Mechanistic studies on the formation of linear polyethylene chain catalyzed by palladium phosphine-sulfonate complexes: experiment and theoretical studies.
Mechanistic studies on the formation of linear polyethylene chain catalyzed by palladium phosphine-sulfonate complexes: experiment and theoretical studies.
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DOI：
10.1021/ja9047398
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发表时间：
2009-09
影响因子：
15
通讯作者：
Shusuke Noda;A. Nakamura;Takuya Kochi;L. Chung;K. Morokuma;K. Nozaki
中科院分区：
文献类型：
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作者：
Shusuke Noda;A. Nakamura;Takuya Kochi;L. Chung;K. Morokuma;K. Nozaki
从实验和理论上研究了Pd膦磺酸盐配合物Pd（CH（3））（L）（Ar（2）PC（6）H（4）SO（3））（L = 2，6-二甲基吡啶，Ar = o-MeOC（6）H（4）（2a）和L =吡啶，Ar = Ph（2b））引发的线性聚乙烯增长。在实验上，用Pd（CH（3））（L）（Ar（2）PC（6）H（4）SO（3））配合物2a和2b得到了高线性聚乙烯。以甲基钯配合物为原料，在钯中心上进行乙烯齐聚反应，生成长链烷基取代的钯配合物（3）。此外，合成了定义明确的乙基和丙基配合物（6（Et）和6（Pr））作为稳定的正烷基钯配合物。尽管存在β-氢，β-氢化物消除生成1-烯烃在所有情况下都非常缓慢或可忽略不计。另一方面，在甲基钯/膦-磺酸盐络合物2a存在下的1-己烯的异构化表明该催化剂体系实际上经历β-氢化物消除和再插入以释放内烯烃。在理论方面，基于起始模型配合物Pd（n-C（3）H（7））（pyridine）（o-Me（2）PC（6）H（4）SO（3））（8），计算了链增长、链行走和链转移的中间体和过渡态的相对能量。首先，提出了Pd/膦磺酸盐体系的Berry准旋转顺反异构化过程。磺酸基的第二个氧原子作为缔合配体参与异构化反应，这是磺酸基最独特的性质之一。链增长发生在较不稳定的烷基Pd（乙烯）配合物10'上，其TS为27.4/27.7（（E+ZPC）/G）kcal/mol。建议在低浓度乙烯下发生可能的β-氢化物消除：取代β-氢化物消除的最高能量过渡态是>37.4/25.3 kcal/mol（TS（9-12））或29.1/27.4 kcal/mol（TS（8 '-9'）达到12 '）。通过28.6/29.1 kcal/mol的TS（TS（14 ′-15 ′））允许乙烯插入异烷基钯物质（14 ′），其能量略高于正烷基钯物质（TS（10 ′-11 ′））。如果β-氢化物消除至12'确实发生，则建议从更稳定的PdH（烯烃）络合物12'进行容易的链转移。因此，在乙烯压力下生产具有高分子量的线性聚乙烯表明顺式和反式PdH（烯烃）（膦-磺酸盐）络合物（12和12 ′）仅在过量乙烯存在下可接近。
Linear polyethylene propagation starting from Pd phosphine-sulfonate complexes, Pd(CH(3))(L)(Ar(2)PC(6)H(4)SO(3)) (L = 2,6-lutidine, Ar = o-MeOC(6)H(4) (2a) and L = pyridine, Ar = Ph (2b)), was studied both experimentally and theoretically. Experimentally, highly linear polyethylene was obtained with Pd(CH(3))(L)(Ar(2)PC(6)H(4)SO(3)) complexes 2a and 2b. Formation of a long alkyl-substituted palladium complex (3) was detected as a result of ethylene oligomerization on a palladium center starting from methylpalladium complex. Additionally, well-defined ethyl and propyl complexes (6(Et) and 6(Pr)) were synthesized as stable n-alkyl palladium complexes. In spite of the existence of beta-hydrogens, the beta-hydride elimination to give 1-alkenes was very slow or negligible in all cases. On the other hand, isomerization of 1-hexene in the presence of a methylpalladium/phosphine-sulfonate complex 2a indicated that this catalyst system actually undergoes beta-hydride elimination and reinsertion to release internal alkenes. On the theoretical side, the relative energies were calculated for intermediates and transition states for chain-growth, chain-walking, and chain-transfer on the basis of the starting model complex Pd(n-C(3)H(7))(pyridine)(o-Me(2)PC(6)H(4)SO(3)) (8). First, cis/trans isomerization process via the Berry's pseudorotation was proposed for the Pd/phosphine-sulfonate system. The second oxygen atom of sulfonate group is involved in the isomerization process as the associative ligand, which is one of the most unique natures of the sulfonate group. Chain propagation was suggested to take place from the less stable alkylPd(ethylene) complex 10' with the TS of 27.4/27.7 ((E+ZPC)/G) kcal/mol. Possible beta-hydride elimination was suggested to occur under low concentration of ethylene: the highest-energy transition state to override for beta-hydride elimination was either >37.4/25.3 kcal/mol (TS(9-12)) or 29.1/27.4 kcal/mol (TS(8'-9') to reach 12'). The ethylene insertion to the iso-alkylpalladium species (14') is allowed via a TS of 28.6/29.1 kcal/mol (TS(14'-15')), slightly higher in energy than that for the normal-alkylpalladium species (TS(10'-11')). Easy chain transfer was suggested to proceed from the more stable PdH(olefin) complex 12' if beta-hydride elimination to 12' does take place. Thus, the production of linear polyethylene with high molecular weight under ethylene pressure suggests that the cis and trans PdH(alkene)(phosphine-sulfonate) complexes (12 and 12') are merely accessible in the presence of excess amount of ethylene.