Nickel-chloro-{2,5-bis((tertbutyl-thiolato)methyl)pyrrolide} (23)

Scheme 56.Synthesis of compound 23 making use of the transmetallation reaction.

After the successful preparation and analysis of the main group compounds, proving them to participate in π-donor acceptor interactions with the heteroaromatic ligand, a related transition metal compound was prepared. It should display the landmark for strong metal–ligand π-interaction. Therefore a transition metal with d⁸-configuration was chosen with empty d-orbitals as π-acceptor and occupied ones, suitable for π-back donation. As the nickel(II) ion is regarded to be a rather soft Lewis-acid, the pincer ligand comprising the soft Lewis-bases as side arm donor atoms namely sulfur were used in the synthesis.

Figure 60. Crystal structure of nickel-chloro-{2,5-bis((tertbutyl-thiolato)methyl)pyrrolide} (23). Thermal ellipsoids are depicted at the 50% probability level, hydrogen atoms are omitted for clarity.

23 crystallizes in the orthorhombic space group Pbcn with half a molecule in the asymmetric unit. It is completed by a C2-axis located in the N1–Cl1 axis. Due to the considerable longer methylene–sulfur bonds (183.09(12) pm) in comparison to the previously described methylene–nitrogen bonds (147.62(12) pm/147.60(14) pm) in 15

the donor–metal–donor arrangement becomes more linearly shaped. Thus, the geometry at the nickel(II) ion is almost perfectly squared planar with N1–NI1–Cl1 and S1–NI1–S1A angles of 180.0° and 170.375(17)°, respectively.

Investigating the crystal structure of 23 it becomes apparent that the N1–

Ni1 bond is significantly shortened compared to other nitrogen coordinated Ni–Cl fragments (Figure 61). With a bond length of 182.20(13) pm the Ni–Cl bond is almost as short as the N–Al bond in 10, although the aluminium(III) ion has a drastically smaller ion radius. Thus, additional interactions with respect to the N–Al interaction must be present in 23. As the nickel(II) ion contains filled and an empty d-orbital, a N–Ni π-donor-acceptor interaction becomes likely. Structures comprising a similar N–Ni bond length than 23 all consist of a π-system, involving a nitrogen atom.¹³⁹ Within those, the compound reported by Yamamoto et al. stands out (Scheme 57).^139e The crystal structure includes a pincer compound (24), very similar to that in 23 but with a fully delocalized π-system between the {SNS} donor atoms. Bond lengths within the pyrrole heterocycle of

24 clearly indicate a π-back donation from the nickel atom into the unoccupied pyrrole π-orbital. With C1–C2 and C3–C4 bond lengths of 140.6 pm and 141.3 pm, respectively, and 138.9 pm for the C2–C3

bond, the bonding situation in the pyrrole moiety has been inverted. This is in sharp contrast to the situation in 23. The heterocyclic C–C bonds resemble the motif already observed in the main group complexes reported earlier in this thesis. The C1–C2 bond is 138.23(16) pm, and the C2–C2A bond 142.4(3) pm long. Hence, 23 does not comprise a considerable amount of π-back donation. Another approach was conducted by Wayland et al. They studied the impact of a coordinated transition metal species (palladium and platinum) on the pyrrole π-system of the 2,5-bis(α-pyridyl)-pyrrolate (PDP) ligand

Figure 61. Result of a CSD search for N-coordinated Ni–

Cl fragments.

Scheme 57. Crystal structure of the {SNS}NiCl pincer complex (24) prepared by Yamamoto et al.

(Scheme 58).3^c PDP displays quite a rigid type of pincer with a delocalized π-system including the entire backbone and all donor atoms. Compound (25) displays a medium π-back donation in comparison with 23 and 24. The C–C bond lengths of the pyrrole moiety in 25 are 140.2(8) pm and 140.0(13) pm for C6–C7 and C7–C7A.

Table 33. C-C Bond length in the pyrrole moieties in 23 and related compounds from literature (24 and 25).

Bond length [pm] {SNS}NiCl (23) {SNS}NiCl (24) [PDP]PdCl (25)

C1–C2 138.23(16) 140.6 140.2(8)

C2−C3 142.4(3) 138.9 140.0(13)

C3−C4 --- 141.3 ---

N1−C1 136.88(13) 135.8 134.1(6)

ΔSB-DB 4.2 −2.05 −0.2

Table 33 summarizes the C–C bond lengths of the compounds discussed above. It is apparent that there is almost no metal→ligand π-back donation from the nickel(II) ion present in 23 as the C2–C3 bond is elongated and not shortened compared to the lithium pyrrolide species (8). Nonetheless, all NPyrrole–metal bonds have similar lengths. This must be to a counterbalancing effect. A stronger σ- or π-donation from the ligand towards the nickel atom could be the reason for the N–Ni bond shortening. This additional interaction, with respect to 24 and 25, is possible as the negative charge is concentrated in the pyrrole moiety in 23 and cannot be delocalized towards the side arms. The C2–C3 bond in 23 is slightly elongated compared to the lithiated compound (8) hinting to a weak ligand→metal π-donation. A quantification of the σ-donation is considerably more challenging than for the π-donation. Basically, the N1-C1 bond length can give a hint as it is directly bonded to the σ-donating pyrrole nitrogen atom and in fact the N1–C1 bond is longer in 23 than in the related compounds (Table 33). Thus, the withdrawal of σ-electron density caused by N1sp2→Ni1 donation affects the N1–C1 bond in a more severe way than in 24 and 25. In combination, the rather weak π-donation together with the stronger σ-donation compensates the π-back donation present in 24 and 25 and equalizes the pyrrole–metal bond length. Taking the N–C bond length in pyrrole into account can be misleading. It seems to work fine for compounds 23, 24 and

Scheme 58. Crystal structure of the [PDP]PdCl complex (25) prepared by

Wayland et al.

25, however, the aluminium compound (10) comprises a shorter N1-C1 bond length although it is much more Lewis-acidic than nickel(II) or palladium(II).

The reason for the absence of the π-back donation in 23 can be found in the molecular orbitals computed on the HF/6-31g* level of theory¹¹⁴ (Crystal Explorer)⁸⁰. The unoccupied π-orbital can be found in the rather diffuse LUMO+8. It is too high in energy to have any kind of interaction with the metal centered d-orbitals. The related compounds 24 and 25 show that a derivatization of the ligand backbone or the side arm residues can affect the orbital energies and therewith provide the requirements for a π-back donation.

The variation of the donor atoms seems to have no effect on the π-system. Going from nitrogen donor atoms to sulfur atoms does neither increase the energy of an occupied pyrrole π-orbital nor does it lower the energy of the LUMO of the pyrrole π-system.

Furthermore, its effect on the energy level of the metal centered d-orbitals is marginal as well. As could be shown by compound 24 an enlargement of the heterocyclic π-system or a substitution of the metal bonded chlorine atom are the methods of choice to control the HOMO-LUMO gap.

Figure 62. LUMO+8 of compound 23, computed using the HF/6-31g* level of theory.¹¹⁴

3.5.2