Engineering a homogeneous alkane conversion catalyst: A porphyrin based approach to alkane activation and functionalization

Abstract

The design of a metal mediated catalytic system capable of carrying out both alkane C-H bond activation and functionalization depends critically upon engineering the appropriate ligand environment. The rational for the synthesis of 2,3,7,8,12,13,17,18-octafluoro-5,10,15,20-tetrakis(pentafluorophenyl)porphinato rhodium [(F₂₈TPP)Rh, Chapter 1] relies upon differential effects on the homolytic and heterolytic bond cleavage processes central to alkane conversion. Here we report that increasing the effective electronegativity of the rhodium metal by placing it at the center of a perfluorinated porphyrin ligand has little effect on the metals ability to activate carbon-hydrogen (C-H) and hydrogen-hydrogen (H-H) bonds.

We hypothesized that increasing the metal's effective electronegativity would stabilize the Rh(I) oxidation state and allow the nucleophilic removal of the methyl group bound to the rhodium center following the C-H activation reactions. To verify this hypothesis (Chapter 2), we subjected the (F ₂₈TPP)RhCH₃ complex to a variety of nucleophiles, and found that triphenylphosphine (PPh₃) reacts with (F₂₈TPP)RhCH ₃ to cleanly produce the methyl triphenylphosphonium salt of the Rh(I) porphyrin [(F₂₈TPP)Rh]⁻ [CH₃PPh ₃]⁺ as the sole isolatable product.

The electrochemical behavior of Rh(F₂₈TPP) was explored to provide a thermodynamically more efficient pathway for the regeneration of the Rh(II) catalyst (Chapter 3). We show here the first unambiguous reversible one electron redox couple for four coordinate Rh(II) porphyrins. These studies have also shown that the disproportionation of Rh(II) porphyrins is facilitated by coordination and ion pairing events, and that the ability to maintain a stable Rh(II) complex relies on a careful choice of reaction environment.

In an effort to accelerate C-H activation, we prepared a series of heavily fluorinated diporphyrin dimers (Chapter 4). By tethering the two rhodium centers necessary for C-H activation, we hoped to improve upon the reaction rates observed for the monomeric, termolecular activation step. These new fluorinated rhodium(II) porphyrin dimers exhibit 200 fold rate enhancements for the activation of methane C-H bonds when compared to (F₂₈TPP)Rh, while maintaining the electronic properties necessary for alkane conversion.