Site-specific synthesis of oxo-bridged mixed-valence binuclear complexes on mesoporous silica

We report the site-specific synthesis of mixed valence Ti-IV-O-Fe-II complexes within the pores of ordered mesoporous silica (SBA-15). By using 6-di-tert-butylpyridine as the selective activator of tripodally linked Ti-IV-OH groups of Ti-grafted SBA-15, the FeCl2 center dot 4H(2)O complexes reacted selectively with the nucleophilic Ti-IV-O- groups. The formation of Si-O-Fe-II byproducts, due to the reaction with the abundant Si-OH groups, was successfully restricted and the selectivity for forming the Ti-IV-O-Fe-II complexes exceeded 80%. The metal-metal interaction of Ti-IV-O-Fe-II complexes was confirmed by the appearance of Ti-IV/Fe-II -> Ti-III/Fe-III metal-to-metal charge transfer band, and their coordination, valency, and spin state were characterized by diffuse transmission UV-vis, Fourier transform IR, and Fe K-edge X-ray absorption fine structure measurements. It was also confirmed that the present methods can be extended to other metal combinations of Ti-IV-O-Ni-II and Ti-IV-O-Mn-II. The electron transfer processes occurring under photoinduced metal-to-metal charge transfer of oxo-bridged mixed valence complexes on silica supports have recently been proven as a new class of visible-light-sensitive redox centers. Thus, the present synthetic procedure allows the fabrication of a variety of photochemical reaction centers according to the molecular-level design.