The rates of nitrile hydration reactions were investigated using [Ru(η6-p-cymene)Cl2(PR2R′)] complexes as homogeneous catalysts, where PR2R′ = PMe2(CH2P(O)Me2), PMe2(CH2CH2P(O)Me2), PPh2(CH2P(O)Ph2), PPh2(CH2CH2P(O)Ph2), PMe2OH, P(OEt)2OH. These catalysts were studied because the rate of the nitrile-to-amide hydration reaction was hypothesized to be affected by the position of the hydrogen bond accepting group in the secondary coordination sphere of the catalyst. Experiments showed that the rate of nitrile hydration was fastest when using [Ru(η6-p-cymene)Cl2PMe2OH]: i.e., the catalyst with the hydrogen bond accepting group capable of forming the most stable ring in the transition state of the rate-limiting step. This catalyst is also active at pH 3.5 and at low temperaturesconditions where α-hydroxynitriles (cyanohydrins) produce less cyanide, a known poison for organometallic nitrile hydration catalysts. The [Ru(η6-p-cymene)Cl2PMe2OH] catalyst completely converts the cyanohydrins glycolonitrile and lactonitrile to their corresponding α-hydroxyamides faster than previously investigated catalysts. [Ru(η6-p-cymene)Cl2PMe2OH] is not, however, a good catalyst for acetone cyanohydrin hydration, because it is susceptible to cyanide poisoning. Protecting the −OH group of acetone cyanohydrin was shown to be an effective way to prevent cyanide poisoning, resulting in quantitative hydration of acetone cyanohydrin acetate.

The mechanism of the nitrile-to-amide hydration reaction using [Ru(η 6 -arene)Cl 2 (PR 3 )] complexes as catalysts was investigated (η 6 -arene = C 6 H 6 , p-cymene, C 6 Me 6 ; R = NMe 2 , OMe, OEt, Et, iPr). Experiments showed that the mechanism involves the following general sequence of reactions: substitution of a chloride ligand by the nitrile substrate, intermolecular nucleophilic attack by water to form an amidate intermediate, and dissociation of the resulting amide. The effects of secondary coordination sphere interactions on the rates and yields of the hydration reaction were investigated. Ligands that are capable of acting as hydrogen bond acceptors with the entering water molecule result in faster rates and higher yields than nonhydrogen-bonding ligands. The faster rates are attributable to the H-bonding-facilitated deprotonation of the water as the oxygen of the water bonds to the coordinated nitrile. DFT calculations on the proposed H-bonding intermediates support this interpretation. Most homogeneous catalysts will not hydrate cyanohydrins because of the equilibrium amounts of cyanide that are present in solutions of cyanohydrins; the cyanide poisons the catalyst. Because of its increased catalytic reactivity due to secondary coordination sphere effects, the [Ru(η 6 -arene)Cl 2 (P(NMe 2 ) 3 )] catalyst gives significant yields of cyanohydrin hydration products with glycolonitrile, lactonitrile, acetone cyanohydrin, and mandelonitrile. A Taft plot showed that an increase in the steric bulk of the nitrile results in a decrease in the hydration rate, and a Hammett plot showed that electron-withdrawing groups facilitate nitrile hydration. The decrease in rate as the size of the cyanohydrin increases is likely due to both increased steric bulk and to the addition of electron-donating groups on the nitrile. The [Ru(η 6 -arene)Cl 2 (PR 3 )] catalysts are initially less susceptible to cyanide poisoning than other homogeneous nitrile hydration catalysts because [Ru(η 6 -p-cymene)(CN)(Cl)(P-(NMe 2 ) 3 )] forms in the presence of cyanide. The electron-withdrawing cyanide ligand facilitates nucleophilic attack of water on a coordinated nitrile in this molecule.

Redox-active ligands bring electron- and proton-transfer reactions to main-group coordination chemistry. In this Forum Article, we demonstrate how ligand pK values can be used in the design of a reaction mechanism for a ligand-based electron- and proton-transfer pathway, where the ligand retains a negative charge and enables dihydrogen evolution. A bis(pyrazolyl)pyridine ligand, PzP, reacts with 2 equiv of AlCl to afford [(PzP)AlCl(THF)][AlCl] (1). A reaction involving two-electron reduction and single-ligand protonation of 1 affords [(HPzP)AlCl] (2), where each of the electron- and proton-transfer events is ligand-centered. Protonation of 2 would formally close a catalytic cycle for dihydrogen production. At -1.26 V versus SCE, in a 0.3 M BuNPF/tetrahydrofuran solution with salicylic acid or (HNEt) as the source of H, 1 produced dihydrogen electrocatalytically, according to cyclic voltammetry and controlled potential electrolysis experiments. The mechanism for the reaction is most likely two electron-transfer steps followed by two chemical steps based on the available reactivity information. A comparison of this work with our previously reported aluminum complexes of the phenyl-substituted bis(imino)pyridine system (IP) reveals that the pK values of the N-donor atoms in PzP are lower, which facilitates reduction before ligand protonation. In contrast, the IP ligand complexes of aluminum are protonated twice before reduction liberates dihydrogen.

The catalytic hydration of cyanohydrins to their corresponding α-hydroxyamides provides a route to industrially useful α-hydroxy amides, α-hydroxy esters, α-hydroxy carboxylic acids, and their acrylic derivatives. However, until now, no homogeneous nitrile hydration catalyst has been capable of complete conversion of cyanohydrins to their corresponding amides because cyanohydrins degrade to produce cyanide, which poisons the catalyst. Because the cyanohydrin degradation is an equilibrium process, it was hypothesized that a faster nitrile hydration catalyst would be capable of hydrating the cyanohydrin before degradation occurs. Secondary coordination sphere effects were used to develop a faster catalyst based on the [Ru(η 6 -arene)Cl 2 (PR 3 )] scaffold. A series of [Ru(η 6 -p-cymene)Cl 2 (PR 3 )] complexes, where R = NMe 2 , OMe, Et, was synthesized, and their activity toward cyanohydrin hydration was determined. The complex [Ru(η 6 -pcymene)Cl 2 (P(NMe 2 ) 3 )] is an excellent catalyst, and the unprecedented complete conversion of a cyanohydrin to its corresponding amide using a homogeneous catalyst was achieved with glycolonitrile and lactonitrile.

The terphenyl tin(II) hydride [ArSn(μ-H)] (1) (Ar = CH-2,6(CH-2,6-Pr)) was shown to form an equilibrium with the distannyne ArSnSnAr (2) and H in toluene at 80 °C. The equilibrium constant and Gibbs free energy for the dissociation of H are 2.23 × 10 ± 4.9% and 5.89 kcal/mol ± 0.68%, respectively, by H NMR spectroscopy and 2.33 × 10 ± 6.2% and 5.86 kcal/mol ± 0.73%, respectively, by UV-vis spectroscopy, indicating that the hydride 1 is strongly favored. Further heating of 2 at ca. 100 °C afforded the known pentagonal-bipyramidal Sn cluster Sn(SnAr) (3). Mechanistic studies show that 3 is formed from distannyne 2, which is generated from 1. The order of the reaction for the conversion of 2 into 3 was found to be zero, and the rate constant is 1.77 × 10 M s at 100 °C. Hydride 1 was further characterized by cyclic voltammetry, and its pK was found to be 18.8(2) via titration with 1,8-diazabicyclo[5.4.0]undec-7-ene. The bond dissociation free energy was estimated to be 51.1 kcal/mol ± 3.4% on the basis of its pK and reduction potential. Studies with deuterium indicate ready exchange of D with the hydrides in 1.