Condensation reaction through base assistance within (Ph2SiO)8[AlO(OH)]4

When the polycyclic alumosiloxane (Ph₂SiO)₈AlO(OH)]₄, which may be isolated as the diethyl ether adduct (Ph₂SiO)₈AlO(OH)]₄·4OEt₂, is allowed to react with the double N-methylpiperidine (nmp) adduct of monochloroalane, AlH₂Cl·2nmp (1) (crystal structure analysis), the polycycle (Ph₂SiO)₈AlO(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 (Ph₂SiO)₈AlO(O)_0.5]₄·2nmp (2) can be derived from (Ph₂SiO)₈AlO(OH)]₄ by substituting the central Al₄(OH)₄ motif through an Al₄O₂ entity which consists of a central Al₂O₂ ring coordinated to two further aluminum atoms through almost trigonal planar oxygen atoms. Using tris(ethylene)diamine (ted) as base and reacting it with (Ph₂SiO)₈Al(OH)]₄, we have been able to isolate and completely characterize an intermediate on the way to these formally condensed alumosiloxane polycycles like in (Ph₂SiO)₈AlO(O)_0.5]₄·2nmp (2). It has the composition (Ph₂SiO)₈AlO(O)_0.25]₄·(OH·ted)₂·(OH₂·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 (Ph₂SiO)₈AlO(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 (Ph₂SiO)₈AlO(OH)]₄.