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 {Cr(μ-OH)[μ-PO(OBu)(O-2,6-Bu-4-MeCH)-κO:κO'][PO(OBu)(O-2,6-Bu-4-MeCH)-κO][HOPO(OBu)(O-2,6-Bu-4-MeCH)-κO]}·2CH, [Cr(CHOP)(CHOP)(OH)]·2CH, denoted (1)·2(heptane), [μ-bis(2,6-diisopropylphenyl) phosphato-1κO:2κO']bis[bis(2,6-diisopropylphenyl) phosphato]-1κO,2κO-chlorido-2κCl-triethanol-1κO,2κO-di-μ-ethanolato-1κO:2κO-dichromium(Cr-Cr) ethanol monosolvate or {Cr(μ-OEt)[μ-PO(O-2,6-Pr-CH)-κO:κO'][PO(O-2,6-Pr-CH)-κO]Cl(EtOH)}·EtOH, [Cr(CHO)(CHOP)Cl(CHO)]·CHO, denoted (2)·EtOH, and di-μ-ethanolato-1κO:2κO-bis{[bis(2,6-diisopropylphenyl) hydrogen phosphato-κO][bis(2,6-diisopropylphenyl) phosphato-κO]chlorido(ethanol-κO)chromium}(Cr-Cr) benzene disolvate or {Cr(μ-OEt)[PO(O-2,6-Pr-CH)-κO][HOPO(O-2,6-Pr-CH)-κO]Cl(EtOH)}·2CH, [Cr(CHO)(CHOP)(CHOP)Cl(CHO)]·2CH, denoted (3)·2CH. Complexes (1)-(3) have been synthesized by an exchange reaction between the in-situ-generated corresponding lithium or potassium disubstituted phosphates with CrCl(HO) 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)·2CH, whose structures have the monoclinic P2, orthorhombic P222 and triclinic P-1 space groups, respectively. All three complexes have binuclear cores with a single Cr-Cr bond, i.e. CrOP in (1), CrPO in (2) and CrO in (3), where the Cr atoms are in distorted octahedral environments, formally having 16 ē per Cr atom. The complexes have bridging ligands μ-OH in (1) or μ-OEt in (2) and (3). The organophosphate ligands demonstrate terminal κO coordination modes in (1)-(3) and bridging μ-κO:κO' coordination modes in (1) and (2). All the complexes exhibit hydrogen bonding: two intramolecular O...H-O interactions in (1) and (3) form two {H[PO(OR)]} associates; two intramolecular Cl...H-O hydrogen bonds additionally stabilize the CrO core in (3); two intramolecular O...H-O 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)] 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.