Tetradentate bis(hydroxamate) and hydroxamate-diketonate ligands and their titanium(IV) complexes

A 2,2'-bis(methylene)biphenyl-bridged bis(hydroxamic acid) (HoxH(2)) is prepared by reaction of 2,2'-biphenyldiacetyl chloride with 2 equiv of N-methylhydroxylamine. Use of 1 equiv of CH(3)NHOH gives the cyclic diacylhydroxylamine, which is selectively ring-opened to give a mixed monohydroxamate-monodiketonate ligand HobH(2). Both ligands are metalated by Ti(O(i)Pr)(4) to give the corresponding LTi(O(i)Pr)(2) complexes as exclusively the cis-alpha, (R)-Lambda/(S)-Delta isomers, similar to the previously prepared bis(diketonate) analogues (Bob)TiX(2). The carbonyl oxygens of the hydroxamates in the Hox ligand are constrained to be cis to each other, and the crystal structure of (Hob)Ti(O(i)Pr)(2) suggests that the carbonyl oxygen is a slightly weaker donor than the diketonate oxygen, based on a modest difference in their trans influences. A differential trans effect is also manifest in the observation of only a single geometric isomer of (Hob)Ti(O(i)Pr)(O(3)SCF(3)) and in a 15.6:1 preference for the isomer of (Hob)Ti(OCH(2)CMe(2)CO(2)) in which the alkoxide is trans to the hydroxamate ligand.