Structures and redox properties of metal complexes of the electron-deficient diphosphine chelate ligand R,R-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)(3)Cl (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(center dot-) and 3(center dot-). In contrast, the reduction to 2(center dot-) 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 pi system with rather small contributions from the metals or the phosphorus nuclei.