One to Find Them All: A General Route to Ni(Ⅰ)-Phenolate Species

摘要

The past 20 years have seen an extensive implementation of nickel in homogeneous catalysis through the development of unique reactivity not easily achievable by using noble transition metals. Many catalytic cycles propose Ni(Ⅰ) complexes as potential reactive intermediates, yet the scarcity of nickel(Ⅰ) precursors and the lack of a general, non-ligand-specific protocol for their synthesis have hampered progress in this field of research. This has in turn also limited the access to novel, well-defined Ni(Ⅰ) species for the development of new catalytic reactions. Herein, we report a simple, general route to access a wide variety of Ni(Ⅰ)- phenolate complexes via an unusual example of an olefinic Ni(Ⅰ) complex, [Ni(COD)(OPh*)] (COD = 1,5-cyclooctadiene, OPh* = O(~tBu)_3C_6H_2). This route has proven to be highly efficient for several coordination numbers and ligand classes enabling access to the following complexes: [Ni(IPr)(OPh*)] (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), [Ni(dcype)(OPh*)] (dcype =1,2-bis(dicyclohexyl-phosphino)ethane), [Ni(dppe)(OPh*)] (dppe = l,2-bis(diphenylphosphino)ethane), and [Ni(terpy)(OPh*)] (terpy = 2,2':6',2"-terpyridine). Moreover, reacting [Ni(dcype)(OPh*)] with trimethylsilyl triflate has led to the isolation of a unique example of a cationic binuclear Ni(Ⅰ)-arene complex. All these complexes have been characterized by single-crystal X-ray, DFT, and EPR analyses, thus providing crucial experimental and theoretical information about their coordination environment and confirming a d~9 electronic structure for all complexes involved. Overall, this new synthetic approach offers exciting opportunities for the discovery of new stoichiometric and catalytic reactivity as well as the mechanistic elucidation of Ni-based catalytic cycles.