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)2C6H2CH=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(II) 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 ¹H, the 195Pt NMR and various 13C NMR chemical shifts, and the substituent parameters sF and sR of R' at the aniline site. In common with organic benzylidene anilines, the azomethine ¹H NMR chemical shift showed anomalous substituent behavior. The 195Pt NMR chemical shift of the platinum center can be used as a probe for the electronic properties of the delocalized p-system of the benzylidene aniline framework, to which it is connected. The dual substituent parameter treatment of the azomethine 13C 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. [ABSTRACT FROM AUTHOR]