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N-heterocyclic metallocenes of the lanthanides based on the 4-azapentalenyl anion

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Wise, LE 2009 , 'N-heterocyclic metallocenes of the lanthanides based on the 4-azapentalenyl anion', PhD thesis, University of Tasmania.

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Abstract

This thesis describes synthetic and density functional (DFT) investigations of organometallic complexes of the 4-azapentalenyl anion featuring Group 1, 3 and 4 metals. Included are ligand development methods and organometallic complexes of various extended ring systems, built by benzarmulation of the parent 3H-pyrrolizine ring system. Emphasis was directed towards applications in alkene polymerisation.
The ring extension of the bicyclic 3H-pyrrolizine core led to the successful synthesis and characterisation of 5H-pyrrolo[2,1-a]isoindole and 6H-isoindolo[2,1- a]isoindole. Both were prepared using intramolecular Heck coupling reactions. The attempted synthesis of two other systems, 9H-pyrrolo[1,2-a]indole, an isomer of 5H-pyrrolo [2, 1 -a] isoindole, and a methylated derivative of 5H-pyrrolo [2, 1 -a] isoindole were unsuccessful. Subsequent reactions of the modified heterocyclic systems with Group 1 metals led to the identification via NMR spectroscopy of formation of the corresponding anions in the case of lithium, sodium and potassium salts. Crystalline salts of the novel potassium 5H-pyrrolo[2,1-a]isoindenyl complexes with coordinated Lewis bases were isolated and characterised by NMR spectroscopy, microanalysis and single crystal X-ray diffraction studies. The structural studies revealed the anion to display some cyclopentadienyl-like tendencies, as well as allowing for further metal-ligand interactions, in the same way that the indenyl and fluorenyl anions relate to the cyclopentadienyl anion itself. Other complexes proved too unstable to isolate via decomposition routes described below. Similarly, attempts to prepare Group 3 and 4 metal complexes led to reprotonation of the heterocycle or reductive coupling outcomes.
The effects of extending the ring system of the 4-azapentalenyl anion by benzannulation was investigated using DFT. Studies of the deprotonated 5H-pyrrolo[2,1-a]isoindole indicated an uneven charge distribution over the three ringed heterocyclic system. When comparing the highest occupied molecular orbitals (HOMO) of this system it was observed that a large HOMO was present at position 5 of the deprotonated 5H-pyrrolo[2,1-a]isoindole. The combination of these findings, along with calculated pK\(_a\) values, aided in the understanding of the instability of many of the Group 1, 3 and 4 metal complexes with respect to reprotonation and reductive coupling. This was supported via reacting a potassium salt of 5H-pyrrolo[2,1-a]isoindole with mercuric chloride resulting in the reductively coupled dimer bis(5H-pyrrolo[2,1-a]isoindole), which was identified by \(^1\)H NMR spectroscopy and GC-MS analysis.
DFT studies of unsolvated complexes of 5H-pyrrolo[2,1-a]isoindole Group 1 metal salts indicated that the size and nature of the metal centre influences the type of bonding interaction observed and the extent to which the ligand can "fold" along the N-C bridgehead vector of the anion in allowing for higher hapticity of the metal-ligand interaction in the complex. Similar studies were carried out on Group 1, 3 and 4 metal complexes of the parent 4-azapentalenyl anion system. These complexes either, i, exhibited a folding of the anion along the bridgehead vector, with dihedral angles between 17 - 26 ° for potassium and sodium, respectively, varying in accord with the size of the metal to achieve increased hapticity, ii, or resulted in η\(^5\)- interactions with a non-folded anion for lithium as, presumably, the necessarily increased extent of folding that would be needed in the more symmetrically bound structure comes at too great an expense of aromatic behaviour of the fused ring system of the anion.
DFT studies on the insertion of ethylene into the scandium - hydride bond of bis(4-azapentalenyl)scandium hydride showed the ability of the 4-azapentalenyl anion to alter its binding mode via a η\(^8\)-folded/η\(^5\)-p1anar fluctional process during the insertion process in accord with the binding of catalysis substrates. This process results in the ethylene polymerisation chain growth step having a very low energy barrier, indicating that such fluctional processes are potentially of major interest to this industrially important reaction. However, synthetic access to such transition metal complexes remains a challenge for future work on this interesting figand system that has been shown by this study to have high potential in a niche area of major industrial importance.

Item Type: Thesis - PhD
Authors/Creators:Wise, LE
Keywords: Metallocenes, Rare earth metals
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Copyright 2009 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

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Thesis (PhD)--University of Tasmania, 2009. Includes bibliographical references. 1. Introduction. 2. Synthesis of 3H-Pyrrolozine systems -- 3. Group 1 metal modified 4-azapentalenyl complexes -- 4. Density functional studies of unsolvated group 1,3 and 4 metal 4-azapentalenyl complexes -- 5. Computational studies of catalytically relevant systems -- pKA studies of modified 3H-pyrrolizines

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