Chromium complexes bearing disubstituted organophosphate ligands and their use in ethylene polymerization

The crystal structures of three unusual chromium organophosphate complexes have been determined, namely, bis(μ-butyl 2,6-di-tert-butyl-4-methylphenyl hydrogen phosphato-κO:κO')di-μ-hydroxido-bis[(butyl 2,6-di-tert-butyl-4-methylphenyl hydrogen phosphato-κO)(butyl 2,6-di-tert-butyl-4-methylphenyl phosphato-κO)chromium](Cr-Cr) heptane disolvate or {Cr₂(μ₂-OH)₂[μ₂-PO₂(OBu)(O-2,6-^tBu₂-4-MeC₆H₂)-κO:κO']₂[PO₂(OBu)(O-2,6-^tBu₂-4-MeC₆H₂)-κO]₂[HOPO(OBu)(O-2,6-^tBu₂-4-MeC₆H₂)-κO]₂}·2C₇H₁₆, [Cr₂(C₁₉H₃₂O₄P)₄(C₁₉H₃₃O₄P)₂(OH)₂]·2C₇H₁₆, denoted (1)·2(heptane), [μ-bis(2,6-diisopropylphenyl) phosphato-1κO:2κO']bis[bis(2,6-diisopropylphenyl) phosphato]-1κO,2κO-chlorido-2κCl-triethanol-1κ²O,2κO-di-μ-ethanolato-1κ²O:2κ²O-dichromium(Cr-Cr) ethanol monosolvate or {Cr₂(μ₂-OEt)₂[μ₂-PO₂(O-2,6-ⁱPr₂-C₆H₃)₂-κO:κO'][PO₂(O-2,6-ⁱPr₂-C₆H₃)₂-κO]₂Cl(EtOH)₃}·EtOH, [Cr₂(C₂H₅O)₂(C₂₄H₃₄O₄P)₃Cl(C₂H₆O)₃]·C₂H₆O, denoted (2)·EtOH, and di-μ-ethanolato-1κ²O:2κ²O-bis{[bis(2,6-diisopropylphenyl) hydrogen phosphato-κO][bis(2,6-diisopropylphenyl) phosphato-κO]chlorido(ethanol-κO)chromium}(Cr-Cr) benzene disolvate or {Cr₂(μ₂-OEt)₂[PO₂(O-2,6-ⁱPr₂-C₆H₃)₂-κO]₂[HOPO(O-2,6-ⁱPr₂-C₆H₃)₂-κO]₂Cl₂(EtOH)₂}·2C₆H₆, [Cr₂(C₂H₅O)₂(C₂₄H₃₄O₄P)₂(C₂₄H₃₅O₄P)₂Cl₂(C₂H₆O)₂]·2C₆H₆, denoted (3)·2C₆H₆. Complexes (1)-(3) have been synthesized by an exchange reaction between the in-situ-generated corresponding lithium or potassium disubstituted phosphates with CrCl₃(H₂O)₆ in ethanol. The subsequent crystallization of (1) from heptane, (2) from ethanol and (3) from an ethanol/benzene mixture allowed us to obtain crystals of (1)·2(heptane), (2)·EtOH and (3)·2C₆H₆, whose structures have the monoclinic P2₁, orthorhombic P2₁2₁2₁ and triclinic P-1 space groups, respectively. All three complexes have binuclear cores with a single Cr-Cr bond, i.e. Cr₂O₆P₂ in (1), Cr₂PO₄ in (2) and Cr₂O₂ in (3), where the Cr atoms are in distorted octahedral environments, formally having 16 ē per Cr atom. The complexes have bridging ligands μ₂-OH in (1) or μ₂-OEt in (2) and (3). The organophosphate ligands demonstrate terminal κO coordination modes in (1)-(3) and bridging μ₂-κO:κO' coordination modes in (1) and (2). All the complexes exhibit hydrogen bonding: two intramolecular O_phos...H-O_phos interactions in (1) and (3) form two {H[PO₂(OR)₂]₂} associates; two intramolecular Cl...H-O_Et hydrogen bonds additionally stabilize the Cr₂O₂ core in (3); two intramolecular O_phos...H-O_Et interactions and two O...H-O intermolecular hydrogen bonds with a noncoordinating ethanol molecule are observed in (2)·EtOH. The presence of both basic ligands (OH^- or OEt^-) and acidic [H(phosphate)₂]^- associates at the same metal centres in (1) and (3) is rather unusual. Complexes may serve as precatalysts for ethylene polymerization under mild conditions, providing polyethylene with a small amount of short-chain branching. The formation of a small amount of α-olefins has been detected in this reaction.