Subtle variation of stereo-electronic effects in rhodium(I) carbonyl Schiff base complexes and their iodomethane oxidative addition kinetics

Abstract

Rhodium(I) carbonyl complexes of the form [Rh^I(N,O-ScBa)(CO)(PR₃)] (R = Ph or Cy), with N,O-ScBaH a mono anionic bidentate Schiff base ligand, 2-(cyclopentyliminomethyl)-5-methylphenol (5-Me-Sal-CyPH), and bearing different phosphine ligands on Rh(I), are reported. N,O-ScBaH was also varied to be 2-(cyclohexyliminomethyl)phenol (Sal-CyH) and 2-(isopropyliminomethyl)-5-methylphenol (5-Me-Sal-IPropH) to investigate the effects of different substituent groups on the periphery of the Schiff bases on metal center reactivity. The structural characterization of two sample complexes is described and an extensive spectroscopic kinetic-mechanistic study utilizing UV/vis, infrared and ³¹P NMR spectroscopy of the iodomethane oxidative addition is presented. Iodomethane oxidative addition led exclusively to Rh^III-alkyl complexes as final products, while an associative activation is inferred from the large negative ΔS^≠ value. The relative reactivity of the [Rh^I(N,O-ScBa)(CO)(PR₃)] complexes when stepwise varying PR₃ (PPh₃, PPh₂Cy, PPhCy₂ and PCy₃) as tertiary phosphine ligands follow a surprising similar reactivity relationship as observed for a range of isostructural [Rh^I(X,O-Bid)(CO)(PR₃)] complexes, with X = O, N or S in corresponding oxinate, acetylacetonate and thioureate mono anionic X,O-Bid ligands. This suggests systematic and responsive dynamic behavior of the PR₃ ligands, independent of the bidentate chelate at the Rh(I) metal center. A variation of more than two orders-of-magnitude in reactivity was observed.

Graphical Abstract

Keywords:

• Structure/reactivity relationships
• Schiff base ligands
• rhodium phosphine
• kinetics

Acknowledgements

Financial assistance from the University of the Free State and the SA NRF is gratefully acknowledged. Opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the SA NRF. Drs. D.V. Kama and O.T. Alexander are acknowledged for the single crystal data collections and assistance with solving the structures.

Disclosure statement

The authors declare no conflict of interest.

Additional information

Funding

We also express our gratitude towards SASOL, Dr. Gerdus Kemp and PETLabs Pharmaceuticals, the South African National Research Foundation (SA-NRF/THRIP), in particular funding under the Swiss-South Africa joint research programme (SSAJRP) from the SA NRF (UID: 107802) and the Swiss National Science Foundation (Project IZLSZ2_170856) and and the University of the Free State for financial support of this project.