Browsing by Author "Mandal, Mukunda"

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    Modeling Homo- and Heterogeneous Catalysis with Applications Ranging from Hydrocarbon Activation to the Synthesis of Sustainable Polymers
    (2020-08) Mandal, Mukunda
    The diverse chemical properties of compounds containing metal atoms are inherently highly tunable. In terms of size, they may be small molecular complexes consisting of only 1 or 2 metal atoms or, at the other extreme of scale, they may be crystalline solids with clusters of metal ions periodically dispersed in the structure of a metal–organic framework (MOF). In both cases, relevant chemistries can be tailored to fit specific needs, especially with respect to catalytic reactivity. For a molecular catalyst, the combination of metal atom and its ancillary ligand can be chosen carefully to optimize selectivity and activity, while in three-dimensional MOFs, the metal clusters (called nodes) and connecting organic molecules (called linkers) can be ‘mixed and matched’ to best suit a particular application. Because of the inherent complexity of these systems, experimental characterization of structure/activity relations can often be challenging. Starting from a set of reactants, obtaining a mechanistic picture of the steps leading to the product formation is also demanding since isolation of an intermediate does not necessarily guarantee involvement of the species in the ‘productive’ pathway of the mechanism. Theory and computation can be immensely helpful in these instances to gain molecular-level understanding of the reaction mechanism(s). One can explore multiple pathways that yield the product and then evaluate the feasibility of each pathway by comparing computed energetics. Mechanistic knowledge can then be exploited to establish a structure/property relation, thereby fostering the design of subsequent generations of the catalytic species, ideally having improved performance, and helping to further refine the overall model. This dissertation demonstrates the use of computational methodologies, primarily quantum mechanical density functional theory, to explore the electronic structures of various metal-organic systems and the roles they play in carrying out targeted catalytic processes. In particular, computational modeling efforts are presented that illuminate (i) mechanisms of sustainable polymer production and principles for designing new catalysts (Chapter 2), (ii) site-selective C–H bond functionalization having relevance in drug discovery and chemical biology (Chapter 3) (iii) C–H activation reactions in light hydrocarbons using bio-mimetic copper-complexes having the potential to address challenges in fuel liquefaction (Chapter 4), and (iv) MOF-based single-site heterogeneous catalysts capable of oxidation reactions having industrial relevance (Chapter 5).
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    Supporting Data for “Why So Slow? Mechanistic Insights from Studies of a Poor Catalyst for Polymerization of ε-Caprolactone”
    (2017-05-18) Tolman, William, B; Cramer, Christopher, J; Stasiw, Daniel E; Mandal, Mukunda; Neisen, Benjamin D; Mitchell, Lauren A; wtolman@umn.edu; Tolman, William, B.
    These files contain data along with associated output from instrumentation supporting all results reported in Stasiw, D. E.; Mandal, M.; Neisen, B. D.; Mitchell, L. A.; Cramer, C. J.; Tolman, W. B. Why so slow? Mechanistic insights from studies of a poor catalyst for polymerization of ε-caprolactone. Inorg. Chem., 2016, 56, 725–728. Polymerization of ε-caprolactone (CL) using an aluminum alkoxide catalyst (1) designed to prevent unproductive trans binding was monitored at 110 °C in toluene-d8 by 1H NMR and the concentration versus time data fit to a first-order rate expression. A comparison of t1/2 for 1 to values for many other aluminum alkyl and alkoxide complexes shows much lower activity of 1 toward polymerization of CL. Density functional theory calculations were used to understand the basis for the slow kinetics. The optimized geometry of the ligand framework of 1 was found indeed to make CL trans binding difficult: no trans-bound intermediate could be identified as a local minimum. Nor were local minima for cis-bound precomplexes found, suggesting a concerted coordination–insertion for polymer initiation and propagation. The sluggish performance of 1 is attributed to a high-framework distortion energy required to deform the “resting” ligand geometry to that providing optimal catalysis in the corresponding transition-state structure geometry, thus suggesting a need to incorporate ligand flexibility in the design of efficient polymerization catalysts.. Corresponding author for experimental data is William B. Tolman (wtolman@umn.edu). Corresponding author for computational data is Christopher J. Cramer (cramer@umn.edu).