Vanadocalixarenes on Silica: Requirements for Permanent Anchoring and Electronic Communication

摘要

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-Bu~t-calix4- (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 ~(51)V NMR spectroscopy and analysis of the UV/vis spectrum of materials after calixarene removal via calcination. ~(13)C 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 information using lower-rim 1,3-disubstituted calixarene substituents via RO-metal interactions (R = lower-rim substituent). In addition, the critical necessity of covalent interactions between support and metal for maintaining isolated and redox stable vanadium-containing active sites is demonstrated by comparison with weakly physisorbed V(V)-calixarene complexes at similar surface densities. These rely on the noncovalent interactions of molecular precursor p-Bu~t-calix4-(OMe)(O)3V=0 (1n) with the surface of partially dehydroxylated silica yielding material 2n. Unlike the redox and configurationally stable sites in material 2c, vanadocalixarene species in 2n aggregate and undergo reduction, as observed via EPR spectroscopy, ultimately leading to mixed V oxidation states as detected via near-infrared spectroscopy.