Synthesis, structure and spectroscopic properties of 2,3-bis(diphenylphosphino)quinoxaline (dppQx) and its copper(I) complexes

Phosphinoquinoxalines were prepared by treatment of 2,3-dichloroquinoxaline (3) with phosphorus nucleophiles. The Arbuzov reaction of 3 with PPh(O-i-Pr)2 gave a mixture of diastereomers of 2,3-(PPh(O)(O-i-Pr))2quinoxaline (6); the crystal structure of rac-6 was determined, but attempts at reduction to yield bis(phenylphosphino)quinoxaline 7 resulted in P–C cleavage and formation of phenylphosphine. The bis(secondary phosphine) 7 could be generated from 3 and LiPHPh(BH3), but was not isolated in pure form. Copper-catalyzed coupling of PHPh2 with 3 gave 2,3-bis(diphenylphosphino)quinoxaline (4, dppQx), whose coordination chemistry was investigated, with comparison to data for the analogous 1,2-bis(diphenylphosphino)benzene (dppBz) complexes. Reaction of dppQx with [Cu(NCMe)4][PF6] gave [Cu(dppQx)2][PF6] (8); CuCl yielded [Cu(dppQx)Cl]2 (9). Reaction of [Cu(NCMe)4][PF6] with one equiv of DPEphos, followed by one equiv of dppQx, gave [Cu(dppQx)(DPEphos)][PF6] (10). Ligand 4 and copper complexes 8 and 9 were crystallographically characterized. The UV–Vis spectra of dppQx and its copper complexes were red-shifted from those of the dppBz analogs; in contrast to results for the dppBz complexes, those of dppQx were not luminescent in solution.