Understanding thermodynamics and kinetics of organometallic complexes for tailoring analogous immobilized metal catalysts.

摘要

We use isothermal titration calorimetry (ITC) to study the metal-ligand binding equilibria for a series of monodentate P ligands and bidentate P-P, N-N, and P-N ligands to PdCl2(solv)2 in various solvents at 25°C. For all ligands, we discuss differences in the obtained thermodynamic parameters (K, ΔG, Δ H, and ΔS) and differences in binding modes with the Pd center. For bulky ligands and poor electron donating ligands (monodentate), only one equivalent of ligand was able to bind to the Pd center. Two equivalents of all other ligands were able to bind the Pd center. Strong electron donating solvents such as pyridine coordinated to the Pd center and allowed only one equivalent of ligand to bind. For bidentate ligands, we observed large, exothermic enthalpies accompanied by large, negative (unfavorable) entropies upon binding to the Pd center. We discuss solvent reorganization for the binding of bidentate ligands to PdCl2(solv)2 and how it manifests itself as enthalpy-entropy compensation.;The next phase of our study focuses on the homogeneous activity of Wilkinson’s catalyst for terminal olefin hydrogenation in different solvents. We characterize the resulting Rh species in each solvent using 31P NMR spectroscopy and solution calorimetry in order to determine possible changes in the active Rh structure and test each solvent system for its activity toward the hydrogenation of 1-heptene and 1-octene in a differential batch reactor at 0°C. We interpret the kinetic results (i.e. turnover frequencies) in terms of the Rh species present in each solvent as well as the electron donating and accepting properties of each solvent. Strong electron donating solvents such as pyridine inhibited the hydrogenation reactions due to coordination to the Rh centers.;We transition to studying homogeneous and heterogeneous catalytic results for Wilkinson’s catalyst and a series of supported analogs. First, we graft different functional groups (primary amines, secondary amines, and diphenylphosphines) onto SBA-15 silica surfaces. We then immobilize Wilkinson’s catalyst on each of the three types of functionalized SBA-15 and rigorously characterize each supported Rh species using a variety of spectroscopic techniques, most notably 2D 31P{1H} HETCOR (heteronuclear correlation) NMR. Using these spectroscopic results, we propose structures for the most likely surface Rh species. When used for hydrothiolation reactions, these catalysts exhibit switches in stereoselectivity (though retaining high regioselectivity for the anti- Markovnikov product) based on the type of functional group (and, therefore, the local Rh structure). Conversely, for hydrosulfonation, all three supported Rh catalysts are regioselective in favor of the Markovnikov product. We then focus on the phosphine-functionalized SBA-15 silica as a control surface in order to examine the effect of the nature of the grafting solvent on the resulting grafted Rh species. We compare the homogeneous activity and selectivity of Wilkinson’s catalyst in each grafting solvent (acting as the reaction solvent) for the hydrothiolation of phenylacetylene by thiophenol as a model reaction to the heterogeneous activity and selectivity of each supported Rh catalyst (grafted in each of the different solvents). We found solvents that were highly active for homogeneous hydrothiolation (e.g. THF) produced heterogeneous Rh catalysts that were barely active and poorly selective when used as grafting solvents. We also observed solvents that were not as active for homogeneous hydrothiolation (e.g. toluene, DCE), yet produced highly active, regio- and stereoselective catalysts when used as grafting solvents.