Homotropic Cooperativity in Iron-Catalyzed Alkyne Cyclotrimerizations.

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- by the substrate in controlling the catalytic activity of the two-coordinate iron complex [Fe(2,6-XylCH)] (, Xyl = 2,6-MeCH). 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≡CH]/[]), referred to as the ([RC≡CH]/[] < 40) and ([RC≡CH]/[] > 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 regime, whereas up to four are involved in the regime. The second-order rate dependence on indicates that binuclear complexes are the catalytically competent species in both regimes, with that in the one being 6 times faster than that involved in the one. Moreover, Eyring plot analyses revealed two different catalytic cycles, with a rate-determining step more endergonic in the regime than in the one, but with a more ordered transition state in the regime than in the one.