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, [Fe(III)((tame-N(3))S(2)(Me2))](+) (1), [Fe(III)(Et-N(2)S(2)(Me2))(py)](1-) (3), and [Fe(III)((tame-N(2)S)S(2)(Me2))](2-) (15). The anionic bis-carboxamide, tris-thiolate 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 spin state 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-translationally 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(3))S(2)(Me))(NO)](+) (18, nu(NuO) = 1865 cm(-1)), an analogue of NO-inactivated NHase, and [Fe(II)((tame-N(3))S(2)(Me))(CO)] (16; nu(CO) stretch (1895 cm(-1)). Anions (N(3)(-), CN(-)) are shown to be unreactive toward 1, 3, and 15 and neutral ligands unreactive toward 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(2)(Me2))](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.

Figure 1

The post—translationally modified NHase active site.

Scheme 1

Scheme 2

Scheme 3

Figure 2

ORTEP of the anion of dimeric (NMe₄)₂[Fe^III((Et—N₂)S₂^Me2)]₂•2MeOH (2) showing atom labeling scheme. All hydrogen atoms have been removed for clarity.

Scheme 4

Figure 3

ORTEP of the cation of [Fe^III((tame—N₃)S₂^Me2)](PF₆)•PhCN (1), and the anions of (Me₄N)[Fe^III((Et—N₂)S₂^Me2)(Py)]•2MeOH (3) and (NMe₄)₂[Fe^III((tameN₂S)S₂^Me2)]•MeCN (15), showing 50% ellipsoids and atom labeling scheme. All hydrogen atoms have been removed for clarity.

Figure 4

Inverse molar magnetic susceptibility 1/χ_m vs temperature (T) plot for (Me₄N)[Fe^III((Et—N₂)S₂^Me2)(Py)]•2MeOH (3) and (NMe₄)₂[Fe^III((tame—N₂S)S₂^Me2)]•MeCN (15) each fit to an S = 3/2 spin-state with Curie constants 2.05 (3) and 1.91 (15).

Figure 5

Low temperature (7 K) X-band EPR spectrum (black) of (NMe₄)₂[Fe^III((tame—N₂S)S₂^Me2)]•MeCN (15) in MeOH/EtOH (9:1) glass, fitted (red) to E/D = 0.107.

Figure 6

Cyclic voltammogram of [Fe^III((tame—N₃)S₂^Me2)](PF₆)•PhCN (1) and (NMe₄)₂[Fe^III((Et—N₂)S₂^Me2)]₂•2MeOH (2), both in MeCN at 298 K (0.1 M (Bu₄N)PF₆, glassy carbon electrode, 150 mV/sec scan rate). Peak potentials versus SCE indicated.

Figure 7

ORTEP of reduced [Fe^II((tame—N₃)S₂^Me2)(CO)]•MeCN (16), the cations of [Fe^III((tame—N₃)S₂^Me2)(NO)](PF₆) (18) and [Fe^III((tame—N₃)S₂^Me2)(MeCN)](PF₆)•MeCN (19) showing 50% ellipsoids and atom labeling scheme, as well as the weakly interacting MeCN (Fe—N(4)= 2.63 Å) in 19.

Figure 8

ORTEP of the anion of (NEt₄)₂[Fe^III((tame—N₂SO₂)S₂)]•MeCN (20) showing 50% ellipsoids and atom labeling scheme.