulphur or other donor atom) interactions. The aforementioned systematic surveys have shown that it is possible to control the formation of secondary interactions by varying the steric bulk of organic groups, either/or in a remote substituent of a ligand or directly bound to the metal centre, as in organometallic derivatives [34], [35], [36], [37]. It is the determination of the prevalence and, when formed, the persistence of Te⋯S secondary interactions operating in the crystal structures of tellurium 1,1-dithiolates that is one of two major motivations for the present survey of their crystallographically determined structures.
In addition to comments on structural aspects of tellurium compounds in the aforementioned reviews focussing on 1,1-dithiolate ligands [20], [21], [22], [23], more generally, the structural chemistry of tellurium has been reviewed periodically over the years by Abel et al. [40], Sudha and Singh [41], with the latest comprehensive survey, appearing in 1994, by Haiduc et al. [42].
The complementary focus of this survey of tellurium 1,1-dithiolate structures is a systematic investigation of the coordination environments in these molecules which may be influenced by two lone pairs of electrons in the tellurium(II) structures or one lone pair in the case of tellurium(IV) species. The pivotal role of stereochemically active lone pairs of electrons in rationalising geometries is readily appreciated by the wide applicability of the Valence Shell Electron Pair Repulsion (VSEPR) model for organic molecules [43], [44]. Stereochemically active lone pairs are also well known to influence coordination geometries in main group element compounds but in certain examples containing the heavier atoms, it is well known that the lone pair can be stereochemically inert [45], [46], [47]. This comes about as often greater coordination numbers are found in compounds of the heavier elements, meaning a diminishing contribution of the s-orbital to the overall bonding. An additional consequence of crowded coordination geometries is that it can sometimes be difficult to definitely detect the influence of a putative lone pair of electrons upon the molecular structure. A useful concept relating to the detection of lone pairs in high coordinate molecules involves considering the topological distribution of donor atoms about the central atom [48]. Here, a holodirected arrangement shows the ligand donor atoms to be distributed uniformly on the surface of sphere about the central donor which contrasts a hemidirected distribution whereby an obvious gap is apparent on the sphere that is ascribed to the presence of a lone pair of electrons [48]. Systematic studies of the heavier main group element compounds examining the influence of the lone pair of electrons are probably most developed for lead(II) compounds [49], [50], [51] with less attention been directed towards antimony(III) [35], [52], [53], [54] and bismuth [55], [56]. The role of lone pairs of electrons obviously has attracted considerable attention in tellurium structures [20], [21], [22], [23], [40], [41], [42] with the classic example of “stereochemical inertness” for tellurium(IV) compounds found in the [TeCl6]2− salts [57], [58].
With varying oxidation states, chemical compositions, and the putative influence of stereochemically active lone pairs, it is no surprise that a wide variety of coordination geometries are possible in the structures described herein. Table 1 summarises the more frequently encountered coordination geometries, and their designations. Coordination geometries are analysed in terms of formal covalent bonds along with the secondary Te⋯X interactions, whenever present. The criterion used to determine the “significance” of a Te⋯X interaction is simply based on the sum of the van der Waals radii, as tabulated by Bondi [59], e.g. 2.06A for tellurium, 1.80A for sulphur, etc.
In this survey, the structural chemistry of the tellurium(II) 1,1-dithiolates are described first followed by those of tellurium(IV). Within each division, binary compounds are discussed first, followed by ternary derivatives, usually containing one or more halide (pseudo-halide) donors. Finally, organotellurium compounds are described in order of increasing number of tellurium-bound organic substituents. All structural data were extracted from the Cambridge Crystallographic Data Centre [60]. Particularly relevant to the present survey are the 1,1-dithiolate ligands shown in Fig. 1, namely (I) N,N′-dialkyl(aryl)dithiocarbamate; (II) O-alkly(aryl)dithiocarbonate (xanthate); (III) O,O′-dialkyl(aryl)dithiophosphate; and (IV) dialkyl(aryl)dithiophosphinate, as all of these form compounds with tellurium(II)/(IV) that have been crystallographically determined. Within each of the specified categories, the structures are described in order of those containing (I), the most numerous, (II), (III) and finally, (IV), the least represented. When applicable, mixed 1,1-dithiolate ligand structures are discussed at the conclusion of each category. All crystallographic illustrations are original and were drawn with the aid of DIAMOND programme [61] using arbitrary spheres; hydrogen atoms are not illustrated to aid clarity.
Section snippets
Tellurium(II) compounds
Tellurium(II) 1,1-dithiolates are less numerous that their tellurium(IV) counterparts but, arguably display a greater diversity of coordination geometries and more varied supramolecular association patterns. An obvious explanation for the greater disparity in their coordination environments is explained in terms of the presence of two lone pairs of electrons around the tellurium(II) centre rather than one pair as for the tellurium(IV) structures.
Tellurium(IV) compounds
Tellurium(IV) is a reducing centre and 1,1-dithiolate ligands can be readily oxidised, e.g. −S2CNEt2 to disulfiram, Et2NC(S)SSC(S)NEt2, −S2COEt to dixanthogen, EtOC(S)SSC(S)COEt, and −S2P(OEt)2 to (EtO)2P(S)SSP(S)(OEt)2, etc. [19]. Under these circumstances, the only binary 1,1-dithiolate compounds to have been characterised with tellurium(IV) are those containing dithiocarbamate ligands which are renowned for their ability to stabilise high oxidation states. Mixed ligand compounds, for example
Summary and conclusions
The foregoing description of the structural chemistry of crystallographically characterised tellurium 1,1-dithiolate compounds has focussed upon coordination geometries and supramolecular aggregation patterns. In terms of the former, significant influence exerted by lone pairs of electrons is evident. In the tellurium(II) species, the presence of two lone pairs of electrons is indicated in all coordination geometries. By contrast, often the lone pair of electrons in the tellurium(IV) structures
Acknowledgements
We thank CNPq, FAPESP and CAPES for financial support.
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