Synthesis of New Mixed Phosphine∼Iminophosphorane Bidentate Ligands and Their Coordination to Group 10 Metal Centers

Abstract

The selective monobromation of a symmetrical bidentate diphosphine (dppm = bis-diphenylphosphinomethane) yielding a highly reactive intermediate, 2 (P∼PBr+·Br-), is reported. Two methods of trapping were devised to produce mixed phosphine−aminophosphonium salts 3 (P∼PNHR+·Br-). The first method relies on the reaction of 2 with 2 equiv of primary amine to give 3a−c (P∼PNHR+·Br-, R = p-Me-Bn, p-MeO-Bn, Ph). The second method utilizes 1 equiv of primary amine and DABCO as trapping agent to give 3a−e (P∼PNHR+·Br-, R = p-Me-Bn, p-MeO-Bn, Ph, nBu, α(+)-Me-benzyl). These salts were then deprotonated quantitatively to yield the desired new phosphine−iminophosphorane ligands 4a−e (P∼PNR). This simple strategy allows for a wide variation of the R substituent at the nitrogen donor group (R = alkyl, aryl, benzyl). In particular, the optically pure ligand 4e (R = α(+)-Me-benzyl) was obtained in one pot from commercially available α(+)-Me-benzylamine. Reaction of 4 with Pd(COD)Cl2 affords the complexes 5 via coordination to Pd(II) centers and revealed the chelating behavior of these ligands. X-ray crystal structures of 5a (presented in ESI), 5c, and 5e are reported. Complexes of platinum(II), 6c and 6e, were also synthesized and characterized crystallographically. The complex of nickel(II), 7a, adopts a tetrahedral geometry as shown by X-ray analysis and consistent with a lack of NMR signal.