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-activationby 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≡CH]0/[1]0), referred
to as thelow([RC≡CH]0/[1]0< 40) andhigh([RC≡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 thelowregime, whereas
up to four are involved in thehighregime. The second-order
rate dependence on1indicates that binuclear complexes
are the catalytically competent species in both regimes, with that
in thehighone being 6 times faster than that involved
in thelowone. Moreover, Eyring plot analyses revealed
two different catalytic cycles, with a rate-determining step more
endergonic in thelowregime than in thehighone, but with a more ordered transition state in thehighregime than in thelowone.