C-13 chemical shifts in the aryl complexes PdX(p-RC6H4)(TMEDA). Is the PdX(TMEDA) fragment a substantial pi-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 pre pared, together with the derivatives PdI(C6F5)(TMEDA) and PdI(2-Me-C6H4)(TM EDA). Their C-13 NMR spectra were recorded and showed that (i) the ipso car bon, C-1, with delta = 107.6-162.0, is always shifted to higher frequencies , relative to the model organic compound HC6H4R, (ii) C-1 is markedly affec ted by the nature of the p-R substituent, with electron-withdrawing groups producing high-frequency shifts, (iii) there is a linear correlation betwee n delta C-1 and the Hammett constant sigma(p+) with a correlation coefficie nt R = 0.998 and (iv) there is no compelling evidence for a large pi - dono r effect from palladium into the aryl ring. Screening constants for C-1 in a series of phenyllithium compounds, p-RC6H4Li, were calculated. There is a n excellent correlation (R = 0.972) of Delta delta C-1 for 1 with Delta sig ma C-1 for p-RC6H4Li. Copyright (C) 1999 John Wiley & Sons, Ltd.