Hydroxypyridine/Pyridone Interconversions within Ruthenium Complexes and Their Application in the Catalytic Hydrogenation of CO 2 .

Reaction of a new ligand 6-D i PPon (6-diisopropylphosphino-2-pyridone) with 0.5 equiv of [RuCl 2 ( p -cymene)] 2 resulted in the formation of a mixture of [RuCl 2 ( p -cymene)(κ 1 - P - 6-D i PPon )] 2 ( 1 ) and [RuCl( p -cymene)(κ 2 - P,N - 6-D i PPin )]Cl ([ 2 ] Cl ) (where 6-D i PPin = 6-diisopropylphosphino-2-hydroxypyridine). The ratio between the two products can be controlled by the nature of the solvent. The similar reaction between 6-D i PPon and [RuCl 2 ( p -cymene)] 2 in the presence of AgOTf and Na[BArF 24 ] (where BArF 24 = [{3,5-(CF 3 ) 2 C 6 H 3 } 4 B] - ) resulted in the formation of the complexes [RuCl( p -cymene)(κ 2 - P,N - 6-D i PPin )]OTf, ([ 2 ] OTf ) and [RuCl( p -cymene)(κ 2 - P,N - 6-D i PPin )]BArF 24 ([ 2 ] BArF 24 ), respectively. Reactions between complex [ 2 ] Cl , [ 2 ] OTf , or [ 2 ] BArF 24 and a base (either DBU or NaOMe) resulted in the deprotonation of the hydroxyl functional group to form a novel neutral orange-colored dearomatized complex, 3 . The identity of complex 3 was confirmed as [RuCl( p -cymene)(κ 2 - P,N - 6-D i PPon * )], where 6-D i PPon * is the anionic species (6-diisopropylphosphino-2-oxo-pyridinide), which contains the deprotonated moiety. The new 6-D i PPon ligand and its corresponding air stable half-sandwich derivative ruthenium complexes 1 , [ 2 ] OTf , [ 2 ] BArF 24 , 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-D i PPon , 6-D i PPin , and 6-D i PPon * offer the potential for novel secondary sphere interactions and proton shuttling reactivity. The consequences for this have been explored in the activation of H 2 and the subsequent catalytic hydrogenations of CO 2 into formate salts in the presence of a base.