Controlling the Formation of Two Concomitant Polymorphs in Hg(II)Coordination Polymers

Controlling the formation of the desired product inthe appropriate crystalline form is the fundamental breakthrough ofcrystal engineering. On that basis, the preferential formationbetween polymorphic forms, which are referred to as differentassemblies achieved by changing the disposition or arrangement ofthe forming units within the crystalline structure, is one of the mostchallenging topics still to be understood. Herein, we have observedthe formation of two concomitant polymorphs with generalformula {[Hg(Pip)2(4,4 '-bipy)]middotDMF}n(P1A, P1B;Pip=piperonylic acid; 4,4 '-bipy = 4,4 '-bipyridine). Besides, [Hg-(Pip)2(4,4 '-bipy)]n(2) has been achieved during the attempts toisolate these polymorphs. The selective synthesis ofP1AandP1Bhas been successfully achieved by changing the syntheticconditions. The formation of each polymorphic form has been ensured by unit cell measurements and decomposition temperature.The elucidation of their crystal structure revealedP1AandP1Bas polymorphs, which originates from the Hg(II) cores andintermolecular associations, especially pinpointed by Hgmiddotmiddotmiddot pi and pi middotmiddotmiddot pi interactions. Density functional theory (DFT) calculationssuggest thatP1B, which shows Hg(II) geometries that are further from ideality, is more stable thanP1Aby 13 kJmiddotmol-1per[Hg(Pip)2(4,4 '-bipy)]middotDMF formula unit, and this larger stability ofP1Barises mainly from metalmiddotmiddotmiddot pi and pi middotmiddotmiddot pi interactions betweenchains. As a result, these structural modifications lead to significant variations of their solid-state photoluminescence.