Aryl, bi-functionalised imidazo[4,5-f ]-1,10-phenanthroline ligands and their luminescent rhenium(I) complexes

.


Introduction
2,4,5-triphenylimidazole [1] (lophine) based chromophores are a very well-known class of molecule [2] which are of interest in several areas of application due to their fluorescence and chemiluminescence properties [3].Various synthetic approaches have been described [4] and the fluorescent properties of these species can be altered via the addition of different groups to the imidazole core [5].Because of the convenient syntheses that have been described for such species, it is also possible to deploy these interesting molecular systems in the context of ligands for coordination chemistry.For example, imidazo[4,5-f]-1,10phenanthroline derivatives, where the chelating unit is integrated into the 2,4,5-triarylimidazole core, continue to attract attention as they can be easily adapted for coordination chemistry [6], as well as a variety of other potential applications [7].
In addition to our own contributions [25], the reports of imidazo [4,5-f]-1,10-phenanthroline complexes with Re(I) are far less extensive, but examples have been described [26].These include recent work on aggregation-induced emission using 2-(2-thienyl)imidazo[4,5-f]-1,10phenanthroline species [27], and complexes for use in colourimetric and fluorimetric based sensors [28].In this current work we describe the synthesis and investigation of a range of new imidazo[4,5-f]-1,10-phenanthroline ligands which incorporate two different R groups (one on the apical site and one linked to the nitrogen in the imidazole-like ring) giving multi-functionality.The possibility of introducing bulky aryl groups such as trityl, azodye substitutents and a pendant terpyridine unit into a phosphorescent complex were investigated.These different types of ligand substituent were of interest because of the potential future work that could be envisaged, including highly lipophilic complexes for bioimaging studies, and building blocks for multimetallic photoactive assemblies.
The characterisation of compounds 1-5 was initially investigated using 1 H and 13 C{ 1 H} NMR spectroscopies.The incorporation of different aromatic substituents and the unsymmetrical nature of 1-5 generally led to detailed 1 H NMR spectra, particularly in the aromatic region.However, characteristic peaks were often identified allowing confirmation of the proposed structures.For example, the 1 H NMR spectrum of 1 displayed a singlet at 1.27 ppm consistent with the butyl moiety, and two furthest downfield doublets (at 9.22 and 9.18 ppm) consistent with the protons at the 2-and 9-positions of the phenanthroline group; this latter pattern was typically observed for 1-5.The 1 H NMR spectra of 2 showed two singlets at 3.78 and 3.83 ppm consistent with the inequivalent methoxy groups of the apical substituent.revealed a singlet at 2.43 (6H integral) suggesting free rotation of the ring, while the 13 C{ 1 H} NMR spectrum clearly identified the C--O resonance at 164.9 ppm consistent with retention of the ester functionality within the ligand structure (Fig. S1, ESI).1-5 were further characterised using HRMS, IR and UV-vis.absorption spectroscopies (see later discussion).Full characterisation details are presented in the Experimental section.
For example, comparison of the spectra of 5 and fac-[ReBr(CO) 3 ( 5)] showed approximately + 0.2 ppm shifts of the phenanthroline protons (particularly clear for resonances > 9 ppm) together with smaller shifts for the other aromatic protons (see Fig. 1).Interestingly, while the 1 H NMR spectrum of ligand 5 revealed equivalent methyl groups (at 2.43 ppm) associated with the dimethylaniline group (implying free rotation of the substituent), the corresponding spectrum for fac-[ReBr(CO) 3 (5)] suggested a subtle inequivalence imparted by complexation with Re(I).Two singlets were observed at 2.48 and 2.50 ppm, both of which are slightly downfield from the corresponding resonance in 5 and therefore implies an inhibition of rotation of that substituent once the ligand is coordinated.Characteristic aliphatic resonances were also indicative of the successful formation of fac-[ReBr(CO) 3 (2)], where the two methoxy resonances showed a subtle downfield shift from the free ligand to 3.71 and 3.77 ppm.In most cases, and where solubility allowed, 13 C{ 1 H} NMR spectra were used to identify three downfield resonances ca.189-197 ppm, which correspond to the coordinated carbonyl ligands.In the case of fac-[ReBr(CO) 3 (5)] these downfield resonances were supplemented by the ester carbonyl of the ligand at 164.9 ppm.
The 1 H NMR spectrum of the terpyridine adorned fac-[ReBr (CO) 3 (4)] could only be acquired in d6-DMSO due to limited solubility in other common NMR solvents.Comparison of the spectrum with that of the free ligand (4) showed significant shifts in some of the peaks which indicated coordination of the rhenium centre.Hypothetically, there are two sites of coordination: the 1,10-phenanthroline or the 2,2 ′ :6 ′ ,2 ′ '-terpyridine unit (previous studies have shown that under relatively mild conditions 2,2 ′ :6 ′ ,2 ′ '-terpyridine species are likely to coordinate to Re(I) in a bidentate fashion) [29].For fac-[ReBr(CO) 3 (4)], the NMR data showed, through comparison with the other complexes, that the furthest downfield shifts were those associated with the 1,10phenanthroline moiety, and therefore suggestive as the most likely site of complexation (rather than the terpyridine terminus).Finally, the presence of only one signal for the aliphatic tert-butyl group was consistent with the formation of a single complex species with one coordination mode.
Solid-state IR spectroscopic studies were also undertaken on the complexes to reveal either two or three ν(C--O) bands between and 1850 cm − 1 , the two lower frequency absorptions occasionally overlapping around 1920-1880 cm − 1 (e.g.Fig. 2).This is consistent with a local symmetry (with respect to the coordinated carbonyl ligands) that can be approximated to a C s point group, as expected [25] for the facially capped coordination geometry at Re(I).fac-[ReBr(CO) 3 ( 5)], which contains the benzoate group also displayed the characteristic peaks that were observed in the free ligand, with the C--O ester group visible at around 1700 cm − 1 (Fig. 2).High resolution mass spectrometry data was obtained for all complexes revealing the protonated parent ion (e.g.Fig. 3) or cationic sodium adduct in each case.

X-ray crystal structures of the complexes
Suitable crystals of two complexes, fac-[ReBr(CO) 3 ( 1)] and fac-[ReBr (CO) 3 ( 5)], were obtained by the slow diffusion of hexane into a toluene solution of the complex, allowing single crystal X-ray diffraction studies to be undertaken.The parameters associated with the data collection for both complexes are shown in Table S1, with selected bond length and angle data in the corresponding figure captions.fac-[ReBr(CO) 3 ( 5  crystallised as the toluene solvate.Both X-ray structures (Figs. 4 and 5) show the anticipated mode of coordination for the ligands, chelating trans to the two carbonyl ligands, with the third CO ligand and bromide donors occupying the axial positions at the Re atoms.All bond lengths and angles associated with the coordination sphere are typical for a [ReBr(CO) 3 (N^N)] type complex with a chelating diimine [25,30].For both structures, the steric congestion arising from the 1,2-diaryl substituted imidazole groups results in these groups mutually rotating away from planarity to the ligand core.The apical p t butylphenyl substituent of fac-[ReBr(CO) 3 ( 1)] deviates from planarity by a torsion angle of 35.3(2) • , whilst the proximal 4-substituted aryl group of the azobenzene sidearm has a torsion angle of 77.4(2) • and is observed in its lower energy E-configuration.Interestingly, the distal phenyl group of the azobenzene moiety is approximately co-planar with the ligand core.The structure of fac-[ReBr (CO) 3 (5)] is observed with the 4-benzoyloxyphenyl group displaying a torsion angle of 41.86 (12) • relative to the ligand core and the distal phenyl ring of the benzolyloxy moiety being approximately co-planar to the imidazo[4,5-f]-1,10-phenanthroline core.The R ′ group, 3,5-dimethylphenyl, displays a torsion angle of 71.69 (12) • .In the case of fac-[ReBr(CO) 3 (1)] no significant contacts were observed in the packing diagram whilst with fac-[ReBr(CO) 3 (5)] an approximately graphitic slipped π-stacking interaction (3.558(10) Å) was observed between adjacent 4-substituted aryl groups of the 4-benzoyloxyphenyl moieties.

UV-vis Absorption and luminescence spectroscopy
Firstly, chloroform solutions of the free ligand 1-5 were investigated using UV-vis.absorption spectroscopy.Generally, the electronic spectra of 1-5 showed a composite of bands in the UV region with large molar absorption coefficients (ε > 3 × 10 4 M − 1 cm − 1 ).These various bands are therefore likely to be associated with different allowed π → π* transitions within the aromatic components of the ligands.The spectra for the azo derivatives 1 and 2 show stronger absorption between 350 and 400  nm and a weaker shoulder at 400-450 nm may be attributed to the presence of a formally n → π* transition localised within the azobenzene moiety of these species (Fig. 6).
Ligands 1-5 were then investigated using steady state and timeresolved luminescence spectrocopy.Following excitation in the UV region, each of the ligands demonstrated emission in the blue part of the visible region at 415 (for ligand 1), 414 (2), 414 (3), 415 (4) and 403 nm (5).The spectral appearance of the emission bands often revealed a hint of vibronic coupling (e.g.Fig. 6) typical of a 1 (π-π*) excited state in a rigid chromophore.The invariance of the emission maxima sugggests that the imidazo[4,5-f]-1,10-phenanthroline core is primarily responsible for the emission properties, which is consistent with a lack of extended conjugation arising from the differing substituents.With associated emission lifetimes (λ ex = 295 nm) of 3.7, 3.6, 3.7, 3.6, and 2.6 ns, for 1-5 respectively, these results also indicate emission from a spin-allowed relaxation process (i.e.fluorescence) in each case which probably originates from a 1 (π-π*) excited state.
The UV-vis.absorption properties of the Re(I) complexes showed a range of bands in the UV and visible regions.Typically, the shorter wavelength features were associated with the expected intraligand transitions (mainly originating from π → π* transitions discussed earlier) that are affiliated with the different aromatic constituents within the ligand framework.For example, fac-[ReBr(CO) 3 ( 3)] has a relatively intense feature at 274 nm which can be attributed to the summative absorption of the trityl unit with the core of the ligand (Fig. 7).In the visible region a new, characteristically broad peak at 400-500 nm is apparent for the complexes (e.g.Figs.7 and 8) and is consistent with a metal-to-ligand charge transfer (M Re LCT) transition, in accordance with numerous previous studies on diimine complexes of Re(I) [31].The magnitude of the molar absorption coefficients (typically ~ 7000 M − 1 cm − 1 ) suggests a spin-allowed process (i.e. S 0 → S 1 ) associated with this M Re LCT transition.It is also possible that a spin forbidden transition ( 3 MLCT; S 0 → T 1 ) may contribute to the weaker, lower energy shoulder of this absorption band, as has been noted in other heavy metal complexes (for example, those based upon iridium) that possess significant spin orbit coupling [32].In two cases, fac-[ReBr(CO) 3 ( 1)] (red), fac-[ReBr(CO) 3 (2)], the ligand based azobenzene n → π* transition is also expected to overlap with the more intense 1 MLCT feature (see Fig. 7).The invariance in the energies of the 1 MLCT transitions suggests that the structural differences of the coordinated ligands (1)(2)(3)(4)(5) impart little influence upon the electronics of the complex in each case.

Table 1
Selected IR, UV-vis.and luminescence spectroscopic data for the Re(I) complexes.
Complex reasons for this observation are currently unclear, further investigations will consider subtle evolutions of the ligand structure to try to unravel the underpinning factors.Finally, Fig. 10 shows the comparison of the excitation spectrum of fac-[ReBr(CO) 3 ( 3)] with the absorption spectrum.The spectra show that the excitation between 350 and 450 nm, which includes both ligandcentred and the 1 MLCT band, is by far the most efficient means of generating the 3 MLCT emission.The shorter wavelength absorption feature, predominantly associated with the trityl moiety, is notably absent in the excitation profile.

Conclusions
In summary, a series of substituted imidazole[4,5-f]-1,10-phenanthroline derivatives, containing varying types of functionalised aryl substituent, including azo dye, trityl and terpyridine pendants, have been isolated and fully characterised.These ligands were demonstrated to be effective chelates for Re(I) to form complexes of the type fac-[ReBr (CO) 3 (N^N).The resultant complexes were isolated as air stable powders.Two of the Re(I) complexes were structurally characterised using X-ray crystallography and showed the expected coordination sphere features as well as integrity of the functionalised ligand forms.While each of the free ligands demonstrated fluorescence in the blue region, the corresponding Re(I) complexes displayed bathochromically shifted visible emission in the range 558-585 nm which is likely due to an emitting 3 M Re LCT state.
While this current work has shown the scope of functionalisation that can be introduced into imidazole[4,5-f]-1,10-phenanthroline type ligands, and their tolerance to coordination chemistry studies, further work will focus upon the potential uses and applications of such species.For example, the successful isolation of fac-[ReBr(CO) 3 ( 4)] invites further investigation as a luminescent building block for multimetallic assemblies by employing the pendant 2,2 ′ :6 ′ ,2 ′ '-terpyridine unit as a secondary (and bridging) binding site.A further area of investigation could focus upon the photochromic, and photoisomerisation, behaviour of the azo dye containing species, especially if one considers the integration of the photoactive complexes into host materials.In addition, previous work has shown that a wide range of luminescent Re(I) complexes have already been successfully investigated as biologically compatible, cell imaging agents [33].An ability to enhance the lipophilicity of such systems, as demonstrated in the current study, is important and the scope displayed here to add extremely bulky lipophilic groups such as trityl moieties could be useful in such a context.

Experimental
All reactions were performed with the use of vacuum line and Schlenk techniques.Reagents were commercial grade and were used without further purification unless otherwise stated.[ReBr(CO) 5 ] was prepared according to the literature procedure [34].1,[35] 4-(benzoyloxy)benzaldehyde [36], and 4-[2,2 ′ :6 ′ ,2 ′ '-terpyridin]-4 ′ -ylbenzamine [37] were prepared as previously reported.1 H and 13 C{ 1 H} NMR spectra were run on NMR-FT Bruker or 250 spectrometers and recorded in CDCl 3 .1 H and 13 C{ 1 H} NMR chemical shifts (δ) were determined relative to internal TMS and are given in ppm.Low-resolution mass spectra were obtained by the staff at Cardiff University.High-resolution mass spectra were carried out by at the EPSRC National Mass Spectrometry Service at Swansea University.UV-vis studies were performed on a Jasco V-570 spectrophotometer as CHCl 3 solutions (10 − 5 M). Potophysical data were obtained on a JobinYvon-Horiba Fluorolog spectrometer fitted with a JY TBX picosecond photodetection module as aerated CHCl 3 solutions.The pulsed source was a Nano-LED configured for 295 or 372 nm output operating at 1 MHz. Luinescence lifetime profiles were obtained using the JobinYvon-Horiba FluoroHub single photon counting module and the data fits yielded the lifetime values using the provided DAS6 deconvolution software.

Data collection and processing
X-ray diffraction datasets were measured on a Rigaku FRE + diffractometer equipped with VHF Varimax confocal mirrors and an AFC12 goniometer and HG Saturn 724 + detector [38] using Crystal Clear software for data collection CrysAlisPro software for data reduction [39].

Structure analysis and refinement
The structures were solved by dual methods using SHELXT 2018/2 [40] and refined on F o 2 by full-matrix least-squares refinements using SHELXL 2018/3 [41] within the OLEX2 suite [42].All non-hydrogen atoms were refined with anisotropic displacement parameters, and all hydrogen atoms were added at calculated positions and refined using a riding model with isotropic displacement parameters based on the equivalent isotropic displacement parameter (U eq ) of the parent atom.Geometric and thermal restraints were applied to disorder components to model appropriately atomic displacement parameters.

Scheme 2 .
Scheme 2. Structures of the isolated complexes.