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The role of dispersion type metalπ interaction in the enantiotropic phase transition of two polymorphs of tris-(thienyl)bismuthine.

Two polymorphs of tris(thienyl)bismuthine Bi(2-C4H3S)3 (1) were isolated upon crystallization from n-hexane at different temperatures. The high temperature form 1-HT crystallized at 269 K in the trigonal space group R3[combining macron], whereas the low temperature form 1-LT crystallized at 245 K in the triclinic space group P1[combining macron]. An enantiotropic phase transition was observed at 250 K showing a transition energy of 1.4 kJ mol-1. Both polymorphs reveal the formation of centrosymmetric dimers that are based on London dispersion type bismuthπ heteroarene interactions. These primary building units show additional dispersion type interactions between neighbouring dimers and as a result 2D networks are formed. DFT calculations on the model systems BiX3π thiophene (X = Cl, Me) verify the hypothesis of a soft and shallow binding potential of the London dispersion type bismuthπ heteroarene interaction, providing an explanation for the reversibility of the phase transition.