Protonation of [tpmRu(PPh3)(2)H]BF4 [tpm = tris(pyrazolyl)methane] - Formation of unusual hydrogen-bonded species

Protonation of [tpmRu(PPh3)(2)H](BF4) with excess HBF4Et2O in CD2Cl2 yielde d, in a straightforward manner, the dicationic eta(2)-dihydrogen complex [t pmRu(PPh3)(2)(H-2)](BF4)(2). which, as expected, is more acidic than its mo nocationic Tp [Tp = hydrotris(pyrazolyl)borate] analog [TpRu(PPh3)(2)(H-2)] BF4 (pK(a): 2.8 vs. 7.6). The complex [tpmRu(PPh3)(2)(H-2)](BF4)(2) is unst able towards H-2 loss at ambient temperature. However, acidification of [tp mRu(PPh3)(2)H]BF4 with excess aqueous HBF4 or aqueous triflic acid in [D-8] THF gave very interesting results. Variable-temperature H-1- and P-31-NMR s tudies revealed that the aqueous acid did not fully protonate the metal hyd ride to form the dihydrogen complex, but a hydrogen-bonded species was obta ined. The feature of this species is that the strength of its Ru-H ... H-(H 2O)(m) interaction decreases with temperature; this phenomenon is unusual b ecause other complexes containing dihydrogen bonds show enhanced M-H ... H- X interaction as the temperature is lowered. Decrease of the dihydrogen-bon d strength with temperature in the present case can be attributed to the de cline 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. A t higher temperatures, facile H/H exchange occurs in Ru-H ... H-(H2O)(m) vi a the intermediacy of a "hydrogen-bonded dihydrogen complex" Ru-(H-2)...(H2 O)(m). To investigate the effect of the H+(H2O), cluster size on the streng th of the dihydrogen bonding in [tpmRu(PPh3)(2)H](+), molecular orbital cal culations at the B3LYP level have been performed on model systems, [tpmRu(P H3)(2)H](+) + H+(H2O) and [tpmRu(PH3)(2)H](+) + H+(H2O)(2). The results pro vide further support to the notion that the formation of larger H+(H2O)(n) clusters weakens the Ru-H ... H(H2O)(n) dihydrogen bonding interaction.