Grafting of [Mn(CH₂tBu)₂(tmeda)] on silica and comparison with its reaction with a silsesquioxane

The reaction of [Mn-(CH₂rBu)₂(tmeda)] (1) and a silica partially dehydroxylated at 700°C (SiO_2/700) yields a single surface species [(≡SiO)Mn(CH₂tBu)(tmeda)] (2a; tmeda = tetramethylethylendiamine), while a mixture of 2a and [(≡SiO) ₂Mn-(tmeda)] (2b) is obtained by using SiO_2/200, SiO _2/300, or SiO_2/500 as evidenced by mass balance analysis, and IR and EPR spectroscopy. The reaction of 1 and (c-C₅H ₉)₇Si₇O₁₂SiOH (3), a soluble silanol that is a molecular model for a silica support, generates the bis-siloxy complex 4, [{(c-C₅H₉)₇Si₇O ₁₂SiO)₂Mn(tmeda)₂], in a quantitative yield; compound 4 was characterized by single-crystal X-ray diffraction. These reactions exemplify the limitation of considering molecular silanol derivatives as straightforward and reliable homogeneous models for silica, and address the need for thorough characterization of surface species by the use of surface-science techniques. These studies show the possibility of preparing coordinatively and geometrically unique surface species that would be difficult to prepare by solution chemistry methods; insights into the chemical and physical properties of these surface species are also gained.