Effects of the heterocycle and its substituents on structure and fluxionality in rhodium(I) and iridium(I) complexes with the hindered thiolates 6-tert-butylpyridine-2-thiolate and 1-alkyl-4-tert-butylimidazole-2-thiolate (alkyl = methyl and tert-butyl)

Article abstract of DOI:10.1021/om060317t

Three new sterically hindered heterocyclic thiolate ligands are studied (HetS = 6-tert-butylpyridine-2-thiolate, ^tBu-PyS; 1-methyl-4-tert-butylimidazole-2-thiolate, Me-^tBu-ImS; 1,4-di-tert-butylimidazole-2-thiolate, ^tBu₂-ImS). Related Rh(I) and Ir(I) metal complexes with molecular formulas [M(HetS)(COD)] _n, [M(HetS)(CO)₂]_n, and [M(HetS)(CO)(PPh ₃)]_n (where n = 1 or 2) were made to assess the steric and electronic effects of heterocycle (pyridine vs imidazole) and bulky substituents on the ring. A combination of solution-phase molecular weight determination, infrared spectroscopy, variable-temperature NMR, and solid-state X-ray diffraction studies were used to determine the molecularity of the complexes (value of n) and the coordination modes of the ligands. In the pyridine series, no evidence was found for nitrogen coordination; the presence of the tert-butyl group makes the heterocyclic thiolate behave like a nonheterocyclic derivative. In the imidazole series, three coordination modes were found, all of them including complexation through both the thiolate sulfur and the basic ring nitrogen. Evidence for fluxional and dimer-monomer interconversions was found for several of the imidazole derivatives, and the size of the 1-alkyl group played a significant role in determining the structures of the complexes.