Single-Site Cobalt-Catalyst Ligated with Pyridylimine-Functionalized Metal–Organic Frameworks for Arene and Benzylic Borylation
Rajashree Newar, Wahida Begum, Neha Antil, Sakshi Shukla, Ajay Kumar, Naved Akhtar, Balendra Balendra, Kuntal Manna
Abstract
We report a highly active single-site heterogeneous cobalt-catalyst based on a porous and robust pyridylimine-functionalized metal–organic frameworks (pyrim-MOF) for chemoselective borylation of arene and benzylic C–H bonds. The pyrim-MOF having UiO-68 topology, constructed from zirconium-cluster secondary building units and pyridylimine-functionalized dicarboxylate bridging linkers, was metalated with CoCl2 followed by treatment of NaEt3BH to give the cobalt-functionalized MOF-catalyst (pyrim-MOF-Co). Pyrim-MOF-Co has a broad substrate scope, allowing the C–H borylation of halogen-, alkoxy-, alkyl-substituted arenes as well as heterocyclic ring systems using B2pin2 or HBpin (pin = pinacolate) as the borylating agent to afford the corresponding arene- or alkyl-boronate esters in good yields. Pyrim-MOF-Co gave a turnover number (TON) of up to 2500 and could be recycled and reused at least 9 times. Pyrim-MOF-Co was also significantly more robust and active than its homogeneous control, highlighting the beneficial effect of active-site isolation within the MOF framework that prevents intermolecular decomposition. The experimental and computational studies suggested (pyrim•−)CoI(THF) as the active catalytic species within the MOF, which undergoes a mechanistic pathway of oxidative addition, turnover limiting σ-bond metathesis, followed by reductive elimination to afford the boronate ester.