| Abstract
| - The reactions ofNb(C5H3RR‘)2Cl2with Red−Al followed by hydrolysis yieldNb(C5H3RR‘)2H3 (R= R‘ = H, 1;R = H, R‘ = SiMe3, 2; R = R‘ =SiMe3, 3). These compounds react with Lewisacidic coinage cationic species,namely, [Cu(MeCN)4]PF6,AgBF4, and “Au(THT)PF6”, preparedin situ from AuCl(THT) and TlPF6 in a 2 to1ratio to yield the adducts[{Nb(C5H3RR‘)2H3}2M]+(M = Cu, R = R‘ = H, 7; R = H, R‘ =SiMe3, 8; R = R‘= SiMe3, 9; M = Ag, R = H, R‘ =SiMe3, 10; R = R‘ = SiMe3,11; M = Au, R = R‘ = H, 12; R = H, R‘=SiMe3, 13; R = R‘ = SiMe3,14). Like 1, but unlike the correspondingtantalum derivativesTa(C5H3RR‘)2H3(R= R‘ = H, 4; R = H, R‘ = SiMe3,5; R = R‘ = SiMe3, 6),2 and 3 show exchange couplings in theirhigh-field1H NMR spectra due to a hydride tunneling phenomenon.The magnitudes of exchange couplings are larger inthe cases of 2 and 3 than in the case of1 as a result of the decrease of electron density uponincreasing thenumber of SiMe3 substituents on the Cp ring. Theaddition of a Lewis acidic cation results in the observationofan AB2 pattern for the hydrides at room temperature, whichsplits at low temperature into an ABC one in agreementwith a fluxional behavior of the cation which binds to two hydrides ofeach niobium center. The activationenergy of these fluxional processes are close to 42−45kJ·mol-1 in the case of Cu adducts, 37kJ·mol-1 in thecase of Ag adducts, and 40 kJ·mol-1 in the case ofAu adducts. The magnitude of exchange couplings isreducedupon addition of copper cation to 1−3, is of the sameorder of magnitude after addition of a silver cation, andisgreatly increased by addition of a gold cation. A model isproposed to explain these variations which involvestwo isomeric states that are close in energy, one involving twobridging and one terminal hydrides on niobiumand one involving one bridging hydride and a dihydrogen molecule.A line shape analysis experiment carriedout on 14 allows determination of the parameters of theclassical exchange, the coupling constants at varioustemperatures which reach 550 Hz at 347 K, and the parameters of thequantum mechanical exchange accordingto our proposed model. The structure of 14 has beenstudied by X-ray diffraction. The structure has beensolvedfrom diffractometer data by Patterson method and refined by blockedfull-matrix least squares on the basis of3082 observed reflections to R and Rwvalues of 0.0346 and 0.0381, respectively. The structure shows thepresenceof two bridging hydrides between the niobium and gold atoms; one ofthem is found close to the terminal hydride.
- The niobocene trihydridesNb(C5H3RR‘)2H3(R = R‘ = H, 1; R = H, R‘ = SiMe3,2; R = R‘ = SiMe3, 3) showexchange couplings in their high-field 1H NMR spectra dueto a hydride tunneling phenomenon. These compounds react with aLewis acidic coinage cation to yield the adducts[{Nb(C5H3RR‘)2H3}2M]+.The X-ray diffraction structure of a gold adduct shows thepresence of two bridging hydrides between the niobium and gold atoms;one of them is found close to the terminalhydride.
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