Unexpected rearrangement of palladacycles derived from 2-oxopropionaldehyde phenylhydrazones. Synthesis of palladium complexes containing a bianionic tridentate C, N, O ligand

The synthesis of bidentate monoanionic C-N derivatives of 2-oxopropionaldehyde phenylhydrazones, compounds 3, [Pd(2-{NH-N=CH-C(O)Me}-3-RC6H3)Cl(PPh3)] is described. The structure of 3b (R = Me) was determined by X-ray diffraction. The bond distances and angles are similar to those reported for related metallacycles. The distance N(2)... O(1) (2.583(7) Angstrom) shows the existence of a strong intramolecular hydrogen bond between the NH and COCH3 groups. The C(9)-O(1) (1.224(9) Angstrom), C(8)-C(9) (1.46(1) Angstrom), C(8)-N(1) (1.307(8) Angstrom), and N1-N2 (1.325(7) Angstrom) bond lengths indicate that 3b exists mainly in the keto-hydrazo form. The action of NaMeO on 3 in MeOH afforded deep violet compounds which did not contain chlorine atoms. The analytical data, the infrared spectra, and the NMR studies, including H-1-H-1 COSY and NOESY experiments, showed that deprotonation of the N-H bond took place to give 4, [Pd(2-{N=N-CH=C(O)Me}-3-RC6H3)(PPh3)] by means of an unexpected hydrazo-azo tautomerization. The structure of 4c (R = Et) was determined by X-ray diffraction. The C(10)-O(1) (1.32(1) Angstrom) C(10)-C(9) (1.37(1) Angstrom), C(9)-N(2) (1.33(1) Angstrom) and N(1)-N(2) (1.25(1) Angstrom) bond lengths indicate that 4c exists mainly in the azo-enol form. The action of HCl (0.5 N) on the tridentate metallacycles 4 regenerated the bidentate derivatives 3, showing the reversibility of this process.