Sh. Yim et al., A NEW MODEL FOR THE REDUCED FORM OF PURPLE ACID-PHOSPHATASE - STRUCTURE AND PROPERTIES OF [FE2BPLMP(OAC)(2)](BPH4)(2), Bulletin of the Korean Chemical Society, 19(6), 1998, pp. 654-660
[(FeFeBPLMP)-Fe-II-B-III(OAc)(2)](BPh4)(2) (1), a new model for the re
duced form of the purple acid phosphatases, has been synthesized by us
ing a dinucleating ligand, 2,6-bis methyl-2-pyridylmethyl)amino)methyl
-4-methylphenol (HBPLMP). Complex 1 has been characterized by X-ray di
ffraction method as having (mu-phenoxo)bis(acetato)diiron core. Comple
x 1 was crystallized in the monoclinic space group C2/c with the follo
wing cell parameters: a=41.620(6) Angstrom, b=14.020(3) Angstrom, c=27
.007(4) Angstrom, beta=90.60(2)degrees, and Z=8. The iron centers in t
he complex 1 are ordered as indicated by the difference in the Fe-O bo
nd lengths which match well with typical Fe-III-O and Fe-II-O bond len
gths. Complex 1 has been studied by electronic spectral, NMR, EPR, SQU
ID, and electochemical methods. Complex 1 exhibits strong bands at 592
Mn, 1380 nm in CH3CN (epsilon=1.0x10(3), 3.0x10(2)). These are assign
ed to phenolate-to-Fe-III and intervalence charge-transfer transitions
, respectively. Its NMR spectrum exhibits sharp isotropically shifted
resonances, which number half of those expected for a valence-trapped
species, indicating that electron transfer between Fe-II and Fe-III ce
nters is faster than NMR time scale. This complex undergoes quasirever
sible one-electron redox processes. The Fe-2(III)/(FeFeIII)-Fe-II and
(FeFeIII)-Fe-II/Fe-2(II), redox couples are at 0.655 and -0.085 V vs S
CE, respectively. It has K-comp=3.3x10(12) representing that BPLMP/bis
(acetate) ligand combination stabilizes a mixed-valence (FeFeIII)-Fe-I
I complex in the air. Complex 1 exhibits a broad EPR signal centered n
ear g=1.55 which is a characteristic feature of the antiferromagnetica
lly coupled high-spin (FeFeIII)-Fe-II system (S-total=1/2). This is co
nsistent with the magnetic susceptibility study showing the weak antif
erromagnetic coupling (J=-4.6 cm(-1), H=-2JS(1). S-2) between Fe-II an
d Fe-III center.