284472-96-4Relevant articles and documents
Protonation of [tpmRu(PPh3)2H]BF4 [tpm = tris(pyrazolyl)methane]- formation of unusual hydrogen-bonded species
Chu, Hei Shing,Xu, Zhitao,Ng, Siu Man,Lau, Chak Po,Lin, Zhenyang
, p. 993 - 1000 (2007/10/03)
Protonation of [tpmRu(PPh3)2H](BF4) with excess HBF4Et2O in CD2Cl2 yielded, in a straightforward manner, the dicationic η2-dihydrogen complex [tpmRu(PPh3)2(H2)](BF4)2, which, as expected, is more acidic than its monocationic Tp [Tp = hydrotris(pyrazolyl)borate] analog [TpRU(PPh3)2-(H2)]BF4 (pK(a): 2.8 vs. 7.6). The complex [tpmRu(PPh3)2(H2)](BF4)2 is unstable towards H2 loss at ambient temperature. However, acidification of [tpmRu(PPh3)2H]BF4 with excess aqueous HBF4 or aqueous triflic acid in [D8]THF gave very interesting results, Variable-temperature 1H- and 31P-NMR studies revealed that the aqueous acid did not fully protonate the metal hydride to form the dihydrogen complex, but a hydrogen-bonded species was obtained. The feature of this species is that the strength of its Ru-H···H-(H2O)(m) interaction decreases with temperature; this phenomenon is unusual because other complexes containing dihydrogen bonds show enhanced M-H···H-X interaction as the temperature is lowered. Decrease of the dihydrogen-bond strength with temperature in the present case can be attributed to the decline of acidity that results from the formation of larger H+(H2O)(n) (n > m) clusters at lower temperatures; steric hindrance of these large clusters also contribute to the weakening of the dihydrogen bonding interactions. At higher temperatures, facile H/H exchange occurs in Ru-H···H-(H2O)(m) via the intermediacy of a 'hydrogen-bonded dihydrogen complex' Ru- (H2)···(H2O)(m). To investigate the effect of the H+(H2O)(m) cluster size on the strength of the dihydrogen bonding in [tpmRu(PPh3)2H]+, molecular orbital calculations at the B3LYP level have been performed on model systems, [tpmRu(PH3)2H]+ + H+(H2O) and [tpmRu(PH3)2H]+ + H+(H2O)2. The results provide further support to the notion that the formation of larger H+(H2O)(n) clusters weakens the Ru-H····H(H2O)(n) dihydrogen bonding interaction.
