A luminescent linear trinuclear magnesium complex assembled from a phosphorus-based tris-hydrazone ligand

Abstract

A novel linear trinuclear magnesium complex {P(S)[N(CH₃)N=CHC₆H₄-o-O]₃}₂Mg₃ was prepared by the reaction of P(S)[N(CH₃)N=CHC₆H₄-o-OH]₃ with MgCl₂·6H₂O in the presence of triethylamine. The trinuclear magnesium complex is fluorescent in solution as well as in the solid-state.

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Among the various biologically important metal ions, Mg²⁺ occupies a prominent position. The favorable charge–size ratio of Mg²⁺ coupled with the dramatic change in its ionic radius (0.65 Å) vis-à-vis its hydrated radius (4.76 Å) is believed to be responsible for the many crucial roles of Mg²⁺ ions in various biological processes.¹ These include not only catalytic processes such as phosphate ester hydrolysis but also a structural role in which the Mg²⁺ ion is important for stabilizing the conformations of biological molecules. Thus, it is believed that Mg²⁺ helps to fold RNA into specific structures and assists ribozyme activity.² In view of the above, a study of the coordination chemistry of Mg²⁺ in general and that of multi-nuclear magnesium complexes in particular, assumes considerable importance. We have recently designed a tris-hydrazone ligand, P(S)[N(CH₃)N=CHC₆H₄-o-OH]₃ (LH₃), built on a phosphorus support and have shown its efficacy towards divalent transition metal ions.³ Herein we report the synthesis, structure and spectroscopy of a novel linear trinuclear magnesium complex {P(S)[N(CH₃)N=CHC₆H₄-o-O]₃}₂Mg₃ (L₂Mg₃). Apart from its structural novelty, this compound shows luminescent properties both in solution as well as in the solid-state and is therefore of interest in view of the recent thrust in new organic light emitting diodes (OLEDs).⁴

The tris-hydrazone ligand LH₃ reacts with MgCl₂·6H₂O in a 2 ∶ 3 stoichiometric ratio in the presence of Et₃N to afford the linear trinuclear magnesium complex, L₂Mg₃. The ³¹P NMR chemical shift of this diamagnetic complex appears at 69.1 ppm, which is deshielded with respect to the ligand (³¹P NMR of LH₃: 71.7 ppm). L₂Mg₃ shows a prominent molecular ion peak (M⁺: 1088) in its FAB mass spectrum.

The molecular structure of L₂Mg₃ was determined by single-crystal X-ray crystallography and is shown in Fig. 1.‡ Two molecules of the tris-hydrazone ligand are completely deprotonated and provide a ligand environment with a total of six negative charges (O⁻). The two trianionic ligands cumulatively bind a linear array of three magnesium ions each of which is present in an oxidation state of +2. This generates a neutral trinuclear magnesium complex. The terminal magnesium ions in this complex (Mg_t) are encapsulated in a facial N₃O₃ coordination environment. The geometry around the terminal metal centers can be described as distorted octahedral. The central magnesium ion is also present in a distorted octahedral environment. However, its ligand environment is composed of only phenolic oxygen atoms (6O). The disposition of the aromatic groups in the metal complex L₂Mg₃ gives rise to a paddle wheel type of structural arrangement when the molecule is viewed through the inter-metal axis, Fig. 1(d). There is a noticeable change in the bond parameters of the complex when compared with the ligand. Thus, the N–N_(avg) bond distance increases from 1.388(5) Å (LH₃) to 1.437(4) Å in L₂Mg₃. The other metric parameters (average values) are as follows: Mg_t–N_(im) = 2.187(3) Å, Mg_t–O = 2.033(3) Å and Mg_c–O = 2.085(2) Å. The bond parameters are comparable to those of other Mg²⁺ complexes.⁵ The inter-metal distance (Mg_t–Mg_c) is 2.7828(13) Å. The perfect linearity of the trinuclear magnesium array can be gauged by the Mg_t–Mg_c–Mg_t angle, which is 180°.

Fig. 1 (a) The ligand LH₃. (b) The molecular structure of the complex L₂Mg₃. (c) Side view of the core of L₂Mg₃. (d) Paddle wheel type of arrangement of L₂Mg₃ viewed along the inter-metal axis.

A further investigation of the L₂Mg₃ complex shows weak intermolecular C–H⋯S=P and π–π interactions in the solid-state leading to supramolecular architectures. In comparison to other weak intermolecular hydrogen bonds, C–H⋯S=P contacts are rare and only three examples are reported thus far to the best of our knowledge.^3,6,7 The imino protons (CH=N) interact with the terminal P=S units leading to a double-bridged zig-zag one-dimensional polymeric chain (Fig. 2). In addition to these contacts, intermolecular π–π interactions between the aromatic six-membered rings are also observed which augment the one-dimensional polymeric chain⁸ (see ESI†).

Fig. 2 Intermolecular C–H⋯S=P interactions in the L₂Mg₃ complex. All the hydrogen atoms which are not involved in the hydrogen bonding have been omitted for clarity. The bond parameters for the C–H⋯S contact are C–H 0.94(1) Å, H⋯S 3.013(54) Å and C⋯S 3.909(65) Å and C–H⋯S 159.92(20)°. The symmetry is −x, 2−y, −z.

The optical behavior of L₂Mg₃ is quite interesting. Although the ligand LH₃ does not show any fluorescence, the complex shows emission behavior both in solution and in the solid-state. The emission spectra of L₂Mg₃ in solution (CH₂Cl₂; λ_ex = 354 nm) and in the solid-state (powder in a quartz tube; λ_ex = 325 nm) are shown in Figs. 3(a) and (b). In dichloromethane solution, an intense emission with a peak maximum at 442 nm is observed. In comparison, the solid-state emission peak occurs at 448 nm.

Fig. 3 Emission spectra of the L₂Mg₃ complex (a) in solution state (b) in solid-state (c) in thin film (d) in doped polymer.

The excited state emission spectrum of L₂Mg₃ obtained by different excitation wavelengths is similar to its UV–vis absorption spectrum with intense peaks around 275 and 365 nm, indicating that the emission is from the vertically excited state and that the molecular structure of the complex remains invariant in the excited state⁹ (see ESI†). The luminescent properties of L₂Mg₃ were investigated in its thin film as well as in a polymer matrix. Thus, a spin-coated thin-film sample of L₂Mg₃ was prepared from its dichloromethane solution and its emission spectrum was studied [Fig. 3(c)]. We also prepared doped polymer thin films from a solution of 90% polystyrene and 10% L₂Mg₃ complex in dichloromethane. The emission spectrum of such a polymer film is shown in Fig. 3(d). The luminescent properties of both the solid thin films and the polymer-doped thin films are nearly similar to that observed in solution and in the solid-state. It is observed from TGA that the L₂Mg₃ complex starts to decompose at 425 °C (see ESI†). This clearly indicates that the L₂Mg₃ complex is thermally quite stable.

The ability of L₂Mg₃ to be uniformly dispersed in a polymer medium and the retention of its luminescent properties in such a matrix augurs well for exploring its OLED applications. To the best of our knowledge, this represents the first time that luminescent behavior has been observed for multi-nuclear magnesium complexes. Hitherto, fluorescent probes have been utilized for detecting magnesium ions. In the present instance the ligand itself is non-fluorescing and it is only on the formation of the trinuclear complex that the fluorescence behavior is manifested. The tunability of the ligand in terms of electronic and steric properties should allow the assembly of other trinuclear complexes with varying emission properties.

We are thankful to the Council of Scientific and Industrial Research, New Delhi, India for financial support and also to Prof. Y. N. Mohapatra, Department of Physics and Samtel Research Center for Display Technologies, IIT Kanpur for fluorescence measurements in the solid-state.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: synthesis, UV–vis spectrum, excitation spectrum, TGA of the L₂Mg₃ complex. See http://www.rsc.org/suppdata/cc/b4/b414353a/

‡ Crystal data for L₂Mg₃: C₅₁H₅₁Cl₉Mg₃N₁₂O₆P₂S₂. M = 1446.08, triclinic, P1̄ (no. 2), a = 11.065(3), b = 11.168(3), c = 15.436(3) Å, α = 72.67(3), β = 83.03(3), γ = 61.40(2) °, V = 1598.3(6) Å, Z = 1, F(000) = 740, ρ = 1.502 g cm⁻³, λ = 0.71073 Å, T = 213(2) K, μ = 0.596 mm⁻¹, 10130 reflections collected, 4733 unique (R_int = 0.0374), R = 0.0532 [I > 2σ(I)], R_w = 0.0628 (all data). CCDC 251534. See http://www.rsc.org/suppdata/cc/b4/b414353a/ for crystallographic data in .cif or other electronic format.

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