Elsevier

Polyhedron

Volume 197, 15 March 2021, 115025
Polyhedron

Cationic rhenium(I) complexes bearing a π-accepting pyridoannulated N-heterocyclic carbene ligand: Synthesis, photophysical, electrochemical and theoretical investigation

https://doi.org/10.1016/j.poly.2021.115025Get rights and content

Highlights

  • A novel series of cationic rhenium(I) complexes is described that bears a pyridoannulated N-heterocyclic carbene ligand;

  • The compounds display long-lived triplet ligand-centered red luminescence;

  • Electrochemical investigation shows one irreversible oxidation and reduction process;

  • Optical and electronic properties were further elucidated by computational investigation.

Abstract

A novel family of photoactive cationic tris-carbonyl rhenium complexes of general formula fac-[Re(CO)3(pyipy)(L)]PF6, where pyipy is the 2-(pyridyl)-imidazo[1,5-a]pyridyl-3-ylidene ligand and L = pyridine (3·PF6) or PPh3 (4·PF6), is herein presented. Compounds 3·PF6 and 4·PF6 are straightforwardly synthetized from the corresponding chloro-derivative fac-[ReCl(CO)3(pyipy)] via halogen abstraction with silver(I) salt, followed by coordination of the neutral monodentate L ligand. The target complexes were characterized by 1H and 13C NMR as well as FT-IR spectroscopy and high-resolution mass spectrometry (HR-MS). In addition, X-ray diffractometric analysis was carried out by solving the single-crystal structure of compound 3·PF6. For both complexes, dilute samples in CH3CN solution at room temperature display a structured photoluminescence profile in the red region that is ascribed to a long-lived excited state (τ = 19.3–30.0 μs in degassed condition) with mainly triplet ligand-centered (3LC) character. This assignment is further corroborated by the minor hypsochromic shift of the emission profile observed for samples in 2-MeTHF at 77 K glassy matrix, in agreement with previous studies on the corresponding neutral counterparts. Electrochemical characterization by cyclic voltammetry (CV) shows two irreversible processes, one in the positive and one in the negative bias, which are associated to the metal-NHC moiety and the chelating NHC ligand, respectively. Finally, both electronic and optical features are further studied by means of computational investigation at density functional theory (DFT) and time-dependent DFT (TD-DFT) level of theory, which supports the attribution of the electronic transitions involved.

Graphical abstract

Two novel cationic rhenium tris-carbonyl complexes bearing a pyridoannylated N-heterocyclic carbene have been prepared and characterized jointly by experimental and computational investigation. The compounds display long-lived red photoluminescence ascribed to an excited state with triplet ligand-centered character.

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Introduction

The photophysics and photochemistry of metal carbonyl complexes have been matter of intense investigation due to their interesting optical and redox features,[1] since early works of Wrighton,[2] Rillema,[3] and Demas[4]. A particularly appealing class of photoactive complexes is represented by the family of mononuclear Re(I) fac-triscarbonyl derivatives bearing N-donor heteroaromatic ligands with accessible π* orbitals featuring π-accepting properties. The interest in such compounds was mainly driven by their appealing application in photocatalysis[5] (notably for CO2 reduction and H2 production), imaging probes in bio-medicine,[6] and emitters in organic light emitting diodes (OLEDs).[7]

In this framework, derivatives of general formula [Re(N^N)(CO)3L]n+, where N^N is a bidentate N-heterocyclic ligand such as 2,2′-bipyridine, 1,10-phenanthroline and related scaffolds, and L is either an anionic monodentate ancillary ligand (L = halogen, cyanide, alkoxy, alkynyl, etc.; n = 0) or a neutral ligand such as pyridines, phosphines, isonitriles (with n = 1), are the most investigated ones by far. Several studies have shown that these compounds display broad and featureless photoluminescence in the green-to-red portion of the electromagnetic spectrum with excited state lifetime ranging from hundreds of nanoseconds to a few microseconds time scale. The nature of the emissive excited state may vary from purely triplet metal-to-ligand charge transfer (3MLCT) to ligand-to-ligand charge transfer (3LLCT) and up to (sizable) triplet ligand-centered (3LC) character depending on the nature and electronic properties of both the N^N and the ancillary ligand, with often a certain degree of mixing between the two states.[2], [3] Hence, a metal–ligand-to-ligand character is more often associated to the emissive triplet manifold (3MLLCT). Photoluminescence quantum yield (PLQY) values largely varies as well, being cationic rhenium(I) complexes typically more efficient than neutral counterparts with values almost one order of magnitude higher for the former.[4] Furthermore, neutral dinuclear species of general formula [Re2(μ-X)2(CO)6(μ-diaz)], where X  = halogen and diaz = 1,2-diazine type of ligands, have shown superior properties and efficient electroluminescence, as reported by some of us.[8]

Surprisingly, the vast majority of the phosphorescent derivatives containing the Re(CO)3 unit investigated so far features poly-pyridine type of ligands. On the other hand, limited attention has been devoted to investigate alternative ligands, in particular those containing stronger σ-donating moieties, such as N-heterocyclic carbenes (NHCs). This is in spite of the fact that NHC are outstanding ligands in organometallic chemistry,[9] and have demonstrated to play pivotal role in the preparation of efficient emitters with other transition metals,[10] including Ir(III),[11] Pt(II),[12] Au(I),[13] and Cu(I)[14].

In this respect, Che and co-workers firstly described a series of [Re(N^N)(CO)3(NHC)]X complexes featuring an NHC as the ancillary ligand.[15] It is only in 2011 that Massi and co-workers[16] described the first examples of photoactive Re(I) tris-carbonyl complexes bearing an NHC as the chromophoric ligand, namely [ReX(CO)3(C^N)] with X  = Cl-, Br- and C^N = 3-butyl-1-(2′-pyridyl)benzimidazolin-2-ylidene NHC ligand. The complexes displayed 3MLCT emission in the green-yellow region with PLQY ≤ 1%. In related derivatives, energetic stabilization of the π* orbitals located onto the heteroaromatic ligand, achieved through extension of the π-system and/or introduction of additional heteroatoms,[17] gave rise to a bathochromic shift of the 3MLCT emission wavelength, as expected. This shift was accompanied in some cases by a prolongation of the excited state lifetime up to τ = 1.07 μs and an increase of the PLQY, with values falling in the range 0.1–13%. On the other hand, employment of the 3-(pyrid-2-yl)dimethylthiazol-2-ylidene as the NHC ligand gave a yellow phosphorescent complex displaying PLQY = 4% and τ = 399 ns.[18] Concomitantly, Zheng and co-workers described related derivatives with either π-conjugated or methylene-bridged C^N ligands containing pyridyl-benzimidazolin-2-ylidene, pyridyl-imidazolin-2-ylidene and 2-pyridyl-1,2,4-triazoline-5-ylidene NHC scaffolds.[19] They found that the rupture of the π–conjugation between the π-accepting pyridyl group and the carbene moiety is detrimental for the emission properties of the final complex. Whereas, conjugated counterparts display photophysical properties that agrees with that earlier reported by Massi.[16], [17], [18]

Prompted by our interest in the photophysics of novel rhenium(I) carbonyl compounds and encouraged by our previous results,[20] we aim herein at investigating the effect of neutral vs. cationic nature of the complexes onto both optical and redox properties in this family of tris-carbonyl rhenium(I) complexes bearing the pyridyl pyridoannelated NHC ligand, namely [pyipy]PF6. The variation of the overall charge of the complex is known to have profound effects on the photophysical properties of diimine-based counterparts.[2], [3], [4] However, to the best of our knowledge, it has been overlooked for Re-NHC complexes. Herein, the synthesis, chemical, electrochemical and photophysical characterization of a series of [Re(CO)3(pyipy)(L)]PF6, where L = pyridine, PPh3, is presented along with the experimental data, which have been thoroughly rationalized also by means of computational approaches.

Section snippets

Synthesis and X-ray characterization

The synthetic pathway employed for the synthesis of the target cationic complexes 3·PF6 and 4·PF6 is displayed in Scheme 1. The tris-carbonyl rhenium halo-complex 12 from [pyipy]PF6 and [ReX(CO)5] (X = Cl, Br), previously described by us elsewhere,[20] represent the starting material to synthesize the novel Re complexes. De-halogenation procedure is carried out by treatment with an Ag(I) source and metathesis with a PF6- salt in a coordinating solvent, such as CH3CN, yielding the intermediate

Electrochemistry

The electrochemical behaviour of 3·PF6 and 4·PF6 was assessed by cyclic voltammetry (CV) in N,N-dimethylformamide (DMF)/0.1 M tetra-n-butylammonium perchlorate (TBAP) and showed the same pattern previously observed for neutral parental complexes 1 and 2.[20] In fact, as displayed in Fig. 2, compound 3·PF6 and 4·PF6 showed an irreversible oxidation process (O1) in the positive bias, which is commonly observed for Re carbonyl complexes, at EO1 = +1.340 V and +1.391 V vs SCE, respectively. In the

Photophysics

The photophysical properties of complexes 3·PF6 and 4·PF6 were investigated at concentration of 2 × 10-5 M in both air-equilibrated and degassed acetonitrile solution at room temperature as well as at 77 K in 2-MeTHF glassy matrix. The electronic absorption and the normalized emission spectra are displayed in Fig. 3 and the corresponding photophysical data are listed in Table 2. In the UV range, an intense absorption band in observed with λabs,max = 237 nm (ε = 2.61 × 104 M−1 cm−1) and λabs,max

Computational investigation

To support the interpretation of the photophysical and electrochemical data, the electronic structures of the cationic complexes 3+ and 4+ and their absorption and emission spectra were computed in acetonitrile employing density functional and time-dependent density functional theory along with the polarizable continuum model (PCM) of solvation (see experimental section). Molecular orbital plots and electron density difference maps (EDDM) computed for compound 3+ are displayed in Fig. 4 and

General considerations

[ReCl(CO)5] was purchased from Acros. The synthesis of complex 1 was carried out following a procedure reported elsewhere by us.[20] All procedures involving rhenium complexes were carried out under an argon atmosphere using standard Schlenk techniques. Silica gel for column chromatography was purchased from Sigma-Aldrich. Nuclear magnetic resonance spectra were recorded using a Bruker Avance III HD 500 spectrometer equipped with a N2 cryo-probe CPPBBO Prodigy at 298 K. 1H and 13C{1H} NMR

Conclusion

Two novel cationic tris-carbonyl rhenium complexes bearing the pyridoannulated NHC ligand pyipy is reported. For one of the derivatives, namely compound 3·PF6, atom connectivity was unambiguously ascertained by solving the single-crystal X-ray structure. Both compounds display long-lived emission with structured profile in the red portion of the electromagnetic spectrum. The photoluminescence is attributed to an exited state with mainly 3LC character as jointly supported by photophysical

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

A. B. and M. M. gratefully acknowledge the Université de Strasbourg and CNRS for financial support. The International Centre for Frontier Research in Chemistry (icFRC), and the Labex CSC (ANR-10-LABX-0026 CSC) within the Investissement d’Avenir program ANR-10-IDEX-0002-02 is also acknowledged for funding the PhD fellowship of A. B. M. M. kindly acknowledges the French Agence Nationale de Recherche (ANR) for the grant ANR-18-CE06-0007-01. The Institut Carnot MICA is kindly acknowledged for

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      Many light-induced molecular processes that involve Re(I) complexes implicate an ultrafast intersystem crossing (ISC) due to SOC [6,12,14]. In this way, Heydová, R. et al. [1] observed that the lowest excited state of [Re(X)(CO)3(bpy)] (where X are halide ligands) has a predominantly triplet character by ISC, but containing some contributions from the singlet emissive states depending on ligand; the photophysical properties and reactivity of these complexes can be also sensitive to the nature of X ligand via halogens or pyridine (py) derivatives [12,22]. Regarding the emission energies, through a simple modification of the ligands, the Re (I) complexes can modulate the emissive excited states to obtain different emission colors.

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    These authors contributed equally to the work.

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