Sequential Coupling Approach to the Synthesis of Nickel(II) Complexes with N-aryl-2-amino Phenolates

A sequential multicomponent coupling approach is a powerful method for the construction of combinatorial libraries because structurally complex and diverse molecules can be synthesized from simple materials in short steps. In this paper, an efficient synthesis of nickel(II) complexes with N-aryl-2-amino phenols via a sequential three-step coupling approach is described, for potential use in nonlinear optical materials, bioinspired catalytic systems, and near-infrared absorbing filters. Seventeen N-aryl-2-amino phenolates were successfully synthesized in high yields based on the coupling of 3,5-di-tertbutylbenzene-1,2-diol with a pivotal aromatic scaffold, 4-bromo-2-iodo-aniline, followed by sequential Suzuki-Miyaura coupling with aryl boronates. A total of 16 analytically pure nickel(II) complexes with N-aryl-2-amino phenolates were obtained from 17 complexation trials. The procedure allowed us to assemble 4 components in high yields without protection, deprotection, oxidation or reduction steps. Various building blocks that included electron-donating, electron-withdrawing, and basic were used, and readily available, nontoxic and environmentally benign substrates and reagents were employed with no generation of toxic compounds. No strict anhydrous or degassed conditions were required. Absorption spectroscopic measurement of the synthesized nickel(II) complexes revealed that the ortho-substituent Ar-1 exerted more influence on the absorption wavelength of the complexes than the para-substituent Ar-2. On the other hand, both substituents Ar-1 and Ar-2 influenced the molar absorptivity values. These observations should be useful for the design of new and useful nickel(II) complexes as near-infrared chromophores.