Protonation of metal-sulfur complexes :synthetic and mechanistic studies

Abstract

Nickel and iron are present in certain metalloenzymes (e.g. nitrogenases, hydrogenases and carbon monoxide dehydrogenase) and also in some industrial catalysts (e.g. isomerization and hydrocyanation catalysts) where the substrate transformations also involve reactions with acids. Understanding the protonation of simple nickel and iron complexes is important in establishing the mechanisms of these natural and industrial catalysts, this thesis will focus on kinetic studies of the protonation mechanisms of various nickel and iron complexes. The complexes [Ni(S2CR)(triphos)]BPh4 {R = Me, Et or Bun; triphos = PhP(CH2CH2PPh2)2} and [Ni(S2CC6H4R′-4)(triphos)]BPh4 (R′ = H, Me, Cl or OMe) were synthesised and characterised by spectroscopy, and for R = Et; R′ = Me, H, Cl or OMe the X-ray crystal structures were determined. The crystallography shows that the nickel is trigonal bipyramidal with the dithioate ligand being bidentate and spanning equatorial and axial positions. The protonation reactions of all [Ni(S2CR)(triphos)]+ and [Ni(S2CC6H4R′-4)(triphos)]+ with HCl to form [Ni(HS2CR)(triphos)]+, [Ni(HS2CC6H4R′-4)(triphos)]+ in MeCN have been studied . In all cases the rates of protonation are slow and the rate law is complicated. The kinetics are consistent with two coupled equilibria where initially the HCl binds to a sulfur through hydrogen bonding, and this is followed by intramolecular proton transfer to the sulfur. For the alkyl derivatives the rates of protonation are dependent on the electronic nature of the R group while the affect of the 4-Rʹ-substituent on the aryl derivatives is small. The overall isotope effects for transfer of the proton from HCl to [Ni(S2CR)(triphos)]+ {( 𝐾1𝑅𝑘2𝑅)H / (𝐾1𝑅𝑘2𝑅)D} are all small (0.94 – 0.25). The protonation of each sulfur in [Ni(S2CR)(triphos)]+ (R = Me or Ph) has been explored using molecular mechanics calculations (GAUSSIAN09 package, geometries optimized at the B3LYP/Lanl2dz levels of theory). The calculations indicate that the protonation of the sulfur in the equatorial position results in chelate ring-opening, but protonation of the sulfur in the axial position does not. The calculated pKaRs of the coordinated carboxydithioic acid are rather insensitive to the R-substituent (pKaR = 7.2 – 8.3). The analogous [Ni(O2CR)(triphos)]BPh4 (R = Et or Ph) have also been prepared and studied. The protonation of these complexes is much more rapid than the dithioate 5 analogues and so the kinetics of the protonation reactions were studied with the weak acid lutH+ (lut = 2,6-dimethylpyridine). . The complexes [Ni(OC6H5R-4)(triphos)]BPh4, [Ni(SePh)(triphos)]BPh4 and [Ni(XRʹ)(triphos)]BPh4 (R = H, CH3, Cl or OCH3X = S, R = Et, But or Cy) have been synthesised and characterised. For [Ni(SeC6H5)(triphos)]BPh4 and all [Ni(XRʹ)(triphos)]BPh4 the X-ray crystal structures have been determined. The protonation reactions of all these derivatives have been studied using the weak acid lutH+. The kinetics of the protonation reactions is consistent with an equilibrium mechanism in which the acid initially hydrogen bonds to the O or Se site and then undergoes intramolecular proton transfer. In the reaction of [Ni(SePh)(triphos)]+, the hydrogen bonded intermediate accumulates at high concentrations of lutH+, but for the reactions of [Ni(OC6H4R-4)(triphos)]+ the hydrogen bonded intermediate never attains a sufficient concentration to complicate the kinetics. The reason for the slow and similar rates of protonation of [Ni(XPh)(triphos)]+ (X = O, S or Se) have been studied using DFT calculations which indicate that the slow proton transfer is because of steric interactions between lutH+ and the phenyl group on triphos. [Ni(SRʹ)2(dppe)] (Rʹ = Et, But or Cyh) have been synthesised and characterised using spectroscopy. The kinetics of the protonation reactions of all the derivatives with lutH+ in MeCN is described. The rate law is complicated and the kinetics is consistent with two coupled equilibria where firstly the acid hydrogen bonds to sulfur and this is followed by intramolecular proton transfer to the sulfur. In the last section of this thesis, the substitution reactions of two types of Fe-S-based clusters [Fe4S4(OR)4]2- and[{MoFe3S4(OPh)3}2(-SPh)3]3- have been studied with PhS- in the presence of NHEt3+. The clusters were characterised using 1H NMR spectroscopy. The kinetics of the reactions of the clusters with PhS- is consistent with a mechanism involving an initial rapid protonation step followed by the dissociation of phenoxide in the rate-limiting step. The mechanistic interpretations of these results are discussed in the light of studies on other Fe-S-based clusters and recent DFT calculations which suggest there is an elongation of a Fe-S bond after protonation.