We report that pincer-ligated iridium complexes undergo oxidative addition of the strained CâC bond of biphenylene. The sterically crowded species (tBuPCP)Ir (RPCP = κ3-1,3-C6H3(CH2PR2)2) initially reacts with biphenylene to selectively add the C(1)âH bond, to give a relatively stable aryl hydride complex. Upon heating at 125 °C for 24 h, full conversion to the CâC addition product, (tBuPCP)Ir(2,2â²-biphenyl), is observed. The much less crowded (iPrPCP)Ir undergoes relatively rapid CâC addition at room temperature. The large difference in the apparent barriers to CâC addition is notable in view of the fact that the addition products are not particularly crowded, since the planar biphenyl unit adopts an orientation perpendicular to the plane of the RPCP ligands. Based on DFT calculations the large difference in the barriers to CâC addition can be explained in terms of a âtiltedâ transition state. In the transition state the biphenylene cyclobutadiene core is calculated to be strongly tilted (ca. 50°â60°) relative to its orientation in the product in the plane perpendicular to that of the PCP ligand; this tilt results in very short, unfavorable, non-bonding contacts with the t-butyl groups in the case of the tBuPCP ligand. The conclusions of the biphenylene studies are applied to interpret computational results for cleavage of the unstrained CâC bond of biphenyl by (RPCP)Ir.
我们报道了钳式连接的
铱配合物对联亚苯基的应变 C–C 键进行了氧化加成。空间拥挤的物种(tBuPCP)Ir (RPCP = γ3-1,3-
C6H3(CH2PR2)2)最初与
联苯反应选择性地添加C(1)–H键,得到相对稳定的芳基
氢化物络合物。在125℃下加热24小时后,观察到完全转化为C-C加成产物(tBuPCP)Ir(2,2-
联苯)。不太拥挤的 (iPrPCP)Ir 在室温下经历相对快速的 C→C 加成。由于平面
联苯单元采用垂直于 RPCP
配体平面的方向,因此加成产物并不是特别拥挤,因此 C→C 加成的表观势垒的巨大差异是值得注意的。根据 DFT 计算,C-C 添加势垒的巨大差异可以用“倾斜”过渡态来解释。在过渡态下,
联苯撑
环丁二烯核被计算为相对于其在垂直于 PCP
配体平面的平面中的产物方向强烈倾斜(约 50° - 60°);在 tBuPCP
配体的情况下,这种倾斜会导致与叔丁基的非常短、不利的非键合接触。
联苯撑研究的结论可用于解释 (RPCP)Ir
联苯的无应变 C–C 键断裂的计算结果。