Synthesis, Structure, and DFT Analysis of the THF Solvate of 2‐Picolyllithium: A 2‐Picolyllithium Solvate with Significant Carbanionic Character

Previous studies of different solvates of 2-methylpyridyllithium (2-picolyllithium) have uncovered electronic structures corresponding to aza-allyl and enamido resonance forms of the metallated pyridine-based compounds. Here, we report the synthesis and characterization of [2-CH2Li(THF)2C5H4N], a new THF solvate. X-ray crystallographic studies reveal a dimeric arrangement featuring a non-planar eight-membered [NCCLi]2 ring, in which the primary cation-anion interaction is between the central Li atom and the C atom of the deprotonated methyl group [length, 2.285(2) Å], suggesting a new carbanionic resonance structure for this 2-picolyllithium series. The significant carbanionic character of [2-CH2Li(THF)2C5H4N] was confirmed by gas-phase DFT calculations [B3LYP/6-311+G(d)] with the calculated electron density interrogated by means of quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analyses. For comparison these computational analyses were also performed on the literature structures of [2-CH2Li(2-Picoline)C5H4N] and [2-CH2Li(PMDETA)C5H4N]. In a reactivity study, [2-CH2Li(THF)2C5H4N] was found to undergo nucleophilic addition to pyridine to generate dipyridylmethane in a good yield

A regioselectively 1, 1',3 ,3'-tetrazincated ferrocene complex displaying core and peripheral reactivity

Regioselective 1,1′,3,3′-tetrazincation [C-H to C-Zn(tBu)] of ferrocene has been achieved by reaction of a fourfold excess of di-t-butylzinc (tBu2Zn) with sodium 2,2,6,6-tetramethylpiperidide (NaTMP) in hexane solution manifested in the trimetallic iron-sodium-zinc complex [Na4(TMP)4Zn4(tBu)4{(C5H3)2Fe}], 1. X-ray crystallographic studies supported by DFT modelling reveal the structure to be an open inverse crown in which two [Na(TMP)Zn(tBu)Na(TMP)Zn(tBu)]2+ cationic units surround a {(C5H3)2Fe}4- tetraanion. Detailed C6D6 NMR studies have assigned the plethora of 1H and 13C chemical shifts of this complex. It exists in a major form in which capping and bridging TMP groups interchange, as well as a minor form that appears to be an intermediate in this complicated exchange phenomenon. Investigation of 1 has uncovered two distinct reactivities. Two of its peripheral t-butyl carbanions formally deprotonate toluene at the lateral methyl group to generate benzyl ligands that replace these carbanions in [Na4(TMP)4Zn4(tBu)2(CH2Ph)2{(C5H3)2Fe}], 2, which retains its tetrazincated ferrocenyl core. Benzyl-Na π-arene interactions are a notable feature of 2. In contrast, reaction with pyridine affords the crystalline product {[Na·4py][Zn(py∗)2(tBu)·py]}∞, 3, where py is neutral pyridine (C5H5N) and py∗ is the anion (4-C5H4N), a rare example of pyridine deprotonated/metallated at the 4-position. This ferrocene-free complex appears to be a product of core reactivity in that the core-positioned ferrocenyl anions of 1, in company with TMP anions, have formally deprotonated the heterocycle

The development of alkali-metal zincate chemistry for application in synthesis and catalysis

The work reported within this thesis focuses on two main projects. With these projects involving the chemistry of the metal zinc. The first of these projects was to investigate the chemistry an unprecedented tetrazincated ferrocene complex (4) and the second project was to investigate the primarily interactions in the zincate [{K(HMDS)2ZnBn}∞]. The first project is a continuation of a previous study that resulted in the formation of a tetrazincated ferrocene complex (4), with the intention of using this as an intermediate for the synthesis of tetrasubstituted ferrocenes.;These functionalizations were attempted using three different electrophilic reagents, specifically I2, D2O and CO2. The results from this were often confusing and inconclusive. However, when using D2O for a quench of the tetrazincated ferrocene complex, this did in fact seem to produce the desired D4-ferrocene product. Furthermore, the reactivity of the tetrazincated ferrocene complex towards aromatic substrates was probed to provide understanding of how this species behaves as a base.;Previous studies had shown that when pyridine is added to compound 4, a polymeric γ-metallated pyridine species (6) is formed. However, this was only acquired in an isolated yield of 5%. Here, the reaction was optimized, improving the crystalline yield to 29%. This study has also revealed that this process is more complicated than originally thought, via electrophilic quenching reactions with I2, as it seems to also metallate at the β-position of pyridine and not exclusively the γ-position as previously thought.;During the course of this project, an alternative zinc reagent to tBu2Zn was desired in order to avoid safety issues. A synthesis of isomeric iBu2Zn has been successfully developed and was found to be reproducible to a yield of 78%. This compound was then tested in a series of reactions in order to form a comparison with tBu2Zn and its related bimetallic 'ate' bases.;The second of these two projects, focuses on a recent publication by Guan et al. which has shown that the polymeric potassium zincate [{K(HMDS)2ZnBn}∞] (8) can be used for the direct catalytic functionalization of the benzylic C-H bond of diarylmethanes. One of the main aims of this project was to achieve the isolation and characterisation of the proposed intermediate in the investigation performed by Guan et al., which was the metallated diphenylmethyl system [{KZn(HMDS)2(CHPh2)}∞]. Despite there being promising signs of achieving this, due to time constraints, this ultimately was not achieved.;The second main aim of this section was to investigate how the potassium atom in [{K(HMDS)2ZnBn}∞] primarily interacts with the zincate component by disrupting its polymeric nature. This was achieved through the use of the highly effective monomerizing agent Me6TREN which revealed that the monomeric unit should be considered as (HMDS)2ZnBnK rather than K(HMDS)2ZnBn as suggested by Guan. This can be rationalized due to the π-philicity of the heavier alkali metal which preferentially coordinates to the delocalised charge within the aryl ring rather than the localised charge of the amido nitrogen centres of the HMDS ligands when the hemisolvating Me6TREN ligand forces it to choose only one.The work reported within this thesis focuses on two main projects. With these projects involving the chemistry of the metal zinc. The first of these projects was to investigate the chemistry an unprecedented tetrazincated ferrocene complex (4) and the second project was to investigate the primarily interactions in the zincate [{K(HMDS)2ZnBn}∞]. The first project is a continuation of a previous study that resulted in the formation of a tetrazincated ferrocene complex (4), with the intention of using this as an intermediate for the synthesis of tetrasubstituted ferrocenes.;These functionalizations were attempted using three different electrophilic reagents, specifically I2, D2O and CO2. The results from this were often confusing and inconclusive. However, when using D2O for a quench of the tetrazincated ferrocene complex, this did in fact seem to produce the desired D4-ferrocene product. Furthermore, the reactivity of the tetrazincated ferrocene complex towards aromatic substrates was probed to provide understanding of how this species behaves as a base.;Previous studies had shown that when pyridine is added to compound 4, a polymeric γ-metallated pyridine species (6) is formed. However, this was only acquired in an isolated yield of 5%. Here, the reaction was optimized, improving the crystalline yield to 29%. This study has also revealed that this process is more complicated than originally thought, via electrophilic quenching reactions with I2, as it seems to also metallate at the β-position of pyridine and not exclusively the γ-position as previously thought.;During the course of this project, an alternative zinc reagent to tBu2Zn was desired in order to avoid safety issues. A synthesis of isomeric iBu2Zn has been successfully developed and was found to be reproducible to a yield of 78%. This compound was then tested in a series of reactions in order to form a comparison with tBu2Zn and its related bimetallic 'ate' bases.;The second of these two projects, focuses on a recent publication by Guan et al. which has shown that the polymeric potassium zincate [{K(HMDS)2ZnBn}∞] (8) can be used for the direct catalytic functionalization of the benzylic C-H bond of diarylmethanes. One of the main aims of this project was to achieve the isolation and characterisation of the proposed intermediate in the investigation performed by Guan et al., which was the metallated diphenylmethyl system [{KZn(HMDS)2(CHPh2)}∞]. Despite there being promising signs of achieving this, due to time constraints, this ultimately was not achieved.;The second main aim of this section was to investigate how the potassium atom in [{K(HMDS)2ZnBn}∞] primarily interacts with the zincate component by disrupting its polymeric nature. This was achieved through the use of the highly effective monomerizing agent Me6TREN which revealed that the monomeric unit should be considered as (HMDS)2ZnBnK rather than K(HMDS)2ZnBn as suggested by Guan. This can be rationalized due to the π-philicity of the heavier alkali metal which preferentially coordinates to the delocalised charge within the aryl ring rather than the localised charge of the amido nitrogen centres of the HMDS ligands when the hemisolvating Me6TREN ligand forces it to choose only one