Synthesis, Structures and Ethylene Polymerisation Behaviour of Low Valent β‐Diketiminato Chromium Complexes

The preparation, reaction chemistry and ethylene polymerisation behaviour of low valent β‐diketiminato chromium complexes are described. [(DDP)CrCl(µ‐Cl)] 2 ( 1 ) [DDPH = 2‐{(2,6diisopropylphenyl)amino}‐4‐{(2,6‐diisopropylphenyl)imino}pent‐2‐ene] is formed by treatment of CrCl 3 (THF) 3 with LiDDP. Alkylation of 1 with AlMe 3 results in the formation of the binuclear dimethyl complex [(DDP)CrMe(µ‐Cl)] 2 ( 2 ). In contrast, the attempted alkylation of 1 with benzylmagnesium chloride results in reduction to form the dichromium(II) complex [(DDP)Cr(µ‐Cl)] 2 ( 3 ). Depending on the conditions of crystallisation, 3 can be obtained as the THF adduct [ 3(THF) 2 ·THF ] or co‐crystallised with a molecule of dibenzyl [ 3·Bz‐Bz ]. Cleavage of the dimeric unit in 1 can be achieved by the addition of carboxylates or β‐diketonates to give [(DDP)CrCl(O 2 CR)(THF)] (R = Me 4a , Ph 4b ) and [(DDP)CrCl({O(R)C} 2 CH)] (R = Me 5a , Ph 5b ), respectively. Single crystal X‐ray diffraction studies have been performed on 1 , 2 , 3(THF) 2 ·THF , 3·Bz‐Bz , 4a , and 5b . Complexes 1 , 2 and 3(THF) 2 ·THF are dimeric and have molecular C 2 h symmetry. Complex 3·Bz Bz is also dimeric but has its potential C 2 h symmetry removed by a significant tetrahedral distortion of the chromium coordination geometry. Compound 4a has an octahedral chromium centre coordinated to a single bidentate diketiminate ligand, a bidentate acetate, a chloride and a THF molecule. Complex 5b has a square pyramidal chromium with apical chloride and basal η 2 diketiminate and diketonate ligands. The complex contains strong intramolecular C−H ··· π stabilising interactions. All the complexes are active in ethylene polymerisation on treatment with suitable aluminium activators, affording high molecular weight polyethylene.