Concerted Electron Transfer in Iminopyridine Chromium Complexes: Ligand Effects on the Polymerization of Various (Di)olefins

A study of reactions
among CrCl2, CrCl3(THF)3, and iminopyridine
ligands differing in the nature of the
substituents at the iminic carbon and at the ortho positions of the
aryl ring (2,6-R12C6H3NCR2(C5H3N) (R1 = R2 = H (L1); R1 = iPr, R2 = H (L2); R1 = H, R2 = CH3 (L3)) but featuring close electron-accepting
properties has provided a new example of the redox chemistry of chromium
complexes. The reactions of unsubstituted aniline L1 and
of L3 with CrCl2 give rise to [(L1•)­CrIIICl2(THF)]− (Cr1) and [(L3•)­CrIIICl2(THF)]− (Cr3) complexes, respectively, containing chromium in the physical trivalent oxidation state and the ligand in the monoanionic radical
state (L•)− as a result of a one-electron
transfer from the metal to the ligand. In contrast, the reactions
of CrCl2 with the ortho-substituted L2 and
of CrCl3(THF)3 with the unsubstituted L1 give rise to [(L2)­CrIICl2(THF)]0 (Cr2) and [(L1)­CrIIICl3(THF)]0 (Cr4) having
the chromium in the divalent and trivalent oxidation states, respectively,
and the unperturbed ligand in the neutral state. All four complexes
were used, in combination with methylaluminoxane (MAO), as catalyst
precursors for the polymerization of ethylene, cyclic olefins (i.e.,
norbornene and dicyclopentadiene), and 1,3-butadiene. A chromium to
ligand synergy, coupled with a good stability of the active intermediate
in the presence of the Al activator, proven particularly effective
in the polymerization of ethylene, especially for Cr1, giving high molecular weight linear poly­(ethylene)­s. The formalism
in the metal oxidation state does not affect the reactivity toward
the cyclic olefins and 1,3-butadiene, while ligand steric effects
emerge clearly. The use of bulky ortho substituents shuts down the
activity in the polymerization of cyclic olefins, particularly for
the bulkier dicyclopentadiene, and reverses the catalyst chemoselectivity
in the polymerization of 1,3-butadiene.

一项关于不同取代基性质的亚胺基碳和芳环邻位（2,6-R12C6H3N=CR2(C5H3N) (R1 = R2 = H (L1); R1 = iPr, R2 = H (L2); R1 = H, R2 = CH3 (L3)）配体以及具有相似电子接受特性的CrCl2、CrCl3(THF)3和iminopyridine的研究，为铬络合物的氧化还原化学提供了新的例证。未取代苯胺L1与L3与CrCl2的反应分别生成了含有铬处于物理三价氧化态和配体处于单负离子自由基态的[(L1•)­CrIIICl2(THF)]− (Cr1)和[(L3•)­CrIIICl2(THF)]− (Cr3)络合物，这是由于金属向配体转移了一个电子。相反，CrCl2与邻位取代的L2以及CrCl3(THF)3与未取代的L1的反应则产生了分别含有铬处于二价和三价氧化态的[(L2)­CrIICl2(THF)]0 (Cr2)和[(L1)­CrIIICl3(THF)]0 (Cr4)络合物，其中配体保持中性状态。所有四种络合物均与甲基铝氧烷（MAO）结合，用作乙烯、环烯烃（例如，环戊二烯和二环戊二烯）和1,3-丁二烯聚合反应的催化剂前体。铬与配体的协同作用，加上活性中间体在铝活化剂存在下的良好稳定性，特别是在乙烯聚合中，尤其是对于Cr1，提供了高相对分子质量线性聚（乙烯）的生成。金属氧化态的公式不影响对环烯烃和1,3-丁二烯的反应活性，而配体的立体效应则明显显现。使用庞大的邻位取代基关闭了环烯烃聚合中的活性，特别是对于更庞大的二环戊二烯，并在1,3-丁二烯聚合中反转了催化剂的化学选择性。