Synthesis of Electrophilic Rhodium and Iridium Complexes and Investigation of Reactivity for C-H Bond Activation and Functionalization

Abstract

Direct methods for the transformation of alkanes into alcohols, alkenes, amines and other functionalized products would have a great impact on industrial processes. In this thesis the work described is aimed at developing electrophilic Rh and Ir systems to promote C-H bond activation and functionalization of alkanes and arenes. Several approaches have been taken to prepare the target complex [Tp*Ir(OH₂)₃]<super>+</super> (Tp* = hydridotris(3,5-dimethylpyrazoyl)borate). While this species has not been synthesized, C-N coupling between ethylene and N-iodosuccinimide has been observed during the course of this study and is promoted by Tp*Ir(C₂H₄)₂. Electrophilic Rh and Ir complexes bearing dimethylbutadiene (DMB) ligands have been found to be poor species for C-H activation due to low stability at elevated temperatures. Interestingly, formation of the unique five-coordinate [(DMB)Ir(COE)Cl]₂ dimer has been observed. Alkane dehydrogenation using (<super>dm</super>Phebox)Ir(OAc)2(OH2) (<super>dm</super>Phebox = 2,6-bis(4,4-dimethyloxazolinyl)-3,5-dimethylphenyl) is promoted at 200 °C and results in quantitative formation of olefin and (<super>dm</super>Phebox)Ir(OAc)H. At early reaction times 1-octene is the major product, supporting terminal C-H activation by the Ir center. Oxygen can be utilized in this system as a hydrogen acceptor to promote regeneration of the Ir bisacetate species at room temperature. This is the first example of regeneration of a complex for dehydrogenation using oxygen. Unfortunately, the system is not catalytic as the reaction with oxygen is not compatible with the high temperatures required for C-H bond activation. In order to expand the number of examples of alkane dehydrogenation at Ir<super>III</super> centers, a second system utilizing the <super>tBu</super>NOCON (<super>tBu</super>NOCON = 4,6-di-tert-butyl-(1,3-bis(2-pyridyloxy)benzene)) pincer ligand has been developed. Complexes (<super>tBu</super>NOCON)M(OAc)₂OH₂ (M = Rh or Ir) were synthesized and explored for C-H bond functionalization. The Ir analogue promotes alkane dehydrogenation at 200 °C, but is not stable under the reaction conditions. Arene functionalization has also been explored using Ir-aryl complexes. The product of benzene C-H activation, (Phebox)Ir(OAc)Ph has been investigated for arene functionalization using hypervalent iodide reagents and CO. Reactions utilizing C₆F₅I(TFA)₂ are promising and may potentially yield functionalized arene. Coordination of CO to the Ir center has been observed and isomerization of the <italic>trans</italic> product, (<super>dm</super>Phebox)Ir(CO)(OAc)Ph, occurs at room temperature to yield the <italic>cis</italic> isomer (<super>dm</super>Phebox)Ir(OAc)(CO)Ph.