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) 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 BaN(SiHMe(2))(2)(thf)(4) and AlEt(3) to yield the hybrid material AlEt(3)@BaN(SiHMe(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(OtBu)(3). The reaction of equimolar amounts of {BaN(SiHMe(2))(2)}(n) and HOSi(OtBu)(3) produced a mixed silylamido/siloxide cluster of Ba(3)OSi(OtBu)(3)N(SiHMe(2))(2) 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)-HOSi(OtBu)(3) and Mg(AlMe(4))(2)-HOSi(OtBu)(3) reactions, leading to the isolation of complexes of Ba(AlEt(4))(2) (toluene) and MgOSi(OtBu)(3))(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(OtBu)(3) was studied. While putative MgOSi(OtBu)(3) and MgOSi(OtBu)(3) could not be crystallized from the reaction mixtures, cluster complexes Mg(5)(O)OSi(OtBu)(3)Me(3) and Mg(4)(OH)(2)OSi(OtBu)(3) could be unambiguously identified by X-ray crystallography.