Emissive Monocopper Amidophosphine Complexes and Lewis Acid-Assisted Reductive Coupling of Carbon Monoxide

Citation

Miller, Alexander James Minden (2011) Emissive Monocopper Amidophosphine Complexes and Lewis Acid-Assisted Reductive Coupling of Carbon Monoxide. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/SPH6-R695. https://resolver.caltech.edu/CaltechTHESIS:12072010-151616905

Abstract

Two major themes are presented, in roughly chronological order: the synthesis and characterization of photoluminescent copper complexes are described, followed by studies on the selective conversion of synthesis gas (CO and H₂) to oxygenates. With the latter comprising the majority of the work, it is the subject of the introductory Chapter 1. In Chapter 2, the photoluminescent copper chemistry is introduced, and the synthesis and photophysics of monomeric amidophosphine complexes of copper is presented. The copper complexes are exceptional luminophores, with quantum efficiency up to 70% and lifetimes up to 150 μs. In Chapter 3, homogeneous CO hydrogenation is pursued using a strategy reliant on the incorporation of pendent Lewis acidic groups into the secondary coordination sphere of a metal carbonyl complex. This design feature promotes facile C–H and C–C bond formation, with transition metal hydrides as the hydrogen source. A structure-function study investigating the specific role of the Lewis acid determined that thefirst C–H bond formation is not particularly sensitive to the acid, whereas the second C–H bond formation and C–C coupling are both highly sensitive to the length of the tether between the metal and the borane. In Chapter 4, this chemistry is extended to utilize dihydrogen directly as a reductant, in a "frustrated Lewis pair" (FLP) mechanism. A strong phosphazene base is too bulky to interact with the pendent borane, but can heterolytically cleave dihydrogen in concert with the borane to generate a borohydride that transforms the carbonyl ligands into a metal-bound C₂ organic fragment. In Chapter 5, Lewis acidic boranes are again employed as promoters of reductive chemistry, this time for CO₂ reduction. The same late transition metal hydrides that were employed for CO reduction, such as [HNi(dmpe)₂][BF₄] (dmpe = 1,2-bis(dimethylphosphino)ethane), are able to reduce CO₂ gas when used in concert with the appropriate borane, affording a borane-formate adduct. In Chapter 6, the "frustrated Lewis pair" concept is extended to a different problem: the dehydrogenation of amine-boranes, which are candidates for hydrogen storage applications. Treatment of amine-boranes with the FLP ^tBu₃P/B(C₆F₅)₃ effects rapid and quantitative transfer of H₂ from the amine-borane, forming cyclic aminoborane products along with [^tBu₃PH][HB(C₆F₅)₃]. Appendices are provided, including early work on Brønsted acid-assisted CO reduction, speciation of trialkylborohydrides, tabulating NMR impurities in deuterated solvents of interest to the organometallic chemist, and crystallographic tables.

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