Properties of Square-Pyramidal Alkyl−Thiolate Fe^III Complexes, Including an Analogue of the Unmodified Form of Nitrile Hydratase

Abstract

The syntheses and structures of three new coordinatively unsaturated, monomeric, square pyramidal thiolate-ligated Fe(III) complexes are described, (15). The anionic bis-carboxamide, tristhiolate N 2 S 3 coordination sphere of 15 is potentially similar to that of the yet-to-be characterized unmodified form of NHase. Comparison of the magnetic and reactivity properties of these reveals how anionic charge build-up (from cationic 1 to anionic 3 and dianionic 15) and spin-state influence apical ligand affinity. For all of the ligand-field combinations examined, an intermediate S= 3/2 spinstate was shown to be favored by a strong N 2 S 2 basal plane ligand-field, and this was found to reduce the affinity for apical ligands, even when they are built-in. This is in contrast to the post-translationaly modified NHase active site, which is low-spin, and displays a higher affinity for apical ligands. Cationic 1 and its reduced Fe II precursor are shown to bind NO and CO, respectively, to afford [Fe III ((tame-N ((tame-N 3 )S 2 Me )(CO)] (16; ν CO stretch (1895 cm -1 ). Anions (N 3 -, CN -) are shown to be unreactive towards 1, 3 and 15, and neutral ligands unreactive towards 3 and 15, even when present in 100-fold excess, and at low-temperatures. The curtailed reactivity of 15, an analogue of the unmodified form of NHase, and its apical-oxygenated S= 3/2 derivative [Fe III ((tame-N 2 SO 2 ) S 2Me2 )] 2-(20) suggests that regioselective post-translational oxygenation of the basal plane NHase cysteinate sulfurs plays an important role in promoting substrate binding. This is supported by previously reported theoretical (DFT) calculations. 1

Use of Metallopeptide Based Mimics Demonstrates That the Metalloprotein Nitrile Hydratase Requires Two Oxidized Cysteinates for Catalytic Activity

“…13,15,16,106 We note that the thiolates located in the equatorial plane of [Co III NHase-m1] are the likely candidates for oxidation. A computational analysis suggests that the oxidation of the axial thiolate, as has been observed experimentally in Fe III and Co III complex with similar ligand sets, 34,109 leads to the sulfienate dissociation stabilizing a four-coordinate Co III N 2 S 2 species (supporting information), experimentally derived structures of which have been previously reported. 31,110 …”

Metal-Assisted Oxo Atom Addition to an Fe(III) Thiolate

“…A similar increase in Fe–S bond lengths (from 2.189 to 2.219 Å) is seen upon replacement of the neutral imines in [Fe III (tame-N 3 )S 2 Me2 )] + with anionic carboxamides in [Fe III -((tame-N 2 S)S 2 Me2 )] 2 −. 17 The S–Fe–S angle (144°) in 8 is significantly wider than that of 2 (121°), as well as imine-ligated [Fe III (S 2 Me2 N 3 (Et,Pr))] + (105°). 61 …”

“…51, 53–59 Although an open coordination site is required for O 2 binding, thiolates are known to intercept open coordination sites by forming intermolecular M-SR-M bridges, resulting in oligomerization. Despite this, we, 17, 51, 54, 59–67 and others, 2, 3, 16, 18, 68–70 have synthesized a variety of coordinatively unsaturated mononuclear thiolate-ligated iron complexes that are capable of binding small molecules. For example, thiolate-ligated [Fe II (N 4 S Me2 -(tren))] + , 54, 71 and bis-thiolate-ligated [Fe III (S 2 Me2 N 3 (Pr,Pr))] ( 2 , 63 Scheme 3), are five-coordinate, and contain flexible ligands capable of accommodating a sixth ligand, 62, 63, 72 as well as a variety of metal ions, in multiple oxidation states.…”

Iron Complexes of a Proton-Responsive SCS Pincer Ligand with a Sensitive Electronic Structure

“…80,[83][84][85][86][87][88] These examples all also have near square planar or square pyramidal geometries. Mössbauer spectra of ferric S = 3/2 complexes generally have isomer shifts around 0.3 mm/s and large quadrupole splitting 63.…”