Hydroxypyridine/Pyridone Interconversions within Ruthenium Complexes for their Catalytic Application in the Hydrogenation of CO2

Reaction of a new ligand 6-DiPPon {where 6-DiPPon = 6-diisopropylphosphino-2-pyridone} with half equivalent of [RuCl2(p-cymene)]2 resulted in the formation of a mixture of [RuCl2(p-cymene)(κ1-P-(6-DiPPon)]2 (1) and [RuCl(p-cymene)(κ2-P,N-(6-DiPPin)]Cl ([2]Cl) {where 6-DiPPin = 6-diisopropylphosphino-2-hydroxypyridine}. The ratio between the two products can be controlled by the nature of the solvent. The similar reaction between 6-DiPPon and [RuCl2(p-cymene)]2 in the presence of AgOTf or Na[BArF24] {where BArF24 = [{3,5-(CF3)2C6H3}4B]−} resulted in the formation of the complexes, [RuCl(p-cymene)(κ2-P,N-(6-DiPPin)]OTf, ([2]OTf) and [RuCl(p-cymene)(κ2-P,N-(6-DiPPin)]BArF24, ([2]BArF24), respectively. Reactions between the complexes [2]Cl, [2]OTf or [2]BArF24 with a base (either DBU or NaOMe) resulted in the deprotonation of hydroxyl functional group to form a novel neutral orange coloured dearomatised complex, 3. The identity of complex 3 was confirmed as [RuCl(p-cymene)(κ2-P,N-6-DiPPon*)], where 6-DiPPon* is the anionic species, [6-diisopropylphosphino-2-oxo-pyridinide], which contains the deprotonated moiety. The new 6-DiPPon ligand and its corresponding air stable half-sandwich derivative ruthenium complexes, 1, [2]OTf, [2]BArF24 and 3 were all isolated in good yields and fully characterized by spectroscopic and analytical methods. The interconversions between the neutral and anionic forms of the ligands 6-DiPPon, 6-DiPPin and 6-DiPPon* offer the potential for novel secondary sphere interactions and proton shuttling reactivity. The consequences for this have been explored in the activation of H2 and the subsequent catalytic hydrogenations of CO2 into formate-salts in the presence of a base.