Ligand Binding to Four-Atom-Linked Capped Porphyrins

Equilibrium measurements and kinetic behavior from seconds to picoseconds are described for the binding of O₂ or CO to the four-atom-linked capped porphyrin systems Fe(Por)(base), where Por = OC₂OPor (1) or OC-(CO)NPor (2) and base = 1-methylimidazole (1-Me₂Im) or 1, 2-dimethylimidazole (1, 2-Me₂Im). Binding of O₂ or CO to the amide-linked system Fe(2)(base) would impose nonplanarity on the amide linkages; this does not occur as this system does not bind O₂ or CO even with gas pressures of 100 atm. In contrast, the ether-linked system Fe(1)(base), with its sterically crowded pocket, is able to bind CO, but with exceptionally high values of P_½^CO of 100, 40, and 17 Torr at +25, 0, and -20 °C, respectively, for Fe(1)(1-MeIm) and ~5 x 10³ Torr at 25 °C for Fe(1)(1, 2-Me₂Im). The thermodynamics of binding of CO to Fe(1)(1-MeIm) involves a normal entropy term of ΔS° = -29(3) eu but a less negative than normal enthalpy term of ΔH° = -5.8(8) kcal/mol. Binding of CO to Fe(1)(1-MeIm) is not isosbestic below -30 °C. The value of P_½^O₂ of Fe(1)(1-MeIm) is 61 Torr at -43 °C; above that temperature the binding is not reversible. Kinetic analysis of Fe(1)(1-MeIm)(CO) yields the exceptionally low value of the association constant k_B^CO of 18.5(5) M^-1 s^-1 and a normal value of the dissociation constant k_B^-CO of 0.044(2) s^-1. In that system, after subpicosecond photolysis, there is evidence that there may be ~15% geminate recombination occurring with a half-life of 14 ps. This is the first hint of trapping in a model system, but it is still far from the type of trapping seen in proteins, which persists for hundreds of nanoseconds. For the 1-MeIm complex at high pressures of CO, and especially for the 1, 2-Me₂Im complex with CO, which can only be detected at high pressures, kinetics observed following photodissociation are complex.