Hindrance, Donor Ability of Men∩NN Chelates and Overall Stability of Pentacoordinate [PtCl2(η2-CH2=CH2)(Men∩NN)] Complexes as Observed by η2-Olefin 1JPt,C Modulation: An NMR Study

A series of pentacoordinate d8 PtII complexes, of the type [PtCl2(η2‐CH2=CH2)(Men∩NN)], where Men∩NN = bis‐nitrogen ligand, with a variable number of Me groups (i.e., 2,2′‐bipyridyl; 1,10‐phenanthroline; 6‐methyl‐2,2′‐bipyridyl; 2,9‐dimethyl‐1,10‐phenanthroline; 2,9‐dimethyl‐1,10‐phenanthroline; N,N′‐trimethyl‐ethylenediamine; N,N,N′,N′‐tetramethyl‐ethylenediamine; N,N,N′,N′‐tetramethyl‐1,2‐diaminocyclohexane) was studied. The compounds are characterized by variable steric hindrance, due to the variable number (n) of Me substituents, on or ortho to the N‐donors of aliphatic diamines or aromatic diimines, respectively. This approach was developed to investigate the interaction of substituents with the metal coordination sphere. With this aim, we analyzed the NMR properties of the considered complexes, with respect to modulation of the metal electron density (195Pt NMR signal frequency) by alkyl groups close to the N‐donors. 1H and 13C NMR analysis of the η2‐olefin signals has revealed, for each kind of bis‐nitrogen ligand, a positive or negative chemical shift variation that is proportional to the number of Me groups geminal or vicinal to the N‐donors. Interestingly, the 1JPt,C values increase by approximately 45 Hz for each additional Me on the series of diamine or diimine bis‐nitrogen ligands. A rationale for the stability changes observed in such pentacoordinate complexes is suggested, based on the NMR spectroscopic data analysis. The effect of steric hindrance around bis‐nitrogen donors on the metal–olefin bond, according to the Dewar–Chatt–Ducanson model, is described. Low hindrance around bis‐nitrogen donors corresponds to a weaker metal–olefin interaction, whereas high steric hindrance around bis‐nitrogen donors corresponds to a stronger metal–olefin interaction.