Acceptor Properties and Redox Stability of Chelating

Article pubs.acs.org/IC

Gauging Donor/Acceptor Properties and Redox Stability of Chelating Click-Derived Triazoles and Triazolylidenes: A Case Study with Rhenium(I) Complexes Lisa Suntrup, Sinja Klenk, Johannes Klein, Sebastian Sobottka, and Biprajit Sarkar* Institut für Chemie und Biochemie, Anorganische Chemie, Freie Universität Berlin, Fabeckstraße 34−36, D-14195 Berlin, Germany S Supporting Information *

ABSTRACT: Bidentate ligands containing at least one triazole or triazolylidene (mesoionic carbene, MIC) unit are extremely popular in contemporary chemistry. One reason for their popularity is the similarities as well as differences in the donor/acceptor properties that these ligands display in comparison to their pyridine or other Nheterocyclic carbene counterparts. We present here seven rhenium(I) carbonyl complexes where the bidentate ligands contain combinations of pyridine/triazole/triazolylidene. These are the first examples of rhenium(I) complexes with bidentate 1,2,3-triazol-5-ylidene-containing ligands. All complexes were structurally characterized through 1H and 13C NMR spectroscopy as well as through single-crystal X-ray diffraction. A combination of structural data, redox potentials from cyclic voltammetry, and IR data related to the CO coligands are used to gauge the donor/acceptor properties of these chelating ligands. Additionally, a combination of UV−vis−near-IR/IR/electron paramagnetic resonance spectroelectrochemistry and density functional theory calculations are used to address questions related to the electronic structures of the complexes in various redox states, their redox stability, and the understanding of chemical reactivity following electron transfer in these systems. The results show that donor/acceptor properties in these bidentate ligands are sometimes, but not always, additive with respect to the individual components. Additionally, these results point to the fact that MIC-containing ligands confer remarkable redox stability to their fac-Re(CO)3-containing metal complexes. These findings will probably be useful for fields such as homogeneous- and electro-catalysis, photochemistry, and electrochemistry, where facRe(CO)3 complexes of triazoles/triazolylidenes are likely to find use.

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INTRODUCTION The copper-catalyzed azide−alkyne cycloaddition reaction (click reaction), 1 which selectively delivers only one regioisomer of the formed triazoles, is now widely used in several branches of chemistry.2 The same reaction can be used for generating chelating triazole-containing ligands, the coordination chemistry of which has been explored in recent years.3 Additionally, such ligands can be easily converted to the corresponding triazolylidenes, which are arguably the most prominent class of the so-called mesoionic carbene (MIC) ligands.4 Such chelating MIC ligands are now well established in organometallic chemistry.4a−e These chelating MIC ligands are related to their 1,3,4-triazol-2-ylidene counterparts.4f,g However, the chelating 1,3,4-triazol-2-ylidene ligands are known to be much poorer donors.4e Metal complexes of both chelating triazoles and MICs have displayed intruiging properties in homogeneous catalysis,5 photochemistry,6 electrocatalysis,7 and electron-transfer research.8 One argument that is often used for the popularity of such ligands is their exceptional donor and/or acceptor properties.3,4 Thus, a monodentate 1,2,3-triazole is considered to be a poorer σ donor compared to both the corresponding MIC and the famous pyridine, even though the σ-donor strength of the triazole and pyridine is not © 2017 American Chemical Society

very different. On the other hand, a pyridine is considered to be a better π acceptor than a 1,2,3-triazole. We are not aware of any direct comparison between the π-acceptor properties of monodentate pyridine, 1,2,3-triazole, and the corresponding MIC. Since all three classes of the above-mentioned donors are now widely used as part of chelating ligands,5−8 the obvious question that arises is the additive nature (or not) of these trends in the donor/acceptor properties. This question is far from trivial because steric effects, chelate bites, and so on are also known to affect the donor/acceptor strengths of chelating ligands. Rhenium(I) carbonyl complexes with an additional bidentate ligand are intriguing chemical platforms for several reasons. Many of them display excellent photochemical and electrocatalytic properties.9 The CO coligands can be used as good reporter ligands in IR spectroscopy to investigate the electronic nature of the metal complexes. Additionally, chemical/electrochemical reduction is usually centered on the chelating bidentate ligand, with the reduction potential then providing a good indirect measure for the lowest unoccupied molecular Received: February 13, 2017 Published: April 21, 2017 5771

DOI: 10.1021/acs.inorgchem.7b00393 Inorg. Chem. 2017, 56, 5771−5783

Article

Inorganic Chemistry orbital (LUMO) energy.10a−h The latter point is particularly interesting as reduction in mixed-ligand [Ru(bpy)2(L-L)]2+ (bpy = 2,2′-bipyridine) complexes, which otherwise display excellent electron-transfer properties, are often bpy-centered and hence do not provide any good measure for the position of the L-L-centered orbital.8a All of the above-mentioned properties and phenomena were investigated with the complex [ClRe(CO)3(bpy)], which is perhaps the most striking example of a rhenium(I) complex.10a,e,h Accordingly, corresponding rhenium(I) complexes with chelating pyridyltriazole ligands and other triazole-containing chelating ligands were reported.11 To the best of our knowledge, there is only one report of a rhenium(I) complex with a 1,2,3-triazol-5-ylidene-containing ligand: one that was obtained through an indirect route via methylation of the rhenium-bound triazolide complex.12 Additionally, electronic structures of such complexes in their various redox states were also never investigated. In view of the importance of bidentate triazole- and triazolylidene-containing units as ligands,3,4 we have now turned our attention to the ligands shown in Scheme 1 and,

ligands, the redox stability of the corresponding metal complexes, and the electronic structures of the complexes in various redox states and to investigate the influence of electron transfer on the chemical reactivity of these complexes.

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RESULTS AND DISCUSSION Synthesis, Characterization, and Crystal Structures. The syntheses of the seven ligands or ligand precursors used in this work were performed either by following reported procedures or by adapting them to the present ligands (see the Experimental Section).5h,m,13 As expected, the syntheses of the rhenium complexes 1, 2, 6, and 7 with all-nitrogen-donating chelating ligands were straightforward. However, for complexes 3−5, which contain one or more MIC type donors, the syntheses were more involved and time-consuming. The use of NEt3, much longer reaction times, and more complex isolation and purification procedures were necessary to get those compounds in the pure form (Scheme 1 and Experimental Section). The poor yield for some of these complexes (17%; see Scheme 1) is likely related to the aforementioned difficulty. In complexes 1−5, the substituents on the click-derived ligands are the same (Dipp = 2,6-diisopropylphenyl). Additionally, complexes 6 and 7, which have different substituents (ethyl) on the 1,2,3-triazole rings, were synthesized to investigate possible effects of the substituents on the various properties of these complexes. This series thus allows us to probe the effect of changing one or both of the pyridine rings in bpy for either a triazole or a triazolylidene ring (Scheme 1). The new ligands and the complexes were characterized by 1H and 13C NMR spectroscopy, mass spectrometry, and IR spectroscopy (see the Experimental Section and Supporting Information). It was possible to grow single crystals of all seven rhenium complexes and investigate these by single-crystal X-ray diffraction. While complexes 1, 2, 3, and 7 were crystallized as solvates, the other three complexes were crystallized in the solvent-free form (Table S6). Crystallographic details are given in Table S6. The rhenium(I) centers in all complexes display a distorted octahedral geometry (Figures 1 and S43−S48), as would be expected for such a low-spin d6 metal ion. The chelating ligands are trans to two of the CO ligands of the facRe(CO)3 unit in all cases. The bond lengths within the triazole and triazolylidene rings are within the expected range (Tables S7−S13).5−8 In complexes 1 and 6, containing pyridyltriazole ligands, the Re−N1(triazole) bond lengths [2.153(4) and 2.136(7) Å for 1 and 6, respectively] are slightly shorter than the Re− N2(pyridine) bond lengths [2.193(4) and 2.204(7) Å for 1 and 6, respectively; Table 1]. Such a trend has been observed previously,5h and one is tempted to take these as indications of the better donor ability of triazole in comparison to pyridine. However, bond lengths do not always correlate with the donor strengths of the ligands, and hence such correlations might not always work. For complexes 2 and 7 containing bitriazole ligands, the Re−N(triazole) bond lengths observed are similar to the corresponding bond lengths in complexes 1 and 6 (Tables S7 and S12). For the triazolylidene-containing complexes 3−5, the Re−C(MIC) bond lengths are in the range 2.135(3)−2.169(7) Å. In the pyridyltriazolylidene complex 3, the Re−N2(pyridine) distance is 2.237(2) Å, which is slightly longer than the corresponding distances in complexes 1 and 6 (Tables S9−S11). This effect might be related to the stronger binding of the rhenium center to the MIC donor in 3, resulting in a more asymmetric bonding

Scheme 1. Ligands Used in This Work (Top) and Syntheses of the Complexes (Bottom)

in particular, to their rhenium(I) complexes, 1−7. The ligands have been systematically varied to incorporate varying numbers of triazoles/triazolylidenes/pyridines. Additionally, the ligands L6 and L7 were used to determine the possible effects of different substituents on the ligand backbone on the properties that were investigated. In the following, we present the synthesis, X-ray structural characterization, IR/UV−vis−nearIR (NIR)/electron paramagnetic resonance (EPR) spectroelectrochemical data and density functional theory (DFT) calculations on these complexes. This combined approach is used to decipher the donor/acceptor abilities of these chelating 5772


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Inorganic Chemistry

Figure 1. ORTEP views of complexes 1, 3, and 5. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity.

frequencies, and this is exactly what is observed for all seven complexes (Figures 2 and S19−S25).

Table 1. Bond Lengths (Å) of the Re−Donor Bonds in Complexes 1−7 Re1−N1 Re1−N2

1

2

6

7

2.153(4) 2.193(4)

2.164(8) 2.177(9) 3

2.136(7) 2.204(7)

2.179(6) 2.181(6)

Re1−C1 Re1−N2

4

2.135(3) 2.237(2)

2.151(5) 2.205(4) 5

Re1−C1 Re1−C2

2.165(6) 2.169(7)

situation and leading to a longer Re−N(pyridine) distance. Similarly, the Re−N(triazole) bond length in 4, which contains an additional MIC donor, is the longest Re−N(triazole) distance observed in all of the investigated complexes (Table S10). In 5, which contains a bitriazolylidene ligand, the Re− C(MIC) distances are slightly longer compared to the corresponding distances in 3 and 4. This effect is likely related to the presence of two bulky Dipp groups adjacent to the carbene donors in 5. The Re−C(CO) and C−O bond distances in all complexes are in the expected range (Tables S7−S13).11 IR Spectroscopy, Cyclic Voltammetry, and Donor/ Acceptor Strengths. The presence of the three CO groups in a fac-Re(CO)3 arrangement makes the characterization of complexes containing that fragment through IR spectroscopy particularly fruitful.10a−h Not only do the number of bands observed in the IR spectrum give a perfect indication of the local symmetry in the complexes but also their positions additionally provide valuable information regarding the overall donor situation of the other ligands. For complexes of type [XRe(CO)3(L-L)], the number and resolution of the bands observed in the IR spectrum is critically dependent on the geometry around the rhenium center and the difference in the donor properties between the ligands X and the chelating ligands L-L.10a−h For the present cases, if the geometry about rhenium is fac, then the presence of Cl− as the axial ligands and nitrogen- or carbon-type donors in the chelating equatorial ligands should give rise to three well-resolved CO stretching

Figure 2. IR spectra of complexes 1−5 in THF. Bands corresponding to the CO stretching are shown.

Even though the numbers of bands observed in the IR spectrum of the complexes are identical, their positions are critically dependent on the type of chelating ligand (Figure 2 and Table 2). Thus, for complex 5, which contains a MIC− MIC type of chelating ligand, the bands corresponding to the Table 2. IR Data for the CO Stretching Frequencies (Measured in THF) ν̃/cm−1

compound 1 2 3 4 5 6 7 Re(bpy)(CO)3Cl14 5773

2023 2026 2011 2017 2006 2022 2025 2019

1924 1928 1913 1921 1913 1921 1925 1917

ν̃average/cm−1 1894 1896 1880 1880 1865 1892 1892 1895

1946 1950 1934 1939 1928 1945 1947 1943


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Inorganic Chemistry

the ligands might not always be straightforward. Such assumptions will work best if the oxidation step is metalcentered and the reduction step is ligand-centered. Complexes 1−5 display one oxidation step and one or more reduction steps (Figures 3−5 and S26−S33 and Table 3). As

CO stretching frequencies appear at 2006, 1913, and 1865 cm−1, with an average value of 1928 cm−1. At the other extreme is complex 2, which contains a bitriazole chelating ligand. The values for this case are 2026, 1928, and 1896 cm−1, with an average of 1950 cm−1. When the CO stretching frequencies of complex 1 are compared with those of 6 and those of 2 with those of 7 (same donors with different substituents on the ligand backbone), it is seen that the effect of the substituents on the ligand backbone is marginal. Hence, the properties of complexes 6 and 7 will not be discussed further in this manuscript. Even though it is always tempting to correlate data obtained from the CO stretching frequencies with the σ-donor capacity of the coligands, this might not necessarily be correct. The differences in the CO stretching frequencies for such cases finally are measures of the electron density on the metal center. Hence, both the σ-donor and π-acceptor (π-donor contributions are not likely to play any significant role for the ligands used here) capacities of the ligands are likely to play a role in modulating the electron density at the metal center. Thus, from the average CO stretching frequencies (Table 2), a trend based on the overall donor ability (considering both σ-donor and πacceptor contributions) of the chelating ligands can be set up (Scheme 2). The scale of the donor strengths for the ligands discussed here, together with the well-known bpy ligand, is as follows.

Figure 3. Cyclic voltammogram of 3 in DMF at 100 mV/s, with NBu4PF6 as the supporting electrolyte.

Scheme 2. Ligands in Comparison with Regard to Their Overall Donor Abilitya

Figure 4. Cyclic voltammogram of 3 (first reduction) in DMF at various scan rates. An additional reoxidation peak appears at −2.05 V at higher scan rates.

a

The overall donor ability as judged from the CO stretching frequencies of the corresponding complexes is a result of both the σ-donor and π-acceptor properties of the ligands. See the text for more details.

The introduction of one or more MIC donors clearly leads to an increase in the overall donor capacity of the ligands. Even though the introduction of a triazole unit in these chelating ligands leads to a poorer donor ability, the attenuation in the donor ability in comparison to pyridine is only marginal. Apart from the IR spectroscopy of (normally) COcontaining complexes, redox potentials measured from cyclic voltammetry are also often used for gauging the donor/ acceptor properties of ligands in metal complexes. [Ru(bpy)3]2+-type complexes were often studied with the aforementioned goals in mind.15 In those complexes, oxidation is usually reversible and ruthenium-centered, and hence that potential is taken as a measure of the donating abilities of the ligands. The first reduction is bpy-centered, and that redox potential is usually taken as a measure of the π-accepting capability of the ligands. It should however be borne in mind that in contrast to CO probes in IR spectroscopy, which is a local probe, redox potentials are global probes, and hence the correlation of such potentials with the donor/acceptor ability of

Figure 5. Cyclic voltammogram of 5 in DMF at 100 mV/s, with NBu4PF6 as the supporting electrolyte.

shown below, the potentials of these redox steps, as well as their reversibility are critically dependent on the types of ligands used. The oxidation step for the complexes appear in the range 0.57−0.79 V versus ferrocene/ferrocenium (Fc/Fc+), with only the oxidation step for complex 5 being completely reversible (Figure 5). The observation of reversible oxidation steps in facrhenium(I) complexes is rare,10a−h and details of this phenomenon are discussed in the next section. As mentioned above, the oxidation potentials are sometimes used as measures for the overall ligand donor properties. The reversibility of the oxidation step is often rendered a critical point for such 5774


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Inorganic Chemistry Table 3. Redox Potentials of Complexes 1−5 versus Fc/Fc+a compound

Epa/V

EpcI/V

EpcII/V

1 2 3 4 5 Re(bpy)(CO)3Cl10a,16

0.61 0.59 0.78 0.79 0.57c 0.95

−2.20 −2.69 −2.13 −2.44 −2.37 −1.83

b −2.84 −2.87 b −3.14 −2.25

However, from a qualitative point of view, such trends are not always simply additive of the two components. Spectroelectrochemistry, Redox Stability, and DFT Calculations. In order to test for either pure electrochemical reversibility or reversibility following an EC mechanism, IR, UV−vis−NIR and EPR spectroelectrochemical measurements were carried out on the complexes. Of all of the measured compounds, reversibility, as judged either from the cyclic voltammograms or from the regeneration of the starting spectrum after running a complete redox cycle in spectroelectrochemistry, was observed for the reduction step of 3 and for the oxidation step of 5. Hence, those two cases will be discussed below in detail. When the IR spectrum of 3 upon one-electron reduction is monitored, it is seen that the original bands at 2009, 1909, and 1878 cm−1 (in DMF/0.1 M Bu4NPF6) disappear and new bands at 1967 and 1845 cm−1 appear upon completion of the reduction step (Figure 6). Both

a

Measured in DMF/0.1 M NBu4PF6 at 100 mV/s. bNot observed because of the solvent window. cStandard potential for reversible oxidation.

comparisons. However, more fundamental than that is the assumption that oxidation is metal-centered for all cases where such a comparison is made. This assumption might not always be true if one changes the fundamental ligand type (pyridine vs MIC), and hence such correlations might not work. Upon a comparison of the oxidation potentials shown in Table 3 with the correlations on the overall donor strengths of the ligands obtained from IR spectroscopy (vide infra), it is seen that the trends do not match. This discrepancy is related to the fact that redox potentials are global measures, and the sites of oxidation in these complexes are not uniform. Thus, the oxidation potentials of the metal complexes are not good indicators of the donor properties for these classes of compounds. Reduction in the types of rhenium(I) complexes studied here is known to be centered on the chelating ligands, and hence the reduction potentials are often taken as indicators for the πaccepting capacity of the chelating ligands.10a−h All complexes display at least one one-electron reduction, and the responses of complexes 1 and 3−5 are indicative of the operation of an electron-transfer chemical-reaction (EC) type of mechanism. For example, the reduction step of 3 at −2.13 V is coupled with reoxidation steps at −1.83 and −2.05 V (Figure 4). The overall reversibility of those responses is critically dependent on the ligand type, as discussed in the next section. Because reduction in these complexes is centered on the chelating ligand (see below), the reduction potentials can be used as a first approximation to gauge the π-acceptor capacity of these ligands. Upon comparing the data given in Table 3, we can thus set up the following trends in the π-acceptor capacities of these ligands (Scheme 3): bpy > py−MIC (L3) > py−triaz (L1) > MIC−MIC (L5) > triaz−MIC (L4) > triaz−triaz (L2). Thus, the inclusion of pyridine rings seems to drastically improve the π-acceptor capability of these chelating ligands, with triazoles (in the absence of pyridines) providing the poorest π-accepting ligands. Inclusion of the MIC units makes the π-accepting ability intermediate of the two former cases.

Figure 6. Changes in the IR spectrum of 3 in DMF/0.1 M Bu4NPF6 during the first reduction.

of these bands are broad, with the band at 1845 cm−1 likely being a result of the overlap of two nearly degenerate bands, as was previously observed for other rhenium(I) complexes.10a−h Intriguingly, at least one band at 1990 cm−1 corresponding to an intermediate species appears and then eventually disappears upon completion of reduction (Figure 6). These observations are indicative of the operation of an EC mechanism. Upon reduction of [ClReI(CO)3(L3)] (3) to [ClReI(CO)3(L3)]•− (3•−), Cl− release is plausible from the electron-rich reduced species, which would give rise to a solvent-coordinated neutral species [(dmf)ReI(CO)3(L3)]•. The two reoxidation steps observed in the cyclic voltammogram of 3 could then be associated with reoxidation of the chloride-containing anion radical 3•− and reoxidation of the dmf-bound neutral radical [(dmf)ReI(CO)3(L3)]• (Scheme 4). This final reoxidation step could then lead to the release of dmf and “re-binding” of the chloride anion to regenerate the starting complex 3. The small absolute shift of the IR bands to lower wavenumbers is in line with an L3 ligand-centered reduction, as has been formulated above. Alternatively, upon reduction, the formation of a chloride-free five-coordinated rhenium complex is also plausible (Scheme 4). In fact, the strongly donating MIC ligands might even stabilize such a species, as has been previously observed for rhenium complexes with strongly donating ligands.10i The IR spectra of 3, 3•−, [(3-Cl)DMF]•−, and [3-Cl]•− were also calculated by theoretical methods (Figure S38 and Table S2). While there are some discrepancies in the absolute calculated values compared to the experimental values, the differences between the CO stretching frequencies of the native and reduced forms are reproduced reasonably well by the

Scheme 3. Ligands in Comparison with Regard to Their πAcceptor Capacities

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Inorganic Chemistry Scheme 4. Two Possible Pathways of [3]•− undergoing Ligand Loss/Exchange

calculations (Table S2). The experimental data for the reduced species do not fit completely with any one of the formulated reduced species. Hence, it is reasonable to assume that reduction leads to chloride release and to the formation of more than one species in equilibrium. The experimentally observed broad IR bands for the reduced species would also support such an assumption. To further probe the reduced species, EPR spectroscopy was performed together with structure-based spin population calculations. The in situ electrochemically generated species 3•− displays an isotropic signal at room temperature with a g value of 2.004 (Figure S37). Unfortunately, no hyperfine coupling to any of the nuclei with nuclear spin was resolved in that spectrum. The intensity of the signal was seen to increase when measurements at −60 °C were performed (Figure S36). A look at the Löwdin spin-population analysis of 3•− shows that more than 90% spin is centered on the L3 ligand (Figure 7),

Figure 8. Changes in the UV−vis−NIR spectroelectrochemistry of 3 in DMF/0.1 M Bu4NPF6 during the first reduction.

predict this to be a highest occupied molecular orbital (HOMO−1) to LUMO+1 transition (Figure 9). This

Figure 7. Calculated Löwdin spin-population distribution for 3•−.

Figure 9. TD-DFT-calculated select contributing orbitals for 3 for the main lowest-energy absorption band.

hence supporting a formulation 3•− for that reduced species. While these investigations do not provide information regarding the Cl− abstraction process, they do provide clear evidence for a ligand-centered spin. We also calculated the spinpopulation distribution for the chloride-free five-coordinated species [ReI(CO)3(L3)]• (Figure S42), and for that case, about 37% spin is seen to be located at the rhenium center. Thus, the removal of chloride and the formation of a five-coordinated species seems to shift the electron density to the metal center. The reduced species was also characterized by the UV−vis− NIR spectroelectrochemical method. The native form of 3 displays the lowest-energy band at 351 nm (Figure 8). Timedependent DFT (TD-DFT) calculations reproduce the position of this band with absolute accuracy (Table S4) and

transition can thus be assigned to a metal−ligand-to-ligandcharge-transfer (MLLCT) transition from a Re−Cl-based filled orbital to a L3-centered empty orbital. One-electron reduction of 3 to 3•− leads to the appearance of three new bands at 944, 644, and 452 nm (Figure 8). With the help of TD-DFT, the band at 944 nm, which is broad, is assigned to a mixture of HOMOα to LUMOα and HOMOα to LUMO+1α transitions (Figure 10). This transition is then a mixture of intraligand charge transfer (ILCT) and ligand-to-ligand charge transfer (LLCT), as would be expected for a L3•− radical ligand. The band at 644 nm is assigned to a HOMOα to LUMO+5α transition and thus has mixed ILCT and ligand-to-metal chargetransfer (LMCT) character. Finally, the band at 452 nm is 5776


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Figure 10. TD-DFT-calculated select contributing orbitals for the reduced form [3]•− for the main absorption bands. For TD-DFT data on the reduced and DMF-coordinated and the reduced five-coordinated rhenium complex, see the Supporting Information.

the oxidation step for this compound (see above). As discussed below, IR, and UV−vis−NIR spectroelectrochemical measurements further support this fact, as can be seen through regeneration of the starting spectra upon completion of the oxidation cycle.10g The strongly donating bi-MIC ligand likely compensates for the electron density lost at the rhenium center through oxidation, thus making that step reversible for complex 5. Compound 5 displays IR bands at 2002, 1903, and 1865 cm−1 in DMF/0.1 M Bu4NPF6. Upon one-electron oxidation to 5•+, these bands are shifted to 2075, 2027, and 1967 cm−1 (Figure 11). The much larger shift (compared to the reduction case discussed above) to higher wavenumbers is already an indication of a more direct involvement of the rhenium center in the oxidation step.10g The DFT-calculated IR spectra also deliver a higher absolute shift of the IR bands for the oxidation step compared to the reduction step discussed above (Figure

assigned as a mixture of HOMOα to LUMO+7α and HOMOβ to LUMOβ transitions and can thus be assigned to have mixed LLCT and metal-to-ligand charge-transfer (MLCT) character. As the three aforementioned bands of the reduced species “continuously rise” during the reduction process, the detection of isosbestic points is impossible for such a case (Figure 8). Thus, the final spectrum that we observe is likely a mixture of various species where Cl− is coordinated to the rhenium(I) center in one case and DMF in the other and a further chloridefree five-coordinated species (see discussion of IR Spectroscopy). We also performed TD-DFT calculations on the species [(3-Cl)DMF]•− and [3-Cl]•− (Table S4 and Figure S41). Whereas the calculated spectrum for [(3-Cl)DMF]•− is very similar to that calculated for 3•−, calculations on [3-Cl]•− provide a better match with the experimentally observed spectrum in the visible region (experimental 452 nm and calculated 469 nm; Table S4). The data presented here do not allow us to distinguish between those several species. However, the spectroelectrochemical measurements unequivocally prove the overall reversibility of the reduction step, as seen from regeneration of the starting spectrum after completion of the reduction and reoxidation cycles (Figure S34). For compounds containing the fac-ReI(CO)3 unit, the oxidation steps are usually known to be irreversible.10a−h This is also the case for all of the compounds investigated here except for 5. The irreversibility is usually because of CO loss from the oxidized fac-ReII(CO)3 fragment due to decreased πback-bonding from the rhenium(II) center. Improved reversibility of the oxidation step in rhenium(I) compounds is, however, sometimes observed for complexes that do not contain the three CO ligands on rhenium(I).10i The cyclic voltammogram of 5 already points to the reversible nature of

Figure 11. Changes in the IR spectrum of 5 in DMF/0.1 M Bu4NPF6 during the first oxidation. 5777


Inorganic Chemistry S39 and Table S3). However, DFT seems to underestimate the absolute amount of shift upon one-electron oxidation of this complex. The species 5•+ turned out to be EPR-silent down to liquid-nitrogen temperatures. A DFT-calculated Löwdin spinpopulation analysis of 5•+ delivers more than 50% spin on the rhenium center, with the remaining spin being distributed among the other ligands including the bi-MIC ligand L5 (Figure 12). Such a large amount of spin on a heavy metal such as rhenium will open up fast relaxation pathways, resulting in immense line broadening and the observed EPR silence of 5•+.10g

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CONCLUSION

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EXPERIMENTAL SECTION

We have presented here seven Re(I) complexes with chelating ligands that contain various combinations of py/triaz/MIC donors and compared these to their famous bpy analogue. All complexes were characterized through single-crystal X-ray diffraction. The substituents on the triazole rings (Dipp vs ethyl) do not seem to influence the donor/acceptor properties of these ligands in any significant way. By analyzing the IR bands of the rhenium carbonyl fragments, we have shown that the bidentate ligands containing two MIC units are the strongest donors, while the ones containing two triazole units are the poorest donors. The chelating ligands containing one or more MIC units are all better donors than bpy. Reduction potentials obtained from cyclic voltammetry of the rhenium complexes help us in gauging the π-acceptor capacities of these chelating ligands. While bpy is a better π-acceptor than all of the chelating ligands investigated here, the ligand with two triazole units have the poorest π-acceptor capacity. Importantly, inclusion of even one pyridine ring seems to drastically improve the π-acceptor character of these chelating ligands. We have also confirmed here that using redox potentials of metal complexes for judging the donor/acceptor properties of the corresponding ligands only works well if such redox steps are predominantly localized on either the metal or the ligands. Results from IR/EPR/UV−vis−NIR spectroelectrochemistry and DFT calculations were used to probe the redox stability of these ligands in their complexes. It was shown that the pyridylMIC-containing complex 3 displays reversible reductive electrochemistry, and the bi-MIC-containing complex 5 shows reversible oxidative electrochemistry. The latter observation of a reversible oxidation step in a rhenium(I) complex is remarkable and shows the ability of these strongly donating bi-MIC ligands to stabilize oxidized species. The results presented here thus allow us to categorize these increasingly popular chelating ligands with respect to their donor/acceptor properties. The inclusion of MIC ligands seems to increase the redox stability of the corresponding oxidized metal complexes. This fact is likely to be useful while using these ligands as components of electro- and photocatalysis. It is perhaps no surprise that MICs as well as other kinds of chelating Nheterocyclic carbene ligands are establishing themselves as privileged ligand classes in electro- and photocatalysis.7,9a−c

Figure 12. Calculated Löwdin spin-population distribution for 5•+.

Complex 5 displays two close-lying bands at 365 and 318 nm in its UV−vis spectrum (Figure 13). The band at 365 nm is

Figure 13. Changes in the UV−vis−NIR spectrum of 5 in DMF/0.1 M Bu4NPF6 during the first oxidation.

assigned through TD-DFT calculations to a HOMO to LUMO +1 transition (Figure S43 and Table S5) and the band at 318 nm to a mixture of HOMO−1 to LUMO+2 and HOMO to LUMO+2 transitions. Just as was seen for 3 above, all of these transitions are of predominantly MLLCT character. Upon oneelectron oxidation to 5•+, weak bands appear at 725 and 422 nm. TD-DFT predicts the band at 725 nm to be a transition from HOMO−4β to LUMOβ (Figure S44 and Table S5), and hence this band can be assigned to an LMCT transition. The band at 422 nm is assigned with the help of TD-DFT calculations to a mixture of HOMO−17β to LUMOβ and HOMO−10β to LUMOβ, also pointing to LMCT character for these transitions. It is remarkable that the bi-MIC ligand confers such redox stability to the oxidized rhenium(I) center, to make an oxidation step, which is otherwise almost always irreversible, reversible for the present case. We have thus presented here an example of the spectroscopic signature for the oxidized form of a carbene-containing metal complex.

Caution! Compounds containing azides are potentially explosive. Although we never experienced any problems during synthesis or analysis, all compounds should be synthesized only in small quantities and handled with great care! General Remarks and Instrumentation. All reactions were carried out using standard Schlenk-line techniques under an inert atmosphere of nitrogen (Linde, HiQ Nitrogen 5.0, purity ≥99.999%). Workup occurred open to air and moisture because of the stability of the compounds. Commercially available chemicals were used without further purification. Methanol was distilled from magnesium methanolate; other solvents were available from a MBRAUN MBSPS-800 solvent system. All solvents were degassed by standard techniques prior to use. 1H and 13C{1H} NMR spectra were recorded on Jeol ECS 400, Jeol ECP 500, and Bruker Avance 700 spectrometers at 20 °C. Chemical shifts are reported in ppm (relative to the tetramethylsilane signal) with reference to the residual solvent peaks.17 Multiplets are reported as follows: singlet (s), doublet (d), triplet (t) quartet (q), quintet (quint), and combinations thereof. Highresolution (HR) mass spectrometry was performed on an Agilent 6210 electrospray ionization time-of-flight spectrometer. Mid-wave5778


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Inorganic Chemistry

reported.5h,m,13 The syntheses of the ligands [HL3]BF4 and [HL4]BF4 have been adapted from the ethyl derivatives published by Hohloch et al. in 2015.5h 1-(2,6-Diisopropylphenyl)-3-methyl-4-(pyridin-2-yl)-1H-1,2,3-triazol-3-ium Tetrafluoroborate ([HL3]BF4). In a 100 mL round-bottom flask, L1 (1.00 mmol, 306 mg, 1 equiv) and m-chloroperoxybenzoic acid (2.00 mmol, 344 mg, 2 equiv) were suspended in chloroform (15 mL) and refluxed for 30 min. After cooling to room temperature, the mixture was poured into dichloromethane (100 mL) and washed with aqueous KOH (1 m, 3 × 50 mL). The organic phase was dried over Na2SO4 and the solvent removed under reduced pressure to give the corresponding N-oxide as a white solid, which was directly used for methylation. Therefore, the N-oxide was transferred to a 100 mL Schlenk flask and dissolved in dichloromethane (10 mL). Me3OBF4 (4.00 mmol, 588 mg, 4 equiv) was added, and the mixture was stirred at room temperature for 4 days. The solvent was evaporated and the residue suspended in dry ethanol (40 mL). Mo(CO)6 (1.00 mmol, 264 mg, 1 equiv) was added, and the mixture was refluxed for 1 h. After cooling to room temperature, the mixture was filtered and all solvents were removed. The crude product was then purified by column chromatography (SiO2 and 10:1 dichloromethane/methanol) to give the desired product as a light-brown solid (64%, 261 mg, 0.64 mmol). 1 H NMR (400 MHz, CDCl3): δ 9.28 (s, 1H, triazolium 5H), 8.79 (m, 1H, pyridine H), 8.78 (d, J = 7.9 Hz, 1H, aryl H), 8.08 (t, J = 7.6 Hz, 1H, aryl H), 7.64 (t, J = 8.1 Hz, 1H, aryl H), 7.54 (dd, J = 7.4 and 2.8 Hz, 1H, aryl H), 7.39 (d, J = 7.4 Hz, 1H, aryl H), 4.82 (s, 3H, NCH3), 2.28 (p, J = 6.6 Hz, 2H, CHCH3), 1.28 (d, J = 6.8 Hz, 6H, CH3), 1.18 (d, J = 6.8 Hz, 6H, CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 149.8, 145.5, 142.5, 141.9, 138.9, 133.1, 132.1, 130.8, 126.5, 126.34 124.8 (all Caromatic), 42.0 (NCH3), 29.0 (CHCH3), 24.6 (CH3), 23.6 (CH3). HR ESI-MS: m/z 321.2017 ([M]+). Calcd: m/z 321.2024. 1,1′-Bis(2,6-diisopropyl)-3-methyl-1H,1′H-4,4′-bis(1,2,3-triazol-3ium) Tetrafluoroborate ([HL4]BF4). In a 100 mL Schlenk flask, L2 (0.50 mmol, 228 mg, 1 equiv) and Me3OBF4 (0.50 mmol, 73 mg, 1 equiv) were dissolved in dichloromethane (10 mL) and stirred under nitrogen for 3 days. The crude mixture was poured into n-hexane (200 mL), and the precipitate was collected by filtration. The desired product was obtained as a white solid (72%, 201 mg, 0.36 mmol). 1 H NMR (400 MHz, CDCl3): δ 9.23 (s, 1H, triazolium 5H), 9.10 (s, 1H, triazole H), 7.64 (t, J = 7.8 Hz, 1H, aryl H), 7.54 (t, J = 7.8 Hz, 1H, aryl H), 7.39 (d, J = 7.9 Hz, 2H, aryl H), 7.32 (d, J = 7.9 Hz, 2H, aryl H), 4.88 (s, 3H, NCH3), 2.35−2.18 (m, 4H, CHCH3), 1.29 (d, J = 6.8 Hz, 6H, CH3), 1.21−1.15 (m, 18H, CH3). 13C{1H} NMR (100 MHz, CDCl3): δ 146.0, 145.5, 135.5, 133.3, 132.2, 131.7, 131.7, 130.7, 124.9, 124.2 (all Caromatic), 41.7 (NCH3), 29.1 (CHCH3), 28.8 (CHCH3), 24.8, 24.5, 23.8, 23.6 (all CH3). HR ESI-MS: m/z 471.3229 ([M]+). Calcd: m/z 471.3231. 1. In a 50 mL Schlenk flask equipped with a reflux condenser and a gas bubbler, Re(CO)5Cl (0.15 mmol, 54.3 mg, 1 equiv) and L1 (0.15 mmol, 46.0 mg, 1 equiv) were refluxed in 10 mL of methanol for 12 h. After cooling to room temperature, the mixture was filtered over a pad of Celite and the solvent removed. The resulting white solid was recrystallized two times from dichloromethane and n-hexane to give the desired product as a white powder (40%, 0.06 mmol, 36.9 mg). Crystals suitable for X-ray diffraction were obtained by evaporation of the solvent from a concentrated solution of complex in chloroform. 1 H NMR (700 MHz, CDCl3): δ 9.10 (bs, 1H, pyridine H), 8.16 (s, 1H, triazole 5H), 8.04 (dt, J = 7.8 and 1.6 Hz, 1H, aryl H), 7.86 (d, J = 7.8 Hz, 1H, aryl H), 7.59 (t, J = 8.0 Hz, 1H, aryl H), 7.48 (dt, J = 7.8 and 0.7 Hz, 1H, aryl H), 7.37 (t, J = 8.8 Hz, 1H, aryl H), 2.32 (p, J = 6.7 Hz, 1H, CH(CH3)2), 2.26 (p, J = 6.7 Hz, 1H, CH(CH3)2), 1.24− 1.21 (m, 9H, CH3), 1.15 (d, J = 8.2 Hz, 3H, CH3). 13C{1H} NMR (176 MHz, CDCl3): δ 197.4, 195.2, 189.5 (CCO), 153.149.4, 149.0, 146.8, 145.5, 139.3, 132.3, 132.0, 126.1, 125.3, 124.7, 124.3, 122.3 (all Caromatic), 29.0 (CH(CH3)2), 24.9, 24.4, 24.1, 23.8 (all CH3). HR ESIMS: m/z 635.0863 ([M + Na]+). Calcd: m/z 635.0831. IR (THF, cm−1): ν̃ 2023, 1924, 1894. 2. In a 50 mL Schlenk flask, equipped with a reflux condenser and a gas bubbler, Re(CO)5Cl (0.15 mmol, 54.3 mg, 1 equiv) and L2 (0.15

length IR (MIR) spectroscopy was carried out on a Nicolet NEXUS 670/870 Fourier transform infrared (FT-IR) spectrometer using a thin-layer IR cell (CaF2 windows). Electrochemistry. Cyclic voltammograms were recorded with a PAR VersaStat 4 potentiostat (Ametek) by working in anhydrous and degassed DMF (99.8% extra dry, Acros Organics) with 0.1 M NBu4PF6 (dried, >99.0%, electrochemical grade, Fluka) as the supporting electrolyte. Concentrations of the complexes were about 1 × 10−4 M. A three-electrode setup was used with glassy carbon as the working electrode, a coiled platinum wire as the counter electrode, and a coiled silver wire as the pseudoreference electrode. The Fc/Fc+ couple was used as the internal reference. Spectroelectrochemistry. MIR spectra were recorded with a Nicolet NEXUS 670/870 FT-IR spectrometer with a PAR VersaStat 4 potentiostat (Ametek). UV−vis−NIR spectra were recorded with an Avantes spectrometer consisting of a light source (AvaLight-DH-SBal), a UV−vis detector (AvaSpec-ULS2048), and a NIR detector (AvaSpec-NIR256-TEC). Spectroelectrochemical measurements were carried out in an optically transparent thin-layer electrochemical18 cell (CaF2 windows) with a platinum-mesh working electrode, a platinummesh counter electrode, and a silver-foil pseudoreference electrode. Anhydrous and degassed DMF (99.8% extra dry, Acros Organics) with 0.1 M NBu4PF6 as the electrolyte was used as the solvent. EPR. EPR spectra at the X-band frequency (ca. 9.5 GHz) were obtained with a Magnettech MS-5000 benchtop EPR spectrometer equipped with a rectangular TE 102 cavity and a TC HO4 temperature controller. The measurements were carried out in synthetic quartz glass tubes. For EPR spectroelectrochemistry, a three-electrode setup was employed using two Teflon-coated platinum wires (0.005 in. bare and 0.008 in. coated) as the working and counter electrodes and a Teflon-coated silver wire (0.005 in. bare and 0.007 in. coated) as the pseudoreference electrode. Single-Crystal X-ray Diffraction. X-ray data of 2, 4, and 5−7 were collected on a Bruker Smart AXS system at 140(2) or 130(2) K using graphite-monochromated Mo Kα radiation (λα = 0.71073 Å). The strategy for data collection was evaluated using the SMART software. The data were collected by the standard ω-scan techniques and scaled and reduced using the SAINT+ and SADABS software. Xray data of 1 and 3 were collected on a Bruker D8 Venture system at 100(2) K using graphite-monochromated Mo Kα radiation (λα = 0.71073 Å). The strategy for the data collection was evaluated by using the APEX2 software. The data were collected by ω- and φ-scan techniques and scaled and reduced using the APEX2 and SADABS software. The structures were solved by intrinsic phasing or direct methods using SHELXS-97, SHELXT-2014/7, or SHELXT-2016/4 and refined using SHELXL-2014/7 or SHELXL-2016/4 by full-matrix least squares, refining on F2. Non-hydrogen atoms were refined anisotropically.19 In the case of 4, disordered solvent molecules were taken into account by using PLATON/SQUEEZE.20 CCDC 1442818 (1), 1442811 (2), 1442848 (3), 1442889 (4), 1442807 (5), 1442817 (6), and 1442816 (7) contain the supplementary crystallographic data for this paper. DFT Calculations. DFT calculations were done with the ORCA 3.0.0 program21 package using the BP86 and B3LYP functionals for the geometry optimization and single-point calculations, respectively.22 All calculations were run with the empirical van der Waals correction (D3).23 Convergence criteria were set to default for the geometry optimizations (OPT) and tight for the self-consistent-field (SCF) calculations (TIGHTSCF). Relativistic effects were included with the zeroth-order relativistic approximation (ZORA).24 Triple-ζ-valence basis sets (def2-TZVP)25 were employed for all atoms. Calculations were performed using the resolution of the identity approximation26 with matching auxiliary basis sets. Low-lying excitation energies were calculated with TD-DFT. Solvent effects were taken into account with the conductor-like screening model (COSMO).27 Spin populations were calculated according to the Löwdin population analysis.28 Molecular orbitals and spin populations were visualized with the Molekel 5.4.0.8 program.29 Synthetic Procedures. Synthesis of Ligands. The syntheses of the ligands L1, L2, [H2L5]BF4, L6, and L7 have been previously 5779


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mL of toluene. An excess of triethylamine (0.3 mL) was added and the mixture refluxed for 2 days. After cooling to room temperature, nhexane and acetonitrile were added to the resulting yellow solution. The acetonitrile phase was separated, washed with water, and then extracted three times with dichloromethane. The organic phases were collected and dried over Na2SO4, and the solvent was evaporated under reduced pressure. The crude product was then purified by column chromatography (SiO2 and 95:5 dichloromethane/acetone as the eluent) to give the pure product as a yellow solid (17%, 12.6 μmol, 12.1 mg). Crystals suitable for X-ray diffraction were obtained by the slow diffusion of n-hexane into a concentrated solution of the complex in dichloromethane. 1 H NMR (700 MHz, CDCl3): δ 7.54 (t, J = 7.8 Hz, 2H, aryl H), 7.35 (ddd, J = 18.7, 7.8, and 1.3 Hz, 4H, aryl H), 4.57 (s, 6H, NCH3), 2.74 (p, J = 6.8 Hz, 2H, CH(CH3)2), 2.53 (p, J = 6.8 Hz, 2H, CH(CH3)2), 1.30 (dd, J = 6.7 Hz, 6H, CH3), 1.28 (d, J = 6.8 Hz, 6H, CH3), 1.09 (d, J = 6.8 Hz, 6H, CH3), 1.05 (d, J = 6.9 Hz, 6H, CH3). 13 C{1H} NMR (176 MHz, CDCl3): δ 196.8, 189.3, 186.1 (all CCO), 146.8, 145.3, 143.6, 134.7, 131.4, 124.4, 123.7 (all Caromatic), 39.8 (NCH3), 28.85 (CH(CH3)2), 28.62 (CH(CH3)2), 25.98, 25.87, 22.77, 22.50 (all CH3). ESI-MS: m/z 755.2815 ([M − Cl]+). Calcd: m/z 755.2814. IR (THF, cm−1): ν̃ 2006, 1913, 1865. 6. The synthesis was analogous to that of 1. The desired product was isolated from Re(CO)5Cl (0.20 mmol, 72.3 mg, 1 equiv) and L6 (0.20 mmol, 38.5 mg, 1 equiv) as a white solid (64%, 0.13 mmol, 64.0 mg). 1 H NMR (500 MHz, CDCl3): δ 8.95 (ddd, J = 5.6, 1.5, and 0.6 Hz, 1H, pyridine H), 8.59 (s, 1H, triazole H), 8.13 (dt, J = 7.8 and 1.5 Hz, 1H, aryl H), 8.02 (ddd, J = 8.0, 1.3, and 0.3 Hz, 1H, aryl H), 7.52 (ddd, J = 7.6, 1,4, and 0.7 Hz, 1H, aryl H), 4.59 (dq, J = 7.4 and 1.4 Hz, 2H, CH2), 1.61 (t, J = 7.4 Hz, 3H, CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 154.1, 150.3, 141.3, 127.1, 125.4, 123.4, 100.9 (all Caromatic), 48.5 (CH2), 15.1 (CH3). HR ESI-MS: m/z 502.9877 ([M + Na]+). Calcd: m/z 502.9891. IR (THF, cm−1): ν̃ 2022, 1921, 1892. 7. The synthesis was analogous to that of 2. The desired product was isolated from Re(CO)5Cl (0.23 mmol, 86.2 mg, 1 equiv) and L7 (0.23 mmol, 41.6 mg, 1 equiv) as a white solid (83%, 0.19 mmol, 90.9 mg). 1 H NMR (500 MHz, CDCl3): δ 8.32 (s, 2H, triazole H), 4.57 (dq, J = Hz, 2H, CH2), 1.59 (t, J = Hz, 3H, CH3). 13C{1H} NMR (126 MHz, CDCl3): δ 141.1, 122.9 (all Caromatic), 48.4 (CH2), 15.2 (CH3). HR ESI-MS: m/z 521.0113 ([M + Na]+). Calcd: m/z 521.0109. IR (THF, cm−1): ν̃ 2025, 1925, 1892.

mmol, 68.5 mg, 1 equiv) were refluxed in 10 mL of methanol for 12 h. After cooling to room temperature, the mixture was filtered over a pad of Celite and the solvent removed. The resulting white solid was recrystallized from dichloromethane and n-hexane to give the desired products as a white powder (54%, 82.0 μmol, 62.2 mg). Crystals suitable for X-ray diffraction were obtained by the slow diffusion of nhexane into a concentrated solution of the complex in dichloromethane. 1 H NMR (400 MHz, CD2Cl2): δ 8.21 (s, 2H, triazole 5H), 7.63 (t, J = 7.7 Hz, 2H, aryl H), 7.41 (d, J = 7.9 Hz, 4H, aryl H), 2.32 (dp, J = 27.3 and 6.8 Hz, 2H, CH(CH3)2), 1.22 (dt, J = 11.3 and 6.7 Hz, 24H, CH3). 13C{1H} NMR (100 MHz, CD2Cl2): δ 146.9, 146.3, 140.9, 132.7, 132.6, 125.1, 124.8, 124.6 (all Caromatic), 29.4 (CH(CH3)2), 25.0, 24.5, 24.3, 24.0 (all CH3). HR ESI-MS: m/z 785.2110 ([M + Na]+). Calcd: m/z 785.1987. IR (THF, cm−1): ν̃ 2026, 1928, 1896. 3. In a 50 mL Schlenk flask, equipped with a reflux condenser and a gas bubbler, Re(CO)5Cl (0.05 mmol, 18.1 mg, 1 equiv) and [HL3]BF4 (0.05 mmol, 20.0 mg, 1 equiv) were suspended in 5 mL of toluene. Excess of triethylamine (0.3 mL) was added and the mixture refluxed for 2 days. After cooling to room temperature, nhexane and acetonitrile were added to the resulting yellow solution. The acetonitrile phase was separated and washed with water and then extracted three times with dichloromethane. The organic phases were collected and dried over Na2SO4, and the solvent was evaporated. The crude product was then recrystallized from dichloromethane and nhexane to give the product as yellow crystals (67%, 33.5 μmol, 21.0 mg). Crystals suitable for X-ray diffraction were obtained by the slow diffusion of n-hexane into a concentrated solution of the complex in dichloromethane. 1 H NMR (500 MHz, CD2Cl2): δ 9.11 (ddd, J = 5.5, 1.6, and 0.9 Hz, 1H, pyridine H), 8.08 (td, J = 7.9 and 1.6 Hz, 1H, aryl H), 7.92 (dt, J = 8.1 and 1.1 Hz, 1H, aryl H), 7.59 (t, J = 7.8 Hz, 1H, aryl H), 7.45−7.38 (m, 3H, aryl H), 4.55 (s, 3H, NCH3), 2.69 (p, J = 6.8 Hz, 1H, CH(CH3)2), 2.47 (p, J = 6.8 Hz, 1H, CH(CH3)2), 1.31 (d, J = 6.8 Hz, 3H, CH3), 1.25 (d, J = 6.7 Hz, 3H, CH3), 1.11 (d, J = 6.9 Hz, 3H, CH3), 1.06 (d, J = 6.9 Hz, 3H, CH3). 13C{1H} NMR (126 MHz, CD2Cl2): δ 199.3 (Ccarbene), 197.5, 189.9, 188.6 (all CCO), 156.5, 150.4, 149.9, 147.1, 145.9, 139.5, 135.1, 131.9, 126.2, 124.8, 124.2, 121.2 (all Caromatic), 39.6 (NCH3), 29.3 (CH(CH3)2), 28.9 (CH(CH3)2), 25.9, 25.9, 22.9, 22.8 (all CH3). HR ESI-MS: m/z 649.0980 ([M + Na]+). Calcd: m/z 649.0987. IR (THF, cm−1): ν̃ 2011, 1913, 1880. 4. In a 50 mL Schlenk flask, equipped with a reflux condenser and a gas bubbler, Re(CO)5Cl (0.20 mmol, 72.3 mg, 1 equiv) and [H2L4](BF4)2 (0.20 mmol, 112 mg, 1 equiv) were suspended in 5 mL of toluene. An excess of triethylamine (0.3 mL) was added and the mixture refluxed for 2 days. After cooling to room temperature, acetonitrile was added to the resulting yellow solution. The mixture was first washed with water and then extracted with dichloromethane. The organic phases were collected, and the solvent was evaporated. The crude product was purified by column chromatography (SiO2 and DCM as the eluent) to give the desired product as light-yellow needles (28%, 55.0 μmol, 42.7 mg). Crystals suitable for X-ray diffraction were obtained by the slow diffusion of n-hexane into a concentrated solution of the complex in chloroform. 1 H NMR (700 MHz, CD2Cl2): δ 8.09 (s, 1H, aryl H), 7.65 (dt, J = 10.8 and 7.8 Hz, 3H, aryl H), 7.45 (td, J = 7.9 and 1.7 Hz, 6H, aryl H), 4.47 (s, 3H, NCH3), 2.76 (p, J = 6.8 Hz, 1H, CH(CH3)2), 2.58 (p, J = 6.8 Hz, 1H, CH(CH3)2), 2.43 (p, J = 6.8 Hz, 1H, CH(CH3)2), 2.31 (p, J = 6.8 Hz, 1H, CH(CH3)2), 1.38 (d, J = 6.9 Hz, 3H, CH3), 1.33−1.26 (m, 9H, CH3), 1.24 (d, J = 6.9 Hz, 6H, CH3), 1.18 (d, J = 6.8 Hz, 3H, CH3), 1.12 (d, J = 6.9 Hz, 3H, CH3). 13C{1H} NMR (176 MHz, CD2Cl2): δ 197.8 (Ccarbene), 197.7, 190.3, 186.3 (all CCO), 147.1, 146.2, 145.9, 143.0, 141.4, 135.3, 132.6, 131.9, 125.1, 124.8, 124.8, 124.2, 122.4 (all Caromatic), 38.6 (NCH3), 29.5, 29.4, 29.3, 28.9 (all (CH(CH3)2), 26.0, 25.9, 25.2, 24.6, 24.2, 23.9, 23.0, 22.9 (all CH3). HR ESI-MS: m/z 799.2199 ([M + Na]+). Calcd: m/z 799.2144. IR (THF, cm−1): ν̃ 2017, 1921, 1880. 5. In a 50 mL Schlenk flask, equipped with a reflux condenser and a gas bubbler, Re(CO)5Cl (0.1 mmol, 36.2 mg, 1 equiv) and [H2L5](BF4)2 (0.10 mmol, 66.0 mg, 1 equiv) were suspended in 5

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00393. NMR, IR, and EPR spectra, cyclic voltammograms, spectroelectrochemistry, DFT, and crystal structures and crystallographic data (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.S.). ORCID

Biprajit Sarkar: 0000-0003-4887-7277 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Fonds der Chemischen Industrie for financial support (Kekulé-Stipendium for S.K.). Dr. S. Hohloch 5780


Article

Inorganic Chemistry

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is kindly acknowledged for solving the crystal structure of compound 6.

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DEDICATION Dedicated to Prof. Dietrich Gudat on the occasion of his 60th birthday, with our very best wishes.

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