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The Presence and Absence of Agostic Electrostatic Si−H···M Interactions in [M{Me2Si(H)N-t-Bu}] (M = Li, Mg1/2) Species

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journal contribution
posted on 23.12.1997, 00:00 by Bernd Goldfuss, Paul von Ragué Schleyer, Sandra Handschuh, Frank Hampel, Walter Bauer
Agostic Si−H···Li contacts (i.e. significantly less than ∼3.0 Å) are not present in the solvent-free X-ray structure of [Li{Me2Si(H)N-t-Bu}]3; shorter (t-Bu)CH3···Li distances (∼2.8 Å) are observed instead. The situation in solution is different. While the 1H−6Li HOESY spectrum of [Li{Me2Si(H)N-t-Bu}] in toluene at −80 °C evidently detects the same trimeric species (lacking Si−H···Li interactions, but with short (t-Bu)CH3···Li contacts), two major species with strong Si−H···Li interactions also are present. In the Li−HN−SiH3 computational model system, Si−H···Li interactions are favored energetically and result in increased Si−H distances and decreased Si−H frequencies. Agostic Si−H···metal contacts in the solid state are found in the X-ray crystal structure of [Mg2{Me2Si(H)N-t-Bu}4], where Li“+” is replaced by the more highly charged Mg“2+”. The two short agostic Si−H···Mg interactions (2.2, 2.5 Å) which result also are shown by the two low ν(Si−H) frequencies (2040, 1880 cm-1) in the IR spectrum (Nujol mull) of [Mg2{Me2Si(H)N-t-Bu}4].