A Structural and Functional Model for Dioxygenases with a 2‐His‐1‐carboxylate Triad

Abstract

An attractive model: The iron complex shown on the left models the 2‐His‐1‐carboxylate active sites of Rieske dioxygenases both in terms of structure and function. 18O‐labeling studies of olefin dihydroxylation support the involvement of a high‐valent iron‐oxo species.

Mimicry of the 2‐His‐1‐Carboxylate Facial Triad Using Bulky N,N,O‐Ligands: Non‐Heme Iron Complexes Featuring a Single Facial Ligand and Easily Exchangeable Co‐Ligands

“…The minor complex is proposed to be either an isomer or a dinuclear complex. The 19 F NMR spectrum shows a somewhat broadened peak (fwhm = 73 Hz) for the triflate anion, indicating that it is at least partially coordinating to an iron center. In the case of FeL 2 type complexes with non-coordinating triflate counterions, a sharp fluorine signal typical for free triflate ions is observed.…”

“…The series of 19 F NMR spectra using both types of neutral ligands range from a very broad signal with a full width half maximum (fwhm) larger than 2000 Hz when iron triflate is in excess, to a somewhat sharper signal (fwhm around 500 Hz) when the ligand is in excess (Figures S2 and S5). The observation of a single signal indicates exchange on the NMR timescale, with the broadening being due to relatively slow exchange and/or to paramagnetic broadening.…”

“…Furthermore, the equilibrium might include coordination and dissociation of the triflate counterions and the coordinating carbonyl group. Finally, the 19 F NMR spectrum shows a broad (fwhm = 1581 Hz) signal at -66 ppm typical for a coordinating or exchanging triflate ion.…”

“…The mono-ligand iron(II) complex [Fe(L1)(X)] (X = OTf or Cl), featuring a bidentate carboxylate within a facial ligand setting, closely resembles the facial triad coordination sphere, as reported by Que et al (Figure 1). [19] This complex is active in the epoxidation as well as in the cisdihydroxylation of olefins, although catalytic turnover was not achieved. Studies by Rutledge et al focused on the use of mixed N,O-ligands based on a central pyridine donor (H2-R; Figure 1).…”

“…[1,[6][7][8][9][10][11][12][13][14][15][16][17] Highly active and selective catalysts for the epoxidation of olefins with turnover numbers over 120 and high stereoselectivity (97 % ee) [18] been described. [10,[19][20][21][22][23][24][25] Most of these complexes are structurally distinct from the typical non-heme enzyme active site in the sense that these lack a carboxylate, or in general an Odonor moiety and that the N-donors often stem from a pyridine ring instead of imidazole rings.…”