Use of 73Ge NMR Spectroscopy for the Study of Electronic Interactions

• Published in: Inorganic chemistry 2008-11, Vol.47 (22), p.10765-10770
• Main Authors: Yoder, Claude H, Agee, Tamara M, Schaeffer, Charles D, Carroll, Mary J, Fleisher, Adam J, DeToma, Alaina S
• Format: Article
• Language: English
• ISSN: 0020-1669; 1520-510X
• DOI: 10.1021/ic801341t

The lack of understanding of the structural and electronic factors that affect the often difficult to observe germanium resonance has been a major deterrent to studies of bonding interactions at germanium. We utilized the symmetrical system GeR4 to determine what structural factors inherent in the R group affect the shape and position of the 73Ge resonance. The 73Ge resonances of symmetrical tetrakis germanium compounds of the type GeR4 (R = alkyl, aryl), GeX4 (X = F, Cl, Br, I), Ge(OR)4 (R = alkyl, methoxyalkyl, dimethylaminoalkyl), Ge(NR2)4 (R = alkyl), and Ge(SR)4 (R = alkyl, dimethylaminoalkyl) were examined for evidence of intramolecular coordination. Although many of these compounds have sharp resonances due to idealized tetrahedral symmetry with relatively long relaxation times, others have broad or no observable resonances due to fast quadrupolar relaxation. We hypothesize that the perturbation of symmetry by even weak Lewis interactions or conformational changes causes broadening of the resonance before the interaction can become sufficiently strong to cause the significant low-frequency shift generally associated with hypercoordination in most nuclei. Intermolecular coordination to GeCl4 is believed to be responsible for the low-frequency shifts in 73Ge resonances and the associated changes in peak widths in mixtures with bases such as tributylphosphine oxide (TBPO) and triethylphosphine oxide (TEPO). Adduct formation with these bases is confirmed by broad 31P resonances that are resolved into five peaks at −40 °C. The exchange-broadened resonances due to the 1:1 and 1:2 TEPO adducts are also observed at −40 °C in the 73Ge spectrum. Thus, relatively strong bonding to the germanium in GeCl4 results in both low-frequency shifts and broadening of the resonance. The broad 73Ge resonances that occur in some compounds may be in part due to exchange as well as quadrupolar relaxation.