The syntheses and reactivities of niobium cyclopentadienyl arylimido complexes bearing the 2,6-di-iso-propylphenyl (Dipp) imido substituent

Abstract

The work in this thesis focusses on the syntheses and reactivities of niobium cyclopentadienyl arylimido complexes featuring the 2,6-di-iso-propylphenyl (Dipp) imido substituent. The steric and electronic effects of the DippN2– arylimido moiety are exploited in order to achieve new reactivities of half-sandwich and bis(cyclopentadienyl) niobium complexes. CHAPTER 1 outlines the structure, bonding and resulting typical reactivities of RN2– ligands of transition metal imido complexes, with particular focus on systems where the imido substituent R is an aryl group. The potential reactivities and applications of niobium cyclopentadienyl imido systems [Cp’2NbV(NR)]+ and Cp’NbIII(NR) towards small molecule activation are outlined based on those reported for analogous zirconocene systems Cp’2ZrIV(NR) and Cp’2ZrII, respectively.


The syntheses of the cyclopentadienyl complexes CpNbCl2(NDipp) (2; 39% yield) and Cp2NbCl(NDipp) (3; 71% yield) from NbCl3(NDipp)(DME) (1; DME = 1,2-dimethoxyethane) are described in CHAPTER 2. The subsequent reduction chemistry of the mono(cyclopentadienyl) complex 2 is explored in the absence of strong σ-donors and π-acceptors, with the envisaged coordination of dinitrogen. The synthesis of the corresponding mono(fluorenyl) derivative FluNbCl2(NDipp) (6) is discussed.


The work in CHAPTER 3 explores the removal of the chloride ligand from the niobocene imido complex 3, both via electrophilic abstraction and nucleophilic displacement, with the syntheses of the derivatives Cp2Nb(ClGaCl3)(NDipp) (9), Cp2NbMe(NDipp) (11) and Cp2Nb(NDipp)(PCO) (12) being described. The reaction between complex 3 and GaCl3 (3 equivalents) is demonstrated to form the unusual
aryl–gallium π-bonding species Cp2Nb(ClGaCl3)(N) (10).


The reactivity of the d0 niobocene arylimido complex 3 towards electrophilic addition is investigated in CHAPTER 4. Protonation of complex 3 exclusively occurs at the N-atom to afford the amido species [Cp2NbCl(NHDipp)]+ (13+), isolable as either its triflate salt (75% yield) or
tetrakisborate (68% yield) salts. The reaction of complex 3 with the larger trityl (Tr+) electrophile is found to result in electrophilic aromatic substitution at the para-position of the aryl imido substituent, with the formation of the niobocene imido complex Cp2NbCl(NDippTr) (17; DippTr = 2,6-iPr2C6H2-4-Tr; 29% yield).


The unusual oxidative coupling of the niobium(V) complex 3 is probed in CHAPTER 5, where the reaction with AgBArF4 (ArF = 3,5-bisphenyl) is demonstrated to afford the diphenoquinoidal diketimido salt [2(N2N)][BArF4]2 ([18][BArF4]2;
91% yield). The paramagnetic character of [18][BArF4]2 is investigated both computationally (DFT) and experimentally (Evans’ method). Additionally, the reduction of the diketimido complex [18][BArF4]2 to the neutral biphenyl diimido species 2(N2N) (20; ~30% yield) is outlined.


The overall conclusions of this thesis are presented in CHAPTER 6 along with areas for future study. Finally, the experimental details (CHAPTER 7), APPENDICES and REFERENCES are presented.