Diiron Enzymes: Synthesis, Characterization and Hydrogen Peroxide Reactivity of Diiron(Ill) Complexes with the Syn N-Donor Ligand H2BPG 2DEV.
- B. Wong, S. Friedle, S. J. Lippard
- J. Am. Chem. Soc
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 9triptycenecarboxylate 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(4CNPy)2] (11) exhibit solvent-dependent thermochromism in coordinating solvents that was studied by variable temperature UV-vis spectroscopy. 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 0 4/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, 2,6di(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-(pMeOPhO)Py, respectively) decompose by a known bimolecular pathway to form mixed-valent diiron(ll,lll) species at low temperature. Use of 2-(pyridin-2yloxy)phenol as the ligand L resulted in a doubly bridged diiron complex (4) and