Hydrogen activation by arene ruthenium complexes in aqueous solution. Part 2. Build-up of cationic tri- and tetra-nuclear ruthenium clusters with hydrido ligands

Abstract

The low-pressure hydrogenation of the hydrolysis mixture of [Ru₂(η⁶-C₆H₆)₂Cl₄] in water (1.5 atm, 20 °C) led, in the presence of NaClO₄, to the oxo-capped trinuclear cluster cation [Ru₃(η⁶-C₆H₆)₃(µ-Cl)(µ₃-O)(µ-H)₂]⁺1 which crystallized as the perchlorate salt. The chloro-capped trinuclear cluster cation [Ru₃(η⁶-C₆H₂Me-1,2,4,6)₃(µ₃-Cl)H₃]²⁺2, crystallized as the dichloride, was accessible from the durene derivative [Ru₂(η⁶-C₆H₂Me-1,2,4,6)₂Cl₄] by high-pressure hydrogenation (60 atm, 55 °C) in water. In hot aqueous solution, the chloro-capped cluster 2 underwent hydrolysis to give the oxo-capped cluster [Ru₃(η⁶-C₆H₂Me₄-1,2,4,6)₃(µ₃-O)H₃]⁺3. In the presence of NaBF₄, the low-pressure hydrogenation (1.5 atm, 20 °C) of the hydrolysis mixture of [Ru₂(η⁶-C₆H₆)₂Cl₄] led to the tetranuclear tetrahydrido cluster cation [Ru₄(η⁶-C₆H₆)₄H₄]²⁺4 which precipitated as the tetrafluoroborate salt. Under high-pressure conditions (60 atm, 55 °C) and in the absence of an additional salt, the hexahydrido cluster cation [Ru₄(η⁶-C₆H₆)₄H₆]²⁺5 was formed and obtained as the dichloride. By analogy, the p-cymene derivative [Ru₂(η⁶-C₆H₄MePrⁱ-p)₂Cl₄] gave [Ru₄(η⁶-C₆H₄MePrⁱ-p)₄H₆]²⁺6. In contact with air, the hexahydrido clusters 5 and 6 were transformed into the corresponding tetrahydrido clusters [Ru₄(η⁶-C₆H₆)₄H₄]²⁺4 and [Ru₄(η⁶-C₆H₄MePrⁱ-p)₄H₄]²⁺7. The hexahydrido cluster 5 is capable of hydrogenating fumarie acid to give succinic acid and 4; the latter adds molecular hydrogen to regenerate the hexahydrido species 5. The crystal and molecular structures of the cluster salts [Ru₃(η⁶-C₆H₆)₃(µ-Cl)(µ₃-O)(µ-H)₂]ClO₄(cation 1), [Ru₃(η⁶-C₆H₂Me₄-1,2,4,6)₃(µ₃-O)H₃]BF₄(cation 3), [Ru₄(η⁶-C₆H₆)₄H₄]Cl₂(cation 4) and [Ru₄(η⁶-C₆H₄MePrⁱ-p)₄H₆][ClO₄]₂(cation 6) have also been determined.