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Disappearing polymorphs in metal-organic framework chemistry: unexpected stabilization of a layered polymorph over an interpenetrated 3-dimensional structure in mercury imidazolate.

The "disappearing polymorphs" phenomenon is well-established in organic solids, and has had a profound effect in pharmaceutical materials science. We now report the first example of this effect in metal-containing systems in general, and in coordination network solids in particular. Specifically, attempts to mechanochemically synthesize a known interpenetrated diamondoid (dia) mercury(II) imidazolate metal-organic framework (MOF) yielded a novel, more stable polymorph based on square-grid (sql) layers. Simultaneously, the dia-form was found to be highly elusive, observed only as a short-lived intermediate in monitoring solvent-free synthesis and not at all from solution. The destabilization of a dense dia-framework relative to a lower dimensionality one is in contrast to the behavior of other imidazolate MOFs, with periodic density functional theory (DFT) calculations showing that it arises from weak interactions, including structure-stabilizing agostic C-H···Hg contacts. While providing a new link between MOFs and crystal engineering of organic solids, these findings highlight a possible role for agostic interactions in directing topology and stability of MOF polymorphs.