Surface Organobarium and Organomagnesium Chemistry on Periodic Mesoporous Silica MCM‐41: Convergent and Sequential Approaches Traced by Molecular Models

The alkaline earth metal alkyl complexes [Ba(AlEt 4 ) 2 ] n and Mg(AlMe 4 ) 2 were directly grafted onto periodic mesoporous silica MCM‐41, which had been dehydroxylated at 270 °C (specific surface area a s : 1023 m 2  g −1 ; pore volume V p : 1.08 cm 3  g −1 ; main pore diameter 3.4 nm). Alternatively, barium alkyl surface species were generated by sequential grafting of MCM‐41 with Ba[N(SiHMe 2 ) 2 ] 2 (thf) 4 and AlEt 3 to yield the hybrid material AlEt 3 @Ba[N(SiHMe 2 ) 2 ] 2 (thf) 4 @MCM‐41. For a better understanding of the surface chemistry, AlEt 3 @MCM‐41 was also accessed. All hybrid materials were analyzed by diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, elemental analysis, nitrogen physisorption, and solid‐state NMR spectroscopy; this clearly revealed distinct surface chemistry for the alkylaluminate‐treated materials [Ba(AlEt 4 ) 2 ]@MCM‐41 and Mg(AlMe 4 ) 2 @MCM‐41. In an attempt to mimic the surface chemistry, the organometallic precursors were treated with HOSi(O t Bu) 3 . The reaction of equimolar amounts of {Ba[N(SiHMe 2 ) 2 ] 2 } n and HOSi(O t Bu) 3 produced a mixed silylamido/siloxide cluster of Ba 3 [OSi(O t Bu) 3 ] 3 [N(SiHMe 2 ) 2 ] 3 with bridging‐only siloxide ligands as well as one bridging and two terminal silylamido ligands. The Schlenk equilibrium was found to govern the [Ba(AlEt 4 ) 2 ] n –HOSi(O t Bu) 3 and Mg(AlMe 4 ) 2 –HOSi(O t Bu) 3 reactions, leading to the isolation of complexes of [Ba(AlEt 4 ) 2 (toluene)] 2 and Mg[OSi(O t Bu) 3 )] 2 (AlMe 3 ) 2 , respectively. Allowing for a donor‐induced cleavage of Mg(AlMe 4 ) 2 , the reaction of [MgMe 2 ] with one or two equivalents of HOSi(O t Bu) 3 was studied. While putative Mg[OSi(O t Bu) 3 ](Me) and Mg[OSi(O t Bu) 3 ] 2 could not be crystallized from the reaction mixtures, cluster complexes Mg 5 (O)[OSi(O t Bu) 3 ] 5 Me 3 and Mg 4 (OH) 2 [OSi(O t Bu) 3 ] 6 could be unambiguously identified by X‐ray crystallography.