Cation Binding with Nickel Pincer-Crown Ether Complexes (2016)

Undergraduates: Stewart Kerr, Jacob Smith

Faculty Advisor: Alexander Miller
Department: Chemistry

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Previous research in the Miller lab has shown that the rates of cation-accelerated dihydrogen activation using an iridium monohydride cation ligated in a pentadentate fashion by an aza-crown ether containing aminophosphinite (NCOP) pincer ligand can be modulated by over two orders of magnitude using cationic alkali metal additives (Li+ or Na+). In an effort to better understand these rate enhancements and cation binding properties of this ligand framework, a series of analogous nickel(II) complexes supported by NCOP pincer-crown ether ligands with aza-15-crown-5 (15c5NCOP) or aza-18-crown-6 (18c6NCOP) macrocycles have been synthesized and their alkali metal cation binding abilities investigated using 1H NMR. The complex (15c5NCOP)NiBr interacts strongest with Li+ cations (Ka = 101 M-1, LiPF6, CD3CN, 293 K) and has weak interactions with Na+ (Ka = 3 M-1, NaBArF, CD3CN, 293 K) and K+ (Ka = 5 M-1, KPF6, CD3CN, 293 K). The stoichiometry of cation binding by (15c5NCOP)NiBr was determined to be 1:1 using the method of continuous variation. The (18c6NCOP)NiBr complex with the larger crown ether interacts strongest with Na+ cations (Ka = 1517 M-1, NaBArF, CD3CN, 293 K) and more weakly with Li+ (Ka = 35 M-1, LiPF6, CD3CN, 293 K) and K+ (Ka = 54 M-1, LiPF6, CD3CN, 293 K) cations. The complexes (15c5NCOP)NiX (X = Cl, I) were synthesized and their Li+ binding abilities were compared to (15c5NCOP)NiBr. Each of these complexes interact similarly with Li+ cations.