Condensation reaction through base assistance within (Ph 2SiO) 8[AlO(OH)] 4

Abstract

When the polycyclic alumosiloxane (Ph 2SiO) 8[AlO(OH)] 4, which may be isolated as the diethyl ether adduct (Ph 2SiO) 8[AlO(OH)] 4·4OEt 2, is allowed to react with the double N-methylpiperidine (nmp) adduct of monochloroalane, AlH 2Cl·2nmp ( 1) (crystal structure analysis), the polycycle (Ph 2SiO) 8[AlO(O) 0.5] 4·2nmp ( 2) is obtained. Compared to the starting material and apart from the coordinating bases, the compound formally has lost two water molecules. The structure of (Ph 2SiO) 8[AlO(O) 0.5] 4·2nmp ( 2) can be derived from (Ph 2SiO) 8[AlO(OH)] 4 by substituting the central Al 4(OH) 4 motif through an Al 4O 2 entity which consists of a central Al 2O 2 ring coordinated to two further aluminum atoms through almost trigonal planar oxygen atoms. Using tris(ethylene)diamine (ted) as base and reacting it with (Ph 2SiO) 8[Al(OH)] 4, we have been able to isolate and completely characterize an intermediate on the way to these formally condensed alumosiloxane polycycles like in (Ph 2SiO) 8[AlO(O) 0.5] 4·2nmp ( 2). It has the composition (Ph 2SiO) 8[AlO(O) 0.25] 4·(OH·ted) 2·(OH 2·ted) ( 3) and has, compared to the starting material, the same number of hydrogen, oxygen, aluminum and silicon atoms within the inner molecular framework. Nevertheless, its structure is very different: whereas half of the molecule is structurally similar to (Ph 2SiO) 8[AlO(OH)] 4, with OH-groups forming hydrogen bridges to the nitrogen atoms of ted and connecting two aluminum atoms, the other half contains a unique oxygen atom which is in an almost planar trigonal bonding mode to three aluminum atoms. Furthermore, this part of the molecule has an aluminum atom to which a water molecule is coordinated, one of the hydrogen atoms being involved in hydrogen bonding to a further tris(ethylene)diamine (ted). This structure gives some important insights in the possible mechanism of the “condensation reaction” within (Ph 2SiO) 8[AlO(OH)] 4.