[(η⁵-pentamethylcyclopentadienyl)(3-fluoro-N-methylbenzylamine-к¹,N)dichlorido]iridium(III)

Abstract

A half-sandwich iridium(III) complex containing 3-fluoro-N-methylbenzylamine ligands has been obtained by reaction of one equivalent of [(η⁵-Cp*)IrCl₂]₂ (Cp* = pentamethylcyclopentadienyl) with two equivalent of 3-fluoro-N-methylbenzylamine in very good yield. The structure of this complex was confirmed by X-ray crystallography, ¹H-NMR, ¹³C-NMR spectroscopy, and elemental analysis.

1. Introduction

Organometallic half-sandwich iridium (Ir) complexes containing amines or imine ligands have received considerable attention in the field of catalytic chemistry [1,2,3,4,5] and medicinal chemistry [6,7,8,9], as these ligands can be readily modified with appropriate substituents. Most of these iridium complexes comprise cyclopentadienyl ligand, amine or imine chelating ligand, and a monodentate halide ligand. However, the Ir complexes bearing N-monodentate ligands are much less developed [10,11]. In the field of biology, the N-monodentate complexes exhibit a variety of properties that are different from those of the bidentate compounds. For example, the N-monodentate complexes can undergo double hydrolysis [12]. In this contribution, Ir complex containing secondary amine 3-fluoro-N-methylbenzylamine as N-monodentate ligand was prepared and characterized.

2. Results and Discussion

The title complex was synthesized according to the modified procedure of the reported literature [4]. As shown in Scheme 1, treating 3-fluoro-N-methylbenzylamine with 7.5 equiv of sodium acetate in dichloromethane at room temperature for 4 h, then adding [(η⁵-Cp*)IrCl₂]₂ (0.5 equiv) to the mixture at room temperature for 8 h resulted in the form of the title complex in high yield, up to 85.1%, without other side products. The addition of sodium acetate did not result in the C-H activation of aromatic ring. In addition, we found that the same product would be obtained in absence of sodium acetate. The product was characterized by ¹H-NMR spectroscopy (see Supplementary Materials, Figure S1), ¹³C-NMR spectroscopy (see Supplementary Materials, Figure S2), elemental analysis, and X-ray crystallography (see Supplementary Materials, Table S1).

Scheme 1. Synthesis of [(η⁵-Cp*)Ir(C₆H₄FCH₂NHCH₃)Cl₂].

In CDCl₃, the characteristic peak in the ¹H-NMR for product is at ca. δ 3.93 ppm, corresponding to the NH group. The benzylic CH₂ displays two signals, i.e., a doublet peak (δ 4.91 ppm) and a doublet of doublets (dd) peak (δ 3.48 ppm). As shown in Scheme 2, H_b-H_c is coupled to form doublet peak (J_Hb-Hc = 12.8 Hz). However, H_b-H_a and H_b-H_c are separately coupled to form doublet of doublets peak (J_Hb-Hc = 12.8 Hz; J_Ha-Hc = 11.9 Hz).

Scheme 2. The mode of H-H coupling for the benzylic CH₂ group.

The recrystallization of this compound in dichloromethane/diethyl ether solution at 289 K gave single crystals suitable for X-ray diffraction. The molecular structure of the product is shown in Figure 1. It is clear that only nitrogen atoms and iridium link, forming the title complex, and no C,N-chelating iridium complex was obtained. The title complex adopts piano-stool configuration, with Cp* acting as the seat and 3-fluoro-N-methylbenzylamine ligand and chloride groups as the legs. The crystal packing of the title complex is orthorhombic. The distance between iridium to the centroid of bound η⁵-cyclopentadienyl ligand is 1.7852 Å. The bond length of Ir-N1 is 2.164(6) Å. The angle of C1-N1-Ir1 and C2-N1-Ir1 are 116.8(7)° and 113.0(7)°, respectively. The Cp* group and the F atom attached to C₅ showed disorder. Only one form remains with Figure 1. It had been reported that a prerequisite for the occurrence of the cyclometallation reaction of the palladium complexes was that the nitrogen had to be trisubstituted by alkyl or aryl groups (tertiary amines) [13,14]. The rational explanation for this was that the steric bulk of the substituents would weaken the N-Pd bond to such an extent that the electrophilicity of Pd(II) would remain high enough to induce the substitution of a proton [13,14]. The formation of chelated iridium complexes through C-H activation displays a process similar to the above-mentioned palladium complexes. The cyclometallation reaction of iridium(III) complexes can occur when tertiary amines was employed [15]. As a result, it seems that the production of monoligated complexes in this system is ascribed to the small size of secondary amines compared to tertiary amines.

Figure 1. X-crystal structure of [(η⁵-Cp*)Ir(C₆H₄FCH₂NHCH₃)Cl₂] hydrogen atoms, except C-H, which have been omitted for clarity. Displacement ellipsoids are shown at the 50% probability level. (Ir1: orange; N1: blue; H1: light blue; F1: yellow; Cl1 and Cl2: green; C: gray). H atoms attached to carbon are omitted, as are the minor components of the Cp* and 3-fluorophenyl ring disorders.

3. Materials and Methods

3.1. General Methods and Physical Measurements

All other reagents were purchased from commercial sources and used without purification. ¹H-NMR spectra were captured in 5 mm NMR tubes at 298 K on Bruker DPX 500 (¹H = 500.13 MHz) spectrometers (Bruker, Karlsruhe, Germany) using TMS as an internal standard and CDCl₃ as solvent. ¹³C-NMR spectra were referenced to the residual solvent (CHCl₃, 77.16 ppm) for chloroform-d₁. Elemental analysis was performed by the Analytical Center of the University of Science and Technology of China. X-ray diffraction data were collected at 298(2) K on a Bruker Smart CCD area detector (Bruker, Karlsruhe, Germany) with graphite-monochromated MoKα radiation (λ = 0.71073 Å). The structures were solved by direct methods, and further refinement with full-matrix least-squares on F² was obtained with the SHELXL program package [16,17], using SHELXS (TREF) with additional light atoms found by Fourier methods.

3.2. Synthesis of [(η⁵-Cp*)Ir(C₆H₄FCH₂NHCH₃)Cl₂]

The Ir(III) dimer [(η⁵-Cp*)IrCl₂]₂ was prepared according to reported methods [18]. Complexes [(η⁵-Cp*)Ir(C₆H₄FCH₂NHCH₃)Cl₂] were synthesized according to the modified procedure in this work. Under a nitrogen atmosphere, a mixture solution of 3-fluoro-N-methylbenzylamine (0.12 mmol, 16.7 mg), NaOAc (0.9 mmol, 122.5 mg), and CH₂Cl₂ (20 mL) was stirred at temperature for 4 h, after which [(η⁵-Cp*)IrCl₂]₂ (0.06 mmol, 47.8 mg) was added and stirred 8 h. Filter and CH₂Cl₂ were removed under reduced pressure and recrystallized from dichloromethane/diethyl ether. Yield: 54.8 g 85.1%. ¹H-NMR (500.13 MHz, CDCl₃) δ 7.34 (dd, J = 13.8, 7.8 Hz, 1H), 7.16 (d, J = 7.5 Hz, 1H), 7.05 (dd, J = 21.9, 8.8 Hz, 2H), 4.93 (d, J_Hb-Hc = 12.8 Hz, 1H), 3.93 (s, 1H), 3.48 (dd, J_Hb-Hc = 12.8 Hz; J_Ha-Hc = 11.9 Hz, 1H), 2.74 (d, J = 6.1 Hz, 3H), 1.71 (s, 15H). ¹³C-NMR (125.8 MHz, CDCl₃) δ 162.81 (d, J¹_C-F = 247.8 Hz), 116.52 (d, J²_C-F = 21.3 Hz), 138.44 (d, J³_C-F = 6.8 Hz), 130.58 (d, J³_C-F = 8.2 Hz), 125.24 (d, J⁴_C-F = 2.8 Hz), 115.51 (d, J²_C-F = 21.1 Hz), 84.90 (s), 60.00 (s), 39.35 (s), 9.26 (s). Anal. Calcd. for C₁₈H₂₆Cl₂FIrN: C, 40.15; H, 4,87; N, 2.60; Found: C, 40.17; H, 4.85; N, 2.62.

Single crystal X-ray diffraction for C₁₈H₂₅Cl₂FIrN (M_r = 537.49): Orthorhombic, space group P2(1)2(1)2(1), a = 9.0825(18) Å, b = 12.552(3) Å, c = 17.516(4) Å, α = 90°, β = 90°, γ = 90°, V =1996.9(7) ų, Z = 4, T = 293(2) K, μ(MoKα) = 6.961 mm⁻¹, Dcalc = 0.001788 g/cm³, 11,650 reflections measured (−11 ≤ h ≤ 8, −15 ≤ k ≤ 14, −21 ≤ 1 ≤ 21), 3899 unique (Rint = 0.0602), which were used in all calculations. The final R1 was 0.0419(I > 2σ(I)) and ωR2 was 0.1028 (all data). The Cp* ring and the 3-fluorophenyl ring showed disorder over two positions. The site occupancies were refined to 0.696(17):0.304(17) for the Cp* ring and 0.80(2):0.20(2) for the 3-fluorophenyl ring. CCDC 1842677 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.