Synthesis, Structure, and C-C Cross-Coupling Activity of (Amine)bis(phenolato)iron(acac) Complexes

A series of (amine)bis(phenolato)iron(III) acac complexes has been prepared and characterized. Reaction of Fe(acac) 3 with the diprotonated linear tetradentate proligand N, N'-bis(4,6di-tert-butyl-2-methylphenol)-N, N'-dimethyl-1,2-diaminoethane, H-2[L1], and tripodal tetradentate ligand precursors dimethylaminoethylamino-N, N-bis(2-methylene-4,6-di-tert-butylphenol), H-2[L2], dimethylaminoethylamino-N, N-bis(2methylene-4-methyl-6-tert-butylphenol), H-2[L3], 2-methoxyethylamino- N, N-bis(2-methylene-4,6-di-tert-butylphenol), H-2[L4], 2-methoxyethylamino-N, N-bis(2-methylene-4-methyl-6-tert-butylphenol), H-2[L5], and 2-methoxyethylamino- N, N-bis(2-methylene-4,6-dimethylphenol), H-2[L6], produces the distorted octahedral Fe-III complexes [L1]Fe(acac) (1), [L2]Fe(acac) (2), [L3]Fe(acac) (3), [L4]Fe(acac) (4), [L5]Fe(acac) (5), and [L6]Fe(acac) (6). In all of these complexes, the phenolato oxygen atoms are cis-oriented. The paramagnetic Fe-III complexes 1-6 were also characterized by UV/Vis and IR spectroscopy, mass spectrometry, cyclic voltammetry, and magnetic measurements. Single crystal X-ray molecular structures have been determined for complexes 1, 2, 3, 5, and the proligand H-2[L6]. Preliminary investigations of complexes 1-6 for catalytic cross-coupling reactions of aryl Grignard reagents with cyclic and acyclic secondary alkyl halides and benzyl halides were performed. While the activity for cyclohexyl chlorides and bromides was high, cross-coupling of benzyl halides was moderate and 2-bromo- and 2-chlorobutane gave poor yields of cross-coupled product.