Chemistry of [RuL2(PPh3)2]o, +(HL = 8-hydroxyquinoline): synthesis, structure, metal redox behaviour and isomer selectivity

Abstract

The reaction of [Ru(PPh₃)₃Cl₂] with 8-hydroxyquinoline (HL) has afforded [RuL₂(PPh₃)₂] in two isomeric forms, one red and one yellow-green, the RuN₂O₂P₂ co-ordination sphere being cis,trans,cis and trans,trans,trans respectively. Chemical or electrochemical oxidation of either isomer affords exclusively red-brown trans,trans,trans-[RuL₂(PPh₃)₂]⁺ isolated as the PF₆^– salt. The latter complex is one-electron paramagnetic (low-spin t₂⁵) and in frozen glass (77 K) displays a rhombic EPR spectrum. Reduction of trans,trans,trans-[RuL₂(PPh₃)₂]PF₆ by chemical or electrochemical means yields only trans,trans,trans-[RuL₂(PPh₃)₂]. X-Ray crystallography has been performed on cis,trans,cis-[RuL₂(PPh₃)₂] and trans,trans,trans-[RuL₂(PPh₃)₂]PF₆·CH₂Cl₂. The Ru–O and Ru–N bond lengths decrease upon metal oxidation but the Ru–P bond length increases by ca. 0.15 Å(strong 4dπ–3dπ Ru^II–P back-bonding). The reduction potential (Ru^III–Ru^II) order E_½(cis,trans,cis) > E_½(trans,trans,trans) corresponds to superior back-bonding in the cis- as compared to the trans-Ru(PPh₃)₂ motif. The stability of the observed geometry–valence combinations has been rationalised in terms of back-bonding, phosphine crowding and phenolato oxygen repulsion.