Mechanistic Insight from Energy and Volume Profiles for CO Binding to a Lacunar Iron(II) Cyclidene Complex

Abstract

The binding of CO to a lacunar Fe(II) cyclidene complex is characterized by very negative values for the activation entropy and activation volume, whereas the reverse dissociation reaction is accompanied by small positive activation entropy and activation volume data. The constructed volume profile suggests that, in the transition state, CO disappears completely into the ligand pocket during partial Fe−CO bond formation, followed by a high-spin to low-spin transition on Fe(II) during which the metal center moves into the ligand plane and accounts for the subsequent volume decrease.

Mechanistic elucidation of small molecule – transition metal interactions by kinetic techniques

“…In another study the binding of CO to lacunar Fe(ii) complexes was studied in detail as a function of temperature and pressure in acetonitrile. 12,13 The overall reaction for the binding of CO to [Fe II (PhBzXy)](PF 6 ) 2 can be summarised as in Scheme 1. The volume profile for this reaction is presented in Fig.…”

Reverse Spin‐Crossover and High‐Pressure Kinetics of the Heme Iron Center Relevant for the Operation of Heme Proteins under Deep‐Sea Conditions

“…This is due to the positive volume contribution caused by desolvatation, which is more prominent for “unprotected” porphyrins,11a such as [Fe(P3)]. The volume profile for the reaction of [Fe(P1)] (Figure 1 a) is contrary to that obtained for [Fe(P2)], [Fe(P3)] and a non‐heme cyclidene complex13 that is the only complex with the confined pocket for which pressure‐dependent binding of CO was reported (see related literature information in the Supporting Information). The striking feature of the overall addition reaction is its positive reaction volume ( Δ V ≠ CO =Δ V ≠ CO −Δ V ≠ −CO =40 cm 3 mol −1 Figure 1 a).…”

“…Consequently, going from the product to the transition state there is a significant volume increase (positive Δ V ≠ −CO ). By way of comparison, the partial molar volume of CO is about 33 cm 3 mol −1 ,13 and the low‐spin‐to‐high‐spin transition of the iron center is usually coupled to a volume change of about 15 cm 3 mol −1 14. The overall reaction volume for the formation of carbonyl complexes, starting from the five‐coordinate species, is smaller (about −35 cm 3 mol −1 and −25 cm 3 mol −1 for [Fe(P2)] and [Fe(P3)], respectively) than the sum of these two effects (Figure 1 b and Figure S45).…”

“…In general, CO binding to heme models and proteins was characterized by negative activation volume, because of the either solely existence of the negative Δ V ≠ Fe‐CO and Δ V ≠ SPIN factors or its domination in the rate determining event 2d. 13…”

Mechanistic Information on the Reversible Binding of NO to Mono‐ and Dinuclear FeII Complexes of a Biomimetic S4N Ligand

“…[9a] Several studies have been reported on various iron(II) complexes that exist in high-spin/low-spin electronic state equilibria, [10,[13][14][15][16][17] and these have shown that the volume changes associated with such a spin transition of d 6 iron(II) fall in the range from -4 to -22 cm 3 mol -1 , depending on the solvent and ligand structure around the metal. [13][14][15] The volume of activation found for the binding of NO in the present study lies within this range of values and supports that Fe-NO bond formation can involve a contribution from the high-spin to lowspin transition.…”