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Homogeneous Catalysis of Nickel Hydride Complexes Bearing a Bis (Phosphinite) Pincer Ligand

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Homogeneous Catalysis of Nickel Hydride Complexes Bearing a Bis (Phosphinite) Pincer Ligand Homogeneous Catalysis of Nickel Hydride Complexes Bearing a Bis(phosphinite) Pincer Ligand A Dissertation Submitted to the Graduate School of the University of Cincinnati in Partial Fulfillment of the Requirements for the Degree of DOCTOR OF PHILOSOPHY (Ph.D.) in the Department of Chemistry of McMicken College of Arts and Sciences by Sumit Chakraborty Master of Science (M. S.), Chemistry Indian Institute of Technology, Kanpur, India, 2007 Bachelor of Science (B. S.), Chemistry Hooghly Mohsin College, Burdwan University, India, 2005 Dissertation Advisor: Hairong Guan, Ph.D. Homogeneous Catalysis of Nickel Hydride Complexes Bearing a Bis(phosphinite) Pincer Ligand ABSTRACT 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 such as reduction of carbonyl functionalities present in aldehydes, ketones and CO2, dehydrogenation of formic acid, isomerization of terminal olefins, and cyanomethylation of aldehydes. 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; in each cycle the formal oxidation state of CO2 is reduced by 2, followed by the consumption of one equivalent of borane. A catalyst with more bulky substituents on the phosphorus atoms of pincer ligand is a more effective catalyst than those containing smaller substituents. This i 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. Several mechanistic possibilities such as heterogeneity, cooperativity, and formation of other reactive intermediates have been experimentally tested to understand the unusual kinetic behavior. A mechanistic investigation pertaining to NiH-catalyzed isomerization of terminal olefins such as 1-butene has been carried out. Two major pathways for olefin isomerization reaction, namely (a) metal hydride addition-elimination and (b) π-allyl metal hydride have been ruled out on the basis of reactivity studies of branched nickel butyl complex and labeling experiments. An H-atom transfer pathway has been proposed based on the radical inhibitor studies. In addition to the olefin isomerization, the branched nickel butyl complex isomerizes to the linear nickel butyl complex possibly via a unique intramolecular pathway. Nickel hydrides efficiently catalyze the cyanomethylation of aldehydes to produce β- hydroxy nitriles. This reaction does not require an external base which is mandatory for other related systems. The (POCOP)Ni-CH2CN complex, a potential intermediate in this reaction, has been isolated and characterized. This compound reacts directly with benzaldehyde to produce the β-hydroxy nitrile. The nickel alkoxide complexes that result from the insertion reaction between nickel hydrides and aldehydes, act as internal bases in this system. Varieties of aldehydes including base-sensitive aldehydes have been successfully coupled with acetonitrile using nickel hydride catalysts. ii iii Homogeneous Catalysis of Nickel Hydride Complexes Bearing a Bis(phosphinite) Pincer Ligand TABLE OF CONTENTS Chapter 1: Introduction 1.1 Reduction of Carbonyl Functionalities with First-Row Transition Metals 2 1.2 Catalysis Involving the Insertion of Carbonyl Substrates into an M-H Bond 6 1.3 Our Mechanistic Hypothesis 10 1.4 Nickel Hydrides in Stoichiometric and Catalytic Studies 12 1.5 Goals of the Research Project 15 Chapter 2: Catalytic Hydrosilylation of Aldehydes and Ketones 2.1 Introduction 17 2.2 Synthesis of Nickel Pincer Hydride Complexes 20 2.3 Stoichiometric Reduction of Aldehydes and Ketones with Nickel Hydride Complexes 24 2.4 Catalytic Hydrosilylation of Aldehydes and Ketones with Nickel Hydride Complexes. 28 2.5 Conclusions 35 2.6 Experimental Section 35 Chapter 3: Catalytic Reduction of CO2 to Methanol with Simple Boranes 3.1 Introduction 44 3.2 CO2 Insertion into Ni-H Bonds of Nickel Pincer Hydride Complexes 47 3.3 Regeneration of Nickel Hydrides from Nickel Formate Complexes 51 3.4 Catalytic Reduction of CO2 with Catecholborane 54 3.5 Comparative Studies of CO2 Reduction with Different Nickel Catalysts and Boranes 57 3.6 Reactions of Nickel Hydride Complexes with Boranes 61 iv 3.6.1 Reactions with BH3•THF 64 3.6.2 Reactions with 9-BBN 73 3.6.3 Reactions with HBcat 79 3.7 Rationalization of Catalytic Activities 84 3.8 Conclusions 85 3.9 Experimental Section 85 Chapter 4: Catalytic Decomposition of Formic Acid to Release Dihydrogen 4.1 Introduction 103 4.2 Stoichiometric Steps Related to Ni-Catalyzed Dehydrogenation of HCO2H 108 4.3 Catalytic Decomposition of HCO2H 109 4.4 Kinetic Studies of Catalytic Decomposition of HCO2H 111 4.4.1 Heterogeneity 112 4.4.2 Metal-Metal Cooperativity 113 4.4.3 Formation of Other Reactive Intermediates 114 4.4.4 Dual Processes Catalyzed by Nickel Hydrides 115 4.5 Conclusions 117 4.6 Experimental Section 118 Chapter 5: Mechanistic Insight of Nickel-Catalyzed Olefin Isomerization Reaction 5.1 Introduction 123 5.2 Catalytic Isomerization of 1-Butene to 2-Butene 124 5.3 Metal Hydride Addition-Elimination Pathway 125 5.4 π-Allyl Metal Hydride Pathway 130 5.5 Hydrogen Atom Transfer Pathway 132 5.6 Isomerization of Branched Nickel Alkyl Complex to Linear Nickel Alkyl Complex 135 5.7 Conclusions 139 5.8 Experimental Section 139 v Chapter 6: Catalytic Cyanomethylation of Aldehydes 6.1 Introduction 146 6.2 Transition Metal-Catalyzed Cyanomethylation of Aldehydes and Ketones 147 6.3 Catalytic Cyanomethylation of Aldehydes with Nickel Hydride Complexes 149 6.4 Synthesis of Nickel Cyanomethyl Complex 152 6.5 Reactivities of Nickel Cyanomethyl Complex 153 6.6 Conclusions 158 6.7 Experimental Section 159 vi Homogeneous Catalysis of Nickel Hydride Complexes Bearing a Bis(phosphinite) Pincer Ligand LIST OF FIGURES Chapter 1: Introduction Figure 1 Iron-Based Achiral and Chiral Catalysts for the Reduction of Carbonyl Groups 4 Chapter 2: Catalytic Hydrosilylation of Aldehydes and Ketones t Figure 1 X-ray Crystal Structure of [2,6-( Bu2PO)2C6H3]NiCl (2b) 21 t Figure 2 X-ray Crystal Structure of [2,6-( Bu2PO)2C6H3]NiH (3b) 23 Chapter 3: Catalytic Reduction of CO2 to Methanol with Simple Boranes i Figure 1 X-ray Crystal Structure of [2,6-( Pr2PO)2C6H3]NiOC(O)H (9a) 49 t Figure 2 X-ray Crystal Structure of [2,6-( Bu2PO)2C6H3]NiOC(O)H (9b) 50 c Figure 3 X-ray Crystal Structure of [2,6-( Pe2PO)2C6H3]NiOC(O)H (9c) 50 Figure 4 1H NMR for the Reduction of 9b-13C with HBcat 53 Figure 5 13C{1H} NMR for the Reduction of 9b-13C with HBcat 53 Figure 6 Possible Products from the Reaction of a Metal Hydride Complex with a Borane 62 i 2 Figure 7 X-ray Crystal Structure of [2,6-( Pr2PO)2C6H3]Ni(η -BH4) (10a) 69 t 2 Figure 8 X-ray Crystal Structure of [2,6-( Bu2PO)2C6H3]Ni(η -BH4) (10b) 70 c 2 Figure 9 X-ray Crystal Structure of [2,6-( Pe2PO)2C6H3]Ni(η -BH4) (10c) 71 c Figure 10 X-ray Crystal Structure of [2,6-( Pe2PO)2C6H3]Ni(µ-H)2[BC8H14] (11c) 77 Figure 11 Variable-Temperature 1H NMR Spectra of the Mixture of 3b and HBcat 80 Figure 12 Variable-Temperature 1H NMR spectra of the 1 : 1 Mixture of 3a and HBcat 82 Figure 13 Temperature-Dependent 11B NMR Study of a Mixture of 3a and HBcat 83 vii Chapter 4: Catalytic Decomposition of Formic Acid to Release Dihydrogen Figure 1 Selected Examples of Homogeneous Catalysts for the Decomposition of Formic Acid 105 Figure 2 Kinetics of Ni-Catalyzed Dehydrogenation of HCO2H 112 Figure 3 Structures of Potential Reactive Intermediates 114 Figure 4 Successive-Addition Kinetics of Ni-Catalyzed Dehydrogenation of HCO2H 115 Chapter 5: Mechanistic Insight of Nickel-Catalyzed Olefin Isomerization Reaction i sec Figure 1 X-ray Crystal Structure of [2,6-( Pr2PO)2C6H3]Ni(Bu ) (14a) 127 viii Homogeneous Catalysis of Nickel Hydride Complexes Bearing a Bis(phosphinite) Pincer Ligand LIST OF TABLES Chapter 2: Catalytic Hydrosilylation of Aldehydes and Ketones i Table 1 Stoichiometric Reactivity of [2,6-( Pr2PO)2C6H3]Ni(OCH2Ph) (4a) toward Different Silanes 28 Table 2 Comparing the Catalytic Activities of 3a-c in Hydrosilylation Reaction 29 Table 3 Hydrosilylation of Aldehydes Catalyzed by Nickel
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