Reversible Sigma C–C Bond Formation Between Phenanthroline Ligands Activated by (C5Me5)2Yb

The electronic structure and associated magnetic properties of the 1,10-phenanthroline adducts of Cp*2Yb are dramatically different from those of the 2,2′-bipyridine adducts. The monomeric phenanthroline adducts are ground state triplets that are based upon trivalent Yb­(III), f13, and (phen•– ) that are only weakly exchange coupled, which is in contrast to the bipyridine adducts whose ground states are multiconfigurational, open-shell singlets in which ytterbium is intermediate valent (J. Am. Chem. Soc 2009, 131, 6480; J. Am. Chem. Soc 2010, 132, 17537). The origin of these different physical properties is traced to the number and symmetry of the LUMO and LUMO+1 of the heterocyclic diimine ligands. The bipy•– has only one π*1 orbital of b1 symmetry of accessible energy, but phen•– has two π* orbitals of b1 and a2 symmetry that are energetically accessible. The carbon pπ-orbitals have different nodal properties and coefficients and their energies, and therefore their populations change depending on the position and number of methyl substitutions on the ring. A chemical ramification of the change in electronic structure is that Cp*2Yb­(phen) is a dimer when crystallized from toluene solution, but a monomer when sublimed at 180–190 °C. When 3,8-Me2phenanthroline is used, the adduct Cp*2Yb­(3,8-Me2phen) exists in the solution in a dimer–monomer equilibrium in which ΔG is near zero. The adducts with 3-Me, 4-Me, 5-Me, 3,8-Me2, and 5,6-Me2-phenanthroline are isolated and characterized by solid state X-ray crystallography, magnetic susceptibility and LIII-edge XANES spectroscopy as a function of temperature and variable-temperature 1H NMR spectroscopy.二(五甲基环戊二烯基)镱(Cp*₂Yb)的1,10-邻菲啰啉(1,10-phenanthroline)加合物，其电子结构与相关磁性质，与2,2′-联吡啶(2,2′-bipyridine)加合物存在显著差异。单体邻菲啰啉加合物的基态为三重态，其基于三价镱(III)（电子构型f¹³）与自由基阴离子邻菲啰啉(phen•–)，二者之间仅存在弱交换耦合；而联吡啶加合物的基态则为多组态开壳层单重态，其中镱处于中间价态（参考文献：J. Am. Chem. Soc 2009, 131, 6480；J. Am. Chem. Soc 2010, 132, 17537）。上述物理性质差异的根源，在于杂环二亚胺配体的最低未占据分子轨道(Lowest Unoccupied Molecular Orbital, LUMO)与次低未占据分子轨道(LUMO+1)的数目与对称性。自由基阴离子联吡啶(bipy•–)仅存在一个能量可及的b₁对称性π*₁轨道，而自由基阴离子邻菲啰啉(phen•–)则拥有两个能量可及的、分别为b₁与a₂对称性的π*轨道。碳的pπ轨道具有不同的节点性质与轨道系数，且其能级存在差异，因此轨道布居会随环上甲基取代的位置与数目发生改变。电子结构变化带来的一个化学结果是：二(五甲基环戊二烯基)镱(邻菲啰啉)[Cp*₂Yb(phen)]从甲苯溶液中结晶时为二聚体，但在180–190 ℃下升华时则为单体。当使用3,8-二甲基邻菲啰啉时，加合物Cp*₂Yb(3,8-Me₂phen)在溶液中以二聚体-单体平衡形式存在，其吉布斯自由能变(ΔG)趋近于零。本研究针对3-甲基、4-甲基、5-甲基、3,8-二甲基以及5,6-二甲基取代的邻菲啰啉所形成的加合物，通过固态X射线晶体学、磁化率测量、LⅢ边X射线吸收近边结构(XANES)光谱（随温度变化）以及变温氢(¹H)核磁共振光谱进行了分离与表征。