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

When the polycyclic alumosiloxane (Ph2SiO)8[AlO(OH)]4, which may be isolated as the diethyl ether adduct (Ph2SiO)8[AlO(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)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 (Ph2SiO)8[AlO(O)0.5]4·2nmp (2) can be derived from (Ph2SiO)8[AlO(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)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 (Ph2SiO)8[AlO(O)0.5]4·2nmp (2). It has the composition (Ph2SiO)8[AlO(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)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 (Ph2SiO)8[AlO(OH)]4.

The structurally characterized (Ph2SiO)8[AlO(O)0.25]4·(OH·ted)2·(OH2·ted) (3) represents the intermediate of the condensation reaction from the eight-membered ring (Ph2SiO)8[AlO(OH)]4 to the dehydrated product (Ph2SiO)8[AlO(O)0.5]4·2nmp (2) suggesting a stepwise mechanism.Figure optionsDownload as PowerPoint slide