Halogen-elimination photochemistry and oxygen-activation chemistry of late transition-metal complexes

Thesis (Ph. D. in Inorganic Chemistry)--Massachusetts Institute of Technology, Dept. of Chemistry, 2012.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references.Multi-electron reaction chemistry, from both ground- and excited-state species, is at the heart of many topics in renewable energy and catalysis. In this thesis, two classes of reactions central to the themes of energy conversion and multi-electron chemistry are studied on mono- and bimetallic late transition-metal platforms. In the early chapters, studies of photochemical halogen elimination, the key energy-storing step in photocatalytic hydrogen production from HX (X = Cl, Br), are described. In the latter sections of the thesis, the oxygen-activation and reduction chemistries of rhodium and iridium hydride complexes are highlighted. In Chapters 1 and 2, photochemical halogen elimination from a variety of late transition-metal complexes is described. Studies of phosphine-terminated gold(III) halide complexes demonstrated that efficient halogen photoelimination can be promoted by ligand-to-metal charge-transfer (LMCT) excitation, in complexes devoid of a formal metal-metal interaction. In addition, gold was partnered with rhodium and iridium in a series of heterobimetallic complexes, and these complexes were also shown to cleanly eliminate halogen when illuminated, with additional electronic structural insights and reactivity trends emerging from this latter suite of compounds. In Chapters 3-6, small-molecule reactivity studies of rhodium and iridium complexes, with a particular slant towards oxygen reduction, are disclosed. A new class of two-electron mixedvalent dirhodium and diiridium complexes is described. Featuring a coordinatively unsaturated M0 center, these complexes display an expansive reactivity with numerous small-molecule substrates. A dirhodium hydride complex, prepared by HCl addition to the mixed-valent precursor, mediates the reduction of oxygen to water. Studies on iridium model complexes, coupled with detailed kinetic studies, produced a clear mechanistic understanding of this chemistry. In particular, the preparation and reactivity of a diiridium hydroperoxo complex gave many key insights into the activation of O2 and the subsequent release of water. Analogous oxygen-reduction chemistry was also demonstrated to occur on a monorhodium platform, which will facilitate detailed mechanistic studies enabled by systematic ligand alteration.by Thomas S. Teets.Ph.D.in Inorganic Chemistr

Cyclometalated Iridium(III) Complexes with Deoxyribose Substituents

Fundamental study of enzymatic nucleoside transport suffers for lack of optical probes that can be tracked noninvasively. Nucleoside transporters are integral membrane glycoproteins that mediate the salvage of nucleosides and their passage across cell membranes. The substrate recognition site is the deoxyribose sugar, often with little distinction among nucleobases. Reported here are nucleoside analogues in which emissive, cyclometalated iridium(III) complexes are “clicked” to C-1 of deoxyribose in place of canonical nucleobases. The resulting complexes show visible luminescence at room temperature and 77 K with microsecond-length triplet lifetimes. A representative complex is crystallographically characterized. Transport and luminescence are demonstrated in cultured human carcinoma (KB3-1) cells

Cyclometalated Iridium(III) Complexes with Deoxyribose Substituents

Fundamental study of enzymatic nucleoside transport suffers for lack of optical probes that can be tracked noninvasively. Nucleoside transporters are integral membrane glycoproteins that mediate the salvage of nucleosides and their passage across cell membranes. The substrate recognition site is the deoxyribose sugar, often with little distinction among nucleobases. Reported here are nucleoside analogues in which emissive, cyclometalated iridium(III) complexes are “clicked” to C-1 of deoxyribose in place of canonical nucleobases. The resulting complexes show visible luminescence at room temperature and 77 K with microsecond-length triplet lifetimes. A representative complex is crystallographically characterized. Transport and luminescence are demonstrated in cultured human carcinoma (KB3-1) cells

Halogen photoelimination from dirhodium phosphazane complexes via chloride-bridged intermediates

Halogen photoelimination is a critical step in HX-splitting photocatalysis. Herein, we report the photoreduction of a pair of valence-isomeric dirhodium phosphazane complexes, and suggest that a common intermediate is accessed in the photochemistry of both mixed-valent and valence-symmetric complexes. The results of these investigations suggest that halogen photoelimination proceeds by two sequential photochemical reactions: ligand dissociation followed by subsequent halogen elimination.Chemistry and Chemical Biolog

Integrating GWAS and Transcriptomics to Identify the Molecular Underpinnings of Thermal Stress Responses in \u3cem\u3eDrosophila melanogaster\u3c/em\u3e

Thermal tolerance of an organism depends on both the ability to dynamically adjust to a thermal stress and preparatory developmental processes that enhance thermal resistance. However, the extent to which standing genetic variation in thermal tolerance alleles influence dynamic stress responses vs. preparatory processes is unknown. Here, using the model species Drosophila melanogaster, we used a combination of Genome Wide Association mapping (GWAS) and transcriptomic profiling to characterize whether genes associated with thermal tolerance are primarily involved in dynamic stress responses or preparatory processes that influence physiological condition at the time of thermal stress. To test our hypotheses, we measured the critical thermal minimum (CTmin) and critical thermal maximum (CTmax) of 100 lines of the Drosophila Genetic Reference Panel (DGRP) and used GWAS to identify loci that explain variation in thermal limits. We observed greater variation in lower thermal limits, with CTmin ranging from 1.81 to 8.60°C, while CTmax ranged from 38.74 to 40.64°C. We identified 151 and 99 distinct genes associated with CTmin and CTmax, respectively, and there was strong support that these genes are involved in both dynamic responses to thermal stress and preparatory processes that increase thermal resistance. Many of the genes identified by GWAS were involved in the direct transcriptional response to thermal stress (72/151 for cold; 59/99 for heat), and overall GWAS candidates were more likely to be differentially expressed than other genes. Further, several GWAS candidates were regulatory genes that may participate in the regulation of stress responses, and gene ontologies related to development and morphogenesis were enriched, suggesting many of these genes influence thermal tolerance through effects on development and physiological status. Overall, our results suggest that thermal tolerance alleles can influence both dynamic plastic responses to thermal stress and preparatory processes that improve thermal resistance. These results also have utility for directly comparing GWAS and transcriptomic approaches for identifying candidate genes associated with thermal tolerance

Highly Luminescent Cyclometalated Iridium Complexes Generated by Nucleophilic Addition to Coordinated Isocyanides

In this work, we report a new class of blue-emitting cyclometalated iridium complexes supported by acyclic diaminocarbene (ADC) ancillary ligands. These neutral, tris-chelated complexes are not obtainable via traditional synthesis routes and instead are generated through metal-mediated nucleophilic addition to a metal-bound isocyanide, which is followed by orthometalation of the ADC under mild conditions. Importantly, four of the variants exhibit efficient phosphorescence when immobilized in PMMA matrix, achieving quantum yields of 79% for blue emitters with a 2-(2,4-difluorophenyl)­pyridine (F₂ppy) C^N ligand and 30–37% for orange emitters with a 2-phenylbenzothiazole (bt) C^N ligand. Electrochemical studies demonstrate significantly higher-lying HOMO levels in the ADC complexes relative to the NHC analogues, a phenomenon that results in enhanced charge-transfer character in the excited states of the ADC complexes. This study demonstrates that ADC ancillary ligands not only give rise to new structures for Ir­(III)-based phosphorescent emitters but also are promising targets for use in light-emitting devices and other thin-film optical applications

Highly Luminescent Cyclometalated Iridium Complexes Generated by Nucleophilic Addition to Coordinated Isocyanides

In this work, we report a new class of blue-emitting cyclometalated iridium complexes supported by acyclic diaminocarbene (ADC) ancillary ligands. These neutral, tris-chelated complexes are not obtainable via traditional synthesis routes and instead are generated through metal-mediated nucleophilic addition to a metal-bound isocyanide, which is followed by orthometalation of the ADC under mild conditions. Importantly, four of the variants exhibit efficient phosphorescence when immobilized in PMMA matrix, achieving quantum yields of 79% for blue emitters with a 2-(2,4-difluorophenyl)­pyridine (F₂ppy) C^N ligand and 30–37% for orange emitters with a 2-phenylbenzothiazole (bt) C^N ligand. Electrochemical studies demonstrate significantly higher-lying HOMO levels in the ADC complexes relative to the NHC analogues, a phenomenon that results in enhanced charge-transfer character in the excited states of the ADC complexes. This study demonstrates that ADC ancillary ligands not only give rise to new structures for Ir­(III)-based phosphorescent emitters but also are promising targets for use in light-emitting devices and other thin-film optical applications

A Thermodynamic Analysis of Rhenium(I)−Formyl C−H Bond Formation via Base-Assisted Heterolytic H_2 Cleavage in the Secondary Coordination Sphere

Conversion of synthesis gas, a mixture of carbon monoxide and hydrogen, into value-added Cn≥2 products requires both C–H and C–C bond-forming events. Our group has developed a series of molecular complexes, based on group 7 (manganese and rhenium) carbonyl complexes, to interrogate the elementary steps involved in the homogeneous hydrogenative reductive coupling of CO. Here, we explore a new mode of H2 activation, in which strong bases in the secondary coordination sphere are positioned to assist in the heterolytic cleavage of H2 to form a formyl C–H bond at a rhenium-bound carbonyl. A series of cationic rhenium(I) complexes of the type [ReI(PB:-κ1-P)(CO)5]n (n = 0, +1), where PB: is a phosphine ligand with a tethered strong base, are prepared and characterized; measurement of their protonation equilibria demonstrates a pronounced attenuation of the basicity upon coordination. Formyl complexes supported by these ligands can be prepared in good yield by hydride delivery to the parent pentacarbonyl complexes, and several of the free-base formyl complexes can be protonated, generating observable [ReI(PBH-κ1-P)(CHO)(CO)4]n complexes. Intramolecular hydrogen bonding is evident for one of the complexes, providing additional stabilization to the protonated formyl complex. By measuring both the hydricity of the formyl, ΔG°H–, and its pKa, the overall free energy of H2 cleavage is calculated from an appropriate cycle and found to be thermodynamically uphill in all cases (in the best case by only about 8 kcal/mol), although significantly dependent upon the properties of the supporting ligand

Oxygen Reduction Reactions of Monometallic Rhodium Hydride Complexes

Selective reduction of oxygen is mediated by a series of monometallic rhodium­(III) hydride complexes. Oxidative addition of HCl to <i>trans</i>-Rh^ICl­(L)­(PEt₃)₂ (<b>1a</b>, L = CO; <b>1b</b>, L = 2,6-dimethylphenylisocyanide (CNXy); <b>1c</b>, L = 1-adamantylisocyanide (CNAd)) produces the corresponding Rh^III hydride complex <i>cis</i>-<i>trans</i>-Rh^IIICl₂H­(L)­(PEt₃)₂ (<b>2a</b>–<b>c</b>). The measured equilibrium constants for the HCl-addition reactions show a pronounced dependence on the identity of the “L” ligand. The hydride complexes effect the reduction of O₂ to water in the presence of HCl, generating <i>trans</i>-Rh^IIICl₃(L)­(PEt₃)₂ (<b>3a</b>–<b>c</b>) as the metal-containing product. In the case of <b>2a</b>, smooth conversion to <b>3a</b> proceeds without spectroscopic evidence for an intermediate species. For <b>2b/c</b>, an aqua intermediate, <i>cis</i>-<i>trans</i>-[Rh^III(OH₂)­Cl₂(L)­(PEt₃)₂]Cl (<b>5b/c</b>), forms along the pathway to producing <b>3b/c</b> as the final products. The aqua complexes were independently prepared by treating peroxo complexes <i>trans</i>-Rh^IIICl­(L)­(η²-O₂)­(PEt₃)₂ (<b>4b/c</b>) with HCl to rapidly produce a mixture of <b>5b/c</b> and <b>3b/c</b>. The reactivity of the peroxo species demonstrates that they are plausible intermediates in the O₂-reduction chemistry of hydride complexes <b>2a</b>–<b>c</b>. These results together show that monometallic rhodium hydride complexes are capable of promoting selective reduction of oxygen to water and that this reaction may be controlled with systematic alteration of the ancillary ligand set