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

Abstract

A series of organometallic 4,4′-substituted benzylidene aniline complexes 4-ClPt-3,5-(CH₂NMe₂)₂C₆H₂CHNC₆H₄R′-4′, abbreviated as PtCl[NCN(CHNC₆H₄R′-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(CHO)-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 σ_F and σ_R 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 π-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.