Structural comparison of the prepared group 14 species

In the first decade of the 21st century many discoveries were made in the area of low valent heavy group 14 compounds and their specific dissimilarities were investigated in a very detailed process, as described in the introduction. However, besides the reactivity towards small molecule substrates or intermetallic orbital interactions, the interaction with the ligand carrying such a metal species was neglected although it could provide useful information about the nature of these metal species in general and not merely towards selected molecules. Having a well-studied π-system like pyrrole embedded in the ligand makes a study of the π-interactions feasible.

Table 29. Comparison of selected structural properties of the prepared group 14 halide species and the lithium pyrrolide 8. Lewis-acidity among these metal(II) species should be in a similar scale and thus should not be primarily responsible for the different ΔSB-DB values observed. A π-donation from the N1pZ orbital towards metal centered orbitals would elongate the C2–C3 bond and shorten the formal double bonds. Thus, this π-donation seems to be an explanation for the observed differences in bond lengths. Assumption of an increasing degree of this π-donation ascending group 14 would be in good agreement with the obtained bond lengths. Bond lengths observed in compound 8, which does not show significant amounts of π-interaction, further confirms the theory of an increasing π-donation going from lead to germanium. Without this assumed loss of electron density via the N1pZ→M donation the C2–C3 bond in 8 is quite short and the C1–C2 and C3–C4 bonds are in a similar range like in the group 14 complexes. The electron withdrawal effect which

should be considerably weaker for lithium than for the group 14 metals seems to be decreased in the same magnitude as the lacking π-donation. Thus the C1-C2 and C3-C4 bonds have approximately the same length in all the species described in Table 29.

The N1pZ→M donation would further shorten the N1-M bond as bonding orbitals in-between N1 and M are populated (HOMO-2 of compound 15). Comparison with the M(hmds)2 species, which form rather short N-M bonds with a coordination number as low as two, confirms group 14 pincer complexes to form short bonds between pyrrole and the metal ion.

As can be seen in Figure 57 the molecular orbitals suggest a stronger interaction between germanium and pyrrole than for its heavier homologues.

In the germanium species, the orbital overlap is clearly visible at an isolevel of 0.045 au. In contrast, there is hardly any overlap detectable for the tin species, however, the orbitals at N1 and Sn1 have the same algebraic sign and would overlap at a lower isolevel. Within the lead compound, there is no π-overlap detectable, even at a very low isolevel. Remarkably, the ordering of the molecular orbitals is changed. This is due to a decrease in energy of the lone pair going from germanium to lead. For germanium it is mainly located in the HOMO-1 (-5.915 eV). In the tin and lead species, it is located in the HOMO-2, with energy values of -6.403 eV and -6.637 eV, respectively. This observation is not surprising as the ability of the heavier elements to undergo sp hybridization decreases with increasing atomic number of the tetrele element ending up at lead with an energetically low lying lone pair with mainly s-character This has already been observed by Lappert et al. in 2007 when comparing a three coordinate lead species with its lighter congeners.¹³⁷ They computed the orbital character of the metal centered lone pair in a LPbCl (s^0.918, p^0.082), LSnCl (s^0.861, p^0.139) and LGeCl (s^0.816, p^0.184) species.

Figure 57. Frontier molecular orbitals of the prepared group 14 species, computed at the M06¹³³/cc-pVTZ⁹⁷

level of theory.¹¹⁹

With the increasing divergence of the C-C bond lengths within the pyrrole heterocycle when ascending the group of the tetrele elements, the aromaticity should be reduced. To quantify the differences in aromaticity the NICS(1)ZZ values, computed by D. M. Andrada, have been taken into account.¹¹⁹ They confirm the hypothesis that an increased divergence of bond length (ΔSB-DB) decreases the aromaticity of pyrrole, however, the differences are marginal (Table 30) and can hardly be used as an evidence.

Table 30. Computed NICS(1)ZZ values for the prepared group 14 species.

Compound NICS(1)ZZ [ppm]

{NNN}Ge (15) −25.0 / −24.9 {NNN}Sn (18) −25.2 / −25.1 {NNN}Pb (21) −25.6 / −25.5

The NBO analysis⁹⁹ in contrast creates a picture with more distinct differences between each element. Most noticeable are the obtained values for the acceptor-donor interaction energy ΔE⁽²⁾ (Table 31). For the metal→ligand π-back donation the obtained energy values are not significant and are neglected in the investigation. The donation from the N1pZ orbital towards the metal centered orbitals, however, clearly display the expected differences of the investigated species. The π-interaction within the lead compound is worth 4.11 kcal/mol. Going to tin the ligand→metal π-interaction is increased by 5.36% and further increased by 72.27% going from lead to germanium.

Surprisingly, the values for the tin and lead compounds are almost identical. That is in sharp contrast to the experimentally observed results as well as to the computed molecular orbitals. This may be due to the intermolecular Pb–Cl interaction in 21 which was not taken into account for the computational investigations and could have had an influence on the experimentally observed bond lengths in 21.

Table 31. NBO results for compounds 15, 18 and 21. Partial charges (Q) (in au) and occupation numbers (LP) (in au), Wiberg bond order (BO) and second-order acceptor-donor interaction energies (ΔE⁽²⁾) (in

Among the methods used above for explaining the ligand-metal interactions there is another very useful experimental tool to prove the capability for metal ligand π-interaction of each single group 14 metal. The chemical shift of the protons at the 3- and 4- position of the pyrrole moiety directly depends on the π-electron density of the heterocycle. The ring current effect is deshielding the protons, however, loss of electron density in the pyrrole π-orbitals weakens this effect and the corresponding protons are high-field shifted. Figure 58 shows extracts from the ¹H-NMR spectra of the prepared compounds {NNN}Ge (15), {NNN}Sn (18), {NNN}Pb (21) and the lithium pyrrolide (8).

The spectra have been recorded using crystalline material of the corresponding compounds, dissolved in toluene-d8. The chemical shift of the protons in 3- and 4-position of pyrrole is in perfect agreement with the inferences drawn from the experimentally observed bond lengths in the heteroaromatic cycle. Most remarkable, displaying a chemical shift of 6.13 ppm the signal for the tin compound is much closer to the chemical shift of the germanium compound (6.09 ppm) than to the lead species (6.27 ppm). The chemical shift of the lead compound on the other hand is similar to that of the lithium pyrrolide species (6.30 ppm). The same trend but much less pronounced is witnessed for the ¹³C-NMR spectra. Although the concentrations of the samples vary (Sn vs Li) which could affect the resulting chemical shifts, the observed differences between the single compounds are too distinct to be caused by a different sample

Figure 58. Extract from the ¹H-NMR spectra of the prepared group 14 compounds in the oxidation state +2 and the lithium pyrrolide compound (8), focusing on the signal of the pyrrole C–H protons.

concentrations. The computational results concerning the tin and lead species are somehow in contradiction to the experimental results, which have been proven by the high resolution X-ray data and by NMR spectroscopy. With some limitation the NICS(1)ZZ

values confirm the experimental results as well. However, the NBO analysis⁹⁹ does not fully support these results which may be due to some problems with the model in particular as the intermolecular interactions have not been taken into account, which may affect the final result.²³

Finally, it can be stated that the pyrrole–metal π-interaction decreases descending group 14. However, the change is not proportional to the atomic number of the corresponding elements. According to the discussed experiments it decreases in the following order Ge > Sn ≫ Pb (≥ Li).