Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides.

The kinetics of hydride transfer from Re(Rbpy)(CO)3H (bpy = 4,4'-R-2,2'-bipyridine; R = OMe, tBu, Me, H, Br, COOMe, CF3) to CO2 and seven different cationic N-heterocycles were determined. Additionally, the thermodynamic hydricities of complexes of the type Re(Rbpy)(CO)3H were established primarily using computational methods. Linear free-energy relationships (LFERs) derived by correlating thermodynamic and kinetic hydricities indicate that, in general, the rate of hydride transfer increases as the thermodynamic driving force for the reaction increases. Kinetic isotope effects range from inverse for hydride transfer reactions with a small driving force to normal for reactions with a large driving force. Hammett analysis indicates that hydride transfer reactions with greater thermodynamic driving force are less sensitive to changes in the electronic properties of the metal hydride, presumably because there is less buildup of charge in the increasingly early transition state. Bronsted α values were obtained for a range of hydride transfer reactions and along with DFT calculations suggest the reactions are concerted, which enables the use of Marcus theory to analyze hydride transfer reactions involving transition metal hydrides. It is notable, however, that even slight perturbations in the steric properties of the Re hydride or the hydride acceptor result in large deviations in the predicted rate of hydride transfer based on thermodynamic driving forces. This indicates that thermodynamic considerations alone cannot be used to predict the rate of hydride transfer, which has implications for catalyst design.

Synthesis of Triarylmethanes via Palladium-Catalyzed Suzuki-Miyaura Reactions of Diarylmethyl Esters.

The synthesis of triarylmethanes via Pd-catalyzed Suzuki-Miyaura reactions between diarylmethyl 2,3,4,5,6-pentafluorobenzoates and aryl boronic acids is described. The system operates at mild conditions and has a broad substrate scope, including the coupling of diphenylmethanol derivatives that do not contain extended aromatic substituents. This is significant as these substrates, which result in the types of triarylmethane products that are prevalent in pharmaceuticals, have not previously been compatible with systems for diarylmethyl ester coupling. Further, the reaction can be performed stereospecifically to generate stereo-inverted products. On the basis of DFT calculations, it is proposed that the oxidative addition of the diarylmethyl 2,3,4,5,6-pentafluorobenzoate substrate occurs via an SN2 pathway, which results in the inverted products. Mechanistic studies indicate that oxidative addition of the diarylmethyl 2,3,4,5,6-pentafluorobenzoate substrates to (IPr)Pd(0) results in the selective cleavage of the O-C(benzyl) bond in part because of a stabilizing η3-interaction between the benzyl ligand and Pd. This is in contrast to previously described Pd-catalyzed Suzuki-Miyaura reactions involving phenyl esters, which involve selective cleavage of the C(acyl)-O bond, because there is no stabilizing η3-interaction. It is anticipated that this fundamental knowledge will aid the development of new catalytic systems, which use esters as electrophiles in cross-coupling reactions.

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.

The Role of Proton Shuttles in the Reversible Activation of Hydrogen via Metal-Ligand Cooperation.

The reversible activation of H2 via a pathway involving metal-ligand cooperation (MLC) is proposed to be important in many transition metal catalyzed hydrogenation and dehydrogenation reactions. Nevertheless, there is a paucity of experimental information probing the mechanism of this transformation. Here, we present an in-depth kinetic study of the 1,2-addition of H2 via an MLC pathway to the widely used iron catalyst [(iPrPNP)FeH(CO)] (1) (iPrPNP = N(CH2CH2PiPr2)2-). We report one of the first experimental demonstrations of an enhancement in rate for the activation of H2 using protic additives, which operate as "proton shuttles". Our results indicate that proton shuttles need to be able to both simultaneously donate and accept a proton, and the best shuttles are molecules that are strong hydrogen bond donors but sufficiently weak acids to avoid deleterious protonation of the transition metal complex. Additionally, comparison of the rate of H2 activation via an MLC pathway between 1 and two widely used ruthenium catalysts enables more general conclusions about the role of the metal, ancillary ligand, and proton shuttles in H2 activation. The results of this study provide guidance about the design of catalysts and additives to promote H2 activation via an MLC pathway.

Structure and dynamic NMR behavior of rhodium complexes supported by Lewis acidic group 13 metallatranes.

Monovalent Rh was installed into the group 13 metallatranes, M[N(o-(NCH2P(iPr)2)C6H4)3] (where M = Al and Ga, abbreviated as ML) to generate Rh → M bonds in the parent complexes, Cl-RhAlL (1-Cl) and Cl-RhGaL (2-Cl). The electron-withdrawing nature of the group 13 metalloids was probed by cyclic voltammetry, and Rh-Ga was found to be more electron-deficient than Rh-Al (Epc = -2.07 and -1.95 V vs. Fc+/Fc for 1-Cl and 2-Cl, respectively). Both 1-Cl and 2-Cl were further functionalized through metathesis reactions using MeLi to generate 1-CH3 and 2-CH3, respectively, or using LiHBEt3 to form 1-H and 2-H, respectively. The solid-state structures of all Rh-M bimetallics feature Rh-M bond lengths that are less than the sum of the covalent radii of Rh and M (Rh-M: 2.50-2.54 Šfor 1-X and 2.49-2.46 Šfor 2-X, where X = Cl, CH3, and H). In the Rh-M structures, the Rh center is distorted from square pyramidal geometry due to steric interactions between X and the isopropyl substituents of L. Finally, all the Rh-M bimetallics exhibit fluxionality that involves phosphine exchange. Of note, the two phosphines cis to the X ligand become inequivalent at low temperature. The activation barrier to exchange these two phosphine donors is: 14.9, 14.2, 10.9, and 11.5 kcal mol-1 for 1-Cl, 2-Cl, 1-H, and 2-H, respectively. The activation barriers for 1-CH3 and 2-CH3 are both >15.2 kcal mol-1. At high temperature, 2-Cl was also found to exchange all three phosphine donors. Mechanisms for the different types of phosphine exchange are proposed.