Organoarsine Metal-Organic Framework as a Solid-State Ligand for Rhodium(I) Olefin Hydroformylation Catalysis

Abstract

A new triaryl arsine (Ar₃As)-based metal-organic framework (MOF) named AsCM-102 has been prepared by the reaction of As(C₆H₄-4-CO₂H)₃ with Co(BF₄)₂ and 4,4'-bipyridine. AsCM-102 contains pairs of staggered As donors that function as trans-chelators for the facile incorporation of organometallic Rh^I species via a single crystal-to-single crystal transformation. Coordination of Rh^I is achieved by soaking crystals in a solution of [Rh(CO)₂Cl]₂ at 70 °C. The originally closed and offset As₂ pockets expand to facilitate the trans-As₂ chelation of Rh^I. The resulting metalated MOF displays trans-[(Ar₃As)₂Rh(CO)Cl_n]^(1-n)+ complexes inside uniquely confined micropore reaction environments. Installation of the As-Rh-As moieties significantly enhances the internal porosity of the MOF. Crystalline Rh^I-AsCM-102 is an air-stable and recyclable hydroformylation catalyst, which is more active than its phosphine-based analogue. It is also selective toward the formation of iso-aldehydes over n-aldehydes with various C₆-C₈ olefin feedstocks. By leveraging the absolute atomic coordinates of Rh^I-AsCM-102 obtained from single-crystal X-ray diffraction analysis, density functional theory (DFT) explains the experimentally observed iso-favored hydroformylation regioselectivity due to pore confinement. Rh^I-AsCM-102 is resistant toward leaching of As into solution under forcing reaction conditions (40 atm of CO/H₂, 70 °C). This work demonstrates the premise that incorporation of organo(arsines) into MOF scaffolds is a safer and more convenient strategy for their deployment in catalysis, by alleviating M-As bond lability and As toxicity issues, which prevents their widespread use in homogeneous catalysis.