Fundamental Studies of Early Transition Metal-Ligand Multiple Bonds: Structure, Electronics, and Catalysis

Citation

Tonks, Ian Albert (2012) Fundamental Studies of Early Transition Metal-Ligand Multiple Bonds: Structure, Electronics, and Catalysis. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/FYGA-KK18. https://resolver.caltech.edu/CaltechTHESIS:12192011-112528810

Abstract

Two major topics are covered: the first section is focused on the structure, electronics, stoichiometric reactivity and catalysis of nonmetallocene early transition metal complexes that often contain metal-ligand multiple bonds (Chapters 2-4); the second section is dedicated to the development of hydrazide(2-) ligands for group 5 elements, which were heretofore unexplored as ligands for group 5 (Chapters 5-6).

A series of tantalum imido and amido complexes supported by a pyridine linked bis(phenolate) (ONO) ligand has been synthesized. Characterization of these complexes via X ray crystallography reveals both Cs and C2 binding modes of the bis(phenolate)pyridine ligand. DFT calculations and molecular orbital analyses of the complexes have revealed that the preference for Cs symmetric ligand binding is a result of Ta-O π bonding: in cases where Ta-O π bonding is overridden by stronger Ta N π bonding, C2 symmetric ligand binding is preferred because this is the lowest energy geometric conformation.

Titanium and zirconium complexes supported by a related pyridine bis(anilide) ligand (NNN = pyridine 2,6 bis(N-mesitylanilide)) have been synthesized. The ligand geometry of these complexes is dictated solely by chelate ring strain rather than metal-ligand π-bonding. These complexes were tested as propylene polymerization precatalysts, with most complexes giving low to moderate activities (10^2-10^4 g/mol*h) for the formation of polypropylene.

(ONO)TiX2 complexes are highly active precatalysts for the intermolecular hydroamination of internal alkynes with primary arylamines and some alkylamines. (ONO)TiBn2 also cyclotrimerizes dimethylacetylene. During the cyclotrimerization reaction the Ti(IV) precatalyst is reduced to Ti(II), which is the active species for catalysis. The mechanism of formation of TiII has been investigated and an (ONO)Ti(II) species has been trapped by ethylene and crystallographically characterized.

Hydrazide complexes (dme)TaCl3(NNPh2) and (dme)NbCl3(NNPh2) (dme = 1,2 dimethoxyethane) were synthesized. Unlike the corresponding imido derivatives, (dme)TaCl3(NNPh2) is dark blue due to an LMCT that has been lowered in energy as a result of an Nα-Nβ antibonding interaction that raises the HOMO. Reaction of (dme)TaCl3(NNPh2) with a variety of neutral, mono and dianionic ligands generates the corresponding ligated complexes retaining the k-1 bound [TaNNPh2] moiety.

Furthermore, a series of colorful terminal hydrazide complexes of the type (dme)MCl3(NNR2) (M = Nb, Ta; R = alkyl or aryl) or (MeCN)WCl4(NNR2) have been synthesized. Perturbing the electronic environment of the β nitrogen significantly impacts the lowest-energy charge transition in these complexes, and in the W complexes leads to metal based reduction. The photophysics of these complexes highlights the importance of the difference in reduction potential between metal centers, and could lead to differences in ligand- and/or metal-based redox chemistry in early transition metal hydrazidos, especially in the context of N2 fixation.

Finally, the hydroxy-bridged dimer [(COD)IrOH]2 (COD = 1,5-cyclooctadiene) cleanly C-H activates indene and cyclopentadiene to form (COD)Ir(η3-indenyl) and (COD)Ir(η5-C5H5), respectively. The kinetics of the formation of (COD)Ir(η3-indenyl) has been investigated, and the mechanism involves coordination of indene to the dimeric [(COD)IrOH]2 followed by rate determining C-H activation from the dimer-indene unit.

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