Synthesis and structures of complexes with axially chiral isoquinolinyl-naphtholate ligands

Abstract

The synthesis of axially chiral ligands 1-(3′,6′-di-t-butyl-2′-hydroxy-1′-naphthyl)-3-R-isoquinoline (R = H, Prⁱ, Bu^t) (L^R-H) is described. Ligands with unsubstituted isoquinolinyl rings tend to give 1:2 metal complexes. The syntheses and crystal structures of Li₂(L^H)₂(THF)₂ (9), (L^H)₂Ti(OPrⁱ)₂ (12), Zn(L^H)₂ (13) and [Mg(L^H)₂]₂ (14) are reported. Complex formation is highly stereoselective; the ligands in 1:2 complexes have the same stereochemistry (i.e. R,R and S,S but not R,S), whereas in the binuclear magnesium compound 14 the bridging and non-bridging ligands L^H have opposite stereochemistry. The reaction of L^H-H with Pd(acac)₂ afforded the N,O chelate Pd(acac)(L^H) (10), whereas towards K₂PtCl₄ the same ligand acts as an N-donor only, to give trans-PtCl₂(L^H-H)₂ (11) in which the OH groups are hydrogen-bonded to one of the two chloride ligands. The more bulky ligand with a t-butyl substituent in the 3-position of the isoquinolinyl ring reacts with zinc and magnesium bis(amides) to give the mixed-ligand species (L^Bu)ZnN(SiMe₃)₂ and (L^Bu)MgN(SiMe₃)₂, respectively, which catalyse the ring-opening polymerisation of ε-caprolactone (CL) and rac-lactide (LA).