1H,13C, and73Ge NMR spectral analysis of substituted aryltrimethylgermanes

Abstract

NMR chemical shifts of 1H, 13C, and 73Ge are reported for a series of monosubstituted aromatic trimethylgermanes of the type XC6H4Ge(CH3)3; X = p-N(CH3)2, p-OCH3, p-OC2H5, p-C(CH3)3, p-Si(CH3)3, p-Ge(CH3)3, p-Sn(CH3)3, p-CH3, m-CH3, -H, m-OCH3, p-Cl, p-Br, m-F, m-CF3, p-CF3, o-OCH3, and o-CH3. The relatively narrow 73Ge resonances show a strong correlation with Hammett sigma constants, with a correlation coefficient of 0.976 and 0.876 for 73Ge chemical shifts in meta- and para-substituted derivatives, respectively. The 13C chemical shifts of the methyl carbons bonded to germanium also display a relationship, with correlation coefficients of 0.904, 0.993, and 0.911 for para-, meta- and all derivatives, respectively. Comparisons of the Hammett plots for the homologous series XC6H4M(CH3)3; M = C, Si, Ge, Sn, show that, in general, correlation coefficients decrease while slopes increase significantly down the group, presumably reflecting the corresponding increase in chemical shift range of the group 14 atom. The Hammett constant derived for the p-Ge(CH3)3 group of +0.13 compares with the NMR-derived constants of -0.12 for p-C(CH3)3, +0.14 for p-Si(CH3)3, and -0.14 for p-Sn(CH3)3. The indication of electron release by carbon and tin can be rationalized through traditional hyperconjugative arguments for carbon and by the low electronegativity and consequent inductive effect of tin. The small electron attraction suggested by the positive constants for silicon and germanium can be simply, and perhaps naively, attributed to pi-acceptor interactions with the benzene ring.

Use of73Ge NMR Spectroscopy and X-ray Crystallography for the Study of Electronic Interactions in Substituted Tetrakis(phenyl)-, -(phenoxy)-, and -(thiophenoxy)germanes

“…Table displays the results of regression analyses for the 13 C and 73 Ge chemical shifts of para -substituted phenyl derivatives versus Hammett σ and Taft σ R ° constants . In general, the correlation coefficients for the regression of the carbon shifts versus normal Hammett constants were similar to those reported previously for the substituted phenyltrimethylgermanes but significantly lower for the germanium shifts . Use of σ R ° gave a good correlation for the C1 chemical shifts of the seven tetrakis( para -phenyl) derivatives (Figure ).…”

“…As part of our continuing interest in electronic effects in group 14 compounds, we have also explored the effects of substituents on the NMR chemical shifts of the tetrakis(phenyl) compounds. The 13 C chemical shifts of the ipso carbon (C1) for the substituted tetrakis(phenyl)germanes are 4−6 ppm lower than the shifts for the ipso carbons of the analogous substituted phenyltrimethylgermanes . For example, the 13 C chemical shift of the ipso carbon of (C 6 H 5 ) 4 Ge is 136.1 ppm, whereas the analogous shift for C 6 H 5 Ge(CH 3 ) 3 is 142.5 ppm.…”

“…Carbon-13 chemical shifts for all derivatives are reported in Table , where the ipso ring carbon is labeled C1. Assignments of 13 C resonances were based on (1) additivity relationships; (2) off-resonance proton-coupled spectra; (3) comparison of 13 C chemical shifts with those reported in previous studies of substituted tert -butylbenzenes, phenyltrimethylsilanes, phenyltrimethylgermanes, , and phenyltrimethylstannanes; , and (4) established trends in magnitude of J ( 19 F− 13 C) coupling constants in some of these systems. Literature values of tetraphenylgermane − and tetrakis( p -tolyl)germane − are in excellent agreement with those determined here, with the exception of the 141.5 ppm carbon-13 chemical shift reported for tetraphenylgermane by Batchelor et al for C1 (both Vaickus et al and Dräger et al report 136.2 ppm).…”

“…1 H and 13 C NMR spectra were obtained on Varian UNITY 300 MHz and INOVA 500 MHz spectrometers using conditions described previously. , Samples were contained in 5 mm o.d. tubes.…”

What is special about silicon in functionalised organic semiconductors?

“…Previous studies have highlighted the ability for both silicon and germanium to accept electrons by resonance due to the presence of empty d-orbitals. 15,16 This effect is expected to be weaker for germanium due to an elongated C-Ge bond and a mismatch in size and energy of the available d-orbitals with the Scheme 1 Synthesis of TEC-FADT. (a) 3.5 eq.…”