Chemistry of Ferro- and Ferriverdins. Iron Redox and Geometrical Stereodynamism

Abstract

Synthetic ferroverdins, Fe(RQ)3-(2, R = Me,lBu, Cl, Br), have been isolated as green sodium salts via iron(II)-promoted nitrosation of phenols. Bivalent ions (M2+) displace Na+ affording trinuclear MFe2(RQ)6, e.g., MgFe2(MeQ)6. The 1,2-quinone 2-oximato formulation (2) and fac geometry (3a) of Fe(RQ)3- in NaFe(RQ)3 are revealed by1H NMR spectra. Natural ferroverdin liberated by a species of Streptomyces differs from 2 only in the substituent R. Both Fe3+ and Fe(bpy)33+ rapidly and quantitatively oxidize ferroverdin to ferriverdin, FeII(RQ)3-FeIII(RQ)3, with retention of fac geometry. The fac-Fe(RQ)3 isomer (4a) so formed is however labile and spontaneously isomerizes to me r-Fe(RQ)3(4b; equilibrium population > 85%). In contrast, the equilibrium concentration of mer-Fe(RQ)3-(3b) in solutions of ferroverdin is estimated to be < 0.5%. Both ferro- and ferriverdin are low spin: S = 0 and1/2, respectively. The EPR spectra of the latter in frozen-acetonitrile-toluene glass (77 K) are diagnostic of geometry: axial for fac (4a) and rhombic for mer (4b). The average axial and rhombic distortion parameters are 3800 and 0 cm-1, respectively, for 4a and 4900 and 2800 cm-1 for 4b. The ground-state electronic configuration is e4 a1 corresponding to gx, gy < gz. In 4b, an optical transition between two Kramers doublets is experimentally observable near 5600 cm-1. The cycle of redox and isomerization equilibria [formula omitted] has been electrochemically mapped. It is possible to electrogenerate the Fe(RQ)3z (z = -1, 0) species in a predetermined isomericconfiguration at low temperature. The equilibrium constants [3a]/[3b] and [4b]/[4a] lie in the ranges 200-300 and 6-8, respectively, at 298 K. The formal potential (0.46-0.75 V vs SCE) of the fac couple, 4a-3a, is more positive than that (0.27-0.55 V) of the mer couple, 4b-3b. This correlates with the larger axial distortion of 4b compared to 4a. The rates of the isomerization reactions 3b -* 3a and 4a -* 4b have been followed spectrophotometrically for R = Me. Both processes are intramolecular and obey the first-order rate law. Their activation parameters (kcal mol'1; ASeu) are 26.3 and 11.3 and 23.2 and 5.5, respectively. Mechanisms are unclear, but a twist pathway is chemically reasonable. © 1989, American Chemical Society. All rights reserved.