Spontaneous H2 Loss through the Interaction of Squaric Acid Derivatives and BeH2

The most stable complexes between squaric acid and its sulfur‐ and selenium‐containing analogues (C₄X₄H₂; X=O, S, Se) with BeY₂ (Y=H, F) were studied by means of the Gaussian 04 (G4) composite ab initio theory. Squaric acid derivatives are predicted to be very strong acids in the gas phase; their acidity increases with the size of the chalcogen, with C₄Se₄H₂ being the strongest acid of the series and stronger than sulfuric acid. The relative stability of the C₄X₄H₂ ? BeY₂ (X=O, S, Se; Y=H, F) complexes changes with the nature of the chalcogen atom; but more importantly, the formation of the C₄X₄H₂ ? BeF₂ complexes results in a substantial acidity enhancement of the squaric moiety owing to the dramatic electron‐density redistribution undergone by the system when the beryllium bond is formed. The most significant consequence of this acidity enhancement is that when BeF₂ is replaced by BeH₂, a spontaneous exergonic loss of H₂ is observed regardless of the nature of the chalcogen atom. This is another clear piece of evidence of the important role that closed‐shell interactions play in the modulation of physicochemical properties of the Lewis acid and/or the Lewis base.