Chlorido­bis­[2-(di­phenyl­phosphanyl)ethanamine-²P,N](tri­phenylphos­phane-P)ruthenium(II) chloride toluene monosolvate

There has been recent inter­est in the ruthenium amino­phosphane complexes dichloridobis[2-(di­phenyl­phosphanyl)ethanamine-κ²P,N]ruthenium(II), Cl₂Ru(Ph₂PCH₂CH₂PNH₂-κ²P,N)₂ (Morris et al., 2002), chloridobis[2-(di­phenyl­phosphanyl)ethanamine-κ²P,N]hydro­ruthenium(II), HClRu(Ph₂PCH₂CH₂PNH₂-κ²P,N)₂ (Guo et al., 2004), and bis­[2-(di­phenyl­phosphanyl)ethanamine-κ²P,N]di­hydro­ruthenium(II), H₂Ru(Ph₂PCH₂CH₂CH₂PNH₂-κ²P,N)₂ (Jia et al., 2009). In part, this has been motivated by the formal similarity of the coordination environment of the ruthenium to that in Nayori diphosphane/di­amine complexes. The ruthenium amino­phosphane complexes have been investigated in this context and have been shown to be excellent hydrogenation catalysts for ketones and imines. In closely related complexes containing 1-alkyl-2-alkyl-1-amino-2-(di­phenyl­phosphanyl)ethane, Ph₂PCHRCHR'NH₂ (where R and R' are alkyl groups), and related chiral ligands, enanti­oselective hydrogenation has been observed (Guo et al., 2004; Abdur-Rashid et al., 2005; Dahlenburg & Kuehnlein, 2005, 2008). A number of these complexes have been structurally characterized and in all cases the two chlorides, or the hydride and the chloride, or the two hydrides, are trans, and the two amino­phosphane ligands are bound in an equatorial fashion with cis phosphane and cis N atoms, as illustrated by (I) in Scheme 1 (Morris et al., 2002; Guo et al., 2004; Jia et al., 2009; Abdur-Rashid et al., 2005; Dahlenburg & Kuehnlein, 2005, 2008)

We have found that [RuCl(PPh₃){Ph₂P(CH₂)₂NH₂}]⁺ possesses a different stereochemistry, in which the two Ph₂P– groups in the two chelating Ph₂P(CH₂)₂NH₂ ligands are trans and the two –NH₂ groups are cis (see Scheme 1).

Ignoring potential, yet reversible, chirality from conformations arising from the –CH₂CH₂- linkages present in both, compound (I) is achiral while the [RuCl(PPh₃){Ph₂P(CH₂)₂NH₂}]⁺ cation is chiral, and the latter is the only bis-amino­phosphane complex of Ru where the two chelating ligands are not in the same plane and the only example where the nonchelating ligands are cis.

The syntheses of [RuCl₂(PPh₃)₃] and Ph₂P(CH₂)₂NH₂ were carried out as described in the literature (Jia et al., 2009; Hallman et al., 1970).

For the preparation of [RuCl(PPh₃){Ph₂P(CH₂)₂NH₂}₂]Cl, (II), the reaction was carried out under argon at room temperature. A 50 ml round-bottomed flask was charged with RuCl₂(PPh₃)₃ (0.250 g, 0.026 mmol) dissolved in anhydrous toluene (10 ml) and a solution of Ph₂P(CH₂)₂NH₂ (0.141 g, 0.615 mmol) in toluene (~2 ml) was added to the flask. The color changed instantaneously from dark brown to transparent light brown. The solution was stirred for 20 min, during which time a precipitate formed. After filtration, crude (II) (yield 0.12 g, 0.11 mmol, 52%) was obtained. Light-orange X-ray quality crystals of (II) were obtained by slow evaporation of a saturated toluene solution over a period of 4 d.

The ³¹P spectrum of (II) is reported relative to an H₃PO₄ standard and was simulated using Spinworks 3.1.8.1 (Marat, 2011; Quirt & Martin, 1971).

Crystal data, data collection and structure refinement details are summarized in Table 1. The N-bound H atoms were located from a difference map and constrained, with U_iso(H) = 1.2U_eq(N). N—H distances were restrained to be similar within 0.02 Å. C-bound H atoms were placed in calculated positions, with U_iso(H) = 1.5U_eq(C) for methyl H atoms or 1.5U_eq(C) otherwise [These are both the same - should they be different?]. Five reflections were omitted as outliers having ΔF²/e.s.d. > 10 or F_c/F_c(max) > 0.5.

The reaction of Ph₂P(CH₂)₂NH₂ with Cl₂Ru(PPh₃)₃ displaces two PPh₃ ligands and a Cl^- anion to form [RuCl(PPh₃){Ph₂P(CH₂)₂NH₂}₂]Cl, (II) (Scheme 2).

The asymmetric unit of (II) in the crystal structure contains two [RuCl(PPh₃){Ph₂P(CH₂)₂NH₂}₂]⁺ cations, two Cl^- anions and two toluene solvent molecules. In both cations, the Ru atom is o­cta­hedrally coordinated by two chelating Ph₂P(CH₂)₂NH₂ ligands, a PPh₃ ligand and a Cl^- ligand (Figs. 1 and 2). The two Ph₂P– groups in the Ph₂P(CH₂)₂NH₂ ligands are trans to one another, while the –NH₂ groups are cis, with one being trans to Cl^- and the other trans to PPh₃. The two Ph₂P– groups are displaced slightly away from PPh₃. This probably reflects the steric bulk of PPh₃ and constraints imposed by the chelate ring. The three P donor atoms coordinate in a meridional fashion.

While (II) crystallizes in the achiral space group P2₁/n, the [RuCl(PPh₃){Ph₂P(CH₂)₂NH₂}₂]⁺ cation is chiral and crystallizes as a racemic mixture. Within the asymmetric unit, both cations show minor differences in bond lengths and angles (Fig. 2 and Table 2). A molecular fitting (Spek, 2009) shows that both symmetry-unique cations have approximately the same conformation (Fig. 3), with differences mainly in the positions of the phenyl groups. The most prominent difference is a twist of 18.2 (2)° between the relative mean planes of the phenyl groups containing atoms C28A and C28B. While these bond lengths and angles are typical (Bruno et al., 2004), the chiral stereochemistry of the cation is unique in ruthenium–amino­phosphane chemistry.

As predicted from the X-ray structure, which shows that the two Ph₂P(CH₂)₂NH₂ ligands are not equivalent, the ³¹P NMR spectrum contains three unique phospho­rus resonances (Fig. 4). Since the spectrum is second order, chemical shifts and coupling constants cannot be extracted directly. However, it can be closely simulated as an ABX spin system using P_A = 24.8 p.p.m., P_B = 28.8 p.p.m. and P_X = 42.7 p.p.m., and J_AB = 288.4 Hz, J_AX = 30.0 Hz and J_BX = 26.9 Hz. Coupling constants of about 300 Hz are typical of trans-coordinated phospho­rus, while 30 Hz is characteristic of cis coordination (Nelson, 2002). This allows the assignment of P_X to PPh₃. The assignment of P_A and P_B is ambiguous. Because of the non-equivalence of the two ligands and because the cation is chiral, all 12 CH₂ and NH₂ H atoms are non-equivalent. Reflecting this, the ¹H NMR spectrum contains a number of broad, sometimes overlapping, resonances which we cannot assign unambiguously.