Diiodido-bis{N-[2-(diphenylphosphino)benzylidene]benzylamine-κ²N,P}dicopper(I)

Abstract

The one-pot template reaction between 2-(diphenylphosphino)benzaldehyde, benzylamine and copper(I) iodide yields the dinuclear copper complex (P∩N)₂Cu₂I₂, as revealed by single-crystal X-ray diffraction.

1. Introduction

P∩N ligands are used in many areas of coordination chemistry due to their advantageous and tunable ligand properties. Many architectures are feasible. Besides the length of the bridge between the phosphorus and nitrogen atoms, the properties of the N-donor atom can vary: for example, the nitrogen atom can be part of a N heterocycle or it can be found as part of an amine, imine, or amide group. A huge number of variations in P∩N ligands have been realized, and their use covers practically all areas of coordination chemistry from basic synthetic and structural research to more applied fields like catalysis and optoelectronics [1,2,3,4,5,6,7,8]. Many P∩N ligands, e.g., Ph₂P(s-py) (with s-py = substituted 2-pyridyl-type moiety), are commercially available and, hence, broadly used.

An easy variation in P∩N ligand properties can be realized by imine formation, according to Scheme 1. Commercially available 2-(diphenylphosphino)benzaldehyde is a particularly attractive platform as it can give access to a wide variety of P∩N ligands. Imine formation is a very efficient process, which does not require demanding reaction procedures. In principle, all types of primary amines—aromatic or aliphatic—react in high yields under mild conditions [9,10,11,12,13,14,15,16,17,18,19,20,21]. In this context, of particular interest might be the use of oligo-amines, which lead to the formation of polydentate P∩N ligands [22,23], or the use of chiral amines [24].

Our interest in P∩N copper(I) complexes comes from the fact that many of them feature interesting luminescence properties. For example, Ph₂P(s-py)-type ligands form complexes of the type (P∩N)₃Cu₂X₂ (X = Cl, Br, I) with a 3:2 stoichiometry with one bridging bidentate P∩N ligand and two monodentate P∩N ligands bound via the P atoms [25]. The halides build the bridge between the two copper atoms. It should be noted that other stoichiometries are also known for Ph₂P(s-py)-type ligands [2,25,26,27]. These complexes often show extremely high emission quantum yield with comparable low emission lifetimes due to thermally activated delayed fluorescence, which makes them interesting candidates for OLED applications. It has been shown that OLEDs using those emitters are, indeed, promising alternatives to expensive emitter molecules based on iridium [1,28,29,30,31,32].

To investigate whether ligands bearing an imine group, as shown in Scheme 1, also form photophysically interesting complexes, the synthesis of a copper(I) iodide complex was undertaken with R = benzyl. The ligand with R = Bn is not new but has already been used for the complexation of Re(I) [33], Fe [34], Co [35], Pd [36,37,38,39,40], Rh [41], Ru [42], Au [9,43], Pt [9,44], Ir [45], and Cu [46].

2. Results and Discussion

The original idea was to synthesize a complex with a 3:2 stoichiometry, like those of the type (P∩N)₃Cu₂X₂ (vide supra). Therefore, the stoichiometry, as described in Section 3, follows this 3:2 relation.

The title compound was synthesized in a very simple one-pot reaction (Scheme 2). Such a template synthesis facilitates the procedure even more as it saves the isolation/purification step of the P∩N ligand. First, 2-(diphenylphosphino)benzaldehyde and benzylamine were stirred for 3 h under ambient conditions in dichloromethane [33]; then, solid copper(I) iodide was added. Immediately, a dark-orange solution was formed. After stirring overnight, the formed orange-red powder was isolated by decantation and dried in vacuo. Not untypical for copper halide complexes bearing phosphane ligands, the resulting compound was only very slightly soluble in standard, non-coordinating solvents. In donating solvents like hot pyridine and acetonitrile, the complex is sparingly soluble; however, it is doubtful that the complex also remains in its dimeric form in solution. Because of this low solubility in non-coordinating solvents, no NMR spectra were recorded.

The complex could be recrystallized from hot acetonitrile, yielding well-shaped red crystals. The single-crystal X-ray diffraction analysis revealed that instead of the intended 3:2 stoichiometry, a dinuclear complex with a 1:1 composition was formed, as shown in Figure 1. The copper atom is surrounded by a nitrogen atom, a phosphorus atom, and two iodine atoms in a distorted tetrahedral environment with the iodine atoms in a bridging binding mode. The central {Cu₂I₂} core is somewhat asymmetric with two similar but yet different Cu–I bond lengths of 2.579(1) Å (Cu1–I1) and 2.698(1) Å (Cu1–I1ⁱ). The nitrogen atoms are in a cis-configuration, rendering the whole complex inversion symmetric. The overall structure is very similar to a copper(I) iodide complex, bearing a closely related P∩N ligand [47]. The Cu1–N1 bond length (2.127(3) Å) is in the range typical for sp²-hybridized nitrogen atoms [2,25]. The Cu1–P1 bond length (2.227(1) Å) is in the expected range. The bite angle of the P∩N ligand is 89.79(7)°, and the Cu1–I1–Cu1ⁱ angle is 70.10(2)°. Cuprophilic interactions do not play a considerable role in the formation of the dimeric structure, as the Cu^…Cu separation is 3.032 Å and, thus, well beyond the sum of the van der Waals radii of 2.8 Å [48]. All lengths and angles are, thus, as expected for a complex with a {Cu₂I₂} structure [49,50,51].

In the ESI mass spectrum, the fragment [L₂Cu]⁺ (L = ligand) can be detected as the most intense signal. Other fragments could not be assigned with high confidence (cf. Figure S2). The IR spectrum features signals at 1628 and 1432 cm⁻¹, which is characteristic of the C=N stretch vibration and the P–phenyl group, respectively [12,52,53,54,55,56]. Contrary to many other related dinuclear complexes, 1 does not feature any observable photoluminescence at room temperature upon excitation with a UV light. This can be explained by an absence of low-lying π* orbitals, including the imine nitrogen atom, as the luminescence of such dinuclear copper(I) complexes is often based on a d→π* metal-to-ligand-transfer-excited state [56].

3. Experimental Methodology

3.1. General

All solvents and starting materials were commercially available and used without further purification. Elemental analyses were carried out by the Center for Chemical Analysis of the Faculty of Natural Sciences of the University Regensburg. The IR spectrum was recorded on a Bruker Alpha II spectrometer equipped with a Platinum ATR module. Single-crystal structure analysis was carried out on an STOE-IPDS diffractometer (STOE & Cie GmbH, Darmstadt, Germany) with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structure was solved by direct methods (SIR-97 [57]) and refined with olex2.refine [58] in Olex2 [59]. The H atoms were calculated geometrically, and a riding model was applied in the refinement process.

3.2. Synthesis of Diiodido-bis{N-[2-(diphenylphosphino)benzylidene]benzylamine-κ²N,P}dicopper(I), 1

2-(Diphenylphosphino)benzaldehyde (0.10 g, 0.34 mmol) and benzylamine (37 mg, 0.34 mmol) were dissolved in 30 mL of dichloromethane and stirred for 3 h. Copper(I) iodide (44 mg, 0.23 mmol) was added in one portion. The color of the solution turned immediately dark orange. The reaction mixture was stirred overnight. The orange-red powder formed was isolated by decanting and was dried in a vacuum. As soon as the complex precipitated from the reaction mixture, it was not sufficiently soluble in common weakly or non-coordinating solvents to perform NMR spectroscopy. Nicely shaped, red single crystals suitable for single-crystal X-ray diffraction were obtained by cooling a hot-filtrated solution of the title compound in acetonitrile.

Yield: 0.11 g, 84% based on CuI. Elemental analysis calc. for C₅₂H₄₄Cu₂I₂N₂P₂ (1139.79 g·mol⁻¹): C 54.79%; H 3.89%; N 2.46%; found: C 55.67%; H 3.96%; N 2.50%. MS(EI) (dcm/MeOH + 10 mM NH₄Ac): m/z 821.3 [L₂Cu]⁺. IR (ATR, cm⁻¹): 3034, 3000, 2883, 2843, 1628, 1477, 1432, 1092, 1024, 766, 744, 691, 513, 480, 430. The IR spectrum can be found in the Supplementary Materials.

Crystal data for C₅₂H₄₄Cu₂I₂N₂P₂ (M = 1139.73 g/mol): monoclinic, space group P2₁/n (no. 14), a = 15.916(3) Å, b = 9.226(1) Å, c = 15.939(3) Å, β = 95.94(2)°, V = 2328.0(7) ų, Z = 2, T = 297 K, μ(CuK_α) = 2.346 mm⁻¹, D_calc = 1.626 g/cm³, 24633 reflections measured (2.8° ≤ 2Θ ≤ 25.91°), 4505 unique (R_int = 0.042, R_sigma = 0.023), which were used in all calculations. The final R₁ was 0.0321 (I > 2σ(I)) and wR₂ was 0.0860 (all data). Further crystallographic details can be found in Table S1. CCDC 2334274 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 (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +44 1223 336033; E-mail: [email protected]).