Cationic rhenium(I) complexes bearing a π-accepting pyridoannulated N-heterocyclic carbene ligand: Synthesis, photophysical, electrochemical and theoretical investigation
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.
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 1–2 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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