Pore Environment Effects on Catalytic Cyclohexane Oxidation in Expanded Fe₂(dobdc) Analogues

Metal-organic frameworks are a new class of heterogeneous catalysts in which molecular-level control over both the immediate and long-range chemical environment surrounding a catalytic center can be readily achieved. Here, the oxidation of cyclohexane to cyclohexanol and cyclohexanone is used as a model reaction to investigate the effect of a hydrophobic pore environment on product selectivity and catalyst stability in a series of iron-based frameworks. Specifically, expanded analogues of Fe₂(dobdc) (dobdc^4- = 2,5-dioxido-1,4-benzenedicarboxylate) were synthesized and evaluated, including the biphenyl derivative Fe₂(dobpdc) (H₄dobpdc = 4,4'-dihydroxy-[1,1'-biphenyl]-3,3'-dicarboxylic acid), the terphenyl derivative Fe₂(dotpdc) (H₄dotpdc = 4,4″-dihydroxy-[1,1':4',1″-terphenyl]-3,3″-dicarboxylic acid), and three modified terphenyl derivatives in which the central ring is replaced with tetrafluoro-, tetramethyl-, or di-tert-butylaryl groups. Within these five materials, a remarkable 3-fold enhancement of the alcohol:ketone (A:K) ratio and an order of magnitude increase in turnover number are achieved by simply altering the framework pore diameter and installing nonpolar functional groups near the iron site. Mössbauer spectroscopy, kinetic isotope effect, and gas adsorption measurements reveal that variations in the A:K selectivities arise from differences in the cyclohexane adsorption enthalpies of these frameworks, which become more favorable as the number of hydrophobic residues and thus van der Waals interactions increase.