Size selectivity of a copper metal-organic framework and origin of catalytic activity in epoxide alcoholysis.

{Cu(bpy)(H(2)O)(2)(BF(4))(2)(bpy)} (Cu-MOF; MOF=metal-organic framework; bpy=4,4'-bipyridine), with a 3D-interpenetrated structure and saturated Cu coordination sites in the framework, possesses unexpectedly high activity in the ring-opening reaction of epoxides with MeOH, although the reaction rate drops remarkably with more bulky alcohols. This (apparent) size selection and the single Cu(2+) sites in an identical environment of the crystalline matrix resemble zeolites. The real nature of active sites was investigated by attenuated total reflection infrared (ATR-IR), Raman, EPR, and UV/Vis spectroscopies. Cu-MOF has highly dynamic structural properties that respond to MeOH; its framework dimensions change from 3D to 2D by restructuring to a symmetric coordination of four bpy units to Cu. This interaction is accompanied by the partial dissolution of Cu-MOF as multi-Cu clusters, in which Cu(2+) ions are connected with bpy ligands. Although both molecular and surface catalysis contribute to the high rate of alcoholysis, the soluble oligomeric species (Cu(m)bpy(n)) are far more active. Finally, addition of diethyl ether to the reaction mixture induces the reconstruction of dissolved and solid Cu-MOF to the original framework structure, thereby allowing excellent recyclability of Cu-MOF as an apparent heterogeneous catalyst. In contrast, the original Cu-MOF structure is maintained upon contact with larger alcohols, such as iPrOH and tBuOH, thus leading to poor activity in epoxide ring opening.