Oxo- and Hydroxo-Bridged Heme-Copper Assemblies Formed from Acid−Base or Metal−Dioxygen Chemistry

Abstract

The iron−copper dinuclear active site in heme-copper oxidases (e.g., cyctochrome c oxidase) has spurred the development of the inorganic chemistry of bridged heme-copper complexes, including species possessing (porphyrinate)FeIII−O(H)−CuII−L moieties. We describe here the synthesis, by two routes, of [(F8TPP)FeIII−O−CuII(MePY2)]+ (5) {F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate; MePY2 = N,N-bis[2-(2-pyridyl)ethyl]methylamine}. First, 5-(CF 3 SO 3 ) was generated by reaction of [(MePY2)CuII](CF3SO3)2 (3-(CF 3 SO 3 ) 2 ) and [(F8TPP)FeIII−OH] (4) with triethylamine in THF or CH3CN in 65−70% yield. The complex was also prepared by reduction of O2 by a 1:1 mixture of copper(I) and iron(II) complexes, [(MePY2)CuI(CH3CN)](BArF) (1-(BArF)) (BArF = tetrakis(3,5-bis-trifluoromethylphenyl)borate) and (F8TPP)FeII (2) in O2-saturated THF or acetone, at −80 °C with subsequent warming to room temperature. Preliminary stopped-flow kinetics on the O2 reaction with the 1:1 mixture show the formation of at least two intermediates (i.e., a superoxo species (F8TPP)Fe−O2 first, and then a presumed peroxo-bridged Fe−O2−Cu species) prior to the formation of the final μ-oxo complex [(F8TPP)FeIII−O−CuII(MePY2)]+ (5-(BArF)). The 1H NMR spectrum of 5-(CF 3 SO 3 ) in CD2Cl2 exhibits a pyrrole peak at 67.7 ppm (corroborated by 2H NMR), while downfield (23.4 and 18.9 ppm) and dramatically upfield-shifted resonances (−87.7, −155.4 and −189.4) have been assigned to hydrogens of the MePY2 moiety, by specific deuteration. The μ-hydroxo complex [(F8TPP)Fe−(OH)−Cu(MePY2)](OTf)2 (6-(CF 3 SO 3 ) 2 ) was synthesized by addition of 3-(CF 3 SO 3 ) 2 to 4 in CH3CN, or by protonation of 5-(CF 3 SO 3 ) with CF3SO3H. In a 1H NMR-spectroscopic protonation titration (CF3SO3H), the pyrrole 67.7 ppm resonance for 5-(CF 3 SO 3 ) progressively converts to 70.3 ppm, diagnostic of 6-(CF 3 SO 3 ) 2 . The protonation is slow on the NMR time scale. The 1H NMR spectral properties are consistent with antiferromagnetically coupled high-spin iron(III) and Cu(II) ions (S = 2 spin state), and the interaction is weaker in 6-(CF 3 SO 3 ) 2 (5-(CF 3 SO 3 ), μeff = 5.05 μB; 6-(CF 3 SO 3 ) 2, μeff = 5.60 μB; Evans NMR method). By titration using a series of organic acids, the pK a for 6-(CF 3 SO 3 ) 2 has been estimated to be 16.7 < pK a < 17.6 (CH3CN solvent), or 9.6 ± 2 (aqueous). Plots of δ vs 1/T for both μ-oxo and μ-hydroxo complexes 5-(CF 3 SO 3 ) and 6-(CF 3 SO 3 ) 2 have been obtained, showing linear Curie (for downfield resonances) or anti-Curie (for upfield peaks) behavior.

Nitrogen Oxide Atom-Transfer Redox Chemistry; Mechanism of NO(g) to Nitrite Conversion Utilizing μ-oxo Heme-FeIII–O–CuII(L) Constructs

“…The compounds (F 8 )Fe II , 13b,38 , (TMPP)Fe II , 12 , [(tmpa)-Cu I (MeCN)][B(C 6 F 5 ) 4 ], 11,39 [(MePY2)Cu I (MeCN)] [B(C 6 F 5 ) 4 ], 13b [(AN)Cu I ][B(C 6 F 5 ) 4 ], 40 [(AN)Cu II (NO 2 )](SO 3 CF 3 ), and (F 8 )-Fe III (NO) 37b,41 were synthesized and characterized following literature methods. (TMPP)Fe III (Cl) was purchased from TriPorTech (Germany).…”

Models of Copper Enzymes and Heme-Copper Oxidases

“…7.11) and O2 results in the formation of a ^i-oxo Fe(M)-0-Cu(II) complex; ironsuperoxo and heme-Cu peroxo intermediates are detected or implicated. 188 Section 3.3: J. P. (Tollman and co-workers have developed still newer generations of heme-Cu complexes, which as models for cytochrome c oxidase, effect the electrocatalytic reduction of dioxygen. 189 ] + species is high-spin, the first example of such a hemecopper dioxygen (peroxo) complex.…”

“…192 An Fe(II)-Cu(I) complex with binucleating ligand employing a tethered heme axial pyridyl donor with pyridyl-amine tridentate chelate (for Cu) forms a peroxo level Cvadduct. 193 …”

Superoxo, μ-peroxo, and μ-oxo complexes from heme/O 2 and heme-Cu/O 2 reactivity: Copper ligand influences in cytochrome c oxidase models

“…To unambiguously identify pyrrole resonances, which aid iron oxidation and spin-state assignments (27), complementary 2 H NMR spectroscopic investi-**A minor 2͞O2 reaction is observed in CH2Cl2͞6% EtCN, but it does not affect the 1͞2͞O2 kinetics described in this report. ) S ϭ 2 systems, as previously described (14,15,17,28).…”