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?

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, NHase(dark). [(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 P2(1)/c with a = 11.9521(14) (A) over circle, b = 11.3238(13) (A) over circle, c = 26.624(3) (A) over circle, beta = 98.280(2)degrees, and Z = 4. The upsilon(NO) stretching frequency of 1856 cm(-1) and the Mossbauer parameters, delta = 0.06 mm/s and DeltaE(q) = 1.75 mm/s, compare favorably with those of NHase(dark). The similarities of the iron-sulfur bond distances to the thiolate, 2.284(2) (A) over circle and 2.291(2) (A) over circle, versus thioether, 2.285(2) (A) over circle, 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.