Palladium(II) Complexes of Oxybenziporphyrin

Stępień, M.; Latos-Grażyński, L.; Lash, T. D.; Szterenberg, L.
Inorg. Chem. 2001, 40, 6892-6900 | view at publisher's site

8,19-Dimethyl-9,13,14,18-tetraethyloxybenziporphyrin coordinates palladium(II) to form the four-coordinate anionic complex [(OBP)PdII]-. The NMR data provide evidence for the retention of macrocyclic aromaticity and coordination via a carbon σ-donor. Protonation of the external oxygen atom to give [(HOBP)PdII] switches the molecule to a less aromatic phenol-like state, which is manifested by a significant reduction of the macrocyclic ring current. [(AcOBP)PdII] and [(TsOBP)PdII], two ester derivatives of [(OBP)PdII]-, are similar to the protonated species, and their benzenoid character is more pronounced. However, reaction of [(OBP)PdII]- with methyl iodide leads to selective methylation of the coordinating C(22) atom to form a novel organopalladium complex (OBPMe)PdII. The strong shielding of the inner Me(22) (δ(1H) −2.00 ppm in CDCl3) indicates that the aromaticity of the macrocycle has been retained. At the same time the 13C chemical shift of C(22) (44 ppm) shows that the palladium-bound carbon has undergone a drastic hybridization change. Alkylation with n-BuI yields a mixture of the O-substituted [(n-BuOBP)PdII] and the C-substituted [(OBP-n-Bu)PdII], which confirms the ambident nucleophilicity of [(OBP)PdII]-. DFT calculations carried out for six tautomers of oxybenziporphyrin and the 22-methylated palladium species provide further insight into the electronic structure of the ligand and its complexes. Relative energies of the tautomers, increasing in the order [CH,NH,N,NH,O] < [CH,N,NH,N,OH] < [CH2,N,NH,N,O] < [CH2,N,N,N,OH], have been used to estimate the accessibility of four limiting delocalization modes postulated for oxybenziporphyrin and its derivatives. The state of macrocyclic aromaticity observed experimentally in the free base and the phenolic aromaticity of the O-protonated tautomer are the most favorable, and the latter has its energy higher by only 13 kcal/mol. The peculiar bonding situation in (OBPMe)PdII, which can be inferred from the NMR data, is also predicted by the DFT methods, which show a strongly distorted tetrahedral environment of C(22).