A rhenium(I) complex which reacts with aldehydes and ketones in aqueous media: A structural,spectroscopic and theoretical analysis

Abstract

A Re(I) complex of 3,3′-diamino-2,2′-bipyridine reacts irreversibly with aldehydes and unhindered ketones in water to form bis-aminal cyclised derivatives with the solid-state structure of the reaction with benzaldehyde reported. This reaction produces a Re(I) complex which is significantly more emissive than the starting compound.

Introduction

In recent years, transition metal complexes have been utilised as sensors or probes for volatile organic compounds (VOCs). Such systems have generally been limited to solid state arrays of Pt(II),^1–10 Ag(I) or Au(I)^11–13 complexes where intermolecular metal–metal interactions form the basis of the signalling response, be it either colourimetric and/or emissive. Consequently, the reversible sorption of the VOC into the lattice of the complexes modulates the metal–metal interactions inducing a change in colour or luminescence output. The nature of some of these species has allowed thin films to be applied to optical fibres allowing transduction of the optical response of selected vapochromic materials.^14,15 The aim of this investigation was to access a responsive metal-based molecule which possesses reactivity to simple organic compounds in the solution state, preferably aqueous media. Herein, we report a Re(I) complex which differentially reacts with both aldehydes and unhindered ketones in aqueous media, which in the process dramatically modulates both the UV–Vis absorption properties and the luminescence characteristics.

Results and discussion

The rhenium-containing complex 1 was prepared by heating 3,3-diamino-2,2′-bipyridine ¹⁶ with [ReCl(CO)₅] in CHCl₃ to reflux and after filtration, pure 1 was obtained as a pale yellow powder. This complex reacts readily and irreversibly with acetaldehyde to give the corresponding cyclised species 2a, where the aldehyde has added across the diamine unit, with the loss of water, to give the seven-membered diaza macrocycle. While the complex 1 readily reacts with acetaldehyde, no reaction with either aromatic aldehydes or ketones was observed under the same conditions. However, reaction with 4-nitrobenzaldehyde, acetone and benzaldehyde occurs virtually instantaneously upon addition of a catalytic amount of acid, to give 2b, 2c and 2d, respectively, although even under these conditions no reaction with 4-bromoacetophenone was observed (Scheme 1).

Scheme 1.

Aminal derivatives of compound 1. 2a R = CH₃– and R′ = H; 2b R = 4-O₂NC₆H5– and R′ = H; 2c R = R′ = CH₃–; and 2d R = Ph and R′ = H.

The X-ray single crystal structure of the reaction product formed with benzaldehyde was obtained (Figure 1) (X-ray crystallography data for 2d has been deposited with the Cambridge Crystallographic data Centre CCDC 615248). As would be expected, the structure shows the 3,3′-diamino-2,2′-bipyridine unit coordinated to the rhenium centre via the bipyridine nitrogen atoms. The aldehyde has added across the 3,3′-diamino unit such that the benzaldehyde moiety lies in the equatorial plane of the bipyridine ring system.^17–19 Analysis of the isolated crystalline material by ¹H NMR shows double the amount of signals that would be expected for the complex 2d with one set of signals attributable to a minor component (~30%). Correspondingly, there are four possible isomers formed upon reaction with benzaldehyde (Figure 2) with the phenyl unit reacting to give the equatorial or axial isomers (Figure 2, i and ii). This is coupled with potential conformers of i and ii which are formed dependent upon the relative position of the axial chloride ligand (e.g. adopting a cis- or trans-oid conformation with respect to the axial chloride ligand iii and iv). The concentration of one of the isomers in the ¹H NMR of 2d is a product of selective crystallisation in the isolation process as in the ¹H NMR of 2b equal amounts of both isomers are observed. Heating a sample of 3b in (CD₃)₂SO (298–353 K) does result in altering the ¹H NMR spectrum, with both a change in the chemical shift and multiplicity of selected signals, but the spectrum still contains signals corresponding to two isomers. It seems only reasonable to assume that this change in the ¹H NMR upon heating is due to ring flipping of the animal ring and it will interconvert isomers i–iv and ii–iii. However, as two isomers are still present at this elevated temperature, it seems highly likely that the two isomers arise from the cis- or trans-oid conformation of the 4-nitrophenyl substituent with respect to the axial chloride. Cooling the sample back to room temperature returns the spectrum to its original chemical shift and multiplicity discounting any heat-induced chemical modification (e.g. displacement of the chloride ligands with the donor solvent). The ¹H NMR of 2c (acetone derivative) gives one set of signals for the 2,2′-bipyridine ligand and two signals for the aminal methyl units. As there are only two possible isomers (cis- or trans-oid isomers generated by the axial chloride ligand), a simplified spectrum is observed, with two methyl signals arising from cis- or trans-oid conformation with respect to the axial chloride ligand.

Figure 1.

Molecular structure of 2d (anisotropic displacement parameters are shown at a 50 % probability level). Selected bond lengths: Re(1)–C(1) 1.913(3), Re(1)–C(3) 1.919(3), Re(1)–C(2) 1.920(3), Re(1)–N(2) 2.148(2), Re(1)–N(1) 2.160(2) and Re(1)–Cl(1) 2.5048(6) Å.

Figure 2.

Structures of the four possible isomers of 2d.

The advantage of a fluorescent probe of the type of complex 1 is that the metal-to-ligand charge-transfer (MLCT) excited state is likely to be localised on the bipyridine ligand and consequently electronically influenced by alterations in the periphery of the ligand. Therefore, we anticipated that reactions of the 3,3′-diamino group with aldehydes and ketones would modulate the emission properties of the Re(I) complex.

Initially, luminescence measurements were conducted on the purified complexes. Although 1 itself is not highly soluble in water, it was encouraged into solution by dissolving in a drop of methanol and then diluting with water so that the resultant solution was >95% aqueous. Complex 1 was weakly emissive in aqueous solution, whereas the corresponding cyclised derivative (2c) was strongly fluorescent at 540 nm following excitation at 420 nm. Having assessed the photophysical properties of the purified complexes the in situ, in cuvette solution state responsiveness of 1 was assessed by both UV–Vis absorption and luminescence studies. The electronic spectrum of 1 showed a very weak absorption in the visible region which is likely to be associated with MLCT transitions. This band was not altered upon addition of acid and therefore precludes significant N-lone pair participation. A drop of acetone was then added to the cuvette resulting in the immediate formation of a yellow coloured solution, which deepened in colour over the duration of 5 min. This colourimetric response to acetone is shown in Figure 3, which compares the absorption profile before and after the addition of acetone. It is clear that the absorption band responsible for the visible colour change is the broad transition at ~450 nm, which is both more intense and red-shifted in comparison with 1. This dramatic change in optical properties is probably a consequence of the change of the torsion angle of the bipyridine ligand. In the diamine precursor 1, the two pyridine units adopt a non-planar conformation due to the unfavourable steric interactions between the 3,3′-diamino substituents. However, upon reaction with aldehydes and ketones, the ligand becomes more planar (NCCN torsion angle reduced to <1.5°) allowing a more conjugated π system and thus enhancing the energetic accessibility of a bipyridine π* orbital. UV–Vis absorption spectra were also obtained on 1 and the four compounds (2a–2d) in MeCN (10⁻⁵ M). First, complex 1 revealed a composite of bands between 250 and 450 nm, wherein intra-ligand-based transitions are expected to dominate at higher energies. In common with other rhenium(I) diimine complexes of this type, there is likely to be a spin-allowed MLCT-based transition in the lower energy region of the spectrum, which we tentatively assign to the weaker shoulder feature at 437 nm. Upon reaction with the various aldehyde and ketone substrates, the absorption spectra again show common features. Most notably, a bathochromic shift of a strong (ε ~ 10000) band to 411–424 nm, which may be indicative of a ligand-based absorption that is likely to be superimposed upon the expected ¹MLCT band.

Figure 3.

‘In cuvette’ spectroscopic analysis of a slight excess of acetone addition to 1, in the presence of acid. Left: UV–Vis absorption spectra. Right: Emission spectra (λ_ex = 420 nm). All spectra recorded in water (10⁻⁵ M).

Preliminary density functional theory (DFT) calculations are consistent with this. Geometry optimisations of the diamine precursor 1 and both cis- and trans-oid isomers of the cyclised derivative 2c show the expected reductions in py–py interannular dihedral angle on cyclisation and in each case the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) show significant metal (d_xz/yz) and ligand (π*) contributions, respectively (Figure 4). Furthermore, estimated vertical transition energies to the bipy-centred excited state undergo a red shift of ca. 50 nm on going from 1 to the cis- and trans-oid isomers of the cyclised derivative 2c.

Figure 4.

Surface plots of the (a) HOMO (−5.307 eV) and (b) LUMO (−2.377 eV) for trans-oid 2c.

Luminescence studies revealed a ‘switching on’ of an intense emission band at 540 nm (as observed for the purified cyclised complex 2c), following excitation λ_ex = 420 nm (Figure 3). The lifetime of this band was determined as <1 ns suggesting that there may be a significant LC contribution to the ³MLCT excited state or a quenching mechanism occurring such as electron transfer from the bis-aminal lone pairs.

Having established the ‘in cuvette’ reaction chemistry of 1 with acetone, a comparative study was conducted with acetaldehyde as the analyte. In this case, reaction appeared to be very rapid (addition resulting in an immediate colour change) and did not require the addition of acid to give the cyclised species 2a. As in the case of acetone, the luminescence studies showed a very large intensity increase in an emission peak at 540 nm, which is reasonable given the structural similarity to 2a. Consequently, we assessed several ketones and aldehydes of various structural forms. The luminescence studies were in agreement with the structural analysis and an electronically deactivated ketone such as 4-bromoacetophenone was found to be unreactive even in the presence of acid. Benzaldehyde was reactive in the presence of acid giving 2d, generating the typically yellow solution which was highly fluorescent at 525 nm

In summary, we have demonstrated how the spectroscopic properties of a Re(I) complex of 3,3′-diamino-2,2′-bipyridine can be modified by the addition of ketones and aldehydes in aqueous solution. Furthermore, the selectivity can be modulated by varying the pH and initial studies suggest that aldehydes of different structural form can be discriminated upon by analysis of the emission wavelength.

Experimental

All reactions were carried out in air and all chemicals and solvents were reagent grade and used without further purification.

Synthesis of 1

A suspension of 3,3′-diamino-2,2′-bipyridine (0.10 g, 0.54 mmol) and pentacarbonylchlororhenium(I) (0.19 g, 0.54 mmol) was allowed to reflux in chloroform (20 mL) for 8 h. After which time the solution was cooled and filtered to give (1) as a pale yellow solid (0.11 g, 42%). ^{17 1}H NMR (500 MHz, (CD₃)₂SO) δ_H 8.38 (dd, J = 5.0, 1.2 Hz, 2H, py), 7.58 (dd, J = 8.5, 1.2 Hz, 2H, py), 7.40 (dd, J = 8.5, 5.0 Hz, 2H, py), 6.19 (s, 4H, NH₂); 13C (240 MHz, (CD₃)₂SO) 199.10 (C=O), 191.99 (C=O), 144.0 (py, quaternary), 142.35 (py, CH), 139.96, (py, quaternary), 128.30 (py, CH), 125.81 (py CH); IR (ATR, νCO) 2017, 1910 and 1863 cm⁻¹; ESI-MS: m/z 457 [M–Cl]⁺.

Reaction of 1 with aldehydes and ketones

In a typical experiment, a small vial was charged with 1 (5 mg, 0.010 mmol) and CH₃CN (0.7 mL), to this was added both benzaldehyde (1 mg, 0.010 mmol) and a catalytic amount of (+/−)-10-camphorsulfonic acid (≈1.0 mg, 3.8 × 10⁻³ mmol) and the spectra recorded. Concentration and cooling of the CH₃CN solution gave 2d as a dark yellow solid which was isolated by filtration (3 mg, 51%). ¹H NMR (400 MHz, (CD₃)₂SO) δ 8.48 (dd, J = 5.0, 0.7 Hz, 2H)^a, 8.46 (dd, J = 5.0, 0.7 Hz, 2H)^b, 7.83 (d, J = 2.6 Hz, 2H, –NH)^a, 7.75 (d, J = 2.3 Hz, 2H, NH)^b, 7.71 (m, overlapping)^c, 7.57–7.45 (m, overlapping)^c, 7.30 (dd, J = 8.5, 5.1 Hz, 2H)^b, 7.27 (dd, J = 8.45, 5.1 Hz, 2H)^a, 5.27 (t, J = 2.3 Hz, 1H)^b, 5.16 (t, J = 2.6 Hz, 1H)^a. ^acorresponds to the minor isomer, ^bcorresponds to the major isomer (in both cases the integration has been normalised for each particular isomer). ^coverlapping peaks corresponding to both isomers; IR (ATR, υCO) 2014, 1911, 1856; ESI-MS: m/z 545 [M–Cl⁻].

2a (acetylaldeyde derivative). Isolated by column chromatography (silica, 10% MeOH in CH₂Cl₂), (3.7 mg, 71%). ¹H NMR (400 MHz, CD₃CN) δ 8.50 (dd, J = 5.1, 1.2 Hz, 2H), 8.49 (dd, J = 5.4, 1.2 Hz, 2H), 7.35 (dd, J = 8.4, 1.2 Hz, 2H), 7.36 (dd, J = 8.4, 1.2 Hz, 2H), 7.16 (dd, J = 8.4, 5.1 Hz, 2H), 7.12 (dd, J = 8.4, 5.1 Hz, 2H), 6.01 (br s, 4H), 4.39 (m, 1H), 4.33 (m, 1H), 1.56 (d, J = 6.2 Hz, 3H), 1.54 (d, J = 6.2 Hz, 3H). IR (ATR, υCO) 2015, 1884, 1847sh cm^–1. ESI-MS: m/z 541 [M + Na⁺].

2b (4-nitrobenzaldehyde derivative). Precipitated directly from CH₃CN solution as yellow crystals (5 mg, 80%); ¹H NMR (400 MHz, (CD₃)₂SO) δ 8.48 (m, overlapping 4H, py), 8.31 (d, J = 8.2 Hz, 4H, Ar), 8.01 (d, J = 3.2 Hz, 2H, NH), 7.95 (d, J = 3.1 Hz, 2H, NH), 7.79 (d, J = 8.9 Hz, 2H, Ar), 7.76 (d, J = 8.8 Hz, 2H, Ar), 7.67 (m, overlapping 4H, py), 7.33 (dd, J = 4.1, 1.6, 2H, py), 7.31 (dd, J = 4.0, 1.7 Hz, 2H, py), 5.55 (t, J = 3.1 Hz, 1H, (NH)₂PhCH), 5.47 (t, J = 3.2 Hz, 1H, (NH)₂PhCH); IR (ATR, υCO) 2026, 1920, 1858sh cm⁻¹. ESI-MS: m/z 589 [M–Cl⁻].

2c (acetone derivative). Isolated by column chromatography (silica, 10% MeOH in CH₂Cl₂), (4 mg, 75%). ¹H NMR (400 MHz, (CD₃)₂SO) δ 8.44 (dd, J = 5.0, 1.3 Hz, 2H), 7.54 (dd, J = 8.5, 1.3 Hz, 2H), 7.36 (s, 2H, NH), 7.29 (dd, J = 8.5, 5.0 Hz, 2H), 1.52 (s, 3H), 1.42 (s, 3H). 13C (240 MHz, (CD₃)₂SO) 199.16 (C=O), 192.10 (C=O), 145.63 (py, quaternary), 143.94 (py, CH), 140.91, (py, quaternary), 129.45 (py, CH), 125.45 (py CH), 65.91 (C(CH₃)₂), 28.59 (C(CH₃)₂), 27.08 (C(CH₃)₂); IR (ATR, υCO) 2013, 1891sh, 1875 cm⁻¹; ESI-MS: m/z 497 [M–Cl⁻].

Crystallographic data

Crystals of 2d suitable for analysis by X-ray analysis were obtained as follows: To a solution of 1 (≈3 mg) in MeCN (2 mL) was added an excess of benzaldehyde and a catalytic amount of (+/−)-10-camphorsulfonic acid. The reaction was then gently heated and the hot solution filtered through a plug of cotton wool. The reaction was then allowed to evaporate slowly in the dark. After 24 h, the solution had concentrated and small dark yellow crystals had formed. Crystal data for 2d C₂₀H₁₄ClN₄O₃Re: M = 580, monoclinic C2/c, a = 25.5636(6), b = 11.5038(3), c = 14.5909(3) Å, β = 119.5030(10), V = 3734.47(16) Å ³ , Z = 8, ρ(calc) = 2.063 Mg⁻³, F(000) = 2224, crystal dimensions 0.15 × 0.14 × 0.12 mm, μ(Mo-κ_α) = 6.682 mm⁻¹, T = 150(2) K. A total of 19,086 reflections were measured in the range 2.803 ⩽ θ ⩽ 33.187° (hkl range indices: −39 ⩽ h ⩽ 39, −14 ⩽ k ⩽ 17, −22 ⩽ l ⩽ 22), 7108 unique reflections (R_int = 0.0194). The structure was refined on F2 to R_w = 0.0512, R = 0.0257 (6526 reflections with I > 2σ(I)) and GOF = 1.152 on F2 for 262 refined parameters, largest difference peak and hole 2.188 and −1.824 eÅ⁻³.

Computational details

All DFT calculations were performed using the Jaguar programme package. The calculations were implemented at the B3LYP level of theory, ²⁰ with standard 6-31G* basis sets on all atoms except the Re centres, for which the Jaguar triple-ζ form of the standard Los Alamos ECP basis set (LACV3P*) was used. Solution-phase ground- and excited-state energies were obtained using restricted-open and unrestricted DFT, respectively. Solvent effects were simulated using the Poisson–Boltzmann continuum solvent model as implemented in the Jaguar programme. Vertical transition energies for the MLCT bands in complexes 1 and cis- and trans-oid 2c were estimated from the difference in (single-point) energy between the ground and first excited (triplet) states.

Supplemental Material

CHL917888_Supplementary_Information_CLN – Supplemental material for A rhenium(I) complex which reacts with aldehydes and ketones in aqueous media: A structural, spectroscopic and theoretical analysis

Supplemental material, CHL917888_Supplementary_Information_CLN for A rhenium(I) complex which reacts with aldehydes and ketones in aqueous media: A structural, spectroscopic and theoretical analysis by Lindsay P Harding, Asuka AT McRobbie, T Riis-Johannessen, Simon JA Pope, Craig R Rice, Donna Rollinson and Martina Whitehead in Journal of Chemical Research