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

Abstract

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]Cl2 only for L–L = 1,2-(Me2As)2C6H4 (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 31P{1H} 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]Cl2·0.5dmf·H2O has been determined. The mean Ru–As bond length, 2.4468(15) A, is significantly longer than for typical trans-[RuII(diars)2] moieties (2.425 A; 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) A, compared with 2.31 A for published structures containing trans-[RuII(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 EPAca.+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 (EL). 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) A, 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.