Synthesis, Structural Analysis, and Catalysis for Suzuki–Miyaura Coupling Reactions of Tetradentate N^C^C^N and Tetracarbene C^C^C^C Nickel–N-Heterocyclic Carbene Complexes

Four novel Ni(II)–N-heterocyclic carbene (Ni–NHC) complexes were synthesized and characterized by NMR, ESI–MS, SEM, TEM, EDS mapping, XPS, and single-crystal X-ray diffraction (XRD) analysis. Ni–NHC complexes 7 and 8 are intramolecular tetradentate N^{^}C_NHC^{^}C_NHC^{^}N-cyclometalated [1L + 1M] complex featuring coplanar of azole and quinoline rings, and Ni–NHC complex 11 is an interligand tetracarbene C_NHC^{^}C_NHC^{^}C_NHC^{^}C_NHC-cyclometalated [2L + 1M] complex with two six-membered, conformation of boat-shaped, C_NHC^{^}C_NHC-cyclometalated structural units. Complex 9@Ni–NHC is a softly fluffy black powder with the surface morphology of a tightly arranged honeycomb-like pore structure and may be an interligand tetracarbene C_NHC^{^}C_NHC^{^}C_NHC^{^}C_NHC-cyclometalated [2L + 1M] structure containing a very small amount of Ni⁰ state. DFT calculations suggested enhanced stability for tetracarbene C_NHC^{^}C_NHC^{^}C_NHC^{^}C_NHC-cyclometalated complexes 9@Ni–NHC and 11. Cyclic voltammetry studies shown that complexes 7, 8, and 9@Ni–NHC have similar redox properties of two pairs of redox peaks, and complex 11 displays only one reversible redox process with half-wave potentials at E_1/2 = −0.864 V. All complexes were found to be efficient catalysts for the Suzuki–Miyaura cross-coupling reaction, and complexes 7 and 8 display better catalytic activity than complexes 9@Ni–NHC and 11 and exhibit catalytic activity comparable to Pd(OAc)₂ catalyst, which may be attributed to the intraligand chelating tetradentate coordination and the coplanar conjugate effect between the azole and quinoline rings.