An NMR study of mixed, tartrate-containing TiIV complexes

Abstract

The reactions of amines and amino alcohols with diisopropyl or diethyl R,R- or S,S-tartrate and Ti(OiPr)4 were examined by 1H and 13C NMR to obtain and characterize nonfluxional complexes with the tartrate units in novel binding modes. The mildly acidic 8-hydroxyquinoline and N-phenyl-N-benzoylhydroxylamine selectively formed the products of a double OiPr substitution, Ti2(tartrate)2(ligand)2(OiPr)2, and the products of double tartrate substitution, Ti(ligand)2(OiPr)2, while 2,4-pentanedione formed only the latter Basic amino alkanols formed diastereomerically pure Ti2(tartrate)2(aminoalkoxide)(OiPr)3 species. N,N-Dimethyl-2-aminoethanol (Hdmae) also and uniquely formed monomeric Ti(tartrate)2(Hdmae)2 species that could be described as doubly zwitterionic. Secondary or tertiary amines formed triply C2-symmetric Ti3(tartrate)4(amine)2(OiPr)4 assemblies. Some minor components were believed to be μ-OiPr species. All mixed complexes except Ti(tartrate)2(Hdmae)2 contained chelating and bridging tartrate units, without coordination by ester carbonyls. A nonchelating, nonbridging tartrate unit was also present in the amino alcohol cases. Primary amines, aromatic amines, and hydrazines all failed to provide identifiable complexes. As well, N,N-dibenzylhydroxylamine failed to generate in solution the complex that had previously been characterized by X-ray crystallography. Amidst the rich chemistry of TiIV-tartrate systems, the evident selectivities in product formation were ascribed to macro-ring closures that are specifically directed by the electronic nature of the addend. Transient OiPr-bridged intermediates were also implicated. Keywords: mixed TiIV alkoxides, chiral TiIV alkoxides, enantiospecific complexation.