[(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).

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).

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.

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.