Properties of square-pyramidal alkyl-thiolate Fe(III) complexes, including an analogue of the unmodified form of nitrile hydratase.
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.
Lugo-Mas
Priscilla
P
The Department of Chemistry, University of Washington, Box 351700 Seattle, Washington 98195-1700, USA.
Taylor
Wendy
W
Schweitzer
Dirk
D
Theisen
Roslyn M
RM
Xu
Liang
L
Shearer
Jason
J
Swartz
Rodney D
RD
Gleaves
Morgan C
MC
Dipasquale
Antonio
A
Kaminsky
Werner
W
Kovacs
Julie A
JA
D018360:Crystallography, X-Ray; D004583:Electrons; D006836:Hydro-Lyases / Q000737:chemistry* / Q000378:metabolism; D007501:Iron / Q000737:chemistry*; D008280:Magnetics; D009942:Organometallic Compounds / Q000737:chemistry* / Q000378:metabolism; D010084:Oxidation-Reduction
D009942:Organometallic Compounds; D007501:Iron; D006836:Hydro-Lyases; C048831:nitrile hydratase