Ligand field factors in promoting S=3/2 {FeNO}(7) nitrosyls

The EPR spectra of {FeNO}(7) iron nitrosyls are of interest as models for nitrosylated nonheme proteins which exhibit the unusual S = 3/2 spin state. Few models of such S = 3/2 species are known, although [Fe(edta)NO](2-) adopts this spin behavior. The present study examines [Fe-II L] and [Fe-II L(NO)] complexes of polyaminopolycarboxylates and pyridylmethylamines which are derived from nta(3-) and edta(4-) as models of such systems. The series of L = nta(3-) (nitrilotriacetate), uedda(2-) (N,N'-ethylenediaminediacetate), pida(2-) (2-pyridylmethyliminodiacetate), tpa (tris(2-pyridylmethyl)amine), tpen (N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine), and edta(4-) (ethylenediaminetetraacetate) was chosen to provide ligands of increasing ligand field strength for both six-coordinate and seven-coordinate [FeL(NO)] complexes in order to investigate the effect on the net complexes near 1:1 methods ligand/Fe-II ratios are [Fe(pida)(H2O)(2)] and [Fe(uedda)(H2O)(2)] having E(1/2) values of 0.39 V and 0.26 V vs NHE (mu = 0.1, T = 22 degrees C). [Fe(tpa)(H2O)Cl](+) and [Fe(tpen)](2+) have waves at 0.547 V and 0.84 V. Bis complexes occur at 20:1 ratios for pida(2-) and uedda(2-). [Fe-II(NO)] complexes are spontaneously formed by admitting NO to Ar purged Fe-II L solutions. EPR spectra of frozen samples (77-105 K) show that [Fe(nta)NO](-) (g = 4.36, 4.02, 2.00), [Fe(pida)(H2O)(NO)] g = 4.02 (axial), 2.00), [Fe(tpda)Cl(NO)](+) (g = 4.00, 2.00) are S = 3/2 complexes. [Fe(tpen)NO](2+) (g = 2.03, 1.97, 1.96) is a low-spin S = 1/2 complex with no N-shf coupling, suggestive of a seven-coordinate structural analogue of the [Fe(edta)NO](2-), S = 3/2 complex. MO diagrams for six- and seven-coordinate [FeL(NO)] complexes as a function of increasing ligand field strength, which incorporate the spin-polarization effect for weak fields, are presented to explain the change from S = 3/2 to S = 1/2 of the nonheme protein model complexes. The strong-field limit yields the Enemark-Feltham order. Cyclic voltammetry and differential pulse polarography studies support the prior Rhodes-Barley-Meyer conclusion for [Fe(edta)NO](2-) that, for the [FeL(NO) complexes, the oxidation [Fe-II L(N over dot O] -1e(-) under right arrow [Fe-II L(NO+)] occurs coincidentally at the same, or nearly the same, potential as their [Fe-II L(H2O) -1e(-) under right arrow> [Fe-III L(H2O)] complex. The [Fe-II L(NO)] complexes of nta(3-), pida(2-), and edta(4-) are electrochemically silent at glassy cal-bon, whereas those of tpa and tpen are electrochemically reversible. Consistent with the MO predictions, six-coordinate S = 1/2 {FeNO}(7) complexes have both metal d(xz) and ligand NO N character in the HOMO, and exhibit N-shf in the EPR spectrum. Seven-coordinate S = 1/2 {FeNO]7 complexes have purely metal-based d(xy) HOMO's and exhibit no N-shf coupling. The absence of stable CO adducts for many nonheme proteins and their model complexes is explained by the MO orders and the reduction of spin polarization as a stabilizing factor. (C) 1997 Elsevier Srience Inc.