Organometallic benzylidene anilines: donor-acceptor features in NCN-pincer Pt(II) complexes with a 4-(E)-[(4-R-phenyl)imino]methyl substituent

A series of organometallic 4,4'-substituted benzylidene aniline complexes 4-ClPt-3,5-(CH2NMe2)(2)C6H2CH=NC6H4R'-4', abbreviated as PtCl[NCN(CH=NC6H4R'-4']-4], with R' = NMe2, Me, H, Cl, CN (1-5, respectively), was synthesized via a Schiff-base condensation reaction involving reaction of PtCl[NCN(CH=O)-4] (7) with the appropriate 4-R'-substituted aniline derivative (6a-e) in toluene. The resulting arylplatinum(n) products were obtained in 75-88% yield. Notably, product 2 was also obtained in 68% yield from a reaction in the solid state by grinding solid 7 with aniline 6b. The structures of 2, 4, and 5 in the solid state (single crystal X-ray diffraction) showed a non-planar geometry, in particular for compound 5. The electronic interaction between the donor benzylidene fragment PtCl(NCN-CH) and the para-R' aniline substituent through the azomethine bridge was studied with NMR and UV/Vis spectroscopy. Linear correlations were found between the azomethine H-1, the Pt-195 NMR and various C-13 NMR chemical shifts, and the substituent parameters sigma(F) and sigma(R) of R' at the aniline site. In common with organic benzylidene anilines, the azomethine H-1 NMR chemical shift showed anomalous substituent behavior. The Pt-195 NMR chemical shift of the platinum center can be used as a probe for the electronic properties of the delocalized pi-system of the benzylidene aniline framework, to which it is connected. The dual substituent parameter treatment of the azomethine C-13 NMR shift gave important insight into the unique behaviour of the Pt-pincer group as a substituent. Inductively, it is a very strong electron-withdrawing group, whereas mesomerically it behaves like a very strong electron donating group.