Ligand derivatization

Within the compounds 1, 2 and 3, 1 is superior, serving as key intermediate for the substitution of the nitrogen donor atoms. By addition of two equivalents of iodomethane the bis-ammoniumsalt is prepared which evolves trimethylamine when treated with a nucleophile. Using P-, O- and S-nucleophiles the {NNN} chelating ligand can be converted into a {PNP}, {ONO} or {SNS} pincer ligand system (Scheme 27). This variation of donor atoms increases the variety of possible target metals either to harder ({ONO}) or to softer metals ({PNP} and {SNS}). However, the method is limited to highly nucleophilic and non-basic substrates.

Scheme 27. Synthesis of {PNP}-, {ONO}- and {SNS}-pincer ligands.

With a rather weak base like a sodium thiolate, the reaction follows a SN2 mechanism with participation of the neighboring aryl (pyrrolyl) group. It is known that substitution reactions at the benzylic position (phenyl) follow the SN2 mechanism.⁷⁹ Similar assump-tions can be made for pyrrole as aryl group. Furthermore it should be even more acti-vated due to the stabilization of the intermediate (Scheme 28).

Scheme 28. Mechanism of the SN2 reaction with the neighboring group effect of pyrrole.

If the nucleophile is basic enough to deprotonate the pyrrole amine, the heterocycle becomes highly electron rich causing a very dominant neighboring group effect. The addition of the former pyrrole N–H proton to the nucleophile weakens its nucleophilicity and the intermediate decomposes to unidentifiable products. The use of four equivalents of nucleophile, two as base for the deprotonation and the remaining two equivalents for

the nucleophilic attack does not show any improvement of the reaction. The intermedi-ate then decomposes in an unknown pathway to an unidentifiable black tar.

3.1.5 {SNS}-Pyrrole based pincer ligand

The {SNS}-pyrrole based pincer ligands were prepared following the procedure ex-plained in chapter 3.1.4 and summarized in Scheme 29.

Scheme 29. Synthesis of the {SNS}-pyrrole based pincer ligands.

3.1.5.1 2,5-Bis((tertbutyl-thiolato)methyl)pyrrole (5)

5 has been prepared following Scheme 29 and was obtained as a yellow oil. Unfortu-nately it was impossible to obtain single crystals of 5, thus its presence was proven by NMR-spectroscopy. The ¹H-NMR spectrum is very much alike the related free ligand spe-cies. 5 was used within this thesis for the synthesis of complexes with rather soft late transition metals.

3.1.5.2 2,5-Bis((thiophenolato)methyl)pyrrole (6)

6 has been prepared along a protocol similar to 5. After recrystallization, single crystals suitable for X-ray diffraction experiments were obtained.

Figure 10. Crystal structure of 2,5-bis((thiophenolato)methyl)pyrrole (6). Thermal ellipsoids are de-picted at the 50% probability level. Hydrogen atoms, besides H100, which was freely refined, are omitted

for clarity.

6 crystallizes in the orthorhombic space group Pnma with half a molecule in the asymmetric unit. The molecule is completed by a mirror plane going through N1 and H100, being perpendicular to the heterocyclic plane. 6 seems to be perfectly suited as a

reference for the protonated pyrrole based pincer ligand system, as there are no hydro-gen bondings present, which could vitiate the resulting C–C bond length of the pyrrole heterocycle.

A very useful tool to detect these weak interactions is the CrystalExplorer⁸⁰ program.

Starting from a cif file, it calculates the promolecule density of the selected compound.

The resulting output is a surface which includes the space that is dominated (>0.5) by the electron density of the selected molecule. The intermolecular close contacts can be mapped onto this surface by taking the distance of the enclosed atoms to the surface (di), the distance of the external atoms to the surface (de) and the van der Waals radii of the involved atoms into account (Equation 1). The resulting value is the normalized distance dnorm describing the distance of an atom inside the surface from an atom outside the surface normalized to their van der Waals radii.⁸¹

Equation 1. The normalized contact distance.

The dnorm value is calculated for each pixel of the surface, negative values are labeled in red (indicating a possible close contact), positive are values are labeled in blue. The resulting colored surface is named the Hirshfeld surface⁸² and is a powerful tool to detect intermolecular interactions within a crystal structure.

A closer investigation of the crystal structure of compound 6 using the Hirshfeld sur-face tool within the Crystal Explorer⁸⁰ program revealed a η⁵-N–H–π interaction that can be considered rather strong (Figure 11). The bond lengths and angles at H100 hint to the strength of this interaction. A CSD search for hydrogen–centroid distances to pyrrole and cyclopentadienide between 100 pm and 400 pm yielded a mean value of 353 pm, with the shortest distance being 240 pm⁸³ long. With a hydrogen–π-system distance of only 244(4) pm, a H–centroid distance of 248 pm and an N–H–centroid angle of 173.3°

the N–H–π interaction in 6 is among the strongest reported in the CSD until today.

Theoretical calculations rank N–H–

π interactions as being between 0.7 kcal/mol and 17.3 kcal/mol (hypothetical alanine–benzene interaction).⁸⁴ However, Mohan et al.

recognized a strong dependency on the N–H polarization. The values for protonated alanine vary between 10.7 kcal/mol and 17.3 kcal/mol, whereas the range for neutral alanine is given by 0.7 kcal/mol and 4.7 kcal/mol. Similar observations were made by Tsuzuki et al., showing that substituted methyl moieties have higher C–H–

π interaction energies than methane.⁸⁵ Furthermore, he stated that the interaction energy is orientation dependent, with the maximum interaction energy at a donor–H–

acceptor angle of 180°.⁸⁶ With an angle close to the ideal 180°, and the short hydrogen–

π-plane distance in combination with the rather acidic pyrrole N–H proton, the N–H–π interaction found in 6 is considered to be among the strongest present in literature until today (Figure 12). According to Mohan et al. the interaction is worth between 5 kcal/mol and 10 kcal/mol, which is a wide range, however, these values strongly depend on the

Figure 12. Results of a CSD search for N-H⋅⋅⋅π interactions. X-axis: H–centroid distance [pm]; Y-axis:

N-H⋅⋅⋅centroid angle [°].

Figure 11. Hirshfeld surfaces for compound 6. Left: N–H–π interaction forming a chain like arrangement (green dashed lines). Right: Interconnection of these chains (red dashed lines) via Ph–H–

S interaction.

used computational method and rather precise determinations of non-covalent interac-tions are associated with an enormous computational effort.⁸⁴

This interaction can be regarded as structure determining effect as selected mole-cules arrange themselves to chains via this N–H-π interaction. The planes of the respec-tive pyrrole heterocycles are tilted within a chain by 66.6(3)°. These chains are further connected by phenyl–H–S interactions forming a two-dimensional network in the crys-tal. The hydrogen–sulfur distance is 288.0 pm long and the C–H⋅⋅⋅S angle measures 165.9°.