Vanadocalixarenes on Silica: Requirements for Permanent Anchoring and Electronic Communication

Abstract

Grafted V(V)−calixarene complexes that are 1,3-disubstituted at the lower rim have been synthesized using an extraordinarily mild procedure, using the V(III)−calixarene precursor p-But−calix[4]−(OMe)2(O)2V−Cl (1c), which is subsequently grafted onto silica and, afterward, oxidized to yield material 2c. The calixarene acts as a sterically bulky chelating ligand and enforces isolated pseudo-octahedral vanadium centers on silica, as ascertained by 51V NMR spectroscopy and analysis of the UV/vis spectrum of materials after calixarene removal via calcination. 13C CP/MAS NMR spectroscopy of 2c reveals lack of a downfield-shifted ipso carbon resonance, which is strongly indicative of a high d-orbital electron density for the V metal centers in 2c, a result that is expected on the basis of a proposed simple geometric model that is consistent with single-crystal X-ray crystallographic data of early transition metal metallocalixarenes. This result motivates future programming of grafted metal centers with chemical ...