Different acidity and additive effects of zirconium metal-organic frameworks as catalysts for cyanosilylation

The Zr(iv) metal-organic framework with 1,4-benzenedicarboxylate (UiO-66) in different forms was studied as a solid catalyst for carbonyl cyanosilylation. The anhydrous material (UiO-66-A) obtained after calcination has open Lewis-acid sites and acts as a heterogeneous and size selective catalyst for the reaction of aldehydes and trimethylsilylcyanide (TMSCN). Notably, it was found that the as-synthesized hydrous form (UiO-66-H) shows comparable activity to UiO-66-A, so UiO-66 can be used as a catalyst for cyanosilylation with no need of high-temperature activation. With a number of intentionally designed control experiments, we demonstrated that the acetic acid enclosed in UiO-66-H during synthesis serves as a Brønsted acid to promote the reaction, though acetic acid is inactive by itself. The different acidity between UiO-66-H and UiO-66-A was confirmed by using the isomerization of α-pinene oxide as a probe reaction. Both UiO-66-H and UiO-66-A are recyclable without significant degradation in framework integrity and catalytic activity. In addition, it was unexpectedly found that pyridine, which is inactive alone, acts as co-catalyst, rather than a Lewis acid poison, to dramatically accelerate the catalytic reaction over UiO-66-H or UiO-66-A. A synergistic mechanism was suggested, in which the Lewis or Brønsted acid activates the aldehyde substrate while pyridine acts as a Lewis base to activate TMSCN.