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

Abstract

The complexation of rhodium(II) tetraacetate, tetrakistrifluoroaceate and tetrakisoctanoate with a set of diamines (ethane-1,diamine, propane-1,3-diamine and nonane-1,9-diamine) and their N,N'-dimethyl and N,N,N',N'-tetramethyl derivatives in chloroform solution has been investigated by (1) H and (13) C NMR spectroscopy and density functional theory (DFT) modelling. A combination of two bifunctional reagents, diamines and rhodium(II) tetracarboxylates, yielded insoluble coordination polymers as main products of complexation and various adducts in the solution, being in equilibrium with insoluble material. All diamines initially formed the 2 : 1 (blue), (1 : 1)n oligomeric (red) and 1 : 2 (red) axial adducts in solution, depending on the reagents' molar ratio. Adducts of primary and secondary diamines decomposed in the presence of ligand excess, the former via unstable equatorial complexes. The complexation of secondary diamines slowed down the inversion at nitrogen atoms in NH(CH3 ) functional groups and resulted in the formation of nitrogenous stereogenic centres, detectable by NMR. Axial adducts of tertiary diamines appeared to be relatively stable. The presence of long aliphatic chains in molecules (adducts of nonane-1,9-diamines or rhodium(II) tetrakisoctanoate) increased adduct solubility. Hypothetical structures of the equatorial adduct of rhodium(II) tetraacetate with ethane-1,2-diamine and their NMR parameters were explored by means of DFT calculations.

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

“…4), the formation of hydrogen bonds was expected to cause significant low frequency shifts of the signal of relevant nitrogen atom. Observed 15 N chemical shifts in the solid state were in agreement with our previous calculations [16] concerning 1 : 2 equatorial adducts of diamines with Rh 2 (AcO) 4 .…”

“…The combination of a rhodium core with bifunctional flexible ligand‐like diamines yields a large number of possible structures. In our previous work, some hypothetical structures of the 1 : 1 and 1 : 2 equatorial adducts of Rh 2 (AcO) 4 with 1 have been analysed using the DFT approach. These structures were expected to display a wide 15 N chemical shift range, from −377 to −422 ppm, depending on the position of a nitrogen atom in a molecule and intramolecular hydrogen bonds pattern.…”

“…In contrast to the ligands previously explored, diamine molecules are flexible and display conformational variety. Depending on the combination of rhodium salt and diamines, we observed in CDCl 3 solution the formation of various adducts, axial and equatorial, exhibiting 2 : 1, 1 : 1 or 1 : 2 stoichiometries . Nevertheless, most of the products precipitated on the bottom of NMR tubes, as insoluble polymeric material.…”

“…Depending on the combination of rhodium salt and diamines, we observed in CDCl 3 solution the formation of various adducts, axial and equatorial, exhibiting 2 : 1, 1 : 1 or 1 : 2 stoichiometries. [16] Nevertheless, most of the products precipitated on the bottom of NMR tubes, as insoluble polymeric material. In the present work, we reported the results of investigation of these materials by elemental analysis, 13 C and 15 N CPMAS NMR spectroscopy and density functional theory (DFT) molecular modelling.…”

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

“…Previously, we applied NMR techniques to the study of complexes of three rhodium tetracarboxylates, Rh 2 (AcO) 4 , Rh 2 TFA 4 , and Rh 2 Mosh 4 (Figure 1) with ligands bearing nitrogen atoms, such as pyridine, methylpyridines, azoles, and amines 55–62 . Complexes of some tertiary amines revealed interesting features.…”