Modeling the active sites of non-heme diiron metalloproteins with sterically hindered carboxylates and syn N-Donor ligands
Author(s)Friedle, Simone, 1976-
Massachusetts Institute of Technology. Dept. of Chemistry.
Stephen J. Lippard.
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Chapter 1. Different Synthetic Approaches to Modeling the Active Sites of Carboxylate-Bridged Non-Heme Diiron Enzymes Carboxylate-bridged non-heme diiron enzymes activate dioxygen to perform a variety of biological functions. Synthetic model compounds have been prepared to gain insight into the intricacies of dioxygen activation in these enzymes. In this introductory chapter, the challenges and advances of different diiron systems with terphenyl and dendrimer-appended carboxylates, nitrogen-rich, and syn Ndonor ligands are highlighted. Chapter 2. 9-Triptycenecarboxylate-Bridged Diiron(ll) Complexes: Capture of the Paddlewheel Geometric Isomer The synthesis and characterization of diiron(ll) complexes supported by 9- triptycenecarboxylate ligands (-O2CTrp) is described. The interlocking nature of the triptycenecarboxylates facilitates formation of quadruply bridged diiron(ll) complexes of the type [Fe 2(-O 2CTrp) 4(L) 2] (L = THF, pyridine or imidazole derivative) with a paddlewheel geometry. A systematic lengthening of the Fe-Fe distance occurs with the increase in steric bulk of the neutral donor L, resulting in values of up to 3 A without disassembly of the paddlewheel structure. Reactions with an excess of water do not lead to decomposition of the diiron(ll) core, indicating that these quadruply bridged complexes are exceptionally stable. The red-colored complexes [Fe2(f-O2CTrp) 4(4-AcPy) 2] (10) and [Fe2(M-O2CTrp)4(4- CNPy)2] (11) exhibit solvent-dependent thermochromism in coordinating solvents that was studied by variable temperature UV-vis spectroscopy.(cont.) Reaction of [Fe 2(1-O2CTrp)4(THF)2] with N,N,N',N'-tetramethylethylenediamine (TM EDA), tetra-n-butyl ammonium thiocyanate, or excess 2-methylimidazole resulted in the formation of mononuclear complexes [Fe(O 2CTrp) 2(TMEDA)] (13), (nBu 4N)2[Fe(O2CTrp) 2(SCN)2] (14), and [Fe(O 2CTrp)2(2-Melm) 2] (15) having an 04/N2 coordination sphere composition. Chapter 3. Synthesis, Characterization, and Oxygenation Studies of Carboxylate-Bridged Diiron(ll) Complexes with Aromatic Substrates Tethered to Pyridine Ligands and the Formation of a Unique Trinuclear Complex In this study, diiron(ll) complexes were synthesized as small molecule mimics of the reduced active sites in the hydroxylase components of bacterial multicomponent monooxygenases (BMMs). Tethered aromatic substrates were introduced in the form of 2-phenoxypyridines, incorporating hydroxy and methoxy functionalities into windmill-type diiron(llII) compounds [Fe 2(M-0 2CArR)2 - (O2CArR) 2(L) 2] (1-4), where -O2CArR is a sterically encumbering carboxylate, 2,6- di(4-fluorophenyl)- or 2,6-di(p-tolyl)benzoate (R = 4-FPh or Tol, respectively). The inability of 1-4 to hydroxylate the aromatic substrates was ascertained. Upon reaction with dioxygen, compounds 2 and 3 (L = 2-(m-MeOPhO)Py, 2-(p- MeOPhO)Py, respectively) decompose by a known bimolecular pathway to form mixed-valent diiron(ll,lll) species at low temperature. Use of 2-(pyridin-2- yloxy)phenol as the ligand L resulted in a doubly bridged diiron complex (4) and 5 an unprecedented phenoxide-bridged triiron(ll) complex (5) under slightly modified reaction conditions. Chapter 4.(cont.) Modeling the Syn-Disposition of Nitrogen Donors in Non-Heme Diiron Enzymes. Synthesis, Characterization and Hydrogen Peroxide Reactivity of Diiron(lll) Complexes with the Syn N-Donor Ligand H2BPG2DEV In order to model the syn disposition of histidine residues in carboxylate-bridged non-heme diiron enzymes, we prepared a new dinucleating ligand, H2BPG 2DEV, that provides this geometric feature. The ligand incorporates biologically relevant carboxylate functionalities, which have not been explored as extensively as nitrogen-only analogs. Three novel oxo-bridged diiron(lll) complexes [Fe 2( - O)(H20) 2(BPG2DEV)](CI0 4)2 (6), [Fe 2(p-O)(-O02CAriPro)(BPG 2DEV)](CIO 4) (7), and [Fe 2(,1-O)(-CO 3)(BPG 2DEV)] (8) were prepared. Single crystal X-ray structural characterization confirms that two pyridines are bound syn with respect to the Fe-Fe vector in these compounds. The carbonato-bridged complex 8 forms quantitatively from 6 in a rapid reaction with gaseous CO02 in organic solvents. A common maroon-colored intermediate (max = 490 nm; e= 1500 M- 1 cm - 1) forms in reactions of 6, 7, or 8 with H20 2 and NEt 3 in CH 3CN/H 20 solutions. Mass spectrometric analyses of this species, formed using 180-labeled H20 2, indicate the presence of a peroxide ligand bound to the oxo-bridged diiron(lll) center. The Mossbauer spectrum at 90 K of the EPR-silent intermediate exhibits a quadrupole doublet with 6 = 0.58 mm/s and AEQ = 0.58 mm/s. The isomer shift is typical for a peroxodiiron(lll) species, but the quadrupole splitting parameter is unusually small compared to related complexes.(cont.) These Mossbauer parameters are comparable to those observed for a peroxo intermediate formed in the reaction of reduced toluene/o-xylene monooxygenase hydroxylase (ToMOH) with dioxygen. Resonance Raman studies reveal an unusually low-energy 0-0 stretching mode in the peroxo intermediate that is consistent with a short diiron distance. Although peroxodiiron(lll) intermediates generated from 6, 7, and 8 are poor O-atom transfer catalysts, they display highly efficient catalase activity, with turnover numbers up to 10,000. In contrast to hydrogen peroxide reactions of diiron(Ill) complexes that lack a dinucleating ligand, the intermediates generated here could be reformed in significant quantities after a second addition of H20 2, as observed spectroscopically and by mass spectrometry. Appendix 1. Supporting Tables and Figures for Chapter 2 Appendix 2. Supporting Information for Chapter 4 Appendix 3. Synthesis of Triptycene Carboxylate-Bridged Dimetallic Complexes with First Row Transition Metals The synthesis and structural characterization of dimetallic complexes of the type [M2(1t-02CTrp) 4(THF)2] (M = Mn, Co, Ni, Cu, Zn) supported by triptycenecarboxylate ligands (-O2CTrp) is described. Appendix 4. Synthesis and Structure of a Molecular Ferrous Wheel, [Fe(0 2CH)(O 2CArPro)(1,4-dioxane)]6 The structural characterization of a novel, hexanuclear iron(ll) compound with the carboxylate ArPrOCO2- is described.
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2009.Cataloged from PDF version of thesis. Vita.Includes bibliographical references.
DepartmentMassachusetts Institute of Technology. Department of Chemistry
Massachusetts Institute of Technology