Structural and dynamic NMR characterization of [Pd(bipy)(R-thiourea)2]2+ and [Pd(phen)(R-thiourea)2]2+ cations

The synthesis and characterization of 10 complexes, [Pd(bipy)(R-TU)2]Cl2 and [Pd(phen)(R-TU)2]Cl2 (bipy = 2,2′-bipyridyl; phen = 1,10-phenanthroline; R-TU = N-alkyl substituted thioureas), is presented. The conformational and dynamic behavior in solution, analyzed by several NMR techniques, is compared to that of the free thiourea ligands. Spectra at variable temperatures of the free thioureas are consistent with hampered rotation around the C–N bonds. Mono-alkyl derivatives, in methanol, show equilibria between syn and anti conformers, whereas di-alkyl thioureas show equilibria between the syn–anti and syn–syn conformers (syn and anti indicate the position of the alkyl chain with respect to the S atom over the two amino-branches). Syn protons of the alkyl-chains are converted into the corresponding anti protons by C–N rotation, which also exchanges external (close to the S atom) to internal N–H (on the opposite side with respect to the S atom) within the NMR timescale. This is also observed for the corresponding PdII complexes, which, according to the enhancement of the double C–N bond character, present slower syn/anti exchanges. Moreover, metal coordination selects anti conformers for the mono-alkyl thioureas, and syn–anti conformers for the di-alkyl thioureas.

A thorough multinuclear NMR analysis has shed light on a topic, so far, neglected: structural features and dynamic behavior of thiourea complexes in solution. Coordination to metal at rt freezes rotation about the C–N bond and selects, by a kind of molecular recognition, few among the possible thiourea conformers.Figure optionsDownload as PowerPoint slideHighlights
► Synthesis and structural characterization in solution of Pd thiourea complexes by NMR spectroscopy.
► Conformational analysis of free and metal coordinated thioureas.
► Comparative thermodynamic analysis of the rotational motion about C–N bonds of free and coordinated thioureas.
► Thioureas conformational selection by PdII coordination.