Pulsed fourier transform NMR of substituted aryltrimethyltin derivatives : III. Proton data at 100 MHz and the derived Hammett σ-constant of the trimethyltin group

Abstract

Proton NMR data at 100 MHz are reported for thirteen para- and meta-substituted phenyltrimethyltin compounds, XC 6H 4Sn(CH 3) 3, where X = para-N(CH 3) 2, para-OCH 3, para-OC 2H 5, para-CH 3, meta-CH 3, -H, para-F, meta-OCH 3, para-Cl, para-Br, meta-F, meta-Cl and para-Sn(CH 3) 3. Correlation coefficients with Hammett σ-constants of greater than 0.95 are obtained with the methyltin proton chemical shifts and coupling constants to carbon [ 1 J( 13C 1H)] and tin [ 2 J(SnC 1H)]. Solvent effects and other extraneous factors invalidate comparisons of ϱ values in terms of the relative attenuation of the transmission of substituent effects through homologous carbon, silicon, germanium and tin systems, but coupling constant data reflect a diminution of ca. one tenthfold per bond in the order ϱ[C(1)Sn] > ϱ [SnC] > ϱ [CH]. Satisfactory correlations ( r > 0.95) are obtained in this series of closely-related compounds among the conventionally recorded two-bond, 2 J(SnC 1H) and the constituent, one-bond 1 J (Sn 13C) and J( 13C 1H) coupling constants, but the correlation coefficient for the comparison between the two one-bond couplings, 1 J(Sn 13C) and 1 J( 13C 1H) is lower ( r = 0.872). Changes in the couplings at the methyltin carbon bond tin-119 atoms are interpreted in terms of isovalent hybridization; a model based upon effective nuclear charges is tested with respect to both NMR coupling constants and 119Sn Mössbauer Isomer shifts at tin and is invalidated. Proton and carbon-13 NMR, chemical shift and coupling constant data are used to derive a Hammett σ-constant for the para-trimethyltin group of −0.14, and the significance of this value is discussed.