Nickel(II) Complexes of Monofunctionalized Pyridine-Azamacrocycles: Synthesis, Structures, Pendant Arm “On-Off” Coordination Equilibria, and Peroxidase-like Activity

Novel nickel(II) complexes of pyridine-azamacrocycles (PyMACs) with pendant arms have been synthesized using simple, direct, and selective mono-N-functionalization of PyMACs. These complexes have been characterized by spectroscopy and X-ray crystallography. Nickel(II)-PyMAC complexes with a flexible pendant arm bearing a tertiary amine, a carboxylic acid, or an amide group exhibit structural and color changes due to “on-off” arm coordination to the metal center. Five- or six-coordinate complexes with the arm bound to the nickel(II) center are high-spin, while their four-coordinate “arm-off” counterparts are low-spin. Synergistic axial coordination of acetonitrile and the amide group from the pendant arm was observed. Coordination to the nickel(II) center lowers the pKa of the functional group attached to the macrocycle via a propylene linker by up to 4−5 orders of magnitude. Varying hydrogen bonding and proton-donating properties of the pendant arm affects the peroxidase-like activity of Ni(II)-PyMAC complexes in the oxidation of ABTS with hydrogen peroxide.

本研究通过对吡啶氮杂大环（pyridine-azamacrocycles, PyMACs）进行简便、直接且具有选择性的单N-官能化反应，成功制备了一系列带有侧臂的新型吡啶氮杂大环镍(II)配合物。该类配合物均通过光谱学方法与X射线晶体学完成了结构表征。带有含叔胺、羧酸或酰胺基团的柔性侧臂的镍(II)-PyMAC配合物，会因侧臂与金属中心发生“开-关”式配位作用，出现结构与颜色的变化。当侧臂配位于镍(II)中心时，形成的五配位或六配位配合物呈高自旋态；而侧臂脱离配位的四配位对应物则为低自旋态。实验中还观察到乙腈与侧臂酰胺基团之间存在协同轴向配位作用。与镍(II)中心的配位作用可使经由丙烯连接臂连接于大环的官能团的pKa值降低4~5个数量级。侧臂的氢键作用与质子供体性质的差异，会影响镍(II)-PyMAC配合物在过氧化氢氧化ABTS反应中的类过氧化物酶活性。