Tetradentate Bis(hydroxamate) and Hydroxamate-Diketonate Ligands and Their Titanium(IV) Complexes

A 2,2′-bis(methylene)biphenyl-bridged bis(hydroxamic acid) (HoxH2) is prepared by reaction of 2,2′-biphenyldiacetyl chloride with 2 equiv of N-methylhydroxylamine. Use of 1 equiv of CH3NHOH gives the cyclic diacylhydroxylamine, which is selectively ring-opened to give a mixed monohydroxamate-monodiketonate ligand HobH2. Both ligands are metalated by Ti(OiPr)4 to give the corresponding LTi(OiPr)2 complexes as exclusively the cis-α, (R)-Λ/(S)-Δ isomers, similar to the previously prepared bis(diketonate) analogues (Bob)TiX2. 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(OiPr)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(OiPr)(O3SCF3) and in a 15.6:1 preference for the isomer of (Hob)Ti(OCH2CMe2CO2) in which the alkoxide is trans to the hydroxamate ligand.

通过2,2'-联苯二乙酰氯与2等份N-甲基羟胺的反应，制备得到2,2'-联苯-双(亚甲基)桥连的双(羟肟酸)（HoxH2）。使用1等份的CH3NHOH可得到环状二酰基羟肟胺，该化合物经过选择性开环反应，生成混合的单羟肟酸单二酮配体HobH2。这两种配体均通过Ti(OiPr)4进行金属化，得到相应的LTi(OiPr)2配合物，且仅以cis-α, (R)-Λ/(S)-Δ异构体的形式存在，类似于先前制备的双(二酮)类似物(Bob)TiX2。在Hox配体中，羟肟酸的羰基氧原子被限制为相互处于顺式，而(Hob)Ti(OiPr)2的晶体结构表明，基于它们在反式效应上的适度差异，羰基氧原子相较于二酮氧原子是一个稍弱的供体。在(Hob)Ti(OiPr)(O3SCF3)的单一几何异构体观察以及(Hob)Ti(OCH2CMe2CO2)异构体中以15.6:1的比例偏好中，也显现出差异性的反式效应，其中醇氧原子与羟肟酸配体处于反式。