Claire Newton

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 CrCl3(THF)3 with LiDDP. Alkylation of 1 with AlMe3 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(O2CR)(THF)] (R = Me 4a, Ph 4b) and [(DDP)CrCl({O(R)C}2CH)] (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 C2h symmetry. Complex 3·Bz Bz is also dimeric but has its potential C2h 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 η2 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.

Low valent chromium complexes bearing N,O-chelating pyridyl-enolate ligands [OC(But)(2-CHN5H3Me-x)]− (x = 3–6)

Treatment of [CrCl2(THF)] (THF = tetrahydrofuran) with the sodium salt of HOC(But)(2-CH2NC5H3Me-x)2 (x = 6) in refluxing THF results in fragmentation of the L2X (L = neutral donor, X = anionic donor) ligand to afford the square planar bis(pyridyl-enolate) chromium(II) complex [Cr{OC(But)(2-CHNC5H3Me-6)}2] (1) and the by-product trans-[CrCl2(2,6-dimethylpyridine)2] (2). On prolonged heating in refluxing toluene, oxidation of 1 occurs yielding the octahedral tris(pyridyl-enolate) chromium(III) complex [Cr{OC(But)(2-CHNC5H3Me-6)}3] (3). Similar use of the sodium salts of HOC(But)(2-CH2NC5H3Me-x)2 (x = 3–5) also results in ligand fragmentation but gives solely the chromium(III) tris-chelated complexes [Cr{OC(But)(2-CHNC5H3Me-x)}3] [x = 3 (4), 4 (5), 5 (6)]. Surprisingly, prolonged heating in a sealed tube of a mixture of [CrCl2(THF)] and the sodium salt of HOC(But)(2-CH2NC5H3Me-x)2 (x = 6) in the presence of diphenylacetylene furnishes the bimetallic chromium(II) complex [ClCr{(μ-OC(But)(2-CH2NC5H3Me-6)2}CrCl] (7), in which the two intact L2X ligands bridge the Cr–Cr vector (2.92 A), one acting as a bidentate ligand and the other as a tridentate ligand. Single crystal X-ray diffraction studies have been performed on 1, 2, 3, 6 and 7.

A five-coordinate chromium alkyl complex stabilised by salicylaldiminato ligands

The five-coordinate bis(salicylaldiminato)chromium(III) complex [3,5-(t-Bu)2(O)C6H2CHN(2,6-i-Pr2C6H3)]2CrCl (2) reacts with triethylaluminium in THF at room temperature to give the air sensitive ethyl species [3,5-(t-Bu)2(O)C6H2CHN(2,6-i-Pr2C6H3)]2CrEt (3); in the presence of aluminium activators, 2 and 3 are moderately active ethylene polymerisation catalysts.

CHROMIUM ETHYLENE POLYMERISATION CATALYSTS BEARING REDUCED SCHIFF-BASE N, O-CHELATE LIGANDS

Treatment of CrCl3(THF)3 with the lithium and sodium salts of the reduced Schiff-base ligand 3,5-(tBu)2-2-(OH)C6-H2CH2NH(2,6-Me2C6H3) affords bis-helate chromium(II) and mono-chelate chromium(III complexes respectively; both give active ethylene polymerisation catalysts upon treatment with alkylaluminium activators.