Synthesis and X-ray structures of (biphenoxide)Ti species: four and five coordinate complexes that crystallize as mixtures of diastereomers

A set of novel Ti(IV) crystal structures are described, where two diastereomers prefer to crystallize together in a 1:1 ratio in the unit cell. The achiral ligand 4,4′,6,6′-tetra-tert-butyl-2,2′-biphenol (1) reacted with chiral Ti(IV) complexes of the type Ti(Oi-Pr)2(OR*)2 to give (biphenoxide)Ti(OR*)2 [OR*=mentholate (4a) or borneolate (4b)] as a mixture of diastereomers (4a and 4a′, 4b and 4b′), in which the bis(phenoxide) ligand has the R and S configuration. An attempt to separate diastereomers 4a and 4a′ by crystallization resulted in a 1:1 mixture of these compounds in the unit cell. Diastereomers of 4a and 4b crystallized in the presence of CH3CN to give five coordinate trigonal bipyramidal complexes (biphenoxide)Ti(OR*)2(NCCH3) (5a, 5a′, 5b, 5b′). Compounds 5a and 5a′, and 5b and 5b′ again crystallized as a 1:1 mixture of diastereomers.