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) Å, is significantly longer than for typical trans-[Ru^II(diars)₂] moieties (2.425 Å; 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) Å, compared with 2.31 Å 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^Aca.+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) Å, 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.