Sulfur versus iron oxidation in an iron-thiolate model complex.
In the absence of base, the reaction of [Fe(II)(TMCS)]PF6 (1, TMCS = 1-(2-mercaptoethyl)-4,8,11-trimethyl-1,4,8,11-tetraazacyclotetradecane) with peracid in methanol at -20 °C did not yield the oxoiron(IV) complex (2, [Fe(IV)(O)(TMCS)]PF6), as previously observed in the presence of strong base (KO(t)Bu). Instead, the addition of 1 equiv of peracid resulted in 50% consumption of 1. The addition of a second equivalent of peracid resulted in the complete consumption of 1 and the formation of a new species 3, as monitored by UV-vis, ESI-MS, and Mössbauer spectroscopies. ESI-MS showed 3 to be formulated as [Fe(II)(TMCS) + 2O](+), while EXAFS analysis suggested that 3 was an O-bound iron(II)-sulfinate complex (Fe-O = 1.95 Å, Fe-S = 3.26 Å). The addition of a third equivalent of peracid resulted in the formation of yet another compound, 4, which showed electronic absorption properties typical of an oxoiron(IV) species. Mössbauer spectroscopy confirmed 4 to be a novel iron(IV) compound, different from 2, and EXAFS (Fe═O = 1.64 Å) and resonance Raman (ν(Fe═O) = 831 cm(-1)) showed that indeed an oxoiron(IV) unit had been generated in 4. Furthermore, both infrared and Raman spectroscopy gave indications that 4 contains a metal-bound sulfinate moiety (ν(s)(SO2) ≈ 1000 cm (-1), ν(as)(SO2) ≈ 1150 cm (-1)). Investigations into the reactivity of 1 and 2 toward H(+) and oxygen atom transfer reagents have led to a mechanism for sulfur oxidation in which 2 could form even in the absence of base but is rapidly protonated to yield an oxoiron(IV) species with an uncoordinated thiol moiety that acts as both oxidant and substrate in the conversion of 2 to 3.
Department of Chemistry and Center for Metals in Biocatalysis, 207 Pleasant Street SE, University of Minnesota, Minneapolis, Minnesota 55455, USA.
Aidan R
AR
McDonald
Michael R
MR
Bukowski
Erik R
ER
Farquhar
Timothy A
TA
Jackson
Kevin D
KD
Koehntop
Mi Sook
MS
Seo
Raymond F
RF
De Hont
Audria
A
Stubna
Jason A
JA
Halfen
Eckard
E
Münck
Wonwoo
W
Nam
Lawrence
L
Que
D007501:Iron / Q000737:chemistry*; D010084:Oxidation-Reduction; D013057:Spectrum Analysis; D013438:Sulfhydryl Compounds / Q000737:chemistry*; D013441:Sulfinic Acids / Q000737:chemistry; D013455:Sulfur / Q000737:chemistry*
D013438:Sulfhydryl Compounds; D013441:Sulfinic Acids; D013455:Sulfur; D007501:Iron