Rhenium Tricarbonyl Complexes of Azodicarboxylate Ligands

The excellent π-accepting azodicarboxylic esters adcOR (R = Et, iPr, tBu, Bn (CH2-C6H5) and Ph) and the piperidinyl amide derivative adcpip were used as bridging chelate ligands in dinuclear Re(CO)3 complexes [{Re(CO)3Cl}2(µ-adcOR)] and [{Re(CO)3Cl}2(µ-adcpip)]. From the adcpip ligand the mononuclear derivatives [Re(CO)3Cl(adcpip)] and [Re(CO)3(PPh3)(µ-adcpip)]Cl were also obtained. Optimised geometries from density functional theory (DFT) calculations show syn and anti isomers for the dinuclear fac-Re(CO)3 complexes at slightly different energies but they were not distinguishable from experimental IR or UV–Vis absorption spectroscopy. The electrochemistry of the adc complexes showed reduction potentials slightly below 0.0 V vs. the ferrocene/ferrocenium couple. Attempts to generate the radicals [{Re(CO)3Cl}2(µ-adcOR)]•− failed as they are inherently unstable, losing very probably first the Cl− coligand and then rapidly cleaving one [Re(CO)3] fragment. Consequently, we found signals in EPR very probably due to mononuclear radical complexes [Re(CO)3(solv)(adc)]•. The underlying Cl−→solvent exchange was modelled for the mononuclear [Re(CO)3Cl(adcpip)] using DFT calculations and showed a markedly enhanced Re-Cl labilisation for the reduced compared with the neutral complex. Both the easy reduction with potentials ranging roughly from −0.2 to −0.1 V for the adc ligands and the low-energy NIR absorptions in the 700 to 850 nm range place the adc ligands with their lowest-lying π* orbital being localised on the azo function, amongst comparable bridging chelate N^N coordinating ligands with low-lying π* orbitals of central azo, tetrazine or pyrazine functions. Comparative (TD)DFT-calculations on the Re(CO)3Cl complexes of the adcpip ligand using the quite established basis set and functionals M06-2X/def2TZVP/LANL2DZ/CPCM(THF) and the more advanced TPSSh/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) for single-point calculations with BP86/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) optimised geometries showed a markedly better agreement of the latter with the experimental XRD, IR and UV–Vis absorption data.

Herein, we report on the reactions of five azo dicarboxylate ester ligands adcOR (R = Et, iPr, tBu, Ph, Bn, Scheme 1) and an amide derivative (adcpip, Scheme 3) with [Re(CO)5Cl] that we carried out in order to obtain mono-and dinuclear Re(CO)3Cl complexes (Scheme 3). For the sake of comparison, we also 2 ynthesized the phenylazocarboxylic ethyl ester (pacOEt) [54] model ligand which exclusively forms mononuclear complexes (Scheme 1, right). We geometry-optimised the structures using density functional theory (DFT). The fit with the experimental XRD and IR data was used for benchmarking of functionals and basis sets. We also studied the electrochemical properties, EPR spectroscopy and UV-Vis-NIR absorption spectroscopy and combined the experiments with DFT and TD-DFT calculations to probe for the electronic properties of the complexes. Comparison will be drawn between the Re(CO) 3 Cl complexes of these adc ligands with those of the similar bridging µ-κ 2 ,κ 2 chelate ligands with very low-lying π* orbitals apy, bptz, bpip and bpym (Scheme 2) [43][44][45][46][47][48][49][50][51][52][53].

Syntheses
When stirring the adc ligands with [Re(CO) 5 Cl] in toluene/CH 2 Cl 2 mixtures at ambient T, no reaction occurred. At higher T, starting from 60 • C, the reaction mixture turned reddish, indicating the formation of complexes. For the dinuclear complexes, the reaction mixtures then gradually turned dark and after a few hours dark-coloured reaction mixtures were obtained. After evaporation, the resulting dark materials were recrystallised from CH 2 Cl 2 /n-hexane and the resulting material extracted with toluene (for details, see Section 4).
This procedure allowed us to obtain dark green to black dinuclear complexes [{Re(CO) 3 Cl} 2 (µ-L)] from reactions using 1:2 (ligand:Re) ratios for the ligands containing low yields for R = Et (20%) and iPr (26%) complexes and a moderate yield of 54% for the pip derivative (Scheme 4). For R = tBu, Ph, Bn, the dark materials could not be analysed satisfactorily. The residual material from these reactions contained large amounts of the previously reported dinuclear complex [Re 2 (µ-Cl) 2 (CO) 8 ] [55][56][57][58] and non-defined organic material, presumably from thermal decompositions of the adc ligands. The formation of [Re 2 (µ-Cl) 2 (CO) 8 ] is not a dead-end as this dinuclear complex is also a reasonable Re(CO) 3 Cl precursor [55,56]. However, as [Re(CO) 5 Cl], it requires thermal activation.

Syntheses
When stirring the adc ligands with [Re(CO)5Cl] in toluene/CH2Cl2 mixtures a ambient T, no reaction occurred. At higher T, starting from 60 °C, the reaction mixture turned reddish, indicating the formation of complexes. For the dinuclear complexes, the reaction mixtures then gradually turned dark and after a few hours dark-coloured reaction mixtures were obtained. After evaporation, the resulting dark materials were recrystallised from CH2Cl2/n-hexane and the resulting material extracted with toluene (for details, see Experimental Section).
From this, we conclude that the adc-OR ligands with the bulky tBu, Ph and Bn substituents coordinated to slow due to their steric strain and decomposition of the ligands and their complexes is fast under the thermal activation conditions. For R = E and iPr and even more so the adcpip ligand, the complex formation is fast enough to compete with the decomposition. This means that this thermal synthesis method is on a razor's edge between the necessary thermal activation of the Re precursors and the thermal decomposition of the azo-dicarboxylates. We also tried microwave and sonochemical activation, but the results were similar in that sizeable amounts o [Re2(µ -Cl)2(CO)8] were formed and yields of the complexes remained low. When using 1:1 ligand to Re ratios, for R = Et or iPr the dinuclear complexe [{Re(CO)3Cl}2(µ -L)] were also obtained. In contrast to this, for the adcpip derivative, the mononuclear dark violet complex [Re(CO)3Cl(adcpip)] was obtained in 88% yield from such a reaction solution. Under the same condition, we also obtained the dark red [Re(CO)3Cl(pacOEt)] (Scheme 1) in 75% yield. From this, we conclude that the adc-OR ligands with the bulky tBu, Ph and Bn substituents coordinated to slow due to their steric strain and decomposition of the ligands and their complexes is fast under the thermal activation conditions. For R = Et and iPr and even more so the adcpip ligand, the complex formation is fast enough to compete with the decomposition. This means that this thermal synthesis method is on a razor's edge between the necessary thermal activation of the Re precursors and the thermal decomposition of the azo-dicarboxylates. We also tried microwave and sonochemical activation, but the results were similar in that sizeable amounts of [Re 2 (µ-Cl) 2 (CO) 8 ] were formed and yields of the complexes remained low.
When using 1:1 ligand to Re ratios, for R = Et or iPr the dinuclear complexes [{Re(CO) 3 Cl} 2 (µ-L)] were also obtained. In contrast to this, for the adcpip derivative, the mononuclear dark violet complex [Re(CO) 3 Cl(adcpip)] was obtained in 88% yield from such a reaction solution. Under the same condition, we also obtained the dark red [Re(CO) 3 Cl(pacOEt)] (Scheme 1) in 75% yield.
Based on previous experiences with the similar complex [{Re(CO) 3 Cl} 2 (µ-apy)] [47], we recorded EPR spectra during the reaction of [Re(CO) 5 Cl] and adcpip (2:1) and observed  Figure S2) in the assumed radical complex [{Re(CO) 3 Cl} 2 (adcpip)] •− . The observation of these radicals already in the reaction mixtures is in agreement with the very low-lying π* LUMOs of the ligand [22,23,[25][26][27][28][29] allowing reduction during the synthesis procedure (although the reductant is not clear). However, this seems to occur only to a small part of the material as we obtained the binuclear complexes in their neutral, diamagnetic form after recrystallisation of the reaction product.

Exchange Reactions for the Chlorido Coligand
When trying to exchange the chlorido coligands in [{Re(CO) 3 Cl} 2 (µ-adcpip)] through PPh 3 , we recorded an IR spectrum after a few minutes' reaction time, that clearly showed the three resonances for the [Re(CO) 3 ] fragment markedly shifted to lower energy and two bands in the range 1700 to 1800 cm −1 , indicating one coordinated and one uncoordinated C=O function of the adcpip ligand (Supplementary Figure S3A). The EPR spectrum of the reaction solution did not show the 11-line pattern typical for a dinuclear Re complex but a 9-line pattern representing presumably the mononuclear [Re(CO) 3  Based on previous experiences with the similar complex [{Re(CO)3Cl}2(µ -apy)] [47] we recorded EPR spectra during the reaction of [Re(CO)5Cl] and adcpip (2:1) and observed an 11-line EPR pattern in keeping with two Re centres of nuclear spin I = 5/2 ( 185,187 Re; Supplementary Figure S2) in the assumed radical complex [{Re(CO)3Cl}2(adcpip)] •− . The observation of these radicals already in the reaction mixtures is in agreement with the very low-lying π* LUMOs of the ligand [22,23,[25][26][27][28][29] allowing reduction during the synthesis procedure (although the reductant is not clear) However, this seems to occur only to a small part of the material as we obtained the binuclear complexes in their neutral, diamagnetic form after recrystallisation of the reaction product.

Exchange Reactions for the Chlorido Coligand
When trying to exchange the chlorido coligands in [{Re(CO)3Cl}2(µ -adcpip)] through PPh3, we recorded an IR spectrum after a few minutes' reaction time, that clearly showed the three resonances for the [Re(CO)3] fragment markedly shifted to lower energy and two bands in the range 1700 to 1800 cm −1 , indicating one coordinated and one uncoordinated C=O function of the adcpip ligand (Supplementary Figure S3A). The EPR spectrum of the reaction solution did not show the 11-line pattern typical for a dinuclea Re complex but a 9-line pattern representing presumably the mononuclea The IR spectra of the mononuclear and the dinuclear complexes are markedly different (Tables 1 and S4 and Figures S1 and S7). The expected three CO stretching vibrations for the mononuclear [Re(CO)3Cl(adcpip)] are markedly higher in energy than those for the dinuclear [{Re(CO)3Cl}2(µ -adcpip)]. This shift is in keeping with the enhanced π backbonding in the dinuclear complex [27,29,32,44,53]. For the mononuclea [Re(CO)3Cl(adcpip)], a further band at 1704 cm −1 represents the uncoordinated adcpip CO group. Generally, the IR spectra of the Re(CO)3Cl complexes agree with those o previously reported fac-[Re(CO)3X] (X = any ligand) complexes showing three CO stretching resonances [47,[49][50][51][52][53][59][60][61][62][63], the two at lower energy are sometimes merged into one broad band (Table 1). The IR spectra of the mononuclear and the dinuclear complexes are markedly different (Table 1 and Table S4 and Figures S1 and S7). The expected three CO stretching vibrations for the mononuclear [Re(CO) 3 Cl(adcpip)] are markedly higher in energy than those for the dinuclear [{Re(CO) 3 Cl} 2 (µ-adcpip)]. This shift is in keeping with the enhanced π backbonding in the dinuclear complex [27,29,32,44,53]. For the mononuclear [Re(CO) 3 Cl(adcpip)], a further band at 1704 cm −1 represents the uncoordinated adcpip CO group. Generally, the IR spectra of the Re(CO) 3 Cl complexes agree with those of previously reported fac-[Re(CO) 3 X] (X = any ligand) complexes showing three CO stretching resonances [47,[49][50][51][52][53][59][60][61][62][63], the two at lower energy are sometimes merged into one broad band (Table 1).
When comparing different bridging ligands in dinuclear Re(CO) 3 Cl complexes, the CO stretching energies of the adc complexes do not differ markedly from those of related NˆN bridging ligands, only the merging of bands II and III is more pronounced ( Table 1).
Within our study, we made comparative DFT calculations on the Re(CO) 3 Cl complexes of the adcpip ligand using the quite established basis set and functionals M06-2X/def2TZVP/LANL2DZ/CPCM(THF), and the more advanced TPSSh/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) for single-point calculations with BP86/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) optimised geometries (see later, Table 2 and Table S2) and IR spectra ( Figures S8 and S9, Table 1, Tables S3 and S4). The latter method gave markedly better agreement of calculated data with experimentally observed spectra. The CO stretching bands of the mononuclear [Re(CO) 3 Cl(adcpip)] ([Re]) complex are predicted at slightly lower energies than observed experimentally, with a systematic shift to lower wave numbers by about 30 cm −1 . In contrast, the M06-2X/def2TZVP/LANL2DZ/CPCM(THF) method predicts vastly higher energies above 2100 cm −1 for these vibrations. For the dinuclear complexes ([Re] 2 ), the situation is less simple, as a higher number of distinct vibrational modes were predicted, especially in the lower wavenumber range around 1900 cm −1 where one merged band is observed experimentally. While the assignment of individual predicted vibrations to the observed maxima is difficult, a slight redshift of all predicted modes compared to the experimental data is observed. Importantly, although we are sure that the sample contains both syn and anti isomers of [Re] 2 , the IR spectra gave no evidence for two species.  No NMR data of the binuclear complexes and [Re(CO) 3 Cl(pacOEt)] were obtained due to paramagnetic species impairing the measurements. From the mononuclear [Re(CO) 3 Cl (adcpip)], 1 H NMR spectra showed the protons in the 1,5 positions of the piperidyl groups split into six components in agreement with the nuclear spin of 185,187 Re of 5/2 ( Figure S5). The 13 C spectrum confirms the two non-equivalent piperidine groups ( Figure S6).

Structures from X-ray Diffraction and DFT Calculations
By diffusion of n-heptane into a dilute solution of the mononuclear complex [Re(CO) 3 Cl (adcpip)] in EtOAc, we obtained single crystals of suitable quality and were able to solve the crystal structure in the monoclinic space group P 2 1 /n (see Figure 1A, crystal data in Table S1, metrics in Tables 2 and S2). In contrast to this, crystallisation of the dinuclear adcpip compound failed.
No NMR data of the binuclear complexes and [Re(CO)3Cl(pacOEt)] were obtain due to paramagnetic species impairing the measurements. From the mononucl [Re(CO)3Cl(adcpip)], 1 H NMR spectra showed the protons in the 1,5 positions of piperidyl groups split into six components in agreement with the nuclear spin of 185,18 of 5/2 ( Figure S5). The 13 C spectrum confirms the two non-equivalent piperidine grou ( Figure S6).

Structures from X-ray Diffraction and DFT Calculations
By diffusion of n-heptane into a dilute solution of the mononuclear comp [Re(CO)3Cl(adcpip)] in EtOAc, we obtained single crystals of suitable quality and w able to solve the crystal structure in the monoclinic space group P 21/n (see Figure 1 crystal data in Table S1, metrics in Tables 2 and S2). In contrast to this, crystallisation the dinuclear adcpip compound failed. Thus, we embarked on quantum chemical calculations using density functional theory (DFT) to model the dinuclear structures for the adcpip derivatives using two different methods as mentioned above. The experimental molecular structure parameters of [Re(CO) 3 Cl(adcpip)] (Table 2), the IR data (Table 1) as well as the UV/Vis absorption spectra (to be presented later) allowed unequivocal benchmarking in favour of BP86 optimised geometries and frequency calculations including TPSSh single-point property calculations and triple zeta basis sets for all elements (Figure 1). We thus present in the following only the results from this type of calculation. The key data from both the calculations using BP86/def2-TZVP(+def2-ECP for Re)/CPCMC(THF) or TPSSh/def2-TZVP(+def2-ECP for Re)/CPCMC(THF), and M06-2X/def2TZVP/LANL2DZ/CPCM(THF) are listed in the Supplementary Table S3.

Both the mononuclear [Re(CO) 3 Cl(adcpip)] and the dinuclear [{Re(CO) 3 Cl} 2 (µ-adcpip)]
complex show the expected facial (fac) configuration of the three CO ligands (Figure 1). The geometry of the dinuclear [{Re(CO) 3 Cl} 2 (µ-adcpip)] is quite unsymmetric in both anti and syn forms. No centre of inversion was found for the anti isomer and the Cl-Re ... Re-Cl dihedral angles were 157 • (anti) and 42 • (syn), respectively. The calculated total energies say that the syn isomer is slightly more stable by 0.2 eV. The Re ... Re distance was calculated at 4.81 Å (anti) and 4.77 Å (syn), respectively.
The essential metrics of the [Re(CO) 3 Cl(NˆO)] entities around the Re centres (Table S3A)

Electrochemistry, EPR and DFT-Calculated Frontier Orbitals
Electrochemistry: Both mononuclear and dinuclear complexes show a first oneelectron reversible reduction and a second irreversible reduction (Figure 2, data in Table 3). The irreversible character has been identified as being due to rapid cleavage of chloride after electrochemical reduction in comparable [Re(CO) 3 Cl(NˆN)] (NˆN = diimines) derivatives [46][47][48][49][50]59,61,[64][65][66][67][68][69]. The dinuclear complexes exhibit very high reduction potential, for the adcpip complexes the potential of the mononuclear species lies lower by 0.3 V. For the uncoordinated adc ligands, we found two reduction processes at around −1 V and −1.9 V vs. the ferrocene/ferrocenium couple (Table S5). Assuming ligand-centred processes for these reductions, the coordination of Re(CO) 3 Cl causes massive anionic shifts, e.g., of more than 1 V upon coordination of one Re fragment to adcpip and more than 1.3 V when two Re fragments are coordinated.  Irreversible oxidation waves were observed for all complexes slightly above 1.1 V For the complex [{Re(CO) 3 Cl} 2 (µ-adcOEt)], we found evidence for a split-wave for the first reduction ( Figure S12) that we attributed to the syn and anti isomers based on the very similar behaviour of the complex [{Re(CO) 3 Cl} 2 (µ-apy)] (Table 3). For the other derivatives, we did not observe a splitting of the reduction waves. Interestingly, this phenomenon was first overseen for the Cl complex of the apy ligand [53], but was later found very pronounced for the F derivative and essentially absent for the Br and I congeners [44].
Irreversible oxidation waves were observed for all complexes slightly above 1.1 V and the dimeric [Re 2 (µ-Cl) 2 (CO) 8 ] was oxidised reversibly at 1.32 V by two electrons ( Figure S16).

UV-Vis-NIR Absorption Spectroscopy
The UV-Vis-NIR absorption spectra of the three dinuclear [{Re(CO) 3 Cl} 2 (µ-adc)] complexes all show quite intense long-wavelength bands in the range 720 to 850 nm (Figures 4 and S22, data in Table 4), which show pronounced negative solvatochromic behaviour (Table S7). The mononuclear [Re(CO) 3 Cl(µ-adcpip)] shows an intense band centred at 538 nm in keeping with the violet colour, while [Re(CO) 3 Cl(pacOEt)] shows a slightly red-shifted maximum at 502 nm. slightly red-shifted maximum at 502 nm.
This places the adc complexes at the "red end" of the series of dinuclear Re(CO)3Cl complexes with bridging diimine ligands ( Table 4). The long wavelength bands of the azopyridine (apy), the tetrazine bptz and the pyrazine (bpip) derivatives lie in the same range. Their optical band gaps are all below 2 eV and increase along the series adcOEt < adcOiPr < apy < bptz < adcpip, while the complexes of the established bpip and bpym lie markedly over 2 eV.    This places the adc complexes at the "red end" of the series of dinuclear Re(CO) 3 Cl complexes with bridging diimine ligands ( Table 4). The long wavelength bands of the azopyridine (apy), the tetrazine bptz and the pyrazine (bpip) derivatives lie in the same range. Their optical band gaps are all below 2 eV and increase along the series adcOEt < adcOiPr < apy < bptz < adcpip, while the complexes of the established bpip and bpym lie markedly over 2 eV.
The TD-DFT-calculated long-wavelength absorption bands of the dinuclear anti-[Re] 2 and syn-[Re] 2 were found at 585 and 754 nm (anti) and 642 nm (syn), respectively ( Figure 5, data in Tables S8 and S9). Combining the individual spectra of the two syn and anti isomers gives a broad band with maxima around 700 nm and extending beyond 1000 nm in the NIR range. This prediction is in good agreement with the experimentally observed data, which show a broad band in this region with the maximum at about 730 nm. A second intense band was calculated at around 420 nm, while the experimental spectrum shows this band at about 450 nm. Thus, the calculated spectrum is overall in very good agreement with the experimental data and systematically blue-shifted by only a small offset of~30 nm.
For the mononuclear [Re], the calculated long-wavelength band lies at 501 nm, while the experimental value was found at 538 nm, which is again a small blue-shift for the calculated data. Further absorption maxima are predicted at 317 and 379 nm, which are visible as shoulders in the experimental data. Overall, the calculated spectrum is also in very good qualitative agreement for [Re]. In addition, both calculated spectra show resolved bands in the UV range, while in the experimental spectra bands in the UV are not resolved and were merged into the solvent UV cutoff.
Trials to monitor the UV-Vis absorption spectra while changing the electrochemical potentials (spectroelectrochemistry) allowed us to confirm for [Re] 2 that we have indeed isolated the neutral complex and the spectrum in Figure 4 represents [Re] 2 and not the reduced form. Unfortunately, further efforts to generate the radical anionic complexes (mono-or dinuclear) failed and we observed rapid decomposition for all examples. two syn and anti isomers gives a broad band with maxima around 700 nm and extending beyond 1000 nm in the NIR range. This prediction is in good agreement with the experimentally observed data, which show a broad band in this region with the maximum at about 730 nm. A second intense band was calculated at around 420 nm, while the experimental spectrum shows this band at about 450 nm. Thus, the calculated spectrum is overall in very good agreement with the experimental data and systematically blue-shifted by only a small offset of ~30 nm. For the mononuclear [Re], the calculated long-wavelength band lies at 501 nm, while the experimental value was found at 538 nm, which is again a small blue-shift for the calculated data. Further absorption maxima are predicted at 317 and 379 nm, which are visible as shoulders in the experimental data. Overall, the calculated spectrum is also in very good qualitative agreement for [Re]. In addition, both calculated spectra show resolved bands in the UV range, while in the experimental spectra bands in the UV are not resolved and were merged into the solvent UV cutoff.
Trials to monitor the UV-Vis absorption spectra while changing the electrochemical potentials (spectroelectrochemistry) allowed us to confirm for [Re]2 that we have indeed isolated the neutral complex and the spectrum in Figure 4 represents [Re]2 and not the reduced form. Unfortunately, further efforts to generate the radical anionic complexes (mono-or dinuclear) failed and we observed rapid decomposition for all examples.
In order to obtain an idea why the reduced complexes are so labile, we embarked on calculating the optimised geometries of [ DFT potential energy surface scans along the Re-Cl vector ( Figure 6) reveal a markedly labilised Re-Cl bond in [Re(CO)3Cl(adcpip)] n upon one-electron reduction from n = 0 to n = −1.  Complex fragmentation along the Re-Cl bond is thus much more favourable in one-electron reduced anionic state. This redox-dependent weakening of the Re-Cl bo in [Re] •− and the dinuclear [Re]2 •− complexes is in line with the observed rapid liga exchange (Cl vs. N donors) observed for these radicals and the irreversible characte the first reduction in the CV. This is also in agreement with the frequent experimen observations of halide loss after reduction in [Re(CO)3X] complexes [44,[47][48][49][50]59,61,69].
We further assume that the cleavage of the Re-Cl bond and replacement by solv molecules obviously trigger the following loss of one [Re(CO)3(solv)] + fragment from reduced dinuclear complexes [Re]2 •− . Complex fragmentation along the Re-Cl bond is thus much more favourable in the one-electron reduced anionic state. This redox-dependent weakening of the Re-Cl bond in [Re] •− and the dinuclear [Re] 2 •− complexes is in line with the observed rapid ligand exchange (Cl vs. N donors) observed for these radicals and the irreversible character of the first reduction in the CV. This is also in agreement with the frequent experimental observations of halide loss after reduction in [Re(CO) 3 X] complexes [44,[47][48][49][50]59,61,[64][65][66][67][68][69].
We further assume that the cleavage of the Re-Cl bond and replacement by solvent molecules obviously trigger the following loss of one [Re(CO) 3 (solv)] + fragment from the reduced dinuclear complexes [Re] 2 •− .

Conclusions
Reactions of [Re(CO) 5 Cl] with the azodicarboxylic esters adcOR (R = Et, iPr, tBu, Bn (CH 2 -C 6 H 5 ) and Ph) and the piperidinyl amide derivative adcpip were attempted and led to successful isolation of dinuclear Re(CO) 3  The expected excellent π-accepting properties are mirrored in the easy reduction of the complexes with redox potentials slightly lower than 0 V vs. ferrocene/ferrocenium, which means that they are positive on the SCE and NHE scales. This is reflected in the observation that solutions of [{Re(CO) 3 Cl} 2 (µ-adcpip)] contain already measurable amounts of a dinuclear radical anionic complex as observed via EPR spectroscopy. The observed potentials agree quite well with the calculated HOMO-LUMO gaps for the adcpip complexes. As expected, the LUMO is centred on the adcpip ligand with large coefficients at the azo group, showing π* character. The low-lying π* orbitals are also responsible for the very long-wavelength transitions observed in the NIR range (700 to 1100 nm).
Trials to record EPR spectra of the reduced complexes failed due to their inherent instability. Only for the dinuclear adcpip complex did we observe an 11-line signal upon electrochemical or chemical reduction in agreement with ligand-based radical with two Re centres of nuclear spin I = 5/2 ( 185,187 Re). All other attempts led to six-line spectra in agreement with the loss of one Re(CO) 3 fragment from the dinuclear radical complexes. Assuming Cl-cleavage as the first (rapid) decomposition reaction after reduction, DFT calculations showed that Cl − cleavage is faster for the reduced complexes. DFT potential energy surface scans for the Re-Cl bond in [Re(CO) 3 Cl(adcpip)] and [Re(CO) 3 Cl(adcpip)] •− show a marked labilisation of the Re-Cl bond in line with this assumption.
In the context of possible applications in low-energy electron transfer materials in catalysis or low-energy absorbing materials in optoelectronics, the herein studied adcpip complexes [{Re(CO) 3 Cl} 2 (µ-adc)] are promising with regard to their properties, but not their stability. In future studies, we will thus re-consider the [{Re(CO) 3 Cl} 2 (apy)] complex and investigate its substitution stability and general stability under excitation and electrochemical reduction.
Syntheses of the dinuclear complexes [{Re(CO 3 )Cl} 2 (µ-adc)]-general description. First, 0.5 mmol of the adc ligands was heated with 1.1 mmol (398 mg) [Re(CO) 5 Cl] in a mixture of toluene and CH 2 Cl 2 (3:1 v:v) under an Ar atmosphere to 70 • C. After about 30 min, the product formation leads to dark blue-green solutions and after 6 h the reaction was stopped. Further reaction times lead to formation of increasing amounts of the side product [Re 2 (CO) 8 Cl 2 ]. After evaporation of the solvents, the dark black materials were re-crystallised from CH 2 Cl 2 /hexane (1:1) and the residue was extracted using toluene and the extracts dried in vacuo.
[{Re(CO) 3   of PPh 3 was added and the reaction mixture turned brown. EPR spectroscopy of this solution gave strong indication of a mononuclear radical complex. The mixture was stirred for another 30 min, and then all volatiles were evaporated. Dissolving the material in CH 2 Cl 2 and careful precipitation using about 5× the volume of n-hexane gave 26 mg of brown material. Elemental analyses for the assumed mononuclear [Re(CO) 3  Instrumentation: NMR spectra were recorded with a Bruker Avance II 300 MHz spectrometer ( 1 H: 300.13 MHz) equipped with a BBO ATM 5 mm probe head with zgradient (Bruker, Rheinhausen, Germany). Chemical shifts were relative to TMS. The spectral analyses were performed by the BrukerTopSpin 2 software. Elemental analyses were carried out using a Hekatech CHNS EuroEA 3000 Analyzer (Hekatech, Wegberg, Germany). The Cl contents of [(CO) 4 Re(µ-Cl 2 )Re(CO) 4 ] were obtained through dissolution using HNO 3 and aqueous determination of Cl − . IR spectra were measured in ATR mode using a Perkin Elmer 400 or a Thermo Avatar 370 DTGS FT-IR spectrometer (Perkin Elmer, Rodgau, Germany). UV-Vis absorption spectra were recorded on a Varian Cary50 Scan spectrophotometer (Varian, Darmstadt, Germany). EPR spectra were recorded in the Xband with a Bruker System ESP300 or ELEXSYS 500E, equipped with a Bruker Variable Temperature Unit ER 4131VT (Bruker, Rheinhausen, Germany). g values were calibrated using a dpph sample. Spectral simulation was performed using Bruker SimFonia V1. 26. Electrochemical experiments were carried out in 0.1 M n-Bu 4 NPF 6 solutions using a threeelectrode configuration (glassy carbon working electrode, Pt counter electrode, Ag/AgCl pseudo reference) and an EG&G Parc model 175 (EG&G, Gaithersburg, MD, USA), a Metrohm Autolab PGSTAT30 or a Metrohm µStat400 potentiostat (Metrohm, Filderstadt, Germany). Experiments were run at a scan rate of 100 mV/s at ambient temperature, the ferrocene/ferrocenium couple served as internal reference. UV-Vis-spectroelectrochemical measurements in 0.1 M nBu 4 NPF 6 /CH 2 Cl 2 solution were performed using an optically transparent thin-layer electrode (OTTLE) cell [73,74] at room temperature.
Single crystal X-ray diffraction: SC-XRD measurements were performed at 170(2) K, employing a Bruker D8 Venture diffractometer including a Bruker Photon 100 CMOS detector using Mo K α radiation (λ = 0.71073 Å). The crystal data were collected using APEX4 v2021.10-0 [92]. The structure was solved by dual space methods using SHELXT, and the refinement was carried out with SHELXL employing the full-matrix least-squares methods on F O 2 < 2σ(F O 2 ) as implemented in ShelXle [93][94][95]. The non-hydrogen atoms were refined with anisotropic displacement parameters without any constraints. The hydrogen atoms were included by using appropriate riding models. Data of the structure solutions and refinement of [Re(CO) 3 Cl(adcpip)] can be obtained free of charge at https://www.ccdc.cam. ac.uk/structures/ (accessed on 3 November 2022) under the deposition number 2194078, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge, CB2 1EZ UK (Fax: +44-1223-336-033 or e-mail: deposit@ccdc.cam.ac.uk).