N-Arylcyano-β-diketiminate Methallyl Nickel Complexes: Synthesis, Adduct Formation, and Reactivity toward Ethylene

Abstract

The syntheses and structures of new bis-N-phenyl-4-cyano-β-diketiminate (1), bis-N-phenyl-2-cyano-β-diketiminate (2), N-phenyl-4-cyano-N-phenyl-2-cyano-β-diketiminate (3), N-phenyl-4-cyano-N-2,6-diisopropylphenyl-β-diketiminate (4), N-phenyl-2-cyano-N-2,6-diisopropylphenyl-β-diketiminate (5), and methallyl nickel complexes [L1Ni(η3-methallyl)] (6), [L2Ni(η3-methallyl)] (7), [L4Ni(η3-methallyl)] (8), and [L5Ni(η3-methallyl)] (9) from the reaction of deprotonated ligands 1/2, 4/5, and methallylnickel chloride dimer are reported. Subsequent reactions of complexes 6–9 with tris(pentafluorophenyl)boron give rise to new adducts [L1Ni(η3-methallyl)]·2B(C6F5)3 (10), [L2Ni(η3-methallyl)]·2B(C6F5)3 (11), [L4Ni(η3-methallyl)]·B(C6F5)3 (12), and [L5Ni(η3-methallyl)]·B(C6F5)3 (13), where B(C6F5)3 is coordinated to the cyano group. New compounds are characterized by NMR, IR, and mass spectrometry techniques, and crystal structures of most compounds are obtained and described. Complexes 6–9, adducts 10–13, and adducts 10–13 with 5 equiv of B(C6F5)3 have been investigated toward ethylene activation. Complexes 6–9 were inactive toward ethylene even at 60 °C, while adducts 10–13 showed reactivity toward ethylene; adding 5 equiv of B(C6F5)3 to adducts 10–13 leads to an increase in reactivity toward ethylene. Adducts 10 and 11 alone and with 5 equiv of B(C6F5)3 catalyze the oligomer formation, while adducts 12 and 13 by the same conditions catalyze the polyethylene formation. Adduct 13 activity is five times higher with respect to 12 at 50 °C and two times higher at 70 °C. The formation of branched moderate molecular weight PE by 12/B(C6F5)3 and 13/B(C6F5)3 is observed.

The performance of methallyl nickel complexes and boron adducts in the catalytic activation of ethylene: a conceptual DFT perspective

“…1/B(C 6 F 5 ) 3 has a |Δμ| value slightly higher with respect to 2/B(C 6 F 5 ) 3 (lower Δη DA values) and therefore oligomerization is expected to be more favorable for the former adduct. This result is consistent with the experimentally observed formation of oligomers [22,23]. Isomers {3,4} and {3/B(C 6 F 5 ) 3 , 4/ B(C 6 F 5 ) 3 } have quite similar Δμ values; therefore, using the chemical potential alone, it is not possible to distinguish which is the best species for ethylene activation.…”

“…When compounds 1 and 2 with two nitrile functionalities (Scheme 3) were activated with tris(pentafluorophenyl)boron, they produce butene and hexene. On the other hand, complexes 3 and 4, with isopropyl groups near the Ni center and nitrile group with coordinated borane, yield polyethylene (Scheme 3) [22,23]. We explained the differences in performances of systems 1/B(C 6 F 5 ) 3 -4/B(C 6 F 5 ) 3 based on the electronic effect produced by B(C 6 F 5 ) 3 coordinated to the cyano group and steric hindrance near the metal center.…”

“…These complexes have a nitrile group in the ortho or para position of the aromatic ring that can coordinate boron compounds easily [55][56][57]. It was shown previously that, without coordination of B(C 6 F 5 ) 3 , complexes 1-4 were unable to activate ethylene [22,23]. When compounds 1 and 2 with two nitrile functionalities (Scheme 3) were activated with tris(pentafluorophenyl)boron, they produce butene and hexene.…”

“…The aim of this research was twofold, on one hand we want to verify recently reported experimental results [22,23] concerning the reactivity of Ni methallyl complexes toward ethylene with theoretical calculations and, on the other, to identify which of the catalysts was more suitable for olefin oligomerization or polymerization. In addition, complex 4, which showed the best results in ethylene polymerization when activated with B(C 6 F 5 ) 3 , was selected to be also activated with BF 3 in order to analyze the effect of the size of the co-activator.…”

“…Recently, we discovered a series of neutral methallyl nickel complexes (Scheme 2) able to polymerize or oligomerize ethylene using small amounts of co-activators such as B(C 6 F 5 ) 3 [22,23]. While analyzingthesenickel-basedcomplexesandtheiradducts,theimportance of the position of a special group or framework in a complex structurebecameclear.Infact,wefoundthatthereactivityofthesystems under study strongly depended on the position of the CN group and on the co-activator B(C 6 F 5 ) 3 coordinated to it.…”