Nitron-derivative-based palladium carbene complexes: structural characterization, theoretical calculations, and catalytic applications in the Mizoroki-Heck coupling reaction

Abstract

New palladium(0) and palladium(ii) complexes with N-heterocyclic carbene (NHC) ligands derived from nitron and its derivatives were synthesized. The structures of most of these complexes were established by single-crystal X-ray diffraction studies. Among the new complexes, the palladium complex with a monodentate NHC ligand derived from nitron demonstrated the highest efficacy as a catalyst precursor in the Mizoroki-Heck coupling reaction of aryl chlorides with alkenes. Theoretical calculations provide valuable insights into the electronic parameters of both the ligands and the palladium complexes, highlighting the significance of a robust Pd-C bond and the π-accepting property of the NHC ligand in achieving enhanced catalytic activity. Notably, catalyst activation occurred much more rapidly with the preformed palladium(0) complex compared to its palladium(ii) counterpart.

Scheme 1. Representative palladium NHC complexes as pre-catalysts in cross-coupling reactions.

Scheme 2. Palladium NHC complexes derived from nitron.

Scheme 3. Synthesis of palladium(0) NHC (a–c) and palladium(ii) NHC complexes (d).

Fig. 1. The new molecular structures with thermal ellipsoids drawn at 50% probability level. Hydrogen atoms are omitted for clarity. (a) The molecular structure of 1a; selected bond distances and angles (Å and °): Pd1–C1, 2.073(3); Pd1–C21, 2.038(3); Pd1–C41, 2.078(3); Pd1–C42, 2.162(3); C41–C42, 1.437(4); C1–Pd1–C21, 102.86(11); C1–Pd1–C42, 118.59(11); C21–Pd1–C41, 98.96(12); C1–Pd1–C41, 158.15(11); C21–Pd1–C42, 138.46(12). (b) The molecular structure of 1b; selected bond distances and angles (Å and °): Pd1–C1, 2.074(5); Pd1–C12, 2.088(5); Pd1–C25, 2.090(5); Pd1–C26, 2.120(5); C25–C26, 1.424(7); C1–Pd1–C12, 99.78(18); C1–Pd1–C26, 113.8(2); C12–Pd1–C25, 106.77(19); C1–Pd1–C25, 153.3(2); C12–Pd1–C26, 146.20(19). (c) The molecular structure of 1c; only one of the two independent molecules in an asymmetric unit is shown; selected bond distances and angles (Å and °): Pd1–C1, 2.087(5); Pd1–C18, 2.109(5); Pd1–C35, 2.098(5); Pd1–C36, 2.127(5); C35–C36, 1.447(8); C1–Pd1–C18, 111.13(19); C1–Pd1–C35, 99.8(2); C18–Pd1–C36, 108.7(2); C1–Pd1–C36, 139.8(2); C18–Pd1–C35, 148.4(2). (d) The molecular structure of 2; selected bond distances and angles (Å and °): Pd1–C1, 2.101(2); Pd1–C11, 2.078(2); Pd1–C22, 2.113(2); Pd1–C23, 2.114(2); C22–C23, 1.444(4); C1–Pd1–C11, 89.59(9); C1–Pd1–C22, 114.37(9); C11–Pd1–C23, 116.04(10); C1–Pd1–C23, 154.28(10); C11–Pd1–C22, 155.96(10). (e) The molecular structure of 4; selected bond distances and angles (Å and °): Pd1–C1, 1.954(5); Pd1–N5, 2.077(4); Pd1–I1, 2.6067(6); Pd–I2, 2.6000(6); C1–Pd1–I1, 88.91(16); C1–Pd1–I2, 88.11(17); N5–Pd1–I1, 92.97(13); N5–Pd1–I2, 90.30(13), C1–Pd1–N5, 174.4(2); I1–Pd1–I2, 175.59(2).

Fig. 2. (a) The correlation between Pd–carbene and CN–C distances in the X-ray structures. Complexes 1c and 1c′ are the two independent molecules in an asymmetric unit. (b) The correlation between Pd 3d_5/2 binding energies and CN–C distances.

Fig. 3. (a) The linear correlation of the Pd–C distances between the X-ray and optimized structural data. (b) The correlation between Pd–C distances in the X-ray structures and WBIs.

Fig. 4. Frontier orbitals in Pd(0) complex 1a and Pd(ii) complex 4.

Fig. 5. Time-yield curves of complexes 1a and 4 in the catalytic reaction between 4-chloroacetophenone and styrene carried out at 140 °C.