What's in an Atom? A Comparison of Carbon and Silicon‐Centred Amidinium⋅⋅⋅Carboxylate Frameworks**.

Despite their apparent similarity, framework materials based on tetraphenylmethane and tetraphenylsilane building blocks often have quite different structures and topologies. Herein, we describe a new silicon tetraamidinium compound and use it to prepare crystalline hydrogen bonded frameworks with carboxylate anions in water. The silicon‐containing frameworks are compared with those prepared from the analogous carbon tetraamidinium: when biphenyldicarboxylate or tetrakis(4‐carboxyphenyl)methane anions were used similar channel‐containing networks are observed for both the silicon and carbon tetraamidinium. When terephthalate or bicarbonate anions were used, different products form. Insights into possible reasons for the different products are provided by a survey of the Cambridge Structural Database and quantum chemical calculations, both of which indicate that, contrary to expectations, tetraphenylsilane derivatives have less geometrical flexibility than tetraphenylmethane derivatives, that is, they are less able to distort away from ideal tetrahedral bond angles. [ABSTRACT FROM AUTHOR]