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 d(8) Pt-II complexes, of the type [PtCl2((2)-CH2=CH2)(MenNN)], where MenNN = 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 (Pt-195 NMR signal frequency) by alkyl groups close to the N-donors. H-1 and C-13 NMR analysis of the (2)-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 (1)J(Pt,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.