Comparison of crystal structure and DFT calculations of triferrocenyl trithiophosphite’s conformance

Previous electrochemical studies for triferrocenyl trithiophosphite revealed in their cyclovoltammograms three reversible one-electron peaks corresponding to stepwise oxidation of the three ferrocene moieties. It should be noted that the first oxidation potential is almost identical to free ferrocene [6]. Herein we report the crystal structure of triferrocenyl trithiophosphite.

For triferrocenyl trithiophosphite a trans-gauche-gauche configuration with torsion angles of −34°, −40°, and 173°, respectively, has been observed, although a propeller-like gauche-gauche-gauche configuration of alkyl(aryl)thio groups has been observed for trithiophosphites even in the solid state [7] or in the gas phase [8-10].

Triferrocenyl trithiophosphite has nine axes of internal rotation: three P–S bonds, three C–S bonds, and three Fe–cyclopentadienyl axes. The rotation around the P–S bonds results in a totally unsymmetrical structure with three ferrocenylthio groups exhibiting different orientations towards the phosphorus lone electron pair (Figure 1).

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Figure 1: ORTEP representation of triferrocenyl trithiophosphite showing 50% probability thermal ellipsoids.

Several possible conformations of triferrocenyl trithiophosphite have been considered quantum-chemically (Figure 2, Table 1): trans-trans-trans (ttt), gauche-trans-trans (gtt), gauche-gauche-trans (ggt), and gauche-gauche-gauche (ggg). During optimization the ggt conformer adopted a cis-gauche-trans conformation with Fc(C)–S–P lone pair dihedral angles of 8°, −60°, and 173°, respectively (Table 1). The lowest energy has been predicted for the gtt conformer, nevertheless the energy differences between the gtt and cgt conformers are negligible (0.23 kcal/mol). Interestingly, the cgt conformation has been found previously for tricymantrenyl trithiophosphite [19]. The highest relative energy is predicted for the ggg conformer (1.7 kcal/mol). The ferrocene adopts an almost eclipsed conformation in all the models with the dihedral angle between two Cp rings of ≈ 10°. Our previous work indicated that Cp can rotate at room temperature [20]. The Fc(C)–S–P lone pair dihedral angle for the ttt conformer is ≈ 150°, and for the ggg conformer it is ≈ −35°. For the gtt/cgt conformers the trans S–Fc bonds are almost antiparallel to the phosphorus lone pair (LEP): 175°, −161°/173°. The dihedral angle for the gauche S–Fc bond in the gtt conformer is −56°, and a close value is predicted for one of the gauche S–Fc bonds in the tgg conformer (−60°), whereas the second one is almost parallel to LEP (8°).

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Figure 2: Optimized conformations and relative energies of four possible conformers of triferrocenyl trithiophosphite.

Table 1: Calculated relative energies and dihedral angles Fc(C)–S–P=X (°) (X = LEP, O, S) of four possible conformers of (FcS)3P, (FcS)3PO, and (FcS)3PS.

  (FcS)3P (FcS)3PO (FcS)3PS ttt 0.91 0 0.04   149/151/151 149/149/149 149/149/149 gtt 0 0.23 0.20   −56/175/−161 −56/−173/−135 47/174/135 ggt/cgt 0.23 0.52 0.36   8/−60/173 −62/−47/165 46/45/176 ggg 1.73 0.55 0   −37/−35/−36 −52/−34/−53 42/44/44

The energy difference between the considered conformations is quite small, suggesting other factors playing a significant role. The highest energy predicted for the ggg conformer is obviously related to the absence of stabilizing intramolecular CH···π (like in the gtt and cgt cases) or CH···Fe (like in the ttt case) interactions between neighboring fragments in the structure. The latter plays an important role from the electrostatic point of view; the NBO analysis predict a negative charge at the Fe ion and positive charges at hydrogen atoms (Figure 3). Thus the crystal structure of (FcS)3P is defined rather by plural intermolecular interactions than by relative energetics of conformers (Figures S1–S3 in Supporting Information File 1).

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Figure 3: Calculated NBO charges on the Fe ions and hydrogen atoms for the optimized ttg conformer (left) and for two neighboring molecules (right) from X-ray analysis data.

Previously, for triferrocenyl trithiophosphate and triferrocenyl tetrathiophosphate with P=O and P=S moieties propeller-like ggg conformations have been found by X-ray diffraction analysis. Indeed, computations predict the ggg conformer to be the most energetically advantageous for the P=S containing compound, however with very close energies of the ttt and the ggg conformers (Table 1). For the P=O containing compound the ttt conformer is predicted to have the lowest energy. Nevertheless for both P=X compounds computations predict very small energy differences between all four conformers, lower than 0.6 kcal/mol. Thus, one can conclude that in these cases crystal packing influences the conformation. A comparison of the crystal packings for the PLEP, P=O, and P=S containing compounds clearly confirms this conclusion experimentally (Figure 4).

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Figure 4: Molecular structures in the solid state of a) (FcS)3P, b) (FcS)3PO [19], and c) (FcS)3PS [7] as established by single crystal X-ray diffraction analyses. C atoms – grey, Fe atoms – brown, O atoms – red, P atoms – orange, S atoms – yellow.

We compared the crystal packings of three similar compounds: (FcS)3P, (FcS)3PO [19], and (FcS)3PS [7] (Figure 4). All three compounds form crystals belonging to the monoclinic syngony. In all three cases, the molecules in the crystals form a herringbone motif. In (FcS)3P, C–H···π interactions dominate, while in (FcS)3PS and (FcS)3PO, in addition to C–H···π interactions, by one C–H···S and two C–H···O interactions, respectively, are observed. It should be noted that (FcS)3PO crystals contain a solvent molecule that participates in intermolecular interactions. Thus, despite the similarity of the molecular structure of the three compounds and some crystal parameters, the intermolecular interactions differ noticeably from each other.

At the same time one should underline the role of the ferrocene moiety for the crystal structure of the (FcS)3P. The related (PhS)3P molecule with Ph rings instead of Fc units exist in the propeller-like gauche-gauche-gauche configuration [21], forming the C–H···π-bonded dimers (Figures S4 and S5 in Supporting Information File 1). The computations of the relative energies of five possible conformers of (PhS)3P (ggg, ttt, ttg, ggt, ccg) predict the lowest energy for the ccg conformation (Figure 5). The propeller-like ggg conformer found in the solid state has the highest energy. Most obviously the latter is stabilized by intermolecular C–H···π interactions (Figures S4 and S5 in Supporting Information File 1). The bulky Fc moieties do not allow to form such type of dimers.

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Figure 5: Quantum chemically optimized conformations of the (PhS)3P molecule and their relative energies (kcal/mol).

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