Protonation of [tpmRu(PPh3)2H]BF4 [tpm = Tris(pyrazolyl)methane] – Formation of Unusual Hydrogen-Bonded Species

Abstract

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 (pKa: 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.

Brønsted–Lowry Acid Strength of Metal Hydride and Dihydrogen Complexes

“…Similarly the complex [Ru(H 2 )(PPh 3 ) 2 (tpm)] 2+ , where tpm is tris(pyrazolyl)methane, has a pK a DCM 2.8. 182 Several bipyridine complexes have been studied (entries 11,14,15,33,81,136,157) with pK a LAC ranging from -4 to 26. The bipyridine ligand contributes 8 pK a units to a metal hydride complex according to the LAC system.…”

“…While both the HBPz 3 – and tacn ligands provide three nitrogen donors fac on the metal, the use of the neutral tacn ligand results in a more acidic complex. Similarly, the complex [Ru­(H 2 )­(PPh 3 ) 2 (tpm)] 2+ , where tpm is tris­(pyrazolyl)­methane, has a p K a DCM 2.8 …”

Preparation, characterization, and catalytic properties of ruthenium nitrosyl complexes with polypyrazolylmethane ligands

“…Polypyrazolylmethanes, such as bis(3,5-dimethyl-1-pyrazolylmethane) (Bpm * ) and tris(1-pyrazolyl)methane (Tpm), are the neutral analogues of the Bp and Tp ligands [36,37], which are isosteric and isoelectronic with the anionic polypyrazolyborates. Ruthenium complexes with Bpm and Tpm and their derivatives have also been prepared [38][39][40][41][42][43]. However ruthenium nitrosyl complexes of polypyrazolylmethanes are scarcely studied.…”

Solution Chemistry of a Water-Soluble η2-H2Ruthenium Complex: Evidence for Coordinated H2Acting as a Hydrogen Bond Donor

“…16 In contrast to this suggestion, however, several computational papers have shown the plausibility of intermolecular H-bonding to H 2 . [17][18][19][20][21] In this paper, we report the synthesis of a new ruthenium η 2 -H 2 complex that is amenable to a detailed H-bonding study. Among the remarkable properties of this complex is the finding that in aqueous solution the η 2 -H 2 ligand is not substituted by water.…”