Syntheses, characterization and redox properties of homoleptic ruthenium(II)-diphosphine and diarsine complexes:: deviations from ligand additivity

Homoleptic Ru(II)-diphosphine and Ru(II)-diarsine complexes, [Ru(L-L)(3)](2+), have been synthesized by two routes. Treatment of [RuCl2(PPh3)(3)] with excess ligand in EtOH gave [Ru(L-L)(3)]Cl-2 only for L-L = 1,2-(Me2As)(2)C6H4 (diars) 1a and Me2PCH2PMe2 (dmpm) 2a. In the latter reaction, neutral trans-[RuCl2(dmpm-P,P')(dmpm-P)(2)] 3 was also a product (detected by P-31{H-1} NMR spectroscopy). A more widely-applicable synthesis was treatment of the halide-free starting material [Ru(dmf)(6)](OTf)(3) (dmf = Me2NCHO; OTf = CF3SO3-) with excess ligand in EtOH. This gave the triflate salts [Ru(L-L)(3)](OTf)(2) (L-L = diars 1b, dmpm 2b, Me2PCH2CH2PMe2 (dmpe) 4 and Et2PCH2CH2PEt2 (depe) 5. The complexes were characterized by microanalysis, infrared and electronic spectroscopies, multinuclear NMR spectroscopy, and FAB mass spectrometry. The crystal structure of [Ru(diars)(3)]Cl-2.0.5dmf.H2O has been determined. The mean Ru-As bond length, 2.4468(15) Angstrom, is significantly longer than for typical trans-[Ru-II(diars)(2)] moieties (2.425 Angstrom; mean of 14 structures), suggesting steric crowding. X-Ray quality crystals of the diphosphine complexes were not obtained. However, Ru K-edge EXAFS measurements on [Ru(dmpe)(3)](OTf)(2) were fitted well by a shell of six phosphorus atoms at Ru-P = 2.38(1) Angstrom, compared with 2.31 Angstrom for published structures containing trans[ Ru-II(dmpe)(2)], again indicative of steric crowding. Electrochemical studies, in extremely anhydrous media, revealed an irreversible oxidation for [Ru(L-L)(3)](OTf)(2) assigned as Ru(II)/Ru(III), at E-P(A) ca. + 2.3 V vs. ferrocene/ferrocinium. Digital simulation showed that the oxidations are kinetically slow, and the formal Ru(II)/Ru(III) potentials are around + 1.8 V. This is more positive than the value predicted using published electrochemical ligand parameters (E-L). The latter were checked, for diars and dmpe, by synthesizing and measuring the Ru(II)/Ru(III) redox potentials of [Ru(2,2'-bipyridine)(2)(L-L)](OTf)(2)(L-L = dmpe 6 and diars 7). The crystal structure of 7 was determined. The mean Ru-As bond length, 2.4066(3) Angstrom, lends further support to the contention that the homoleptic cation 1 is sterically crowded. Attempts to synthesise pure samples of related complexes with monodentate phosphines (PR3) or aryldiphosphines were unsuccessful.