In the presence of titanium tetraisopropoxide, tridentate salen ligands derived from cis-1-amino-2-indanol have been utilized in the asymmetric addition of trimethylsilylcyanide to benzaldehyde, which gave the cyanohydrins in up to 85% ee. We have examined the reaction of titanium tetraisopropoxide with these salen ligands by NMR spectrometry and X-ray crystallography. Reaction of ligands derived from salicylaldehydes with bulky substituents in the 3-position with titanium tetraisopropoxide gave the L*Ti(O-i-Pr)2 complexes, which are the proposed precatalysts in the asymmetric addition reaction. These ligands give good to very good enantioselectivity in the asymmetric trimethylsilylcyanation reaction. However, with groups smaller than tert-butyl in the 3-position, substantial amounts (up to 73%) of the catalytically inactive L2*Ti species are formed, resulting in large drops in the ee's of the cyanohydrins. The L2*Ti species were formed as mixtures of diastereomers, one of which has been characterized by X-ray crystallography. The crystal structure shows the titanium to be bonded to two ligands with a pseudo-octahedral coordination geometry. The ligands are bound in a meridional fashion and the complex is C 2 -symmetric. Use of 2 equiv of ligand relative to titanium tetraisopropoxide resulted in significant reductions in the ee of the cyanohydrin product, presumably due to formation of increased amounts of the inactive diastereomeric L2*Ti complexes.

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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)(PPh3)] 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.

A nonfunctionalized bis­(imidazole) ligand precursor has been directly metalated using IrCp*­(OAc)2, leading to a mixture of bis­(protic N-heterocyclic carbene) (bisPNHC) complexes (2a,b). Treatment of 2a,b with HCl gas in CH2Cl2 gave a bisPNHC complex (3a), which has been transformed into a hydride bisPNHC complex. Complex 3a underwent ligand and counterion exchange reactions to afford acetonitrile and ethylamine bisPNHC complexes (5 and 6). Furthermore, these bisPNHC complexes have been tested as catalysts in transfer hydrogenation reactions of ketones and unsaturated ketones.

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