Kinetic Effects of Sulfur Oxidation on Catalytic Nitrile Hydration: Nitrile Hydratase Insights from Bioinspired Ruthenium(II) Complexes

Abstract

Kinetic investigations inspired by the metalloenzyme nitrile hydratase were performed on a series of ruthenium(II) complexes to determine the effect of sulfur oxidation on catalytic nitrile hydration. The rate of benzonitrile hydration was quantified as a function of catalyst, nitrile, and water concentrations. Precatalysts L(n)RuPPh3 (n = 1-3; L(1) = 4,7-bis(2'-methyl-2'-mercapto-propyl)-1-thia-4,7-diazacyclononane; L(2) = 4-(2'-methyl-2'-sulfinatopropyl)-7-(2'-methyl-2'-mercapto-propyl)-1-thia-4,7-diazacyclononane; L(3) = 4-(2'-methyl-2'-sulfinatopropyl)-7-(2'-methyl-2'-sulfenato-propyl)-1-thia-4,7-diazacyclononane) were activated by substitution of triphenylphosphine with substrate in hot dimethylformamide solution. Rate measurements are consistent with a dynamic equilibrium between inactive aqua (L(n)Ru-OH2) and active nitrile (L(n)Ru-NCR) derivatives with K = 21 ± 1, 9 ± 0.9, and 23 ± 3 for L(1) to L(3), respectively. Subsequent hydration of the L(n)Ru-NCR intermediate yields the amide product with measured hydration rate constants (k's) of 0.37 ± 0.01, 0.82 ± 0.07, and 1.59 ± 0.12 M(-1) h(-1) for L(1) to L(3), respectively. Temperature dependent studies reveal that sulfur oxidation lowers the enthalpic barrier by 27 kJ/mol, but increases the entropic barrier by 65 J/(mol K). Density functional theory (DFT) calculations (B3LYP/LanL2DZ (Ru); 6-31G(d) (all other atoms)) support a nitrile bound catalytic cycle with lowering of the reaction barrier as a consequence of sulfur oxidation through enhanced nitrile binding and attack of the water nucleophile through a highly organized transition state.

Singlet Oxygen Generation, Quenching and Reactivity with Metal Thiolates†

“…Oxidized thiolate ligands are found in several metalloproteins which actually require the oxidized thiolate ligand for their activity: Nitrile hydratase is a well-studied example (10,11). Thiolate oxidation may also lead to labilization of the oxidized ligand so that it can readily be substituted; this effect is believed to be important for Ru anticancer predrugs where the oxidized thiolate of the predrug is replaced by a nucleotide base (12)(13)(14)(15).…”

“…Several metalloproteins such as nitrile hydratase possess oxidized thiolate ligands (both sulfenate and sulfinate) at the active cobalt site. This has inspired several groups to investigate the chemistry of cobalt thiolate complexes with molecular oxygen (10,11,103,104). Most of these studies have used ground state molecular oxygen (or single oxygen atom donors such as H 2 O 2 ).…”

Metal-centered oxidation decreases nitrile hydration activity of bioinspired (N2S3)Ru-PPh3 precatalysts

“…[1][2][3][4][5][6] The active site of NHase contains a mononuclear low-spin non-heme iron(III) or non-corrin cobalt(III) center in an unusual N 2 S 3 X type donor motif that contains two carboxamido nitrogens and three cysteine derived sulfur donors in distinct oxidation states; thiolate (RS -), sulfenate/sulfenic acid (RSO¯/RSOH) and sulfinate (RSO 2¯) as shown in Scheme 1. [7,8] Although asymmetric sulfur-oxidation is crucial for the catalytic activity of NHase, [9] the Co(III) peptide maquettes of Shearer [10] and our Ru(II) bioinspired catalysts [11,12] are the only two systems to have successfully implemented a mixed sulfenato/sulfinato donor set into a catalytically active synthetic system. In fact, the total number of functional NHase complexes remains small and limited to low-spin d 6 Co(III) complexes, with the exception of our low-spin d 6 Ru(II) catalysts.…”

“…In fact, the total number of functional NHase complexes remains small and limited to low-spin d 6 Co(III) complexes, with the exception of our low-spin d 6 Ru(II) catalysts. [11][12][13][14][15][16] To date, no functional NHase models with low-spin d 5 iron(III) centers are known despite its natural occurrence in the enzyme. Previously, we noted that aerobic conditions decreased the hydration activity of our Ru(II) catalyst 1, which was attributed to metal-centered oxidation since ferrocenium hexafluorophosphate (FcPF 6 ) had the same effect.…”

Sulfur-oxidation enhances nitrile hydration in bioinspired ruthenium complexes : catalytic, kinetic, and DFT investigations.

“…Since, p-toluamide can potentially bind to Ru as an inhibitor it was employed externally. 133 As in the prior study, high reaction temperatures (368 -398K) were maintained to ensure complete dissociation of the triphenylphosphine donor from the precatalysts L n RuPPh3.…”