Title: Metal−Hydride and Hydride−Hydride Distances from Measurements of Selective and Nonselective <sup>1</sup>H <i>T</i><sub>1</sub> Relaxation Times of Hydride Ligands: Relaxation in NbCp<sub>2</sub>H<sub>3</sub>
Abstract: The detailed 1H NMR relaxation study, performed for Cp2NbHX2HA (1) and its partially-deuterated derivatives in toluene-d8, has demonstrated how T1 and T1sel measurements can be used for accurate determination of the hydride−hydride and metal−hydride distances in Nb complexes. The T1sel method was independently supported by the relaxation studies of the partially deuterated derivatives of 1. The application of the T1sel approach is limited by the presence in the HX/HA pair of the slow hydride/hydride site exchange characterized by saturation transfer experiments: k296 = 1.34 s-1, ΔG⧧(298 K) = 17.3 kcal/mol, ΔH⧧ = 13.1 kcal/mol, and ΔS⧧ = −14.2 eu. The exchange does not show a pronounced isotopic effect, and k(H)/k(D) = 1.1 at 295 K. It has been suggested that the T1sel approach can be used for indication of slow hydride/hydride exchanges. The experimental estimations have shown Nb−H dipole−dipole interactions to be very important for the hydride relaxation in 1. The Nb−H contribution shortens significantly the hydride T1 times, providing, for example, up to 64% of the HX relaxation rate. The relaxation HX−HA and Nb−H distances have been discussed.
Publication Year: 1998
Publication Date: 1998-01-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 18
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