Stoichiometric and catalytic small molecule activation reactions are vital for the synthesis of new materials. The activation of phosphorus-hydrogen or phosphorus-phosphorus bonds allows for the facile synthesis of new phosphorus-containing molecules for a wide variety of applications.1 An investigation of the P-H dehydrocoupling reaction was undertaken utilizing two rhodium(I) based catalysts. Over the course of this investigation it was found that the Rh(I) systems were also active catalysts for the reverse reaction: phosphorus-phosphorus bond hydrogenation (and hydrosilylation). This reaction was exploited for the synthesis of novel phosphines from P-P bound species. Molecules with P-P bonds were reacted in a stoichiometric fashion with the catalyst precursor, producing a variety of novel species with interesting bonding features which shed some light on the reaction mechanism. Following the discovery in 2006 that a linked phosphine-borane system could reversibly activate hydrogen2 a tremendous effort has been put forth to understand and expand this unprecedented reactivity.3,4 This new archetype for metal-free small molecule activation, containing a bulky Lewis acid and Lewis base which are unable to bond directly due to steric repulsion, has been termed a “frustrated Lewis pair” (FLP).3,4 The FLP concept is expanded to include bulky P-P bound species, pyridines and P-O bound Lewis bases as partners for B(C6F5)3. In some cases small molecule activation produced ion pairs or zwitterions related to those found for reactions with tertiary phosphines,3,4 but in others novel reaction pathways were discovered including phosphorus-phosphorus bond cleavage, catalytic hydrogenations and the formation of novel intramolecular FLPs. An unexpected situation was observed for the pair of 2,6-lutidine with B(C6F5)3, where adduct formation was observed along with free Lewis acid and base, but H2 activation by the FLP proceeded smoothly. Covalently bound phosphinoboranes of the general formula R2PB(C6F5)2 are synthesized. While systems with small R groups dimerized, monomers existed for cases with bulkier R groups. These monomers were found to exhibit extraordinarily short phosphorus-boron bonds yet were still capable of H2 activation analogous to bimolecular phosphine-borane systems. These systems also showed unique reactivity with Lewis acids and Lewis bases. This work further demonstrates the broad and general utility of the FLP concept in the synthesis of new materials and in catalytic transformations.

Stoichiometric and catalytic small molecule activation reactions are vital for the synthesis of new materials. The activation of phosphorus-hydrogen or phosphorus-phosphorus bonds allows for the facile synthesis of new phosphorus-containing molecules for a wide variety of applications.1 An investigation of the P-H dehydrocoupling reaction was undertaken utilizing two rhodium(I) based catalysts. Over the course of this investigation it was found that the Rh(I) systems were also active catalysts for the reverse reaction: phosphorus-phosphorus bond hydrogenation (and hydrosilylation). This reaction was exploited for the synthesis of novel phosphines from P-P bound species. Molecules with P-P bonds were reacted in a stoichiometric fashion with the catalyst precursor, producing a variety of novel species with interesting bonding features which shed some light on the reaction mechanism. Following the discovery in 2006 that a linked phosphine-borane system could reversibly activate hydrogen2 a tremendous effort has been put forth to understand and expand this unprecedented reactivity.3,4 This new archetype for metal-free small molecule activation, containing a bulky Lewis acid and Lewis base which are unable to bond directly due to steric repulsion, has been termed a “frustrated Lewis pair” (FLP).3,4 The FLP concept is expanded to include bulky P-P bound species, pyridines and P-O bound Lewis bases as partners for B(C6F5)3. In some cases small molecule activation produced ion pairs or zwitterions related to those found for reactions with tertiary phosphines,3,4 but in others novel reaction pathways were discovered including phosphorus-phosphorus bond cleavage, catalytic hydrogenations and the formation of novel intramolecular FLPs. An unexpected situation was observed for the pair of 2,6-lutidine with B(C6F5)3, where adduct formation was observed along with free Lewis acid and base, but H2 activation by the FLP proceeded smoothly. Covalently bound phosphinoboranes of the general formula R2PB(C6F5)2 are synthesized. While systems with small R groups dimerized, monomers existed for cases with bulkier R groups. These monomers were found to exhibit extraordinarily short phosphorus-boron bonds yet were still capable of H2 activation analogous to bimolecular phosphine-borane systems. These systems also showed unique reactivity with Lewis acids and Lewis bases. This work further demonstrates the broad and general utility of the FLP concept in the synthesis of new materials and in catalytic transformations.

Combinations of sterically encumbered Lewis acids and Lewis bases are precluded from dative bond formation, failing to yield classical Lewis acid-base adducts. These unique systems are referred to as frustrated Lewis pairs (FLPs) and demonstrate a plethora of unique small molecule activations.Herein, the syntheses of phosphonium alkoxy- and aryloxyborate salts in addition to phosphonium thioborate salts are detailed. The scope of Lewis acid and base, alcohol and thiol, that are effective in this chemistry, is also detailed. The syntheses of novel borate and boronate esters of the form B(OR)3 and (C6R4O2)B(C6F5) are detailed and applied in metal-free heterolytic dihydrogen activation with phosphines. The further study of a variety of perfluoroboranes in the activation of H2 with tertiary phosphines is also detailed. Derivatization of triarylboranes, boronate esters, borinic esters and chloroboranes by reaction with one or two equivalents of alky- or aryldiazomethane is achieved yielding the corresponding RR’C insertion products. The solid-state structures of the free boranes, in addition to Lewis base adducts of a sampling of these species, are detailed. Reactivity of the resulting sterically encumbered boranes in imine hydrogenations, H2 and XeF2 activation are also detailed. Combinations of intermolecular frustrated Lewis pairs are found to react collaboratively to activate greenhouse gases such as carbon dioxide (CO2) and nitrous oxide (N2O), yielding the corresponding zwitterionic compounds R3P(CO2)BR2R’ and R3P(N2O)BR’3. Atom connectivity has been established via X-ray crystallographic studies and molecular structures and parameters are discussed. Subsequent exchange chemistry of both CO2 and N2O species with transition metal and other d-block Lewis acids are investigated and yield zwitterionic compounds and ion pairs which are otherwise unobtainable employing more conventional methods. Transition metal containing Lewis acids are employed in conjunction with tri(tert-butyl)phosphine for the FLP-mediated direct activation of N2O.

Combinations of sterically encumbered Lewis acids and Lewis bases are precluded from dative bond formation, failing to yield classical Lewis acid-base adducts. These unique systems are referred to as frustrated Lewis pairs (FLPs) and demonstrate a plethora of unique small molecule activations.Herein, the syntheses of phosphonium alkoxy- and aryloxyborate salts in addition to phosphonium thioborate salts are detailed. The scope of Lewis acid and base, alcohol and thiol, that are effective in this chemistry, is also detailed. The syntheses of novel borate and boronate esters of the form B(OR)3 and (C6R4O2)B(C6F5) are detailed and applied in metal-free heterolytic dihydrogen activation with phosphines. The further study of a variety of perfluoroboranes in the activation of H2 with tertiary phosphines is also detailed. Derivatization of triarylboranes, boronate esters, borinic esters and chloroboranes by reaction with one or two equivalents of alky- or aryldiazomethane is achieved yielding the corresponding RR’C insertion products. The solid-state structures of the free boranes, in addition to Lewis base adducts of a sampling of these species, are detailed. Reactivity of the resulting sterically encumbered boranes in imine hydrogenations, H2 and XeF2 activation are also detailed. Combinations of intermolecular frustrated Lewis pairs are found to react collaboratively to activate greenhouse gases such as carbon dioxide (CO2) and nitrous oxide (N2O), yielding the corresponding zwitterionic compounds R3P(CO2)BR2R’ and R3P(N2O)BR’3. Atom connectivity has been established via X-ray crystallographic studies and molecular structures and parameters are discussed. Subsequent exchange chemistry of both CO2 and N2O species with transition metal and other d-block Lewis acids are investigated and yield zwitterionic compounds and ion pairs which are otherwise unobtainable employing more conventional methods. Transition metal containing Lewis acids are employed in conjunction with tri(tert-butyl)phosphine for the FLP-mediated direct activation of N2O.

This research focused on the synthesis of novel ogranoboron reagents in efforts to perform a variety of synthetic transformations, and additionally, the development of new methodologies to generate drug-like scaffolds. Initially, three novel tripod ligands were synthesized, and two were effectively chelated to boron to provide the desired organoborates. Such organoborates were employed in nucleophilic additions where they were found to be ineffective, whereas some activity was observed in Suzuki-Miyaura cross-coupling reactions. An additional project on organoboron compounds was conducted and focused on the development of organoboron frustrated Lewis pairs (FLPs) to facilitate the storage and transfer of hydrogen, nucleophilic addition reactions, and Claisen rearrangements. A new method for synthesizing a pyrrolidine diol unit was accomplished, and this intermediate was utilized to synthesize two FLPs. The reactivity of the FLPs with small molecules was assessed, and the pyrrolidine diol unit was subsequently evaluated for its ability to undergo a multicomponent reaction (MCR) to yield compounds possessing beneficial biological activity. Further research in this area was conducted, and a 5-aminoimidazole scaffold was synthesized employing a new MCR which is more efficient than previously reported methodologies. 5-Aminoimidazoles are frequently found in compounds which possess desirable biological activity, and this novel method was employed to generate a library of eleven 5-aminoimidazoles. Additionally, two post condensation modification reactions were developed. During initial studies, a side product was observed which was identified as a dihydrotriazine, which is another biologically appealing chemotype. Therefore, an enhanced method of synthesizing this product was developed, and a library of eleven dihydrotriazines was produced. In summary, novel organoboron reagents were synthesized, and their activity was evaluated. The pyrrolidine diol utilized to synthesize FLPs was applied towards an MCR. Furthermore, a novel MCR was developed for the synthesis of 5-aminoimidazoles, and an enhanced protocol for the synthesis of dihydrotriazines was found.

A series of new “frustrated Lewis pairs” (FLPs), including chiral versions, with a predefined spatial relationship between the basic and acidic centers is proposed. Several synthetic protocols toward the targets were investigated: through an aryllithium-haloborane coupling; using organotin reagents and a chiral diazaborolidine; and through organoboranes RBH₂ as the boron component. Further development of the project is discussed in light of the discovered data. The intermolecular system consisting of 8-bromo-2-methylquinoline and (C₆F₅)₃B was shown to exist in the form of an FLP. This FLP is not capable of heterolytic H-H bond cleavage with formation of an isolable adduct either at 1 atm or at 4 atm of H₂.

With hundreds of papers published since 2006 on frustrated Lewis pair (FLP) chemistry the development of innovative Lewis acids and Lewis bases is a quickly expanding field. This thesis is separated into two parts. The first part investigates the incorporation of tridentate ligands on ruthenium alkylidene species and their subsequent reactivity with Lewis acids and FLPs in an attempt to synthesize innovative olefin metathesis catalysts. The second part explores the use of ligands coordinated to ruthenium metal centers as a method for expanding the Lewis acid and base functionalities in the field of FLP chemistry. Ruthenium-alkylidene complexes of the general formula ((PEP-Cy)RuX2(CHPh) (E = O, NH, NMe; X = Cl, Br)) were obtained via the reaction of Grubbs 1st generation catalysts with the tridentate ancillary ligands O(CH2CH2PCy2)2 [POP-Cy], HN(CH2CH2PCy2)2 [PN(H)P-Cy] and MeN(CH2CH2PCy2)2 [PN(Me)P-Cy]. Subsequent treatment of GaX3 with these alkylidene species resulted in the formation of cationic alkylidyne-hydride complexes, ([(PEP Cy)Ru(H)(X)(CPh)][GaX4] where E = O, NH, NMe; X = Cl, Br; however, when E = NH, a mixture of alkylidyne-hydride and cationic alkylidene species was observed. Conversely, when E = NMe, alkylidyne-hydride formation was followed by transfer of the alkylidene fragment to the Me of the ligand framework. Subsequently, Ru-acetylide complexes [LRu(PPh3)2CCPh] were reacted with the B(C6F5)3 to afford the para-attacked products [LRu(PPh3)2C=C(Ph)((C6F4)B(F)(C6F5)2)] (L = Cp, Indenyl). Further studies revealed that [IndRu(PPh3)2CCPh] and ER3 (E = B, R = C6F4H; E = Al, R = C6F5) formed FLP mixtures that effected the activation of CO2 between the nucleophilic β-acetylide carbon and the Lewis acid to form [IndRu(PPh3)2C=C(Ph)C(O)O-ER3] or [IndRu(PPh3)2C=C(Ph)C(O ER3)2] when E = Al and R = C6F5. Similarly, these FLP combinations were used to activate phenylacetylene and benzaldehyde forming the species [IndRu(PPh3)2C=C(Ph)C(Ph)=C(H)(ER3)] and [IndRu(PPh3)2C=C(Ph)C(Ph)O-ER3], respectively. Lastly, the synthesis of [((Ph2PC6H4)2BCl)(η6-Ph))RuCl] was achieved via the reaction of (Ph2PC6H4)2BPh with (Ph3P)RuCl2. Subsequent halide abstraction with K[B(C6F5)4] resulted in the formation of [((Ph2PC6H4)2B)(η6-Ph))RuCl][B(C6F5)4] which was used as a carbon-based Lewis acid in adduct formation with Lewis bases to yield products of the general form [((Ph2PC6H4)2B)(η5 C6H5 o-LB))RuCl][B(C6F5)4] (LB = PPh3, PCy3, PMe3, SIMes, etc). The activation of H2 was also achieved when combinations of [((Ph2PC6H4)2B)(η6-Ph))RuCl][B(C6F5)4] and bulky phosphines were employed, resulting in the products [((Ph2PC6H4)2B)(η5 C6H6))RuCl][B(C6F5)4], [((Ph2PC6H4)2B)(η6 C6H5))RuCl][B(C6F5)4] and [R3PH][B(C6F5)4] (R = Mes, tBu, Cy). Similarly, [((Ph2PC6H4)2B)(η6-Ph))RuCl][B(C6F5)4] was used to catalytically hydrogenate sterically encumbered aldimines at room temperature with catalyst loadings as low as 1 mol% via a FLP-type mechanism.

With hundreds of papers published since 2006 on frustrated Lewis pair (FLP) chemistry the development of innovative Lewis acids and Lewis bases is a quickly expanding field. This thesis is separated into two parts. The first part investigates the incorporation of tridentate ligands on ruthenium alkylidene species and their subsequent reactivity with Lewis acids and FLPs in an attempt to synthesize innovative olefin metathesis catalysts. The second part explores the use of ligands coordinated to ruthenium metal centers as a method for expanding the Lewis acid and base functionalities in the field of FLP chemistry. Ruthenium-alkylidene complexes of the general formula ((PEP-Cy)RuX2(CHPh) (E = O, NH, NMe; X = Cl, Br)) were obtained via the reaction of Grubbs 1st generation catalysts with the tridentate ancillary ligands O(CH2CH2PCy2)2 [POP-Cy], HN(CH2CH2PCy2)2 [PN(H)P-Cy] and MeN(CH2CH2PCy2)2 [PN(Me)P-Cy]. Subsequent treatment of GaX3 with these alkylidene species resulted in the formation of cationic alkylidyne-hydride complexes, ([(PEP Cy)Ru(H)(X)(CPh)][GaX4] where E = O, NH, NMe; X = Cl, Br; however, when E = NH, a mixture of alkylidyne-hydride and cationic alkylidene species was observed. Conversely, when E = NMe, alkylidyne-hydride formation was followed by transfer of the alkylidene fragment to the Me of the ligand framework. Subsequently, Ru-acetylide complexes [LRu(PPh3)2CCPh] were reacted with the B(C6F5)3 to afford the para-attacked products [LRu(PPh3)2C=C(Ph)((C6F4)B(F)(C6F5)2)] (L = Cp, Indenyl). Further studies revealed that [IndRu(PPh3)2CCPh] and ER3 (E = B, R = C6F4H; E = Al, R = C6F5) formed FLP mixtures that effected the activation of CO2 between the nucleophilic β-acetylide carbon and the Lewis acid to form [IndRu(PPh3)2C=C(Ph)C(O)O-ER3] or [IndRu(PPh3)2C=C(Ph)C(O ER3)2] when E = Al and R = C6F5. Similarly, these FLP combinations were used to activate phenylacetylene and benzaldehyde forming the species [IndRu(PPh3)2C=C(Ph)C(Ph)=C(H)(ER3)] and [IndRu(PPh3)2C=C(Ph)C(Ph)O-ER3], respectively. Lastly, the synthesis of [((Ph2PC6H4)2BCl)(η6-Ph))RuCl] was achieved via the reaction of (Ph2PC6H4)2BPh with (Ph3P)RuCl2. Subsequent halide abstraction with K[B(C6F5)4] resulted in the formation of [((Ph2PC6H4)2B)(η6-Ph))RuCl][B(C6F5)4] which was used as a carbon-based Lewis acid in adduct formation with Lewis bases to yield products of the general form [((Ph2PC6H4)2B)(η5 C6H5 o-LB))RuCl][B(C6F5)4] (LB = PPh3, PCy3, PMe3, SIMes, etc). The activation of H2 was also achieved when combinations of [((Ph2PC6H4)2B)(η6-Ph))RuCl][B(C6F5)4] and bulky phosphines were employed, resulting in the products [((Ph2PC6H4)2B)(η5 C6H6))RuCl][B(C6F5)4], [((Ph2PC6H4)2B)(η6 C6H5))RuCl][B(C6F5)4] and [R3PH][B(C6F5)4] (R = Mes, tBu, Cy). Similarly, [((Ph2PC6H4)2B)(η6-Ph))RuCl][B(C6F5)4] was used to catalytically hydrogenate sterically encumbered aldimines at room temperature with catalyst loadings as low as 1 mol% via a FLP-type mechanism.

This thesis describes the activation of small molecules using frustrated Lewis pairs, in particular investigating their use to reduce CO₂ to methanol, thus producing a new route towards a renewable fuel. Chapter One summarises the requirement for a renewable fuel source, the alternative methods currently available and previous research conducted into converting CO₂ to methanol using FLPs and other reducing agents. Chapter Two describes the synthesis of a new family of electron-deficient tris(aryl)boranes, B(C₆F₅)_3-x(C₆Cl₅)_x (x = 1-3), allowing the electronic effects, resulting from the gradual replacement of C₆F₅ with C₆Cl₅ ligands, to be studied. The novel Lewis acids have been fully characterised and their Lewis acidities have been determined using NMR spectroscopy, electrochemistry and DFT studies. Chapter Three discusses the synthesis of nine novel FLPs and their use to successfully split H2. Each borohydride salt has been spectroscopically fully characterised and five of the salts have been characterised using single crystal X-ray diffraction. To determine the exact positions of the H atoms, single crystal neutron diffraction and DFT experiments were carried out on [1-H][H-TMP]. Chapter Four details attempts to use the borohydride salts, synthesised in Chapter Three, to reduce CO₂ to methanol. Each experiment was been fully investigated and their catalytic viability was determined. The X-ray crystal structure of [1-OCHO][H-TMP] is described and each formatoborate and methoxyborate salt were fully characterised. Chapter Five describes experimental procedures and characterisation data.

Three classes of bifunctional compounds related to frustrated Lewis pair chemistry were studied. The first class, alkynyl-linked phosphonium borates, was strategically synthesized and the corresponding neutral alkynyl-linked phosphine boranes generated in solution. They were reacted with THF, alkenes and alkynes to undergo either ring-opening or multiple bond addition reactions, giving rise to zwitterionic macrocycles. In two select alkynyl-linked phosphonium borates, thermolysis resulted in unique rearrangements transforming the phosphino- and boryl-substituted alkynyl moieties into C4 chains. The alkynyl-linked phosphine boranes were further demonstrated to coordinate as η3-BCC ligands in Ni(0) complexes. The rigid nature of the coordination was confirmed by dimerization without cleavage of the Ni–B interaction upon the addition of acetonitrile or carbon monoxide. Moreover, reactions with Al-, Zn- and B-based Lewis acids prompted hydride transfer within the alkynyl-linked phosphonium borate and interesting functional group transfer reactions. The second class of the bifunctional systems, a series of gem-substituted bis-boranes, was subjected to reactions with tBu3P and CO2. The O-linked bis-borane was shown to coordinate the phosphino-carboxylate moiety with one B, while the methylene-linked bis-boranes were demonstrated to chelate the carboxyl group. The third bifunctional system class, vinyl-group tethered boranes, was examined to elucidate the mechanism of the frustrated Lewis pair addition reaction to olefins. Using a bis(pentafluorophenyl)alkylborane, the close proximity of the olefinic protons and the ortho-fluorine nuclei were evident by both NOE measurements and DFT calculations. Moreover, its reactions with phosphine bases suggested that an initial interaction between the highly electrophilic borane and the olefinic fragment precedes such frustrated Lewis pair addition reaction. Furthermore, a bis(pentafluorophenyl)alkoxyborane was synthesized and reacted with P-, N-, C- and H-based nucleophiles, demonstrating the wide range of Lewis bases that can be applied in olefin addition reactions with complementary regioselectivity.