Sterically and Electronically Flexible Pyridylidene Amine Dinitrogen Ligands at Palladium: Hemilabile cis/trans Coordination and Application in Dehydrogenation Catalysis

Abstract

Ligand design is crucial for the development of new catalysts and materials with new properties. Herein, the synthesis and unique hemilabile coordination properties of new bis-pyridylidene amine (bis-PYE) ligands to palladium, and preliminary catalytic activity of these complexes in formic acid dehydrogenation are described. The synthetic pathway to form cationic complexes [Pd(bis-PYE)Cl(L)]X with a cis-coordinated N,N-bidentate bis-PYE ligand is flexible and provides access to a diversity of Pd^II complexes with different ancillary ligands (L=pyridine, DMAP, PPh₃ , Cl, P(OMe)₃ ). The ¹ H NMR chemical shift of the trans-positioned PYE N-CH₃ unit is identified as a convenient and diagnostic handle to probe the donor properties of these ancillary ligands and demonstrates the electronic flexibility of the PYE ligand sites. In the presence of a base, the originally cis-coordinated bis-PYE ligand adopts a N,N,N-tridentate coordination mode with the two PYE units in mutual trans position. This cis-trans isomerization is reverted in presence of an acid, demonstrating a unique structural and steric flexibility of the bis-PYE ligand at palladium in addition to its electronic adaptability. The palladium complexes are active in formic acid dehydrogenation to H₂ and CO₂ . The catalytic performance is directly dependent on the ligand bonding mode, the nature of the ancillary ligand, the counteranion, and additives. The most active system features a bidentate bis-PYE ligand, PPh₃ as ancillary ligand and accomplishes turnover frequencies up to 525 h^-1 in the first hour and turnover numbers of nearly 1000, which is the highest activity reported for palladium-based catalysts to date.

Keywords: cis/trans coordination; formic acid dehydrogenation; hemilability; palladium; pyridylidene-amines.

Figure 1

(a) Limiting resonance structures of PYA/PYE ligands featuring an anionic or a neutral coordination site; (b) examples of mono‐, bi‐, and tri‐dentate PYA/PYE ligand precursors; (c) representative PYA complexes with catalytic activity.

Scheme 1

Synthesis of ligand precursors 1 and 2, and complexes 3–4.

Figure 2

ORTEP representation of 3 c (a) and 4 (b) at 50 % probability level (all hydrogen atoms and BPh₄ anion of 3 c omitted, rings below the coordination plane in lighter shade). Selected bond distances (Å) and bond angles (deg) for complex 3 c: Pd1−Cl1 2.3054(10), Pd1−P1 2.2757(9), Pd1−N1 2.044(3), Pd1−N2 2.068(3), N1−C18 1.432(5), N2−C7 1.412(5), N1−C1 1.313(5), N2−C19 1.322(5), C1−C2 1.425(6), C1−C5 1.423(6), C2−C3 1.351(6), C4−C5 1.351(6), C19−C23 1.430(5), C19−C20 1.432(6), C23−C22 1.356(6), C20−C21 1.347(6), P1−Pd1−Cl1 89.87(4), N1−Pd1−N2 82.90(12), N1−Pd1−P1 96.12(9), N2−Pd1−Cl1 90.04(9), C1−N1−C18 121.1(3), C7−N2−C19 121.8(3). For complex 4: Pd1−Cl1 2.3285(5), Pd1−Cl2 2.3168(6), Pd1−N1 2.0202(18), Pd1−N2 2.0256(18), N1−C7 1.427(5), N2−C19 1.422(3), N1−C1 1.320(3), N2−C13 1.322(3), C1−C2 1.430(3), C1−C5 1.435(3), C2−C3 1.350(3), C4−C5 1.353(3), C13−C17 1.433(3), C13−C14 1.422(3), C14−C15 1.346(3), C17−C16 1.349(3), Cl1−Pd1−Cl2 93.73(2), N1−Pd1−N2 82.89(7), N1−Pd1−Cl2 91.67(5), N2−Pd1−Cl1 91.74(5), C1−N1−C7 121.69(18), C13−N2−C19 121.81(18).

Scheme 2

Synthesis of complexes 3 b, 5, 6 and 7.

Figure 3

¹H NMR spectra (CD₂Cl₂, 300 MHz) of 1 and 2, and complexes 3 b, 4, 6 and 7 (trans refers to N−CH₃ group of the PYE site trans to ancillary ligand L).

Scheme 3

Xantphos‐type reversible swapping of the two PYE coordination sites from cis to trans, triggered by deprotonation of 4 and protonation of 8.

Figure 4

Time‐dependent gas evolution profile from palladium‐catalyzed formic acid dehydrogenation using complex 3 b (circles), 4 (diamonds), 6 (squares), 7 (triangles) and 8 (inverted triangles). Condition: catalyst (1 mol%), FA (1 mmol), Et₃N (0.91 mmol), and 1,4‐dioxane (1 mL) at 80 °C.

Scheme 4

Synthesis of complexes 9, 10, and 12 for comparison with bidentate ligated bis‐PYE complex 3 b and the tridentate ligated bis‐PYE complex 8, respectively.

Figure 5

ORTEP representation of 10 (a) and 12 (b) at 50 % probability level (all hydrogen atoms and BPh₄ anion of 10 omitted, rings below the coordination plane in lighter shade). Selected bond distances (Å) and bond angles (deg) for complex 10: Pd1−Cl1 2.3076(3), Pd1−P1 2.2703(3), Pd1−N2 2.0724(10), Pd1−N4 2.0274(10), N4−C19 1.4227(14), N2−C7 1.4182(15), N4−C20 1.3221(15), N2−C4 1.3212(15), P1−Pd1−Cl1 88.830(11), N4−Pd1−N2 82.63(12), N4−Pd1−P1 95.3(3), N2−Pd1−Cl1 92.45(3), C20−N4−C19 121.20(10), C7−N2−C4 122.15(10). For complex 12: Pd1−Cl1 2.3372(4), Pd1−N1 2.0225(12), Pd1−N2 1.9661(12), Pd1−N3 2.0400(11), N3−C15 1.4247(18), N3−C16 1.3466(16), Cl1−Pd1−N1 94.62(4), N1−Pd1−N2 82.07(5), N2−Pd1−N3 81.65(5), N3−Pd1−Cl1 101.93(3), C16−N3−C15 119.39(12).