Synthesis, structures and ethylene polymerisation behaviour of low valent beta-diketiminato chromium complexes

The preparation reaction chemistry and ethylene polymerisation behaviour of low valent beta -diketiminato chromium complexes are described. [(DDP)CrCl (mu -Cl)](2) (1) [DDPH = 2-((2,6-diisopropylphenyl)amino)-4-((2,6-diisoprop ylphenyl)imino)-pent-2-ene] is formed by treatment of CrCl3(THF)(3) with Li DDP. Alkylation of 1 with AlMe3 results in the formation of the binuclear d imethyl complex [(DDP)CrMe(mu -Cl)](2) (2). In contrast, the attempted alky lation of 1 with benzylmagnesium chloride results in reduction to form the dichromium(II) complex [(DDP)Cr(mu -Cl)](2) (3). Depending on the condition s 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 P-dik etonates to give [(DDP)CrCl(O2CR) (THF)] (R = Me 4a, Ph 4b) and [(DDP)CrCl( {O(R)C}(2)CH)] (R = Me 5a, Ph 5b), respectively. Single crystal X-ray diffr action 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- 2h sym metry. Complex 3 Bz Bz is also dimeric but has its potential C-2h sy mmetry removed by a significant tetrahedral distortion of the chromium coor dination geometry. Compound 4a has an octahedral chromium centre coordinate d to a single bidentate diketiminate ligand, a bidentate acetate, a chlorid e and a THF molecule. Complex 5b has a square pyramidal chromium with apica l chloride and basal eta (2) diketiminate and diketonate ligands. The compl ex contains strong intramolecular C-H . . . pi stabilising interactions. Al l the complexes are active in ethylene polymerisation on treatment with sui table aluminium activators, affording high molecular weight polyethylene.