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-κ O :κ O ′)di-μ-hydroxido-bis[(butyl 2,6-di- tert -butyl-4-methylphenyl hydrogen phosphato-κ O )(butyl 2,6-di- tert -butyl-4-methylphenyl phosphato-κ O )chromium]( Cr — Cr ) heptane disolvate or {Cr 2 (μ 2 -OH) 2 [μ 2 -PO 2 (OBu)(O-2,6- t Bu 2 -4-MeC 6 H 2 )-κ O :κ O ′] 2 [PO 2 (OBu)(O-2,6- t Bu 2 -4-MeC 6 H 2 )-κ O ] 2 [HOPO(OBu)(O-2,6- t Bu 2 -4-MeC 6 H 2 )-κ O ] 2 }·2C 7 H 16 , [Cr 2 (C 19 H 32 O 4 P) 4 (C 19 H 33 O 4 P) 2 (OH) 2 ]·2C 7 H 16 , denoted ( 1 )·2(heptane), [μ-bis(2,6-diisopropylphenyl) phosphato-1κ O :2κ O ′]bis[bis(2,6-diisopropylphenyl) phosphato]-1κ O ,2κ O -chlorido-2κ Cl -triethanol-1κ 2 O ,2κ O -di-μ-ethanolato-1κ 2 O :2κ 2 O -dichromium( Cr — Cr ) ethanol monosolvate or {Cr 2 (μ 2 -OEt) 2 [μ 2 -PO 2 (O-2,6- i Pr 2 -C 6 H 3 ) 2 -κ O :κ O ′][PO 2 (O-2,6- i Pr 2 -C 6 H 3 ) 2 -κ O ] 2 Cl(EtOH) 3 }·EtOH, [Cr 2 (C 2 H 5 O) 2 (C 24 H 34 O 4 P) 3 Cl(C 2 H 6 O) 3 ]·C 2 H 6 O, denoted ( 2 )·EtOH, and di-μ-ethanolato-1κ 2 O :2κ 2 O -bis{[bis(2,6-diisopropylphenyl) hydrogen phosphato-κ O ][bis(2,6-diisopropylphenyl) phosphato-κ O ]chlorido(ethanol-κ O )chromium}( Cr — Cr ) benzene disolvate or {Cr 2 (μ 2 -OEt) 2 [PO 2 (O-2,6- i Pr 2 -C 6 H 3 ) 2 -κ O ] 2 [HOPO(O-2,6- i Pr 2 -C 6 H 3 ) 2 -κ O ] 2 Cl 2 (EtOH) 2 }·2C 6 H 6 , [Cr 2 (C 2 H 5 O) 2 (C 24 H 34 O 4 P) 2 (C 24 H 35 O 4 P) 2 Cl 2 (C 2 H 6 O) 2 ]·2C 6 H 6 , denoted ( 3 )·2C 6 H 6 . Complexes ( 1 )–( 3 ) have been synthesized by an exchange reaction between the in-situ -generated corresponding lithium or potassium disubstituted phosphates with CrCl 3 (H 2 O) 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 )·2C 6 H 6 , whose structures have the monoclinic P 2 1 , orthorhombic P 2 1 2 1 2 1 and triclinic P \overline 1 space groups, respectively. All three complexes have binuclear cores with a single Cr—Cr bond, i.e. Cr 2 O 6 P 2 in ( 1 ), Cr 2 PO 4 in ( 2 ) and Cr 2 O 2 in ( 3 ), where the Cr atoms are in distorted octahedral environments, formally having 16 ē per Cr atom. The complexes have bridging ligands μ 2 -OH in ( 1 ) or μ 2 -OEt in ( 2 ) and ( 3 ). The organophosphate ligands demonstrate terminal κ O coordination modes in ( 1 )–( 3 ) and bridging μ 2 -κ O :κ O ′ 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[PO 2 (O R ) 2 ] 2 } associates; two intramolecular Cl...H—O Et hydrogen bonds additionally stabilize the Cr 2 O 2 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.