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Protonation of [tpmRu(PPh 3 ) 2 H]BF 4 [tpm = Tris(pyrazolyl)methane] – Formation of Unusual Hydrogen‐Bonded Species
Author(s) -
Chu Hei Shing,
Xu Zhitao,
Ng Siu Man,
Lau Chak Po,
Lin Zhenyang
Publication year - 2000
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(200005)2000:5<993::aid-ejic993>3.0.co;2-3
Subject(s) - chemistry , dihydrogen complex , protonation , steric effects , hydride , triflic acid , aqueous solution , hydrogen bond , inorganic chemistry , hydrogen , medicinal chemistry , metal , molecule , crystallography , stereochemistry , catalysis , organic chemistry , ion
in CD 2 Cl 2 yielded, in a straightforward manner, the dicationic η 2 ‐dihydrogen complex [tpmRu(PPh 3 ) 2 (H 2 )](BF 4 ) 2 , which, as expected, is more acidic than its monocationic Tp [Tp = hydrotris(pyrazolyl)borate] analog [TpRu(PPh 3 ) 2 (H 2 )]BF 4 (p K a : 2.8 vs. 7.6). The complex [tpmRu(PPh 3 ) 2 (H 2 )](BF 4 ) 2 is unstable towards H 2 loss at ambient temperature. However, acidification of [tpmRu(PPh 3 ) 2 H]BF 4 with excess aqueous HBF 4 or aqueous triflic acid in [D 8 ]THF gave very interesting results. Variable‐temperature 1 H‐ and 31 P‐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–(H 2 O) 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 + (H 2 O) 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–(H 2 O) m via the intermediacy of a “hydrogen‐bonded dihydrogen complex” Ru–(H 2 ) ··· (H 2 O) m . To investigate the effect of the H + (H 2 O) m cluster size on the strength of the dihydrogen bonding in [tpmRu(PPh 3 ) 2 H] + , molecular orbital calculations at the B3LYP level have been performed on model systems, [tpmRu(PH 3 ) 2 H] + + H + (H 2 O) and [tpmRu(PH 3 ) 2 H] + + H + (H 2 O) 2 . The results provide further support to the notion that the formation of larger H + (H 2 O) n clusters weakens the Ru–H ···· H(H 2 O) n dihydrogen bonding interaction.