First {Fe−NO}6 Complex with an N2S3Fe−NO Core as a Model of NO-Inactivated Iron-Containing Nitrile Hydratase. Are Thiolates and Thioethers Equivalent Donors in Low-Spin Iron Complexes?

Abstract
The spectroscopic and structural properties of [(bmmp-TASN)FeNO]BPh4 (1) (bmmp-TASN = 4,7-bis(2‘-methyl-2‘-mercaptopropyl)-1-thia-4,7-diazacyclononane) have been determined and are compared with the nitric oxide inactivated form of iron-containing nitrile hydratase, NHasedark. [(bmmp-TASN)FeNO]BPh4 is prepared from the addition of NO to (bmmp-TASN)FeCl followed by addition of sodium tetraphenylborate. [(bmmp-TASN)FeNO]BPh4 crystallizes from acetonitrile−methanol solutions upon ether vapor diffusion as dark blue plates in the monoclinic space group P21/c with a = 11.9521(14) Å, b = 11.3238(13) Å, c = 26.624(3) Å, β = 98.280(2)°, and Z = 4. The νNO stretching frequency of 1856 cm-1 and the Mössbauer parameters, δ = 0.06 mm/s and ΔEq = 1.75 mm/s, compare favorably with those of NHasedark. The similarities of the iron−sulfur bond distances to the thiolate, 2.284(2) Å and 2.291(2) Å, versus thioether, 2.285(2) Å, are attributed to the low-spin configuration of the iron. The relationship between this structural observation and the spectroscopic properties of the complex are discussed.

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