Preparation of a Series of Supported Nonsymmetrical PNP-Pincer Ligands and the Application in Ester Hydrogenation

Abstract

In contrast to their symmetrical analogues, nonsymmetrical PNP-type ligand motifs have been less investigated despite the modular pincer structure. However, the introduction of mixed phosphorus donor moieties provides access to a larger variety of PNP ligands. Herein, a facile solid-phase synthesis approach towards a diverse PNP-pincer ligand library of 14 members is reported. Contrary to often challenging workup procedures in solution-phase, only simple workup steps are required. The corresponding supported ruthenium-PNP catalysts are screened in ester hydrogenation. Usually, industrially applied heterogeneous catalysts require harsh conditions in this reaction (250-350 °C at 100-200 bar) often leading to reduced selectivities. Heterogenized reusable Ru-PNP catalysts are capable of reducing esters and lactones selectively under mild conditions.

Keywords: catalyst immobilization; heterogeneous catalysis; hydrogenation; pincer ligands; solid-phase synthesis.

Figure 1

Representative examples of nonsymmetrical PNP pincer ligands.

Figure 2

Representative examples of pincer‐based ruthenium catalysts used in ester hydrogenation.

Scheme 1

Synthesis of PN building blocks 1 a–h.

Scheme 2

Solid‐phase synthetic approach towards supported pyridine‐based PNP‐type pincer ligands L₁–L₁₄.

Figure 3

Solid‐phase synthesis of supported PNP pincer ligand L₆ monitored by ³¹P NMR.

Figure 4

Complete library of supported PNP pincer ligands L₁–L₁₄.

Scheme 3

Solid‐phase synthesis of resin‐bound Ru‐PNP complexes C₁–C₁₄.

Figure 5

Solid‐phase synthesis of supported Ru‐PNP complex C₆ monitored by ³¹P NMR.

Figure 6

a) ³¹P MAS NMR, b) ¹H MAS NMR and c) ¹³C CP/MAS NMR spectrum of C₁₄. Rotational sidebands are denoted by asterisks (*) and (#).

Figure 7

Selected solid‐state (left) and solution‐phase NMR signals (right) of supported Ru‐PNP complex C₁₄ and homogeneous analogue XVII.28

Scheme 4

Synthesis of homogeneous Ru‐PNP complex 5.

Figure 8

ORTEP representation of 5. Only one molecule of the asymmetric unit is depicted. Displacement ellipsoids correspond to 30 % probability. C‐bound hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (°) (values of the second molecule of the asymmetric unit are given in square brackets): P1−Ru1=2.3094(6) [2.3175(6)], P2−Ru1=2.3357(6) [2.3494(6)], N1−Ru1=2.1631(16) [2.1514(16)], Cl1−Ru1=2.5183(6) [2.5371(6)], C8−Ru1=1.830(2) [1.826(2)], C8−O1=1.153(3) [1.160(3)]; N1‐Ru1‐P1=80.86(5) [80.41(5)], N1‐Ru1‐P2=81.75(5) [82.14(5)], N1‐Ru1‐C8=172.60(8) [171.45(8)], N1‐Ru1‐Cl1=89.17(4) [87.16(4)], P1‐Ru1‐P2=161.86(2) [160.82(2)].

Figure 9

³¹P{¹H} NMR spectra of supported complex C₆ (black) and 5 (red).

Figure 10

Substrate scope for ester hydrogenation using supported complexes C₆ and C₉ (conversion and selectivity indicated below structures). For conditions see Table 1, [a] Substrate (0.25 mmol), C₆ (1.0 mol %), KOtBu (10 mol %), THF (1 mL), 80 °C, H₂ (50 bar), 24 h, [b] C₆ (2 mol %), 100 °C.