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<^>CNHC<^>CNHC<^>N-cyclometalated [1L + 1M] complex featuring coplanar of azole and quinoline rings, and Ni-NHC complex 11 is an interligand tetracarbene CNHC<^>CNHC<^>CNHC<^>CNHC-cyclometalated [2L + 1M] complex with two six-membered, conformation of boat-shaped, CNHC<^>CNHC-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 CNHC<^>CNHC<^>CNHC<^>CNHC-cyclometalated [2L + 1M] structure containing a very small amount of Ni0 state. DFT calculations suggested enhanced stability for tetracarbene CNHC<^>CNHC<^>CNHC<^>CNHC-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 E1/2 = -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.