Complexation of rhodium(II) tetracarboxylates with aliphatic diamines in solution: 1H and 13C NMR and DFT investigations

Complexation in situ of 1-methylpiperidine, 1,2-dimethylpyrrolidin, and 1,2-dimethylpiperidine with rhodium(II) tetracarboxylates: Nuclear magnetic resonance spectroscopy, chiral recognition, and density functional theory studies

Complexation in situ of 1-methylpiperidine, racemic 1,2-dimethylpyrrolidin, and racemic 1,2-dimethylpiperidine with rhodium(II) tetracarboxylates in chloroform was studied by ¹ H and ¹³ C nuclear magnetic resonance (NMR) spectroscopy and density functional theory (DFT) methods. As substrates, three dirhodium(II) compounds were applied, tetraacetate, tetrakistrifluoroacetate, and a derivative of optically pure Mosher's acid. Due to conformational flexibility, free and complexed ligands can adopt potentially various conformations. The NMR titration experiments revealed the subsequent formation of 1:1 and 1:2 complexes, depending on the molar ratio of substrate to ligand. Conformations of free and complexed ligands were examined by the comparison of experimental and DFT gauge-independent atomic orbital (GIAO) calculated chemical shifts and by the analysis of the internal energy of the compounds. For some ligand and substrate combinations, a mixture of complexes differing in ligand conformations was formed. Complexes of Mosher's acid derivative of rhodium(II) with racemic 1,2-dimethylpyrrolidin and 1,2-dimethylpiperidine exhibited NMR chiral recognition phenomenon, manifested by splitting of signals in ¹³ C NMR and ¹ H,¹³ C HSQC spectra.

Polymeric adducts of rhodium(II) tetraacetate with aliphatic diamines: natural abundance 13C and 15N CPMAS NMR investigations

Complexation properties of dimeric rhodium(II) tetracarboxylates have been utilised in chemistry, spectroscopy and organic synthesis. Particularly, the combination of these rhodium salts with multifunctional ligands results in the formation of coordination polymers, and these are of interest because of their gas-occlusion properties. In the present work, the polymeric adducts of rhodium(II) tetraacetate with flexible ligands exhibiting conformational variety, ethane-1,2-diamine, propane-1,3-diamine and their N,N'-dimethyl- and N,N,N',N'-tetramethyl derivatives, have been investigated by means of elemental analysis, (13)C CPMAS NMR, (15)N CPMAS NMR and density functional theory modelling. Elemental analysis and NMR spectra indicated the axial coordination mode and regular structures of (1 : 1)n oligomeric chains in the case of adducts of ethane-1,2-diamine, N,N'-dimethylethane-1,2-diamine N,N,N',N'-tetramethylethane-1,2-diamine and N,N,N',N'-tetramethylpropane-1,3-diamine. Propane-1,3-diamine and N,N'-dimethylpropane-1,3-diamine tended to form heterogeneous materials, composed of oligomeric (1 : 1)n chains and the additive of dirhodium units containing equatorially bonded ligands. Experimental findings have been supported by density functional theory modelling of some hypothetical structures and gauge-invariant atomic orbital calculations of NMR chemical shifts.