Within this thesis two new ligand species namely the {NNN}-pyrrole based pincer ligand with increased steric demand and the {SNS}-pyrrole based pincer ligand could be prepared and were proven to be highly suitable for the coordination of a variety of metal species.
These complexes with metal ions as small as germanium(IV) (39 pm) and big as lead(II) comprising an ion radius of 119 pm, convey the coordination flexibility of this type of ligand. Consequently, the distance of both side arm nitrogen donor atoms varies between 440.0 pm for aluminium(III) (10) and 491.2 pm for lead(II) (21) (Figure 65). The ligand flexibility is further mirrored by the variable coordination mode which can be tridentate or bidentate in the presence of another Lewis-acidic molecule as shown in the palladium compound (26).
An example for this coordination diversity is the dimeric lithium pyrrolide species (7-9).
The pyrrole nitrogen atom coordinates to both lithium ions in the µ2-bridging mode. This could be expected for sp3-hybridized nitrogen atoms153 as they comprise a tetrahedral geometry, however, this coordination mode was not explained for aromatic nitrogen atoms showing a similar tetrahedral motif.92 High resolution X-ray diffraction data for compound 7 together with an extensive computational study confirmed the assumption of a lithium-π interaction in dimeric structures of aromatic lithium amides (Figure 66).
Apart from the prove of ligand flexibility, it could be shown that the alkyl chains bonded to the side arm donor functionalities are stereochemically active. Even the slight
Figure 66. Pyrrole→lithium interaction in compound 7. Energy values depicted in the
figure belong to σ- and π- interaction energies of N1 in compound 7.
Figure 65. Superposition plot of compounds 10 (dark gray) and 21
(light gray).
increase in steric bulk from dimethylamino- to pyrrolidino-groups drastically affects the reactivity of NPyrrole bonded atoms. In compound 11 the bulkier pyrrolidine groups, with respect to dimethylamine, prevent the pyrrole N–H from being deprotonated by the rather basic trimethylaluminium compound. Replacing the pyrrolidine groups by dimethylamine moieties leads to a quantitative deprotonation of the pyrrole heterocycle. Both products could be confirmed by crystallization. The N-metallated species symbolizes the thermodynamic product and the N-protonated compound, forming a C–H⋅⋅⋅N interaction between pyrrole and a trimethylaluminium molecule represents the kinetic product.
This reactivity can be transferred to the {NNN}GeCl species (15). Reacting 15 with methyllithium exclusively yielded the [{NNN}Li]2
lithium pyrrolide species (8) (Figure 67). By increasing the size of the lithiumorganic compound from methyllithium to TMS-methyllithium which is similar in size to trimethylaluminium it should be possible to synthesize the desired {NNN}Ge-alkyl species.
Further investigation of the tetrele complexes afforded an absolutely unknown phenomenon in metal organic chemistry. The silicon compound {NNN}HSiCl2
crystallizes in two different connectivity modes. Depending on the crystallization conditions, the ligand can coordinate as a tridentate ligand yielding an octahedral environment at the silicon atom or the ligand can act as a bidentate species with a trigonal bipyramidal surrounding at the silicon atom. This observation of thermodynamic vs. kinetic crystallization product was confirmed by a computational investigation showing a difference in energy of only 7.8 kJ/mol for both isomers.
Descending group 14 the interaction of the heavier elements with the pyrrole π-system was focused on. By analyzing the C-C bond lengths of the pyrrole heterocycle in combination with a computational investigation and a NMR-spectroscopic study on the heavy group 14 pincer complexes a decreasing interaction of the pyrrole π-system with the tetrele element going from germanium to lead was noticed. It could be clearly pointed out that within the heavy tetrele elements, tin is much more similar to
Figure 67. Intermediate species in the reaction of 15 with MeLi, explaining the
formation of compound 8.
germanium than it is to lead. Lead does not show a significant interaction with the π-system. Evaluation of the data yielded a similar metal-π interaction for lead than for lithium in 7 and 8.
Unexpectedly, none of the prepared group 14 compounds contained a metal→ligand π-back donation. To visualize the consequences of a back donation on the pyrrole π-system a transition metal complex with nickel(II) was prepared. The molecular orbitals computed for this compound do not comprise an overlap between the unoccupied pyrrole π-orbital and a d-orbital of the nickel(II) ion. Related compounds3c,139e hint to the fact that the empty pyrrole π-orbital is too high in energy which could be an explanation for the lacking π-back donation in the tetrele complexes as well.
A way to tune the orbital energies is to replace the remaining metal bonded substituent. It was shown, that replacement of chlorine by methyl elevates the HOMO and thus narrows the HOMO-LUMO gap by approximately 1 eV. Another approach can be the substitution of the side arms as the compounds with pyridyl side arms3c clearly show. By enlarging the pyrrole π-sytem they contain a LUMO with equal contributions of the pyrrole π-system and a metal centered d-orbital (Figure 68). By varying the metal bonded substituent it should be possible to obtain pyrrole based pincer ligands containing a rather small HOMO-LUMO gap, which makes π-back donation likely. Those species will contain new properties with a quasi-open-shell orbital configuration and a stronger metal ligand bond. This stronger ligand metal bond should make new reactions feasible, like the reduction of a metal species, yielding germanium in the oxidation state +1, which was not possible with compound 15. These yet unknown properties will open a new field of chemistry in the area of pyrrole based pincer ligands. With the properties of the Frustrated Lewis Pairs57 combined at a single atom, similar to the metalylenes but
Figure 68. Molecular orbitals of {NNN}GeCl (15). HOMO-1 clearly shows the metal centered lone pair, whereas the empty pyrrole π-orbital shows only small orbital coefficients in the LUMO. It seems to be higher in energy, indicating an even larger gap between the metal centered lone pair and the empty
pyrrole π-orbital.
rather convenient to synthesize, they comprise high potential in molecule/bond activation.