PhD, University of Cincinnati, 2019, Arts and Sciences: Chemistry

Several new cobalt complexes bearing bis(phosphinite)-based (POCOP) pincer ligands have been synthesized through direct C–H bond activation. Various cobalt complexes or salts including Co2(CO)8, Co(PMe3)4H, Co(PMe3)4Me and CoCl2 have been tested as the precursors. A series of cobalt(I) POCOP pincer complexes were isolated from the reaction of Co2(CO)8 with various POCOP pincer ligands. The resulting pincer complexes, (RPOCOP)Co(CO)2 (R for substituents on the phosphorus donors) for short, can be oxidized by I2 to yield Co(III) complexes. Another series of cobalt(I) POCOP pincer complexes featuring two PMe3 ligands have been synthesized through the reaction of the POCOP ligands with Co(PMe3)4H or Co(PMe3)4Me. The dissociation of one PMe3 ligand enables the cobalt center to react with phenylsilane to form a cobalt(III) complex. CoCl2 has also been demonstrated to be an effective precursor to activate the central C–H bond of a POCOP pincer ligand to afford a cobalt(II) pincer complex. In addition to the POCOP pincer system, a bis(phosphinomethyl)pyrrolide-based (PNP) cobalt(I) pincer complex has also been synthesized using PMe3 as an ancillary ligand to stabilize the metal complex. Among these newly synthesized cobalt pincer complexes, complexes (RPOCOP)Co(CO)2 prove to be efficient catalysts for the catalytic hydrosilylation of aldehydes with (EtO)3SiH at elevated temperatures (=100°C) in a closed system or at 50 °C in an open system. Mechanistic investigation has suggested that the reaction is initiated by CO dissociation, which is kinetically feasible even at room temperature but thermodynamically unfavorable. Detailed mechanistic studies focusing on the cobalt dicarbonyl complexes catalyzed hydrosilylation of aldehydes have been conducted. The cobalt dicarbonyl complexes are shown to be pre-activated by an aldehyde at elevated temperatures in an open system. The “same tube” and “separate tubes” competition experiments support either a decomposition (open full item for complete abstract)

Committee: Hairong Guan Ph.D. (Committee Chair); Michael Baldwin Ph.D. (Committee Member); David Smithrud Ph.D. (Committee Member) Subjects: Chemistry

PhD, University of Cincinnati, 2018, Arts and Sciences: Chemistry

This dissertation focuses on the hydrogenation of challenging substrates, such as esters, amides and nitriles catalyzed by earth abundant metals including iron and cobalt. In this work, well-defined iron hydride complexes bearing a iPrPNHP pincer ligand (iPrPNHP = HN[CH2CH2P(iPr2)]2) have been synthesized and employed as efficient catalysts for the hydrogenation of esters and amides; cobalt complexes with the iPrPNHP ligand and cobalt particles have been shown to catalyze the hydrogenation of nitriles for selective formation of imines and primary amines, respectively. Hydrogenation of esters is vital to the chemical industry for the production of alcohols, especially fatty alcohols that find broad applications in consumer products. In this dissertation, an iron-based complex (iPrPNHP)FeH(CO)(BH4) has been used as an efficient catalyst for the hydrogenation of esters under relatively mild conditions. This catalytic system is also effective for the conversion of coconut oil derived fatty acid methyl esters to detergent alcohols without adding any solvent. With the same catalytic system, amides have also been tested as the substrates for the hydrogenation study. It has been found that the catalytic reactions are limited to secondary amides in which an aryl group is attached to the amide nitrogen. In order to improve the thermal stability of the iron pincer complexes bearing CO as the ancillary ligand, both mono(isonitrile) and bis(isonitrile) iron pincer complexes have also been synthesized. For the bis(isonitrile) iron complexes, all the three possible isomers (e.g., cis, anti-, cis, syn- and trans-isomers) of [(iPrPNHP)FeBr(CNtBu)2][BPh4] and two isomers (e.g., cis, anti- and cis, syn-isomers) of [(iPrPNHP)FeH(CNtBu)2][BPh4] have been isolated. The reactivity of the [(iPrPNHP)FeH(CNtBu)2][BPh4] complexes towards benzaldehyde proves that the NH hydrogen facilitates the C=O insertion to the Fe H bond, which is possible with the cis, syn-isomer. The mono( (open full item for complete abstract)

Committee: Hairong Guan Ph.D. (Committee Chair); William Connick Ph.D. (Committee Member); Allan Pinhas Ph.D. (Committee Member) Subjects: Chemistry

PhD, University of Cincinnati, 2017, Arts and Sciences: Chemistry

This dissertation research focuses on the development of tridentate ligand supported nickel complexes for both catalytic and stoichiometric bond activation reactions. One of the main objectives of this research is to understand how the flexibility of ligands impacts the reactivity of transition-metal complexes. To this end, three different ligands with varying degree of flexibility have been studied. Chelating ligands have been used to enhance the stability of transition-metal complexes because compared to monodentate ligands these ligands are less likely to dissociate from the metal. Guided by this hypothesis, a chelating secondary phosphine oxide (SPO) ligand has been synthesized and shown to act as either a bidentate or a pseudo tridentate ligand depending on the electronic demand and geometry at the nickel center. Nickel(0) species supported by this SPO is a more effective catalyst than Ni(0)–Ph2P(O)H for cross-coupling of aryl iodides with aryl thiols. Both electron-donating and electron-withdrawing groups including OMe, CF3, CN, pyridyl or ester groups are tolerated in this catalytic system. However, low isolated yields are observed with sterically crowded aryl iodides and aryl thiols. Nickel complexes bearing a tridentate pyridine bis-amide ligand have also been studied for carbon-sulfur cross-coupling reaction of aryl iodide with aryl thiol. One of the nickel chloride complexes in which (o-Me)2C6H3 group is the substituent on the amide nitrogens has been used as the catalyst. Harsh reaction conditions including high catalyst loadings, high temperatures and long reaction times are required to obtain nearly quantitative yields. Two cationic nickel hydride complexes containing a PNHPiPr ligand have been synthesized. For the cationic hydride with Br- as the counter anion, NaBH4 is the main hydride source whereas alcohol and residual water are the proton sources for NH hydrogen. These cationic hydrides have been studied for the reduction of CO2. The rate of CO2 i (open full item for complete abstract)

Committee: Hairong Guan Ph.D. (Committee Chair); William Connick Ph.D. (Committee Member); Allan Pinhas Ph.D. (Committee Member) Subjects: Chemistry; Inorganic Chemistry; Organic Chemistry

PhD, University of Cincinnati, 2013, Arts and Sciences: Chemistry

A series of platinum(II) complexes with the formulae Pt(R2NCN)(tpy)+, (tpy=2,2′:6′,2″-terpyridine; R2NCN- =2,6-bis(cyclic amine-methyl)phenyl anion; R = cyclic amine = piperidyl (pip); morpholinyl (mor); methylpiperizinyl (mepiz); thiomorpholinyl (tmor); and methylpiperidinyl (mepip)) was prepared. The tpy ligand is tridentate, whereas the R2NCN- ligand is monodentate, bonded through the central aryl group with the cyclic amines dangling above and below the axial coordination sites. A series of experiments were undertaken in order to evaluate the influence of axial Pt···N axial interactions on the reactivity and redox properties. 195Pt NMR chemical shifts are very sensitive to variations in the basicity of the amine groups, providing strong evidence for the existence of non-covalent axial Pt··· interactions in solution. The axial Pt···N interaction were detected and assessed by measuring the barriers to inversion for the dangling cyclic amines. The difference between the barrier to inversion of the free ligand and complex was interpreted as a measure of the extent of interaction between the dangling amines and Pt metal center. Structural, spectroscopic and electrochemical results suggest that changes in the donor properties of the dangling nucleophiles has remarkable influence on the reactivity of the Pt(R2NCN)(tpy)+ complexes. The electrochemical properties of these complexes were characterized by a combination of cyclic voltammetry, bulk electrolysis and spectroelectrochemistry. Cyclic voltammetric measurements demonstrated that changing the donor properties of the dangling groups tunes the potential by ~100 mV. The scan rate dependence of cyclic voltammograms of Pt(R2NCN)(tpy)+ presented here are consistent with a stepwise electron-transfer in which the second electron transfer step is slightly more favorable than the first (i.e., E2°′>E1°′). The axial Pt···N interactions preorganizes the complex for electron transfer, thereby increasing the rate of het (open full item for complete abstract)

Committee: William Connick Ph.D. (Committee Chair); Michael Baldwin Ph.D. (Committee Member); James Mack Ph.D. (Committee Member) Subjects: Inorganic Chemistry

PhD, University of Cincinnati, 2012, Arts and Sciences: Chemistry

This dissertation is focused on the synthesis of well-defined nickel hydride complexes bearing a bis(phosphinite) pincer ligand (commonly known as a POCOP ligand) and utilities of these metal complexes in varieties of useful organic transformations. Aldehydes insert cleanly and selectively into the Ni-H bonds of (POCOP)NiH complexes to form nickel alkoxide complexes. These nickel alkoxide compounds further react with phenylsilane to regenerate (POCOP)NiH and produce silyl ethers. Based on these observations, an efficient and chemoselective hydrosilylation process has been developed utilizing nickel hydrides as catalysts. The process is highly compatible with varieties of functional groups in aldehydes. A nickel hydride complex with smaller substituents on the POCOP ligand proves to be more effective hydrosilylation catalyst. In case of ketones, partial hydrosilylation occurs. (POCOP)NiH complexes also react with CO2 to produce nickel formate complexes. When stoichiometric amounts of boranes are used, nickel hydrides are cleanly reformed. The use of catalytic amounts of nickel hydrides and excess of boranes reduces CO2 to the corresponding methanol derivatives. The initial reduction products can be further hydrolyzed to yield methanol. The overall transformation is comprised of three catalytic cycles. A catalyst with more bulky substituents on the phosphorus atoms of pincer ligand is a more effective catalyst than those containing smaller substituents. This phenomenon has been rationalized by invoking more favorable dihydridoborate adduct formation between less bulky nickel hydrides and boranes. One of such dihydridoborate adduct has been successfully isolated and its influence on the catalytic CO2 reduction has been demonstrated. (POCOP)NiH complexes have been found be active catalysts for the decomposition of formic acid to release dihydrogen. When the kinetics the reaction is monitored by in-situ IR spectroscopy, a unique sigmoidal pattern is observed. Seve (open full item for complete abstract)

Committee: Hairong Guan PhD (Committee Chair); William Connick PhD (Committee Member); Allan Pinhas PhD (Committee Member) Subjects: Chemistry

Doctor of Philosophy, Case Western Reserve University, 2007, Chemistry

Diphosphine ligands 2,6-(2-CH2PR2C6H4)2C6H3Br (R = Ph, Cy, tBu), diimine ligands 2,6-{2-R'N=C(H)C6H4}2C6H3I (R' = Ph, 2,6-Me2C6H3, 2,4,6-Me3C6H2, Cy, (S)-α-methylbenzyl) and the diamine ligand 2,6-(2-Me2NCH2C6H4)2C6H3I have synthesized and characterized. The reaction of these ligands with Pd2(dba)3 afforded new kinds of PCP and NCN pincer complexes. These pincer complexes containing m-terphenyl scaffolds have been characterized by NMR spectroscopy and single crystal X-ray crystallography. Structure analyses of these pincer complexes reveal a C2 symmetric environment. In addition, this system shows the greatest “twist” angle to date for pincer complexes. There has been no evidence of the interconversion of possible atropisomers even at elevated temperature, which indicates the high rigidity of these pincer complexes. Further more, the introduction of chiral imine groups was utilized for resolution of chiral pincer complexes having high degree of non-fluxionality.

Committee: John Protasiewicz (Advisor) Subjects: