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Paper | Special issue | Vol 40, No. 2, 1995, pp.559-576
Published online, 1st January, 1970
DOI: 10.3987/COM-94-S38
The π-Donor Ability of Vinamidine Chelates (Co, Ni, Cu, Zn, Pd) Assessed by X-Ray, IR and NMR

Rudolf Knorr, Reinald Zölch, and Kurt Polborn

*Institute of Organic Chemistry, University of Munich, Karlstrasse 23, D-80333, München, Germany


Formyl groups were used as reporter substituents attached to positions with high Π-electron densities in the vinamidine moieties of a 2,3-dihydro-1,4-diazepinium cation (7a) and of five metal complexes (9a-e). X-Ray structure analyses of the latter indicated significant Π donation to the coplanar formyl groups but insignificant changes of the formyl geometry. Infrared wave numbers in solution revealed a weak Π donation from the vinamidinium system (7a) and a uniformly strong one from the metal bis(chelates) (9a-e), but infrared intensities were inconclusive. However, clear evidence emerged from the rates of aldehyde rotation at the vinamidine moieties, as measured for these diamagnetic or paramagnetic heterocycles by 1H nmr: The Π-donor chararacters were energetically quantified by comparisons of literature data with the rotational barriers 11.0 kcal/mol at vinamidinium (in 7a) and 14.7-15.8 kcal/mol at vinamidin(id)e (in 9a, b, d and e).