Nitrosonium Complexes of Resorc[4]arenes: Spectral, Kinetic, and Theoretical Studies

Resorc[4]arene octamethyl ethers 1−3, when treated with NOBF4 salt in chloroform, form very
stable 1:1 nitrosonium (NO+) complexes, which are deeply colored. The complexation process is reversible,
and the complexes dissociate and bleach upon addition of methanol or water, to give the starting
macrocycles. Resorc[4]arenes 1 and 2 are in the same cone conformation, but with different side-chains,
whereas 3 possesses a different conformation (chair), while bearing the same side-chain as 2. Kinetic and
spectral UV−visible analysis revealed that NO+ interacts with resorc[4]arenes 1 and 2 both outside and
inside their basket, leading to complexes with two absorption patterns growing at different rates, one featuring
high-energy bands (HEB) within the near-UV zone, and the other one low-energy bands (LEB), attributed
to charge-transfer interactions, within the visible range. The presence of ester carbonyl groups in 2 strongly
drives the NO+ cation outside the resorcarene. Resorc[4]arene 3 showed a spectral pattern pointing out a
clear involvement of the ester moieties in the NO+ entrapping, beside the formation of significant charge-transfer interactions. 1H NMR spectroscopy and molecular modeling clearly supported these findings.