Synthesis and Catalytic Activity in Suzuki Coupling of Nickel Complexes Bearing <i>n</i>-Butyl- and Triethoxysilylpropyl-Substituted NHC Ligands: Toward the Heterogenization of Molecular Catalysts

Abstract

Cyclopentadienyl N-heterocyclic carbene (NHC) nickel complexes of general formula [Ni(R-NHC-nBu)XCp] [R-NHC-nBu = 1-butyl-3-methyl-, 1-isopropyl-3-butyl-, 1-phenyl-3-butyl-, 1-(2,4,6-trimethylphenyl)-3-butyl-, 1-(2,6-diisopropylphenyl)-3-butyl-imidazol-2-ylidene; X = Cl or I; Cp = η5-C5H5], which bear an n-butyl side-chain attached to one of the nitrogen atoms of the NHC ring, were synthesized as models for trialkoxysilylpropyl-substituted complexes. They were prepared by the direct reactions of nickelocene with the corresponding imidazolium salts (R-NHC-nBu·HX). The new complexes [Ni(Me-NHC-nBu)ClCp] (1a), [Ni(iPr-NHC-nBu)ClCp] (1b), [Ni(Ph-NHC-nBu)ICp] (1c), [Ni(Mes-NHC-nBu)ICp] (1d), and [Ni(iPr2Ph-NHC-nBu)ICp] (1e) were obtained in moderate to good yields and were fully characterized by standard spectroscopic techniques, and in the cases of 1a,b,d,e by single-crystal X-ray crystallography. The bulky electron-rich pentamethylcyclopentadienyl derivatives, [Ni(Mes-NHC-nBu)ICp*] (2d) and [Ni(iPr2Ph-NHC-nBu)ICp*] (2e) (Cp* = η5-C5Me5), were prepared from reactions of in situ prepared [Ni(acac)Cp*] with the corresponding carbene precursors. Both Cp* complexes were also fully characterized spectroscopically, and their structures were established by single-crystal X-ray crystallography. All new complexes catalyzed the Suzuki–Miyaura cross-coupling of phenylboronic acid with aryl halides in the absence of cocatalysts or reductants. However, the small dialkyl-substituted species 1a and 1b proved to be the least efficient. In addition, in contrast to our previous results with the closely related diaryl-substituted species [Ni(Ar2NHC)LCp†] (L = Cl–, NCMe (PF6–); Cp† = Cp, Cp*), in which complexes that bear the electron-rich Cp* ligand were much more active than those bearing the Cp ligand, no substantial catalytic behavior differences were observed between the Cp complexes 1d,e and their Cp* counterparts 2d,e. A TOF of up to 352 h–1, a so far unprecedented rate for nickel(II) complexes under similar conditions, was even observed with the Cp complex 1d. In view of these encouraging results, the triethoxysilylpropyl-substituted analogue of 1d, [Ni(Mes-NHC-TES)ClCp] (1d-TES) (Mes-NHC-TES = 1-(2,4,6-trimethylphenyl)-3-[3-(triethoxysilyl)propyl]imidazol-2-ylidene), was prepared, fully characterized, and tested catalytically. As it showed similar catalytic activity to 1d, it was heterogenized on alumina to give 1d-Al. The latter species, however, exhibited a greatly reduced catalytic activity compared to 1d and 1d-TES. Possible reasons for both the excellent activities of 1d and 1d-TES and the disappointing activity of 1d-Al are discussed.