1H, 13C and 15N NMR studies on adducts of some amines with dirhodium complexes
Institute of Organic Chemistry, Polish Academy of Sciences, Warsaw, Poland
Adducts formation of rhodium(II) acetate dimer and rhodium(II) trifluoroacetate dimer with some amines (Scheme 1), in CDCl3 solution, has been investigated by use of 1H, 13C and 15N nuclear magnetic resonance spectroscopy.
Scheme 1
NMR titration experiment shows that typically two adducts, 1 : 1 and 1 : 2, are formed in solution, depending on amine to rhodium complex ratio. At low temperature, 233 K - 253 K, the NMR signals of two adducts and signals of free ligand have been observed separately. Stepwise adduct formation has been confirmed using VIS spectroscopy.
15N NMR data are the most interesting result of work. Observed 15N adduct formation shifts, Δδ = δadduct - δamine, cover the range from –9 to – 39 ppm. For example, the values of Δδ = -24.0 ppm (1 : 1 adduct) and Δδ = –11.1 ppm (1 : 2 adduct) have been measured for rhodium(II) acetate dimer–piperidine system. The values found for amines differ from the results obtained for pyridine derivatives, where much bigger 15N adduct formation shift, from -50 to -75 ppm, has been observed.
Deshielding Δδ values can be used to identify the complexation site in the ligand containing a few heteroatoms.
The work was supported by the State Committee for Scientific Research under Grant No. 4 T09A 060 25