Structures and Redox Properties of Metal Complexes of the Electron-Deficient Diphosphine Chelate Ligand <i>R</i>,<i>R</i>-QuinoxP

The air-stable chiral diphosphine chelate ligand R,R-QuinoxP (L; 2,3-bis(tert-butylmethylphosphino)quinoxaline) as developed by Imamoto et al. has been used to obtain the crystallographically characterized complexes (L)PtCl2 (1), (L)PdCl2 (2), and (L)Re(CO)3Cl (3). Coordination was found to occur via the P donor atoms, as indicated by crystal structures and NMR studies; the quinoxaline N donors do not participate in any coordination to the metals. The stereochemical arrangements observed illustrate the enantioselectivity reported for catalysis involving complexes of L. Electron acceptance by the quinoxaline heterocycle is responsible not only for the improved stability of L toward air but also for rather facile reduction of the complexes to the persistent radical anions 1•− and 3•−. In contrast, the reduction to 2•− proceeds irreversibly even at 243 K in the absence of excess chloride. EPR, UV–vis, and IR spectroelectrochemistry was used, when possible, to establish the spin location in the quinoxaline π system with rather small contributions from the metals or the phosphorus nuclei.由Imamoto等人开发的空气稳定手性双膦螯合配体R,R-QuinoxP（L；2,3-双(叔丁基甲基膦基)喹喔啉），已被用于获得经晶体学表征的配合物(L)PtCl₂(1)、(L)PdCl₂(2)及(L)Re(CO)₃Cl(3)。晶体结构与核磁共振（Nuclear Magnetic Resonance, NMR）研究表明，配位通过磷供体原子发生，喹喔啉的氮供体未参与与金属的配位。所观察到的立体化学排布，佐证了该配体L参与的催化反应所报道的对映选择性。喹喔啉杂环的电子接受能力，不仅提升了配体L的空气稳定性，还使得配合物易于还原为稳定的自由基阴离子1•−和3•−。与之相反，即便在243 K且无过量氯化物存在的条件下，还原生成2•−的过程仍不可逆。研究人员在可行条件下采用电子顺磁共振（Electron Paramagnetic Resonance, EPR）、紫外-可见（Ultraviolet-Visible, UV–vis）光谱及红外（Infrared, IR）光谱电化学技术，确定了自旋定域于喹喔啉π体系中，金属或磷核的贡献极小。