Influence of Steric Hindrance on the Core Geometry and Sulfoxidation Chemistry of Carboxylate-Rich Diiron(II) Complexes

Abstract

The asymmetric terphenyl-2'-carboxylate ligand 3,5-dimethyl-1,1':3',1' '-terphenyl-2'-carboxylate, -O2CArPh,Xyl, was prepared in high yield. This ligand facilitates the assembly of the diiron(II) complexes [Fe2(micro-O2CArTol)2(O2CArPh,Xyl)2(THF)2] [2, -O2CArTol=2,6-di-p-tolylbenzoate], [Fe2(micro-O2CArTol)2(O2CArPh,Xyl)2(pyridine)2] (5), [Fe2(micro-O2CArPh,Xyl)2-(O2CArPh,Xyl)2(THF)2] (3), and [Fe2(micro-O2CArPh,Xyl)2(O2CArPh,Xyl)2(pyridine)2] (6), all of which have a windmill geometry. The iron-iron distance of 3.355[10] A in 6 is approximately 1 A shorter than that in the analogue [Fe2(micro-O2CArTol)2(O2CArTol)2(pyridine)2] (4) and similar to the approximately 3.3 A metal-metal separation at the active site of the reduced diiron(II) form of the soluble methane monooxygenase hydroxylase enzyme (MMOHred). A series of ortho-substituted picolyl-based ligands, 2-picSMe, 2-picSEt, 2-picStBu, 2-picSPh, 2-picSPh(Me3) (Ph(Me3)=mesityl), and 2-picSPh(iPr3) (Ph(iPr3)=2,4,6-triisopropylphenyl), were prepared and allowed to react with [Fe2(micro-O2CAr)2(O2CAr)2(THF)2] to produce [Fe2(micro-O2CAr)3(O2CAr)(picSR)] (7-13, Ar=ArTol or ArPh,Xyl) complexes in 45-87% yields. The substrates tethered to the pyridine N-donor ligands picSR, where R=Me, Et, tBu, or Ph, coordinate to one iron atom of the diiron(II) center by the nitrogen and sulfur atoms to form a five-membered chelate ring. The Fe-S distance be-comes elongated with increasing steric hindrance imparted by the R group. The most sterically hindered ligands, 2-picSPh(Me3) and 2-picSPh(iPr3), bind to the metal only through the pyridine nitrogen atom. The reactions of several of these complexes with dioxygen were investigated, and the oxygenated products were analyzed by 1H NMR spectroscopy and GC/MS measurements following decomposition on a Chelex resin. The amount of sulfoxidation product is correlated with the Fe...S distance. The ratio of oxidized to unoxidized thioether substrate varies from 3.5, obtained upon oxygenation of the weakly coordinated 2-picSPh ligand in 10, to 1.0, obtained for the bulky 2-picSPh(iPr3) ligand in 12, for which the iron-sulfur distance is >4 A. External thioether substrates were not oxidized when present in oxygenated solutions of paddlewheel and windmill diiron(II) complexes containing 1-methylimidazole or pyridine ligands, respectively.

Molecular Determinants of the Regioselectivity of Toluene/ o -Xylene Monooxygenase from Pseudomonas sp. Strain OX1

“…Certainly, an understanding of the molecular determinants of BMM substrate specificity, regioselectivity and enantioselectivity properties is preliminary to the rational design of new, improved catalysts, as proved by the large number of studies on BMM catalytic mechanisms and on synthetic analogues capable of catalyzing reactions similar to those catalyzed by BMMs (14,28,39).…”

Synthesis, Characterization, and Oxygenation Studies of Carboxylate‐Bridged Diiron(II) Complexes with Aromatic Substrates Tethered to Pyridine Ligands and the Formation of a Unique Trinuclear Complex

“…The Fe–S distance in the thioether complex is 3.090 Å, which indicates a very weak interaction between the two atoms made possible by the larger size and more diffuse orbitals of sulfur. [25] In the case of the phenoxypyridine complex, however, no bonding interaction is feasible at an Fe–O distance larger than 3 Å and the complex therefore adopts the typical windmill structure. It is unlikely that the use of different carboxylates, – O 2 CAr Tol vs. – O 2 CAr 4-FPh , would affect the coordination geometry.…”

“…Unlike previous diiron compounds with tethered substrates, the phenyl ring is located farther from the diiron center, as found by X-ray crystallographic structural analysis, and might be sterically unable to approach the oxygenated diiron center. [22-25,29] The oxidation of tethered phosphine and sulfide groups in analogous diiron systems revealed that the extent of oxidation diminishes when the substrate is systematically moved away from the diiron center. [23-25] In order to test this hypothesis, the tethered substrates would need to be redesigned to bring them closer to the diiron center.…”

“…To circumvent this problem, the substrates were tethered to ancillary neutral donor ligands. With the use of this approach, C–H activation of benzylic moieties in benzyl- and ethylpyridines,[22] oxidation of sulfides and phosphines,[23-25] and oxidative N -dealkylation reactions were achieved. [26-29] The extent of oxidation reflected the proximity of the substrate to the diiron center, and little or no oxidation was observed when a substrate moiety was installed in the meta or para position of the pyridine ligand.…”

Evolution of strategies to prepare synthetic mimics of carboxylate-bridged diiron protein active sites

“…This fortuitous discovery that high-valent diiron terphenylcarboxylate complexes could be intercepted by tethered substrates inspired subsequent studies of the reactivity of oxygenated intermediates toward organic moieties held in close proximity to the diiron center. Attachment of benzyl [84, 85], ethyl [85], ethynyl [86], phenoxyl [87], phosphido [84, 88], or sulfido [88, 89] units to an amine or pyridine ligand afforded a series of tethered substrates that could be easily incorporated into 13 , affording the corresponding diiron(II) compounds. After exposing the substrate/diiron(II) complex to O 2 , the reaction products were analyzed by gas chromatography-mass spectrometry (GC-MS).…”