Light-controlled switching of the spin state of iron(III)

Abstract

Controlled switching of the spin state of transition metal ions, particularly of FeII and FeIII, is a prerequisite to achieve selectivity, efficiency, and catalysis in a number of metalloenzymes. Here we report on an iron(III) porphyrin with a photochromic axial ligand which, upon irradiation with two different wavelengths reversibly switches its spin state between low-spin (S = 1/2) and high-spin (S = 5/2) in solution (DMSO-acetone, 2:598). The switching efficiency is 76% at room temperature. The system is neither oxygen nor water sensitive, and no fatigue was observed after more than 1000 switching cycles. Concomitant with the spin-flip is a change in redox potential by ~60 mV. Besides serving as a simple model for the first step of the cytochrome P450 catalytic cycle, the spin switch can be used to switch the spin-lattice relaxation time T1 of the water protons by a factor of 15.

Recent Implementations of Molecular Photoswitches into Smart Materials and Biological Systems

“…The molecular spin switch has been applied for photoswitching of magnetic resonance imaging (MRI) contrast by a factor of 6.7; the change is clearly visible with a clinical MRI scanner . According to the same principle, an iron(III)–porphyrin complex with a photochromic axial ligand, upon irradiation with two different wavelengths, showed reversible switching between the low‐ ( S= 1/2) and high‐spin ( S= 5/2) states in solution, constituting a simple model for the first step of the cytochrome P450 catalytic cycle …”

Dendronised Ni(ii) porphyrins as photoswitchable contrast agents for MRI

“…Further studies are devoted to improving the switching efficiency and relaxivity by the replacement of the pyridine ligand by imidazole, 24 or by replacing Ni 2+ with Fe 3+ within the porphyrin core. 28,29…”

Unusual KIE and dynamics effects in the Fe-catalyzed hetero-Diels-Alder reaction of unactivated aldehydes and dienes

“…10). However, compared with the observed admixture states 54 , this method was found to significantly overstabilize the high-spin states 56 than the quartet state (ΔG S-Q =~−11.8 kcal mol −1 ). Whereas the PBE0 method was a good method to describe this energy gap 57 and also supported the SMD B3LYP-D3 results in this system.…”

“…In addition, the sixcoordinate mode pathway via 4 TS1A endo has the lowest reaction barrier, which is just lower by~0.3 kcal mol −1 than that for the five-coordinate mode pathway via 4 TS2A endo by the SMD B3LYP-D3 method, due to the lower carbonyl LUMO energy in 4 IVA. Notably, driven by the above-mentioned larger carbonyl LUMO activation caused by the high-spin metal and the nature of admixed spin states 54 , the reaction barriers of these two pathways in the sextet state are also slightly higher than those of their quartet counterparts by 0.3-1.4 kcal mol −1 by the SMD B3LYP-D3 method. Finally, the six-coordinate mode pathway to form product 4 IIIA endo is exothermic by 2.6 kcal mol −1 .…”

“…The computed freeenergy difference between 4 IA and 6 IA can be reduced to 1.3-4.8 kcal mol −1 by the SMD PBE0-D3//B3LYP-D3 and SMD B3PW91-D3//B3LYP-D3 methods ( Supplementary Table 3A). Since several Fe(III)-porphyrin complexes with two axial oxygen ligands were experimentally found to have admixed spin states (quartet and sextet states) 54 , the B3LYP-D3 method modestly overestimates the stability of 4 IA relative to 6 IA. In addition, 4 IA is more stable than the five-coordinate intermediate 4 IVA with one axial PhCHO ligand by 4.1-4.8 kcal mol −1 by the SMD B3LYP-D3//B3LYP-D3 and SMD B3LYP-D3 methods.…”