(η⁵-Pentamethylcyclopentadienyl)iridium{1,3-bis(2,3,4,5,6-pentafluorobenzyl)imidazolin-2-ylidene}dichloride

Abstract

In this paper, we report (η⁵-pentamethylcyclopentadienyl)iridium{1,3-bis(2,3,4,5,6-pentafluorobenzyl)imidazolin-2-ylidene}dichloride, 2, synthesized using a silver transfer agent generated from 1,3-bis(2,3,4,5,6-pentafluorobenzyl)imidazolium bromide. Compound 2 was characterized using ¹H and ¹⁹F NMR spectroscopy and mass spectrometry, and its structure was determined in a single-crystal X-ray diffraction study.

1. Introduction

Group 9 metal complexes of N-heterocycle carbene (NHC) ligands bearing polyfluoroaryl substituents have provided a fertile area of research recently. Importantly, these complexes possess catalytic properties [1,2], including for base-free dynamic resolution in combination with an enzyme [3], and display unusual reactivity [4,5,6,7], including carbon–fluorine bond activation [8,9]. The structures of some of the complexes have been determined in single-crystal X-ray diffraction studies [5,6,7,8,9,10,11,12,13], but of these, most complexes contain a monofluorinated substituent, and only four bear a perfluorinated aryl substituent. The crystal structure of the dichloromethane solvate of the complex Cp*IrCl₂{κC-C₆H₅CH₂NC₃H₂NCH₂C₆F₅} (1) (Figure 1) [13] display a number of short intermolecular distances that suggest significant attractive interactions: π–π stacking between the phenyl ring of molecule of 1 and the pentafluorophenyl ring of another, hydrogen bonding between the hydrogen atoms of the imidazolin-2-ylidene ring of one molecule of 1 and the chloride ligands of another, hydrogen bonding between the hydrogen atoms of a dichloromethane molecule and the chloride ligands, and an interaction between one of the chlorine atoms of dichloromethane and the C₅ ring of the pentamthylcyclopentadienyl ligand of 1. Here, we report the analogous iridium complex of the NHC ligand bearing two pentafluorobenzyl substituents (2) (Figure 1).

2. Results

The title compound (2) was synthesized in two steps from 1,3-bis(2,3,4,5,6-pentafluorobenzyl)imidazolium bromide [14]. Treatment of this salt with silver oxide afforded the silver carbene complex [AgBr{κC-1,3-(C₆F₅CH₂)₂N₂C₃H₂}], which was added to (η⁵-pentamethylcyclopentadienyl)iridium dichloride dimer (Scheme 1). As expected, the ¹H NMR spectrum is simple, displaying just four resonances: two singlets arising from the methyl (δ 1.76) and imidazolin-2-ylidene ring hydrogen atoms (δ 6.52), and two mutually coupled doublets assigned to the diastereotopic methylene hydrogen atoms (δ 6.20 and 5.51). The ¹⁹F NMR spectrum is also simple, possessing three resonances assigned to the ortho (δ −140.39), meta (δ −159.64) and para (δ −150.91) fluorine atoms. The mass spectrum shows the cation on loss of chloride (see Supplementary Materials).

Compound 2 crystallized as a monosolvate from chloroform in the orthorhombic space group Pnam. The molecules of both 2 and chloroform lie on the crystallographic mirror plane (Figure 2). Selected distances and angles are given in

Table 1. Selected distances (Å) and angles (°) of 2.CHCl₃.

Distance/Å		Angle/°
Cp*—Ir	1.809(5)	Cp*—Ir—Cl(1)	123.2(2)
Ir—Cl(1)	2.4263(10)	Cp*—Ir—C(11)	132.9(2)
Ir—C(11)	2.049(6)	Cl(1)—Ir—Cl(1)	87.71(5)
		Cl(1)—Ir—C(11)	88.35(12)
Cp*···Cl(1S)	3.547(5)	Ir—Cp*···Cl(1S)	174.8(2)
C(6)···Cl(2S)	3.795(5)
C(7)···Cl(2S)	3.830(5)	Ir—Cl(1)···C(1S)	95.1(3)
Cl(1)···C(1S)	3.458(8)	Ir—Cl(1)···C(12)	103.9(1)
Cl(1)···C(12)	3.449(5)	Cl(1)···C(12)—N(1)	138.1(3)
		Cl(1)···C(12)—C(12)	107.1(3)

. The Cp*—Ir, Ir—Cl and Ir—C(NHC) distances and Cp*—Ir—Cl, Cp*—Ir—C and Cl—Ir—C(NHC) angles are consistent with those of 1.

The crystal structure comprises columns of alternating molecules of 2 and chloroform parallel to the b axis. There are short distances between the molecules of 2 and adjacent chloroform molecules evident in the crystal structure (Figure 3). One molecule of chloroform is positioned close to the pentamethyl cyclopentadienyl ring with the plane defined by the three chlorine atoms almost parallel (ca. 1.7°) to that of the C₅ ring. The Cl(1S)···Cp*, Cl(2S)···C(6) and Cl(2S)···C(6) distances (Table 1) are 0.1, 0.3 and 0.4 Å greater than the sum of the van der Waals radii of carbon and chlorine (3.45 Å) [15]. The other molecule of chloroform is positioned close to the chloride ligands. The C(1S)···Cl(1) distance suggests bifurcated hydrogen bonding between the molecules of chloroform and 2. A similar interaction involving one of the hydrogen atoms of the dichloromethane molecule is present in the crystal structure of 1.CH₂Cl₂.

The columns are linked parallel to the a axis by short distances between the chloride and imidazolin-2-ylidene ligands of molecules of 2 (Figure 4). The Cl(1)···C(12) distances are the same as the sum of the van der Waals radii of carbon and chlorine [15], which, together with the associated angles, strongly suggest two hydrogen bonds between the two molecules of 2. A similar interaction is evident in the crystal structure of 1.CH₂Cl₂. Thus, the chloride ligands of 1 and 2 are hydrogen bond acceptors to two species with hydrogen bonds that are mutually perpendicular and also perpendicular to the Ir—Cl(1) bond.

3. Materials and Methods

Silver(I) oxide (Aldrich, Milwaukee, WI, USA) was used as supplied. [(η⁵-C₅Me₅)IrCl(μ-Cl)]₂ [16] and 1,3-bis(2,3,4,5,6-pentafluorobenzyl)imidazolium bromide [14] were prepared as previously described. The mass spectrum was recorded on a Bruker Daltonics micrOTOF spectrometer (Bilerica, MA, USA). The ¹H and ¹⁹F NMR spectra were recorded using a Bruker DPX400 spectrometer (Bilerica, MA, USA). ¹H was referenced internally using the residual protio solvent resonance relative to SiMe₄ (δ 0), and ¹⁹F externally to CFCl₃ (δ 0). All chemical shifts are quoted in δ (ppm), using the high frequency positive convention, and coupling constants in Hz.

3.1. Synthesis of (η⁵-Pentamethylcyclopentadienyl)iridium{1,3-bis(2,3,4,5,6-pentafluorobenzyl)imidazolin-2-ylidene}dichloride (2)

A mixture of silver(I) oxide (0.16 g, 0.69 mmol) and 1,3-bis(2,3,4,5,6-pentafluorobenzyl)imidazolium bromide (0.50 g, 1.16 mmol) in dichloromethane (10 cm³) was stirred in the absence of light. After 6 h, the mixture was twice filtered through celite, followed by removal of the solvent from the filtrate by rotary evaporation to afford {1,3-bis(2,3,4,5,6-pentafluorobenzyl)imidazolin-2-ylidene}silver bromide as a black solid. A portion of this (0.25 g, 0.40 mmol) was added to [(η⁵-C₅Me₅)IrCl(μ-Cl)]₂ (0.17 g, 0.21 mmol) in dichloromethane (50 cm³) in the absence of light. After 16 h the reaction mixture was filtered through celite and the solvent removed under reduced pressure. Recrystallization from dichloromethane afforded the product as a yellow crystalline solid. Yield 0.26 g (79%). Anal. Calc. for C₂₇H₂₁Cl₂F₁₀IrN₂.CH₂Cl₂: C, 36.90; H, 2.54; N, 3.07. Found C, 36.39; H, 2.58; N, 3.24%. MS(ESI): [M − Cl]+ found m/z = 790.8945; [C₂₇H₂₁³⁵ClF₁₀IrN₂]⁺ requires m/z = 791.0936. ¹H NMR (400.14 MHz, CDCl₃): δ 6.52 (2H, s, HC=CH), 6.20 (2H, d, ²J_H–H′ = 15.5 Hz, CHH′), 5.51 (2H, d, ²J_H–H′ = 15.5 Hz, CHH′), 1.76 (15H, s, C₅(CH₃)₅). ¹⁹F NMR (376.46 MHz, CDCl₃): δ −140.39 (F_ortho, dd, ³J_F–F = 22.2 Hz, 8.0 Hz), −150.91 (F_para, t, ³J_F–F = 22.2 Hz), −159.64 (F_meta, dt, ³J_F–F = 22.2, 7.5 Hz) (A, M and X components of an AA′MXX′ spin system).

3.2. Single-Crystal XRD Determination

Crystals of 2 suitable for single-crystal X-ray diffraction were grown from chloroform. Diffraction data of a single crystal with dimensions 0.19 mm × 0.06 mm × 0.02 mm were collected at 100.0(1) K on an Agilent SuperNova (Santa Clara, CA, USA), single source at offset, Atlas diffractometer using with graphite-monochromated Cu—Kα radiation (λ = 1.54184 Å). The index ranges are: –18 ≤ h ≤ 15, –11 ≤ k ≤ 12, –25 ≤ l ≤ 23. A total of 9716 reflections were collected in the range θ 4.34° to 73.97°. There are 3239 independent reflections (R_int = 0.0386, R_sigma = 0.0372), of which 2825 are observable reflections with I > 2σ(I). Using Olex2 [17], the structure was solved with the Olex2.solve [18] structure programme using Charge Flipping and refined with the Olex2.refine [18] refinement package using Gauss–Newton minimization. The non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atom positions were added in idealized positions and a riding model with fixed thermal parameters (Uij = 1.2Ueq for the atom to which they are bonded (1.5 for CH₃)) was used for subsequent refinements. The function minimized was Σ[w (|F_o|2 − |F_c|2)]. Crystal Data for C₂₈H₂₂Cl₅F₁₀IrN₂ (M = 945.96 g/mol): orthorhombic, space group Pnam (no. 62), a = 15.1950(3) Å, b = 10.4051(3) Å, c = 20.3737(5) Å, V = 3221.22(12) Å3, Z = 4, μ(CuKα) = 12.599 mm⁻¹, D_calc = 1.950 g/cm³. The final R₁ was 0.0338 (I > 2σ(I)) and wR₂ was 0.0873 (all data).