Metal-Organic Framework-Confined Iron Pincer Complex for Highly Efficient and Selective Borylation of Methane and Ethane.

We report the design and development of a heterogeneous NNP-pincer ligand, 6-((diisopropylphosphaneyl)amino)-(2,2'-bipyridine), which is integrated within a porous zirconium metal-organic framework (mNNP-UiO-67) to synthesize highly active earth-abundant metal catalysts for alkane functionalization. The mNNP-UiO-67 MOF-supported mononuclear iron(II)-dihydride (mNNP-UiO-FeH2) exhibits high efficiency in catalyzing selective monoborylation of methane and ethane. mNNP-UiO-FeH2-catalyzed borylation of methane using pinacolborane (HBpin) produces CH3Bpin in 76% yield, while significantly suppressing the formation of thermodynamically favored over-borylated products. The catalyst remains active over six recycling cycles, achieving a cumulative turnover number (TON) of 10,793, a methane utilization efficiency of 5.5%, and a space-time productivity of 13.5 gL-1h-1. The catalyst is also effective for ethane activation, yielding 36% of C2H5Bpin with 100% selectivity and a TON of 774. In stark contrast, the analogous homogeneous control is barely active, giving a TON of only 8 in methane borylation due to its rapid decomposition. Experimental results indicate that the stabilization of NNP-FeIIH2 active sites and the catalytic intermediates such as NNP-Fe0 and NNP-FeIIH(Bpin) within the MOF through active-site isolation is crucial for the remarkably high activity and selectivity. Mechanistic studies suggest the σ-bond metathesis between the Fe─Bpin bond of NNP-FeIIH(Bpin) and the C─H bond of methane as turnover-limiting in the catalytic cycle.