Aluminum Hydroxide Secondary Building Units in a Metal–Organic Framework Support Earth-Abundant Metal Catalysts for Broad-Scope Organic Transformations

The intrinsic heterogeneity of alumina (Al 2 O 3 ) surface presents a challenge for the development of alumina-supported single-site heterogeneous catalysts and hinders the characterization of catalytic species at the molecular level as well as the elucidation of mechanistic details of the catalytic reactions. Here we report the use of aluminum hydroxide secondary building units (SBUs) in the MIL-53(Al) metal–organic framework (MOF) with the formula Al(μ 2 –OH)(BDC) (BDC = 1,4-benzenedicarboxylate) as a uniform and structurally defined functional mimic of Al 2 O 3 surface for supporting Earth-abundant metal (EAM) catalysts. The μ 2 –OH groups in MIL-53(Al) SBUs were readily deprotonated and metalated with CoCl 2 and FeCl 2 to afford MIL-53(Al)-CoCl and MIL-53(Al)-FeCl precatalysts which were characterized by powder X-ray diffraction, nitrogen sorption, elemental analysis, density functional theory, and extended X-ray fine structure spectroscopy. Activation with NaBEt 3 H converted MIL-53(Al)-CoCl to MIL-53(Al)-CoH which effectively catalyzed hydroboration of alkynes and nitriles and hydrosilylation of esters. X-ray photoelectron spectroscopy and X-ray absorption near-edge spectroscopy (XANES) indicated the presence of Al III and Co II centers in MIL-53(Al)-CoH while deuterium labeling studies suggested σ-bond metathesis as a key step for the MIL-53(Al)-CoH-catalyzed addition reactions. MIL-53(Al)-FeCl competently catalyzed oxidative Csp 3 –H amination and Wacker-type alkene oxidation. XANES analysis revealed the oxidation of Fe II to Fe III centers in the activated MIL-53(Al)-FeCl catalyst and suggested that oxidative Csp 3 –H amination occurs via the formation of Fe III –O t Bu species by single electron transfer between Fe II centers in MIL-53(Al)-FeCl and ( t BuO) 2 with concomitant generation of 1 equiv of t BuO· radical, C–H activation through hydrogen atom abstraction to generate alkyl radicals, protonation of Fe III –O t Bu by aniline to generate MIL-53(Al)-Fe III -anilide, and finally C–N coupling between the Fe III -anilide and alkyl radical to form the Csp 3 –H amination product and regenerate the Fe II catalyst. These highly active single-site MOF-based solid catalysts were readily recovered and reused up to five times without significant decrease in catalytic activity. This work thus demonstrates the great potential of using the aluminum hydroxide SBUs in MOFs to support EAM catalysts for important organic transformations.