Two simple precursor 2,2′-bisoxazoline complexes of ruthenium

Abstract

Syntheses, X-ray crystal structures and some electrochemical properties of Ru^IIL₂Cl₂, Ru^IIIL(dimethylformamide)Cl₃ and [Ru^IIL₂Cl₂]PF₆, where L is 4,4,4′,4′-tetramethyl-2,2′-bisoxazoline, are described. It is found that the first two complexes serve as very good precursors to synthesise a host of mixed-ligand complexes of Ru(II) and Ru(III) containing L.

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In the last decade, various substituted chiral bisoxazolines of the types 1 and 2 have emerged as an efficient class of ligands in metal-catalysed asymmetric syntheses.^1–4 While a number of metal complexes of 2 (Evans's ligand) have been prepared and characterised structurally in various contexts,^1–5 very little attention has been paid to the transition metal chemistry of 1. This is possibly because in most cases, the bisoxazolines of type 2 have been found to be superior to those of type 1 in producing a greater enantiomeric excess. In an extensive review on the metal complexes of oxazolines⁵ that appeared in 1999, Gómez et al. write that “unfortunately, to date, no X-ray structures containing chelate” 1 “have been determined.” After the appearance of this review, Walther et al. have reported the X-ray crystal structures of octahedral Ni(1a)(acetylacetonate)₂ and square planar Ni(1a)(mes)(Br) (mes≡mesityl) and Pd(1a)(mes)₂ in connection with their studies on the catalytic transformations brought about by Pd(II) and Ni(II) complexes.⁶ Recently, we have undertaken a project to develop the transition metal chemistry of the bisoxazolines of type 1. Elsewhere we have reported⁷ the photophysics and structure of a copper(I) dimer of a derivative of 1. Herein we make a preliminary report of our studies on its ruthenium chemistry. The syntheses, structures and some properties of two simple ruthenium complexes of 1a, from which a host of mixed-ligand complexes of ruthenium(II) and ruthenium(III) can be prepared, are described here.

Reaction of 1 mol of RuCl₃·3H₂O with 2 mol of the ligand 1a in hot dimethylformamide (DMF) in the presence of 0.5 mol of metallic zinc and an excess of the chloride ion leads to the isolation of Ru(1a)₂Cl₂ in 25% yield; after allowing the filtrate to stand in air for several days, Ru(1a)(DMF)Cl₃ is obtained in 40% yield. The use of zinc metal in our synthesis is novel. For example, Ru(bpy)₂Cl₂ or Ru(phen)₂Cl₂ (bpy≡2,2′-bipyridine; phen≡1,10-phenanthroline) are synthesised in a similar manner without adding metallic zinc to the reaction mixture.^8,9 The role of zinc here is to reduce ruthenium(III) to ruthenium(II); no trace of metallic zinc can be found in the crude product of Ru(1a)₂Cl₂. Incidentally, no bpy or phen analogue of Ru(1a)(DMF)Cl₃ is known.

The structures of Ru(1a)₂Cl₂ and Ru(1a)(DMF)Cl₃ as determined by X-ray crystallography are shown in Fig. 1 and 2, respectively. The ligand 1a, which is a non-aromatic 1,4-diimine, is found to bind ruthenium through the N ends and the DMF molecule in Ru(1a)(DMF)Cl₃ binds through the O atom. In both the complexes, the metal atom has a slightly distorted octahedral environment; the distortion is mainly caused by the chelate bites [77.6(2)° in Ru(1a)₂Cl₂ and 78.0(4)° in Ru(1a)(DMF)Cl₃]. The chlorine atoms in Ru(1a)₂Cl₂ are mutually cis and those in Ru(1a)(DMF)Cl₃ span a meridian. The Ru–Cl and the Ru–N bond lengths in Ru(1a)(DMF)Cl₃ are somewhat shorter than those in Ru(1a)₂Cl₂, indicative of the differences between the radii of Ru(III) and Ru(II). The bidentate ligands are slightly distorted from planarity with deviations of individual atoms from the 5-membered ring planes being less than 0.15 Å; the two rings intersect at angles of 5.2 and 8.1° in Ru(1a)₂Cl₂ and 1.9° in Ru(1a)(DMF)Cl₃.

Fig. 1 X-Ray crystal structure of Ru(1a)₂Cl₂ with ellipsoids at 15% probability. Selected bond distances (Å) and angles (°): Ru1–N1A 2.098(5), Ru1–N1B 2.102(5), Ru1–N10A 2.091(5), Ru1–N10B 2.082(4), Ru1–Cl2 2.421(3), Ru1–Cl3 2.416(3), N1A–Ru1–N1B 178.2(2), N10A–Ru1–Cl3 172.2(1), N10B–Ru1–Cl3 88.0(2), N1A–Ru1–Cl3 94.8(2), N1B–Ru1–Cl3 85.8(1).

Fig. 2 X-Ray crystal structure of Ru(1a)(DMF)Cl₃ with ellipsoids at 15% probability. Selected bond distances (Å) and angles (°): Ru1–N1 2.063(10), Ru1–N10 2.061(9), Ru1–Cl2 2.367(4), Ru1–Cl3 2.330(4), Ru1–Cl4 2.353(4), Ru1–O31 2.109(8), N1–Ru1–Cl2 176.6(3), N10–Ru1–Cl2 98.6(3), Cl2–Ru1–Cl4 91.4(2), Cl2–Ru1–Cl3 91.3(2), Cl3–Ru1–Cl4 176.7(1), N1–Ru1–O31 94.9(4), N10–Ru1–O31 172.9(4), O31–Ru1–Cl2 88.5(3).

In the ¹H NMR spectrum of 1a in CDCl₃, the methyl protons appear as a singlet at 1.37 ppm and the methylene ones as another singlet at 4.13 ppm. The presence of a twofold axis of symmetry in 1a is reflected in its ¹³C NMR spectrum also, in which four types of C atoms are indicated [methyl C, 28.42; C(CH₃)₂, 68.80; methylene C, 80.10; imino C, 153.67 ppm with reference to TMS]. However, this symmetry is lost in Ru(1a)₂Cl₂, as revealed by its ¹³C NMR spectrum in CDCl₃ where 10 distinct resonances are observed (see Experimental), indicating that none of the ten carbon atoms in a particular ligand fragment in the complex are magnetically equivalent. The ¹H NMR spectrum of Ru(1a)₂Cl₂ is very rich and we have not yet been able to decipher it.

The electrochemical behaviour of Ru(1a)₂Cl₂ and Ru(1a)(DMF)Cl₃ has been examined by cyclic voltammetry at a glassy carbon electrode in acetonitrile under N₂ atmosphere. Ru(1a)₂Cl₂ shows (Fig. 3) a quasireversible Ru^III/II couple at 0.18 V vs. SCE (saturated calomel electrode). This Ru^III/II couple appears at −0.34 V vs. SCE in Ru(1a)(DMF)Cl₃ (Fig. 3). On the positive side of SCE, Ru(1a)(DMF)Cl₃ displays a quasireversible Ru^IV/III couple at 1.22 V vs. SCE. For comparison, we note that the Ru^III/II couple in cis-Ru(bpy)₂Cl₂ and cis-Ru(phen)₂Cl₂ appears^10,11 respectively at 0.31 and 0.33 V vs. SCE in acetonitrile at a platinum electrode.

Because of the very low potential of the Ru^III/II couple, Ru(1a)₂Cl₂ is easily oxidised by air. We have found that it is possible to isolate [Ru(1a)₂Cl₂]PF₆, the ruthenium(III) counterpart of Ru(1a)₂Cl₂, just by refluxing a methanolic solution of Ru(1a)₂Cl₂ in air and by subsequent addition of excess NH₄PF₆. It should be noted that an oxidising agent like chlorine is needed in the synthesis of [Ru(bpy)₂Cl₂]PF₆ from Ru(bpy)₂Cl₂.¹² In X-ray crystallography, [Ru(1a)₂Cl₂]PF₆ is found to consist of discrete [Ru(1a)₂Cl₂]⁺ cations and PF₆⁻ anions with both the cation (Fig. 4) and anion having crystallographic C_s symmetry. The chlorine atoms in [Ru(1a)₂Cl₂]⁺ are found to maintain the cis configuration. While the Ru–Cl bonds in [Ru(1a)₂Cl₂]PF₆ are significantly shorter than those in Ru(1a)₂Cl₂, the Ru–N bonds in the complexes Ru(1a)₂Cl₂ and [Ru(1a)₂Cl₂]PF₆ are comparable. The two 5-membered rings of 1a in [Ru(1a)₂Cl₂]PF₆ intersect at an angle of 5.6°. In cyclic voltammetry at a glassy carbon electrode in acetonitrile under N₂ atmosphere, the Ru^III/II couple in [Ru(1a)₂Cl₂]PF₆ is found to be lower by 40 mV in potential and much less reversible than that in Ru(1a)₂Cl₂ (Fig. 3). In this context, we mention that the Ru^III/II couple in [Ru(bpy)₂Cl₂]⁺ is reported¹³ to be lower by 20 mV in potential but somewhat more reversible than that in Ru(bpy)₂Cl₂.

Fig. 3 The cyclic voltammograms of Ru(1a)₂Cl₂ (○, c = 1.09 mM), Ru(1a)(DMF)Cl₃ (----, c = 1.38 mM) and [Ru(1a)₂Cl₂]PF₆ (––, c = 0.42 mM) in acetonitrile (0.1 M in tetraethylammonium perchlorate) at a glassy carbon electrode. Scan rate ν = 50 mV s⁻¹. Under the same experimental conditions, the ferrocene–ferrocenium couple appears at 0.35 V vs. SCE with a peak-to-peak separation of 80 mV at ν = 50 mV s⁻¹.

Fig. 4 X-Ray crystal structure of [Ru(1a)₂Cl₂]PF₆ with ellipsoids at 15% probability. Selected bond distances (Å) and angles (°): Ru1–N1 2.113(4), Ru1–N10 2.101(3), Ru1–Cl1 2.329(3), N1–Ru1–N10 77.2(2), N1–Ru1–N1 178.7(2), N10–Ru1–Cl1 89.1(1), 168.5(1).

In Scheme 1 we show how¹⁴ a number of mixed-ligand complexes of ruthenium(II) and ruthenium(III) containing 2,2′-bisoxazoline can be synthesised by replacing the chlorine atoms in Ru(1a)₂Cl₂ and the DMF molecule in Ru(1a)(dmf)Cl₃. Thus, we envisage a ruthenium chemistry of 2,2′-bisoxazoline that will be as rich as that of bipyridine.

Scheme 1

Experimental

Methods and materials

The ligand 1a (4,4,4′,4′-tetramethyl-2,2′-bisoxazoline) was synthesised by following a reported procedure.¹⁵ Microanalyses were performed on a Perkin–Elmer 2400II elemental analyser. Molar conductance was determined by a Systronics (India) direct reading conductivity meter (model 304). FTIR spectra (KBr disc) were recorded on a Nicolet Magna-IR spectrophotometer (Series II), UV-VIS spectra on a Shimadzu UV-160A spectrophotometer and NMR spectra (in CDCl₃) by a Brucker DPX300 spectrometer. Cyclic voltammetry was performed using an EG&G PARC electrochemical analysis system (model 250/5/0) in purified acetonitrile under a dry nitrogen atmosphere in conventional three-electrode configurations. A planar EG&G PARC G0229 glassy carbon milli electrode was used as the working electrode. Magnetic susceptibility was determined at room temperature by a PAR 155 vibrating sample magnetometer. The magnetometer was calibrated with Hg[Co(SCN)₄] and the susceptibility data were corrected for diamagnetism using Pascal's constants. In the calculation of the magnetic moments of the two ruthenium(III) complexes, Ru(1a)(DMF)Cl₃ and [Ru(1a)₂Cl₂]PF₆, we have ignored the temperature-independent paramagnetism (TIP) of the Ru³⁺ ion.

Syntheses

Ru(1a)₂Cl₂. RuCl₃·3H₂O (0.26 g, 1 mmol), 0.39 g (2 mmol) of 1a, 0.29 g (6.7 mmol) of LiCl and 0.03 g (0.5 mmol) of zinc dust were taken up in 3 ml of DMF. This was heated at ∼100 °C until a microcrystalline compound started appearing (almost 1 h was needed). The solution was then cooled to room temperature, 10 ml of acetone was added and the solution kept in the refrigerator overnight. The next day the violet crystalline compound that had appeared was filtered off, washed with 10 ml of water and dried in vacuo over fused CaCl₂. It was recrystallised from a dichloromethane–hexane mixture. Yield 0.15 g (25%). Anal. calcd for C₂₀H₃₂Cl₂N₄O₄Ru: C, 42.53; H, 5.72; N, 9.92; found: C, 42.65; H, 5.66; N, 10.01%. UV/VIS (CH₃CN) λ_max/nm (ε/dm³ mol⁻¹ cm⁻¹): 532 (8000), 464 (4300), 247 (6500), 220 (10,100). ¹³C NMR (300 MHz, CDCl₃, TMS): δ 15.23, 25.07, 27.22, 29.57 (four methyl C), 65.81, 71.44 (two alkyl quaternary C), 82.52, 83.88 (two methylene C), 155.10, 157.59 (two imino C).

Ru(1a)(DMF)Cl₃. The dark red filtrate obtained above after filtering Ru(1a)₂Cl₂ was kept in air for a week. The dark yellow crystalline compound that deposited was filtered off, washed with 10 ml of water and dried in vacuo over fused CaCl₂. Yield 0.20 g (40%). Anal. calcd for C₁₃H₂₃Cl₃N₃O₃Ru: C, 32.72; H, 4.86; N, 8.81; found: C, 32.64; H, 4.82; N, 8.91%. FTIR (KBr) ν/cm⁻¹: 1638vs (C[double bond, length half m-dash]O). μ/μ_B: 2.07 (at 300 K). UV/VIS (CH₃CN) λ_max/nm (ε/dm³ mol⁻¹ cm⁻¹): 235 (10,500), 324 (2250), 379 (5900).

[Ru(1a)₂Cl₂]PF₆. Ru(1a)₂Cl₂ (0.14 g, 0.25 mmol) was dissolved in 20 ml of methanol and refluxed for 48 h. The red reaction mixture was then cooled to room temperature and filtered. To the filtrate, 0.5 g of NH₄PF₆ dissolved in 3 ml of water was added dropwise with constant stirring and the solution left in air for 6 h. The yellow compound that precipitated was filtered off, washed with a few drops of cold water and dried in vacuo over fused CaCl₂. Yield 0.02 g (15%). Anal. calcd for C₂₀H₃₂Cl₂N₄O₄RuPF₆: C, 33.83; H, 4.55; N, 7.89; found: C, 33.72; H, 4.60; N, 7.84%. FTIR (KBr) ν/cm⁻¹: 842vs, 558s (PF₆). μ/μ_B: 2.05 (at 300 K). Λ_M (CH₃CN): 132 Ω⁻¹ cm² mol⁻¹ (1 : 1 electrolyte). UV/VIS (CH₃CN) λ_max/nm (ε/dm³ mol⁻¹ cm⁻¹): 464 (1850), 356 (5500), 241 (9900).

X-Ray crystallography

Single crystals of Ru(1a)₂Cl₂, Ru(1a)(DMF)Cl₃ and [Ru(1a)₂Cl₂]PF₆ were grown by direct diffusion of hexane into a dilute dichloromethane solution of the respective complexes. Data were collected with Mo-Kα radiation using the MARresearch Image Plate System. The crystals were positioned at 70 mm from the Image Plate. One hundred frames were measured at 2° intervals with a counting time of 2 min. Data analyses were carried out with the XDS program.¹⁶ The structures were solved using direct methods with the SHELXS-86 program.¹⁷ In all the structures the non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms bonded to carbon were included in geometric positions and given thermal parameters equivalent to 1.2 times those of the atom to which they were attached. The structures were refined on F² using SHELXL-93.¹⁸

Crystal data. Ru(1a)₂Cl₂: C₂₀H₃₂Cl₂N₄O₄Ru, M_w = 564.47, monoclinic, space group P2₁/a, a = 15.42(2), b = 9.832(14), c = 16.076(19) Å, β = 96.67(1)°, U = 2421(6) ų, Z = 4, D_c = 1.549 g cm⁻³, μ = 0.901 mm⁻¹, 6185 reflections collected, 3926 unique, R(int) = 0.031, final residuals: R1 = 0.0457 and wR2 = 0.1207 for observed data, R1 = 0.0857 and wR2 = 0.1429 for all data. Ru(1a)(DMF)Cl₃: C₁₃H₂₃Cl₃N₃O₃Ru, M_w = 476.76, orthorhombic, space group P2₁2₁2₁, a = 8.607(14), b = 14.48(2), c = 15.73(2) Å, U = 1960(5) ų, Z = 4, D_c = 1.615 g cm⁻³, μ = 1.223 mm⁻¹, 2046 unique reflections collected, final residuals: R1 = 0.0581 and wR2 = 0.1598 for observed data, R1 = 0.0994 and wR2 = 0.1835 for all data. [Ru(1a)₂Cl₂]PF₆: C₂₀H₃₂Cl₂N₄O₄RuPF₆, M_w = 709.44, orthorhombic, space group P22₁2₁, a = 8.545(12), b = 12.001(16), c = 14.002(20) Å, U = 1436(4) ų, Z = 4, D_c = 1.641 g cm⁻³, μ = 0.860 mm⁻¹, 5008 reflections collected, 2750 unique, R(int) = 0.027, final residuals: R1 = 0.0354 and wR2 = 0.0780 for observed data, R1 = 0.0479 and wR2 = 0.0832 for all data.

CCDC reference numbers 174511–174513. See http://www.rsc.org/suppdata/nj/b1/b109294b/ for crystallographic data in CIF or other electronic format.

Acknowledgements

M. G. B. D. thanks EPSRC and University of Reading for funds for the Image Plate System. D. D. thanks the Department of Science and Technology, New Delhi, India for financial support.

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