Four- and five-coordinate high-spin iron(II) complexes bearing bidentate soft/hard SN ligands based on 2-aminopyridine

Group VIII coordination complexes of bidentate P^N ligands bearing π-extended quinoline or phenanthridine N-heterocycles

Group 8 complexes of π-extended N-heterocyclic ligands present some unexpected results. Benzannulation shifts the lowest energy excitation to the blue by impacting the character of the transition rather than affecting frontier orbital energies.

Structures, Spectroscopic Properties, and Dioxygen Reactivity of 5- and 6-Coordinate Nonheme Iron(II) Complexes: A Combined Enzyme/Model Study of Thiol Dioxygenases

The synthesis of four new FeII(N4S(thiolate)) complexes as models of the thiol dioxygenases are described. They are composed of derivatives of the neutral, tridentate ligand triazacyclononane (R3TACN; R = Me, iPr) and 2-aminobenzenethiolate (abtx; X = H, CF3), a non-native substrate for thiol dioxygenases. The coordination number of these complexes depends on the identity of the TACN derivative, giving 6-coordinate (6-coord) complexes for FeII(Me3TACN)(abtx)(OTf) (1: X = H; 2: X = CF3) and 5-coordinate (5-coord) complexes for [FeII(iPr3TACN)(abtx)](OTf) (3: X = H; 4: X = CF3). Complexes 1-4 were examined by UV-vis, 1H/19F NMR, and Mössbauer spectroscopies, and density functional theory (DFT) calculations were employed to support the data. Mössbauer spectroscopy reveals that the 6-coord 1-2 and 5-coord 3- 4 exhibit distinct spectra, and these data are compared with that for cysteine-bound CDO, helping to clarify the coordination environment of the cys-bound FeII active site. Reaction of 1 or 2 with O2 at -95 °C leads to S-oxygenation of the abt ligand, and in the case of 2, a rare di(sulfinato)-bridged complex, [Fe2III(μ-O)((2-NH2) p-CF3C6H3SO2)2](OTf)2 ( 5), was obtained. Parallel enzymatic studies on the CDO variant C93G were carried out with the abt substrate and show that reaction with O2 leads to disulfide formation, as opposed to S-oxygenation. The combined model and enzyme studies show that the thiol dioxygenases can operate via a 6-coord FeII center, in contrast to the accepted mechanism for nonheme iron dioxygenases, and that proper substrate chelation to Fe appears to be critical for S-oxygenation.

Synthesis, coordination behavior and structural features of chiral iron(ii) PNP diferrocene complexes

New chiral PNP and PNHP ferrocene ligands and the corresponding Fe(ii) complexes of PNP ligands with the general formula [Fe(PNP)X₂] (X = Cl, Br) were synthesized.

Synthesis and reactivity of TADDOL-based chiral Fe(II) PNP pincer complexes-solution equilibria between κ(2)P,N- and κ(3)P,N,P-bound PNP pincer ligands

Treatment of anhydrous FeX2 (X = Cl, Br) with 1 equiv. of the asymmetric chiral PNP pincer ligands PNP-R,TAD (R = iPr, tBu) with an R,R-TADDOL (TAD) moiety afforded complexes of the general formula [Fe(PNP)X2]. In the solid state these complexes adopt a tetrahedral geometry with the PNP ligand coordinated in κ(2)P,N-fashion, as shown by X-ray crystallography and Mössbauer spectroscopy. Magnetization studies led to a magnetic moment very close to 4.9μB reflecting the expected four unpaired d-electrons (quintet ground state). In solution there are equilibria between [Fe(κ(3)P,N,P-PNP-R,TAD)X2] and [Fe(κ(2)P,N-PNP-R,TAD)X2] complexes, i.e., the PNP-R,TAD ligand is hemilabile. At -50 °C these equilibria are slow and signals of the non-coordinated P-TAD arm of the κ(2)P,N-PNP-R,TAD ligand can be detected by (31)P{(1)H} NMR spectroscopy. Addition of BH3 to a solution of [Fe(PNP-iPr,TAD)Cl2] leads to selective boronation of the pendant P-TAD arm shifting the equilibrium towards the four-coordinate complex [Fe(κ(2)P,N-PNP-iPr,TAD(BH3))Cl2]. DFT calculations corroborate the existence of equilibria between four- and five-coordinated complexes. Addition of CO to [Fe(PNP-iPr,TAD)X2] in solution yields the diamagnetic octahedral complexes trans-[Fe(κ(3)P,N,P-PNP-iPr,TAD)(CO)X2], which react further with Ag(+) salts in the presence of CO to give the cationic complexes trans-[Fe(κ(3)P,N,P-PNP-iPr,TAD)(CO)2X](+). CO addition most likely takes place at the five coordinate complex [Fe(κ(3)P,N,P-PNP-iPr,TAD)X2]. The mechanism for the CO addition was also investigated by DFT and the most favorable path obtained corresponds to the rearrangement of the pincer ligand first from a κ(2)P,N- to a κ(3)P,N,P-coordination mode followed by CO coordination to [Fe(κ(3)P,N,P-PNP-iPr,TAD)X2]. Complexes bearing tBu substituents do not react with CO. Moreover, in the solid state none of the tetrahedral complexes are able to bind CO.