Towards new coordination modes of 1,2,3-triazolylidene: controlled by the nature of the 1^st metalation in a heteroditopic bis-NHC ligand

Abstract

An unusual effect of the nature of the first metal coordination of a heteroditopic N-heterocyclic carbene ligand (L2) towards the coordination behavior of 1,2,3-tzNHC is explored. The first metal coordination at the ImNHC site (complexes 3 and 4) was noted to substantially influence the electronics of the 1,2,3-triazolium moiety leading to an unprecedented chemistry of this MIC donor. Along this line, the Rh^III/Ir^III-orthometalation in complexes 4 makes the triazolium C₄–H more downfield shifted than C₅–H, whereas a reverse trend, although to a lesser extent, is observed in the case of the non-chelated Pd^II-coordination. This difference in behavior assisted us to achieve the selective activation of triazole C₄/C₅ positions, not observed before, as supported by the isolation of the homo- and hetero-bimetallic complexes, 5, 6 and 7–9via C₅- and C₄-metalation, respectively. Furthermore, the %V_bur calculations eliminate any considerable steric influence and the DFT studies strongly support the selectivity observed during bimetalation.

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Introduction

N-heterocyclic carbenes (NHCs), with their characteristic strong σ-electron donating nature¹ coupled with the tuneable stereoelectronic properties,² have proven to be a versatile class of ligands to anchor one, two or more metals for diversified applications.³ Along with the very common Arduengo type imidazolylidenes, triazole derived 1,2,4-triazolylidenes have also been widely utilized for various applications.⁴ After Bertrand's early discovery of 1,2,4-triazol-3,5-diylidene supported organometallic polymer,⁵ the Peris group successfully utilized this bis-NHC system to access diverse homo- as well as heterometallic complexes.^3c,6 Later in 2008, the group of Albrecht introduced 1,2,3-triazolylidenes into the NHC family as an abnormal congener (mesoionic carbenes, MICs) of normal NHCs.⁷ They exhibit superior donor properties, due to less heteroatom stabilization, which influence the reactivity of their transition metal complexes.⁸ Furthermore, easy accessibility of the precursor triazolium salts via a simple [3 + 2] cycloaddition reaction of azides and alkynes (‘click reaction’)^8a,9 have made 1,2,3-triazolylidenes attractive in organometallic chemistry.¹⁰ Among them, the 1,2,3-triazol-5-ylidenes derived from the corresponding C₄-protected triazolium salts are well explored,^10c,11 whereas the related MICs, generated from the analogous C₄- and C₅-unsubstituted triazolium salts, are rarely studied (Fig. 1).^12,13 This might be due to the availability of two backbone carbons (C₄ and C₅), which could, in principle, be deprotonated to yield either C₄- or C₅-ylidenes restricting the selective metalation.¹² This was indeed supported by the formation of a mixture of C₄- and C₅-coordinated products during the metalation of unsubstituted 1,2,3-triazolylidenes.¹² Moreover, the reported complexes of unsubstituted MICs are only of the cyclometalated type and they possess the metal center at the triazolylidene C₅-position (Fig. 1B).^12,13 The additional stability offered by chelation could possibly be the driving force for such metalation at the C₅-position.

Fig. 1 Comparison of previous reports on 1,2,3-triazolylidene complexes with the present work.

Based on these literature reports, we engaged ourselves in developing strategies for selective activation of the backbone protons (C₄– and C₅–H) of the unsubstituted 1,2,3-triazolium salt as this will open up a new family of metal-NHC complexes for various applications including catalysis (activities of these regioisomeric metal complexes are expected to be different). We hypothesized that either some electronic or steric modulation could help in achieving this. In this line, along with our interest^14a towards hetero-bimetallic complexes,^14a,b we have now designed an unsymmetrical bis-azolium salt [L2-H₂]Br₂ which possesses a triazolium group substituted with an N-phenyl-para-imidazolium moiety. The first metalation occurs at the imidazolium end as its C₂–H acidity is more compared to that of the triazolium backbone protons and thus, provides us the unique opportunity to control the subsequent metalation either at the C₄- or C₅-position of the triazolium group. Accordingly, mono-metalation of [L2-H₂]Br₂ with Pd^II- and Ir^III/Rh^III precursors yielded non-orthometalated (3) and orthometalated (4a and b) analogues, respectively which were detected to impart an unprecedented influence on the triazolium C₄/C₅–H chemical shifts and thus, essentially on their reactivities. This effect was utilized for the synthesis of their bimetallic counterparts (5, 6 from 3 and 7–9 from 4) via selective activation of either the C₄– or C₅–H which was unequivocally supported by detailed NMR analyses along with the X-ray crystallographic studies.

Results and discussion

Previous reports on unsubstituted 1,2,3-triazolium salts with Ir^III- and Pt^II-centers^12,13 suggest that metalation occurs at the C₅-position of the triazole ring. However, it should be noted that these complexes are of the cyclometalated type which might have some influence on the metalation behavior. To understand this in detail, at the outset, we intended to study the chemistry of simple C₄/C₅-backbone unsubstituted mesoionic carbene precursors such as the click derived [L1-H]Br with Pd^II and Au^I centers (Scheme 1a) as they do not generally prefer the chelate complex formation via orthometalation. After synthesis, the NMR analyses of [L1-H]Br reveal that the C₄– and C₅–H resonances have very close chemical shift values (δ = 10.05 and 10.16 ppm, respectively), which might pose difficulties to its selective deprotonation cum metalation.

Scheme 1 (a) Synthesis of [L1-H]Br and its palladium (1) and gold (2) complexes: (i) NaNO₂, HCl/H₂O (10% solution), 0 °C, 1 h; (ii) NaN₃, 0 °C-RT, 12 h; (iii) vinyl acetate, reflux, 24 h; (iv) EtBr, CH₃CN, reflux, 24 h; (v) [Pd(CH₃CN)₂Cl₂], Cs₂CO₃, KBr, CH₃CN/pyridine, 70 °C, 24 h; (vi) [Au(SMe₂)Cl], Cs₂CO₃, CH₃CN, 70 °C, 24 h; (b) molecular structures of 1 and 2 with ellipsoids at a 50% probability level. Hydrogen atoms except H8 and Me moieties of the N–Et groups are omitted for clarity. Pyridine is shown in capped stick.

Nevertheless, [L1-H]Br was first reacted with [Pd(CH₃CN)₂Cl₂] under suitable conditions (Scheme 1a) and the ¹H NMR spectrum with only one triazolylidene backbone proton along with a Pd^II-MIC ¹³C{¹H} signal at δ = 139.5 ppm suggest the formation of a monopalladium complex. The X-ray crystallographic analysis disclosed the structure of complex 1 (Scheme 1b) in which the non-cyclometalated Pd^II-center is attached at the C₅-position of MIC ligand L1. To ascertain whether this is the preferred coordination mode of the ligand under consideration (L1) or not, we then attempted the synthesis of an analogous Au^I-complex (Scheme 1a). We first proceeded with the most common transmetalation strategy via Ag^I–NHC complex formation using Ag₂O. To our surprise, formation of a mixture of C₄- and C₅-ylidene coordinated Au^I-complexes¹⁵ was observed even at room temperature, which could be due to the formation of both C₄– and C₅–ylidene coordinated Ag^I-complexes having similar stability and/or comparable carbene transfer efficiency.¹² This observation clearly suggests that both the backbone protons (C₄/C₅–H) are susceptible towards deprotonation. However, a C₅–ylidene coordinated Au^I-complex 2 was exclusively obtained in good yield (80%) via a Cs₂CO₃ assisted metalation strategy and the multinuclear NMR data along with the ¹H–¹H NOESY spectrum (Fig. S10†) confirmed the coordination of Au^I at the C₅-position of L1. This conclusion was established by the molecular structure determination via X-ray crystallography (Scheme 1b).

After studying the coordination behavior of the simple MIC ligand L1, we started investigating the possibilities of selective triazolium backbone activation. In this direction, as per our postulated electronic modulation, we designed an unsymmetrical bis-azolium salt [L2-H₂]Br₂, containing an imidazolium group along with a 1,2,3-triazolium moiety, the precursor for a biscarbene ligand. [L2-H₂]Br₂ was synthesized as an air stable white powder in excellent yield (94%) following the multistep procedure as detailed in Scheme 2. The ¹H NMR analysis reveals the most downfield shifted imidazolium N–CH–N proton resonance at δ = 10.20 ppm, whereas the two triazolium backbone N–CH–C protons were observed at δ = 9.79 (C₅–H) and 9.37 (C₄–H) ppm (confirmed by 2D NMR spectroscopy).

Scheme 2 (a) Synthesis of bisazolium salt, [L2-H₂]Br₂: (i) NaNO₂, HCl/H₂O (10% solution), 0 °C, 1 h; (ii) NaN₃, 0 °C–RT, 12 h; (iii) vinyl acetate, reflux, 24 h; (iv) imidazole, K₂CO₃, CuO, DMSO, 150 °C, 48 h; (v) EtBr, DMF, reflux, 24 h. (b) Molecular structure of [L2-H₂]Br₂ with ellipsoids at a 50% probability level. Hydrogen atoms except H5, H12, and H13, counterions, solvent of crystallization and Me moieties of the N–Et groups are omitted for clarity.

It is worth mentioning that the difference in chemical shifts of the triazolium backbone protons became more prominent after the installation of the imidazolium moiety in [L2-H₂]Br₂ as compared to that in [L1-H]Br. This indicates some electronic influence of the installed imidazolium moiety on the triazolium unit, which would probably be beneficial for the selective activation of its backbone protons. Moreover, the relatively higher acidity of the imidazolium N–CH–N proton than that of the triazolium ones implies that the first metalation should happen preferably at the imidazolylidene site, which would provide us with a unique opportunity to fine tune the electronics of the triazolium moiety via first metalation. Similar behavior was observed previously with a related unsymmetrical bis-azolium salt and was attributed to the difference in acidities^16a of the azolium moieties.^16b Finally, the structure of [L2-H₂]Br₂ was confirmed by X-ray crystallographic analysis (Scheme 2b).

With the well characterized bis-azolium salt [L2-H₂]Br₂ in hand, we proceeded to study its metalation behavior. Initially [L2-H₂]Br₂ was treated with [Pd(CH₃CN)₂Cl₂] in the presence of Cs₂CO₃ under the conditions shown in Scheme 3a. The NMR analyses confirmed the formation of complex 3via coordination of a Pd^II-center to the imidazolylidene moiety, suggested by the absence of the most downfield shifted proton in the ¹H NMR spectrum as well as the Pd^II-bound ImNHC ¹³C{¹H} NMR signal at δ = 150.2 ppm.^17c,d The chemical shift values of the backbone protons (δ = 9.92 (C₄–H) and 10.24 (C₅–H) ppm) were assigned from the NOESY spectrum (Fig. S15†). Finally, the X-ray crystallographic analysis confirmed the coordination of Pd^II to the imidazolylidene donor (Scheme 3b). Before proceeding towards the synthesis of bimetallic complexes from 3, we also synthesized the Ir^III-complex of [L2-H₂]Br₂, 4a/a′ in good yields of 78–80%. The absence of the imidazolium proton of [L2-H₂]Br₂ and the observed integration of 3 instead of 4 aryl protons in the ¹H NMR spectrum along with the ¹³C{¹H} NMR signal for the Ir^III-ImNHC at δ = 165.8 ppm^17a,b suggest the attachment of Ir^III to the imidazolylidene donor and the phenyl ring in an orthometalated fashion in complex 4a. This is further substantiated by the diastereotopic nature of the imidazolium N–CH₂ protons (two multiplets of one proton intensity at δ = 4.24 and 4.40 ppm). The X-ray crystallographic analysis of a single crystal of 4a′ establishes the structure of the monoiridium complex (Scheme 3b) as concluded from NMR analyses. Interestingly, the 2D-NOESY (Fig. 2) spectrum of 4a reveals that the triazolium C₄–H is significantly downfield shifted (δ = 9.86 ppm) compared to C₅–H (δ = 9.05 ppm).

Scheme 3 (a) Synthesis of palladium (3), iridium (4a and a′), and rhodium (4b) complexes: (i) [Pd(CH₃CN)₂Cl₂], Cs₂CO₃, KBr, CH₃CN/pyridine, RT, 12 h. (ii) [M(Cp*)Cl₂]₂ (M = Ir/Rh), NaOAc, K₂CO₃/Cs₂CO₃, KBr (for 4a and b) or KI (for 4a′), CH₃CN, 75 °C, 24 h. (b) Molecular structures of 3 and 4a′ with ellipsoids at 50% probability level. Hydrogen atoms except H15/H16 in 3 and H20/H21 in 4a′, counterions, and the Me groups of N–Et moieties are omitted for clarity. Pyridine and Cp* moieties are shown in capped stick.

Fig. 2 2D-NOESY NMR spectrum of 4a showing the interaction of triazolium C₄–H with the N–CH₂ protons of the ethyl group.

In order to confirm this switching of triazolium backbone proton chemical shifts upon orthometalation, we also synthesized the analogous Rh^III-complex, 4b (Scheme 3). The NMR analysis confirmed that the Rh^III-center also coordinates to the ImNHC in a similar way to Ir^III and importantly, has a comparable influence on the triazolium proton chemical shifts (δ = 9.83 and 9.09 ppm for C₄–H and C₅–H, respectively), in sharp contrast to that observed in the case of the non-orthometalated Pd^II-complex 3. This may be attributed to the planar orthometalated Rh^III/Ir^III-center, which possibly attracts some electron density from the triazolium ring making the C₄–H more acidic than C₅–H.

All the above findings from the 1^st metalation of ligand [L2-H₂]Br₂ ascertain that the nature of metal coordination offers substantial electronic influence on the triazolium moiety and thus, governs the chemical shifts, which would essentially control the activity of its backbone protons. It is in line with previous observations by several research groups that the coordination mode of a metal centre and its orientation after metalation strongly influence the site-selective C–H activation due to some electronic effect in the resulting system.¹⁸

With this definite idea about the electronic effect of the first metal coordination, we focused on the synthesis of the corresponding bimetallic complexes from 3 and 4. First, the monopalladium complex, 3 was reacted with [Pd(CH₃CN)₂Cl₂] (Scheme 4a) and the product was isolated in good yield (85%). The NMR spectroscopic analyses provided primary evidence for the homobimetallic complex (5) formation. First of all, the ¹³C{¹H} resonances at δ = 150.6 and 140.4 ppm, concluded to be Pd^II-ImNHC and Pd^II-MIC, respectively from the HMBC spectrum (Fig. S31†), confirm the attachment of two Pd^II-centers to ligand L2, which was also supported by the ESI-mass analysis. Furthermore, a resonance at δ = 7.74 ppm in the ¹H NMR spectrum was assigned to the triazole backbone C₄–H based on the NOESY spectrum (Fig. S32†), indicating the Pd^II-coordination at the triazolylidene C₅-position. Finally, the single crystal X-ray crystallographic analysis confirmed the coordination of the second Pd^II-center to the C₅-position of 1,2,3-tzNHC (Scheme 4b), which was observed to have a more downfield shifted proton in 3 as per the detailed NMR analyses. To validate this further, we also synthesized another heterobimetallic (Pd^II–Au^I) complex 6 from 3 (Scheme 4) and the ¹H NMR spectrum of the obtained complex suggested the formation of the expected complex. Further support for the formation of the heterobimetallic complex, 6 was obtained from the ESI-MS analysis, exhibiting the most intense peak at m/z 729.8502 for [M–Cl–py]⁺ (calcd. m/z = 729.8540) with the isotopic patterns matching perfectly. Coordination of Au^I to the 1,2,3-tzNHC C₅-position was finally established via single crystal X-ray crystallographic analysis of 6. All the above results reinforce that the more downfield shifted triazolium proton is activated during the second metalation of a Pd^II–NHC complex (3) of [L2-H₂]Br₂.

Scheme 4 (a) Synthesis of the bimetallic complexes 5 and 6 from 3: (i) [Pd(CH₃CN)₂Cl₂], Cs₂CO₃, KBr, CH₃CN/pyridine, 60 °C, 24 h; (ii) Ag₂O, DCM, RT, 12 h; (iii) [Au(SMe₂)Cl], RT, 12 h. (b) Molecular structures of 5 and 6 with ellipsoids at a 50% probability level. Hydrogen atoms except H3 in 5, H15 in 6, and the Me groups of N–Et moieties are omitted for clarity. Pyridine groups are shown in capped stick.

Keeping this in mind, we proceeded with the second metalation of the orthometalated Ir^III-complex 4a with the anticipated activation of the triazolium C₄–H. In this direction, complex 4a was reacted with [Pd(CH₃CN)₂Cl₂] using Cs₂CO₃ as the base in the CH₃CN/pyridine solvent mixture and the desired Ir^III–Pd^II bimetallic complex 7 was obtained in 64% yield (Scheme 5a). The ¹H NMR spectrum of 7 unveils that one of the triazolium protons of the precursor complex 4a is missing as expected and the 2D NMR data confirmed the Pd^II-coordination to the C₄- instead of the C₅-position. This establishes the deprotonation of carbon having a more downfield shifted proton in 4a during sequential metalation and notably, the C₅–H resonance is upfield shifted to δ = 7.93 ppm in 7 from 9.05 ppm in 4a. Furthermore, the attachment of Pd^II to the mesoionic carbene was supported by the ¹³C{¹H} NMR carbene signal at δ = 136.3 ppm, which was upfield shifted compared to the corresponding Pd^II-bound C₅-MIC signal at 140.4 ppm in 5. Eventually, the X-ray crystallographic studies authenticate the attachment of Pd^II to the triazole C₄ position (Scheme 4b).

Scheme 5 (a) Synthesis of the heterobimetallic complexes 7–9: (i) [Pd(CH₃CN)₂Cl₂], Cs₂CO₃, KBr, CH₃CN/pyridine, 70 °C, 24 h. (ii) Ag₂O, DCM, RT, 12 h and then [Au(SMe₂)Cl], RT, 12 h. (b) Molecular structures of 7 and 8 with ellipsoids at a 50% probability level. Hydrogen atoms except H20 in 7 and H21 in 8, and the Me moieties of N–Et groups are omitted for clarity. Pyridine and Cp* moieties are shown in capped stick.

In order to affirm the activation of C₄- rather than the C₅-position of the triazolium moiety during the second metalation of the monoiridium complex 4a, we further synthesized the related Au^I complex, 8 in 61% yield, following the transmetalation procedure (Scheme 5a). The HMBC spectrum, displaying a correlation of the Au^I-MIC ¹³C{¹H} resonance with the Tz–N–CH₂ resonance (Fig. S41†), provides strong evidence for Au^I-coordination at the triazolylidene C₄ position which was ultimately established by the X-ray diffraction analysis (Scheme 5b). Furthermore, to prove the generality of this finding, we also utilized the mono-Rh^III-complex, 4b for the synthesis of a Rh^III–Pd^II bimetallic complex 9 following a similar procedure used for the synthesis of complex 7 (Scheme 5). Multinuclear NMR spectroscopic along with mass spectrometric data analyses reveal the formation of the expected heterobimetallic complex, 9. Moreover, the HMBC NMR spectrum (Fig. S45†) confirms the coordination of the Pd^II-center to the triazole C₄ position by exhibiting a correlation between the Pd^II bound tzNHC ¹³C{¹H} signal at δ = 136.5 ppm and the triazole Tz–N–CH₂ protons at δ = 5.09 ppm, as observed in the case of the complex 7. All of the above results establish that the orthometalated coordination of the Ir^III/Rh^III-center has a distinct electronic impact on the triazolium moiety in 4a and b which plays a crucial role in achieving the selective activation of the triazole backbone C₄–H proton during the second metalation. It is worth mentioning that the complexes 7–9 represent the first ever isolated metal complexes of a C₅-unprotected 1,2,3-triazol-4-ylidene donor.

More insight into the contrasting influence of the 1^st Pd^II- and Ir^III-metal centres on the second metalation was obtained from the %V_bur analysis and DFT calculations. The %V_bur calculated at the triazole C₅ position of the monometallic complexes, 3 and 4a′ was found to be essentially similar (50.1 vs. 50.4, calculated with a sphere radius of 3.5 Å, see the ESI†). This observation clearly suggests that the selectivity observed during the formation of complexes 5/6 from 3 and 7/8/9 from 4 is primarily controlled by electronic factors. Furthermore, the difference of DFT calculated ground state energy between the representative bimetallic systems 5 with its C₄-analogue, 5′ and 7 with its C₅-analouge, 7′ was noted to be significant (0.7 kcal mol⁻¹ and 1.4 kcal mol⁻¹, respectively, see the ESI†) and is in favour of the experimentally observed regioisomers. Thus, DFT calculations also endorse the isolation of C₅- and C₄-bound bimetallic systems (5 and 7, respectively) based on the nature of the 1^st metal coordination to L2.

Conclusions

In conclusion, we have uncovered an unprecedented electronic influence of the 1^st metal coordination on altering the reactivity/metalation behavior of the C₄/C₅-unprotected 1,2,3-triazolium moiety of a bis-azolium salt, [L2-H₂]Br₂. Importantly, the impact of monometalation in a non-chelated (complex 3) or chelated (complexes 4a and b) fashion on the triazole backbone (C₄/C₅) protons could be ascertained easily by 2D NMR analysis. Crucially, these changes in the electronic nature of triazole protons assisted us to access the first ever selective metalation at either the C₄- or C₅-position of 1,2,3-triazolylidene as undoubtedly supported by the synthesis of bimetallic complexes 5–9. Furthermore, the %V_bur calculations suggest that the observed selectivity is primarily controlled by the electronic nature of the first metal coordination. Moreover, the DFT studies of selected complexes (5 and 7) strongly support the exclusive formation of a particular regioisomer as observed experimentally. The tandem catalytic activity studies of the synthesized complexes are underway in our laboratory.

Data availability

Experimental data including experimental procedures and characterization data (NMR, ESI-MS, and single crystal X-ray); the NMR and ESI-MS spectra of the new compounds; detailed computational studies are available in the ESI.†

Author contributions

A. R. conceived and designed the project. P. M. I. and C. S. T. performed the experiments. P. M. I. and A. R. wrote the manuscript and all authors approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully acknowledge SERB, India (Grant No. CRG/2020/000780) and IITM (IRDA) for financial support and the department of chemistry/SAIF, IIT Madras for the instrumental facility. P. M. I and C. S. T thank IIT Madras for a research fellowship. We also thank Dr P. K. S. Antharjanam and Mr Shashi Kumar for X-ray structure analyses and theoretical calculations, respectively.

Notes and references

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15. The formation of a mixture of C₄/C₅-bound AuI-MIC complexes (in a 0.45:0.55 ratio), obtained by transmetallation of the in situ generated Ag-carbene complex of L1, was confirmed from the multinuclear (Fig. S46 and S47†) and 2D (HMBC) NMR data (Fig. S48†).
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Footnote

† Electronic supplementary information (ESI) available: Experimental details, characterization data, and NMR and mass spectra of the synthesized compounds. CCDC 2176097–2176105. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d2sc05024b

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