Zirconium and Titanium Propylene Polymerization Precatalysts Supported by a Fluxional <i>C</i>₂-Symmetric Bis(anilide)pyridine Ligand

Titanium and zirconium complexes supported by a bis­(anilide)­pyridine ligand (NNN = pyridine-2,6-bis­(N-mesitylanilide)) have been synthesized and crystallographically characterized. C2-symmetric bis­(dimethylamide) complexes were generated from aminolysis of M­(NMe2)4 with the neutral, diprotonated NNN ligand or by salt metathesis of the dipotassium salt of NNN with M­(NMe2)2Cl2. In contrast to the case for previously reported pyridine bis­(phenoxide) complexes, the ligand geometry of these complexes appears to be dictated by chelate ring strain rather than metal–ligand π bonding. The crystal structures of the five-coordinate dihalide complexes (NNN)­MCl2 (M = Ti, Zr) display a C1-symmetric geometry with a stabilizing ipso interaction between the metal and the anilido ligand. Coordination of THF to (NNN)­ZrCl2 generates a six-coordinate C2-symmetric complex. Facile antipode interconversion of the C2 complexes, possibly via flat C2v intermediates, has been investigated by variable-temperature 1H NMR spectroscopy for (NNN)­MX2(THF)n (M = Ti, Zr; X = NMe2, Cl) and (NNN)­Zr­(CH2Ph)2. These complexes were tested as propylene polymerization precatalysts, with most complexes giving low to moderate activities (102–104 g/(mol h)) for the formation of stereoirregular polypropylene.由双(苯胺基)吡啶配体（bis(anilide)pyridine ligand，其中NNN=吡啶-2,6-双(N-均三甲苯胺基)）负载的钛及锆配合物已被合成，并通过晶体学方法完成表征。C2对称的双(二甲基酰胺)配合物可通过两条路径制备：其一为将M(NMe2)4与中性双质子化的NNN配体进行氨解反应，其二为将NNN的二钾盐与M(NMe2)2Cl2进行复分解反应。与此前已报道的吡啶双(苯氧基)配合物不同，此类配合物的配体几何构型似乎由螯合环张力所主导，而非金属-配体π键。五配位二卤化物配合物(NNN)MCl2（M=钛(Ti)、锆(Zr)）的晶体结构呈现C1对称构型，且存在金属与苯胺基配体间的稳定ipso相互作用（ipso interaction）。四氢呋喃(THF)与(NNN)ZrCl2配位后，可得到六配位C2对称配合物。针对(NNN)MX2(THF)n（M=Ti、Zr；X=NMe2、Cl）以及(NNN)Zr(苄基)2，研究人员通过变温1H核磁共振波谱法（variable-temperature 1H NMR spectroscopy）探究了C2配合物可能通过平面C2v中间体进行的快速对映体互变过程。将此类配合物作为丙烯聚合预催化剂进行测试后发现，多数配合物可实现低至中等活性（102–104 g/(mol·h)），产物为无规聚丙烯（stereoirregular polypropylene）。