Characterization of [nickel(cyclam)] CB[8] as an outer-sphere redox agent

Long-range electron transfer reactions play a key role in the life-giving processes of photosynthesis and respiration; however, much is still unknown about electron transfer occurring over extended distances. The focus of this project is to design and study synthetic systems in which electron transfer will be forced to take place over larger than normal distances due to steric bulk surrounding a redox active metal centre. Two approaches are followed – (1) complexing bulky pendant arm macrocycles with redox active metals and (2) encapsulating a redox active metal macrocycle within a bulky host molecule—cucurbit[8]uril.

The metal pendant arm macrocycles proved to be insoluble in aqueous solutions, excluding them from kinetic studies. The inclusion complex, [Ni(cyclam)]@CB[8], however, is water-soluble at the low concentrations needed for kinetic analysis. Furthermore, the inclusion complex is readily oxidized to the nickel(III) form by the oxidant hexaaquacobalt(III). Upon oxidation, the [NiIII (cyclam)]@CB[8] complex can be used as an oxidant for the benzenediols, hydroquinone and catechol. The results of the above kinetic studies can be directly compared to previous studies involving free NiII(cyclam) as the reductant/oxidant. In this way, the steric effect of the bulky cucurbit[8]uril ring on the rate of electron transfer can be directly accounted for. The results indicate that encapsulation of the Ni(II) macrocycle within cucurbit[8]uril results in a factor of 2 decrease in the rates of electron transfer as compared to that of the free Ni(II) macrocycle. This indicates that steric bulk has a significant effect on the rate of electron transfer, as expected.

A novel "one-pot" synthesis of the inclusion complex [Ni(cyclam)]@CB[8], as well as the crystal structure of [Cu(cyclam)]@CB[8], showing cyclam in the rare trans-I configuration, will also be reported.