Isolation of a structural intermediate during switching of degree of interpenetration in a metal-organic framework

A known pillared layered metal-organic framework [Co₂(ndc)₂(bpy)] is shown to undergo a change in degree of interpenetration from a highly porous doubly-interpenetrated framework (2fa) to a less porous triply-interpenetrated framework (3fa). The transformation involves an intermediate empty doubly-interpenetrated phase (2fa') which has been isolated for the first time for this kind of phenomenon by altering the conditions of activation of the as-synthesized material. Interestingly, all the transformations occur in single-crystal to single-crystal fashion. Changes in degree of interpenetration have not been explored much to date and their implications with regard to the porosity of MOFs still remain largely unknown. The present study not only provides a better understanding of such dramatic structural changes in MOF materials, but also describes an original way of controlling interpenetration by carefully optimizing the temperature of activation. In addition to studying the structural mechanism of conversion from 2fa to 3fa, sorption analysis has been carried out on both the intermediate (2fa') and the triply-interpenetrated (3fa) forms to further explain the effect that switching of interpenetration mode has on the porosity of the MOF material.