13C chemical shifts in the aryl complexes PdX(p-RC6H4)(TMEDA). Is the PdX(TMEDA) fragment a substantial π-donor?

A series of aryl complexes of the form PdX(p-RC6H4)(TMEDA)(1, R=OMe, Me, H, CO2Me, CN, NO2, TMEDA=tetramethylethylenediamine, X=Br, I) were prepared, together with the derivatives PdI(C6F5)(TMEDA) and PdI(2-Me-C6H4)(TMEDA). Their 13C NMR spectra were recorded and showed that (i) the ipso carbon, C-1, with δ=107.6–162.0, is always shifted to higher frequencies, relative to the model organic compound HC6H4R, (ii) C-1 is markedly affected by the nature of the p-R substituent, with electron-withdrawing groups producing high-frequency shifts, (iii) there is a linear correlation between δC-1 and the Hammett constant σp+ with a correlation coefficient R=0.998 and (iv) there is no compelling evidence for a large π−donor effect from palladium into the aryl ring. Screening constants for C-1 in a series of phenyllithium compounds, p-RC6H4Li, were calculated. There is an excellent correlation (R=0.972) of ΔδC-1 for 1 with ΔσC-1 for p-RC6H4Li.