Synthesis and spectroscopic properties of dihydrogen isocyanide niobocene [Nb(η5-C5H4SiMe3)2(η2-H2)(CNR)]+ complexes. Experimental and theoretical study of the blocked rotation of a coordinated dihydrogen
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Synthesis of stable hydride isocyanide derivatives Nb(η5- C5H4SiMe3)2(H)(CNR) has been achieved through the formation of coordinatively unsaturated 16-electron species Nb(η5-C5H4SiMe3)2H by thermolytic loss of H2 followed by the coordination of an isocyanide ligand. Low-temperature protonation with a slight excess of CF3COOH leads to the η2-dihydrogen complexes [Nb(η5-C5H4SiMe3)2(η2-H2)(CNR)]+. NMR spectra of these H-H complexes and their monodeuterated H-D isotopomers present a single high-field resonance at room temperature. By lowering the temperature to 178 K, decoalescence of the signal was observed for the H-D complexes but not for the H-H ones. By combining DFT electronic structure calculations with a monodimensional rotational tunneling model, it has been shown that the absence of decoalescence of the H-H signal is due to the existence of a very large exchange coupling. Conversely, for the H-D isotopomer, the difference in zero point energy corresponding to two nonequivalent (H-D and D-H) positions leads to a slight asymmetry which dramatically reduces the exchange coupling, allowing decoalescence to be observed. Therefore, the H-D classical rotation and the quantum exchange processes will not be practically observed for this complex, whereas only the classical process for the H-H species is quenched out on the NMR time scale.
Antiñolo, A., Carrillo-Hermosilla, F., Fajardo, M., Garcia-Yuste, S., Otero, A., Camanyes, S., Maseras, F., Moreno, M., Lledós, A., & Lluch, J. M. (1997). Synthesis and spectroscopic properties of dihydrogen isocyanide niobocene [Nb(η5-C5H4SiMe3)2(η2-H2)(CNR)]+ complexes. Experimental and theoretical study of the blocked rotation of a coordinated dihydrogen. Journal of the American Chemical Society, 119(26), 6107-6114. https://doi.org/10.1021/ja9640354