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

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(μ-O2CArTol)2(O2CArPh,Xyl)2(THF)2] [2, -O2CArTol = 2,6-di-p-tolylbenzoate], [Fe2(μ-O2CArTol)2(O2CArPh,Xyl)2(pyridine)2] (5), [Fe2(μ-O2CArPh,Xyl)2-(O2CArPh,Xyl)2(THF)2] (3), and [Fe2(μ-O2CArPh,Xyl)2(O2CArPh,Xyl)2(pyridine)2] (6), all of which have a windmill geometry.
The iron−iron distance of 3.355[10] Å in 6 is ∼1 Å shorter than that in the analogue [Fe2(μ-O2CArTol)2(O2CArTol)2(pyridine)2] (4) and similar to the ∼3.3 Å 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(μ-O2CAr)2(O2CAr)2(THF)2] to
produce [Fe2(μ-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 Å. 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.