4.8 Article

Homotropic Cooperativity in Iron-Catalyzed Alkyne Cyclotrimerizations

Journal

ACS CATALYSIS
Volume 13, Issue 10, Pages 6610-6618

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/acscatal.3c00764

Keywords

cooperativity; iron; alkynes; cyclotrimerization; kinetic studies

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Enhancing catalytic activity through synergic effects is a current challenge in homogeneous catalysis. In addition to the well-established metal-metal and metal-ligand cooperation, we showcase an example of self-activation by the substrate in controlling the catalytic activity of a two-coordinate iron complex. This behavior was observed in the regioselective cyclotrimerization of aryl acetylenes. Two kinetically distinct regimes were observed dependent upon the substrate-to-catalyst ratio, showing positive substrate cooperativity.
Enhancing catalytic activity through synergic effects is a current challenge in homogeneous catalysis. In addition to the well-established metal-metal and metal-ligand cooperation, we showcase here an example of self-activation by the substrate in controlling the catalytic activity of the two-coordinate iron complex [Fe(2,6-Xyl2C6H3)2] (1, Xyl = 2,6-Me2C6H3). This behavior was observed for aryl acetylenes in their regioselective cyclotrimerization to 1,2,4-(aryl)-benzenes. Two kinetically distinct regimes are observed dependent upon the substrate-to-catalyst ratio ([RC equivalent to CH]0/[1]0), referred to as the low ([RC equivalent to CH]0/[1]0 < 40) and high ([RC equivalent to CH]0/[1]0 > 40) regimes. Both showed sigmoidal kinetic response, with positive Hill indices of 1.85 and 3.62, respectively, and nonlinear Lineweaver-Burk replots with an upward curvature, which supports positive substrate cooperativity. Moreover, two alkyne molecules participate in the low regime, whereas up to four are involved in the high regime. The second-order rate dependence on 1 indicates that binuclear complexes are the catalytically competent species in both regimes, with that in the high one being 6 times faster than that involved in the low one. Moreover, Eyring plot analyses revealed two different catalytic cycles, with a rate-determining step more endergonic in the low regime than in the high one, but with a more ordered transition state in the high regime than in the low one.

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