Synthesis, structures, and aggregation properties of N-Acylamidines

Wigbers C, Prigge J, Mu Z, Fröhlich R, Chi LF, Würthwein E

Abstract

N-Acylamidines 1a-q, which are important ligands for metal ion complexation, were easily prepared either by acylation of amidines 2 or by treatment of N-acylimidates 3 with amines. Additionally, the reaction of amidinium salts with aroyl halides in the presence of sodium hydroxide gave the aryl derivatives 1r-1ad and three azobenzene derivatives 1ae-1ag. The respective substitution pattern of these novel derivatives was selected with respect to possible steric and electronic effects in coordination reactions as well as with regard to the aggregation properties. N-Acylamidines 1 may exist as three types of tautomers, which were examined in solution by NMR spectroscopic analysis and by quantum chemical DFT and SCS-MP2-calculations for single molecules in the gas phase. Internal rotations require activation energies between 3-17 (C-N) and 15-27 (C=N) kcal/mol. Due to their excellent ability to form intra- and intermolecular hydrogen bonds, these molecules show very variable aggregation behavior in the solid state (X-ray analysis) as well as on surfaces, depending on the substitution pattern. Thus, dimeric (1c, 1d, and 1u) and tetrameric (1f) aggregates with strong NH···OC hydrogen bonding, as well as polymeric chain structures (1x and 1z) were observed in the solid state. Furthermore, a dimer 1k involving only the amidine subunits, rather than the carbonyl function, was found. Monolayers of molecules 1u and 1ab physisorbed at the liquid/HOPG (high orientated pyrolytic graphite) interface in 1-phenyloctane solution were examined by Scanning Tunneling Microscopy (STM).