Thermodynamic and kinetic hydricity of transition metal hydrides.

The prevalence of transition metal-mediated hydride transfer reactions in chemical synthesis, catalysis, and biology has inspired the development of methods for characterizing the reactivity of transition metal hydride complexes. Thermodynamic hydricity represents the free energy required for heterolytic cleavage of the metal-hydride bond to release a free hydride ion, H-, as determined through equilibrium measurements and thermochemical cycles. Kinetic hydricity represents the rate of hydride transfer from one species to another, as measured through kinetic analysis. This tutorial review describes the common methods for experimental and computational determination of thermodynamic and kinetic hydricity, including advice on best practices and precautions to help avoid pitfalls. The influence of solvation on hydricity is emphasized, including opportunities and challenges arising from comparisons across several different solvents. Connections between thermodynamic and kinetic hydricity are discussed, and opportunities for utilizing these connections to rationally improve catalytic processes involving hydride transfer are highlighted.

Bathochromic Shifts in Rhenium Carbonyl Dyes Induced through Destabilization of Occupied Orbitals.

A series of rhenium diimine carbonyl complexes was prepared and characterized in order to examine the influence of axial ligands on electronic structure. Systematic substitution of the axial carbonyl and acetonitrile ligands of [Re(deeb)(CO)3(NCCH3)]+ (deeb = 4,4'-diethylester-2,2'-bipyridine) with trimethylphosphine and chloride, respectively, gives rise to red-shifted absorbance features. These bathochromic shifts result from destabilization of the occupied d-orbitals involved in metal-to-ligand charge-transfer transitions. Time-Dependent Density Functional Theory identified the orbitals involved in each transition and provided support for the changes in orbital energies induced by ligand substitution.

Aqueous Hydricity from Calculations of Reduction Potential and Acidity in Water.

Hydricity, or hydride donating ability, is a thermodynamic value that helps define the reactivity of transition metal hydrides. To avoid some of the challenges of experimental hydricity measurements in water, a computational method for the determination of aqueous hydricity values has been developed. With a thermochemical cycle involving deprotonation of the metal hydride (pKa), 2e- oxidation of the metal (E°), and 2e- reduction of the proton, hydricity values are provided along with other valuable thermodynamic information. The impact of empirical corrections (for example, calibrating reduction potentials with 2e- organic versus 1e- inorganic potentials) was assessed in the calculation of the reduction potentials, acidities, and hydricities of a series of iridium hydride complexes. Calculated hydricities are consistent with electronic trends and agree well with experimental values.

Solvent-Dependent Thermochemistry of an Iridium/Ruthenium H2 Evolution Catalyst.

The hydricity of the heterobimetallic iridium/ruthenium catalyst [Cp*Ir(H)(µ-bpm)Ru(bpy)2]3+ (1, where Cp* = η5-pentamethylcyclopentadienyl, bpm = 2,2'-bipyrimidine, and bpy = 2,2'-bipyridine) has been determined in both acetonitrile (63.1 kcal mol-1) and water (29.7 kcal mol-1). Hydride 1 features a large increase in the hydride donor ability when the solvent is changed from acetonitrile to water. The acidity of 1, in contrast, is essentially solvent-independent because 1 remains strongly acidic in both solvents. On the basis of an X-ray crystallographic study, spectroscopic analysis, and time-dependent density functional theory calculations, the disparate reactivity trends are ascribed to substantial delocalization of the electron density onto both the bpm and bpy ligands in the conjugate base of 1, [Cp*Ir(µ-bpm)Ru(bpy)2]2+ (3). The H2 evolution tendencies of 1 are considered in the context of thermodynamic parameters.

Aqueous Hydricity of Late Metal Catalysts as a Continuum Tuned by Ligands and the Medium.

Aqueous hydride transfer is a fundamental step in emerging alternative energy transformations such as H2 evolution and CO2 reduction. "Hydricity," the hydride donor ability of a species, is a key metric for understanding transition metal hydride reactivity, but comprehensive studies of aqueous hydricity are scarce. An extensive and self-consistent aqueous hydricity scale is constructed for a family of Ru and Ir hydrides that are key intermediates in aqueous catalysis. A reference hydricity is determined using redox potentiometry and spectrophotometric titration for a particularly water-soluble species. Then, relative hydricity values for a range of species are measured using hydride transfer equilibria, taking advantage of expedient new synthetic procedures for Ru and Ir hydrides. This large collection of hydricity values provides the most comprehensive picture so far of how ligands impact hydricity in water. Strikingly, we also find that hydricity can be viewed as a continuum in water: the free energy of hydride transfer changes with pH, buffer composition, and salts present in solution.

Isolation of saccharides in dairy and soy products by solid-phase extraction coupled with analysis by ligand-exchange chromatography.

The present study reports an improved method to quickly and reproducibly isolate the saccharides from a variety of dairy and soy products utilizing reversed-phase solid-phase extraction to quantitatively remove fats, fatty acids, and lipids followed by desalination and deproteinization by ion-exchange solid-phase extraction with no loss of saccharides during extraction. Analysis of the isolated saccharides was performed by ligand-exchange HPLC. The method presented requires no prolonged heating (thus protecting the saccharides from hydrolysis or isomerization), uses benign reagents, and realizes a significant time savings over existing methods. The isolation and analysis of monosaccharides (glucose, galactose and fructose), disaccharides (lactose and sucrose), and polysaccharides (raffinose and stachyose) from dairy products (whole, reduced fat, and lactose-free milk and yogurt), infant formula (powdered and premixed), and soy beverages were studied in this investigation with recoveries ranging from 88% to 110% in all products studied. We also applied the method to quickly discriminate authentic soy milk from a soy beverage, branded as soy milk.